QT*A^ nniiiiiiwiiiiiiiiiiMiHiiMiiilliMliiiM^ Ill ■ Digitized by the Internet Archive in 2010 with funding from Open Knowledge Commons (for the Medical Heritage Library project) http://www.archive.org/details/principlesofhuma1915star PRINCIPLES OF HUMAN PHYSIOLOGY PRINCIPLES OF HUMAN PHYSIOLOGY BY ERNEST H. STARLING M.D. (Lond.), F.R.C.P., F.R.S., Hon. M.D. (Breslau) Hon. Sc. D. (Cambridge and Dublin) Jodrell Professor of Physiology in University College, London SECOND EDITION With 566 Illustrations, 10 in Colour PHILADELPHIA LEA & FEBIGER 706 SANSOM STREET 1915 Printed in Great Britain PREFACE TO THE SECOND EDITION During the short time that has elapsed since the pubhcation of the first edition of this work physiology has made some considerable advances, and I have found it necessary to re-write much of the sections dealing with ^'oluntary muscle and with the circulation, in addition to making many modifications in other parts of the work. New sections have also been intro- duced dealing with, the nutrition of the brain and Tvath the innervation of the bronchi. I am much indebted to many friends, knowni and unkno\M^i, who have pointed out mistakes and omissions in the first edition. I shall be glad to receive any suggestions as to points in which this text-book may be made more useful to students. UxivERsnY College, Loxdon PREFACE TO THE FIRST EDITION Physiology, though deaUng with the pheuomena of hving organisms, has to use the same tools, whether material or intellectual, as the sciences of physics and chemistry. Any advances which are made in these sciences not only increase our powers of attack upon physiological problems but at the same time alter the intellectual standpoint from which we view them. On the other hand, the investigation of the phenomena of hving beings is continually attracting our attention and that of workers in the other branches of science to unexplored regions in physics and chemistry. This mutual stimulation and co-operation among the different sciences have as their result a continual modification of our attitude with regard to the fundamental problems of physiology. The present time has seemed to me, therefore, fitting for the production of a textbook which, while not neglecting the data of physiolog}% should lay special stress on the significance of these data, and attempt to weave them into a fabric representing the principles which are guiding physiologists and physicians of the present day in their endeavours to extend the bounds of the known and to increase their powers of control over the functions of living organisms. In a science such as physiology, based on so wide a discipline and with so diverse a technique, it is almost impossible for any one man to attain to a personal acquaintance with all its branches. In the present book I have therefore not hesitated to avail myseK of the work of masters of the science in fields which I had not myself explored. Thus, in the physiology of the nervous system, which has been transformed and built up on a new basis by the researches of Sherrington, I have endeavoured to follow this author as closely as possible. I am also deeply sensible of my obhgations to the writings of Tigerstedt, Leathes, and Lusk on general metabolism, of Abderhalden and Plimmer on physiological chemistry, of Bayliss on general physiology, as weU as to various authors of articles in the " Ergebnisse der Physiologic," in Nagel's " Handbuch der Physiologic,'' and in Dr. L. E. Hill's " Recent and Further Advances in Physiology," Although I have endeavom-ed to confine my demands on the previous knowledge of the student within the narrowest possible hmits, I should recommend him in every case to read some primer on physiology in order to obtain a bird's eye view of the subject before beginning the study of this work. He will then be able to vary the order of chapters in this book according to the part of physiology which he is hearing about in his lectures or working at in his practical classes. Those of my readers who are entirely unacquainted with physiology might do well on a first perusal to omit Book I., dealing with the general concepts of the science. viii PREFACE I have deemed it a hopeless and indeed a useless task to give any full account of the multifarious methods employed in the experimental investiga- tion of the different organs, of the body. In most cases I have consigned to small type a description of one or two typical methods, which would suffice to show how the questions may be approached from the experimental side. Throughout the work I have sought to show that the only foundation for rational therapeutics is the proper understanding of the working of the healthy body. Until we know more about the physiology of nutrition, Cjuacks will thrive and food faddists abound. Ignorance of physiology tends to make a medical man as credulous as his patients and almost as easily beguiled by the specious puffings of the advertising druggist. I trust, therefore, that the following pages will be found of value not only to the candidate for a university degree but also to the practitioner of medicine in equipping him for his struggle against the factors of disease. In the selection of diagrams for the illustration of this book I am especi- ally indebted to Professor Schafer and to his publishers, Messrs. Longmans, for the permission to make use of a large number from Quain's " Anatomy " and from Schafer's " Essentials of Histology." I must also express my obligation to Professor Wilson for the use of certain figures from his admirable work on the cell, to the publishers of Cunningham's " Anatomy," and to many physiological friends, especially to Dr. Mott and Dr. Gordon Holmes, for the use of original diagrams. The index was kindly made for me by Mr. Lovatt Evans. ERNEST H. STARLING University College, London May 1912 C O T^ T E N T S CHAPTER I PAGK INTRODUCTION 1 BOOK I GENERAL PHYSIOLOGY CHAPTER II THE STRUCTURAL BASIS OF THE BODY ].3 CHAPTER III THE MATERIAL BASIS OF THE BODY SKCTION I. The Elementary Constituents of Living Cells 3G II. The Proximate Constituents of the Animal Body 45 III. The Fats 53 IV. The Carbohydrates 59 V. The Proteins 7I VI. The Mechanism of Organic Synthesis 109 CHAPTER IV THE ENERGETIC BASIS OF THE BODY I. The Energy OF Molecules IN Solution 123 11. The Passage of Water and Dissolved Substances across Membranes 131 III. The Properties of Colloids I39 IV. The Mechanism of Chemical Changes in Living Matter. Ferments 153 V. Electrical Changes IN Living Tissues 170 BOOK II THE MECHANISMS OF MOVEMENT AND SENSATION CHAPTER V THE CONTRACTILE TISSUES I. The Structure of Voluntary Muscle 1 77 II. The Excitation of Muscle 185 III. The Mechanical Changes that a Muscle undergoes when it Con- tracts 1 g.jt ix X CONTENTS CHAPTER V (continued) SECTION PAGE IV. The Conditions affecting the Mechanical Response of a Muscle 205 V. The Chemical Chajstges rsr Mhscle 212 VI. The Pkodtjction of Heat in Muscle 219 VII. Electrical Changes in Muscle 224 VIII, The Intimate Natuke op Muscular Contraction 234 IX. Voluntary Contraction 239 X. Other Forms of Contractile Tissue 243 CHAPTER VI NERVE FIBRES (CONDUCTING TISSUES) I. The Structure of Nerve Fibres 250 II. Propagation along Nerve Fibres 253 III. Events accompanying the Passage of a Nervous Impulse 256 IV. Conditions affecting the Passage of a Nervous Impulse 258 V. The Excitation of Nerve Fibres 262 VI. The Conditions which Determine Electrical Stimulation 270 VII. The Neuro-Muscular Junction 275 VIII. Polarisation Phenomena in Nerve 280 IX. The Nature of the Excitatory Process 284 CHAPTER VII THE CENTRAL NERVOUS SYSTEM I. The Evolution and Significance of the Nervous System 288 11. The Nervous System op Vertebrates 297 III. General Characteristics of Reflex Actions 303 IV. Nature op the Connection between Neurons 307 V. Functions of the Nerve-Cell 312 THE SPINAL CORD VI. Structure of the Spinal Cord 315 VII. The Spinal Cord as a Reflex Centre 322 VIII. The Mechanism of Co-ordinated Movements 338 IX. Trophic Functions of the Cord 359 X. The Spinal Cord as a Conductor 351 THE BRAIN XL The Structure of the Brain Stem 361 XIL The Functions of the Brain Stem 392 XIII. The Functions of the Cerebellum 397 XIV. Visual Reflexes 406 XV. Summary op the Connections and Functions of the Cranial Nerves 410 CONTENTS XI THE CEREBRAL HEMISPHERES SEOTION PAQB XVT. General Stkuctural Arrangements of the Cerebrum 416 XVII. The Fitnctions of the Cerebral Hemispheres 434 XVIII. The Nutritive and Vascular Arrangements of the Central Nervous System 461 THE AUTONOMIC SYSTEM XIX. The Visceral or Autonomic Nervous System 466 CHAPTER VIII THE PHYSIOLOGY OF SENSATION I. On the Relation of Sensation to Stimulus 478 II. Cutaneous Sensations 486 III. Sensations of Smell and Taste 498 IV. Auditory Sensations 505 V. Voice and Speech 520 VISION VI. Dioptric Mechanisms of the Eyeball 528 VII. The Retinal Changes involved in Vision 558 VIII. Visual Sensations 568 IX. Movements of the Eyeballs 585 X. Visual Judgments 589 XL The Nutrition of the Eyeball 694 THE ORGANIC SENSATIONS XII. Sensations of Movement and Position 598 XIII. The Labymnthtne Sensations 603 BOOK III THE MECHANISMS OF NUTRITION CHAPTER IX THE EXCHANGES OF MATTER AND ENERGY IN THE BODY (GENERAL METABOLISM) I. Methods employed in Determining the Total Exchanges of the Body 614 II. The Metabolism during Starvation 624 III. The Effect of Food on the Metabolism of the Body 631 IV. The Effect of Muscular Work on Metabolism 637 V. The Significance of the Food-stuffs 642 VI. The Normal Diet of Man 648 Xll CONTENTS III. The Histoey of Fat in the Body IV. The Metabolism of Carbohydrates CHAPTER X THE PHYSIOLOGY OF DIGESTION SECTION PAGE Changes undergone by the Food-stuffs in the Alimentary Canal I. Digestion in the Mouth 661 II. The Passage of Food from the Mouth to the Stomach 676 in. Digestion in the Stomach 683 rv. The Movements of the Stomach ^ 697 V. The Pancreatic Juice 702 VI. The Liver and Bile "7 13 VII. The Intestinal Juice 718 VIII. Functions of the Large Intestine 722 IX. Movements of the Intestines 725 X. The Absorption of the Food-stuffs 733 XL TheF^ces '^^^ CHAPTER XI THE HISTORY OF THE FOOD-STUFFS I. Protein Metabolism 756 11. NucLEiN OR Purine Metabolism 774 783 796 CHAPTER XII THE BLOOD General Characters of the Blood 810 T. The White Blood -Corpuscles ^1"^ 11. The Red Blood-Corpuscles ^1^ III. The Blood-Platelets IV. The Coagulation of the Blood V. The Quantity and Composition of the Blood in Man 854 836 839 CHAPTER XIII THE PHYSIOLOGY OF THE CIRCULATION I. General Features of the Circulation 11. The Blood-Pressure at Different Parts of the Vascular Circuit 874 III. The Velocity of the Blood at Different Parts of the Vascular System IV. The Mechanism of the Heart Pump 868 88() 891 CONTENTS xiii CHAPTER XIT {continued) SECTION PAGE V. The Flow OF Blood THROUGH THE Arteries 918 VI. The Flow of Blood in the Veins 932 VII. The Pulmonary Circulation 935 VIII. The Causation of the Heart-Beat 939 IX. The Nervous Regulation op the Heart 969 X. The Nervous Control of the Blood-Vessels 982 XI. The Effect of Muscular Exercise on the Circulation 1006 XII. The Influence on the Circulation op Variations in the Total Quantity op Blood 1009 CHAPTER XIV LYMPH AND TISSUE FLUIDS 1012 CHAPTER XV THE DEFENCE OF THE ORGANISM AGAINST INFECTION I. The Cellular Mechanisms op Defence 1021 II. The Chemical Mechanisms op Defence 1030 CHAPTER XVI RESPIRATION I. The Mechanics of the Respiratory Movements 1039 II. The Chemistry op Respiration 1051 III. The Regulation of the Respiratory Movements 1076 The Chemical Regulation op the Respiratory Movements 1079 The Reflex Nervous Regulation op Respiration 1089 IV. The Effects on Respiration of Changes in the Air Breathed 1098 V, The Mechanisms of Oxidation in the Tissues 1105 CHAPTER XVII RENAL EXCRETION I. The Composition and Characters op the Urine 1110 IT. The Secretion op Urine 1131 in. The Physiology OF Micturition 1152 CHAPTER XVI 11 THE SKIN AND THE SKIN GLANDS 1161 XIV CONTENTS CHAPTER XIX SECTION PAGE THE TEMPERATURE OF THE BODY AND ITS REGULATION 1167 CHAPTER XX THE DUCTLESS GLANDS 1178 BOOK rv REPRODUCTION CHAPTER XXI THE PHYSIOLOGY OF REPRODUCTION I. The Essential Features of the Sexual Pkocess 1199 11. Development and Hebedity 1212 III. Repkoduction in Man 1217 IV. Peegnancy and Parturition 1230 V. The Secretion and Properties of Milk 1137 INDEX 1247 CHAPTER I INTRODUCTION Physiology in its widest sense signifies the study of the phenomena pre- sented by living organisms, the classification of these phenomena, and the recognition of their sequence and relative significance. Such a range would include many studies which are not generally grouped under the term physiology, and would in fact correspond to the comprehensive science of biology. Thus the study of the relations of living beings to one another and to their surroundings is the special object of the science of cecology. The aims of physiology in its restricted sense are the description, analysis, and classification of the phenomena presented by the isolated organism, the allocation of every function to its appropriate organ, and the study of the conditions and mechanisms which determine each fmiction. The fundamental phenomena of life are essentially identical throughout the whole series of Hving organisms. This continuity of function is the necessary correlation of the continuity of descent, which brings into relation all members of the animal and vegetable kingdoms. No living organism can therefore be regarded as outside the sphere of our investigations. The interest of mankind in this subject was, however, naturally awakened in connection with his o\^al body, and the science, growing up as ancillary and preliminary to medical studies, has always taken man as its chief tjrpe of study. In the present work the elucidation of the functions of man will also be our first concern, and this for two reasons. In the first place, in physiology, as in all other sciences, the motive of man's activity is his social instinct to increase the power of his race in the struggle for existence, by the acquisition of control, either over the external forces of nature, which may be turned to his own benefit, or over the factors, intrinsic and extrinsic, which tend to his enfeeblement or extirpation by disease and death. Consciously or unconsciously, all our researches on physiology, whether on the higher animals or on the lowest protozoa, have the welfare of man as their ultimate object. In the second place, the choice of the higher animals as our chief objects of study receives justification from the fact that whereas morphology, or the science of structure, must proceed from the lowest to the highest organisation, the science of function presents its problems in their simplest form in the most highly dift'erentiated organisms. In the unicellular animal all the essential functions which we associate with living beings are carried out, often simultaneously, in one little speck of proto- plasm. An analysis of these functions, the determination of their conditions 1 1 2 PHYSIOLOGY and mecliamsm, is obviously impossible under such circumstances. It is only when, as in the higher animals, one part of the hving body is difierentiated into an organ which has one function and one function only as the outlet for its acti\dties, that it becomes possible to peer into the details of the function with some chance of discovering its ultimate mechanism. Our especial preoccupation with the physiology of man will not prevent our employment of examples from any part of the animal or vegetable kingdom, when light can be thrown by their study on fundamental physio- logical phenomena common to the whole of the living world. In many cases such a study will enable us to separate the essential features in a process from those which have been added as auxihary, with increasing complexity of the structures concerned. What are the fundamental phenomena which are wrapt up in our con- ception of living beings ? When deahng mth the higher animals, we are inclined to lay greater stress on the phenomena involving a discharge of energy. Thus we should say that a man was alive if his body were warm and if he were presenting spontaneous movements, such as those of respira- tion or of the heart. The life of a man in the ordinary sense of the term is made up of those movements which place him in relationship with his environ- ment. For the production of these movements, as for the maintenance of a constant body-temperature, a continual expenditure of energy is necessary. Experience teaches us that these movements come to an end in the absence of food or of oxygen, and that an increased call on the energies of the body must always be met by a corresponding increase in the air and in the food supplied. Two further processes must therefore be included among those making up our conception of life, viz. the function of assimilation (the taking in and digestion of food), and the function of respiration, in which oxygen is absorbed and carbon dioxide is excreted into the surrounding atmosphere. The substances which make up our food-stuffs are all capable of oxidation. Composed chiefly of carbon and hydrogen, with some oxygen, nitrogen, and sulphur, they yield on complete combustion carbon dioxide, water and small amounts of ammonia or allied bodies, and sulphates. In this process of oxidation there is liberation of heat. In the body a similar oxidation occurs, the products of oxidation being discharged into the surrounding medium. An amount of energy is thus set free which is available for the activities of the living organism. Before we can make any accurate in- vestigations of the conditions which determine these activities, we must know whether the two great laws of chemistry and physics, viz. the conser- vation of mass and the conservation of energy, hold good for the processes within the living body. The many experiments which have been made on this point have decided the question in the affirmative. Thousands of experiments have been made, both on man and on animals, in which the total income of the body, viz. food and oxygen, has been weighed, and compared with the total output, viz. carbon dioxide, water, and bodies allied to ammonia (urea, &c.). In every case complete equality has been INTRODUCTION 3 obtained, and we can be certain that any matter found in the body must have been derived from without. There is no creation or destruction of matter in the body. The determination of the equation in the case of the total energy of the body is rather more difficult. We have, in the first place, to measure the total income and output of the body, and to determine the total heat which would be evolved by the oxidation of the food-stufis taken into the carbon dioxide, water, &c., that are given out. We must then compare the figure so obtained with the actual output of energy by the body. The latter can be measured in terms of heat by placing the animal inside a calorimeter. Many practical difficulties arise in the performance of the experiment, in consequence of the necessity of providing the animal with a constant supply of air to breathe, and of allowing for the continual loss of water by evapora- tion which is going on at the surface of the animal. The first accurate experiments of this nature were made by Rubner. This observer deter- mined by means of the calorimeter the total heat loss of dogs. In the same animals the material income and output of the body were measured, and a calculation was made as to the amount of energy which would be set free in the body by the processes of oxidation involved in the change of material observed. The following Table represents a summary of Rubner's results : Calculated Heat loss deter- Duration Dog. Condition of dog. heat mined calori- of production. mctricaliy. experiment. Cal. Cal. Days. 1. Fasting . 259-3 261-0 5 2. ,, . . . 545-6 528-3 o 3. Fed with meat . 329-9 333-9 1 4. Fed with fat . 302-0 299-1 5 5. Meat and fat 332-1 330-0 12 6. jj jj • 311-6 331-0 8 7. Fed with meat . 375-0 379-5 6 8. " » • • 683-0 681-3 7 It will be seen that the average difference between the calculated and observed results amounts only to 1-01 per cent. — an amazing agreement considering the extreme difficulties of the experimental methods involved. The important deduction to be drawn from these observations is that the food-stuffs which are oxidised in the body develop in this process exactly the same amount of energy as when they are burnt up outside the body. From one aspect, therefore, the animal body may be looked upon as a machine for the transformation of the potential energy of the food-stuff's into kinetic energy, represented by the warmth and movements of the body as well as by other physical changes involved in vital processes. In the living organism we cannot, however, distinguish between the source of energy and the machinery, as we can in the case of our engines. When we endeavour to trace the food-stuffs after their entry into the body, we lose sight of 4 PHYSIOLOGY them at the point where they are built up to form apparently an integral part of the living framework. During activity there is a discharge of the products of oxidation of the food-stuffs from this living matter, which there- fore becomes reduced in mass. This reduction, or disintegration of the living matter, associated with activity, is always followed by a period of increased integration, during which the organism grows by the assimilation of more food. Om' conception of life must therefore involve the idea of a constantly recurring cycle of processes, one of building up, repair, or inte- gration, and the other, associated with activity, of destruction or disinte- gration. If the former process predominates, we obtain a steady increase in the mass of the organism, an increase which we speak of as growth, and in many cases, as in that of plants, it is this power of growth which we take as our criterion of the existence of life. In fact, the possession by the green parts of plants of the power of utiHsing the energies of the sun's rays for the integration of food-stuffs, such as starch, with a high potential energy, is the necessary condition for the existence of all higher forms of hfe on this earth. Closely associated with the property of growth is the power possessed by all living organisms of repair, i.e. the replacement by newly formed healthy Uving material of parts which have been damaged by external events. The process of growth does not, in the individual, proceed indefinitely. At a certain stage in its life every organism divides, and a part or parts of its substance are thrown off to form new individuals, each of them endowed with the same properties as the parent organism, and destined to grow until they are indistinguishable from the organism whence they were derived. In the lowest forms of Hfe, the unicellular organisms, these processes of growth and division may go on until brought to an end by some change in the environment which will not allow the necessary conditions of life, viz. assimilation and disintegration, to proceed. In all the higher forms, how- ever, after the process of reproduction has been completed, the parent organism begins to undergo decay, and the processes of assimilation and repair no longer keep pace with those of destruction, however favourable the environment, until finally death of the organism takes place. All these phenomena, viz. assimilation, respiration, activity associated with the discharge of energy, growth, reproduction, and death itself, are bound up in our conception of life. All have one feature in connnon, viz. they are subject to the statement that every living organism is endowed with the power of adaptation. Adaptation may indeed receive the definition which Herbert Spencer has applied to life — " the continuous adjustment of internal relations to external relations." A living organism may be regarded as a highly unstable chemical system which tends to increase itself continuously under the average of the conditions to which it is subject, but undergoes disintegration as a result of any variation from this average. It is evident that the sole condition for the survival of the organism is that any such act of disintegration shall result in so modifying the relation of the INTRODUCTION 5 system to the environment that it is once more restored to the average in which assimilation can be resumed. Every phase of activity in a hving being must be not only a necessary sequence of some antecedent change in Its environment, but must be so adapted to this change as to tend to its neutralisation, and so to the survival of the organism. This is what is meant by ' adaptation.' Not only does it involve the teleological conception that every normal activity must be for the good of the organism, but it mui^t also apply to all the relations of living beings. It must therefore be the guiding principle, not only in physiology with its special preoccupation with the internal relations of the parts of the organism, but also in the other branches of biology, which treat of the relations of the living animal to its environment, and of the factors which determine its survival in the struggle for existence. The origin of new species and the succession of the different forms of life upon this earth depend on the varying perfection of the mechanisms of adaptation. We may imagine that the first step in the evolution of life was taken during the chaotic chemical interchanges which accompanied the cooling down of the molten mass forming the earth, when some compound was formed, probably with absorption of heat, endowed with the property of continuous polymerisation and growth at the expense of surrounding material. Such a substance could continue to exist only at the expense of the energy derived from the surrounding medium, and would undergo destruction with any stormy change in its environment. Out of the many such compounds which might have come into being, only such would survive in which the process of exothermic disintegration tended towards a condition of greater stability, so that the process would come to an end spontaneously, and the organism or compound be enabled to await the more favourable conditions necessary for the continuance of its growth. With the con- tinued cooling of the earth, the new production of endothermic compounds would become rarer and rarer ; and in all probability the beginning of life, as we know it, was the formation of some complex substance, analogous to the present chlorophyll corpuscles, with the power of absorbing the newly penetrating sun's rays and utilising them for the endothermic forma- tion of further unstable compoimds. Once given an unstable system, such as we have imagined, the great principle laid down by Darwin, viz. survival of the fittest, will suffice to account for the production from it by evolution of the ever-increasing variety of living beings which have appeared in the later history of this globe. The ' adaptation,' i.e. the reactions of the primitive living material to changes in its environment, must become ever more and more complex, since only by means of increasing variety of reaction is it possible to provide for the stability of the system within greater and greater range of external conditions. The difference between higher and lower forms is therefore one of complexity of reaction, or of range of adaptation. In all the physiological processes which we shall study in the course of this work, adaptation will be found the constant and guiding quality. The 6 PHYSIOLOGY naked protoplasm of the plasmodium of Myxomycetes, if placed on a piece of wet blotting-paper, will crawl towards an infusion of dead leaves, or away from a solution of quinine. It is the same property of adaptation, the deciding factor in the struggle for existence, which impels the greatest thinkers of our time to spend long years of toil in the invention of the means for the offence and defence of their community, or for the protection of man- kind against disease and death. The same law which determines the down- ward growth of the root in plants is responsible for the existence to-day of all the sciences of which mankind is proud. This " adjustment of internal to external relations " is possible, however, only within strictly defined limits, limits which increase in extent with rise in the type of organism, and in the complexity of its powers of reaction. Some of these limiting conditions we shall have to study in the next chapter. Among the chief of them are temperature, and the presence of food material and of oxygen. At the present time the limits of temperature may be placed between 0° and 50° C, Many organisms, however, are killed by the alteration of only a few degrees in the temperature of their environment. Every shifting of a cold or warm current in the Atlantic, in consequence of storms on the surface, leads to the destruction of myriads of fish and other denizens of the sea. In the higher animals a greater stability in face of such changes has been accomplished by the development of a heat-regulating mechanism, so that, provided sufficient food is available, the temperature of the body is maintained at a constant level, which represents the optimum for the discharge of the normal functions of the constituent parts of the body. The presence of food material in the environment of the living organism is a necessary condition for its continued existence. In some cases, and this we must assume to be the primitive condition, the food material must be of a given character and form a constant constituent of the sur- rounding medium. In the higher forms, the development of a complex digestive system has enabled the organism to utilise many different kinds of food, while the storage of any excess of food as reserve material, either in the form of fats or carbohydrates, provides for a constant supply of food to the constituent cells of the body, even when it is quite wanting in the environment. Since plants depend for their food in the first place on the carbohydrates produced within the chlorophyll corpuscles out of the atmospheric carbon dioxide by the energy of the sun's rays, necessary conditions for their existence will be sunlight and the presence of this gas in the surrounding atmosphere. One other necessary condition for the existence of life is the presence of water. Although this substance cannot furnish any energy to the complex molecules of which the living matter is composed, it is an essential con- stituent of all living matter, and takes part in all the changes which deter- mine the transformations of matter and energy in the organism. This short summary of the chief characteristics of living beings would be incomplete without the mention of what is perhaps their distinctive feature, namely, organisation. Although little marked in the lowest members INTRODUCTION 7 of the living kingdom, where we can detect only a speck of structureless material containing a few granules, of which one or more, in consequence of its reaction to stains, is distinguished by the name of a nucleus, in the higher members this organisation becomes more and more marked. This increased complexity of organisation, which we often speak of as histological differentiation, runs parallel with increasing range of power of adaptation, and with increasing efficiency of adaptive reactions attained by the setting apart of special structures (organs) for the performance of definite functions. This parallelism between the development of function and structure justifies us in the assumption generally, though often only tacitly, made by physio- logists, that the structure is the determining factor for the fimction. We might regard the histological differentiation as representing merely a con- tinuation of the increasing molecular complexity, which we assumed nmst accompany and determine every widening in the range of the adaptive power of the organism. To sum up : — our objects in the study of physiology include the descrip- tion of the chief reactions of the body to changes in its environment, the analysis of these reactions into the simpler reactions of which they are made up, and the assignment to each differentiated structure of the organism its part in every reaction. We must determine the conditions under which each reaction takes place, so that we may learn to evoke any part of it at will by application of the appropriate stimulus, i.e. by effective change of environment. A reaction involves expenditure of energy, and this can be derived only from chemical change in the reacting organ, and ultimately from the dis- integration or oxidation of the food-stuffs. Our next task must be, there- fore, the analysis of the energetic and material changes, with a view to determining the whole sequence of events, from the occurrence of the external exciting change to the finished reaction, which will alter in the direction of protection the relation of the organism to its environment. In short, it is the office of physiology to discover the routine sequence of events in the living organism under all manner of conditions. In attacking this problem our methods cannot differ fundamentally from those of the physicist and chemist. In every case our experiments will consist in the observation and measurement of movements of one kind or another which we shall interpret in terms of mass or energy. Physiology, if it could be completed, would therefore describe the how of every process in the body. It would state the sequence of events and would summarise these as so-called ' laws.' These laws would, however, no more explain the phenomena of life than does the ' law of gravitation ' explain the fact that two masses tend to move towards one another with uniform acceleration. Nor can we hope to explain physiological phenomena by reference to the laws of physics and chemistry, since these themselves are only expressions of sequences, and not explanations. With every growth in science, however, its generalisa- tions become wider and its laws summarise ever more extensive groups of phenomena. We have no reason for asserting that, in the course of research, 8 PHYSIOLOGY we may not finally succeed in describing vital phenomena in the '' conceptual shorthand " * used by the physicist, involving his ideal world of ether, atom, and molecule. At present we are far from such a consurmnation. The principle of adaptation is the only formula which will include all the pheno- mena of Hving beings, and it is difficult to see how this principle can be expressed by means of the concepts of the physicist. This diflB.culty, which must be felt with greater force the more deeply the physiologist endeavours to peer into the processes within the living cells, has led some, even at the present day, to the assumption of some special quality in hving organisms which is designated as ' vital force ' or ' vital activity.' Such views are classified together under the term vitalism. From his beginning man has been accustomed to draw a sharp line of dis- tinction between those phenomena which by their constant occurrence seemed to him natural, and therefore explicable, and those phenomena of which he could not see the determining antecedent, and which were to him, therefore, anomic and capricious. To the latter he set up graven images, and not perceiving his own springs of action, endowed them with a self-determining personahty such as he imagined himself to possess. This procedure, though possessing certain advantages in allowing him to perform his common duties free from the ever-lurking fear of supernatural interference, suffered from the great drawback that it fenced off unknown phenomena as unknowable and not to be known. It has therefore acted as a continual check on the growth of man's knowledge and control of his environment. Such a graven image is vitalism. As a working hypothesis it must be sterile. Just as the hypothesis of special creation would impede all research into the relationships of animals and plants, so vitalism would stay the hand of the physiologist in his endeavours to determine the changes which occur within the living organism. In many cases, however, the terms ' vitalism ' and its antithesis ' mechanism ' are used unjustifiably. The production of energy within the body is due to the oxidation of Mie food-stuffs. In certain functions it is not yet fully established whether the changes involved take place at the expense of the energy, hydrostatic pressure or otherwise, of the fluids outside the cells, or whether energy is supplied to the process by the cells themselves at the expense of oxidative or other changes occurring in their living sub- stance. Both views are possible, but the adoption of either by a physiologist does not justify the statement that he is a ' vitaHst,' ' neo-vitalist,' or ' mechanist.' The office of the physiologist is the determination of the changes which occur in the living body and the establishment of the causal nexus {i.e. the routine of sequences) between them. For such a man to describe himself as a vitalist or mechanist is as germane to the subject as if he were to call himself a Trinitarian or a Plymouth Brother. Throughout this chapter we have assumed no necessary dividing line between the different classes of phenomena in the conceptual universe, although in the present state of our knowledge we are far from being able to include the whole of them under the same general laws. It might be * Karl Pearson, "Grammar of Science," p. 328 et seq. (2nd ed.) INTRODUCTION 9 objected that in taking up this attitude we had left out of account one supreme fact, viz. the existence of consciousness in ourselves. As a com- parative and objective study, however, physiology is concerned, not with the study of consciousness, but with the conceptions in consciousness of the fhenomena presented by living beings. Consciousness, in fact, we know only in ourselves. From the actions of other living beings similarly organised, we infer in them the existence of a similar consciousness. Again, from the fact that the reactions of the higher mammals are evidently determined, not by immediate impressions, but largely by stored-up impressions of past stimuli we credit them also with a certain but lower degree of consciousness. As we descend the scale of animal Hfe, evidence of the existence of consciousness, as we know it, rapidly diminishes and finally disappears, though it is im- possible to draw a sharp line between those animals which possess conscious- ness and those in which it is absent. That it is a necessary accompaniment of life is certainly not the case. A man is living though he is asleep, anaes- thetised, or stunned, and it would be absurd to speak of the consciousness of a cabbage. Consciousness is, in fact, comiected with the possession of a highly developed central nervous system, and its activity is in proportion to the complexity of this system. Since the brain with all the other organs of the body is derived from a simple cell, the fertilised ovum, similar in its absence of differentiation to the lowest organisms, it might be argued that all types of life are endowed with something which is not consciousness, but which has the potentiality of developing into consciousness. To such a hypothetical property Lloyd Morgan has given the name ' metakinesis.' We have, however, no means of judging of the presence or absence of this hypo- thetical quality and still less of determining whether it is a property only of living substance, or is shared also by the atoms of so-called dead material. BOOK I GENERAL PHYSIOLOGY CHAPTER II THE STRUCTURAL BASIS OF THE BODY THE CELL All the liigher animals and plants, when submitted to microscopic examina- tion, are seen to consist of structural units which are spoken of as cells. In each organ we find a mass of these cells closely resembhng one another in all respects, and we may therefore regard the function of any organ as the sum of the functions of its constituent cells. Indeed, any given reaction of the whole body is the resultant of the reactions of the unlike cells of which the body is composed. The cell is therefore the physiological as well as the structural unit, and it is necessary to commence our study of the functions of the animal body with some consideration of the functions and reactions which are common to all the structural units. This composite structure is pecuUar to the higher forms of life. Amongst the lower forms, both animal and vegetable, an immense number of organisms consist only of a single cell. In this cell are represented all the phenomena of life, all the adapted reactions which we associate with the life of the higher organisms. That the unicellular condition represents the more primitive stage from which the higher organisms have been evolved in the course of ages is indicated by the fact that every one of these higher organisms in the course of its development passes through a unicellular stage, namely, the fertilised ovum. We may assume that the series of changes attending the development of the higher organism from the egg is a repetition in sum- mary of the changes which have determined the evolution of the species from the primitive unicellidar type.* The general characteristics of the cell present important similarities, whether we are considering a cell which forms the whole of an organism or a cell which is but an infinitesimal part of a highly developed animal. The name ' cell ' was first applied by botanists to the structural units found by them in plant tissues, and involved therefore the idea of certain qualities which do not enter into our present conception of the term. A section through the stem of a growing plant shows it to be made up of an aggregation of cells in the etymological sense of the word, i.e. small sacs bounded by a wall of cellulose and containing cell sap. Immediately inside the cellulose wall is a thin layer, the primordial utricle, which encloses at one point a spherical or oval structure known as the nucleus. If the section be * This assumption is often spoken of as the ' law of rc-cai)itulation.' 13 14 PHYSIOLOGY taken from the growing tip of a plant (Fig. 1), the cell sap is found to be wanting and the cells consist only of the substance known as protoplasm, which later on will form the primordial utricle. This with a nucleus is enclosed in a dehcate cellulose wall. The wall is not an essential constituent, since it is absent from many vegetable cells at some period of their hfe and from animal cells generally. A better conception of the essentials of a cell can be obtained by the study Fig. 1. General view of cells in the growing root-tip of the onion, from a longitudinal section, enlarged 800 diameters. (Wilson.) a, non-dividing cells, with chromatin-network and deeply stained nucleoli ; h, nuclei preparing for division (spireme-stage) ; c, dividing cells showing mitotic figures ; e, pair of daughter-cells shortly after division. of a unicellular animal such as an amoeba (Fig. 2). This is an organism frequenting stagnant pools, of varying size (from 0-1 to 0-3 mm. in diameter), apparently of a semi-fluid consistence. When first examined it is generally spherical, but in a short time begins to change its form, putting out processes known as pseudopodia. By shifting the distribution of its material among these processes, it is able to move about and also to ingest particles of food or pigment with which it may come in contact. Near its centre a differentiated portion can be distinguished which is known as the nucleus. The rest of the amoeba, the protoplasm or cytoplasm, often presents further differentiation into an outer clear layer and an inner finely granular substance. The latter may contain coarser granules, some of food material, others apparently formed in situ by the surrounding protoplasm, and often small vacuoles (' contractile vacuoles ') which are continually altering their size and serve to keep up a circulation of fluid in the interstices of the cytoplasm. In all cells, whether animal or vegetable, with which we are acquainted, this THE STRUCTURAL BASLS OF THE BODY 15 twofold structure is also found. So we may define a cell as a small mass of protoplasm containing a nucleus. Doubt has often been expressed whether a nucleus is to be regarded as essential to our conception of a cell. In many of the lowest forms of animals and plants, such as the Flagellata among the former and the Cyanophycete and Bacteria among the latter, no distinct nucleus can be demonstrated. In many of these forms the dimen- sions of the whole organism are too minute to allow of any definite statement being ■''"^■-'■-^'^•'•''v'^'^.o^'i-; ",> Fig. 2. Amoeba proteus, an animal consisting of a single naked cell, X 280 (From Sedg- wick and Wilson's Biology.) n, the nucleus ; ivv, water- vacuoles ; cv, contractile vacuole ; fv, food-vacuolc. made as to the presence or absence of nuclear material. In the larger of them, how- ever, the cytoplasm of the cell contains numerous scattered granules which stain with dyes in the same way as do the nuclei of the cells of higher animals, and these granules possess the resistance to the action of certain digestive fluids which is typical of nuclei. They may therefore be taken as representing the nucleus in the higher forms. Even in the latter, at certain stages, namely, during the divisicn of the cell, the nucleus breaks up into discrete parts, and there is no reason for believing that such a scattered condition of the nuclear material may not last throughout the whole life of the cell. We have defined a cell as a small mass of protoplasm containing a nucleus. Since we shall have to use the term ' protoplasm ' on many occasions in the course of this work, we must have a definite conception of what we mean by it. The term is often used by histologists as implying a substance of certain definite chemical and staining characters. When employed by physiologists it general!/ implies any material which we can, on a study of its behaviour to changes in its environment, regard as endowed with life. Huxley has defined it as " the physical basis of life." Though it may be con- venient to have a word such as protoplasm signifying simply 'living material,' it is important to remember that there is no such thing as a single substance — protoplasm. The reactions of every cell as well as its organisation are the resultant of the molecular structure of the matter of which it is built up. The gross methods of the chemist show him that the composition of the 16 PHYSIOLOGY ' protoplasm ' of the muscle cell is entirely different from that of a leucocyte or white blood corpuscle. The finer methods of the physiologist show him that every sort of cell in the body has its own manner of fife, its own pecu- Harities of reaction to uniform changes in its surroundings. No individual will react in exactly the same manner as another individual, even of the same species, and the reactions of the whole organism are but the sum of the Attraction-sphere enclosing two centrosomes /' Plasmosome or true nucleolus Chromatin- network Nucleus < Linin-network Karyosome net-knot, or chromatin- nucleolus Plastids lying in the cytoplasm Vacuole Passive bodies (metaplasm or paraplasm) suspended in the cytoplasmic mesli- work Fig. 3. Diagram of a cell. Its basis consists of a meshwork containing numerous minute granules [microsomes) and traversing a transparent ground-substance. (Wilson.) reactions of its constituent cells. There is not one protoplasm, therefore, but an infinity of protoplasms, and the use of the term can be justified only if we keep this fact in mind and use the word merely as a convenient abbreviation for any material endowed with life. Even in a single cell there is more than one kind of protoplasm. In its chemical characters, in its mode of life, and in its reactions, the nucleus differs widely from the cytoplasm. Both are necessary for the life of the cell and both must be considered, according to our present ideas, as ' living.' In the cytoplasm itself we find structures or substances which we must regard as on their way to protoplasm or as products of the breakdown of protoplasm ; but in many cases it is impossible to say whether a given material is to be regarded as lifeless or as reactive living matter. Even in a single cell we may have differenttation among its different parts, one part serving for the process of digestion while others are employed for the purpose of locomotion. Here again there must be chemical differences, and therefore different protoplasms. In this work the term protoplasm will be used in its broadest sense, namely, as the physical basis of living organisms. STRUCTURE OF THE CELL. In every cell can be distinguished the two parts — nucleus and cytoplasm. The nucleus is generally an oval or THE STRUCTUEAL BASIS OF THE BODY 17 spherical body lying near the centre of the cell and bounded by a definite contour or nuclear membrane. In its interior it contains masses or filaments of a material known as chromatin, which are strung, so to speak, on a fine network of material kno^vn as linin. Besides the granules of chromatin, other masses are sometimes found which stain in a different manner and are called nucleoli. The material filling up the meshes of the network is the nuclear sap or nucleoplasm. The cytoplasm, which varies greatly in extent in different cells, varies also in its appearance, being sometimes homogeneous, sometimes alveolar, sometimes granular in structure. In it can be often distinguished differentiated parts which may be regarded as organs of the cell. Thus in the amoeba we have the contractile vacuoles already men- tioned. In the green parts of plants the cytoplasm contains green gi-anules, the cliloroplasts, whose special function it is to assimilate carbon dioxide from the atmosphere, and by means of the energy of the sun's rays to convert this into starch with the evolution of oxygen. Other parts of the plant have similar granules, the leucoplasts, whose office it is to build up sugar into starch, and it is possible that other kinds of these ' plastids ' with special chemical functions are present in the cytoplasm of many cells. In addition to these cell organs, the cytoplasm may contain granules which represent stages in the metabolism of the ceU and are either food material which is being assimilated or products of the disintegration of the protoplasm, formed either for the service of the cell itself or, in the case of the multi- cellular animals, for the service of other cells of the organism. Others of these granules may represent reserve material, i.e. excess of noimshment which has been put aside by the cell in an insoluble form, to serve for its subsequent needs in times of scarcity. THE PHYSICAL STRUCTURE OF PROTOPLASM. Owing to the close similarities which exist between the fundamental properties of all living organisms, histologists have sought to discover some corresponding uniform morphological organisation of the physical basis of these phenomena, namely, protoplasm. It is often impossible, even under the highest powers of the microscope, t(i make out any structure whatsoever in the cytoplasm, which is spoken of then as hyaline. In most cases examination of a cell, even unstained, shows some differentiation between a more or less regular framework or meshwdik and a more fluid portion filling up its interstices, and these appearances arc still more manifest when the cells have been fixed by various hardening fluids. All the results obtained in this manner must be regarded with some suspicion, since, as has been shown by Fischer and by Hardy, it is possible to imitate artiticially the various structures, which have been assigned as characteristic of protoplasm, by hardening a homogeneous colloidal solution such as egg-white by difl'erent methods and with different agents. The theories of protoplasmic structure can be classified under three heads : 1 . The Granular Theory of Altmann. By the use of certain hardening reagents, a dense mass of spherical or rod-shaped gi-annles may be demon- strated in almost all cells of the body (Fig. 4). These granules have been 18 PHYSIOLOGY regarded by Altmami as the elementary particles of life, and he locates in them the various vital functions, the sum of which make up the life of the cell. According to Altmann these granules can only arise from the division of pre-existing granules, and he has formulated the phrase omne granulum e granulo, which is a further extension of Virchow's sentence omnis cellula e cellula. It is probable that a number of different kinds of structures of vary- ing importance are included among Altmann's granules. In some cases they Fig. 4. Section of liver stained to show granules. (Altmann. are the products of the activity of the cytoplasm and, as in secreting cells, will be later on cast out with water and salts as the specific secretion. In other cases they may be cell organs or plastids with the special metabohc functions assigned to all granules by Altmann. In some cases no treatment whatever will display the existence of granules. 2. The Fibrillar Theory. By the employment of appropriate methods of hardening, it is easy in most cells to demonstrate a network or clusters of fibrils which form, so to speak, a denser part of the cell. This fibrillar network has been named the ' spongioplasm ' in contra-distinction to the structureless material filling its meshes known as ' hyaloplasm.' A network is, however, one of the commonest pseudostructures produced in the coagu- lation of an albuminous fluid by any means whatever, and it is probable that in most cases the network which is seen in hardened cells is simply an artefact. Sometimes a large portion of the protaplasm may take a fibrillar form which can be detected even in the unstained and unfixed cell, and there is no doubt that, in certain phases at any rate, the fibrillar structure of the protoplasm is really present. 3. The Alveolar Theory op Bijtschli. This theory may be looked upon as corresponding morphologically to the granular theory of Altmann. If we imagine a hyaline protoplasm which is continually manufacturing metaplasmic products and storing them up in its protoplasm, these products will be deposited as spherules gradually increasing in size, so that the proto- THE STRUCTURAL BASIS OF THE BODY 19 Fig. 5. Diagram of a cell, highly magnified. (Schafer.) p, protoplasm, consisting of hyalo])lasm and a network of spongioplasm ; ex, cxoplasm ; end, endoplasm, with distinct granules and vacuoles; c, double centrosome; n, nu- cleus ; n', nucleolus. plasm between them will be converted into alveolar partitions between the droplets. In many an egg cell, where there is a growth of protoplasm from this building up of food into reserve materials, the development of such an alveolar structure can be followed in the living protoplasm, and such cells when mature show a marked alveolar structure whether ex- amined fresh or in the hardened and stained condition. Such a protoplasm would be prac- tically an emulsion of one fluid in another and according to Biitschli artificial emulsions, made by mixing rancid oil with sodium carbonate solutions, may show under the microscope a very close resemblance to cell protoplasm (Fig. 6), and may even exhibit amoeboid changes of form in consequence of the diffusion currents set up at the surface of the drop between its contents and the surrounding water. Most histologists are in accord that none of the above theories can be regarded as applicable to all forms of protoplasm, but that during the life of a cell its protoplasm, as observed under the microscope, may be either hyaline and structureless or may present any of the structural modi- fications described above, according to its state of nutrition and the form in which its metabolic products are laid down in the cell. Of course it is possible that, even in the apparently hyaline protoplasm, a structural differentiation is still pre- sent, but is invisible owing to the minute size of its constituent parts or an identity of refractive index between the alveolar walls and their contents. The fact that every chemical dift'ereutiation occurring within the colloidal mass will tend to cause differences of surface ten- sion, and therefore formation of droplets, shows that an alveolar structure, i.e. one in which there is a large number of surfaces separat- ing heterogeneous mixtures inside the cells, must be of very common occurrence, even in cases where it is not detectable under the microscope. Such a structure must be present, at any rate, in those cases where, apart Fio. 6. A, protoplasm of an epidermal cell of the crayfish ; B, foam-like appearance of an emulsion of olive oil. (BiJTSCHLi.) 20 PHYSIOLOGY from the existence of a solid cell wall, the cell presents a certain degree of rigidity and resistance to deforming stress. ULTRAMICROSCOPIC STRUCTURE OF PROTOPLASM. Since the study of the behaviour of the cell shows that it must possess a much moi"e complex structure or organisation than that which is revealed by the microscope, one, that is to say, which permits of the spatial differentiation of the different chemical processes that may occur at one and the same time in the protoplasm, many theories have been put forward of an ultramicroscopic cell structure. Though Spencer in 1864 spoke of physiological units out of which protoplasm could be regarded as made up, and Darwin (1868) conceived ultramicroscopic particles — gemmules — which might be discharged from every cell in the body and, passing into the reproductive organs, serve as the material basis of heredity, the first elaborate conception of such a structure was worked out by Nageli (1884). According to Nageli all organised structures are made up of miccllEe, minute particles arranged in definite order and surrounded with water. For growth to take place it was necessary that the system should be in a condition of ' turgor,' which was determined by the amount of water between the micellae. These micellae arose in every case from the division of pre-existing micellae, and the vital properties of the protoplasm were to be regarded as the sum of the changes taking place in the individual micellae. Similar conceptions have been put forward by numerous other observers, each of whom has applied a different name to the elementary living particle, such as ' pangene,' ' plasome,' ' biophor,' ' biogen-molecule,' and many others. The resemblance of these theories to that of Altmann is obvious, though the latter regarded the elementary particle as in many cases of microscopic size and capable of demonstration by appropriate methods of staining. That the cell possesses organs of smaller dimensions than itself, which may give rise to like organs by division, is shown by Schimper's observations on the plastids of plant cells. These apparently are not formed by a process of differentiation of the protoplasm, but are continuous from one generation to another and are reproduced by division. There is no doubt, however, that most of the granules to be observed in the cytoplasm are not of this character, but are elaborated by the general cytoplasm out of the foodstuffs which are supplied to it ; and though conceptions such as those of De Vries and Verworn are " often of value as a means of describing certain phenomena in the life of the cell and have played a great part in the description of the phenomena of heredity, they cannot be regarded as having any serious justification in fact. At the present time our know- ledge of the properties of the colloidal and capillary systems, which must play so great a part in the organisation and reactions of living protoplasm, is much too meagre to justify weight being laid on any theory of the ultramicroscopic structure of protoplasm that can at present be put forward. One question which has been much discussed relates to the physical condition of protoplasm. Is it to be regarded as a viscous fluid or as a soft sohd ? The perfect potential mobility of the protoplasm of many cells, as instanced by the flow of a substance of an amoeba into its pseudopodia, or the occurrence of rapid streaming movements in the threads of protoplasm found in many plants, e.^. the root hairs of tradescantia, indicates a fluid character for the protoplasm. Against such a character has been urged the fact that in protoplasm we may have shape, organisation, and power of resistance to deformation — qualities which are generally associated with the possession of solidity. It must be remembered, however, that the absence of resistance to deformation, which is characteristic of a liquid, applies only to the internal molecules, and that the surface of any liquid is in a condition of tension which not only limits deformation, but presents considerable resistance to any enlargement of the surface. Small water animals take THE STRUCTURAL BASIS OF THE BODY 21 advantage of this resistance to run freely over the surface of water, although their specific gravity may be greater than that of water. The continued existence of protoplasm in a watery environment shows that not only must its composition be different from that of its environment, but that there must bo a distinct surface separating the two. The superficial layers of the proto- plasm must therefore be in a condition of tension, and exercise pressure on the internal portions of the cell, which will tend to diminish the surface of the cell to the smallest possible extent, i.e. to bring it into the spherical form. This form is characteristic of free cells in their conditions of inactivity, and the smaller the mass of protoplasm, supposing it to be homogeneous, the greater will be the pressure exerted by its surface layer on its contents and the greater resistance will it present to deformation of the spherical form. A fluid drop, if suspended in a fluid with which it is immiscible, will present greater rigidity the smaller its dimensions. Almost any degree of rigiditv can also be imparted to larger masses of fluid protoplasm if their interior has undergone chemical differentiation so as to be made up of two or more im- miscible fluids arranged as droplets within alveoU, as in Biitschh's theory. In such a case every droplet will present resistance to deformation and every surface will resist penetration or extension. The resistance of the surface in colloidal fluids is still further increased by a property common to all these fluids, namely, the aggregation in the surface of a greater concentra- tion of the dissolved substance than is present in the underlying fluid. If, foi' instance, we take a beaker containing egg-white diluted 100 times, and drop a steel magnetised needle on to the surface, it will float although it is much heavier than the fluid, in consequence of the resistance of the surface. If the needle be greasy the same thing will occur on a glass of water. In this case the needle will lie N. and S. On the albumen solution, however, the needle will lie in the position in which it has been dropped. The aggregation of the albumen molecules on the surface of the fluid is such that it is practi- cally solid and resists any turning of the needle. In consequence of the sur- face aggregation and solidification of the colloidal molecules, it is possible to throw out the greater part of the albumen in a sohd form from a solution of this substance, if it be shaken up in a bottle with a httle air so as to make a surface. As the fluid is shaken fresh surfaces are always being formed, and the albumen aggregating in each of these surfaces has not time to redissolve before a fresh aggregation occurs on a new surface, and the films thus pro- duced gradually collect to form a solid mass of insoluble protein. Proto- plasm may be regarded as essentially fluid in character, the form and rigiditv which are acquired by most cells being due to chemical and physical differen- tiation occurring in the fluid. THE SURFACE LAYER OF CELLS. Since it is by means of its surface layer that the organism enters into relation with its emnronment, this layer acquires a prime importance for the hfe of the cell, and we may there- fore consider here at greater length some of the properties of this layer, the PlasmaJiauf, as it has been called. The superficial layer of the protoplasm is not to be confounded with the 22 PHYSIOLOGY cell wall. The latter, which plays a great part in the building up of vegetable tissues, is formed by a process of secretion from the hving protoplasm and is situated altogether outside the superficial Plasmahaut. The cell wall differs considerably in its chemical composition from the protoplasm out of which it has been formed. In most plants it consists of cellulose, a substance belonging to the carbohydrate group, and with a composition represented by some multiple of the formula CgHioOg. In other cells the wall may be built up from calcium carbonate or other Hme salts, from silica, fromchitin. In many cases it is perforated to allow the passage of communicating strands of protoplasm between adjacent cells. It is generally freely permeable to all kinds of solutions, and in this case plays no part in regulating the inter- changes of the cell with the environment. The superficial layer of protoplasm represents that part of the hving substance which stands in immediate relationship to the environment. Every change in the latter can only influence the living cell through this layer, and it is through this layer that substances must pass on their way into the cell for assimilation, or out of the cell for excretion. The retention of an individuality by the cell must be determined by chemical and physical differences between this layer and the surrounding fluid. Since it differs from the rest of the protoplasm in the changes to which it is subject, it must also differ in its chemical composition, apart altogether from the factors which, as we saw above, determine molecular differences between the surface and the interior of any colloidal solution. On this account one must assume the existence of a definite boundary layer of the protoplasm, even where it is impossible to see any differentiation between this layer and the deeper parts under the highest powers of the microscope. A (living) cell, which leads its life in a liquid environment, must take up the greater part of its food material in the form of solution, and it is the permeability of the superficial protoplasm which will determine the passage of food substances from the surrounding medium into the body of the cell. The immiscibility of the protoplasm with the surrounding fluid shows that the permeability of the membrane must be a limited one. The qualitative permeabihty can be easily studied in vegetable cells. These present within a cellulose wall a thin layer of protoplasm (the primordial utricle), enclosing a cell sap. If the root hairs of tradescantia be immersed in a 10 per cent, solution of glucose or in a 2 to 3 per cent, solution of salt, a process of plasmolysis takes place. The cell sap diminishes in amount by the diffusion of water outwards so that the primordial utricle shrinks (Fig. 7). On im- mersing the cells in distilled water, water passes into the cell sap until the further expansion of the protoplasmic layer is prevented by the tension of the surrounding cell walls. This behaviour can be explained only on the assumption that the protoplasm is impermeable both to sugar and to salt, but is freely permeable to molecules of water, i.e. it behaves as a semi- permeable membrane. Similar experiments can be made on animal cells. The most convenient for this purpose are the red blood corpuscles. These also shrink when immersed in salt solutions with a greater molecular THE STRUCTUEAL BASIS OF THE BODY 23 concentration than would correspond to the plasma of the blood from which the corpuscles were derived, whereas if placed in weak salt solutions or distilled water they swell up and burst, discharging their haemoglobin in solution into the surrounding fluid. By comparison of various salts it is found that the strength of each salt solution which is just necessary to cause plasmolysis or haemolysis, as the case may be, is determined entirely by its molecular concentration, i.e. a decinormal solution of sodium chloride will be equivalent 12 3 4 Fig. 7. Vegetable cells, showing varying degrees of plasmolysis. (De Vrtes.) in its effect on the cells to a decinormal solution of potassium nitrate or of potassium chloride. The impermeability of the plasma skin does not apply to all dissolved substances. Overton has found that, whereas this layer is practically impermeable to salts, sugars, and amino-acids, it permits the easy passage of monatomic alcohols, aldehydes, alkaloids, &c. All these substances are more soluble in ether, oil, and similar media than they are in water. The passage of dissolved substances through a membrane wetted by the solvent depends on the solubility of these substances in the membrane, and Overton therefore concludes that the superficial layer of protoplasmic cells must itself partake of a ' lipoid ' character, and that cholesterin and lecithin probably enter largely into its composition. Thus only those aniline dyes which are soluble in a mixture of melted lecithin and cholesterin have the property of penetrating the living cell, and only these dyes, such as methylene blue, neutral red, can be used for intra vitam staining. For the same reason substances which have the power of dissolving lecithin and cholesterin, such as ether or bile salts, also act as hsemolytic agents, i.e. they cause a destruction of the red blood cells by dissolving the superficial layer which is necessary for their preservation from the solvent effects of the sur- rounding fluid. The semi-permeability of the plasma skin can be altered by changes in the saline concentration or other factors of the surrounding medium. Over- ton has sho^vn that, whereas a 7 per cent, solution of saccharose produces plasmolysis in living cells, no plasmolysis is observed if they are treated with a solution containing 3 per cent, methyl alcohol phis 7 per cent, cane sugar. The superficial layer, therefore, is able to dissolve a mixture of methyl alcohol and cane sugar, although it has no solvent power on cane sugar in 24 PHYSIOLOGY pure watery solutions. It is possible that, in order to serve the nutrient needs of the cells, more extensive changes may take place in the permeabihty of the surface layer under hmited conditions of time and space. There is no doubt, for instance, that dextrose, to which the surface layer is apparently impermeable, can yet serve as a very efficient food for the ceU, and one might ascribe the fact that the cell assimilates only the food which it requires and no more, to such hmited changes in permeabihty. An important factor in the process of assimilation, at any rate by lowly organised cells, must be the relative solubihty of the absorbed substances in the cell and its surrounding medium respectively. When a watery solution of iodine is shaken up with chloroform, the latter sinks to the bottom, carrying with it the greater part of the iodine. If a watery solution of organic acid be shaken with ether, the latter fluid will extract the greater quantity of the acid. In no case will the extraction be complete, but there wiU be a definite ration between the amount dissolved by the ether and the amount dissolved by the water, the so-called ' coefficient of partage,' depending on the different solubihties of the dissolved substance in the two menstrua. In the same way a mass of protoplasm will tend to absorb from the surrounding medium and to con- centrate in itseK aU those substances which are more soluble in the colloidal system of the protoplasm than in the surrounding fluid, and this process of absorption may be carried to a very large extent, if the dissolved substances meet in the cell with any products of protoplasmic activity with which they form insoluble compounds so that they are removed from the sphere of action. It is probably by such a process as this that we may account for the accumulation of calcium or sihcon in such large quantities in connection with the bodies of various minute organisms. Whereas assimilation by a living cell is ultimately conditioned by the permeabihty of the surface protoplasm, its form is determined by the tension of this layer. If the tension is uniform at all parts of the surface the form of the cell will be spherical. Any diminution of the surface tension at one point must tend to cause a bulging of the fluid contents at this point, just as on distending a rubber tube with one weak spot in its wall this suddenly gives way with the production of a large balloon, which rapidly extends in size and ruptures unless the pressure be diminished. Diminution of the sur- face tension at one point of the cell wiU be attended by a contraction of all the rest of the surface and a driving out of the contents through the weak part. This process will not as a rule result in destruction of the cell ; the resulting protrusion wiU be hmited by the distortion of the internal alveolar structure of the protoplasm caused by any alteration of the spherical form of the cell. Change of form in hving structures thus depends ultimately on alterations in surface tension, return to normal being affected by the elastic reaction of the structural arrangement of the protoplasm. This point we shall have to consider more fully when deahng with muscular contraction. At present it is sufficient to see how any shght alteration in the chemical environment, such as might be due to the presence of a particle of food-stuff, may cause local variations in the surface tension of the plasma skin and THE STRUCTURAL BASIS OF THE BODY 25 thus result in the protrusion of pseudopodia and the ingestion of the food particle. VITAL PHENOMENA OF CELLS. A. Assimikition. The activity of every living being, whether uni- or multicellular, can be regarded as com- pounded of two phases, assimikition and dissimilation. By assimilation we mean the building up of the living substance at the expense of material obtained from the external world. In this process substances are formed of high potential energy, and this energy can be obtained only at the expense either of energy imparted to the system at the moment of assimilation, as, e.g. in the assimilation of carbon from carbon dioxide under the influence of the sun's rays, or of energy contained in the food-stuffs themselves. In all living organisms, except those provided with chlorophyll corpuscles, it is the latter method which is adopted, and a food-stuff therefore connotes some substance which can be taken in by the cell and can serve to it as a source of chemical energy. The evolution of energy, which is required for the movements and other vital activities of the cell, is derived from a disintegra- tion or dissimilation of the protoplasm and is generally associated with the process of oxidation. In assimilation, besides the building up of living protoplasm, there may also be a synthesis of more complex from less complex compounds, without their necessary entry into the structure of the living molecule. In the absence of any definite criteria by which we may judge as to the living or non-Hving condition of parts of the cell, it is a little dangerous to draw any hard-and-fast distinction between these two sets of processes. Assimilation requires the ingestion of food into the organism, and in the second place its digestion, i.e. its solution in the juices of the cells. These two processes are succeeded, through stages which we cannot trace, by an actual growth in the living material. In naked cells ingestion may occur either at any part of the surface, as in the amoeba, or at a specialised portion, so-called ' mouth,' as in many of the infusoria. Digestion is apparently effected in most cases by the production and secretion around the ingested food particle of solutions containing ferments, i.e. agents which have the power of hydrolysing the different food-stuffs and rendering them soluble. In the vast majority of living organisms the energy for their activities is derived from the oxidation, ultimately of the food-stuff's, but immediately of molecules attached to the living pTotoplasm. A necessary condition, therefore, for the life of these cells is the presence of oxygen in the surromiding medium, from which it is taken up in the molecular form. We may there- fore speak of an assimilation of oxygen ; but it is still a matter of dispute whether the oxygen is built up as such in the living molecule (so-called intra- molecular oxygen) to be utiUsed for the formation of carbon dioxide when a discharge of energy is necessary, or whether it is only taken in at the moment when the combustion of the carbon and hydrogen constituents of the food or protoplasm is necessary for the supply of energy. However this may be, products are formed as a result of this oxidation which are of no further value to the cell and are therefore excreted, i.e. turned out of the cell. The chief of these are the products of oxidation of carbon and hydrogen, namely, 26 PHYSIOLOGY carbon dioxide and water. There are also many substances resulting from the oxidation of the nitrogenous portions of the protoplasm, which have to be excreted in the solid or dissolved form. Although the assimilation of oxygen is so general a quality of living protoplasm, the presence of this gas, at any rate in the free form, does not seem to be necessary for all kinds of life. Thus a number of the bacteria are known which are anaerobic, i.e. exist only in the absence of oxygen. Examples of such are bacillus tetanus, and the bacillus of malignant oedema. In order to cultivate them it is necessary to dis- place all the air in the cultivating vessels by means of a current of hych-ogen. It has been supposed that the ultimate source of the energy of these organisms is also derived from a process of oxidation, and that they differ from other organisms in being able to utilise for this purpose oxygen which is built up into the structure of their food sub- stances. It is possible, however, that these organisms derive the energy for the building up of their protoplasm, for their movements, &c., not from a process of oxidation at all, but from processes of disintegration of the substances which they utilise as food. It is by such means that in all probability the intestinal worms, fairly highly organised animals, are able to exist in the intestine in a medium containing no oxygen, but rich in carbon dioxide. Here they are plentifully supplied with food-stuffs and can afford to adopt a wasteful method of nutrition, in A^hich only a small fraction of the energy is obtained which would be produced by a total oxidation of the food. B. The Phenomena of Dissimilation. The activities of a living cell or organism can be regarded in every case as dependent originally on en- vironmental change, and are adapted to this change, i.e. are of such a nature that they tend to preserve the organism intact, to favour its groM^th, or pre- vent its destruction. The property of reacting in such a manner to changes in the environment is fundamental to all protoplasm and is spoken of as excitahility, and the change which will influence an organism and cause a corresponding adaptive change in it is known as a stimulus. Stimuli may be of various kinds. Thus mechanical, thermal, chemical, electrical changes, light, and so on, may act as stimuli. The reactions which they evoke involve in every case chemical changes in the protoplasm, i.e. changes in the metabol- ism of the cell. Sometimes this change may be assimilatory in character, leading to an increased growth of the protoplasm, or at any rate to a cessation of dissimilation. In such a case the stimulus is spoken of as inhibitory, because it diminishes or prevents the output of energy by the organism. The frequent result of a stimulus is an increased output of energy, which may appear in the form of movement, in the form of heat, or as chemical change. A common feature of all dissimilatory changes evoked by the appUcation of a stimulus is that the energy of the reaction is always many times greater than the energy represented by the stimulus, the excess, of course, being suppUed at the expense of the potential energy of the food material which has been stored up in or built up into the hving protoplasm. This dispropor- tion between stimulus and reaction can be well illustrated on an excitatory tissue such as muscle. Thus in one experiment the gastrocnemius muscle of a frog was loaded with a weight of 48 gms. The nerve running to the muscle was placed on a hard surface and a weight of half a gramme was allowed to fall upon it from a height of 10 mm. The muscle contracted in response to this mechanical stimulus applied to the nerve and raised the weight 3-8 mm. THE STRUCTURAL BASIS OF THE BODY 27 In this case the work performed by the muscle was 48 x 3-8 = 1824 grm. mm., while the potential energy of the stimulus represented only 0-5 X 10-0 = 5-0 grm. mm. Thus the work performed by the muscle was thirty-six times larger than the energy of the stimulus applied to the nerve. In the case of unicellular organisms, definite classes of motor reaction to stimulus ha%'c been described. The ordinary retraction of a imicellular organism, such as the vorticella, in response to a touch is called thigmotaxis. Certain cells are influenced by gravity, tending to rise or fall in the surroimding medium according to the conditions which favour their existence. A similar sensitiveness to gravity is observed in the growing parts of plants, where the root always grows downwards and the stem up- wards. This reaction to gravity is known as qeotaxis, which is distinguished as ' nega- tive ' or ' positive ' respectively, according as the plant grows in opposition or in obedience to the gravitational attraction. If growing plants be placed on the rim of a wheel and rotated so that the centrifugal force is greater than that of gravity, the stems all grow towards the centre of the wheel while the rootlets grow outwards. In the same way the reaction of micro-organisms to light is laio^\Ti as pJiototaxis, some organisms seeking the light while others shun it. Among the primitive reactions of cells pcrhajjs the most important in the life of higher animals are those grouped mider the term chemiotaxis. The fertilisation of the ovum in the prothallus of ferns is effected by the penetration of the antherozoids produced in the male organs at some little distance from the female organs. It was shown by Pfeffer that the movement of the antherozoids towards the ova is effected in response to a chemical stimulus, probably malic acid, since he fomid that antherozoids suspended in a fluid will always swim towards any locality where there is a greater concentration of this acid. In the same way aerobic bacteria are attracted by the presence of oxygen. If such bacteria are present in a solution with an alga, on exposure of the fluid to light there is an evolution of oxygen by the green alga, and a consequent congregation of the bacteria round the seat of production of the oxygen. The movements of the white corpuscles of the blood of the higher animals are also largely determined by their chemical sensibility, and various substances can be divided into (a) those which exercise positive and (h) those which exercise negative chemiotactic influence on the leucocytes. Thus the intro- duction under the skin of an animal of a capillary tube containing a solution of sub- stances of the first class, such as peptone, tissue extracts, or the chemical products of certain bacteria, leads to an accumulation within the tube of leucocytes which pass to it from all the surrovmding tissues. Other substances, such as quinine, exert a nega- tive chemiotaxis. Tubes filled with these, after introduction into the subcutaneous tissue of a mammal, ^ill be foimd many hours later to contain no leucocytes at all. THE RELATIONS OF THE NUCLEUS TO THE CYTOPLASM. The universal existence in living cells of a differentiated nucleus indicates that the life cycle of assimilation and dissimilation must depend on an interaction between the nucleus and cytoplasm, and that each plays a distinct part in the sum of the changes Avhich make up the life of the cell. The different staining reactions of nucleus and cytoplasm suggest a corresponding difference in their chemical composition, a suggestion which is confirmed by analysis. In the building up of protoplasm proteins play an important part. They are not present, however, as simple proteins, but built up with other complex bodies to form conjugated proteins. Whereas in the cytoplasm these conjugated proteins consist chiefly of compounds of protein and lecithin, in the nucleus the chief constituents belong to the class of nucleo-proteins. The nucleo- proteins are of varying composition, and are distinguished chiefly by the 28 PHYSIOLOGY large amount of phosphorus in their molecule. A nucleo-protein can be broken down into nuclein and protein. Nuclein can be broken down into nucleic acid and a protein-hke substance, protamine. Nuclei differ among each other and at different periods of their existence or in different conditions of activity according to the greater or less amount of protein which is com- '^;j Fig. 8. Nucleated and non-nucleated fragments of Amaba. (Wilson after Hofeb.) A, B. An Aync^a divided into nucleated and non-nucleated halves, five minutes after the operation. C, D. The two halves after eight days, each containing a eon- tractile vacuole. bined with the nuclein. The latter seems to be the essential constituent of cell nuclei and to be present in only small quantities in the cytoplasm. The properties and reactions of these bodies will be dealt with at greater length in the next chapter. In order to appreciate the part played by the nucleus in the ordinary cell processes, we must study the behaviour of cells or parts of cells deprived of a nucleus and compare it with that of similar cells or parts of cells still containing a nucleus. By means of a fine needle it is possible to divide the larger protozoa into two pieces, one with and one without a nucleus. Hofer, experimenting on the amoeba, found that the fragment containing the nucleus quickly regenerated the missing part and pursued a normal existence. On the other hand, the non-nucleated fragments showed no signs of regeneration. They might, indeed, live as long as fourteen days after the operation (Fig. 8). THE STRUCTURAL BASIS OF THE BODY 29 Their movements continued for a short time and then ceased, though the pulsations of the contractile vesicle were but little affected. The power of digestion of food was completely lost. Other observers have shown that Stentor, an infusorium which possesses a fragmented nucleus, may be broken up into fragments of all sizes. Nucleated fragments as small as one-twenty-seventh the volume of the entire animal are still capable of Fiu. 9. Kcgcneration in the unicellular animal Stentor. (From Grubbk after Balblvni.) A. Animal divided into three pieces, each containing a fragment of the nucleus. B. The three fragments shortly afterwards. C. The three fragments after twenty-four hours, each regenerated to a perfect animal. regeneration. The wound quickly heals and the special organs — the mouth, with its surrounding cilia, and the contractile vacuole — are regenerated, but all non-nucleated fragments C[uickly perish (Fig. 9). Many similar observations have shown that the non-nucleated cytoplasm, though it may survive for some time and perform normal movements in response to stimuli, such as those of ingestion of food particles, loses entirely the power of digestion, secretion, and growth. In animals possessing a shell, a small secretion of the Ume salts may occur on the surface, but this process rapidly comes to an end as the store of material in the cytoplasm is exhausted. In vegetable cells it is possible to break up the protoplasm by means of plasmolysis into nucleated and non-nucleated parts. The nucleated part quickly forms a new cell wall. The non-nucleated part is unable to effect this formation, and soon dies unless it is in connection with an adjacent cell containing a nucleus by means of j&ne threads of protoplasm which pass through pores in the intercellular septa (Fig. 10). In the higher animals 30 PHYSIOLOGY we have, in the case of the nerve-cell, an example of the necessity of the nucleus for growth. Here division of the nerve fibre causes degeneration of the whole fibre separated from the cell containing the nucleus, and regenera- tion of the fibre, when it occurs, is eft'ected by a down-growth of that part of the fibre which is still in connection -svith the nucleus. All these facts show W J A C D Fig. 10. Formation of membranes by protoplasmic fragments of jilasmolysed cells. (Wilson after Townsend.) A. Plasmolysed cell, leaf -hair of Cucurhita, showing protoplasmic balls connected by strands. B, Calyx-hair of Oaillurdia ; nucleated fragment with membrane, non- nucleated one naked. C. Root-hair of Murchantia ; all the fragments, connected by protoplasmic strands, have formed membranes. D. Leaf-hair of Cucurhita ; non- nucleated fragment, with membrane, connected with nucleated fragment of adjoining cell. :f that the power of morphological as well as of chemical synthesis depends on the presence of a nucleus. On this account the nucleus, as we shall learn later on, must be regarded as the especial organ of inheritance. The trans- mission of the paternal quahties from one generation to the next is eft'ected by the entrance simply of the nuclear material of the male cell, the spermatozoon, into the ovum. In the words of Claude Bernard, " the functional phenomena in which there is expenditure of energy have their seat in the protoplasm of the cell {i.e. the cytoplasm). The nucleus is an apparatus for organic syn- thesis, an instrument of production, the germ of the cell." Similar conclusions may be drawn from a study of the changes in the nucleus which accompany different phases in the activity of the whole cell. THE STRUCTURAL BASIS OF THE BODY 31 Thus in growing plant cells the nucleus is always situated at the point of most rapid growth. In the formation of epidermal cells the nucleus moves towards the outer wall and remains closely apphed to it so long as it is growing in thickness. When this growth is finished the nucleus moves to another part of the cell. In the formation of root hairs the outgrowth always takes place in the immediate neighbourhood of the nucleus, which is carried forward and remains near the tip of the growing hair. The active'growth of cyto- plasm, which accompanies the activity of secreting cells, is always associated with changes in the position and in the size of the nucleus. Where the nutritive activi1\ of the cell is very intense, as in the silk glands of various lepidopterous larvae, the nucleus is found to be very large and much branched (Fig. 11) so as to present the greatest possible extent of surface through fig. il. Branched nucleus from which interchanges can go on between *^« ^P/^^^.g §1*°^ ^^ butterfly ^ ^ larva {Pteris). (Koeschelt.) nucleus and cytoplasm. The important changes which the nucleus undergoes in the process of cell division we shall have to consider more fully in the later chapters of this work. In the function of assimilation it is natural to assume that it is those constituents of the nucleus which are pecuhar to it both morpho- logically and chemically, namely, the chromatin filaments, which are most directly concerned. This assumption receives support from the changes which have been observed to occur in these filaments during various phases of nutritive activity of the cell. The staining powers of chromatin are in direct proportion to the amount of nuclein it contains. In the eggs of the shark it has been shown that the chromosomes undergo characteristic changes during the entire growing period of the egg. At first they are small and stain deeply with ordinary nuclear dyes, but during the period of growth they undergo a great increase in size and at the same time lose their staining capacity,* their surface being increased by the development of long threads which grow out in every direction from the central axis. As the egg ap- proaches its full size, the chromosomes diminish in size and are finally reduced to minute intensely staining bodies which take part in the first division of the egg preparatory to its fertilisation (Fig. 12). We must conclude that whereas the processes of destructive metabolism or dissimila- tion, which determine the activity of the cell, have their immediate seat in the cytoplasm, the processes of constructive metabolism which lead to the formation of new material, to the chemical and morphological building up of the cell, are carried out in or by the intermediation of the nucleus. HISTOLOGICAL DIFFERENTIATION OF CELLS. Even within the limits of a single cell, differentiation of structure can take place by the setting apart of distinct portions of the cell for isolated functions. Thus in an organism such as vorticella the cell is shaped somewhat like a wine-glass, * Piiickert, cited by Wilson. 32 PHYSIOLOGY the stem being composed of a spiral contractile fibre which has the function of withdrawing the rest of the organism when necessary towards its point of attachment. The main portion of the cell presents at its free extremity a part which is the seat of ingestion of food, and is therefore spoken of as the ' mouth.' This is surrounded by a circle of cilia whose function it is to set up currents in the surrounding fluid and so favour the passage of food particles towards the mouth. Food when ingested at this end passes only a short ^^x'X^I. '^»»s»iw«^^^,-as#^ Fig. 12. Chromosomes of the germinal vesicle in the shark Pristiurus, at different periods, drawn to the same scale. (Rtjckbrt.) A. At the period of maximal size and minimal staining-capacity (egg 3 mm. in diameter). B. Later period (egg 13 mm. in diameter). C. At the close of ovarian life, of minimal size and maximal staining-power. distance into the body of the vorticella. Here fluid is secreted around it which serves for its digestion. This portion of the cell may therefore be regarded as the alimentary canal or stomach. The indigestible residue of the food is excreted in close proximity to the mouth. In addition to these organs we have the usual differentiation of the protoplasm into an external and internal layer, and the development within the protoplasm of contractile vacuoles which serve to keep up a circulation of fluid and therefore to pass the products of digestion through all parts of the cell body. Within the limits of the single cell which forms the vorticella we may therefore speak of organs for contraction, for digestion, for circulation, and so on. The organs which are thus formed in unicellular animals or plants can be divided into two classes, namely (1) temporary organs, which are formed out THE STRUCTUEAL BASIS OF THE BODY 33 of a common structural basis and can therefore be replaced at any time by the cytoplasm if destroyed. Examples of such organs are the ciha, the commonest motor apparatus of unicellular organisms ; the pseudopodia, which, as we have seen, can be made and destroyed at will ; the mouth of animals such as Volvox or Vorticella ; and the stinging cells or nectocysts, which surround the mouth of many of these animals and serve to paralyse or kill the smaller hving organisms which are brought by the ciha within reach in order that they may serve as food. In contradistinction to these organs are (2) a number of others which must be regarded as permanent. These cannot be formed by differentiation from the cytoplasm of the cell, but are derived by the division of pre-existing organs of the same character, and are therefore transmitted from one generation to another. As examples of such cell organs may perhaps be mentioned the nucleus, \Ndth its chromo- somes, and the plastids, of which the chloroplasts of vegetable cells are the most conspicuous. Certain cell organs may fall into either class. Thus, the contractile vacuoles are sometimes derived by the division of the pre- existing vacuoles in a previous generation, at other times are certainly formed out of the common cytoplasm. The centrosome, a small particle generally situated in the C}i:oplasm, which plays an important part in cell division, is generally derived by the division of a pre-existing centrosome, but under certain conditions and in some organisms can be developed in situ in the cytoplasm itself. The possibiUty of histological differentiation and of the adaptation of structure to definite fmictions becomes much more pronounced as we pass from the unicellular to the multicellular organisms or metazoa. The lowest of the metazoa, such as the sponges, consist of Uttle more than an aggregation or colony of cells. All the cells are still bathed with the outer fluid, and any differentiation of structure or function seems to be entirely conditioned by the position of the cell. In the coelenterata the differentiation is already much more marked. The hydra, one of the simplest of the group, consists of a sac formed of two layers of cells and attached by a stalk to some firm basis. Round the mouth of the sac is a circle of tentacles. The inner layer, or hypoblast, represents the digestive and assimilatory layer, while the epiblast, or outer layer, is modified for the purposes of protection, of reception of stimuh, and of motor reaction. In the jelly-fish the differentiation of the outer layers leads to the formation of the first trace of a nervous system, i.e. a system fitted especially for the reception of stimuli and for their trans- mission to the reactive tissues, namely, the muscles. In all these classes of animals the external medium of every cell forming the organism is the sea- water or other medium in which they live. This can penetrate through the interstices between the cells, and every cell is there- fore exposed to all the possible variations which may occur in the composition of the surrounding medium. A great step in evolution was accomplished with the formation of the ccelomata, the class to which all the higher animals belong. In these, by the formation of a body cavity containing fluid, an internal medium is provided for all the working cells of the body. The com 2 34 PHYSIOLOGY position of this internal medium is maintained constant by the activity of the cells in contact with it, and the stress of sudden changes in the chemical com- position of the surrounding medium is borne entirely by the outer protective layer of epiblast cells. These are rendered more or less impermeable by the secretion on their surfaces of a cuticular layer, and only such of the con- stituents of the surrounding medium are allowed to enter the organism as can be utihsed by it for building up its hving protoplasm. Out of the coelom is later on formed a circulatory system which, by the circulation of the coelo- mic fluid or of blood throught the whole body, can procure a still more perfect uniformity in the chemical conditions to which every cell is exposed. It is not till much later that the organism achieves an independence of external conditions of temperature. In the mammalia, by means of the reactive nervous system, the heat produced in every vital activity by the chemical changes of combustion and disintegration is so balanced against the heat lost through the external surface to the environment that the temperature of the internal fluid is maintained practically constant. One of the main results of the differentiation of function and structure is therefore a gradual setting free of the majority of the cells of the body from the influence of variations in the environment ; and in the highest type of all animals, in man, this inde- pendence of external conditions is carried to a much further extent by . conscious adaptations, such as the use of clothes, dwellings, artificial heating, and so on. The differentiation of the cells which compose the organs of the body is determined in the first place by the different conditions to which they are exposed in virtue of their positions in the course of development. All the higher animals may be considered as built in the form of a tube, the external surface of which is modified for the purpose of defence and for adaptation to changes in the environment. From this layer there are developed not only the protective cuticle, but also the organs of motor reaction, namely, the special senses and the nervous system. The internal surface of the tube is modified for purposes of ahmentation. From it are developed all those structures which serve for the digestion of the food-stuffs, for their absorption into the common circulating fluid, for their elaboration after absorption, and their preparation for utihsation by other cells of the body. Between these two surfaces are situated the supporting tissues of the body as well as the organs for the conversion of the potential energy of the body into motion and work, namely, the muscles. Here also is the coelom or body cavity, repre- sented in the higher animals by the pleural and peritoneal cavities. The alimentary canal projects for a considerable part of its course into this coelom, being attached to the body wall only by one side. From the ccelom is also developed the blood vascular system, surrounded by contractile and con- nective cells which maintain a constant circulation of the blood throughout the body. By this differentiation the body becomes divided into a number of organs, each of which is composed of like cells, modified for a common function and bound together by connective tissue, the latter serving also to carry the blood-vessels which convey the common medium for the working THE STRUCTURAL BASIS OF THE BODY 35 cells. In the study of physiology our task consists, firstly, in the description of the special part taken by each organ in the general functions of the body, and, secondly, in the determination of the hmiting conditions of such func- tions and of the physical and chemical factors which determine them. Finally, we have to endeavour to form a complete conception of the chain of events concerned in the discharge of each function and of their causal nexus. We have compared the higher animal in the foregoing hues to a colony of cells, and we often speak of an isolated cell of the body as if it were an independent elementary organism. A better term for such an aggregation of cells as presented by the higher animals is not however ' cell colony,' but ' cell state,' since, just as in the state pohtic, no cell is independent of the activities of the others, but the autonomy of each is merged into the Hf e of the whole. With increasing differentiation there is increasing division of func- tion among the various members of the state, and each therefore becomes less and less fitted for an independent existence or for the discharge of all its vital functions. The more highly civiHsed a man becomes and the greater his speciahsation in the work of the community, the smaller chance would he have of existing on a desert island. Thus the Hfe of the organism is essentially composed of and determined by the reciprocal actions of the single elementary parts. It is evident that, if the process of speciahsation has gone far enough, a discussion whether each unit has or has not an independent life is beside the mark, since it cannot possibly exist apart from the activities of the other cells. Of late years histologists have brought forward evidence which seems to imply that an actual structural interaction exists, in addition to the functional dependence which is a necessary resultant of speciahsation. Even in the case of plant cells with their thick cellulose walls, fine bridges of protoplasm can be made out passing from one cell to another through pores in the cellulose wall. In animals protoplasmic bridges are known to exist joining up adjacent cells in unstriated muscle, epithehum and cartilage cells, and in some nerve-cells. The conclusion has therefore been drawn that the morphological unit is not the cell, but the whole organism, and that the division of the common cytoplasm into cells is merely a question of size and convenience. There can be no doubt that the determining factor in the division of cells is their growth ; the cell divides because it grows. With increased mass of living substance it is necessary to provide for increase of surface both of cytoplasm and of nucleus. Whether all the tissues of the higher animals remain in structural continuity by protoplasmic bridges, &c., must be to us a matter of indifierence, since all that is necessary for the interdependent working of the different cells of the body is a functional continuity, and this in the higher animals is effected by the presence of a common circulating fluid and a reactive nervous system connected by con- ducting strands with all the cells of the body. CHAPTEE III THE MATERIAL BASIS OF THE BODY SECTION I THE ELEMENTARY CONSTITUENTS OF PROTOPLASM The material basis of whicli living organisms are built up is derived from the surrounding medium, and the elements which compose the framework of the body must therefore be identical with those found in the earth's crust. Not all the elements are so utihsed in the formation of hving matter. Every hving organism without exception contains the following elements : carbon, hydrogen, oxygen, nitrogen, sulphur, phosphorus, chlorine, potassium, sodium, calcium, magnesium, and iron. In addition to these twelve elements others are found in certain organisms, sometimes to a large extent, but it is not known how far they are necessary to the proper development of these organisms, and it is certain that they do not form an integral constituent of all organisms. Of these elements we may mention especially sihcon, iodine, fluorine, bromine, aluminium, manganese, and copper. Deahng with the first class, i.e. those which are essential to all forms of life, we find that their relative proportions in hving organisms have httle or no relation to their proportions in the environment of the organisms. Their presence, however, in the latter is a necessary condition of life. In the case of plants which have a fixed habitat and cannot move in search of food, the growth of the plant is hmited by the amount of the necessary element which is present in smallest quantities in the surrounding medium. This is what is meant by the agriculturist's ' Law of the Minimum.' Of the elements derived from the earth's crust, those present in the smallest amounts in most soils are potassium, nitrogen, and phosphorus. The growth of a crop in any given soil is determined by the amount of that one of these three substances which is present in smallest quantities, and the aim of agriculture is to supply to every soil the ingredient which is present in minimal amount. Carbon forms the greater part by weight of the solid constituents of living protoplasm. The proximate constituents of living organisms are piactically all carbon compounds, so that organic chemistry, which was originally the chemistry of substances produced by the agency of hving organisms, has come to be synonymous with the chemistry of carbon com- 36 THE ELEMENTARY CONSTITUENTS OF PROTOPLASM 37 pounds. The carbon compounds which make up the living cell are com- bustible, i.e. they can unite with oxygen to form carbon dioxide with the evolution of heat. Li the inorganic world practically all the carbon occurs in a completely oxidised form, namely, carbon dioxide. A small amount, 4 parts in 10,000, is present in the atmosphere, while vast quantities are buried in the crust of the earth as carbonates of the alkaline earths, &c., in the form of chalk and limestone. In this condition the carbon dioxide is practically removed from the life cycle, the whole of the carbon contained in the tissue of Uving beings, whether plant or animal, being derived from the minute proportion of carbon dioxide present in the atmosphere. The energy for the conversion of carbon dioxide into the oxidisable forms with high potential energy, which make up the tissues of plants and animals, is furnished by the sun's rays. The machine for the conversion of the radiant energy into the potential chemical energy of the carbon compounds is represented by the chlorophyll corpuscles in the green parts of plants. In these corpuscles, under the influence of the sun's rays, the carbon dioxide of the atmosphere, together with water, is converted into carbohydrates, viz. starch (CgHioOs), and the oxygen liberated in the process is set free into the surrounding atmosphere. 6CO2 + 5H2O = CeHioOg + 6O2. In this process a large amount of energy is absorbed, an energy which can be set free later by the oxidation of the starch to carbon dioxide. lu the oxidation of one gramme of starch about 4500 calories are evolved, and this represents also the measure of the solar energy which must be absorbed by the chlorophyll corpuscle in the process of formation of starch from the carbon dioxide of the atmosphere. By this means the world of Ufe is pro- vided with a source of energy. At the expense of the energy of the starch further synthetic processes are carried out. By the oxidation of a part of the carbohydrates, sufficient energy may be supphed to deoxidise other portions of the carbohydrates with the production of fats. Thus SCgHiaOe — 8O2 = CigHggOa (Glucose) (Stearic acid) The potential energy of a fat is still greater than that of a carbohydrate, one gramme of fat giving on complete combustion to carbonic acid and water as much as 9000 calories. By the introduction of ammonia groups (NHg) into the molecules of fatty acids, amino-acids may be formed, from which the complex proteins are built up to form the chief constituents of the living protoplasm. The synthesis of carbon compounds from the inert carbon dioxide of the atmosphere can be effected only by chlorophyll corpuscles. All animals take in carbon, hydrogen, nitrogen, oxygen, and sulphur in the form of tlu^ carbohydrates, fats, and proteins which have been built up in the living plants. In the anima lorganism these food-stuft"s serve as sources of energy. They undergo a gradual oxidation, and finally leave the l)0(lyin the form of 38 PHYSIOLOGY carbon dioxide, water, ammoma or some related compound, and sulphates. A sharp distinction has therefore often been drawn between the metabolism of plants and animals, plants being regarded as essentially assimilatory in character while animals are dissimilatory, utihsing the stores of energy which have been accmnulated by the plant. There is, however, no definite hne of demarcation. Although, generally speaking, the green plant breaks up carbon dioxide, giving off oxygen and storing up carbon compounds, and the animal taking in carbon compounds oxidises them with the help of the oxygen of the atmosphere to carbon dioxide, which is redischarged into the surrormding medimn and is available for further assimilation by plants, yet this process of respiration is common to all hving organisms, whether plants or animals. In the green plant it may be masked by the assimilatory process occurring under the influence of the sun's rays, but in the dark all parts of the plant, and in the hght all parts which are free from chlorophyll, display a process of respiration, i.e. they are constantly taking up oxygen from the atmosphere and using it for the oxidation of carbon compounds in their tissues, with the production of carbon dioxide. The sum total of the processes of hfe tend, therefore, to maintain a constant proportion of carbon dioxide and oxygen in the atmosphere, the decomposition of carbon dioxide by the green plants being balanced by the oxidation of the carbon compounds and the continual discharge of carbon dioxide by animals. It is not certain, however, that this balance wiU be maintained throughout all time. As Bunge has pointed out, there are cosmic factors at work which are apparently tending to cause a constant diminution in the quantity of carbon dioxide in the atmosphere, which alone is of value to the plant. One of these factors is the variable affinity of the sihca and carbon dioxide respectively for the chief bases of the earth's crust. At a high temperature sihca can displace carbon dioxide from its compounds. Thus chalk heated with sihca will give rise to calcium sihcate with the evolu- tion of carbon dioxide. At an early geological epoch, therefore, it is probable that the greater part of the sihca was present in combination with bases and that the proportion of carbon dioxide in the atmosphere was very much higher than it is now. At temperatures at present ruling on the earth's surface carbon dioxide is a stronger acid than silica. The action of water charged with carbon dioxide on a sihcate is to cause its gradual decomposition with the formation of carbonate and sihca. Both these products, being in- soluble, are deposited as part of the earth's crust, the sihca in the form of sandstone, the carbonate as chalk or limestone. The carbon dioxide is being constantly removed by water from the atmosphere and being locked up in this way in the earth's crust, the process of separation of calcium car- bonate being aided to a marked extent by the agency of living organisms themselves. The whole of the extensive deposits of limestone and chalk have been separated from the sea-water by the action of hving organisms. With the cooling of the earth's crust which is supposed to be going on, the discharge of carbon dioxide by volcanoes must get less and less, so that one can conceive a time when the whole of the carbon'dioxide will be bound up THE ELEMENTARY CONSTITUENTS OF PROTOPLASM 39 with bases in the earth's crust, and Hfe, without any source of carbon, must become extinct. Hydrogen exists ahnost exclusively in the form of water. \\\ this form it is taken up by plants and animals, with the exception of a small proportion absorbed in the form of ammonia. In this form too it is discharged by Hving organisms. Oxijgen is the only element which, in all the higher organisms at any rate, is taken up in the free state. It forms one-fifth of the atmosphere and, as the oxides of the various metals, a considerable fraction of the earth's crust. It takes a position apart from the other food- stuffs in that its presence is the essential condition for the utihsation of their potential energy. In the living cells it combines with the oxidisable compounds formed by the agency of the hving protoplasm, with the production of carbon dioxide and water, and the evolution of energy. This process is spoken of as respira- tion. Like the three elements we have already considered, nitrogen is also derived directly or indirectly from the surrounding atmosphere. In conse- quence of its feeble combining power for other elements and the instability of its compounds, very Httle nitrogen is to be found in the combined state in the earth's crust, whereas it constitutes four-fifths of the atmospheric gases. It can be taken up by most plants only in the form of ammonia, nitrites, or nitrates. To animals these compounds are useless, and the only source of nitrogen to this class is the protein which has been built up by the agency of the plant cell. Since nitrogen in the free state is useless to nearly all living organisms, the existence of life must depend on the amount of com- bined nitrogen which is available. In view of the small tendency presented by this element to enter into combination, it becomes interesting to inquire into the source of the combined nitrogen which is the common capital of the living kingdom. There are certain cosmic factors which result in the pro- duction of combined nitrogen. The passage of electric sparks or of the silent discharge through moist air leads to the production of ammonium nitrite. N.2 + 2H.,0 - NHjNOa. Every thunderstorm, therefore, will result in the production of small quan- tities of ammonium nitrite, which will be washed do\vn with the rain and serve as a source of combined nitrogen to the soil. Every decaying vegetable or animal tissue serves as a source of ammonia, so that from various causes the soil may contain nitrogen in the form of ammonia or of ammonium nitrite. These forms of combined nitrogen are not, however, suitable for all classes of plants. Most moulds can assimilate ammonia as ammonium carbonate or as amino-acids or amines, provided that they are supplied at the same time with sugar, the oxidation of which will serve them as a source of energy. Some moulds, many of the higher plants, and especially the Grani- ineso, which include the food-producing cereals, require their nitrogen in the condition of nitrates. It is necessary, therefore, that the ammonia or nitrites in the soil shall be converted into this highly oxidised form. This 40 PHYSIOLOGY conversion is effected by a group of micro-organisms. There are a number of bacteria (bacterium nitrosomonas) which have the power of converting ammonia into nitrites. Others (bacterium nitro- monas) convert nitrites into nitrates. If sewage matter rich in ammonia is allowed to percolate through a cylinder packed with coke and the process be continued for several weeks, it is found after a time that in its passage through the filter the fluid has lost its ammonia and contains the whole of its nitrogen in the form of nitrate. If the cyhnder be tapped (Fig. 13) half-way down, say at K, the fluid will be found to contain, not nitrates, but nitrites. In this conversion the two kinds of microbes men- tioned above are concerned. At the top of the cyhnder the nitrous bacterium is present, in the bottom of the cyhnder the nitrate bacterium is present. The conversion of ammonia into nitrates by the agency of bacteria has been made the basis of a method of treatment of sewage which is now very largely employed. These different bacteria play an important part in all soils in preparing them for the cultivation of crops. Is the total capital of combined nitrogen which is worked over by these bacteria and utilised by the whole living world confined to the small quanti- ties produced by atmospheric discharges ? Of late years definite evidence has been brought forward that such is not the case and that organisms exist which can utilise and bring into combination the free atmospheric nitrogen itself. Thus certain soils have been found to undergo a gradual enriching in nitrogen althougli no nitrogenous manure has been applied to them. Winogradsky has shown that this fixation of nitrogen by soils is effected by a distinct micro-organism, which may be isolated by growing it on gelatinous silica free from any trace of combined nitrogen, so that the organism ha 3 to procure its entire nitrogen from the atmo- sphere. Under such conditions the numerous other micro-organisms of the soil die of nitrogen starva- tion, and only the microbe survives which is able to utihse free nitrogen. This organism, which he called Clostridium pasteurianum, grows well on sugar solution if free from ammonia and enriches the solution with combined nitrogen. It is anaerobic, i.e. only grows in the absence of oxygen. In the soil, where oxygen is constantly present, it occurs associated in a sort of symbiosis with two species of bacteria which are aerobic and protect it from Fig. 13. Arrangement for studying the nitrifica- tion of sewage. (Miss H. Chick.) THE ELEMENTARY CONSTITUENTS OF PROTOPLASM 41 the surrounding oxygen. The mechanism by which this organism is able to fix free nitrogen, and the nature of the first product of the assimilation are not yet ascertained. Such an assimilation will serve to the organism as a source of energy, since the application of heat is necessary for the dissociation either of ammonium nitrite or of nitrous acid into nitrogen and water, as is seen from the following equation : HNOgAq. + 308 Cal. = H + N + Og + Aq. NH^NOaAq. + 602 Cal. = 2N + 4H + 20 + Aq. In addition to this spontaneous fixation of nitrogen by humus, a method has long been known to farmers by which the fertility of a soil can be in- creased without the apphcation of nitrogenous manures. If a plot of land is to be left fallow it is a very usual custom to sow it with some leguminous crop such as sain- foin. Careful experiments by Boussingault, Lawes and Gilbert, and others have shown that the growth of almost any leguminous crop in a soil poor in nitrogen may result not only in the production of a crop containing much com- bined nitrogen, but also in an actual increase of nitrogen in the soil from which the crop is taken. It was then sho^vn by the last two observers, as well as by Schloesing and Laurent, that the power of a leguminous crop to enrich the soil with nitrogen was dependent on the presence on the roots of certain small nodules which had been described long before by Malpighi (Fig. 14). They showed also that the production of these nodules took place only as a result of infection. Beans grown in steriUsed sand produced a plant free from nodules, which, however, grew very scantily unless nitrogenous manure were added to the sand. Such a crop derived the nitrogen for its growth from the added nitrogen, the total amount of which in the soil was therefore diminished by the crop. If, however, the sterihsed sand were treated with an infusion of root nodules from another plant wthout the addition of any combined nitrogen at all, the beans developed nodules on their roots and grew luxuriantly, and at the termination of their growth the soil was richer in nitrogen than at the commencement. On microscopic examination the protoplasm which makes up these nodules is found to be swarming with small rods (Fig. 15), and it was shown by Beyerinck that these rods are bacteria and can be cultivated in media apart altogether from the plant. We have thus an example of a class of bac- teria which, like those of humus, are able to assimilate the free nitrogen of the atmosphere, but, unlike them, can only effect this assimilation in a condition of symbiosis, i.e. Hving in the gro-^ing tissues of a leguminous plant. Similar nodules have been described on the roots of other plants which can grow in a Fig. 14. Root of vetch with nod- ules. 42 PHYSIOLOGY soil free from combined nitrogen, e.g. conifers, but it is in the leguminosse that their presence is most widespread. The source of the combined nitrogen, which can be built up by plants into proteins and utihsed in this form by animals, is thus not only the am- monium nitrite produced by the agency of electric discharges in the atmo- __s WV^" ^ ,^ o \ Fig. 15. Section of a root nodule of Dorychnium. (VtriLLBMm.) a, cortical tissue ; h, cells containing bacteria. sphere, but also the free nitrogen of the atmosphere assimilated by various types of bacteria. Sulphur is found in all soils in the form of sulphates, generally of lime. As sulphates it is taken up by plants. In the plant cell a process of deoxida- tion takes place at the expense of the energy derived either from the starch or, in the case of bacteria, from other ingredients of their food-supply. It is built up, together with nitrogen, carbon, and hydrogen, to form sulphur derivatives and amino-acids such as cystine, and these, together with other amino-acids, are synthetised to form proteins. Practically the whole of the sulphur taken in by animals is in the form of proteins. It shares the oxida- tion of the protein molecule in the animal body which it leaves in the form of sulphates. The output of sulphates by an animal can therefore be regarded, like the nitrogen output, as an index of the protein metabolism. It is returned to the soil in the form in which it was taken by the plant, and the cycle can be continuously repeated. Iron, although forming but a minute proportion of the material basis of hving organisms (the whole body of man contains only six grammes), is nevertheless indispensable for the maintenance of life. It is necessary, for instance, in two important functions, viz. the formation of chlorophyll in the green plant and the respiratory process in the higher animals. Although iron forms no part of the chlorophyll molecule, plants grown in the absence of this THE ELEMENTARY CONSTITUENTS OF PROTOPLASM 43 substance remain etiolated, but form chlorophyll if the smallest trace of iron is added to the soil ijj -which they are growing or even if the leaves are washed with a very dilute solution of an iron salt. In animals iron forms an essential constituent of haemoglobin, the red colouring-matter of the blood, whose office it is to carry oxygen from the lungs to the tissues. It is probable too that the minute traces of iron in protoplasm exercise an important function in the processes of oxidation which are continually going on. Even in the inorganic world iron plays the part of an oxygen carrier. In the earth's crust it occurs as ferrous salts and as ferric oxide. The ferrous siUcate, for instance, may be decomposed by water containing carbon dioxide into sihca and ferrous carbonate ; the latter then absorbs oxygen from the atmosphere, liberating carbon dioxide and forming ferric oxide. In the presence of decomposing organic matter, the ferric oxide parts with its oxygen to oxidise the organic substances and is converted once more into ferrous carbonate, and this may be decomposed by the oxygen of the air as before. In the presence of sulphates and decomposing organic matter ferrous sulphate, which is first formed, undergoes deoxidation to ferrous sulphide, and this may again be oxidised to sulphates and ferric salts on exposure to the atmosphere, so that both the sulphur and the iron act as oxygen carriers between the atmosphere and the organic matter. Iron is obtained by plants from the soil as ferrous or ferric salts. In the protoplasm it is built up into highly complex organic compounds, and in this form is taken up by animals. It is probable that the main requirements of the animal for iron, which are very small, may be satisfied entirely at the expense of these organic compounds, but there can be little doubt that the animal can, if need be, also utihse the iron salts present in its food. The animal proceeds extremely economically with its supply of iron. Any excess of iron above that needed to supply the iron lost to the body, as well as the latter, is excreted almost entirely with the fseces in the form of sulphide. In the soil this undergoes oxidation and returns once more to the form in which it was originally taken up by the plant. PJiospJiorus is absorbed by the plant as phosphates. In the cell proto- plasm it is built up with fatty acids and other organic radicals to form com- plex compounds such as lecithin, a phosphorised fat, and nuclein, a com- bination of phosphorus with nitrogenous bases of great variety. Both leci- thin and nuclein are essential constituents of living protoplasm. Practically the whole of the phosphorus income of animals is represented by these lecithin and nuclein compounds. After absorption into the animal body they are broken down by processes of dissociation and oxidation, with the pro- duction, as a final result, of phosphates, which are excreted with the urine or faeces and return to the soil. Chlorine, 'potassium, sodium, calcium, and magnesium are taken up by the plants in the form of salts. Although plapng an essential part in all vital processes, they do not seem to be built up into organic combination with the protein and other constituents of the cell protoplasm. They are therefore taken up also by animals in the form of salts, and as such are again excreted with the urine. 44 PHYSIOLOGY Little is known about the significance, if any, of the other elements which I have mentioned as occasional constituents of living beings. Silicon, which is of universal distribution, is assimilated as sihca, probably in colloidal solution, and is distributed in minute quantities through all plant and animal tissues. It forms a very large percentage of the mineral basis of grasses, but even here it does not. seem to be indispensable, since these will grow in a medium devoid of sihca as luxuriantly as under normal con- ditions. Fluoritie is found in the enamel of the teeth and in minute traces in other tissues of the body. Bromine, though present in quantity in some seaweeds, appears to play no part in the economy of higher animals. Iodine is found in large quantities in many seaweeds and is present as an organic iodine compound in the skeleton of certain horny sponges. An organic iodine compound is also found in the thyroid gland of the higher animals, and may possibly be the active principle by means of which these glands are able to affect the nutrition of the whole body. Iodine, therefore, would seem to be an essential constituent of the higher animals. Aluminium is found in large quantities in certain lycopods. Whether it is essential to their growth is not known. Copper is certainly not a necessary constituent of a large number of plants and animals. In one class, the cephalopods, it appears to take the part of iron in the formation of a blood pigment. The haemocyanine, which was described by Fredericq, plays the same part in the blood of cephalopods that is played by haemoglobin in the blood of vertebrates. When oxidised it is of a blue colour, but gives off its oxygen and is reduced to a colourless compound on exposure to a vacuum. Among these elementary constituents of the body, a definite line of demarcation can be drawn between the carbon and hydrogen on the one hand and all the other constituents on the other. The first two elements are built up in a deoxidised form into the living structure of the protoplasmic molecule. The products of their complete oxidation are volatile, namely, carbon dioxide and water, and leave the body in these forms. The nitrogen set free by the breaking down of the proteins will pass off as free nitrogen or as ammonia. The sulphuric acid formed by the oxidation of the sulphur combines with the bases to form non- volatile salts. We may therefore divide the ultimate constituents of the body into those which are combustible and are driven off on heating, and those which are left behind as the ash. SECTION II THE PROXIMATE CONSTITUENTS OF THE ANIMAL BODY In spite of the enormous variety of the proximate constituents of hving organisms, they are all members or derivatives of three classes of compounds. Since living organisms form the entire food of the animal kingdom, a study of these proximate constituents includes the study of all the food-stuffs. These classes are : {a) Proteins, containing the elements carbon, hydrogen, nitrogen, oxygen, and sulphur ; in some cases also phosphorus. (6) Fats, containing carbon, hydrogen, and oxygen. (c) Carbohydrates, containing carbon, hydrogen, and oxygen, the two latter elements being present in the proportions in which they form water. THE CHIEF TYPES OF ORGANIC COMPOUNDS OCCURRING IN THE ANIMAL BODY The full consideration of the various modifications undergone by these three classes of food-stuffs in the body, especially if we include the by-products occurring both in plants and in animal metabolism, involves a ■nade knowledge of organic chemistry which indeed at its origin was simply the chemistry of the products of li^^ng {i.e. organised) beiags. The most important substances 'svdth which we shall have to deal belong to a comparatively restricted number of groups. For the convenience of the reader a short summary of the relationships of these gi'oups to one another and to the hydrocarbons is given here. THE HYDROCARBONS (Fatty Series). These form a continuous homologous series, and may be saturated or unsaturated. Examples of the saturated series are : CH4 methane C2H6 ethane C3H3 propane C4Hip butane, and so on, the general formula for the group being C„H2„+2- These paraffins, the lower members of which are gaseous, whih' the h.igher membens form the petroleum ether, the heavy petroleums, vaseline, and the paraffin wax with which we are all familiar, are entirely inert in the aniiiial body. If taken with the food they pass through the alimentary canal unchanged. In order to render them accessible to the action of the living cell they must fii-st undergo oxidation. The unsaturated hvdrocarbons have the general formuhe C„Ho„. C„Ho„ o, C.>H,,...,. &c. 45 46 PHYSIOLOGY Examples of the first two groups are ethylene CH2 II ■ ■ CH2 and acetylene CH !il CH Derivatives of all these groups occur in the body. THE ALCOHOLS, The first product of the oxidation of hydrocarbons is the series of bodies known as the alcohols. Examples of these are : CH3OH methyl alcohol C2H5OH ethyl C3H7OH propyl C4H9OH butyl C5H11OH amyl CgHigOH capryl „ and so on, the general formula for the group being CuH2n + lOH. Tn all these alcohols the OH group is, so to speak, more mobile than the other atoms connected with the carbons, and can therefore be replaced by other substances or groups with comparative ease. In this respect therefore an alcohol can be compared to water HOH or to an alkaline hydroxide NaOH or KOH. The best-known example of the group is ethyl alcohol, the ordinary product of fermentation of sugar. In these alcohols the H of the OH group can be replaced by Na. Thus, water with metallic sodium gives sodium hydroxide and hydrogen as follows : 2H0H + 2Na = 2NaOH + Hg. In the same way alcohol treated with metallic sodium gives off hydrogen, and the remaining fluid contains sodium ethylate, thus : 2C2H5OH + 2Na = 2C2H50Na + H2 (sodium ethylate) On the other hand, the OH group may be replaced by acid radicals. Thus, if ethyl alcohol be treated with phosphorus pentachloride, ethyl chloride is formed together with phosphorus oxychloride and hydrochloric acid. Thus : Et.OH + PCI5 = POCI3 + HCl + Et.Cl (ethyl chloride) With concentrated sulphuric acid the reaction is similar to that which obtains between sodium hydrate and this acid, and we have formed ethyl hydrogen sulphate and water. Thus: Et.OH + H2SO4 = Et.HSOi + HOH If alcohol be warmed with acetic acid and strong sulphuric acid, among the products of the reaction is ethyl acetate, which is volatile, and therefore passes off. Thus : Et.OH + HC2H3O2 = Et.C2H302 + HOH. These compoimds of the hydrocarbon group of the alcohol, such as methyl, ethyl, propyl, &c., with an acid, in which the ethyl takes the part of a base, are known as esters. An ester treated with an alkali is decomposed with the formation of an alkaline salt of the acid, and the corresponding alcohol which, being volatile, is given off on warming the mixture. Thus : Et.C2H302 + NaHO = NaC2H302 + Et.OH. (ethyl acetate) (potassium acetate) (alcohol) This process of (k^'omposition of an ester with the formation of the alkaline salt of an acid is often spoken of as saponification, i.e. soap formation, though the term ' soap ' PROXIMATE CONSTITUENTS OF THE ANIMAL BODY 47 is applied only to the compounds of alkalies with the higher fatty acids. The series of alcohols we have just dealt with containing one OH group replaceable by metals or acid radicals are known as monatomic alcohols. If in the molecule of the paraffin two or more atoms of hydrogen have been replaced by the group OH, we speak of diatomic or polyatomic alcohols. Thus, derived from the paraffin propane C'sHs we may have the monatomic alcohol C3H7OH, propyl alcohol, or the triatomic alcohol C3H5(OH)3, which is known as glycerin, or glycerol. Other alcohols of physiological imjiortance arc cholesterol and cetyl alcohol. Cholesterol is a monatomic alcohol with the formula C27H45OH. It is very complex in structure, and belongs to the aromatic series. Recent work points to an affinity of cholesterol with the terpenes, wliich have hitherto been found only as the product of the metabolism of plant cells. Cholesterol is a constant constituent of protoplasm. It occurs in large quantities in the medullary sheath of nerves ; it is a normal constitu- ent of bile and may form concretions (biliary calculi) in the gall bladder. In combina- tion with fatty acids it is an important constituent of sebum and of wool fat. CH3 Another alcohol — cetyl alcohol — Ci8H34^0 = (CH2)i4 occurs in the feather glands of I CHoOH the duck and forms an important constituent of the was, spermaceti, obtained from a cavity in the skull of the sperm whale. ALDEHYDES. By oxidation of any of the alcohols we obtain another group of compomids — the aldehydes. From ethyl alcohol, for instance, by warming with potas- sium bichromate and dilute sulphuric acid, ethyl aldehyde is produced and given oii. In H /H I these aldehydes the group C,^— H is converted into the group C = O, and it is the possession of this group which determines the aldehyde character of any compound, as well as the reactions wliich are typical of this class of compounds. Some of the typical reactions of aldehydes may be here shortly summarised : (1) They act as reducing agents, the CHO group being converted into the group COOH, which is distinctive of an acid. We may therefore say that on oxidation aldehydes are converted into the corresponding fatty acids as follows : CH3 CH3 CHO COOH (ethyl aldehyde) (acetic acid) On account of the ease with which this oxidation takes place, aldehydes act as strong reducing agents. Warmed with an alkaline solution of cupric hydrate, they take up oxygen, reducing the cupric to a red precipitate of cuprous hydrate. If warmed with an ammoniacal solution of silver {i.e. silver nitrate solution to M^hich ammonia has been added mitil the precipitate first formed is just redissolvcd), they reduce the silver nitrate with the formation of a mirror of metallic silver on the surface of the glass vessel in which they are heated. (2) On warming with phenyl hydrazine, they give the typical compounds, hydra- zones and osazones, which are also given by the sugars and will be mentioned in con- nection with these bodies. (3) They also form addition products. With ammonia, they yield the group of compoimds knoAMi as aldehyde ammonia. Thus : CH3 CH3 1 -r NH3 = i /NH, CHO ^OH 48 PHYSIOLOGY With sodium hydrogen sulphite the following reaction takes place : CH3 CH3 I + NaHSOs = I /OH CHO CH<; \s03Na These compounds of aldehydes with sodium sulphite can be readily obtained in a crystalline form and furnish a convenient means of separating the aldehydes from their solutions. (4) All the aldehydes possess a strong tendency towards polymerisation. Ethyl or acetic aldehyde treated with strong sulphuric acid gives the compound paraldehyde. Thus : 3C2H4O - C6H12O3. (acetic aldehyde) (paraldehyde) If warmed with strong potash the polymerisation occurs to a still further extent with the formation of resinous substances of unknown composition, but at any rate of a very high molecular weight, the so-called ' aldehyde resin.' Formic or methyl aldehj'^de, CH2O, may in the same way undergo polymerisation with the formation of a mixture of substances belonging to the group of sugars, namely, the hexoses, as follows : 6CH2O = CeHiaOg. This formation of sugar from formic aldehyde probably plays an important part in the assimilation of the carbon from the carbonic acid of the atmosphere by the green parts of plants. ACIDS. By the oxidation of the group CHO of the aldehydes we obtain the group COOH which is characteristic of an organic acid, Thus, formic aldehyde on oxidation gives the compound HCOOH, formic acid. Ethyl or acetic aldehyde, CH3CHO, with an atom of oxygen, gives the compound CH3COOH, acetic acid. CH3 CH3 1+0=1 CHO COOH. Since these acids are derived from the paraffins a whole series of them exists corre- sponding to the series of paraffins, and known as the fatty acids. Examples of this group are : Formic acid Acetic, acid Propionic acid Butyric acid HCOOH CH3 CH3 CH. COOH CH2 CHo I I COOH. CHo COOH. In addition to these fatty acids, there are also unsaturated acids, derived from the imsaturated hydrocarbons. DERIVATIVES OF THE FATTY ACIDS AMINO-ACIDS are derived from the fatty acids by the replacement of one atom of hydrogen by the group NHg. Thus from propionic acid we may have : CH2NH2 CH3 CH2 or CH.NHo COOH COOH. The second form, the ((-amino acid, is the only one which occurs in the body. I PROXIMATE CONSTITUENTS OF THE ANIMAL BODY 49 OXYACIDS ;irc foiuiocl by the replacement of one H atom by the group OH. Thus : CH3 CHOH is oxypropionic acid or lactic acid. COOH KETO- ACIDS. Oxyacids are formed by the oxidation of the group CHg or CH3. If at the same time the Hg group be removed by oxidation a keto-acid may be formed. This is probably the maimer in which such acids arise in the body, though it is more usual to regard a keto-acid as the result of oxidation of a ketone. Thus : CH3 CH3 1 CH3 1 CO 1 1 CH3 CH2 OH COOH (acetone) (pyruvic acid — a keto-acid) ACID AMIDES are formed from a fatty acid by replacing the - OH of the - COOH group by - NH2, e.g. : CH3 CH3 I from I CO.NH2 COOH. (acetamide) (acetic acid) AMINES. These may be regarded as formed from ammonia NH3 by replacing one or more of the H atoms by an organic radical. Thus we may have : /CH3 /CH3 /CH3 N^H NeeH3 N,^CH3 \H ^H ^CH3 (methylamine) (dimethylamine) (triinethylamine) Under the action of living organisms primary amines may be formed from a -amino acids by a process of decarboxylation. Thus : CH3 CH3 II CH.NH2 - COo = CH2.NH2 I COOH (a-amino-propiouic acid) (ethylaminc) AROMATIC COMPOUNDS These all contain a nucleus, made up of six carbon atoms, which is extremely stable, so that processes of oxidation, reduction, &e., can be carried out in the compound without destruction of the nucleus. The simplest aromatic compoimd is benzene CeHg. It behaves as a saturated compomid. It is represented as a hexagon with a hydrogen atom at each angle. H H 50 PHYSIOLOGY All the hydrogen atoms are of equal value. They may be replaced by other grouj)s, such as OH, CI, NH2, or by more complex groups belonging to the fatty series, e.g. CH3, C2H5, &c. Monosubstitution derivatives exist only in one form : CgH^.X Disubstitution compounds exist in three forms, according to the relative position of the substituted H atoms. These are known as the ortho, meta, and para compounds, and have the formulae : XXX H H H H H H H ortho- H .^y X H meta- H H '\/. H X para- The following are some of the most important monosubstitution derivatives of benzene : Nitrobenzene CgHg . NO2. Aniline CgHs.NHa. Benzene sulphonic acid CgHg.SOsH. Phenol CeHj.OH. Toluene CgHs.CHg. Benzyl alcohol CgHg . CHgOH. Benzylaldehydc CsHj . CHO. Benzoic acid CgHs . COOH. Of the disubstitution compounds, we need only mention the following : The dihydroxyhenzenes : Pyrocatechin or catechol Resorcinol Hydroquinone OH OH OH OH ' ortho meta OH \/ OH para- Salicylic acid (o-hydroxybenzoic /OH acid) CgH^/ ^COOH. Tyrosin (para hydr oxyphenyl alanine ): OH CH2.CH(NH2)C00H. Examples of trisuhstitution derivatives of benzene are : OH Pyrogallol OH OH PROXIMATE CONSTITUENTS OF THE ANIMAL BODY OH 51 Homogentisic acid OH CH2.COOH Adrenaline OH OH CH.OH 1 CH2.NH(CH3) Picric acid NO2 OH NO2 NO, OPTICAL ACTIVITY Most of the compounds produced by the agency of living organisms exhibit optical activity, i.e. have the property of rotating the plane of polarised light either to the right or to the left. In an ordinary wave of light the vibrations of the waves take place in all planes perpendicular to the direction of its propagation. When such a ray is passed through a Nicol's prism (made of Iceland spar) it emerges as a plane polarised beam, i.e. waves in one plane only are transmitted. Another Nicol's prism will allow such a ray to pass only if it is parallel to the first prism. If it is rotated through a right angle, no light will pass, A Nicol's prism may thus be used to determine the plane of polarisation of any beam of light. In the polarimeter two Nicol's prisms moimted parallel to one another are employed. One of them (the polariser) is fixed ; the other (the analyser) can be rotated round hi D ^DjDt ==4];ic c^B ^.'.''. Fig. 16. Diagram of polarimeter, B, polariser ; n, analyser ; o, tube containing solution under examination. the axis of the beam of light passing through the first. When both prisms are parallel light passes through the analyser. On interposing a solution of an optically active substance between the two prisms, the plane of polarisation of the beam is rotated, so that the light passing through the analyser is diminished. The light may be brought to its original intensity by rotating the analyser either to the right (clockwise) or to the left. In this way the direction and degree of the optical activity may be determined. Optical activity is connected with the molecular arrangement of the substance exhibiting this property, and depends on the presence of one or more ' asymmetric carbon atoms ' in the molecule. CH3 CH3 I 1 Thus in hictic acid H.COH, or in alanine HC.NHo, the middle carljon atom is COOH COOH asymmetric, i.e. it is unequally loaded on the four sides. 52 PHYSIOLOGY We can imagine such a carbon atom as occupying the interior of a tetrahedron. A B R3 R3 Fig. 17. In this tetrahedron, if we represent the four groups combining with the carbon by Rx, R2, Rsj R4, they can be arranged either as in A or B. It is evident that no amount of turning about will convert the tetrahedron A into tetrahedron B, but that, if we hold A before a mirror, its image in the mirror will be represented by B. One arrange- ment is therefore the mirror image of the other, and a compound containing one such carbon atom will be capable of existing in two forms, namely, one form corresponding to A, the other form corresponding to B. It is found that the unequal loading of the carbon atom, which is present in such an asymmetric arrangement, causes the com- pound containing the asymmetric carbon to have an action on polarised light. One of the varieties will rotate polarised light to the right, while its mirror image will rotate polarised light to the left. A mixture of equal parts of the two compotmds will rotate equally to left and right, i.e. will have no action on polarised light. The variety rotating to the right is dextrorotatory, and the other Isevorotatory,* while the mixture of the two is known as the racemic or inactive variety. The three forms are said to be stereoisomeric, and are distinguished as the d, I, and i forms re- spectively. If two asymmetric carbon atoms are present in a compound, we may have four stereoisomers ; and generally if there are n asymmetric atoms in a molecule, there will be 2" possible stereoisomers. These will not all be necessarily optically active, since the dextrorotation due to one asymmetric carbon atom may be exactly neutralised by the laevorotation due to another, so that ' internal compensation ' takes place and the substance io optically inactive. Thus in tartaric acid four forms are known, namely, d, I, racemic or i, and mesotartaric, also inactive, in which internal compensation occurs. These four varieties may be represented as follows : COOH COOH HCOH HOCH HOCH HCOH COOH COOH cZ-tartaric acid Z-tartaric acid COOH HCOH HCOH COOH mesotartaric acid inactive tartaric acid Several methods may be employed to separate the racemic form into its two optically active components. One of these methods, first employed by Pasteur, is to grow moulds in the solution. One of the optical isomers is destroyed, leaving the other unchanged. Another method is the fractional crystallisation of the salts with alkaloids, e.g. strychnine in the case of lactic acid. * The specific rotatory power of a substance is equal to the number of degrees through which the plane of polarisation is rotated when it passes through a 100 per cent, solution of the substance in a tube 1 decimetre long. Thus polarised light passing through such a tube of 10 per cent, glucose solution would show a rotation of 5-25 degrees, i.e. its specific rotatory power is + 52-5. SECTION III THE FATS These substances are widely distributed throughout the animal and vege- table kingdoms. In the higher animals they form the main constituents of the fatty or adipose tissue lying under the skin and between the muscles and often forming large accumulations around the viscera. In the marrow of bones they may amount to 96 per cent. They also occur in fine particles distributed through the protoplasm of cells and probably also in combination with the other constituents which make up protoplasm. Large amounts are also found in certain members of the vegetable kingdom, as, for instance, in the fatty seeds and nuts, e.(j. linseed, olives, Brazil nuts. CHEMISTRY OF THE FATS The fats are esters of glycerol and the fatty acids. Glycerol is a trihydric or triatomic alcohol and can therefore form esters with one, two, or three of its hydroxyl groups ; thus with acetic acid the following compounds are known : (1) (2) CH2OH CH2OH I I CHOH CH— 0— OC.CH3 I I CH2O— OC.CH3 CH2OH a-rnonacetin /3-raonacetin monoglycerides (3) (4) (5) CH2— 0— OC.CH3 CH2OH CH2— 0— OC.CH3 I I I CHOH CH— 0— OC.CH3 CH— 0— OC.CH3 CH.,— 0— OC.CH3 CH2— 0— OC.CH3 CH2— 0— OC.CH3 a, a diacetin a, /3 diacctin triacctin ' Tr"i " Hi trifflvccride diglycendes In these compounds the phenomenon of isomerism occurs owing to the presence of primary and secondary alcohol groups in glycerol. In the case 53 54 PHYSIOLOGY of the diglycerides and the triglycerides mixed esters, in which the fatty acid radical varies, are possible : (6) (7) CH,— 0— OC.CH, CH,OH Lg \J vyv^.vy-LXg V^XigV CHOH CH— 0— OC.CH CH2— 0— OC.CH2.CH3 CH2— 0— OC.CHaCHg (8) CH,—0— OC.CH, L2 \J vy^.v^J-13 CH— 0— OC.CH2.CH3 CH2— 0— OC.CH2.CH2.CH3 The glyceryl esters which compose the fatty material of hving matter — whether animal or plant — are mainly triglycerides, the monoglycerides and diglycerides being seldom found in nature. The natural fat is usually found to consist of a mixture of triglycerides ; these triglycerides, instead of being mixed esters as in formula (8), are generally simple esters as in formula (5). The differences in the composition of the natural fats depend therefore on the variety of the fatty acid radical combined with the glycerol. The fatty acids which enter into the composition of the triglycerides belong to two main homologous series : A. The saturated fatty acids, namely : Formic acid, H.COOH Acetic acid, CH3.COOH Propionic acid, CHg.CHg.COOH Butyric acid, CH3.CH2.CH2.COOH Valerianic acid, CH3.(CH2)3.C00H Caproic acid, CH3.(CH2)4.C00H Capryhc acid, CH3.(CH2)6.COOH Capric acid, CH3(CH2)8.C00H Laurie acid, CH3(CH2)io.COOH Myristic acid, CH3(CH2)i2-COOH Palmitic acid, CH3(CH2)i4.C00H Stearic acid, CH3(CH2)i6.COOH Arachidic acid, CH3(CH2)18.COOH Behenic acid, CH3(CH2)2o.COOH Lignoceric acid, CH3(CH2)22-COOH B. The unsaturated fatty acids, namely : (1) Acryhc series, e.g. oleic acid (CjjH2„_202) (2) Linoleic series, e.g. hnoleic acid (C^H2n_402) (3) Linolenic series, e.g. linolenic acid {Q'^^n^^^^i THE FATS 55 Of the long list of fatty acids given above only a few occur to any extent in the animal body. In milk, although the greater part of the fat consists of the triglycerides of oleic, palmitic, and stearic acids, other members of the series given above are present in small amounts. On the other hand, the adipose tissue, strictly so called, consists almost exclusively of the fats de- rived from the fatty acids palmitic, stearic, and oleic, i.e. tripalmitin, tri- stearin, and triolein. The great differences in the appearance of the fat of different animals are due to the varying amounts in the relative quantities of these three fats which may be present. While triolein is liquid at 0° C, tri- stearin and tripalmitin are solid at the temperature of the body. According to the relative amounts of these three substances, therefore, we may have a fat which, like mutton suet, is solid at the body temperature, or a fat con- taining much olein which is still fluid and runs away when the body is opened after death, even when it has already cooled. PROPERTIES OF THE FATS. The fats are colourless substances devoid of smell. They are insoluble in water, in which they float. They are soluble in warm absolute alcohol, but separate out into crystalline form on cooling. They are easily soluble in ether. If they are strongly heated with potassium bisulphate they give off pungent vapours of acrolein derived from the decomposition of the glycerin of their molecule. C3H.5(OH)3 - 2H2O = C3H4O If they are heated with water or steam or submitted to the action of certain ferments, they imdergo hydrolysis, taking up three molecules of water, and are spht into three molecules of fatty acid and one molecule of glycerin, e.g., C3H5(CieH3i02)3 + 3H2O = 3HCieH3,0, + C3H5(0H)3 (neutral fat — tripalmitin) (palmitic acid) (glycerin) This process may occur spontaneously when fat is left exposed to the air. Fat which has been ai-tificially spht in this way is said to be rancid. Most natural fats generally contain a small amount of fatty acid which gives them an acid reaction. On boiling a neutral fat for a long time with an aqueous solution of potassium or sodium hydrate, or better still with an alcohohc solution of potassium or sodium ethylate, the fat undergoes saponification, giving the alkaline salt of a fatty acid and glycerin. The former compomid is spoken of as a soap. In water the soaps form a sort of pseudo-solution on heating which sets to a solid jelly on cooling. From a dilute watery solution the soap can be thrown down in the solid form by the addition of neutral salts. Fats are insoluble in and non-miscible with water. If shaken up with water the droplets rapidly run together and rise to the surface, forming a continuous layer of the oil or fat. The same thing happens if an absolutely neutral fat be shaken up with a dilute solution of sodium carbonate. If, however, the fat be slightly rancid, i.e. if fatty acid be present, the latter combines with the alkaU with the expulsion of CO.^ to form a soap. The presence of soap in colloidal solution in the water at once diminishes or aboUshes the surface 56 PHYSIOLOGY tension between the neutral fat and the water. Like many other colloidal solutions, a soap solution presents the phenomenon of surface aggregation, i.e. the concentration of the soap at the surface is increased to such an extent as to form practically a solid pellicle of molecular dimensions on the surface of the fluid. The same pellicle formation occurs at the surface of every oil globule, so that on shaking up rancid oil with dilute sodium carbonate, the whole of the oil is broken up into fine droplets which show no tendency to run together again, and remain in suspension in the water. The suspension of fine oil droplets, which has the appearance of milk, is spoken of as an emulsion. It can be at once destroyed by adding acid. This decomposes the soap, setting free the fatty acids, which are insoluble in the water. The pelhcle around each globule is destroyed, and the globules run together as neutral oil would in pure water. In order to characterise any given animal fat or mixture of fats the following reactions are made use of : (1) The 'acid number ' of the fat, i.e. its content in free fatty acids, is determined N by titrating it in ethyl alcohol solution with ^r. alcoholic solution of potash, using phenol - phthalein as an indicator. (2) The ' saponification number.' This represents the number of milligrammes of potassium hydrate necessary to saponify completely one gramme ot fat. (3) The percentage of volatile fatty acids is determined by saponifying the fat, then treating it with a mineral acid to set free the fatty acids and distilling over the volatile acids in a current of steam. (4) The iodine number is the amount of iodine which is taken up by a given weight of fat. It is a measure of the amount of tmsaturated fatty acid present, i.e. in ordinary fat, oleic acid. Besides the glycerides, a certain number of substances occur in the body derived, not from a combination of fatty acids with glycerol, but from a formation of esters of the fatty acids and other alcohols, e.g. cholesterol or cetyl alcohol. Thus, spermaceti is a mixture of cetyl palmitate with small quantities of other fats. The fatty secretion of the sebaceous glands in man and the higher animals, which furnishes the natural oil of hair, wool, and feathers, consists, with small traces of glycerides of cholesterol esters. Lanohne, which is purified wool fat, consists almost entirely of cholestpryl stearate and palmitate. These cholesterol fats are attacked with extreme difiiculty by ferments or micro-organisms. It is probably on this account that they are manufactured in the body for protective purposes. So far as we know, when once formed, they are incapable of further transformation in the body. They are not appreciably altered by the digestive ferments of the ahmentary canal, and the cholesterol is said to pass through the latter unaltered.* Cholesterol is also found in combination with fatty acids in every living cell. Whenever protoplasmic structures are extracted with boihng ether, a certain amount of cholesterol is present with the fats which are so extracted. In view of the great stabihty of this substance when exposed to the ordinary mechanisms of chemical change in the body, it seems * According to Gardner, chplesterpl jnp,y be absorbed from the intestine. THE FATS 57 probable that the part played by cholesterol is that of a framework or skeleton, in the interstices of which the more labile constituents of the proto- plasm can undergo the constant cycle of changes which make up the phe- nomena of life. PHOSPHOLIPINES OR PHOSPHATIDES The fats form the chief constituent of the deposited and reserve fat throughout the animal kingdom and are also contained in the protoplasm of the living cell. The chief fatty constituents of protoplasm differ from the above fats in the following particulars : they contain phosphoric acid and an amine. On this account they have been called phosphorised fats. Thudi- chum, who isolated various compounds of this nature from brain, suggested the term phosphatides as a general name for them. The term lipoid has also been used, but it includes all the substances composing a cell which are soluble in ether, e.g. cholesterol, cetyl alcohol, and the fats. Leathes has suggested the term phospholipine for those compounds, for it denotes that the com- pound is partly fat (hp), that it contains phosphorus, as well as a nitrogenous basic radical (ine). The phosphohpines comprise the substances lecithin, cephalin, cuorine, sphingomyeline. In brain and other tissues similar com- pounds, which contain no phosphorus, occur, and in the place of glycerol we may find galactose. Leathes has proposed calling these compounds Hpines and galactohpines. Lecithin, the chief phosphohpine, is an ester compounded of two fatty acid radicals, phosphoric acid, glycerol, and the amine, chohne. The various lecithins may be distinguished, according as they contain different fatty acid radicals, as oleyl-lecithin, stearyl-lecithin. The following formula represents distearyl-lecithin : CH2-0-OC.(CH2)ieCH3 CH— 0— OC.(CH2),6CH3 HO^' "^O.CH.^.CH^.NiCH^), OH On warming with baryta water lecithin is broken down into fatty acid, glycerophosphoric acid, and chohne. The latter base, which is trimothvl- [C2H4OH oxethyl-annnonium hydrate, N \ (OH-j)., must be distinguished from [oh neurine, N - (CH.,).; wliicli is tiiinetlivl-vitivl-aininoiiiiim hydrate, ami is [oh much more poisonous t ha lU'lioli lie. C'iioliue forms a salt with liydroi'lijorieacid, which, with platinum chloride, yields a double salt of characteristic crystalline 58 PHYSIOLOGY form, insoluble in absolute alcohol. The universal distribution of lecithin seems to indicate that it plays an important part in the metabolic processes of the cell. There is no doubt that it may serve, inter alia, as a source of the phosphorus required for building up the complex nucleo- proteins of cell nuclei. It seems to represent an intermediate stage in the utilisation of neu- tral fats by protoplasm, and its occurrence in the brain as a constituent of more complex molecules, which contain also a carbohydrate nucleus (galacto- sides, such as cerebrin), might be interpreted as indicating some share also in the metabohsm of carbohydrates. Lecithin may be extracted from tissues by boihng with absolute alcohol. On coohng the alcohohc extract in a freezing mixture, the lecithin separates out as granules or semi-crystalline masses. When dried in vacuo, it forms a waxy mass, which melts at 40° to 50° C. In water it swells up to form a paste, which, under the microscope, is seen to consist of oily drops or threads, the so-called myehn droplets. In a large excess of water it forms an emulsion or a colloidal solution. Its power of taking up water on the one hand, and its solubihty in alcohol and similar media on the other, give it an intermediate position between the water-soluble crystalloids and the insoluble fats, and enable it to play an important part both as a vehicle of nutritive substances and as a constituent of the hpoid membrane, which bounds and determines the osmotic relationships of all Hving cells. The phospholipines are provisionally classified according to the proportions of N and P in their molecule, as follows : (a) Mono-aminc-monophosphatides, N : P = 1 : 1 (including lecithin and cephalia). (6) Diamino-mono-phosphatides, N : P = 2 : 1 {e.g. sphingomyelin). (c) Mono-amino diphosphatides, N : P = 1 : 2 {e.g. cuorin, a lipine extracted from heart muscle by Erlandsen). {d) Diamino-diphosphatides, N : P = 2 : 2. (e) Triamino-monophosphatide, N : P = 3 : 1 (an example has been reported as occurring in egg yolk). All these bodies (except cuorin) are obtained by the extraction of the brain or of nerve fibres. Many also occur in egg yolk. The galacto-lipines include two substances extracted from the brain, viz. phrenosin and kerasin. Both these on decomposition yield galactose, a nitrogenous base called sphingosine and a fatty acid. We know little or nothing of their significance. SECTION IV THE CARBOHYDRATES The carbohydrates are a group ot bodies oi wide distribution and great importance in both the vegetable and animal kingdoms. In plants the first product of assimilation of carbon is a carbohydrate, and in animals these substances form one of the most important sources of energy. They consist of the elements carbon, hydrogen, and oxygen, the two last-named being almost invariably in the proportions necessary to form water. It is on this account that the term carbohydrate has been given to the group. Their general formula might be expressed C^HgnOn- Certain derivatives of the group, obtained by the substitution of methyl and other radicals for a hydrogen atom, though necessarily classified with carbohydrates on account of their reactions, do not conform to this general formula, e.g. rhamnose, CgHiaOg.' All the carbohydrates which are of importance in the animal economy contain six carbon atoms or a multiple of this number. Analogous substances, however, can be prepared containing less or more than this number of carbon atoms. A series of compounds exist which contain in their molecule 2, 3, 4, 5, 6, 7, 8, 9 carbon atoms, and are termed dioses, trioses, tetroses, pentoses, hexoses, heptoses, and so on ; the termination ' ose ' with the Greek numeral prefixed, indicating the number of carbon atoms, gives them a distinct designation. These are all oxidation products of pol3'atomic alcohols, being either ketones or aldehydes of these alcohols. Thus from COH glycerol we may obtain glyceryl aldehyde CHOH and dioxyacetone CH2OH CH2OH CO. Both these substances behave as sugars and beloiig to the group of CH2OH trioses. They are generally obtained together and are called glycerose. " CH2OH From the hexatomic alcohol (CHOH), we may obtain either the aldehyde CH2OH 59 60 PHYSIOLOGY CH2OH COH CO (CH0H)4 or the ketone (CH0H)3. These two oxidation products of the CH2OH CH2OH polyatomic alcohols are known as aldoses and ketoses respectively. All these compounds are distinguished by the termination ' ose.' It is convenient to call those compounds containing six carbon atoms the sugars, because it is to this group that the natural sugars belong. Stereoisomerism in the Sugars. It will be noticed that of the six carbon CH2OH atoms contained in the sugar molecule, e.g. the aldose (CII0H)4, four COH are asymmelric, i.e. their four combining affinities are saturated with groups of different kinds, viz. several carbon atoms, one H atom, and one OH group : ^ H— C— OH C They must therefore present many stereoisomeric forms. If n represent the number of asymmetric carbon atoms in a compound, the possible number of stereoisomers is 2". Thus an aldehexose with four asymmetric carbon atoms (CH0H)4 must present 2* isomers, i.e. sixteen isomeric compounds, so that there must be sixteen sugars all possessing the formula CH20H(CHOH)4COH, in addition to the inactive sugars obtained by a mixture of two oppositely active members of this group. Of the sixteen possible sugars of this formula, as many as fourteen have been found or have been artificially prepared. Only a small number are, however, of any physiological importance. These include the aldoses, glucose, mannose, and galactose, and the ketose, fructose or levulose. All the other sugars are unassimilable by the animal cell and are not manufactured by plants. Since these sugars can be divided into the optically active and the inactive varieties, an obvious mode of designation would be to represent them as d-, 1-, and i- varieties respectively, i.e. dextro-rotatory, Isevo-rotatory, and inactive. On Fischer's suggestion, however, this mode of nomenclature has been altered in favour of representing, by the letter prefixed, not the optical qualities of the substance in question, but its relation to other substances, especially glucose. Thus, d-fructose means that fructose is the ketose corre- sponding to the dextro-rotatory glucose, d-fructose itself being leevo-rotatory, though its active asymmetric C atoms are identically arranged with those in THE CARBOHYDRATES 61 glucose. With this hmitation one may say that it is only the d-hexoses of a particular form which are assimilable, and therefore of physiological im- portance. The small differences in the configuration of the four d-sugars can be readily seen if their graphic formulae be compared : CHO CHO CH2OH CHO H.C.OH HO.C.H CO H.C.OH HO.C.H HO.C.H HO.C.H HO.C.H H.C.OH H.C.OH H.C.OH HO.C.H H.C.OH H.C.OH H.C.OH H.C.OH CH2OH CH2OH CH2OH CH2OH d-ghicose d-mannose d-fructose d-galactose THE PENTOSES. C5H10O5 These bodies occur largely iu plants in the form of complex polysaccharides, the pentosanes, wliich give pentoses on hydrolysis with acids. Two forms of pentose have been found in the animal body, namely, i-arabinose, which has been isolated from the urine in cases of pentosuria, and 1-xylose (or d-ribose, Levene), which occurs built up into the nucleic acid molecule of the pancreas and perhaps other organs. The pentoses can apparently be utilised by herbivora as food-stuffs. We know nothing as to the part they play in the animal body or as to the causation of the rare condition of jientosuria. Since, however, they are reducing substances and the presence of pentose in urine might therefore lead to a suspicion of diabetes, it is necessary to mention the tests by which the presence of pentoses may be detected. The two following are the chief tests for pentoses : (1) The solution supposed to contain a pentose is mixed with an equal volume of concentrated hydrochloric acid. To the mixture is added a small quantity of solid orcin and the whole is heated. If pentose is present the solution becomes at first reddish-blue and later bluish-green. The colour can be extracted on shaking the fluid with amyl alcohol, the solution, on spectroscopic examination, showing an absorption band between C and D. (2) Instead of adding orcin, we may add phloroglucin to the mixture of hydrochloric acid and pen'ose. The solution on heating becomes first cherry red and then cloudy. On shaking with amyl alcohol a red solution is obtained which shows a band between D and E. THE HEXOSES AND THEIR DERIVATIVES The most important of the carbohydrates belong to this class and are either hexoses or formed by a combination of two or more hexose molecules. They are divided into three main groups : (1) MonosaccJiarides, with the fornnda CgHjaOe, examples of which are glucose, fructose, &c. (2) Disaccharides, which are derived from two molecules of a monosac- charide with the elimination of a molecule of water, as follows : ^CgHiaOg — Hp = CiaHgaOii- (Examples, maltose and cane sugar.) 62 PHYSIOLOGY (3) Polysaccharides, composed of three or more molecules of a mono- saccharide. The number of molecules which are associated in the com- pounds of this group may be very large. We can regard their general forma- tion as represented by the following equation : nCeHi^Oe - nH^O = {C,B.r,0,),. (Examples, starch, dextrin, &c.) THE MONOSACCHARIDES Only four hexoses out of the large number which have been synthetised are assimilable by the animal body. These are mannose, glucose, galactose, and fructose, the three former being aldoses, while the last is a ketose. All of them are derivatives of d-glucose. They may be synthetised in several ways. The most interesting, because it probably represents the mechanism of synthesis of hexoses in plants, is the formation from formaldehyde. In alkahne solutions formaldehyde polymerises with the formation of a mixture of hexoses known as acrose. From this mixture a-acrose can be isolated by the formation of its osazone and the reconversion of this osazone into sugar. It is found to be identical with i- fructose. If a solution of this i-fructose be treated with yeast, d-fructose is fermented, leaving 1-fructose behind. For the preparation of d-fructose it is necessary to convert the inactive sugar into the corresponding acid, mannonic acid. This with strychnine or morphia forms salts which can be separated into the d- and 1- groups by fractional crystallisation. From the d- modification d-mannose can be obtained, and this by conversion into the osazone and reconversion into a sugar gives d-fructose. All the monosaccharides, however many carbon atoms they contain, present certain general reactions determined by their chemical composition. (o) Like ordinary aldehydes and ketones, the sugars act as strongly reducing substances, and, hke aldehydes, reduce ammoniacal solution of silver to metalhc silver, and many of the higher oxides of metals to lower oxides. On this behaviour is founded the commonest of all the tests for the presence of reducing sugar- — Trommer's test. (6) On oxidising a monosaccharide the COH group becomes converted to COOH. Thus, glucose on oxidation gives gluconic acid : COH(CHOH)4CH20H -f 0 = C00H(CH0H)4CH20H. On further oxidation the end group CHgOH also is affected, and we obtain a dibasic acid. Thus glucose gives saccharic acid. (c) By means of nascent hydrogen the monosaccharides can be reduced to the corresponding polyatomic alcohol. Thus the three hexoses, glucose, fructose, and galactose, give the corresponding three alcohols, sorbite, man- nite, and dulcite C6H14O6. {d) Another important general reaction of the monosaccharides depending on the COH or the CO group is the reaction with phenyl hydrazine. On warming a solution of sugar with a solution of phenyl hydrazine in acetic THE CARBOHYDRATES 63 acid, the following reactions take place. The first reaction results in the production of a hydrazone : CH20H(CHOH)3CHOHCHO + H^N.NH.CeH^ = CH20H(CHOH)3CHOH.CH : N.NH.CeHs + H.p. The hydrazone then reacts with another molecule of phenyl hydrazine with the production of an osazone : CH2(OH)(CHOH)3CHOH.CH : N.NH.CeH^ + H^N-NHCeHs = CH20H(CH0H)3C.CHN.NH.CeH5 II N.NH.CcH^ + H2O + H2. The hydrogen formed in this reaction acts upon a second molecule of phenyl hydrazine, splitting it into aniline and ammonia. On this account it is always necessary to have an excess of phenyl hydrazine in the operation. The osazones form well-defined crystalUne products which are generally yellowish in colour and difier in their melting-point and in their crystalline form. They are therefore of extreme value in the separation and identifica- tion of different carbohydrates. They can be also used for the artificial preparation of certain sugars. Under the influence of acetic acid and zinc dust they form osamines, which on treatment with nitrous acid are recon- verted into the corresponding sugar, generally a ketose. GLUCOSE, DEXTROSE or GRAPE SUGAR, is the chief constituent of the sugar of fruits, especially of grapes. It occurs in the body as the end- product of the digestion of starch. When pure it forms white crystals which melt at 100° C, and lose the one molecule of water of crystalHsation at 110° C. It is easily soluble in water, and the solution shows bi-rotatiou. Its final specific rotatory power at 20° C. is 52-74. TESTS FOR GLUCOSE. Trommer's test depends on the power possessed in comnion with the other sugars of reducing cupric hydrate to cuprous oxide. The sugar solution is made alkaline with caustic potash or soda, and a few drops of coppex sulphate solution added. On heating the blue solution thus obtained to boiling, it turns yellow, and a yellowish-red precipitate of cuprous hydrate is prodiiccd. This test is generally performed with Fehling's solution, which consists of an alkaline solution of cupric hydrate in Rochelle salt. The proportions in making the solutions are so arranged that 10 c.c. of Fehling's solution are completely reduced by -05 gramme glucose. This reaction is made use of for the quantitative determination of glucose in solution. The determination may be carried out either volumetrically, as in Fehling's or Pa\'y"s method, or gravimetrically, as in Aliihn's method. Moore's Test. A solution of glucose treated with a little sti'ong caustic potash or soda and warmed, becomes first yellow and then gradually dark brown, and gives off a smell of caramel. With ordinary yeast, glucose solution-s ferment readily, giving off COo, and form alcohol with small traces of amyl alcohol, glycerin, and succinic acid. With phenyl hydrazine glucose gives well-marked needles of glucosazone. These are precipitated when the liquid is still hot, the precipitate being increased as the liquid cools. The crystals form bundles of fine yellow needles which are almost in- soluble in water, but are soluble in boiling alcohol. When purified by recrystallisation they melt at 204-205° C. On treating a watery solution of glucose with benzoyl cJiloride and caustic soda 64 PHYSIOLOGY and shaking till the smell of benzoyl chloride has disappeared, an insoluble precipitate is produced of the benzoic ester of glucose. This method has been often used for isolating glucose from fluids in which it occurs in minute quantities. Molisch's Test. On treating 0-5 c.c. of dilute glucose solution with one drop of a 10 per cent, alcoholic solution of a-naphthol, and then jDouring 1 c.c. of concentrated sulphuric acid gradually down the side of the tube, a purple ring is produced at the junction of the two fluids, which on shaking spreads over the whole fluid. This reaction depends on the formation of furfurol from the glucose. In order to identify glucose in a normal fluid, the following tests may be applied, after removing any protein which may be present : (1) Reduction of cupric hydrate or Fehling's solution. (2) Estimation of reducing power of solution. (3) Estimation of rotatory power of solution on polarised light. (4) Formation ©f osazone crystals with phenyl hydrazine. These crystals must come down while the fluid is still hot. They must be purified and their melting-point taken. A determination by combustion of their nitrogen content will give direct information whether the sugar is a monosaccharide or disaccharide. (5) The solution is made acid and boiled for some time. It is then made up to its former volume and its reducing power and efliect on polarised light once more taken. In the case of a disaccharide, which would be converted into monosaccharide by boiling in acid solution, these two readings would be altered, whereas neither the rotatory power nor the reducing power of glucose would undergo any change. (6) Fermentation with ordinary yeast. A positive result would exclude glycuronic acid. D-FRUCTOSE, or LEVULOSE, occurs mixed with dextrose in honey and in fruit sugar. It is also, with glucose, formed by the digestion or inver- sion of cane sugar. It is crystallisable with difficulty. Its watery solution is Isevo-rotatory, and reduces Fehling's solution somewhat less strongly than glucose, its reducing power being 92, if we take that of glucose as 100. It ferments readily with yeast ; with phenyl hydrazine it gives the same osazone as is formed from glucose. GALACTOSE is formed by the digestion or hydrolysis of milk sugar or lactose. It is also obtained on hydrolysing cerebrin, a constituent of the brain, with dilute mineral acids, and by the hydrolysis of certain vegetable gums. It is much less soluble in water than glucose. It is dextro-rotatory and shows marked bi-rotation. With ordinary yeast it ferments but ex- tremely slowly. One species of yeast is known, namely, saccharomyces apicu- latus, which, while fermenting d-fructose and glucose, has no effect on galac- tose. This yeast can therefore be used to isolate galactose from a mixture of the monosaccharides. It reduces Fehling's solution, its reducing power being somewhat less than that of glucose. Yeasts can be trained to ferment galactose. MANNOSE. Mannose, though an assimilable sugar, is of such rare occurrence in our food-stuffs that it plays practically no part in animal physiology. It is dextro- rotatory, reduces Fehling's solution, ferments easily with ordinary yeast, and gives ■m osazone which is identical with that derived from glucose. DERIVATIVES OF THE HEXOSES Two 'derivatives of glucose are of considerable physiological importance, namely, glucosamine and glycuronic acid. THE CARBOHYDRATES 65 Glucosamine, CgHiaNOs, has the structural formula : CH2OH (GH.0H)3 CH.NH2 CHO It is obtained from chitin, which forms the exoskeleton of large numbers of the invertebrata, by boiling this with concentrated hydrochloric acid. It is stated to have been obtained as a decomposition product of certain proteins and their derivatives, such as the mucins. It is of special interest as affording an intermediate product between the carbohydrates and the oxy- amino acids which can be obtained by the disintegration of proteins. In solution it is dextro-rotatory, reduces FehHng's solution, and gives an osazone resembling that derived from glucose. GLYCURONIC ACID, CgHioO,, may be regarded as one of the first results of oxidation of the glucose molecule. The group which has undergone oxidation is not the readily oxidisable CHO group, but the CHgOH group at the other end of the molecule. The formula of this acid is therefore : COOH t I (CH.OH)^ CHO. In the free state it does not occur in the animal body. It is constantly found in the urine after administration of certain drugs such as phenol, camphor, or chloral, and then occurs as a conjugated acid with these substances. These conjugated acids are Isevo-rotatory, though the free acid is dextro-rotatory. In the free state it reduces FehHng's solution and gives an osazone which is not sufficiently characteristic to distinguish from glucosazone. It does not undergo fermentation with yeast. This test is therefore the best means of distinguishing the acid in urine from glucose. THE FORMATION OF GLUCOSIDES Tlio graphic forinuhi? given on p. Gl do not explain all the possible modes of arrange- ment of the groups of the sugar molecules. Many of these sugars, when dissolved in water, present the phenomenon known as multi-rotation. K their rotatory power be taken immediately after solution, it is found to be greater or less than the rotatory power taken some hours or days later. Glucose, for instance, immediately after solu- tion, has a high specific rotatory power, which diminishes rapidly if the solution be boiled, and more slowly if it be allowed to stand. Finally, the specific rotatory ix)wer becomes constant at + 52-5° D. This change in rotatory power seems to be associated with a change in the arrangement of the groups, the aldose, for example, assuming, by the shifting of a mobile oxj-gcn atom, what is known as a lactone arrangement. o 66 PHYSIOLOGY Thus glucose COH(CHOH).CHOH.CHOH.CHoOH becjmes CHOH . (CHOH)o . CH . CHOH . CHoOH 0 This change in the arrangement of the molecule renders a further stereoisomer it m possible, owing to the fact that now the end group which was formerly COH becomes H I 0— (>-0H so that now there are five instead of four asymmetric ca -bon atoms. The two isomers of glucose, which are thus rendered possible, are represented by the following structural formulse : H— C— OH OH— C— H HCOH HCOH CH2OH CH2OH In these molecules the OH attached to the end group can be replaced by other radicals, including other sugar molecules. In this way we get the formation of glucosides. Thus, if glucose be dissolved in methyl alcohol and be treated with hydrochloric acid, we obtain a and ft methyl glucosides, the formulae of which would b3 represented as follows : H— C— OCH3 CH3O— Cc-H HCOH HCOH CH2OH CH2OH Instead of methyl we might insert other groups, and even other hexose groujjs, such a? glucose or galactose, obtaining the, two sugars maltose and lactose, which may thus b3 regarded as glucosides — maltose as the a glucoside of glucose, lactose as the ft galac- toside of glucose. The mode of combination of the two hexose groups to form these disaccharides may be represented as follows : H H OH H H ^ CH2OH — C— C — C— C — C glucose rest OH \ HOH ^ maltose. HO H HO HO OHC — C^C — C — C-^ CH, glucose rest H OH H H THE CARBOHYDRATES H CHoOH — C /OH h\ — C — C — C — C galactose r3st OH H H OH 0 HO H HO HO OHC — C — C — C — C — CHo glucose rest H OH H H 67 lactose A very large number of glucosides occur as plant products. Among these we may mention amygdalin, salicin, phloridzin. indican, &c. THE DISACCHARIDES The disaccharides are formed by the union of two molecules of mono- saccharides with the elimination of one molecule of water, and can be re- garded, according to the manner in which the molecules are combined, as glucosides, galactosides, &c. On hydrolysis, e.g. on heating with acids, they take up one molecule of water and are spUt up into the corresponding mono- saccharides. Thus, cane sugar gives equal parts of glucose and fructose, maltose gives equal parts of glucose and glucose, while milk sugar or lactose gives equal parts of glucose and galactose. CANE SUGAR, sometimes known as saccharose, is widely distributed throughout the vegetable kingdom, and forms an important article of diet. It has no reducing power on Fehhng's solution. It is strongly dextro-rota- tory and has a specific rotatory power of + 66-5°. On hydrolysis it is converted into equal molecules of glucose and fructose. Owing to the fact that fructose rotates polarised light more strongly to the left than glucose does to the right, the mixture of the two monosaccharides so obtained is- laevo-rotatory. On this account the change from free cane sugar to the mixture of monosaccharides is known as inversion, and the mixture is often spoken of as ' invert sugar.' The term ' inversion ' has since been loosely applied to the process of hydrolysis itself, so that we often speak of the inversion of maltose or of lactose, meaning thereby the hydrolysis of these sugars with the production of their constituent monosaccharides. With yeast, cane sugar first undergoes inversion by a special ferment present in the yeast {invertase), and the mixture of fructose and glucose is then fermented. MALTOSE is formed during the hydrolysis of starch by acids or by digestive ferments, and is also the chief sugar in germinating barley or malt. It is strongly dextro-rotatory, ferments easily with yeast, and reduces Fehling's solution ; its reducing power is about 70 per cent, of that of glucose. With phenyl hydrazine it gives phenyl maltosazone, which forms definite yellow crystals with a melting-point of 206° C. MILK SUGAR or LACTOSE is found only as a constituent of milk. It forms colourless rod-like crystals, which are much less soluble in water than are the two other disaccharides. On account of this solubility it is much less sweet than either cane sugar or maltose. It is dextro-rotatory and 68 PHYSIOLOGY shows bi-rotation. It is not fermented by ordinary yeast. Before fermen- tation can occur the lactose must be spht by the agency of acids or by a ferment, lactase, which occurs in the animal body and in certain moulds, into the monosaccharides glucose and galactose. Lactose reduces Fehling's solution and gives with phenyl hydra2dne lactosazone, which is easily soluble in hot water and therefore does not come down mitil the fluid is cold. THE POLYSACCHARIDES These play an important part throughout the whole vegetable kingdom, where all the supporting tissues of the plants, their protective substances, and many of their reserve materials consist of members of this group. In the animal body, where the supporting tissues are composed chiefly of deriva- tives of proteins, the sole significance of polysaccharides hes in their value as food-stuffs. In plants, anhydrides both of hexoses and pentoses occur in bewildering variety. Here, however, we may confine our attention to those members of the group of polysaccharides which are important as food-stufls. STARCH (CgHioOg) is present in large quantities in nearly afl vegetable foods, and is an important constituent of the cereals, from which flour and bread are derived, as well as of tubers, such as the potato. In the plant cells it occurs as concentrically striated grains within minute protoplasmic structures — ^the amyloplasts, the office of which it is to manufacture starch from the glucose present in the cell sap. When freed, by breaking up the cells and washing with water, it forms a white powder consisting of micro- scopic grains, each of which presents the characteristic concentric striation. It is insoluble in cold water. In hot water the grains swell up and burst, forming a thick paste, which sets to a jelly on coohng. This semi-solution, as well as the original starch-grains, gives an intense blue colour on the addition of iodine. On treating starch with cold alkahes or cold dilute acid, it is converted into a soluble modification, the so-called soluble starch or amylodextrin, which also gives a blue colour with iodine. This modification is also produced as the first stage of the action of diastatic ferments upon starch. On boihng with dilute acids, starch is converted first into a mixture of dextrins, then into maltose, and finally into glucose. On acting upon starch with various ferments, such as the diastase which may be extracted from malt or germinating barley, or with the amylase occurring in sahva or pan- creatic juice, it undergoes hydrolysis, the final result of the action being a mixture of four parts of maltose to one part of dextrin. As to the inter- mediate stages in this reaction opinions are still divided. The first product is soluble starch, amylodextrin, giving a blue colour with iodine. This breaks up into a reducing sugar, and another dextrin, erythrodextrin, which gives a red colour with iodine, and this dextrin, on further hydrolysis, yields reducing sugar and achroodextrin, which is not coloured by the addition of iodine. Thus there are a series of successive hydrolytic decom- positions of the molecule, each resulting in the splitting off of a molecule of sugar and the production of a lower dextrin. The DEXTRINS are ill-defined bodies which are difiicult to separate. THE CARBOHYDRATES 69 They are amorphous white powders, easily soluble in water, forming solutions which, when concentrated, are thick and adhesive. They are insoluble in alcohol and ether. With cupric hydrate and caustic alkah they form blue solutions, which reduce shghtly on boiUng. They are not precipitated by saturation with ammonium sulphate. On boiUng with dilute acids, they are converted entirely into glucose. The changes undergone by starch during its hydrolysis by means of diastase have been used by Brown and his co-workers as a method of arriving at some idea of the size and structm'e of the starch molecule. Proceeding from the discovery that the end-products of this reaction consisted of 81 per cent, maltose and 19 per cent, dextrin, they concluded that starch must consist of five dextrin-like groups, four of which are arranged symmetrically round the fifth. At each stage one of these groups is split off and hydrolysed to form malto-dextrin : - L^i ^u ""/-> "\ \ ^^^ molecule of water being ■^ "^ l(Cl2H2oOio)2J taken up. The malto-dextrin group is then changed into maltose by the further assimilation of two molecules of water. The central dextrin-like group is attacked with great difficulty by the ferment, and therefore remains at the end of the reaction as achroodextrin. The malto-dextrin, the penultimate stage in the action of diastase, can be regarded as formed by the condensation of three molecules of maltose attached by the oxygen of two CHO groups, so that one CHO group remains free and determines the reducing power of the malto-dextrin molecule. Its formula may therefore be repre- sented as follows : K /C12H20O9 the sign,-' being used to denote the open terminal CHO group. They further found that the stable dextrin remaim'ng at the end of the diastatic hydrolysis of starch probably had the formula of 4OC6H10O5H0O, and might be regarded as a condensation of forty glucose molecules with the elimination of thirty-nine molecules of water. The starch molecule caimot be less than five times that of the stable achroo- dextrin. Since the latter has a molecular weight of 6498, the molecular weight of starch cannot be less than 32,400, and its empirical formula can be represented by : IOOC12H20O10, or (8OC10H00O10.4OC6H10O5). INULIN. Another kind of starch, known as inuUn, occurs in dahUa tubers. It is easily hydrolysed by weak acids, and is entirely converted into d-fructose, or levulose. GLYCOGEN, or animal starch, is found in the Hver, muscles, and other tissues of the body, and occurs in large quantities in all foetal tissues. It is a white powder, soluble in water, forming an opalescent solution. It is precipitated from its solution on the addition of alcohol to 60 per cent., or by saturation with solid ammonium sulphate. On boiling with acids, it is entirely converted into glucose. It is affected by the ferments diastase and amylase, in the same way as vegetable starch, giving first dextrins and finally a mixture of maltose and dextrin. With iodine it gives a mahogany-red colour, which, like the blue colour produced in starch, is destroyed by boiling, to return again on coohng. We shall have occasion to consider its properties more fully when we are deaUng with the functions of the Hver. 70 PHYSIOLOGY THE CELLULOSES. Cellulose (C6Hio05)^ is a colourless, insoluble material, or mixture of materials, which compose the cell walls of the youPxger parts of plants, and therefore forms a constituent of most of our vegetable foods. It is insoluble in water or dilute acids or alkalies, its only solvent being an ammoniacal cupric oxide solution. On boiUng with strong acids, it gradually undergoes hydrolysis and yields sugar, the nature of which varies according to the source of the cellulose. In herbivorous animals cellu- lose undergoes digestive changes and forms an important constituent of their food. The solution of the cellulose in this case is effected by the agency, not of ferments secreted by the wall of the gut, but of micro-organisms which swarm in the paunch of ruminants and in the caecum of other herbivora. In some cases the effective agent is a cytase present in the vegetable cells themselves. Since this ferment is destroyed by boiUng, cooked hay is much less digestible than in the raw condition. In certain invertebrata it seems probable that a true cellulose- digesting ferment, or cytase, is secreted by the walls of the ahmentary canal. In man cellulose undergoes practically no change in digestion, and serves merely by its bulk to promote peristalsis and the normal evacuation of the bowels. A further consideration of its chemical properties, as well as of the closely allied vegetable materials, gums, pectins, mucilages, derived for the most part from the condensation of pentose molecules, may be dispensed with here. SECTION V THE PROTEINS As sources of energy to the organism all three classes of food-stuffs are valuable in proportion to their heat equivalents, and it is often a matter of indifference whether the main bulk of the energy required is supphed at the expense of fat or at the expense of carbohydrate. The proteins, however, form the most important constituent of hving protoplasm. On this account protein must always be present in the food to supply the material necessary for building up new protoplasm in the growing animal and for replacing the waste of hving material which is taking place in the discharge of its normal functions. Regarding the complexity of reaction presented by hving protoplasm as determined in the first instance by the chemical and physical complexity of this material itseK, we should expect to find that the proteins forming its main constituents, would themselves partake of some of this quahty. The carbohydrates and fats, although in many cases made up of huge molecules, are nevei-theless built up on a very simple type. Starch, for instance, with a molecular weight of over 30,000, is formed simply by the polymerisation of glucose molecules. The ordinary fats, stearin and palmitin, consist of fatty acids with long straight chains of CHg groups combined with the glyceryl radical. Their molecular weight is very large, but their molecules are simple in structure. When, however, we break up a protein molecule we meet with a great number of subsidiary groups, the presence of which is essential to the making of a nutritive protein. Owing to this complexity of structure it is not easy to give a simple definition in chemical terms of what we mean by the term ' protein.' It is necessary rather to describe certain of the quahties presented by this group, the possession of which we regard as essential to the conception of a protein. Elementary Composition. All proteins contain oxygen, hydrogen, nitro- gen, carbon, and sidphur. The proportion of these elements in the various proteins may be represented as follows : C 50-6-54-5 per cent. H 6-5- 7-3 „ „ N 15-0-17-6 „ „ S 0-3- 2-2 „ „ 0 21 •5-23-5 „ „ Nearly all the proteins contain a small trace of phosphorus varying from 71 72 PHYSIOLOGY 0-4 to 0*8 per cent. It is doubtful, however, how far this phosphorus forms an integral part of the protein molecule. Physical Characters. The proteins are amorphous indiifusible substances belonging to the class of bodies known as colloids. Most of them are soluble either in water, weak salt solutions, or in dilute acids or alkalies. They are inert bodies and tasteless. Although they form compounds with various metalhc salts, acids, or alkahes, these compounds are but ill defined, and the relative proportions of the ingredients vary according to the conditions under which the compound was formed. As is the case with most colloids when in solution or pseudo-solution, they can be brought into an insoluble form by various simple agencies, such as shaking, change of temperature, alteration of reaction, or addition of neutral salts. Coagulation by heat forms a dis- tinguishing feature of a number of members of this class, which are therefore spoken of as ' coagulable proteins.' For instance, white of egg is a solution of different proteins. On diluting it with weak salt solution no precipitation takes place. If, however, the solution be heated to about 80° C. a precipitate of coagulated protein is formed. If a strong solution be boiled the whole fluid sets to a sohd white mass (hydrogel). This change is irreversible, i.e. it is not possible by lowering the temperature to bring the white of egg again into solution, and many properties of the protein have been changed in the act of coagulation. With certain proteins and their allies the coagulation on change of temperature is a reversible process. Thus an alkaline solution of caseinogen, the chief protein of milk, if treated with a Httle calcium chloride and heated, undergoes coagulation and sets into a jelly, but on cool- ing the mixture the coagulum once more enters into solution. Ordinary gelatin, which is closely allied to the proteins, with water forms a solid jelly below 20° C, and a fluid solution above this temperature. If a protein be heated in a current of air or oxygen it undergoes combustion. In all cases a certain amount of incombustible material is left, consisting of inorganic salts which were closely attached to the protein molecule. If a solution of protein be subjected to long- continued dialysis, the proportion of ash may be diminished very largely, but in no case has any experimenter succeeded in obtaining a preparation of protein absolutely ash-free. On this account it has been thought that the salts of the ash must be in chemical combination with the protein ; but having regard to the physical character of colloidal solutions, which we shall study in the next chapter, and the power of adsorption of substances possessed by such solu- tions, there is no need to regard these salts as essential constituents of the protein. Crystallisation of Proteins. Although the indiffusibihty of protein solu- tions differentiates them from the crystalloid substances such as sugar or sodium chloride, under certain conditions it is possible to obtain crystals consisting, largely at any rate, of proteins. Thus in the seeds of certain plants, e.g. hemp seeds, Brazil nut, pumpkin and castor-oil seeds, the so-called aleurone crystals may be seen under the microscope enclosed in the proto- plasm of the cells. These crystals consist of proteins belonging to the class of THE PROTEINS 73 globulins. By chemical means they can be separated from the surrounding tissues and, after washing, dissolved in a solution of magnesia. Drechsel showed that on dialysing such a solution against alcohol, the fluid undergoes gradual concentration, and crystalline granules of the magnesia compound of the protein separate out. These crystals contain 1 4 p.c. MgO. A better method of obtaining such crystals has been devised by Osborne. The ground seeds are extracted with 10 per cent, sodium chloride solution, and filtered. The filtrate is diluted with water heated to 50° or 60° C. until a shght turbidity forms. After warming the diluted solution until this turbidity disappears, and then allowing it to cool slowly, the protein separates in well-developed crystals. It is possible also to obtain crystals of animal proteins. Haemoglobin, the oxygen- carrying protein of the red blood corpuscles, can be made to crystalhse with extreme ease. With some animals, such as the rat, it is only necessary to bring the haemoglobin into solution, by the addition of a httle distilled water and ether to the blood, to cause the crystalUsation of the hberated haemoglobin. Egg albumin and serum albumin may also be crystalhsed with ease by a method devised by Hofmeister and improved by Hopkins. If, for instance, we wish to crystalhse egg albumin, white of eggs is treated with an equal bulk of saturated solution of ammonium sulphate in order to precipitate the globulin. It is then filtered, and the filtrate is treated with saturated ammonium solution until a shght permanent precipitate is produced. This precipitate is then just redissolved by the cautious addition of water, and dilute acetic acid (10 per cent.) is added drop by drop until a shght precipitate is produced. The flask is now corked and allowed to stand for twenty-four hours, when the precipitate, which will have in- creased in quantity, will be found to consist entirely of acicular crystals. A similar method may be used for serum albumin. In each case the crystals contain a considerable proportion of ammonium sulphate. This may be replaced by sodium chloride by washing the crystals with a saturated solution of this salt. By absolute alcohol the crystals may be coagulated and may be then washed practically free from salt, but it is not possible to obtain crystals of coagulable protein free from the presence of some salt. Although by repeated crystalhsation of egg albumin a product may be obtained which is absolutely constant in both its physical and chemical characters, we cannot ascribe to crystalhsation the same importance in securing purity and homogeneity of the substance that we can when we are dealing with inorganic salts. This is due to the fact that these crystals take up other colloids with great ease. When haemoglobin, for instance, is crystal- hsed from blood, the first crop of crystals, although thoroughly washed from their mother liquor, always contain a considerable proportion of serum albumin. Indeed, the presence of colloidal material seems to render the pro- duction of the so-called mixed crystals much more easy. Thus Schultz has shown that in urine mixed inorganic crystals can be obtained. Human urine is allowed to stand twenty-four to forty-eight hours with dicalcium phosphate and then filtered. On allowing the filtrate to evaporate slowly, 3* 74 PHYSIOLOGY a crystalline precipitate is produced consisting of whetstone-shaped crystals which are doubly refracting. On treating these crystals with dilute acetic acid this acid extracts calcium phosphate from the crystals. The original shape of the crystals is, however, retained. The only difierence under the microscope consists in the fact that they have now lost their doubly refracting power on polarised hght. They consisted of a mixture of calcium sulphate and calcium phosphate, from which, on treatment with acid, only the calcium phosphate was dissolved out. The Molecular Weight of Proteins. We may arrive at an approximate idea of the minimum size of the protein molecule in various ways, though in all cases om' calculations are apt to be vitiated by the difficulty of obtaining a preparation which is homogeneous, i.e. chemically pure, and by the ease with which molecules of the size which we must assume for proteins form adsorption combinations in varying proportions with other substances. If we assrnne that each molecule of the respective protein contains only one atom of sulphur, we can calculate its molecular weight. It is evident that the protein which contains 1 per cent, of sulphur will have a molecular weight of 3200. In this way the following molecular weights have been arrived at (Abderhalden) : Sulphur per cent. Molecular weight, Edestin 0-87 3680 Oxyhsemoglobin . . 0-43 7440 (horse) Serum albumin . 1-89 1700 (horse) Egg albumin . 1-30 2460 Globulin , 1-38 2320 The greater part at any rate of the sulphur in the protein molecule occurs as a constituent of a substance, cystine, each molecule of which contains two atoms of sulphur. Each molecule of protein must also contain two atoms of sulphur, and we must regard double the molecular weight given, in this Table as the minimum molecular weights of these various proteins. Some idea of the molecular complexity represented by these weights may be gained by writing out the empirical formulae of the various proteins, e.g., Egg albumin C204H322N52O66S2 Protein in hsemoglobin (from horse) , . . C680H1098N210O241S2 Protein in haemoglobin (from dog) . . . C725H1171N194O214S2 Crystalhsed globuhn (from pumpkin seeds) . 039211481^2008382 With some proteins we may make use of other elements to arrive at an idea of the approximate molecular weight. Thus, oxyhsemoglobin contains between 04 and 0-5 per cent. iron. If we assume that each molecule of oxyhsemoglobin contains one atom of iron, its molecular weight must be from 11,200 to 14,000. Attempts have been made to solve the same question by studying the compounds of proteins with inorganic salts or oxides. Thus, the crystals of .globuhn from pumpkin seeds prepared with magnesia contain 1-4 per cent. THE PROTEINS 75 MgO. Assuming that one molecule of protein has coml)ined with one molecule MgO, the molecular weight of the protein must be about 28(J(j. (If a; be the molecular weight X 100 — 1-4 40 1-4 .-. X = 2817) Harnack has shown that many proteins are precipitated from their solutions as copper compounds by the addition of copper sulphate. Harnack found that this precipitate of copper contained either 1-34 — 1-37 Cu. or 2-48 — 2-73 per cent. Cu. The smaller percentage would correspond to a molecular weight of 4700, while the second number might be accounted for on the hypothesis that each molecule of protein was combined wath two atoms of copper. Similar attempts have been made by determining the amount of acid or alkali necessary to keep certain types of protein in solution. We shall see later on, however, that the amovmts vary largely with the physical condition and previous history of the colloidal substance. We are dealing here not with compounds in the strict chemical sense of the term, but with adsorption compounds, where the quantities taken up are determined not only by the chemical nature of the protein itself, but by the state of aggrega- tion of its molecules. It is therefore impossible to lay any great stress on the determinations of the molecular weight which have been effected in this way. Some clue to the size of the protein molecule is afforded by determinations of the osmotic pressure or molecular concentration of their solutions by physical methods. When w^e determine the freezing-point or boiling-point of protein solutions, the depression of freezing-point, or elevation of boiling- point is so small that it falls within the hmit of experimental error or is no greater than can be accounted for by the inorganic salts present in the solution. Since, however, colloidal membranes, such as films of gelatin or vegetable parchment, are impervious to proteins, we can directly deter- mine the osmotic pressure of their solutions. In many cases no osmotic pressure whatever is found. In other cases, e.g. egg albumin, or serum, the colloidal constituents of these solutions are found to give an osmotic pressure of such a height that 1 per cent, protein corresponds to about 4 mm. Hg. pressure. Such an osmotic pressure would indicate a molecular weight for the serum proteins of about 30,000. A determination of the osmotic pressure of haemoglobin by Hiifner gave a molecular weight about 16,000. These results, however, must be received with caution, since we are not justified in applying to these gigantic molecules data derived from a study of smaller molecules such as salt or sugar. Even if we accept these determinations of osmotic pressure as indicating the molecular weights I have just quoted, it is evident that a very shght degree of aggregation of the molecules into larger complexes will bring the osmotic pressure below the point at which it is measurable, and would transform the solution into a suspension of particles in which one could not expect to find any osmotic pressure whatsoever. 76 PHYSIOLOGY THE STRUCTURE OF THE PROTEIN MOLECULE We can arrive at some idea of the manner in which the protein molecule is built up only by breaking it down bit by bit, employing methods which, while resolving the large molecule into its proximate constituents, will not act too forcibly in changing the whole arrangements of these constituents. The relation of the starches or polysaccharides to the sugars was found by studying the hydrolysis of the former, and it is by the hydrolysis of the pro- teins that we have arrived at most of our present knowledge of their con- stitution. Contributory evidence may also be gained by the use of oxidising agents or by employing the refined methods of analysis possessed by certain hving organisms — ^bacteria, by which means we can effect hmited oxidations or reductions or can replace an NHg group by H, or a COOH group byH. ACID HYDROLYSIS OF PROTEINS. For this purpose rather stronger acids are used than for the hydrolysis of starch. The protein is heated for twenty-four hours in a flask fitted with a reflux condenser either with con- centrated hydrochloric acid or with a 25 per cent, sulphuric acid. Hydro- chloric acid was first made use of by Hlasiwetz and Habermann, who added a certain amount of stannous chloride to the mixture in order to prevent any oxidation taking place. We obtain in this way an acid fluid containing an extremely complex mixture of various substances, all of which belong to the class of amino-acids, and must be regarded as the proximate constituents of the protein molecule. A similar hydrolytic change may be effected by the use of digestive ferments obtained either from the ahmentary canal of higher vertebrates or from certain plants. Thus we may use pepsin, the active constituent of the gastric juice, trypsin, the proteolytic ferment secreted by the pancreas, papaine, or other vegetable ferments obtained from papaya, from pineapple juice, and so on. These ferments are all milder in their action than the strong acids. Pepsin, for instance, only efiects a partial decomposition of the protein molecule. Its action results in the formation of substances which still present all the protein reactions d,nd are classified as hydrated proteins or as proteoses and peptones. Trypsin carries the protein a stage further and gives a mixture of amino-acids. Certain groups, however, of the protein molecule present a considerable resistance to the action of trypsin, so that when its action is complete these groups are still found not yet broken down into their constituent amino-acids. The putrefactive processes determined by the process of bacteria in solutions of proteins are somewhat too comphcated in their results to throw much illumination on the structure of the protein molecule itself. This method is, however, of extreme value when it is appHed to isolated con- stituents of the proteins. Under the action of these bacteria we may have a process of deamination which may be accompanied by simple hydrolysis or by reduction. In the former case an amino-acid may be converted into an oxyacid, in the latter case into a fatty acid. THE PROTEINS 77 Thus tyrosine under the action of bacteria of putrefaction may spUt up into ammonia and oxyphenyl propionic acid. OH.C6H4.CH2.CHNH2.COOH + H2 = HO.C6H4.CH2.CH2.COOH + NH3 Under the action of yeasts an amine may become an alcohol. C5H11.NH2 + H2O = C5H11.OH + NH3 (amylamine) (amylalcohol) On the other hand, the effect of the bacteria may be to spUt off carbon dioxide from the amino-acids. Thus, the diamino-acid, lysine, CH2NH2 CH2NH2 CH2 CHa I I CH2 becomes CHg pentamethylene diamine. CH2 CH2 CH.NH2 CH2NH2 I COOH Tyrosine becomes p. oxyphenylethylamine, a substance ha^^ng marked physiological effects, and an important constituent of ergot. Phenylalanine C6H5.CH2.CH.NH2.COOH, becomes phenylethylamine CeHs.CHa.CHg.NHj. These reactions are therefore of value in determining the exact grouping of the atoms in the more complex of the proximate constituents of the proteins. Since all the known disintegration products of the proteins belong to the class of amino-acids, it may be of value to point out some of the distinguishing features of this class of bodies. PROPERTIES OF AMINO-ACIDS. An amino-acid is derived from an organic acid by the replacing of one atom of hydrogen by the amino group NHg. Thus from the acids, acetic acid propionic acid CH3 CH3 I I COOH CH, I " COOH we may obtain the mono-amino-acids, amino-acetic acid alanine or a-amino-propionic acid CH2NH2 CH3 COOH CH.NH2 COOH 78 PHYSIOLOGY It will be noticed that in the fatty acids with more than two atoms of carbon the position of the NHg group may be varied. Thus, instead of alanine we may have another amino-propionic acid, namely : CH2NH2 CH2 COOH This acid would be spoken of as ^-amino-propionic acid, alanine being a-amino-propionic acid. This nomenclature is always used to distinguish the position of the NHg group, so that we may have mono-amino-acids «, l3, y, S, € • • • and so on. Practically all the amino-acids which occur as constituents of the protoplasmic molecule belong to the a group. On inspection of the formula of glycine it is evident that only one isomer of this body is possible. In alanine, however, the carbon atom to which NHg is attached, is asymmetric, since its four combining affinities are each attached to different groups. Thus : C ' H— C— NH2 C In this case, therefore, there is a possibility of stereoisomerism, and alanine must have an influence on polarised hght. If the compound CH3 I HCNH2 I COOH is dextro-rotatory, then its stereoisomer CH3 I HgNCH •' COOH will be laevo-rotatory, and it will be possible to obtain a racemic modification without any influence on polarised light by mixing equal molecules of these two isomeric forms. All the amino-acids derived from proteins are optically active, whereas those obtained by synthesis are inactive, and special means have to be devised in order to obtain from the artificially formed racemic amino-acid either the d- or Z-amino-acid. If more than one hydrogen atom in an organic'acid be replaced by NHg THE PROTEINS 7d we obtain diamino- and triamino-acids. Thus, ornithine, obtained by the spHtting up of arginine, one of the commonest disintegration products of protein, is a-o-diamino- valerianic acid. CH2NH2 CH2 I CH2 I CH.NH2 COOH The presence in the amino-acids of the basic radical NHg and of the acid group COOH lends to these bodies a double character. In themselves devoid of strong chemical quahties, possessing neither acid nor alkaline reaction, they are able in the presence of strong acids or bases to act either as base or acid. When in solution by themselves it is possible that there is an actual closing of the ring by a soluble union between the NHg group and the COOH group, so that, e.g. the formula of glycine xnory be : CH2— NH3 CO — 0 When such a neutral compound is treated with acid this bond is loosed and we have the salt of the amino-acid. Thus, with hydrochloric acid, glycine forms glycine hydrochlorate : CH2NH2HCI I COOH a salt which still possesses an acid group and which is therefore capable of com- bining with ethyl to form the hydrochlorate of the ester of the amino-acid. Thus : CH2.NH2HCI COOC2H5 With bases the amino-acids form salt-Like compounds such as potassium amino-acetate : CH2NH2 COOK Amino-acids also combine with one another. This power of combination much increases the difficulty of separating the constituents from a mixture of amino-acids. Amino-acids, which singly are extremely insoluble, are readily soluble when in the presence of other amino-acids. Ou account of the dual nature of the amino-acid molecule, these sub- 80 PHYSIOLOGY stances act as feeble conductors of the electric current, i.e. as electrolytes. The charge carried by an amino-acid and its ionisation depends upon the conditions in which it is placed. Since it may act either as the cation or the anion, it is spoken of as an amphoteric electrolyte. One reaction of the amino-acids is of special interest in connection with the respiratory functions of the body, namely, the formation of carbamino- acids. If a stream of carbon dioxide be passed into a mixture of an amino- acid, e.g. glycine, with lime, the carbon dioxide is taken up. On filtering the mixture a clear liquid passes through which gradually in course of time deposits a precipitate of calcium carbonate. The filtrate first obtained contains a compound of calcium, calcium glycine carbonate. The formula is as follows : CHa.NH \o.co coo Ca METHODS OF SEPARATING AMINO-ACIDS, By the hydrolysis of protein by means of acid or of trypsin, we obtain a complex mixture of amino-acids. From this mixture certain amino-acids are separated with ease. Thus, tyrosine, which is extremely insoluble, crystallises out on concentrating the fluid, and further concentration leads to the separation of leucine. The other acids, which keep each other mutually in solution, are however very difl&cult to isolate. We owe to Fischer the first general method for their separation. We may take one experiment as an example. Five hundred grammes of casein are heated for some hours under a reflux con- denser with IJ litres of strong hydrochloric acid. The liquid is then saturated with gaseous hydrochloric acid and allowed to stand for three days in the ice-chest. Crystals of hydrochlorate of glutamic acid separate out. The filtrate from these crystals is evaporated at 40° C. under diminished pressure to a syrupy consistence, and is then dissolved in 1|^ litres of absolute alcohol. Hydrochloric acid is then passed into the solution to complete saturation, the mixture being warmed for a short time on the water bath, and the mixture is once more evaporated to a syrupy consistence. By this treatment all the amino-acids have been converted into the hydrochlorates of their esters, e.g. : CH2NH2HCI C2H4NH2HCI 1 I COOC2H5 COOCgHs &c. From the hydrochlorates the esters are set free by the addition of potassium carbonate, the mixture being cooled in a freezing mixture. By this means the esters of aspartic and glutamic acids are separated and are extracted by shaking with ether. The remaining esters are then liberated by the addition of 33 per cent, caustic soda together with potassium carbonate, and are again extracted by ether. The combined ethereal solutions are dried by standing over fused sulphate of soda and then evaporated, when a residue containing the free esters is obtained. These esters are then separated by fractional distillation under a very low pressure obtained by means of the Fleuss pump, the second receiver of the apparatus being cooled in liquid air. The various fractions of aminoesters obtained in this way are hydrolysed- — the lower fractions by boiling for some hours with water, the higher fractions by boiling with baryta. The THE PROTEINS 81 acids obtained by the hydrolysis can then be further purified by means of fractional crystallisation. THE DISINTEGRATION PRODUCTS OF THE PROTEINS By the methods just described the following substances have been isolated from proteins : A. FATTY SERIES (1) Mono-amino-acids {Monobasic) GLYCINE or GLYCOCOLL. This, the simplest member of the group, is amino-acetic acid : CH2NH2 COOH It occurs in considerable quantities among the disintegration products of gelatin and to a shght extent among those derived from certain of the pro- teins. Like the other a-amino-acids, it has a sweetish taste, whence its name was derived {yXvKoa- = sweet, /coAXrj = glue). ALANINE is a-amino-propionic acid : CH3 CH.NHo COOH It is optically active, the alanine derived from proteins being dextro- rotatory. Closely allied to alanine is the amino-acid SERINE, which was first ob- tained by the hydrolysis of silk and has since been found as a constituent of a large number of proteins. Its formula is : CH2OH CH.NH2 COOH i.e. it is amino-oxypropionic acid. Its special interest Ues in the fact that it was one of the first of the amino- oxyacids to be isolated, and it is possible in these acids that we must seek the intermediate stages between carbo' hydrates and proteins. AMINO-VALERIANIC ACID has the formula CH3 CH3 CH CH.NH2 COOH It occurs only in small quantities in the protein molecule. 82 PHYSIOLOGY LEUCINE, one of the oldest known members of the group of amino-acids, is obtained in large quantities from the disintegration of nearly all the animal proteins, of which in some cases it may form as much as 20 per cent. It has the formula CH3 CH3 \/ CH I CH.NH2 COOH i.e. it is amino-isobutyl acetic acid. On evaporating a tryptic digest of protein, impure leucine crystalHses out in the form of imperfect crystals, the so-called ' leucine cones.' Lately another isomer of leucine has been discovered, namely, a -amino -methyl ethyl propionic acid. This is called isoleucine. (2) Mono-amino Derivatives of Dibasic Acids Of these two are known, namely, aspartic and glutamic acids. ASPARTIC ACID is a-amino-succinic acid : COOH I CH.NH2 CH, I COOH and glutamic acid is the next homologue, namely, a-amino-glutaric acid : COOH CH.NH2 CH., CH^ I COOH Owing to the possession of two carboxyl groups these amino-acids have a much more pronounced acid character than is the case with the other members of the group which we have been studying. THE PROTEINS 83 Aspartic acid was first found in the shoots of asparagus in the form of I lie amide, asparagine : COOH I CHNHg I CONH2 This substance is very widely distributed throughout the vegetable kingdom and is present in seedlmgs in very large quantities, as much as 25 per cent, of the dried weight. In plants it apparently serves either as a reserve material or as the form in which the greater part of the nitrogen is conveyed from the reserve organs to be built up into the protoplasm of the growing parts of the plant. (3) Diamino-acids Of these two are known, namely, lysine and ornithine. Owing to the presence of two NHg groups in their molecule, they possess marked basic characters, and are precipitated from the acid solution obtained by the hydrolysis of proteins on adding phosphotungstic acid. Since lysine, argi- nine, and histidine (another amino-acid which will be described later) all contain six carbon atoms in their molecule, these three bodies were classed together by Kossel as the ' hexone ' bases. Apart, however, from their high content in nitrogen, the chemical resemblance between these bodies is no closer than between them and the other members of the amino-acid series. Another body isolated by Fischer in small quantities is supposed to belong to this class and to have the composition diamino-trioxydodecoic acid. LYSINE CgHi^NgOa is a-e-diamino-caproic acid having the formula CH,NH, (CH,)3 CH.NH2 COOH ARGININE, which was first discovered in plants (the cotyledons of lupins), is not a simple amino-acid, but a compound of an amino-acid with guanidin. If boiled with baryta water it spUts up into urea and a substance reacting as a base which was called ornithine.* ORNITHINE, diamino- valerianic acid, has the formula CH2NH2 I (CH^)^ CH.NH2 COOH * Ornithine had b?en previousl}^ discovered in the urine of fowls, after tlie admini- stration of benzoic acid, in the form of an acid known as ornithuric acid. 84 PHYSIOLOGY The constitution of arginine is analogous to that of creatine, one of the most abundant nitrogenous extractives of muscle, which has the formula HN = C H^N -N(CH3)CH2C00H It is methyl guanidine acetic acid. On boihng creatine with baryta water it takes up a molecule of water and sphts in the situation of the dotted line in the formula, giving H^N \C0 (urea) and NH(CH3)CH2COOH (methyl glycine). _ hX This later substance is known as sarcosine and is derived from glycine by the replacement of one atom of hydrogen by a methyl group CHg. Arginine has a similar formula. On the left-hand side of the dotted line the formula would be identical with that of creatine. On the right-hand side the sarcosine group is replaced by a diamino-acid of the fatty series, diamino-valerianic acid or ornithine. DIAMINO-TRIOXYDODECOIC acid is, as its name imphes, a derivative of a twelve carbon acid. Its constitutional formula has not yet been made out. B. AMINO-ACIDS CONTAINING AN AROMATIC NUCLEUS The best known of these is TYROSINE, which has the formula OH CeHi CH2CH.NH2COOH It is paraoxyphenyl a-alanine. It is one of the first of the amino-acids to be spht off from the protein molecule under the influence of hydrolytic agents. Owing to its insolubihty it rapidly separates out as bundles of fine needle-shaped crystals at the sides and bottom of the vessel. When tyrosine is treated with an acid solution of mercuric nitrate containing a httle nitrous acid, a precipi- tate is produced, and on boihng, the precipitate and the supernatant fluid assume a deep red colour. This re- action is given by all benzene derivatives in which one atom of hydrogen in the ring is re- placed by one OH group. This Fig. is. Tyrosine crystals. (Plimmeb.) THE PROTEINS 85 is known as Hoffmann's test, but is identical with Millon's reaction, which is given by all proteins containing tyi'osine in their molecules. Closely alhed to the foregoing compound is another aromatic aniino-acid, namely, PHENYL a-ALANINE : C«H. CH2CH.NH2COOH It is an almost constant constituent of proteins. TRYPTOPHANE was known long before it had been isolated, owing to the fact that with bromine water it gives a rose-red colour. It had long been observed that this substance was to be obtained at a certain stage in the digestion of proteins by pancreatic juice, but nothing was known about its constitution until Hopkins succeeded in isolating it by precipitation with mercuric sulphate dissolved in 5 per cent, sulphuric acid. Cystine is also precipitated by this reagent, but comes down with a less concentration of the salt than trytophane, so that it is possible to separate the two substances by a species of fractional precipitation. Tryptophane can be isolated by decomposing the mercury salt with sulphuretted hydrogen, and is obtained in a crystalhsed form. On distillation it gives an abundant yield of indol and skatol, bodies also obtained during the putrefaction of proteins. Tr}^to- phane itself is indol amino-propionic acid : C.CH2CHNH2.COOH CH CsH, . NH C. AMINO-ACIDS OF HETEROCYCLIC COMPOUNDS Three of the disintegration products of proteins can be grouped in this class. Two of them contain the pyrrol ring, namely, prohne and ox}-proline. PROLINE, which was first isolated by Fischer, is a-pyrrohdin carboxyhc acid and has the formula CH2-CH2 CH2 CH.COOH NH OXYPROLINE is the oxy-derivative of this body and has the formula C5H9NO3, the exact position of the oxy- group having not yet been deter- mined. Doubts have been expressed whether the pyrrol group is present as such in the protein molecule, or whether prohne, for example, is 86 PHYSIOLOGY not formed by the closing of an open chain of a compound belonging to the amiuo-acids in the fatty series. Thus from an oxy-amino-valeri- anic acid CH2OH.CH2.CH2.CH.NH2.COOH we can by dehydration make the compoimd CH2CH2.CH2.CH.COOH, the formula of which will be seen NH to be identical with that given for proUne. The third member of this group contains the iminazol ring : CH— NH. Il >CH CH ^N^ and is known as HISTIDINE. Its structural formula is as follows : CH— NH I! >CH c w CH2.CH.NH2.COOH i.e. it is iminazol a-amino-propionic acid or iminazol alanine. Since it occurs in the phosphotungstic precipitate from the products of acid disintegration of proteins and contains six carbon atoms, it was formerly classified with lysine and arginine as a hexone base. D. SULPHUR-CONTAINING AMINO- ACIDS Sulphur forms an integral part of the molecule of aU classes of proteins except protamines. In some substances alhed to proteins, such as keratin, it may occur to the extent of 3 per cent. On boihng proteins with caustic potash or soda, a portion of the sulphur is spht off to form a sulphide, which gives a black precipitate on addition of copper salts. On this account it was formerly thought that the sulphur must be present in two forms, the oxidised and the unoxidised, in the protein molecule. Recent investigation has shown, however, that practically the whole of the sulphur is present in the form of CYSTINE, and that this body on boihng with alkahne solutions gives up only a little more than haM its content in sulphur. This substance, which has been known for many years as the chief con- stituent of a rare form of urinary calculus and as occurring in the urine in certain cases of disordered metabohsm, is again a derivative of the three- carbon propionic acid. On reduction it gives a body known as cysteine, which is a-amino-thiopropionic acid. CH2SH CH.NH2 COOH THE PROTEINS 87 Cystine itself is compounded of two cysteine molecules joined together by their sulphur atoms and has the formula C H 2 — S^ — S^ — ^CH 2 CH.NH2 CH.NH2 I I COOH COOH E. OTHER CONSTITUENTS OF THE PROTEIN MOLECULE When we add together the total amino-acids obtainable by the acid disintegration of any given protein, a considerable proportion of the original protein remains imaccounted for. This remainder must have a greater content in hydrogen and oxygen than the amino-acids enumerated above, and it has been suggested that among the missing unascertained con- stituents of proteins may be oxyamino-acids, of which serine would form one of the lowest members. The isolation of such substances would present considerable interest, in that it would supply the intermediate stages between the constituent groups of the protein molecule and the carbohydrates, the first product of assimilation by living organisms. Only one such intermediate body has so far been isolated, namely, glucosamine, an amino-derivative of glucose. It was first shown by Pavy that from the products of disintegration of a protein such as egg-white it was possible to obtain a reducing substance and to isolate an osazone resembhng in its characters those derived from the sugars. Since then various observers have shown that this reducing sub- stance is most probably glucosamine : CH2OH (CH0H)3 CH.NH2 CHO Although this substance may be obtained from crystaUised egg albumin or crystalhsed serum albumin, authorities are not yet con\nnced that it forms an integral part of these proteins. Both egg-white and serum contain proteins belonging to the class of mucins, ovomucoid and serimi mucoid, each of which yields on acid hydrolysis from 16 to 30 per cent, glucosamine. Since various observers have obtained results varying from 1 to 16 per cent, gluco- samine for crystalhsed egg albumin, it seems possible that in every case the crystals carried down with, them some of the carbohydrate-rich mucoid, and that the varying results were due to the difierent amomits of nuicoid present in the crystals. By our ordinary methods it is impossible to prepare a specimen of either egg albumin or serum albumin which is entirely free from this amino-derivative of carbohydrate. Connected with this group of proteins may be reckoned the diamino- 88 PHYSIOLOGY trioxydodecoic acid already mentioned as occurring among th.e disintegration products of proteins. THE BUILDING UP OF THE PROTEIN MOLECULE By simple hydrolysis the protein molecule may be broken down into a large number of amino-acids. Analyses of various proteins show that these amino-acids are present in difierent proportions in the individual proteins, so that in many cases a large number of identical amino-acid groups must be present in the protein molecule with smaller numbers of other groups. In endeavouring to form an idea of the manner in which the amino-acids can be hnked together into one gigantic molecule, Hofmeister first put forward the idea that the hnkage follows the general formula : — CHa— NH— CO— or — NH— CHa— CO— NH— This theory o£ the constitution of proteins was based on the fact that a similar grouping was known to occur in leucinimide, obtained by the con- densation of two molecules of leucine, C4H9 /CH\ NH CO CO NH \ch/ C4H9 and also by the fact that only a small proportion of the NHg groups present in the separated amino-acids exist free in the protein molecule. By the action of nitrous acid the terminal NHg groups are split off and replaced by OH. When proteins are treated with nitrous acid only a small proportion of the total nitrogen is spHt off in this way. The linking of the amino groups must therefore take place by means of the nitrogen, i.e. by NH groups. Synthetic experiments have fully confirmed this hypothesis. In 1883 Curtius obtained a substance giving the biuret reaction, the so-called ' biuret base,' by the spontaneous polymerisation of glycocoll ester. This base has been shown by recent researches to consist of four glycine molecules arranged together in an open chain. The clue to the structure of this base was given by Fischer, who has devised a number of ingenious methods for combining together amino-acids of any character and in any number. Thus from two molecules of glycine we may obtain the compound glycyl glycine, as follows : NH2.CH2.COOH -f HNH.CH2.COOH - H2O = NH2.CH2.CO.NH.CH2.COOH THE PROTEINS 89 This may be prepared in various ways. In one method glycine is converted into its ester CH2.NH2.CO.OCH3. In a watery solution this undergoes spontaneous conversion into glycine anhydride which belongs to the class of bodies known as diketo- piperazins, as follows : yCHa— C0\ 2NH2.CH2CO.OCH3 - 2CH3OH + NH< methyl alcohol >NH CO— CH/ On treating this with dilute alkali it takes up water, splitting in the situation of the dotted line and forming glycyl glycine, NH2CH2CO . NH . CHgCOOH. More general methods have been devised by Fischer for the same purpose, depending on the use of the halogen acyl clilorides. Thus chloracetylcliloride and alanine yield chloracetalanine : Cl.CHo.COCl + NHo.CH(CH3).C00H = CI.CH2.CO - NH.CH(CH3)C00H + HCl. By the subsequent action of ammonia, the halogen group is replaced by the amino group, and a dipeptide results : CI.CH2.CO -NH.CH(CH3)C00H + 2NH3 = NH2.CH2.CO - NH.CH(CH3)C00H + NH4CI. Different halogen acyl chlorides are used for introducing the various amino-acid radicals, e.g. chloracetylchloride for glycyl, o-bromopropionylchloride for alanyl, &c. By various such methods Fischer has succeeded in combining compounds contaiuing as many as eighteen amino-acids, e.g. alanyl leucine, glycyl tyrosine, dialanyl cystine, dileucyl cystine, leucyl pentaglycyl glycine, and so on. The last named would be built up out of one molecule of leucine and six molecules of glycine. These compounds have been designated by Fischer as polypeptides, to signify their close connection with the peptones produced by the agency of digestive ferments on the proteins. He dis- tinguishes di-, tri-, tetra-, &c., peptides according to the number of indi^ddual amino-acids taking part in the formation of the compound. The poly- peptides resemble in all respects the peptones. Most of them, even if derived from relatively insoluble amino-acids, are soluble in water, insoluble in absolute alcohol. They dissolve in mineral acids and in alkaUes with the formation of salts, thus resembling in their behaviour the amino-acids. They have a bitter taste, although the amino-agids from which they are formed have a shghtly sweet taste, in this way again resembhng the natural peptones. The higher members of the series give certain reactions, such as the biuret reaction, which are regarded as characteristic of peptones, and like the latter are precipitated by phosphotungstic acid. Their behaviour with trypsin depends on the optical behaviour of the amino-acids from which they are formed. If synthetised from the amino-acids identical with those occurring in the disintegration of natural proteins, they resemble the pep- tones in undergoing hydrolysis and disintegration into their constituent amino-acids. Trypsin, however, has no influence on polypeptides com- pounded of the inactive amino-acids, or of the amino-acids which are the optical opposites of those which form the constituents of normal proteins. Though most of the amino-acids which occur natm-ally are Isevo-rotatory, the polypeptides formed from them are generally strongly dextro-rotatory. 1.90 PHYSIOLOGY Thus in the building up of the protein molecule there is an almost indefi- nite coupHng up of numerous amino- acid groups, the connecting element in each case being the nitrogen. Of the two or more optical isomers possible of each amino-acid containing more than two carbon atoms, only one is made use of for this purpose. A still further fiexibihty in its reactions to its environment is conferred on the protein molecule by changes occurring with great readiness in the intra-molecular grouping of its constituent atoms. Thus, if we take the simplest member of the class of polypeptides, glycjl glycine, four structural formulae are possible, namely : (1) NH2CH2CO - NH.CH2.COOH (2) NH.CH2.CO \ C0.CH2.NH3^ (3) NH2.CH2.C(0H) = N.CH2.COOH (4) N.CH2.CO II >o C(0H)CH2.NH3/ (2) and (4) being the intramolecular form of the formulae (1) and (3). (3) and (4) are sometimes spoken of as the enohc form. If we consider that perhaps some hundred of the amino-acid groups may go to making up a single protein molecule, it is possible to form some conception of the enormous variabiUty in reaction possible to such a compound. THE CONSTITUTION OF DIFFERENT PROTEINS All the proximate constituents of proteins, so far as we know, are amino- acids. Of these the following have been isolated, namely, glycine, alanine, amino-valerianic acid, leucine, isoleucine, prohne, oxyprohne, serine, phenyl alanine, glutamic acid, aspartic acid, tyrosine, tryptophane, cystine, lysine, histidine, arginine, and ' di-amino-trioxydodecoic ' acid. The question now arises whether aU the diSerent varieties of protein owe their pecuMarities to the presence of different amino-acids or whether the greater number of the amino-acids above mentioned are present in all pro- teins, the differences between the latter being determined by differences in the arrangement and relative amounts of their proximate constituents. A large number of analyses of different proteins have been made by Abderhalden, Osborne, and others, utiUsing the methods for the isolation of amino-acids devised by Fischer. The constitution of some representative proteins as determined in this way are given in the Table opposite. These results show that all the proteins contain a very considerable proportion of the total number of amino-acids which have as yet been isolated from acid digests of proteins. The differences in various proteins cannot therefore be determined by quahtative differences in their constituent molecules, but must depend on the relative amounts of the amino-acids which are present and on their arrangement in the whole molecule. As regards relative amounts of amino-acids we find very striking differences. Thus, THE PROTEINS 91 s'i Hi « Edestin (hemp seeds) ■3 3 a to o 1 3 o 3 "a O Keratin (from horse ihair) Glycine 0 (J 3-8 0-9 0 0 0 lG-5 4-7 Alanine 2-7 8-1 3-6 2-7 1-5 4-2 — — 0-8 1-5 Serine 0-6 — 0-33 012 0-5 OG 7-8 — 0-4 0-6 Amino - valeri - anic acid — — present 0-3 7-2 — 4-3 — . 1-0 0-9 Leucine 20-0 71 20-9 6-0 9-35 29-0 0 — 21 7-1 Proline 1-0 2-25 1-7 2-4 6-70 2-3 110 — 5-2 3-4 Oxyproline — — 2-0 — 0-23 1-0 — — 3-0 — Gtlutamic acid . 7-7 8-0 6-3 36-5 15-55 1-7 — — 0-88 3-7 Aspartic acid 31 1-5 4-5 1-3 1-39 4-4 — — 0-56 0-3 Phenylalanine . 31 4-4 2-4 2-6 3-2 4-2 — — 0-4 0 Tyi'osine . 2-1 11 2-1 2-4 4-5 1-5 — — 0 3-2 Tryptophane present present present 1-0 1-50 present — — 0 — Cystine 2-3 0-2 0-25 0-45 ? 0-3 — — — ov. 10 Lysine — 2-15 1-0 0 5-95 4-3 0 120 2-75 M Arginino . — 2-14 11-7 3-4 3-81 5-4 87-4 58-2 7-62 4-5 Histidine . " " 11 1-7 2-5 11-0 0 12-9 0-4 0. glutamic acid, which forms 8 per cent of egg albumin and only 1-7 per cent of globin (derived from haemoglobin), amounts to 36-5 per cent, in ghadin, the protein extracted from wheat flour. Striking differences are also notice- able in the relative proportions of the diamino- acids and bases, the so-called hexone bases. Whereas in casein they form about 12 per cent, of the total molecule, in globin they form about 20 per cent., and in the protamines, salmine and sturiue, about 85 per cent, of the total molecule consists of these bodies. On this account the two last-named bodies have a strongly basic character. From these figures it is evident also that certain of the amino- acids must occur many times over in the protein molecule. Thus in globin, if we assume the presence of one tyrosine molecule, there must be at least thirty-two leucine and ten histidine molecules. On these data the molecular weight of ha)moglobin would come out at about 14,000, a figure which agrees with that derived from a study of the amounts of sulphur and iron respec- tively in its molecule. THE DISTRIBUTION OF NITROGEN IN THE PROTEIN MOLECULE Attempts have been made to differentiate among the proteins by a metho.d which, while less laborious than the isolation and recognition of the individual amino-acids, may yet throw some light on the manner in which the nitrogen is combined within the molecule, and on the relative amoimts of the different classes of nitrogen groups which may be present. One method, which was devised by Hausmann, is carried out as follows. One gramme of the protein is dissociated by boihng with strong hydrochloric acid. The nitrogen, which has been split ofi as ammonia and is present in the solution 92 PHYSIOLOGY Table I. Group Protein Source N per cent. Amide N Amino Basic N Humin* Protamines 1 Salmine i^Sttirine Salmon-roe Sturgeon-roe — 0 0 87-8 83-7 Histones Histone Thymus — 3-3 38-7 Albumins \ and Ovalbumin Egg-white 15-51 8-64 68-13 21-27 1-87 phospho- Caseinogen Milk 15-62 10-36 66-00 22-34 1-34 proteins , Globulins /Globulin i^Edestin Wheat Hemp seed 18-39 18-64 7-72 10-08 53-40 57-83 37-10 31-70 1-52 0-64 Alcohol- soluble proteins Izein 1 Gliadin [Prot- Maize Wheat and rye Witte's 16-13 17-66 18-40 23-78 77-56 70-27 3-03 5-54 0-99 0-79 Albumoses albumose Hetero- peptone Witte's — 7-14 68-17 25-42 — albumose peptone — 6-45 57-4 38-93 — Table II. — Distbibution oe the Niteogen est Vabious Pboteins (Van Slyke) Gliadin Edestin Hair (dog) Gelatin Fibrin Hsemo- cyanin Ox hsemo- globin Ammonia N . 25-52 9-99 10-05 2-25 8-32 5-95 5-24 Melanine N . 0-86 1-98 7-42 0-07 3-17 1-65 3-60 Cystine N 1-25 1-49 6-60 0 0-99 0-80 0? Arginine N 5-71 27-05 15-33 14-70 13-86 15-73 7-70 Histidine N . 5-20 5-75 3-48 4-48 4-83 13-23 12-70 Lysine N 0-75 3-86 5-37 6-32 11-51 8-49 10-90 Amino N of the filtrate 51-98 47-55 47-50 56-30 54-30 51-30 57-00 Non-amino N of the filtrate (proline, oxyproline, \ tryptophane) 8-50 1-70 3-10 14-90 2-70 3-80 2-90 Sum 99-77 99-37 99-85 99-02 99-58 100-95 100-00 as ammonium chloride, is then distilled ofi with magnesia and received into decinormal acid, where its amount can be determined by titration. This nitrogen is variously designated as amide nitrogen, ammonia nitrogen, or * When a protein is boiled for a long time with strong acid, a black precipitate may occur which contains Nitrogen. This is known as humin nitrogen. THE PROTEINS 93 easily displaceable nitrogen. The remaining fluid, freed from ammonia, is precipitated with phosphotungstic acid. By this means all the diamino-acids and bases are thrown down. The nitrogen in the preciptate is determined by Kjeldahl's method and is called diamino- or basic nitrogen. In the remaining fluid, which contains mono-amino-acids, the total nitrogen, the mono-amino-nitrogen, is determined by Kjeldahl's method. Table I., p. 92, gives some of the results obtained in this manner, and shows that there are considerable differences in the distribution of the different kinds of nitrogen among the various classes of proteins. The method is, however, only a rough one as compared with the separation of the individual amino-acids. An improved means of determining the distribution of nitrogen in the protein molecule has been devised by Van Slyke. Some of his results are given in Table II., p. 92. TESTS FOR PROTEIN A. COLOUR REACTIONS OF THE PROTEINS These are of importance since in many cases they are an indication of the nature of the groups present in the protein molecule. (1) THE BIURET REACTION. When a solution of a protein is made strongly alkaline with caustic potash or soda, and dilute copper sulphate added drop by drop, a colour varying from pink to violet is produced. In the case of the proteoses and peptones (the hydrated proteins) the coloiu- is pink ; in the case of the coagulable proteins, violet. According to Schifi this colour is given by all compounds containing the following groups : .CO.NH2 NH<^ \C0.NH2 .CO.NH2 CHgv ^CO.NHa CO— NH2 I CO— NH2 and the group I I (NH2)C— CO— NH— C We have already seen that this grouping is typical of the protein molecule. (2) THE XANTHO-PROTEIC REACTION. On adding strong nitric acid to a solution of protein and boiling, a yellow colour is produced which turns to a deep orange when excess of caustic alkali or ammonia is added. The production of this reaction points to the existence of benzene derivatives in the protein molecule, and it is therefore a general test for the presence of aromatic groups. 94 PHYSIOLOGY (3) MILLON'S REACTION. Millon's reagent is a solution of mercuric nitrate in water containing free nitrous acid. On adding a few drops of this to a protein solution a white precipitate is produced which turns a brick-red colour on boihng. It depends on the presence in the protein of a hydroxy- derivative of benzene, and is determined in the protein by the tyrosine, which is oxyphenylalanine. (4) SULPHUR REACTION. On warming a solution of protein with caustic soda in the presence of lead acetate a black colour is produced owing to the precipitation of lead sulphide. The depth of coloration gives a rough indication of the amount of sulphur in the protein under investigation. (5) THE HOPKINS-ADAMKIEWICZ REACTION. It was stated by Adamkiewicz that on the addition of acetic acid and concentrated sulphuric acid to protein, a violet colour was produced. Hopkins and Cole showed that the success of this reaction depended on the presence of glyoxyhc acid CHO.COOH as an impurity in the acetic acid used. The test is therefore performed now as follows : Glyoxyhc acid is prepared by the action of sodium amalgam on a solution of oxahc acid. A few drops of this solution are added to the solution of protein, and strong sulphuric acid poured down the side of the tube. A bluish violet colour is produced at the junction of the two fluids. This re- action is due to the presence in the protein of tryptophane. The so-called Liebermann's reaction has been shown by Cole to be essentially a modification of the above, and is due also to the presence of tryptophane. In this test the protein is precipitated by alcohol, washed with ether, and heated with con- csntrated hydrochloric acid, when a blue colour is produced, glyoxylic acid being derived from the alcohol and ether. (6) REACTIONS INDICATING THE PRESENCE OF CARBOHYDRATES. Mohsch's test is apphed as follows. A few drops of alcohohc solution of a-naphthol and then strong sulphuric acid are added to a protein solution. A violet colour is produced, which on addition of alcohol, ether, or potash turns yellow. The reaction is determined by the presence, either as an impurity or a constituent part of the molecule, of a carbohydrate radical which, under the influence of strong sulphuric acid, is converted into furfurol. The furfurol gives the colour reaction with the a-naphthol. Another test for the carbohydrate radical is the orcin reaction. A small quantity of the dried albumin is added to 5 c.c. of fuming hydrochloric acid, and the mixture is then warmed. When the albumin is nearly all dissolved a httle sohd orcin is added on the point of a knife, and then a drop of ferric chloride solution. After warming this mixture for some minutes a green colour is produced which is soluble in amyl alcohol and gives a definite absorption spectrum. B. METALLIC SALTS The. following metallic salts form double insoluble compounds with pro- teins, ^nd therefore cause a double precipitation when added to solutions of these bodies : ferric chloride, copper sulphate, mercuric chloride, lead acetate, zinc acetate. THE PROTEINS 95 C. ALKALOIDAL REACTIONS Proteins, like the polypeptides and the amino-acids of which they are composed, may function either as weak acids or as weak bases, according as they are treated with bases or acid radicals respectively. In the presence of strong acids, therefore, proteins act like organic bases, and are thrown down in an insoluble form by the various alkaloidal precipitants. With certain proteins, such as the protamines, where there is a preponderance of basic groups, it is not necessary to add mineral acid in order to ensure the precipita- tion. The following are the principal alkaloidal precipitants which may be employed : (a) Phosphotungstic acid. (b) Phosphomolybdic acid. (c) Tannic acid. (d) Potassium mercuric iodide. (e) Acetic acid and potassium ferrocyanide. (f) Trichloracetic acid. (In order to precipitate all the coagulable proteins from a solution it is treated with an equal volume of 10 per cent, trichloracetic acid, well shaken and filtered.) {g) Metaphosphoric acid. (7i) Salicyl-sulphonic acid. These two latter are generally employed in a 5 per cent, solution. {{) Picric acid. A mixture of picric and citric acids is largely employed, under the name of Esbach's reagent, as a precipitant for coagulable proteins in the urine. D. TESTS DEPENDING ON THE COLLOIDAL CHARACTER OF THE PROTEIN (1) HEAT COAGULATION. On boiling proteins in a very shghtly acid solution some are coagulated and form an insoluble white precipitate. This test is applicable to albumins, globulins, and under certain conditions to the derived albumins. In order that the separation of protein in this way may be complete it is necessary to provide for the presence of neutral salts and also for the maintenance of a slight acidity. The best method of carry- ing out this test, therefore, is to boil the protein in slightly alkaline or neutral solution after the addition of 2-5 per cent, of sodium chloride or sodium sulphate. While the solution is in active ebullition 1 per cent, acetic acid is added drop by drop until the reaction is just acid to litmus. By this means a nearly perfect separation of all the coagulable proteins mav be effected. (2) HELLER'S TEST. On pouring a solution of protein carefully down the side of a test-tube containing strong nitric acid so as to form a layer on the top, a white layer of coagulated protein is produced at the junction of the two fluids. A similar coagulative effect is produced by other strong mineral acids. 96 PHYSIOLOGY (3) PRECIPITATION BY NEUTRAL SALTS. On addition of a neutral salt in excess to a colloidal solution the relation between the solvent and the particles which are in suspension or pseudo-solution are altered. It is therefore possible in many cases by the addition of neutral salts to separate out the dissolved colloid without otherwise altering its characters in any way, so that, on collecting the precipitate and separating the salt carried down with it, it can be dissolved again by adding water. Some classes of proteins can be salted out very readily, while others require a much higher concentra- tion of salt before they are precipitated. The salts which are generally employed for salting out proteins have been divided by Schryver into three classes : Class I. Class II. Class III. Sodium chloride. Potassium acetate. Ammonium sulphate. Sodium sulphate. Calcium chloride. Zinc sulphate. Sodium acetate. Calcium nitrate. Sodium nitrate. Magnesium sulphate. The two calcium salts are, however, rarely employed, as they tend to render the precipitated protein insoluble. The salts of the first class require much higher concentration for the pre- cipitation of the proteins than those of the second, and these than those of the third. Since the degree of concentration of any salt necessary for the precipitation of any particular protein is characteristic for this body, it is possible to employ a fractional process of salt precipitation in order to separate mixtures of proteins into their components. Owing, however, to the tenacity with which different colloids adhere to one another it is diffi- cult, even after many repetitions of the process of fractional salting out, to obtain products which can be regarded as free from admixture. For the purpose of fractional precipitation the salts most frequently employed are those of the third class, namely, ammonium sulphate and zinc sulphate. We shall have to deal with results obtained by this method when treating of the separation of albumoses and peptones. The precipitabihty of different proteins with neutral salts serves also as the basis of the ordinary classification of these bodies. THE CLASSIFICATION OF PROTEINS It is possible that in the future, when we know all the disintegration products of the various proteins and the manner in which they are arranged in the molecule, the classification of these bodies will be based on their con- stitution. At the present time it is obviously impossible to make any classi- fication on such a basis, since the necessary knowledge is wanting, and we have therefore to use a purely artificial classification, such as that adopted by the Chemical and Physiological Societies in 1907, based chiefly on the solubilities of the various proteins in water and salt solutions. We shall here only indicate the characters of the main groups into which proteins THE PROTEINS 97 are conventionally divided, and leave the closer study of the individual proteins to be dealt with in connection with the organs or tissues in which they occur. (1) THE PROTAMINES. These occur in the body only in combination with other groups. They are obtained from the ripe spermatozoa of certain fishes, where they occur in combination with nucleic acid. They are charac- terised by the very large amomit of bases contained in their molecule, amounting to 85 per cent, of the total substance. It was formerly thought by Kossel that the protamines contained only diamino-acids and bases, but it has been shown later that a small proportion of mono-amino-acids may also be obtained from their disintegration {v. Table, p. 91). On account of their constitution they possess strongly basic characters and form well- marked salts, e.g. sulphates and chlorides, as well as double salts vnth. platinum chloride. They contain no sulphur and do not coagulate on heating. (2) HISTONES. This class of proteins, like the protamines, only occurs in combination with other groups, such, for instance, as nuclein and hsematin. They may be obtained from red blood-corpuscles, where they form the globin part of the hseraoglobin molecule, or from the leucocytes of the thymus gland, or from the spermatozoa of fishes. The histones are precipitated from their watery solutions by addition of ammonia, but are soluble in excess of this reagent. In the presence of salts they are coagulated on boiling. With cold nitric acid they give a precipitate which dissolves on warming, but is thrown down again on cooling. The most characteristic feature of this class of bodies is, however, the high proportion of diamino-acids and bases contained in their molecule. (3) ALBUMINS. These are soluble in pure water and are precipitated by complete saturation with ammonium sulphate, zinc sulphate, or sodio- magnesium sulphate. Egg Albumen forms the greater part of the white of egg. It gives the ordinary protein tests, coagulates on heating at about 75° C, and is precipitated from its solutions if shaken with a drop of dilute acetic acid in excess of ether. It is laevo-rotatory, its specific rotatory power being -35-5°. Serum Albumen occurs in large quantities in the blood plasma, serum, lymph, and tissue fluids of the body. It coagulates at 75° C, and is dis- tinguished from egg albumen by its greater specific rotatory power, —56°, and by the fact that it is not precipitated by ether and sulphuric acid. Some vegetable proteins belong to this class, e.g. the leucosin of wheat. (4) GLOBULINS. These bodies are insoluble in pure water and require the presence of a certain amount of neutral salt to dissolve them. They are precipitated from their solutions by complete saturation with magnesium sulphate or by half-saturation with ammonium sulphate. The chief members of this class are : Crystallin, obtained from the crystalline lens by passing a stream of caibon dioxide through an aqueous extract of this body. 4 98 PHYSIOLOGY Serum Globulin or Paraglobulin, a constituent of blood plasma and blood serum. Fibrinogen, wMcli occurs in blood plasma and is converted into fibrin when the blood clots. Paramyosinogen, a normal constituent of muscle. Midway between these two groups may be placed the muscle protein, myosin (or myosinogen), which, though soluble in pure water, resembles the class of globuhns in the ease with which it is precipitated by the addition of neutral salts. In addition to the members of the globuhns named above and derived from the animal body, proteins alUed to this class form an important con- stituent of plants, and are found in large quantities in many seeds used as articles of food. These are vegetable globulins. Prominent members of the group are the edestins, which may be obtained from hemp seeds, cotton seeds, and sunflower seeds, zein from maize, legumin from beans. (5) GLI ADINS, contained in cereals, and soluble in alcohol. (6) GLUTELINS, proteins also obtained from cereals and soluble in weak alkahes. (7) DERIVATIVES OF PROTEINS. A. METAPROTEINS. These may be regarded as compounds of the protein molecule or of part of the molecule with acid or basic radicals. Acid Albumin, or acid metaprotein, is formed by the action of warm dilute acids or of strong acids in the cold on any of the preceding bodies. If a weak alkah be added so as to nearly neutrahse the solution of acid metaprotein, this latter is precipitated. If the precipitate be suspended in water and heated, it is coagulated and becomes insoluble in dilute acids or alkahes. Alkali Albumin, or alkahne metaprotein, is formed by the action of strong caustic potash on white of egg or on any other protein, or by adding alkah in excess to a soluion of acid metaprotein. It is precipitated on neutrahsation of its solution. In close association with this group may be included the proteins as they occur in combination with the metallic salts, such as copper sulphate. On splitting off the copper moiety from these compounds, the protein left is practically free from ash, and behaves in many respects like an albuminate, being insoluble in absolutely pure water, but easily dissolved by the addition of a trace of free acid or alkali. A group of protein derivatives described by Hopkins is produced by the action of the free halogens on protein solutions. We get in this way two definite classes of compounds. One class, which contains the largest percentage of halogen, is obtained by treating a protein solution with chlorine, bromine, or iodine, dissolving up the resultant precipitate in alcohol and pouring the alcoholic solution into ether, when the halogen compound is thrown down as a fine white precipitate. By dissolving this precipitate in weak soda and precipitating with acid, we obtain a series of compounds containing only about one-third as much of the halogen as is contained in the first precipitate, suggesting that the halogen forms both substitution and additive compounds with the protein molecule. Albumins, globulins, and metaproteins are often associated together as the coagulable proteins, since they may be thrown down entirely from their solution on boihng in shghtly acid medium in the presence of neutral salts. THE PROTEINS 99 B. HYDRATED PROTEINS. When proteins are subjected to the action of superheated water or steam, or heated ^^^th acids, or acted on at the body temperature by certain ferments, e.g. pepsin, trypsin, or papain, they undergo a change which is attended by the addition of a number of molecules of water to the protein molecule (hydrolysis). This action, when carried to its end, results in the production of the amino-acids which we have already dealt with. These hydrolytic changes proceed by a series of stages, so that the intermediate products still present many of the protein reactions. The hydrated proteins are divided into two groups, proteoses and peptones. The formation of these intermediate products is especially marked with the proteolytic ferments. Pepsin with hydrochloric acid, the ferment of the gastric juice, for example, only breaks down the protein molecule as far as the proteoses and peptones. Trypsin also gives rise to both proteoses and pep- tones as intermediate products. The action of these ferments on proteins is in fact closely analogous to the action of diastase on the great polysaccharide molecule of starch. In this case, as intermediate products we have first dextrins of various complexity, secondly maltose, and finally, if the ferment maltase be also present, dextrose. The monotony of the starch molecule determines a great similarity of composition between its various disintegra- tion products. It may be regarded as an anhydride of many (100 or more) molecules of a hexose, and the intermediate stages in this hydrolysis are also hexoses and their anhydrides. The protein molecule is distinguished bv the variety of the groups which enter into its formation, and this heterogeneous character of the molecule renders possible a nmch greater variety of inter- mediate products than we find in the starches. Thus a protein molecule may consist of the groups A, B, C, D, E, F, G, H, &c. When hydrolysis occurs it may result in the immediate sphtting off, say, of part of group A, while the residue breaks up into a series of proteoses whose composition may be represented as ABF, ABC, DFG, BDEF, &c. With further hydrolysis these groups are broken into still smaller ones, and the penultimate stages of the hydrolysis will be polypeptides similar to those which have been synthetisod by Fischer from the ultimate products of protein hydrolysis. No sharp dividing line can be drawn between the proteoses, peptones, and poly- ])eptides. Of the last group we have already seen that the higher members give the biuret reaction as well as the other protein reactions, if the necessary groups, e.g. tyrosine, tryptophane, are present in the molecule. The prote- oses and peptones are, however, ill-defined bodies. We have at present no satisfactory means of isolating the dift'erent members of these groups and obtaining them in a state of chemical purity. Their classification is there- fore, like that of the proteins generally, a conventional one, depending on their solubilities and their precipitability by neutral salta, especially ammo- nium sulphate. Both proteoses and peptones give the xanthoproteic and Millon's reactions common to all proteins, and, like these, are precipitated by such reagents as mercuric chloride, potassio-mercuric iodide, or phospho- tungstic acid. On adding excess of caustic potash and a drop of dilute 100 PHYSIOLOGY copper sulphate to solutions of either of these classes of bodies, a pink colour is produced which deepens to a violet on addition of more copper (the biuret reaction). Their solutions can be boiled without undergoing coagulation. Many of them may be thrown down from their solutions by absolute alcohol, but are not rendered insoluble even by prolonged standing under the alcohol. The characters of the different members of these groups will be considered at greater length when deahng with the changes undergone by the proteins during the process of digestion. At present we may merely summarise the distinguishing features of these two classes. (a) Pkoteoses, e.g. albumose from albumin, caseose from casein, elastose from elastin. All of these are precipitated from their solutions on saturation with ammonium sulphate. In the presence of a neutral salt they give a precipitate on the addition of nitric acid. This precipitate is dissolved on heating the solution, but reappears on cooling. All, with the exception of heteroalbumose, are soluble in pure water, and all are soluble in weak salt solutions or dilute acids or alkahes. They are shghtly diffusible through animal membranes. (&) Peptones, e.g. fibrin peptone, gluten peptone. These are all soluble in pure water, diffuse fairly readily through animal membranes, but other- wise give the same reactions as albumoses. From the latter class peptones are distinguished by the fact that they are not precipitated on saturation of their solutions either in acid or alkahne reaction with ammonium sulphate or any other neutral salt. Many of them are soluble in alcohol. (8) THE PHOSPHOPROTEINS. In this class may be grouped a number of substances of very diverse properties which, however, resemble one another in containing phosphorus as an integral part of their molecule. When subjected to digestion with pepsin and hydrochloric acid they are dissolved, but a small quantity of a phosphorus- containing complex may remain behind undissolved. This residue has been called paranuclein or pseudonuclein. It is in reahty derived from nucleoprotein, which is present in the phosphoprotein as impurity and should be called simply nuclein. The phosphoproteins have markedly acid characters. They are insoluble in pure water, easily soluble in alkahes and ammonia from which the original body is thrown down again on addition of acid. Their solutions in alkali are not coagulated by heating. To this class belong caseinogen, the chief protein of milk, vitelhn, the main protein in the yolk of egg, and the vitelhns in the eggs of fishes and frogs. The vitellins are generally associated with a large amount of lecithin. The phosphoproteins differ from the nucleoproteins, which also contain phosphorus, in the facts that they are readily decomposed by caustic alkali with the liberation of phosphoric acid, and do not contain purine bases. The phosphorus of the nucleoproteins is not split of! by alkali (1 per cent.), and on hydrolysis the nucleic acid constituent gives rise to purine bases. (9) CONJUGATED PROTEINS. Various complex bodies which play an important part in building up cells and in the various processes of the body make up this group of compounds. They resemble one another only in the THE PROTEINS 101 fact that in each of them a protein radical is combined with some other body, often spoken of as the prosthetic group.* {a) Chromoproteins. Of this class, consisting of a colouring- matter combined with a protein, the most important is hcemoglobin. This substance, which is the red colouring-matter of the red corpuscles of the blood and plays an important part in the processes of respiration, acting as an oxygen carrier from the lungs to the tissues, is composed of the protein, globin, united with an iron-containing body, haematin. Oxyhsemoglobin contains from 4-5 per cent, hsematin (C32H32N404Fe). It is easily crystallisable, and its physical and chemical characters have therefore been more precisely determined than is the case with most other members of the group of conjugated proteins. We shall have to deal more fully with its properties in the chapters on Blood and Respiration. [b) The Nucleoproteins. These are formed by the combination of a phosphorised organic acid, nucleic acid, with a protein which may belong to any of the classes we have enumerated above. Some of the best-marked members of this group consist of compounds of nucleic acid with basic histones or protamines. The combination between protein and the prosthetic group seems to take place in two stages. If a nucleoprotein be subjected to gastric digestion a large amount of the protein goes into solution as proteose or peptone, leaving an insoluble remainder. This precipitate is not, however, nucleic acid, but still contains a protein group, the compound being spoken of as nuclein. From the latter nucleic acid can be spUt off by heating with strong acids or other means. The nucleoproteins are soluble in water and salt solutions, and are easily soluble in dilute alkalies. They have acid characters and are precipitated by the addition of acids. The nuclei ns, on the other hand, are insoluble in water and salt solutions, but are easily dissolved by dilute alkahes. The nucleins and nucleoproteins form the chief and invariable constituent of cell nuclei. They may be therefore prepared from the most diverse organs. The heads of the spermatozoa of the salmon consist entirely of nuclein. Miescher and Schmiedeberg found that the nuclein obtained from this source contained 60-5 per cent, nucleic acid and 3r)-56 protamine, and was in fact a nucleate of protamine. The nuclein derived from the spermatozoa of echinoderms has been found to be a com- pound of nucleic acid and histone. From organs rich in cells, such as the thymus and the pancreas, and from nucleated red blood-corpuscles, nucleo- proteins may be obtained which can be broken down into nuclein and protein, the nuclein again being composed of a protein residue with nucleic acid. As tirst extracted from the animal cell the nucleoproteins are associated with a considerable proportion of lecithin, and in this labile compound form the ' tissue iibrinogen ' of Wooldridge. To prepare this substance an organ rich in cells, such as the thymus, is minced and extracted with water or normal salt solution. After * By the Germans the term ' protcid ' is often applied to this group. In English, however, the term ' proteid ' has been generally used for the shnple protein known to the Germans as ' Eiweisskorper.' On account of the confusion which has risen from this double use of the term ' proteid,' I have attempted to avoid it altogether in this volume. 102 PHYSIOLOGY separating the cells by means of the centrifuge, the clear fluid is decanted off and acidified with acetic acid. A precipitate is produced consisting of ' tissue fibrinogen.' This substance is soluble in excess of acid and is easily soluble in alkalies. All the tissue fibrinogens are highly unstable bodies and undergo changes in the mere act of precipitation and re-solution. When injected into the blood they cause intravascular clotting. On digestion with gastric juice they yield a precipitate of nuclein, and this precipitate contains a large proportion of the lecithin present in the original substance. In the nucleoproteins nucleic acid is combined with proteins in two degrees, a large portion of the protein being separable by gastric digestion, while the remainder needs stronger reagents for its dissociation. The relation of the two portions -of the nucleo- protein may be represented therefore by the following schema : Nucleo-protein Protein Nuclein Protein Nucleic acid (generally histone or protamine) By various means, all of wliich involve hydrolysis, the nucleic acid may be broken up into its proximate constituents. These differ according to the source of the nucleic acid. Whatever the source, the disintegration products belong to closely alUed groups of substances. These may be grouped as follows : (1) Pliosflioric Acid. The proportion of phosphorus varies within but narrow hmits in the different nucleic acids, the average being about 10 per cent. It is probable that the phosphoric acid represents, so to speak, the combining medium for the groups contained in the nucleic acid molecule, as is the case with the various groups which make up the lecithin molecule. (2) The Purine Bases. Among the products of disintegration of nucleic acid we find constantly one of the bases adenine, C5H5N5, and guanine (C'sHgNgO). These substances, with the products of their oxidation, xan- thine, C5II4N4O2, hypoxanthine, C5H4N4O, have long been known to be ch)sely alhed to uric acid, C5H4N4O3, but their true relationships have only been thoroughly known since the researches of Fischer on this group. According to Fischer they can be all regarded as derivatives of the body purine, iN=«CB: I I II II >H« 3X 4(J ]^9 /^ Each group in this purine ring is generally designated with a number indicated in the structural formula, in order that it may be possible to represent the position of any substituted groups in its derivatives. Uric acid itself is 2-f)-8-trioxypurine with the following formula : HN— CO I I 00 C— NH^ I II >co HN— C— NH/ THE PROTEINS • 103 It can be synthetised by fusing together in a sealed tube trichlorolactamide and urea. Thus : NHg CONH2 NH— CO II II CO + CHOH + NH2 = CO C— NH + NH^Cl. + 2HC1 II /CO I II /CO NH, CCI3 NH2" NH— O-NH-^ The relation of xanthine, hypoxanthine, guanine, and adenine to uric acid is shown by the following formulse : NH— CO HN CO II II CO C— NH. CO— C^NH I II >C0 I II )CH NH— C— NH^ HN C— N ^ Uric acid Xanthine 2-6-8-trioxypurine 2-6-dioxypurine HN— CO N = C.NH2 NH— CO 1 1 HC C — NHx II 11 >H N— C — N ^ HC II N- C— NHx - O-N ^ I 1 NHgC O-NHx II 11 >CH N— C^-N ^ Hypoxanthine Adenine Guanine 6-oxypurine 6-amino-puriae 2-amino 6-oxypurine Closely allied to this group of bodies are the chief constituents of tea, coffee, and cocoa, namely caffeine, which is trimethvl diox^^urine, and theobromine, which is dimethyl dioxypurine. From the structural formulae given it will be seen that the purine radical contains two nuclei. The nucleus N C C C I I N— C is spoken of as the pyrimidine nucleus, pyrimidine having the formula iN= «CH I I ''HC ''CH I II »N— "CH The other is the radical which we have met vnth. already in histidine, a • lisintegration product of proteins, namely iminazol : HC— NH II >CH HC— N ^ Besides the purine bases proper, we find among the disintegration products of nucleic acid a series of bases derived from the pyrimidine ring. These are uracil, thymine, and cytosine. Uracil is 2-6-dioxypyrimidine, NH — CO I I CO— CH NH— CH 104 PHYSIOLOGY Thymine is 5-methyl uracil, NH— CO 1 I CO C.CH3 I II NH— CH while CYTOSiNE is 6-amino-2-oxypyrimidine, N ^C.NHs I I CO CH I II NH— CH Besides these two groups of nitrogenous compounds derived from the purine and pyrimidine rings, many nucleic acids yield on hydrolysis a carbo- hydrate. Thus, Hammarsten has isolated a pentose, from the nucleo- proteins of the pancreas. It is supposed that the nucleic acid of the thymus gland contains a hexose, since it is possible to spht off from it Isevuhnic acid, which is one of the first products of the decomposition of a hexose. The com- plex constitution of the nucleic acids and nucleoproteins may be rendered clearer from the following schema : on digestion yields Nucleo -protein nuclein proteoses and peptones dissolved in alkali and precipitated with hydrochloric acid yields nucleic acid acid derivatives of protein, histones or protamines hydrolysed yields phosphoric acid reducing sugar pentose or hexose purine bases adenine guanine pyrimidine bases uracil thymine cytosine It must not be imagined, however, that all these disintegration products are present in all nucleic acids. Thus the nucleic acid derived from the pancreas, the so-called guanyhc acid, yields of the purine bases only guanine, and of the pyrimidine bases only thymine and uracil, and every variety is met with as we analyse the nucleic acids of different origin. The fact that nucleic acid is a characteristic and necessary constituent of all nuclei adds interest to the divergence of its constituent radicals from those which dis- tinguish the proteins of the cell protoplasm. Further importance is lent to this section of the chemistry of the body by the close relationship which we shall have to study later between the nuclein metabolism of the body and the production and excretion of uric acid. (c) The Glycoproteins. In the glycoproteins the prosthetic group is represented by a carbohydrate radical, generally containing nitrogen, such as glucosamine or galactosamine. They are spht into their two constituents. THE PROTEINS 105 protein and carbohydrate radical, on prolonged boiling with dilute mineral acids or by the action of alkaUes. They may be divided into the two main groups of mucins and mucoids. The mucins play a large part in the animal kingdom as protective agents. They form the shmy secretion which covers the inner surface of the mucous membranes and the outer surface of many marine animals, and is secreted either by the goblet cells of the epithelium or by special groups of cells collected together to form a mucous gland. They may be precipitated from their solutions or semi-solutions by the addition of acids, and after precipita- tion need the addition of alkalies for their re-solution. They are not coagu- lable by heat. The presence of their protein moiety causes them to give the various typical protein tests, such as the xanthoproteic, Millon's, the biuret reaction, and so on. Prolonged boiling with acids sphts the molecule, with the production of acid metaprotein and albumoses and glucosamine. From the mucin of frogs' eggs a similar treatment results in the production of galactosamine. With the mucins may be classified certain bodies wliich have been derived from ovarian cysts, namely, pseudomucin and paramucin. Pseudomucin occurs as a con- stituent of the colloid material from ovarian tumours. It forms slimy solutions wliich do not coagulate by heat and are not precipitated by acetic acid. It is precipitated Ijy alcohol, the precipitate being soluble in water even after standing a long time under the ak'oliol. On boiling with acid it gives a reducing substance. Paramucin differs from the above in reducing Fehling's solution before boiling with acids. Otherwise it resembles jiseudomucin. Leathes, in investigating this body, isolated from it a reducing substance which apparently was an amino-derivative of a disaccharide, |)cnha])s in combination with glycuronic acid. The mucoids include a number of substances which may be extracted from various tissues by the action of weak alkalies, e.g. from tendons, bone, and cartilage. The best studied example of this group is the chondromucoid which, with collagen, forms the ground substance of cartilage. Chondro- mucoid is especially rich in sulphur and gives protein by long treatment with weak alkali. On boiling for a short time with acid it is decomposed into sulphm-ic acid and chondroitin, and this latter, on further action of the acid, is converted into a substance chondrosin, which is certainly an amino- derivative of a polysaccharide containing the elements of glycuronic acid and an amino-disaccharide. Chondroitin-sulphuric acid occurs not only in cartilage but also in bone, yellow elastic tissue, white fibrous tissue, and as a constant constituent of the lardacein or amyloid substance wliich occurs as a deposit in the middle coat of the blood-vessels as the result of syphihs or long-continued suppuration, and gives rise to the condition known as ' lar- daceous disease." Another example of this class of nuicoids is ovomucoid, which is a constituent of egg-white. In order to prepare ovonuicoid the globulin and albumin are precipitated by boiling diluted egg-white. From the iiltrate ovomucoid can then be thrown down by alcohol. A similar body has ))een prepared from blood serum. Both these mucoids yield a large amount of reducing substance on hydrolysis. Thus from 100 grm. of ovo- mucoid it is possible to prepare 30 grm. of glucosamine. 4* 106 PHYSIOLOGY (10) THE ALBUMINOIDS OR SCLERO-PROTEINS. Under this heading are grouped a number of diverse substances which play an important part in building up the framework of the body. Their value as skeletal tissue seems to be determined by their insoluble character. On this account it is practically impossible to speak of purifying them. In every case we can simply take the residue of a skeletal tissue which is left after extraction of the soluble constituents. When broken down by the action of strong acids they yield a series of disintegration products which are included among those we have already studied as the disintegration products of proteins. Their difference from the proteins which are employed in metabohsm for their nutritive value is caused either by the absence of certain groups common to all the nutritive proteins, by the presence of an excess of one or two groups, or by the presence of certain polypeptides which present considerable resist- ance to the action of digestive ferments. This class plays the part in the animal economy which in the vegetable kingdom is filled by the anhydrides of the hexoses and pentoses, e.g. the celluloses, Hgnin, the pentosanes, &c. Collagen forms the main constituent of white fibrous tissue and the ground substance of bone and cartilage. It is insoluble in water, hot or cold, and in trypsin. Under the action of acids or when subjected to prolonged boihng with water, especially under pressure, it is converted into gelatin, which is soluble in hot water, forming a colloidal solution hquid at high temperatures, but setting to a jelly when cold. When subjected to acid hydrolysis it gives a series of amino-acids from which tyrosine and tryptophane are wanting. On this account gelatin does not give any reaction either with Millon's reagent or with glyoxyhc acid. On the other hand, there is a preponderance of such groups as glycine and phenylalanine, and it is probable that glycine, phenylalanine, and leucine are joined together, perhaps with other amino- acids, to form a polypeptide which is not attacked by digestive ferments, and therefore determines the resistance of the original collagen molecule to solu- tion. Gelatin is precipitated by tannic acid, but not by acetic acid. It is dissolved with hydrolysis by gastric juice or by pancreatic juice, whereas collagen, its anhydride, is imafEected by the latter. On prolonged boihng in water it is converted into a modification which does not form a jelly on cooling. Under the action of formaldehyde it is converted into an insoluble modification which does not melt on warming. Reticulin. This name has been applied to the tissue which forms the supporting network of adenoid tissue, and has also been described in the spleen, the mucous mem- brane of the intestine, liver, and kidneys. It differs from collagen in resisting chgestion by gastric juice, and also in containing phosphorus in organic combination. According to Halliburton there is no essential difference between reticulin and collagen. The keratins are produced by the modification of epithehal cells and form the horny layer of the skin as well as the main substance of hairs, wool, nails, hoofs, horns, and feathers. They are distinguished by their insolubihty in water, dilute acids or alkahes, and in the higher animals pass through the ahmentary canal unchanged. Although differing in their elementary composition, according to the tissue from which they are pre- THE PROTEINS 107 pared, they are all distinguished by the very large amount of sulphur present in their molecule. The greater part of this sulphur is in the form of cystine, of which as much as 10 per cent, can be extracted from keratin. They also yield, on acid hydrolysis, tyrosine in larger quantities than is the case with the ordinary proteins. Neurokeratin, which forms the basis of the neuroghal framework of the central nervous system, must be grouped by its general behaviour as well as by its origin \\^th the keratins. It resembles the other members of this class in its insolubility and in its high content in sulphm-. It is extracted from nervous tissues by boiling these with alcohol and ether and then submitting the tissue to prolonged tryptic digestion, which leaves the neurokeratin unaffected. Elastin is a constant constituent of the connective tissues, where it forms the elastic fibres. In some locaHties, as in the Hgamentum nuchse, practically Fibroin Keratin Keratin Keratin of Elastin from from from Gelatin sUk horn horsehair feathers Glycine .... 36-0 25-75 0-45 4-7 2-6 16-5 Alanine 21-0 6-6 1-6 1-5 1-8 0-8 Amino-valerianic acid 0-0 1-0 4-5 0-9 0-5 10 Proline .... |. resent 1-7 5-2 Leucine .... 1-5 ^ 21-4 15-3 71 8-0 21 Phenylalanine 1-5 3-9 1-9 0-0 0-0 0-4 Glutaroic acid 0-0 0-8 17-2 3-7 2-3 0-88 Aspartic acid present present 2-5 0-3 11 0-56 Cystine .... — — 7-5 — — Serine .... 1-6 11 0-6 0-4 0-4 Tyrosine 10-5 0-34 3-6 3-2 3-6 0-0 Tryptophane . — — — — — 0-0 Lysine .... traces — 0-2 11 — 2-75 Arginine 1-0 0-3 2-7 4-5 — 7-62 Histidine small amount — — 0-6 — 0-4 Oxyproline — • — — — 30 the whole tissue is made up of these fibres. Elastin is insoluble in water, alcohol, or ether, or in dilute acids and alkahes. It is slowly dissolved on ■prolonged treatment with gastric juice, but is practically unaffected in the alimentary canal. It gives the xanthoproteic and Millon's tests. Other members of this group are fibroin, which forms the main j substance of silk, spongin, the homy framework of sponges, conchiolin, the ground substance of shells, and perhaps the amyloid substance or lardacein which we have already mentioned in connection with the mucoids. All these sclero- proteins present considerable differences in their quahtative and quantitative composition in aniino-acids. Their proximate composition is shown in the Table given above (Abderhalden). We have finally to mention a miscellaneous collection of bodies which are aUied to the proteins and are distinguished by their extreme insolubility. 108 PHYSIOLOGY They are often designated as albmnoids. Of their composition we know practically nothing. Under this name are grouped such substances as those forming the membrana propria of glands, the sarcolemma of striated muscle, the albumoid of the crystalhne lens, the ground substance of the chorda dorsahs, the organic basis of fish scales, and many similar substances. In every case the substance is characterised necessarily according to its place of origin, httle or nothing being known as to its chemical composition. SECTION VI THE MECHANISM OF ORGANIC SYNTHESIS THE ASSIMILATION OF CARBON The building up of protoplasm from the material which is available at the earth's surface must be an endothermic process. The food presented to the plant contains the necessary elements, but as a rule in a state of complete oxidation. The energy of the hving plant, as of animals, is derived almost entirely from the oxidation of its constituents. The building up of un- organised into organised material must therefore be effected at the expense of energy supplied from without. The source of this energy is the sun's rays. The machine for the conversion of solar radiant energy into the chemical potential energy of protoplasm is the green leaf. Here a deoxidation of the carbon dioxide of the atmosphere takes place, with the production of carbo- hydrates, generally in the form of starch. The formation of starch must be regarded as the first act in the life-cycle, since this substance serves as a source of energy to the already formed protoplasm in its work of building up all the other constituents of the living cell. It is the solar energy captured by the green leaf which is utihsed by all plants devoid of chlorophyll, as well as by the whole animal kingdom. There are one or two exceptions to this statement. Thu.s the bacterium nitrc- somonas, described by Winogradsky, grows on a medium devoid of all organic con- stituents, and derives the energy for its constructional activity from that set free in the conversion of ammonia into nitrites. The sulphur bacteria apparently derive their energy from the decomposition of hydrogen sulphide and the liberation of sulphur. The fundamental importance of this process of assimilation for the whole of physiology justifies some accomit of the researches which have been directed to the elucidation of its mechanism. The production of oxygen by the green plant was disovered by Priestley in 1772, and a few years later Ingenhaus showed that this production occurred only in the light and was effected only by green plants. De Saussure (180-4) pointed out that the essential process concerned was a setting free of the oxygen from the carbon dioxide of the atmosphere, and recognised that the co-operation of water was also necessary. Mohl in 1851 observed the formation of starch grains in the chlorophyll corpuscles, and regarded these as the first products of 109 110 PHYSIOLOGY assimilation. The organs of carbon dioxide assimilation are the chloroplasts. These, which are responsible for the green colour of plants, are generally small oval bodies embedded in the cytoplasm, but sometimes, as in spirogyra, may have the form of spiral bands. In a plant which has been kept for some time in the dark, or in an atmosphere free from carbon dioxide, they present no enclosed granules. Within three to five minutes after exposure to hght in the presence of carbon dioxide, starch granules make their appearance within them, and grow rapidly, assuming the typical laminated structure. Engelmann has pointed out a means by which it can be proved that the chloroplasts carry out this process without the co-operation of the rest of the cytoplasm. Certain bacteria have a great avidity for oxygen and present movements only in the presence of this gas. If a filament of spirogyra be placed in a suspension of these bacteria and be examined under a microscope, the bacteria will be seen to congregate in the immediate neighbourhood of the chlorophyll bands. The same phenomenon is observed in the case of chlorophyll corpuscles isolated by breaking up the cells in which they were contained. These corpuscles therefore take up carbon dioxide and water, and form carbohydrate and oxygen, as follows : n(6C02 + 5H2O) = (CgHioOs), + n{QO,) The whole structure of the green leaf is directed to the furthering of this process. Its cells contain chlorophyll corpuscles, which change their position according to the intensity of the illumination. A free supply of air to all the cells is provided by means of the stomata on the under surface of the leaf. Horace Brown has shown that the rate at which carbon dioxide diffuses through such fine openings is as great as if the whole leaf were an absorbing surface. We get, therefore, optimum absorption of carbon dioxide by the leaf, with the maximum protection of the absorbing tissue and the necessary hmitation of loss of water by transpiration. In Adew of the very small amount of carbon dioxide in the atmosphere, the extent of the assimilatory process is remarkable. One square metre of leaf of the catalpa can lay on 1 grm. of sohd per hour, using up for this pur- pose 784 ccm. carbon dioxide. The rapidity of assimilation is increased within Mmits by increasing the intensity of the hght falhng on the plant, though an over- stimulation of the process is prevented by the movements of the chloroplasts just mentioned. It is also increased by raising the per- centage of carbon dioxide in the atmosphere supplied to the leaf. The optimum percentage of carbon dioxide will of course vary with the other conditions of the leaf. In certain experiments Kreusler found the optimum to be about 1 per cent. Taking the amount of assimilation in normal air with •03 per cent, carbon dioxide at 100, the assimilation in an atmosphere con- taining 1 per cent, was 237, and was not increased by raising the percentage of carbon dioxide to 7 per cent. Owing to the decomposition of the organic matter of the soil, the percentage of carbon dioxide near the ground is always greater than in the higher strata of the atmosphere — a fact which is taken THE MECHANISM OF ORGANIC SYNTHESIS 111 advantage of by the low-growing plants and herbage. Other necessary con- ditions of assimilation are the presence of water and the maintenance of a certain external temperature. The absorption of the sun's rays by the leaf raises the temperature of the later above that of the surrounding medium, and so quickens the process of assimilation. The assimilation of carbon dioxide, the formation of starch, and the evolution of oxygen will go on in the isolated chloroplast. In the absence of chlorophyll, as in an etiolated leaf, the formation of starch will take place if the plant be supplied with a sugar such as glucose, and this conversion represents the main function of the leucoplasts present in all the cells of the reserve organs of plants. In the absence of chlorophyll no decomposition of carbon dioxide takes place, so that this pigment is evidently essential for the utilisation of the sun's energy. Chlorophyll may be extracted from leaves by means of absolute alcohol. A solution is thus obtained which is green by transmitted and red by reflected light, i.e. chlorophyll is a fluorescent substance. It presents four absorption bands, the chief being an intense black band between Fraunhofer's lines B and C. If the chlorophyll is the means of conversion of the solar into chemical energy, the conversion must take place at the expense of the Ught which is absorbed by the pigment. One would expect, therefore, the process of assimilation to be most pio- iiounced in those parts of the spectrum corresponding to the absorption bands — an expectation which has been realised by experiment. As to the exact chemical changes effected by these absorbed rays physio- logists are still undecided. There can be no doubt that an early product of the process is a hexose, which is rapidly converted into cane sugar or into starch. It was suggested by Baeyer in 1870 that carbon dioxide was reduced to formaldehyde, which later by condensation yielded sugar. We know that formaldehyde easily polymerises to form a mixture of hexoses, but until recently no evidence had been brought forward of its presence as an lutei'mediate product in the assimilatory process. For most plants, indeed, formaldehyde is extremely poisonous, though certain algae, as well as the water-plant, Elodea, can stand a solution containing -001 per cent, formaldehyde. Bokorny stated that spirogyra could form starch out of such derivatives of formaldehyde as sodium oxymethyl-sulphonate, or from methylal. The difficulty in these cases is that possibly a spontaneous formation of sugar from the formaldehyde had taken place in the solution and that the plants were using up the sugar rather than the formaldehyde as the source of their starch. One nnist assume, with Timiriazeft', that the function of chlorophyll in the process of assimilation is that of a sensitiser. Just as the addition of eosin to the emulsion used for coating photographic plates will render these sensitive to the red and green parts of the spectrum, i.e. will excite change in the silver salt when light from these parts of the spectrum falls upon it, so the chlorophyll serves as a means by which the absorbed solar energy can be utilised for the production of chemical change in the chloroplast. Attempts 112 PHYSIOLOGY have been made to imitate this process outside the plant. Thus Bach passed a stream of carbon dioxide through a 1 -5 per cent, solution of a fluorescent substance, uranium acetate, in sunhght. As a result there was a precipitate of uranium oxide and peroxide, with the formation of traces of formaldehyde. Usher and Priestley, on treating a solution of carbon dioxide with 1-5 per cent, uranium acetate or sulphate in bright sunlight, obtained uranium peroxide and formic acid, but no formaldehyde. The formation of peroxides in these conditions suggests that the first change in the chloroplast may be as follows : CO2 + 3H2O = 2H2O2 + CH2O Such a reaction must be regarded as reversible since the hydrogen per- oxide first formed would tend to oxidise the formaldehyde again. Moreover it would have a destructive influence on the chlorophyll itself, which is easily oxidised. In order, therefore, that the reaction should go on in one direction only, i.e. that of assimilation, means must be present in the chlorophyll cor- puscles for the removal of both hydrogen peroxide and formaldehyde as soon as they are formed. The removal of the hydrogen peroxide can be efiected by a catalase, which is fairly widely distributed in plants and has been shown by the last-named authors to be present in the chloroplasts. In order to demonstrate the production of the first result of assimilation, i.e. formalde- hyde, the further stages in its conversion must be stopped by kilhng the plant and the catalase it contains. They therefore placed leaves, which had been boiled, in water saturated with carbon dioxide and exposed them to bright sunlight. The leaves were bleached by the oxidation of the chlorophyll, and some substance of an aldehydic nature was produced, as shown by the red colour obtained on placing them in rosanihne, previously decolorised with sulphurous acid. Two proofs were brought forward that this substance was formaldehyde : (a) Some of the bleached leaves were soaked for twelve hours in aniline water. The chloroplasts under the microscope were seen to contain crystals resembling methylene aniline. (b) The leaves were distilled in a current of steam. The distillate was shown to contain formaldehyde by the formation of methylene aniline crystals on treatment with aniline, and by the preparation from it of the characteristic tetrabrome derivative of hexamethylenetetramine. Usher and Priestley conclude that the first products of the photolysis of carbonic acid are hydrogen peroxide and formaldehyde. Both these substances are rapidly removed from the reaction. The hydrogen peroxide is broken up by the catalase into water and oxygen which is turned out by the plant. The formaldehyde is at once polymerised in the protoplasm of the chloroplast with the formation first of a hexose and then of starch. The formaldehyde, if not removed in this way, destroys the catalase. The hydrogen peroxide, if not broken up by the catalase, destroys the chlorophyll. THE MECHANISM OF ORGANIC SYNTHESIS 1 13 The relations between the various factors in this process may be dia- grammatically expressed thus : Carbon dioxide + Water (// 7iot removed, destroys) -> Chlorophyll ^ Hydrogen peroxide + Formaldehyde / ^(// not removed, foisons) Enzyme Living protoplasm Oxygen Carbohydrates In thus reducing certain of the stages in the assimilation of carbon to phenomena which can be imitated outside the Hving organism we have made considerable strides in the ' understanding ' of the process. The stage for which the vitality of the chloroplast is absolutely essential is the formation of starch from formaldehyde. Outside the body, our polymerisation of formaldehyde results in the formation of a mdxture of sugars which are optic- ally inactive. The same process, in the hving cell, leads to the production of optically active sugars which are connected stereochemically and mutually convertible one into the other, e.g. fructose and glucose. The derivatives of protoplasm, containing asymmetric carbon atoms, are in the same way optically active, and it seems that the asymmetry of the protoplasmic molecule conditions a corresponding asymmetry in the substance which it builds on to itself. The protoplasm furnishes, so to speak, a mould in which polymerisation of formaldehyde can result only in the production of sugars of certain definite stereochemical configurations. Few, if any, chemical reactions are pure. Nearly all are attended with by-reactions, so that the yield of end product never attains 100 per cent, of the theoretical yield. Even if the above mechanism be regarded as the chief one, it is probable that side reactions take place at the same time, so that we may have the formation of substances such as glyoxylic acid and other derivatives of the fatty acid series. Such by-products might play an im- portant part in the other synthetic activities of the cell, and especially in the formation of fats and proteins. THE FORMATION OF PROTEINS Our knowledge of the mechanism by which proteins are synthetisod in plants is still more incomplete than that of the synthesis of carbohydrates, and we are reduced in most cases to a discussion of the possible ways in which, from our knowledge of the chemical behaviour of the constituents of the protein molecule, we might conceive of its formation. We can at any rate state the problems which have to be solved and study the conditions under which the synthesis of protein is possible in plants and in animals. 114 PHYSIOLOGY We know that plants are independent of any organic food for building up their various constituents, whether carbohydrate, protein, or iat, pro- vided only that they possess chlorophyll corpuscles and so are able to utihse the energy of the sun's rays. Most plants will grow in the dark if supphed with sugar and with combined nitrogen either in the form of ammonia or of nitrates. The higher plants are especially dependent on the presence of nitrogen in the latter form, and it is on this account that the nitrifying bac- teria of the soil acquire so great an importance for agriculture. From the carbon dioxide of the atmosphere or from the hexose formed by the assimila- tion of carbon, and from nitrogen, in the form either of ammonia or nitrates, together with inorganic sulphates, the plant cell is able to build up all the various types of protein which are distributed throughout the vegetable kingdom. Our study of the disintegration products of proteins has shown that this class of bodies contains a large number of the most diverse groups, having as a common character the possession of nitrogen in their molecule, generally as an NHg or NH group. These disintegration products can be classified as follows : (a) Open chain amino-acids. (6) HeterocycHc compounds, including : (1) P}Trol derivatives. (2) Pyi-imidine derivatives. (3) Iminazol derivatives. These two last groups co-exist in all the purine compounds. {c) Benzene derivatives. [d) Indol derivatives. The first step in the synthesis of proteins is probably the formation of these constituent groups. Just as in digestion the protein molecule is taken to pieces with the formation of the difierent amino-acids, so in the synthetic action of protoplasm the reverse process of dehydration occurs, resulting in a couphng up of the different groups, as has been effected by Fischer in the case of the polypeptides. Wherever transport of protein from one part of the organism to another is necessary the protein is carried, not in its original form, but in the hydrolysed condition of amino-acids. Thus the germination of seeds which contain rich stores of protein is accompanied by a liberation of proteolytic ferments -wdthin the cells of the seeds, and the breakdown of the reserve protein into its constituent amino-acids. As amino-acids it is transported into the growing tip and leaves of the seedhng, analysis of the latter showing a very large percentage of nitrogen in the form of amino-acids. This is especially the case if the synthetic functions of the gromng tip are hindered by interference with assimilation, as, e.g. by keeping the plant in the dark. Under these circumstances, asparagine may form as much as 25 per cent, of the total dried weight of the seedhng. In animals the greater part of the protein of the food is broken down into its constituent amino- acids in the intestine. These are absorbed and probably carried to the THE MECHANISM OF ORGANIC SYNTHESIS 115 different organs of the body, where they are resynthetised, generally in different proportions from those of the original protein, into the protein specific for the organ or tissue. The same process of hydrolysis and subsequent synthesis occurs whenever the transport of protein is neces- sary from one organ to another. We shall later on have to discuss the possibihty of synthesis of the different amino-acids in animals. AVe need, therefore, at present only deal with the possible methods by which, from the glucose or substances produced in the assimilation of carbon and from the ammonia or nitrates derived from the soil, the plant is able to make the different groups which go to the building up of the protein molecule. All the amino-acids contain the NHj group in the « position. We can therefore consider them as formed by the interaction of an «-oxyacid and ammonia. Thus : CH3 CH3 CH.OH + NH3 = CH.NH2 + H-P COOH COOH lactic acid alanine This particular example, namely, the formation of alanine, may occur at the expense of the glucose produced as the first product of assimilation of carbon dioxide. If a solution of glucose together with lime be exposed to sunlight for a considerable time it undergoes decomposition with the forma- tion of lactic acid. Thus : CeHi^Os = 2C3He03 glucose lactic acid This change of glucose to lactic acid under the catalytic influence of the alkaline calcium hydrate probably occurs by means of a shifting of the elements of the water, a process which in many long chains seems to occur with considerable facility, and is dependent on the spatial configuration of the molecule involved. Thus the change of sugar to lactic acid is readily eft'ected by means of many micro-organisms in the case of glucose, fructose, and mannose, but with considerable difficulty in the case of galactose. In the three former sugars the atoms lound th(^ two middle carbon atoms of the chain are disposed thus : OH.C.H H.C.OH I ^^^ I H.C.OH OH.C.H 116 PHYSIOLOGY When either of these arrangements reacts with water, thus : CH2OH CH2OH CHOH OH.C.H HOOH I CHOH COH OH + H CHOH COH + H2O CH2OH CHOH COH we obtain two molecules of glyceric aldehyde, which then by a further shifting of the OH and H groups becomes CH3 CH.OH COOH lactic acid Lactic acid with ammonia and some dehydrating agent will give amino- propionic acid or alanine. The formation of the higher amino-acids in- volves a process of reduction of the sugar first formed in the chlorophyll- granules. It is possible, however, that the starting-point for the amino-acid synthesis may be, not a hexose itself, but some other substances, formed, so to speak, as by-products in the assimilation of sugar from carbon dioxide. We have seen reason to believe that the first result of the action of the sun's rays within the chlorophyll corpuscle is formaldehyde. This substance in the presence of calcium carbonate when exposed to the hght gives a mixture of glyceryl aldehyde and dihydroxyacetone. If we can assume that acetone is formed from the latter by a process of reduction, we might possibly derive leucine from an interaction of this substance with lactic acid and ammonia. Thus : CH3 CH3 CH;, CH5, CO -F CH.OH + NH, + H, CH, COOH \ch/ CH, + 2HoO CH.NH, COOH THE MECHANISM OF ORGANIC SYNTHESIS 117 As an intermediate product in the synthesis of starch, glyoxylic acid CHO I has been described as occurring in the green parts of plants. This COOH substance with ammonia gives formyl glycine, and by the splitting oft of formic acid, glycine or amino-acetic acid. Why nitrates are necessary for certain forms of plants is not at present understood. In the proteins nitrogen always occurs in an unoxidised form as NH or NHg, and the nitrates taken up from the soil must therefore undergo reduction before they can be built into the protein molecule. It is supposed that they may pass through a series of reductions, namely : HNO3 HNO2 HNO H2N— OH nitric acid nitrous acid hyponitrous acid hydroxylamine and that the latter substance then reacts with formaldehyde or other sub- stance derived from the carbon dioxide assimilation to form amino-com- pounds. In general we may say that the probable mechanism of formation of amino-acids is the production of a-oxyacids, which then react with ammonia to form the amino-acids of the protein molecule ; but of the exact steps in this process we are at present ignorant. Knoop's work would point to the ketonic acids as forming one step, and as interacting with ammonia, A\ath simultaneous reduction, to form amino-acids. The pyrrol ring which occurs in prohne and in oxyproUne may possibly be derived from an open chain amino-acid, and it has, in fact, been suggested that the proUne fomid in the products of the acid digestion of proteins is derived from ornithine by a process of condensation with the loss of ammonia. Thus : CH2NH2.CH2.CH2.CH.NH2COOH becomes CH,.CH<,.CH,.CH.COOH or, as it is generally written NH CHo— CH., CH CH.COOH 2 NH Its pie-existence in the protein molecule is, however, practically assured, and it plays an important part in the building up both of chlorophyll and of hsematin, the prosthetic group of haemoglobin. CH— NH Iminazol || ')CH CH— N -^ occurs in histidine (which is iminazol alanine), and can be formed fairly readily by the action of certain catalytic agents on a mixture of glucose and ammonia. Thus, if a solution of glucose with ammonia and zinc oxide be 118 PHYSIOLOGY exposed to light, methyl imiuazol is formed in large quantities. Windaus and Knoop imagined that in this process glyceric aldehyde and formaldehyde are first formed, and that these then interact with ammonia to form methyl iminazol. CH3 C — NH 11 >H CH— N ^ It is interesting to note that if we attach to this compound carbamide or urea we obtain a body belonging to the class of purines. Xanthine, for instance, would have a formula NH— CO CO C— NH I II >CH NH— CH— N^ Thus by the action of simple catalytic agencies on sugar and ammonia we can obtain the iminazol nucleus, and by easy transitions pass through this to the purine nucleus with its contained ring, the pyrimidine nucleus, found in the bases cytosine, uracil, &c., which occur in the nucleins. With regard to the formation of the aromatic constituents of the protein molecule, i.e. those containing the benzene and indol rings, we have at present very Uttle indication even of the lines along which it might be possible to prosecute our researches. It has been suggested that inosite may represent some stage in the formation of the benzene ring from the open chain found in the carbohydrates. Inosite has the same formula as glucose, namely, CgHigOe, but is a saturated ring compound : CHOH CHOH CHOH CHOH CHOH CHOH and may be expected to be formed as a result of polymerisation of formalde- hyde. We have no evidence, however, of the possibihty of such a formation, and the relations of this substance with the benzene compounds are by no means intimate. It is of such universal occurrence, "both in plants and animals, that it is difficult to refrain from the suspicion that it may play some part as an intermediate stage between the fatty and the aromatic series. Since plants are able to manufacture all these varied substances out of the products of assimilation of carbon and ammonia or nitrates, they must also find no difiiculty in transforming one amino-acid into another, and we know that most plants can procure their nitrogen from a solution of a single amino-acid as well as from a nutrient fluid containing the nitrogen in the form THE MECHANISM OF ORGANIC SYNTHESIS 119 of ammonia. In animals the power of transforming one amino-acid into another, of one group into another, is probably strictly limited. So far as we know, nearly all the amino-acids utilised in the building up of the animal proteins are derived directly from those contained in the food. On the other hafid, we have evidence in the animal body of synthesis of the purine bodies, and therefore of the pyrimidine and iniinazol rings. The hen's egg at the beginning of incubation contains very Uttle nuclein, nearly the whole of its phosphorus being present in the form of phosphoproteins and lecithin. As incubation proceeds these substances disappear, their place being taken by the nucleins which form the chief constituent of the nuclei of the developing chick. In the same way the ovaries and testes of the salmon are formed during their sojourn in fresh water at the expense of the skeletal muscles, especially those of the back. Here again there is a transformation of a tissue poor in purine bases into a tissue which consists almost exclusively of nucleins and protamines. Whether in this case there is a direct conversion of the niono-amino-acids of the muscle proteins into the diamino-acids and bases typical of protamines, we do not know. It is more probable that only diamino-acids and bases previously existing in the muscle are utilised for the formation of the generative glands, the other amino-acids being oxidised and utilised for the ordinary energy requirements of the animal. THE SYNTHESIS OF FATS In some plants fat globules have been stated to appear as the first products of the assimilation of carbon dioxide under the influence of sunlight, but there is no doubt that as a rule the formation of fats as reserve material in seeds or fruits occurs at the expense of carbohydrates. In the higher animals, too, although a certain amount of the fat of the body is derived from the fat taken up with the food, the organism can also manufacture neutral fat out of the carbohydrates presented to it in its food. The problem, therefore, of the synthesis of the fats is the problem of the conversion of a sugar such as glucose into glycerin and the fatty acids. Although this conversion is apparently so easily effected by the Hving organism, it is one which from the chemical standpoint involves considerable difficulties. On account of the fact that the higher fatty acids consist largely of oleic and stearic acids, i.e. acids containing eighteen carbon atoms in their chain, it has been thought that the synthesis might be brought about by the linking together of three molecules of a hexose. Such a change would involve a series of difficult chemical transformations. For instance, no less than sixteen out of the eighteen oxygen atoms present in the three glucose molecules would have to be dislodged in order to convert the chain into stearic acid. Moreover, although these two acids contain a multiple of six carbon atoms, a whole array of fats are foimd both in plants and animals which could not be derived by a simple aggi-egation of glucose molecides, and it is worthy of note that, of all the fatty acids which occur in nature, all those with more than five carbon atoms contain an even number of carbon atoms. Thus in milk, in addition to the three common fats, tristearin, tripalmitin, and triolein, we find the 120 PHYSIOLOGY glycerides of caproic, capiylic, capric, lauric, and myristic acids, i.e. acids with 6, 8, 10, 12, and 14 carbon atoms. In all cases these acids are the normal acids with straight unbranched chains. It seems probable that in the transformation of carbohydrate into fatty acid the latter is built up, not by six carbon atoms, but by two carbon atoms at a time. It has been suggested by Magnus Levy and by Leathes that the transformation may occur by way of lactic acid. We have seen already that glucose and the sugars of analogous composition may be converted under the influence either of sunlight or of micro-organisms into lactic acid. Lactic acid breaks down with readiness into aldehyde and formic acid. CH, CH. CHOH = CHO+H COOH COOH Aldehyde undergoes condensation to form aldol. CH. CHO CH3 CHOH CH, CHO aldehyde aldol Aldol reacts with water and undergoes a shifting of its OH and H groups, in a manner with which we are already familiar as occurring in the conversion of glucose into lactic acid, forming butyric acid. We may represent the reaction in the following way, placing the water molecules opposite those groups of the aldol molecule with which they react : CH3 H HO :CH H OH gives CH2 0 c |h oh CH3 CH^ i CH2 COOH + 2H,0 THE MECHANISM OF ORGANIC SYNTHESIS 121 It will be seen that although water must enter into reaction there is no addition of water to the aldol in order to form the butyric acid. It has been suggested that similar reactions might account for the forma- tion of the higher fatty acids, in which case one molecule of acetic aldehyde would be added to the fatty acid in order to build up the acid which is next highest in the series. Although certain of the higher acids have been pre- pared in this way, proof is still wanting that a continuous series of syn- theses may be effected by the continuous addition of aldehyde. Such a li)^othesis is, however, more probable tban the direct conversion of three molecules of sugar into one molecule of stearic acid. The latter change would be associated with a very great absorption of energy, whereas a con- tinuous building up of fatty acids by the addition of aldehyde obtained through lactic acid from the disintegi-ation of hexose molecules only requires a small expenditure of energy, which could be obtained by the combustion of the formic acid formed as a by-product in the process. If we suppose that the synthesis of the higher fatty acids from sugar is carried out in this way, the energy equations would be as follows (Leathes) : 1 g. mol. glucose ) ^/2 g. mols. aldehyde + 2 g. mols. formic acid. 677-2 cals. J \ 2 x 275-5 + 2 x 61-7 = 674-4 cals. 2 g. mols. aldehyde ^ , / 1 g- r>iol. aldol ) f 1 §• ^o^- butjTic acid. 551 cals. " /■ ^t 546-8 cals. / ^\ 517-8 cals. Or, tracing the same change on as far as palmitic acid : 4 g. mols. glucose "j fl g. mol. palmitic acid + 8 g. mols. formic acid. 2708 cals. / ^\ 2362 cals. + 494 cals. = 2856 cals. In the first stage of the synthesis, the reaction leading to butyric acid, the net result would be, supposing the formic acid to be oxidised, that some 160 calories, or nearly 25 per cent, of the whole energy, would be rendered avail- able for other purposes. In the latter stages leading to pabnitic acid some of the energy derived from the oxidation of the formic acid Avould be required for effecting the synthesis, and only about 12-5 per cent, of the original amount contained in the sugar would be set free. It is worth noting that in the butyric fermentation of sugar by micro-organisms there is a production first of lactic acid, and this substance then disappears to give place to butyric acid. At the same time carbonic acid and hydrogen are evolved, both gases being derived from the decomposition of the formic acid. In the process a certain amount of caproic acid is always produced, and the crude butyric acid of fermentation is used as the source from which commercial caproic acid is derived. Attempts to produce the higher fatty acids by the condensation of successive molecules of aldehyde have so far only resulted in the production of branched chains of carbon atoms, whereas the nornuil fatty acids of the body are straight chains ; though Raper has sliown that the normal caproic acid may be formed by the condensation of aldol with itself. Miss Smedley has suggested that a more probable line of synthesis lies through pyruvic acid. Pyruvic acid, which may be produced iu the body fi'om 122 PHYSIOLOGY lactic acid, and so from carbohydrate, is fermented by yeast with the production of acetaldehyde and carbon dioxide, by means of a ferment carboxylase. If we assume the existence of a similar ferment in the cells of the body, it would split this acid into aldehyde and 002- Aldehyde, however, combines with a molecule of pyruvic acid to form a higher keto=acid, which might either be oxidised to the fatty acid containing one carbon atom less, or might be again transformed by enzymes into an aldehyde capable of reacting with another molecule of pyruvic acid. These changes are represented in the following equations : CH3CO.COOH = CH3CHO + CO2 CH3CHO + CH3CO.COOH = CH3CHOH.CH2.CO.COOH CH3CHOH.CH2.CO.COOH +-0 = CH3CHOH.CH2COOH + CO2 /3-oxyacids would thus be a normal stage in the building up as well as in the breaking down of fatty acids. The glycerin which enters into the formation of the ordinary neutral fats can be synthetised by both plants and animals, and there is every ground for believing that it, hke the fatty acids, may be derived from carbohydrates. We have already seen that in the conversion of glucose into lactic acid the first step is the formation of glyceric aldehyde, CH2OH CH2OH CHOH CHOH CHOH CHO *'CH0H* OH^OH CHOH CHOH CHO CHO and it is easy to understand how by a process of reduction the aldehyde is converted into the corresponding alcohol, namely, glycerin. The synthesis of the neutral fat from glycerin and fatty acid is a change which can be accomplished by many ferments. It is one involving practically no absorp- tion or expenditure of energy. The change is a reversible one, and we find both in plants and animals that a hydrolysis of neutral fat into fatty acid and glycerin always occurs when a transport of the fat is required, while the laying down of fat as a store of energy is always preceded by a resynthesis of the neutral fat. We shall have occasion to deal in greater detail with these questions when we have to discuss the formation and fate of the fat in the animal body. CHAPTER IV THE ENERGETIC BASIS OF THE BODY SECTION I THE ENERGY OF MOLECULES IN SOLUTION RvERY vital act involves at the same time a transformation of the material liasis of the living cell and a transformation of energy. The ultimate source of the energies displayed by the animal organism is the chemical energy of the substances taken in as food. In all the changes undergone by either matter or energy in the body there is neither destruction nor creation. The living organism may therefore be regarded in one sense as a )nachine, that is to say, a system for the conversion of one form of energy into another. Thus the steam-engine converts the potential energy of over- heated steam into mechanical work ; a gas-engine the chemical energy of an explosive mixture of gases into heat and mechanical energy ; in a battery there is a transformation of chemical into electrical energy ; in a dynamo, of mechanical into electrical energy, and so on. In the living cell the chemical energy of the food may undergo conversion into any of the other forms mentioned above, i.e. heat, work, electrical difference of potential, or it may be used for the production of other chemical substances possessing perhaps as nmch potential energy as or more than the food-stuffs themselves. Protoplasm, which is the seat of all these changes in both plants and animals, is active only within fairly narrow limits of temperature, approxi- mately between 5° and 40° C. In consistence it is slimy and wet, water forming from 70 to 95 per cent, of its bulk. No substance introduced into the protoplasm has any influence on it, unless it be soluble, and the first stage in the preparation of food-stuifs for assimilation always consists in a process of solution. The sole source of energy to the body being that con- veyed with the food, it follows that all the energy with which we have to deal is the energy of molecules in watery solution, the playgiound of whose activities is a jelly-like mass of colloidal material, heterogeneous yet struc- turally continuous. It is important, therefore, at the outset to inquire into the nature of this energy and the methods by which it may be measured. OSMOTIC PRESSURE. If we place two gas jars together, mouth to mouth, as in Fig. 19, the upper jar containing hydrogen and the lower jar some heavier gas, such as oxygen or carbon dioidde, within a very short time 123 124 PHYSIOLOGY fc4 the gases will have become intimately mixed, and each jar will contain an equal amomit of both gases. We say that each gas has difiused into the other, and ascribe the diffusion to the movement of the gaseous molecules. In closed vessels the rapidly moving molecules are continually impinging on the walls of the vessels and rebounding, and it is this bombardment by the gaseous molecules which is responsible for the pressure exerted by a gas on its containing walls. If we double the amount of gas in a given space, we double the amount of molecules which strike a unit area of the wall in unit tune, and therefore double the pressure exerted by the gas on the vessel wall. In this way we may explain the law of Boyle that the pressure of a gas is inversely proportional to its volume, or the product of pressure and volume at a given temperature is a constant, PV = C, or since the energy of the molecules is proportionate to the absolute temperature, PV = RT, the famiUar gas equation. The molecules of substances in solution behave, within the limits of the solution, in a manner precisely similar to the free molecules of a gas. Thus, if a vessel be half filled with a 10 per cent, solution of sugar and be then filled up by carefully pouring distilled water, so as to form a distinct layer on the heavier sugar solution, the sugar at once begins to move upwards into the distilled water. In consequence of the resistance offered to the movement of the sugar molecules through the water, this process of diffusion is slow, but if the vessel be left undisturbed and free from any agitation for two or three months the sugar will be found to spread gradually throughout the liquid, so that at the end of this time all parts of the fluid contain a uniform amount of sugar. This process of difiusion, hke that of gases, must be ascribed to a con- tinuous translatory movement of the dissolved molecules. Since the mole- cules possess mass and are endowed with a velocity, it is evident that they can exercise a pressure on any membrane or dividing surface which tends to hinder their free passage within the hmits of the solvent. Thus if we take a pig's bladder containing a 20 per cent, solution of dextrose and immerse it in distilled water, water will pass in and distend the bladder to such an extent that it may burst from the rise of pressure in its interior. This swelUng of the bladder is due to the fact that the molecules of sugar pass through it only with difficulty, and therefore in their passage outwards towards the confines of the water exert a pressure on the walls, driving them apart and so causing a distension of the bladder. It is impossible, however, by this means to obtain the full osmotic pressure due to the pressure exerted by the sugar molecules, since the bladder wall itself is not absolutely im- permeable to sugar. If we imagine the sugar solution confined in a cyhnder and covered with a layer of distilled water, the movement of the sugar Fig. 19. THE ENERGY OF MOLECULES IN SOLUTION 125 molecules will cause them to wander from the lower to the upper part, and this process of diffusion will cease only when the concentration has become the same in all parts of the solution. Supposing, however, the two fluids are separated by a piston, p (Fig. 20), which is ' semi-permeable,' i.e. allows free passage to water, but not to the dissolved sugar, the molecules of sugar will now exert a pressure on the piston similar to that exerted on the walls of the containing vessel, and will tend to drive it upwards. The force which it is necessary to apply to the piston to prevent its upward movement will be the measure of the osmotic pressure of the sugar in the solution. If the piston be pressed down with a greater force, the sugar molecules alone are pressed together, since water can pass freely through the surface of the piston, and the sugar solution is therefore rendered more concentrated. Since force nmst be applied to the piston in order to press it down, work is done in the process, so that the concentration of any solution involves the performance of an amount of work determined by the initial and final osmotic pressures of the solution. If, on the other hand, a weight be apphed to the piston which is less than the osmotic pressure exerted by the sugar solution, the piston with its weight ^^A\\ be moved upwards, and the solution will undergo dilution until its osmotic pressure exactly balances the weight on the piston. We see that the osmotic pressure of a solution represents a certain amount of potential energy, which can be utiUsed in an osmotic machine, such as that represented in the diagram, for the performance of work. THE MEASUREMENT OF THE OSMOTIC PRESSURE. By a method differing but httle from the one just sketched out, Pfeffer succeeded directly in measuring the Osmotic pressure of certain solutions. For this purpose Pfeffer took advantage of the fact, discovered by Traube, that various pre- cipitates, if deposited in the form of membranes, were impermeable to the substances producing them as well as to some other dissolved substances, though allowing a free passage of water. Thus, if a drop of a concentrated solution of potassium ferrocyanide suspended to a glass rod be introduced carefully into a more dilrrte solution of copper sulphate, it will be observed that at the junction of the drop and the surrounding fluid there is a brown membranous precipitate of copper ferrocyanide. In consequence of the greater concentratioir of the fluid in the drop, a constant passage of water takes place from without inwards through the membrane, and the drop therefore grows continually in size, sometimes sending out branches as a result of slight currents in the fluid set up by accidental vibrations. Sugar intro- duced into such a drop, although quickening its rate of growth, does not pass out into the surrounding copper sulphate solution, nor is there any passage of copper sulphate inwards or potassium ferrocyanide outwards. Pfeft'er con- ceived the idea of depositing such a semi-permeable membrane within the interstices of a clay cell. Strengthened in this way, it is able to afford a resistance to pressure, and therefore to permit of the contained fluid reaching 126 PHYSIOLOGY its full osmotic pressure. For this purpose a porous jar carefully cleansed and containing a solution of sugar mixed with, a little copper sulphate is dipped into a weak solution of potassium ferrocyanide. A semi-permeable membrane of copper ferrocyanide is thus produced in the pores of the filter, and this, while allowing the passing of water, is impermeable to the sugar. The tube is then fitted with a cork provided with a closed mercurial mano- meter and is immersed in distilled water, when it is found that water passes into the cell until the pressure within the latter is equal to the osmotic pressure of the dissolved substances. By this means Pfeffer obtained the following results with a 1 per cent, solution of cane sugar at different tem- peratures : Temp. °C. Pressure in atmospheres Calculated 6-8 13-7 22-0 32-0 36-0 Atm. 0-664 0-691 0-721 0-716 0-746 Atm. 0-665 0-681 0-701 0-725 0-735 It is always possible to calculate the pressure of a gas when its nature, its mass, and its volume are known. By Avogadro's hypothesis, equal volumes of gases at the same pressure contain equal numbers of molecules. On this account the molecular weight of any gas can be reckoned directly from its density. The figures obtained by Pfeffer show that the same laws apply to the osmotic pressure of substances in solution as to the pressure of gases in their free state. It is therefore possible to reckon the osmotic pressure which would be exerted by 1 per cent, sugar in solution at a given temperature. This calculation is carried out as follows : A gramme molecule of any gas at 0° C. and 760 mm. Hg has a volume of 22-4 litres, therefore 342 grammes of cane sugar (the molecular weight of CVaHaoOn = 342), if it could be converted into a gas at 0° C. and 760 mm. Hg, would have a volume of 22-4 litres. One gramme of sugar therefore at 22-4 the same temperature and pressure would have a volume of ^ — — litres = 65-5 c.c. In PfefEer's experiment the gramme of sugar was dissolved in 100 grammes of water, making a total volume at 0° C. of 100-6 c.c. The gaseous pressure of .the sugar molecules 65-5 in this solution will therefore amount to — 0-651 atmosphere 100-6 ^ At a temperature of 6-8 the pressure would be 0-667 atmosphere, as against the observed 0-664 atmosphere. Pfefier's method is difficult to carry out and is not applicable to all dissolved substances, since the cupric ferrocyanide membrane is permeable for many substances, such as potassium nitrate or hydrochloric acid. Other THE ENERGY OF MOLECULES IN SOLUTION 127 indirect methods have therefore been apphed to the comparison of the osmotic pressures of different solutions. DETERMINATION OF THE OSMOTIC PRESSURE BY PLASMOLYSIS. Solu- tions which have the same osmotic pressure are spoken of as isosmotic or isotonic. The method of plasmolysis, which we owe to the botanist De Vries, coasists essentially in the comparison of the osmotic pressure of solutions with that of the cell sap of certain plant cells, and depends on the fact that the ' primordial utricle,' the layer of protoplasm enclosing the cell sap, while freely permeable to water, is impermeable to a large number of salts and other crystalloids, such as sugar. It is therefore, so far as concerns these substances, 'semi-permeable.' The cells which have been most used for this purpose are the cuticular cells on the mid-rib of the lower surface of the leaves of tradescantia discolor. If some of these cells are brought into a concentrated salt solution, which is ' hypertonic ' as compared with the cell sap, water passes out of the cell into the salt solution, until the contents of the cell attain a molecular concentration equal to that of the surrounding medium. The protoplasmic layer therefore shrinks, leaving a space between it and the cell wall (Fig. 7, p. 23). If the outer solution has a smaller molecular concentration than the cell sap, water passes into the cell and causes here a rise of pressure which simply presses the protoplasm still more closely against the cell wall. If we determine the concentration of the salt solution at which the shrinkage of the proto- plasm, the plasmolysis, just occm's, and another smaller concentration at which plas- molysis is absent, we know that the concentration of the cell sap lies between those of the two salt solutions. Thus, if plasmolysis occurs in a solution containing 0-6 per cent, sodium chloride and is absent in a solution containing 0-59 per cent, of the same salt, the concentration of the cell sap must be about equivalent to a 0-595 jjer cent. NaCl solution. Solutions of different salts, in which plasmolysis just occurs, must also be isotonic wdth one another. Thus a 1-01 per cent, solution of KNO3 is found to be isotonic with a 0-58 per cent. NaCl solution. DETERMINATION BY HAMBURGER'S BLOOD-CORPUSCLE METHOD. The limiting external layer of red blood-cori:)uscles resembles the primordial utricle of plant cells in being impermeable to a number of dissolved substances. If, therefore, it be placed in a solution of smaller concentration than the corpuscle contents, it mil swell up and, since it has no supporting cell wall, the increase in size will go on until the corpuscle bursts, and its contained red colouring-matter, haemoglobin, passes into solution in the surrounding fluid. If the corpuscles be then allowed to settle or be centrifuged, the fact that haemolysis has occurred is shown by the red colour of the clear supernatant fluid. With a given sample of blood, the concentration of a potassium nitrate solution is found at which the first traces of haemolysis occur. In order to determine the osmotic pressure of a solution, saj^, of sugar or of sodium chloride, these are also added in various dilutions to blood corpuscles until we get solutions in which haemolysis just occurs. These solutions will then be isotonic with the first determined ])otassium nitrate solutions. As an example of this method may be adduced the following results : Concentration of the solution in which the blood corpuscles do not lose hfemoglobin Concentration of the solution in which the blood corpuscles begin to lose hsemoglobin Mean concentration Potassium nitrate Per cent. I 04 Per cent. 0-9f. Per cent. lOt) Sodium chloride 0-60 0-56 0-585 Cane sugar 6-29 5-63 5-96 Potassium iodide 1-71 1-67 1-64 Sodium iodide . 1-54 1-47 1 -605 Potassium bromide . 1-22 113 M7 128 PHYSIOLOGY OSMOTIC PRESSURE OF ELECTROLYTES. It will be noticed in the last Table that the isotonic solutions of different salts contain these salts in the proportion of their molecular weights, i.e. each solution contains the same number of molecules of dissolved salt. For the term isotonic we might therefore employ equimolecular. When, however, these salts are compared with solutions of sugar, it is found that the osmotic pressures of the salt solutions are double or nearly double those of equimolecular solu- tions of sugar. The osmotic pressure of a sugar solution is equal to the pressure which its molecules would exert if they occupied the same space in a gaseous form. A dilute salt solution therefore acts as if every one of its molecules were doubled. This deviation of salt solutions from solutions of sugar is bound up with the power of the former to conduct an electric current. A sugar solution conducts electricity little better than pure water. On the other 2 3 4 5 6 7 H ■ II ■ 11 ■ V/////////////////////////^//////////////////////////////////^^ Fig. 21. Diagram to illustrate Barger's method of determining osmotic pressure. The upper figure shows the capiUary tube with nine alter- nate drops of cane sugar and the substance under investigation. hand, the smallest trace of salt added to distilled water enormously increases its conducting power. As Arrhenius has shown, this increase of the osmotic pressure of a salt solution is determined by the dissociation which all these salts undergo in watery solution. A dilute solution of sodium chloride con- tains, not the molecule NaCl, but an equal number of the ions Na and CI, Na carrying a positive charge while the CI ions carry a negative charge. As regards osmotic pressure and various other properties, each of these charged ions acts as a whole molecule. It is the existence of these ions which confers on the salt solution the power of conducting electricity — a power the exercise of which is attended with a dissociation (an electrolysis) of the salt into its constituent ions, the electro-positive ion being deposited at the negative pole while the electro-negative ion is deposited at the positive pole. The molecular weight of NaCl is 58-5. The molecular weight of glucose is 180. If there were no dissociation, a 0-58 per cent, solution of NaCl would be isotonic or isosmotic with a 1 -8 per cent, solution of glucose. On account of the ionic dissociation or ionisation, it is actually isosmotic with a glucose solution of about 3-5 per cent. THE ENERGY OF MOLECULES IN SOLUTION 129 INDIRECT METHODS OF MEASURING OSMOTIC PRESSURE. Equi- molecular solutions have the same osmotic pressures. Since the osmotic pressure of a solution is therefore directly dependent on the number of molecules it contains in unit space, any method which will give us informa- tion as to the number of molecules present will also enable us to determine the osmotic pressure. Other properties of solutions which, like the osmotic pressure, are fmictions of the number of molecules present, are vapour-tension, boihng-point, freezing- point. The presence of a substance in solution in water diminishes its vapour-tension at any given temperature, raises its boihng-point, and depresses its freezing-point, and the extent of the deviation from distilled water is proportional to the number of dissolved molecules present. The determination of the rise of boihng-point, though much employed by chemists, is of very httle value in physiology, OTAing to the fact that nearly all the fluids of the body are seriously modified in character by a rise of tem- perature to 100° C. On the other hand, Barger has suggested an ingenious method in which the altera- tion of vapour-tension is made the basis of a method for determining the osmotic pressure of small quan- tities of fluids at ordinary temperatures. And this }nethod may find important appHcations in phy- siology. BARGER'S METHOD. Drops of the fluid, the vapour- tension of which it is desired to ascertain, are drawn up into a tube (1-5 mm. in diameter), so as to alternate with small drops of cane sugar solution of kno^vii content (Fig. 21 ). Water in a state of vapoiu: will pass from the solution of which the vapour-tension is the higher. By observing the edge of a drop under a magnification of 65 diams., it can he easily seen whether it has grown or diminished in size. If the edge of the drop remains stationary, it shows that the vapour-tension and the osuiotic pressure of the two fluids .".re equal. A scries of trials is made with different strengths of salt solution until this equality is established. In this method only minimal quantities of material are required, and the determination of the aqueous tension is made at ordinary temperatui'es. The method, however, which is of greatest value in physiology is the measurement of the depression of freezing-point. The depression of freezing-point can be converted directly into osmotic pressure by multiplying the depression of freezing-point observed by the factor 122-7. Thus a 1 per cent, solution of sodium chloride with A =0-61 will have an osmotic pressure of 0-61 x 122-7 = 74-84:7 metres of water. The determination is carried out in a Beckmann's apparatus with a thermometer reading to ^i^° C. (Fig. 22). A solution freezes at a lower temperature than pure Fig. 22. Beckmann's apparatus for determina- tion of freezing-point. 130 PHYSIOLOGY water, and the depression of freezing-point is proportional to the number of molecules present. Thus the freezing-point of a 1 per cent, solution of NaCl is —0-61° C. The depression of freezing-point is generally represented by the Greek letter A- This method has the advantage that the fluids are in most cases in no wise altered by the process of freezing, and it can be applied to solutions containing coagulable proteins which would be irretrievably altered by any considerable rise of temperature. Every substance in solution possesses, therefore, a certain amount of potential energy in the form of osmotic pressure. This pressure is inde- pendent of the nature of the substance dissolved and is determined merely by its molecular concentration. It can be used as a driving force for the movement by diffusion of the molecules themselves, or, by the use of appropriate mechanisms or ' machines,' for the performance of mechanical work, or, as will be seen later, for the production of electrical differences of potential. In addition to this osmotic or volume energy every molecule in solution can be regarded as endowed with a chemical energy, which is dependent not only on the number of molecules present, but also on the nature of the molecules. In the case of electrolytes and of substances which are susceptible of ionisation, the potential or intensity of the chemical energy of each molecule is capable of measurement. On the other hand, the chemical energy of a substance such as glucose cannot be definitely expressed apart from consideration of the conditions under which it is present. If we take the whole course of transformations undergone by glucose in the body, we may speak of it as having a potential energy, which is measured by the total heat energy given out by this substance on its complete combustion with oxygen to carbon dioxide and water. In the intermediate changes which it undergoes during its metabohsm in the cells of the body, this energy is probably set free by degrees, but its chemical energy in any given phase cannot be measured unless the conditions and the end results of the chemical changes which it is undergoing are known. This chemical energy may be utiUsed for the production of heat, for the performance of chemical work in the building up of other substances, or by the multiphcation of the number of molecules in a solution, for the production of increased osmotic pressure, which in its turn may be con- verted into the energy of movement either of masses or of molecules. SECTION II THE PASSAGE OF WATER AND DISSOLVED SUBSTANCES ACROSS MEMBRANES We have already seen that if, in a solution, the concentration of the dis- solved substance or solute is not uniform, there is a movement of the sub- stance from the place of higher to the place of lower concentration, and this movement proceeds until the concentration is equal throughout the fluid. This movement of dissolved substances through a fluid is spoken of as diffusion, and is analogous in all respects with the process by which the inter- mixture of gases is attained. The movement in the case of dissolved sub- stances, as of gases, takes place from the region of higher to the region of lower (osmotic) pressure. It can therefore be ascribed to differences of pressure, or rather to the factor which we regarded as responsible for the production of the pressure, viz. the movement of the molecules themselves. The rate of diffusion is not the same for all substances. In gases the rate varies inversely as the square root of the density of the gas. Thus hydrogen (density = 1) diffuses four times as rapidly as oxygen (density = 16). We find similar differences between the rates of diffusion of dissolved substances — differ- ences which also are determined in all probabihty by the weight and size of the individual molecules, although the relation between molecular weight and rate of diffusion is not so simple as the ratio between these two quantities in gases. The dift'usibihty of a substance is given by its diffusion coefficient. The amount of dissolved substance, which diffuses in a unit of time across a given area of fluid, is proportional to the difference between the osmotic pressures at two cross-sections of the column of fluid at an in- finitesimally small distance apart. If we take a cyhndrical mass of solution which is one centimetre long and has a sectional area of one square centimetre (Fig. 23), and maintain a constant difference of concentration between A and B = I, the diffusion coefficient is the amount of substance which diffuses in a unit of time from A to B. Thus the statement that the diffusion co-efficient of urea is 0-810 at 7-5° C. denotes that if A be con- tinually filled with a 1 per cent. solution of urea, while in B a constant current of distilled water is kept up so as to maintain the concentration at zero, in the course of a 131 132 PHYSIOLOGY day 0 -810 gramme of urea will pass from A to B through the cyhnder of one centimetre in length and one square centimetre in cross-section. The deter- mination of these diffusion coefficients presents many difficulties. The task is, however, rendered easier by the fact, first ascertained by Graham, that diffusion of salts occurs as rapidly through a sohd jelly of gelatin or agar-agar as through water. It is therefore possible to make the plug in the diagram soHd by the admixture of one of these two substances, and to maintain a con- stant concentration on the two sides of it by the circulation of fluid without affecting the rate of diffusion through the cyhnder by setting up accidental currents. More important from the physiological point of view than diffusion through fluids is the exchange of fluids (water and dissolved substances), which may take place across membranes. Such processes are of constant oc- currence in all parts of the body and are concerned in such functions as the formation and absorption of lymph, the absorption from and secretion into the intestines, absorption from serous cavities, and so on. In many of these functions we shall have to consider later whether the transference across the membrane is determined solely by the nature and concentration of the fluids on the two sides of it or is effected by the active intervention, involving the expenditure of energy, on the part of living cells forming constituent elements of the membrane itself. It is worth our while, there- fore, to consider at some greater length the purely physical factors which may be concerned in the passage of water and dissolved substances across membranes. In the case of fluids containing only one substance in solution, the ex- change across the membrane will be determined entirely by the osmotic pressures. Thus, if two watery solutions, with the same osmotic pressure, are separated by a membrane through which diffusion can take place, no change in volume occurs on either side of the membrane. If the solutions on either side of the membrane are of unequal osmotic pressure, water passes from the side where the pressure is lower to the side where it is higher, and there is a simultaneous passage of the solute from the side of greater to the side of less concentration. If, however, the solutions on the two sides contain dissimilar substances, with different diffusion coefficients, the conditions are more comphcated, and may tend even to produce a movement of fluid in apparent opposition to the difference of osmotic pressure. Under these circumstances the nature of the membrane itself is all-important. We may therefore shortly consider the various modes in which interchanges may take place across membranes of varying permeabihty. We shall see that the close analogy which exists between substances in solution and gases, when deahng with ' semi- permeable ' membranes, is also borne out by experiment when used to predict the behaviour of solutions separated by such permeable membranes as occur in the body. The simplest case is that in which two fluids are separated by a perfect semi-permeable membrane that permits the passage of water but is absolutely PASSAGE OF WATER AND DISSOLVED SUBSTANCES 133 impermeable to dissolved siibstances. In this case the transference of water from one side to the other depends entirely on the difference of osmotic pressure between the two sides. If we suppose two vessels, A and B (Fig. 24), separated by such a mem- brane, A containing a solution of a and B a solution of [3, water will pass from A to B so long as the osmotic pressure of [3 is greater than the osmotic pressure of the solution of a. If B be subjected to a hydrostatic pressure greater than the osmotic difference between the two fluids, water will pass from B to A until the force causing filtration or transudation (the hydrostatic pressure) is equal to the force causing absorption into B (the difference of osmotic pressures). Under no circum- stance wdll there be any transference of salt or dissolved substance between the two sides. Such semi-permeable membranes as this, however, rarely Fig. 24, occur in the body over any extent of surface. The external layer of the cell protoplasm may resemble the protoplasmic pelhcle of plant cells in possessing this ' semi-permeabihty * ; but in nearly all cases where we have a membrane made up of a number of cells, it can be shown to permit the free passage of at any rate a large number of dissolved substances. Let us now consider what will occur when the two solutions A and B are separated by a membrane which permits the free passage of salts and water. If the osmotic pressure of B be higher than A at the commencement of the experiment, the force tending to move water from A to B will be equal to this osmotic dift'erence. But there is at the same time set up a dift'usion of the dissolved substances from B to A and from A to B. The result of this diffusion must be that there is no longer a sudden drop of osmotic pressure from B to A, and the result of the primary osmotic difference on the move- ment of water willbe minimised in proportion to the freedom of diffusion which takes place through the membrane. Now let us take a case in which A and B represent equimolecular and isotonic solutions of a and [3. It is evident that the movement of water into A will vary as A;; — Bp * = 0. But diffusion also occurs of a into B and of [3 into A. Now the amount of sub- stance diffusing from a solution is proportional to the concentration, and therefore to its osmotic pressure, as well as to its diffusion coefficient. Hence the amount of a diffusing into B will vary as A y.y.h (when h is the diffusion coefficient). In the same way the amount of ^ diffusing into A will vary as B/?. (Bk'. Hence, if a^ is greater than [3k', i.e. if a is more diffusible than [3, the initial result nuist be that a greater number of molecules of a will pass into B than of [3 into A. The solutions on the two sides of the membrane will thus be no longer equimolecular, but the total number of molecules of x -t- (i in B will be greater than the number of molecules of a + (i in A, and this differ- * Ap = osmotic pressiu'e of A, &c. 134 PHYSIOLOGY ence will be most marked in the layers of fluid nearest the membrane. The result, therefore, of the unequal diffusion of the two substances is to upset the previous equahty of osmotic pressures. The layer of fluid on the B side of the membrane will have an osmotic pressure greater than the layer of fluid in immediate contact with the A side of the membrane, and there will thus be a movement of water from A to B. Hence if we have two equimolecular and isotonic solutions of different substances separated by a membrane permeable to the solutes, there will be an initial movement of fluid towards the side of the less diffusible substance. We have an exact parallel to this in Graham's famihar experiment, in which a porous pot filled with hydrogen is connected by a vertical tube with a vessel of mercury. In consequence of the more rapid diffusion outwards of the hydrogen than of atmospheric air inwards, the pressure within the pof sinks below that of the surrounding atmosphere and the mercury rises several inches in the tube. We must therefore conclude that, even when the two solutions on either side of the membrane are isotonic, there may be a movement of fluid from one side to the other with a performance of work in the process. In fact, osmosis may occur from a fluid having a higher towards a fluid having a lower osmotic pressure. If, for example, equimolecular solutions of sodium chloride and glucose be separated by a peritoneal membrane, the osmotic flow will take place from the fluid having the higher osmotic pressure — sodium chloride.* We might compare with this experiment the results of separating hydrogen at one atmosphere's pressure from oxygen at two atmospheres' pressure by means of a plate of graphite. In this case the initial result will be a still further increase of pressure on the oxygen side of the diaphragm — a movement of gas against pressure taking place in consequence of the greater diffusion velocity of hydrogen. So far we have considered only the behaviour of solutions when separated by a membrane, the permeability of which to salts is comparable to that of water ; so that the passage of salts through the membrane depends merely on the diffusion rates of the salts. There can be no doubt, however, that we might get analogous movements of fluid against total osmotic pressure determined, not by the diflusibihty of the salts, but by the permeabihty of the membrane for the salts — a permeability which may depend on a state of solution or attraction existing between membrane and salts. We have an analogue to such a condition of things in the passage of gases through an india-rubber sheet. If two bottles, one containing carbonic acid, the other hydrogen, be separated by a sheet of india-rubber, carbon dioxide passes into the hydrogen bottle more quickly than hydrogen can pass out into the carbon dioxide bottle, so that a difference of pressure is created, and the rubber bulges into the carbon dioxide bottle. We might, in the same way, conceive of a membrane which permitted the passage of dextrose more * In consequence of ionic dissociation of the sodium chloride, a decinormal solution of this salt will have an osmotic pressure nearly twice as great as that of a similar solution of the non-ionised glucose. PASSAGE OF WATER AND DISSOLVED SUBSTANCES 135 easily than that of urea. The importance of the membrane in determining the direction of the osmotic passage of fluid is well illustrated by Raoult's experiments. When alcohol and ether were separated by an animal mem- brane, alcohol passed into the ether, whereas if vulcanite were employed for the diaphragm, the osmotic flow was in the reverse direction, and an enormous pressure was set up on the alcohol side of the diaphragm. * The next point to be considered is the passage of a dissolved substance across membranes, in consequence of differences in the partial pressure of the substance in question on the two sides of the membrane. Stress has been laid by Heidenhain and others on the fact that in the peritoneal cavity, as well as from the intestine, salt may be taken up from fluids containing a smaller percentage of this substance than does the blood plasma, and they regard this absorption as point- ing indubitably to an active inter- vention of living cells in the process. A B This argument requires examination. Let us suppose the two vessels A and B (Fig. 25) to be separated by a ' i^r'"25~ membrane which oft'ers free passage to water and a difficult passage to salts. Let A contain 0-5 per cent, salt solution and B a solution isotonic with a 1 per cent. NaCl, but containing only 0-65 per cent, of this salt, the rest of its osmotic tension being due to other dissolved substances. If the membrane were absolutely ' semi-permeable,' water would pass from A to B until the two fluids were isotonic, i.e. until A contained 1 per cent. NaCl (we may regard volume B as infinitely great to simphfy the argument). If, however, the membrane permitted passage of the dissolved substances, the course of events might be as follows : At first water would pass out of A, and salt would diffuse in until the percentage of NaCl in A was equal to that in B. There would now be an equal partial pressure of NaCl on the two sides of the membrane, but the total osmotic pressure of B would still be higher than A. Water would therefore still continue to pass from A to B more rapidly than the other ingredients of B could pass into A. As soon, however, as more water passed out from A, the percentage of NaCl in A would be raised above that in B. The extent to which this occurs will depend on the impermeability of the membrane. As the NaCl in A reaches a certain concentration it will pass over into B, and this will go on until equilibrium is estabhshed between A and B. Extending this argument to the conditions obtaining in the living body, we may conclude that neither the raising of the percentage of a * Here we have a possible clue to the explanation of some phenomena of cell activity, to which the term ' vital ' is often assigned. In the swimming-bladder of fishes, for instance, we find a gas which is extremelj- rich in oxygen, and tlie oxygen is said to have been secreted by the cells lining the bladder. It is, however, possible that the processes here may be analogous to Graham's atmolysis, and that the bladder may represent a perfected form of Graham's india-rubber bag. 136 PHYSIOLOGY salt in any fluid above that of the same salt in the plasma, nor the passage of a salt from a h5rpotomc fluid into the blood plasma, can afiord in itself any proof of an active intervention of cells in the process. In the case of the pleura, for example, we seem to have a membrane which is very imperfectly semi-permeable. It is permeable to salts, but presents rather more resistance to their passage than to the passage of water. Hence on injecting 0-5 per cent. NaCl solution into the pleural cavity, water passes from the pleural fluid into the blood, until the percentage of sodium chloride in the fluid is raised perceptibly above that in the blood plasma. The limit of the resistance of the pleural membrane to the passage of salt is, however, soon reached, and then salt passes from pleural fluid into blood ; but in every case this passage is from a region of higher to a region of lower partial pressure. Hence at a certain stage of the experiment we find a higher percentage of salt in the pleura than in the blood-vessels, although the total amount of salt in the pleural fluid is less than that originally put in, or, in other words, salt has been absorbed. We have already seen that the effective osmotic pressure of a substance, i.e. its power of attracting water across a membrane, varies inversely as its difiusibihty, or as the permeabihty of the membrane to it. What, then, will be the effect if on one side of the membrane we place some substance in solution to which the membrane is impermeable ? We will suppose that A and B both contain 1 per cent. NaCl, but that B contains in addition some substance x to which the membrane is im- permeable. Since the osmotic pressure of B is higher, by the partial pressure of X, than that of A, fluid will pass from A to B by osmosis. But the conse- quence of this passage of water will be to concentrate the NaCl in A, so that the partial pressure of this salt in A is greater than in B. NaCl will therefore diffuse from A to B, with the result that the former difference of total osmotic pressure will be re-estabhshed. Hence there will be a continual passage of both water and salt from A to B, until B has absorbed the whole of A. This result will be only delayed if the osmotic pressure of A is at first higher than B, in consequence of a greater concentration of NaCl in A. There may be at first a flow of fluid from B to A, but as soon as the NaCi concentration on the two sides has become the same by diffusion, the power of X to attract water from the other side will make itself felt, and this attrac- tion will be proportional to the osmotic pressure of x. We shall have occasion to discuss a specific instance of this case when deahng with the mechanism of absorption of fluid by the blood-vessels from the connective tissue spaces. A more famihar example is afforded by the process known as dialysis. Many animal membranes, all of which are colloidal in character, and others such as vegetable parchment, while freely permeable to salts, are impermeable to dissolved colloids. If, therefore, a fluid containing both colloids and crystalloids in solution, e.g. blood-serum, be enclosed in a tube of vegetable parchment, which is hung up in a large bulk of distilled water (Fig. 26), all the salts diffuse out, and if this be frequently changed, we obtain finally a fluid within the dialyser free from salts and other crystalloid substances, but containing the whole of the colloidal proteins originally present. PASSAGE OF WATER AND DISSOLVED SUBSTANCES 137 Thus the transference of fluids and dissolved substances across membranes is determined not only by the osmotic pressure of the solutions, but also by the diffusion coefficient of the solutes and the permeability of the membrane. This permeabiUty may be of the same character as the permeabiHty of water, in which case the rates of passage of the dissolved substances across the Fig. 26. Dialyser, consisting of a tube of parchment paper immersed in a vessel through which a constant stream of sterile distilled water can be passed. (Wf.obleski.) membrane vary as their diffusibiUties, and are therefore probably some func- tion of their molecular weights. On the other hand, the membrane may exhibit a certain attraction for, or power of dissolving, some of the solutes to the exclusion of others, in which case there will be no relation between the diffusibiUties and the rates of passage of the dissolved substances. In a recent paper Bayliss has drawn attention to certain other factors which may determine permanent inequaUty of distribution of a salt on the two sides of a membrane permeable to the salt. If Congo red, which is a compound of an indiflusible colloid acid with sodium, be placed in an osmo- meter which is immersed in water, a certain osmotic pressure is developed. On adding sodium chloride either to the inner or outer fluid, there is a fall in the osmotic ])i'cssure if time be allowed for equihbrium to be estabhshed. At this ])()int it is found that the outer fluid, which is free from dye, contains a larger percentage of sodium chloride than the inner solution of dye. This difference is permanent and is more marked the greater the concentration of the dye salt. In the following Tabic is given the concentrations of the two 5* 138 PHYSIOLOGY fluids with difierent percentages of salt. The numbers indicate the litres to which each gramme molecule of the salt is diluted. Apparently the difference depends on the fact that the non-dissociated salt must be equal on Dye CWorine Inside Outside 30 30 30 100 52 465 <5500 32-9 30 73-6 180 29-5 the two sides of the membrane and that the dissociation is much impeded on the inner side on account of the presence there of another salt of sodium. A sodium salt of any other indiffusible substance, e.g. of a protein such as caseinogen, would behave in a precisely similar fashion. SECTION III THE PROPERTIES OF COLLOIDS Although the chemical changes involved in the various vital phenomena occur between substances in watery solution, the solution in every case is bound up within the meshes or adsorbed by the surfaces of a heterogeneous mass of colloids. The complex chemical molecules which make up protoplasm itself are all colloidal in character. The participation of colloids in chemical reactions introduces conditions and modes of reaction differing widely from those which have been studied in watery solutions. Our knowledge of these conditions is still very imperfect, but the important part played by colloids in the processes of hfe renders it necessary to discuss in some detail their properties and modes of interaction. The term colloid, from koWij, glue, was first introduced by Thomas Graham, Professor of Chemistry at University College from 1836 to 1855. Graham divided all substances into two classes, viz. crystalloids, including such substances as salt, sugar, urea, which could be crystalhsed with ease, diffused rapidly through water, and were capable of diffusing through animal membranes ; and colloids, which included substances such as gelatin or glue, gum, egg-albumin, starch and dextrin, were non-crystalUsable, formed gummy masses when their solutions were evaporated to dryness, diffused with extreme slo\\aiess through water, and would not pass through animal membranes. The process of dialysis was therefore introduced by Graham for the separation of crystalloids from colloids. Although the broad dis- tinction drawn by Graham between colloids and crystalloids still holds good, some of the criteria by which he distinguished the two classes are no longer strictly appUcable. For instance, it has been shown that many typical colloidal substances, such as haemoglobin, can be obtained in a crystalUne form. On the other hand, all gradations exist between substances, such as egg-albumin, which are practically indiifusible, and those, such as connnon salt, which are very diffusible. Graham pointed out that colloids exist under two conditions : (1) In a state of solution or pseudo-solution, in which they form sols, and are distinguished as hydrosols, when the solvent is water ; and (2) In a soHd state, in which a relatively small amoimt of the colloid sets with a large amount of a fluid, such as water, to form a jelly. This solid form is known as a gel. The most famihar instance is the jelly which is obtained on dissolving a little gelatin in hot water and allowing the mixture 139 140 PHYSIOLOGY to cool. Siich a jelly is known as a hydrogel. In many of these gels the water can be replaced by other fluids, such as alcohol, without any alteration in the appearance of the sohd, which is then known as an alcogel. An example of an alcogel is the jelly which can be made by dissolving soap in warm alcohol and allowing the mixture to cool. A number of these colloidal substances can be shown on purely chemical grounds to consist of monstrous molecules. Thus the molecular weight of haemoglobin is at least 16,000, and one must ascribe similar high molecular weights to such substances as egg-albumin and globuhn. Still greater must be the molecular size of such substances as the cell proteins, which may be made up of more than one type of protein built up with various nucleins, with lecithin and cholesterin, to form a gigantic complex, to which it would probably not be an exaggeration to ascribe a molecular weight of over 100,000. This chemical complexity is not, however, a necessary condition of the colloidal state, as is shown by the existence of colloidal sihca, of colloidal ferric hydrate and alumina, and even of colloidal metals. On neutralising a weak solution of sodium silicate or water-glass by means of HCl, we obtain a solution which contains sodium chloride and silicic acid. On dialysing this mixture for some days against distilled water, the whole of the NaCl passes out, and a solution of silicic acid or colloidal silica is left in the dialyser. This solution can be concentrated over sulphuric acid. When concentrated to a syrupy consistence it becomes extremely unstable. The addition of a minute trace of sodium chloride or other electrolyte to the solution causes it to set at once to a solid jelly (gelatinous silica), the change being accompanied by an appreciable rise of temperature. The change is irreversible, in that it is not possible to bring the silicic acid into solution again by removal of the electrolyte by means of dialysis. If, however, it be allowed to stand with weak alkali for some time, it gradually passes into solution. Analogous methods are employed for the preparation of colloidal FcgOs and AI2O3. Of special interest are the colloidal solutions of the metals. Faraday long ago pointed out that, on treating a weak solution of gold chloride with phosphorus, it underwent reduction with the formation of metallic gold. The gold was not precipitated, but remained in suspension or pseudo- solution, giving a deep red * or a blue hquid, according to the con- ditions under which the reaction was effected. This solution was homo- geneous in that it could be filtered without change, and could be kept for months without deposition of the gold. The latter was, however, thrown down on addition of mere traces of impurity, though greater stabihty could be conferred on the solution by adding to it a little ' jelly,' i.e. a weak solution of gelatin. In 1899 Bredig showed how similar hydrosols might be prepared from a number of different metals, viz. by the passage of a small arc or electric sparks between metallic terminals submerged in distilled water. If, for example, the terminals be of platinum, the passage of the current is seen to be accompanied by the giving off of brown clouds, which spread into the surrounding fluid. These clouds consist of particles of platinum of all sizes. The larger settle at the bottom of the vessel, the smaller — * Ruby glass is a colloidal ' solid ' solution of gold in a mixture of silicates. THE PROPERTIES OF COLLOIDS Ul which are ultra-microscopic in size, i.e. from 5 /x/>i to 40 /x/ot* — -remain in sus- pension, and \ye obtain a bro^vn fluid which can be filtered through paper or even through a Berkefeld filter without losing its colour. It may be kept for months without any deposit taking place. The addition of minute traces of electrolytes precipitates the platinum particles, leaving a colourless fluid. "We shall have to return later on to the consideration of the behaviour of these metalUc sols. PROPERTIES OF GELS. A typical hydrogel is the firm mass in which a solution of gelatin sets on coohng. It is clear, hyaUne, apparently structure- less, and possesses considerable elasticity, i.e. resistance to deforming force. It may be regarded as formed by the separation of the warm pseudo-solution of gelatin into two phases : first a solid phase, rich in gelatin and forming a tissue or mesh work, in the interstices of which is embedded the second phase, consisting of a very weak solution of gelatin. If the process be observed under the microscope, according to Hardy, minute di'ops first appear, which, as they enlarge, touch one another and form networks. In stronger solutions the first structures to make their appearance consist, not of the more con- centrated iihase, but of droplets of the dilute solution of gelatin ; the stronger solution collects round these drops and solidifies to a honeycomb structure. In many cases the more fluid part of the gel is practically pure water. In such a case immersion in alcohol causes a diffusion outwards of the water, which is replaced by alcohol with the formation of an alco-gel. In a dry atmosphere the gel loses water and becomes shrivelled and dry, but in some cases, e.g. gelatin, it can resume its former size and characters on immersion in water. Other gels, such as sihcic acid or ferric hydrate, lose the power of swelhng up after drying. The change in them is therefore irreversible. A gel adheres to the last traces of water with extreme tenacity. In consequence of its structure, it presents an enormous extent of surface on which adsorption can take place. At this surface the vapour-tension of fluids is diminished, as well as the osmotic pressure of dissolved substances. On this account gelatin must be heated for many hours at a temperature of 120° 0. in order to be thoroughly dried. When dry, it, as well as other solid colloids, can exert a considerable amount of energy when caused to swell up by moistening. This energy was made use of by the ancient Egyptians in the quarrying of their stone blocks by the insertion of wedges of wood ; water was poured on the wood, and the swelhng of the wedges split the rock in the desired direction. "j" On account of the extent of surface it is practically impossible to wash out the inorganic constituents from a gel. The diminution of the osmotic pressure of many dissolved substances at surfaces causes the concentration at the surface of the solid phase to be greater than that in the surrounding medium. Thus, if dry gelatin be immersed in a salt solution it will swell * One ft is one-thousandth of a millimetre ; one ^y^ is one-thousandth ^, i.e. onc- millionth of a millimetre. t According to Rodewald, the maximal pressure with which dry starch attracts water amounts to 2073 kilo, per sq. cm. 142 PHYSIOLOGY up, but the solution which it absorbs will be more concentrated than the solution in which it is immersed, so that the proportion of salt in the latter will be diminished. When, however, equihbrium is estabhshed between a gel and the surrounding fluid, it is found to present no appreciable resistance to the passage of dissolved crystalloids. Thus salt or sugar diffuses through a column of sohd gelatin as if the latter were pure water. On the other hand, gels are practically impermeable to other colloids in solution. This impermeabihty is made use of in the separation of crystalloids from colloids by dialysis, membranes used in this process being generally irreversible and heterogeneous gels {i.e. vegetable parchment, animal membranes). Other gels, such as tannate of gelatin or copper ferrocyanide, are not only impermeable to colloids, but also to many crystalloid substances. These membranes, therefore, were used by Pfeffer for the determination of the osmotic pressure of such crystalloids as cane sugar. PROPERTIES OF HYDROSOLS. Substances such as dextrin or egg- albumin may be dissolved in water in almost any concentration. If a solution of egg-albumin be concentrated at a low temperature, it becomes more and more viscous and finally sohd. But there is no distinct point at which the fluid passes into the sohd condition. Such solutions are known as hydrosols. Much discussion has arisen whether they are to be regarded as true solutions or as pseudo-solutions or suspensions. The chief criterion of a true solution is its homogeneity. In a solution the molecules of the solute are equally diffused throughout the molecules of the solvent, and it is impossible, without the apphcation of energy, to separate one from the other. Thus filtration, gravitation leave the composition of the solution unchanged. It is true that, by the employment of certain kinds of mem- brane, e.g. the semi-permeable copper ferrocyanide membrane, it is possible to separate solute from solvent, but in this case the force required to effect the filtration is enormous and grows with every increase in the strength of the solution. The measure of the force required is the osmotic pressure of the solution, and it becomes natural therefore to regard the possession of an osmotic pressure as a distinguishing criterion of a true solution. Is there any evidence that colloid solutions also display an osmotic pressure ? I have shown that it is possible to determine the osmotic pressure of colloidal solutions directly, taking advantage of the fact that colloidal membranes, while permitting the passage of water and salts, are im- permeable to colloids in solution. The method originally adopted was as follows : In order to determine the osmotic pressure of the colloidal constituents of blood-serum, 150 c.c. of clear filtered serum are filtered under a pressure of 30-40 atmospheres through a porous cell which has been previously soaked with gelatin. The first 10-20 c.c. of filtrate, which contain the water squeezed out of the meshes of the gelatin and have also lost salt in consequence of absorption by the gelatin, are rejected. The filtration is allowed to go on for another twenty-four hours, when about 75 c.c. of a clear colourless filtrate is obtained, perfectly free from all traces of protein, but possessing practically the same freezing-point as the original serum. (Although the colloids, if they possess an osmotic pressure, must THE PROPERTIES OF COLLOIDS 143 also cause a dei^ression of the free zing- jx)mt, any such depression would be within the errors of observation, since a pressure of 45 mm. Hg. would correspond only to 0-005° C.) The concentrated serum left behind in the filter is then put into the os- mometer, the filtrate being used as the inner fluid. The construction of the osmometer is shown in the diagram (Fig. 27). The tube BB is made of silver gauze, connected at each end to a tube of solid silver. Round the gauze is wTapped a piece of peritoneal membrane, as in making a cigarette. This is painted all over with a solution of gelatin (10 per cent.) and then a second layer of membrane applied. Fine thread is now twisted many times round the tube to prevent any disturbance of the membranes, and the whole tube is soaked for half an hour in a warm solution of gelatin. In this way one obtains an even layer of gelatin between two layers of peritoneal membrane and supported by the wire gauze. The tube so prepared is placed -ndthin a wide tube, AA, which is provided with two tubules at the top. One of these, 0, is for filling the outer tube ; the other is fitted -nith a mercurial manometer, M. Two small reservoii's, CC, are connected -ndth the outer ends of BB, by means of rubber tubes. The whole apparatus is placed in a wooden cradle, DD, pivoted at X, and provided with a cover so that it may be filled with fluids at different temperatiu-es if necessary. The colloid solution is placed in AA, and the reservoirs, CC, and imier tube, BB, are filled with the filtrate, i.e. with a salt solution approximately or absolutely isotonic mth the colloid solution. The apparatus is then made to rock continuously for days or weeks by means of a motor. In this way the fluid on the two sides of the membrane is continually renewed, and the attainment of an osmotic equilibrium facilitated. With this apparatus I found that the colloids in blood-serum, containing from 7 to 8 per cent, proteins, had an osmotic pressure of 25 to 30 mm. Hg., which would correspond to a molecular weiglit of about 30,000. A more couvenieiit form of osmometer has been devised by B. Moore, using parchment paper as the membrane. AVith this osmometer, the existence of an osmotic pressure in colloidal solutions has been definitely established both by Moore in the case of haemoglobin, proteins, and soaps, and by Bayliss in the case of colloidal dyes, such as Congo red. The osmotic pressure of haemoglobin was found by Hiifner to correspond to a molecular weight of about 16,000, i.e. a molecular weight already deduced from its composition and its combining powers with oxygen. Often, however, the osmotic pressure is very much smaller than would be expected from the molecular weiglit of the substance, owing to the fact that colloids in solution 144 PHYSIOLOGY may be in many different conditions of aggregation. Thus the molecule of colloidal sihca must be many, probably thousands of times larger than the molecule as represented by HgSiOg. The osmotic pressure being proportional to the number of molecules in a given volume of solution, the larger the aggregate the smaller would be the total number of molecules, and the smaller therefore the osmotic pressure of the solution. It is in consequence of the huge size of the molecular aggregates that colloidal solutions, such as starch or glycogen, and probably globulin, display no appreciable osmotic pressure. We cannot divide colloidal solutions into two classes, viz. those which form true solutions and present a feeble osmotic pressure, and those which only form suspensions and therefore exert no osmotic pressure. In inorganic colloids, such as arsenious sulphide, Picton and Linder have shown that all grades exist between true solutions and suspensions. With increasing aggregation of the molecules, the suspension becomes coarser and coarser until finally the sulphide separates in the form of a precipitate. The measurement of the osmotic pressure of the colloids of serum points to their having a molecular weight of about 30,000. Chemical evidence shows that haemoglobin has a molecular weight of about 16,000, and we have every reason to beheve that the much more complex molecules forming the cell proteins may have molecular weights of many times this amount. When, however, we arrive at molecular weights of these dimensions, the disproportion between the size of the molecules and those of the solvent, water, becomes so great that a homogeneous distribution of the two sub- stances, solute and solvent, is no longer possible. The size of a molecule of water has been reckoned to be -7 x 10 ~ ^ mm. A molecule 10,000 times as large would have a diameter of -7 X 10 "~ ^ mm. = -07 ^a, a size just within the hmits of microscopic vision. Long before molecules attained such a size they would no longer react according to the laws which have been derived from the study of the behaviour of the almost perfect gases, but would possess the properties of matter in mass. They have a surface of measurable extent, and their relations to the molecules of water or solvent will be determined by the laws of adsorption at surfaces rather than by the laws of interaction of molecules. As a matter of fact we find that such solutions present an amazing mixture of properties, some of which betray them as mechanical suspensions, while others partake of the nature of the chemical reactions such as those studied in the simpler compounds usually dealt with by the chemist. OPTICAL BEHAVIOUR OF HYDROSOLS. Nearly all colloidal solu- tions present what is known as the Faraday-Tyndall phenomenon. When a beam of hght is passed through an optically homogeneous fluid, the course of the beam is invisible. A beam of sunlight falling into a dark room is rendered visible by impinging on and illuminating the dust particles in its course. Each of these particles, being illuminated, acts as a centre of dis- persion of the Hght, so that the course of the beam is apparent to a person THE PROPERTIES OF COLLOIDS 145 standing on one side of it. Tyndall showed that, if the particles were sufficiently minute, the light dispersed by them at right angles to the beam was polarised. This can be easily tested by looking at the beam through a NicoFs prism. If the prism be slowly rotated, it will be found that, while at one position the hght is bright, in the position at right angles to this it becomes dim or is extinguished. The production of the Tyndall pheno- menon may therefore be regarded as a test for the presence of ultra-micro- scopic particles, varying in size from 5 to 50 yu/z. The phenomenon is perhaps too sensitive to be taken as a proof that a fluid presenting it is a suspension rather than a solution. It is shown, for instance, by solutions of many bodies of high molecular weight, such as rafiinose (a tri-saccharide) or the alkaloid brucine (Bayhss). A particle having a diameter less than half the wave-length of hght, i.e. about 300 X or -3 ^t, cannot be clearly distinguished under any power of the microscope. The fact that an ultra-microscopic particle may serve as a centre for dispersal of light may be used for rendering such particles visible under the microscope. For this purpose a strong beam of hght is passed in the plane of the stage of the microscope through a cell containing the hydrosol, which is then examined under a high power. The arrangement for this purpose was first devised by Zsigmondy and Siedentopf. On examining with this apparatus a dilute gold sol, we see a swarm of dancing points of hght, ' hke gnats in the sunhght,' which move rapidly in all directions, rendering it almost impossible to count their number in the field. The coarser particles present shght oscillations similar to those long known as the Brownian movements. The smallest particles which can be seen show a combined movement, consisting of a translatory move- ment, in which the particle passes rapidly across the field in one-sixth to one- eighth of a second, and a movement of oscillation of much shorter period. The representation of the course of such a particle is given in Fig. 28. The size of the smallest particles seen in this way may amount to -005 //, Not all colloidal solutions show these particles in the ultra-microscope. In some cases this is due simply to the small size of the particles, and the addition of any substance, which causes aggregation and therefore increase in the size of the particles, will bring them into view. In others the absence of optical inhomogeneity may be due to the coincidence of the refractive indices of the two phases of the hydrosol, or to the absence of any surface tension and therefore dividing surfaces between the two phases. ELECTRICAL PROPERTIES OF COLLOIDS In the case of many hydrosols the ultra-microscopic particles of which they are composed carry an electric charge which, according to the nature of the solution, may be either positive or negative. On this account, the particles move if placed in an electric field, and the direction of their move- ment reveals the nature of their change. Thus colloidal ferric hydrate is 146 PHYSIOLOGY electro-positive and travels from anode to cathode. Silicic acid, in the presence of a trace of alkah, is electro-negative, and the same is true of a hydrosol of gold. When a current is passed through these hydrosols, the colloidal particles travel to the anode, where they are precipitated. In certain coUoids the charge varies according to the conditions under which they are brought into solution. If, for instance, egg-white be diluted ten times with distilled water, filtered and boiled, no precipitate occurs, but A. Fig. 28. Movements of two particles of india-rubber latex in colloidal solution, recorded by cinematograph and ultra -microscoiie. (Hei>^ri.) ■we obtain a colloidal suspension of albumin. When thoroughly dialysed, this protein is insoluble in pure water, but is soluble in traces of either acid or alkah. In acid solution the protein particles carry a positive charge, whereas in alkahne solution their charge is negative. The charged condi- tion of these particles must play a considerable part in keeping them asunder and therefore in preventing their aggregation and precipitation. This is shown by the fact that any agency which will tend to discharge them will cause precipitation and coagulation. Among such agencies is the passage of a constant current, just mentioned. To the same action is due the coagulative or precipitating effects of neutral salts. Thus any of the colloids we have mentioned, ferric hydrate, sihca, gold, or boiled albumen, are thrown down by the addition of traces of neutral salts, and it is found that in this process they carry with them some of the ion with the opposite charge to that of the colloidal particle. Thus, in the precipitation of the electro-positive ferric hydrate the acid ion of the salt is the determining factor, the coagulative power increasing rapidly with the valency of the acid. On the other hand, in the precipitation of a gold sol the electro -positive ion is the effective agent, and here again the coagulative effect is enormously increased by a rise in valency. This is shown in the following Tables, where it will be seen that, in the coagulation of gold, barium chloride, with the divalent Ba", is seven times as powerful as K2SO4, containing the univalent K'. On the other hand, in the precipitation of the electro-positive THE PROPERTIES OF COLLOIDS 147 ferric hydrate, KgSOj, with a divalent SO./', is 400 times as effective as BaCla. Amount of Salt xecessary to precipitate Colloidal Solutions To coagulate Fe203 To coagulate Gold K2SO4 1 g niol. in 4,000,000 c.c BaCla 1 g. mol. in 500,000 c.c. MgSOi >, ,, ^. 4,000,000 „ NaCl „ „ „ 72,000 .. BaCla „ „ „ 10,000 „ KpSOi „ „ „ 75,000 „ NaCI „ „ „ 30,000 „ The presence of a charge is not, however, a necessary condition for the stabiUty of a colloidal solution. Thus the proteins of serum, globuhn in a weak sahne solution, or gelatin, present no drift when exposed to a strong electric field. In such cases one must assume the stabihty of the solution to be determined by the absence of any surface tension between the two phases in the solution, or between the particles of solute and the solvent. Thus no force is present tending to cause aggregation of the particles. The charged condition of a colloidal particle makes it behave in an electric field in much the same way as a charged ion of an electrolyte, and this similarity extends also to its chemical behaviour, so that we have a class of compounds formed resembhng in many respects chemical com- binations, but differing from these in the absence of definite quantitative relations between the reacting substances. This class of continuously varying chemical compounds has been designated by Van Bemmelen absorp- tion compounds. Since, however, the interaction must take place at the surface layer bounding the charged particles, it will be perhaps better, as Bayhss has done, to use the term adsorption. The huge molecules or aggre- gates of molecules which distinguish the colloidal state form a system with a considerable inertia, so that we have a tendency to the estabhshment of conditions of false equihbrium. Once a configuration is estabUshed, it is necessary, in consequence of the inertia, to overstep widely the conditions of its formation in order to destroy it. Thus a 10 per cent, gelatin solution sets at 21° C, but does not melt until warmed to 29-6° C. Solutions of agar in water set at about 35° C, but do not melt under 90° C. A gel of gelatin takes twenty-four hours after setting to attain a constant melting- point. The factors involved in the formation of adsorption or absorption com- binations are therefore : (1) Extent of surface. In a colloidal solution this must be enormous in proportion to the mass of substance in solution. Thus a 10 c.c. sphere with a surface of 22 sq. cm., if reduced to a fine powder consisting of spherules of -00000025 cm. in diameter, will have a surface of 20,000,000 sq. cm., i.e. nearly half an acre. At the whole of this surface adsorption may take place, invohnng the concentration of dissolved electrolytes, ions, or gases. (2) Chemical nature of particle. (3) Electric charge on the surface. The sign of this may be determined 148 PHYSIOLOGY by the chemical nature of the colloid and its relation to the electrolytes in the surrounding medium. Another factor which may determine the character of the charge on the particles has been pointed out by Coehn. This observer finds that, when various non-con- ducting bodies are immersed in fluids of different dielectric constants, they assume a positive or negative charge according as their own dielectric constants are higher or lower than the fluid with which they are in contact. For instance, glass (5 to 6) is negative in water (80) or alcohol (26), whereas in turpentine (2-2) it is positive. In water, as Quincke has found, nearly all non-conducting bodies take on a negative charge. Among these are cotton-wool and silk. Particles of these in water, exposed to an electric field, move towards the anode. The same is true, as Bayliss has shown, of paper. The conditions which determine the formation of these adsorption com- pounds can be studied in their simplest form on the adsorption of dyestufEs by substances such as paper. If we take a series of solutions of a dye, such as Congo-red, in progressively diminishing concentration, and place in each solution the same amount of filter-paper, we find that a part of the dye is taken up by the paper, and the proportion taken up is larger the more dilute the solution. This relation has been spoken of by Bayhss as the law of adsorption. This is illustrated by the following Table of results of such an experiment : Concentration of Proportion of dye Proportion of dye solution in solution in paper Initial Final Per cent. Per cent. 0-014 0-0056 40 60 0-012 0-0024 20 80 0-010 0-0009 9-3 90-7 0-008 0-0003 4 96 0-006 0-00008 1-3 98-7 0-004 — trace practically all 0-002 — trace practically all If put into the form of a curve, where the ordinates represent the per- centage of dye left in solution, and the abscissae the original concentration of the solution, the curve only approaches the axis {i.e. zero concentration) asymptotically. In other words, however dilute the original solution may be, there will always be a certain amount of the dye left unabsorbed by the paper. Similar relations are found to exist between proteins and electro- lytes. By continuously washing a protein or gelatin with distilled water, the removal of electrolytes becomes slower and slower, but it is practically impossible within finite time to get rid in this way of the last traces of ash. Although the chemical behaviour of colloids is largely determined by surface phenomena, it presents at the same time analogies with more strictly chemical reactions, since it is conditioned by the chemical structure of the colloid molecule as well as by the charge carried by the latter. A good example of these adsorption combinations is presented by globulin, the behaviour of which has been studied by Hardy. This may be obtained from diluted blood-serum by precipitation with acetic acid. Four states THE PROPERTIES OF COLLOIDS 149 can be recognised in both the soKd condition and in solution, viz. globuhn itself, compounds with acid or with alkali, and compounds with neutral salt. The amount of acid and alkah combining with the globulin is in- determinate, the effect of adding either acid or alkali to the neutral globulin being to cause a gradual conversion of an oqaqvie, milky suspension into a hmpid, transparent solution. On drying HCl globulin, the dried solid is found to contain all the chlorine used to dissolve it. The acid may therefore be regarded as being in true combination. Both acid and alkah globulins act as electrolytes, the globulin being electrically charged and taking part in the transport of electricity. In order to produce the same extent of solution, the concentration of the alkah added must be double that of the acid. The relation of globulin to acids and alkalies is similar to that of the so-called amphoteric substances, such as the amino-acids. An amino-acid, such as glycine, can react as a basic anhydride with other acids, thus : .NH2 .NH2HCI CH/ + HCl - CH< ^COaH CO2H or as an acid anhydride with bases : CH2.NH, CH2.NH., I +NaHO- I +H2O COOH COONa Like these too, globulin forms soluble compounds with neutral salts. An amphoteric electrolyte thus acts as a base in the presence of a strong acid, and as an acid in the presence of a strong base. From true electrolytes, colloidal solutions differ in the fact that their particles are of varying size according to the conditions in which they exist and carry varying charges of electricity, whereas an ion such as Na or CI has a mass which is constant for the ion in question, and always carries the same electric charge. The charged particles of an acid- or alkali-globulin may be distinguished therefore as pseudo-ions. In these adsorption combinations, although the chemical nature of the colloidal molecules is concerned, there is an absence of definite e(|ui]ibvium points, such as we are accustomed to in most chemical reactions. The inertia of the system and the large size of the molecules determine the occurrence of false equilibria and of delayed reaction, so that the condition and behaviour of a colloidal system at any moment are determined, not entirely by the quantitative relations of its components, but also by the past history of the system. COMBINATIONS BETWEEN COLLOIDS Besides the compounds between colloids and electrolytes, combination, or at least interaction, takes place between different colloids. Many colloids are precipitated by other colloidal solutions. This effect is always found to occur when the colloidal solutions carry different charges. Thus ferric hydrate in colloidal solution is precipitated by colloidal silica or colloidal 150 PHYSIOLOGY gold, both colloids being thrown out of solution. On the other hand, certain colloids may exercise a protective influence on other colloidal solutions Thus, as Faraday first showed, colloidal gold is much more stable in the presence of a httle gelatin. The colloids of serum can dissolve a considerable amoimt of purified globuhn. Although the latter in solution shows a drift in the electric field, the resulting solution is quite unaffected by the passage of a current through it. In these cases the protective colloids carry no charge, but the same protective effect may be observed if a large excess of, e.g. an electro-positive colloid be added to an electro-negative colloid. This interaction between different colloids probably plays an important part in many physiological phenomena. We have reason to beheve that the reactions between toxin and antitoxin, and between ferment and substrate, which we shall study later, are of this character, and that the compounds formed belong to the class of adsorption combinations. THE COAGULATION OF COLLOIDS Most colloidal solutions are unstable, and the relations between the suspended particle or molecule and the surrounding fluid may be upset by shght changes of reaction or the presence of minute traces of salts. As a result the hydrosol is destroyed, the suspended particles aggregating to form larger complexes. These aggregations may settle to the bottom of the fluid as a precipitate, or may form a species of network, the result varying according to the nature of the colloid and its concentration. Thus gelatin changes from the condition of hydrosol to hydrogel with fall of temperature. The same is true of agar. On the other hand, by adding calcium chloride to an alkahne solution of casein, we obtain a mixture which sets to a jelly on warming, but becomes fluid again on coohng. Other agen- cies may lead to the production of changes which are irreversible. Thus a strong solution of colloidal silica sets to a sohd jelly on the addition of a trace of neutral salt, and it is not possible to reform the hydrosol, however long the jelly is submitted to dialysis. Two methods of bringing about coagulation of hydrosols deserve special mention. The first of these is heat-coagulation. If a solution of egg- albumin or globulin be heated in neutral or shghtly acid medium and in the presence of neutral salt, the whole of it is thrown down in an insoluble form. This coagulated protein is insoluble in dilute acids or alkahes. The same coagulative effect of heating is observed in the metalhc sols. With con- centrated solutions of protein, heat coagulation results in the formation of a gel, i.e. a network of insoluble protein, containing water or a very dilute solution of protein in its meshes. In dilute solutions the result is the pro- duction of a flocculent precipitate. Another method is the so-called mechanical coagulation. If a solution of globuhn or albumin be introduced into a bottle, which is then violently shaken, a shreddy precipitate makes its appearance in the solution, and this precipitate increases, so that by prolonged shaking it is possible to throw down 80 or 90 per cent, of the dissolved protein in the coagulated form. THE PROPERTIES OF COLLOIDS 151 Ramsden has shown that this mechamcal coagulation is a surface pheno- menon. It depends on the fact that a large number of substances in solution (viz. any which lower the surface tension of their solutions) undergo concen- tration at the free surface of the fluid. Such substances are proteins, bile- salts, quinine, saponin, &c. In the case of proteins the concentration reaches such an extent, and the molecules at the surface are so closely packed together, that they form an actual solid pelHcle, which hinders the movement of any object, such as a compass needle, suspended in the surface. When the solution is violently shaken, new surfaces are constantly being formed, and as the older surfaces are withdrawn into the fluid, the solid pelhcle on them is rolled up into a fine shred of coagulated protein, and this process will con- tinue until there is no protein left to form a pellicle. We must conclude that colloidal solutions, although differing so widely from true solutions in many of their properties, are connected with these by all possible grades. In a solution of an ordinary crystalloid or electrolyte the molecules of the dissolved substance are distributed equally and homo- geneously among the molecules of the solvent. In the various grades of solution a colloid solution or hydrosol may be assumed to begin when the size of the molecule is increased out of all proportion to that of the molecules of the solvent. The ' dissolved ' molecules now begin to have the properties of matter in mass and to present surfaces with all their attendant attributes. The same sort of solution may be formed with smaller molecules, such as Si02, when these are aggregated together with adsorbed water into huge molecular complexes, or, as in metallic sols, by the division of the sohd metal into ultra-microscopic particles. The distinguishing features of a colloidal solution are due to this lack of homogeneity, and to the fact that in every solution there are two phases — a fluid phase, and a second phase, which is either sohd or a concentrated or supersaturated solution of the colloid. The huge size of the molecules and the development of surface not only determine the formation of adsorption combinations, but, on account of the inertia of the system, cause a delay in changes of state, and tend to the formation of false equihbria dependent on the past history of the system. IMBIBITION All colloids, even those such as starch or gelatin, which are insoluble in cold water, exhibit a phenomenon, viz. ' Quellung ' or imbibition, which in many cases it is impossible to distinguish from the process of solution. This phenomenon, which was long ago studied by Chevreul and has lately been the subject of a series of careful experiments by Overton, is exhibited by all animal tissues and all colloids. Thus elastic tissue dried in vacuo absorbs from a saturated solution of common salt 36-8 per cent, of water and salt. If dried colloids be suspended in a closed vessel over various solutions, they will take up water in the form of vapour from the solution, and the osmotic pressure of the solution in question will inform us as to the amount of work which would be necessary in order to separate the water again from the colloids. 152 PHYSIOLOGY Thus it lias been reckoned that to press out water from gelatin containing 28-4 parts of water to 100 parts of dried gelatin would require a pressure of over two hundred atmospheres. The imbibition pressure of colloids in- creases rapidly with the concentration of the colloid and at a greater rate than the latter. In this respect, however, imbibition pressure resembles osmotic or indeed gaseous, pressure. At extreme pressures the pressure of hydrogen rises more rapidly than its volume diminishes. In solutions this effect is more marked the larger the size of the molecule. Thus a 6-7 per cent, solution of cane sugar has the same vapour-tension, and therefore the same osmotic pressure, as a -67 per cent. NaCl solution. A 67 per cent, cane-sugar solution has, however, the same osmotic pressure as an 18| per cent, solution of common salt. It is impossible to draw any hard line of distinction between imbibition pressure and osmotic pressure, or to say where a fluid ceases to be a solution and becomes a suspension. All grades are to be found between a solution such as that of common salt with a high osmotic pressure and optical homogeneity, and a solution such as that of starch, which scatters incident light and is therefore opalescent, and has no measurable osmotic pressure. The close connection between the processes of imbibition and of solution is shown also by the fact that the latter occurs only in certain media, the nature of the media being dependent on the chemical character of the sub- stances in question. Thus all the crystalhne carbohydrates — such as grape sugar and cane sugar — are easily soluble in water, only slightly soluble in alcohol, and practically insoluble in ether and benzol. The amorphous carbohydrates, which must be regarded as derived by a process of condensa- tion from the crystalhne carbohydrates — e.g. starch, cellulose, gum arable, &c.^ — have a strong power of imbibition for water. This power may be hmited, as in the case of cellulose, or may be unlimited, as in the case of gum arable, so that a so-called solution results. On the other hand, they swell up but shghtly in alcohol, and are unaffected by ether and benzol. In the same way proteins all take up water, and in many cases form a so-called solution, but are unaffected by ether and benzol — a behaviour which is repeated in the case of the amino-acids, out of which the proteins are built up, and which are easily soluble in water, but are practically insoluble in ether and benzol. On the other hand, india-rubber and the various resins take up ether, benzol, and turpentine often to an indefinite extent, while they are un- touched by water. With this behaviour we may compare the easy solubility of the hydrocarbons, the aromatic acids, and esters in ether and benzol, and their insolubility in water. As Overton has pointed out, the power of amorphous carbohydrates to take up fluids is modified by alteration of their chemical structure in the same direction as the solubility of the corresponding crystalline carbohydrates. Thus, if the hydroxyl groups in the sugars be replaced by nitro, acetyl, or benzoyl groups, they become less soluble in water, while their solubility in alcohol, acetone, &c., is increased. In the same way the replacement of the hydroxyl groups in cellulose by NO 2 groups diminishes the power possessed by this substance of taking up water, but renders it capable of swelhng up or dissolving in alcohol and acetone. SECTION IV THE MECHANISM OF CHEMICAL CHANGES IN LIVING MATTER. FERMENTS All the events which make up the hfe of plants and animals are accompanied and conditioned by chemical changes of the most varied character. In a previous chapter we have endeavoured to form an idea of the ways in which some of the synthetic processes that occur in the hving body may be effected. We saw that, although it was possible to imitate in many respects the vital syntheses by ordinary laboratory methods, the imitation fell far short of the process as it actually occurs in the li^dng cell, both in completeness of the reaction and in the ease wnth which it could be effected. We can, for instance, by passing carbon dioxide over red-hot charcoal, convert it into carbon monoxide, and this gas, acting on dry potassium hydrate, forms potassium formate. Formate of Hme, on dry distillation, gives a small proportion of formaldehyde which, under the influence of dilute alkahes, will condense to the mixture of sugars known as acrose. The green leaf in sunhght absorbs the minimal quantities of carbon dioxide present in the atmosphere and converts it almost quantitatively into starch wathin a few minutes, and this change is effected in the absence of any concentrated reagents and at the ordinary temperature of the atmosphere. Many of the chemical transformations effected by Hving cells we have so far been quite unable to imitate. The problem of the synthesis of camphor, of the terpenes, of starch, of cellulose, is still unsolved, and even in the case of those sub- stances which we can manufacture outside the living cell our methods involve the use of powerful reagents and of high temperatures, and result in most cases in the production of many side reactions, besides that which it is our special object to imitate. The distinguishing characteristics of the chemical changes wrought by the living cell are : (1) The rapidity with which they are effected at ordinary tempera- tures. (2) The specific direction of the process, which is therefore almost complete, with a surprising absence of the side reactions which interfere to such an extent with the yield of the methods employed in a chemical laboratory. This second characteristic may, however, be regarded as a consequence of the first, since an increase in the velocity of any given reaction will deter- mine a preponderance of this reaction over all other possible ones. A funda- 153 154 PHYSIOLOGY mental question, therefore, in physiology must relate to the manner in which the cell is able to influence the velocity of some specific reaction. In spite of the enormous diversity of chemical reactions occurring in the body, they may be divided into a relatively small number of types. Nearly all the reactions are reversible. The chief types of chemical change are as follows : (1) HYDROLYSIS. In most cases this involves the taking up of water and a decomposition into smaller hiolecules. Thus the proteins are broken down in the intestine into their constituent amino-acids. The disaccharides, such as maltose or lactose, take up one molecule of water and giv6 rise to two molecules of monosaccharide. The fats take up three molecules of water with the formation of fatty acid and glycerin. Hippuric acid is broken down into benzoic acid and glycine. The reverse change, that of dehydra- tion, is also effected, apparently with equal facihty, by the hving cell, the hexoses losing water and being converted into a complex starch or glycogen molecule, while the amino-acids are built up first into polypeptides, and these again into the complex proteins of the cell. Besides the reactions in which there is a difierence in the amount of free water on the two sides of the equation, it seems probable that hydrolysis and simultaneous dehydrolysis at different parts of the molecule determine a number of chemical transforma- tions, which at first sight seem to involve a simple sphtting of the molecule. An example of such a process is afforded by the conversion of glucose into lactic acid described on p. 116. (2) DEAMINATION. This process involves the sphtting off of an NH2 group from an amino-acid as ammonia, and its replacement by H or OH. Many tissues of the body appear to have this power. In most cases the nature of the change in the remaining fatty moiety of the molecule has not yet been ascertained. If, for instance, to a mass of liver cells some amino- acid, such as glycine, alanine, or leucine, be added, ammonia is set free in proportion to the amount of amino-acid which was added. This ammonia is therefore assumed to be derived from the amino-acid, and it has been sug- gested that here also the process of sphtting off ammonia is a hydrolytic one and that the NHg gTOup is replaced by OH. Thus — CH3 CH3 CH.NH2 + H2O = CH.OH + NH3 COOH COOH (alanine) Recent work by Neubauer tends to show that deamination is accompanied in the first place by oxidation, so that the first intermediate product formed is not an a oxy-acid, but an a ketonic acid. A second atom of oxygen is then taken up, and carbon dioxide is split off, with the production of the next lower acid of the series. CHEMICAL CHANGES IN LIVING MATTER. FERMENTS 155 We might represent these changes as follows : (1) CH3 CH3 I I CHNH. + 0 = CO + NH3 I " I COOH COOH (2) CH3 I CH3 CO +0=1 + CO2 I COOH COOH . Is the reverse change ever effected in the animal body ? If it were possible to replace the OH group in an oxy-fatty acid by NHg or the 0 in an a ketonic acid by HNHg, it ought also to be possible to nourish an animal from a mixture of carbohydrates and ammonia, or at any rate by supplying him with a mixture of the appropriate oxy-acids or ketonic acids and am- monia. Until recently there was no evidence that the animal body is able to utilise nitrogen, except in organic combination as amino-acids or the complex aggregate of amino-acids known as proteins. In the plant the process of synthesis of protein from ammonia and a carbohydrate such as hexose is continuously going on, and it is probable that the formation of amino-acid occurs by a process the reverse of that which we have just been studying. Knoop has shown that the same reversed change may occur even in a mammal, and that here again the intermediate substance is an a ketonic acid. On administering benzylpyruvic acid (C6H5.CH2.CH2.CO.COOH) to a dog, a certain amount of benzylalanine (CeHg.CHa.CHa.CHNHg.COOH) appeared in the urine. The first phase of the oxidative deamination of amino-acids is thus a reversible one and may be represented : R R R I I /OH I CHNH, + 0 ;=: C( ;=: CO + NH3 COOH COOH COOH (3) DECARBOXYLATION. Many amino-acids when subjected to the agency of bacteria lose a molecule of carbon dioxide and are converted into a corresponding amine. For instance, lysine, which is diamiiio-caproic acid, is converted into pentamethylene diamine or cadaverine. Thus : CH.,.NH., CH,..XH., I " " r " CH., CHo I I C'Ho becomes CHo I I " CH„ CH2 ' I I ^ CH.NH2 CH.2.NH2 I COOH 156 PHYSIOLOGY In the same way ornitliine derived from the breakdown of arginine is con- verted by putrefactive bacteria into tetra-methylene diamine or putrescine. Other examples of this process of decarboxylation are : Isoamylamine from leucine, (CH3)2.CH.CH2.CH2.NH2. [3 phenylethylamine from phenylalanine, C6H5.CH2.CH2.NH2. Para-oxyphenylethylamine from tyrosine, OH.C6H4.CH2.CH2.NH2. A similar process has been supposed to take place as a step in the suc- cessive oxidation of the carbon atoms in the long chain fatty acids or carbo- hydrates, but a thorough study of this process as it occurs in the higher animals is still wanting, and its very existence is indeed still hypothetical. In the case of the fats the oxidation takes place chiefly or entirely in the P position. On the other hand, decarboxylation certainly takes place in substances such as the a amino-acids, where the first oxidation change occurs in the a group, and probably closely follows this oxidation change. The reverse reaction, namely, the insertion of the group CO.O at the end of the long carbon chain, is not known to take place, but would furnish a means by which the organism with apparent simphcity could build up long carbon chains and so imitate the process which in the laboratory is generally effected by attaching a ON group to the end of the molecule. In the case of the fats the building up, hke the oxidative breakdown, appears to occur by two carbon atoms at a time ; hence all the fatty acids met with in the body have an even number of carbon atoms in their chain. It is worthy of note that all the changes which we have been considering — changes which not only account for the greater part of the chemical re- actions of the hving body, but may lead to the production of the most complex substances known — are performed with httle expenditure or evolu- tion of energy. This is evident if we examine the heat evolved by the total combustion of one gramme molecule of the initial and final substances in a number of typical reactions. In the following Table these are given for the substances involved in typical instances of the three classes of chemical change that we have just been considering : (1) Hydrolysis Initial substance Heat of com- bustion per gram molecule Final substance Heat of combustion Maltose . 1350 2 Glucose . 1354 Glucose 677 2 Lactic acid 659 Hippuric acid . 1013 / Glycine . t Benzoic acid 2351 1008 773/ ^^^^ (2) Dea] VIINATION Initial substance Heat of combustion Final substance Heat of combustion Alanine 389-2 Lactic acid 329-5 Leucine 855 Caproic acid 837 Aspartic acid 386 Succinic acid 354 CHEMICAL CHANGES IN LIVING MATTER. FERMENTS L57 Initial substance Alanine Leucine (3) Decarboxylation Heat of Final combustion substance 389 855 Ethylamine . Isoamylaniine . Heat of combustion 409 867 (4) OXIDATION AND REDUCTION. The fourth class of chemical reactions differs from those just described in being attended with a very considerable energy change. This class involves the processes of oxidation and reduction. In almost every hving cell by far the largest amount of the energy available for the discharge of the functions of the cell is derived from the oxidation of the food-stuffs, and even in the plant the energy is obtained from the oxidation of the food-stuffs, built up in the first instance at the cost of the energy of the sun's rays. If we take the final changes in the food-stuffs, the very large evolution of energy attending their oxida- tion is at once apparent. Thus in the conversion of glucose into COg and water there is an evolution for each gramme molecule of 677 calories. In the combustion of glycerin 397 calories are evolved. In the oxidation of a fat such as trimyristin there are 6650 calories evolved. The change does not, in the Hving cell, occur all at once, but the molecule is oxidised step by step. In each step the heat change will, however, be probably greater than the heat changes in the other types of chemical change which we have been considering. Since the mechanism of oxidation in the animal body will have to be discussed at length in a subsequent part of this work, we may at present confine our attention to the other types of chemical change. Of these, all which involve a sphtting of a large molecule into smaller ones with the taldng up of one or more molecules of water, as well as, in all probabihty, those in which the reverse change of dehydration and synthesis occur, are effected in the body by means oi ferments. To the same agency are also ascribed the process of deamination, which takes place in many organs of the body, and, though with less certainty, the processes which involve decarboxylation. FERMENTS Under the name ferments we include a number of substances of indefinite composition whose existence is chiefly known to us by their action on other substances. A ferment has been defined as a body which on addition to a chemical system is able to effect changes in this system without supplying any energy to the reaction, without being used up, and without taking any part in the formation of the end products. It differs therefore from the reacting substances in the absence of any strict quantitative relation- ships between it and the substances included in the system in which its effects are produced. Minimal quantities of ferment can induce chemical changes involving almost indefinite quantities of other substances, the only result of increasing the quantity of ferment being to quicken the rate of tlie change. Since they are effective in minimal doses they occur in living tissues in minute quantities, and it is partly due to this fact that it has 158 PHYSIOLOGY hitherto proved impossible to obtain any preparation of a ferment which could be regarded as a pure substance. The difficulty in their isolation is increased by the fact that all of them are colloidal or semi- colloidal in character, and present, therefore, the tendency common to all colloids of adhering to other colloidal matter as well as to surfaces such as those pre- sented by a precipitate. A common method of isolating, or rather obtaining a concentrated preparation of a ferment is to produce in its solution an inert precipitate such as cholesterin or calcium phosphate. The ferment is carried down on the precipitate and may be obtained in solution on washing the precipitate with water. A further difficulty in their preparation hes in the unstable character of many members of the group. Although they are not coagulated by alcohol, they are nevertheless gradually changed, so that every act of precipitation of a ferment tends to rob it of some of its powers, i.e. of the only characteristic by which we can establish its identity. Of these ferments a large number have already been described as taking part in the ordinary chemical processes of hfe. So wide is their dominion in cell chemistry that many physiologists have thought that the whole of hfe is really a continual series of ferment actions. The following hst repre- sents some of the ferments whose existence has been definitely estabhshed in the animal body. The greater part of them are involved in the processes of digestion in the ahmentary canal. The preponderance, however, of digestive ferments in the hst is due to the fact that we know more about digestion than about the other chemical processes taking place within the cells of the body. List of Ferments Ferment Converting Into Amylase (of saliva, pancreatic Starch Maltose and dextrin juice, liver, blood serum, &c. ) Pepsin ..... Proteins . Proteoses and pep- tones Trypsin ..... Proteins . Peptones and amino- acids Enterokinase .... Trypsinogen Trjrpsin Erepsin ..... Proteoses Amino-acids Lipase (of pancreatic juice, liver, Neutral fats Fatty acid and &c.) glycerin Maltase . . . ^ . Maltose Glucose Lactase . . . .'' . Milk sugar Glucose and galactose Invertase or sucrase . Cane sugar Glucose and levu- lose Arginase ..... Arginin Urea and ornithine Urease ..... Urea Ammonium carbo- nate Lactic acid ferment . Glucose Lactic acid Zymase ( ? present in the body) . Glucose Alcohol and COg Deaminating ferment (?), v. p. 154 Amino-acids Oxy-acids(?) r CHEMICAL CHANGES IN LIVING MATTER. FERMENTS 159 Many other ferments will probably be distinguished with increase in our knowledge of cellular metaboUsm. The long hst which is here given suffices to show how great a part these bodies must play in the normal processes of hfe. A study of the conditions of ferment actions is therefore essential if we would form a conception of the chemical mechanisms of the living cell. It is important to note that all the changes wrought by ferments can be effected by ordinary chemical means. Thus the disaccharides can be made to take up a molecule of water and undergo conversion into mono- saccharides. If a solution of maltose be taken and bacteria be excluded from the solution, it undergoes at ordinary temperatures practically no change. If the solution be warmed, a slow process of hydration takes place which is quickened by rise of temperature, so that if the solution be heated under pressure to, say, 110° C, hydrolysis occurs T\dth considerable rapidity. If, however, a little maltase be added to the solution, the change of maltose into glucose takes place rapidly at a temperature of 30° C. In the same way a solution of protein may be kept almost indefinitely without undergoing hydrolysis, which, however, can be induced by heating the solution under pressure. The action of the ferments in these two cases is to quicken a process of hydrolysis which without their presence would take an infinity of time for its accompUshment. In this respect their action is similar to that of acids, and indeed of a whole class of bodies which are spoken of as catalysers or catalysts. A catalyser is a substance which will increase (or diminish) the velocity of a reaction without adding in any way to the energy changes involved in the reaction, or taking any part in the formation of the end-products. Since the catalyser is unchanged in the process, a very small quantity is able to influence reactions involving large quantities of other substances. By adding acids to a watery solution of the food-stuffs, the process of hydrolysis is quickened in proportion to the strength and concentration of the acid. The effective catalytic agents in this process appear to be the hydrogen ions of the free acid. There are many other bodies, besides the free acids, which may act as catalysers, and a study of the conditions under which catalysis takes place may throw some Ught on the essential nature of the action of ferments. The velocity of almost any reaction in chemistry can be altered by the addition of some catalytic agent, and there are few of the ordinary reactions in which catalysis does not play some part. Among such processes we may instance the action of spongy platinum on hydrogen peroxide. Hydrogen peroxide undergoes slow spontaneous decomposition into water and oxygen. If a little spongy platinum be added to it, it is at once seen to decompose rapidly with the evolution of bubbles of oxygen, and the action does not cease until the whole of the hydrogen peroxide has been destroyed. Spongy platinum is able in the same way to quicken a very large number of chemical reactions. Thus sulphur dioxide and oxygen when heated together will combine very slowly ; the combination becomes rapid if a mixture of the 160 PHYSIOLOGY two gases be passed over heated platinum. The same reaction, namely, the combination of sulphur dioxide with oxygen, may be quickened by the addition of a small trace of nitric oxide, and this fact is made use of in the manufacture of sulphuric acid on a commercial scale by the ordinary lead- chamber process. Hydrogen peroxide and hydriodic acid slowly interact with the formation of water and iodine. This reaction may be quickened by the addition of many substances, among which we may mention molybdic acid. There is moreover a specificity in the action of catalysers, though not so well marked as with ferments. Whereas all the disaccharides are con- verted by acids into the corresponding monosaccharides, a ferment such as invertase acts only on cane sugar, and has no action on maltose or lactose, each of which requires a specific ferment (maltase, lactase) to effect their ' inversion.' But we find many examples of a restricted action even among inorganic catalysers. Thus potassium bichromate will act as the catalyser for the oxidation of hydriodic acid by bromic acid, but not for the oxidation of the same substance by iodic acid. Iron and copper salts in minute traces will quicken the oxidation of potassium iodide by potassium persulphate, but have no influence on the course of the oxidation of sulphur dioxide by potassium persulphate. Tungstic acid increases the velocity of oxida- tion of hydriodic acid by hydrogen peroxide, but has no effect on the velocity of oxidation of hydriodic acid by bromic acid, and these examples may be multiphed to any extent. One cannot therefore regard the limitation of action of the ferments as justifying any fundamental distinction being drawn between the action of this class of substances and catalysts. Whereas the influence of most catalysers on the velocity of a reaction increases rapidly with rise of temperature, in the case of ferments this in- crease occurs only up to a certain point. This point is spoken of as the oftimum temperature of the ferment action. If the mixture be heated above this point the action of the ferment rapidly slows ofi and then ceases. This contrast, again, is only apparent. The ferments are unstable bodies easily altered by change in their physical conditions, and destroyed in all cases at a temperature considerably below that of boiUng water. Thus ferment actions, hke catalytic actions, are quickened by rise of temperature, but the effect of temperature is finally put a stop to by the destruction of the ferment. The same apphes to those inorganic catalysers whose physical state is susceptible, like that of the ferments, to the action of heat. Thus the colloidal platinum ' sol ' exerts marked catalytic effects on various reactions, e.g. on the decomposition of hydrogen peroxide and on the combination of hydrogen and oxygen. The reaction presents an optimum temperature, owing to the fact that the colloidal platinum is altered, coagu- lated, and thrown out of solution when this is heated to near boihng-point. We may therefore employ either class of reactions in trying to form some conception of the processes which are actually involved. Very many theories have been put forward to account for this action of catalysers or of ferments. Many of them are merely transcriptions in CHEMICAL CHANGES IN LIVING MATTER. FERMENTS 161 words of the processes which actually occur, and fail to throw any hght on their real nature. The essential phenomena involved fall directly into two classes. In the first class we must place those which are determined by the influence of surface. In many cases the combination of gases can be hastened by increasing the surface to which they are exposed, as by passing them over broken porcelain or over powdered charcoal. This cata- lytic effect is certainly connected with the power of a solid to condense gases at its surface, and is therefore proportional to the extent of surface exposed. Thus the efficacy of platinum in hastening the combination of hydrogen and oxygen is in direct proportion to its fineness of sub-division, and is best marked when the metal is reduced to ultra-microscopic dimen- sions, as in the colloidal solution of platinum. Every colloidal solution must be regarded as presenting an enormous surface in proportion to the mass of substance in solution. There is therefore a direct proportionality between the power of a substance to condense a gas on its surface and its power to quicken the velocity of chemical changes in which the gas is in- volved. The same process of condensation may occur with dissolved sub- stances. In all cases where the presence of a substance in solution diminishes the surface tension of the solvent, there is a diffusion of dissolved substances into the surface, i.e. a concentration of dissolved substances at the surface of contact. It was suggested by Faraday that the catalytic property of surfaces was due to this condensation of molecules, and the consequent bringing of the two sets of molecules within each other's sphere of influence. Whether this is the sole factor involved is doubtful, since mere compression of gases or increased concentration of solutions does not in the majority of cases result in such a quickening of the velocity of reaction as is brought about by the eft'ect of the surface. It is possible that this condensation effect or adsorption may be in every case combined with the second factor which we must now consider, namely, the formation of intermediate products. If we boil an alkaline solution of indigo with some glucose, the indigo is reduced with oxidation of the glucose. The mixture therefore becomes colourless. On shaking up with air the colourless reduction product of the indigo absorbs oxygen from the atmosphere, and is re-transformed into indigo. These two processes can be repeated until the whole of the glucose is oxidized, and the process can be made continuous if air or oxygen be bubbled through a heated solution of glucose containing a small trace of indigo. In this case the indigo does not add to the energy of the reaction. It appears unchanged among the final products and a small amount may be used to eft'ect the change of an infinite quantity of glucose. It therefore may be said to act as a ferment or catalytic agent. Instead of an alkaUne solution of indigo, we may use an ammoniacal solution of cupric oxide for the purpose of carrying oxygen from the atmosphere to the glucose. This is reduced to cuprous hydrate on heating with the sugar, but cupric hydrate can be at otice re-formed by shaking up the cuprous solution with air. It has been thought that many or all of the catalvtic reactions occur in the same wav by two stages, i.e. 6 162 PHYSIOLOGY by the formation of an intermediate product. Thus, in the old lead chamber process for the manufacture of sulphuric acid, the nitric oxide may be sup- posed to combine with the oxygen of the air to form nitrogen peroxide. This interacts with sulphur dioxide, giving sulphur trioxide and nitric oxide once more. The nitric oxide, which we alluded to before as the catalyser, may in this way be regarded as the carrier of oxygen from air to sulphur dioxide. It has been suggested that the action of spongy platinum or colloidal platinum rests on the same process, and that in the oxidation of hydrogen, for instance, PtO or PtOg is formed and at once reduced by the hydrogen with the formation of water. There is a certain amount of experimental evidence in favour of this hypothesis. According to Engler and Wohler,* platinum black, which has been exposed to oxygen, in virtue of the gas which it has occluded, has the power of turning potassium iodide, and starch blue. This power is not destroyed by heating to 260° in an atmosphere of CO2, or by washing with hot water. On exposure of the platinum black to hydrochloric acid, a certain amount is dissolved, and the substance loses its effect on potassium iodide. The amount dissolved corresponds with the amount of iodine liberated from potassium iodide, and also with the amount of oxygen occluded, the (soluble) platinum and oxygen being in the proportions necessary to form the compound PtO. But why should a reaction take place more quickly if it occurs in two stages instead of one ? As Ostwald has pointed out, the formation of an intermediate compound can be regarded as a sufficient explanation of a catalytic process only when it can be demonstrated by actual experiment that the rapidity of formation of the intermediate compound and the rapidity of its decomposition into the end-products of the reaction are in sum greater than the velocity of the reaction without the formation of the intermediate body. In the case of one reaction this requirement has been fulfilled. The catalytic effect of molybdic acid on the interaction of hydriodic acid and hydrogen peroxide has been explained by assuming that the first action which takes place is the formation of permolybdic acid, and that this then interacts with the hydrogen iodide to form water and iodine. Now it has been actually shown — (1) that permolybdic acid is formed by the action of hydrogen peroxide on molybdic acid ; (2) that permolybdic acid with hydriodic acid produces water and iodine ; (3) that the velocity with which these two reactions occur is much greater than the velocity of the inter- action of hydrogen peroxide and hydriodic acid by themselves. Although we may find it difficult to explain why a reaction should occur more quickly in the presence of a catalyser by the formation of these intermediate bodies, certain simple analogies may help us to comprehend how a factor which introduces no energy can yet assist the process. Thus a man might stand to all eternity before a perpendicular wall twenty feet high. Since he cannot reach its top at one jump, he is unable to get there at all. The introduction of a ladder will not in any way alter the total energy he must expend on raising his body for twenty feet, but will enable him to attain the top. Or we might imagine a stone perched at the top of a high hill. The passive resistance of the system, the friction of the stone, and its inertia will tend to keep it at rest, even though it be on a sloping surface and therefore tending to slide or roll to the bottom. If, however, it be rolled to a point where there is a sudden increase in the rapidity of slope, it may roll over, and having once started * Quoted by Mellor, " Chemical Statics." CHEMICAL CHANGES IN LmNG MATTER. FERMENTS 163 its downward course, its momentum will carry it to the bottom. The amount of energy set free by the stone in its fall will not vary whether the course be a uniform one, or whether it falls over a precipice at one time and rolls down a gentle slope at another. It is evident that by a mere alteration of the slope, or, in the case of a chemical reaction, of the velocity of part of its course, a change in the system may be initiated and brought to a conclusion which without this alteration would never take place. Since the action of ferments, like that of catalysts, consists essentially in the quickening up of processes which would otherwise occur at an in- finitely slow velocity, it is possible that in their case also the formation of an intermediate compound may be involved in the reaction. Light may be thrown upon this question by a study of the velocity of the reaction in- duced by the action of a ferment. It Ls well known that the velocity of a reaction depends on the number of molecules involved. As an illustration, we may take first the case of a reaction involving a change in one substance. If arseniuretted hydrogen be heated, it undergoes decom- position into hydrogen and arsenic. This decomposition is not immediate, but takes a certain time, and the velocity with which the change occurs depends on the tempera- ture. At any given temperature the amount of substance changed in the unit of time varies with the concentration of the substance. If, for instance, one-tenth of the gas be dissociated in the first minute, in the second minute a further tenth of the gas will also be dissociated. Thus, if we start with 1000 grammes of substance, at the end of the first minute 100 grammes will have been dissociated, and 900 of the original substance will be left. In the second minute one-tenth again of the remaining substance will be dissociated, i.e. 90 grammes, leaving 810 grammes. In the third minute 81 grammes will be dissociated, leaving 729 grammes. The amount changed in the unit of time will alwaj^s bear the same ratio to the whole substance which is to be changed, and will therefore be a function of the concentration of this substance. Put in the form of an equation, we may say that , the amount changed in the unit of time, will be equal to KC, where K is a constant varying with the substance in question and with the temperature, and C represents the concentration of the substance. The equation <^ = KC applies to a monomolecular reaction. If two substances are involved, the equation will be rather different. In this case the amount of change in a unit of time will be a function of the concentration of each of the substances, and the form of the equation will be (^ = K(C^ x Cj.). In the case of the unimolecular reaction, halving the concentration of the substance will halve the amount of substance changed in the unit of time. In the case of a bimolecular reaction, halving each of the substances will cause the amount of change in the unit of time to be reduced to one-quarter of its previous amount. If now either a unimolecular or a bimolecular reaction be quickened by the addition of a catalyser or ferment, and the ferment enter into combination with one of the substances at some stage of the reaction, it is evident that our equation must take account also of the concentration of the ferment or catalyser. In the case of the catalytic effect of molybdic acid on the inter- action between hydrogen peroxide and HI, there was definite evidence of a reaction taking place between the molybdic acid and the peroxide, resulting in the formation of an intermediate compound, namely, permolybdic acid. Brode has shown that the interaction of the molybdic acid is revealed in the equation representing the velocity of the reaction. Without the addition of molybdic acid the equation would be : (^ = K(C„20o ^ Chi). After the addition of molybdic acid, the equation becomes : 0 = K(C„202 + y C molybdic acid)CHi, when y is another constant depending on the molybdic acid. If ferments act in n, similar way by the formation of intermediate compounds, this fact should be revealed by a study of the velocity at which the ferment action takes place. 164 PHYSIOLOGY Various methods may be adopted for the study of the velocity of ferment action. If, for instance, we are investigating the action of diastase upon starch, we should take solutions of starch and of diastase of known concen- trations, keep them in a water bath at 38° C, and at a certain point add, say, 20 c.c. of ferment solution to every 100 c.c. of the starch solution. At periods of five or ten minutes after the addition had been made, 5 c.c. of the mixture might be withdrawn by a pipette and at once run into boiling Fehling's solution. The precipitated cuprous oxide would be dried and weighed, and would give directly the amount of sugar formed by the action of the ferment. After obtaining a series of data in this way, a curve could be drawn, showing the amount of change of starch which had occurred in each unit of time. In the case of the action of invertase on cane sugar the investigation is still easier. Since the change from cane sugar to invert sugar is accompanied by a change in the rotatory power of the solution on polarized light, it is only necessary to put the mixture of ferment and cane sugar into a polarimeter tube, which is kept at a constant temperature by means of a water jacket, and read off at intervals of a few minutes the change in the rotatory power of the solution. From this change can be easily calculated the percentage of cane sugar still present, and there- fore the total amount which has been converted into fructose and glucose. In investigating the action of proteolytic ferments, as, e.g. that of trypsin on caseinogen, samples are taken at five-minute intervals and run into some substance such as trichloracetic acid, which will precipitate all the unchanged protein, but will leave in solution the products of hydration of the protein. From the amount of nitrogen in the filtrate from the precipi- tate can be determined the total amount of protein which has undergone hydration in the sample under observation. Or the amount of albumoses and peptones present in each sample may be estimated by the intensity of the biuret reaction which can be obtained. This method, however, suffers from the drawback that the albumoses and peptones, at any rate in the action of trypsin, are formed merely as a stage in the process, and the in- tensity of the reaction will first rise to a maximum and then gradually dis- appear. A very convenient method is that employed by Henri and by Bayliss in the investigation of the kinetics of tryptic action, namely, the determination of the conductivity of the solution. In the disintegration of the molecule caused by the action of the ferment, there is a continuous increase in the conductivity of the solution, and this increase can be regarded as an index to the rate of change in the substances undergoing disinte- gration. By such methods it has been found that, when the quantity of ferment employed is very small in comparison with the substrate (the substance acted upon), the amount of change in a given time is proportional to the amount of ferment present, and is (within limits) independent of the con- centration of the substrate. This is well shown by the two following Tables representing the action of lactase upon lactose (E. F. Armstrong) : CHEMICAL CHANGES IN LIVING MATTER. FERMENTS 165 Proportions Hydro lysed in 100 c.c. of a 5 per cent. Solution of Lactose Solutions containing — lo hours 20 hours 45 hours 1 C.C. lactase 10 c.c. „ 20 c.c. „ 0-15 1-6 3-2 2-2 23-3 45-8 3-9 38-6 Amoitnt op Sugar (Lactose) Hydrolysed Solutions containing — 24 hours 46 hours Proportion ^Yeight Proportion Weight 10 per cent, lactose 20 „ 30 „ U-2 7-0 4-8 1-42 1-40 1-44 22-2 10-9 7-7 2-22 2-18 2-21 Moreover, if we take only the earlier stages of the ferment action, it is found that, with small proportions of ferment, equal amounts of substrate are changed in successive intervals of time until about 10 per cent, has been hydrolysed. This is shown in the following Table : 2 per cent. Lactose with Lactase Time Amount hydrolysed ^ hour 3-2 6-4 1 „ 9-6 2 hours 16-4 3 „ 20-8 These results can be interpreted only by assuming that the first stage in the reaction is a combination of ferment with substrate. It is only this compound which represents the active mass of the molecules, i.e. the molecules of substrate which are undergoing change. This compound, as soon as it is formed, takes up water and breaks down, setting free the hydrolyzed substrate and the ferment, which is at once ready to combine with a further portion of the substrate. In such a case the velocity of reaction must be directly proportional to the amount of ferment, and the same absolute quantity of substance will continue to be changed in succeeding units of time. Supposing, for instance, we had a load of bricks at the bottom of a hill which had to be transferred to the top. and five men to effect the trans- ference. The rate of transference would be directly proportional to the number of men employed ; we could double the rate by doubling the men. 166 PHYSIOLOGY Moreover, the number of bricks carried in each unit of time would be the same. Five men would carry as many bricks in the second ten minutes as they would in the first, and so on. On the other hand, the velocity with which the transference was effected would be independent of the number — that is, the concentration — of the bricks at the bottom of the hill. The active mass of bricks could be regarded as that number carried at any moment by the transferring factor, namely, the men. The equation of change would be (^ = KG, where G is the concentration of the ferment. This concen- tration is always being renewed, and kept constant by the breaking down of the intermediate product, so that the rate of change would be continuous throughout the experiment. On the other hand, when the amount of ferment is relatively large, the rate of change, though at first very rapid, tends continuously to diminish. This is shown by the following Table representing the rates of change, during succeeding intervals of ten minutes, in a caseinogen solution to which a strong solution of trypsin had been added (BayUss) : Velocity of Trypsin Reaction N . 8 per cent, caseinogen + 2 c.c. 10 AmHO + 2 c.c. 2 per cent tryps 5in at 39^ 'C 1st 10 minutes K =0-0079 2nd 0-0046 3rd 0-0032 4th 0-0022 5th 0-0016 7th 0-0009 &c. &c. The cause of this rapid diminution in the velocity of change is probably complex. One factor may be an auto-destruction of the ferment, which is known to occur in watery solution. That this is not the only, or even the chief, factor involved is shown by the fact that, when the action of trypsin on caseinogen has apparently come to an end, it may be renewed by further dilution of the mixture or by removal of the end-products of the action by dialysis. It is evident that, in this retardation of the later stages of ferment action, the end-products are concerned in some way or other, and the retarda- tion can be augmented by adding to the digesting mixture the boiled end- products of a previous digestion. The retarding effect of the end-products resembles in many ways that observed in a whole series of reactions which are known as reversible. As an example of such a reaction we may take the case of methyl acetate and water. When methyl acetate is mixed with water, it undergoes decomposition with the forma- tion of methyl alcohol and acetic acid. On the other hand, if acetic acid be mixed with alcohol, an interaction takes place with the formation of methyl acetate and water. These changes are represented by the equation : MeCgHgOa + HOH :== MeOH + HCgHgOj. methylacetate water methylalcohol acetic acid Each of these changes has a certain velocity constant, and, since they are in opposite directions, there must be some equilibrium point where no change will occur, and CHEMICAL CHANGES IN LIVING MATTER. FERMENTS 167 there will be a definite amount of all four substances present in the mixture, namely, water, alcohol, ester, and acid. This equilibrium point can be shifted by altering the amount of any of the four substances. Thus the interaction of methyl acetate and water can be diminished to any desired extent by adding to the mixture the products of the interaction, namely, methyl alcohol and acetic acid. There is evidence that some of the ferment actions are reversible. Thus maltase acts on maltose with the formation of two molecules of glucose. If the maltase be added to a concentrated solution of glucose, we get a reverse efiect, with the production of a disaccharide which has been desig- nated as isomaltose or revertose. To this reverse action may be due a certain amount of the retardation observed in the action of trypsin on coagulable protein. A more important factor is probably the combination of the ferment itself with the end-products and the consequent removal of the ferment from the sphere of action. Several facts speak for such a mode of explanation. Thus the action of lactase on milk sugar is not re- tarded by both its end-products, namely, glucose and galactose, but only by galactose. In the same way the action of invertase on cane sugar is retarded by the end-product fructose, but not at all by the other end-product, glucose. So far, therefore, a study of the velocity of ferment actions would lead us to suspect that the ferment combines in the first place with the substrate, and that this combination is a necessary step in the alteration of the sub- strate. In the second place, the ferment is taken up to a certain extent by some or all of the end-products, and this combination acts in opposition to the first combination, tending to remove the ferment from the sphere of action, and therefore to retard the whole reaction. Other facts can be adduced in favour of these conclusions. Thus it has been shown that invertase ferment, which is destroyed when heated in watery solution at a temperature of 60° C, can, if a large excess of its substrate, cane sugar, be present, be heated 25° higher without undergoing destruction. The same protective effect is observed in the case of trypsin. Trypsin in watery or weakly alkaline solutions undergoes rapid decomposition. At 37° C. it may lose 50 per cent, of its proteolytic power within half an hour. If, on the other hand, trypsin be mixed with a protein such as egg albumin or caseinogen, or with the products of its own action, namely, albumoses and peptones, it can be kept many hours without undergoing any considerable loss of power. It has been found that, whereas maltase splits up all the a-glucosides, it has no power on the /3-glucosides ; that is to say, maltase will lit into a molecule of a certain configuration, but is powerless to affect a molecule which ditTers from the first only in its stereochemical structure. On the other hand, emulsin. which breaks up /3-gluco- sides, has no influence on n-glucosides. This specific affinity of the ferments for optically active groups of bodies suggests that the ferment itself may be optically active. We cannot of course isolate the ferment and determine its optical behaviour ; but that it is optically active is rendered probable both by these results and certain results obtained by Dakin on lipase, tlic fat -splitting ferment. Dakin carried out his experi- ments on the esters of mandelic acid. Mandelic acid is optically inactive, but this optically inactive modification consists of a mixture of equal parts of dextro-rotatory 168 PHYSIOLOGY and lee vo -rotatory mandelic acid. The esters prepared from the optically inactive acids are themselves optically inactive. Dakin found that, when an optically inactive mandelic ester was acted upon by a lipase prepared from the liver, the final results of the action were also inactive ; but if the reaction were interrupted at the half-way point, the mandelic acid which had been liberated was dextro-rotatory, while the remainder of the ester was lee vo -rotatory. Thus the rate of hydrolysis of the dextro- component of the ester is greater than that of the Isevo-component, a result which can be best explained by the assumptions (a) that the enzyme or a substance closely asso- ciated with it is a powerfully optically active substance ; (b) that actual combination takes place between the enzyme and the ester undergoing hydrolysis. Since the additive compounds thus formed in the case of the dextro- and lee vo -components of the ester would not be optical opposites, they would be decomposed with unequal velocity, and thus account for the liberation of the optically active mandelic acid. We may conclude that in the action of ferments on the food substances, whether carbohydrate or protein, an essential factor is the combination of the ferment with the substrate. Only the part of the substrate, which is thus combined with the ferment, can be regarded as the active mass and as undergoing the hydrolytic change. What is the nature of this combina- tion ? Ferments, which are all of a colloid or semi-colloid character, cannot be dealt with in the same way as the catalysts of definite chemical com- position, such as molybdic acid or nitric oxide. In many cases the sub- strate, e.g. starch or protein, is also colloidal, and the combination therefore falls into the class of combinations between colloids. In this we have an interaction between two substances in which the adsorption by the surfaces of the molecules of one or both substances plays an important part, though this adsorption is itself determined or modified by the chemical configuration of the molecules. The combination of ferments with their substrates be- longs, therefore, to that special class of interactions, not entirely chemical and not entirely physical, but depending for their existence on a co-operation of both chemical and physical factors, which we have discussed earlier under the name of adsorption compounds. FERMENTS AS SYNTHETIC AGENTS If maltase, obtained from yeast, or from the so-called takadiastase (prepared from Aspergillus oryzce), be added to a solution of maltose, the latter is hydrolysed to glucose. The process of hydrolysis stops short of complete inversion at a point varying with the concentration of the sugar solution. Thus in a 10 per cent, solution of maltose, inversion proceeds until 98 per cent, of the maltose is converted into dextrose, whereas in a 40 per cent, solution the change stops short when 85 per cent, sugar has undergone inversion. Croft Hill showed that if the maltase were added to a 40 per cent, solution of dextrose, a change took place in the reverse direction, which proceeded until 85 per cent, of the glucose was left. The sugar formed, which is a disaccharide, was regarded by Croft Hill as maltose. According to Emmerling, however, it is the stereoisomeric sugar, iso-maltose, which is formed ; and Croft Hill in his later papers spoke of the sugar as revertose. In the same way it has been shown by Castle and Loewenhart that the CHEMICAL CHANGES IN LIVING MATTER. FERMENTS 169 hydrolysis of esters by lipase is a reversible reaction, the action of lipase being simply to hasten the attainment of the equilibrium point between the four substances — ester (or neutral fat), water, fatty acid, and alcohol. Similar reversible effects have been described for other ferments. Thus the addition of pepsin to a strong solution of albumoses causes the appear- ance of an insoluble precipitate, which is called plastem, and has been re- garded as produced by the resynthesis of the original protein molecule. If all ferment actions are in this way reversible, a possibihty is opened of regarding the S5nithetic processes occurring in the hving cell, as well as the processes of disintegration, as determined by the action of enzymes. It must be noted that these effects are only obtained with distinctness when dealing with concentrated solutions. The degree of synthesis which would be produced in the very dilute solutions of glucose, &c., occurring in the animal cell would therefore be infinitesimal. But if a mechanism were provided for the immediate separation of the synthetical product from the sphere of reaction, either by removing it to a different part of the cell or by building it up into some more complex body which was not acted on by the ferment, the process of synthesis might go on indefinitely, and the infinitesimal quantities be summated to an appreciable amount. Some experiments by Bertrand on fat synthesis have been interpreted as showing that the process of synthesis by ferments is not the mere attain- ment of an equilibrium point in a reversible reaction. It has long been known that watery extracts of the fresh pancreas split neutral fats into the higher fatty acids and glycerine. This observer has shown that if the pancreas be dried with alcohol and ether and powdered, addition of the dry powder to a mixture of the higher fatty acids and glycerine brings about a rapid synthesis of neutral fat. The process of synthesis is at once stopped by the addition of water. In this case either there are two ferments present, one a synthetising, the other a hydrolysing, ferment, differing in their con- ditions of activity, or there is one ferment which may act either as a fat- sphtting or fat-forming agent according to the conditions under which it is placed. In the latter case the effect of the addition of water would be simply to alter the equilibrium point of the mixture. It has been shown that in all reversible reactions the equihbrium position is the same from which- ever side it be approached. The action of the ferment is to hasten the attainment of equihbrium, the position of the latter being determined by the relative concentration of the reacting molecules. 6* SECTION V ELECTRICAL CHANGES IN LIVING TISSUES The material composing living cells and tissues is permeated throughout with water containing electrolytes in solution. All salts, as we have seen, undergo ionic dissociation in watery solution — a dissociation which, in the concentrations occurring in the animal body, must be nearly complete. When an electric current passes through the living tissues it is carried by the charged ions formed by the dissociation of the salts. Thus, n/10 solution + of sodium chloride contains almost entirely Na and CI ions. In addition to these charged inorganic ions, the cell protoplasm contains in solution or suspension various colloidal particles which in many cases are themselves charged. By the presence of these colloidal particles marked differences may be caused in the distribution of the inorganic ions owing to the power of adsorption possessed by the colloids for many inorganic salts. It is evident that any unequal distribution of the charged ions or colloidal particles in a tissue or on the two sides of a membrane may give rise to corresponding unequal distribution of electric charges, and therefore differences of potential between different parts of the tissue, which under suitable conditions may find their expression in an electric current. It is therefore not surprising that practically every functional change in a tissue has been shown to be associated with the production of differences of electrical potential. Thus all parts of an uninjured muscle are isopotential, and any two points may be led off to a galvanometer without any current being observed. If, however, one part of the muscle be strongly excited, as, for instance, by injury, so that it is brought into a state of lasting excitation, it will be found that, on leading off from this point and a point on the uninjured surface to a galvanometer, a current flows through the latter from the uninjured to the injured surface. Every beat of the heart, every twitch of a muscle, every state of secretion of a gland, is associated in the same way with electrical changes. In most cases the electrical changes associated with activity have the same general character, the excited part being found to be negative in reference to any other part of the tissue which is at rest. The uniform character of the electric response in different kinds of tissues suggests that an accurate know- ledge of the changes in the distribution of charged ions responsible for the response ought to throw important light on the intimate nature of excitation generally. It may be therefore advisable to consider more closely the 170 ELECTRICAL CHANGES IN LIVING TISSUES 171 conditions which determine differences of potential in a complex system of electrolytes. As a simple case we may take an ordinary concentration cell. Two vessels (Fig. 29), A and B, are united by a glass tube C. A contains a 10 per cent, solution of zinc sulphate and B a 1 per cent, solution of the same salt. A rod of pure zinc is immersed in each limb. On connecting the zinc by a zinc wire to a galvanometer a current is observed to flow from A to B through the galvanometer, and therefore from B to A through the cell. A ^ 1 i i 10% /»/ 1 / « B ®Zn In® ®Zn ®2n ®Zn ©Z/7 © Zn © Zn Fig. 29. Fig. 30. solution of zinc sulphate contains partly undissociated ZnS04 and partly dissociated Zn and SO4 ions. If a rod of zinc be immersed in a watery fluid the zinc tends to dissolve. The Zn passing into the fluid is, however, directly ionised, and therefore carries a positive charge into the fluid, leaving the zinc negatively charged (Fig. 30). This process of solution will rapidly come to an end, since the positively charged ions in the fluid will repel back into the zinc any ions which may be escaping from the zinc. The amount of zinc actually dissolved in the fluid is infinitesimal, the process of solution ceasing when the pressure (osmotic pressure) of the Zn ions in the fluid equals what may be called the ' electrolytic solution pressure ' of the zinc. The continued solution of the zinc is therefore only possible when means are supplied for the Zn ions in the fluid to get rid of their positive charges. In an ordinary Daniell cell the Zn ions which leave the zinc are dis- charged by combining with the SO4 ions passing to the zinc from the copper sulphate in the outer cell. It is a well-known fact that pure zinc docs not dissolve in acid until some other metal, such as copper, is brought into con- tact with it, so as to set up an electric couple, i.e. to provide means for the discharge of the Zn ions passii\g into the solution. When the zinc is immersed in the two solutions of zinc sulphate in the concentration battery, the same change will occur. The ZnSO.i solution in the two limbs of the 172 PHYSIOLOGY concentration cell already contains Zn ions. Since their pressure in the 10 per cent, solution is greater than in the 1 per cent, solution, fewer Zn ions will leave the zinc in A than in B. The negative charge on the Zn in A will therefore be less than that on the rod in B, and positive electricity will there- fore flow from A to B. This will disturb the equihbrium at the surface both of B and A, so that Zn ions will be deposited from the fluid on the surface of the zinc in A and will continue to pass from zinc into solution in B. At the same time there is a movement of SO4 ions, set free at the surface of A towards B. The ultimate result, therefore, is that the zinc in B dis- solves and the same amount of zinc is deposited on A. The solution of zinc sulphate on A becomes progressively weaker, while that in B be- comes stronger, until finally the concentrations in the two limbs are identical and the current ceases. In this process no chemical energy is involved, the energy set free by the conversion of zinc into zinc sulphate in B being exactly balanced by the energy lost by the deposition of zinc from zinc sulphate in A. Yet the current which is produced has a certain amount of energy which can be utihsed for heating a wire through which it is made to pass. Since this energy must be taken from the cell, the cell is cooled during the passage of the current. We have here a close analogy with ' the case of compressed gases. If the 10 per cent, and 1 per cent, solutions were mixed together in a calorimeter, no change of temperature would be produced, since no work is done in the process. In the same way no cooHng effect is observed if compressed gas be allowed to expand into a vacuum. If, however, the compressed gas be allowed to expand from a narrow orifice against the pressure of the external air, so that it does work in the process, it is cooled, and this cooKng eflect is made use of in the working of refrigerating machines or for the hquef action of gases. We may therefore regard the concentration battery as a machine for making the substances in solution do work as they expand from a strong into a dilute solution. The differences of potential obtained from an ordinary concentration cell are very small and would not ^ suffice to account for such a high electromotive force as is set up, e.g. in the contraction of a muscle. We have seen earher, however, that even in isosmotic solutions differences of u\/ B UV pressure may be brought about by differences in diffusibihty of the sub- stances in solution, especially if the two solutions be separated by a mem- brane. Very large differences may be produced if this membrane be practic- ally impermeable to one or other of the dissolved substances. In the same way a semipermeable membrane, i.e. a membrane with different permea- bihties for the diflerent ions of the two solutions, may suffice to bring the 31. ELECTRICAL CHANGES IN LIVING TISSUES 173 differences of potential of a concentration cell up to and beyond the extent which is observed in living tissues. Supposing we have (Fig. 31) two solutions, A and B, each containing an electrolyte, UV, in different concentrations separated by a membrane m. If u represents the velocity of transmission of U through m, and v the velocity of V, then the electromotive force of the cell is given by the formula 5i^l^ 0-0577. log.io - Volt. U + V C'l If V is taken as very small, the membrane maybe regarded as semipermeable for the corresponding ion V. Supposing we take potassium chloride as the solution, we should have to make the concentration in B eight times that in A, in order to get a current of a strength equal to that obtained from the olfactory nerve of the pike, for example. Macdonald has made such an assumption in order to explain the normal nerve current. He suggests that the axis cyhnder contains an electrolyte which is equivalent to a 2-6 per cent, solution of potassium chloride. It is unnecessary, however, to assume such great differences of concentration if we regard the membrane as itself a solution of electrolytes, as has been suggested by Cremer, or if we take different substances on the two sides of the membrane. In the case of two electrolytes, U^Vi, UgVa (L^ being the cation in each case), separated by a membrane with varying permeabihty for the different ions, "the electro- motive force of the cell is given by the following formula : 0-0677 log,. !^^ where m^, v^, Wgj Vj, are the velocities of the corresponding ions. We assume that the concentrations of the two solutions are identical. Now it is evident that by making Wg and Vi very small, the expression log-^, — may be made to attain any quantity, and in the same way by making u^ + v, infinitesimally small the electromotive force of the combination wiW also become correspondingly small. The thickness of the membrane does not come into the formula, so that membranes of microscopic or even ultra - microscopic thickness, which we have seen reason to assume as present in and around cells and their parts, could perform all the fmictions required of the hypothetical membrane in the above example. This is also the case when Vj is the same as V., — that is to say, there is a common anion or a connnon cation on the two sides of the membrane. It must be remembered that the passage of a current through a membrane impermeable to one or other ion in the surrounding fluid will cause an accu- mulation of the ion at the surface of the membrane, so that this will become polarised. Such an accumulation at any surface will naturally alter the properties of the surface, including its surface tension. The construction of the capillary electrometer depends on this fact. When mercury is in contact with dilute acid or mercuric sulphate solution it takes a positive charge from 174 PHYSIOLOGY the fluid, and the state of stress at the surface of contact between the mercury and the negatively charged fluid diminishes the surface tension of the mercury. If the mercury be in the form of a drop in a tube drawn out to a capillary, the mercury will run down the capillary and the drop will be deformed until the surface tension tending to pull the mercury into a spherical globule is just equal to the force of gravity tending to make the mercury run out through the end of the capillary (Fig. 32). If the mercury be immersed in sul- phuric acid it will descend to a lower level in the capillary owing to the diminution of its surface tension. If now the acid and the mercury be connected with a source of current so as to charge the mercury negatively, the effect will be to ^^^ diminish the charge previously taken up by the mercury. ^^^^k The state of tension at the contact with the acid is therefore ^^I^H diminished, the surface tension is increased, and the mercury i^^^^l / withdraws itself from the point of the capillary. If, however, \^^^V/ the mercury be connected with the positive pole, its charge ^^^m/ will be increased and its surface tension correspondingly \^^/ diminished, so that the meniscus will move towards the \ W / point of the capillary. The movement of the meniscus to or n I away from the point may thus be used, as in the capillary U U • electrometer, to show the direction and amount of any Fig. 32. moderate electric change occurring in a tissue, two points of which are connected with the mercury and the acid re- spectively. It is possible that this electrical alteration of surface tension may be a determining factor in many of the phenomena of movement observed in the animal body. We shall have occasion to discuss this question more fully when endeavouring to account for the ultimate nature of muscular contraction. BOOK II THE MECHANISMS OF MOVEMENT AND SENSATION CHAPTER V THE CONTRACTILE TISSUES SECTION I THE STRUCTURE OF VOLUNTARY MUSCLE The most striking features in the continual series of adaptations to the environment, which make up the hfe of an individual, are the movements carried out by contractions of the skeletal muscles. In fact, all the mechan- isms of nutrition can be regarded as directed to the maintenance of the neuro-muscular apparatus, i.e. of the mechanism for adapted movement. With the growth of the cerebral hemispheres, which determines the rise in the scale of animal life, the skeletal muscles become more and more the machinery of conscious reaction. Even the highest of the adaptations possessed by man, those involving the use of speech, are impossible ■\^'ithout some kind of movement. A man's relation to his fellows, and his value in the community, are determined by these higher nuiscular adaptations. It is not, therefore, surprising that the organs of the body which present in the highest degree the reactivity characteristic of all hving things should have early attracted the attention of physiologists and have been the object of numberless researches directed to determining the ultimate nature of the processes generally described as vital. The movements of the muscles are carried out in response to changes aroused in the central nervous system by events occurring in the environ- ment and acting on the surface of the body. Every movement of an animal is thus in its most primitive form a reflex action, and involves changes in a peripheral sense organ, in an afferent nerve fibre, in the central nervous system, and in an efferent nerve fibre, before the actual process of contrac- tion occurs in the muscle itself and gives rise to the resultant movement (Fig. 33). If we are to determine the nature of the changes involved in this reflex action, we must be able to study them as they progress along the different elements which make up the reflex arc. This analysis is facihtated by the fact that we are able to arouse a condition of activity in the different parts of the arc, even when isolated from one another. Thus we can excite any given reflex movement by stimulation of the periphery of the body, or of the afferent nerve passing from the surface to the central nervous 177 178 PHYSIOLOGY system. We can proceed further and cut the efferent nerve away from the central nervous system and still succeed in exciting a condition of activity in the efferent nerve or in its attached muscle. All parts of the reflex arc possess the property of excitability, and we are thus able to arouse the activity of each part in turn, to study its conditions, its time relations, and the physical and chemical changes concomitant with the state of acti^dty. It will be convenient for our analysis to begin with the tissue whose V \ 1 Central Nervous 'i^n.'^nry 11^ :>ensoryj^erve y ' I— \Sysfem Fig. 33. Diagram of a reflex arc. reaction forms an end link in the reflex chain, namely, the muscle, and to proceed from that to the consideration of the processes occurring in the conducting, strand between central nervous system and muscle, namely, the nerve fibre, postponing to a future chapter the treatment of the more complex processes associated with the central nervous system. In the higher animals we may distinguish several varieties of muscle. All movements that require to be sharply and forcibly carried out are effected by means of striated muscular tissue, and as these movements are in nearly all cases under the control of the will the muscles are generally spoken of as voluntary. Unstriated or involuntary muscles from sheets or closed tubes surrounding the hollow viscera. By their slow, prolonged contractions they serve to maintain and regulate the flow of the contents of these organs. Such fibres are found surrounding the blood-vessels, the ahmentary canal, the bladder, &c. Intermediate in properties as well as structure between these two classes is the heart muscle. This, like voluntary muscle, is striated, but presents considerable variations both in structure and function from ordinary skeletal muscle. Many of its properties will be considered in treating of the physiology of the heart. The properties of contractile tissues have been most fully investigated in the voluntary muscles, almost exclusively on the muscles of cold-blooded animals, such as the frog. The choice of skeletal muscles for this purpose is justified by the fact that a function is most easily investigated in the organs in which it is most highly developed. The choice of cold-blooded animals is guided by the fact that it is possible to isolate the muscle from the rest of the body and to study its reactions during a considerable time THE STRUCTURE OF VOLUNTARY MUSCLE 179 without the research being interfered with by the death of the tissue. We may therefore deal at length with the properties of the skeletal muscles, pointing out incidentally in what respects the heart muscle and involuntary muscle differ from the skeletal muscle. The voluntary or striated muscles form a large part of the body, and are known as the flesh or meat. Each muscle is embedded in a layer of connective tissue, and is made up of an aggregation of muscular fibres, which are united into bundles by means of areolar connective tissue. The individual fibres vary much in length, and may be as long as 4 or 5 cm. At each end of the muscle the fibres are firmly united to tough bundles of white fibres, which form the tendon of the muscle, and are attached as Fig. 34. Muscular fibre of a mammal, examined fresh in scrum, highly magnified. (Schafer.) a rule to bones. Running in the connective tissue framework of the muscle we find a number of blood-vessels, capillaries and nerves. On examination of a living muscle, each fibre is seen to consist of a series of alternate light and dark striae, arranged at right angles to its long axis, and enclosed in a structureless sheath — the sarcolemma. Lying mider the sarcolemma are a number of oval nuclei embedded in a small amount of granular protoplasm. Li some animals these nuclei occupy a central position in the fibre. Each band may be considered to be made up of a number of prisms (sarcomeres) side by side, with interstitial substance (sarcoplasm) between them. The muscle prisms of adjacent discs are connected to form long columns (primitive fibrillae, or sarcostyles). Each muscle prism is more transparent at the two ends than in the middle, thus giving rise to the appeareance of hght and dark stria?. Li the middle of the hght band is a line or row of dots (often appearing double), called Krause's membrane. The development of this regular cross and longitudinal striation is closely connected with the evolution and speciahsation of the muscular function, i.e. contraction. Contractility is among others a function of all undift'erentiated protoplasm. Undift'erentiated cells, such as the ama?ba, can eftect only slow and weak contractions. Directly a speciahsation of function is necessary and some cell or part of a cell has to contract rapidly in response to some stimulus from within or without, we find a difierentiation both of form and of internal structure. In many cases, as in the developing muscle of the embryo or the adult muscles of many invertebrates, this difterentiation aliects only part of the cell, so that while one part presents the ordinary granular appearance, the other half is finely and longitudinally 180 PHYSIOLOGY striated, the striation being apparently due to the development of special contractile fibrillee. In the slowly contracting unstriated muscle of the vertebrate intestine, the longitudinal striation is with difficulty made out, but as the muscle rises in the scale of efficiency, the longitudinal striation becomes more apparent, and in the striated muscle of vertebrates, and still more in the wonderful wing-muscles of insects, which can perform three hundred complete contractions in a second, the longitudinal is associated Fig. 36. Fig. 35. Fig. 35. Muscle fibre of an ascaris. a, the differentiated contractile portion of the cell. (After Hbrtwig.) Fig. 36. Muscle fibres from the small intestine, showing the fine longitudinal stria- tion. (SCHAFER.) with and often apparently subordinated to a transverse striation, due to the regular segmentation of the contractile fibrillee or sarcostyles. Every muscular fibre, which presents any trace of histological differentiation, may be said to consist of contractile fibrillae (sarcostyles), each composed of a series of contractile elements {sarcous elements or sarcomeres), and embedded in a granular material known as sarcoplasm. The great divergence in the aspect of muscular fibres from different parts of the animal kingdom is THE STRUCTURE OF VOLUNTARY MUSCLE 181 largely conditioned by the varying relations, spatial and quantitative, of the sarcoplasm to the sarcostyles. Thus in the higher vertebrates, two types of voluntary muscular fibre are distinguished, according to the Fici. 37. Transverse sections of the pectoral muscles of a, the falcon, h, the goose, and c, the domestic fowl. It will be noticed that the relative amount of granular or red fibres present varies directly as the bird's power of sustained flight. (After Knoll.) amount of sarcoplasm they contain : one rich in sarcoplasm, more granular in cross-section, and generally containing hsemoglobin ; and the other poor in sarcoplasm, clear in cross-section, and containing no haemoglobin. From the fact that the granular fibres are B found chiefly in those muscles which have to carry out long-continued and powerful contractions, it seems reasonable to regard the interstitial sarcoplasm as the local food-supply of the active sarcostyles, although some authors have endowed the sarcoplasm ^^^th a contractile power of its own, differing only by its extremely prolonged character from the quick twitch of the sarcostyles. The connection between structure and activity of the muscle-fibres is well shown by Fig. 37. In some animals, such as the rabbit, we find muscles consisting almost entire!}^ of one or other of these varieties ; but iu most animals (amongst which we may reckon frog and man) the two varieties occur together in one muscle, so that what we have to say about the properties of voluntary muscle, which rests nearly entirely on experiments with frog's-'^''"' muscle, really has reference to a mixed muscle, i.e. muscle containing both red and white fibres. 8 i 5 ,. ■ 38. Fibrils of the wing-muscles of a wasp, prepared by Rollet's method. Highly magnified. (E. A. Schafer.) A, a contracted fibril, b, a stretched fibril, with its sarcous elements separated at the line of Hcn.sen. c, an uncon- tracted fibril, showing the porous struc- ture of the sarcous elements. Since the sarcous element repre- sents the contractile unit of the muscle, a knowledge of its intimate structure should be of great im- portance for the theory of nuiscular contraction. Unfortunately, how- 182 PHYSIOLOGY ever, we are here at the hniits of the demonstrably visible. It becomes difficult to determine how far the appearances observed under the microscope are due to actual structural differences or are produced by the unequal diffraction of hght by the various elements of the muscle fibre. All observers are agreed that the essential contractile element is the row of sarcous elements forming the muscle fibril or sarcostyle. Schafer, working on the highly differentiated wing-muscle of the wasp, concludes that each sarcostyle is divided by Krause's membranes (the hues in the middle of each hght stripe) into sarcomeres. Each sarcomere contains a darker sub- stance near the centre divided into two parts by Hensen's disc. At each end of the sarcomere the contents are clear and hyaline. In the act of contraction, the clear material flows, according to Schafer, into tubular pores in the central dark material. Most histologists agree in assigning to the middle part of the sarcous element (the sarcomere) a denser structure than to the two ends. According ^ 3 to Macdougall, however, the hghter appearance at each end of the sarcomere is an optical illusion. He regards the sarcous element as a cyHndrical bag with homogeneous contents, crossed only by one or three dehcate transverse Fig. 39. Diagram of a sarcomere in membranes. Krause's membrane would a moderately extended condition, })q rigid, while the lateral wall of the A, and in a contracted condition, B ; , , . , -i i i • K, K, membranes of Krause; h, sarcous element IS extensible, and is Hne or plane of Hensen ; SE, folded longitudinally, SO that it can poriferous sarcous element. , , , , , , . , (Schafer.) bulge out and produce a shortemng and thickening of the whole sarcous element if by any means the pressure be raised in its interior. In favour of a differentiation within the sarcomere itself is the fact that under certain conditions it is possible to produce a precipitate, hmited only to central part, ■i.e. to the sarcous element to which Schafer assigns a tubular structure. When a muscle fibre, killed by osmic acid or alcohol, is examined under the micro- scope by polarised light, it is seen to be made up of alternate bands of singly and doubly refracting material. The doubly refracting (anisotropous) substance corresponds to the dark band, and the singly refracting (isotropous) to the light band. If the living fibre be examined in the same way, it is found that nearly the whole of it is doubly refracting, the singly refracting substance appearing only as a meshwork with long parallel meshes corresponding to the muscle prisms. In short, in a living fibre the muscle prisms are anisotropous, the sarcoplasm isotropous. When a muscle fibre contracts, there is an apparent reversal of the situations of the light and dark stripes, owing to the fact that the interstitial sarcoplasm is squeezed out from between the bulging sarcomeres, and accumulates on each side of the mem- branes of Krause. The accumulation of sarcoplasm in this situation makes the pre- viously light striae appear dark, and the dark striae by contrast lighter than they were before. That there is no true reversal of the striae is shown by examining the muscle by polarised light, the two substances, isotropous and anisotropous, retaining their relative positions. Every skeletal muscle is connected with the central nervous system by nerve fibres, some conveying impressions from the muscle to the centre. THE STRUCTURE OF VOLUNTARY MUSCLE 183 the others acting as the path of the motor impulses from the centre to the muscle. These latter — the motor nerves — end in the muscular fibre itself, by^ means of a special end-organ — the motor end- plate. The neurilemma of the nerve fibre becomes continuous with the sarcolemma, the medullary sheath ends suddenly, while the axis cyhnder ramifies in a mass of un- differentiated protoplasm, containing nuclei, and lying in contact with the contractile substance of the muscle immediately under the sarcolemma (Fig. -10). This mass of protoplasm is known as the ' sole plate.' It is not marked in all animals. Thus in the frog the axis cylinder ends in a series of branches at right angles to one another, dis- tributed over a considerable length of the muscle fibre. The sole plate in this case seems to be limited to scattered nuclei lying in close contact with the terminal branches of the nerve fibre. So far as we can tell at present, the ultimate ramifications of the axis-cyhnder end freely and do not enter into organic connection with the contractile substance itself. Most of our knowledge on the subject of muscle has been derived from the study of the gastroc- nemius and sartorius muscles of the frog. The position of these muscles is shown in the accompany- ing diagram (Fig. 41). The gastrocnemius, which, with the attached sciatic nerve, is most frequently employed as a nerve-muscle preparation, forms a thick belly immediately under the skin at the back of the leg, and arises by two tendons from the lower end of the femur and the outer side of the knee- joint. The two tendons converge towards the centre of the muscle, uniting about its middle, and from them a number of short muscular fibres arise, passing backwards and dor.sally to be inserted into a fiat aponeurosis covering the lower half of the muscle, which ends in the tendo Achillis. On account of this irregular arrangement of the muscular fibres, the gastrocnemius can only be employed when the contraction of the muscle as a whole is the object of investigation. The eft'ective cross-area of the fibres is much greater than the actual cross-section of the muscle, so that, while the actual shortening of the gastrocnemius is but small, its strength of contraction is considerable. The sartorius muscle consists of a thin band of muscle fibres running parallel from one end of the muscle to the other. It lies on the ventral surface of the thigh, arising from the symphysis pubis by a thin fiat tendon, and is inserted by a narrow tendon into the inner side of the head of the tibia. On account of the regularity with which its fibres are disposed, this muscle is of especial value in experiments on the local conditions of a muscle fibre accompanying its activity. When a greater mass of ap- proximately parallel fibres is necessary, recourse may be had to a ]irei)aration consisting of the gracilis and semi-membranosus muscles together. This latter muscle lies dorsally to the gracilis muscle which is shown in the illustration. if Fig. 40. Motor end-organ of a lizard, gold preparation. (Kuhxe.) 72, nerve fibre dividing as it ap- proaches the end-organ: r, ramifi- cation of axis cylinder upon b, gran- ular bed or sole of the end-organ; m. clear substance surrounding the ramifications of the axis cylinder. 184 PHYSIOLOGY Other muscles in the frog used for particular purposes are the mylohyoid and the dorsocutaneous muscles. The mylohyoid muscle of the frog, which lies on the ventral surface of the tongue, has the advantage that its fibres lie in close contact with a Cruralis Add. luagn. — Tib. aut. loui Tendo Achillis Fig. 41. Muscles of hinder extremity of frog. (After Ecker.) lymph-space occupying the centre of the tongue. If any drug be injected into this lymph-space it acts with extreme rapidity on the muscle fibres, so that the tongue- preparation of the frog is a useful one for the study of the action of different substances on muscle fibres. SECTION II EXCITATION OF MUSCLE A MUSCLE may be caused to contract in various ways. Normally it con- tracts only in response to impulses starting in the central nervous system and transmitted down the nerves. But contraction may be artificially excited in various ways in a muscle removed from the body. If we make a muscle-nerve preparation {i.e. a muscle \\ath as long a piece of its nerve as possible attached to it), such as the gastrocnemius of the frog with the sciatic nerve, we find we can cause contraction by various forms of stimuli — mechanical, thermal, or electrical — -applied to the muscle or the nerve (direct and indirect stimulation). Thus the muscle responds with a twitch if we pass an induction shock through it or its nerve, or pinch either with a pair of forceps. Or we may use chemical stimuli, and cause contraction by the application of strong glycerin or salt solution to the nerve. These experiments do not prove conclusively that muscle itself is irritable. It might be urged that, when we pinched or burnt the muscle we stimu- lated, not the muscle substance itself, but the terminal ramifications of the nerve in the muscle, and that these in their turn incited the muscle to contract. But the independent excitability of muscle is shown clearly by the following experiment by Claude Bernard. A frog, whose brain has been previously destroyed, is pinned on a board, and the sciatic nerves on each side exposed. A ligature is then passed round the right thigh underneath the nerve, and tied tightly so as effectually to close all the blood-vessels supplying the limbs, without interfering with the blood-supply to the nerve. Two drops of a 1 per. cent, solution of curare are then injected into the dorsal lymph-sac. After the lapse of a quarter of an hour it is found that the strongest stimuli may be applied to the left sciatic nerve without causing any contraction of the muscles it supplies. On the right side, stinuilation of the nerve is as efficacious as before. Both gastrocneniii I'espond readily to direct stimulation, showing that the muscles are not affected by the drug. Since both sciatic nerves have been exposed to the influence of the curare, it is evident that the difference on the two sides cannot be due to any deleterious effect on them by the curare. We have also excluded the nuiscles themselves : so we nnist conclude that the curare paralyzes the nuiscles by affecting the terminations of the nerve within the nuiscle. and probably the end-plates themselves. 185 186 PHYSIOLOGY This experiment teaches us that muscle can be excited to contract by direct stimulation, even when the terminal ramifications of the nerve within it are paralyzed, so that stimulation of them would be without effect. The same fact may be demonstrated in a different way by means of chemical stimuH. It is found that whereas strong glycerin excites nerve fibres, it is without effect on muscle fibres, while on the other hand weak ammonia is a strong excitant for muscle, but is without effect on nerve. If the frog's sartorius be dissected out and the lower end dipped in glycerin, no twitch is produced. On snipping off the lower third of the muscle and then immersing the cut end in glycerin, a twitch at once occurs. The lower end contains no nerve fibres (Fig. 42), and it is only when a section containing nerve-fibres is exposed to the action of glycerin that contraction takes place. On the other hand, mere exposure of muscle to the vapour of dilute ammonia causes con- traction (and subsequent death), although the nerve to the muscle can be immersed in the solution without any excitation being produced. Of all the different stimuli capable of exciting muscular contraction, the electrical is that most frequently employed. It is easy, using this form, to graduate accurately the intensity and duration of the stimulus. At the same time the stimulus Fig. 42. The ramification j^^^y be apphed many times to any point on the oi the nerve fibres within *^ ^^ . "^ . i • i i the sartorius muscle of muscle or nerve Without kilung the part stimulated, the frog showing the free- whereas with other forms of stimulus it is difficult to dom oi the lower portion . . .... of the muscle from nerve obtain excitatory effects Without injuring to a fibres. (KuHNE.) greater or less extent the part stimulated. METHODS EMPLOYED FOR THE STIMULATION OF MUSCLE AND NERVE. The two commonest forms of electrical stimuli employed are (1) the make and break of a constant current, (2) the induction currents of high intensity and short duration obtained from an induction coil. (1 ) Constant Current. As a source of constant current a Daniell's cell is generally employed. This consists of an outer pot containing a saturated solution of copper sulphate, in which is immersed a copper cylinder. To the cylinder at the top a binding screw is attached, by which the connection of the copper with a wire terminal is effected. Within the copper cylinder is a second pot of porous clay, filled with dilute sulphuric acid, in which is immersed a rod of amalgamated zinc. In this cell the zinc is the positive and the copper the negative element. Hence the current flows (in the cell) from zinc to copper, and if the binding screws of the two elements are connected by a wire, the current flows in the wire (outer circuit) from copper to zinc, thus completing the circuit. Since in the outer circuit the current flows from copper to zinc, the terminal attached to the copper is called the positive 'pole, and that to the zinc the negative pole. When the current is required to be very constant, the zinc may be immersed in a saturated solution of zinc sulphate instead of dilute sulphuric acid. A Daniell's cell, though very constant, gives only a small current, owing to its small electromotive force and high internal resistance. When a stronger current is required it is best to use a storage battery. In this, EXCITATION OF MUSCLE 187 when charged, the two elements are lead and lead oxide, PbOa- It has the advantage that it may be used over and over again, being recharged through a resistance from the electrical mains when it has run down. Another useful type of cell is the Leclanche cell. This consists o* a glass jar con- taining a solution of sal-ammoniac. Into this dips an amalgamated rod of zinc, which is the positive plate. A piece of gas carbon forms the negative plate. This is surrounded by peroxide of manganese (MnOo) which is kept in contact with the surface of the carbon by being placed in a porous pot. In some forms of Leclanche the manganese and carbon are ground up together and pressed into a cylinder which surrounds the zinc rod. When the cell is on open circuit — that is, when the terminals are not con- nected and no current is passing — very little action takes place ; but when the circuit is closed and the current passes, the zmc dissolves in the sal-ammoniac, forming a double chloride of zinc and ammonia, while ammonia gas and hydrogen are liberated at the carbon pole. The nascent hydrogen reduces the jjeroxide of manganese and so polarisa- tion is prevented. On account of its great solubility in water the ammonia has no polarising action. The Leclanche is a convenient form of cell, as when once set up it requires a minimum of attention. If it is worked through a considerable resistance, it will keep in order for some time, particularly if the work is intermittent ; but if it is used with a small resistance in cu'cuit it polarises very rapidly. The E.M.F. of one Leclanche cell is 1-4 volt in the external circuit. The positive current is conventionally said to run from the zinc to the carbon in the cell, and from the carbon to the zinc in the circuit outside. The wire attached to the carbon is the positive pole, that to the zinc the negative pole. Dry cells are usually Leclanche cells, in which the solution of sal-ammoniac is prevented from spilling by absorption with sawdust or plaster of Paris. The E.M.F. is the same as the Leclanche, but they polarise much more readily. If the poles of a Darnell's cell be connected by \vires with a nerve or muscle of a nerve-muscle preparation (as in Fig. 43), the ciu-rent will flow from copper to the nerve at A, and along the nerve from a to k. At K the current will leave the nerve to flow to the zinc of the battery, so com- ])leting the circuit. The point at which the current enters the nerve {i.e. the point of the nerve connected with the positive pole of the battery) is called the anode, and the point at which the current leaves the nerve is called the cathode. The wires by which the current is conducted to and from the nerve are called the electrodes. As electrodes we generally employ two platinum wires mounted together on a piece of Aiilcanite. For the purpose of making or breaking the current at will, various forms of keys are employed. The ordinary make and break key consists of a hinged wire dipping into a mercury cup. When the wire is depressed so that it dips into the mercury, the circuit is complete. On raising the wire by means of the handle, the circuit is broken. Du Bois Reymond's key consists of two pieces of brass, each of which has two l)iiKling screws for the attachment of wires. These are connected by a third piece, or bridge, which is jointed to one of the two side bits, so that it may be raised or lowered at pleasure {v. Fig. 44). It may be used either as a simple make-and-brcak key. or. as is more usual, as a short-circuiting key. In the first case one brass bank is attached to one terminal, the other to the other terminal. If the bridge be now lowered, the connection is made and the current passes. If the bridge be raised, the current is broken. Fig. 44 A and B show the way in which the key is arranged for short-circuiting. It will be seen tliat four wires are attached to the key ; two going (o the l)at(ery, and two we may suppose going to a nerve. When the bridge is down, as in Fig 44 a, the current from the cell on coming to the key has a choice of two routes. It may either go through the brass bridge, or through the other wii-es and nerve. The resistance of J Ntrc/e. 188 PHYSIOLOGY the nerve however is about 100,000 ohms, whereas that of the bridge is not the thousandth part of an ohm. When a current divides, the amount of current that goes along any branch is inversely proportional to the resistance. Here the resistance in the nerve- circuit is practically infinite compared with that in the brass bridge, and so all the Fig. 44. Du Bois key, closed. Du Bois key, open. current goes through the bridge and none through the nerve. We say then that the current is short-circuited. It is often necessary to reverse the direction of a current through a nerve-muscle preparation or a galvanometer in the course of an experiment. For this purpose Pohl's reverser may be used. It consists of a slab of ebonite or paraffin or other in- sulating material, in which are six small holes filled with mercury. A binding screw is in connection with the mercury in each of these holes. Two cross-wires (not in contact with one another) join two sets of pools together, as shown in Fig. 45. A cradle con- sisting of two wires joined by an insulating handle carries two arcs of wire by which the'pools at a and h may bs put into connection with either x and y, or the corresponding pools on the opposite side. It will be seen that with the cradle tipped to one side, as in Fig. 45 A, the current from the battery enters the reverser at a ; this proceeds up the wire of the cradle, down towards the right, then along the cross-wire to the pool at x. x is therefore the anode, and y the cathode. In Fig. 45 B the cradle has been swung over to the other side. Here the cross-wires are not used at all by the current, which passes from a up the sides and down the curved wire to y. In this case y is now the anode and x the cathode, and the direction of the current through the circuit connected with x and y is reversed. By taking out the cross-wires, Pohl's reverser may be used as a simple switch, by which the current may be led into two different circuits in turn. With this form of reverser difficulty is often experienced owing to dirt accumulating on the mercury and forming an insulating layer be- tween it and the binding screw or copper wire. Several improved forms of reverser are now made where the mercury poles are replaced by brass banks, and these are gene- rally to be preferred in practice. (2) Induced Currents. In using these the muscle or nerve is stimulated by the current of momentary duration produced in the secondary circuit of an induction-coil by the make or break of a constant current in the primary. The construction of the induction-coil or inductorium is founded on the fact that if a coil of wire in connection with a galvanometer be placed close to (but insulated from) Diagram of Pohl's reverser. EXCITATION OF MUSCLE 189 Fig. 46. Diagram of inductoiium. r^. primary ; Rg, secomlary coil, tn, electro- magnet of Wagner's hammer, w, Helmholtz'.s side wire. another coil through which a current may be led from a battery, it is found that on make and break of the current of the second coil a momentary current is induced in the first. The induced current on make is in the reverse direction, that on break in the same direction as the primary current. The electromotive force of the induced current is proportional to the number of turns of wire in the coils. The induction-coil consists of two coils, each containing many turns of wire. The smaller coil (r^. Fig. 46), consisting of a few turns of comparatively thick wire, is the primary coil, and is put into connec- tion with a battery. It has within it a core of soft iron wires, which has the effect of attracting the lines of force, concentrating them, and so increasing its power of in- ducing secondary currents. The secondary coil, R2, of a large number of turns of very thin wire, is arranged so as to slide over the primary coil. It is provided with two terminals, which may be con- nected with the nerve or other tissue that we wish to stimulate. Since the electromotive force of the induced current is proportional to the number of turns of wire, it is evident that the electromotive force of the current delivered by the induction coil may be many thousand times that of the battery current flowing through the primary coil. The induced currents increase rapidly in strength as the coils are approached to one another ; the strength of these therefore may be regulated by shoving the secondary up to or away from the primary coil. A short-cu'cuiting key is always placed between the secondary coil and the nerve to be stimulated. If only single induction shocks are to be used, a make-and-break key is put in the primary battei'y circuit, and the two wires from the battery and key are attached to the two top screws of the primary coil {c and d. Fig. 46). It is then found that the shock given by the induced current on break of the primary current is much stronger than that on make. In endeavouring to explain this difference in the intensity of the make-and-break induction shocks, it must be remembered that the intensity of the momentary current induced in the secondary coil at make or break of the primary current is proportional (I ) to the number of turns of wire in each coil ; (2) inversely to the mean distance between the coils {i.e. the nearer the coils, the stronger the induced current) ; (3) to the rate of change in strength of the primary current. Now, when a current is made through the primary coil, induction takes place, not only between primarj' and secondary coils, but also between the individual turns of the primary coil itself. This current of self- induction, being opposed in direction to the battery current, hinders and delaj's the attainment by the latter of its full strength, and so slows the rate of change of current in the primary coil. Hence the intensity of the momentary current induced in the secondary coil is less than it would have been without the retarding effect of self-induc- tion. At break of the current, an extra current is also produced in the primary coil in the same direction as the battery current, and therefore tending to reduce the rate of change of the current from full strength to nothing. In this case, however, the primary circuit being broken, the current of self-induction cannot pass without jumping the great resistance offered by the air, so that its retarding effect on the rate of dis- appearance of the primary current may be practically disregarded. In Fig 47 the line. a, b, c, d, will represent the changes occurring in the jirimary current at make and break, a b corresponding to the make and c d to the break. The lower Hup represents 190 PHYSIOLOGY the momentary currents induced in the secondary circuit, m being the current of low intensity and long duration produced by the make, and b the shock of high intensity and short duration caused by the sharjj break of the primary current. When we desire to use faradic stimulation — that is, secondary induced shocks rapidly repeated 50 to 100 times a second — we make use of the apparatus attached to the coil, known as Wagner's hammer (Figs. 48a and 48b). In this case the wires from the battery are connected to the two lower screws (o and h. Fig. 46). Fig. 48a shows the direc- tion of the current when Wag- ner's hammer is used. The cur- rent enters at a, runs up the pillar and along the spring to the screw X. Here it passes up through the screw, and through the primary coil Kj^. From the primary coil it passes up the small coil m, and from this to the terminal b and back to the battery. But in this course the coil m is converted into an electro-magnet. The hammer h attached to the spring is attracted down, and so the spring is drawn away from the screw X, and the current is therefore broken. The break of the current destroys the magnetic power of the coil, the spring jumps up again and once more makes circuit with the screw x, only to be drawn down again directly this occurs. In this way the spring is kept vibrating, and the primary circuit is continually made and broken, with the production at each make-and-break of an induced current in the secondary coil. It is evident that, when the primary current is made and broken fifty times in the second, there will be a hundred momentary currents produced during the same period in the secondary coil. Every alternate one of these produced by the break of current and the relaxation y^y^ second. Thus a single muscle-twitch is completed in about yL second. It must be remembered, however, that this number is only approximate, and varies with the temperature of the muscle and its condition, being much longer in a fatigued muscle. Moreover, it is almost impossible to avoid some deforma- tion of the curve due to defects of the recording instruments used. Thus the relative period during which no mechanical changes are taking place in the muscle must always be shorter than is apparent from a curve obtained THE MECHANICAL CHANGES OF MUSCLE 199 by the foregoing method. The elasticity and extensibility of the muscle must prolong the apparent latent period, since the first effect of contraction of any part of the muscle will be to stretch the adjacent part, and only later to Fig. 57. Burdon Sanderson's method for photographic record of muscle-twitch. The exciting shock is sent into the muscle by the wires d and d' . move the tendon to which the lever is attached. Thus if we have a weight supported by a rigid wire, and suddenly pull the upper end of the wire so as to raise the weight, the latter will rise instantaneously. If, however, the weight be suspended by a piece of elastic, it will not follow the pull exactly, but will lag behind, the first part of the pull being occupied with stretching the india-rubber, and only when this is stretched to a certain degree will the weight begin to rise. The same re- tardation of the pull would be observed if, instead of india- rubber, we used a piece of Uving muscle. It is possible to obviate this instrumental inertia by employing solely photographic methods for the record and magnification of the muscle- twitch. Thus in the experi- ments of Sanderson and Burch the thickening of the muscle at the point stimulated was recorded graphically by photo- graphing the movement on a Fu;. ')S. Photograjjhic record of muscle-twitch. «K4- /I?;™ KTN V I," J 1 • 1 (B. Sanderson.) The upper curve is the move- Sht (Fig. 57), behmd which Wnt of the muscle, the middle curve the signal was a moving sensitive plate. showing the moment of excitation, and the lower mi • T 11 • i - 1 curve is that of a tuning-fork vibrating .300 times inus avoiding all instrumental a second. inertia, and diminishing the inertia of the muscle to a minimum, the mechanical latent period was found to be only 0-0025 second (Fig. 58). This figure we can take as the average latent period for the skeletal muscle of the frog at the ordinary tem- perature of the laboratory (about 16° C). We shall have occasion later on TBOLH^i^^^^ ^^^^^^^M^^^^^HI 200 PHYSIOLOGY to consider the changes which occur in the muscle between the application of the stimulus and the moment at which the first mechanical change makes its appearance. The relaxation of muscle is helped by a moderate load, and in a normal condition is complete. It is not active — that is to say, is not due to a con- FlG. 59. V. Kries' apparatus for taking ' after-loading ' and ' arrested con- traction curves.' traction in the transverse direction — but is a passive effect of extension and elastic rebound. This may be shown by allowing a muscle to contract while floating on mercury. The subsequent lengthening on relaxation is very incomplete. Even with the most careful arrangements for securing isotonicity in the record of the contraction there is probably a certain amount of over-shoot of the lever whenever, as at high temperatures, the contraction is sufficiently rapid. The effect of this is that one cannot assume the existence of an actual pull on the lever during the whole time of the ascent of the latter. We can therefore speak of a period during which there is contractile stress — that is to say, when the muscle is actually pulhng on the lever, which will occupy only a part of the ascent of the curve. The dura- tion of this period of contractile stress may be shown by re- cording what is known as ' arrested ' contractions. One mechanism for this purpose is shown in the figure (Fig. 59). The stop Su is used AAAAAAAAAAAAAAAAAAAAA FiQ. 60. Curves of isotonic and arrested contractions of an unloaded muscle. (Kaiser.) THE MECHANICAL CHANGES OF MUSCLE 20L simply for after-loading the muscle so that the weight shall not act upon the muscle until it begins to contract. The stop So may be regu- lated so that it suddenly checks the movement of the lever at any desired height above the base line. We may thus get a series of contractions such as those shown in Fig. 60. It will be seen that at the points x , x", and x"' the muscle was still pulling on the lever, and therefore held it up against the stop. At the point \ the arrested twitch returns rapidly to r D ID i ■|^^ 1 1 1 1 1 1 ■ \ Tension Fig. CI. Diagram to show the relation between the initial length of a muscle and the tension developed in it during excitation (as measured by the isometric method). The tension developed at each initial length is measured by the horizontal distance between the two thick lines, the left line representing the resting muscle, and the curved thick line on the right the contracted muscle. (From Blix.) the base hne, showing that the movement of the lever in the unarrested curve above this point was due to the inertia of the mo^^ng parts and not to the actual pull of the muscle. THE ENERGY OF CONTRACTION. When a muscle contracts we may conceive of it as converted into a body with elastic properties other than those which it possesses during rest. Directly after it has been excited it possesses potential energy which can be measured as tension by the isometric method and which will degenerate in a few hundredths of a second into heat, or can be turned into work by allowing the muscle to shorten and to raise a weight, as in the isotonic method of recordino; muscular contractions. Under the conditions of an ordinarv physiological experiment, a contracted muscle loaded only by a light lever is shorter than the non-contracted, but can be stretched to the 202 PHYSIOLOGY length of the latter by a certain weight, when it will be in a con- dition of tension. In their natural position in the body muscles may possess any length between extreme shortening and extreme elongation whether they are in a resting or in an excited condition. Since the relaxed muscle requires only a minimal force to extend it to the maximal length possible in its natural relationships in the body, it is usual to speak of the different lengths of an excited and unexcited muscle, the lengths being in this case those which are impressed on the muscle by a minimal load. When we measure by means of the isometric method the maximum energy set free in a muscle as the result of excitation, we find, as Bhx first pointed out, that this energy depends on the length of the muscle fibres during the period of contractile stress set up by the excitation. With increase in the length of the muscle the tension developed on excitation increases until the length of the muscle is somewhat greater than that which it possesses in its normal relationships in the body. To lengthen the muscle beyond this point a certain stretching force must be apphed to it which rapidly increases. The tension developed on excitation however soon begins to diminish. These relationships are shown by the diagram (Fig. 61), where the ordinates repre- sent the length of the muscle and the abscissae the tension on the muscle. The left-hand thick line represents the muscle in a state of rest, the right- hand curved line the muscle in a state of excitation. The horizontal distance between the two fines gives the increase of tension (as measured by the isometric method) produced when the muscle passes from the resting into the excited state as the result of stimulation by a single induction shock. Since the tension set free on excitation depends on the length of the muscle fibres during the production of the condition of tension, the tension developed will be diminished if the muscle be allowed to shorten before its maximum tension has been reached. This is the case with all isotonic records of muscular contraction, so that it becomes difficult to give any exact expression for the total energy changes in a muscle which is allowed to shorten. As A. V. Hill has pointed out, a muscle is a machine primarily for developing tension, and the potential energy thus set up may be used for the production of work to any degree the conditions of loading allow. The work done by a muscle when it contracts is measured by multiplying the weight hfted by the height through which it is hfted, w x h. Since however, the result will vary according to the conditions of loading of the muscle, a much more useful quantity is obtained by measuring the tension produced in a muscle which is stimulated but not allowed to shorten. The potential energy available due to the new elastic conditions of the fibres is found to be approximately J Til, where T is the maximum tension developed in the twitch and I is the length of the muscle (A. V. Hill). THE MECHANICAL CHANGES OF MUSCLE 203 ^ THE EXTENSIBILITY OF MUSCLE Living muscle in a perfectly normal condition is tlistinguislied by its slight but perfect elasticity ; that is to say, it is considerably stretched by a shght force (in the longitudinal direc- tion), but returns to its original length when the extending weight is re- moved. The length to which muscle Fig. 62. Extensibility of india-rubber (n) is stretched is not proportional to compared with that of a frog's gastroc- ^]^q weight used, but any given in- nemius muscle (o). '^ . . '^ . . crement of weight gives rise to less elongation the more the muscle is already stretched. The accompanying curves show diagrammatically the elongation of muscle as compared with a piece of india-rubber when the weight on it is uniformly increased. Dead muscle is less extensible and its elasticity is less perfect. A given weight appUed to a dead muscle will not stretch it so much as when the muscle was alive, but the dead muscle does not return to its original length when the weight is removed. A contracted muscle, on the other hand, is more extensible than a muscle at rest. A gramme applied to a contracted gastrocnemius will cause greater lengthening than if it were apphed to the n same muscle at rest. ■ ^ The relation between the excitabihty of a muscle under the two conditions of contrac- tion and rest are shown in the diagram in Fig. 63. At the Point y the muscle is unable to shorten at all against a weight. It is evi- dent from this diagram that the height of contraction of a muscle diminishes as the load is increased, very rapidly if the muscle is after-loaded, less rapidly if the weight applied to the muscle be allowed to extend it at rest. It is evident however that in either case the diminution in height is not in proportion to the load and that the work done by the muscle, w x h, as the weight is increased, rises at first quickly, then more slowly to a maximum to sink finally to zero. By inspection of diagram (Fig. 63) it will be seen that O.ho < lO.hi < 20.h2 < 30.h3 > 40.h,, > SO.hj, so that in this case the maximal mechanical work is obtained when the muscle is loaded with about 30 gms. Fig. 63. Curve showing the length of a muscle under various loads in the contracted condition bjj, and uncontracted condition cy. The double lines a b, &c., represent the con- tracted muscle, while the long single lines a c, &c., show the lentfth of the inactive muscle. 204 PHYSIOLOGY PROPAGATION OF CONTRACTION. THE CONTRACTION WAVE The whole muscle does not as a rule contract simultaneously. When excited from its nerve the contraction begins at the end-plates and spreads in both directions through the muscle. The rate of propagation of the con- traction wave can only be measured by employing a curarised muscle, so as to avoid the wide spreading of the excitatory change by means of the intra- muscular nerve-endings. For this purpose a curarised sartorius muscle is taken, stimulated at one end, and the thickening of the muscle recorded by means of two levers placed, one near the exciting electrodes and the second at the other end of the muscle, as shown in the diagram (Fig. 64). The Fig. 64. Diagram of arrangement for recording the contraction wave in a curarised sartorius. difference between the latent periods of the two curves represents the time taken by the contraction wave in travelUng from a to h. By measurements carried out in this way it is found that the rate of propagation of the con- traction in frog's muscle is 3 to 4 metres per second ; in the muscle of warm-blooded animals it may amount to 6 metres. The actual duration of the shortening at any given point is necessarily smaller than that of the whole muscle, and amounts in frog's muscle to only 0-05-0 -09 sec, about half the duration of the contraction of a whole muscle of moderate length. The length of the wave is obtained by multiplying the rate of transmission by the duration of the wave at any one point. It varies therefore in frog's muscle between 3000 x -05 (= 150) and 4000 X -09 ( — 360) milUmetres. Thus the muscle fibres in the frog are much too short to accommodate the whole length of the wave, and the contraction of the whole muscle must be made up of the summated effects of the contraction wave as it passes from point to point. Hence the longer the muscle, the more must the contraction be lengthened by the time taken up in propagation from one end to another. SECTION IV THE CONDITIONS AFFECTING THE MECHANICAL RESPONSE OF A MUSCLE STRENGTH OF STIMULUS. If a series of single break-shocks be applied to a muscle or nerve at intervals of not less than five seconds, it will be found that beyond a certain distance of the secondary from the primary coil no effect at all is produced. The shocks are said to be subminimal. On pushing the secondary coil nearer the primary a point will be reached at which a small contraction will be observed. On then pushing in the coil a millimetre at a time the contrac- tion will become greater for the next couple of centimetres {e.g. as the coil is moved from 12 to 10 cm. distance). Further increase of current by approximation of the coils is without effect, although the current actually used may be increased a hundred times in moving the coil from 10 to 0. It was formerly thought that this limited gradation of the muscular response according to strength of stimulus was due to a similar gradation in the response of each indi\adual muscle fibre of which the muscle is composed. It seems more probable, however, that, when a minimal or subminimal response is obtained, not all the fibres making up the muscle are contracting. A mini- mal contraction is in fact a contraction in which some fibres of the whole muscle are stimulated. A maximal contraction is one in which all the fibres are stimulated. So far as con- cerns each individual muscle fibre every contraction is a maxi- mal contraction. The fibre either contracts to its utmost or it does not contract at all. The rule of ' all or none ' which was first enunciated for heart-muscle is probably true for every con- y^^^. ,;;, tractile element. The difference between skeletal and heart muscle lies in the fact that in the former the excitatory process does not spread from one fibre to its neighbours. If, for instance, we take a curarised sartorius and split its lower end, as in Fig. 05, the stimulus applied to a causes a contraction only of the left-hand side of the muscle, while a stimulus applied to B is in the same way limited to the right-hand side. If a piece of ventricu- lar or auricular muscle of the frog or tortoise were treated in the same way, a stimulus appUed at a would cause a contraction which would travel across the bridge at the upper end and extend to b. 205 206 PHYSIOLOGY It was shown by Gotch that, if each of the three roots which make up the sciatic nerve and send fibres to the gastrocnemius be stimulated in turn, it is often impossible to evoke a maximal contraction of the gastrocnemius, however strongly each root be stimulated. Keith Lucas has shown that if stimuli in gradually increasing strength be applied to the motor nerve (containing only seven to nine fibres,) which supplies the dorso-cutaneus muscle of the frog, the contraction of the muscle increases, not gradually, but by a series of steps. This can be explained only by assuming that the smallest effective stimulus excites perhaps four out of the seven nerve fibres, those immediately in contact with the electrodes. With increasing strength of current the stimulus becomes effective for the three fibres lying next to these, and finally still further increase of current may excite all the fibres making up the nerve (Fig. 66). Fig 66. Curve showing relation of height of contraction of dorso-cutaneus muscle to strength of stimulus. Ordinates = height of contraction ; abscissa = strength of stimulus. (K. Ltjcas.) THE REPETITION OF STIMULUS SUMMATION. The response of a muscle fibre to a single shock, whether measured by the isotonic or the isometric method, i.e. as shortening or as tension, is independent of the strength of stimulus and varies only with the length of the fibre during the rise of the excitatory condition. If, however, a second shock is sent in during this period a further evolution of energy is possible, and the effect is still further increased by putting a series of stimuU into the muscle or its attached nerve before the development of the contractile stress due to the first stimulus has reached its maximum. If two shocks at intervals of one hundredth of a second be sent into a muscle, the response, whether shortening or rise of tension, will be greater than that produced by one shock. If a series of shocks be sent in, the excitatory condition is main- tained, so that instead of a simple muscle twitch rising to a maximum and then falhng, the muscle lever rises to a given point, which in the muscle con- tracting isometrically may be double that due to a single stimulus, and then remains at this height during the continuation of the repeated excitations. If the muscle be allowed to contract isotonically, the continued contraction produced by a series of stimuli may with a heavy load be three or four times as considerable as that produced by a single stimulus. This condition of apparently continued stimulation brought about by continued apphcation of stimuli is said to be summated. REFRACTORY PERIOD. If the interval between two stimuh sent into a muscle be successively shortened in a series of observations we finally arrive at a point at which summation is no longer apparent, i.e. the effect of THE MECHANICAL RESPONSE OF MUSCLE 207 Fig. 67. Muscle curves showing summation of stimuli, r and r' , the points at which the stimuli were sent into the nerve. From the first stimulus alone the curve abc would bo obtained. From r' the curve rfe/ is obtained. These two curves are summated to form the curve aghik when both stimuli are sent in at the interval r r'. the two stimuli is no greater than the effect of a single stimulus. This means that the second stimulus has become ineffective, and this ineffective- ness we must ascribe to the condition set up in the muscle as the result of the first stimulus. For a very short period of time after stimulation a muscle is inexcitable to a second stimulus. The period during which it is inexcitable is known as the refractory period and amounts in skeletal muscle to about -0015 second. The same phenomenon is better marked in certain other excitable tissues, such as the heart muscle, but it seems to be a common property of excitable tissues generally. When a loaded muscle is made to record its contractions isotonically we may get summation of effects, though the interval between the stimuli is greater than that which corresponds to the duration of the rise of contractile stress. Thus if the interval is just so long that the second becomes effective just as the contraction due to the first has com- menced to die away, the second con- traction seems to start from the point to which the muscle has been raised by the first (Fig. 67). By repeating these stimuli in a heavily loaded muscle, the contraction may be made three or four times as extensive as a single twitch. With slow stimuli the summation is, however, rather mechan- ical than physiological. The period of contractile stress, which lasts only about •03 second, is so short that it has no time to raise the weight to the maximum height before it has passed away. This is shown by the fact that if the muscle be after- loaded, so that the lever is raised to the top of the curve of a single twitch, application of the stimulus will make it shorten still more, and by repeated after-leading in this fashion, it is possible to make the muscle raise a weight in response to a single stimulus to the same height that it would if excited by a series of stimuli. This mechanical factor in sum- mation is showni in Fig. 68. It will be noted, however, that the tetanus is not a steady one and is probably due to stimuli Fig. 68. Contractions of a frog's muscle. Two single twitches repeated at intervals of are followed by a tetanus, which is almost twice as high as a about -^- of a second If single contraction. After two more single twitches, the drum ', ^" t- i' • was made to rotate more slowly, and single shocks employed, ^"^ ^'"^^^ °^ stimulation at the same time as the ' after-loading ' was continually were increased to 50 or increased. It can bo scon that the curve obtained in this way 100 per second a tetanus is as high as the original tetanus. (V. Frey.) would be produced and the curve would be probably twice as high as that represented in the figure. We thus see that for the overcoming of a resistance a single twitch is not economical. It is doubtful whether any contractions of muscles which occur in the body are other than tetani of varying duration. TEMPERATURE. Speaking generally, the eft'ect of warming a muscle is to quicken all its processes. The latent period becomes shorter and the muscle curve steeper and shorter. 208 PHYSIOLOGY It is very often observed that the height of contraction of the warmed muscle is greater than that obtained at ordinary temperatures. It seems that this apparent increase in height is really instrumental in origin, the quicker-moving muscle jerking the lever beyond the real extent of the contraction. If proper means are taken to eliminate this overshooting of the lever, it is found that the height of contraction is unaltered between 5° and 2C° C, the only change being in the time-relations of the curves. This is especially well shown in the so-cal'ed ' arrest ' curves (Fig. 69). Fig. 69. Isotonic and ' arrest ' curves of muscle-twitch : (1) unloaded at 14° C. ; (2) at 25° C. ; (3) at 0° C. ; (4) loaded at 14° C. Note that the arrest curves attain the same height throughout. (Kaisee.) If a muscle be heated gradually (without stimulation) up to about 45° C, it begins to contract slowly at about 34° C, and this contraction reaches its maximum at 45° C, at which point the muscle has entered into pronounced rigor mortis. Cold has the reverse effect. The intra-molecular processes which he at the root of the muscular activity are slowed, so that the latent period and the contraction period are prolonged. The action of cold on the excitability of muscle is to increase it, so that any form of stimulus is more effective at 5° C. than at 25° C. Moreover, when maximal stimuh are being used, and the muscle is heavily loaded, the first effect of the application of cold may be to increase the height as well as the duration of contraction, for the same reason that a gentle push is more efficacious in closing a door than would be a heavy blow with a hammer. If, however, a muscle be cooled for a short time to zero or a httle below, it loses its irritabihty, which returns if the muscle be gradually warmed again. Prolonged exposure to severe cold irrevocably destroys its irritability. Warming the muscle will now simply . bring about rigor mortis. FATIGUE. A muscle will not go on contracting indefinitely. If it be repeatedly stimulated, changes soon become apparent in the curve of contraction. The latent period is prolonged, as well as the length of the contractions ; the absolute height and work done are diminished. At the same time the muscle does not return to its original length ; the shorten- ing which remains is spoken of as ' contraction remainder.'' After an initial rise during the first few contractions, these diminish uniformly in height THE MECHANICAL RESPONSE OF MUSCLE 209 till they are no longer apparent, so that the muscle is now said to have lost its irritabihty. At the same time there is a great prolongation of the curve, occasioned almost entirely by a retardation of the relaxation, so that after forty or fifty contractions several seconds may elapse before the lever returns to the base hne (Fig. 70). The fact that the relaxation part of the muscle curve is affected by various conditions, especially fatigue, apparently independently of the contraction part, led Fick to put forward a theory that two distinct processes were concerned in the response of a Fiu. 70. Muscle curves showing fatigue in consequence of repeated stimulation. The first six contractions are numbered, and show the initial increase of the first three contractions. (Brodie.) muscle to excitation, one process causing the active shortening and the other the relaxation. (It must be noted that this is not the same as saying that the lengthening is an active jirocess, a statement negatived by the behaviour of a muscle when caused to contract on mercury.) He suggested that the disintegration associated with activity might be conceived as occurring in two stages : the first resulting in the production of sarcolactic acid and the active shortening of the muscle ; the second in the further conversion of the acid into CO2, with a consequent relaxation. A retardation of this second phase would cause the prolonged curve with ' contraction remainder ' observed in a fatigued muscle. We shall return to this point when discussing the chemical and heat changes which accompany contraction. If left to itself, the muscle which has been exhausted by repeated stimula- tion will recover. The recovery is hastened by passing a stream of blood, or even of salt solution, through the blood-vessels of the muscle. Recovery in a muscle outside the body is never complete. The phenomena of fatigue probably depend on two factors : (1) The consiunption of the contractile material or the substances avail- able for the supply of potential energy to this material. (2) The accumulation of waste products of contraction. Among these waste products the lactic acid is probably of great importance. Fatigue may be artificially induced in a muscle by ' feeding " it with a dilute solution of lactic acid, and again removed by washing out the muscle with normal saline solution containing a small percentage of alkali. 210 PHYSIOLOGY THE ACTION OF SALTS The action of sodium salts on muscle is of considerable interest. We are accustomed to use a 0-6 per cent, solution of NaCl as a ' normal fluid ' to keep muscle preparations moist. If, however, the solution be made with distilled water, it has a distinctly excitatory efiect upon the muscle, Fig. 71. a. Tracing of the contraction of a frog's sartorius, poisoned with veratrin, in response to a momentary stimulus. The time-marking indicates seconds. B. Tetanic contraction of normal sartorius in response to rapidly interrupted stimuli. (The duration of the stimulus is indicated by the words ' on ' and ' off.') It will be noticed that the two curves are practically identical. (Miss Btjchakan.) SO that single induction shocks may cause tetaniform contractions. The same excitatory effect is still better marked with solutions of NagCOg. If a thin muscle, such as a frog's sartorius, be immersed in a solution con- taining 0-5 per cent. NaCl, 0-2 per cent. Na2HP04, and 0-04 per cent. NagCOg (Biedermann's fluid), the muscle enters into a series of frequent contractions, so that it may wriggle from side to side, or may even ' beat ' for a time with the regu- larity of heart-muscle, though at a much greater rate. This excitatory action of sodium salts is neutrahsed by the addition of traces of calcium salts. Hence the normal saHne used in the laboratory should always be made with tap water, containing calcium salts. Potassium salts, although form- Excitation. ^_aj^\J.jLA.a.AJLa_a.jlJI_A_/l/_(UlA_aaaji Seconds. Fig. 72. Tracing of the contraction of a . . . muscle poisoned by the injection of a strong Hlg SO important a constituent ot solution of veratrin, showing the double ^^^ ^^^ ^f muscle, act as muscle contraction due to unequal poisoning ot • i i i different fibres. (Biedeemann.) poisons, quickly and permanently destroying its irritabihty. If a muscle be transfused with normal fluids containing minute traces of potas- sium salts, it at once shows all the signs of fatigue, signs which may be removed by washing out the potassium salts by means of 0-6 percent. NaCl solution. It is possible that the setting free of potassium salts may be one of the factors involved in the development of the normal fatigue of muscle. THE MECHANICAL RESPONSE OF MUSCLE 211 THE ACTION OF DRUGS Of the drugs that ba\^e a direct action on muscle, the most remarkable is veratrin, which causes an excessive prolongation of a muscular contraction (produced by a single stimulus). Thus the 'twitch' of a muscle poisoned ^vdth veratrin may last fifty or sixty seconds, instead of the normal one-tenth of a second (Fig. 71). Barium salts have a similar, though less marked effect. In order to carry out the poisoning with veratrin, very weak solutions (1 in 100,000 or 1 in 1,000,000 of normal saline) should be used and the muscle exposed to its action for some time. We get then on a single stimulus a respMDiise lasting many seconds and exactly similar in height and form to a tetanus obtained by discontinuous stimu- lation. If stronger solutions be used, the action of the drug is apt to affect the fibres unequally, so that we may have a sharp normal twitch preceding the prolonged con- traction (Fig. 72). If the muscle be excited several times immediately after the prolonged contraction has passed away, it responds with twitches like those of a normal muscle, but if allowed to rest a few minutes, stimulation is again followed by the peculiar long-drawn-out contraction. SECTION V CHEMICAL CHANGES IN MUSCLE CHEMICAL COMPOSITION OF VOLUNTARY MUSCLE Voluntary muscle consists of elongated cells, the muscle fibres being embedded in a connective tissue framework, and, as in all cellular tissues, proteins form its chief chemical constituents. The contents of the fibres are semi-fluid and can be expressed from the finely divided muscle as a viscous fluid known as muscle plasma. Muscle plasma is obtained in the following way. The living muscle of frogs is frozen, minced with ice-cold knives and pounded in a mortar with four times its weight of sand containing -6 of common salt. The mixture is then thrown on to a filter kept at 0° C. when an opalescent fluid filters through. The filters soon become clogged and therefore must be frequently changed, and their temperature must not be allowed to rise above 2° to 3° C. If the temperature of the muscle plasma be allowed to rise, clotting takes place, the clot later on contracting and squeezing out a serum, as is the case with blood plasma. The muscle-plasma is neutral or shghtly alkaline. When coagulation takes place, however, it becomes distinctly acid, and this acidity is due to the formation of sarcolactic acid in the process. Arguing chiefly from analogy with the blood-plasma, the muscle-plasma has been said to contain a body, myosinogen, which is converted when clotting takes place into myosin. The exact nature of the proteins in muscle-plasma, as well as of the protein con- stituent of the clot, which we have called myosin, is still a subject of debate. Kiihne, to whom we owe our first acquaintance with muscle-plasma, described the clot as consisting of myosin, a globulin, soluble in 5 per cent, solutions of neutral salts, such as NaCl or MgS04, precipitated by complete saturation with MgS04, and coagulated on heating to 56° C. In the muscle-serum, obtained after separation of the clot, he found three proteins, one coagulating at 45° C, one he called an albumate {i.e. a derived albumen or metaprotein), and the third coagulating about 75* C, and apparently identical with serum albumen. Halliburton extended these researches to the muscles of warm-blooded animals. He described four proteins as existing in muscle-plasma, of which two, paramyosinogen and myosinogen, gave rise to the clot of myosin. In no case, however, is it possible entirely to dissolve up the clot when once formed, and it seems that the so-called solution in dilute salt solutions was merely an extraction of still soluble protein in the meshes of the clot. Von Fiirth has shown that if the muscles of a mammal are washed free of adherent lymph and blood, the plasma obtained by extraction with normal salt solution contains only two proteins. These proteins are extremely unstable, and are gradually transformed on standing into insoluble 212 CHEMICAL CHANGES IN MUSCLE 213 protein, giving ri.se to a precipitate in dilute solutions, or forming a jelly-like clot in strong solutions. The properties of these proteins may be summarised as follows : (1 ) Myosin (paramyosinogen of Halliburton). A globulin, coagulating at about 47°- 50°C., precipitated byhalf saturation with ammoniumsuljjhateorondialy&is. Transformed slowly in solution, rapidly on precipitation, into an insoluble protein, mj^osin fibrin. ■ (2) Myogen (myosinogen of Halliburton). A protein allied to the albumens in that it is not precipitated by dialysis. Coagulates on heating at 55°-6G° C. It changes slowly into an insoluble protein, myogen fibrin, but passes through an intermediate soluble stage called soluble mj'Ogen fibrin. This latter body coagulates on heating to 40° C, being instantly converted at this temperature into insoluble myogen fibrin. It does not seem that any ferment action is associated with these changes, which we may represent by the following schema : Muscle-plasma. myosin or paramyosinogen. 4 myogen (myosinogen of Halliburton, albumate of Kiihne). I Soluble mj'ogen fibrin. I Mj'osin fibrin. Insoluble myogen fibrin. Muscle clot. Soluble myogen fibrin, which in mammalian muscle-plasma forms only on standing, exists apparently preformed in frog's muscle. Hence the instantaneous clotting of frog's muscle-plasma on warming to 4C° C. The residue left after the expression of the muscle-plasma consists chiefly of connective tissue, sarcolemma, and nuclei, and as such contains gelatin (or rather collagen), mucin, nuclein, and adherent traces of the proteins of the muscle-plasma itself. The muscle-seium contains the greater part of the soluble constituents of muscle. OTHER CONSTITUENTS OF MUSCLE. A number of other sub- stances are found in muscle in small quantities, those which are soluble being contained to a great part in the muscle serum. It will suffice here to enumerate the chief of these. (a) Colouring-matter. All red muscles contain a considerable amount of hanno- globin. A special muscle pigment allied to ha-moglobin has been described by MacMunn as myohaematin. The only evidence for its existence is spectroscopic. (b) Nitrogenous extractives. Of these, the most important is creatine (C4H9X3O.,) of which 0-2 to 0-3 per cent, may be found in muscle. Its significance will be the subject of consideration later. Other nitrogenous bodies occurring in smaller quantities are hypoxanthine, xanthine, and traces of urea and amino-acids. (f) Non-nitrogenous constituents. Fats, in variable amount. Glycogen. This substance is invariably found in healthy muscle. Fresh skeletal muscle contains about 1 per cent. In the embryo the muscles may contain many times this quantity of glycogen. Glucose is present in fresh muscle in minimal quantities, about -01 }>er cent. When muscle is allowed to stand, especially in a warm place, the glycogen under- goes partial conversion into glucose, so that the latter increases at the expense of the former. 214 PHYSIOLOGY Inosit (CgHjaOg SHjO) or ' muscle sugar ' occurs in minute traces in muscle. It does not belong to the group of carbohydrates at all, being a hexahydrobenzene. It is nonfermentable and does not rotate polarised light nor does it reduce Fehling's solution. Its significance is quite miknown. [d) Inorganic constituents. Muscle contains about 75 per cent, of water. Ash forms 1 to 1-5 per cent, and consists chiefly of potassimn and phosphoric acid, with traces of calcium, magnesium, chlorine and iron. RIGOR MORTIS All muscles after removal from the body, or if left in the body after general death, lose after a time their irritabihty, and this loss is succeeded by the phenomenon known as rigor mortis. The muscle, which was pre- viously flaccid, contracts, though the shortening is not very powerful and can be prevented by a moderate load on the muscle. Whereas the hving muscle is translucent, supple, and extensible, it becomes in the process of rigor opaque, rigid and inextensible. When rigor has been estabUshed, the reaction of the muscle is also found to have changed from a shghtly alkahne to a distinctly acid one, the acid being due to the presence of sarco- lactic acid. From this condition of rigor there is no recovery. There can be no doubt that the change in consistence of the muscle and probably also its shortening in rigor are due to the coagulation of the muscle proteins. Both changes can be imitated by heating the muscle, as is indicated by Brodie's experiments. This observer found that, if a hving muscle be hghtly loaded and then warmed very gradually, a series of stages in the heat-contraction could be distinguished corresponding to the coagulation temperatures of the different proteins described by von Fiirth in muscle-plasma. It seems Hkely, however, that the main contraction at all events, that which comes on spontaneously after death or immediately on warming the muscle to 45° C, has another component. In the coagulation of the separated muscle- proteins there is no evidence of any appreciable formation of sarcolactic acid, whereas the formation of this substance seems to bear an important relation to the occurrence of rigor. Thus after severe muscular fatigue, as in hunted animals, where there has already been a considerable formation of the waste products of muscular contraction, rigidity may come on almost immediately after death. If a thin hving muscle be plunged into boihng water, it undergoes instant coagulation, but no chemical change. The re- action of the scalded muscle, hke that of fresh muscle, is shghtly alkahne to htmus. No sarcolactic acid or carbonic acid is produced. On the other hand, in surviving muscle, after the cessation of the circulation, there is a steady formation of lactic acid which accumulates in the muscle. The actual coagulation of the muscle-proteins occurring in rigor is largely, if not entirely, determined by the increasing acidity of the muscle thereby pro- duced. In fact, it is the production of the acid which causes the onset of rigor, and not the rigor which causes a sudden formation of acid. Hence if the accumulation of lactic acid be prevented by perfusing the muscle with salt solutions, the onset of rigor may be postponed indefinitely, and the muscle may begin to putrefy without having undergone rigor. CHEMICAL CHANGES IN MUSCLE 215 THE PRODUCTION OF LACTIC ACID IN SURVIVING MUSCLE The lactic acid formed in muscle {.sarcolactic acid) is a physical isomer of the lactic acid formed in the fermentation or soui'ing of milk. They both have the formula CH3 . CH(OH) . COOH, i.e. they are ethylidene lactic acids. The lactic acid of fermenta- tion is optically inactive ; sarcolactic acid rotates polarised light to the right ; while a third isomer which is Isevo-rotatory is produced by the action of various bacilli and vibriones on cane sugar. The sarcolactic acid can be extracted from the muscle b}^ means of alcohol. It was pointed out by Hopkins and Fletcher that most of the methods previously used for the extraction of lactic acid from muscle caused the formation of lactic acid in this tissue. To obviate this difficulty, they adopted the precaution of cooling the muscles before cutting them out of the body and then dropping them into alcohol cooled to 0° C. While in this ice-cold alcohol they were finely divided with scissors and then pounded up in a cooled mortar. In this way the tissue was destroyed at a temperature which did not allow of the changes responsible in sur- viving muscle for the production of lactic acid. It is generally separated in the form of the zinc sarcolactate, by boiling its partially purified solution with zinc carbonate. Its presence may be tested for by means of Uflfelmann's reagent, which is made by the addition of ferric chloride to dilute carbolic acid. The purple solution thus pro- duced is at once changed to yellow by the addition of even traces of lactic acid. A much more definite colour reaction for lactic acid has been introduced by Hopkins. The test is carried out in the following way. About 5 c.c. of strong sulphuric acid are placed in a test-tube together with one drop of saturated solution of copper sulphate, which serves to catalyse the oxidation that follows. To this mixture a few drops of the solution to be tested are added, and the whole well shaken. The test-tube is now placed in a beaker of boiling water for one or two minutes. The tube is then cooled under a water-tap, and two or three drops of a very dilute alcoholic solution of thiophene (ten to twenty drops in 100 c.c.) are added from a pipette. The tube is replaced in the boiling water and the contents immediately observed. If lactic acid is present the fluid rapidly assumes a bright cherry red colour, which is only permanent if the tube be cooled the moment after its appearance. A study of the lactic acid content of muscle by Fletcher and Hopkins, using the precautions described above, has shown that fresh muscle contains only minimal amounts of lactic acid, the quantity being smaller, the greater the care that is taken to avoid injury to the muscle and to keep its temperature low until sufficient time has elapsed for its vital chemical processes to be destroyed by the action of the cold alcohol. If the muscle be left in the body after the death of the animal or be excised, a steady formation of lactic acid takes place, which is more rapid in the first few hours after death, but continues until the muscle passes into rigor. With the complete onset of rigor, frog's muscles are found to contain about 4 per cent, lactic acid. After this time the amount does not increase. The onset of rigor and the rate of production of lactic acid are quickened if the muscle be kept warm. It is interesting to note that the amount of lactic acid found in rigid muscle is almost invariable whatever the previous history of the muscle. Thus, if the muscle be finely minced and then extracted with cold alcohol, it is found to contain about -2 per cent, lactic acid. If. how- ever, it be allowed to stand after mincing, there is a slow production of lactic acid up to the maximum -4 per cent. Again, a muscle which has been tetanised to exhaustion contains about -2 per cent, lactic acid. "When allowed to undergo rigor, the amount rises to about 4 per cent. 216 PHYSIOLOGY It has long been known that the onset of rigor is associated with an evolu- tion of carbonic acid by the muscle. Fletcher has shown that this increased output of carbonic acid by a surviving muscle is due simply to the driving off of carbonic acid from the carbonates in the muscle as a result of the production of lactic acid. There is no evidence of a new formation of carbonic acid in the dying muscle as a result, for instance, of oxidative changes. THE CHEMICAL CHANGES WHICH ACCOMPANY ACTIVITY The principle of the conservation of energy teaches us that the energy of the contraction of muscle must be derived from chemical changes, probably processes of decomposition and oxidation, occurring in the muscle itself. In seeking out the nature of these changes three methods are open to us : (1) We can examine the changes in the muscle itself, avoiding so far as possible reintegrative changes by working on excised muscles. (2) We can investigate the changes in the medium surrounding the muscle. Muscle may be exposed in a vacuum or in a confined space of air, and its gaseous interchanges during rest and activity compared. Or we may lead a current of defibrinated blood through excised muscles, and determine the change in the composition of the blood in passing through the muscle under various conditions. (3) A method which, although apparently complex, has rendered the utmost service to the physiology of muscle is to use the changes in the total metabohsm of the animal during rest and muscular work as a clue to the muscular metabohsm itself. In such a case the respiratory exchanges of the animal are determined (viz. its oxygen intake and its COg output), and the urine and faeces are carefully analysed, in order to judge of the action of muscular work on the carbon and nitrogen metabohsm of the body. By the third of these methods we may show that muscular exercise increases largely the intake of oxygen and the output of carbon dioxide by the body. No corresponding changes are found in the nitrogenous metabohsm, so that ultimately we may regard the energy of the muscular contraction as derived from the oxidation of the food-stuffs and especially the carbohydrates. That it is this class of bodies which is the immediate, or at any rate the most accessible, source of muscular energy, is shown by the rise in the respiratory quotient which occurs during muscular exercise. When the exercise is moderate there is no evidence of the production of any other substance than carbon dioxide as a result of the muscular metabohsm, but with violent exercise it can be shown that lactic acid is not only pro- duced in the muscle, but appears in the blood and is excreted in the urine. It has been shown by Ryflfel that normal urine contains 3—4 mg. of lactic acid per hour. In one expsriment the urine passed after running one third of a mile with the production of severe breathlessness contained 454 mg. of lactic acid. In another experiment blood obtained before running contained 12-5 mg. per 100 c.c, and that obtained immediately after running one third of a mile contained 70 mg. lactic acid per 100 c.c. On the other hand, the examination of the urines of competitors in a twenty-four hours track walking race showed no increase in the output of lactic acid above the normal 4 mg. per hour. CHEMICAL CHANGES IN MUSCLE 217 The appearance of lactic acid thus seems to be attendant on a relative deficiency in the oxygen supply to the contracting muscle. The same conclusion may be drawn from experiments made many years ago by Araki, in which lactic acid was observed in quantities in the urine in cases where the oxidative processes of the body were interfered with by CO poisoning. Similar results are obtained when we investigate the chemical changes accompanying the contraction of excised muscles of the frog. If frogs' muscle be hung up in an atmosphere of nitrogen and stimulated repeatedly with single shocks, it will give a series of contractions gradually diminishing in size {v. p. 209). After a time the muscle is completely fatigued, and no further response can be ehcited on stimulation. On now examining it, it is found to be acid in reaction and to contain about -2 per cent, lactic acid. There is no evidence that under these conditions any carbonic acid is produced, though a certain amount may be hberated in consequence of the acidification of the muscle. Almost the same results are obtained when the muscle is stimulated in ordinary atmospheric air. The penetration of oxygen from the air through the body of the muscle is so slow that all the muscle except the thin layer on the surface may be regarded as cut ofi from the action of oxygen. By hanging the muscle, especially a thin muscle such as the sartorius, in an atmosphere of pure oxygen, the results are quite different. In the first place the muscle does not fatigue so soon. More- over, a muscle which has been stimulated to exhaustion in an atmosphere of nitrogen, if restored to one of pure oxygen, will rapidly recover its power of contraction. In pure oxygen no lactic acid is produced, and a muscle stimulated to exhaustion contains very httle more lactic acid than does resting muscle. On the other hand, the intake of oxygen and the output of carbonic acid by the muscle is increased at each contraction. We thus find that a muscle during contraction may produce lactic acid or carbonic acid according as oxygen is absent or present. In both cases contraction takes place apparently normally, but fatigue supervenes much more rapidly in the absence of oxygen. The question arises whether we should regard the formation of lactic acid and carbonic acid as alternative processes, or whether lactic acid is first formed and is then removed under the action of oxygen, undergoing partial or complete oxidation to carbonic acid in the process. The evidence is distinctly in favour of the second h}^othesis. Thus Hopkins and Fletcher have fomid that muscle possesses in itself a chemical mechanism for the removal of lactic acid. If a fatigued muscle be exposed to pure oxygen, 30 per cent, of the lactic acid present in the muscle may disappear within two hours and 50 per cent, within six to ten hours. Thus, even apart from the circulation which of course would remove large quantities of any lactic acid which might be produced in the muscles, these can deal with this metabolite locally. It has been found that a muscle may be fatigued several times and then placed in oxygen to recover, so that lactic acid is produced and removed also several times. If at the end the muscle be allowed to undergo rigor, it is found to contain -4 per cent, lactic acid, i.e. exactly the same amount as if it had 218 PHYSIOLOGY given no contractions at all. Fletcher and Hopkins interpreted this result as showing that under the influence of oxygen, lactic acid is put back into the precursor from which it arose, and would assume that part of the lactic acid is completely oxidised to carbonic acid and water, the energy so evolved being employed in the building up of the precursor from the rest of the lactic acid. On the other hand it is possible that the lactic acid produced in the initial stage of contraction may be under normal circumstances completely removed by oxidation, and that the energy or part of the energy so made available is used to build up some precursor substance, not out of the lactic acid, but out of the glycogen already present in the muscle (Parnas). It is certain that prolonged activity of muscle, especially in the presence of oxygen, may be associated with a diminution in the glycogen store of the muscle. We cannot, however, discuss this question further without refer- ence to the total energy changes in muscle contracting with or without oxygen, and the clue to these changes is given by a study of the heat pro- duction in muscle. SECTION VI THE PRODUCTION OF HEAT IN MUSCLE The experience of everyday life teaches us that muscular exercise is asso- ciated with increased production of heat. Thus a man walks fast on a frosty day to keep himself warm. In large animals the production of heat in muscular contraction can be easily shown by inserting the bulb of a thermometer between the thigh muscles, and stimulating the spinal cord. The rise of temperature produced in this way may amount to several degrees. This observation is confirmed when we investigate the contraction of an isolated muscle outside the body. If a frog's muscle is tetanised, its tem- perature rises from 0-l-l° to 0-18° C, and for each single twatch from 0-001° to 0-005° C. It is evident that such small changes in temperature as 0-001° cannot be estimated by ordinary thermometric methods. By converting a heat change into an electrical change, however, we can estimate differences of temperature with much greater accuracy and fineness than by the use of a thermometer. Two main principles are employed in measuring cool temiDerature by electrical methods. The thermo- /^ electrical method depends on the fact that, when o the junctions of a circuit made of two metals are ntimon ■ | | at different temperatures, a current of electricity ^^^r"^/ generally flows through the circuit. This current can ■'^ be measured by means of a galvanometer, and is '^ proportional to the difference of temperature between ^^^ ^^• the two junctions. Thus in the circuit (Fig. 73) composed of two metals, antimony and bismuth, if the upper junction be cooled, there will be a cm-rent flowing from antimony to bismuth in the direction of the arrow, and this current will within limits be proportional to the difference of temperature. To measure the production of heat dming muscular contraction, a small flat thermo- pile (containing four or six elements composed of iron and German silver, or copper and ' constantan ') is fixed with one of its ends between two frog's gastrocnemii. Another exactly similar pile, but reversed, is placed between two other gastrocnemii, which are kept resting and at a perfectly constant temperature. So long as the two piles are at the same temperature no current flows ; but, with a sensitive galva- nometer, the slightest difference of temperature, such as that caused by the contraction of one pair of muscles, at once causes a deflection of the galvanometer, the extent and direction of which enable us to estimate exactly the seat and amount of heat produced. When we are using such delicate detectors of temperature difference, we are met by the difliculty that every junction in the circuit tends to become the seat of an electro- motive force in consequence of slight changes of tcni])crature due to currents of air. &c. It is therefore advisable to use a plan adopted by Blix, of placing all the apparatus, the muscle included, within the galvanometer case. The arrangements of such an experiment as emiilovcd bv A. V. Hill are shoAvn in the diagram (Fig. 74). 219 220 PHYSIOLOGY In this instrument the junction of copper with the alloy constantan constitutes a thermo-electric couple. The magnet and mirror chamber is entirely separated off from the rest of the instrument by the walls of the tube containing the magnet. The grooves are usually filled with plasticine, and into them fit the edges of an outer case of brass constituting the walls of the muscle chamber. The inside of this case is lined with wet blotting-paper. The copper coil consists of many more turns than are shown in the figure : its ends, aa and bb, are separated by the celluloid plate, and are con- nected by the constantan plug ; the points where the copper meets the constantan constitute the thermo-electric jmictions. The tube containing the magnet hangs Magnet & Mirror Chambe. Constantan Plug Muscle Quartz Fibre Mirror Groove ^ Broca Magnet System Copper Coll Electrode Celluloid Plate Electrode Fig. 74. down through holes bored in the broad copper coil. The two semimembranosus muscles ride astride of the celluloid plate, one in contact with each end of the constantan plug. The small piece of bone at their upper ends which has been left connecting the two muscles is placed exactly on the top of the celluloid plate at x and held in position by a clamp (not shown in the figiure). The cojiper terminals of the coil are coated with celluloid varnish to prevent short circuiting of the thermo-electric currents, and to prevent poisoning of the muscles. Each muscle is in contact with a pair of electrodes, made of fine platinum wire ; the muscle lies over the upper and beneath the lower of these electrodes, as shown in the figure. The tendons at the lower ends of the muscles are tied to silk threads which pass through holes in the base of the instriunent. These are then attached to recording levers which write on a drum beneath the table. When all is ready three heavy soft-iron cylinders are placed over the instrument : the latter is screwed to a wooden block which is fixed to a thick iron plate attached to the table. In the cylinders holes are bored to admit and let out after reflexion the light from a Nernst lamp. The lamp, which is about three metres away, shines upon the mirror, and a line in it, after reflexion, is focussed on to the screen, which is also three metres away. The line is brought on to the scale by the small field exerted by a control magnet placed outside the cylinders at a suitable position on the table : its position may be read easily to half a millimetre, and the movement due to a twitch is usually of the order of 80 mm. These soft-iron cylinders cut off entirely all external magnetism sufficient to cause harmful disturbance during an experiment. They lower very largely the strength of the constant external field in which the magnet lies, and leave it chiefly PRODUCTION OF HEAT IN MUSCLE 221 Fig. 74;A. Arrangement of apparatus for measuring small differences of temperature. supported in any position by the quartz fibre. Thus all the movements set up in the magnet by the thermo-electric cm-rents are working against little more than the torsion of a quartz fibre only 6 fj, thick. This explains the great sensitivity of the instrument. A second method depends on the fact that rise of temperature increases the resistance of a wire to the passage of an electric current. A current detector consists of a small grid of fine platinum wire which is i)laced against the muscle between two muscles. This grid is then made one limb of a Wheatstone's bridge (Fig. 74a). a small current is passed through the circuit, and the resistances are so adjusted that no current flows through the galvano- meter. Any alteration in temperature of the grid will alter the balance of the re- sistance and will cause a current to flow through the galvanometer in a direction which will vary according as the resistance in the grid is increased or diminished. It is possible to calibrate the arrangement so that a deflection of the galvanometer over one degree will correspond to a certain fraction of a degree of difference in temperature of the grid. This method is employed in Callender's recording thermometers, and has been made by Gamgee the basis of an arrangement for the continuous record of the temperatvu-e of the human body. Most of the earlier work on the development of heat in muscle had as its leading motive the discovery of the relation between the heat produced and the work performed by a muscle mider varpng conditions of load. When a loaded muscle contracts, however, it is not easy to analyse its mechanical conditions, since part of the shortening of the muscle during contraction can be regarded merely as a recovery from the condition of extension induced by the weight, and the amphtude of the excursion may be largely conditioned by the inertia of the weight moved. Working on these lines, Heidenhain discovered that the heat production in muscle during contraction is not an invariable quantity, but varies according to the condition of the muscle and especially according to the tension developed in it during contraction. It was therefore at its maximum under isometric conditions when it was not allowed to shorten at all during contraction. As we have seen, the muscle changes, as the result of excitation, from a body having certain elastic properties to one having other elastic properties. The whole energy of the contraction is converted for a short period into a state of tension which can be used to do work by raising a weight. If it be not allowed to shorten, the state of tension passes off and the whole energy which has been set free must appear as heat. The potential energy developed in a muscle twitch is approximately equal to i. TL where T is the tension developed and / the length of the muscle, and it is this amount which must be compared with the heat production measured in the muscle 222 PHYSIOLOGY by one of the metliods described. A. V. Hill has shown that the heat production in a contracting skeletal muscle occurs in two phases, a rapid production of heat which apparently is synchronous with the contraction itself, and a slow production of heat which continues for some time after the muscle has relaxed. The second phase of heat production depends on the presence of oxygen, and is observed at its best when the muscle is kept in pure oxygen. If the muscle be allowed to contract in nitrogen only the initial heat production is observed. The heat production of the second phase is stated by Hill to be approximately equal to that in the first phase. These results have been interpreted as showing that the initial change in muscular contraction is the development of lactic acid. The appearance of this lactic acid in some way changes the muscle and sets up potential energy at the surface of its ultimate fibrils, which will result in a shortening of the muscle if any movement of its ends be allowed. A comparison of the energy of the tension set up with the actual heat evolved in the initial stage when a muscle is not allowed to contract shows that the two quantities are approximately equal. In a series of experiments Hill found that the ratio -J- Tl : H in the sartorius muscle under low initial tensions and in comparatively weak contractions approximated to the value 1, the mean value being -91. Under high initial tensions and in strong contractions of the sartorius muscle, it is lower, being roughly from 04 to 0-6. He concludes from this that under certain conditions the initial process of con- traction consists largely, if not entirely, of the hberation of free potential energy manifested as tension in the muscle. This potential energy may be used for the accomplishment of work or for the production of heat. The efficiency of the initial stage of contraction is therefore almost 100 per cent. If, however, a muscle is to go on contracting without rapidly showing signs of fatigue, it must be kept in oxygen, so that the processes of replace- ment or of removal of the lactic acid may take place. Under these circum- stances there is a further evolution of heat after the contraction, equal to that set free during the initial stage. So that the total efficiency of a muscle kept in oxygen would not be more than 50 per cent. This is assuming that the process of oxidation of the lactic acid and its replace- ment in whole or in part in the muscle molecule is completely carried out diu-ing the time of the observation. It is improbable that such is the case, and it seems possible that the evolution of heat during the so-called recovery stage of the muscle has been imder-estimated. If a series of observations of the heat production and tension developed during isometric contractions be made with varying initial tension on the muscle, it is found that while the ratio of tension developed to heat produced is approximately constant, both these quantities first increase and then finally diminish. The optinmm of the heat production in some experiments seems to fall later than the optimum of the tension developed. It seems probable that in this case the essential factor is not so much the tension exerted on the muscle previous to its excitation, but the length of the muscle fibre during the time that it is excited. The longer the muscle fibre, within PRODUCTION OF HEAT IN MUSCLE 223 Umits, when it is excited, the greater the tension and the greater the heat production developed, i.e. we may assume that increased length of muscle fibre increases the chemical changes, ensuing on excitation, which are respon- sible both for the development of mechanical energy and the production of heat. The significance of these results for the essential nature of muscular contraction we shall discuss in a later chapter. SECTION VII ELECTRICAL CHANGES IN MUSCLE Fig. 75. Diagram of non- polarisable electrode. a.covered wire; &, amal- gamated zinc rod; c, glass tube ; d, saturated If a current from a battery be passed between two plates of platiniun immersed in acidulated water or salt solution, electrolysis of the water takes place, bubbles of oxygen appearing on the positive plate (anode), and bubbles of hydrogen on the negative plate (cathode). If now we remove the battery, and connect the two plates (electrodes) by wires with a galvanometer, a cm-rent passes through the galvanometer and water in the reverse direction to the previous battery current. This current is called the polarisation current, and is due to the electrolysis of the water that has taken place. The vessel in which the electrodes are immersed has in fact become a galvanic cell, the platinum covered with oxygen bubbles being the positive element, and that covered with hydrogen bubbles the negative element. Exactly the same process of electrolysis or polarisation takes place when we pass currents through the tissues of the body by means of metallic electrodes. Hence before we can study accm'ately the delicate electrical changes that may occur normally in living tissues, it is necessary to have some form of electrodes in which this polarisation will not occur. The ' non-polari- sable ' electrodes which are most generally used for this ZnSO^ solution ;erplugo'f Pu^'POse are made in the following way. A glass tube zinc sulphate clay ;/, plug (Fig. 75) is closed at one end with a plug of kaolin made of normal saline clay. into a paste with a saturated solution of zinc sulphate. The rest of the tube is filled with a similar solution. Dipping into the zinc sulphate solution is a rod of pure zinc, amalgamated. Just before use, a plug of china clay made with normal saline solution is put on the end of the tube, so as to effect a connection between the zinc sulphate clay and the nerve or muscle which it is desired to stimulate or lead off. In these electrodes there is no contact of metals with fluids that can produce dissimilar ions {e.g. hydrogen or oxygen) at the surface of contact, and hence they may be regarded as practically non-polari- sable. A more convenient form is that employed by Burdon Sanderson, in which the glass tube is bent into a U (Fig. 76). The mouth of the tube is closed by a smaller glass tube plugged with clay, and bearing a plug of normal saline clay. In such electrodes the conduction of the cur- rent through the nerve or muscle to the metallic part of the circuit may be represented as shown on the opposite page (see Fig. 77). If a muscle such as the sartorius be removed from the body, and two non- polarisable electrodes connected with a dehcate galvanometer be apphed to two points of its surface, there will be a deflection of the mirror attached to the galvanometer, showing the presence of a current in the muscle from 224 Fig. 76. U-shaped non-polarisable electrodes. ELECTRICAL CHANGES IN MUSCLE 225 the ends to the middle, and in the external circuit from the middle (or equator) to the ends. It was formerly thought that this current was always present in all normal muscles, and it was spoken of as the ' natural muscle current ' ; the muscle was said to be made up of a series of electromotive molecules, the equator of each molecule being positive to the two poles (du Bois Raymond). It has been conclusively shown, however (by Hermann and others), that this Zn + Zn Na ^ Na Zn <— < 50^ a a 50^ Zn FiG. 77. Fic. 1?. Cm-rent of rest. current of resting muscle is not a natural current at all, but is due to the effects of injury in making the preparation. The less the preparation is injured, the smaller is the current to be obtained from it, and in some con- tractile tissues, such as the heart, there may be absolutely no current during quiescence. Hermann describes the fact of the existence of currents of rest thus : " In partially injured muscles every point of the injured part is negative towards the points of the uninjured surface." Fig. 78 shows the direction of the current in a muscle with two cut ends When the whole muscle is quite dead, this cur- rent of rest, or ' demarcation current ' (Her- niaim), disappears. The current is due to the electrical differences at the junction of living and dying (not dead) tissue. If the sartorius of the frog be cut out and immersed for twenty- four hours in 0-6 per cent. NaCl solution made with tap water {i.e. con- taining hme), all the injured fibres die, and the uninjured fibres are then found to be iso-electric and therefore currentless. The existence of this current may be demonstrated without using a galva- nometer. If the nerve of a sensitive muscle-nerve preparation {a, Fig. 80) be allowed to fall on an excised muscle h, so that two points of the nerve are in contact with the cut end and with the surface of the second muscle 6, the muscle a will contract each time the nerve touches h so as to complete the circuit. Whatever be the explanation of this current of resting muscle, there is no doubt that a very definite electrical change occurs in a muscle when it contracts. To show this change, we may lead off two points, one on the cut end and one on the surface of the muscle of a nniscle-nerve pre- paration, to a galvanometer. We shall then obtain a deflection of the mirror Fic. 70. Rhcoscopic frog. 226 PHYSIOLOGY of the magnet, due to the current of rest or demarcation current. If now the nerve be stimulated with an interrupted current so as to throw the muscle into a tetanus, the ray of hght from the galvanometer mirror swings back towards the zero of the scale, showing that the current which was present before is diminished. When the excitation of the nerv^e is discontinued, the galvanometer indicates once more the original current of rest. This diminution of the current of rest during activity of a muscle is spoken of as the ' negative variation.' In carrjdng out this experiment it is usual to compensate the demarcation current by sending in a small fraction of the current from a constant cell. The arrangement of the apparatus is represented in the accompanying diagram. Two non-polarisable D Fig. 80. electrodes np are applied to the surface and cross-section of a muscle m. These are connected with the shunt of the galvanometer, one of the wires, however, being con- nected with a Pohl's reverser P, and this in its turn with the shunts. The two end- terminals of the reverser are coimected with a rheochord, through the wire of which ah a constant current is passing from the Daniell cell D. By means of the rider c the fraction of current passing through the reverser can be modified to any extent. The key k being open, the muscle is connected with the shunt and galvanometer, and the direction and extent of the swing noticed. The key h is then closed, and by means of the reverser the current is sent through the galvanometer in the opposite direction to the demarcation current, and the rider c shifted until the two cm'rents exactly b9,lance one another, and the needle of the galvanometer returns to zero of the scale. This adjustment is first made, using only y^j^ of the total current, and then by means of the shunt, Jjj, and finally the whole current is thrown into the galvanometer. If this precaution be not taken, much too large a current may in the first case be sent through the galvanometer, to the detriment of the instrument. If we know the difference ac of potential between the two ends of the wire, the proportion —r will give us the E.M.F. of the demarcation current. The galvanometer needle having by compensation been brought to zero, stimulation of the nerve at e by interrupted currents causes the needle to swing at once in the opposite direction to the first variation. This swing is the measure of the negative variation or current of action. In order to study the electrical changes accompanying a single muscle twitch, it is necessary to employ some instrument which can react much more rapidly than the ELECTRICAL CHANGES IN MUSCLE 227 ordinary galvanometer. For this purpose we may employ either the capillary electro- meter or the string galvanometer of Einthoven. The capillary electrometer is an instrument for recording and measuring difference of potential. That is to say, if connected with two points, it measures the force which would make a current flow between these two points if they were comiected by a wire. Its structure is very simple. It consists of a glass tube di'awn out to a fine capillary point. This tube with the capillary is filled with mercury. The point dips into a wide tube containing dilute sulphuric acid, at the bottom of which is a little mercury. Two platinum wires fused into the glass and dipping into the mercury serve as terminals. When the instrument is used, the meniscus of the merciu"y in the capillary at its junction with the acid is observed under the microscope, or a magnified image of it is thrown on a screen with the aid of the electric light. If now the capillary and acid be con- nected with two points, it will be observed that any difference in the potential of these two points causes a movement of the meniscus. If the point comiected to acid be negative as compared with the point comiected to mercury in capillary, the meniscus moves towards the point of the cajiillary. If the acid be positive as compared with the capillary, the meniscus moves away from the point. The extent of the excursion is proportional to the difference of potential. Since the capillary electrometer appears to have no latent period, and is free from instru- mental vibrations, it is extremely useful in recording the quick changes in potential occurring in the diphasic electrical changes that accompany every Capillary electrometer. (Burcii.) contraction-wave in the body. The excursions lend themselves well to photography, so that we may obtain a graphic record of every electrical variation, and thus determine its extent and its time-relations. It must be remembered that this instrument is an electrometer (measurer of diifer- ence of potential), and not a galvanometer (current measurer). When the electrometer Fig. 81. R Fig. 82. Fig. 83. is connected with two points at different potential, current passes into it for a fraction of a second, and polarises the surface of the mercury, so that it takes up a new position 228 PHYSIOLOGY Diagram showing diphasic variation of uninjured muscle. in the capillary. This polarisation causes an electromotive force which exactly balances the E.M.F., setting up the polarisation so no current passes the surface. Hence the use of non-polarisable electrodes is not so essential in experiments with this instrument as when we make use of the galvanometer. In the D'Arsonval galvanometer (Fig. 82) the current is sent through a coil of fine wire hung between the poles of a permanent magnet. The same principle is made use of in the string galvanometer of Einthoven (Fig. 83). In this a very delicate thread of silvered quartz or of platinum is stretched between the poles of a strong magnet. The poles of the magnet are pierced by holes so that the thread may be illumined by an electric light from one side, and from the other may be observed by means of a microcope ; or a magnified image of the thread may be thrown upon a screen. When- ever a current passes through the thread it moves laterally, and the lateral movement may be photographed on a moving photographic screen. Owing to the minute dimensions of the thread the instrument is one of extreme delicacy. It will detect very minute currents and will respond accurately to very rapid changes in potential. If a perfectly uninjured regular muscle (Fig. 84), such as the sartorius, be stimulated with a single in- duction shock at one end, x, and two points, a and 6, be led off to a capillary electro- meter, each stimulus appHed at X gives rise to an excursion of the meniscus of the electro- meter, known as a ' spike,' and shown in Eig. 85. Knowing the constants of the instrument used, we can analyse this spike, and we find that it represents a diphasic change. Our study of the mechanical changes in muscle has shown that, when the muscle is stim- ulated at X, a contraction wave commences which travels down the muscle through a and h. The electrical investigation of the muscle shows that exci- tation of X arouses an electrical change which also passes down the muscle at the same rate as the mechanical change which it precedes. If we are leading off from X and a, the electrical change ensues immediately upon stimulus, i.e. there is no latent period to the elpctrical chano^e. On leading off from a Fig. 8.5. A typical electrometer record from a sar- T ■/. T • 1 - J. -J torius muscle excited by a single induction shock. and 6 there is a latent period Time-marking = 200 D.V. (Keith Lucas.) between the stimulus and the first change, representing the time taken from the change to travel from X to a. When the change reaches a this becomes the seat of an electro- ELECTRICAL CHANGES IN MUSCLE 229 motive force of such a direction that the current would pass in the outer circuit from b to a. We may say, therefore, that a is negative to b. A fraction of a second later the excitatory change has passed on to b and has died away at a. Now b is negative to a* and the current therefore passes in the opposite direction. Between a and b, therefore, there is a diphasic current, the first phase representing negativity of a to b, and the second phase ""A" ♦•04 •03- M T 02- \^ ^ ..0, \\ \ T -J 1- i \ - 01- \ [^ - 02- / -■03- - CM- j O 1 V. .,,,.-. . . 1 • ' ■ ■ Fiii. 86. Diphasic response of uninjured sartorius (obtained by analysis of curves such as Fig. 85). A, at 8° C. ; B, at 18° C. (Keith Lucas.) representing negativity of b to a. A diphasic change is thus also a' sign of a propagated change. Every excitation of a normal muscle gives rise to a diphasic variation of such a direction that the point stinmlated first becomes * The statement that the excited portion of the muscle becomes ' negative/ tliough sanctioned by long usage, is not very exact and may give rise to misconception. When we lead oft the terminals of a cojiper-zinc couple or cell to a galvanometer, a current flows outside the cell from copper to zinc and inside the cell from zinc to copper. In this case the zinc is said to be electropositive to the copper, and in the same way we must assume that the excited portion of a muscle is electropositive to the unexcited portions. When, therefore, we speak of any part of a tissue being negative, we are using a conventional expression to indicate the direction of the current in the outer circuit, and not the electrical condition of the tissue itself. In order to avoid the con- fusion which might result from an attempt to replace the loose expression ' negative ' by the more correct expression ' electroijositive,' Waller has suggested the employ- ment of the term 'zincative' to indicate the electrical condition accompanying excitation. This term would also serve to emphasise the fact that tlie excited portion, like the zinc in a zinc-copper cell, is the chief seat of chemical change. 230 PHYSIOLOGY negative to all other points of the muscle, and this ' negativity ', to use a loose but convenient expression, passes as a wave down the muscle, preceding the wave of contraction and travelhng at the same rate. If one leading-ofi point be injured, e.g. at h, the change accompanying excitation is absent at that point. A single stimulus apphed at x will in this case give only a monophasic variation in which a is relatively negative to h. When we study the time relation of the electrical variation ensuing on a single stimulus, we find that the electrical change under the electrodes begins at the moment that the stimulus is apphed. It takes about -0025 sec. to attain its culminating-point. At this point the mechanical change or contraction of the muscle begins. These time-relations vary with the temperature of the muscle. We have already seen that the efiect of lowering the temperature is to increase the latent period of the contraction. In the same way it slows the rise of the electrical change and the rate of propagation of the wave of electrical change. This is shown in Fig. 86, in which are given the diphasic response of the sartorius first at 8° C. and secondly at 18° C. We are therefore justified in regarding the electrical change as an index to the chemical changes evoked in the muscle as the direct result of the stimulus. The flow of material, which is responsible for the change in form of each contracting unit, is secondary to these changes. As the result of stimulation, a chemical change is aroused at the point of excitation and travels thence along the muscle fibres at a rate of about three metres per second, i.e. the same rate as that of the following wave of mechanical change and, hke this, varying with the temperature. Under certain conditions an excitatory condition may be propagated without the presence of a visible contraction. Thus, if the middle third of the sartorius be soaked for a time in water, it passes into a condition known as ' water rigor,' in which it is incapable of contracting, although capable of transmitting an excitation from one end of the muscle to the other. The connection of a diphasic current of action with an excited condition of the tissues passing as a wave from one end to the other is shown still more clearly on a slowly contracting tissue, such as the ventricle of the frog or tortoise. Fig. 87, a, is a photographic record of the variation obtained from the tortoise ventricle, which is led off to a capillary electrometer, one (acid) terminal being connected with the base of the ventricle, the other (mercury) with the apex. Each part of the ventricle remains contracted for a period of \\ to 2 seconds, and then the contraction passes of!, first at the base and later at the apex. The electrical events are an exact replica of the mechanical. Directly after the stimulus has been applied, the base becomes negative and the column of mercury moves up. A moment later the excitatory condition extends to the apex. There is thus a sudden equahsation of potential between the two terminals, and the mercury comes back quickly to the base line. Here it stays for 1| to 2 seconds. During this time the whole heart is in an excited condition. Both base and apex are equally excited, and there can be no difference of potential between them. The excitatory condition ELECTRICAL CHANGES IN MUSCLE 231 then passes off, first at the base and then at the apex. There is thus a small period of time in which the apex is still contracted or excited while the base is relaxed, and the apex is therefore negative to the base. This terminal negativity of the apex is shown on the photograph by the excursion of the column of mercury away from the point of the capillary. If one terminal, e.g. the apex, be injured, we obtain quite a different variation, which is shown in Fig. 87, b. It is evident from this figure that the electrical sign lasts practi- FiG. 87. Electrometer records of the electrical variations in a tortoise ventricle, excited to beat rhythmically by single shocks. A, Ventricle uninjured, b. One leading oflf spot injured. (B. Sajidersox.) cally as long as the mechanical sign of the excited state, and that we are not justified in regarding the first spike of the diphasic variation as indicative of an excitatory wave attended by an electrical change which is independent of the succeeding mechanical change. The only difference between the electrical changes in this case and in that of voluntary muscle is that in the latter all processes are very much quicker, so that as a rule the point a (Fig. 81) has ceased to be negative before the negativity of h has attained its full height, and there is thus no prolonged equipotential stage. Although iu the case of the slowly contracting ventricle of the tortoise, the record obtained of the electrical changes accompanying its contraction by means of the capillary electrometer shows with great clearness the diphasic nature of the variation, and there- fore the wave character of the electrical change, considerable difficulty is experienced sometimes in recognising that the ' spike ' record of the electrical change in voluntary muscle or in nerve is also due to a diphasic variation. In this case the electrical change 232 PHYSIOLOGY at any spot lasts only about ^1^ second, and there is not a prolonged equipotential period, as in the case of the heart. The natm-e of the variation is, however, obvious, if we compare the electrometer record of an intact and therefore currentless muscle with that of a muscle in which one of the leading-off points has been injm-ed, so as to give rise to a demarcation current. The two curves are given in Fig. 88, the upper shadowy tracing being that obtained from the injured muscle. It will be seen that the dis- tinguishing character of an electrometer record of a diphasic variation in the rapidly contracting striated muscle consists in the fact that the down- stroke of the image of the meniscus is as rapid as the upstroke, whereas the monophasic variation of the injured muscle presents a slow fall pro- duced by the gradual leakage of the charge imparted to the instrument back tlu?ough theelectrodes and muscle. When such a record is analysed, we obtain a curve similar to those in Fig. 89, which retire- sent monophasic variations of a sartorius injured at one end, under different conditions of temperature. A similar curve to the diphasic variation can be obtained by putting Fig. 88. Superimposed photographs of the J electrical varia- tion of the sartorius in response to a single stimulus. (Btjedon Sanderson.) then in a reverse direction for another -^Iq- second. It must be remembered that a diphasic variation does not mean that one part of a muscle changes from normal in one direction, and then swings back past the normal in another direction, but that a change in one direc- tion at one electrode dies away and is succeeded by a similar change in the same direction, which also dies away, at the second electrode : that is to say, a diphasic variation implies the pro- gression of a wave of electrical change between the leading- off points. Using a string galvanometer, which reacts much more rapidly, the diphasic nature of the varia- tion is immediately apparent from the photographic record, even with voluntary muscle. Fig. or nerve. STIM / j 89. Monophasic variations of an injured sartorius. A, at 18° C. ; B, at 8° C. (Keith Lucas.) The electrical variation obtained by leading off a heart beating normally is a much more complex affair, and even now physiologists are not agreed as to its interpretation. Gotch has suggested that the complex character of the variations obtained both from the spontaneously beating frog's heart as ELECTRICAL CHANGES IN MUSCLE 233 well as from the mammalian heart is due to the twisting and alteration in direction of the fibres and of the course of the contraction wave which have occurred in the evolution of the heart from a simple tube. The question will be discussed more fully in chapter xiii. THE DEMARCATION CURRENT OR CURRENT OF INJURY According to Hermann, muscle or nerve may become negative under two conditions : (1) During activity ; (2) when dying as the result of injury. It is doubtful, however, whether these two conditions are really distinct. Section or injury of a muscle causes a prolonged stimulation of the adjacent parts of the muscle fibres. These parts, therefore, being excited, must be negative to the unexcited parts which are further away from the seat of injury so that a demarcation current is really an excitatory current. We thus come to the conclusion, only paradoxical in terms, that the so-called currents of rest are really currents of action and are due to excitation around the injured spot.* SECONDARY CONTRACTION. RHEOSCOPIC FROG The negative variation of one muscle may be used to make another contract. If the nerve of the preparation a (in Fig. 90) be laid so as to touch at two points the cut end and surface of the muscle h, and the nerve of h then stimulated with single induction shocks, every contraction of h will be attended by a contraction of a, excited by the negative variation of the current passing through its nerve from the point touching the cut end to that in contact with the equator of h. . . Fio. 90. If the nerve of h is tetanised, a as well as h Rheoscopic frog. enters into a continued contraction. This ' secondary tetanus ' is of interest as showing that, although the contractions of h are fused, the excitatory process and negative variations are still quite distinct. * If the demarcation current is really only due to excitation, -we should expect to find it weaker than the action current obtained by exciting the whole muscle to contract. And this is the case. The E.M.F. of the demarcation current of a sar- torius equals about 0-05 of a Daniell cell. The action cm'rent of the same muscle may attain to an E.M.F. = 0-08 of a Daniell cell (Gotch). 8* SECTION VIII THE INTIMATE NATURE OF MUSCULAR CONTRACTION Experiments on the metabolism of the body as a whole show that the energy of muscular work is derived from the oxidation of the food-stuffs. In man the performance of work involves an increase of the oxidative processes of the body with a corresponding evolution of energy, of which four-fifths will appear as heat while one-fifth may be transformed into mechanical work. In this respect the physiological mechanisms for the production of mechanical energy resemble the greater number of the machines employed by man for the same purpose. In nearly all these the prime source of energy is the oxidation of carbon ^nd hydrogen in the form of coal or oil. In the steam-engine and internal-combustion engine the whole energy set free by the process of oxidation appears first as heat, and then a certain portion of the heat is converted into mechanical work. There is a hmit to the efficiency of such heat engines, depending on the maximum, differences of temperature available between the two sides of the working part of the machine. The efficiency of any heat engine is expressed by T - Ti the formula E = — ^ — , where T is the highest temperature (in absolute measurement) obtained by the working substance and T^ is the lowest temperature of the same substance. Ordinary engines rarely attain more than half this ideal efficiency, but it is evident that the greater the difference of temperature available the greater will be the efiiciency of the machine. Internal- combustion engines, such as the gas engine or the oil-engine, therefore give a greater percentage of the total energy of the fuel out as mechanical energy than is the case with the steam-engine. Engelmann has maintained that in muscle there is a similar transforma- tion of heat into mechanical energy. He has found that non-living sub- stances, which contain doubly refractive particles and possess the property of imbibition {e.g. catgut) when soaked with water, will contract on heating and relax again on coohng. He has constructed a model in which a thread of catgut in water, surrounded by a platinum coil, can be made to simulate muscular contractions and relaxations by passing a heating current through the platinum coil. He imagined that the chemical changes in the muscle liberate heat and that the effect of this heat upon the doubly refractive particles is to make them imbibe the surrounding water so that they change 234 THE INTIMATE NATURE OF MUSCULAR CONTRACTION 235 from an oval to a spherical shape. It would be impossible, however, for any large changes of temperature to take place in the muscle without en- tirely destroying its chemical character, and with small differences of tem- perature it would be impossible to attain the efhciency of 12 to 20 per cent, which characterises muscle. Under certain conditions we may obtain by a machine almost the entire energy of a chemical change. The condition is that the chemical change shall be susceptible of taking place in a galvanic battery. We may use, for instance, a series of Daniell cells to drive an electric motor and allow the motor to perform mechanical work. Under these circumstances we could theoretically obtain 100 per cent, of the total chemical energy avail- able, and in conditions of practice the efficiency of the machine may attain to 70 or 80 per cent. A similar arrangement might be present in the ultimate contracting elements of the muscle fibre. The mechanism in the fibre must be one which will provide for a more or less direct transformation of chemical energy into mechanical energy without a previous conversion of the chemical into heat energy. In the living body, where everything is in solution, all the energies may be reduced to one of two kinds, osmotic energy and surface energy. The contractile machine must therefore be one which employs one or other, or both, of these forms of energy. We might with Macdougall, regard the contractile element as a cylindrical structure differing in its contents from the surrounding sarcoplasm. When the muscle is at rest the contents of the muscle prism will be in equihbrium with the surrounding sarcoplasm. We might imagine the excitatory process to consist in a sudden chemical change occurring in the contents of the muscle prism. The production of a number of new molecules within the muscle prism {e.g. of lactic acid) would raise tlie osmotic pressure within the prism and occasion a rapid flow of water from the sarcoplasm. As a result the pressure in the muscle prism would rise and cause a bulging of its lateral wall and a shorten- ing of the whole element. The subsequent phase of relaxation may be due either to a secondary change, e.g. oxidation, leading to the formation of a substance to which the walls of the prism are freely permeable, or to the gradual leak of the primary products of oxidation or disintegration into the sarcoplasm. The substance or substances giving rise to the osmotic differ- ences which determine contraction may be either products such as lactic acid and carbon dioxide, which are formed during contraction, or may possibly be of the nature of neutral salts set free from some condition of combination with the proteins of the sarcous element. Macdonald has brought forward micro-chemical evidence of the appearance of potassium salts in the sarcous element during the state of activity of the muscle. On the other hand, Bernstein has suggested that the changes during muscular contraction arc determined by alterations in surface tension. If a little mercury be spilt on a plate tlie particles form globules. They are kept from spreading themselves out in a thin liltu under the influence of gravity inconsequence of tlie surface tension of the mercury. Any modification of the surface will alter the tension, and therefore state of expansion, of the 236 PHYSIOLOGY globule. Thus, if the globule be in sulphuric acid it undergoes a certain amount of polarisation, and becomes positively charged. By altering the charge of such a globule we can change its shape, as is shown diagram- matically in Fig. 91. If b represents the shape of the globule lying on the plate in some weak sulphuric acid, a will represent the shape of the globule when it is connected with the negative pole of a battery, while c will repre- _ sent its shape when it is connected ^ with the positive pole of a battery, the other pole in each case being connected with the acid. If we consider muscle as made up of a series of chains of oval particles, a chemical change in the surface of these particles, causing an increase of surface tension, will tend to make them assume the globular shape, and will therefore cause a shortening and thickening of the whole fibre. According to Schafer, contraction is associated with a, flow of the outer hyaline contents of the sarcous element into the tubular structiu'e forming the middle portion. Such a flow may be determined either by osmotic differences between the centre and periphery of the sarcous element, or by a change in the surface tension obtaining between the isotropic fluid at the ends and the anisotropic structures in the centre of the muscle prism. The tendency of recent investigation is all in favour of the second hypo- thesis, namely, that the essential factor in the processes of excitation and contraction is an alteration of surface. In the first place the electrical changes accompanying the excitatory process denote a polarisation or accumulation of ions on the surfaces situated in the excited area. The chemical change which is responsible for the current of action, or the negative charge at the excited spot, takes place almost instantaneously and disappears somewhat more slowly. It would seem that the excitatory process consists essentially in the setting free of certain ions on the surface or surfaces in the contractile tissue, and that the passing away of the excitatory state is due to the disappearance of these ions, either by diffusion away into the surrounding fluid or by further chemical changes, such as oxidation. A study of the development of tension and of heat production in a muscle on excitation has shown that in both cases the yield of energy on excitation is increased by lengthening and diminished by shortening the muscle. Now alteration in length of the muscle will not alter its volume, but will alter the extent of its longitudinal surfaces, and it appears therefore that the production of heat as well as of mechanical energy is not a volume, but a surface effect. Finally the work of A. V. Hill on the heat production in muscle seems to show that the rise of tension in a muscle on excitation is due to the hberation of chemical bodies, of which lactic acid is certainly one, in the neighbourhood of certain longitudinal surfaces or membranes, and that the presence of these bodies changes the tension at such surfaces and thereby the longitudinal tension of the fibre. The extent and intensity of the production of these bodies must depend on the area of the chemically THE INTIMATE NATURE OF MUSCULAE CONTRACTION 237 active surfaces. The muscle reacts at the end of the excitatory stage^ not by any active process of lengthening, but by neutrahsation, or simply physical diffusion of the active chemical bodies away from the interfaces or mem- branes. Later on, lactic acid is removed or replaced by its previously unstable precursor under the influence of oxygen with the production of some carbon dioxide and a certain amount of heat. We have seen already that the efficiency of the initial chemical change in which lactic acid is set free may approximate 100 per cent. It must be noted that although the oxidative processes are responsible ultimately for all the energies of the higher animal, no oxidative change is involved in the production of lactic acid from e.g. glucose, nor is the presence of oxygen necessary for the contraction of muscle to take place. On the other hand, if we wish to obtain the maximum amount of work from a muscle, we must supply it richly with oxygfen, the presence of which seems essential not to the contractile process but to the stage of recovery. In this stage a certain amount of heat is evolved, set free by the oxidation of the lactic acid, and we must assume that part of the energy so available is utihsed for building up the precursor from which the lactic acid is derived. It is as if the process of oxidation furnished the energy for winding up a spring, whereas excitation removed a catch and allowed the spring to run down, setting free this energy for the performance of work or for conversion into heat.* For many years it was imagined, as a result of expeiiments by Hermann, Pfliiger, and othei's, that the oxygen sujiplied to a muscle was built up with its other constituents, especially carbohydrates, into a complex 'inogen' molecule. On stimulation this mole- cule underwent an explosive rearrangement, the carbohydrate and oxj'gen parts of the molecule combining to form carbonic acid, another product of the decomposition being lactic acid. The careful experiments of Fletcher have shown, however, that in the absence of oxygen there is no e\idence of the formation of carbonic acid during contraction, and therefore no reason to assume the presence of oxygen in the muscle in an intramolecular form. Everything points to oxygen being taken in and applied forthwith to the pm-posesof oxidation, so that the output of carbon dioxide and water keeps pace with the intake of oxygen. It is at present quite impossible to come to any conclusion as to the nature of the precursor from which the lactic acid is derived. The immediate precursor cannot be glucose or glycogen since the heat evolved in the initial stage of contraction is two or * Peters has shown that if a muscle be stimulated to exhaustion under anaerobic conditions, about 0-2 per cent, lactic acid is formed with the evolution of -9 calories per gramme of muscle substance. The production of 1 gm. of lactic acid is therefore accompanied by the evolution of 450 calories. According to A. V. Hill the ' recovery heat production ' in oxj'gen is of about the same order as the initial heat production, so that in the oxidative removal of 1 gm. of lactic acid there would also be an evolution of about 450 calories. The oxidation of 1 gm. of lactic acid produces 3700 calories, about eight times as much as the quantity observed. Hill considers this amount far too large to have escaped detection in his experiments, and therefore concludes that the lactic acid is not oxidised but replaced in its previous position under the influence and with the energy of the oxidation either ; [a) of a small part of the lactic acid itself, or {h) some other body. He regards the latter alternative as the more probable, and concludes therefore, that the lactic acid is part of the machine ard not part of the fuel of the muscle. 238 PHYSIOLOGY three times as great as could be derived from the mere conversion of either of these substances into lactic acid. We must, therefore, conclude that the oxidation of lactic acid which goes on during the process of recovery is used to yield the energy necessary for building up the active molecules, which are the precm'sors of lactic acid and which have a higher potential energy than glucose itself, so that when it rapidly decomposes sufficient energy is set free to account for the observed heat production. Some such utilisation of the energy of oxidation of the lactic acid is indicated by the results of Parnas, who found that the heat evolved dm'ing this recovery process corresponded to only about one half the heat which would be evolved by the formation of the carbon dioxide output of the muscle during the same time as a result of the oxidation of lactic acid. SECTION IX VOLUNTARY CONTRACTION The whole of our analysis of the processes accompanying the contraction of a skeletal muscle has so far had reference merely to the contractions evoked by artificial stimuli, mainly electric. These contractions have either been the simple twitch, with a duration of about one- tenth of a second, evoked by a momentary stimulus, or the tetanus, a continued contraction composed of a number of single twitches, summated and fused together. Under normal circumstances the contraction of skeletal muscles is brought about either reflexly, or in response to some stimulus descending from the cerebral cortex, the so-called ' voluntary contraction.' These contractions may have a duration of almost any extent. The quickest contractions carried out by man have a duration of about 0-1 sec. Considerable effort and training are required to reduce a muscular movement to this degree, and nearly all contractions, even the rapid ones, last considerably over 0-1 sec. Since we have no certain means of producing contractions of any given length, except by means of repeated stimuh, it is natural that physiolo- gists have regarded voluntary contractions as similar to the artificial tetanus, and as, like this, composed of fused single contractions, and have endeavoured to determine the number of contractions per second, i.e. the natural rhythm of the tetanus. If, however, every muscular contraction in the body is to be regarded as of the nature of a tetanus, effected by rapidly repeating stimuli sent down the motor nerve from the central nervous system, we must assume a similar discontinuity for the process underlying the normal tone of muscles, and for the continued contraction of unstriated muscles, e.g. of the arteries. Is this discontinuity of muscles really essential for the production of a prolonged contraction ? So far as our present knowledge of the intimate nature of muscular contraction goes, it would seem quite possible that the continuous state of contraction is dependent on a continuous evolution of energy in the muscle. We have seen reason to regard the chemical processes in a contracting muscle as presenting two phases, namely, (1) the production of a substance which increases the osmotic pressure within the sarcous elements, or raises the surface tension of the ultimate contractile elements of the muscle, thus causing a shortening and thickening of those elements ; and (2) the further change of this substance into one which can escape by diffusion, or into a substance with a low surface tension, so that now the muscle relaxes and can be stretched by any extending 239 240 PHYSIOLOGY force. If these two phases went on continuously, but the Jfirst phase kept ahead of the second one, a continuous state of contraction would be produced in the muscle. Since the contraction of the muscle only occurs in response to impulses from the central nervous system, we should have to imagine also a continuous stream, e.g. of negatively charged ions, descending the nerve and evoking an excitatory change in the muscle fibres as they impinge on the neuro -muscular junction. We have evidence that a state of excita- tion of a nerve, which is apparently continuous, may excite a correspondingly continuous state of excitation in the muscle attached. During the passage of a constant current through muscle there is a continuous contraction in Fig. 92. Continued contraction followed by rhythmic contractions of a muscle in response to a constant stimulas. (Biedbemenn.) The muscle was excited by the passage of a constant current, the cathoda end having been moistened with a weak solution of NaC03. the neighbourhood of the cathode. If the irritabihty of the muscle at this point be increased by the apphcation of a solution of sodium carbonate, Biedermann has shown that this excitation is propagated to the rest of the muscle, and on closure of the current we obtain a prolonged contraction followed by rhythmic contractions (Fig. 92). Moreover in frogs, the ex- citabihty of which has been heightened by keeping them at 2° to 3° C. for some days, the closure of a descending current through the sciatic nerve causes a prolonged contraction of the gastrocnemius ; and in the same way there may be a prolonged contraction produced by the opening of an ascend- ing current through the nerve. The question, however, can only be decided by experiment. If a volun- tary or reflex contraction is of the nature of a tetanus, we should be able, by a study of the mechanical and electrical phenomena combining the contraction, to obtain distinct evidence 'of this causation. It was shown by Wollaston that, on hstening to a contracting muscle, a low sound was heard which, according to him, corresponded to a vibration frequency of 36 to 40 per second. The same observation was made by Helmholtz, and can be repeated by any one who will place the end of a stethoscope on a muscle, e.g. the biceps, and listen to the sound produced when it contracts, Helmholtz pointed out, however, that the tone heard corresponded to the resonance tone of the external ear, and was the same as that noted when listening to any irregular sound of low intensity. Thus the roar of London that we hear in the middle of Hyde Park has the same pitch as the muscle sound of the contracting biceps. The muscle sound, therefore, teaches us nothing as to the pitch or number of contractions per second making up the VOLUNTARY CONTRACTION 241 voluntary tetanus. It merely points to an irregularity or discontinuity in this contraction. By bringing ^^brating reeds of different frequency in contact with, the contracting muscles of the frog, Helmholtz came to the conclusion that the chief element in the muscle sound was the first over-tone of a sound with a vibration frequency of 18 to 20 per second, which, according to him, was to be taken as representing the number of single contractions in every voluntary muscular contraction. Nearly all voluntary contractions present a certain degree of irregularity, and the same irregularities are observed when a tetanic spasm in the muscle of the body is caused by strong excitation of the cerebral cortex, as in epilepsy. On taking a record of such contractions, Schaefer and Horsley showed that in nearly all cases the tracing presents superposed undulations repeated at the rate of eight to twelve per second. These observers concluded that this was the normal rate at which the impulses descend the nerve to arouse a voluntary contraction. One difficulty in this conclusion is that when human muscle is excited by eight to twelve stimuU per second, we obtain, not a tetanic contraction with a few irregularities superposed on it, but a series of single contractions, the so-called clonus. In order to produce a nearly continuous contraction we must employ a vibration frequency of about 30 per second. It has been suggested to get over this difficulty that under normal circumstances the discharge does not travel along all the nerve fibres at the same time, so that the difierent muscle fibres composing the muscle will be in different phases of contraction, and there will be never any large degree of relaxation between the individual contractions of the whole muscle. Von Kries has found that the duration of a muscle twitch may be lengthened by lengthening the duration of the electrical change used to excite the nerve, and has suggested that the normal excitatory process may resemble the prolonged electrical change which can be produced electro-magnetically, rather than the short sudden shock represented by the induced current of an induction-coil. Attempts have been made to decide the question by recording the electrical changes accompanying the natural contractions of a muscle, i.e. those excited reflexly from the central nervous system. It was long ago shown by Loven that a certain discontinuity could be seen in records of the electrical changes obtained from a frog's muscle in the tetanic spasms produced by an injection of strychnine, but according to Burdon Sanderson this discontinuity represents a series of spasms discharged from the central nervous system. Each discharge produces, not a t'svitch, but a continued contraction of short duration. On photographing the electrical changes of strychnine spasm as obtained by a capillary electro- meter, he found that each individual spasm could only be compared to a short tetanus. The most recent investigations of the question we owe to Piper, who made use of the string galvanometer, an instrument much more delicate in the reproduction of rapid changes than is the capillary electro- meter. Piper led off two points in the fore-arm, one electrode being placed about two inches below the bend of the elbow, and the other about'four inches above the wrist. A single stimulus of the median nerve was found 242 PHYSIOLOGY by him to give a typical diphasic variation in the muscles. When the muscles were contracted voluntarily, well-marked oscillations of the galvano- meter wire were obtained, indicating the existence in the muscle of forty- eight to fifty complete diphasic variations in the second (Fig. 93). Piper obtained similar records on leading off other muscles of the body when these were placed voluntarily in a state of contraction, and he concludes therefore that each voluntary contraction, short or long, is a tetanus composed of about fifty fused twitches per second. These results would indicate that Fig. 93. Electrical variations produced by voluntary contractions of human muscle. (Piper.) the impulse, which normally travels down the motor nerve from the anterior cornual cell to the muscle, is discontinuous, and therefore that on leading off a motor nerve to a galvanometer we ought to obtain electrical oscillations of fifty distinct stimuh per second. Dittler has investigated by means of the string galvanometer the electrical changes accompanying the ordinary contractions of the diaphragm, and also those occurring in the phrenic nerve. He finds that both in the muscle and in the nerve there is evidence that each contraction is a fused series of single contractions, evoked by the discharge along the nerve of between fifty and seventy excitations per second. So far therefore the evidence is in favour of the view that voluntary contraction, and, one must add, the tonic contractions of all skeletal muscles, are discontinuous in nature and analogous to the tetanus which we may evoke artificially by rapid stimulation either of muscle or of its motor nerve. SECTION X OTHER FORMS OF CONTRACTILE TISSUE SMOOTH OR UNSTRIATED MUSCLE The little we know about the physiology of unstriated muscle is derived chiefly from experiments on the intestine, ureter, bladder, and retractor penis.* This tissue differs from voluntary muscle in containing numerous plexuses of nerve fibres (non-medullated) and ganglion-cells, so that in all our researches it is difficult to be certain whether the results are due to the muscle fibres themselves, or to the nerves and nerve-cells which are so inti- mately connected with them ; especially as we have as yet no convenient drug Uke curare, by aid of which we might discriminate between action on muscle and action on nerve. The differences between unstriated and voluntary muscle, although at first sight very pronounced, on further investigation prove to be in most cases differences of degree only, quahties and reactions which are marked in involuntary muscle being also present in a minor degree in the more highly differentiated tissue. The contraction of smooth muscle is so sluggish that the various stages of latent period, shortening, and relaxation can be easily followed with the eye. The latent period may be from 0-2 to 0-8 second, and the contraction may last from three seconds to three minutes. Smooth muscle preserves many of the properties of undift'erentiated protoplasm, especially an automatic power of contraction, which is regulated by the condition of the muscle. Thus whereas the voluntary muscle is intimately dependent on its connection with the central nervous system, and in the absence of this is reduced to a flabby inert tissue, the smooth muscle, isolated from all its nervous connections, presents in many cases rhythmic contractions, and can carry out a peripheral adaptation to its environment. These rhythmic contractions are almost invariably observed * The retractor penis, which is found in the dog, cat, horse, hedgehog (but not in rabbit or man), is a thin band of longitudinally arranged unstriated muscle, which is inserted at the attachment of the prepuce, and is continued backwards in a sheath of connective tissue to the bulb, where it divides into two slips, which pass on eitlier side of the anus. It is innervated from two sources, the motor fibres being derived from the lumbar sympathetic and running to the muscle in the internal pudic nerve, while the inhibitory fibres run in the pelvic visceral nerves (nervi erigentes) and are derived from the second and third sacral nerve-roots. •243 244 PHYSIOLOGY if the muscular tissue be subjected to a certain amount of tension, after separation from the central nervous system. The rhythm of the contrac- tions may vary from one (spleen) to twelve (small intestine) contractions in the minute. The stimuli for smooth muscle are essentially the same as for striated. As we should expect, however, from the sluggish response of this kind of contractile tissue, the optimum rate of change of current which excites is very much slower than in the case of striated muscle. Thus in many in- stances a single induction shock, even if very strong, is powerless to excite contraction, and the make-induction shock of long duration and low intensity is always more efficacious than the short sharp break-induction current. A still better stimulus is the make or break of a constant current. When the latter form of stimulation is used, response occurs at the make sooner than at the break, and, just as in the voluntary muscle, the make excitation starts from the cathode and the break excitation from the anode. An apparent exception to this statement is afforded by the behaviour of certain forms of involuntary muscle. In the intestine, in the skin of worms, and in many other muscular tubes the smooth muscle-fibres are arranged in two different sheets, one consisting of longi- tudinal, the other of circular fibres. If non-polarisable electrodes, connected with a constant source of ciurent, be applied to the surface of the small intestine, when the current is made K A there will be apparently a strong con- FiG. 94. At the cathode'K'there is a small line traction of the circular coat at the of constriction, surrounded by an area of anode, which spreads up and down the relaxation. At the anode itself the muscle intestine, and a linear contraction of IS relaxed, but is strongly contracted on each . side of the anode, so that on rough observation the longitudinal coat at the cathode, it would be thought that contraction occurred The same result is observed in the at the anode itseK. earthworm and leech. But careful observation shows in each case that the irregularity is really only apparent, and that in the immediate neighbourhood of the anode there is relaxation of both coats, with a contraction of the circular coat on each side, and that at the cathode there is a contraction of both coats. The accompanying diagram (Fig. 94) will serve to show the condition of the circular coat at each electrode. As a matter of fact, in con- sequence of the arrangement of the fibres, we have in the neigh- bourhood of the anode a number of places (virtual cathodes) where the currc nt is leaving the muscle- cells to enter inert conducting — tissues, and in the same way ^ there will be in the neighbom'- hood of the cathode a number of virtual anodes (Fig. 95). Thus if we take the ureter and lead a current through it while it is slung up in thread loops serving as electrodes, there is contraction of both coats at the cathode and relaxation of both at the anode. If, however, the ureter be packed in a pulp of blotting-paper 95. Diagram to show the spread of current which occurs when a current is led through a tube such as the ureter by means of two electrodes applied to its surface. It will be noticed that while + E is the anode, there are immediately below and around it a number of cathodes, E^, E^,, E,^,, E,^,, due to the current leaving the muscle to flow through indifferent tissues. (Biedermann.) OTHER FORMS OF CONTRACTILE TISSUE 245 moistened with normal saline, thus allowing the current to leave the contractile tissues anywhere along the ureter, we get the same aberrant results of stimulation as are obtained with the intestine. In voluntary muscle, if one stimulus follows another at an interval which is not too large, a summated contraction is produced which is greater in amphtude than that due to a single stimulus. This summa- tion may be mechanical or physiological, the former being observed when the stimulus is repeated during the decline of the excitatory process and being due simply to the after-loading of a muscle by the first contraction. It is best marked when the muscle is heavily loaded. If, however, the stimuU be sent in at sufficiently short intervals so that two stimuli fall within the period of rise of contractile stress, an increased height of contraction is obtained under all conditions, and under isometric conditions the tension developed is greater than that with a single stimulus. If the interval between two stimuh be so short that the second falls within what we have called the refractory period due to the first stimulus, no summation is obtained, the second stimulus being ineffective. In the slow contraction of involuntary muscle we could hardly expect mechanical summation to come into play. Most types of this tissue show, however, the true summation, i.e. the increased hberation of energy due to repetition of the stimulus during the rise of the excitatory condition. As might be expected the refractory period is also longer in involuntary muscle, since all the processes of this muscle are slowed in comparison with those of voluntary muscle. In certain types of tissue, and especially in heart muscle, the refractory period lasts during the whole of the period of contraction. During this time therefore a second shock ^^^U be ineffective. As the contraction dies away the muscle fibre gradually recovers its sus-- ceptibility to stimulation, but it does not recover its full irritabiUty until it has entirely relaxed. On this account it is impossible to obtain summa- tion in or to tetanise heart muscle, the apphcation of interrupted currents to this tissue producing only a series of rhythmic contractions. In all involuntary muscle we may observe summation of the effects of stimuli even when the individual stimuh are insufficient to produce any excitation. Thus in a muscle such as the retractor penis, we may find a strength of induction shock which, appUed singly, is just insufficient to evoke any response. If, however, the shocks are repeated at intervals of a second, it will be found that the first three or four stimuh are ineffective and then the muscle enters into a contraction which increases with each succeeding stimulus until it has attained its maximum. There is thus summation before any contraction has occurred, a summation of stimuli. Each stimulus, in fact, alters the state of the contractile tissus and makes it more ready to respond to the next stimulus, so that the stimuli become more and more effective. If time is allowed for the muscle to relax between successive stimuh, this summation is evidenced by a continually increasing height of contraction, the so-called ' staircase.' The same initial increase of 246 PHYSIOLOGY efiect is observed when voluntary muscle is excited by continually recurring stimuU {v. Fig. 70, p. 209). We shall meet with other examples of this summation of stimuh when deahng with the physiology of the central nervous system. It is indeed a fundamental phenomenon in the physiology of excitation. CHEMICAL STIMULATION. Strong salt solution excites contractions just as in the case of skeletal muscle. Many drugs, such as physostigmine, ergot, salts of lead and barium, digitalis, may act directly on smooth muscle and cause contraction. As one would expect, however, from the greater independence of the smooth muscle, the action of these drugs varies from organ to organ, muscle-fibres, which apparently are histologically identical, reacting diversely according to their origin. MECHANICAL STIMULATION. Smooth muscle may react to a local pinch or blow with a local or a general (propagated) contraction. The most important form of mechanical stimulation is that produced by tension. The efiect of increasing the tension on smooth muscle may be twofold : causing in the first place relaxation and in the second excitation with in- creased contraction. These two efiects may be illustrated by taking the case of the bladder. If this viscus (which is surrounded by a complete coat of smooth muscle) has all its connections with the central nervous system severed, it is when empty in a state of tonic contraction. If fluid be injected into it rapidly there is a great rise of pressure in its cavity, due to the forcible distension. If, however, the fluid be injected slowly the bladder muscle relaxes to make room for it, so that a considerable amount of fluid may be accommodated in the bladder without any great rise of pressure. This process of relaxation has its limit. If the injection of fluid be continued the walls begin to be stretched passively, and this increased tension acts as a stimulus causing marked rhythmic contractions of the whole bladder. In the same way the response of a smooth muscle to an electrical stimulus is much increased by previous increase of the tension on the muscle fibres. PROPAGATION OF THE EXCITATORY STATE, OR WAVE OF CONTRACTION. On stimulating any part of a voluntary muscle fibre, a wave of contraction is started which travels to each end of the fibre, but no further. There is no propagation from muscle fibre to muscle fibre, the synchronous contraction of the whole muscle being brought about by simultaneous excitation of all its fibres. It is doubtful whether this isolation of the excitatory state is found in smooth muscle. As a rule a stimulus applied to any part of a sheet of smooth fibres may travel all over the sheet just as if it were a single fibre. It seems probable indeed that there is protoplasmic continuity by means of fine bridge-like processes between adjacent muscle cells. And even in the absence of such bridges the propaga- tion of the contraction could be easily accounted for. Although in the case of voluntary muscle the rule is isolated contraction, yet a very small change in the muscle, such as that produced by partial drying or by pressure, is sufficient to cause the contraction to spread from one fibre to another. OTHER FORMS OF CONTRACTILE TISSUE 247 Indeed by clamping two curarised sartorius muscles together, as in the diagram (Fig. 96), it is found that stimulation of the muscle a causes con- traction of the muscle b. The current of action of a in this case has served to excite a contraction in b. a! It must be remembered that in all unstriated muscle the fibres are sur- rounded by a network of non medullated nerve fibres. Some physiologists are inclined to ascribe to these fibres an important part in the propagation of the contraction wave. In the case of the heart muscle, however, it can be shown almost conclusively that thein'opagation take splace independently of nerve fibres, and i^robably the same is true for many kinds of involuntary muscle. INFLUENCE OF TEMPERATURE. Smooth muscle is extremely sus- ceptible to changes of temperature ; as a rule warming causes relaxation, while appUcation of cold causes a tonic contraction. The condition of the muscle at any given time depends not only on its actual temperature, but also on the rapidity with which this temperature has been reached. Thus a rapid coohng of the retractor penis muscle of a dog from 35° to 25° may cause a contraction as extensive as would be produced by a slow coohng to 5° C. On warming a muscle from 30° to 50° C. it lengthens gradually up to about 40°, and it may then undergo a marked heat contraction (varying in degree in different muscles) at about 50° C, which may pass off at a somewhat higher temperature. It is killed somewhere between 40° and 50° C. It seems very doubtful whether any true rigor mortis occurs in smooth muscle. The hard contracted appearance of the smooth muscle in a recently dead animal is chiefly conditioned by the fall of temperature. On excising the muscle and warming it up to body temperature it may again relax and show signs of irritabihty two or three days after the death of the animal. Different smooth muscles, however, vary very much in their tenacity of life. DOUBLE INNERVATION. Voluntary muscle is absolutely dependent for its activity on the central nervous system. Cut of? from this it is flabby and motionless. Its sole fmiction is to contract efficiently and smartly on receipt of impulses arriving along its nerve. It is only necessary therefore that these impulses should be of one character — motor, and we know that each fibre of a muscle, such as the sartorius, receives one ett"erent nerve fibre terminating in an end-plate. In the case of smooth muscle we have a tissue which has an activity and reactive power of its own, and apart from its innervation may be at one time in a state of relaxation, at another in a state of tonic contraction. Fig. 97. Tracing from tho retractor penis nivisole of the dog. showing lengthening (inhibition) on stimulation of the nervus erigens, and a smart contraction on stimulating the pudic (motor) nerve. (Movements of muscle re- duced I.) 248 PHYSIOLOGY In order that the central nervous system should have efficient control over such a tissue, it must be able to influence it in two directions : it must be able to induce a contraction or increase a contraction already present, and it must also be able to put an end to a spontaneous contraction, i.e. to induce relaxation. In order to carry out these two effects, smooth muscle receives nerve fibres of two kinds from the central nervous system, one kind motor, analogous to the motor nerves of skeletal muscle, the other kind inhibitory, causing relaxation or cessation of a previous contraction. All these fibres belong to the visceral or ' autonomic ' system. They are connected with gangHon-cells in their course outside the central nervous system, and their ultimate ramifications in the muscle are always non-medullated, A typical tracing of the opposite effects of these two sets of nerves is given in Fig. 97. In the invertebrata many ' voluntary ' striated muscles probably possess a double innervation. Thus in the crayfish the adductor muscle of the claw consists of striated muscular fibres, every fibre of which is supplied with two kinds of nerve fibres. By exciting these fibres one may get, according to the conditions of the experiment, either contraction of a relaxed muscle or relaxation of a tonically con- tracted muscle (Fig. 98). AMCEBOID MOVEMENT Fig. 98. Tracing of contraction of adductor muscle of claw of crayfish, showing inhibition re- Amceboid movement is Seen in the uni- sulting from stimulation of its ti i • i ,■, r. ^ nerve (at b) by means of a con- cellular orgamsms such as the amoeba and stant current. The break of the in the white blood corpuscles. It can OCCUr current causes a second smaller , .,,. ,. ■,■ -, r , inhibition. (BiEDEEMANN.) ouly Within certain hmits of temperature (about 0° C. to 40°) ; within these limits it is the more active the higher the temperature. At about 45° the cell goes into a condition resembhng heat rigor. The fluid in which the corpuscles are suspended is of great importance. Distilled water, almost all salts, acids and alkaUes, if strong enough, stop the action and kill the cell. The movements are also stopped by CO 2 or by absence of oxygen. Artificial excitation, whether electrical, chemical, or thermal, causes universal contraction of the corpuscle, which therefore assumes the spherical form. CILIARY MOVEMENT Ciha are met with in man in nearly the whole of the respiratory passages and the cavities opening into them, in the generative organs, in the uterus and Fallopian tubes of the female, and the epididymis of the male, and on the ependyma of the central canal of the spinal cord and its continuation into the cerebral ventricles. The ciha (Fig. 99) are dehcate tapering filaments which project from the hyaline border of the epithehal cells. There are about twenty or thirty to each cell. The hyaline border is really made up of the enlarged basal portions of the ciha. -j OTHER FORMS OF CONTRACTILE TISSUE 249 111 action the cilia bend suddenly down into a hook or sickle form, and then return slowly to the erect position. This movement is repeated many (twelve to twenty) times a second, and thus serves to move forward mucus, dust, or an ovum, as the case may be. The movement seems to be entirely automatic, and it is quite unaffected by nerves, at any rate in all the higher animals. There is, however, a functional connection between all the cells of a cihated epithelial surface, so that movement of the ciha, started in one cell, spreads forward as a wave, just as, when the ■\\and blows, waves of bending pass over a field of corn. The conditions of ciliary action are the same as those for amoeboid movement of naked cells. The minuteness of the object has up to now prevented us from deciding whether the cilium is itself actively contractile, or whether it is simply passively moved by the action of the basal part situated in the hyaline border of the cell. rro^ nV rn." Fig. 99. Ciliated columnar epithelium from the trachea of a rabbit mucus-secreting (SCHAFER.) cells. CHAPTER VI NERVE FIBRES (CONDUCTING TISSUES) SECTION I THE STRUCTURE OF NERVE FIBRES ON stimulating the nerve of a nerve-muscle preparation at any part by electrical, thermal, or mechanical means, the stimulus is followed, after a very short interval, by a contraction of the muscle. This observation illus- trates the two functions of nerve fibres, irritability and conductivity — that is to say, a suitable stimulus can set up changes in any part of the nerve, which are trans- mitted down the nerve without any visible effects occurring in it, and it is not until this nervous change has reached the muscle that a visible effect takes place in the shape of a contraction. In the animal body a direct excitation of the nerve fibre in its course never takes place under normal circumstances. The only func- tion the nerve fibre has to perform is that of conducting impulses from the sense organs at the periphery to the central nervous system, and efferent impulses from this to the muscles and other of its servants. Hence it is absolutely es- sential that there should be vital continuity along the whole length of the fibre. Damage to any part, such as by crushing, heat, or any other injurious condition, infallibly causes a block to the passage of an im- pulse. A nerve fibre is essentially a long pro- cess or arm of a nerve-cell (Fig. 100). The Fig. 100. Diagram of a motor nerve- ^ell may either be situated on the surface of cell with its nerve fibre. (After , , t . ■ ,t ^ ■ ^ Bakkbr.) the body or, as m most cases m the higher a.h, axon hillock ; d, dendrites ; animals, may be withdrawn from the a.x, axis cylinder ; m, medullary c • , • i n j_- <• n ^ sheath- n.ii, node of Kanvier. surtace mto a special collection 01 cells sucn 250 THE STRUCTURE OF NERVE FIBRES 251 as the posterior root ganglion, or may be one of the mass of cells and inter- lacing processes making up a central nervous system. All nerves are alike in possessing as their conducting part the continuous strand of protoplasm produced from the nerve-cell and known as the axon or axis cyhnder. By special methods the axon may be shown to be made up of fibrillse or neuro-fibrils, embedded in a more fluid material (Fig. 101). These neuro- FiG. 101. Medullcatcd nerve fibres, showing continuity of the neuro-fibrils across the node of Ranvier. (Bethe.) a, longitudinal ; h, transverse section. fibrils are supposed to be continuous throughout the cell and the axis cyhnder and to represent the essential conducting constituents of the nerve. In the course of growth the nerves develop certain histological differences, which appear to bear some relation to the nature of the processes they con- duct or to the character of their parent cell. Thus all the fibres which are given off from and which enter the central nervous system, i.e. the brain and spinal cord, belong to the class known as medullated. In this type the conducting core or axis cyhnder is surrounded with a layer of apparently insulating material known as myehn, forming the medullary sheath, or the sheath of Schwann. This sheath consists of a fatty material composed largely of lecithin, and staining black with osmic acid, supported in the interstices of a network formed of a horny substance knOwn as neurokeratin. The medullary sheath is surrounded by a structureless membrane, the primitive sheath or neurilemma. At regular intervals a break occurs in the medullary sheath, the neurilemma coming in close contact with the axis cyhnder. This 252 PHYSIOLOGY break is the node of Ranvier, the intervening portions of medullated nerve being the internodes. In each internode, lying closely under the neurilemma, is an oval nucleus embedded in a little granular protoplasm. The medullated nerve fibres vary considerably in diameter, the largest fibres being distributed to the muscles and skin, the smallest carrying impulses from the central nervous system to the viscera. The latter all come to an end in some collec- tion of ganghon-cells of the sympathetic chain or peripheral ganglia, the impulses being carried on to their destination by a fresh relay of non- medullated nerve fibres. The non- medullated fibres (Fig. 102) differ from the medullated simply in the absence of a medullary sheath. They possess, in many cases at any rate, a primitive sheath, under which we find nuclei lying closely on the side Fig. 102. Non-meduUated nerve fibres. (Schaper.) of the fibre and bulging out the sheath. In their ultimate ramifications they tend to form close networks or plexuses and appear to lose the last traces of a sheath. The medullated nerves are bound together by connective tissue (endo- neurium) into small bundles, which are again united by tougher connective tissue into larger nerve-trunks. These fibres as a rule branch only when in close proximity to their destination, and then the branching always occurs at a node of Ranvier. As to the functions of the myehn sheath in the medullated nerve fibre very httle is known. It does not make its appearance until the axis cylinder is formed, and is apparently derived from a series of cells which grow out from the spongioblasts of the central nervous system and form a chain surrounding the out-growing axons. In the regeneration of a nerve fibre after section the myehn sheath appears later than the axon in the peripheral part of the nerve. It has been supposed by some to act as a sort of insulator ensuring isolated conduction within any given nerve fibre. We have, however, no proof that equally isolated conduction is not possible in the non- medullated fibres of the visceral system, although it is certainly true that a finer ordering of movements is required in the skeletal muscles than in the visceral unstriated muscles. Moreover in the central nervous system the main tracts cannot be shown to be functional before the date at which they acquire their medullary sheaths, suggesting that previously any impulse making its way along the tract underwent dissipation before arriving at its destination. It is possible too that the myehn sheath may serve as a source of nutrition to the enclosed axis cylinder, which, in the greater part of its course, is far removed from its trophic centre, namely, the cell of which it is an outgrowth. This trophic function of the myehn sheath has a certain basis of fact in that the myelin sheath is as a rule larger in those fibres which take the longer course. SECTION II PROPAGATION ALONG NERVE FIBRES The velocity of propagation along a nerve fibre may be measured, although in early times it was thought to be as instantaneous as the lightning flash. To measure the velocity of propagation in a motor nerve, a frog's gastroc- nemius is prepared, with a long piece of sciatic nerve attached. The muscle is arranged (Fig. 103) so that its contraction may be recorded on a rapidly moving surface, on which are also recorded, by means of electro-magnetic Fig, 103. Diagram of arrangement of experiment for the determination of the velocity of transmission of a motor impulse down a nerve. The battery current passes through the primary coil of the inductorium c, and a ' kick over ' key k. By means of the switch s, the break shock in the secondary cu'cuit can be sent through the nerve n, either at b or at a. The muscle m is arranged to write on the blackened surface of a trigger or pendulum myograph, and is excited during the passage of the recording surface by the automatic opening of the key k. (The time-marker is not shown.) signals, the moment at which the stimulus is sent into the nerve, and also a time-marking showing ., 1 ,, sec. Tracings are now taken of the contraction of the muscle : first, when the nerve is stimulated at its extreme upper end ; secondly, as close as possible to the muscle. It will be found that the latent period, which elapses between the point at which the stimulus is sent into the nerve and the point at which the lever begins to rise, is rather longer in the first case than in the second. The dift'erence in the two latent periods gives the time that the nervous impulse has taken to travel down the length of nerve between the two stimulated points. Calculated in this way, the velocity of propagation in frog's nerve is about 28 metres per second. In man and in warm-blooded animals the velocity has been variously estimated at from 60 to 120 metres per second. The higher of these figures is probably nearer the truth. 253 254 PHYSIOLOGY On the other hand, in invertebrata the velocity of propagation along nerve fibres may be quite slow. The following Table represents the velocity of transmission along a number of different fibres, as determined by Carlson, compared with the duration of a single muscle-twitch in the same animal. Species Muscle Nerve Contrac- Rate ot 1 Muscle tion time in seconds Nerve the impulse in metres per second Frog Gastrocnemius . \0-10 Sciatic (medullated) 27-00 Snake Hyoglossus 0-15 Hyoglossal (medullated) 14-004 Lobster . Adductor of 0-25 Ambulacral 12-00 (Homarus) forceps (non-meduUated) Hagfish . Retractor of jaw 0-18 Mandibular (non-medullated) 4-50 Limvdus . Adductor of forceps 1-00 Ambulacral (non-medullated) 3-25 Octopus . Mantle 0-50 Pallial (non-medullated) 2-00 Slug (Limax) . Foot 4-00 Pedal (non-medullated) 1-25 Limulus . Heart 2-25 Nerve plexus in heart (non-medullated) 0-40 The velocity of propagation in sensory nerves is more difficult to deter- mine owing to the fact that a sensory impulse, on arrival at the receiving organ — i.e. some part of the central nervous system — does not at once give rise to some definite recordable mechanical change, such as a muscular con- traction. There is another method of determining the velocity of conduction, which may be used also with sensory fibres. The passage of a nerve- impulse down a nerve, just as the passage of a wave of contraction along a muscle fibre, is immediately preceded or accompanied by an electrical change, which also travels along the nerve as a wave of ' negativity.' The velocity of propagation of this wave may be measured, and is found to give the same numbers as the velocity determined by the preceding method. The existence of this electrical change enables us to show that a nerve- impulse, excited at any point in the course of a nerve fibre, travels in both directions along the fibre. The power of nerves to transmit impulses in either direction is shown further by the experiment known as Kiihne's gracihs experiment. The gracihs muscle of the frog is separated into two portions by a tendinous intersection, so that there is no muscular continuity between the two halves. The nerve to the muscle divides into two branches, one to each half, and at the point of junction there is division of the axis cylinders themselves. If the section a in the diagram (Fig. 104), which is quite isolated from the rest of the muscle, be stimulated, as by snipping it with scissors, PROPAGATION ALONG NERVE FIBRES 255 the whole muscle contracts. If the portion of the muscle which is free from nerve fibres be stimulated in the same way, the contraction is limited to the fibres directly stimulated, showing that in the first case the stimulus excited nerve fibres which transmitted the impulse up the nerve to the point of division and then down again to the other half of the muscle. Since nerves have this power of conduction in both directions, it might be thought that a single set of nerve fibres might very well subserve both afferent and efferent functions, at one time conducting sensory impulses from periphery to cord, at another time motor impulses from cord to muscles. But this is not the case. As a matter of fact we find in the body a marked differentiation of function between various nerve fibres. Thus Bell and Majendie showed that the spinal roots might be divided into afferent and efferent, the anterior roots carrying only impulses from spinal cord to periphery, while the posterior roots carried impulses from periphery to central nervous system. The law known by the name of these observers states indeed that a nerve fibre cannot be both motor and sensory. We may find both kinds of fibres joined together into a single nerve-trmik, but the fibres in each case are isolated and conduct impulses only in one or other direction. Under normal conditions the afferent fibres are excited only at their endings on the surface of the body, while the eft'erent fibres are excited only at their origin from the spinal cord. The difference in the function of different nerve fibres depends therefore not so much on the structure of the nerve fibre itself as on the connections of the fibre. We can show this experimentally by grafting one set of nerve fibres on to another. If the cervical sympathetic be united to the Ungual nerve, stimulation of the spnpathetic, instead of causing, as usual, constriction of the vessels of the head and neck, will cause dilatation of the vessels of the tongue and secretion of watery saliva. In the same way the finer functional differences between the various forms of sensory nerves seem to be deter- mined by their connections within the central nervous system. Stimulation of the optic nerve by any means whatsoever evokes a sensation of light. One and the same stimulus applied to different nerves will evoke different sensations, e.g. a tuning-fork applied to the skin will give a sensation of vibration, to the ear a sensation of sound. We shall have occasion to return to this question of the restricted function of nerve fibres when we deal with Miiller's ' law of specific irritability ' in the chapter on Sensations. Fig. 104. Kiihne's gracilis experiment. SECTION III EVENTS ACCOMPANYING THE PASSAGE OF A NERVOUS IMPULSE In muscle we saw that the passage of an excitatory wave was accompanied or followed by electrical changes, production of heat, and mechanical change, all pointing to an evolution of energy from the explosive breaking-down of contractile material. In nerve, however, which serves merely as a conducting medium, we should not expect so much expenditure of energy, or in fact any expenditure at all. All that is necessary is that each section of the nerve should transmit to the next section just so much kinetic energy as it has received from the section above it. And experiment bears out this conclusion. The most refined methods have failed to detect the slightest development of heat in a nerve during the passage of an excitatory process, and we know already that there is no mechanical change in the nerve. The only physical change in a nerve under these circumstances is the development of a current of action. A nerve becomes, when excited at any point, negative at this point to all other parts of the nerve, and, just as in muscle, this ' negativity ' is propagated in the form of a wave in both directions along the nerve. That the excitatory process in nerves is probably accompanied by certain small chemical changes is indicated by the facts that, in the complete absence of oxygen, the nerve fibres lose their irritability, and that this loss of irritability is hastened by repeated stimulation of the nerve. When the irritability has been abolished by stimulation in the absence of oxygen, it may be restored within a few minutes by readmission of oxygen to the nerve. If we connect a galvanometer to two points of an uninjured nerve, no current is observed, all points of a living nerve at rest being isoelectric. On making a cross-section of the nerve at one leading-ofi point, a current is at once set up, which passes from the surface through the galvanometer to the cross-section. This is a demarcation current, set up at the junction between living and dying nerve. This current rapidly diminishes in strength and finally disappears, owing partly to the fact that the dying process started in the nerve by the section extends only as far as the next node of Ranvier and there ceases, so that after a short time the electrode apphed to the cross- section is simply leading off an intact living axis cyhnder through the dead portion of the nerve, which acts as an ordinary moist conductor. On making a fresh section just above the previous one, the process of dying is again set 256 EVENTS ACCOMPANYING A NERVOUS IMPULSE 257 up, and the demarcation current is restored to its original strength. If, while the demarcation current is at its height, we stimulate the other end of the nerve with an interrupted current, the needle of the galvanometer swings back towards zero, i.e. there is a negative variation of the resting current. In order to demonstrate the wave-hke progression of the electrical change from the excited spot along the nerve, it is necessary, as in the case of muscle, to make use of a very sensitive capillary electrometer or a string galvano- meter. It is then found that the change progresses along the nerve at the same rate as the nervous impulse, i.e. 28 to 33 metres per second in the frog. But it lasts only an extremely short interval of time at each spot, ^^z. six to eight ten-thousandths of a second. Thus the length of the excitatory wave in nerve is about 18 mm. SECTION IV CONDITIONS AFFECTING THE PASSAGE OF A NERVOUS IMPULSE TEMPERATURE. Below a certain temperature the propagation of the excitatory process in the nerve is absolutely abohshed. The exact tempera- ture at which this occurs varies according as we use a warm- or a cold- I blooded animal. In the frog it is necessary to cool the nerve below 0° C. before con- i ^ .^ ^ iL duction is abohshed, whereas in the mammal I I it is sufficient to cool the nerve to somewhere t— I between 0° and 5° C. Since coohng the nerve does not excite it, this procedure forms a convenient method for blocking the passage of impulses along a nerve without using the irritating procedure of section. On warming the nerve again the conductivity returns. The rapidity with which the excita- tory process is propagated along either a nerve or a muscle fibre depends on the temperature. Thus the mean rate of conduction in the frog's nerve at 8° to 9° C. is about 16 metres per second. The temperature coefficient of the velocity of nerve propagation, i.e. velocity at Tn + 10 , , - tit —_ has been found by Lucas velocity at Tn to be about 1-79. The same value was found by Maxwell for conduction in molluscan nerve, and in frog's striated muscle Woolley found the temperature coefficient for con- duction of the excitatory process to vary between 1-8 and 2. An ingenious method (Fig. 105) has been used by Keith Lucas for the determination of the conduction rates in nerve at different temperatures. The glass vessel repre- sented in the figure is filled with Ringer's solution, in which the whole nerve-muscle preparation is immersed. The muscle used was the flexor longus digitorum, so that the whole length of the sciatic, tibial, and sural nerves could be used. The nerve is passed up through the constrictions in the inner glass vessels at c and D, and is attached to the thread e. f, i, and G are three non-polarisable electrodes composed of porous 258 CONDITIONS AFFECTING A NERVOUS IMPULSE 259 aii|||| iiiifiiiiiiiii iiiiifiirin c clay, containing saturated zinc sulphate, in which a zinc rod is immersed. If the current is passed in at G and out at F the effective cathode is at the lower end of the constriction c, and similarly if the current is passed in at i and out at G, the effective cathode is at D. The tendon of the muscle A is attached by a thin glass rod H to a very light recording lever, the movement of which is magnified by placing it in the focal plane of a projecting eye-piece and recording its image on a moving sensitive plate. The whole apparatus, with the exception of the glass rod at H, can be immersed in a water bath at any given tempera- ture. Two records are taken with the whole apparatus, first stimulating at c, and secondly stimulating at d. The difference between the latent periods in these two cases is the time taken for the excitatory wave to travel from d to c. The rate of propagation is similarly recorded when the water bath is i-aised to 18° C. or to any desired tempera- ture. Since we are only dealing with differences in latent periods the effect of the rise of temperature on the latent period of the muscle itself does not affect the determinations. THE INFLUENCE OF FATIGUE. In the description of the phenomena of mus- cular fatigue given in ^j^ iqq the last chapter it was assumed that the muscle was being ex- cited directly. The same phenomena are observed when the muscle is excited through its nerve, though in this case fatigue comes on much more quickly. If, after the muscle has been excited in this way until exhausted, it be excited directly, it will respond with a contraction nearly as high as at the beginning of the experiment. We see therefore that the nervous structures are more susceptible to the influences causing fatigue than the muscle itself, and it can be shown that the weak point in the nerve- muscle preparation is not the nerve, but the end-plates. In fact it is not possible to demonstrate any phenomena of fatigue in the nerve-trunk.* This fact can be shown in mammals by poisoning the animal with curare, and then stimulating a motor-nerve continuously while the animal is kept alive by means of artificial respiration. As the effect of the curare on the end- plates begins to wear off in consequence of its excretion, the muscles supplied by the stimulated nerve enter into tetanus. The action of the curare may be cut short at any time by the injection of salicylate of physostigmine, when the muscles will at once begin to react to the excitation. The same fact may be shown on the excised nerve-muscle preparation of the frog. The gastrocnemii of the two sides with the sciatic nerves are dissected out, and an exciting circuit is so arranged that the interrupted * Unless it be asphyxiated by total deprivation of oxygen. Curve of muscle-twitch obtained by forcgoinc method. (Keith Lucas.) moment of excitation, b = movement of muscle, c = time-marker. 260 PHYSIOLOaY secondary currents pass through the upper ends of both nerves in series (Fig. 107). At the same time a constant cell is connected with two non-polarisable electrodes {np, np) placed on the nerve of b, so that a current runs in the nerve in an ascending direction. The effect of passing a constant current through a nerve is to block the passage of impulses through the part traversed by the current. When the con- stant polarising current is made, the muscle may give a single :^ twitch, and then remains quies- cent. The exciting current is then sent through both nerves by the electrodes e, and e,. The 1 "-L^"^ '^2- muscle A enters into tetanus, which gradually subsides owing to "fatigue." When A no longer responds to the stimulation, the constant current through the nerve of B is broken. B at once enters into tetanus, which lasts as long as the contraction did in the case of A, and gradually subsides as fatigue comes on. Since both nerves have been excited through- out, it is evident that the fatigue does not affect the nerve-trunk. We have already seen that a muscle will respond to direct stimulation when stimulation of its nerve is without effect, and must therefore conclude that the first seat of fatigue is the junction of nerve and muscle, i.e. the end-plates. In the normal intact animal the break in the neuro-muscular chain which is the expression of fatigue occurs still higher up, i.e. in the central nervous system, and is probably due to some reflex inhibition of the central motor apparatus from the muscle itseK. Thus after complete fatigue has been produced in a muscle so far as regards voluntary efforts, direct stimulation of the muscle itself or its nerve will produce a contrac- tion as great as would have been the case at the beginning of the experiment. THE INFLUENCE OF DRUGS. The most important drugs with an influence on nerve fibres are those belonging to the class of anaesthetics. Of chloroform, and alcohol. The action of any of these substances on the excitability and conductivity of a nerve maybe studied by means of the simple apparatus represented in Fig. 108. The nerve A Fig. 107. Arrangement of experiment for demon- ^ strating the absence of fatigue in medullated nerve fibres. EC, exciting circuit ; cp, polarising circuit. Fig. 108. these we may mention carbon dioxide, ether, CONDITIONS AFFECTING A NERVOUS IMPULSE 261 of a nerve-muscle preparation is passed through a glass tube which is made air-tight by plugs of normal saline clay surrounding the nerve at the two ends of the tube. By means of two lateral tubulures a current of CO2, or air charged with vapour of ether or other narcotic, can be passed through the tube. The nerve is armed with two pairs of elec- trodes which are stimulated alternately, the pair within the tube serving to test the action of the drug on the excitability, while the pair outside the tube show the presence or absence of any effect on the conducting potver of the nerve below it. Of the gases and vapours mentioned above, COg and ether both diminish and finally aboUshTthe excitability and conductivity of the nerve fibres. The conductivity, however, persists after all trace of excitability has dis- appeared, before in its turn being also abohshed. On removing the gas Fig. 109. Tracing to show the effect of ether on excitability and conduclivity of nerve. Nerve excited by single induction shocks alternately within and above ether. cbanib:'r. The vertical lines indicate contractions of the muscle (gastrocnemius.) The lower line indicates the periods during which the nerve was exposed to the action of ether. A, disappearance of excitability ; B, reappearance of excitabilit;/ ; c, disappearance of conductivity ; n, reappearance of conductivity.' (From a tracing kindly lent by Prof. Gotch. ) or vapour by blowing air over the nerve, the conductivity and excitabihty gradually return in the reverse order to their disappearance (Fig. 109). Alcohol is said to increase the excitabihty or leave it unaffected, while diminishing the conductivity of the nerve. Chloroform rapidly abolishes both excitabihty and conductivity. It is a much more severe poison than the drugs just mentioned, so that in many cases its effects are permanent, and no, or only a very partial, recovery of the nerve is obtained on removal of the chloroform vapour from the apparatus. SECTION V THE EXCITATION OF NERVE FIBRES Many different forms of stimuli may be used to arouse the activity of an excitable tissue such as muscle or nerve. Thus we may use thermal, mechani- cal, or chemical stimuH. If the temperature of a motor nerve be gradually raised, no effect is noticed till about 40° C. is reached, when the muscle may enter into weak quivering contractions. Sudden warming of the nerve always gives rise to excitation. At about 45° C. the nerve loses its irritabihty and dies. On the other hand, a nerve may be rapidly cooled without any excitation taking place. A nerve may be excited mechanically by crushing or cutting. These methods destroy the nerve. It is possible to excite a nerve mechanically, without any serious injury to it, by carefully graduated taps, and this method has been used in investigating the phenomena of electronus. All chemical stimuh apphed to the nerve have a speedy effect in destroying its irritabihty. The chemical stimuli most used are strong salt solutions, glycerin, or weak acids. If any one of these be appUed to a motor nerve, the muscle enters into an irregular tetanus, which lasts till the irritabihty of the nerve is destroyed at the part stimulated. None of these forms of stimuH can be adequately controlled either as to strength or duration. Moreover, owing to their destructive effects, any repetition of the stimulus will fall on a nerve or muscle more or less altered by the first stimulus. We are therefore justified in the use of electrical stimuH not only for arousing the activity of excitable tissues, but also for determining the conditions of excitation of muscle and nerve. For this pur- pose we may use either the make and break of a constant current, the induced current of short duration produced in a secondary coil of an inductorium by the make or break of the primary circuit, or the discharge of a condenser. The last-named method of stimulation is especially useful when we desire to deter- mine the total amount of energy involved in the electrical stimulation of a nerve or muscle. The arrangement of such an experiment is shown in Fig. 110. By means of the switch S the condenser can be put into connection either with the battery from which it receives its charge or with the nerve through which it can discharge. By knowing the capacity of the condenser and the electromotive force by which it is charged, we can estimate the energy of the charge sent through the nerve. E (energy in ergs)* = 5FV^ (F = capacity in microfarads ; V = electromotive force in volts). * An erg is the amount of work producad or energy expended by the action of on3 dym through oub cantimstre. A dyne is the force which will give to a mass of on3 gram an acceleration of one centimetre per second per second. 262 THE EXCITATION OF NERVE FIBRES 263 .^^^ In this way it has been found that the energy of a minimal effective stimulus for frog's nerve is about jo^j-jj of an erg. The amount of energy necessary to excite the nerve will vary with the rate at which the condenser is allowed to discharge through the nerve. Its rate can be modified by altering the resistance in the discharging circuit or by altering the electromotive force of the charge. Tliis method has been adopted by Waller in determining the rate of change at which excitation is obtained with a minimal exj^en- diture of energy, which he calls the " characteristic " of the tissue in question. To this point we shall have occasion to refer later. When using the make and break of a constant current as a stimulus, the first fact of importance is the relation of the seat of excitation to the poles by which the current is led into or out of the ex- citable tissue. We have already seen that when a current is passed through a muscle or nerve the muscle contracts only at make or at break of the current, no propagated excitatory effect being produced during the passage of the cur- rent. The excitation at make is obtained with a smaller current than the excitation at break. Besides this difference in intensity, there is a difference in the point from which excitation starts. A make contraction starts from the cathode, a break contraction from the anode. This is well shown by the two following experiments : {a) A curarised sartorius muscle of the frog (Fig. Ill), with its bony insertions still attached, is fastened at the two ends to two electrodes, which are able to swing when the muscle contracts, and are attached by threads to levers which serve to record the contraction. The middle of the muscle is then fixed by clamping it lightly. A circuit is arranged so that a con- stant current can be sent through the electrodes and the whole length of the muscle. It is found, on making the ciu:rent, that the lever attached to the cathode— that is, to the electrode by which the current leaves the muscle- — rises before the other lever. On the other hand, on breaking the current the lever at the anode rises first, showing that the anodic half of the muscle contracts before the cathodic half. (6) The irritability of a muscle, i.e. its power of responding to a stimulus by contracting, is intimately dependent on the hfe of the muscle. If the muscle be injured or killed at any spot, its irritability at this spot will be therefore diminished or destroyed. Hence, if we stimulate a muscle at the injured spot, no contraction will ensue. This fact may be used to demon- FiG. 110. Arrangement of apparatus for the excitation of a nerve by means of condenser discharges. B, battery ; K, rheochord; c, rider of rheochord ; s, switch (Pohl's re- verser without cross wires) ; c, con- denser ; ?i, nerve ; 7n, muscle ; e, non- polarisable electrodes. ,^-<5 Fia. 111. Sai-torius clamped in middle and attached to levers at either end. 264 PHYSIOLOGY strate the production of excitation at cathode on make, and at anode on break of a constant current. A muscle with parallel fibres, such as the sartorius, is injured at one end, and a constant current passed, first from the injured to the uninjured end, and then in the reverse direction (Fig. 112). It is found in the former case, when the anode is on the injured part (which is therefore less excitable), I vu J -,„^^»/;^'....„j\ that break of the current is ineffec- Uathode ^ anoae(injurecl) tive, and in the latter, when the contraction at make. ,n i • ,l■•^ £ cathode is on the injured surface, that the make stimulus is inefiec- , , tive, shomnff that the part excited anode m kathode (injured) j ? xi, xi, j ^ i ===—-=— ^ corresponds to the cathode at make \S I ^ ~ n ,30 < r> a rna. e. ^^^ ^^ ^^^ anode at break. ^^1'--^'^ With a current of very short .^ , , „ ^ ■'. , , ^ . p , n duration no excitation is produced Fig. 112. Diagram to show the effect oi local . , . , , injury on the excitability of a muscle, b, at break. Every induction shock battery; m, muscle. The arrows indicate ^^^^ ^^ therefore regarded as a the direction oi the current. . " make stimulus, and when such a shock is led through a muscle the contraction in each case will start at the cathode, i.e. the point at which the induction shock leaves the muscle. The results of stimulating nerve fibres are similar to those obtained by stimulating muscle fibres directly. Under normal circumstances, if a constant current be passed through the nerve of a nerve-muscle preparation for a short time, the muscle responds only at the make and the break of the current, remaining perfectly quiescent all the time the current is passing. If the nerve be in a very excitable condition, it is possible that the muscle may be thrown into a tetanus or continued contraction during the whole time that the current is passing (' closing tetanus '). On the other hand, if a strong ascending current be passed through the nerve for a considerable time, the muscle when the current is broken may go into continued contraction, which may last some time. Normally, however, the muscle simply responds mth a single twitch at the make and break of the current, although, on investigating the condition of the nerve during the passage of the current, we find that it is considerably modified. This modification in the condition of the nerve is spoken of as electrotonus, and includes changes in its irritabihty and its electrical condition. To investigate these changes the following apparatus is necessary : two constant batteries, induction-coil, a reverser and keys, a pair of non-polarisable electrodes, and a pair of ordinary platinum electrodes. Fig. 113 represents roughly the arrange- ment of the experiment. A constant current from the battery is led tlirough a part of the nerve by means of non-polarisable electrodes, which are about one inch apart. In this circuit we put a reverser, by means of which the direction of the current of the nerve may be changed at will, and a key to make or break the current. This is the polarising circuit. The other battery is arranged in the primary circuit of the coil, a key being interposed, so that we may use make or break induction shocks, which are ap- plied to the nerve by means of the small platinum electrodes. The tendon of the muscle is cormected by a thread with a lever, which is arranged to write on a smoked surface, so that the height of the contraction can be recorded. ^HE EXCITATION OF NERVE FIBRES 265 We first find the position of the secondary coil, at which the break induction shock is a submaxiiual stimulus, and we employ this strength of stimulus throughout the experiment. The make induction shock is prevented from acting on the nerve by closing a shortcircuiting key in the circuit of the secondary coil. The nerve is now stimulated at various points with a single break induction shock, and the contractions recorded. The heights of these contractions serve to indicate the irritability of the nerve at the point stimulated. We now throw the polarising current into the' nerve. At the make of this current the muscle will prob- ably respond with a twitch which is not recorded. We then test once more the irritability of different points of the nerve, and we find that, when the stimulus is ai^plied near a, the point where the current enters the nerve (anode), the stimulus, which before gave a moderately large con- traction of the muscle, now has either no eft'ect or else produces a very weak contraction. On the other hand, in the region of the cathode the stimulus, which before was submaximal, has now become maximal, as is shown by the increase in the height of the con- traction evoked by the induction shock. We now reverse the direction of the polarising current, so that the current of the nerve runs from k to a. a reversal of the changes in the nerve Fig, 113. Arrangement of apparatus for showing electrotonic changes in irritabiUty. e, exciting current ; p, polarising current ; r, Pohl's reverser. With this reversal of current there is also ; that is to say, the normally submaximal stimulus is maximal when applied near a, and minimal when applied near k. On break of the polarising current the condition of the nerve retui'ns to normal, and the submaximal stimulus is once more submaximal throughout. This retm'n to normal conditions, however, is not immediate, since the fii'st effect of breaking the current is a swing-back, so to speak, past the normal, the diminished irritability at the anode giving place to an increased irritabilitj^ which only gradually subsides. In the same \vaj% immediately after the polarising current has ceased to flow, the neighbourhood of the cathode acquires a condition of diminished irritability, and this only gradually gives place to a normal condition. This experiment teaches us that, when a constant current is passed through a nerve, there is increase in the irritabihty in the nerve near the cathode, and a diminution in irritabihty near the anode. These conditions of increased and diminished irritabihty are spoken of as catelectrotomis and anelectronus respectively. In muscle we have seen that a make contraction always starts from the cathode, and a break contraction from the anode. Now the event that takes place at the cathode on make and at the anode on break of a constant current is, as the last experiment shows us, a rise in irritability, in the former case from normal to above normal, in the latter from subnormal to normal. Hence we may say that the excitation is caused by a sudden rise of irritability, which may be due either to a sudden appear- ance of catelectrotomis, or a sudden disappearance of anelectrotonus. I have said sudden because the steepness of the rise of irritability is a necessary factor in causing excitition. If the polarising current passing through a nerve be slowly and grcidiuilly increased to considerable strength, it will 9* 266 PHYSIOLOGY give rise to no contraction. The degree of suddenness of the rise, which is most beneficial in causing contraction, varies with the nature of the tissue stimulated. Thus it is more rapid in nerve than in muscle, and in pale muscle than in red muscle, and in voluntary muscle than in unstriated muscle. It is evident that there must be, somewhere between the anode and cathode, an indifierent point- — that is to say, a region where the irritability Fig. 114. Diagram to show the variations of irritability in a nerve during the passage of polarising currents of different strengths. The degree of change is represented by the distance of the curves from the base line ; the part of the curve below the line signifying decrease, that above the line increase of irritability. A, anode ; b, cathode ; ?/i, effect of weak current ; y^, medium current ; y^, strong current. It will be noticed that the indifferent point, x, where the curve crosses the horizontal line, approaches nearer and nearer the cathode as the current is increased in strength. (From Foster, after Pflttgbr.) is neither increased nor diminished. We find experimentally that this in- difierent point is nearer the anode when the polarising current is weak, and approaches the cathode as the current is strengthened, so that with very strong currents nearly the whole intrapolar length is in a condition of an- electrotonus (Fig. 114). When a strong polarising current is used, the de- pression of irritabihty at the anode is so marked that no impulse can pass ascending current make excitation blocked at anode. kath. break excitation at anode blocked at kathode. Fig. 115. Diagram to show the blocking effect of a strong constant current passed through the nerve of a nerve-muscle preparation. this region. Thus if we send a very strong ascending current through the nerve, there is no contraction at make. This is owing to the fact that the impulse started at the cathode on make of the current cannot reach the muscle, its passage down the nerve being blocked in the region of the anode (Fig. 115, A). The results of stimulating motor nerves by means of constant currents were studied by Pfliiger and, embodied in a Table, make up what is known as Pfliiger's law. The result of stimulating varies with the strength of a current. THE EXCITATION OF NERVE FIBRES Law of Contraction 267 strength of current Ascending Descending Make Break Make Break Weak . c 0 c 0 Medium . c c C c Strong . 0 Cor T CorT 0 c = contraction. C = strong contraction. T = tetanus. O = no eli'ect. With the weakest currents excitation occurs only at make, since a make-stimulus, i.e. the rise of catelectrotonus, is always more effectual than a break-stimulus, i.e. the disappsarance of anelectrotonus. With currents of moderate strength excitation occurs both at make and break, bsing better marked at make, especially in the case of Fig. 116. Arrangement of experiment to demonstrate Pfliiger's law of contraction. descending currents. With very strong currents we get a contraction at make only when the current is descending, since, when the cm'rent is ascending, the excitation started at the cathode cannot pass the block at the anode. For the same reason a break contraction is obtained only with an ascending current, since at the break of a descending current there is a swingback of the nerve at the cathode to a condition of diminished irritability, which effectually blocks the excitation started higher up the nerve at the anode. The arrangement of the experiment for demonstrating Pfliiger's law is shown in Fig. 116. The strength of the current is graduated by means of the rheochord, the current being led into the nerve by means of nonpolarisable electrodes. It is extremely important in these experiments to avoid any injury or drying of the nerves at either of the two electrodes, since the excitatory effect either at make or break would be abolished by local injury. These results, worked out chiefly on motor nerves, have been con- firmed as far as possible experimentally on sensory nerve, and on muscle and contractile tissues generally, and probably hold good for all irritable Uving tissues. It is said that an anelectrotonus takes some time to attain its full height, and a catelectrotonus reaches its maximum almost directly after the current is made, and that it is on this account that a current of very short duration excites only at the make, the break occurring before the anelectrotonus is developed enough for its disappearance to cause a stimukis. Other things being equal, a current of given strength causes a stronger 268 PHYSIOLOGY on excitation has been suggested by Gotch. s excitation the greater the length of nerve that it flows through. It must be remembered, however, that the nerve oilers considerable resistance to the passage of the current, and so, to keep the current constant while increasing the length of intrapolar nerve, we must largely increase the electromotive force employed. A very convenient method of showing the effect of the length of intrapolar nerve The two sciatic nerves of a frog are dissected out, one of them being in connection with the gastrocnemius. These are first arranged as in Fig. 117. a. h, and c are three non-polarisable electrodes, the terminals of a constant battery being connected to a and c. The position of the rider on the rheo- chordis then ascertained at which make of the current just excites contraction in the muscle of nerve 2, the current in this case passing from a to 6 along nerve 1, and from 6 to c along a small piece of nerve 2. We will suppose that eleven units of current are necessary to produce excita- tion, h is then withdrawn and the nerve 2 laid on a (Fig. 117, b), so that the current can now pass from a to c entirely through a long stretch of nerve 2. On again seeking the minimal stimulus, it will be found that a smaller current is sufficient to excite, contraction being obtained with seven units. Since the length of nerve traversed, and therefore the resistance to the current, are the same in both cases, it is evident that a current is more effective the greater the length of excited nerve that it traverses. A nerve cannot be excited by currents passed transversely across it, since in such cases the anode and cathode he so close to one another in a nerve- fibril, as it is traversed by a current, that their effects counteract one another. ELECTRICAL STIMULI AS APPLIED TO HUMAN NERVES When we attempt to apply the results gained on frog's nerves to man, we are met at once by the difficulty that we cannot dissect out the nerves and apply stimuh to them directly. So usually unipolar excitation is used, one electrode, either anode or cathode, being apphed to the skin over the nerve to be stimulated, and the other to some indifferent point, such as the back. It is evident under these circumstances that the current is concentrated as it leaves the anode and reaches the cathode, and diffuses widely in the body, seeking the lines of least resistance. Thus it is impossible to get pure anodic or cathodic effects. If the anode be applied over the nerve, the current enters by a series of points (the polar zone), and leaves by a second series (the peripolar zone). The polar zone will thus be in the condition of anelec- trotonus, and the peripolar zone in that of catelectrotonus. The current, however, will be more concentrated at the polar than at the peripolar zone, and so the former effect will predominate. These restrictions in the apphca- THE EXCITATION OP NERVE FIBRES 269 tion of the current cause slight apparent irregularities in the law of contrac- tion as tested on man. In stimulating the nerv^es of man for the purpose of determining the con- ditions of the different muscles, we may use either induced currents (generally called faradic stimulation) or the make and break of a battery current (galvanic stimulation). The electrodes are covered with chamois leather moistened with salt solution in order to diminish the resistance of the skin. When it is desired to stimulate any given muscle the stimulating electrode is brought as nearly as possible over the spot where the muscle receives its motor nerve. These ' motor points ' have been mapped out, and reference is generally made to a diagram in determining the point for any given muscle. By reversing the current the stimulating electrode may be made either anode or cathode. It is found that stimulation occurs most easily on closure of the current when the stimulating electrode is the cathode ; with the greatest diffi- culty when the current is broken and the stimu- lating electrode is the cathode. These different contractions are generally represented by capital letters, and the usual relationship is ex- pressed by the statement that CCC is obtained most easily, then ACC and AOC, and finally COC. CCC == cathodal closing contraction. , ACC = anodal closing contraction. AOC = anodal opening contraction. COC == cathodal opening contraction. Fig. 118. Electrodes applied to the skin over a nerve-trunk. In A the polar area is anelectrotonic and the peripolar catelectro- tonic. The former condition therefore preponderates, since the current here is more con- centrated. In B the conditions arc reversed, the polar zone corresponding in this case to the cathode. (Wallee.) When the motor nerve to a muscle has under- gone degeneration the muscle also begins to degenerate, and we find certain alterations in its response to artificial stimu- lation. In the first place, the muscle may fail to respond to induction shocks, while it may show an increased irritability for galvanic shocks. In the .second place, qualita- tive alterations in irritability may be present, so that ACC may be obtained witli a smaller current than CCC. These alterations are spoken of as the ' reaction of degenera - tion.' SECTION VI THE CONDITIONS WHICH DETERMINE ELECTRICAL STIMULATION Foe, every tissue traversed by a current there is a minimum rate of change at which the current through the tissue must be increased or diminished in order to cause excitation. If instead of suddenly making and breaking the current passing through an irritable structure we carry out the change gradually, no excitatory effect is produced, even although the current may finally attain a considerable strength. This fact may be demonstrated by the use of an appara- tus known as the rheonome. A useful form of rheonome is that devised by Lucas (Fig. 119). Two zinc plates d and E, immersed in a saturated solution of zinc sulphate contained in a rectangular cell, are separated from one another by a vulcanite diaphragm. In the diaphragm is a hole G by which the two sides of the vessels are connected. This hole can be closed at any desired rate by a shutter f. When the hole is closed no current can pass between the plates, and the amount which can pass will depend on the extent to which the shutter has been raised. By giving the hole the right shape it is possible to diminish the resistance of the apparatus regularly. If this rheonome be placed in circuit with a battery and an excitable tissue, such as the nerve of a nerve-muscle preparation, we can make a current or break a current through the tissue at any desired rate. Thus the course of the current through the tissue will be represented, not by a vertical line, but by a sloping line which may be given any desired degree of steepness (Fig. 120). If the current be slowly increased through the nerve or be slowly cut off from the nerve, no excitatory efiect takes place, while quickly opening or closing the slmtter will cause excitation. It might be concluded that the excitatory effect of a current increases with 1. The intensity of the current. 2. The rate of change of the current. The second of these conditions needs, however, some correction. As we increase the rate of change of current, by employing in the case of induced currents more and more rapid alternations, we find that the excitatory effect, 270 Fig. 119. Rheonome of Keith Lucas. ELECTRICAL STIMULATION 271 instead of increasing, begins to diminish, and finally disappears, so that high- frequency currents of enormous tension can, as in Tesla's experiments, be led through the body without any apparent physiological effect. On the other hand, by using more sluggish forms of irritable tissue, we may find that even induction shocks are too rapid for effective excitation. Thus the red muscles of the slow-moving tortoise react better to the slow make than to the sudden break induction shock, and many forms of unstriated muscle are unaffected by either make or break shock. There is in fact for each tissue an optimum rate of change varying with the character of the tissue, at which the current necessary to produce a response is at a minimum. This optimum rate of change is spoken of by Waller as the ' characteristic ' of an irritable tissue. A further investigation of the time relations of electrical stimuli by Keith Lucas has thrown important light on the character of the excitatory Tig. 120. String galvanometer records of the change of current obtained by opening the diaphi'agm in the rheonome (Fig. 119) at different rates. (K. Lucas.) response itself. The difference between various excitable tissues is perhaps best brought out by finding the minimum strength of current which will excite at make and then determining how much this current must be in- creased when it is broken at a very short interval of time after it has been made. The following Table represents the relation between duration and strength of current necessary to stimulate in the case of the sciatic nerve of the toad : Duration of current (sec.) Strength of current (volts) CO •086 ■0070 •091 ■0035 •119 •00087 •179 •00043 •245 If we slightly alter the use by Waller of the word ' characteristic ' we may take as the characteristic of the tissue the duration of the stimulus at which the current necessary to stimulate was just double the minimum. In the above case the minimal stimulating current was approximately doubled when the duration of the current was limited to about -001 sec. From a number of experiments of this description, Lucas gives the following as the characteristic, or what he terms the " excitation times," of muscle and of nerve : 272 PHYSIOLOGY Muscle . -017 sec. Nerve fibre ....... -003 ,, Nerve-ending (or intermediary substance) . . -00005 ,, Similar differences are obtained when we attempt to determine by means of the rheonome the minimal gradient of current necessary to excite a nerve. In the case of the toad's nerve the minimal gradient must be ten times as steep as in the case of the toad's muscle, and is such that in one second the current must reach a strength forty-five times the minimal strength which is necessary to excite when the current is made instantaneously. In the frog's nerve the minimal gradient is still steeper, so that in one second the current must reach sixty times the strength of the minimal exciting current. We may interpret these results as signifying that the excitatory state produced in an irritable tissue involves the production of some change which passes away spontaneously. The rate of production of the change, and still more the rate of its spontaneous disappearance, differ from tissue to tissue. If the gradient of a current which is made through the tissue is too slight, the spontaneous disappearance of the excitatory change goes on as rapidly as the production in consequence of the rise of current. No excitation therefore takes place. The " excitation time " of the tissue will thus be proportional to the duration of the excitatory change produced in the tissue as a result of the stimulus. We may compare the excitation time of three tissues with the duration of the electrical change produced in the same tissues by a single stimulus. The excitation times were : Frog's nerve fibre ..... -003 sec. Muscle fibre of sartorius . . . . -017 „ Ventricular muscle of frog .... 2-000 „ In the case of muscle, according to Burdon Sanderson, the electrical change reaches its culminating-point in -0025 sec, and may take perhaps eight times this interval before it dies away. In the cardiac muscle of the tortoise Sanderson found the electrical change to last between two and three seconds. SUMMATION OF STIMULI. Closely associated with the excitation time of the tissues are the phenomena of ' summation of stimuli ' and ' re- fractory period.' If two subminimal stimuli are sent in within a sufficiently short interval of time, their effect is summated, so that two stimuh, each of which would be ineffective, may together produce an excitation. In the case of striated muscles, in order that mechanical summation of con- traction may take place, the second stimulus must become efiiective before the muscle has completely relaxed ; the second contraction, that is to say, starts from the height to which the first contraction has brought the muscle. A similar condition of things appears to hold for summation of stimuli, if we substitute for mechanical change in muscle the molecular change which accompanies the excitatory state. For summation of two stimuh to take place, the second stimulus must occur at a time before the condition excited by the first stimulus has passed away. The maximum time at which summa- tion of two stimuli can take place will therefore vary from tissue to tissue and will bear a relation to what we have designated the ' excitation time ' of the tissue and also to the rapidity of current gradient necessary to excite the tissue. This will be evident if we compare the maximum summation intervals for different tissues with the excitation time of the same tissues. ELECTRICAL STIMULATION 273 stimulating current 5 ';' above minimal stimulus Summation interval '.' Excitation time" sec. sec. •OUUo •U03 •0015 •017 •0080 2-000 Frog's nerve ,, sartorius „ heart REFRACTORY PERIOD. The phenomenon of a refractory period has long been known in connection with the heart muscle and has often been regarded as characteristic of this muscle. If, in the isolated ventricle, a beat be evoked by a single minimal stimulus, subsequent repetition of the stimulus during the course of the contraction is ineffective, and becomes effective only when the contraction has passed away. The heart is said to be refractory to stimuli during this period. The duration of the refractory period is a question of the strength of the stimulus used. With strong stimuli the heart may be made to contract when the relaxation has only progressed half way, and with very strong stimuli one contraction may be made to follow the last at such a short interval that hardly any trace of relaxation is observable between the beats. The phenomenon seems to be common to all excitable tissues. Thus if two stimuli are applied to a nerve within a sufficiently brief interval, the second stimulus is ineffective, so far as can be determined by the response of an attached muscle or by means of a capillary electrometer. The period is longer the lower the temperature and varies from •OOOG sec. at 40° C. to ^002 sec. at 12° C. This critical interval is lengthened if the irritabihty of the nerve is depressed by narcotics. We may ascribe it to the second stimulus being appUed before the excitatory change due to the first stimulus has reached its culminating-point. THE EFFECT OF TEMPERATURE ON EXCITABILITY. It was found by Gotch that the excitabihty of a nerve within certain hniits was increased by cooHng the nerve and diminished by raising its temperatm-e (Fig. 121). Thus, if a frog be cooled to 2° C. or 3° C. for a day, it will be found that simple section of the sciatic nerve may suffice to send the gastroc- nemius into continued contraction, and under these circumstances ' closing tetanus ' may be obtained with the greatest ease. This increase of excita- bihty does not apply to all kinds of stimuh. In the case of nerve its irri- tability was found to be increased by warming, and diminished by cooling for induction shocks and for all galvanic currents of less duration than •005 sec. In skeletal muscle Gotch found the excitabihty for all forms of stimuli increased by cooUng. Lucas has show^n that these paradoxical effects in nerve, namely, increase of excitabihty towards currents of long duration and the simultaneous decrease towards currents of short duration, are conditioned by two opposed changes in the tissue. The fall of tempera- ture delays the subsidence of the excitatory process, but at the same time renders more difficult the initiation of a propagated distm-bance. The first of these effects reduces the current required for excitation in a ratio which is greater the greater the duration of the cm-rent. The latter increases the 274 PHYSIOLOGY current required in the same ratio for all durations. If then the change of temperature is such that the two opposite effects are exactly balanced at a certain medium duration of current, it follows that for currents of longer duration the net result will be to reduce the current required for excitation, while for currents of shorter duration the net result will be to increase the current required. The effect of temperature therefore on the minimum exciting current will vary from tissue to tissue according as the two factors, rate of subsidence of excitatory change and the initiation of a propagated Fig. 121. Tracing of muscle contractions to show effect of cooling a nerve on its excitability. The lower line indicates the changes in temperature of the excited part of the nerve. The muscle responded only when the nerve was cooled, the stimulus becoming ineffectual when the nerve was warmed. (GOTCH.) disturbance as a result of the excitatory change, are relatively affected by change of temperature. THE EFFECT OF INJURY. The irritabihty of the nerve of a muscle- nerve preparation is not equal in all parts of its course, but is greater at the upper end, probably in consequence of the proximity of the cross-section. Some time after a motor nerve is divided the increased irritabihty at bhe upper end gives way to a decreased irritabihty, and this decrease goes on till the nerve is no longer excitable. The diminution in excitabihty gradually extends down the nerve fibre, so that the part of the nerve nearest the muscle remains excitable the longest. This progressive change in the irritabihty of a nerve after section is spoken of as the Ritter-Valli law. It is soon followed by definite histological changes in the nerve, which we shall describe later. SECTION VII THE NEURO-MUSCULAR JUNCTION The excitatory process travelling down a motor nerve has to be transmitted to the muscle by the intermediation of the nerve-ending or end-plate. We have learnt to regard the axis cylinder as the seat of the propagated ex- citatory process. In the end-plate, however, the axis cylinder comes to an end. When stained by methylene blue or by impregnation with chromate of silver or mercury, the axis cylinder, after passing through the sarcolemma of the muscle fibre, is seen to break up into a number of branches (in some cases forming a typical end- arborisation), which lie on or are embedded in a small amount of imdifi'erentiated protoplasm con- taining nuclei (the ' sole plate '). A similar break in structural continuity seems to occur in the central nervous system wherever an impulse is propagated from the axon process of one nerve-cell to the body or dendrites of another nerve-cell. The end-pro- cesses of the axon come in contact with the next member in the chain of neurons, but no anatomical continuity is to be made out, at any rate in the higher animals. In the central nervous system the area of contiguity, where an impulse passes from one neuron to another, is spoken of as a synapse. The presence of the synapse, or end-plate, between muscle and nerve imposes certain new conditions on the conduction of the excitatory impulse. One of the most important of these hes in the fact that the conduction across the end-plate, and probably across the synapse of the central nervous system, is irreciprocal. An excitatory process started in the nerve travels easily across the end-plate to the muscle. On the other hand, an excitatory process started in the muscle does not extend through the end- plate to the nerve fibre. This fact may be shown on the frog's sartorius. If the lower tibial end of the muscle be spht, as in Fig. 122, a mechanical stimulus, such as a snip with the scissors, apphed to the lower nerve-free end of one of the limbs, e.g. at A, causes a contraction of the corresponding half of the muscle, which does not extend to the other half. On snipping the muscle a httle higher up at b, where nerve-endings are present, the resulting 275 i'lu. 122. 276 PHYSIOLOaY contraction involves the whole of the muscle, owing to the fact that the excitation started in the nerve- endings spreads in both directions through the branching nerve fibres. It is possible that this irreciprocity of conduc- tion may be of comparatively late appearance in evolution. So far as we know, an excitatory process in a sheet of muscle and nerve fibres, such as we find in lower invertebrata, e.g. in medusa, may travel with equal facihty in all directions. We are probably not warranted from our experiments on skeletal muscle in concluding that the contraction of a cardiac muscle- cell may not set up an excitatory process in the surrounding network of nerve fibres. It is impossible, however, to put such a suggestion to experimental test, since in the heart there is no portion of muscle fibre sufficiently removed from nerves to allow of an excitation being applied which might not at the same time affect the nerve fibres. There is evidence that the transmission of the excitatory condition across the end-plate, from nerve to muscle, involves a special excitatory process and the expenditure of energy. Thus there is a period of delay between the arrival of an excitatory impulse at the terminations of the motor nerve and the beginning of the electrical change which marks the moment of stimulation of the muscle fibre. If we compare the latent period of a muscle stimulated directly with its latent period when excited through the nerve, we find that there is an increased period of delay in the latter which is not wholly accounted for by the time taken for the impulse to travel from the stimulated spot down the nerve fibres to the muscle. The extra delay is due to the processes occurring in the end-plate. This end- plate delay amounts to -0013 sec. We may take it roughly at a thousandth of a second. The end-plate seems to be the weakest point in the neuro- muscular chain. We have already seen that, when a nerve of a nerve- muscle preparation is stimulated repeatedly, the muscle very soon shows signs of fatigue, and that the seat of this fatigue is not in the nerve, nor in the muscle, but in the end-plate. It has been suggested that the excitation of muscle through nerve depends on an electrical change or discharge at the nerve-ending. This , discharge must originate in the terminations of the axon and must influence, in the first instance, the substance which forms the intermediary between the axon and the contractile material of the muscle. We have indeed direct evidence of the existence of a third substance, neither nerve nor muscle, at the point of junction of these two tissues. Thus, curare is generally said to paralyse the end-plates. Evidence for this statement has been given in the previous chapter. Kiihne has shown that when the irritabihty of the frog's sartorius is tested at different points it is greater in the situation of the end-plates. This might be ascribed to the presence of the more irritable nerve-fibres passing into the muscle-fibres at these points. The unequal distribution of irritabihty is not, however, changed when the muscle is fully poisoned with curare, so as to block entirely the passage of any impulse from the nerve to the muscle. We must therefore regard curare as acting, not on the axon terminations, but on the substance THE NEURO-MUSCULAR JUNCTION 277 intervening between these terminations and the contractile substance of the muscle. Additional evidence of the existence of such a " receptor " substance, as he calls it, has been furnished by Langley. Nicotine resembles curare in blocking the passage of impulses from the motor nerve to skeletal muscle, though inferior to curare in this respect. If 4 mg. of nicotine be injected into the vein of an anaesthetised fowl, the hind hmbs become gradually stiff and extended in consequence of a tonic contraction of all their muscles. The effect slowly passes off, but can be reinduced by a second dose of nicotine. It is worthy of note that the stimulating effect of nicotine occurs even when sufficient is given entirely to paralyse the motor nerves. It might be thought that the stimulating effect of nicotine was a direct one upon the muscle fibre, but experiment shows that curare has a marked antagonising action on the contraction produced by nicotine, A sufficient dose of curare annuls the contraction produced by a small amount of nicotine and diminishes that caused by a large amoimt. The point of action of the nicotine must therefore be the same as that of the curare. After a muscle has been relaxed by curare it can be still made to contract by direct stimula- tion. On the other hand, nicotine will produce its stimulating effect when injected into a bird in which degeneration of all the nerve fibres of the muscle has been produced by previous section of the nerve-trunks. It is evident therefore that nicotine, hke curare, acts, not on the axon termina- tions, but on a receptor substance, an intermediary substance intervening between the axon terminations and the contractile substance of the muscle. Evidence in favour of such an intermediary substance has been brought by Keith Lucas from an entirely different standpoint. In determining the optimal electrical stimuli or the ' characteristic ' of muscle and nerve by the condenser method {v. p. 203), Lucas finds that, even after moderate doses of curare sufficient to abohsh the possibihty of excitation through the nerve-trunk, the muscles show two optimal stimuli, pointing to the existence in them of two excitatory substances, one of which is not paralysed by moderate doses of curare. This result was confirmed when the tissue was investigated by determining the relation of current duration to the liminal current strength necessary to excite. In a normal sartorius he finds three substances, each distinguished by its own ' excitation time,' In the pelvic nerve-free end of the sartorius there is only one substance with an excitation time of -017 sec. This may be regarded as the muscle substance proper. In the sciatic nerve-trunk there is a second substance with a much steeper characteristic and with an excitation time of -003 sec. On experi- menting on the middle region of the sartorius we find not only these two substances but a third substance, which Lucas calls the substance [3, with an extremely rapid excitatory process. Its excitation time is •0(^'K)5 sec. The presence of these three substances in the middle part of the toad's sartorius is sliowu in the diagrams (Fig, 123), which represent the relation of streugtli to duration of the currents necessary to evoke a contraction. In this cuivo (t represents the muscle material, y the nerve material, and [i the curve of the intermediary substance. 278 PHYSIOLOGY Similar conditions are found in the visceral neuro-muscular system. Here the nerve fibres leaving the central nervous system do not pass direct to the muscle fibres, but end in arborisations round gangUon-cells, which are collected to form the gangha of the sympathetic chain or gangha situated more peripherally and nearer the reacting tissue. Eelays of fibres, for the most part non-meduUated, arise from these ganghon-cells and pass to the unstriated muscles of the blood-vessels and viscera, where they end in plexuses or networks among the muscle fibres, possibly connected by short branches with the fusiform muscle fibres themselves. No structure is 'V' ; V a. u.j-i^.1-.. , 1, , , , Fig. 123. present at the periphery exactly analogous to the end-plate, and it is possible that as Elhott suggests, the end-plate is really homologous with the whole of the sympathetic ganghon with its post-ganglionic fibres passing to the visceral muscles. At any rate, the action of curare and of nicotine on these peripheral gangha is very similar to their action on the skeletal end-plates, nicotine, however, having a relatively stronger action than curare. In- jection of nicotine stimulates and then paralyses the peripheral nerve-cells of the visceral system ; curare in sufficiently large doses paralyses them. More instructive in relation to the presence of receptor substances is the action of adrenahn. This substance, which is produced by the medulla of the suprarenal glands, has a specific action on all tissues innervated by the sympathetic system. It causes almost universal constriction of the blood-vessels, dilatation of the pupil, acceleration of the heart, and inhibition of the intestinal muscles, with the exception of the ileo-cohc sphincter, which causes it to contract, all of which effects can also be produced by stimulation of branches of the sympathetic nerve. On the other hand tissues which are not innervated from the sympathetic, such as the blood- vessels of the brain, are unaffected by the drug. This fact, together with the opposite effects of adrenalin on different unstriated muscles, shows that its action cannot be a direct one on muscle-fibre. It presents a marked contrast, for instance, to barium salts, which produce a contraction of every unstriated muscle-fibre in the body. On the other hand, we cannot THE NEURO-MUSCULAE JUNCTION 279 ascribe this action to a stimulation of the sympathetic nerve- endings, since adrenalin is equally effective if applied after the whole of these nerve-endings have been made to degenerate by section of the post-ganghonic sympathetic nerve-trunks. Its action therefore must he at the junction between nerve and muscle, and must be on some intermediate or receptor substance de- veloped at the myoneural junction, and having for its function the trans- ference of the excitatory process from the nerve fibre to the contractile substance of the muscle fibre. Similar receptor substances may act as intermediaries in every case of propagation of an impulse across a synapse of whatever description, and may by their properties determine the peculiar quahties of the synapse. We may compare them to the fulminating cap which in a shell is used to transfer the process of combustion from the slow- match to the bursting charge. Their existence is of especial importance when we endeavour to investigate the mode of action of drugs. It is probable that they will be found to play a great part in determining the differential action of drugs on various tissues in the body. SECTION VIII POLARISATION PHENOMENA IN NERVE ELECT ROTONIC CURRENT. If a constant current be passed through a nerve fibre through the electrodes x and y, — x being the anode and y the cathode — and the extrapolar portions of the nerve ah, cd be connected with galvanometers, the needles of both are deflected, and the direction of the deflection shows the existence of a current in the extrapolar portions of the nerve a to h, and from c to d. We thus see that a passage of a current through a part of a nerve gives t i y G' G2 Fig. 124. Diagram showing electrotonic currents, p, polarising circuit ; Gi, g2, galvanometers. The galvanometers will indicate, before the passage of the polarising current, the ordinary demarcation current of the nerve resulting from the cross-section at . the upper end. This current flows, in the outer circuit, from equator to cut end, and therefore in the nerve fibre from a to h, and from d to c. The effect of closing the polarising current will be to increase the current of rest between a and 6, and to diminish that between c and d, rise to a current flowing through a considerable portion of the nerve fibre on each side of the polarising current and in the same direction. This current is called the electrotonic current. It must not be confounded with the current of action, which originates at one of the poles, only at make or break of the current, and is transmitted thence in the form of a wave with a measurable velocity (in the frog) of about 30 metres per second The electrotonic current is developed instantaneously, and lasts the whole time that the current is flowing through the nerve. Its production is dependent 280 POLARISATION PHENOMENA IN NERVE 281 on the occurrence of polarisation between the sheath and the conducting part of the nerve fibre and may be exactly reproduced on a model consisting of a core of zinc or platinum wire in a casing of cotton soaked with ordinary salt solution. Although thus physical in origin, its production is dependent on the vitality of the nerve, and so is not to be confounded with the simple spread of current. The polarisation phenomena resulting from the passage of a constant current through a medullated nerve can be studied on a model made up of Glass tube containing 0-6/^ Na CI. Pt.wire Fig. 125. Apparatus for imitating the polarisation phenomena in medullated nerve {' Kernleiter ' model). a glass tube filled with normal salt solution, containing a platinum or zinc wire stretched through it (Fig. 125). On leading a current through a and b, and connecting c and d with a galvanometer, a current will be observed in the extrapolar portion of the model in the same direction as in the intrapolar. That this spread of current is due to polarisation is shown by the fact that, if the model be made of zinc wire immersed in saturated zinc sulphate solu- tion, so that no polarisation can occur, the spread of current to the extra- polar area is also wanting. If we examine the phenomena taking place at the anode, we see that a current passes here through an electrolyte to the conducting core. Every passage of a current through an electrolyte must be accompanied by dissociation, the current being carried by the ions. We get therefore a movement of negative ions up into the electrode, and a deposition of electropositive ions on the core (Fig. 127, a). In the same way at the cathode there will be a deposit of electro negative ions on the core (Fig. 126, d), so we may say that the core is posi- tively polarised '^at the anode and negatively polar- ised at the cathode. This ^'7- J 26. IMagram to show polarisation at the surface . , . . between conducting core and electrolj-tc sheath in a polarisation while opposing ' Kovlciier.' the primary current, will set up currents in the surrounding electrolytic sheath, as shown by the arrows in Fig. 127, the current passing from a to h and from b to c in the electrolyte, returning towards a in the core. Hence if we lead off from the sheath in the neighbourhood of the anode from a and c, a current will pass in the galvanometer from a to c, that is along the core in the same direction as the intrapolar current. The same factors will cause an extrapolar current in the cathodic area, the catelectrotonic current. -ill il,+ ++++++ + + -+.♦- c b a d e f 282 PHYSIOLOGY This polarisation will not disappear at once on breaking the polarising current. The nerve or nerve-model will still be positively polarised at the anode and negatively polarised at the cathode. On connecting therefore these two points with the galvanometer, we shall get a current in the opposite direction to the previous polarising current, viz. from anode to cathode (Fig. 128). This is the so-called negative polarisation of nerve. Similarly in the extrapolar regions of the nerve we shall have currents in the same "^ Polarising *^Tf' Positive polarisation Fig. 129. Diagram to show direction of the Fig. 130. Diagram of arrange- positive polarisation current, due to a break ment for showing paradoxical excitation at the anode. contraction. tissue, and therefore if we connect a to k, there must be a current outside the nerve from h to a, and in the nerve from a to k, viz. in the same direction as the polarising current. We see therefore that negative polarisation is due to polarisation occurring between an electrolytic sheath and a con- ducting core, whereas positive polarisation is hardly a polarisation effect at all, but is a current of action. POLARISATION PHENOMENA IN NERVE 283 PARADOXICAL CONTRACTION. If the sciatic nerve of a frog be dissected out, and one of the two branches into which it divides be cut, and the central end of this branch stimulated, the muscles supphed by the other half of the nerve contract to each stimulus. Ligature or crushing of the nerve x (Fig. 130) between the points stimulated and the point which joins the main trunk puts a stop to this effect, showing that it is not due to a mere spread of current. The fibres passing down n are in fact stimulated by the electrotonic current developed in x during the passage of the exciting current. SECTION IX THE NATURE OF THE EXCITATORY PROCESS Under this heading we have really two questions to discuss, namely, (a) the nature of the change excited at the stimulated spot in an excitable tissue, and (b) the propagation of the excitatory change away from the excited spot, e.g. down a nerve fibre. That these two phenomena are more or less in- dependent and may be dealt with separately is shown by the result of passing a constant current through a parallel-fibred muscle, such as the sartorius. In this case, as we have seen (p. 214), at make of the current an excitatory change occurs at the cathode and is transmitted throughout the whole length of the muscle, giving rise to a twitch of the muscle. During the passage of the current there is still an excitatory change at the cathode, but limited to a region within one or two milhmetres of the cathode. An attempt has been made by Boruttau and other physiologists to explain the nerve process, not as a wave of electrical change affecting the substance of the axis cylinder itself, but as a propagated catelectrotonic current. This observer found that, by working with a ' platinum core model' {' Kernleiter') (Fig. 125) of considerable length, the catelectrotonic current was developed at one end of the model some appreciable time after a current had been sent in at the other end, thus resembhng a current of action. It is, however, impossible to explain all the electrical phenomena of nerve as due simply to polarisation. We might go so far as to assume that the excitatory effect at the cathode is due to negative polarisation, and that excitation at break, i.e. at the anode, is caused by the sudden coming into existence of a negative polarisation current ; but then it would be difficult to understand how the excitation, so produced at the anode, should give rise to a current so much exceeding the current which produced it that it would appear in our external circuit as a current of positive polarisation. The same objection would hold to the comparison of a nerve-fibre with a submarine cable. An electric disturbance produced at any part of a cable [i.e. a conducting wire in an insulating sheath) is propagated along the cable at a certain finite velocity which can be calculated when we know the conductivity of the core, the capacity of the cable, and the di-electric constant of the sheath. In all these cases there must be a decrement of the change as it is transmitted away from its seat of origin, a decrement for the existence of which there is no evidence in a nerve fibre or other excitable tissue.* Moreover the phenomenon of propagation of an excitatory process * It might be urged, on the other hand, that one would not expect to find any appreciable decrement in a cable only 1 to 3 inches long. 284 THE NATURE OF THE EXCITATORY PROCESS 285 is equally well marked in tissues, such as muscle and non-medullated nerve fibres, which show very httle of the electrotonic effects described in the last section. The absence of decrement in the excitatory process has been taken as an indication that the axis cyhnder of the nerve is the seat of energy changes which may be let loose under the influence of chemical or electrical changes, just as the energy of a contracting muscle is set free by the exertion of an infinitesimal force applied as a stimulus. The nerve on this view does not simply transmit the energy which is imparted to it, like a telegraph wire, but itself furnishes the energy of the descending nerve-process. Against this view might be urged the absence of phenomena of fatigue in nerve, as showing that nervous activity is not accompanied by any ex- penditure of energy or using up of material. But it must be remembered that this absence of fatigue holds good only for medullated nerve fibres and is not found in non-medullated nerves,* and even in medullated nerves the persistence of irritability is dependent on the continual supply of a certain small amount of oxygen. It may therefore possibly be explained by a continual process of restitution taking place at the expense of the sheath. Fatigue is absent, not because nothing is used up, but because the assimilative changes exactly balance and make good the dissimilation involved in the propagation of a nervous impulse. There is thus a certain amount of justification in the comparison of a nerve fibre to a chain of gunpowder, though in the nerve fibre the impetus to disintegration, imparted from each particle to the next in order, consists, not in a rise of temperature at the point of ignition, but in all probability in an electrical change ; and the total evolution of energy is so small that it cannot be measured as heat by the most sensitive methods at our disposal. The excited condition at any segment of a nerve is associated with a develop- ment of electromotive forces at the junction of the segment with the adjacent resting segments. The current of action thereby produced can pass by the sheath of the nerve, so that it must enter the axon at the excited spot, and leave it at the adjacent unexcited segment. Hermann has suggested that in this way the current of action at any excited spot may excite the adjacent segments or molecules, causing them to become negative and thus setting up a current of action which in its turn excites the succeeding segments. In this way the excitatory process may travel the whole length of the nerve. Propa- gation would thus involve the successive setting up of an excitatory process all along the nerve or excitable tissue, though it is difiicult to see why on this theory every excitatory state should not give rise to a propagated change. We are as yet a long way from a comprehension of the changes involved in the process of excitation, though we are able to form some idea of many of the factors which must be involved. Any theory of the excitatory process must take into account the following phenomena : (1) The excitatory state is attended wdth an electrical change of such * This statement is based chiefly on experiments on the olfactory nerve of the pike. Halliburton andBrodie foimd no signs of fatigue in the non-medullated fibres of the sympathetic supply to the spleen, even after several hours stimulation. 286 PHYSIOLOGY a nature that the excited spot is negative to adjacent unexcited spots. This electrical change rises rapidly to a maximum and dies away more slowly, the rate of its rise, and still more of its subsidence, varjang largely according to the nature of the tissue under investigation. (2) The excitatory change is aroused only at the poles of a current passing through the tissue, i.e. at those places where polarisation can occur in consequence of the electrical movement of ions. (3) Excitation only occurs at the cathode at make of the current, and only occurs if the current attains a sufficient strength within a certain period of time, the relation of strength of current to rate of change varying in different tissues. (4) All hving tissues are made up of colloids, divided into compartments by membranes of various permeabihties and permeated with salts and other electrolytes in solution. Disregarding for the moment all considerations of structure, it is possible to form a hypothesis of the nature of electrical excitation which takes into account the facts just mentioned and enables us to give a quantitative or mathematical expression to the factors involved. An electrical current passing through a tissue containing membranes, impermeable to the dis- solved ions, will set up differences of concentrations at and near the mem- branes. Nernst, on the supposition that these differences of concentrations, when sufficiently large, would cause an excitation, arrived at a formula connecting the lowest current required to excite with its duration, and another formula connecting the lowest amplitude of an electrical current with its frequency. The mathematical investigation of the question has been continued by A. V. Hill in conjunction with Keith Lucas. For this purpose we may suppose that the excitable unit is represented by a cyhndrical space closed at its two ends by the mem- branes A and B (Fig. 131) and filled with a solution of electro- lytes. If a current be passed from B to A the positively charged ions will move towards A and Fig. 131. tend to accumulate there. The accumulation of the ions near the membranes will^be^Hmited by the tendency of the ions to equahse their con- centration in all parts of the cell by diffusion. If we suppose that a necessary condition for excitation is that the concentration of the ions in the neigh- bourhood of one of the membranes shall reach a certain definite value, it becomes possible to calculate under what conditions of strength, duration, &c., an electrical current will just produce excitation. The rise of the excitatory state would here be determined by the rate at which the ions accumulate, the subsidence of the excitatory state by the rate of dispersal of the ions by diffusion. The formula arrived at by these observers has this form : . ^ THE NATURE OF THE EXCITATORY PROCESS 287 where i is the smallest current which will excite, t is duration of the current. while \, fx, 0 are constants which depend on : (1) The distance between the membranes. (2) The distance from the membrane at which the concentration changes are being considered. (3) The diffusion constant of the ion, (4) The number of ions by which a given quantity of electricity is carried. (5) A constant expressing in general terms the ease with which a propagated disturbance is set up. Investigation on these hnes may give us in future sufficient infor- mation to form a material conception of the factors involved in excitation, factors which in the above formula have only a symbolic existence. Thus a determination of the distance between the membranes would give us some clue to the size of the ultimate excitatory units in the tissue involved.* The constant fx has reference only to the position relative to the membranes at which the changes of concentration are effective. From Lucas's experiments it would seem that the changes of concentration occur in the immediate neighbourhood of one of the membranes. Macdonald has brought forward evidence that the passage of a current through a nerve involves the setting free of certain inorganic ions. The subsidence of the excitatory state depends on the rate of dift'usion of ions. If, however, we compare the rates of subsidence of the excitatory state in different tissues we find much greater divergence than would be possible on the assumption that the diffusion is one affecting inorganic ions. Thus between the substance ^ (the intermediate substance) of the frog's sartorius and the ventricular muscle fibre of the same animal, the rate of subsidence of the excitatory state changes in the ratio 4000 : 1. If the ions concerned were simple ions, such as H-, Ca--, Na-, Cr, &c., it would be impossible to account for this wide variation, since their velocities differ in the ratio of 10 : 1 at most. Moreover the effect of rise of temperature on the rate of subsidence is greater than the effect of a similar rise on ionic velocities. It is evident therefore that the theory is one for use as a working hypothesis only. That excitation is associated with accumu- lation of ions in the region of the exciting electrode, that the subsidence of of the excitatory state is due to disappearance by diffusion or otherwise of these ions, there can be little doubt. But the questions as to the nature of these ions, and their relation to the colloidal constituents of the excitatory tissue, or to other possible substances, changes in which may form an integral part in the excitatory state, must be left for future investigation. * It would be premature at present to give any histological significance to Hill and Lucas's diagrammatic cylinder. As Hardy has pointed out, the nerve cannot consist of a row of such cylinders, otherwise excitation would occur throughout the whole intrapolar region, and not be confined to the cathode at make and the anode at break. It may be that we are dealing here again with the jwlarisable sheath of the 'Kenileiter.' and that the membrane a corresponds to the surface of the axis cylinder or of its neuro-fibrils. CHAPTER VII THE CENTRAL NERVOUS SYSTEM SECTION T THE EVOLUTION AND SIGNIFICANCE OF THE NERVOUS SYSTEM Every vital phenomenon maybe regarded as a reaction conditioned by some change in the environment of the animal and adapted to its preservation. In the community of cells forming the whole organism, the defence of any one part must involve the co-operation of the whole community ; no change in a cell of the body can be regarded as a matter of indifference to any of the other cells. For this subordination of the activities of each part to the welfare of the whole, as for the co-operation of all parts in maintaining the welfare of each, a means of communication is necessary between the various cells. For some of the lower functions the channel of communication is the blood, which serves as a medium for carrying food material from one part of the body to another, or for the transmission of chemical messengers, which, elaborated by one set of cells, may affect the metabohsm of cells in distant parts of the body. This method of correlating different activities would, however, be too slow and clumsy for the quick adaptation of the organism to sudden changes of environment. Such a rapid correlation can be effected only by a propagation of some molecular change from the seat of incidence of the stimulus either to all parts of the body, or to some mechanism controlling all parts of the body. The medium for the propagation of a state of excitation is furnished by the nervous system. We have seen that stimuh of various kinds, involving such various forces as thermal, chemical, and electrical energy, are transformed by a muscle or nerve fibre into what we call a state of excitation, which is propagated along the fibres, whether nerve or muscle, at a certain definite rate, its passage in the case of the muscle being followed by a wave of contraction. In unicellular animals, such as the amoeba and vorticella, there is no difiierentiation of any structure which can be regarded as pecuharly nervous. A stimulus appHed to any part of the amoeba may evoke responsive activity in all other parts. A slight touch applied to any point on a vorticella will cause an excitation which is rapidly propagated to the stalk, causing this to 288 EVOLUTION OF THE NERVOUS SYSTEM 289 contract and so withdraw the organism from any possible injury. In the lowest metazoa, such as the sponges, we find no special nervous structures. The cells forming the sponge may react to changes in their environment by contraction or by alteration of their relative positions. Many of the cells can move from one part of the sponge to the other in response to chemical changes occurring in the body of the sponge. So far, however, no cells have been distinguished as endowed above their fellows ^vith the property of irritabiUty or the power of reaction to stimulus. It is in the next class, that of the Coelenterata, where we first find a definite nervous system. The object B m.c. Fig. 132, Diagrammatic representation of evolution of a nervous system. (Modified from Foster.) ec, epithelial cell ; mp, muscular process ; sc, sensory cell ; wp, nerve process or fibre ; mc, muscle-cell ; sn'p, sensory nerve process ; mwp, motor nerve process ; cc, central cell. of a nervous system is to ensure the co-operation of the whole organism in any reaction to changes in its surroundings. At its first appearance therefore we should expect a nervous system to be developed in connection with that layer of the animal which is in immediate relation to the environment, namely, the efiblast or external layer. In some species of hydra, though no typical nervous tissues have been detected, many of the epithehal cells lying on the surface are prolonged at their inner ends into a long contractile process (Fig. 132, a), so that stimuH apphed to the surface and acting on the epithelial cells can cause, as an immediate response, a contraction of the underlying nmscular processes. We may easily conceive that in such an animal, among the cells forming the epiblast, certain cells might become endowed with a special sensitiveness to external changes, other cells being developed, like those of the hydra just mentioned, into special contractile structures. If in the course of development the protoplasmic continuity between these two sets of cells had not become interrupted (and we have no ground for assuming that such an interruption occurs under normal circumstances), it is evident that we should have so produced the simplest form of a reflex arc (Fig. 132, b), namely, a sensory cell, which is stimulated by slight physical changes in its surroundings and is thereby tliro\\^i into a state of activity similar to that which we have already studied in nmscle and nerve. This state of activity would be propagated by the protoplasmic channels to the muscular cell and 10 290 PHYSIOLOGY / arouse there the specific function of the muscle, namely, contraction. In such a simple reactive tissue, Hnes of less resistance would be rapidly laid down through the protoplasmic continuum, and these hnes, acquiring a specific structure or composition, would form a network uniting sensory and muscular cells. Thus a stimulus apphed to any sensory cell would spread to the ad- jacent sensory and muscular cells, and the response of the muscle-cells would be greatest near the stimulated spot, gradually djang away as the area of the excitation widened. A further step in the development of such a hypotheti- cal elementary nervous system would occur when certain of the sensory cells (Fig. 132, c) developed a special sensitiveness, not to mechanical changes in the environment, but to the protoplasmic excitatory process arriving at them along the nerve network. These cells would act as relays of force, picking up the excitations arriving from ^^- , the undifferentiated sensory cells, and sending them on with increased vigour along the nerve network. In such a manner a stimulus applied at one point could be sent on in successive relays from cell to cell throughout the whole reactive tissue on the surface of the body. We cannot point to any par- ticular animal as presenting in- stances of either of the two types of elementary nervous system just described. If such exist, they have not yet been investigated, or the undifferentiated character of their nervous tissues has thwarted the efforts of zoologists to display their specific characters by staining reagents. In the lowest definite nervous system with which we are acquainted, namely, that of the jelly-fish, all three 1 types of cell, the sensory cell, the reactive or central cell, and the motor cell, are already developed and have undergone among themselves a considerable degree of differentiation. In a jelly-fish or medusa, such as aureha or sarsia (Fig. 13.3), the reactive tissue of the body is confined to the under-surface of the so-called umbrella with the tentacles and manubrium. A section through the umbrella shows a layer of epithelium containing differentiated sense-cells, below which is a plexus or rather network of fine nerve fibres with a certain number of nerve- cells at the nodes of the network. From this network fibres pass more deeply to end in a finer network situated among a layer of muscle fibres formed, hke the sensory cells, by a differentia- tion of the primitive epithelium or epiblast (Fig. 134). Besides this diffuse Fig. 133. Diagrammatic view of a jelly-fish (Hertwig.) TJ, umbrella ; m, manubrium; t^, T2, tentacles V, velum ; N, nerve ring ; r, ' marginal body.' EVOLUTION OF THE NERVOUS SYSTEM 291 nervous system, there is a continuous ring of nerve fibres round the margin of the umbrella, thickened at intervals by the accumulation of nerve-cells, which are in close relation to special collections of sensory cells in the ' marginal bodies.' These sensory cells present a differentiation among them- selves, some being apparently determined for the reception of mechanical Flu. 134. Diagram of subepithelial plexus of nerve fibres and nerve-cells, communicat- ing on the one side with the sensory epithelium, and on the other side with the sub- umbrellar sheet of muscle fibres. (After Bethe. ) stimuli, others for the reception of light stimuh, while others again are found in close relation with Httle masses of calcium carbonate crystals, by the direc- tion of the weight of which the cells are able to react to changes in the position of the animal in space. In the jelly-fish therefore the nervous or reactive system has already acquired a considerable degree of differentiation. I'lu. l;}o. Figure of a ji'lly-lisli in wliicli all the marginal bodies exccjit one have boon removed, and wliich lias been incised in various directions so as to divide the nerv(> ring and all the " long paths,' so that only the diffuse nerve network remains functional. (Romanes.) We may study the behaviour of a more primitive systeni if wo remove the special sense-organs of the medusa by cutting off the whole of the marginal ring with its contained marginal bodies (Fig. 135). We have then a layer 292 PHYSIOLOGY of contractile tissue, innervated by a nerve network, and covered by a layer of epithelium containing sense-cells. To this network is attached the manubrium, which represents the mouth and stomach of the animal. In such a mutilated jelly-fish it is easy to show that a stimulus apphed to one spot on the surface travels outwards from the excited spot to all parts of the bell. The stimulus is propagated also to the manubrium, which in some species bends in the direction of the excited spot — that is to say, in the direc- tion which represents the shortest possible path from the excited spot to the manubrium. This preparation rarely presents any automatic activity. It may react to a constant stimulus by a rhythmic series of contractions, but remains perfectly motionless in the absence of stimulus. The unmutilated Fig. 136. Schema showing the utility of the multiplication of neurons and their grouping in central ganglia. (Cajal.) A, an ideal invertebrate with only cutaneous ' sensory ' neurons. B, invertebrate, such as a medusa, with sensory and motor neurons, but no central nervous system. c, invertebrate {e.g. Annelid) in which the motor neurons are concentrated in central ganglia. a, sensory neuron ; b, muscle ; c, motor neuron. jelly-fish presents rhythmic contractions of its sub-umbrella tissue which are inaugurated in any or all of the marginal bodies and serve to drive the animal onwards through the water in which it is immersed. The rhythmic contractions may be initiated, augmented, or diminished, in response to stimuh of hght, mechanical irritation, or changes in the position of the whole animal acting on the marginal bodies. In the reaction of an animal to ex- ternal stimuli it must be an advantage if the energies of the whole can be concentrated in defence of any one part and be evoked by a stimulus applied at one point. Such a co-operation of the whole for the benefit of the part involves the existence of direct paths from the stimulated point to all parts of the animal if the reaction is to take place with any promptitude. In the medusa we find a beginning of such ' long paths.' The general direction of the fibres of the network is radial, and there is a concentration of such fibres in the neighbourhood of the marginal bodies, so that an excitation can pass more readily from a sense-organ to the menubrium than it can laterally along the circumference of the animal. Moreover, a stimulus which is too shght to excite a reflex contraction of the muscular tissue may travel along the nerve tissue to each of the marginal ganglia and arouse these to a discharge of motor impulses. We have therefore in the medusa sensory cells of different sensibihties ; central cells specially adapted to reacting to and reinforcing a EVOLUTION OF THE NERVOUS SYSTEM 293 nerve stimulus started by a small change in the environment ; a general nerve network propagating the excitatory changes in all directions, but with special ease in certain directions ; and a reactive muscular tissue which carries out movements at the end of the chain of excitation, all the elements forming the chain being derived from epiblastic cells. A further differentiation of a nervous system, such as that just described, must in the first place involve the laying down of more ' long paths ' and the collection of the special ' central ' cells into closely comiected masses (gangha), so as to concentrate the control of the reactions of the body, and to permit of the ready subordination of every part to the needs of the whole. A special direction is given to this development by the evolution of animals, such as worms and crustaceans, which are segmented and capable of locomotion. The fact that these animals are segmented determines the collection of the central cells into a chain of gangha, one ganghou or pair of gangha being provided for each segment. In the act of locomotion it is of advantage to the animal that those sense-organs or sensory cells which are projicient, i.e. which are stimulated by changes in the environment originating at a distance from the animal, should be collected together near that part which goes first, namely, the head end. Thus the projected sensations of sight, those which are excited by chemical changes in the surrounding medium and represent the sense of smell, and those which are specially aroused by vibrations in the surrounding medium and correspond to those which we call the sense of sound, are in the majority of these animals sub- served by organs situated near the head end. The wisdom of a man is measured by his fore- sight. The chances of an animal in the struggle for existence are determined by the degree t-o which the reactions of the animal to its immediate en- vironment are held in check in response to stimuli arising from approaching events. An animal with- out power to see, smell, or hear its enemy will re- ceive no impulse to fly until it is already within its enemy's jaws. It must therefore be of advantage to segmented animal that the activities of the whole chain of segmented gangha should be subservient to those central nerve-cells which are in direct con- nection with the projicient sense-organs at the head. The influence exerted by the head ganglia will be in the first place inhibitory of the direct reaction Fig. 137. View of central nervous system of craj-- fish. (After Yung and VOGT.) a, cerebral ganglion. b, commissure, e, suboesophageal ganglion. g, first abdominal ganglion. /, oesophagus. m, optic nerve, p, antennary nerve. s, stomato-gastric nerve. 294 PHYSIOLOGY excited in each segment by stimulation of its surface, and, for this influence to be propagated, long tracks must be laid down, joining up ganglion to ganghon and propagating impulses from the head gangha to the most distant part of the chain. As a type of such a system we may refer to the crayfish. In this animal the central nervous system (Fig. 137) consists of a chain of thirteen ganglia, namely, six abdominal ganglia, six thoracic ganglia, and one supraoesophageal or cerebral ganglion. In the abdomen and thorax the ganglia form a longitudinal series situated in the middle line of the ventral aspect of the body close to the integu- GAMGLION - CNA Fig. 1.38. Diagram of nervous system of a segmented invertebrate (earthworm or crayfish). (From Schafek, after Retzius.) s', sensory cells; s, afferent nerve fibres ; m, motor neuron ; i, central or intermediate cell. ment. All give origin to a variable number of nerves, which are distributed partly to the muscles, partly to the skin and sense-organs. They are connected by longitu- dinal bands of nerve fibres or commissures, which are double, each ganglion being bilobed. The most anterior of the thoracic ganglia, which is the largest, is marked at the side by notches, as if it were made up of several pairs of ganglia fused together. From this ganglion two commissures pass forward round the gullet to unite in front of this tube, behind the eyes, with the transversely elongated mass of ganglion cells and fibres called the supraoesophageal ganglion. This ganglion consists of three fused pairs of ganglia, which have been termed the protocerebron, the deuterocerebron, and the trito- cerebron. The most anterior gives origin to the optic nerves, which run by the optic stalks to the eyes. From the middle ganglion on each side a tegumentary nerve passes to ramify in the integument and from the inferior surface the antennulary nerves pass to the internal antenna?. From these small branches are distributed to the organ of hearing. The posterior protuberance of the brain gives origin to the antennary nerves which pass to the large external antennae of the animal. The first thoracic, EVOLUTION OF THE NERVOUS SYSTEM 295 or suboesophageal, ganglion gives origin to ten pairs of nerves which are distributed to the mandibles, to the jaws or maxillae, to the maxillipedes, and to the branchial appendages of the latter. When we investigate the structural basis of such a nervous system we find that, as in medusa, the starting-point of the reflex arc is in certain neuro- epithelial cells (Fig. 138) lying on the surface of the body. These cells are spindle-shaped, and have one short process passing to the surface, and a long process which runs in a nerve fibre or collection of fibres towards the ganglion of the segment. Arrived at the ganglion it divides into two branches, which pass towards the two ends of the body and become lost in Fig. 139. Diagram of a reflex arc in a (ncuro- fibrillar) invertebrate nervous system. (Bethe.) The efferent paths are coloured red, the afferent black. the granular material forming the inner part of each ganghon. The ganglia themselves consist internally of this punctated substance or granular material, and externally of a capsule of ganglion-cells. Each of the ganglion- cells sends one thick process towards the centre, which rapidly divides, some of the branches passing into the granular material, while one branch passes outwards in a nerve to end in a network of fine fibrils within the muscles on the surface of the body. The nervous impulse excited in the sensory cell on the periphery travels therefore up a nerve fibre into the grami- lar substance of the ganglion. From this granular substance it is collected by the fine branches of the ganglion-cells and is transmitted by them along the motor nerve fibre to the muscles. The central granular material consists entirely of a close felt- work of fibres, which may be regarded as processes either of the sensory nerve fibres or of the nerve-cells. The typical reflex arc in this case therefore is formed by two nerve-cells with their processes. Such a nerve-cell with its processes is spoken of as a neuron. The first neuron the recipient neuron, or receptor, is represented by the sensory cell with its 296 PHYSIOLOGY two processes in the granular material. The second neuron is formed by the ganghon-cell with its finely branched dendritic processes in the granular matter and its motor axon, which passes into the muscle fibres. As to the manner in which the impulse passes from the branches of one cell into those of the other, opinions are still divided. The question will have to be more fully considered when we come to deal with the vertebrate nervous system. Many beheve that there is no anatomical continuity between the two neurons, and that the excitatory change is transmitted by a mere contiguity, a change m one set of nerve-endings exciting a corresponding change in another set of nerve- endings in immediate contact with them. By certain methods, however, it is possible to show the existence of an anatomical continuum throughout the whole nervous system in these invertebrate animals. Apathy and Bethe have demonstrated the presence of a continuous system of neurofibrils, much smaller than an individual nerve fibre, which, starting in a sensory cell, pass into a network of fibrils forming the greater part of the central granular matter. From this network neurofibrils run along the dendrites into the ganghon-cells, forming there a small network through the centre of which a neurofibril is continued down the nerve processes again, and passes out along the motor nerve to end in a network of fibrils among the muscle fibres. In a system so constituted it is evident that, although an excitatory process passing along a given fibril may find certain paths easier than others, and so maintain a constant prescribed path through the nerve system, yet it will be possible, by sufficiently increasing the strength of the excitatory process, to cause it to travel in all directions in the central nervous system and to evoke in this way a general activity of all parts of the body, a condition in fact found to obtain in the normal animal. It is significant that, although a great number of fibrils pass into the bodies of the ganghon-cells, yet in many cases, especially in crustaceans, fibrils are to be found sweeping from the neuropilem or nerve network of the granular substance into a nerve process, and thence into its motor axon without at any time entering the body of the cell (Fig. 139). SECTION II THE NERVOUS SYSTEM OF VERTEBRATES In these, as in the invertebrata, the central nervous system is developed by an involution of the epiblast, reveahng thereby its primitive relations to the surface of the body. At an early period in foetal hfe, shortly after the formation of the two layers of epiblast and hypoblast, a thickening is ob- served in the epiblast. This thickening soon gives place to a groove, the neural groove (Fig. 140), and the walls of the groove folding over form a Fig. 140. Transverse section of human embryo of 2-4 mm. to show developing neural canal. (T. H. Bkyce.) »c. neural canal ; mc, muscleplatc ; ;«y, outer wall of somite ; sc, sclerotome. canal, the neural canal, which is dilated at the head end of the embryo to form three enlargements known as the cerebral vesicles. When first formed the canal is oval in cross-section, its wall being made up of a layer of columnar cells between the outer extremities of which are seen smaller rounded cells. The internal layer of columnar cells send a process peripherally which branches at the end so as to form a close meshwork of fibres. These fibres branch more and more as development progresses, and eventually form the supporting tissue of the adult central nervous tissue, known as the neuroglia. As the wall of the canal grows in thickness, some of the cells may wander outwards and form neurogUa-cells with numerous radiating branches. In the adult nervous system little is left of those colls except their nuclei, so that the neuroglia appears as a close felt-work of fibres, to which here and there nuclei nro attached. These cells 297 1») * 298 PHYSIOLOGY are formed from the most superficial layer of the invaginated epiblast, and are spoken of as S'po7igioUasts. The deeper layer of cells, which are to give rise to the permanent nerve-cells, and are therefore known as neuroblasts, rapidly divide and form a thick layer surrounding the internal layer of spongioblasts, through which pass the peripheral processes of the latter. When first formed these cells have no processes. Later on each neuroblast acquires a pear shape, the stalk of the pear having a somewhat bulbous extremity (Fig. 142). The stalk continually elongates, and the elongated process may leave the spinal cord altogether and grow outwards to any part of the body of the embryo, or may pass to other parts of the central nervous sys- tem. This long process of the developing nerve- cell is known as the axon. Some time after its formation other processes grow out from the cell, which soon branch and end in the immediate neighbourhood of the cell. The axons of the cells near the ventral part of the neural tube grow out 11 1 to the different muscles of the body, ■ J 1 «L where they end in close connection with Mm. f^ 1 /"^ *^® muscular fibres by an arborisation 1^ I 0% ^l IV^K "^^^^ forms the end-plate. They provide %^ \' / I' * ; an efferent path for impulses running from the central nervous system to the musculature of the body. The afferent channel is formed in a somewhat different manner. Even before the neural groove has closed in, a thickening of the epiblast is seen immediately external to the groove on each side. This thickening becomes divided into a series of collections of cells lying immediately under the epiblast on the lateral and dorsal surface of the neural canal. The cells, which are at first round or oval, send off two processes in opposite directions so that they become bi-polar (Fig. 142). One process passes into the central nervous system, where it divides, some of its branches being distributed in the nervous system at the same level, while others run a con- siderable distance towards the head immediately outside the tube of nerve- cells. The other process grows downwards, along with the processes from the ventral cells of the tube, towards the periphery of the body, where it ends in close connection with the surface in the various sense-organs of the skin and muscles. These collections of bi-polar cells form the posterior root ganglia. In fishes they retain their primitive character throughout life, but in mammals the bi-polar cell is to be found only in the spiral and vesti- bular ganglia which give origin to the fibres of the eighth nerve. In all the other ganglia the shape of the cell becomes modified by an approxima- tion of the points of attachment of the two processes until finally the cell Fig. 141. Neuroblasts from the spinal cord of a chick embryo. (CaJal.) A, three neuroblasts stained to show neuro-fibrils ; a, a bi-polar cell. B, a neuroblast showing the ' incre- mental cone ' c. THE NERVOUS SYSTEM OF VERTEBRATES 299 becomes iini-polar, giving off one process wliicli divides by a T-shaped junction into two, one of which runs towards the spinal cord, while the other takes a peripheral course as the afferent nerve fibre. The central nervous system thus becomes provided with a ' way in ' and a ' way out ' for the chain of impulses concerned in a nervous reaction or reflex action. The further development of the spinal cord is mainly determined by the extension of the Fig. 142. Section through developing spinal cord and nerve-roots from chick embryo of fifth day. (C.ajal.) A, ventral root ; b, dorsal root ; c, motor nerve-cells ; d. sympathetic ganglion- cells ; E, spinal ganglion cells still bi-polar ; f, mixed nerve ; h, c, d, motor nerve fibres to /, developing spinal muscles ; e, a sensorj' nerve-trunk. axons of the cells outside the tube of cells themselves, and by the provision of the ' long paths ' which are a necessary condition of increased efficiency of the reacting organ. Some time after the outgrowth of the axon a medullary sheath is formed, apparently by the agency of the axon itself, so that each group of axons leaving or entering the cord forms a bundle of medullated nerve fibres. The long branches of the posterior or dorsal roots running up towards the head form a mass of fibres behind the tube of cells known as the posterior columns. Fibres starting in the spinal cord itself run upwards and downwards to end in other parts of the cord, or in the more anterior divisions of the central nervous system forming the brain, and surround the neural tube on its ventral and lateral aspects with a sheath of white matter. To these white fibres are added others, which take origin in the brain and pass all the way down the cord. Meanwhile the cells themselves become separated 300 PHYSIOLOGY by the ramifications between them of the branches of axons entering the cord, as well as of the dendrites of the cells themselves. Thus, in its adult form, the spinal cord consists of a central mass of nerve-cells and fibres, known as the grey matter, which is encased in a sheath of white matter formed of medullated nerve fibres. The cord itself is cyhndrical in shape, and is divided into two symmetrical halves by the anterior and posterior fissures. In each half of the cord the grey matter on cross-section is cres- centic in shape, presenting an anterior or ventral horn and a posterior or dorsal horn, and is connected with the corresponding mass in the other half of the cord by grey matter known as the anterior and posterior grey com- missures. Between the two grey commissures is the central canal, relatively very minute when compared with the condition in the f CBtus and lined by a single layer of columnar cihated epithehum, the cells of which are directly descended from the neural epithehum lining the medullary canal. THE STRUCTURE OF NERVE-CELLS In the adult animal a typical nerve-cell, such as those forming a prominent feature in the anterior horn of the spinal cord, is a large cell with many branches. It has a large vesicular nucleus with very little chromatin. Fig. 143. Nerve-cell from the spinal cord, stained by Nissl's method. a, axis-cylinder process or axon ; h, proto- plasm of cell, consisting of c, fibrillated ground substance, and e, the granules of Nissl ; d, nucleus. (Lenhossek.) Fig. 144. The point of origin of the axon, the ' nerve- hillock,' highly magnified, to show absence of Nissl's granules from the origin of the process. (Held.) which may be collected into one or two nucleoh. The body of the cell presents dift'erent appearances according to the manner in which it has been treated for histological examination. When separated from the sur- rounding tissues by means of dissociating fluids it may present traces of striation, the individual striae running from one process to another of the cell. When treated fresh with methylene blue, or hardened by alcohol or corrosive sublimate and stained with methylene blue or toluidine blue, THE NERVOUS SYSTEM OF VERTEBRATES 301 the protoplasm is seen to contain angular masses which are deeply coloured with the dye (Fig. 143). These masses are known as the Nissl granules or bodies. By other methods it is possible to demonstrate that the whole protoplasm of the cell between the Nissl bodies is pervaded by fine fibrils, which enter the cell from the processes and may run out of the cell by the axon or may run into some of the other shorter processes (Fig. 146). The processes of the cell, as is evident from their development, are of two kinds. The axon which becomes continuous with the axis cylinder of the medullated Figs. 145 and 146. Nerve-cells from siainal cord. (Bethe.) Fig. 145, showing Golgi network, and neurofibrils : d, c, /, junctions of axons with Golgi network. Fig. 146,. showing neurofibrils and Nissl bodies. nerve fibre arises from a part of the cell body known as the axon hillock, which is the only part of the cell free from Nissl bodies (Fig. 144). The other processes, which may be very numerous, are known as the dendrites. They are generally thicker than the axon at their origin from the cell, but rapidly diminish in size as they give off branches, the branches apparently terminating freely in the grey matter in the immediate neighbourhood of the cell. In specimens stained by the Golgi method the dendrites may sometimes present a somewhat serrated outline. The Nissl bodies of the cell extend some way into the dendrites. A nerve cell with all its processes, axon, and dendrites is spoken of as a neuron. From the development of the central nervous system in vertebrates, it is evident that the nervous path of every reaction must be made up of two or more neurons. If we take, for example, the simplest possible reaction which might be eifected through a single segment of the spinal cord, we see that the afferent impulse might be started by some stimulus appUed to the ramifications in the skin of the distal processes of the posterior root ganglion- cell {cf. Fig. 132). The nerve impulse so started is carried by the nerve fibre past the T-shaped junction in the posterior root ganghon into the cord. 302 PHYSIOLOGY. and along a branch of the entering nerve fibre which runs right across the cord to terminate in the neighbourhood of the anterior horn-cells. Here the impulse must be transmitted in some way to the dendrites or body of one of the large motor nerve-cells in the anterior horn, whence it is carried along the axon of the cell, leaving the cord by the anterior root and passing down a peripheral nerve to the end- plate on a muscle fibre. Here again by some means the arrival of the impulse excites the muscle to contract. This reaction never takes place in the contrary sense, i.e. no impulse started in the motor nerve can travel back through the spinal cord and along the sensory nerve. Although an impulse excited in the nerve passes easily to the muscle, an excitatory process started in the muscle itself is confined to this tissue and never extends to the nerve fibre. Apparently the same ■ rule holds good ^dthin the grey matter of the central nervous systein, where two neurons come into relation with one another. An impulse passes easily from the axon of one into the dendrites and cell of the other neuron, but, so far as we are aware, it is impossible by exciting an axon to cause a retrograde wave of excitation to pass through its corresponding cell and into t he terminations of the axons in immediate contact with the cell. This statement has been called by Sherrington the ' Law of Forward Direction.' It might be also spoken of as the irrecifrocal conduction of the nerve arc. The character of a reaction to any stimulus, apphed to the surface of the body, is determined by the course which the impulse, excited in the afferent nerves, takes on entrance into the central nervous system. This course is laid down by the connections of the neurons through which the nerve impulse passes. In the central nervous system therefore, more than in any other part of the body, function is directly dependent on struc- ture. Theoretically if we had a perfect knowledge of the connections of the neurons in the central nervous system and knew the nerve fibres affected by any given stimulus, we should be able to prophesy exactly the result of such stimulus. In the case of the simpler reactions this is already possible, but in the higher parts of the nervous system the enormous complexity of the systems of neurons excludes any possibility of our forming more than a general idea as to the nerve paths traversed in any given reaction ; and the variations which exist from individual to individual must always prevent in the intact animal an absolute prediction of the results of any stimulus. SECTION III GENERAL CHARACTERISTICS OF REFLEX ACTIONS There are certain features common to all reactions, carried out through the intervention of the central nervous system, which must be regarded as determined by the properties of the neurons, i.e. the conducting links in the chain of excitatory tissues intervening between the stimulated spot on the exterior and the reacting tissue, muscle or gland. These charac- teristics may be roughly classified as follows : (1) LOCALISATION. In a simple system of neurons a given stimulus will nearly always produce the same reaction. In a frog possessing only a spinal cord, the upper parts of the central nervous system having been destroyed, any harmful stimulus applied to a toe will cause a lifting of the leg. If the motor nerve to the gastrocnemius be excited, the whole muscle contracts. If one of the nerve-roots entering into the formation of the sciatic nerve be excited, only certain fibres of the gastrocnemius contract, the locahty of the reacting fibres being determined by their connection with the excited nerve fibres. In the same way the contraction of certain muscles of the leg, in response to a stimulus applied to the skin of the foot, is deter- mined by the fact that the nerve fibres, which carry the impulses from the toe into the spinal cord, divide there and make connections with the motor neurons, whose axons are distributed to the several muscles involved in the reaction. The connection of the sensory with the motor neuron may be direct, but in most cases the impulse has to pass through intermediate neurons before arriving at the motor neurons. The path of the impulse, however, in spite of its enormous extension, is as definite as is the path from an excited motor nerve-root to a muscle fibre. (2) DELAY. Instead of one nerve-ending intervening between the stimulated nerve and the reacting tissue, there will be, in the case of the reflex action, two, three, or more nerve-endings interpolated in the path of the impulse. These nerve-endings are the fields of conjunction, the si/napses, between the axon of one neuron and the dendrites and cell body of the neuron next in the chain. We have seen that there is a distinct difference between the latent period of a muscle excited through its nerve as compared with the latent period when excited directly, and we ascribed this latent period to a delay in the motor end-plate. We should expect therefore to find that the delay or 303 304 PHYSIOLOGY latent period in the case of a reflex action, i.e. the lost time in the conversion of an afferent into an efferent impulse in the central nervous system, would be appreciable and would increase with the complexity of the response — that is, with the number of neurons involved in the reaction. Such is indeed the case. In determining the actual ' lost time ' in the central nervous system for any given reflex, it is necessary to subtract from the total delay, inter- posed between the apphcation of the stimulus and the resultant movement, the time taken by the impulse in travelling to and from the central nervous system, as weU as the latent period of the muscles themselves. The re- mainder is known as the ' reduced reflex time.' Wundt found in the frog, when a reflex contraction of the gastrocnemius was excited by a stimulation of a posterior root of the same side, that the reduced reflex time was -008 sec. For a crossed reflex the delay was increased by -004 sec. If we assume that one additional neuron is involved in the crossed reflex, the lost time at a synapse would be -004' sec. ; if two cells are intercalated, the synapse delay would be only -002 sec. Since the uncrossed reflex has a delay of -008 sec, at least two, and possibly four, synapses are involved in the path of this simple reflex. The bhnking excited by stimulation of the eyelid has a reduced reflex time of -047 sec. (3) SUMMATION. When contractile tissues, such as striated or un- striated muscle, are excited by single shocks, a certain minimal strength of stimulus is necessary in order to produce a contraction. Weaker stimuH are spoken of as sub-minimal, and when apphed singly have apparently no efiect on the muscle. In dealing with the properties of involuntary muscle we saw that a sub-minimal stimulus is not necessarily devoid of efiect because it fails to evoke a contraction, since, if repeated at sufficiently frequent intervals, a summation of stimulus occurs, so that at the fifth or sixth apphcation a stimulus, which was previously inefiective, becomes effective and a contraction results. The muscle will now continue to respond to each stimulus, but, if the excitations be discontinued for a time, reapphca- tion of a stimulus of the same strength becomes once more inefiective. This summation of stimulus is a prominent feature in all reflex actions, so much so that it may be often impossible to evoke a reaction to a very strong single induction shock, whereas the application of a tetanising current too weak to be felt on the tongue may produce a marked reaction. We shall have occasion later on to deal with special examples of this summation of stimulus. (4) FATIGUE. In the muscle-nerve preparation the weakest point and that which soonest sufiers from fatigue is the end-plate, or rather the field of conjunction of nerve fibre and muscle fibre. In the central nervous system the synapses of the different neurons are equally susceptible, and since several of such synapses are involved in every reflex action, we should expect to find that the central nervous system would show signs of fatigue before the peripheral structures. If a given reaction be repeatedly eh cited by applying a stimulus to a certain area of the surface, the reaction becomes CHARACTERISTICS OF REFLEX ACTIONS 305 feebler and finally disappears altogether long before any signs of fatigue in the motor apparatus can be detected by stimulation of the motor nerve itself. This fatigue is produced equally well if the reaction be excited by stimulating a sensory nerve directly, and since we know that it is practically impossible to fatigue nerve fibres, we must conclude that the seat of fatigue is in the grey matter of the spinal cord itself. (5) ' BLOCK ' OR RESISTANCE. In the central nervous system there is an absolute block to the passage of an impulse backwards through a synapse, i.e. from a nerve-cell or its dendrites into the end ramifications of an axon. The phenomena of fatigue show that there is a certain degree of resistance at the synapse to the passage of an impulse in the normal direction, and that this resistance is rapidly increased under the conditions which produce fatigue. When we study the structure of the central nervous system more fully, we find that although there are certain shortest possible paths, i.e. ones involving few neurons, for every impulse arriving at the central nervous system, yet so extensive is the branching of the entering nerve fibres and so complex are the neuron systems with which they come in connection that an impulse entering along one given fibre could spread to practically every neuron in the spinal cord and brain. Such a result is indeed observed in animals poisoned by strychnine. In such animals the slightest stimulus applied to any part of the skin excites strong tonic spasms in the whole musculature of the body. Every single nerve fibre, that is to say, can discharge into every motor neuron of the cord. That this result does not ensue on locahsed stimulation in a normal animal is dependent on the varying resistance to the passage of an impulse into the several neurons with which the entrant fibre comes in relation. A small stimulus will dis- charge therefore only along the few neurons where the resistance is lowest. Increase of the stimulus, either by increase of its strength or by summation of weak stimuh, will enable the impulse to spread along more neurons and therefore will elicit a more widespread response. Only when the blocks are entirely removed by the administration of strychnine, or when the stimuli are abnormally powerful and long continued, will the impulse spread to all regions of the central nervous system, so that response becomes general and inco-ordinate instead of local and adapted to the stimulus. (6) FACILITATION OR ' BAHNUNG.' The passage of a nervous impulse across a synapse or series of synapses in the central nervous system has a twofold eft'ect. If the passage be too often repeated phenomena of fatigue are produced, and there is an increase of the block at each synapse. If, however, the stimulus be not excessive and the reaction not too frequently evoked, the effect of passage of an impulse once is to diminish the resistance, so that a second application of the stimulus evokes the reaction more easily. The process of summation in fact is chiefly in the direction of removal of block. We have a close analogy to this process of facilitation in the ' stair- case phenomenon ' observed in cardiac and unstriated muscle. In these tissues the repetition of a sub-minimal stinuilus renders it in time effective, and then repetition of the now eft'ective stimulus causes a gradually 306 PHYSIOLOGY increasing height of contraction, which depends on the state of the contracting tissue itseK and cannot be evoked by changes in the strength of the stimulus. This process of faciHtation or ' Bahnung ' is of great interest in connection with the development of ' long paths ' in the central nervous system, and more especially with the acquirement of new reactions by the higher animals. The Law of Facihtation is really the Law of Habit. When an impulse has passed once through a certain set of neurons to the exclusion of others it vnll tend, other things being equal, to take the same course on a future occasion, and each time that it traverses this path the resistances in the path will be smaller. Education is the laying down of nerve- channels in the central nervous system, while still plastic, by this process of ' Bahnung ' along fit paths, combined with inhibition (by pain) in the other unfit paths. Memory itself has the process of ' facilitation ' as its neural basis. (7) INHIBITION. The constant occurrence of a reaction in response to a given stimulus is obtained only if care be taken to isolate the segment of the central nervous system involved from the entry of other afferent stimuli. As a rule, if two stimuh be apphed simultaneously at different points, the reaction which ensues will not be a combined one, the resultant of the reactions which would be normally conditioned by each single stimulus, but will be a response to one of the stimuh, which we must therefore regard as the more effective. The reaction to the other stimulus is either abohshed altogether or comes on after a considerable period of delay. The central nervous system can apparently attend to only one thing at a time. In physiological terms we should say that every effective reaction inhibits every other reaction. In the spinal cord of the frog the normal withdrawal of the foot in response to stimulation of the toe of the same side can be inhibited by strong stimulation of the other sciatic nerve, by stimulation of the spinal cord at a higher level, or by stimulation of the optic lobes. Immediately after pithing the brain of the frog, the whole animal becomes flaccid and motionless, and for the next few minutes it is impossible to eHcit any reaction by stimulation, however strong, apphed to the skin of the body. In the production of this condition of ' shock ' the inhibition of all the spinal centres, produced by the strong stimulation of the injury to the brain and medulla, plays at any rate an important part. We may say that the passage of an impulse through a chain of neurons diminishes the block for subsequent impulses at each synapse that it traverses, but increases during its passage the block in all the adjacent synapses. In dealing with the special reactions of the spinal cord we shall have occasion to refer more fully and in greater detail to many of these properties which are characteristic of all reflexes. Before, however, treating of the functions of the separate parts of the central nervous system in the higher mammals, it may be of interest to consider the exact nature of the structure intervening between neuron and neuron at each field of conjunction or synapse, as well as the significance of the two chief elements of the central nervous system, nerve-cell and nerve fibre, in the production of co-ordinated purposive reactions. SECTION IV NATURE OF THE CONNECTION BETWEEN NEURONS The study of the development of the central nervous system in higher animals has shown that this system is made up of neurons, the connections of which determine the possible paths of impulses in the adult cord. The first stage in the development of the neuron is a single cell without processes, and it is only by the growth of these processes out from the cell that the spinal cord becomes capable of serving as an aggregate of conducting paths. Moreover the deferred acquisition of an influence of one neuron on the next neuron in the line of impulse, or at any rate on the peripheral tissue which receives the end arborisation of its axon, is shown by the fact that entire destruction of the spinal cord in the embryo at an early stage in its develop- ment does not prevent in any way the development of the voluntary muscles (Harrison) ; although, after birth, a severance of the connection between spinal cord and skeletal muscle leads to a rapid degeneration and atrophy of the latter. In the nmscle-nerve preparation there is an apparent break of structure at the termination of the nerve in the muscle fibre, any con- tinuity between nerve- ending and contractile substance being subserved by undifferentiated protoplasm. There is therefore no difficulty in con- ceiving a propagation across a similar nerve-ending or synapse, between the axon of one neuron and the cell body or dendrites of another neuron. If, however, the conception we have formed above of the evolution of a nervous system from a continuous conducting protoplasmic network, by a process of facilitation or ' Bahnimg ' attended by histological differentiation, be correct, we should expect to find in the fully developed brain and spinal cord some traces at any rate of continuity throughout the whole system of neurons. The question as to the existence of anatomical continuity from neuron to neuron has been hotly discussed both for vertebrates and in- vertebrates. In the case of the latter, evidence in favour of the continuity of neuro-fibrillse from sensory surface to reacting tissue is very strong. Many observers, especially Apathy, Bethe, and Held, have described a similar continuity in the nervous system of mammals. The last-named observer regards this continuity as a product of later development and as due to a process of concrescence occurring between the axon terminations and the bodies of the nerve-cells with which they come in contact. It is easy to show the existence of a fibrillar structure both in the nerve-cell and in the 307 308 PHYSIOLOGY nerve fibre (Fig. 147). The axis cylinder of the nerve fibre can be regarded as made up of fine fibrillae embedded in an interfibrillar substance. At the nodes of Ranvier the interfibrillar substance is interrupted, the fibrillse alone extending into the next internode and representing the continuous structure which determines the conducting power of the nerve fibre. In the nerve- cell the fibrillse occupy all the space between the Nissl bodies, passing from dendrite to dendrite, and many of them from all the dendrites and all parts of the cell sweeping through the axon hillock to form the fibrillse of the nerve fibre. The existence of these fibrillar structures in nerve-cell and nerve ^. Ax, .7 A Fig. 147. Part of an anterior cornual cell from the calf's spinal cord, stained to show neuro fibrils. (Bethe.) Ax, axon; a, b, c, dendrites. Fig. 148. Arborisation of collaterals from the pos- terior root-fibres round the cells of the posterior horn. (Ram()]s- y Cajal.) fibre is accepted by most histologists. The question, however, of the con- nection between the fibrillee of one axon and those of the next neuron, i.e. the histology of the synapse, presents much greater difficulties and has excited much difference of opinion. If we examine a nerve-cell such as a cell of Purkinje of the cerebellum, or a cell of Clarke's column in the cord, we find that it is surrounded by a thick basket-work of fibres which are the arborisations or end terminations of the axons which pass to the cell to enter into functional relationships with it (Figs. 148 and 149). This peri- cellular network is of great extent and may equal in total diameter the diameter of the cell itself. Whether the basket-work is really a network, NATURE OF CONNECTION BETWEEN NEURONS 309 or merely a felt-work in which the fine fibres intertwine among each other without becoming actually continuous at any points, is difficult to make out. On the periphery of the cell itself another network has been described and is known as the Golgi network (Fig. 150). This has been displayed both by the process of impregnation with silver chromate (Golgi method), as well as by staining with methylene blue. Some authors have regarded this network as an artefact due to precipitation of albuminous fluids on the surface of the cell. According to Bethe, however, the Golgi network on the one hand receives fibres from the encircling pericellular basket-work of axons, Fig. 14!). Baskot-vvorkof^fibrcs around two cells of Purkinjc. (Cajal.) a, axis-cylindoi' or ncrve-fibro process of one of the cor])uscles of Purkinje; h, fibres jirolonged over the beginning of the axis- cylinder process ; c, branches of the nerve-fibre processes of cells of the molecular layer, felted together around the bodies of the coi'puscles of Purkinje. Fig. 1.50. Superficial network of Golgi surrounding two cells from the cerebral cortex of the cat ; Erlieh's method. (Cajal.) A, large cell ; b, small cell ; a, a, folds in the network; 1), a ring-like condensation of the network at the poles of the larger cell ; c, spinous projections from the surface. and on the other hand gives off towards the interior of the cell fine fibrils, which are continuous witli the neurofibrillar of the cell and pass out in its axon. The diagrammatic course of a nerve impulse according to Bethe is represented in the accompanying diagram (Fig. 151). An impulse starting from the periphery of the body travels up the distal process of the posterior root ganglion-cell, passes either through the cell or directly to the central process, and travels along this to the terminations of the posterior root fibres round a posterior horn-cell. Here it passes into the peri-cellular basket-work or axon network, thence into the Golgi network and along the fibrillar of the cell out by the fibrillse of the axon and so to a fresh synapse with a cell of the anterior horn. There are certain physiological difficulties in the acceptance of this 310 PHYSIOLOGY doctrine of continuity through the central nervous system. Even if it be true, it would not in any way upset the importance of the neuron theory. Every plant or animal individual must be regarded as a protoplasmic con- tinuum. With growth of the hving matter, its metaboHc functions demand the dispersion of nuclear material through the protoplasm, and this is effected by division of the nucleus. Considerations of strength and rigidity demand the division of the protoplasm into compartments or cells, which, at j&rst at any rate, remain in protoplasmic continuity. This division has probably a further advantage in that lesions of parts of the individual entail merely the death of the cells immediately affected and do not necessarily Fig. 151. Schema of the neurofibrillar continuum, involved in an ordinary reflex act, in a vertebrate nervous system. (Bbthe.) spread to the whole organism. Thus in the central nervous system injury to one axon causes degeneration of the axon below the point of section, but the degeneration stops short at the end arborisation and does not spread into the next neuron. If we assume that, in consequence of the straitness of the path, the propagation through the fibrillse is especially difficult in the synapse, most of the phenomena described above as characteristic of the reactions which take place in the central nervous system can be easily ex- plained on the theory of continuity of the fibrillse. The serious difficulty in the acceptance of this theory is, however, the ' Law of Forward Direction,' i.e. the fact that an impulse will pass from an axon to the next neuron, but will not pass backwards across the synapse from the cell body to the con- tiguous axon. Bethe suggests that this rule of Forward Direction, which is possibly present only in the more highly developed nervous systems, may be due to a species of " polarity " of the nerve-fibril, of such a nature that an impulse is strengthened and so assisted on its passage in the normal direction, but is diminished and finally abolished when it passes in the opposite direction. Such an explanation is unsatisfactory, since there is absolutely no experimental evidence of the existence of such polarity in a NATURE OF CONNECTION BETWEEN NEURONS 311 nerve fibre ; all the evidence that we have at present points to a nerve fibre having the power of propagating equally well in either direction. It is certainly more useful to regard a synapse as of the nature of a motor nerve- ending, in which an impulse arriving along the branches of an axon excites a fresh impulse in the excitable tissue, i.e. the nerve-cell, with which the branches of the axon come in contact. Moreover the neurons are formed without any structural connection with the future destination of their axons. These grow out as processes with thickened amoeboid extremities. Harrison has shown that the growth of the axo]i from the cell may be observed under the microscope in a neuroblast separated altogether from the body, and kept on a warm stage in a thin layer of coagulated lymph. It is possible that we may have to distinguish two types of nervous system, viz. : (a) A neurofibrillar type, peculiar to invertebrata, with conduction in all directions. (6) A synaptic type, in which the Law of Forward Direction holds, of later evolution, and forming the greater part of the nervous system of vertebrata. SECTION V FUNCTIONS OF THE NERVE-CELL When a unicellular organism, containing a single nucleus, is cut into two parts, both continue to live for some time, each performing active move- ments and evincing all the phenomena which we associate with activity and therefore with destructive katabolism. For the continued existence of a cell the processes of constructive metabolism, or anabolism, must take place pari passu with those of disintegration, and for this the presence of the nucleus is necessary. Hence, in a few days, the half cell with the nucleus has repaired its loss and become once again a normal individual, whereas the half without a nucleus undergoes degeneration and death. The axon of a nerve- cell can be regarded as analogous to a long pseudopodium of an amoeba. Like this, if cut away from that part of the cell containing the nucleus, though capable for a time of discharging its active function of propagation of excitatory impulses, yet it finally dies, death of the nerve fibre occurring in the mammal within three to five days after separation of the axon from the cell. Every nerve-cell therefore may be looked upon as a trophic centre of the nerve fibre proceeding from it as well as of the medullary sheath, which is practically a product or secretion of the axis cylinder. But has the nerve-cell any more important functions to dis- charge ? It has long been customary to endow the nerve-cell with all the properties which are distinctive of a nervous system, and to ascribe to it the active part in the origination of automatic actions, in the reflection of afferent impulses, and in the supply of energy to all nervous processes. That the passage of impulses through the nerve-centres requires the ex- penditure of energy by these centres can be proved in various ways. In the first place, we have the fact that in all nervous systems, at any rate of the higher animals, arrangements are made for their free supply with oxygen Very short deprivation of oxygen causes a complete block throughout the system, in many cases preceded by a short period of increased excitability or ease of transmission. If, in the rabbit, the thoracic aorta be clamped for a few minutes, the hind limbs become paralysed, and if the obstruction be continued for half an hour, there is widespread degeneration and death of the cells with their fibres in the grey matter of the lumbar and sacral cord. In the second place, the ready production of fatigue of the nervous system points to a considerable using up of material as a condition of the passage of nerve impulses. In many instances, moreover, an infinitesimal stinmlus travelling up a few nerve fibres may excite widespread activity of the whole central neivous system with the discharge of impulses along .312 FUNCTIONS OF THE NERVE-CELL 313 practically every nerve of the body. Thus the presence of a crumb on the larynx will excite impulses travelling up the superior laryngeal nerve, which in themselves can involve but httle expenditure of energy. The result, however, of their arrival at the central nervous system is the discharge of impulses along the motor nerves causing spasmodic contractions of almost every muscle in the body. It seems beyond doubt then that energy is evolved in the central nervous system as a result of metabolic changes, and that energy may be added to impulses passing through the central nervous ^W^^' "t^^- '^"' r.lob^"^ j XvW/ Fig. 152. Diagrammatic representation of the brain of Cacrinus to show the parts involved in Bethe's experiment. The dotted line x shows the incision employed to isolate the neuropilem of the ganglion of the second tentacle. system, which therefore acts as a relay of force. But this activity does not necessarily require the presence or co-operation of nucleated cells. In dealing with the nature of a nerve impulse we had reason to conclude that there may be an actual, though minimal, liberation of energy in the axis cylinder with the passage of each nerve impulse. The non-nucleated parts of a cell, whether the axon or the cell body, are equally capable of this evolution of energy, and we might conceive therefore of a nervous system which, existing for a few days, might act as a normal reflex centre in the entire absence of the nucleated cell bodies. This conception has been realised by Bethe in an experiment on the crab (Carcinus menas). In this animal the reflex movements of the tentacle are carried out by a gang- lion situated at its base. As in the other Crustacea, the cell bodies in this ganglion lie outside the mass of neuro-fibrils in the centre, forming a sort of capsule (Fig. 152). Bethe was able, under the dissecting microscope, to remove the cell bodies without interfering with the nerves entering or leaving the central mass of fibrils. All the nerve processes with their connections were therefore left intact. In animals, operated in this way, Bethe found that for two or three days the tentacle reacted normally to stimuli applied to its surface. The reflex functions of the ganglion were not in any way affected by the removal of the nucleated bodies of the cells. A similar 314 PHYSIOLOGY experiment would be impossible in the central nervous system of vertebrates, since impulses must of necessity pass through the cell body on their way from the termination of one axon to the begimiing of the next. In the spinal root ganglion, however, most of the cells lie on the surface. In the rabbit Steinach exposed a posterior root ganglion, separating it from all its vascular supply, but leaving its nervous attachments intact. The wound was opened every day for the next few days and an instrument passed under the ganglion so as to divide any newly forming vessels. As a result of the deprivation of blood-supply the ganglion-cells died. But Steinach found that nerve impulses were still conducted perfectly well through the ganglion at a time when microscopic examination showed a complete atrophy of all cells. It is therefore only in virtue of the fact that the nerve-cell is the seat of the nucleus, and therefore of the assimilative functions of the neuron, that any pre-eminent importance can be ascribed to it in the building up of a reactive nervous system. Prominent among the functions with which the nerve-cell has been endowed is that of automaticity of actiofi in the absence of stimulus other than that supplied by its own metabohsm or by the fluids which bathe it. A priori there is no reason to deny to the neuron a property which is pos- sessed by other cells of the body, such as the muscular cells of the heart, and is a fundamental quahty of undifferentiated protoplasm. The purpose, however, for which these cells have been evolved and differentiated is that of reaction, of adapting the organism to changes in its environment, and it is doubtful whether, in this differentiation, it has retained any auto- matic properties whatsoever. In the absence of any afferent impulse the whole central nervous system would probably be inert. In a frog retaining only the spinal cord Hering divided all the posterior roots. The frog re- mained flaccid and motionless. Injection of strychnine was powerless to evoke the usual tetanic spasms. In such a strychninised frog, however, it was only necessary to open the wound and touch one of the divided posterior roots to throw the whole bod}^ into convulsions. As shown by Sherrington and Mott, division of all the afferent nerves coming from the upper limb in monkey or man entirely abohshes all such contractions of the limb, as are usually affected through the intermediation of the cerebral cortex. Cutting off the major portion of the afferent impulses to the respira- tory centre does not, it is true, abohsh all respiratory discharges, but converts the rhythmic respirations into a series of inspiratory spasms which are repeated at long intervals and are entirely inadequate for the proper aeration of the blood. According to Sherrington a repetition on the mammal of Hering's experiment does not lead to the same results, since a spasmodic discharge is produced from the isolated spinal cord as a result of asphyxia. But it is doubtful whether in this case there was not some continuous excita- tion of the cord going on, as a result of the closure of the demarcation current in the cut ends of the posterior roots by the body fluids. It is possible that the neurons possess some automatic power, i.e. some power of initiating nervous processes, as a result of changes in the fluids surrounding them. This automaticity, however, is not a prominent feature of the nervous system, which has been evolved as a purely reactive mechanism to the afferent impulses resulting from the material changes continually taking place in the environment of the animal. SECTION VI STRUCTURE OF THE SPINAL CORD In the higher representatives of the invertebrate class the central nervous system consists, as we have seen, of a chain of gangha, each ganglion or pair of ganglia presiding over the reactions of its own segment, but connected by long paths with the other ganglia and with the head ganglia. The latter, being especially developed in connection with the organs of special sense which are projicient in function, acquire a control over the rest of the ganglia (Fig. 153). The vertebrate spinal cord may be looked upon as a chain of ganglia which DORSAL Spinal caual 'Neui-en.teric canal /n/undifculuTa VENTRAL DORSAL a^opKaja. VENTRAL (T- Co.n. Fio. 153. Vertebrate central nervous system compared with that of tlie arthrojiocl. (Gaskell.) (Note that according to Gaskell the ventricles of the hrain and the primitive neural canal corres])ond to the invertebrate stoTnach ami intestine.) have become fused concurrently with a dimiinition in the importance of the local segmental reactions and with a growth in the solidarity of the whole system ; so that in the higher vertebrates, at any rate, little trace of the primitive segmental arrangement is evident in the internal structure of the cord. Some remains of this arrangement still persist, however, in the origin from the cord of nerve roots, which are distributed roughly within the area of the corresponding segment of the body. In man the spinal cord is an elongated cylindrical structure slightly flattened from before backwards and about eighteen inches long. It gives ol^ a series of nerve-roots, which are ananged in thirty-one pairs and are distributed symmetrically to the two sides of the body. Each 315 316 • PHYSIOLOGY nerve arises by two roots, an anterior and a posterior, the anterior being composed of a series of rootlets spread over a considerable area of the cord, while the posterior roots arise as a compact bundle from a groove on the postero-lateral aspect of the cord. The posterior nerve-roots pass through a gangHon and join the anterior roots in the intervertebral foramina to form the mixed nerve. On section the cord is seen to consist of a core of grey matter surrounded on all sides by white matter. The white matter is made up of meduUated nerve fibres which are devoid of a neurilemma, and run within tunnels or tubes in the supporting neurogha. The grey matter has roughly the form of a letter H, and consists, in cross-section, of a comma-shaped mass on each side of the cord, joined across the middle hne by a band of grey matter. On the anterior aspect of the cord is a furrow, the anterior fissure, which contains a process of the enveloping membrane of the cord, the pia mater, and is hmited at its bottom by a band of white matter, the anterior white commissure, which unites the anterior columns of white matter. On the hinder aspect of the cord is another fissure, the posterior fissure, which is very narrow and is built up chiefly by neuroglia. A third fissure at the point of origin of the posterior nerve-roots serves to divide the white matter of the cord into an antero-lateral column and a posterior column, and the former is imperfectly separated by the spread-out anterior rootlets into anterior and lateral columns. The cord in cross-section (Fig. 154) is circular in the dorsal region and oval in the cervical and lumbar regions. It presents two marked enlargements, namely, the cervical enlargement, corresponding to the outflow of the nerves going to the upper hmb, and the lumbo-sacral enlargement, which gives off the nerves to the lower limb. In the sacral region it rapidly tapers off to a blunt point. In the centre of the band of grey matter, connecting the two masses on each side of the middle Hne, is the central canal of the cord, the remains of the primitive neural canal of the embryo. The grey matter in front of it is called the anterior grey commissure, that behind the posterior grey commissure. The comma- shaped mass of grey matter on each side of the cord presents in front the broad anterior comu, and behind the narrower posterior cornu, which extends up to the postero-lateral groove in the Hne of emergence of the posterior roots. In the dorsal region of the cord the grey matter projects into the lateral column of white matter to form the lateral horn. The grey matter consists of a supporting tissue of neuroglia in which are embedded nerve-cells and their processes and the endings of nerve fibres. The neurogha is formed of a thick felt- work of fibres with here and there nuclei applied to the fibres. Occasionally we may meet cells provided with a very large number of branches and representing the cells from which all the fibres of the neurogha have been derived. The neurogha is present in specially large amount in two situations, namely, immediately around the central canal and as a capsule to the enlargement of the posterior cornu, known as the head or cafut cornu posterioris. In this latter situation the neurogha con- tains a large number of small richly branched nerve-cells and is spoken of as the substantia gelatinosa of Rolando. The nerve-cells are arranged in STRUCT UEE OF THE SPINAL CORD 317 distinct groups. In the anterior horn we may distinguish three groups, a median group of cells near the middle line, many of which send their processes across to the other side in the anterior white commissure, and an external Cervical. Dorsal. Lumbar. i'lt;. Ip4. Sections of human .spinal cor.l from]thc lower cervical, mid-clor.^^al. and mid-Iurabar regions, showing the principaf groups of nerve-cells, and on the right side of each section the conducting tracts as they occur in the several regions (magnified about 7 diameters). (E. A. Schafer.) a, b, c, groups of cells of the anterior horn ; d, cells of the lateral horn ; e. middle group of cells ; /, cells of Clarke's column ; g. cells of posterior horn ; cc, central canal ; ac, anterior commi.ssure. 318 PHYSIOLOGY group often subdivided into a postero- external and an antero-external. The latter group is especially developed in the regions of the cervical and lumbar enlargements and consists of very large multipolar cells with many dendrites which send their axons into the anterior roots and by these to the muscles of the Kmbs. Another group of rather smaller cells is found in the lateral horn, in that region of the cord where this is marked. A very definite group of cells may be seen in the dorsal region of the cord in the inner aspect of the root of the posterior horn. This, which is known as Clarke's column, is formed by large cells elongated in the longitudinal direction of the cord. Besides these definite columns a number of nerve-cells are distributed irregu- larly through the grey matter, especially of the posterior horn. Direct pyraniicH^^^ Ant. lot. J asc. tract. Posterior Soots, with coUakrcds. Fig. 155. Spinal cord. (After Lbnhossek.) On left side of figure are shown the nerve-cells with their axis-cylinder processes. On the right side the dis- tribution of the chief collaterals. 1, motor cells ; 2, cells of the columns ; 2a, cells of Clarke's column, sending processes across into direct cerebellar tract ; 3, 4, and 5, commissural cells. According to the destiny of their axons these nerve-cells may be divided into four groups (Fig. 155). (1) THE MOTOR CELLS, the largest of all, which send their axons into the anterior roots, where they run to supply skeletal muscle fibres. A i a sub-group of these cells we may class the somewhat smaller cells of the lateral horn, which in all probability send their axons by the anterior roots to supply visceral muscles. Their axons can be distinguished from the motor axons by the smaller diameter of the nerve fibres they form. They pass later from the mixed nerve along a white ramus communicans into the sympathetic system, in the ganglia of which they end. (2) CELLS OF THE COLUMNS. As typical of these cells we may take those which form Clarke's column. Their axons do not leave the central nervous system, but pass out into the white matter to some other part of the central nervous system, contributing thus to form the white columns of the cord. (.3) COMMISSURAL CELLS. These cells send their axon across the middle line to the op])ositc side of the cord, making up a great part of the anterior white com- missure. (4) CELLS OF GOLGI. These cells are found chiefly in the posterior horn. They are multi-polar and are distinguished from all the other cells by the fact that their STRUCTURE OF THE SPINAL CORD 319 axon does not pass far from the cell, but rapidly breaks up into a number of branches which terminate in the near neighbourhood of the cell giving off the axon. They may be regarded as association cells, i.e. as serving to establish a functional connection between many different cells at any given level of the grey matter. The white matter of the cord is divided by the fissures already described into anterior, lateral, and posterior columns. The nerve fibres of which it is composed are all of them axons of nerve-cells situated at different levels of the central nervous system or outside the cord. Since the whole object of the study of the anatomy of the cord is the tracing out of the systems of neurons of which it is made up, and therefore of the possible paths of any reflexes or nerve impulses through the cord, a mere anatomical differentiation of different columns is quite useless unless we can determine in each column the origin and destination of the fibres of which it is composed. For tracing out the course of the different axon systems in the central nervous system several methods are available. (a) HISTOLOGICAL. Two methods may be employed for staining a nerve-cell with all its processes, namely, the intravitam staining with methylene blue and the impregnation method invented by Golgi. In the latter method, of which there are many modifications, the nervous tissue is hardened in some chromate or biclu-omate, and is then soaked in a solution of silver nitrate or mercuric chloride. In this waj^ a precipitate of silver or mercuric chromate is formed within the nerve-cells and their processes ; but for some unexplained reason the impregnation is not general, and is confined to a small jjercentage of the neurons. If the precipitate were diffuse, even a thin section would be absolutely opaque ; since it is partial, thick sections may be cut and, after clearing, allow the tracing of the processes of the few impregnated nerve-cells through the whole thickness of the section. We may in this way get sections 0-1 mm. thick at the point of entrance of a posterior nerve-root and trace out the course and ending of a large number of the fibres composing the nerve-root, or we may in a section involving the anterior nerve-root trace the course of an axon of an anterior cornual cell out of the cord into the root. This method is of no use in tracing any given nerve fibre through the whole length of the cord. For this purpose, however, several methods are availal)le. (&) MYELINATION METHOD OF FLECHSIG. Nerve fibres at their first formation as axons of a nerve-cell are non-medullated, the medullary sheatli being formed later with the beginning of function of the nerve. It has been shown by Flechsig that the myelination does not occur simultaneously through all parts of the central nervous system, but that it is later in proportion as the nerve fibre is more recent in the phylogcnetic history of the animal. The cord in its most primitive form can be regarded fibres, as a series of ganglia ])residing over the different segments of the body. The most primitive fibres therefore would be those wliii-Ii run from the perii)hery of tiie body to each segment and from each segment out to the muscles, and so a medullary sheath is first formed in a number of the fibres entering and leaving the cord in the nerve-roots. Next in order of myelination are those rA— Fig. 15(5. Section through the vw- vioal s])inal cord of a new-born cliild. stained by Weigert"s method, to show absence of mcdullation in pyramidal tract, crt, anterior commissure ; Fp. crossed pyramidal tract ; /■'< . ilin-rt cerebellar tract ; Zip, jiostorior root zone ; rp'. iwsterior root- (Bechterew.) 320 PHYSIOLOGY fibres which connect different segments of the cord, the internuncial or intra-spinal fibres. Next come those fibres which connect the spinal cord with the cerebellum. Last of all to receive a medullary sheath are the fibres which take a direct course from the cerebral cortex to the spinal cord. These are called the pyramidal tracts, and in man are not medullated until the first month after birth (Fig. 156). (c) THE WALLERIAN METHOD. A nerve fibre, when cut off from the nerve- cell of which it is a process, degenerates. This degeneration is marked by a breaking up of the medullary sheath and a conversion of the phosphorised fat, myelin, of which it is composed, into ordinary fat. Later on the fat is absorbed and the nerve becomes replaced by a strand of fibrous tissue in the case of peripheral nerves, of neuroglia in the central nervous system. If the white matter of one half of the spinal cord be divided in the dorsal region, and the animal be killed about three weeks after the opera- tion, sections of the cord both above and below the lesion will show the presence of degenerated fibres. In order to display these fibres pieces of the cord are hardened in a solution containing bichromates and are then immersed in a mixture of osmic Fig. 157. Cells from the oculo-motor nuclei thirteen days after section of the nerve on one side. a, cell from healthy side ; h, cell from side on which nerve was divided. ( Flatatj. ) acid and bichromate. By this method ordinary fat is stained, but myelin is left un- stained (Marchi's method). Degenerated fibres are therefore stained black in virtue of their content in fat. The black staining has different distribution according as we take a section of the cord above or below the lesion. The existence of the degeneration shows that those fibres which are degenerated in the cervical region are axons of nerve cells situated below the lesion, while the fibres in the lumbar cord which are degenerated must have their nerve-cells in some part of the nervous system which is above the lesion. If the animal be kept alive for a considerable time, six months or more, before being killed, the occurrence of degeneration in any given area of the cord will be shown by the absence of normal nerve fibres in this area. In such a case some method of staining the medullary sheath, such as that of Weigert or Heller, is employed, when the degenerated area will be evident owing to its inability to take the stain. This method, however, is not so satisfactory as the Marchi method, since it is impossible in this way to detect in a section the presence of one or two degenerated nerve fibres, whereas by the use of the Marchi method they would appear as black dots in the unstained section {cp. Fig. 164). {d) METHOD OF RETROGRADE DEGENERATION. When a nerve fibre is divided there is no degeneration as a rule in the part of the nerve fibre central to the lesion. The nerve-cell is, however, affected, and the extent to which this occurs is more pronounced according as the lesion is nearer to the cell (Fig. 157). V, for instance STRUCTURE OF THE SPINAL CORD 321 an anterior root be divided and three weeks later the animal be killed and sections made of the corresponding segment of the cord and stained with toluidine blue or methylene blue, a striking difference will be observed between the cells of the anterior horn of the two sides of the cord. On the side of the lesion the nucleus of the cells will be somewhat swollen, and may be displaced towards the perijjhery of the cell. The Xissl granules are no longer distinct, but the whole cell is diffusely stained blue. In some cases this change may go on to complete atrophy of the cell and consequent degenera- tion of the whole of its axon. Generally, however, the cell gradually recovers, so that six months after the lesion no difference will be observable between the cells on the two sides of the cord. This method must be used with some caution as a means of tracing out the connections of any given neurons in the central nervous sy.stem, since it has been shown by Warrington that somewhat similar changes may be produced in the anterior horn-cells by division of the posterior roots, thus cutting off those im- pulses by which their activity is normally excited. Here we have a lesion applied to one neuron exciting a histological change in the cell body of another neuron which is next in the chain of the nervous arc. The structure of the cord is closely connected with and determines its two- fold function, namely, as a series of reflex centres for the different segments of the body, and as a means of communication between the trunk and limbs and the higher parts of the central nervous system. An examination of the relative area of the white matter at different levels of the cord shows a steady increase from the lower to the upper end. The increase is not, how- ever, proportional to the number of fibres which enter or leave the cord in the various spinal nerve-roots. Of these fibres therefore a certain proportion are destined to serve merely the local segmental reflexes, while others are con- tinued directly upwards to the brain or are connected with cells which them- selves send their axons up to the brain (cells of the columns). All the motor fibres in the nerve roots arise from cells in the spinal cord near the point of origin of the root. Any direct influence of the brain on the motor mechan- isms of the body is therefore effected through the intermediation of the segmental neural mechanisms of the grey matter of the cord. We will consider the function and related structure of the cord in these two aspects : first, as a reflex centre, and, secondly, as a conductor of impulses to the higher parts of the central nervous system. 11 SECTION VII THE SPINAL CORD AS A REFLEX CENTRE Iisr the evolution of the cord the primitive segmental arrangement has been especially interfered with by the development of the four limbs. Since the reactions of the hmbs transcend in importance and complexity those of the rest of the body, a great enlargement of the cord has occurred in the region of the nerve-roots which supply the limbs. Each limb must be considered as produced by the fusion of a number of body segments, in which the morphological segmental arrangement has entirely given place to a physio- logical one. Thus no single muscle of the limbs is innervated from one nerve-root, every muscle being formed from elements belonging to several segments and innervated from several nerve-roots. The segmental arrange- ment of the cord is hidden moreover by the increasing complexity of the spinal reflexes and the consequent involvement of many segments in even the simplest reactions. As we shall see later, practically no reflex can be evoked, even by stimulation of one nerve fibre or nerve-root in any of the vertebra ta, which does not involve in its response elements belonging to many segments. Since the reactions, which can be carried out by any part of the nervous system, depend on the neurons of which the part is composed, it is necessary, before treating of the reactions of the spinal animal, to consider the " way in " to and the " way out " of the centre, as well as the connections between the entering and issuing paths. Each segment of the cord gives off a pair of nerve-roots, subdivided into an anterior and a posterior root (Fig. 158). In mammals it is easy to show that the posterior root is exclusively afferent in function. Section of the root, either distal or proximal to the ganghon, pro- duces no paralysis of any description. It may cause diminished sensation in the area supplied by it, and if two or three adjacent posterior roots be divided, complete anaesthesia results in the central part of the skin area supphed from these roots. Stimulation of the central end of a divided posterior root evokes in a conscious animal signs of pain. In an animal possessing only spinal cord and bulb, reflex effects are produced, i.e. movements of skeletal muscles as well as effects on visceral muscles, such as constriction of blood- vessels, relaxation of intestinal muscle, and so on. On the other hand, section of an anterior root causes paralysis of muscles or parts of muscles. Section of all the anterior roots going to a limb will produce complete motor paralysis of the hmb. Stimulation of the central end of a divided 322 THE SPINAL CORD AS A REFLEX CENTRE 323 anterior root has no effect. Stimulation of the peripheral end evokes con- traction of muscles, and if the root experimented on be in the upper dorsal region of the cord, certain visceral effects, e.(j. dilatation of the pupil or augmentation of the heart beat, may result. To this general law, the law of Bell and Magendie, which affirms the purely afferent function of the posterior roots and the purely efferent function of the anterior roots, certain exceptions must be noted. In the first place, in the lower vertebrata the separation of afferent from efferent fibres seems to be not so complete as in the higher vertebrates. Thus in the chick Cajal and others have described fibres given off as axons from the cells of the grey matter and leaving the cord by the posterior root. The function of these fibres is unknowTi. In the frog Steinach has stated that visceral Fig. 158. Figures (from Yeo) to illustrate the degree and direction of degenera- tion as a result of section of the spinal roots. I, division of whole nerve below ganglion. II. division of anterior root. Ill, division of posterior root above ganglion. IV, division of posterior root above and below ganglion. effects may ensue on stimulation of the lower posterior roots. This statement is controverted by Horton-Smith, who, however, has noticed contractions of fibres of voluntary muscles as the result of stimulating these roots. In a class by themselves we must place the vasodilator effects observed bj' Strieker, Dastre and Morat, and Bayliss to follow excitation of the peripheral ends of the posterior roots. Bayliss has shown that the fibres, through which the vasodilatation is produced, must have their cell-station in the posterior root ganglia. It seems therefore that the same fibres provide for carrying both afferent impulses from skin to cord, and vaso- dilator impulses from the cord to the vessels of the skin. Bayliss has designated the impulses which effect the vasodilatation as antidromic, since they are opposed in direc- tion to the normal impulses of the nerve fibre. Of the same nature are the curious trophic impulses which extend along the posterior roots and which must come into play when eruptions of erythema or herpes occur as the result of inflammation or haemorrhages in the substance of the posterior loot ganglia. Both these phenomena are at present but imperfectly understood ; and their anomalous character is only intensified hy the further fact elicited by Bayliss, viz. that it is possible, by stimulation of afferent nerves, to excite reflexly vasodilatation through ihe intermediation of the posterior roots. Unless this reflex dilatation is simply an example of an ' axon reflex ' {v. p. 275) it would fiu-nish an exception to the otherwise universal law of forward direction in the mammalian nervous sj'stem. A third exception to the law of Bell and Magendie is only apparent. It is sometimes found that excitation of the peripheral end of a divided anterior root gives rise to mani- festations of pain or to reflex movement. This 1 as been shown by Schiff to be due to the presence, in the sheaths cf the anterior roots, of fine fibres derived from the posterior roots and taking a recmrent course to end probably in the membranes of the cord. This recurrent sensibility is at once abolished by section of two or three adjacent posterior roots. 324 PHYSIOLOGY THE WAY IN We may now consider the possible ways open to a nerve impulse entering the cord. Each posterior root on entering the cord divides into two bundles. The smaller bundle passes to the outer side of the tip of the posterior horn, where its fibres bifurcate (Fig. 159), giving rise to fibres which pass up and down the cord in a small longitudinal band of fibres known as Lissauer's tract. The fibres run only a short distance before turning into the grey matter, and terminate in arborisations round the cells of the substantia gelatinosa in the head of the posterior horn. By far the greater number of the posterior root- fibres pass to the inner side of the posterior horn into Burdach's or the postero- external column. Here they also divide into two main branches, one running up and the other down in the white matter. The descending branch passes through two or three segments before turning into the grey matter of the posterior horn of a lower segment. Of the ascending branches, some end at different levels of the cord, but a cer- tain proportion of the fibres from every root traverse the whole length of the cord in the posterior columns to terminate in the posterior column nuclei (nuclei graciUs and cuneatus) in the medulla oblongata. As we proceed up the cord the entering posterior root fibres displace the long fibres of those below towards the middle line, so that in a section through the cord in the upper cervical region the posterior median column, or column of Goll, is made up almost exclu- sively of fibres from the hind limb, while the postero- external column consists of fibres from the fore limb. Besides these distant connections, every entering nerve fibre makes connection with all parts of the grey matter in and about its level of entrance by means of collaterals (Fig. 160). Five groups of these collateral branches can be distinguished, i.e., ( 1 ) Fibres which arborise round cells in the posterior horn of the same side. (2) Fibres which pass through the dorsal grey commissure to the grey matter of the opposite side of the cord. Fig. 159. Longitudinal section of spinal cord of chick, showing bifurcation of dorsal root- fibres, and the passage of their collaterals into the grey matter. Three cells of the dorsal horn are also seen sending their axons into the dorsal columns. (Cajal.) THE SPINAL CORD AS A REFLEX CENTRE 325 (3) Fibres terminating round the median group of cells of the anterior horn. (4) Fibres which end in a rich basket-work round the cells of Clarke's column. (5) The sensori-motor bundle, which passes forwards through the grey matter to end round the cells in the anterior horn of the same side of the cord. Each entering posterior root fibre, besides these collaterals in the neighbour- hood of its entrance, gives but few to higher segments of the cord before it terminates in the pos- terior column nuclei. Sherrington suggests that the cells of Clarke's column receive fibres mainly from the ascending branches of the nerve roots from the posterior limb, a corresponding station for the nerve fibres of the anterior limb being represented by the cells of the nucleus cuneatus. That several different systems of fibres are included in these roots is shown by the different periods at which they acquire their myelin sheath. Among the earliest to acquire a sheath are the fibres which end in the pos- terior horn and those which pass to the anterior horn, while the long fibres in the dorsal columns do not become medullated until much later in foetal life. Since the nerve fibres of the central nervous system do not become functional until they have acquired a medullary sheath, we must conclude that the reflex responses affecting the segment in which the fibres enter are developed earher than those which involve also the activity of the cerebellum and medulla. The primitive segmental character of the central nervous system is retained in its pure form only in the segmentation of the dorsal spinal root ganglia. Each of these ganglia or afferent roots consists of the fibres from the sense-organs in a segmental area of the body surface as well as from the nuiscular and visceral apparatus in the same segment. Section of one dorsal posterior nerve-root will cause a diminution of sensibility over a band-like area corresponding to the distribution of the fibres of the root, though to produce complete insensibility the two adjacent nerve-roots must jitt FiG. 160. Chief collaterals of dorsal column fibres from new-born mouse. (C'aJal.) A, intermediate nueleus ; B, anterior (ven- tral) cornu ; c, dorsal or posterior cornu ; c, substance of Rolando. 326 PHYSIOLOGY be divided, in consequence of the overlap of fibres at the periphery. In the hmbs the segmental distribution of the sensory fibres is made out with more Fig. 161. Transverse section of spinal cord, showing collaterals terminating in a rich arborisation round the cells of Clarke's column (a, b), as well as others passing to the anterior cornua, and through the commissures. (CaJal.) difficulty. Each hmb must be regarded as made up from a series of fused segments, from five to seven in number. The accompanying diagram (Fig. 162) Tcdl aboysaZ or verutj-aL me/lvan. lUit- oftrvunkj (jf* tJve. kJT£^^ B.e£xxl Fig. 162. from Sheriington shows the manner in which the skin fields of these segments are combined to make up the total skin area in the hind Hmb of the monkey. THE SPINAL CORD AS A REFLEX CENTRE 327 THE WAY OUT Primitively the motor nerves also represent fibres passing from a col- lection of ganglion-cells to the muscles of the corresponding body segment. In the dorsal region this segmental arrangement of motor nerve fibres is still traceable in the adult animal. In all other parts the morphological has become subservient to a physiological arrangement. Every muscle of the hmbs contains elements from several segments, and is innervated there- fore from several anterior spinal roots. Hence it follows that stimulation of one anterior root produces no definite movement of a group of muscles, but partial contraction of a number of muscles which do not normally con- tract simultaneously. Thus stimulation of a sensory nerve may evoke either flexion or extension of a hmb, but not both simultaneously. Stimula- tion of the motor roots will cause simultaneous contraction of both flexor and extensor muscles. It is this subordination of morphological to physiological arrangement in the hmbs which has necessitated the formation of limb plexuses. The nerve-root is a mor- phological collection of fibres ; the nerve issuing from a limb plexus and passing to a group of muscles is a physiological collection. When it is stimulated it evokes a contrac- tion of a group of muscles which are normally synergic, i.e. co- operate in various movements. The fibres passing to the skeletal muscles are large, about 14 yu to 19 /x in diameter, and their axis cylinders represent the axons of large nerve- cells in the anterior horn. In the dorsal region of the cord in man, from the second dorsal to the second lumbar nerve-roots, the anterior roots contain, besides these coarse fibres, a number of fine fibres about 1-8 ^t to 3-6 /u in diameter (Fig. 163). These fine fibres were shown by Gaskell to leave the nerve shortly after the junction of the two roots, to pass as a white ramus commimicans to the sym- pathetic. Excitation of the white rami evokes various visceral eftects, such as dilatation of the }3upils, augmentation of the heart, contraction of blood- vessels, inhibition of the gut, erection of hairs, &c. Gaskell pointed out that the outflow of these fine fibres coincided with the existence of a prominent lateral horn in the grey matter, and suggested that cells of the lateral horn might be regarded as the origin of the visceral nerve fibres. This suggestion has been confirmed by Anderson, who has shown that section of the white rami communicantes brings about an alteration in the cells of the lateral horn as a result of retrocrrade degeneration. Fig. 163. Section across the second thoracic ventral nerve-root of the dog (stained with osmic acid) to show varying sizes of the con- stituent fibres. (Gaskell.) 328 PHYSIOLOGY CENTRAL PATHS OF SPINAL REFLEXES The impulse entering the cord is thus able to affect immediately a number of systems of neurons, namely, cells in the anterior horn, in the posterior horn, in Clarke's column, in the substantia gelatinosa, in the lateral column of the same side of the cord, and the corresponding groups of cells on the oppo- site side of the cord either directly by crossing collaterals or indirectly through III.T. ILL. V.S, Fig. 164. Cross-sections of spinal cord of a dog, showing the descending nerve- tracts originating in the first three thoracic segments (method of ' successive degeneration '). The eighth cervical segment had been excised and 568 days later a cross-cut was macle at level of the third thoracic nerve. The extent of the lesion is shown in the first figure (III. T). The other sections show the degenera- tions as revealed three weeks later by Marchi's method. (Shebrington.) cells which send their axons across the middle line. Through the ascending and descending fibres of the posterior columns it can also set into action the reflex mechanisms of adjacent segments of the cord. In addition to this direct spread of afferent impulses up and down the cord there is an anatomical basis for a co-ordination between the grey matter of different levels. This co-ordination is effected through the intermediation of the internuncial or intra-spinal fibres which pass up and down the cord from segment to segment. The course of the descending fibres may be studied by carrying out a total transection of the spinal cord at the sixth cervical vertebra, and six months later, when all the fibres degenerating as a result of the section have disap- peared, carrying out a further transection or hemisection a few segments below the first transection. If the animal be killed two or three weeks after the second operation it will be found that a number of fibres in the white matter are degenerated below the second section (Fig. 164). These fibres THE SPINAL CORD AS A REFLEX CENTRE 329 therefore must be derived from cells of the grey matter situated between the levels of the first and second sections, and they can be traced down the cord through a large number of segments. Analogous methods may be used for tracing the course of the ascending intra-spinal fibres. These intra-spiual fibres occur in the following situations : (1) In the lateral columns immediately outside the grey matter, in the bay between the anterior and posterior horns. (2) Close to the grey matter in the anterior basis bundle. (3) In the posterior columns, united with the descending branches of the entering posterior roots in the comma tract, and also in the immediate periphery of the cord and abutting on the posterior fissure in the septo- marginal tract. (4) Mingled with the fibres of the pyramidal tract. All these tracts are mixed, i.e. contain both ascending and descending fibres. As a rule, the longer the course of a fibre the more peripherallv does it he in the cord. The shortest of the fibres may onlv unite segment to segment, while the longest fibres may run through the greater part of the cord. THE SPINAL ANIMAL An animal possessing only a spinal cord contains a reflex neural apparatus which can be excited to activity by impulses of various qualities and from any part of the skin. Thus the afferent impulse may correspond to what in ourselves we call tactile and be provoked by mechanical stimulation, or may result from changes of temperature and correspond to those producing sensations of heat and cold. Strong stimuli of any kind may give rise also to afferent impulses which in the intact animal would have the quahty of pain. Since these stimuli are such as to produce injury if continued, they mav be named, when applied to the spinal animal, pathic or nocuous. The spatial distribution of the stimulus \\\\\ determine the situation and number of nerve fibres set into action, so that there will be a great variation in the distribution of the excited neurons of the central grey matter according to the quahty, distribution, and intensity of the stimulus. The efferent part of the reflex is provided for by the connection of the anterior cornual cells to the whole skeletal musculature of the body, as well as by the distribution of the axons of the lateral horn-cells to the sympathetic system and through this to the viscera. On the other hand, if the spinal cord be separated from the medulla oblongata and higher parts of the brain, it is deprived of all connection with the most highly elabai-ated sense-organs of smell, sight, hearing, and equili- bration, and also of the important afferent and efferent impulses which pass between brain and viscera through the vagus nerves. In studving the reaction of the isolated spinal cord we are studying a nervous system cut off from its most complex components, but at the same time deprived of the initiation and guidance which it must normally be continuallv receiving from the higher sense-organs through the brain. A study of the spinal animal will therefore be instructive as a study of the mammalian nervous svstem in its simplest possible aspect. It will, however, in all cases be the study of 11 * 330 PHYSIOLOGY an incomplete and maimed system, the incompleteness increasing as we ascend the scale of animals in our experimentation, owing to the increasing subordination of the lower to the higher centres, and of the immediate reflexes to the educated reactions of the anterior part of the brain. SPINAL SHOCK If the spinal cord of the frog be divided just below the medulla, for some minutes after the section all four hmbs are perfectly flaccid, and it is impos- sible to evoke any reaction by the apphcation of the strongest stimuh. If the animal be left to itself for half an hour there is a gradual return of reflex tone ; the animal draws up its legs and assumes a position not far removed from that of the normal frog, the head being lower than under normal conditions. We may say that the phenomena of shock in the frog last only a short time. With increasing complexity of the nervous system the phenomena of shock become more lasting, so that among laboratory animals it is in the monkey that spinal shock is most apparent. It is in- teresting to note, as pointed out by Sherrington, that shock appears to take effect only in the aboral direction. Thus, even in the monkey, section through the lower cervical region, though causing profound paralysis of the lower hmbs and part of the trunk, apparently has no influence at all on the reactions of the nervous system above the section. " The animal imme- diately after the section will contentedly direct its gaze to sights seen through the window, or, if the section have been below the brachial region, may amuse itself by catching flies on the pane. This is the more remarkable since the profound depression of the nerve-centres below the point of section extends also to the blood-vessels and viscera, so that there is a great fall of blood pressure and diminished production with increased loss of heat. The sphincters are flaccid or patulous, the skeletal muscles are toneless, and no reaction is evoked by the strongest stimulus to the skin or to a sensory nerve." Much discussion has arisen as to the duration of shock. Goltz and others imagined that the phenomena of shock may persist for months or even years. According to Sherrington, in the higher animals the phenomena of shock are comphcated by the onset of an " isolation dystrophy " which may occur before the condition of shock has entirely disappeared. In order therefore to examine the capabihties of the isolated spinal cord at their best, a time must be chosen when the sum of shock and isolation dystrophy together is at its minimum. The occurrence of shock after complete transection of the cord in the cervical region cannot be ascribed to the fall of blood pressure which ensues as a result of the severance of the efferent vaso- motor tracts from the vaso-motor centre in the medulla. The centres above as well as those below the transection are equally exposed to the effects of the lowered blood pres- sure, but it is only those below the section which show signs of shock. Nor can it be regarded as operative shock due to the severity of the lesion ; such an operative shock would be effective in either direction, and we do not THE SPINAL CORD AS A REFLEX CENTRE 331 find that the method of transection, whether by tearing across the cord or cutting it with a minimum disturbance, alters appreciably the amount of shock displayed by the segment of the cord situated below the lesion. On the other hand, if in a dog, which has undergone transection of the cord in the lower cervical region and has been allowed sufficient time to recover from the shock, a second transection be carried out two or three segments below the site of the first operation, the influence of the second section is hardly notice- able on the lower segment of the cord. Apparently then the chief factor in determining shock in all those centres situated aborally of the lesion is the cutting off of the impulses which are continually streaming down from the higher centres and from the great sense-organs connected with the anterior portions of the nervous system. With every rise in the animal scale the im- pressions received by the special senses take an increasing part in the deter- mining of all the reactions of the body, so that we might expect the effect of cutting off the impulses from the higher centres to be greater, the higher in the scale of animal hfe is the animal on which the experiment is carried out. The state of profound shock produced in the spinal cord by the operation passes off gradually. The blood pressure, which may have fallen to 40 or 50 mm. Hg., rises -within two or three days to its normal height, i.e. 90 to 110 mm. Hg. The sphincter muscles of the anus gradually recover their tone, and within a short time the reflex evacuation of the bladder and rectum may occur as in a normal animal. The skeletal muscles recover their tone within a few days, and after a short time co-ordinated movements can be brought about in the trunk and limbs by appropriate stimulation of sensory surfaces. At first the reactions thus produced are feeble and the reflex is rapidly fatigued. Of these reflexes those excited by nocuous or painful stimuli are the first to make their appearance ; a little later are seen those due to stimuli affecting the tactile organs in the skin, or the sense-organs of deep sensibihty situated round the bones and joints and excited by deep pressure or changes in posture of the limbs. In a dog which has undergone complete cervical transection two or three months previously, the tone of the muscles is somewhat increased. Although the dog is unable to walk, if it be raised and given a little push forward, so as to stretch the extensor muscles of its hind limbs, it may take two or three steps forward before its legs collapse. Although the locomotor apparatus is present, the nexus is lacking which determines the regulation of these move- ments through the organs of static sense, so that the spinal movements are insufficient to maintain the animal in such a position that a line drawn verti- cally from its centre of gravity shall fall between its points of support. On the other hand, swimming movements may be carried out regularly. The frog deprived of its brain can swim like a normal animal, but in consequence of the depression of its head tends to swim ever deeper in the water. If a ' spinal ' dog be held up by the fore limbs, the hind limbs nearly always enter into alternating movements of flexion and extension (' mark time ' move- ments), the two limbs acting alternately as in normal progression. The stimuli in this case seem to be started by the stretching of the skin and other 332 PHYSIOLOGY THE SIMPLE REFLEX stnictures at the front of the thighs. In such animals three reflexes, amongst others, can be excited almost invariably, viz. : (1) Scratch reflex. Gentle stimulation, mechanical or electrical, of any point over a saddle-shaped area on the dorsum behind the shoulders (Fig. 165) causes rhythmic movements of flexion and extension of the hind limb of the same side, the effect of which would be to scratch away the irritant object. These movements are re- peated at the rate of about four per second. [2) Flexor reflex. Nocuous stimuli, such as the prick of a needle appHed to any part of the foot, causes flexion of the leg and thigh, often accom- panied by extension of the opposite hind limb. (3) Extensor or ' stej)- ping' reflex. Gentle pres- sure appHed to the plantar surface of the hind foot, especially if the limb is somewhat flexed, causes a movement of ex- FiG. 165. A. The receptive field, whence the scratch tension of the hmb accom reflex of the left hind limb can be evoked. B. Diagram of spinal arcs involved. L, afferent path from left foot ; b, afferent j)ath from right foot; Ka, e/3, receptive paths from hairs on 'scratch area'; fc, final },• j ]• ] common path (motor neuron); pa, p/3, proprio-spinal neu- JHHCl umo rons. (Sheeringto^t.) panied sometimes by a flexion of the opposite In such an animal the carrying out of the vis- ceral reflexes may be very efficient. The blood pressure has attained its normal height and may be altered reflexly in very much the same way as in a normal animal, although the medullary vaso-motor centre can no longer be concerned. Thus in the diagram (Fig. 166) is represented the effect on the blood pressure of exciting the central end of the digital nerve in a spinal dog. The pressure rises from 90 to 208 mm. Hg. — a pressor effect as great as any which can be obtained in an animal still possessing all the connections of the vascular system with the vaso-motor centre. The height of the rise shows that as regards the influence on the blood pressure the spinal cord must be acting as a whole. No effect on the blood-vessels confined to the segment, or segments, adjacent to that of the nerve stimulated would suffice to cause a rise of more than a few mm. Hg. The reflex apparatus for other visceral functions seems to be equally perfect. The urinary bladder, when sufficient urine is accumulated, con- THE SPINAL CORD AS A EEFLEX CENTRE 333 tracts forcibly, the contraction being accompanied by relaxation of the sphincter and followed by rhythmic contractions of the urethral muscles ; accumulation of faeces in the rectum leads to their normal evacuation. With a httle assistance impregnation may be effected in or by such a maimed aoo mm Hg. 150 mm. Hg. 100 mm. HS b. p. Signal Time in 2" Fig. 166. Blood pressure tracing from a spinal dog. The signal indicates the time during which the afferent nerve was .stimulated. (Sherrington.) animal, and in the female may terminate at full term in normal parturition. Pregnancy is accompanied by hypertrophy of the mammary glands and is followed by secretion of milk, so that the young may be suckled as in a normal animal. Similar phenomena have been observed in the human sub- ject. Such an animal furnishes us with an opportunity of analysing the factors which are involved in the maintenance of muscle tone, as well as in the carry- ing out of the simplest reflexes involving contractions of the skeletal muscles. MUSCULAR TONE Every muscle in the body is in a condition of shghtly continued contrac- tion which keeps it tense, so that when it contracts in response to a stimulus there is, so to speak, no ' slack " to be taken up before the muscle begins to pull on its attachments. This tone is seen in the retraction undergone by muscles or tendons when they are divided in the living animal. If a frog possessing only spinal cord be hung up by its jaw, the limbs will be observed to occupy a position which is short of complete extension. The tone of the muscles which is concerned in the maintenance of this attitude is at once abolished by the destruction of the spinal cord. It may be abolished on one side by section either of the anterior roots going to the muscles, or of the posterior roots coming from the nuiscles (Fig. 1()7). In the intact animal muscle tone is diminished by disease and may be abolished during profound anaesthesia, as it is indeed in the condition of shock. Much light has been thrown on the factors which determine muscular tone by a study of the ' tendon phenomena ' of which the knee-jerk is the 334 PHYSIOLOGY most familiar example. If the leg is allowed to hang loosely in a jpositioil of slight flexion at hip and knee and the patellar tendon be struck, the ex- tensor muscles of the thigh contract and raise the leg. This phenomenon is known as the knee-jerk. Similar ' tendon reflexes ' can be obtained in other muscles, such as the tendo Achillis, the triceps, and the extensor muscles of the wrist, but with not so great ease as is the case with the knee. The knee-jerk is not altered by rendering the tendon anaesthetic by section .,-._.., , of all its nerves. The essential feature is a slight passive increase of the tension to which the muscle is already subjected. Since the jerk is abolished by sever- ance at any point of the reflex are, viz. muscle spindle, cord, muscle, it was thought at first to be of the nature of a reflex action. The interval, however, which elapses between the moment at which the tendon is struck and the response of the muscle is generally considered to be too short to allow of an impulse to travel from the tendon, or muscle, up to the cord and back again to the muscle. The interval was found by Gotch to be about -005 sec, whereas the latent period of contraction which ensues upon direct stimulation of the vastus internus is also •005 sec. On the other hand, the latent period when the nerve to the muscle was stimulated was '01 sec. The lost time of the knee-jerk is less than one-quarter of that of the briefest reflex time. The contraction has therefore been thought to be due to the direct stimula- tion of the muscle by the sudden stretching produced on striking its tendon. Mere tension of the muscle is not, however, the only factor. The tone which is reflexly maintained in the muscle is necessary for this response to direct stimulation to take place, and it seems to keep the muscles in a state of wakefulness ready to respond ]W.^ Tlfe posterior ^^ *^^ shghtest local stimulation. The knee-jerk is roots of the nerves therefore of special importance as an index to the tonic limb \ave been condition of the muscles concerned, being brisk and divided. easily elicited when the tonus is pronounced, and shght KEw.) ^^ absent when the tone of the muscle is depressed. Especially interesting is the relation shown by Sherrington to exist between the tonic condition of antagonistic muscles, e.g. between the ham- strings and the vastus internus of the quadriceps extensor muscle. Section of the hamstring muscles (so as to relax them) or even section of their nerve causes at once great increase in the jerk elicited by tapping the patellar tendon. On the other hand, the knee-jerk is abolished by stretching the ham- string muscles, or by weak stimulation of the central end of the cut nerve to the hamstrings (Fig. 168). In this way a voluntary flexion of the knee by contraction of the ham- strings automatically abolishes the resistance which would be offered by Fig. 167. Hind part of a spinal frog, THE SPINAL CORD AS A REFLEX CENTRE 335 L.3 the tonic contraction of the extensor muscles. In the absence of such an arrangement every movement of a joint, by stretching the antagonistic muscles, would automatically increase their tone, and thus set up a resistance to itself. The subject would thus be muscle-bound. Very great exaggeration of the tendon phenomenon is observed in cases where the pyramidal tracts are degenerated, and indicates a heightened reflex excitability of the lower spinal centres, perhaps reinforced by impulses from the cerebellum. The importance of the latter impulses in determining the myotatic irritabihty of the muscles is especially marked in man, where total transverse lesion of the upper part of the spinal cord often abolishes permanently the tone of the muscles innervated from the lower portion of the cord, and especially the knee- jerk. How far this absence of tone is due to collateral changes in the cord has not yet been determined. In animals com- plete transverse section of the cord of the cervical region is followed by increase of the knee- jerk, which in the rabbit may be ehcited within a quarter of an hour after the section has been carried out. In the increased myotatic irritabihty observed after . removal of the cerebral cortex, or after degeneration of the pyramidal tracts coming from the motor cortex, a single tap on the patellar tendon may evoke a series of contractions of the extensor nmscles of the thigh, giving rise to what is known as knee clonus. In the same way forcible flexion of the ankle causes a series of rhythmic contractions of the calf muscles (ankle clonus), varying in rhythm from six to ten per second. The heightened tone of the muscles under these conditions, and the ease with which any shght increase in their tension gives rise to clonic contractions, cause such patients to liave a peculiar dancing gait, characteristic of p}Ta- midal degeneration, and known as the ' spastic ' gait ; it is generally associ- ated with a certain loss of voluntary control of the movements of the Hmbs, so that the whole complex of symptoms is called ' spastic paraplegia." The value of the tendon phenomena as a means of diagnosis has tended to obscure their great importance in the normal individual. Every joint is protected by inextensible ligaments and by muscles. A sudden strain on a ligament either will have no effect, or will rupture some of its fibres and per- haps injure the adjacent joint surfaces. An ordinary reflex contraction Fig. 168. Diagram to show muscles and nerves concerned in Sherrington's experiment on the reciprocal innervation of antagonistic muscles. l3, l4, l5, third, fourth, and fifth Uimbar roots ; si, s2, first and second sacral roots. 336 . PHYSIOLOGY would be powerless to prevent this, since the mischief would be done before the reaction could take place. But the central nervous system confines itself to keeping the muscles awake, so that they themselves may react to any sudden increase in their tension by an equally sudden contraction, which saves the joint before the central nervous system has even become aware of the strain. The tone of the muscles, as well as the consequent tendon phenomena, is dependent on the integrity of the reflex arc governing the muscles in question. It has been shown by Sherrington that the afferent part of the arc is represented by the afferent nerves from the muscle itself, and that these nerves receive their sense impressions from the special nerve-endings charac- teristic of muscle — the ' muscle-spindles.' Even in the purely muscular nerves a large proportion of the fibres are afferent in function, and, after section of the appropriate posterior roots distal to the ganglia, as many as 40 per cent, of the fibres going to a muscle may be found degenerated. Though most of these have the muscle-spindles as their destination, a certain number pass to the tendon and aponeuroses connected with the muscle, where they end in the end- organs known as the organs of Golgi and the organs of Rufl&ni. After section of the motor nerves the muscle fibres degenerate, with the exception of the modified fibres, which, enclosed in a connected tissue sheath, are concerned in the formation of the muscle-spindles. Muscle tone and tendon phenomena may therefore be abolished by lesions of afferent nerves, which leave a considerable part of the cutaneous sensibility of the limb intact. In man the spinal reflex mechanism connected with the knee- jerk is situated in the third and fourth lumbar segments. The jerk may be abolished by section of the third and fourth posterior nerve-roots, although to render the whole hind limb anaesthetic it would be necessary to divide all the roots from the second lumbar to the fourth sacral inclusive. Recent researches by Snyder and by Jolly indicate that the reflex nature of the knee-jerk cannot be entirely excluded. Jolly, using the string galvanometer, has taken the current of action in the vastus internus muscle as an index of the commencing contraction of this muscle in the knee-jerk. He has also by the same method, by leading off the afferent and efferent nerves respectively, measured the lost time in the sense-organs and in the motor end-plates of the muscle. In the spinal cord he obtained the following electrical latencies in one case : Latency of knee-jerk ..... 5-3o-* Afferent endings ..... 04o- Nerve conduction . . . . .1-4 Motor endings and action current . . l-S 3-1 Synapse time .... 2-2o- In this case the shortest latency determined for nerve-endings has been deducted from the shortest latent period obtained from the knee-jerk in the spinal cat. On the other hand, some decapitated preparations have been found to present considerably longer latent periods, e.g. 11 and 12a. This variation in the latent period supports the view that the knee-jerk is a reflex of which the synapse time is very short, about 2(r, * o- = -001 sec. THE SPINAL CORD AS A REFLEX CENTRE 337 and that in certain cases there maybe increased delay in the spinal cord. When the latencies of the knee-jerk and the homonymous flexor reflex are compared by the electrical method, it is fomad that the latter is roughly double the former, the average latency of the knee-jerk in the spinal cat being 6-6cr, and of the homonymous flexor reflex 13-2o-. Jolly suggests that this difference may be due to the fact that the knee- jerk mechanism involves only one spinal synapse or set of synapses, while the flexor reflex may invoh^e two. In these estimates the rate of conduction in mammalian nerve has been taken at 120 metres per second. SECTION VIII THE MECHANISM OF CO-ORDINATED MOVEMENTS The detailed study of the chief reflexes obtainable from a spinal animal has, in Sherrington's hands, yielded much information as to the linking of the various events which are concerned in the carrying out of every co-ordinated movement, and as to the conditions which determine the sequence and extent of the activities involved. We may take, as the type of such a reflex, the flexion of the leg and thigh which ensues on the application of a painful stimulus to the ball of the foot, such as pricking with a needle, or the application of the faradic current. Of course, in the spinal animal no pain can result from stimulation of any part below the level of section of the cord, and it is better therefore under such circumstances to speak of nocuous or pathic stimuli, since all stimuli which cause pain are such that, if their operation continued, they would result in damage to the material structure of the animal. This flexor reflex is also easily obtainable in the frog as a result of stimulating one of its toes by mechanical or chemical stimuli, but it is easier to analyse the diflerent events involved in the reaction in the case of the larger animal. The effect varies with the strength of the stimulus. The minimal effec- tive stimulus causes simply movement of the foot. As its strength is in- creased this movement is attended by flexion of the leg on the thigh, and finally by flexion of the thigh on the body. With still further increase there is a spread to the opposite hind limb, which, however, performs the opposite movement of extension. Increase in the strength of stimulus causes not only an increase in the strength of contraction of the reacting muscles, but also an extension of the reaction to more and more muscles, or groups of muscles. The spread occurs always in definite order. The stimulus when represented by the prick of a needle can affect only one or two nerve fibres. The impulse carried along these fibres through a posterior root to the cord spreads in the cord, affects the motor neurons of the anterior horns, and causes these to discharge. The first discharge is as a rule Hmited to those in the immediate proximity of the entering impulses, but even when minimal, involves the simultaneous action of more than one anterior root. We may say that the motor response is determined to a certain extent by the spatial proximity of the afferent to the efferent tracts, but that it is always pluri- 338 THE MECHANISM OF CO-ORDINATED MOVEMENTS 339 segmental, the most important determining factor being the adaptation of the movement to the stimukis which is apphed. The gradual spread of the response with increasing strength of stimulus is spoken of as ' irradiation.' The nature of the response is determined by the locus or place of application of the stimulus and by the quahty of the latter. While a painful stimulus causes flexion of the leg, deep pressure on the plantar surface of the paw causes extension — the ' stepping ' reflex. However extensive the irradiation, the muscles which are set into action are always such that their actions co-operate towards a given end. Thus, when the impulse spreads to the opposite hmb and produces an extension, the reaction is such as would ensue when the dog steps on a sharp point and immediately retracts the irritated limb away from the injurious agent while it extends the other limb in the first act of progression or movement away from the dangerous spot. A superficial study of this reflex would therefore lead us to the conclu- sion that, by the varying resistance in the different synapses on the course of the connections of the stimulated aft'erent nerves, the impulses are directed so as to affect solely and exclusively the muscles whose activity will co- operate and aid the primary reflex. Such a description would, however, only represent one half of the process. Every muscle in the body is in a state of tone varying with its extension. If this tone is not to interfere with the carrying out of a reflex movement, there must be some means by which it can be inhibited. Such an inhibition we have seen occur as the result of con- traction of antagonistic muscles ; but the remarkable fact has been brought out by Sherrington that the impulses, which start on the surface of the body and set loose a chain of motor impulses resulting in the co-ordinated contraction of certain muscles, spread at the same time to the motor mechanisms governing the muscles antagonistic to the movement, and exer- cise on these an inhibitory effect. This inhibition can be easily shown in a spinal animal in the following way. The anterior thigh muscles are cut away from their attachments to the tibia and the patellar tendon is connected by a thread with a recording lever. On then exciting the flexor reflex by nocuous stimulation of the foot, the lever attached to the patellar tendon falls (Fig. 169b), showing that the extensor muscles have undergone actual elongation. The same eftect is observed even when the hamstrings, the flexors of the knee, have been divided. The inhibition of the extensor tone is thus not only determined by the in- creased tension of the flexors, but is a direct result of the primary cutaneous stimulus. From a broad standpoint the function of the nervous system is the co-ordination of all the activities of the body so that these may be com- bined to one common end, viz. the preservation of the organism. For this purpose there must be no clashing between opposing activities of different parts. If one part is engaged in any action this action must be the policy of the body as a whole. Yet the surface of the body is being continually played upon by ever-changing stimuli, tending to excite 340 PHYSIOLOGY first one reflex and then another, and the activities so excited would pro- duce confusion in the conduct of the animal, if there were not some means by which at any one time only one reaction should be in the act of being carried out. The imperative stimulus should dominate the actions of the body as a Fig. 169, a and b. (Sherrington.) The flexion reflex observed as reflex contraction (excitation) of the flexor muscles of the knee (a), and as reflex relaxation (inhibition) of the extensor muscle (b). The stimulus was a series of weak break induction shocks applied to a twig of the internal saphenous nerve below the knee. Observation B was made four minutes after A. Note the summation of stimuli, in each case six stimuli being required before the reaction was evoked. whole. Just as, in the mental world, attention must be undivided if we are to avoid confusion of judgment, so in the lower nervous activities there must always be concentration on one act or another. There may be a struggle of different stimuU, but one must finally be prepotent and annul altogether the influence of the others. The study of the spinal animal shows that this concentration of energy is obtained by the process of inhibi- THE MECHANISM OF CO-ORDINATED MOVEMENTS 341 tion. Every successful reflex, i.e. one which actually occurs, inhibits all other reflexes which are not co-operative with the one which is taking place. We may, for instance, stimulate the area of skin which gives rise to the scratch reflex, and at the same time apply a painful stimulus to the foot. The result is not a movement compounded of the two reflexes, but, as a rule, the flexor reflex preponderates. If, for instance, the scratch reflex be proceeding and then the foot be pricked, the scratch reflex immediately comes to an end, and the flexor reflex occurs. When this in its turn has come to an end, the scratch reflex may be once more re- sumed (Fig. 170). One stimulus may reinforce another if the reactions ensuing on the two stimuli are allied — i.e. tend to co-operate one with another. In every other case, however, an afferent impulse entering the cord and spreading to a motor mechanism, so as to produce a co-ordinate contraction of various muscles, causes at the same time inhibition of the muscles antagonistic to the movement, and a block, or inhibition, in all other reflex arcs of the cord. The anatomical basis of the various events involved in the carrying out of such a reflex as that just studied is shown in the diagram (Fig. 171). In this diagram the nerve-fibre a represents the pain- receiving or nociceptive nerve from the skin of the foot. This passes by a posterior root into the spinal cord, where it divides and gives off a number of collaterals. These collaterals, as we have already seen, pass in various directions ; some to the neighbouring grey matter, some to the centres in the higher parts of the nervous system. Neglecting the latter and any intermediate neuron which may be intercalated between the aft'erent fibre and the motor cell, we see that those collaterals which affect the motor cells of the muscles of the two hind limbs can be divided iifto two sets, one of which always produces during activity excitation in certain efferent neurons, whilst the other produces inhibition of the efferent neurons of the antagonistic muscles. The single afferent nerve fibre is therefore, with regard to one set of its central terminal branches, specifically excitor, and, in regard to another set of its central endings, specifically inhibitor. In the case in point the central terminal branches of the nerve a are excitor for the flexor muscles of the same side and inhibtor Fig. 170. Scrateli lefle.K tempo- rarily inhibited In- application of a pathic .stimulus to foot. Signal A, stimulation of scratch area. Signal n. stimulation of paw by strong induction shock. 342 PHYSIOLOGY for the extensor muscles of the same side and for the flexor muscles of the opposite side. The ascending branches of the nerve fibre in the same way will have endings w^hich, while inhibitor for the greater number of other possible reflex changes, will be excitor in a shght degree for certain efferent neurons whose action is allied to that of the primary reflex. The diagram shows also that the contraction of the flexor muscle, set up as the result Fig 171 Diagram indicating connections and actions of two afferent spinal root-cells a and a', in regard to their reflex influence on the extensor and flexor muscles of the two knees. The sign + indicates an excitatory effect, the sign — an inhibitory effect. (Sherrington.) of stimulating a, itself initiates a secondary reflex process from muscle up the nerve fibre a and back again to the muscle by the efferent neuron. This muscular afferent nerve also has central terminations of two signs — excitor to itself and inhibitor to the antagonistic muscles. For the sake of clearness the diagram omits a number of other channels coming from other regions of the cord, or from other efferent nerves, the sign of which would be negative, i.e. which would tend to inhibit the activity of the whole reflex arc. We see therefore that from every sensitive point on the surface of the body impulses can be initiated which will set into action whole chains of neurons, and will have a widespread influence throughout the central nervous system. It is important to note that the efferent path innervating, say, the flexor muscles of one side is common to many reflexes. It is used, for instance, by mutually antagonistic reflexes such as the scratch reflex and the flexor or THE MECHANISM OF CO-ORDINATED MOVEMENTS 343 pain reflex. We must assume therefore that the mutual inhibition of differ- ent reflexes occurs, not in the ' final common path ' — i.e. in the motor neurons, which must always remain open — but further back in the arc, probably near its afferent side. We have reason to believe that the propagation of impulses through the central nervous system involves expenditure of energy, and that the seat of this expenditure may be located in all probability at the synapses. It follows that the result of any particular sensory stimulation will not be absolutely invariable, but that the spread of the nerve process in the nervous system, Fic .172. ■ Mark-time ' reflex in spinal dog, inhibited by slight stimulation of the tail (duration of stimulation shown by signal). Note the augmentation of the mark-time reflex following the inhibition (successive spinal induction). (Shekrington.) and the degree of block presented by the various synapses and determining the potency of any given reaction, will depend on the condition of the various synapses at the time of the stimulation. This condition may be altered in various ways. Repeated excitation causes in the synapses, just as in the nerve-endings of the skeletal muscle, a condition of fatigue. Stimulation confined to a single point in the " scratch area ' of the spinal dog excites a scratch reflex which rapidly dies away. On shifting the exciting electrodes a little to one side the reflex act begins again, often with greater force than at first, and a very prolonged reaction can be induced by gradually moving the electrodes along the surface of the skin. A reflex arc therefore rapidly shows signs of fatigue, and the minute change in locus of stimulus which is required to reinduce a practically identical action shows that the seat of fatigue must lie chiefly on the afferent side of the arc ; perhaps in the first synapses through which the impulse has to pass. This easy incidence of fatigue tends to cut short any given reaction and to render it easier for other reactions to take its place. 344 PHYSIOLOGY Just as excitation causes fatigue and therefore furnishes a hindrance to repetition of the same act, so the reverse process of inhibition, which is a large component of every reaction, is followed by a condition of increased excitabihty, or diminished resistance to the passage of impulses. In each case there is a tendency for a ' swing-back ' to take place from inexcitability, to over- excitability, from excitation to inexcitability. This ' successive spinal induction,' as it has been termed, may be seen on inhibiting some moveinent by the excitation of an independent reflex. Thus the scratch reflex is excited, and then while the excitation is still continued the reaction is inhibited by excitation of the extensor or stepping reflex. As soon as the ' stepping ' reflex has passed off, the scratch reflex returns with an intensity greater than before. This successive spinal induction explains the tendency which exists in the spinal cord to an alternation of response ; every act tend- ing to come to an end by fatigue and, as a result of negative spinal induction, inducing the opposed or antagonistic act. Thus if a spinal dog be held up in the vertical position, so that the hind limbs hang freely, these latter execute a series of alternate movements of flexion and extension. The starting-point of these is the stretching of the anterior thigh muscles. Once started they continue of themselves, each act exciting the alternating antagonistic act. A reflex act has often been distinguished from other reactions, described as conscious or purposive, by its fatality — i.e. by the invariability with which it results on a given stimulus, whether the reaction be for the good of the animal as a whole or not. Thus a decapitated eel will wind itself with ecjual readiness around a stick or a hot poker. All reactions are, however, purposive. The machinery for them has been evolved and the paths laid down in the spinal cord under the action of natural selection, so that they must act, at any rate in the average of cases, towards the well-being of the animal as a whole. Since the nerve path involved in any reaction includes a number of synapses, each of which may be influenced from other parts of the body in a positive or negative direction, an absolute uniformity of response cannot be predicated for any one reaction. There will be changes in the facility with which it is evoked and changes in its extent, and these will become the more operative the greater the complexity of the arc, and the larger the number of other impulses to which it may be subject. The fatality of response is therefore only shown at its best in the very simplest of reflexes, or the most lowly organised nervous systems. The purposive character of the reflexes obtained from the spinal frog has some- times led writers, especially in pre-Darwinian days, to endow the spinal cord with a guiding intelligence. At the present time we recognise that every reaction of a living being must be purposive, in the sense of being adapted to the preservation of the species, if the latter is to survive in the struggle for existence. The question as to whether we are justified in predicating the existence of even a germ of consciousness or volition in the spinal animal must be decided in the negative. " Associative memory would seem to be a postulate for the very existence of perception. Where even simplest ideas are not, there cannot be consciousness. Animal movements that are appropriate not only for an immediate but also for a remote end indicate associative memory. THE MECHANISM OF CO-ORDINATED MOVEMENTS 345 The approach of a dog in answer to the calling of its name, the return of an animal when hungry to the place where it has been wont to receive food, such movements may be taken as indicative of consciousness since they indicate the working of associative memory. Examined by this criterion all purelj^ spinal reactions fail to evince features of consciousness " (Sherrington). THE PART PLAYED BY AFFERENT IMPRESSIONS IN THE CO- ORDINATION OF MUSCULAR MOVEMENTS. Every reflex act is initiated in the first place by some form of sensory stimulus. In the carrpng out of the muscular contractions and the resultant movements of the hmbs, other impulses are set up in the structures which subserve deep sensibility, in- cluding those of muscles, which in their turn aftect the excitability and the activity of the motor neurons. These secondary afferent impulses are im- portant whether the movements be aroused by immediate sensory stimulation of the surface of the body, or through the higher parts of the brain, as in volitional movements. Their significance is shown by the marked disorders of movement pro- duced in a limb by section of some or all of its afterent nerves. Thus if all the posterior roots supplying one hind limb of the frog be divided the posture of the desensitised limb is abnormal, whether the frog be suspended or be in a sitting posture. Such a frog generally swims with the desensitised hmb in permanent extension. The complete absence of muscular tone under these circumstances has already been mentioned. When a contraction of the cjuadriceps extensor is induced b}^ a single shock applied to the intact motor nerve, the curve obtained shows a relaxation Hne much slower and more prolonged than when the cut nerve is similarly excited. In the latter case, or when the posterior roots alone are divided, the lever at the end of re- laxation dips below the base hne with an inertia fling, which is never present while the nerve is intact. The contraction of the muscle, when its afterent path is intact, seems to develop reflexly in the muscle itself a condition of tone which damps the inertia swing of the contraction. In the dog, after section of the afferent nerves of one hind hmb, this limb is not at first used for walking ; it is kept more or less flexed at hip and knee, and later, when it is employed in walking, it is lifted too high with each step. After divasion of the afferent fibres of both limbs these appear as if they were affected with motor paralysis. At first, during walking, the fore limbs simply drag the hind limbs after them, though later, as the hind limbs are drawn along, they make alternate movements and may ultimately afford a certain amount of support to the body. Still more striking effects are observed in complete apa?sthesia of the fore limb in monkey or man. The Hmb is permanently paralysed ; it is never used in climbing, or in the taking of food. That the peripheral motor mechanism is intact is shown by the fact that stimulation of the appropriate area of the cerebral cortex in such animals elicits at once a perfectly normal movement of the hand or limb. It seems, however, impossible for the cortex to initiate such movements in the absence of all aff"erent impulses arriving from the limb. Similar paralysis was observed by Chas. Bell in the upper lip of the ass after 346 PHYSIOLOGY section of the corresponding branches of both fifth nerves, and was interpreted by him as indicating a possible motor function for these nerves. In these phenomena of sensory paralysis we are deahng with the effects produced by the deprivation of two distinct classes of afferent impressions, viz. those from the skin, and those from the deep structures and muscles. The phenomena due to these two factors may be studied separately If in the monkey all the afferent brachial roots except the last cervical, which supplies cutaneous sensations to the whole hand, be divided, the monkey uses the arm and hand both in chmbing and in taking food. A marked ataxy of the movement is, however, observed. Whereas the normal monkey, in taking grains of rice out of the observer's hand, exhibits perfect precision of move- ment so that he rarely touches the hand on which the grains are lying, the monkey with only cutaneous sensibility remaining grasps clumsily with the whole hand, and the arm sways as it is put out, often missing the object aimed at altogether. Cutaneous insensibihty of the hind limb causes very httle disturbance of locomotion, the alternate movements of which seem to be started by the stretching of the structures at the front of the thigh. On the other hand, a patient affected with such a loss may be the subject of ' static ataxy,' i.e. he is unable to stand with his feet together and his eyes shut. The afferent impressions from the skin of the feet appear therefore to be neces- sary for the maintenance of static equiUbrium. In the carrying out of co-ordinated movements, such as those of loco- motion, the impressions from the muscles play a more important part. Division of the afferent nerves from the muscles gives rise to a condition of tonelessness, and the passive mobihty of the joints is greater than usual, so that the hip with the Hmb extended at the knee may be flexed to an abnormal extent. The effect of this loss of tone is more apparent in the case of certain muscles. The disturbance of co-ordination resulting from the cutting off of afferent muscular impressions is well seen in cases of tabes dorsahs, or locomotor ataxy, in man, and to a slighter extent in cases of peripheral neuritis affecting chiefly the sensory nerves of muscles. The ataxic gait of such patient is characteristic. There is no loss of power in the muscles, but there is loss of control. The patient is unaware of the position of his Hmbs and has to guide his walk by visual impressions ; even then the movements are inco-ordinated. The contraction of every muscle is exaggerated, so that in walking the leg is first raised too high and then is brought down on to the ground with a stamp. As the disease progresses the loss of control becomes more and more pronounced, so that attempts to walk simply give rise to a profusion of disordered movements, the legs being thrown in all directions with the patient's efforts, but with no effective result. The centres are no longer informed of the degree to which each muscle is con- tracted, and the impressions are wanting which should cut short the con- traction of a muscle when it has attained its optimum, and which should inhibit the antagonists during the contraction and induce activity of the antagonists in successive alternation to those of the other muscles. In such a patient therefore walking finally becomes impossible, and, with well- THE MECHANISM OF CO-ORDINATED MOVEMENTS 347 nourished muscles and a motor path which is intact, he is condemned to pass the rest of his days in bed. THE EFFECT OF POISONS ON THE SPINAL CORD The reflex functions of the spinal cord may be abolished by the same drugs, such as ether, chloral, &c., which abolish conductivity in a nerve fibre. The central effect of these drugs is obtained with much smaller con- BODY , prostnotonic NECK . - turning BODY. ^ opisthof-onic NECK retraction FACE \ Fig. 173. Diagram by Sherrington to show influence of tetanus toxin on the response to excitation of the motor area of the cortex in the monkey. A, normal animal. B, after poisoning with tetanus, f and/ = flexion of leg and arm respectively, e and e signify extension. < signifies opening of mouth ; = signifies closing of mouth. centrations than is the case with the peripheral nerves and is the cause of their anaesthetic effect. More interesting from the point of view of the physiologist is the action of such a drug as strychnine, or the somewhat similar action of the toxin formed by the tetanus bacillus. If a small dose of strychnine be injected into a spinal frog, after a short period of heightened irritability the slightest stimulus applied to the surface will cause spasms, which may affect every 348 PHYSIOLOGY muscle iu the body. Pinching the foot, instead of causing it to be drawn up now causes the legs, arms and back to be rigidly extended. The extension is not a co-ordinated act, but is associated with strong contraction of the flexors, the final position of the limbs being determined by the preponderating strength of the extensor muscles. The real meaning of this condition is seen if, in a spinal mammal, the extensor muscles be connected with a lever and the flexor muscles cut. On exciting the flexor reflex by pricking the foot, there is instantaneous relaxation of the extensor muscles. A small dose of strychnine is now given, insufficient to cause general convulsions. It is now fomid that on pricking the foot the extensor muscles respond, not with inhibition, but with a contraction. Strychnine acts by abolishing the in- hibitory side of every co-ordinated act and converting the process of inhibition into one of excitation. Co-ordination therefore becomes an im- possibility, and stimulation of any spot excites contractions not only of the appropriate muscles but also of the antagonists of these muscles, the direction of the resulting movement being determined simply by the relative strength of the two sets of muscles. The same effect is produced by tetanus toxin, and, since the action of this toxin may be conflned in its early stages to one limb, it is possible to show the abolition of the inhibitor side of the reflexes in this one limb while the limb of the other side reacts normally to the stimulus. The same abolition of inhibition is found whether the response be excited by stimulation of the skin or by voluntary excitation from the cortex of the brain. Thus in the monkey, on stimulating the cortex, opening of the mouth may be excited from all the spots marked " • - S fl s THE SPINAL CORD A8 A CONDUCTOR 359 one segment of the cord to another through grey and white matter alter- nately. It is supposed that all of the ascending tracts may convey afferent impulses from the posterior spinal roots to the brain, although evidence as to the part taken by each tract is very conflicting. The following account represents the views which may be regarded as the most probable (Page May) (Fig. 176) : Pain impulses, on entering the cord by the posterior roots, cross to the other side at once, and then pass up, chiefly in the antero-lateral column, by the spino-thalamic fibres,. as far as the optic thalamus. Sensations of heat and cold take a very similar course. Hence they are generally affected by lesions of the cord in the same way as pain sensations. Impulses of touch and pressure, after entering the cord, pass up in the posterior column of the same side for four or five segments, then cross gradually and pass up in the opposite anterior column. Impulses serving muscular sensibility, including the impulses from joints and tendons, take two courses. Those which do not reach consciousness, and are involved in the involuntary guidance of mus- cular movements, run up chiefly in the anterior and posterior cerebellar tracts of the same side. Those w^hich furnish the material for conscious sensations and give information as to the position of the Umbs, &c., are entirely homolateral, and travel up in the posterior columns of the same side of the cord. All impulses which reach the brain cross finally to the optic thalamus and thence to the cerebral cortex of the opposite side. Hemisection of the cord on one side, as was first pointed out by Brown Sequard, causes the following symptoms : (1) Paralysis of the voluntary motor conductors on the same side. (2) A paralysis also of the vaso-motor conductors on the same side, and, as a consequence, a greater afflux of blood, and a higher temperature. There may be some degree of hypersesthesia on this side. (3) There is anaesthesia affecting all kinds of sensibihty, excepting the muscular sense, in the opposite side to that of the lesion, owing to the fact that the conductors of sensitive impressions from the trunk and limbs decussate in the spinal cord ; so that an injury in the cervical region of that organ in the right side, for instance, alters or destroys the conductors from the left side of the body. (4) There is some degree of anaesthesia also on the side of the lesion, in a very limited zone, above the hypera^sthetic parts, and indicating the level of the lesion in the cord. This anaesthesia is due to the fact that the con- ductors of sensory impressions, reaching the cord through the posterior roots, at the level or a little below the seat of the alteration, have to pass through the altered part to reach the other side of the cord. The only direct unbroken cortico-spinal fibres are those contained in the pyramidal tracts. Motor impulses, which start from the cerebral cortex on one side, pass down that side till they reach the lower part of the medulla. Here the greater number of the fibres cross over in the pyramidal decussation to run down in the crossed pyramidal tract on the other side of the cord. The few fibres which do not cross over in the pyramidal decussation are continued as the direct or anterior pyramidal tract. These, however, also 560 PHYSIOLOGY cross to the other side in their passage down the cord before becoming con- nected with the anterior cornual cells. Hemisection therefore of the spinal cord in the dorsal region will produce paralysis of motion and loss of or impaired muscular sensation in the parts supplied by the nerves on the same side below the lesion. A great part of the white matter of the cord is concerned then in main- taining connection between the brain and higher parts of the nervous system and the periphery, through the intermediation of the cells of the grey matter of the cord. Corresponding to this function we find a gradual increase in the number of fibres in the white matter as we ascend from the sacral part of the cord to the medulla, the white matter being continually reinforced as it ascends the cord by fibres establishing connection with the ganglion- cells forming the nuclei of the nerve-roots. Vaso-motor impulses to the limbs travel down the lateral columns of the cord on the same side. THE BRAIN SECTION XI THE STRUCTURE OF THE BRAIN STEM The physiology of the brain falls naturally into two main divisions, namely, that of the brain stem, including the medulla, the pons, Sylvian iter, corpora quadrigemina and third ventricle, and that of the cerebral hemispheres. It is usual, in treating of the structure of the brain stem, to consider it as a prolongation forwards of the spinal cord and as consisting, like this, of a central tube of grey matter surrounded by a tube of white matter. Like the spinal cord, the brain stem may be regarded as originating primitively by the fusion of a series of ganglia presiding over the local reactions of their respective somites. The modifications in this segmental arrangement, which have occurred in the course of evolution, have been so profound that little trace of the primitive segmental arrangement is to be observed. At the fore end of the body have been developed the organs of special sense, which are the most important in determining the reactions of the animal in response to present or approaching changes in its environment. Indeed the whole course of evolution is conditioned by the evolution of the brain stem in the first place, and of its outgrowth, the cerebral hemispheres, in the second. Hence we cannot expect to find in the brain stem the regularity of arrange- ment of grey and white matter that we have studied in the cord. The typical division of the grey matter into cornua becomes altogether lost. While some nerves take their origin from or terminate in the central tube of grey matter, in other cases the collections of nerve cells and fibres forming the nuclei of the cranial nerves have become more or less separated from the central axis. Moreover the central grey matter is by itself quite inadequate to deal with the flood of afferent impressions entering the central nervous system through the organs of special sense, or to co-ordinate these with one another or with those arriving from the skin and lower part of the body. Masses of grey matter, which have no representative in the cord, make their appear- ance, and may be regarded as additional sorting stations or fields of conjunc- tion for the afferent and efferent impulses which determine the nervous activities of the animal. The general features of the structure of the bi'ain will he best umlerstood by reference to the mode of development of this part of the central nervous 3G1 12 ♦ 362 PHYSIOLOGY system. At the front end of the body, the primitive neural tube, formed by the invagination and growing over of the epiblast, is somewhat enlarged and is marked off by two constrictions into the three primitive cerebral vesicles, which are named respectively the fore-, the mid-, and the hind-brain, or the prosencephalon, the mesencephalon, and the rhombencephalon (Fig. 177). At their first formation the walls of these vesicles are composed of simple epithehal cells, and show no trace of nervous structure. A httle later the cells forming the walls present a difierentiation into neuroblasts and spongioblasts, the former developing into nerve- cells, while the latter form the neuroglial supporting tissues of the brain and probably also furnish the cells of the sheath of Schwann to the outgrowing cranial nerves. In some places the wall of the vesicles remains un- differentiated ; no nervous tissues develop in it, and FiGf. 177. Diagram of the j^ forms a layer of epithehum known as ependi/ma. cerebral vesicles oi the / ^ _ . . brain of a chick at the By the varying growth of nervous tissue in different second day. (Cadiat.) ^^^^ ^f ^^^ ^^jj ^^^ . -^^ structure of the adult 1, 2, 3, cerebral vesi- , . . -, ^ ^ ~J: _^^ _, . , , • t cles ; 0 optic vesicles. Dram IS brought about (Fig. 178). Ihus m the hmd- brain, or rhombencephalon, the roof of the neural canal posteriorly fails to develop, so that in the adult brain there is merely a layer of epithehum covering the expanded central canal, here known as the fourth ventricle. This back part of the hind-brain is often called the myelencephalon, the anterior portion being'the metencephalon. The floor of the myelencephalon undergoes •^ ^ . . ° f/^(^ pin considerable thickening and forms the future medulla ob- longata. In the metencephalon, nervous tissue is developed all round the canal, the floor of the canal forming the pons Varohi, while the cerebellum is developed by an outgrowth of the dorsal ^ ^.,., ., ,,.. , ■ _ ^ . Fig. 178. Longitudinal section through brain oi wall. In the region of the con- chick of ten days. (After MiHALKOVicz.) Striction between the hind- and olf olfactory lobes ; h, cerebral hemisphere ; •j-u-1 i 1 • ii Iv, lateral ventricle; pin, pineal gland; bo, cor- mid-bram known as the isthmus, p^^a bigemina ; cbl, cerebellum ; oc, optic com- the roof or dorsal wall forms the missure ; pit, pituitary body ; pv, pons Varolii ; T „ T T , mo, medulla oblongata ; v^, v*, third and fourth superior cerebellar peduncles at ventricles. the side, and between them a thin layer of nervous matter known as the valve of Vieussens, or superior medul- lary velum. The cavity of the third vesicle corresponds in the adult brain to what is known as the fourth ventricle. The mesencephalon, or second cerebral vesicle, takes a relatively small part in the formation of the adult human brain, though very conspicuous in many of the lower types of brain. The whole of its wall is transformed THE STRUCTUKE OF THE BRAIX STEM 363 into nervous tissue, the roof or dorsal wall forming the corpora quadrigeniina , while the two crura cerebri are developed in its ventral wall. The cavity of the second cerebral vesicle is retained as a narrow canal known as the aque- duct of Sylvius, and connects the fourth ventricle with the third ventricle. Very soon after its first appearance the first cerebral vesicle is modified by the formation of lateral expansions, known as the optic vesicles, which later on are constricted off from the central part of the cavity so as to be connected with this by two short tubular passages, the optic stalks. From the optic vessels are ultimately developed the retinae of the eyes. By the development of nerve-cells in the optic cup the ganglion-cell layer of the retinae is produced, and from these cells fibres grow back along the optic stalk and make connection with the grey matter developed in the lateral wall of the fore-brain and w^ith the adjacent parts of the mid-brain, viz. the superior corpora quadrigemina. The large masses of nervous tissue de- veloped in the lateral walls of the fore-brain are the optic thalami, which represent the head ganglia of the brain stem. The front portion of the first cerebral vesicle expands in a forward and downward direction, and from the upper and lateral aspects of the outgrow^th thus formed the cerebral hemi- spheres are produced as two hollow pouches. The original back part of the fore-brain is sometimes spoken of as the diencephalon, while the anterior part of the cerebral hemisphere growing from it is the telencepha[lon. The floor or ventral wall of the fore-brain undergoes moderate thickening to form the nervous structures which occupy the ' interpeduncular space ' at the base of the brain, viz. the posterior perforated spot, the corpora mammillaria and the tuber cinereum. The roof of the first cerebral vesicle remains thin and in its primitive epithehal condition, like the roof of the back part of the fourth ventricle. In the course of development the cerebral hemisplieres become larger than the whole of the rest of the brain put together, growing backwards over, the latter as far as the middle of the cerebellum (Fig. l79). Their dorsal and lateral walls become much thickened and consist of white matter internally and grey matter externally. The part of the hemisphere which lies over the first cerebral vesicle is undifferentiated and remains as a simple epithelial layer. This becomes closely applied to the similar layer forming the roof of the third ventricle, from which it is separated only by a process of the pia mater carrying numerous blood-vessels (the velum interpositum). Tn the adult brain the cavities of the cerebral hemispheres are known as the lateral ventricles, the remains of the first cerebral vesicle receiving the name of the third ventricle. The lower and outer part of the hemispheres, i.e. the part which is first formed, becomes much thickened and forms the corpus striatum, which is closely applied to the front and outer part of the optic thalamus. In the corpus striatum two masses of grey matter are developed, namely, the nucleus caudatus and the nucleus lenticularis. A layer of nerve fibres ascends from the brain stem to be distributed throughout the whole of the cerebral hemispheres. This forms a sort of capsule to the optic thalamus, lying between this body and the corpus striatum beliind. but in 364 PHYSIOLOGY It is called the front piercing the corpus striatum between its two nuclei, internal capsule. The development of the different parts of the brain stem from the three cerebral vesicles and their gradual subordination and overshadowing in the course of development by the cerebral hemispheres is well shown if we Lamina terminalis Optio recess Optic nerve Optic commissure Hypophysis Anterior commissure Foramen of Monro 3rd nerve Corpus mammillare 3rd ventricle Cerebral peduncle / / Pons ' / / / / / V \r-w- suprapineal recess / / / / / / ^\ ,, , „ Cerebellum / ' \ Medulla oblongata Pineal bodv / / / ' 4th ventricle I ■' Superior medullary velum Cerebral aqueduct Corpora quadrigemina Fig. 179. Median section of an adult human brain. (J.Symington.) compare the brain of a fish with that of a reptile and again with that of a mammal (Fi g . 1 80) . Man's position in the scale of animal life is determined not by increasing complexity of the structures forming his brain stem, but by the gradual subordination of these to the latest formed cerebral hemispheres, and the enormous growth of his capacity to adapt himself to a varying environment consequent on the increase in size of his cerebral hemispheres. THE STRUCTURE OF THE BRAIN STEM 365 Ammocietes Telkustka Amphibia ,CB CB Mammalia ^ jy, CO / P i 1 R I A ,*^ri^y(/^P Fig. 180. Diagrammatic view of the brain in different classes of vertebrates. (Gaskei.l.) tB,eercbelhnn; ft. pituitary body : pn. pinea body; c.str, corpus striatum ; GHR, right ganglion babenulje ; i, olfactory ; n. optic nerves. 366 PHYSIOLOGY THE HIND-BRAIN It will be convenient to trace first the modifications undergone by the axial part of the nervous system in the brain, and then to deal with the new masses of grey matter which have no homologies in the spinal cord, as Fig. 181. Section through the lower border of the medulla oblongata, at the pyramidal decussation. (Bechterew.) fla, anterior fissure ; d, decussation of the pyramids ; F, anterior columns ; Ca, anterior cornu ; cc, central canal ; S, lateral columns ; fr, formatio reti- cularis ; ce, neck, and g, head of the posterior cornu ; rpCl, posterior root of first cervical nerve ; nc, beginning of nucleus cuneatus ; ng, nucleus gracilis ; fP, funiculus gracilis ; H^, funiculus cuneatus ; sip, posterior fissure. well as the long tracts of white matter serving to connect different levels or different sides of the brain. In examining successive sections from the spinal cord up through the medulla, the first change which makes its appearance is due to the decussa- tion of the pyramids (Fig. 181). Throughout the spinal cord, fibres have been crossing from one side to the other through the anterior white com- missure, many of them belonging to the pyramidal system. But at the lower border of the medulla we see a large mass of fibres crossing between the anterior columns and the postero -lateral columns, at first cutting off the head of the anterior horn and later on breaking this up altogether, so that the only definite collection of grey matter left in this situation is a small part of the lateral column of grey matter known as the lateral nucleus. In this way are formed the big anterior columns of the medulla, which are known as the pyramids, and contain all the fibres that in the cord are represented by the direct and crossed pyramidal tracts. The next change is due to the ending of the posterior columns (Fig. 182). These are the central ascending branches of dorsal nerve roots, having there- fore an origin outside the cord. On their way up the cord they send in collaterals to end in the grey matter of the posterior horn. The main mass terminates in the medulla, just above the pyramidal decussation, in two collections of grey matter — the nucleus gracilis and the nucleus cuneatus — THE STRUCTURE OF THE BRAIN STEM 367 which are formed by a great hypertrophy of the grey matter at the root of the posterior horn. The effect of this development in the dorsal region of the medulla is to push the head of the posterior horn outwards. At the same time this mass of gelatinous substance becomes enlarged, so that in section we have three grey masses from within outwards, the nucleus gracilis, the nucleus cuneatus, and the nucleus of Rolando. Funiculus gracili Funiculus cuiicita'""' Sp. root of 5th I) — y^f Formatio reticuliii j Lower end of oli\ m eminence nucleus ■Cuneate nucleus ^ubst. gel. Rolandi Decussation of fillet Int. access olivary n. >fcrve XII. Fig. 182. Transverse section through medulla of foetus, immediately above pj'ramidal decussation. (Cunningham.) Stained by Pal- Weigert method. The fibres of the postero- median column, which are derived chiefly from the lower limb, end in arborisations round the cells of the micleus gracihs, while those of the postero- external column, or column of Burdach, of which the majority is formed by fibres from the upper limbs, terminate in the grey matter of the nucleus cuneatus. The cells of these two masses of grey matter of course give off axons, which can carry on the impulses brought to them by the fibres of the posterior columns. These axons speedily leave the dorsal aspect of the medulla, bending round, as the arcuate fibres, to the deeper parts of its structure. Thus nothing is left to take the place of the posterior columns on the posterior aspect of the cord. With the dis- appearance of these columns and the development of the pyramids we get a practical obliteration of the anterior fissure and a displacement of the central canal towards the dorsal surface. A little higher up (Fig. 183) the canal opens out altogether, forming the fourth ventricle, covered on its dorsal surface only by a thin layer of ependyma, a simple epithelium representing all that is left of the dorsal wall of the primitive cerebral vesicle. The appearance of the section is now modified by two structures. In the first place, a new mass of grey matter, consisting of a thin layer shaped like a Hask with its orifice directed inwards, is developed in the lateral part of the medulla, between the pyramids in front and the tubercle of Rolando behind. This is the olivary body, and has on its inner and dorsal sides two little grey masses which are the accessor// olivary bodies. The other feature is the new 368 PHYSIOLOGY relay of sensory fibres which start from the dorsal nuclei, the nuclei gracihs and cuneatus. These fibres run outwards and forwards from the nuclei right round the medulla. Some fibres pass into the restiform body of the same side. A larger number, forming the superficial arcuate fibres, pass superficially to the ohve to join the restiform body of the opposite side, while others, the deep arcuate fibres, pass deeply to the olives, and crossing in the median raphe turn upwards in the broken mass of grey and white ■Posterior longitudinal fasciculus Substantia gelatinosa Kolandi Spinal root of fifth nerve Xucleus ambiguus t erebello-olivary fibres wgr JJorsal accessory olivary nucleus Vnterior superficial arcuate fibres J ilkt ,r M( -,1 il accessory olivary nucleus -'Inferior olivary nucleus — Pyramid Arcuate nucleus Anterior superficial arcuate fibres Fig. 1 83. Transverse section through the middle of the olivary region of the human medulla. (Cunningham.) matter which hes between the olives and the superficial grey matter of the fourth ventricle. This decussation, which is known as the ' decussation of the fillet ' or the sensory decussation, takes place immediately above the level of the decussation of the pyramids. In its upward course it forms a conspicuous strand of fibres, lying close to the mesial plane and separated from its fellow of the opposite side simply by the median raphe. To this collection of fibres is given the name of the jillet or lemniscus. It is perhaps the most important of the afferent tracts of the brain stem, receiving as it does continuations of the posterior columns of the cord as well as contributions from the various sensory cranial nerves. It may be traced forwards as far as the thalamus and subthalamic region, where its fibres terminate. The region corresponding to the anterior column of the spinal coid is thus invaded in the medulla by two great longitudinal tracts of fibres namely, the pyramids and the tracts of the fillet. The region corre- THE STRUCTURE OF THE BRAIN STEM 369 sponding to the anterior basis bundle, i.e. that part of the anterior colmnns occupied chiefly by intra-spinal fibres, is thus pushed further backwards and finally comes to lie immediately beneath the grey matter of the floor of the fourth ventricle. Immediately dorsally to the fillet is to be seen another Fibres Subdel.Bol.-V- FiG. 184. Diagram to show the sources of the fibres making up the reatiform body. Ar.N, arcuate nucleus ; Ar fibres, arcuate fibres ; Pyr, pyramid ; C.Sp. Tract, direct cerebellar tract; C.Ol fibres, cerebello-olivar}- fibres; Pl.B, posterior longitudinal bundle ; DN. nucleus of Deiters ; NB, nucleus of Bech- terew ; Ro.N, roof nuclei ; Vest. N, vestibular nerve. well-marked bundle of longitudinal fibres, knowai as the posterior longi- tudinal bundle. These fibres, which serve to connect the nuclei of many of the cranial nerves, can be regarded as analogous to the constituent fibres of the anterior basis bundle in the cord, and can in fact be traced into this part of the anterior columns in the first and second cervical segments of the cord. The fourth ventricle is covered in by the cerebellum, which is attached to the axial part of the brain by three peduncles, the inferior peduncles or restiform bodies, the lateral peduncles, which form the great mass of transverse fibres known as the pons Varolii, and the superior 370 PHYSIOLOGY peduncles, which run forward to the posterior corpora quadrigemina. The restiform bodies can be regarded as the direct continuation forwards of the lateral columns of the cord, minus the pyramidal tracts, the chief remaining tract therefore being the posterior or direct cerebellar tract. In the region of the dorsal nuclei, however, it receives accession of fibres from the gracile and cmieate nuclei of the same side and, through the superficial arcuate fibres, from the nuclei of the opposite side, and thus passes as a thick white bundle into the cerebellum. Among these arcuate fibres are also a number derived from the olivary body of the opposite side, known as the cerebello- ohvary fibres. On its way it is joined by a smaller bundle, the ' internal restiform body,' which conveys fibres from the vestibular division of the eighth nerve and also serves to connect Deiters' nucleus with the cerebellum. The restiform body is thus made up of the following fibres (Fig. 184) : (1) The direct. or posterior cerebellar tract, derived from the cells of Clarke's column on the same side of the cord. (2) The posterior superficial arcuate fibres, derived from the gracile and cuneate nuclei of the same side. (3) The anterior superficial arcuate fibres, from the gracile and cuneate nuclei of the opposite side. (4) The cerebello-olivary fibres. (5) The veatibulo-cerebellar fibres. A section through the pons shows the fourth ventricle widely dilated, with a floor formed of grey matter as in the medulla. The chief difference in the appearance of the section is due to the great masses of transverse fibres which pass into the pons by the lateral peduncles of the cerebellum, cross by the median raphe, anct turn either upwards or downwards on the opposite side or end in connection with the nerve-cells which are scattered throughout the white fibres. The pyramids can still be seen as thick longi- tudinal bundles on each side in the midst of the transverse fibres. They are considerably larger than in the medulla and become larger as we trace them up towards the mid-brain, owing to the presence of a number of fibres which are derived from the cortex cerebri and end in the grey matter of the pons. The tract of the fillet hes on each side of the middle line dorsally to the transverse fibres. A little to the outside of the fillet is seen a special mass of grey matter, known as the superior olive. The nervous mass lying behind the transverse fibres of the pons, between them and the grey matter of the fioor of the fourth ventricle, is known as the formatio reticularis. It is divided into a lateral and mesial part by the fibres of the hypoglossal nerve. In the lateral portions there is a considerable quantity of grey matter, which can be regarded as continuous with the grey matter of the lateral horns of the cord. The ' lateral nucleus ' is simply a condensed part of this grey matter, lying between the olive and the gelatinous substance of Rolando. The mesial part of the formatio reticularis is almost free of nerve- cells. The reticular appearance of this part of the pons is due to the inter- section of fibres which run longitudinally and transversely. The transverse fibres are a continuation of the deep arcuate fibres. The longitudinal fibres THE STRUCTUKE OF THE BRAIN STEM 371 in the outer part of the formatio reticularis are the representative of the lateral columns of the cord after the removal of the direct cerebellar and the crossed pyramidal tracts. They include therefore the antero-lateral ascend- ing tract (tract of Gowers) and a number of other fibres corresponding to the Supr. ccr. pedunclf lit ot otli II Valve of Vieussens Alotor nucleu-. ot jtli ii Hot or root of 5tli 11 __ Jfp^' , T^' ■ • •' Supr. olive --^^ _i^J> I' ' /'.^ Sensory root ..^ / : ■> n>---^ ^ , ~ S/ f ofStlin. 7' ~~ - yj^^^ :i-^~^//i ' Sensory iiuduiis of 5tli n. — . r- ^^i--Form. reticularis r orpus irapczoides [iddic pedum ot cerebellum Fig. 185. Transverse section through middle of pons Varolii of orang on level of nuclei of fifth nerve. (Cunningham.) lateral basis bundle in the cord. In the mesial part of the formatio reticu- laris the longitudinal tracts are the tract of the fillet and the posterior longi- tudinal bundle on each side of the middle line. In the upper part of the pons Varolii a well-marked collection of transverse fibres are to be seen lying dorsally to the tracts of the fillet. This collection is called the corpus trapezoides and is made up of ascending fibres derived from the nuclei of the cochlear nerve, the auditory part of the eighth nerve. A little further forward a section will escape the cerebellum altogether, being bounded ventrally by the upper or anterior part of the pons and dorsally by a thin mass of grey matter, the valve of Vieussens (Fig. 18()). On each side of the valve of Vieussens may be seen the superior peduncles of the cerebellum. As these peduncles are traced upwards they sink gradually deeper into the pons until they lie on the outer side of the tegmental region or formatio reticularis. They are made up of fibres which run from the 372 PHYSIOLOGY dentate nucleus of grey matter in the cerebellum to the mid-brain, where they decussate below the Sylvian iter and end in the red nucleus and in the thalamus of the opposite side. They also contain the continuation upwards of the antero- lateral ascending tract, which, passing up in the superior peduncles, bends dorsally round the fourth nerve and then, turning back- wards, ends in the superior vermis of the cerebellum. In a section through the upper part of the pons the division into the formatio reticularis or 4th ventricle Meseuc . root of 5th n Postr. long, bundk Valve of Vieussens ,"'''lo"r of 4+h ventricle 'Supr ceiebcUar peduncle 'Lateral fillet Form, reticularis Nucleus of lateral fllkt Commencing de- cust;atinnof supr. cerebellar ped. Mesial fillet Trinsverse fibres P J ramids Fig. 186. Section across upper part of pons Varolii of the orang. (Cunningham.) tegmentum and the part made up of transverse and longitudinal fibres, the pedal portion, is well marked {v. Fig. 186). The fourth ventricle has now become constricted to a narrow canal triangular in section and closed above by the valve of Vieussens. It is surrounded, especially on its ventral side, by gi'sy matter containing the cells of origin of the fourth nerve. In the tegmental portion we may distinguish on each side the superior cerebellar peduncle. Outside the longitudinal fibres of this peduncle are a number of transverse fibres derived from the corpus trapezoides seen in the previous section. To these fibres is given the name of the ' lateral fillet.' They are on their way to end in the roof of the mid-brain in the posterior corpora quadrigemina. The posterior longitudinal bundle hes near the middle hne, immediately under the grey matter of the floor of the fourth ventricle, while the longitudinal fibres of the fillet, now called the mesial fillet, form a distinct mass in the ventral portion of the formatio reticularis. The pedal portion contains the longitudinal fibres of the pyramids, now much increased in amount, cut up into bundles by transverse fibres derived from the middle peduncles of the cerebellum. The cerebellum, which covers in the fore part of the fourth ventricle, will have to be described in greater detail later on. At present it will suffice THE STRUCTURE OF THE BRAIN STEM 373 to say that it consists of a middle and two lateral lobes. The surface of the middle lobe turned towards the fourth ventricle is known as the inferior vermis, the dorsal surface forming the superior vermis. Each vermis and each lateral lobe is subdivided into a number of smaller lobes. The intimate structure of all parts of the cerebellum is, however, very uniform. It consists of a mass of white matter internally, covered by a layer of grey matter, the Inf. corpus (juadrigeminum Grey matter - Aqueduct of Sylvius- Mesenc. root of 5th n. Nucleus of 4th nerve luf ., brachium ._!l^l'usti.long f^fs,;^- -.X J Substantia uiara Crusta Fii!. 1,S7. Transverse section through human mid-brain, on level of the inferior corpora quadrigcmina. (Cunninghaji.) extent of grey matter being largely increased by the formation of numerous parallel and more or less curved grooves or sulci which give the whole organ a laminate appearance. In the mass of white matter, which forms the core of each lateral hemisphere, is an isolated nucleus of grey matter known as the corpus dentatum. In the white matter of the middle lobe is another mass of grey matter known as the roof nucleus or nucleus fastigii. Be- tween the nucleus fastigii and the nucleus dentatum are two other nuclei, the nucleus globosus and the nucleus cmhoUJormis. THE MID-BRAIN A little further forward the fourth ventricle comes to an end, and the section passes through the mid-brain (Fig. 187), the cavity of the second cerebral vesicle being represented by the narrow Sylvian a(iueduct. bounded dorsally by the corpora quadrigomina and ventrally by the crura, the stalks 374 PHYSIOLOGY of the biain. The crura are divided by an irregular mass of grey matter, the substantia nigra, into two parts. The ventral portion is known as the 'pes or crusta. It is composed ahnost entirely of longitudinal white fibres, among which is the continuation forwards of the pyramids of the medulla. The pyramids, however, form only about two-fifths of the total mass of white fibres, the rest consisting of fibres which rim from the difierent parts of the Superior quadri- geminal body Extl. gen. body Infr. brachium Intl. gen. body \ ucleus of 3id nerve Mesial fillet Crusta Optic tract Supr. cerebellar peduncle 3rd nerve Substantia nigra ^ Corpus mamniillare Fig. 188. Transverse section through human mid-brain at the level of the superior corpus quadrigeminum. (Cunningham.) cerebral cortex, especially from the frontal and temporal lobes, to end in the formatio reticularis of the pons, probably in relation with the grey matter in this situation and with the endings of the transverse fibres derived from the cerebellum and forming the middle peduncles of the cerebellum. The dorsal part, the tegmentum, is a direct prolongation forwards of the formatio reticu- laris of the medulla and pons, and hke this contains much scattered grey matter. On a level with the inferior corpora quadrigemina a. number of decussating fibres are to be seen in the tegmentum, which are derived from the superior cerebellar peduncles. Their decussation is complete at the level of the upper border of the inferior corpora quadrigemina. Here each peduncle turns upwards, and a large proportion of its fibres end in the red nucleus (Fig. 188), a mass of grey matter forming a conspicuous feature of sections through the anterior part of the mid-brain. Many of the fibres pass round the red nucleus, forming a sort of capsule over it, to the ventral THE STRUCTURE OF THE BRAIN STEM 375 part of the optic thalamus, in which they probably end. It is possible that a certain proportion pass through the optic thalamus and run straight to the cerebral cortex of the Rolandic area. The lateral fillet has disappeared fiom the region of the tegmentum and passed into the inferior corpora quadri- gemina. The mesial fillet forms a flat band lying to the outer side of the red nucleus and comes into close relation with a ganglion of the fore-brain, known as the internal geniculate body. The roof of the mid-brain is formed by the corpora quadrigemina. The inferior corpora quadrigeniina are composed of central grey matter encapsulated by white matter, derived chiefly from the lateral fillet. The superior corpora quadrigemina are composed of several layers of grey matter traversed by nerve fibres, derived partly from the fillet, partly from the optic tract, and partly from the occipital lobe of the cerebral hemisphere. THE FORE-BRAIN In the fore-brain the most important feature is the optic thalami, the two head ganghonic masses of the brain stem (Fig. 189). In this region the central neural canal, which in the mid-brain forms the Sylvian iter, widens out to the third ventricle, in the lateral walls of which are developed the two optic thalami. It is a narrow cleft, rapidly deepening in depth fi'om behind forwards. As we trace sections forwards we see that the two crura cerebri diverge from one another. The floor of the third ventricle is thus left thin. It is formed from behind forwards by a thin layer of grey matter with numerous vessels, the locus perforat us posticus, two small eminences, the corpora mammillaria, and in front of these another lamina of grey matter known as the tuher cinereum. In front of the tuber cinereum is the infun- dihulum, which leads to the posterior lobe of the pituitary body. In front of the infundibulum the optic chiasma is closely attached to the lowest part of the anterior wall of the ventricle. The front wall is formed by a thin layer of nervous matter, the lamina cinerea, at the upper border of which, project- ing shghtly into the ventricle, is a strand of white fibres connecting the an- terior parts of the two optic thalami and kno^vn as the anterior commissure. The roof of the third ventricle is formed entirely of epithelium, the ependyma, along the upper surface of which is the layer of pia mater, the velum inter- positum. The roof is invaginated into the ca^'ity by two delicate vascular fringes, the choroid plexuses. At the back part of the roof is attached the stalk of the pineal body, and behind this stalk, between the anterior parts of the anterior corpora quadrigemina, is a small space knowni as the trigonum habenulcp, which contains a well-marked collection of nerve-cells known as the ganglion habenulce. The lateral walls are formed entirely by the optic thalami. The upper surface of the optic thalamus looks into the lateral ventricle of the cerebral hemispheres, from which it is separated by the velum interpositum and by the ependyma, the epithelium completing the inferior wall of the lateral ventricle in this region. It consists of three masses of grey matter — the anterior nucleus, the lateral nucleus (the largest of the three), and the mesial nucleus. Its outer surface is in contact with the laver of nerve fibres formed bv the crusta of each cms cerebri as it diverges 376 PHYSIOLOGY from its fellow to pass up into the cerebral hemispheres. Into this layer, ' the internal capsule/ fibres proceed from all parts of the thalamus to pass to the cerebral cortex. The anterior extremity of the thalamus, known as the anterior tubercle, forms a marked projection into the lateral ventricle. In Corpus callosum Lateral ventricle Nucleus caudatus Internal capsule Thalamus Nucleus lentiformis Anterior commissre CoUiculus superior Inferior brachium CoUiculus inferior 4th nerve TrigonuxQ lemnisci 5th nerve, Brachium conj unctivum Pons 8th nerve Hestiform body 9th nerve 10th nerve OUve 7th nerve 12th nerve Fig. 1 89. Right lateral aspect of brain stem, with a part of the cerebrum. (J. Symington.) front of this, the foramen of Monro leads from the third ventricle into the lateral ventricle. This foramen is bounded anteriorly by a strand of fibres, known as the ' anterior pillar of the fornix,' which hes just behind the anterior commissure and forms a conspicuous feature in the anterior part of the lateral wall of the third ventricle. It passes in the wall down to the corpus mammillare. From the corpus mammillare a well-marked bundle of fibres passes up into the optic thalamus to end round the large cells in the anterior nucleus of the thalamus. The posterior extremity of the thalamus forms a definite prominence, the jmlvinar. To the outer and back part of the pulvinar two bodies are developed, known as the geniculate THE STRUCTURE OF THE BRAIN STEM 377 bodies. These may be regarded as special outgrowths of the grey matter of the optic thalamus, one of which, the external geniculate body, is in close connection with the fibres from the optic tracts, while the other, the internal geniculate body, receives fibres from the lateral fillet ultimately derived from the organ of hearing. In a section through the fore part of the mid-brain (Fig. 190) these two bodies maybe seen lying to the outer side of the anterior corpora quadrigemina, so that the fore-brain, to a certain extent, enfolds the anterior part of the mid-brain. Below the thalamus at its back part is the prolongation forwards of the tegmentum of the crus. This is often spoken of as the subthalamic region. The red nucleus is a conspicuoiLS object in /?M Fig. 190. Transverse section through upper part of mid-brain. Th, thalamus ; hrs. ))rachium superior ; cq^. anterior (or superior) corpus quadrigeminum ; ccjl, ccjc. internal and external geniculate bodies ; /, fillet ; .«, aque- duct; pi, posterior longitudinal bundle; r, raphe; ///. third nerve; nil I. its nucleus; I pp. posterior perforated space; sn. substantia nigra; cr. crusta ; //. optic tract ; J/, medullary centre of the hemisphere ; iic. nucleus caudatus ; .«/. stria tcrminalis. sections through the back part of this region, but gradually diminishes as we proceed forwards, and disappears before the level of the corpora mammillaria is reached. The mesial fillet, which in the mid-brain lies on the lateral and dorsal aspect of the red nucleus, is prolonged upwards together with fibres from the superior cerebellar peduncle into the ventral part of the thalamus, where probably all of the fibres end in connection with the thalamic cells. The substantia nigra gradually disappears. Before it has disappeared we may see on its outer side a special collection of grey matter called the nucleus of Luys or the corpus snbthalaniicum. In addition to the anterior and posterior commissures already described as connecting the two optic thalami at the front and back of the third ventricle, the two sides are connected about the middle of the cavity by the middle or soft commissure. The optic thalamus is often described together with the corpus striatum as forming the basal ganglia. The corpus striatum is, however, genetically, and 378 PHYSIOLOGY probably functionally, part of the cerebral hemisplieres, and its connections ^^dll therefore be best dealt with when describing the latter bodies. THE AXIAL GREY MATTER In the spinal cord we could distinguish between the anterior grey matter giving origin to the motor nerves, the posterior grey matter serving as an end station for a number of the sensory posterior root fibres, and a lateral horn, , X. fVACUS] . \ ARCUATE NUCLEUS XII. [hypoglossal] Fig. 191. Cross-section of medulla showing nuclei of nerves x and xii. (Cunningham.) less well marked, probably giving origin to the visceral system of nerves. As the central canal widens out to form the fourth ventricle, the relative position of these various parts becomes altered, the anterior grey matter being now nearest the median line, while the posterior grey matter lies more laterally. Part of the lateral grey matter seems to lie deeper than the rest, from which it is separated by the tangle of fibres and cells known as the formatio reticularis. All the cranial nerves from the third to the twelfth arise or end in the axial grey matter, or in close proximity to it. So great, however, is the complexity of this part of the nervous system, and so in- volved are the genetic relations of the various nerves, that it is difficult or impossible in many cases to state definitely the spinal analogies of these nerves. The cranial nuclei (of origin or termination) may be roughly classed as follows : ( 1 ) Motor Somatic Nuclei. These consist of an almost continuous column of multipolar cells, lying close to the middle line on each side in the floor of THE STRUCTUEE OF THE BRAIN STEM 179 the fourth ventricle, the Sylvian iter, and the back part of the third ventricle. From below upwards these groups of cells give origin to the fibres of : (a) The hypoglossal nerve. (6) The sixth nerve. (c) The fourth nerve. (d) The third or oculo-niotor nerve. (2) SphncJinic Sensory Nuclei. Immediately outside the column of motor cells is a column of grey matter which receives the terminations of Fig. 192. Diagram showing tiie brain connections of the vagus, glosso-pharytigeal auditory, facial, abducent, and trigeminal nerves. (Cunningham after Obkr- STEINER.) the afferent fibres belonging to the ninth, tenth, and eleventh nerves, and is sometimes called the vago-glossopharyngeal-accessory nucleus. This grey matter of course does not give rise to the fibres of these nerves, which, like other sensory nerves, are axons of ganglion-cells lying outside the central nervous system. (3) Splanchnic Motor Nuclei. These lie more deeply at some distance from the middle line, and include the 7iuclciis ombiguus for the eft'erent fibres of the vago- glossopharyngeal, the nucleus of the seventh or facial nerve 380 PHYSIOLOGY (originally splanchnic or branchial, now typically somatic), and the motor nucleus of the fifth nerve with its prolongation into the mid-brain. (4) Sensory Somatic Nuclei. The chief representative of this group is the great sensory root of the fifth nerve. The fibres of this nerve arise from the Gasserian ganglion, pierce the fibres of the pons Varohi, and run to the dorso-lateral part of the pons, where they divide into ascending and descending fibres. These fibres form a cap to the substantia gelatinosa, the descending branches, which are longer, being conspicuous in sections of FIBRES TO NUCL.LEMNISCI &CORPORA QUADRIGEMINA I! \s.a. PYRAMID NERVE-ENDINGS iN ORGAN OF CORTI Fig. 193. Plan of the course and connections of the fibres forming the cochlear root of the auditory nerve. (Schafer.) r restiform body; V, descending root of the fifth nerve; tuh.ac, tuberculum acusticum ; n.acc, accessory nucleus ; s.o, superior olive ; n.fr, nucleus of trape- zium ; n.VI, nucleus of sixth nerve ; VI, issuing root-fibre of sixth nerve. the medulla as low down as the first or second cervical nerve. This nerve gives common sensation to practically the whole of the head. It is doubtful in what group we should place the fibres of the eighth nerve. This nerve really consists of two parts very different in function, the cochlear or auditory nerve, and the vestibular or labyrinthine nerve. The fibres of each are derived from ganghon-cells in the internal ear, pass to the medulla at its widest part, and then, dividing into two, terminate in masses of grey matter situated at the extreme lateral part of the floor of the fourth ventricle. The branches of the cochlear nerve (Fig. 193) make connection with two collections of cells, the dorsal nucleus, apparently embedded in the fibres of the root itself, and the accessory nucleus, a little triangular mass of grey matter situated in the angle between the cochlear and vestibular nerves. From these nuclei fibres are given off which take two courses. Some, follow- ing the previous course of the cochlear nerve, pass across the surface of the fourth ventricle as the stricB medullares or stricB acousticce, and then bending inwards pass into the tegmentum of the opposite side. Others pass deeply and form a mass of transverse fibres in the ventral part of the tegmentum, the THE STRUCTUEE OF THE BRAIN STEM 381 corpus trapezoides or trapezium. After making connections with the superior oUvary body and a special nucleus, they join the superficial set of fibres, and run up in the tegmentum to the inferior corpora quadrigemina, forming the lateral fillet. The vestibular nerve (Fig. 19-1) also has two nuclei of termination, the median nucleus with small cells, and the lateral or Deiters' nucleus with large cells. Some fibres pass also to the nucleus of Bechterew, which is in close relation with the roof nuclei of the cerebellum. The descending fibres end TO VERMIS FIBRES O VESTIBULA ROOT NERVE ENDINGS IN MACUI->£ ?. AMPULL/E Fn;. 194. Plan of the course and connections of the fibres forming the vestibular root 'of the auditor}^ nerve. (Schafer.) /■, restiforni body ; y. descending root of fifth nerve ; p, cells of principal nucleus of vestibular root ; d. fibres of descending vestibular root ; nd. a cell of the descend- ing vestibular nucleus; d, cells of nucleus of Deiters; B. cells of nucleus of Bech- terew; ill, cells of luicleus tccti (fastigii) of the cerebellum; jylb. fibres of posterior longitudinal bundle. No attempt has been made in this diagram to rejiresent the actual positions of the several nuclei. Thus a large part of Deiters' nucleus lies dorsal to and in the immediate vieinitv of the restiform bodv. chiefly in the median nucleus, while the ascending fibres end in Deiters' nucleus. From the latter a distinct band of fibres passes up to the cere- bellum, forming the median division of the restiform body, while other fibres run across to the tegmentum of the opposite side, where they take part in the formation of the posterior lomjitudinal bundle. In a section through the fourth ventricle through the middle of the pons, a group of large cells is seen in the position occupied by the nucleus of the hypoglossal below. These cells give rise to the fibres of the sixth nerve. Another group is seen lying laterally and more deeply, evidently belonging to the lateral horn system. This is the nucleus of the seventh or facial nerve, the fibres of which pass dorsally and anteriorly, looping round the sixth nerve-nucleus, before issuing as tiie root of the seventh nerve. In the upper part of the pons we find the fifth nerve (Fig. 195) %\nth its two roots. The fibres of the sensory root derived from the cells of the 382 PHYSIOLOGY Gasserian ganglion bifurcate. The upper divisions, which are short, end in a mass of grey matter at the lateral part of the formatio reticularis, the so-called sensory root, while the descending divisions form a long strand of white fibres passing down as far as the second cervical nerve and lying over the substantia gelatinosa of Rolando, around the small cells of which the fibres finally terminate. The motor fibres arise partly fi-om the motor nucleus, a mass of cells lying internally to the sensory nucleus, and belonging probably to the lateral horn system. A large number are derived from a long column of cells, which stretches forward from the nucleus as far as the level of the anterior corpora quadrigemina. These fibres are known as the descending motor root of the fifth nerve. In the region of the mid-brain, besides the root of the fifth nerve just mentioned, we find only the motor nuclei of the third and fourth nerves, which are situated near the median line in the ventral part of the central grey matter, corresponding in situa- tion to the sixth and twelfth nerves lower down. INTERMEDIATE GREY MATTER OF THE CEREBRAL AXIS The masses of grey matter which are found throughout this region may be re- garded as extra shunting stations (or associa- tion centres for various systems of nuclei and conducting paths), which have arisen in con- i'lG. 195. Diagram showing cen- tral connections of fifth nerve. (Cajal.) A, Gasserian ganglion ; b, acces- sory motor nucleus ; c, main motor sequence of the great complexity of reaction nucleus ; d, facial nucleus ; e, nucleus ■ i £ •■! x. • of hypoglossal; f, sensory nucleus of reqmred of the nerve mechanisms m connec- fifth nerve ; g, cerebral tract (fillet) tion with the organs of special sense. We of fifth nerve. , r ^ t . Vi-i must confine ourselves iiere to little more than the enumeration of the chief masses, though we shall have occasion to refer to some in more detail when dealing with the co-ordinating mechanisms of the cerebral axis. From below upwards we may enumerate the following grey masses : In the medulla is the large olivary body, with the accessory olive lying on its inner side. Each ohve sends fibres across the middle fine to the opposite cerebellar hemisphere, and must be regarded as connected with this body in its functions, since atrophy or removal of one side of the cerebellum is followed by atrophy of the opposite olive. In the pons we find a similar but smaller body, the superior olive, in the neighbourhood of the nucleus of the seventh nerve. The superior ohve is THE STRUCTURE OF THE BRAIN STEM 383 closely connected with the co-ordination of visual and auditory impressions with the eye movements. Deiters' nucleus, which occurs in the same region, although described as one of the nuclei of the eighth nerve, might equally well be included in this class owing to its manifold connections with both afferent and efferent mechanisms. In close connection with Deiters' nucleus are a number of grey masses in the cerebellum, the roof nuclei in the roof of the fourth ventricle. In the mid- brain we must mention the superficial grey matter covering the corpora cpadrigemina. On the ventral side of the Sylvian iter are the various masses of gi-ey matter in the crura, the red nucleus, a large mass in the tegmentum just below the oculo-motor nucleus, and the substantia nigra, which divides each crus into two parts, the dorsal tegmentum and the ventral pes or crusta. Finally at the fore part of the cerebral axis we come to the great ganglionic mass already described, the optic thalamus and the geniculate bodies. The geniculate bodies may be regarded as outgrowths of the optic thalamus which have developed in connection with the terminations of the auditory and the optic nerve fibres. The optic thalamus is connected by fibres with all parts of the cortex and represents the termination of the whole tegmental system, so that in many ways it may be regarded as a sort of foreman of the central nervous system, controlling the activities of the lower level centres and bringing all parts of this system in relation with the supreme cerebral cortex. THE CHIEF LONG PATHS IN THE BRAIN STEM In dealing with the spinal cord we were able to treat it as one organ, very largely on account of the uniformity of the afferent and eft'erent mechanisms connected with its various segments. Every afferent impulse arriving at the cord has many possible paths open to it, on account of the branching of the nerve fibres as they enter the cord and the connection of these branches with different neurons of varying destination. The exact path taken by any given impulse under any given set of circumstances is determined by the varying resistance at the synapses which intervene between the terminations of the afferent fibres conveying the impulse and the next relay of neurons. These resistances in their turn are altered by the processes of facilitation and inhibition, which may be due to contemporaneous or previous events. A conspicuous example of these conditions is aft'orded by the phenomena of simultaneous and successive spinal induction. The uniformity of afferent and efferent mechanisms disappears when we include the brain stem with the spinal cord. The main efferent channel of impulses is still through the spinal cord, since here are found the efterent mechanisms for all the skeletal muscles of the trunk and limbs, the chief servants of the central nervous system in the daily events of life. Other efferent channels are added, which acquire special importance with the gi'owth of the upper brain or cerebral hemispheres. These mechanisms 384 PHYSIOLOGY include those for the movements of the eye muscles, those concerned in facial expression, and those responsible for the movements of the mouth in mastica- tion and deglutition, and in man, in speech. Important visceral efferent fibres are also contained in the vago-glossopharyngeal nerves, which leave the brain stem at its hindmost part in the region of the medulla oblongata, and influence the condition of the heart and the ahmentary canal with its accessory organs. On the other hand, the afferent mechanisms of the brain stem far transcend in importance, i.e. in their influence on the reactions of the animal, those of the spinal cord. Among these afferent mechanisms are those which we have spoken of as ' projicient ' sense organs or organs of foresight, the impulses from which must predominate over all reactions determined by the immediate environment of the animal. Into the medulla oblongata are poured the impulses from the greater part of the alimentary canal and from the heart (the chief factor in the circulation) and the lungs. At the junction of the medulla and pons is the great eighth nerve, really con- sisting of two, one of which, the cochlear nerve, carries impulses from the projicient sense-organ of hearing, while the other, the vestibular nerve, has its terminations in the labyrinth, the sense-organ of equihbration. To the impressions received from this organ all the complex co-ordinating motor mechanisms of the spinal cord have to be subordinated, in order that they may co-operate in the maintenance of the equihbrium of the body as a whole. Into the pons enters the fifth nerve, carrying sensory impressions from the whole of the head, while in the mid- and fore-brain we find the endings of the optic tracts derived from the eyes and carrying visual impressions. From the front of the fore-brain are produced the olfactory lobes. At each segment or level in the brain stem the afferent fibres from these various sense-organs enter and join afferent tracts, carrying impulses on from the spinal cord— impulses originally derived from the muscles and skin of the trunk and hmbs. At each level there may be an immediate ' reflection ' back to the cord, so that the spinal afferent impressions may co-operate with the cranial afferent impressions in the production, through the spinal cord, of reactions affecting the viscera or the skeletal muscles. On the other hand, both kinds of afferent impressions may pass on up the brain stem to involve higher centres, and, mingling with impulses from other afferent nerves or from the projicient sense-organs, may result at some higher level in an efferent discharge, which may include reactions not represented in the cord, or reactions of far greater complexity than are possible in the purely spinal animal. In consequence of the endless complex interminghng of afferent im- pulses, any diagrammatic representation of tracts is apt to be misleading unless it be remembered that at each break or synapse in the chain of neurons there are numerous possibilities of branching discharge, and that in our diagrams we can only give the course of such impulses as, by the frequency of repetition in the average hfe of the animal, have involved the grouping of a large number of nerve paths of similar function into tracts. The con- stituent elements of these tracts will present similar destinations and possi- THE STRUCTUEE OF THE BRAIN STEM 385 bilities of interruption, i.e. of reactions involving the motor mechanisms at the different levels in the brain stem. It is thus much more difl&cult in the brain stem than in the spinal cord to describe a ' way in ' and a ' way out.' In a chain consisting, say, of six neurons, a, h, c, d, e, f (Fig. 196), though a is certainly afferent and /efferent, it must always be more or less a question of words whether we regard neurons c and d as afferent or efferent in character. It is usual in our classifications to be guided chiefly by the direction of such impulses in relation to the cerebral hemispheres. All tracts going up to the cerebral hemispheres may be involved more or less in the production of such changes in the nervous matter of these hemispheres as are associated with conscious sensation. In the same way there is a possibiUty that the chains of neurons which carry impulses in a descending direction may be involved in the production of voluntary move- ment. It is therefore usual to classify these two sets of tracts as ascending and descending, or as afferent and efferent. If we adopt such a classifi- cation it must be with a distinct reservation that tracts which apparently are going downwards may play a greater part in the determination of sensation than in the determination of movement, and that there may, and indeed must, be a reverberation of impulses through these ascending and descending tracts, so that it must be difficult to dissociate the various elements in the extremely complex neural events which are involved, say, in the simplest kind of conscious sensation. As we trace out the evolution of the brain we find an ever-increasing subordination of the lower to the higher centres, so that in man himself many reactions which in the lower animals are carried out by the spinal cord alone, involve the educated co-operation of the cerebral hemispheres. With this increased control there is a corresponding increase in the develop- ment of long paths. In the brain of a fish, for instance, the cerebral hemi- spheres are connected only with the fore-brain ; a httle higher up there are connections between the hemispheres and the mid-brain as well. The chief long tracts are those which run between the thalamus, the mid-brain or the hind-brain, and the spinal cord. With the huge development of the cerebral hemispheres in man there is also development of long paths, the pyramidal tracts, from the hemispheres down to all the motor mechanisms of the cord, and of tracts which connect all parts of the cortex with the grey matter of the pons and indirectly with the cerebellum. The tracts which in the lower animals were of supreme importance in determining subordination of lower to higher centres, of immediate reactions to those determined by the organs of foresight, dwindle therefore in importance. Those tracts, such as the thalamo-spinal, tecto-spinal, vestibulo-spinal, which form the main mass 13 Fig. 196. 386 PHYSIOLOGY of the white matter of the brain stem in lower types of vertebrates, become reduced to a few scattered fibres in the brain of man and are insignificant as compared with the great cerebro-bulbar and cerebro- spinal tracts. ASCENDING TRACTS The Teacts of the Fillet. The fibres which enter the spinal cord by the posterior roots pass into the posterior columns and along these to the dorsal column nuclei, the nucleus gracihs and the nucleus cuneatus, where they end by arborisations among the cells composing these nuclei. From these nuclei the axons of the cells pass in various directions, the chief mass of them forming the deep arcuate fibres. These emerge from the inner side of the nuclei and pass through the raphe to the other side of the medulla where they join the spino-thalamic fibres and form the definite collection of longitudinal fibres, lying dorsally to the pyramids, which is known as the main tract of the fillet, or, often, the mesial fillet. As these fibres traverse the pons they are joined at the outer side by a number of bundles which are derived from the central continuation of fibres connected with those derived from the cochlear nerve. This part is known as the lateral fillet. The cells of the accessory and lateral nuclei of the cochlear nerve send their axons by the trapezium to the s;uperior ohvary nucleus and other small masses of grey matter on the other side. In these nuclei the fibres for the most part terminate, but a fresh relay of neurons carries on the impulses and forms the main part of the lateral fiUet. These pass up, getting more dorsal as they ascend, and finally terminate in the inferior corpora quadrigemina. The mesial fillet, which we can regard as a continuation of certain spinal tracts upwards, is reinforced throughout the whole extent of the medulla and pons by fibres originating from the masses of grey matter in which the sensory cranial nerves terminate. Certain of these fibres may form a distinct tract in the forma tio reticularis, known as the central or thalamic tract of the cranial nerves. Another similar tract in the formatio reticularis is derived from the central termina- tions of the fifth nerve, and is known as the trigemino-thalamic tract. All these fibres pass up in the tegmentum of the mid-brain and finally end, partly in the grey matter of the subthalamic region and partly in the grey matter of the thalamus itself. To the thalamus are also continued a few fibres from the lateral fillet. By this means the head ganghon of the fore-brain is in a posi- tion to receive, so to speak, samples of the afferent impressions derived from every sense-organ of the body. The Visual Paths. Two classes of afferent impressions which arrive at the optic thalamus are probably of equal importance to all the other afferent impressions taken together. These are impulses derived from the organs of vision and of smell. The greater part of the fibres composing the optic nerves arise as axons of the ganghon-cells of the retinae. Passing backwards, the nerves of the two sides join in the optic chiasma, which is closely attached to the floor of the third ventricle. After joining in the chiasma the optic nerves are apparently continued round the crura cerebri as the optic tracts. THE STRUCTURE OF THE BRAIN STEM 387 Corpus CaIIoSUiti ^^ Thilamo-CorrKil FibrfS __ _.CI&usrrum 5ubst&.nri6 Nipr6^ Peduncle Ctrebellum --' Sfiino-CerebelUr Tracfs (Co- ordination &■ Muscular Tone PyrevfTiid Dee\> A/cuaffe Fibres Dorsal Column (direct) /stnst of posiTion ■) y " •• movemenf/ Sf)in^l Nerve _ Sylvian Iftr Medl^.n Flllet- ^tntd.tt Nucleus Crossed Sensory Fibres /Pdin. Hei.t& Cold\ vTouch & Pressure I Inferior Olivi ^sceMPiMG neRve TRACTS. Fio. 197. Diagram of ascending tracts between the spinal conl and brain (Gordon HoLSiES), with the probable path of sensory impulses. 388- PHYSIOLOGY These pass round on each side and can be seen to make connection with the back part of the thalamus, the external geniculate body, and the superior corpus quadrigeminum. Part of the tract, which is sometimes called the mesial root, passes into the internal geniculate body. This part of the tract has probably nothing to do with vision and forms a commissure running in the optic chiasma connecting the internal geniculate bodies of the two sides. The course of the optic fibres is shown in the diagram (Fig. 198). In man and in some other mammals, e.g. dog, monkey, the nerve fibres decussate incompletely in the chiasma. The uncrossed bundle is derived from the outer half of the retina of the same side, whereas the crossed bundle is derived from the mesial half of the retina on the other side. The right optic nerve thus carries all the impulses originating in the right eye. The right optic tract carries all the impulses originating from stimuh occurring in the left field of vision. It must be remem- bered that vision in man is bin- ocular, both retinae being concerned in the perception of each field of vision. The external and internal geniculate bodies may be regarded as extensions of the optic thala- mus, the former in special relation with the organ of vision, the latter with the organ of hearing. The olfactory bulb is also con- nected by tracts with the thalamic region, probably through the column of the fornix and the bundle of Vicq d'Azyr. Since, however, the chief connections of the olfactory lobe are with the more primitive portions of the cerebral hemi- spheres, the olfactory tracts will be more conveniently'] treated of in connection with the latter. The Cerebellar Paths. We have already traced out the course of spinal fibres which terminate in the cerebellum. They may be shortly summarised as follows : (!) The posterior or direct cerebellar tract, originating in Clarke's column of cells of same side, passing up in the lateral columns and by the restiform body into the superior vermis of the middle lobe of the cerebellum. (2) The anterior cerebellar tract or tract of Gowers, originating in the grey CORP, GEN. INT. Fig. 198. Diagram- matic representa- tion of the optic tracts and their connections. (Cunningham. UOB* THE STRUCTURE OF THE BRAIN STEM 389 matter of both sides of the cord and passing in the lateral columns through the lateral part of the medulla and pons, and finally attaining the superior vermis through the superior cerebellar peduncles. (3) The posterior columns, ending chiefly in the homolateral posterior column nuclei. From these nuclei, though the great mass of fibres passes into the fillet, a certain number from the nuclei of both sides join the resti- form body to pass into the middle lobe of the cerebellum. In the medulla these afferent tracts of the cerebellum are joined by the following sets of fibres : 1. The ohvo-ccrebellar. 2. The vestibulo-cerebellar. 3. A few fibres from the chief sensory nuclei, including those of the vago glossopharyngeal nerves. AU these fibres terminate in the cortex, chiefly of the middle lobe. From the cortex of this lobe fibres pass to the central and roof nuclei of the cere- bellum, namely, the nucleus dentatus, the nucleus embohformis, the nucleus fastigii, and the nucleus globosus. The efferent tracts of the cerebellum start from these central nuclei, no fibres which originate in the cortex of the cerebellum apparently leaving the precincts of this organ. Some of these efierent fibres of the cerebellum will be better described \^'ith the descending tracts of the brain stem. Of those which take an ascending direction, the great bulk are contained in the superior cerebellar peduncles. These origin- ate for the most part in the dentate nucleus and the nuclei embohformis and globosus. As the superior peduncles run forwards they sink below the posterior corpora quadrigemina, and in the tegmentiun, below the Sylvian iter, decussate with the tract of the opposite side to pass to the red nucleus. In the red nucleus many of the fibres end, some, however, passing through the nucleus together with fibres derived from the cells of the red nucleus itself to end in the thalamus and in the grey matter of the subthalamic region. DESCENDING TRACTS The chief descending tracts having their origin in the brain stem are the rubro-spinal bundle or bundle of Monakow, the complex system of fibres known as the posterior longitudinal bundle, and the vestibulo-spinal fibres from the upper part of the medulla. (1) The RUBRO-SPINAL FIBRES Originate in the red nucleus. They cross the median fine and rim down, at first in the tegmentum and later in the lateral column of the medulla oblongata and cord. In their passage they communicate with the various motor nuclei of the cranial nerves. They can be traced to all segments of the cord, where they terminate in connection with the anterior horn-cells. (2) The POSTERIOR longitudinal bundle. This bundle is to be seen in all sections through the brain stem below the level of the oculo-motor nucleus. It consists of fibres, some of which pass upwards, while others pass do\\ni- wards. Most of the fibres take origin in the cells of Deiters' nucleus and of the reticular formation of the pons, medulla, and niid-braiu, as well as from 390 PHYSIOLOGY Optic ThjilMDuS lniernd>l ) CApsuiej LenticuUrl ^ Nucleus j Rubro-jpin&l Tracf — DiiUfs Nucleus -j'M^-Oinfd^h Nucleus' Inj^erior Olive Vestibule Spinal Tract Crossed Pvramidal Tract >H^U^^""^ Pyramidal Tract D65C6HDING N£RV£ TRACTS. Fia. 199. Schema of course taken by chief descending tracts of brain stem. (Gordon Holmes). The tract in red, to the right of the rubro-spinal tract, includes the posterior longitudinal bundle, together with the fibres of the thalamo-spinal and tecto-spinal tracts. THE STRUCTURE OF THE BRAIN STEM 391 certain cells in the sensory nucleus of the fifth nerve. The fibres traced upwards can be seen to send collaterals to end in the various parts of the nuclei of the third, fourth, and sixth nerves. Lower down it becomes con- tinuous with the anterior basis bundle of the spinal cord and merges in the internuncial fibres which serve to connect the various levels of the cord. Some of the fibres, which are descending, are derived from a small nucleus, the so-called nucleus of the posterior longitudinal bundle, which is found in the grey matter at the side of the posterior part of the third ventricle. This bundle also receives fibres from the superior ohvary body. It is one of the earhest to undergo myehnation in the foetus {cp. also Fig. 205, p. 408). (3) The VESTIBULO-SPINAL TRACT takes origin for the most part in the cells of Deiters' nucleus. The fibres pass down in the anterior part of the spinal cord and terminate in the anterior horns. They are sometimes known as the antero-lateral descending tract. It is probably through this tract that the cerebellum is able to affect indirectly the activity of the motor mechanisms of the cord. Two other descending tracts which are important in the lower vertebrates are insignificant in man. These are the thalamo-spinal tract, consisting of descending fibres derived from the optic thalamus, and the tecto-spinal tract, containing fibres derived from the roof of the mid-brain. In the mid- and hind-brain these fibres run in the tegmentum. In the cord they are found in the anterior columns. The olivo-spinal tract, which is supposed to originate in the olivary body, forms a small tract in the cervical region near the surface, opposite the lateral angle of the anterior horn. SECTION XII THE FUNCTIONS OF THE BRAIN STEM The brain stem may be taken to include all those parts lying between the cerebral hemispheres and the spinal cord, from the optic thalamus in, front to the medulla oblongata behind. The brain may be divided into the follow- ing parts from before back : (1) Thalamencephalon, including the corpus striatum, the cerebral hemispheres and rhinencephalon, or olfactory lobes. (2) Diencephalon, i.e. the fore-brain, especially the optic thalamus. (3) Mesencephalon, or mid-brain, including the quadrigemina, the iter of Sylvius, and the crura cerebri. (4) Metencephalon, composed of the pons Varohi, the upper part of the fourth ventricle, and cerebellum. (5) Myelencephalon, or bulb, consisting of the medulla oblongata. We may get some idea of the part played by these different regions of the brain in determining the reactions of the individual as a whole by examining the behaviour of the animals in whom all the rest of the brain in front of the part in question has been removed. If, however, we take into account the numberless connections existing between the different levels in the central nervous system, the interdependence between the different portions, and the subordination, especially in the higher animals, of the functions of the lower to those of the higher levels, we must acknowledge that such experiments can only give us an imperfect idea of the possibihties of each level when in connection with all other portions of the nervous system. THE FUNCTIONS OF THE MEDULLA OBLONGATA OR MYELENCEPHALON The possibihties of any given nervous centre are determined by the afferent impressions which enter it, and by the connections made by the nerves carrying these impulses with the motor tracts within the centre. The bulb receives afferent impressions of ' taste ' from the tongue through the nervus intermedins, from the ahmentary canal as low as the ileocohc sphinc- ter, from the lungs, the heart, and the larger blood-vessels, i.e. from the most important of the viscera of the body, by the fibres of the vago-glossopharyn- geal nerves. Its only skeleto-motor centre is that for the muscles of the tongue (the hypoglossal). It sends also efferent fibres to the viscera, which arise from cells in the nucleus ambiguus. These fibres carry motor impulses 392 THE FUNCTIONS OF THE BRAIN STEM 393 to the muscles of the larynx and bronchi, and to the oesophagus, stomach, and intestines, secretory fibres to the stomach aiid inhibitory fibres to the heart. At the upper border of the bulb enter also the fibres of the eighth nerve, carrying important impressions from the organ of hearing and the organ of static sense. These will be in all probability divided or injured in isolating the bulb from the higher portions of the brain. While in connection with the upper portions of the brain, the bulb receives also afferent impressions from the skin of the face, and the mucous membrane of the nose and mouth through the descending branches of the root of the fifth nerve, which pass down superficially to the tubercle of Rolando. When in connection with the cord the medulla receives afferent impressions from the whole surface of the body and from all the muscles and joints through the posterior column nuclei. The bulbo-spinal animal, i.e. one in whom a section has been carried out at the upper boundary of the medulla, differs from the spinal animal chiefly in the maintenance of the nexus between the visceral functions and the skeleto-motor functions of the body. After removal of all the brain in front of the bulb, the animal still continues to breathe regularly and auto- matically. The blood pressure and the pulse rate remain normal, and all three mechanisms, respiration, pulse rate, blood pressure, may be affected reflexly by appropriate stimuli, or may be altered in consequence of central stimulation of the medulla. In addition to the reflex mechanisms of locomotion, which are evident in the spinal animal, the bulbo-spinal animal shows a greater degree of solidarity in its responses. It is easier to evoke movement of all four limbs-. In the frog, if the eighth nerve has been left intact, there is a certain power of equilibration left, and the animal when laid on its back tries to right itself and usually succeeds. It is in this portion of the central nervous system that have been located the great majority of the so-called centres. By a statement, that the centre of such-and-such movement or function is situated in the medulla, we mean merely that the integrity of the medulla, or certain parts of it, is essential for the carrying out of the function. Every function, for instance, in which impulses passing up the vagus nerves are involved, is necessarily dependent on the integrity of these nerves and their central connections, and, since these are situated in the medulla, the centres for these functions are also located in this region. From a broad standpoii\t the medulla or bulb may be looked upon as a ganglion, or a collection of ganglia, whose main office is to guard and preside over the working of the mechanisms at the anterior opening of the body ; by means of which food is seized, tasted, taken into the alimentary canal, and finally digested. The respiratory apparatus belongs to the same system and is innervated through the same nerve channels. Hence the various events in alimentation, such as deglutition, vomiting, mastication,' or in the allied respiratory functions, such as phonation, coughing, and' respiration itself, are endowed with centres in this part of the brain. In 13* 394 PHYSIOLOGY connection with the termination of the vagus nerves of this part of the brain is the location here of the chief vaso-motor centre, i.e. in a region which is in close proximity to the endings of the chief afferent nerves from the heart and larger blood-vessels and to the nucleus of the efferent controlhng nerve to the heart. THE METENCEPHALON (PONS VAROLII AND CEREBELLUM) Destruction of the brain at the front of the fourth ventricle and just behind the posterior quadrigemina will leave the animal with a central nervous system, which is in connection by efferent nerves with the whole musculature of the body (with the exception of certain eye muscles) and which receives impressions through the spinal cord from the whole surface of the trunk and limbs, and through the fifth nerve from the face and head, and also the higher speciahsed impressions from the organ of hearing and the organ of static sense. The impressions from the two great projicient senses of smell and sight would be wanting. Such an animal presents considerable advance in the complexity of its reactions above one possessing only spinal cord and bulb. The frog, for instance, after such an operation, can still walk, spring, and swim ; when placed on a turntable it reacts to passive rotation by turning its head in the opposite direction. On stroking its back it croaks. If the cerebellum be also removed the animal becomes spontaneously active and crawls about until it is blocked by some obstacle. In this condition there is great activity of the swallowing reflex. Anything which touches the mouth is snapped at. If placed on its back the frog at once rights itself. In the mammal a similar increase of reflex activity is observed though the power of progression is not retained. THE MESENCEPHALON OR MID-BRAIN A section in front of the anterior corpora quadrigemina would leave the animal with the nervous system receiving all normal sensory impressions, with the exception of the olfactory, and with efferent paths to all the muscles of the body, including those of the eye. In the mammal such an operation brings about a condition known as ' decerebrate rigidity.' Though respira- tory movements continue normally, the whole musculature is in a cataleptic condition, the elbows and knees being extended and resisting passive flexion ; the tail is stiff and straight, the neck and head retracted. This condition seems to depend on an over-activity of the reflex tonic functions of the lower centres. That it is reflex is shown by the fact that the rigidity is at once abolished in a limb on dividing the appropriate posterior roots. The position of the limbs may be also modified by sensory stimuh. A similar condition of increased tonus is observed in the frog. The apparatus for emotional expression is still intact though somewhat modified, and an impression which would give rise to pain in the intact animal may cause vocalisation in an animal in whom the brain above the mesencephalon has been destroyed. THE FUNCTIONS OF THE BRAIN STEM 395 THE BRAIN STEM AS A WHOLE (INCLUDING THE THALAM- ENCEPHALON, OR OPTIC THALAMI) The iutroductiou of the head gangha of the brain stem, viz. the optic thalami, completes in the lower animals at all events the apparatus for im- mediate response to stimulus. The powers of such an apparatus may be studied by examining the behaviour of an animal in whom the cerebral hemispheres have been destroyed. The result of this operation varies according to the type of animal chosen, though all types present certain common features. When a frog's cerebral hemispheres have been excised, a casual observer would not at first notice anything abnormal about the animal. It sits up in its usual position, and on stimulation may be made to jump away, guiding itself by sight, so that it avoids any obstacles in its path. Movements of swallowing and breathing are normally carried out. The animal thrown on to its back, immediately turns over again. If put into water, it swims about until it comes to a floating piece of wood or any support when it crawls out of the water and sits still. If it be placed on a board and the board be inchned, it begins to crawl slowly up it, and by gradually in- creasing the inchnation may be made to crawl up one side and down the other. But a striking difference between it and a normal frog is the almost entire absence of spontaneous motion — that is to say, motion not reflexly provoked by changes immediately taking place in its environment. xA.ll psychical phenomena seem to be absent. It feels no hunger and shows no fear, and will suffer a fly to crawl over its nose without snapping at it. " In a word, it is an extremely complex machine, whose actions, so far as they go, tend to self-preservation ; but still a machine in this sense, that it seems to contain no incalculable element. By applying the right sensory stimulus to it we are almost as certain of getting a fixed response as an organist is when he pulls out a certain stop." According to Schrader and Steiner, if care be taken not to injure the optic thalami, spontaneous movements may be occasionally observed after removal of the cerebral hemispheres. On the approach of winter such a frog has been observed to bury itself in order to hibernate, and with spring to resume activity and to feed itself by catching insects. The behaviour of such decerebrate animals depends on the part taken in the initiation of movement and adapted reactions by stimuli entering through the higher sense-organs. Thus an ordinary bony fish after ablation of the cerebral hemispheres maintains its normal equiUbrium in water. It is continually swimming about, stopping only when it reaches the side of the vessel or when worn out by fatigue. Here, again, if the thalami and optic lobes be intact the fish has been observed to show very httle difference from a normal animal and to possess the power of distinguishing edible from non-edible material. On the other hand, in the elasmobranch fishes, which depend mainly upon their olfactory apparatus as a guide to movement, the removal of the cerebral hemispheres with the olfactory lobes, or of the latter alone, produces complete immobility and absence of 396 PHYSIOLOGY spontaneous morement, even though the optic thalami and optic lobes may be intact. In the bird the cerebral hemispheres may be removed with ease. A decerebrate pigeon, if its optic lobes be intact, walks about avoiding all obstacles, and may even fly a short distance. In the dark, i.e. in the absence of visual impressions, it remains perfectly still. The bird, however, is unable to recognise food, or enemies, or individuals of the opposite sex ; it shows no fear and responds to stimuh like the brainless frog described above. Goltz has succeeded in the dog in removing the whole of the cerebral heruispheres in three operations. The dog was kept ahve for eighteen months after the final operation. It was able to walk in normal fashion and spent the greater part of the "day in walking up and down its cage. At night it would sleep and then required a loud sound to awaken it. It reacted to stimuh in a normal fashion, shutting its eyes when exposed to a strong Hght, shaking its ears in response to a loud sound. On pinching its skin it attempted to get away, snarhng or turning round and biting clumsily at the experimenter's hand. It had no power to recognise food and had to be fed by placing food in its mouth, though, if this food were mixed with a bitter substance, such as quinine, it was at once rejected. The dog never showed any signs of pleasure, or recognition of the persons that fed it, or of fear. Removal of the hemispheres had thus produced loss of all understand- ing and memory. There was no sign of conscious intelligence, and all the actions of the animal must be regarded as reflex responses to immediate excitation. With the development of the cerebral hemispheres in the higher mammals there is a considerable shifting of motor reactions from those which are immediate and ' fatal ' or inevitable to those which are educatable. The cerebral hemispheres in man take a large part in the determining of even the common reactions of everyday life. Ablation of the hemispheres therefore, or even part of the hemispheres, in the ape and man gives rise to much more lasting symptoms than is the case in the animals we have just studied. These defects we shall have to consider more fully later. The results, however, obtained on the lower animals, from the dog downwards, show that the brain stem, from the head ganghon of the optic thalamus back to the medulla, with the spinal cord, represents a complex mechanism which can be played upon by impulses received through all the sensory apparatus of the body, and is able to adjust the motor and visceral reactions to the immediate environment of the animal. Certain of these immediate reactions are susceptible of further physiologi- cal analysis. We have seen that the spinal cord contains the co-ordinated mechanism for the movement of the limbs. We may now discuss how the movements of the limbs are co-ordinated with those of the trunk and head in the maintenance of the unstable position of the animal in standing and in locomotion. For this purpose there has been developed the great mass of nerve matter in the roof of the metencephalon, viz. the cerebellum. SECTION XIII THE FUNCTIONS OF THE CEREBELLUM The carrying out of co-ordinated movements is associated with and regu- lated by afferent impressions which can be divided into two main groups. In the first group may be placed those due to the changes in the environ- ment of the animal, working on sensory structures or ' receptors,' of varpng qualitative sensibility, in the surface of the body. These receptors may be excited by the mechanical stimuli of pressure, by changes of temperature, or by nocuous or harmful impressions, such as Avould, in the presence of consciousness, give rise to pain. At the fore end of the body we have in addition the special receptor organs excited by waves of light or of sound. The action of any of these impressions, if of sufl&cient intensity, is to evoke an appropriate reflex movement, such as the flexor reflex in response to nocuous stimulus applied to the foot, or the stepping, or extensor, reflex excited by steady pressure on the sole of the foot. The integrity of the nerve paths carrying these afferent impressions and of the motor paths to the muscles is not, however, sufficient. A secondary set of afferent impulses is essential in order to guide and regulate the extent of the resultant discharge. These secondary afferent impulses start in the deep tissues, viz. the muscles, joints, and ligaments, which are provided with special sense-organs capable of being stimulated by the mechanical changes of tension or pressure set up by the movements themselves. The importance of these impressions for the carrying out of muscular movements is shown by the ataxia which is the result of injury to the corresponding afferent nerves. Degeneration of the nerves to muscles, or section of the afferent roots, causes marked ataxia of the movements of the limb, whereas no such result follows section of all the cutaneous nerves supplying the surface of the limb with sensibility. To this system of aft'erent nerves Sherrington has given the name of the ' proprioceptive ' system, since it is excited, not directly by changes in the environment, b\it by alteration in the animal itself. It is responsible for reactions difioring in many respects from those which are the immediate result of stimulation of the other system, the ' exteroceptive,' which is distributed over the surface of the body. Since it is excited by the movement of the nuiscles themselves, i.e. by the first result of the reaction to external stimulus, it serves as a governing mechanism to regulate the extent of each motor discharge. Its excitation not only prevents over-action of the muscles, but may evoke a compensatory reflex in an opposite direction to the reflex inmiediately excited from the skin. A marked feature of this 397 398 PHYSIOLOGY system is its tendency to continued or tonic activity. The steady sliglit con- traction, or ' tone,' which is observable in most skeletal muscles, is inde- pendent of the surface sensibihty and depends entirely on the proprioceptive system of the muscles and their accessory structures. In the decerebrate animal the rigidity of a limb disappears at once after section of its afferent roots, though it is unaltered by division of the main skin nerves. This tonus does not affect all muscles to an equal degree. In every hmb there is a predominance of tonus in certain muscles, so that the result on the whole Hmb is an attitude or posture which is typical of the limb or the animal. Thus the spinal frog takes up an attitude which is very different from that which would be impressed on it by gravity in the absence of muscular activity. If one of its hind hmbs be extended gently, it soon draws it up to reproduce the same crouching position. The posture of the hmb is therefore a result of afferent impressions continually ascending its proprioceptive nerves and exciting a tonic activity which predominates in certain definite muscles. This posture, as carried out by the spinal cord, is a segmental response. It determines the relation of the Hmb to the trunk, and to a less extent of the four Hmbs to one another. It is not concerned with the relation of the animal as a whole to its environment, and only to a slight extent with the maintenance of equihbrium in the presence of the continually acting force of gravity. In the evolution of the nervous system there^ has been a continual subordination of the hinder parts to the head end, in consequence of the development at this end of the aU-important distance receptors, the impulses from which take a predominating part in determining the reactions of the body as a whole. In fact the subordination of one part of the central nervous system to another is in direct relation to the importance of the afferent impulses arriving at each portion of the system. Thus the vaso-motor centres segmentally distributed throughout the spinal cord are subject to the vaso-motor centre in the medulla, which is developed at the point of entry of the vagus nerves, i.e. the chief afferent nerves from the heart and large blood-vessels. The collections of grey matter presiding over the segmental reactions of the intercostal muscles are entirely subordinated to the grey matter in the medulla around the entry of the vagus fibres from the lungs. This subordination of the hinder to the anterior sense-organs is paralleled in the case of the proprioceptive system. Entering the hind-brain at the upper border of the medulla is the eighth nerve, composed of two parts which differ widely in functions, viz. the cochlear division and the vestibular division. The former is entirely concerned with the reception of sound waves, and is therefore the auditory nerve. The vestibular nerve, which is distributed to the rest of the membranous labyrinth, must be assigned to the proprioceptive system. The labyrinth is practically a double organ. The primitive auditory sac arises as a simple involution of the surface. In the CQurse of development the front part is modified to form the canal of the cochlea, which is set apart entirely for the reception of sound. From the THE FUNCTIONS OF THE CEREBELLUM 399 back part there are formed two sacs— the saccule and utricle — and the three semicircular canals. The saccule and the utricle, which receive each a larg ; branch of the vestibular nerve, represent the otohth organ, which is fomid in almost all classes of animals. The crayfish, for instance, at the base of its antennae presents a small sac lined with hairs and richly supplied with nerves. In this sac a small calcareous particle rests on the hairs. It is evident that the incidence of the pressure of the small stone or otohth on the hairs will vary according to the position of the animal (Fig. 200), so that any change in the position of the head will be attended by alteration in the nerve fibres which have been stimulated by the ^ Mm ^ mm' ot Fig. 200. Diagram of an otolith organ, to show how alterations in its position will cause the weight of the otolith (of.) to press on different sense-cells, and therefore to affect different nerve fibres. pressure of the otohth, and therefore in the nature of the impulses flowing to the central nervous system. The importance of these impulses in regulating the locomotion and the maintenance of the equihbrium of the animal is well shown if the otolith be replaced by a small fragment of iron. Under normal circumstances the iron particle will act quite as well as an otohth. If, however, a powerful magnet be brought in the neighbourhood of the animal, the pressure of the particle will not be determined simply by gravity and therefore by the position of the animal, so that there will be a dissonance between the impulses arriving from the otolith organ and those arising from the sense-organs of the body, and marked disorders of equilibrimn are the result. In the saccule and utricle the vestibular nerve ends in similar otohth organs known as the maculae acousticac. These are small elevations covered with long hairs and supplied with nerves. One or two calcareous secretions or otoliths are embedded in the hairs, so that any change in position will cause a corresponding change in the nerve fibres which are being excited by the weight of the otoliths. The semicircular canals, which he in the three planes of space, are also provided wnth end-organs, somewhat similar in structure to the maculae acousticae, but devoid of otoliths. They are excited by mass movements of the fluid endolymph, filling the canals, which are set up by rotation of the head. Since the nervous apparatus of the labyrinth is excited not by changes in the environment, from which it is carefully shielded, but by changes in the 400 PHYSIOLOGY animal itself, we are justified in assigning it to the proprioceptive system, of which indeed it represents the most important receptor. Just as the pro- prioceptive nerves of a hmb are responsible for the tonus of the limb muscles, so, as Ewald has shown, each labyrinth is responsible to a considerable degree for the tonus of the corresponding side of the body. Extirpation of one labyrinth causes a lasting loss of tone in the muscles of the same side. A further functional resemblance lies in the part played by the labyrinth in the determination of posture. The resultant effect of the impulses arising in it is to maintain a reflex posture of the head and eyes, so that the optic axes in a position of rest are directed towards the horizon. Stimulation of the labyrinth causes therefore movements of the eyes which may or may not be associated with correlated movements of the head. As in the case of the other sense-organs of the anterior end of the body, the reflexes excited from the labyrinth dominate over those evoked by pro- prioceptive impulses from the hinder portions of the body. At the entry of its nerve into the brain stem a mass of grey matter is developed, which must be regarded as the head ganghon of the proprioceptive system, and the chief co-ordinating organ of all the reflex systems which determine posture of the hmbs and of the whole animal, and therefore the maintenance of equilibrium both at rest and during locomotion. This organ is the cerebellum, associated with the grey matter in the upper part of the fourth ventricle at the point of entry of the vestibular nerves. The cerebellum commences in early foetal life as a small elevation in the dorsal wall of the neural tube, where the eighth nerve enters. Simple in structure and small in extent in most of the fishes and amphibia, it grows in extent with increasing complexity of the animal's motor reactions, and attains its greatest develop- ment in the mammaha. In this class the cerebellum, hke the cerebrum, is most highly developed in man and the higher apes. It is generally described in man as consisting of a middle lobe, composed of the superior and inferior vermis, with two lateral hemispheres, and these are subdivided by anatomists according to the situation of the chief sulci. From the physiological point of view the structure of the organ is relatively simple, as is shown by the uniformity of its structure throughout all parts. It may be considered as formed of two main structures, viz. the cortex and the central or roof ganglia. The surface of the cerebellum is increased by being thrown into folds or laminae, so that a section of this organ has a tree-like appearance. A section through a lamina shows three distinct zones : an outer molecular layer presenting a granular appearance with a few nuclei ; internal to this a granule layer composed of many nuclei of nerve - cells ; and most deeply a central core of white matter. Between the molecular and granular layers are situated the cells of Purkinje, large flask -shaped cells each with one apical dendrite, distinguished above all other dendrites of the central nervous system by the richness of its branching, and with one axon, which leaves the base of the cell and passes down into the central white matter, giving off collaterals in its course. In preparations made by Golgi's method we are able to distinguish the various elements composing these layers and their relations. The molecular layer, besides neuroglia- ccUs and the branching dendrites of the cells of Purkinje, contains certain star -shaped cells {a Fig. 201), which give off an axon running parallel with the surface in the molecular layer. From this axon branches dip down towards the cells of Purkinje, THE FUNCTIONS OF THE CEREBELLl^ 401 where they end in a rich basket-work of fibres around the body and beginning of the axon of these cells. The nuclear or granular layer presents two kinds of cells. The most numerous is a small cell with a few short dendrites, each of which terminates in a claw-shaped arborisation, and a single long axon, which passes straight up into the molecular layer, where it bifurcates. The two branches run parallel with the surface in a direction at right angles to the plane of expansion of the dendrites of Purkinje's cells, apparently resting against the serrations on the edges of these processes. The second kind of cell in the granular layer is the so-called Golgi's cell — a large cell Central white matter. Fig. 201. Schema of constituent elements of cerebellum. (Modified from Bom and Daviuoff.) On the left is a section of the cortex as it appears when stained by ordinary methods. The middle portion represents diagramniatically a section at right ansles to the lamina^, while to the right of the dotted line the section is taken in the same plane as the laminre. a, star-shaped cells of molecular lnyeT ; b, b, cells of Purkiiije; c, ' Golgi cell ' ; d. small cells of nuclear layer ; e, ' tendril fibre ' ; /, ' moss' fibre ' ; g, axon of cell of Purkinje. with many dendrites and an axon which terminates by frequent branches in the neigh- bouring grey matter. The Hbri's making up the white matter are of three kinds — two afferent and one efferent. The nto.s-s- Jibres, so called from the curious thickenings they present in the nuclear layer, pass up into the grey matter and terminate by frequent branches in this layer. The tendril Jibres, also afferent, end in a rich arborisation which surrounds the distal part of the bodies and the bases of the dendrites of the cells of Purkinje. The efferent fibres arc represented by the axons of the cells of Purkinje, which acquire a medullary sheath and run down into the white matter. This slight sketch of the anatomy gives us a conception of the extreme complexity of choice presented to nervous impulses traversing the cerebellar cortex. Thus a discharge along an axon of the cell of Purkinje may be excited (1) by an impulse ascend- ing the tendril fibres ; or (2) by one aseending the moss fibres through the granule cells, and then i)assing by their bifurcating axon to the dendrites of the cells of Purkinje; or (3) by the star-shaped cells of the molecular layer and their basket-work round the body of Purkinje's cells. The rouj gaivjlia consist of the nuclei fastigii near the middle line, the nuclei em- 402 PHYSIOLOGY boliformes situated just dorsal to these, and the nuclei dentati, large crenated capsules of grey matter lying in the middle of each lateral lobe. The cells composing the grey matter of the central nuclei are large and multipolar, resembling those foimd in the nuclei of motor nerves. The cerebellum receives fibres from all the receptor apparatus of the body which can be classed in the proprioceptive system. The greater number of these fibres run directly to the cortex, especially of the vermis, and there is no evidence of the passage of any efferent fibres from the cortex directly to the motor apparatus of the cord. The connections of the cerebellum are established by means of its three peduncles, and may be classified as follows : AFFERENT TRACTS. Inferior Peduncle. By this peduncle afferent fibres pass to the superior vermis : (1) From Clarke's column of the same side by the posterior cerebellar tract. (2) From the dorsal column nuclei, viz. the nucleus gracilis and nucleus cuneatus of each side, so that connection is estabished in this way with the prolongations of the posterior sensory roots which run into the posterior columns of the cord. (3) By the internal restiform body from the vestibular division of the eighth nerve, part of the fibres passing through, and perhaps making connections with, Deiters' nucleus. (4) A strong band of fibres passes from the inferior olivary body into the opposite cerebellar hemisphere. Atrophy of one side of the cerebellum induces a corresponding atrophy in the opposite olivary body. Middle Peduncle. The broad mass of fibres making up these peduncles is partly afferent and partly efferent. Many fibres originate in the cells in the formatio reticu- laris of the pons, cross the middle line, and pass up into the lateral cerebellar hemi- sphere of the opposite side. Fibres also pass from the cerebellum to the pons to end round cells in the same region. By this means connection is established between the cerebellar hemispheres and the corticopontine fibres which pass by the crura cerebri between the pons and the frontal and temporal portions of the cerebral cortex of the opposite side. On account of this connection there is a close association between the development of each cerebellar hemisphere and the contralateral cerebral hemisphere. Atrophy of one half of the cerebrum brings about atrophy of the opposite hemisphere of the cerebellum. The Superior Peduncle. By this path fibres from the superior corpora quadri- gemina, i.e. from the terminations of the optic nerve, pass into the cortical grey matter of the cerebellum (Fig. 202). EFFERENT TRACTS. The cerebellar cortex must be regarded as a receiving rather than as a discharging station. Stimulation of it has little effect unless strong currents are employed, and a inotor response is obtained more easily the deeper the electrodes are sunk below the grey matter. The fibres which form the axons of the cells of Purkinje pass partly towards the pons by the middle peduncle, largely, how- ever, towards the roof nuclei, where they terminate. These nuclei form the efferent stations of the cerebellum. From them fibres pass in various directions. A large bundle leaves the dentate nucleus, runs into the superior peduncle, or brachium, and passing deeply across to the tegmentum of the opposite side, traverses the red rmcleus to end in the subthalamic region of the opposite side of the brain. A certain number of fibres, chiefly derived from the central nuclei, such as the nucleus fastigii, pass forward to the corpora quadrigemina chiefly on the same side. From the cerebellum itself no direct tract runs into the spinal cord. The nuclei of Deiters and of Bechterew (the paracerebellar nuclei), which are connected with the endings of the vestibular nerve, are, however, closely associated with the roof nuclei, and give rise to descending fibres which pass into the antero -lateral region of the cord as the vestibulo-spinal tract. The cerebellum is a receiving station not only for impulses which arise in the skin and eyes, i.e. on the surface of the body, but especially for those THE FUNCTIONS OF THE CEREBELLUM 403 which have been defined as proprioceptive, and originate either in the muscles and tendons or in the labyrinth. Activity of this apparatus is roused as a rule by the movement of the organism itself, and is only a secondary result of the environmental stimulation which provoked the original movement. By its efferent tracts starting in the roof- and paracerebellar nuclei, the cerebellum is able to affect the musculature of the same side of the body by a direct influence on the anterior horns. It also enters to a much greater extent into relation with the opposite cerebral hemi- spheres, so that it is in a position to control or modify the activity of these, whether excited on their sensory or on their motor sides. STIMULATION OF THE CEREBEL- LUM. It was first shown by Ferrier that movements of the same side of the body can be excited by stimulation either of the cerebellar hemispheres or of the superior vermis. These results have been confirmed by subsequent observers, and point to each half of the cerebellum being connected functionally with the skeletal muscular apparatus of the corresponding side of the body. The cortex cerebelh is not excited with ease. To evoke movements much stronger stimuU are necessary than, e.g. for the excitation of the motor area of the cerebral cortex. This again is in accordance with what we should expect from the anatomy of the organ, knowing as we do that the cortex is an end-station for a number of afferent paths, but has no direct efferent paths from it to the lower motor mechanisms of the cord. On the other hand, move- ments are excited by minimal stimuli from the intrinsic nuclei of the cerebellum. As a result of his experiments Horsley has concluded that the cortex cerebelh must be regarded as an aft'erent receptive centre from which axons pass to the ventrally placed efferent nuclei, viz. the nuclei dentati, fastigii, emboliformes, as well as Deiters' nuclei. Whereas excitation of the roof nuclei produces more especially movements of the eyes and head, the paracere- bellar {e.(]. Deiters' nucleus) are responsible more especially for the move- ments of the trunk and limbs. The movements of the body which are thus evoked are those concerned in maintaining equihbrium and are involved in every alteration in the position of the body. Fig. 202. Diagram of aflfcrent and efferent tracts of csrebellum. (After V. Gehuchten.) OT, optic thalamus ; rn, red nucleus ; POT, posterior cerebellar tract; act, anterior cerebellar tract : v, fifth nerve. 404 PHYSIOLOGY EFFECTS OF ABLATION OF THE CEREBELLUM. Complete unilateral extirpation of the cerebellum, after the irritative effects of the lesion itself have passed away, brings about a condition of the animal charac- terised by : ■ (1) Shght loss of power on the same side of the body. (2) Considerable loss of tone on the same side. (3) Tremors or rhythmical Sup.Vermis \---C.V.T. - - -,s.c movements of the muscles on the same side accompanying any willed movements. These three symptoms are denoted by Luciani as asthenia, atonia, and astasia. At first C.R.V-t/7 CB)(5P \ the animal is quite unable to stand, and lies on the side of the lesion with neck and trunk curved in the same direction ; when it attempts to stand it always falls to the same side. After two or three weeks the power to stand is regained, though when it attempts to walk the hindquarters drag and tremors accompany every effort. The animal attempts to correct the tendency to fall towards the side of the lesion by an exaggerated abduction of the hmbs to that side, and Tig. 202a. Schema of connections of Deiters' jg always ready to take ad- nucleus. (Bkuce.) , f , 1 . p n CR,restiformbody; kk, roof nuclei ; sf, sagittal vantage of the support of a wall fibres from cortex to roof nuclei ; cvt, cerebello- to enable it to maintain its vestibular tract; dn, Deiters' nucleus; iii, vi, ■■.■-. ■ ^ . . . , nuclei of third and sixth nerves; plf, posterior eqmhbnum. Swimmmg IS much longitudinal bundle; viii, vestibular division of • better carried out than walking, eighth nerve ; sc, semicircular canals ; vsT, vesti- , . , , r , i , -n ,i bulo spinal fibres. the contact 01 the water with the skin furnishing guidance to the spinal mechanism which is lacking when the animal attempts to walk. When the whole cerebellum is removed the animal is unable to walk, sometimes for months. After a time it gradually learns to walk, but this is carried out by an alteration of the method of progression. The disorders of locomotion are quite distinct from the spinal ataxia observed after interfer- ence with the afferent tracts from the muscles. The difficulty now is that each diagonal movement of the limbs in progression tends to throw the centre of gravity to one side or other of the basis of support, and it is the mechanism for maintaining the right position of the centre of gravity, i.e. the posture of the body as a whole in relation to its environment, which is at THE FUNCTIONS OF THE CEREBELLl^M 405 fault. The animal, in the case of the dog, therefore attempts to correct the tendency to fall to one side or other at each step by making its basis of support as wide as possible, and gradually acquires a peculiar gait, consisting of a series of springs, in which the two fore limbs and two hind limbs act together, the diagonal movements of the fore limbs being practically abandoned. In lesions of the cerebellum in man the most marked symptoms are the cerebellar ataxy and the occurrence of tremors, ' astasia,' on the perform- ance of willed movements. The ataxy has the same origin as that in the dog ; each spinal act of locomotion tends to throw the centre of gravity outside the line of support, and the tendency to fall thus brought about is voluntarily compensated by abduction of the corresponding limb. A staggering gait is thus produced, which is practically identical with that of a drunken man, and presents no trace of the over-action of muscles so characteristic of spinal ataxy. That the compensation, which is slowly acquired after extirpation of the cerebellum, is of cerebral origin is shown by the fact that extirpation of the cerebral hemispheres, or even of the motor areas of the hemispheres, after extirpation of the cerebellum, at once abohshes the power of movement which has been reacquired, and after the motor areas are destroyed on both sides the loss of power of progression is permanent. These experiments show that the cerebellum, in Sherrington's words, must be regarded as the head ganghon of the proprioceptive system, acting as a centre where arrive the afferent impulses from the cord, the fifth nerve and especially from the labyrinth. It influences, through the superior peduncle, the cerebral cortex and furnishes the subconscious basis for the guidance of the motor functions of the latter organ. Through its connections with the nuclei of the bulb and the efferent tracts arising therefrom, it aug- ments the tonic activity of all the muscles of the body, an effect which is especially marked in the absence of the cerebral hemispheres and is respon- sible for the condition known as decerebrate rigidity. As a centre of conjunc- tion for the afferent impressions from the muscles and those from the laby- rinth it co-ordinates the segmental reflexes, which determine the relative posture of each limb, with those originating in the labyrinth and determining the position of the head. Thus the whole mechanism provides for a mainten- ance of equilibrium of the body as a whole, and for the proper balancing of the reflex movements of the different hmbs with those of the trunk during all the changes in the position of the centre of gravity attending locomotion. The view liere put forward really includes the various descriptions of the functions of the cerebellum which have been given by different authorities. Thus Luciani describes the cerebellum as an organ wliich by unconscious processes exerts a continual reinforcing action on the activity of all the spinal centres. Munk ascribes to the cerebsllum the function of maintaining bodily equilibrium. Lewandowsky regaids the cerebellum as the central organ of the muscular senses. Huglilings Jackson ex- pressed many years ago an important characteristic of the cerebellum when he WTote that the cerebellum is the centre for continuous movements, and the cerebrum for. changing movements. All these descriptions come under Sherrington's conception of the cerebellum as head ganglion ot the proprioceptive system. SECTION XIV VISUAL REFLEXES Foremost among the afferent impulses determining the reactions of higher animals are those arising in the eyes. Each retina, or rather the two retinae acting together as a single organ, can be regarded as a sensory surface, every point of which corresponds to a point, or series of points, lying in a given direction outside the body. Each optic nerve contains about half a milhon nerve fibres, i.e. as many as enter the cord by the posterior roots from the whole of the body. The two optic nerves coming from the retinae meet together in the floor of the fore-brain and form the chiasma. A.t the chiasma a decussation of fibres takes place, which, in animals such as the rabbit, with no fusion of the fields of the two eyes, is practically complete. In man only those fibres which arise in the mesial half of each retina cross the mesial plane ; these, together with the uncrossed fibres from the temporal half of the other retina, form the optic tract of the opposite side (Fig. 203). The optic tract passes backwards across the crus cerebri and finally divides into three branches, in the roof of the mid- and fore-brain, which end in the giey matter of the anterior corpora quadri- optifdecus'St?o'n(!hL7maT;'o^% gemina and in the external geniculate body optic tract ; NC, nucleus caudatus; , ,, 1 . p ,1 J.- xT- 1 LN, lenticular nucleus ; Th, optic and the pulvmar of the optic thalamus, thalamus; G, external geniculate Kunning in the optic tract are also fibres body; AQ, anterior corpus quadri- ,.■,■, • 1,1 r eeminum ; P, pulvinar ; OpR, optic which are simply commissural; these form radiations running to 00, the occi- the mesial root of the optic tract. They cross pital cortex; llln nucleus of third -, . .1 nerve in floor of Sylvian aqueduct ; m the optic chiasma and serve to connect the ly, fourth ventiicle. two internal geniculate bodies. In addition to the afferent fibres from the retina to the brain the optic tract contains a certain number of efferent fibres which pass out and end in the retinae. It is evident from these connections that whereas section of one optic 406 Fig. 203. Diagram to show con- nections of optic tracts. (After Sherbington.) VISUAL REFLEXES 407 NUCL. EOfNGEt;- NUCL. LAT. MT (DARKSCHEWITSCM^ NUCt.OORS IL'CL.VENT.I. f«NT.).__ •JUCl-.DORS.Il/POSTl (V. GUDOENJ NUCL. CEfJTRAUS--' NUCt.VENT.Il.(p0ST.) nerve, say the right, will only cause loss of vision in the right eye, section of the right optic tract Avill cli^^de the fibres coming from the right halves of both retinae. This portion of the retina in each eye is stimulated in the normal position of the eyes by rays of hght coming from the objects lying to the left of the field of vision. Section of the right optic tract therefore causes bhndness to all objects to the left of the median line, left hemianopia. Section of both optic tracts of course causes complete blindness. Every movement of the head involves compensatory movements of the eyes, and conversely, in any change in the environment of the animal which demands its attention, there is a movement of the eyes so as to turn the gaze on to the origin of the disturbance as an antecedent to any body movement. In the absence of normal regulative im- pulses from the skin, or from the semicircular canals, the afierent impressions from the eyes may serve for the maintenance of fairly well co-ordinated move- ments— a compensation which is rendered possible by the power of the cerebral cortex to learn new reactions by experience. The centres for the eye move- ments are contained in the grey matter in the floor of the back part of the third ventricle and of the iter of Sylvius. Here we find the nucleus of the third or oculo-motor nerve. The oculo- motor nucleus consists of several divisions, viz. a lateral part containing large motor cells, a superficial median nucleus with small cells, and a deeper median nucleus with large ceUs. By localised stimulation it has been found possible to differentiate the functions of the different parts of the micleus (Fig. 204). Stimulation of the back part of the third ventricle causes contraction of the cihary muscles, and a little behind this contraction of the pupil. On stimulating the floor of the iter, from before backwards we obtain contractions in order of the rectus internus, the rectus superior, the levatoi- palpebrae superioris, the rectus inferior, and the inferior obhque muscle. On stimulating more laterally, or exciting the corpora quadrigemina, dilatation of the pupil was obtained. It seems probable that the optic thalamus and the closely related external geniculate body are mainly concerned Nvith the reception of visual impulses and their forwarding to the cerebral cortex. On the other hand, the anterior or superior corpora quadrigemina are mainly concerned with the co-ordination Fig. 204. Diagram to show origin of the different fibres of the third and fourtla nerves from the oculo-motor nuclei. 408 PHYSIOLOGY of visual impressions and visual movements with the movements of every part of the body, and especially with the complex mechanism we have already studied in connection with the labyrinth and cerebellum. Stimulation of the corpora quadrigemina therefore evokes movements of the eyes and of the head ; extirpation of this part, though it may interfere with co-ordination, does not necessarily involve loss of sight, even when the extirpation is bilateral. The multifarious intercourse which is continually taking place between AntC.Quad. Optic Tract •A A/. ^••■" ^\////.Vestn o. U. m^^\Ant'basf5 bundle Fig. 205. Diagram of connections of posterior longitudinal bundle. Ant.C.Quad, anterior corpus quadrigeminum ; oc.m.n, oculomotor nucleus ; IV.n, nucleus of fourth nerve ; VI. n, nucleus of sixth nerve ; D.N, Deiters' nucleus ; S.O, superior olive ; VIII. Vest.n, vestibular nerve ; p.l.b, posterior longitudinal bundle ; Ist c.n, first cervical nerve. the eye centres and those for the movements of the body, and between afferent impressions from the eyes and those from the semicircular canals and the proprioceptive system generally, is effected to a large extent through the intermediation of the posterior longitudinal bundle, which extends through- out the whole length of the mid-brain and the hind-brain, and in the spinal cord becomes cojitinnous with the anterior basis bundle of the anterior VISUAL REFLEXES 409 columns. Receiving fibres above through the anterior commissure from the optic thalamus, and from the superior corpora quadrigemina, it is associated in its course with the three motor nuclei that give origin to the nerves supplying the muscles of the eyeball, viz. the third, fourth, and sixth nerves. Fibres enter the posterior longitudinal bundle from the auditory system, and from the superior olive, and connections are also estabhshed between this bundle and the facial nucleus, and the nucleus of Deiters, representing the central station of impulses from the labyrinth. The general connections of the bundle are shown in Fig. 205. SECTION XV SUMMARY OF THE CONNECTIONS AND FUNCTIONS OF THE CRANIAL NERVES Cranial nerves. The cranial nerves are generally reckoned as twelve in number : 1st, olfactory ; 2nd, optic ; 3rd, oculo-motor ; 4tli, or trochlear ; 5th, or trigeminus ; 6th ; 7th, or facial ; 8th, auditory ; 9th, glossopharyn- geal ; 10th, vagus or pneumogastric ; 11th, spinal accessory ; 12th, hypo- glossal. Of these the first two stand on a different footing from the rest, which, hke the spinal nerves, are outgrowths of nerve fibres from the central tube of grey matter surrounding the neural canal or from gangha corresponding to the spinal posterior root ganghon. The olfactory bulb and the retinae, from which the majority of the fibres forming the olfactory tract and the optic nerve respectively take their origin, are analogous rather to lobes of the brain than to peripheral sense- organs. Thus in the retina there are three relays of neurons through which the visual impulse must pass before it arrives at the optic nerve. The olfactory tract and optic nerve are thus comparable with the association or commissural fibres connecting different parts of the central nervous system. The connections of these sensory fibres have already been fully dealt with, and the structure of the peripheral sense-organ will be treated of under the physi- ology of the special senses. Among the cranial nerves proper we may therefore reckon the third to the twelfth. The third or oculo-motor arises from an extensive nucleus which extends on either side along almost the whole length of the ventral part of the aqueduct of Sylvius close to the middle line, the most anterior part lying in the back part of the third ventricle (Fig. 204). The anterior part is com- posed of small cells which give origin to the fibres innervating the intrinsic muscles of the eye, namely, the cihary muscle and the sphincter pupillae. The rest of the nucleus is made up of large multipolar cells, arranged in groups, and gives origin to the fibres passing to most of the extrinsic muscles of the eye. The fibres of the third nerve pass through the tegmentum to emerge at the inner margin of the crusta of the same side. The fibres from the posterior large-celled nucleus supply the following muscles : levator palpebrarum, superior rectus, inferior rectus, internal rectus, and inferior obhque. Stimulation of the trunk of the third nerve causes the eyeball to look 410 CONNECTIONS AND FUNCTIONS OF CRANIAL NERVES 411 upwards and inwards, with contraction of the pupil and spasm of accommo- dation. The nucleus of the fourth nerve is situated, just behind that for the third, in the floor of the Sylvian aqueduct, on a level with the inferior corpora quadrigemina. The fibres run from here down towards the pons, then turn sharply backwards to pass into the valve of Vieussens, which they cross hori- zontally, decussating with the nerve of the opposite side. The superficial origin is therefore from the valve of Vieussens. This nerve supphes the superior oblique muscle of the eyeball. Its stimulation causes the eyeball to look downwards and outwards. The sixth nerve, the motor nerve for the external rectus muscle of the eyeball, arises from a group of large multipolar cells lying on each side of the middle fine in the floor of the fourth ventricle. The fibres of the nerve pass directly outwards to emerge from the anterior ventral surface of the medulla between the pyramids and the ohvary eminence, at the lower border of the pons. Stimulation of this nerve causes the eyeball to look directly outwards. All these three oculo-motor nuclei receive collaterals from the fibres forming the posterior longitudinal bundle, many of which are axons of cells in Deiters' nucleus. It is by this means that the contractions of the muscles moving the eyeball are co-ordinated. Sherring- ton has shown that although the third, fourth, and sixth nerves arise directly from the brain stem and have no ganglion on their course, they are really mixed afferent- efferent nerves. Their afferent fibres, which must arise from the cells in the central nervous system itself, run to the receptor nerve endings with which all the extrinsic eye muscles are richly provided. They are exclusively proprioceptive, and supply no organs outside the muscles innervated by the motor fibres. The occurrence of afferent fibres in these nerves explains the fact previously observed by Sherrington, that after total desensitisation of the eyeball by means of cocaine, or by section of the first division of the fifth nerve, the ocular movements are carried out with as much precision as in the normal animal. As we have seen, such precision of movement requires the co-operation of afferent impressions from the muscle, and the only possible channels for these impressions are the pro- prioceptive sense-organs and the afterents of the third, fourth, and sixth nerve pairs themselves. The fifth nerve, or trigeminus, resembles a spinal nerve in that it has a motor as well as a sensory root. The motor root is much the smaller of the two. The fibres of the sensory root take their origin in the cells of the Gasserian ganglion, which is in all respects similar to the ganglion of a posterior spinal nerve-root. The sensory root represents the somatic afferent part of all the motor cranial nerves from the third to the hypoglossal and has a correspondingly wide field of ending in the brain stem. The aft'erent fibres of the fifth nerve, as they enter the pons, bifurcate, like a spinal afferent nerve, into ascending and descending branches. The ascending branches are short and pass to an upper sensory nucleus, situated below the lateral part of the fourth ventricle in the upper part of the pons. The 412 PHYSIOLOGY descending branches, wldch. are much longer, are collected into one or more bundles which pass downwards in the lateral part of the reticular formation, accompanied by the downward extension of the sensory nucleus known as the substantia gelatinosa. The descending root can be traced down in the upper part of the cervical cord, its fibres in this region forming a cap to the gelatinous substance of Eolando. From the "cells of the sensory nucleus fibres pass towards the median raphe, crossing to the other side to take part in the formation of the tract of the fiUet (the trigemino- thalamic tract). The efferent fibres forming the motor root arise from two nuclei. The chief motor nucleus consists of large pigmented multipolar cells situated just below the surface of the lateral margin of the fourth ventricle at the upper part of the pons. The accessory or mesencephahc nucleus is composed of large unipolar ceUs, situated in the central grey matter along the lateral aspect of the anterior end of the fourth ventricle, and in a corresponding position in mid-brain as far as the upper border of the inferior corpora quadrigemina. The fifth nerve is the motor nerve for the muscles of mastication, and for the tensor tympani and tensor palati muscles. It is the sensory nerve for the whole of the face (including eyeball, mouth, and nose). It also contains dilator fibres to blood-vessels derived from the chorda tympani, and is said to have trophic functions. The latter conclusion is from the fact that section of the fifth nerve in the skull is followed by ulceration and sloughing of the cornea, and finally by destructive changes involving the whole eyeball. Since, however, these results may be prevented by carefully shielding the eye from all dust and deleterious influences, it is probable that the ulceration is merely a secondary consequence of the anaesthesia. The cornea being anaesthetic, foreign objects that fall on its surface are allowed to remain there, and so give rise to injurious changes and ulceration. The fifth is also said to be the nerve of taste for the anterior third of the tongue, but it is probable that the taste fibres which run in the fifth are derived from the glossopharyngeal or from the nervus intermedins. The eighth nerve and its connections have been discussed already on several occasions. We may here briefly summarise what has already been stated. In describing the eighth nerve it is necessary to consider separately its two divisions, the dorsal or cochlear division and the ventral or vestibular nerve. The fibres of the cochlear nerve originate in the bipolar cells of the spiral ganghon of the cochlea. They carry impulses from the auditory end- organ. On entering the medulla they bifurcate into ascending and descend- ing branches which terminate in two nuclei, the ascending branches in the ventral nucleus, the descending branches in the dorsal nucleus. The ventral or accessory nucleus Hes between the cochlear and vestibular divisions ven- trally to the restiform body. The dorsal nucleus, often called the acoustic tubercle, forms a rounded projection on the lateral and dorsal aspects of the restiform body. From these two nuclei new relays of fibres start, pass to the other side, by crossing the median raphe (where they form the trapezium) to lUfl up in the lateral fillet of the opposite side. From the ventral nucleus tlie fibres pass directly to the opposite side, forming the greater part of the CONNECTIONS AND FUNCTIONS OF CKANIAL NERVES 413 trapezium, makiiig connection on their way with the nucleus of the trapezium and with the superior olive. From the dorsal nucleus most of the axons pass dorsally, forming the striae acousticse at the middle of the floor of the fourth ventricle. On arriving at the middle line they dip down and join the fibres of the trapezium of the opposite side. The further course of these fibres up to the internal geniculate body, the posterior corpora quadrigemina, and the auditory radiations of the cerebral cortex, have been described on p. 380. The ventral division of the eighth nerve, or vestibular nerve, originates in the bipolar cells of the vestibular gangUon or ganglion of Scarpa. These cells, like those of the spiral ganglion, retain the primitive bipolar character. The fibres divide into ascending and descending branches which become connected with two nuclei. The dorsal or vestibular nucleus, or principal nucleus, which receives the ascending fibres, is a mass of grey matter lying laterally of the vago-glosso-pharyngeal nucleus and corresponding to the lateral triangular area, the trigonum acoustici, which is seen on the surface of the fourth ventricle outside the ala cinerea. The descending vestibular nucleus, receiving the descending branches of the vestibular nerve, lies below but continuous with the principal nucleus. The fibres of the vestibular nucleus send also collaterals to the nucleus of Deiters and the nucleus of Bechterew, two accumulations of large multipolar cells lying ventrally and internally to the vestibular nucleus, both nuclei being in close relation to the roof nuclei of the cerebellum. Many fibres of the vestibular nerve pass apparently through these various nuclei on the inner side of the restiform body into the cerebellum, where they make connection with the roof nucleus or nucleus fastigii. By the nuclei of Deiters and Bechterew the vestibular nerve is connected through the dorsal longitudinal bundle and the descending vestibulo-spinal tract with the motor nuclei of the cranial and spinal nerves. The use of the vestibular nerve is entirely connected with the function of equihbrium. It is probably not concerned in conveying auditory im- pressions, all its nerve fibres being derived ultimately from the nerve-endings in the saccule and utricle and semicircular canals. The seventh cranial nerve or facial nerve emerges from the brain at the inferior margin of the pons, lateral to the point of exit of the sixth nerve. It is almost entirely a motor nerve, but carries also some sensory fibres for taste and general sensibility which it receives from the nervus intermedius of Wrisberg. The motor nucleus of the seventh nerve hes in the reticular formation, dorsally to the superior olive, at some depth below the floor of the fourth ventricle. From this nucleus the fibres first pass inwards and dorsally towards the floor of the ventricle, where they collect to form a bundle which runs upwards in the grey matter for a short distance and then turns sharply in a ventro-lateral direction to emerge on the lateral aspect of the pons. The fibres from the motor nucleus supply the muscles of the face, the scalp, and the ear. Secretory fibres also run in the chorda tympani, which is a branch of the facial. These, however, are probablv derived, like the sensory fibres, from the nerve of Wrisberg. The sensory fibres of the nerve of Wrisberg originate in the nerve-cells of the geniculate ganglion, and 414 PHYSIOLOGY passing inwards with the main root of the facial, divide into ascending and descending branches and end in the upper part of the column of grey matter which receives also the sensory fibres of the ninth and tenth cranial nerves. The ninth and tenth cranial nerves arise by a series of bundles of nerve fibres from the side of the medulla. Both the ninth and tenth are mixed visceral sensory and motor nerves. The sensory nucleus is a column of grey matter lying laterally to the hypoglossal nucleus just below the promin- ence on the floor of the fourth ventricle known as the ala cinerea. The descending fibres of these nerves form a well- marked bundle of white fibres known as the fasciculus soli- tarius, or sometimes, from its supposed connection with the regulation of respiration, the ' respiratory bundle of Gierke.' It may be traced down as far as the uppermost part of the cer- vical cord, its fibres losing themselves on their way down among the cells of the enclosing Plan of the origin of the tenth and grey matter. The efferent fibres of the ninth and tenth nerves pyramid ; nXII, nucleus of hypoglossal ; are derived partly from the Fig. 206. twelfth nerves. pyr, pyramia ; nXII, nucleus of XII, hypoglossal nerve ; diiX, XI, dorsal nucleus of vagus and accessory ; n.amh, nucleus ambiguus ; dorsal nucleus of the vagUS and js, fasciculus solitarius (descending root of vagus and , . glossopharyngeal); fsn, its nucleus; X, crossing accessory nerveS lymg extern- motor fibre of vagus ; g, cell in ganglion of vagus ally to the nucleus of the twelfth giving origin to a sensory fibre ; rfF, descending root •' of fifth ; cr, corpus restiforme. nerve, and partly from the nucleus ambiguus, a mass of grey matter lying deeper in the medulla (Fig, 206). The ninth or glossopharyngeal nerve supplies motor fibres to the muscles of the pharynx and the base of the tongue, and secretory fibres to the parotid gland. The sensory fibres convey impulses from the tongue, the mouth, and pharynx, the fibres originating outside the central nervous system in the gangHon-cells of the ganglion petrosum and the ganghon superius. It also contains inhibitory fibres to the respiratory centre. The tenth nerve, vagus or pneumogastric, is joined by the accessory part of the spinal accessory, so that the two nerves may be considered together. It has both afferent and efferent functions : Efferent functions : Motor to levator palati and three constrictors of pharynx. Motor to muscles of larynx. Inhibitory to heart. Motor to muscular walls of oesophagus, stomach, and small intestine. CONNECTIONS AND FUNCTIONS OF CRANIAL NERVES 415 Motor to unstriated muscle in walls of bronchi and bronchioles. Secretory to glands of stomach and to pancreas. Afferent functions : Regulate respiration. Stimulation of central end may quicken respiration and promote inspiration, or may inhibit inspiration. Stimulation of central end of superior laryngeal branch causes stoppage of inspiration, expiration, cough. Depressor and pressor (from heart to vaso-motor centre). Reflex inhibition of heart. Its afferent fibres arise from cells in the gangha on the trunk of the vagus, namely, the jugular ganghon and the ganglion trunci vagi. The spinal accessory nerve arises partly in connection with the vagus, partly by a series of roots from the lateral region of the spinal cord as low as the sixth cervical segment. The spinal portion of the nerve is purely motor and supplies fibres to the sterno-mastoid and trapezius muscles. The tivelfth or hypoglossal nerve arises from a collection of large multi- polar cells in the floor of the fourth ventricle at its lower end close to the middle line. The nerve-trunk issues from the ventral part of the medulla in the groove between the anterior pyramid and the olivary body. The hypoglossal is purely motor in function, supplying the muscles of the tongue, the extrinsic muscles of the larynx, as well as those moving the hyoid bone. Since the integrity of the nuclei of the cranial nerves is a necessary con- dition for the carrying out of various reflex acts in which these nerves are involved, the grey matter of the fourth ventricle and aqueduct is often spoken of as if it were cut up into a series of centres distinct for every act. The chief of these are the respiratory and the vaso-motor centres. Other centres that may be enumerated are : Centres for movements of intrinsic and extrinsic ocular muscles. Cardiac inhibition. Mastication, deglutition. Sucking. Convulsive (connected with respiratory). Vomiting. Diabetic (connected with vaso-motor). Salivary. Centres of phonation and articulation. We shall have to consider the action of these centres more fully in treating of the several functions of the body. It must be remembered, however, that when a dozen or more centres are enumerated as being situated in the fourth ventricle, it is not meant that we can anatomically distinguish a group of cells for each act or group of actions named. When we say that a part of the nervous system is a centre for any action, we merely mean that this part forms a necessary link, or meeting of the ways, in the complicated directing of nerve impulses that takes place in every co-ordinated act. THE CEEEBKAL HEMISFHEEE8 SECTION XVI GENERAL STRUCTURAL ARRANGEMENTS OF THE CEREBRUM The cerebral hemisplieres form the most important part of the brain. It is to the development of this part that is due the rise in type in vertebrates. In development they are formed as two diverticula from the front part of an outgrowth of the first cerebral vesicle. In the lowest vertebrates these outgrowths are connected entirely with the olfactory sense-organs, and we may regard the olfactory w part of the brain as a fun- damental part on which has been built up all the rest of the cerebral hemi- spheres. In a cartilaginous fish the whole of the upper brain is connected with the organ of smell, and consists of a thickening in the floor of the out- growth from the fore-brain. The roof of the outgrowth is formed of 1 simple ^epithehum. With the development of the visual sensations in the bony fishes there is still very little corresponding growth of the fore-brain, most of the fibres from the optic nerves going to the roof of the mid-brain (the optic lobes). The beginning of the cerebral hemispheres is associated with the development of nervous tissue in the roof of the prosen- cephalon. At its first appearance this higher brain material still receives chiefly olfactory impressions. But the structure of the cerebral cortex thus laid down differs from that of the centres forming the brain stem or the ol- factory lobe itself in that it provides for a very rich association of impulses between all its parts. The fibres entering the cortex break up into a fine mesh- work of fibres which run tangentially to the surface and come in contact with innumerable dendrites of nerve-cells situated at some little distance below the surface (Fig. 207). We have here the first germ of an apparatus in which the nerve-paths can be determined by education, i.e. in consequence of inhibitions by pain, rather than by the limits set by heredity. In the amphibian brain, and still more in the brain of the reptile, the cerebral 416 Tig. 207. Section through cerebral cortex of the frog. (After Edingee.) STRUCTURAL ARRANGEMENTS OF CEREBRUM 417 cortex extends over the whole of the roof of the cerebral hemispheres, though even here a very large proportion of it is devoted to the association of olfactory impulses. The importance of these olfactory association fibres is well shown in the figure (Fig. 208) of a diagrammatic section through a Uzard's brain. Above the reptiles there is a divergence in the course of development. The wider reactive powers of birds are based chiefly on an enormous development of the corpus striatimi, whereas in mammals the corpus striatum remains relatively small and the chief development occurs in the roof of the cerebral hemispheres, the so-called pallium or mantle. With the increased entry of fibres from the optic thalamus into the cerebral hemispheres, carrying mipulses from the eyes, ears, and all the other sense- ortrans of the body, the olfactory part of the brain diminishes in importance, ' ""^^^^^^^Tf-^ Fig. 208. Schematic section through brain of lizard showing the chief nerve-tracts. (After Edinger.) and in the higher mammals and man is altogether overshadowed by the newly formed part of the pallium. On this account those parts of the cerebral hemispheres in special connection with the olfactory sense-organs are often spoken of as the archipallium, in distinction to all the rest of the more newly formed brain substance, known as the neopallium. In man the cerebral hemispheres form a great ovoid mass exceeding in size all the rest of the brain put together. The two hemispheres are separated by a deep fissure, the great longitudinal fissure. Before and behind, this fissure extends to the base of the cerebrum, but in the middle the two hemispheres are connected by a mass of transverse fibres known as the corpus callosum. On the outer side each cerebral hemisphere presents a deep cleft, the Sylvian fissure. The whole surface of the brain is thrown by fissures (or sulci) into convolutions, by which means a very large increase of the surface grey matter is obtained. By these fissures the brain surface is divided into lobes. The general arrangement is shown in Figs. 209 and 210. The chief lobes are the frontal, the parietal, the occipital, the temporal, the insular, the limbic, and the olfactory. On the inner side, from' before baclvwards, we have the marginal, the paracentral, the pre-cuneus, the cuneus ; and in close proximity to the corpus callosum, the cingulum or U 418 PHYSIOLOGY S.precentralis Inferior 5_preeentralls superior ,,,,,.. ^ . I S.centralis (Rolandi) S.fronUds inferior l S.postcentralis inferior S. frontalis Superior ^ - ' S.frontalls medius , ^ su?c'^' Ramus ant. horizontalis ' Ramus ant.ascendens S. diagonal is Ramus post, of Sylvian f. S.postcentralis intermedius '. postcentralis superior S. intraparietalis S.temporalis medius S.occipitalis lateralis S.occipitalis transi/ersus Fig. 209. Left cerebral hemisphere of man, lateral aspect. (Symington.) S.precentralis mesial is S.cenf rail's (Roland, fiars marainalis s.cinduli S-parietalis superior S.pariefo-occipitalis nfull S-Corporh callosi 5. rosfralis Incisura iemporalis S.subparietalis 5. calcarinus fr-,^^-^ dentata S.collaterall: \ • S.collanrslis S. temporalis inferior Fig. 210. Left cerebral hemisphere of man, from the mesial aspect. (Symington.) STRUCTURAL ARRANGEMENTS OF CEREBRUM 419 supra- callosal convolution above, and the hippocampal convolution and the uncus below. The chief fissures separating these are the Sylvian fissure, the A Forebfaln \':. Midbrain Cerebellum £yer Cortex -■■ Occipital cortex Midbrain .^^ Cerebellum Cerebellum Fio. 211. Diagrams from Monakow, showing the evolution of the neopallium, and the gradual shifting of the visual sensory tracts from the mid-brain to the fore- brain, and thence to the occipital cortex. A, a bonj' fish. B, brain of a lizard. C. brain of a mammal (cat). central sulcus or fissure of Rolando, the parieto-occipital fissure, the calcarine fissure, the collateral fissure, and the calloso-inarginal fissure. Each of the main lobes (or gyri) mentioned above is further subdivided 420 PHYSIOLOGY by smaller fissures. The extent of these secondary fissures varies from brain to brain, the higher types of brain being richer in convolutions than those of the more primitive races. The gradual evolution of the cerebral cortex, and the concomitant shifting of the chief afferent impulses, arising in the projicient sense-organs, from the lower gangha to the higher educatable cortex, is well shown in the diagrams from Monakow (Fig. 211, p. 419). In the lower fishes practically all the reactions to visual impressions are carried out by the optic lobes. In the higher types the reflexes through these lobes become subordinated, first to the more complex organ of the optic thalamus (where representatives from all the afferent tracts of the body assemble), and later to the still more complex occipital cortex, when the reactions are determined not only by inherited nerve-paths but also by the various blocks and facihtations imprinted on the nerve-paths by the experience of the individual himself. The original cavities of the hemi- spheres form the lateral ventricles, each of which, in the adult brain, is prolonged into the main divisions of the hemispheres as the anterior horn, the posterior horn, and the inferior horn. Each lateral ventricle is roofed over by the corpus callosum and the adjoining white matter of the hemispheres. On opening the ven- tricle we see on its floor the body of the fornix, a flattened tract of white matter with longitudinal fibres, which in front bifurcates into two cylin- drical bundles which pass vertically downwards in front of the foramen of Monro into the mesial part of the sub- thalamic tegmentum. Internal to the fornix is a layer of pia mater, including the choroid plexus. On removing this the third ventricle is opened, so that in this region the wall of the cerebral hemispheres, like the roof of the third ventricle, is Hmited to a simple layer of ependyma. At the margin of the choroid plexus can be seen a part of the superior surface of the optic thalamus, separated, however, from the cavity of the ventricle by a layer of ependyma. Outside and in front of the optic thalamus are the Fig. 212. Horizontal section through the optic thalamus and corpus striatum, the ' basal ganglia.' (Natural size.) (QuAnsr.) vl, lateral ventricle, its anterior cornu ; cc, corpus callosum; si, septum lucidum ; af, anterior pillars of the fornix ; vi, third ventricle ; th, thalamus opticus ; st, stria meduUaris ; nc, nucleus caudatus, and nl, nucleus lenticularis of the corpus stria- tum ; ic, internal capsule ; g, its angle or genu ; nc, tail of the nucleus caudatus appearing in the descending cornu of the lateral ventricle ; cl, claustrum ; /, island of Reil. STRUCTURAL ARRANGEMENTS OF CEREBRUM 421 masses of nervous material constituting the corpus striatum. These present two nuclei of grey matter, known as the nucleus caudatus and the nucleus lenticularis (Fig. 212). The crusta of the crura cerebri as it ascends to the cerebral hemispheres passes behind between the optic thalamus and the corpus striatimi, and in front between the nucleus lenticularis and nucleus caudatus of the corpus striatum. Outside the corpus striatum we find another mass of white fibres, known as the external capsule, and this is separated from the white matter of the cortex cerebri by a thin layer of grey matter known as the claustrmn. In a horizontal section through the brain, the part of the internal capsule which pierces the corpus striatum forms an angle with the posterior part separating the optic thalamus from the lenticular nucleus. The part where the two limbs come in contact is known as the rjenu of the internal capsule (Fig. 212). THE OLFACTORY APPARATUS OF THE BRAIN In man the olfactory sense is but feebly developed, and the parts of the brain connected therewith are inconspicuous in comparison with those en- gaged in the reception of impressions from the other two main projicient sense-organs, namely, sight and hearing. On this account it is not easy to make out the connections of the olfactory lobe proper, the rhinencephalon, with the primitive part of the cortex, the arch i pall ium, subserving the olfac- tory sense and probably the allied sensations derived from the mouth ca\nty. The wide connections of the olfactory sense-organs with the different parts of the brain in the lower vertebrate are shown in the diagrammatic figure of the brain of a reptile (Fig. 208, p 417). It is interesting to note that the olfactory nerve fibres are derived from cells situated actually on the surface of the body. These aie bilateral, spindle-shaped cells, lying in the olfactory mucous membrane at the upper part of the nasal cavity. The peri- pheral process is short and passes towards the sm'face, wliile the deep process passes as a non-medullated nerve-fibre through the cribriform plate of the ethmoid to sink into the olfactory bulb. The bulb, in man, is a greyish enlargement at the anterior end of the olfactory tract. In sections stained by Golgi's method of impregnation it may be seen that the olfactory fibres terminate in an arborisation in close connection with a thick end arborisation derived from a dencbite of a large nerve-cell, known as a mitral cell. The synapses between these two sets of fibres are prominent objects in a section through the olfactory bulb and form the ' olfactory glomeruli ' (Fig. 213). The axons of the mitral cells pass back in the olfactory tracts. Each olfactory tract divides posteriorly into two roots, the mesial root wliich curves inw^ards beliind Broca's area and passes into the end of the callosal gyrus, and the lateral root which runs backwards and over the outer part of the anterior perforated spot. Its fibres pass into the uncinate extremity of the hippocampal gj'rus. The small triangular field of grey matter between the diverging roots of the olfactory tract is known as the olfactory tubercle. The primitive rhinencephalon includes in the adult human brain the olfactory bulb and tract, together with the anterior perforated space, the anterior part of the uncinate gyrus, the subcallosal gyrus, the septum lucidum, and the hii^ix)- campal convolution. The two sides of the rhinencephalon are united by fibres passing through the anterior commissure. Other tracts subserving tliis apparatus include the habenula passing from the fornix to the ganglion of the habenula, the fast'ieulus rctrollexus passing from this to the interpeduncular ganglion, and the corpus mammillare wliich is connected with the column of the fornix on the one hand and throughthe bundle of Vicq d'Azyr with the thalamus on the other. 422 PHYSIOLOGY THE CHIEF TRACTS OF THE CEREBRAL HEMISPHERES We may divide the tracts of the upper brain or cerebral hemispheres into three classes : I. Tracts comiecting the brain with lower levels of the central nervous system. II. Tracts connecting different parts of the cortex of one hemisphere and serving as a means of association between these different parts. III. Tracts (commissural) connecting the two cerebral hemispheres together. I. THE PROJECTION FIBRES These are the fibres which connect the cerebral cortex with the different lower levels of the central nervous system. They form a great part of the Fig. 213. Schema of course of olfactory impulses. (Ram6n y Cajal.) A, olfactory mucous membrane ; b, olfactory glomeruli ; c, mitral cells ; E, granule cells ; D, olfactory tract ; l, centrifugal fibres. fibres of the corona radiata and are condensed at the base of the brain into the broad band of fibres known as the internal capsule. A few of the fibres of the projection system may gain the cortex through the lenticular nucleus and by the external capsule. The projection fibres may be divided into two groups according as they conduct impulses to or away from the cerebral cortex : the afferent or corticipetal, and the efferent or corticifugal. A. AFFERENT TRACTS OF THE CEREBRUM. (1) The thalamo-cortical. From all parts of the optic thalamus fibres arise as axons of the cells of its grey matter and streaming out from its outer and under surfaces pass to every part of the cortex. Although there is no division of them into distinct groups as they leave the thalamus, they are often described as constituting a frontal, a parietal, an occipital, and a ventral stalk. The front fibres pass through the anterior limb of the internal capsule to reach the cortex of the frontal lobe, many of the fibres, however, termina- ting in the caudate and lenticular nuclei. The parietal fibres issuing from the lateral surface of the thalamus pass through the internal capsule to be distributed chiefly to the parietal lobe. The occipital fibres issue from STRUCTURAL ARRANGEMENTS OF CEREBRUM 423 the outer part of the pulvinar and the external geniculate body and constitute the so-called ' optic radiation,' passing outwards and backwards to be distributed to the cortex of the occipital lobe. The ventral fibres pass downwards and outwards below the lenticular nucleus and end partly in the latter nucleus and partly in the cortex of the temporal lobe and of the insula or island of Reil. (2) The fillet system of fibres. This great mass of ascending fibres has been already described {cjj. Fig. 197) as gathering up the impulses from the different sensory nerves of the cerebro-spinal > o®' system and terminating in the thalamus and subthalamic region. (3) The superior cere- bellar PEDUNCLE. These fibres, from the central gangha of the cerebellum, terminate for the most part in the thalamus and sub- thalamic region. It is pos- sible that some of them may pass through the hinder end of the internal capsule, with- out interruption in the thala- mus, to end in the Rolandic area. (4) The optic radiation. These diverging fibres in the back part of the corona- radiata are mixed up with fibres which are partly cortici- fugal. The corticipetal fibres arise in the pulvinar and the external geniculate body and end in the occipital cortex. (5) The auditory radiation. These fibres consist of the axons of cells situated in the internal geniculate body. They pass through the posterior limb of the internal capsule under the lenticular nucleus to end in the temporal lobe. Fig. 214. ■^ LOBE. Schema of projection fibres of cortex- (Cunningham.) B. THE EFFERENT PROJECTION FIBRES. (1) The pyramidal tract. This is composed of fibres which arise from the large Betz cells in the ascending frontal convolution, the ' motor area.', They pass through the corona radiata into the internal capsule, where they occupy the genu and the anterior two-thirds of the posterior limb. Hence they pass into the crusta, where they occupy the middle two-fifths of this 424 PHYSIOLOGY structure, and are continued as the pyramids of the pons and medulla to she upper part of the spinal cord, where most of them decussate to the other tide to form the crossed pyramidal tracts. Some of the fibres do not cross at the pyramidal decussation, but are continued down in the same position in the anterior columns of the spinal cord of the same side, forming the direct or anterior pyramidal tracts. These fibres cross for the most part lower down in the cord, so that the direct pyramidal tract is not seen below the cervical region. The pyramidal tracts are not found in lower vertebrates, and make their first appearance in the mammaha. Their development corresponds with the gradual increase in the direct interference of the cerebral cortex in the reactions of the organism as a whole and are an in- dex to the gradual shifting of these reac- tions from the inevitable to the educated reflex. The fibres of the pyramidal tract end at various levels of the spinal cord and can be traced to the lower end of the sacral region. According to Schafer they end in the posterior cornua, so that their action is to set going a reaction which could otherwise be elicited by stimulation of the afferent fibres entering by the posterior root at the level of the cord where they end. (2) The fronto-pontine fibres. These arise from cells in the cortex Diagrammatic representation of the frontal lobe, and pass down in the anterior hmb of the internal capsule to gain the mesial part of the crusta of the crus cerebri. The fibres end in the grey matter of the formatio reticularis of the pons, the nucleus pontis. (3) The temporo-pontine fibres. These arise from the two upper temporal convolutions, especially from that area which is associated with hearing. They pass inwards under the lenticular nucleus through the hinder limb of the internal capsule to gain the outer part of the crusta. In this situation this tract passes down into the pons, where it ends in the nucleus pontis. As part of these projection fibres we ought probably to reckon the fibres which take origin or end in the corpus striatum. The afferent fibres of this body are derived chiefly from the thalamus, forming the thalamo-striate .fibres. Other fibres arise in the nuclei of the corpus striatum and pass down in the dorsal portion of the crusta to end for the most part in the pons, the strio-pontine fibres. The relative position of these various fibres in the internal capsule Fig. 215 _ of the internal capsule, as seeen in hori zontal section. (Cunningham.) STRUCTURAL ARRANGEMENTS OF CEREBRUM 425 and ill the ciusta is shown in the accompanying diagrams (Figs. 215 and 21f3). The fronto-pontine and temporo-pontine fibres, which end in the nucleus pontis, come there in relationship with the fibres forming the middle peduncles of the cerebellum and derived chiefly from the lateral lobes of the cerebellum. These fibres may therefore be regarded as the eft'erent side of the great cerebro-cerebellar connections of which the afi'erent side is represented by the fibres — efferent so far as concerns the cerebellum — which pass from the cere- bellar cortex to the dentate nucleus and thence by a fresh relay in the superior cere- bellar peduncles to the red nucleus, optic thala- mus, and cortex of the opposite side. The development of these fibres, as of the lateral lobes of the cerebellum, is largely proportional to the growth of the cerebral hemispheres. In cases where there has been congenital atrophy of one cerebral hemisphere the crusta of the same side and the lateral lobe of the cerebellum of the opposite side also fail to develop. Fig. 216. Transverse section through mid-brain to show position of fillet and pyramid. AQ, anterior corpus quadri- geminum ; dV, descending root of fifth nerve; F, fillet (I, lateral, and m, mesial fillet) ; Pyr, pyra- mid ; Fr. fibres from frontal lobe to pons; TO, fibres from tem- poral and occipital lobes to pons ; Ne, fibres from nucleus caudatus to pons ; III, root of third nerve ; S, Sylvian iter; Rn, red nucleus. II. ASSOCIATION FIBRES These fibres serve to unite different por- tions of the cortex of the same hemisphere and may be classified into short and long association fibres. The short association fibres pass round the bottom of the sulci in U-shaped loops connecting adjacent convolutions. Fig. 217. Chief association bundles of the cerebral hemispheres. (Cukninqham. ) \. Outer aspect of hemisphere. B. Inner aspect of hemisphere. These fibres are some of the latest to acquire a medullary sheath and probably first become functional as associated activity between the various portions of the cortex is gradually acquired by education. The long association fibres may be divided into five groups as follows : (a) The uncinate fasciculus passes from the orbital convolutions of the frontal lobe to the front part of the temporal lobe round the stem of the Sylvian fissure (Fig. 217). 14* 426 PHYSIOLOGY (h) The cingulum is closely associated with those parts of the cerebral cortex known together as the limbic lobe. In front it originates in the neighboiu"hood of the anterior perforated space, passes round the genu of the corpus callosum, and then is carried backwards over the upper surface of this body to its hinder end, where it turns round and is distributed to the hippocampal gyrus and to the temporal lobe. (c) The longitudinal superior fasciculus lies in the base of the frontal and parietal lobes, and passing from before backwards connects the frontal occipital, and temporal parts of the cerebral cortex. {d) The longitudinal inferior fasciculus runs along the whole length of the occipital and temporal lobes, being situated behind on the outer aspect of the optic radiation. (e) The occipito-frontal fasciculus lies on the inner aspect of the corona radiata in intimate relation to the caudate nucleus, and projects out over the upper and outer aspect of the lateral ventricle immediately outside the ependyma. III. THE COMMISSURAL FIBRES These are arranged in three groups : (a) The corpus callosum forms a great mass of white fibres passing trans- versely in both directions between the two hemispheres. Its fibres are Fig. 218. Schematic section through cerebral hemispheres, to show chief classes of nerve tracts. (After Ramon y Cj4Jal.) A, corpus callosum ; b, anterior commissure ; c, pyramidal tract ; a, cell giving off projection fibre ; 6, cell giving off commissural fibre ; c, cell with axon forming association fibres. derived from every part of the cerebral cortex with the exception of the olfactory bulb and the hind and fore parts of the temporal lobe. As the fibres cross the middle line they become gradually scattered, so that they tend to connect wholly dissimilar parts of the cortex of opposite hemispheres. Each callosal fibre arises in one hemisphere and ends by fine arborisations in the opposite hemisphere. It may represent either the axon of one of the cortical cells or a collateral from a fibre of association or a collateral from a projection fibre (Fig. 218). (b) The anterior commissure is situated in the anterior wall of the third ventricle in front of the two pillars of the fornix. It connects together the two olfactory lobes and portions of the opposite temporal lobes. In lower vertebrates it is almost entirely olfactory in function, but in man the olfactory STRUCTURAL ARRANGEMENTS OF CEREBRUM 427 fibres only form a small proportion of the total number making up the bundle. (c) The psalterium or hippocampal commissure is a thin lamina formed of transverse fibres filhng up the small triangular space on the under surface of the hinder part of the corpus callosum formed by the divergence of the posterior pillars of the fornix. Like the anterior commissure, the hippo- campal commissure is closely associated with the sense of smell. Its fibres arise from the pyramidal cells in the cornu ammonis or hippocampus and pass for the greater part to the cornu ammonis of the opposite side. MINUTE STRUCTURE OF THE CEREBRAL CORTEX The cortex of the cerebral hemispheres consists of a layer of grey matter covering a central mass of white fibres. With the growth in size of the brain, which accompanies the development of increased intelHgence and powers of adaptation the necessary increase in cortex is rendered possible by the folding of the surface into convolutions and fissures. The chief of these convolutions have already been indicated in the sketch of the anatomy of the brain (Fig. 209). ^ On section the grey matter is seen to consist of many layers of nerve-cells embedded in neurogha and nerve fibres, both medullated and non-medullated. The nerve-cells vary in size and shape ; one kind of cell is, however, typical of this part of the central nervous system. This is the pijramidal cell (Fig. 219), a cone-shaped or pear-shaped cell with one large apical dendrite which runs towards the surface and breaks up in the most superficial layer into a number of branches. Dendrites are also given oft' from the sides and lower angles of the cell. The axon, which arises from the axon hillock in the middle of the base of the cell, passes downwards into the white matter, giving of? collaterals in its course. Some of these axons pass by the corona radiata into the internal capsule and into the crura cerebri, including those which form the pyramidal tracts ; others, or their collaterals, may pass into the adjacent regions of the cortex, or across by the corpus callosum into the opposite hemisphere. Although varying in structure at different parts, it is generally possible to distinguish four or five layers in the cortex. (1) The most superficial layer, known as the outer fibre lamina, or molecular layer, contains very few cells. It is composed generally of the den- drites of cells from the deeper layers. It contains a few cells which are spindle-shaped and are provided with several processes running parallel to the surface. These are sometimes called association cells. It is probable that aft'erent fibres, entering the cortex, pass up towards the surface and end for a large part in this molecular layer. (2) Below this is a layer of pyramidal cells, the outer cell lamina, which is divided by some observers, e.g. Campbell, into three, viz. : (a) The small pyramidal cells. (b) Medium-sized pyramidal cells. (c) Internal layer of large pwamidal cells. 428 PHYSIOLOGY (3) Below the pyramidal layer we find a stratum of small cells, most of which are stellate in form. This is known as the stellate or granule layer, or middle cell lamina. (4) Internal to the granule layer is the inner fibre lamina. In the motor cortex and in certain other parts of the brain this contains large sohtary cells, which in the motor area receive the name of the cells of Betz. (5) Most internal of all, lying next to the white matter, is the poly- lit' ' 1 I ! , mr..r Fig. 219. Schematic representation of the neuro- fibrillar apparatus of a cortical pyramidal cell. (After CaJal. ) a, axon ; dh, dendrites. morfhous layer or inner cell lamina, composed of many types of cells, among which spindle-shaped cells predominate. Other cells are also found resem- bling pyramidal cells of the more superficial layer, but directed in the reverse direction, so that their axons take a course towards the surface. These are the cells of Martinotti. We also find Golgi cells with a freely branching axon, which terminates in the adjacent grey matter. STRUCTURAL ARRANGEMENTS OF CEREBRUM 429 If sections of the cortex be stained by some method such as Weigert's, which displays medullated nerve fibres, sheaves of radial fibres may be seen running from the white centre towards the surface and giving off a rich Fig. 220. Diagrammatic section of cerebral cortex. (From Barker after Starr. Strong, and Leamino.) I. inoli'cular layer with a, bi-pnlar cell ; II, layer of small pyramidal cells ; III, layer of large pjTamidal cells ; I\', polymorplunis layer : \'. white matter. mesh work of fibres to the intervening portions of the grey matter. In addition bands of tangential fibres are seen running parallel to the surface in certain situations, viz. : 430 PHYSIOLOGY (a) A layer of very fine fibres just under the surface of the cortex. This layer is especially marked in the hippocampal convolution and is but slightly developed in other regions of the cortex. (6) A layer between the molecular layer and the layer of pyramidal cells, known as the outer line of Baillarger. (1) Molecvilar or outer fibre lamina. 0-34 mm. (2) Pyramidal or outer cell lamina. 0-90 mm. (3) Granular layer or middle cell lamina. 0-22 mm. (4) Inner fibre lamina. 0-22 mm. (5) Polymorphic or inner cell lamina. 0-31 mm. a. Tangentiallayer. b: Outer line of Bail- larger. c. Inner line of Bail- larger. Fig. 221. Motor leg area. (c) Internal to the granule layer is another zone of fibres, the inner line of Baillarger, giving its name to the inner fibre lamina. {d) In the part of the occipital cortex, distinguished as the visuo-sensory area, which receives fibres of the optic radiations, a special layer of tangential fibres is observed running through the middle of the granular layer and dividing it into two parts. This is known as the line ofGennari (Fig. 222). A careful study of the distology of the different parts of the cortex in man enables us to distinguish certain types of structure characteristic of various regions of the grey matter. In attempting by such means an histological localisation of functions we have to take into account : (a) The thickness of the cortex. STRUCTURAL ARRANGEMENTS OF CEREBRUM 431 (6) The relative thickness of the various layers. (c) The character of the cells found in the various layers. (d) The arrangement and degree of development of the systems of medullated fibres, both radial and transverse. The possibilities in such a method are at once apparent if, as in Figs. 221, 222 and 223, we compare the structure of the cortex from, e.g. the pre-central (:!) (I-21 TTTJ 4 r ■^ ">)^: (5) 0-29 FiLi. 222. Visuo-sensoiy. Tiu. 223. Visuo-psj-chic. motor convolution, the visuo-sensory area of the occipital convolution, and the ' visuo-psychic ' area. The finer differences are not so readily perceptible without careful study and measurement of the various layers and the elements constituting them. By this method we can mark out the cortex into areas, which agree closely with the localisation of functions as arrived at by experimental methods or by a study of the systems of fibres in the brain or of the functional defects observed under pathological conditions. The localisation arrived at in this way is represented in the diagrams taken from Campbell (Fig. 224). Thus in the motor area, the precentrul 432 PHYSIOLOGY or ascending frontal convolution, we find below the granular layer the well- marked pyramidal cells or Betz cells, which are larger than any other element in the cerebrum. The layer of pyramidal cells is also very thick, while Fig. 224. Human brain showing outer (A) and mesial (B) surfaces, and the situation of the chief motor and sensory areas. The different shading represents the extent of each of these areas as determined by a study of the histological structure of the cortex. (Campbell.) the granular layer is but slightly developed. In this area the actual average thickness of the different layers is as follows : Molecular or outer fibre lamina . Pyramidal or outer cell lamina . Granular or middle cell lamina . Betz or inner fibre lamina Polymorphous layer or inner cell lamina 0-34 mm. 0-90 mm. 0*22 mm. 0-22 mm. 0"31 mm. STRUCTURAL ARRANGEMENTS OF CEREBRUM 433 In the visuo-sensory area the granular layer is the thickest, and is divided into two layers by the band of tangential fibres forming the line of Gennari. In the association areas, both those, such as the intermediate precentral and visuo- psychic, which are normally associated with motor or sensory processes, as well as the higher association centres of Flechsig, i.e. the frontal, parietal and temporal lobes, the most marked feature in the section is the great development of and the large number of cells observed in the outer cell lamina or pjTamidal cell layer. It will be noticed, too, that the audito-sensory area is but small in extent and lies almost entirely within the lips of the fissure of Sylvius, while the greater part of the superior temporal convolution, which on the left side is associated with the auditory images and associations necessary for the comprehension of speech, partakes of the character of an intermediate or psychic sensory area. The same method may be applied to a comparison of the relative develop- ment of the cerebral functions in different types of animals. A comparison of the brain of the dog, ape, and man shows that while the absolute amount of brain substance devoted to the elementary functions of movement and sensation remains practically the same throughout, in man these areas are, relatively to the whole brain, very much diminished in size, the greater part of the brain surface being taken up with the nervous material of the type which is connected with the functions of association involved in the higher processes of reflection, intelligence, and vohtion. If we draw still lower animals into the sphere of our observations we are enabled to form some idea as to the relative significance of the various elements of the cortex. Thus in an animal, such as the rabbit, the poly- morphous layer is three times the thickness of the p}Tamidal layer ; whereas in man, ^v'ith an infinitely greater range of reaction, it is only one-third of the thickness of this layer. If we may roughly assign a function to each of the types of cells found in the cortex, we may say that the pyramidal cell layer is generally associative in functions. The large pyramidal cells of Betz are motor ; the granular layer is sensory, while the polymorphous layer presides over the lowest cortical functions, such as those concerned in the getting of food, the sexual instincts, and so on. The description given above applies to the whole of the neopallium. In the more primitive part of the brain, the archipallium, represented by the hippocampus, we find only two cell lamina} which are homologous with the middle (granule) and the inner polymorphous cell layers of the neopallium. SECTION XVII THE FUNCTIONS OF THE CEREBRAL HEMISPHERES In an animal possessing cerebral hemispheres it is impossible to foretell with certainty what particular reaction may be evoked by any stimulus. The animal which has been deprived of its hemispheres can, as we have seen, be played on at will, whereas the intact animal is an individual whose actions — to judge by our own experience — are guided by intelligence, and influenced by motives or by feehngs of fear, hunger, pain, and the like. In short, its behaviour is analogous to that which in man we associate with conscious feehng and vohtion. This association of the voKtional manifesta- tions w^th the cerebral hemispheres has long been assumed, and is borne out by the exact paralleHsm existing between the degree of intelhgence with w^hich an animal is endowed and the extent of development of its cerebral hemispheres. Moreover in man himself there is a proportionahty between the average size of the brain, i.e. of the cerebral hemispheres, and the average intelhgence of the race. Earher attempts to analyse the factors entering into the sphere of consciousness and to associate with these factors locaUsed parts of the brain failed, largely on account of a faulty psychological analysis and the absence of any proper experimental groundwork for the conclusions put forward. Gall, the founder of phrenology, recognised more clearly than previous authors that the cerebral hemispheres must be regarded as the material basis of consciousness. Impressed, however, by the fact that there was no proportionahty between the acuteness of the senses and the degree of development of the cerebral hemispheres, he considered that any division of fmictions among different parts of the hemispheres must relate to highly complex psychical conditions, and therefore on very slender grounds allotted to parts of the brain functions such as those of intelhgence, memory, judg- ment, amativeness, and so on. These conclusions of Gall were overthrown by Flourens on both theoretical and experimental grounds. In the first place, Flourens pointed out that the mental faculty of man cannot be divided up into a number of different independent quahties or faculties, such as those proposed by Gall. In the second place, he showed that in the pigeon although loss of the whole cerebral hemispheres destroyed intelligence and, associative memory with the actions founded on such endowments, removal of portions of the brain caused simply a lowering of these functions, and it 434 FUNCTIONS OF THE CEREBRAL HEMISPHERES 435 COR was a matter of indifference whether the brain substance was taken from the anterior or from the posterior portions of the hemispheres. Flourens there- fore concluded that the cerebral hemispheres acted as a whole as the seat of the will and intelligence. There is no doubt that Flourens was so far per- fectly correct, since all parts of the brain must co-operate in determining the psychical condition of any individual in any given moment. He was, however, as later researches showed, in error in thinking that no difference could be distinguished between the parts contributed by the various con- volutions of the brain to the organic whole which is called consciousness. As we have seen, histological evidence, which in the case of ^5^ the cerebellum displays a marked uniformity throughout the whole cortex, in the case of the cerebrum reveals strik- ing differences between its various areas. The demarca- tion of the cerebral cortex into areas according to the histo- logical structure of their grey matter agrees with, and in many cases supplements, the results procured by an experi- mental inquiry into the func- tions of the different parts. That there is a localisation of function in the cortex so far as concerns the movements of the two sides of the body was known to Galen, who men- tions the occurrence of paralysis on one side of the body as a re- sult of lesions in the brain of the opposite side. In 1861 a French physician, Broca, confirming older statements by Dax and Bouillaud, maintained that aphasia, i.e. loss of power of speech, when it occurred in right-handed people was always associated with a lesion of the third frontal convolution of the left hemisphere, which has ever since that time been known as Broca's convolution. Hughlings Jackson in ISG-I drew attention to the connection of locaUsed spasms (Jacksonian epilepsy) with lesions of certain parts of the central convolutions. On anatomical grounds Meynert considered that the posterior portions of the hemispheres were probably more nearly connected with sensation, and the anterior with the power of movement ; but direct evidence of motor localisation was first brought by Fritsch and Hitzig in 1870. These observers pointed out, in contradiction of the then received idea, that the grey matter of the cortex was excitable, and that it was possible to evoke co-ordinated movements of the limbs on stimulating the Fig. 225. Upper surface of dogs brain, showing results of excitation. (Fritsch and Hitzig.) A, neck muscles ; -I-, movements of fore limb ; =**=, movements of hind limb ; O, movements of face; ASG, anterior sigmoid g}Tus; PSG, posterior sigmoid gyrus ; COR, coronary fissures ; Scr, crucial sulcus. 436 PHYSIOLOGY B. front part of the hemispheres in dogs with weak currents. The results of their experiments are shown in Fig. 225. These experiments were soon after repeated and confirmed by Ferrier, who extended his observations to the monkey, and more lately by Horsley, Schafer, Beevor, Sherrington, and others in the higher apes and man. It was formerly a subject of dispute whether the movements resulting from stimulation of the cortex were due to the excitation of the grey matter or of the underlying white matter. The following facts show that the seat of the excitation is in the grey matter : (1) A smaller strength of current is required to excite the grey matter than the underlying white matter, after removal of the grey matter. (2) In animals poisoned by chloral the grey matter is inexcitable, though movements can still be aroused on stimulating the white matter. A similar inexcitability of the grey matter can be produced by paint- ing it with cocaine. (3) The latent period elapsing between the beginning of the stimu- lation and the occurrence of the movement in the corresponding limb is longer when the grey matter is excited than when the stimulus is applied to the white matter. The results obtained by Fran9ois Franck give a latent period of -065 sec. for the grey matter and -045 sec. for the white matter (Fig. 226). Fig. 226. Tracings to show latent periods of movements obtained by stimulating : A, grey matter; B, underlying white matter g^of cortex. Time-marking Whether the stimulus acts directly on the pyramidal cells of the'' cortex, or whether, as seems more likely, it is the endings of the afferent nerves to the cortex which are really excited by the stimulus, we cannot at present determine. (F. Franck.) When we compare different animals, such as the dog, monkey, and man, we find there is a much finer difierentiation of movements evoked by stimula- tion of the cortex in the higher than in the lower type. Whereas in the dog the excitable areas shade into one another, in the higher ape and man the areas are much more circumscribed and are often separated from adjoining areas by an inexcitable zone. The locahsation of motor functions in the cortex of the chimpanzee is indicated in the accompanying diagrams by Sherrington (Figs. 227, 228). It will be seen that the motor cortex is hmited, on the convex side of the brain, to the precentral convolution, or ascending frontal convolution, situated immediately in front of the fissure of Kolando. On the inner aspect of the hemispheres only the corresponding part of this convolution gives motor responses on excitation. We may say broadly that, from above downwards, by stimulation of the precentral convolution we get movements of the leg, arm, and face ; though, as is shown in the diagram, FUNCTIONS OF THE CEREBRAL HEMISPHERES 437 within these larger areas smaller areas can be distinguished for definite co-ordinated movements of the different parts of the body. NATURE OF MOVEMENTS EXCITED. The movements obtained by excitation of these areas resemble in every respect the co-ordinated move- FiG. 227. /)/7us & i/agina Toes Ankle"--. Knee Hip .., Shoulder Elbow Finders & fhSmb Sulcus , . . centralis Abdomen ^Chest Ear-' ■•' / Eyelid, ■-'Closure Nose °^j^ ^ Op ening \ of jaw l/ocal cords MasticaTJon Sulcus centralis Sulccalioso Svbic.parifto occtp. Fig. 228. SuLcCentral. ^"u^i Vkgina, Sitle./nvcencr.marg. Sulccalicarin. CSS. del. Fig. 227, outer surface ; Fig. 228, inner surface of brain of cliimpanzcc, showing movements obtained by excitation of the motor areas. (Sherrington.) ments observed during the normal willed or spontaneous activity of the animal. Like the movements evoked by stinuilation of a sensory surface they involve therefore the reciprocal innervation of antagonistic nuiscles. Never do we find simultaneous contractions of antagonists, even where two 438 PHYSIOLOGY opposing centres are excited simultaneously ; one reaction is prepotent, as is the case with cutaneous excitation, and this reaction is attended and brought about by ordered contraction of certain muscles accompanied by an ordered relaxation of their antagonists. Thus the movement of opening the jaw, which can be excited from a fairly large area of the cortex, involves a relaxation of the normal tone of the masseter muscle. Flexion of the leg demands relaxation of the extensor muscles. As in the case of the spinal reflexes, this relaxation, or inhibition, can be abolished under the action of strychnine, or the toxin of tetanus. After administration of either of these it is impossible to evoke inhibition of any muscle. Excitation of the cortical centre for the movements of the jaw causes contraction of both closers and openers of the jaw, i.e. a strife in which the stronger masseter muscles predominate, so that the jaw is firmly closed. The part played by muscular relaxation in the response to cortical stimula- tion is also well seen in the case of the eye muscles. Stimulation of the centre for eye movements on the convex surface of the frontal lobes on the right side causes ' conjugate deviation ' of both eyes to the left. This movement involves contraction of the right internal rectus and left external rectus, and a simul- taneous inhibition of the tone of the right external rectus and left internal rectus. If all the muscles of the right eye be divided except the external rectus, this eye looks permanently towards the right side, i.e. a right external strabismus or squint is produced. On now exciting the right cortex both eyes move to the left, although the right internal rectus is divided. The movement of the right eye stops at the middle line, and is brought about simply by a relaxation of the tone of the right external rectus muscle (Sherrington). This movement of both eyes on stimulation of one side of the brain shows that the function of each hemisphere is not entirely unilateral with regard to the muscles of the body. As a rule the response to excitation of the motor area for limbs is strictly unilateral. In the case of those movements, how- ever, which are normally carried out by co-operation of the muscles of the two sides, such as the movements of the trunk, neck, and eyes, stimulation of the motor area in one hemisphere evokes a movement involving the muscles of both sides of the body, i.e. the cortical representation is one of movement rather than one of muscles. Where an action is carried out by similar con- tractions of corresponding muscles on the two sides, the movement itself is bilaterally represented in the cortex. Types of such reactions are found in closure of the mouth, contraction of the abdominal muscles, erection or flexion of the trunk. It seems that under such circumstances there is a free communication between the lower motor centres of the two sides, since the bilaterahty of the response is not altered by extirpation of the cortex of the opposite hemispheres to that which is being stimulated. CORTICAL EPILEPSY. When electrical excitation, of any strength over the minimum effective stimulation, is applied to the motor area of the cortex, the movements evoked tend to persist for a short time beyond the duration of the stimulus. On still further increasing the strength of the FUNCTIONS OF THE CEREBRAL HEMISPHERES 439 current, the contraction spreads to adjoining muscles, and finally may affect all parts of the body, giving rise to the phenomenon known as an epileptic convulsion. The same effect may often be caused by weak stimuli, if the irritability of the cortex be raised in consequence of repeated previous stimulation. A typical fit consists of two parts. The first effect of the stimu- lation is a strong tonic contraction ; this outlasts the stimulus for some time, and then gives way to a series of clonic contractions, repeated at first at intervals of from six to ten per second, but gradually getting slower as the fit dies away. The tracing of such a contraction is given in Fig. 229. Fig. 229. Tracing of muscular contractions during an epileptic convulsion aroused by strong stimulation of the motor area. (Hoksley and Schafer.) The main phenomena of a fit, due to irritation of any portion of the motor area, were described by Hughlings Jackson in 186-i, even before the experimental proof of cortical locahsation had been brought forward by Fritsch and Hitzig. A similar condition may occur in the human subject as a result of irritative lesions of this part of the cortex, such as that due to the presence of a tumour or a spicule of bone pressing on the brain. Jackson showed that in this condition the convulsive movements follow a certain order or ' march.' Thus if the thumb area be the seat of stimulation, the fit begins by a contraction of the thumb muscles, then spreads to the muscles of the hand, fore-arm and shoulder muscles of the same side, and then to the face, trunk, and leg. If it begins in the toes the order would be up the leg and down the arm. The same ' march ' is observed in artificial stimulation of the motor area. If the convulsions are very strong they spread to the leg of the opposite side and then to the w^hole body. The spread to the other side of the body is not prevented by division of the corpus callosum, nor by isolating the centres from one another, so that the sequence seems to be maintained through the mediation of the sub-cortical centres. Complete excision of the cortical centre for any given movement excludes this move- ment from participation in the fit. In man this U'^e of epilepsy is, in the milder cases at any rate, generally unattended with loss of consciousness. In animals epileptic convulsions can be excited by stimulation of any portion of the cortex, though it is obtained by a weaker stimulus applied to the motor cortex than to any other part. Jacksonian epilepsy is often preceded by a sensation of numbness or tingling, the ' aura,' in the part in which such convulsions begin. In ordinary idiopathic epilepsy tactile or visual sensory aurae may precede the attack ; but in this case loss of consciousness is always a prominent symptom, even in th(> milder form of the disease. Universal 440 PHYSIOLOGY epileptic con\n.ilsions can be excited in animals by the injection of absinthe into a vein. Dining the convulsion there is a rise of blood pressure and a quickening of the pulse ; the respiration is very often stopped during the tonic part of the spasm, so that the patient becomes Hvid. The universal con- dition of excitation affects also the centres from which the secretory nerves originate, so that there is an excessive flow of saHva, which, in the idio- pathic case, is responsible for the characteristic frothing at the mouth. EFFECTS OF ABLATION OF THE MOTOR CENTRES We have seen that a dog may preserve complete power of movement after a total ablation of both cerebral hemispheres. We should not expect therefore to find any lasting paralysis as a result of extirpation of portions of the brain, such as the motor centres. Ablation of the motor areas in these animals, during the first few weeks after the operation, gives rise to considerable disorders of movement, the muscles on the side of the body opposite to the lesion being markedly weaker than those on the same side. These symptoms, however, gradually pass off, so that after a time not only are both hmbs employed in the ordinary automatic movements of progression, but the animal can be taught new movements in the Kmb, the cortical centre for which has been excised. We must conclude therefore that in the dog all the movements, including those which are voluntary and conscious, can be carried out in the absence of the motor centres, although destruction of these centres may impair the accuracy with which some of the finer movements are regulated. In the monkey (Macacus) the effect of ablation is more marked, corre- sponding to the greater degree of locahsation in these animals. If the whole of the motor area on the external surface of the brain be excised, e.g. on the right side, there will be almost complete paralysis of the left arm and the left side of the face, and weakness of the muscles of the left leg. The animal will continue to use the leg in walking and in chmbing. If the lesion extends to the medial side of the hemisphere paralysis of the leg is more marked, and the muscles of the left side of the trunk are also affected. Many of these symptoms disappear in the course of time. In a monkey in which Goltz had destroyed the greater part of the left side of the cerebral hemispheres it was found that the right arm and hand could be still employed alone for such purposes as taking food, although the movements were much more awkward than those of the left hand. Still less complete is the recovery from lesions of the motor area in man. We possess now a considerable number of typical histories of cases in which part of the motor cortex has been destroyed by disease or by operation, and the seat of the lesion verified by post-mortem examination. In all these cases there has been a loss of voluntary movement corresponding in distribu- tion to the seat of the lesion and proportionate in its severity to the extent of the lesion. On the other hand, equally extensive lesions outside the as- cending frontal convolution have been shown to have no effect on voluntary movements. The loss of movement is chiefly confined to those which we FUNCTIONS OF THE CEREBRAL HEMISPHERES Ul regard as volitional. Although, for instance, the arm may be paralysed, it can be still raised in association with a movement involving the other arm. A certain degree of recovery from the immediate effects of the lesion may be observed, but the recovery is never complete. The difference in the reaction of various animals to lesions of the motor cortex is connected with the gradual shifting of functions from the sphere of fatal necessary reactions to the sphere of educatable adaptations {i.e. from the lower centres to the cerebral cortex), which is a characteristic of the evolu- tion of the higher type of nervous system, and is a concomitant of the in- creased adaptability which distinguishes man from all the lower animals. In the animal without hemispheres the motor mechanisms for all the movements of the body are present and can be set into action from any point on the sensory surface of the body. The first effect of adding the cerebral hemi- spheres to this mechanism is to increase the range of reactions, to modify them or to inhibit them, by diverting the stream of nervous impulses into channels which have to a large extent been laid down in the cortex by the past experience of the individual. In the frog and bird we notice an auto- maticity and a ' conscious ' adaptation of movements to purpose, although the hemispheres have no direct connection with the motor centres of the cord, and present no areas which we can designate as motor. In the dog, although a portion of the brain is in direct connection with the spinal motor centres, and can therefore initiate movements without making use of the mid-brain motor machinery, these movements play only a small part in the motor life of the animal, and the removal of the corresponding centres takes away but little of the conscious functions of the animal. In man the enor- mous power of acquisition of new movements is rendered possible by the shifting of one motor function after another to the sphere of influence of the cerebral hemispheres. Almost every act of life in man has become one involving co-operation of the cerebral cortex. For many years after birth man is helpless and far inferior, as a reactive organism, to animals much lower in the scale. Even the lower motor fmictions, such as those of loco- motion or defence, have to be painfully learnt, and this learning implies the laying down of paths (Bahmmg) in the cortex. On this account the sub- cortical centres in man are no longer complete. Acting in every instance of life as a subordinate or adjunct to the cerebral hemispheres, they are unable to carry out even the simpler motor reactions of the body after removal of those portions of the hemispheres especially engaged in the control of voluntary movement. The motor defect therefore which is brought about in man, as a result of destruction of one or more of the motor centres, is to a large extent permanent. If the lesion in man be strictly limited to the motor areas in the ascending frontal convolution, it is impossible to detect any loss of sensation in the affected parts of the body. On the other hand, some loss of sensation is often found where the paralysis is widespread and occasioned by extensive lesion in the neighbourhood of the Rolandic area. Moreover, even in localised lesions in man, an epileptic fit may be preceded by a sensory aura in the part 442 PHYSIOLOGY which is the starting-poiut of the convulsive movements. Much discussion has taken place as to the exact significance to be assigned to these slight sensory phenomena. By some observers, e.g. Munk, it has been thought that the motor centres were the end- stations of the fibres subserving muscular sensations, and that the movements resulting from their stimulation were due to the revival of such sensations. Bastian insisted on the important part played in voluntary actions by afferent impressions, and these centres have sometimes been spoken of as ' kinsesthetic ' or sensori-motor. The discussion has, however, now resolved itself practically into one of terms. There is no doubt that, when the lesion is strictly locahsed in the motor area, paralysis may be present without any loss of sensation whatsoever. The paralysis therefore cannot be classed with the sensori-motor paralysis dis- tinguished earlier as the result of division of sensory roots. On the other hand, when we say that this part of the brain represents a ' centre for voluntary movements,' we do not mean that the vohtional motor impulses arise de novo from the pyramidal cells in its grey matter. Every neuron in the nervous system is part of an arc, and it is generally difficult to label any given neuron as definitely sensory or motor. In a reaction involving a chain of neurons we can assign the name of motor to that neuron which sends its axon to the muscle, and of sensory or afferent to that neuron which receives the impulses at the periphery of the body. Where in the chain we are to draw the dividing fine and to say these neurons are sensory and those motor, it is difficult to decide. The motor areas in the cortex give origin to the long fibres of the pyramidal tract, which passes right through the central nervous system to the segmental centres of the cord. We know that the integrity of these tracts is essential for the carrying out of voluntary move- ment. It is therefore convenient to speak of them as motor or efferent tracts, and their origin as motor centres ; although these tracts have the same relation to the motor- cells of the spinal segment as have the afferent fibres from the posterior roots by which similar movements may be evoked. On the other hand, the activity of the pyramidal cells of the cortex, hke those of the motor- cells of the spinal cord, is determined by the arrival at them of afferent impressions. In the absence of these afferent impressions no spontaneous discharge of motor impulses takes place. Thus in the spinal frog we have seen that complete inactivity is brought about by section of all the posterior roots. In the same way paralysis of the arm is induced by section of all its posterior roots, although it can be shown that the motor cortex is still excitable, and that the apphcation of an induced current to the motor centres of the arm evokes a movement as easily as in the normal animal. The motor- cells in the cortical motor centres are normally played upon and aroused by impressions arriving at them from all other parts of the brain and nervous system, and determined originally by impressions falhng on the surface of the body. FUNCTIONS OF THE CEREBRAL HEMISPHERES 443 THE FUNCTIONS OF THE CORPUS STRIATUM The mass of grey matter known as the corpus striatum, which consists of the nucleus lenticularis and the nucleus cordatus, is the basal part of the outgrowth from which each cerebral hemisphere is formed and in the lowest vertebrata represents almost the whole of the telencephalon. For many years the corpus striatum was classed with the optic thalamus as the ' basal ganglia,' and these two ganglia were regarded as relay stations between the cerebral cortex and the lower parts of the ct5ntral nervous system. This view was correct so far as concerns the optic thalamus, in which end all the afferent tracts and from which afferent impressions are carried on by fresh relays of fibres to the cortex. In the higher mammals the motor cortex has a direct connection with the motor nuclei of the bulb and spinal cord through the pyramidal tracts, which are not interrupted anywhere on their course. On destroying the corpora striata degenerated fibres are found running to the optic thalamus, to the red nucleus, and from the latter to the posterior longitudinal bundle. On the other hand the corpus striatum receives fibres from the olfactory tracts and from the optic thalamus. These connections would tend to show that the corpus striatum serves in no way as an inter- mediary between the cortex and the lower parts of the central nervous system, but is an independent mass of grey matter, receiving impulses from the same source as the cortex and sending impulses which may join in the stream of impressions which play upon the lower motor mechanisms of the bulb and cord. Isolated excitation of the caudate and lenticular nuclei has no visible effect, provided the current is not so strong as to spread to the adjoining pyramidal fibres in the internal capsule. A study of the evolution of the central nervous system in different classes of animals points to a diminishing importance of these bodies in the normal hfe of the animal. In the carti- laginous fishes it probably serves to a greater or less degree the same functions in the determination of educated reflexes as the cerebral hemispheres in mammals. In birds the corpus striatum attains its greatest relative develop- ment, the increased powers of adaptation in these animals being apparently procured by development of the corpus striatum instead of as in mammals by increased development of the palHum or cerebral hemispheres. In the monkey Kinnear Wilson found no definite results to follow destruction of the grey matter in these bodies. The animals were, however, only allowed to survive the operation of destruction three weeks, and the same observer has pointed out that destruction of the corpora striata in man may give rise to a morbid condition, characterised by tremor in the execution of willed movements and increased tonicity of the muscles. He therefore ascribes to these bodies, or rather to the sensori-motor mechanism which has its chief meeting-place in their nuclei of grey matter, a stead}'ing efi'ect on the motor system, and places this system by the side of the other systems wliich we have already studied, namely, the vestibular, the cerebellar, and the pyramidal system. 444 PHYSIOLOGY According to Meyer and Barbour, the anterior part of the corpus striatum plays an important part in the regulation of body temperature. In the experiments a metal tube, closed at one end, was introduced through the brain so as to he in or on the corpus striatum. Through this tube water at any temperature could be passed. It was found that coohng the water gave rise to shivering and increased heat production in the body with a rise of body temperature, while warming the water had the reverse effect. He is therefore inclined to regard this part of the nervous system as representing the chief thermo-taxic mechanism of the body. THE LOCALISATION OF SENSORY FUNCTIONS IN THE CORTEX It was pointed out by Ferrier that movements might be obtained on electrical excitation of regions of the cortex cerebri other than those we have described as motor. Thus excitation of the superior temporal convolution on the right side causes the animal to turn its head and eyes to the left and to prick up its ears. In the same way stimulation of the right occipital lobe causes movement of both eyes and head to the left side. These portions of the brain cannot be regarded as having a direct relationship to the motor mechanisms involved in the above movements, since their ablation leads to no defect of movement, but does, in many cases, lead to defect of sensation. Thus excision of the right occipital lobe in the monkey, though leaving the eye movements intact, causes a loss of power to discern objects lying to the left of the middle hne. The obvious explanation therefore of the movements, obtained on excitation of this portion of the cortex, is that they are due to the revival or arousing of sensory impressions, that these portions of the cortex represent the cortical receiving-stations for the impulses from definite sense-organs, and that the movements obtained are simply those which are normally associated with a corresponding sensory excitation. This conclusion is borne out by the fact that it requires a greater strength of stimulus to excite movement on stimulation of the sensory areas than is necessary if the stimulus be applied to the Kolandic area. Moreover Schafer has shown that the latent period which intervenes between the stimulus and the resulting movement is considerably longer when the stimulus is apphed to the sensory centre than when it is applied to the motor centre, suggesting that more neurons are interpolated between the point of stimulus and the discharging motor neuron in the first case than in the latter. Thus in one experiment the latent period between the stimulus and the resulting move- ment of the eyes amounted to 0-2 sec. when the frontal lobes were stimulated and 0-4 sec. when the occipital lobes were stimulated. Finally the anatomi- cal investigation of the course of the fibres in the white matter of the cerebral hemispheres points to a concentration of sensory fibres from different sense- organs towards certain regions of the cortex. The diagrams (Figs. 230 and 231) show those portions of the brain to which the endings of the sensory tracts of the central nervous system are directed. From the purely anatomical standpoint we may designate as ' sensory areas ' of the cortex : FUNCTIONS OF THE CEREBRAL HEMISPHERES 445 (1) An area including both central convolutions, i.e. the ascending frontal and the ascending parietal, and spreading forward into the frontal lobes. (2) An area occupying the hinder portion of the occipital lobe and the greater part of its inner surface. ' Tactile * area Visual area Auditory aroa Fig. 230. Outer .side of right cerebral hemisphere, according to Flechsig. The dotted surface indicates the regions where the majority of the afferent (sensory) fibres end. (3) An area occupying the superior temporal convolution and extending well into the fissure of Sylvius. (4) An area on the inner side of the hemisphere, occupying the hippo- ' Tactile' area Olfactory area Fig. 231. Inner .surface of the same hemisphere. (Fr.ECHSio.) campal gyrus and the margin of the gyrus fornicatus close to the corpus callosum. Let us see how far experimental evidence bears out this localisation. 446 PHYSIOLOGY (a) TACTILE AND MOTOR SENSIBILITY. A lesion limited to the ascending frontal convolution may produce paralysis of definite movements or groups of muscles without any detectable interference with sensation. When, however, in man a widespread injury, involving both the Rolandic area and the adjacent portions of the brain, occurs as the result of some morbid condition, such as blockage of the middle cerebral artery, the resulting hemiplegia is almost always associated with a greater or lesser degree of hemiancBSthesia. We are therefore justified in locating tactile and muscular sensibility somewhere in the region LE.FT RETINA richt RETINA Qf ^]^g ccutral couvolutious, and it is probable that while it may include the motor area its chief representation is to be found in the post-central gyrus, i.e. the ascending parietal convolution. The sensory aura which pre- cedes an attack of Jacksonian epilepsy points to the motor area itseif having some degree of sen- sory functions, and it has been observed that faradisation of the central convolution in man may produce tinghng sensations in the part of the body which is the seat of the muscular contractions induced by stimulation. No pain is, however, felt as a result of the stimulation. The impulses which subserve cutaneous and muscular sensibihty travel up to the brain in the mesial fillet. This tract comes to an end in the ventro-lateral portion of the thalamus and the subthalamic region. The new relays of fibres, which carry on impulses to the cortex, arise in the thalamus and pass through the hinder limb of the internal capsule to be distributed to the central convolutions. Their area of distribution is, however, much wider than the area of origin of the pyra- midal fibres. We may therefore conclude that tactile and muscular sensi- bihty are chiefly subserved by the central convolutions, including the motor area, but are especially dependent on the integrity of the post-central gyrus. Flechsig has shown that fibres from the thalamus, which may probably be regarded as continuations of the fillet system, are also distributed to other portions of the cortex, i.e. the temporal, the frontal, and the occipital lobes. It is therefore not surprising that the hemiansesthesia produced by lesions in the cetral convolutions is rarely or never complete. Fig. 232. Diagram showing the probable relations between the parts of the retinse and the visual area of the cortex. (Schafer.) FUNCTIONS OF THE CEREBRAL HEMISPHERES 447 VISUAL IMPRESSIONS Each optic tract, carrying impulses arising as a result of events occurring in the opposite field of vision, ends in the pulvinar of the optic thalamus, the external geniculate body, and the superior corpora quadrigemina. The last named is apparently not concerned in vision, but represents a centre for the co-ordination of visual impressions with those from other regions of the body in influencing bodily movements. From the pulvinar and external geniculate body arises a sheaf of fibres, which pass through the extreme hinder end of the posterior limb of the internal capsule and diverge in the centrum ovale to be distributed to the occipital lobes, being here known as the optic radiations. The anatomical connection of the occipital lobes with vision is confirmed by evidence derived from experiment. Movements of the eyes result from stimulation of almost any part of this lobe. If the upper surface of the right occipital lobe be stimulated, both eyes move downwards and towards the left. Excitation of the posterior part causes movement of the eyes up and to the left ; while between these two parts there is an intermediate zone, most marked on the mesial surface, stimulation of which evokes a purely lateral deviation of the eyes to the left. It is therefore con- cluded not only that there is representation of visual impressions in the occipital lobes, but that there is a certain amount of localisation within the visual area itself, as is represented in the diagram (Fig. 232). These conclusions are fully borne out by the results of ablation. While extirpation of the whole occipital lobe on one side in animals causes crossed hemianopia, i.e. has the same effect as division of the corresponding optic tract, extirpation of these lobes on both sides causes complete bhndness. It seems that the fovea centralis — the region of distinct vision — is bilaterally represented, so that central vision is usually retained in both eyes after destruction of one occipital lobe (Fig. 233). The area connected with vision seems to be smaller in man than in the ape, and in the ape than in the dog. Thus in man complete blindness has been observed as the result of localised bilateral lesions of the internal sur- faces of the occipital lobes, and we find the same relative limitation of area as we proceed from lower to higher forms in the case of the other sensory areas of the cortex. THE AUDITORY AREA Anatomical study indicates a connection of auditory sensations with the superior temporal lobe. The impulses, started by the arrival of sound waves at the ear, travel by the cochlear nerve to the medulla. From the two audi- tory nuclei a well-marked set of fibres passes across to the opposite side in the corpus trapezoides, then turns up into the tegmentum of the oppsite side to form the tract known as the lateral fillet. The fibres of this tract end partly in the inferior corpora quadrigemina, partly in the internal geniculate body. From the latter, fibres pass into the internal capsule, and thence as ' auditory radiations ' directly to the superior temporal convolution. In the monkey stimulation of the upper two-thirds of this lobe of the 448 PHYSIOLOGY brain causes pricking of the opposite ear, dilatation of the pupils, and rotation of the head and eyes to the opposite side. It was stated by Ferrier that ablation of the superior temporal convolution causes deafness, but Schafer found that, even after extirpation of the superior temporal convolutions of both sides, monkeys showed signs of hearing quite distinctly, and of under- standing the nature of the sounds heard. One must conclude therefore that the function of auditory perception is not entirely confined to the temporal lobe, though its focal point may be located in the superior temporal con- volution, especially in that part which is seated within the fissure of Sylvius. Fig. 233. Perimeter charts from right and left eye, showing the limitation of the field of vision (right hemianopia) produced by a lesion of the left occipital cortex. (Bechtekew.) This conclusion is strengthened by the results of clinical evidence in man, in whom cerebral lesions, which have produced disturbances of auditory per- ception, are found almost invariably to be closely associated with the superior temporal convolution. SMELL AND TASTE The course of the fibres from the olfactory lobe may be used to throw light upon the localisation of olfactory sensation in the cerebral cortex. There is a great divergence between different animals in the degree to which the olfactory sense is developed, and with this divergence we find corre- sponding variations in the development of certain portions of the brain. In those species with highly developed olfactory sense the following parts of the brain show special growth : (1) The olfactory lobe, including the olfactory bulb, and the olfactory tract. (2) The posterior part and the inferior surface of the frontal lobe. (3) The hippocampal gyrus and the dentate convolution. (4) A convolution termed the gyrus supracallosus and forming that part of the gyrus fornicatus closely encircling the corpus callosum. . (5) The anterior commissure. FUNCTIONS OF THE CEREBRAL HEMISPHERES 449 The olfactory lobe is connected almost exclusively with the cerebral hemispheres of the same side. Ferrier found that electrical excitation of the hippocampal region causes contortion of the lip and nostril on the same side, i.e. a reaction such as that actually induced in these animals by applica- tion of an irritative or pungent odour direct to the nostril. Ablation ex- periments have not yielded very definite evidence on the question of localisa- tion of the olfactory sense. So widespread are the connections of the olfac- tory tract throughout the brain that it would be extremely difl&cult, if not impossible, to extirpate all those parts which receive fibres from this tract. It is usual to regard the sense of taste as associated with that of smell, but here again experiment and clinical evidence have yielded very little that is definite. GENERAL CHARACTERISTICS OF CORTICAL MOTOR FUNCTIONS The motor phenomena, which may be observed as the result of artificial excitation of the motor and sensory areas in the cortex, constitute a very small fraction of the activities which must be associated with the cerebral hemispheres. An animal with its cerebral hemispheres intact differs markedly from a decerebrate animal in the variety of combined movements which it may exhibit, either spontaneously or in response to external stimuh. When, however, we excite the motor areas directly, we obtain movements which are practically identical with those which we may elicit from a bulbo- spinal animal by appropriate peripheral stimulation. The movements thus excited from the skin may be looked upon as variations from the tonic postural activity of the musculature of the body. We have seen that from the end- organs subserving deep and muscular sensibihty (the proprioceptive system), as well as from the labyrinth, impulses are continually arising which travel up to the spinal cord, bulb, cerebellum, and mid-brain, and excite a tonic activity of these centres. The normal attitude of the animal depends on the tonus thereby produced in certain muscles. Muscular tone is indeed a quality specially found in certain groups of muscles. If the cerebral hemi- spheres be removed, as by a section through the crura cerebri or in front of the mid-brain, this postural tonus is increased and the animal enters into the condition of ' decerebrate rigidity.' Destruction of one labyrinth diminishes the tone on the same side of the body ; section of all the afferent nerves from a hmb abolishes the tone in that limb, so that its posture thereafter depends entirely on gravity. The movements which are excited in such animals by cutaneous stimula- tion involve as a necessary factor inhibition of the postural tone as well as co-operative inhibition of the antagonistic muscles. In the same way excitation of the motor area of the cortex has as its most essential feature an inhibitory action on the postural tonus in addition to its excitatory action on the muscles concerned in the movement. A certain antagonism is evident between the total action of the cerebral hemispheres and that of the proprioceptive part of the central nervous system. Whereas in the decere- brate animal there is increased tonus in the masseters, in the neck muscles, 15 450 PHYSIOLOGY the muscles of the trunk, and the extensor muscles of the hnibs, stimulation of the cortex produces opening of the mouth, flexion of the fore hmb or of the hind hmb, more easily than any other movements. That an essential part of this action is inhibitory is shown by the effects of exciting the motor area of the cortex after exhibition of strychnine or during the local action of tetanus toxin. Whereas in the normal animal closure of the jaw and exten- sion of the fore Hmb are only obtain- able from one or two points on the surface of the brain, after the injec- tion has taken place, every part of the jaw area gives closing of the jaw, every part of the arm area gives ex- tension of the hmb {cp. Fig. 173), Since the predominant influence of the motor cortex is therefore inhibitory of the stronger muscles of the body, as well as of the tonus, which is con- tinually and reflexly maintained, it is not surprising that excision of both hemispheres should give rise to de- cerebrate rigidity, or that destruction or division of the chief direct tracts from the cortex to the motor spinal mechanisms, viz. the pyramidal tracts, should determine increased tonus and rigidity of the limbs — the so-called ' spastic ' condition observed in cere- bral paralyses. Two separable systems of motor innervation appear thus to control two sets of musculature. One system exhi- bits the transient phases of heightened reaction which constitute reflex move- ments; the other maintains that steady tonic response which suppHes the muscular tension necessary to attitude. Hughlings Jackson long ago called attention to this contrast between the two systems. He pointed out that while the cerebrum innervates the muscles in the order of their action from the most voluntary movements (the limbs) to the most automatic (trunk), the cerebellum, or, as we should say now, the whole proprioceptive system, innervates them in the opposite order. The cerebellum therefore he regarded as the centre for continuous move- ments and the cerebrum for changing movements. The increased tone of the paralysed muscles, observable after hemiplegia, he ascribed to unbalanced cerebellar influence. While there is no doubt that the cerebellum must play, and does play, a considerable part in the production of decerebrate rigidity Fig. 234. Diagram (from Mott after Mon- AKOW) to show the interaction of the different levels in the central nervous system in the production of co-ordinated ' volitional ' movements. s, sensory neuron ; b, bulb ; th, thala- mus ; MA, motor area ; p, pyramidal fibre ; c, cerebello-pontine nuclei ; vs, vesti- bular neuron (Deiters' nucleus) FUNCTIONS OF THE CEREBRAL HEMISPHERES 451 and of the spastic condition of hemiplegia, it is not the only element involved ; nor is it essential, since decerebrate rigidity may continue after extirpation of the cerebellum and an exaggerated knee-jerk may result from section of the spinal cord in the lower cervical region. HIGHER ASSOCIATIVE FUNCTIONS OF THE CORTEX The simple and uncomplicated nature of the movements elicited on cortical stimulation shows that we cannot regard these motor centres as responsible for the whole, or even the greater part, of the motor functions of the cortex. They are in fact simply the starting-point for the motor impulses which run down the long pyramidal tracts, but which result from the activities of the cerebral hemispheres as a whole. In the lower mammals they do not even represent the only starting-point, as is shown by the almost perfect recovery of vohtional motor power in a dog deprived of its motor cortex. The distinguishing feature of the response of an animal possessing cerebral hemispheres is that it is not determined solely and exclusively by the nature and position of the peripheral stimulation, but involves elements connected vdth. the past experiences of the animal, and including therefore the results of previous stimulation of many of the sense-organs, either directly, or indirectly as a result of reflex movements. The animal's reactivity is determined by the past history of the animal, and this modifying influence on the brain must involve parts connected with all its sense-organs. In any conscious motor act we may say therefore that the brain acts as a whole, or nearly as a whole. In endeavouring to arrive at some idea of the neural processes concerned in volitional movements, i.e. movements of the intact animal, we are deahng with events which in ourselves come ^^^thin the sphere of consciousness, so that some assistance is derived by appealing to our own mental experiences. Especially is this necessary in the case of the sensations. It might be imagined that a simple sensation would ensue as the result of local stimula- tion, say of the visual centre on one side. Our knowledge of the properties of the systems of neurons composing the central nervous system would teach us that no excitatory process could remain confined to one portion of the brain, but nnist diverge in many directions. It is true that excision of the occipital lobes on one side causes bUndness to objects in the opposite half of the field of vision. This is, however, merely a result of localisation of the end of visual fibres, and the same effect can be brought about by division of the right optic tract, or damage to the right half of both retinte. On the other hand, an appeal to our own experience shows that no sensation can be regarded as simple, i.e. as following merely stimulation of visual fibres or visual centres. Thus the sensation of a luminous point has comiected with it not only luminositv but also colour and intensity. Moreover the apparent position of the luminous point comes into conscious- ness at the same time as the consciousness of the luminosity itself, and this location of the stinnilation involves muscular impressions from the eyeballs and an association between certain points on the retina and certain corre- 452 PHYSIOLOGY sponding muscular movements of the eye muscles, of the head and neck, and even of the body and arm — movements which would be necessary to bring the image of the spot on to the fovea centralis and to approach the whole body to the site of the stimulating object. As the visual sensation becomes more complex the associated sensations and experiences which it evokes become more numerous. Thus the image of a chair falhng on the retina excites a long train of nervous processes. At once we become aware not only of a visual impulse but of an object which possesses colour, extension, or size in three dimensions, soHdity, hardness, distance or position in space, &c. These qualities are founded on past ex- periences— visual, muscular, and tactile. Moreover we are at once aware of the uses of the chair, and of its name both spoken and written, a mental activity connoting revival of higher visual and auditory sensations. The higher in the scale of intelhgence, the greater is the development of the cerebral hemispheres and the m-ore extensive are the associations arising in connection with any single sense impression. Besides the portions of the brain which send out the motor paths and which receive the endings of the sensory paths, there may be whole regions taken up by the interconnecting neurons which subserve the association of the activities of all parts of the cerebral hemispheres, and the higher the animal is in the scale of intelligence the larger must be the relative amount of brain substance set apart for these functions of association. This is very evident if we compare the brain of three animals, such as the dog, the ape, and man. Although as we ascend to man there is an absolute increase in the amount of brain substance involved — say in the motor areas or in the sensory areas — the increase is very small as compared with that in those portions of the brain which give no response on stimulation, and in man these ' silent ' parts of the brain form the greater part of the cerebral cortex. Although every phase of cerebral activity, every conscious event, involves co-operation of a large number of distant portions of the brain substance, in most of them there will be some seat of sense impressions which "will be predominant, and a train of ideas may be specially visual, or auditory, or tactile. It is therefore not surprising that in the immediate neighbourhood of the cortical areas which receive the endings of the sensory tracts associa- tion areas are developed which may be labelled according to the sense-organ with which they are most nearly in relation. Thus we may speak of the visual-sensory and the visual association, or psychical area, the auditory- sensory and the auditory-psychical, and so on. The hmits of these areas are indicated in Fig. 224, p. 432. Conditioned reflexes. Until recently, our study of the processes of association and therewith all the higher functions of the cerebral hemispheres was chiefly carried out in man, and in most cases by the introspective method. Even when carried out on other men, it was chiefly by using speech as an index to the introspective experi- ences of those who were being investigated. During the last few years a method has been introduced by Pawlow, for investigating the higher cerebral functions by an objective method which is capable of very wide application. When an animal which is hungry is shown food, we say that ' its mouth waters,' i.e. there is a secretion of FUNCTIONS OF THE CEREBRAL HEMISPHERES 453 saliva; and if the animal be provided with a salivary fistula the extent of the emotion of appetite may be gauged in cc. of saliva flowing from the fistula. It is found in such an animal that a flow of saliva may be excited, not only by the sight or adminis- tration of food, but also by any other event which has become associated as the result of experience with the taking of food. Thus we may use this method in order to deter- mine the sensitiveness of the animal's perception of pure tones. Thus if we wish to know whether the animal can recognise the difference between middle C and middle Qjt, as produced by tuning-forks, we can for some days or weeks allow him to hear both these sounds frequently but always follow up one of them, say C, by giving him a piece of meat. After a time it is found that not only can he distinguish between the two sounds, but that he has a memory of the absolute pitch, so that whenever the note middle C is sounded or any note differing from it by not more than 8 d.v. per second, there is a flow of saliva from the fistula, whereas the note C^ is heard without producing any response. Such an acquired reaction is designated by Pawlow, a ' conditioned reflex ' and the method has been applied by him to study the association between the most widely different impressions and the condition which we can regard as appetite and which is associated psychically with the idea of food. THE FUNCTION OF SPEECH The acts of a conscious individual, i.e. one possessing cerebral hemispheres, are determined by his experience. The Avider the range of past sense impressions which can be called up and taken into the chain of processes involved in any reaction — the more, that is to say, the individual weighs his acts in the light of past experience — the more fitted will these acts be to his maintenance amid the ever-changing stresses of the environment. In this guiding of behaviour by experience man, as well as the higher mammals, may profit also from accumulated racial experience. The increased complexity of the neural processes concerned in every reaction of the body, and the excessive lost time brought about by the intercalation of one neuron after another in the chain of the excitatory process, would finally counteract the advantages derived from the growth in complexity of the brain, were it not that, as a result of education or training, sliort cuts are laid down, by means of which reactions adapted to the maintenance of the individual can be carried out immediately, without thought and without correlated calling up of numberless sense impressions. Education in fact consists in laying down these ' short cuts ' which, as habits, are the basis of the behaviour of the animal. The more complex the central mechanism and the wider the range of environmental change to which adaptation is necessary, the longer must be the time involved in this process of road-making within the cerebral hemi- spheres. The behaviour of man is therefore a product of many years' training, durijvg which time he is in a state of subjection and unfit, from the absence of habit, to maintain himself as a unit in the human connnunity. The neural short cuts of habit are, however, only of advantage to the individual in dealing with those events which are of everyday occurrence. Every novel circumstance must involve a revival of past sense impressions and a calling up of ai?tivities of the most diverse ]wrtions of the brain in order to arrive at the safest or most advantageous mode of action adapted to the circumstances. Here again the complexity of the process would, by the very delay involved, put a stop to a further 454 PHYSIOLOGY rise in intellectual, i.e. associative, capacities, were it not for the invention of Speech. In speech we have a symbohsm which acts as an economy of thought or of cerebral activities. An object, such as a table, with its associated properties of colour, consistence, spatial extension, and resistance, with the connoted acts associated with its use, can now be evoked as a word, involving com- paratively simple auditory and motor processes, which itself may be em- ployed as a unit of thought and brought into connection with other words, each of which in the same way is the symbol for a whole series of sensory and motor processes. The training of the cultivated man consists in a constant extension of the range of this symbolism, and the acquisition of words including wider and wider groups of neural processes, so that finally we arrive at those short verbal collections which, as the so-called natural laws, sum- marise the experience not only of the individual but such as is common to the whole race of mankind. All science may in fact be regarded as an extension of the process of representation of neural experience in symbolic shorthand, which in the child begins with the utterance of such a simple word as ' mamma,' and from which speech has arisen. A study of the nervous mechanisms involved in speech is therefore of interest in its relations to the development of the intelligence, and helps us to reahse more completely the conditions which determine the activity and functioning of the cerebral hemispheres. Much light is thrown upon this mechanism by the study of disorders in man grouped together under the name Aphasia. It has been usual to divide the disorders of speech known as aphasia into various groups, as follows : (1) Motor aphasia, or aphasia of Broca. In this condition, which was de- scribed fully by Broca and referred by him to a lesion of the third left frontal convolution, the patient is miable to speak, although he understands what is said to him and has been stated to suffer from no impairment of his intelUgence. (2) Sensory aphasia, or aphasia of Wernicke. This condition was con- nected by Wernicke with the existence of lesions in a fairly wide area, known as the area of Wernicke, which involves the supramarginal and angular gyri and the hinder portions of the first and second temporo-sphenoidal convo- lutions. In these cases there may be hmited power of speech, but there is serious impairment of the intelligence and especially of the power of appreciation of spoken words, so that the patient does not understand what is said to him. This condition may or may not be attended with alexia, loss of power to read. Any impairment of the motor processes of speech which is present is due rather to the inability of the patient to appreciate what he himself is saying, so that there is here a species of sensory paralysis in the higher sphere of neural processes. (3) Anarthria. This is a condition in which there is marked impairment of the motor powers of expression, although intelligence and appreciation of speech, both spoken and written, may be unaltered. This condition is generally associated with lesion of the white matter of the external capsule as it passes round the lenticular nucleus. FUNCTIONS OF THE CEREBRAL HEMISPHERES 455 There are, however, considerable difficulties in the acceptation of this traditional classification. Microscopic examination of Broca's convolution shows a type of cortex entirely different from that part, viz. the psycho- motor area of the ascending frontal convolution, which is concerned \vith the higher cerebral processes resulting in movement. Its structure is in fact identical with that described by Campbell as the ' intermediate precentral area ' and regarded as characteristic of the association areas. Moreover it is difficult to comprehend how a function such as speech, with its enormously complex mechanism, could be hmited to so small a portion of the brain as Broca's convolution. The neural basis of language must in fact be co- extensive with the sensory centres (the projection spheres) and ^\^th the whole region of lower association. We might indeed speak of auditory and visual word-centres as located in the visuo-psychical and auditory psychical centres. There is probably, however, no word, still less a collection of words, expressing an idea, which does not involve the activity of practically all parts of the cerebral cortex. As Bolton * points out, " a word, such as ' mouse,' at once sets in effect processes of association which pass to every projection sphere with the solitary exception of the gustatory, and even this may be aroused in a person who has eaten a fried mouse in the hope of thereby recovering from an attack of whooping-cough." A careful examination of an extensive series of cases by Marie has shown, in fact, that Broca's aphasia does not exist as a result of lesions of Broca's convolution. This part of the brain may be destroyed without any disorder of speech. The cases described by Broca of motor aphasia are really cases of sensory aphasia from lesion of Wernicke's area, combined with anarthria due to subcortical injury of the fibres of the internal capsule. The statement that there is no loss of intelhgence in these cases of so-called motor aphasia does not bear investigation. Although as patients they may comport themselves reasonably, as soon as they have to perform any duties which have been learnt by them in connection with their ordinary avocations , they show their deficiency. They are incapable of transacting ordinary busi- ness, at any rate to the extent to which they were before the lesion. The amount of impairment of intelligence will vary in different cases according to the extent of the lesion. Thus softening generally affecting the occipital lobe may, with hemianopia, cause ' word-blindness," or alexia, a loss of power of appreciating the meaning of pronounced or written words. In most individuals, and certainly in the uneducated, this power may be cut out altogether without iiitei'fering considerably with the mental powers. On the other hand, from babyhood upwards we have learnt the meaning of words and their grouping by auditory impressions. If the whole of the auditory associations be destroyed by an extensive lesion in the first and second tem- poral convolutions the resulting loss of word appreciation, sensory aphasia, will be attended with great diminution of mental powers. It must be remembered tliat the area of Wernicke is not a sensory centre, but a centre of association between the various sense-impressions, especially those of * 111 his admirable article iu Hill's " Further Advances in Physiology." 456 PHYSIOLOGY hearing and sight. It may therefore be spoken of as an intellectual centre. Pure motor aphasia of course exists, but is always anarthria and is due to a lesion in the lenticular zone, i.e. in the lenticular nucleus and its neighbour- hood, in the anterior part and the genu of the internal capsule, and possibly in the external capsule. Amenta Dementa Fig. 235. Types of lesions giving rise to deficient intellectual power. In amentia, the deficiency is due to failure of development ; in dementia, to atrophy of the cells (especially small pyramidal) previously present in the cortex. (Mott.) It is important to make a distinction between loss of sanity and loss of intellectual powers. The essential factor of sensory aphasia is the exist- ence of intellectual impairment, though in his behaviour the patient may appear perfectly normal. On the other hand, in insanity there may be perfect retention of the intellectual processes, which depend on the proper working of the lower association centres. The personality of the individual, and therefore finally his behaviour, involves a further association on a higher plane of these intellectual processes and therefore control in accordance with the relation, past, present, or future, of the individual to his environment. FUNCTIONS OF THE CEREBRAL HEMISPHERES 457 The prefrontal region is in all probability the seat of this highest plane of association. Insanity always involves alteration of personality and depends on failure of development or on disintegration processes (subevolution or dissolution of this region) (Fig. 235). In monkeys and cats Franz has found that destruction of the frontal lobes causes a loss of recently formed habits. He concludes from his experiments that the fro/atal lobes are the means by which we are able to learn and to form habits, i e. to regulate our behaviour in accordance with the needs of our position in society. THE TIME RELATIONS OF CENTRAL NEURAL REACTIONS In the spinal animal a stimulus of any particular quality and localisation always evokes an appropriate reaction. A certain period of time necessarily elapses between the moment at which the stimulus is applied and the moment at which the resulting reaction takes place. This interval is spoken of as the simple reaction time, and in the spinal animal is entirely independent of consciousness. Many reactions, even in the intact animal, are also, as we may say, involuntary and are not modified perceptibly by our consciousness of their occurrence : such reflexes as the withdrawal of the hand when it comes in contact with a hot surface, the shutting of the eyehd when the con- junctiva is touched, the drawing up of the leg when the sole of the foot is tickled. Not only are these carried out in the absence of voluntary impulses, but in many cases it is almost, if not quite, impossible to check the reaction by any effort of the will. When the leg is drawn up in response to a painful or nocuous stimulus applied to the foot, a certain amount of time is involved in each of the following links in the chain of processes which determine the reaction : (1) The conversion in the peripheral sense-organ of the mechanical stimulus into a nerve process. (2) The passage of a nerve impulse up the nerve from the end organ to the spinal cord. (3) The passage of the impulse across two or more synapses in the gi'ey matter of the cord. (4) The passage of the impulse down the motor nerve fibres from the spinal cord to the muscles. (5) The processes occurring in the end-organs of the muscle. (6) The latent period in the muscle fibre itself. With a weak stimulus No. 1 is impossible to measure. With a strong stimulus it may be so short as to be practically negligible. (2), (4), (5), and (6) represent (juantities for the measurement of whicli wo have all the necessary data. In any given reflex therefore we may add these periods together and subtract them from the total reaction time ; we thus get a ' reduced re- action time,' which represents the time involved in the passage of tlie impulse through the central nervous system, and in the conversion of an afferent impulse into an aggregate of co-ordinated motor impulses. It is found that the reduced reaction time accounts for the greater part of the total 15* 458 PHYSIOLOGY reaction time. Since we have no reason to assume that the rate of passage of an impulse through the intra-spinal course of a nerve fibre differs appreciably from the rate at ^Yhich it is conducted by the same nerve fibre outside the cord, the extra delay which occurs in the passage of the impulse through the cord must take place either in the nerve-cells themselves, or in the synapses, through which the impulse passes from one neuron to the next in the chain of reflex elements. The rate of passage of an impulse through the nerve-cell can only be determined in one part of the body, viz. in the posterior spinal root gangUa, Fig. 236. Arrangement of apparatus for determination of reaction time. (Alcock and Ellison.) K, coil ; E, exciting electrodes ; f, tuning-fork ; a, b, keys ; s, t, electro- magnetic signals ; d, drum. since only in these is it possible to detect the moment of passage of an im- pulse across a given section of a nerve fibre on both sides of the ganglion- cell in which the nerve fibres arise. Experiments on this point have been made by Steinach and by Moore. In each case the time occupied in the passage of the impulse through the ganghon was not appreciably longer than if the impulse had passed through a corresponding stretch of uninterrupted nerve fibre. We are therefore justified in concluding that the relatively great delay in the passage of an impulse through the central nervous system has its seat in the synapses across which the impulse has to pass. This con- clusion is in accordance with our experience on the latent period of muscle, the greater part of which is due to changes occurring in the nerve-endings, i.e. in the synapses between motor nerve and muscle. The greater the number of synapses involved in any given reaction, i.e. the greater the com- plexity of the reaction, the longer will be the period which elapses between the moment of application of the stimulus and the moment at which the FUNCTIONS OF THE CEREBRAL HEMISPHERES i59 response takes place. Especially is this the case when the complex mesh- work of neurons of the cerebral hemispheres is involved, or when the occur- rence of the reaction is associated with the conscious processes of sensation and vohtion. In the latter case the determination of the reaction time has the added interest that it gives information as to the time-relations of the psychical processes which are the representation in consciousness of the physiological changes occurring in the neurons of the central nervous system. Many methods are employed for the measuring of the reaction time of conscious processes. In most methods the ai^plication of the stimulus is arranged so as to close the circuit of a cm-rent which flows tluough an electro-magnet activating a lever which writes on a rapidly moving blackened surface. The reaction of the individual who is the subject of experiment is arranged so that the resulting movement activates a key by which the same current is opened. We thus obtain a tracing on the blackened surface showing the moment of application of the stimulus and the moment at which the reaction takes j)lace. Thus, if the reaction time for an auchtory stimulus is to be determined, the electric current is arranged so as to pass through : (1) A spring contact key which can be pressed so as to make a noise. (2) An electric signal -na-iting on a rapidly moving surface. (3) A second key, which the subject will release as soon as he hears the noise of the first key and so break the ciu-rent. The recording surface may be a drum, a pendulum myograph, or a spring myograph, such as the ' shooter ' of du Bois-Reymond. If the sensory impression is to be from the skin the current may be made to pass through the primary coil of an inductorium and wires be taken from the second coil to some part of the sm-face of the skin. In this case the signal may be started by opening the circuit, and the subject of the experi- ment will respond by closing the circuit by means of a spring key directly he feels the shock caased by the break of the primary circuit. If the reaction period is to be determined for sight a white piece of paper may be placed on an electro-magnet in the primary circuit and the person will respond directly he sees this move. Many other instruments have been devised for the same purpose. The average reaction times obtained with the different senses are as follows : Electrical Sight Hearing stimulation of skin 0-186 to 0-222 sec. 0115 to 0-182 sec. 0-117 to 0-201 sec. The two figures given for each case are the extremes obtained in difterent series of observations. The times vary according to the condition of the person that is the subject of the experiment. They are lengthened by fatigue ; they are shortened up to a certain point by continued practice. Within limits also they are shortened by increase of the strength of the stimulus. DILEMMA. When the subject has to make'a deliberate choice between the parts of the body stinuilated the reaction time is considerably longer. To show this, the wires from the secondary coil are connected by a switch to two pairs of electrodes which are applied, one to the right and one to the left half of the body. It is agreed beforehand that the subject shall react only to stimulation, say, of the right side. The switch is removed from the observation of the subject and the stimulus is applied irregularly to one side or to the other. It is found that the additional neural processes 460 PHYSIOLOGY involved in determining whether the stimulus is on the right side, and there- fore should be followed up as agreed, adds considerably to the length of the reaction time (on an average -066 sec). It is possible to complicate the dilemma to almost any extent. Thus the experiment may be so arranged that either a red or a white disc appears and the subject has to react with the right hand to the red disc and with the left hand to the white disc. In such an experiment the reaction time was found to be 0-154 sec. longer than the simple reaction time. A still more complex process would be involved in the experiment in which a word was spoken, and the subject had to speak some other word which had some association with the word which formed the stimulus, e.g. horse — mammal ; paper — pen, &c. In such an experiment the reaction time was found to be as long as 0'7 to 0-8 sec. We see that the recording of the time of occurrence of any physical event can only occur after a certain lost time, which represents the observer's reaction time for the stimulus in question. This only applies, however, to movements carried out in response to single stimuli, or to stimuli repeated at irregular intervals. When the stimuli are rhythmic the lost time only applies to the first one or two of the stimuli. The observer or subject is conscious of the interval elapsing between the physical event and his reaction, and anticipates the later stimuh so that his reaction becomes synchronous with the stimulus. This synchronism of stimulus and reaction characterises all ryhthmic movements, such as dancing or the playing of an orchestra in time with the beat of the conductor's baton. SECTION XVIII THE NUTRITIVE AND VASCULAR ARRANGEMENT OF THE CENTRAL NERVOUS SYSTEM The brain and spinal cord are enclosed within three membranes or meninges, named from without inwards, the dura mater, the arachnoid membrane, and the pia mater. The dura mater consists of a strong fibrous membrane, smooth and hned with endothelial cells on its inner surface. In the head its outer surface is closely attached to the bones forming the cranium, of which it represents the periosteum. Strong fibrous partitions are sent from the dura mater into the cavity of the cranium to support the chief parts of the brain. One of these, the falx cerebri, supports the two cerebral hemi- spheres, a second, the tentorium cerebelli, forms a horizontal division between the cerebral hemispheres and the cerebrum, and a smaller one. the falx cerebelli, passes a short distance inwards between the cerebellar hemispheres. In the spinal canal the bones have their own periosteum and the dura mater, which is closely attached round the margins of the magnus, forms a loose sheath round the spinal cord, being slung up in the vertebral canal by the tubular prolongations which it sends along each nerve root to form the outer sheath of the nerve. The dura mater in the cranium may be separated with greater or less difficulty into two layers, and between these two layers are found the venous sinuses, which receive the whole of the blood returned from the brain. These venous sinuses are angular clefts, the chief of which lie along the attached margin of the falx cerebri and the tentorium cerebeUi. Most of the blood leaves the skull by the internal jugular veins. In the spinal cord the place of these venous sinuses is taken by a plexus of thin- walled veins, imbedded in fat, lying on the outside of the dura. Under the dura mater we find the subdural space, which is rather potential than actual. It can be regarded as a large lymph space and any contents are drained oft' into the lymph spaces of the nerve roots and adjoining tissues. The arachnoid is a delicate transparent membrane which covers the whole of the surface of the brain and spinal cord. Superficially it is covered with endothelial cells which bound the subdural space. On its deep surface it is connected with the pia mater by fine fibres. It bridges over the inequalities in the surface of the brain so that in various localities a space is left which is filled with cerebro-spinal fluid and is known as the subarachnoid space. In certain situations it sends prolongations into the fissures of the brain. Thus a marked expansion passes by the transverse fissure between the 461 462 PHYSIOLOGY cerebral hemisplieres and the third ventricle, sending prolongations into the lateral ventricles. This layer of connective tissue is covered on one surface by the ependyma of the ventricles, on the other surface by the ependyma forming the roof of the third ventricle. It carries a rich plexus of blood vessels known as the choroid plexus, and the ependyma covering the vascular fringes which dip into the cerebral ventricles consist of clear columnar or cubical cells, often spoken of as the epitheHum of the choroid plexus. Similar vascular fringes are found in the roof of the fourth ventricle. The pia mater is a layer of connective tissue which serves to carry the blood-supply to the whole surface of the brain. It is closely apphed to the surface and follows all the irregularities of the latter, dipping down into the various fissures and crevices on the brain. In the spinal canal the pia mater sends out a series of processes on each side of the spinal cord, the ligamentum denticulaium, the outer extremities of which are attached to the dura mater and serve to shng the spinal cord in its dural sheath. The brain is richly supplied w^ith blood. Its chief supply is derived from the two carotids and the two vertebral arteries. The vertebrals unite on the lower surface of the bulb to form the basilar artery, which divides again at the anterior extremity of the pons varolii into two branches which unite \x\th. the two carotid arteries to form the circle of Willis, so that the pressure in this arterial circle can be maintained indifferently by any three out of the four arteries by which it is supplied. From these vessels three main arteries, the anterior, middle and posterior cerebral, pass up to supply the correspond- ing regions of the outer surface of the brain, while the inner parts of the brain, e.g. the corpus striatum, optic thalamus, &c., are supplied from artsries arising from the circle of Wilhs and passing straight into the sub- stance of the brain. The connection between the vascular supply of the different parts of the brain is shght and effected only by the capillaries ; hence obstruction of any one vessel, such as the middle cerebral, perma- nently cuts oft' the blood-supply to the greater part of the area supphed by it and the result is death and softening of the brain substance. The arteries supplying the surface of the brain divide up into arterioles and capillaries within the pia mater, and the capillaries run into the brain substance sur- rounded by a so-called lymphatic sheath, which apparently communicates with the subarachnoid space. In certain cases of disease these perivascular sheaths may be found to contain leucocytes often filled with products of disintegration of the nervous tissues. THE CEREBRO-SPINAL FLUID The subarachnoid space contains a thin transparent colourless flviid, known as the cerebro-spinal fluid. In composition this fluid resembles blood plasma minus its protein constituents. It contains a mere trace of coagulable proteins but it has the same molecular concentration as the blood plasma and its salts are identical with those of the blood plasma. It also contains other diffusible constituents of blood plasma, e.g. small traces of sugar and of urea. It may be collected by introducing a cannula through the THE CENTRAL NERVOUS SYSTEM 463 atlanto- occipital membrane into the ample subarachnoid space lying over the fourth ventricle. Another method is to introduce a glass cannula through a slit in the sheath of a nerve root up into the subarachnoid cavity of the spinal canal. In man it may be obtained in small quantities for examination by introducing a hollow needle directly into the spinal canal in the lumbar region between the laminae of the vertebrae. On introducing a cannula into the subarachnoid space, the fluid may spurt out, showing that it is under a certain pressure (about 100 nun. H2O). After the first rush the fluid begins to drop away, at first rapidly, but becoming slower with lapse of time. If the fluid be allowed to drain oiS for some hours, signs of interference with the functions of the central nervous system are evinced. The cerebro-spinal fluid appears to be formed chiefly in the neighbourhood of the choroid plexus. Although its composition would suggest that it was merely a transudation from the blood, the amount formed does not seem to run parallel with the pressure in the capillaries of the brain. Moreover, it has been shown by Dixon and Halhburton that a considerable increase in the flow of cerebro- spinal fluid may be brought about by injecting an extract of the choroid plexus itself. It has therefore been concluded that this fluid is really a secretion by the modified ependyma cells covering the fringes of the choroid plexus. Although the method of formation of the cerebro-spinal fluid is still not clear, there is no question that its removal from the subarachnoid space is brought about by simple physical factors. The subarachnoid space com- municates with the ventricles by means of openings in the roof of the fourth ventricle. The pressure of the fluid is ordinarily about equal to the pressm-e in the venous sinuses of the cranium. If salt solution be injected into the subarachnoid space, it escapes with extreme ease and it is found that its channel of escape is into the veins and especially into the venous sinuses of the dura mater. Its removal by these sinuses is facilitated by the existence of pecuUar structures, known as the Pacchionian bodies. Each of these bodies is a bulbous protrusion of the arachnoid membrane into a blood sinus. It remains connected with the arachnoid by a narrow pedicle, through which a continuation of the sub-arachnoid space is prolonged into the interior of the sinus. It is therefore a Httle sac of arachnoid membrane separated from the blood stream only by an invagination of the endothelium lining the sinus. Filtration of the cerebro-spinal fluid will occur into the venous sinuses whenever the pressure of the fluid rises above that of the blood in the sinus. The fluid can also escape, but with greater difticulty, along the sheaths of the spinal nerve roots, by which it will pass into the lymphatic space outside the spinal canal. THE NUTRITION OF THE BRAIN. The grey matter of t4u^ brain is very richly supplied with blood vessels. Any interference with the blood flow through the brain rapidly checks the functions of the central nervous system in consequence of deprivation of oxygen. Although so susceptible to slight deprivation of oxygen it does not seem that the brain tissues have a very rapid gaseous metabohsm ; that is, they need oxygen supply at high tension but do 464 PHYSIOLOGY not deprive the blood of any very large amount of the oxygen which it con- tains. Nor does it seem probable that the brain requires a large supply of food material. It must be remembered that in all parts of the brain a peri-vascular lymphatic intervenes between the capillary and the brain tissue. Since these ' lymphatics ' communicate with the subarachnoid space they must contain a fluid which differs httle if at all from the compo- sition of the cerebro-spinal fluid obtained from the subarachnoid space. The nutrient fluid of the brain is therefore practically salt solution with a trace of sugar and possible minute traces of amino acids. Our study of the events which accompany the propagation of a nervous impulse down a nerve fibre has prepared us for the conclusion that very httle energy is involved in ordinary nerve activity. It is true that extreme fatigue causes changes in the initial granules of the nerve cells and is therefore asso- ciated with the using up of some material constituent. But even though material changes in the nerve-cells and in the synapses may be' larger in amount than those in nerve fibres, they are probably not to be compared in extent with those taking place in a contracting muscle or in an active liver cell. THE CEREBRAL CIRCULATION In all higher animals the brain is enclosed in a rigid case formed by the bony cranium. In the child, before the cranial vault is fully ossified, part of this vault consists of membrane, known as the anterior fontanelle. It is easy to see that the fontanelle pulsates with each heart-beat as well as with rise of venous pressure, such as that produced during strong expiratory efforts. When ossification is complete, such alterations in the volume of the cranial contents are impossible. And yet the pressure in the arteries within the cranium must be still pulsatile, the rise of pressure at each heart-beat must make the arteries expand, but room for this expansion has to be found by contraction of some other part of the cranial contents. We find that each arterial beat is associated with a corresponding expulsion of some of the contents of the veins and a contraction of these vessels. If, for instance, a cannula be introduced through the occipital bone into the torcular Herophih, the venous blood is seen to pulsate and to be pressed out with each beat of the heart. If there is a rise of arterial pressure, although the arteries may expand somewhat at the expense of the veins, there can be no dilatation of the whole organ. The only effect of the rise of pressure will be to cause an increased pressure fall in the cranial vascular system, and therefore augmented velocity of flow through the system. A prolonged rise of pressure may cause a certain amount of dilatation of the vessels, but only at the expense of the cerebro-spinal fluid. Since this is only small in amount, any expansion of the brain due to vascular causes must be very Hmited. BRAIN PRESSURE. If by means of a trephine an opening be made into the cranial vault, the brain bulges into the opening. By screwing a tube covered with a membrane into the trephine opening, we can find the pressure necessary to force the brain back to its previous position. This is known as the brain pressure and is approximately equal, as might be expected, to the THE CENTRAL NERVOUS SYSTEM 465 cerebro-spinal pressure and to the pressure in the venous sinuses. It is closely dependent on the latter. Forced expiratory efforts, such as may occur in the convulsions of strychnine poisoning, may raise the pressure from 30 to 50 mm. Hg. In the vertical position in man the pressure may be slightly negative in consequence of the tendency of the venous blood to run downwards towards the heart. REGULATION OF THE BLOOD-SUPPLY TO THE BRAIN No satisfactory evidence has been brought forward of the existence of vaso-motor nerves controlling the calibre of the cerebral blood vessels. Nor indeed are such nerves necessary. The brain, as the master tissue of the body, controls through the medullary centres the circulation through all other parts of the body. It is therefore able to regulate the blood-supply through its arteries by allowing less or more blood to pass through other parts of the body. For the exercise of its normal functions it requires a certain blood-supply, which again will depend simply on the pressure in the carotid arteries and circle of Willis. If this pressure fails, the functions of the brain are affected and loss of consciousness rapidly ensues. This is what occurs when a person who is weak from long illness faints on suddenly getting up from bed. In the normal individual the change in the circulation ^\'ith altera- tion of bodily position, which would be produced by the action of gravity, is at once counteracted through the vaso-motor system. The splanchnic area is contracted or dilated according to the necessities of the case, but the pres- sure in the carotid and the circulation of the brain remains unaltered. Even when the heart in consequence of disease is scarcely able to carry on the circu- lation, the arterial pressure undergoes little or no alteration. Any other tissue of the body, even the heart itself, may suffer, but the brain at all costs must receive its proper supply of blood. SECTION XIX THE VISCERAL OR AUTONOMIC NERVOUS SYSTEM In the medulla oblongata it is easy to differentiate the central grey matter connected with the peripheral nerves into two categories, viz. splanchnic and somatic. Each of these two sets of nerves possesses both afferent and efferent fibres. Gaskell has suggested that the same arrangement would hold for any typical segmental nerve, which would therefore have four roots, viz. two somatic — the motor and sensory roots distributed to the skin and skeletal muscles — and two splanchnic roots, also motor and sensory, and composed of small fibres distributed to the viscera or structures which are visceral in origin {e.g. developed from the branchial arches). In the medulla the somatic efferent fibres, such as the sixth and twelfth nerves, arise from the column of large cells lying in the floor of the fourth ventricle close to the middle hue. The splanchnic fibres, e.g. those of the facial and vago-glosso- pharyngeal nerves, arise from a column of cells — the nucleus ambiguos and facial nucleus, lying more laterally and deeper, below the surface of the ventricle. The motor root of the fifth would also belong to the same system. In the spinal cord the visceral fibres arise in the cells of the lateral horn, i.e. from a situation corresponding to the splanchnic motor nuclei of the pons and medulla. Whereas, however, the splanchnic afferent nerves, such as the glossopharyngeal, and perhaps the sensory nucleus of the fifth, form a well- marked splanchnic system of nuclei in the medulla, in the cord the afferent fibres from the viscera pass in with the other afferent somatic fibres, and their immediate connections in the cord are as yet unknown. The autonomic system of nerves includes the sympathetic system (properly so called) and some of the cranial and sacral nerves. The sym- pathetic system (Fig. 237) is composed of a chain of gangha lying each side of the vertebral column, there being as a rule one ganghon to each spinal nerve-root. In the cervical region these gangha are condensed into two, the superior and inferior cervical gangha, united by the cervical sympa- thetic trunk ; and the upper three or four thoracic ganglia on each side are condensed to form the ' stellate ' ganghon. At the bottom of the chain there is only one coccygeal ganglion for the coccygeal vertebrae. In the abdomen is a second system of gangha, in special connection with the abdominal viscera, lying in front of the aorta and surrounding the origins of the large arteries to the ahmentary canal. These are the semilunar or solar gangha, the superior mesenteric and the inferior mesenteric gangha. 466 THE AUTONOMIC NERVOUS SYSTEM 467 Sup.cerv. g.' Inf.cerv.g • H Stellate g/ ' S^mp. chain <^ Semilunar g.^ Sup.mes.g.^, > Head & Neck , Abdominal Viscera Hypogastric n ^g^^jscn FT(i. 2:?7. Diagrammatic rciirescntation of the distribution of the sympathetic system. 'rh(> l)laci< lines represent the mecluHated pre-ganglionic fibres, .such as those making up the white rami eoininunicantcs, while the imst-ganglionic fibres are printed in red. On the extreme right of tlie figure is indicated the general distribution of the white rami arising from the several nerve-roots, while the double brackets jjoint to the nerve- roots making up the limb plexuses. H, heart ; .s, .stomach ; i. small intestine ; c, colon ; B, bladder. 467 468 PHYSIOLOGY In the organs themselves we find a third system of ganghon-cells, either scattered or collected to form small ganglia. These isolated ganglion cells as a rule have no connection with the fibres of the sympathetic system, but, as we shall see later, he on the course of the impulses descending by other nerves of the autonomic system, e.g. the vagus or the pelvic visceral nerves. The three systems of ganglia have been distinguished as the lateral, collateral, and terminal gangha. Fig. 238. Diagram of spinal segment with its nerve roots, somatic and visceral. (G. D. Thane.) (The visceral roots are represented in red.) The gangha of the sympathetic chain are connected with all the spinal nerves, just after they have given off their posterior divisions, by means of the rami communicantes. These rami communicantes are of two kinds : white rami consisting of small medullated fibres, and grey rami composed ahnost exclusively of non- medullated nerves. It has been shown by Gaskell that the white rami are formed by fibres which have their origin in the spinal cord and perhaps in the posterior root ganglia ; whereas the grey rami represent fibres which, arising in the sympathetic gangha, run back to join the spinal nerves. The visceral outflow represented by the white rami is limited to a distinct region of the cord, viz. from the first thoracic to the third or fourth lumbar nerve-roots ; whereas the grey rami pass from the sympathetic to all the spinal nerve-roots. It is found by experiment that stimulation of a hmited number of white rami produces all the effects that can be evoked by stimulation of the grey rami, showing that the impulses leaving the cord pass upwards and downwards in the sympathetic system and are broken some- where in their course, being transferred to a fresh relay which, by means of non-medullated nerves, carries them on to their destination. Finally, in certain organs of the body are to be found sheets of nerve structures, including both ganglion cells and fibres, which must be regarded as local nerve-centres, capable of carrying out co-ordinated acts in response to stimuli, independently of the central nervous system. It seems probable that these systems are to be regarded as analogous rather to the diffuse neuro- fibrillar system of an animal, such as the medusa, than to the synaptic ner- vous structures characteristic of the central nervous system of vertebrates. THE AUTONOMIC NERVOUS SYSTEM ^69 In the latter the direction and effect of any impulses are determined by the synapses intervening between various systems of neurons and allowing the passage of the impulse only in one direction. This law of forward direction has not been proved to hold good for the primitive nerve systems ; an impulse apparently spreads equally well in either direction. As a type of this peripheral diffuse nerve system may be cited the Auerbach's and Meissner's plexuses in the wall of alimentary canal. How far we are to regard the nerve-nets in other viscera, such as the heart and the bladder, as conforming to this type is still a moot point, and will be discussed in dealing with the origin of the heart-beat. Posc'root - Ant' root — Pre-ganglionic fibre - -Symp. gangl. -..J " ■ , ' ^ ^ Post-qanqlionic fibre IVIade-up spinal nerve ' Fig. 239. Diagram (after Langley) to show tlic manner in which a spinal nerve is completed by the entry of a grey ramus, containing fibres derived from cells in the sympathetic chain. p.pr.d, posterior primary division. (The post-ganglionic fibres are represented red.) The relationships of the white and grey rami are strikingly illustrated in the case of the pilomotor systems of nerves. These in the cat arise from the cord by the anterior roots from the fourth thoracic to the third lumbar inclusive. Passing by the white rami to the sympathetic system, they travel upwards and downwards and end by arborisations in the various gangha of the main chain. From the cells of each ganglion a fresh relay of fibres starts, which runs as a bundle of non-medullated nerves (the grey ramus) to the corresponding spinal nerve, with which it is distributed to its peripheral destination. Each grey ramus causes erection of the hairs above one vertebra, whereas stimulation of one white ramus causes erection over three or four vertebrae, showing a distribution of the fibres of the white ramus to the cells in several successive ganglia. These pilomotor fibres in the cat have the following distribution : (1) For the head and upper part of the neck the fibres arise by the fourth to the seventh thoracic anterior roots, and have their cell stations in the 470 PHYSIOLOGY superior cervical ganglion. They travel as small medullated nerve fibres from the white rami up the sympathetic chain, through the stellate ganghon and ansa Vieussenii and up the cervical sympathetic. (2) The next set of nerve fibres have their cell station in the stellate ganghon. The white rami arise from the fifth to the eighth thoracic nerves, while the grey rami pass to the nerve-roots from the third cervical nerve to the fourth thoracic nerve. (3) The remaining nerves, supplying all the rest of the body and tail, arise by the white rami from the seventh thoracic to the third or fourth lumbar nerve, and are distributed as grey rami to all the spinal nerves below the fourth thoracic. We thus see that, in speaking of the functions of a spinal nerve-root, we must clearly distinguish whether we mean the root as it arises from the spinal cord, in which case its visceral functions will include those of its white ramus, or whether we mean the made-up or complete spinal nerve after it has received its grey ramus (Fig. 239). In the latter case the visceral functions of the root will be more restricted than in the former case, and will have a different distribution. In stimulating the nerve-roots in the spinal canal it is sometimes possible, by weak stimuli, to display the functions of the corresponding white ramus, and then by increasing the stimulus to get superadded the effects due to the excitation of the grey ramus in the made-up nerve, in consequence of the spread of current. "When, for example, the eleventh thoracic anterior roots are stimulated in the spinal canal with weak shocks, a fairly long strip of hairs in the lumbar region will be erected, the maximum movement of the hairs being near the middle of the strip. This marks the area of distribution of the pilomotor nerves given by the eleventh thoracic nerve to the sympathetic. If then the strength of the shock be increased to a certain point, the hairs in the long strip will of course be erected as before, but in addition there will be energetic erection of hairs in a short strip a little distance above the long strip, and separated from it by a quiescent region. This short strip is the same as that affected by stimulating the grey ramus or the dorsal cutaneous branch of the eleventh thoracic nerve. It marks the area of distribution of the pilomotor fibres received by the spinal nerve from the sympathetic." (Langley.) We may now indicate briefly the main course and functions of the fibres of the sympathetic system. (1) The head and neck are supplied by fibres leaving the spinal cord by the first five dorsal nerves (chiefly by the second and third). They all have their cell station in the superior cervical ganghon. They convey : Vaso- constrictor impulses to the blood-vessels. Dilator fibres to the pupil. Secretory fibres to the salivary glands and sweat glands. Vaso-dilator fibres to the lower lip and pharynx (?). (2) The thoracic viscera (heart and lungs) are supplied by the same five nerve-roots. The cell station of these fibres is, however, situated in the stellate ganglion. They convey : Accelerator or augmentor impulses to the heart. (3) The abdominal viscera receive fibres from the lower six dorsal nerves THE AUTONOMIC NERVOUS SYSTEM 471 and the upper three or four kimbar. Most of these fibres run through the sympathetic chain without making any connection with the gangha, and have their cell stations in the collateral ganglia of the solar plexus, the semi- lunar and superior mesenteric ganglia. On their way to these gangha they form the greater and lesser splanchnic nerves. Their functions are : Vaso-constrictor for stomach and small intestine, kidney, and spleen. Probably vaso-dilator for the same viscera. Inhibitory for both muscular coats of stomach and small intestine. Motor for ileocohc sphincter. (4) The pelvic viscera are supplied by the lower dorsal and upper three or four lumbar nerve-roots. These fibres also pass by the main chain to •rn A rttr 4 Spinal cord Sympathetic chain Solar ganglion Fig. 240. Figure (after Laxgley) to show the probal>le mode of connection of the fibres of the splanchnic nerve with nerve-cells. A, usual type, all the fibres i^assing through the lateral chain to end in the collateral ganglia of the solar ])lcxus ; b, alternative condition, in which a small minority of the fibres have their cell-stations in the sym- pathetic chain. The prc-ganglionic fibres are black, the post ganglionic red. make connections with the cells chiefly in the inferior mesenteric gangha. They convey : Vaso-constrictor impulses to pelvic viscera. Inhibitory fibres to colon (both coats). Motor and also inhibitory fibres to bladder. Motor fibres to retractor penis. Motor and inhibitory fibres to uterus and vagina. (5) The fore hmb receives nerves from the white rami of the fourth to the tenth thoracic nerves. All these fibres are connected with cells in the stellate ganglion. They convey : Vaso-constrictor impulses to the blood-vessels. Secretory nerves to the sweat glands. (6) The hind limb is supplied by the nerve-roots from the eleventh 472 PHYSIOLOGY thoracic to the third lumbar inclusive. The cell stations of these fibres are situated in the sixth and seventh lumbar and first sacral ganglia. They convey : Vaso-constrictor impulses. Secretory nerves to the sweat glands. Every fibre of the sympathetic system is thus in some point of its course interrupted by a nerve-cell, and Langley has shown that this is the only cell- break in the fibre, i.e. every fibre is connected with one cell and one cell only. This law apphes not only to the sympathetic fibres but also to the fibres of the other visceral nerves. Each fibre therefore can be regarded as made up of two sections — a pre-ganglionic fibre arising in the central nervous system and passing down to a ganglion as a fine medullated nerve fibre, and a post-ganglionic fibre arising in this ganglion and continued generally as a non-medullated fibre to its peripheral distribution. This transference from one system to another involves the passage across a synapse and a nerve-cell. The situation of this nerve-cell may be easily revealed by utilising the action of nicotine, first studied by Langley. If nicotine be apphed to a sympathetic ganghon, it first stimulates and then paralyses any junction between axon termination and nerve-cell which may he in the ganglion. Intravenous injection of nicotine therefore causes a primary general excitation of all visceral ganglion-cells. There is an enor- mous rise of blood pressure, which may be accompanied by other sympathetic effects, such as dilatation of the pupil, secretion of saliva, erection of the hairs, and so on. This rise rapidly passes off, and it is then found impossible to evoke any reflex visceral effects or any contraction, e.g. of the blood-vessels, by stimulation of the spinal cord ; the passage of the impulses is blocked in every one of the visceral ganglia. By observing the effects of stimulation of a nerve before it enters a ganglion and then painting the ganglion with nico- tine and again trying the effects of excitation, it is easy to determine whether the nerve fibres which were excited in the first case form any connections with the nerve-cells of the ganglion. In the strength in which it is usually applied nicotine is without effect on nerve fibres. THE CRANIAL AUTONOMIC FIBRES In the course of development the greater part of the fibres of the facial nerve have lost their special visceral functions and have the aspect and functions of ordinary somatic fibres. Visceral fibres are contained in the following cranial nerves — the third, seventh, ninth, tenth, and eleventh. Third nerve. The visceral fibres of this nerve pass to the ciliary ganglion in the orbit where they end ; the post-ganglionic fibres from this ganglion form the short ciliary nerves which innervate the sphincter pupillse and the ciliary muscles. Seventh nerve. The autonomic fibres of the facial arise from the medulla in the intermediate nerve of Wrisberg, which is practically the anterior con- tinuation of the ninth, tenth, and eleventh nerves. From the seventh nerve is derived the chorda tympani nerve, which supplies vaso-dilator nerves THE AUTONOMIC NERVOUS SYSTEM 473 to the tongue, the submaxillary and sublingual glands, and secretory fibres to these glands. The cell stations of these nerves lie peripherally, those of the sublingual gland in the so-called submaxillary ganglion ; those of the submaxillary gland in the hilus of this organ. It is probable that the seventh sends also pre-ganglionic visceral fibres to the spheno-palatine ganglion, whence a fresh relay of fibres — post-ganglionic — run with the branches of the fifth nerve, to supply secretory fibres and possibly vaso-dilator fibres to the mucous membrane of the nose, soft palate, and upper part of the pharynx. The glossopharyngeal or ninth nerve sends fibres, which evoke secretion as well as vaso-dilatation in the parotid gland, via the otic ganglion. Probably also dilator fibres leave this nerve to supply vessels at the back of the tongue. THE VAGUS The efferent visceral fibres of the tenth and eleventh nerves arise in the same column of cells as the two nerves just considered. Most of the fibres run in the vagus. They include motor fibres to the oesophagus, stomach, and small intestines as far as the ileocolic sphincter ; inhibitory fibres to the heart, motor fibres to the unstriated muscles of the bronchi, and secretory fibres to the gastric glands. The cell stations of these fibres are apparently situated peripherally, the jugular ganghon, and the ganglion of the trunk of the vagus being in all likelihood responsible only for the afferent fibres in this nerve. Nicotine therefore aboHshes any effect of stimulating the vagus in the neck, though inhibition of the heart can still be produced on excitation of the post-ganglionic fibres arising from the cells in the sinus venosus. SACRAL AUTONOMIC FIBRES These all run in the pelvic visceral nerve, also called nervus en'gens. This nerve is connected with a collection of ganglia lying in the hypogastric plexus at the base of the bladder. It has the following functions : Dilator to vessels of the penis (hence its name of nervus erigens). Motor to bladder, colon, and rectum. Inhibitory to sphincter muscle of bladder. Inhibitory to retractor penis. It will be observed that in many cases the viscera get their nerve-supply from both sets of visceral nerves, and that in such cases the two sets of nerves are antagonistic in function. It is impossible, however, to draw a sharp line between the functions of the two sets, since the same nerve may be motor for one set of muscular fibres and inhibitory for another set in the same viscus. Thus the colonic branches of the inferior mesenteric ganglion are motor (constrictor) for the blood-vessels and inhibitory for the muscular walls of the colon. While the sympathetic nerve-supply is inhibitory for nearly the whole intestinal muscles, it produces strong contraction of the band of muscle forming the ileocolic sphincter. In the bladder there is no doubt that the sympathetic supply includes both inhibitory and 474 PHYSIOLOGY motor fibres, the predominating effect on excitation of the nerve varying from one species of animal to another. FUNCTIONS OF THE SYMPATHETIC AND PERIPHERAL GANGLIA These gangha consist of a mass of nerve-cells embedded in connective tissue, each cell being surrounded by a special capsule of endothehal cells. The nerve-cells, though in section resembhng those in a posterior root ganghon, differ from these in being multipolar, each cell probably possessing one axon and several dendrites. The dendrites end in httle arborisations round adjacent cells. Since the main nervous system is characterised by the possession of nerve-cells, it was formerly thought that any collection of nerve-cells must partake of the co-ordinating and reflex functions of the central nervous system, i.e. must act as local nervous centres. All efforts have failed, how- ever, to prove the existence of such a function, and we must conclude that the sole use of these ganglia is to serve as distributing- centres. We may assume that one pre-ganglionic fibre divides, and the branches arborise round several cells (Fig. 240), whence new fibres arise to carry the impulse to the periphery — an impulse in the case of which there is no need for .any minute localisation. Indeed the essential part of a nerve-centre is not the nerve-cells at all, but the presence of a complex tangle of fibres, rendering possible the passage of impulses in all directions, the passage of an individual impulse being limited by reason of the varying strength of the impulse and the varying resistance of the many possible tracts. In many invertebrata the nervous system consists of a punctated material composed of a dense interlacement of fibrils, while the cells lie outside the centres and have one thick process dipping into the nervous mass, from which process both axon and dendrites arise. In this case, as we have seen, extirpation of the cell bodies does not destroy the capacity of the remaining fibrillar substance to act as a reflex centre. Such a complex of fibres is found in mammals in the plexuses of Auerbach and Meissner, which act as local nerve-centres for the intestine. But all such mechanism is wanting in the sympathetic ganglia, which con- tain neither association fibres between different cells of a ganglion nor com- missural fibres between the cells of adjacent ganglia. All the fibres in a sympathetic ganglion have either entered it from the white rami or are destined to leave it as fibres of grey rami. Several reflexes formerly described in peripheral ganglia, as, e.g. the ' submaxillary ' ganglion, have been proved to be fallacious. There is, however, a certain group of phenomena which can be elicited in sympathetic ganglia, and which have been termed by Langley and Anderson pseudo- reflexes, or, better, axon reflexes. If, for instance, we divide all the nerves going to the inferior mesenteric ganglion, leaving the bladder connected with the inferior mesenteric ganglion only by the hypogastric nerves, and then after dividing the left nerve stimulate its central end, we obtain a contraction of the right half of the bladder. This effect is abohshed by THE AUTONOMIC NERVOUS SYSTEM 475 painting the inferior mesenteric ganglion with nicotine, showing that the activity of the cells of this ganglion is involved in the process. It has been shown, however, by Langley and Anderson that this is not a true reflex, but is rather analogous to Kiihne's gracilis experiment {cf. p. 255). A pre- ganglionic fibre arriving at the inferior mesenteric ganglion branches, one branch ending round the cells of the ganglion, while the other bi'anch passes down in the left hypogastric nerve to a cell situated near the base of the bladder (Fig. 241). When therefore we stimulate this nerve we are stimula- Sp.cord Inf. mes. g- ^ Post-ganglionic fibre Pre -ganglionic fibre -Hypogastric nerves Diagram to illustrate Langley and Anderson's explanation of the hypo- gastric reflex as an axon reflex. Fig. 241 The division of the axon where the propagation or '-reflexion ' takes place is at X ting a pre-ganglionic fibre, and the excitation spreads up to the point of junction of the two branches and then down the other branch to excite the cell in the inferior mesenteric ganglion. We thus obtain an apparent motor reflex by stimulation of a nerve which is itself motor. Similar pseudo- reflexes can be obtained along the abdominal chain on the pilomotor nerves but furnish no groimds for ascribing the property of reflex centres to peri- pheral ganglia. On the other hand, these axon reflexes are very similar to the spread of the excitatory process which occurs in the diffuse nerve network of an animal such as the medusa. Irreciprocity of conduction would seem therefore to be the most useful criterion of a true reflex. INHIBITION IN PERIPHERAL GANGLIA The existence of ganglion-cells in the eour.se of the nerves to visceral muscles has often been supposed to account for cerUiin peculiarities in the innervation of visceral, as conapared with skeletal, nuLscle. Chief among the differences between these two kinds of muscle i& the frequency with which inhibition may be brought about in visceral muscle by stimulation of pcriplieral efferent nerves. In skeletal muscle inliibition is only known as the result of alteration of the activity of the motor centres from which it is supplied. It has therefore been throught that the peripheral ganglia of vi.sceral mu-jcle play the part of the motor spinal centres of skeletal muscle, and that when wo excite an inhibitory norvo, say to the intestine, we are interfering with and diminishing 476 PHYSIOLOGY a tonic state of activity which has its seat in a peripheral ganglion-cell connected with the visceral muscle. This view, which was first put forward by Claude Bernard, has been specially defended by Dastre and Morat. These observers point out that vaso-dilator action diminishes or disappears as nerve-strands are stimulated more and more peripherally, and con- clude that the vaso-dilator fibres run to the sympathetic ganglion and inhibit their tonic action. The untenability of this view has been demonstrated by Langley. Thus the chorda tj^mpani fibres run to a local nerve-' centre ' in the hilum of the submaxillary gland. On Bernard's theory stimulation of the fibres peripherally to the centre should cause contraction of the arteries ; but it is found that, after paralysis of the ganglion by nicotine, stimulation of the post-ganglionic fibres causes dilatation, so that the nerve fibres given o& from the local centre are not vaso-constrictor but vaso-dilator. Moreover it can be shown that the sympathetic fibres which do cause constriction make no connection with the cells in the hilum of the gland, but run on the walls of the arteries to their distribution. These fibres are connected, not with cells of the submaxillary ganglion (the local nerve-centre), but with cells in the superior cervical ganglion. We must conclude that the inhibitory nerves in these visceral structures exert their influence directly on the peripheral tissue, and not by a diminution of activity of a tonically acting peripheral centre. AFFERENT FIBRES AND THE AUTONOMIC SYSTEM (Referred Pain) The supply of afferent fibres to the viscera is very small in proportion to the supply to the outer surfaces of the body. According to Langley, in a visceral nerve, such as the hypogastric, only about one-tenth of the medul- lated fibres are afferent, and the proportion of afferent fibres in the splanchnic nerves is probably not very different from this. At the two ends of the ahmentary canal, i.e. at the mouth and anus, the afferent visceral fibres become of more importance, since through their means co-operative somatic reflexes have to be excited as well as the simple visceral reflexes. Thus in the pelvic visceral nerve about one-third of the fibres are afferent, and the vagi contain a large number of afferent fibres from the lungs as well as from the other viscera innervated by it. According to Dogiel, afferent visceral fibres arise from sensory cells of the sympathetic ganglia and, passing in to the posterior spinal ganglia, divide and form pericellular endings round a special type of posterior root-cells, the axon from which divides again into a number of branches which end in connection with typical unipolar cells. Thus, according to this observer, a few afferent sympathetic fibres can stimulate a considerable number of posterior root-cells. Langley, however, has not been able to obtain any experimental confirmation of the origin of branching afferent fibres from cells of the sympathetic system. It is probable that the afferent fibres of the visceral nerves arise, like those of the somatic system, from ganghon-cells of the posterior spinal ganglia. Every white ramus contains afferent fibres, stimulation of which may evoke a rise of blood pressure as well as movements of the skeletal muscles. In spite of the supply of afferent fibres to the viscera, most of these organs are very insensitive to ordinary stimulation such as handhng or cut- ting. In operations on man for resection of the gut, if the abdominal cavity be opened under local or general anaesthesia, cutting and suturing may be THE AUTONOMIC NERVOUS SYSTEM 477 conducted without any anaesthetic and without causing any pain to the patient. On the other hand, impulses may arise in the afferent nerve- endings of the viscera, as a result of disease or certain forms of stimulation, e.g. stretching or compression, which may reach consciousness and give rise to the sensation of pain. This pain is not localised in the viscera, but is referred to certain parts of the surface of the body. When the afferent autonomic fibres of a nerve are the seat of pain, the primary referred pain is in the area of the cutaneous somatic fibres of the nerve. It has been shown by Mackenzie and by Head that visceral disease may cause hyper-sensiti\aty of the corresponding areas of the skin, and a method has been elaborated by these observers for utilising this referred pain or skin tenderness as a means of locahsing the site of the disease. CHAPTER VIII THE PHYSIOLOGY OF SENSATION SECTION I ON THE RELATION OF SENSATION TO STIMULUS Up to the present we have dealt with the central nervous system as a machine for the conversion of afferent into appropriate efferent impulses, and have regarded the sense-organs simply as mechanisms by means of which stimuli of various quahty, arising from events in the environment of the animal, could give rise to nerve impulses. We have seen reason to assign these afferent impressions to various classes, according to the physical character of the stimulus involved and according to the quahty of the response. The nature of the physiological processes evoked in the organism depends on the physical nature of the stimulus and the locus of its incidence on the surface of the body. Thus a nocuous stimulus applied to the foot causes flexion of the leg, whereas steady pressure on the sole evokes a ' stepping ' reflex with extension of the leg. A beam of light falhng on the eye calls forth movements involving contractions of the intrinsic and ex- trinsic ocular muscles. The same beam of Mght falling on the skin is devoid of effect unless it is so strong as to burn the skin, when a reflex is excited similar to that evoked by a nocuous stimulus. As our study of the adaptive nerve mechanisms becomes more detailed, and especially when we take into account the activities of the association centres in the cortex, it becomes more and more difl&cult to follow the chain of processes which lead to any given reaction ; in order to advance further in our knowledge of the activity of the receptor organs, we have frankly to abandon the objective method which has served us in the study of all the other functions of the body, and appeal to our own consciousness for information as to the effects of their excitation. Each one of us is aware that stimulation of an afferent nerve may cause a change in consciousness which we denote as a sensation, and we attribute to other living organisms, presenting similar reactions to ourselves, similar changes in consciousness in consequence of like stimuh. Certain sensations differ one from the other to such an extent that com- parison among them becomes impossible. Thus in the skin and underlying parts we have, as a result of stimulation, sensations of touch and pressure, sensations of heat and of cold, and sensations of pain. The contact of 478 RELATION OF SENSATION TO STIMULUS 479 certain dissolved substances with the end-organs of the gustatory nerves excites in us a sensation of taste. Other substances diffused in the air and carried by it to the olfactory terminations give as sensations of smell. Vibrations of a certain frequency, transmitted by the air and by the auditory ossicles to the endings of the auditory nerve, produce sensations of sound, while Ught falling on the retina evokes visual sensations. Besides these sensations resulting from stimulation of the exteroceptive system of nerves, we are aware of the existence of a number of organic sensations — some derived from the viscera (enteroceptive), others caused by stimulation of the proprioceptive system. As examples of the latter we may mention the muscular sense, by which we judge of the amount of tension exerted by a contracting muscle ; the sense of position of the hmbs ; and the sense of position of the head, resulting from stimulation of the labyrinthine organ. How the physiological excitatory process in nerve fibres, \\dth its concomi- tant chemical and electrical phenomena, is able on arrival at the brain to excite a conscious sensation we are unable to decide, or even to discuss, since we are dealing here with processes of two different orders. We should not arrive any nearer to the solution of this riddle if we were able to follow out the whole of the events occurring in the body as the result of the application of any given stimulus to its surface. We might under these circumstances be able to predict with certainty the behaviour of any animal, if we knew its past history and the comparative resistance of every path in its central nervous system which might possibly be traversed by any given nerve impulse. Such knowledge would be purely objective and could not be used to explain the ' epi-phenomenon ' of consciousness. One might in fact imagine a machine which would react hke a Uving animal, but would be perfectly devoid of self-consciousness, and we should be unable in such a case to decide whether consciousness were or were not present. Each one of us only knows consciousness as it exists in himself. Before therefore we can employ our conscious sensations as a means of throwing light on the conditions of action of the receptor organs of the bodv. we must have some idea as to how far our sensations correspond to the stimuli. i.e. the physical events by which they have been evoked. Appeal to our own experience shows the existence of two kinds of difference between various sensations. The greatest difference is found between those sensa- tions which are normally evoked by different sense-organs. Thus we are all aware of the meaning attached to such qualities or such sensations as sweet, red, hard, high-pitched (of sound), &c. It would be absolutely impossible to compare these sensations among themselves. We cannot say, for instance, that this sound is louder than that colour is red. Such fundamental differing quaUties of sense are spoken of as the modality of the sensation. On the other hand, within the sensation evoked by any one sense-organ we find differences of quality which are more comparable among themselves. Thus we can compare the pitch of various sounds, or the colour of various objects seen with the eye. We can even say that the bitter taste of any 480 PHYSIOLOGY substance is more marked than the sweet taste of another. The question arises how far these differences in sensation correspond to and are a measure of differences in the physical events by which they have been evoked. A very little consideration suffices to show that there is no resemblance between a sensation and the stimulus, and that one and the same physical event applied to different sense-organs will evoke absolutely distinct sensations ; while different modes of stimuh apphed to one sense-organ will always evoke the same sensation. Thus if light falls on the retina it causes a sensation of light. If the same radiant energy, consisting of transverse vibrations in the ether, be allowed to fall on the skin, it either produces no sensation at all, or, if concentrated by means of a burning-glass, may give rise to a sensation of warmth, heat, or pain. If we take a tuning-fork which is vibrating 100 times per second and apply it to the surface of the skin, we get simply a sensation of vibration, i.e. a series of tactile impressions repeated at rapid intervals. If the same tuning-fork be appHed to the head, its vibrations are imparted to the bones of the skull and thence to the auditory nerve-endings and arouse in our consciousness a tone-sensation of a certain note. The same thing happens if the vibrations of the tuning-fork are conducted by the ear to the auditory nerve- endings in the ordinary way through the external and middle ear. On the other hand, a sensation of light may be aroused not only by the incidence of radiant energy of a certain wave length on the retina, but also by electrical or mechanical stimulation of the retina. If the eye be turned inwards and the finger be pressed on the eye through the outer canthus of the lids, a sensation of hght is aroused and we see a coloured circle which we refer to some spot lying to the nasal side of the eye stimulated. The character of the sensation bears therefore no resemblance to the physical events by which the sensation is evoked, but depends entirely on the nature of the sense-organ which is stimulated. A sensation of hght may be produced by any stimulation of the retina, or of the optic nerve, or of the terminations of the optic nerve in the brain. In the same way stimulation of an auditory nerve or its intracranial endings gives rise to sensations of sound. Where the question has been investigated it has not been found possible to evoke different quahties of sensation by different modes of stimulation of nerve fibres, and it has therefore been concluded that the quality of any sensation depends simply and solely on the termination of these nerves in the central nervous system, and that where sensations of different quahty are produced there must be also difference of nerve fibres. This idea was formulated by Miiller, and is often alluded to as Miiller's ' law of specific irritabihty.' The law states that every sensory nerve reacts to one form of stimulus and gives rise to one form of sensation only, though if under abnormal conditions it be excited by other forms of stimuh, the sensation evoked will be the same. Although the different forms of sensation must be regarded as dependent on the integrity of the brain, and of its connections with the peripheral sense- organs, sensations are not referred to the brain, but are localised as proceed- RELATION OF SENSATION TO STIMULUS 481 ing from some part of the body or from some region outside of the body. Thus the sensation of taste is always locaHsed in the mouth ; sensation of touch at the skin or surface of the body ; while the sensations of hearing and of sight are ' projected,' i.e. are interpreted as coming from the en- vironment outside ourselves. Even the organic sensations of posture or fatigue are referred to the peripheral reacting parts of the body and not to the central nervous system. A sensation therefore cannot be interpreted as a reproduction of external events, but as a symbol of these events evoked by stimulation of the sense-organs of the body. It is indeed impossible by a purely intuitive study of sensations to arrive at any correct idea of their origin or of the factors concerned in their produc- tion. No sensation is the immediate and sole product of a stimulus appUed to the peripheral end of a nerve fibre, but the simplest sensation involves a judgment, i.e. complex neural activities which are the resultant of innumer- able past and present streams of nervous impulses aroused by peripheral events and poured into the central nervous system. It is important therefore not to regard a sensation as in any way constituting an elementary unit, by the aggregation of a number of which a conscious state is produced. As we have seen, the primitive function of the whole nervous system is reaction. The neural life of an animal is composed of a series of reactions, some simple, some complex, and becoming ever more complicated as we ascend the animal scale. The first reactions of a baby, for instance, will be those by which it procures nourishment and satisfies a need. The earhest event in its dawning consciousness will be, not a sensation of sweetness or of colour, but that of a thinij which can satisfy its needs. It will have had to try many gustatory experiments before, out of the sum of its material experiences, it will be able to choose a number of hke factors which can be grouped together as ' sweet.' Judgment of quahty of sensation involves a power of abstraction and of classifying similar elements in different neural events or reactions and the referring of these elements to the external world. It is very difficult, how- ever, to divest ourselves of the mental standpoint reached as the result of many years' continual trials, successes and failures, and constant care has to be exercised if we are not to fall into the common conception of the ego, the personality, or soul, as a sort of sentient god sitting somewhere in the brain, or, as Descartes suggested, in the pineal gland, and receiving by means of one part or other of his servile material brain a blue sensation from the eyes, or an auditory impression, or a tactile impression, and then, if he feels so inchned, pressing the stop in a pyramida cell to let out a volun- tary motor response. An elementary miit in psychical life, as in neural hfe, must be a complete reaction. It is from the reaction and not from the sensation that a constructive psychology will have to be built up. . Although any given sensation may be jjroduced by many forms of stimulation of the sense-organ, under normal circumstances each sense-organ is so arranged and protected that it is only stinnilated by one kind of physical process, i.e. by the one for which its liminal or threshold excitability is at a minimum. Thus the retina, though it may be stimulated mechanically 482 PHYSIOLOGY or electrically, is in the normal individual very thoroughly protected from the possibiHty of such excitation, so that all impulses arising in the retina may be almost certainly referred to changes in the light- waves which fall on the eye, and by means of the dioptric mechanism of this organ are thrown in a distinct pattern upon the retina. Although the sensation is not a repro- duction of the stimulus, it is a symbol of the stimulus, and can be used to inform us of events occurring in the world around. Like stimuli, falhng on the same end- organ, always evoke like sensations, other conditions being equal. An orderly sequence of sensations may therefore be interpreted as indicating a corresponding orderly sequence of physical occurrences in the world around us. THE QUANTITATIVE RELATIONSHIPS BETWEEN STIMULUS AND SENSATION Since our sensations are merely symbols of the physical conditions which give rise to them, it is important to inquire how far they correspond quanti- tatively to differences in the energy of the afferent stimuli, i.e. how alterations in the strength of stimulus will affect the intensity of the resulting sensation. Whatever form of stimulus be appHed and whatever sense-organs be affected, a certain minimum intensity of stimulus is necessary for it to be effective, i.e. to produce a minimum sensation. This strength, which varies with different sense-organs, is spoken of as the ' liminal intensity,' or ' threshold value ' of stimulus or sensation respectively. As the strength of the stimulus is increased above this minimal amount the resulting sensation also increases. The change in intensity of sensation is not, however, indefinite. When the stimulus is increased to a certain amount the resultant sensation becomes maximal, and a further increase in the stimulus evokes no further increase in sensation. In fact fatigue of the sense-organs or recipient centres of the brain rapidly sets in, so that the sensation diminishes even with increasing strength of stimulus. In each sense-organ we can measure the amount of energy which must be applied to it in order to evoke a minimum sensation. This figure varies considerably with the physiological condition of the animal. In deahng with reflexes we have seen that the motor result of stimulation of a receptor organ varies in the same manner. Thus a minimal stimulus is more effective if repeated a few times at definite intervals (summation of stimulus) : the stimulus which is subminimal may become minimal and effective as a result of repetition. Another factor which intervenes is that known as ' adaptation ' — a process associated to a certain extent with the phenomenon of fatigue. Adaptation is best studied in the case of the eye. Here the dark-adapted eye, i.e. one that has been kept from light for half an hour, will react, and give a visual sensation, to a strength of stimulus which is only one-fiftieth of the minimal stimulus required to evoke sensation in the eye that has been lately exposed to light. Another phenomenon which may alter the strength of the liminal intensity RELATION OF SENSATION TO STIMULUS 483 of stimulus is that known as ' contrast.' A finger plunged into mercury feels a ring of constriction at the level of the surface of the mercury, i.e. where there is a contrast between the pressure of the mercury and the absence of pressure as the finger emerges into the air. The strength of the effective stimulus depends also on the number of nerve-endings simultaneously excited. Thus when dealing with tactile sensations, or sensations of pressure, in determining the minimal stimuliLS we must take into account the area stimulated, and we express the stimulus, just sufficient to produce a threshold sensation, as ' weight per square milli- metre of surface.' Moreover the rapid ' fatigabihty ' or adaptation of all sense-organs makes the rate at which the stimulus is applied of considerable importance. Thus when weight was applied to the skin of the ball of the thumb at the rate of 0-75 gramme per second over a surface of 21-2 scj. mm., the minimum load necessary to evoke a distinct sensation was 2-5 grammes. When, however, the rate of application of the weight was increased to 5 grammes per second, distinct sensation was produced with a load of 0-25 gramme. It is evident that the larger the area of the skin stimulated the greater will be the minimal weight required, since this has to be dis- tributed over all the nerve-endings contained in the area of skin which is subjected to pressure, and the larger the area the smaller will be the stimulus apphed to each nerve-ending. The following figures were obtained by von Frey on different regions of the skin : stimulated surface 21-2 mm. 2 Volar side of wrist (Subject K) Ball of thumb (Subject K) Volar side of wTist (Subject F) Stimulated surface 3 5 mm. 2 Ball of thumb (Subject F) Tip of finger (Subject F) . Volar sid;- of wrist (Subject F) Rate of loading 17 grm. per second. Threshold value of stimulus per mm.- 0-024 - 0-038 grm. . >0-lS9 - 0-039 „ . >0-236 - 0-055 „ Rate of loading 3 grm. per second Threshold value of stimulus. 0-200 - 0-045 grm. 0-170 - 0-028 „ 0-640 - 0-028 „ It will be noticed that when the excited surface is small much gi'eater variations are found from spot to spot in the size of the minimum stimulus. This is probably connected with the fact that the sense-organs for pressure are distributed in points or spots over the surface of the skin, so that when the stimulated surface is small the threshold value of the stimulus will be deter- mined by the number of the tactile spots which are included in the stimulated area. The minimal effective stimuli in the case of the other senses have been determined as follows : (a) HEARING. A musical tone can be heard when the variations of pressure in the air ammount to -00000059 mm. Hg. with an amplitude of vibrations of •(X)(X)()006G mm. It has been calculated that the intensity of the work performed on the drum of the ear by such a minimal tone repre- sents an average of 5*1 x 10 "'•'' orgs. In the case of noise the amount of energy required to produce a inininuuu sensation is still smaller. A distinct 484 PHYSIOLOGY sound was heard when a weight 6-7 milUgrammes was allowed to fall a distance of 1-2 mm. on to an iron plate at a distance of 500 mm. (6) VISION. The minimum intensity of light necessary to arouse sensa- tion in a dark-adapted eye is, according to Aubert, equal to about one three-hundredth of the intensity of the hght reflected from a piece of white paper which is being lit by the hght of the full moon. The amount of energy involved in such a stimulus is much smaller even than that deter- mining a sensation of touch or hearing. WEBER'S LAW It is an interesting question how far the strength of sensation may be regarded as an index to the strength of stimulus. Although it is easy to measure in absolute terms the intensity of a stimulus, i.e. of a purely physical process, there is no means by which we can express in absolute measure the strength of a sensation. We cannot even compare the strengths of two sensations differing in quality or modahty ; and although we can say that such and such a hght is stronger than another Hght, it is impossible to say that the sensation resulting from the stronger is two, three, or more times that of the weaker. In measuring the effect on sensation of increasing the stimulus we are therefore reduced to using the smallest appreciable increase of sensation as our unit of sensation. The question as to the relation between the intensity of stimulus and the intensity of sensation resolves itself into an inquiry as to what increase in a given stimulus is necessary in order that it may evoke an appreciable increase in sensation. Weber's law states that the increase of stimulus which is necessary to produce an appreciable increase in sensation must always bear the same ratio to the whole stimulus. Thus if we found that we could just distinguish the difference between a weight of 10 oz. and a weight of 9 oz., it would not be sufficient to add one ounce to a weight of 10 lb. in order to produce a distinct difference in sensa- tion. In the latter case we should not be able to appreciate any difference until we had added a pound, i.e. one-tenth of the whole stimulus to the weight. We can distinguish between 10 oz. and 11 oz., or between 10 lb. and 11 lb., but not between 10 lb. and 10 lb 1 oz. Several methods have been proposed for testing the limits of the apphcabihty of this law. Of these the most important are : (1) The method of minimal diflerence. (2) The method of average error. In the first method we find by trial how much a given stimulus must be increased in order to evoke an appreciable increase of sensation, and this determination is made for a number of stimuli of different intensity. In the second method it is sought to find a strength of stimulus which is just equal to another stimulus of given intensity. It will be found that errors will be made on both sides, and the average error is taken as representing the minimum difference, which is just sufficient to cause a distinct difference of sensation. In all sense-organs Weber's law is only apphcable between hmits, which RELATION OF SENSATION TO STIMULUS 485 vary with each sense-organ, and it does not hold either for very weak or for very strong stimuli. Within these hmits the ratio which an increase of stimulus must bear to the whole stimulus to produce an increase of sensation may be given approximately as follows for the different sense-organs : When weights are placed on corresponding points of two sides of the body, e.cj. on the two hands, we can appreciate differences of about one- third ; if the contrast be successive, i.e. if the weights be placed on the same spot in succession, we can appreciate differences between one-fourteenth and one-thirtieth. The range over which this amount of accuracy is attained ex- tends from 50 to 1000 grammes. In judging of weights with the help of movement (the method one ordinarily adopts) the limit of accuracy is about one-twentieth ; for sounds the appreciation of difference amounts to about one- ninth. The organ which is most susceptible to shght changes of intensity is the eye ; by this organ we can appreciate diff'erences of one one-hundredth to one one-hundred-and-twentieth in the total illumination. FECHNER'S LAW. Fechner has endeavoured to give a mathematical expression to the facts described under Weber's law. According to Weber the proportion between the increase of stimulus necessary to cause increase of sensation and the whole stimulus is a constant for all intensities of excitation. Thus if C is a constant R where k represents the smallest appreciable increase of sensation evoked by the minimal increase of stimulus, R is the stimulus, and AR is the minimal increase of stimulus. If the same relation may be allowed to hold for infinitesimally small differences of sensation and infinitely small differences of stimulus, this formula may be expressed by the equation : dE=C^ R By integration we obtain the expression : . E = C log. nat. R. i.e. the sensation is proportional to the natural logarithm of the stimulus, which is Fechner's psycho-physical law. In view of the fact, however, that Weber's law only holds good between certain limits, not much practical value can be attached to such a matliematical expression. Moreover Fechner's calculation is based on the unprovable and unjustifiable assi nip- tion that, within the limits of applicability of Weber's law, the smallest appreciable increase in sensation is always the same, i.e. that the increased sensation which is evoked by the addition of 6 grammes to a weight of 100 grammes is identical with the increased sensation called forth by adding 60 grammes to an initial weight of 1000 gra,mnies. Such an assumption does not, as a matter of fact, agree with om- own exiierienee ; and it is probably premature here, as in many other departments of biology, to attempt to include the complex of variable phenomena presented by animal function^ within the Procrustean bed of a mathematical foriiiula. SECTION II CUTANEOUS SENSATIONS The skin, being the outermost layer of the body, represents the tissue or organ by which the organism is brought into relationship with its environ- ment. In the widest sense of the term the skin is protective. This function it discharges by virtue not only of its physical properties but also of its rich endowment with sense-organs, by means of which the intracorporeal events can be correlated with those occurring outside and immediately affecting the organism. We are accustomed to distinguish several qualities of sensation among those having their origin in the skin, the chief of which are the sense of touch, including that of discrimination, the sense of pain and the sense of tempera- ture. The very different quahties of sensation included under these three classes suggest that there may be a special mechanism, or class of mechanism, for each sense, and a careful investigation of the sensory quahties of the skin surface bears out this idea. Isolated stimulation of minute areas on the skin does not excite all the sensations together, but only a sense of touch or of pain, or a sense of cold or warmth. We are therefore justified in deahng with each of these sensations separately. THE TEMPERATURE SENSE. By means of the skin we can appreciate that a body coming in contact with the skin is either cold or warm. If the body is at the same temperature as the skin, as a rule no sensation of tempera- ture is excited. It was formerly thought that the sensations both of heat and cold were determined by the excitation of one and the same end-organ. Warming of this end-organ would produce a sensation of warmth, while a diminution of its temperature would produce the sensation of cold. Careful investigations by Blix and Donaldson of the distribution of the temperature sense has shown that this opinion cannot be maintained. If a small surface warmed to a few degrees above the temperature of the skin be moved ovei: any part of the surface of the body, e.g. the back of the hand, it is found that the warmth of the instrument is not appreciable equally at all parts of the surface of the skin. At some points the sensation of warmth will be very pronounced, but between these points the sensation of warmth may be entirely wanting and the instrument may be judged to be of the same tem- perature as the hand itself. In this way a series of ' warm points ' may be mapped out. On now cooling the instrument a few degrees below the temperature of the surface of the body and then moving it over the surface in 486 CUTANEOUS SENSATIONS 487 the same way, it will be found again that the coolness of the instrument is only appreciated at certain points which can be regarded as ' cold points ' and as containing the nerve-endings by the excitation of which the sensation of cold is produced. If the warm points be pricked out in red ink and the cold points in blue ink it will be seen that they do not in any way correspond. A convenient instrument for this purpose is the one invented by Miescher, con- sisting of two tubes cemented together and communicating at a small flattened extremity, which is applied to the surface of the skin ; through the tubes water can be led at any desired temperature, which is read off by a thermometer placed within the tube. Having mapped out the warm spots it may be shown that they are excitable by means of mechanical or electrical stimuli and that the sensation produced is the same as if they had been excited by their adequate stimulus, viz. rise or fall of temperature. Cold spots. Heat spots. Fig. 242. Heat and cold spots on part of pahn of right hand. The sensitive points are shaded, the black being more sensitive than the lined, and these than the dotted parts. The unshaded areas correspond to those parts where no special sensation was evoked. (Goldscheider.) The mapping out of the spots is rendered difficult by the irradiation of the sensation produced so that it is difficult to refer the sensation of warmth or cold definitely to the point stimulated. An investigation of the topography of these warm and cold spots shows that the apparatus for the appreciation of cold is much more extensively distributed over the body than that for the appreciation of warmth, as is evidenced from the diagram (Fig. 242) giving the topographic distribution of the cold and warm sense- organs on the palm of the hand. The temperature sense is best marked in the following regions of the body : the nipples, chest, nose, the anterior surface of the upper arm and the anterior surface of the fore-arm, and the surface of the abdomen. It is much less marked on the exposed parts of the body, such as the face and hands, and is but slight in the mucous membranes. Thus it is possible to drink hot fluid, such as tea, at a temperature which would be painful to the hand, and still more to any other part of the body. The scalp is also very insensitive to changes of temperature. The acuteness of the temperature sense varies considerably with the condition of the skin and with the previous stimulation of the sense-organs. The sense is most acute at about ordinary skin temperature, i.e. between 27° and 32° C. At this temperature the skin can appreciate a difference of 1° C. When the skin is very cold or very hot the temperature sense is not nearly so delicate. 488 PHYSIOLOGY This sense presents the phenomenon of adaptation in a marked degree. It is a famihar experience that on coming from the external air on a cold day into a warm room a sensation of warmth is experienced all over the body. In a few minutes this sensation wears ofi. On now leaving the room to go outside again, the sensation of cold is at once appreciated, to disappear in its turn after a few minutes. The effect of adaptation is still better shown by the experiment of taking three basins of water a, h, and c : a contains cold water, h tepid water, c hot water. The left hand is immersed in the cold water and the right hand in the hot water for a few minutes. On now placing both hands into the basin of tepid water it feels hot to the left hand and cold to the right hand. Such experiences as this led Weber to the conclusion that the essential stimulus for the temperature sense was not the actual temperature to which the sense-organs were subjected, but the fact of a change of temperature. He imagined that while the temperature sense- organs were being warmed a sensation of warmth was produced, and when their temperature was being lowered, a sensation of cold. Such a theory would not, however, account for the fact that, above a certain tempera- ture, water may feel warm and the feehng may continue so long as the skin continues to be stimulated. On a cold day the air may feel cold to the face and the feeling may last the whole time that the face is exposed. Moreover we have in the temperature sense conditions which remind one of the after images which occur in the eye and v/hich will form the subject of a later section. If a penny be pressed on the forehead and then removed the sensa- tion of cold lasts some httle time after the penny has been removed. In this case a sensation of cold is produced although the end-organs are being gradually warmed up after the removal of the penny. In order to account for these facts Hering, at a time when the differentiation of hot and cold spots had not yet been effected, suggested that the temperature sense-organs could be regarded as having a zero-point at which no sensation was produced. If their temperature was raised above this point a sensation of warmth was produced and vice versa. The zero-point, however, was not a fixed one, but could move upwards to a certain extent on prolonged exposure to high temperature, or downwards on prolonged exposure to a low temperature. In the light of the researches of Blix and Goldscheider we should have to apply Hering's theory of a zero-point to each of the temperature end-organs separately. A cold pencil passed over a warm spot evokes no sensation whatsoever. If, however, a pencil considerably warmer than the skin be passed over a cold spot this may be excited, so that the paradoxical result is produced of a sensation of cold as the result of stimulation with a warm body. It is a familiar fact that the immediate effect of entering a hot bath is very much the same as that of entering a cold bath, viz. a rise of blood pressure and contraction of the unstriated muscles of the skin and hair follicles with the production of ' goose skin.' It has been suggested that the distinctive quality of a sensation of hot as compared with that of warm is due to the simultaneous stimulation of warm spots and cold spots. When testing the CUTANEOUS SENSATIONS 489 distribution of the temperature sense it is found that the sense of cold is evoked more promptly than that of warmth. This is interpreted as showing that the end- organs for the warm sense are situated more deeply than those for cold. We have no evidence as to the histological identity of these organs. THE SENSE OF TOUCH By means of the sense of touch we arrive at a conclusion a^ to the qualities, such as shape, texture, hardness, &c., of the bodies with which the skin is in contact. In this judgment, however, very many other sensations are involved besides those which can be regarded as strictly tactile. Thus the hardness of an object signifies its resistance to deformation, besides its power of deforming the skin surface with which it is in contact ; the former quahty, i.e. of resistance, is one which involves the muscular sense, since we judge of it by the extent to which we can move our muscles without causing any alteration of the surface of the body. The tactile sensibihty of the skin as a whole, like its temperature sensi- bility, is due to the presence in it of a number of touch spots, i.e. small areas which are extremely sensitive, separated by areas almost or entirely insensitive to pressure. The tactile sensibihty of any part is proportional to the nmnber of such touch spots present. If the calf of the leg be shaved and then tested by pressing on it mth a fine bristle or hair it "will be found that the minimal stimulation used evokes sensation only at certain definite points, the ' touch spots.' In a square centimetre of such skin there may be about fifteen touch spots. On thrusting a fine needle into one of these spots a sharply locahsed sensation of pressure is produced unaccompanied by any painful quality and often described as having a ' shotty ' character, as of a little hard object embedded in the skin and there pressed upon. These touch spots are arranged chiefly around the hairs, lying usually on the side from which the hair slopes. They vary in number according to the part of the body which is the subject of investigation. Thus the dorsal surface of the finger contains about seven times as many touch spots as an equal area between the shoulders. In some regions, such as the skin over subcutaneous surfaces of bone, as much as one centimetre may intervene between two neighbouring touch spots. They have no relation to the warm and cold spots ; they are entirely absent from tlio cornea, the glans penis, and the conjunctiva of the upper lid. The adequate stimulus for these tactile nerve-endings is not so much pressure as deformation of surface. It appears to matter little whether the surface be deformed by pulling it or by pushing an instrument into it. The ineffectiveness of mere pressure is shown by dipping the finger into a vessel of mercury. The sensation of pressure is only noted at the point where the finger passes through the siirface of the mercury, and this is the only part where there is an actual deformation of the skin, due to the sudden passage from the pressure of the mercury to tlie negligible pressure of the outside air. The tactile apparatus is smarter in its response than any other of the sense-organs. On this account stimuli are still percei\'ed as discrete, 16* 490 PHYSIOLOGY when they are repeated at a rhythm which would result in complete fusion in the case of any of the other sense-organs. Thus if a bristle be attached to a tuning-fork and allowed to press on the skin, the vibrations of the fork are perceived by the ear as a continuous sound and by the skin as a series of discofitinuous taps. Faradic currents when applied to the skin can be perceived as separate when repeated at the rate of 130 per second. The sensations evoked by placing the finger against the edge of a cog-wheel do not become continuous until the wheel is revolving at such a rate that the stimulation on the skin by the serrations occurs at a greater rate than 5C0 or 600 per second. The tactile apparatus resembles all the other skin sense-organs in showing adaptation. A stimulus after continuing for some time may become ineffective. We are usually entirely unaware of the stimulation of our skin by the pressure of the clothes, and even an unwonted stimulation, such as that of the mucous membrane of the mouth by a plate carrying artificial teeth, though almost unbearable 3 Fig. 243. Hair mounted on a wooden handle, and used by von Frey for testing tactile sensibility. during the first day, rapidly becomes less, and in a few days it is not perceived at all. In order to test the sensitiveness of touch we may use the method in- troduced by Hensen, viz. the bending of a glass-wool fibre. We could determine the pressure at which any given fibre will bend, and if we find by trial the fibre which just evokes sensation when pressed on the skin, we know exactly the force which we are applying to the skin. Von Frey employed hairs of difierent thickness for the same purpose (Fig. 243). The following represents the minimal excitabihty of the surface of different parts of the body when tested in this way. Tongue and nose . Lips . Finger-tip and forehead Back of finger Palm, arm, thigh Fore-arm Back of hand Calf, shoulder Abdomen . Outside of thigh , Shin and sole Back of fore-arm . Loins Gim. per sq. mm, 2 2-5 3 5 7 8 12 16 26 26 28 33 48 CUTANEOUS SENSATIONS 401 The sensitiveness of the sense-organs in the skin is probably much greater than that of the nerve-trunks themselves. Thus Tigerstedt found that the minimal mechanical stimulus necessary to excite the exposed nerve amounted to 0'2 grm. moving at 140 mm. per second. For the touch spots von Frey found that 0*2 grm. moving at 0*17 mm. a second is an adequate stimulus. In testing the sensibiUty of any surface it is important to remember that the hairs themselves form very effective tactile organs. The touch spots are distributed in greatest profusion around hair follicles, and there is a rich plexus of nerve fibres round the root of each hair. A shght touch appHed to the hair acts on these as on the long end of a lever, the hair being pivoted at the surface of the skin, so that pressure on the hair is transmitted, increased five or more times in force, to the hair foUicle and the surromiding nerve-endings. The actual sensibility of any part is therefore much dimin- ished by removal of the hairs. On 9 sq. mm. of the skin, from which the hairs had been shaved, the minimal stimulus necessary to evoke a tactile sensation was found to be 36 mg., whereas on the same surface before it was shaved 2 mg. was effective. WEBER'S LAW. The smallest increment or decrement of stimulus which determines a perceptible difference of sensation must, according to Weber's law, always bear the same ratio to the whole stimulus. In measuring such differences it is best to apply the stimulus successively to the same surface of the skin rather than simultaneously to adjoining areas. The time interval between two successive stimuli should not be more than five seconds and the duration of the stimuli should be equal. Weber found that in the terminal phalanx of the finger the minimal perceptible difierence was about one-thirtieth, but the ratio was not the same for all regions of the skin nor for all individuals. The following represents the liminal difference in various skin regions : Forehead, lips, and cheeks . . . l/30th to l/40th Back of fore-arm, of leg, and of thigh ; ^ back of hand, and first and second - 1/lOth to l/20th phalanx of finger, &c. . .J All parts of the foot, surface of leg, and thigh ..... more than 1 /10th THE SPATIAL QUALITY OF TOUCH. DISCRIMINATION. If any part of the skin be stimulated the subject of the experiment can tell at once the exact situation of the excited spot. If two points be stinuilatod simul- taneously excitation is perceived as double, i.e. as proceeding from two points, provided the distance between the points exceeds a certain amount, varying in different parts of the body. The power of discrimination, i.e. of judging whether a stimulus is single or double, can be tested by arming the points of a pair of compasses with small pieces of cork and then seeing how far apart the points must be when pressed on the skin in order that the 492 PHYSIOLOGY stimulus may be perceived as double. The following Table represents this distance for various regions of the body : Distance in mm. Skin region mm Tip of tongue . . . 11 Volar surface of finger tip . 2-3 Dorsum of third phalanx 6-8 Palm of hand ..... 11-3 Back of hand . . . . 31-6 Back of neck ..... 54-0 Middle of back, upper arm, and thigh 67-1 When touch spots are sought out for stimulation with the points of a compass, the distance at which the excitation is perceived as double is much diminished, as is shown by the following Table of distances for the touch spots in millimetres : Skin region Distance of touch spots Volar side of finger tips 0-1 Palm of hand 0-1 Fore-arm (flexor side) . . . 0-5 Upper arm .... 0-6 Back ..... 0-4 The compass points are perceived to lie apart with a special distinctness when they are appUed to touch spots lying on difierent lines which radiate from the hair folhcles. The figures given in the first Table have no relation to touch spots, but show the average distance over which an excitation can be perceived as double. The delicacy of discrimination of any part is largely associated with its mobihty. Thus in the arm the dehcacy increases continuously from the shoulder to the finger-tip. If the localising power for touch on the shoulder be taken as 100, that of the finger-tips will be represented by 2582. In the same way there is a continuous decrease of the distances of discrimina- tion as we pass along the cheek from the ear to the hp, i.e. from the non- mobile to the mobile part. The power of discrimination is increased to a certain extent by practice and largely diminished by fatigue. Any factor which diminishes the tactile sensibility of the part, such as cold, will also diminish the power of discrimination. The fact that we can localise the point of stimulation shows that every factile sensation derived from the surface of the body, besides the qualities of intensity and extensity, has also associated with it a characteristic quality dependent on its position. This localised quality of a tactile sensation was called by Lotze ' local sign.' Among psychologists there has been much discussion as to how far this ' local sign ' is an inborn attribute of the sensa- tion of every point on the body surface, or how far it is acquired by ex- perience and based on memory of movements and muscular impressions. In the retina we have a sense-organ which, like the skin, possesses local sign, but in far higher degree, the power of discrimination of the retina being three thousand times as great as that of the most sensitive part of CUTANEOUS SENSATIONS 493 the skin. Cases of congenital cataract occur in which the subjects have been bhnd from birth. By extraction of the cataract we can give such persons the power of sight. It is found that at first there is no power of locaUsing visual impressions. The ' local sign ' is only developed in re- sponse to experience, by comparing simultaneous visual, tactile, and motor sensations. By analogy we might ascribe the local sign of cutaneous sensations to a similar causation. Our study of the spinal animal has indeed given us a physical or histological conception of local sign. We know that stimulation of any part of the body evokes an appropriate reaction, the nature of which is determined by the central connections of the entering nerve fibres. A fibre entering at one segment must therefore come into relation with a different set of motor cells from those which are set into action by a fibre entering one segment lower down. Every nerve fibre from the skin will therefore have an appropriate complex of motor paths in functional connection with its central endings, and when the activity of these reflex paths comes to be represented in consciousness it is evident that the sensation derived from each point must differ from that derived from any other point of the skin by virtue of the differing motor events actually or potentially excited from the two points. In ascribing therefore ' local sign ' to coincident muscular sensations, and to the memory and experience of past movements, we are giving but an imperfect explanation ; since the difference between the sensations from different parts, which are at the bottom of our powers of locaHsation, has its origin in the structure of the central nervous system itself and is present from the very beginning of the evolution of a reactive nervous system. PROJECTION OF TOUCH. Since the alterations in the surface of the skin which give rise to tactile sensations are habitually caused by contact with external objects, we come to regard the sensations themselves, not as changes in the skin, but as quahties of the object which touch the skin, i.e. we project the sensation. The projection is, however, not so great as in the case of visual sensations. Cutaneous sensations we always consider as quahties of an object immediately aff'ecting and altering the condition of ourselves, whereas the visual sensations are referred at once to objects lying right away from ourselves, so that we are not aware that any change has taken place in our bodies as a result of the entering of rays of fight into the eye. It is remarkable to what extent projection of touch sensation may occur. Thus a surgeon actually lengthens his fingers by using a probe. When he is probing for dead bone he feels the grating of the bone, not at his finger- tips, but he projects the sensation to the end of the probe. In the same way tactile sensations evoked by the contact of bodies with the insentient endings of hair are referred to the ends of the hairs rather than to the hair follicles where the nerve impulses actually come into being. The dependence of local sign on habitual experience is well shown by the various tactile illusions, such as the well-known experiment of Aristotle. If we cross the first and middle fingers and bring them in this 494 PHYSIOLOGY position in contact with a pea, if the eyes are shut, we should at once say that two peas lay under the fingers. This is especially marked if the pea be rolled between the fingers. The two sides of the fingers which come in contact with the pea usually touch two different objects, and these parts of the skin would have to be re-educated, i.e. their local sign would have to be changed in accordance with the changed conditions, before the pea would be perceived in its true state as single. THE PAIN SENSE When the pressure of a hard object on the skin is increased beyond that necessary to evoke a tactile sensation, at a certain pressure the quahty of sensation changes and it becomes painful. For the evolution of the race as well as for the preservation of the individual this pain sense is all- important ; it is the expression in consciousness of the reflexes of self- preservation which can be evoked in the spinal animal by stimuh which are nocuous, i.e. calculated to do actual damage to the tissues of the body. Thus when a sharp point is pressed on the skin the sensation becomes painful just before the pressure is sufficient to cause penetration. The so-called trophic lesions which occur in parts devoid of sensation are determined for the most part by the lack of the pain sense and the consequent failure of the preservative reflexes of the part. It is remarkable that pain may result from changes in organs which are devoid of ordinary sensibility. Thus the intestine may be cut, sewn, or handled without arousing any sensation whatsoever. A strong contraction of the muscular wall or increased dis- tension of the gut will, however, evoke a griping pain. In the same way the ureters, which are devoid of sensation, can give rise to excruciating agony when they are contracted firmly on a retained calculus. We are accustomed to distinguish many difierent quahties of pain, but on analysis it will be found that these quahties depend on the nature of the sense-organ which is simultaneously stimulated. Thus a burning pain denotes simultaneous stimulation of the pain sense and of the nerve- endings to the warm spots. A throbbing pain results when the vessels of the part are dilated and the part is tense with effused lymph, so that each pulse of the vessels causes an exacerbation of the painful stimula- tion and perhaps also stimulation of the tactile end-organs. The sense of pain has often been ascribed to over-maximal stimula- tion of any form of sensory nerve. Although it is true that over-stimulation of the auditory or optic nerve by a loud sound or a bright hght may be extremely unpleasant, the sensations evoked do not partake of the characters of painful sensations such as would be produced by pricking or burning the skin. Moreover a careful investigation of the sensory points on the skin brings out the fact that there are besides the tactile and temperature spots, other spots from which only painful sensations can be evoked. We have seen already that over-stimulation of a touch spot does not, as a matter of fact, cause pain. The pain spots which are distributed among the touch and temperature spots are insensitive to a low grade of stimulus. As the CUTANEOUS SENSATIONS 495 strength of the stimuhis is increased a point is suddenly reached at which the sensation evoked is painful. Moreover in parts of the body tactile and temperature sense are entirely wanting, though painful impressions can be easily evoked. The best example of this is seen in the cornea, minimal stimulation of which evokes pain, but nothing which can be regarded as a tactile sensation. The specific quahty of pain sensation is shown more- over by the fact that in many cases of disease the sense of pain may be abolished without the sense of touch. Such a patient is said to suffer from analgesia, but not anaesthesia. When pricked on an analgesic part the patient can say that he is pricked, but has no objection to any amount of repetition of the stimulus, since the sensation is entirely devoid of painful character. THE WORK OF HEAD ON CUTANEOUS SENSIBILITY In a Ions series of researches on man Head has shown that three different classes of sensations may be evoked by stimuh appUed to the surface of the body. In order to study the functions of the afferent nerves Head has investigated not only the condition of patients, the subjects of accidental division of cutaneous or other nerves, but also (in conjunction with Rivers) the effects of nerve section on himself. In the first place, it is necessary to differentiate deep sensibihty from cutaneous sensibiHty proper. After desensitisation of any given area of the skin it is still possible in this area to appreciate deep pressure and pain, and the locaHsation of the situation of the pressure is fairly accurately carried out. On the other hand, the sensations of Hght touch, as well as of temperature and the pain evoked by a light pin prick, are absent. The sensations of pressure, as well as of deep pain or pressure pain, are therefore carried by the nerves of deep sensibility. These nerves are not the cutaneous nerves, but are derived from the sensory elements in the muscular nerves. To the fingers, for instance, they run in the tendons of the muscles. Simultaneous division, as by a circular- saw cut, of the cutaneous nerves and tendons to the fingers will abolish deep as well as superficial sensibility. Deep sensibility must there- fore be classified, anatomically at any rate, with the ' organic sensations ' of muscular effort and of position, which will be dealt with in a subsequent section. Cutaneous sensibility proper Head divides into two categories, namely, protopathic and epicritic sensibility. These two forms of sensibility may be studied separately on an area of skin, which has been desensitised by section of its cutaneous nerves, during the process of regeneration of these nerves. PROTOPATHIC SENSIBILITY returns to the skin at an interval of seven to twenty-six weeks after the nerve section. At this time it is possible to appreciate in the area under investigation the sensation of pain, and to recognise roughness of an object rubbed on the skin. Localisation is still somewhat diffuse and inaccurate, so that the sensation evoked by stimulation of the protopathic area may be referred to some adjoining normal 496 PHYSIOLOGY part of the skin. The temperature sense is also present, but of a low grade. Thus heat over 38° C. and cold under 24° C. can be appreciated as such, but the intervening temperatures produce no sensation. Sensations evoked in the protopathic area are strongly endowed with what may be termed ' affective ' character. Thus painful stimulation is much more unpleasant when apphed to this area than would a similar stimulation be when apphed to a normal area of skin. In contradistinction to the deep sensibihty which is diffuse, protopathic sensibihty is distributed in spots, so that heat and cold spots, for instance, may be distinguished as on the normal skin. It is interesting that the glans penis is normally provided only with protopathic sensibihty. EPICRITIC SENSIBILITY does not return to the desensitised area until one to two years have elapsed since the division of the nerves. With its return the affective character of the protopathic sensations at once disappears and is replaced by an accurate discrimination of the nature and extent of the stimulus ; the tactile sense proper, i.e. the appreciation of the Hghtest touch apphed to the skin and its accurate locahsation, belonging entirely to the epicritic sensations. The power of discriminating the distance between two points applied to the skin simultaneously is also a function of the epicritic sensibihty. With the discriminating tactile sense returns also the power of appre- ciating fine differences of temperature, i.e. differences between 26° and 37° C. This classification may be summed as follows : Deep sensibility . . . including J Pressure sense \^ Pressure pain (Skin pain Heat over 38° C. Cold under 24° C. Tactile sense proper Pain localisation Discrimination Heat and cold between 26° and 37° C. Epicritic sensibility (accurately localised) Head and Thompson have shown that on entering the cord these various sensations imdergo a new grouping. Thus the pain impulses, which arise in and are carried by the muscular nerves, the nerves of deep sensibihty, unite with those which run in the protopathic system, so that a lesion of the cord which abolishes the sense of pain will abohsh all forms of pain, whether arising from the skin or from the underlying tissues. In the same way all temperature sensations, whether the fine ones of the epicritic system or the coarser ones of the protopathic system, run together in the cord. If the heat sense is affected by a lesion of the cord all forms and all degrees of the sensation are affected in like measure, and the same applies to the sensations of cold. CUTANEOUS SENSATIONS 497 THE HISTOLOGICAL CHARACTER OF THE ELEMENTS INVOLVED IN CUTANEOUS SENSATIONS A very large number of different forms of sensory nerve-endings have been described in relation to the skin. Their exact allocation among the different cutaneous senses presents considerable difficulties. As regards touch, two kinds of elements are probably involved. In the first place, the most sensitive tactile apparatus are the follicles of the short hairs. Around these follicles we find a sheaf of nerve fibres, some of which end in the hair papilla and others form a ring near the level of the openings of the sebaceous glands. The other tactile end- organ is Meissner's corpuscle. The distribution of these in the skin is not, however, dissimilar to that of the power of discrimination, with which they may be specially comiected. Other end-organs which are supposed to be stimulated by changes of pressure, and therefore to be tactile, are the organs of Ruffini which occur in the papillae of the palm and fingers, and, lying more deeply, the elastic tissue spindles as well as the Golgi corpuscles and the Pacinian corpuscles in the subcutaneous tissue. As regards pain, we known that in the cornea, which possesses only the pain sense, the sensory nerve- endings are in the form of branches of axis cylinders among the epithehal cells. Similar free nerve-endings occur in the epidermis all over the body, and it is therefore imagined that these have the special function of subserving the pain sense. We have at present no evidence as to the histological character of the organs by which the sensations of heat and cold are aroused. SECTION III SENSATIONS OF SMELL AND TASTE Every living organism shows a susceptibility, i.e. a power of reaction, to chemical stimuli. Thus the plasmodium of myxomycetes, placed on a strip of filter-paper of which one end is immersed in an infusion of dead leaves and the other in distilled water, will crawl along the paper towards the infusion of leaves. If the infusion of dead leaves be replaced by a weak solution of quinine, the plasmodium will be repelled and will travel along towards the vessel of water. These movements of attraction and repulsion are spoken of as positive and negative chemiotaxis respectively. A similar chemical sensibility accounts for the clustering of aerobic bacteria towards the surface of a fluid, i.e. where the density of oxygen is greater, or around chlorophyll-containing algse which are giving off oxygen in the sunlight. The aggregation of leucocytes round microbes or other foreign particles in the tissues is also determined by their chemiotactic sensibihty. Chemiotaxis then represents the faculty by means of which these minute organisms are able to adapt themselves to chemical changes in their environ- ment and to react to chemical substances at a considerable distance from themselves. If we could endow these elementary organisms with con- sciousness and with a sense of their surroundings, we should have to say that they became aware of the presence of some harmful or attractive material at some distance from themselves. The sensation they received from these distant objects would be therefore a projected sensation. On the other hand, a chemical sensibihty of the body surface or part of it furnishes the criterion by which particles are accepted and ingested as food or rejected as useless or harmful. Consciousness in this case would be of something affecting and in contact with some part of the organism itself. The sensation would not be projected further than the periphery of the body. These two kinds of chemical sense — the projected and the surface sense — are found throughout almost all classes of the animal kingdom, and in the higher animals at least are known as the senses of smell and taste. The former sense in many animals attains a high degree of com- plexity and is prepotent in determining the behaviour of an animal in response to the changes in its surroundings. In the elasmobranch fishes the olfactory lobes form the greater part of the higher brain, and extirpation of them produces a loss of spontaneity and of delayed reactions similar to 498 SENSATIONS OF SMELL AND TASTE 499 .•Sfi> If4^- that which can be brought about in higher types by extirpation of the whole of the cerebral hemispheres. The sense of taste, on the other hand, is only used for sampling the nature of substances taken into the mouth and determining their ingestion or re- jection. It is therefore much simpler in its extent and more susceptible of analysis. THE SENSE OF TASTE The end-organs which subserve the function of taste are represented by the taste-buds. These are oval bodies (Fig. 244) embedded in the stratified epithelium, which occur scattered over the tongue, a few being also found on the hard palate, the anterior pillars of the fauces, the tonsils, the back of the pharynx, the larynx, and the inner surface of the cheek. On the tongue they are found chiefly in the grooves around the circumvallate papillae of man, and in the grooves of the papillse foliatse of rabbits. A few are also present on many of the fungiform papillae. They consist of medullary and cortical parts, the former being composed of columnar or sustentacular cells, the latter of thin fusiform cells, the taste- cells proper. The nerve fibres concerned with taste end in arborisations among these taste- cells. The peripheral end of the fusiform cell projects as a delicate process through the orifice of the taste-bud, so that it can come in contact with the fluids contained in the cavity of the mouth. A sapid substance, to stimulate these organs, must be in solution ; hence quinine in powder is almost tasteless, owing to its shght solubility in neutral or alkaline fluids. The number of difl'erent tastes is very hmited. We distinguish four primitive taste sensations, viz. sweet, sour, bitter, and salt, some authors adding to this an alkaline taste and a metallic taste. Many substances owe their distinctive character when taken into the mouth to the fact that they stimulate not only the taste-nerves but also the nerve- endings of common sensation. Thus acids, when in weak solution, have an astringent character besides their sour taste, and if strong produce a burning sensation. The primitive taste sensations can affect one another if excited simultaneously. With weak stimulation one taste may practically annul another. Thus a dilute solution of sugar is rendered almost taste- less by the addition to it of a few grains of common salt. If the primitive taste sensations are more strongly excited we get a mixed sensation, in which the components can still be distingushed. Thus, adding sugar to lemon juice not only diminishes its acidity but produces a mixed sensa- tion, the quality of which is pleasant and in which the components, sour and sweet, can be easily distinguished. We get no such fusing of sensations Fig. 244. Two taste-buds from the tongue, e, Stratified epithelium ; p, opening or pore of taste- bud ; s, gustatory cells; st, sustentacular cells. (KOLLIKER.) 500 PHYSIOLOGY as in the eye, where a sensation of white Kght may result from stimulation of the retina by two complementary colours. Stimulation of one kind of taste- organ heightens the sensibihty of the other taste- organs. Thus after the apphcation of salt, distilled water may taste sweet. That these primitive taste sensations are served by different nerve- endings is shown by the following facts (a) The tongue is not equally sensitive at all points to all four tastes. Thus the back of the tongue is more sensitive to bitter, while the tip and sides of the tongue react more easily to sweet and sour substances. A differ- ence may be detected between even the circum vallate papillse themselves ; a mixtui'e of quinine and sugar appUed to one papiUa may excite chiefly a bitter taste, while with an adjacent papilla a sweet taste may predominate. (6) By certain drugs we can depress the sensibihty of the taste-organs, and we then find that the various tastes are affected to different degrees. Thus on painting the tongue with cocaine the first effect is a diminution of tactile and pain sensibihty, so that the application of acid evokes a very sour taste without any of the astringent or stinging sensations normally aroused by the contact with the acid. After this point the taste sensations are also abohshed. The bitter sensation disappears first, then the sweet, and then the sour, while the taste of salt appears to remain unaffected. On the other hand, if the leaves of Gymnema sylvestre be chewed, the sensations of bitter and sweet are abohshed, leaving intact the acid and salt tastes, and also the general sensibility of the mucous membrane. There is no doubt that the stimulating effect of any chemical substance on the taste-nerves has relation to its chemical constitution. Thus a sour taste is determined by the presence of H ions ; the alkaUne taste by that of OH ions. The fact that certain acids, e.g. acetic, have a stronger sour taste than would correspond to their dissociation, i.e. to the number of H ions present, is due to the fact that these acids penetrate more easily into the gustatory cells than the mineral acids with a larger dissociation co- efficient. All the a-amino-acids have a sweet taste. On the other hand, the polypeptides produced by the combination of these amino-acids, as well as the peptones derived from the hydrolysis of proteins, have a bitter taste. Most of the alcohols and sugars have a sweet taste, while the metalhc derivatives of these substances are bitter. We do not yet understand the law which determines whether any given substance shall have a taste at all, and what its taste should be. The nerves of taste are the glossopharyngeal, which suppKes the back part of the tongue, and the lingual branch of the fifth nerve and the chorda tympani, which supply the front part. All these fibres are probably con- nected with a continuous column of grey matter in the brain stem, which represents the splanchnic afferent nucleus of the fifth nerve, the nervus intermedins, and the glossopharyngeal. Some authors have stated that all the taste fibres of the fifth nerve are derived from the glossopharyngeal by the communication through the tympanic plexus and the chorda tympani nerve, while Gowers has recorded a case of complete unilateral loss of taste SENSATIONS OF SMELL AND TASTE 501 in which there was a lesion destroying the fifth nerve, the glossopharyngeal being intact. It seems possible that the actual region of the taste-nerve may vary, the fibres running to the splanchnic column of grey matter being contained sometimes in the fifth, sometimes in the glossopharyngeal, and sometimes in both. Most of our so-called tastes should rather be designated flavours, and are dependent, not on the gustatory nerves, biit on the sense of smell. Gasserid^n Ganglion Fig. 245. Diagram showing origin and course of the nerve fibres of taste. When the olfactory sense is destroyed very httle difference is to be perceived between an onion and an apple. The epicure with a fine palate has really educated his sense of smell and would be but httle satisfied with the simple sensations derived from his four sets of gustatory end- organs. THE SENSE OF SMELL The psychical analysis of olfactory sensations is rendered difficult by the fact that this sense in man plays but a small part in his usual adapta- tions. We have thus to deal with a sense which is in many respects vestigial. We see traces of great complexity in its possibilities of performance, but are baffled in our endeavours to reduce the whole of the phenomena to the simpler factors of which they are composed. Moreover, like all vestigial func- tions, the extent to which the sense is developed varies from one individual to anotlier. Many, for instance, are unable to appreciate the smell of vanilla, of hydrocyanic acid, or of violets. On the other hand, in animals, such as the dog, the olfactory sense seems to play a great part in determining behaviour, and the nervous associations, which are the physiological basis of ideas, must in these animals be largely connected ^^^til olfactory im- pressions. Another factor which diminishes the importance of olfactory sensations in man is the ease with which the sense-organ becomes fatigued. It often happens that the inmates of a room are perfectly comfortable 502 PHYSIOLOGY and may perceive no fault in the ventilation, although a new-comer from the outside at once remarks that the air is foul. The organ of smell is situated at the upper part of the nasal cavities. Here the mucous membrane covering the superior and middle turbinate bones and the corresponding part of the septum is different from that covering the rest of the nasal passages. Over the lower parts of the nasal cavities the mucous membrane is of the ordinary respiratory type, and is composed of cihated columnar epithehum containing a number of goblet- cells. In the olfactory part the epithehum is much thicker, of a yellow colour, and apparently composed of a layer of columnar cells resting on several layers of nuclei. These nuclei belong to the olfactory cells proper, true spindle-shaped nerve-cells with one process extending towards the mucus covering the free surface, while the other is continued along channels in the bone, and through the cribriform plate as one of the non-medullated olfactory nerve fibres. These nerve fibres dip into the olfactory lobes, where they terminate by a much-branched arborisation or end basket in the so-called olfactory glomeruh, in close connection with a similarly branched dendrite of the large ' mitral ' cells of the olfactory lobe. The axons from these latter carry the olfactory impulse towards the rest of the brain. In the connective tissue basis (dermis) of the mucous membrane are a number of small mucous or serous glands (Bowman's glands) whose office it is to keep the surface of the membrane constantly moist. In ordinary respiration the stream of air never passes higher than the anterior inferior border of the superior turbinate bone, so that it does not come in contact with the olfactory mucous membrane. The sensations of smell which are aroused during ordinary respiration depend on diffusion from the respiratory air into the still air of the upper olfactory portion of the nasal cavity. The direction of olfactory attention is achieved by sniffing ; in this act the nostrils are dilated and the direction of the anterior part of the nasal respiratory chamber altered, so that the stream of entering air is directed towards the upper olfactory portion of the cavity. The fact that we are able to perceive smells when breathing normally shows that the odorous substance must be diffusible, i.e. gaseous in form. The amount of substance necessary to excite sensation is extremely minute. Thus -01 mg. of mercaptan diffused in 230 cubic metres of air is still distinctly perceptible. In this case a htre of air would contain only -00000004 mg. of the substance, and the amount actually in contact with the olfactory epithehum would be still smaller. It is possible, however, to show the presence of these odorous substances in air by physical means. Tyndall pointed out that air containing a small proportion of odorous substances absorbed radiant heat to a much greater degree than did pure air. Thus in one experiment air containing patchouli absorbed radiant heat thirty- two times as strongly as the pure air. Most odorous substances possess large molecules and have therefore high vapour densities. On this account the smell tends to hang about objects, the rate of diffusion of the vapour being only small. SENSATIONS OF SMELL AND TASTE 503 Since the endings of the olfactory cells are bathed in fluid, it is evident that the odorous substances must be dissolved by this fluid before they can excite the olfactory nerve fibres, and in the case of aquatic animals we know that the projected chemical sense, which we call smell, can only be aroused by substances in solution. It is difficult to show in man that the nerve- endings can be excited by solutions. Most of the experiments have been made with solutions which had an injurious effect upon the oKactory epithelium. According to Arousohn it is possible to excite sensations of smell if the nasal cavity be filled with normal sahne fluid, containing a very small proportion of the odorous substance. To this experiment it has been objected that it is almost impossible to fill the nasal cavities without leaving some air spaces so that the olfactory sensation obtained might have been due to stimulation of the olfactory cells in such a space. There is, however, no a -priori reason to deny the probabihty of Aronsohn's conclusions. Many olfactory stimuh owe their pecuhar character to the simultaneous stimulation of other kinds of nerve- endings. Thus a pungent smell, as that of ammonia, chlorine, &c., involves stimulation of the nerves of common sensibiUty, i.e. the fifth nerve, besides stimulation of the olfactory nerve. No satisfactory classification of smells has yet been made. The follow- ing facts tend to show that there are a number of primitive sensations of smell, as of other sensations : Fig. 246. Zwaardemaker's [a) Certain individuals, whose olfac- olfactometer. tory sense is in other respects normal, ' have no power of distinguishing some odours. (6) The olfactory sense is easily fatigued. If it be fatigued so as to be absolutely insensitive for one kind of smell, it is still normally excitable for other smells. (c) It is possible by mixing odoriferous substances in certain proportions to annul their eft'ect on the olfactory organ. Thus 4 grm. of iodoform in 200 grm. of Peruvian balsam is almost odourless, and the same neutralisa- tion of odours is obtained if the odour of each substance be allowed to act separately on each side by tubes inserted into each nostril. For this purpose we may use the in3trument invented by Zwaardeinaker, called the olfactometer. This consists of a porous cylinder into which is inserted a tube. The porous cylinder is first immersed in the fluid whose porous qualities are to be tested, and when it is thoroughly soaked it is taken out, dried outside by a cloth, and indde by drawing air through it for a short time. One end of the bent tube is then inserted into the cylinder, which it must accurately lit, while the other end is placed in one nostril. The small wooden screen shown in Fig. 246 serves to shut off the smell of the fluid from the other nostril. When the observer breathes through the bent tuba the amount of vapour taken up from the cylinder will depend on the amount 504 PHYSIOLOGY of surface exposed, and therefore can be diminished or increased by pusliing the bent tube further in, or by drawing it out. If the tube is pushed in so far that the smell is only just perceptible the length of the tube may be measured and taken as the liminal intensity of stimulus for the given substances, in its action on the olfactory nerve- endings. This unit was called by the inventor of the instrument an ' olfactie.' By this means it is possible to make quantitative estimations of the olfactory sense on one individual and to compare them with observations made on other individuals. By using two such instruments it is possible to present different smells to the two nostrils. One obtains in this way combination effects which can be compared to the phenomenon which we shall study later in dealing with binocular contrast. SECTION IV AUDITORY SENSATIONS By means of our auditory sensations we are made aware of such changes in our environments as are capable of giving rise to a disturbance which can be propagated through the surrounding elastic medium, the air, to our ears. Any sudden jar given to a sohd body sets up vibrations which are propagated to the surrounding air as sound waves, i.e. a series of acts of condensation and rarefaction spreading out from the centre of disturbance, like the waves which are caused on the surface of a pond by throwing a stone into its middle. With a dehcate tambour we can record these changes of pressure and convert them, by means of a lever writing on a blackened sur- face, into movements at right angles to the direction of movement of the surface. The ampHtude of vibration of the membrane will be proportional to the amount of compression and expansion occurring at each wave. These waves travel through the air at the rate of 1100 feet per second, their wave length varying with the number of vibrations per second. It is of course possible to get vibrations of almost any number per second. Only when the number of vibrations fall within distinct limits are they effective in producing a sensation of sound. Before we can discuss the physiological mechanism of hearing we must have a clear idea of the character of the physical change — the stimulus — which is effective in evoking an auditory sensation and determining its quality. Sounds may be divided into noises and musical tones. If the vibrations or series of vibrations arriving at the ear are irregular in character, such as those produced by striking the table or the floor with a stick, we speak of the resultant sensation as a noise. If, on the other hand, the vibrations follow one another at a regular sequence and possess a rhythm — if, for instance, a series of vibrations be imparted to the air by a tuning-fork vibra- ting at 100 times per second — the effect on consciousness is that of a musical tone. Of course there is no hard-and-fast line between the two kinds of sound ; even when the prevalent impression is that of a noise it is often possible to pick out some series of vibrations which predominate among the irregular ones with which they are accompanied. When we strike a single stick with a hammer the effect is that of a noise. If, however, we take a series of sticks of different lengths and strike them in succession, it will be noticed that the sound produced by each stick corresponds to a distinct note, and tmies may be played on such a collection of sticks. On the other hand, 505 506 PHYSIOLOGY the tone of a musical instrument may be so harsh that there is very httle difference between it and a noise. In a musical tone we can distinguish various characters or quahties : (1) The loudness of a tone is determined by the amplitude of the vibra- tions of which it is composed. If a violin string be bowed forcibly the excursion of its string at each vibration is greater than when it is bowed gently, and the amphtude of the corresponding alternating waves of sound varies in proportion to that of the vibrating body by which they are started. By attaching a pointed shp of paper to the end of a tuning-fork and so record- ing its vibrations on a blackened surface, it is easy to see the connection which exists between the amphtude of vibrations and the loudness of the sound produced by the vibrating fork. (2) The pitch of a musical tone depends on the frequency of the vibrations of which it is composed. By means of a siren we can determine the number of vibrations corresponding to each note. As the speed of revolution of the siren is increased, and therefore the number of interruptions per second of the stream of air passing through the holes in its disc, the note appears to us to rise continuously. If w^e take two tuning-forks, one vibrating at 100 times per second and the other vibrating 200 times per second, the pitch of the latter is observed to be considerably higher than that of the former. In fact it forms the octave. If the number of vibrations is less than about thirty per second no musical tone is produced, the individual vibrations being perceived as a series of pulses in the surrounding air, and it is only when we increase the number to aboutf orty per second that we are able to appreciate the pitch of the note produced. As the number of vibrations per second is increased the note rises steadily without break till we arrive at 40,000 to 50,000 vibrations per second. Above this number of vibrations the human ear is incapable of perceiving any note at all, though it is probable that small animals can perceive notes still higher in the scale. In music neither the lowest nor the highest tones are used. The lowest tones of the large organs, that of the sixty- four foot pipe, is 16 vibrations per second, and one can hardly speak of its effect as that of a musical tone. The highest notes employed in music are a4 and c5 with 3520 and 4224 vibrations per second on the piano, and d5 with 4752 vibrations on the piccolo flute. In music therefore we only employ between 40 and 4700 vibrations per second, i.e. about seven octaves. The manner of construction of the musical scale will be dealt with later. (3) Timbre or quality of musical sounds. When the same note is sounded on different instruments, i.e. tuning-fork, violin, piano, trumpet, human voice, every person, whether he has an educated musical ear or not, can say at once what kind of instrument is being used. This fact shows that the sound wave produced by these instruments must differ, altogether apart from any differences in amplitude or in number of vibrations per second, and if the sound waves produced by these instruments be recorded an actual difference is found in the shape of the curve. AUDITORY SENSATIONS 507 If a stretched wire be plucked so as to set it into transverse vibrations it will give out a certain note, dependent on its length, its thickness, and the tension to which it is subjected. If its length be halved it will give out a note which is of double the number of vibrations per second. If only one- third of the wire be set into vibrations the sound wave produced will have three times the number of vibrations of that of the whole string. When the string is free to vibrate as a whole the segments of it tend to vibrate even while the whole string is vibrating. If therefore we take the note given out by the whole string, the ' fundamental tone,' as corresponding to 132 vibra- tions per second, there will also be a series of notes superadded to the funda- mental tone with vibrations per second in the ratio of 1, 2, 3, 4, 5, and 6, &c. Thus if the fundamental tone be c, the overtones, or harmonics, will be produced as is shown below : 123456789 10 Vibrations per Second 132 2 X 132 3 >; 132 4 x 132 5 X 132 6 x 132 7 x 132 8 x 132 9 x 132 10 x 132 Nearly all musical instruments, as well as the apparatus for producing the human voice, resemble a stretched wire in giving out overtones in addition to the fundamental tones, and the difference in the quality of various instruments is chiefly determined by the varying predominance of the different overtones. In some the higher overtones may be most marked, in others only the lower overtones. The tuning-fork is practically the only instrument the note of which is pure, i.e. free from harmonics or overtones. It must be remembered that these different tones arrive at the external ear simultaneously. We do not have some particles of air vibrating at one rate and other particles at another rate, but all the simple vibrations of which the component tone is composed are combined together to form a compound wave, the shape of which differs according to the constituent vibrations of which it is made up. Thus in the diagram (Fig. 247) the wave shown by the continuous line is compounded of the series of simple vibrations represented by the different dotted lines. The components of such a compound curve can be detected if the sound be analysed by allowing it to act on resonators, i.e. on instruments which can be set into vibration by certain simple tones of which the com- ponent tone is made up. The stretched strings of a piano may be used as a battery of such resonators. If the dampers of the strings be raised, by depressing the loud pedal, and a note be then sounded into the piano, it may be noticed that the piano gives back the sound, and on attaching pieces of straw to the various strings it will be seen that only certain straws vibrate, i.e. those on the strings which are vibrating to the fundamental tone or overtones contained in the sound received by the piano. For the analysis 508 PHYSIOLOGY of sounds the resonators devised by Helmholtz are generally employed. Ttese consist of hollow vessels, with an opening at one end, made of different Fig. 247. d, a compound sound wave, which may be analysed into a, the fundamental tone, and h and t, the first and second overtones. (Hensen.) sizes, so that each will resound only to a definite number of vibrations per second. BEATS AND DISSONANCE. The overtones of any sound, at any rate the lower ones, are at considerable distance from one another on the musical scale, and therefore differ considerably in the number of vibrations of which they are composed. If two tuning-forks be sounded, the vibrations of which differ only by one or two per second, the phenomenon known as ' beats ' is produced. This is due to the summation or interference of the waves from the two tuning-forks. Supposing we have tuning-forks vibrating one at 100 and the other at 101 times a second, and they start vibrating together. At first the waves of compression started by each fork will coincide, so that the total compression of the air at each beat will be the compound effect of the compression produced by the two forks. The two forks therefore will reinforce one another. After the lapse of half a second the tuning-forks will be at different phases of their excursion. The 101 fork will be moving in one direction while the 100 fork is moving in the other, so that the compression produced by one fork coincides with the expansion of the air produced by the moving backwards of the other fork. The sound produced by one fork is therefore diminished by the sound produced by the other fork, and the total sound is less than either of the two forks. At the end of one second, the phases of the two forks once more corresponding, we shall get the sound increased in loudness ; thus there is an alternate waxing and waning of the sound which recurs once a second and is spoken of as a ' beat.' The number of beats per second may be used to determine the differences in the vibration frequencies of two forks. Thus two forks vibrating one at 100 and the other at 110 will give ten beats per second. As the number of beats increases the effect produced on the ear becomes more arid more dis- AUDITORY SENSATIONS 509 agreeable, just as the rapid alternation of illumination produced by a flicker- ing light is disagreeable to the eye. This objectionable character of the sound is most marked when the beats recur at about thirty-three times per second ; the individual beats are not then distinguished, but we speak of the sound as discordant or dissonant. CONSONANCE. The opposite condition of consonance or harmony in- volves therefore, in the first place, an absence of beats, i.e. of rhythmic oscillations of ampUtude of sound waves which reach the ear. The con- stituent tones and overtones must be capable of being combined into a compound wave of regular amplitude and rhythm. In the most complete consonance the component notes are identical as concerns at any rate the gi'eater number of their overtones. The most complete consonance is attained when the two notes which are sounded together are identical. Almost as complete is the consonance obtained when a note is sounded together w^th its octave. The other consonant intervals which are employed in music are as follows : 1 :2 Octave 2: .3 Fifth .3 : 4 Fourth 4: 5 Major third 5: 6 Minor third 5: 8 Minor sixth 3: 5 Major sixth It will be noticed that in all these consonant combinations the vibration frequencies of the notes are in proportion to small whole numbers. If we put down not only the fundamental tones of these notes but also their over- tones, we shall see that there is considerable identity as regards the latter. In the case of the octave the two are almost identical, the only difference being the ground tone of the lower note, and the identity diminishes as we pass from the octave through the thirds to the sixths. The overtones which are identical are shown by black type : Fand '.mental tone Octave 1:2. Fifth 2:3. Overtone I 1 . 2 . 3 . 4 . 5 . 6 . 7 . 8 . 9 . 10 12 4 6 8 10 ( 2 . 4 . 6 . 8 . 10 . 12 . 14 . IG . 18 . 20 3 6 9 12 15 18 ^ ,, 0 * I 3 . 6 . 9 . 12 . 15 . 18 . 21 . 24 . 27 . 30 ^^^''^^'■^■■^- ■[ ^ 8 12 IG 20 24 28 Major thid 4:5 .' * " § • 12 ■ 16 . 20 . 24 . 28 . 32 . 3a .40 •* ( 5 10 15 20 25 30 35 40 Minor third 5:G ' ^ ' '^^ ' ^\- ^^ " -' ' ^^ ' '"'^ ■ ^0 . 45 . 50 t G 12 18 24 30 36 42 48 rajorsixth3:5 .( ^ " « " 9 • 1^ • 15 • IS . 21 . 24 . 27 . 30 •' ^ • ' • i 5 10 15 20 25 30 510 PHYSIOLOGY Fundamental tone Second 8 : 9 Seventh 8 : 15 Overtone r 8 . 16 . 24 . 32 . 40 . 48 . 56 . 64 . 72 . 80 •\ 9 18 27 36 45 54 63 72 / 8 . 16 . 24 . 32 . 40 . 48 . 56 . 64 . 72 . 80 •\ 15 30 45 60 75 90 In the second and seventh, which are discordant, it is only the eighth overtones which are identical, while the fundamental tones will, as a rule, be so close together that beats will be produced of a number calculated to give dissonance. Since the phenomenon of beats depends on the absolute number of vibrations per second they are more easily produced by two notes near together at the lower end of the scale than at the upper end. Thus the dis- sonance is quite perceptible in a major third at the lower end of the piano, but disappears at the upper part, since here the beats produced are so rapid that they become imperceptible. The various notes used in music are obtained by employing the consonant intervals which we have given above. The major chord is composed of the fundamental tone, the major third and the fifth. If we take ' c ' as the fundamental tone, the notes of the chord are c, e, g, with vibration fre- quencies corresponding to 1, |^, |, i.e. 4, 5, 6, the major chord from g i^ g, h, d, i.e. three notes with vibration frequencies corresponding to -|, ^^-, ^, i.e. 4, 5, 6. The major chord from the fourth, /, is /, a, c, with the vibration frequencies ^, ^, ^-, i.e. 4, 5, 6. The C major scale is therefore as follows : c D E F G A B C 1 9 5 4 3 5 15 2 8 4 3 2 3 8 Different instruments are tuned to one normal note, i.e. to A with 440 vibrations per second (this note varies somewhat in different countries). Taking this as the normal, the vibration frequencies of the various notes used in music are given in the following Table : Notes Vibrations per second c . . 33 66 132 264 528 1056 2112 D 37-125 74-25 148-5 297 594 1188 2376 E 41-25 82-5 165 330 660 1320 2640 F 44 88 176 352 704 1408 2816 G 49-5 99 198 396 792 1584 3168 A 55 110 220 440 880 1760 3520 B 61-875 123-75 247-5 495 990 1980 3960 COMBINATION TONES. If two tuning-forks, with an interval of one-fifth between them, are sounded together, we may hear a weak lower tone, the pitch of which is an octave below that of the lower fork. This is known as a ' combination tone.' The combination tones are divided into two classes : AUDITORY SENSATIONS 511 (1) ' difference tones/ in which the frequency is the difference of the frequen- cies of the generating tones ; (2) ' summation tones,' which have a pitch corresponding to the sum of the vibrations of the tone of which they are composed. By means of appropriate resonators these tones can be rein- forced, showing that they have an objective existence and are not produced in the ear itself. Not only can the ear appreciate difierences between different musical instruments, dependent on the varpng overtones present in the sound pro- duced by each instrument, but, when a number of these instruments are sounded simultaneously, the ear can pick out from the compound sound the notes due to the individual instrument, and a person with a trained ear can with ease name notes composing any chord struck on an instrument such as the piano. This power of analysis, which is possessed by the ear, or at any rate by the auditory apparatus, may be stated in the form of the law, known as Ohm's law, which is as follows : " Every motion of the air which corresponds to a composite mass of musical tones is capable of being analysed into a sum of simple pendular vibra- tions, and to each single vibration corresponds a simple tone, sensible to the ear and having a pitch determined by the periodic time of the corresponding motion of the air." THE PHYSIOLOGY OF HEARING The organ of the ear may be considered as consisting of an accessory part and an essential part. The latter is formed by the terminal expansion of the auditory nerve. The accessory part is constructed so as to bring the waves of sound to act on the end organs. The ear is divided anatomically into three parts — the external ear, consisting of the pinna with the external auditory meatus ; the middle ear, consisting of the tympanic cavity ; and the internal ear, consisting of the osseous and membranous labyrinths with the terminal branches of the auditory nerve. The external ear in the lower animals is fashioned so as to collect sound waves from different directions. To this end it is provided with muscles and in many cases is very movable. In such animals the immediate response to a slight sound is a pricking up of the ears and a direction of their orifices towards the source of sound, a reflex direction of attention which in man is replaced by a conjugate deviation of the two eyes towards the side from which the sovmd comes. The collecting fmiction of the pinna in man is rudimentary ; in fact a man can hear almost as well with his ear cut off as normally. The form of the pimia in man may have a slight inttuenoe in the judgment of the direction from which sounds iiroceed. It has been noticed that a compound tone changes slightly in quality as its position in relation to the ear is altered. This is partly due to the fact that the auricle may reflect a fundamental tone more strongly than the partial or the converse. According to Rayleigh this difference in quality is determined chiefly by the fact that diffraction of the sound waves occurs as they piss round the head to the ear remote from the source of the sound, so that the partial tones reach the two ears in different degrees of intensity anil determine a difference in quality of the sound as heard b^^ the two cars. 512 PHYSIOLOGY The external auditory meatus in man is about one inch long and directed forwards, inwards, and sHghtly upwards. Its general function, other than as a mere conductor of the sound vaves, is to protect the dehcate vibrating menibrana tijmpani which closes its inner end. Opening on the skin of the meatus are special sebaceous glands which secrete a yellow wax {cerumen) with bitter taste and peculiar odour. The wax not only protects the cuticle of the ear and the membrana tympani from drpng, but, together with the hairs at the orifice of the meatus, serves to repel insects and prevent their entering. B}^ the length of the meatus moreover the drum is protected from draughts and its temperature is maintained constant. Fig. 248. Diagrammatic view of auditory organ. (Aiter Schafer.) 1, auditory nerve ; 2, internal auditory meatus ; 3, utricle ; 5, saccule ; 6, canalis media of cochlea ; 9, vestibule containing perilymph ; 12, stapes ; 13, fenestra rotunda ; 19, incus ; 18, malleus ; 17, membrana tympani ; 16, external auditory meatus ; 14, pinna of external ear ; 23, Eustachian tube. The sound waves which pass down the external meatus impinge on the drum of the ear and set this into vibration. The vibrations are thence transmitted by a chain of three small bones, the auditory ossicles, across the cavity of the tympanum to the fluid which bathes the terminations of the auditory nerve in the internal ear. Since the drum of the ear has to pick up and transmit vibrations of every frequency, and to reproduce accurately in its movement the finest variations of pressure in the course of the wave, it is essential that it should be devoid of any periodicity, i.e. a tendency to vibrate at a certain frequency. If such periodicity were present the ear would pick out and magnify some particular overtone present in the com- pound tones reaching the ear and magnify it to the exclusion of the other AUDITORY SEXSATIONS 513 overtones. The perfect aperiodicity of the tympanic membrane is secured by its structure and attachments. The membrane is composed of a thin layer of fibrous tissue covered externally with skin and internally by the mucous membrane of the tympanum. To its inner surface is attached the handle of the malleus, the first of the auditory ossicles, along its whole length. This attachment of an elastic membrane to a mass of bone would itself tend to damp any vibrations of the membrane. By the attachment of the tendon of the tensor tympani muscle to the inner surface of the handle of the malleus the middle of the membrane is drawTi inwards, so that it forms a cone whose walls are convex outwardly. The membrane is built up of circular and radial fibres, the circular being best marked towards the periphery. By the dragging inwards of its central part it follows that the tension of its constituent fibres varies from point to point so that each bit of the membrane would have a different periodicity, and the membrane as a whole is aperiodic. By exposing the tympanum from above it is possible with a micro- scope to observe the actual movements of the handle of the malleus when sound waves fall on the tympanic membrane. The maximum movements at the apex of the cone maybe taken as about '04 mm., but sounds are easily audible which would produce movements of the tympanic membrane quite imperceptible under this method of examination. On the inner side of the tympanic membrane is the tympanic cavity, which is connected in front with the pharynx by means of the Eustachian tube. This is opened by each movement of swallo\\4ng, so that the pressure in the tympanum is kept equal to that of the outside air. When the Eustachian tube is blocked or diseased the air in the tympanum is gradually absorbed and the patient becomes deaf on that side. Stretching across the tympanum, from the membrana tympani to the outer wall of the internal ear, is a chain of ossicles, which are named respectively the malleus, the incus, and the stapes. These ossicles are articulated together, so that a movement inwards of the malleus causes a movement inwards of the base of the stapes. The malleus, or hammer bone, consists of a thickened head, from which two processes nm, \"iz. the manu- brium, which is attached to the tympanic membrane, and the processus gracilis, by which it is anchored to the walls of the tpnpanic cavity. By means of three ligaments it is so fixed that it is capable only of rotating around a horizontal axis, which passes through the anterior hgament, the head of the malleus, the body of the incus, and the short process of the incus When the manubrium is pushed inwards, a part of the malleus above this axis must move outwards. The incus, sometimes known as the anvil bone, is articulated with both the stapes and the malleus, and a ligament passes from its short process to the posterior wall of the tympanic cavity. The posterior surface of the rounded head of the malleus fits into the saddle- shaped cavity on the anterior surface of the incus, while the tip of the long process of the incus is articulated with the stapes. Movement inwards or outwards of the head of the malleus causes rotation of the incus round an axis which passes from the tip of the short process through its body. Thus when the handle of the malleus moves inwards the greater part of the body of 17 514 PHYSIOLOGY the incus and of the head of the malleus move outwards together, while the long process of the incus moves inwards. The stapes, or stirrup bone, is fixed in the fenestra ovalis of the internal ear, in the inner surface of the tympanum, by the annular Mgament. It is placed almost at right angles to the long process of the incus, and therefore is pressed into the foramen ovale when this process moves inwards. The breaking up of the connection between the tympanic membrane and the foramen ovale into three bones, connected by joints, must tend to prevent any propagation of sound by direct continuity of substance. The vibrations of the membrane result in actual movements of the whole bones, which represent a chain of levers reproducing exactly the movements of the membrane. In the transmission of a sound wave from the membrana tympani to the labyrinth there is a change in the amount of force as well as in the amphtude of the movement. The three bones can be regarded as forming a lever with two arms, one of which is the manubrium of the malleus, and the other the long process of the incus. The length of the former is to that of the latter as 3 to 2, so that the movements transmitted from the tympanic membrane to the base of the stapes are diminished in the proportion of 3 to 2 and have their force increased in the proportion of 2 to 3, Moreover, as the drum of the ear has an area which is about twenty times that of the foramen ovale, the energy of its movements is concentrated on an area twenty times smaller. Hence the pressure of a sound wave acting on the tympanic membrane is increased thirty-fold ( ^ x 20) when it acts on the base of the stapes. The computation of the actual energy, involved in the movement of these structures by sound waves, which are just perceptible to the ear, yield striking results as to the extreme sensitiveness and efficiency of this apparatus. Lord Rayleigh has estimated that the amplitude of the movement of an aerial particle involved in the propagation of sound at the limits of audibility is less than one ten-milhonth of a centimetre. By other methods it has been calculated that the ear is affected by vibrations of molecules of the air not greater than -0004 mm., which is equal to 0-1 of the wave length of green light. These results show that the amounts of energy, required to influence the eye and the ear respectively, are of the same order of magnitude. Two muscles are found in the tympanum, viz. the tensor tympani and the stapedius muscles. When the tensor tympani contracts it draws the handle of the malleus inwards and so increases the tension of the tympanic mem- brane. Direct observation has shown that a contraction of this muscle occurs whenever sounds fall on the membrane, and that this reflex contraction is bilateral even when the stimulation of the ear is unilateral. The stapedius muscle tilts the base of the stapes and at the same time draws it shghtly outwards, so relaxing the tympanic membrane. It acts therefore as an antagonist to the tensor tympani. It is not yet known what exact part this muscle plays in audition. THE END-ORGANS OF HEARING The movements of the stapes are communicated to a fluid, the perilymph, and by this to the endolymph, which immediately bathes the end-organ of AUDITORY SENSATIONS 515 hearing. The internal ear consists essentially of a membranous sac, formed originally by an involution of the epithehum covering the surface of the embryo. In the course of development the sac, which is filled with the endolymph, becomes much modified in shape, forming from before back- wards the scala media of the cochlea, the saccule, the utricle, and the three semicircular canals. At certain parts of its inner surface thickenings of the epithehum occur, which become connected ^vith the terminations of the eighth nerve. The membranous labyrinth hes inside a bony case, the osseous Flu. 249. The membranous labyrinth. CM, canalis or scala media of the cochlea ; 5, saccule ; «, utricle ; sc, semi- circular canals. Fig. 250. Vertical section through the cochlea. labyrinth, from which it is separated by the perilymph. The osseous laby- rinth is formed from before backwards by the cochlea, the vestibule, and the semicircular canals. The utricle, the saccule, and the semicircular canals are concerned with the functions of equihbration. At present we have only to deal with the structure of the cochlea and the end-organs contained in the scala media. The cochlea is a spiral tube of bone 20 to 30 mm. long, divided by the scala media into two parts, viz. scala vestibuh and scala tympani, which are continuous at the apex of the spiral (hehcotrema). The essential part of the organ of hear- ing is contained in the scala media. The sound waves falling on the ear and striking the membrana tympani are transmitted with diminished amplitude but increased force by the chain of ossicles to the fenestra ovahs ; through these they are communicated to the perilymph which fills the vestibule. The vibrations travel from the vestibule to the scala vestibuli. Every rise of pressure in this canal will cause an actual movement of fluid and will push the scala media towards the scala tympani. The increased pressure thus communicated to the scala tympani causes a bulging of the membrane closing the foramen rotundum. Movement inwards of the stapes causes therefore a movement outwards of this membrane and vice versa, and the wave of pressure in passing from one aperture to the other must be connnunicated to the scala media with all the sensitive structures which it contains. The scala media is triangular in cross-section, ha\ang its apex at the spiral lamina and its base at the outer wall of the cochlea. It is separated by the membrane of Reissner from the scala vestibuli and by the basilar mem- brane from the scala tympani. The basilar membrane is composed of a number of elastic fibres, which pass in a radial direction from the spiral 516 PHYSIOLOGY lamina to the spiral crest on the outer wall of the cochlea. The length of these fibres increases from 0'041 mm. at the base of the cochlea to 0495 mm. at the helicotrema. n sp.i Fig. 251. Vertical section of the first turn of the human cochlea. (G. Retzius.) s.v, scala vestibuli ; s.t, scala tympani ; d.c, canal or duct of the cochlea ; sf.l, spiral lamina ; 7i, nerve fibres ; l.sp, spiral ligament ; sir.w, stria vascularis; s.Sf, spiral sulcus ; R, section of Reissner's membrane ; I, limbus laminae spiralis ; m.t, membrana tectoria ; iC, tunnel of Corti ; 6.m, basilar membrane ; li.i, h.e, internal and external hair-cells. The end-organ of the auditory nerve is represented by the organ of Corti, which rests on the basilar membrane (Fig. 252). It consists of a double m.t B.M Fig. 252. Section through the end-organ of. the auditory nerve in the cochlea (organ of Corti). BM, basilar membrane ; c, canal of Corti ; rc, rods of Corti ; ih and oh, inner and outer hair-cells ; so, sustentacular cells ; An, auditory nerve ; mt, membrana tectoria. row of stiff cells, the inner and outer rods of Corti, which run throughout the whole length of the scala media and are surrounded by sense epithelium, the hair-ce]]s. On the inner side of the rods of Corti there is a single row, on the outer side three rows of hair-cells. Between the hair-cells are the sustenta- cular cells, or cells of Deiters, the peripheral processes from which join AUDITORY SENSATIONS 517 together so as to form a reticulate membrane over the hair-cells, the hairs themselves projecting through orifices in the membrane. Resting on the upper surface of the membrana reticularis is the membrana tectoria. To this membrane is often ascribed a damping effect on the vibrations of the struc- tures below. Any movement of the basilar membrane would be transmitted to the rods of C'orti, and by these to the overlying hair-cells. With every vibration these would move in the line of their long axis so that their hairs would move up and down in the membrana reticularis and possibly strike against the under surface of the membrana tectoria. The fibres of the auditory nerve pass up through the column of the cochlea, through the bipolar gangUon-cells which form the spiral ganghon, and then out along grooves in the spiral lamina to end in arborisations, partly in the inner hair- cells and partly among the outer hair-cells. The complexity of the structure above described suggests that a large amount of discriminating and analysing power possessed by the ear for sounds of different qualities is determined by the differentiation of the end- organ itself. Not only are we able to appreciate differences in ampUtude and pitch of the sound waves which arrive at the ear, but we are also capable of analysing the compound sounds and determining the simple tones out of which they have been composed. This power of analysis must be due either to the presence of some battery of resonators in the end-organs of the auditory nerve, or to the existence of a large number of different nerve fibres, each of which is excited only by a distinct number of vibrations per second, or, finally, Ave must assume that the end-organ of hearing is affected as a whole and that the nerve fibres transmit to the brain the dift'erent forms of wave caused by various complex sounds, the analysis being carried out in the cerebral cortex itself. This last hypothesis, the relegation of the powers of analysis to the cerebral cortex, is, at the present time at any rate, equivalent to giving up any attempt to explain the power of analysis possessed by the organ of hearing. On the other hand, the complex structure of the organ of Corti suggests that here we have an actual battery of resona- tors, by means of which sense waves are analysed into their components. This is Helmholtz's theory of the function of the cochlea. It assumes that in the organ of Corti there are vibrating structures tuned to frequencies within the limits of hearing, viz. from 30 vibrations to about 4000 vibrations per second. We can distinguish notes in the middle of the musical scale which differ from one another only by 0-3 to 0-5 vibration per second. Within the limits of this scale we should therefore require about 4200 resonators in the ear. In order to account for the sensitiveness of the ear to sounds below 40 vibrations and above 4000 per second, we might allow another 300 vibrators, so that 4500 different resonators would be necessary altogether. Helmholtz at first thought that these resonators were represented by the arches of Corti, but on Hensen pointing out that the basilar membrane was composed of fibres varying from 0*041 to 0-495 nun. in length, he concluded that it was probably the breadth of the basilar membrane which deternu'ned the tuning to any particular note. This membrane from its structure behaves like 518 PHYSIOLOGY a system of stretched strings bound together by a semi-fluid substance. The parts of the membrane near the fenestra rotunda would be adapted for the higher notes, while those near the helicotrema would vibrate to deep tones. Each fibre would, by its vibration, set the overlying part of the organ of Corti into vibration, so that each nerve fibre composing the auditory nerve would be stimulated by a different note. Miiller's law of specific irritabihty is thus observed, each nerve fibre transmitting an impulse which excites one quahty, and only one quahty, of sensation. When a compound wave falls on the organ of Corti, it is actually resolved into its simple component waves by the fibres of the basilar membrane, and we therefore get stimulation of a number of nerve fibres and a sensation produced which is a true mixed sensa- tion compounded of a number of simple tone sensations. By an effort of attention therefore it is possible to pick out from a mixed sensation its different components, and in this way we may explain the analytic powers of the ear. Certain observations on fatigue of the auditory fibres support the notion that each fibre reacts to notes of one pitch, and only to these notes. If the vibrations of a tuning-fork be conducted by two ' telephones to both ears the sound appears to come from somewhere in front of the middle fine of the body. If the sound be transmitted to one ear for some time so as to produce a condition of shght fatigue, and then the other telephone be held up to the other unfatigued ear, the sound is at once referred to the unfatigued side. In this locahsed projection of the sound waves we have a simple means of judging of the presence or absence of the apparatus in one ear or the other. It was pointed out by Bonders that, when one ear was fatigued by a note of 360 vibrations per second, and immediately afterwards a note of 365 vibra- tions per second was conducted equally to both ears, no trace was per- ceptible of fatigue, the sound being located exactly in the median line. We are therefore justified in concluding that the end-organ of the nerve fibre which carries the nerve impulses corresponding to a vibration frequency of 360 per second is not the same as the nerve fibre or end apparatus which evokes the sensation corresponding to a note of 365 vibrations per second. If this theory is correct, destruction of the lower part of the cochlea should abohsh the power of appreciation of high notes, while damage to the region of the hehcotrema should impair sensibility to low notes. Certain results of experiments on dogs afford confirmation of this view, although all such results involving judgment of the powers of appreciation of high or low sounds respectively possessed by animals must be received with caution. A few isolated cases have been recorded in man in which atrophy of the nerve fibres supplying the lower whorl of the cochlea was attended by total want of perception of high tones. According to Rutherford, the whole of the basilar membrane, and therefore of the auditory hairs, vibrates equally to every note, just as the plate of a telephone receiver reproduces faithfully the shape of the vibrations impinging on the transmitter of the telephone. This ' telephone theory,' as it has been called, relegates the whole work of analysis to the central nervous system, and gives us no clue as to how such an act of analysis may take place. One would imagine that a much simpler apparatus than AUDITORY SENSATIONS 519 that represented by the organ of Corti would be sufficient, if no analysis of the stimulus took place in the peripheral organ. When a bow is drawn against the edge of a plate the vibrations afifect different parts of the plate unequally, so that lycopodium powder sprinkled on such a plate assumes a complicated pattern. Waller suggests that the basilar membrane vibrates as a whole to every tone, but that it presents nodal and internodal points, like the vibrating plate. Since the hair-cells move with the basilar membrane they produce what may be called ' pressure patterns ' against the membrana tectoria, so that different combinations of nerve fibres are stimulated according to the pattern, i.e. according to the shape of the compound wave. A somewhat similar hjrpothesis has been put forward by Ewald, but neither of these theories presents any advantages over the resonator theory of Helmholtz, nor does it account satisfactorily, as the Helmholtz theory does, for the remarkable powers we possess of analysing all kinds of complex sounds. The cochlea becomes more elaborate in structm'e as we ascend the animal scale, and there is no doubt that this elaboration attains its greatest height in man, who possesses greater powers of analysing sounds than are possessed by any other animal. SECTION V VOICE AND SPEECH The development of the analytical powers of the auditory apparatus is so closely connected with that of the faculty of speech that we may conveniently deal with the latter at this point rather than relegate it to a chapter on special muscular mechanisms. We may first consider the mechanism of production of voice, which man shares with many other animals, before discussing the mechanism of the wholly human faculty of speech. Voice is produced in the larynx, a modified portion of the wind-pipe, by the vibrations of two elastic bands, the vocal cords, which are set into action by an expiratory current of air from the lungs. In many respects the larynx resembles a reed instrument, in which a current of air is caused to vibrate by the vibrations of an elastic tongue. Whereas, however, the period of the vibrations in such an instrument, and therefore the note, is deter- mined by the length of the tube which is attached to the reed, in the larynx the note produced by the blast of air is modified partly by alterations in the tension of the vocal cord, and partly by varjang the strength of the blast of air. ANATOMICAL MECHANISM OF THE LARYNX. The essentia] framework of the larynx is formed by four cartilages, viz. the cricoid, the thjrroid, and the two arytenoid cartilages. The cricoid cartilage, which lies immediately over the upper- most ring of the trachea, is shaped like a signet ring, the small narrow part being directed forwards and the broad plate backwards. The thyroid cartilage consists of two parts or alse, joined together in front and forming the prominence known as Adam's apple ; beliind, it presents four processes or cornua, the superior of which are attached by ligaments to the hyoid bone, while the inferior cornua articulate with the postero- lateral portion of the cricoid cartilage. By means of this articulation very free move- ment is permitted between the two cartilages, the general direction of movement being one of rotation of the cricoid cartilage on the thyroid, round a horizontal axis directly through the two articular siu'faces between the two cartilages, while movements of the thyroid upon the cricoid are also possible in the upward, downward, forward, and back- ward directions. The two arytenoid cartilages are pyramidal in shape. By their bases they articulate at some distance from the middle line with convex articular surfaces .situated in the upper margin of the plate of the cricoid cartilage. The anterior angle of the base is the vocal process, wliile the external angle is the muscular process of the arytenoid. The crico-arytenoid joints permit of two kinds of movements of the arytenoid cartilages, viz. : (1) Rotation on their base around their vertical long axis, so that the anterior vocal process is rotated outwards and the muscular process backwards and inwards or conversely. 620 VOICE AND SPEECH 521 (2) Sliding movements of the whole arytenoid cartilage either outwards or inwards, so that their inner margins may be drawTi apart or approximated. The larynx is covered internally by a mucous membrane continuous with that of the trachea. It is lined with ciliated epithelium, except over the vocal cords, where the epithelium is stratified. The two vocal cords, or thyro-arytenoid ligaments, con- sist of elastic fibres which rim from the middle of the inner angle of the thjToid carti- lage to be inserted into the anterior angle of the arytenoid cartilages. Their length in man is about 15 mm., in woman about 11 mm. The cleft between them is known as the glottis, or rima glottidis. Two ridges of mucous membrane above and parallel to the vocal cords are the false vocal cords (Fig. 253). Between the true and the false vocal cords on each side is a recess known as the ventricle of Morgagni. This ventricle permits the free vibration of the vocal cords. The false cords take no part in phonation, but help to keep the true cords moistened by the secretion of the numerous mucous glands with which they are provided. The position and tension of the vocal cords are determined by the action of the various intrinsic muscles of the larynx. The part taken by the various muscles in each movement cannot be directly ascertained. We can in most cases only study the direction of the fibres, and judge, from this direction and consequent isolated action of the muscles, the part taken by any given muscle in the production of voice. The chief muscles (Fig. 254) are as follows : (1) The crico-thyroid muscle is a short triangular muscle attached below to the cricoid cartilage and above to the inferior border of the thyroid carti- lage ; the fibres pass from below upwards and backwards. When this muscle contracts, the cricoid cartilage is drawn up under the anterior part of the thyroid cartilage so that its broad expansion behind with the arytenoid cartilages, is drawn downwards and backwards, thus putting the vocal cords on the stretch. This muscle is probably the most important in determining the teasion of the vocal cord. (2) The posterior crico-arytenoid muscle arises from a broad depression on the corresponding half of the posterior sm'face of the cricoid cartilage its fibres converging, to be inserted into the outer angle of the arytenoid cartilage. These muscles rotate the outer angle of the arytenoid cartilages backwards and inwards. They thus cause a movement outwards of the anterior angles, so that the glottis is widened. During every act of inspiration there is a widening of the glottis, which is probably effected by contraction of these muscles. If they are paralysed the vocal cords are approximated and tend to come together during inspiration, so that dyspncea may be produced. (3) The lateral crico-arytenoid muscle arises from the upper border of the cricoid cartilage and passes backwards to be inserted into the muscular process of the arytenoid cartilage. These muscles when they contract pull the muscular process of the arytenoid cartilage forwards and dowaiwards, thus approximating the vocal cords at their posterior ends and antagonising the action of the jxjsterior crico-arytenoid muscles. (4) The ari/tenoid muscles consist of transver.se fibres, .some of which decussate, 17* Fig. 253. Anterior half of the larynx, .seen from behind. The section on the right side is somewhat in front of the left side. e, epiglottis ; e', cushion of epi- glottis ; t, thyroid cartilage ; s, s', ventricle of larjnx ; h, great coruu of hyoid bone ; t a, thjTO-arytenoid muscle; t'/, vocal cords. Above the ventricles are the false vocal cords, r, first ring of trachea. (A. Thomson.) It passes upwards and outwards. 522 PHYSIOLOGY miiting the posterior sui'face of the two arytenoid cartilages. When they contract they draw the arytenoid cartilages together. (5) The thyro-arytenoid muscles consist of two portions. The outer fibres rise in front from the thyroid cartilage and pass backwards to be inserted into the lateral border and the muscular process of the arytenoid cartilage. Some of the fibres pass obliquely upwards towards the aryteno-epiglottidean folds. These are often spoken of as a separate muscle, the thyro-epiglottidean. By their action they tend to draw Fig. 254. Muscles of the larynx. (Sappey.) A, as shown in a view of the larynx from the right side. 1, hyoid bone ; 2, 3, its cornua ; 4, right ala of thyroid cartilage ; 5, posterior part of the same separated by oblique line from anterior part ; 6, 7, superior and inferior tubercles at ends of oblique line ; 8, upper cornu of thyroid ; 9, thyro-hyoid ligament ; 10, cartilago triticea ; 11, lower cornu of thyroid, articulating with the cricoid; 12, anterior jDart of cricoid ; 13, crico-thyroid membrane ; 14, crico-thyroid muscle ; 15, posterior crico- arytenoid muscle, partly hidden by thyroid cartilage. B, as seen in a view of the larynx from behind. 1, posterior crico-arytenoid ; 2, arytenoid muscle ; 3, 4, oblique fibres passing around the edge of the arytenoid cartilage to join the thyro-arytenoid, and to form the aryteno- epiglottic, 5. the arytenoid cartilages forwards and to relax the vocal cords. The upper fibres may also assist in depressing the epiglottis. The inner fibres are called the musculus vocalis. They arise from the lower half of the angle of the thjrroid cartilage, and passing back- wards in the vocal cords are attached to the vocal processes and to the adjacent parts of the outer surfaces of the arytenoid cartilages. Many fibres do not run the whole distance, but end in an attachment to some part of the vocal cord. Although their action must be to draw the arytenoid cartilages forwards, yet, since they are contained in the vibrating portion of the vocal cords, they cannot by their contraction relax these cords. It is probable that they play a great part in determining the tension of the vocal cords after these have been put on the stretch by the action of the crico- thyroid muscles. They may possibly act as a sort of fine adjustment of the tension, the coarse adjustment being represented by the crico-thyroids. VOICE AND SPEECH :m THE PRODUCTION OF VOICE In order to study the changes in the larynx which are associated with voice production we must make use of the laryngoscope. The principle of this instrument is very simple. A large concave mirror with a central aperture is fixed before one eye of the observer, sitting in front of the patient or person to be observed. The latter is directed to throw his head slightly backwards and to open his mouth. In order to keep the tongue out of the way the patient is made to hold the end of it by means of a towel. The mirror is then so arranged as to reflect light from a lamp into the cavity of the mouth. A small mirrer fixed in a handle is then warmed, so as to prevent the condensation of the patient's breath, and passed to the back of the mouth until it rests upon and shghtly raises the base of the uvula. By this mirror the light reflected into the mouth from the large mirror is again reflected down on to the larynx, and a reflection of the larynx and trachea is seen in the mirror. By laryngoscopic examination we can see the base of the tongue, behind which is the outhne of the epiglottis. Behind this again in the middle line are seen the two vocal cords, white and shining (Fig. 255). The cords appear to approximate posteriorly ; between them is a narrow chink, the diameter of which varies with each respiration, being wider during inspiration. On each side of the true vocal cords are seen the pink false vocal cords. In some cases the rings of the trachea, and even the bifurcation of the trachea itself (Fig. 255, c), may be seen in the interval between the vocal cords. In order that the vocal cords may be set into vibration, they must be put into a state of tension and the aperture of the glottis narrowed, so as to afford resistance to the current of air. In the dead larynx it is possible to produce sounds by forcing air from bellows through the trachea, after the vocal cords have been put on the stretch by pulling the arytenoid cartilages backwards. By experimenting on patients on whom tracheotomy has been performed, it has been found that the pressure of air in the trachea, necessary to cause production of voice, is, for a tone of ordinary loudness and pitch, between 140 and 240 mm. of water, and with loud shouting the pressure rises to as much as 945 mm. of water. This pressure is furnished by the contraction of the expiratory muscles, i.e. of the abdomen and of the thorax. Since the pitch of the note produced rises with increasing force of the blast, while the tension of the cords remains constant, it is evident that, in the act of ' swelhng ' on a note, the increased pressure necessary for the crescendo must be associated with diminishing tension of the cords. It is the failure to secure this nuiscular relaxation that so often causes a singer to sing sharp when swelling on any given note. The voice, like the sound produced on any musical instrument, may vary either in pitch, loudness, or in quality or timbre. The range of any individual voice is generally about two octaves. The pitch of the voice usually employed is determined chiefly by the length of the vocal cords. Thus in children the voice is high-pitched. Before and at puberty there 524 PHYSIOLOGY is a considerable development in the size of the larynx in both sexes. This is especially marked in the male, and accounts for the sudden drop in pitch (' breaking ') of the voice. In the female the increased size of the larynx is chiefly perceptible in the increase in fulness and richness of the voice which occurs at this age. Even when we take all the voices together, Fig. 255. Three laryngoscopic views of the superior aperture of the larynx and surrounding parts in different states of the glottis during life. (From Czermak. ) A, the glottis during the emission of a high note in singing. B, in easy or quiet inhalation of air. C, in the state of widest possible dilatation, as in inhaling a very deep breath. The diagrams A', B', C have been added to Czermak's figures to show in horizontal sections of the glottis the position of the vocal liga- ments and arytenoid cartilages in the three several states represented in the other figures. In all the figures so far as marked, the letters indicate the parts as follows, viz. : I, the base of the tongue ; e, the upper free part of the epiglottis ; e', the tubercle or cushion of the epiglottis ; p h, part of the anterior wall of the pharynx behind the larynx ; in the margin of the aryteno-epiglottidean fold w, the swelling of the membrane caused by the cuneiform cartilage ; s, that of the corniculum ; a, the tip of the arj^enoid cartilages ; c v, the true vocal cords or lips of the rima glottidis ; CVS, the superior or false vocal cords ; between them the ventricle of the larynx ; in C, < r is placed on the anterior wall of the receding trachea, and b indicates the commencement of the two bronchi beyond the bifurcation, which may be brought into view in this state of extreme dilatation. bass, tenor, alto, and soprano, the total range for ordinary individuals does not exceed three octaves. In singing the voice may be produced in various ways, i.e. in different registers. Thus we distinguish the chest register, the middle register, and the head register. The deeper notes of any individual voice are always produced in the chest register. Observation of the vocal cords shows that when producing such notes the glottis forms an elongated slit, all the muscles which close the glottis and increase the tension of the cords being in action. The vocal cords are relatively thick and broad and can be seen to vibrate over their whole extent. When singing with the head voice VOICE AND SPEECH 525 the vibrations of the cord are apparently confined to their inner margins ; the aperture of the glottis is wider in front than behind, so that more air escapes during phonation by this method than in the production of the chest voice. In order to change the pitch of the note the following means are probably employed in the larynx : (1) Alteration in the tension of the vocal cords. (2) Alteration in the length of the part of the vocal cords which is free to vibrate, which can be accompUshed by the approximation of the arytenoid cartilages to one another, or by their approximation to the thyroid cartilage. (3) The alteration in the shape of the vocal cords, which is determined by the activity of the different portions of the internal thyro- arytenoid muscles. (4) The varying pressure of the blast of air passing through the glottis. The loudness of the tone produced is practically proportional to the force of the blast of air employed. The quality or timbre of the voice depends not so much on the vocal cords as on the accessory resonating apparatus, represented by the trachea and chest and by the cavity of the mouth. The greater part of the education involved in voice training is directed to the modification of the shape of the mouth cavity, so as to secure the greatest possible fulness, i.e. richness in overtones, of the tone produced in the larynx. THE MECHANISM OF SPEECH The sounds employed in speech, viz. vowels and consonants, are produced by modifying the laryngeal tones by changes in the shape of the mouth and nasal cavities. In whispering speech there is no phonation at all, but the sound is produced by the issue of a blast of air through a narrow opening between the Ups, between the tongue and soft palate, or between the tongue and the teeth. The vowel sounds are continuous, whereas the consonants are produced by interruptions, more or less complete, of the outflowing air in dift'erent situations. The vowel sounds, u, o, a, e, i (pronounced oo, oh, ah, eh, ee), are tones, i.e. are produced by a regular series of vibrations. These tones take their origin in the mouth cavity, as can be shown easily by the fact that we can whisper these soimds distinctly without any phonation what- ever. To each of them corresponds one or two distinct notes, the pitch, i.e. the resonance, of which is regulated by the shape of the cavity in which they are produced. It is possible to determine these notes by means of resonators. The pronunciation even of the simplest vowel sounds differs in dift'erent individuals. For instance, those pronounced by a Londoner dift'er from those pronounced by a man from Manchester or from Yorkshire, and the French vowels differ somewhat in pitch from those employed by the German, and these again from those employed by the average English- man. 526 PHYSIOLOGY The characteristic notes were given by Helmholtz as follows : U = f 0 = b' A = b" E = f, b"' 1 = f, d'^' a] U i^^i: o E If the five vowels are whispered loudly, the gradual rise in pitch of the tone is easily perceptible. We do not in this way, however, note the lower component of the sound in the E and I ; this can be brought out by a simple device (Fig. 256). If we place the mouth in the position necessary to produce these different vowels, and then percuss over the cheek, we obtain the typical note for each vowel, the air in the mouth cavity being set into vibration by the percussion. Now shift the finger, which is to be percussed, so that it lies over the pharynx, just behind the angle of the jaw, and percuss again. The note will be observed to rise with U, 0, A, A (ah) U{oo) 1 (ee) Fig. 256. Shape of the oral cavity in the production of the vowel sounds, A, U, I. (Grutzner.) and then fall with E. I. With the three vowels U, 0, A, we have a single cavity formed by the hps, the palate, and the tongue ; this cavity is longest and narrowest with U and shortest and most open with A. With E and I the dorsum of the tongue comes up against the front part of the soft palate, so that the mouth cavity is divided into two, the anterior short narrow cavity, and the posterior broader cavity between the soft palate and the base of the tongue. We therefore have two notes produced, one in each cavity. The change in shape of the mouth cavity is shown in the figures. With U and A the cavity seems to be single ; with I the development of a pharyngeal resonating cavity is well shown. Diphthongs are produced by changing the form of the mouth cavity from that of one vowel sound to another, thus AI (the Enghsh i) = ah-ee run together and abbreviated. Consonants are sounds produced by a sudden check being placed in the course of the expiratory blast of air by closure of some part of the pharynx VOICE AND SPEECH 527 or mouth. They are classified, into labials, dentals, or gutturals, according as the check takes place at the lips, between teeth and tongue, or between back of tongue and soft palate. Each of these again can be divided into soft and hard consonants as they are accompanied or not with phonation. Thus when we pronounce D the production of the laryngeal sounds goes on during the check of the sound produced at the teeth, whereas ^^^th T there is an absolute interruption of phonation during the pronunciation of the consonant. It is thus practically impossible to make any marked difEerence between hard and soft consonants when whispering. In the production of nasal sounds such as NG- the mechanism is the same as for the production of B, D, G, except that the posterior opening of the nares is not kept shut by the soft palate, so that part of the sound comes continually through the nasal passages, when it acquires a peculiar resonance. These sounds are on this account often spoken of as ' resonants.' The aspirates are produced by the passage of a simple blast of air through a narrow opening which may be at the throat as in H, between tongue and teeth as in TH, or between lips and teeth as in PH or F. The vibratives, such as R, are formed by placing the tip of the tongue or the uvula, or the lips, in the path of the blast of air so that they are set into vibration by the blast. In Enghsh the vibrative R employed is entirely due to the tongue. The sibilants, which may be voiceless as in ' S ' or accompanied with phonation as in ' Z,' consist of continuous noises produced by a narrowing of the path of the air between the tongue and the hard palate. They are therefore similar in production to the aspirates. In the production of the sound ' L ' the tongue is applied by its edge to the alveolar process of the upper jaw, so that the air or voice escapes by two small apertures in the region of the first molar and between the inner side of the cheek and the teeth. The acoustic characters of these various consonants are stiU but imperfectly studied. VISION SECTION VI DIOPTRIC MECHANISMS OF THE EYEBALL When light falls on any object a certain proportion of it is reflected and scattered, and will affect any organism in the neighbourhood possessing sensibiUty to light. Mere sensibility of the surface to hght would not, however, suffice to arouse projected sensations, since the rays of hght from a number of different objects would interfere with one another. An animal with such sensibiUty would be aware of or be able to react to differences of hght and darkness, but could not direct its movements in accordance with the nature of the objects from which the hght proceeded. For this purpose there must be not only a surface sensitive to hght impressions but also dioptric mechanisms, by means of which a real image of external objects, in their proper spatial relationships, is thrown on to an extended sensory surface. Each point in this surface will correspond to a point lying outside the body and serving as a source of hght, and the sensations evoked, since they correspond to the rays of hght coming from external objects, can be projected, and referred to the objects themselves lying outside and at some distance from the body. The organ of vision, the eye, consists of two parts, viz. : (a) The sensory surface or retina, composed of a number of areas, each of which can be separately stimulated by hght. (6) A dioptric mechanism for projecting a real image of external objects on this sensory surface. We have in fact an arrangement very analogous to a photographic camera, where a real image is thrown by a lens on to a sensitive plate, each point of which undergoes chemical change in proportion to the amount of incident hght, so that a photographic record is the result. THE FORMATION OF AN IMAGE BY A LENS We may confine our attention to the case of a bi-convex lens, and we may assume in the first place that the thickness of the lens is neghgible. In Fig. 257 c and c' are the centres of the sperical surfaces bounding the lens ; the hue joining them is the optical axis of the lens. The surface at q is parallel to the surface at r, so that a ray of hght, such as pq falhng on the 528 DIOPTEIC MECHANISMS OF THE EYEBALL 529 lens at Q, will leave the lens at r in direction rs parallel to pq. The point o where the ray cuts the optical axis is known as the optical centre of the lens. This optical centre in a bi-convex lens lies within the lens, its distance from the two surfaces being practically as their radii. Fig. 257. Since we are neglecting the thickness of the lens the line pqrs may be regarded as straight, so that we may say that the rays which pass through the centre of a lens do not deviate. If a pencil of parallel rays falls upon the lens, while those rays which pass through the optic centre undergo no deviation, all the others on leaving the lens wiU be convergent towards a point which is known as the principal focus of the lens (Fig. 258). Con- Fiu. 258. Diagram of the course of parallel rays through a bi-couvex Icus by which they are converged to the principal focus, f. Fig. 259. The rays of light from a converge on passing through the lens to the secondar}' focus, f. f and a arc conjugate foci. versely, if a point of hght be placed at the principal focus the rays of light passing through it to the lens will take the reverse course and leave the lens as a bundle of parallel rays. Any point of light situated between infinite distance and the principal focus Avill have a corresponding point on the other side of the lens to which its rays will converge. Such corre- sponding points are known as a conjugate foci (Fig. 259). In a thin lens, 530 PHYSIOLOGY with the same media on each side, the anterior and posterior focal distances are the same, so that from which ever direction a parallel beam falls on the lens the point to which its rays are converged on the other side of the lens is constant. . If instead of a point of hght we have a series of points such as that coming from a bright hne in the plane of the paper (as in Fig. 260), each of these points will have a corresponding point on the opposite side of the lens. Thus from the point p three rays may be taken, viz. : Fig. 260. (1) The ray po, which passes through the centre of the lens and does not deviate. (2) The ray pe, which is parallel to the axis and therefore is converged to the principal focus (j)^. (3) The ray pg, which passes through the focus on the front surface of the lens 0^ and therefore takes a course on the other side of the lens parallel to the axis. The intersection of any two of these three hues will be the situation of the image p to the point p. In the same way all the other points in the object will have a corresponding image on the opposite side, so that an inverted real image of the luminous object is formed on this side. The size of the image as compared with the object will depend on the distance of the object from the lens. It is greater than the object if the latter is less than 2/ (twice the focal distance) from the lens, equal if the distance is 2/, and diminished if the distance is greater than 2/. A lens with a focal distance of 1 metre is taken as unit strength. Such a lens is said to have a strength of one dioptre ; a lens of two dioptres would have a focal distance of \ metre ; of four dioptres one of |- metre, and so on. DIOPTRIC MECHANISMS OF THE EYEBALL The eye is not directly comparable to a camera in its optical arrange- ments, since the image is formed, not in air, but in the fluids of the eye itself. Moreover the conditions are complicated by the facts that it is DIOPTRIC MECHANISMS OF THE EYEBALL 531 impossible to neglect the thickness of the refractive media, and that these are many in number. If we take the simplest case, where there are only two media separated from one another by a spherical surface of contact, we can easily determine the course taken by any ray in passing from the first to the second medium. In Fig. 261 (from Landois) let L be the hrst {e.g. air) and G the second {e.g. glass). These are separated by the spherical surface ab, with its centre at m. Since all the a 1 n ffmmiiiiiiiiiiiiiiiiiiiiiiiiiiiiliiiiiiiiiiiiiiiiiiiiilii h a L g:: ;;::■;;;;: :::::i ::::::::: h 1 a h 2 n p <^^^^-^^^ ■■■ -"-"^ "|'r-:-:J::::||;;;:: :.,^, jl 2 c "'"a ^^^-^^ ■■ --■■-•■■■■■■■-■"■'^'''Itir'-^'''^'' ^ h 3 a ^— -^^^^ 3 ....-'■'■■'■"""..■■■■■'"" 1 1 3 V-'; S> ^. ..--•■'■"" 4 h c '" 2 ^^ UilllHIIIillllllllllllillllllllllllllllllllllllllllllllllH S B 0 Q Fig. 261. radii drawn from m to ah are perpendicular to the surface all rays falling in the direction of the radii must pass unrefracted through m. All rays of this sort are called rays or lines of direction ; m, as the point of intersection of all these, is called the ncdal point. The line wliich comiects m with the vertex of the spherical siirface, .r, and which is prolonged in both directions, is the optic axis, OQ. A plane (ef) in x, perpendicular to OQ, is called the principal plane, and in it x is the principal point. The following facts have been ascertained : (1 ) All rays {a to a^), which in the first mechum are parallel with each other and with the optic axis, and fall upon ab, are so refracted in the second medium that they are all again united in one point {pi) of the second medium. This is called the second principal focus. A plane in this point, perpendicular to OQ, is called the secoml focal plane (cd). (2) All rays (c to Co). which in the first medium are parallel to each other, but not parallel to OQ, reunite in a point of the second focal plane (r), where the non-refracted directive ray (c-i???.r) meets this. (In this case the angle formed by the rays c to Co with OQ must be very small.) Tlie propositions 1 and 2, of course, may be reversed ; the divergent rays proceeding from pk towards ah pass into the first medium parallel to each other, and also with the axis oq (o to 05) ; and the rays proceeding from r pass into the fii'st medium parallel to each other, but not parallel to the axis oq (as c to (•2)- (3) AH rays, which in the second medium are parallel to each other {h to 65) and with the axis oq, reunite in a ])oint in the first medium {p) called the first focal point ; of course, the converse of this is true. A plane in this point pcrj)endicular to oq is called the first focal plane (ab). The radius of the refractive surface (»i:r) is equal to tlie difi'erenee of the distance of both focal points (;> and /)i) from the princi})al focus (;r) ; tluis mx = piX — px. In compound systems composed of several refractive media witli splierieal surfaces of contact, such as the eye. we may proceed from medium to medium with the same methods as those just described. Since, however, sucli a procedure would be very tedious, the method first ^jroposed by Gauss is usually adopted. Gauss showed that if the several media are ' centred ' — i.e. if all have the same optic axis — then the refractive indices of such a centred system may be represented by two equally strong refractive 532 PHYSIOLOaY surfaces at a certain distance apart. The rays falling upon the first surface are not refracted from it, but are regarded as projected forwards parallel with themselves to the second surface. Refraction is then considered to take place at the second surface just as if that were the only surface present (represented by the dotted line II in Fig. 262). Fig. 262. The position of the cardinal points in the schematic eye. (Helmholtz.) h„ h,,, principal points ; Ic,, k,„ nodal points ; p^„ posterior focus. All such systems possess three pairs of cardinal points, viz. two principal foci, two principal points, and two nodal points. These points may be defined by the following principles : [a) Rays which pass through the principal focus are, after refraction, parallel to the optic axis. (6) Rays which pass through the first principal point, after refraction, pass through the second. (c) Rays which pass through the first nodal point, after refraction pass through the second, and the direction of the refracted ray is parallel to the direction of the incident ray. In Fig. 262 the situation of these cardinal points is shown in the human eye. The rays of Hght pass through the following media, cornea, aqueous humour, lens, and vitreous humour, and may be refracted at the following surfaces ; anterior and posterior surfaces of the cornea, anterior and posterior surfaces of the lens. Since, however, the refractive indices of cornea, aqueous humour, and vitreous humour are practically identical, we can reduce the refractive media to two, viz. cornea, aqueous, and vitreous in the one case, and lens in the other ; and the refractive surfaces to three, viz. anterior surface of cornea, anterior surface of lens, and posterior surface of lens. In order to determine the path of the rays in the eye we have to deter- mine the following data, viz. : (1) The radius of curvature of the anterior surface of the cornea. (2) The distance of the anterior surface of the cornea from the anterior surface of the lens. DIOPTRIC MECHANISMS OF THE EYEBALL 533 (3) The radius of curvature of the anterior surface of the lens. (4) The thickness of the lens. (5) The radius of curvature of the posterior surface of the lens. In addition we must know the refractive index of the cornea and aqueous or vitreous humour, and also the average refractive index of the lens. There is a considerable difference between the refractive indices of the outer and the inner portions of the lens, the inner part being much denser and having a higher refractive index than the periphery. The following Table represents the constants of a human eye as determined by Helmholtz : Refractive index of aqueous and vitreous humours 1-3365 Total refractive index of lens ...... 1-4371 mm. Radius of curvature of cornea ...... 8 „ ,, anterior surface of lens 10 „ ,, posterior surface of lens 6 Distance from anterior surface of cornea to anterior surface of lens . 3-6 „ „ „ posterior surface of lens 7-2 Anterior focus of cornea ....... 23-692 Posterior focus of cornea 31-692 Focus of lens .... 43-707 Posterior focus of eye 19-875 Anterior focus of eye . 14-858 Distance from anterior siurface of corne X to : First principal point 1-9403 Second principal point 2-3565 First nodal point 6-957 Second nodal point 7-373 Anterior focus of eye 12-918 Posterior focus of eye . 22-231 It will be seen that both the principal points lie in the anterior chamber, while the nodal points fall in the back part of the lens. The posterior focus of the eye falls upon the retina. For many purposes we may simplify our calculations by running the two principal points and the two nodal points into one. In such a reduced eye the single principal point is situated 2-3 mm. behind the anterior surface of the cornea, and the single nodal point 0'47 mm. in front of the hinder surface of the lens. If a circle be drawn from the single nodal point as a centre through the single principal point as a circumference we get a surface II in the figure, which represents the anterior refracting surface of such a reduced eye. A reference to the Table of Constants of the human eye shows that, whereas the anterior focal distance of the cornea is 23 mm., that of the whole eye is 15 mm. and that of the lens 44 mm. It is evident from this that the anterior surface of the cornea is the most important refractive surface of the eye, and that, in the convergence of rays necessary for the formation of an image on the retina, it is the refraction at this surface which plays the 534 PHYSIOLOGY greatest part. Under water this refraction is of course aboKshed, since the refractive indices of the aqueous humour and cornea are practically identical with that of water. The eye therefore becomes long-sighted ; it is impossible to get any clear image of near objects, and distant objects can only be seen with a strong eSort of accommodation. A smaller effect is produced by removal of the lens, an operation often undertaken when this body has become opaque as a result of cataract. Such an operation not only aboHshes the power of accommodation of the eye, but diminishes the refractive power of the eye at rest by ten dioptres, so that a lens of this power has to be placed at the front of the eye in order to render possible clear vision of distant objects. PART OF THE RAYS IN THE FORMATION OF THE RETINAL IMAGE In the reduced eye the construction of the path of the rays is very simple. When the eye is focused for the object which is being looked at, the rays from any point of the object come to a point on the retina. All that is necessary therefore is to draw lines from points of the object through the single nodal point to the retina, as is shown in Fig. 263. The image thus produced, like Fig. 263. Path of the rays in the formation of an image on the retina. that produced by a bi-convex lens, is real, inverted, and diminished in size. The further the distance of the object from the retina the smaller will be its image on the retina. The angle formed at the junction of two hues drawn from the extreme points of an external object to the nodal point is the ' visual angle.' In the schematic eye a visual angle of sixty seconds corresponds to a distance on the retina between the two ends of the image of -00438 mm. Few people can distinguish two points of light the line joining which sub- tends a smaller visual angle than sixty seconds. If we measure the histo- logical elements in the centre of the retina which are responsible for distinct vision, viz. the cones, we find that in the yellow spot each cone is about -002 to -005 mm. thick. The hmits of our power of distinguishing two luminous points is approximately in agreement with the diameter of each end- organ of vision. In order that the images of the two points may give rise to dis- tinct sensations their images must fall upon different cones. This fineness of vision exists only at the yellow spot, the accuracy of vision in the peripheral parts of the retina being very much lower. Not only is the image less DIOPTRIC MECHANISMS OF THE EYEBALL 535 perfectly focused, but the number of sensory elements in a given area of the retina diminishes steadily as we pass from centre to periphery. OPTICAL DEFECTS OF THE EYE The normal eye is so constructed that parallel rays come to a point on the retina. Rays which are divergent when they fall on the anterior surface of the cornea are brought therefore to a focus behind the retina. In such an eye any object lying at a distance nearer than five metres would give rise to an image behind the retina, were it not for the fact that the eye possesses means of altering its focus and so of bringing divergent rays to a focus on the retina ; this means is known as accommodation. In a photographic camera the process of focusing is carried out by altering the distance between the lens and the sensitive plate. The same method is adopted in the eyes of certain animals. In the mammalian eye, however, ac- commodation for near objects, i.e. the focusing of divergent rays on to the retina, is accomplished by a change in the curvature of the lens. The lens becomes more convex on its anterior surface, so that its refractive power is increased and the hinder focus of the dioptric mechanism of the eye is there- fore diminished. Every eye possesses a certain definite range of vision, which, in a normal person, has its far point at infinite distance and its near point at a distance from the eye which depends on the elasticity of the lens and the range through which this can alter its curvature. The normahty of an eye is determined by the fact that parallel rays come to a focus on the retina when the apparatus of accommodation is at rest. Two classes of de\aation from this normal, or enmietropic, eye are common. In the first class parallel rays come to a focus in front of the retina. In such eyes, which are designated myopic, it is impossible to get a clear image on the retina of distant objects. In order that the rays may be focused on the retina they must be divergent, so that even when accommodation is para- lysed the far point of distant vision Hes at some finite distance from the eye, varying with the extent of the disorder. If in such eyes the range of accommodation is normal, the near point will be much nearer to the eye than in the emmetropic eye (Fig. 26J:). The second class of abnormal eyes are known as hypermetropic. These eyes can be regarded as too short for their refractive media. Parallel rays Fig. 264. Diagrams of course taken by parallel rays in entering normal (emmetropic) eye (a), hyper-metro- pic eye (b), and myopic eye (c). 536 PHYSIOLOGY , are brought to a focus at a point behind the retina. Persons so afiected can see objects at a distance, but always with some effort of accommodation. If accommodation be paralysed by means of atropine everything will appear blurred. Since accommodation is required even for infinite distance, the greatest possible effort will be insufficient to bring near objects into accurate focus, and the near point of such eyes will be greater than normal. Persons with hypermetropia, or long-sightedness, can therefore see objects at a dis- tance perfectly well, but are unable to read small print, since the point of near vision is too far from the eye to allow the small letters to subtend a sufficiently large angle. Both these disorders can be corrected by suitable spectacles. In the case of the myopic eye we need lenses which will convert the parallel rays into divergent rays ; such cases are treated therefore with concave lenses. Con- versely, hypermetropia is treated with convex lenses, which will aid the too feeble refractive power of the eye, and so bring parallel rays to a focus on the retina without any effort of accommodation. The degree of myopia or hypermetropia is denoted by the refractive power of the lens which is necessary to make the eye emmetropic. In order to determine the refractive power of any eye it is usual to employ Snellen's test type. This consists of a series of letters which are placed at a distance of five metres from the eye. At this distance the visual angle subtended by each of these letters is so small that a clear retinal image is necessary for their recognition. This is easy in the case of a normal eye. After allowing the patient to attempt the recogni- tion of the type without spectacles he is then made to regard it through a weak convex lens. If the patient can now read as well as or better than before he is hypermetropic, since it is only the hypermetropic eye which is able to unite convergent rays of light on to the retina. If, on the other hand, the reading of the type is made more difficult the patient is either normal (emmetropic) or myopic. In the latter case a concave lens is tried. If the reading is rendered more easy by this means the patient is myopic. In prescribing the lenses for hypermetropia, the strongest lens with which the patient is able to see represents the degree of hypermetropia. Since now the mechanism for accommodation mtLst be relaxed as far as is possible, the strength of such a lens serves as a measure of the degree of hypermetropia. On the other hand, in myopia the degree of the disorder is determined by the weakest lens, by means of which the patient is able to see distant objects. In a perfect dioptric mechanism the media through which the hght passes must be perfectly transparent, and the centres of curvature of the various refracting surfaces must he in one straight hne, i.e. the system must be properly centred. In neither of these respects can the eye be regarded as perfect. If a strong beam of Hght be thrown into the eye, the refraction of the beam caused by the shght difference in structure between adjacent portions of the cornea and lens makes these objects immediately visible, and the field of vision is filled with diffused light arising from the illuminated points in these structures. Under normal circumstances, however, these shght differences in the regularity of the refracting media do not make any appreciable difference to our vision. More easily detected are the opacities due to structures in the vitreous humour. These opacities can be seen, when the eyes are turned towards a uniformly illuminated surface, as small dark DIOPTRIC MECHANISMS OF THE EYEBALL 537 points or strings of beads, which, since they alter their position with changes in the direction of the eyes, are often spoken of as muscce volitantes. The centring of the eye is also never perfect. In the horizontal meridian the optic axes of the cornea only diverge about 0 3° from the axis of the lens, but in the vertical meridian there is as much as 1*3° difference between the two axes. Moreover the visual axis does not correspond exactly with the optic axis of the eye. The fovea centralis, the point of distinct vision on which the image of any object must be brought in order to see it as distinctly as possible, always hes outside and somewhat below the point at which the optic axis strikes the retina. The angle between the two axes is often spoken of as the angle a. This angle in the horizontal meridian varies between 3*5° and 7°, and in the vertical meridian is about 3-5°, Although this divergence of the axes causes a certain amount of astigma- tism {v. p. 538), it is too small to interfere appreciably with the sharpness of vision. SPHERICAL ABERRATION. When a beam of parallel rays falls on to the surface of a spherical lens those rays which pass through the circum- ference are converged to a focus which lies nearer to the lens than the focus of those rays which pass through its centre. In an optical instrument the blurring of the image thus produced is counteracted in two ways : (1) By making the curvature in the middle of the lens greater than at its periphery. (2) By stopping out the peripheral rays by means of a diaphragm, or by using only a cyhnder cut from the centre of the lens. It is famihar to every photographer that where sharpness of definition is required it is necessary to use a small stop, giving a corresponding increased exposure. In the eye spherical aberration is diminished by both these means. The curvature of the lens is greater towards its centre than at its circumference, and the peripheral rays of hght are shut out by a circular diaphragm, the iris, the diameter of the aperture in which varies according to the amount of light falhng into the eye, and according to the nearness of the object which is the point of regard. CHROMATIC ABERRATION. The refraction of hght in passing from a lighter to a denser medium is due to the fact that in the latter the velocity of propagation of the light is less than in the former. This diminution of the velocity of the propagation affects rays of various wave-lengths differently, so that of the various rays which make up white hght those at the red end with a long wave-length are refracted least, and those at the violet end with a short wave-length are refracted the most. On this account, when light passes into a prism it is split up into its component rays with the production of a spectrum. The same splitting up of rays occurs when hght passes through a simple lens. As is shown in Fig. 265, the violet rays come to a focus at a point nearer the lens than the red rays. A screen held at v will therefore show a bluish- violet centre with a red margin ; at r the centre will be reddish and the margin \'iolet. In optical instruments this chromatic aberration is corrected by 538 PHYSIOLOGY combining glasses of different powers of dispersion, with the production of so- called achromatic lenses. In the eye achromatism is practically uncorrected. The difference in the focus of red and violet rays in the eye amounts to about 0-5 mm. ; hence, if we are looking at a red and a violet spot situated close together, it requires a greater act of accommodation to bring the image of the red spot on the retina than is the case with the violet spot. The red spot therefore looks nearer, i.e. more prominent. It may be this special V ^ effort of accommodation necessary for the appreciation of red that makes this colour stand out, so to speak, and be conspicuous as compared with the other colours of the spectrum. The fact that as a rule we do not see coloured fringes around every object that we look at is due, not to the optical, but to the physiological quahties of the eye. When white hght falls on the eye and is focused by the latter on to the retina, it will be the rays of medium refrangibiHty which come to a point at the retina ; these optically will be surrounded with a halo composed of the red and violet rays, as is shown in the figure at/. The retina is, however, comparatively insensitive for rays at the two extreme ends of the spectrum. Moreover the strong illumination produced by the middle rays of the spectrum, i.e. about the yellow, at the centre of the illuminated spot, by contrast depresses the excitabihty of the surrounding parts of the retina, so that the halo due to the red and violet rays is neglected and does not come into consciousness. ASTIGMATISM. We have assumed so far that the refracting surfaces of the eye are practically spherical ; this does not apply strictly either to the cornea or the lens. The small differences between the curvatures of the cornea in the horizontal and vertical meridian towards its centre do not as a rule give rise to appreciable disorders of vision. In many cases, however, the asymmetry in the anterior surface of the cornea is sufficient to cause a considerable difference in the refraction of rays in the different meridians, and this disturbance is known as astigmatism. The curvature of the vertical meridian of the cornea is nearly always somewhat greater than that of the horizontal meridian. When this difference is sufficiently pronounced it becomes impossible for a definite image of a point of light to be formed on the retina, since the rays which diverge from the luminous point in the vertical plane are brought to a focus sooner than those in a horizontal plane. Such an eye will therefore possess two posterior foci, one for the vertical meridian, the other for the horizontal meridian. The manner in which such rays diverge is shown in Fig. 266. DIOPTRIC MECHANISMS OF THE EYEBALL 539 The ett'ects of astigmatism are especially noticeable when the patient is trying to see clearly objects composed of horizontal and vertical lines, such as print. A vertical hne can be conceived as made up of a series of points each of which sends out a horizontal sheaf of rays. In the same way a horizontal line is distinguished as a series of points sending out flat sheafs of vertical rays. If the curvature of the cornea in the vertical meridian is greater than in the horizontal it will need a greater effort of accommodation to bring the vertical Hnes to a focus on the retina than it does to bring the horizontal lines. In reading therefore there is a constant shifting of the focus of the eye, and the mechanism of accommodation becomes rapidly tired and Fig. 266. Diagram showing course of rays in an astigmatic eye. (Waller.) The curvature of the cornea is greater in the vertical meridian vvv than in the horizontal meridian hhh. Hence the rays of light coming from the point p and passing through the vertical meridian come to a focus at f^, while those through the horizontal meridian come to a focus at/^. There is thus no point behind the cornea at which all the rays from p will come to a focus, and the image of the point must be blurred, being elongated in a horizontal direction at/i, and in a vertical direction at /^. strained, with the production of pain in the eyes or of headache. In order to correct astigmatism it is necessary to find out first the curvature of the corner in the different meridians, and then to reinforce the curvature of the weakei meridian by means of a cylindrical lens. If the eye is myopic the cyhndrical lens may be concave and placed so that its curvature counteracts that of the cornea in the meridian in which the curvature is greatest. ACCOMMODATION The rays falling on the eye from a point of light at a distance greater than five metres from the eye may be regarded as practically parallel, and are converged in the normal eye to a focus on the retina. As the point of light is moved nearer to the eye the latter is still able to focus the rays on the retina through a considerable range. On approximating the points of light to a distance which is less than the near point of distinct vision, the rays are no longer converged to a point on the retina, and a blurred image is the result. This near point of vision may be determined in any eye by finding out the smallest distance from the eye at wliich small print can be easily dis- tinguished. The distance is measured by means of a graduated rod between the eye and the printed object. This ' accommodation,' by which the eye is able to focus divergent rays on to the retina, imphes either a change in the distance of the refracting surfaces from the retina, or an increase in the total refractive powers of the eye. In man and the higher animals it is by the latter means alone that 540 PHYSIOLOGY Fig. 267. Diagram of phakoscope. accommodation is effected. The fact that accommodation, as was shown by Young, may be carried out under water, i.e. under conditions in which the curvature of the cornea does not cause any appreciable deviation of the rays passing through it, shows that the change in the combination cannot be located in the cornea. It was shown by Helmholtz that the essential process in accommodation is an altera- tion in the curvature of the lens, the anterior surface be- coming more convex when the eye is accommodated for near objects. This may be shown by means of the phakoscope (Fig. 267). This is simply a box, blackened inside, with holes at a, 6, c, and d. At a is the observer's eye ; at 6 the observed eye. Across the middle of (Z a wire is stretched. A candle is placed at c. The observer at a then sees three reflections of the candle from the eye at 6 : a bright erect image from the anterior surface of the cornea ; a larger but dimmer erect image from the anterior surface of the lens ; and a small very dim inverted image from the posterior surface of the lens. These images must be observed first when the eye at h is accommodated for a distant object, and then when it is accommodated for the wire stretched across the opening d. It will be noticed that the change of accom- modation from far to near objects is accompanied by a change in the second image (that from the anterior surface of the lens), which becomes smaller. The change in this image is more easily seen if the candle be made to throw two images on the eye by interposing a double prism at c. Then, as the lens becomes more convex to accommodate for near objects, the two images of the candle reflected from its anterior surface approach one another (Fig. 268). By measuring the size of the image of the candle-flame produced by the anterior surface of the lens, and knowing the size of the flame itself and the distance from the observed eye, it is possible to calculate the curvature of the lens in the living body. The radius of curvature of a reflecting surface is given approximately by the follow- ing formula : R =2-— a b to a b c Fig. 268. Diagram of reflected images from cornea and lens surfaces seen in phakoscope. a, from anterior surface of cornea ; h, from anterior surface of lens ; c, from posterior surface of lens. 1, during accom- modation for distance ; 2, during accom- modation for near objects. DIOPTRIC MECHANISMS OF THE EYEBALL 541 where R is the radius of curvature, a tha distance of the object, b the size of the image, C the size of the object. The object generally used is the distance between two lights or two white objects called mires ; the ' image ' being the distance between their images. Owing to the movements of the eye the latter cannot be accurately measiired by the usual method, employed by physicists, of looking at the images thi'ough a telescope which has a micrometer at the focus of the object. This difficulty is overcome by doubling the image. For this purpose Helmholtz devised the ophthalmometer, in which the doubling is brought about by two plane glass plates set at a variable angle to one another. The principle of the instrument can be gathered from the diagram (Fig. 269). We may suppose it is necessary to measm-e the line ah, which may be taken to repre- sent an image reflected from the anterior surface of the cornea or lens. If we look at this line through a plate of glass the plane of which is at right angles to our line of sight, no distortion of the line ab takes place. If, however, the plate be placed Fig. 269. Diagram to illustrate principle of ophthalmometer. (After Schenck.) obliquely, as at g^ g^, there will be an apparent shifting of the line sideway to cd. In the ophthalmometer there are two glass discs, g^ g^, and g.^ i72» one immediately over the other, so placed that the image ah is looked at thi'ough the junction between the two plates. The plates are then turned, as in the diagram, until ab appears as two distinct lines ec and cd just touching one another at c. At this point each image of the line ah has been shifted through one half the length of ab. Knowing the thickiiess of the plates and their refractive index, it is easy to calculate, from the angle through which the plates have been turned, the apparent shifting, of the line ah. This lateral movement amounts to ac, i.e. to — , and we have merely to double this result in order Ji to obtain the actual size of the image on the cornea or lens. A table is generally supplied with the instrument giving the actual size of the image corresponding to the angle through which the plates have been turned ; the eye always being placed at a constant distance from the instrument and the luminous object always being the same size. The size of the image is calculated in the following way * : "Let aa (Fig. 270) be one of the plates, AB the incident, CD the refracted lay. Then, since the refracted ray is parallel to the incident ray, the angle ABN is equal * Parson's " Elementary Ophthalmic Optics. 542 PHYSIOLOGY to the angle DON' ( = a). Similarly the angle of refraction CBn is equal to the angle BCn' ( = /3). Let h be the thickness of the glass plate. Produce DC backwards to A'. It is required to find the perpendicular distance between A and A' ( = x). X Now -g^ = sin BGA' = sin {n'CA' - n'CB) = sin (a — /3). And -gp = cos j3. sin (a — /3) Therefore x = ft.. g — . cos /j If there are two such plates, arranged as in Fig. 269, then ab = 2x cos fi The angle a is measured by the instrument ; the angle /3 is calculated from the formula for refraction, sin a = n. sin /3 ; and the thickness of the plates, h, is known. Therefore the distance between the images can be calculated." In the normal eye in a position of repose, i.e. focused for parallel rays, the curvatures of the three principal refracting surfaces are as follows : mm. Anterior surface of cornea .... 8 Anterior surface of lens .... 10 Posterior surface of lens .... 6 During maximum accommodation the radius of curvature of the anterior surface of the lens changes to 6 mm. At the same time there is a slightly increased curvature at the periphery of the posterior surface, but the effects of this change are neghgible as compared with those produced by the altera- tion of the anterior surface. In order to determine the method in which this change in curvature of the anterior surface of the lens is brought about we must refer in some detail to the structure of the anterior half of the eye and the manner in which the lens is hung up between the aqueous and vitreous humours. The outermost layer of the eye is formed by the sclerotic coat, a strong tough membrane of white fibrous tissue. In front this is continuous with the cornea, which, having a smaller radius of curvature than the globe of the eye, protrudes like a watch-glass from its anterior surface. The main substance of the cornea is formed, hke the sclerotic, by white fibrous tissue, modified however in its consistence so as to be perfectly transparent instead of white and opaque hke the rest of the sclerotic. Internal to the sclerotic is the choroid coat, a mem- brane with a double pigmented internal layer, and supphed with blood- vessels which furnish the vascular supply to the whole eye. In front the choroid coat presents a series of folds, arranged in a circle around the anterior part of the cavity of the vitreous and known as the cihary processes. In front of the ciliary processes the choroid leaves the sclerotic and hangs as a curtain into the cavity of the aqueous humour, between the cornea and the lens. The curtain, which is known as the iris, presents a circular orifice at DIOPTRIC MECHANISMS OF THE EYEBALL 543 its centre, the fwpil, and is provided with muscular fibres by means of which the pupil may be constricted or dilated. The posterior surface of the iris, like the inner surface of the choroid generally, is Hned by a pigmented epithelium. The posterior layer of the cornea is formed by a tough elastic membrane {Descemet's membrane) which is covered posteriorly by a layer of cubical epithelial cells. At the circumference of the cornea Descemet's membrane, or the posterior elastic lamina, breaks up into a number of fibres, which pass outwards and backwards to be inserted into the anterior part of the choroid and root of the iris. These fibres are the ligamentum pectinatum iridis. Between the fibres of the ligamentum pectinatum are a number of spaces lined with endothelium continuous with the anterior chamber and known as the spaces of Fontana, and outside these spaces, in the corneo-sclerotic junction, is a circular sinus continuous with the venous system, the ca7ial of Schlemm or sinus venosus. The iris rests at its inner margin on the lens, so dividing the cavity of the aqueous humour into two parts. In front is the anterior chamber and behind the posterior chamber. The latter is a small annular space, triangular in cross-section, and bounded by the iris in front, the lens behind, and the cihary processes externally. The crystalline lens, made up of radiating lens fibres, each of which is produced by the modification of an epithehal cell, is biconvex, the posterior surface being more convex than the anterior. It is surrounded by a tough structureless membrane, the capsule of the lens, and rests in a cavity hollowed out of the anterior surface of the vitreous humour. At its circumference it is hung up and fastened to the cihary processes by the suspensory ligament, or zonule of Zinn [zonula ciliaris). (Fig. 271.) This hgament is formed in the follo'v^ang way : The vitreous humour is bounded externally by the hyaloid membrane, which separates it from the retina. In front the hyaloid membrane passes behind the lens, but as it lies on the ciliary processes is closely adherent to these structures and sends of? fibres which pass radially from the cihary pro- cesses to the capsule of the lens and form the zonule, or suspensory ligament. The greater part of the suspensory ligament, i.e. from the ora serrata of the retina to the edge of the cihary processes, is closely attached to these pro- cesses. From their edge a number of fibres pass and fuse at their inner extremities with the lens capsule. These fibres are arranged in three groups : (1) The anterior group passing to the anterior capsule of the lens. (2) A middle group passing to the equator of the lens. (3) A posterior group lying close to the hyoid membrane and passing to the posterior lens capsule. The suspensory ligament is always in a condition of tension. If the finger be pressed on the outside of the eyeball, it will be felt that this organ presents a resistance to deformation which cannot be ascribed simply to the firmness of the sclerotic coat, but must be determined by the existence of a positive pressure in the fluid filling the eyeball. This pressure, which is known as the infra-ocuhr pressure^ may be measured by means which we shall have to discuss later, and amounts to about 25 mm. Hg. As a result of 544 PHYSIOLOGY this pressure the membranes which confine the fluids of the eyeball are dis- tended, i.e. pressed outwards, and this pressure keeps the bases of the cihary processes pressed against the choroid coat and thus enables them to withstand the pull exerted by the tense suspensory ligament. The pull exerted by the suspensory ligament affects mainly the tough an- terior surface of the lens capsule and so has a constant flattening effect on the anterior surface of the lens. This may be proved by measuring the curvature of the lens in a recently excised eye, and then removing the lens altogether from the eyebaU and measuring the curvatures of its surfaces again. It Sinus venosus Conjunctiva Retina Fig. 271. Section through anterior part of eyeball to show mode of suspension of lens. (After Mekkel and Kallitjs.) will be found that, as soon as the lens is free from its attachments in the eyeball, the curvature of its anterior surface is increased. The change therefore in the lens, which is responsible for the alterations in its refractive power determining accommodation, may be effected by any means which will relax the suspensory ligament, such, for instance, as approximation of the ciliary processes to the margin of the lens. This movement of the ciliary processes is effected by the ciliary muscle. The attachments of this muscle are shown in Fig. 271 . It forms a circle of unstriated muscle fibres, triangular in cross- section, and extending round the whole circumference of the eyeball. The fibres of the muscle are divided into three groups : (a) The meridional fibres, which run from the corneo- sclerotic junction backwards and outwards to be attached to the anterior part of the choroid coat behind the ciliary processes. (6) The radial fibres, which pass from the margins of the canal of Schlemm, DIOPTRIC MECHANISMS OF THE EYEBALL 545 and from the fibres of the hgamentum pectinatum to be attached behind to the whole extent of the cihary processes. (c) The circular bundle, which forms a ring-muscle, composed of fibres running around the circumference of the eye in the inner part of the ciliary processes. This bundle is best marked in hypermetropic eyes ,„ and is almost absent in myopic eyes. When this muscle contracts it draws the anterior part of the choroid and the cihary pro- cesses forwards and inwards, while the ring-fibres approxi- mate the ciliary processes to the margin of the lens. By this approximation of the cili- ary processes to the lens the suspensory ligament is relaxed, and the anterior surface of the lens bulges, i.e. becomes more convex as a result of its inherent elasticity (Fig. 272). This explanation of accommoda- tion, which was first put forward by Helmholtz, is almost universally accepted. According to some the change of shape of the lens dm'ing accommodation is brought about by the actual pressm'e of the ciliary processes on its margin by which the middle of its anterior surface is pressed forwards. According to Tscherning this effect is produced, not by relaxation, but by a tightening of the suspsnsory ligament through the contraction of the ciliary muscle. There is no doubt, however, that during forced accommodation, such as can be brought about by instillation of eserine into the eye, which produces spasm of the ciliarj^ muscle, the suspensory ligament is so relaxed that the lens lies loosely in the eyeball. Bending the head downwards causes therefore an actual change in the position of the lens, which may drop as much as 1mm. forwards towards the cornea. Under the same conditions a quick movement of the head causes the lens to shake, and the quiver of the lens can be seen by an external observer and proved by the subjective oscillation of external objects which is noticed after such a movement. Moreover, if a needle be passed through the sclerotic so that its point lies in the ciliary processes, stimulation of the ciliary muscle causes a movement of the outer part of the needle backwards, showing that the point ot the needle which is in the ciliary processes has been moved forwards (Fig. 273). The loosening of the lens dm-ing spasm of accommodation is well sho\vninrare cases where there is congenital absence of the whole iris. In such cases the shaking of the patient's head is seen to cause an oscillation of the lens within the eyeball. During acconamodation the increased curvature of the anterior surface of the lens causes an approximation of this surface to the cornea, which may 18 Fig. 272. Diagram of mechanism of accommoda. tion. (TiGEKSTEDT after Schon.) The dotted line shows the form of the lens during accommodation for near objects. 546 PHYSIOLOGY be directly observed {cp. Fig. 273), especially in people with somewhat prominent eyes. No change takes place in the intraocular pressure, in either aqueous or vitreous cavities, as the result of acconmaodation. The passage of fluid takes place with such ease between the fibres of the suspensory Fig. 273. Accommodation in the cat's eye. b, distance ; a, for near vision. (After BebPv.) Two needles have been passed through edge of cornea into ciliary bodies, to show forward movement of latter during accommodation. ligament that a slight movement of the lens forwards or backwards does not upset the equaUty of pressures in the two chambers. THE RANGE OF ACCOMMODATION. Assuming that, as is the case with the normal eye, the tension of the suspensory Ugament is sufficient to keep the eye focused for parallel rays, the near point of vision will be determined by the unconstrained shape of the lens, i.e. the amount by which its curvature can increase when the suspensory hgament is completely relaxed. The shape of the lens varies with age, its convexity, being greatest directly after birth and diminishing steadily from that time to the age of sixty or seventy (Fig. 274). In consequence of the Of h c small size of the eyeball, the eye at birth is generally somewhat hypermetropic, but from the age of ten onwards we find that the point of near vision re- cedes continuously with advancing age. The range of accommodation is measured by the strength of the lens which will give to rays coming from the near point of distinct vision the same direction as if they came from the far point. In a normal eye the far point is at infinite distance and the rays are parallel. The change in the range of accommodation with advancing years is shown in the following Table : Fig. 274. Lens from human eye at different periods of life. (Allen Thomson.) a, at birth ; b, adult ; c, old age. Age 10 20 30 40 60 60 70 Range of accommodation in dioptres 14 10 7 4-5 2-5 1 0-25 This gradual diminution in the range of accommodation gives rise finally to disturbances of vision which are known as presbyopia. The ordinary reading I DIOPTRIC MECHANISMS OF THE EYEBALL 547 distance is about ten inches. A smaller distance than this is rarely made use of even in youth, when the point of near vision is only three or four inches from the eye, on account of the effort required to converge the axes of the two eyes to this extent. The effect of the gradual dimination in the elas- ticity of the lens is noticed as soon as the point of near vision recedes beyond ten or twelve inches, i.e. 25 to 30 cm. This occurs as a rule between forty- five and fifty, when we begin to experience difficulty in reading small print, since the visual angle subtended by such print at a distance over ten or twelve inches is too small to allow of distinct vision. The condition of presbyopia, is remedied by employing reading glasses, i.e. by wearing spectacles which converge the rays of Hght and so enable small objects to be brought nearer to the eye than its near point. It is evident that these glasses must be strengthened continually with advancing age. No trouble is experienced in seeing distant objects, the refraction of the eye at rest remains as before. The dimness of vision in extreme old age for distant as well as near objects is due to changes, not in the refractive power of the eye, but in the transparency of the refractive media, such as cataract, i.e. opacity of the lens, opacity of the cornea, and so on. The former condition can often be remedied by extracting the lens. Strong convex glasses (ten dioptres) must then be used to take the place of the lens. THE COMPARATIVE PHYSIOLOGY OF ACCOMMODATION The mechan'.sm of accommodiition which we have studied in man is found with very little modification throughout the whole group of mammalia, though, in the domestic animals at any rate, the range of accommodation is very much less than in man. On examining other types of animals we meet, as was shown by Beer, an amazing variety of methods by which the range of the eye may be altered. In order to bring distinct images of objects at various distances on to the retina, practically every possible focusing method is made use of in one type or another of the animal kingdom. The following details are taken from Beer's papers. In birds the eye, like that of man, is normally focused for distant objects, and accom- modation for near objects is accomplished by a change in cmvatm'e of the anterior surface of the lens. Whereas, however, in man the suspensory ligament is relaxed by a drawing forwards of the choroid membrane, in the bird's eye this relaxation is effected by a drawing backwards of the posterior lamina of the cornea, where it breaks up into the ligamentum pectinatum iridis. In these eyes the main attachment of the suspensory ligament is to the ligamentum pectinatum ; the retraction of this ligament is effected by a special muscle known as Cravipton' s muscle, which corresponds to the ciliary muscle in man, but unlike this consists of striated voluntary muscle. This movement of the posterior elastic lamina of the cornea can be easily sIioaati by passing two needles through the corneo-sclerotic junction until their points lie in the anterior chamber. On exciting Crampton's muscle electrically, the outer end of the needle moves forwards, showing that the deeper part of the corneo-sclerotic junction is being pulled backwards towards the ciliary portion of the eye (Fig. 275). The histological character of the muscle of accommodation in birds seems to be connected with the rapid accommodation that is necessary when a bird swoops down towards the ground to pick up some food insect. Moreover, since binocular vision is not present in many birds, and convergence of the optic axes must be minimal, it is probable that the con- tractions of Crampton's muscle play a great part in guiding the movements of the bird, and especially in aiding it to judge distances. In oiu- selves such judgment is very faulty without the co-operation of the two eyes. 548 PHYSIOLOGY In amphibia and snakes, wliich at rest are also focused for distance, active accom- modation for near objects is effected, not by change in curvatmre of the lens, but by an increase in the distance between the lens and the retina. In amphibia the ciliary muscle, which lies between the root of the iris, the sclerotic and choroid, causes a rise of pressure in the vitreous cavity, and the lens, being the most movable part of the boundary wall of this cavity, is pushed forwards towards the cornea. The aqueous humour, which is displaced by this forward movement of the lens, finds a place in the lateral angle of the eye, which is increased in depth by the pull of the muscle fibres. In snakes the same action is effected by a muscle, often cross-striated, which is situated in the root of the iris. In both these cases the movement of accommodation is unaffected by opening the aqueous cavity, whereas in mammals it is at once rendered impossible if the aqueous cavity be laid open. Fig. 275. Accommodation in a bird's eye. (Beer.) K, rest ; A, accommodation for near objects. Most of the teleostean fishes are short-sighted, i.e. at rest they are focused for near objects. Active accommodation in these animals diminishes the refractive power of the eye, so that accommodation occurs for distant objects. In the fish's eye there are no ciliary processes, ciliary muscle, zonule of Zinn, or spaces of Fontana, such as are foimd in the higher vertebrata. The iris only approaches the margin of the lens, and does not shut out its peripheral rays. The lens, which is spherical (Fig. 276), is hung up by means of a flat band attached to its uppei pole. This is known as the ' suspensory ligament,' but is quite different in structure and mechanism from the sus- pensory ligament or zonule of Zinn of the vertebrate eye. From the lower and inner pole of the lens a dark pigmented structure passes backwards ; this was described by its first discoverer as the campanula, but since it is muscular in character is better named the ' retractor lentis.' On stimulation this muscle pulls the lens backwards, and so lessens the distance between it and the retina, in this way accommodating the eye for distance. The eye of the cephalopod mollusc, such as sepia, is also short-sighted, and active accommodation, as in the fish's eye, is accommodation for distance. The mechanism is, however, quite different. The globe of the cephalopod's eye has the shape shown in the diagram (Fig. 277). The most resistant part of the globe is formed by a strong ring of cartilage wliich passes round the equator of the eye. The rest of the sclerotic is formed of delicate membrane, which is thinnest in the ring just behind the cartilagin- ous ring. In the anterior wall of the eye is a strong muscular ring, composed of meridional fibres, which run from the cartilaginous ring to be inserted into the ciliary processes or corpus ciliare, which is closely attached to the equator of the lens. When this muscle contracts it pulls back the whole anterior wall of the eye together with the lens, approximating it to the retina. This movement is of necessity accom- panied by a rise of ocular pressure, but room for the displaced fluid is found by a bulging of the walls of the eyeball at their thinnest part, i.e. just behind the cartilaginous DIOPTRIC MECHANISMS OF THE EYEBALL 549 ring, so that there is an actual diminution of the distance between the lens and the retina. In every class of animals, except in the cephalopod and in birds, species are found which possess no power of accommodation at all, or only to a very slight extent. This A Fig. 276. Diagrams from Beer to show mode of accommodation (for distance) in a fish. A, vertical section of the eyeball ; B, view of eye from front ; L, lens ; Ls, suspensory ligament ; J, iris ; Rl, retractor lentis or ' campanula ' ; C, changes in position of lens when eye is accommodated for an object at varying distances. Fi(i. 277. Aceoiiimodatiou in cyo of sepia. (Beek.) R, at rest ; a, during accommodation (for distance). is the case in frogs, alligators, vipers, and in many rodents. Man}- of these animals arc distinguished by nocturnal habits, and in daylight their pupils may be constricted to such an extent aS to render accommodation unnecessary. In many of them, too, the exact form of an object is not so important as the power to follow its movements. In such cases the movement of the extrinsic ocular muscles or of the head are more important than the exact focusing of the object on the retina. 550 PHYSIOLOGY THE FUNCTIONS OF THE IRIS The iris is the forward prolongation of the pigmented choroid coat. It is covered anteriorly by a layer of epithehum continuous with Descemet's epithelium, and behind by a thick layer of pigmented epithehum which is prolonged forwards from the retina. It is composed of delicate connective tissue, attached at its circumference to the fibres of the Hgamentum pec- tinatum, and contains two sets of unstriated muscular fibres. The one set, the sphincter fwpillce, is composed of fibres which run a circular course around the margin of the pupil. The other set, the dilatator pupillcB, forms a flattened layer of radiating fibres, which he close to the posterior surface and extend from the attachment of the iris nearly to the rim of the pupil. The pigment in the iris and choroid serves the same purpose as the blackened fining, which^is supphed to every optical instrument, in preventing reflection and dispersion of the incident light, and therefore preventing any hght falhng on the retina except those rays which pass through the pupil and refractive surfaces of the eye. The pigment in the iris has an additional importance in that it ena,bles this organ to act as a diaphragm. It not only shields the retina, the sensory apparatus of the eye, from the effects of any excess of illumination, but, by stopping out the rays of light passing through the periphery of the lens, it diminishes spherical aberration and enables a clear image of external objects to be formed on the back of the eyeball. The diameter of the pupil is continually varying, according to the amount of hght falhng into the eye and the condition of the mechanism of accommodation. Contraction of the pwpil occurs under the following circumstances : (1) When hght falls on the retina. This movement, which is known as ' the hght reflex,' is determined by a contraction of the sphincter pupillee, together with a relaxation of the dilatator muscle. The contraction ensues within a period of 0*04 to 0-05 sec. after the moment at which the hght has access to the retina, and attains its maximum within 0-1 sec. In man as well as in other animals which have binocular vision, and in which there is a partial decussation of the fibres of the optic nerves in the optic chiasma, the reflex is bilateral, i.e. light falhng into one eye causes simultaneous con- traction of both pupils. In the higher animals this reaction of the pupil to hght demands the integrity of the nervous paths between the eye and the brain ; but in many of the lower animals, e.g. in the frog and eel, the reflex nervous mechanism is aided by a local sensibihty of the iris to hght. In these animals the contraction of the pupil in response to illumination takes place even in the excised eye, and seems to be determined by a direct stimulation of the pigmented contractile fibres of the sphincter pupillse by means of the hght. The effect of light on the pupil varies considerably according to the condition of adaptation of the eye. The dilatation of the pupil is maximal when the eye has been in the dark for some time and may amount then to 7*3 to 8 mm. In one experiment, on exposing the eye to a feeble hght, e.g. 1-6 candles at a moderate distance, the pupil diminished in size to 6 3 mm. ; DIOPTEIC MECHANISMS OF THE EYEBALL 551 with an illumination of 50 to 100 candles the size of the pupil was 3-7 mm., and with 500 to 1000 candles, 3-3 mm. This efiect was obtained by a rajiid change of the illumination of the eye. When the change in illumination is sufficiently slow no alteration of the pupil takes place, and when the illumina- tion, which has at first caused a maximal constriction of the pupil, is con- tinued the pupil gradually relaxes with the adaptation of the retina to light. This relaxation occurs within three or four minutes after exposure to light has taken place. The same influence of adaptation will be observed if two individuals are brought into a moderately Ughted room, one from bright dayhght and the other from a dark room. The pupils of the first will dilate widely, while those of the second will constrict to their maximimi extent. In each case the change will pass off gradually, so that at the end of five or ten minutes there will be no difference observable between the eyes of the two persons. (2) When vision is directed to a near object the contraction of the ciliary muscle which results in accommodation is accompanied by con- vergence of the visual axes, brought about by contraction of the two internal rectus muscles. With these two movements is always associated a third, viz. contraction of the sphincter pupillae, the increased sharpness of the image obtained by this means being an indispensable condition for the fineness of vision which we desire when we examine any object closely. The fact that the amount of light which will fall into the eye from any given object in- creases inversely as the square of the distance of the object from the eye ensures that sufficient light wiU pass through the constricted pupil for the appreciation of the finer details of the object. (3) In sleep the pupils are always contracted. This fact seems at first in opposition to the other conditions regulating the size of the pupil, since during sleep no light is falling into the eye. If the eyelid of a sleeping person be raised the pupil will be found to be constricted ; as the person wakes up, in consequence of the interference, the pupil dilates and may then constrict again if the light is held so as to fall into the eye. This behaviour of the pupil may enable us to distinguish feigned from real sleep. The constriction of the pupil is really, like that which accompanies accommodation, an associated condition, and depends on the fact that during sleep the axes of the eyeballs are directed upwards and inwards. (4) Contraction of the pupil is a marked eft'ect of the action of certain drugs, especially opium and its alkaloid, morphia, as well as of the alkaloids eserine, or physostigmine, and pilocarpine. Contraction of the pupil also occurs in general excitatory conditions of the central nervous system and is therefore found during the stage of induction of chloroform and ether anaesthesia. Dilatation of the pupil occurs : (1) On the removal of light stimulus from the eye. If the removal is complete the pupil remains dilated, but if there is any light at all the pupil gradually constricts again as the eye becomes dark-adapted. (2) Dilatation of the pupil can be reflexly excited by the stimulation of 552 PHYSIOLOGY many sensory nerves, and is constantly observed as a result of severe pain. The presence or absence of dilated pupils may serve therefore as a means of testing how far an emotional expression of pain is to be credited to a physical cause. (3) The pupils are often dilated in emotional conditions such as fear. (4) Dilatation of the pupils occurs in every condition of extreme exhaus- tion, when the activities of the nervous centres are lowered. It is therefore seen during the third stage of chloroform anaesthesia, or in the comatose condi- tion produced by excess of alcohol. Among the drugs which cause dilatation of the pupil the beUadonna alkaloids, atropine and homatropine, are the best known. These alkaloids will produce dilatation of the pupil when simply dropped into the conjunctival sac, and are therefore largely used to dilate the pupil as a prehminary to ophthalmo- scopic investigation of the eye. INNERVATION OF THE INTRINSIC MUSCLES OF THE EYE The eyeball is supphed by the short ciliary nerves, which come from the lenticular or cihary ganghon and pass- ing forwards pierce the sclerotic coat about half-way between the equator and the posterior pole of the eyebaU. The lenticular ganglion has three so- called roots, by which it is connected with or receives fibres from : (1) The third or oculo-motor nerve, by the ' short root.' (2) With the sympathetic plexus oc.m sym '' ' Fig. 278. Nerve-supply to the eyeball. (After Foster.) l,g, lenticular ganglion with its three roots, viz. : r.h, radix brevis or short root ; r.l, radix longus or long root ; sym, sympathetic root ; F. opth. oph- thalmic division of F nerve ; /// oc.m, oculo-motor nerve ; II, optic nerve ; l.c, long ciliary nerves ; s.c, short ciliary nerves. lying in the cavernous sinus, and, through the fibres lying on the internal carotid artery, with the cervical sympathetic nerve. (3) With the nasal branch of the ophthalmic division of the fifth nerve by means of the ' long root.' The eyeball is also supplied by the two long ciliary nerves (Fig. 278) which come direct from a branch of the ophthalmic division of the fifth nerve and pass forwards on to the eyeball, piercing the sclerotic coat in front of the point at which this coat is penetrated by the short cihary nerves. There are thus three nerves by means of which the activity of the DIOPTRIC MECHANISMS OF THE EYEBALL 553 muscular fibres forming the ciliary muscle, the sphincter, and the dilatator iridis can be influenced, viz. the third nerve, the fifth nerve, and the sym- pathetic nerve. On exciting the root of the third nerve we obtain : {a) Constriction of the pupil. (b) Contraction of the ciliary muscle, i.e. spasm of accommodation. The same effects are produced by stimulating the lenticular gangUon or the short cihary nerves. Excitation of the long cihary nerves of the ophthalmic division of the fifth nerve, or of the Gasserian ganghon, causes dilatation of the pupil, but is without influence on the cihary muscle. Stimulation of the sympathetic in the neck causes maximal dilatation of the pupil accompanied by constriction of the vessels of the iris and the eyeball generally. If the superior cervical ganghon be extirpated so as to cause degeneration of aU the sympathetic fibres passing up to the eye, it will be found a fortnight later that stimulation of the Gasserian ganghon has no longer any influence on the size of the pupil. We may therefore come to the following conclusions as to the functions of the nerves suppl}aug the interior of the eyeball : The third nerve supphes fibres which run through the lenticular gan- ghon and the short cihary nerves and cause constriction of the pupil and con- traction of the cihary muscle. These fibres arise in the oculo-motor nucleus, which is situated at the back part of the floor of the third ventricle, immedi- ately below the anterior corpora quadrigemina. The sympathetic nerve sends fibres which pass to the eye along two routes. A certain number which run on the external carotid artery in the cavernous sinus pass by the sympathetic root to the ganghon and by the short cihary nerves to the eyeball and cause contraction of the blood-vessels. Other fibres pass from the superior cervical ganghon to the Gasserian ganghon of the fifth nerve, along the nasal branch of its first division and then along the long cihary nerves to the eyeball. These fibres carry impulses which dilate the pupil. The sympathetic fibres to the eyeball arise in the cord, probably from ceUs of the lateral column in the lower cervical or uppermost dorsal region. They leave the cord by the first two dorsal anterior roots, pass through the stellate ganghon, the ansa Vieussenii, and up the cervical sympathetic to the superior cervical ganglion where they terminate. New relays of fibres start in this ganghon and travel direct to their destination in the eyeball. Excitation of the cervical spinal cord easily evokes dilatation of the pupil, and it was on this account that Budge located in this part of the cord a ciho- spinal centre. The fibres derived from the fifth nerve itself must be looked upon as chiefly afferent or sensory in fimction. Some observers have ascribed to them a dilatator effect on the blood-vessels of the eye, but confirmation for this view is wanting. The cihary muscle is normaUy at rest and is only set into activity as a result of vohtional or reflex efforts to direct the gaze to near objects. The iris is imder the influence of tonic impulses which arrive at it along both sets of nerve fibres, oculo-motor and s}Tnpathetic, Section 18» 554 PHYSIOLOGY therefore of the sympathetic nerve causes constriction, and section of the third nerve dilatation of the pupil. These tonic influences are probably- reflex in origin, since it is foimd that, after cutting off afferent impressions from the retina by division of the optic nerve, section of the third nerve produces no further dilatation of the pupil. Since the dilatator muscle is often difficult to demonstrate under the microscope, the view has been put forward that dilatation of the pupil on stimulation of the sym- pathetic nerve is due merely to the relaxation of the tonic contraction of the sphincter Fig. 279. Effect on iris of cat of local stimulation. The first effect, as in a, is to cause contraction of the constrictor pupil! se below the electrodes, and this is succeeded in b by a strong localised contraction of the radiating fibres. (Langley and Andekson.) pupillse. The following experiments by Langley and Anderson showed definitely the erroneousness of this view : On stimulating the corneo-sclerotic junction so as to excite a limited portion of the iris, a well-defined local dilatation of the pupil is produced. If the dilatation were due to the relaxation of the sphincter, the dilatation could not be local, but would have extended to the whole circumference of the pupil (Fig. 279). In another experi- ment they isolated a sector of the iris by two radial cuts ; on exciting this sector it shortened, and the same effect was produced by excitation of the sympathetic in the neck, although any action of the sphincter must have been abolished by the mode of preparation. Section of the sympathetic in the neck causes lasting constriction of the pupil, and the same effect is produced by extirpation of the superior cervical ganglion. After the lapse of some time, however, the muscles, freed from their nervous connections with the ganglion, enter easily into a condition of hypertonus, so that the pupil on the side of the lesion may be more dilated than on the normal side. This hypertonus is especially marked when a slight amount of asphyxia or rise of blood pressure is present.* THE OPHTHALMOSCOPE By means of this instrument we are enabled to obtain a magnified picture of the back of the eyeball, as well as to judge of the presence and degree of abnormalities in the refracting apparatus of the observed eye. * Probably on account of the escape of adrenaline into the circulation, and its sympatho-mimetric action on the iris. DIOPTRIC MECHANISMS OF THE EYEBALL 555 When light falls on the eye through the pupil the greater part of it is absorbed by the pigment of the retina and the choroid coat. A small amount, however, is diffusely reflected, and is sent out by the "way it came, viz. through the pupil. If the vision is directed on the luminous point at the source of the rays, the reflected rays leaving the eye will be converged to a point and form an image which will coincide with the source of illumination. It is on this account that the pupil always appears black. When we look at a person's eye, we necessarily interpose oiu- head and eye between the observed eye and the source of light, so that no reflected light can come back to our eyes. Only in albinos, where the pigment of the choroid coat and retina Fig. 280. Indirect ophthalmoscopy. A, course of raj's from source of light E to observed eye ; o, observer's eye ; M, miri'or ; l, lens. B, course of rays from an illuminated spot on the retina of the observed eye to the observer's eye. is lacking, do we get a red appearance, due to the reflected light passing through the vascular tissues of the choroid and iris. In a hypermetropic eye at rest only those rays are brought to a focus on the retina which are convergent as thej'- enter the pupil. Light reflected from the retina of such an eye will therefore be divergent as it leaves the pupil, and we may obtain a ' red reflex ' by direct observation of the eye. In order that we may obtain an image of the interior of a normal eye we must arrange that our eye coincides with the soirrce of illumination. For tliis purpose we use the device invented by Helmholtz, viz. a slightly concave mirror M-ith a hole in the centre. By means of this mirror light is converged on to the pupil, and the light reflected by the retina is brought to a focus at the centre of the mirror, where is placed the observer's eye. This ophthalmoscope may be used in one of two ways : (a) INDIRECT OPHTHALMOSCOPY. In carrying out ophthalmic observations the examination is much facilitated by instilling atropine into the ob.^erved eye, so as to dilate the pupil to the widest extent and paralyse the mechanism of accommodation. If a beam of light be tlu'OAvn into the pupil, the emergent rays from the eye -will be parallel and will give rise to a red reflection seen by the observer's eye at the centre of the opht hal- moscopic mirror. If the ej^e be myopic, the issuing rays viill he convergent and will therefore be brought to a focus at some point in front of the eye, giving rise to a real image of the retina. If the eye be hypermetropic, the issuing rays vriW be divergent, and the observer will see the red reflection of light from the back of the retina. If now a lens of low power, say about ten dioptres (4 in. focus), be held a few centi- 556 PHYSIOLOGY metres in front of the observed eye (Fig. 28Cb), the reflected rays issuing from the pupil will be brought to a focus at a point between the observer and the lens, so that at this point will be formed a real inverted image of the back of the eyeball. This image, in the case of the normal eye, will lie at the focus of the lens. If the eye be myopic, the convergent rays will be brought to a focus nearer to the lens than its principal focus, while the divergent rays from the hypermetropic eye will give rise to an image in a plane between the principal focus and the observer. From the figxire (Fig. 281) it is evident that -— - = — , i.e. the "■ ^ ' AB AO magnification of the image will be propor- tional to the focal length of the lens used divided by the posterior focal length of the eyeball. If we are using a bi-convex lens of 10 cm. local length and the eye be assumed to have a posterior focal length of 1-5 cm., the real inverted image that we see in the Fig. iSl. To illustrate how the rays from an illuminated point of the retina form a parallel beam on leaving the eye, and are brought to a focus at b by interposing the lens L. bi-convex lens will be — , 1-5 i.e. 6-7 times as large as the retinal structures represented. (&) THE DIRECT METHOD. In this method the observer places himself close to the observed eye, throwing light into the latter from the mirror, and relaxes by an effort of will his accommodation absolutely.* Fig. 282. Path of rays in examination by the direct method. A, path of illuminating rays ; b, path of rays from iUuminated retina to observer's eye. If both the observer's eye and the observed eye are normal and unaccommodated, i.e. focused for distance, the rays of light, is-uing from any point on ihe retina of the observed eye, will leave the corneal surface as a beam of parallel rays, which on entering the observing eye will in turn be focused to a point on its retina. The observer there- fore sees an erect magnified image of the retina of the observed eye. If we take the * In the use of the opththalmoscope it is very difficult to relax accommodation when trying to see something which is quite close. The student will find it an advan- tage to try to imagine that he is looking tlu-ough a telescope at an object at a con- siderable distance off. He will then find the picture at the back of the eyeball Fuddc nly come into view. DIOPTRIC MECHANISMS OF THE EYEBALL 557 focus of the eye as 1-5 cm. the magnification of the image is equivalent to that which 20 would be produced by a lens of the same focus and is equal to — , i.e. about thirteen times. Since this method gives us a highly magnified image of the back of the eyeball, it is of extreme value in judging of the existence of pathological conditions of the retina or choroid. It is also of value in enabling the oculist to determine by objective methods the existence of any errors of refraction in a patient's eye. On examining the eye by the direct method, if the eye be myopic and the rays leaving it con- vergent, it will be impo-ssible for the observing eye to bring them to a focus, and it will be necessary to place a concave lens in front of the hole in the ophthalmoscopic mirror in order to bring the back of the observed eyeball into view. The weakest di vergent lens through whi ch an image of the observed eye can be obtained will give the degree of myopia of the eye. On the other hand, the rays from a hypermetro- pic eye, being divergent, will need a certain effort of accommodation to bring them to a focus in the observer's eye, and here the degree of hypermetropia will be given by the focus of the strongest convex lens through which it is just possible to obtain a clear image of the retina and retinal vessels. By the same means we may judge of the existence of astigmatism and form an idea of the meridians in which the refractive power of the eye is faulty. For this purpose observations are taken of the focus of the eye — firstly, for horizontal retinal vessels ; secondly, for vessels which are rumiing vertically. On examining the back of the eyeball by either of these methods, the most prominent object is the optic disc or optic nerve-papilla, which marks the point of entrance of the optic nerve. It is seen as a pale oval disc surrounded by a deep red background (Fig. 283). From the middle of the papilla the retinal vessels pass into the eyeball, and they are seen diverging from the papilla to ramify over the rest of the retina. The arteries can be distinguished from the veins by their brighter red colour as well as hy the stronger reflection of light from their sm'faces. The yellow spot is very difficult to see, except in atropinised eyes, since it only comes into view when the observed eye is looking straight into the ophthalmoscope. Under these conditions there is a strong ' light reflex,' and the pupil contracts up to a pin-point, unless paralj'sed by means of atropine. In order to see the blind-spot, or optic disc, the observed eye must be directed inwards ; thus if A is looking at the right eye of B, B must be told to look over A's right sioulder. Fig. 283. Ophthalmoscopic view of fundus of eye, showing the optic disc, or point of entry of the optic nerve, with the retinal vessels branchinc; from its centre. SECTION VII THE RETINAL CHANGES INVOLVED IN VISION In nearly all sense-organs the essential constituent is a bipolar nerve-ceU lia^ang one process extending towards the surface and ending between epithelium-cells covering that sui'face, and a central process, which runs towards the central nervous system, where it forms synapses with the pro- cesses of other nerve-cells (Fig. 284). In some cases, such as the oKactory cells and the sense-cells embedded in the epidermis of worms and other 7^il "-mm ^'^^^^'J'r^^^'. B C Fig. 284. A, olfactory sense-cell ; b, auditory sense-cell ; c, connections of gustatory fibres (taste-bud); d, nerve-ending in skin or corneal epithelium (probably pain fibres). invertebrata, the peripheral process is quite short. In other cases, as in the ordinary posterior root ganghon-cell, the peripheral process may be several feet in length. The retina, however, cannot be regarded as a simple sense- organ, but is homologous with a complete lobe of the brain. It is formed, like the cerebral hemispheres, as a hoUowed outgrowth from the fore-brain or anterior cerebral vesicle. The stalk of this outgrowth narrows so as to produce an optic vesicle connected with the rest of the fore-brain by the optic stalk. As the vesicle grows towards the surface its anterior wall is invaginated so that an ' optic cup ' is formed, at the mouth of which the lens and other parts of the eye are developed at the expense of the over-lying epiblast and the surrounding mesoblast. The posterior wall of the cup develops into the retinal pigment, while the nervous elements which make up the retina are formed by division and differentiation from the anterior 558 RETINAL CHANGES INVOLVED IN VISION 559 wall. That part of the cup originally derived from the external surface of the body is turned towards the posterior layer or retinal pigment. From it is developed the special end-organ of vision, viz. the rod and cone layer of the retina. Besides this special sensory epithehum, the retina presents two other sets of neurons through which impulses generated in the sensory epi- thehum must pass before they arrive at the optic nerve. The three relays of nervous elements in the retina have the following arrangement : (1) The first relay — the sense epithe- himi — consists of rods and cones with their nuclei (Fig. 285), the latter being situated in the outer nuclear layer. Each rod presents an external (a) and an in- ternal limb {b). The former, in the eye which has been kept in the dark, has a purphsh colour from the presence of rJwdopsin or visual 'purple. From the inner end of the inner hmb a fine fibre c passes to its nucleus in the outer nuclear layer, and from the nucleus a central process g passes into the outer molecular layer where it ends freely in a little knob e. The cones, which are thicker than the rods, also possess outer and inner hmbs. From the inner hmb a thick process containing a nucleus, passes through the external nuclear layer and ends with a broad base e in the outer molecular layer, from which short fibres are given ofi to come in contact with the bipolar cells of the imier nuclear layer. (2) The second relay is formed by the bipolar cells of the inner nuclear layer. Each of these sends ofi one fibre peripherally to make contact with endings of the rod and cone fibres in the outer molecular layer, and another process which passes centrally into the inner molecular layer. Here the process of the rod bipolar forms an arborisation aromid the body of a cell in the ganghon-cell layer, while the processes of the cone bipolars end at various levels in the inner molecular layer, forming synapses with the dendrites of the ganghon-cells. (3) The ganghon-cells, which represent the third relay, receive the impulses from the more peripheral parts of the retina and send them towards Fig. 2S5. I. a rod ; II, a cone of mammalian retina; /(. externa] limiting membrane. (R. Greeff.) 560 PHYSIOLOGY tlie brain along the fibres of the optic nerve, each of which is the axon of one of the ganghon-cells. These axons, which form the inner layer of the retina, the so-called ' nerve-fibre layer,' are non-mednllated as they pass over the surface of the retina, but acquire a medullary sheath as they pass out of the eyeball through the cribriform plate of the sclerotic and join to form the optic nerve. Most of the bipolar cells are connected with several rods or cones ; only in the fovea centraHs do we find a special bipolar cell provided for every cone. a TO b Fig. 286. Schema of retina. (From Bohm and Davidoff after Cajal.) 1, nerve-fibre layer ; 2, ganglion-cell layer ; 3, inner molecular layer ; 4, inner nuclear layer ; 5, outer molecular layer ; 6, outer nuclear layer ; c, cone ; r, rod ; b, bipolar cells ; S, spongioblast ; am, amacrine cell ; c.n, centrifugal nerve fibre ; M, fibre of Miiller ; n.M, nucleus of fibre of Miiller ; n, neuroglia ; o, outer limiting membrane. Every ganghon-cell comes into connection with and receives the impulses from a considerable number of bipolar cells, so that the number of fibres in the optic nerve running centrally is not so great as the number of sense elements in the rod and cone layer of the retina. Besides these cells situated on the direct path of the visual impulse, other cells of a nervous nature are found in the inner nuclear layer (the outer and inner horizontal cells), and also in the inner molecular layer, the so-called ' amacrine ' cells. These cells have been imagined to serve as a means of connection or association between different parts of the retina, and may be taken as analogous to the association- cells found in the cerebral cortex. The analogy of the retina with a lobe of the brain is illustrated by the fact that, in addition to the fibres originating in the retina and passing towards the brain, a considerable number of fibres pass from the central nervous system into the retina, where they end chiefly in the two molecular layers. These may have as one of their functions the correla- tion of processes occurring in the retinae of the two eyes and may be asso- ciated with phenomena such as those of binocular contrast, which we shall have to study later on. RETINAL CHANGES INVOLVED IN VISION 561 Important differences are found in the structure of the retina in its different parts. At the point of entrance of the optic nerve — ^the optic disc — the only elements present are the nerve fibres, which diverge from this point over the whole inner surface of the retina. A short distance externally to the optic disc is found the macula lutea with a small depression in the middle, the central spot oi fovea centralis (Fig. 287). When we fix our gaze on any object the visual axes are so directed that the image of the object falls on the fovea centrahs. At this spot the retina is thinned by the gradual disappearance of all its layers except the outermost. The outermost layer is moreover distinguished by the fact that the rods have disappeared and that \\\\\\ mm. \\\\\\\\\\\\\m\mmmmmmM iiMUiinUiUIUUW Fig. 287 Section through half the fovea centralis, and GoLDiNG Bird.) only the cones are present, and are very much larger than the cones in any other part of the retina. The fibres fi'om those cones, passing to the inner nuclear layer, diverge as they leave the fovea centralis, all the layers of the retina being displaced towards the circumference in order to allow the light to fall on the cones without having to pass through any of the other layers of the retina. As we pass from the centre to the periphery of the retina the cones become fewer and the rods more nmnerous. At the extreme margin the rods also are scattered more diffusely, and at the ora serrata, which hes a short distance behind the cihary processes, the special nervous elements come to an end, and the retina is continued forwards over the ciliary pro- cesses and the posterior surface of the iris as a layer, two cells thick, closely packed with pigment granules (the uvea). The following facts show that the layer of the rods and cones represents the end-organ of vision, and that for distinct vision to take place the image of an external object must be formed in this layer : 562 PHYSIOLOGY (a) The point of entry of the optic nerve, where the whole thickness of the retina is composed of nerve fibres, is absolutely insensitive to hght and constitutes the blind-spot (Fig. 288). If the hght of a small flame be Fig. 288. directed, by means of a mirror, into the eye so that it falls only on to the optic disc, the individual receives no sensation of hght. The existence of the bhnd-spot is more easily shown by the following experiment : On closing the left eye and gazing fixedly with the right eye at the white cross in the figure, on approxi- mating the book to the eye a point will be found, when the book is at a distance of eight inches from the eye, at which the white circle becomes invisible and the whole figure appears to be covered by the black ground. By measuring the apparent size of the bhnd-spot and its distance from the point of fixation, we find that its situation on the retina corre- sponds exactly to the point of entry of the optic nerve. The bhnd-spot is so large that ii V) Fio. 289. Diagram of the path of the rays of light in the for- mation of Purkinje's figures. V represents a retinal vessel. When this is lluminated from a, a shadow is formed on the hinder layers of the retina at a'. This is projected along a line passing through the optic axis, and ap- pears to come from a point {a") on the wall. On moving the light from a to b, the image of the vessel appears to move from a" to b". at a distance of about six feet the image of the head of a man will fall on it and therefore be invisible. (6) At the point of most distinct vision, i.e. the fovea centralis, all the layers of the retina are absent except the outermost, i.e. that of the cones. (c) ' Purkinje's figures.' If a strong light be focused by means of a lens on to the sclerotic just outside the cornea, and the eye be made to stare fixedly at a dull background, an arborescent image of the retinal vessels will appear on the background. On moving the illumination the image of the vessels will move in the same direction. Knowing the dimensions of the eyebaU and the distance of the background from the eye as well as the angle through which the light is moved and the apparent displacement of the image of the vessels, the distance of the sensory part of the retina behind the vessels may be calculated (Fig. 289). Direct measurements in this way have shown RETINAL CHANGES INVOLVED IN VISION 563 that the distance between the vessels and the sensitive elements of the retina must amount to between 0-17 and 0-36 mm. Anatomical measurements of the thickness of the retina show also that the average distance between the vessels and the layer of rods and cones varies between 0-2 and 0-3 mm., showing that it is in this layer that the actual transformation of a hght stimulus into a nerve impulse must take place. On spreading out the retina under the microscope and looking at its external surface we see that the rods and cones form a sort of mosaic, the thicker cones being surrounded by the smaller circles representing the cross-sections of the rods. Since each of these is a terminal sense-element the image thrown by the dioptric mechanism of the eye on to the retina must be converted into a mosaic-hke expanse of small isolated pictm-es, and our impression of external objects must be formed by a synthesis of the elementary sensations produced by the stimulation of every single rod or cone cell. DIRECT AND INDIRECT VISION If we fix our attention on to an object, we direct our eyes so that the image of the centre of the object falls exactly on the fovea centraHs of each retina. The diameter of the central spot is about 1 to 1-5 mm., which corresponds to a visual angle of 4:° to 6°. This angle therefore repre- sents the extent of the visual field in which we have distinct vision. The hght which falls into the eye forms an image of external objects which extends over the whole of the retina. The sensations excited by the stimulation of the periphery of the retina are much more in- distinct than those excited by the image on the central spot. The appreciation of external objects, by means of the image they throw on the external parts of the retina, is spoken of as indirect vision in contradistinction to direct vision, which implies fixation of the object and the formation of an image of it on the fovea centralis. The whole extent of the objects which we can see by direct and indirect vision is spoken of as the visual field. In order to determine the visual field we make use of a perime'^er (Fig. 290). ThLs instrument consists of a band of metal forming the arc of a circle of about 35 cm. radius. At one end this arc is fastened to a pillar, and can be turned through the axis imssing through the i^illar so as to lie in various meridians. At the centre of the circle is another pillar, which provides a chin-re^t, so arranged that the eye Fig. 293. Priestlov Suuth's perimeter. 564 PHYSIOLOGY of the observed person lies exactly at the centre of the circle at the top of the pillar. At the point round which the arc rotates is a small white disc. In using the instru- ment the person, whose field of vision is to be determined, places his eye at the top of the pillar and gazes fixedly at the white disc ; another small white disc is then moved along the curved arc and the point noted atwhich it is no longer visible, while theobserved person is gazing fixedly at the white disc on the axis of rotation. The arc is then moved 20° and the same experiment carried out, and this is continued until the limit has been determined in every meridian of the visual field. The rotating arc is graduate d, the graduations showing the visual angles subtended by any portion of the arc. As xn Fig. 291. Perimeter chart showing the field of vision in a normal (right) eye. the readings are made they are marked down on a chart, such as that shown in Fig. 291, so that finally a graphic representation of the visual field is obtained. The visual field is more extensive on the outer than on the nasal side of the eye, the latter being contracted by the cutting off of some of the rays falling on the outer side of the retina by means of the nose. Although stimulation of the peripheral parts of the retina does not give us much idea of the nature of the things vfe are looking at, yet it is of great importance in informing us of the relation of the object, which is the immediate point of attention, to its surroundings. It therefore plays a great part in regulating the movements of the body. Its importance will be at once appreciated if one eye be closed and the stimulation of the peri- pheral parts of the retina in the other eye be excluded by allowing this eye only to look through a tube. Although we can then see the objects to which we direct our gaze perfectly distinctly, we find that on trying to move towards any given object our movements are uncertain and misdirected. CHEMICAL AND PHYSICAL CHANGES IN THE RETINA When light falls on the retina chemical and physical changes take place ; these either originate or accompany the transmutation of the ether vibrations into the nerve impulses, which ascend the optic nerve. If a frog that has been RETINAL CHANGES INVOLVED IN VISION 565 in the dark for some time be killed, an eye taken out, bisected, and the retina removed and examined by a weak light, it will be found to have a purplish-red colour. On microscopical examination this colour is seen to be confined to the outer limbs of the rods. After a very short exposure to diffuse daylight the colour disappears. The colouring- matter {rhodopsin) may be dissolved out by means of a solution of bile salts. The purple-red solution thus formed also bleaches rapidly on exposme to hght. By means of this rhodopsin photographs or ' optograms ' of external objects may be taken on the retina. The rabbit's eye is cut out and placed in front of a window. After some time the eye is bisected and plunged into a 4 per cent, solution of alum, which partially fixes the optogram, and an inverted picture of the window with its cross-bars is obtained on the retina. If a retina, which has been bleached by exposure to light, be replaced on the pigment layer Kning the choroid, in a short time the colour will be Fig. 2'J2. Sections of the frog's retina. A, kept in the dark ; b, after exposure to the light, showing retraction of the cones, and protrusion of the pigmented epithelium between the outer limbs of the rods. (Engelmann.) restored. On examining sections through the retina it is found that, in one which has been exposed to light, the cells of the layer of pigmented epithehimi send up fine processes full of pigmented granules between the outer hmbs of the rods. In an eye which has been kept in the dark, on the other hand, the cells of the pigment layer are quite fiat, so that the front part of the retina, including the rods and cones, can be removed without any difficulty (Fig. 292). Thus the function of the pigmented epithelium is to supply visual purple to the outer hmbs of the rods as fast as the pigment already there is bleached by light. It might be thought that this chemical change was the active agent in producing excitation of the optic nerve fibres ; but the fact that in the fovea centrahs,* the region of most distinct vision, we find only cones which contain no visual pmple indicates that this chemical process is not essential for the conversion of hght-waves into a nervous impulse. * According to Edridge Green visual purple diffuses into the fovea centralis, and p'.iys an es^eatial part in vision as a sensitiser of the cones. 566 PHYSIOLOGY When light falls upon the retina the cones are retracted, and lie close upon the external hniiting membrane ; whereas in an eye that has been kept in the dark they extend down between the rods as far as the pigmented layer. II. ^tapltttl^ itt lid Ir ▼^. : ■ ; ' r^^WB^*|to|l»^irj^^ i .; t ! i . ■■:^u]-\]rii\i-,.,.,..\,Awm m ^^^i^ i ,-L-^ ;,,::.;; ::-Mii:±rtliiiii!.U!:i. Fig. 293. Electrical variation in frog's eye as recorclcd by the string galvanometer. (EiXTHOVEN and Jolly.) I, on exposure to a single flash ; II, on exposure to light of moderate duration ; III, effect on a light eye of momentary darkening. The falling of light on the retina is also accompanied by an electrical change, which may be regarded as analogous to the current of action in nerve. It is, however, much more complicated than the latter. The eyeball of the frog led off from its anterior and posterior surfaces shows a current directed in the eyeball from behind forwards (the resting or demarcation current). It was first shown by Holmgren that this resting current undergoes modification when light is allowed to fall on the eyeball. Of late years the nature of this modification has been studied especially by Waller with the galvanometer, by Gotch with the capillary electrometer, and by Einthoven and Jolly with the string galvanometer. The nature of the response varies according to the EETINAL CHANGES INVOLVED IN VISION 567 strength and duration of the stimulus, and to the conditirn of the eye, whether fatigued or fresh, light- or dark-adapted. A typical response to a momentary flash is shown in Fig. 293, I. Within a very short latent period after the incidence of the flash, i.e. after a latent period of not more than -01 sec, there is a small short negative variation of the resting cm-rent, which is immediately followed by a large positive variation, i.e. the resting current is increased. This is followed immediately by a diminution and II. Fig. 294. Diagrammatic representation of the reactions to light of the three hypothetical substances a, b, and c. (Eintho\ten and Jolly.) I, the three effects shown separately ; II, the three effects combined to form a single curve, a is the lighting and a' the darkening effect of the iirst substance. I, light ; d, darkness. then, after a considerable latent period, by a second slow prolonged increase of the ciurent. When the duration of the stimulus is longer the moment of shutting oflE the light is seen to be followed immediately by a second positive vaxiation. This . is shown in Fig. 293, II. It is possible to obtain this response to darkness bj'^ shutting ofE for a sliort period of time the light falling into the eye. The result of such an ex- periment is shown in Fig. 293, III. Einthoven and Jolly explain these results by the assumption that three separate processes are concerned when the retina is stimulated. For convenience they speak of the changes in tliree distinct substances, A, b, and c. Of these a reacts more rapidly than the other two, and its action is specially marked in a 'light' eye, appearing almost isolated on sudden darkening of short duration (a flash of darlaiess). On lighting, it develops a negative, on darkening a positive potential difference. The substance B reacts less rapidly than A. On lighting, it develops a positive, on darkening a negative potential difference. Its action is especially marked in a dark eye wliich is illuminated for a short time with a weak light. Sub- stance o reacts in the same sense as B, but much more slowly. Its action is wanting in a completely light eye. The actions of these three substances are represented diagrammatically in Fig. 294, I. and II. It is interesting to note that the latent period of the photo-electric reaction agrees with the latent period of the light perception of the human eye, and may vary from •01 sec. with strong stimuli to as much as 2 sec. with very weak stimuli. SECTION VIII VISUAL SENSATIONS The retinal changes which we have just described as occurring in the retina on .exposure to Hght give us very little information as to the nature and conditions of the physiological activity excited in this organ by the physical stimulus of Hght. We are therefore driven to use as our criterion of these physiological processes the changes excited in consciousness, and the greater part of our knowledge of the physiology of vision is derived from examination of such of our own sensations as have their primary origin in the retina. How far these sensations can be regarded as having their seat in the retina, how far they are determined by physiological changes in the visual and ad- jacent portions of the brain, it is not possible to say. We are only able to deal with the sensations as they spring ready formed into our consciousness. NATURE OF THE STIMULUS The word ' hght,' as employed by physicists, imphes a particular kind of energy, which, arriving at the retina in a certain way,, excites in us a sensation of hght. The conception is therefore primarily physiological. Every material substance is endowed with a certain amount of internal energy, the index to which is its temperature. In virtue of this energy it is constantly radiating energy at a greater or less rate through the sur- rounding ether, its internal energy at any given moment being determined by the balance between the amount of energy it gives off and the amount of energy which it receives from surrounding bodies. This radiant energy is transmitted through space as transverse oscillations of the ether at the rate of about two hundred thousand miles per second and with very variable wave-length and rate of oscillation. The whole energy available to us on the surface of the earth is derived from that portion of the radiant energy of the sun which is intercepted by the earth. Since the velocities of transmission of rays of various wave-lengths differ as these rays pass through a dense medium, such as glass, it is possible to break up the compound waves of radiant energy arriving at us from the sun, or emitted by any hot body, by allowing them to pass through a prism. When the luminous solar rays are passed in this way through a prism we get, as is known, a spectrum, the rays which are refracted the least being red, while those which are most refracted are violet. Between these two extremes we have rays of the following colours — orange, yellow, green, blue, indigo, which merge one into the other without any perceptible break. The 568 VISUAL SENSATIONS 569 different parts of the visible spectrum, when obtained from the sun, show vertical dark hues, which are known as Fraunhofer's lines. These are due to the fact that certain rays emitted by the glowing centre of the sun are absorbed in passing through the gaseous envelope which surrounds the sun. The hues are distinguished by certain letters and have all been assigned to the existence of known elements in a gaseous form in the solar envelope. Any part of the spectrum is distinguished according to its relation to these hues, since each of them has a constant wave-length. The visible spectrum ex- tends from the hne A at the hmit of the red, which has a wave-length of 760 milHonths of a milhmetre, to the hne H at the end of the violet with a wave-length of 397. The visible part of the spectrum does not, however, include even the majority of the rays which arrive at the earth from the sun. Beyond the red we get the ultra-red rays, which have a large amount of energy, so that their presence can be easily detected by their warming effect on blackened bodies, such as the blackened bulb of a thermometer or a thermo-junction, held in this part of the spectrum. In the same way, beyond the violet end there is a long extent of rays with high refrangibihty and small wave-length. Though not perceptible to the eye, they reveal their existence by the marked influence they exert on salts of silver ; they are therefore often spoken of as the actinic or photographic rays. If the investigation of the constituent rays of the solar spectrum were carried out at a considerable altitude above the sea and by means of c^uartz prisms and lenses, the extent of the imasible spectrum would be found to be largely increased, since the ultra-red and ultra-violet rays are absorbed by the constituents, especially the aqueous vapour, of the atmosphere. Why can we not see these rays as well as those in the middle of the spectrimi ? Is it that they are absorbed by the media of the eye, through which the ray of light has to travel before it reaches the retina, or is their invisibility due to an actual insensibility of the retina to rays of high and low wave-lengths ? On testing the absorption of these rays by the transparent media of the eye, we find that the absorption of the ultra-red rays is but slight and that a considerable proportion of these rays must be always arriving at the retina, so that their invisibiUty must be determined by the fact that they are unable to excite the special sensory elements of the retina. On the other hand, the absorption of the ultra--saolet rays by the eye media is practically complete, although these rays on arriving at the retina have the power of evoking sensation. Thus it has been observed that after extraction of the lens for cataract the visibility of the spectrmn, which in the normal eye only extends to the line H with a wave length of 397, is increased on the violet side so that the spectrum may be seen as far as a point corresponding to a wave length of 313. It is evident from this that in the absorption of the ultra-violet rays by the eye the lens takes a preponderating part. LUMINOSITY OF DIFFERENT PARTS OF THE SPECTRUM The physiological nature of om* conception of light is shown by the fact that the spectrum dift'ers in its luminosity, i.e. in its total stimidating''eftect 570 PHYSIOLOGY on the retina in its different parts, and that the relative luminosities of different parts of the spectrum bear no relation to the amount of radiant energy represented by each kind of wave length. The greatest energy is attained by the ultra-red rays and there is a gradual diminution from here to the violet end of the spectrum. The yellow part of the spectrum, however, appears much brighter than any other part. The relative luminosity of different parts is shown in the following Table by Vierordt : Part of spectrum Red'B' . Orange ' C ' Reddish yellow ' D ' Yellow ' D ' to ' E ' Green ' E ' Bluish green ' F ' Blue ' G ' . Violet ' H ' Luminosity 22 128 780 1000 370 128 8 1 The limited excitabihty of the retina and its special sensitivity to rays in the middle of the spectrum present considerable advantages for the normal functioning of the optical apparatus. No pro^asion is made for securing achromatism. The dispersion of the ultra-red and ultra-violet rays is so great in passing through the refracting surfaces of the eyeball that, if they all arrived at the retina, and this organ were sensitive to both kinds of rays, it would be impossible to obtain any clear image of external objects. The image formed by the ultra-red rays would be far behind the retina when the ultra-violet rays were focused on the retina and vice versa. As it is, the retina is unstimulated by the two ends of the spectrum, and its stimulation by the red as well as by the blue rays is only minimal, so that, for the excita- tion of a mosaic of spots on the retina in spatial extension and arrangement corresponding to that of the objects from which the light reaches the retina, practically only the middle part of the spectrum is of importance, and the distance of the image formed by the reddish-yellow rays from that formed by the green rays will not be great enough to cause any appreciable distortion of the exciting image. THE RELATION OF THE INTENSITY OF SENSATION TO THE STRENGTH OF STIMULUS Weber's law, viz. that the increase of stimulus necessary to give an in- crease of sensation always bears the same ratio to the whole stimulus, holds good for visual sensations. This ratio in the case of white hght is about j- 1^ . We can thus distinguish between two lights of 20 and 20 1 candle-power, both of them at the same distance from the eye, or between two of 99 and 100 candle-power. If the illumination be excessive the law no longer holds good, and we should be unable to tell the difference between two lights of the latter power if held close to the eye, or between two arc lamps at a con- siderable distance, even though one might be much stronger than the other. According to some authors our power of distinguishing differences in lumin- osity varies with different colours. VISUAL SENSATIONS 571 TIME RELATIONS OF THE EXCITATORY PROCESS When a stimulus of short duration, such as an induction shock, is applied to a muscle, the response of the latter bears no hkeness in its intensity and its time-relations to the exciting stimulus. The muscle after a short latent period begins to contract when the stimulus has abeady ceased to act. It contracts rapidly at first, then more slowly, and then relaxes. In the same way the sensation evoked by a momentary light stimulus takes a certain time to attain its maximum and then dies away slowly, persisting, that is to say, during a certain interval after the stimulus has been entirely withdrawn. This persistence of the visual sensation is experienced whenever we look at a brilliant source of hght, such as a candle or lamp, and then either shut the eyes or direct the gaze on to a blackened surface. We then see on the dark background a bright image of the candle or lamp, which gradually fades This phenomenon is often spoken of as a ' positive after-image.' If the object has been very bright the image in fading becomes coloured. It first appears greenish blue, which changes later to violet, rose colour, and finally orange or green. The slow rise and fall of the sensation evoked by a momentary stimulus, or by a change in the intensity of hght falling on the retina, are responsible for the blurred outhne of any object that is regarded while in rapid motion. If a disc with alternate sectors of black and white, such as is shown in Fig. 295, be caused to rotate slowly, it is easy to distinguish both black and white sectors. As the speed of rotation increases the margins of the sectors become blurred, and the fact that both margins of each white sector are blurred shows that the sensations produced both by the appli- cation and by the shutting off of the stimulus, and due to the white light reflected from the surface, are gradual yio. 295. and not instantaneous. With further increase in rate the whole disc takes a grey colour, which, however, oscillates or ' flickers,' and with a still further increase the ' flicker ' disappears and the disc has a uniform gi-ey appearance. The phenomenon is analogous to the phenomenon observed in muscle as the result of intermittent excitation. Each single shock gives rise to a contraction which is more prolonged than the shock itself. When the shocks are repeated we obtain, according to their frequency, a series of single contractions, a partial fusion of contrac- tions, so that an imperfect tetanus is produced (' flicker '), or finally — with a certain rate of interruption of the stimulus— complete fusion of contrac- tion, i.e. complete tetanus. We see therefore that when a portion of the retina is excited for a certain 572 PHYSIOLOGY period by rays of a given intensity during a period A, and is then unillumin- ated during a period B, if A + B is sufficiently small, e.g. in the case of the disc if the rotation is sufficiently rapid, the sensation evoked is a continuous one and is equal to that which would be produced by a continuous stimulation A which is equal to — . This fact is spoken of as Talbot's law. It enables A + B ^ us to produce a grey of any desired intensity by varying the relative sizes of black and white sectors on a rotating disc. For complete fusion to take place the period A + B need not be less than -04 sec. if we are using light of moderate strength. With low intensity of iUumination this value rises, i.e: complete fusion is obtained with a lower rate of rotation of disc than when we are using bright illumination. The value also varies according to the colour of the light. Different colours require different times for their action on the retina in order to produce the maximum sensation. If a spectrum be exposed to the eye for a very short period of time it appears colourless and shortened at the red end. If the period of exposure be increased the red and blue ends are seen, but no other colour is perceptible. The sensations due to the incidence of red rays attain their maximum in the shortest time, then come blue rays, while the green take the longest time to attain their maxi- mum. This difference between the time-relations and the sensations evoked by the various rays accounts for the fact that on rotating a disc of alternate white and black sectors at a certain rate the sectors appear blurred and bounded by coloured fringes. FATIGUE If a constant stimulus be prolonged, the intensity of the resulting sensa- tion rapidly diminishes, i.e. the apparatus concerned in the production of the sensation shows signs of fatigue. This diminution in the intensity of sensation may be observed so early as one-fifth of a second after the beginning of the stimulus. Connected with this fatigue of the retina is the phenomena known as the ' negative after-image.' If we look at a bright spot or source of hght for some seconds and then turn our gaze to a uniformly illuminated white surface, we see in the middle of the white surface, i.e. at the point of fixation, a dark image of the bright object which we had previously looked at. The stimulus applied to all parts of the retina in this case is uniform. Certain elements, however, i.e. those which had been previously stimulated by the bright object, are fatigued. Their response is therefore less than that of the surrounding untired retinal elements, and the resulting sensation is a dark image, which is referred to that part of the white surface from which proceeds the light falhng on the fatigued elements of the retina. ADAPTATION It is common experience that our eyes have the power of adapting their sensitiveness according to the degree of illumination. When we pass from dayUght into a dark room, such as the developing chamber of the photo- grapher, it is at first impossible to distinguish any objects even by the dim red light coming from the photographer's lamp or the window. After a short VISUAL SENSATIONS 573 time, however, we begin to distinguish objects more clearly. Any slight defects in the dark chamber begin to make themselves apparent, such as entry of light under the door and through cracks or nail-holes in the ceihng or walls. By direct measurement it can be shown that within ten minutes after passing from dayhght into complete darkness the sensitiveness of the retina increases twenty-iive-fold, and after two hours' exposure to complete darkness thirty-five-fold. On the other hand, on coming out of the dark room we are at first dazzled by the flood of hght with which we appear to be surrounded ; the pupils constrict to their utmost, and accurate vision is impossible on account of the excess of hght which seems to pour into our eyes. Very shortly this condition passes off, and within five minutes vision is once more normal, and the ordinary size of the pupils re- estabUshed. The process of adaptation affects not only the quantitative relation between the intensity of the stimulus and the resulting sensation, but deter- mines also a qualitative alteration in the reaction of the retina to hght. This is especially marked in the case of colours. On going into a flower-garden on a summer morning, when dawn is just beginning, although all objects in the garden can be clearly distinguished, there is a striking difference in its colom-tone as compared with that which it presents in dayhght. The scarlet geraniums have disappeared. On close examination this disappear- ance is found to be due to the fact that the flowers are dark, i.e. the hght from them does not stimulate the retina at all. The other coloured flowers can be distinguished, but only in shades of grey. With a very Uttle increase in illumination the blue flowers come into evidence and the prevailing tone of the garden is cold, made up as it is of greens, blues, and greys. With in- creasing illumination the reds finally make their appearance. After long exposm-e to the darkness of night the eyes have become dark-adapted. The same behaviour of the dark-adapted eye may be demonstrated in the laboratory. If a person who has been in a dark room for half an hour ob- serves a spectrum of low intensity the whole spectrum appears colom'less, its red end being cut off. The distribution of luminosity over the spectrum is also altered. Whereas the spectrum to the normal Hght-adapted eye appears brightest in the yellow between the hnes D and E, the spectrum of low intensity to the dark-adapted eye has its point of greatest luminosity in the green. This striking difference between the light-adapted and the dark-adapted eye does not apply to small objects the image of which, when the vision is directed towards them, will fall entirely on the fovea centrahs of the retina. In the dark-adapted eye, the sensitiveness of the central spot of the retina is not nearly so great as that of the more peripheral portion. On a dark night we are often able to distinguish a star which, however, disappears as soon as we turn our eyes so as to bring its image on the central spot of the two retinae. Moreover the qualitative change in relation to the colours observed in the dark-adapted eye does not apply to the fovea centralis. In a dark room a small spot of light, whatever its colour, when the visual 574 PHYSIOLOGY axes are directed on it, is seen in its true colour as soon as its intensity is suffi- cient for it to be seen at all. Before this takes place it may be seen by the peripheral parts of the retina, but as a colourless spot of light which dis- appears as soon as the gaze is directed on it. This marked difference between the beha^dour of the fovea centrahs and the more peripheral parts of the retina in the dark-adapted eye has been attributed to the difference we have already studied in the anatomical structure of these parts. The only per- cipient elements foimd in the fovea centrahs are the cones. In the adjacent portion of the retina we find also the rods which increase in relative number as we pass from the central spot towards the periphery. Schultz long ago pointed out that in the retinae of many night animals, such as the owl, the mouse, the cat, the rods are the predominating element, the cones being absent or very few in number. Von Kries has suggested that in aU proba- bihty the retina is endowed with two kinds of vision. {a) Vision by means of rods, which are colour-bhnd, so that on stimula- tion by any rays of the retina a sensation of white or grey is produced. The rods are chiefly excited by the more refrangible rays of the spectrum, being totally unaff'ected by the red rays. They show great power of adapta- tion. This form of rod vision may be connected with the visual purple. In the dark-adapted eye this pigment is found pervading the whole of the outer limbs of the rods ; it rapidly fades on exposure of the eye to light, so that it must be absent in the hght-adapted eye. On examining its absorption spectrum we find that its absorptive power is greatest for the rays in the green part of the spectrum, and that it allows the red rays to pass almost without absorption, i.e. it absorbs just those rays which experiment shows us have the greatest effect in producing a sensation of hght in the dark-adapted eye. (6) The cones, on this view, would represent a more highly differentiated apparatus of vision. They alone are present in that part of the retina which we use exclusively for distinguishing the finer details of surrounding objects ; they are sensitive to all colours, and when stimulated by all the rays of the spectrum simultaneously give rise to a sensation of white light. Their sensitiveness to illumination is, however, inferior to that of the rod apparatus. According to this theory, therefore, whereas in a dim hght we determine the position of surrounding objects and differences in their luminosity by means of the rods, the greater part of our visual impressions, including all that we obtain by dayhght and our knowledge of the finer visual qualities of things, are brought to us by the intermediation of the cones. COLOUR VISION If a ray of white hght be passed through a prism it is widened out into a bright coloured band or spectrum, the red rays, which are least refrangible, being at one end, and the blue rays at the other. It is usual to divide the colours of the spectrum into seven — red, orange, yellow, green, blue, indigo, violet ; but the division is an arbitrary one, and the colours shade into one another so gradually that no two observers would agree exactly on the limits between them. The difference of wave-length necessary to give a dis- VISUAL SENSATIONS 575 tinct difference in colour varies according to the part of the spectrum which is under observation. In the middle of the spectrum it is between 0-7 /x/a and 2-0 /w/i. At the red end a difference of 4-7 /x/x is required to evoke a new quahty of sensation, and at the extreme end, both red and violet, there is a section of the spectrum over which no variation in colour can be perceived. By passing the rays forming a spectrum through a similar prism placed in the reverse direction, all these rays can be recomposed to form white hght. The sensation of white hght is therefore due to the simultaneous incidence on the retina of all the rays of the spectrum. That we have no suspicion of the existence of these rays when we experience a sensation of white shows that our eye does not possess any resolving or analysing apparatus such as exists in the internal ear for the compound wave of sound. Any part of the spectrimi, or any coloured object, may be characterised in three different ways : (1) LUMINOSITY. The luminosity of different parts of the spectrum varies, being greatest in the yellow for the hght-adapted eye. We could, however, match the luminosity of the red of one spectrimi with that of the yellow of a second spectrum by increasing the intensity of the beam of hght used to produce the first spectrum. (2) SHADE OR COLOUR. It has been reckoned by Konig that in the spectrum we can distinguish 165 different shades of colom*. Edridge-Green has shown that when a normal observer screens off the rest of the spectrum until the part left appears monochromatic, and repeats this operation through the whole length of the spectrum, he wiU mark off' not more than eighteen to twenty-seven ' monochromatic patches.' There are certain colours which can be appreciated by the eye which are not present in the spectrum, such as the varpng shades of purple. (3) SATURATION. When we look at a colom-ed surface, e.g. red, our eye is stimulated partly by the white light which is reflected in toto from the surface, partly by the red rays which are specifically reflected and give the colour of the object. According as these red rays are free from niixtme with white rays, so their satm'ation is said to increase. The degree of saturation of any colour can be determined by regarding it through a spectro- scope. A completely saturated red would give only rays at the red end of the spectrum. We can, however, speak not only of a physical but of a physio- logical saturation. xVccording to the condition of the retina and nature of the stimuh to which it has been previously exposed, so does the saturation of any given colour vary. It might at first be thought that the retina could respond with a simple sensation to a stimulus by any part of the spectrum, a low number of ether vibrations per second producing a sensation of red, a number rather higher a sensation of orange ; so that there might be as many simple coloiu-sensa- tions as we can appreciate different shades in the spectrum. But a simple analysis of our own sensations seems to show that some of the spectral colours, although simple in so far as the stimuli are concerned, are compomid so far as the sensation is concerned. Thus most people would say at once that orange 576 PHYSIOLOGY is a mixture of red and yellow, and, as a matter of fact, we find that on mixing rays from the red with others from the yellow part of the spectrum we do obtain a sensation of orange. The stimulus obtained by mixing the red and yellow rays is not the same as a stimulus caused by rays from the orange part of the spectrum. In the former case compound waves made up of the two wave-lengths, 656 ijljjl and 564 ixfx, are falling on the retina, in the latter case a simple wave with a length of 608 /x^t, and yet the sensations produced are identical. Experience shows that there are relations between the physiological effects produced by different parts of the spectrum which have no physical analogue in the stimuli themselves. Such, for instance, are the phenomena known as the coloured after-image. We have seen that, as the result of fatigue, stimulation of any part of the retina by a bright object produces, when the stimulation is removed, a dark after-image, which has its seat in the previously stimulated portion of the retina. If we look stead- fastly for a minute in a bright light at a red disc on a white ground and then look away at a uniform white surface, we see an after-image of the disc on the sm'face. This after-image is, however, green, and the white background takes on a reddish tinge. If the disc in the first instance has been a greenish blue the after-image is red, and to every colour in the spectrum we find there corresponds another which represents the after-image evoked by stimulation with the first colour. If the first disc has been green the after-image will be purple, and vice versa. We can therefore arrange the spectrum into a series of pairs of colours which are known as complementary. The following is a list of such pairs, with wave-lengths of the rays involved, as determined by Helmholtz : Colours Wave-lengths Red — ^greenish blue .... 656 — 492 Orange — blue Bright yellow — blue Yellow— indigo Greenish yellow — violet 608—490 574—482 567-^65 564—433 If the retina be stimulated by any of these pairs of colours simultaneously the effect is not that of colour, but of white. White light, or the sensation of white, can thus be due either to simultaneous stimulation of the retina by all the rays of the spectrum, or to stimulation of the retina by pairs of colours which are known as complementary. If these pairs be taken rather nearer in the spectrum a colour is obtained representing a part of the spectrum situated between the two constituents of the pair. If the rays are further away than would correspond to the complementary colours we obtain again a coloured sensation, which, however, is unsaturated, being mixed with a certain amount of white light. By taking three colours, such as red, green, and Aaolet, or four colours, such as red, yellow, green, and violet, it is possible by mixing them in various proportions to form either white light or any colour of the spectrum, besides the various purples which do not occur in the spec- trum. These experiments of mixing colours can be carried out in various ways : In every case we aim at stimulating the retina simultaneously with rays of different VISUAL SENSATIONS 577 wave-lengths, or successively at such short intervals of time that there is complete fusion of the seasations resulting from the individual excitations. In order to deter- mine fine differences in shade a coloured surface is always provided with which a colour produced by the fusion of the different rays under experiment may be compared : (1) The most exact method of mixing colours is to employ a couple of spectra and by means of prisms to bring different parts of the spectra on one and the same white surface on which the result of the mixture can be compared with sample colours. (2) In Maxwell's ' colour top ' discs with a radial slit are placed one over the other on a disc which can be made to revolve with considerable rapidity. By means of the slit two or three discs of different colours can be slid one into the other, so that the disc is composed of sectors of variable extent of the two or three colours wliich we wish to mix. These discs are generally mounted on a background of a larger disc, which can be used as a sample colour to compare with the result of the mixture of the colours in the centre. If we are determining the relative amount of different colours necessary to produce white, the outside disc would be partly white and partly black, so that on rotation the effect of grey is produced, i.e. a weak white. Thus in one experiment the small discs in the centre were red, green, and blue. It was found that when the sectors were chosen in the following proportion : 165 red, 122 green, and 73 blue, a grey colour was produced equal to the grey obtained in the outer disc by mixing 100 white with 260 black. These methods and all others in which coloured surfaces are used suffer from the defect that no pigments give perfectly pure coloiu'-sensations. On looking at a red painted surface, for instance, in a bright light with a spectroscope, it will be found that the spectrum contains yellow and green rays as well as red. On this account no information as to the effect of mixing colour- sensations can be obtained by mixing the pigments themselves. Thus a familiar way of producing a green pigment is to mix a blue and a yellow pigment. A mixture of blue and yellow rays will give a sensation of white. The fact that blue and yellow pigment mixed together give a green pigment is due to the fact that the blue pigment cuts off the red and yellow rays, and reflects, or allows to pass, the blue and green rays, while the yellow pigment retains the blue and violet rays, but reflects the red, yellow, and green rays. When we mix the two we get, not an addition, but a subtraction ; the blue pigment absorbs the red rays which the yellow pigment allows to pass, and the yellow pigment absorbs the blue rays which the blue allows to pass. The only rays left over are the green, and the mixture of pigment has a green colour. THEORIES OF COLOUR VISION These facts show that in all probabiUty the primitive colour-sensations are few in number, and that the various sensations excited by the different parts of the spectrum are not simple, but are compounded of mixtures of these primary sensations. The two theories which have obtained most vogue, and which enable us to account for a certain number of the phenoniona associated with colour- vision, are those known as the Young-Helmholtz theory and the Hering theory. {a) THE YOUNG-HELMHOLTZ THEORY. This theory, which was first put forward by Young and elaborated by Helmholtz, assmnes that there are three primary colour-sensations— red, green, and violet — which are represented by three separate sets of elements in the retina, or in each 19 578 PHYSIOLOGY cone, or by separate substances, each of which is afiected by one of these colours. Thus the rays with a longer wave-length excite chiefly the red fibres or elements ; those of medium wave-length the green percipient element ; those of short wave-length the violet percipient element of the retina. The excita- bihty of these three elements by the rays of different parts of the spectrum are represented in the figure (Fig. 296). At the extreme red and violet end Gr. JBl. V Fig. 296. Curves showing sensitiveness of the three varieties of nerve fibres to different parts of the spectrum. 1, red fibres ; 2, green fibres ; .3, violet fibres. of the spectrum alterations of the wave-length cause no alteration in colour, showing that these rays only excite either the red or the violet fibres. All other parts of the spectrum excite the three sets of fibres simultaneously, but to varying degrees. Thus the greater part of the red, e.g. about ' C,' excites the red percipient element strongly, the other two only weakly ; the result is a sensation of red. The yellow rays at ' D ' excite almost equally the red and the green percipient elements, but the violet only shghtly. Yellow is therefore a mixed sensation. The green rays excite the green per- cipient element strongly, the other two shghtly, with green sensation as a result. Blue rays excite green and violet percipient elements to a moderate extent, and the red rays to a somewhat less extent ; blue is therefore a mixed sensation composed chiefly of green and violet. Simultaneous excitation of all three elements evokes a sensation of white or grey, according to the intensity. The cases of defective colour-vision which occur — the so-called colouc- bhndness— are regarded under this theory as due to the absence of one or other of the elements which determine the primary colour-sensations. The normal eye is spoken of as trichromatic since it has three primary colour- sensations. Two kinds of dichromatic eyes are described — those in which the red sensation is lacking, and those in which the green sensation is lacking. In either of these cases the subject confuses red and green. The ripe cherry on a tree they may distinguish from the leaves, not by their colour, but by form or difference in their luminosity. It is only when they are tested by means of the spectrum that we find that whereas in the first case (red blind- ness) there is insensibihty to the red end of the spectrum, in the second case the red end of the spectrum is seen as well as by the normal person, so that VISUAL SENSATIONS 579 the defect must be located towards the middle of the spectrmn. Theoreti- cally, of course, violet bhndness ought also to exist, but cases of this nature are so rare that their very existence is doubted. Cases have also been recorded with total colour-bhndness (so-called monochromatic vision). Such cases have been supposed to be endowed only with vision similar to that found in the dark-adapted eye, i.e. with sensations only of white and black rather than vision determined only by the existence of the violet-perceiving constituent of the cones. Indeed the phenomena we have studied under the heading of dark adaptation would tend to show that, if we accept the Young-Helmholtz theory, we should add to the primary visual sensations those of hght and dark, which have their seat in the rods. (6) THE HERING THEORY. According to Hering there are four, and not three, primary colour-sensations, viz. red, yellow, green, and blue. In this theory the sensations of white and black are also regarded as primary visual sensations. These sensations are placed in three groups — ^red and green, yellow and blue, white and black. For each pair of sensations he con- siders that there is a special substance in the retina, dissimilation or cata- bolism of which gives rise to one colour-sensation ; anabohsm or assimilation to the other. Thus if white light falls on the retina, it causes a breaking down or cataboUsm of the white-black substance. This breaking down excites certain fibres of the optic nerve, and produces in consciousness a sensation of white. If the light be now removed, this breaking down gives place to anabolism or building up of the white-black substance, which excites the same nerve fibrils in a different way, giving rise to a sensation of black. The white-black substance is affected not only by white light but also by the colours red, green, yellow, blue, and their mixtures. The other two ^nsual substances are affected only by red and green or by yellow and blue respec- tively. Hence even the spectral colours do not give rise to pure sensations, there being always some mixture of a sensation of white with the proper colour-sensation. Most of the phenomena of colour-^dsion that we have mentioned above can be equally well explained on either theory. Thus the fact that blue and yellow together give rise to a sensation of white may be explained on the Young-Helmholtz theory by saying that the stimulation of all three sets of fibrils is equal, as wiU be seen by adding together the ordinates of each curve in Fig. 296 at yellow and at blue. Adopting Hering's h}^othesis, we may say that, anabolism and catabo- lism being equally excited in the yellow-blue substance, no change in it takes place, and the sole sensation is that produced by the stimulation of the white- black substance. The pairs of colours that we have distinguished are there- fore, according to this theory, not in the strict sense of the word comple- mentary, but antagonistic. The fact that white hght appears to us as a simple sensation and gives us no suspicion of the coloured rays of which it may be composed is in favour of Hering's theory. Cases of colom--blindness would be reduced by Hering to two classes, viz. those in which the red-green sub- stance is lacking and those in which the yellow-blue substance is lacking. 580 PHYSIOLOGY Most of the data with regard to colour-blindness have been worked out with reference to the Young-Helmholtz theory, and have therefore been inter- preted in accordance with this hypothesis. It is very difficult, however, to harmonise the facts of colour-blindness either with this or with Hering's hypothesis. It is better therefore to abandon hypotheses altogether and to adopt a purely empirical classification of colour- vision, as has been done by Edridge-Green. This observer points out truly that very marked colour- bhndness may be present mthout any interference with the appreciation of the luminosity of any part of the spectrum. A person may be able to see the spectrum up to its extreme red end, and yet distinguish in the spectrum only two colours, which we may caU red and violet. We may, in fact, regard discrimination of colour differ- ence as superadded to and evolved later than the appreciation of light. Discrimination may show various degrees of deficiency without any inter- ference with the appreciation of luminosity. According to Edridge-Green a normal individual will name six distinct colours in the spectrum — red, orange, yeUow, green, blue, violet. Such an individual, when made to map out the spectrum in the manner indicated on p. 575, will distinguish about eighteen monochromatic patches. A few individuals will place another colour, which has been called indigo, between the blue and the violet, and will mark out from twenty-two to twenty-nine monochromatic patches. ' Colour-bhndness ' may be brought about by one of two conditions : (a) a shortening of the red or violet end of the spectrum ; (b) absence of power to discriminate between the colours in the spectrum. The former condition may be present with complete power of discrimination between the different parts of the spectrum which are visible. Thus in normal individuals the hmit of the visible red spectrum is between X 760 and X 780. In a certain number of cases it is foimd that the spectrum is not visible beyond X 700 with bright light, or beyond X 620 with dim light. Such cases may be said with truth to suffer from red bhndness, and they will be unable to see a red lantern or appreciate its colom' imless the red fight is mixed with a considerable amount of orange. They may be detected by testing their power of mixing colours. A rose colour consists of a mixture of a violet and red light. In an individual with a shortened red end of the spectrum only the violet element of the rose would be visible, so that he would be inclined to class it with the blues rather than with the reds. Cases also occiu- in which there is a shortening of the violet end of the spectrum, but they have little or no practical importance. Of the second class of cases, distinguished by deficiency of power of discriminating colours, all grades are known. A very large proportion of individuals, as much as 20 per cent., present a power of distinguishing colours which is below normal, and Edridge-Green distinguishes these various classes (calling the normal person hexachromic) as pentachromic, tetra- chromic, trichromic, and dichromic. If we regard a spectrum in very dim light it appears grey. With a sfight increase in luminosity we can make out two colours, red at one end and violet at the other. On fiuther increasing VISUAL SENSATIONS 581 the luminosity the spectrum appears trichromic, being composed of the colours red, green, and violet. In colour-blind individuals this limitation of the colours distinguished applies to all strengths of luminosity. Thus the dichromic sees only red and violet, the trichromic sees red, green, and violet. It is the dichromic cases to which the name of ' colour-blind ' has been chiefly applied, the trichromic cases being often missed, or classified simply as ' colour weak.' The name of ' anomalous trichoraats ' has been applied to people -who, while able to discriminate most of the spectral colours, use abnormal proportions of the red and green, when they mix these colours to form yellow. The ordinary ' red-blind ' person is generally a dichromic with shortening of the red end of the spectrum. The ' green-blind ' person is a dichromic without shortening of the red end. It is an instructive experience to make either a dichromic or a trichromic mark out on a spectrum a monochromatic patch. In a dichromic, such a patch at the red end will include red, orange, yellow, and green. In a trichromic, red, orange, and yellow will probably be included in the patch. This method is the most accurate way of determining diminution of the power of colour discrimination and shows with ease even the minor degrees of colour blindness. CONTRAST PHENOMENA Simultaneous Contrast. If a grey disc be placed on a piece of red paper, and the whole covered with tissue paper, the disc will take on a greenish tinge. If the groimd colour be green, the disc will appear red ; if blue, the disc will appear yellow ; in fine, whatever be the ground colom-, the colour of the disc wiU be complementary to it. These eftects are spoken of as simultaneous contrast. Successive Contrast. If, after gazing steadily for some time at a red disc on a white surface, the eyes be turned towards a plain white sm'face, a negative after-image of the disc is seen on the paper coloured green, i.e. the complementary colour of the red disc. Sm'rounding this the paper appears red. If we look at the sun for some time, and then tm-n our eyes away, there is at first a positive after-image, and we see a bright sun wherever we look. In a short time this disappears and gives way to a black sun (a negative after-image). Thus we may say that stimulation of any part of the retina with any colour is followed by a colour sensation referred to the same part of the visual field and complementary to the first. It has been much discussed whether these phenomena are simply effects of judgment, or whether they are produced by definite changes taking place in the retina. Helmholtz explains them by the fii'st hypothesis, and looks upon them as cerebral processes. Hering, on the other hand, has extended his theory so as to embrace these phenomena, and ascribes them to definite changes in the retina, or at any rate in the peripheral part of the visual mechanism. A corollary to his theory that we mentioned above is that if dissimilation of a visual substance be excited at any point of the retina, assimilation of the same substance is set up in the carts of the retina 582 PHYSIOLOGY immediately adjoining that point. To this process the name of ' retinal induc- tion ' has been given. In this way the phenomena of simultaneous contrast may be explained. Thus if a ray of red light falls on any spot, it may be supposed to excite dissimilation of the red- green substance at this spot. This sets up assimilation of the same substance in the adjoining parts of the retina, and the red object is therefore surrounded with a green halo, which at once becomes evident if we increase our appreciation for slight colour- tones by diminishing the total amount of light by means of tissue-paper. It has been much debated whether the contrast phenomena depend upon psychical or retinal events. There is no doubt that the question must be answered in the latter sense, and that these phenomena are quite independent of the judgment of the individual. This is shown clearly by two experiments. A box (Fig. 297 ) is di-^dded into Purple Purple Yellow Green Purple Purple Fig. 297. two long compartments, a h and c d. At a the compartment is closed by a red glass- plate and at c by a blue glass-plate. Apertures are provided at b and d for the observer's eyes. At + and -I- two small grey crosses are fixed about the middle of the com- partment on sheets of transparent glass. On looking through the openings b and d and converging the eyeballs, so as to fix the line o, we get a fusion more or less complete of the two colours red and blue, so that the background appears purple ; or there may be a struggle between the colours, at one time blue, at another red predominating. To the judgment, however, there is one background and not two, and therefore, accord- ing to the theory of Helmholtz, the grey crosses should by contrast both acquire the same induced colour, which would be complementary for purple. But it is found that the two crosses are perfectly distinct in colour, that which is seen by the eye against the blue ground being yellow while that on the red ground is green, showing that the phenomena of simultaneous contrast are peripheral and not centra'l in their causation. The same fact is very definitely established by the following experiment devised by Sherrington. The disc (Fig. 298) presents two rings, each half-blue and half -black. The outer ring is intensified when at rest by simultaneous contrast, the black half being seen against the surrounding yellow, while the luminosity of the blue half is increased by the effect of the surrounding black. In the inner ring the blue half is darkened by contrast with the surrounding yellow, while the black half is not evident at all. If the disc be rotated, we get two concentric rings on an apparently homogeneous field. It is found, however, that the outer ring flickers long after complete fusion has taken place in the inner ring, showing that the stimulation of the retina by the outer ring is increased under the influence of contrast. VISUAL SENSATIONS 583 On this theory successive contrast phenomena are analogous to certain phenomena we have already studied in other tissues. If extensive breaking down of the visual stuff has been occurring, when the stimulus is removed there will be a swing-back of the condition of the protoplasm of the nerve- endings in the opposite direction, and the catabolic will be replaced by anabolic changes ; just as, on breaking a constant current that has been flowing through a nerve, the condition of raised irritabiUty at the cathode gives place to a condition in which the irritability is depressed below the normal. The improving effect on the heart of stimulation of the vagus is also analogous to a successive contrast effect. During stimulation of the vagus the breaking down of the contractile substance is stopped or checked, so that building up or anabohsm can go on without interruption. When Fig. 298. the excitation of the vagus ceases there is an extra store of contractile material in the muscle- cells. This causes the beat to be more vigorous, and we may say that the increased anabohsm has been followed by a period of increased catabolism, just as strong stimulation of a part of the retina with green (anabohsm) gives rise to a red after-image (catabolism). It seems probable that, as McDougall has pointed out, the examples of simultaneous contrast depend on inhibitory processes analogous to those which we have studied in the spinal cord in deahng with reciprocal innerva- tion and the conditions for the isolation of any effective reflex. Just as the flexor reflex, due to nocuous stimulation of the paw, inhibits the extensor or stepping reflex, by blocking the synapses of those elements on the sensory path which would pom- their impulses into the final common path of the extensors, so stimulation of any portion of the retina will tend to inhibit processes of a similar kind occurring in adjacent parts of the retina or their central connections. Stimulation of one part of the intestine causes inhibition of the acti\aty of the intestine below the point of stimulation. If this inhibition occurred 584 PHYSIOLOGY on both sides of the stimulated spot we should have a phenomenon exactly analogous to the process of simultaneous contrast. In the same way the increased briskness of the antagonistic reflex which foUows the temporary inhibition accompanying the primary reflex can be compared to the negative after-image resulting on prolonged stimulation of any point in the retina, while alternating after-images, positive and negative, which may succeed a single positive stimulation, have their analogue in the alternating rhythmic movements of flexion and extension of the two legs which may follow single stimulation of one of them. The phenomena of binocular contrast show that the retinse are not alone concerned in the production of these pheno- mena, but that we are dealing also with the central connections of the optic nerves, the activities of which must be regulated by the same laws as those determined from our study of the more lowly activities of the spinal cord. SECTION IX MOVEMENTS OF THE EYEBALLS In order to obtain stinct vision of any object, an image of it must be formed on the fovea centralis. Visual attention, i.e. the fixing of the gaze on any object, involves the adjustment of the visual axes by movements of the eyeball, in order that the image of the object of attention shall fall on the central spot in each eye. The eyeball rests on a pad of fat surrounded by a denser capsule of connective tissue, the capsule of Tenon, from which it is separated by a lymph space. Within this space it is able to rotate around axes which pass through a point almost in the middle of the eyeball. These movements of rotation are carried out by the six ocular muscks, the four recti and the two oblique. The four recti musclcs^ — the superior, inferior, external, and internal — arise from a continuous tendinous oval rin^ which is attached at the back of the orbit to the margin of the optic foramen and sphenoidal fitsure. From this common origin the muscles pass forwards as flat bands close to the walls of the orbit ; they come in contact with the eyeball at its equator, passing through the surromiding lymph space, and are attached to the eyeball about 7 mm. behind the margin of the cornea, their positions being indicated by their names. Of these muscles the internal rectus is the thickest and strongest, the superior rectus is the thinnest and weakest. The superior oblic^ue muscle arises in a short tendon attacheel to the back part of the orbit in front of and internal to the optic foramen. The muscle runs forward in the upper and inner angle of the orbit and ends in a round tendon, which passes through a fibrous pulley at the upper and imier angle behind the anterior margin of the orbit. The tendon then makes a sharp bend and passes outwards, backwards, and down- wards between the superior rectus muscle and the eyeball, and is attached to the latter just below the outer edge of the superior rectus, behind the attachments of the rectus mascles and about half-way between the anterior and posterior poles of the eyeball. The inferior oblique muscle rises from the orbital plate of the sui^erior maxilla, just witliin the anterior margin of the orbit. The muscle forms a flat band and passes upwards, backwards, and outwarels between the inferior rectus and the wall of the orbit, and ends in a tenelinous expansion, which is inserted under the external rectus muscle into the posterior and outer part of the eyeball, somewhat behind the line of attach- ment of the superior oblique muscle. In discussing the actions of these muscles we may assume the eyes to be in what is known as their primary position, i.e. with the visual axes parallel and directed to a point on the horizon. It is evident that if the muscles rotate the eyeball about an axis in a plane at right angles to the visual axes, the pupils will move inwards, outwards, or in any direction, but will not rotate. On the other hand, if the axis of rotation of a movement produced by any muscle lies in a plane which is not vertical to the visual axes, then 585 19* 586 PHYSIOLOGY a movement of the pupil will be associated with rotation of the eyeball. The j&rst of these conditions is practically fulfilled by the external and internal rectus muscles, as is shown in the diagram (Fig. 299). These muscles by their contraction will move the pupil directly outwards or directly inwards. The axis of rotation in the diagram would run vertically through the eyeball. The other four muscles produce movements with axes of rotation which are not at right angles to the visual axis. The rectus superior, for instance, produces rotation round the axis joining r.sup. and r.inf., and will therefore produce movement of the eyeball upwards and inwards with a certain amount of rotation of the eyeball on its antero-posterior diameter. The rectus inferior in the same way moves the eyeball downwards and inwards with oiLsup. r.iTif r.ext. T.int r.ext. T.snp. r.inO r.ijd:. Fig. 299. Diagram to show points of attachment and liens of action of extrinsic ocular muscles. Fig. 300. Diagram to show direction in which pupil will move under the action of the various ocular muscles. rotation. The obhquus superior moves the eyeball downwards and out- wards, and the obhquus inferior upwards and outwards, in both cases with some rotation on its antero-posterior axis (Fig. 300). The only movements of the eyeball therefore which can be carried out by the action of one muscle, or by the reciprocal action of a pair of muscles, are the movements outwards and inwards, which are involved in the common actions of convergence of the visual axes and conjugate deviation of both eyeballs. Movements upwards or downwards will require the co-operation of at least two muscles. In order to direct the gaze upwards a contraction of the superior rectus which moves the eyeball upwards and inwards must be associated with a contraction of the inferior oblique muscle which rotates the eyeball upwards and outwards. In the same way the inferior rectus muscle will be associated with the superior obhque. As we have seen, four out of six muscles when acting singly cause'rotation of the eyeball on its antero-posterior axis. The question arises whether MOVEMENTS OF THE EYEBALLS 587 these movements of rotation ever occur under normal circumstances. This may be tested in the following way : The gaze is first directed at a brilliant line of light, e.g. a straight electric incandescent filament. The gaze is then directed to a uniform white surface. On this white surface a negative after- image of the vertical line is seen. It is found that in whatever direction the eyes be tm-ned, upwards, downwards, or obliquely to right or left, the after- image always retains its vertical direction. If there were rotation of the eyeballs on their antero-posterior diameters, the stimulated portions of the retina, i.e. those which are the seat of the after-image, would lie obliquely, and the apparent direction of the negative after-image would be also obHque. We see therefore that, under normal circumstances, no rotation of the eyeball on its antero-posterior diameter takes place. The actions of the difierent muscles are always so co-ordinated that all movements of the eyeballs take place round axes, which he in a plane passing through the centre of rotation of the eyeballs and at right angles to the visual axes. In man movements of the eyes are always bilateral and take place in such a way that an image of one and the same object will fall on the central spot of each retina. These movements are simple in character and are of only two kinds, viz. : (1) Movements of both eyes with maintenance of parallehsm of the visual axes ; to this class belong the movements of conjugate de\nation employed in following the passage of an object across the field of vision from right to left, or vice versa, as well as less extensive upward and downward movements of both eyeballs. (2) The movement of convergence of the axes of both eyes, which is always associated with accommodation for near objects, and therefore with contraction of the cihary muscles and of the constrictors of the pupils. Other movements can be effected, but only with effort. Thus we can converge the axes of the eyes so as to look at a near object lying to one side of us. Such an action is, however, associated with considerable effort, and in nearly all cases is replaced by movements of the head. Whenever we wish to examine an object closely we turn the head so that the object lies between the two eyes, and a simple movement of convergence serves to bring the image of the object on to the two foveas centrales. BINOCULAR VISION— CORRESPONDING POINTS When we fix om* gaze on any object, although an image of the object is formed on each retina, the object appears as single and not as double. On the other hand, gentle pressm-e on one eyeball, so as to shift its gaze slightly from that imposed on it by the co-ordinated action of the ocular muscles, at once causes the object to appear double. This shows that the single appear- ance of an object seen with the two eyes is due to the fact that the image from this object must fall upon points in the retina, simultaneous stimidation of which produces only a single sensation. These points are known as ' corres-ponding 'points.' It is evident that to each point in one retina only one point can exist in the other retina which corresponds to it, since for every 588 PHYSIOLOGY position of the eyes a luminous point can only throw an image on a definite point in each retina. Such corresponding points are, in the first place, the central spots in each retina. When we look at any spot the axes of the above eyes are so directed that the image of the spot falls upon the fovea centralis of the two retinse. The image of all points to the right of this spot will fall on the nasal side of the right retina and on the temporal side of the left retina, and vice versa. If the right retina were cut out and placed on the left, the corresponding points of the two retinae would be nearly over one another. This statement is not absolutely accurate, as is shown by a careful investigation of the corresponding points by means of the haploscope. In this instrument a white screen is placed vertically at the far point of vision of the eyes, which are made some- what myopic by means of a convex lens. Each eye looks through a cylindrical tube, the axis of which coincides with the visual axes. On each half of the white field is a mark which, however, appears single, since its image falls on the fovea centralis of the two retinae. If in the left field of vision a line be drawn vertically upwards from the mark, and in the right field of vision another line vertically downwards, the two lin:s appear as a single line passing through the mark. It will be seen that the upper half of the line is not absolutely continuous with the lower half, but appears to form a slight angle with it, showing that the general statement made above is not absolutely correct. By means of this instrument we can determine the extent and situation of all the points in the two retinse which correspond. The totality of all the points in space, the lines from which to the eyes will fall on corresponding points, is known as the horopter. Its determination is, however, only of theoretical interest. It might be thought that single vision with the two eyes was due to the fact that the nerve fibres from corresponding points in the two retinse pass to and terminate in identical nervous structiu'es in the brain. This is not the case. Single vision is largely the result of experience, and we can show by experiment that both elements are present in the fused sensation obtained from the two eyes. Thus if the left eye is directed on to a red surface and the right eye on to a blue surface, we may obtain either a fusion of the fields with the production of a purple colour, or a struggle of the two fields so that the whole field appears sometimes red and sometimes blue. In the same way if two figures, lying side by side, one ruled with vertical lines and the other with horizontal lines, be looked at so that the image of the right figure falls on the right retina and of the left figure on the left retina, we do not see a single figure with the lines crossing one another so as to form a network, but we again obtain a struggle so that first one figure is seen with the vertical lines prominent, and then this wanes and is succeeded by the other figure in which the horizontal lines are prominent. Which figure we see most easily seems to be dependent on the direction of the eye movement. If the eyes are moved vertically upwards and downwards the figure with the vertical lines comes more into prominence, while the figure with the horizontal lines is seen when tlie eyes are moved from side to side. SECTION X VISUAL JUDGMENTS LOCALISATION AND PROJECTION. Much discussion has been wasted on the question why we see things upright while the images on our retina are inverted. The answer is a simple one. We do not look at nor are we con- scious of the image on our retina. When we say that we see anything we are not expressing merely a sensation, but we are giving an interpretation of certain sensations in the light of long experience which has involved a large number of sensations besides that of vision. Thus a new-born child sees, i.e. receives images on its retina which excite impulses in the brain, but it is unable to interpret anything that it sees. In the first few months there is indeed no connection between the visual sensations and eye movements ; it is only about the third month after birth that the child will follow a lighted candle or bright object with its eyes, and this association of ocular movements with retinal impressions gradually extends also to many other movements. The continual and at first apparently aimless movements of the infant bring in a flood of muscular and tactile impressions w^hich only after many trials are recognised as corresponding with sensations arriving from the eyes. It at first finds that with the right hand it can touch objects lying on the right side of the field of vision. It becomes conscious therefore, not of the left side of its retina, but of a series of objects which have distinct relations to its right hand, and of a certain thing seen outside itself. The projection and localisa- tion of visual impressions are therefore not intuitive or innate qualities attached to stimulation of each point of the retina, but are the result of experience, the testing and comparing of visual sensations with tactile and muscular sensations from all parts of the body. From these experiences we learn to associate stimulation, say, of the right side of the retina with the presence of objects lying in front of and to the left of the body, and to project our visual sensations in this direction. If, for instance, we press the finger, with closed eyelids, on the outer side of the right eyeball, a luminous ring, or phosphene, will be seen apparently towards the left, i.e. the region whence the pressed-upon part of the retina will be normally stimulated by rays of light. The projection of visual impressions is well shown in Scheiner's experiment. Two needles are placed one behind the other on a wooden rod, one at 18 cm. and the other at a distance of 60 cm. from the eye. One eye is closed, and then a card is held before the other. In the card two small holes are pierced by means of a needle at a distance 580 590 PHYSIOLOGY from one another less than the diameter of the pupil. On accommodating now for one needle, the other needle appears double. Thus if the eyes are accommodated for the distant needle F (Fig. 301, II), the image of N is formed behind the retina, and since only a very narrow bundle of rays can pass through the holes in the card two images of the needle are formed on the retina. In the same way, if the eye be accommodated for the near needle the image of F falls in front of the retina, and therefore there will be again two images of it on the retina. In the former case, if the hole /3 be covered with a card, the left-hand image disappears. In the latter case, on covering ^ the right-hand image disappears, showing that the apparent position of the object depends on the relation of its image in the retina to the point of fixation, i.e. to the fovea centralis. JUDGMENT OF SIZE. The apparent size of an object is determined in the first place by the magnitude of its image formed on the retina, and therefore by the visual angle which the object sub- tends at the optical centre of the eye, as will be evident from the diagram (Fig. 302). The apparent size of any given object varies inversely in proportion to the distance. Thus the size of an image on the retina of an object two inches long at a distance of one foot is equal to the image of an object four inches long at a distance of two feet. An object can be seen if the visual angle subtended by it (the angle AcB in Fig. 302) is not less than sixty seconds. This is equivalent to an image on the fovea centrahs about 4^ across, which is about the diameter of a cone. The visual angle is, however, by no means the most important factor in our judgment of size. Thus as a man walks away from us his size does not appear to vary, although the visual angle subtended by him on our retina is continually diminishing. Where the size of an object is known to us, as in the instance just mentioned, it is used as a means of judging the size of surrounding objects. Where the size of the object is unknown our judgment of its size is determined by a comparison of its apparent size, as judged from the size of the retinal image, with the muscular effort of the convergence and accommodation which are present at the same time. Thus if we gaze at the sun for a minute so as to gain a negative after-image the size of this image will be constant. Its apparent size, however, will vary according to the distance of the surface on which we direct our gaze : on looking at a piece of paper held near, it may be about one inch across ; on looking at a distant wall, it may be Fig. 301. Diagram to illustrate Scheiner's experiment. F, the far needle ; N, the near needle ; a andjS, two pin-holes in a piece of card. The continuous lines indicate the path of the rays for which the eye is accom- modated. VISUAL JUDGMENTS 591 Fig. 302. several feet across. If we look through a piece of coarse wire gau^e at a distant window, the meshes of the gauze appear to be in the neighbourhood of the window and extremely large ; on directing the gaze then to an object held just in front of the wire gauze, the mesh will look extremely small. On the other hand, if we cut out the movements of accommodation and convergence by looking at a piece of wire gauze through a minute pinhole in a card, ^ ' the size of the meshes will increase as the gauze is brought near to the eye. In this case we judge of their size entirely by the visual angle they subtend. In the muscular elements which contribute to this judgment of size, the convergence of the axes is more important than the accommodation of each eye, so that judgment is but little affected if we paralyse accommoda- tion altogether by dropping atropine into the eye. The visual axes may be regarded as practically parallel for any object at a greater distance than five metres ; for such objects no act of accommodation is necessary. In judging of the size of any object beyond this distance we have only the visual angle to go by, which of course gives by itself no information unless we know the distance of the object. Here the obscuration of the outlines of the object in consequence of the deficient transparency of the atmosphere plays a large part in our judgments and may be upset in either direction by changes in transparency of the atmosphere. Thus when walking on the Downs in foggy weather a gigantic object may be seen looming through the mist, which, on advancing a couple of paces, is seen to be a sheep. On the other hand, in the clear air of the Alps the traveller continually under-estimates the size of distant objects, and takes a mass of rocks of the size of St. Paul's for a traveller wending his way up the snow arete. ILLUSIONS OF SIZE The distance between two points appears longer if a number of points be interposed between the two. Thus if two equal quadrilateral figures be divided — one by horizontal and the other by vertical lines — the one divided by horizon- - — tal lines will appear elongated vertically, and that divided by vertical lines elongated hori- FiG. 303. zontally (Fig. 303). Appar- ently it requires a somewhat less effort to pass directly from one point to the other than when the gaze has to follow an interrupted line. The eye muscles probably make a separate effort of movement at each interruption of the line. To these 592 PHYSIOLOGY movements is due the illusion in Fig. 304, where parallel lines seem to diverge or converge. Of two equal lines, one of which is vertical and one horizontal, the vertical line seems the longer. When the eye is moved upwards, the tendency of the superior rectus to move the eye inwards has to be counteracted by a pull of the inferior oblique muscle turning the eye outwards. A greater effort is therefore required than when the eye is moved either inwards or outwards, and it has been suggested that it is this greater muscular effort which is responsible for our over- valuation of the length of vertical as compared with that of hori- zontal lines. It must not, however, be imagined that an actual movement of the eyes is necessary in order to judge of the size and distance of any object. If a white thread be hung up in a dark room and be illuminated for an instant of time by an electric spark it is impossible for an observer in the dark room to move his eyes so that the r////////////////x /////////////////J r////////////////x /////////////////J Fig. 304. Fig. .305. The eyes arc directed to the point b. A thread hung obliquely at a under these circumstances gives rise to the images shown in the ujiper figures • — i.e. two images which do not lie on corresponding points. Nevertheless the thread is seen as single. image of the thread shall fall on the corresponding points of the two retinae before the illumination has disappeared. In spite of the fact, however, ihat the image of the thread falls on non-corresponding points, as will be seen from the diagram (Fig. 305), the thread is seen, not as double, VISUAL JUDGMENTS 593 Fig. 306. but as single, and a very correct impression may be obtained of its size and position. In this judgment or interpretation a number of separate processes must be involved. The fact that the eyes are not accommodated for the thread evokes at once the associated movements which would be necessary, if time allowed, to bring the non-corresponding images on to corresponding points of the retina. We do not therefore assume a double- ness of an object, even when its images fall on non-corresponding points, unless our gaze is voluntarily directed on the object. THE J UDGMENT OF SOLIDITY. The fusion of non-corresponding images to a single visual conception is responsible for our appreciation of solidity. If we look at any object which is not too far from the eyes, first with the right and then with the left eye, we shall see that the images in the two eyes are not identical. If we look, for instance, at a truncated pyramid, we shall see with the right eye rather more of the right side of the pyramid and with the left eye rather more of the left side (Fig. 306). If these two images a __ S__^ and 6 bB so arranged that they fall on corre- sponding points of the two retinae, there is no confusion of sensation, but the resulting impres- sion is that of a solid object. This is the principle involved in the stereoscope, which allows us to combine two images of this character so that they fall on corresponding points of the two retinae. In Brewster's stereoscope (Fig. 307), which is almost invariably used at the present time, the combination of the two pictm'es is effected by means of two half-lenses with convex surfaces, and their thimier margins directed in- wards so that they act as prisms. A vertical black screen is placed between the two lenses. On looking through the lenses towards the point s, the direction of the rays is changed by the prisms so that the image of the pictures B and b' fall on corresponding points of the retinae — B into the left eye and b' into the right eye. The ordinary photo- graphs for these stereoscopes are taken by means of a double camera with lenses at a distance apart considerably greater than that between the two eyes. On this account there is actually an exaggeration of the solidity of the combined images. When only one eye is used the external world has a much flatter appear- ance. Some idea of solidity is still gained from the fact that the accommo- dation has to be altered in order to bring different parts of the solid objects into focus. The judgment is also aided by the eft'ects of light and shade. The inadequacy of such means in default of the stereoscopic images in binocular vision is at once appreciated when we try to dissect or to carry out any fine manipulation using only one eye. Fio. 307. Brewster's stereoscope. SECTION XI THE NUTRITION OF THE EYEBALL The eyeball is protected in front by the eyelids. These are Uned internally with a delicate mucous membrane continuous with the conjunctiva covering the anterior surface of the eyeball. This membrane is kept constantly moist by the secretion of the lacrymal gland, a small acino- tubular gland built up on the type of a serous gland, situated at the upper and outer angle of the orbit. The secretion, ' the tears,' has a slightly alkaline reaction and contains about 98-2 per cent, water and 1 -8 per cent, total solids, viz. 0-5 per cent, coagulable protein and 1 "3 per cent, inorganic salts, of which sodium chloride is the most important constituent. Since the tears are constantly flowing over the eyes they serve not only to moisten the surface but to wash away any irritant material or bacteria which may be deposited from the air. The tears have a bactericidal power which is lost if the fluid be boiled for two or three minutes. Our knowledge as to the nervous mechanism of the secretion of the tears is still incomplete. Stimulation of the conjunctiva evokes an increased secre- tion of lacrymal fluid which can also be induced by emotional conditions. It is stated that lacrymal secretion can be produced by the stimulation of the cervical sympathetic as well as by stimulation of certain cranial nerves, e.g. facial and fifth nerve. Structural changes analogous to those to be described in the salivary glands have also been found in the lacrymal gland as a result of secretion. The excess of fluid is drawn ofl from the conjunctival sac by the nasal duct, which leads to the nasal cavity on the same side. If the eyes be kept open for some minutes, the conjunctiva covering the eyeball becomes dry and irritation is set up. Normally the membrane, and especially that over the cornea, is kept moist and trans- parent by involuntary movements of the eyelids, which close or blinlc about twice a minute, so distributing the lacrymal secretion over the whole conjunctival surface. This blinking is a reflex act, the afferent channels being fibres of the fifth nerve, and the efferent the fibres of the facial nerve supplying the orbicularis palpebrarum. It is spoken of as the ' conjunctival reflex,' and is one of the last reflexes to disappear in chloroform or ether narcosis. Just below the mucous membrane of the lids we find a series of specialised sebaceous glands, the ' Meibomian glands.' The fatty secretion of these glands is poured out at the edge of the lids, keeping these and the eyelashes greasy, and so preventing their being wetted by the tears. Any overflow of tears from the conjunctival sac is thereby prevented, unless the secretion 594 THE NUTRITION OF THE EYEBALL 595 becomes excessive ; so that the whole of the fluid under normal circumstances is kept within the sac and flows away only through the nasal ducts. INTRAOCULAR PRESSURE. The eyeball is formed of a tough in- extensible capsule, the sclerotic, filled with fluid or semi-fluid contents. In order that the eyeball may be sufficiently rigid to maintain the normal relations of the various refractive media, and to afford a fixed point for the action of the ciliary muscle, this fluid must be under pressure. On connecting Fro. 308. Arrangement of apparatus for measurement of intraocular pressure. (Henderson and Starling.) G is a piston-recorder for recording graphically the changes in pressure. a small manometer with the anterior chamber, care being taken to prevent any escape of the intraocular fluid, it is found in the normal eye that this pressure is about 25 mm. Hg. The problem of measuring intraocular pressure is analogous to that of measuring the intracranial pressure. The eyeball represents a cavity into which fluid is continually being poured and from which it is being absorbed, the intraocular tension determining the exact balance between the processes of secretion and absorption. It is therefore necessary in determining the amount of this pressure to take care that no fluid either enters or leaves the eyeball. For tliis purpose we can make use of the arrangement represented in the accompanying diagram (Fig. 308). The steel needle a is connected to a capillary glass tube, cb. This has a lateral opening, through which a bubble of air can be introduced into the tube. By means of a T-piece the capillary is connected with a water manometer e, and with a reservoir f, containing 0-9 per cent, salt solution. The pressure in the apparatus is now raised to about 25 cm. of water. Wliile the fluid is dropping from the end of the needle, it is tlu-ust through the lateral part of the cornea, so as to lie in the middle of the anterior chamber. A bubble is then intro- duced by the side tube, d, into the capillary tube, and the reservoir adjusted to such a height that the bubble remains stationary. We know then that the pressure inside the eye exactly balances the pressure of the fluid in the reservoir, and we have also provided that there shall be no appreciable escape of fluid from the eye or entry of fluid into the eye. If the bubble remains stationary for three or four minutes we know that equilibrium is attained, and we can read off the height of the intraocular pressure on the manometer e comiected with the reservoir. 596 PHYSIOLOGY On making an opening into the cornea the fluid drains away and the eye- ball becomes soft and collapsed, the cornea being folded, and the eye being naturally useless as an optical instrument. The fluid which flows away, and which forms the aqueous humour and also fills the interstices of the gela- tinous tissue of the vitreous, contains only a minute trace of protein, con- sisting, in every 100 parts, of 98-7 parts water and 1-2 to 1-3 total solids, of which only 0-08 to 0-12 part consists of protein. If a cannula be kept in the anterior chamber this fluid rapidly alters its character, becoming coagulable, and containing 3 to 4 per cent, of proteins. The intraocular fluid is continually being renewed. The eyeball receives a rich vascular supply, which forms a close network of vessels and capillaries I.O.P mm. Hg. mm. He Art. B.P. mm. Hg. mm. Hs. 10 sec. Fig. 309. Curve showing effects on the intraocular pressure (in the dog) of mechani cal interference with the circulation. Blood- pressure measured in left carotid, intraocular pressure in right eye. From p to s the descending thoracic aorta was occluded. From q to K the right vertebral and subclavian arteries were also occluded. (Henderson and Starling.) in the choroid coat, with its prolongations the ciliary processes and iris. The chief seat of formation of the intraocular fluid is the ciliary processes. Here there is a constant transudation of fluid from the blood-vessels into the anterior part of the vitreous cavity, the amount of the transudation varying with the pressure in the blood capillaries (Fig. 309), being increased by any rise in the capillary blood pressure or by any fall in the intraocular pressure. Of the fluid poured out by the ciliary processes a very small proportion (perhaps one-fiftieth) passes backwards into the vitreous humour and gradually drains out of the eyeball by the lymphatic spaces of the optic nerve. By far the larger amount passes forward through the fibres of the suspensory ligament into the posterior chamber (the annular cavity between the iris in front and the lens and ciliary processes behind), and thence round THE NUTRITION OF THE EYEBALL 597 the margin of the iris into the anterior chamber. From the anterior chamber it passes into the spaces of Fontana at the outer angle of the chamber, whence, under pressure, it can filter slowly between the endothehal cells lining the canal of Schlemm into this vessel and so drain away into the venous system. A considerable resistance is offered to the passage of fluid into the canal of Schlemm. Hence the constant transudation of fluid from the ciUary processes raises the intraocular pressm-e to 25. mm. Hg, and a con- tinuous production of about 6 cubic milhmetres of fluid per minute suffices to maintain the pressure at this height. If the formation of intraocular fluid be increased by rise of blood pressure, the intraocular pressure must also rise until a point is reached at which the increased filtration through the anterior angle is just equal to the increased production resulting from the rise of blood pressure. Within \dde hmits therefore the intraocular pressure varies with the blood pressm-e. This is shown by the following records of both pressures in dift'erent animals (Henderson and Starling) : Dog Arterial pressure Intraocular pressure. 128 mm. Hg. . . 26 mm. Hg. 158 mm. Hg. . . 34 mm. Hg. 180 mm. Hg. . . 40 mm. Hg. 70 mm. Hg. . . 23 mm. Hg. Conversely the intraocular pressure may be altered by increasing or diminishing the ease with which the fluid escapes through the filtration angle. If the anterior angle becomes blocked as the result of inflammatory changes, or other causes, the intraocular pressm-e rises gradually mitil it attains a height far above normal. The eyeball to the finger feels stony hard, and the increased pressm-e affects seriously the circulation of blood through the retinal vessels, so that atrophy of the retina is produced together with dis- turbance of the nutrition of the whole eyeball. This condition of raised intraocular tension occurs in the disease known as glaucoma. The constant renewal of the intraocular fluid is important not only for the maintenance of the intraocular pressure but also for the nutrition of the structures, such as the lens, suspensory ligament, and vitreous humour, which do not receive any vascular supply. THE ORGANIC SENSATIONS SECTION XII • SENSATIONS OF MOVEMENT AND POSITION In studying the phenomena of reflex movements, as presented by the spinal animal, om: attention was drawn to the importance of the afferent impulses transmitted to the central organ by means of a special system of sense- organs, called by Sherrington the proprioceptive system. These afferent impressions intervene at a later period in every reflex action than do the initiating sensory (exteroceptive) impulses. They arise as a result of the reflex movement itself, and serve to regulate the extent of this movement as well as the co-ordinated changes in the other muscles of the body. Whether they be synergic or antagonistic, the abolition of the impulses arising in this system has an effect similar to that of the destruction of the governor of an engine. The movements excited by peripheral stimulation become excessive and conflicting ; there is no longer the give-and-take of the antagonistic muscles surrounding the joint, and the result is a state of disorder and inco-ordination, termed ataxy. Of the proprioceptive impulses a certain proportion reach the cerebral cortex and arouse states of consciousness which we speak of as sensations of position, movement, or resistance, and which form the basis of judgments as to these conditions. In consciousness they are contrasted with the sensa- tions arising from the other sense-organs in the same way as they are in the subconscious regulation of the motor adaptations of the body. All the senses which we have so far considered give us information of things, i.e. of a material world which can affect ourselves, but which we conceive of as existing altogether apart from our sensations of it. Indeed the visual and auditory sensations we project to distances remote from the body. The sensations, on the other hand, which are aroused through the intermediation of the proprioceptive system we refer entirely to ourselves. By them we receive information of the condition of the material ' me,' i.e. of ourselves as things apart from the objects which surround us and the changes in which ordinarily excite our activity. Consciousness we have seen to be developed in proportion to the differen- tiation of the educatable association centres, which are responsible for our powers of ideation, and by means of which the different reflex movements 698 SENSATIONS OF MOVEMENT AND POSITION 599 which we call volitional are carried out, guided, augmented, or inhibited, according to the past experience of the individual. Volitional movement is therefore a movement determined by previous neural events, of which a part at any rate is represented in consciousness as feehng, emotion, or desire. Where an act is involuntary, i.e. does not need the guidance of experience, individual or racial, for its performance, the afferent impulses which arouse it are also, as a rule, devoid of representation in consciousness. Thus we have no sensation of the passage of a bolus along the oesophagus. The proprioceptive impulses also only afiect consciousness where they are necessary for the guidance of vohtional movement. The tactile and gusta- tory impressions from the tongue have a very full representation in conscious- ness. Volition, however, only interferes for the rejection or acceptance of the food taken into the mouth, and is not required for the minute direction of the movements of mastication and deglutition. The muscular sensibihty of the tongue, and therefore our voluntary control of its movement, is extremely slight, although there must be a continual flow of afferent impressions from the tongue to the lingual motor centres to guide the complex movements both of mastication and deglutition. In the case of the palate muscles, as of the oesophagus, muscular sensibility is entirely wanting. It has been suggested that afferent impressions from the muscles can play only a subordinate part in our sensations of movement, since we are not aware of the part taken by each individual muscle in any given move- ment. Such a statement is absurd. We have no objective phenomenal experience of our muscles. All that we are aware of and can judge of by our other senses is the movement as a whole, and our sensation of movement is therefore referred to the whole movement and not to the individual muscles. The sensations arising in the proprioceptive system can be divided into two main classes : (1) The sensation of the relative positions of parts of the body, (2) The sensations which inform us of the position of the head, with regard to its surroundings, i.e. with regard to the direction of the pull of gravity, (It must be remembered that ' downwards ' always means towards the centre of the earth, ' upwards ' away from the centre of the earth, i.e. against the gravitational forces.) This orientation sense depends on the in- tegrity of a special sense-organ contained in the labyrinth of the internal ear. It is therefore sometimes spoken of as the labyrinthine sense. THE SENSE OF RELATIVE POSITION, INCLUDING THE MUSCULAR SENSE Without using our eyes we are able at any moment to tell the position of our limbs. If one arm be moved passively into any position we can without difficulty move the other arm into an exactly similar position. We thus know the extent to which we move the limb and the static position attained as the result of the movement. If the movement is resisted, we are able to 600 PHYSIOLOGY adjust the force of the muscular contraction to the resistance, and to form therefore a fair idea as to the strength of the resistance. (a) PASSIVE MOVEMENTS. A large number of different sense-organs contribute to the formation of these judgments. In the appreciation of passive movement the chief end-organs involved are those in connection with the joints and their ligaments, though it is probable that the deeper sense- organs in the soft parts around the joints also contribute to the total sensa- tion. Cutaneous sensations apparently play but little part in the judgments of passive movement. It is true that the alternating movements of the hind hmbs, which occur in a spinal animal when it is held up by the hands under the fore hmbs, are started, partly at any rate, by the stretching of the skin of the thighs ; but this effect is one rather of initiation of movement, and can hardly be regarded as proprioceptive in character. The strength of the sensation of passive movement depends on the extent of the movement as well as on the rate with which it is carried out. The dehcacy of perception varies in different joints. Thus in some joints a movement of 0-25° per second is appreciated as a movement, while in other joints the movement must be as extensive as 1-4° per second. It is more easily appreciated when the joint surfaces are pressed together than when they are pulled apart, showing that the nerve- endings in the joint surfaces play a part in the origination of the sensations. (b) THE SENSE OF MOVEMENT (MUSCULAR SENSATION). This term is applied to those sensations by which we judge of the extent and force of any active movement which we may have carried out. Many authors have ascribed an important part in this act of judgment to the so-called ' sense of innervation,' i.e. a sense of the actual energy which is being discharged from the motor cells of the central nervous system to the muscles, and have thought that when we raise a weight we judge of its amount, not by the degree of stretching of the muscle or pressure on sensory nerves in the muscle, but by the amount of force we voluntarily put out to raise the weight. The fact, however, that we can judge of weights, when the muscles are made to contract by electrical stimuli and not by voluntary impulses, shows that this sense is in large part, if not entirely, peripheral. It is, however, very com- plex in nature, and is served by a whole array of different end-organs in the skin, joints, tendons, and muscles. The muscles themselves are known to be well supplied with afferent nerves. Stimulation of the central end of a muscular nerve may reflexly excite or inhibit movements of other muscles. Sherrington has shown that, after section of the motor roots, over one-third of the fibres in a muscular nerve remain undegenerated, proving their connection with the posterior root ganglia. The sensory nerve- endings in the muscle are represented partly by the tendon nerve- endings and partly by the muscle-spindles. The former are richly branched end- arborisations of nerve fibres on the surface of the tendon bundles. The muscle-spindles consist of one or more muscle fibres, often continuous with normal fibres, enclosed in a sheath composed of several layers of fibrous tissue with intervening lymph-spaces. One or more nerve fibres pierce this SENSATIONS OF MOVEMENT AND POSITION 601 sheath and, after making many spiral turns round the muscle fibres, branch freely and terminate in httle knobs on the surface of the fibres (Figs. 310,311 ). The cross-striation of the muscle fibres within the spindle is but faintly marked. It is evident that the continuity of these sense-organs with the contracting muscle ensures in the best possible way that the organs should Fig. 310. A neuro- muscular spindle of the cat. (Ruffini.) c, capsule; pr.e, primary ending; s.e, secondary ending; 'pl.e, plate ending (all these are probably sensory in function). s a d - Fig. 311. Part of a muscle-spindle more highly magnified. n, nerve fibres passing to spindle ; a, annular endings of axis cylinders ; s. spiral endings; c?, dendritic endings ; s/i, connective-tissue sheath of spindle. (Ruffini.) be affected by the shghtest change of tension of the muscle, and should transmit information of the state of tension to the central nervous system. THE PSYCHOLOGICAL SIGNIFICANCE OF SENSATIONS OF MOVE- MENT. Not only are these organic sensations of importance as afi'ording us information of the condition of om" own bodies as distinct from the objects in the world around, but they enter into and qualify om- judgments derived from all the sensations which arise in the special sense-organs. When we regard the continuous aimless activity of a healthy baby, we see that all ideas of space, of extension, of relative position are wanting, or at any rate are not present to guide the movements. Bit by bit muscular experience is acquired. The child learns that a given movement of the right arm will bring the hand in contact with something which is exciting the left 602 PHYSIOLOGY side of his retina. The surface of the thing, if of sufficient extension, can excite tactile sensations in all the fingers of the right hand. By moving one finger over the object the tactile sensations are found to be continuous ; by moving the whole hand forwards the thing is found to possess extension in a direction away from the body, and therefore in the third plane of space. Thus gradually are acquired not only ideas of extension, distance, and space, but certain movements are correlated with stimulation of definite regions of the skin or of the retina. Tactile and retinal impressions therefore acquire local sign, and power is acquired of moving the limbs to a degree and in a direction adapted to stimuh arising from any part of the tactile or retinal surfaces. The child gradually acquires the power of following a bright object with its eyes, i.e. of contracting the ocular muscles so as to keep the retinal image of the object on the fovea centralis, and up to adult age we are still engaged in this balancing of muscular movement against sense impressions — a balancing in which the muscular sensations are the constant guide and criterion of success. Only by the muscular sensations are we informed whether our willed movement has been carried out or not. It is in virtue of the muscular and alHed sensations that we are able to clothe our visual and tactile sensations with properties of extension, sohdity, and resistance, which create them in consciousness as parts of a material world. SECTION XIII THE LABYRINTHINE SENSATIONS Throughout almost the whole of the animal kindgom, and in practically all freely moving metazoa, we find a sense-organ which has often been designated as an auditory organ. This organ, which is situated in the integu- ment, is in the form of a small sac generally open to the exterior, and lined by cells provided with hairs and richly supplied with nerves. Resting among the hairs is a small concretion, generally of carbonate of lime, which is known as an otolith. These sacs have generally been regarded as auditory in function, hence the term otolith applied to the concretion. The evidence for audition, i.e. the power of appreciating vibrations in the elastic medium surrounding them, is scanty. Thus in fishes this power has been stated to be absent unless the vibrations are of sufficient amplitude to affect the sense- organs of the skin.* On the other hand, there is evidence that these otohth organs are connected with equihbration. Section of the nerves going to them in the crajl&sh causes disturbance of locomotion. Steinach has succeeded in the crayfish in replacing the concretion by a small particle of iron. The animal's behaviour and movements were perfectly normal until it was brought within a powerful magnetic field. Under the influence of this field the effect of gravity on the iron particle was annulled and replaced by a force of attraction in another direction, and the effect was at once seen as pronounced disorders of locomotion, the animal swimming in an abnormal position. From a sac, such as that present throughout the lower animals, the organ of hearing in the higher vertebrata is developed. Arising as a pit in the epiblast in the neighbourhood of the hind-brain, the auditory sac becomes shut off from the exterior, and then, by an outgrowth in various directions, forms the complex membranous labyrinth of the internal ear. This mem- branous lab}T.'inth, as we have seen, can be divided into two parts, viz. the canalis media of the cochlea in front, and the saccule, utricle, and semi- circular canals behind. The canalis media of the cochlea is concerned with the reception and analysis of soimd waves. In the lower vertebrates in which auditory sensations are wanting the cochlea is absent, and in fishes is represented merely by a small diverticulum knowni as the lagena. With the development of air-breathing vertebrates we see the first signs of a special * On the other hand, Piper has succeeded in detecting an electrical variation in the eighth nerve of fishes in response to a sound stimulus. 603 604 PHYSIOLOGY organ of hearing. Thus a primitive cochlea is present in the amphibia, and especially in the anm-a, and in some of the reptiles as well as in birds it ac- quires a bend and shows the beginning of a spiral arrangement. Only in the mammals does it attain a degree of development at all comparable with that found in man, and characterised by the formation of one and a half to four spiral turns in the cochlea as well as in the canahs media. This development of auditory functions cannot involve any abrogation of the important part played by the otolith organ throughout all the lower classes of the animal kingdom. In man, as in the crayfish, it is the otoHth organ which determines his behaviour in relation to the force of gravity, and is there- fore responsible not only for the maintenance of equilibrium but also for the sensations which enable him consciously to orientate himself and to know the position in which he happens to be at any given moment. With the increasing importance of visual sensa- tions in determining the be- haviour of the animal, close connections are established be- tween the central connections of the nerves running from the otolith organ and the parts of the brain concerned with the innervation of the eye muscles. By this means the position of the eyes is constantly adapted to the position of the head. The auditory part of the inter- nal ear has already been described. That part of the labyrinth which represents the primitive otolith organ consists of a bony framework containing perilymph, in which is contained the membranous labyrinth with the endings of the vestibular division of the eighth nerve. The osseous labyrinth consists of a cavity, thes vestibule, into which open behind the three bony semicircular canals. In the vestibule are contained two little membranous sacs, the utricle and saccule, the cavities of which are connected by means of the saccus endolymphaticus. Into the utricle open the three semicircular canals, the three canals having five openings. These semicircular canals are arranged in three planes, each of which is at right angles to the other two, so that in the organ are represented the three planes of space. We may distinguish on each side an external or horizontal canal, an anterior or superior vertical canal, and a posterior vertical canal. The two outer canals lie always exactly in the same plane, which is practically horizontal in the normal position of the head. Each posterior vertical canal lies in a plane which is parallel to that of the superior vertical canal of the opposite side. We thus see that these semicircular canals form Fig. 312. Figure from Ewald showing the situ- ation of the three semicircular canals in the skull of the pigeon. THE LABYRINTHINE SENSATIONS 605 together three planes — one horizontal and two vertical, the two latter being at righ» angles to one another (Fig. 312). The membranous canal lies within the osseous canal, a considerable sjxxce intervening between the two canals. At one end the osseous canal is dilated and the membranous canal undergoes a corresponding dilatation so as to fill up the whole bony canal. In this dilatation, which is knowTi as the ampulla, we find the ending of a branch of the vestibular nerve in a special sense epithelium forming the crista acustica (Fig. 313). The crista is composed of hair-cells with sus- tentacular cells between them. The fibres of the vestibular nerve end in arborisations Fig. 313. End-organ of vestibular nerve in ampulla of semicircular canal (' crista acustica '). among the hair-cells, the hairs of which project into the endolymph filling the ampulla. In the utricle and saccule we also find sjiecial sense-organs, known as the macula acustica, the structure of which is very similar to that of the crista in the ampullaj. Among the hairs, however, of the macula is found a small concretion of carbonate of lime, the otolith. The first accurate experimental investigation of the functions of these different parts we owe to Flourens. This observer showed that, whereas extirpation of the cochlea caused deafness, extirpation of the vestibule and semicircular canals left the auditory sense intact, but caused marked dis- orders of equilibration. That the peculiar arrangement of the semicircular canals in the three planes of space was connected in some way with the functions of these structm-es was also indicated by Flourens' observation that destruction of the horizontal canals on each side gave rise to continual nodding movements of the head in the plane of the injured canals. By many physiologists the results obtained by Flourens were ascribed to continued irritation of the peripheral sense-organs or of the central parts of the brain in consequence of the lesion. The accurate experiments of Goltz, and especially 606 PHYSIOLOGY -'F' those of his pupil Ewald, showed that these effects might last twelve to eighteen months, or be permanent, and must therefore be regarded as an Ausfallserscheinung, i.e. as due to abolition of a function and not to the arousing of a function by abnormal stimulation. Most of the experiments on this subject have been carried out on pigeons on account of the easy accessibility of their semicircular canals. Confirma- tory observations have, however, been made on mammals. After destruc- tion of all the canals or of the whole membranous labyrinth on both sides, disturbances of equihbrium are aroused which may last for a considerable time. The animal can neither stand, nor fly, nor maintain any fixed attitude, but is constantly moving about incoherently and often so violently that it is necessary to pad its cage in order to prevent it from injuring itself. Although the movements are so violent, very Kttle guidance sujB&ees to stop them altogether. Any support given by the hand enables the animal to rest quietly. After some months these disorders gradually disappear, and the animal learns to guide its movements by sensations of touch and sight alone ; but they are instantly brought back in all their severity if the eyes be bandaged, so as to deprive the co- ordinating centres of the guiding visual sensations. The same effect is produced if that part of the brain which alone is edu- catable, viz. the cerebral cortex, be excised. Extirpation of the cerebral hemispheres in pigeons causes no dis- orders of equilibrium, but extirpation, after destruction of the labyrinth, brings back the disorders which were noted during the first days after the operation, and these disorders are now permanent. Recovery even in the presence of the cerebral hemispheres is, however, never really complete. Although the animal may be able to walk and fly very fairly, it suffers from a loss of power and loss of tone which affect all its muscles, but especially those moving the trunk and neck. If the labyrinth has been extirpated only on one side, then this loss of tone is noticed chiefly on the opposite or contralateral side of the body (Fig. 314). Loss of tone after complete destruction is well shown in the following experiment devised by Ewald : A small lead bullet is hung by a thread to the beak of the pigeon. As the bird moves about the bullet swings, the head following its movements ; finally the bullet happens to fall over the beak of the animal — the head is now found to be fixed in the position shown in the figure (Fig. 315). The anterior muscles of the neck are too weak and toneless to restore the head to its Fig. 314. Abnormal posture of pigeon, in which the labyrinth had been ex- tirpated on one side five days pre- viously. (Ewald.) THE LABYRINTHINE SENSATIONS 607 normal position against the weight of the bullet. No such phenomena aie presented by a normal bird. The same absence of tone is seen in mammals. A dog with both laby- rinths destroyed may jump down from a table once, but will not repeat the experiment, since the muscles of the fore hmbs are too toneless to support the head against the shock of the jump, and he knocks his head against the ground as his legs collapse imder him. If only one canal be put out of action, as, for instance, by stopping it with dentist's amalgam, the head is thrown into oscillations in a corresponding plane, or perhaps rather we should say that when the head oscillates in this plane there are no corresponding sensa- tions set up which tend to inhibit the movements. The same effect may be produced temporarily by painting any one of the canals with cocaine so as to paralyse its nerve-endings. The converse experiment of isolated stimula- tion of one canal has also been effected by Ewald. For this purpose Ewald, by means of a dentist's bmT, opened one bony canal at two spots. By the hole furthest away from the ampulla he introduced an amalgam stopping, so as to prevent any current of fluid backwards through the canal. Over the second hole he fixed, by means of plaster of Paris, a tube which was connected by a flexible rubber tube with a rubber ball. By this means, while the bird was sitting quietly on its perch, he could suddenly blow upon the exposed membranous canal with- out disturbing the bird in any way. By the air pressure thus produced on the canal a stream of endolymph was caused in the direction of the ampulla. Every time this was done he fomid that the animal moved its head and eyes in the direction of the current and always exactly in the plane of the canal which was being stimulated. By this means proof was brought of the correctness of the theory put forward by Breuer and Mach, viz. that the specific stimulus of the nerve-endings in the ampulla is afforded by the current of the endolymph in the semicircular canals. Since the endolymph is a fluid with inertia it will not immediately follow a rotational movement of the bony walls of the semicircular canals. Thus a sudden tiurning of the head from left to right will cause movement of endo- lymph towards, and therefore increased pressure on, the ampullary nerve- endings of the right horizontal canal, and movement of endolymph away from, and therefore diminished pressure on, the corresponding ampulla of the left side. In this way, for movement in any given plane, the two corresponding semicircular canals of the two sides are synergic, and unite in sending impulses which guide the equilibrating centres, and inform us of Fig. 315. 608 PHYSIOLOGY the position of our head in space. " One canal can be affected by and trans- mit the sensation of rotation about one axis in one direction only : and for complete perception of rotation in any direction about any axis six semi- circular canals are required in three pairs, each pair having its two canals parallel (in the same plane), and with their ampullae turned opposite ways. Each pair would thus be sensitive to any rotation about a line at right angles to its plane or planes, the one canal being influenced by rotation in the one direction, the other by rotation in the opposite direction " (Crum Brown). These reflex movements of head and eyes are the invariable result of move- ments set up in the endolymph, and occur equally well in the absence of the cerebral hemispheres. If an animal or man be placed on a turntable and rotated, his first tendency will be to turn his head and eyes in the opposite direction to that of rotation. If the rotation be continued, the endolymph gradually takes up the movement of the surrounding parts of the head, and if the eyes be closed, no movement of head or eyes is observed. If now the rotation is stopped, the endolymph will tend to go on moving, and the effect will be the same as if a movement of rotation were suddenly begun in the opposite direction. Head and eyes will now be turned, without any volun- tary impulse, in the direction of the previous rotation, and in consciousness there will be an actual sensation of rotation in the opposite direction. This sensation is in opposition to the sensations derived from other parts, and hence the feehng of giddiness and the actual disorders of equilibrium which are its concomitants. That this feehng of giddiness on rotation is due to impulses started in the semicircular canals is shown by the fact that, in a large number of deaf-mutes where these organs are imperfectly developed, it is impossible to produce giddiness and the associated eye movements by passive rotation. THE FUNCTION OF THE OTOLITHS The semicircular canals are, as we have seen, a higher development of the otolith organ. The primitive part of this organ is represented by the maculae in the utricle and saccule. It is to these organs that we must ascribe our powers of appreciating the static position of the head, as well as, to a slight degree, movements, not of rotation, but in one plane forwards or backwards. A consideration of the structure of the otolith organ shows at once that the incidence of the weight of the otoliths on the hairs of the macula will vary according to the position of the head. Thus in the diagram (Fig. 200, p. 399) in A (normal position) the chief weight of the otolith falls on the hairs from h to c, whereas, when the head has been rotated round a right angle so that the man, for instance, is lying on his right side, the chief weight of the otoliths will fall on the hairs at c. The nerve-endings stimulated by the weight of the otoliths will therefore vary according to the position of the head. The cerebellum and its associated structures represent a mechanism for the regulation of the movements of the trunk as a whole and the position of its centre of gravity in relation to the position of the head. THE LABYRINTHINE SENSATIONS 609 The beginning and ending of all movements of the head, or any change in the rate at which it is moving, must cause a momentary alteration of the incidence of pressure of the otoliths on the sensory hairs. Any translatory movements of the head must therefore excite a set of nerve fibres which will be constant for each direction. We are therefore justified in ascribing to these organs the functions possessed by the otolith organ throughout the animal kingdom, viz. the transmission of impulses to the central nervous system which are aroused by the position or movements of the head, and, hke the sensations from the muscles, regulate and govern any motor reaction to a sensory stimulus. Of these afferent impulses a certain proportion will arrive at the cerebral cortex, and in consciousness will inform us of our position in space and of the direction and extent of any movement, active or passive, of the head. 20 BOOK III THE MECHANISMS OF NUTRITION CHAPTER IX THE EXCHANGES OF MATTER AND ENERGY IN THE BODY GENERAL METABOLISM All the energy which leaves the body as heat or work is derived from pro- cesses of oxidation, the carbon, hydrogen, nitrogen, and sulphur of the food- stuff's uniting with oxygen in the body and being eliminated in the form of carbon dioxide, water, urea and other substances, and sulphates. In a starving animal this discharge of energy must be associated with a loss of body substance. The necessity for taking food is determined by the need of replacing this loss. The food-stuSs cannot, hke the coal or fuel of a steam- engine, be utilised directly as a source of energy, but must be built up to a greater or less degree into the structm'e of the living protoplasm. The total amount of living material in the body, though maintained fairly constant in the adult animal, may yet undergo alterations under varying conditions, and these alterations are naturally more marked in the growing animal. We have in this chapter to inquire into : (1) The nature and amount of the substances which may serve as food- stuffs and are necessary for maintaining the weight of the body constant or providing for its growth ; (2) The relation between the total amount of material taken up by the body and the total amomit given out ; (3) The variations in the total chemical exchanges determined by variations in the output of energy by the body ; and (4) The significance of the various classes of food-stuffs as sources of energy and in the replacing of tissue waste. We have therefore to make balance-sheets of two kinds, namely : (1) an accurate comparison of the ingesta (food and oxygen) and the egesta (carbon dioxide, water, urea, &c.) ; and (2) one showing the amomit of potential energy introduced into the body c jmpared with the amount of energy set free in the body. 613 SECTION I METHODS EMPLOYED IN DETERMINING THE TOTAL EXCHANGES OF THE BODY The determination of the material exchanges of the body involves an ac- curate comparison of its income and output. The income consists of the food-stuffs and oxygen. The food-stuffs may be divided into two classes, namely, (1) the organic food-stuffs, which on oxidation may serve as sources of energy, and (2) the inorganic food-stuffs, such as salts and water. The latter class neither add to nor subtract from the total energy of the organism, but their presence is a necessary condition of all vital processes, and as they are contained in the various excreta a corresponding amount must be present in the food in order to make good this loss. In spite of the bewildering complexity of the nature of the foods taken by man, their essential constituents can always be confined to the three classes, proteins, fats, and carbohydrates, and any analysis of the food must give the relative amounts present of these three classes of substances. The approximate analysis of the food-stuffs presents little difficulty. The nitrogen is determined by Kjeldahl's method. The figure thus obtained is multiplied by the factor 6-25, and the resulting figure is taken to represent the total protein in the food. Of course such a valuation may give too high a value when the food-stuff is one that is rich in nitrogenous extractives. The total fat is determined by extracting the food in a Soxhlet apparatus with ether. It is advisable to precede this extraction by an extraction with boiling alcohol. The total ethereal and alcoholic extract obtained is reckoned as fat. The amount of water is determined by drying the food-stuff at 110° C, and the amount of inorganic constituents by ashing the dried remainder. Carbohydrates may be determined directly by boihng the food with dilute acids in order to convert all its disaccharides and polysaccharides into hexoses, which are then reckoned as glucose, and estimated by their copper- reducing power. In most cases, however, the total protein, fat, and ash are subtracted from the dried weight of the food and the remainder is taken as carbohydrate. Although the methods for the analysis of food-stuffs are by no means diffi- cult, the total analysis of the food during a metabolism experiment may become extremely tedious on account of the very large number of analyses Avhich have to be performed. The labour is lightened by the fact that nearly all the ordinary food-stuffs have been subjected to analysis and their average 614 THE TOTAL EXCHANGES OF THE BODY 615 composition published by the Agricultural Board of the United States. Since, however, the foods vary in composition, especially in water content, from time to time, a calculation of the total income of proteins, fats, and carbohydrates from data given by workers in other lands must present a considerable margin of error. In order to attain greater accuracy some observers have made a complete food in the form of biscuits or of preserve which is prepared in large quantities at the beginning of the experiment and used as the sole food throughout the experiment. Pfliiger, for instance, con- verted the horse-flesh, with which he desired to feed his dogs in a metabolism experiment, into sausage meat which was sealed up in cases and sterihsed. The sausage meat having been analysed at the begimiing of the experiment, it was only necessary thereafter to weigh the amount eaten by the dog in order to know accurately the total amount of protein, fat, and carbohydrate ingested by the animal. In experiments on man it has been endeavoured to obtain the same result by limiting the food to a few articles of diet which could be accurately analysed in each case. The monotony of such a diet tends to interfere with the success of the experiment, since the subject of the experiment loses his appetite and his processes of nutrition are not normally carried out. It is usually possible to steer a middle course between the two extremes of too much and too little variation of diet, and so to obtain values for the composition of the iugesta which cannot differ very largely from their true composition. The material output of the body consists of the products of combustion of the food-stuffs, which are turned out by the various channels of excretion, namely, the kidneys, the alimentary canal, the lungs, and the skin. These excreta must therefore be collected and analysed. In addition to the main sources of excretion, small quantities of material are lost by the shedding of the cuticle, by the growth and cutting of the hair and nails, and so on. In most cases the losses in this way are so small that they may be disregarded. The nitrogen of the food-stuffs and that derived from the disintegration of the tissues of the body is excreted almost exclusively in the urine, a small amount being thrown out by the alimentary canal. The total nitrogen must be therefore determined both in the faeces and in the urine. The nitrogen in the fa3ces is derived from two sources. Part represents those nitrogenous constituents of the tissues which have resisted the digestive processes of the alimentary canal. There is in addition a certain amount derived from the in- testine itself. During complete starvation faecal masses are formed in the intestine, and it has been calculated that in a normal individual about one gramme of nitrogen a day is excreted by the mucous membrane of the gut and contributes to the formation of the faeces. It is usual therefore to regard one gramme of the nitrogen of the faeces as belonging to the output of the body and representing the result of nitrogenous metabolism, while the balance is taken as belonging to undigested food-stuifs, and is subtracted from the total nitrogen of the latter in reckoning the real income of the body. A small amount of nitrogen is also lost by sweat, but this can be disregarded unless the sweating is profuse, when the loss of nitrogen by this channel may 616 PHYSIOLOGY rise to as much as 4 per cent, of the total nitrogenous output of the body. Although a trace of ammonia has been described as occurring in the expired air, the amount is so minute that any loss of nitrogen by the lungs can be neglected. That the loss both by lungs and skin under ordinary circum- stances can be disregarded is shown by the fact that it is possible to account directly for the whole nitrogen of the body by a comparison of the compo- sition of food with that in the urine and faeces. If, for instance, an animal is kept on a sufficient diet which contains a perfectly regular amount of nitrogen, after a few days a condition known as nitrogenous equilibrium is set up, i.e. the total nitrogen of faeces and urine is exactly equal to the total nitro- gen of the food. The same thing apphes to the sulphur, as is shown in the following Table (quoted by Tigerstedt) : Days of experiment Nitrogen of food Nitrogen excreted Per cent, difference Sulphur ingested Sulphur excreted 1-7 8-17 . 154-81 213-72 153-02 213-26 -0-51 -0-21 12-77 12-79 In order to express the nitrogenous metabolism in terms of protein, we use the factor employed in estimating the amount of protein in the food, i.e. we multiply the total nitrogen of the excreta by 6-25. This will give the total protein which has been broken down during the period of the experiment. Much more important from the energy standpoint is the deter- mination of the total processes of oxidation of the body, information on which is given by a comparison of the oxygen intake with the output of carbon dioxide and water. The estimation of these substances presents much greater difficulties than the investigation of the nitrogenous exchange and involves the use of some form of respiration apparatus. The follov/ing are the chief methods which have been employed for this purpose : I. THE METHOD OF HALDANE. This method is extremely convenient when dealing with the gaseous exchanges of small animals, such as mice, rats, guinea-pigs Soda Lime H^SC, Fig. 316. Haldane-Pembrey respiration apparatus, c, chamber for animal ; m, gas meter. or rabbits. The animal is placed in the chamber c, which may be simply a wide- mouthed bottle (Fig. 316). This chamber is supplied with a thermometer, and can be kept at any desired temperature by immersion either in warm or cold water. On the inlet .side of the bottle is a series of tubes or bottles, some of which contain sulphuric acid and pumice-stone, while the others contain soda lime. On the outlet THE TOTAL EXCHANGES OF THE BODY 617 ^W L WATER ADDED WATER eSORBED 1_J n ^CARBON DIOXIDE AB50fiB£0 hi n Fig. 317. Air circuit in Benedict's respiration apparatus. side of the vessel is a corresponding series of vessels for the absorption of water and of carbon dioxide. On the further side of these vessels is a gas meter. During an ex- periment air is sucked through the wliole apparatus by means of an aspirator or a water pump, the amount of air passing througli the apparatas being measured by the meter. The animal is thus supplied with pure air freed from water vapour and from carbon dioxide. Any water or carbon dioxide produced by the animal is absorbed by the vessels interposed in the course of the outgoing air. These vessels are weighed at the beginning of the experiment and at the end, and the difference in weights will there- fore give the amounts of carbon dioxide and water which have been discharged by the animal. The intake of oxygen by the animal is determined indirectly. Since it gives off only carbon dioxide and water, and absorbs only oxygen during its stay in the chamber, the loss of weight of the animal during its stay in the chamber, subtracted from the total amount of carbon dioxide plus water it gives off, will represent the amount of oxygen absorbed. The advantage of this apparatus is that it can be fitted up in any labora- tory, and is accurate for the purposes to which it is applied. It is not, how- ever, appropriate for long-continued experiments or for experiments on larger animals or on man *liimself. Most of the data with regard to the respiratory exchange under various cir- cumstances have therefore been obtained by one of the three following methods : II. THE METHOD OF REGNAULT AND REISET. The principle of this method consists in placing the animal that is to be the subject of investigation in a closed chamber containing a given volume of air. The carbon dioxide produced bj^ the animal is absorbed by means of caustic alkali, and the oxygen consumed by the animal is made good by allowing oxygen to flow into the chamber from a gasometer. The inflow of oxygen is regulated so as to keeji the pressure of air in the chamber constant. At the end of the experiment the alkali is titrated and the amount of carbon dioxide absorbed thus determined. The air in the chamber is also analysed so as to be certain that it contains an excess neither of carbon dioxide nor of oxygen. The amomit of oxygen absorbed by the animal is knoAvn already, the oxygen which has been allowed to flow in having been measm'ed. A modification of this method has been devised by Benedict and is especially applicable to clinical pm-poscs. In this method the individual who is the subject of the experiment breathes through a nose-piece into a wide metal tube, the mouth being kept closed. The metal tube forms part of a closed system tlu-ough which a current of air is maintained by means of a ])ump. In the course of the current of air are inter- posed vessels for the absorption of carbon dioxide and of water, and the volume of gas in the system is maintained constant by admitting oxygen to it in projiortion as the oxygen of the system is used up in respiration. In Fig. 317 is given a diagrammatic scheme of the air circuit, and in Fig. 318 a diagram of the arrangement of the whole respiration apparatus, showing the nose-piece for breathing, the tension equaliser, the air-purifying apparatus, and the oxygen cylinder. The tension equaliser, A, is attached to the ventilating pipe near the point of entrance of the air into the Imigs. It consists of a pan with a rubber diaphragm (which may be conveniently made from a lady's bathing-cap). As the air is dra\\ii into the hmgs the rubber diaphragm sinks, to rise again with expiration. The respiratory movements can thus proceed without altering appreciably the jiressure within the closed system of tubes. By the admission of oxygen the supply of oxygen iis adjusted so as to keep the bag from becoming either too much distended or too uuich flattened. As the air leaves the lungs and passes 20* INTRODUCED 618 PHYSIOLOGY into the constantly moving current of air, it is carried along by the pump and flows through two Wolff's bottles containing strong sulphmic acid and pumice for the removal of water vapour. It then passes through a brass cylinder, c, filled with soda lime for the absorption of carbon dioxide. From here it passes again through sulphuric acid in a Kipp generator for the absorption of water given off by the soda lime. Since the air so deprived of moisture would be uncomfortable to breathe, it is then carried through another Kipp generator containing water with a trace of sochum carbonate for the neutralisation of any acid fumes which may be given off by the sulphuric acid. It then passes back to the tube from which the subject is breathing. In tin's way it is possible to determine very accurately the amount of oxygen used up and the amount of carbon dioxiele given off in the course of an experiment lasting one to three hours or longer. The oxygen consumption is measm'cd by weighing the cylinder of this gas, chosen small for this purpose, before anel after the experiment. III. PETTENKOFER'S METHOD. In the apparatus designed by Pettenkofer the animal or man was placed in a chamber through which a constant cur- rent of fresh air was passed. The amount of air passing through the chamber was measured by means of a meter. Through- out the experiment continuous samples both of the air entering the chamber and of the air leaving the chamber were taken. The analyses of these samples served to show the composition of the whole air entering and leaving the cham- ber, and therefore the changes in the air caused by the presence of the animal. The advantage of this apparatus is that an adequate ventilation can be kept up, and the apparatus can be built of any size. In the apparatus of Tigerstedt built on this plan the chamber had a capacity of 100-6 cubic metres, and was, in fact, a small room. A small respiratory apparatus has been built by Atwater. IV. ZUNTZ AND GEPPERTS METHODS. For mtny purposes the methods devised by Zuntz and Geppert present many aelvantages, especially when it is desired to take the respiratory exchanges in man or any animal during a limited perioel of time. The subject of the experiment has his nostrils clamped and breathes into and out of a face-piece. This face-piece is provided with valves either of aluminium or of animal membrane, which serve to separate the in-going from the out-going current of air. In the course of the out-going current is placed a very delicate gas meter which presents practically no resistance to the air current. A branch from the efflux tube passes to a gas analysis apparatus. By an ingenious method it is arranged that an aliquot part of the whole of the out-going air is drawn off into this apparatus, so that the experiment can be interrupted at any time, and the analysis of this sample will give the average composition of the expired air, and therefore, on multiplicaticn by the total gas passing through the gas meter, the total output of carbon dioxide eluring the course of the observation. One advantage of this method is that the apparatus is portable, and can be applied to the investigation of the respiratory exchanges of ])atients in hospitals or of man or animals while they are walking about. It has been ufed, for instsnce, by Zuntz and his pupils in an interesting series of researches on the gaseous metabolism of men at high altitudes. Fig. 318. Arrangement of apparatus in Benedict's method for determination of respiratory exchange. N, tubes inserted into nostrils of patient ; A, tension equaliser ; c, cyhnder contain- ing soda lime for absorbing 002- THE TOTAL EXCHANGES OF THE BODY 619 By means of one or more of these methods we may arrive at a correct idea of the total income and output of an individual for periods of many days. The following details by Tigerstedt may serve as an example of the results obtained in such an experiment. The experiment lasted two days. The subject was a man of twenty-six years of age, weighing about 65 kilos, who had previously taken no food for five days. The following Tables represent his material income and output. Total Income Food 3 3 N. c. a '3 2 P4 Fat 5| Ash Brccad . 373 7-3 36 337 46 4 278 9 _ Butter . 388 0-4 37 351 3 337 4 7 — Cheese . 116 4-3 56 60 27 35 — 5 — Salt meat 26 11 16 10 9 — — 2 — Milk 2313 11-3 2047 266 71 85 95 16 — Broth . 658 11-8 580 78 74 — — 9 — Beer 1413 1-2 1273 77 8 — 67 b 59 Beef steak 700 20-6 533 167 129 33 — 7 — Potatoes 452 0-9 359 93 6 1 82 5 — Water 2335 — 2335 — — — — — — Totals 8773 59-3 831-6 7275 1439 371 497 525 61 59 Total Output Total amount N. C. Water Solids Protein Fat Carbo- hydrate Ash Respiration Urine Faeces 2701 2564 455 41-5 4-8 -!5:-0 32-5 43-8 2248 2490 363 91-6 30 20 27 21 15 Totals . 5720 46-3 529-3 5101 91-6 30 20 27 36 As we should expect in a man who had fasted five days, this balance-sheet shows a marked retention of the food taken in, i.e. a marked excess of income over output. Thus of the nitrogen ingested, 1.3 grm., which is equivalent to 81-3 grm. of protein, was retained ; of the carbon, 302 grm. was retained. Of this 302 gi'm., 42-7 grm. would be contained in the 81-3 grm. of protein, so that the rest of the carbon, namely, 259-6 grm., was probably laid down in the form of fat. This would correspond to 339 grm. of fat. Of the salts contained in the ash of the food, 25 grm. were retained in the body. The carbon and nitrogen reappearing in the excreta serve as an index of the amount of metabolism of the food-stuffs which had occurred during the two days. In order to supply the energy requirements of the body during the time of the experiment, -498 grm. of carbohydrate, 59 grm. of alcohol, 620 PHYSIOLOGY and 138 grm. of fat had been completely oxidised. The amount of protein used up during this time can be obtained by multiplying the nitrogen of the urine plus 1 grm. of nitrogen of the faeces by the factor 6-25, and is found to amount to 271-9 grm. THE ENERGY BALANCE-SHEET OF THE BODY The energy income of the body is measured by the potential energy of the food-stujEEs, i.e. the amount of energy which can be evolved, either as heat, work, or in any other form, by the oxidation of the food-stufis to the end- products which occur in the body. Since it is convenient to have a uniform method of expressing the total potential energy of a food-stuff, we generally express it in calories, and speak of the heat-valae of a food-stuff. The heat- value of any given food is the amount of large calories * which it evolves on complete combustion with oxygen, and is determined by burning a weighed quantity of the dried food-stuff in oxygen in the bomb calorimeter. The following heat values have been obtained for different food-stuffs : Substance Heat value Lean meat . 5-656 (or 5-345 Rubner) Lard . . . . . 9-423 Butter .... . 9-231 Grape sugar . 3-692 Cane sugar . 4-116 Starch . . . . . 4-191 In the case of some food-stuffs it is necessary to draw a distinction be- tween the absolute heat-value and the physiological heat-value. Since carbohydrates and fats undergo complete oxidation in the body to carbonic acid and water, their physiological heat- values, i.e. the values of these food- stuffs to the organism, are identical with their absolute heat- values. Pro- teins, however, do not undergo complete oxidation. When they are oxidised in the bomb calorimeter the nitrogen is set free in a gaseous form. In the animal body no nitrogen is eliminated in the gaseous form, the whole of it being excreted as urea and allied substances still endowed with a considerable store of potential energy, which can be set free when their oxidation is com- pleted in a calorimeter. In order to determine the physiological heat- value of protein we must subtract from its absolute heat- value the heat- value of the excretory products in the form of which it leaves the body. The physiological heat value of proteins has been determined by Rubner in the following way : A dog was fed with the same protein which had served for the determination of the absolute heat- value. While the dog was receiving this food its urine was collected, dried, and its heat-value determined by combustion in the calorimeter. It was fomid that for each gramme of protein which had undergone disintegration in the body an amount of urine was passed corresponding to a heat- value of 1 -0945 calories. The heat- value * A calorie is the amount of heat necessary to raise a gramme of water from 0°C. to 1° C. A large calorie is the heat required to raise a kilogramme of water from 0" C. to 1° C, and is therefore equal to 1000 small calories. THE TOTAL EXCHANGES OF THE BODY 621 of the faeces formed under the same diet was 0 • 1854 calorie for each gramme of protein. Rubner fiuther reckoned that a certain amomit of heat would be required for the solution of the proteins and of the urea, and reckoned this at 0-05 calorie. The reduced or physiological heat- value of protein is there- fore equal to 5-345 — (1-0945 + 0-1854 + 0-05) = 4-015 calories. A determination of the heat-values of the various food-stuifs shows minute differences between individual members of the same class. Since it is impossible to reckon out accurately the relative amounts of the different kinds of protein, carbohydrate, &c., contained in each diet, Rubner has calculated the average physiological heat- values of the three classes of food- stuffs. These figures have been universally adopted, and are as follows : 1 grm. protein = 4-1 calories 1 grm. fat = 9-3 „ 1 grm. carbohydrate = 4-1 „ Careful experiments have shown that just as there is no loss of matter in the body, so also the sum of the energies put out by the body is equal to the sum of the energy obtained by the oxidation of the tissues and of the food-stuffs in the body during the same time. In an earlier chapter I have quoted the results of an experiment by Rubner on a dog, which demonstrated this equivalence, as proving the important fact that the fundamental doctrine of the Conservation of Energy applies to the organised as to the inanimate world. Similar results have been arrived at by Atwater in a series of experiments with a special calorimeter on man. It may be sufficient here to give the figures from one such experiment : a b C d e f cr 0 h i Date Cals. Gals. Cals. Cals. Cals. Cals. Cals. Cals. % Dec. 9-10 . 2519 110 142 -85 + 3 2349 2414 -1-65 + 2-8 10-11 . 2519 110 133 -25 -44 2345 2386 -f-41 + 1-7 11-12 . 2519 no 132 -21 -93 2391 2413 + 22 -t-0-9 12-13 . 2519 110 1.33 -14 -55 2345 2375 -1-30 4-1-3 Total 4 days 10076 440 540 -145 -189 9430 9588 -f-158 Average \ one day 1 2519 110 135 -3G -47 2357 2397 -f-40 + 1-7 3 a -i- ^--j "o o J2 ^ St3 O w •a §^ ^ ^ r g a o 3 .a a 8 a o 1 -a g 3 p d licat of CO protein gainc d lieat of of fat gaine °5 eo s ^ S + ■3 -S " B §3 tc - 1? =5 a i - u o a °« o =3 O "ot S .2 "S a 0 .3 •3 ^ s gS t^ s-g H a .2 5 2^ "■S 3 i. a _ -' 1 9 "Z ■ If " a « ■^^ ° w- = .f; ^ 3 a ° " ;^ — 622 PHYSIOLOGY It we take into account the great difficulties of such an experiment, we cannot but be impressed with the closeness of agreement between the total output of energy reckoned as heat and measured by the warming of a given volume of water and the total income of energy as estimated from the chemi- cal reactions involved in the metabolic changes which had taken place during four days of the experiment. The important result which comes out in such experiments is that the food-stuffs produce the same amount of energy ' iiijiiiiiiiiiiiiiiiiiiiimiiiiniiiiiiiii i Double Window illllllllllllllllllllllllllllllllllllllllll vC. Calorimeter Chamber. I -A, i^ — mw^^": T j CO^ I f Water ' removed ' ' removed /r L ''■>' f 1 ''->' poda-Lime H;S04. WatertCOj: deficient in Onygen \J Air minus CO, and Water; deficient in Oxyge Oxygen enters.* Fig. 319. Diagram to show the principle of the Atwatcr -Benedict calorimeter. (After Halliburton.) when oxidised in the body as when burnt to the same end-products outside the body, so that it becomes easy in any given research to sum accurately the energy income of the body. The Atwater calorimeter has been improved to such an extent by Benedict and his fellow workers that it has practically replaced all other forms for physiological purposes. It consists of a room or chamber with double non-conducting walls. All round the inner wall of the room are fitted coils of pipes through which a stream of water flows. The pipes are fitted with discs so as to take up rapidly heat produced in the room. The current of water is accurately adjusted so as to maintain the temperature of the inner wall constant. As the inner wall and outer wall are kept at the same tempera- ture, no heat is lost to the exterior, the whole of the heat produced by the animal or individual in the chamber being communicated to the water passing through the chamber. The temperatures of the entering and leaving water are taken by accurate thermometers reading to a hundredth or a thousandth of a degree Centigrade. Knowing the amount of water that has passed through in a given time and the difference in temperature during the same time, it is easy to calculate the amount of heat given off by the animal under investigation. It is generally convenient to maintain a constant difference of temperature between the entering and leaving water by appro- priate adjustment of the amount of water passing through the apparatus. The equality of temperature between the inner and outer casing is recorded THE TOTAL EXCHANGES OF THE BODY 623 by electric thermo couples, any difference of temperature being at once compensated by electrically warming the cooler part. The chamber con- tains a bicycle or other arrangement for the performance of mechanical work. It is adequately ventilated by a current of air passing through an apparatus similar to that of Benedict, described on p. 617. It is thus possible to estimate simultaneously the total heat production of an individual as well as the respiratory exchanges, including both carbon dioxide output and oxygen intake. The general principle of the calorimeter is shown in the diagram (Fig. 319). The calorimeter is also supplied with bed, table, chair, &c., and food can be introduced through a double window so that an experiment may be continued over two or three days on one and the same individual. SECTION II THE METABOLISM DURING STARVATION It will tend to simplify our task if we deal first with the results of the experiments which have been made on the metaboHc exchanges of animals during starvation, i.e. dming a period when the whole energy involved in the maintenance of the movements of respiration and circulation, and in the maintenance of the body temperature, &c., is derived from the animal's own tissues. It must be remembered that the tissues of an animal comprise two distinct classes. In the first class must be placed the Hving machinery of the body, generally composed of proteins or their near alhes. In the second class are the fatty tissues of the body, which form no part of the ordinary machinery, but function simply as a storehouse of material which can be utihsed for the production of energy. In addition to the store of fat there is, in a well-fed animal, a certain reserve of carbohydrate in the form of glycogen, deposited in the liver and muscles of the body. This store of glycogen is drawn upon to a large extent at the beginning of a period of starvation. The total amount of glycogen present at any time is so small in comparison with the possible amount of fat that it cannot provide the energy necessary for the prolonged period during which the maintenance of hfe is possible in a complete state of inanition, although it plays an important part during the first one or two days of a period of starvation. Contrary to general belief, the condition of an animal which is completely deprived of food is not a painful one. For this statement we have not only such evidence as can be derived from inspection of animals placed in this condition, but also evidence derived from men who have voluntarily or involuntarily been deprived of food for considerable periods. Especially instructive in this connection are the cases of the so-called professional ' fasting men,' two of whom, Succi and Cetti, have been subjected to complete metabohc investigation during the period of their starvation. During the first day or two there is a craving for food at meal-times. This, however, passes off, and during the later portions of the experiment even the desire for food may be entirely absent. As might be expected, the restriction of food is followed by a diminution in the amount of water required by the animal. The essential characteristic of the state of inanition is an ever-increasing weakness, accompanied by a strong disinclination to undertake any mental or physical exertion whatsoever. The animal passes its time in a state of 624 THE METABOLISM DLHING STARVATION 625 sleep or semi-stupor. In the case of Succi, who fasted for thirty days, considerable muscular exertion was undertaken on the twelfth and on the twenty-third day of starvation without any appreciable ill-effects. A strong effort of the will must have been necessary in his case to overcome the auto- matic instinct to preservation of life by the utmost economy in the expendi- ture of energy. The pulse-rate and the body temperature remain nearly normal until a few days before death, which is ushered in by an increase in the somnolent condition of the animal and by a gi-adual slowing of respiration and fall of temperature. The urine is naturally diminished with diminution in the output of urea and in the amount of water consumed. Some faeces are formed, and may be voided during or at the close of the starvation period. In Succi their amount varied from 9-5 to 22 grm. a day and contained from 0-3 to 1-0 grm. nitrogen. On microscopic examination they consisted of an amorphous material enclosing a number of crystals of fatty acids. During the whole of the starvation period energy is being used up in the body for the maintenance of its temperature and the Aatal movements of respiration and circulation. Since this energy is derived from the destruction and oxidation of the tissues of the body, it is evident that starvation must be associated with a constant and steady loss of body weight. In experiments on man the daily loss of weight during the first ten days amounts to between 1 and 1 -5 per cent, of the original total weight. This loss of weight does not affect all parts of the body alike. It might be imagined that, since the loss of weight is determined by the using up of the tissues of the body for the production of energy, those organs which are most active should show also the greatest loss of weight. The very reverse of this is the case, as will be seen from the following Table : Percentage Loss of Weight of Different Organs and Tissues during Starvation. (Voit.) Ti?=ue Percentage loss of weight of fresh tissue Percentage loss of dry tissue Fat 97 Spleen 67 G3 Liver . 54 57 Testes 4(1 — Muscles 31 30 Blood 27 IS Kidneys 20 21 Skin and lia rs 21 — • Intestineii IS — Lungs 18 19 Pancreas 17 — Heart 3 — Brain and spinal cord 3 0 626 PHYSIOLOGY Those organs of the body which are most necessary for the maintenance of hfe, the brain, the heart, the respiratory muscles, such as the diaphragm, undergo very httle loss of weight. Of the other tissues the fat, which is a mere reserve to provide for such contingencies, is drawn upon first, and during starvation 97 per cent, of the total fat of the body may be consumed. The nitrogen needs of the body during starvation seem to be supphed chiefly at the expense of the muscles and glands, which waste to a very marked degree. The muscles being used simply as reserve material, it is easy to understand the condition of lethargy and muscular inactivity which charac- terises the state of inanition. During starvation all tissues of the body undergo a process of autolysis or disintegration, giving up the products of this process to the common circulating fluid. The nutritional demands of a tissue are determined by its activity. Hence the active tissues of the body take up the material set free from all the other cells of the body and so maintain their weight at the expense of all other parts. A similar pre- dominance of the nutrition of active over inactive tissues is to be observed in cases of partial starvation, i.e. where the deprivation of food only applies to a single food constituent. Thus Voit, in a series of experiments, fed pigeons on a food which, while normal in all other respects, contained a deficiency of calcium salts. On kilhng the birds after a certain length of time, it was found that while the bones used in the necessary movements of the animals presented a normal appearance, the others, such as the sternum and skull, showed a marked deficiency of lime salts and had undergone a process of rarefaction giving rise to the condition known by pathologists as osteo- porosis. Many other instances of the sacrifice of a temporarily useless tissue on behalf of tissue of high physiological value are known. Thus the salmon and its congeners, which five part of the year in the sea, lay their eggs and undergo their early development in the fresh water of the upper reaches of rapid streams. An adult salmon leaves the sea in the early summer months in a magnificent state of muscular development, fit to perform the prodigious feats of swimming which are required in order to get it over the rapids of the river which it has to ascend. It takes no food. In the upper reaches of the stream or river there is a growth of the genital glands, ovaries, or testes. The whole material for the growth of these large organs is derived from the atrophy of the skeletal muscles. In this case we have the growth of an active tissue at the cost of an inactive one, the activity, however, being determined, not by the direct call upon it from the environment, but by what we may speak of as the ' physiological habit ' of the animal. The animal organism, in the complete absence of food, deals with the resources of its bodily tissues with the utmost possible economy. The total metabolism therefore sinks rapidly during the first two days of starvation, and then remains practically constant. There is indeed a shght continuous diminution with the fall in body weight, but if we reckon out the total metabohsm per kilo body weight, we find that till within a day or two before death it is a constant quantity. This fact is shown in the following Table of THE METABOLISM DURING STARVATION 627 the output of energy in man during a five days' period of starvation (Tigerstedt) : Metabolism durdjg Starvation (]VIa>-) Day of experiment Xitrogen output Fat oxydised Total calories Calories per kilo body weight 1 . . . 12-16 2U4-8 2231 33-3 2 . . . 12-85 190-3 2112 32-1 3 . . . 13 -02 179-9 2032 31-3 4 . . . 13-67 176-4 2003 31-3 5 . . . 11-44 180-0 1979 31-4 Although in one and the same individual the total metabolism during hunger varies directly with the body weight, this rule does not apply when we compare the metabolism of different animals or different examples of the same species. We find, in fact, that in larger animals the metabolism is relatively less than in smaller animals, so that if we take the evolution of calories per kilo body weight the result is inversely proportional to the body weight. This is shown in the following Table, which represents the total metabolism of a number of animals of diff'erent sizes (Rubner) : Body weight, Calories per kilo klos body weight Man 70-6 32-9 Dogl . 30-4 35-3 Dog 2 . 17-7 45-0 Dog 3 . 3-1 85-3 Rabbit 1 2-9 50-2 Rabbit 2 2-05 58-5 Guinea pig 0-672 223-1 On account of the gi-eater relative metabolism of smaller animals, their resistance to starvation is less than that of larger animals. A rat or a mouse will only stand total abstinence from food for two or three days. The differ- ence is determined by the fact that a smaller animal has a relatively larger surface per unit body weight than is the case with a larger animal. The greater part of the energy set free during starvation is required for the main- tenance of the body temperature. A larger amount of energy per kilo is required in those animals with a relatively larger body surface through which heat loss may occur. That the difference in relative surface is the deter- mining factor for the differences in total metabolism per kilo body weight is shown by the fact that, if we reckon out the amount of surface presented by each of the animals in the above list, we find that the output of energy per square metre of body surface is approximately identical in all cases. This is shown in the following Table, in which the calorie 628 PHYSIOLOGY output per square metre of surface has been reckoned for a number of animals of different weight : Body weight Calories per square metre body surface Animal 1 2 '„ 3 „ 4 „ 5 „ 6 „ 7 30-40 23-70 19-20 17-70 10-90 6-45 3-10 977 1069 1135 1040 1109 1054 1091 Speaking roughly, we may say that a warm-blooded animal during starvation requires the daily expenditure of 1000 calories per square metre body surface in order to maintain its temperature and carry out such motor processes as are essential to life. THE METABOLISM OF CARBOHYDRATE, FAT, AND PROTEIN DURING STARVATION Since during starvation no energy is supplied to the body from without in the shape of food-stuffs, we can regard the whole expenditure of the animal during its period of starvation as occurring at the expense of its capital. The amount of carbohydrate which can be stored up as glycogen or other forms is strictly limited. In many experiments the glycogen meta- bolism has therefore been entirely disregarded, and it has been estimated that the chemical capital of the body consisted entirely of proteins and fats. The glycogen metabolism, however, during the first day of a period of starva- tion may form a considerable fraction of the total metabohsm of the body, and can hardly be excluded without introducing serious errors. The relative parts played by protein, carbohydrate, and fat respectively in the chemical exchanges of a starving animal may be determined in the following way : The amount of protein consumed is given by estimating the total nitro- gen of the excreta by Kjeldahl's method and multiplying the result by the factor 6-25. The loss of weight of the body minus the protein consumed may be roughly taken as equivalent to the fat plus carbohydrate consumption. But this is only a rough method, since the quantity of water in the tissues may undergo considerable variations, and so affect the total weight of the body. For any accurate results the respiratory exchanges must be measured including both oxygen intake and carbon dioxide output. After deducting the carbon dioxide due to the combustion of the carbon of the proteins which does not appear in the urine in combination with nitrogen, the remainder of the carbon dioxide is derived entirely from carbohydrate and fat metabolism. Knowing the respiratory quotient and the total amount of carbohydrate and fat metabolism, it is possible to come to a conclusion as to how much of the THE METABOLISM DURING STARVATION 629 carbon dioxide is derived from oxidation of glycogen and how much from the oxidation of fat. A very efficient check on this calculation is furnished if the individual or animal can be placed at the same time in an accurate calorimeter, as in Benedict's experiments, o^^dng to the fact that a gramme of fat when converted into carbon dioxide and water produces more than double the amount of heat which would be evolved by the complete oxidation of glycogen. In Benedict's experiments the heat- value of the metaboUsm calculated by the above method agreed with the heat as actually measured by the calorimeter within 0-5 per cent., whereas if the total carbon of the first day had been reckoned as fat, the discrepancy would have been as high as 5 per cent, in many cases. The influence of glycogen metaboHsm on that of protein during the first and second days of fasting is shown in the following experiments (Benedict) : First day Second day Glj'cogcn metabolised X. eliminated Glycogen metabolised X. eliminated Total Per kilo Total Per kilo S.A.B. S.A.B. S.A.B. H.C.K. H.R.D. 181-6 135-3 64-9 165-6 32-8 3-15 2-31 1-09 2 -.33 0-59 5-84 10-29 12-24 9-39 13-25 29-7 18-1 23-1 44-7 41-6 0-52 0-31 0-39 0-64 0-76 11-04 11-97 12-45 14-36 13-53 The total metabolism per kilo body weight very soon attains a constant level. The relative part taken in the production of the total energy by fats and proteins respectively may vary from animal to animal according to the amount of fat available in the body. If the animal has been receiving previous to the experiment a diet rich in protein the excretion of nitrogen and urea in the urine diminishes rapidly during the first days of starvation. During the first two days therefore a considerable proportion of the necessary energy is obtained at the expense of protein. Between the third and fifth day, however, the nitrogenous excretion reaches a minimum, at which point it remains approximately constant to within a day or two before death. If the animal has been receiving a diet very poor in protein the excretion of nitrogen may be low throughout the whole course of the experiment. These facts are illustrated by the curves in Fig. 320, which show the output of urea in three experiments on a dog under different conditions of nutrition. In the first experiment the dog, before the experimental period, had been receiving 25(30 grm. of meat daily ; in the second it had been receiving 1500 grm. of meat, and in the third only a small quantity, which was not accurately measured. Although there is a great difference between the urea output during the first day of the experiments, the urea output during the sixth to eighth davs is identical. In many cases for a few davs before 630 PHYSIOLOGY death there is a rise of protein metabolism. This rise is synchronous with a practically complete disappearance of fat from the body. The animal now has to supply all its requirements at the expense of the protein tissues, which .ol CM, UREA Fig. 320. Three experiments on the output of urea during starvation (dog). (TiGERSTEDT after VoiT.) In (1) (thin line), the dog received 2500 grm. meat per day before the ex- periment ; in (2) (thick line), the diet was 1500 grm. meat ; and in the third experiment the meat was reduced to a minimum. therefore waste rapidly and account for the increased excretion of nitrogen. This is shown in the following experiment of Rubner on a rabbit : Days 1-3 4-5 6-8 Average daily out- put of nitrogen 1-67 grm. . 1-46 „ . 3-21 ., Average amount of fat oxidised daily lC-3 grm. 10-3 „ 2-4 „ We see therefore that during starvation, apart from the first day or two, the animal derives the main portion of its necessary energy from the com- bustion of fats, provided that there is a sufficient store of these substances in the body. A certain consumption of protein is unavoidable. Since protein comes from the working tissues of the body they are spared so far as possible, and it is only when the stored fat is used up that any large call is made on the tissue-protein. SECTION III THE EFFECT OF FOOD ON THE METABOLISM OF THE BODY A MARKED contrast exists between protein and the other two classes of food-stuffs in relation to nutrition. Whereas it is possible in the case of many animals to maintain life with a diet consisting of proteins, salts, and water, such as is contained in the leanest possible meat, a diet of pure fat or carbohydrate, or a mixture of the two, is almost equivalent to an absolute abstinence from food, the animal on such a diet surviving only a few days longer than during complete starvation. The primary importance of pro- teins in nutrition therefore indicates that it is advisable to deal first with the effects on metabolism of a diet consisting of this food-stuff alone. In an animal which has been starved for five or six days the nitrogenous output of the body has attained a practically constant level, varying with the size of the animal and with the relative content of its body in fat. Let us assume that such an animal is excreting 5 grm. of nitrogen daily, corresponding to a protein metabohsm of 31-25 grm. of protein. It might be thought that this loss of protein to the body would be met if we administered to the animal as food a similar amount, i.e. 31-25 grm. of protein. On trying the ex- periment, however, we find the effect of giving protein food is to increase largely the nitrogenous output of the body, so that after receiving this amount of protein the animal's nitrogenous excretion will amount to nearly 10 grm. The waste of tissue-protein in the body therefore proceeds. In order to stop this waste, i.e. to ensure that the animal does not lose more nitrogen than it receives in its food, we must give an amount of protein corresponding to be- tween two and a half and five times the amount of protein which undergoes disintegration during starvation. The reason for this is obvious on reference to p. G27. There we see that a man on the fifth day of starvation excreted 11-44 grm. of nitrogen, corresponding to 71-5 grm. of protein. This protein metabolism did not, however, represent the sole source of the energv output of the body. The total energy output was 1979 calories. Of this amount only 293 calories could be obtained from the combustion of the 71 grm. of protein, the balance being due to the oxidation of fat stored up in the body. This signifies that only one-fifth of the total energy requirements of the body were suppHed at the expense of protein. We cannot therefore expect to stop loss of bcxly substance by giving an amount of protein food which would correspond only to one-fifth of the energy requirements. lu most cases, G31 632 PHYSIOLOGY if we are dealing with an animal with a considerable store of fat in its body, nitrogenous equilibrium, i.e. an equivalence between income and output of nitrogen, is attained with a quantity of protein in the food which is less than five times the amount lost during starvation. In such a case the total energy requirements of the body are met not only at the expense of the protein food but also at the expense of the fat of the tissues. The animal will continue to lose weight and to become thin, although he is in a state of nitrogenous equihbrium. The protein taken in with the food on a pure protein diet has a twofold function to perform. In the first place, every functional activity of the living tissues is probably associated with a certain amount of wear and tear, and results in the production of disintegration products which are not in a condition to be resynthetised into living working protoplasm. We know, for instance, that from every mucous surface dead cells are being continually cast off and that a constant disintegration of red blood corpuscles goes on, resulting in the production of the bile pigments ; and we are warranted in extending the operation of these changes of which we have ocular evidence to the case of other cells, such as those of the liver and of the muscles, where direct proof of destruction of tissue during normal metabolism is more difiicult to obtain. It is certain that some portion of the nitrogen excreted during complete starvation must come from this source, and that one of the functions of protein food is the replacement of tissue which has been lost in this way. When, however, we are feeding an animal on a pure protein diet, by far the larger portion of the food is utihsed for meeting the energy requirements of the body. In this function protein food, apart from accidents of digestibility and structural adaptation of the animal's digestive arrangements to its habits of fife, presents no apparent advantages over the other two classes of food-stuffs. Its value to the animal is repre- sented by its physiological heat- value. It may be represented therefore numerically as 4-1, and is equivalent to the value of carbohydrate * and is far inferior to the value of fats with a heat equivalent of 9-3. If, instead of giving to the starving animal a pure protein diet, we administer a mixed diet containing a sufficient quantity of fat or carbohydrate, or of both substances, to meet the normal energy requirements of the body, we can restrict the utilisation of protein more nearly to the replacement of tissue waste in the body, and are therefore able to attain nitrogenous equihbrium with a much smaller proportion of protein than is possible when this substance furnishes the whole diet. In carnivora, which have the habit of supplying a large proportion of their energy needs at the expense of protein, the amount of carbohydrate and fat which must be added to protein in order to attain nitro- genous equihbrium with a nitrogenous output equal to that in starvation is very large and corresponds to an energy- value in excess of the total energy expenditure during starvation. In omnivora, such as man, it is easy to attain nitrogenous equilibrium on a mixed diet with a smaller nitrogen turnover * This may be expressed by saying that protein is isodynamic with an equal weight of carbohydrate. THE EFFECT OF FOOD ON METABOLISM 633 than is found during starvation. In the experiment given on p. 627 the average nitrogen output during starvation was about 12 grm. of nitrogen. In Succi, the fasting man, the nitrogen output varied from 11-19 grm. on the fifth day to 2 -82 grm. on the twenty-first day. Daily Nitrogen Excretion of Succi in Starvation Day N. Day N. Day N. 1 17-0 8 . 9-74 15 . 505 2 . 11-2 9 . 10-05 16 . 4-32 3 . 10-55 10 . 7-12 17 . 5-4 4 10-8 11 . 6-23 18 . 3-6 5 . 1119 12 . • . 6-84 19 5-7 6 . 1101 13 . 5-14 20 . 3-3 7 8-79 14 . 4-66 21 2-82 Chittenden has shown that in man a perfectly normal nutrition may be maintained on a mixed diet containing about 7 grm. of nitrogen daily. In the cases investigated by Chittenden the energy output of the men could be regarded as normal, corresponding to 32-35 calories per kilo body weight. If the amount of fat and carbohydrate be very largely increased it is possible to maintain nitrogenous equilibrium on even smaller quantities of protein. Thus nitrogenous equilibrium was attained by Siven on a diet containing 33 grm. of protein daily (= 5 grm. of nitrogen), but in this case the carbo- hydrates and fats were increased to such an extent that the man was taking in 78-5 calories per kilo per day. These results suggest that the quahtative metabolism of the body is determined by the relative amount of food-stuff supplied to the body and circulating in its juices at any given time, and that preponderance of one food-stuff will tend to excite the cells of the body to the utilisation of this food-stuff at the expense of the other two. That such is the case is shown by a study of the effect of increasing each class of food on the metabolism of the body as a whole. EFFECTS OF VARIATIONS IN PROTEIN Most of the experiments on the influence of variations in the quantity of protein food have been made on carnivora, such as the dog and cat. Within very wide limits the output of nitrogen is proportional to the intake. This is shown in the Tables (p. 634) by Voit, representing two experiments on dogs. In Experiment I the animal had been fed for some days with 500 grm. of meat per diem. The fact that he was excreting nitrogen corresponding to 547 grm. shows that this amount was insufficient and that he was not yet in a condition of nitrogenous equilibrium. Each day he was using up 47 grm. of the protein of his body in addition to the 500 grm. supplied in the food. On increasing his food three-fold to 1500 grm. the nitrogenous output was also increased, but a state of nitrogenous equilibrium was not reached until the eighth day of the experiment. During the six days intervening 778 grm. of meat had been retained in the body, i.e. there had been a retention of 634 PHYSIOLOGY protein, probably in the form of increased muscular substance. The amount is too great to be accounted for by retention of the disintegration products of the protein in the body. It must have been stored up in the form of protein and probably, to a large extent at any rate, as actual living tissue. Experiment I EXPERIMENT II Day Daily meat ration riesl) loss per day Day Daily me^t ration Flesh loss per day 1 . 2 . 3 . 4 . 5 . 6 . 7 . 8 . 500 1500 1500 1500 1500 1500 1500 1500 547 1222 1310 1390 1410 1440 1450 1500 1. . 2 3 . 4 . . 5 . 6 . 1500 1000 1000 1000 1000 1000 1500 1153 1086 1088 1080 1027 In the second experiment the diminution of the protein of the food was followed by a loss of protein from the body, the output being greater than the income. The excess, however, was rapidly diminishing and equilibrium had been practically attained on the last day of the experiment. During this time the animal had excreted 14-8 grm. of nitrogen more than it had received in its food, which would correspond to a diminution of the protein store of its body, reckoned as muscular substance, by 434 grm. Many such experiments have been performed, and they all agree in showing that in carnivora a very appreciable storage of nitrogen can take place in the body. In cats it is sometimes possible to double the body weight by administration of a large protein diet. Since no fat is laid on at the same time and the animals are in a fine healthy condition, one must conclude that the greater portion of the storage takes place by a growth of muscle substance. The degree to which the storage can take place is, however, variable and is generally smaller in dogs than in cats. However much protein is given, the limit is finally arrived at where no further laying on of protein tissues of the body is possible, and the animal then enters into a state of nitrogenous equihbrium, when he excretes a quantity of nitrogen exactly equal to that taken in. This equivalence of income and output signifies that the extent of the total metabolism of the body is affected by the amount of protein supplied in the food, and, as a matter of fact, the total energy output of the body rises and falls with the quantity of protein in the food. This is shown in the Table (p. 635) by Pettenkofer and Voit, in which the figures have been recalculated by Pfliiger. We see therefore that carnivorous animals can satisfy their total energy requirements at the expense of protein. When the protein income is in excesss of their requirements a small amount is laid on, probably in the shape of increased muscular tissue. The most marked effect is, however, an THE EFFECT OF FOOD ON METABOLISM 635 increased metabolism which rises in proportion to the nitrogenous income. The hmit to this increase is set by the powers of the alimentary canal to digest the protein. The rise in metabolism consequent on protein food is very rapid and afi'ects the gaseous exchanges as well as the output of nitrogen. Magnus Le\y and Falk found that a large protein meal might increase the respiratory exchanges 40 per cent., an increase which lasted seven hours. Xitrogcnin food Nitrogen output Fat gain or loss Total calorics 1 0 5-61 -98 1067 2 . 17 20-37 -61 1106 3 , 34 36-69 -43 1360 4 51 51-00 -24 1552 5 61 59-74 -36 1893 6 . 68 69-50 + 8 1741 7 . 85 85-41 + 4 2181 The nitrogenous output also rises immediately after a protein meal, so that 50 per cent, of the nitrogen of the ingesta may appear in the urine within seven hours after the meal. The whole of these results cannot be strictly applied to omnivorous animals, such as man. In these it is impossible to supply all the energy requirements of the body on a pure protein diet. Even if a man eats as much meat as he can, he will be unable to obtain sufficient energy for his daily requirements. Whereas the average daily requirements of a man amount to about 3000 calories, 1 lb. of meat would yield only about 400 calories, and even if he took 4 lb. of meat daily, an amount which is im- possible for most individuals, he would only be obtaining about 1600 calories. The cures for obesity, in which a large protein diet plays an important part, owe their efficiency to this fact. They are in all cases practically equivalent to a state of semi-starvation. Many experiments have been made on the influence of variations in the quantity of protein in a mixed diet. Within wide limits the output of nitrogen is strictly proportional to the intake. A normal adult man seems to be unable to store up protein in any form, and differs in this respect from carnivora, such as the dog or cat. The only way in which protein can be laid on in the body is by furnishing a physiological stimulus to the growth of muscle, i.e. by constant exercise. Without this it is not possible to produce growth of the muscles of the body, however much protein we may give in the diet. The conditions are, however, different when dealing with an individual in whom from some cause or other the muscular tissues have not attained their full development. Thus in growing individuals a certain amount of the protein of the food is always retained in the body and laid on as tissue-protein. In convalescence after severe fever, during which a great wasting of the muscles has taken place, forced feeding with large amounts of protein has been found to give rise to a considerable retention of protein in the body. This process only goes on until the muscles have attained their normal condition of development. When the tissues 636 PHYSIOLOGY have, so to speak, reached ' par,' the possibihty of laying on protein tissues ceases. On the other hand, protein food has in man, as in animals, a specific stimulating effect on metabolism, so that the respiratory exchanges are largely increased as a result of a heavy protein meal. This effect has been named by Rubner the ' specific dynamic effect ' of protein. We shall have occasion later to discuss its significance. THE INFLUENCE OF FATS AND CARBOHYDRATES If either fats or carbohydrates be given to a starving animal a certain sparing of the fat of the body takes place, but this effect, according to some observers, is accompanied by a distinct increase in the metabolic exchanges of the body. As regards the protein metabolism, Cathcart finds that while administration of fat increases the nitrogen output during starvation, carbohydrate food causes a diminution in the nitrogen output, and thus exercises a marked sparing effect on the proteins of the body. Voit found that during starvation or with an insufficient protein diet addition of fat to the food increased the total metabolism. When sufficient protein was being supplied, the addition of fat caused no increase in the total metabolism, the whole of the fat in the food being laid on as fat in the body. The stimulating effect of fat on metabolism is but slight. Magnus Levy found that the increase in the metabolism on the administration of fat to a starving animal was minimal and never exceeded 10 per cent. Carbohydrates have a somewhat greater influence on metabolism. The administration of a large meal of carbohydrates to a starving animal may raise the respiratory exchanges from 20 to 30 per cent., and the increase may last four hours after the meal. This stimulating influence on metabolism is, however, much less than that observed on the administration of large doses of protein. If carbohydrate be given in excess of the daily energy requirements the greater proportion of it remains in the body, being stored up to a small extent as glycogen, but mainly in the form of fat. SECTION IV THE EFFECT OF MUSCULAR WORK ON METABOLISM When an animal performs muscular work its total output of energy must be increased, and this must take place at the expense either of an increased intake of food or an increased destruction of the tissues of the body. For many years a theory put forward by Liebig received universal acceptance among physiologists. According to this view the food- stuffs could be divided into two classes, namely, the proteins, which were the plastic food substances and were concerned in the building up of tissues, and, secondly, the fats and carbohydrates, which took no part in the building of the tissues, but were oxidised for supplying heat to the body. Muscular work was sup- posed to be attended with a breakdown of the muscular tissue, and therefore to be performed at the expense of an increased protein metabolism, which had to be made good by adding to the protein intake in the food. If this view were correct one would expect to find a great increase in the nitrogenous metaboHsm of the body with every increase in muscular work. Such an increase has, in fact, been found by Pfliiger in dogs which were nourished on a purely protein diet. In these animals the sole source of energy to the organism w^as protein, and therefore any additional call on the energies of the body must be associated with an increased intake of food or an increased loss of material from the body in the shape of protein. In an animal which is enjoying a normal mixed diet, or has a store of carbonaceous material in its tissues in the shape of fat, there is no increase of nitrogenous metabolism during muscular w^ork w^hich w^ould correspond in any way to the amount of work done. This was shown long ago by Voit in experiments on the dog. The following Table represents the nitrogenous metabolism, i.e. the amount of muscular material in the shape of protein metabolised during the day, under varying conditions of rest and activity, during starvation and after food : Doc. I Food Flesh metabolism Condition of dog Gnu. 0 .... 164 . . Rest 0 .... 167 Work 0 .... 149 .. Rest 637 638 PHYSIOLOGY Dog II Food Flesh metabolism Grm. Condition of dog 1500 grm. lean meat 1522 Eest 1500 „ „ 1625 Work 1500 „ „ 1526 Rest 1500 „ „ 1583 Work 1500 „ „ 1535 Rest During the work days the animal performed about 1500 kilogramme metres in the day. The differences in the protein metabolism between the rest and the work days are thus practically insignificant, the nitrogenous output being determined, not by the work done, but by the amount of protein administered in the food. In the second experiment there is an average increase of protein metabohsm during the work days amounting to 86 grm. of flesh, a quantity which is insufficient to furnish the energy of the work done. The same results were arrived at in a classical experiment performed by Fick and Wishcenus on themselves, in which they measured their total nitrogenous metabohsm during an ascent of the Faulhorn from the Lake of Brienz. The vertical distance traversed was 1956 metres. During the seventeen hours before the experiment, the six hours of the ascent, and the seven subsequent hours they ate food practically free from nitrogen. The urine passed during the ascent and during the next seven hours was collected in each case and its nitrogen determined. Fick passed 5-74 grm. of nitrogen, which, if the energy of the protein were totally converted into work, would correspond to 63,378 kilogramme metres. In Wislicenus the amount was 5-55 grm. of nitrogen, equivalent to 61,280 kilogramme metres. Fick, who weighed 66 kilos, in raising himself to a height of 1956 metres, had performed 129,096 kilogramme metres, and Wislicenus, with a weight of 76 kilos, had performed 148,656 kilogramme metres. Even if we assume the possibihty of a conversion of the total energy of the protein metabolised during the experiment into mechanical energy, we cannot account for more than half of the total work done. All subsequent experimenters have con- firmed the deductions which were drawn from these two researches, namely, that muscular work, while practically without influence on nitrogenous metabolism, increases enormously the carbonaceous metabolism of the body, so that, except in the rare cases where the diet consists of pure protein and the body is practically free from fat, the additional energy output during work is derived from the oxidation of carbon and hydrogen to carbon dioxide and water. The general nature of the changes in the metabolism of the body during work is well illustrated by the results obtained by Atwater on man. The total energy output of a man was reckoned as heat by means of the calori- meter. The heat equivalent of the external muscular work performed by the man was also reckoned as heat. In the Table (p. 639) we give the total output of energy per day during rest and work, the latter being also expressed in calories.* * One large calorie, or ' kilo-calorie,' is equivalent to 425 kilogrammetres. EFFECT OF MUSCULAR WORK ON METABOLISM Energy per Day 639 Nature of experiment Heat eliminated External worl£ in calories Total in calories By radiation and conduction In urine and faeces In water vaporised from lungs and skin Rest with food (average of four daj's) Rest fasting (four experi- ments) (average of five days) Work (fourteen experi- ments) (average of forty-six days) 1850 1005 3802 26 21 29 521 561 743 546 2397 2187 5120 If we compare the energy- value of the work done with the excess of the total expenditure of the body over that found during the rest experiments, we find that the performance of muscular work involves an increase in the total energy- expenditure of the body by an amount equal to about five times that of the work done. Of course a certain proportion of this excess of energy over work done is accounted for by the increase in the work which must be performed by the respiratory muscles and heart in the state of greater activity which is imposed upon them by the external work, and is necessary for the proper provision of the active muscles with increased food- supply and oxygen. Even if we neglect these factors altogether, we see that the efficiency of the body as a machine corresponds to between 16 and 20 per cent., an efficiency which exceeds that of the best of our steam- engines and is only equalled by certain internal- combustion engines. A comparison of the excreta of the same individual whose energy ex- changes are given in the above Table during rest and activity will ^ive us information as to the source of the increased energy put out during the per- formance of muscular work. Thus during a period of rest and starvation the average output of carbon dioxide during six hours amounted to 189-6 grm. ; during a rest experiment with food the average output for a period of six hours was 230-4 grm. of carbon dioxide. During work the averat^e output in the same individual during six hours rose to 705 grm. of carbon dioxide on a carbohydrate diet, and to 634-8 grm. on a diet containing a large amount of fat. The oxidation of carbon was therefore increased more than threefold as a result of muscular work. If we compare in the same wav the protein metabolism of the same individul during these experiments no such alteration is observed. Thus the average output per day during rest and starvation corresponded to 82 grm. of protein. During rest and with an approximately sufficient amount of food the average amount of protein consumed was 98-8 grm. During a work day in which he received practically 640 PHYSIOLOGY the same amount of protein in the food and a somewhat insufficient amount of carbohydrates and fats, his consumption of protein amounted to 109-4 grm. Here, as against a three- to fourfold increase of the gaseous metabohsm of the body, we have only a 10 per cent, increase of the protein metabohsm. A still larger difference between the respiratory changes in the resting or working condition is found when the exchanges are taken over shorter intervals. The following Table represents the respiratory exchanges in a trained muscular subject during complete rest and when doing moderate or severe work, namely, riding a bicycle with a brake. Each observation lasted from 10 to 15 minutes. Condition COg eliminated per minute Oxygen absorbed per minute Respiratory quotient Pulse rate Rate Lying Moderate work Severe work 200 1720 2227 243 1834 3265 .83 .94 ■ .98 56 150 166 20 32 38 These last figures represent a maximum since the subject worked with great difficulty and was exhausted when he came off the machine. There is another method by which we can arrive at some idea of the nature of the material which is furnishing by its oxidation the necessary energy for the performance of muscular work. It is evident, if we compare the formulae of a carbohydrate and a fat respectively, that it will require a relatively larger amount of oxygen to oxidise the fat than is necessary in the case of the carbohydrate. In the latter there is sufficient oxygen to combine with all the hydrogen present and form water. The whole of the oxygen therefore which is taken in is employed in the oxidation of the carbon, and one volume of oxygen will produce one volume of carbon dioxide. Thus : CgHiaOs + 6O2 = 6H2O + 6CO2. If the whole of the animal's energy requirements were furnished by the oxidation of carbohydrates, the output of carbon dioxide expired would be exactly equal in volume to the oxygen inspired, and the respiratory quotient of the animal, namely, ■ _ ^ . — —. — -, O2 inspired would be equal to unity. In fats the amount of oxygen is only sufficient to combine with four atoms of the hydrogen of the molecule. When fats undergo oxidation, of the oxygen used only a portion is devoted to the formation of carbon dioxide, the rest being employed in the oxidation of hydrogen to water. In an animal using only fats the carbon dioxide output of the body would be considerably less than the oxygen intake and its respira- tory quotient would be less than unity. The respiratory quotients for pro- tein, fats, and carbohydrates are given in the following Table (Atwater) : EFFECT OF MUSCULAR WORK ON METABOLISM 641 Material Respiratory quotient CO. Starch 10 Cane sugar 10 Glucose 10 Animal fat 0-711 Protein 0-809 The respiratory quotient in an animal at any given time is therefore determined by the nature of the substances which are undergoing oxidation in its body. If the performance of muscular work involved special chemical processes, a metabolism of one of the main constituents of the body in preference to either of the others, this sudden change in the quality of the metaboUsm should show itself in the respiratory quotient. According to Speck and Lowy, moderate muscular work which is not associated with dyspnoea, although attended by a large increase in the carbon dioxide output and the oxygen intake of the body, does not alter the respiratory quotient. This is probably correct if the respiratory changes are taken over a sufficient length of time, when the respiratory quotient must depend on the food which is furnished to the body however the energy of this body is expended. When however, as in Benedict's and Cathcart's experiments, the observations are of short duration, muscular exercise is almost always fomid to be associated with a rise in the respiratory quotient, which lasts too long to be accounted for by a mere washing out of the carbon dioxide from the blood and the body tissues. After the cessation of the exercise the respiratory quotient sinks below normal. The respiratory quotient however rarely rises even during exercise to 1 as it would if the muscular work were performed solely at the expense of carbohydrates. These results suggest that while muscular work can be performed at the expense of any of the food-stuffs or of the three classes of constituents of the body, carbohydrate is the immediate or the most readily available source of muscular energy. When the body passes suddenly from a resting to an active condition the first call is therefore on the carbohydrates of the blood and those stored up in the muscles and liver, whereas after exercise these carbohydrate stores are slowly replenished probably at the expense of the proteins. The fact that the body can draw on its fat stores for the perform- ance of muscular work suggests that this substance may also serve as a source of muscular energy, but there is no evidence that the body is able to convert fats under any circumstances into carbohydrates, so that we must assume that under certain conditions the fats or their decomposition products may be directly utihsed by the muscles. 21 SECTION V THE SIGNIFICANCE OF THE FOOD-STUFFS When the proteins are taken as food they are rapidly and ahnost completely metabolised, so that the energy output of the body increases fari passu with the increased protein food. With a large excess of protein, a certain Hmited storage of this material is possible, but the stored-up protein rapidly disappears on deprivation of food. On this account the course of the meta- bolism during starvation is the same after the first two or three days, whether the animal has previously received large or small amounts of protein in its diet. In man, even this limited power of storing nitrogen is apparently absent. Under no circumstances can we produce a laying on of fat in the body, even in carnivora, however much the protein income is increased. The metabolism of fats and carbohydrates, on the other hand, is deter- mined, not by the amount of these substances in the food, but by the energy requirements, i.e. the functional activity of the living tissues. Any excess * of either of these foods simply gives rise to their storage in the body almost entirely in the form of fat. How are we to regard the particular position taken up by the proteins in metaboHsm ? Voit, whose laborious observa- tions form the foundation of all our present knowledge of metabolism, drew a sharp contrast between the proteins which were built up to form parts of the living cells, the tissue or morphotic protein, and those which underwent rapid oxidation in the tissue juices without ever forming an integral con- stituent of the living protoplasm. The latter he designated circulating protein. The rapid fall in the nitrogenous excretion during the first two days of starvation he ascribed to the using up of the circulating protein. As soon as this was exhausted the animal was reduced to living on its own tissues, so bringing into the metabohc cycle the tissue-proteins themselves. This theory has been energetically attacked of late years by Pfliiger, according to whom the whole of the protein, which is broken down and oxidised to urea, must at one time have formed an integral part of a living cell, so that tissue- protein would be the sole source of the urea. He explains the rapid excretion of nitrogen which follows the ingestion of a protein meal by the special avidity of the animal cell for protein. When enough of this is presented to it it feeds upon nothing else, and only when there is a comparative lack of pro- tein will it make use of carbohydrate or fat for its needs. Thus while a dog * That is, assuming that the animal is able to digest and absorb the excess. On this factor probably depends the possibility of fattening an animal. 642 THE SIGNIFICANCE OF THE FOOD-STUFFS G43 is fed on a rich mixed diet it lives practically on protein alone, storing up the fats and carbohydrates of the food as fat. If food be now withdrawn the animal must live either at the expense of its own living tissue (proteins) or must attack the stored-up fats in its body. The latter alternative, as a matter of fact, takes place. The animal spares the precious protein and lives on the fat of its own body. Hence comes the great fall in the excretion of urea that is observed in starvation, the consumption of proteins sinking to the indispensable minimum. If now a protein meal be given, the cells of the body return to their former way of living, and satisfy as much of their needs as possible at the expense of protein, so that the urea excretion rises almost in proportion to the food given. In order to attain nitrogenous equilibrium on a purely protein diet, it is necessary to give the cells enough protein for their total requirements, i.e. three to five times as much as would corre- spond to the nitrogenous excretion during hunger. If a larger amount of protein be given than is necessary for the maintenance of nitrogenous equilibrium, a certain amount of nitrogen is retained in the body, probably as protein, giving rise to an increase in the total living material of the body, and the animal increases in weight. The amount of urea excreted by an animal is proportional not only to the quantity of protein taken in with the food but also to the weight of the animal ; so the animal which has grown heavier in consequence of increased supply of nitrogenous food will need a larger amount of protein to maintain its nitrogenous equilibrium, which will be produced with the same amount of protein as soon as the animal has increased in weight to a certain extent. In order therefore to maintain the increase in weight, it is necessary to give ever-increasing quantities of protein, and the stuffing process is finally put an end to by the refusal of the digestive organs to digest any more. There can be little doubt, however, that Voit was substantially correct in assigning a twofold fate to the ingested protein, and that, as Speck has pointed out, we can consider protein metabolism under two headings, viz. tissue or nutritional metabohsm and energy metabolism. Since the proteins form the main constituent of living protoplasm, the death and destruction of cells, which are constantly going on in the body, nmst result occasionallv in the production of substances which are not available for resynthesis, and are therefore turned out of the body with the urine.* A certain proportion of the proteins of the food must therefore be applied to replacing this waste of tissue. The proportion will be larger in cases where a growth of the nitrogen- ous tissues is occurring, as in the young animal or during convalescence from a wasting disorder. On the other hand, a certain proportion of the nitrogen in the urine will be derived from the breakdown of tissues, and this in its turn * Tiger&tedt suggests that the irreducibk^ minimal protein consumption may be due, not to a special metabolic cycle of the protein on its way into and out of the living cell, but to the fact that the circulating fluids of the body contain large amounts of protein as essential and constant ingredients, and the living cells therefore, which are bathed by these fluids, cannot refrain, even in times of protein scarcity, frrm feeding to a certain extent on these the predominant constituents of their nutrient medium. 644 PHYSIOLOGY will be increased under any conditions which bring about an augmented tissue disintegration, such as the toxaemia of fevers, poisoning by arsenic or phos- phorus, or partial asphyxia by deprivation of oxygen, as after inhalation of carbon monoxide gas. In this function protein cannot be replaced hj either of the other food-stuffs. With regard to its second fate, viz. the furnishing of energy to the body, protein stands on the same level as carbohydrates or fats, its relative value as compared with these two classes of substances being represented by its physiological heat- value. Owing to the inability of the body to store protein to any marked extent, any protein which is absorbed in the alimentary canal in excess of the nutritional requirements of the body is at once broken down and oxidised to satisfy the energy requirements of the cells. The NH and NHg groups in the protein molecule add little or nothing to its chemical or potential value. The energy value of the protein depends on its carbon and hydrogen content, and it seems probable that the greater part of the nitrogen is split off as ammonia from the protein molecule during or shortly after its absorption from the alimentary canal. It is on this account that an increased excretion of urea is the almost immediate consequence of the ingestion of protein. The point made by Pfliiger against Voit, viz. that no oxidation occurs in the lymph or blood, is really beside the mark. The living cell is a complex system which may include food in all degrees of oxida- tion or chemical change, without this food material necessarily forming part of the living framework. Every histologist distinguishes the paraplasma from the more active and essential protoplasm, and we mast assume that it is in the ultra-microscopic interstices of the foam-like protoplasm that the chief processes of chemical change and oxidation occur. A proof therefore that oxidation depends on the condition of the cells and not on that of the blood does not justify the conclusion that the whole nitrogenous metaboHsm of the body is confined to the living protoplasm itself. Polin has lately brought forward a number of facts which point not only to a twofold origin of the nitrogen of the urine but also to a qualitative difference in the two orders of protein metabolism. Whereas in the urine of man on a normal diet the urea nitrogen forms 85 per cent, or more of the total nitrogen, a reduction of the protein ration to the minimum necessary to meet the nutritional requirements of the body causes not only an absolute diminution of the urea but a large relative diminution when compared with the other constituents of the urine, such as creatinin. He concludes therefore that the nitrogenous end-products of nutritional metabolism are different from those of the energy metabolism. As Speck has pointed out, there is also a difference in the time-relations of the two orders of metabolism. Whereas the nitrogen, which furnishes no energy to the body, is rapidly eliminated when protein is being utilised for the supply of energy to the body, the occurrence of increased tissue waste causes a rise of nitrogenous excretion, which comes on slowly, often after the lapse of a day, and may last two or three days. The process of protoplasmic disintegration appears therefore to occur in a series of stages, which occupy a considerable time and end in the production of substances qualitatively distinct from that THE SIGNIFICANCE OF THE FOOD-STUFFS 645 substance, urea, which is the almost exclusive nitrogenous end-product of the energy metabolism of protein. THE FOOD-VALUE OF CERTAIN SUBSTANCES ALLIED TO PROTEINS PROTEOSES AND PEPTONES. In the digestion of the naturally oc- curring proteins the first products of hydration consist of a mixture of substances known as proteoses and peptones. In the further processes of digestion, under the influence of the ferments of the pancreas and small intestine, these substances are converted into the amino-acids which we have learnt to regard as the proximate constituents of the protein molecule. Many experiments have been performed in order to determine the nutritive value of these digestive products. In nearly all cases it has been found that the meat in the diet of an animal can be replaced by a corresponding quantity of the products of digestion of the same meat without interfering with the nitrogenous equilibrium of the animal, and Loewi and others have shown that the same result may be attained by feeding an animal on the products of pancreatic digestion of protein, i.e. a mixture consisting almost entirely of amino-acids. Since the proteins of the body differ in their composition from the majority of the proteins of the food, it is evident that each food-protein molecule has to be taken to pieces and reconstructed before it can take its place in the body fabric, and it is therefore only natural that, so far as metabolism is concerned, the results should be identical whether we feed the animal with the ordinary food- protein or with the products of its metabolism. This mode of feeding cannot, however, be regarded as presenting any advantages. Under normal circumstances the food molecules are broken down by degrees. Their products of hydrolysis are set free in small quantities at a time and can be therefore absorbed and disposed of in proportion as they are set free. On the other hand, a sudden flooding of the alimentary canal with a large quantity of the products of digestion intro- duces an abnormal factor which must tend to produce wastage of nitrogen in the body and to disturb the normal chain of processes involved in the regular course of digestion. COLLAGEN AND GELATIN, Among the various sclero-proteins or albu- minoids which occur as normal constituents of our foods these two are the only substances which undergo digestion and solution in the alimentary canal to any appreciable extent, other substances, such as elastin and keratin, reappearing for the greater part in the fseces. As we have seen, gelatin, the flrst product of hydration of collagen, represents, so to speak, an imperfect protein. When it is hydrolysed by acids its disintegration products include the ordinary amino-acids of the fatty series, including a considerable amount of glycine and also a certain amount of phenyl alanine and proline. The oxy- phenyl group which occurs in all the food-proteins in the form of tyrosine, as well as the indol-containing group tryptophane, are absent. On this account purified gelatin does not give either Millon's test or the Hopkins-Adam- kiewicz test with glyoxylic acid. As might be expected from its composition. 646 PHYSIOLOGY gelatin cannot entirely replace the proteins of the food, but is able to take the place of part of the proteins. If nitrogenous equilibrium has been attained on a certain amount of protein together mth a mixed diet, a considerable proportion of the protein, but not all, can be replaced by gelatin. In an experiment on a dog, in nitrogenous equilibrium on a mixed diet containing 0-6 grm. protein per kilo, it was found that fully five-sixths of the protein could be replaced by gelatin without any disturbance of the nitrogenous metabohsm. Physiologists have succeeded in maintaining animals for a short time in a state of nitrogenous equilibrium on a diet containing no protein, but in its place a mixture of gelatin with tyrosine and tryptophane. It is doubtful, however, whether such experiments could be continued indefinitely. In most cases the animal after a time refuses to eat the gelatin. A somewhat similar behaviour is found in the case of zein, the crystallised protein from maize, which yields no tryptophane or tyrosine on hydrolysis. Hopkins has shown that animals fed with zein, together with a small proportion of tryptophane, live longer than those fed with zein alone. But in no case could he maintain life on this diet for a period greater than forty-five days. There are evidently other groups in the protein molecule which are essential for the maintenance of life and which were not represented in the mixture of zein and tryptophane. Experiments have been carried out in order to ascertain whether asparagine, which forms so important a nitrogenous constituent of young plants, can be directly utilised by animals. There is evidence that this substance has a real nutritive value for certain herbivora. The utilisation is not, however, a direct one. The asparagine appears to be taken up by the bacteria which swarm in the paunch or caecum of these animals. It is built up by these micro-organisms into protein, and it is the protein of the micro- organisms and not the asparagine itself which is digested, absorbed, and utilised by the mammal. OTHER CONSTITUENTS OF THE FOOD CELLULOSE. This substance, which forms the cell walls of plants, furnishes an important food-supply to the herbivora. Its digestion is not, however, carried out by the action of juices secreted by the intestinal canal itself. The solution of the cellulose is effected partly under the influence of a cellulose or cytase present in the plant cell themselves, partly under the influence of the micro-organisms living in the paunch or caecum. Under the influence of these bacteria cellulose is dissolved with the production of carbon dioxide, methane, and butyric and acetic acids. In man the greater part of the cellulose of the food is undigested, and its chief value appears to be that of lending bulk to the indigestible material and so aiding the normal movements of the intestines. In the case of young plant cells, such as those of lettuce or carrots, a certain percentage of the cellulose undergoes solution in the intestine. Here, again, the digestion is probably effected by the agency of putrefactive organisms. ALCOHOL. When alcohol is taken by man in moderate quantities the greater part of it undergoes oxidation, and leaves the body as carbon dioxide and water. About 10 per cent, which escapes oxidation is excreted unaltered THE SIGNIFICANCE OF THE FOOD-STUFFS 647 by the lungs and urine. This oxidation of alcohol is a result of true utihsa- tion, since the addition of a certain amount of alcohol to the food does not result in an increase of the output of carbon dioxide. In small quantities therefore alcohol can act as a food. This function, however, is quite un- important, and is overshadowed by the poisonous action of this substance. A man unaccustomed to its action cannot take more than 16 to 25 grm. without experiencing its poisonous effects. This amount of alcohol only represents a total heat- value of 112 to 175 calories, i.e. only about 5 per cent, of the total energy requirements of the body. Only very rarely therefore can we be justified in administering alcohol as a food. Its value in a diet is entirely that of an accessory or adjuvant in exciting appetite by its taste and smell, an advantage which is largely counterbalanced by the danger of introducing a poison into the body which on long continuance tends to set up various degenerative changes in the tissues, and if taken in any quantity at one time causes a temporary abolition of those processes of inhibition and control which have been the determining factors in the survival of the race throughout the struggle for existence. THE INORGANIC FOOD-STUFFS If an animal be fed with the proper quantities of fats, proteins, and carbohydrates, from which all the salts have been removed as completely as possible, it rapidly shows a distaste for the food, becomes, ill, and dies in a shorter time than if it were receiving no food at all. Part of the symptoms which occur in these cases are due to the production of acid substances, e.g. sulphuric acid, in the course of metabolism of the proteins. It is possible to obviate this acid intoxication by administering sodium carbonate with the food. This admixture, however, only suffices to prolong the life of the animal for a short time. It is evident therefore that the inorganic constitu- ents of the food, although yielding no energy to the body, are as essential for the maintenance of fife as the energy-yielding food-stuffs, namely, proteins, carbohydrates, and fats. In the course of this work we shall have occasion to study the intimate dependence of the functions of various tissues, such as skeletal and heart muscle, on the presence of salts in normal proportions in the fluids with which they are bathed. Animals in a state of salt hunger show by the disorders of digestion which occur that the presence of salts is equally requisite for the due performance of the processes of secretion and absorption. Towards the end of the experiment the animal vomits its food, which shows no signs of digestion even when it has lain some hours in the stomach. Forster has shown that in salt-hunger the body is continually giving ofi inorganic constituents in the urine. The amount of these is smallest when it is supplied richly with organic food-stuffs. It seems that the salts of the body exist in a state of unstable combination with the tissue constituents, especially the proteins. If tl\e amount of food supplied is insufficient, the animal fives on its own tissues, thus setting free salts which appear in the urine. The loss of salts to the body will therefore be in direct proportion to the degree in which the animal is living at the expense of its own tissues. SECTION VI THE NORMAL DIET OF MAN The adult man has to take a certain quantity of food every day in order to furnish an amount of energy equal to that lost from the body as heat and mechanical work, and to replace the waste of tissue. This income is repre- sented, not by the total food taken into the ahmentary canal, but by the pro- portion of the food which is absorbed from the canal. This will vary with the digestibihty and nature of the diet, and in any experiments instituted to determine the metabohsm of man the first question that must be decided is as to the proportion of food-stuSs actually utihsed. Food which is not absorbed will be excreted from the body in the faeces. The degree of utihsa- tion of food-stuffs will therefore be given by an analysis of the fseces passed daily on any given diet. In order to be certain that the faeces passed during a given time correspond to and are derived from the food taken in at the same time, it is usual to give at the beginning and end of the experiment a capsule of lampblack, so as to colour the faeces and dehmit those formed during the period of the experiment. The faeces during starvation contain a certain proportion of nitrogen, carbohydrates, and fats, and in judging of the degree of utihsation of any given food-stuff the amounts of these substances excreted by the alimentary canal during starvation must be deducted from the total amount found in the faeces. The excretion of nitrogen by the intestines varies between 0-54 and 1 -36 grm. per day. If we find an amount of nitrogen in the faeces not exceeding these figures, we are justified in concluding that the utihsation of the nitro- genous constituents of the food is practically complete. This is the case when the man receives as food the animal proteins, such as meat, eggs, or milk. In experiments carried out with these materials the amount of nitro- gen of the faeces varied between 0-14 and 1-9 grm. Only when excessive amounts of milk are given is the utihsation less complete and the nitrogen in the faeces increased above this amount. If, however, the protein be given in the form of vegetable food, the wastage of nitrogen by the faeces is much greater. It may rise as high as 48 per cent, and amount to 9 grm. per day. Nearly always it exceeds 15 per cent. This greater wastage of the nitrogen of vegetable food depends partly on the fact that certain non-protein and indigestible nitrogenous constituents of seeds and grains are reckoned as protein, partly on the fact that a considerable proportion of the protein may be enclosed in indigestible envelopes, and partly on the greater stimulant 648 THE NORMAL DIET OF MAN r,49 action of the vegetable diet on the movements of the ahmentary canal, so that the food is hurried through the intestine before the processes of digestion and absorption have had time to attain their Hmit. This last factor may interfere also with the digestion of animal protein on a mixed diet. The fats and carbohydrates of the ordinary diet of man are also utilised to a very large extent. The fteces passed on a fat-free diet always contain between 3 and 6 grm. of ethereal extract which is reckoned as fat. When the fat of the food consists largely of olein and is fluid at the temperature of the body, it is almost totally absorbed, the absorption becoming less as the melting-point of the fat rises. Ordinary carbohydrates are also very well absorbed, but here very large variations may be produced by altering the condition in which they are presented in the food. The following lable shows the relative digestibihty of the different food-stuffs in a healthy individual on a normal diet : Percentage of Food-stuff Absorbed Protein Fat Carbohydrate Ash Total energj' Average of fivej ^^.^^ g^ g^.^ ^„.^ g^.. experiments ( In judging therefore of the sufficiency of any given dietary, it is important to remember that on the average only about 90 per cent, of the total energy of the food is available for use by the body. If, for instance, the body requires an amount of energy equivalent to 3000 calories per day, it would be necessary to give food corresponding to 3333 calories per day. If, as is the case with the poorer classes, the food consists mainly of vegetable pro- ducts it may be necessary to increase still further this allowance, since on a diet such as rye bread the loss of energy in the faeces may amount to as much as 35 per cent, of the total energy of the food. The quantity of food which is necessary to keep an adult man in a state of health, without loss or gain of weight, is represented by that amount which is sufficient after absorption to supply the total daily output of energy. This output will vary considerably not only from individual to individual but also with the weight and size of the man, and above all with his state of muscular activity. Thus in the case of a woman weighing 4945 kilos, in a state of hysterical sleep, the total output of energy during twenty- four hours amounted to 1228 calories, i.e. 24-8 calories per kilo body weight. Under more normal conditions, with the increased tone of muscle which is present during waking hours, the evolution of energy by the body is increased above this amount. Pettenkofer and Voit found that a resting individual had an output of 2300 calories during starvation and 2G70 calories on a normal diet. A series of experiments by Tigerstedt on individuals kept in a state of rest, but on normal diet, gave results varying between 26 and 36 calories per kilo for the twenty-four hours. We may take therefore 30 calories per kilo body weight as the average requirements of a man in a state of rest. This would correspond to 2100 calories for a man weighing 70 kilos. When muscular work is performed the energy output is at once largely increased, and with it the food requirements of the body. The attempts which have been 21* 650 PHYSIOLOGY made to arrive at some idea of the average amounts of food required by a working man have been based not so much on scientific experiment as on an analysis and comparison of the diets in general use among different classes of men, the amounts of which are determined by the instinct of the man himself, while its quahty is hmited by his daily earnings. Tigerstedt divided the different diets which have been investigated into six classes, according to the total amount of energy which each of them represents. These are as follows : Group Protein Fat Carbo- hydrate Calories gross Calories net Calories per kilo body weight I II . III . IV . V VI . 84 88 130 141 167 152 56 39 64 71 89 139 599 512 520 677 774 1062 2483 2825 3257 4020 4685 6269 2235 2538 2932 3618 4218 5642 32 36 42 52 60 81 In view of this great variation among the diets of different individuals it becomes difficult to arrive at any conclusion as to the diet which should be regarded as the average and should guide us in drawing up dietary scales for pubUc institutions. Voit, from experiments on ordinary workmen per- forming eight or nine hours' labour a day, such as a bricklayer or carpenter, has laid down the following as an average diet, namely : Protein Pat Carbohydrate 118 grm. 56 „ 500 „ This would correspond to a total calorie value of 3055, or, subtracting 10 per cent, for the food which is not utihsed in the ahmentary canal, to 2749 calories. The fat in this diet is rather low, preference being given to the carbohydrate on account of its greater cheapness. It is not advisable to reduce the fat below this limit, since the human body has a need for a certain proportion of fat, in the absence of which the utihsation of the other food-stuffs does not proceed normally. The presence of fat in the diet is especially important in the case of infants and young children, many of the disorders of nutrition prevalent among the children of the lower classes being largely determined by the absence of the proper quantity of fat from the diet. In the case of severe work this diet may not be adequate to supply the necessary quantity of energy. Thus, for soldiers, Voit's ration during manoeuvres consists of : Protein Fat Carbohydrate 135 gim. 80 „ 500 „ THE NORMAL DIET OF MAN with a total calorie value of 3348. His ration for war-time consists of 651 Protein Fat Carbohydrate 145 grm. 100 „ 500 „ corresponding to 3575 calories. Even this may be insufficient to supply the energy needs during a period of intense muscular activity. In one experi- ment by Atwater, in which the individual performed muscular work for a period of sixteen hours out of the twenty-four in a calorimeter, the total energy given out amounted to over 9000 calories in the course of the twenty- four hours. It is probable, however, that in all cases where such excessive calls on the energies of an individual are made, one or even two rest days would follow the day of exertion, so that the deficiency of the food on the day of exertion would be made good by an increased intake of food on the following days. Thus three days' intake of 5000 calories would yield suffi- cient energy for the output of 9000 calories on one day of exertion and of 3000 calories, the normal amount, on each of the two succeeding rest days. The relative part played by the different constituents of a diet in yielding energy to the body has been determined by many observers and especially in a long series of researches by Atwater. In the following Table are given details of the daily food in one such experiment on a man weighing 76 kilos : Food Weight per day grm. Water Protein Fat Carbo- hydrate grm. N. c. H. Heat of com- material grm. grm. grm. grm. gmi. grm. bustion calories Beef 35 22-0 11-5 1-0 _ 1-84 6-73 0-99 76 Butter . 245 27-2 2-9 208-3 — 0-47 1-54 25-28 1929 Bread 350 148-7 28-3 3-9 165-5 4-97 90-69 13-55 913 Gingersnaps 60 2-4 3-6 3-4 49-2 0-63 25-74 3-98 259 Shredded wheat . 40 2-5 41 0-6 32-2 0-72 16-70 2-54 164 Sugar 60 — — — 60-0 — 25-26 3-89 238 Cereal coffee 1200 1191-6 0-7 — 7-2 0-12 3-96 0-60 41 Whole milk 1355 1149-0 52-9 74-6 69-2 8-40 108-00 16-93 1247 Total ration per day J3345 2543-4 104-0 291-8 383-3 17-15 431-08 67-76 4867 Very few accurate experiments have been made on the daily requirements of women. Since the average weight of a woman is less than that of a man and the work performed less severe, she will require a smaller amount of food both to meet the energy expenditure of the body and to provide for the repair of her tissues. Voit, under the assumption that the body weight of woman is four-fifths that of man, and that her energy require- ments are diminished in the same proportion, has given the following as the daily requirements of a woman engaged in manual labour : 652 PHYSIOLOGY Protein . . . . .94 grm. Fat f 45 „ Carbohydrate .... 400 „ equivalent to a gross calorie value of 2444. « VITAMINES ' The statement that a diet composed of the five classes of food-stuffs, proteins, fats, carbohydrates, salts and water, is all that is necessary for the maintenance of life is not strictly true. It is found that if rats, for instance, be fed on milk or bread and milk, they can live and grow in a normal manner. If, however, the proteins, fats and carbohydrates be extracted from the milk, purified, and then mixed in the same proportion as they were in the original milk, the animals, if young, cease to grow, and after some days or weeks die. The ordinary nourishing effect may be restored by adding to the artificial mixture an alcoholic extract of milk, or of yeast cells, or of many other living tissues. It was thought by Stepp, who first called attention to this fact, that certain lipoids must be an essential ingredient of the food. Further light has been thrown on the question by the researches of Hopkins and of Funk. The latter has investigated especially the condition known as beri-beri, which is a state of mal-nutrition brought on by an exclusive diet of polished rice, i.e. a rice which has been deprived of its outer coating. This condition does not occur if to the diet be added the dust rubbed off the rice in the process of polishing. A similar con- dition can be produced in fowls and is rapidly cured by addition to the diet of rice polishings or of an alcoholic extract of these polishings, of yeast cells, or of lime juice. It seems that in addition to the five classes of food-stuffs, minimal quantities of certain other substances are necessary in order that the processes of life may proceed normally. How these substances act we do not know, but we must imagine that they have a drug- like effect on some organs of the body and take the place of or give rise to some of the hormones which are essential for the orderly working of the different organs of the body. Funk has given to these substances the name of ' vitamines.' RELATIVE PROPORTIONS OF THE DIFFERENT FOOD- STUFFS IN THE DIET OF MAN Since we have exact determinations at our disposal of the total energy output of a man under various conditions, it is easy to assign a total diet to each class which shall satisfy these energy requirements. In such a diet fat and carbohydrate are mutually replaceable in proportion to their calorie value, though it seems that in most individuals for the conservation of perfect health a certain minimum amount both of fats and carbohydrates is necessary. Some observers, however, have described an increased output of carbon dioxide as the result of the ingestion both of carbohydrates and of fats, pointing to a stimulating effect on metabolism of these food-stuffs them- selves. If these results are generally applicable, we cannot regard the total energy output of a man on a given diet as affording a criterion of the amount of food necessary for him per day, since it is possible that with a smaller amount of food the stimulating effect on metabolism might be wanting and tnat the functions of the body might be normally performed with a greater economy of material. The stimulating effect of fats and carbohydrates on metaboUsm has not, however, been universally observed, whereas in the case of proteins every worker has noted an increased metabolism in propor- THE NORMAL DIET OF MAX or.S tion to the amount of protein in the diet. Especially is this marked in man, where the power of storing protein in the body seems to be minimal or absent in the normal adult. As we have seen, the protein taken in the food has a twofold destiny. Part of it, probably the smaller portion, is needed to be built up into the tissues, and to form living protoplasm in replacement of wear and tear. The other, the larger portion, serves, like the fats and carbo- hydrates, for meeting the energy requirements of the body. The amino- acids produced by the disintegration of the proteins in the alimentary canal are rapidly absorbed and apparently undergo deamination in the body. The nitrogen so split off is at once excreted in the urine, while the non- nitrogenous moiety is rapidly oxidised to carbon dioxide and water. How- ever much protein is ingested, so long as the digestive powers of the animal are not overtaxed, all that is not required for replacing tissue waste undergoes this fate, and we can therefore attain nitrogenous equilibrium on a diet containing 50 grm. or 150 grni. of protein daily. The amount of protein taken in the food, and digested and oxidised in the body by any given individual, affords no clue to the amount \^'hich is absolutely necessary for the maintenance of life and for the normal discharge of the bodily functions. It is therefore not surprising to find the greatest possible divergences between various classes of men i2i the quantity of protein taken in their daily food. An average individual in affluent circumstances, eating three meat meals a day, probably takes in from 100 to 160 grm. of protein daily, corresponding to a nitrogen content of 16 to 25 grm. On the other hand, there is no doubt that an individual can lead a perfectly normal existence with a nitrogen intake as low as 5 or 6 grm. a day. It is not possible to explain these differ- ences as determined by individual idiosyncrasies, nor is the appetite of the individual to be taken as a safe guide to the relative composition of the foods. The average diet of any race has been determined up to the present not so much by the physiological requirements of the body as by the nature of the food available. Hence, whereas the races living in tropical climates are mainly herbivorous or frugivorous, the northerners, who have developed their intellectual and bodily superiority in their harder struggle for food amidst more inclement surroundings, have been perforce obliged to satisfv their energy requirements at the expense of animal food. In these days when the products of all climes are at the disposal of civilised man, his food-supply is no longer dependent on the country in which he lives, and it is possible to regulate the composition of his food according to the results of physiological experience. Since the proteins represent the most costly constituents of the food, it becomes impoitant to inquire how much of this class of food-stuffs is essential to the maintenance of health and whether any advantage is given by taking proteins in excess of the physiological mininnim. The fact that those nations which hold the highest place in the world are mainly flesh- eaters cannot be regarded as any proof of the advantage of a flesh diet over a diet poorer in piotein. It is the hard struggle for existence which in the northern races has eliminated the weakling and resulted in the production of a superior race. The fact that he is a flesh-eater can also be ascribed to the 654 PHYSIOLOGY exigencies of plimate and does not necessarily prove that a large flesh diet is responsible for his greater efficiency. Of late years a number of careful experiments have been made to deter- mine the minimum amount of protein which must be present in the daily ration of man. I have mentioned above two experiments in which nitrogen- ous equihbrium was obtained with much smaller amounts of protein than those given in the normal diets. In these experiments, in one of which the man received 43-5 grm. of protein daily, and in the other only 33 grm. of pro- tein, it was found necessary to give at the same time amounts of carbo- hydrates and fats far exceeding those in the normal diet, so that whereas, e.g. the normal individual takes in between 32 and 35 calories per kilo, the man on 43-5 grm. of protein needed 47-5 calories per kilo, and the one on 33 grm. of protein needed the huge amount of 78-5 calories per kilo. Other experimenters have, however, succeeded in maintaining perfect health and nitrogenous equihbrium for a considerable time on a diet con- taining a much smaller amount of protein than has been generally considered necessary without adding to the ration abnormal quantities of fats or carbo- hydrates. Thus Siven, in an experiment on himself, found that he could maintain nitrogenous equilibrium for thirty-two days on a diet containing only 6-26 grm. of nitrogen. The total heat- value of the food per day was only 2444 calories. Folin, in individuals with an insufficient amount of protein containing only 2-1 to 2-4 grm. of nitrogen, found that the nitrogen output per diem was only 3 to 4 grm. It would seem therefore that a healthy adult man, having a sufficient intake of non-nitrogenous food, need not metabohse more protein than suffices to yield 3 to 4 grm. of nitrogen per day, i.e. between 25 to 35 grm. of protein. Other observations have been made on vegetarians, showing that individuals can maintain perfect health on a diet containing only about 34 grm. of protein a day, and with a total calorie value of 1400 to 2000. A series of experiments were conducted by Chittenden with a view to determining how far such a diet is suitable to the average individual, and especially whether it can be continued for long periods of time without interfering with the well-being of the subject of the experiment. The general results of these experiments show that the physiological needs of the body can be met by greatly reduced protein intake with the estabhshment of continued nitrogenous equilibrium on a far smaller amount of protein food than is contained in the ordinary dietary tables, and that on this diet the individual in some cases, far from suffering in health, has his physical and mental efficiency increased. The experiments were made on various classes of men : instructors and students in the university, soldiers, and athletes. In the case of Chittenden himself the average daily diet contained about 6 gi-m. of nitrogen and had a heat- value of about 1600 calories. In the case of another individual the intake of nitrogen per day was 9-5 grm. and the heat- value of the food 2500 calories. It is thus possible to reduce the total energy of the food from about 3300 calories to about 2500 calories. Of the protein taken in by a normal individual therefore a certain amount which is not needed by the organism is at once broken up and serves THE NORMAL DIET OF MAN 655 simply to increase the total metabolism of the body without serving any useful physiological purpose other than heat production* Many ailments, especially of middle age, have been ascribed to an excess of protein in the food. It has been thought that the kidneys and other organs may suffer from the strain of eliminating excess of nitrogenous waste products. But the energy metabolism of proteins results almost entirely in the formation of urea — an innocuous substance which can have little harmful effect on the kidneys, even if we assume (an assumption hardly justifiable) that these organs (unlike other organs of the body) suffer as a result of their normal functional activity. There is no doubt that many disorders of middle life may be put down to over-feeding and lack of muscular exercise, but there is just as much reason to ascribe these evils to the carbohydrates as to the proteins of the diet. It is indeed possible that an ahnost exclusive protein diet might be more suitable than carbohydrates for a sedentary life, where the normal stimulus to oxidation of the food-stuffs, viz. muscular exercise, is absent, and that the ' stimulant ' effect of proteins on metabolism might have a real value to the organism. The hmitation of protein diet to a minimum is only justifiable in adult healthy men. Where from any cause there has been a loss or destruction of nitrogenous tissue, as after infective diseases, the body possesses the power of storing up nitrogen in the form of flesh or muscular tissue, and it is important under such circumstances to give in the food an excess of protein which can be used for this purpose. Moreover, when a rapid growth of muscle is going on, as during training, an excess of protein in the food is also desirable, though nothing is gained in pushing this administration of protein to an inordinate extent. The fact that in many cases greater economy and efficiency of nutrition are attained by diminishing the protein of the diet affords no argument for accepting or rejecting any given class of foods. Thus the normal requirements of the individual can be obtained by the administration of a diet containing meat, eggs, vegetables, and cereals, or by a diet derived entirely from the vegetable kingdom. A purely vegetable diet is rendered possible in civilised countries by the ease with which the products fi'om warmer chmates can be obtained. A diet composed only of the products of temperate chmates would tend to be deficient in fats and oils. In such climates it is therefore necessary to import the oily fruits grown in warmer lands, or to supplement the diet with such animal food as milk or butter or eggs. In drawing up a purely vegetarian diet it is important to remember that its constituents, especially its protein, are digested with greater difficulty than the corre- sponding ingredients in animal food. A larger quantity therefore has to be given in a vegetable diet in order to allow for the greater loss by the fa}ces. Any general reform of diet which may be indicated by recent physiological experiments would seem to lie rather in the direction of hmitation of the * But heat production is a very important function of the food, and on the Chittenden diet tends to be deficient ; so that individuals on tliis regime 'feel the cold ' more than they did when on an ordinary diet. 656 PHYSIOLOGY quantity of difEerent articles of food, perhaps especially of those rich in protein, than in a limitation to foods obtained from the animal or vegetable kingdom. In certain individuals the increased bulk of indigestible residue which is afforded on a vegetarian diet presents a distinct advantage, since it serves to promote peristalsis and the normal evacuation of the large bowel. There is no scientific evidence that for the ordinary person any advantage is to be gained by adherence to a strictly vegetarian diet. THE DIET OF THE GROWING ANIMAL Since the young animal is smaller than the adult, it presents a greater surface in proportion to its bulk, and therefore will need more energy per kilo body weight than is the case with the adult. The processes of growth are attended moreover with an active metabolism, so that the metabolism is more energetic in the young animal than in the adult, even if we take into account the relative surface in the two cases. In the following Table is given a number of observations showing the output of carbon dioxide per square metre body surface at different ages in boys and men : Effeci OF Age on Met \BOLisM (Man) Ag e 13t d/ weight kUos CO., per kilo body weight per hour CO J per square metre per hour lO.V . 30 Ml 28-2 14 45 1 -DO 27-6 17 56 0-81 24-2 23 05 0-58 18-6 45 77 0-48 16-3 58 85 0-41 14-2 Between the ages of fourteen and nineteen years in boys the carbon dioxide output is absolutely greater than at any other age. It is during these years that the growth of the boy is most marked. A boy between nine and thirteen years old requires just as much food as a full-grown man, and between the ages of fourteen and nineteen he will require more. This great increase between the fourteenth and nineteenth years is not noticed in females. There is a gradual absolute increase in girls up to the eleventh year. At this age and henceforward the total carbon dioxide output and therefore the total food requirements remain constant, being about equal to that of a woman of thirty. The total energy requirements of the growing animal are not, however, the only factor which we have to consider. The protein of the food has not only to replace the wear and tear of tissue, but also to provide material for the growth of new tissues. The protein needs there- fore of the growing animal are relatively greater than those of the adult, and absolutely greater during the period of most rapid growth, namely, in boys between the ages of fourteen and nineteen. It is a popular belief that in a working-class family the worker or wage-earner needs a greater protein supply than the rest of the family. This is a mistake. The adult worker can obtain his energy equally well from carbohydrates and fats, whereas THE NORMAL DIET OF MAX 657 an excess of protein is absolutely necessary in the case of the children to provide the material for their proper development and growth. The relation between rate of growth and protein content of food is well illustrated by a comparison of the composition of the milk in different animals. In the following Table (Proscher) it will be seen that the more rapidly an animal grows the greater is the protein content of the milk with which it is supplied : Time iii which the body weight of the uew-boiii animal was doubled. Days 100 parts of Milk contain Protein Ash Lime rilUSljIldlic acid Man . 180 1-6 0-2 0-328 0-473 Horse . 60 2-0 0-4 1-240 1-310 Cow . 47 3-5 0-7 1-600 1-970 Goat . 19 4-3 0-8 2-100 3-220 Pig . 18 5-9 — — — Sheep . 10 6-5 0-9 2-720 4-120 Dog . 8 71 1-3 4-530 4-930 Cat . 7 9-5 — — — We should expect then that the milk which is the sole food of the growing infant should contain a relatively greater proportion of protein than is neces- sary in the case of the adult. In an experiment by E. Feei-, ([uoted by Bunge, a child weighing 8226 grm. at the thirtieth week took 951 grm. of milk. Human milk contains : Protein ' . . 1 •() per cent. Fat . . 3-4 „ Sugar . • • (M „ Ash . • 0-2 „ The child was therefore receiving daily : Protein • • 15-2 grm. Fat 32-3 „ Sugar . 58-0 „ Ash . . 1-9 „ According to the same p ro )ortions a man of 70 kilos would take in : Protein 129 grm. Fat 275 ., Sugar . . 494 ,. A.«h . . 16 „ It is interesting to note that the protein of this diet differs but little from that in the diets ordinarily accepted as standard, but there is a large excess in the fat, and in the total calori value, as would be expected from the more rapid metabolism and the velntively larger body surface of the young child. CHAPTER X THE PHYSIOLOGY OF DIGESTION CHANGES UNDERGONE BY THE FOOD-STUFFS IN THE ALIMENTARY CANAL The use of the process of digestion is to alter the food-stuffs so as to fit them for absorption into the blood, by means of which they may be carried to all parts of the body. In most cases the food-stuffs cannot be utilised in their original form by the living cells. When we nourish ourselves at the expense of an animal or plant, we are taking in not only the current coin of the organism which is being used for the supply of energy to its vital processes, but also, and to a much larger extent, the framework forming ■ the machinery of the organism as well as its stores of carbohydrate or fat. The food-stuffs as we ingest them are in the most inactive form possible. Practically all are colloidal, neutral, and tasteless, and present no tendency to unite with oxygen or, indeed, to undergo any change whatsoever, apart from the interference of living organisms such as bacteria. In a starving animal the stores of carbohydrate and fat and the protein structure of the inactive living cells have to be converted into a soluble form — transformed, so to speak, into currency — ^before they can be utihsed by other Hving cells, such as those of the heart, for the discharge of their normal functions and the maintenance of the hfe of the animal. In the same way, when we take these colloidal or insoluble substances into our alimentary canal, they have to be rendered soluble or diffusible, in order to allow of their easy transference across the wall of the gut into the blood and their transport to the tissue-cells. The cells of the body cannot deal with all kinds of carbohydrate. Most animal cells will starve when presented with starch, dextrin, or any of the disaccharides, such as maltose, lactose, or cane sugar. It is necessary there- fore that all the carbohydrates shall be reduced in the aUmentary canal or in its walls to the form of monosaccharides. As regards proteins, the processes of digestion have a different significance according as we are deahng with their value as givers of energy or their value as builders up of the living protoplasm. If the proteins of the food are to be oxidised and utilised as a source of energy, they must be rendered soluble so as to enable them to be absorbed and carried to those parts of the body where they may undergo deamination and complete oxidation. If they are to be built up as integral parts of living cells, to take the place of molecules which have been destroyed in the 668 THE PHYSIOLOGY OF DIGESTION 659 wear and tear of functional activity, the change must be almost equally profound. The proteins of the cells from different parts of the body have different molecular constitutions. Not only do they differ among themselves, but they differ very largely from many of the proteins which may be taken in with the food. A child is able to obtain material for the growth of his brain- cells, his muscle-cells, or his hver-cells, from a diet containing protein in the form of caseinogen, or of vegetable gluten, or of meat fibrin. A reference to the Table on p. 91 will show the striking difference in composition between the various proteins of the food and the proteins which have to be formed from them in the living tissues. It is evident that to form serum albumin, for instance, out of wheat ghadin, an entire reconstruction is necessary. This can only be accomplished by taking the protein molecule to bits, and by selecting certain of its constituent parts and building these up in the proper proportions to form a new protein molecule. For the purposes of nutrition therefore the changes in the protein molecule must be greater the more variation there is in the composition of the protein of the food from the composition of the proteins of the tissues. In primitive ahmentary canals every cell lining the canal may be en- dowed with amoeboid properties and capable of devouring the food particles, the subsequent changes in the latter to fit them for their journey through the rest of the body being performed in the body of the cell itself. In all the higher animals, however, including ourselves, the greater part of the prepara- tion of the food is accomphshed extracellularly in the lumen of the ahmentary canal, and the changes are effected by means of special digestive juices, which are formed by the activity of masses of cells produced as outgrowths from the wall of the canal. The digestive juices attack the food-stuffs by means of ferments, and in every case the action of these ferments is hydro- lytic, the food-stuffs taking up one or more molecules of water and under- going dissociation into simpler molecules. Since each class of food-stuff requires a different ferment, a great variety of ferments is concerned in the processes of digestion. As the end-result of digestion the many kinds of food taken by man are reduced to a fairly small number of simpler bodies. These end-products are : (1) Carbohydrates. The monosaccharides : glucose, fructose or levulose, and galactose. (2) Fats. Fatty acids, or (in alkaline medium) soaps, and glycerin. (3) Proteins. Here we have a great variety of mono- and diamino-acids, which may be enumerated as follows : MONO-AMINO-ACrDS Glycine (aminoacetic acid) .... Alanine (amino propionic acid) Serine or oxyalanine (oxyaminopropionic acid) . . I Monobasic acids Aminovalerianic acid j of fatty series Leucine (aminoisobutylacetic acid) Isoleucine (aminocapi'oic acid) 660 PHYSIOLOGY Mono -amino -acids — continued Aspartic acid .... Glutamic acid .... Phenylalanine Tyrosine (oxy phenylalanine) Proline (pyrrolidine carboxylie acid) Oxj'proline (oxypyrrolidine carboxylie acid) Tryptophane (indolaminopropionic acid) DiAMINO-ACIDS AND THEIR COMPOUNDS Lysine (diaminoca])roic acid) Arginine (guanidinaminovalerianic acid) Histidine (iminazolalanine) .... ' Diaminotrioxydodecoic acid ' . . . Cystine (derived from aminothiopropionic acid) Diabasic acids Benzsne (aromatic) derivatives Heterocyclic compound^ I The j ' hexone bases ' derived from a 12-carbon acid f Sulphur-containing I bodv Those constituents of the food which undergo no oxidation in the body, such as the' water and salts, are practically unchanged in the alimentary canal, and are absorbed in their original form into the blood. SECTION I DIGESTION IN THE MOUTH It is a common experience that when food is taken into the mouth there is a flow of a liquid, ' saliva,' into this cavity. Saliva is the product of secretion of three pairs of large salivary glands situated in the neighbourhood of the mouth and pouring their secretions into this cavity hy means of ducts. It is possible to collect the fluid secreted by each of these glands separately, and it is found that the saliva varies in properties according to the gland from which it is derived. In addition to these large glands the whole mucous membrane of the mouth is beset with small glands. The saliva is in most cases a mixture of the secretions of all three pairs of salivary glands as well as of the small glands of the mucous membrane. When collected it forms a colourless cloudy liquid, slimy in character. The cloudiness is due to the presence of a number of formed elements consisting of desquamated epithelial cells, disintegrating leucocytes, and gland-cells, as well as coagu- lated clumps of mucin. Its reaction is in healthy individuals slightly alkaline. Its specific gravity varies between 1002 to 1008 coagulable proteins, nuicin, and in some cases a diastatic ferment, ptyalin, and traces of jiotassium sulphocyanate. Its average composition is as follows : Fig. a. .32 J. Dissection to display the salivary glands, sub-lingual gland ; h, sub- maxillary gland ; c, parotid gland; d, common opening of ducts of sub-maxillary and sub- lingual glands ; i, opening of duct of parotid gland. Its chief constituents are 100 parts mixed yaliva contain : Total solids 0-5 to 1-0 Inorganic solids ........ O-l to 0-6 Organic solids (mucin, serum albumin, scrum globulin) . 0-1 to 0-4 Potassium sulphocj'^anate ...... 0-00 to 0-016 Freezing-point (A) = - 0-07 to - 0-34 Potassium suli)hocvanate is an almost constant constituent of human saliva, though it is often absent in that of other animals, such as tlie dog. It is generally present to the extent of -01 per cent, so that on the addition of a drop of ferric chloride to 661 662 PHYSIOLOGY saliva a definite red colour is obtained. So far as we know it is formed in the body when- ever cyanides or organic nitriles in small quantities make their appearance in the circulating fluid, either as the result of administration or perhaps as by-products in the normal processes of metabolism. The conversion of the poisonous cyanides into the almost innocuous sulphocyanates seems to be a means by which the organism protects itself against the poisonous effects of the former. The channels of excretion of the sulphocyanate are by the salivary glands, the kidneys, and possibly by the gastric juice. THE USES OF SALIVA The main function of saliva is to moisten the food and so facilitate its mastication and deglutition. The presence of the mucin is of special value for the latter process since it renders the mass of food sHppery. In animals, such as dogs, where the saliva is devoid of any digestive ferment, this must represent its sole function. In man and some of the herbivora the saliva exerts a well-marked digestive effect on one of the food-stuffs, namely, starch. If a warm solution of starch be taken into the mouth, kept there for one minute, and then expelled into a test-tube, the starch will be found to have entirely disappeared, its place being taken by a reducing sugar. The stages of the action of saliva on boiled starch can be followed more easily if its action is retarded by keeping the mixture cool, or at a temperature not above 25° C. The first change is a conversion of the opalescent gelatinising starch solution into a clear solution which no longer sets on cooling, but still gives a blue colour with iodine. The fluid contains what is known as soluble starch. The soluble starch then undergoes hydrolytic dissociation into a dextrin, which gives a red colour with the iodine and is therefore known as erythro- dextrin, together with maltose. The erythrodextrin is then hydrolysed into an achroodextrin (giving no colour with iodine) and maltose, and the achroo- dextrin is still further broken up into dextrin and maltose. The conversion of starch into maltose is never complete, though if the maltose be removed by dialysis as it is formed, it was found by Lee to be possible to convert as much as 95 per cent, of the starch into reducing sugar. The stages in the conversion are represented in the following Table : Starch I soluble starch erythro-) dextrines Maltose (achroo- ) dextrines maltose The process by which the huge starch molecule is converted into dextrins and maltose is a very compHcated one, and a number of intermediate com- pounds of dextrins and maltose can exist between those whose presence is revealed by their varying reaction to iodine. Ptyalin is most active in a neutral medium, so that the addition of minute traces of acid to the saliva increases its diastatic power. In the presence of free mineral acid ptyaUn is rapidly destroyed, -003 per cent. DIGESTION IN THE MOUTH 663 hydrochloric being sufficient for this purpose. It acts most rapidly at the body temperature. At 0° C. its action is stiU just perceptible./ If heated to 60° G. it is destroyed. We have seen that boiled starch solution is changed by saliva when kept only a few seconds in the cavity of the mouth. When the starch is in the solid condition, as in biscuits and most farinaceous foods, its stay in the mouth during the normal process of mastication is not long enough to allow of any considerable hydrolysis occurring. When a meal is taken the food which is swallowed forms a mass lying in the fundus of the stomach. This mass is penetrated only with difficulty by the acid gastric juice secreted by the mucous membrane of the stomach within five minutes of the taking of food. Even half an hour after a meal the interior of the mass of food in the stomach may be still found to be neutral or slightly alkahne. The food therefore, thoroughly moistened by and mixed with saliva, remains in the stomach for thirty to forty minutes before the salivary ferment is destroyed by the penetration of the acid gastric juice. During this time the ptyahn continues to exert its efEect, so that we may say that the chief part of the sahvary digestion occurs actually in the stomach, and results in an almost complete alteration of the starch into dextrins and maltose. Unboiled starch is attacked with extreme slowness by the diastatic ferments either of the saliva or the pancreatic juice, so that, if taken by man, large quantities are unutilised and reappear in the fseces. Thirty to forty minutes after a meal the food becomes thoroughly soaked with the acid gastric juice, and saUvary digestion gives place to gastric digestion. THE SECRETION OF SALIVA The mucous membrane of the mouth, especially on the under surface of the tongue, presents a number of small glands which contribute by their secretions to the moistening of the mouth. The greater part of the saliva is, however, formed in man by three pairs of glands, viz. the sub-lingual and the sub-maxillary glands, situated in the floor of the mouth below the jaw, and the parotid gland, lying in the cheek over the ramus of the inferior maxilla. The ari'angement of these gland^^, especially of those in the floor of the mouth, varies somewhat in different animals. In the dog and cat the sub-lingual gland is wanting, its place being taken by a gland situated somewhat further back and known as the retro-lingual gland. In the pig both retro-lingual and sub-lingual glands are present in addition to the sub-maxillary, and one may sometimes find traces of the retro- lingual gland in man. Many animals, e.g. the dog. also possess a gland situated in the orbit, which pours its secretion into the mouth — the orbital gland. These glands can be divided, according to their structure and the nature of the secretion which they form, into several classes. Among the salivary glands of the mucous membrane we may distinguish two types, the mucous gland and the serous gland. In specimens hardened and stained by the orchnary methods the mucous gland is distingiiishcd by the fact that its short duct opens into wide alveoli, the lining cells of which are distended with mucin and therefore present a clear imstaincd space in the section. In the other type, the serous gland, the duct lined with colunmar cells branches into a series of acini which present a well-marked lumen and are lined \nt\\ small granular cells with a very distinct and well -staining nucleus. The same general distinction can be made out in the large salivary glands. The parotid gland in man and in 664 PHYSIOLOGY all the higher mammalia is a typical serous gland, though here and there A, mucous cell may be occasionally seen. The orbital gland of the dog represents practically one of the mucous glands of the general mouth cavity on a large scale. The sub-lingual and sub-maxillary glands in man represent a third type. Most of the alveoli are mucous in character. At the ends of the alveoli are seen crescent-shaped cells between the mucin-distended cells and the basement membrane. These are known as the demilune cells, or the Crescent cells of Gianuzzi. In some cases these mucous alveoli with demilunes may be found along.^ide of typical serous alveoli. B V Fig. 322. a, serous gland ; b, pure mucous gland from mouth. a, ducts ; /, fat-cells. (KoLLIKER. Thus in man the sub-maxillary gland is usually a mixed gland, the serous alveoli predominating. The sub-lingual gland is also mixed, but with a predominance of the mucous alveoli. In the monkey the sub-maxillary gland is almost entirely serous. In the dog the sub-maxillary gland is a pure mucous gland with demilunes, while the retro-lingual and sub-lingual gland when present are of the mixed type. In the rabbit the sub-maxillary gland is serous, while the sub-lingual gland is mucous. In the cat the sub-maxillary is mucous, the retro-lingual is mixed, and the sub-lingual, when present, is mixed, with predominance of the mucous type. The normal behavioar of the sahvary glands during digestion is best studied by aid of a method used long ago by de Graaf and reintroduced with considerable elaboration of late years by Pawlow. It is possible without any disturbance of the animal's nutrition to transplant the papilla on which the duct opens to the outside so that the saliva from any particular gland shall flow externally instead of into the cavity of the mouth. By attaching a small funnel to the fistulous opening, it is possible to collect the pure saliva unmixed with the secretion of any other of the glands. By this method it has been found that as soon as food is introduced into the mouth there is a secretion of saliva, the relative extent to which different glands are involved varying according to the nature of the stimulation. Thiis with meat there is only a small amount of secretion, which is derived chiefly from the sub-maxillary and sub-lingual glands, and is rich in organic constituents. When dry material, such as dry powdered meat, is introduced the flow of juice is more copious and more watery. The same effect may be produced in the dog by psychic excitation. Thus salivation may be induced by showing food to the dog, or even by the suggestion that dry powder is to be introduced into the mouth. A comparison of the juices ob- tained from different glands shows that the serous and mucous glands differ, as might be expected, in the nature of their secretion. A serous gland, such as the parotid, gives a thin watery secretion almost free from mucin, but containing small traces of coagulable protein. The mucous gland delivers DIGESTION IN THE MOUTH 665 a secretion which is viscid from the presence of mucin, and contains also a small trace of coagulable protein. Both in parotid and mucous saliva the percentage of salts is very low, so that the freezing-point of these fluids is considerably higher than that of the blood-plasma. In the dog the saliva is free from any ferments. In man ptyalin — the starch-sphtting ferment — is found in the saliva from both kinds of gland, though it predominates in that obtained from the parotid gland. The total amount of saliva which may be obtained varies, of course, in different animals. Each gland may, however, in the course of the day give an amount of juice far exceeding, e.g. ten or twelve times, its own weight. In man it is reckoned that over one litre of Fig. 323. Diagram of nerve-supply to sub-maxillary gland, Sm.G, sub-maxillary gland ; N.L, lingual nerve ; Ch.T, chorda tympani ; Sm.Gl, sub-maxillary ganglion ; Sm.D, \\'hartou"s duct ; V.J. jugular vein ; C.A, carotid artery ; G.C.S, superior cervical ganglion ; N.S, sympathetic fibres ramifying on facial artery. (After Foster.) saliva may be formed every twenty- four hours, and in the herbivora, such as the horse, the total diurnal production must amount to many htres ; 500 grammes of hay alone may evoke the secretion of a litre of sahva. The intimate dependence of the secretion of sahva on the events occurring in the mouth shows that the activity of the salivary glands must be excited by reflex means. The afferent nerves in this reflex are those that supply the mucous membrane of the mouth, i.e. the fifth nerve and the glossopharyngeal. The efferent channels of the reflex were discovered by Ludw^g. Each one of the large salivary glands receives nerve fibres from two sources, viz. from the cerebro-spinal and from the sympathetic system. It is probable that the cerebrospinal supply is derived always from one part of the cerebral axis, namely, from the filaments which make up the nervus intermedins. From this point they diverge in their course to the glands. The fibres to the sub-maxillary, the sub-lingual, and the retro-lingual glands pass into the facial nerve, and then from this nerve along the chorda tympani to the lingual division of the fifth nerve. The lingual nerve passes below the duct, and just before it crosses the two ducts of the sub-maxillary and retro- or sub- lingual glands it gives off a small branch backwards, namely, the chorda tympani, which runs along the sub-maxillary duct to be distributed to the glands, and in its course gives olf fibres also to the retro-lingual (Fig. 323). 666 PHYSIOLOGY The fibres are apparently finally distributed to the secreting alveoh, where they end freely on the secreting cells just below the basement mem- brane. They do not, however, take an uninterrupted course. By means of the nicotine method Langley has shown that all the fibres to the sub-Hngual and the sub-maxillary glands end somewhere near the glands in connection with ganghon-cells. From the ganghon-cells fresh relays of fibres are given off which pass to the gland-cells themselves. The fibres going to the sub-maxillary gland are connected with scattered cells lying in the substance of the gland itself ; in the cat those passing to the retro-lingual gland are connected for the most part with the ganglion- cells which make up the Mylohyoid ngual . N Hyog"lossus Geniohyoid Fig. 324, Diagram of the arrangement of the nerve-supply to the sub-maxillary gland, as exposed in an actual experiment. Duct,Wh, Wharton's duct (of sub-maxiUary) ; Duct.R.L, retro-lingual duct ; Ch.Ty, chorda tympani nerves. (Alcock and Ellison.) so-called ' sub-maxillary ganghon.' The fibres to the sub-hngual gland in man probably take a similar course. The fibres to the parotid gland pass along the glossopharyngeal nerve and its tympanic branch (nerve of Jacobson) to the tympanic plexus. Hence the fibres are carried by the small superficial petrosal nerve to the otic ganglion, and thence by the auriculo-temporal, which is a branch of the third division of the fifth nerve. The sympathetic supply to all these glands is contained in fine filaments on the walls of the arteries with which they are supphed. The fibres are derived ultimately from the spinal cord, whence they issue by the upper three anterior dorsal nerve-roots. They pass into the stellate ganglion, round the ansa Vieussenii to the inferior, and so to the superior cervical ganglion of the sympathetic. Here the fibres end around the cells of the ganghon, and a fresh relay of fibres, chiefly non-medullated, arise from the cells and travel on the walls of the branches of the external carotid artery to their destination. The efliect of stimulating the peripheral ends of the cerebrospinal nerves going to the glands presents a general resemblance, whichever be the gland involved. Within a period of half to two seconds after the stimulation has been applied, a secretion of saliva is produced, presenting similar characters DIGESTION IN THE MOUTH 667 to that which would be obtained from the gland under normal conditions if it were provided with a permanent fistula. The concentration of the sahva as well as its rate of secretion depends on the strength of the stimulus. The following Table (Heidenhain) shows the effect on the amount and composition of sub- maxillary sahva obtained by weak and strong stimulation of the chorda tympani nerve : strength of stimulus Quantity in one minute Per cent, of organic solids Per cent, of salts Weak . Strong . Weak . 0-17 0-72 0-17 0-84 2-OG 1-67 0-20 046 0-26 With the strong stimulus the amount of saliva was increased over four- fold, while the percentage of organic substances in the sahva was raised from 0-84 to 2-06 per cent. There was at the same time an increase in the per- centage of salts. If the excitation be continued for a considerable time, there is a gradual rise in the percentage of inorganic salts and a fall in the percentage of organic matter. The cranial nerves going to these glands have another important effect, namely, vaso-dilatation. It was shown by Claude Bernard that if the outflow from the vein of the sub-maxillary gland were measured, on exciting the chorda tympani the flow might increase four to eight times, and indeed to such an extent that the blood passing through the gland did not stay there long enough to lose its oxygen. Moreover the dilatation of the arterioles removes the normal resistance which serves to damp and obliterate the pulse between the arteries and the veins. As the result of exciting the chorda therefore the blood coming from the vein may show distinct pulsation, and may have a brilhant scarlet hue just as if it were derived from an artery. \ The same dilatation has been observed to attend excitation of the cranial ' supply to the parotid gland. The effects of exciting the sympathetic nerve-supply differ according to the gland and the animal which is the subject of experiment. In the dog excitation of the cervical sympathetic causes the secretion of a few drops of thick viscid saliva from the sub-maxillary gland. In this animal no secretion is obtained at all from the parotid gland on exciting the sympathetic, but the influence of the excitation is sho^^■n by the occurrence of histological changes in the gland-cells. In the cat the sub-maxillary sahva obtained on sympathetic excitation may be as copious as and even more watery than the saliva obtained from the sub-maxillary on stimulation of the chorda tympani. We shall have later on to discuss how far these results are to be ascribed to a fundamental difference in the point of attack of impulses carried to the secreting cells by the two sets of nerve fibres, and how far to the varying effects of the cranial and sympathetic nerves respectively on the blood-vessels. It must be remembered that the sympathetic nerve Duct. 668 PHYSIOLOGY carries the vaso-constrictor fibres to most or all of the vessels of the head and neck, and therefore in most cases we should expect, and we find, that stimu- lation of this nerve causes vascular constriction in the gland affected. Before attempting to decide this point we must study in somewhat greater detail the changes which occur in the gland concomitantly with the secretion. CHANGES IN THE GLAND ACCOMPANYING SECRETION The fact that a sub-maxillary gland of the dog under favourable condi- tions will secrete its own weight of sahva in five minutes, and will continue to secrete for many hours afterwards, shows that there must be a continual renewal of the fluid which is turned out in the secretion. The source of this fluid must be the blood which is circu- lating through the gland. If we refer to the diagrammatic representation of the elements which make up a secreting lobule ' and which may be involved in the act of secretion (Fig. 325), we see that between the lumen of the duct and the blood, which must be regarded as the source of the fluid, the following layers of cells intervene : (1) the endothelium of the blood capillaries ; (2) the basement mem- brane ; (3) the epithehal cells of the gland proper. We have, in the first place, to decide to which of these elements can be ascribed the chief part in the act of secretion. The secretory activity of the sub- maxillary gland, whether evoked reflexly by the introduction of acids into the mouth or directly by the injection of pilocarpine or by stimulation of the peripheral end of the chorda tympani nerve, is always associated with a considerable dilatation of the vessels of the gland, and a consequent large increase in the blood flow through the gland, which may amount to between three and eight times the quantity passing during rest. Such an increase in the supply of blood is necessary in order to afford a source for the large quantity of fluid which is turned out in the sahva. Another effect of the dilatation will be to raise the pressure in the capillaries of the gland. We cannot, however, regard this rise of pressure in the capillaries as an essential factor in the act of secretion. If [ atropine be administered excitation of the chorda causes the same vaso- dilatation as before, though no secretion is produced. Moreover Ludwig showed that the force with which the secretion is turned out into the ducts of the gland is greater than that represented by the blood pressure. The blood pressure in the capillaries must be considerably lower even with full vascular dilatation than the pressure in the carotid artery. If two mano- meters be connected, one with the carotid artery and the other with the duct, it will be found that on stimulating the chorda tympani nerve the secretion will be excited, and the mercury will be driven up by the pressure Lymph spaces , -Secreting cells. - Blood capillary. Basement membrane Fig. 325. Diagram to show relation of the secreting cells of a gland to the blood and lymph supply. DIGESTION IN THE MOUTH 669 ''Uk.UO^^^^^^J^- T,^»n«JU*-Z9Mi. . [lofxtX , Fig. 326. Tracing of volume of sub-maxillary gland, showing effect of stimulation of the chorda after administration of 10 mg. atro- pine. The blood-pressure (lowest line) was unaltered by the stiumlation. (Bttnch.) of the secretion in the corresponding manometer until it attains a height which may be double that of the mercury in the manometer connected with the carotid artery, and therefore must be still greater than the pressure in the capillaries of the gland. This ex- periment, which is easy to repeat, showed the impossibihty of the act of secretion being in any way determined by a process of filtra- tion. We have now further evi- dence that work is done in the production of the salivary secre- tion, evidence which was not available when Ludwig first car- ried out the experiment just de- scribed. When a fluid containing salts in solution is filtered through a porous membrane the filtrate has the same content in salts as the original fluid. We can effect a separation of dissolved salts from a fluid by filtration under pressure through some membrane which is impermeable to the salts, e.g. a membrane of copper ferrocyanide — a so-called semipermeable mem- brane. Under these circumstances a very large pressure is necessary in order to cause the filtration of any fluid at all, a pressure which is equal to the osmotic pressure exerted by the substances in solu- tion. Thus if we were filtering a 1 per cent, solution of NaCl through a semipermeable mem- brane, we should have to exert a pressure of about seven atmo- spheres in order to obtain a filtrate free from sodium chloride. To obtain a filtrate containing half the amount of sodium chloride, if such were possible, would therefore need a pressure of about three and a half atmospheres. On comparing the osmotic pressures of sahva and blood respectively — and for this pmpose we can employ the depression of freez- ing-point as our index — we find that the molecular concentration of saliva, and therefore its osmotic pressure, is always very much less than that of the blood plasma, and may vary between half and three-quarters of the latter. Supposing the membrane separating the lumen of the duct from the Fio. 327. Tracing of volume of sub-maxillary gland showing decrease on excitation of chorda. (BuNcn.) 670 PHYSIOLOGY blood-vessels could be regarded as endowed with the properties of a semi- permeable membrane, we should need, in order to effect the separation, a pressure ten to twenty times as large as the arterial blood pressure. We must conclude then that work, both osmotic and mechanical, is performed in the separation of the fluid from the blood and its transference in the form of saliva to the duct. A very simple experiment will suffice to show that this work must be effected, not by the endothehal cells of the blood- vessels, but by the gland- cells themselves. The fluid passes from the blood-vessels first into the lymphatic spaces, whence it is taken up by the secretory cells. If the first act in secretion consisted in an increased exudation of fluid from the blood into the lymph spaces the first effect of exciting the chorda tympani nerve should be to cause an accumulation of fluid in the lymph spaces, some increase in the lymph flow from the gland, and the swelling of the whole gland. By placing the gland in a plethysmo- graph we can record the actual changes in its volume which ensue on ex- citation of the chorda tympani. If all secretion be prevented by the previous administration of atropine, stimulation of this nerve produces, as might be anticipated, an increased volume of the gland in consequence of the dilatation of its vessels (Fig* 326). If, however, the gland be allowed to secrete we obtain, in spite of the simultaneous increase in size of the vessels, an actual diminution in the size of the gland itself, showing that the first effect of the stimulation is on the cells of the alveoh (Fig. 327). Under the stimulus these empty themselves of the fluid they contain, replenishing their loss at the expense of the fluid in the lymph spaces. The increased passage of fluid from blood to lymph space is therefore a secondary and not a primary effect of the nerve stimulation, and the first effect on the gland is a diminution of volume and not an increase, as one would expect if the vascular endothelium were primarily responsible for the act of secretion. HISTOLOGICAL CHANGES DURING SECRETION The process of secretion is associated with marked changes in the structure of the cells composing the secretory alveoli. The changes are of the same general character whatever class of glands we investigate, though the ease with which they are to be demonstrated varies with the reactions of the variou-s glands to the hardening fluids usually employed. If a small frag- ment of a mucous gland be teased in blood serum or in 2 per cent. NaCl solution, the cells are found to be packed with a mass of coarse highly refrac- tive granules (Fig. 328). If a corresponding specimen be made from a serous gland (Fig. 329) the cells are also packed with granules, which, how- ever, are much finer in structure. On making similar specimens from glands which have been forced to secrete for six or seven hours, the individual cells are found to be much smaller and the protoplasm of the cell is absolutely or relatively increased in amount, while the granules are much fewer and are now confined almost entirely to the inner margin of the cell. Activity is thus associated certainly with a discharge of granules, and probably with some increased building up of protoplasm. We may regard the act of DIGESTION IN THE MOUTH 671 secretion as determined by the alteration of the granules and their discharge, together with water and salts, to form the specific secretion of the gland. During rest the granules are re-formed by precipitation in or modification of the protoplasm surrounding the nucleus. We have evidence that although the granules form the secretion, they represent, not the secretion itself, but a precursor of some at any rate of its constituents. Thus if acetic acid be added to the sahva obtained from the sub-maxillary gland, the mucin is pre- cipitated as threads and films. If the granules in the secreting cells also consist of mucin we should expect acetic acid to have a coagulating effect upon them. We find, on the contrary, that on allowing acetic acid to flow over a section of the fresh gland the granules at once sw^ell up and burst. Fig. 3i8. Mucous cells from a fresh subma xillary gland of a dog. (Lang- ley.) a, mucous cell examined fresh from a resting gland ; a', the same cell treated with weak alcohol ; b and b\ cells from a discharged gland before and after treatment with weak alcohol. Fig. 329. Acini of a serous salivary gland. (Langley.) A, resting condition ; b, discharged condition. We must regard these granules therefore, not as mucin, but as a precursor of mucin, mucigen. The effect of ordinary hardening reagents, such as dilute alcohol up to 70 per cent, or Miiller's fluid, is to cause these granules to swell up so that the cells become filled with a mass of mucin giving the typical hyahne appearance of ordinary sections of these glands. In the case of the serous gland the granules (Fig. 330) are apparently protein in nature. Where ptyalin is a constituent of the saliva we are probably justified in assuming that it is contained either pre-formed or more probably as a precursor in the granules. In the glands of the stomach we have evidence that the granules are not pepsin — the characteristic ferment of gastric juice — but a precursor of this substance, namely, pepsinogen. It is very customary therefore to speak of the granules in a secreting gland as zymogen granules, i.e. the precursors of zymins or ferments. It is probable that we ought to regard these granules not merely as material precursors of the constituents of the secretion, but as little machines or cell laboratories in which proceed a whole series of chemical and osmotic changes which determine the production of the fully formed secretion directly from the protoplasm and indirectlv from the ordinary constituents of the surrounding lymph. 672 PHYSIOLOGY a--'^'i From an examination of the stained specimens of glands the following stages in the production of secretion have been described : (1) In the neighbourhood of the nucleus, and probably with its active co-operation, a differentiation of the protoplasm occurs with the production in most cases of a basophile substance which in hardened specimens generally takes the appearance of filaments. Since these filaments are sometimes regarded as the working part of the protoplasm, they have been given the name of ergas- toplasm (Fig. 331, c and d). (2) By a modification of the ergastoplasm granules are formed. After their first appearance these granules undergo gradual modification, as is shown by the fact that the staining reactions of the granules near the base of the cell differ from those of the gran- ules at the free margin. (3) When the secretion is ex- cited, the fully formed granules take up water and salts in vary- ing proportions, swell up, and discharge their contents into the lumen of the alveolus as the secretion proper to the gland. Fig. 330, Sub-maxillary gland of rabbit. (ScHAFER after E. Mtjller.) The cells, all serous, are in different functional states : a, a loaded cell ; b, a discharged cell ; c, a secretory canaliculus penetrating into a cell. i D Fig. 331. Cells of pancreas, showing succes sive stages in activity, a,b,C',d. a, resting D, discharged gland. (Mathbws.) ELECTRICAL CHANGES Every locahsed chemical change in a system permeated by electro- lytes must give rise to electrical difiierences of potential. It is therefore natural that electrical changes should accompany the intense chemical activity which is associated with secretion. The interpretation of these changes is difficult, owing to the simultaneous operation of another factor which may determine electrical differences of potential, namely, the movement of fluids through porous membranes. If the hilum of the sub-maxillary gland and its outer surface be connected with a galvanometer, a resting difference of potential is nearly always found, generally in such a direction that the outer surface is positive to the hilum. The current through the gland is therefore from within out. On exciting the chorda tympani nerve a diphasic efiect is generally obtained, DIGESTION IN THE MOUTH 673 the resting difference being first increased and later on diminished. On excitation of the sympathetic nerve we generally obtain a purely negative variation of the resting difference. These results were interpreted by BayHss and Bradford as due to the co-operation of the two factors, chemical change in the gland-cells and movement of fluid through the cells. The positive variation, i.e. the current from within out, was ascribed to the movement of fluid, whereas the negative variation of the resting differencft was thought to be due to the chemical changes in the gland-cells. THE SIGNIFICANCE OF THE DOUBLE NERVE-SUPPLY TO THE GLANDS Iccording to Heidenhain, although the parotid gland gives Httle or no | secretion on stimulation of the sympathetic nerve, prolonged stimulation \ of this nerve causes histological changes in the gland even more marked than those produced by the cranial nerve. Similar histological changes were found by him in the sub-maxiUary gland. He was therefore led to put forward the hypothesis that the salivary glands are supplied by two funda- mentally different classes of fibres, namely : (1) trophic fibres, which deter- mine the chemical changes in the gland responsible for the production of the specific constituents of the secretion, and (2) secreto-motor fibres, excitation of which causes the cells to take up water and salts from the lymph and blood, and pass them in large quantities into the duct. According to this ^ view the sympathetic nerve-supply to the gland would consist almost entirely of trophic fibres, whereas secreto-motor fibres would predominate in the cranial nerve- supply. The action of atropine would appear at first sight to favour this hypothesis. In minute doses it entirely annuls the action of the chorda tympani nerve or the corresponding nerve to the parotid, while it is without efl'ect on the sympathetic nerve-supply imless given in huge doses. The preponderating effect of the sympathetic on the histological structure of the gland-cells has not been confirmed by later observers, and the varying effects of atropine on the two sets of nerve fibres may be con- ditioned by morphological rather than by functional differences between their nerve- endings. According to Langley and Carlson, the difference in the action of the chorda tympani and of the sympathetic on the submaxillary gland is due to the synchronous action of these nerves on the blood-supply to the gland, the sympathetic causing vaso-constriction, while the chorda tympani causes vaso-dilatation. In confirmation of this explanation they have shown that clamping the carotid artery during chorda stimulation diminishes the amount of saliva secreted but increases the percentage of solids in the fluid. This theory is however inadequate to explain the differences observed in the secretion of saliva reflexly aroused by introduction of substances into the mouth. In a dog with a permanent submaxillary fistula a copious flow of saliva may be caused by the introduction of -25 per cent, hydrochloric acid or of meat powder. The amount of saliva secreted under the two circumstances is approximately the same, but that evoked by the introduction of meat powder contains about t^^'^ce as much solid 22 674 PHYSIOLOGY contents as that which follows the introduction of acid into the mouth. Babkin has shown that the same differences are foimd after complete section of the sympathetic and that there is the same acceleration of the circulation through the gland whether the secretion is aroused by introduction of meat powder or of acid. It is impossible therefore to explain the difference in the composition of the saliva obtained under these two circumstances as due to differences in the blood-supply to the gland, and we must conclude either that the chorda tympani contains different kinds of fibres which are excited to varying extent according to the nature of the reflex stimulations, or that one and the same nerve fibre can convey specifically different impulses. The latter explanation would not be in accord with the generally accepted Miiller's law of specific irritability but is the explanation to which Babkin himself inchnes. At any rate it is certain that according to the nature of the reflex stimulus . either the secretion of water and salts or the secretion of organic sohds may preponderate, altogether apart from changes in the circulation simultaneously evoked. THE ENERGY INVOLVED IN THE ACT OF SECRETION The source of the energy must be sought in the processes of oxidation occurring in the cells of the gland, and Barcroft has attempted to determine the total amount of energy put out by the gland in the act of secretion by measuring its respiratory exchanges under the conditions of rest and activity. He found that the resting submaxillary gland in a small dog took up 0-25 c.c. of oxygen per minute and put out 0-17 c.c. COg, while during active secretion it absorbed 0-86 c.c. Og and gave off 0-39 c.c. COg. Assuming that the total oxygen taken up is employed in the oxidation of a food substance, such as glucose, and that the whole of the energy of the chemical changes is set free in the form of heat, we find that a resting gland weighing about six grammes produces about 1-1 calories per minute. We know, however, that a certain amount of external work is performed in the secretion of a sahva containing less salts than the original blood, and also, when there is any resistance to the flow of saliva through the duct, in raising the hydro- static pressure of the saliva in the duct to a height greater than that in the blood capillaries. Can we from all these data form a conception of the total changes occurring in the gland and involved in the formation of the secretion ? Even during rest, changes are going on in the gland-cells, changes which involve the taking up of food material and its assimilation under the influence of the nucleus, perhaps into the nucleus itself, and certainly into the undifferen- tiated cytoplasm. In this cytoplasm a further change occurs, leading to its transformation into granules. When activity is excited by the stimulation of secretory nerves, rhe primary change appears to involve simply the granules. These structures must absorb water, apparently against osmotic pressure. Those nearest the lumen swell up, become converted into spheres containing water and salts in smaller proportion than exists in the lymph bathing the cells (and presumably in the protoplasm surrounding the granules), and in this swollen form are discharged or ruptured on the periphery of the cell DIGESTION IN THE MOUTH 675 into the lumen, so giving rise to secretion. This discharge of a fluid with a smaller molecular concentration than the cell or surrounding blood plasma must lead to an increased concentration in the remaining parts of the cell. The increased concentration would naturally induce a flow of water from lymph into cell, and the consequent concentration of the lymph would in the same way cause a flow of water from blood to lymph. This pull of water by the cell from the blood is still further increased in another way. The act of secretion, involving as it does the expenditure of energy, can be carried out only at the expense of chemical changes in the cell. These chemical changes, as in all other metabolic processes of the body, will result in the formation of a number of small molecules from the great coUoid molecules of the proto- plasm. The products of metabolism, or metabolites, will therefore accumulate in the cell, pass into the lymph, and increase the concentration of the latter. The increased concentration will call forth an increased transudation of fluid, e.g. water, from the blood-vessels, and the transudation thus evoked will be greater than that necessary to provide the water of the saHva, and will therefore produce a distension of the lymphatic spaces of the gland and an increased discharge of lymph along its efferent lymphatics. As a second- ary result of the activity, perhaps in consequence of the removal of the products of the resting metaboHsm of the gland, there is increased growth of protoplasm, increased activity of the nucleus, and therefore a tendency to increased assimilatory changes and a preparation of the cell for further secretory changes either immediately or hereafter. In the gland, however, as in muscle, when we attempt to form a concep- tion of the mechanism of the chemical machine in the Uving cell, we are brought up against insuperable difficulties. One might perhaps conceive of the secretory granules being bounded by a membrane impermeable to inter- mediate metabohtes and salts, but permeable to carbon dioxide. If the" first effect of stimulation of the secretory nerves were to produce an explosive disintegration of the complex molecules making up the granules, we should have a sudden multiplication of molecules within the granules. This woidd cause a large rise of the osmotic pressure in these granules and the consequent absorption of water from the surrounding protoplasm. This process, how- ever, could only result in the production of a fluid in the granules having the same osmotic pressure as the surrounding medium, whereas we know that saUva has a molecular concentration which is only one half of that of the blood or lymph. We should therefore have to make a second assumption, namely, that, before the extrusion of the solution from the granules, there is a further breakdown of the metabolites by a process of oxidation, with the production of carbon dioxide which diffuses into the surrounding protoplasm. We have, however, no evidence of either of these processes or for any of these assumptions, and I have only adduced them in order to show how far we are still from the actual comprehension of the events occurring in every living cell, and underlying its conditions of rest and activity. SECTION II THE PASSAGE OF FOOD FROM THE MOUTH TO THE STOMACH The food after mastication is carried to the stomacli by a complex series of co-ordinated movements involving the muscles of the pharynx and the oesophagus (Fig. 332). Various methods have been used to study the process of deglutition in man and the higher animals. Important information can be obtained by allowing a man or animal to swallow either a fluid or solid with which bismuth is mixed and observing the passage of the opaque substance under the Rontgen rays. The sub- nitrate of bismuth may be mixed with milk for a fluid or with bread and milk for a semi-solid substance, or may be enclosed in a cachet and swallowed as a solid bolus. The time of entry of the food into the stomach may be determined by aus- cultating with a stethoscope over the region of the cardiac orifice. Since a certain amount of air is always swallowed at the same time as the food, the escape of this air through the small cardiac orifice gives rise to a bubbling noise which can be easily heard. In the horse the move- ment of a bolus down the oesophagus can be seen or felt from the outside of the neck. The relative time- relations of the events at different parts of the oesophagus may be Fig. 332. Dissection to show muscles employed in deglutition. h, styloid process, from which arise 1, the styloglossus; 2, the stylohyoid; 3, the stylo- pharyngeus muscles ; c, section of lower jaw ; d, hyoid bone ; e, thyroid cartilage ; 17, isth- mus of thyroid gland ; 4, cut edge of mylo- hyoid muscle ; 5, 6, 7, 8, muscles of tongue ; 9, 10, 11, superior, middle, and inferior constrictors of pharynx ; 12, oesophagus. (Allen Thomson.) obtained by passing sounds provided with rubber balloons to different levels in the tube and connecting these sounds with recording tambours 676 PASSAGE OF FOOD FROM MOUTH TO STOMACH 677 or piston recorders. This method has been employed both in men and in animals. When a mouthful of water is taken two sounds may be heard on ausculta- ting over the oesophagus. The first sound immediately follows the beginning of the act of swallowing and is probably due to the impact of the fluid against the posterior pharyngeal wall brought about by the sudden con- traction of the mylohyoid and other muscles which throw the fluid from the back of the tongue across the pharynx. The second sound is heard best by listening over the epigastrium. It begins from four to ten seconds after the first sound, and lasts for two or three seconds. The interval between the two sounds is not constant, and may vary in the same individual. If the observation be carried out on a man lying on his back, the trickling second sound is changed into a series of sounds which have been described as squirts, which vary from two to five in number, each lasting about one second. The second sound may be absent when a solid bolus is swallowed. On observing the process by Rontgen rays very much the same time-relations are obtained. If a mouthful of milk mixed with bismuth carbonate be swallowed, it will be seen passing rapidly down the oesophagus to the cardiac orifice of the stomach. Here the passage becomes slow, and the fluid escapes slowly in a narrow stream into the stomach. The average time which elapses between the beginning of deglutition and the disappearance of the last trace of fluid from the oesophagus is about six seconds. The same course of events is induced when the food swallowed is semi-solid. If, however, the bolus be dry, such as a cachet of bismuth carbonate, it may pass down the oesophagus with extreme slowness and may take as much as fifteen minutes to reach the cardiac orifice, although the individual who has swallowed it is quite unaware of its continued presence in the oesophagus. If, as would normally be the case, the cachet be well moistened with sahva or water before swallowing, it passes much more rapidly, the total time taken being between eight and eighteen seconds. It has been customary since the time of Magendie to divide the act of swallowing into three stages, during the first of which the bolus of food is carried past the anterior pillars of the fauces ; during the second through the pharynx, past the openings of the nasal cavities and of the larynx ; and during the third through the oesophagus into the stomach. There is, how- ever, no pause between these various stages. The act of deglutition is one, and the initiation of the first stage inevitably involves the completion of the third. The food is masticated, and is collected as a bolus on the dorsum of the tongue. A pause then takes place in the movements of mastication, a slight movement of the diaphragm usually occurs known as ' respiration of swallowing,' and then a sudden elevation of the tongue throws the bolus back through the anterior pillars of the fauces. In this movement the chief factor is the contraction of the mylohyoid muscle, which presses the tongue against the palate and pushes it backwards. The backward movement of the tongue may also be aided by the contraction of the styloglossus and palatoglossus muscles which pull the base of the tongue suddenly backwards. 678 PHYSIOLOGY These muscles, especially the palatoglossi, serve to close the isthmus faucium, thus preventing any return of the food towards the mouth. As the food is passing through the upper part of the pharynx it traverses a region common to the respiratory as well as the digestive passages. Its passage through this region is therefore rapid, and is associated with a closure of the two openings of the air passages into the pharynx. The nasal cavity is shut off by a simultaneous contraction of the levator-palati and palato-pharjmgeus muscles and azygos uvulae, by which means the soft palate is raised (Fig. 333) and the posterior pillars are proximated to the uvula. ^Ma ir r^x Fig. 333. Diagram (after Tigekstedt) to show the jDosition of the soft palate. I, during rest ; II, during the act of swallowing. The upper and back wall of the palate is thus formed into a tense sloping roof which guides the bolus down the pharynx. More important is the shutting off of the lower air passages. The con- traction of the mylohyoid muscles which initiate deglutition is followed almost immediately (at an interval of 0-47 sec.) by an elevation of the larynx, and this elevation is accompanied by closure of the glottis as well as of the superior opening of the larynx. The laryngeal opening is bounded in front by the epiglottis, behind by the tips of the arytenoid cartilages, and at the sides by the aryteno-epiglottidean folds. When deglutition takes place the arytenoid cartilages, which normally lie against the posterior wall of the pharynx, are rotated and move inwards and forwards, so that the laryngeal opening assumes the form of a tri-radiate fissure, the vertical limb being short, while the transverse limb is rounded owing to the pulhng inwards of the margins of the epiglottis. At the same time both the true and false vocal cords are approximated, while the movement of the dorsum of the tongue backwards enables the closed laryngeal orifice to he directly under the back part of the tongue. The muscles which are actively involved in this closure PASSAGE OF FOOD FROM MOUTH TO STOMACH 679 of the lower air passages are the external thyro-arytenoid, arytenoid, ary- epiglottidean, and the lateral crico-arytenoid muscles. Since the approxi- mation of the posterior to the anterior boundary of the laryngeal opening is only rendered possible by the elevation of the whole larynx under the hyoid bone, the act of deglutition cannot be carried out unless the larynx is free to move. The two openings from the back of the pharynx into the air passages being thus closed, the bolus is shot rapidly past them into the region of the middle and inferior constrictors of the pharynx. If the bolus be liquid or semi-fluid, the movement of the back part of the tongue may be sufficient to propel the substance past the constrictors through the lax oesophagus to its lower end. It is on this account that, when corrosive fluids are swallowed by accident, we very often find the damage to the oesophagus Hmited to the three points where it is narrowed and where, therefore, there is a slight hindrance to the onward flow of fluid. If the bolus be large and solid or semi-solid, it is seized in the grasp of the middle constrictors on passing through the upper part of the pharynx, and is thrust by successive con- tractions of this muscle and of the inferior constrictor gradually down the oesophagus. The walls of the cervical part of the oesophagus are composed of striated muscle. In the thorax striated and unstriated muscles are asso-. ciated together, while the lower third, in the neighbourhood of the stomach, consists almost entirely of unstriated muscle. Corresponding to these differences in structure, Kronecker and Meltzer have found differences in the duration and rapidity of propulsion of the contractional waves in each part. The following Table shows the time-relations of the chief muscles engaged in deglutition as determined by Kronecker and Meltzer and by Marckwald : Time from commencement Interval Muscle movement Duratioii of contraction — — Mylohyoid 0-6 sec. — 0-03 sec. Respiration of swallowing — — 0-07 sec. Elevation of larjiix 0-8 see. ()•;} 8t'c-. 0-2 sec. Constrictors of pharynx 1-0 to 2-0 sec. — 0-9 sec. First section of oesophagus 2-(> to 2-5 sec. 3-0 sec. 1-8 sec. Second section of oesophagus 0-0 to 7-0 sec. 6-0 sec. 3 0 sec. Third section of oesophagus about 10 sec. The free passage of food down the oesophagus under the influence of the propulsive force exercised by the mylohyoid muscles shows that the walls of this tube must be lax, and in fact one must assume that the first act of deglutition, so far as concerns the oesophagus, is an inhibition initiated 680 PHYSIOLOGY reflexly with the beginning of the act of deglutition. When a second act of deglutition succeeds the first with a sufficiently short interval, the reflex inhibition due to the second act may prevent the development of any wave of contraction in the oesophagus. This tube thus remains in a lax condition Fig. 334. Curves obtained during swallowing by placing two rubber balloons, one (the upper curve) in the pharynx, the other (lower curve) in the oesophagus. In A the second balloon was 4 cm. ; in B 12 cm. ; and in c 16 cm. from the upper end of the oesophagus. In each curve it will be noticed that the excursion of the upper lever is followed immediately by an excursion of the lower lever (due to passage of the swallowed fluid and transmitted rise of pressure), and then, after an interval of time varying with the distance between the balloons, by another rise due to the peristaltic contraction of the wall of the oesophagus. and allows the free rapid passage of the food downwards until the movements of deglutition have come to an end, when a peristaltic contraction occurs and sweeps all remaining adherent particles of food into the stomach. The circular fibres of the lower end of the oesophagus which form the cardiac sphincter of the stomach are normally in a state of tonic contraction. When one mouthful of food is swallowed it may be either squirted directly into the stomach, or may remain at the lower end of the oesophagus until the following PASSAGE OF FOOD FROM MOUTH TO STOMACH 681 peristaltic wave forces it through the orifice. When several acts of deglu- tition succeed one another, the cardiac sphincter shares in the inhibition of the oesophageal walls, and offers no resistance to the direct propulsion of food from the mouth to the stomach. Cannon has shown that the relaxation of the cardiac orifice which accompanies swallowing extends also to the cardiac end of the stomach. This relaxation lowers the pressure within the stomach, and makes room for the incoming food. If the stomach be filled with a fluid such as starch solution, the cardiac sphincter may be seen to relax rhythmically, allowing of the regurgitation of Fig. 335. Tracings of ^respiratory movements' to show the effect of stimulating the central end of the glossopharyngeal nerve. (Makckwald.) The point of stimulation is marked with a cross. Note that the stoppage may occur at any phase of the respiratory movement. the stomach contents into the lower part of the oesophagus. Their entry into this tube is at once followed by a peristaltic contraction of this part of the oesophagus (apparently entirely unconscious), which drives the fluid back into the stomach. These movements of regurgitation become more and more infrequent as the gastric contents become acid, and are not observed at all if the stomach be filled with a fluid already acid. This phenomenon has been spoken of as the ' acid control of the cardia.' THE NERVOUS MECHANISM OF DEGLUTITION Deglutition is a reflex act. When we swallow voluntarily we supply the necessary initial stimulus either by touching the fauces with the tongue or by forcing a certain amount of saliva into the fauces. The afferent channels of the reflex are contained in the second division of the fifth nerve, the glosso- pharyngeal nerve, and the pharyngeal branches of the superior laryngeal nerve. We can excite a single act or a whole series of acts of deglutition by electrical stimulation of the central end of the last-named nerve. The efferent fibres which determine the contraction of muscles engaged in the act of deglutition travel by the hypoglossal nerve to the muscles of the 22* 682 PHYSIOLOGY tongue, by the fifth to the mylohyoid, by the glossopharyngeal, the vagus, and the spinal accessory nerves to the muscles of the fauces and pharynx. The closure of the larynx is effected by impulses which travel through the superior and inferior laryngeal branches of the vagus. The centre for the act is situated in the medulla oblongata, and can be considered as consisting of a chain of centres stimulation of one of which involves the firing off of all the others in orderly sequence. Thus, as Meltzer has shown, the propulsion of the contraction down the oesophagus is determined by the intracentral nervous connections, and does not require the integrity of the muscular tube itself. If the oesophageal nerves be divided, the act of deglutition is abohshed, the upper part of the oesophagus becoming permanently relaxed, while the lower part, including the cardiac sphincter, enters into a state of tonic contraction. On the other hand, the oesophagus may be ligatured or cut across without interfering with the propulsion of the wave of contraction. On the other hand, the oesophagus may be ligatured or cut across without interfering with the propulsion of the wave of contraction, started in the pharynx, from one end of the tube to the other. Stimulation apphed to the mucous surface of the oesophageal tube is wdthout effect. There is an important interdependence between the functions of respira- tion and deglutition. If an inspiratory or expiratory movement were going on during the act of deglutition, food might be drawn into the lungs or driven into the nasal cavities. Such an accident is prevented by the fact that every act of swallowing inhibits a respiratory movement. This in- hibition is effected reflexly through the glossopharyngeal nerve. Stimula- tion of the central end of this nerve at once causes cessation of respiration in whatever phase it may happen to be (Fig. 335). This cessation lasts for five or six seconds, i.e. a sufficient length of time for a whole series of acts of deglutition. Respiration then recommences, and the inhibition cannot be prolonged by continuing the stimulation of the glossopharyngeal nerve. This inhibition of the activity of the respiratory centre can be shown on oneself. If the breath be held until the feehng of dyspnoea, i.e. the need to breathe, becomes insistent, relief is at once experienced by swallowing, and the feehng of relief will last for three or four seconds. SECTION III DIGESTION IN THE STOMACH GASTRIC JUICE Within five minutes of the taking of food into the mouth a secretion of gastric juice begins from the multitude of tubular glands which make up the greater part of the mucous membrane of the stomach. As the food, masticated and thoroughly mixed with saliva, is swallowed in successive portions, it accumu- lates in a mass in the fundus of the stomach, and the mass thus formed is penetrated with difficulty by the juice which is continually being poured out by the walls of the stomach, so that salivary digestion can be continued for a considerable time. The gastric juice, which is so poured out, can be obtained in various ways, most of them yielding it mixed more or less with the food-stuffs. In cHnical practice it is the custom to give a definite meal, and then at a given interval after the meal to wash out the stomach, so obtaining a mixtm'e of gastric juice and partially digested food. A method of obtaining the juice in a perfectly pure condition has been devised by Pawlow. A case had been previously described by Richet in which, as the result of the accidental taking of a corrosive alkali, the oesophagus had become occluded by the cicatrisation of the ulcer pro- duced. In order to preserve the individual from starvation, it was neces- sary to perform gastrostomy, i.e. to make an artificial opening into the stomach through which he could be fed. Although in this patient the pas- sage of the saliva from mouth to stomach was completely prevented, it was observed that merely taking food into the mouth was followed by the secre- tion of gastric juice. Pawlow produced this condition artificially in dogs. The oesophagus was divided and the two ends brought to the surface of the neck. At the same time an opening was made into the stomach. The animals could be fed either through the opening of the oesophagus in the neck, or with solid food through the gastric fistula. They could eat also and swallow food as usual, but the food thus swallowed simply fell out of the opening in the neck without passing into the stomach. Under these circumstances it is found that the taking of food is quickly followed by a secretion of gastric juice, which can be collected in vessels connected with the fistulous opening. If taken from a fasting animal, such a juice is per- fectly free from admixture, and can be regarded as pure gastric juice. It is quite clear, strongly acid, wnthout smell. It contains about 0-3 to 0-6 per 683 684 PHYSIOLOGY cent, total solids ; it contains no peptone, but traces of protein. The following Table represents its average composition : Hydrochloric acid . . . 0-46 to 0-58 per cent. ChloriBe 0-49 to 0-62 „ Total solids .... 0-43 to 0-60 „ Ash 0-09 to 0-16 „ If the juice be allowed to stand in the ice chest for a day it becomes cloudy and deposits a fine granular precipitate, which apparently represents the active agent of the juice, and may perhaps be regarded as pepsin in a pure form. The actions of gastric juice are due partly to the acid, partly to the combined action of the acid and the ferments. The acid of the gastric juice, when obtained free from admixture, is entirely hydrochloric acid. Dog's juice contains on the average about 0-6 per cent. HCl ; human gastric juice probably contains less, about 0-2 per cent. When, however, we examine the gastric contents, composed of a mixture of gastric juice and semi-digested food, we always find, besides the hydrochloric acid, other acids present, among which the most prominent is lactic acid. So constantly is this latter acid present that it was formerly thought by some physiologists to be the chief acid of the gastric juice. It is produced by processes of fermentation occurring in the food. Whenever we take carbohydrates we swallow at the same time micro-organisms, and these in the warm moist mass quickly attack the carbohydrates, converting them into sugar and then into lactic acid. As the gastric juice gradually soaks into the food and renders it acid, it stops this lactic acid fermentation, so that whereas in the early stages of gastric digestion both acids are present in considerable quantity, towards the end of gastric digestion lactic acid is almost entirely absent. In some pathological conditions free hydrochloric acid may be entirely wanting from the gastric juice, and the detection of this acid in gastric juice becomes therefore a matter of considerable clinical importance. For this purpose we can employ various indicators, which change colour in the presence of a free strong acid such as HCl, but are unaffected by weak acids such as lactic acids, or the fatty acids. Chief among these indicators are Congo red, which turns blue with mineral acids, and a slaty colour with lactic acid ; and tropaolin 00, which turns a brilliant red in the presence of a free mineral acid, but is imaltered by lactic acid. The reagent which is most employed is Gunzberg's reagent. This is a mixture of phloroglucin and vanillin dissolved in absolute alcohol. A drop of this is evaporated to dryness in a porcelain capsvde. A drop of the fluid suspected to contain free acid is then added, and also evaporated to dryness. If free HCl be present, the residue on drying becomes a brilliant red colour, an effect which is not produced by the presence of free lactic or free fatty acids. Great stress has been laid on the determination of the actual amount of free H ions present, and for this purpose the acidity of gastric juice or of digestive mixtures has been tested by determining its inverting power on cane sugar, or its power to hasten the saponification of ethyl acetate. The acidity estimated in this way is diminished considerably by the presence of albumens, and still more by the presence of albumoses or peptones. But it does not seem that the adjuvant action of the acid on the proteolytic DIGESTION IN THE STOMACH 685 powers of the gastric ferment is in any way affected by the diminution of its acidity caused by the presence of peptone. The coloured indicators men- tioned above, however, serve as trustworthy indications of the amount of free acid present, considered with regard to its digestive functions. In order to determine quantitatively the amount of free HCI, the following pro- cedure is employed (Morner and Sjoqvist) : Ten cubic centimetres of the gastric juice are neutralised with barium carbonate (litmus being employed as an indicator). The mixture is dried in a platinum dish, incinerated, and the ash extracted with warm water and filtered. In this process the organic acids are destroyed and converted into barium carbonate. The solution therefore contains merely the barium which was taken up to combine with the free HCI. Estimation of the barium in the filtrate gives the amount of BaCl present, and therefore the amount of hydrochloric acid in the gastric juice. The barium is determined by titrating in presence of sodium acetate and acetic acid with potassium bichromate solution, ' tetra paper ' being used as an indicator. This turns deep blue in the presence of free bichromate in solution. THE ACTIONS OF GASTRIC JUICE ON FOOD-STUFFS By the action of the hydrochloric acid certain changes are induced in the food-stuffs. Cane sugar is inverted to glucose and fructose ; some proteins, such as blood fibrin, are swollen up to form a jelly-like mass. The caseinogen of milk is precipitated, the collagen of the connective tissues is swoUen up. It is possible that a certain small amount of hydrolysis also takes place in the dextrins and maltose produced by the action of ptyalin on starch. The chief digestive function of the gastric juice is dependent on the action of the ferment pepsin. This substance, which is inactive in neutral medium, needs the co-operation of an acid, hydrochloric acid being the most effective, though its place may be taken by phosphoric, sulphuric, or lactic acid. Its main effect is on the proteins of the food. The stages in its action may be best studied by taking as an example its action on blood fibrin. If fibrin be immersed in 0-4 per cent, hydrochloric acid, it swells up to a gela- tinous mass. On then stirring in an extract of gastric mucous membrane, or any preparation of pepsin, the gelatinous mass rapidly undergoes solution. If the mixture be boiled and neutraHsed, immediately after solution has occurred, nearly the whole of the protein is thrown down in a coagulated form. The first effect therefore is the production of coagulable soluble proteins from the insoluble fibrin. If the action be allowed to proceed for some hours, a whole series of products of hydrolysis are found in the mixtm'e. On neutrahsing the fluid, a precipitate may be thrown down consisting chiefly of acid albumen. The greater proportion of the protein remains in solution. This remainder may be purified from any unaltered coagulable protein by boihng in slightly acid solution and filtering. The filtrate contains a mixture of bodies belonging to the class of hydrated proteins, viz. proteoses and peptones. By means of fractional precipitation with ammoniimi sulphate or zinc sulphate, these mixtures can be subdivided into various substances, although in no case can we be certain that we are dealing with chemical individuals. The Table on p. 686 represents the chief bodies obtained by Pick by this 686 ^3 g ^ r* 1 s o tS 3 S ^ ^ 'S. PHYSIOLOGY PM < < 1>^ !h eg !-< n o o o S SP fl Ph f^^ Pm ■S iJ S ?g g PM ■3 ^ -2 g 3 to O m CM CO TJH (N C5 1— 1 t- o t^ CO CO tH in 1—1 l-H 1-H l> CO 1— 1 1— 1 o 00 o CO o CO tc CO C5 ^ o ■-* CO CO CO t^ t^ CD t- CD lO CO -dS o fl fl rO < -^ W) += i^s fl o 0) P.t^ Fh rO ^fl £ m <1 rS ^ 5§M O) 1 ■S""'^ r£! ,5 >=-^ <1 " fl ■fl ig 'S-t^ ^ i t>> rO ,^ ^ ^ ^ CD 't O) 'J' -go -^o ^OQ $,m ^^ .&^ " w o lij ^^ k& •+= o o 1^ 12; < pp fl fl o o Ph p^ Oj 03 PM PM DIGESTION IN THE STOMACH 687 method from ' Witte's peptone,' a commercial preparation containing proteoses and peptones. The following careful analysis of the constituents of protoalbumose and heteroalbumose (or protoproteose and heteroproteose) respectively shows that the different proteoses really correspond to different groupings of the amino-acids making up the original protein molecule : Results of the Complet ^E Hydrolysis of Hetero- and Protoalbumose Heteroalbumose Protoalbumose Glutaminic acid ...... 9-51 0-63 Leucine 305 5-79 Isoleucine . 2-96 1-62 Valine 3-54 0-76 Alanine 3-39 2-50 Valine-alanine mixture 1-86 0 00 Proline 4-27 4-96 Phenylalanine 2-45 4-35 Aspartic acid 4-73 2-98 Glycocoll . 015 1-44 Tyrosine . 3-48 4-58 Arginine . 7-30 7-72 Histidine . 3-90 2-77 Lysine 8-90 8-40 Cystine 1-36 0-68 Ammonia . 1-28 0-92 The results obtained by Pick by hydrolysis of these different bodies show that in the breakdown of protein produced by gastric juice there is really a division of the complex molecule into smaller molecules, which are quahta- tively different. Thus of the fractions which he obtained, some contain the greater part of the sulphur originally present in the protein molecule, another contains the greater part of the carbohydrate group, while others are free altogether from the tryptophane group which is responsible for Hopkins' reaction obtainable in the original protein. Proceeding from primary through secondary albumoses to peptones, there is probably a continuous diminution in the size of the molecule. During the time which gastric juice has to exert its influence, a maximum, say, of twelve hours, the breakdown of proteins never passes beyond the albumose and peptone stage, and it is in this form that the proteins of the food pass on through the pyloric orifice into the small intestine. ACTION ON THE CONNECTIVE TISSUES AND OTHER FOOD-STUFFS ALLIED TO PROTEINS Collagen. The connective tissues are made up chiefly of white fibres more or less modified, which consist of collagen. This substance forms the main basis of areolar tissue, of white fibrous tissue, and of bone. On prolonged boiling, it is converted into gelatin. The gastric juice dissolves 638 PHYSIOLOGY collagen, converting it, probably tbrough the stage of gelatin, into gelatoses and gelatin peptones, bearing the same relation to the original substance as is borne by the proteoses and peptones to the proteins. On account of this action, adipose tissue (which consists of protoplasmic cells distended with fat, and bound together by connective tissue) is broken up into its constituent cells. The protoplasmic pellicle is dissolved, and the fat floats freely in the gastric juice. Elastin, which also occurs in varying amounts as the chief constituent of the elastic fibres of connective tissues, is slowly acted upon by gastric juice. Under the conditions of natural digestion, however, it may be regarded as indigestible. Mucin, which forms a considerable proportion of the ground substance of connective tissues, is converted by gastric juice into peptone-like sub- stances, and into reducing bodies probably allied to glycosamine. The NUCLEO-PROTEiNS, the chief constituents of cells, and therefore in- gested in large amounts with food-stuffs such as sweetbreads, are first dissolved by the acid of the gastric juice, and are then broken up into two moieties. The protein half is converted into proteoses and peptones, while the nuclein moiety is precipitated in an insoluble form. On PHOSPHO-PROTEiNS gastric juice acts in a somewhat similar manner. The protein of milk, caseinogen, undergoes special changes in the stomach. The first effect of gastric juice, even in neutral medium, is to convert the caseinogen into an insoluble casein. This action is generally ascribed to the presence of a distinct ferment of the gastric juice, named rennin, or rennet ferment. But, according to some authorities, it is due directly to the pepsin, i.e. rennin and pepsin are identical. For the conversion of caseinogen into the solid clot of casein the presence of Ume salts is necessary. The addition of remiet to an oxalated milk apparently produces no effect, but clotting ensues if a soluble hme salt, such as calcium chloride, is then added to the mixture. Under the action of the acid gastric juice the solid clot of casein is dissolved, but a precipitate is left containing a small proportion of the original phosphorus of the caseinogen. This precipitate is sometimes spoken of as para-nuclein, or fseudo-nuclein. It does not yield purine bases on hydrolysis with acids, but contains phosphoric acid in organic com- bination. By prolonged digestion with strong gastric juice it is possible to dissolve the whole of this precipitate. It is therefore thought that in the clotting of milk the caseinogen under the action of the rennet first undergoes a conversion into a soluble casein, or perhaps a splitting into a soluble casein and some other protein. The soluble casein then, under the influence of the lime salts, forms an insoluble casein which is precipitated, and causes the sohdification of the milk. In the absence of lime salts, the conversion or splitting of caseinogen takes place, but the second stage of the process cannot occur until the lime salts are added. DIGESTION IN THE STOMACH 689 THE EFFECT OF GASTRIC JUICE ON CARBOHYDRATES On account of the fact that cane sugar undergoes inversion into equal molecules of glucose and fructose in the stomach, it has been sometimes thought that gastric juice contains a ferment inveitase. It seems, however, that the inversion which takes place in the stomach can be completely accounted for by the action of hydrochloric acid present, and that there is no need to assume the presence of a special ferment. In the same way inuhn, the variety of starch which gives rise to the Isevorotatory sugar fructose on hydrolysis, and is found in dahlia tubers and certain other reserve structures of plants, is converted by the acid of gastric juice into fructose. The inulin is therefore completely utilised in the ali- mentary canal of animals, although there is no definite ferment inulase provided for its hydrolysis. THE EFFECT OF GASTRIC JUICE ON FATS The chief action of this juice on fats is the solution of their connective- tissue framework and protoplasmic envelopes, so as to set the fat free in the stomach contents. After a fatty meal it is found moreover that a consider- able proportion of the fat in the stomach has undergone hydrolysis and con- version into free fatty acid. In this hydrolysis two factors are involved, viz. (1) the action of the warm dilute hydrochloric acid ; (2) the action of a special fat-splitting ferment or lipase, which is secreted by the walls of the stomach, and acts especially at the beginning of gastric digestion before the contents have attained a high degree of acidity. The action of this ferment is only marked if the fat be present in a finely divided form, e.g. as yolk of egg. The chief digestion of fat takes place in the next segment of the ahmentary canal, namely, in the duodenum. THE SECRETION OF GASTRIC JUICE Pawlow has shown that if an animal provided with gastric and oesophageal fistulae be given food, when hungry, it will eat with avidity, and since the food cannot reach the stomach and so satisfy its hunger, it will continue to eat for two or three hours. Five minutes after the beginning of this sham feeding, gastric juice begins to drop from the fistulous opening ; and in this way large quantities of juice, free from any admixture with other substances, can be easily obtained. By this means we obtain a secretion of gastric juice, which is excited by the presence of food in the mouth. This method does not , however, enable us to determine whether the character of the juice will be altered in any way by the changes which the food undergoes in the stomach itself. In order to form an idea of the normal course of secretion of gastric juice, when food is taken into the stomach in the ordinary way, Pawlow has devised another procedure. A small diverticulum representing about one- tenth of the whole stomach is made at the cardiac or pyloric pjid, in direct muscular and nervous continuity with the rest of the stomach, but shut off from the main part of the viscus by a diaphragm of mucous membrane. The method in which this operation is carried out will be evident by reference to 690 PHYSIOLOGY the diagram (Fig. 336). In a dog treated in this way it is found that the amount of juice secreted by the small stomach always bears the same ratio to the amount secreted by the large stomach, while the digestive power of the juice obtained from the small stomach is equal to that obtained from the large. This is shown in the following Table : * Secretion from Gastric Fistula after Sham Meal Hours Small stomach | Large stomach Quantity 1 strength t Quantity strength 1 2 3 7-6 c.c. 4-7 c.c. 1-1 c.c. 5 -88 mm. 5-75 mm. 5-5 mm. 68-25 c.c. 41-5 c.c. 14-0 c.c. 5-5 mm. 5-5 mm. 5-38 mm. Total 13-4 c.c. — 123-75 c.c. — Fig. 336. Diagram to show Pawlow's method of making a cul-de-sac of the cardiac end of the stomach, with vascular and nerve supply intact. In A the line of the incision into the stomach wall is shown. B represents the operation as completed. In A ; 0, oesophagus ; R.v, L.v, right and left vagus nerves ; P, pylorus ; C, cardiac portion of stomach ; A, B, line of incision. In B : V, main portion of stomach ; 8, cardiac cul-de-sac ; A, abdominal wall ; e, e, mucous membrane reflected to form diaphragm between the two cavities. In this case a fistulous opening had been estabhshed into the large stomach, so that the juice could be obtained simultaneously from both sections of this organ. Secretion was excited by a sham meal, in which the food taken by the animal dropped out of an opening in the neck, and was not allowed to reach the stomach. It will be seen that the secretions in the two sections of the stomach run parallel to one another, while there is an almost exact equivalence between the strengths of the juices obtained from each * Pawlow, " The Work of the Digestive Glands " (translated by W. H. Thompson, M.D.), p. 80. t The strength of the juice was determined by measuring the number of millimetres of coagulated egg-white (in Mett's tubes) which were digested in eight hours. DIGESTION IN THE STOMACH 69] section. We may therefore regard the secretion obtained from the small stomach as a sample of that produced by the large, and from the changes in this small stomach judge of the effects occurring in the whole organ. By this method it is possible to study the effects of a normal meal in which the food is swallowed, or of a sham meal in which the food is merely masticated in the mouth, or of a meal in which the food is directly introduced into an opening into the large stomach. The method which we must adopt for the collection of gastric juice shows that we have to do, in the first place, with a reflex nervous mechanism, since an active secretion is excited by the presence of food in the mouth and by its mastication. Moreover a secretion, which is at least as vigorous as that produced by a sham meal, can be evoked by merely arousing in the dog the idea of a meal. If the animal be hungry, it is sufficient to show it the food to produce a secretion. In the experiment from which the following Table is taken, the dog was co:i':inually excited by showing it meat during a period of an hour and a half. At the end of this time the animal, which had an oesophageal fistula, was given a sham meal. It will be observed that the psychical secretion obtained during the first period of the experiment was rather greater than the secretion produced by the introduction of food into the mouth. Psychical Secretion of Gastric Juice (Pawlow) Time Quantity 8 minutes lOc.c. 4 10 „ 4 10 „ 10 10 10 „ 10 „ 8 10 „ 8 10 „ 19 „ 10 „ 9 „ • 3 „ Sham Feeding Time Quantity 17 minutes 10 c.c. 9 10 „ 8 10 „ The afferent channels for this reflex may be therefore either the afferent nerves from the mouth, or, when the idea of food is involved, any of the nerves of special sense, such as sight, smell, or hearing, through which these ideas are called forth. The efferent channels can only be one of two nerves, viz. the vagus and the sympathetic, since these are the only two which are distributed to the stomach. That it is the former of these nerves which is involved is shown by the fact, recorded by Pawlow, that psychical secretion, as well as the results of a sham meal, is entirely abolished by division of both vagi. On this account division of both vagi may give rise to entire absence of gastric digestion, and death of the animal may ensue from inanition, or from poisoning by the products of decomposition of food in the stomach, even 692 PHYSIOLOGY when care has been taken to avoid injury to the pulmonary and tracheal branches of these nerves. The converse experiment of exciting secretion by direct stimulation of the vagus presents greater diflS.culties. Stimulation of the vagus in the neck causes stoppage of the heart, and consequent anaemia of the mucous membrane of the stomach. Moreover, the stomach seems to be much more susceptible than the sahvary glands to the action of poisons, such as anaes- thetics. Its activity is also easily affected by inhibitory impulses arising in the central nervous system as the result of either painful impressions or emotional states of the animal. In order to avoid these disturbing factors Pawlow proceeded as follows : An animal with fistulse of oesophagus and stomach had one vagus nerve divided. A thread was attached to the peri- pheral end of the cut vagus and allowed to hang out through the wound. Four days after the operation the vagus was drawn out of the wound by carefully pulling on the thread, so as not to hurt or frighten the animal in any way, and its peripheral end stimulated by means of induction shocks. No effect was produced on the heart, owing to the degeneration of the cardio-inhibitory fibres, which is well known to occur within this period after section. Five minutes after the commencement of the stimulation the first drop of gastric juice appeared from the gastric cannula, and a steady secretion of juice was obtained with continuation of the stimulation. This experiment furnishes the decisive and final evidence that the secretory nerves to the stomach run in the two vagi. There is one marked difference, however, between the action of these nerves and the action of the chorda tympani nerve on the submaxil- lary gland, namely, the great length of the latent period before gastric secretion occurs. The length of this latent period has not yet been satisfac- torily explained. It cannot be due to delay occurring between the vagus fibres and the local nervous mechanism in the stomach. It may be that the chemical changes finally resulting in secretion require a longer period for their accomphshment than is the case in the sahvary gland. Physiologically there is, indeed, no special need for a rapid secretion of gastric juice, whereas in the mouth it is essential that the introduction of food should be immedi- ately followed by the production of sahva, for the tasting and testing of the food and for its subsequent mastication or rejection. These experiments show conclusively that an important — probably the most important — part of the gastric secretion is determined by a nervous mechanism. This nervous secretion does not, however, account for the whole of the gastric juice obtained as the result of a meal. If an animal provided with two gastric fistulse, one into a diverticulum and the other into the main stomach, has both its vagi divided, it is found that the introduction of meat into the large stomach is followed, after a period of twenty to forty-five minutes, by the appearance of a secretion of gastric juice from the small stomach. Moreover, when an animal is given a normal meal and is allowed to swallow the food after mastication, the total amount of gastric juice obtained is greater than that produced by the sham feeding alone and the flow is of longer duration. In fact, we may say that the gastric juice secreted DIGESTION IN THE STOMACH 693 in response to a normal meal consists of two parts, viz., (1) a large amount, the secretion of which begins within five minutes of the taking of the food and is determined by the reflex nervous mechanism described above ; and (2) a smaller portion, the secretion of which is excited by the presence of the food in the stomach. This combined character of the gastric juice produced by a normal meal is shown in the following Table (Pawlow) : Secretion of Gastric Jxhce Hours Normal meal. 200 grm. meat into stomarh 150 grm. meat direct into stomach Sham meal Sum of two last ex- periments Quantity c.c. Strength mm. Quantity c.c. Strength mm. Quantity c.c. Strength mm. Quantity c.c. 1 12-4 5-43 5-0 2-5 7-7 6-4 12-7 2 13-5 3-63 7-8 2-75 4-5 5-3 12-3 3 7-5 3-5 6-4 3-75 0-6 5-75 7-0 4 4-2 3-12 ,5-0 3-75 0 () 5-0 In the first column is given the result of a normal meal on the secretion from the gastric diverticulum. In the second column are given the amount and digestive power of the juice which is excited by the direct introduction of 150 grm. of meat into the large stomach of the animal, care being taken not to excite in any way the nervous reflex mechanism. In the third colmun are given the amount and digestive power of the juice which is evoked by a sham meal of 200 grm. of meat. In the fourth column is given the sum of the last two experiments. It will be seen that the total effect of the sham meal plus the direct introduction of meat into the stomach is almost identical with the secretion obtained when the food is taken in a normal way and allowed to pass through the oesophagus into the stomach. The second phase of the gastric secretion cannot be ascribed to the inter- vention of the reflex vagal mechanism. Since it occurs after cutting off the stomach from its connections with the central nervous system, it must have its causation in the gastric walls themselves. That it cannot be due to mechanical stimulation is shown by the fact, previously mentioned, that it is impossible by local stimulation of the mucous membrane, by rubbing, or introduction of sand, or any other method, to evoke a secretion. Moreover it is not produced by all sorts of food. The introduction of white of egg, of starch, or of bread into the stomach causes no secretion. On the other hand, if bread be mixed with gastric juice and allowed to digest for some time, the introduction of the semi- digested mixture into the stomach evokes a secre- tion. We have already seen that meat produces a secretion ; still more potent than meat, however, is a decoction of meat, or bouillon, or Liebig's extract of meat, or certain preparations of peptone. Pure albumoses and peptones have no effect, so that the exciting mechanism must be some 694 PHYSIOLOGY chemical substances present in meat, and produced in various other foods under the action of the first gastric juice secreted in response to nervous stimuh. Popielski has shown that this secretion occurs after complete severance of the stomach from the central nervous system, as well as after destruction of the sympathetic nervous plexuses of the abdomen. Since the injection of bouillon directly into the circulation has no effect, this author concludes that the second phase of secretion is determined by the stimulation of the local nerve plexus, and that we have here, in short, a peripheral reflex action, the centres of which are situated in the walls of the stomach itself. There is, however, another possible explanation for this second phase of secretion. Although the peptogenic substances, those substances which evoke gastric secretion on introduction into the stomach, have no effect on the gastric glands when injected directly into the blood stream, it is possible that they may have an influence on the cells which line the cavity of the stomach, and that they may produce, in these cells, some other substance which is absorbed into the blood, and acts as a specific excitant of the gastric glands. A process of this nature is known to occur in the next segment of the ahmentary canal, viz. the duodenum, where it determines the secretion of the pancreatic juice and the bile. Edkins has carried out a series of experiments to determine whether such a chemical mechanism may not also account for the secretion of gastric juice, which is excited by the introduction of substances into the stomach. Edkins' experiments were carried out in the following way : The animal, dog or cat, having been anaesthetised, the abdominal cavity was opened, and a ligature passed round the lower end of the oesophagus so as to occlude the cardiac orifice and effectually crush the two vagus nerves. A glass tube was then introduced through an opening in the abdomen into the pyloric part of the stomach, and fixed in this position by a ligature tied tightly round the pylorus. The glass tube was connected by means of a rubber tube with a reservoir containing normal salt solution at the temperature of the body. By means of this reservoir, a certain amount of fluid was introduced into the stomach and kept there at a constant pressure ; the quantity of fluid intro- duced varied from 30 to 50 c.c. It has been shown by Edkins, as well as by von Mering, that no absorption of water or saUne fluid occurs in the stomach. It is therefore possible to recover the whole of the fluid an hour after it has been introduced, by simply lowering the reservoir below the level of the animal's body. If secretion of gastric juice has occurred into the cavity of the stomach, the fluid will be increased in amount, and will contain hydro- chloric acid as well as pepsin. In a series of control observations Edkins showed that the mere introduction of this fluid into the stomach caused no secretion of gastric juice, the fluid removed at the end of an hour having the same bulk and the same neutral reaction as the fluid which had been injected. Edkins then tried the influence of injecting substances into the blood stream. The injection of peptone, of acid, of broth, or of dextrin into the blood stream produced no secretion of gastric juice. If, however, in the course of the hour during which the fluid was allowed to remain in the DIGESTION IN THE STOMACH 695 stomach, a decoction made by boiling pyloric mucous membrane wdth acid, or with, water, or with peptone was introduced in small quantities every ten minutes into the jugular vein, the fluid removed at the end of the hour was found to be distinctly acid in its reaction and to possess proteolytic properties. The injection of these substances had therefore caused the secretion of a certain amount of gastric juice containing both hydrochloric acid and pepsin. In order to produce this positive effect, it was necessary to employ pyloric mucous membrane, extracts made by infusing or boiling cardiac mucous membrane with any of these substances being without effect. Edkins concludes therefore that the secondary secretion of gastric juice is determined, not, as Pawlow and Popielski imagined, by a local stimulation of the reflex nervous apparatus in the gastric wall, but by a chemical mechanism. The first products of digestion act on the pyloric mucous membrane, and produce in this membrane a substance which is absorbed into the blood stream, and carried to all the glands of the stomach, where it acts as a specific excitant of their secretory activity. This substance may be called the gastric secretin or gastric hormone. It is noteworthy that it is produced in that portion of the stomach where the process of absorption is most pronounced. The normal gastric secretion is therefore due to the co-operation of two factors. The first and most important is the nervous secretion, determined through the vagus nerves by stimulation of the mucous membrane of the mouth, or by the arousing of appetite in the higher parts of the brain. The second factor, which provides for the continued secretion of gastric juice long after the mental effects of a meal have disappeared, is chemical, and depends on the production in the pyloric mucous membrane of a specific substance or hormone, which acts as a chemical messenger to all parts of the stomach, being absorbed into the blood and thence exciting the acti^^ty of the various secreting cells in the gastric glands. It is still a moot point whether this gastric hormone is formed only in the pyloric mucous membrane, or whether it may not be also produced in the lower sections of the gut. Popielski has stated that the introduction of bouillon into the small intestine excites a secretion of gastric juice in animals, even after extirpation of the abdominal sympathetic plexuses and division of both vagi. On the other hand, introduction of the same substance into the large intestine has no influence on gastric secretion. Popielski ascribes this secretion again to a local reflex ; but it is more probable that the mechanism in this case is the same as that involved in the secretion which is excited by the presence of semi-digested food in the stomach itself. Pawlow has sho\vn that the second phase of the gastric secretion is largely influenced by the character of the contents of the stomach. Thus the inges- tion of large quantities of oil diminishes considerably the amount of gastric juice secreted, and Pawlow has suggested the administration of oil or oily foods as a possible remedy in cases where the production of gastric juice, and especially of hydrochloric acid, is in excess. It has long been imagined that the secretion of gastric juice was stimulated by the taking of alkahes. 696 PHYSIOLOGY This idea has been shown by Pawlow to be erroneous. Whereas the forma- tion of gastric juice is increased by the administration of acids, especially after a meal, it is largely diminished by the administration of alkahes such as sodium bicarbonate. In fact, sodium bicarbonate diminishes the activity of the digestive glands throughout the ahmentary tract, and can be used as a means of diminishing the secretion of gastric juice as well as of pancreatic juice. A further important question has been propounded by Pawlow, namely, whether there is any alteration in the constitution and amount of gastric juice with variations in the character of the food. So far as concerns the first phase of secretion, the psychical or ' appetite ' juice, this observer has shown that, whatever the previous diet of the animal, the juice always has the same characters, the same digestive power, and the same percentage of hydrochloric acid. He finds, however, that in the case of the second, or what we may call ' chemical ' secretion, i.e. that produced by local changes in the stomach, there is considerable variation in the nature of the juice. Whereas the secretion of juice is greatest in amount after a meal of meat, the digestive power of the juice is greatest after one of bread, and Pawlow regards these difierences in the juice as determined by the variations in the stimulus apphed to the gastric mucous membrane. It is doubtful, however, whether these results justify us in ascribing a number of specific sensibihties to the gastric mucous membrane. We have seen that the psychical juice depends merely on appetite, and therefore will be greater in amount the more welcome the food is to the animal. On the other hand, the juice secreted in the second phase must vary according to the quantity of gastric hormone produced in the pyloric mucous membrane, and therefore with the nature and amount of the substances produced in the prehminary digestion of the gastric contents by means of the psychic juice. The amount of juice may vary also with the salts contained in the food, according to their alkahne or acid character, and the percentage of pepsin in the juice may vary with the intensity of stimulus as well as with the quantity of fluid available for the formation of the gastric juice. These factors will co-operate in determining the characters of the whole juice secreted after any given meal, and it seems possible to explain the variations, observed on such different diets as meat and bread, without having recourse to the difficult assumption of a specific sensibihty of the gastric mucous membrane to such inert substances as dextrin or egg albumin. SECTION IV THE MOVEMENTS OF THE STOMACH These can be best studied by Cannon's method — that is, by direct observa- tion of the movements in a living unanaesthetised animal by means of the Rontgen rays. In order to make the shape of the stomach %asible, the food — bread and milk — is mixed with a quantity of bismuth subnitrate or bismuth oxychloride (Hertz). The presence of this salt does not interfere with the processes of digestion, but renders the gastric contents opaque to the Rontgen rays. On examining by this means the stomach of a cat which has just taken a meal, the whole of the food is seen to be hnng in the fundus. It is marked off by a strong constriction, the ' transverse band,' from the pyloric portion. In about twenty to thirty minutes faint waves of contraction begin a little to the cardiac side of the transverse band and travel slowly towards the pylorus. These waves succeed one another so that the pyloric part of the stomach may present a series of constrictions. Their effect is to force towards the pylorus the food which has been digested by gastric juice and detached from the surface of the mass in the fundus. The pylorus remaining closed, the food cannot escape, and therefore is squeezed back, forming an axial reflux stream towards the cardiac end. These contractions last throughout the whole period of gastric digestion, and become more marked as digestion proceeds. Their eff'ect is to bring the whole of the food in close contact wdth every particle of pyloric mucous membrane and to cause a thorough mixture of food and gastric juice. At varying periods after a meal, accord- ing to the nature of the food taken, the arrival of one of these waves of contraction at the pylorus causes a relaxation of the orifice, and a of gastric contents are 697 Fig. 3.37. Shadow sketches of the outlines of the stomach of a cat, imme- diately after a meal (11.0), at various intervals after- wards( 12.0,2.0.3.30,4.30). c, situation of o?sophageal opening ; yz, ' transverse band ' ; icr, junction of cardiac and pyloric por tions. (W. B. Caxxox.) few cubic centimetres squirted into the first part of the duodenum. 698 PHYSIOLOGY While these movements of the pyloric mill are going on, the cardiac portion of the stomach is exercising a steady pressure on its contents in consequence of a tonic contraction of its muscular wall, so that each successive portion of the food mass which is loosened by the digestive action of the gastric juice is forced on into the pyloric mill. As digestion proceeds the opening of the pylorus becomes more frequent. The stomach empties itself more and more, until finally the whole of the viscus has the shape of a curved tube (Fig. 337). Fig. 338. Sketch of human stomach, in erect position, shortly after a bismuth meal. (Hertz.) F, fundus ; F, umbilicus ; lA, incisura angularis ; PC, pyloric canal ; o, oesophagus. At the very end of digestion the pylorus may open to allow the passage even of undigested morsels of food. Very similar are the changes in the human stomach, as shown by Hertz. The term fundus is by him limited to that part of the stomach situated above the cardiac orifice (in the erect position). The hody of the stomach is marked off, more or less, by the incisura angularis on the lesser curvature corresponding to what we have called the transverse band. The pyloric portion consists of the pyloric vestibule and the pyloric canal, the latter being a tubular portion about 3-0 cm. in length, especially well marked in infants. When a small quantity of food has been swallowed (in the erect position) its weight is sufficient to overcome the resistance of the contracted gastric wall, and it rapidly passes to the pyloric part. Peristalsis begins almost at once, each constriction starting near the middle of the stomach, and deepening as it slowly progresses towards the pylorus (Fig. 338). About one inch from the pyloric canal it is so marked that part of the pyloric vestibule becomes almost completely separated from the rest of the stomach. THE MOVEMENTS OF THE STOMACH 699 The part thus cut off then diminishes in size in every direction, part of its contents being forced through the pyloric canal, while th3 remainder escapes back as an axial reflux stream into the stomach. The waves recur at regular intervals of fifteen to twenty seconds, and three or four are present simul- taneously. They continue without cessation until the stomach is empty — from one to four hours after the meal according to its bulk and composition. The foregoing descriptions apply especially to the events which succeed the taking of a considerable meal. If warm fluid alone, e.g. water, be swal- lowed, the opening of the pylorus occurs within a very short time after the fluid has reached the stomach. Thus if a large draught of water be taken to quench thirst it may arrive in the duodenum within a minute or two after being swallowed, and it is from the duodenum and small intestine that any absorption takes place. When a meal is undergoing digestion, there is a dis- tinct relation between the amount of acid present in the gastric contents and the opening of the pylorus. One may indeed say that acidity of the gastric contents exercises a direct inhibitory stimulus on the pyloric sphincter. These movements of the two portions of the stomach may be observed also on anaesthetised animals and even on a stomach which has been excised and placed in warm salt solution. They must therefore have their origin in the walls of the stomach itself. Although the co-ordination between the two parts of the stomach, between the tonic contractions of the fundus and the rhythmic contractions of the pyloric part, may be carried out by the local nervous system — Auerbach's plexus — situated between the layers of the muscular coat, it is probable that the advancing waves of contraction observed in the antrum are myogenic, i.e. directly originated in and deter- mined by the muscle fibres themselves. Cannon has shown that these movements persist after complete division of Auerbach's plexus by 2 to 6 circular incisions carried through the entire muscular coat of the stomach ; and it is evident that they do not partake of the nature of a true peristalsis, since they are not preceded by a wave of relaxation. The opening of the pylorus, on the other hand, which occurs at increasingly frequent intervals at the end of a wave, must be ascribed to a nervous mechanism. Although the local mechanism probably plays the greater part in this act of relaxation, the normal emptying of the stomach is also largely dependent on the integrity of the connection of this viscus with the central nervous system. If both vagus nerves be divided in a dog below the point at which they give off their branches to the lungs and heart, a large amount of food may remain in the stomach in an undigested condition. The secretion of gastric juice is deficient, and the opening of the pylorus is not easily carried out. Such dogs therefore tend to die of sapra?mia, being poisoned by the absorption of pro- ducts of putrefaction from the gastric contents. Pawlow has shown that animals can be kept alive for months after di^'ision of both vagi if a gastric fistula be made, the animals be carefully fed, and care be taken to wash out adherent non-digested portions of food from the stomach. The opening of the pylorus depends not only on intragastric events but also on the condition of the duodenum. It has been sho^^^l by Serdjukow 700 PHYSIOLOGY that the pylorus remains firmly closed so long as the contents of the duo- denum, are acid. If alkaline fluid be introduced into the stomach, this is rapidly passed into the duodenum. If, however, some acid be introduced at the same time into the duodenum by means of a duodenal fistula, the pylorus remains firmly closed, and no fluid passes into the duodenum until the acid LSyCh G.SpM L.Sp.N Fig. 339. Distribution of the vagus in the abdomen of the dog. _(M. H. Naylor.) RV, LV, right and left vagi. The right vagus runs behind the oesophagus (Oe) and stomach (St), and in those places is represented by a discontinuous line. Cb, connecting branch between right and left vagi ; P, pancreas ; Dd, duodenum ; FD J, flexura duodeno-jejunalis ; I, I, I, intestine ; L, liver ; K, kidney ; A, suprarenal capsule ; RG, LG, right and left cura of diaphragm ; L.Sy.Ch, left sympathetic chain ; 12 D, 13 D, twelfth and thirteenth dorsal ganglia; 3 L, third lumbar ganglion; G.Sp.N, L.Sp.N, great and small splanchnic nerves ; S.G, left semilunar and superior mesenteric ganglia ; D.A, dorsal aorta. which was placed there has been neutralised by the secretion of pancreatic juice and succus entericus. We have probably in the walls of the ahmentary canal a local nervous mechanism for the movements of the pyloric sphincter. This may be played upon by impulses starting either in the stomach or in the duodenum, probably by the contact of acid with the mucous membrane. Increasing acidity on the side of the stomach causes relaxation of the orifice, whereas acidity on the duodenal side causes contraction of the pyloric THE MOVEMENTS OF THE STOMACH 701 sphincter. The exact parts, however, played in this mechanism by the local system and by the central nervous system respectively have not yet been thoroughly made out, though there is no doubt that these movements may proceed independently of any connection with the central nervous system. Stimulation of the peripheral end of the vagus nerves may exercise vary- ing effects on the stomach wall as well as on its sphincters. In the normal animal stimulation of the peripheral end of the vagus as a rule causes strong contractions of the oesophagus as well as of the cardiac sphincter. After the administration of atropine, stimulation of the same nerve will occasion dilatation of the cardiac sphincter. On both cardiac and pyloric portions of the stomach the vagus exercises inhibitory as well as augmentor effects. So far as concerns the musculature of the fundus or body of the stomach, the most usual effect is an inhibition during stimulation of the vagus succeeded by an augmented tonus immediately the stimulus is removed. If the vagus be excited a number of times the tonus of the muscular wall augments with each stimulus. On the pyloric portion stimulation of the vagus also causes inhibition, followed by contraction. The inhibition may, however, be very short and in rare cases altogether absent, so that during the excitation this inhibition is followed by a series of large rhythmic contractions. The pre- vaihng motor effect of the vagus therefore is in the fundus increased tonus, in the pyloric portion augmented peristaltic waves. On the pylorus itself we may obtain from vagal stimulation either increased or diminished contrac- tion. The conditions under which each of these may be evoked have not yet been definitely ascertained. Whether the splanchnic nerve, i.e. the sym- pathetic system, has a direct influence on the movements of the stomach has been disputed. According to Page May any effect produced by stimulation of this nerve, generally consisting in diminished motor activity, is probably due to the simultaneous influence on the vascular supply to the organ ; the blood-vessels being constricted, an artificial anaemia is produced which in itself is sufficient to account for diminished activity. Other observers regard the splanchnic as having an influence on the stomach similar to its action oa the intestine, and regard it as the chief inhibitory nerve to this organ. It is possible that the extent to which the stomach is brought under the control of the sympathetic system may vary in different species of animals. Carlson has shown that the ' pangs of hunger ' are associated with and probably due to rhythmic contractions of the stomach wall, which come on about meal time especially if this be delayed. INTESTINAL DIGESTION The products of gastric digestion, after being worked up in the pyloric half of the stomach, are passed at intervals into the first part of the duo- denum. Here they meet the secretions of three glands, namely, the pancreas, the liver, and the tubular glands of the intestine. In addition to these must be mentioned the secretion of Brunner's glands, which are situated at the very beginning of the duodenum. The glands of Brunner extend only over about half to one and a haK inches in the carnivora, such as the dog or cat, but in the herbivora they may be found occupying the upper six inches of the intestine. The secretion of these various juices is practically simultaneous and is aroused by the very act of entry of the acid chyme into the duodenum. Although they co-operate in their action on the food-stuffs, it will be con- venient to deal separately with each both as regards their action and the mechanism of their secretion. SECTION V THE PANCREATIC JUICE Pure pancreatic juice can be obtained either from an animal with a per- manent fistula or from one with a temporary fistula by the injection of secre- tin into the animal's veins. A flow of pancreatic juice may also be produced by the administration of pilocarpine. This drug acts, however, as a poison on many tissues of the body, not confining its action to the pancreas or even to the secreting glands. It is not to be wondered at therefore that the pancreatic juice obtained by its injection differs in quahty from that obtained by the more natural method of injection of secretin. The average com- position of pancreatic juice is shown in the Table on p. 703. It is a clear or shghtly opalescent fluid, strongly alkahne from the presence N N of sodium carbonate, its alkalinity varying between — and — NaaCOs. It is therefore about as alkaline as gastric juice is acid, and it will be found that equal quantities of gastric juice and pancreatic juice when added together practically neutrahse one another. The proteins of the juice may be roughly divided into three groups, a small amount of nucleo-protein precipitated on acidification, a protein coagulating at 55° C, and another at about 75° C. The juice tends to become poorer in proteins and richer in alkali as secre- 702 THE PANCREATIC JUICE 703 A B c Alkalinity : («) (^^) Number of c.c. — NaOH equal 1 j 12-7 12-4 9 5-5 to 10 c.c. juice I.e. in terms of Na in 100 c.c. Total solids in 100 c.c. J 0-2921 1-6 1 1-56 ( 0-2852 2-25 0-2587 1. { 0-1166 6-38 6-40 Total proteins in 100 c.c. . Ash in 100 c.c. ( I ( 1 0-5 1-00 \ 0-92 J 1-00 1-00 4-8 1-3 Chlorides in 100 c.c. . 0-28081 0-2966 ( — — 0-2695 Total nitrogen .... — — 0-735 A. Secretin juice from three dogs. Sp. gr. 1014. B. Secretin juice, specimen collected at beginning (a), and at end (6). C. Pilocarpine juice. tion proceeds. The concentrated juice obtained by injection of pilocarpiu, which may contain as much as 6 per cent, total sohds, is always considerably less alkahne than the more dilute juice got by injection of secretin. The most interesting and important constituents of the juice are its ferments or precursors of ferments. The juice on arrival in the intestine has, or develops, an effect on all three classes of food-stuffs, namely, proteins, fats, and carbohydrates, ACTION ON PROTEINS Although the digestive action of pancreatic juice on proteins was pointed out by Corvisart, little attention was paid to this action either by Claude Bernard or subsequent authorities until Kiihne subjected the action of extracts of the gland to a thorough investigation. The neglect of this action by Claude Bernard must be ascribed to the fact that he worked with pancreatic juice. It has been shown more recently that pancreatic juice as secreted is free from proteolytic effects, and that for the development of this power it is necessary that some change should be brought about in the juice itself, namely, a conversion of trypsinogen into trypsin. This change under normal circumstances is brought about directly the juice enters the gut, by the action of a substance — enterokinase — contained in the succus entericus. The pancreatic juice thereby acquires a proteolytic activity superior to that of any other digestive juice, so that the proteins of the food undergo a very thorough disintegration. The different constituents of the protein molecule show a varying resistance to the action of tr}^sin. The greater part of the molecule is rapidly broken down into its proximate constituents, namely, amino-acids, and the same change is undergone by the proteoses and peptones resulting from the gastric digestion of proteins. Within a few minutes therefore after the chyme has reached the small intestine a certain amount of 704 PHYSIOLOGY amino-acids will have been formed. Some of the groups present, however, a resistance to disintegration. After tryptic digestion for a few hours the mixture will be found still to contain a considerable quantity of peptone, which in consequence of its resistance to further alteration was designated by Kiihne ' antipeptone.' The antipeptone of Kiihne certainly included some of the diamino-acids, which at that time had not been isolated. There is always a part, however, which gives the biuret reaction and is only sKghtly broken down after the prolonged action of trypsin into the amino-acids. Even when the trypsin has acted for weeks and the biuret reaction has entirely disappeared, the mixture will be found to contain, in addition to the separate amino-acids, some members of the polypeptide class, composed of two or more molecules of amino-acid united together. One of these poly- peptides has been isolated by Fisher and Abderhalden from the products of tryptic digestion of the protein of silk, and has been found to contain glycine, alanine, and proHne. The stages therefore in tryptic digestion, e.g. on fibrin, may be set out as follows : (1) After one hour's digestion — soluble coagulable protein, deutero- albumose, peptone, amino-acids, with a small amount of alkah metaprotein produced by the action of the alkali of the juice. (2) After digestion for one day — deutero-albumose, ' antipeptone,' amino-acids, polypeptides. (3) After digestion for one month — amino-acids, polypeptides. Among the amino-acids tyrosine is one of the first to be spht off, and this substance, with leucine, was among the earhest known products of pancreatic digestion. The action of trypsin is thus seen to resemble very closely the action of boihng concentrated hydrochloric acid. Like the latter it attacks the protein molecule at the — CO — ^NH — couphng, introducing water at this point and therefore breaking up the polypeptide groupings into simple amino-acids. Why it always leaves a certain remnant of the polypeptides unattacked is not at present explained. The investigation of its action on the polypeptides has shown that very minute differences in the grouping of the molecule may determine whether or not the molecule is attacked by trypsin. Apparently it will only attack such molecules as are present in the naturally occurring proteins. Thus under the action of trypsin the following polypeptides undergo hydrolytic dissociation : alanyl glycine, alanyl alanine, alanyl leucine A ; while the closely similar polypeptides glycyl alanine, glycyl glycine, alanyl leucine B are left untouched. CONDITIONS OF TRYPTIC ACTIVITY Since the pancreatic juice is strongly alkahne it might be expected that trypsin would be most effective in an alkaline medium. It must be remem- bered, however, that the alkahne juice when secreted meets the correspond- ijigly acid contents discharged from the stomach, and that the resulting mixture is practically neutral. This neutrality exists throughout the small intestine, the reaction of the contents of the gut being similar to that of a fluid containing alkali which has been saturated by the passage of carbonic THE PANCREATIC JUICE 705 acid, viz. alkaline to such indicators as methyl orange, and acid to such indicators as phenolphthalein. On investigating the action of trypsin outside the body, it is found that, at any rate as concerns its earlier stages, this ferment is more active in the presence of sodium carbonate. It is usual to make up an artificial digestive mixture by dissolving commercial trypsin in 0-2 to 0-3 per cent, sodium carbonate. The optimum amount of sodium carbonate depends on the strength of the solution in trypsin : the more trypsin present the higher is the optimum amount of sodium carbonate. It is stated that although an alkahne reaction is more advantageous for the earlier stages of tryptic activity, the later stages take place best in a neutral medium. This result is probably due to the fact that trypsin in alkaline medium is extremely unstable, so that when prolonged digestions are carried out the trypsin would be rapidly destroyed if the medium were strongly alkaline. The destructibihty of trypsin, as well as its action, is largely affected by the presence of proteins or their digestion products in solution. Bayliss has adduced evidence to show that when trypsin acts upon protein it enters into some form of combination with the protein molecule. This combination protects the trypsin from the destructive action of alkali. The velocity of the reaction, which takes place under the influence of trypsin, gradually diminishes, owing probably to a combination of the trypsin' with the products of digestion, e.g. with the peptones or amino-acids, and its consequent removal from the sphere of action. If by any means the amino- acids be removed the action of the trypsin is renewed. This destruction of ferment occurs in the intestine itself. If the intestinal contents be collected by means of a fistula at the lower end of the ileum, they show little or no proteolytic activity. The trypsin is therefore an extremely active ferment which carries out its function of protein hydrolysis at the upper part of the gut and is destroyed before reaching the lower end. THE ACTIVATION OF PANCREATIC JUICE It was observed by Kiihne that extracts of the fresh pancreas did not develop their full activity for some considerable time, the development being aided by prehminary treatment with a weak acid. When a pancreatic fistula is made according to Pawlow's method, the juice obtained always presents some proteolytic activity. It was shown by Pawlow and Chepo- walnikofE that the development of the activity of the juice was due to the action of a constituent of the succus entericus which they named enterohinase, and it has since been found that if care be taken to avoid contact of the juice with the mucous membrane surrounding the orifice of the duct, it is, when secreted, entirely inactive. The enterokinase acts like a ferment on a body, trypsinogen, present in the juice as secreted, converting this into trypsin. Pawlow therefore called this body the ' ferment of ferments.' This view of the action of enterokinase lias been challenged, especially by Delezcnne, according to whom there is an actual combination between the enterokinase and the trypsinogen. trypsin itself being a mixture or combination of the two bodies. He compared the reaction to that of the h.vmolvsins, which, as is well knowii. involve in 23 706 PHYSIOLOGY their action the co-operation of two bodies, the amboceptor and the complement. If this "were correct there should always be a proportionality between the quantities of trypsinogen and enterokinase respectively which are necessary to form trypsin. It has been shown by Bayliss and Starling that this proportionality is not present. The smallest quantity of enterokinase is sufficient to activate any amount of trypsinogen if sufficient time be allowed. The effect of increasing or diminishing the amount of enterokinase is not to alter the total amount of trypsin finally produced, but merely the time taken for its production. This behaviour characterises a ferment, and we may therefore conclude that the view originally put forward by Pawlow is correct, namely, that trypsin is produced from trypsinogen under the action of a ferment enterokinase. If pancreatic juice be allowed to stand, even with the addition of toluol to prevent bacterial infection, it gradually acquires a certain degree of activity. If, however, sodium fluoride be used as an antiseptic the juice remains permanently inactive. The spontaneous activation of the juice may be hastened by neutralisation. The most potent means next to enterokinase is the addition of lime salts. If a few drops of 10 per cent, calcium chloride solution be added to fresh pancreatic juice, the calcium being in such a quantity as to suffice to combine with all the carbonate present in the juice, complete activation of the juice occurs within a couple of days, no further increase in its digestive powers being obtained on subsequent addition of enterokinase. It has been suggested that the action of calcium is in some way to assist in the pro- duction of an enterokinase from some precursor of this body already present in the juice. According to Mellanby, the calcium acts simply by neutralising the juice and thus allowing minute traces of enterokinase already present in the juice to exert their effect. It is not likely that this calcium activation plays any part in the normal pro- cesses of digestion, since for its completion it needs twelve to sixteen hours, whereas the enterokinase present in the succus entericus will effect the activation of the juice within a few minutes. THE ACTION OF PANCREATIC JUICE ON MILK On tlie addition of pancreatic juice to milk a clot is produced which speedily redissolves. If re-solution takes place too rapidly the production of a formed clot may be missed. In every case, however, on heating the milk a few minutes after the addition of the trypsin a clot is obtained. How far this action is to be ascribed to the proteolytic ferment trypsin, or how far it is due to the presence of a free rennet-like ferment in the juice, is not yet definitely settled. Since the rennet action is parallel to the proteolytic activity of the juice, it is probable that we must regard the clotting of milk as the first stage in its proteolysis. THE ACTION OF PANCREATIC JUICE ON CARBOHYDRATES The pancreatic juice, as well as fresh extracts of the pancreas itself, contains a strong amylolytic ferment, diastase, amylase, or amylopsin. If a few drops of pancreatic juice be added to a 1 per cent, solution of boiled starch, within a few seconds the solution clears, and in half a minute, on the addition of iodine, a red colour is obtained, showing the presence of erythro- dextrin. At the end of a few minutes no colour is obtained with iodine, and the solution contains maltose. The stages in the hydrolysis of starch brought about with pancreatic juice are exactly similar to those effected by ptyalin. If the juice be neutralised, the process of hydrolysis goes on to the formation of dextrose or glucose. This further conversion is due to THE PANC!REATIC JUICE 707 the presence in the juice of a second ferment — maltase — which converts the disaccharide maltose into the monosaccharide glucose. The juice in the gut is therefore able to effect the further digestion of the products of salivary digestion. On the other disaccharides pancreatic juice is without effect. It contains no invertase, nor does it, in spite of certain statements to the contrary, ever contain lactase. It has therefore no effect on either cane sugar or milk sugar. THE ACTION OF PANCREATIC JUICE ON FATS Fresh pancreatic juice contains a strong lipase or fat-sphtting ferment, by means of which, in the presence of water, neutral fats, e.g. the triglycerides of palmitic, stearic, and oleic acids, are broken up into glycerin and the corre- sponding fatty acids. This ferment is active either in alkaline, neutral, or very slightly acid reaction. If the reaction be alkahne the fatty acids produced by the lipolysis combine with the alkali present with the formation of soaps. The ferment may be obtained from extracts of the fresh gland, but is rapidly destroyed if active trypsin be present. It is also contained in some of the dried commercial preparations of trypsin. It is apparently insoluble in distilled water, and is therefore found in the residue after extracting these commercial preparations with water. It is easily soluble in glycerin. The velocity with which lipolysis occurs is much increased (four to five times) by the addition of bile. This adjuvant action of bile is not destroyed by boihng, and is due entirely to the bile salts. These act in two ways. In the first place, by their physical qualities they diminish the surface tension between water and oil, so enabling a closer contact to be effected between the watery solution contained in the juice and the oil which is presented to it. Moreover they may aid in the solution of the ferment itself. In the second place, bile salts have a solvent action on soaps as well as on fatty acids in slightly acid medium. Bile may be regarded therefore as a favourable excipient or medium for the interaction of the lipase and the neutral fats. The lipase of pancreatic juice will also hydrolyse the esters of the fatty acids, such as ethyl butyrate or monobutyi'in. On the phosphorised fats or phosphatides, such as lecithin, its action is still a subject of doubt. According to certain authors extracts of the pancreas have the power of splitting off chohne from lecithin. It is not known whether the same property is present in pancreatic juice itself, or whether any other dissociations are brought about in the complex molecule of lecithin under the action of this digestive fluid. THE SECRETION OF PANCREATIC JUICE In order to study the relation of the secretion of pancreatic juice to the other processes of digestion, observations must be carried out on an animal with a permanent pancreatic fistula. Such a fistula was established bj" Claude Bernard hj bringing the duct of the pancreas to the surface and inserting into it a lead or silver tube. The arrangement was, Jiowever. imsatisfactory since after a few days the tube dropped out and the natm-al course of the duct from pancreas to intestine was restored. In order to avoid the disadvantages 708 PHYSIOLOGY of this proceeding Heidenhain and Pawlow independently devised another method to enable us to determine the causes of pancreatic secretion. The pancreas in most cases possesses two ducts, the upper one opening along with the bile duct, the lower one a short way down. The relative sizes of these two ducts vary in different animals, the lower one being larger in the dog, while in man and the cat the upper one is larger. In order to establish a pancreatic fistula in a dog, a small quadrilateral piece of the duodenal wall is exsected, having the papilla of the lower duct opening in the middle of its mucous siu-face. The integrity of the gut is restored by suturing in a single line of stitches the margins of the woimd in the duodenum, and the exsected piece is brought to the surface and stitched in the middle of the abdominal wound. The greater part of the pancreatic secretion will escape by the fistula, and can be collected either by the insertion of a cannula into the duct or by attaching a glass funnel below its orifice. Great care has to be taken in the after treatment of such animals. The pancreatic juice, which flows over the papilla, acquires in so doing strong proteolytic powers, and tends therefore to dissolve and irritate the adjacent abdominal wall. This can be prevented by taking care to collect all the juice, and to allow none to flow away over the surface of the body. Another drawback is that the continual loss of pancreatic juice in many cases seriously affects the animal's health. This may be obviated to a certain extent by keeping the animal on a milk diet with the addition of sodium bicar- bonate to replace the loss of this salt by the juice. With great care Pawlow has succeeded in keeping such animals in a perfectly healthy condition. In the fasting condition there is, as a rule, no secretion of juice, though the escape of a few drops may be observed at long intervals. If a meal be administered to the animal, a flow of juice begins in one to one and a half minutes. From this time there is a steady, slow rise of the rate of secretion, which lasts for two to three hours, and then gradually diminishes. The greatest increase in flow is observed at the time when the first portions of digested food escape from the stomach into the duodenum. The secretion must therefore be determined in some way by the entry of this food into the duodenum. By experiments on dogs possessing a gastric as well as a pancreatic fistula, it has been shown that the introduction of acid, e.g. 0-4 per cent. HCl, into the stomach evokes, as soon as it passes into the duodenum. a rapid flow of pancreatic juice. A similar, but smaller, effect is produced by the passage of oil from the stomach into the duodenum. The introduction of alkahes is without effect. Weak acids are also effective exciters of secretion if they be introduced directly into the duodenum itself or into a loop of small intestine. The effect gradually diminishes as the loop which is chosen comes nearer to the caecum, and as a rule the injection of dilute acid into the lower foot or eighteen inches of ileum is without effect on the pancreas. The striking resemblance between the secretion thus evoked and that produced in the salivary glands by injection of acid into the mouth suggests that we have here to do with a reflex of the same kind as that which affects the salivary glands. In the search for the channels of this reflex Heidenhain showed that stimulation of the medulla oblongata occasionally produced a flow of pancreatic juice. He was unable, however, to obtain any' secretion on stimulation of the vagus nerve. The pancreas receives fibres from the vagi as well as from the splanchnic nerves (sympathetic system). According to Pawlow the ill success of Heidenhain's experiments was due to the fact that in any operation a gland is played upon by reflex impulses THE PAXCREATIC JUICE 709 partly of au inhibitoiy, partly of a secretory nature, iu which the iuhibitory predominate, and by the further fact that the pancreas is extremely susceptible to alterations in its blood-supply, so that any stimulation of the vagus which caused inhibition of the heart would ipsojacto prevent the effect of simultaneous excitation of secretory fibres from making its appearance. Pawlow noticed that if in an animal Avith a permanent fistula the vagus on one side were cut and left for four days in order to allow the cardio-inhibitory fibres to degenerate, repeated stimulation of the peripheral end of the nerve evoked a flow of pancreatic juice. He obtained the same results by stimu- lating this nerve below the point at which it had given ofE its cardio-inhibitory fibres, in animals in which the reflex inhibitions from the operation itself were prevented by total section of the medulla. Under certain circumstances he obtained also secretion on stimulation of the splanchnic nerves, and there- fore concluded that these two nerves — splanchnics and vagi — were the efferent channels in the reflex secretion set up by the introduction of the acid into the duodenum. It was shown later, however, independently, both by Popielski, a pupil of Pawlow, and by Wertheimer, that the injection of acid into a loop of small intestine was followed by secretion of juice even after section of both vagi and destruction of the sympathetic gangha at the back of the abdominal cavity. On repeating these experiments Bayhss and Starling found that a secretion of juice was produced even when the acid was introduced into a loop of the small intestine entirely freed from any possible nervous connections with the rest of the body. It was evident therefore that the stimulus or message from the intestine to the pancreas which causes the secretion of the latter must be carried, not by the nervous system, but by the blood stream. Since the injection of acid into the portal vein was without effect on the pancreas, it was concluded that something must be produced in the epithehal cells of the gut under the influence of acid, and that this product of the epithehal cells was absorbed in the blood stream and was the active agent in exciting the pancreas. On pounding up some scrapings of the intestinal mucous membrane vnth dilute hydrochloric acid and filtering, and injecting the filtrate, a copious flow of pancreatic juice was produced. This chemical messenger or hormone from the intestine to the pancreas is called ' secretin,' or ' pancreatic secretin ' to distinguish it from possible other members of the same class. It is produced in the mucous membrane from a precursor— pro-secretin. The latter has not been isolated, but that it is present in the mucous membrane is shown by the fact that secretin can be extracted by the action of acids from mucous membrane which has been killed by heat or by the prolonged action of alcohol. Secretin itself is not a ferment. In order to prepare it the mucous membrane is ground up with sand, boiled with O-i per cent, hydrochloric acid, and then neutraHsed while boihng by the cautious addition of sodium hydrate. The coagulable proteins are in this way precipitated, and the filtered solution contains the secretin. It is not precipitated by the ordinary alkaline reagents, and diffuses slowly through animal membranes. Though stable in acid solutions, it is very rapidly destroyed iu alkahne or neutral 710 PHYSIOLOGY solutions, especially under the influence of bacteria. It is apparently oxidised with extreme ease. A similar, or more probably the same, body may be produced from intestinal mucous membrane by treating this with solutions of soap. In this secreting mechanism we have a very striking example of a correlation between the activities of two different portions of the body efiected by chemical means. The strongly acid chyme enters the first part of the duodenum. Immediately a certain amount of secretin is produced by the acid in the cells of the mucous membrane. The secretin is carried by the blood stream to the cells of the pancreas and excites there the secretion of strongly alkahne pancreatic juice. As soon as sufficient juice has been secreted to neutralise the acid chyme the formation of secretin, and therefore the further secretion of pancreatic juice, comes to an end. If the stomach still contains food the process is, however, renewed in virtue of the local reflex mechanism which we have just studied regulating the opening and closure of the pylorus. So long as the contents of the duodenum are acid the pylorus remains firmly closed. As soon as these are neutralised the pylorus relaxes and allows the entrance of a further portion of acid chyme. Thus the formation of secretin proceeds afresh, and the whole chain of processes goes on until the stomach is empty and all its contents have passed into the intestine. In view of the efficacy of this chemical reflex mechanism, the question arises whether the results first obtained by Pawlow were really due in some way to the formation of secretin. Stimulation of the vagus may cause con- traction of the stomach, opening or closing of the pylorus, and it seems possible that under its action there might have been an escape of acid gastric contents into the intestine, and therefore the formation of secretin, which would suffice to arouse the pancreatic secretion. Later experiments by this observer, in which the escape of any gastric contents was effectively pre- vented by ligature of the pylorus while the stomach itself contained an alkahne solution, have shown that even with these precautions a flow of juice may be obtained on stimulation of the vagus nerve. The flow, however, is very small in comparison with that obtained by injection of secretin, and one must conclude that, although the nervous system may play a small part in the excitation of the activity of this gland, the main factor involved is the chemical mechanism which has just been described. The amount of pancreatic juice obtained after a meal varies with the nature of the latter. The Table on p. 711 represents the results obtained on an animal fed with 60O c.c. of milk, 250 grm. of bread, and 100 grm. of meat respectively. The difierences between these results seem, however, largely determined by the duration of gastric digestion, and therefore the amount of acid secreted in the stomach and passed on to the duodenum. It was suggested by Walther that apart from this quantitative adaptation there was a quahtative alteration in the constitution of the juice according to the nature of the food ingested, that, e.cj., excess of protein causes an increase of the trypsin, THE PANCREATIC JUICE 711 while excess of carbohydrate would cause an increase in the amylase of the juice. Later researches have failed to confirm this view. Apparently when the pancreas is excited to secrete, it turns out its various ferments in constant proportion, depending on the amounts of these already present and stored up in the gland. Secretion of Pancreatic Juice (Walther) Hours after meal 600 c.c. milk 250 grm. bread 100 grm. meat 1 8-5 C.C. 36-5 c.c. 38-75 c.c. 2 7-6 C.c. 50-2 c.c. 44-6 c.c. 3 14-6 C.C. 20-9 c.c. 30-4 c.c. 4 11-2 c.c. 14-1 c.c. 16-9 c.c. 5 3-2 c.c. 16-4 c.c. 0-8 c.c. 6 1-0 c.c. 12-7 c.c. — 7 — 10-7 c.c. ♦ — 8 — 6-9 c.c. — THE STRUCTURAL CHANGES IN THE PANCREAS WHICH ACCOMPANY SECRETION The ease vnih which secretin may be prepared and used to arouse the activity of the pancreas has rendered it possible to study more closely the changes which in this gland accompany activity. Kiihne and Sheridan Lea succeeded in observing the gland of the rabbit in a living state under the A B Fig. 340. A terminal lobule of the pancreas of the rabbit. (Kuhne and iSheriban Lea.] A, in resting condition ; b, after active secretion. microscope. They noted that activity, excited by pilocarpine, was associated with a discharge of granules, a clearing up of the cells, and a diminution in size and the appearance of a lumen to the gland alveoli (Fig. 340). A normal resting gland is of an opaque, yellowish- white colour, and of firm consistence. On section it is seen to consist of numerous secreting alveoli which open into narrow intercalary tubules, and these in their turn into wide collecting tubules. The lining epithelium of the intercalated tubules is often con- tinued into the secreting part, where they lie internal to the secreting cells, as the so-called centro-acinar cells. The secreting cells themselves present 712 PHYSIOLOGY two well-marked zones, a narrow peripheral zone in which the nucleus is embedded, which is strongly basophile, and a central part which is turned towards the lumen, occupying two- thirds or three-quarters of the cell, and is closely packed with highly refractive granules strongly acidophile and presumably containing or composed of the precursors of the various con- FiG. 341. Alveoli of dog's pancreas. (Babkest, Rubaschkin and Saavitsch.) A, resting ; b, after moderate secretion with discharge of granules. stituents of the pancreatic juice (Fig. 341). If the activity of the gland be aroused by injection of secretin and the injection be contimied until the rate of secretion evoked by each injection diminishes considerably, i.e. the gland shows signs of fatigue, marked changes are observed both macroscopically and under the microscope. The gland is now pink and transparent in appear- ance, moist and soft in consistence. On section the lumen of each alveolus is enlarged, the cells are shrunken, and the granules are found to lie only along the border of the cell turned towards the lumen, the rest of the cell, which is much reduced in size, being made up of the basophile protoplasm. Similar effects are observed after long continued stimulation of the vagus (Fig. 341 B). SECTION VI THE LIVER AND BILE The liver, the largest gland in the body, is, Uke the other glands associated with the ahmentary tract, formed in the embryo by an outgrowth of the hypoblast Uning the ahmentary canal. At first it resembles in structuie other secreting glands, such as the pancreas, being composed of branch tubules which pour their secretion into a common duct. In the adult, however, the relation of the Uver cells to the ducts is entirely subordinated to their relation to the blood-vessels of the liver, and it requires special histological methods to make out the relations between the hver cells and the bile-ducts. The liver, on section, is seen to be divided off into lobules composed of columns of polygonal cells, radiating from the centre hke the spokes of a cart-wheel. The portal vein, which drains the blood from the ahmentary canal, breaks up into branches which lie at the periphery of the lobules, forming the interlobular veins, and send off numberless capillaries which pass inwards between the columns of cells to join the intralobular vein lying at the centre of the lobule. From the intralobular the blood passes by the large sublobular vein into the hepatic veins and inferior vena cava. In an injected specimen it is easy to see that every hver cell is connected with at least one blood capillary, and the Hver thus forms a blood gland, lying as it does at the gate of entrance of blood from the alimentary canal into the general circulation. The portal vein conveys only venous blood to the hver. In order to supply oxygen to the working Uver cells, this organ receives a second supply of arterial blood by the hepatic artery derived from the coehac branch of the aorta. The branch of the hepatic artery runs with the branches of the portal vein in the connective tissue of Glisson's capsule surrounding the lobules, and breaks up into capillaries which are in free communication with the capillaries derived from the portal system and pour their blood finally into the hepatic vein. As might be expected from its structure, the secretory functions of the liver represent but a small proportion of its activities in the body. The liver is, in fact, the greatest chemical factory of the body, receiving by the portal vein the products of digestion as they are absorbed from the alimentary canal. It converts these into other substances, breaking them down or building them up according to the needs of the body as a whole. Thus, when carbohydrates are being absorbed in quantity it converts the glucose contained in the portal blood into glycogen which it stores up, reconverting 713 23* 714 PHYSIOLOGY the latter into glucose and letting it loose into the circulation when this sub- stance is required by the body tissues. In the complete absence of carbo- hydrate from the food the liver may, as we shall see later, actually convert the products of protein digestion into sugar. In the same way the liver plays an important part in the metabohsm of proteins and of fats, so that its functions will have to be dealt 'odth in the various chapters concerned with the fate of the difierent food-stuffs and difierent constituents of the animal body. In this chapter we are merely concerned wth its action as a secreting gland. The fact that its secretion is in so many animals poured into the intestine by an orifice common to it and the pancreatic juice suggests that these two fluids co- operate in their actions on the ingested food-stuffs, and points to a direct use of the bile in the processes of digestion. In addition to this function, the bile must also be regarded as an excretion, representing as it does the channel by which the products of disintegration of haemoglobin — the red colouring- matter of the blood — are got rid of from the organism. As an ex- cretion the production of bile must be continuous, and related, not to the processes of digestion, but to the intensity of destruction of the red corpuscles. On the other hand, bile, as a digestive fluid, is needed in the gut only during the period that digestion is going on. The exigencies of the body therefore require a continuous excretion of bile by the liver, but a discontinuous entry of this fluid into the small intestine. This discontinuity in the entry of a continuous secretion into the intestine is secured, in the majority of animals, by the existence of the gall-bladder, a diverticulum from the bile-ducts, in which all bile, secreted during the intervals between the periods of digestive activity, is stored up. In the horse, where the gall-bladder is absent, its place is taken to some extent by the great size of the bile ducts. Moreover in such an animal the process of digestion is much more continuous in character than it is in carnivora. Since the bile accumulates in the gall- bladder during the whole time that digestion is not going on, and is only poured into the gut during digestion, in a fasting animal the gall-bladder is distended, whereas in an animal some hours after a meal the gall-bladder is practically empty. During the period that the bile remains in the gall-bladder it under- goes certain changes, as is shown by comparison of the composition of bile obtained from the gall-bladder with that obtained from a fistula of the bile-duct. Analyses of Bile (Human) From a biliary fistula (Yec ) and Herroun) in Prom the gt ill-bladder (Hoppe- .?eyler) in 100 parts 100 parts Mucin and pigments 0148 Mucin . 1-29 Sodium taurocholate 0055 Sodium taurochohxte . 0-87 Sodium glycocholate 0 165 Sodium glycocholate . . 303 Cholesterin . .0-038 Soaps . 1-39 Lecithin Cholesterin . 0-35 Fats Lecithin . . 0-53 Inorganic salts 0-840 Fats . . 0-73 Water . 98-7 THE LIVER AND BILE 715 During its star in the bladder the bile is concentrated by the loss of water and by the addition to it of mucin or nucleo- albumen, derived from the cells lining the bladder. Of the other constituents of bile, the pigments must be regarded simply as waste products, and an index to the disintegration of haemoglobin. Their mode of origin will be discussed in deahng with the history of the red blood corpuscles. They pass into the intestine and are there converted by the processes of bacterial reduction into stercobilin, which is excreted for the most part with the fseces, a small proportion being ab- sorbed into the blood-vessels and turned out in a more or less altered condi- tion as the pigments of the urine. From the point of view of digestion, the important constituents of bile are the bile salts, with the lecithin and cholesterin held in solution by these salts. The time-relations of the secre- tion, as well as of the flow of bile into the intestine in connection "u-itli the processes of digestion, can be learnt from animals in which the bile is conducted to the outside of the body by means of a permanent fistula. For this purpose Pawlow has devised the following operation : In the dog the abdomen is opened, and the common bile-duct sought as it passes through the intestinal wall. The orifice of the duct, with a piece of the surroimding mucous membrane is cut out of the wall of the intestine, and the aperture thus made sutured. The ex- cised portion of mucous membrane, with the opening of the duct, is then se'mi on to the surface of the duodenum, and the loop of duodenum at this point is stitched into the abdominal woimd. After healing, the natural orifice of the bile-duct is thus made to ojien on the surface of the abdomen. In an animal treated in this way the flow of bile from the fistula is found to run parallel to the pancreatic secretion. Although smaller in amount, it rises and falls \-vith. the latter. Thus a meal of meat produces a large flow of bile, a meal of carbohydrates only a small flow. Moreover, beginning almost immediately after taking food, it attains its maximum ^^^th the pancreatic juice in the third hour and then rapidly declines. In the production of this flow of bile two factors may be involved : (1) the emptying of the gall-bladder ; (2) an increased secretion of the bile. In order to determine the relative importance to be ascribed to each factor, we must compare the results obtained on an animal possessing a Pawlow fistula with, those obtained on an animal provided with a fistulous opening into the gall-bladder, the common bile-duct in the latter having been ligated to ensure that the total secretion of bile passed out by the fistula. In such animals we find, as we should expect, that the secretion of bile is a continuous process, but that, synchronously with the great outpouring of bile into the intestine during the third hour after a meal, there is an increased secretion of this fluid. The passage therefore of the semi-digested food from the stomach into the duodenum causes not only a slow contraction and emptying of the gall-bladder but also an increased secretion of bile by the liver. What is the mechanism involved in the production of these two eft'ects ? The muscular wall of the gall-bladder, as has been shown by Dale, is imder the control of nerves derived both from the vagus and from the sympathetic, the former conveying motor and the latter inhibitory impulses. It is usual to suppose that the entry of acid chyme into the duodenum provokes reflexly 716 PHYSIOLOGY the contraction of the gall-bladder, but the exact paths and steps in this reflex act have not yet been fully determined. The increased secretion of bile, which is produced by the passage of the acid chyme through the pylorus, can be also evoked by the introduction of acid, such as 04 per cent. HC], into the duodenum, and occurs even after division of all connection between the liver and the central nervous system. Since the presence of bile is necessary for the full development of the activities of the pancreatic juice, and its secretion is initiated by the same sort of stimulus, i.e. acid apphed to the mucous membrane of the gut, the question naturally arises whether the mechanism for the secretion of bile may not be identical with that for the secretion of pancreatic juice. In order to decide this point we must make a temporary bihary fistula by inserting a cannula into the hepatic duct. A solution of secretin is then prepared from an animal's intestine. In making this solution we must be careful to avoid any contamina- tion by bile salts, which may possibly be adherent to the mucous membrane of the gut and would in themselves, on injection, evoke an increased secretion of bile. It is therefore better to extract the pounded mucous membrane with boihng absolute alcohol, until this fluid, evaporated into a small bulk, shows no trace of bile salts. The dried and powdered gut is then boiled with dilute acid. On injecting the solution of secretin so obtained into the animal's veins, an increased flow of bile is at once produced. In one experiment, for instance, the injection into the veins of 5 c.c. of a solution of secretin, pre- pared in this way, increased the secretion of bile by the liver from twenty- seven drops in fifteen minutes to fifty-four drops in fifteen minutes. The rate of secretion was therefore doubled. We may conclude that the mechan- ism, by which the increased secretion of bile is produced at the time when this fluid is required in the intestine, is identical with that for the secretion of pancreatic juice, and that in each case one and the same substance — secretin — ^is formed by the action of the acid on the cells of the mucous membrane, and, on absorption into the blood stream excites both the fiver and pancreas to increased activity. THE DIGESTIVE FUNCTIONS OF THE BILE Bile contains a weak amylolytic ferment. Its uses in digestion are dependent, however, not on the presence of this ferment, but on the peculiar action of the bile salts on the fermentative powers of the pancreatic juice. It was shown long ago by Williams and Martin that the amylolytic power of pancreatic extracts is doubled by the addition of bile or of bile salts. Pawlow has stated that the same holds good oi the proteolytic power of this juice. Most important, however, is the part played by the bile in the diges- tion and absorption of fats. The fat-splitting action of pancreatic juice is trebled by the addition of bile, whether boiled or unboiled. This quickening action of the bile probably depends, like its function in the absorption of fats, on the pecuhar physical properties of the bile salts, with those of the lecithin and cholesterin which they hold in solution. Not only does such a solution diminish the surface tension between watery and oily fluids, so THE LIVER AND BILE 717 promoting the closer approach of the lipase of the pancreatic juice to the fats on which it is to act, but it has also the power of dissolving fatty acids and soaps, including even the insoluble calcium and magnesium soaps. It is probable that it aids also in holding in solution, and bringing in contact with the fat, the Upase of the pancreatic juice. It has been shown by Nicloux that the lipase contained in oily seeds, such as those of the castor plant, is insoluble in water, but soluble in fatty media. The dried ferment obtained from the pancreas in many cases yields no hpase to water, but gives a strongly lipolytic solution when extracted with glycerin. The digestive function of bile therefore lies in its power of serving as a vehicle for the suspension and solution of the interacting fats, fatty acids, and fat- splitting ferment. This vehicular function plays an important part in the absorption of fats. These pass through the striated basilar membrane bounding the intestinal side of the epithelium, not, as was formerly thought, in a fine state of suspension (an emulsion), but dissolved in the bile in the form of fatty acids or soaps and glycerin. On the arrival of these products of digestion in the epithelial cells, a process of resynthesis is set up. Droplets of neutral fat make their appearance in the cells, whence they are passed gradually into the central lacteal of the villus and so into the lymphatics of the mesentery and into the thoracic duct. The bile salts thus released from their function as carriers are absorbed by the blood circulating through the capillaries of the villi, and carried by the portal vein to the Hver. On arrival they are once more taken up by the liver-cells and turned out into the bile. Owing to the fact of their ready excretion by the liver-cells, bile salts are the most reliable cholagogues with which we are acquainted. By this circulation of bile between liver and intes- tine the synthetic work of the liver in the production of the bile salts is reduced to a minimum, and it has oidy to replace such of the bile salts as undergo destruction in the alimentary canal, under the influence of micro- organisms, and are lost to the organism by passing out in the faeces as a gummy amorphous substance known as dyslysin. Further investigation is still wanted as to the exact method in which secretin acts on the liver-cells, and especially as to whether it actually excites in them the manufacture of fresh bile salts, or whether it simply hastens the excretion of such bile salts as have been formed by the spontaneous activity of the liver-cells or have arrived at them after absorption from the alimentary canal. Such questions can only be decided by studying the action of secretin on animals possessing a permanent biliary fistula. The effect of various diets on the secretion of bile has been studied by Barbera. He finds that, whereas the secretion of bile is greatest on a meat diet, it is somewhat less on a diet of fat, and is insigniticant on a purely carbohydrate diet. That is to say, the secretion of bile is greatest on those diets the digestion of which is attended by the passage of a large anioiuit of acid chyme or of oil into the duodenum. Oil is almost as efficacious as acid in promoting the production of secretin in the living duodenum, the production in this case being probably determined by the formation of soap from the oil, and tlie direct action of the soap on the prosecretin in the epithelial cells of the gut. SECTION VII THE INTESTINAL JUICE For the development of one of its most important properties, namely, that of proteolysis, the pancreatic juice is dependent on the co-operation of the intestinal juice or succus entericus. Besides this activating power on the pancreatic juice, the intestinal juice has numerous other functions to discharge in the digestion of the food-stuffs. In spite of the great similarity which obtains between the microscopic structure of the wall of the gut at different levels from duodenum to ileocohc valve, functionally there are many differences between the upper, middle, and lower portions of the gut. Speaking generally, we may say that, whereas the processes of secretion are better marked in the upper portions of the gut, the processes of absorption predominate in the lower sections, i.e. in the ileum. Much of the divergence in the statements which have been made with regard to the factors determin- ing secretion and absorption in the small intestine is due to the failure to appreciate this great difference between the activity of the mucous membrane at various levels. The process of secretion in the small intestine may be studied by isolating loops by means of ligatures, and determining the amount of secretion formed in these loops in the course of a few hours' experiment on an anaesthetised animal. Better results, however, may be obtained by establishing permanent fistulse. These fistulse are of two kinds. Thiry's original method of establishing a fistula consisted in cutting out a loop of intestine, and restoring the continuity of the gut by suturing the two ends from which the loop had been severed. The upper end of the loop itself is closed and the lower end is sutured into the abdominal woimd. For some purposes it is better to make a Tliiry-Vella fistula. In this case the continuity of the gut is restored as in the simple Thiry fistula, but both ends of the excised loop are left open and brought into the abdominal wound. In such a fistula it is easy to introduce substances into the upper end and determine the flow of juice from the lower end, the constant empty- ing of the loop being provided for by the peristaltic contractions of its muscular coat. In animals with intestinal fistulse a number of different conditions have been found to give rise to a flow of succus entericus, and so far no qualitative difference has been recorded between the upper and lower ends of the gut, A condition which will cause a free flow of juice from a fistula high up in the intestine mil generally cause a scanty flow from a fistula made from the ileum. In all cases it is found that a flow of juice is produced in consequence of a meal. If a dog with a fistula, which has been starved for twenty-four hours, be given a meal of meat, a flow of juice may begin within the next 718 INTESTINAL JUICE 719 ten minutes. The flow remains very slight for about two hours and then suddenly increases in amount during the third hour, corresponding thus very nearly to the flow of pancreatic juice excited by the same means. In this post-prandial secretion of juice it is not probable that the nervous system takes any very large share, though its intervention in the secretion has not been excluded by direct experiment. There are certain facts which .seem indeed to speak for an action of the central nervous system on the process of intestinal secretion, not in the direction of augmenta- tion, but in the direction of inhibition of secretion. Thus it has been observed, on many occasions, that extirpation of the nerve plexuses of the abdomen or section of the splancluiic nerves causes a condition of diarrhoea which may last for two or thrco days. This conchtion might be determined either by an increased motor activity of the gut, or by removal of inhibitory impulses normally arriving at the intestinal glands, Such a view receives support from an experiment first performed by Moreau. The abdomen of a dog is opened under an anaesthetic, and three contiguous loops of small intestine are separated by means of ligatures from the rest of the gut. The middle loop is then denervatcd by destruction of all the nerve fibres lying on its blood-vessels, as they course through the mesentery, care being taken not to injure the blood-vessels themselves. The loops are then replaced in the abdomen and the animal left from four to sixteen hours. On killing the animal at the end of this time, it is often found that the middle loop, i.e. the denervated loop, is distended vrith. fluid having all the properties of ordinary intestinal juice, whereas the other two loops are empty. A series of comparative experiments by Mendel and by Falloise have sIiotsti that the secretion begins about four hours after the operation, increases for about twelve hours, and then rapidly declines, so that at the end of two daj'S all three loops will be fomid empty. This has often been interpreted as due to the removal of inhibitory impulses passing from the central nervous system to the local secretory mechanism, and we have no direct evidence which can be adduced against such a view. It is possible, however, that the hyperaemia of the gut, which is produced by the process of denervation, may be sufficient to account for the increased formation of intestinal juice, since the hyperfemia will tend to pass off as the vessels recover a local tone. It is not possible to explain the flow of intestinal juice which follows a meal by any assumption of nervous impulses transmitted through the local nerve plexuses of the gut, since these have been divided in the making of the fistula. It we exclude a nervous reflex action by the long paths, namely, through the spinal cord and the sympathetic or vagus nerves, the flow which attends the passage of food into the first part of the duodenum must be excited by the formation of some chemical messenger. As to the existence of such a chemical messenger or hormone for the intestinal secretion there can be no doubt, but the evidence as to its precise nature is at present conflicting. It is stated by Pawlow that the most effective stimulus to the flow of succus entericus is the presence of pancreatic juice in the loop of gut. No evidence has yet been brought forward that injection of pancreatic juice into the blood stream will cause any flow of intestinal juice. On the other hand, Delezenne and Frouin have shown that in animals provided with a permanent fistula involving the duodenum or upper part of the jejunum, intravenous injection of secretin always causes a secretion of intestinal juice. In the upper part of the gut therefore the sinuiltaneous presence of the three juices necessary for complete duodenal digestion is 720 PHYSIOLOGY ensured by one and the same mechanism, namely, by the simultaneous activity of the secretin, produced in the intestinal cells by the action of the acid chyme, on pancreas, liver, and intestinal glands. A further chemical mechanism for the arousing of intestinal secretion has been de- scribed by Frouin. According to this observer, the flow of juice can be excited by intravenous injection of intestinal juice itself. Since this juice is alkahne, and does not produce any effect on the pancreas, it must be free from pan- creatic secretin. It would seem therefore that the flow of juice in the upper part of the gut, excited by the pancreatic secretin, causes also a production of a different secretin or hormone, which can be absorbed from the lumen of the gut, travel by the blood stream to other segments of the small intestine, and there excite a secretion in preparation for the on-coming food. Further experiments are, however, necessary on this point. The glands of the small intestine can also be excited by direct mechanical stimulation of the mucous membrane. The easiest method of exciting a flow of intestinal juice from a permanent fistula is to introduce into the intestine a rubber tube. The presence of the sohd object in the gut causes a secretion, and within a few minutes drops of juice can be obtained from the free end of the tube. The object of such a sensibihty to mechanical stimuh is obvious ; it is of the highest importance that the onward passage of any sohd object, especially if it be indigestible, shall be aided by the further secretion of juice in the portions of gut which are immediately stimulated. This mechanical stimulation probably acts on the tubular glands of the intestine through the intermediation of the local nervous system, the plexus of Meissner. It is stated by Pawlow that a juice obtained by mechanical stimulation differs from that produced by the introduction of pancreatic juice into the loop in containing httle or no enterokinase, so that the pancreatic juice excites the secretion of the substance which is necessary for its own activation. CHARACTERS OF INTESTINAL JUICE The intestinal juice obtained from a permanent fistula has a specific gravity of about 1010. It is generally turbid from the presence in it of migrated leucocytes and disintegrated epithehal cells. It contains about 1-5 per cent, total solids, of which 0-8 per cent, are inorganic and consist chiefly of sodium carbonate and sodium chloride. It is markedly alkahne in reaction, but less so than the pancreatic juice. The organic matter, besides a small amount of serum albumin and serum globuhn, includes a number of ferments, all of which are adapted to complete the processes of digestion of the food-stuffs commenced in the stomach and duodenum. Of these ferments two are concerned in proteolysis. Enterokinase we have already studied in detail. Possessing no action itself on proteins, it is a necessary condition for the development of the full proteolytic powers of the pancreatic juice. In addition to this ferment another ferment has been described by Cohnheim under the name ' erepsin.' Erepsin or some similar ferment is present in the fresh pancreatic juice and in almost all tissues of the body. It INTESTINAL JUICE 721 is distinguished by the fact that, although it has no power of digesting coagu- lated protein or gelatin, and only slowly dissolves caseinogen and fibrin, it has a rapid hydrolytic effect on the first products of proteolysis, converting albumoses and peptones into amino- and diamino-acids — their ultimate cleavage products. The other ferments of the intestinal juice are connected with the digestion of carbohydrates. In all mammals the intestinal juice is found to contain invertase, which transforms cane sugar into glucose and levulose or fructose, and maltase, which converts maltose into glucose. In young mammals, as well as in those in whom the milk diet is continued throughout fife, the intestinal mucous membrane also contains lactase, i.e. a ferment converting milk sugar into galactose and glucose. Such a ferment can be extracted from the mucous membrane of all young animals, but may be very slight or even absent in the intestines of older animals, when it is no longer needed for the ordinary processes of nutrition. By means of these three ferments, coming as they do after the digestion of the starches by the amylase of the saHva and pancreatic ju.ice, it is provided that all the carbohydrate food of the animal is transformed into a hexose, in which form alone carbohydrate can be taken up and assimilated by the cells of the body. The seat of origin of these various ferments has been the subject of special investigations by Falloise. Whereas secretin can be obtained from the whole thickness of the mucous membrane, and is probably therefore contained in the form of prosecretin in the epithelial cells covering the villi as well as in those lining the follicles of Lieberkuhn, a superficial scraping of the mucous membrane, which removes only the epithelial cells covering the villi with the adherent mucus and intestinal secretion, gives a much more active solution of entsro- kinaise than a deeper scraping. The most active solutions of enterokinase are, however, to be obtained from the fluid found in the cavity of the intestine after the injection of secretin. It seems therefore that enterokinase is not present as such in the epithelial cells, but is first produced in the proce?s of secretion and formation of the intestinal juice. The other ferments, namely, erepsin, maltase, invertase, and lactase, probably pre-exist as such in the epithelial cells, especially in those lining the tubular gland'^ of the gut, since pounded mucous membrane in water yields a solution of these ferments which is generally more powerful in its action than the succus entericus itself. So great is the difference, in fact, that many physiologists have suggested that the chief action of these ferments occurs, not in the lumen of the gut, but in the passage of the food-stufis through the epithelial cells of the small intestine on their way to the blood-vessels. SECTION VIII FUNCTIONS OF THE LARGE INTESTINE Great differences are found in the structure of the large intestine of different animals, differences which depend, not on the zoological position of the animal, but entirely on the nature of its food. In the carnivora the large intestine is short and narrow and possesses little or no caecum. In herbivora the large intestine is well developed with sacculated walls, and the csecum, i.e. that part of the large gut distal to the opening of the ileum into the colon, is very large. Man occupies a somewhat intermediate position be- tween these two classes. The differences between the total length of the ahmentary canal in various animals are largely determined by the varying development of the large intestine. The relation of these differences to the diet is seen if we compare the length of the intestine with the length of the animal. Thus in the cat the intestine is three times the length of the animal, in the dog from four to six times, in man from seven to eight times, in the pig fourteen times, and in the sheep twenty-seven times. The great develop- ment of the large intestine in vegetable feeders is due to the fact that in this class of food all the nutritious material is enclosed in cells surrounded by cellulose walls. In order that the food-stuffs — e.g. proteins, starch, &c. — may be dissolved by the digestive juices and absorbed by the wall of the gut, these cellulose walls must be disintegrated. In none of the higher verte- brates do we find any cellulose digesting ferment, cytase, produced in the ahmentary canal. The cellulose has therefore to be dissolved either by the agency of bacteria or by means of cellulose- dissolving ferments which may be present in the vegetable cells themselves. Thus in ruminants the masses of grass and hay are first received into the paunch, where they are kept warm and moist with saliva. In the paunch opportunity is thus given for the development of huge numbers of micro-organisms which can dissolve cellu- lose. From time to time portions of the sodden mass are returned to the mouth, chewed and then swallowed again to be subjected to the action of the proper digestive juices. In the horse and rabbit the chief part of the digestion of the cellulose occurs in the csecum. Even after abstinence from food for some time the caecum is still found to contain food material. In the caecum, under the action of bacteria, the cellulose is dissolved and the cells are opened up so as to allow their contents to escape. The products of digestion of cellulose include a number of organic acids, chiefly of the lower fatty acid series, as well as methane, carbon dioxide, and hydrogen. In the paunch the acids accumulate so that fermentation occurs in an acid medium, 722 FUNCTIONS OF THE LARGE INTESTINE 723 whereas in the caecum the acids are neutrahsed by the secretion of alkalies and the reaction remains practically neutral. The products of digestion of cellulose, as well as the contents of the vegetable cells set free by the solution of the cell walls, are gradually absorbed by the walls of the large gut. In carnivora the large intestine has very unimportant functions to discharge in digestion and absorption. The proteins of meat are practically entirely absorbed by the time that the food has arrived at the ileocolic valve, and the same applies to fat. In man the importance of the large intestine will vary with the nature of his food. Under the conditions of civilised life the food material is almost entirely absorbed by the time that it reaches the lower end of the ileum. If, however, a large quantity of vegetable food be taken, such as fruit or green vegetables, or cereals roughly prepared and coarsely ground, a considerable amount of material may reach the large intestine unabsorbed. A certain proportion of this may undergo absorption in the large intestine, while the rest will pass out with the faeces, increasing their bulk. It is hardly possible to speak of a secretion by the mucous membrane of the large intestine. In herbivora alkahne carbonates are secreted to neutrahse the acids produced in the bacterial fermentation of the food, but the processes of absorption and secretion keeping pace, there is no accumu- lation of the products of secretion in the intestine. A section of the mucous membrane shows a number of simple tubular glands. The greater number of the cells lining these glands are typical ' goblet ' cells and contain plugs of mucin. The secretion of mucus not only aids the passage of the faeces along the gut, but probably impedes the propagation of the bacteria which are present in comitless numbers in the faeces. This may accoimt for the fact that although bacteria are so numerous in the faeces, it is difficult to cultivate any large numbers most of them being dead. As an absorbing organ the large intestine of man is of little importance. From observations on fistulae in man it has been calculated that about 500 c.c. of water pass the ileocohc valve in the twenty- four hours. Of these about 400 c.c. undergo absorption in the large intestine. The absorption of any of the food substances by this part of the gut is much slower than that which takes place on introduction by the mouth. Feeding by nutrient enemata is thus always very inadequate. In some cases after the introduction of large enemata into the large intestine a certain amount may escape back- wards into the ileum and may there undergo absorption. The isolated large intestine of man is able to absorb only about six grammes of dextrose per hour and about 80 c.c. of water. If egg albumin or caseinogen solutions be introduced by the rectum no absorption can be detected after several hours. In observations extending over a considerable time some disappear- ance has been observed of proteins and emulsified fats, as well as of boiled starch. This was due, however, to the action of bacteria on these substances, and was probably of very little value for the nourishment of the individual. Feeding by nutrient enemata is thus merely a method of slow starvation. If it is employed it should be limited to administration of water, Salines, or solutions of glucose. 724 PHYSIOLOGY The chief value of the large intestine in carnivora and in civilised man would seem to be as an excretory organ, since it plays an important part in the excretion of lime, magnesium, iron, and phosphates. Lime salts are ex- creted partly with the faeces, partly in the urine. The path taken by the hme under different conditions varies with the character of the other con- stituents of the food. If phosphates are present in large quantities the greater part of the hme will be excreted by the large intestine and escape with the faeces as insoluble calcium phosphate. If acids be administered, such as hydrochloric acid, the amount of hme in the urine will increase, that in the faeces will diminish. Thus in herbivora normally only about 3 to 6 per cent, of the lime is excreted with the urine, whereas in carnivora with an acid urine the proportion leaving the body by this channel rises to 27 per cent. The excretion of magnesium is determined by very similar con- ditions. Its phosphates are somewhat more soluble than those of hme. In man about 50 per cent, of the magnesium leaving the body is contained in the urine, whereas the amount of lime in the faeces is ten to twenty times as much as that contained in the urine. It must be remembered that the whole of this difference is not due to excretion of hme into the gut, since a certain proportion of this substance may be precipitated as an insoluble phosphate or carbonate in the upper part of the small intestine and pass through the gut without undergoing absorption. The absorption of iron takes place in the duodenum and upper part of the jejunum. Only 1 or 2 milhgrammes appear in the urine, all the rest being excreted in the large gut and appearing in the faeces, chiefly as sulphide of iron. Of the acid radicals phosphates may pass out either with the urine or with the faeces, the exact path taken being determined by the relative amount of calcium and alkahne metals present in the food. If there is an excess of calcium most of the phosphates will leave with the faeces. The large intestine is the main channel of excretion for certain substances which cannot be regarded as normal constituents of the food, e.g. the heavy metals, such as bismuth and mercury. If bismuth be administered sub- cutaneously the faeces will be found to contain this substance, and the wall of the large intestine will be stained black from a deposit of sulphide of bismuth. This deposit stops short at the ileocohc valve. The excretion of mercury by the wall of the large intestine may account for the frequent presence of ulceration of this part of the gut in cases of poisoning by mercury. SECTION IX MOVEMENTS OF THE INTESTINES The movements of the intestines can be investigated either by observation of the exposed gut, or by the shadow method introduced by Cannon, in which the nature of the movements is judged from the shadows of food containing bismuth which are thrown on a sensitive screen by means of the Routgen rays. These movements have been the subject of experimental investigation for many years, but with varpng results. The great discrepancy which obtained between the statements of earher observers is due to the fact that they failed to exclude the many disturbing impulses which can play on any segmsnt of the gut, either reflexly through the central nervous system, or from other parts of the ahmentary canal itself through the local nervous system. In order to observe the normal movements of the gut, it is neces- sary to exclude the distm'bing influences due to reflexes through the central nervous system either by extirpation of the whole of the nerve plexuses in the abdomen, or by division of the splanchnic nerves, or by destruction of the lower part of the spinal cord from about the middle dorsal region. If the abdomen of an animal which has been treated in this way be opened in a bath of warm normal salt solution, so as to exclude the disturbing influence of drying and cooling of the gut, the small intestine wdll be seen to present two kinds of movements. In the first place, all the coils of gut imdergo swaying movements from side to side — the so-called pendular movements. Careful observation of any coil will show that these movements are attended with slight waves of contraction passing rapidly over the sui'face. If a rubber balloon, filled with air and connected with a tambour, be inserted into any part of the gut, it wU reveal the existence of rhythmic contractions of the circular muscle repeated from twelve to thirteen times per minute. By means of a special piece of apparatus (the ' enterograph ') it is possible without opening the gut to record the movements of either circular or longitudinal muscular coats ; and it is then found that both coats present rhythmic contractions at the same rate, the two coats at any point con- tracting synchronously. When the contractions are recorded by means of a balloon, the constriction which accompanies each contraction is seen to be most marked at the middle of the balloon, i.e. at the point of greatest tension, and the amphtude of the contractions is augmented by increasing the tension on the walls of the gut. These movements are unafl'ected by the direct application of drugs such as nicotine or cocaine, which we might 725 726 PHYSIOLOGY expect to paralyse any local nervous structures in the wall of the gut. Bayhss and Starhng concluded that these rhythmic contractions were myogenic,* that they were propagated from muscle fibre to muscle fibre, and that they coursed down the gut at the rate of about 5 cm. per second. Since, however, they may apparently arise at any portion of the gut which is subject to any special tension, it is not easy to be certain that a contraction recorded at any point is really propagated from a point two or three inches higher up. These contractions must cause a thorough mixing of the contents of the gut with the digestive fluids. Oii' examining under the Rontgen rays the intestines of a cat which has taken a large meal of bread and milk mixed with bismuth some hours previously, a length of gut may be seen in which the food contents form a continuous column. Suddenly movements occur in this ^ (P CD O CD CD a5 m Fii. 343. Diagram of the 'segmentation' (pendular) movements of the intestines as observed by the Rontgen rays, after administration of bismuth. (C'AiifNON.) I. A continuous column, intestinal movements being absent. 2. The column broken up into segments. 3. Five seconds later, each segment divided into two, the halves joining the corresponding halves of adjacent segments. 4. Condition (2) repeated five seconds later. column, which is split into a number of equal segments. Within a few seconds each of these segments is halved, the corresponding halves of ad- jacent segments uniting. Again contractions recur in the original positions, dividing the newly formed segments of contents and re-forming the segments in the same position as they had at first (Fig. 343). If the contraction is a continuous propagated wave, it is evidently reinforced at regular intervals down the gut, so as to divide the column of food into a number of spherical or oval segments. The points of greatest tension immediately become the points which are midway between the spots where the first contractions were most pronounced. The second contractions therefore start at these points of greatest tension, and divide the first formed segments into two parts, which join with the corresponding halves of the neighbouring segments. In this way every particle of food is brought successively into intimate contact * Magnus has shown that strips of the longitudinal coat, pulled off from the small intestine of the cat, may continue to beat regularly in oxj-genated Ringer's solution. He stated that these contractions only occurred if portions of Auerbach's plexus were still adherent to the muscle fibres, and concluded therefore that the rhythmic, like the peristaltic, contractions, were neurogenic. Gunn and Underbill however have obtained well-marked rhythmic contractions from strips of muscle entirely free from any remains of the nerve plexus, thus confu-ming the view enunciated above. MOVEMENTS OF THE INTESTINES 727 with the intestinal wall. These movements have not a translatory effect, and a column of food may remain at the same level in the gut for a considerable time. The onward progress of the food is caused by a true peristaltic contrac- tion, i.e. one which involves contraction of the gut above the food mass and relaxation of the gut below. If a balloon be inserted in the lumen of the exposed gut, it will be found that pinching the gut above the balloon causes an immediate relaxation of the muscular wall in the neighbourhood of the balloon. This inhibitory influence of the local stimulus may extend as much as two feet down the intestine towards the ileocaecal valve. On the other hand, pinching the gut half an inch below the situation of the balloon causes a strong continued contraction to occur at the balloon itself^^(Fig. 344). Fig. 344. Intestiiical contractions (balloon method). In this dog all the abdominal ganglia had been excised, and both vagi cut. Sho^ving proimgated effects oi mechanical stimulation, above and below the balloon. (1) pinch above, (2) pinch below, (3) pinch below balloon. Stimulation at any portion of the gut causes contraction above the point of stimulus and relaxation below the point of stimulus (the ' law of the intes- tines '). The same efiect is produced by introduction of a bolus of food, especially if it be large or have a direct irritating efiect on the wall of the gut (Fig. 345). In this case the contraction above and the inhibition below cause an onward movement of the bolus, which travels slowly down the whole length of the gut until it passes through the ileocaecal opening into the large intestine. The peristaltic contraction involves the co-operation of a nervous system. Whereas in the oesophagus it is the central nervous system which is involved, the peristaltic contractions in the small intestine occur after severance of all connection with the brain and spinal cord. On the other hand, they are absolutely abolished by painting the intestine with nicotine or with cocaine. They must therefore be ascribed to the local nervous system contained in Auerbach's plexus, which we can regard as a lowly organised nervous system ^vith practically one reaction, namely, that formulated above as the ' law of the intestines.' An anti- peristalsis is never observed in the small intestine. Mall has shown that, if a short length of gut be cut out and reinserted in the opposite direction, a species of partial obstruction results, in consequence of the fact that the peristaltic waves, started above the point of operation, cannot travel down- 728 PHYSIOLOGY wards over the reversed length of gut. The intestine above this point there- fore becomes dilated. If however the reactions of the local nervous system be paralysed or inhibited, a reflux of intestinal contents is quite possible, since the contractions excited at any spot by local stimulation of the muscle have the effect of driving the food either upwards or downwards ; the direction of movement of the food will be that of least resistance. : The .movements of the small intestine are also subject to the central nervous system. Stimulation of the vagus has the effect of producing an initial inhibition of the whole small intestine, followed by increased irrita- bility and increased contractions (Fig. 346). On the other hand, stimulation of the splanchnic nerves causes complete relaxation of both coats of the small gut (Fig. 347). It seems that the splanchnics normally exercise a tonic Fig. 345. Passage of bolus. Contractions of longitudinal coat (enterograph). Tlie bolus (of soap and cotton- wool) was inserted into the intestine four inches above the recorded spot at a. The figures below the tracing indicate the distance of the middle of the bolus from the recording levers. As the bolus arrived two inches above the levers there is cessation of the rhythmic contractions and inhibition of the tone of the muscle. This is followed, as the bolus is forced past, by a strong contraction in the rear of the bolus. ■ inhibitory influence on the intestinal movements, which can be increased by all manner of peripheral stimuh. On this account it is often impossible to, obtain any movements in the exposed intestine so long as these remain in connection with the central nervous system through the splanchnic nerves. The relaxed condition of the gut which obtains in many abdominal affections is probably also reflex in origin, and is due to reflex inhibition through the splanchnic nerves. As a result of the two sets of movements described above, the food is thoroughly mixed with the digestive juices, and the greater part of the products of digestion are brought into contact with the intestinal wall and absorbed. What is left — a proportion varying in different animals according to the nature of the food — is passed on by occasional peristaltic contractions through the lower end of the ileum into the colon, or large intestine. The lowest two centimetres of the ileum present a distinct thickening of the cir- cular muscular coat, forming the ileocoHc sphincter. This sphincter relaxes in front of a peristaltic wave and so allows the passage of food into the colon. On the other hand, it contracts as a rule against any regurgitation which might be caused by, contractions in the colon. Although thus falhng into MOVEMENTS OF THE INTESTINES 729 line with the rest of the muscular coat, as concerns its reaction to stimuli arising in the gut above or below, it presents a marked contrast to the rest of the gut in its relation to the central nervous system. It is unaffected by stimulation of the vagus. Stimulation of the splanchnic, however, which Fig. 346. Effect of stimulation of right vagus on intestinal contractions. ■"Fig. 3-47. Excitation of both splanchnic nerves. Balloon method, returned to abdomen. Intestine causes complete relaxation of the lower part of the ileum with the rest of the small intestine, produces a strong contraction of the muscle-fibres forming the ileocolic sphincter (ElUott). MOVEMENTS OF THE LARGE INTESTINE By means of the occasional peristaltic contractions, accompanied by relaxation of the ileocolic sphincter, the contents of the small intestine are gradually transferred into the large. In man these contents are con- siderable in bulk, aie semi-fluid, and probably fill the ascending as well as the transverse colon. 730 PHYSIOLOGY The large intestine is supplied with nerves from the central nervous system. These run partly in the sympathetic system along the colonic and inferior mesenteric nerves, partly in the pelvic visceral nerves or nervi erigentes, which come off from the sacral cord and pass direct to the pelvic viscera. In addition it possesses a local nervous system, presenting the same structure as that found in the small intestine. The movements of the large intestine differ considerably in various animals, as has been shown by EUiott, according to the nature of the food and the part played by this portion of the gut in the processes of absorption. In the dog the process of absorp- tion is almost complete at the ileocohc valve, whereas in the herbivora a very large part of the processes of digestion and absorption occurs in the colon and caecum. Man takes an intermediate position as regards his large intestine between these two groups of animals. ElUott and Barclay Smith divide the large intestine into four parts, according to their functions, viz. the caecum, and the proximal, intermediate, and distal portions of the colon. Of these the dog possesses practically only the distal colon. We may take Elhott's account of the movements as they probably occur in man. They agree very closely with those observed by Cannon under normal circum- stances in the cat by means of the Rontgen rays. The food as it passes from the ileum first fiUs up the proximal colon. The effect of this distension is to cause a contraction of the muscular wall at the junction between the as- cending and transverse colon. This contraction travels slowly over the tube in a backward direction towards the caecum, and is quickly succeeded by another, so that the colon may present at the same time several of these advancing waves. These waves are spoken of as anti-peristaltic ; but as they do not involve also an advancing wave of inhibition, they must not be regarded as representing the exact antithesis of a peristaltic wave, as we have defined it. The effect of these waves is to force the food up into the caecum, regurgitation into the ileum being prevented partly by the obHquity of the opening, partly by the tonic contraction of the ileocolic sphincter. As the whole of the contents cannot escape into the caecum, a certain portion will sHp back in the axis of the tube, so that these movements have the same effect as the similar contractions in the pyloric end of the stomach, causing a thorough churning up of the contents and its close contact with the intestinal wall. The movements are rendered still more effective by the sacculation of the walls of this part of the large intestine. The distension of the caecum caused by this anti- peristalsis excites occasionally a true co-ordinated peristaltic wave, which, starting in the caecum, drives the food down the intestine into the transverse part. These waves die away before they reach the end of the colon, and the food is driven back again by waves of anti- peristalsis. Occasionally more food escapes through the ileocolic sphincter from the ileum, so that the whole ascending and transverse colon may be filled with the mass undergoing a constant kneading and mixing process. The result of this process is the absorption of the greater part of the water of the intestinal contents, as well as of any nutrient material, and the drier part of the intestinal mass collects towards the splenic flexure, where it may be MOVEMENTS OF THE INTESTINES 731 separated by transverse waves of constriction from the more fluid parts which are being driven to and fro in the proximal and intermediate portions. By means of occasional peristaltic waves these hard masses are driven into the distal part of the colon. The distal colon must be regarded as a place for the storage of the faeces and as the organ of defsecation. In the transverse colon,, in the descending and iliac-colon, the anti- peristaltic movements and consequent churning of the contents are probably slight. These therefore represent the intermediate colon, with propulsive peristalsis as its chief activity. The descending colon is never distended, and Elliott therefore Fig. 348. Skiagram to show normal position of colon in man, and the position attained by its contents at different periods after a meal containing bismuth. The bismuth meal was taken at 8 a.m. The times of arrival at different levels are marked on the colon. (Hertz.) regarded it as a transferring segment of exaggerated irritabihty. The storage of the waste matter takes place chiefly in the sigmoid flexure. This with the rectum represents the distal portion of the colon. The dis- tinguishing feature of the distal colon is its complete subordination to the spinal centres. It probably remains inactive until an increasing distension excites reflexly through the pelvic visceral nerves a complete evacuation of this portion of the gut. Stimulation of these nerves in an animal, such as the cat, produces a rapid shortening of the distal part of the colon, due to contraction of the recto-coccygeus and longitudinal fibres of the gut, followed after some seconds by a contraction of the circular coat. This originates at the lower limit of the area of anti-peristalsis, i.e. probably at the upper end of the sigmoid flexure, and, spreading rapidly downwards, empties the whole of this segment of the gut. In man the emptying of the rectum itself is largely assisted by the contractions of the voluntary nniscles of the abdominal walls and pelvic floor. The last section of the rectum is emptied at the close of the act by a 732 PHYSIOLOGY forcible contraction of the levator ani and the other perinseal muscles, and this contraction also serves to restore the everted mucous membrane. The carrying out of this reflex act is dependent on the integrity of a certain part of the lumbar spinal cord. If this ' centre ' be destroyed, the tonic con- traction of the sphincter muscles disappears. This centre may be either excited to increased action, or be inhibited by peripheral stimulation of various nerves, or by emotion, such as fear. Apphcation of warmth to the region of the anus causes reflex relaxation of the sphincter ; application of cold increases its tonic contraction. In man, as Hertz has shown by the skiagraphic method, the pelvic colon becomes filled with faeces from below upwards, the rectum remaining empty till just before defsecation. In individuals whose bowels are opened regularly every morning after breakfast the entry of fseces into the rectum gives rise to a sensation of fulness and acts as the call to defsecation. If no response be made, the desire to defsecate passes away, since the rectum relaxes and the fsecal mass no longer exercises pressure on its wall. Hertz has shown that the minimal pressure required to produce the call to defsecate varies from 30 to 40 mm. Hg, according to the length of the gut which is the seat of distension. '■'-i SECTION X THE ABSORPTION OF THE FOOD-STUFFS THE ABSORPTION OF WATER AND SALTS The intake of water, and probably of salts, by the alimentary canal, in accordance with the requirements of the organism as a whole, seems to be regulated almost entirely by the central nervous system, the higher parts of this system, viz. those concerned with appetite, being particularly in- volved in the process. Thus in man any large loss of fluid to the body, as by sweating, diarrhoea, haemorrhage, gives rise to an intense thirst that has its natural reaction in increased intake of water by the mouth. On the other hand, the property possessed by the ahmentary canal of absorbing water and weak saline fluids contained in its interior is very little influenced by the state of depletion, or otherwise, of the water depots of the body. It is practically impossible, however large the quantities of fluid ingested, to evoke the production of fluid motions, the greater part of the ingested fluid being absorbed on its way through the ahmentary canal. Thus a man may keep himself in perfect health and maintain the water content of his body constant whether he take one litre or six litres of water daily. The whole process of regulation, apart from that determined by appetite, is carried out at the other end of the cycle, viz. by the kidneys. As concerns absorption of water there is no chemical solidarity between the ahmentary surface and the rest of the body. Whenever water is presented to the surface it is absorbed and passes into the circulation. The absorption of water in the stomach may be regarded as nil. Although from this viscus alcohol, and possibly peptone and sugar, may be absorbed to a slight extent, water or saline fluids introduced into it are passed through the pylorus either without change or increased by the secretion of fluid from the gastric glands. In no case is there a diminution of fluid in the stomach. The chief absorption of water occurs in the small intestine. It is on this account that the salient features of cases of dilatation of the stomach with stenosis, absolute or relative, of the pyloric orifice can be nearly all referred to the starvation of the body in water, and can be often relieved by the administration of water either subcutaneously or by the rectum, i.e. by the channels through which absorption is still possible. The introduction of water into the stomach simply increases the dilatation, but does not relieve the intense thirst of the patient. Water that has been swallowed 733 734 PHYSIOLOGY to quench tkirst has first to be passed from the stomach into the small intestine before it can be absorbed and reheve the needs of the tissues. The intestinal contents at the ileocsecal valve contain relatively nearly as much water as they do at the upper part of the jejunum. Their absolute bulk is, however, much smaller, so that only a small proportion of the water that has been taken in by the mouth remains to be absorbed in the large gut — an amount probably much less than that which has been added to the contents of the small intestine in the form of secretion by the stomach, liver, pancreas, and intestinal tubules. The main problem before us is therefore the mechanism of absorption of water and sahne fluids by the viUi of the small intestine. By means Epithelium of villus Vein Artery Lieberkiihn's follicle Central lacteal '^M^.'c^-. Mucosa Muscularis mucosse Submucosa Lymphatic plexus Circular muscle Lymphatic plexus Longitudinal muscle Fig. 349. Diagrammatic section through wall of small intestine to show vascular and lymphatic arrangements of mucous membrane. (After Mall.) of these structures the absorbing surface of the intestine is largely increased. It has been calculated that each square milhmetre of intestine represents an absorbing surface of 3 to 12 mm.^ Each villus (Fig. 349) consists of a framework of reticular tissue containing many leucocytes in its meshes, separated from the lumen of the gut by a continuous layer of columnar epithehal cells. These cells rest on an incomplete basement membrane and present on the side turned towards the lumen of the gut a striated basilar border. The villus offers two channels by means of which material, which has passed through the epithelium, may be carried into the general circu- lation. In the centre of the villus is the central lacteal, a club-shaped vessel bounded by a complete layer of delicate endothehal cells. This leads into a plexus of lymphatics placed superficially to the muscularis mucosae. From the superficial plexus communicating branches pass vertically to a corre- sponding plexus lying in the submucosa. The central lacteal and the super- ficial plexus are free from valves, which, however, are present in abundance in the deeper plexus, so that fluid can pass easily from the lacteal to the THE ABSORPTION OF THE FOOD-STUFFS 735 deeper plexus, but not in the reverse direction. From the muscularis mucoscB unstriated muscle fibres pass up through the villus to be attached partly to the outer surface of the central lacteal, partly by expanded extremities to the basement membrane covering the surface of the villus. Contraction of these muscle fibres mil tend to empty the central lacteal into the deep plexus of lymphatics and may also cause an expulsion of the contents of the spaces of the retiform tissue of the villus into the central lacteal. The alimentary canal represents one of the few localities where a formation of lymph is constantly proceeding, even in a condition of complete rest. On placing a cannula in the thoracic duct of a dog an outflow of lymph is obtained which may vary in different animals between 1 c.c. and 10 c.c. in the ten minutes. The greater part of this lymph is derived from the ahmentary canal, so that any of the intestinal contents which have made their way into the spaces of the villus might be entrained in this lymph current and carried away with it into the thoracic duct and so into the general blood system. The other possible channel of absorption is by the capillary blood- vessels of the villus. Each villus is supphed with blood from one or two arterioles which break up into a rich plexus of capillaries lying close under the basement membrane of the villus. The return blood is collected into one or two veins, which join the radicles of the portal vein in the submucosa and in the mesentery. In these capillaries the blood is circulating rapidly, so that a considerable amount of material may pass into them from the spaces of the villus within, say, one hour without altering appreciably the percentage composition of the blood. On the other hand, it must be remem- bered that the blood in these vessels is at a high pressure, probably not less than 30 mm. Hg., so that any absorption into the blood stream must occur against this pressure. It is probable therefore that in explaining any absorption by the blood-vessels we shall have to place out of court any possi- bihty of the passage occurring in consequence of hydrostatic differences of pressure, i.e. by a process of filtration. When salt solutions are introduced into the small intestine they are rapidly absorbed without the production of any corresponding increase in the rate of lymph flow from the thoracic duct. On the other hand, the absorption of large amounts of fluid may cause an actual diminution of the solids of the plasma, so that we are justified in regarding the capillary network of blood-vessels at the surface of the villi as solely responsible for the absorption. What are the forces which cause this transference of fluid and dissolved substances from one side to the other of the membrane composed of epithelial cells ylus capillary endothelium ? Like other cells, those of the intestinal epithelium are probably bounded on their free surface by a ' lipoid ' mem- brane, i.e. one containing some complex of lecithin and cholesterin and per- meable only by such substances as are soluble in lipoids. On the other hand, the cement substance between the cells may be of a dift'erent character and possibly permeable to water-soluble substances. The question has been 736 PHYSIOLOGY propounded whether the greater part of the substances which enter the blood plasma from the gut pass between the cells or through the cells. Water could, of course, pass in either way. Most of the inorganic salts, such as sodium chloride, as well as the very important constituents of the food, the sugars, are insoluble in hpoids, and would have to pass between the cells. When the question is investigated by the use of dye-stuffs, soluble or insoluble in Hpoids, it is found that the hpoid-soluble dye-stuffs, such as neutral red or toluidin blue, pass into the cells, whereas the dye-stuffs which are insoluble in such substances pass into the intercellular spaces. Too much stress, how- ever, must not be laid on these experiments. All these dye-stuffs are ab- normal so far as the body is concerned. We cannot imagine that at any time in the course of evolution of the properties of the intestinal epithehum the cells were ever presented with or had to discriminate between different dye-stuffs. The fact that absorption of these dye-stuffs is determined by the physical conditions of the cell membrane is no proof that the absorption of the normal food constituents is determined in the same way. In fact, it is quite legitimate to assume that the lipoid membrane or hmiting layer round every cell has as its main office, not the regulation of the access of food- stuffs to the cell, but its protection from any of the food-stuffs which it does not require for its metabohsm. If it were not for such a membrane the assimila- tion of a salt would be determined entirely by its concentration in the imme- diate surroundings of the cell, whereas we know that the assimilation of any living organism, whether uni- or multi- cellular, is regulated in the first place by the activity of the organism itself. According to this activity and the needs thereby induced the uptake of food material may be large or small whatever its concentration in the surrounding medium. It would indeed be strange that the whole absorbing surface of the intestine should be covered by a membrane, of which the greater part was useless for the absorption of the common food-stuffs, as would be the case if these could only penetrate the membrane by the narrow chinks between the cells. It seems more probable that the absorption of the different food-stuffs, and probably also of the normal salts of the body, is effected by the cells themselves, in accordance with their nutritional needs, and this view is strengthened when we come to examine into the absorption even of normal saline solutions. If 50 c.c. of normal sodium chloride solution be introduced into a loop of intestine, it is absorbed steadily, so that at the end of an hour not more than about 20 c.c. may be recoverable. The absolute amounts absorbed differ in various experiments, but are fairly uniform for repeated observations on one and the same animal. The absorption of such a solution could be ascribed to the osmotic pressure of the colloids in the blood plasma or lymph within the spaces of the vilh. If, instead of using isotonic solutions, hypertonic solutions are employed, e.g. a 2 or 3 per cent. NaCl solution, absorption still takes place, but may be preceded by an interval in which there is an actual in- crease of the fluid contained in the gut. Here, again, we might ascribe the absorption to the physical factors present, were it not that absorption is found to commence before the fluid in the gut has attained isotonicity with THE ABSORPTION OF THE F00D-8TLTFS 737 the blood. In fact, employing a 1-5 per cent, salt solution, absorption may- occur from the very beginning of the experiment. If such a solution is passed through the epithelial membrane into the blood plasma with a smaller tonicity, it is evident that work must be done in the process, work which can only be furnished by the cells of the epithehum. When sugar solutions are employed they behave in somewhat similar fashion to sodium chloride solutions, provided that the sugar is one of the absorbable hexoses, both sugar and water being rapidly absorbed. It is important to note that dex- trose is absorbed from the gut almost as rapidly as sodium chloride, and quite as rapidly as sodium iodide, although its diffusibihty is very consider- ably less than either of these salts. Moreover great differences are found between the rate at which different sugars are absorbed, differences which are not referable to the diffusibihty of the sugars in question. Thus the monosaccharides glucose, fructose, galactose are absorbed with double the rapidity of solutions of cane sugar and maltose, and it seems that in the absence of hydrolytic splitting of the disaccharides absorption from the gut would be entirely aboUshed. Thus lactose disappears from the intestine much more slowly than either of the other two disaccharides, so that large doses may give rise to a laxative effect. In animals devoid of lactase, the lactose-sphtting ferment, in their intestinal epithehum milk sugar is apparently not absorbed at all. The most cogent argument perhaps in favour of an active intervention of the cells of the gut in the process of absorption is furnished by the study of the absorption of blood serum. It has been sho^vn that if an animal's own serum be introduced into a loop of its intestine the serum undergoes absorp- tion. This absorption affects the water and salts more than the protein, so that the percentage of the proteins in the fluid remaining in the intestine is increased. Finally, however, the whole of the serum is absorbed. In this case the fluid within the gut is identical with the fluid within the blood-vessels. There are no differences in concentration, quality of salts, or osmotic pressure of proteins. Nevertheless water passes through the cells of the gut from their inner to their outer sides, entraining with it the salts of the serum and a certain proportion of the indift'usible proteins. It is impossible to explain this result as due to the digestion of the proteins and their conversion into diffusible products, since the intestinal loops were washed free of any trypsin that they contained, and serum has itself a strong antitryptic action which would prevent its being attacked by even a strong solution of trypsin. The active intervention of the cells in the absorption of salt solutions, as well as of serum, can be abohshed by any means which diminishes or destroys their vitahty, such as the addition of sodium fluoride to the fluid to be absorbed, or destruction of the epithelium by previous temporary occlusion of the blood-vessels supplying the loop of intestine. We must conclude that, when a fluid is introduced into the intestine, an active transference of water from the lumen into the blood stream is effected by the intermediation of forces having their origin in the metabolism of the cells themselves. This work of absorption of the cells may be aided or 24 738 PHYSIOLOGY hindered according to the physical conditions present. If these act against the cells, e.g. if the fluid be hypertonic, the absorption is eflected more slowly, while with hypotonic solutions the physical conditions concur with the vital acti^dty of the cells in bringing about a very rapid transference of fluid from the gut into the blood-vessels. Among these physical conditions we must reckon the nature of the salts present in the solution. If these can pass easily into and through the cells, e.g. anunonium salts, sodium chloride, absorption is carried out rapidly. If, on the other hand, the salts in the intestinal contents are but shghtly diffusible or have very Httle power of penetrating into the cells, the absorption of water by the cells causes an increased concentration of the salts, and therefore an increased osmotic pressure, which offers a resistance to any further absorption ; and the process comes to an end, when the absorptive power of the cells is exactly balanced by the increased osmotic pressure, or attraction for water, of the intestinal contents. Cushny and Wallace, as the result of their experiments on the relative absorbability of salt solutions from the gut, divide the salts into four main classes as follows : I II Ill IV Sodium chloride, Ethyl sulphate, Sulphate, phosphate, Oxalate, bromide, iodide. nitrate, lactate, sali- ferrocyanide, capry- fluoride. formate acetate, cylate, phthalate. late, malonate, succi- propionate, butyrate, ' nate, malate, citrate. valerianate, caprate. tartrate. Of these the first class contains those salts which are absorbed with great ease from the intestine. The second group of salts are absorbed with somewhat greater difficulty. The third group are absorbed so slowly, i.e. the salts retain the water in which they are dissolved so long, that they increase peristalsis and act as laxatives or purgatives. The members of the fourth class are not absorbed at all. It is evident that this classification is independent of the diffusibility of the salts. Sodium acetate has a much smaller dissociation value and a lower diffusibility than sodixun chloride or iodide, and yet is absorbed at approximately the same rate as these two salts. There is, however, as Cushny pointed out, one physical or chemical character which apparently determines the non-absorbability (relative or absolute) of the members of the tlfird and fourth classes. All these salts form insoluble compounds with calcium. This common character is not an explanation of the permeability of the cell wall, but is simply a general statement of one of the conditions which affect the power of the cells to take up salts from their solutions, tliis power being absent in the case of salts which furnish an insoluble calcium compound. THE ABSORPTION OF FATS Fats administered to an animal in excess of its diurnal requirements are stored up in the body in the form in which they are administered. Each cell of the body probably possesses in itself the mechanism for the utihsation of these neutral fats, and for eflecting in them the various changes involved in the successive stages of their disintegration and oxidation through which they are finally converted to CO2 and water. The problem therefore of fat absorption is ultimately one of the simplest with which we have to deal, and involves merely the transference of the neutral fat of the food to the circulatiug fluids in such a form that it can be carried by them to the place THE ABSORPTION OF THE FOOD-STUFFS 739 where it is required for the metaboHsm of the body or where it may be stored up as a reserve substance. The processes of digestion of fat result in the production of glycerin and fatty acids, if the reaction be neutral or slightly acid. H the reaction of the gut be alkahne the alkah will combine with the fatty acids to produce soaps. Analyses of the contents of the gut after a fatty meal show that the greater proportion of the fats are present as a mixture of fatty acids and soaps, the amount of these substances as compared with unchanged fat increasing as we descend the gut. In studying the absorption of fats the investigator is able to take advantage of the fact that the micro-chemical detection of this substance is usually very easy. Globules of fats or fatty acids containing any proportion of the unsaturated fatty acids have the property of reducing osmic acid, and therefore of being stained black by this reagent. Practically all the fats which occur in the food or in the cells of the body contain oleic acid or the glyceride of this acid in association with palmitic or stearic acid, and therefore give the typical micro- chemical fat reactions. In many cases it is useful to employ the specific stains for fats, such as Sudan red or alkanna red. It is important to remember that the intensity of the fat reaction given by a cell is only an expression of the fat or fatty acid contained in a free state in the cell, and is no criterion of the total amount of fat which may be present. Thus a normal heart muscle in section gives only a diffuse light brown coloration with osmic acid. After poisoning by phosphorus or by diphtheria toxin, every muscle- cell may be found studded with minute black granules of fat. Chemical analysis shows, however, that the normal heart muscle contains as much fat as the degenerated muscle. Our micro- chemical methods will therefore throw no hght on the amount of fat which is actually in combination with the cell protoplasm. If an animal be examined a few hours after the administration of a meal rich in fats, the lymphatics of the intestine are seen to be distended with a milky fluid — chyle — and the same fluid is found filling the cisterna hjmphatica magna and the thoracic duct. The lymph from the thoracic duct will also be milky, and chemical analysis shows that the opacity is due to the presence of minute granules of neutral fat. The fat in such chyle may amount to over 6 per cent., so that in a moderate-sized dog 12 grammes of fat may be carried in the com"se of an hour from the intestine to the blood by tliis means. This great access of fat to the blood during fat absorption introduces corresponding changes in the blood. The plasma itself becomes milky, and if the blood be allowed to clot, the sermn expressed from the clot is also milky. On standing, a layer of fat globules hke cream may rise to the surface of the serum. Fat is found in a free state in this finely divided condition in the blood plasma so long as it is being absorbed in the intestine. During btarvation it disappears entirely, the serum becoming perfectly clear. Thus part, at any rate, of the fat which is absorbed from the gut is carried thence by the lymphatic channels in the form of neutral fat to the blood stream, by which it is distributed to the various tissues of the body, gradually 740 PHYSIOLOGY leaving the blood stream in a manner which at present has not been deter- mined. Not all the fat which is absorbed takes this path by way of the lymphatics and the thoracic duct. Ligature of the thoracic duct, if effective, certainly impedes the absorption of fat, but does not abohsh it. If the thoracic duct lymph be collected during the absorption of a given quantity of fat from the intestine, not more than 60 per cent, of the fat which has disap- peared from the gut can be recovered from the lymph. What happens to the remainder we do not know. Apparently it does not reach the blood in a finely divided condition. If the thoracic duct be ligatured the per- centage of fat in the blood rapidly falls to a minimum which remains con- stant, even during starvation. If now fat be administered, although a considerable proportion of it may be absorbed, the percentage of fat in the A B i •V Fig. 350. Columnar epithelium from small intestine of frog stained with osmic acid to show fat-absorption. A, five hours after a meal of olive oil ; B, three hours later. It should be noticed that the fat globules first formed grow in size in the course of digestion, pointing to a gradual deposition of fat on the globules from solution in the protoplasm. (SCHAJ'ER.) blood is not raised. If therefore the fat is absorbed directly into the blood it cannot be in the particulate condition, and it must be in such small quantities at a time that it is at once removed from the blood by the tissues through which this fluid flows. It is difficult to imagine that any large proportion of this lost fraction of the fat is absorbed into the blood stream in the form of soaps, since, as Mmik has shown, soaps injected into the blood- stream act as poisons and give rise to a great fall of blood-pressure, incoagulability of the blood, and a condition of coma. We must therefore leave out of account for the present the mechanism of absorption of this lost fraction and endeavour to trace the course of the absorption of that part of the fat which makes its way into the lymphatics. Microscopic examination of a section of the villus during fat absorption shows that the absorption occurs for the most part through the epithelial cells. These are found closely packed with fat granules (Fig. 350), which, small at the beginning of the process of absorption, rapidly enlarge till they occupy the greater part of the cell lying between the nucleus and the basilar striated border. Most observers are agreed that no fat globules are to be seen within the border itself. THE ABSORPTION" OF THE FOOD-STUFFS 741 According to Altmann the fat granules found in the cells during absorption are them- selves produced by a transformation of fuchsinophile granules -which are present in the cell even during the fasting condition. At an early stage the small fat granules can be stained so as to show a distinct fuchsinophile envelope. Altmann intei'prets this ap- pearance as showing that the epithelial cells take up the fat in a dissolved form, probably in a hych'olysed condition, and that a process of synthesis then occirrs in the granules leading to the formation and accumulation of fat. When the process of absorption is proceeding actively the meshes of the villus contain a number of free fat granules, and the leucocytes in these meshes are generally found also full of these granules. According Fig. 351. a. Vertical section through intestinal epithelium of a rat during fat absorption. B. Horizontal section through deeper parts of the cells, showing excretion of fine fat globules into the interccUular clefts. (Retjtek.) to Zawarykin and Schafer an important function in the transfer of the granules from epithelial cells to central lacteal was performed by the leucocytes. These were supposed to take up the fat granules extruded bj' the epithelial cells at the base of the villi, to wander into the central lacteal where they broke down, fmnishing in this way the molecular basis of the chyle as well as its protein constituents. This view was strongly combated by Heidenhain, who pointed out that many of the granules staining darkly with osmic acid were not necessarily fat, and that the number of leucocytes within the villi were hardly sufficient to account for the amount of material observed. According to Renter the epithelial cells take up fat in a dissolved condition through the striated border, and deposit it as granules of neutral fat in the inner portion of the protoplasm. From here the fat is passed on by the protoplasm by the side of the nucleus and extruded in the form of very fine granules in the deeper parts of the inter-epithelial clefts, which thus function as true excretory chamiels for the epithelial cells (Fig. 351). It is probable that the muscular mechanism of absorption described many years ago by Briicke plays an important part in the absorption of fats, but it is difficult to furnish any experimental proof of the manner in which this mechanism works. Repeated contractions of the muscle fibres of the villus would tend to empty the spaces into the central lacteal, and this in its turn into the submucous plexus of lymphatics, so that the hniiph in the spaces is constantly renewed and passes laden with absorbed fat particles into the valved lymphatics of the mesentery. 742 PHYSIOLOGY It was long considered that the fats were taken up by the epithehal cells from the intestine as fine particles of neutral fat, the chief use of the pan- creatic juice being to aid the formation of an emulsion of fat in the intestines. There seems to be httle doubt that this was an error, and that the fats are absorbed, dissolved in the bile, either as soap or as fatty acid. The arguments for this view can be shortly summarised as follows ; (1) Although the bile does not dissolve neutral fats, it has a strong solvent action on fatty acids, on soaps, and even on the insoluble calcium soaps. This solvent power is greatest in the case of oleic acid, of which bile can dis- solve 19 per cent. It is very small in the case of pure stearic acid, but the solubihty of the latter acid is largely increased if it be associated as usual with oleic acid. Moore has shown that this solvent action is chiefly conditioned by the bile salts, aided by the lecithin and cholesterin also present in the bile, a solution of lecithin and cholesterin in bile salts having a greater solvent power than the salts alone. (2) The presence of bile in the intestine is essential for the normal absorption of fat. If the bile be cut off by occlusion of the bile ducts or by the establishment of a biliary fistula, the utihsation of fat sinks from about 98 per cent, to about 40 per cent., the unabsorbed fat appearing in the faeces. This large undigested residue of fat hinders also the absorption of the other food-stufls by covering them with an insoluble layer, so that nutrition as a whole may suffer considerably. (3) Absorption may also be interfered with by ligature of the pancreatic duct. This result is probably due to the absence of the fat-splitting ferments of the pancreatic juice from the intestine. If the faeces be analysed it is found that a very large proportion of the fat has been spht into fatty acids in the course of its passage through the ahmentary canal. This lipolysis has, how- ever, been carried out by the agency of micro-organisms, i.e. in the lower segments of the gut where the greater part of the bile has been already re- absorbed into the portal circulation. If fat be given to animals deprived of their pancreas, in a finely divided form, such as cream or milk, a certain proportion of it is absorbed. Under these conditions a considerable degree of hpolysis may occur in the stomach itself, so that the fats would be already hydrolysed when they came in contact with the bile in the duodenum. (4) It was shown by Schiff, by means of his amphibohc fistula, that the bile which is poured into the gut undergoes a circulation, being re-absorbed from the lower parts of the digestive tube, carried to the hver by the portal vein, and re-secreted in the bile. The same quantity of bile salts may therefore be used over and over again as a vehicle for the transfer of the fatty acid and soaps from the lumen of the gut into the epithelial cells. (5) Substances which are physically almost identical with fats, e.g. petroleum or paraffin, are not absorbed even when introduced into the intes- tine in the finest possible emulsion. If neutral fat be melted with a soft paraffin and the resulting mixture made into a fine emulsion and administered it is found that the intestine rejects the paraffin, but takes up the neutral fat. This result can only be explained by assuming that the fat in the THE ABSOKPTION OF THE FOOD-STUFFS 743 particles has been actually dissolved out by the digestive juices and has been absorbed in a state of solution. We may sum up the processes involved in digestion and absorption of fat as follows. Neutral fat is hydrolysed into fatty acid and glycerin under the action of the gastric juice, the pancreatic juice, and the succus entericus, the efiect of the gastric juice being, however, extremely Umited unless the fat be presented to it in a finely divided condition. The Hpolytic action of the pancreatic juice and succus entericus is largely aided and increased by the simultaneous presence of bile, which, in virtue of the bile salts lecithin and cholesterin it contains, enables the pancreatic juice to enter into close relation with the fat, and dissolves the products of the activity of the ferment, and so enables it to attack renewed portions of the neutral fat. As the result of this hpolysis there are formed glycerin, which is soluble in water, and fatty acids or soaps, according as the reaction of the medium is acid or alkahne. The alkaline soaps are soluble in water, the soaps of magnesium and calcium are soluble in bile, free fatty acids are soluble in bile acids. The fat is thu5 reduced to" a condition in which it is soluble in the intestinal contents whatever their reaction. In this state of solution its constituents are taken up by the cells of the intestinal mucosa. Within the cells a process of sjTithesis takes place, the soaps being spht and the fatty acids thus set free or absorbed, being combined with glycerin with the elimination of water to form neutral fat, which appears as fine granules in the cell proto- plasm. By an active process of excretion these granules are extruded in a somewhat more finely divided form into the intercellular clefts and into the spaces of the villus, whence by the contractions of the musculature of the villus they are forced with the lymph transuding from the capillary blood- vessels into the central lacteal, and thence along the mesenteric lymphatics to the thoracic duct. This description would apply to about 60 per cent, of the fat which is absorbed. It is probable that all the fat which is absorbed is taken up in a dissolved condition, but whether the remaining 40 per cent, enters the blood stream, or is utihsed and broken down in the tissues of the intestinal wall itself, we have no means of judging. Under normal circum- stances the utilisation of fat is almost complete. By the time the intestinal contents have arrived at the lower end of the ilemn 95 per cent, of the fat has been absorbed. Removal of the whole large intestine was found by Vaughan Harley not to afiect fat absorption. THE ABSORPTION OF CARBOHYDRATES As a result of the action of the various digestive juices all the carbo- hydrate constituents of the normal diet of man are reduced to the state of monosaccharides. The absorption of these digestive products may take place at any part of the ahmentary canal, the greatest part in the act of absorption being taken by the small intestine. By the time that the food has arrived at the ileocsecal valve practically the whole of the carbohydrate constituents of the food have been absorbed. All experimenters are agreed that the carbohydrates pass into the body by way of the vessels of the portal 744 PHYSIOLOGY system. The lymph from the thoracic duct contains no more sugar than does the arterial blood taken at the same time, whereas several observers have obtained an increased percentage of sugar in the portal blood during the absorption of a big carbohydrate meal. Of the carbohydrates of the food, some, Hke starch, dextrin, glycogen, are colloidal and indiffusible ; others, such as the disaccharide3 cane sugar, milk sugar, and maltose, are soluble and diffusible, and the products of the action of digestive ferments on these two classes, namely, monosaccharides, mannose, fructose, glucose, galactose, are also soluble and diffusible. The problem as to the mechanism involved in the passage of these substances across the intestinal waU into the blood-vessels has been already dealt with in treating of the absorption of water and salts. The most striking fact is the relative impermeabihty of the intestinal wall to the disaccharides as compared with the monosaccharides. The intestinal wall is apparently only able to take up in any quantity such sugars as can be utilised by the cells of the organism. For this purpose the disaccharides are useless ; cane sugar or lactose introduced into the blood-vessels or subcutaneously is excreted quan- titatively in the urine, and, as might be expected, does not increase in any way the glycogen of the Hver. When maltose is injected in the same manner a certain proportion of it is utihsed owing to the fact that the blood and fluids of the body contain a ferment, maltase, capable of converting the disaccharide into the monosaccharide, glucose. The absorption of these disaccharides occurs therefore much more slowly from the intestine than does the absorption of monosaccharides, the process of absorption being always preceded by and waiting for the process of hydrolysis. Thus huge doses of cane sugar may be taken without causing the appearance of cane sugar in the blood or urine. It has been found that sugar does not appear in the urine until as much as 320 grm. of cane sugar have been ingested, whereas any quantity of glucose over 100 grm. may give rise to glycosuria. Lactose is absorbed still more slowly, and in animals whose intestine is free from the ferment lactase, is not absorbed ; large doses of lactose in such animals therefore give rise to diarrhoea. The behaviour of the intestinal wall to the non-assimilable sugars of artificial origin has not yet been suflB.ciently investigated. It would be interesting to inquire whether the rate of absorption of the different sugars was in any way determined by their stereomeric configuration, whether, for instance, Z- glucose would be absorbed as rapidly as the ordinary d-g\\\cose. THE ABSORPTION OF PROTEINS In very few departments of physiology has there been so great a revo- lution in our ideas as in that relating to protein absorption, especially as to the form in which it is absorbed from the alimentary canal, and its fate after absorption. As to the channel by which it obtains entry into the circulation, practical agreement reigns that it is absorbed by the blood-vessels. Almost every physiologist who has occupied himself with the investigation of the lymph flow from the thoracic duct has been impressed by the fact that the THE ABSOKPTION OF THE FOOD-STUFFS 745 variations in the amount of lymph to be obtained in this way bear no relation to the condition of the animal as regards the state of digestion. Nor do we find any appreciable increase in the amount of lymph flow or in the amount of proteins contained in this lymph during digestion. The small increase observed by Asher and Barbara would be sufficiently accounted for by the increased blood-supply to the intestines during digestion, and is insufficient to account for the absorption of any appreciable quantity of the protein which is being taken up from the alimentary canal. Moreover it was shown by Schmidt Miilheim that the absorption of proteins was not inter- fered with as the result of Hgature of the thoracic duct, and that after this duct had been ligatured the ingestion of proteins was followed at the usual interval by the increased output of urea which is the invariable concomitant of protein absorption and assimilation. We must therefore conclude that the products of protein digestion are taken up by the epithehal cells and passed on by these into the blood-vessels. During the absorption of a protein meal changes have been described by various observers in the structures of the villus. In nearly every case there is marked increase in the number of mitotic figures in the epithelium lining the follicles of Lieberkiihn. According to Hofmeister there is during absorption an increase in the number of leucocytes in the villi, and this observer ascribed an important function to these ecUs in the absorption of protein. Heidenhain showed that this increase of leucocytes was not constant in all animals, and bore no relation to the amount of absorption that was taking place, and was quite inadequate to account for the total absorption that was carried on. On the other hand, several observers have described changes in the epithelium as the result of protein digestion. According to Renter the epithelial cells become swollen, their protoplasm stains less deeply, and at their basal ends the cells' limits disappear, the protoplasm being apparently distended -nith hyaline coagulable material (Fig. 352). Reuter regards this appearance as a direct expression of the taking up of proteins in a dissolved form and their conversion near the bases of the cells into coagulable proteins ; but further evidence on this subject is necessary before we can attach much importance to such an interpretation of the appearances observed. Under the influence of the gastric juice the proteins of the food are resolved during their stay in the stomach into albumoses and peptones. In the small intestine the process of hydration is carried further, the tr}^sin of the pancreatic juice carrying the proteins through the stage of secondary albumoses and peptones, and converting them into a mixture of amino-acids and polypeptides. The same end-products result from the action of the erepsin of the intestinal wall on the albumoses and peptones produced by gastric digestion. The digestive juices finally reduce the proteins therefore to a mixture of amino-acids, with a certain remainder of polj'peptides con- sisting of two or three of the amino-acids associated together, wliich do not undergo further disiritegration under the action of the intestinal ferments. The final products give no biuret test. The first question we have to decide is to what extent the proteins are reduced to their ultimate hydration pro- ducts before absorption. We have evidence that protein may be absorbed by the small intestine without having imdergone any hydration whatsoever. The absorption of serum protein has been discussed already in dealing with the mechanism of absorption of salt solutions from the gut. In a series of 24* 746 PHYSIOLOGY experiments made by Friedlander tke absorptions of various proteins were compared after their introduction into loops of the small intestine which had been washed free from ferment. During a period of three hours this author found that 21 per cent, of the proteins of egg white or of blood serum were absorbed. During the same period, of alkah-albumen which had been intro- duced into the loops, 69 per cent, was absorbed. On the other hand, when ■N^ n ^ »i€ Fig. 352. Figures (from Reuter) showing changes in intestinal epithelium induced by absorption of protein. I, epithelium of fasting rat ; II, initial stage ; III, later stage of protein absorption. syntonin and casein were introduced into the intestine, no absorption what- ever was observed. As to the condition in which such unchanged protein reaches the blood stream our knowledge is still imperfect. Foreign proteins, such as egg albumin, or the serum of other species introduced into the blood stream, may cause poisonous effects, and give rise to albuminuria, to lowering of blood pressure, or to alteration of the coagulability of the blood. If injected in small quantities they excite, as a reaction on the part of the organism, the production in the blood serum of a 'precipitin, and the presence of the precipitin may be looked upon therefore as a test by which we may decide whether these proteins have passed through the intestinal wall unchanged. In most cases it is found that, however abundant the amount of protein administered in the soluble form, none of it appears in the urine, THE ABSOEPTION OF THE FOOD-STUFFS 747 nor is any precipitin formation aroused. Ascoli has, however, observed such events occasionally to follow the administration of large doses of egg white, and it has been showTi that there is a difference in the behaviour of animals to the introduction of soluble protein into their ahmentary canal, according as they are new born or are more than a few days old. It seems that during the first few days of life the cellular lining of the alimentary canal is permeable to foreign proteins, whereas later on any protein which is taken up unchanged from the gut does not arrive in the same unchanged condition in the blood stream. The absorption, however, of unchanged proteins can play but a small part in the assimilation of protein as a whole. Animals very rarely take coagulable proteins in a condition in which they will arrive at the small intestine in a state of solution unchanged. Even in the carnivora the Hving tissues taken into the stomach will undergo coagulation by the acid, and wiU then be dissolved by the gastric juice. In man practically all the proteins of the food Are either insoluble or are rendered insoluble by the process of cooking. For absorption to take place it is therefore necessary that this insoluble or coagulated protein should be brought into solution, and this process is accomphshed, together with hydration, by means of the ferments of the gastric and pancreatic juices. This process of solution has long been regarded as the chief object of the digestive ferments. Although both Kiihne and Schmidt MiiHieim were aware of the production of amino-acids, such as leucine and tyrosine, as the result of digestion, they regarded their pro- duction as evidence of a waste of material. Proteoses and peptones are soluble, diffusible, and rapidly absorbed from the alimentary canal, and there is no doubt that a large proportion of the products of protein digestion are taken up by the absorbing membrane in this form. For many years phy- siologists were occupied with the problem as to the fate of these peptones and proteoses after their entrance into the mucous membrane. ' They do not pass as such into the blood. The injection of small quantities of proteose and peptone into the blood gives rise to the excretion of these substances by the kidneys ; injection of larger quantities has pronoimced poisonous effects, which were first studied by Schmidt Miilheim and Fano. If samples of blood be taken either from the portal vein or from the general circulation after a heav)'' protein meal, no trace either of proteose or of peptone is to be found in the blood. The observations of Hofmeister and others to the contrary depend on the fact that these observers employed a method for the separation of coagulable protein, as an antecedent to the testing for pro- teoses, which was in itself capable of producing small traces of these sub- stances. Hofmeister showed that during the absorption of a protein meal the mucous membrane either of the stomach or of the intestine, if rapidly killed by plunging into boiling water directly it was taken from the animal, always contained a considerable amount of peptone, and similar observations were made by Neumeister. If, however, the mucous membrane was kept warm for half an hour after removal from the body, the peptone disappeared. Salvioh, imder Ludwig's guidance, introduced peptone into a loop of gut 748 PHYSIOLOGY wliich was kept alive by passing defibrinated blood through its vessels. At the end of some hours the loop was found to contain a certain amount of coagulable protein, but no trace of peptone, nor was any trace of the latter substance found in the blood which had been passed through the vessels. These observations were interpreted as pointing to a regeneration in the intestinal wall of coagulable protein from the proteose and peptone taken up from the gut, and opinions were divided whether the most important part of this regeneration was to be ascribed to the leucocytes of the vilU (Hofmeister), or to the epithelial cells of the mucous membrane itself. It is evident that such a conclusion was not justified by the experiments. AU that these experiments showed was that the proteoses and peptones disappeared, i.e. were converted into something which did not give the biuret test. The discovery of the ferment erepsin by Cohnheim led this observer to repeat the experiments of Hofmeister and Neumeister with a view to testing the conclusions drawn by these physiologists. Cohnheim found that, although it was perfectly true that proteose and peptone disappeared when intestinal mucous membrane and peptone were placed together in the presence of either blood or of Ringer's fluid, this disappearance was due, not to a regeneration of coagulable protein, but to the fact that the erepsin of the mucous membrane carried the process of hydrolysis a step further, con- verting the proteoses and peptones into the ultimate crystalHne products of protein hydrolysis. Similar observations were made by Kutscher and See- mann, who showed that at anytime after a protein meal these end-products, especially leucine, tyrosine, lysine, and arginine, were to be found in the contents of the small intestine. A repetition of SalvioU's experiment by Cathcart and Leathes deprived this also of much of its significance. It was found that the artificial circulation, although sufficient to maintain the activity of the muscular wall of the intestine, as evidenced by the peristaltic movements, was insufficient to keep the mucous membrane ahve. After one hour's experiment the loop contained a mass of epithelial cells mixed with the products of the action of erepsin on the introduced peptone solution. In no case was there any diminution in the amount of uncoagulable nitrogen, i.e. there was no formation of coagulable protein, while the processes of absorption had been brought by the desquamation entirely to a standstill. These additional experiments caused a complete revolution in the attitude of physiologists towards the problem of protein absorption. All this evidence went to show that protein, however introduced, whether as coagulated protein or as albumose and peptone, underwent complete hydrolysis either in the gut or in the wall of the gut before entering the blood stream. If this were the case it should be possible to feed an animal on a diet in which the neces- sary protein had been replaced by the corresponding amount of ultimate products of protein hydrolysis, i.e. by a mixture which would give no biuret reaction. Such a possibility had previously been negatived on theoretical grounds by Kiihne and by Bunge. It was thought by these observers either that the animal body lacked the power of synthesis of proteins from these crystalHne products (hydration products), or that any complete THE ABSORPTION OF THE FOOD-STUFFS 749 hydration occurring in the intestine would involve such a loss of energy to the body as to be unteleological. Neither of these theoretical objections is justified in fact. We know from the researches of Fischer and others that although the difi'erent proteins in our food present a marvellous quahtative simihtude, in that all of them yield on hydrolysis the same kinds of amino- acids, there are great differences in the relative amounts of these amino-acids contained in different proteins. Thus, in gelatin, glycine is contained in considerable quantities, but is absent in many of the other proteins. Caseinogen is distinguished by the large amount of leucine that it yields, while ghadin, the chief protein of wheat flour, contains very large amounts of glutamic acid. It is difiicult to imagine how, for instance, muscle protein could be formed from wheat protein, a process continually occurring in the growing animal, unless we assumed that the protein molecule is first entirely taken to pieces, and that its constituent molecules are then selected by the growing cells of the body and built up in the order and proportions which are characteristic of muscle protein. Moreover, when we measure the amount of energy change involved in the hydrolysis of the proteins, we find it is rela- tively small. There is not a loss of 5 per cent, of the total energy available i.e. the heat of combustion of the products of pancreatic digestion would differ from that of the original protein submitted to digestion by less than 5 per cent. The energy of the protein as evolved in the body Hes, not in the couphng of the amino-acids with one another, or indeed in the coupUng of the nitrogen to the carbon, but, hke that of the other food-stuff s, in the carbon itself, and is derived from the combustion of the carbon of the molecule under the influence of the oxidising processes of the body into carbon dioxide. The experimental decision of this question was first attempted by 0. Loewi, who found that it was possible to keep a dog in a state of nitrogenous equiU- brium on a diet containing fat, starch, and a pancreatic digest of protein which contained no substances which would give the biuret test. These results have been confirmed for carnivora by Henderson, by Liithje, by Abderhalden and Rona, and by Hemiques and Hansen. According to Abderhalden, it is possible to keep an animal alive when the nitrogen in his food is represented entirely by the end-products of pancreatic digestion. The same result cannot be attained by the administration of the products of acid hydrolysis of protein, but this may be due either to the racemisation of the amino-acids under the action of the strong acid, or to the fact that the acid spHts up certain pol}^eptide groupings which are still contained in the tr}^sin digest, and which possibly cannot be synthetised by the cells of the body. We are justified therefore in concluding that while a certain small proportion of the proteins of the food may be absorbed imchanged, a much larger proportion is taken up as proteoses and peptones or as amino-acids. The proteoses and peptones are, however, rapidly changed in the mucous membrane itself into amino-acids, which we may regard as the form in which practically all the protein of the body is presented to the absorbing mechanisms of the alimentary canal for absorption and for passing on into the circulating fluids. 750 PHYSIOLOGY THE FATE OF THE AMINO- ACIDS AFTER ABSORPTION BY THE INTESTINAL EPITHELIUM. During a condition of starvation the normal protein requirements of the body, or rather of the active tissues, are met at the expense of the less active tissues. The protein characteristic of any ■ tissue can be taken down, removed to another part of the body, and built up into the protein characteristic of some other active tissue. It is diflS.cult to conceive that such a transference and transformation could occur in any other way than by a more or less thorough disintegration of the protein molecule at one place and its synthesis at the other, and we know from the researches of Hedin and others that every tissue contains intracellular ferments which are capable of effecting the disintegration of the protein molecule, and are responsible for the autolytic degeneration of tissues after death. If therefore the normal interchange of protein between the tissues is accompHshed, as we know it to be in plants, by the disintegration of the proteins into their constituent amino-acids and their subsequent reintegra- tion, there is no a priori reason to beheve that the blood carries the proteins from the ahmentary canal to the tissues in any other form than that of amino-acids. The experimental proof of this conclusion was hardly possible before the invention of a rehable method for the detection of small quantities of amino-acids in the blood and tissues. This is rendered possible by van Slyke's method, in which after the separation of coagulable proteins by alcohol the amino-acids are determined by measuring the nitrogen evolved on addition of nitrous acid. Van Slyke has shown that the blood always contains a certain amount of amino-acids even during fasting. After a pro- tein meal there is a considerable increase in the amount of amino-acids. Thus the blood of fasting animals contains from 3-1 to 5*4 milligrams amino- acid nitrogen per 100 c.c. Blood taken after food contains 8*6 to 10*2 milligrams amino-acid nitrogen per 100 c.c. of blood. The question of the fate of amino-acids thus absorbed from the intestine to the blood is decided by an estimation of the amino-acid content of the different tissues after the injection of amino-acids into the blood. Van Slyke has found that after the injection of amino-acids only a certain proportion is excreted with the urine, and that the rest of the amino-acids rapidly disappeared from the blood and are taken up by the tissues without undergoing any immediate chemical change, though in the case of certain tissues, such as the muscles, a definite saturation point exists which sets the limit to the amount of amino-acids that can be absorbed. On the other hand the capacity of the internal organs, and especially of the hver, for the absorption of amino-acid is much greater. It is worthy of note, however, that the absorption of amino-acids by the tissues from the blood is never complete, i.e. the amino-acids of the blood must be in a state of equilibrium with those of the tissues, although the con- centration in the latter may be much greater than in the former. If several hours be allowed to elapse after the injection of amino-acids before the analysis of the tissue is undertaken, it is found that the amino-acid nitrogen content of the hver may have returned to normal although the concentration THE ABSORPTION OF THE FOOD-STUFFS 751 in the muscles has suffered no appreciable fall. Since we have evidence that the circulation of amino-acids through the liver gives rise in this organ to the formation of urea, we must conclude that this organ is especially ■ responsible for the breakdown of the products of protein digestion which are not directly required for replacing tissue waste. This breakdown must involve a process of deamination. We may therefore conclude that the amino-acids normally produced by a protein digestion are absorbed without further change into the blood stream. They then circulate throughout the body, a certain proportion of them being built up in each tissue into the proteins characteristic of that tissue in order to replace the waste caused by wear and tear. The rest, probably the major part of the protein, is taken up by the liver, where it undergoes deamination, the nitrogen moiety being rapidly converted into urea and excreted by the kidneys, while the non- nitrogenous moiety is carried to the working tissues to which it serves as a ready and immediate source of energy. The fact that not only the blood but also the tissues contain amino-acids, even after complete starvation for some days, shows that these substances are intermediate steps not only in the synthesis but in the breaking do^vn of body proteins. Free amino-acids are thus the protein currency of the body, just as glucose is the carbohydrate currency. In the fasting body we must regard the processes of autolysis as the main source of the amino-acids found in the tissues, and it is by autolysis that the proteins of the resting tissues are made available in starvation for those whose continued working is essential for the maintenance of hfe. The fact that high protein feeding does not appreciably increase the amino-acid content of the tissues, shows that any storage of nitrogen in the organism must take place, not in the form of amino-acids, but as body protein. It was formerly thought that the deamination of amino-acids occvirred on a large scale in the wall of the alimentary canal, on the gromids that a larger amount of ammonia was present in the portal blood than in the arterial blood. It seems, probable, however, that the sovu-ce of this excess of ammonia is to be found in intestinal bacterial changes, and that the major portion of the amino-acids is actually absorbed unchanged. The view of Abderhalden that the amino-acids are synthetised in the intestinal wall to serum proteins, and absorbed in that form into the blood-stream, need here only be mentioned, since it lacks experimental support. THE ACTUAL COURSE OF DIGESTION In a recent series of papers London describes the com'sc of digestion of meals of Various characters in dogs wliich had been provided with fistulas in one of the following places : (a) gastric fistula (into the fundus of the stomach) ; (b) pyloric fistula (on the duodenal side of the pylorus) ; (c) duodenal fistula (about one foot below the pylorus) ; {d) jejunal fistula (about the middle of the small intestine) ; (e) ileum fistula (just above the caecum). We may take as an example the course of digestion of a meal comixjsed of 200 grm. of bread. This is eaten by the animal, mixed with the saliva and swallowed. On arriving at the stomach it gives rise to the secretion of gastric juice. In a series of special experiments London found that on the average 200 grm. of bread evoked the secretion of 20 grm. of saliva, about 10 grm. of mucus from (he coats of the stomach, and about 315 grm. of gastric juice. The secretion of gastric juice is continuous dui'ing 752 PHYSIOLOGY the whole time that the food remaiBS in the stomach. In the animal Tvith a pyloric fistula, one to two minutes after the meal had been taken, a few drops of alkaline fluid were extruded from the opening. Erom three to eight minutes after the conclusion of the meal small quantities of clear acid gastric juice were repeatedly extruded. The first admixture of the food with the outflow from the fistula occurred at eight to twelve minutes after the completion of the meal, and after this time the pylorus continued to open at regular intervals of ten to forty seconds, discharging each time a small amount of fluid composed of particles of undigested bread mixed with gastric juice. One and a half houi"s later the pylorus began to open less regularly and the fluid became of a more pasty consistence, devoid of lumps of undigested bread. In the fourth, fifth, and sixth hom's after the meal the pylorus opened only once every one or two minutes, and towards the end of this period the fluid extruded was clear. The follow- ing Table shows the percentage amount of food taken which had left the stomach at the end of each hoiu' after the meal : First hoiu* . 32-6 perc Second hour . . 17-9 Third hour . 29-5 Foiu'th hour . 1-87 Fifth hour 6-66 Sixth hour . 4-21 The large proportion of the ingested food leaving the stomach during the first two or three houi's can hardly be regarded as normal. Since in these experiments there was a free outflow from the pylorus and the food was not allowed to enter the duodenimi, the local reflex, evoked by the presence of acid in the duodenum, was absent. The gastric contents obtained in this way were analysed in order to find what changes had been wi'ought on the food by the gastric juice. It was found that 32 per cent. of the bread had been brought into solution. This solution had aftected the proteins more than the carbohydrates. Thus 67 per cent, of the nitrogen had been brought into soluble form, consisting chieflj' of proteoses and peptones. Ko amino-acids were formed. Only 25 per cent, of the starch of the bread had been rendered soluble, and of this, 21 per cent, was in the form of dextrine and 4 per cent, in the form of sugar. Xo absorption, however, either of the digested proteins or of the digested carbohydrates was ever foimd to take place in the stomach. DUODENAL DIGESTION. The influence exerted by the pancreatic juice, bile, and succus entericus, poui'ed out on the food in the duodenum, was studied by analybis of the intestinal contents leaving the intestine by a fistula, either at the lower end of the duodenum, or in the jejimum, or in the ilemn. From the duodenal fistifla Ihe expulsion of food occurs at repeated inteiwals, but in a somewhat irregiflar fashion, its movements being determined partly by the contractions of the stomach and partly by those of the duodenal wall. Usually a large gush is followed by a series of small gushes. Although only a foot intervenes between the duodenal fistifla and the pyloric fistula, a great difference is observed in the character of the intestinal contents obtained in the two cases. The outflow fi-om the duodenmn, being mixed with the pancreatic juice and the bile is yellow in colom- and increased in amoimt. With a meal of 200 grm. there is secreted on the average 130 grm. of bile and 140 grm. of pancreatic juice. During its passage through the duodenum the carbohydrates of the food undergo con- siderable changes, so that even one foot below the in'lorus we find that one-half to three-fifths of the carbohydi'ates have been converted into dextrine and sugar. A further digestion of the proteins also takes place amomiting to about one-tenth of the whole protein taken with the food. On deducting the amount of juices which have been added to the food it is found that even in this short length of intestine absorption has taken place of about one-sixth of the ingested food, about one-fourth of the carbohydi'ates having been absorbed and about one-eighth of the proteins. In a dog A\ith a fistifla about the middle of its small intestine, the outflow began six to fiifteen minutes after the meal, and lasted six or seven hours. The outflow was THE ABSORPTION OF THE FOOD-STUFFS 753 by small gushes repeated at intervals of five to ten seconds separated by intervals of one to five minutes, dm-ing which nothing appeared at the orifice of the cannula. The material obtained was quite different in character from that flowing from the duodenal fistula. The pasty character had disappeared, the material forming a frothy, orange- yellow, even jelly-like mass with practically no trace of undigested bread. From a fistula in the ileum the outflow occurred at long intervals of three to fifteen minutes and was much scantier than that obtained from the jejunal fistula, consist- ing of a thick jelly-like, orange-coloured mass. Both proteins and carbohydi'ates were entirely digested, and in the case of the former the chief products of digestion consisted of amino-acids. Thus in one experiment, after four large meals of 500 grm. of meat each had been given, in order to obtain sufficient quantity for analysis, 175 grm. of soluble substances were obtained. From this were isolated tyrosine, leucine, alanire, aspartic acid, lysine, and traces of arginine and histidine. From a fistula in the caecum there was no outflow until fom: or five hours after the meal had been taken. The material from the gut was then extruded in foecal like masses at long intervals of one half to one hour. This regular outflow lasted lor about six hours. The reaction of the contents was strongly alkaline, with no food particles, and the material contained merely debris of cells, with small traces of sugar, dextrine and unaltered starch. The absorption of the food-stuffs is thus practically complete by the time that the food has reached the lower end of the small intestine. The following Table gives the total amounts obtained in a series of experiments from the different fistulse after administration of 200 grm. of bread, and also the per- centage amoimt of food-stuffs which had been absorbed before the food had arrived at the level of the fistula in question : Total amounts obtained Absorbed from 200 grm. of bread per cent. Pyloric fistula 691 gr m. 0 Duodenal fistula . 691 „ 17-45 Jejunal fistula 585 ,. 37-77 Ileum fistula 412 .„ 67-65 Csecal fistula 80 „ 94-34 SECTION X THE F^CES The fseces are often regarded as representing the undigested or indigestible constituents of the food which have escaped solution and absorption in their passage through the aUmentary canal. This view is hardly correct as appMed to man or to the carnivora. In these the absorption of the constituents of a meal, whether consisting of fats, proteins, or carbohydrates, is practically complete by the time that the food has arrived at the lower end of the ileum. The faeces, in fact, are not derived from the food, but are produced in the ahmentary canal itself. This is shown by the fact that on analysing the faeces no soluble carbohydrates or proteins, albumoses, peptones, or amino-acids are to be found. After a meal of meat microscopic examination of the faeces reveals no trace of striated muscle fibres. Moreover animals in a state of complete starvation form faeces which do not differ in their composition from the fasces which are found after feeding with meat, eggs, sugar, or cooked starch, though the amount is less in a state of inanition than under normal circumstances. In one experiment Hermann isolated a loop of gut, joining its ends together so that a continuous ring was formed. The continuity of the gut was then restored by suturing the two free ends. After some weeks the isolated loop was found to contain a semi-sohd material similar to faeces in appearance, consistence, and chemical composition. It contained a large amount of phosphoric acid, Kme, and iron. So long as vegetables or coarsely ground cereals are excluded from the diet, the nature of the latter does not alter the chemical constitution or appearance of the faeces. Under these circumstances the faeces have the following composition : Water . . . . 65 to 67 per cent. Nitrogen . ... . 5 to 9 „ Ether extract . . . 12 to 18 ,. Ash 11 to 22 „ The ash consists chiefly of lime and phosphoric acid with some iron and magnesia. The ethereal extract contains fatty acids and a small amount of lecithin. Neutral fat is present in very small proportions. The faeces also contain small (Quantities of cholahc acid and its products of decomposition, dyslysin, and coprosterin, a body allied to cholesterin, and a certain amount of purine bases consisting of guanine, adenine, xanthine, and hypoxanthine. On the average the faeces contain about 0-11 grm. of purine bases per diem, 754 THE F^CES 755 about seven times as much as is contained in the urine passed in the same time. The material basis of the faeces seems to be largely desquamated epitheHal cells from the intestinal wall, and bacteria, of which countless numbers, chiefly dead, are present. It has been reckoned that as much as 50 per cent, of the faeces may consist of the dead bodies of bacteria. Very different is the composition of faeces if the food contains a large amount of cellulose. Not only does the ingested cellulose pass unchanged into the faeces, but large quantities of other substances enclosed in the cellu- lose walls may also escape digestion and absorption. Moreover the in- creased bulk of the undigested residue stimulates peristalsis, and thus quickens the passage of the food through the gut to such an extent that the digestive ferments have not time to exert their full action on the digestible constituents of the food. The influence of the character of the food is well illustrated by a comparison of the amount and composition of the faeces on different kinds of bread (Rubuer) : Eind of bread Weight of moist fasces Weight of faeces dried Percentage of ingested food Nitrogen (grm.) Bread from fine flour Bread from coarse flour . Bro'mi bread . 132-7 252-8 317-8 24-8 40-8 75-79 4-03 6-66 12-23 2-17 3-24 3-80 The following Table is also instructive. In this Table Rubner calculates the amount of faeces which a man would pass in twenty-four hours if he satisfied his energy requirements at the expense of one only of the difl'erent kinds of food enumerated, material which would be excreted in the faeces The numbers refer to the amount of organic Meat . . 26 grm. Rice . . 60 grm Eggs . . 26 „ Maize . 51 „ Macaroni . . 27 „ Turnips . 101 „ Wheaten bread . 36 „ Potatoes . 133 „ Milk . • 42 „ Coarse brown bread . 146 „ The indigestible cellulose in the food is not without value. It has been shown previously that the peristaltic contractions of the intestine are roused primarily by the mechanical stimulus of distension. If the food is capable of entire digestion and absorption the amount of faeces formed is limited to that produced by the intestinal wall itself. The small bidk exercises very Httle stimulating effect on the intestine, and the movements of the latter will therefore tend to be sluggish, especially in the absence of the mechanical stimulus determined by muscular exercise. The presence of a certain amount of cellulose in the diet may therefore be of considerable advantage by giving bulk to the faeces and ensuring the proper regular evacu- ation of the lower gut. It is probable that the constipation which is so common a disorder in civiHsed communities is due as much to the refinement in the preparation of the food as to the prevalence of sedentary occupations incident on the working of such a community. CHAPTER XI THE HISTORY OF THE FOOD-STUFFS SECTION I PROTEIN METABOLISM A PEOTEiN consists of the elements carbon, hydrogen, oxygen, nitrogen, and sulphur. In the oxidation which these bodies, in common with the other food-stuiis, undergo in the body, the carbon and hydrogen are converted to carbon dioxide and water. A certain proportion escapes this complete oxidation, being excreted by the kidneys in combination with nitrogen as the essential constituents of the urine (chiefly urea). When proteins are oxidised in the body there is a definite relation between the carbon dioxide . CO2 which is produced and the oxygen consumed. This respiratory quotient -jr- in the case of proteins equals 0-81. Since the respiratory quotient on a pure consumption of fats is only 0-71 and on carbohydrates equals 1, we may, by a study of the respiratory exchanges, arrive at some idea of the extent to which protein metabolism is responsible for the energy exchanges of the body as a whole. Such information by itself will always be somewhat uncertain, since it is possible, by a combination of fat and carbohydrate metaboUsm in proper proportions, to produce a respiratory quotient identical with that obtaining when the metaboHsm is purely of protein. On the other hand, the specific constituents of proteins, namely, nitrogen and sulphur, are excreted entirely by the urine (if we exclude the small traces which may leave the body in the sweat, or as scales of epidermis, hair, nails, &c.). It has therefore been customary to take the nitrogen output in the urine as an index of the protein metabohsm. This proceeding is only justified if we remember that we are deahng in the urine merely with the nitrogen and sulphur of the protein molecule, and that a large proportion of the carbon and hydrogen moiety of this molecule is being left unregarded. Since we cannot follow all the stages in the changes undergone by proteins on their way through the body, it will be convenient to take the nitrogenous end-products of protein metabolism such as appear in the urine, and to deter- mine, where possible, their precursors, and the conditions which determine their formation in the body. The chief nitrogenous constituents of urine are urea, ammonia, uiic acid, creatinine, hippuric acid. There is a small residue 756 PROTEIN METABOLISM 757 of undetermined nitrogen which may include traces of purine bases, such as xanthine and hypoxanthine, traces of amino-acids, small amounts of pig- ment and of nucleo-protein from the wall of the bladder. The relative pro- portions in which these bodies occur are not invariable, but differ according to the nature of the protein foods taken and also according to the proportion which the protein metabohsm bears towards the total energy requirements of the body. In the following Tables (Folin) are given the average composi- tion of two specimens of urine from the same individual, one on a diet con- taining the ordinary proportion, and the other on a diet containing only a minimal amount of protein : TABLES I AND II Distribution of Nitrogen in Urine on Various Diets July 13 July 20 Ordinary diet Low protein diet Vol. of m'ine 1170 c.c. 385 c.c. Total nitrogen 16-8 grm. 3-60 grm. Urea . 14-70 grm. = S7-5 % 2-20 grm. = 61-7% Ammonia . 0-49 grm. = 3-0 % 0-42 grm. = 11-3% Uric acid 0-18 grm. = M % 0-09 grm. = 2-5 % Creatinine . 0-58 grm. = 3-6 % 0-60 grm. = 17-2 % Undetermined 0-85 grm. = 4-9 % 0-27 grm. = 7-3 % Total SO3 . 3-64 grm. 0-76 grm. Laorganic SO3 3-27 grm. = 90-0 % 0-46 grm. = 60-5 % Ethereal SO3 0-19 grm. = 5-2 % 0-10 grm. = 13-2 % Neutral S • 0-18 grm. = 4-8 % 0-20 grm. = 26-3 % 111 dealing with the metabolism of the body as a whole we saw reason to beheve that the proteins taken in with the food might be regarded as having a twofold destiny. One part, and under normal circumstances the greater part, is apphed to the production of energy, in this respect discharging a function which might equally well be performed by the fats and carbohy- drates of the food. In its second function protein cannot be replaced by any other food-stuff, since it alone contains the necessary elements as well as the groupings of these elements which are essential for the building up of the living tissues. We saw reason to believe that this tissue metabolism onlv accounted, however, for a small part of the nitrogen of the food. On this account it is "possible to ensure health and a condition of nitrogenous equili- brium with amounts of protein in the diet of man which might varv between 40 and 200 grm. per diem. The more protein that is taken in with the food the greater is the relative amount which is apphed to the energy needs of the body. If therefore we would attempt to find out what are the end- products of the tissue metabolism we should confine the energy metabolism of proteins within the smallest possible limits by reducing the quota of pro- tein in the diet to its minimum. Folin has shown that if we compare'the com- position of the urine obtained under these two conditions, namely, on a diet containing a normal quantity of protein and on a diet coutaioiug a minimal 758 PHYSIOLOGY amount, we find evidence of a qualitative difference between the two kinds of metabolism. The difference is well brought out in the Tables just quoted. On a large diet the greater part of the nitrogen can be regarded as derived directly from the food, whereas on a small diet a relatively larger proportion of it must come from protein which has been previously built up into the tissues. Fohn distinguishes these two sources of the nitrogen of the urine as exogenous, i.e. that from the food, and endogenous, i.e. derived from the tissues. Two facts stand out in comparing these two urinary 24H0L Pig. 353. The hourly variation in the excretion of nitrogen after a meal. The meal was given at 0. The thick line represents the average ab- sorption of the food from the alimentary canal. The thin-lined curves represent the N. excretion (1) after a meal of 1000 grm. meat ; (2) after 500 grm. meat and 150 grm. fat ; (3) after a meal of 500 grm. meat ; (4) and (5) both represent the excretion in a fasting animal. (From Tigebstedt after Feder.) analyses. In the first place, on a normal protein diet the urea accounts for 87 per cent, of the total nitrogen of the urine. On an excessive protein diet this percentage may rise to 90 or 95. On the low protein diet the percentage of nitrogen appearing as urea is reduced to 60. On the other hand, practi- cally identical amounts of creatinine are obtained under the two conditions, so that whereas on the full diet it amounts only to 3-6 per cent., on the low protein diet it forms as much as 17 per cent, of the total nitrogen output. We are therefore justified in regarding urea as to a large extent exogenous in origin, and as derived directly from the nitrogenous moiety of the protein molecule, which may not at any time have formed part of the living tissues of the body. On giving a large protein meal to a dog the urea in the urine rapidly rises, and at the end of four or five hours 50 per cent, of the total nitrogen taken in with the food has appeared in the urine as urea (Fig. 353). If we take into account that the digestion of a meat meal in this animal may go on for eight hours, we are justified in the statement that by far the greater PROTEIN METABOLISM 759 poi-tion of the protein nitrogen taken with the food is excreted almost directly after absorption as urea in the urine. Urea is therefore to be re- garded in the first place as an index to the amount of protein absorbed. We have seen that the end-products of protein digestion in the intestine are the amino-acids ; and that these are the immediate precursors of the urea is shown by the fact that the administration of these bodies is followed very rapidly by the appearance of the whole of their nitrogen in the urine as urea. Many attempts have been made to explain the method by which urea may be derived from the amino-acids. F. Hofmeister succeeded in preparing urea by oxidising amino-acids with potassium permanganate in the presence of ammonia and ammonium sulphate. He assumes that urea is formed by an oxidation synthesis. The first step would be the formation, by oxidation of protein or amino-acids, of the group CONHg, and this at the moment of formation would combine with the NH2 left over in the oxidation of the ammonia to form urea, According to Drechsel and Nencki, the immediate precursor of the urea is probably ammonium carbamate, which loses a molecule of water, thus : C0<^H.-H,0-CO<™= Schroder, on the other hand, suggested that the urea was formed from ammonium carbonate by the loss of two molecules of water. °°Tic acid led to the excretion of the corresponding amino-acid in the urine. Since keto-acids occur as the ordinary products of the breakdown of amino-acids and also as the inter- mediate products of oxidation of oxy-acids, e.g. lactic acid, it is evident that the animal body can assimilate ammonia and form amino-acids, provided only that it is supphed with the proper non- nitrogenous acids. These latter need not be derived from proteins at all, but, hke lactic acid, be a result of carbohydrate metabolism. Thus, if the fitting non-nitrogenous food be given (e.g. oxy-fatty acids, or carbohydrates, from which these bodies may be formed), part of the nitrogen set free by protein disintegration might be recombined with the formation of amino-fatty acids without gi^^ng rise to urea or appearing in any way in the nitrogen balance-sheet of the body. This possibility enjoins the necessity of caution in interpreting the results of metabolism experiments where the nitrogen excreted is taken to represent the total protein metabohsm of the body. The fate of the nitrogen does not, however, matter much to the energy balance-sheet of the body, since so far as regards energy the residue of the protein molecule or the amino-acid molecule which is left behind after the process of deainination has taken place, has lost only from one-fifth to one-tenth of the total energy of the original molecule. This is shown in the following Table of the heat equivalents of some of the amino-acids and their corresponding fatty and oxy-acids : Calories Substance Leucine per grm. molecule 855 Isobutylacetic acid . Alanine .... 837 389 Propionic acid . Lactic acid 367 329 Pyruvic acid not dstermined Even in the case of the smallest molecule the loss of energy attendant on simple deamination and conversion into the corresponding oxy-acid only amounts to about 20 per cent. We thus come to the conclusion that the urea output in the urine after a protein meal teUs us nothing whatever about the fate of that part of the protein which contains 80 to 90 per cent, of the total energy of the protein food. So far as concerns the output of energy, the exogenous protein metabolism may be regarded as practically non-nitrogen- ous. The rise in the rate of excretion of urea after a protein meal was regarded both by Voit and Pfliiger as a sign that the cells of the body pre- ferred to use proteins for all their requirements if this substance were available. We see now that the big output of urea after a protein meal affords no basis for this view, but is rather a sign that the body has no need for all the nitrogen contained in its food and that this must be got rid of before the really valuable part, the energy-giving part of the protein molecule, is admitted into the metabolic cycle of the cells. The important problem in the energy metabolism of protein is thus, not 764 PHYSIOLOGY the origin of the urea, but the nature of the substances that are left after deamination and their subsequent fate in the body. Since they contain only the elements carbon, hydrogen, oxygen, one would expect to find that they could replace either fat or carbohydrate. So far as concerns the pro- duction of energy this is true. Moreover, as we shall see in deahng with the metaboHsm of carbohydrates, we have definite evidence that part of this non- nitrogenous moiety of the protein molecule may be converted into sugar or glycogen. Thus, of the amino-acids formed by the digestion of proteins, glycine, alanine, aspartic acid and glutamic acid can be converted quanti- tatively under appropriate circumstances into glucose. On the other hand, leucine, phenylalanine and tyrosine yield no glucose, even in the diabetic animal, but may in the Hver undergo conversion into aceto-acetic acid, which is a stage in the oxidative disintegration of fats. In spite of this latter fact we have no evidence that fat may be formed from this part of the protein molecule ; at any rate, no fat which can be stored in the body and give rise to the production of adipose tissue. On the other hand, the proteins, more than either of the other two food-stufis, cause a direct augmentation of the respiratory exchanges of the body. This is shown in the following Table by Rubner, in which isodynamic quantities of proteins, fats, and carbo- hydrates were administered during a period of starvation : Day Food Calories Energy metabolism N. grm. Fat grm. Carbohydrate grm. Total calories Per kilo body weight calories 2 . . _ _ _ _ 969 40-2 3 . . 56-8 — — 1543 1072 44-8 4 . . — — — 947 39-9 5 . . — 167 — 1536 963 40-9 6 . . -^ — — • — • 922 39-6 7 . . — — 411 1446 982 42-3 8 . . — • — — — 977 42-1 It will be seen that the metabolism, i.e. the caloric output, of the body on administration of protein increased 11 -9 per cent., whereas with fat the increase amounted only to 1-2 per cent., and with carbohydrate to 4-7 per cent. In another similar experiment the animal received 57-4 calories protein, 54*2 calories fat, and 57 calories carbohydrate respectively per kilo body weight. During hunger the total metabolism per kilo body weight amounted to 37-5 calories ; with meat, to 46 calories ; with fat, to 39-4 calories ; with carbohydrates, to 39-4 calories. Compared with the metabohsm during starvation the rise per cent, with protein was 24-3, and with fat and carbohydrates 5-1. This surplus output of energy resulting from the administration of protein cannot be ascribed to increased work thrown on the digestive organs. There is no evidence that this is greater in the case of proteins than it would be with carbohydrates or fats ; PROTEIN METABOLISM 765 and even if the capacity of these organs be strained to their utmost by administration of large quantities of bones, the increase in the carbon dioxide output which results is not so great as that following a large protein meal. It seems therefore that the CHO moiety of the protein undergoes oxidation more rapidly than either dextrose or the ordinary fats of the diet, and that, as we concluded in an earUer chapter, the metabolism of these substances is really to a considerable extent dependent on the quantity presented to the organ- ism rather than on the actual needs of the cells of the body. It is this rise in metabolism and respiratory exchanges after protein ingestion which justifies to a certain extent the idea that the proteins, more than any other food-stuff, have a stimulant action on metabohsm. The reason why the CHO remainder of the protein molecule is so prone to oxidation and does not, like an excess of carbohydrates, undergo conversion into fats in the body, we shall have to consider in greater detail in deahng with the fate of this latter class of substances. We need, however, considerably more evidence as to the extent to which deamination occurs and as to its conditions and end-products before we can hope to determine the cause for the rapid breakdown of these end-products in the body. ARE THE AMINO-ACIDS INTERCONVERTIBLE i Although the animal organism is apparently capable of synthetising amino-acids from ammonia and the corresponding keto- or oxy-fatty acid, it is unable to convert one amino-acid into another. On this account many proteins are inadequate as food substances since they do not contain the necessary amino-acid groups. Life cannot be supported on such bodies as zein or gelatin, which are lacking in the tr}^tophane and tyrosine groups. The failure in these cases is not, as has been generally supposed, owing to an inabiUty to assimilate, i.e. synthetise, nitrogen as ammonia, but to the fact that in the animal the apparatus is wanting for the manufactm*e of some of the oxy-fatty acids and other radicals which form the non-nitrogenous part of the amino-acids. This view receives confirmation from the fact that the simplest of the amino-fatty acids, namely, glycine, can be easily manu- factured in the body, acetic acid being one of the latest stages in the oxidation of most carbohydrates and fats. It has been shown that alanine too can be easily manufactured by the body, by the amination of the three carbon acids or oxy-acids derived from the breakdown of glucose or glycogen. THE EXCRETION OF AMMONIA A large proportion of the urea appearing in the urine after a protein meal is exogenous and is derived by a rapid separation of ammonia from the proteins or their disintegration products ahnost immediately after their absorption. The greater part of the ammonia is converted in the hver into urea, which is excreted by the kidney. A certain small proportion of the nitrogen in the urine is generally turned out in the form of ammonia. This proportion is not increased by the administration of ammonium carbonate. If ammonium chloride be given to a starving rabbit it appears in the urine 766 PHYSIOLOaY "imclianged, and so increases the proportion of ammonia in this fluid. If, however, the ammonimn chloride be administered at the same time as the animal is receiving its ordinary vegetable diet there is no increase in the ammonia in the urine, the whole of the ammonium chloride being converted into urea. The factor which determines the proportion of ammonia in the urine is the relative proportion of acids and bases which have to be ehminated from the body. The normal reaction of urine, though acid as regards certain indicators, can be regarded as neutral, since it contains no free hydrogen ions, the ' acidity ' being due to the presence of such substances in solution as acid sodium phosphate. If the fixed alkaUes in the food are sufficient to combine with the whole of the acids excreted from the body, then the ammonia will be completely converted into urea and ehminated as such. If, however, a dose of mineral acid be administered to an animal, this must be excreted in com- bination with a base. If the fixed alkahes available do not suffice for this purpose, the neutrahsation of the acid is efiected by couphng with ammonia. The ammonia of the urine is therefore an index to the amount of acids which are excreted. These acids may be introduced directly with the food, as when mineral acids are administered by the mouth, or may be the product of abnormal metabohc processes occurring in the body. Thus under certain circumstances, e.g. in complete carbohydrate starvation, there is a failure in the last stages of the oxidation of fats, and oxy- fatty acids, viz. oxybutyric acid and aceto-acetic acid, are produced in the body in large quantities, but cannot undergo further disintegration. The alkalescence (electrical neu- trahty) of the fluid media of the body is a necessary condition for the con- tinuance of the Hfe of the cells and especially of the normal processes of oxidation. It is therefore essential for the preservation of Hfe that the acids thus formed and accumulating as a result of the impaired oxidative processes should be neutralised, carried to the kidneys, and excreted by them in combination with some base. When these acids are produced in large quantities the alkahes of the food and of the tissues do not suffice for their neutrahsation. Ammonia, which is a constant intermediate stage in the production of urea, is then utihsed for this purpose and the acids appear in the urine together with the corresponding amount of ammonia. The ammonia therefore of the urine gives valuable information, not as to the total nitro- genous exchanges of the body, but as to the formation of acids in abnormal quantities during the processes of metabohsm. There is one other method in which urea may be formed by a rapid alteration of the proteins taken in with the food. Nearly all the ordinary proteins contain arginine as an integral part of their molecule. This sub- stance can be regarded as formed by a couphng of guanidine with amino- valerianic acid and as analogous to the most prominent extractive of muscle, namely, creatine, which is methyl guanidine acetic acid. On heating either of these substances with baryta water it undergoes hydrolysis and is decom- posed with the formation of urea and, in the case of arginine, a-^-diamino- valerianic acid ; in the case of creatine, methyl amino-acetic acid or sarco- sine. It has been shown by Dakin and Kossel that the same change may be PROTEIN METABOLISM 767 efiected under the agency of a ferment, arginase, which is contained in extracts of the intestinal wall or of the hver. We have every reason to beheve therefore that a certain small proportion of the urea which appears in the urine after the ingestion of protein is due to this hydi'olytic sphtting of the arginine contained in the protein molecule. The other moiety of the arginine, namely, the diamino-valerianic acid, probably undergoes the same changes as the other amino-acids, such proportion of it as is not required for the building up of the tissues of the body being deaminised and giving rise to urea and some CHO group in the manner already discussed. THE ENDOGENOUS OR TISSUE METABOLISM OF PROTEINS On comparing the output of the various nitrogenous excreta given in Fohn's Tables quoted above (p. 757), we see that on a low protein diet, when the exogenous or energy metabohsm of this food-stuff is reduced to a mini- mum, the only substance which does not undergo simultaneous diminution is the creatinine. Whereas on an ordinary diet free from meat it only ac- counts for about 3 per cent, of the total nitrogen output, on the low diet it contains as much as 17 per cent. The conclusion at once suggests itself that creatinine, more than all the other constituents of the mine, must be regarded as an index of the tissue metabohsm of protein. Let us see what facts can be adduced in favour of this view. Creatinine has the formula : NH = C.N(CH3).CH2 I I NH CO and may be regarded as derived by a process of dehydration from creatine (methyl guanidine acetic acid). NH =C.N(CH3).CHoC00H I NHg It may be formed from this latter substance by boihng for three hours with strong hydrochloric acid. Creatine has long been known as the most abimdant nitrogenous extractive in the body. It exists in relatively large quantities in muscle, and in meat extracts, such as Liebig's, it occurs to the extent of 10 or 12 per cent. It has been calculated that the body of a man at any time contains about 90 grm. of this substance. On boihng creatine with baryta water it undergoes hydrolysis A\ith the formation of urea and sarcosine or methyl glycine. CHa CH3 NH I NH, I ^C.N.CHoCOOH + HoO = >C0 + HN.CH2COOH NHg/ NHg/ Creatine Urea Methyl glyci no Owing to the ease with which this formation of urea from creatine may be brought about outside the body, it was natural that this substance should be regarded as an important precm'sor of the urea in the urine. The view 768 PHYSIOLOGY was held till recently, however, on the ground of experiments by Voit, that creatine administered in the food appeared in its entirety as creatinine in the urine, so that if creatine were hberated from the muscles in their normal processes of metabohsm it would pass to the kidneys and be excreted as creatinine without undergoing further decomposition. On this account too the creatinine in the urine was regarded as derived almost exclusively from the creatine taken in with the food. The analyses given in Fohn's Tables show that in one respect at any rate this view was incorrect. Creatinine is excreted in considerable quantities even when the man is on a creatine-free diet, or even when his food is almost free from protein. It has been found moreover by Fohn that creatine administered by the mouth may disappear in the body. This is especially the case if the animal or man is on an in- sufficient protein diet, but there is no evidence of a corresponding increase in urea formation. If a larger amount be given creatine appears as such in the urine. In most cases a certain minute proportion escapes and causes an increase in the quantity of creatinine. Under abnormal circumstances, e.g. during illness, when the physiological activities of the body are lowered, a portion of the creatine may be found in the urine in an unchanged con- dition. If creatinine is to be regarded in any way as the index of tissue metabohsm its amount ought to vary with the extent of this metabohsm. Thus it should be increased when there is an exaggeration of the disintegrative processes in the tissues, and should be diminished when the nutritive changes in these tissues, especially in the muscles, are reduced to a minimum. The end-products of tissue metabohsm therefore should be increased under the following conditions : (1) Increased motor activity involving increased wear and tear of the muscular tissues. (2) In fevers, especially in those where there is severe toxsemia and rapid wasting of the muscles of the body. On the other hand, it should be diminished where the activity of the muscular tissue is reduced to a minimum, as under the influence of sleep or soporifics, or where the bulk of the muscular tissue is reduced as well as its activity, as in cases of widespread muscular atrophy and paralysis. The excretion of creatinine has been investigated under these various conditions by Van Hoogenhuyze and Verploegh, and their results fully bear out the view expressed above as to the intimate relation of creatinine with the tissue metabohsm of protein. L; During protein starvation the uric acid output, though diminished, does not show a change which is at all proportional to that shown by the urea. This substance also might therefore represent an end-product of tissue metabohsm. Since, however, uric acid is an outcome of the metabohsm of a special group of bodies, the nucleins and purine bases, we shall have to devote a complete section to its consideration. Although the urea is diminished in protein starvation, it still remains the most abimdant nitrogenous constituent of the urine. We are therefore not justified in excluding this substance from the products of tissue metabohsm. PROTEIN METABOLISM ^6D If any creatine undergoes complete oxidation in the body during protein starvation a certain proportion of the urea might be derived in this way. We shall see later that uric acid may possibly also undergo further oxidation with the formation of urea. Even during complete protein starvation some of the urea which is turned out may be the expression of a utilisation of pro- tein through deamination for the energy needs of the body. The active cells are bathed everywhere with a tissue fluid in which prciceins form a prepon- derating constituent, and it is possible that, even in the times of greatest protein need, these cells utilise the proteins of their surrounding medium, though in a reduced degree, for the production of energy. In this case the active cell would initiate the utilisation by throwing off that part of the protein molecule, namely, NHg, which is useless to the cell as a source of energy, so that deamination would be carried out in the working tissues, and not, as in the rapid formation of urea after a heavy meal, in the liver. SULPHUR Sulphur occurs in the urine in three forms, namely, as ordinarv inorganic sulphates, as ethereal sulphates (indoxyl- and skatoxyl-sulphates), and in an unoxidised condition often termed neutral sulphur. There is no doubt that part of the latter consists of cystine, part of sulphocyanates, and in some animals mercaptan compounds. The excretion of the inorganic sulphates rises fari passu with that of the urea, so that very soon after the throwing off of the NH2 group there must be also a removal and oxidation of the greater part of the sulphur contained in the cystine group of the ^^rotein molecule. So far as regards the metabolism of the body as a whole, the ethereal sulphates may be classed with the inorganic sulphates. They arc excreted in varying quantity accordiiig to the extent of the decomposition processes which are occurring in the intestine. Under the influence of these processes the trypto- phane, produced in the pancreatic digestion of proteins, is converted into indol and skatol. These two substances after absorption are deprived of their poisonous qualities by oxidation and conjugation with sulphuric acid to form the indoxyl- and skatoxyl-sulphates of the urine, both of which are innocuous. If the processes of putrefaction are increased, as in intestinal obstruction, the relative amount of sulphate appearing in the conjugated form is also increased. On administration of phenol a large proportion of the sulphate appears in the urine conjugated with phenol or with products of its oxidation. If the normal putrefactive processes which go on in the intestine are abolished by the administration of intestinal antisejitics such as naphthalene or calomel, the ethereal sulphates practically disappear from the urine. We cannot therefore regard the absence or diminution of the ethereal sulphates during protein starvation as throwing any light on the endogenous protein metabolism. On the other hand, the fact that the neutral sulphur undergoes no decrease suggests that this part of the sulphur output of the organism may be connected with tissue metabolism. Further observations on the output of neutral sulphur during fever or wasting diseases are necessary before a definite conclusion can be arrived at on this point. 770 PHYSIOLOGY THE FATE OF THE AROMATIC AND OTHER CYCLIC GROUPS IN THE PROTEIN MOLECULE A t5rpical protein such as can be utilised as a complete food-stuff con- tains, in addition to the amino-acids of the fatty series, a number of other nitrogenous derivatives of cycHc compomids, including benzene, indol, pjnrrol, and iminazol. Substances such as gelatin, from which some of these groupings are absent, cannot, as we have seen, entirely replace protein in the food. So far we are acquainted with three compounds of the aromatic series among the products of disintegration of the protein molecule. These are tyrosine, phenylalanine, and tryptophane. Since these substances are also contained in the protein constituents of the tissues we may assume that, after they have been set free by the digestive hydrolysis of proteins, they are absorbed and built up again with the other amino-acids in appropriate groupings. Like these they are susceptible of complete oxidation in the body, so that they can contribute to the supply of energy. Any one of these substances, administered with the food or subcutaneously, is entirely de- stroyed, with the production of urea, carbon dioxide, and water. In this respect they present a marked contrast to abnost all other compounds of the aromatic series. In these we find that the benzene ring is extremely stable, so that, although changes may occur in its side-chains, the benzene ring itself appears intact in the urine, and is not broken up in the body. Thus benzoic acid, benzylalcohol, and phenyl propionic acid, when administered, are passed in the urine as hippuric acid (benzoyl glycine). Indol and skatol, which are closely alhed to tryptophane, undergo oxida- tion in the body without further modification and appear in the urine as conjugated aromatic sulphates. Some light is thrown on the conditions of breakdown of these aromatic bodies by the study of a rare disorder in metaboHsm, which may occur in certain famiUes and is known as alcaptonuria. In this condition, which is congenital and lasts throughout "hfe, the urine darkens considerably when made alkahne and exposed to the air. It has the power of reducing Fehhng's solution, so that the presence of sugar might be suspected. On analysis the pecuharities of the urine are found to be due to the presence in it of a substance known as homogentisic acid. This is dioxyphenyl acetic acid. HO /\ CH2COOH The amount of this substance in the urine bears a constant ratio to the nitrogen excreted. It does not disappear during starvation, and is much increased on a large protein diet. It must therefore be derived from the disintegration of proteins both exogenous and endogenous. If tyrosine or phenylalanine be administered to patients affected with this disorder, both substances are quantitatively converted into homogentisic acid. The ratio PROTEIN METABOLISM 771 of this acid to the total nitrogen indicates that the wliolc of the tyrosine and phenylalanine of the protein molecule, whether set free in the alimentary canal or in the tissue metabohsm, is converted into homogentisic acid. It is not possible to conceive of the direct conversion of tyrosine °« HO into homogentisic acid OH CHoCOOH CH2.CHNH2.COOH The tyrosine must first be reduced to phenylalanine CH0.CHNH..COOH and then this substance must undergo oxidation into homogentisic acid. Since phenyl lactic acid and phenyl pyruvic acid, but not phenyl acetic acid, are also converted in alcaptonuric patients to homogentisic acid, it has been suggested that these two substances form stages in the conversion of phenylalanine into homogentisic acid. Thus OH CH,CHOH.COOH CHgCO.COOH CH,COOH Phenyl lactic Phenyl pyruvic Homogentisic It is further thought that under normal circumstances the phenyl deriva- tives, tyrosine and phenylalanine, are oxidised to homogentisic acid as in the alcaptonuric patient. In the normal individual, however, the introduction of two hydroxyl groups into the benzene ring leads to some process, perhaps of a ferment character, which breaks up the ring. This ferment is absent in the alcaptonuric, so that the transformation of the phenyl derivatives stops short at the stage of homogentisic acid (Garrod). The eminently specific character of this process is shown by the fact that although these various substances undergo complete oxidation in the body, a shght modifi- cation in the chain of the processes renders the change impossible. Thus if the side group in phenyl lactic or phenyl pyi-uvic acid be converted to acetic acid before the introduction of the two OH groups into the phenyl ring, the phenyl acetic acid thus produced is incapable of undergoing further oxidation. TjTosine in the intestine undergoes dcamination to form ox}^henyl propionic acid and oxyphenyl acetic acid. These cannot be further oxidised, but appear in the urine as such or, after conversion into kresol or phenol, as sulphuric acid esters. Somewhat similar conditions apply to the oxidation of tr^'ptophane. 772 mY^IOLOGY This body is an indoi derivative and consists of a benzene ring and a pyrrol ring having two of their carbon atoms in common. Its formula is HC C C.CH2CHNH2.COOH I II II HC C CH i.e. it is indol amino-propionic acid. It undergoes, like tyrosine, complete oxidation in the body. On the other hand, a very slight alteration in the molecule renders it incapable of this change. Thus the tryptophane set free by the tryptic digestion of proteins under the influence of the putre- factive bacteria of the intestine may undergo deamination and reduction with the production of indol propionic acid, and this by oxidation may be converted to indol acetic acid. The latter substance by decarboxylation may be converted into skatol, or by oxidation nearer the chain and further loss of carbon dioxide into indol. Of these products of bacterial change, indol acetic acid may be found in the urine, and indol and skatol are oxidised to the corresponding phenols and pass into the urine conjugated either with sulphuric acid or with glycuronic acid. Apart, however, from these putre- factive changes due to bacteria, no indol derivatives pass into the urine. The amount of the indol and skatol esters serves therefore merely as an index of bacterial decomposition in the alimentary canal, and gives us no clue to the total tryptophane metabolism of the body. If putrefaction be prevented by the administration of calomel or other intestinal antiseptic these esters may entirely disappear from the urine. On the other hand, the partial obstruction to the onward passage of food, caused by dividing the small intestine in two places a few inches apart and replacing the intervening length of intestine the wrong way round, causes the indican excretion to be increased twenty or thirty fold. Subcutaneous injection of tryptophane in rabbits does not increase the indoxyl and skatoxyl sulphates (urinary indican) in the urine, whereas a considerable increase is brought about by subcutaneous injection of indol. The pyrrol ring which occurs in proteins as proline and oxyproline {i.e. pyrrohdine carboxylic acid and oxypyrrolidine carboxylic acid) appears to undergo complete disintegration in the body. The steps in this conversion are unknown, though it is possible that the ring may be unlinked so as to produce from the pyrrol ring amino- valerianic acid, which would then under- go the process of deamination with which we are already famihar. This ring is of interest since it appears to take an important part in the building up of the molecule of hsematin, the essential prosthetic group of the haemoglobin molecule. Another ring grouping, iminazol, occurs in histidine, which is iminazol c(-amino-propionic acid. This too undergoes complete oxidation in the body. It is important to bear in mind that this ring may be produced synthetically by very simple means, i.e. by the action of zinc oxide and PROTEIN METABOLISM 773 ammonia on glucose, which results in a rich yield of methyl iniinazol {v. p. 117). The same grouping is found in creatinine, as is seen by comparing the formula) : H2C-N0H OC-NH HC-N^ Creatinino IMcthyl-iminaxoI and it is possible that this may furnish a clue to the mode of formation of creatinine in muscle. Creatine has generally been regarded as the primary product of muscular metabolism, but it is possible that the ring-grouping is the original one and that creatine is produced by hydrolysis occurring in this ring. The iniinazol group is at present chiefly interesting in that it contributes to the formation of the complex ring compounds known as the purines. Since the purine metabolism is closely connected with the question of the origin of uric acid we may consider these questions together. SECTION II NUCLEIN OR PURINE METABOLISM In an undifferentiated cell the proteins, as such, form but a small part, the mass of the cell being composed of conjugated proteins. The nucleo-proteins are especially abundant constituents of nuclei, and therefore occur to a greater or less extent in all the ordinary animal foods, eggs and milk excepted. Just as the metabolism of proteins is the metabohsm of the amino-acids, so the metabolism of the nucleo-proteins and nucleins is essentially comprised in the history of its main constituents, i.e. the purines. The nucleo-proteins themselves are bodies of very varjdng composition. If any cellular tissue such as thymus or liver be extracted with water or salt solution, a fluid is obtained from which a precipitate can be thrown down by the addition of acid. This precipitate as a rule is soluble in excess of acid or in alkahes. If subjected to gastric digestion it undergoes solution, leaving behind a residue of nuclein which is rich in phosphorus. The amount of this residue varies with the strength of the artificial gastric juice employed, so that the method cannot be looked upon as in any way quantitative, and the question arises whether the original nucleo-protein is to be regarded as an association or a combination of nuclein with ordinary protein. The most convenient source for the preparation of nucleins is the heads of fish spermatozoa. All nucleins are associations or compounds of nucleic acids with proteins belonging to the class of protamines or histones. The nucleins of fish spermatozoa contain protamine as one of their constituents. On separating off the protamine, nucleic acids can be isolated. These acids have been named either according to their source or according to the purine base which is their most prominent constituent. Only from inosinic acid, the nucleic acid of muscle, has it been found possible to prepare crystalline deriva- tives, so that in all other cases it is difficult to decide whether we are deahng with chemical individuals or with mixtures. On hydrolysing any of the nucleic acids by heating with strong mineral acid, they are broken down into a series of bodies belonging to the following four groups : (1) phosphoric acid, (2) purine bases, (3) pyrimidine bases, (4) a carbohydrate. The chief purine bases obtained from the hydrolysis of nucleic acid are guanine and adenine. Hypoxanthine and xanthine are often obtained as products of decom- position of nucleic acid, but are generally formed by the deamination and oxidation of guanine and adenine. Fischer has shown that all these bodies 774 NUCLEIN OR PURINE METABOLISM 775 are derivatives of a base purine. They contain a central chain of three carbon atoms to which is attached on each side a urea group, so that they may be regarded as diureides. Purine itself has the formula N=CH I I HC C~NH II II \CH N— C— nX Purine The relation of the purine bases obtained from disintegration of nucleic acid to purine itself has been given on p. 103. From these formulae we see that adenine and hypoxanthine are related to one another, adenine being 6-aminopurine, while hypoxanthine is 6-oxypurine. In the same way guanine and xanthine are related, guanine being 2-amino-6-oxypurine, while xanthine is 2- 6-dioxypurine. The investigation of the relationships of these bases was of interest to physiologists since it brought to light the close relation which they have to uric acid, a substance which has been known as a constituent of urine and urinary calculi for a long time, having been discovered in 1776 by Scheele. Uric acid is 2-6-8-trioxypurine and has the formula HN— CO I I CO C— NH. I II >o HN— C— NH/ Uric acid = 2-6-8-trioxypurine In uric acid the two urea groups are attached to a central 3-carbon chain, and it is interesting to note that one of the first syntheses of uric acid, namely, that by Horbaczewski, was accomplished by melting together trichlorlactamide and urea in a sealed tube. Belonging to the same group of purines are the two bases which form the essential constituents of tea, coffee, and cocoa. In tea and cofEee is found caffeine, which is l-3-7-trimethyl-2-6-dioxypurine, and in cocoa occurs the closely allied theobromine, which is 3-7-diinethyl-2-6-dioxypuriue. Caffeine is thus methyltheobromine. CH3.N— CO HN— CO I I /CH3 [ I /CH3 CO C— N( CO C— N< I II >CH I II >CH CH3.N— C— N^ CH3N— C— N^ Caffeine = 1-3-7-trimethyl- Theobromine = 3-7-dimethyl- 2-6-dioxypurine 2-6-dioxypurine The pyrimidine bases which are also obtained from the hydrolysis of nucleic acid are derived from a pyi'imidine nucleus which is, so to speak, half a purine nucleus, consisting of a C. ^ chain joined to a 3-carbon 770 PHYSIOLOGY chain. Three pyrimidine bases have been isolated from the decomposition products of nuclein, namely, thymine, cytosine, and uracil. NH— CO XH — CO N = C.XHa CO CH CO C.CH3 CO CH I II I II I II NH — CH NH — CH NH — CH Uracil 2-6-diosy- Thymine 5-niethyl- Cytosine 6-amino- pyrimidine uracil 2-oxyi3yrimidine Cjiiosine is easily converted by oxidation into uracil. After separation of the purine and pyrimidine bases and phosphoric acid a substance is left over which gives the reactions of a carbohydrate, TMs carbohjTlrate differs in different nucleic acids. In plant nucleic acid, as "well as in guaiwlic acid from the pancreas and inosinic acid from muscle, the carbohydrate is a pentose, d-ribose. Most nucleic acids of animal origin yield Isevulinic acid en hydrolysis and must therefore contain a hexose. The researches of Levene, Jones, and others have shown that the nucleic acids vary in complexity and consist of one or more so-called nucleotides linked together. The simplest nucleic acids are mono- nucleotides. Examples of these are inosinic and guanylic acid. Each consists of phosphoric acid and a pm'ine or pyrimidine base linked together by carbohydrate. Upon hydrolysis with boiling mineral acid they are decomposed into their three constituents : HO. 0=P0 - C5H8O3 - C5H4X5O + 2H2O = H3PO4 + C5H10O5 + CsHgNsO ^guanylic acid ' d-ribose guanine HO. 0=P0 - C5H8O3 - C5H3N4O + H2O = H3PO4 -f CgHioOs + C5H4N4O / inosinic acid d-ribose hypoxanthine When submitted to neutral hydrolysis at 175° C under pressure, decomposition occurs in a different way, the phosphoric acid is split off and a glucoside-like body is left which is called a nucleoside. Thus guanylic acid yields guanosine, inosinic acid yields a body known as inosine or hypoxanthosine. Under the action of ferments, known as nucleases, which may occur in animal tissues, the mono-nucleotides may be split in one of two ways. The phosphonuclease removes the phosphoric acid, leav- ing a compound of purine and carbohydrate, while the pm'ine nuclease sets free the ])urine base and leaves a hexose- or pentose-phosphoric acid. Of the more complex nucleic acids which occur in cell nuclei, yeast nucleic acid has been the most carefully studied. According to Levene this nucleic acid is a tetranucleotide, having a structure represented by tlie following formula : HO. O = PO.C5H8O3.C5H4N5O / guanine group \ 0= PO.C5H803.C5H,N5 adenine group 0/ O = PO . C'sHgOs . C4H3X2O., y" uracil group 0= PO.C5HSO3.C4H4N3O / cytosine group NUCLEIN OR PURINE METABOLISM 777 If this is subjected to neutral hydrolysis, it loses the whole of its phosphoric acid and sets free four nucleosides, viz. : guauosine, adenosine, uridine, and cytidine. FORMATION OF NUCLEINS IN THE BODY In the case of the proteins we saw reason to beheve that in the higher animals, at any rate, there was no power of converting one amino-acid into another (with the exception of the lowest member of the series, namely, glycine), and that on this accomit the food had to contain representatives of every amino-acid (or perhaps of the corresponding oxy-fatty acid) necessary to the building up of the tissue proteins. The nucleins, on the other hand, can certainly be synthesised by the animal. This is shown by the fact that the hen's egg before incubation contains practically no nuclein or purine bases. During incubation tissues are formed, and there is a rapid increase in the number of nuclei, so that the chick just before it is hatched contains a considerable amount of nuclein from which purine bases can be extracted. This nuclein must have been formed by a synthesis from the phospho-proteins and phosphatides (phosphorised fats) which form so prominent a constituent of the egg- yolk, and in the same way the purines must have been formed by a process of synthesis. This synthesis may occur by a conjugation of two urea molecules with the 3-carbon chain which is so prominent a feature in the proximate principles of the body {e.g. in lactic acid, alanine, and all the compoimd amino-acids of which alanine is a constituent). Methyliminazol, representing one-half of the purine ring, can be formed simply by allowing ammonia and glucose to stand in contact with zinc hydroxide. The power of synthesis of purines possessed by the body must comphcate the question of their fate after ingestion, since it is evident that they can either be destroyed and excreted in some other form or that the products of their destruction may be built up into fresh purine or nuclein molecules. In the same way, in the growing child there is a rapid increase in the nuclein of the body, although the only food ingested is milk, which contains but an insignificant amount of nuclein. FATE OF NUCLEINS IN THE BODY Nucleins and nucleic acids are dissolved by the pancreatic juice, but no digestion of the nucleic acid occurs in the alimentary tract other than by the action of micro-organisms. We must assume therefore that the nucleic acid is taken up by the cells of the intestinal wall unchanged. Ingestion of nucleic acid is followed by an increased excretion of uric acid in the urine, so that we regard this substance as the end-product of nuclein metabolism in the body. It is evident that the uric acid of the urine may be derived either from the nucleins of the food or from the nucleins of the tissues of the body, the uric acid in these two cases being spoken of as exogenous and endogenous respectively. By digestion of nucleic acids with animal tissues or extracts of animal tissues under varying conditions, it is possible to bring about all the changes involved in the conversion of the purine base contained in tliem into uric acid. In the 25* 778 PHYSIOLOGY intestinal wall, or after absorption into other tissues of the body, the nucleic acid is subjected to hydrolytic changes by the agency of ferments which may be classed as nucleases. These are, however, of different kinds, the phosphonuclease sphtting off the phosphoric acid and leaving the nucleo- sides, while the purine nucleases, which are more effective in a sHghtly alkahne medium, split off the purines, leaving the phosphoric acid com- bined vnth. the carbohydrate. The purines set free in this way undergo further changes. The hypoxanthin derived from inosinic acid is converted under the action of an oxidase first into xanthine and then into uric acid. This was one of the eai'liest facts discovered in the metabolism of purines. Horbaczewski showed that if spleen pulp be digested with blood for some time, it is possible to extract a considerable amount of xanthine from the mixture. If, however, oxygen be bubbled through the fluid, the xanthine disappears, its place being taken by uric acid. From the more complex nucleic acids the amino-purines, adenine, and guanine, are set free. These first undergo deamination under the action of special ferments, adenase and guanase, and are thus converted into hypoxanthine and xanthine respectively. These bodies then, under the action of oxidases, may be converted into uric acid. All these changes occur in the living body, though not necessarily in the order just set out. Thus when the pyrimidine derivatives are administered to dogs, they pass out unchanged. If, however, free nucleic acid be administered to the animal, no trace of these derivatives can be found in the urine, so that they must have undergone com- plete oxidation. In the same way the dog's liver is able to deaminise completely the adenine group of nucleic acid, converting it into hypoxanthine, but is without effect on free adenine. It is evident, therefore, that the various ferments which have been described act partly on the whole nucleoside molecule, partly on the products of its decomposition, and that the results of the action of the body ferments are not the same in the two cases. If we make this reservation, namely, that the constituent parts of the nucleic acid molecule may undergo changes while still bound to the other parts, we may represent diagrammatically the formation of uric acid from nucleic acid as follows : Ferments Nuclease Nucleic acid Phosphoric acid guanosine adenosine m-idine cytidine deaminase xanthosine inosine fate unknown hydrolysis xanthine hypoxanthine oxidase oxidase oxidase (uricase) xanthine uric acid allantoin (in dogs) NUCLEIN OR PURIXE METABOLISM 779 or nucleic acid I auclcase 4 mononucleotides \ purine nuclease | III pentose-phosphoric acid guanine adenine pyrimidine bases deaminase | I (guanase and adenase) xanthin hypoxanthine oxidase xanthine . . I oxidase | ui'ic acid The question arises whether the uric acid excreted by a man represents the whole of the nucleins which have been destroyed in the body. Although complete equivalence has been found between the amount of hypoxanthine ingested and the amount of uric acid excreted, the same equivalence has not been established in the case of nucleic acid, and the important question arises whether uric acid once formed is stable or whether it may undergo further changes before being excreted. In many animals, such as the dog, the amount of uric acid in the urine is only minute, the chief purine derivative in this fluid being allantoin. AUantoin is formed when uric acid is oxidised with potassium permanganate, the follo^^ing changes taking place : NH— CO NH— CO CO C— NH^ + 0 + H2O = CO XH2 + COo ^CO I >C0 NH— C— XH/ NH— €H— NH The same transformation can be effected by extracts made fi"om the liver of the dog and probably of other animals. The ferment carrying out this change is known as uricase. No such ferment is found in human hver or any human organs, and, according to Jones and others, uric acid once formed in the human organism is not further oxidised. The small trace of allantoin which may occur in himaan urine is directly derived from the food. Modern research does not confirm the idea which was "formerly held that a portion of the uric acid formed might undergo further oxidation in man with the production of urea. On the other hand it is important to bear in mind the possibility that some of the uric acid which occurs in human urine may be formed by a process of synthesis. We have seen already that in the bird the greater part of the uric acid is formed not from purines at all but by a process of synthesis from lactic acid and ammonia, and though we have no evidence of a similar change occurring in the mammal, we are not able definitely to exclude its possibility. EXCRETION OF URIC ACID The complexity of these various processes in man renders it a difficult task to form a clear idea of the origin of the urinary uric acid and of the conditions which determine the variations in the amount excreted at 780 PHYSIOLOGY different times. Under ordinary circumstances a man excretes about half a gramme of uric acid per day. In addition the urine contains a certain small amomit of purine bases, the ratio of these bases to the uric acid being generally about 1:6. From 10,000 Htres of human urine Kriiger and Salomon succeeded in isolating the following purine bases ; Xanthine Hypoxanthine Adenine 10-1 grm. 8-5 „ 3-5 „ The same urine would probably have contained about 500 grm. of uric acid. As we should expect, the amount of uric acid in the urine varies with the diet. The following Tables from Bunge give the composition of the urine secreted (1) on a mixed diet, (2) on a diet mainly composed of meat, (3) on a diet mainly composed of bread : Twenty-four hours Mixed Meat Bread Quantity c.c. . . Urea .... grm. 1500 1672 1920 33 67 20 Uric acid ,, •55 1-3 •25 Ammonia ,, •9 2-1 •9 Creatinine ,, •77 •9 •4 Hippuric acid . ,, •4 — — Sulphates 2 4-6 1-2 Sodium chloride ,, 16-5 7-5 8^2 Phosphates ,, 3-16 3-4 1-6 Potassium » 2-5 3-3 1-3 Calcium, ma gnesii ini, ir on, colo uring-matter, gases, fermeni s. An attempt has been made to arrive at the amount of uric acid produced endogenously, i.e. from the breakdown of the tissues, from a study of the quant'ty of uric acid in the urine under varying conditions of food. During starvation, when the man is living on his own tissues, one might expect the uric acid to be increased in consequence of disintegration of the tissues. It has been suggested that the amount of the endogenous uric acid in the urine would be obtained by an analysis of the urine from patients taking a diet free from purine bases, but containing sufficient nitrogen to maintain nitrogenous equilibrium. It is impossible, however, to arrive at any constant figure for the endogenous uric acid. Even in the entire absence of purine derivatives from the diet the amount of uric acid increases' with the total nitrogenous metabolism. This fact is well shown in the Tables by Folin (already quoted) of the composition of the urine on a low and a high protein diet respectively. Although in each case care was taken to exclude purine-containing bodies from the food, the output of uric acid on the high nitrogenous diet was double as much as on the low diet. All we can say i§ that uric acid is constantly NUCLEIN or purine metabolism 781 being derived from the tissue disintegration, but that it varies under different conditions of nutrition as well as under different conditions of activity of the body. There are two main conditions which give rise to a marked increase in the output of endogenous uric acid. These are (1) severe muscular activity, (2) febrile states accompanied by increased nitrogenous metabolism. Since both these conditions are associated with an increased breakdown of muscle substance we may regard the uric acid as derived especially from the hypoxanthine or its precursors, such as inosinic acid, contained in the muscle. The foods which are especially effective in causing increase in the exogenous uric acid are those rich in nuclein, such as sweetbreads or liver, . _J.. ■' — '■ -n "" n i -^h- h-J I "] 1 1 1 "^ .♦T 1 i "■ 2\AlA V 1 1 ' 1 1 -J^X- . 1 1 t ^ -L 1 1 { 1 ^ ^ XT y- Si i 1 1 ir s^i' S ', >': 1 1 / ' . ] ', i 1 t 2^ N U- :\ ', i 1 \, \ !'.! ~- -X-1- i_ ,' \ r» _j l\ l_ ^ ' \ ' 1 1 z 7 \ V 1 1 • -+•! s 1 "\ ^^iv^ / -' [ .-•' "''M^J*^-!^ -' N 1 "^ y K 1 _ 1 r^\ a 1 1 a - in 1 ±:_l^ ! Ki 1 1 1 1 U 12 I 2 3 4 0 3 7 ( 9 10 11 12 Fig. 355. Curves showing the hourly excretion of uric acid and urea after a single meal. (Hopkiss.) The continuous line = uric acid output; the dotted line = urea output. and those rich in hypoxanthine or its precursors, such as meat or meat extract. When these foods are taken, or when nucleic acid itself is administered, a con- dition of leucocytosis is generally produced, the number of leucocytes in the blood being increased as much as three times. It has been suggested that the uric acid is actually formed by a disintegration of the newly formed leucocytes and not by a direct conversion of the purines of the food. It is quite possible, as suggested by Schittenhelm, that the leucocytes play a part in the transference of the nucleins from the intestine to the circulation. But the absence of any absolute proportionality between the degree of leucocytosis and the amount of uric acid excreted points to the probability of a direct conversion of the purines of the food into uric acid. URIC ACID IN GOUT Gout is a condition in which deposits of urate of soda occui- in the cartilages of the joints, the great toe joint being the seat of predilection for this disorder. The deposit is generally associated with an acute inflammation of the joint. In normal individuals the amouilt of uric acid in the blood is too small to be detected. Uric acid is readily excreted by the healthy kidneys. If the production of uric acid be largely increased by the administration in large quantities of food-stuffs rich in pm-ines, it becomes 782 PHYSIOLOGY possible to demonstrate the actual presence of uric acid in the blood. In gout there is constantly an increased amount of uric acid in the blood, probably in the form of sodium m-ate, even when the patient is on a pmine-free diet, so that gout may be re- garded, from one point of view at any rate, as a uricaemia of endogenous origin. On the other hand, the output of uric acid in the urine is not increased, and may in fact be somewhat smaller than normal. It might be thought that the presence of uric acid in the blood must therefore be due to diminished power of excretion of this sub- stance by the kidneys. This view is difficult to reconcile with the fact that if uric acid be injected subcutaneously into gouty subjects it is stated to be excreted in the urine exactly in the same way and as rapidly as in normal persons. It has been suggested that gout consists essentially in a disturbance in the various fermentative mechanisms which are responsible for the changes imdergone by the purines, so that there is an increased amount not only of uric acid itself but of various intermediate products in its formation from the pm-ine bases of the food and of the tissues. The deposit of the uric acid in the joint cartilages characteristic of acute gout appears to be simply a crystallisation of lu'ate of soda from a supersaturated solution of this substance in the blood. The whole question of the pathology of gout and of the disordered metabolism which may precede or intervene between actual acute attacks of the disease is in need of further investigation. Especially is it important to determine the influence on this condition not only of the nucleins and proteins of the food, but of the other constituents, such as carbohydrates and fats. Speaking broadly, gout is a disease of the well-to-do, of the person who, while pursuing a sedentary or no occupation, is not limited in his food-supply. It is almost unknown in the labom'ing class, where hard manual work is combined with a bare sufficiency of food. It seems therefore that it is not so much the supply of purines in the diet which must be controlled as the general conditions of nutrition which determine the fermentative changes in the piu'ines, either of the food or tissues, under normal conditions of metabolism. SECTION III THE HISTORY OF FAT IN THE BODY Fat is found in the body in various situations. In a fat animal the largest amount occurs in the panniculus adiposus in the subcutaneous tissues. Large quantities are also found surrounding the abdominal organs and between the layers of the mesentery and great omentum. In this adipose tissue the fat is enclosed within and distends connective-tissue cells, the protoplasm of which is reduced to a thin pellicle round the fat globule. Fat is also found in the form of granules in more highly specialised cells, such as the secreting cells of the liver or the muscle-cells. The condition of these cells is often spoken of as fatty infiltration, or fatty degeneration, according to the circumstances which are responsible for bringing about the deposition of fat. We shall have to discuss later on how far we are justified in assuming any real distinction between these two processes. From the physiological standpoint the most important intracellular depot of fat is in the liver. If this organ be deprived of glycogen and fat by starvation, a fatty meal gives rise to a great deposition of fat in its cells. There is apparently an antagonism between the processes which lead on the one hand to the deposition of glycogen and on the other to the deposition of fat. Thus an excessive carbohydrate diet, which induces great deposition of fat in the subcutaneous tissues, only causes the formation of glycogen in the liver. The glycogen must be got rid of before it is possible to cause the deposition of fat. On this account, the normal content in fat of the livers of dift'erent animals varies with their ordinary diet. Fishes, e.g. the cod, which take but little carbohydrate in their food, have generally a very large quantity of fat in their livers. Herbivorous animals, as a rule, have practically no fat in the liver. Fat also occurs in certain secretions, e.g. the milk and the sebum, its function in the latter case being mainly protective. Besides the visible deposit of fat found in adipose tissue and in other situations a large amount of fat is always present built up into the proto- plasm of the cells in such a condition that its presence cannot be detected by histological means. The presence or absence of visible fatty globules affords very little clue to the total quantity of fat in the cells. Thus in one case the heart muscle, which had undergone extreme fatty degeneration and was loaded with fat globules, contained 19 per cent, of its dried weight of fat. A heart muscle taken from a normal animal at the same time, presenting no visible fat globules, contained 17 per cent, of fat. 783 784 PHYSIOLOGY COMPOSITION OF FAT The fats occur generally in the form of triglycerides of various fatty acids. In adipose tissue the acids are chiefly stearic, palmitic, and oleic, the consistency of the fat depending on the relative amount present of triolein, with its low melting-point. In certain animals the glycerides of more unsaturated fatty acids occur. Thus lard contains about 10 per cent, of fats belonging to the hnoleic series. The fats of cows' milk, though consisting chiefly of the three above mentioned, include also the esters of butyric and caproic acids in fair amounts, and traces of the intermediate acids, capryhc, capric, lauric, and myristic acids. The ' fat ' extracted from the tissues {e.g. heart muscle) includes a considerable amount of ' phosphatides ' (lecithins, &c.). It also contains a much larger proportion of unsaturated fatty acids, of the linoleic and even lower series, so that its ' iodine value ' is generally found relatively high (120 as compared with 40 to 60 in adipose tissue). FUNCTIONS OF FAT First and foremost must be mentioned the significance of fat as a reserve food store. The power of the organism to store up reserve carbohydrate is strictly limited. The hver of man can probably not accommodate more than 150 grm. of glycogen, and assuming that the muscles of the body may contain an equal amount, 300 grm. represents the extreme limit of storage of carbohydrates in the body. On the other hand, in most animals there is practically no limit to the amount of fat which can be laid down, and over-feeding, whether with carbohydrates or fats, leads to the deposition of fat. This fat does not enter into the normal metabolism of the body, but is available for use whenever the needs of the body are increased above its income. As to the part taken by fat, especially the hidden fat of the working cells, in the chemical processes which determine the life of the cell, our knowledge is still very scanty. Fats enter into the constitution of the complex bodies lecithin and myelin, which form important constituents of the limiting membrane of every hving cell. As constituents of the mem- brane itself, fatty substances therefore have a protective action, and also regulate the passage of substances into the cell across the membranes. The presence of lecithin as an integral constituent of all protoplasm, and of the first products of disintegration of protoplasm, suggests that this substance may play a part in the normal transformations which occur within the cell, and may represent, so to speak, the currency into which fat is transformed in order to participate in the vital processes, and that it is in this form that the energy of fat is utilised for the needs of the cell. ORIGIN OF FAT IN THE BODY Fat formation is the result of an excess of income over expenditure. As soon as the latter exceeds the former the fat store is drawn upon, so THE HISTORY OF FAT IN THE BODY 785 that adipose tissue is the one which presents the greatest loss during starva- tion. As much as 97 per cent, of the total fat of the body may disappear during this process. We have therefore to consider what part is played by each class of food-stuffs in the formation of fat. Can this substance be formed from all three classes of food-stuffs ? FORMATION FROM THE FAT OF THE FOOD. Experiment has shown that the composition of the fat of any animal is by no means constant and can be varied within ^vide limits by alterations in the nature of the fat presented in the food. This dependence of the composition of the fat on the fats of the food is shown strikingly in an experiment performed by Lebedeff. Two dogs, after a ;^reliminary period of starvation, were fed, one on a diet containing a large amount of linseed oil, and the other on a diet containing much mutton suet. After some weeks, when the animals had put on a large amount of fat, they were killed, and it was found that whereas the fat of the dog which had been fed on mutton suet was solid at 50° C, that of the dog fed on oil was still fluid at 0° C. It has been shown moreover that by feeding animals with fatty acids not usually found in the body these are laid down in the adipose tissue. Thus colza oil contains a glyceride of erucic acid, and an animal, as Munk has shown, fed on colza oil lays on fat in which erucic acid is present. The same physiologist has observed that after the administration of various fatty acids to a man with a chylous fistula, the glycerides of the corresponding fatty acids made their appearance in the chyle, whether these fatty acids were those normal to man or consisted of substances, such as erucic acid, not generally found in human fat. One must conclude therefore that the fats taken with the food, if not immediately required for the energy needs of the body, are laid down without change in the adipose tissues, as well as in the cells of the body. The mechanisms involved in the translation of fat from the alimentary canal to the tissues are of the simplest possible description and only involve changes of hydrolysis and dehydrolysis. The fats are hydrolysed in the gut and are resynthetised to a certain extent in their passage into the epithelium. In the chyle and blood they probably wander chiefly as neutral fats, to be rehydrolysed for their passage into the cells of the body, which they may enter either in the form of soaps or possibly as fatty acids dissolved in some of the constituents of protoplasm. FORMATION OF FAT FROM CARBOHYDRATES. It has long been the experience of farmers that animals might be fattened on a diet in which carbohydrates predominate. The chemical difficulty involved in the trans- formation of carboliydrates into fats has often led to a doubting attitude on the part of chemists towards this transformation. Voit put forward the view that when fats are formed in the body as a result of an excessive carbohydrate diet, they are formed, not directly by a transformation of carbohydrate, but from the proteins of the food, the role of the carbo- hydrates of the food being simply to protect the proteins from disintegration and oxidation, so that the whole of their carbon can be utilised for the formation of fat. 786 PHYSIOLOGY Definite evidence has, however, been brought forward, especially by Lawes and Gilbert, for the transformation of carbohydrates into fats. In these experiments two young pigs, ten weeks old, of the same litter, with approximately equal weights, were taken. One was killed and the fat and total nitrogen in the body estimated. From the amount of nitrogen the maximum possible quantity of proteins present was calculated. The second was fed on barley for four months. The barley was measured and analysed, as well as the amount of undigested fat and protein that passed through the animal. At the end of the four months the second animal was killed and analysed. It was found that the animal contained 1-56 kilos more protein and 8-6 kilos more fat. It had taken up with the food 7-49 kilos more protein and 0-66 kilo fat. If we subtract the protein added to the body (1-56) from that taken up with the food (749) there is a remainder of 5-93 kilos which might possibly have given rise to fat. But 7-9 kilos of fat had been added in the body — a far larger amount than could possibly have arisen from the maximum amount of protein left over for the purpose. At least 5 kilos of fat in this experiment must have been derived from the direct conversion of the carbohydrates of the food. We must conclude that fat can be formed directly from carbohydrates, although how and where this conversion takes place is at present quite unknown. The fats formed on a carbohydrate diet are deposited chiefly in the sub- cutaneous tissue. For the reasons already given the Uver is found free from fat under these conditions. In the fat formed from carbohydrate the two saturated acids, palmitic and stearic acid, predominate. On this account the fat has a firm consistency and a high melting-point. The fats of low melting-point, such as olein, are absorbed more readily from the intestine than those of high melting-point. Where the fat of the body is chiefly derived from the fat of the food it tends to be of the more fluid acids and contains a larger percentage of olein. Although it is impossible to trace out all the steps in the process of conversion of sugar into fatty acid, we are acquainted with certain reactions which may throw some light on the nature of the changes involved. If we compare the formula of dextrose with that of the corresponding fatty acid, caproic acid, CHg CH2OH CH2 caoH CH2 OHOH CH2 CHOH CH2 CHOH COOH CHO we see that the conversion involves a considerable loss of oxygen. In order to convert three molecules of glucose, CgHiaOe, into one molecule THE HISTORY OF FAT IN THE BODY 787 of stearic acid, CigHgeOa, it is necessary to split off 16 atoms of oxygen. That this setting free of oxygen actually occurs in the transformation of carbohydrate into fat is shown by the study of the respiratory exchanges of animals which are rapidly laying on a store of fat at the expense of a carbo- hydrate food. Thus the marmot, towards the end of summer, eats large quantities of carbohydrate food and very rapidly lays on a thick layer of subcutaneous fat to last it during the winter. If glucose were entirely oxidised in the body the amount of oxygen absorbed would be exactly equal to the amount of carbon dioxide evolved. Thus CgHiaOe + GOo = 6CO2 + 6H2O. In this case the respiratory quotient would be 6CO2 _ ^ If, however, oxygen is being set free by the conversion of part of the carbohydrate into fat, this oxygen will be available for the oxidation of other portions of the carbohydrate. The animal Avill not need to take in so much oxygen from outside for the production of the same amount of carbon dioxide, and the carbon dioxide output of the animal will therefore be greater than its oxygen intake. Pembrey has shown that under these conditions the respiratory quotient may be as high as 1-5. We caimot assume, however, that the process of conversion of glucose into fatty acids takes place by this simple process of deoxidation. The change is probably a more complex one, and occurs in separate stages. Glucose easily breaks up under the action of ferments into two molecules of lactic acid, and lactic acid can be equally easily converted into aldehyde and formic acid, thus : CgHioOg = •2C3He03 lactic acid, and CH3" I CH3 H CHOH =1 +1 I CHO COOH COOH Now aldehydes possess a marked tendency to combine with other molecules of other or the same substance, i.e. to undergo polymerisation. Thus from two molecules of aldehyde we get one molecule of aldol, CH3 I CH3 CHOH 2| = 1 CHO CH2 I CHO which by a simple transposition of oxygen would give but^Tic acid, or by oxidation would give p-oxybutyric acid, a substance which occurs during various abnormal conditions of metabolism. The fats occurring in the body, e.g. in milk, include only the fatty acids 788 PHYSIOLOGY with, an even number of carbon atoms {v. p. 54). We may probably agsiim6 from this fact that the building up, as well as the breaking down, of fatty acids occurs by two carbon atoms at a time. Although heating aldehyde or aldol with potash or any other polymerising agent gives rise to a mixture of many substances, it is probable that under the catalytic agencies at the disposal of the li\^ng cell these synthetic changes are directed entirely in one direction, so that from butyric acid we shall have hexoic, capryhc, capric acid, and so on. The process would seem to take place more easily through pyruvic acid, as described on p. 121. Why the process comes to an end with the formation of the 16 and 18 carbon atoms it is difficult to see.* Possibly with the formation of acids whose melting-point is higher than that of the body temperature, a certain stability is imparted to them which prevents their further circulation and ready synthesis to the still higher acids. With regard to the glycerine which is a necessary constituent of the neutral fats laid down in the body, there is no difficulty in accounting for its formation from the carbohydrates. By a simple splitting of glucose, we may obtain two molecules of glyceraldehyde, CHoOH CH2OH = 2 CHOH CHO CHOH I CHOH I CHOH I CHOH I CHO which by reduction is readily converted into the corresponding alcohol glycerine, CH2OH . CHOH . CH^OH. We may conclude then that fats are formed by the body with ease from carbohydrates, and that in all probabihty this change involves a building up of the fatty acid from the lower members by the successive addition of a group containing two atoms of carbon. The whole change, as Leathes has shown {v. p. 121), is an exothermic one. For the formation of one molecule of palmitic acid, four molecules of glucose would be required, and 12-5 per cent, of the total energy of the glucose would be set free as heat. THE FORMATION OF FAT FROM PROTEINS. Among the decom- position products of proteins the amino-derivatives of the fatty acids take a prominent part. Of these some may be converted into carbohydrate in the body, while others such as leucine and tyrosine may give rise to aceto- acetic acid. It seems therefore that these latter might in their turn be built up by the process we have just discussed into the higher members * From the fats extracted from the kidney Dunham has isolated carnaubic acid, CaiH^.Oo. THE HISTORY OF FAT IN THE BODY 789 of the series. For many years, as a result of the investigations of Voit, the proteins were indeed regarded as the chief, if not the sole, source of the fats of the body, and it needed the energetic assaults of Pfliiger on this doctrine, in 1891, before it could be clearly examined by physiologists. Let us see what are the grounds for assuming a formation of fat from protein. In the first place, there is a well-known experiment by Voit. A dog was fed with large quantities of lean meat for a considerable time. Voit found that the whole of the nitrogen of the intake was excreted, but that a certain percentage of carbon was retained in the body, and that the percentage of this carbon was greater than could be accounted for by the deposition of glycogen in the hver and muscles. He therefore assumed that it must have been laid down as fat. Pfliiger showed that these con- clusions were not justified by Voit's results, and were really based on the fact that too high a figure had been assumed for the carbon of the meat. Whereas Voit found that the animal had laid on 56 grm. of fat during one day of the experiment, a recalculation of the same results by Pfliiger shows that the animal could not have put on more than 3-9 grm. of fat, an amomit which might quite well be accounted for by the fat and glycogen present in the meat. Pfliiger has shown moreover that an animal may be fed for weeks on the leanest meat that it is possible to procure, in any quantities, without putting on any fat at all, and, as we have seen, in- creasing the ration of protein increases simply the nitrogenous and general metabolism of the body. Although therefore we must assume that the healthy body does not normally form fat from protein, there are certain pathological conditions which seem at first to tell in favour of such a conversion. Thus durins certain diseases, such as diphtheria, pernicious anaemia, and as the result of poisoning by phosphorus, the majority of the organs of the body imdergo acute fatty degeneration. The liver may be enlarged and aU its cells are studded with fat granules which are apparently formed by a change in the protoplasm of the cells. This change was long interpreted as due to a direct conversion of protein into fat. More exact analyses have shown that during fatty degeneration the total fat in the body is not increased. Thus one observer took 124 pairs of frogs and poisoned one of each pair with phosphorus. The animals were then killed, and the whole of them analysed. The difference in the content of fat between the poisoned and unpoisoned animals fell within the limits of experimental error, so that there had been no increase in the fat of the body as the result of the poisoning. In some of these cases the liver is actually enlarged, but this deposition of fat in the cells is due to the immigration of the fat from other parts of the body and not to conversion of the protein of the cells. This is shown by the facts that the composition of the fat in the degenerated liver varies according to the composition of the fat in the rest of the body, ai\d that, if abnormal fats are given with the food, such as erucic acid or iodine fats, these are found in the fat extracted from the liver. In fatty Regeneration two processes are at work : one is the immigi-ation of fats 790 PHYSIOLOGY from other parts of the body ; the second, and probably the more important one, is a change in the relation of the fat to the protoplasm of the cell. It was long stated that the fat of milk was not increased by feeding with fats, but only by feeding with proteins. More recent researches have given contrary results. The dependence of the composition of milk fat on the composition of the fat present in the body or administered in the food is shown by the fact that cows fed on oilcake may produce a butter which is useless for commercial purposes owing to its low melting-point. In one experiment, when a cow was fed on linseed oil, the iodine number of the milk fat rose from 30, its normal figure, to 70-4. After the introduction of iodine fat subcutaneously, iodine fats are found in the milk. In another experiment a bitch which had been fed with mutton suet and had deposited in its tissues a fat of high melting-point produced a milk the iodine number of which was the same as that of the mutton suet. In this case the fat of the milk had evidently been derived from the tissues, since during the lactation the animal was being fed on meat which was poor in fat. The same dependence of fatty secretion on diet has been found in geese, where the composition of the oil secretion of the feather glands has been altered by giving abnormal fats, such as sesame oil, with the food. We must conclude that the protein of the food does not give rise to fat in the body. A nearer consideration of the composition of the proteins, taken in connection with our discussion as to the mechanism by means of which the fat is built up in the body, might help to account for this fact. The fatty acids formed by the disintegration of proteins are chiefly the lower acids of the series, such as acetic and propionic, which would undergo rapid oxidation in the body. Butyric acid has not yet been found among the products of disintegration of the proteins, and the 6- carbon acid, derived from leucine, is not the normal acid, but is a branched chain, viz. isobutyl- acetic acid. THE UTILISATION OF FATS IN THE BODY The constant presence of fat, and bodies allied to fat, in protoplasm, from whatever source obtained, suggests that these substances can enter directly into the chemical changes on which the life of the cell depends and that they play an essential part in vital phenomena. The direct utiHsation of fat for the needs of the body is also indicated by the results of experiments on man and the lower animals. After a few days' starva- tion the body may be regarded as practically free from stored carbohydrate. The sole source of the energy which is evolved under these circumstances must be fats and proteins, and it is possible to determine by an estimation of the nitrogen output the exact fraction of the total energy evolved which is to be ascribed to protein metabohsm. Thus in the case of Cetti, the professional faster, it was found that the nitrogenous metabolism per unit of body weight remained fairly constant between the fifth and tenth days of starvation, and corresponded to an average of 1 grm. of protein per kilo body weight daily, In order to convert this amount of protein into THE HISTORY OF FAT IN THE BODY 791 urea, carbonic acid, sulphuric acid, and water, nearly 2 grm. of oxygen would be required in the twenty-four hours, i.e. about 1 c.c. per minute. Cetti's total oxygen consumption was at the rate of 5 c.c. per kilo per minute, so that four-fifths of the oxygen absorbed was required for the oxidation of non- nitrogenous substances, and these, as we have seen, could only have been fats. In animals with a large store of fat the pro- portion of the energy obtained at the cost of the fats may be still greater. In dogs Rubner and Voit reckoned that only 10 to 16 per cent, of the total energy was derived from proteins, the rest, i.e. 8i to 90 per cent., being obtained from the oxidation of fats. The oxidation of .fats supphes energy not only for the production of heat but also for the performance of mechanical work, and it seems probable that the utihsation of the fat occurs in the muscular tissues themselves. Fat is found as a normal constituent of all muscle fibres, and the amount of this substance is greater in proportion to the activity of the muscles concerned. Thus the ever-active heart muscle, and the red muscles of the diaphragm, contain larger amounts of fat than the pale voluntary muscles which only have to undertake short periods of activity. In the human heart muscle 15 per cent, of the sohds are soluble in ether, and more than one-half of the ether extract is composed of fat, and is suffi- cient to supply the energy of the contracting heart for six or seven hours' work. The degree to which the muscles during contraction call upon each class of food-stuffs may be judged from the respiratory quotient. If the body has -previously supplied the greater part of its needs at the expense of fats, it will continue to do so during muscular work. This is well shown in the following Table, in which the oxygen consumption and respiratory quotient are compared in' a man resting and working on three different diets, one principally fat, one principally carbohydrate, and the other principally protein : Diet principally Resting Working m. kg. of work done Per m. kg. of work c.c. oxy- gen used per min. Resp. quo- tient c.c. oxygen used per min. Resp. quo- tient c.c. oxy- gen used Cal. Fat . Carbohydrate Protein 319 277 306 0-72 0-90 0-80 1029 1029 1127 0-72 0-90 0-80 354 346 345 2-01 2-17 2-38 9-39 10-41 11-35 We may conclude then that the tissues of the body are able to obtain their energy by the direct utilisation of the fats which they contain. The changes in the fat molecules which are involved in the utihsation of their energy are still to be determined. The energy of fat is only available on its oxidation. The transformation of fats into fatty acids or glycerine, or the synthesis of fats from aldehydes or from carbohydrates, which we 792 PHYSIOLOGY have discussed in the previous section, do not involve any large changes of energy. Weight for weight, butyric acid with its 4 carbon atoms has practically the same heat- value as stearic acid with its 18 carbon atoms, or stearine T\ith its 57 carbon atoms. We have therefore to determine what changes the great fat molecule undergoes before it is brought into a condition in which it may undergo oxidation and set free the energy required for the purposes of the body. The general tendency of metabohc research of recent years is to show that the hving cell is in a position to effect all changes which do not involve a large evolution or absorption of energy in either direction. In the plant cell, at any rate, the fatty acids may be converted into amino-acids, or the latter may be deaminised, as occurs in the liver, into fatty or oxyacids. Dextrose may pass into maltose and starch, or starch may be converted into maltose or dextrose. If therefore fats are constantly being made from carbohydrates, or from the lower molecules such as aldehyde, by a process of repeated addition of a group containing two carbon atoms, it is probable that the same change wiU go on in a reverse direction when fats are broken down previous to oxidation. In the germination of oily seeds the utiHsation of the fat is preceded by the spHtting of the higher fatty acids into acids of lower molecular weight. Although we cannot trace out in the animal body the stages in the breakdown of a large fatty acid, such as stearic acid, we can, by a certain artifice much used in metabohc experimentation, bring forward evidence in favour of the view that the breakdo'wn, hke the building up of fats, occurs by two carbon atoms at a time. When, in the process of breaking down, a fat finally arrives at the four- or two-carbon stage, it is quickly oxidised and is therefore not traceable in the excretions or in the fluids of the body. This end stage may, however, be preserved from oxidation by hanging it, so to speak, on to an aromatic ring. If acetic acid or ethyl alcohol be administered in small quantities, it is entirely oxidised. If, hoAvever, these bodies be attached to a benzene ring and be administered as a phenacetic acid or phenylethyl alcohol, they are excreted in the oxidised form of phenaceturic acid, which is simply a combination of phenacetic acid with glycine. In the same way benzoic acid and benzyl alcohol are •excreted in the form of hippuric acid, thus : CgHs . COOH + NHg . CHg . COOH = C6H5 . CO . NH . CHg . COOH + H2O Benzoic acid Glycine Hippuric acid Phenacetic acid, CeHg.CHgCOOH, is excreted as Cgllj.CHa.- CO.NH.CH2COOH. In each case the fatty side-chain is protected from further oxidation by its attachment to the benzene ring and by the tacking on of the glycine molecule. With phenylpropionic acid two carbon atoms of the side-chain are oxidised, and the remaining benzoic acid excreted as hippuric acid. Phenyl- butyric acid undergoes a similar change ; two carbon atoms are oxidised away, leaving phenylacetic acid, which is excreted as phenylaceiuric acid. THE HISTORY OF FAT IN THE BODY 793 If phenyl valerianic acid be given, four carbon atoms are oxidised away and benzoic acid is left, and appears in the urine as hippuric acid. In each case the oxidation of the side-chain occurs by two carbon atoms at a time, and it seems probable that a similar change will occur in the ordinary fatty acid, the last stages, in the absence of any protective ring compound, being oxidised like the earlier groups and therefore not detectable in the excretions. Evidence in the same direction is afforded by certain cases in which the oxidative power of the body for fats is inadequate, either by reason of morbid changes in the oxidative powers of the body, or as the result of what we may call an overstrain of the fat-oxidising powers. Such a con- dition is found in the acetonuria of acute acidosis, such as occurs in the end stages of diabetes. The oxybutyric and diacetic acids occurring in the urine in this condition were formerly thought to be derived from the carbohydrates of the food, or from sugar abnormally produced in the body. The condition of acidosis, however, is often brought on directly as the result of putting the patient on a strict anti-diabetic diet, i.e. one consisting chiefly or exclusively of fats and proteins, and may be produced in a healthy man by simple starvation, when the body has only at its disposal its stored-up fats and proteins. It occurs in a marked degi-ee on the administration of a diet consisting almost entirely of fats. Thus in one experiment a healthy man took as his sole diet for five days a daily ration of 250 grm. of butter, 200 grm. of oil. and a little wine. The result was an intense acidosis, such as is only found in the severest cases of diabetes, diacetic acid, oxybutyric acid, and acetone being found in the urine in large quantities. On the last day of the experiment these acids caused so much of the nitrogen in the urine to appear as ammonia that of the 5-8 grm. total nitrogen excreted only 2-7 grm. were in the form of urea, while as much as 2-1 grm. were present as ammonia. If, during a period of starvation in man, a day is interpolated on which 100 grm. of protein are taken, the amount of acetone excreted falls below that obtained on the other days when the individual is living chiefly at the cost of his own fat. These facts indicate that the chief source of the p-oxybutyric acid and the diacetic acid is the fats of the food or of the body. The condition of acidosis is more easily brought about bv ingestion of butyric acid than of the higher acids, such as palmitic or stearic, sug- gesting that whatever fatty acid is given it is finally reduced to butp-ic acid before its oxidation, and that in the condition of acidosis it is merelv the last stages of this oxidation which are at fault. We are thus justified in concluding that the oxidative breakdo^vn of fats occurs always by an oxidation in the p position. We take, for instance, the 6-carbon stage : CH3 . CH., . CHo . CH., . CFo . COOK the first change which probably occurs is the oxidation : CH9.CHg.CHg.CHOH.CHg.COOH 794 PHYSIOLOGY A further change is the complete oxidation of the last two groups and the production of butyric acid : CH3.CH2.CH2.COOH This then undergoes again oxidation in the p position, with the production of p-oxy butyric acid : CH3.CHOH.CH2.COOH and then again is converted to diacetic acid, CH3.CO.CH2.COOH In the normal individual this last stage undergoes complete oxidation, both oxybutyric acid and diacetic acid given to a healthy person being completely destroyed in the body. It is only under the abnormal conditions which we have mentioned above that these last stages fail of complete oxidation, and are excreted unchanged in the urine. THE QUESTION OF THE FORMATION OF SUGAR FROM FAT The ease with which the animal body performs the difficult chemical operation of transforming carbohydrate into fat suggests that under appropriate conditions it might effect the reverse change. Is there any evidence that in the animal body sugar may be derived from fat ? Such a conversion is of normal occurrence during the germination of fatty seeds, starch sugar and cellulose being formed at the expense of the stored-up fats of the seeds. If such seeds be allowed to germinate over mercury in a confined volume of oxygen, they are found, like seeds containing chiefly carbohydrate reserves, to absorb oxygen and to give off carbon dioxide. Whereas, however, in the latter case the amount of carbon dioxide evolved is almost equal to the oxygen absorbed, in the case of the fatty seeds much less carbon dioxide is given out than would correspond to the volume of oxygen absorbed, so that the total volume of gas above the seeds diminishes. The same change in the relation of oxygen intake to carbon dioxide output is found under certain conditions in animals. During hibernation, as Pembrey has shown, the marmot has a very low respiratory quotient, which may be not greater than 0-3 or 0-4. This means that the animal takes in more oxygen than the carbon dioxide which it gives out, and this intake of oxygen can be so marked as to cause an appreciable increase in the weight of the animal, which under such circumstances is literally living on air. This retention of oxygen can only be explained by assuming that there is a conversion of substances containing a small amount of oxygen into substances containing a larger amount of oxygen going on in the body, such a conversion as that of fats into carbohydrates. Just as the high respiratory quotient obtained from a marmot during the period of putting on fat was shown to be associated with a conversion of carbohydrate into fat, so does the abnormally low quotient obtained during hibernation indicate the reverse change of fat into carbohydrate. The same conversion has been alleged to take place in certain cases of THE HISTORY OF FAT IN THE BODY 795 diabetes. In many cases when the diabetic animal is Hving on a purely protein diet, a uniform ratio has been found to exist between the glucose or dextrose and the nitrogen excreted. — equals generally 2'8. In certain other cases a constant D : N ratio of 3-65 has been found. The former represents a conversion of 45 per cent., the latter of 58 per cent., of protein into sugar. In a few cases, however, even during complete starvation, the ratio D : N has been found to be much greater than that given above and to amount to as much as 10 or 12. These animals are stated to be practically free from carbohydrates, so that the sugar excreted in the urine can only come from the breakdown of proteins or fats. It is impossible by any means whatever to break up a protein molecule so as to get from it ten times as much dextrose as corresponds to the nitrogen, and Pflliger concludes that in cases where such a high D : N ratio exists the dextrose must have been derived by a conversion of the fats of the body. This conclusion is by no means generally accepted. If correct, it would bear out the general statement made above, namely, that in the living body practically all the chemical changes are reversible, and that the living cell can so regulate the conditions of the reaction that the reversible reaction becomes practically complete in either direction, the direction being deter- mined by the needs of the body at the time. Accepting this generahsation, the chemical mechanism by which fats are converted into carbohydrates must be the reverse of that by which carbohydrates are changed into fats. The 2-carbon group spHt oS from the large fatty molecules are probably utilised for the building up of the sugar molecule. We know that such a synthesis can take place from such simple groups as formic, glycolhc, or glyceric aldehyde. Though it is impossible to deny to any cell of the body the power of effecting the con- version of fats into carbohydrates, or carbohydrates into fats, the chief centre for such conversions is probably the chemical factory of the body, namely, the liver. It is significant that in the course of fatal diabetes, in which the fat disappears entirely from the body, and there is wasting of practically all the tissues, the liver is the only organ which retains its weight unchanged. During this disease there has been an enormous amount of work done in the conversion of proteins and possibly of fats into carbohydrates which could not be utilised by the body, and the large size of the hver at death suggests that the work of transformation has been performed by this organ. SECTION IV THE METABOLISM OF CARBOHYDRATES All the carboliydrates whicli are taken in with the food are ultimately transformed in the alimentary tract, or in its walls, into the three mono- saccharides, glucose, fructose, and galactose. These three, together with mannose, are the only sugars which are directly fermentable and directly assimilable by higher animals. A consideration of their structural formulae shows that they are fairly easily interconvertible, galactose presenting the greatest divergence from the general type. This conversion actually takes place in watery solution. If a solution of any one be left for some months, it Mill be found to contain all four at the end. Since these monosaccharides, for the greater part glucose, must enter the blood in large quantities during the absorption of a heavy carbohydrate meal, one would expect to iind a greater proportion in the blood during such a meal than during a period of starvation. The amount of reducing sugar in the blood, however, is practically constant, and varies between 0-1 and 0-15 per cent. Searching for the origin of this constant proportion of reducing sugar, Claude Bernard found that the blood of the hepatic vein in a fasting animal contained more sugar than the blood taken at the same time from the portal vein. Although the reliabihty of this experimental result has been put in doubt by more recent investigators, it was important in that it attracted Bernard's attention to the liver. If the liver be taken from an animal which has been dead for some time, and extracted with water, the extract is found to contain a large quantity of reducing sugar (glucose). If, however, it be removed immediately the animal is dead, its vessels washed out with ice-cold saline fluid, and be then cut up and thrown into boihng water, ground and extracted, the extract, after separation of the coagulable proteins, contains hardly a trace of sugar, and no more than is present in the blood. The fluid is, however, opalescent ; and Bernard found that this opalescence was due to the presence of a substance at that time new to science, belonging to the class of polysaccharides. This substance he called glycogen, i.e. the sugar-former. After a carbohydrate meal, glycogen may be present in very large amounts in the liver, up to 12 per cent, of the weight of the fresh liver. From its solution in water it can be thrown down by the addition of alcohol to 60 per cent When collected and dried, it forms a snow-white powder, 796 THE METABOLISM OF CARBOHYDRATES 797 tasteless and odourless, with a formula identical with that of starch, viz. CgHinOj. Like starch, it is hydrolysed by the action of acids and super- heated water, or of amylolytic ferments, into dextrins, maltose, and finally glucose. It gives with iodine a mahogany-red colour, which disappears on boiling, but returns again on cooling. It is not possible to extract the whole of the glycogen from a tissue by merely boil- ing it with water. Kiilz introduced on this account the method of dissolving the tissues in caustic alkali, then throwing down the protein with phospliotungstic acid, and in the filtrate i^rocipitating the glycogen with alcohol. This method has been modified by Pfliiger as follows : 100 grm. of the tissue (liver or muscle) are heated with 100 c.c. caustic potash containing 60 to 70 per cent. KHO for twenty-four hom-s in the water bath. The solution is then cooled, diluted with 200 c.c. of water, and treated with 800 c.c. alcohol of 96 per cent. The precipitate of glycogen is filtered ofE and washed several times with 66 per cent, alcohol. The precipitate of glycogen is now washed with a little water into a small beaker, neutralised carefully with acetic acid, and then introduced into a 100 c.c. flask. To the solution 5 c.c. of hydi'ochloric acid of 1*19 sp. gr. are added, and the mixture is made up to 85 c.c. The flask is then heated in the water bath for thi-ee hours. By this means the whole of the glycogen is converted into glucose, which can be estimated by Fehling's method or by Allihn's method. Li practice it is more accm-ate to estimate the glycogen in the form of sugar than to weigh it directly. If large quantities of glycogen are expected in the tissue, the inversion of the glycogen must be carried out in a larger beaker, and only an aliquot portion taken for titration. The large amount of sugar foimd in the liver which has been left in the body is due to the conversion of glycogen into glucose. This conversion has been variously ascribed to the activity of the surviving hver- cells, or to the action of an amylase ferment present in the liver-cells. That it is really a ferment action is proved by the fact that the liver may be dehydrated with alcohol, dried and powdered, and kept for months in this condition without any alteration occurring in the glycogen. If, however, the coagulated liver be mixed with water and allowed to remain at the temperature of the body for some hours, the glycogen is found to disappear and give place to glucose. FORMATION OF GLYCOGEN Glycogen is most readily formed from the carbohydrates of the food. In order to obtain a large amount from the liver, the animal is fed twelve to twenty-four hours previously on a meal which is rich in carbohydrates. Not all carbohydrates will give rise to the formation of glycogen. Only those which we have mentioned as directly assimilable, i.e. which will give rise in the alimentary tract to mannose, glucose, fructose, or galactose, will cause an increased formation of glycogen. The conversion involves a direct polymerisation of the glucose, produced either directly from the foods or by a molecular rearrangement taking place in one of the other three of these monosaccharides. Glycogen can also be formed from the proteins of the food, or from the products of their disintegration, the amino-acids. By means which we shall consider shortly, it is possible to free the liver of animals entirely from glycogen : if such animals be fed on a diet of washed fibrin or of pure 798 PHYSIOLOGY caseinogen, or even on the ultimate products of pancreatic digestion of proteins (containing therefore only amino-acids), and be killed shortly after- wards, the liver is found to contain glycogen. It does not seem to be possible for the hver to manufacture glycogen out of fats. At any rate, that is the interpretation which is generally placed on experiments on feeding with fats. In these experiments it is found that if fats be administered to an animal after the hver has been freed from glycogen, although the liver may store up fats it does not store up any glycogen. If an animal be starved, the glycogen gradually disappears from the Uver, although even at the end of ten or twelve days' complete deprivation of food small traces of glycogen may still be found in this organ. If starva- tion be combined with hard work ; if, for instance, a dog be made to drag about a milk-cart on the second day of the starvation period, its liver becomes quite free from glycogen. The same disappearance of glycogen may be produced by any means which evoke an increased muscular activity, e.g. poisoning with strychnine. Of the various reserve materials which are available the carbohydrate is the first to be called upon to meet the increased needs of the tissues during functional activity, such as muscular work or increased heat production. Thus the glycogen rapidly disappears from the Hver of a rabbit which has been immersed in a cold bath. The glycogen of the hver represents a reserve material analogous to the reserve carbohydrates stored up as starch in different parts of plants. When the blood is loaded with carbohydrates, a considerable proportion is laid down as the inert polysaccharide glycogen. As soon as the supply of sugar to the blood is withdrawn, the tissues continue to use the sugar of the blood, which is made up at the expense of the glycogen in the hver. In every liver- cell therefore a twofold process is always going on, namely, a building up of glycogen by the activity of the hver-cells, and a breaking down of glycogen under the action of the ferment formed in the liver-cells. Which of these two processes preponderates depends, in the normal animal, on the percentage amount of sugar in the blood which is circulating through the organ. On account of the importance of glycogen as a reserve material it is produced and stored up in almost all growing tissues, to be utihsed in their subsequent development. Thus it is found in large quantities in the placenta during a certain period, in foetal muscles, and in various other situations. It is found in yeast, in oysters, and in the muscles of the body generally. In foetal muscles it may amount to as much as 40 per cent, of the total dried sohds. The glycogen of the adult muscle is apparently utihsed during muscular work, and diminishes in amount with activity of the muscle. In adult muscles it never reaches anything like the percentage which is found in the hver. The average amounts found by Schondorf in the different tissues were as follows : THE METABOLISM OF CARBOHYDRATES 709 Maximum per cent. Minimum percent. of fresh tissue of fresh tissue Liver .... 18-69 7-300 Muscle . 3-72 0-720 Heart . 1-32 0-104 Bone 1-90 0-197 Intestines 1-84 0-026 Skin . 1-68 0-090 Blood . 0-0066 0-0016 THE UTILISATION OF SUGAR IN THE BODY Arterial blood is always found to contain between 0-12 and 0-15 per cent, of sugar in the form of glucose. The same amount is fovmd whether the blood be taken from an animal after a heavy carbohydrate meal or from one in a condition of complete starvation. The constancy of the sugar content of the blood suggests that this substance is a necessary con- stituent of the circulating fluid, necessary, that is to say, for the nutrition of the tissues. That it is being used up in all the processes of the body is shown by the immediate alteration in the respiratory quotient which occurs when the food is changed from a mixed diet to one consisting mainly of carbohydrate. An important factor in the maintenance of a constant sugar content in the blood is the reconversion of the stored-up glycogen of the Hver into sugar. The glycogen is not, however, the sole source of the sjgar, since in complete starvation the sugar content of the blood remains constant even after the last traces of glycogen have disappeared from the Hver. If the liver be cut out of the body or removed from the circulation, during the few hours that the animal survives there is a steady diminution in the blood- sugar, pointing to the hver being the chief, if not the sole, source of the blood sugar. In some animals, e.g. the carnivora, it would seem that the liver can continue to supply sugar to the blood on a diet which includes only proteins and fats, and we have already seen that in such animals glycogen itseK can be stored up at the expense of protein. It is doubtful whether a perfectly normal existence is possible in man in the total absence of carbo- hydrates from the food, though there is no doubt that in the northern nations, e.g. the Eskimos, the amount of carbohydrate consumed is very smaU in comparison with the fats and proteins. During muscular exercise the increased output of energy is associated ■svith a corresponding increase in the absorption of oxygen and in the output of carbon dioxide, pointing to a consumption of carbohydrate and fat in the contracting muscles. We might therefore assume that sugar is being normally released by the hver into the blood-stream so as to maintain the proportion of this substance in the blood at a certain level, and that the sugar is as constantly being taken up and oxi- dised in the muscles, where it serves as a source of energy. According to Chauveau and Kaufmann the venous blood flowing from a contracting muscle contains less sugar than the arterial blood flowing to the muscle. A 800 PHYSIOLOGY similar consumption of glucose occurs in the isolated contracting mammalian heart when fed with Einger's fluid containing a small trace of glucose. A heart, fed with blood and performing a normal amount of work, may use about 4 mg. sugar per gramme of heart muscle per hour. That the question of utilisation of sugar by the tissues is highly complex is shown by a study of the conditions under which sugar may appear in the urine. We learn thereby to appreciate to some extent the significance of carbohydrates both as sources of energy and as foods for the tissues, though we are still a long way from unravelling all the changes which the sugar must undergo in the cell before it appears once again in the oxidised products, carbon dioxide and water. GLYCOSURIA Normal urine always contains a small proportion of sugar, about 1 part per 1000, i.e. about the same as the blood itself. For the detection of these small traces of sugar in the urine special methods are necessary. The term glycosuria is not employed unless sugar appears in quantities large enough to give a reaction wdth Fehling's solution or with the phenylhydrazine test. Such a condition may easily be brought about by the injection of sugar subcutaneously or intravenously. It is then found that any trace of the di- saccharides, cane sugar or lactose, introduced in the circulation, is excreted in the urine. A rather larger quantity of maltose may be injected slowly without appearing in the urine, since the blood-serum contains a ferment maltase, which converts the maltose into glucose. Glucose, fructose, man- nose, or galactose, if introduced slowly into the circulation, are stored up as glycogen in the liver. If, however, the percentage of sugar in the blood rises above 2 parts per 1000, the sugar (generally glucose) appears in the urine. When this condition of hyperglycsemia (excess of sugar in the blood) is set up, the concentration of the sugar in the urine no longer corresponds to that in the blood. If the blood contains, e.g. 4 parts per 1000, the urine may contain from 2 to 7 per cent, of sugar. Up to a certain point, then, blood-sugar is kept back by the kidneys as a necessary food material for the tissues. Any excess above the normal apparently acts as a foreign substance and is excreted by the kidneys in a concentration much greater than that in which it exists in the blood- serum. (1) ALIMENTARY GLYCOSURIA. A state of hyperglyca^mia may be induced by the administration of abnormally large quantities of glucose by the alimentary canal. The amount has to exceed in a healthy individual 100 grm. in order that it shall appear in the urine. In certain individuals the power of assimilating glucose may be deficient so that an alimentary glyco- suria may be caused by any over-indulgence in carbohydrate food. In the healthy person it is hardly possible to produce glycosuria by the administra- tion of starchy foods, since the liver can store up the excess of glucose as fast as it is produced from the starch by digestion and absorbed into the blood stream. (2) DIABETIC PUNCTURE. It was shown by Claude Bernard that puncture of the floor of the fourth ventricle in rabbits is often followed THE METABOLISM OF CARBOHYDRATES 801 immediately by an excessive secretion of urine and the, appearance of sugar in this fluid. The glycosuria may last from twenty-four to thirty hours. If at the end of this time the animal be killed, the liver is found to be free from glycogen. A sample of blood taken during the height of glycosuria may contain from 3 to 4 parts of sugar per 1000. In order that the experiment may succeed it is important that the animal be previously well fed. If the puncture or ' piqure ' be carried out on an animal that has been starved or whose Uver has been freed by any means from glycogen, no glycosuria is produced. It is evident that the effect of the puncture has been to cause a rapid conversion of the glycogen previously stored up in the liver into glucose. The glucose so formed escapes into the blood, raising the sugar content of this fluid above the normal, and the excess is immediately excreted by the kidneys together with an increased amount of water. A similar temporary hyperglycaemia and glycosuria may be brought about by fright, strugghng or the administration of ansesthetics ; but the effect is absent, if both splanchnic nerves have been previously divided above the supra- renals. It has been shown (Elliott, Cannon) that all these conditions are associated with an increased discharge of adrenahn from the medulla of the suprarenals into the circulation. Since the injection of adrenaline itself causes a condition of diabetes similar in all its limitations and aspects to ' puncture diabetes,' it is now generally believed that the two conditions are identical, and that the diabetic puncture acts through the splanchnic nerves on the suprarenals, setting free adrenahn, which passing to the liver causes a rapid ' mobilisation ' of the stored-up glycogen, and a consequent hyper- glycaemia and glycosuria, lasting as long as the glycogen store holds out. !■ ; (3) PHLORIDZIN DIABETES. Phloridzin is a glucoside extracted from the root cortex of the apple-tree. It may be decomposed into a sugar and phloretin. When phloridzin or phloretin is administered by the mouth or subcutaneously it gives rise to glycosuria, unaccompanied, at first at any rate, by any other symptom. The urine may contain from 5 to 15 per cent, of glucose. The glycosuria induced in this way differs from the forms already described in the fact that it is not due to hyperglycsemia. Analysis of the blood shows that the sugar is shghtly diminished rather than increased. The excretion of glucose seems to be due to a specific effect of the drug upon the kidneys. If a cannula be placed in the two ureters so as to collect the urine from each kidney separately and a small dose of phloridzin be then injected by a hypodermic syringe into the left renal artery, the urine flowing from the left ureter will in two minutes be found to contain sugar, while the urine from the right kidney will not contain any sugar for another five or ten minutes. The effect therefore is rapidly to drain off sugar from the blood. In order to maintain the sugar content of the blood at its normal height the liver must rapidly manufacture fresh sugar to take the place of that lost by the kidneys. In the first instance the liver will utilise its stored-up glycogen for this purpose. If a dose of phloridzin be given to each of tw(; animals and one animal killed as soon as the excretion of sugar is coming to an end, the liver W'ill be found free from glycogen. If now a second dose of phloridzin 20 802 PHYSIOLOGY be given to the other, which may be regaided as glycogen-free, glycosuria is produced as before, and the excretion of sugar can be continued indefinitely by repeated administration of the drug. So long as suiiicient food is given, including carbohydrates, the loss of sugar does not entail any increase in the destruction of the tissues ; but if the drug be administered to starving animals the waste of sugar has to be made good at the expense of material other than carbohydrate. The source of the sugar excreted under these circumstances is the protein of the tissues. The nitrogen excreted in the urine rises in amount in proportion to the quantity of sugar excreted, and there is a constant ratio between the amount of nitrogen and the amount of sugar excreted in the urine. In different experiments this ratio D : N varies from 2-8 : 1 to 3-6 : 1. If meat be administered to such starving animals with glycosuria, the D : N ratio does not alter ; the amount of nitrogen in the urine increases, but the sugar increases in the same proportion. The sugar production is therefore proportional to the protein metabohsm and must be derived from protein. The source of the sugar is the amino-acids of which the protein is composed. It has been shown by Lusk, Embden and Dakin that the following amino-acids yield large amounts of glucose when administered to a phloridzinised animal : glycine, alanine, serine, cystine, aspartic acid, glutamic acid, ornithine, proMne and arginine. We must assume that these amino-acids produced in digestion or by the autolysis of the tissues undergo deamination and that the sugar is formed by a process of s}Tithesis from the oxyacids thereby produced. On the other hand leucine, tyrosine and phenylalanine give no increase in the output of sugar. It is however just these amino-acids which seem to follow the line of fat metabohsm, since they are converted into aceto-acetic acid when perfused through a dog's Hver — and the administration of fats to phloridzinised dogs is also without effect on the sugar excretion. The drain of sugar from the organism determined by the action of phloridzin on the kidneys thus necessitates a continued breakdown of the nitrogenous tissues of the body in the effort to maintain a normal supply of sugar to the tissues, and unless excessive feeding be employed the animal must waste. The great increase in the nitrogenous output resulting from the condition of phloridzin diabetes is shown in the following Table (Lusk) : Goat Dog D N D:N D N D:N Fasting . Fasting . Fasting and diabetic . Fasting and diabetic . Fasting and diabetic . Fasting and diabetic . 20-33 26-08 23-39 1901 3-72 3-71 4-90 8-83 8-06 6-84 4-15 2-95 2-90 2-78 63-55 65-30 65-84 64-60 4-04 4-17 12-66 18-76 18-57 17-29 5-02* 3-38 3-54 3-74 * The high D : X ratio on the first day is evidently due to the conversion of the glycogen still present in the body. THE METABOLISM OF CARBOHYDRATES 803 The constant drain of sugar will in time involve a relative carbohydrate starvation of the tissues, which will make good their energy requirements as much as possible at the expense of protein and fat. The administration of meat will diminish the fat metabolism to a certain extent, but since it does not alter the D : N ratio it would appear that the latter does not depend in any way on the quantity of fat undergoing oxidation. This is shown in the following respiration experiment (Mandel and Lusk) on a dog with phloridzin glycosuria, in which the metabolism dming starvation and after ingestion of meat was determined : rJ:X Calories from proti'iii Calorics from lat Calorics total Fasting 300 grm. meat 3-69 3-55 80-2 161-9 274-4 261-7 354-6 423-6 The enormous waste of energy involved in such a constant loss of sugar will be apparent if we consider that a D : N ration of 3-65 means that 52-5 per cent, of the energy in the protein taken as food or set free from the tissues is lost to the organism in the form of glucose. According to Rubner 28-5 of the energy of meat protein is not utilised in the body, but is liberated simply as heat. This stimulating effect of protein on metabolism or on the processes of oxidation in the body is described by Rubner as the ' specific d^aiamic action ' of proteins. If we accept this view and add this 28-5 per cent, lost as heat to the 52-5 per cent, lost as sugar there would remain a balance of only 19 per cent, actually available for the vital activities of the tissues. It is not to be wondered at that the nitrogenous metaboHsm may be increased three- to five-fold as a result of the artificial induction of the diabetic condition. The carbohydrate starvation has other deleterious effects, since we have evidence that a certain amount of carbohydrate food is a necessary con- dition for both fat and protein metabohsm. The necessity of carbohydrate for the assimilation of protein is brought out in an experiment by Cathcart. It has long been known that carbohydrate administration has a sparing effect on protein metabolism. If an animal or man be starved, the nitrogenous output sinks to a certain level and there remains practically stationary. If now pure carbohydrate food be administered sufficient to meet the energy requirements of the animal or man (about 35 calories per kilo), there is at once a rapid drop in the output of nitrogen and therefore in the protein waste of the tissues. Fat has a much slighter or no sparing effect on the nitrogenous metabohsm. Indeed in certain experiments by Cathcart the administration of fat caused an actual rise in the nitrogenous output. The importance of carbohydrates is borne out by the results of feeding animals with proteins which have been digested ^vith pancreatic juice imtil the biuret reaction has disappeared. After Loewi had sho^^^l that it was possible to maintain nitrogenous equilibrium in dogs with such a digest. 804 PHYSIOLOGY Lesser was unable to confirm his results ; but it has been pointed out that the essential difference between the two observers was that Loewi gave an abundant supply of carbohydrates with the digest, while Lesser omitted carbohydrates altogether and administered fats and protein digest alone. The evidence that the carbohydrates play a necessary part in the meta- bolic history of fats has already been mentioned {v. p. 793). We have seen that in the absence of carbohydrates the last stages in the oxidation of fats make default so that the partially oxidised fatty acids, oxybutyric acid and aceto-acetic acid, accumulate in large quantities and are excreted as such or as acetone in the urine. Not only does this involve a loss of energy to the body, but these organic acids require other bases for their neutralisation. Up to a certain point they will be excreted in the urine in combination with the fixed alkaHes. When these are no longer present in sufficient quantity they will be excreted in combination with ammonia, so that the ammonia of the urine is largely increased. If the condition of carbohydrate starvation be continued, this mechanism of neutrahsation is insufficient and the pheno- mena of acidosis, dyspnoea and coma, ensue, resulting in the death of the animal. Another effect of continued administration of phloridzin is fat infiltration of the Uver. This is merely a result of the carbohydrate starvation. A similar condition of fat infiltration can be brought about by feeding with a pure protein plus fat diet. The liver seems to be able to act as a storehouse either of fat or of carbohydrate, so that there is an inverse ratio between the amormt of glycogen and the amount of fat stored up in the Hver at any given time. It has been shown that the fat in the liver under these circumstances is simply fat which has been transferred to this organ from the ordinary fat depots, subcutaneous tissues, &c., of the body. (4) PANCREATIC DIABETES. Von Mering and Minkowski found that total excision of the pancreas gives rise to a severe and rapidly fatal diabetes, which presents many similarities to the severer cases of diabetes in man. Owing to the fact that the tissues of a diabetic are extremely prone to in- fection, it is often difficult after total excision of the pancreas, when diabetes has been set up, to procure healing of the wounds without suppuration. The operation is therefore usually carried out in two stages. In the first stage one small portion of the tail of the pancreas is transplanted under the skin of the abdomen, while the rest of the gland is excised. Such animals do not get diabetes and therefore recover quickly from the operation. When the wounds are quite healed the transplanted portion is removed through a simple skin incision. The second operation is followed in a few hours by the appearance of a large amount of sugar, 5 to 10 per cent, in the urine. The glycosuria persists, the animal rapidly wastes, and finally dies at the end of two to three weeks from diabetic coma. From the nature of the operation it is evident that the condition of diabetes observed under these circumstances has nothing to do with the presence or absence of the pancreatic secretion from the intestine, since this secretion is cut off at the first operation and diabetes does not make its appearance until the second small portion of the THE METABOLISM OF CARBOHYDRATES 805 gland is removed. Moreover ligature of the ducts of the pancreas or obstruc- tion of the ducts by the injection of melted paraffin does not give rise to diabetes. The excretion of sugar by the kidneys is due to an increase in the sugar content of the blood. The blood-sugar may amount to between 4 and 5 parts per 1000. This state of hyperglycaemia and the excretion of sugar in the urine persist even when the animal is completely starved or is fed on a pure protein or protein plus fat diet. Moreover, as in phloridzin glycosuria, we find a constant ratio between the sugar and the urinary nitrogen, the D : N ratio being usually about 2-8. The administration of protein food to an animal previously starved increases the output of nitrogen, but increases at the same time the output of glucose. No similar increase in the glucose excretion is observed as a result of the administration of fat. We must conclude therefore that in the absence of carbohydrate from the diet the excess of sugar in the blood as well as that escaping by the urine is derived from the breakdown of the proteins of the tissues. On the other hand, the power of the animal to assimilate or utilise carbohydrate is diminished and sometimes entirely abohshed, so that glucose administered to a starving animal with pancreatic diabetes may appear quantitatively in the urine. The amount taken by the alimentary canal is simply added to the amount which would have been excreted if no food had been given. In most cases, at any rate during the first week after total extirpation, there is apparently still some power of carbohydrate assimilation, since adminis- tration of glucose causes a transitory rise in the respiratory quotient (Moor- house). Glycogen disappears entirely from the liver ; but the muscles, especially the heart, may contain a normal or an increased amount of glycogen. There is a rapid wasting of all the tissues of the body, including the fats and proteins, and finally the animal is destroyed by the accumulation of the products of imperfect oxidation of the fatty acids. It is still very difficult to say definitely why removal of the pancreas brings about this condition or what disturbance of metabolism is primarily responsible for it. Two views have been put forward. According to one, the primary disturbance is the diminished or absent power of utilisation of sugar by the tissues ; according to the second, an increased production of sugar by the liver. There is no doubt that in the diabetic animal the power of utilising carbohydrates is deficient. This is shown by the low respiratory quotient and by the fact that administration of glucose to the animal causes an almost corresponding increase in the amount of glucose excreted in the urine. But the loss of power of utilisation is not absolute, at any rate in the first week of the disorder. Administration of glucose causes a slight and temporary rise in the respiratory quotient, and if 20 gms. of glucose be administered, it is often possible to recover only about 15 to 18 gms. from the urine. Moreover the increased amount of glycogen in the heart muscle of diabetic dogs points to a persistent power of assimilation of sugar by this organ. The heart from a diabetic animal, if fed with its own blood, can be shown to use up not only the sugar circulating in the blood but also its store of glycogen, and this utilisa- 806 PHYSIOLOGY tion is especially marked if the heart be made to work excessively by raising the arterial resistance and administering adrenahn ; but taken as a whole the power of utihsing glucose is very inferior to that possessed by normal animals. One of the most striking features of the condition caused by total extirpation of the pancreas is the rapid diminution of the fat depots of the body, attended by a marked condition of lipaemia and accumulation of fat in the Hver. The blood is so full of fat globules that it has been compared in appearance to strawberries and cream. One of the first effects of extirpation of the pancreas is therefore a rapid fat mobihsation, and the respiratory quotient agrees with that obtained when the metabolic needs of the body are being mainly satisfied at the expense of the fat. The sugar of the urine, after the depletion of the glycogen store of the Uver, is derived from the protein, and the protein tissues of the body therefore diminish as rapidly as the fat stores. On the theory of deficient utihsation, it is thought that these tissues suffer from carbo- hydrate starvation, even though they are bathed in a medium containing an increased amount of sugar, and that the liver in response to some call from the tissues turns first its glycogen and later on the proteins of the body into sugar to supply this lack — all to no purpose however, since the tissues are unable to avail themselves of the sugar or ferment. According to the second view, the primary disorder affects only the Hver. This organ is freed from some restraining influence on its power of manufacturing sugar from glycogen and from protein, so that the blood is flooded with sugar, which is therefore excreted in the urine. Any deficient utihsation of the sugar would be regarded as secondary to a poisoning of the tissues by this overloading of their nutrient fluids with sugar. It is certain that the sugar production in diabetes is excessive, as is shown by the rapid wasting of the protein tissues to give rise to the sugar, and that this over-production takes place in the hver is shown by the fact that extirpation of the liver in the diabetic animal causes a rapid disappearance of the sugar from the blood. According to the Vienna school (Eudinger, Falta, and others), a close interaction exists between the thyroid, the suprarenals, the pancreas, and the liver, the thyroid to a slight extent, the pancreas still more, inhibiting the glycogenic functions of the liver, while the suprarenals through their excretion of adrenalin stimulate this function. Glycsemia and glycosuria caused by extirpation of the pancreas would therefore be ascribed to an unchecked activity of the suprarenals. An important difference however seems to exist between the two conditions. Adrenalin glycosuria comes to an end when the glycogen store of the liver is exhausted, whereas pan- creatic diabetes continues until the death of the animal, long after all traces of glycogen have disappeared from the liver. We do not yet know how the pancreas affects sugar production or utilisation in the normal animal. It is generally assumed that it secretes into the blood stream a hormone which may, according to the view of the nature of diabetes which we adopt, pass to the tissues and enable them to utilise sugar, or pass to THE METABOLISM OF CARBOHYDRATES 807^ the liver and inhibit the sugar production in this organ. A very small portion of the pancreas is suificient for this purpose, but we have been unable to imitate the action of the pancreas still in vascular connection with the body by injection or administration of extracts of this organ. Even connection of a healthy animal with a diabetic animal by means of its blood-vessels, so as to allow the healthy blood, presumably provided with the products of secretion of the pancreas, to circulate through the diabetic animal, does not abolish the condition of hyperglycsemia in the latter, though connection of the portal vein of the healthy animal with that A B ■:•:•••■•'• ■■-'• ... ,_. -■ ..a flF> G i^ <^^ i .V- ?i,i ' ■ \ ..^..'i^i'i^ '/5 ^# \a^^f^> .-'^ '.V. Fig. 356. (A) and (B) show an islet with the surrounding tissue in a resting gland (A) and after exhaustion with .secretin (B). In (A) the secreting acini are charged with symogen granules. In ( B) these have entirely disappeared. On the other hand no change is noticeable in the cells of the islet. In the latter the granular cells are the h cells, and the clear hyaline cells are the a cells, (m) showing what are called Minkowski granules. The granulation of this cell is regarded by Bensley as due to postmortem changes. of the diabetic animal has, according to Hedon, had the effect of stopping the condition of glycosuria. Further work is required on this point. We thus see that the pancreas has a two-fold function, namely, the secretion of a digestive juice into the intestine and the exercise by some means or other of an influence on general metabolism, the absence of which is followed by the supervention of diabetes. Corresponding with this two-fold fimction, two kinds of structures are present in the gland, the secreting acini and the islets of Langerhans. These latter, though arising in connection with the ducts, are solid masses of cells and have no communication with the lumen of tlie ducts. According to Bensley and Lane the islet cells may be divided into two varieties which have been given the name of A and B cells, according as their granules are fi.xed respectively by alcoholic or watery solutions. It has been shown both by Bensley and by Homans that these cells undergo no alterations when the gland is excited to secrete by the injection of secretin. On the other hand, if four-fifths 808 PHYSIOLOGY of the pancreas be removed, the remaining part may gradually become inadequate to prevent diabetes, and Homans has shown that when under these circumstances diabetes supervenes, the granules disappear from the B cells. Changes have also been found in the islets of Langerhans in fatal cases of diabetes in man. It seems therefore probable that what we may term, for lack of a better word, the antidiabetic functions of the pancreas, are associated with and dependent on the integrity of the islets of Langerhans. (5) DIABETES IN MAN. In its severer forms the diabetes of man resembles very closely that produced in the dog by total extirpation of the pancreas. The output of urine is largely increased and the frequency of micturition is often the first symptom noticed. On examination the urine, though hght in colour, is of a high specific gravity, 1030 to 1035, and may contain from 5 to 10 per cent, of sugar. The appetite is largely increased, but in spite of the large amount of food taken the body wastes. The excessive quantity of fluid lost by the body gives rise to a constant thirst. The patient may die after some months or years in a condition of diabetic coma. Warning of the onset of this condition is given by the rise of ammonia in the urine and by the appearance of oxybutyric and diacetic acids. The breath may smell of acetone, and this substance may also be present in the urine. On the other hand, the diabetic state is attended by diminished resistance of the tissues to infection. A pimple may become a carbuncle ; a slight sore on the foot may give rise to a rapidly spreading gangrene of the lower extremity ; tubercular infection of the lungs spreads rapidly to the whole organ so as to stimulate pneu- monia. The patient may thus die of some such intercurrent infection before the onset of coma. In a few cases the pancreas is found to be atrophied or diseased, but in the large majority no marked pathological change is to be observed in this organ. Yet the condition is essentially similar to that which occurs in pancreatic diabetes. The radical defect is the inability, relative or complete, of the organism to assimilate carbo- hydrate. We may find all grades between such cases and those in which there is still a considerable power of assimilation. In order to determine the grade of the disorder it is usual to give a test diet with a certain pro- portion of carbohydrate, e.g. 100 grm. of bread with meat, bacon, eggs, butter, green vegetables, cheese, lettuce, coffee and wine. If the urine remains free from sugar on this diet, the diabetes is mild in character. More bread may then be added to the diet from time to time until sugar appears in the urine and the limit of tolerance for carbohydrate has been reached. In many cases the sugar will disappear from the urine on the administration of a diet consisting entirely of proteins and fats. When this has been effected carbohydrates may be added in small proportions to the diet until the limit is found at which the assimilatory powers of the patient are reached. It seems that administration of any carbohydrate in excess of this limit is of disadvantage to the patient and hastens the progress of his disorder. When the power of assimilating carbohydrates is entirely abolished the prognosis is almost absolutely fatal. This point THE METABOLISM OF CARBOHYDRATES 809 may be determined in two ways. In the first place, a patient with no power of carbohydrate assimilation will continue to excrete sugar in the urine on a pure protein fat diet, and the D : N ratio will be 2-8 or higher. Information may also be obtained from a study of his respiratory quotient. The production of dextrose from protein involves the absorption of oxygen. Oxygen ^^^ll therefore be taken in which will not reappear as carbon dioxide in the expired air. In severe cases of diabetes therefore the respiratory quotient will faU below that representing fat metaboHsm, i.e. below 0-7. In most cases of diabetes, where there is still some power of assimilating carbohydrate and of storing up glycogen, the respiratory quotient wiU be found approximately normal. A very low respiratory quotient is a sign of the severity of the disorder. This study of the conditions of carbohydrate metabohsm shows how aU three classes of food-stuffs co-operate in the maintenance of the chemical processes which lie at the root of the existence and the activities of Kving organisms. We see how fallacious were the ideas that the proteins alone were necessary for life and that protoplasm w^as simply living protein. Protoplasm, i.e. the material substrate of life, must be regarded as a complex in which proteins, fats, carbohydrates, nucleins, salts, and water all play a part and of which each is an essential constituent. In the higher animals proteins are necessary to furnish the proteins of the tissues, and the food must contain just those amino-acids which are requisite for the building up of the proteins characteristic of each separate tissue. Moreover certain groups of the protein molecule appear to be destined to serve as mother- substances of hormones and other chemical compounds which play a dynamic rather than static part in the phenomena of life, and supply conditions of activity rather than material for the production of energy. The carbohydrates not only act as sources of energy, but are necessary to the building up of the proteins into the protoplasmic complex. Without them moreover this complex cannot properly utiUse the fat contained in itself or supphed in its food. On the other hand, the carbohydrates by themselves are not available as food, but require some connecting hnk, which may be protein or nitrogenous in character, to enable their associa- tion with the active part of the protoplasm and their utihsation by oxidation. At the same time there is a certain possibility of interconversion between these different substances ; sugar may be formed from proteins, fats from carbohydrates. On the other hand, the formation of fats from proteins is apparently impossible in the cells of the higher animals, and the evidence for the formation of sugar from fat is Umited to the study of the respiratory quotient in hibernating animals. With the exception of a few cases quoted by Pfliiger and von Noorden, no support for such a conversion is obtained from the conditions observed in the glycosuria caused by the administration of phloridzin or by extirpation of the pancreas. 26* CHAPTER XII THE BLOOD In the unicellular animals and in the lowest metazoa the cells are bathed by the medium in which the organisms live, and are therefore exposed to all the changes in the composition of this fluid which may be brought about by cosmic events. With the evolution of a body cavity filled with fluid the tissue- cells are set free from the necessity of adapting their meta- bohsm to wide ranges of chemical composition, being bathed by an internal medium which is maintained practically constant in its characters for any given tj^pe. With increasing differentiation the fluid of the coelom, which may be called blood, becomes enclosed in branching systems of tubes, and its circulation is provided for by the development of contractile chambers at some point or points of the tubes. In all the higher animals, the blood, the common medium and means of exchange for all parts of the body, circulates through a closed system of tubes, a constant flow being kept up by the action of the heart. It is separated from the tissue elements themselves by the walls of the blood-vessels. The free interchange of material between blood and tissues is facilitated by the tenuity of the vascular wall. The interstices of the tissues contain a fluid, the ' tissue fluid,' any excess of which is drained off by special channels known as lymphatics and carried back to the blood. Interchange between the blood and the tissue-cells can be effected partly by diffusion, partly by a direct exudation or filtration of the fluid parts of the blood with certain of its constituents through the capillary walls. Since the function of the blood is to act as the common nutritive medium of all parts of the body, it has to convey food materials from the digestive organs and oxygen from the lungs to the tissues, From these it receives in exchange their waste products, namely, carbon dioxide and the results of nitrogenous metabolism, and carries them away to the excretory organs, such as the lungs and kidneys, by which they are ehminated. It is evident that the composition of the blood must vary from time to time and place to place according to the condition of activity and the function of the organ which it is traversing. The organs of the body are adjusted to respond to very minute changes in the composition of the circulating fluid, and add to or subtract from its constituents according as these are present in deficiency or excess. The changes are therefore kept within infinitesimal limits ; in most cases they are within the limits of errors of analysis, 810 THE BLOOD 811 and we may therefore treat the blood as a fluid of approximately constant composition and qualities. Blood obtained from a mammal is an opaque fluid varying in tint according to the vessel from which it is derived, being scarlet when taken from an artery, purplish in colour when taken from a vein, the difference being determined by the degree of oxygenation of the blood. On shaking venous blood with air it takes up oxygen and acquires the scarlet colour characteristic of arterial blood. If examined in a thin layer under the microscope its opacity is seen to be due to the fact that it is not homo- geneous, but consists of a number of corpuscles of different kinds sus- pended in a light yellow transparent fluid. In order to make out the characters of these corpuscles the blood should be diluted with some ' normal ' fluid, such as 0-9 per cent. sodium chloride. It is then seen to contain two classes of corpuscles. Much the most numerous are the (J 1 J m, Tis ■ Fig. 357. "^[[Non-micleated red blood-discs red corpuscles. These difEer m of 1,^^^^ blood. On the right of the appearance according as the blood figure the corpuscles are seen on edge. . , . , » l J. (SW.VLE ViNCEST.) IS derived from a mammal or from one of the lower orders of vertebrates. In all the latter it is a nucleated cell. In the frog, for instance, it is an oval bi-convex disc containing an oval nucleus in the centre. In man and other mammals the red corpuscle is a bi-concave circular disc (Fig. 357), varying in size in different species. The average sizes of the corpuscles in man are given in the follo^^^ng Table : Diameter ...... 7-1 to 7-8 /m Thickness (at periphery) . . . 2-5 fi Thickness (at centre) . . . . 1 -0 to 2 -0 /x In the blood of man there is an average of five million red blood-discs in every cubic millimetre of blood. The other kind of formed element, the white corpuscle, or leucocyte, is present in much smaller numbers than the red corpuscle, there being in human blood an average of one leucocyte to every 500 red corpuscles. These leucocytes are colourless cells, somewhat larger than the red blood-discs of man, presenting one or more nuclei and a granular or hyaline protoplasm. When examined on the warm stage they are seen to be amoeboid, and many of them, like the amoeba, have the power of ingesting granules of carmine, food, or dead bacteria with which they may come in contact. In addition to these two classes, a third body is generally described under the name of ' blood-platelets ' or hsematoblasts. These are especially well seen when the blood has been received directly into an excess of osmic acid. It is still doubtful whether they are pre-existent in the circulating blood or are formed in the plasma by a process of precipitation. 812 PHYSIOLOGY Unless special precautions are taken, the examination of blood obtained from a blood-vessel is interfered with by the process of clotting, which ensues shortly after the blood has left the blood-vessels. If blood be received into a beaker it is at first perfectly fluid, so that it can be poured from one vessel to another. After a space of time varying from three to eight minutes it begins to be viscous, and if poured out of the beaker leaves an adherent layer on the sides of the vessel. A minute later the whole mass of the blood becomes sohd and the beaker can be inverted without spiUing its contents. If a section be made of this blood-clot it is found to owe its soHdity to a network of fine threads of a protein substance named Fig. 358. Network of fibrin, after washing away the corpuscles from a filiu of blood that has been allowed to clot ; many of the filaments radiate from little clumps of blood-platelets. (Schafee.) fibrin, which have formed throughout the plasma and enclose the corpuscles in their meshes (Fig. 358). On leaving the clot for some hours, drops of yellow fluid appear on its surface and run together. The whole clot contracts, and finally there is a reduced clot floating or suspended in a yellowish fluid known as serum. If after the blood has left the vessels it be whipped with a bunch of twigs, or stirred with a glass rod, the fila- ments of fibrin as they are formed are deposited on the twigs. After three or four minutes the twigs can be withdrawn and the spongy fibrin collected. The blood which is left consists only of the corpuscles, plus serum, and wiU not clot, since its fibrin has been removed. It is known as defibrinated blood. Since the corpuscles are apparently unchanged in the meshes of the clot and clotting can be produced in blood-plasma entirely separated from corpuscles, we must look upon the process of coagulation as deter- mined in the main by changes in the blood-plasma. We can regard the blood therefore as a tissue consisting of a fluid matrix, which is extremely unstable and undergoes change when it leaves the vessels, and as having. embedded in its matrix, formed elements or cells of various kinds. SECTION I THE WHITE BLOOD-CORPUSCLES Amoeboid cells are a constant constituent of the coelomic fluid in all classes of animals. Even in the lower metazoa, where there is not yet a body cavity, wandering mesoderm cells are present which apparently discharge functions analogous in all respects to those of the white blood- corpuscles of mammals. On carefully examining a specimen of human blood, either fresh or in the form of a thin stained film, several varieties of these cells are seen to be present. In a fresh specimen we can distingmsh the following varieties : (a) A cell with a lobed nucleus and finely granular protoplasm ; Fig. 359. Various forms of leucocytes. a, eosinophile corpuscle ; h, ordinary polynuclear leucocyte (' ncutrophile ') ; c, hyaline corpuscle ; d, lymphocyte. (6) A small cell consisting almost entirely of a nucleus surrounded by a thin layer of protoplasm ; (c) A cell with a single nucleus and clear hyahne protoplasm ; ((Z) A cell with a lobed or reniform nucleus, the cytoplasm being beset with large coarsely refracting granules. These four types are known as the finely granular or polymorpho- nuclear cell, the lymphocyte, the hyaline corpuscle, and the coarsely granular corpuscle. The dift'ereutiation of the various types of leucocytes is more easily made if recourse be had to staining with mixtures of aniline dyes. This method was introduced by Ehrhch, who classified leucocytes according to the staining characters of their granules, dividing the latter into : (a) Those staining with acid dyes, such as eosin — acidophile or eosino- phile granulation ; (6) Those staining with basic dyes — basophile ; 813 814 PHYSIOLOGY (c) Those staining only with a mixture of the acid and basic dyes and therefore spoken of as neiitrophile. An acid dye is generally a salt in which the colouring-matter plays the part of an acid radical. Thus eosin is the sodium salt of the coloured acid tetrabrom-fluorescein. Basic dyes possess basic colour radicals. An example of this class, methylene blue, is the chloride of the coloiu'ed base tetramethyldiphenthiazine. Neutral dyes, accord- ing to Ehrlich, are those in which a colour base is combined with a colovu? acid, such as the eosinate of methylene blue, or the picrate of rosaniline. In preparations stained with mixtures of these dyes we may distinguish the following types : (1) The polymorphonuclear cells. These preesnt a lobed nucleus, and their protoplasm contains abundant fine neutrophile granules. They form about 70 per cent, of the total leucocytes. If the specimen be overstained with eosin the granules may take on a red stain. (la) A few cells are sometimes seen with a horseshoe or hour-glass nucleus and presenting a few neutrophile granules. These are spoken of as transitional cells, and have been supposed to represent an intermediate stage between large mononuclear or hyaline cells and the polymorpho- nuclear leucocyte. They do not form more than 1 per cent, of the leuco- cytes. (2) The lymphocytes are small cells with a round nucleus surrounded by a thin layer of hyahne protoplasm which is free from granules. These form 23 per cent, of the leucocytes. (3) Large mononuclear or hyahne corpuscles. These cells are two or three times the size of a red corpuscle, and possess a large oval nucleus which stains feebly with basic dyes. In normal blood not more than 2 per cent, of the leucocytes are of this type. (4) In every well-stained blood-fihn the eosinophile corpuscle is very evident, although not forming more than 3 per cent, of the white corpuscles. The nucleus is generally single, but is often crescent-shaped or reniform. The protoplasm is crammed with large discrete highly refractive granules which stain deeply with eosin and give micro- chemical reactions for iron as well as phosphorus. The granules, which in man, dog, and rabbit are spherical, are cuboidal in the horse, and in birds have the shape of short rods. (5) A cell which is found with difficulty, but is apparently a normal constituent of human blood, is the basophile leucocyte. This, which is somewhat smaller than the polymorphonuclear cell, has a lobed or tri-lobed nucleus and presents a number of granules in its protoplasm which stain deeply with basic dyes. It is sometimes spoken of as a ' Mast ' cell, the German term for the cell being used without translation. It does not form more than 0-5 per cent, of the total leucocytes of the blood. The granules are practically invisible in fresh specimens, in this respect pre- senting a contrast with the eosinophile granules. The leucocytes as a whole undergo variations in number according to the physiological state of the animal and are increased during digestion, specially of a protein THE WHITE BLOOD-CORPUSCLES 815 meal. They vary from one in 300 to one in 600 red corpuscles, or, taken as a whole, from 18,000 to 9000 per cubic millimetre of blood. FORMATION OF THE LEUCOCYTES In classifying the white corpuscles of the blood it is essential to know whether the different varieties we have described represent phases in one and the same corpuscle or a number of different cells of separate origin. The question as to the specificity of each kind of leucocyte cannot be regarded as settled. According to some observers, Gulland and others, all the leucocytes are derived from one kind of cell, namely, the lymphocyte. Ehrlich and his school, on the other hand, regard each type as forming a tissue sui generis, originating in separate localities and from distinct kinds of cells. Since division of the leucocytes in the blood itself appears to be an occurrence of the utmost rarity, we must locate the original seat of forma- tion of these cells in two tissues. Lymphocytes are derived from the adenoid tissue forming the lymphatic glands and the lymph nodules sur- rounding so many of the mucous cavities. These lymphatic nodules present towards their centre a clearer zone, consisting of cells rather larger than those of the periphery and known as the ' germ centre.' The nuclei in these cells present a well-marked reticular arrangement, and nuclear figures are often to be seen. By the division of these cells lymphocytes are formed, pushing towards the periphery of the nodule, where they make their way into the lymph-sinus and are carried slowly by the Ijmaph into the blood. Some of these lymphocytes may possibly pass directly through the capillary wall into the blood-stream. The other tissue concerned in the formation of leucocytes is the bone- marrow. This is the chief blood-forming tissue of the body, since it is re- sponsible also for the production of all red blood- corpuscles which are formed during adult life. In the red marrow are seen a number of cells known as myelocytes. These contain a single rounded nucleus and a well-marked protoplasm which may be non-granular or may contain granules, generally eosinophile in character, but sometimes basophile. It is stated that all intermediate stages are to be found in the bone-marrow between these ' myelocytes ' and the polymorphonuclear leucocyte as well as the eosino- phile leucocyte. It is certain that in the disease leukaemia, which is associated with an increased number of leucocytes in the blood, there may be an increase either of eosinophile cells or of the neutrophile cells, and either condition is associated with changes in the red bone-marrow. We may therefore provisionally arrange the leucocytes of the blood according to their origin as follows : (1) Small lymphocyte derived from lymphoid tissue. (2) Large mononuclear or hyaline corpuscle : doubtful whether derived by a growth of (1) or from a myelocyte. (3) Polymorphonuclear leucocyte formed in bone-marrow. (4) Eosinophile cell derived from similar cells in the bone-marrow. This origin of the eosinophile corpuscle is rendered more probable by the 816 PHYSIOLOGY fact that the shape of the granule, which differs from one species to another, is the same whether the cell be derived from the blood or the bone-marrow. The intermediate or transitional ceU may be derived either from the lymphocyte or from a myelocyte. In many cases of leuksemia the myelocyte passes into the blood in large numbers without undergoing the changes necessary to convert it into the typical blood-cell. We find then mono- nuclear cells which are either free from granules or contain eosinophile or basophile granules. FUNCTIONS OF THE LEUCOCYTES PHAGOCYTOSIS. We have seen that the leucocytes from whatever animal they be taken present two phenomena, viz. that of amoeboid move- ment and that of ingesting foreign particles which may be presented to them. On account of this power of eating up foreign particles they are frequently spoken of as ' phagocytes,' in this respect resembhng unicellular organisms and the undifierentiated cells of many kinds of tissue. All the phenomena connected with the process of inflammation in higher animals are directed to the assemblage of leucocytes at the spot which is the seat of injury or of infection, so that they may devour and remove either the injured tissue or the invading micro-organisms. This process plays there- fore an important part in determining the immunity of any animal against infection ; though in the higher animals it is assisted by a number of other mechanisms directed towards the same end, which we shall have to discuss in a subsequent chapter. The use of phagoc3rtosis is not, however, confined to the protection of the organism against infection. Wherever any efiete or dead tissue has to be cleared away, whether as the result of injury or in the course of metamorphosis of organs, the leucocytes play an important part. Thus in the great rearrangement of tissues which occurs in the larval state of insects, the removal of the muscle fibres which are no longer required is effected by the accumulation of phagocytes around them. The phagocytes may send processes into the muscle substance, which dissolve this tissue and then take it up. The absorption of the tail of the tadpole is effected in the same way by means of phagocytes. In mammals, including man, the moulding of the long bone which occurs in the process of growth is effected by continual and coincident processes of absorption and new formation of bone. The absorption is carried out by means of special phagocytes formed by the aggregation of a number of leucocytes, the well-known ' giant cells ' or myeloplaxes which form so prominent a constituent of bone-marrow. The blood-corpuscles represent the wandering phagocytes of the body. There are fixed phagocytes of which the myeloplaxes just mentioned may be regarded as a type. Other members of this class are the endothelial cells (Kupfer's ' Sternzellen ') which line the capillaries of the liver. If a suspension of carmine or of micro-organisms be injected into the blood- stream these endothelial cells are found a little later to have taken up large numbers of the foreign bodies. Under normal circumstances these THE WHITE BLOOD-CORPUSCLES 817 cells as well as some similar cells in the spleen take up'efEete red blood- corpuscles and destroy them. During the process of degeneration of a peripheral nerve brought about by its separation from the ganghon- cells of which its fibres are the processes, a marked proliferation of the nerve- nuclei takes place. These become surrounded with protoplasm and act the part of phagocytes, loading themselves with the fat globules set free by the degeneration of the myelin sheath. To the same class of fixed phagocytes ma}^ possibly be ascribed certain of the plasma-cells of the connective tissues. That the polymorphonuclear leucocytes are endowed with these phagocytic properties is universally acknowledged, but some doubt still exists as to how far the other types of leucocytes which we have described can function as phagocytes. It is probable that the lymphocytes, and certainly the large mononuclear or hyaline corpuscles, are endowed with these properties. The granular corpuscles, namely, eosinophile and baso- phile, are thought by some to function as unicellular glands and to react to infection, not by englobing the micro-organisms, but by discharging substances stored up in their granules which have a poisonous effect on the micro-organisms, and so prepare them for subsequent ingestion by the polymorphonuclear leucocytes. The other functions which have been ascribed to leucocytes are un- important as compared with their role as phagocytes, and are all of them questionable. Thus some authors ascribe to leucocytes an important part in the taking up of fat from the intestine and its carriage into the lymphatic system. In the coagulation of the blood the leucocytes have been supposed to act by the discharge of substances which may act as precursors of the fibrin ferment. In the invertebrata the wandering mesoderm cells not only remove the injured tissue but apparently give rise to new connective tissues. The same function was formerly assigned to the leucocytes of mammals by Ziegler, and Metchnikoff still beheves that after the removal of any injured tissue the emigrated leucocytes undergo elongation to form so-called fibroblasts, by the further division of which are produced the white fibres of the connective tissues as well as the branched connective-tissue cells. Most authors at the present time have come to the conclusion that the work of the leucocytes is complete with the removal of dead or injured tissue, and that the process of re- generation is carried out by the plasma-cells of the connective tissue, which enlarge, undergo division, and form the fibroblasts of the developing tissue. These plasma-cells can change their position and act as phagocytes, eating up and digesting the polymorphonuclear leucocytes which have prepared the way for their regenerative activity. Their amoeboid power is shown by the fact that if two sterile cover-glasses be introduced under the skin, new connective tissue is formed between the cover-glasses, and this method has been adopted by Ziegler for the study of the cellular changes involved in the new regeneration of this tissue. SECTION II THE RED BLOOD-CORPUSCLES The red blood-corpuscles, or erythrocytes, in man and in mammals are nucleated bi-concave discs, about 7 to 8^ (32^00 ^^•) ^^^ diameter and about one-third of this in thickness. The colour of a single corpuscle when viewed under the microscope is yellow, the red colour being only apparent when larger numbers are seen together. The red corpuscles form about 50 per cent, of the total mass of the blood, there being about 5,000,000 red corpuscles in every cubic millimetre of blood. They are soft, flexible, and elastic, so that they can readily squeeze through apertures and canals narrower than themselves without undergoing permanent distortion. Each red corpuscle consists of a framework or stroma, composed chiefly of protein material, containing in its meshes or in a state of loose chemical combination a red colouring-matter, haemoglobin, to which is due the colour of the corpuscles and of the blood itself. It is only in mammalia that the red corpuscles are of the character described. In the camel they are oval in shape, but otherwise resemble the corpuscles of other mammals. In all other classes of vertebrata the red corpuscles are oval, nucleated cells. The hsemoglobin is diffused through the protoplasm of the cell-body and does not extend to the nucleus. During the early part of foetal life the corpuscles of mammals are also nucleated, but in the adult condition the erythrocytes, except under abnormal conditions, lose all traces of the nucleus before entering the blood stream. The small size and great number of the red corpuscles determine that a very large area of surface of red corpuscles is exposed to the plasma. The volume of each corpuscle has been estimated as -0000000722 mm.^, and its surface as -000128 mm. 2, so that the total surface of red corpuscles in the blood of a man weighing about 70 kilos (assuming his total blood as ^^ of the body weight) would be about 3000 sq. metres, or 1500 times the surface of the body itself. This great extent of surface is of importance in facilitating the exchange of material, especially oxygen, between the corpuscle itself and the surrounding plasma. On treating the blood with weak solutions of tannic or boracic acid a separation occurs between the hsemoglobin and the stroma, the former appearing as a small ball near the centre of a colourless disc or being extruded so as to lie just outside the stroma. Briicke, who first observed this appear- ance, gave the name of ' zooid ' to the mass of hsemoglobin and of ' oecoid ' to the stroma. 818 THE EED BLOOD-CORPUSCLES 819 OSMOTIC RELATIONSHIPS OF THE RED CORPUSCLE. If the blood- plasma be concentrated by evaporation or by the addition of neutral salts its osmotic pressure rises and water diffuses from the corpuscles into the plasma in order to equahse the osmotic pressure within and without the corpuscle. The latter therefore becomes -^Tinkled or crenated. On the other hand, dilution of the plasma diminishes its osmotic pressure below that of the corpuscle, and water therefore passes into the latter, which swell up and become spherical, and if the plasma be made sufficiently dilute the corpuscles burst with the liberation of the haemoglobin they contain. The corpuscles of mammalian blood neither gain nor lose volume in a solution containing 0-9 per cent, sodimn chloride. The osmotic pressure, as deter- mined by the freezing-point, of such a solution is identical with that of the blood. For frogs' blood such a solution would be too concentrated and bring about crenation. The salt solution which is normal for frogs' blood only contains 0-65 per cent, sodium chloride. Although the average molecular concentration of blood-plasma in mammals is equivalent to tliat of a 0*9 per cent, sodium chloride solution, it may vary even in one animal within fairly wide limits, as is shown by the following determinations of the freezing-point of blood-serum taken from animals under various circmnstances : Man(heahhy) -0-56 to -0-600 Dog -0-55 to -0-645 Ox -0-55 to -0-662 Rabbit -0-55 to -0-620 The behaviour of the red corpuscles when immersed in solutions of sodium chloride of different concentrations shows that its limiting membrane or most external layer is impermeable to sodium chloride. If this salt be added to defibrinated blood and the crenated corpuscles separated by the centrifuge, practically the whole of the added sodium chloride remains in the plasma or serum. The red corpuscle is impermeable to most neutral salts as well as to cane sugar and glucose. We may therefore make ' normal ' solutions with sodium chloride, sodium sulphate, potassium nitrate, or cane sugar, taking care that each of the solutions shall be isotonic ^\'ith a 0-9 percent, solution of sodium chloride. On the other hand, a solution of urea behaves towards the corpuscles hke distilled water. If some red corpuscles be added to a 1 per cent, solution of urea in normal salt solution, they neither shrink nor swell, and if the mixture be centrifuged and the corpuscles and super- natant fluid examined separately, the percentage of urea in the two cases will be found identical, though there would be a great preponderance of sodium chloride in the supernatant fluid. If a 1 or 2 per cent, solution of urea in water be added to defibrinated blood, the corpuscles will swell up and burst just as if distilled water had been added. There are a large number of substances to which the corpuscles are permeable, e.g. alcohol, chloroform, ether, &c. In their permeability the corpuscles resemble most other vege- table and animal cells in permitting the passage of all those substances which are soluble in fats and the allied substances, cholesterin, lecithin, and protagon, which are invariable constituents of all living cells. According 820 PHYSIOLOGY to Orerton the external limiting pellicle of tlie red corpuscles, as in most living cells, is formed by a lecitliin-ch-olesterin compound, whose solvent power determines the permeabihtj of the ceU by foreign substances. If therefore we wish to stain the Hving ceU we must choose some dyestuif, such as methylene blue or neutral red, which is soluble in such lipoid bodies. CHEMISTRY OF THE RED BLOOD-CORPUSCLES The red corpuscles consist of two parts, haemoglobin and stroma, probably in a state of loose chemical combination. By various means it is possible to destroy this combination and to dissolve out the haemoglobin, leaving the colourless swoUen-up stroma floating in the plasma. At the same time the blood becomes darker but more transparent, and is spoken of as ' laked ' blood. It has been thought by Schwann, Schafer. and others that the red corpuscle con- sists of a solution of haemoglobin contained within an envelope which contains lecithin and cholesterin and forms the stroma. Though the haemoglobin can be separated from the stroma by very simple means, it is difficult to believe that it is in watery solution. Thus the blood-corpuscles contain a greater percentage of solids than any soft tissue of the body. The blood-corpuscles contain 36-7 per cent, solids, as against muscular tissue with 25 per cent, or nervous with 22 per cent, solids. Of these solids, 95 per cent, consist of haemoglobin, so that the solution would have to contain at least 30 per cent, haemoglobin. No solution of haemoglobin of this strength can be prepared. In many animals, such as the rat and guinea-pig, it is sufficient merely to ' lake ' the blood, i.e. to bring the haemoglobin into solution in the surrounding plasma or serum, in order to make the haemoglobin crystallise out. Some form of combination is there- fore necessary in the corpuscles if merely to keep the haemoglobin from separating out in a crystalline form. Blood may be ' laked ' by any of the following means : (a) Addition of a small amount of ether. (6) Free dilution with water. (c) Alternate freezing and thawing of the blood. (d) Addition of bile-salts. (e) The action of foreign blood-serum or of various haemolysins whose nature we shall have to discuss later. From such laked blood we may prepare either haemoglobin or stroma. In order to separate the stroma from the haemoglobin, blood which has been de- fibrinated or prevented from clotting by the addition of a little sodium oxalate is centrifuged matil all the formed elements form a solid cake at the bottom of the tube. The tube is then filled up with normal saline fluid and again centrifuged, and this process repeated twice in order to wash away adherent plasma or serum. After the final washing two volumes of distilled water saturated with ether are added to one volume of caked corpuscles. The corpuscles swell up and their haemoglobin passes into solution into the surrounding fluid. The blood is laked. The fluid is once more centrifuged in order to throw down white blood corpuscles. A 1 per cent, solution of acid sodixun sulphate is now added drop by drop until the solution acquires the opaque appearance presented by ordinary blood. The action of this salt, as of dilute acids, is to precipitate the swollen-up stromata, which reacquire the power of reflecting light from their surfaces and restore the opacity to the blood. On centrifuging, the stromata are thrown down, and can be collected and washed with distilled water several times on the centrifuge. THE RED BLOOD-CORPUSCLES 821 The stroma protein only forms about 4 per cent, of the total solids of the corpuscle. It is insoluble in dilute acids, but easily soluble in dilute alkahes. On gastric digestion the greater part dissolves, leaving a residue which is rich in phosphorus, and has been called nuclein. Stroma protein is therefore spoken of as a nucleo-protein. Wooldridge, who devised the method given above for the preparation of pure stromata, regarded the protein as a compound of protein and lecithin. The substance certainly contains a large quantity of lecithin, the greater part of which is present in the precipitate obtained on gastric digestion. According to Wright the nuclein residue yields purine bases on hydrolysis, and is therefore rightly classed with the other nucleins from tissue-cells and contained in nuclei. From the laked solution of corpuscles oxy haemoglobin can be obtained in a crystalline form with varying readiness according to the animal from which the blood is derived. Thus in the case of the rat, the guinea-pig, the dog, and the horse it is sufficient merely to cool the laked blood, preferably in a freezing mixtme to about — 10° C. in order to obtain a large crop of hsemoglobin crystals. Crystallisation is facilitated by the addi- tion of 25 per cent, of absolute alcohol to the mixtme, though the use of alcohol certainly tends to interfere with the subsequent piu-ification and solubility of the haemoglobin. Oxyheemoglobin can be recrystallised by dissolving it in weak alkali at 35° C, cooling the solution to 0° C, and then adding cold alcohol to 25 per cent, and allowing the mixture to stand for some days at a temperatm'e of — 5° to — 10° C. In the case of those bloods which yield oxyhaemoglobin crystals with greater difficulty, it is better to add to the laked blood an equal volume of a saturated solution of ammonium sulphate. The precipitate of globulins is filtered off and the filtrate allowed to stand in a cool place. Crystals of hagmoglobin then come down in quantity. PROPERTIES OF HAEMOGLOBIN The crystals thus obtained are as a rule microscopic in size. Most animals yield an oxyhaemoglobin which crystallises in rhombic prisms or needles belonging to the rhombic system. In the guinea-pig the crystals are tetrahe- dral in form, while the oxyhsemoglobin of the squirrel crystallises normally in the form of six-sided plates. On recrystalli- sation, however, a squirrel's haemoglobin can be obtained as a mixture of rhombic prisms with rhombic tetrahedra. The water of crystallisation of oxyhsemoglobin varies in different animals between 3 and 9 per cent. The solubihty of the crystals differs according to the animal from which they have been derived, and is in direct proportion to the difficulty with which the crystals are obtained. They are more soluble in highly diluted solutions of ammonia and the caustic alkalies and their carbonates than in water. A solution of haemoglobin will not diffuse through parchment paper ; the elementary analysis of oxyhaemoglobin crystals gives somewhat varying Fig. 360. Crystals of oxyhsemoglobin. 1. From rat. 2. From guinea-pig. 3. From squirrel. 822 PHYSIOLOGY results according to the animal employed. In the case of the oxyhaemo- globin of the dog Jaquet obtained the following figures : In 100 part 3 c . 53-91 54-97 H . . . 6-62 7-22 N . . . . 15-98 16-38 Fe . . . . 0-333 .. 0-336 S . . . . 0-54 0-568 0 . . . 22-62 20-93 Fe per cent. Authority 0-336 . Jaquet. 0-335 . Zinoffsky 0-336 . . Hiifner. 0-336 . Jaquet. The chief differences between different animals appear to have re- lation to the sulphur. Haemoglobin from the hen contains 0-857 per cent, sulphur. All specimens are alike in containing a constant proportion of iron, as is shown in the following Table : Oxyhsemoglobin of Dog Horse . . . Ox Hen On the assumption that each molecule of oxyhsemoglobin contains one atom of iron, its molecular weight would be 16,660, and this result is borne out by the volume of oxygen or carbonic oxide which can enter into combina- tion with haemoglobin. It has been suggested by Bunge that the enormous size of the haemoglobin molecule finds a teleological explanation ; if we consider that iron is eight times as heaAr^^ as water, a compound which would float easily along with the blood- current through the vessels could only be secured by the iron being taken up by so large an organic molecule. Oxy- haemoglobin is a compound in definite proportions of oxygen and haemo- globin or reduced haemoglobin. It can be easily dissociated and is split up by such simple means as exposure to a vacuum. If, for instance, some arterial blood or solution of oxyhaemoglobin be introduced into a Torricellian vacuum, the fluid is seen to give off bubbles of gas, and the colour changes from a brilliant scarlet to a dull bluish red. In this process each gramme of oxyhaemoglobin gives off 1-34 c.c. of oxygen. The same change can be effected by treating a solution of oxyhaemoglobin with reducing agents such as an alkaline solution of ferrous tartrate (Stokes's fluid) or ammonium sulphide ; in the latter case reduction is aided by gently warming the solution. Another reagent of value for effecting the reduction of oxyhaemo- globin is a solution of hydrazine. The oxygen in oxyhaemoglobin can be replaced by equivalent quantities of other gases. Thus if carbon monoxide gas be led through a solution of oxyhaemoglobin, oxygen is given off and its place is taken by an equal volume of carbon monoxide with the formation of a more stable compound, carbon monoxide haemoglobin. This body is only dissociated with extreme slowness and is unaffected by the addition of reducing agents. By using special precautions to prevent oxidation of the gas the carbon monoxide can be replaced in this compound by nitric oxide, THE RED BLOOD-CORPUSCLES 823 NO. We have therefore a series of three compounds which can be arranged in order of stabihty, thus : NO-haemoglobin. CO-hsemoglobiu. Oa-hsemoglobin. The poisonous properties of carbon monoxide are due to its power of turning out the oxygen from the oxyhsemoglobiu, thus depriving the tissues of the oxygen which is normally carried to them by the red corpuscles. Haemoglobin and its derivatives give well-marked absorption spectra. Thus dilute solutions of oxyhaemoglobin placed in front of the sHt of a spectroscope show two well-marked absorption bands between Fraunhofer's Fig. 361. The spectra of oxyhsemoglobin in different grades of concentration, of (reduced) haemoglobin, and of carbon monoxide haemoglobin. (After Peeyer and Gamgee.) 1 to 4. Solution of oxyhaemoglobin containing (1) less than -01 per cent., (2) "09 per cent., (3) -37 per cent., (4) -8 per cent., (.5) solution of (reduced) haemoglobin containing about '2 per cent., (G) solution of carbon monoxide haemoglobin. In each of the six cases the layer brought before the spectroscope was 1 cm. in thickness. The letters (^4, a. Sec.) indicate Fraunhofer's lines and the figures wave-lengths expressed in xWiTrnr millemetre. lines D and E. The centre of the band nearest to D corresponds to X 579, and is often spoken of as the band a, while the second band, the one next to E, which can be called the band [3, is broader, has less sharply defined edges, and its centre corresponds approximately to X 544. On concentrating the solution or using thicker layers a point is reached at which the two bands fuse into one, and with a still stronger solution it will be found that the whole of the spectrum is absorbed with the exception of the red end. The above figure shows the spectrum of oxyhaemoglobin in varying concentrations, a stratum one centimetre thick being examined. If a reducing agent be added to the solution of oxyhaemoglobin the two bands disappear and their place is taken by a more diffuse band hnng midway between the two (Fig. 361, 5), its centre corresponding to \ 555. This is the 824 PHYSIOLOGY absorption spectrum of haemoglobin or reduced haemoglobin. The spectrum of carboxyhaemoglobin is very similar to that of oxyhaemoglobin, the bands, however, being shifted shghtly towards the red end. This solution is of a brighter red than oxyhaemoglobin. Its tint is best observed on diluting the blood to a large extent, when oxyhaemoglobin acquires a yellowish tint, while the pink colour of CO-haemoglobin is retained so long as any colour is visible. The fact that CO-haemoglobin is not altered by reducing agents can be shown by adding ammonium sulphide to CO-haemoglobin and examin- ing with the spectroscope, when no change is observed. All these derivatives of haemoglobin, besides their absorption bands in the visible spectrum, have characteristic absorption of light in the ultra-violet spectrum, as has been shown by Gamgee. In the case of oxyhaemoglobin this absorption causes a band (Soret's band) which occupies the greater part of the spectral region between Fraun- hofer's lines G and H. In reduced haemoglobin this band is displaced towards the visible part of the spectrum. Another compound of haemoglobin with oxygen is methcBmoglohin. This substance, although not of normal occurrence in the body, is found in urine and in blood whenever there is a sudden breaking down of red blood- corpuscles with the setting free of haemoglobin in the blood-plasma. It may be prepared by the addition of a ferricyanide, permanganate, or nitrite, or other oxidising or reducing agents to the laked blood or to solutions of oxy- haemoglobin. It is a chocolate-brown substance, crystalHsable, and gives a distinct absorption band in the red between Fraunhofer's lines C and D. It is unaltered by exposure to a vacuum. On treatment with reducing agents, however, such as Stokes's fluid, the methaemoglobin is converted into haemoglobin, from which by shaking with air oxyhaemoglobin can be re- formed. The fact that methaemoglobin cannot be reduced by exposure to a vacuum indicates that it is a compound of oxygen with haemoglobin in which the oxygen is in a different state of combination. According to Buck- master methaemoglobin only contains half the oxygen contained by oxyhae- moglobin, so that the composition of the two bodies might be represented. Hb^ I (oxyhaemoglobin) and Hb = 0 (methaemoglobin). The change from oxyhaemoglobin to methaemoglobin is not affected, however, by a simple shifting of the oxygen groups, but must be assumed to involve two distinct events. The whole of the oxygen in loose combinatioii with haemoglobin is given off, and the oxygen in the methaemoglobin molecule is derived from the oxidising agent added, so that ferricyanide of potash, for instance, is converted into ferro-cyanide.* Since the whole of the oxygen in the oxyhaemoglobin is given off on the addition of potassium ferricyanide, we may use this fact in order to determine the total amount of oxygen in combination in the blood {v. p. 859). * When the change is effected by reducing agents we must assume that the oxygen of the water or air is the source of that required for the oxidation of the reduced haemo- globin to methaemoglobin. THE RED BLOOD-CORPUSCLES 825 DERIVATIVES OF HEMOGLOBIN. Haemoglobin is a compound of an iron-containing coloured group (the prosthetic group) with a protein, which probably varies somewhat in different animals. The prosthetic group is identical in every case where it has been examined. A separation of the prosthetic group from the protein moiety can be effected with extreme ease, and occurs whenever the haemoglobin is treated with weak acids, with alkaUes, or is heated above 70° C. The protein group is known as globin. In order to separate globin, oxyhsemoglobin crystals are dissolved in water and treated with small quantities of very dilute hydrochloric acid. A precipitate of pig- ment forms, which, if the haemoglobin used be free from inorganic salt, rapidly dissolves in excess of the acid. Alcohol and ether are then added in such relative quantities that the ether separates rapidly from the aqueous solution. The colouring-matter (hsematin) dissolves in the ether, whilst the protein (globin) remains in solution in the water. The solutions are separated by a separating funnel and ammonia added carefully to ^ ^''JT- the aqueous solution. This throws down a pre- / ^ j^ ^Jk -^■»?' cipitate of the protein, which is soluble in acids ^^^ r^ -^a ^ and alkalies and coagulable on heating ; the coagu- s, ^ / ^ '-/' jW^ ' \ lum, however, is soluble in acids. It is precipitated A -v-^*~ \ * ^ '^^ :^ bj^ ammonia in the presence of ammonium chloride. a 't ^^ * N VT \ K It contains as much as 16-89 per cent, nitrogen, j>& k / ;S "^^ ' ^""^ and yields a considerable amount of the basic 'jS ^ A !) ^ * ^ derivatives on hydrolysis. It is therefore classified / ' _, \.if -^f , with the histones. ^TiC'^^ '' ^=^ ■i'-fi W- Haemoglobin yields about 94 per cent, of '-'v-<^jx. '" ^^ ^ ^ "^ globin and about 4-5 per cent, of the chromo- ^ ^ f >-\ '^ genie group, haematin. In order to obtain -r. o^o u "^ .*■ ^ o & _r' .... Fig. 362. Hsemin crystals. hcematin in a pure condition it is usual to start with the crystalline derivative of haemoglobin known as hmmin. When some dried blood is heated with a crystal of common salt and placed in acetic acid on a slide, a residue is obtained in which a nmnber of reddish-brown needles are embedded known as Teichmaim's crystals or haemin crystals (Fig. 362). The preparation of these crystals is often used as a convenient test for the identification of blood. In order to obtain them in large quantities the following method, devised by Chalfejew, is employed. One volume of defibrinated blood is added to four volumes of glacial acetic acid previously heated to 80° C. As soon as the temperature has fallen to 60° C. the liquid is again warmed, and then allowed to cool. Crystals are formed which are allowed to stand for twelve hours and are then separated and washed by decantation, first with distilled water and then with graduated strengths of alcohol. In order to purify these crystals the crude material is shaken for fifteen minutes with a mixture of chloroform and pyridine. The solution is filtered and then thrown into glacial acetic acid previously saturated with sodium chloride and heated to 105° C. A few drops of concentrated hydrochloric acid are then added and the mixtm'e allowed to stand for twenty-four hours. The crystals which separate out are filtered off, washed Avith dilute acetic acid, and then ch'ied. Haemin crystals have been regarded as hydrochloride of hannatin. Elementary analysis shows that they have the following formula (Will- statter) : C33H3204N4Cl.Fe. By dissolving haemin in alkahes and throxN-ing 826 PHYSIOLOGY the solution into an excess of acid a precipitate is obtained which is hsematin. Hsematin forms a powder of bluish-black colour and metallic lustre. It is in- soluble in water, alcohol, or ether, but is shghtly soluble in glacial acetic acid and in absolute alcohol. It is easily soluble in concentrated sulphuric acid, but undergoes decomposition, losing its atom of iron and being transformed into hwmatoporphyrin, which forms a deep purple solution. The formula of hsematin has not yet been ascertained with certainty. It is probably £C Fig. 363. Absorption spectra of haemoglobin and its derivatives. 1. Oxybsemoglobin. 2. Reduced hsemoglobin. 3. Methsemoglobin. 4. Alkaline methsemoglobin. 5. Acid hsematin in ether. 6. Alkaline hsematin in rectified spirit. 7. Reduced hsematin. 8. Acid hsematopor- phyrin. 9. Alkaline hsematoporphyrin. (From MacMunn.) C33H3204N4Fe.OH. Its compounds with acids and alkahes are spoken of as acid and alkaline hsematin, and each gives a characteristic absorption spec- trum (Fig. 363). The alkaline solutions exhibit one indistinct absorption band between C and D, the acid solutions an absorption band also between C and D but nearer to C, and resembling somewhat the band presented by methse- moglobin. According to Hoppe-Seyler and Gamgee perfectly pure solutions of hsematin in alkalies are quite unaffected by reducing agents ; in the pres- ence of certain foreign matters, however, alkaline hsematin, when treated with reducing agents, exhibits a spectrum known as that of reduced alkaline hsematin, which is identical with that of hsemochromogen. The same THE RED BLOOD-CORPUSCLES 827 change is further observed when alkahne ha3matin made by the action of alkalies on ordinary blood is treated with reducing agents such as ammonium sulphide. Since this substance haemochromogen is responsible for the respiratory functions of the haemoglobin, i.e. the power of its molecule to form unstable compounds with oxygen, its preparation merits fuller consideration. Hcemochromogen is prepared by the action of caustic alkalies on haemoglobin in the absence of oxygen. For this purpose a test-tube con- taining a solution of sodium or potassium hydrate is placed in a bottle with two necks containing a solution of haemoglobin, care being taken not to spill any of the alkaline solution. Hydrogen is then passed through the larger bottle until the haemoglobin is entirely reduced and all the air is replaced by hydrogen. The bottle is then inverted so as to mix its contents with the caustic alkali, when haemochromogen is formed and can be recog- nised by its characteristic colour and spectrum. The haemochromogen in solution has a^ cherry- red colour, and when sufficiently diluted shows two well-marked absorption bands identical with those given by reduced alkahne haematin (Fig. 363, 7). Of the two absorption bands which are situated between D and E, that nearest to D has very sharply defined borders ; the position of the two absorption bands may be given in terms of their wave- lengths as follows : \ 567 to 547 and X 532 to 518. The band nearest D is given by haemochromogen solutions diluted so that there is only one part of the pigment in 25,0(X) parts of water, so that the formation of reduced alkahne haematin is an even more delicate test for blood than the spectrum of oxyhaemoglobin itself. When CO-haemoglobin is treated in the same way with alkali in the absence of oxygen, a body CO-haemochromogen is formed which contains exactly the same volume of CO in combination as the original CO-haemoglobin. This fact, combined with the possibility of reducing ordinary alkahne haematin by the action of ammonium sulphide or Stokes's fluid, indicates that the group of atoms which in haemoglobin is responsible for taking up oxygen or carbon monoxide gas passes unchanged into the haemochromogen molecule. Haemochromogen therefore represents an iron- containing coloured radical which can combine with protein bodies to form haemoglobin, and is responsible for the oxygen-combining powers of the latter. We may assume therefore that oxyhaemoglobin and CO-haemoglobin contain oxyhaemochromogen and CO-haemochromogen respectively. HcBmato'por'phjrin. If haemoglobin, haematin, or haemin be mixed with concentrated sulphuric acid, it dissolves forming a purple-red solution. On pouring this solution into a large quantity of water, haematoporphp'in is thrown down in the form of a brown precipitate. In order to prepare haematoporphyrin, pure crystallised haemin is added to a saturated solution of hydrobromic acid in glacial acetic acid. The whole is allowed to stand for three or four days and then thrown into distilled water. The resulting mixture is filtered and the haematoporphyrin thrown down by careful neutralisation of the hydrobromic acid with caustic soda. Haemato- porphyrin is easily soluble in alkalies and somewhat less readily so \n acids. 828 PHYSIOLOGY forming alkaline and acid haematoporphyrin respectively. The formula of hsematoporphyrin has been given by Nencki and Sieber as CigHisNaOg, and is according to them isomeric with the chief bile-pigment, bihrubin. According to Willstatter its formula is C33H38lSr406. An alcohohc solution of hsematoporphyrin acidulated with hydrochloric acid shows two absorption bands : one, the fainter, between C and D, and the other, broader and more defined, midway between D and E. Solutions of alkaline hsemato- porphyrin show four absorption bands : a weak band between C and D, another between D and E, a more strongly marked band nearer to E, and a fourth band, darkest of all, between h and F, It wall be observed that in the formation of haematoporphyrin from hsematin the iron of the latter has been spht ofi by the action of the strong acid. Laidlaw has found that the sphtting off of iron occurs much more readily in the absence of oxygen. If reduced haemoglobin be taken, or defibrinated blood which has been allowed to stand rmtil it is thoroughly reduced, it is sufficient to add 15 per cent, hydrochloric acid in order not only to convert the greater part of the haemoglobin to haematin but to split off the iron of the latter and form haematoporphyrin. Haemotoporphyrin occurs in minute quantities in normal urine and in larger quantities in certain toxic conditions, especially in poisoning by sulphonal, when the urine may have a bright purple colour. It is important to remember that although urine is acid from the presence of acid sodium phosphate, urinary haematoporphyrin is always alkahne haematoporphyrin and gives the spectrum of this body. CHEMICAL RELATIONSHIPS OF H^MATIN. Hsematin, or hsemochromogen, is widely diffused through the animal kingdom, occurring in the form of haemoglobin in a large number of the invertebrata, as well as in all the vertebrata except, perhaps, Amphioxus. Since the respiratory function of haemoglobin depends on the power of its iron-containing radical to combine with a molecule of oxygen, forming an easily dissociable compound, it becomes of interest to try whether by a study of its disinte- gration products we can throw any light on its chemical relationships and on the con- ditions of its formation in the living organism. When hsematin is oxidised with sodium bichromate and acetic acid two new acids are formed, called the haematinic acids. One of these has the formula CgHgO^N, and the other CgHgOj. The first acid is con- verted into the second by the action of alkalies. The relationship of the two haematinic acids can be represented by the following formulae : — CO /^% C5H7/ >0 C5H7 NH -CO/ \rn/ CO- COOH COOH If, on the other hand, haemin or haematoporphyi'in be reduced by the action of hydriodic acid dissolved in acetic acid with the addition of phosphonium iodide, and the product be distilled with steam, the distillate contains a niixtmre of substituted pyrroles formerly known as hsemopyrroes. The mixtm-e readily oxidises to a red substance on exposure to the air. If ammonia be added to the colom'ed solution the colour changes to yellow, which, on the addition of an aipmoniacal solution of zinc chloride, changes to pink with a green fluorescences. These reactions are also given by urobilin, one of the ui'inary pigments and the chief pigment of the faeces, as well as by hydi'obilirubin, a substance obtained by the action of tin and sulphuric acid on an alcoholic solution of hsematin. THE RED BLOOD-CORPUSCLES 829 The hsemopyrroles, according to Willstatter are three in number and have the following formula : — Cryptopyrrole CH3, jC2Hg NH Phyllopyrrole CHs ^C2H5 'cHa ca NH Isohaemopyrrole CH3, ^^C2H6 CH3V yTL NH The same substances can be obtained from the chlorophyll of plants. On treatment with acid chlorophyll loses magnesium, and is converted into phacophytin. From this three COOH groups can be sjDlit off leaving a substance setioporphjTin. It is interesting that hsematoporphyrin can be readily converted into the same substance. On treatment with pyiidin and alcoholic potash it is converted into haemoporphyrin, and this heated with soda lime gives setioporphyrin (C31H36X4), Thus the same group forms the basis both of the substance which is responsible in the plant for the assimilation of carbon from carbon dioxide, and of the pigment which in the animal is the carrier of oxj^gen between the tissues and the surrounding medium. According to Willstatter, setioporphyrin and hfematoporphyrin are both built up of four substituted pyrrole rings. Thus hiBmatopoi"j)hyi'in has the following structmal formula : — ■ CH3.C- OH.C2H4.C- COOH.CaH^Ct CHs.a -CH )n -C =c \xH =C.CH3 C- N< C- c= -C.CH=CH.OH -CH ^C.CsH^.COOH HX \ CH3.C= -C.CH3 and the same worker suggests the following foiuiula for hannin : — CH3C — CH CH -C.CH3 COOH.C2H4.C CH,.0 C.C2H4.COOH THE SYNTHESIS OF THE BLOOD-PIGMENTS. Chemists have not yet succeeded in the artificial formation of h^matoporphp'in. Given hsematoporphyrin, however, evidence has been brought forward both by Menzies and Laidlaw of the possibiUty of forming artificially both ha?matin and haemoglobin, or some substance indistinguishable from the latter. 830 PHYSIOLOGY Reduced haemoglobin is a compound of haemochromogen and a protein, globin. The spMtting off of the prosthetic chromatogenic group — ^hsemo- chromogen — can be effected either by acid or alkali. When the latter is employed we obtain a red solution which is fairly stable, and can be con- verted by shaking up with air into ordinary alkahne hsematin. With acids the decomposition is easily carried further. Even with 2 per cent, hydro- chloric acid a certain amount of haematoporphyrin is formed, and if the strength of the acid be increased to 15 per cent, the whole of the iron is spht off and the haemochromogen is converted entirely into haemato- porphyrin. If oxyhaemoglobin be treated in the same way it yields acid or alkaline haematin directly, so that haematin must be regarded as an oxyhaemo- chromogen. The distinction drawn by Hoppe-Seyler between haemo- chromogen and reduced alkaline haematin had its chief ground in the fact that pure haematin is not reduced to haemochromogen by the action of such reducing agents as ammonium sulphide. The conversion can, how- ever, be easily effected by using a strong reducing agent, such as hydrazine hydrate. Whether the haematin contains the whole of the oxygen of the oxyhaemoglobin is doubtful. According to Ham and Balean, when oxyhaemoglobin is converted by means of acids into acid haematin, exactly half of the oxygen of the oxyhaemoglobin is given off, so that haematin would only contain one-half of the oxygen of the oxyhaemoglobin. There is a marked difference between the stabihty of haematin and haemochromogen. In the oxidised form of haematin the iron is firmly bound and can only be spUt off by using strong sulphuric acid, concentrated hydrochloric acid being insufficient for the purpose. It has been shown by Laidlaw that the change in the reverse direction, i.e. the combination of iron with haematoporphyrin to form haemochromogen, may be effected with equal ease. One gramme haematoporphyrin prepared by Nencki's method is dissolved in dilute ammonia and warmed in a flask on the water-bath. Some Stokes's fluid, prepared from about 2 grm. ferrous sulphate, and a few drops of a 50 per cent, hydrazine hydrate solution are added. At the end of one or two hours the solution is seen to be of a bright red colour when examined in thin layers, and on dilution shows the typical absorption spectrum of haemochromogen, which changes to that of alkaline haematin on shaking with air. Strong potash is added, and the ammonia is boiled off in an evaporating dish with free exposure to the air. The hydrazine is decomposed, and a solution of haematin remains, and can be precipitated by acidification with hydrochloric acid. The pigment obtained in this way agrees in every respect with that prepared from oxyhaemoglobin. Analysis of the product gave 9-58 per cent, of iron, which agrees with Nencki's formula for haematin, CggHgoN^OgPe. A pigment occurring in the wing feathers of certain birds, called turacin, was shown by Church to contain copper, and to yield, on treatment with strong sulphui'ic acid, a substance indistinguishable from haematoporphyrin. Laidlaw has succeeded in synthetising this pigment by treating ordinary hsematoporphjrrin obtained from blood THE RED BLOOD-CORPUSCLES 831 with ammoniacal copper solution, showing that it is a compound corresponding to hsematin, in which the place of iron is taken bj^ copper. It was stated some years ago by Menzies that a solution of impure hsemochromogen, prepared by the action of ammonium sulphide on alkaUne hsematin obtained in the ordinary way from blood, on standing for some days was reconverted into reduced haemoglobin. Ham and Balean have confirmed this observation, and have shown in addition that haemo- chromogen, prepared by the action of ammonium sulphide on an alkaUne solution of pure hsemin, though perfectly stable by itself, was rapidly reconverted into haemoglobin if a solution of globin were added to the mixture. The same change took place if egg-white were used instead of globin. The haemoglobin thus formed was changed into oxyhaemoglobin on shaking with air. xA.lthough in these experiments the oxyhaemoglobin was not separated in the crystalline form, its colour and spectral characters are so very distinctive that we are justified in concluding not only that it is possible to effect a recombination of the haemochromogen and globin, but also that other proteins can take the place of globin in the haemoglobin molecule. THE LIFE-HISTORY OF THE RED BLOOD-CORPUSCLES The growth of the embryo as well as of the young animal must be attended with a continual increase in the number of red blood-corpuscles present in the body. In the developing embryo the first formation of red corpuscles occurs in the vascular area. In the chick, about the twentieth hour of incubation, the area opaca, which surrounds the blastoderm, and will later become the area vascidosa, presents on examination under the low power a network of anastomising strands more opaque than the rest of the area. On section these strands are seen to be made up of cellular masses, the ordinary mesenchyma, mth branched cells and amoeboid corpuscles lying between. The cells in these cords are continually multi- plying by indirect di^^sion. Those on the outer side of the cord become the endothelimn of dilated blood-vessels, while those in the interior acquire a yellowish colour from the laying down of haemoglobin in their cytoplasm. The cords become canalised, and, as soon as a connection is estabhshed with the vascular system of the embryo, the newly formed blood- corpuscles move slowly on into the general circulation. The red corpuscles in the bird are true erythrocytes, i.e. are nucleated cells. The leucoc}i;es seem to arise by the immigration of wandering cells from the surrounding mesenchyma. Other places in the foetus Avhere a similar growth of corpuscles proceeds throughout foetal Hfe are the liver and the spleen, and later on the bone-marrow. In the mammal the nucleated erythrocytes, though forming the majority of the red corpuscles in early foetal life, become fewer and fewer in number as gestation advances, so that at birth practically the whole of the cor- puscles are of the non-nucleated type. These, however, can be shown to be derived from nucleated rod corpuscles by a process either of extrusion 832 PHYSIOLOGY or of degeneration and solution of the nucleus (Fig. 364). The formation of red corpuscles does not cease with the end of foetal life or even with the attainment of full stature by the animal. We have definite proof that a continual formation of red corpuscles can proceed and is proceeding throughout the whole of adult hfe. In an adult the total volume of blood and the total number of corpuscles remain approximately constant. By bleeding an animal we can diminish the total amount of corpuscles. The / eiv -_. : . . - . J Fig. 364. Part of a blood-vessel from the yolk-sack of the rabbit embryo, showing the changes which occur in the formation of erythrocytes. (From Schafer after Maximow. ) a, megaloblasts ; h, normoblasts changing into erythroblasts ; c, erythroblasts, in which the nuclei are disappearing ; d, an erythrocyte fully formed, but not yet disc- shaped ; tn, phagocytic endothelial cells ; I, lymphocytes ; Ic, a divided lymphocyte ; n, erythroblasts, shrunken with atrophic nucleus. first effect of such a bleeding is that the fluid parts of the blood are made up, so that the volume of the blood is restored to normal and the blood therefore becomes relatively poor in corpuscles. In a few weeks, however, the corpuscular content of the blood is found to be once more normal, showing that the loss of corpuscles has been followed by a compensatory regeneration. The fact that the pigments constantly leaving the body with the urine and fseces, namely, urochrome and urobilin or stercobihn, are derived by means of the liver from haemoglobin, shows that a constant destruction of red corpuscles must be proceeding. Since the number of corpuscles remains unaltered, this loss of haemoglobin must be made THE RED BLOOD-CORPUSCLES 833 good by a continual regeneration of fresh haemoglobin and new red cor- puscles. The seat of the formation of red corpuscles in the higher verte- brates is the bone-marrow. Here we have a structure protected from pressure where the capillaries and veins are dilated and thin-walled, and allow a slow passage of blood and the entry of newly formed corpuscles through the imperfect walls into the blood-stream (Fig. 365). That the marrow is the tissue involved in the process is shown by the fact that it is the only tissue of the body which undergoes an alteration in appearance when blood formation is stimulated by such means as repeated bleeding (or destruction of corpuscles by the injection of toxic agents. Under such fi^. 'Mra.'i^Hm' . is the number of cc. of seruminlOO c.c. of blood ; \ (&), \(s), the conductivity respectively of the blood and serum (both measured at or reduced to 5° C. and expressed in reciprocal Ohms x 10^). A reciprocal Ohm is the conducti^aty of a mercury column 1 -063 metres long and 1 square millimetre in section. THE GENERAL COMPOSITION OF THE BLOOD The general composition of the blood has been determined by Karl Schmidt in man, and by Abderhalden in the horse and bullock. The results are given in the Tables on pages 863 and 864. The important points to be drawn from these analyses may be sum- marised as follows. Blood contains from rather over one-third to one-half of its weight of corpuscles. It contains from 20 per cent, to 25 per cent, solids. Blood-plasma is resolved by clotting into serum and fibrin. The fibrin forms only 0-2 to 0-4 per cent, of the total weight of blood. The serum contains in 100 parts 8 to 9 parts of soUds, of which 7 to 8 parts consist of proteins, while the salts make up about 1 part. The chief salt present in the serum is sodium chloride, which constitutes 60 per cent, of the ash. Next to this comes sodium carbonate, about 30 per cent., and besides these two we find' traces of potassium, sodium, and calcium chlorides and phosphates. Traces of fats, cholesterin, lecithin, dextrose, urea, and other nitrogenous extractives are constantly found in the serum. The fats are much increased after a meal rich in them and may give the serum a milky appearance. The red corpuscles contain from 30 to 40 per cent, total sohds. Of the solid constituents haemoglobin forms nine-tenths ; the other tenth corresponds to the stroma consisting of stroma protein (nucleo-protein), lecithin, cholesterin, and salts. There is a striking con- trast between the salts of the corpuscles and those in the serum, the former consisting chiefly of potassium phosphate, the latter of sodium chloride, which in some animals is entirely wanting in the corpuscles. QUANTITY AND COMPOSITION OF BLOOD 863 Q O O CO O W o I— I OQ >H -Si ft'S (-1 1— o »-- ^>> ft_=s 2 '=' o o O o 61315 386-84 315-08 56-78 0-388 3-973 1 0-095 4-935 CO CO 1 1— 1 Magnesia . 0-0809 Chlorine . . 1-949 Phosphoric acid 1-901 Inorganic phos- phoric acid . 1-458 Water Solids Haemoglobin Protein Svigar Cholesterin Lecithin . Fat . Phosphoric acic as nuclein Soda Potash O 4^ 'S .g C3 fl O C^ O O 9Cppp pOt;- CjOCOOiOOC^lO OC^C^I TjH lo CO CO t-- into its peripheral cut end. The tube d is tilled with PHYSIOLOGY oil and b with salt solution or defibrinated blood. On clamping the artery, blood flows into a and drives the contained oil over into b, the contents of b being meanwhile forced into the peripheral end of the artery. When blood has com- pletely filled the bulb a, the two bulbs are reversed, and the blood now entering the artery displaces the oil in b, and forces the blood which had entered a on into the peripheral end of the artery. Knowing the capacity of the bulbs and the number of times it has been necessary to turn them in the course, say, of one minute, we know also the amount of blood which has passed across the section of the artery under experiment. In order to determine from this volume the velocity of the blood across the section, i.e. through the artery, the total volume passing in the minute must be divided by the cross- section. This will give the velocity per minute. Many modifi- cations of this apparatus have been devised. A simple form of cm-rent measurer is shown in Fig. 388. The whole apparatus is constructed of glass. The tube a is connected with the Fig. 387. Diagram of central end of a cut artery, and the tube p with the peripheral Ludwig's ' Stro7mihr.' end. The blood flows into B and fills it. As soon as it is full and its level rises over the level of the bend of the siphon ^ ^ tube s, the blood is rapidly siphoned ofi into c whence it flows along p into the peripheral part of the artery. The side tube E is connected with a mercury or membrane manometer. Every time that B is emptied into c a depression is pro- duced on the manometer tracing, which thus records not only the average pressm-e but also the average velocity of the blood in the artery. Each instrument has to be calibrated in order to know how much blood passes from b to c each time that siphonage occurs. None of these methods give any infor- mation of 'any rapid changes occurring in the velocity of the blood, e.g. dm'ing a single pulse-wave. For this purpose we must have recourse to some instrument such as Chauveau's hsemadromograi^h or Cybulski's photohsematachometer. The hcemadromograph (Fig. .389) consists of a pendulum which is hung in a tube, through which the blood is allowed to flow, placed in the course of the artery. The deviation of this pendulum from the vertical will be in proportion to the velocity of the current, and if its upper end be connected, as in the diagram, with a tambour, the variations in velo- city can be recorded on a blackened surface by means of a lever. The phofo- hcBmatachometer is based on an interesting application of Pitot's tubes. If a current of blood be directed along the tube ab possessing two vertical side tubes c and d the pressure at c will be greater than that at d, since at c the from Artery to Recorder FxG. 388. A simple blood-current measurer. (IsHiKAWA and Stakling.) (Fig. 390), VELOCITY OF BLOOD IN VASCULAR SYSTEM 889 momentum of the moving mass of blood is added to the lateral pressure of the fluid. A tube of this shape is connected with an artery, such as the carotid, and the tubes 4 pi Fig. 389. Diagram showing the construction of Chauveau's hfemadromograph. Fig. 390. Diagram to show principle of construction of Cybulski's photo-hasmata- choraeter. h and h' are attached at the points c and d. These two tubes are united at their upper extremities. In this case so long as the blood flows from a to h the fluid in /( will rise higher than in /;.', and the difference in height of the fluid in the two tubes will be pro- Fi(i. 391. Record of blood- velocity in the carotid artery of the rabbit. (Cybulski.) portional to the velocity of the blood. A graphic record of this diftercnce of pressure is obtained by allowing a narrow beam of light to throw an inuige of the menisci of the two columns of fluid through a slit on to a moving photographic plate. Such a record is given in Fig. 391. The width of the black space at any point is proportional to the 890 PHYSIOLOGY velocity of the blood at the moment at which this part of the record was being taken. Of course this instrument has to be calibrated if we wish to determine the velocity of the blood in absolute measure. In Fig. 391 the velocity at the points 1 and 1', corre- sponding to the cardiac systole, was 248 mm. per second. At 2 and 2', corresponding to the dicrotic elevation, the velocity was also 248 mm. At 3 and 3', towards the end of diastole, the velocity sank to 127 mm. The velocity of the blood in the capillaries can be measured by direct observation of the capillaries under the microscope, and noting the time it takes for a blood-corpuscle to move from one edge of the field to the other. THE VELOCITY IN DIFFERENT PARTS OF THE VASCULAR SYSTEM During systole the velocity of the blood in any part of the arterial system must be greater than during diastole ; thus in the carotid of the horse the following figures were found : Velocity per second During systole . . . . . 520 mm. During diastole . . . . . 150 mm. The following figures have been obtained from experiments on dogs (Tigerstedt) : Body weight Artery Volume per second Linear velocity per second Diameter of artery B.P, Remarks kg. 14-6 14-1 Crural. Crural. Carotid. CO. 0-63 1-69 1-95 mm. 128 275 241 mm. 2-5 2-8 3-3 mm.Hg. 77 88 93 Nerves un- injured Nerves cut Nerves un- injured SECTION IV THE MECHANISM OF THE HEART PUMP In the mammal the two sides of the heart are only in communication by means of the blood-vessels of the systemic and pulmonary area. Each side consists of an auricle into which the veins open, and a ventricle which receives the blood from the auricle and discharges it into the arterial trunk — either aorta or pulmonary artery. Since the auricles have to act merely as a receptacle for fart of the blood which enters during the relaxation or diastole of the heart, their cavities are smaller than those of the ventricles, and their walls are thin, corresponding to the small amount of work thrown on them in propelling blood into the relaxed ventricle. The ventricles have the office of carrying on the main work of the circulation and of forcing blood through the peripheral resistance. Their walls are much thicker than those of the auricles. The right ventricle has a wall which is only about one- fourth the thickness of the left ventricle, in conformity with the much heavier work to be done by the latter. On cutting a section through the two ven- tricles in a contracted condition the thin wall of the right ventricle is seen to lie in the form of a crescent round the circular left ventricle. The capacity of both ventricles is approximately equal, and amounts in man to about 1-40 c.c. for each ventricle when the heart is completely relaxed. The auricles are separated from the ventricles by a fibrotendinous ring. From this ring take origin most of the muscular fibres of the heart walls. The muscular fibres of the auricles run in both circular and longitudinal directions, the circular fibres being continued round both auricles, and special rings of circular fibres surrounding the openings of the great veins. From the fibrotendinous ring between the auricle and the left ventricle and from the sides of the aorta the muscular fibres forming the superficial layer of the ventricular wall pass obliquely downwards to the left towards the apex of the ventricle. Here they loop round into the interior of the ventricle and pass up near its inner surface to end either in the papillary muscles or in the auriculo-ventricular ring of fibrous tissue. Between these two layers we find a third inedian layer of muscular fibres which is in the form of a muscular cone. The fibres of this layer form complete loops round the left ventricle. The middle layer is connected by many strands of muscular fibres with both inner and outer layers. Mall divides the muscular fibres of tlie inniuiualian heart into four It consists of a tube armed with a stilette. A, which tits it accm'ately. At o is a tap which, when opened, will permit the passage of the stilette, and can close the tube en- tirely when the stilette is withdrawn. About 2 cms. above the lower extremity of the tube is a small drum-like enlargement, closed on one side by a thick membrane, e. On the edge of this membrane is fixed by means of shellac a minute mirror, F, 1 nmi. in diameter. With the stilette protruding, the manometer is thrust directly into the cavity of the heart, and fixed in position by a pm-se-string suture tlu:ough the super- ficial part of the heart muscle, tied tightly round the end of the manometer. The stilette is then withdrawn and the tap turned off, but alterations in pressure in the cavity of the heart causes minute oscillations of the menibrane,wluchcan be recorded and magnified to any desired extent by means of a beam of light reflected from the mirror on to a moving photographic plate or paper. The advantage of this optical method of registration is that the magnification can be increased to any extent without altering the mass moved. The ' figiu:e of merit ' of this manometer, i.e. its own period of vibration, when tilled with fluid, is about 250 per second with a tliick membrane, so that it canrecord with perfect accuracy all such rapid changes of pressure as may occur even in the left ventricle. 29 898 PHYSIOLOGY On registering the endocardiac pressure by the optical method it is found that the curves vary in form according to the condition of the heart. Fig. 398. Endocardiac preissure tracings, taken with Piper's manometer A, Simultaneous tracings from left ventricle and left auricle. To be read from left to right. B and C taken from left ventricle, C at a faster rate of recording surface thanB. 'Jo be read from right to left. Ki= closure of A.V. valves; Pj opening of aortic valves; 83, elastic oscillation or wave; K2, opening of A. V. valves. In order to interpret these curves we must utiHse the knowledge obtained from a simultaneous record of the pressures in the auricle and ventricle, or in. .the ventricle and aorta. Figm'e 398 represents the different forms THE MECHANISM OF THE HEART PUMP 899 of curve obtained from the left ventricle. Very often, as in a, the curve is not unlike a single muscle twitch or the curve of contraction of the fi'og's heart muscle. Nearly always however it is possible to see on the upstroke one or two elevations, the most noticeable being the elevation marked Si- This can be shown to correspond to the opening of the aortic valves. This is still better marked in b, where the heart was beating very forcibly and rapidly under the influence of adrenalin, and is also very exadent in c. In some cases, as in a, the rise of pressure occm's distinctly more slowly after Sj than before. In b there is a further rapid rise of pressure after Sj before the curve begins to slope away, and at the change of velocity of rise, there is a second wave at the point marked S^. This is also marked in c. The slope of the curve after S^ or Sg varies considerably according to the Aorta l/entrLcle Auricle amount of blood the heart is sending out. In c the intraventricular pressure curve runs almost horizontal for a time and this part of the curve is known as the systoUc plateau, but, as is evident from a, a plateau in the strict sense of the term is not always present. At the end of the ' plateau ' the pressure rapidly falls, and the period where the lines thin out, i.e. the point at which the fall is occurring most rapidly, corresponds to the closure of the aortic valves. The average course of the changes of pressure in the heart during each beat is shown diagrammatically in Fig. 399 (Piper). The cardiac cycle begins with the contraction of the auricle at 1 which may or may not give rise to a shght rise of pressure in the ventricles. As the auricular contraction dies away, the ventricular contraction begins at 2, This causes a very rapid rise of pressure. Almost immediately after the beginning of the rise, or 900 PHYSIOLOGY sometimes synchronously, the auriculo- ventricular valves close at the point marked 3. The pressure then rises rapidly in the ventricular cavity. Directly it exceeds the pressure in the aorta, the aortic valves open at the point marked 4, and the aortic pressure thereafter rises with the ventricular pressure. During the whole duration of the ventricular contraction the aortic pressure remains somewhat below the ventricular pressure, showing that blood is flowing continuously from ventricle into aorta. The rise of pressure in the aorta may be at first rapid and then slow ofi gradually. With the change of velocity of the rise of pressure vibrations may be set up at 5 especially in the aorta and also, but to a less extent, in the ventricle. These vibrations are however often absent. At the end of the plateau the pressure in the ventricle falls rapidly as this organ begins to relax. Its pressure therefore falls below that of the aorta and the aortic valves close, the closure of the valves being followed by the so-called dicrotic elevation or incisure at 7. The pressure in the ventricles then continues to fall, first rapidly and then more slowly, until it reaches the fine of zero pressure, and remains at or near this line during the greater part of diastole. With a big inflow there may be a shght rise towards the end of diastole which may be accentuated by the auricular contraction. If the chest is opened the pressure in the ventricle never sinks below zero during any part of diastole. The time relations of these events naturally varies with the frequency of con- traction of the heart. In a dog's heart beating about 100 times a minute the following phases in the ventricular tracing were determined by Hiirthle. (1) A small rise of pressure due to contraction of the auricles, lasting about '05 seconds. (2) A very steep ascent rather above the middle of which the aortic valves open. The beginning of the rise is generally marked by a sharp secondary wave or by a sudden change in the direction of the curve. This point is about -02 to -04 seconds after the beginning of the ventricular contraction. (3) A prolonged stage, lasting about two-tenths of a second during which the ventricle is contracting. During this time the tracing may show a flattened top, the ' plateau,' or a rounded summit. Kear the beginning of this plateau, a second wave may make its appearance if the rate at which the pressure rises falls off suddenly. This second wave is therefore most marked with a considerable output, and a heart which beats forcibly and rapidly. (4) A rapid fall due to the relaxation of the ventricular muscle. Near the upper part of this fall the aortic valves close, but it is generally difficult, without comparison with the simultaneous aortic pressure curve to make out the exact point of closure on the ventricular pressure curve. (5) A period lasting about two-tenths of a second during which the ventricle remains relaxed and the pressure is approximately zero, in some cases rising shghtly towards the end. The period of outflow of blood lasts from the moment at which the aortic valves open to the moment at which in the relaxing ventricle the pressure falls below that in the aorta and the aortic valves close. It therefore corresponds with the duration of that part THE MECHANISM OF THE HEART PUMP 901 of the curve which has been called the ' plateau.' The majdmum pressure attained in the left ventricle naturally depends on the height of the aortic pressure and is always greater than this. Under normal conditions in the dog, with an average aortic pressure of 100 mm. Hg., the pressure in the ventricle may be 130 or 150 mm. Hg. The difference between the average pressure in the aorta and the maximum pressure attained during contraction of the ventricle will naturally be greater the larger the amount of blood which is thrown out at each contraction. Thus in one case in a dog of 10 kilos, with an average aortic pressure of 100 mm. Hg., the maximum pressure in the left ventricle was 145 mm. Hg. with an output of 2040 c.c. per minute, and 115 mm. Hg. with an output of 650 c.c. per minute. On the right side the maximum pressure is much less, corresponding to the low resistance of the pulmonary system of blood-vessels. Otherwise the general course of the curves is very similar on the two sides of the heart. The maximum pressure in the right ventricle may be taken as varying between 25 and 35 mm. Hg. under ordinary conditions. CHANGES OF PRESSURE IN THE AURICLES Owing to the absence of valves between the right auricle and the venae cavffi, changes of pressure within this cavity are transmitted along the veins. The venous pulsation is especially marked in circumstances which give rise to high venous pressure, so that the veins are not entirely emptied at any part of the cardiac cycle. The most superficial observation shows that the jugular vein pulsates twice for each heart-beat. The exact form of the pressure tracing in the auricles varies considerably according to the inflow of blood and the state of filling of their cavities. A typical tracing with a moderate inflow of blood is given in Fig. 398 A^ p. 898, and in this figure the relations of the different elevations in the auricular tracing to the intraventricular events can be made out. A somewhat different curve is given in Fig. 400, but it will be noted that the essential features of the curves are identical. In every case the auricle curve presents the following features : (1) The first positive wave (pre-systolic wave) corresponding to the auricular systole. (2) The second positive wave l^ (first systolic wave) occupying the begin- ning of the ventricular systole. This is caused by the sharp closure of the mitral valve. (3) A third positive wave (second systolic wave) which may present secondary undidations. This rise of pressure is due to the gradual filhng of the auricles while the auriculo- ventricular valves are still shut. (4) A negative wave which corresponds to the ' post-systolic vacuum ' of Chauveau and Marey. At this point the ventricle is entirely relaxed and the auriculo-ventricular valves open, so allowing the blood to flow freely from the auricle into the ventricle. This negative wave is not always well marked, and is represented only by a series of small undulations in Fig. 398 A. 902 PHYSIOLOGY The pressure rises in the left auricle somewhat higher than in the right auricle. In the latter case the big veins act as a supplementary reservoir to the auricle, so that in no period of the cardiac cycle need the pressure in Fig. 400. Curve of pressures in left auricle of cat. (STKAtrB.) I, II, III Ton = 1st, 2nd, and 3rd heart sounds. the latter chamber rise to any extent. In both the auricular tracings given the heart sounds are apparent as small oscillations in the curve. NEGATIVE PRESSURE. When the flow of blood through a tap is suddenly interrupted, the momentum of the fluid tends to make it continue its movement so that a diminution of pressiu-e is produced in the rear of the moving column. In the tracings obtained by means of the older instrrunents, such as those of Hlirthle, the lever at the end of the ventricular systole even descended below the base line, thus suggesting that for a short period of time there was actually a negative pressure in the ventricles. If the manometer be comiected with the heart by a tube provided with a valve allowing the movement of fluid only in one direction, it becomes a maximum or minimtmi mano- meter according to the direction of the valve. This tube, however, increases the inertia of the whole system, so that the use of the minimum manometer tends to exaggerate the apparent negative pressure. According to some statements there may be a negative pressure of 40 to 60 mm. Hg. in the ventricle at some period during the cardiac cycle. Many explanations were put forward to account for this negative pressure, but they are all rendered unnecessary by the fact revealed by more perfect graphic methods that at no period in the cardiac cycle is there a negative pressiu'e in the ventricles. Naturally under normal conditions the pressm'e in the chest outside the heart is negative owing to the elastic retraction of the lungs, and may vary from — 3 to — 30 mm. Hg., according as it is measiu-ed during normal expiration or during forced inspiration. This negative I^ressure may be transmitted to the interior of the heart, but the pressiure within the ventricle never falls below the piessm'e obtaining outside the ventricles. CHANGES IN FORM OF THE HEART As the heart walls are perfectly flaccid during diastole, the shape of this organ as a whole will depend upon the position in which the heart is lying and the direction of its support. Thus if the chest and the pericardium be opened and the animal be in the supine position, the heart during diastole will be flattened from before backwards as a result of the simple weight of its contents. In this position therefore systole will be accompanied by a THE MECHANISM OF THE HEART PIBIP 903 shortening in the lateral and vertical directions and a lengthening in the sagittal direction. During systole, when the heart becomes tense and all its fibres are firmly contracted, the heart, whatever its previous condition, takes the form of a truncated cone. Under normal circumstances the heart in the unopened chest lies in the pericardium, which is attached above to the great vessels and below to the central tendon of the diaphragm. It is supported laterally by the lungs, which, however, owing to their elasticity, have very little influence on its shape during diastole. When the heart is freed from the pericardium, the obhquity of its fibres causes the apex to move forwards and to the right during systole ; this movement is normally prevented by the attachment of the pericardium to the central tendon of the diaphragm, so that the most movable part of the heart comes to be the base. If three needles be passed through the chest wall so that their points lie, one in the base, one about the centre of the ventricles, and one in the apex of the ventricles, each ventricular systole is found to be accompanied by a movement of the needle in the base of the heart down- wards, a slighter movement in the same direction of the needle in the middle of the ventricles, and practically no movement at all of the needle which is thrust into the apex. During systole the base of the heart moves downwards towards the apex. This movement is determined partly by the shortening of the fibres of which the ventricular wall is composed, partly by the lengthen- ing of the great arteries as blood is forced into them under pressure from the ventricles. The changes in the shape of the cavities of the heart during contraction have been studied in the stage of extreme contraction produced by heat rigour. In such hearts it is found that the cavities are never entirely obhterated, though the right ventricle is reduced to a narrow sht ^Nidening out slightly in the neighbourhood of the auriculo-ventricular orifices, while in the left ventricle a distinct cavity is left between the mitral valves and the free ends of the papillary muscles. During normal activity it is probable that the emptying of the cavities rarely proceeds to so great an extent. THE APEX BEAT The movement of the heart at each contraction is communicated to the chest wallr over a limited area of which it may be felt and seen, except in fat individuals. The region where the pulsation of the chest wall is most marked lies in the fifth intercostal space, a little to the median side of the left nipple. The pulsation is spoken of as the ' apex beat,' and was formerly thought to be due to the twisting forward of the apex at each systole. The apex of the heart is really situated lower down, and, as we have already seen, so long as the pericardium is intact is relatively motionless. During diastole the ventricles form a flabby flattened cone lying against the chest wall and slightly deformed by the latter. In systole the ventricles contract forcibly on the contained fluid and become hard and rigid, assuming the form of a rounded cone. This sudden recovery of shape and hardening of the 904 PHYSIOLOGY ventricular wall pushes out the part of the chest wall in immediate proximity to the ventricles and so gives rise to the ' apex beat.' The cardiac impulse may be registered by means of a cardiograph. In nearly all forms of this instrument a button resting on the chest wall Fig. 401. A cardiograph. This is strapped round the chest, the central button is applied to the ' apex beat,' and its pressure on the chest wall regulated by means of ths three screws at the sides. The tube at the upper part of the instrument serves to connect the drum of the cardiograph with a registering tambour such as that shown in Fig. 395. transmits the movement of the latter to a tambour, which again is connected by a tube to a registering tambour. One such instrument is shown in Fig. 401. The curves so obtained, which are known as cardiograms, may vary considerably in the same subject according to the pressure employed and the exact spot at which the tambour is applied. Their interpretation often presents difficulties owing to the fact that their form is conditioned by two factors, viz. (1) the actual size (antero- posterior diameter) of the ventricles, Fig. 402. Cardiogram. (Hxjrthle.) (2) the resistance to distortion {i.e. the tension) of the ventricular wall ; this factor will increase in importance with increasing pressure of the cardiograph button on the chest wall. Fig. 402 represents a cardiographic tracing or cardio- gram which may be spoken of as typical. In order to interpret this curve we must record at the same time either the intraventricular pressure in animals or the heart sounds in man. This cardiogram presents considerable similarities to the endocardiac pressure curve ; in both there is an ascending limb, a plateau, and a descending limb, and in many cases a small elevation occurs at the beginning of a curve during the contraction of the auricle. The exact point at which the auricular passes into the ventricular contraction THE MECHANISM OF THE HEAET PUMP 905 varies from case to case and may be altered by altering the degree of pressure put on the recording button. In the first figure given the auricular systole finishes before the main rise of the lever occurs. In many cases, however, the elevation due to the auricular systole may take up the greater part of the ascending hmb of the cm"ve, as in Fig. 403. In the experiment from which this figure was taken the heart sounds were recorded at the same time as the apex beat. It will be seen that the 1 .2 Fig. 403. Cardiogram (b) with'simultaneous record of heart-sounds (a), (Hi'RTHLE.) 1, position of fii'st heart-sound ; 2, position of second heart-sound. first heart sound, corresponding to the ventricular systole, begins, not at the commencement of the rise of the cardiogram, but at the notch near the top of the ascent. The first part of the ascent is therefore caused by the increase in the volume of the ventricle, due to the sudden contraction of the auricles, the ventricular systole being marked by the notch near the top of the curve. Owing to the co-operation of the volume and pressure factors in the production of the cardiogram, the curve generally begins to decHne with the diminution in volume which follows the sudden opening of the aortic valves. Here again, however, the effect will vary with the pressure of the button. If an actual deformation of the ventricular muscle can be effected, as in thin patients, the plateau of the curve may last during the whole of the cardiac cycle. Other forms of curves may be obtained which show con- siderable deviation from the endocardiac pressure tracing ; these are spoken of as atypical, and are generally conditioned by a faulty position of the cardiograph, the button being applied to the chest wall in the immediate vicinity of the apex beat instead of to the apex beat itself. THE HEART SOUNDS If we apply our ear to the front of a person's chest (it is more convenient to use the stethoscope for the purpose) we hear two distinct sounds accom- panying each beat of the heart, followed by a pause corresponding to the diastole. The sounds are compared to the syllables lubh, dup, the first sound being low-pitched and prolonged, the second sound high and sharp. Thus the heart sounds may be represented : lubb, dup (pause), lubb, dup (pause). The causation of the second sound is very simple, and may be considered first. It is heard just over the second right costal cartilage, i.e. the place where the aorta lies nearest the surface. It comes at the end of the systole, 29* 906 PHYSIOLOGY as determined by the hardening of the apex of the heart, felt as the apex beat, and can be shown to be synchronous with the closure of the aortic valves. It is, in fact, caused by the sudden shutting and stretching of these valves that occur directly the heart ceases to contract and to force the blood into the aorta. If the valves be hooked back in an animal by means of a wire passed down a carotid artery, the second sound disappears and is replaced by a murmur caused by the blood rushing back into the ventricle at the end of the systole. The same disappearance of the normal second sound is observed in cases where the valves are prevented from closing by diseased conditions. The pulmonary and aortic valves generally close simultaneously. In some cases, however, the aortic may close slightly before the pulmonary, giving rise to a ' reduplicated second sound.' The pulmonary element of this sound is best heard over the second left cartilage, or in the second left intercostal space. The first sound has probably a twofold origin, viz. from the sudden closure of the auriculo- ventricular valves and from the contraction of the thick muscular wall of the ventricle. If the veins going to the heart be clamped so that the heart can no longer be distended with blood nor the valves put on the stretch, the first sound is altered in character, but not abolished. The first sound may indeed be heard on listening with a stethoscope to the beat of an excised heart. It is said that two notes may be detected in the first sound — a high note of short duration due to closure of the valves, and a low-pitched note due to the muscular contraction. The muscular element of the first sound has the same pitch as the sound produced by contracting voluntary muscle, and therefore as the resonance-tone of the ear. This consideration prevents our arguing from the tone that a cardiac contraction is a tetanus. As we shall show later on, each ventricular contraction is analogous to a simple muscle- twitch and not to a tetanus. THE THIRD HEART SOUND. A number of observers have described a third heart sound as occurring in certain individuals during the diastole, a short time after the second sound. It is softer and of a lower pitch than the second sound, and is heard most distinctly over the apex beat. It is probably due to the vibrations set up in the fluid itself or in the auriculo-ventricular valves by the sudden inrush of blood from auricles to ventricles at the beginning of the diastole. The sound is shown objectively by the vibrations on the endocardiac pressure curve given in Fig, 400 (Straub). It has also been registered electrically by Einthoven. CARDIAC MURMURS When a fluid escapes through a narrow orifice into a wider space vibrations are set up in the fluid and may be transmitted by any elastic medium to the ear, giving rise to the sensation of sound. Such a sound is produced when water is allowed to run from a tap into a vessel of water, or when air is blown out between the partially closed lips. The formation of such vibrations forms, indeed, the basis for the construction of many musical instruments. The same sort of vibration may be set up in the large vessels or in the heart, whenever the blood passes rapidly through a narrow orifice into a wider space. THE MECHANISM OF THE HEART PUMP 907 In the normal individual sounds produced in this way are so slight that they may be neglected ; under abnormal conditions, as after diseases affecting the valvular orifices of the heart, this vibration may occur during every heart cycle and be heard with ease on applying the ear to the chest. These murmurs, or bruits as they are called, are of paramount importance in enabhng the medical man to form a judgment as to the condition of the different valves of the heart. Thus injury to an aortic valve, so as to allow of leakage during diastole, involves the squirting of a small amount of fluid under high pressure from the aorta into the relaxed ventricle. On hstening to the chest of a man with such a lesion this regurgitation during diastole is heard as a rushing sound occurring in the place of or continuing the second sound up to the beginning of the next first sound which denotes the beginning of systole. In many cases the disease which occasioned the inadecjuacy of the valve is followed by processes of repair and cicatrisation in which the valves become puckered and contracted and perhaps adherent, so that the orifices can never become thoroughly patent or thoroughly closed. Under such circumstances vibrations will be set up in the cm-rent of blood as it escapes through the narrow orifice into the aorta during systole, and on listening to the chest over the second right costal cartilage a ' to and fro ' bruit is heard composed of a systolic immediately followed by a diastoHc murmur. In the same way incompetency of the mitral valve or dilatation of the mitral orifice, in con- sequence of weakness of the cardiac muscle, gives rise to a murmur which lasts during the whole of the ventricular contraction and is therefore systolic in character. Such a murmur is heard best over the apex beat, and is also transmitted backwards so that it can be heard on listening at the back of the patient. A narromng of the mitral orifice in consequence of contraction of the valves will set up a resistance to the flow of blood from left auricle to left ventricle. The auricle becomes hypertrophied, its contraction prolonged, and the escape of blood through the contracted orifices gives rise to a murmur which is heard on listening over the apex beat as a presystolic bruit. This bruit is easily distinguished from a systolic murmur by noticing that it runs up to and ends with the apex beat, whereas a systolic murmur does not begin until the elevation of the apex commences. Several physiologists have succeeded in recording heart sounds graphically. Hiirthle's method consists in an ajjplication of the microphone. A special form of stethoscope is so arranged that by its means the vibrations corresponding to the heart sounds are transmitted to a contact between silver and carbon. Througli this contact a strong curi'ent is passing. This also passes through an electro-magnet, which attracts an iron disc attached to the membrane of a Marey's tambour. Any vibration transn)itted to the carbon-silver contact alters its resistance, and so the strength of the current passing through the electro-magnet. In this way the heart sounds can alfect tlie pull exerted by the elect ro-magnet on tlie membrane of the tambour, and tlie change in the volume of the contained air is recorded by means of an ordinary registering t;>mbour. Similar results have been obtained by Einthoven, who has allowed tlie variations in the current passing through the microphone to be recorded directly by means of a very delicate capillary or string elect rometer. 908 PHYSIOLOGY TIME-RELATIONS The time-relations of the various events of the cardiac cycle are indicated in the accompanying diagram (Fig. 404). In man the heart beats on the average about seventy-two times in the minute, so that each cardiac cycle — i.e. systole flus diastole — can be regarded as occupying 0-8 sec. During five-tenths of a second the ventricles are relaxed ; during the first four- tenths of this period, which corresponds to the diastole of the heart as a whole, blood is flowing in a steady stream from the veins through the auricles into the ventricles, so that the heart is gradually increasing in size. The systole of the Blood nto vent] from : lowin| aurilcles and icles veins. Systole of Aur- icles Systole Diastole. 01 0 2 0 3 04 0 5 0 6 0 7 08 0 9 10 sees. Heajt Sounds — — aup Fig. 404. Lubb dup Diagram of events constituting a cardiac cycle. auricle then occurs and lasts about 0*1 sec. This is followed by the ventricu- lar systole, the immediate effect of which is to close the auriculo- ventricular valves on both sides of the heart. The whole ventricular contraction lasts 0-3 sec, ; during the first period of this the ventricle is getting up pressure, the pressure rapidly rising until it equals the aortic pres- sure. This period, during which the ventricle is simply contracting isometrically on its contents without any flow of blood occurring, lasts between -02 and -04 sec, and is, of course, longer the higher the pressure in the aorta. Directly the intraventricular pressure rises above this point the aortic valves open and blood is driven into the aorta. The outflow of blood continues throughout the whole of the ventricular systole, and may be taken as lasting about 0-2 sec. The ventricle then suddenly relaxes, the period of relaxation occupying about 0-5 sec. ; the ' plateau ' of the endocardiac pres- sure curve on the average lasts about 0-18 sec, and according to the condition of the heart and the peripheral resistance may present a gradual ascent or descent. Directly relaxation commences the ventricular pressure falls below the aortic pressure or the puLmonary pressure and the semilunar valves close, giving rise to the second sound. Systole is now at an end and the dias- toHc period of filling recommences. The first sound is synchronous with commencement of the ventricular contraction, and the same event is signalled by the occurrence of the apex beat. THE MECHANISM OF THE HEAET PUMP 909 Although, the pulse frequency may undergo considerable variations according to the condition of the individual, being higher during activity or under conditions of mental excitement, the greater part of the difference in duration of the cardiac cycle thereby induced falls upon the diastohc period. Thus to take \vide limits the pulse-rate may vary between 32 and 124 beats in the minute, while under the same circumstances the period occupied by the systole only varies between 0-382 and 0-190 sec. — and these, of course, are extreme limits. Variation therefore in the time occupied by each cardiac cycle is determined mainly by variation in the time occupied by the diastole. FILLING OF THE HEART IN DIASTOLE Since the heart is perfectly flaccid during diastole it is unable to exert any suction force on the blood in the veins. Its filling during diastole depends on the existence of a positive pressure within the veins, or at any rate of a pressure greater than that in the auricles and ventricles ; the greater the pressure within the large veins the more rapidly will the blood enter the heart during diastole and the larger the amount of blood in this viscus when it begins its contraction. In this process an important part is played by the mechanical conditions existing in the thoracic cavity. Owing to the elasticity of the lungs and the fact that they are constantly tending to contract, the pressure in the thorax is less than that of the external atmosphere by the amount which is required to distend the lungs to fill the cavity. At the end of expiration this difierence amounts to about 5 mm. Hg. rising to 9 mm. Hg. at the end of inspiration and to 30 mm. Hg. at the end of a forced inspiration. On the other hand, the veins outside the thorax are exposed to a pressure which is a httle above that of the atmosphere. When the thorax is at rest the veins and auricles are therefore expanded and the flow of blood into them rendered more easy. The respiratory movements, by causing aji alternating suction on the walls of the great veins, act Hke an accessory pimip and cause an aspiration of blood into the veins of the thorax with each inspiration. It is evident that if the pressure within the thorax be sufficiently raised so as to cause a positive pressure on the big veins and auricles, the return flow of blood to the heart must come to an end. Thus during extreme muscular efforts the glottis is fixed and a positive pressure is produced in the thorax. The deficient circulation and the deficient aeration of the blood thereby induced are showTi by the engorgement of the superficial veins and the blue- ness of the surface. Weber showed that by a forcible expiration, with the glottis closed, the pulse might disappear at the wrist and the circulation be brought for a time to a standstill, so that even loss of consciousness might supervene. Since the heart during its systole diminishes its own volume by the expulsion of blood from the thorax, it becomes smaller, and the space thus provided in the chest cavity is taken up by an expansion of the veins, auricles, and lungs. To this systolic 910 PHYSIOLOGY diminution of intrathoracic pressure is due the ' cardiao-pneumalic ' movements. These are recorded by attaching one nostril to a delicate tambour by means of a tube, while the other nostril and the mouth are kept closed. If a carotid pulse tracing be taken at the same time it will be found that there is a fall of the lever attached to the nasal cavity synchronous with the rise of the pressure in the arteries, due to the expulsion of blood from the heart. The normal filling of the heart during diastole can be prevented by any- thing which hinders its expansion, such as the presence of fluid in the pericardial cavity. The same effect may be produced experimentally. If oil be allowed to flow into the pericardium under pressure, when the pressure of the oil rises to about 60 mm. the pressure of the vena cava rises to a height just above that obtaining in the pericardial cavity. On increasing the pressure, a point is finally reached at which no more blood can be driven from the veins to the heart, so that the arterial blood-pressure falls to zero and death ensues. In order to maintain the arterial pressure it is necessary that the amount of blood driven into the arterial system by the contraction of the left ventricle should be exactly equal to that leaving the arteries to pass into the capillaries during the period which elapses between each systole. Over-filHng of the heart is prevented to a certain extent by the resistance of its walls. The danger of over-filling is therefore most marked in the right ventricle. An important part is played moreover by the pericardium in this regard. Even when beating normally, the heart during diastole tends to protrude through a sht made in the pericardium, and Barnard has shown that the right auriculo-ventricular valve ceases to be entirely efficient when the pericardium has been freely opened, the closure of this valve being dependent on the support aflorded to the heart by the pericardium. SYSTOLIC OUTPUT OF THE HEART The amomit of blood which passes through the whole body and is avail- able for the metabolic exchanges of all the tissues depends on the amount of blood which leaves the heart each minute. The height of the arterial pressure also depends on the relation between the amount of blood leaving the arterial system by the capillaries and that entering from the heart. The determina- tion of the output of the left ventricle is therefore one of the most important problems in physiology. The output of the right ventricle must be equal to that from the left ventricle, otherwise the blood would accumulate on one or other side of the heart and bring the circulation to a standstill. It is therefore immaterial on which side of the heart the output be determined. The methods which have been devised for determining the cardiac output fall into two classes. In the first class it is sought to determine the total volume of blood leaving the right or left ventricle in the course of a given time, say one minute. If this amount be divided by the number of heart-beats in the same time, the output of each ventricle per beat is at once obtained. A second method consists in the determination of the volume changes in the ventricles at each beat of the heart. During diastole THE MECHANISM OF THE HEART PITUP 911 the ventricles are receiving blood and increase in volume, during systole they expel blood and therefore diminish in volume. The change in volume at each beat gives therefore the combined output of right and left ventricles and must be divided by half in order to give the output of either ventricle separately. METHODS OF DETERMINING OUTPUT. In a method devised by the author it is possible to deteiuiine the output of the left ventricle under all manner of conditions Fig. 40.J. Arrangement of apparatus for working on the isolated mammalian heart. (Kxowi.tox and Starling.) and to vary at will the arterial resistance, the venous pressure, the filling of the heart, or the tein])erature of the blood-supply to the heart. The arrangement of the apparatus isshowniuFig. 40.'). Artihcial respiration being maintained, the chest isopencdunder an ansesthetic. The arteries coming from the arch of the aorta — inthecat.the innomi- nate and the left subclavian— are thenligatured, thus cutting off the whole blood-supply to the brain, so that the anaesthetic can be discontinued. Cannula? are placed in the inno- minate artery and the superior vena cava. The cannula; are tilled beforehand with a solu- tion of hiruelin in normal salt solution so as to prevent clotting of the blood eluring the experiment. The descending aorta is closed by a ligature. The only path left for the blood is by the ascending aorta and the camiula CA in the innominate artery. 912 PHYSIOLOGY The arterial cannula communicates by a T-tube with a mercurial manometer M' to record the mean arterial pressure, and passes to another T-tube, v, one limb of which projects into a test-tube B. The air in this test-tube will be compressed with a rise of pressure and will serve as a driving force for the blood through the resistance. It thus takes the part of the resilient arterial wall. The other limb of the T-tube passes to the resistance R. This consists of a thin- walled rubber tube {e.g. a rubber finger- stall) which passes through a wide glass tube provided with two lateral tubulures w, w. One of these is connected with a mercurial manometer Jf 2 and the other with an air reservoir ^, into which air can be pumped by the elastic bellows S. When air is in- jected into the outer tube the tube B collapses, and will remain collapsed until the pressure of the blood within it is equal or superior to the pressure in the air surrounding it. It is thus possible to vary at will the resistance to the outflow of the blood from the arterial side. From the peripheral end of B the blood passes at a low pressure and is collected in a vessel N, which is provided with a siphon, and can be made of such dimensions that the blood is siphoned oS as soon as 10, 20, or 30 c.c. have collected in the vessel. A lateral branch on the siphon tube leads by a rubber tube to a tambour D. Every time that siphonage occurs there is a change of pressure within the tambour which can be registered by the lever on a blackened smrface. The siphon discharges the blood into a reservoir F, which is kept immersed in a vessel of water maintained at any desired temperature by some source of heat. From the spiral below F, an india-rubber tube leads to a cannula GV, which is placed in the superior vena cava, all the branches of which have been tied. This cannula is provided with a thermometer to show the temperature of the blood supplied to the heart. A tube placed in the inferior vena cava and coimected with a water manometer shows the pressure in the right auricle. On the recording surface we thus have a record of the arterial pressure, of the output of the whole system, as recorded by the tambour, and of the pressure within the right auricle. If desired the simple cm^rent measurer described on p. 888 can be inserted in the arterial circuit at X, so as to give immediately the output of the left ventricle. This method, although of considerable importance in giving information as to the conditions which determine the output of the left ventricle and the maximum capacity of the heart as a pump, tells us nothing as to the output of the left ventricle under normal conditions in the intact animal. For this purpose some indirect means must be adopted which can be used on the intact animal and if possible on man him- self, so that the output can be measured under different conditions of rest and activity. Moreover, the output as measured on the other side of the artificial arterial resistance represents the ventricular output minus the blood flow through the coronary arteries. It is possible, however, to insert a cannula into the coronary sinus, and so to measure the blood-flow through the heart muscle. The coronary circulation must be added to the flow through the arterial resistance in order to arrive at the correct total output of the left ventricle. The two chief methods for the determination of the ventricular output in the intact animal are those of Zuntz and of Krogh. ZUNTZ'S METHOD. This is based on a comparison of the differences in gases contained in the arterial and venous blood and the actual amount of oxygen taken from the air in the lungs. Thus in one case he found that in a horse weighing .360 kilos 2733 c.c. of oxygen were taken up in he lungs per minute, while the arterial blood contained 10-33 per cent, more oxygen than the venous blood. Since therefore every 100 c.c. of blood that passed through the lungs had taken up 10-33 c.c. of oxygen, and 2733 c.c. had been taken up in the coiirse of a minute, it is evident that 100 X 2733 10-33 =^M57c.c. of blood must have passed through the lungs in the time. This therefore was the output of blood by the right ventricle in a minute and was equivalent to -00122 of the body- weight per second. In a similar experiment on a dog the output per second of the right ventricle was found to be -00157 of the body-weight. In order to get the output at each beat it will THE MECHANISM OF THE HEAET PUMP 913 be necessary to divide the output per minute by the number of heart-beats in the same time. From the results of determinations made in this way Zuntz concluded that the output of the right ventricle in man at each beat varies between 50 and 100 c.c. and may be taken on an average at 60 c.c. KROGH'S METHOD. In Krogh's method an endeavour is made to determine the volume of blood flowing tlirough the lungs in a given time by determining how much nitrous oxide is taken up from a mixtmre of nitrous oxide and air, with which the lungs are filled. Nitrous oxide is chosen because it can be breathed in considerable proportions without injury, and is itself very soluble in water or in the blood. The estimation is carried out in the following way. A small recording spirometer is filled with about 4^ litres of a gas mixture containing 10 to 25 per cent. N2O and 20 to 25 per cent, oxygen. The subject, seated in a chair or on a bicycle ergometer, expires to the greatest possible extent, and then takes a deep inspiration from the spirometer. He holds his breath for five to fifteen seconds, breathes out sharply into the spirometer, 4-10 , 2-24 28-/ sec Fig. 406. (Krogh,) expiring at least one litre. At the end of this sharp expiration, a sample of his alveolar air is taken by comiecting the tube from his face-piece with an evacuated glass bulb, as in Haldane's method of determining alveolar air. The breath is now held for a period, time varying between six and twenty-five seconds. He then makes a final sharp ample expiration into the spirometer, a sample of his alveolar air being taken at the end of this expiration. The excursions of the spirometer indicate exactly what volume of air he has breathed in and breathed out at each part of the experiment. These are recorded on a travelling siu-face, so that the duration of the experiment is represented by the horizontal distance between the lines showing the moments of sampling (Fig. 406). By comparison of the composition of ordinary alveolar air with the alveolar air ob- tained after the first sharp expiration, the amount of residual alveolar air is determined, so that the total volume of gas contained in the lungs at each part of the experiment is also kno%vn. Diu'ing the time when the breath is being held, nitrous oxide is being taken up in solution by the blood as it passes through the lungs, its solubility being such that 1 c.c. of blood, if exposed to an atmosphere of pvu-e nitrous oxide will take up 0-43 c.c. of this gas. From the data obtained in this way, the amoimt'of blood passing through the limgs diu'ing the period between the two expirations can be calculated. The following record of one experiment may serve as an example. The volume of air in the lungs at the begiiuiing of the experiment was 3 "25 litres and contained 12 per cent, nitrous oxide, so that the total quantity of nitrous oxide in the air of the lungs was 3250 c.c. X -^jfjj = 390 c.c. At the end of the period the total volume of air in tlie lungs was three litres, containing only 10 per cent, nitrous oxide, so that the lungs now contained only 300 c.c. nitrous oxide, 90 c.c. nitrous oxide having been taken up by the blood. This 90 c.c. was taken up from an air in which the mean pressure of this gas was = 11 per cent. During the period of observation, from a gas containing at atmospheric pressure 11 per cent, of nitrous oxide, each c.c. of blood will take up = 0-047 c.c. In order to take up 90 c.c. therefore 1-9 litres of ^ 100 blood must have passed through the lungs during the time of the observation. The 914 PHYSIOLOGY experiment lasted twenty-eight seconds. The amount of blood passing through the lungs per minute was therefore 4-2 litres. This figure represents the output from the right ventricle during one mimite, and if the pulse rate is 70 per minute, the output per beat will be — — ^~ = 60 c.c. per beat. The figure arrived at in this way for the ^ 70 average output of each ventricle in man during rest, thus agrees with the figure obtained by Zuntz. The output of both ventricles is, of course, the same. According to Krogh, the ventricular output per minute in man may vary from 2-8 litres to 21 litres of blood per minute. The latter is an extreme figure and was obtained in a powerful athelete doing hard work. In the case of Krogh himself, the maximum output was about 12 litres per minute. It is interesting to note that the same perform- ance may be obtained from a dog's heart in the heart-lrmg preparation, allowing for the difference in size between the hearts of the dog and man respectively. CARDIOMETRIC METHOD, Of the various methods which have been devised for recording plethysmographically the changes in the volume of the heart at each beat (as first carried out by Roy), the simplest is that devised by Henderson. The chest and pericardium being opened, a glass cardiometer, of the shape shown in Fig. 407, is slipped Fig. 407. Henderson's glass cardiometer. over the heart. This cardiometer consists of a glass sphere with a wide opening. To the margin of the opening is tied a rubber diaphragm with a hole in it which accurately fits the heart as it lies in the auriculo-ventricular groove. The tube of the cardiometer is connected with some form of piston recorder or a tambour with a slack membrane. The disadvantage of this method is that the graphic record of rapid and ample changes in volume is one of the most difficult problems in experimental physiology, the inertia and friction of the moving piston tending to deform the shape of the ciu've obtained. Straub has therefore used a soap bubble as the volume measurer, photographing its edge and using the record as an index to the change in volume. It is possible, how- ever, to obtain a pistol recorder moving sufficiently freely to give a fairly correct re- production of the volume changes of the heart, provided that these do not occur with too great rapidity. It has been suggested by Piper to convert the volume changes into small pressure changes, and to record these latter by one of the methods described above. The factors which determine the output of the left ventricle are best studied in the heart lung preparation. In this it can be shown that, pro- vided the venous inflow remains constant, the output is also constant and is unaffected by considerable alterations of arterial resistance and of the rate of the heart. Thus with a moderate venous inflow the output remains constant whether we maintain the average arterial pressure at 60 mm. Hg. or at 160 mm. Hg. It is also unaffected by altering the rate of the heart from 80 beats per minute up to 160, or even 200, beats per minute. On THE MECHANISM OF THE HEART PUMP 9lb the other hand, the output is at once altered by alterations in the venous inflow and, as already stated, can be altered in a heart weighing 50 gnis. from a few c.c. up to 3000 c.c. per minute. The only essential in this preparation is that the output from the left ventricle shall be sufficient to maintain a circulation through the coronary vessels and so keep the active muscle properly supplied with blood. With increasing inflow of blood into the heart the large veins, auricles, and ventricles naturally become more filled during diastole, and during systole of the ventricles, when the auriculo- ventricular valves are closed, the blood rushing in from the venous system must accumulate in the big veins and auricles to a still greater extent. The venous pressure therefore rises with increased venous inflow. In so far then as venous pressm'e is an index of venous inflow, we may say that the output of the heart increases with, the venous pressure so long as the heart is functionally capable of deahng with the blood it receives during diastole. But although the ventricular output is practically independent of the frequency of the heart- beat and a constant venous inflow, the venous pressure tends to fall as the heart-beat increases in rate. The optimum venous pressm'e is that which fills the ventricle during its diastole to the maximum extent to which it is able to respond. As the rate of the heart increases the inflow of blood can also be increased without causing over distension of the ventricles. The increase of heart rate therefore is an important factor in enabling this organ to deal with the maximum amomit of blood. Although increase of rate does not alter the output with constant venous inflow, it does increase the maximum amount of infloAving blood which the heart is able to expel. We thus see that alterations iu the vigour of the circulation depend in the first instance on the venous circulation. The greater the volume of the blood that is brought up. to the heart by the accessory factors of the cir- culation, the greater ^^•ill be the output of this organ. The changes in rate and force of the heart which accompany its increased activity and increased output, e.g. during exercise, represent merely the means by which this organ is able to deal in the most advantageous manner with the increased inflow. THE WORK OF THE HEART The energy of the ventricular contraction is expended in two ways : first, in forcing a certain amount of blood into the already distended aorta against the resistance presented by the arterial blood-pressure, which itself is directly conditioned by the resistance in arterioles and capillaries ; and secondly, in imparting a certain velocity to the mass of blood so thrown out. Thus the energy of the muscular contraction is converted partly into potential energy in the form of increased distension of the arterial wall and partly into the kinetic energy represented by the momentum of the moving column of blood. The work done at each beat may be calculated from the formula : ^j., W = QR + -j- -!7 916 PHYSIOLOGY where W stands for work, w for the weight, and Q for the quantity (volume in c.c.) of blood expelled at each contraction ; E is the average arterial resistance or pressure during the outflow of blood from the heart, and V is the velocity of the blood at the root of the aorta. In this equation QR. is the work done in overcoming the resistance,* and — — is the energy expended in imparting a certain velocity to the blood. If we take 60 c.c, as the average output of each ventricle, 100 mm. Hg. as the average pressure at the beginning of the aorta, and 500 mm. per second as the velocity imparted to the blood thrown into the aorta, we can calculate the work done by the human heart at each beat. QR = 60 X 0-100 m. X 13- 6 = 81 • 6 grammetres, or roughly 80 grammetres. On the other hand, the expression wY-^ 60 X (0-5)2 2g 2 X 9-8 = 0*7 grammetres. It is evident that this latter factor is neghgible, and that for all practical purposes we may regard the work of the heart as proportional to the output multiphed by the average arterial blood-pressure. Taking the average pressure in the pulmonary artery at 20 mm. Hg., the work of the right ventricle at each beat would amoimt to about 16 grammetres, a total for the two ventricles of about 100 grammetres per beat, which is equivalent to about 10,000 kilogram metres in twenty- four hours for a man at rest. This work is done by a contraction of the muscle fibres surrounding the cavities of the ventricles. It is important to remember that the strain or tension which is thrown on these fibres and which resists their contraction will simply be determined not by the blood-pressure which has to be overcome, but also by the size of the ventricle cavities. Since the pressure in a fluid acts in all directions, the tension caused by any given pressure on the walls of a hollow vessel will increase with the diameter of the vessel. Thus if we take a sphere with a radius of 10 cm. filled with fluid at a pressure of 10 cm. Hg. there will be a pressure on each square centimetre of the inner surface of the sphere of 136 grm. The total distending force, i.e. the pressure on the whole of the inner wall of the sphere, will be equal to this pressure multiplied by the area, i.e. to 136 X 477^2 = 136 X 47r X 100. If by a contraction of the walls the radius be reduced to 5 cm., the total pressure on the internal surface will be reduced to 136 X 47r X 25, i.e. will be one quarter of the previous amount. Moreover in the case of the heart, with increasing distension the wall becomes thinner and the number of muscle fibres in a given area fewer, so that the larger the heart the more strongly will each fibre have to contract in order to produce a given tension in the contained fluid. At the beginning of systole the distended heart must therefore contract more strongly than at the end of the systole, in order to raise the blood it contains to a pressure sufficient to overcome that in the aorta. * This expression, QR, is only approximately correct. Supposing the pressvire in the aorta at the beginning of systole is 50 mm. Hg. and at the end of systole 150 mm., the work could not be deduced accurately from the average pressure, but would need a simple application of the integral calculus for its determination. The expression employed above deviates from the real value only by about 10 per cent., and is therefore sufficiently accurate for our purpose. THE MECHANISM OF THE HEAET PUMP 917 It is evident that an unrestricted diastolic filling of the heart is not of unqualified advantage to this organ. If during diastole the heart be too forcibly distended, as may easily occur after opening the pericardium, or in cases of enfeeblement of the heart's action by chloroform poisoning or otherwise, the muscle fibres of the heart may be quite unable to contract against the distending force represented by a pressure in the heart equal to that in the aorta. Under such conditions we may have sudden heart failure, which can only be reUeved by diminishing the diastolic distension, as, e.g., by letting blood from the veins opening into the heart. SECTION V THE FLOW OF BLOOD THROUGH THE ARTERIES THE PULSE. Owing to the elasticity and distensibility of the arterial wall the rhythmic rise of pressure corresponding to each heart-beat causes an expansion, which can be felt by the finger placed on any exposed artery, such as the radial, and is spoken of as the pulse. Just as the blood-pressure diminishes from heart to periphery, so the amplitude of the pulse decreases as we go farther away from the heart. If the arterial system were perfectly rigid the increased pressure due to the forcing of the blood into the arterial system at each ventricular systole would occur practically simultaneously at every point. The arteries are, however, elastic and distensible, so that the first effect of the flow of blood into the aorta is to distend the section of the aorta nearest to the heart. The elastic reaction of this forces a portion of the blood into the nearest section, so that the increased pressure is transmitted from segment to segment of the arteries in the form of a wave at the velocity of about seven metres per second. It is important not to confuse the velocity of the pulse- wave with that of the blood-flow ; the latter is never greater than 0-5 metre per second, and is very much less than this in the smaller arteries. Perhaps the differ- ence between the two quantities may be made clearer by illustration : If the hindmost of a row of billiard-balls be struck sharply with a cue the foremost ball flies off and the others stop still ; in this case the energy imparted to the first ball by the stroke has been transmitted from ball to ball, just as the effect of the ventricular contraction is transmitted from section to section of the arterial blood- stream. If the balls are struck so that the cue continues pressing on the hindmost after the stroke is delivered, the front ball flies off, while the others move slowly along in the direction of the stroke. In the arteries this continuous pressure is furnished by the elastic reaction of the arterial wall, and we see how the impact of the blood may travel quickly as a wave of increased pressure while the blood itself is moving slowly along, impelled by the reaction of the arterial wall. If we imagine a rigid tube ab (Fig. 408) provided with a piston at the end A, and filled with an incompressible fluid, an inward movement of the piston at a will cause a simultaneous outflow of fluid at the end b. If the end B is closed the piston at A cannot be moved at all. Pressure applied to the piston will raise the pressure simultaneously at all points in the 918 FLOW OF BLOOD THROUGH THE ARTERIES 919 tube AB. The increased pressure applied at a is therefore transmitted with practically no loss of time to all parts of the tube ab. This immediate spread of the wave of pressure only applies to an incompressible fluid within a rigid tube. If the fluid were compressible, if it consisted, €.(j. of air, a sudden movement inwards of the piston at a would not be felt imme- diately at B. The propagation of the wave of pressure from a to b would take a finite period of time, its velocity being identical with that of the velocity of propagation of a wave of sound in air, i.e. 1100 feet per second. B Fig. 408. The same retarding effect will be produced if we have an incompressible fluid within a tube whose wall is distensible and elastic. If we imagine (Fig. 409) an elastic tube bc filled and distended with water and connected at B to a rigid tube, which is provided with a piston, the first effect of a rapid movement of fluid driven in by the piston ^^dll be a rise of pressure at the point immediately in front of the piston, viz. at a. The wall being distensible, and pressure being propagated along the fluid in every direction, the rise of pressure at a will be spent partly on the particles of fluid in Fig. 409. front of it, viz. at h, but also on the walls of the tube, so that this is stretched and the cross-section of the tube enlarged. The distended segment at a will then exert a pressure on the contained fluid, driving this backwards and forwards. The fluid on its side towards the piston ^^^ll tend to come to a stop, while that towards the distal end of the tube will be accelerated. The distended wall therefore returns to its original diameter, and the next segment at h is stretched in its turn, so that a wave of increased pressure is propagated along the tube in the direction of the arrow. The velocity ^\^th which this wave is propagated depends on the density of the fluid, i.e. its inertia, and on the resistance of the walls of the tube to distension, since this will determine the rapidity of its recovery. The velocity of propagation of the wave of increased pressure, or the wave of expansion of the artery, is expressed by the following formula : v = k\ gea 920 PHYSIOLOGY where v is the velocity per second, g, the acceleration due to gravity, e, the elastic coefficient of the wall, a, the thickness of the wall, d, the diameter of the tube, D, the density of the fluid, k, a constant. 5ovWAAAAnAiA/V/\/\/\A/\ A Fig. 410. Pulse-curves described by a series of sphygmographic levers placed at intervals of 20 cm. from each other along an elastic tube, into which fluid is forced by the sudden stroke of a pump. The pulse- valve is travelling from left to right, as indicated by the arrows over the primary (a) and secondary (6, c) pulse-waves. The dotted vertical lines drawn from the summit of the several primary waves to the tuning-fork curve below, each complete vibration of which occupies tjV sec, allow the time to be measured which is taken up by the wave in passing along 20 cm. of the tubing. The waves (a') are waves reflected from the closed distal end of the tubing ; this is indicated by the direction of the arrows. It will be observed that in the more distant lever (vi) the reflected wave, having but a slight distance to travel, becomes fused with the primary wave, so that the rise of pressure in VI is actually greater than that in V. (From Foster, after Marey.) If the end c of the tube is closed, the wave of a positive pressure on arriving at B will be reflected back as a positive reflected wave. If a Fig. 411. FLOW OF BLOOD THROUGH THE ARTERIES 921 tracing be taken of the oscillations or variations of pressure in the tube, two waves at least are seen, one of which is the primary wave due to the movement of fluid caused by the piston ; the other is the secondary wave reflected back from the periphery. The fact that the secondary wave is a reflected one is shown by the fact that the nearer to the peripheral resist- ance the pulse is recorded, the nearer is the secondary to the primary wave, as is seen in Fig. 410. If the tube bc be widely opened a reflected wave is also observed, but this time of reversed sign, i.e. the wave is one of negative pressure. The production of this wave is dependent on the momentum of the moving column of fluid. If in the tube ab, with a tap at c and a manometer m (Fig. 411), the current of fluid be suddenly checked by turn- ing the tap c, the column in front of the tap, having a certain momentum, will tend ^ to go on moving and therefore produce a suction or negative pressure behind it. When a wave of positive pressure arrives at the open end of a tube there is a sudden increase in the velocity of output, and the momentum of the mass of fluid which is thrown out causes a similar suction or negative pressure, which travels back the whole length of the tube. If the end of the tube is only partially closed, every primary positive wave will be transformed into a reflected one which is partly positive and partly negative. Since both these reflected waves travel through the tube with the same velocity and will mutually interfere, the result may be either a positive or a negative wave or nothing at all, according to the degree of constriction. In a branching system of tubes, such as the arterial system, reflection of waves must take place at every dividing place. All the conditions for the origin of reflected waves and interference of such waves are present in the arterial system. It is impossible a 'priori, however, to say whether any reflected wave will form a marked feature on the pulse-tracing. It is possible that the multitudinous reflections which must occur in every part of the arterial system may interfere with one another to such an extent that they mutually annul each other. The origin of any secondary wave in the pulse-tracing must therefore be determined by experiment. To study the pulse more fully it is necessary to obtain a graphic record of the expansion of the arteries, or, what comes to the same thing, of the exact changes in pressure which produce this expansion. The curve obtained with the mercurial manometer shows elevations corresponding to the pulse ; but the instrument is far too sluggish to record the finer variations of pressure. For this purpose a manometer which has very little inertia, such as Hiirthle's or Piper's, must be used. The expansion 922 PHYSIOLOGY of the artery is registered by means of a lever, which may be made to rest more or less heavily upon the artery, and the movements of which are recorded on a blackened surface. Such an instrument is called a sphygmo- graph. Of the many forms of sphymographs, Marey's or Dudgeon's is perhaps the most convenient for chnical purposes. The principle of Marey's sphygmograph is shown in Fig. 412. The button b is adjusted so as to press on the radial artery. Its movements are transmitted to a lever m. The screw on this works on a small cogged wheel at o, which is also the axis of the Fig. 412. writing lever I. The movements of the button b thus transmitted to a point near the axis of I are reproduced by this lever highly magnified, and as such are recorded on a blackened surface. The pressure on the artery can be adjusted by means of a screw. Dudgeon's sphygmograph (Fig. 413) is rather easier to use than Marey's, and is therefore largely employed for clinical purposes. It is provided with a dial by which the pressure on the artery can be graduated, and has a small clockwork arrangement for moving along the slip of smoked paper on which the records are taken. The Fig. 413. Dudgeon's sphygmograph, showing its mode of application to the radial artery. arrangement of the levers in this form of sphygmograph is shown in Fig. 414, where F is the (adjustable) spring bearing by its button p on the artery. The up-and-down movements of p are transmitted to s, being much magnified and converted into side- to-side movements. The point of s rests on the blackened surface represented in section at A, and scratches on this, when moving, a magnified record of the expansion of the artery under the knob p. In all these sphygmographs, even the most perfect, the moving parts have a con- siderable amount of inertia, so that the curve they give is always more or less deformed. This fact must be borne in mind when comparing the pulse-curves obtained by means of a sphygmograph with those given by the more perfect forms of manometer, such as Frank's or Piper's. FLOW OF BLOOD THROUGH THE ARTERIES 923 Either form of sphygmograph is generally applied to the radial artery, since this is near the surface and is supported by bone, and the arm is well adapted for the application of the sphymograph. The pulse-curve obtained by means of a sphymograph varies according to the artery employed and the force with which the lever presses on the artery, but all the curves present the same general features. The velocity of the pulse can be measured by taking simultaneous tracings from two arteries separated by some distance from one another, such, as the femoral artery and the dorsalis 'pedis, or from the carotid and radial arteries. In a healthy individual the velocity varies between 7 and 10 metres per second. The more rigid the arteries the greater will be the velocity, so that the velocity of propagation gradually increases wth advancing age, and is higher in the arteries of the lower extremities than in the more distensible arteries of the arm. The length of the pulse-wave can be found by multiplying the velocity of transmission by the time occupied by the wave in passing any given point. The duration of the wave at any point corresponds to the time of a cardiac cycle, viz. 0-8 sec, so that if the velocity of transmission be taken as 7 metres per second, the length of the wave is about 5-6 metres. The pulse-wave thus reaches the periphery long before it has been com- pleted in the aorta. Fig. 415 represents a pulse-curve taken from the radial artery. The elevation due to the expansion of the artery is rapid and uninterrupted. We have alreadv explained that this is due to the sudden pumping of blood into the first part of the aorta, whence the impulse is transmitted as a wave along the arteries. The curve descends gradually till the next beat occurs, since the elastic reaction of the arteries, which tends to keep up the pressure, acts more constantly and steadily than the heart-beat. On this descending part of the curve occur two or three secondary elevations : h is the primary or ' percussion ' wave, c the pre- dicrotic or ' tidal ' wave, and e the dicrotic wave. Elevations may occur on the curve after e which are called post-dicrotic waves. It is better to class the elevations before the dicrotic notch d as systolic elevations, and those afterwards, including the dicrotic elevation itself, as diastoUc. For the exact understanding of these elevations it is necessary to compare the pulse tracings taken from a small arteiy with the variations in pressure which occur at the same time in the aorta and in the left Fig. 414. Diagram of arrangement of re- cording lever in Dudgeon's sphygmo- graph. Fig. 415. Pulse-curve from radial artery. 924 ventricle (Fig. 416) PHYSIOLOGY We are enabled in this way to dissociate the waves caused by the ventricular systole from those which have their origin in the arterial system. In Fig. 416 is given somewhat diagrammatically typical tracings of the intra-auricular, intraventricular, and aortic pressures during one heart-beat, and in dotted hues is represented approximately the sort of a curve which would be given by a sphymograph applied to the aorta, taking into account the greater inertia of the latter instrument. The auricular systole begins at the ordinate 1. It gives a shght rise of Aorta I/entrlcle Auricle 5 4 5 Fig. 416. pressure in the ventricle, but as a rule is not transmitted to the aorta, though often some small traces of it can be seen. As the auricular con- traction is dying away, the ventricular contraction begins at 2. The first effect of this rise of pressure is to close the auriculo-ventricular valves, as is shown by the elevation at 3 in the auricular curve, and the shock of the closure is occasionally transmitted to the aorta. The pressure in the ventricles then rapidly rises. At the point 4 it surpasses the pressure in the aorta and then rapidly rises above it. Since the aortic valves offer no resistance to the flow of blood from ventricles to aorta, they must open as soon as the intraventricular exceeds the aortic pressure, and this is shown by the rise of pressure in the aorta at 5. The shock of the inrush of blood may give rise to a distinct secondary wave at this point. The pressure then continues for a time to rise rapidly both in the ventricle and in the aorta, blood flowing from the heart into the arterial system. As the first rush of blood diminishes and as the blood begins to escape more rapidly under the influence of the rise of pressure from the peripheral end of the arterial system, the rise of pressure in the ventricle and aorta slows off. and the junction between these two periods at 5, where the rise of FLOW OF BLOOD THROUGH THE ARTERIES 925 pressure becomes suddenly slower, may be marked in the aortic curve by one or two secondary waves. It must be remembered however that all these secondary waves shown on the aorta at 4 and 5 may be absent, the one at 5 being the one which is most frequently seen. From 6 to 7 the ventricle is still contracting and forcing blood into the aorta. The curve of pressure is generally rounded. It may present a flat top, the plateau, or the top may be rounded with an inchnation to fall or to rise (cf. Fig. 398). At 6 the ventricle relaxes, the intraventricular pressure falls rapidly, and at 7 falls below the aortic pressure. The aortic valves must now close since the pressure is greater on their aortic side. The pressure in the ventricle now continues to fall until it becomes zero. In the aorta however there is a sharp elevation immediately after 7, i.e. immediately after the closure of the aortic valves. This is known as the dicrotic elevation, the previous depression being the dicrotic notch. It is at this point that the second sound of the heart is heard and is evidently due to the vibrations which are represented graphically in the record of intra-aortic pressure. There are several factors at work tending to produce a secondary wave at this point. With the sudden cessation of the inflow of blood from the ventricles at the end of the ventricular contraction a negative wave must be produced at the beginning of the aorta, which, transmitted along the arterial system, will tend to produce a reflux of blood towards the heart. The movement so caused is reinforced by the elastic reaction of the arterial wall so that the returning blood is driven up against the aortic valves, closing them tightly and putting them on the stretch. Even in a rigid tube the sudden cessation of flow causes a negative wave, followed by a positive wave in the opposite direction in the aorta ; this positive wave is increased by the elastic reaction of the stretched aortic valves. The blood is driven up against them by the wave of positive pressure and then rebounds, hke a bilhard ball from the elastic cushion, and gives rise to the dicrotic elevation. That the dicrotic elevation is for the most part a positive wave running in a centrifugal direction is shown by the fact that the distance between it and the primary wave does not alter appreciably from whatever part of the arterial system the tracing be taken. If it were a reflected wave the distance between it and the primary elevation of the pulse-curve ought to be less the nearer the periphery the pulse tracing is taken. The predicrotic waves in the pulse tracing are evidently due to the instrimiental exaggeration of the wave which may occasionally be seen even in a perfect pressure tracing at 5. The rapid rise of pressure in the aorta or in the more peripheral artery, which follows the opening of the aortic valves sets up a tendency to secondary oscillations at this point. The greater the inertia of the instrument, the greater is the exaggeration of these waves. As is shown by the dotted Une in Fig. 416, the lever of the sphygmograph is jerked up, so that it practically leaves the artery, and then falls and rebounds again, so that the simple rounded top becomes resolved by instrumental error into a curve with two waves, which have 926 PHYSIOLOGY been called the percussion wave and the predicrotic wave. In the same way the inertia of the instrument will tend to exaggerate the dicrotic elevation and possibly to give rise to shght postdicrotic waves. The general form of the pulse-curve varies with changes in the heart, in the arteries, and in the peripheral resistance. Thus some curves may present secondary elevations on the ascending part, and are called anacrotic, w^hile in others all secondary elevations occur on the descending part. The latter type is called catacrotic, and is the tracing usually obtained from a normal radial artery. By comparing these two types of curves with the corresponding intraventricular pressures, we find that in both cases blood is flowing into the aorta during the whole time from the beginning of the primary elevation to the notch just before the dicrotic elevation. This is shown by the fact that the intraventricular pressure is all this time sHghtly higher than the aortic pressure. So long as this is the case blood must flow from ventricle into aorta. (This fact proves that there is normally no part of the cardiac cycle during which the ventricle remains contracted and empty, the ventricle in all cases relaxing before it has completely emptied itself of blood.) Now it is easy to see the conditions which determine whether the systolic plateau shall be ascending or descending, and therefore when the pulse shall be anacrotic or catacrotic. If, after the first sudden rise of pressure in the aorta, the blood can escape more rapidly through the peripheral resistance than it is thrown into the beginning of the aorta, the ' systoHc plateau' will sink, and a catacrotic pulse tracing is obtained. If, on the other hand, the peripheral resistance is high, or an extra large amount of blood be thrown into the aorta at each stroke of the heart {e.g. by prolonga- tion of the diastole), the aortic pressure will rise so long as blood is flowing in, and we get an ascending systohc plateau and an anacrotic pulse. Thus we obtain an anacrotic pulse in old people with Bright's disease, in whom the peripheral resistance is very high, and also in animals when the heart is slowed by vagus action. The production of the dicrotic elevation is favoured by any influence which increases the elastic resihency of the arteries or causes the primary elevation of the pulse to be rapid and sharp. Thus it is much more pro- nounced in young people than in old people, whose arteries have become rigid. When the peripheral resistance is low through relaxation of the 9,rterioles, and the heart is beating forcibly, as in many cases of fever and also to some extent after a good meal with alcohol, the dicrotic elevation becomes very marked. Under such circumstances it may be easily felt with the finger at the wrist, and in many cases the mistake has been com- mitted of taking the dicrotic wave for a normal beat, and so doubhng the rate of the pulse. OTTO FRANK'S WORK ON THE PULSE. In the account given above of the peculiarities of the pulse-curve in different parts of the system I have adopted in the main the views of Marey and Hiirthle, which have been generally accepted for a con- siderable time and have influenced most of the clinical work on this subject. According FLOW OF BLOOD THROUGH THE ARTERIES 927 to these authors all the secondary waves on the pulse-curve are central in origin, and can therefore be traced with slight modification on the curves obtained from the aorta, the large and the small arteries. Although Marey investigated the propagation of reflected waves and showed their presence in the artificial schema of the circulation {vide p. 920), he considered that they could not contribute to the production of the waves on the pulse-curve, owing to the enormous number of points at which reflection might occur, so that the different reflected waves would tend mutually to annul each other's effects. Moreover the distance of the dicrotic notch from the primary elevation was found to be nearly the same at different parts of the system, pointing to a propaga- tion of this wave from the centre to the circumference of the arterial system. Frank points out that the effect of the propagation of the fairly simple wave started Fig. 417, Pulse-pressure curves taken by means of Frank's manometer (Frank), A, B, c, aortic pressure curves at different rates of the heart ; d and e, aortic pressure curve, D, compared with simultaneous record of the pressure in the femoral artery e. in the aorta in an endless system of elastic tubes would be to diminisli the rapidity of onset of each vibration, and therefore to diminish the secondary vibrations on the curve. In a closed elastic system of tubes, such as the arterial sj^stem, there will be factors at work analogous in many respects to those responsible for the deformation of the curve given by an imperfect manonieter. These will be of two kinds, namely, (1) oscillations of the colunui of fluid within the stretched arterial wall, (2) reflections of waves from different points in the periphery. These reflections we should expect , to bo evident in certain cases. Thus in the carotid there should be a reflected wave from the circle of Willis ; in the descending aorta, from the bifurcation of tliis vessel into the two iliac arteries. As a result the pulse in the peripheral arteries, such as the radial or femoral, diverges considerably from the pulse in the aorta. In the figure (Fig, 417, D and e) the primarj'rise of pressure in the femoral artery is even higher than the primary rise in the aorta, i.e. the primary wave must be augmented here by a reflected wave from the peripherj-. Frank acknowledges that the dicrotic depression in tliis curve is due to the propagation of the wave set up by the closure of the aortic valves ; 928 PHYSIOLOGY but he would regard the more pronounced dicrotism of the pulse, of which examples have been given earlier, as due for the most part to the reflection of waves from the periphery. From time immemorial the physician has sought by feehng the pulse to come to some idea as to the condition of the circulation. A number of different quahties have therefore been distinguished. According to the number of beats per minute the pulse is distinguished as frequent or rare. The size of the pulse has reference to the amplitude of excursions of each beat and the pulse is distinguished as large or small. The velocity of the pulse expresses the speed with which the excursion is accomphshed. The quick pulse is one in which the artery presses against the finger suddenly and then disappears suddenly, while in the slow pulse the period during which pressure can be felt is more prolonged. The hardness of the pulse is determined chiefly by the blood-pressure. If the pulse is compressible it is spoken of as soft ; if it can only be obhterated with difficulty it is hard. Certain combinations of these quahties are also described. Thus a large and hard pulse is spoken of as strong, a weah pulse being both small and soft. A small hard pulse is called contracted. If the rhythm of the heart-beat is irregular the pulse is also irregular. An intermittent pulse is one in which one heart-beat is dropped occasionally, i.e. once in every four or eight beats, and may be due to the interposition of a ventricular contraction which is too weak to send the pulse along so far as the radial artery. Judgments as to the conditions of the heart and circulation from the feeling of the pulse oscillations must, however, be made with extreme caution. The pulse-curve may, indeed, give approximate information as to the condition of things in the heart. Thus the period between the beginning of the primary elevation and the dicrotic notch corresponds to the outflow of blood from ventricle to aorta. A large pulse-curve does not necessarily indicate a big output, since the expansion of the artery is determined not only by events occurring in the aorta but also by the tonus of the artery under the finger and the resistance in the peripheral branches. Perhaps the best-marked condition of the pulse is that known as the ' water-hammer ' pulse, which is observed in cases where the aortic valves are injured or diseased so as to allow of regurgitation into the ventricle. The systoHc rise of pressure in the arterial system is followed by an extremely rapid fall, so that towards the end of diastole the pressure in the arteries may be insufficient to keep the arterial system filled. Under such con- ditions, if the arm be held above the head and the wrist of the patient be grasped, the pulse in the arteries of the wrist is felt as a smart blow coinciding with each beat of the heart. THE CIRCULATION THROUGH THE CAPILLARIES The capillary circulation is most easily studied by examining under the microscope the tongue of the frog or the web of the frog's foot. Under a power of about 150 to 180 diameters a network of vessels is seen, FLOW OF BLOOD THROUGH THE ARTERIES 929 consisting of small arteries, capillaries, and veins. The direction of flow in the arteries is opposite to that in the veins. In the capillaries the flow is from arteries to veins, though, on account of the reticular arrangement of these vessels, the direction of the stream through them is not quite constant and may occasionally be reversed. The flow of blood in the arteries is rapid, whereas in the veins it is generally possible to distinguish the individual blood-corpuscles. Through the capillaries the flow is very inconstant. If a group of capillaries be watched for some time the blood may at first hurry through a number of them with great rapidity ; the flow then becomes slower and then may quicken up to a moderate pace again. These variations in the capillary flow are probably associated with spontaneous alterations in the condition of contraction of the small arteries supplpng the group of capillaries. It is easy to observe that the arterial flow is pulsatile, the pulsation disappearing in the capillaries and veins. Another difierence between the circulation in these three kinds of vessels is to be found in the condition of the peripheral zone. In the arteries the blood-stream is divided into two parts, the peripheral stream — about -01 mm. wide, consisting only of colourless plasma with occasionally a stray leucocyte — and an axial stream, in which all the red blood- corpuscles are being hurried along. In the veins there is a similar peripheral plas- matic zone, but here we find regularly scattered leucocytes which travel rather more slowly than the axial stream of red corpuscles. The formation of this axial zone is purely mechanical, and may be imitated in any fluid containing in suspension particles whose specific gravity is somewhat higher than that of the fluid. In the capillaries there is no separation of the two zones, since the lumen of these vessels as a rule allows only the passage of one or two corpuscles abreast, so that they are everywhere in contact with the wall. The corpuscles are e\adently elastic structures, and may be seen to bend if they impinge on the dividing point of two capillaries before they are finally swept off by the stream into one or the other. The capillary wall is composed of a single layer of elongated flattened cells which present little resistance to the passage through them by diffusion of dissolved substances, such as sugar, salts, oxygen, or carbon dioxide. In this way the tissue-cells obtain oxygen from the red blood-corpuscles and nutriment from the plasma, and give off to the circulating blood carbon dioxide and other effete substances as the products of their metabohsm. There is evidence that in some situations the cells forming the capillary wall may be contractile. According to Strieker and others, the cell substance is arranged in strands running from the nuclei around the capillary. By the contraction of these strands the vessel may be narrowed to obliteration. These phenomena have been observed in the nictitating membrane of the frog, but it is doubtful how far they may be extended to the other capillary systems of the body. If the contractile power is at all universally present, it must play an important part in determining the amount of blood-flow through the capillaries of an organ, and will doubtless be largely affected 30 930 PHYSIOLOGY by chemical substances produced as the result of the metabolism of the surrounding tissues. The average length of a capillary is between 0-4 and 0-7 mm. The velocity of blood-flow can be directly determined by observing under the microscope the time taken by any given corpuscle to travel a measured distance on the microscope stage. The mean velo- city determined in this way varies from about 0-5 to 0-8 mm. per second. The blood-pressure in the capillaries may be measured approximately by applying pressure to the outer surface of the skin or mucous membrane and noticing the point at which blanching of the surface is produced. Fig. 418. Apparatus of von Kries for measuring capillary blood-pressure. In von Kries' method a small glass plate, from 2 to 5 sq. mm. in area, is placed on the last joint of the finger. Attached to this glass plate is a small scale pan on which weights are placed until the pressure is just sufficient to blanch the underlying skin. In using this method the cal- culation of the capillary pressure is made as follows : Supposing that the size of the glass plate is 4 sq. mm. and 1 grm. in the scale pan is just sufficient to cause a change of colour in the skin, then a weight of 1 grm. -= 1 c.c. HgO = 1000 c.mm. HgO is present on an area of 4 sq. mm. The height of the column of water supported by . , . 1000 1 sq. mm. is therefore = 250 mm. H,0. The errors of this method are consider- 4 - able, since the pressure thus determined is not the total capillary pressure, but this minus the pressure in the tissue spaces on the outer side of the capillary wall. The result will therefore vary not only with capillary pressure but also with the tension of the skin and the amoimt of fluid in the tissue spaces. The pressure in the capillaries as found by this method necessarily varies with the position of the part under investigation, i.e. with the hydrostatic pressure of the column of blood between it and the heart. The following figures were found by von Kxies : Finger : Mm. HgO Distance of finger below head 328 . . 0 mm. 329 . . 205 mm. 513 . . 490 mm. 738 . . 840 mm. Ear : 20 mm. Hg. Gums of Rabbits : 33 mm. Hg. Frog's Web (Roy) : 100-150 mm. HjO. Capillary venous pressure of brain (Hill) : (1) Animal in horizontal position : 10 mm. Hg. (2) ,, ,, feet-down position : zero or less. (3) During strychnine convulsions : 50 mm. Hg. Owing to the fact that a varying and unknown resistance — that of the arterioles — Ues between the capillaries and the arteries, the pressure in the capillaries must stand in much closer relationship to that in the veins than to that in the arteries. One cannot therefore argue that a fall of FLOW OF BLOOD THROUGH THE ARTERIES 931 arterial pressure necessarily involves a fall of capillary pressure in all parts of the body. We can only judge of changes in the capillary pressure by taking simultaneously the pressures in both the afferent and efferent vessels. If these both rise or fall together we may be certain that the capillary pressure also rises or falls. Where the arterial and venous pressures move in opposite directions it is difficult to say what alterations, if any, will be produced in the capillary pressure. The resistance to the flow of blood through the capillaries is determined by the internal friction, i.e. the viscosity of the blood ; this varies in different animals between three and five times that of water. It has been calculated that the fall of pressure undergone by the blood in passing through any given capillary area is only about 20 to 60 mm. of blood, and at the most is never more than 150 mm. blood, i.e. about 10 mm. Hg. This bears out the conclusion to which we have already come, viz. that the chief seat of the resistance in the vascular system is the arterioles, and it is in this region that the chief fall of pressure occurs. SECTION VI THE FLOW OF BLOOD IN THE VEINS In the veins there is a constant decrement of pressure as we pass from the periphery towards the heart. This decrement of pressure is the conse- quence of the pumping action of the heart, so that the flow through the veins must be ascribed to the same force as that which determines the flow through the arteries, viz. the heart-beat. Owing to the fact that no appreciable resistance lies between the veins and the heart, the difference of pressure necessary to maintain a constant flow through these vessels is very small. Thus in the horizontal position the pressure in the femoral veins may be from 5 to 10 mm. Hg., and in the inferior vena cava from 1 to 5 mm. The pressure in the great veins near the heart is generally negative owing to the aspiration of the thorax, and this negative pressure is naturally increased during inspiration. Opening the thorax therefore causes a rise of pressure in all the large veins. In the latter the pressure depends chiefly on the heart activity, being lowered by vigorous action of the heart pump and raised when this fails in any way. In the peri- pheral veins the pressure is more dependent on the flow through the corresponding arteries. If an artery of a limb be Hgatured the pressure in the small veins of the limb sinks until it is reduced to the pressure in the nearest large trunk in which a flow of blood continues. Each cardiac cycle causes variations in the pressure in the great veins next the heart in two ways : (1) By the transmission along the veins of the alterations in the intra- auricular pressure. (2) By the diminution in the volume of the heart in consequence of the expulsion of its blood along the arteries with each heart-beat. On this account the jugular veins show pulsations with each heart-beat which are somewhat complex in character and resemble closely those occurring in the auricle {vide p. 901). In Fig. 419 a tracing from the wall of the jugular vein is given. It will be seen that each heart-beat gives rise to three variations in pressure within the veins. These three undulations are evidently exactly analogous to those given in Fig. 400 as occurring in the auricular tracing. We should therefore regard a as the auricular contraction, c as the elevation due to the closure of the auriculo- ventricular valves, v as the elevation due to the accumulation of blood in the auricles during the ventricular systole. The curve c is often spoken 932 THE FLOW OF BLOOD IN THE VEINS 933 of as the carotid elevation, and has been ascribed by Mackenzie to direct propagation to the jugular vein from the underlying carotid artery. He has come to this conclusion because he has not found it in tracings of the liver pulse in cases of incompetent tricuspid valves. There is no doubt, however, that the elevation can be seen on tracings from the inferior vena cava. The explanation of its absence from hver tracings is probably to be ascribed to the fact that the great mass of the Hver substance is miable to transmit the very rapid oscillation of pressure due to the closure of the auriculo-ventricular valves. These venous pulsations are much more ac V Ju^.V. Raul. art. V' Fig. 419. Venous pulse-tracing from jugular vein comparod with the arterial pvilse-tracing from the radial arterj'. marked in cases of heart disease, where there is partial failure of the heart pump and overfilKng of the venous system, often combined with incom- petency of the auriculo-ventricular valves. Besides the favourable influences exercised on the circulation through the veins by the aspiration of the thorax and the momentary aspiration of the heart-beat itself, a considerable part is played in the venous circula- tion by the contraction of the muscles of the body as well as by the passive movements of different parts. The adjuvant effect of passive or active movement on the circulation through the veins is rendered possible by the existence in these vessels of valves, which are semilunar folds of the intima projecting into their lumen, and so arranged that they allow the passage of blood only towards the heart. Two such valves are as a rule situated opposite to each other. Every movement of a hmb, active or passive, causes an external pressure on the veins, and therefore empties them towards the heart. Thus, in walking, each time the thigh is moved back- wards the femoral vein becomes empty and collapses, and fills again as soon as the leg is brought forward to its former position or is flexed in front of the body. When muscular movements become general, as in walking or rumiing, the active compression of the veins thus brought about plays an important part in hurrying the blood into the right heart, so that the output of this organ is increased and the arterial blood-pressure is raised. Since the blood in the vessels is subject to the influence of gravity, we should expect to And that the pressure in the veins of the foot was equal to the pressure in the veins, say, of the hand at the level of the heart 'plus the pressure equivalent to the column of blood between these veins and 934 PHYSIOLOGY the heart, i.e. about a metre of blood. On measuring the pressure by von Reckhnghausen's or by Hill's method in these veins, this is not found to be the case. The pressure, indeed, in the veins of the foot is but little higher than that in the veins of the hand. Von Recklinghausen found that after subtracting the distance between the foot and the heart the pressure in the veins was negative by as much as 40 cm. In the same way, as Hill has shown, the pressure in the capillaries of the foot is about the same as in the capillaries of the hand. When a man assumes the upright position the arteries of the leg and foot contract until, under the combined influence of the heart's contraction and gravity, the blood- supply to the capillaries is only sufficient to keep the pressure in these vessels at a certain moderate height. The return of the blood from the dependent parts cannot be ascribed to the heart-beat at all, but is due to the extrinsic mechanism of circulation through the veins, i.e. the contrac- tions of the muscles of the Umb which press all the deep and superficial veins, and in virtue of the valves force the blood contained therein by Poupart's ligament into the abdomen. The fact that circulation through the legs is dependent on the contractions of their muscles explains why it is so difficult to stand still for any length of time without moving, and emphasises the need of moderate exercise for the maintenance of a normal circulation. SECTION VII THE PULMONARY CIRCULATION In the lungs there is an extensive system of wide capillaries presenting very little resistance to the flow of blood. The arterioles are wide and have only a slight amount of muscular fibre in their walls, so that a slight pressure suffices to drive the blood from the right to the left heart. The determina- tion of the normal average pressure in the pulmonary artery presents con- siderable difficulties, but it probably does not exceed 15 to 20 mm. Hg., i.e. about one-sixth of the mean aortic pressure. The capillaries of the lungs may vary passively in size according to the condition under which they may be placed. Thus, whereas at the height of inspiration the blood contained in the lungs is about one-twelfth of the whole blood in the body, this amount is diminished during expiration to between one-fifteenth and one-eighteenth, and by forcible artificial inflation of the lungs may be lessened to one-sixtieth. These changes exercise a considerable effect on the systemic blood-pressure and are largely responsible for the respiratory variations observed in the systemic blood-pressure. On the other hand, the distensibihty of the lung capillaries may play an impor- tant part in enabling the lungs to act, so to speak, as a reservoir for the left side of the heart. If, in consequence of raised arterial pressure or other factor, there is a temporary excess of output on the right side that cannot be dealt with at once by the left heart, the excess is taken up for a time in the lung capillaries. Vaso-motor fibres to the lung- vessels have been described as running in the anterior roots of the third, fourth, and fifth dorsal nerves. Their action is, however, of little importance, and their very existence is questioned by some observers. The fact that injection of adrenalin causes some vaso-con- striction in the lungs, points to the presence of a vaso-motor sympathetic supply to those organs. If we examine a tracing of the arterial blood-pressure, we notice that it presents certain periodic oscillations which accompany the movements of respiration. With each inspiration the blood-pressure rises ; with each expiration it falls. The synchronism of the rise and fall with the respiratory movements is not exact, since the rise continues for a short time after the beginning of expiration before it begins to fall, and the fall continues right into the beginning of the next inspiration, so that the highest point of the curve occurs at the beginning of expiration and the lowest point at the 935 936 PHYSIOLOGY Bespimturj/ tracing Fig. 420. Diagram of blood-pressure curve, showing effects of the respiratory movements on blood-pressure and pulse-rate. (The effects are purposely exaggerated.) beginning of inspiration. During the fall which accompanies expiration the heart-beats, as shown in the diagram (Fig. 420), become less frequent, and an obvious explanation of the fall of pressure would be to ascribe it to a reflex inhibition of the heart. On dividing both vagi, this difference in the pulse-rate during inspiration and expiration disappears, but the main features of the blood-pressure curve remain the same ; so that we must look for some mechanical explanation of the respiratory undulations. We have already seen that under normal conditions the lungs are in a state of over-distension, and that in consequence of this condition they are constantly tending to collapse, and are therefore exerting a pull on the chest wall. As soon as we admit air into the pleural cavity by per- forating the chest wall the lungs collapse. The force with which the lungs tend to collapse amounts to 6 mm. Hg. at the end of a quiet expiration, so we say that in the pleural cavity there is normally a negative pressure of 6 mm. Hg. As the chest expands in inspira- tion it drags the lungs still more open. As these become more distended their pull on the chest wall becomes greater, and hence the negative pressure in the pleura may be increased during forcible inspira- tion to 30 mm. Hg. It must be remembered that the heart and great veins and arteries are in the thorax separated from the pleural cavity only by a thin yielding membrane, so that they are practically exposed to any pressure, positive or negative, which may exist in the pleural cavity. Hence even at the end of expiration the heart and large vessels are subjected to a negative pressure of 6 mm. Hg. Outside the thorax all the vessels are exposed to a positive pressure, conditioned in the neck by the elasticity of the tissues and in the abdomen by the contractions of the diaphragm and abdominal muscles. Blood, like any other fluid, will always flow from a point of higher to a point of lower pressure. There must thus be a constant aspiration of blood from peripheral parts into the thorax. This aspiratory force will not in- fluence arteries and veins ahke. The arteries, having thick, comparatively non-distensible walls, will be very little affected by the negative pressure obtaining in the thoracic cavity, whereas the thin-walled distensible veins will be largely influenced by the same factor. The total result then of the negative pressure in the pleural cavities is to increase the flow of blood from the veins into the heart without affecting to any appreciable degree the outflow of blood from the heart into the arteries. The more pronounced the negative pressure in the thorax, the greater will be the amount of blood sucked into the heart from the veins. During inspiration therefore the heart will be better suppUed with blood than during expiration, and this THE PULMONAEY CIRCULATION 937 factor in itself will tend to raise the arterial blood-pressure. The inspiratory descent of the diaphragm will moreover tend to increase the inflow into the heart by raising the positive pressure in the abdomen, so that blood is pressed out of the abdominal veins and sucked into the heart and the thoracic veins. Another important factor is the influence of the respiratory movements on the circulation through the lungs. In trying to miderstand this influence it must be remembered that the pulmonary capillaries lie in a certain amount of elastic and connective tissue and are separated, on the one side by the alveolar epithelium from air at the ordinary atmospheric pressure, and on the other by the pleural endothehum from the pleural cavity, where the pressure varies from 6 to 30 mm. Hg. below the atmospheric pressure. We may therefore consider the pulmonary capillaries as lying between, and attached to, two concentric elastic bags. Under normal conditions, since these bags are always tending to collapse, the imier one must be pulhng away from the outer one, and the outer one from the chest wall. Hence there must be a negative pressure in the tissues between these two bags — a negative pressure which in the expiratory condition will be something between 0 and - 6 mm. Hg., and in the inspiratory condition between 0 and - 30 mm. Hg. If we regard the average pressure within the pulmonary capillaries as constant, these capillaries must be more dilated in the inspiratory than in the expiratory condition. This dilatation of the pulmonary capillaries will have two effects. Their capacity will be increased and the resistance they present to the flow of blood will be diminished. Let us now consider what effect these changes will have on the general arterial blood-pressure. We will assume that during expiration the pul- monary vessels have a capacity of 25 c.c. and that the beat of the right heart is forcing through them 10 c.c. of blood per second. So long as the chest remains in the expiratory condition 10 c.c. of blood will be floT\-ing into the left heart and into the aorta, so that the systemic blood-pressure will remain constant. Now let us suppose that an inspiratory enlargement of the thorax takes place, the negative pressure in the pleura is increased, the two walls of the Imigs are pulled farther away from one another, and there is a general enlargement of the pulmonary capillaries. We will assume that this enlarge- ment increases the capacity of the pulmonary capillaries from 25 to 30 c.c. Owing to this increased capacity, the first 5 c.c. of blood which flows into the lungs after the beginning of inspiration will not flow out through the pul- monary vein, but will simply serve to bring the capillaries into the same state of distension as before. Hence at the beginning of inspiration the flow through the pulmonary vein will be diminished ; there will be less blood discharged into the left heart, and therefore a fall in systemic pressure. As soon, however, as the increased capacity of the pulmonarry vessels is made up, the dilating effect of the inspiratory movement of these vessels Avail aid the flow through the lungs, in consequence of the diminution of resistance, so that the same force of the right heart which drove 10 c.c. of blood per second through the former resistance during expiration vdW now 30* 938 PHYSIOLOGY drive more, say 12 c.c. of blood. There is thus more blood entering the left heart, and therefore a rise of systemic pressure during the last three- quarters of the inspiratory movement. Expiration will have exactly the reverse effect. At the beginning of expiration there is a diminution of capacity in the pulmonary vessels from 30 to 25 c.c. Hence during the first second of expiration the outflow of blood from the pulmonary vein into the left heart will be 17 c.c. (12 c.c. + 5 c.c). After this, the increased resistance in the pulmonary capillaries in consequence of their constriction will come into play, and the flow of blood through them will fall once more from 12 c.c. to 10 c.c. Hence at the beginning of expiration the inflow of blood from the pulmonary vein into the left heart is greater than at any period. The arterial pressure will therefore rise to its greatest height at the beginning of expiration, and will fall during the last three-quarters of expiration, but will attain its minimum only at the beginning of the next inspiration. In this way the effect of the respiratory movements on the systemic blood- pressure can be entirely explained by the influence they exert on the lung- vessels or lesser circulation. On the other hand, Lewis regards the peri- cardial pressure, i.e. the direct influence of the thoracic movements on the heart, as playing a much more important part than changes in the pulmonary circulation in the production of the respiratory undulations in the blood- pressure. He shows moreover that in man the effect of respiration on arterial blood-pressure may vary according to the type of respiratory movement, a deep intercostal inspiration (not prolonged) causing a pure fall of pressure, while a deep diaphragmatic inspiration gives a pure rise of blood -pressure. In expiration the reverse effects hold. He concludes that it is not possible to make any general statement as to the nature of the blood-pressure response to a particular respiratory act. SECTION VIII THE CAUSATION OF THE HEART-BEAT If the heart be cut out of the body of a cold-blooded animal, such as the frog or tortoise, it wall continue to beat with the normal sequence of its different chambers for hours, or even days, pro\"ided that it be kept cool and moist. In the case of a warm-blooded animal the heart is similarly capable of continuing its rhj^thmic contractions for some little time after excision. The period of sur^^val of the heart is less in warm-blooded than in cold- blooded animals. The fact that in both cases the heart will continue to beat after removal from all its connections with the central nervous system, and when blood is no longer flowing through it, shows that the causation of the heart-beat is to be sought in the walls of the heart itself. The heart wall consists of a muscular tissue resembhng in many respects volmitary muscle ; hke this, it presents longitudinal and transverse striations ; like this, it is capable of contracting in response to direct stimulation. Normally voluntary muscle only contracts in response to impulses from the central nervous system. When Remak described the existence of collections of ganglion-cells in the sinus venosus, it was natural that "physiologists should ascribe to these collections of nerve-cells the same automatic rhythmic fimctions that had been found by Flourens and others to be associated with the grey matter of the medulla oblongata in connection w4th the maintenance of the respiratory movements. ANATOMY OF THE FROG'S HEART The hearts of the frog and of the tortoise have figured so largely in the researches on the causation of the heart-beat that it may be profitable to mention briefly the main points of their anatomy. The frog's heart consists of the sinus venosus, which receives the anterior and l)osterior venaj cavte, two auricles, one ventricle, and the bulbus arteriosus, which opens into the two aorta?. The venous blood from the body flows into the sinus venosus by the three vente cavfe, and thence into the right auricle, while the left auricle receives the blood from the lungs. The ventricle thus receives mixed arterial and venous blood, the arterial blood being directed by the spiral valve of the bulbus aortae so as to flow chiefly towards the head. The muscular fibres of the heart areless highh- developed thanthose of the mammalian heart. They are spindle-shaped, and only dimly cross-striated. The cross-striation becomes more distinctly marked as we proceed from sinus to ventricle, the sinus muscle fibre representing the most primitive condition. There is complete muscular con- tinuity between all the cavities of the heart. The circular ring of muscle at the junction 939 940 PHYSIOLOGY of sinus with auricles and of auricles with ventricles presents only slight traces of cross- striation (Gaskell). The heart is well supplied with nerve fibres and ganglion-cells. The two vagi enter the sinus venosus and branch just under the pericardium. Here they become con- nected with a collection of nerve-cells, known as Remak's ganglion. From the sinus the two vagi, now called septal nerves, pass down in the interauricular septum, one in front and the other behind. Near the auriculo -ventricular groove they enter two col- lections of ganglion-cells, called Bidder's ganglia. From these ganglia non-meduUated fibres are distributed to surrounding parts of the auricle and to the whole of the ventricle. In the upper third of the ventricle occur scattered ganglion-cells attached to the nerve fibres. These are quite absent in the lower half or two -thirds. In the tortoise (Fig. 422) the two auricles are bound together by a flat band of tissue, which serves also to connect the sinus with the ventricle. The septum between L.V.C.S i-A Ventricle Fig. 421. Diagram of frog's heart. (After Cyon.) V, ventricle ; r.a, l.a, right and left auricles (atrium) ; s.v, sinus venosus ; p.v, pulmonary veins ; l.v.c.s and B.v.c.s, left and right su- perior vena cava ; v.c.i, vena cava inferior ; Tr.A. truncus arteriosus. Cor .v. Fig. 422. Tortoise's heart (after Gaskell) as it appears when sus- pended for registering the auricu- lar and ventricular contractions. N, nerve-trunk with fibres con- necting Remak's and Bidder's ganglia ; cor. V, coronary vein. the auricles arises from the central line of this junction wall. The two vagi nerves pass into a large accumulation of ganglion-cells in the sinus, and thence along the basal wall to the auriculo -ventricular groove, lying just under the pericardium. In the groove they pass into a collection of ganglion-cells, whence fibres are given off to both auricles and ventricle. As they leave the sinus, a branch is always given off by the right nerve to accompany the coronary vein, which conveys blood from the ventricular wall to the sinus. Thus the nerves of the tortoise's heart are altogether more accessible than those of the frog's heart. In other points the tortoise's heart is similar to the frog's heart, though considerably larger. THE AUTOMATIC CONTRACTION OF THE FROG'S HEART The frog's heart in the body, or when removed from the body intact, beats regularly, the contraction starting in the sinus, then travelMng to auricles, ventricle, and bulbus. If, however, the heart be removed by cutting it across the sino-auricular junction, or if the auricles be functionally separated from the sinus by a hgature round this junction (Stannius's liga- ture), the auricles and ventricle stop in an uncontracted condition (diastole), while the sinus goes on beating regularly. After the lapse of a period varying from five minutes to half an hour the detached part of the heart begins to beat, at first slowly and then more rapidly, but never attaining the rate of the sinus. The auricles beat first, and then the ventricle. If now the ventricle be cut away by an incision in the auriculo-ventricular groove from the auricles the latter go on beating ; while the former, after a few beats due to the excitation of the incision, stops still, and only after a considerable time may begin again to contract very slowly. THE CAUSATION OF THE HEAET-BEAT 941 On the other hand, a ventricle- apex preparation (that is to say, the lower two-thirds of the ventricle separated functionally from the rest of the heart) never beats again under normal circumstances. To single stimuU it responds with a single beat, not with a series of beats as the whole heart does. If the lower third of the ventricle be separated functionally in the hving frog by crushing the ring of tissue between it and the upper third, it never gives a spontaneous beat again, although it is under the most normal conditions pos- sible in the circumstances. There is thus a descending scale of automatic power in the different parts of the frog's heart — from the sinus, where it is highest, to the lower parts of the ventricle, where it is apparently absent. From this fact it has been thought that the automaticity of the frog's heart is dependent on the gangha present in it. The contraction was supposed to be started by impulses proceeding from the sinus gangUon. If this were cut off, Bidder's ganglia, or the scattered cells in the upper third of the ventricle, could, it was thought, take up its task of originating impulses. The muscle-cells under this hypothesis act as the servants of the ganglion- cells, just as the voluntary muscles wait on the commands of the cells in the spinal cord and brain. The view that the ganghon-cell sends out rhythmic impulses had, how- ever, to be discarded when it was discovered that the muscle forming the lower third of the ventricle either of the frog or the tortoise, though free from ganglion-cells, could be excited by various means to rhythmic contractions. Thus it could be set into rhythmic action when supphed with salt solution under pressure, through a perfusion cannula, or when excited by the passage of a constant current or of weak induction shocks. The fact that the heart muscle responded to continuous stimulation by a rhythmic discharge suggested that the function of the ganglion-cells was to furnish a constant stimulation to the muscle-cells and so maintain these in rh}i:hmic acti\aty. The theory of the ganglionic origin of the cardiac rhythm was seriously affected by a series of researches carried out by Gaskell and by Engelmann. The arguments against the ' neurogenic ' hypothesis may be summarised as follows : (a) The cardiac muscle, free from any gangUon-cells whatsoever, can be excited by various means to rhythmic contraction. When, in the living frog, the apex of the ventricle is crushed off from the base so as to leave only material continuity between the two parts, the circulation of the blood is maintained by the contraction of all the parts of the heart except the apex, which never resumes its activity. If, however, the intraventricular pressure be raised by clamping the aorta the apex begins to beat at its own rhythm, which is independent of the rhythm of the rest of the heart. Moreover a strip can be cut from the apex of the tortoise's ventricle (Fig. 423), free from ganglion-cells, which on keeping in a moist chamber and moistening occasion- ally with normal salt solution enters into rhythmic contractions. (b) In the frog it is possible to excise the inter-auricular septum with its ganglia, and a considerable portion of the ganglia in the sinus venosus and at the base of the ventricles, without interfering in any way with the cardiac 942 PHYSIOLOGY rhythm. This experiment is still easier to carry out in the tortoise's heart where the nerves and gangha run in the basal portion of the auricles. This can be excised, leaving the two auricular appendages in connection with the sinus venosus and with the ventricles. (c) The heart in the developing chick can be seen beating at a time when it is quite free from nerve-cells, which only extend into it at a later date. {d) Remak's gangha are situated at the point where the two vagus nerves enter the heart, and under the microscope can be seen to be connected with the fibres of these nerves. We have now, from the discovery of Langley and Dickinson, a means of judging of the action of gangfion-cells in the drug nicotine, which first stimulates and then paralyses nerve- cells themselves or the synapses between the cells and the nerve fibres in connection with them. Direct apphcation of nicotine to the heart, after a primary period of slowing, leaves the heart-beat practically un- altered, the normal sequence of beat in the various cavities being unaffected. After the apphcation of the drug, however, stimulation of the trunk of the vagus is with- out effect, though slowing or stop- page of the heart may still be produced by excitation of the post ganglionic nerve fibres of the vagus which arise from the cells of Remak's gangha. These gangha must therefore be re- 'garded not as a motor centre for the heart, but merely as a distri- buting-centre for the inhibitory fibres of the vagus. Since tetani- sation of the heart with weak currents also causes local inhibition, it would seem that the finer nerve fibres ramifying throughout the muscular substance are, to a large extent at all events, inhibitory in their function. This is confirmed by the fact that atropine, which paralyses the inhibitory fibres of the vagus, also abolishes the direct inhibitory effect of tetanisation on the heart muscle. Gaskell and Engelmann therefore came to the conclusion that the source of the cardiac rhythm was to be found, not in the ganglia scattered about its cavities, but in the muscular cells themselves. The normal sequence of events — i.e. the subordination of the ventricle to auricles and auricles to sinus, so that the beat always follows in the order, sinus, auricles, ventricle, bulbus — can be ascribed to the difference between the natural rhythms of these different cavities. It is possible to record the con- tractions of each of these parts of the heart separately, after having divided Fig. 423. Tortoise's heart from dorsal surface. (Gaskell.) S, sinus ; J, sino-auricular junction ; A, auri- cles ; C, coronary vein ; V, ventricle. (The dotted line shows how a strip may be cut from the ventricle apex.) THE CAUSATION OF THE HEART-BEAT 943 them, either functionally by crushing the intervening tissue, or by actual section. Under such conditions it is found that there is a descending scale of rhythm from sinus to bulbus, the contractions of the sinus being most frequent, those of the ventricle and bulbus the least frequent. Thus it is impossible for the ventricle to beat at its own rhythm, since before it is ready to beat again after performing one beat it receives an impulse from the auricles causing an excited beat. That the normal sequence of contractions is dependent simply on the natural rhythm of the sinus is shown by the fact that by exciting the ventricle by means of induction shocks repeated at a rhythm shghtly greater than that of the sinus it is possible to excite a reversed rhythm, the order of the beat being now ventricle, auricles, sinus venosus. The dependence of the ven- tricular rhythm on the beat of the sinus may be shown by a simple experiment. The ventricle is con- nected with a lever suspended by a spring so as to record its con- tractions on a drum. A platinum loop connected with a galvanic battery is put round the heart, either round the sinus or round the ventricle (Fig. •424). When a current is allowed to pass through the inner loop the cor- responding part of the heart is warmed. When the ventricle alone is warmed the beats be- come larger, but the rhythm is unaltered. On lowering the loop so as to warm the sinus, the rhythm of the whole heart is quickened, but the size of the ventricular beats is unaffected. The different rhythmic power of these parts of the heart is apparently connected with the histological characters of the muscle fibres at each part. The lowly differentiated sinus cell has well- marked rhythmic power and a quick rhythm of beat, but is not able to exert such force in its contraction. The more highly differentiated ventricle cell has only a shght rhythmic power, but beats forcibly and is a good servant of the sinus. Fig. 424. THE PROPAGATION OF THE WAVE OF CONTRACTION The normal contraction started in the sinus venosus is propagated to the auricles, thence to the ventricle, and thence to the bulbus aorta\ Between the contractions of each of these cavities there is a slight pause, whereas the contraction spreads so rapidly over each cavity that all parts, say of the auricles or ventricle, appear to contract simultaneously. It is obvious that the excitatory wave might be propagated through the heart from one 9M PHYSIOLOGY muscle-cell to another, or by means of nerve fibres, wbich would excite the muscular tissue of each cavity to contract. The distinct pause which intervenes between the contractions of auricles and ventricle was long regarded as evidence for the nervous character of the contraction, and as showing the operation of a nerve-centre in the co- ordination of the contractions of difierent cavities. A contraction wave may, however, be started at any part of the heart and may travel from this to aU other parts. Thus, although the normal direction of the contractions is from sinus to ventricle, it is possible, by stimulating the apex of the ventricle, to excite contractions in the reverse order, viz. from ventricle to sinus. Such a fact is at variance with all our present knowledge of excitation Tig. 425. Heart of tortoise with auricle slit up so as to cause a partial block. (Gaskell.) of motor nerves. Excitation of the nerve going to the sartorius, or any part of the nerve, may excite contractions of all the fibres of which the muscle is composed. On the other hand, excitation of a part of the muscle which is free from nerve fibres causes a contraction which is hmited to the muscle fibres directly excited and does not extend to the nerves. If motor nerves arose from the hypothetical motor ganghon of the heart and passed to the ventricular muscle, one would not expect that contraction of the ventricular muscle could excite these nerves and so cause the propagation of a wave of contraction in the reverse direction. That the propagation cannot be due to any nerve-trunks running from sinus to ventricle is shown by various experiments of Engelmann and Gaskell. Thus, if the auricle is sht up by a series of interdigitating cuts, the contraction wave starting from the sinus travels along the auricular muscle around the end of each section, and finally, on arrival at the ventricle, causes a contraction of this cavity. In the heart of the tortoise the nerve- trunks run, not in the interauricular septum, but in a band of tissue joining THE CAUSATION OF THE HEAKT-BEAT 945 the sinus to the ventricles ; this band can be exercised with all its contained nerves without interfering in any way with the normal sequence of contrac- tions. Moreover the pause observed between the contractions of auricles and ventricles has been shown by Gaskell to be due to the retardation of the excitatory wave which occurs in its propagation through the muscular tissue in the auriculo-ventricular junction. A similar retardation of the wave can be produced at any point either in auricles or ventricle by diminish- ing the conducting muscular tissue to a sufficiently small extent. Thus, if the auricle of the tortoise be divided as in the diagram (Fig. 425), it will be noticed that the sinus first contracts, then the auricular half As ; a distinct pause then occurs while the contractile process is passing over the ' bridge,' and finally Av contracts, followed by the ventricle. The apparent pause between the contrac- tion of the auricles and ventricle is due therefore to a partial ' block ' at the auriculo-ventricular junc- tion. If the block be increased in the experiment just quoted, as, for instance, by allowing the bridge of tissue to dry or by making it still narrower, it may be found that only one out of every two contractions passes across the bridge (Fig. 426), and the slightest increase in the resistance to the propagation of the ^^^: '^^9\ Contraction r j: r> Qf auricles and ventn- wave may lead to the block becoming complete. On cles of tortoise heart, moistening the bridge again every contraction may J^^ grolTe^'h^^^Uen be seen to pass. clamped so as to pro- By the methylene-blue method it is possible to fSrwing ^ onJy' elery demonstrate a close network of non-medullated fibres second contraction to surrounding all the muscle-cells of the heart. It is ^^^^' ^ askell.j obvious that the experiment just quoted would not exclude the possibihty of propagation occurring through such a nerve network. The properties of the network would have to difier from those of any of the nerve tissues with which we are acquainted ; whereas we know that under certain circumstances impulses may be transmitted fi'om fibre to fibre even in striated muscle, and such a mode of propagation is the most obvious explanation of the phenomena observed in the heart. If the auricles be soaked for some time in distilled water they enter into a condition of what is known as water-rigor {Wasserstarre). In this con dition they are incapable of contracting, but can still propagate the wave^from sinus to ventricle. This experiment has been regarded as a demonstration of the part taken by nerve fibres in the propagation of the wave, but such an explanation is not necessary, since a similar condition of water-rigor in a voluntary muscle fibre has been shown to allow the passage of an excitatory wave through the affected part to the normal portion of the muscle, which then responds by a contraction. A series of interesting researches by Carlson on the mechanism of the heart-beat in the king-crab Limulus have been thought to throw light on the vexed question of the automatism of the vertebrate heart. 946 PHYSIOLOGY In Limulus the heart forms a segmented tube of ordinary striated muscular fibres. In large specimens the tube may be from 10 to 15 cm. long and 2 to 2 J cm. broad. Like the hearts of most other invertebrates and of all vertebrates, it has a local system of ganglion-cells, but so situated that they can be cut away entirely from the muscular portions of the organ. The arrangement of the cardiac nervous system in Limulus is shown in Fig. 427. The ganglion-cells are collected chiefly in a dorsal nerve ganglion cord which runs almost the whole length of the heart. From this cord non-medullated nerve fibres pass i2;:22=sd^^,^££2s. Fig. 427. Heart of Limulus from dorsal surface. (Carlson.) mnc, median nerve-cord ; In, lateral nerve-trunks. directly into the substance of the heart, and also send branches to two lateral nerve- trunks, by which fibres are distributed to all parts of the heart. The heart normally contracts about forty times per minute. Each contraction affects all parts practically simultaneously, though in the dying heart the posterior portions apparently contract slightly before the anterior, and may continue to contract after the anterior end has come to a standstill. Division of the muscular tissue leaving the nerve-strands intact does not alter in any way the synchronism of contraction of the two ends of the heart. Division of the nervous cord into two parts, the section being carried between the posterior third and anterior two-thirds, causes complete lack of co-ordination between the two ends ; both ends of the heart continue to contract, but at different rhythms. Extirpation of the Fig. 428. ' Nerve-muscle preparation ' of heart of Limulus consisting of the muscle of the two anterior segments, with the two lateral nerves. (Carlson.) nerve-cord abolishes spontaneous contractions. If the anterior half of the dorsal ganglionic cord be excised, all parts of the heart will continue to contract in unison. If now the lateral nerve-trunks be divided, the anterior half of the heart ceases to con- tract, showing that it was being excited by impulses arising in the posterior part of the ganglionic cord. It is possible therefore to make a nerve-muscle preparation of the anterior part of the heart, consisting of the muscle of the first two segments with a longer stretch of the lateral nerves (Fig. 428). Stimulation of the lateral nerves with a single shock causes a single beat of the anterior segments ; tetanising shocks cause a continued contraction of the muscle preparation. There seems to be no doubt that in this animal the beat of the heart is originated and co-ordinated by the action of the local ganglionic centres. Moreover Carlson has shown that the inhibitory nerve to the heart acts, not by direct influence on the muscle fibres, but by an inhibition of the automatic activity of the ganglionic cells, thus con- THE CAUSATION OF THE HEART-BEAT 947 firming for this special case the general view of inhibition long ago put forward by Morat, but not now generally accepted. The heart muscle docs not show a refractory period, but on direct stimulation with repeated shocks there may be a summation of contractions, which maj' fuse to a com- plete tetanus. The question naturally arises how far the heart of Limulus is to be regarded as a special case, or how far we may transfer results gained from experience on this heart to those of other hearts in which a perfect separation between ganglion- cells and muscle fibres is not so easily attainable. Carlson has sought to show the applicability of his results to the explanation of the cardiac mechanism in vertebrates by a series of observations on other invertebrates' hearts, where the muscular and nervous tissues are not so easily dissociable. Such hearts present phenomena very analogous to those of the frog's heart. According to him the phenomenon of the re- fractory period, the ' all or none ' law of contraction, and the absence of tetanus in the heart of the frog is due, not to the peculiar functions of the muscle fibres, but to the fact that in all our experiments we are affecting muscular and nervous tissues simultaneousl^^ In the absence of more perfect knowledge of the properties of the nerve-nets which surround involuntary and cardiac muscle fibres a decision of the point is not yet possible. The muscle and nerve fibres of Limulus show, however, important differences from the cardiac muscle of the frog in their reaction to chemical stimuli. Acceptation of the neiu"ogenic theory would necessitate the prediction of a type of nervous tissue endowed with properties for which we have no analogy in any of the nerve tissues which have been the subject of exact investigation, whereas the myogenic theory only ascribes to the muscle-cells of the heart properties which are the common attribute of all protoplasm or are displayed in a less marked degree by the ordinarj^ skeletal muscle fibres. It would, at any rate, be premature to transfer unreservedly all the results obtained on the heart of the Limulus, the muscle fibres of which have the structure and behavioiu" of skeletal muscle fibres, to the explanation of the phenomena exhibited by the hearts of vertebrates. THE HEART-BEAT AS A WAVE OF CONTRACTION It the beat of the frog's ventricle, or a strip of mammalian ventricle, be recorded, the curve obtained resembles closely the twitch of a voluntary muscle produced in response to a single excitation. Whereas, however, a single contraction with the subsequent relaxation of voluntary nuiscle only lasts about one-tenth of a second, the contraction of the mammahan ventri- cular muscle lasts three-tenths of a second, of the frog's ventricle about half a second, and of the tortoise ventricle about two seconds.* In the heart, as in a voluntary muscle fibre, the contractile process originates at the stimulated point and travels thence to all other points. The progress of the excitatory wave is well seen if a record be taken of the electrical changes resulting in the frog's heart from a single stinnilation. If the two ends of a strip of ventricular muscle be connected with the two terminals of a capillary electrometer, stimulation at one end causes a diphasic variation, showing that the excitatory process starts at the stimulated end and travels to the other eiid of the heart. Thus if the acid of the electro- meter be connected with the base of the ventricle and the mercury of the capillary be connected with the apex, stimulation at the base causes a wave passing from base to apex. Directly after the stimulation therefore the base * The duration of the contraction depends on the temperature. The figures given are for the mammalian heart at 37° C. and for the amphibian heart at about 15° C. 948 PHYSIOLOGY becomes negative and the column of mercury moves towards the acid ; a moment later the contraction extends to the apex. All parts of the heart are now in a similar condition of excitation : there is no difference of potential between the two terminals and the mercury comes back quickly to the base hne. Relaxation, hke contraction, starts first at the base and proceeds thence to the apex. There is thus a small period during which the apex is still contracted while the base is relaxed and the apex is therefore negative to the base. This terminal negativity of the apex is shown on the capillary electrometer by the excursion of the colunm of mercury away from the point of the capillary (cp. Fig. 87, p. 230). Fig. 429. Electrometer record of variation of spontaneously beating tortoise heart. (Gotch.) Analogous effects are obtained on leading off the spontaneously beating heart in the frog or tortoise (Fig. 429). The conditions are, however, rather more complex, and the most usual variation, as Gotch has shown, is triphasic. In its most primitive form the vertebrate heart is composed of a simple tube in which a contraction starts at the venous end and is propagated in a wave-hke manner along the tube to the arterial end. In the higher vertebrates the heart at its first appear- ance has the same tubular form, but the simple tube very rapidly becomes modified, partly by twisting on itself, partly by the outgrowth of the dorsal or the ventral wall of the tube to form the cavities of the auricle and ventricle. Gotch suggests that the excitatory process follows the course of the original tube and that the typical form of the curve is due to the base becoming excited twice, first at the part in con- tinuity with the auricle, and, secondly, when the wave sweeps up to the bulbus aortse. But it is possible that in the cold-blooded, as in the mammahan, heart there may be a special conducting tissue which THE CAUSATION OF THE HEAKT-BEAT 949 leads the excitatory process to many different parts of the ventricle almost simultaneously. There is no doubt that the ventricular systole is comparable with a simple muscular twatch and cannot be regarded as the summation of several contractions. Since the excitatory process extends in the form of a wave not only to all parts of the same cavity but to all parts of the heart, it is evident that the musculature of the heart is to be compared, not with skeletal muscle composed of many fibres, but to a single muscle fibre in which all parts are in functional continuity. THE BEAT OF THE MAMMALIAN HEART The mammalian heart, like the heart of cold-blooded animals, will beat for some time after it has been cut out of the body, and a perfectly rhythmic activity may be maintained for hours by feeding the heart from the coronary arteries either \^dth defibrinated blood or ^^^th oxygenated Ringer's solution, with or without the addition of glucose. Ganglion-cells are found in the mammalian heart around the openings of the great veins, along the border of the interauricular septum, in the groove between auricles and ventricles, and in the basal parts of the ventricles. The ventricles of mammals are endowed with a greater rhythmic power thaii the corresponding cavities in the frog and tortoise. It is possible to sever or crush all the nervous and muscular connections between auricles and ventricles without destroying their mechanical connection by means of fibrous tissue. Such a procedure does not, even for a moment, stop the contractions of the ventricles, which go on beating at a rh}'i:hm which is independent of and slower than that of the auricles. Porter has shown that a mere fragment of the ventricular wall, perfectly free from ganglion-cells, may maintain rhythmic contractions for some hours if fed by an artificial circulation through a branch of the coronary artery. We may therefore conclude that in the mammalian, as in the amphibian, heart the cause of the rh}i:hm is to be sought in the properties of the muscle fibres themselves, and that every part of the heart-muscle possesses the power of rhythmic activity, the normal sequence of the beats being determined by the greater frequency of the natural rhythm of the venous end of the heart. In the mammalian, as in the amphibian, heart the excitatory condition started at one point in the muscle spreads through the muscle in all directions, and the process of conduction of excitation seems to be independent of nerve fibres. The excitatory process may be conducted not only in the ordinary direction from auricles to ventricles, but also from ventricles to auricles. If the ventricles be excited at a rhythm of higher frequency than the natural beat which is starting at the venous end of the heart, we may obtain a reversed rhythm in which the order of the beats is ventricles — auricles. It is difficult to conceive of an arrangement of neurons which would propa- gate impulses impartially from auricles to ventricles or from ventricles to auricles. Such a condition would seem to be in contradiction to the law of forward direction which obtains throughout the nervous system. On the 950 PHYSIOLOGY other hand the phenomena are easily explained on the assumption that the whole of the musculature of the heart acts in many respects as a single muscle fibre, along which an excitatory process may be propagated in any direction. But in the adult mammahan heart, on superficial dissection, the muscle fibres both of auricles and ventricles are seen to arise from a fibro- cartilaginous ring surrounding the auriculo- ventricular junction, leaving apparently no muscular continuity between the two cavities. On this account it was thought for many years that the propagation of the contrac- tion from auricles to ventricles must occur by means of nerve fibres, and it was only with the discovery by His of a distinct band of modified muscle fibres, passing from the auri- cles to the ventricles, the ' auriculo- ventricular bundle,' that an anato- mical basis was furnished for the physiological behaviour of the heart. The heart is developed from a mus- cular tube in which at the beginning we must assume muscular continuity throughout. The primitive vertebrate heart is formed by a modification of this muscular tube. In this heart, as Keith has shown, we may distinguish five chambers, namely, the sinus venosus, the auricular canal, the auricle, the ventricle and the bulbus (Fig. 430). The musculature of these chambers is continuous throughout. In the adult heart, e.g. of man, the anatomical relations of the different cavities have become considerably modified in the course of develop- ment. The sinus venosus, i.e. the part where in the lower vertebrates the contraction wave takes its origin, is now represented merely by the termination of the superior vena cava and of the coronary sinus in the right auricle. These two veins are derived from the right and left ducts of Cuvier in the embryo. The sinus venosus is also represented by a small amount of tissue under- ling the tcenia terminalis of the right auricle, as well as by the remains of the Eustachian and venous valves. The auricular canal gives rise to the auricular septum and to the auricular ring surrounding the auricu- lar-ventricular orifice, and in some hearts it is prolonged into the ventricle as the intraventricular or invaginated part of the auricular canal. In the adult heart two accumulations of more primitive tissue are found in the region corresponding to the sinus venosus of the embryo which are known Fig. 430. A generalised type of vertebrate heart. (Keith.) a, sinus venosus ; b, auricular canal ; c, auricle ; d, ventricle ; e, bulbus cordis ; /, aorta ; 1-1, sino-auricular junction and venous valves ; 2-2, canalo-auricular junc- tion ; 3-3, annular part of auricle ; 4-4, in- vaginated part of auricle ; 5, bulbo-ventri- cular junction. THE CAUSATION OF THE HEART-BEAT 951 as the sino-auricular node and the auriculo-ventricular node. The sino- aiiricular node (Fig. 431) Ues in the groove between the superior vena cava and the right auricle. The auriculo-ventricular node Ues at the base of the auricular septum on the right side, below and to the right of the opening of the coronary sinus. From this point a bundle of muscular fibres (the bundle of His or the auriculo-ventricular or A.V. bundle) runs along the top of the interventricular septum just below its membranous part and then Superior Vena Cava Aorta InteriorVenet Cava Papillap' JHuscle Fig. 431. divides into the right and left septal divisions, which pass do^^■ll in each ven- tricle on the interventricular septmn into the papillary muscle arising from the septum. Each half of the bundle gives off several branches which break up more and more, fijially forming a reticulated sheet of tissue over the greater part of the interior of the ventricles just below the endocardimn. The fibres composing this tissue are more primitive in character than the rest of the cardiac musculature and have long been distinguished as the ' fibres of Purkinje.' In them the fibrillation is confined to the periphery of the muscle cell. They are distinguished by a high glycogen content. They may be regarded as a part of the muscular wall of the heart specially differen- tiated for the rapid conduction of the excitatory process to all parts of the ventricles (Figs. 432 and 433). Numerous nerve fibres and ganglion-cells are found to accompany the muscle fibres of the auriculo-ventricular bundle. We have, however, no reasons for regarding the uisrvons structures as concerned in the propagation of the excitatory wave. 952 PHYSIOLOGY bundle forms the only continuous muscular The auriculo-ventricular tissue between the auricles and ventricles, and destruction of it causes com- plete abohtion of the normal sequence of beat between auricles and ventricles. s.bv^' Fig. 432. Left ventricle laid open to display the interventricular septum on which the course of the left division of auriculo-ventricular bundle and its ramifications are shown in black. (After Tawaba.) --i^' ^^^ ^^^L ^0^^ g^ffi aue|=jafe€S^^ ^^^ ^^^^2 He mA ^^m K ^^ \a ^nr s^aJ"^ IB m Fig. 433. Fibres of Purkinje, from the subendocardial network. (Taw aba.) By leading off different parts of the heart to the string galvanometer it is possible to determine the time relations of the excitatory process. ^ It is then found that the mass of Purkinje tissue, known as the sino- auricular node, is the starting-point of the excitatory process concerned in each heart- beat. It is therefore spoken of as the ' pace-maker ' o£ the heart. At each beat a contraction starts at the sino- auricular node, spreads a short way up THE CAUSATION OF THE HEART-BEAT 953 the great veins, and along the auricular muscle in all directions. When it arrives at the auriculo-ventricular node, the impulse is carried on to the ventricles along the auriculo-ventricular bundle, spreading along the branches of this bundle to almost all parts of the ventricular muscle. Although normally the sino-auricular node initiates each heart-beat, this node can be put out of action by injury or coohng without stopping the rhythmic sequence of the heart-beat, the office of pace-maker being now taken up by the auriculo-ventricular node. A speciahsation of function accompanies the differentiation in structure which we find in the auriculo-ventricular bundle and its branches. Lewis has showTi that the conduction of the excitatory process along the auriculo-ventricular bmidle of the Purkinje tissue occurs about ten times as fast as the conduction through the ordinary muscular tissue of the heart, the rates being about 5000 mm. and 500 mm. per second respectively. Although all parts of the ventricles receive the impulse to contraction almost simultaneously, the contraction wave, as judged by the electrical changes, is found to commence slightly earher at two points, namely, on the anterior surface near the apex to the right of the groove separating right and left ventricles, and at the extreme apex of the heart where the endocardiac tissue comes very close to the surface. On the other hand, the conus arteriosus is the last part of the heart to begin contracting. The limitation of the muscular continuity to a single narrow bundle which is en- dowed with greatly increased conducting powers, and ends in a network of tissue endowed with similar powers, is e^^dently designed (to use an old-fasliioned but convenient word) to ensure that all parts of the ventricles contract practically simultaneously. If this were not the case the sudden contraction of the muscle fibres near the base of the ventricles would simply bulge out the still uncontracted portion near the apex, and there would be a risk of injury or even ruptui'e of the uncontracted part of the ventricle under the stress of the pressure produced by the contracting part. On the other hand, if all parts of the heart were endowed with a similar rapid power of conduction, any part slightly more excitable or irritated than the rest, might serve as a centre for emitting excitatory waves which would interfere with those transmitted from the auricles, and the tendency to heart delirium would be enormously increased. THE HUMAN ELECTROCARDIOGRAM The passage of the excitatory wave over the different heart cavities is associated with corresponding electrical changes resulting in differences of potential. If we lead oft' any two parts of the heart's surface to a string galvanometer or capillary electrometer, we obtain movements which are caused partly by changes occurring in the muscle just underlying the elec- trode, partly by changes occurring at a distance and transmitted by the inter- vening muscle acting simply as a moist conductor. These two kinds of effect may be alluded to as direct and indirect. If we lead off, not from the heart itself, but from neighbouring tissues in contact with the heart, we shall still obtain the indirect effect of the electrical changes at each heart- beat, and these can be obtained, as Waller has shown, when the intact animal is led off to the electrometer or galvanometer by his limbs. In an animal such as the dog the two fore Umbs may form one lead and the two 954 PHYSIOLOGY hind limbs the other. In man, when the heart hes asymmetrically, it is usual to lead off the right arm, and left arm, the right arm and left leg, or the left arm and left leg, the hand or foot being immersed in salt water con- nected to the galvanometer by a zinc electrode contained in a porous pot full Fig. 434. Electrocardiogram of man, obtained by leading off from the two hands to a string galvanometer. c is the carotid pulse tracing. The different parts of the curve are designated by the letters p, Q, B, s, T, first applied to them by Einthoven. of saturated zinc sulphate solution. By this means an electrocardiogram is obtained similar to that shown in Fig. 434. In view of the mechanism of the propagation of the excitatory wave in the ventricle we should not expect the cardiogram obtained in this indirect fashion to be easy of interpretation, at any rate so far as regards the course of the wave through the ventricular muscle. Such an electro- Had.art. Fig. 435. Simultaneous tracings of the jugular venous pulse and the radial arterial pulse, from a case in which the A.V. bundle was destroyed by disease. The contractions of the auricles are marked by the a waves on the venous pulse. They are more rapid than and quite independent of the ventricular con- tractions. (Mackenzie.) cardiogram however is of considerable use clinically, especially for the determination of the relation between the auricular and the ventricular con- tractions. The different points in a typical tracing, such as that contained in the figure, are designated by the letters P, Q, R, s, t, which were first apphed to them by Einthoven and are retained because they do not involve any theoretical interpretation of the curves. Of these p is certainly due to the auricular contraction and Q marks the beginning of the ventricular contraction. The A.V. interval is given by the distance between p and q, THE CAUSATION OF THE HEART-BEAT 955 the total duration of the excitatory condition in the ventricle by the distance between q and t. The fibres of the auriculo-ventricular bundle may be destroyed by disease. In such cases we get a series of phenomena known under the name of Stokes-Adams's disease, the main characteristic of which is the slow contractions of the ventricle, accom- panied by a rapid venous pulse at a rhythm entirely independent of the ventricular pulse. The automatic activities of auricle and ventricle are in fact dissociated (Fig. 435). At certain intervals, or at certain stages of the disease, the fibres of the bundle may present only a partial block, so that the ventricle responds once to every second con- traction of the auricle. The existence of this disease is shown at once on the electro- cardiogram by the dissociation of the normal relation between the auricular and ven- tricular variations. It may be also sho\vn by a study of the venous pulse (Fig. 435). THE PHYSIOLOGICAL PROPERTIES OF THE CARDIAC MUSCLE THE RESPONSE OF HEART-MUSCLE TO DIRECT EXCITATION When a skeletal muscle is directly stimulated with induction shocks of varying strength, within narrow limits the height of the contraction is proportional to the strength of the stimulus. If the frog's ventricle, rendered motionless by a Stannius ligature, be stimulated with a single induction shock, if it responds at all it will respond with a maximal contraction, no change in the extent of the contraction being obtainable, however the stimu- lus may be increased. There is thus no proportionality in the heart between strength of stimulus and height of contraction. The heart, if it contracts at all, always contracts to its utmost, the height of the contraction being dependent, not on the strength of stimulus, but on other conditions affecting the muscle at the time of its response. Although much stress has been laid on this supposed difference between heart-muscle and voluntary muscle, a renewed investigation of the response of the latter to graded stimuh by Gotch and by Keith Lucas tends to show that the distinction is not so fundamental. According to these observers the fact that the response to a minimal stimulus in skeletal muscle is smaller than the response to a maximal stimulus is simply owing to the fact that in the former case only a small proportion of the muscle fibres is active and that increasing the strength of the stimulus merely increases the number of fibres thrown into contraction. According to this view therefore a maximal con- traction of skeletal muscle would be one involving all the fibres. In the heart-muscle all the muscle fibres are functionally continuous, so that a stimu- lus, if it excites at all, must excite all the fibres, and every contraction must be analogous to the maximal contraction of a skeletal muscle. The existence of the ' all or none ' law in any contractile tissue would be therefore dependent on the existence of functional continuity between all the contractile elements of the tissue. In the retractor penis of the dog it is possible to get graded contractions with graded strength of stimuli, and in this case it is easy to observe that with increasing strength of stimulus a greater extent of the muscle is thrown into the contractile state. Closely connected with this manner of response is the fact that in heart-muscle, under normal circumstances, it is not possible 956 PHYSIOLOGY Fig. 436. Group of pul- sations showing ' stair- case ' character. to get summation of contractions by putting in a stimulus, however strong, before the muscle has returned to rest. If, however, the propagation of the first contraction throughout the heart-muscle be retarded or prevented by a partial death of the tissue, or by stimulus of the vagus nerve, it is possible, as Frank has shown, to obtain an apparent summation of two stimuH, i.e. a curve in which the second contraction is superposed on and rises higher than the first. Such a result, on the explanation given above, would be due to a phenomenon of ' block ' Hmiting the propagation of the first contractile wave, and yielding more to the second, though this is not the explanation given by the original observer. SUMMATION OF STIMULI If an isolated frog's ventricle which is not beating be stimulated with inadequate shocks, it may be found, on repeating these shocks at short intervals of time, that they become adequate and cause a contraction of the ventricle. A stimulus therefore which is subminimal may nevertheless cause some change in the heart- muscle, so that the latter responds more readily to subsequent stimuli. A similar improving effect of previous stimulation on the condition of the heart-muscle may be observed on the contractions themselves. Thus in a Stannius preparation, if the ventricle be excited with single induction shocks, once in every ten seconds, the first four or five contractions form an ascending series, each contraction being rather higher than the preceding one. This is often spoken of as the ' staircase phenomenon ' (Fig. 436). THE REFRACTORY PERIOD At each contraction of the heart-muscle there is a sudden decomposition of contractile material which, so far at least as concerns the incidence of an external stimulus, is maximal, i.e. complete. Directly this has occurred a process of assimilation or re-formation of contractile material begins. This lasts throughout the diastolic period, and the store of contractile material is at its maximum just before the next contraction. A mechanical analogy is furnished by a bucket into which a stream of water is constantly flowing, and which tips up automatically and empties out its contents as soon as the water reaches a certain height. It is evident that the power of the heart-muscle to contract in response to a stimulus (its ' irritability ') must be at a minimum imme- diately after the automatic discharge or decomposition has taken place, and will continually increase from this point as the store of contractile material grows, until it arrives at such a height that the explosive discharge occurs spontaneously. Hence in each cardiac cycle there is a period, known as the refractory period, in which stimuli applied to the heart have no effect. This will be followed by a period in which a stimulus is followed by an extra contraction, but with a prolonged latent period. Just before the next spontaneous contraction the irritability is at its height, and the heart-muscle THE CAUSATION OF THE HEAET-BEAT 957 responds with a contraction to a minimal stimulus. These facts are well shown in Fig. 437. When a tracing is being taken from part of the heart, e.g. the ventricle, which is beating rhythmically in consequence of a stimulus communicated to it from some other part, such as the sinus venosus, an extra contraction is Fit;. 437. Tracings of spontaneous contractions of frog's ventricle, to show refractory period. In each series the surface of the ventricle was stimulated by an induction shock at E. as indicated by the tracing of the signal. In 1, 2 and'3, this stimulus had absolutely no effect, since it fell during the refractory period. In 4, ;">. 6, 7 the effect of the shock was to interpolate an extra contraction in the series, the latent period (shaded part) gradually diminishing from 4 to 7 (diastolic rise of irritability). In 8 the irritabilitj^ of the preparation was already considerable, and the latent period inappreciable. The ' compensatory pause ' after the extra beat is also well shown in 4, 5, 6, 7, 8. (Marey.) followed by a ' compensatory pause,' and in certain cases the first contraction following the pause is considerably augmented. This is due to the fact that one of the impulses arriving from the sinus arrives at the ventricles during the refractory period ensuing on the application of the artificial stimulus ; hence it produces no effect and the ventricle has to wait for the arrival of the next succeeding excitatory wave from the sinus before it gives its next beat. Hence the compensatory pause does not occur when we are testing the effects of artificial stimuli on the sinus venosus. 958 PHYSIOLOGY On account of the refractory period which ensues on the commencement of the contractile process on heart muscle, it is impossible to throw the muscle into a tetanus, since all the stimuh which fall during systole are entirely inefEective. By using very strong stimuh it is possible to intercalate extra contractions before the heart has returned to the base hne, i.e. before diastole is complete. So that in this way one may obtain almost a continuous contraction, presenting, however, waves on its summit, which differs from the tetanus of skeletal muscle in the fact that its height is no greater than the height of a single contraction. Only when the functional continuity of the heart-muscle is impaired by the ' block ' effect of vagal stimulation or the administration of muscarine is it possible to obtain phenomena even superficially analogous to the summa- tion of contractions in skeletal muscle.* FACTORS MODIFYING THE ACTIVITY OF CARDIAC MUSCLE INFLUENCE OF TENSION AND DISTENSION When we examine the behaviour of a heart isolated from the central nervous system and from the rest of the body, as, for instance, in the heart-lung preparation {vide p. 911) we find that it has a marvellous power of adaptation, i.e. of regulating its activity according to the mechanical demands which are made upon it. Thus while we may maintain the venous inflow constant so that the heart is sending out a litre of blood per minute, it makes no difference to the output of the heart whether the average arterial pressure, and there- fore the resistance to the outflow of blood, be maintained at 80 or 160 mm. Hg., although in the latter case the heart must do exactly twice as much work in order to maintain the outflow at the same level. In the same way we may maintain the arterial pressure constant and alter the venous inflow and we find that within very wide hmits the heart is able to expel against the arterial resistance the whole of the blood which flows into it from the veins. In this way we can alter the output of a small heart of 50 gms. from 300 to 3000 c.c. per minute. As we should expect, this variation in the work done by the heart is associated with corresponding variations in the chemical changes which occur at each heart-beat. Evans has shown that the respiratory exchanges of the heart increase 'pari passu with the work it is said to do. Careful investigation of the volume and pressure changes of the heart under varying conditions of arterial resistance and venous filling enables us to throw some light on the mechanism of this power of adaptation. Let us take first the changes in volume as recorded by the cardiometer. A heart is contracting 100 times per minute and forcing out at each beat 10 c.c. of blood into the aorta against an average pressure of 80 mm. Hg., with systolic and diastolic pressures respectively of 100 and 60 mm. Hg. In order that the left ventricle may force 10 c.c. of blood against this resistance, * According to Mines the effect of vagus excitation in enabling the production of summation is due to the shortening of the refractory period which results from vagal stimulation. THE CAUSATION OF THE HEART-BEAT 959 the pressure in its interior must rise at each heart-beat above the maximum systolic pressure in the aorta, e.g. to 110 mm. Hg. The aortic valves w411 open as soon as the pressure rises above 60 mm. Hg. The arterial resistance is now increased so as to raise the average pressure to 120 mm. Hg. The heart now may raise the pressure in its interior to 120 mm. Hg. This will be higher than the diastolic pressure in the aorta and a certain amount of blood will escape, but the outflow of blood will cease as soon as the pressure in the aorta is equal to that in the ventricle. Diastole will then occur, the ventricle will relax before it has emptied out 10 c.c. of blood. Let us assume it has forced out 3 c.c. of blood — it will then contain an excess of 7 c.c. of blood at the end of diastole. Meanwhile the venous inflow is proceeding at the same rate as before so that at the end of diastole it has 7 c.c. more blood than it had at the end of the previous beat, i.e. its diastolic volume will be increased and the heart will be dilated. At the in- creased beat we find that the con- traction of the ventricle is much more forcible. The maximum pres- sure now rises to 130 mm. Hg. and 8 c.c. of blood are sent out into the aorta. At the end of this beat the heart will be still fuller than before, containing an excess of 9 c.c. of blood. The third beat is still more forcible, the intra- ventricular pressure rising to a maximum of 140 mm. Hg., ard 10 c.c. of blood are expelled. After this the heart goes on beating regularly expelling 10 c.c. of blood at each beat, i.e. the same amount as it receives from the veins, and the arterial pressure is maintained constant at an average of 120 mm. Hg. But the heart remains more dilated than it was previously since it contains an excess of 9 c.c. of blood. If now the arterial resistance be suddenly reduced to its previous amount, the first beat after the change may send out 17 c.c. of blood, the second beat 12 c.c. of blood and the third beat 10 c.c. as before. We see therefore that the energy set free at each contraction of the heart is increased by increasing the volume of the heart, but increased volume of the heart means increased length of the muscular Fig. 4.38. Effect or increased arterial pros- sure on the volume changes of the heart, with a stead}' inflow of l.l-t c.e. blood per 10 seconds. C. cardiometer curve. B.P. arterial blood- pressure. V.P. pressure in the inferior vena cava. The lines 100 and 80 show the height of the blood- pressure in mm. Hg. 960 PHYSIOLOGY fibres composing its wall, so we arrive at a statement similar to that made previously for voluntary muscles, namely, that the energy of contraction is a function of the length of the muscle fibres, i.e. to the extent of active surface involved. This reaction of the heart to increasing distension has long been known but was ascribed to the excitatory influence of tension on the muscle fibres. It is evident that in a resting heart increasing distension of its cavities will tend to stretch its muscle fibres and therefore to exert a tension vpjjjMlipjjJj^^ Fig. 439. Effect of alterations in venous supply on volume of heart. Heart, 67 gms. Arterial Venous Output of heart pressure pressure in 10 sees. A ... 124 .. 95 .. 86 B . . . 130 .. 145 .. 140 C . . . 124 .. 55 .. 33 The curved line at the side represents the value of the cardiometer excursions in capacity of ventricles in c.c. on them. By an accurate record of the pressure changes within the con- tracting ventricle under varying conditions it is possible however to exclude the tension on the fibres as the determining factor. In a heart beating regularly the inflow of blood is proceeding during diastole, during the relaxa- tion of the ventricles, i.e. the muscles are giving before the inflowing blood. The latter is therefore able to distend the heart without exercising more than a minimal pressure on its walls and it is found that the pressure in the ven- tricles may be approximately zero at the end of diastole whether the heart is contracting against a resistance of 80 mm. Hg. or a resistance of 120 mm. THE CAUSATION OF THE HEART-BEAT 961 Hg., or whether it is receiving 5 c.c. or 10 c.c. during the period of diastole. With a bigger outflow or a bigger resistance the energy of contraction is increased, although the tension on the heart wall at the beginning of the contraction is not altered. The only condition then which always changes pari passu with the energy of contraction is the distension of the heart cavities, i.e. the length of its muscle fibres, and we are therefore justified in regarding this last factor as the one which determines the energy of the response of the muscle to excitation. Naturally if the distension increased beyond a certain extent it would be associated wdth increased tension on the muscle fibres. But the changes of initial tension and excitatory response are not proportional. It is evident that the capacity of the heart for adapting itself to changes in mechanical demands made upon it will be limited by the inability of the heart to dilate further, as is probably the case in the intact animal, where its dilatation is Hmited by the pericardium, or by the mechani- cal disadvantage at which the further dilated heart acts. The more the heart attains a globular form, the greater the mechanical disadvantage of the muscle fibres in raising the pressure in the interior of the ventricles {vide p. 916) so that by continually increasing the demands on the heart we shall finally arrive at a stage at which this organ is unable to deal with the blood which is applied to it and rapidly fails to expel any of its contents. The physiological condition of the heart is measured by the maximum pressure which it is able to produce in its cavities when it contracts, starting from a certain initial size or length of fibres. As the heart becomes fatigued this pressure falls so that the heart must dilate in order that each contraction shall produce the same maximum pressure as before. Fatigue of the heart is shown therefore, not by failure of it to do its work, but by the fact that it can only do its work when it is undergoing considerable dilatation. Dilatation is therefore a measure of fatigue. What is often spoken of as the tonus of the heart is really synonymous with physiological condition. A heart in good condition has a high tonus. It empties itself almost completely at each beat even when receiving a considerable quantity of blood during diastole. A heart with a low tonus is in the condition of a fatigued heart. It is widely dilated and when it has finished contracting still contains a large amount of residual blood. This property of the cardiac muscle is responsible for the power of ' compensation ' possessed by a diseased heart. We may take as an example the destruction of one aortic valve, a lesion which can be produced experi- mentally in a dog. In this case, immediately after the lesion is established, no additional resistance is offered to the expulsion of the blood, and the ven- tricle will send the normal amount into the aorta. During the succeeding diastole the blood at a high pressure in the aorta will leak back into the ventricle through the damaged valve. The arterial pressure therefore falls rapidly, and the ventricle receives blood from two sides, i.e. by regurgitation through the aortic valves, and in the normal way from the auricles and veins. At the end of diastole the ventricle is therefore overfilled. Increased stretching of its fibres, however, has the effect of exciting an increased con- 31 962 PHYSIOLOGY traction, and the heart at its next systole throws out not only the normal quantity of blood but also that which it has received back from the aorta. The arterial system thus receives at each beat the normal quantity of blood plus the amount which leaks back into the ventricle after each systole ; so that the amount of blood remaining in the aorta and available for passage on to the capillaries is the same as in the normal animal. On this account, after a lesion of the aortic valves has been established, the average of the arterial pressure remains the same as before, although the oscillations of pressure with each heart-beat are increased in extent. The augmented output by the ventricles naturally involves increased work on the part of their muscular walls, which react in the same way as skeletal muscle does to increased work, i.e. by hypertrophy ; the final efiect therefore is a heart bigger than normal, with hypertrophied and thickened walls, but capable of maintaining an adequate circulation throughout all parts of the body ; in other words, in the healthy animal complete compensation has taken place. THE INFLUENCE OF TEMPERATURE ON THE HEART RATE The frequency of the heart varies directly with the temperature. The higher the temperature the greater the frequency. At 40° C. the contrac- tion of the mammalian heart may be four times as frequent as at 25° C. \ Fig. 440. Tracing of contractions of a frog's heart (by Ringer), showing effect of adding a trace of CaCl2 to the NaCl solution used previously for perfusion. The arrow marks the point at which the addition was made. INFLUENCE OF THE CHEMICAL COMPOSITION OF THE SURROUNDING MEDIUM ON THE HEART-MUSCLE The tissues of the heart, hke all other cells of the body, require for the normal display of their functions a definite osmotic environment, i.e. a certain molecular concentration of the fluid with which they are bathed. This is equivalent to a 0-65 per cent, sodium chloride solution for the frog's heart, and to a 0-9 per cent, solution for the mammalian heart. As Ringer first showed, the nature of the neutral salt employed for making up the normal solution is all-important to the heart-muscle. Thus a strip of muscle from the apex of the tortoise's ventricle as a rule does not beat spon- taneously. If, however, it be immersed in a 0-7 per cent, solution of sodium chloride it begins to beat rhythmically after a short latent period. The contractions soon reach a maximum and then gradually die away. Sodium chloride therefore acts as a stimulus to contraction, but is unable to main- tain the beats for any considerable length of time. The strip of muscle ceases contracting in a condition of relaxation. On now adding to the solution a trace of calcium chloride or calcium sulphate, the contractions THE CAUSATION OF THE HEART-BEAT 963 begin again (Fig. 440). Now, however, the relaxations after each contrac- tion become more and more incomplete, until finally the heart stops in a tonically contracted condition. If now a trace of potassium chloride or phosphate be added the contractions begin again and may last for many hours, although the solution contains nothing which can furnish energy to the contracting muscle. It has been suggested that the rhythmic con- tractions of the heart-muscle may be the result of the constant chemical stimulus of the inorganic salts present in the blood-plasma, sodium acting as a stimulus to contraction, while the calcium salts are necessary for the maintenance of the systolic tone, and the potassium salts for the occurrence of relaxation. The exact significance of these different salts for the functions of cardiac and other forms of muscular tissue, though they have been the subject of many detailed investigations, must be still regarded as an open question. Fig. 441. A frog's heart poisoned by excess of calcium salts, recovers its spontaneous rhythm on adding a trace of KCl to the perfusion fluid. (Ringer.) The fluids containing the three salts mentioned above in slightly varying proportions are commonly used to maintain the beat in an excised heart either of a cold- or of a warm-blooded animal. In the case of the latter it is necessary to keep the fluid saturated with oxygen. According to Locke the addition of glucose to the solutions enables the beats to go on for a longer period of time, and will in fact renew the rhythm of a heart which has ceased beating while being fed with pure saline solution. The following represent the fluids most frequently used : Ringer's Fluid (for frog's heart) 1 per cent, sodium bicarbonate . . . , .1 c.c. 1 „ calcium chloride . . . . .1 c.c. 1 ,, potassium chloride . . . . . 0-75 c.c. 0-6 ,, sodium chloride ..... to 100 c.c. Locke's Fluid (for mammalian heart) 0-015 per cent, sodium bicarbonate, 0-024 ,, calcium chloride, 0-042 ,, potassium chloride, 0-92 ,, sodium chloride, 0-1 ,, glucose, in distilled water. The influence of the chemical composition of tiio medium on the contraction of the heart may be investigated in the following ways : One of the simplest methods is that employed by Ciotch. represented in the diagram (Fig. 442). The apparatus consists of a small glass jar with inlet and outlet tubes. 964 PHYSIOLOGY A disc of cork is fixed on to a brass rod so that it can be let down into the fluid. On the upper end of the brass rod is poised a light lever with a paper point. To fix the heart in the apparatus, the top of the ventricle is transfixed by a fine hook to which is attached a thread connected with the lever. The heart is fastened to the cork by a pin through the bulbus aortse. The glass jar is filled with the fluid whose action it is desired to investigate. It is usual to start with Ringer's fluid in order to obtain a normal beat, and then to try in turn the various constituents of this fluid. Another method of investigating the action of the heart of cold-blooded animals is by perfusing the heart cavities with the fluid under investigation. Two forms of perfusion are made use of. In the method first introduced by Williams a double Fig. 442. Gotch's frog heart apparatus. Fig. 443. Brodie's perfusion apparatus for the mamma- lian heart. cannula is tied into the ventricle, the rest of the heart being cut away. The tubes leading to and away from the perfusion cannula are armed with valves so as to allow the fluid only to pass in one direction. The contractions of the ventricle may be recorded either by connecting the outgoing tube with a manometer, which may be a mercurial or a membrane manometer, or by connecting some form of recording apparatus with the vessel in which the heart is contained, so as to register changes in the volume of the ventricle. A large number of different forms of apparatus have been devised for these purposes. In another method the fluid is allowed to flow through the whole heart, passing in by the sinus and out by the aorta. Here again the activity of the heart may be registered either by recording the pulsations in the arterial column of fluid or by connecting a tambour or piston recorder with the vessel in which the heart is contained. The heart of warm-blooded animals can also be investigated by a somewhat similar THE CAUSATION OF THE HEART-BEAT 965 method. It was shown by Porter that the inaininalian heart could be kept alive by transfusing oxygenated blood serum through the coronary vessels, and Locke found that the same results' could be obtained by using oxygenated Ringer's solution, modified so as to have the same tonicity as mammalian blood. A convenient apparatu s for this purpose has been devised by Brodie. The apparatus (Fig. 443) consists of a chamber A to contain the heart, and of a tube B, through which the perfusion fluid is carried to the heart. Both are enclosed in a large outer jacket c, through which is kept flowing a stream of water at body tempera- ture. The chamber A is bell-shaped and is fitted into the jacketing tube c by a ground- glass joint D. Its upper orifice is closed by a piece of rubber tubing of such size that the perfusion tube B slips through it easily. By means of the glass handle F, fused into the tube about half-way down, b can be drawn up or lowered into any desired position. To its lower end the heart camiula is attached by a ground joint. Its upper end is fitted, by a second ground joint, with a small bulb w, with two tubes, r and s, fitted into it. These latter are connected by rubber tubing with aspirators containing the solutions to be perfused. The lower half of the tube B is nearly filled up with a thermo- meter L, the bulb of which projects into the heart cannula T. The upper half is almost filled with a piece of glass tubing sealed at both ends, so that the perfusion fluid passes in a thin layer down the tube and thus offers a large surface for heating purposes. Also by filling the interior of the tube in this way its capacity is reduced to a very small amount. The large outer tube c is kept supplied with warm water, entering through the tube G and overflowing through a side-tube at the top into a wide T-piece N. By raising or lowering this T-piece the level of the water in the jacket is adjusted. The water- supply comes from a cold-water tap, but on its passage to G passes through a metal spiral heated by a Bunsen burner. By varying the rate of flow and the position of the burner the temperature of the water can be regulated with considerable accuracy. The upper end of the supply-tube G is provided with a thermometer so that the tempera- ture of the inflowing water can be seen and regulated. In using the apparatus the heart cannula is removed, and the tube B is then passed through E and pushed down until its lower end issues just below the level of the chamber A. The circulation of the warmed water through the jacket is then started and adjusted to the proper temperature. One of the rubber tubes, s. is next attached to the reservoir containing the main perfusion fluid, and the tube B filled with fluid and left to warm while the heart is being prepared. The heart having been excised and washed well in saline so as to remove as much blood as possible, the cannula is tied into the aorta. The cannula is now held luider the perfusion tube, filled with the warm saline, and at once attached in its proper joosition and the perfusion started. A bent pin to which a long thread is tied is hooked into the apex of the heart, and the perfusion tube pulled up until the heart lies quite within the warm chamber. When thus dra^^^l up the bulb w lies just below the surface of the water in the outer jacket. The tube is held firmly in position by a clamp which fixes one arm of the handle f. The heart cannula is pro\'ided with a side opening v, on to which a long piece of fine rubber tubing is passed. This renders possible the removal of any gas bubbles that may collect in the cannula, or the washing out of the cannula with a stream of fluid if necessary. The beats of the heart are recorded by means of a simple lever attached by the thread previously fixed to the heart. THE SIGNIFICANCE OF THE REACTION OF THE BLOOD FOR THE HEART-BEAT. It was loiio; i\ao shown by Gaskoll that the reaction of the perfusing fluid has a marked influence on the frog's heart. AVhen weak acids are transfused througli this heart, tliere is a gradual diniiiuitiou of toiuis, the beats become smaller and fiiuiUy disappear. A similar relaxation may be obtained as the result of the action of carbon dioxide. Weak alkalies on tl\o other hand produce a gradiud decrease of tonus, so that the 966 PHYSIOLOGY heart is finally arrested in a contracted condition. There will therefore be some reaction intermediate between the weak acid and the weak alkaline fluids which will represent the optimum reaction for the beat of the frog's heart. Mines has shown that this optimum reaction difiers for the difierent cavities of the heart, and also for the hearts from different animals, a shifting of the reaction of the transfusing fluid to the acid side always bringing about a diminished contraction and tonus, while the opposite effects are produced by an increase in the alkahne reaction. In the iMMtiimmm*»,mmmmMMf^^^,,^^ ,,,„...,^M.*iiiiiwM*M^ Fig. 444. Volume curve of ventricles (cat) (lower curve). The upper curve is the arterial pressure, maintained by an adjustable resistance at 130 mm. Hg. Between the arrows the air used for artificial respiration was replaced by a mixture containing 20 per cent. CO2 and 25 per cent, oxygen. Note the dilatation with impaired contraction, followed by increased amplitude of contractions. mammal under normal conditions, the chief factor affecting the reaction of the blood is the tension of carbonic acid in this fluid, and an increase in the carbonic acid of the blood, when suflS.ciently pronounced, always brings about dilatation of the heart. The resistance of the hearts of different animals is however not of the same strength. Thus, a dog's heart is much more susceptible to the presence of a small excess of carbonic acid in the blood than is the cat's heart (cp. Fig. 444). There is probably an optimum tension of carbon dioxide in the blood, varying between 5 and 6 per cent, of an atmosphere, at which the physiological condition of the ventricular muscle is at an optimum, but the tension may be reduced considerably below this without causing any marked change in the action of the heart. Yandell Henderson found that vigorous artificial ventilation of the lungs brought about a condition in which the heart's contraction was very forcible and the heart's cavities almost empty. He ascribed this condition to the hypertonicity of the heart- muscle produced by washing the carbonic acid out of the blood. These results were, however, probably due to the mechanical influence of the respiratory movements on the venous filling of the heart, and there seems no reason to believe that the condition of ' acapnia ' (deficiency of carbon dioxide in the blood) had anything to do with the results observed. The improving effects of administration of carbon dioxide described in the first edition of this work have not been confirmed by more recent and accurate experiments. THE CAUSATION OF THE HEART-BEAT 967 THE NUTRITION OF THE HEART In the frog's heart the muscle fibres are supplied directly by the blood within the cavities, the spongy ventricular wall permitting the access of blood between the fibres. In the mammalian heart the muscular tissue is nourished through the coronary arteries, which break up into a meshwork of capillaries around all the fibres. The flow of blood through the coronary circulation may be measured either in the whole animal or in the heart-lung preparation by introducing a cannula through the wall of the right auricle into the coronary sinus and collecting the blood from the latter outside the body. Another method is to feed a heart from the aorta through the coronary arteries -svith blood and collect the total outflow from the cut pulmonary artery. By a comparison of these two methods it is found in the dog that the blood flow through the coronary sinus forms about three-fifths of the total blood passing through the coronary arteries. It is therefore possible to measure the flow through the coronary sinus in the heart-lung preparation under varjang conditions of pressure and output. The figures so obtained multiplied by f will represent approximately the total flow through the coronary circulation. Blood enters the coronary arteries from the aorta both during systole and diastole, though it is probable that the systole of the ventricles exercises a direct effect in increasing the resistance to the flow of blood through the heart and squeezes out the contained blood into the coronary veins. This may be one reason why the flow of blood through the coronary system is greater in a beating heart than in a heart which is quiescent. The most important factor in determining the flow through the coronary vessels is the arterial pressure. The marked effect of this factor is sho^vn in the following Table : Heart weight, 107 gms., total output per minute, 1400 cc. Arterial Coronary circulation pressure per minute 60 50 100 90 128 124 166 208 190 500 We see from this Table that the heart muscle is supplied with blood in proportion to its needs, since its work and its respiratory exchanges increase continuously with the rise of arterial resistance. Indeed, under the severe test of contracting against an average pressure of 190 mm. Hg. over one-third of the whole blood leaving the heart was passing through its muscular walls, one gramme of muscular tissue being irrigated with 5 cc. of blood per minute. Another important factor in determining the coronary flow is the effect of the metabolites produced by the contracting heart-muscle itself. This is well shown when the heart is asphyxiated. Thus in one experiment while the arterial pressure was maintained constant, the total coronary flow was 56 cc. per minute. Artificial respiration was then dis- continued, and during the succeeding minutes the coronary circulation was 61, 72, 150, 180. The circulation then failed. Carbonic acid produces 968 PHYSIOLOGY also an increase in the flow through the coronary arteries, but it is im- possible with the highest attainable percentages oi carbonic dioxide in the blood to produce such an increase in the coronary flow as is observed during asphyxia. The dilatation of the coronary vessels which occurs in the latter condition must therefore be ascribed to non-gaseous metabohtes produced by the contracting muscle. Thus the heart contains in itself a mechanism for increasing the flow of blood through its tissues whenever this becomes inadequate and the muscle is suffering for lack of oxygen. Mechanical and physiological factors thus co-operate in providing the most important muscle in the body with oxygen sufficient for its needs. If a coronary artery be hgatured, the heart very often beats for one or two minutes with unimpaired force, then a beat is dropped occasionally, and very shortly afterwards the heart stops altogether and the blood- pressure falls to zero. On inspection of the heart immediately after the blood-pressure has fallen, its muscular wall is seen to be in a state of fibrillar contractions, or ' delirium cordis.' All the strands of muscle fibres are contracting more or less rhythmically, but the rhythms of no two parts coincide, so that the heart dilates and is incapable of carrying on the circula- tion. It is probably in this way that sudden deaths occur in cases where the coronary arteries are diseased or calcified. In such cases a man may drop down dead, having previously shown no symptoms of heart mischief. Dehrium cordis may be explained as the result of block, produced by interference with the nutrition of a large part of the heart-wall. The con- tractile wave arriving at this part, in some directions will not spread at all, in others will spread at a lower rate, so that different parts of the heart receive the impulse to contract at different times and a state of inco- ordination results. The same condition can be produced by freezing the apex of the ventricle, so causing a block, or by stimulating the surface of the ventricle at a rate which is greater than can be taken up by the ventricle as a whole, as, e.g., by tetanising currents. Such a condition in the higher animals, as the dog and man, is practically irrecoverable, although in the rabbit, and very rarely in the dog, it is sometimes possible to bring the heart back to a state of rhythmic contraction by kneading it rhythmically. According to Mines delirium cordis is susceptible of a simpler explanation. This condition is easily brought on in the mammalian heart by stimulation of its surface by strong f aradic currents. The effects of increased frequency of contraction on the heart muscle is to decrease the rate of propagation and to decrease the length of the wave of excitation. Ordinarily in the naturally beating heart the wave of excitation is so long and spreads so rapidly, that it excites the whole of the ventricle long before it has ceased in any one part. When, however, the muscle is stimulated more frequently the wave becomes slow and short, so that more than one wave can exist at one time in a single chamber. The main factor then in the production of delirium cordis after obstruction of the coronary artery would probably be a diminished rate of conduction through the affected part. SECTION IX THE NERVOUS REGULATION OF THE HEART In order that the activity of the heart may be adapted to the needs of the body as a whole, its automatic mechanism must be subject to the central nervous system, which must be able to affect the heart in either of two ways, viz. by increasing or diminishing its activity. This subjection to the integrative action of the central nervous system is also necessary for the sake of the organ itself ; otherwise the peripheral adaptation of the heart- muscle to changes in arterial resistance might result iu its exhaustion and permanent damage. The regulation is effected through the intermediation of afferent and efferent nerve fibres connecting the heart mth the central nervous system. The importance of these nerves is shown by the behaviour of animals in which they have been extirpated. Thus a dog in whom all the nerves of the heart had been divided survived the operation for eight months, the pulse reading during the time not having appreciably altered and the animal being in a fair condition of health. Although he regained his normal weight after the operation, he was found incapable of carrying out even a moderate amount of work, such as that represented by running, since the mechanism for in- creasing the action of the heart in response to the needs of the muscles had been lost. THE EFFERENT CARDIAC NERVES The heart in vertebrates is supplied with nerve fibres from two sources, from the medulla oblongata along the vagus nerve, and from the upper dorsal region of the spinal cord through the mediation of the sympathetic system. The fibres which run through the sympathetic system take a somewhat different course in the animals on which the regulation of the heart's acti^^ty has been chiefly studied, viz. the frog and the mammal. In the frog (Fig. 445) the sympathetic fibres leave the spinal cord by the anterior root of the third spinal nerve ; they then pass through the ramus communicans to the corresponding sympathetic ganglion, whence they run up through the second ganglion and the annulus of Vieussens to the first ganglion ; they then pass into the cervical sympathetic strand to the ganglion trunci vagi ; here they join the vague and pass down with the true vagus fibres to the heart. 969 31* 970 PHYSIOLOGY In the dog (Fig. 446) the sympathetic fibres leave the spinal cord by the anterior roots of the second and third dorsal nerves, run in the white rami communicantes to the stellate ganghon, and thence by the anniilus of Vieussens to the inferior cervical gangUon. Cardiac branches convey the sympathetic fibres to the heart and are given ofE from the stellate ganghon, the inferior cervical ganghon, and from the trunk of the vagus. By the nicotine method it is possible to trace out the cell connections of these fibres. As they leave the cord they are medullated nerve fibres, ^Jucj. Ganql. Vagus Vago-symparheHc Subclav. a^•^ N.ll Splanchn. n. Infest arf N.Vii. N.VIII, Fig. 445. Sympathetic chain of frog (right side) to show connection with vagus nerve. The sympathetic ganglia with their branches are black. Of the peripheral branches only the splanchnic nerve is represented. (Modified from EcKER.) similar to the other fibres making up the visceral outflow throughout the dorsal region ; the white fibres pass along the ramus communicans to the stellate ganghon, where they end, forming synapses with the cells of the ganglion. Here fresh relays of fibres, which are non-medullated, start and carry the impulses to the heart along the various cardiac nerves just men- tioned. In the heart these fibres are distributed to the muscle fibres without the intervention of any other ganghon-cells. On the other hand, the fibres which leave the vagus to pass to the heart make connection with the cells of Remak's ganglion, and probably all the other intrinsic cardiac ganglia described above, whence non-medullated fibres carry their impulses to the heart-muscle. THE NERVOUS REGULATION OF THE HEART 'J71 ACTION OF THE VAGUS The action of the vagus fibres on the heart is ahiiost identical in frog and mammal. If in the dog the peripheral end of the cut vagus be stimulated while the arterial blood pressure is being recorded by means of a mercurial manometer, the pulse is seen to become slower, or with a stronger stimulus to G.J. G.h.V- r.Sp.Ac. Fig. 446. Diagram of cardiac inhibitory and accelerator fibres in the dog. (From Foster.) r.Vg, roots of the vagus ; r.Sp.Ac, roots of the spinal accessory ; GJ, ganglion jugulare ; G.h.V, ganglion trunci vagi ; Vg, trunk of vagus nerve ; C.Sy, cervical sympathetic ; GC. inferior cervical ganglion ; AV. annulus of Vieussens ; A.sb. sub- clavian artery ; nc. cardiac nerves ; G.St, ganglion stellatum ; D2, D3, D4. Do, second, third, fourth, and fifth dorsal spinal roots ; G.Th, ganglia of the thoracic chain. cease altogether, and the blood-pressure falls towards zero. On discon- tinuing the stimulus the heart begins to beat again and the pressure rises after a few beats to normal (Fig. 447). If the stimulation of the vagus be prolonged, the blood-pressure, on dis- continuance of the stimulus, may rise above normal owing to the asphyxia of the vaso-motor centres produced by the prolonged cessation of the circulation. Even during the application of the stinmlus the heart often begins to beat again with a slow rhythm. In this case we speak of an 972 PHYSIOLOGY ' escape ' of the heart from the vagus influence. This escape is generally confined to the ventricles and the heart-beats are found on opening the chest to be purely ventricular, the auricles and great veins remaining in a state of diastole. Vagus escape is favoured by distension of the heart cavities, and is often synchronous with the respiratory efforts, which supervene after a certain duration of inhibition as a result of the asphyxia of the respiratory centre. When the arterial system is dilated, so that the mean systemic pressure, and consequently the venous pressure during cardiac inhibition, are low, or when the asphyxial gasps of the animal are prevented by anaesthesia or by a Fig. 447. Blood-pressure tracing from carotid of dog (taken with Hiirthle's manometer), showing effect of excitation of vagus (between the arrows). 0, abscissa line of no pressure. section of the spinal cord, the heart may fail to recover from the inhibition produced even by a transitory stimulation of the vagus. In such cases it is necessary to knead the heart in order to restore its rhythmic action. To study the influence of the vagus on the auricles and ventricles respectively, it is necessary to work with the chest opened and to record separately the contractions of the different segments of the heart. It is then found that the vagus may affect the heart in one of several ways. Its most marked action is on that part of the heart where it enters, viz. the venous end. It may affect that part of the auricle corresponding to the primitive sinus venosus, where the rhythm of the whole heart is determined. In this case the sole effect of the vagus on the auricles and ventricles will consist in an alteration of rhythm. They may cease to beat altogether or they may give beats of normal strength but at a slower rhythm than before. Often indeed under these conditions the beats of the ventricles may be increased in size, since the strength and extent of their contractions are determined, not by the strength of the stimulus arriving from the auricles, but by the length of their fibres, and this will increase with any prolongation of the diastolic period, and consequent increased diastolic filhng of the ventricle. If the vagus acts on the auricles without affecting the sinus part of the auricles (sino-auricular node), the rhythm will be unaltered, but the response of the auricles to the impulses received by them will be diminished, and the amplitude of the excursions of the lever attached to them will therefore be THE NERVOUS REGULATION OF THE HEART 973 considerably reduced. Indeed the auricular contractions may be reduced to such an extent that they cause no movement of the lever. It is only by observing their surface that one may perceive a slight contraction of their fibres. Under such circumstances the rhythm of the ventricles will be unchanged. Generally the vagus absolutely stops the action of all parts of the auricles ; in such cases the ventricles also cease beating. Very often after a short pause the ventricles commence to beat at a slow rhythm, and it is then seen that they are contracting independently of the auricles and sinus. That the ventricle is really inaugurating the beat is shown by the fact that occa- sionally one may observe a reversed beat, i.e. a contraction of the auricle following instead of preceding each ventricular contraction. Whether the vagus has a direct action on the mannnalian ventricle is still doubtful ; its effect is at any rate very slight as compared with that on the venous end of the heart. The fact that stimulation of the vagus causes as a rule tem- porary cessation of the ventricular beat, while functional separation of the ventricles from the auricles causes no such temporary stoppage, would seem to indicate that this nerve has a direct, though sUght, action on the ventricles. Finally the vagus may affect the tissue which conducts the excitatory process from one cavity to another. Under vagus stimulation the auricles may beat at a greater rhythm than the ventricles, a block having been pro- duced in the tissue passing from auricles to ventricles, viz. the auriculo- ventricular bundle. Engelmann has described these effects of vagus excitation as negatively chronotropic (diminution of rhythm), negatively inotropic (diminished strength of contraction), and negatively dromotropic (diminished conductivity), and has distinguished a fourth action, viz. one on the irritability of the muscle to dii'ect stimuli, which he calls negatively bathmotropic. He ascribes these four actions to four different sets of nerve fibres, but it is evident that they are due not so much to the difference in the nature of the impulse as to a difference in the place of incidence of the impulse. Thus, if the vagus fibres which are distributed to the remains of the sinus are speci- ally active, we shall get alterations of rhythm affecting the whole heart. If those which supply the A.V. bundle are excited, the most pronounced effect will be on the propagation of the excitatory process from auricles to ventricles. Practically the same description will apply to the action of the vagus on the frog's heart. Since it is easy in this animal to register the contrac- tions of the empty heart it is possible to show that the vagus has a direct inhibitory action on the ventricles, diminishing the strength of its contrac- tion in response to the stimuli transmitted to it from the venous end. This action of the vagus on the ventricle is not, however, universal, and in the tortoise it is impossible to show any such action. In both these animals the auricles show the same effects as in the mammal, viz. an influence limited to the rhythm when only the sinus is affected, or a diminution of the strength of contraction when the sinus is unaffected and the chief action of the vagus is on the auricular muscle. Ever since the discovery in 1845 by the brothers E. H. and E. F. Weber 974 PHYSIOLOaY of the action of the vagus on the heart, much work has been expended with a ^aew to determining the intimate nature of the inhibitory process. In the former neurogenic theory it was supposed that the vagus altered the activity, perhaps by a process of ' interference,' of the ganghon-cells responsible for the origination of the rhythm. Many facts, however, point to the in- hibitory impulses as being continued to the heart-muscle itself. Thus tetanisation of any portion of the frog's ventricle, especially if it be filled with blood, causes an evident relaxation of the part between the electrodes. Application of nicotine to the heart prevents stimulation of the trunk of the vagus from having any influence on the heart, presumably from paralysis of the cells of Remak's ganghon, which he at the termination of the vagus fibres, or of the synapses between the vagus fibres and the ganglion- cells. It is still possible to inhibit the heart by direct stimulation either of the fibres leaving this ganghon in the sino- auricular junction, or of the nerve-trunks which run in the inter- auricular septum. We must conclude therefore that the inhibition of the heart-muscle is peripheral and depends on the direct action of the nerve fibres on the muscle-cells themselves. These nerve fibres are paralysed by atropine, after administration of which no inhibitory effects can be produced by stimulation of nerve or muscle or any part of the heart. On the other hand, muscarine apparently stimulates the inhibitory nerve-endings, and when applied to the isolated auricle or ventricle causes weakening of the beat and finally complete inhibition, an effect which can be removed by its antagonist atropine. Two views have been held as to the essential nature of the inhibitory process. According to that put forward by Claude Bernard, the natural tendency of any tissue during rest is towards anabolism. Activity in- volves disintegration or breaking down of the living material, and this disintegration must be succeeded by a process of building up or anabolism, which restores the tissue to its previous functional condition. On this view the state of inhibition would merely prolong the period of rest intervening between two periods of activity, so allowing a greater time for restitution to take place, with a corresponding improvement in the functional capacity of the tissue. According to Hering and Gaskell a state of anabohsm can be induced in a tissue comparable to the state of sudden disintegration which is associated with activity. Excitation of the vagus nerve does not merely allow the normal process of building up, which goes on during rest, to take place, but actually hastens this process, just as the excitation of a motor nerve to a skeletal muscle induces an active breakdown of the contractile tissue, or the excitation of the augmentor nerve to the heart induces an increased rate of beat and therefore increased functional activity. If stimulation of an inhibitory nerve induces the opposite chemical change to that occurring during activity, one would expect to find that, just as an active part of a tissue is negative to an inactive part, so a part of the tissue which is under the influence of an inhibitory stimulus should be electio-'positive to any part which is not being so stimulated. According to Gaskell this condition is realised in the heart of the tortoise. The auricles THE NERVOUS REGULATION OF THE HEART 075 are brought to a standstill by separating them from the sinus venosus. The apex of one auricle is then injured by heat, and the injured point and un- injured base are led oft" to a galvanometer. The usual demarcation current dependent on the difference of potential between the injured and uninjured portion is thus observed. If the vagus be now stimulated the auricles remain at rest, but the demarcation current is increased, i.e. a positive varia- tion is produced, an electrical condition opposed in sign to that which would take place when the auricles contract. Doubt still exists, however, as to the exact interpretation to be put on this experiment. It was mentioned above that potassium salts promote relaxation of the ventricle, so acting as antagonists to calcium salts. If potassium salts be present in a sufficient concentration in the circulating fluid, the heart is brought to a standstill in a condition of diastole, as if the vagus mechanism were in action. On removal of the excess of K ions the heart at once starts beating again. Howell has shown that dvuring stimula- tion of the vagus the amount of potassium in a diffusible form in the heart-muscle is increased. He has therefore suggested that the action of the vagus in stopping the heart is effected by the liberation of potassium salts. Potassium normally exi^rts in a large percentage in the heart-muscle, but in a combined form, and Howell assumes that stimulation of the vagus effects a dissociation of this combined potassium, so that the liberated ions are able to exert their inhibitor}^ influence on the heart. THE TONIC ACTION OF THE VAGUS If both vagi of a mammal be divided, the heart as a rule beats more frequently, showing that under normal circumstances tonic impulses are constantly descending the vagi and holding the heart's action in check. The extent of the quickening which is produced by section of the vagi varies in different animals and is apparently associated with the conditions of life of the animal and its powers of carrying out prolonged muscular exertions. Thus in the dog or horse the pulse, which is normally slow, may be doubled in frequency by section of the vagi. In the rabbit, which has a frequent pulse, and is only able to run for a short distance, division of both vagi causes very little alteration in the pulse-rate. It is stated that the tonic action of the vagi is much greater in the hare than in the rabbit. This tonic action may be increased by various conditions of the blood, e.g. the presence of drugs, such as morphia. ACTION OF THE SYMPATHETIC CARDIAC NERVES Stimulation of the sjinpathetic cardiac nerves at any part of their course has an effect on the heart the exact reverse of that produced by stimulating the vagi. In most cases the pulse frequency is increased in consequence of the action of these nerves on that part of the heart from which the rhythm starts. The frequency which is attained by maximal stimulation of the accelerator nerves is independent of the previous rate of the heart-beat. The increase in rate involves a shortening of the time of the cardiac cycle, which chiefly affects the diastolic period. The size of the auricular and ventricular contractions may be increased at the same time as their rate. In fact, like the vagus nerves, the sympathetic fibres of the heart can influence either 976 PHYSIOLOGY rhythm or strength of contraction, or the conduction from auricle to ventricle, according to the part of the heart-muscle which is affected. The augmentor effect on the strength of the ventricular beats is often very marked. The sympathetic fibres are much less easily tired than the vagus fibres, and have a longer latent period. Whereas the latent period Fig. 448. Tracings of ventricular (upper curve) and auricular contractions (lower curve). From X to y the accelerator nerves stimulated. Lowest line = seconds. 4a7". Cjo. ■■'^■(1 ■«llliiifi ii^mmm ji:jffi,#w*#M*lilliiliiiiiiliiiiili Fig. 449. Tracing to show effect of stimulation of the vago-sympathetic nerve on the frog's heart. The rhythm is unaltered, but the beats of auricle and ventricle are much decreased in size. On ceasing the stimulation the beats become augmented. (Gaskell.) Fig. 450. A tracing similar to Fig 449. In this case, however, the stimulation caused complete stoppage (inhibition) of both auricular and ventricular beats. (Gaskell.) of the vagus in the mammal is considerably less than one second, that of the accelerator nerves may amount to ten or even twenty seconds (Fig. 448). Hence if the vago-sympathetic of the frog be stimulated, the first effect is inhibition due to vagus action. The vagus nerve-endings then become fatigued, and the influence of the accelerator fibres makes itself apparent, and the heart commences to beat, and the beats become more rapid and forcible than before (Figs. 449, 450). Like the vagus, the sympathetic nerve fibres appear to exercise a tonic THE NERVOUS REGULATION OF THE HEART 977 influence on the heart, so that after extirpation of the stellate gangUon on each side, the pulse frequently becomes permanently slowed. THE ACTION OF ADRENALIN ON THE HEART The medullary part of the suprarenal glands forms and secretes into the blood-stream a substance, adrenaline, which has a marked action both on the heart and blood-vessels and plays therefore an important part in the regula- tion of the circulation. Whether this secretion is a constant one has not yet been fully ascertained, but there is no question that under certain specified Fig. 451. Intraventricular pressure tracings (left ventricle) from dog's heart (heart-lung preparation). The scale shows pressure in mm. Hg. a. Under influence of adrenaline. b. Under simultaneous influence of adrenaline and COo (15 per cent.) (Patteeson\ conditions there may be a marked influx of this substance into the blood- stream. The action of adrenaline on any part of the body is practically identical with that of excitation of the sympathetic nerve supply to the same part. Its isolated action on the heart is best studied on the perfused heart or in the heart-lung preparation. On adding jJ^ mgm. of this substance to the 500 c.c. of blood circulating through the heart-lung preparation, a maxinmm effect is at once produced and this lasts for 15 to 20 minutes. The action is, like that of the sympathetic nerve, accelerator and augmentor. Through its influence on the sinus or the sino-auricular node the rhythm of the heart is markedly increased, in the dog to about 2-iO per minute. At the same time the energy of each contraction is increased. This is especially shown in a heart which is beginning to fail and is therefore undergoing a certain degree of dilatation. Directly the adrenahne reaches the heart the contractions become extremely energetic so that the heart rapidly diminishes in volume, the venous pressure falls, and the blood flowing into it at each diastole is thrown out with violence into the aorta. The more powerful beat enables the output of the heart at each beat to be maintained or even increased in spite of the shorter duration of the systole. With each beat the maximum 978 PHYSIOLOGY pressure in the ventricle therefore rises to a marked extent {see Fig. 451). The strain on the ventricular wall of this sudden contraction which is necessary to empty the heart, during the period of systole, is often so great that small haemorrhages are produced throughout the substance of the inuscle. The stimulation efiect of adrenaline is shown, moreover, by the considerable rise in the respiratory exchanges of the heart under the influence of this substance, the oxygen intake being increased two or three times above that which obtained before the administration of the adrenahne. The action of adrenalin therefore is in general to enable the heart to cope with a bigger strain either in the shape of arterial resistance or increased venous inflow than it could do without the stimulus of this substance. The wonderful adaptation of the heart to its functions is illustrated, moreover, by the fact that adrenahne, which increases the metabohsm of the heart to such an extent, exercises at the same time a dilator effect on the coronary vessels, so that apart from the high arterial pressure and the metabolites produced by the contracting heart muscle, the vessels are dilated by the action of the same hormone which evokes the need for an increased flow of blood through the working muscle. There is thus a marked antagonism in the influence of the two common hormones on the heart, both of them being produced during general muscular activity. Carbonic acid in excess causes dilatation of the heart, diminished functional activity, slowing of rhythm. Adrenahne causes increased func- tional activity, diminution of cardiac volume, and increased rhythm. The action of adrenahne is so pronounced that it is possible to administer 20 or 30 per cent, carbonic acid to a heart-lung preparation without altering its output if adrenaline be administered at the same time. The heart is slowed by the carbonic acid, but the beat is maintained and contraction is effective in emptying the heart of its content. THE HEART REFLEXES The part of the nervous system chiefly concerned in the central co- ordination of the various afferent impulses which act on the heart is the medulla oblongata. It is in this situation that we find the nerve-cells giving origin to the efferent fibres of the vagus nerves, and also the collection of grey matter in which the afferent fibres of the vagus terminate. Direct stimulation of the vagus centre may cause slowing and stoppage of the heart. The tonic influence of the vagi can be abolished by destruction of this centre. In this region we also find the vaso-motor centre, so that the activity of one can affect that of the other. This cardiac centre may be played upon by impulses arriving at it through various afferent nerves or from the higher parts of the brain and giving rise to the changes of the pulse-rate associated with the emotional conditions, or it may be directly affected by the composition of the blood circulating through its capillaries. The nerve- cells which give off the accelerator or augmentor fibres are situated in the intermedio-lateral tract of the spinal cord, near the point of origin of these fibres. We might therefore speak of an augmentor centre in THE NERVOUS REGULATION OF THE HEART 979 this region ; but it seems probable that the activity of these cells is sub- ordinate to impulses arriving at them from the common meeting-place of visceral impulses, viz. the medulla. The most important of the afierent nerves which affect reflexly the action of the heart are the nerves coming from the heart itself and the aorta. In the mammalian ventricle nerve fibres can be seen running over the surface of the ventricle which are entirely afferent, stimulation of their peripheral ends causing no eft'ect on the heart-beat. Stimulation of their central ends may cause one of four conditions : (a) Slowing of the heart. (b) Rise of blood-pressure from constriction of the splanchnic area. (c) Fall of blood-pressure by dilatation of the arterioles of the body. {d) Reflex movements. The heart does not seem to be provided with the nerves of ordinarv or tactile sensibility. There is no doubt, however, that Sup-. Lar. n. - Depressor I -SCO. --Symp. ,--Vaqus Vagus-- Sup-. lar. n.-j --- Sup-. Cerv. Gang. -Depressor Cerv. symp. n. — Vaqo. symp. RABBIT DOG Fig. 452. Diagrams of the connections of the depressor nerve in the rabbit and dog, according to C!yon. It will be noticed that in the latter animal the depressor nerve runs in the vagus tnink together with the sympathetic nerve, for the greater part of its course. under abnormal circumstances impulses arising in the heart can give rise to sensations of pain which are referred not so much to the heart as to the surface of the body over the left side of the chest and left arm, in the region of the distribution of the cutaneous branches of the second and third dorsal roots. An important afferent nerve coming from the heart, or rather from the beginning of the aorta, is the depressor nerve. In the rabbit this rises by two roots, one from the trunk and the other from the superior laryngeal branch of the vagus, and runs parallel with the vagus to the cardiac plexus (Fig. 452). It is purely afferent, stimulation of its peripheral end causing no effect. On stinmlating its central end fall of blood-pressure (Fig. ir>'^) and reflex slowing of the heart are produced, the latter effect being abolished by section of both vagi. It has been shown by Bayliss that the depressor effect is due to universal dilatation of the blood-vessels of the body, the greater 980 PHYSIOLOGY part, however, being played by the splanchnic area. This nerve is probably brought into action whenever the pressure in the aorta is so high as to con- stitute a serious check to the expulsive action of the heart. It is stated that under these conditions a current of action may be detected in the trunk of the depressor nerve, and that if both depressor nerves be cut when the aortic pressure is high the blood-pressure rises still higher. It presents a means by which the heart can be reheved of a load too great for its powers, and therefore dangerous to its future welfare. In many animals the depressor fibres are bound up with the trunk of the vagus and cannot be excited separately. Fig. 453. Blood-pressure curve from rabbit showing effect of excitation of central end of depressor nerve (mercurial manometer). (Bayliss.) Stimulation of the central end of the vagus generally causes reflex slowing of the heart through the cardiac centre and the other vagus. In this reflex inhibition the chief fibres stimulated are those coming from the lungs (Brodie). Inflation of the lungs causes acceleration of the heart — whether due to diminution of the tonic action of the vagi, or to reflex excitation of the accelerator nerves, is not known. Most sensory nerves of the body when stimulated give either a slowing or a quickening of the heart. Stimulation of the fifth nerve, as in the nasal mucous membrane, always causes reflex inhibition. The rate of the heart-beat in the normal animal is closely connected with the blood-pressure. Increase in blood-pressure due to a large vaso- constriction is associated as a rule (but not invariably) with a slowing of the heart-beat. In fact, ' Marey's law ' states that the ^pulse-rate varies inversely as the hlood-pressure. In this slowing of the heart the vagus nerves are of course active. Whether the blood-pressure acts directly on the cardiac centre in the medulla, or reflexly through afferent nerves distributed to the THE NERVOUS REGULATION OF THE HEART 981 aorta and heart cavities, is not yet fully made out. The reflex slowing of the heart often fails to accompany rise of blood -pressure. Thus the rise in blood- pressure and the increased filling of the heart associated with muscular exercise are attended by an increased pulse-rate. THE PULSE-RATE IN MAN The normal pulse-rate in man is about 72 per minute. It is largely influenced by bodily movements. The pulse-rate varies considerably with age. The following Table represents the average pulse-rate in man at different ages : Age in year.s Pulse-rate per minute 0 . . 136 5 .. 88 10-15 . . 78 15-60 . . 68-72 It must be remembered that marked differences in pulse-rate may be found in different individuals without having any pathological significance. Thus pulse-rates of 30 per minute and 120 per minute have been observed in men who were otherwise perfectly healthy. The pulse-rate is raised by warmth and diminished by cold applied to the surface of the skin. It is also increased somewhat by the taking of food. The act of swallowing causes a reflex quickening of the rate by inhibition of the tonic vagus action. SECTION X THE NERVOUS CONTROL OF THE BLOOD-VESSELS During muscular activity the metabolism of the body as a whole, as judged by its gaseous interchanges, may be increased six- or eight-fold. This increase is due almost exclusively to the additional metabolic changes consequent on muscular activity. The muscles therefore during activity require a greater supply of blood in order to obtain from it the oxygen neces- sary for their contraction, and to get rid of the carbon dioxide, which is the end-result of their activity. In the same way every organ of the body requires an increased blood-supply during activity. Blood must be diverted from the inactive to the active tissues. All parts of the body must co- operate in subordination to the activity of that tissue whose function for the time being is of the greatest importance to the organism. This subordination of the part to the whole, i.e. of every part to the organ whose activity is specially evoked by the needs of the whole organism, is chiefly effected through the central nervous system, though local and chemical mechanisms also play some part in the process. Our knowledge of the nervous control of the blood-vessels dates from the discovery by Claude Bernard that nerve fibres run in the cervical sympathetic to the blood-vessels of the head and neck, and maintain them in a state of tonic constriction. Bernard showed that if in the rabbit the cervical sympathetic on one side be divided, the vessels in the corresponding ear dilate. Vessels come into prominence which were previously invisible, and on account of the greater flow of blood thus produced, the ear on the side of the section becomes warmer than the normal ear. If the head end of the divided sympathetic nerve be stimulated, all the vessels of the ear contract, and the ear becomes colder than that of the other side. The fact that the dilatation of the vessels is produced by section of the cervical sympathetic and lasts for a considerable time after any irritant effect of the section must have passed off shows that the ear- vessels are continually under the influence of tonic constrictor impulses proceeding to them along the nerve fibres of the cervical sympathetic. It can be easily shown that these impulses take their origin in the central nervous system. The paralysis of the ear- vessels, though lessening the resist- ance to the flow of blood there, affects too small a vascular area to have any marked influence on the total resistance of the circulation and therefore on the arterial blood-pressure. If the spinal cord be divided on a level 982 NERVOUS CONTROL OF THE BLOOD-VESSELS 983 with the origin of the first dorsal nerve, the blood-pressure sinks considerably. In the dog it may fall from 120 mm. Hg. to 40 or 50 mm. Hg. The heart after the section beats more rapidly than before, so that the fall of pressure must be ascribed to a change affecting the blood-vessels and lowering the resistance to the flow of blood. Since a maximal effect on the blood-pressure is produced by section of the cord at this level, one may conclude that the tonic constrictor impulses to all the vessels of the body pass through this segment of the cord before leaving it to be distributed to the arterial walls. The source of these impulses may be made out by studying the effect of sections through different levels of the nervous system. Division of the cord at about the first or second lumbar nerve causes no effect on the blood- pressure. On making a section at the sixth dorsal root a considerable fall of pressure is produced, almost, but not quite, as great as that observed after section at the first dorsal segment ; stimulation of the lower end of the cut cord causes almost universal vascular constriction and a large rise of blood- pressure. On the other hand, the fall of pressure is maximal when the section is carried through the first dorsal segment or through any part of the cervical cord. Section of the crura cerebri, or of the brain-stem at the upper border of the fourth ventricle, leaves the blood-pressure unaffected. Destruction of a small region of the medulla situated on each side of the middle line in the neighbourhood of the facial nucleus, i.e. in the forward prolongation of the lateral columns after they have given off their fibres to the decussating pyramids, causes an immediate and maximal lowering of the blood- pressure. We must therefore conclude that all the vessels in the body are kept in a state of tonic contraction by impulses arising in this poi-tion of the medulla oblongata, travelUng down the cord as far as the dorsal region, and then passing out of the cord by the dorsal and upper lumbar nerves. This conclusion is confirmed by the fact that, whereas stimulation of the anterior roots of the cervical and lower lumbar and sacral nerves has no influence on the blood-pressure, a rise of arterial pressure can be obtained by stimulating any of the anterior roots from the first or second dorsal to the second or third lumbar. The same effect is produced by stimulation of the white rami communicantes from these roots to the sympathetic system, or by excitation of the sympathetic system itself. The portion of the medulla concerned with the sending out of the tonic vaso-constrictor impulses is spoken of as the vaso-motor centre. In this region it is exposed to and played upon by afferent impulses from all portions of the body, from the higher centres of the brain and the cortex cerebri, and especially by afferent impulses travelling by the vagi from the viscera of the chest and abdomen. Whether in the absence of all afferent stimuli the centre would be active is doubtful ; all we know is that the sum of the stimuli arriving at the centre produces a state of average continued activity, which is responsible for the maintenance of arterial tone and for the regulation of the arterial blood-pressure. The. centre may also be affected directly by changes in its blood-supply. V 984 PHYSIOLOGY or in the composition of the blood flowing through it. Thus anything which interferes with the gaseous exchanges of the centre, whether obstruction to respiration, absence of oxygen in the air breathed, or a failure of the blood- supply, as by ligature of the cerebral arteries, calls forth an increased state of activity of the centre. This can be best studied by observing the changes in the blood pressure produced in a curarised animal by the cessation of artificial respiration. These changes depend partly on the stimulation of the vaso-motor and vagus centres by the venous blood, and partly on the affection of the heart itself. We will first consider them with both vagi cut, in order to shut out the action of the vagus centre. The blood-pressure is registered by means of a mercurial manometer in connection with the carotid artery. On leaving off the artificial respiration, the blood-pressure may remain at the same height for some seconds, the only change noticed being the absence of the respiratory oscillations. Sooner or later the blood-pressure suddenly rises rapidly (Fig. 454, a), and in another ten seconds may reach a height twice as great as it was previously. The heart beats a little more forcibly in consequence of the increased cardiac tension, but its frequency is almost unaltered. The blood-pressure remains at this height for about a minute, and then gradually falls, the heart-beats becoming smaller and smaller, until the pressure has sunk to a point very little above the abscissa line (level of no pressure). This fall in pressure is due to the failure of the heart. The heart, badly suppHed with oxygen, cannot overcome the high resistance presented by the contracted arterioles ; it gets overfilled, and gradually loses the power of expelling any of its contents. If, when the blood- pressure has sunk to its lowest point, the heart be rapidly cut out of the body, it will begin to beat fairly forcibly, being relieved of the excessive internal tension. The vessels, however, remain constricted until the death of the animal. This is shown by two facts. If, while the pressure is sinking, artificial respiration be recommenced, the heart supplied with oxygen at once begins to beat more forcibly, and the blood- pressure may rise to an even greater height than immediately after the commencement of the asphyxia. Again, if the volume of the kidney be recorded by means of the oncometer, the rise of general blood-pressure produced by asphyxia is seen to be accom- panied by a marked shrinking of the kidney, and this shrinking endures until the animal dies, showing that the fall of blood-pressure following the rise is due, not to a giving way of the arterial resistance, but to failure of the heart. Similar results are obtained when the vessels to the brain are ligatured, or when the animal has to respire an indifferent gas free from oxygen, such as nitrogen (Fig. 454, b) or hydrogen. In the uncurarised animal the rise of blood- pressure is associated with increased respiratory movements and finally with convulsive spasms which may involve practically every muscle of the body. We have spoken above of the phenomena of asphyxia as being due to the circulation of venous blood. There are, however, two factors which may NERVOUS CONTROL OF THE BLOOD-VESSELS 985 be concerned and which may influence the medullary centres and the heart. When the renewal of the lung ventilation is stopped by ligature of the trachea or by cessation of the respiratory movements, the increasing venosity of the blood involves a diminished percentage of oxygen and an increased percentage of carbon dioxide, and when asphyxia is excited by cessation of the circulation through the medullary centres, these centres may suffer at the same time from lack of oxygen and from the accumulation of carbon dioxide. The question arises whether one or both of these factors are con- cerned. It is easy to investigate the action of each separately. A pure oxygen lack may be brought about by allowing an animal to breathe some t Resp.off -100 M I f I f M I A -220 /A J l^off^ -/SO lA^ on '100 \ \ Nitrogen \ \ ] \ \ \ \ \ 1 Fig. 454. Blood-pressure changes in a cat. A. after cessation of respiratory move- ments. B, as a result of artificial respiration with nitrogen. (Mathison.) inert gas, such as nitrogen or hydrogen, or in the curarised animal one of these gases may be administered by the pump used for artificial respiration. The effects of accumulation of carbon dioxide in the blood and tissues may be produced by the administration of gaseous mixtures containing excess of oxygen, i.e. 39 to 40 per cent., with varying percentages of carbon dioxide. In the first case, the tension of the carbon dioxide in the blood will be kept below normal ; in the second case, the tension of oxygen in the blood will be kept above normal. In order to obtain results uncomplicated by the influence of anaesthetics, the experiments may be carried out in animals which have been deprived of consciousness by destruction of the brain above the superior corpora quadrigemina. At different times physiologists have been inchned to ascribe the excitatory phenomenon of asphyxia either to absence of oxygen or to excess of carbon dioxide. Mathison has shown that both'conditions may concur in the production of the rise of blood-pressure in asphyxia. In Figs. -lo-I and 15.") the rise of arterial pressure produced by 986 PHYSIOLOGY a short period of asphyxia is compared with that produced by oxygen lack, by a surplus of carbon dioxide, and by the injection of lactic acid into the circulation. There are certain minor details in these curves which are of interest. When the oxygen of the lungs is rapidly washed out with a neutral gas the asphyxial rise comes on about half a minute later than it would with pure asphyxia. In the latter case it seems that the first rise is due to the accumulation of carbon dioxide. The rise, however, under nitrogen when it occurs is extremely abrupt, and the subsequent fall of blood-pressure, i.e. the heart failure, is earlier in onset and more rapid than with ordinary asphyxia. When excess of carbon dioxide is administered, i.e. 5 to 10 220- t /oo- on CO 2 12"^ per cent O2 30 per cent Fig. 455. Asphyxial blood-pressure changes in curarised cat. A, inhalation of CO2. B, injection of lactic acid. (Mathison.) per cent., a marked rise of pressure occurs which, hke that produced by oxygen lack, is almost entirely conditioned by stimulation of the vaso-motor centres and resulting constriction of the peripheral arterioles. If a loop of intestine be placed in a plethysmograph, it will be seen that the rise of pressure coincides with a shrinkage in volume of the intestine, pointing to a vascular constriction (Fig. 456). The rise of blood-pressure due to the vascular constriction may be maintained for a considerable period, e.g. ten to fifteen minutes, and we do not get the rapid fall of pressure due to failure of the heart that is observed in an ordinary asphyxia tracing. If partial oxygen lack or abnormally increased tension of carbon dioxide be continued for some time, a state of narcosis or paralysis finally ensues which affects not only the higher centres but also those of the medulla, so that death may ensue without convulsions or excessive rise of blood-pressure. Is there any common factor in the two conditions of oxygen lack and carbon dioxide excess which may account for the similarity in their effects ? It has been shown that whenever there is a deficiency of oxygen the metabo- NERVOUS CONTROL OF THE BLOOD-VESSELS 987 / off ♦MJ»%ft«V^"''%y ^^ -100 lism of the tissues undergoes alteration, so that as a result of activity, e.cj. in muscles, lactic acid is formed instead of carbon dioxide. Lactic acid can therefore be detected in the blood whenever violent exercise is taken sufificient to produce dyspnoea, or when the access of oxygen is diminished by poisoning with carbon monoxide, or by reducing the tension of this gas in the air breathed. Oxygen lack can be regarded therefore as synonymous with the production of lactic acid. Lactic acid in- troduced into the blood-stream, as is shown in the curve in Fig. 455, b, is equally effica- cious with oxygen lack or with carbon dioxide excess in the production of a rise of blood- pressure indistinguishable from the asphyxial rise. It seems therefore that the common factor in asphyxia is the increased acidity or H' ion concentration of the blood. We shall have occasion to return to this ques- tion in dealing with the regulation of the respiratory movements. If in the dog, and to a less extent in other animals, the vagi be left intact, the blood- pressure tracing during asphyxia has quite another appearance. At the point of the tracing corresponding to the rapid rise in the previous experiment there is in this case only a slight rise of pressure, but the heart begins to beat very slowly. At each beat it necessarily sends out a greater volume of blood than when it is beating more frequently, and hence the oscillations on the blood-pressure curve caused by the heart-beats become very large. This slow beat is due to the action of the vagus centre, and is at once abolished by section of the two vagi. The sparing of the heart by means of this vagus action enables it to last longer, and th3 final fatal fall of blood-pressure due to heart failure comes on rather later than when the vagi are divided. In the increased vagus action which occurs during asphyxia two factors are probably involved. The cardial inhibitory centre in the medulla probably partakes of the general excita- tion of the medullary centres due in the first place to carbon dioxide excess, in the second to oxygen lack. More important is the direct action of the rise of blood-pressure on the medullary centre. The rise of arterial pressure causes increased intracranial tension, and any increase of the latter excites the vagus centre and produces slowing of the pulse^ The vagus slowing is therefore absent in asphyxia if the arterial blood be allowed to escape through a mercury '^valve so as to prevent any rise of pressure in the brain cavity. CO2 7% on Int. Vol. B.P. Fig. 456. Tracing of arterial blood- pressure and of intestinal volume, to show the influence of a moder- ate increase in the CO2 tension of the blood. (Mathison.) 988 PHYSIOLOGY During the period of increased pressure waves are often observed on the blood- pressure curve. These are of two kinds. In the first place, in completely curarised animals we may observe oscillations of blood-pressure corresponding with the respira- tory rhythm before the administration of curare, or if the vagi are cut, presenting a rhythm similar to that usual in animals with divided vagi. These waves are known as the Traube-Hering curves from the physiologists by whom they were first observed. They are certainly due to irradiation of impulses from the excited respiratory centre to the vaso-motor centre in the medulla. In fact, if the curarisation is not complete a slight twitch of the diaphragm, insufficient by itself to have any mechanical influence on the circulation, may be observed to accompany each rise on the blood-pressure curve. Besides the Traube-Hering curves others are occasionally seen which must arise in a slow rhythmic variation of the constrictor impulses sent out from the vaso- motor centre. These waves are known as the Mayer curves, and are not to be confused with the waves on an ordinary pressure curve due to respiration, being much slower in Fig. 4.57. Blood-pressure tracings showing S. Mayer curves. (C. J. Martin.) their rhythm than the latter. They are observed not only during asphyxia, but may occur in blood-pressure tracings from normal dogs, and are frequent in dogs poisoned with morphia. Fig. 457 represents tracings obtained from a dog under the influence of morphia and curare. The upper curve, taken while artificial respiration was being carried on, shows the three forms of curves — the oscillations due to the heart-beat next in size those due to the respiratory movements, which in their turn are superposed on the slow prolonged curves, i.e. the Mayer curves. The lower curve is taken imme- diately after cessation of the artificial respiration, and shows only the heart-beats and the Mayer curves. The presence of Mayer curves may generally be ascribed to a state of abnormal excitation of the vaso-motor centre. This excitation may arise in various ways. A very frequent cause is the one just described, viz. increased venosity of the blood supplied to the centre. Well-marked Mayer curves are often observed in cases of haemorrhage. In spite of the loss of blood, the vaso-motor centres maintain a normal arterial blood-pressure by means of vascular constriction. As the bleeding continues, this means becomes inadequate, and at this point the ' efforts ' of the centres take on a rhythmic character, giving well-marked Mayer curves, just as the arm of a man holding up a weight begins to shake before he is obliged to give way through fatigue. If the bleeding still continues, the pressure sinks steadily and the curves disappear. The curves may also be often observed during operations involving exposure of the cord. NERVOUS CONTROL OF THE BLOOD-VESSELS 989 and may possibly be ascribed in this case to abnormal irritations ascending the posterior columns. The vaso-motor centre may also be directly atfected by drugs such as digitalis or strophanthus, both of which cause a rise in general blcod-preshure from stimulation of the centre. SPINAL CENTRES The great fall of blood-pressure observed after section of the cord in the lower cervical region is not permanent. After one or two hours the pressure begins to rise, and if the animal be kept alive, may attain a height only a little inferior to that found in normal animals. If the spinal cord of such an animal be destroyed, the blood-pressure sinks practically to zero and the circulation comes to an end, because the animal has been, so to speak, bled to death into its own dilated blood-vessels. In addition to the chief vaso-motor centre in the medulla there is a series of subsidiary centres in the spinal cord, centres which we may probably locate in the portions of grey matter situated in the lateral horns of the cord and giving origin to the fibres which go to make up the white rami communicantes. By means of these spinal centres a certain degree of adaptation is possible between the blood-supply of the various parts of the trunk. The important co-ordination between the state of the blood-vessels and the condition of the central pump, the heart, is, however, wanting, since the blood-vessels are now cut of? from the cardiac centres and from the part of the central nervous system which receives the afferent impulses carried by the vagi. The spinal centres, like the chief vaso-motor centre, are susceptible to changes in the composition of the blood suppUed to them. If an animal be kept alive by means of artificial respiration for a Uttle time after division of the cord just below the medulla, the blood-pressure slowly rises as the spinal centres begin to take on their automatic functions. If artificial respiration be now discontinued the asphyxia excites the centres of the cord. The motor discharge to the skeletal muscles reveals itself in a single prolonged spasm, since the respiratory centre is unable to take any part in directing the motor discharges. Simultaneously with the spasm of the skeletal muscles general constriction of the blood-vessels occurs which outlasts the muscular spasms and causes a considerable rise of blood-pressure (Fig. 4:dS). In this rise of pressure the main factor is lack of oxygen, and precisely similar curves are obtained whether the asphyxia be produced by cessation of artificial respiration oi- by administration of nitrogen. The same eliect may be produced by a very large excess of carbon dioxide, or by the injection of acids into the circulation. There is a striking difference between the sensibility of the spinal centres to these substances as compared with the medullary centres. Thus the medullary vaso-motor centre is readily excited by ventilation with .5 per cent, carbon dioxide, whereas a rise of blood-pressure is only obtained from the spinal animal when mixtures con- taining 25 per cent, and upwards of carbon dioxide are employed. The excitation of the medullary centre conies on about thirty seconds after the 990 PHYSIOLOGY administration of nitrogen has commenced in contrast to that of the spinal centres which does not occur until two minutes or more have elapsed. In the intact animal a maximal stimulation of the vaso-motor centre is pro- duced in the cat by the injection of 2 c.c. N/20 lactic acid, whereas 5 c.c. of N/2 acid are required to excite spinal cord centres. Here therefore, as in Fig. 458. Blood-pressure tracing taken by a mercurial manometer from carotid artery of a dog, three hours after section of the cord, just below the medulla oblongata. At o the artificial respiration was discontinued. A general spasm of the skeletal muscles occurred between x and x. The muscles then relaxed, and were flaccid during the rest of the rise of blood-pressure. the medulla, the common factor is probably increased H ion concentration, the excitation threshold for the medullary centres being only about one- fifth that of the spinal centres. The local spinal centres are connected with the medullary vaso-motor centre on each side by tracts of nerve fibres which descend in the lateral columns of the cord. THE PERIPHERAL TONE OF THE BLOOD-VESSELS Division of the sciatic nerve causes an immediate dilatation of the vessels of the lower limbs in consequence of their severance from the tonic activity of the vaso-motor centres. This dilatation passes off in a day or two and the vessels acquire a tone, i.e. remain in a state of average constric- tion which can be increased or diminished by local conditions. This recovery of tone has been ascribed by many physiologists to the existence of a third set of nerve-centres in the walls of the arteries. In the absence of any direct histological evidence of the existence of such centres it seems more rational to ascribe the tonus to the automatic activity of the muscular fibres themselves. THE COURSE AND DISTRIBUTION OF THE VASO-MOTOR NERVES Since the blood-vessels, like the heart, are the seat of an automatic activity, complete nervous control of these tubes can only be secured by the provision of two sets of nerves : one set — augmentor or motor — which will NERVOUS CONTROL OF THE BLOOD-VESSELS 991 increase the state of constriction of the vessels ; another set — inhibitor or dilator — which will diminish the tone of the arteriole muscle and cause vascular dilatation. Our knowledge of the existence of this second class of nerve fibres to the vessels we owe also to Claude Bernard, who observed that stimulation of the chorda tympani nerve not only evoked secretion from the submaxillary gland but also increased the blood-flow through its vessels five- or sixfold. Subsequent researches have revealed the fact that nearly all the vessels of the body receive vaso-constrictor fibres, and that many receive also vaso-dilator fibres. In order to determine the course and distribution of the vascular nerves it is necessary to have means at our disposal for investigating the condition of the blood-flow through different parts and organs of the body. Let us see what effects will ensue on the local circulation by con- striction or dilatation of the arte- rioles with which it is suppHed. If the arterioles a in the organ b dilate (Fig. 459), the first effect is a diminution of the resistance to the flow of blood into the capil- laries beyond. Supposing that the arterial pressure in the trunk C remain constant, a local diminu- tion of resistance in a will at once determine an increased flow of blood through the arterioles, and the fall of pressure from A to the capillaries will be less than when the arteriole was constricted. If the organ is distensible and elastic, the increased pressure in the arterioles and capillaries will cause dilatation of these vessels, and a consequent dilatation of the whole organ. The same effect on intracapillary pressure, and there- fore on the volume of the part, may be caused by obstruction to the flow of blood from the veins. Provided that there is no obstruction to the flow of blood through the vein, and that the general blood-pressure in c remains constant, dilatation of an organ may be taken as an expression of vaso- dilatation in the arteries with which it is supplied. The diminution of the resistance in A may also increase the velocity of the flow through the part, since the amount of blood flowing in a given period of time through any vessel varies directly as the difference of pressure, and inversely as the resistance in the vessel. We can therefore use the following criteria for the occurrence of a vaso- dilatation in the arterial supply to any part or organ : (1) If the surface of the part is translucent, the increased filling of the blood-vessels will cause redness or blushing. (2) The increased size of the vessels will cause an increase in the volume of the organ concerned. Fig. 459. 992 PHYSIOLOGY (3) An increased velocity of blood-flow will, if the part be normally below the temperature obtaining in the central organs of the body, raise its temperature, and vaso- dilatation can thus be detected by the application of the hand or of a thermometer. (4) Any of the methods mentioned in a previous chapter may be used to determine the velocity in the arteries going to the part, and an increased velocity may be interpreted as due to vaso-dilatation. (5) The increased flow through the part may be detected by cutting the main efferent vein and measuring the total volume of blood which flows from it in a given time. Of these methods the two most used are those based on determination either of the volume of the part, or of the venous outflow from the part. ■ to oncometer Fig. 460. Diagram of oncometer. Fig. 461. Diagram of oncograph. A fallacy may, however, arise, unless means be taken to ensure that the general arterial pressure remain constant during the experiment. A rise of general blood-pressure will cause an expansion of the vessels and of the part supplied, and also increased velocity of blood-flow through the part. In all cases therefore where it is desired to investigate the conditions of the local circulation, it is necessary to combine a determination of the general blood-pressure with some means of estimating changes in the local conditions. We may take as an instance an experiment on the blood-supply to the kidney. For this purpose we may use a kidney plethysmograph or oncometer. The structure of Roy's oncometer is shown in Fig. 460. The oncometer is a metal capsule, the two halves of which are hinged together and come in contact at the whole of their circum- ference except at h, where a small depression is left in each half for the passage of the kidney vessels and ureter. A piece of peritoneal membrane is attached to the rim of each half of the oncometer, the space between this and the brass capsule being filled with warm oil. The kidney rests in the oncometer on this bed of warm oil, from which it is separated by a membrane. A tube leads from the cavity between the brass capsule and membrane to a registering apparatus, or oncograph (Fig. 461), which is a piston recorder containing oil. Any swelling of the kidney will drive oil out of the oncometer into the cylinder of the oncograph and so raise the piston, the excursions of which are recorded by a lever writing on a blackened surface. NERVOUS CONTROL OF THE BLOOD-VESSELS 993 Schafer's plethysmograph (Fig. 462), which can be adapted to ahnost any organ of the body, is made of vulcanite * previously moulded to the size of the organ whose volume is the object of investigation. In one side of the box a depression is left sufficient to accommodate easily the vessels, nerves, or ureter going to the organ. The oncometer is covered with a glass lid which is made air-tight by means of vaseline, the space Fig. 462. Diagram of Schafer's air plethysmograph. between the lid and the vessels being also packed with cotton-wool and vaseline. A glass tube is fixed into one corner of the plethysmograph and leads to a piston recorder or tambour. Every variation in the volume of the organ causes a movement of air into or out of the oncometer and thus gives rise to a corresponding movement of the recording lever ^„«,f"w^t,^.S( ^ '"'r V 1 /■ ( ^ ^ . ^ . hi ^ ^' t r ^ jz Z / jZ / / h ' Vm •J / ^'■^ /^ _ 4^ / "■7 K-A i ^ r f i: ^ -, -^4 J- ± I ' y — u 1 i ^ j3l f / / « r fid T T l!. /' fit i L. f ~t ~L -i L.C. C-,-i ^ i/- ^iU- i^t Ljt*.H" fr^^ -1^^ tui it' t,u -ill ii' i^ S- ^ 1 T « 20 30 40 50 60 80 Fig. 496. Dissociation curve of hsemoglobin in various solvents. I, in water ; II, in 0-7 per cent. NaCl ; III, in 0-9 per cent. KCl. (Barcroft.) — 100 5n im cc 2 » X '' " :^ = ^ ^ ^ ^ ^ ^ — ~ lO -r^ ^ -^ Z^ '^ -^ ^ -^ "^ X 'b- / y / X /' .^ ^ KO 1/ In X —/. / / ^ ^ y ;^ ^ / / ^. / / \> / y 10 L / / ^ ^ y y "*- l_t f 7 / 4 ' <^ /' 60 —i / / i / ^ nj / ^ / ^ z'' 10 ( -L / / / T7 1 / / / -^ *0 jT 1 L / / /" , 5 b / ~ 30 _i /- r / n c Z / /' ?0 Jl J- -7 y / yj /- ^ / ■ffl 1 1h y ^ y ,'/, v/_ ^ 1 , ^ I 7 1 ?. z 0 J 3 4 0 5 n 6 1 7 0 R 1 <» T ^(7 n 7 V« ^ yi n Fia. 497. Effect of varying tensions of COg on the dissociation curve of oxyhapmo- globin. The lowest curve is the dissociation at a COo tension of 420 mm. Hg. from observations by Barcroft. (Bohr.) 1062 PHYSIOLOGY carbon dioxide, . It inust be remembered tKat 40 mm. carbon dioxide repre- sents approximately the normal carbon dioxide tension in the blood. It is true that at 150 mm. oxygen pressure the blood is practically saturated with oxygen whatever (within physiological hmits) the pressure of the carbon dioxide. At lower pressures of oxygen, however, the pressure of carbon dioxide makes a considerable difference. Thus at an oxygen pressure of 20 mm. Hg. the amount of oxyhsemoglobin formed is 67-5 per cent, at a carbon dioxide pressure of 5 mm., whereas at a pressure of carbon dioxide of 40 mm. the amount of oxyhsemoglobin is only 29-5 per cent. - In con- ^ — ■^ " " ^ -^ ^ ^ ^ ^ -^ / / y ^ / / ^ , / / / / / / / " 1 i / / f V / / , J / / // / // // U •// / /// // /// ^ Fig. 498. Dissociation curve of sheep's blood. 1, normal blood ; 2, blood containing 0-04 per cent, added lactic acid ; 3, blood containing 0-08 per cent, added lactic acid. sequence of this fact, in the tissues, where the carbon dioxide tension is high, the oxyhasmoglobin will be dissociated with greater ease, so that oxygen will be set free where it is most wanted. We are now in a position to understand how the oxygen is taken up by the blood as it circulates round the pulmonary alveoli. Arterial blood, such as that which fills the pulmonary veins and the systemic arteries, is very nearly {i.e. about 90 per cent.) saturated with oxygen, and will only take up about 2 volumes per cent, more on shaking it with air at the body temperature. Venous blood requires 8 to 10 volumes per cent, of oxygen to saturate it ; but we have already mentioned that, at a tension of 30 mm. oxygen, the blood becomes nearly saturated. The tension of oxygen in the alveoli is considerably above this. In the trachea the tension of oxygen is about \ of an atmosphere (since the air here contains 16 volumes per cent.), and the tension in the alveoli will be a little lower than this. If we take the oxygen tension in the alveoli at \ of an atmosphere,* it will still be something over 100 mm. Hence the venous blood brought to the alveoli by the pulmonary artery will, on there coming into intimate contact * The oxygen tension in the alveoli has been reckoned at about 12*6 per cent, to 13*5 per cent, of an atmosphere. THE CHEMISTRY OF RESPIRATION 1063 with the atmosphere, take up oxygen from it to saturation, or to a point not far removed from it. The blood, thus laden with oxygen, travels to the left side of the heart, and from there is sent through the arteries to all parts of the body. It must be remembered that neither in the lungs nor in the tissues does the haemoglobin come in actual contact wath the source of the oxygen, nor with the cells which it is to supply. In both cases the interchange is effected through the intermediation of the plasma and, in the tissues, of the lymph as well. Since the tissue-elements are constantly using up oxygen, they absorb any oxygen that is present in the surrounding lymph. There is, in consequence, a descending scale of oxygen tensions from red blood-corpuscle through plasma, vessel wall, lymph, and tissue-element. The cell draws from the lymph, and the lymph from the plasma, so that the oxygen tension in the plasma sinks. This has the same efEect as if we put the red corpuscles in a mercurial pump and lowered the pressure of gas. The immediate result is an evolution of oxygen, which is taken up by the plasma, to be in turn passed on to the lymph and the tissue-cell. Under normal circumstances a blood-corpuscle never stays long enough in the proximity of the tissues to lose its whole store of oxygen. If, how- ever, the further supply of oxygen to the blood be prevented, as in asphyxia, the last traces of oxygen disappear from the blood. The enormous avidity of the tissues for oxygen is shown by the following experiment (Ehrlich). If a saturated solution of methylene blue be injected into the circulation of a living animal and the am'mal be killed ten minutes later, it is found on first opening the body that most of the organs present their natural colour, although the blood is a dark blue colour. On exposure to the atmosphere all the organs acquire a vi^^d blue colour. The a^^dity of the tissues for oxygen has been so great that they have been able to decompose the methy- lene-blue molecule, with the formation of a colourless reduction-product, which on exposure to the air undergoes oxidation again and re-forms methylene blue. If the tissues are able to effect the reduction of a com- paratively stable body Hke methylene blue, it is easy to understand their power of reducing oxyhaemoglobin, which is so unstable that it is decom- posed by simple physical means such as exposure to a vacuum. It was long debated whether the chief processes of oxidation took place in the blood or in the tissues. Our experiences with muscle would alone serve to convince us that, in some tissues at any rate, processes of oxidation take place, and the methylene-blue experiment shows that these processes of oxidation are intense in all the chief organs of the body. It has been found moreover that it is possible to keep a frog alive after substituting normal saline solution for his blood, if he be placed in absolutely pure oxygen, and that in this case indeed the metabolism of the animal goes on as actively as before. As the frog has no blood, it is evident that its metabolic processes, consisting of the taking up of oxygen and the giving out of carbon dioxide, must* have their seat in the tissues. As a result of the oxidative changes in the tissues carbon dioxide is 1064 PHYSIOLOGY produced, and the tension of this gas in the tissues therefore rises. AsBarcroft has pointed out, in cold-blooded animals the dissociation of oxyhsemoglobin with the setting free of oxygen must be largely conditioned by the rise of carbon dioxide tension in the tissues, since at the normal temperature of these animals the evolution of oxygen from haemoglobin is extremely slow. The alteration in reaction of the blood caused by a rise in COg tension or by the presence of small amounts of lactic acid, markedly quickens the rate at which oxyhsemoglobin gives up its oxygen, as is shown in Fig. 499. The Fig. 499. Curves showing the rate at which arterial blood is reduced on bubbling through a gas free from oxygen, and the effect on the rate of the presence of COg and of lactic acid. Ordinates = percentage saturation of oxyhaemoglobin. Abscissae = time in minutes. (Mathison.) carbon dioxide tension in the tissues may be approximately measured by taking the tension of this gas in fluids such as the bile or urine. Here it may amount to 8 or 10 per cent, of an atmosphere, and since the carbon dioxide in venous blood is rarely above 6 per cent, of an atmosphere, there is a descend- ing scale of tensions from tissue to blood, just as there is an ascending scale in the case of oxygen. This gas therefore passes from the tissues through the lymph into the blood by a simple process of diffusion. The carbon dioxide carried by the blood is, Hke the oxygen, chiefly in a state of chemical combination. From dogs' venous blood we may obtain by means of the pump about 50 c.c. of carbon dioxide per 100 c.c. blood. Water at the temperature of the body, if shaken up with an atmosphere of carbon dioxide at a pressure of 760 mm. Hg., would take up about 50 per cent, of the gas, and the plasma as a mere solvent would take up shghtly less. The tension of carbon dioxide in the blood is, however, much less than 1 atmosphere. THE CHEMISTRY OF RESPIRATION 1065 Shaken up with pure carbon dioxide at a pressure of 1 atmosphere, the blood would take up as much as 150 per cent. If we determine the tension of the carbon dioxide in the blood by one of the methods to be described later, we find that in venous blood this gas is at a pressure of only about 5 to 6 per cent, of an atmosphere (about 40 mm. Hg.). Taking the pressure of the carbon dioxide as -^^^jof an atmosphere, and kno\ving that at a pressure of 1 atmosphere the blood 50 IS might dissolve per cent., it that at ^g of sphere the blood an volumes evident atmo- would only dissolve | {} volumes per cent., i.e. about 2|- volumes. All the rest of the carbon dioxide in the blood must therefore be in combination (cp. Fig. 500). The carbon dioxide is contained chiefly in the plasma, though a certain amount is also in combina- tion in the corpuscles. Part of the carbon dioxide must be in combination with some constituent common to both plasma and cor- puscles. When blood- plasma is calcined, the ash is found to be distinctly alkahne and to contain an amount of sodium greater than is necessary to com- bine with the other acid radicals, e.g. CI, SO4, and 75 70 65 o 60 55 50 O AS 40 y y^ y y J ^^ // y y ' / y 7^ ,. / / / ^^ --' r / / > / / X / y ,'' // / -■ //' ,' / - ■'' //' iry^ / 30 40 50 60 70 80 COl vrv nvTM. 9Ca. Fig. 500. Curve of CO2 tension in blood. (Christiaijsen, Douglas and Haldajse) This curve shows the influence of the saturation of the haemoglobin with oxygen, on the amount of CO2 taken up by the blood at various pressures. Upper curve = absorption of COo by human blood in presence of hydrogen and COo. Middle curve = absorption of CO2 by human blood in presence of air and C02. Lower curve = absorption of COo in blood of ox and dog in presence of air ; the tluck line A-B represents the absorption of COo by human blood within the body (supposing the blood is completely deoxj'gcnated in the tissues). It is evident that an increase of 15 c.c. per cent, of COo in the blood as it passes through the tissues would raise the tension of this gas in the blood onlj' 22 mm Hg (from 40 to 62). Under normal con- ditions the rise of COo pressure in the blood on passing through the tissues is not more than 5-7 mm. Hg. 90 PO4, and this excess becomes greater if we consider that a great part of the PO4 and SO4 is derived from the oxidation of the sulphur and phos- phorus present in organic combination in the plasma. We may therefore conclude that a considerable part of the carbon dioxide exists in the plasma as sodium carbonate or sodium bicarbonate. In the same way a certain pro- portion of the carbon dioxide which is held by the corpuscles is probably in combination with sodium. Haemoglobin also has the power of combining with carbon dioxide, and unstable compounds may be formed between carbon dioxide and the proteins of the plasma itself. According to Loewy 34* 1-9 c.c. 6-8 \ 12-0 j ~ 18-8 c.c 7-5 \ 11-8/ 19-3 c.c 1066 PHYSIOLOGY one hundred cubic centimetres of arterial blood from the dog would yield normally about 40 c.c. of carbon dioxide. These 40 c.c. would be divided as f oUows : In simple solution in the plasma and corpuscles . , . , , f(a) in corpuscles . As sodium bicarbonate { ,y. . i y(o) m plasma In organic combination with haemoglobin in corpuscles In organic combination with proteins of the plasma It will be noted that although we are deahng here with arterial blood, in which the tension of carbon dioxide is comparatively low, the carbon dioxide is stated to be in combination as bicarbonate. This is on account of the fact that in no case is the alkalinity of the blood equal to that of a solution of sodium carbonate. On the other hand, if we expose blood to a vacuum the whole of the carbon dioxide is given off. If sodium bicar- bonate be exposed to a vacuum, only half of the carbon dioxide is evolved, sodium carbonate itself not undergoing any decomposition in vacuo. If we attempt to extract the carbon dioxide from blood-serum or blood- plasma, we may obtain nearly all the carbon dioxide present, the last 5 per cent., however, requiring the addition of a weak acid such as oxalic or phosphoric acid in order that it may be given off. How are we to explain the difference between the behaviour of blood and the behaviour of a solution of sodium bicarbonate ? We may artificially make a fluid which behaves to carbon dioxide in the same way as blood, by mixing together sodium carbonate and sodium hydrogen phosphate Na2HP04. From such a mixture the whole of the carbon dioxide may be given off when exposed to a vacuum. On the other hand, a large amount of carbon dioxide will be taken up with a very small difference in tension of the gases. The behaviour of the mixture is due to an interaction which occurs between the acid radicals PO4 and CO3. When the mixture is exposed to a vacuum any sodium bicarbonate present will undergo dissociation, carbon dioxide being given off and the carbonate NagCOs formed. This then reacts with the sodium phosphate in the following way : 2NaH2P04 + NagCOg = 2Na2HP04 + COg + H2O. In this way the whole of the sodium enters into combination with the PO4 and the carbon dioxide previously combined is given off. On exposing the mixture to an atmosphere containing carbon dioxide, the reverse change takes place, and we get once again sodium hydrogen phosphate and sodium carbonate, and finally sodium bicarbonate. It was formerly thought that in the blood- plasma phosphates were an important factor in the evolution of the carbon dioxide. Blood-plasma, however, contains the merest trace of phosphates, and the role of a weak acid competing with the carbon dioxide for the sodium is played chiefly by the proteins of the plasma. We can in fact, by adding proteins to a solution of sodium carbonate and exposing the mixture to a vacuum, obtain the evolution of practically all the carbon THE CHEMISTRY OF RESPIRATION 1067 dioxide previously in combination with the sodium. In the corpuscles both haemoglobin and proteins play the part of a weak acid. When plasma is exposed to a vacuum it is necessary, as we have seen, to add a httle acid in order to obtain the last traces of carbon dioxide from the fluid. Instead of adding a weak acid, haemoglobin or red blood-corpuscles may be employed. In the latter case it seems probable that the action on the carbonates of the plasma is due not so much to the haemoglobin as to an interchange of acid radicals between the corpuscles and the plasma. If carbon dioxide be passed into defibrinated blood the alkalinity of the plasma increases while the chlorides diminish, and it is probably the reverse interchange of radicals between corpuscles and plasma which is responsible for the evolution of carbon dioxide on the addition of corpuscles to plasma in vacuo. THE ALKALINITY OF BLOOD. Blood-plasma is generally described as shghtly alkahne, and its alkahnity is measured in terms of deci- or centi- normal acid. The term alkahnity is relative. Caustic alkali owes its alkahnity to the presence of OH ions. The neutrahty of distilled water is due to the presence of practically equal amounts of H and OH ions in the fluid. We may measure the concentration of H or OH ions in a fluid electrically.* If this electrical method be apphed to blood or blood-plasma it reveals either of these fluids as practically neutral, i.e. there is httle or no greater concentration of H or OH ions in blood than in distilled water. The * By the use of diflferent indicators we may arrive at some conclusion as to the approximate concentration of hydi'ogen ions in any given liquid. In the following Table are set out, from a paj>er by Roaf, the colours of a number of different indicators, and the degree of acidity which is sufficient to change their colours : Indicator Acid colour Transitional colour Alkaline colour Hydrogen ion concentra- tion at whicli colour change begins Dimethylamido- azobenzol [- Red Orange Yellow 1 X 10-3 Congo red Blue 1 Purple and brown 1 Red 1 X 10* Vesuvin brown . Brown — Yellow 1 X 10-" Gallein . Colourless Pink Red 1 X 10-" Na alizarine sulphonate 1 Yellow Orange Red 1 X 10-* Lacmoid . Red Purple Blue 1 X 10-" - 1 X 10-8 Rosolic acid Yellow Orange Red 1 X 10-6 Litmus Red Purple Blue 1 X 10-5 _ 1 X 10-8 Neutral red Red Orange Yellow 1 X 10-8 Alizarine . Yellow Orange Red 1 X 10-8 Phenolphthalein Colourless Pink Red 1 X 10 9 It should be remembered that in distilled water of the highest state of purity the concentration of H and OH ions respectively is about 1 x 10-'. 1068 PHYSIOLOGY reaction of blood as normally taken depends as a matter of fact on the indicator whicti is used. Thus blood-plasma is acid to phenolphthalein, but alkaline to htmus. On the other hand, the carbon dioxide and proteins in combination with the sodium may be easily replaced by stronger acid radicals, and if the carbon dioxide be allowed to escape, very little change in the reaction, i.e. in the concentration of H or OH ions respectively, will be produced. We may thus speak of blood-plasma as actually neutral, though potentially alkahne in that it can neutralise a considerable amount of acid. This potential alkalinity is more important than the actual alkahnity, since on it depends the power possessed by the blood of carrying carbon dioxide from the tissues to the lungs. By adding a fixed acid to the blood we may use up this potential alkahnity and finally arrive at a point at which each addition of acid makes a proportionate increase in the actual acidity, i.e. in the concentration of H ions. From this time forward, however, the plasma has lost its power of binding carbon dioxide and can carry this gas only in simple solution. On exposure of such plasma to carbon dioxide the tension of the gas rapidly rises in the fluid, which becomes saturated when only a relatively small amount of gas has entered into the fluid. Any diminution of the so-called alkalinity of the blood or blood- plasma will seriously impair the function of the blood in respiration. An example of such a condition is the acidosis which occurs in diabetes, or in any other form of carbohydrate starvation. EXCHANGE OF GASES IN THE LUNGS A fluid gives off gas to or takes up gas from any other medium with which it is in contact, according to the relative pressures of the gas. The question arises whether the physical conditions in the lungs are such as to account for the absorption of oxygen and the evolution of carbon dioxide by the blood in its passage through these organs. In order to answer this question we must know the partial pressures or tensions of oxygen and of carbon dioxide in the alveolar air, in the venous blood coming to the lungs, and in the arterial blood leaving the lungs. In the alveoli the pressures are given by the analysis of alveolar air. The determination of the gaseous tensions in the blood presents, however, considerable difficulty. It is necessary to bring the blood in contact with gaseous mixtures containing various proportions of the gas whose tension in the blood it is desired to measure. By making various experiments a gaseous mixture will be found with which the blood is in equilibrium. If we know beforehand the amount of gas in this mixture, we know its tension and therefore the tension of the gas in the liquid. Pfliiger's aerotonometer (Fig. 501) consists of two glass tubes, R and R, contained in a vessel filled with water at the temperature of the body. The upper ends of the tubes are connected by the tube a with the artery or vein in which it is desired to estimate the tension of the blood-gases. If, for instance, we wish to determine the tension of CO2 in venous blood, where we expect the tension to amount to about 4 per cent, of an atmosphere, one tube R is filled with a gaseous mixture containing 3 per cent. CO2, and THE CHEMISTRY OF RESPIRATION 1069 the other tube R with a mixture containing 5 per cent. COo. a is now connected with the distal end of the jugular vein, or with the central end of the carotid artery, and blood is allowed to flow in a thin stream down the walls of the tubes R and R, thus presenting a large surface to the contained gases. The blood collects in the lower narrower jiortions of the tubes, and runs out into the vessels h, b, whence after defibrina- tion it is returned at intervals into the veins of the animal. Bohr's aerotonometer was built on the plan of the Stromaiche devised by Ludwig, and could be inserted in the course either of an artery or of a vein. In using this instru- ment it is advisable to inject some sub- stance like peptone or, better, hirudin, in order to prevent coagulation of the blood. In all these instruments the main difficulty is in obtaining a sufficient surface of the blood exposed to the gaseous mixture. The interchange of gases is thus very slow, and it is difficult to be certain at any time that the blood and the gas with which it is in con- tact are really in equihbrium. Krogh therefore adopted an ingenious de- vice of limiting the volume of air to a small bubble, the superficial area of which is large in proportion to its bulk. This bubble, after it has been in a stream of blood for some minutes, is transferred to a special capillary tube in which its analysis can be carried out with a fair degree of accuracy. The performance of a tonometer may be expressed by the ratio of the surface of blood exj)osed to the volume of the air used. The ' specific surface ' of an aerotonometer area in sq. cm. Fig. 501. Pfliiger's aerotonometer. is represented by The specific surface of Pfliiger's instrument is only volume in c.c. 3-3 and of Bohr's only 5-2. In Krogh's microtonometer the absolute volume of air employed is reduced to a bubble of about 2 mm. in diameter, having a volume of -004 c.c. and a surface of 0-125 sq. cm., so that its specific surface is 30. In such a bubble the equalisation of the tensions takes place with extreme rapidity and only a minute quantity of fluid is necessary. The microtonometer consists of the tonometer proper and the apparatus for the microanalysis of the gas bubble. In the latter the measure- ment of the gas bubble is carried out in a capillary tube, the absorption of carbon dioxide and of oxygen being carried out in the usual way with potash and with pyrogallic acid. The tonometer is represented in Fig. 502. It is kept in a small water-bath at the tem- perature of the blood to be examined. The tonometer is filled with saline solution and contains the gas bubble 2, which can be drawn up by mean.s of the screw 4 into the narrow graduated tube 3, where its volume is measm'ed. The blood from the artery or vein, in which we wish to examine the tension of the gases, passes by a cannula through the tube 1, and enters the tonometer as a fine jet. It forces its way up above the gas bubble, which is pressed a little down by the current, and kept oscillating with great rapidity. From the tonometer the blood flows back througii the tube 7 and is collected in a vessel wliere it can bo measured and afterwards drawn off and reinjected into the 1070 PHYSIOLOGY animal if necessary. Since the total pressure of the gases in the blood is nearly always negative, it is necessary to keep the pressure in the tonometer also negative.* This is accompUshed by means of a mercury valve and can be regulated to any desired pressure. During the comrse of a tonometric experiment the volume of the gas bubble is measured from time to time by di'awing it up into the graduated tube, and the pressure is regulated until the volume of the bubble remains constant. After five minutes gaseous equilibrium vdll have been established between the gas bubble and the surround- ing blood, and it is only necessary then to draw it up into the graduated tube and analyse it in order to determine the tension of the gases in the blood. Clotting of the blood is prevented by the injection of hirudin. B Fig. 502. a, Krogh's microtonometer. b, upper part of microtonometer showing capillary tube into which the bubble is returned for measurement and analysis. In the experiments the tension of the air in the alveoK of the animal's lungs or in the bifurcation of the trachea was determined by taking samples of the air. The results of the experiments show that the tension of the gases in arterial blood follows closely the tension of the corresponding gases in the alveolar air. The tension of carbon dioxide in arterial blood is either identical with or very shghtly above the tension of the gas in the alveolar air. The oxygen tension of the blood is always lower than the alveolar oxygen tension, and the difference is generally 1 to 2 — even 3 to 4: — per cent, of an atmosphere. The results of a series of determinations of the tensions of the gases in the blood and alveolar air respectively are given in Fig. 503. In Fig. 504 a and b (Krogh) the composition of the alveolar air was artificially * Otherwise the whole bubble would gradually go into solution and disappear. THE CHEMISTRY OF RESPIRATION 1071 altered by increasing the percentage of carbon dioxide and of oxygen respectively. It will be seen in each case that there was a corresponding alteration of the tension in the arterial blood, the tension of carbon dioxide 30 •tO 50 20 30 to SO Fig. 503. Tensions of Oo and CO2 in alveoli compared with those in arterial blood of rabbit. The dotted lines represent the tensions in the alveolar air, the uninterrupted lines the tensions of the gases in the arterial blood. (Krogh.) being higher and that of oxygen lower in the blood than iu the air throughout the experiment. We have no direct determinations of the tensions of the gases in the blood of man, though an approximate valuation of these tensions \kl ^OSS yf7 % O2 1$ /n inspired w ; /'' air IT ;/ 16 •1 •1 i '■ it) 1/ li- ' \u 13 \ \ 12 ■ .-.- ^ V " 11 ■ 1 10 0- Oz Z + — ^23^ /JO *o io iO 30 *■ 'a / to / , ^ y' C '' > J^ ^^ I 0 2 0 3 0 4 0 5 0 « 0 7 0 9 0 9 0 Fig. 508. Dissociation curve of oxyhsemoglobin in defibrinated cat's bleed. 1, cat I, after partial occlusion of trachea and fifteen minutes breathing of gas of increasing poverty in oxygen ; 4, cat II, at beginning of experiment ; 3, cat II, after fifteen minutes gas respiration ; 2, after twenty-one minutes ditto. and extending thence to all the spinal centres with the production of muscular spasms. At this stage too there is a discharge of impulses giving contraction of the pupil, and a discharge along the whole sympathetic system, producing the various phenomena of vaso-constriction, erection of hairs, sweating, sahvation, which are generally brought about by stimulation of different parts of this system. The phenomena of the third stage are due to exhaus- tion of the nerve-centres, accompanied or preceded by exhaustion and dilata- tion of the heart, the circulation faihng before the excitation of the lower centres has entirely come to an end. In this third stage it is impossible by the strongest stimuh to evoke any reflex. Considerable discussion has taken place as to the exact nature of the stimulation brought about by want of oxygen. The blood of animals which have been killed by asphyxia is known to contain reducing substances, so that oxygen added to it disappears and cannot be recovered in a vacuum. Pfiiiger therefore suggested that it was these reducing substances themselves which were effective exciting agents. It was shown many years ago by Hoppe-Seyler and his pupils that in conditions of chronic oxygen REGULATION OF RESPIRATORY MOVEMENTS 1087 starvation there was an excessive production of lactic acid in the body, and we have seen that the same is true for the isohxted muscle and that to these substances has been ascribed (Zuntz and Geppert) the excitation of the respiratory centre which occurs in violent muscular exercise. Haldane has suggested that in the hyperpnoea and con- vulsions which occur as the result of breathing mixtures with very low percentages of oxygen the effective stimulus is also lactic acid. Experiments were carried out by Ryffel on individuals who had been subjected in a respiratory chamber to very low oxygen tensions, sufficient to cause cyanosis, so that their oxygen alveolar tension was only about 6 per cent. After an experiment, lasting four hours, there was a definite increase of lactic acid in the blood of the fore arm (up to 23-6 mg. lactic acid per 100 c.c). After one lasting only fifteen minutes, in which the oxygen shortage became very marked, no increase could be detected. When we expose an animal such as a rabbit to low percentages of oxygen, the hyperpncea so produced disappears almost immediately when a larger percentage of oxygen is sui)plied to the animal, whereas that produced by carbon dioxide excess dies away slowly on exjiosure to normal conditions. It would seem that when the exposure to low oxygen tensions is of short duration no lactic acid is pro- duced in the blood. If therefore we ascribe the hyperpnoea to the production of lactic ac'd we must locate the production of this acid in the respii'atory centre itself. There are no inherent improbabilities in such an assumjition, but it is difficult at present to see how it can be put to the test of experiment. In dealing with the question of the blood alkalinity we defined neutrality as a con- dition in which there were equivalent concentration of H and OH ions. In the blood the H ion concentration is about 0-3 x 10"'^iSr. The alkalinity is expressed by concentration OH ions ——-: — TT ■ • The acids and bases of the blood-serum and of the tissue-fluids concentration H ions ^i^uo generally are in such proportions as to maintain the approximate neutrality of these fluids even after considerable additions of acid or alkali. This hydrochloric acid may be added to the extent of -025 N, or NaOH to the extent of -005 X, without causing any marked alteration in the reaction of the blood. Although, however, the change pro- duced by the addition of acids or alkalies is so minute, it is appreciable by electrical methods, and it may still more readily be appreciated by and act as a stimulus for the cells of the body themselves. Thus we have not yet succeeded in determining electric- ally the change in hydrogen ion concentration caused by the change from arterial to venous blood. If, however, blood-serum be saturated with carbon dioxide at a full atmosphere, the concentration of the hych'ogen ions rises to 1 -4 x 10 " "X, while after re- moving the greater part of the carbon dioxide from the same serum by the passage of a stream of aii', the concentration of the hydrogen ions sinks to -008 x 10~"X. As the respiratory centre responds to such minute changes of concentration as would be ex- pressed by a difference of 0-2 per cent, of an atmosphere in the carbon dioxide tension of the circulating blood, it must possess a sensitivity greater than any of our physical means for measuring the concentration of hydrogen ions in a fluid. We may approach this delicacy of reaction by using a large molecule as our indicator. Thus, as we have seen, the dissociation curve of haemoglobin is sensitive to the change in reaction caused by raising the tension of carbon dioxide in the haemoglobin solution bv 10 mm. Hg. (cp. Fig. 497). The regulating factor in the blood is jH-obably not carbon dioxide nor any sijecial acid, but the concentration of hydrogen ions in this fluid or in the cells of the centre itself. Such a conclusion brings under one head all the several factors which we know to act upon the resjju'atory centre, namely, tension of carbon dioxide, presence of acids in the blood — especially lactic — and considerable diminution of oxj'gen supplj' to the cells. The resph'atory centre would then not differ qualitatively from an}' other pjirt of the central nervous system. Its special function would be determined simply by the evolution to a marked degree of a sensibflity to hydrogen ions which is already possessed by the whole of the central nervous system and indeed by practically every tissue of the body. We may conclude that mere lack of oxygen is not to be regarded in itself as an excitatory agent. Its influence will be rather to paralyse all 1088 PHYSIOLOGY activity. On the other hand, excitation is caused by the products of metaboHsm, which vary according as the oxygen supply is ample or in- sufficient for the needs of the cells. In the former case activity results in the production of carbon dioxide, in the latter of lactic acid, and perhaps other substances. Both these are acid substances and their production will therefore raise the concentration of the hydrogen ions in the cells where they are produced as well as in the blood. The nerve-centres are extremely sensitive to minute changes in the hydrion concentration either in themselves or in the fliiids surrounding them, and are thrown into activity by excess of these ions and inhibited, or put to rest, by relative deficiency of the ions. In their relation to H and OH ions respectively the medullary centres have a sensibiHty five times as great as the spinal centres. The condition of apnoea, which is associated not only with cessation of respiratory movements but also with fall of blood-pressure, may be ascribed to relative increase in the OH ions or diminution in the H ions. Since the animal has developed a mechanism by means of which changes in the reaction of the blood can be rapidly adjusted by varying the excre- tion of carbon dioxide, whilst the excretion of other acids is relatively slow, carbon dioxide may be regarded as the normal respiratory hormone, and so far we may agree with Henderson in regarding carbon dioxide as maintain- ing the activities of the various nerve-centres at their normal level. But it is the hydrion concentration which appears to be the essential factor, and the acid substances produced during oxygen lack are equally efficacious, but not so convenient. Thus their production is not a steady process Hke that of carbon dioxide, but, as Mathison points out, commences suddenly at a time when the executive side of the nerve-cell is feehng the effect of oxygen starvation, so that the cell may be too much disorganised to respond to stimulation. " The broad margin of safety protecting the organism against paralysis of its cells by oxygen starvation is assured by the sensitiveness of the medullary centres to hydrogen ion concentration and therefore to carbon dioxide in common with other acids." On the other hand, it must be remembered that excessive production of hydrogen ions may finally result in a condition of paralysis, which in the nervous centres is expressed by narcosis. These effects can only be removed by a free supply of oxygen. The concentration at which these results occur varies, as we have seen, in different parts of the nervous system and also in different tissues. Thus on the heart a slight increase in H ion concentration causes diminished tone, which may lead to dilatation and failure of this organ. The same effect is produced on the unstriated muscle fibre of the blood- vessels. Since in the heart and blood-vessels the reverse effect is produced by increasing the OH ion concentration, it is evident that the fine of ' physiological ' neutrahty at which neither stimulation nor paralysis is produced must vary in different tissues. It is an interesting question whether the electrical excitation of nerves may not be due to a similar alteration in the hydrion concentration at the REGULATION OF RESPIRATORY MOVEMENTS 1089 cathode which is the seat of stimulation. If this were so, all the activities of protoplasm might be regarded as determined by the relative concentration of the H and OH ions within the cells or in the medium surrounding the cells. THE REFLEX NERVOUS REGULATION OF RESPIRATION Although the specific sensibility of the respiratory centre to COg is the most important factor in determining the depth and rhythm of the respiratory movements, these movements and the condition of the respiratory centre §^ Fig. 509. Normal tracing of diaphragm slip (Head's method). itself are modified in a large degree by impulses arriving at the centre along both vagi. Through other sensory nerves of the body the respiratory movements can be altered reflexly, but it is only through the vagi that a con- tinuous stream of impulses passes to the centre under normal circumstances, so that every respiratory movement is modified by these impulses. In studying the nervous mechanism of respii'ation, it is necessary to have some accurate method of recording the respiratory movements. They may be registered by means of a tambour applied to the chest, communicating with another tambour i)rovided with a lever, which is arranged to write on a blackened surface ; or a side tube to a cannula in the trachea may be connected with the registering tambour. In the first case movements of the thorax are registered ; in the second changes of intra-pulmonary pressure. These methods are obviously useless when it is wished to study the effects of artificial distension or collapse of the lungs. In this instance we may use the ingenious method described by Head. In the rabbit a slip of the diaphragm on either side of the ensiform cartilage is so disposed that the end of it maj^ be freed and attached by a thread to a lever without injury to its blood- or nerve-supply. It is found that this slip contracts synchronously with the rest of the diaphragm, so that it serves as a sample of the diaphragm, the contractions of which may be recorded uninfluenced by passive movements of the chest wall or artificial increase of intra-pulmonary pressure. If, while the respiratory movements are being recorded in one of the afore-mentioned ways, both vagi be divided,* a marked change in the respiratory rhythm is at once seen. The first effect is an increased in- spiratory tonus, but this rapidly disappears, and the respiratory move- * The division of the vagi is best effected by putting them on a hooked copper wire, of which the upper end is inserted in a freezing-mixture. In this way complete func- tional division of the nerves is obtained without any excitation. If the nerves be cut, a certain amount of stimulation takes place in consequence of tlie closure of the demarca- tion current produced bv the cross-section. ' 35 1090 PHYSIOLOGY ments become less frequent and are increased in amplitude. If now the central end of one of the vagi be stimulated with an interrupted current, the respiration may be quickened, or, as is more commonly the case, the in- spiratory movements may be increased at the expense of the expiratory, so that finally a condition of inspiratory standstill is produced, and the shp of the diaphragm enters into prolonged contraction. With a very weak stimulus it is sometimes possible to produce augmenta- tion of the expiratory movements or rather inhibition of the inspiratory, and this is the invariable result of passage of a constant current through the vagus in an ascending direction. This effect may be more striking^ly brought about by stimu- lation of the central end of the superior laryngeal nerve, which produces first an inhibition of inspiration, so that the re- spiratory muscles come to a standstill in the position of expiration, and then a forcible contraction of the expiratory mus- cles. This illustration of the presence of expiratory fibres in the superior laryngeal nerve is not confined to laboratory experi- ence, but is constantly occurring in every- day life. The superior laryngeal nerve supphes sensory fibres to the mucous membrane of the glottis, and we know that the sHghtest irritation of these fibres — ^the presence of a crumb or a particle of mucus — causes forcible expiratory spasms, with spasmodic closure of the glottis, which we term a cough.* So we see that the vagus nerve contains two kinds of afferent fibres, or at any rate afferent fibres with two distinct functions. Stimulation of the one kind stops inspiration and produces expiration ; stimulation of the other stops expiration and produces inspiration. Since section of both vagi causes slowing of respiration, impulses which exert some influence on the respiratory centre and quicken respiration must travel up the vagi from the lungs. The respiratory movements cause an alternate distension and contraction of the lungs, and it has long been thought that it is these changes in the volume of the lungs which start the accelerating impulses that travel Fig. 510. Effects of distension- collapse of lung. Both curves are described by a lever attached to a slip of the diaphragm of a rabbit. A contraction of the diaphragm (inspiration) raises the lever ; dur- ing relaxation of the diaphragm the lever falls. In A, the trachea is closed at x, the height of inspiration ; * a pause follows, during which the lever gradu- ally sinks until an inspiration (a very powerful one) sets in. In B, the trachea is closed at the end of expiration, x ; there follow powerful inspirations. (Foster.) * It mu.st not be imagined that the fibres of the superior laryngeal nerves are con- cerned in the reflex maintenance of the normal respiratory rhythm. They are cited here merely because the result of their stimulation resembles that which would be caused by stimulation of the analogous expiratory fibres which run in the trunk of the vagus from the lungs to the respiratory centre. REGULATION OF RESPIRATORY MOVEMENTS 1091 up the vagus nerves. To test the truth of this hypothesis it is necessary to study the two phases of respiration separately ; that is, to see first the result on the respiratory inapulses of distension of the lungs, and, secondly, the result of a sudden collapse or a contraction caused by sucking air out of the lungs. The effects of distension or collapse of the lung may be shown by ?imply closing the trachea at the end of inspiration or of expiration. The results of such an experiment are shown in Fig. 510, A still more marked effect is produced if the lungs, by means of a tube in the trachea, be artificially inflated or if air be sucked out of them. The inflation produces an instantaneous and complete relaxation of the dia- phragm (Fig. 511) which by clamping the tracheal tube may be prolonged Fig. 511. Positive ventilation. (Hkad.) Under the influence of positive ventilation, the inspiratory contractions of the diaphragm become less and less till they disappear completely. Xi'g. vcutilation Jiuplinigm Fig. 512. Negative ventilation. (Head.) At a negative ventilation was commenced. The expiratorj^ relaxation of the diaphragm is seen to become more and more incomplete, until it finally enters into continued contraction, for several sconds, while sucking air out of the lungs causes a tonic contrac- tion of the diaphragm (Fig, 512), Somewhat similar results may be obtained by repeatedly inflating or deflating the lungs (positive and negative ventila- tion). The effects here are comphcated by the fact that one is deahng in both cases with alternating movements of the lungs, of expansion and con- traction, both of which will have an influence on the respiratory centre. Moreover repeated forcible inflation of the limgs increases the ventilation of the pulmonary alveoli, thus lowering the normal carbon dioxide tension of the lungs. As a result of repeated ventilation we may obtain a condition of respiratory standstill. In this condition, however, as we shall see later, the -determining factor is rather chemical than mechanical. These inhibitory and augmentor effects of changes in the volume of the lung must also result from the normal movements of these organs in 1092 PHYSIOLOGY respiration. Let us consider, for instance, what will happen if the influence of the two vagi could be suddenly thrown in after these nerves have been divided. (This experiment can, in fact, be reahsed more or less completely if the functional division of the vagi be effected by cooling or by ether narcosis). The animal would be breathing slowly and deeply. If at the beginning of an inspiration the vagi became functional the expansion of the lungs caused by the inspiratory movement would send inhibitory impulses up to the vagus centre, which would stop the movement of inspiration. The movement of expiration would then begin, and the collapse of the lungs thereby produced would itself send impulses up the vagi which would tend to excite an inspiratory movem.ent. Both inspiration and expiration would therefore be shortened, and the successive movements would follow one another at a shorter interval than if the vagi were not functional. In this way, under normal circumstances, the rhythm of the respiratory centre must be determined reflexly through the agency of the vagi, while the chief factor in determining the total pulmonary ventilation is, as we have seen, the carbon dioxide tension of the blood. In the foregoing account we have spoken of the expiratory and inspiratory effects of the vagus as if they were of equal importance. It seems probable, however, that the inhibitory or expiratory impulses started by the inspiratory movement, the only or the more active part of normal respiration, play a more prominent part in the regulation of respiration than do the inspiratory impulses ; and one observer (Gad) goes so far as to deny altogether the existence of two kinds of respiratory fibres in the vagus. Accord- ing to Gad, the vagus, as regards the respiratory centre, is a purely inhibitory nerve. Hence the primary effect of dividing both vagi is an increased inspiratory tone. This view at first seems paradoxical, in that it explains the final slowing of respiration after section of the vagi as due to the cutting off of previous inhibitory impidses. But inhi- bition in all tissues has a twofold effect. Although the immediate effect is diminution of activity, yet the diminished disintegration necessarily associated with diminished activity means an increase of the anabolic at the expense of the catabolic processes of the tissues. In this way we explained the diminished excitability occurring in a nerve at the anode of a constant current, and it will be remembered that the secondary result of anelectroronus was increased irritability and consequent excitation at break of the constant current. The same sort of process must occur in the respiratory centre. A continued restraint of its rhythmic activity must lead to a heaping up of its irritable material, so that the final result is a state of hyperexcitability in which the centre, so to speak, boils over on the slightest provocation. In this condition a cutting off of the inhibitory impulses must at first increase the activity of the centre, leading to the increased inspiratory tonus already described. But, unchecked by any reign'ng impulses, the centre enters upon a career of spendthrift activity. Each inspiratory contraction is maximal, but the centre, exhausted by the effort, has to wait a considerable time before it can accumulate sufficient energy for the next ; hence the final result of section of both vagi is deepening and slowing of respira- tion. Although Gad has rendered great service in emphasising the importance of the in- hibitory or expiratory impulses which ascend the vagi, there is no doubt that he went too far in denying the existence of inspiratory fibres in the vagus. This is shown by the following experiment of Head. According to Gad's view, collapse of both lungs implies simply a removal of the normal inhibitory impulses ascending the vagi, and is therefore equivalent to division of these two nerves. If in the rabbit the left vagus be divided, a tube can be introduced into the left bronchus, and artificial respiration can be performed by alternate inflation and collapse of the left lung, without in any way affecting the respira- REGULATION OF RESPIRATORY MOVEMENTS 1093 tory centre, all connections with the latter being destroyed {v. Fig. 513). Meanwhile the animal carries out normal respiratory movements, which can be recorded by the diaphragm slip method. While the slip is contracting regularly, the right pleura is opened and the right lung allowed to collapse. The effect of this collapse, carried up by the right vagus to the centre, is an extreme contraction of the diaphragm, and since the onset of asphyxia is prevented by the artificial respiration carried out on the left lung, the tonic standstill of the diaphragm may last over a minute. In this case therefore RT Lung. artif resp app. Lunc Fig. 513. Diagram to illustrate Head's experiment on the effect of collapse of the lung. R.c, respirator}' centre ; R.v, l.v, right and left vagi. the effect of collapse of one lung is enormously greater than that produced by section of both vagi, showang that the effect is due, not to abolition of the ordinary tonic inhibi- tory stimuli, but to excitation of special inspiratory fibres in the vagus by the collapse of the lung. By means of the string galvanometer it is jiossible to show definitely that a collapse o' 1 he lungs does set up a nervous impulse travelling up the vagus nerves. This impulse must be inspiratory in character, ?o that there is no reason to deny the existence of both kinds of fibres in these nerves. The effect of electrical stimulation, especially with an ascending constant current, is also strong evidence in the same direction. After division of both vagi the total piilmouaiy ventilation does not as a rule undergo any marked changes, and in the absence of anaesthesia the aeration of the blood may be carried out almost, if not quite, as well as in the intact animal. The importance of the vagus action for the organism is shown, however, if we put an increased strain on the respiratory mechanism, as, for instance, by increasing the percentage of carbon dioxide in the air breathed. In the intact animal this procedure leads first to increased depth and later to increased frequency of respiration, the total ventilation being thereby augmented to such an extent as to keep the alveolar tension of carbon dioxide almost constant. If the same percentage of carbon dioxide be administered to an animal after section of both vagi the effect is deepening of respiration, but not (|uickeniiig (Fig. 514). Each inspiratory movement, however, is already considerable so that the margin by which increase of 1094 PHYSIOLOGY pulmonary ventilation is possible, by increase of depth of respiration alone, is not so great as in a normal animal. Moreover, since no quickening of rsspiration takes place, the increased ventilation rapidly becomes inadequate for the maintenance of the normal alveolar carbon dioxide tension. In the Fig. 514. Effect of 10'6 per cent. COg in a mixture containing 23'3 per cent. O2 on a rabbit witli both vagi divided. The gas was administered between the arrows. Zero line of blood-pressure is 32 mm. below bottom of tracing. Compare this figure with Fig. 505, p. 1081. (F. H. Scott.) following Table the total amounts of pulmonary ventilation obtained on administration of mixtures containing carbon dioxide to a rabbit before and after section of the vagi are compared. Rabbit, 3 kilos Respirations per minute Vol. of each respiration Total ventilation per minute Respiration with air . „ 4-2 per cent. CO2 . „ 8-6 per cent. CO2. „ air . 72 96 97 72 c.c. 19 25 29 20 1368 2400 2813 1440 Vagi Divided Respiration with air . „ 4-2 per cent. CO2 . „ 8-6 per cent. COg. 45 45 42 29 34 38 1305 1530 1596 Whether we assume that the prevaihng impulses travelling up the vagi are purely inhibitory or are both inhibitory and augmentor, the re- sultant effect, by reining in the activity of the centre, is to economise its energy and the energy of the respiratory muscles. The result of the vagal impulses will therefore be to increase the excitability of the respiratory centre and make it more susceptible to slight changes in the carbon dioxide tension of the blood, while maintaining a sufficient margin of energy to REGULATION OF RESPIRATORY MOVEMENTS 1095 meet the increased needs thrown on the respiratory mechanism by aug- mented metabolism, such as occurs in violent muscular exercise. The important part played by the vagi in the regulation of normal respiration is shown stiU more strikingly if the respiratory centre in the medulla be separated from the higher parts of the brain before the section of the vagi is carried out. Separation of the medulla from the higher parts of the brain, as by section just behind the corpora quadrigemina, has practically no influence on the respiratory rhythm. If now both vagi be divided the normal respiratory movements cease entirely, being replaced by a series of inspiratory spasms, each of which lasts several seconds and is followed by a pause of half to one minute's duration. These spasms are inadequate for the proper oxygenation of the blood. They become gradually less and less frequent, and in about half an hour the animal dies of asphyxia. We must conclude therefore that the medullary respiratory centre with the help of the vagi is able to carry out normal respiratory movements. If both vagi are cut, impulses arrive at the centre from the higher parts of the brain regulating its activity, and enabhng it to carry out modified but sufficient respiratory movements. Removed from both these sources of afierent impulses, the centre discharges only a series of spasms which are totally inadequate for the renewal of the blood-gases, so that the animal dies. We may summarise these results as follows : Respiratory centre with vagi — normal respiration. Respiratory centre wath brain — modified respiration. Respiratory centre alone — inadequate spasmodic contractions of respiratory muscles, afid death of animal. The nature of the suiiplemental action of the mid-brain on the medullary respiratory centre has not yet been made out. It is apparently not dependent on afferent impulses arriving at the brain, since section of no cranial nerve affects in any way the activity of the centres. Certain observers have described 'accessory respiraotry centres ' in the mid-brain, in the region of the posterior corpora quadrigemina. Stinnilation of this part causes increase in the rate of inspiratory movements and finally tonic spasm of the diaphragm. Expiratory effects have been produced by stinmlalion of the an erior corpora quadrigemina, and it would seem that a section has to pass through or behind these bodies in order to produce the results, alieady described, of cutting off the higher centres from the mednlla oblongata after division of the vagi. Other localised spots in the brain from which effects on respiration have been obtained are the inner wall o' the optic thalamus and the root of the olfactory tract. Further experiments are necessary before we can regard any of these centres as normally involved in the maintenance or regulation of the respiratory movements. APNCEA. If artificial respiration be maintained so as to produce a somewhat greater ventilation than occurs by the normal respiratory move- ments of the animal, a standstill of respiration is brought about. This condition is called apnoea. The first explanation of this standstill was that it was due to over- oxygenation of the blood. The fact that it could be produced by artificial ventilation Avith inert gases, sucli as hydrogen and nitrogen, as well as the discovery of the inhibitory influence of distension 1096 PHYSIOLOGY of the lungs on the respiratory centre, led Head to ascribe it to the summation of a series of inhibitory stimuli. In these experiments, however, the fact was forgotten that forced ventilation of the lungs with air or any inert gases vn\\ reduce the carbon dioxide tension in the blood circulating round the pulmonary alveoh and therefore round the respiratory centre. A respiratory pause will therefore ensue and last until the increasing accumulation of carbon dioxide in the blood raises its tension to the normal height, at which the respiratory centre is ' set,' so to speak, to respond by a respiratory dis- charge. If the carbon dioxide content of inspired air be increased to about 4-5 per cent., it is impossible to produce an apnoeic pause, however rapidly the respiratory movements be carried out. It would seem therefore that ordinary apnoea is entirely due to deficiency of carbon dioxide tension in the respiratory centre, and that although the vagus nerve is inhibitory of respiration, it is impossible to summate a series of vagus inhibitions by artificial respiration so as to produce a lasting cessation of respiratory movements. The chief use of the vagi in respiration seems to be for maintain- ing, by frequent inhibitions, the excitability of the respiratory centre at a maximum. M escher distinguished three types of apnoea, viz. : Apnoea vera, due to the washing out of COg from the lungs, and the consequent re- duction of the tension of this gas in the blood. Apncea vagi, a stoppage of respiration caused by stimulation of the inhibitory fibres of the vagi. This stoppage is limited, as we have seen, to the immediate duration of the stimulu? (whether electric or produced by distension of the lungs). Apnoea spuria. Stoppage of respiration by stimulation of other nervous or sensory surfaces. Thus when a duck plunges there is immediate stoppage of respiration, which may last four or five minutes if the animal remains so long under water. The same stoppage may be produced by pouring water on the beak. ' CHEYNE-STOKES ' BREATHING If a man desires to hold his breath for some time he takes first a series of deep breaths. The result is to diminish the carbon dioxide tension in the alveoU and therefore to take away the need and the desire to breathe until the carbon dioxide tension rises to normal as the result of the continued formation of carbon dioxide. By continuing forced respiratory movements for a minute or two the carbon dioxide tension both in the alveoli and in the blood may be brought down to a very considerable extent. As a result there is a prolonged period of apnoea. During this period of cessation of respirations, however, the oxygen is being used up, and the tension of this gas in the alveoli may fall to such an extent that the respiratory centre is excited by lack of oxygen before the carbon dioxide tension in the alveoli has risen to its normal value. As a result of the excitation by oxygen lack, a few breaths are taken and the carbon dioxide tension is once more lowered, and the stimulation due to the oxygen lack, disappears. There is therefore again a cessation of respiration. These periods of cessation alternate with periods of respiration so that we get a condition of periodic breathing REGULATION OF RESPIRATORY MOVEMENTS 1097 which is spoken of as Cheyne-Stokes respiration. During the period of apnoea, resulting on forced breathing, the great diminution of oxygen tension in the alveoli is shown by the fact that the subject of the experi- ment becomes blue, and may indeed lose consciousness. There are at the same time rhythmic changes in the blood-pressure, which rises towards the Fig. 515. Forced breathing of air for two minutes, followed by apnoea for two minutes, and periodic (' Cheyne-Stokes ') breathing for about five minutes. At A, sample of alveolar air contained O2, 11-44 per cent. ; CO2, 5-58 per cent. Second sample at b, Og, 13-55 per cent. : CO,, 5-57 per cent. (Douglas and Hai.dane.) ends of the periods of the apnoea, falhng during the periods of respiration. The first respiration after forced breathing is due to oxygen lack. The period of apnoea may therefore be considerably prolonged, if the onset of oxygen lack be postponed, by increasing the tension of this gas in the alveoli at the commencement of the apnoeic period. By forcibly breathing for a period of two minutes in an atmosphere of oxygen, men have succeeded in holding their breath for as long a period as eight minutes (Vernon). ' Cheyne-Stokes ' breathing is almost invariably observed as one of the effects of exposure to high altitudes, and is then especially marked during sleep. It is often present when the activity of the respiratory centre is depressed, as in cases of lu-ismia or per- nicious anaemia. Under these circumstances it may be temporarily removed by administering either oxygen or carbon dioxide (in small percentage) to the patient. The oxygen improves the condition of the centre ; the carbon dioxide acts as an added stimulus and rouses its activity. 35^ SECTION IV THE EFFECTS ON RESPIRATION OF CHANGES IN THE AIR BREATHED We have already seen that a moderate increase in the carbon dioxide per- centage of -the air breathed {e.g. up to 4 per cent.) causes a proportional increase in the ventilation of the lungs so as to maintain the tension of this gas in the alveoli at the normal level. The same effect is observed whether the mixture breathed contains 18 or 50 per cent, of oxygen, showing that the sHght diminution in oxygen content caused by mixing the air with carbon dioxide is in no way responsible for the effect. If the amount of carbon dioxide be increased to 12 or 15 per cent, it becomes almost impossible to continue the inhalation owing to the spasm of the glottis produced by the irritant effects of the carbon dioxide. If these high percentages be ad- ministered to an animal by a tracheal tube, violent dyspnoea is produced which gradually diminishes, and the animal passes into a condition of narcosis in which the respiratory movements become less and the oxygena- tion of the blood is ineffectively carried out even in the presence of excess of oxygen. The administration of larger percentages, such as 30 or 40 per cent., causes rapid death and failure of the circulation and respiration, often preceded by convulsions. Coincident with the increased respiration brought about by moderate percentages of carbon dioxide there is a rise of blood- pressure, determined partly by vascular constriction, partly by an increased output from the heart. With high percentages of carbon dioxide the curve of blood-pressure obtained resembles that produced by lack of oxygen. Oxygen itself exercises no excitatory effects on the respiratory move- ments. At the normal atmospheric pressure the tension of oxygen in the alveoli is about 107 mm. Hg, a pressure which, as we have seen, is amply sufficient to saturate the hsemoglobin passing through the vessels of the lungs. Since the depth and frequency of respiration are determined by the carbon dioxide tension in the alveoli no alteration in respiration will be produced by increasing the tension of oxygen in the air breathed above its normal amount. The respiratory movements in an atmosphere of pure oxygen will, in the normal individual, remain unchanged. This statement is only true for the healthy individual. If from failure of the heart and circulation, or diminished oxygen tension, or severe loss of blood, the oxygenation of 1098 EFFECTS OF CHANGES IN AIR BREATHED 1099 the blood is already insufficient, marked amelioration of the symptoms may be produced by inhalation of jjure oxygen. Especially is this noticeable where there is failure of the heart. In these cases the heart, already affected, is unable to keep up an adequate circulation and to supply itself with sufficient oxygen. A vicious circle is thus estab- lished in which the heart tends to get steadily worse. By administration of oxygen an adequate supply of this gas to the heart-muscle is assured ; the heart-beat therefore becomes more effective and the whole circulation is improved and therewith the pro- vision of oxygen to the body at large. If a warm-blooded animal be immersed in a chamber and submitted to pure oxygen at a pressure of four atmospheres, it dies as rapidly as if it were in an atmosphere of pure nitrogen. At this pressure the oxidative processes of the body as well as the intake of oxygen into the lungs are absolutely abolished. It is interesting to note that certain other oxidative phenomena, e.g. the spontaneous oxidation of phosphorus, also cease if the tension of the oxygen be sufficiently high. Exposure of an animal over a considerable period of time to a pressure of oxygen of two atmospheres may, as Haldane and Lorrain Smith have shown, set up severe inflammation of the lungs and thereby cause death indirectly. CHANGES IN TENSION OF OXYGEN. If a man breathe a mixture of nitrogen and oxygen free from carbon dioxide, and the oxygen be gradually diminished, no feeling of ' want of breath ' may be experienced. With per- centages of oxygen as low as 12 per cent, there may be no change in the breathing, even though the deficient oxygenation of the blood may be shown by the blueness of the lips and face. If the oxygen be reduced still lower, a certain amount of hyperpnoea may occur, but in many cases the individual experimented on may not feel any ill effects until he suddenly becomes unconscious from lack of oxygen. If fresh oxygen be not supplied this unconsciousness may be followed by convulsive movements and death. If the administration of low percentages of oxygen, e.g. about 10 to 12 per cent, of an atmosphere, be continued for some time, the subject of the experiment may suffer considerable discomfort. One of the signs of oxygen lack is often severe headache, and this may be accompanied by vomiting or nausea and by a feehng of discomfort in the precordial region. Many experiments have been made both on animals and man by submitting them to a lowered atmospheric pressure in chambers specially built for the purpose. The limit to which the pressure may be reduced varies in different individuals, the variations being determined by the type of respiratory movement of the individual in question, since on the depth of respiration depends the relation between the tension of oxygen in the alveoli and that in the inspired air. The lowest limit at which life is possible corresponds to an oxygen tension in the alveoli of 27 to 30 mm. Hg. MOUNTAIN SICKNESS. The phenomena just described as ensuing on exposure of an animal to low oxygen tensions in a respiratory chamber for some length of time are exactly similar to those which are regarded as characteristic of mountain sickness. The following Table shows the diminu- tion in the atmospheric pressure at varying heights above the level of the sea : 1100 PHYSIOLOGY Height above sea level, Barometer Per cent, of an in metres mm. Hg atmosphere 0 760 100 1000 670 88 2000 593 78 3000 524 69 4000 463 61 5000 410 54 6000 367 47 7000 320 42 At a height of 5000 metres the pressure of the air is reduced to little over half an atmosphere and the oxygen tension is therefore only about 11 per cent, of an atmosphere. It must be remembered that in most cases of mountain sickness, in addition to this absolute oxygen lack, there is in- creased consumption of oxygen, owing to the muscular exercise involved in chmbing. Moreover a greater volume of the alveolar air must consist of carbon dioxide if the tension of this gas is to be kept constant (cp. Fig. 506, p. 1082). Since diminished oxygen tension, within fairly wide hmits, does not excite any corresponding increase in the respiratory movements, there must, at these heights, be an actual diminution in the oxygen tension in the alveoh. This diminution in tension is shown by a series of observations carried out by Zuntz on himself and fellow workers at different locahties. It may be noted that on Monte Eosa, where the oxygen tension in the alveoh was reduced to between 37 and 57 mm. Hg, as against the normal 101 to 105 mm. Hg, all the members of the party were suffering from mountain sickness. Height above sea level, metres O2 tension of air Alveolar O2 tension A B c D E F Berlin . Brienz . , Brienzer Rothorn . Col d'Olen . Monte Rosa . 54 500 2130 2900 4560 157 148 121 110 89 105 84-5 68 57 101 94 66 46 80 64 49 105 88 62 60 61 103 86 66 68 37 104 91 71 68 57 As a result of the oxygen starvation there is inadequate supply of this gas to the heart, so that the circulation tends to fail, especially on making the slightest muscular movements. At the same time the oxygen starva- tion of the brain produces failure of judgment and inability to carry out or to co-ordinate muscular movements properly. The symptoms as a rule do not increase until death results, so that, although there is an oxygen starvation of the body, there must be some means by which the respiration is modified EFFECTS OF CHANGES IN AIR BREATHED 1101 so as to obtain a sufficiency of this gas for the lowered requirements of the body. That the adaptation is effective is shown by the fact that most individuals, if they remain at a height, gradually recover from the mountain sickness and may finally be able to carry out muscular movements with almost as gi-eat precision and force as they could previously on the plains. The mechanism by which increased ventilation of the lungs is attained is that already mentioned in dealing with the effects of lack of oxygen, namely, the production of acid substances in the body. The respiratory centre is thus stimulated by these acid substances, especially lactic acid, as well as by the carbon dioxide tension of the blood, and the joint action of these two substances (which probably co-operate in raising the hydrion concentration of the blood) determines the marked increase in the lung ventilation. Since the carbon dioxide is no longer the sole factor responsible for the ventilation, the tension of this gas in the alveolar air is diminished. ACAPNIA. This diminution of carbon dioxide tension in the blood and alveolar air has been regarded by Mosso as the essential factor in the causation of mountain sickness and has been designated acapnia. It may be absent, however, in the most marked cases of mountain sickness, where the respiratory centre has failed to respond to the additional acid stimulation, and may be present to a marked degree in individuals who are experiencing none of the ill eifects of this disorder. Another important means of rapid adaptation is by means of the circula- tion. This is noticeable even in the case of persons sitting quietly in a gas chamber who are subjected to gradually lower pressures. It is evident that a deficient passage of oxygen from the alveoli to the blood may, so far as the tissues and heart are concerned, be accommodated for by increasing the rapidity of the circulation, and this is effected by a quickening of the pulse-rate. The following Table shows the changes in the pulse-rate caused by exposure to varying pressures in a gas chamber : Pulse rff Gas Chamber Pressure . I'ulse 720 64 650 72 424 ....... 84 This quickening of the pulse is to be observed also in the trained mountain soldier, in individuals in whom there is no lowering of the alveolar carbon dioxide tension, so that apparently in such cases the whole adaptation to altered conditions is by means of the circulation. In cases where adaptation fails it is in the circulation that the failure is most marked, so that the symptoms of severe mountain sickness resemble closely those produced by rapid heart-failure. Dilated heart, cyanosis, muscular weakness, vomiting, mental torpor, inco-ordination, delirium, may all be observed in both cases. The disturbance of the central nervous system is shown by the almost invariable occurrence at great heights of Cheyne Stokes breathing. If the animal is able to withstand the immediate effects of exposure to a rarefied atmosphere, a process of adaptation comes into play which finally 1102 PHYSIOLOGY fits him for discharging his functions normally even at the high altitude. From the lack of sensibility of the respiratory centre to small changes in oxygen tension, any diminution in oxygen tension must cause a corresponding diminution in the degree of saturation of the haemoglobin of the blood. This change in oxygen saturation is at once felt by the blood-forming organs. As an immediate effect of change to a region of low atmospheric pressure there is a relative increase in the blood-corpuscles due to a concentration of the blood and a diminution of its plasma. Simultaneously, however, the blood-forming organs enter into a condition of increased activity, so that after a stay of four or five weeks' duration at a height both corpuscles and haemoglobin are considerably increased in total amount. The following Table shows the average number of red corpuscles contained in one cubic millimetre of blood from the inhabitants of regions at varying altitudes : Height above sea level, Red corpuscles metres Christiaiiia . 0 4,970,000 Zxirich . 412 5,752,000 Davos 1560 6,551,000 Arosa . 1800 7,000,000 Cordilleras . 4392 8,000,000 There is of course a limit to the power of adaptation, a limit which varies in different individuals. Thus for some men it is impossible to stay any length of time in the high settlements in the Andes, while others, after two or three weeks' discomfort, become perfectly inured to their new conditions. It seems doubtful, however, whether any of the present race of men could become adapted to permanent residence at a height over 5000 metres, and though for a certain length of time by bringing into play the reserve mechanisms already described, they may raise themselves to a height considerably above 5000 metres, it seems questionable whether without artificial means, such as the inhalation of oxygen, it will be possible for any man to attaio the highest points on the earth's surface, or at any rate to arrive there by his own unaided efforts. The highest summits in the Himalayas have a height approaching that attained by Tissandier with his two companions in his famous balloon ascent, namely, 8600 metres. In this ascent, although oxygen inhalation was used (somewhat ineffectively), two of the party succumbed. The stimulating effect of oxygen lack on the blood-forming organs extends also to the muscular system, so that one of the effects of a residence in high altitudes is increased assimilation of nitrogen. For a time the nitrogen output is less than the nitrogen intake, and there is an actual building up of new tissue. The condition of the individual is similar to that of a growing animal, a fact which may explain the admirable results of a mountain holiday. We can hardly imagine that the power of the organism EFFECTS OF CHANGES IN AIR BREATHED 1103 to react in this way was evolved through generations of mountain climbing. We are probably here making use of an adaptation which has been evolved for the purpose of retrieving loss of blood by haemorrhage, such as must have been of continual occurrence in the struggle of individual against individual, which has resulted in the survival of the animals of to-day. ALTERATIONS IN THE NITROGEN TENSION, The nitrogen of the atmosphere plays no part in the metabohsm of the body, and nuist be regarded as a purely inert gas. It is a matter of indifference whether under normal atmospheric pressure we breathe an atmosphere of pure oxygen or one containing one-fifth part of this gas diluted with four-fifths of nitrogen. The very inertness of nitrogen may be of danger to the body under certain conditions. If a man or an animal be exposed, as in a di\dng-bell, to a pressure of three, four, or six atmospheres, the respiratory functions are unaffected, but the amount of nitrogen dissolved in the fluids of the body is increased in direct proportion to the pressure. If the pressure be now suddenly released, the nitrogen, which cannot be used up by the tissues, is given off from the body-fluids in the form of bubbles, just as carbonic acid gas rises in bubbles from soda-water when the pressure is removed by with- drawing the cork from the bottle. These bubbles occurring in all the capillaries obstruct the flow of blood, and therefore, if the evolution of gas is sufficiently large, the animal dies in convulsions. A similar evolution of gas may occur in the spinal cord, giving rise to destruction of the cord and paralysis (' divers' palsy '). In order to prevent this sudden evolution of gas it is necessary that the change from the high pressure to the ordinary atmospheric pressure should be carried out gradually, so as to give the blood-plasma, supersaturated mth nitrogen, time to get rid of its excess of nitrogen without the formation of bubbles. OTHER GASES. Hydrogen and methane are, like nitrogen, indifferent gases. They may be respired if mixed with 20 per cent, of oxygen, and either of the gases may be used instead of nitrogen to dilute the oxygen that we breathe, without harm or inconvenience. Carbon monoxide is rapidly poisonous by its action on the red corpuscles. It combines with haemoglobin, forming CO-haemoglobin, a compound which is much more stable than oxyhaemoglobin. The blood is therefore deprived of its oxygen carrier, and the animal dies of asphyxia. We have seen, however, that the displacement of oxygen by CO is not absolute, but only relative. Hence, although the avidity of CO for haemoglobin is 140 times that of oxygen, we can convert the CO back into oxyha'moglobin by in- creasing the mass influence of the oxygen. This may be done by giving the poisoned animal pure oxygen to breathe, or even oxygen under pressure. In pure oxygen at a pressure of two atmospheres an animal can breathe and live, even though tlu> \vh()l(> of its haMuoglobin is converted into CO-haemo- globin, the amount of oxygen which is simply dissolved by the blood-plasma being sufficient at this pressure for the respiratory needs of the animal (Haldane). Other gases which have special poisonous properties are hydrocyanic 1104 PHYSIOLOGY acid, sulphuretted hydrogen, phosphuretted hydrogen (PH3), arseniuretted hydrogen, &c. IRRESPIRABLE GASES are those which are so irritating that they pro- duce spasm of the glottis. Such are ammonia, chlorine, sulphur dioxide, nitric oxide, and many others. VENTILATION A point of practical importance is the securing to each individual of sufficient fresh air, so that he may always have a plentiful supply of oxygen, and may be relieved of his waste products. It is found that a dwelling-room becomes unpleasant and stuffy when the percentage amount of COg has reached 0-1 per cent. This stuffiness is supposed to be due to organic exhalations from the skin, lungs, and alimentary canal, some of which have a poisonous effect, giving rise to headache and sleepinesss. Since these cannot be measured, it is taken as a cardinal rule in ventilation that the amount of CO2 should never rise above 0-1 per cent. Since in questions of ventilation we have generally to deal with trades in which the metric measure is not used, it may be convenient to give the data as to carbon dioxide production and the amount of air required in cubic feet. An adult man gives off about 0-6 cubic foot of CO2 every hour. Hence in that time he raises the amount of CO2 in 1000 cubic feet of air from -04 per cent, (the normal amount in the atmosphere) to 0-1 per cent. He must therefore be supphed with 2000 cubic feet of air per hour in order to keep the amount of CO2 down to -07 per cent. (Ordinary air contains -04 per cent. CO2, therefore 2000 cubic feet would contain 0-8 cubic foot CO 2, which with the 0-6 cubic foot given off by the man would be 1-4, which is -07 per cent.) In order that the air may be easily renewed without giving rise to exces- sive draught, a certain amount of cubic space must be allotted to each man. Each adult should have in a room 1000 cubic feet of space, and be supplied every hour with 2000 to 3000 cubic feet of air. SECTION V THE MECHANISMS OF OXIDATION IN THE TISSUES The blood in its passage through the capillaries takes up carbon dioxide from the tissues, giving oxygen to the latter in exchange. This interchange is determined by the differences in tension of the gases on the two sides of the capillary wall. Whereas the tension of oxygen in the plasma varies from 100 mm. Hg in arterial to 25 mm. Hg in venous blood, the tension of oxygen in the tissues outside the vessels in most cases approaches 0, as is shown by Ehrhch's methylene-blue experiment described on p. 1063. On the other hand, the tension of carbon dioxide in the tissues, as judged from the examination of fluids such as bile and urine, varies from 6 to 10 per cent. of an atmosphere. The continuous flow of oxygen into, and of carbon dioxide away from, the tissues points to the constant occurrence of oxidative changes in the tissue-cells. By the blood the tissues receive not only oxygen but also food-stuffs, namely, proteins or amino-acids, fats, and sugars, derived from the alimentary canal, or, in starvation, from other parts of the body. The activity of the tissues, whether motor, as in the case of muscle, or secretory, as in the case of glands, is derived from the energy set free in the partial or complete oxidation of these food-stuffs, which occurs -vv^thin the active cells themselves. A study of the mechanism of oxidation in the body involves therefore a consideration of the processes which take place within the confines of each cell. The question is by no means an easy one. Although we speak of the ' burning ' of food-stuff's, and compare the processes in the body to those which take place in combustion, e.g. in a candle-flame, the analogy is after all a very rough one. In the first place, the food-stuff's, even after absorption, belong to a class of substances which have been designated as dysoxidisahle, since they present no tendency to combine with ordinary atmospheric oxygen. Thus sugars, proteins, or fats, if kept free from microbial infection, may be kept for years exposed to the air without undergoing any change. It is true that in certain cases, e.g. in alkaline solutions of sugar, we may obtain slow absorption of oxygen and oxidation of the sugar. The changes are, however, slight and limited in extent. All these food-stuff's are susceptible of combustion if raised to a sufficiently high temperature, but in the animal body the processes of oxidation have to go on at a temperature varying between 5° and 40° C, and in a solution which is almost neutral in reaction. It might be said that at the temperature of an ordinary flame the combustion of the food-stuff's is immediate and 11(1.5 1106 PHYSIOLOGY complete, whereas in tlie body the oxidation takes place by stages. Recent researcli has tended to remove this point of distinction by pointing out that even in an explosion of a mixture of methane and oxygen there is a series of intermediary products, and that the whole process, if analysed, is made up of stages in which hydrolysis and oxidation go on simultaneously, so that on this account it is difficult to cause a combination, even of hydrogen and oxygen, in the complete absence of any watery vapour. The oxidations in the body are strictly limited both in nature and extent. The mere fact that a substance is readily or even spontaneously oxidisable {autoxidisahle) affords no guarantee that it will undergo oxidation in the animal body. Thus phosphorus or pyrogallol taken by the mouth can be recovered in an unoxidised form from the urine. Carbon monoxide is excreted unchanged. There must apparently be some definite relationship between the molecular structure of the food-stuff and that of the cells of the body. Thus ordinary proteins which undergo complete oxidation contain large quantities of leucine. This substance is Isevorotatory and is designated i!-leucine. If ^leucine be administered to rabbits it is completely oxidised. If its isomer (^-leucine, resembhng it in every particular so far as we can see except in its relation to polarised light, be administered to a rabbit, the greater part of the substance passes through the body unchanged. In the same way there are sixteen sugars of the formula CgHjaOg. Of these only four, namely, glucose, fructose, galactose, and mannose, can be oxidised in the animal body. Other sugars differing in so slight a degree from these four as, e.g., Z- glucose or Z-fructose, cannot be utilised by the body. Not only must there be a distinct relation between the structure of the cell and the molecular structure of the food- stuff supplied, but there must be different mechanisms for the food-stufis and their derivatives. Thus in certain cases of disease or of abnormal nutrition the body may lose absolutely the power of utilising, i.e. of oxidising, a whole class of food-stuffs. In severe diabetes, or after destruction of the pancreas, glucose behaves in the body as if it were one of the artificial unassimilable sugars. The normal oxidation of fats probably proceeds by stages in each of which two atoms of carbon undergo oxidation. The penultimate stage in the oxidation of any of the higher fatty acids is thus oxybutyric acid. In complete carbohydrate starvation, for some reason or other, the body loses its power of completing this last stage, so that the oxybutyric acid undergoes no further oxidation, and either accumu- lates in the body or is excreted combined with bases in the urine. In the normal individual tyrosine, whether administered separately or in combina- tion in protein, is completely oxidised, the benzene ring being broken up. In certain rare cases of disordered metabolism the patient, who is otherwise apparently well, is unable to effect the total oxidation of tyrosine, which is therefore excreted as homogentisic acid, after undergoing only the first stage of its normal transformation in the body. These various mechanisms are adjusted in each case to the functional activity of the cell and are limited therefore, not by the supply of oxygen or of food-stuff to be oxidised, but by the necessities of the cell, i.e. the adaptations induced in it by its environ- MECHANISMS OF OXIDATION IN TISSUES 1107 mental changes. In discussing the mechanism of intracellular oxidation we have therefore to consider in the first place how the dysoxidisable food- stuffs are made to combine with the molecular oxygen diffusing into the cells from the blood in the capillaries, and in the second place the means by which these oxidative changes are strictly limited in accordance with the necessities of the cell, and finally the nature of the specific oxidative mechanisms for each kind of food-stuff and for the various stages in the oxidation of each food-stuff. We are very far as yet from being able to give a definite answer to any one of these questions. Even in the first problem, namely, the oxidation of dysoxidisable substances, we have to confine ourselves almost exclusively to speculation on possibilities. Although these substances will not unite with the oxygen of the air, in which the combining activities of the oxygen are satisfied by the combination of two atoms to form one molecule, many of them readily undergo oxidation if subjected to the action of ' atomic ' oxygen or ' active ' oxygen, and it has been suggested that the problem of the oxidation of the body is really bound up with the question as to the mode of activation of the molecular oxygen derived from the oxyhsemoglobin. Thus Hoppe-Seyler suggested that the activation of oxygen might occur through the intermediation of reducing substances. He supposed that reducing substances might be formed under the influence of ferments by hydrolytic splitting of the food-stuffs. A reducing substance is one that has sufficient affinity for oxygen at the ordinary temperature to tear asunder the bonds which unite two atoms of oxygen to form one molecule, and to combine with one or both of the atoms so set free. If the combination is with only one atom, the other atom of the oxygen molecule is set free in an active form, and is therefore able to oxidise dysoxidisable substances which may be present. Thus, when a mixture of ammonia and pyrogallol is exposed to the atmosphere, the oxygen is rapidly absorbed, forming a dark brown solution, pyrogallol being therefore a reducing agent. But at the same time a certain amount of the ammonia (a dysoxidisable substance) undergoes oxidation with the formation of nitrite. In the slow spontaneous oxidation of phosphorus which occurs on exposing this substance to the atmosphere, ozone, OgO, is always formed. As a type of the formation of reducing substances in hydrolytic fermentations may be adduced the butp'ic acid fermentation, in which sugar is converted into butyric acid, carbonic acid, and hydrogen : ^^^^^^^ ^ ^^^^^^ ^ ^^^^ ^ .,^^ The hydrogen produced in this process would act as a reducing agent. There is no doubt that reducing substances are formed under normal circum- stances in the tissues, as is shown by the methylene-blue experiment, and it is possible that such reducing substances may aid in activating oxygen and in the induction of certain oxidative processes. The activation of oxygen would, however, not explain the specific character of the various oxidations, and the accurate gradation of these oxidations to the necessities of the cell. In mnnv cases reducing substances may themselves act as 1108 PHYSIOLOGY carriers of oxygen, and their action be more or less specific. If, for instance, glucose be boiled with an ammoniacal solution of cupric hydrate, it undergoes oxidation, the cupric being reduced to cuprous hydrate. Cuprous hydrate in ammoniacal solution is a reducing substance ; it absorbs oxygen from the air and is reconverted to cupric hydrate. A small amount of cupric hydrate therefore, in the presence of air, may act as a carrier of oxygen from the air to the sugar and may thus oxidise indefinitely large quantities of sugar. In the same way, if indigo in alkahne solution be boiled with sugar, it undergoes reduction with the formation of a colourless compound. On shaking the decolorised solution with air it absorbs oxygen with the reproduc- tion of indigo, so that here again minute quantities of indigo blue may serve to oxidise large quantities of glucose. The mode of action of these oxygen carriers resembles closely that of the various ferments which effect the transference of water from the menstruum to the substrate {e.g. trypsin, invertase,. &c.). These hydrolytic ferments differ from ordinary hydrolytic agents, such as dilute acids, in the specific character of their action. Trypsin, for instance, will hydrolyse polypeptides of a type corresponding to those which make up the ordinary food-products, but is powerless to hydrolyse polypeptides composed of artificial amino-acids which are the optical isomers of those occurring in the body. It seems possible that we might explain the specific oxidations occurring in the cell by assuming the presence of a number of ferments, oxidases, which would act as oxygen carriers, but each of which would only be able to act on a certain type of food-stufi or on molecules of a given configuration. Such oxidative ferments have been described as existing in many animal and vegetable extracts. Many species of fungus contain a ferment known as tyrosinase, from the fact that, when it is added to solutions of tjncosine in the presence of air, the tyrosine is oxidised with the formation of a brown pigment. The same ferment is able to effect the oxidation of other aromatic substances. The browning of a freshly cut potato or apple on exposure to the air is similarly ascribed to the oxidation of a chromogen by the oxygen of the air, through the intermediation of an oxidase present in the cells. If benzyl alcohol or sahcyl aldehyde be added to a suspension of liver-cells in blood, and air be allowed to bubble through the mixture for some time, the alcohol or aldehyde is oxidised to the corresponding acid. In the same way xanthine (C5H4N4O2) added to a mixture of spleen pulp and defibrinated blood is converted into uric acid (C5H4N4O3). Bach and Chodat have shown that in many cases the oxidase is not a single substance, but a mixture of an organic peroxide with a ferment, peroxidase, which has the property of sphtting off atomic, i.e. active, oxygen from the peroxide. These peroxidases have the same effect on hydrogen peroxide. They must be distinguished from the ferment catalase, which is present in almost all animal and vegetable tissues, and which effects a rapid decomposition of hydrogen peroxide with the formation of molecular oxygen : 2H2O2 = 2H2O + O.,. MFX'HANISMS OF OXIDATION IN TISSUES 1109 In the case of a peroxidase the equation would be represented : H2O2 = H2O + 0'. Many reactions are known in chemistry in which the part of a peroxidase is played by an inorganic catalyst. Thus hydrogen peroxide effects a slow oxidation of many organic substances, but the oxidation is enormously hastened if to the mixture be added a trace of a ferrous salt (Fenton's reaction). The same part may be played by salts of manganese, and it is interesting to note that manganese forms an essential constituent of the peroxidase laccase, which is present in many plants and is responsible for the formation of the Japanese lacquer. It effects a specific oxidation of hydroquinone and pyrogallol. The oxidations carried out by the use of hydrogen peroxide, \\ath or without a catalyst or peroxidase, present a close resemblance to the oxidations occurring in the animal body. Thus Dakin has shown that saturated fatty acids, even the higher members of the series, are gradually oxidised if warmed gently with hydrogen peroxide in the presence of ammonia, and the course of the reaction resembles in many respects that which, on other grounds, we have assumed to take place in the normal metaboHsm of the body. We have no evidence that hydrogen peroxide is formed at any time in the body, though there is some reason to assume its formation in the process of carbon assimilation in the green leaf. If we adopt the views of Bach and Chodat we must assume that every animal cell contains organic peroxides as well as peroxidases, or else that it can under physiological conditions form these substances. Since there is also evidence of the presence of reducing substances in the cells, we may conveniently assume, with Ehrlich, that distinct side-chains of the protoplasmic molecule have specific affinities for oxygen. When all these affinities are saturated, these side-chains will act as peroxides, parting with their oxygen with extreme ease, whereas when the greater number are unsaturated, the resultant effect will be that of a reducing agent. The same protoplasmic molecule may therefore, according to its state of saturation with oxygen, act either as an oxidising or reducing agent, and can effect, probably through the intermediation of specifically adapted oxidases, the oxidation of the various food-stuffs stored up as the paraplasm of the cell. Since the oxidative processes are deter- mined, not by the presence of oxygen, but by the functional activities of the tissue, we must assume that the peroxidases are not preformed in the cell, but exist as precursors, zymogens, from which they can be set free in accordance with the necessities of the cell. It is probable that many of the food-stuffs or other proximate con- stituents are not directly accessible to oxidation, and that the first step in their utiUsation is a process of cleavage or hydrolysis, which itself involves the presence of specific ferments. Thus, so far as we can tell, the amino-acids undergo deamination before oxidation. They can thus be stored up in the cell either free or in the form of protein and present no point of attack to oxygen until the process of hydrolysis and deamination has taken place. This course of events is certainly true for some of the members of the purine group. CHAPTER XVII RENAL EXCRETION SECTION I THE COMPOSITION AND CHARACTERS OF THE URINE The main product of the oxidation of carbon, namely, carbon dioxide, is discharged by the lungs and to a slight extent by the skin. Water, taken as such with the food, but also derived to a shght extent from the oxidation of hydrogen, is got rid of by the lungs, skin, and kidneys. The salts of heavy metals when administered are excreted for the most part by the alimentary canal, e.g. iron, bismuth, mercury, A certain proportion of the pigmentary waste products of the body, derived from the breakdown of the blood-pigment, is also got rid of with the fseces. With these exceptions practically all the waste products resulting from metabolism are excreted in the urine by the kidneys. We have thus to seek in the composition of this fluid the last chapter in the metabolic history of a large number of the constituents of the body. Since, moreover, the kidneys may excrete almost any substance which circulates through their blood-vessels, many of the intermediate metabohtes may be found in minute quantities in the urine and may be isolated by working up large quantities of this fluid. Under pathological conditions these metabolites may appear in the urine in larger amounts and serve then as an index to some inter- ference with the later stages in the metabolism of fats, carbohydrates, or proteins. The composition of the urine must therefore be a variable one, according to the activity of the body, the quantity and nature of the food taken, and the relative amount of water escaping by the kidneys, lungs, and skin respectively. But just as we can describe a normal diet for an adult man of average weight, so we can describe an average composition for the urine. The history of the urinary constituents has been given for the most part in' the chapter dealing with the metabolism of the proximate constituents of the food. It will be useful, however, to enumerate in this chapter the various constituents of the urine and to summarise their properties, preparation, and normal significance. The urine of man is a clear yellow fluid which froths when shaken. On standing, a cloud of mucus is deposited, consisting of a very small amount 1110 COMPOSITION AND CHARACTERS OF URINE 1111 of nucleoprotein derived from the epithelial Hiiing of the bladder and urinary passages. In concentrated urine a deposit occurs on coohng. This deposit dissolves when the urine is warmed, and consists of urates. Under certain circumstances urine is turbid as it is passed, but in this case the turbidity generally consists of earthy phosphates and is not cleared up by heating. The colour of the urine varies with its concentration. After severe sweating the amount of water excreted by the kidneys is small, and the urine is therefore concentrated and of high colour. After copious draughts of Uquid the urine may be very pale and dilute. Ordinary urine has an aromatic odour, but this varies largely with the character of the food. Many food substances give characteristic odours, which may depend on alterations undergone by them in their passage through the body. The specific gravity of the urine is proportional to its concentration. Normally it is 1016 to 1020, though it may rise as high as 10-10 or sink as low as 1002. The molecular concentration of the urine is almost always greater than that of the blood. Its osmotic pressure may be measured by determining the depression of freezing-point. The a of urine normally varies between 0-87 and 2-71 (a of blood = 0-56). After copious draughts of water the depression of freezing-point in the urine may be less than that of serum, and may be as small as 0-25. The reaction of urine is generally described as acid. It is acid to htmus and to phenolphthalein. This is due to the fact that neutral constituents of the food give rise to acid end-products in metaboHsm. The sulphur of proteins is converted into sulphuric acid and the phosphorus of lecithin into phosphoric acid. There is thus a predominance of acid radicals over bases in ordinary urine. This statement, however, only apphes to man and to carnivora. In the food of herbivora there is a predominance of alkahne bases. Vegetable acids, e.g. tartaric, mahc, and citric acids, undergo oxidation to carbonic acid in the body, so that their base? leave the body as alkaline carbonates. The urine of such animals therefore contains an excess of alkaline carbonates, and is alkaline in reaction and froths on the addition of an acid. If a herbivorous animal be starved, so that it has to live on its own tissues, it becomes for the time, so to speak, carnivoroub, and its urine becomes clear and acid. The urine of man can be made alkahne by the ingestion of large quantities of vegetables or fruits. Under such circumstances the urine as passed is generally tm'bid from the presence of precipitated earthy phosphates. In determining the reaction of urine it is usual to adhere to one indicator, e.g. phenolphthalein, and to give the acidity in terms of decinormal acid, naming the indicator used. The acidity {i.e. the concentration of H ions) can also be determined by the electrical method. In this way Hoeber found the acidity of human urine to vary between •I-7 X 10"' and 100 x 10"'. On the average it was 49 X lO-'inthehtre. 1112 PHYSIOLOGY THE AVERAGE COMPOSITION OF THE URINE. Several analyses of the day's urine under varying conditions of food have already been given {v. pp. 757, 780). The following may be taken as a fair average for an adult man on ordinary mixed diet : Total amount of urine = 1500 c.c. This contains about 60 grm. of sohds, of which 25 grm. are inorganic and 35 grm. organic. These are distributed as follows : Inorganic Constituents Organic Constituents Sodium chloride . . 15-0 grm. Urea .... . 30-0 grm Sulphui-ic acid . 2-5 „ Uric acid . 0-7 „ Phosphoric acid . . 2-5 „ Creatinine . . 1-0 „ Potassium . • 3-3 „ Hippuric acid . 0-7 „ Ammonia . . 0-7 „ Other substances . 2-6 „ Magnesia . 0-5 „ Lime . . 0-3 „ Other substances . • 0-2 „ The quantity of urine will naturally vary with the water leaving the body by the kidneys, and therefore according to the habit of the individual with regard to the intake of fluids and with his occupation. Thus after copious sweating the total amount may fall to 400 c.c. in the course of the day. If large draughts of hquid be taken it may rise to 3000 c.c. or more. There are also diurnal variations in the amount secreted, depending probably largely on the circulation through the kidneys. The secretion is at a mini- mum during sleep, and especially between 2 and 4 o'clock in the morning. It is at its maximum during the first hours after rising, and increases generally after each meal. Muscular exercise may also give an initial increase owing to the greater vigour of the circulation associated with exercise. If the exercise is severe enough to cause sweating or is carried to fatigue, there may be a consequent diminution in the amount of urine secreted. THE INORGANIC CONSTITUENTS OF THE URINE (a) ACID RADICALS. The chlorides of the urine are derived almost entirely from the chlorides of the food. Though essential constituents of the body fluids, it does not seem that the chlorides enter into organic com- bination with the constituents of the cells. The output of chlorides, which normally varies from 6 to 10 grm. CI. in the course of the day, will therefore depend on the amount of chlorides taken in with the food. If these be withdrawn altogether, the chlorides may almost disappear from the urine, although the circulating blood contains practically the same amount of chlorides as in the normal individual, showing that the body retains the chlorides necessary for the proper carrying out of the vital processes as long as possible. Chlorides may also disappear from the urine temporarily under various pathological conditions. This is especially marked in cases of acute pneumonia. COMPOSITION AND CHARACTERS OF URINE 1113 Sulphates. The salts of sulphuric acid do not form an important con- stituent of the food. The sulphates of the urine are derived almost entirely from the oxidation of the sulpliur of the protein molecule. The output of sulphates is therefore, like that of urea, an index of protein metabolism. As the nitrogen of the urine goes up, so the sulphates will increase. On an average diet the ratio of urinary nitrogen to SO3 is about 5:1; though, owing to the varying content of different proteins in the sulphur, this ratio will naturally alter with the nature of the protein taken as food. The daily output of sulphuric acid varies between 1-5 and 3 grm. SO3. The greater part of the sulphate is present as sulphates of the alkaline metals. A certain proportion, about 10 per cent., is present in the form of conjugated or ethereal sulphates, chiefly indoxyl sulphate. A small proportion of the sulphur excreted in the urine is present in unoxidised form as so-called neutral sulphur. The neutral sulphur probably includes a number of different bodies, among which sulphocyanates and cystine are the best known. Inorganic sulphates can be precipitated from the urine by the addition of hydro* chloric acid and barium chloride. On filtering off this precipitate, the filtrate contains the ethereal sulphates. On boiling, the hydiochloric acid decomjjoses these substances, setting free sulphuric acid, which combines with the excess of barium i^resent and is precipitated as barium suljihate. This second precipitate therefore, when weighed, gives the amount of ethereal sulphates present. To determine the neutral sulphur the fluid, after the separation of both kinds of sulphates, is treated "with sodium carbonate to precipitate the barium, filtered, and the filtrate evaporated to dryness. The residue is then ignited with potassium nitrate, cooled and extracted with water. By this treatment all the neutral sulphur is converted into sulphates, which can be thrown down from the solution vnth barium chloride and weighed in the usual way. Phosphates. The phosphates of the urine are derived partly from the phosphates of the food, partly from the oxidation of the organic phosphorus- containing constituents of the food and of the tissues, e.g. nuclein, lecithin, &c. If the food contains much calcium and magnesium, the amount of phosphates excreted by the urine diminishes, since these substances are excreted with the faeces as calcium and magnesium phosphates. According to the diet therefore, phosphoric acid may be excreted either by the intestine or by the kidneys. The amount of phosphates, reckoned as P2O5, excreted in the course of the day may vary between 1 and 5 grm. In the urine the phosphates exist as a mixture of the mono- and di-sodium phosphates, the relative amounts of the two varying with the acidity of the urine. If the urine is neutral or alkahne there is very often a deposit of earthy phosphates. Whether this deposit is present or not depends on the varying solubility of the different calcium and magnesium phosphates. Thus the mono-mag- nesium phosphate MgH.,(P04)2 and the mono-calcium phosphate CaH.,(P04)2 are both fairly soluble in water, and their solubility is increased by the presence of neutral salts. With increased acidity of the urine the proportion of the two bases present in these forms is diminished. The di-magnesium and di-calcium phosphates are only slightly soluble in water, and the latter would, if present in the urine, be deposited. One may indeed, in slightly 1114 PHYSIOLOGY acid urine, find the di-calcium phosphate occasionally present as a crystalhne deposit. On heating the urine the di-calcium phosphate breaks up into a mono- calcium phosphate and a tri-calcium phosphate, while the acidity of the urine is increased by the solution of the mono-calcium phosphate. Alkahne urine will always present a precipitate of tri-calcium phosphate Ca3(P04)2. When normal urine is allowed to stand, the urea is converted by the presence of micro-organisms into ammonium carbonate, and the urine becomes alkahne. Under such conditions we may often find a crystal- hne precipitate of ammonium magnesium phosphate, NH4MgP04, the so- called ' triple phosphate.' (6) THE BASES OF THE URINE. The bases include potash, soda, ammonia, magnesia, and hme. The amount of potash excreted in twenty-four hours varies between 1-9 and 3-2 grm., according to the nature of the food taken. With a large meat diet, which contains considerable quantities of potassium, the output of this base is increased. In fasting there is also an increase in the output of potash, owing to the utihsation of the tissues of the body which themselves are rich in potassium. The amount of sodium excreted in the twenty-four hours varies on the average between 4 and 5 grm., but depends very largely on the quantity of sodium chloride taken with the diet. The alkaline earths, lime and magnesia, are invariably present in urine, but in much smaller quantities than the alkahne metals. The average amount of these two bases in the twenty- four hours varies in each case between 0-1 and 0-2 grm. Their output by the urine is no criterion of the amount taken in with the food or absorbed from the intestines, since both these bases may be re-excreted into the gut and appear as insoluble phosphates in the feeces. Normal human urine always contains a small amount of ammonia] on an average between 0-6 and 0-8 grm. in the twenty-four hours. As we have already seen, in dealing with the origin of urea in the body, the quantity of ammonia in the urine is an index to the excess of acids over bases which have to be excreted by this fluid. Thus it is easily possible to increase the proportional amount of ammonia in the urine by the administration of mineral acids. An increase of the proportion of nitrogen excreted as ammonia, apart from the administration of acids with the food, is an im- portant indication of the formation of abnormal acid substances in meta- bolism. Thus in diabetes, when the last stages of fat oxidation are in default, so that the oxy-fatty acids, ^-oxybutyric and aceto-acetic acids, accumulate in the body, there is always a considerable rise in the ammonia of the urine. It is usual to reckon iron among the bases which may be excretec! by the urine. The amount of this substance in the urine is extremely small, as a rule less than 5 mg. in the day. It affords no clue to the iron metabolism of the body, since the main channel of excretion of this substance is the intestine. COMPOSITION AND CHARACTERS OF URINE 1115 ORGANIC CONSTITUENTS OF THE URINE Almost all these constituents contain nitrogen, which in man is dis- tributed among the various urinary constituents as follows : Urea Ammonia Creatinine Uric acid 85-90 per cent. 2-4 „ 3 1-3 „ About 6 per cent, of the urinary nitrogen is in the form of other substances, such as hippuric acid, pigments, &c. /NH OH \«H^ UREA or CARBAMIDE, COc^ Zu^ can be regarded as derived from carbonic acid, CO<^qjj by the replacement of each OH group by an NHj group. It is isomeric with ammonuim cyanate, NH4CNO. If a solution of potassium cyanate and ammonium chloride be warmed together and evaporated, crystals of urea may be obtained in long colourless prisms (Fig. 516) without any water of crystallisation. It is soluble in water and alcohol, and insoluble in ether. Its solutions are neutral in reaction, but it forms crystalUne salts with strong acids. Thus urea nitrate, which is produced by treating strong solutions of urea with concentrated nitric acid, forms microscopic rhombic plates which are extremely insoluble, so that their formation may be used as a test for urea (Fig. 517). With oxalic acid urea solutions yield an insoluble oxalate, also in typical crystals. Urea when heated melts at about 130° C. On further heating, it undergoes decomposition, giving off ammonia and forming biuret, as follows : JNUo G Fig. 516. Urea. (Funke.) 2 (C0<^^) = n;;>NH + NH3. CO C0<: NH, Fig. 517. Urea nitrate. Urea oxalate. At the same time a certain amount of cyanic acid is formed, and this polymerises to cyanuric acid, H3C3N3O3. On heating with, alkalies or with strong acids urea undergoes hydrolysis, with the production of carbon dioxide and ammonia : CONoH, + HoO = COo + 2NH3. The same change is effected in urea by certain micro-organisms, c.y. the 1116 PHYSIOLOGY micrococcus lU'ese, which is responsible for the ammoniacal change which occurs in urine when exposed to the air. Urea, being the chief nitrogenous constituent of the urine, is the most important index to the protein metabohsm of the body. As we have seen, urea may be regarded as partly exogenous, partly endogenous. The greater part of the 30 grm. excreted by a normal indi\^dual in the course of the day is derived directly from the proteins of the foods, as a result of the deamination of the amino-acids, which occurs shortly after their absorption. The ammonia thus formed is combined with carbonic acid and carried to the Hver, where the ammonium carbonate undergoes dehydration with the pro- duction of urea. Its amount will therefore be directly proportional to the amount of protein taken as food. A smaller proportion is derived from the breakdown of the tissues of the body. This endogenous moiety may undergo considerable increase under any conditions which cause a rapid dis- integration of the tissues. Thus there is a large rise in the urea output, even in a starving indi^ddral, in febrile conditions. In order to prepare iirea from urine advantage may be taken of the insolubility of its combination with nitric acid. Urine is concentrated to about one-sixth of its bulk. It is then cooled and treated with twice its volume of pure concentrated nitric acid, care being taken to keep the whole mixture cool. Urea nitrate is precipitated. The precipitate is collected, dried roughly by pressing between filter paper, and then rubbed up with fresh barium carbonate. Barium nitrate is formed and urea set free. The whole mass is dried and treated with hot absolute alcohol, in which the barium nitrate is insoluble. On filtering off the barium nitrate a pure solution of urea is obtained from which the urea will crystalline on allowing the alcohol to evajjorate. CREATININE. Creatinine is a normal constituent of urine, in which it occurs in quantities of 0-8 to 1-3 grm. in the twenty-four hours. It is easily produced from creatine by boihng the latter with strong hydrochloric acid, when a process of dehydration occurs. Creatine has the formula : NHa NH = C— N(CH3)CH2-C00H. On dehydration it is converted into creatinine : NH , KE = C— Kr(CH3)CH2CO Creatinine may be obtained from urine in the following way. The urine is made alkaline -nath milk of lime and treated with calcium chloride and filtered. The filtrate is slightly acidified with acetic acid and evaporated on the water bath to a syrupy consis- tence. A little sodium acetate is added and the mixture extracted with alcohol. The filtered alcoholic extract is now treated wth a concentrated neutral alcoholic solution of zinc chloride. A crystalline precipitate of creatinine zinc chloride is produced. After two days this precipitate is collected on a filter, washed with alcohol, dissolved in water, and then boiled for a quarter of an hour with lead hydrate.' The mixture is then filtered and the filtrate evaporated to dryness. The dry residue is now extracted with cold alcohol and filtered. On allowing the alcohol to evaporate, crystals of creatinine separate out (Fig. 518). It gives the following tests : COMPOSITION AND CHARACTERS OF URINE 1117 (1) WEYL'S TEST. On treating a solution of creatinine with a small quantity of very dilute sodium nitroprusside solution and then with weak caustic alkali, a rich ruby-red colour is produced which gradually changes to yellow. On now adding acetic acid and warming, the solution becomes green and then blue, and finally a precipitate of Prussian blue is formed. (2) JAFFE'S TEST. On treating a solution of creatinine with a few drops of a watery solution of picric acid and dilute caustic potash, an inten.se red colour is produced. This reaction is made use of for the quantitative estimation of creatinine in the urine. Like the other nitrogenous constituents, creatinine can be regarded as partly exogenous, partly endogenous in origin. The exogenous part is derived from the creatine contained in meat. When meat is excluded from Fig. 518. Creatinine. (Funke). Fig, 519. Uric acid. (Funke.) the diet the output of creatinine becomes remarkably constant and is little affected by the total amount of proteins taken, the amount excreted during starvation being practically identical with, that excreted on a full protein diet. It is said to be increased during febrile conditions and as a result of violent muscular exercise. URIC ACID. Uric acid is 2-6-8-trioxypurine. HN— CO N=C(OH) II II OC C— NH, or (HO)C C— NH I II >C0 II II >C(OH) HN— C— NH/ N— C — N/ Uric acid forms small rhombic crystals. The crystalline form varies considerably in the presence of impurities. The different forms of uric acid crystal which may occur in the urine are shown in the accompanying figure (Fig. 519). It is extremely insoluble in pure water, one part of uric acid requiring 39,000 parts of water at 18° C. for its solution. It is easily soluble in concentrated sulphuric acid and alkalies. It may be prepared from human urine or from guano, which consists almost entirely of urates. In order to prepare it from guano, this is dissolved with the aid of heat in 1118 PHYSIOLOGY dilute sodium carbonate, filtered, and the filtrate treated with a few drops of concen- trated hydrochloric acid and boiled. On allowing to cool, the uric acid crystallises out. From urine uric acid may be obtained by adding one-fiftieth of its volume of concen- trated hydrochloric acid and allowing to stand for two days. The uric acid is thrown down in small dark-red or brown crystals. They can be collected on a filter, dissolved in alkali, decolorised by boiling with animal charcoal, and the pure acid thrown down as before by means of hydrochloric acid. A more convenient method of preparation from human urine is based on the fact that ammonium urate is insoluble in concentrated solutions of ammonium chloride (Hopkins). The urine is saturated with crystals of ammonium chloride and a few drops of strong ammonia added. A gelatinous precipitate of ammonium lu^ate is produced. This is collected on a filter, washed oflf with a minimum amount of hot water into a beaker, and a few drops of hydrochloric acid added. The mixture is boiled and then allowed to cool, when the pure acid crystallises out. TESTS FOR URIC ACID (1) MUREXIDE TEST. If a small quantity of uric acid be treated with a little strong nitric acid and the whole evaporated to dryness on the water-bath, an orange- red residue is obtained, which on treatment with ammonia yields a fine purple colour. If a drop of sodium hydrate be now added the purple changes to blue. Instead of nitric acid, bromine water may be employed. (2) SCHIFF'S TEST. If uric acid be dissolved in a little soda and a drop be placed on filter paper previously moistened with silver nitrate, a yellow or brown spot is produced. (3) On boiling uric acid with Fehling's solution for some time, a yellowish precipitate of cuprous hydrate is produced. (4) An alkaline solution of uric acid on treatment with a few drops of a solution of phosphomolyndic acid gives a dark blue precipitate with a metallic lustre, consisting of microscopic prismatic crystals. (5) With sodium hypobromite uric acid is decomposed, giving off about half of its nitrogen as the free gas. URATES. Of the four hydrogen atoms in uric acid two can be replaced by metalhc radicals. Uric acid thus acts as a weak dibasic acid. It forms three orders of salts, namely, the neutral urates, the bi-urates, and the quadri-urates. The neutral urates, M\\J, are very unstable, and only exist in the presence of caustic alkahes. They are decomposed even by the carbonic acid of the atmosphere. The bi-urates, MHU, are the most stable of the urates. They may be prepared by dissolving uric acid with the aid of heat in weak solutions of the alkaline carbonates, from which they separate, on coohng, in stellar crystals. The qaudri-urates have the formula HgU, MHU. They may be pre- pared by boiling uric acid with dilute solutions of potassium acetate. On cooling the mixture the quadri-urate separates as an amorphous precipitate or in crystalhne spheres. The quadri-urates are extremely unstable, and in the presence of water are broken up into the bi-urates and free uric acid. It is probable that under normal conditions the greater part of the uric acid in the urine is present in the form of a quadri-urate (Roberts), and the so- called lateritious deposit, the brick-red amorphous precipitate of urates which occurs in concentrated urine on cooling, consists of these quadri- urates. The exact condition of the urate, however, will depend on the COMPOSITION AND CHARACTERS OF URINE 1119 reaction of the urine. A bi-urate, with acid sodium phosphate, is decom- posed with the formation of uric acid in the following way : MHtJ + MH2PO4 = H2U + M2HPO4. Thus the quadri-urates present in the urine immediately after its secre- tion will tend to undergo spontaneous decomposition into uric acid and the bi-urate, and the latter itself may be decomposed with the formation of uric acid and alkaUne phosphate. We see therefore that when the urine is acid, i.e. there is a predominance of acid phosphates, there will be a tendency to the precipitation of uric acid in the urinary passages. If, however, the di- sodium phosphate be in excess the uric acid may be kept in solution as the quadri-urate or even as the bi-urate. The uric acid of the urine is derived almost entirely from the purine metabohsm of the body. The uric acid may be endogenous or exogenous, i.e. may be derived from the breaking down of the nucleins of the cells or by a direct transformation of the nucleins contained in the food. The amount passed daily varies between 0-4 and 1 grm., according to the nature of the diet. It is not absent from the urine even during complete starvation. It is increased when foods are ingested rich in nucleins, such as Hver or sweet- breads, or in any other precursors of uric acid, e.g. hypoxanthine, such as meat or meat extract. We have no evidence that the urinary uric acid in the mammal is formed by synthesis, though this is the manner in which the greater part of the uric acid excreted by birds and reptiles is formed. Small traces of purine bases also occur in urine, namely, xanthine, hypo- xanthine, and adenine. When tea and coffee are taken the methyl-purines may occur, namely, caffeine, theobromine, and their derivatives. HIPPURIC ACID is a frequent, though not a constant, constituent of human urine. It is derived from benzoic acid or from an aromatic sub- stance which on oxidation can give rise to benzoic acid. In the Icidneys the benzoic acid is conjugated with glycine to form hippuric acid. The amount of hippuric acid excreted in the day may vary between 0-1 and 1 grm. After a diet rich in fruit or vegetables its amount may rise to 2 arm. It is present in considerable quantities in the urine of herbivora and may be most easily prepared from horses' urine. Hippuric acid has the formula : CeHgCO I HNCH2COOH It can be obtained in milk-white crystals (Fig. 520), which are only shghtly soluble in cold water, but easily soluble in alcohol, ether, and acetic acid. It is insoluble in petroleum, ether, and benzol. On heating, it is broken up into benzoic acid and glycine. On heating with concentrated nitric acid it forms nitro-benzol, which can be recognised by its characteristic smell of bitter almonds. In order to extract it from tlie urine, the urine is made alkaline with sodium carbo- nate, filtered, and the filtrate evaporated to a syrupy consistence. This is then treated 1120 PHYSIOLOGY with alcohol, the alcohol evaporated, and the residue repeatedly extracted with acetic ether. The acetic ether is collected, evaporated to dryness, and the residue repeatedly extracted with petroleum ether to remove the benzoic acid and fat. What is left behind is hippuric acid, which can be purified by recrystallisation from alcohol or ether. AMINO- ACIDS. According to Levene and van Slyke amino-acids are always present in the urine, and contribute about 1-5 per cent, of the total nitrogen. OTHER AROMATIC SUBSTANCES. The chief of these is the so-called ' urinary indican,' or potassium-indoxyl-sulphate. This is derived from the indol produced in the intestines from the tryptophane contained in the proteins of the food, the change being effected by the influence of the micro- organisms of putrefaction. The amount of the conjugated sulphates in the urine is thus an index of the extent of putrefaction in the intestines. In dogs, when the intestine has been disinfected by repeated doses of calo- mel, the conjugated sulphates entirely disappear from the urine. Urinary indican has the formula : H C /\ HC HC C -COSOoOH CH Fig. 520. Hippuric acid. (Ftjnke.) C H N H In addition to the tests for conjugated sulphates mentioned earlier, the indoxyl- sulphate can be detected by various methods dependent on the formation of indigo blue. The urine is treated with an equal volume of concentrated hydrochloric acid and several cubic centimetres of chloroform added. A solution of chloride of lime is now added drop by drop, shaking after the addition of each drop. A blue colour is produced which is extracted by the chloroform. It is important not to add too much chloride of lime, as otherwise the blue colour first produced will be destroyed by further oxidation. THE URINARY PIGMENTS. Normal urine gives no definite absorption bands. It owes its colour to the presence of a yellow pigment, urochrome. In order to separate urochrome from urine, the urine is saturated with crystals of ammonium sulphate and filtered. The filtrate, which still contains nearly all the colour of the urine, is shaken up with alcohol, which withdraws the greater part of the colouring-matter. On concentrating the alcohohc solution and pouring it into an equal volume of ether, an amor- phous brown precipitate falls, which is the urochrome. Urochrome, on treatment with aldehyde, yields a pigment closely similar to urobihn. On the other hand, urobilin, treated with potassium permanganate, is converted into a substance practically identical with urochrome. Urochrome must therefore be derived from the same source as urobihn. COMPOSITION AND CHARACTERS OF TRINE 1121 Urobilin is rarely present in normal urine, and then only in the form of a chromogen, from which it must be set free by acidification. In certain pathological conditions, especially in cirrhosis of the liver, urobilin may occur in the urine in considerable quantities. In order to extract urobilin from such urine, the urates are first precipitated by saturation with ammonium chloride, and the filtrate is then saturated \\-ith ammonium sulphate and a droj) of sulphuric acid added. On shaking the fluid up with a mixture of two parts ether and one part chloroform, the urobilin is taken up by the latter. The ether-chloroform solution is separated off and shaken up ^vith caustic soda, when the urobilin passes entirely into the alkaline solution. Urobilin in solution gives a single absorption band between the lines b and F, i.e. at the junction of the green and blue of the spectrum. On treating with zinc chloride and ammonia its solutions show a well-marked green fluorescence. The urobihn of urine is identical vdih stercobilin, the colouring-matter of the faeces. It is formed from bile when the latter decomposes, and is probably produced in the intestines by the action of micro-organisms on bile pigment. Other pigments which may occur in urine are uroerythrin and hsema- toporphyrin. Uroerythrin gives the pink colour to urate sediments. Its chemical nature is not known. It is distinguished by the fact that on addition of caustic soda the pink colour is changed to green. On suspending the red-coloured precipitate of urates in hot water and extracting with amyl alcohol, a pink solution is obtained which shows two absorption bands in the green part of the spectrum. HcBmotoporphjrin is only present in very small amounts in normal uiine, but under certain conditions, especially after poisoning \vith sulphonal, it may occur in such large quantities as to give the urine a deep purple colour. Under these circumstances it is found in the form of alkaline ha^natopor- phyrin and gives the characteristic absorption bands of the latter. Urorosein is a name that has been given to a pigment which is formed when the urine is treated with strong mineral acids. It is probably an indol derivative. It gives a single absorption band between the lines D and E. ABNORMAL CONSTITUENTS OF THE URINE A very large number of substances occur in the urine in minute traces and maj' be detected when large quantities of this fluid are worked up at one time. Most of the so- called pathological constituents may be detected in this way in normal urine. It is only when they occur in casilj' detectable amoimts that their presence becomes of any significance. COAGULABLE PROTEIN. Under normal circumstances urine is free from any coagulahlc protein exeopt the small traces of nnicinous material, nucleoprotein, vhich gives the cloudiness to the urine. If the kidncy-cclls are damaged by disease, by inter- ference with their blood-supply, or by circulating poisons, the glomerular ei)ithelium permits the passage of a certain amount of the proteins of the plasma. Under these circumstances, if small pieces of the kidney be plunged into boiling water, the coagulated protein may be seen in Bowman's capsule. The presence of eoagulable protein (gene- rally spoken of as albumin) in the urine is sigmficantof the pathological conditions of the kidney associated with Bright's disease. A small trace •nnll generally be found in 36 1122 PHYSIOLOGY the urine which is passed shortly after taking muscular exercise. Under this condition the presence of albumin in the urine has no pathognomonic significance. The proteins generally found are identical with those of the blood-plasma and con- sist of serum albumin and serum globulin. Their presence in the urine may be detected Fig. 521. Glucosazone. by the precipitate produced on boiling. In carrying out this test a few cubic centi- metres of saturated salt solution should be added and one or two drops of dilute acetic acid. A more delicate test is that known as Heller's. Some strong nitric acid is placed Fig. 522. Lactosazone. (Plimmer.) in a test-tube and the urine is poured carefully down the side of the tube so as to form a layer on the surface of the nitric acid. If albumin be present a white ring is formed at the junction of the two liquids. SUGAR. Normal urine contains about one part per thousand of glucose. In diabetes the power of assimilating carbohydrates is diminished or destroyed. The COMPOSITION AND CHARACTERS OF URINE 1123 amount of sugar in the blood is increased, and sugar appears in large quantities in the urine. The sugar is practically always glucose or dextrose. Lactose may occur in the urine of nursing women even in conditions of health. Since both these sugars will reduce Fehling's solution it becomes important to be able to distinguish between them. The following tests are used for the detection of abnormal amounts of sugar in the urine : (1) FEHLING'S TEST. The urine is boiled with Fehling's solution (an alkaline solution of copper sul])hate to which Rochelle salt has been added to keep the cupric hydrate in solution). Under the action of glucose or lactose the cupric hydrate is reduced to an insoluble cuprous hydrate, which forms a yellow or red precipitate. (2) The phenylhydrazine test may be carried out as follows : 2 c.c. of .50 per cent, acetic acid, saturated with sodium acetate, and two drops of i)henylhydrazine are added to 5 c.c. of urine. The mixture is evaporated down to .3 c.c, rapidly cooled, and again warmed in a water bath. It is then allowed to cool slowly. Crystals of the correspond- ing osazone separate out in the hot liquid in the case of glucosazone, on cooling in the case of lactosazone (Figs. 521, 522). (3) The most convenient way of distinguishing between lactose and glucose is by adding a little yeast to the urine in an inverted test-tube. If glucose be the sugar present, it is fermented by the yeast with the production of carbon dioxide, which collects at the top of the test-tube. In rare circumstances fructose or Isevulose, or pentose may be found in the urine. The former would be detected by the fact that it rotates polarised light to the left, instead of the right, as is the case with glucose. GLYCURONIC ACID. Small traces of this are present in normal urine. It occurs as a conjugated acid after the administration of various substances, e.g. camphor and chloral. If phenol, indol, or scatol be given to an animal which is receiving very little protein in its diet, these substances will leave the body conjugated, not with sulphuric acid, but with glycuronic acid. Glycuronic acid may be regarded as the first product of oxidation of glucose, having the formula : COOH I ■ (CH0H)4 I CHO It reduces Fehling's solution and rotates the plane of polarised light to the left. OXY-FATTY ACIDS AND ACETONE. These substances occur often as.sociatcd with glucose in diabetes, especially towards the latter stages. They represent the pen- ultimate stages in the oxidation of the fats. Their relation to one another is seen from their formulae : CH3 CH3 CH3 CHOH CO CO I I I CH2 CHo CH3 I I COOH COOH Oxybutyria aciil Accto-acetic acid Acetono They may also occur in any condition of carbohydrate starvation, relative or abso- lute. Thus they are found in the urine during absolute starvation as well as in individuals on a pure fat and protein diet. The two acids are generally found associated in the urine. The presence of aceto-acetic acid may be detected as follows : (1 ) To some urine add ferric chloride as long as a precipitate of ferric phosphate con- tinues to form. Filter this off and to the filtrate add a few more di-o^js of ferric chloride. If the acid be present a claret colour is ]noduced. 1124 PHYSIOLOGY (2) On heating with dilute alkali, aceto-acetic acid is decomposed, with the pro- duction of acetone. This may be detected by its odour or by distilling off a small proportion of the fluid and testing the distillate in the following ways : (a) On the addition of sodium hydrate and iodine and warming, iodoform is formed. (&) Legal's test. A few drops of freshly prepared sodium nitroprusside solution is added and the mixture rendered alkaline with sodium hydrate. A deep red colour is formed. On acidifying with acetic acid this colour is changed to a reddish purple. CYSTINE. This substance, which is a normal product of the hydrolysis of proteins, is found as a constant constituent to the amount of half a gramme a day in the urine of certain individuals. The condition of cystinuria represents, like alcaptonuria, an inborn error of metabolism. It is found in the child and persists throughout life. In such cases the cystine may give rise to urinary deposits or even to a urinary calculus. HOMOGENTISIC ACID. This is an aromatic acid having the composition of dioxyphenyl acetic acid. Its formula is as follows : OH I CHo.COOH OH It occurs as a consitutent of the urine of certain individuals, who are said to be affected with alcaptonuria. The urine of these cases is remarkable for its resistance to putrefactive changes. It slowly darkens on exposure to the air, and on the addition of alkali and shaking with air it becomes rapidly brown or black. It reduces Fehling's solution, so that the presence of sugar may be suspected. Such urine contains homogen- tisic acid in a quantity of 3 to 6 grm. per day. The amount of the acid excreted varies with the protein food taken. It seems that in these cases the power of the organism to break up tyrosine and phenylalanine is entirely absent. If either of these substances be administered by the mouth it is converted almost quantitatively into homogentisic acid, which appears in the urine. Individuals with alcaptonuria continue to secrete homo- gentisic acid during starvation, so that the tjTosine and phenylalanine set free in the course of tissue disintegration undergo the same fate as when they are derived from the food. Alcaptoniu"ics apparently suffer no ill effects as a result of their abnormal metabolism. The tyrosine and phenylalanine can be absorbed and play their part in building up the proteins of the tissues, but the process or ferment is wanting which is responsible for the further break-up of the first product of their oxidation, namely, homogentisic acid. URINARY DEPOSITS In addition to formed elements, such as blood-corpuscles, bacteria, or pus-cells, which may occur in abnor- mal urine, the following deposits may Fig. 523. Various forms of uric acid -i e j crystals. (Frey.) be tound : (a) IN ACID URINE (1) Amorphous urates occur generally as a brick-red amorphous deposit thrown down as the urine cools. It is redissolved on warming the urine, and consists generally of the quadri-urates. The acid urate of sodium and of ammonium may occasionally occur in star-shaped clusters of needles or as spherules with small crystals adhering to them. COMPOSITION AND CHARACTERS OF URINE 1125 (2) Uric acid. Whetstone, dumb-bell, orsheaf-likeaggiegatioiisof crystals, generally deeply pigmented so as to resemble cayenne pepper (Fig. 523). (3) Calcium oxalate (Fig. 524). Colourless, transparent, highly refrac- Fio. 524. Urinary deposit, containing uric acid, sodium urate, and calcium oxalate. Fig. 525. Deposit of ' triple ' phosphate and ammonium urate. (Funke.) tive octahedral crystals (envelope-shaped), in hydrochloric acid. (4) Ammonium magnesium phosphates crystals have been compared to knife-rests or coffin-hds (Fig. 525). They are soluble in acetic acid. (5) Calcium hydrogen phosphate. CaHPO^. These are rare. They form large prismatic crystals often arranged in rosettes. Easily soluble in dilute acetic acid. On adding a solution of ammonium carbonate the crystals are eaten away and form an amorphous deposit. (6) Tyrosine, fine needles in star- shaped bundles, and cystine, in regular hexagonal plates, may occur under very rare circumstances. Insoluble in acetic acid, soluble (in faintly acid urine). The Fig. 520. Aninionivnn urate. (h) IN ALKALINE URINE (1) The commonest precipitate consists of earthy phosphates, amor- phous, easily soluble in dilute acetic acid. (2) Ammonium magnesium phosphate or triple jihosphate is common in urine which has undergone ammoniacal fermentation. (3) Acid ammonium urate (Fig. 52(5) may also occui- in alkaline urine. On treatment with HCl it is dissolved and uric acid in crystals slowly separates out. 1126 PHYSIOLOGY QUANTITATIVE ESTIMATION OF THE CHIEF URINARY CONSTITUENTS It may be useful here to summarise the most trustworthy methods which are employed for the estimation of the chief urinary constituents.* The TOTAL ' ACIDITY ' of the urine is measured by titrating it against decinormal alkali in the presence of an indicator, such as phenolphthalein. The indistinctness of the end-point is due to the presence of calcium salts and ammonium salts. Folin there- foi'e recommends that the titration be carried out in the presence of potassium oxalate, which diminishes the error. Method. To 25 c.c. urine add 15 to 20 grm. potassium oxalate and 1 to 2 drops of phenolphthalein. Shake thoroughly for one or two minutes, and whilst the solution is still cold from the effect of the oxalate, titrate with — NaOH until a permanent pink remains. ^ TOTAL NITROGEN. In all metabolic experiments, the determination of the total nitrogen of the food, the urine, and the fseces is indispensable. In each case Kjeldahl's method is employed. This method depends on the fact that all the nitrogenous sub- stances met with in the body, when heated for a considerable time with concentrated sulphuric acid, undergo oxidation, the nitrogen being finally converted into ammonia. On adding alkali to the mixture the ammonia is set free from its combination with the sulphuric acid and can be distilled off and received into a vessel containing a known amount of decinormal acid. By titrating this acid after the operation we can determine the quantity of ammonia which has been produced. To carry out this method 5 c.c. of m-ine are heated with 20 c.c. sulphuric acid and a small quantity of copper sulphate and potassium sulphate. The copper sulphate is to aid the oxidation of the organic substances, the potassium sulphate is to raise the boiling-point of the mixture. The boiling is continued for half an hour. The flask is then cooled and half filled with dis- tilled water. • A special form of distillation tube (Fig. 527) is now attached by a rubber cork which fits tightly, but just before this is done an excess of strong caustic soda sufficient to neutralise the concentrated sulphuric acid is run in under the acid. The other end of the distillation tube is at once arranged to dip under the surface of a measured quantity of standard acid {e.g. 10 c.c H2SO4), diluted with water, and con- tained in a 600 c.c. Erlenmeyer flask. The flask is then shaken and heated. In about a quarter of an hoiu- the ammonia is completely distilled off, and its amount can be determined by titrating the acid in the flask with — NaOH, methyl orange being used as indicator. UREA. The method usually adopted for estimating the urea is that devised by Hiifner. It depends on the fact that urea is decomposed by an alkaline hypobromite with the production of CO2 and nitrogen. In the presence of an excess of alkali the CO2 is absorbed and the nitrogen may be collected and measured, and serves as an index of the amount of urea present. The reaction which occurs is as follows : C0(NH2)2 + SNaBrO + 2NaOH = 3NaBr + N2 -f Na2C03 + SHgO 60 grm. 22-4 litres = 28 grm. 1 grm. 372 c.c. Actually, however, only 354'33 c.c. nitrogen are evolved by 1 grm. urea. The disadvantage of this method is that other substances, such as ammonia, creati- nine, and uric acid, give off a certain amount of their nitrogen with sodium hypobromite, so that the method is not strictly accurate, though enough so for most clinical purposes. In actually carrying out the method 5 c.c. of urine are treated with 25 c.c. of freshly *Fuller details will be found in Plimmer's " Practical Physiological Chemistry," from which most of the methods here given are taken. COMPOSITION AND CHARACTERS OF URINE 1127 prepared solution of sodiuiu hypobroniite, and the nitrogen evolved is collected in a graduated tube over water. Folin'fi Method. In Kjeid-thrs method all the nitrogenous constituents of the urine are converted into ammonia by boiling with strong sulphuric acid. This conversion occurs with extreme readiness in the case of urea, so that by using a weaker acid and carefully regulating the temperature the hydrolysis may be confined practically to the urea itself. This is the principle of Folin's method of estimating urea. Five cubic centimetres of urine are measured into a 200 c.c. Erlemneyer flask. Five cubic centimetres of concentrated hydrochloric acid, 20 grm. crystallised magnesium chloride, a piece of paraffin the size of a small hazel nut, and finally 2 or 3 drops of a 1 per cent, solution of alizarin red in water are added. A special safety tube is then inserted into the neck of the flask and the mixture boiled until each returning ch'op from the safety tube produces a very perceptible bump. The heat is then reduced somewhat, and the heating is continued for a full hour. The alizarin red is used in order to ensure that the contents of the flask do not become alkaline. At the end of an hour the contents of the Hask are put into a litre flask with about 700 c.c. water and 20 c.c. of a 10 per cent, sodium hydrate, and the ammonia is then distilled off into a measured quantity of acid. The results obtained in this way will give us the total amount of urea together with any ammonia which was preformed in the urine. It is therefore neces- sary also to determine the amount of this pre- formed ammonia. ESTIMATION OF AMMONIA. In Folin's method for the estimation of ammonia, this is set free by the addition of weak alkali (sodium carbonate) and is then removed from the urine at ordinary room temperature by passing a strong current of air thi'ough the liquid. The issuing current of air carrymg the ammonia passes through a measured quantity of decinormal acid. If the air current be sufficiently strong, one and a half hours is sufficient to remove the whole of the ammonia from 25 c.c. of urine. The decinormal acid is then titrated and the amount of the ammonia reckoned. In carrying out the method 25 c.c. of urine is measured into a cylinder 30 to 40 cm. high and about a gramme of sodium carbonate and some petroleum (to prevent foaming) are added. The upper end of the cylinder is then closed by a doubly perforated rubber stopper through which pass two glass tubes, only one of which is long enough to reach below the surface of the liquid. The shorter tube, about 10 cm. in length, is con- nected with a calcium chloride tube tilled with cotton, and this in turn is connected with a glass tube extending to the bottom of a wide-mouthed bottle, capacity about 500 c.c, which contains 20 c.c. decinormal acid in 200 c.c. of water. A more convenient method for the estimation of ammonia is that originally pro- posed by Schitf and recently worked out by Malfatti. It depends on the fact that when a neutral solution of an ammonium salt is treated with formaldehyde, combination occurs with the formation of hexamethylene tetramine (urotropine) and the liberation of a corresponding amount of acid, which can be estimated by titrating with decinormal alkali. The reaction which occurs is as follows : 6CH2O + 2(XH4)2S04 = 6H2O + N4(CH2)r, -f 2H2SO4. Formaldehyde Hexamethylene tetramine In carrying out this method 25 c.c. of urine are measured by means of a piiiette into a flask or beaker and diluted with five times its volume of water. Four or live drojjs of Fig. 527. 1128 PHYSIOLOGY phenolphthalein are then added and decinormal sodium hydrate is run in until there is a slight permanent pink colour. The amount of alkaline solution necessary to produce this colour is a measure of the acidity of the urine. Ten cubic centimetres of formalin, diluted v/ith three volumes of water and previously neutralised to phenolphthalein with decinormal alkali, are then added. The colour disappears owing to the setting free of the acid radicals previously combined with ammonia. Decinormal alkali is then run into the mixture until a permanent pink colour is again obtained. The number of cubic centimetres of the decinormal alkali required in this second case corresponds to the amount of decinormal ammonia previously present in the 25 c.c. of urine. This method gives somewhat higher figures than the method of Tolin just described, owing to the fact that the small traces of amino-acids, which may be present in the urine, react to formalin in a very similar way. The difference does not exceed 10 per cent., so that the method is amply delicate for clinical purposes. CREATININE. In Folin's method for the determination of creatinine, which is now universally employed, advantage is taken of the colour reaction given by creatinine (and by no other normal urinary constituent) with picric acid in alkaline solution (Jafire's reaction), the colour being compared with that of a standard potassium bichromate solution. The reagents employed are decinormal potassium bichromate containing 24-55 grm. per litre, saturated picric acid solution containing about 12 grm. per litre, and a 10 per cent, solution of sodium hydrate. For the comparison of the colours a Duboscq colorimeter is employed. Ten cubic centimetres of urine are measured into a 500 c.c. flask ; 15 c.c. of picric acid and 5 c.c. of sodium hydrate are then added and the mixture allowed to stand for five minutes. Some of the potassium bichromate solution is placed into one of the cylinders of the colorimeter and its depth accurately adjusted to the 8 mm. mark. At the end of five minutes the contents of the 500 c.c. flask are diluted up to 500 c.c. with water and some of the mixture placed into the other cylinder of the colorimeter, and the two colours are then compared. The calculation of the results is very simple. If, for example, it is found that it takes 9-5 mm. of the unknown urine picrate solution to equal the 8 mm. of the bichromate, then the 10 c.c. of urine contains 8*0 10 X — =8-4 mg. creatinine. 9-5 ^ ESTIMATION OF URIC ACID. The best method for this purpose is a slight modi- fication by Folin of the method devised by Hopkins. For this method the following reagents are required : (1) A solution of ammonium sulphate, uranium acetate, and acetic acid, made up as follows : 500 grm. ammonium sulphate, 5 grm. uranium acetate, and 60 c.c. 10 per cent, acetic acid are dissolved in 650 c.c. water. The volume of this solution is almost exactly 1000 c.c. (2) Ten per cent, ammonium sulphate solution. (3) — potassium permanganate solution made by dissolving 1-581 grm. pure potassium permanganate in one litre of water ; 1 c.c. = -00375 grm. uric acid. Measure 200 c.c. urine with a pipette into a 500 c.c. flask and add 50 c.c of the ammonium sulphate and uranium acetate reagent. Mix the solutions and allow to stand for about half an hour so as to allow the precipitate to settle. This precipitate contains a mucoid substance (and phosphates) which, if not thus removed, renders the subsequent filtration and washing of the ammonium urate precipitate very slow. Filter off the supernatant liquid through a dry filter into a dry vessel, and measure out 125 c.c. ( = 100 c.c. urine) of this with j)ipettes into a beaker. Add 5 c.c. concentrated ammonia, mix well, and allow to stand covered with paper for twelve to twenty-four hours. Carefully decant the suijernatant liquid upon a filter, wash the precipitate of ammo- nium urate on to the filter with 10 per cent, ammomum sulphate, and wash this once or twice with the same reagent to remove the chlorides as completely as possible. Remove the filter from the funnel, open it, and with a fine stream of water wash COMPOSITION AND CHARACTERS OF URINE 1129 the ammonium urate precipitate into a beaker. To the ammonium urate precipitate, suspended in about 100 c.c. water, add 15 c.c. strong sulphuric acid and titrate at once, without cooling, with the potassium permanganate solution. At first every small addition of the permanganate is decolorised before it diffuses through the liquid, but towards the end the decolorisation is slower and the permanganate should be added two drops at a time until a faint pink colour is seen throughout the whole solution. The amount of uric acid can then be calculated, 1 c.c of the permanganate solution being equivalent to -00375 grm. uric acid. CHLORIDES. The chlorides of urine are estimated by Volhard's method. The principle of this method consists in precipitating the chlorides by excess of a standard solution of silver nitrate in the presence of nitric acid. The excess of silver is then estimated in an aliquot part of the filtrate with a solution of potassium or ammonium sulphocyanate which has been previously standardised against the silver solution, a ferric salt being used as indicator. The following solutions are required : (1) Standard silver nitrate solution either — or so that 1 c.c. corresponds to -01 grm. NaCl. 10 (2) Potassium sulphocyanate solution (8 grm. per litre). (3) Pure HNO3 free from chlorides. (4) A saturated solution of iron alum. The potassium sulphocyanate solution must be standardised against the silver nitrate solution. This is carried out as follows : Place 10 c.c. AgNOg solution with a pipette in a beaker, add 5 c.c. pure HNO3, 5 c.c. iron alum solution, and 80 c.c. water. Now run in the sulphocyanate solution from a burette until a permanent red tinge is obtained. Note the amount required for the 10 c.c. AgNOg solution. The method of analysis is carried out as follows : Place 10 c.c. urine in a 100 c.c. measuring flask with a pipette. Then add about 4 c.c. pure nitric acid and 10 or 20 c.c. with a pipette of the standard silver nitrate solution. Now fill up to the mark with distilled water, mix thoroughly, and filter into a dry vessel through a dry paper. Take exactly 50 c.c. of the filtrate with a pipette and titrate with the sulphocyanate solution until a permanent red colour is obtained, iron alum having been added before the titra- tion is commenced. Calculation of results : 50 c.c. filtrate = S c.c. KCNS .-.100 c.c. „ =2Sc.c. „ Now X c.c. KCNS = 10 c.c. AgNOg oe 10 X 25 . __ .-. 2yS'c.c. „ = AgNOa This is the excess not utilised to precipitate the chlorides 10 X 2S\ .-. (20 — = amount of AgNOg solution used. X I Hence NaCl in grammes per 10 c.c. in the volume passed in twenty-four hours. ESTIMATION OF PHOSPHATES. The method depends upon the precipitation of all the phosphates by a standard solution of uranium acetate or uranium nitrate in the presence of sodium acetate and acetic acid as (Ur02)HP04. The determination of the end-point, when soluble uranium salt is in solution, is shown by means of potassium ferrocyanide, or by cochineal tincture, which becomes green. The following reagents are required : (1) Acid sodium acetate solution (100 grm. NaAc, 30 grm. HAc, 1000 c.c. HgO). (2) Cochineal tincture (5 grm. cochineal extracted for several days with 150 c.c. alcohol and 100 c.c. water and then filtered). (3) Standarel uranium solution (I c.c. = -005 grm. P2O5 or 5 mg.). This must be prepared by standardising against a standard phosphate solution. Generally sodium phosphate is employed ; about 12 grm. are weighed out and dissolved in 1000 c.c. water ; 50 c.c. of this solution are evaporated to dryness, incinerated, and 36* 1130 PHYSIOLOGY weighed as pyroiDhosphate. From the weight of this the amount of P2O5 in 50 c.c. can be calculated and the remainder of the solution can be diluted, so that 50 c.c. contain 0-1 grm. P2O5. It is simpler to use acid potassium phosphate, KHgPO^, which can be weighed directly and dissolved in water, so that 50 c.c. contain 0-1 grm. P2O5. Fifty cubic centimetres of this solution are titrated with the uranium solution (36 grm. in one litre) in the manner described below, and the uranium solution is then diluted so that 1 c.c. = 5 mg. P0O5. The method of analysis is carried out as follows : Place 50 c.c. urine with a pipette in a 100 c.c. beaker, add 5 c.c. acid sodium acetate solution and a few drops of cochi- neal tincture. Heat the urine to boiling and run in slowly the standard uranium acetate solution from a burette as long as a precipitate is formed. Again heat to boiling and add the uranium solution drop by drop, until the red colour is changed to green. This end-point can also be tested by taking out a drop and placing it in contact with a drop of potassium ferro-cyanide solution or a little heap of finely powdered sub- stance on a white piece of porcelain. A brown colour or precipitate is formed when excess of soluble uranium salt is present in the solution. (A few more drops may be required to reach this point than to turn the cochineal green.) The principle of the estimation of sulphates has already been described. It is not advisable to attempt this volumetrically. SECTION II THE SECRETION OF URINE cl^-M With the single exception of hippiiric acid all the constituents of the urine are formed in parts of the body other than the kidneys. Extirpation of both kidneys leads to an accumulation of these specific urinary con- stituents in the blood and tissues. The work of the kidney is therefore confined to an excretion of preformed constituents. Considered from a broad standpoint, the function of this organ is the preservation of the normal composition of the circulating blood. Whenever the latter contains an abnor- mal constituent or any of its normal constituents are present in abnormal quantities, the kidney excretes the sub- stance in question until the composition of the blood is restored. We have to determine the conditions which influence the quantity and quality of the urine secreted by the kidneys, and to ascribe to each element in these organs its proper share in the total work of the kidney. In no other organ of the body are our views as to function so intimately dejicndent on our knowledge of structure as in the kidney. This organ is a branched tubular gland consisting in man of ten to fifteen nearly equal divisions, known as the Malpighian ])yramids. In certain animals, such as the rabbit and rat, only one pyramid is present. It is divided into an outer portion or cortex, an inner portion, the medulla, and between these the ' boundary layer,' con- taining the larger branches of the renal blood- vessels (Fig. 528). From the outer boundary of the Malpighian i)yramids of the medulla a number of processes, the medullary rays, pass out into the cortex towards the surface of the kidney. All parts of the kiilney are made up of branched tulules embedded in scanty connective tissue and richly supplied with blood-vessels. Each tubule begins by a blind dilated extremity in the cortex, known as Bowman's capsule, which surrounds a bunch of cai)illary blood-vessels, the gloniciulus, the two together forming the Mal- pighian body. From Bownum's capsule a short neck leads into a proximal convol- uted tubule, and this into a U-shaped portion which jmsses down in a medullary ray II 31 Fig. 528. Section of human kidney. (t'ADI.\T.) a, cortex ; h^ medulla or Malpighian pyramids ; c, papilla ; rf, ureter ; <', e, boundary zone. 1132 PHYSIOLOGY into the underlying portion of the medulla, and consists of straight descending and ascending limbs and the loop of Henle. The ascending limb passes into a distal convo- luted tubule, and this by a ' junctional tiibule ' joins with a number of others to form a straight ' collecting tubule.' Several of these unite to form the papillary ducts, which open on the surface of the i^apilla in the expanded ]3art of the renal duct or ureter (Fig. 529). The whole tubule consists of epithelium lying on a basement membrane ; the epithelium varies in structure in different parts of the tubule. The bunch of glomerular caj)illaries is covered with a very thin layer of endothelial cells, and a similar layer forms the lining of Bowman's capsule. The convoluted tubules contain cells which are roughly cubical or cylindrical in cross-section, but do not present very definite cell outlines. These cells, which are similar in the two sets of convoluted Cortex Boundary zone Fig. 529. Diagram showing course of urinary tubules, and the distribution of blood-vessels. (From Yeo.) tubules, have long been distinguished as ' redded epithelium ' (Fig. 530) on account of the ease with which a radial disposition of rods or granules is demonstrated in their protoplasm. As ordinarily prepared, the free margin of these cells, where they abut on the lumen, is irregular. This appearance is due to the readiness with which the cells undergo alteration under the influence of different fixing reagents, especially of such as contain water. When properly fixed it is seen that the rodded structure as described by Heidenhain is due to rows of granules arranged vertically to the basement membrane. Moreover the free margin of the cells, instead of being irregular, consist of a well-marked striated border, formed of a number of very fine hairs closely set together and springing from a row of granules in the peripheral part of the cell (Fig. 531). The hairs, which make up the striated border (sometimes called the ' brush border '), have not been observed to present ciliary movement, and are probably comparable with the similar structures found clothing the free border of the epithelium of the intestinal villus. Such cells are characteristic features of the epithelium lining the urinary organs in all types of animals, and are well marked in the nephridia of worms. Besides these rows of granules other granules are found, especially towards the free margin of the cell and round about the nucleus. Some of the granules appear to be of a fatty, others of a protein character. The descending limb of Henle's loop is narrow, and possesses flattened epithelial cells, while the ascending limb presents an epithelium similar to that of the convoluted THE SECRETION OF URINE 1133 tubules, but with less marked striation. The junctional and collecting tubules are lined with cubical or columnar cells with a clear protoplasm. The marked differences between the structure of these various parts point to a differentiation of function and division of labour among them in the preparation of the fully formed urine. This con- clusion is borne out by a study of the blood-supply of the kidney. The large renal '1 Fig. 530. A portion of convoluted tubulo with rocltled ' epithelium. (HEIDENnAIN.) artery divides in the pelvis into four or five branches, which pass uji to the boundary zone and there give off arteries in different directions ; those which run towards the surface are the interlobular arteries. Each of these, which is an end-artery presenting no anastomoses wath its fellows, gives off on all sides short wide branches, which pass to the glomeruli and constitute the vasa afferentia of these bodies. Each vas afterens has a thick muscular wall. The glomerulus itself consists of a number of anastomosing .-».*,. « ^ Fig. 531. Cross-sections of convoluted tubules from kidney ol nit. (Sauer.) A, during slight secretion ; b, during maximal secretion. wide capillaries invested by an extremely thin wall, which is sometimes said to consist simply of a protoplasmic film devoid of nuclei. The glomerular capillaries are collected together to form an efferent vessel, the ra.s cffrrcns, which is narrower than the vas artVrcns, but, like the latter, presents a well-marked nuiscular coat. The vas efferens breaks up again into a second set of capillaries, which ramify around the tubules of the cortex and communicate with a similar network round the tubules of the medulla. The medullary pyramids are also provided with blood by a plexus of capillaries talcing their 1134 PHYSIOLOGY origin from little bunches of vessels, the vasa recta {v. Fig. 529), which leave the concave side of the arterial arches of the boundary zone to run towards the papilla, and receive also a few vessels which spring from the vasa afferentia of the cortical vessels. From the capillaries of the tubules the blood is collected again into veins, which leave the kidney partly by the cortex and capsular vessels, partly by large venous trunks which join to form the renal vein at the hilum of the kidney. The kidney is richly supplied with nerves, which are chiefly distributed to the muscular walls of its blood-vessels. Some authors have described a fine nerve-plexus suiTounding the tubules and sending branches between and into the cells of the convoluted tubules themselves. The main points in the above description of the structure of the kidney were made out by Bowman in 1840, and suggested the theories of urinary secretion both of Bowman and of Ludwig (1844), theories which have furnished the basis of all our subsequent investigation of the subject. Both observers appreciated the great difference between the membrane covering the glomerular loop and the lining membrane of the tubule, and both drew attention to the difference in the circulation in these two portions of the kidney. The glomerular capillaries, supplied with blood through a short wide artery and drained by an efferent vessel smaller than the afferent, would represent a region of very high capillary blood-pressure, whereas the pressure in the capillaries surrounding the tubules must be low and similar to that in other capillary regions. Bowman therefore suggested that the urine consisted of two parts, namely, one part containing the water and salts produced by a process of filtration through the walls of the glomerular capillaries, and another part, containing the specific urinary constituents urea, uric acid, &c., secreted by the cells probably of the convoluted tubules. To Ludwig, on the other hand, it seemed possible at first to account for the whole process of formation of urine without the assumption of any active intervention on the part of the cells of the tubules. He imagined that the whole of the urinary constituents passed from the blood to the urinary tubule in the glomerulus by a process of filtration. The glomerular transu- date would represent therefore a very dilute urine containing the crystalloids of the blood in the same concentration as in the blood and with no more urea than the blood itself contained. The great difference in urea content between the blood and the fully formed urine he ascribed to a process of concentration taking place in the fluid in its passage through the tubules, in which water and certain of the salts were reabsorbed, a process of reabsorp- tion conditioned by the difference in protein content between the urine within the tubules and the lymph under low pressure on the outside of the tubules. We know now that in its original form the theory of Ludwig is untenable. If a process of concentration takes place within the tubules it must involve the performance of work by the cells lining these tubules, and could not take place as a result of mere differences of colloid content between the two fluids. It was shown long ago by Hoppe-Seyler that, if urine be dialysed against serum, a passage of water takes place, not from urine to serum, but from serum to urine, i.e. the latter is much more concentrated than the former. The movement of water from one fluid to another through a colloid mem- brane depends on the relative osmotic pressures of the two fluids, and this THE SECRETION OF URINE 1135 in turn is determined by the molecular concentration of the two fluids. It is easy to estimate the molecular concentration of any sample of serum or urine. The method which is most convenient is to determine the depression of freezing-point in the two fluids. Whereas serum ordinarily freezes at — 0-56° C. to — 0'59° C, the freezing-point of urine is generally lower and may vary from this figure to as much as - 4-5° C. For the production therefore of urine from blood-plasma, a certain amount of work has to be done, and the seat of this work we can locate only in the cells of the kidney. We may determine the minimum work necessary to form a certain amount of urine of a given concentration by measuring the amount of heat that must be imparted to the blood-plasma in order to reduce it to the same concentra- tion and volume, or we can calculate it if we know the freezing-points of the two fluids. A depression of freezing-point A = - 1° C. corresponds to an osmotic pressure of 122-7 metres of water. To concentrate 100 c.c. of a saline fluid such as urine so as to halve its bulk and double its depression of freezing-point, e.g. from - 1° C. to — 2° C, would therefore require the expenditure of work equivalent to that which would be required to compress 100 c.c. of a gas at a pressure of 122-7 metres of water to half its bulk. The abandonment of Ludwig's view as to the mechanism of the concentra- tion does not, however, place his theory out of court. The question will still have to be discussed whether the chief object of the tubules is the con- centration of the fluid produced in the glomeruli, or whether they add to this fluid by a further secretory process, or whether they may not possibly possess both functions and in their various parts alter the fluid flowing through them either by addition or by withdrawal of water or dissolved constituents. The common point in the two theories is the sharp distinc- tion which is drawn between the nature of the glomerular activity and the nature of the activity of the tubules. The questions which we have to decide by experiment are : (1) The nature of the glomerular activity and the conditions which determine the amount of fluid formed by the glomeruli, and especially whether the energy required for the formation of the glomerular fluid is furnished by the heart through the blood -pressure ^^^thin the capillaries or by the endothelium covering these capillaries. (2) The function of the tubules, whether they secrete or absorb, and what part is played in these processes by the various segments of the tubules which difEer so widely in their histological characters. THE SECRETION OF WATER AND SALTS. FUNCTIONS OF THE GLOMERULI It is generally assumed, as the best explanation of knowi\ facts with regard to the secretion of urine, that a watery exudation free from protein is formed in the glomeruli, and that this becomes concentrated on its way through the tubules either by the absorption of water and certain salts or by the secretion and addition of urea, uric acid. &c., as well as such salts as 1136 PHYSIOLOGY acid phosphates. As to the nature of the glomerular functions two opinions have been held. According to the Ludwig school the process is one simply of filtration, in which, under the pressure of the blood in the glomerular capillaries, the water and crystalloid constituents of the plasma are filtered through the glomerular epithehum, leaving behind the protein constituents. According to Heidenhain the process cannot be regarded as one simply of filtration, but involves the secretory activity of the glomerular epithehum. If the glomerular urine is a filtrate it must resemble blood-plasma in practic- ally all particulars except its protein content, since the blood pressure, which is the only force causing filtration, is too small to effect any appreciable separation of salts. On the other hand, a certain minimum difference of pressure between the two sides of the membrane must be present in order to separate the colloids from the other constituents of the plasma. We have seen in chapter iv (p. 143) that in order to produce a filtrate containing only water and salts from serum a minimum difference of pressure of 30 mm. Hg. is necessary, corresponding to the osmotic pressure of the colloidal con- stituents of the blood-plasma or serum. Thus in order to produce a filtrate, free from protein, from the blood-plasma circulating through the glomerular capillaries, the pressure of the urine in the tubules and ureter must always be at least 30 mm. lower than the pressure of the blood in the glomeruh. A direct determination of the latter figure is not possible. The anatomical arrangements are such as to bring this pressure up to a high point. Not only are the vasa afferentia very short, but the vasa efferentia are only two- thirds of the diameter of the vasa afferentia. Moreover the sudden increase of bed which ensues as the blood passes from the vas afferens to the bundle of capillaries must itself cause a rise of pressure in the latter, due to the transformation of the kinetic energy of the moving fluid into the statical energy represented by pressure on the walls of the vessels. This point can be rendered clearer by the following considerations. If fluid is flowing in a tube of continuous bore ah (Fig. 532) there will be a continuous fall of pressure pressure ""•---,_ ' 1 ~'^ pressure ''""•—, "'"-'-^ c b Fig. 532. from a to h. If, however, in the tube ahc the segment h be of much greater diameter than the segments a and c, although while the fluid is at rest the pressures will be equal at all points of the system, as soon as the fluid moves from a to c, although there is a fall of pressure between a and c, a manometer attached to h may show an actual greater pressure than a manometer inserted at a. Fluid is flowing from a place of lower to a place of higher pressure. The apparent paradox is due to the fact that the energy causing the fluid to move from a to & is of two kinds. It equals \mv^ + P, i.e. represented by the kinetic energy of the moving mass of fluid as well as the difference of pressure THE SECRETION OF URINE 1137 between any two points of the tube. The total energy will diminish continuously from a to c, and is used in overcoming the resistance of the system. We may say then that the sum of these two, namely, |m«" + P, is greater at a than b, and is greater at b than c ; but as the fluid passes from the narrow tube a into the wide tube b there is a sudden fall of its velocity and a consequent diminution of the factor l^v^. In order to provide for a continuous fall in the total energy of the fluid, namely, hmv^ + P, the diminution in the factor Imv^ must cause a corresponding increase in the factor P, i.e. in the lateral pressure exercised by the fluid on the vessel wall. As the total diameter of the bed of the stream in the capillaries may be twenty times that of the bed in the vas afferens the velocity of the blood in these capillaries will be only one-twentieth of that in the artery and the kinetic energy of the blood only one four-hundredth. It is possible therefore that the pressure exercised by the blood on the walls of the capillaries may be even greater than that in the interlobular arteries, and this effect will be still further aided by the narrow diameter of the vas efferens. Although therefore the pressure in the ordinary capillaries of the body is probably not greater than 20 to 30 mm. Hg., the glomerular capillaries might present a pressure little inferior to that in the main arteries of the body. The pressure in the ureter is under normal circumstances approximately nil, whereas that in the glomerular capillaries is probably not more than 20 mm. Hg. below that in the main arteries of the body, so that there is a difference of pressure on the two sides of the membrane more than sufficient to cause a constant filtration of a protein-free fluid from the blood-plasma coursing through these capillaries. On raising the pressure on the tubule side the filtration ought to come to an end when the pressure approaches a figure which is 30 to 40 mm. below that in the glomerular capillaries. A number of observers have found that urinary secretion ceases when the blood pressure falls to between 40 and 50 mm. Hg. The urinary secretion can be stopped by raising the pressure in the tubules by means of ligature of the ureter. On applying the ligature the secretion continues for a time until the pressure in the ureter rises up to a certain point, when the secretion comes to an end. In one experiment the following pressures were obtained in a dog which was secreting urine copiously under the action of diuretin. Manometers were connected both with the carotid artery and with the ureters so that no outflow of urine was possible : Arterial pressure Ureter pressure 140 72 138 92 133 88 In this experiment therefore secretion came to an end with a difference of pressure between ureter and arteries of between 40 and 50 mm. Hg. The absolute pressure attained within the ureter in any given experiment after liga- ture of these tubes will vary with several factors. In the first place, if the minimum secreting pressure is really conditioned by the colloid content of the blood-plasma, it will be less the smaller the projwrtion of colloids in the plasma. In some exiieriments (Magnus) a flow of urine was observed with a blood -pressure as low as 18 mm. Hg., but in this case the blood was extremely dilute as the result of the continuous injection into the blood-vessels of normal salt solution. Barcroft and Knowlton have sliown that the diuresis brought about by injection of saline (Ringer's) solution is inhibited by mixing 1138 PHYSIOLOGY with the saline fluid colloids, such as gelatin and gum, which possess an osmotic pressure. Colloids such as starch, with no measurable osmotic pressure, have no such effect. On the other hand, the ureters, or at any rate the urinary tubules, cannot be regarded as absolutely water-tight. Not only are the cells of these tubules capable of taking up fluid, but it is probable that at high pressures a certain amount of actual filtration takes place between these cells. This process of reabsorption will tend to diminish the actual pressure of the fluid in the ureters, so that the secretion of urine may apparently come to a standstill when there is still a difference of pressure between blood and urine con- siderably over 50 mm. Hg. Under such circumstances the ureter pressure will be higher, and the difference of pressiire between urine and blood less, the more rapid the formation of urine by the glomeruli. In a number of experiments by V. E. Henderson, it was found that the figure B.P. - U.P. tended to approximate 40 mm. Hg. the more rapid the secretion of urine was. With a slow secretion the flow of urine apparently ceased when there was as much as 80 mm. Hg. difference of pressure on the two sides of the glomerular membrane. We may conclude that, for the production of any urine by the kidney, a certain minimum difference of pressure is necessary between the blood in the glomeruli and the urine in the tubule, and that this difference becomes less the smaller the protein content of the blood. Since the only work required in the formation of a protein-free filtrate from the blood is that due to the osmotic pressure of the proteins themselves, and the observed difference of pressure during secretion is greater than this osmotic pressure, we are justified in concluding, provisionally at any rate, that the mechanical factors present at the upper end of the urinary tubule are sufficient to account for the production of a glomerular transudate free from protein, but containing the same proportion of water and salts as the blood-plasma circulating through the capillaries. If the process occurring in the glomeruli is simply one of filtration, three conditions must be realised : (1) The amount of filtrate, so long as the ureter pressure is constant, must depend on the pressure and rate of flow of the blood in the glomerular capillaries, and must fall or rise with the latter. (2) The constitution of the fully formed urine as it appears in the ureters, after modification by addition or subtraction on the part of the tubular cells, must approximate more closely to the supposed glomerular transudate, containing the same proportion of salts as the blood-plasma, the more rapidly the formation of the glomerular transudate takes place : i.e. the quicker the flow of urine the more nearly must its composition, reaction, and osmotic pressure resemble those of the blood-serum. (3) The total quantity of solids excreted in any given time must be increased with any increase in the urinary flow. For, whatever the activity of the tubules, the glomeruli must blindly turn out a certain proportion of solids with every cubic centimetre of fluid that they form. We may deal first with the influence of alterations in the renal blood- supply on the flow of urine. Ligature of the renal vein diminishes and soon stops the flow altogether. Since this procedure must cause a large rise of pressure in the capillaries of the kidney, this result was regarded by Heiden- hain as disproving any possibility of the glomerular process being of the THE SECRETION OF URINE 1139 nature of a filtration. At any given time, however, the glomeruh contain but Httle blood. With total cessation of the renewal of this blood, their contents will rapidly become so concentrated that they will be little more than a mass of red corpuscles. No filtration of water and salts can take place unless there is a continual renewal of the fluid on the blood side of the filter. On the other hand, alterations in the blood-supply to the kidney, determined by changes on the arterial side, have pronounced effects on the amount of urine formed. The pressure in the glomerular capillaries and the rate of flow through these capillaries can be increased in either of two ways : (a) By increase of the driving force, i.e., the general blood-pressure ; (b) By a diminution of the resistance to the flow of blood through the kidneys, as by dilatation of the vessels of this organ. The blood-flow through the kidney can be investigated either by record- ing the total volume of this organ or by determining the amount of blood which leaves it through the renal vein, according to the methods described in chapter xiii. It is necessary at the same time to take a record of the arterial blood- pressure by means of a mercurial manometer. It is evident that an expan- sion of the kidney may be caused by a rise of general arterial pressure, or, the latter remaining constant, by a dilatation of the kidney- vessels ; and, conversely, a fall of kidney volume may be due either to a fall of general blood -pressure or to a constriction of the renal blood-vessels. By taking these two records it is possible to tell whether a given increase of blood-flow through the organ is of local or of general causation, i.e. is active or passive. Thus the volume of the kidney gives us an indirect clue to the pressure in and the flow through the kidney-vessels. The flow through the vessels can be determined directly either by a cannula in the inferior vena cava, all veins other than the renal being clamped, or by Brodie's method already described (p. 994). The results of the experiments carried out by these methods can be represented in the following tabular form : I'rocodiiri' Gnnrral blooil- pri'ssurc l\ona\ vessels Kidney volume Urinary flow Division of spinal cord in Falls to Relaxed Shrinks Ceases neck 40 mm. Stimulation of cord . Rises Constricted Shrinks Diminished Stimulation of cord after Rises Passively Swells Increased section of renal nerves . dilated" vStimulation of renal nerves Unaffected Constricted Shrinks Diminished Stinuilation of splanchnic Rises Constricted Shrinks Diminished nerve Division of one splanchnic nerve : {a) In dog Unaffected Dilated Swells (?) Increased (l>) In rabbit . Falls Relaxed Shrinks (?) Diminished Plethora Rises Dilated Swells Increased Hiemorrhage . Falls Constricted Shrinks Diminislied 1140 PHYSIOLOGY It will be seen that in every case where an increased blood-flow attended with a rise of blood-pressure in the glomerular capillaries, is brought about, the urinary flow is at the same time increased. Another factor, altering the ease with which filtration of watery fluid and salts would take place through the glomerular capillaries, would be the composition of the blood-plasma. Any dilution of this plasma must render filtration more easy, while a concentration would make it more difficult. As a matter of fact hydraemia, and especially hydraemic plethora caused by injection of normal sahne into the circulation, evoke an increased flow of urine. A smaller effect is produced by injection of defibrinated blood, and if the blood has been previously concentrated by depriving the animals of water, there may be little or no increase in flow, in consequence of the high osmotic pressure of the proteins of the plasma injected. Experiments on the action of diuretics have a close bearing on the nature of the process occurring in the glomeruh. A large increase in the urinary flow can be brought about by the intravenous injection of saline diuretics such as sodium sulphate or potassium nitrate, or of neutral crystalloids such as urea or sugar. The question arises whether the mechanical changes thereby induced in the renal circulation are sufficient to account for the diuresis. Three factors might be concerned in promoting an increased glomerular transudation. These are : (1) A rise of pressure in the glomerular capillaries. (2) Acceleration of the blood-flow from the capillaries. (3) Diminution of the amount of proteins in the blood-plasma. When a concentrated solution of salt is injected into the circulation the osmotic pressure of the plasma is raised and water passes from the tissue- cells into the blood-stream, in consequence of the osmotic diflerences between the blood and cells so induced. As a result the total volume of the circulating fluid is increased by the addition to it of water derived from the tissues, i.e. a condition of hydrsemic plethora is set up, just as if a large bulk of normal saline fluid had been injected into the circulation. So long as this hydraemic plethora continues, so long is there a rise both in arterial and venous pressures and an increase in the velocity of the circulating blood. The kidney placed in an oncometer shows a great increase in volume. While the plethora lasts there are mechanical conditions at work in the kidneys, i.e. increased pressure, increased rate of flow, and diminished concentration of plasma — all of which would concur in producing an increased glomerular transudation. With certain salts, such as sodium chloride, the diuresis is coterminous with the hydraemic plethora, but with other members of this class, such as grape sugar, the diuresis outlasts the plethora, so that the con- tinued increased secretion of urine leads to an actual concentration and diminution of the volume of the circulating blood, as is shown in Fig. 533. If the kidney be placed in an oncometer, it is found that the dilatation of the kidney outlasts the plethora, and comes to an end only with the cessation of the increased urinary flow. There must be local influences at work (perhaps the direct effect of the sugar on the blood-vessels) which lead to an THE SECRETION OF URINE 1141 active dilatation of the renal vessels, and a consequent rise of pressure and velocity of the blood in the glomeruli. That the vascular change is really responsible for the increased urinary flow is shown by the fact, determined by Cushny, that if the swelling of the kidney be prevented by means of an 170 leo 150 I+O 130 1-20 no 100 90 so 70 en 50 40 30 20 10 ( A ^^ \, 1 \ \ ■ -h s .^ ^ Art. VtV. mm Wr I 1 Haemoglobin ^^-. - .-- Percent,. 1 T N \ I / 'K \ i a • \ \ r" n V / p N \ iX) o \ ^^ Kidn-v V,',]unip ^ ~~~~~~~ \- . / ^ -^__ frme 1 1 10 '.lo .-^O 4'J ?y ao 'M luu 110 i-jij loo uo i5& Fig. 533. A conaparison of the effects of intravenous injection of 30 grni. glucose in concentrated solution on the arterial blood-pressure, the concentration of the blood, the kidney volume, and the urinary flow. Abscissa = time in minutes. adjustable clamp on the renal artery, no diuresis is produced ; so long as the kidney is kept at its normal size the flow of urine remains at the same rate as before. With regard to the specific diuretics, such as caffeine, the question is not quite so clear. In most cases injection of caffeine in the rabbit brings about a dilatation of the kidney and a proportional increase in the secretion of urine. But cases have been recorded in which expansion of the kidney occurred without any increase in urinary flow, and, on the other hand, augmented urinary flow without any increase in the kidney-volume, or even in the rate of blood-flow through the kidney (at- determined by Brodie's method). The general rule, however, is that a greater rate of blood-flow is 1142 PHYSIOLOGY obtained fciri passu -wdth the increased urinary flow ; and a consideration of certain peculiarities in the renal circulation must prevent us from laying too much stress on apparent exceptions to the rule. To the blood entering the kidneys by the renal arteries two ways are open. The blood may pass through the vasa afierentia, through the glomeruli and tubular capillaries, back to the renal vein. On the other hand, it may escape the glomeruli altogether, and pass through the vasa recta directly into the intertubular capillaries and so into the renal veins. It is a common experience, in injecting the blood-vessels of the kidneys fost-mortem, to find the renal arteries, intertubular capillaries, and veins filled to distension with the injected mass, but hardly any in the glomeruli. One must assume in such a case that there has been spasmodic contraction of the muscular coats of the vasa afferentia (cp. Fig. 534). The normal amount of blood might Bowman's Capsule Fig. 534. Diagram (after Morat) to illustrate the effect of active changes in the vasa afferentia and efferentia on the pressure in the glomerular capillaries. If the vas afferens constricts, the pressure will be represented by the lower dotted line. On the other hand, constriction of the vas efferens would raise the pressure in the glomerulus till it almost equalled that in the renal artery, as is shown by the upper dotted line. A, arteries ; G, glomerular capillaries ; c, tubular capillaries ; v, vein. therefore circulate through the kidney without any flowing through the filtering apparatus, i.e. the glomeruli. On the other hand, a dilatation of the afferent vessels and a shght constriction of the efferent vessels would cause a considerable rise of pressure in the glomerular capillaries, and a consequent increased transudation, without necessarily altering to any marked extent the total circulation of blood through the whole organ. The changes in the afferent and efferent vessels and the glomeruli are, however, beyond our control or powers of observation, so that it is impossible to devise at the present time any crucial experiment which might decide the nature of the process occurring in the glomeruli. THE COMPOSITION OF THE URINE. If the glomerular function is that of mere filtration, we should expect that the more rapidly the process occurs, the more nearly would the urine which is turned out into the ureters resemble the blood-plasma in composition, reaction, and osmotic pressure, since the glomerular filtrate hurried through the tubules would have very little time to undergo any changes resulting in its concentration. If, on the other hand, the diuresis, produced by salt or sugar solutions, is to be ascribed THE SECRETION OF URINE 1143 to a stinmlatiou of the renal epithelium, the differences between blood- plasma and urine should be greatest at the height of the diuresis, when the concentration of the specific stimulant is also at its highest. The following experiment shows that the more rapid the secretion of urine, the more closely does its concentration, as indicated by its osmotic pressure and depression of freezing-point (A), approximate that of the blood-plasma. A dog received -40 grm. of dextrose dissolved in -40 ccm. of water. The following Table represents the relative concentrations of urine and blood- serum at different stages in the diuresis thereby produced : Time Urine Rate of flow A of urine A of blood-serum 11.30-12 10 c.c. 3-3 2-360 0-625 (at 12.0) From 12.0 to 12.7 injecte d 40 grm. dextrose into jugular vein j 12.7 -12.15 35 c.c. 45 1-210 — ■ 12.16-12.20 20 c.c. 50 0-975 0-700 (at 12.16) 12.20-12.30 52 c.c. 52 0-835 — 12.30-12.40 45 c.c. 45 0-825 0-700 (at 12.30) 12.40-12.50 22 c.c. 22 0-830 j 0-675 (at 12.40) 0-675 (at 12.50) A still closer approximation of the concentration of the urine to that of the plasma was obtained by Galeotti in some experiments in which the modifying influence of the tubular epithelium on the glomerular transudate had been prevented by poisoning the animal with corrosive sublimate, which causes destruction of the epithelium, but is said to leave the glomeruli intact. Since the glomerular transudate must have a concentration approxi- mately identical with that of the blood-plasma, it would be impossible for a urine formed by mere filtration to have a concentration less than that of the blood-plasma. It is, however, of frequent occurrence that, after copious potations of tea or light beer, urine is passed with an osmotic pressure and a molecular concentration considerably below that of the blood. In one case Dreser obtained a urine with a freezing-point of A = 0-16° C, and the same result has been obtained on one or two occasions when the diuresis has been produced by the administration of caffeine. If we assume that this hypotonic fluid is formed by the glomeruli, we must at once give up any idea of the process in these structures being essentially one of filtration. But it is possible that the epithelium of the tubules can secrete water as well as solid constituents. The fine adaptation of the kidney to slight changes in the composition of the blood is apparently an endowment of the tubular epithelium, and in those cases where large quantities of hypotonic urine are passed there is not at any time any appreciable change either in the composi- tion of the blood or in its total volume. Water is absorbed from the ali- mentary canal and is almost immediately excreted by the kidneys. When we attempt to produce the same effect by infusion of large quantities of water or hypotonic solutions into the blood-stream, we get a flow of urine apparently determined entirely by the circulation through the kidney and 1144 PHYSIOLOGY having a concentration not inferior to that of the blood. The passage of hypotonic urine can be ascribed to a modification of the glomerular transu- date as it passes through the tubules, a modification due partly to the ab- sorption of salts from the fluid, partly, perhaps chiefly, to a secretion of water or extremely dilute salt solution by the cells of the tubules themselves. Certain other observations accord with our hypothesis that in Bowman's capsule a fluid is transuded having the same molecular concentration as blood-plasma, and there- fore considerably less concentrated than normal urine. Ribbert succeeded in extir- pating the whole of the medullary portion of the kidney in the rabbit, leaving the cortex intact, and found in this case that during the survival of the animal the urine passed was much more dilute than normal. In cases where there is destruction of the tubular epithelium, while the glomeruli remain intact, either in consequence of disease or, as in Galeotti's experiments, as a result of poisons, we are accustomed to obtain a dilute copious urine ; and the continual passage of such urine is in man regarded as a sign of one form of renal disease. The experimental facts which we have passed in review do not therefore negative the view that the glomerular epithehum plays the part of a passive filter in the formation of urine, and that the energy of the process by which ' urine ' is produced in Bowman's capsule is entirely furnished by the heart in driving the blood at a high pressure through the glomerular capillaries. FUNCTIONS OF THE RENAL TUBULES Whatever the nature of the glomerular activity, it is evident that the multiform epithehum of the tubules may alter the glomerular transudate, either by the absorption or by the secretion of water or solid constituents. We may deal with the evidence for the occurrence of these two processes separately. SECRETION BY THE URINARY TUBULES. Although it is impossible to collect the secretion of the glomeruH apart from that of the tubules, the arrangement of the blood-vessels in certain animals enables us to influence separately the circulation to these two parts of the kidney. The amphibian kidney receives a blood-supply from two sources. A number of renal arteries leaving the aorta enter the kidney and supply the whole of the glomeruli, the vasa efferentia from which pass, as in the mammaUan kidney, into the intertubular capillaries. These are also supphed with blood of venous character by the renal portal vein. If all the renal arteries be divided or ligatured, the glomeruli, as was shown by Nussbaum, are entirely cut out of the circulation, though the tubules continue to receive venous blood through the renal portal vein. Nussbaum stated that ligature of all the renal arteries caused cessation of the urinary secretion, which could be reinduced by injection of urea. He concluded that urea with water was secreted by the tubules, whereas peptone, sugar, and haemoglobin were turned out by the glomeruli. Beddard showed that these results of Nuss- baum must have been due to the fact that he had not obstructed the whole of the renal arteries. One or two of these small vessels will suffice to supply blood to a considerable number of the glomeruli. After complete obstruc- THE SECRETION OF URINE 1U5 tion of the arteries, no iirinaiy flow could be induced even with subcutaneous injection of urea. But the cutting oft' of the arterial blood-supply from the tubules caused a rapid destruction of the tubular epithelium, so that the result of the experiment could not be taken as negativing the possibiHty of this epithelium having, when in a normal state of nutrition, some secretory power. He therefore carried out, with Bainbridge, another series of ex- periments of the same description, in which the frogs, after ligature of the renal arteries, were kept in an atmosphere of pure oxygen. Under these circumstances sufficient oxygen diftused into the blood of the renal portal ^a^ boc 1 '\i'A f^ma — Vena cava T•3s^ls / J/J; . "fe^^. /Renal aKt-eries Kidney Vy lo^' Renal poi'l-al— # J^ Anr.abciom.v.-»\ 1> ^ ^- — Femoral v. Fig. 535. The vascular sui^ply to the kidney in the frog. vein to maintain an adequate supply of this gas to the tubules. No desquam- ation of the epithelium resulted, and injection of urea produced a small flow of urine even when, by subsequent injection of the blood-vessels, it was proved that every glomerulus had been cut out of the circulation. The view that a portion of the tubules is secretory in function is further supported by histological examination of these structures under various conditions of activity. In the cells of the convoluted tubules various kinds of granules and of vacuoles may be distinguished. Gurwitsch divides these vacuoles into three classes : (1 ) Large granules staining densely with osmic acid, and probably rich in lecithin. (2) Smaller very numerous granules consisting of some form of protein material. (3) Large vacuoles lying close to the free margins of the cells, whose contents do not undergo coagulation with the ordinary fixing reagents, and therefore are free from protein, fat, or mucin. Tliese vacuoles arc esi)ecially marked in kidneys which are secreting at a great rate, in consequence of the injection of saline diuretics or of large quantities of normal salt solution. They may be regarded as excretory vacuoles, and consist of water or saline Ihiids which have been collected by the cells and are being passed on by them to the lumen of the tubules. As a rule it is impossible to trace any definite constituent of the urine on its way through the cells of the tubules. But if a solution of uric acid in piperazin be injected intravenously into a rabbit, the kidneys, taken twenty to sixty minutes after the injection, present tubules full of uric acid con- cretions. In the medullary portion of the kidney this uric acid precipitate is confined to the lumen of the tubules, but in the convoluted tubules granules 1146 PHYSIOLOGY of uric acid are to be found in the epithelial cells, especially towards their inner border. Since these cells are able to excrete uric acid when present in abnormal quantities in the blood, it is a reasonable assumption that they also undertake the secretion of this substance under normal conditions. Certain observers have in fact described the presence of urate granules in the cells of the convoluted tubules of the bird's kidney. Although the larger number of the urinary constituents must escape detection on their way through the cells, we can throw some light on the excretory functions of the kidney by studying the mechanism by means of which it excretes certain dyestuifs, such as sulphindigotate of soda (' indigo carmine '). If the indigo be injected into the veins, it is excreted in a concentrated form, both by the liver and by the kidney, so that the urine assumes a dark blue colour. If the animal be killed when the excretion of the pigment is at its height, and the kidneys be rapidly fixed with absolute alcohol (which precipitates the dyestuff), all parts of the kidney present a blue colour, which is especially marked in the medulla. Under these cir- cumstances the urine, which is being excreted by the glomeruh, rapidly carries down the dyestuff , wherever it may be turned out, into the tubules of the pyramids. In order to discover the exact locality of the cells in- volved in its excretion, we must stop the glomerular transudate by some means or other. This stoppage of the urinary flow can be effected in two ways, viz. by section of the spinal cord in the neck, so as to reduce the blood-pressure to about 40 mm. Hg., i.e. below the minimum necessary for the production of urine, or by cauterising portions of the surface of the kidney by means of silver nitrate. If the indigo be injected into the veins, after section of the cord, and the animal be killed half an hour later, and the kidneys fixed with absolute alcohol, they are found to be of a bright blue colour, although no urine has been secreted. On cutting into the kidneys the colour is seen to be confined to the cortex, and on making microscopic sections granules of the pigment are found within the lumen and in the epithelial cells of the convoluted tubules. If the kidneys have been cauterised, the stain is confined to the convoluted tubules of the cortex only under those areas which have been cauterised, and where the glomerular functions have been abolished. It has been suggested that the appearances after the injection of indigo are due, not to the secretion, but to the absorp- tion of water in the convoluted tubules. A certain amount of the dyestuff is thus rendered visible by becoming more concentrated, and is precipitated in a granular form, as soon as the salt concentration of the fluid reaches a certain height. The fact that these appearances are wanting after the injection of ordinary carmine, which stains the glomeruli as well as the tubules, combined with the histological facts mentioned in the last paragraph, render this a somewhat forced explanation ; and we must take the results of the injection of indigo-carmine as telling rather in favour of a secretory than of an absorptive function on the part of the convoluted tubules. The question as to the secretory activity of the kidney can be attacked from another side. The glomerular filtrate can contain only those crystal- THE SECRETION OF URINE 1U7 loids of the blood which are diffusible and are not closely combined with its colloidal constituents. Loewi has shown that in this connection a contrast is to be drawn between the behaviour of substances, such as urea or sodium chloride, and certain other constituents of the blood such as phosphates or sugar. Any increase in the rate at which the glomerular secretion takes place must cause a corresponding increase in, the total amount of the solid diffusible constituents of the blood-plasma which are turned out within a given time. Thus every diuresis increases the total output of chlorides and of urea. On the other hand, a diuresis caused, for example, by drinking large quantities of water does not increase the total output of phosphates in a given time, nor does it increase the very small amount of sugar which is normally excreted by the kidneys. If, however, sodium phosphate be previously injected into the blood, then any diuresis increases the output of this salt. The same thing holds for sugar. If an excess of free uncom- bined sugar be present in the blood, either in consequence of intravenous injection of this substance or as a result of previous extirpation of the pan- creas, any form of diuresis will increase the rate at which it is turned out by the kidneys. Loewi concludes that phosphates, which must be present in minimal quantities in the glomerular transudate, are for the most part secreted by the activity of the cells of the convoluted tubules. Under abnormal conditions, e.g. as after administration of phloridzin, the cells of the kidneys can be excited to a similar activity with regard to sugar. After phloridzin injection the urine contains considerable quantities of sugar, but the rate at which the sugar is secreted is not affected in any way by raising the rate of urinary secretion, e.g. by the injection of such substances as sodium sulphate, which increases the rapidity of the glomerular process of transudation. ABSORPTION BY THE RENAL TUBULES. The experiments of Ribbert, mentioned above, in which removal of the medullary portion of the kidney led to the formation of an increased quantity of a more watery urine, points to the possession by the tubules of a power of absorbing water. We have other evidence that this power of resorption is not confined to water, but may affect also the dissolved constituents of the glomerular transudate. It was pointed out by Meyer that, if two salts, such as sodium sulphate and sodium chloride, were present at the same time in the glomerular transudate, any process of resorption should affect chiefly the more diffusible salt, namely, sodium chloride. Such a differential resorption would account for the much greater diuretic power of sodium sulphate as compared with sodium chloride. In certain experiments Cushny produced a diuresis by the injection of equal parts of equivalent NaCl and Na.^SOj solutions into the veins of a rabbit. An increased flow of urine was produced which lasted two hours and a half. The chlorides of the urine rose with the diuresis and reached their maximum at the height of the urinary flow. They then sank, and in some experiments had practically disappeared from the urine towards the end of the observation. The concentration of the sulphates, however, con- tinued to rise in the urine to the end of the experiment. Thus in the first of two identical experiments, when the rabbit was killed at the height of the 1148 PHYSIOLOGY diuresis, the serum contained 0-547 per cent, chlorine and 0-259 per cent, sulphate, while the urine contained 0-372 per cent, chlorine and 0-546 per cent, sulphate. In the second, in which the rabbit was killed when the rate of the urinary flow had considerably diminished, the serum contained 0-493 per cent, chlorine and 0-191 per cent, sulphate, while the urine contained •094 per cent, chlorine and 2-0 per cent, sulphate. These results are illus- trated in Fig. 536. 1 1 1 n \ If V \ \ \ \ ( \ \, ; ( 1 \ \N * \S \, 1 1 \N v^ s jl 1 \ \ N if V Sv, \ ! V ~1^ — N ■^ m m In i *-. .- ^.. ="" =—' /5 30 4-5 60 75 ^0 105 120 135 Fig. 536. Curves showing excretion of urine (thick line), of sulphate molecules (SO / CI -— ^, thinline), and of CI molecules ( , dottedline), after injection of 50 c. c. 96 \35-5 of a solution containing 1 -775 grm. Cl and 4-8 grm. SOj per 100 c.c. The black line along the base marks the duration of the injection. (Cushny.) The difference between the two salts can be made still more striking if the process of resorption be augmented by increasing the pressure within the tubules by partial obstruction of one ureter. Thus in one experiment, where diuresis was produced by the injection of 30 c.c. of a solution con- taining 5-85 per cent. NaCl + 14-2 per cent. Na2S04, the right ureter was partially clamped so as to make the right kidney secrete against a pressure of 31 mm. Hg. The following results were obtained : Urine c.c. 01. g. S04g. 4.37 till 4.47 ) Left kidney .... \ Right kidney .... Difference (absorption) 24 8 16 0-0809 0-0142 0-0677 0-1080 0-0667 0-0413 We must conclude that the tubular epitheUum possesses the power of modifying the glomerular transudate not only by the absorption of water THE SECRETION OF URINE 1U9 but also by the absorption of dissolved constituents, and that the relative permeability of the cells to the constituents is at any rate one factor in deter- mining the substances absorbed. It is not, however, the only factor. The function of the kidney is to preserve the normal constitution of the body fluids by turning out those substances which are abnormal or present in too great an amount. The behaviour of the tubule cells with regard to any given substance will therefore depend to a certain extent on the previous nutritive history of the body. If, for instance, in consequence of the administration of sodium chloride in large quantities to the animal during the few days preceding the experi- ment, the body is overloaded with this salt, it becomes an abnormal con- stituent and the kidney secretes a urine far richer in sodium chloride than is the blood-plasma. Moreover, when diuresis is produced in such an animal by the injection of equivalent quantities of sodium chloride and sodium sulphate, there is no diminution of the NaCl in the urine towards the end of the diuresis, but its percentage rises steadily as the rate of urinary flow diminishes. On the other hand, a total deprivation of sodium chloride extending over several days, although not altering to any large extent the percentage amount of this salt in the blood-plasma, leads to a total dis- appearance of the salt from the urine, the whole of the sodium chloride present in the glomerular transudate being absorbed on its way through the urinary tubules. It has been suggested that the effects of certain diuretics on the kidney, such as caffeine, diuretine, or theocine, may be largely conditioned not so much by their influence on the glomerular circulation as by a paralytic eft'ect on the absorptive functions of the tubules. According to Loewi, on injec- tion of caffeine or diuretine, the increase of total amount of urine is not accompanied by any diminution in the percentage amount of NaCl. Perhaps, however, the strongest evidence in this direction is afforded by an experi- ment of Pototzky. A rabbit had been fed on a diet almost totally devoid of chlorides, and was therefore excreting a urine containing only -08 per cent. NaCl. Under the influence of diuretine the urine was increased and the concentration of the NaCl rose to 0-64 per cent. The same increase in the percentage amount of sodium chloride in the urine has also been observed after the injection of theocine, which has therefore been specially recom- mended as a diuretic in cases of dropsy, where a diminution of the salt content of the body is a valuable means for the diminution of the dropsical fluid present in the tissue spaces. THE RENAL MECHANISM What conclusions can we draw from this mass of experimental data as to the functions of the kidney as a whole, and as to the part played by its various constituent elements in the secretion of urine ? The amazing adaptability of its functions to the needs of the organism has been abund- antly illustrated in the facts with which we have dealt. Its ordinary activity is determined by the production, as a result of the normal processes of meta- 1150 PHYSIOLOGY bolism, of soluble iion- volatile substances in every cell of the body. These substances, together with the excess of water taken in with the food above that lost by respiration and cutaneous transpiration, are turned out by the kidney as urine. The activity of this organ must therefore be determined in the first place by cbemical stimuli. It must react to the slightest deviation from normal of the blood composition by excreting water or dissolved substances. This dehcate sensibihty is displayed in two directions : (1) Under the influence of certain substances, such as urea, uric acid, or water, the cells of the convoluted tubules take up the substance, which is in excess, from the surrounding lymph and accumulate it in vacuoles, which are discharged on the inner surface of the cells into the lumen of the tubules. (2) Besides this specific secretory activity of the cells of the convoluted tubules, the tubules as a whole are endowed with the power of absorbing both water and dissolved substances from the fluid in their lumen. Whether this absorptive power is Hmited to the cells of Henle's loop, as was first suggested by Ludwig, or occurs coincidently with secretion in the cells of the convoluted tubules, as might be imagined from the close analogy between the structure of these cells and that of the intestinal epithehum, we have not sufiicient evidence to decide. We do know, however, that the quality of the absorption is strictly regulated according to the needs of the organism, so that the constituents which are precious are reabsorbed for service in the body, while those which are in excess or are of no value to the organism are allowed to pass out into the ureters. The process of resorption is indeed, as is shown by Cushny's experiments, largely dependent on the physical qualities of the substances undergoing absorption, and especially on the permeabihty of the renal cells to these substances. The physical conditions are, however, subordinated to the physiological, so that a salt so diffusible as potassium iodide is left in the fluid, while sodium chloride may be reabsorbed in large quantities. The necessity for the endowment of the tubular epithelium with a resorp- tive as well as a secretory function is determined by the presence at the beginning of the tubule of a mechanism — the glomerulus, devoid of the fine selective power or chemical sensibihty possessed by the cells of the convoluted tubules. The production of urine by the glomerulus is ap- parently regulated entirely by the pressure and velocity of the blood through its capillaries and by the colloid content of the blood-plasma. We may assume that in Bowman's capsule there is under normal conditions a constant production of a fluid free from protein, but having the same crystalloid concentration as the blood-plasma. With any rise of general blood -pressure the amount of this transudate is increased ; with any fall it is diminished. The small qualitative changes which are constantly occurring in the blood as the result of the taking of food or the activity of different organs, probably produce but little effect on the amount of glomerular fluid. Only indirectly, as the result of these events on the general blood-pressure, or possibly in con- sequence of the production of substances having a vaso-dilator effect on THE SECRETION OF URINE 1151 the renal vessels, will the amount of the urine turned out b}^ the glomeruli be affected. These structures therefore have the twofold function of regulating the total amount of circulating fluid and of providing an in- different fluid, which will, so to speak, flush the kidney tubules and carry- down any constituents excreted in a concentrated form by the cells of these tubules. The constant production of a glomerular transudate might result, especially in terrestrial animals, in the loss to the organism of water, or, under certain nutritive conditions, of substances indispensable as normal constituents of the serum, such as sodium chloride, which could not be made good at the expense of the food. It is for this reason that an absorptive mechanism sensitive to and reflecting the nutritive condition of the whole body, especially as concerns water and salts, should be present in the tubules. As the result of the complementary processes of absorption and secretion in the tubules, the unchanging glomerular filtrate undergoes great modifica- tions in its passage towards the ureter. It receives urea, uric acid, phos- phates, and under certain conditions water, from the cells of the convoluted tubules. It gives up salts, especially sodium chloride, and generally water to the same or other cells of the tubules. So that finally, instead of a fluid isotonic with the blood and containing only about 0-1 per cent, urea, we have a fluid of deep yellow colour, with a molecular concentration four or six times greater than that of the blood, and containing between 2 and 3 per cent. urea. We have at the present time no means of judging the relative amounts of fluid furnished respectively by the glomeruli and the tubules to the fully formed urine. It is probable that, under ordinary circumstances, the processes of secretion and absorption of fluid go on pari passu in the urinary tubules just as they do in the mucous membrane of the small in- testine. The demonstration of secretory powers in the cells of the con- voluted tubules relieves us from the necessity of the assumption made by Ludwig as to the quantity of fluid normally turned out through the glomeruli. On the hypothesis that the sole function of the tubules was one of absorption, and that all the urinary constituents were derived from the glomerular transudate, thirty litres of fluid would have to be filtered through the glomeruli in order to excrete the 30 grm. urea which is the daily output of a man. Of these thirty litres, twenty-eight litres would have to be reabsorbed in tho tul)ules. Since the amount of blood flowing through the two kidneys in a man probably vaiies between 1()00 and 1800 litres in the twenty-four hours, there would be no difficulty in the production of such an amount as thiity litres, which would only represent a concentration in the blood in its passage through the glomeruli of under 2 per cent. The secre- tion and reabsorption of such large (|uantities of fluid seem, however, a clumsy way of arriving at a urine, whose composition should be adopted to the needs of the animal ; and as we have seen, the occurrence of an actual secretion of urea by the cells of the tubules removes the necessity for assum- ing any such wasteful proceeding. It is probabh^ that the actual amount of the glomerular filtrate in the twenty-four hours may not exceed to anv large extent the actual amount of urine formed by the whole kidney in this time. SECTION III THE PHYSIOLOGY OF MICTURITION The urine as it is formed passes through the ureters to the bladder, where it gradually accumulates, and is voided at intervals. The ureters are muscular tubes hned by transitional epithehum. The muscular coat is composed of three layers of unstriated fibres, a middle circular coat lying between external and internal longitudinal coats. If the ureter be exposed in the living animal, contraction waves are seen to pass along its muscular coat from the pelvis of the kidney to the bladder, driving the contained fluid in front of them. The frequency of the con- tractions is increased by warming the ureter, and to a certain extent by distension, so that the waves are more frequent when the secretion of urine is profuse. The ureters enter the bladder at or near its base, at the two posterior angles of the region known as the trigonum. Their entrance is oblique, so that a valvular orifice is formed, which effectively prevents reflux of urine from bladder to ureter. Rhythmic waves of contraction are observed also in the excised ureters, when these are kept warm in normal saline solution. By Engelmann they were regarded as myogenic, since they were present in the middle third of the ureter, which he imagined to be entirely free from ganghon-cells. As a matter of fact ganghon-cells are found throughout the ureter, though in larger numbers at its two ends. The ureters are supplied with nerve fibres from the splanchnic nerves by way of the renal plexus, and at their lower ends from the hypogastric nerves. Stimulation of the latter as a rule increases the rhythm of the contraction presented by the lower end of the ureter. The splanchnic nerves have been stated to produce either acceleration or inhibition of the contractions at the upper end ; their action is, however, uncertain. It is by the rhythmic advancing waves of contraction of the ureter that the urine is continuously passed on to the bladder, so that the pelvis of the kidney is kept empty of fluid whatever the position of the animal. The bladder is lined by transitional epithelium, closely adherent to the underlying muscular coat. It is usual to describe in the latter three layers of muscular fibres : (1) An outer layer composed of bundles running longitudinally from the neck of the bladder to the fundus, sometimes distinguished by the name of the detrusor urince. At the neck of the bladder these bundles send some fibres to be attached to the pubes as the pubo- vesical muscles. On 1152 THE PHYSIOLOGY OF MICTUKITION 1153 the dorsal surface some bundles in the male pass on to the prostate and the urethra, while in the female they end in the tough connective tissue in the urethro- vaginal septum. (2) The middle layer, which is the thickest of the three, is composed of fibres arranged circularly and forming a continuous layer. (3) The inner layer is thin and incomplete, and is composed of anasto- mosing bundles of fibres with meshes in between them which are covered by the folds of the mucous membrane. The bundles of fibres run freely from one layer to the other, and there is no doubt that the name of detrusor ought physiologically to be applied to the whole of the three coats, which act as one in diminishing the capacity of the bladder. At the base of the blad- der the structure of the wall is modified over the triangular region lying between the orifices of the ureters and of the urethra (the trigonum) by the differen- Ureter-- - tiation here of fibres which serve as a sphincter and prevent the escape of _ • rx ^1, 4- • +1 Prosrate- urine. Over the trigonum the mucous membrane of the bladder is smooth Memb.- and closely adherent to the subjacent Bulb muscular fibres, which themselves are Fig. 537, much more closely packed than the rest of the bladder wall. From these muscular fibres the most important sphincter, the sphincter trigoni, is formed. Bundles of muscle fibres pass from the trigonal muscle obhquely forwards and downwards (the individual being con- sidered in the erect posture), and form a loop around the orifice of the bladder, lying on the ventral side of the bladder below and quite distinct from the thick coat of circular fibres belonging to the bladder itself (ss. Fig. 537). This sphincter is the most important mechanism for the retention of urine. If a catheter be passed into the urethra no urine escapes until its orifice has actually entered the bladder. The wall of the urethra is sur- rounded by circular muscular fibres, which, by their tonic contraction, will also tend to prevent the escape of urine along the canal. This urethral muscle is strengthened by two sphincter muscles which are voluntary and composed of striated fibres. The chief one, which has been named by Kalischer the sphincter urogenitalis, but is better known as the compressor urethrce, forms a flat ring around the second part of the urethra, extending in the male from the prostate to the bulb, where its function is taken up by the bulbo-cavernosus. The bladder is therefore supplied with a powerful muscular wall, the contraction of which will cause its evacuation, and with sphincters of two kinds, one involuntary, the sphincter trigoni, at the upper neck of the bladder, and two voluntary, the sphincter urogenitalis and bulbo-cav.^rnosus muscles, which can emptv the lower parts of the urethra. 37 1154 PHYSIOLOGY The nerve-supply of the bladder (Fig. 539) is derived from two main sources, namely, from the upper four lumbar nerves through the sympa- thetic system, and from the second and third sacral nerves by means of Ant. long m. Circular muse. Pubo-vesical m. Symphysis Ant. circular m Ant. long. m. Os pubis Sphincter urogenitalis Circular longitudinal Os pubis Circular coat Longitudinal coat Sphincter trigoni Prostate Circular coat Longitudinal coat Sphincter trigoni Circular coat Longitudinal coat Sphincter trigoni Longitudinal muscle Fibres running to urethro- vaginal st'ijtum Fig. 538. Sagittal sections through neck of bladder. (Metzner after Kalischee.) A, in middle line (male) ; b, slightly to left of middle line (male) ; c, ditto (female). the pelvic visceral nerves or nervi erigentes. The upper lumbar nerves send white rami communicantes to the lateral chain of the sympathetic, and thence to the collateral ganglia, which are grouped round the inferior mesenteric artery to form the inferior mesenteric ganghon. Most of the fibres end in this collection of ganglion-cells, and a new relay of axons passes by two main trunks, the hypogastric nerves, into the pelvis on each side of THE PHYSIOLOGY OF MICTURITION 1155 the rectum and ends in a plexus, the hypogastric plexus, at the base of the bladder. From this plexus fibres pass to the bladder wall. The pelvic visceral nerves are derived from the second and third sacral nerves. They make no connection with the sympathetic system, but pass directly to the J lO'l 3rd lumb. vert. '11 I I ^r- ~- ■ 1 1 ' 't Sup. lufs. ganglion. Sup. ines. uf rVL's Median mcs. nerves Inf. mcs. nerve Inf. mes. ganglion (^ ilyiJDgastric nerve llecf lun .Sacral nerves Ym. 539. Nerve supply to blatklcr of cat. (Nawrocki and Skabitschewsky.) hypogastric plexus and are carried with branches of this plexus to the neck of the bladder. The fibres do not run directly from the spinal cord to their ending in the bladder wall, but make connection with cells situated peri- pherally, partly in the hypogastric plexus, but chiefly in the walls of the bladder itself. Both sets of fibres supply also the rectum and the colon, and carry efi'erent impulses to the bladder. Aft"erent impulses from the bladder travel chiefly in the pelvic visceral nerves. THE FILLING OF THE BLADDER Under normal circumstances the sphincters at the neck of the bladder are in a state of tonic contraction, presenting a resistance to the empt\nng of 1156 PHYSIOLOGY this organ which will vary according to their degree of contraction. Thus it requires a considerably greater pressure in the bladder to overcome the resistance of the sphincters during Hfe than after death of the animal. In some cases after death they may permit the passage of urine when the pressure of the bladder is only about 20 mm. water, whereas in the normal animal the pressure has as a rule to be at least 160 mm. of water before any escape takes place. The urine therefore as it is secreted must accumulate and distend the bladder. The bladder wall reacts to a distending force in the manner which is characteristic of all muscular tissue, especially un- striated. An extending force apphed to an unstriated muscle fibre has a double effect. In the first place, if the stretching force is apphed very slowly, a considerable increase in length of the muscle may occur with the application of a very small amount of force. If, however, the force be applied more rapidly, the sudden increase of tension acts as a direct excitant U.B. _20-' ^ ^/'\AA.VX^AUAAAAA/X\AAAA^^^A^AAVMAV^vAA^/^^AMW Fig. 540. Tracings of rhythmic contractions of urinary bladder. (Sherrington.) to the muscle, causing it to enter into contraction, which may be tonic or rhythmic. The effect of the entry of urine into the empty bladder on the tension in this organ will depend therefore on the rapidity with which the kidneys are secreting. Under normal circumstances micturition occurs in man when the intravesical pressure has risen to about 150 mm. water. Under these conditions the bladder will contain between 230 and 250 c.c. of urine. If, however, the secretion of urine has occurred very rapidly, the same pressure may be attained with a much smaller bladder content, and if the bladder be artificially distended by the injection of fluid through a catheter, 50 c.c. of fluid may suffice to raise the pressure to this level. As the urine is slowly secreted, the bladder wall at first gives to the incoming fluid, so that a considerable amount can be stored without any marked rise of pres- sure. Later on the pressure begins to rise more rapidly, and finally attains a pressure of between 120 and 150 mm. water. At this point the second effect of the stretching of the muscular wall makes its appearance. A mano- meter connected with the bladder shows a series of rhythmic contractions of the muscular wall (Fig. 540), each lasting about a minute, at first slight in extent, but becoming more marked as the distension of the bladder augments. In a bladder entirely cut off' from its connection with the central nervous system these automatic rhythmic contractions gradually THE PHYSIOLOGY OF MICTURITION 1157 increase in force until one of them suffices to overcome the resistance pre- sented by the tonically contracted sphincter. A partial emptying of the bladder therefore takes place, but the pressure falls below that necessary to overcome the resistance of the sphincter before the bladder has been quite emptied, so that there is always under these circumstances a certain amount of residual urine left in the bladder. This is the condition found in animals where the lower part of the spinal cord has been extirpated, or in man where the same part of the central nervous system has been destroyed as the result of accident or disease. THE MECHANISM OF EVACUATION OF THE BLADDER In the denervated bladder the factor finally causing partial evacuation is the gradual increase in the intravesical tension from the accumulation of fluid in this viscus. The same factor is prepotent in determining the onset of normal micturition in an animal with the nervous connections of its bladder intact. Apart from the control of the higher centres, micturition will take place each time that the tension in the bladder has reached a certain height, i.e. about 15 cm. water, the amount of fluid in the bladder at the time depending on the one hand on the rate at which the fluid has entered this organ from the ureters, on the other hand on the irritability of the bladder wall itself and of the nervous centres concerned with its motor innervation. The efi'ect of the gradual accumulation of fluid and rise of tension is twofold. In the first place, it acts on the bladder wall, causing rhythmic contractions of ever-increasing intensity ; in the second place, the mere stretching of the bladder originates impulses in the sensory nerve- endings in its wall, which are reinforced at every rise of tension caused by the rhythmic contractions. These impulses travel up to the spinal centres, and are summated until they result in a sudden discharge of efterent impulses of two kinds, namely : (1) Motor impulses to the whole musculature of the fundus of the bladder (the detrusor in its widest sense) ; (2) An inhibition of the tonic contraction of the sphincter. This in- hibition may be determined by inhibitory impulses travelling to the sphincter and causing its relaxation, or by the central inhibition of the impulses nornially going to the sphincter and maintaining its tonic contraction. The resultant of these two processes, the contraction of the detrusor and the relaxation of the sphincter, is a complete emptying of the bladder, and the act is completed by the contraction of the involuntary and voluntary nniscles surrounding the urethra and causing complete expulsion of the contents of this tube. THE INNERVATION OF THE BLADDER ACTION OF THE PELVIC VISCERAL NERVES. In all animals excita- tion of the peripheral end of one pelvic visceral nervo causes a strong 1158 PHYSIOLOGY contraction of the same side of the bladder, involving all its coats and some- times extending to a slight extent to the contralateral half of the bladder. When both pelvic nerves are stimulated simultaneously contraction of both sides of the bladder causes a considerable rise of pressure in its interior (Fig. 541) which is always sufficient to overcome the resistance of the sphincter and to cause a complete emptying of the bladder. There is no doubt, therefore, that these nerves are the most important for the act of micturition. As to the action of these nerves, however, on the sphincter the results of difierent experimenters are somewhat at variance. In the cat there seems to be no doubt that inhibition of the sphincter may result from stimulation of the pelvic visceral nerves. On the other hand, Fagge, working on the dog, found that although micturition was excited by the stimulation of these nerves, the expulsion of urine did not occur until the Fig. 541. Curve showing rise of pressure in the bladder caused by stimulation of s, sacral nerves ; h, hypogastric nerves. (Fagge.) The scale indicates centimetres of water. intravesical tension had reached the point at which the resistance of the sphincter could be overcome without any alteration of its state of con- traction, i.e. the point at which fluid injected into the bladder through the ureter began to escape from the urethra without stimulation of any nerves whatever. Observations on man would support the view that an active relaxation of the sphincter trigoni is a necessary part of the act of micturition. Thus in experiments by Reyfisch a rigid catheter was introduced into the bladder, which was fully distended with fluid. On withdrawing the catheter until its opening lay just outside the bladder in the posterior urethra, the flow of urine stopped. The man, however, was able to micturats directly he was told to, and to stop again at will. It was impossible in this case for any of the urethral muscles to be concerned, since the rigid catheter impeded their action. The relaxation of the sphincter must therefore be brought about by impulses descending the pelvic visceral nerves, which we may regard as motor to the ' detrusor ' and inhibitory to the sphincter of the bladder. THE PHYSIOLOGY OF MICTURITION Jill) tSection of the nerve on one side causes no abnormality in micturition. After three weeks, stimulation of the intact nerve causes contraction of the whole bladder, owing to the outgrowth of preganglionic fibres from the sound trunk to the decentralised ganglia of the opposite side (Elliott). Section of both nerves paralyses micturition, but power of partial evacuation of the bladder may return in a few weeks. If now the hypogastrics be cut, or even the sacral corcl extirpated, the bladder is not completely paralysed, but its evacuation becomes unconscious and incomplete. ACTION OF THE HYPOGASTRIC NERVES. These nerves, which are derived from the sympathetic system, show marked differences in their action, according to the animal which is the subject of investigation. In the dog the hypogastric nerves cause a strong contraction of the muscle fibres at the base of the bladder, especially of the trigonum and of the sphincter triijoni. When these nerves are stimulated simultaneously with the pelvic visceral nerves a great rise of intravesical tension may be induced without any flow of urine taking place. In some cases prolonged stimulation of these nerves in the dog causes apparently an active relaxation of the sphincter of the bladder. On the other hand, in the rabbit and the cat these nerves cause an inhibition of the bladder wall. In other animals they may excite either contraction or relaxation (or both) of the detrusor. They always contain motor fibres to the sphincter of the bladder as well as to the constrictor fibres surrounding the urethra. Where this effect is tonic micturition must be associated with a central inhibition of their tonic activity. On the other hand, the retention of urine and the distension of the bladder may be aided by a reflex dilatation of the bladder wall and a reflex constriction of the sphincter in each case excited through these nerves. Normally, therefore, both sets of nerves are called into play. The hypo- gastrics play an especially active part during the accumulation of urine in the bladder, while the pelvic visceral nerves are necessary for the complete evacuation of the bladder which occurs at micturition. THE CENTRAL CONTROL OF THE BLADDER The nerve-centre which presides over the toiuis and contraction of the bladder is situated in the lumbo-sacral spinal cord. If this centre and its connections be intact, micturition may be carried out normally even after section of the cord in the doisal region. The centre can be excited reflexly by stimulation of almost any sensory nerve, such as the sciatic or the fifth nerve. In many cases where, in consequence of obstruction to the passage of impulses from the higher parts of the central nervous system, micturition is delayed, this act may be excited by the application of cold or hot sponges to the perineum, and it is well known that almost any irritation of the ])elvic organs in children nuiv give rise to refle.x involuntarv micturition. In the adult the processes of retention and evacuation of urine are modified and controlltMl bv voluntaiv effort. The normal action of the 1160 PHYSIOLOGY sphincter mechanism may be aided by the contraction of the perineal muscles which keep the urethra closed. The reflex process of evacuation may be set in motion by voluntary contraction of the abdominal muscles, by which the pressure in the bladder is increased and the normal sphincter action overcome. It is probable too that the individual has a certain degree of voluntary power over the unstriated muscles of the bladder, and that the contraction of the muscular wall may be directly augmented by impulses proceeding from the cortex to the upper part of the lumbar cord. This view is favoured by the fact that stimulation of the crus cerebri has been observed to cause contraction of the detrusor urinse. In this experiment the abdo- men was opened, so there could be no question of the contraction of the abdominal muscles. CHAPTER XVIII THE SKIN AND THE SKIN-GLANDS In all classes of animals the skin performs two functions. In the first place, it serves to protect the more delicate underlying parts from injury and from penetration or invasion by foreign organisms. In the second place, it serves as a sense-organ, and is richly supplied with nerves, by means of which the activities of the body as a whole may be brought into relation with the changes going on in the environment and affecting the external surface of the body. In warm-blooded animals the skin plays an important part in the regulation of the body temperature, since the loss of heat to the body must occur almost entirely through its surface. In the present chapter we have to deal only wath the first and third of these functions. The development of the skin as an organ of protection shows wide modification in various classes of animals. Thus it may become the seat of formation of horny plates, as in the alhgator ; of poisonous glands, as in the toad ; or of mucous glands, as in many varieties of fishes. In warm- blooded animals the development of hair from the deeper layers of the epidermis serves to diminish the loss of heat. Since the hair-folhcles are richly supplied with nerve fibres, the hairs act also as organs of sensation. In man, where the hairs are rudimentary, except in certain localities, practically only this tactile function is retained. The external layer of the skin in man consists of a tough horny layer formed by the keratinisation of the external layers of cells of the epidermis. The skin is composed of two parts, the epidermis and the cutis (Fig. 542). The epidermis is a stratified squamous epithelium. The deeper layers form the rete mucosum, being soft and protoplasmic, while the superficial layers forming the cuticle are hard and horny. The most superficial layer of the rete mucosum is formed of flattened cells filled with granules of a material staining deeply with ha-ma- toxylin and eosin, known as eleidin. This layer is called the stratum (jranu- losum. Inunodiately superficial to this layer is another in which the cells are indistinct. The cells are clear in section and form what is known as the stratum hicidu))i. The^e two layers evidently form the intermediate stages in the transformation of the cells of the rete mucosum into the horny scales which make up the superficial cuticle. The cutis or corium is composed of dense connective tissue, wliich becomes more open in texture in its deeper part, where it merges into tlic subentaneous connective tissue. The most sirperficial layer of the coilum is prolonged into miiuite papilla? over which llGl 37* 1162 PHYSIOLOGY the epidermis is moulded. These papillae contain for the most part capillary- vessels ; a few contain touch corpuscles, special organs of tactile sensation. The blood-vessels of the skin form a close capillary network immediately at the surface of the cutis, sending up loops into the papillse. All parts of the skin, except the palms of the hands and the soles of the feet, are beset with hair-folhcles. The hair- follicles are small pits which extend downwards into the deeper part of the corium, being down-growths of the rete mucosum. The hair grows from a small papilla of cells at the bottom of the follicle, the stratum corneum Stratum lucidum Stratum sranulosum Eete mucosum r Cutis vera Fig. 542. Vertical section through the skin of the palmar side of the finger, showing two papillse (one of which contains a tactile corpuscle) and the deeper layer of the epidermis. Magnified about 200 diameters. (Schafer). part of the hair lying within the follicle being known as the hair-root. The hair itself consists of long tapering, horny cells, the nuclei of which are still visible, though the cell substance has been almost entirely converted into keratin. In order to keep the cuticle supple and preserving it from the drying effects of the atmosphere, it is kept constantly impregnated with a fatty material known as sebum. This material is formed by the sebaceous glands, which are distributed all over the surface of the skin wherever hair-folhcles are to be found, the mouths of the glands opening into the hair-follicles. A sebaceous gland is a pear-shaped body, consisting of a secreting part and a short neck, opening into the folUcle. The gland proper is composed of a solid mass of cells. The outermost cells are flattened and generally show THE SKIN AND THE SKIN-GLANDS 1163 signs of proliferation. The cells lying internal to these are much larger, and their protoplasm is transformed into a network in the meshes of which are granules which may show the reaction of fat. Further inwards the protoplasmic network diminishes in amount, while the fatty granules increase in size, so that, in the lumen adjoining the duct, we find only a mass of cell debris and masses of fatty material. It has often been thought that the secretion of sebum depended simply on a fatty degeneration of the cells. The granules, however, when they first appear, stain with acid fuchsin rather than osmic acid, and one must regard the formation of sebum as an act of true secretion in which the secretory granules are gradually trans- formed into the special constituents of the sebum. For it must be noted that the sebum is not a true fat, nor does it correspond in composition with the fat found in other parts of the body. It is true that it contains fatty acids, but these are for the most part in combination, not with glycerin, but with higher alcohols, including cholesterol. A somewhat similar material may be extracted from wool, and is known as wool-fat or lanoline, as well as from the feather-glands of water birds, such as the goose and duck. It must be regarded rather as a wax than a fat. It presents many advantages over ordinary fat as a protective salve for the surface of the body. In the first place, it can take up a large amount, as much as 100 per cent., of water. In the second place, it is not attacked by micro-organisms, so that it does not tend to become rancid or to furnish a nidus for the growth of these organisms on the surface of the body. The secretion of sebum is a continuous process, though it is probably Cjuickened in conditions of increased vascularity of the skin. The extrusion of the products of secretion is determined by the presence of unstriated muscle fibres, the arrector piH, which pass from the surface of the cutis obliquely over the outer surface of the sebaceous gland. When these muscle fibres contract the hair is erected and a certain amount of the sebum squeezed out on to the root of the hair and the surrounding skin. This con- traction will occur whenever cold is suddenly applied to the skin. The con- tracted condition of all the muscles of the hair-follicles is shown by the ' goose-skin ' produced under such circumstances. There is no evidence that the secretion of sebum is in any way under the control of the central nervous system. THE SWEAT-GLANDS. Under normal circumstances in temperate climates the greater part of the water taken in with the food in the course of the day is excreted by the kidneys, a smaller proportion leaving by the lungs and by the surface of the skin. On an average we may say that about 700 CO. are got rid of through the skin. The excretion of water by the skin is, however, mainly determined by the need for regulating the temperature of the body, so that the amount leaving in this way depends on the heat production of the body or on the external temperature, and is very little aft'ected by alterations in the quantity of fluid drunk. A certain amount of water is constantly evaporated from the surface of the body as the so-called ' insensible perspiration.' If a man's body be enclosed in a vessel through 1164 PHYSIOLOGY which a current of air is passed, and the temperature of the air gradually raised, it will be noted that the amount of water given off rises slowly up to a certain degree and then rises rapidly. The sudden kink in the curve is due to the setting in of the activity of the sweat-glands, and we are there- fore justified in regarding the insensible perspiration as being determined by evaporation of water^from the surface of the cuticle itself apart altogether from the sweat-glands. The sweat-glands, which are distributed over the whole surface of the skin, are especially abundant on the palm of the hand and on the sole of the foot. They are composed of single unbranched coiled tubes, which he in the subcutaneous tissue and send their ducts up through the cutis, to open on the surface by corkscrew-hke channels which pierce the epidermis. The secreting part of the tube consists of a basement membrane lined by a double layer of cells ; the innermost of these are cubical and represent the secreting cells proper. Between the secreting cells and the basement membrane is a layer of unstriated muscle fibres., The duct of the gland has an epithehum, consisting of two or three layers of cells with a well-marked internal cuticular lining, but there is no muscular layer. The sweat formed by these glands is the most dilute of all animal fluids. As collected it generally contains epithelial scales and some admixture of sebum. After filtration it forms a clear colourless fluid of a specific gravity of about 1003. It contains over 99 per cent, of water. Among the sohd constituents sodium chloride is the most prominent — it may contain from 0'3 to 0-5 per cent, of this salt. It is generally hypotonic as compared with the blood-plasma. It may also contain small traces of protein. This constituent is especially marked in the horse. It generally contains also a small quantity of urea, which may become a prominent constituent in cases of renal disease. The quantity of sweat excreted in the day is very variable. The secretion is under the control of the central nervous system and is almost entirely adapted to the regulation of the body temperature. The nervous mechanism can be set into activity either centrally or reflexly. The most usual factor is a rise of the body temperature. If a man sit in a warm room, e.g. of a Turkish bath, the secretion of sweat commences as soon as the temperature of the body has attained a height of 0-5° to 1° C. above normal. In the case of muscular exercise the temperature will generally be found to be raised if it be taken at the instant that sweating has commenced. The effect of rise of temperature may, however, be either local or central, so that one arm enclosed in a hot-air bath may sweat while the rest of the body is dry. Under ordinary circumstances the central stimulation by the warm blood is the predominant factor. This is shown by an experiment of Luchsinger. In the cat sweating is to be observed only on the hairless pads of the front and hind paws. If one sciatic nerve be cut and the animal be placed in a warm chamber, sweating will commence as the temperature of the animal rises in the three intact paws, while the paw with the nerve cut will be quite dry. Sweating moreover, as has been shown by Kahn, may be induced in the cat's paws by warming the blood passing through the THE SKIN AND THE SKIN-GLANDS 1165 carotid arteries on its way to the brain, at a time when the temperature of the blood circulating through the rest of the body, including the paws them- selves, has undergone no alteration. Sweating may also be aroused by asphyxia, and this result is found even in the spinal cat, i.e. after separation of the spinal centres from the medulla. The secretion of sweat resulting from stimulation of the sweat- nerves, although generally associated with increased vascularity of the skin, is not in any way dependent thereon. Thus even in the amputated limb stimulation of the sciatic nerve may cause the appearance of drops of sweat on the pad of the foot. If the sciatic nerve be stimulated in the intact animal, the secretion of sweat which is produced is associated with constriction of the vessels of the skin, due to simul- taneous stimulation of the vaso-constrictor nerves running in the sciatic nerve. As Langley has shown, the sweat-nerves run entirely in the sympathetic system. Leaving the cord by the white rami communicantes from the second dorsal to the third or fourth lumbar nerves, they pass into the sympathetic chain. Here the first relay of fibres ends in connection with the cells of the sympathetic ganglia, and a fresh relay of fibres, which are non- medullated, pass from the cells along the grey rami into the various spinal nerves, to be distributed to the whole surface of the skin. The secretion of sweat by the sweat-glands may be roused by the injection of pilocarpine even after division of the sweat-nerves, so that this drug must act peripherally on the glands. The action of pilocarpine, as well as the efiects of artificial stimulation of the sweat-nerves, is abolished by the administration of atropine. THE GASEOUS EXCHANGES OF THE SKIN. In any animal with a thin moist skin, such as the frog, the absorption of oxygen and the excretion of CO2 from the skin may be sufficient for the proper aeration of its blood, so that it may continue to live after the extirpation of its lungs. In man there is also a continuous output of CO2 through the skin, but the amount leaving the body in this way is negligible compared with that which is exhaled through the lungs. The loss of CO2 by the skin rises with increase of external temperature. Thus at a temperature of 29° to 33° C. the CO2 output by the skin is about 0-35 grm. per hour, i.e. about 8-4 grm. in the twenty- four hours. When the external temperature rises above 33° C, the COo output increases, so that at 34° it is doubled and at 38-5° it may amount to as much as 1-2 grm. per hour (Schierbeck). It is just at this temperature of 33° C. that a secretion of sweat begins to be noticeable, so it has been suggested that the increased CO2 output may be due directly to the increased work and metabolism of the sweat-glands during their activity. ABSORPTION BY THE SKIN. In order to test the alleged influence of baths containing medicinal substances in solution, many experiments have been made to determine whether absorption is possible by the skin. It may be regarded as established that the uninjured skin is impermeable for watery solutions of salts or other substances. On the other hand, it is possible to produce a certain amount of absorption by the inunction of 1166 PHYSIOLOGY substances dissolved in fatty vehicles. Thus the administration of mercury- is often carried out by the inunction of mercurial ointments, and the fact that mercurial salivation may be produced in these conditions shows that a certain amount of the mercury must have been absorbed. It is difficult to imagine that any appreciable amount of cod liver oil will be available for the nutrition of the infant when this substance is administered by rubbing it on the skin. On the other hand, the moist mucous surfaces, such as the conjunctiva or the mucous membrane of the respiratory passages, as well as raw surfaces of the skin, e.g. which have been deprived of their epidermal layer by the apphcation of bhsters, permit of the rapid passage of substances in watery or oily solution. CHAPTER XIX THE TEMPERATURE OF THE BODY AND ITS REGULATION In dealing with the chemical changes in the body as a whole we have seen that the sum of the metabolic processes is associated with the evolution of heat. In man, under normal circumstances, while doing moderate work, the total energy requirements amount to about 3000 calories. The whole CO 120 110 100 90 y \ / / \ / \ / \ BO 70 SO 50 1 \ / \ / \ / r \ / 40 A i \ / \ 30 y »' \ 20 10 G ' ^ ^ A — A ^ ''n zs 35 40 46 SO 55 60 TCMPERATUftC Fu;. 5-1:5. Effect of tcniperaturo on the COo output of a lui)in seedling. Orclinatcs ~ milligrammes CO2 per hour. Abscissae = temperature in degrees Centigrade. of this is derived from the oxidation of the food, the combination of its carbon and hydrogen with oxygen to form CO.2 and water, with the evolu- tion of the corresponding amount of energy. Of this energy only a small proportion, on the average about one-twentieth, leaves the body as mechani- cal energy, the rest being evolved in the form of heat and being expended in the maintenance of the body temperature, or in the warming of the surrounding medium. 1167 1168 PHYSIOLOGY The evolution of heat is not confined to the higher animals, but is com- mon to all hving beings. It is very evident, for instance, in the germina- tion of peas or barley. The atmosphere of a bee-hive is often ten degrees above that of the surrounding atmosphere. Whenever we can excite increased activity in an organ, we are able to show, except in the single case of the nerve impulse, that such activity is associated with the evolution of heat. This heat is derived from the chemical changes which proceed in the living cells. Since all chemical processes are quickened by rise of tempera- ture, we should expect to find that the heat produced in the metabolic pro- cesses of organisms would tend in itself to quicken these processes. In most chemical reactions a rise of about 10° C. would increase the velocity of reaction from two and a half to three times, and the same rule is, within the hmits of stability of hving tissues, found to hold good for them also. The diagram (Fig. 543) shows the influence of temperature on the chemical changes in a lupin seedhng as measured by the output of CO 2 per hour per 100 grm. of plant. A marked increase in the rate of chemical decomposi- tion is shown to follow a rise of temperature ; but, about 40° C, the rate of change is at an optimum, and thereafter rapidly declines, owing to the fact that the hving tissues are being killed by the excessive temperature. Hence in the animal organism we shall expect to find that the rate of the metabohsm is also proportional to the temperature of the animal. This is universally the case whether we are dealing with warm-blooded or cold- blooded animals. In ' cold-blooded ' animals the temperature of the body, and therefore the rate of its metabohsm and the amount of its heat produc- tion, is proportional to the external temperature (Fig. 544). The following Table gives the average CO2 output per hour of five hzards placed in a chamber which could be maintained at varying temperature : Temperature Temperature of CO3 produced of bath lizards (average) in one hour 5-0° C. 5-5° C. •0246 9-0° a 9-2° C. •0790 15-0° C. 15-2° C. •0981 20-5° C. 20-4° C. •1023 25-0° C. 24-5° C. •1193 30-0° C. 29-3° C. •1440 35-0° C. 34-8° C. •1814 39-0° C. 38-5° C. •5454 It might be thought that such a reaction in change of temperature would result in a vicious circle. Since the animal is continually producing heat and thus raising its temperature above that of its surroundings, one might expect to find that the higher the external temperature the greater would be the difference between this and the temperature of the animal, until finally the latter would rise to such a height that the animal would die of heat-stroke, its tissues being destroyed by the actual temperature attained. A certain THE TEMPERATURE OF THE BODY 1169 protection is afforded to most cold-blooded terrestrial animals by the fact that their surface is moist, and that with a rise of external temperature the rate of evaporation on the surface increases, so that the increase in the rate of cooling bv evaporation more than corresponds to the rate of increase in the heat production, which would tend to raise the body temperature. Most of these animals, however, escape from any extreme rise of external tempera- ture by burrowing underground or taking to the water, while in plants a rise of external temperature assists transpiration to such an extent that the 30' 40 0 10" 20^ ^»>-^ External temp. C° Fig. 544. Effect of alterations in the temperature of the surrounding medium on output of COo in cold-blooded (poikilothermic) animals. (C. J. Martis.) temperature of the plant is generally several degrees below that of the surrounding atmosphere. The extreme variability in the metabohsm of such animals implies a state of dependence of all the activities of the body on the environment, which would prevent the utiUsation to the full of the available sources of energy. An animal whose metabolism was more or less independent of the surrounding temperature must have a great ad- vantage over an animal liable to have his activities reduced and paralysed by a sudden spell of cold weather ; this greater independence of the environ- ment which is characteristic of elevation of type, has been achieved by the warm-blooded animals, including man. Such animals are often spoken of as homoiotliermic, i.e. animals possessing a uniform temperature, in contra- distinction to the cold-blooded animals, which are poikilothennic and possess a temperature varying with that of their surroundings. Amongst the warm-blooded animals the body temperature may be very different according to the species. In birds it is generally from 39° to 40° C. ; in mammals it varies from 35° to 40° C. The temperature of man varies within slight limits about 37° C. (98-4° F.). 1170 PHYSIOLOGY THE DIURNAL VARIATIONS IN THE BODY TEMPERATURE In any animal the seat of heat production, e.g. a contracting muscle, must be warmer than an inactive tissue, and this again than a tissue from which heat is being rapidly abstracted, such as the skin. Owing, however, to the rapidity of the circulation of the blood, the temperature of the internal organs can be regarded as approximately uniform. The temperature of man is usually taken in the mouth, rectum, or axilla. In the case of the mouth the temperature is liable to fluctuation with the rate of breathing, the mouth being cooled by the passage of the air through the nasal cavities. There is also probably loss of heat through the cheeks. In order to determine the temperature in this situation the mouth should be kept closed for a few minutes and then the bulb of the thermometer inserted under the tongue, and the hps kept closed on the stem of the thermometer for five minutes. Except in cases where the cutaneous vessels are much dilated, the tempera- ture of a thermometer in the axilla takes a considerable time to rise to that of a thermometer in the mouth ; it should never be left less than ten minutes in this situation. The following Table represents the temperature of different parts of the body : Surface Skin covered with clothes 33° to 35° C. Naked skin in bath at 5° C 17° G. „ 25°C 26-5° C. Muscles 12 mm. below the Surface In bath at 5° C . 36-3° C. „ „ 25°C 36-9° C. The body temperature of man shows variations of several tenths of a degree according to the time of day at which the temperature is taken. The highest temperature is obtained about six or seven in the evening, and the lowest at about four or five o'clock in the morning. With these diurnal changes in temperature are associated parallel oscillations in the rate of metabohsm as shown by the elimination of carbon dioxide. They are probably determined by the changes in the movement and tension of the muscles occurring during the waking hours. If the habits of a man or animal be reversed, so that he sleeps in the daytime and performs his normal vocation by night, it is possible to reverse also the direction of the diurnal variations in temperature. The temperature may be also affected tem- porarily by various acts, such as the taking of food, or muscular work ; the influence of the latter factor is often very marked. In many individuals a hard game at tennis suffices to raise the temperature to 39° C. (102° F.), and even the healthiest individual will show some change in his temperature as the result of exercise. Pembrey found that the act of marching led to a considerable rise in temperature, a rise which is apparently responsible for the discomfort and fatigue observed under such circumstances. Such a change in the body temperature is merely temporary in its effects, and insignificant in comparison with the wonderful uniformity of temperature THE TEMPERATURE OF THE BODY 1171 observed in men of all races and of all climes under most varying conditions of food and activity. Just as there are limits to the power of the organism to regulate its temperature when there is an excessive formation of heat in the body, so there are limits to the regulation of temperature to severe alterations in the temperature of the external medium. Thus if the body is subjected to excessive external cold, or the loss of heat be increased by absence of clothes, by depriving an animal of its fur, or by immersion in a cold bath, the tempera- ture of the body may sink continuously. This fall of temperature in the higher animals is very soon followed by paralysis of the highest nerve-centres and loss of consciousness. The centres in the medulla are later on affected, so that the respiration is slowed and the blood-pressure falls. If the tem- perature does not fall too low it is possible to re^ave the animal or man by checking the loss of heat or by supplying artificial warmth. Recovery has in fact been observed in men in whom, as a result of exposure, the body tem- perature had fallen to 24° C. In the same way exposure to extreme heat, especially associated with muscular exercise and increased production of heat in the body, may cause a rise of the body temperature. A man, or animal, whose temperature is raised above the normal, is said to be in a state of pyrexia. A rise of 2° or 3° C. is associated with all the phenomena which characterise fever, i.e. quickening of pulse and respiration, malaise, headache, and loss of muscular power. If the temperature rise to a greater degree than this the patient may lose consciousness, and death ensues at a temperature of about 44° C. The very small variations presented by the body temperature in mam- mals, even under the influence of considerable variation in external tempera- ture, or in the production of heat in the body, connotes an accurate adapta- tion between the production of heat in the body and the loss of heat from the body. The regulation of the temperature can be effected either by regulation of heat production, by alteration in the rate of loss of heat, or by a combination of both mechanisms. It will be convenient to deal with these two methods of regulation under separate headings. THE PRODUCTION OF HEAT IN THE BODY The reactions mainly responsililo for heat production in the body are those associated with oxidation ; the processes of disintegration, such as are ciTected by means of hydrolytic ferments, accounting for a very small part of the heat evolved. In these processes of oxidation all the organs participate, the most important, especially in relation to the regulation of heat production, being the skeletal nuiscles. These represent more than half of the total weight of the soft ti8» ■> Eoct'- Cemp. de6. CanC. Flo. 545. Effect of variations in the external temperature on the COo output (per 1000 cm." body surface) of warm-blooded animals. (C. J. MAR-rrM.) same change, though less marked, is observed on exposing the rabbit to a gradual rising temperature. Here, however, the process of regulation is aided by alterations in the heat lost as well as in the heat production (Fig. 545). If the animals be observed, whilst subjected to changes of tempera- ture, it will be evident to any one that the regulation is associated with changes in muscular activity. At 30° to 35° C. the animals will he perfectly flaccid, breathing rapidly, or may go to sleep. On cooling they at once become more vigorous and perform active movements in their cage. The sam3 effects of changes in the external temperature are familiar in our- selves. The slackness and extreme disinclination to violent exercise ob- served in hot moist weather, contrasted with the stringing np of the tone of the muscles which follows exposure to cold, and which may be associated 1174 PHYSIOLOGY with voluntary exercise to keep ourselves warm, are indications of the important part played by the muscles in determining the heat production of the body. As a rule the immersion of a man in a cold bath for a minute or two increases considerably his output of CO 2- It is possible, however, to sit in a bath and by an act of the will keep all the muscles in a state of relaxation. Under these circumstances the temperature of the body rapidly falls, and with it the rate of metabolism, as judged by the output of carbon dioxide. This process of adjustment of the body temperature by variations in the heat production, so long as it represents the only method, is extravagant of energy directly the difference in temperature between animal and environ- ment attains any considerable degree. In the very perfect adjustment of the temperature which is present in the higher mammals, regulation of the heat loss plays a greater part than regulation of heat production. The economy of adjustment by heat loss is well shown if we compare in echidna and rabbit respectively the percentage alteration in COg production when the difference in temperature between animal and environment varies from 10° C. to 20° C. This is for echidna 72 per cent., for mammals 16 per cent. In echidna variations in heat loss can be practically neglected, so that the whole of the work of regulating the body temperature falls on the heat production. As soon as the external temperature falls below a certain degree the mechanism fails, the animal's temperature falls, and it passes into a state of hibernation. THE REGULATION OF HEAT LOSS In all temperate climates, and in fact in all climates except under certain exceptional conditions, the temperature of the warm-blooded animal is higher than that of his environment, so that there must be a con- stant loss of heat from the surface of the body. In the warm-blooded animals in the arctic regions, and in those which have adopted an aquatic existence, the thick layer of fat which underlies the skin protects the active portions of the body, the muscles and internal organs, from excessive loss of heat to the surrounding medium. In most terrestrial animals the loss of heat is also diminished by fur and feathers with which these animals are clothed, and in ourselves the same office is performed by clothes, which are capable of voluntary graduation to external conditions. The value of these different coverings depends on the fact that air is a very bad conductor of heat. In the case of a naked man the layer of air immediately in contact with the surface of the body is warmed, becomes lighter, and rises, its place being taken by fresh cold air. Loss of heat is increased by a draught, which quickens the rate at which the layers of air in contact with the surface are renewed. In the case of clothes the material encloses layers of air and hinders it from free circulation, and it is the air enclosed in the meshes of the garments and between the different layers of garments that plays the greater part in preventing loss of heat. The rapid cooling effect of wet THE TEMPERATURE OF THE BODY 1175 clothes is partly due to the replacement of layers of air by water, which is a much better conducting fluid. In addition to the loss of heat by convection, i.e. by warming the layers of air in contact with the body, heat is also lost by radiation, i.e. by an exchange of heat between the surface of the body and that of surrounding cooler objects. This loss is also prevented by clothing. Since the material of which clothes are made does not allow the passage of radiant heat, they absorb the heat leaving the body and become warm. This warm article of clothing may in its turn act as a centre for the loss of radiant heat, which may again be prevented by putting on another layer. It is a familiar experience that a multiplication of garments is more effective in retaining the heat of the body than merely increasing the thickness of the individual garments. The rate of loss of heat by radiation is diminished by a rise of the amount of watery vapour in the air, since this makes the air more opaque to the passage of radiant energy. Since the loss of heat depends on the difference of temperature between the surface of the body and the surrounding air, or objects, it will be largely affected by the temperature of the skin, and therefore by the amount of blood flowdng through the skin. The blood-flow through the slrin is under the control of the central nervous system, through the vaso-motor and vaso-dilator nerves, and it is by altering the size of these vessels that the central nervous system chiefly acts in regulating heat loss. In cold weather, or when the heat production in the body is low, the vessels are constricted, the skin is cold, and the heat loss is small. On the other hand, if the temperature of the surrounding air is high, or a large amount of heat is being produced in consequence of muscular exercise, the vessels are dilated and the skin is hot. In hot weather the dilatation in the cutaneous vessels is associated with muscular inactivity, so that there is a derivation from the muscles, where the blood is not required, to the skin, where a considerable circulation is necessary. Loss of heat by radiation and convection can only happen when the temperature of the surrounding air is lower than that of the body. The body temperature can, however, be maintained at its normal height in an atmosphere with a temperature much higher than 37-5° C, and this in spite of the fact that the production of heat in the body is still going on. In this case there is a profuse secretion of sweat on the surface of the body. In the evaporation of the sweat, especially if aided by a draught of air, a large amount of heat becomes latent and is abstracted from the body, which is therefore kept at a temperature below that of the surrounding atmosphere. If the secretion of sweat is checked, by depriving an animal or man of water, or if its evaporation be impeded by placing him in an atmosphere already saturated with aqueous vapour, the temperature of the body runs up rapidly and death ensues from hyperpyrexia, or heat-stroke. Although a man can stand exposure to a temperature of 200° or even 250° F. for a considerable time, provided that the air is dry, a temperature of 89° F. is rapidly fatal if the air be saturated with moisture. The same mechanism comes into play when the heat production in the body is very largely increased, as by 1176 PHYSIOLOGY violent exercise. Under these conditions a man may sweat profusely when the temperature of the surrounding atmosphere is at 0° C. The main regulation of heat loss thus takes place by the control of the nervous system over the cutaneous circulation and the sweat-glands. Besides these channels of heat loss, others may play an important part under certain conditions. Heat is lost to the body in warming the food and air which are taken in. It is also lost in respiration in the evaporation of w^ater and the setting free of COg from watery solution into the expired air. The following estimate, by Tigerstedt, represents the proportion of losses in an adult man by these different ways : A. Warming the Food and Air (1) 1500 g. water di'unk at 15° C. and warmed to 37-5° — raised therefore Cal. 22-5° = 33-75 (2) 1500 g. food eaten at 25° C. (mean) and warmed to 37-5°- — raised therefore 12-5; specific heat 0-8 = 15-00 (3) 15,000 g. (= 11,500 1.) air respired at 15° C. and warmed to 37-5°— raised therefore 22-5° ; specific heat 0-237 . . . . . = 79-95 128-70 B. Loss or Water and CO2 in the Breath (4) It is assumed that the inspired air is half saturated with watery vapour at 15° C. and that the expired air is fully saturated at 37-5° C Ap- proximately 450 g. of water would be given off therefore in the form of vapour from the respiratory passages ; the latent heat of the water vapour is 0-537 Cal =241-70 (5) The absorption of heat in the liberation of CO2 from the lungs (800 g.) ; 0-134 Cal. per g = 107-20 348-90 From above . . ... . 128-70 Total . . . . . . . 477-60 The sum of heat losses specified under these five headings amounts to 477-60 Cal. Estimating the total heat loss of an adult man at 2400 Cal., this sum represents only about 20 per cent, of the total. The remaining 80 per cent, (in round numbers) takes place through the skin. If we estimate the total heat loss of an adult man at 2400 calories, we may say that about 5 per cent, of the total heat loss takes place by warming the food and air, about 15 per cent., by the evaporation of water and CO2 in respiration, and about 80 per cent, by radiation and convection and the evaporation of sweat from the skin. The proportion represented by the last factor will increase very largely in the presence of a high external temperature, or of an excessive heat production in consequence of violent muscular exercise. THE NERVOUS MECHANISM FOR HEAT REGULATION The accurate balance between heat production and heat loss which is responsible for the nearly constant temperature of man, indicates the active THE TEMPERATURE OF THE BODY 1177 co-operation of the central nervous system in every step of the process. Whether this function of temperature regulation can be specially localised at any part of the central nervous system, so that it would be possible to speak of a heat-centre in the same way as we speak of a respiratory or vaso-motor centre, is doubtful. Many observers have found that injury to the cor'pus striatum causes a rise of temperature associated with increase both in heat production and in heat loss. On the other hand, injury to or pathological lesions of the pons Varohi often lead to an increased production of heat in the body, which is not compensated for sufficiently by heat loss, and so causes death by hyperpyrexia. It has been suggested that the thermogenic centre, i.e. that responsible for regulating heat production, is situated at a lower level in the nervous system than the thermotaxic system, i.e. that which presides over and determines the balance between heat production and heat loss. The observations of Meyer and Barbour that local coohng of the corpus striatum causes increased respiratory exchanges and heat production, while warming has the reverse effect, certainly point to a locahsation of the thermotaxic centre in this part of the central nervous system. The centres for heat loss must be placed in the medulla, at any rate so far as concerns control of heat loss by alterations in the blood-supply to the skin or in the secretion of sweat. The facts at our disposal are, however, too meagre to warrant any definite locahsation of the heat-regulating function in the central nervous system, or any such accurate analysis of the regulating function as has been just suggested. In many warm-blooded animals the abihty to maintain a constant tem- perature is not fully developed until some time after birth. Pembrey has shown that in the guinea-pig and chick, in which the nervous system is fully functional at birth, the heat-regulating mechanism is also completely adequate, whereas animals such as rats, pigeons, or the human child, which are born in a helpless condition, only acquire the power of regulating their own temperature some time after birth. As we should expect, the develop- ment of the power of regulating heat production runs fari passu with the acquisition of control by the nervous system over the muscles of the body. CHAPTER XX THE DUCTLESS GLANDS INTERNAL SECRETIONS In unicellular organisms, as in the rest of the living world, activity consists in adaptation to external conditions. The changes in the environment which determine the reactions of these organisms may occur at their surface or at some distance. Among the stimuli which, acting from a distance, evoke the reaction of unicellar organisms, probably the most important are those accompanied by chemical changes. The interrelation of micro- organisms with one another is determined almost entirely by such chemical stimuli. Thus the antherozoids of ferns are attracted to the ovule in con- sequence of the production in the tissue surrounding the latter of substances such as malic acid. Among micro-organisms we find some which leave places rich in oxygen, while others move towards any spot in their environ- ment where oxygen is most plentiful. These reactions of unicellular organ- isms to chemical stimuli are classed together under the term chemiotaxis. When the cells are united together to form cell colonies, or when, as in the metazoa, the multicellular aggregate is formed by the failure of the products of division of the ovum to separate one from another, the interrelations between the different cells forming the organism are still largely determined by chemical stimuli. In fact, in the lowest metazoa, svich as the sponge, we know of no other means of correlating the reactions of different parts of the cell aggregate. If a foreign substance be introduced into the living tissue of a sponge it becomes speedily surrounded with a collection of wandering phagocytic cells, called from the surrounding parts by the diffusion of chemical substances from the seat of the lesion into the fluids of the body. The same chemical sensibility determines the aggregation of leucocytes around bacteria or dead tissue, which forms the essential feature of the process of inflammation in higher animals. When the reaction of distant parts of the body to a change occurring in any one part depends on the diffusion of some substance from a stimulated part, the total reaction must require a considerable time for its full develop- ment. A much more effective method of correlation was acquired by the evo- lution of a nervous system, by means of which the consensus f.artium could be maintained by the rapid propagation of molecular changes along differen- tiated paths in the protoplasm. The development of this second mode of 1178 THE DUCTLESS GLANDS 1179 correlation ol activities ditl not. however, do away witii tlie necessity ior the more primitive method. Even in the higher animals, where rapidity of reaction is not required, we find adaptatioiis carried out in response to some change in distant parts of the body, the message having been chemical and not nervous in character {e.g. the secretin mechanism for pancreatic secre- tion). When we speak of the chemical correlation of the activities of the different parts of the body, it is important not to confuse processes which have little or nothing in common. In one sense we may say that every cell in the body is chemically connected with and dependent on all the other cells in the body. This interdependence is a necessary consequence of the differentiation of function associated with increased complexity of the organism. Thus the food-stuffs are digested and absorbed by the cells lining the ahmentary canal and are then transmitted, more or less changed by these cells, to all the other tissues of the body. The liver stores up glycogen and is ready to give of its store to any tissue in need of carbohydrate. All the tissues probably produce urea, which passes to the kidneys and there excites the act of excretion. All tissues produce carbon dioxide, which passes to the lungs to be excreted, but as it traverses the respiratory centre it arouses respiratory movements which are exactly proportioned to the tension of the carbon dioxide and therefore to the need of the whole body to eliminate this waste product. The liver receives ammonia from the alimentary canal and con- verts it into urea, thus shielding all the other tissues from the poisonous effects which would be produced by the entrance of the ammonia into the general circulation. Thus one organ may receive and modify any substance or food-stuff so as to prepare it for more ready assimilation by other tissues. It may shield these other tissues from the poisonous effects of certain waste products, either by converting these into harmless substances or by excreting them from the body. In all these cases the tissues are dealing with some substance which is utilised in bulk, or which, by its accumulation, could exert a toxic influence on other tissues. We are probably justified in treating apart a group of phenomena in which the substance transmitted from one part of the organism to another is significant almost entirely as an excitatory agent, and has little or no value as a source of energy. When the adaptation to a change of a consists in the activity of an organ b, the activity of b can be evoked either by a nerve impulse passing from a to the central nervous system and from this to b, or by the production at a, as a direct consequence of the stinnilus, of a specific chemical substance, which passes into the circulating blood to b, where in its turn it will excite the required state of action. Such chemical messengers are designated hormones, from op/umio, ' I excite.' We have already met with several examples of such bodies. It may be interesting here to consider what must be their general character if they are to fulfil the part of chemical messengers. (1) In the first place, they must not be antigens, i.e. their injection into the blood-stream must not evolve the production of an anti-body. If this were the case, the hormone, on entering the blood-stream, would meet 1180 PHYSIOLOGY its anti-body and would be unable to exert any effect ou the appropriate reacting organ. Practically all the complex colloid bodies allied to the proteins, e.g. ferments, egg albumin, peptone, sera of different animals, when injected into the blood-stream, cause the production of the corresponding anti- body. The hormones must be simpler in character than such substances and probably have a precise and comparatively simple chemical or mole- cular constitution. (2) Since they must be carried by the blood-stream to the reacting organ they must in most cases be susceptible to easy passage through the walls of the blood-vessels if they are to excite a reaction within a fairly short space of time. This consideration would also tend to keep their molecular weight comparatively low. (3) As a rule the chemical messenger must excite a state of activity in response to a change in some other part of the body. When the primary change passes away, the action of the hormone should also disappear. On this account it is necessary that the hormone should either be susceptible of easy destruction, by oxidation or otherwise, in the fluids of the body, or be readily excreted, so that its action may not be continued indefinitely. In previous chapters we have already come across several examples of correlated activities of different tissues effected by chemical means. It is perhaps questionable whether we should regard carbon dioxide, or the lactic acid produced by a contracting muscle, as a hormone in the strict sense of the term, since both these products are produced in large quantities as the final product of oxidation or disintegration of the food-stuffs. Carbon dioxide is, however, rapidly eliminated from the body, and lactic acid is equally rapidly oxidised in the body, and there is no doubt that the activity of the respiratory centre is determined by the presence of this substance in the blood and is thereby perfectly co-ordinated with the activities of the whole of the rest of the organism. In the aUmentary canal the secretion of pancreatic juice at the precise moment when it is required in the duodenum for the digestion of the food arriving there from the stomach is evoked by the production in the cells of the intestinal mucous membrane under the agency of the acid of the gastric juice (the specific stimulus) of a substance secretin. This substance, which is heat stable and diffusible, but is easily destroyed by oxidation, is absorbed into the blood and carried to the pan- creas, where it acts as a specific stimulus for the secretory cells. The same substance excites also the secretion of bile by the liver-cells and the secretion of intestinal juice by the glands of the small intestine. These are perhaps the two best examples of chemical reflexes, i.e. adaptations effected by chemical means rather than by impulses passing along the nerve- channels. There are, however, many other examples of a chemical influence exerted by one organ on another, in which the interaction probably depends on the production of minute quantities of some substance acting in virtue of its excitatory qualities rather than of its value as a source of energy. For many of the organs of the body we know, in fact, no other function than the production of some substance the presence of which in the blood is a necessary THE DUCTLESS GLANDS 1181 conditio)! for tiie carrying out of the normal functions either of growth or activity of many other parts of the body. In other cases an organ may have a twofold function. Thus the pancreas gives an external secretion which is used for the preparation of the food for absorption, and an internal secretion which, passing into the blood, exercises an important influence on the metabolism of the food-stuffs after absorption. Other instances of these chemical correlations may be cited. The secretion of gastric juice, which results from the presence of peptones or other substances in tha stomach, has been ascribed by Edkins to the production in the pyloric nmcous membrane of a gastric hormone, which travels by the blood to the glands of the fundus, where it excites secretion of gastric juice. According to Frouin the injection of boiled succus entericus, free from secretin, provokes the secretion of intestinal juice. In the reproductive system we have many examples of such chemical correlations. The pancreas, by its internal secretion, probably influences not only the oxidation of the carbohydrates but also the assimilation of the food-stuffs by all parts of the small intestine. All these examples are discussed in fuller detail in other parts of this book. There is one class of organs in which a chemical influence exerted on the rest of the body is the only function with which we are acquainted. These are included under the term ductless glands. As examples, we may cite the suprarenal bodies, the thyroid and parathyroids, the thymus and the pituitary body. THE SUPRARENAL BODIES The suprarenal capsules in mammals are two small masses lying on the upper or oral side of the kidneys. They consist of two parts, the cortex and the medulla. The cortex is composed of cells arranged in columns or in a reticular fashion. The outermost layer of cells often presents an alveolar structure, the lumen, however, being but little marked. According to the arrangement of the cells the cortex is divided into three zones, the zona glomerulosa, zona fasciculata, and zona reticulata. The cells themselves are distinguished by the large amount of granules they contain, which give the ordinary reactions for fat, but consist probably of lecithin compounds. In some animals, e.g. the guinea-pig, the cells, especially towards the inner part of the gland, contain many yellow pigment granules. The medulla, much less extensive than the cortex, presents irregularly shaped cells, the outlines of which arc but slightly marked. These cells contain granules which stain darkly with chromates and give a green colour with salts of iron. It is, hence, easy to deUmit the area of the cortex in any section of a gland which has been hardened in a fluid containing chromates. The substance giving this reaction is known as chromophile or chromaffine substance. The suprarenals are richly supplied with blood, especially in the medullary part, the cells of which impinge directly on the endothehal lining of dilated capillaries. They also receive an abundant nerve-supply from the sym- pathetic system, the nerves forming a thick meshwork, especially in the medulla, and presenting at intervals ganglion-cells which may be isolated or combined to form small ganglia. 1182 PHYSIOLOGY A study of the development of the suprarenal glands shows that we have here to do with two distinct tissues, probably differing in the part they play in the animal economy. Whereas the cortex is derived from that portion of the mesoblast, the ' intermediate cell mass,' from which the mesonephros is also developed, the medulla is produced by an outgrowth from the sym- pathetic system and may be said indeed to consist of profoundly modified nerve-cells. In many fishes these two elements of the suprarenal gland remain separated throughout hfe, the cortex being represented by a series of paired interrenal bodies lying on the front of the spinal column, and the medulla by a number of collections of chromaffine cells lying in close juxta- position to the spinal nerves. In some animals accessory suprarenals are not infrequent in which both cortex and medulla may be represented. In all animals we find masses of tissue, the so-called paraganglia, in close associa- tion with the sympathetic system, which present the chromaffine reaction typical of the medulla. Since a watery extract or decoction of these bodies has the same influence on injection into the blood-stream as an infusion of the medulla of the suprarenal body itself, we are probably justified in regarding these bodies as equivalent to accessory medullary portions of the suprarenal. They have the same origin, the same staining reactions, and the same physiological effect as the latter. The functions of the suprarenal bodies were a matter of pure hypothesis before Addison in 1855 drew attention to the coincidence of degenerative destruction of these bodies with a disease which has been known since that time as Addison's disease. The three cardinal symptoms of this disorder are (1) bronzing of the skin, (2) vomiting, (3) excessive muscular weakness and prostration. The disease is almost invariably fatal. Addison's observa- tions have been amply confirmed since that time, but we are not yet in a position to account for the occurrence of all these symptoms as a result of interference with the cortex and medulla of the suprarenals. The experi- mental destruction or extirpation of these bodies has naturally been fre- quently carried out. The operation always leads to the death of the animal within twelve to twenty-four hours. Even when the destruction is carried out by degrees it has been impossible to reproduce the bronzing which is so characteristic of Addison's disease. The one symptom which is observed as a result of the experimental extirpation is the excessive pros- tration, which is attended with muscular weakness and a lowered blood pressure. In a few cases it has been found possible to keep rats alive after total extirpation of these organs, but this result is probably due to the frequent presence in these animals of accessory suprarenals. Schafer and Oliver in 1894 found that a watery extract or decoction of the suprarenal bodies, when injected into the circulation, caused a very great rise of blood-pressure, brought about chiefly by constriction of all the blood- vessels of the body. The active substance responsible for this rise was limited entirely to the medulla, infusions of the cortex being without influence on the blood-pressure. Later on Takamine succeeded in isolating the active substance, to which he gave the name of adrenaline, and since that time THE DUCTLESS GLANDS 1183 physiological chemists have succeeded not only in determining the consti- tution of adrenaline but also in preparing it synthetically. The constitution of adrenaUne is shown by the following formula : HO H0<^ ^— CH(OH)— CH2XHCH3 Since it possesses an asymmetric carbon atom, a substance of this formula may be either Isevo- or dextrorotatory. Both forms, as well as the racemic modification, have been prepared synthetically. The substance wliich occurs in the suprarenal gland is the Isevorotatory modification, and Cushny has shown that it is only this modification which is active, injection of the dextrorotatory compound having only one-twelfth the effect of the laevo- rotatory. Adrenaline is active in excessively minute doses, injection of one four-hundredth of a milligramme per kilo body weight sufficing to evoke a definite rise of blood-pressure. On injecting it into the circulation there is immediately a rise of blood-pressure which, if the vagi are intact, is only moderate in amount, but is accompanied by a marked slowing of the heart. This excitation of the vagus is, however, probably secondary to the rise of blood-pressure and is not due to direct action of the drug on the vagus centre. If the vagi be divided the injection of adrenaline evokes a huge rise of pressure which may amount to 300 mm. Hg. It may indeed be so great that the animal dies from heart-failure or from pulmonary oedema. The rise of pressure is observed even after destruction of the central nervous system. The action is not Hmited to the blood-vessels. It has been shown by Langley and by Elliott that adrenahne injected into the circu- lation arouses every activity which can be normally excited by stimulation of the sympathetic system. A list of the actions of adrenaline is therefore identical with a list of the chief functions of the sympathetic nervous system. In the head it causes dilatation of the pupil, secretion of saliva, and erection of the hairs. On the heart it has a strong augmentor and accelerator influence, so that the heart beats more effectively as a rule even against the enormously increased resistance offered by the constricted arterioles. Whereas a rise of blood -pressure generally causes increased systolic volume of the heart, we may find after an injection of adrenaline and during the height of the rise of blood-pressure that the heart empties itself more effectively than it did before the injection. On the lung-vessels adrenaline has probably a slight constrictor influence. With regard to the vessels of the brain, we find the same divergence of opinion as in the case of excitation of possible vaso-motor nerves to this organ. Some observers, on perfusing the brain with defibrinated blood, have obtained constriction on adding adrenaline to the perfused blood, while others have been unable to obtain any positive results in this direction. In the abdomen intravenous injection of adrenaline causes complete relaxation of the nmsculature of the stomach, small and large intestines, but contraction of the ileocolic sphincter. On the bladder its eifect varies according to the animal studied, 1184 PHYSIOLOGY but ill every case is identical with that obtained by stimulting the hypogastric nerves. It has been shown by Dale that adrenaUne may also excite vaso- dilator fibres or produce vaso-dilator effects when such effects are also obtained from stimulation of the sympathetic nerves. In order to evoke these results it is necessary to paralyse the vaso-constrictors by the injection of ergotoxin, one of the active principles of ergot. This drug, when injected, causes first active vaso-constriction and rise of blood-pressure, followed by paralysis of the vaso- constrictor mechanism. Excitation of the splanchnic nerves or injection of adrenahne will now bring about a fall of blood-pressure due to dilatation of the vessels in the splanchnic area. The point of attack of the adrenahne appears to be in the muscular or glandular tissues themselves, since it may be obtained not only after destruc- tion of the cord and sympathetic plexuses but even after time has been allowed for the peripheral (post-ganghonic) fibres to degenerate as a result of extirpation of the corresponding ganglia. Although the effect is not altered under these circumstances, and it may still produce either relaxation or contraction of muscles according to the original action of the sympathetic on these fibres, we are not justified in regarding it as acting on the contractile material of the cells themselves. Rather must we assume with Langley and EUiott that the action of adrenahne is on some substance mediating between the nerve and the responsive tissue. We may speak of this reactive material as the receptor substance (Langley), or we may locate it in the situation where the nerve joins the muscle or gland- cell and describe adrenaline as acting on the myoneural junction. Each suprarenal receives a number of filaments from the splanchnic nerve on its own side. These pass to the medulla where they end apparently without the interposition of any ganglion cells on their course (Elliott), the cells of the medulla having themselves been developed by a modification of sympathetic ganghon cells. Stimulation of the peripheral end of the splanchnic nerve causes, as we have already seen, a discharge of adrenahne into the blood-stream. This discharge accounts for the secondary rise, often accompanied with quickening of the heart, observed on a blood- pressure tracing as the result of stimulating the splanchnic nerve. Through the splanchnic nerves a discharge of adrenaline can be excited by many general conditions, such as pressure on the brain, puncture of the fourth ventricle, administration of anaesthetics, mental conditions such as excite- ment or fright. Such a discharge is an important element in the reaction to environmental stress and enables the animal to react for the preservation of its life either by offence or flight. If one splanchnic nerve be cut before the administration of anaesthetics or the maintenance of a condition of irritation or fright, the suprarenal gland on the corresponding side will be found to contain two or three times as much adrenahne as the gland which has been left in connection with the central nervous system. It is interesting that no such condition of exhaustion can be produced by electrical stimulation of the peripheral end of the divided splanchnic. It has been suggested, therefore, that the splanchnic nerve contains two sets of fibres, anabolic THE DUCTLESS GLANDS 1185 and katabolic, that only the latter are stimulated by central iriitation, whereas electrical stimulation, exciting both sets of fibres, causes an increased production of adrenaline in the gland, which exactly keeps pace with the increased output. When adrenaline is injected into the blood-stream the efltect is only temporary. It is not excreted in the urine, but rapidly disappears from the blood. Since it is easily oxidised and is extremely unstable in alkaline solution, we may conclude that after performing its excitatory function it is destroyed by oxidation in the fluids. Adrenaline is thus a typical hormone, a body of comparatively low molecular weight, having a drug-like excitatory action on specific tissues of the body, easily diffusible, and rapidly destroyed after discharging its office. Owing to the rai)id destruction of adrenaline, relativelj'^ enormous doses have to be given by the mouth in order to produce any effect on the blood-pressure. There is, however, a whole series of substances, more or less allied to adrenaline in chemical con* stitution, which undergo less rapid destruction and can therefore be administered as drugs in the usual way. Dale and Barger have recently described three such sub- .stances as occurring in infusions of putrid meat and as forming the most important of the active principles of ergot. The constitution of these substances is shown in the following formulae : ^S[*\cHCHoCHoNH, Isoamylamine HO/^ \— CHoCHoNHo p-hydroxyphenylcthylamino / \ — CH2CH2NH2 phenylethylamine HO Hq/ >— CH(0H)CH2NHCH3 adrenaline The formula of adrenaline is placed below in order to show the relation of these substances to the natural hormone. These bodies are produced from the amino -acids of proteins by a process of decarboxylation. Leucine would yield isoamylamine, tyrosine, hydroxyphenylethylamine, and phenylalanine would give phenylethylamine. Such substances may be formed in minute quantities during the normal processes of putrefaction which occur in the alimentary canal. There seems little doubt that we nuist regard adrenaline as a true internal secretion, and therefore must ascribe to the medulla of the suprarenal capsules as well as to the other chromaffine tissue in the body, the function of main- taining the normal constriction of the arterioles and of facilitating in some way or other the functions of the sympathetic system generally. The absence of this secretion in cases of destruction by disease of the suprarenals would serve to account for the weakness, prostration, and lowered blood- pressure of Addison's disease. The two other symptoms of this disease, namely, bronzing aiul vomiting, still remain to be accounted for. It is possible that the latter may be due to some involvement by the morbid process of the mimberless fibres of the solar plexus, which run in close proximity to the suprarenals. We have no knowledge whatsoever of the functions of the cortical portion of these organs. It is possible that future 38 1186 PHYSIOLOGY work may show some connection between the cortex and the destruction of pigment in the body. At present it is only by a process of exclusion that we may guess at a causal relationship between the destruction of the cortex and the bronzing which occurs in Addison's disease. There seems Httle doubt that the rapidly fatal effects of extirpation of both suprarenals is to be ascribed rather to the removal of the cortex than of the medulla. The functions of the latter can be more or less effectively maintained by the other chromaffine tissues found at the back of the abdomen. In the few cases where animals have survived double extirpation small masses of accessory cortical substance have been found embedded in the kidney or elsewhere in the neighbourhood of the suprarenals. THE THYROID GLAND AND THE PARATHYROIDS The thjrroid gland consists of two oval bodies lying on either side of the trachea, joined in many animals across the trachea by an isthmus. Sur- rounded by a capsule of connective tissue^ it is made up of an aggregation of vesicles varying in size from 15 to 150;>i. The vesicles are lined by a single layer of cubical epithelial cells, and are filled with a trans- lucent material known as colloid (Fig. 546). Of the cells, some present granules and resemble the cells of a secreting gland, while others contain masses of colloid, or have undergone colloidal de- generation. Between the vesicles may be seen, here and there, solid masses of cells which by some observers are regarded as destined to replace vesicles the epithelium of which has undergone complete degeneration. The colloid material can be traced between the cells into the lymphatics lying between the vesicles. Since the gland possesses no duct it is supposed that the cells furnish an internal secretion, which makes its way into the blood along the lymphatic efferents of the gland. The thyroid is richly supplied with blood by the superior, middle, and inferior thyroid arteries, and is surrounded with a plexus of veins lying immediately under the capsule. In development the thyroid is formed by an outgrowth from the fore-gut, but the connection with the gut disappears long before the end of foetal life. In rare cases part of the duct may persist, and, becoming gradually filled with fluid, give rise to a hyoid cyst which, lies below the tongue and may require excision by the surgeon. As in the case of the other ductless glands, clinical observations have contributed materially to our knowledge of the functions of the thyroid. Although the gland had been extirpated in animals by Astley Cooper and Fig. 546. Section of thyroid gland of dog. (Swale Vincent.) THE DUCTLESS GLANDS 1187 by Schift", the attention of physiologists and medical men was especially directed to the importance of this organ by the observations of surgeons, especially Kocher, on the untoward and even fatal effects following its complete removal in man in operations for extirpation of goitre. In this country attention had already been called to the connection of a disturbed condition of metabolism known as myxoedema with atrophy of the thyroid. A patient affected with myxoedema presents a gradually increasing blunting of his or her mental activities ; speech is slow, cerebration delayed. With this nervous defect are associated changes in the connective tissues, the subcutaneous connective tissue becoming thickened, so that the face and hands appear swollen and puffy, looking at first sight as if oedema were present. The swelling is, however, due to newly formed connective tissue, and not to the presence of an excess of interstitial fluid in the tissues. The patient often presents a yellow, waxy appearance with a patch of colour on the cheeks. The hair falls out, the pulse is slowed, and the temperature tends to be subnormal owing to the diminution of the rate of metabolism in the body. The intake of food and the excretion of urea are diminished. If the atrophy of the thyroid occurs in early hfe during the period of growth, e.g. in young children, the growth of the skeleton practically ceases. The bones of the limbs may grow in thickness but not in length. There is early synostosis of the bones of the skull and complete cessation of develop- ment of mental powers. Children so affected may hve for many years, but when twenty-five or thirty present still a childish appearance (Fig. 547, c). Stunted, pot-bellied, and ugly, they have the intelligence of a child of four or five. They often present fatty tumours above each clavicle, and similar subcutaneous tumours of fat or loose fibrous tissue are found in cases of myxoedema in the adult. When the thyroid is extirpated in man the result is often the production of typical myxoedema. In some cases, especially in young individuals, the results are more severe, a condition of tetany being set up in which there are tonic spasms of the muscles of the body, especially of the extremities. When the thyroid gland is extirpated in animals the results more closely resemble these acute cases. In certain cases a chronic condition of malnutrition is set up, but a typical myxoedema with thickening of the subcutaneous tissues by new growth of connective tissue has only been described by Horsley in monkeys. The effects are more pronounced in carnivora than in herbivora. In the former a condition of tetany is produced accompanied with muscular tremors and clonic convulsions which come on at intervals and may be accompanied with severe dyspnoea leading to death within fourteen days. In herbivora, wasting, diminution of respiratory exchange, and disorders of nutrition are often the most prominent symptoms. These results were ascribed by Munk to interference with the recurrent laryngeal nerves during the operations, but the observations on man leave very little doubt that they are due entirely to the removal of the chemical influence of the thyroid gland. Many authorities were at first inclined to ascribe these results in man and animals to the circuIatio)i in the blood of 1188 PHYSIOLOGY toxic substances which would normally undergo destruction in the thyroid gland. This theory is put out of court by the results of administration of thyroid gland to patients with myxoedema or to animals deprived of their thyroids. SchifE first showed that the effects of extirpation of the thyroid might be prevented if, at the same time, the thyroid from another animal were transplanted into the subcutaneous tissue to take the place of the one removed. On removing the transplanted thyroid, the typical symptoms of thyroid destruction at once ensued. It was later found that similar good results could be obtained by subcutaneous injection of the expressed juice of the thyroid, and later that even this was not necessary, and that it A EC ^ :*-r FiG. 547. A, a cretin, 23 months old. b, the same child, 34 months old, after ad- ministration of sheep's thyroids for 1 1 months, c, a cretin, untreated, 15 years old. (W. OSLEE.) was sufficient to administer the thyroid gland, either fresh, dried, or partially cooked, by the mouth. The administration of the thyroid gland in this way is indeed one of the therapeutic triumphs of the last twenty years. An ugly and idiotic cretin can be converted in this way into a child of ordinary intelligence with normal powers of growth (Fig. 547). Given to myxoedemic patients, the thyroid gland reduces the swelling of the subcutaneous tissues, causes a new growth of hair, and restores the patient to his or her former state of mental health. Nor is the thyroid gland without influence on the healthy individual. If given in large doses either to man or animals, it quickens the pulse, even causing violent palpitation, and increases the metabolic activities of the body, so that the appetite is increased, the nitrogenous output rises above the intake, and the subcutaneous fat is diminished or disappears. It is possible that a moderate degree of thyroid inadequacy is not infrequent and that the beneficial effects on general health, in removing excessive THE DUCTLESS GLANDS 1189 corpulence and in promoting the growth of hair, which are observed on administering the drug to people of middle life, may be due to the actual replacement of a function which is being insufficiently discharged. The symptoms caused by excessive doses of thyroid gland are strikingly similar to those occurring in the disease known as exophthalmic goitre, where there is a true hypertrophy of the gland associated with cardiac palpitation, proptosis (bulging of the eyes), wasting, and muscular weakness. All these facts warrant us in including the thyroid body among the glands with an internal secretion, the presence of which in the blood-stream is a necessary condition for the normal growth and functions of almost all the tissues of the body. If this secretion is lacking we obtain the condition known as myxoedema in adults, as cretinism in young children. If it be present in excess the symptoms of exophthalmic goitre are produced. The exact character of the internal secretion cannot be regarded yet as definitely established. It seems possible that it is identical with a substance containing iodine in organic combination, which was isolated by Baumann from the thyroid gland and is known as iodothyrin. In certain experiments the results of administration of iodothyrin were found to be identical with those obtained by giving the whole gland. Doubt has been thrown on the specific nature of this body on account of the fact that iodine may be wanting in the thyroid gland in certain animals, though Reid Hunt has shown that the physiological effects of thyroid extract are proportional to the amount of iodine contained therein. SIGNIFICANCE OF THE PARATHYROIDS. The parathyroids are small bodies, varying in number, situated on the border of the thyroid gland or actually embedded in its substance. In histological appearance they differ widely from the thyroid, and consist of sohd masses or columns of epithelial cells surrounded with connective tissue and richly supplied with blood-vessels (Fig. 548). Considerable divergence of opinion still exists as to the significance of these bodies. In some animals, e.g. in the dog, where they are embedded in the gland, they will be necessarily removed in any operation for the extirpatio}i of the thp'oid. In others, such as the rabbit, where they lie outside the gland, it is easy to avoid them in the excision of the thyroid. To this varying distribution of the parathyroids have been ascribed the different results of extirpation of the thyroid in carnivora and herbivora respectively. Forsyth has shown that, in man, the situation of the parathyroids corresponds almost exactly with the places in which are found occasionally accessory thyroids ; and. according to Ednumds, after excision of the thyroid, the parathyroids undergo histological alteration and are converted into thyroid tissue, the cells taking on an alveolar arrangement and producing colloid material. According to this view the parathyroids would represent simply immature thyroid tissue. On the other hand, it has been suggested (Biedl) that the parathyroids have a function entirely dis- tinct from that of the thyroid gland, removal of the thyroids producing simply a condition of cachexia and the changes associated with myxoedema, 1190 PHYSIOLOGY while removal of the parathjrroids is responsible for the nervous disturbances and tetany observed after total extirpation of these organs. The matter cannot yet be regarded as definitely settled. en& end. Fig. 548. Section of parathyroid. (Kohn.) ep, secreting epithelium ; pig, cells containing pigment ; cuf, sinus-like capillaries ; enA, endothelial cells. THE PITUITARY BODY The pituitary body consists of two parts which have separate modes of origin. An outgrowth from the buccal cavity in the embryo meets a hollow extension of the anterior cerebral vesicle. The buccal ectoderm gives rise to the anterior lobe and pars intermedia of the pituitary, while the neural epiblast becomes developed into the posterior lobe (Fig. 549). In some animals the posterior lobe remains hollow and retains its primitive connection with the third ventricle of the brain, but in man it becomes entirely sohd. The anterior lobe in the adult consists of nests of epithelial cells (Fig. 550), many of which are filled with granules, and is richly supplied with large, thin- walled capillary blood-vessels. The anterior lobe is separated from the posterior lobe by a cleft which is the remains of the original hollow outgrowth from the buccal cavity. The epithelial tissue immediately surrounding this cleft differs somewhat from that constituting the anterior lobe. The cells, which present but few granules, are arranged in islets, separated by an intervening tissue continuous with the main mass of the posterior lobe. Many of the islets are hollow and enclose a colloid material. The colloid material has been traced by Herring into the interalveolar connective tissue and into the prolongation of the infundibulum which enters the posterior THE DUCTLESS GLANDS 1191 lobe. One must therefore conclude that the colloid material secreted by the cells of this part passes directly into the third ventricle. The amount of colloid material increases in animals which have undergone extirpation of the thyroid gland. Our first clue to the importance of this organ in the normal processes of the body was furnished by the observations of Pierre Marie, who found that the morbid condition of ' acromecjoly ' is associated with tumours of the Fig. S-tO. Mesial sagittal section through the pituitary body of an adult monkey (semi-diagrammatic). (After Herring.) a, optic chiasma ; h, third ventricle ; c, tongue-like process of pare intermedia ; d, epithelial investment of posterior lobe ; e, anterior lobe ; /, epithelial cleft ; g, pars intermedia ; h, posterior lobe. pituitary gland. This disease consists in an increased growth of certain parts of the skeleton, especially the lower jaw and the extremities of the limbs. Headache is often present, and there may be polyuria and affection of the eyesight. When this disease occurs during the period of active growth there may be an increase in length both of the limb-bones and of the trunk, and most of the giants, which are shown from time to time, are examples of this pathological condition of ' gigantism.' It seems probable that this condition is due to an over-action of the gland. In cases of acromegaly the tumour is generally an adenoma, i.e. an enlargement of the ordinary gland tissue. Total extirpation of the pituitary body is generally followed by death within a few days, death which cannot be attributed to shock or any accidental features of the operation. One must regard the pituitary there- fore as essential to the maintenance of life. It has not been possible by transplantation to replace a removed pituitary body, since the transplanted organ has hitherto always undergone degeneration. In a certain number of cases animals, especially if young, have survived extirpation of the 1192 PHYSIOLOGY pituitary body. In these the operation was followed by arrest of develop- ment— the animals remaining in an infantile condition, small with an excess of fat, and absence of sexual development. The most definite evidence we have as to the mode of action of the different parts of the pituitary gland has been furnished by experiments on administration or injection of the dried gland or its extracts. The posterior lobe seems to be practically inactive, extracts made from this lobe having Fig. 550. Section of cat's pituitary body, passing through the cleft in the gland. (P. T. Herring.) a, pars anterior ; b, cleft ; c, pars intermedia : d, pars nervosa (posterior lobe). the same influence as extracts from nervous tissue generally. If, however, the intermediate epithelial substance is included in the posterior lobe, marked effects may be obtained from the intravenous injection. An extract of the posterior lobe (including pars intermedia) produces, as was shown by Schafer, a rise of blood-pressure and diuresis. The latter result also follows administration of the posterior lobes by the mouth. Dale has shown that the active principle exercises a direct excitatory effect on all unstriated muscle, the effect being unaltered whether the nerve-supply to the muscle be present or not. Thus it produces contraction of the blood-vessels, of the intestinal muscle, and of the uterus, and will act upon muscular tissues, such as the arteries of the lungs or heart, which do not receive con- strictor impulses from the sympathetic system. The active principle is much more stable than the other hormones we have already studied. It is not destroyed by boiling, and after injection into the blood-stream can be THE DUCTLESS GLANDS 1193 recovered from the urine. It is possible that the polyuria, which is not infrequently observed in association with head injuries or tumours of the brain, may be occasioned by an increased escape of this material into the general circulation. Extracts from the anterior lobe have no definite effect when injected into the blood-stream. Schafer found that the addition of the anterior lobe of the pituitary body to the food of young gro\ving animals causes an increased rate of growth. In this experiment eight rats of a htter were taken : four were fed with bread and milk to which the anterior lobes of pituitary bodies had been added, while the other four, which served as controls, received bread and milk with a corresponding quantity of testis or ovary. Later experiments have not however confirmed these results. According to Mackenzie extracts of the pituitary body have a marked excitatory effect on the secretion of milk by the mammary glands. It is evident that much further work is necessary before we can regard the functions of the pituitary body as definitely ascertained. The evidence we have at present would seem to point to the following conclusions : (a) The anterior lobe furnishes some substance to the circulation which promotes growth, especially of the bony and connective tissues of the body. (6) The intermediate part surrounding the cleft between anterior and posterior lobes, in addition to the production of some substance which is a general excitant for unstriated muscle and produces diuresis, also furnishes a colloid secretion which passes directly into the ventricles of the brain and may be assumed to have some influence on the growth or functions of the central nervous system. Schafer regards the principle giving rise to diuresis as distinct from that causing contraction of unstriated muscle, since diuresis may occur without corresponding rise of blood-pressure. The independence of the two phenomena, renal and vascular, cannot be regarded as proved. (c) The posterior lobe consists mainly of neurogha. We have no clue to its functions apart from the masses of intermediate cells which it may contain. Very httle can be said as to the other ductless glands. The thymus forms two large masses in the anterior mediastinum which in man grow up to the second year of life and then rapidly diminish so that only traces are to be found at puberty. It contains a large amount of lymphatic tissue and is therefore often associated with the lymphatic glands as the seat of formation of lymph-corpuscles. The epithelial remains of Hassell's cor- puscles found in the medullary part of its globules have not had any func- tion assigned to them. In certain cases of arrested development or of general weakness in young people the thynuis has been found to be per- sistent. The effect of extracts made from the thymus do not differ from those of extracts made from any other cellular organ. The pineal gland has, so far, not been proved to have any function in metabolism.* It is interesting as a vestigial remnant of a primitive dorsal eye. In certain lizards this organ still presents traces of its original * Cases have been recorded in wliith tumours of the pineal body have been asso- ciated with obesity, premature sexual development and early matiu-ity. 38* 119^ PHYSIOLOGY structure, and is found to conform to the invertebrate type of eye. It is doubtful whether at any time in the history of vertebrates the pineal eye has been functional. The carotid and coccygeal glands have often been grouped with the collections of chromaffine cells already described as associated with the sympathetic system. Their structure resembles more nearly that of the parathyroid bodies or the anterior lobe of the pituitary gland. They con- sist of a small collection of columns or masses of cells bound together by connective tissue with a rich supply of blood-capillaries. Nothing is known as to their function. The lymph and heemolymph glands, and the spleen, are often grouped with these ductless glands. The essential activity of these bodies, however, Hes in the production, not of a diffusible chemical substance, but of formed elements, e.g. lymph-corpuscles, and they do not properly fall within the scope of this chapter. As a matter of convenience, we may deal shortly here with the functions of the spleen. THE SPLEEN This organ is similar in many respects to a lymphatic gland. It is formed of a framework of connective tissue and unstriated muscular fibres, in the interstices of which is contained the splenic pulp. This consists of a Carotid VA./s,,^/v^y\/^..■■^/vW^-A^-^ .■^J'^y^J"vAA.''\AAA./ Dog:8.5 Kilo. all connections with splelen SEVERED EXCEPT one: ARTERY & VEIN -^^ 0 PRESSURE Fig. 551. Pleth3\smographic tracing of spleen (upper curve) from a dog, showing the spontaneous contractions of this organ (reduced from a tracing by Schafer). fine fibrillar network, on the fibrils of which lie endothehal cells. The meshes contain the cells of the splenic pulp, which are fairly large polygonal cells, and leucocytes. Just as in a lymphatic gland the cellular elements of the tissues are bathed by the lymph which flows through the gland, so in the spleen the walls of the capillaries become discontinuous, and the blood is poured out into the interstices of the tissue. The spleen is therefore the only tissue in the body where the blood comes in actual contact with the tissue-elements themselves. The blood from the splenic pulp is collected into large venous sinuses, which run along the trabeculse to the hilum, where they unite to form the splenic vein. The arteries to the spleen are beset in their course along the trabecular with small nodules of lymphoid tissue, which are known as the Malpighian folhcles. THE DUCTLESS GLANDS lli)5 It is evident that the blood must meet with considerable resistance in passing through the close meshwork of the splenic pulp. Li order to ensure a constant circulation through the gland, the muscular tissue of the capsule and trabeculse has the property of rhythmic contraction. If the spleen be enclosed in a plethysmograph, or splenic oncometer, and its volume be recorded by connecting this with an oncograph, it will be seen to be subject to a series of large, slow variations, each contraction and expansion lasting about a minute, and recurring with great regularity (Fig. 551). Superposed on these large waves are smaller undulations due to the respiratory variations of the blood-pressure, and on these again the Uttle excursions corresponding to each heart- beat. The contractile power of the spleen is under the control of the nervous system, and a rapid contraction may be induced by stimulation of the splanchnic nerves. FUNCTIONS OF THE SPLEEN The structure of this organ suggests that the splenic cells must exercise a constant influence on the blood which surrounds them, and that this influence is not purely of a chemical nature. In the liver and kidneys. Fig. 5.52. ("ells from a scraping of the spleen. (Kolliker.) A, splenic pvilp-cell containing red blood-corpuscles, h (k = nucleus) ; b, leucocyte with ])nlynif)rphous nucleus; c, pulp-cell containing disintegrated red corpuscles; n. lyiniihocyte ; E. giant cell ; f, nucleated red corpuscles ; o, normal red corpuscle ; H. multi nuclear leucocyte ; j, eosinophile cell. which exercise so powerful an ofToct on the composition of the blood passing through them, the proper cells of the organs are separated from the blood- stream by the capillary wall. Microscopic examination of the cells of the splenic pulp shows us that these are full of particles of brown pigment or fragments of red corpuscles (Fig. 552). In many cases of infectious disease, such as recurrent fever, the splenic cells are observed towards the end of the attack to be full of the organism— spirillum — which is the cause of the disease. In fact, these cells are so arranged that they can take up solid particles hold in suspension in the blood-plasma. We must indeed look upon the spleen as the great blood-filter, purifying the blood in its passage by 1196 PHYSIOLOGY taking up particles of foreign matter and effete red corpuscles. The process of phagocytosis, which was described under the cellular mechanisms of defence (p. 1022), is in the spleen a normal occurrence, A function has also been assigned to the spleen in the formation of red blood-corpuscles, but the evidence is not sufficient to determine whether such a process occurs normally. Chemical analysis of the spleen reveals the presence of a large number of what are called extractives, such as succinic, formic, butyric, and lactic acids, inosit, leucine, xanthine, hypoxanthine, and uric acid. There is also a protein, combined with iron, as well as several pigments probably derived from the haemoglobin of the red corpuscles destroyed by the cells of the splenic pulp. The fact that, in cases where the spleen is pathologically enlarged as in leucocythsemia, the uric acid in the urine is largely increased points to a connection between the spleen and the formation of uric acid in the body. The numerous extractives which are formed probably owe their origin to the destructive changes effected on the effete constituents of the blood by the agency of the splenic pulp-cells. BOOK IV REPKODUCTION CHAPTER XXI THE PHYSIOLOGY OF REPRODUCTION SECTION I THE ESSENTIAL FEATURES OF THE SEXUAL PROCESS The two fundaniental characteristics of protoplasm, which distinguish it above all others from unorganised matter, are growth and activitij. Growth occurs at the expense of surrounding non-living material, while activity is in every case an adapted reaction to changes in the environment. The second characteristic would seem to involve a limitation of the first, and does, in fact, determine the conditions under which it may occur. In the process of growth of a minute spherical mass of protoplasm its bulk and mass increase as the cube, while the surface only increases as the square, of the radius. Thus the proportion of surface to mass diminishes with in- creased size of the protoplasmic unit, and since activity is a function of the surface the larger the unit the smaller must be its activity. It follows that there must be a limiting size to the living protoplasmic unit, and it is on this account that practically no unicellular animal or plant exceeds a fraction of a millimetre in diameter. If an organism is to attain any larger size, this can only be by a multiplication of units, each presenting the same relative amount of surface as a complete unicellular organism, thougli the surface may be exposed to an internal and not to an external medium. Another factor, limiting the size of the unicellular organism or of the unit of the nmlticellular organism, is the necessity for maintaining a certain proportion between the size of the nucleus and that of the cytoplasm composing the body of the cell. Observations on artificial division of cells have shown us that the functions of digestion, assimilation, and growth depend upon the presence of a nucleus. Hence, when for any reason it is advantageous that a cell should attain a large size, such a cell is almost always found to contain many nuclei. All the ' giant cells ' found in the body of man under normal or pathological conditions are also multi nuclear. Thus the continuous display of the functions of assimilation and dis- similation, of growth and activity, is only possible so long as cell division keeps pace with growth. In unicellular organisms, under favourable condi- tions, this growth and nuiltiplieation occur with prodigious rapidity. It has been computed that a paramccciuni. freelv supplied with food material, 1199 1200 PHYSIOLOGY Avould, by growth and division, in the course of a year form a mass of proto- plasm the size of the earth, assuming of course that no accidents or destruc- tive agencies intervened to destroy the paramoecia which were being formed. This computation, which may seem a fanciful one, is useful as indicating the enormous number of individuals brought under the action of natural selec- tion, which very few survive. In unicellular organisms, such as para- moecium or amoeba, death cannot be regarded as a natural process. They may be eaten by higher organisms or serve as food to vegetable parasites, but so long as conditions are favourable and food-supply sufficient, they will continue to grow and reproduce themselves eternally. In the course of its existence each individual may be brought under many varieties of conditions ; some of these may be so harmful that the individual is destroyed and its race comes to an end. Other individuals, under circumstances of less severity, may undergo modifications in their molecular structure which will serve to neutrahse the effect of the injurious invironment. Any such modification in structure, morphological or molecular, must be transmitted to the next generation, so that with slowly varying external conditions there is a possibihty of a corresponding slow variation in type, which may finally attain a form altogether different from that with which it set out. A new species may in this way be formed by gradual alteration of environment. It is not therefore difficult to understand in the case of such organisms either the maintenance of type by heredity under constant conditions, or the change of type with gradually varying conditions. Reproduction by continuous growth and division is not, however, the only means, even in the unicellular animals, by which new generations may be produced. If protozoa, such as paramoecia, be kept for a long time in nutrient solutions their rapidity of reproduction after a time falls off, while many die, and others become the easy prey to infectious diseases. Under these conditions a new phenomenon makes its appearance, viz. ' conjuga- tion,' which is the analogue of the sexual reproduction of the higher animals. Infusoria contain two kinds of nuclei, a large and a small, known as the macro-nucleus and the micro-nucleus respectively. During conjugation the macro-nucleus breaks up and disappears in two cells, which become closely applied together, while in each the micro-nucleus divides twice to form four spindle-shaped bodies. Three of these degenerate (hke the polar bodies of the ovum), while the fourth divides into two. This is followed by an exchange of micro-nuclei, one micro-nucleus from a passing into b, while one micro- nucleus from b passes into a. The two cells then separate, a single micro-nucleus being formed in each by the amalgamation of the two. This micro-nucleus then divides three times, so that eight nuclei are formed, while the cell itself divides into four, two nuclei passing into each of the daughter cells. Of these one enlarges to form the macro-nucleus, while the other remains as the micro- nucleus. After conjugation has occurred, the colony of infusoria takes on, so to speak, a new lease of life, and there is a rapid production of new generations by simple division of the cells, in which both macro-nucleus and micro-nucleus take part. Conjugation apparently only ESSENTIAL FEATURES OF SEXUAL PROCESS 1201 occurs in the presence of adverse coiulitions, and may be prevented almost indefinitely by maintaining the colonies in as favourable conditions as possible. In certain organisms, especially in Alga>, in which similar pheno- mena take place, each organism after conjugation may surround itself with a thickened wall and remain for a considerable length of time in a state of suspended animation. It is very difficult to understand the advantage of this interchange of nuclear material either to the individual or to the race. Second fission First fission, after separation Differentiation of micro- and macro-nuclei Separation of the gametes > Division of the cleavage-nucleus Cleavage-nucleus Exchange and fusion of the germ- nuclei Oerm-nuclei ► Formation of the polar bodies Union of the gametes Fia. 553. Diagram showing the history of the micro-nuclei during the conjugation of paramcccium. (From Wilson after INIatjpas.) X and Y represent the opposed macro- and micro-nuclei in the two gametes circles represent degenerating and black dots persisting nuclei. It has been suggested that as soon as each individual concerned in the pro- cess receives the nuclear material from organisms which may have been exposed to slightly different circumstances, corresponding changes will be introduced into the tendencies to growth of the product of the union. Some of these tendencies may be more advantageous than before, while others may be the reverse. Increased possibility of variation is, however, intro- duced by this admixture of nuclear material, and this may be the advantage of the process to the race. It should be noted that the half of the micleus lost by each conjugating organism is qualitatively different from that which it retains and probably from that which it receives. A gamete in which the micleus can be represented by ab, and which by simple division will produce similar organisms with nucleus ab, conjugates with an organism 1202 PHYSIOLOGY of slightly dift'ereiit structure, and therefore with a nucleus which can be represented as cd. After conjugation, the ah gamete will contain a nucleus represented by ac, while the cd gamete will contain a nucleus represented by hd. ac or hd may be better or worse combinations than ah or cd. If either of them is better, that organism will survive under the less favourable conditions, and the race will continue with a slight, and to us inappreciable, change of type. REPRODUCTION IN THE METAZOA The numberless cells forming the bodies of the higher animals are all produced by a series of divisions from a single cell, the fertilised ovum. This cell is the result of a process of conjugation between two cells derived from different individuals. With the multiplication of cells forming a single organism there is of course an increased size of the organism. It is doubt- ful whether this of itself would be of any advantage, were it not that the multiphcation of cells goes hand in hand with differentiation, groups of cells being modified structurally and set aside for one or other function of the body. Differentiation of function implies higher functional capacity. As a motor organ or as a means of locomotion, the differentiated muscle- cells, with their attached parts, must be more effective than the undif- ferentiated protoplasm of the amoeba. Specialisation of function involves changes of type in the cells resulting from the division of the primitive un- cUfferentiated ovum. In most cases this change of type is permanent. An epithehal cell such as that forming the epidermis or the liver, when it divides, produces another cell of the same kind. One might almost speak of the evolution of a new species of cell but that it takes place within the short period of the development of the multicellular individual, instead of occupy- ing a long space of time, and involving the destruction of countless indi- viduals, as is the case when a change of type gradually occurs in unicellular organisms. Differentiation necessarily brings with it a limitation of the powers of reproduction. Any one of the descendants of a unicellular organism is in all respects equivalent to its ancestor, and can reproduce the same type of individual. The specialised liver- or muscle-cell can only produce a cell of the same type, one, that is to say, incapable of independent existence or of forming the divergent series of types necessary for the forma- tion of an individual. Differentiation of function therefore involves the setting aside of certain cells, germ-cells, which retain their primitive character and are capable of indefinite division to form new generations each capable of developing into a complete individual. These germ-cells can often be recognised from the very earliest divisions of the fertilised ovum, which lead to the production of the mature individual. Thus, in Ascaris, the progenitor of the germ-cells differs from the somatic cells both by the greater size of its nucleus and in its mode of division (Fig. 554). In the cells destined to produce the somatic cells a portion of the chromatin is cast out into the cytoplasm, where it degenerates, so that only in the germ-cells is the sum total of the chromatin retained. Thus in the two-celled stage, in one cell ESSENTIAL FEATURES OF SEXUAL PROCESS 1203 all the chromatin is preserved, while in the other cell the thickened ends of the chromosomes are cast off into the cytoplasm and degenerate, only the thinner central portions being preserved. When these divide again, the same process is repeated in only one of the daughter cells derived from a germ-cell, and the process is repeated during five or six divisions, after which the chromatin elimination ceases and the two primordial germ-cells Fig. 554. Origin of the primordial germ-cells and casting out of chromatin in the somatic cells of Ascnris. (Wilson and Boveki.) A, two-cell stage dividing ; s, stem-cell, from which arise the germ-cells. B, the same from the side, later iu the second cleavage, showing the two types of mitosis and the casting out of chromatin (c) in the somatic cell, c, resulting four-cell stage ; the eliminated chromatin at c. d, the third cleavage, repeating the foregoing process in the two upper cells. thenceforward give rise only to other germ-cells in which the entire chromatin is preserved. Thus " the original nuclear constitution of the fertilised egg is transmitted, as if by a law of primogeniture, only to one daughter cell, and by this again to one, and so on, \\liile in other daughter cells the chromatin in part degenerates, in part is tran.sformed, so that all of the descendants of these side-branches receive small reduced nuclei " (Boveri, quoted by Wilson). The innnortality, which was the property of all the unicellular ancestors of the metazoa. has in the latter descended only to the germ-cells. All the other cells of the body, which form the nervous and muscular tissues, glands, skin, &c., are mortal. They ])ass through a certain number of divisions ; but although this number is large, it is limited, and on the number 1204 PHYSIOLOGY of divisions which are possible depends the normal duration of life of the organism to which the cells belong. We may thus regard the egg-cell as dividing into two parts. From one part will be formed by differentiation all the complex somatic mechanisms of the adult animal ; the other part will divide, but will remain in an undifferentiated form, until its descendants can conjugate with germ-cells from other individuals and form fertilised egg-cells, destined to undergo the same series of changes. The metazoan individual thus consists of a mortal host holding within itself the immortal sexual cells or gonads. Gaskell has pointed out that the development of the fertilised ovum involves two parallel processes — on the one hand, the elaboration of the elements forming the host ; on the other, of those derived from the free-living independent germ-cells. From the very beginning the somatic part of the organism, the host, is a reacting individual in which the nervous system acts as the integrator of all the activities of the body and as the middleman between the internal and external epithelial surfaces and the muscular system. The host may thus be regarded as a neuro-epithelial syncytium, every step in its evolution and differentiation being attended by increased control of all the units by a central nervous system. The gonads were placed at first within the interstices of this syncytium, and escaped to form a new generation only after the death and disintegration of the host. But differentiation and division of labour affect also the free-living gonads. Some of these form a germ epithelium surrounding the body cavity, of which a few only of the elements pass out of the host as perfect germ-cells, while the others are subordinated to the metabolic needs of these germ-cells and are transformed into various elements, such as nurse-cells, wandering mesoderm cells or phagocytes, yolk-cells, and so on. Gaskell regards the greater part, it not the whole, of the connective-tissue framework of the body, as well as the wandering corpuscles of the blood and tissue-fluids, as derived from these primitive germ -cells. All these tissues, though useful to the host as well as to the finally successful germ-cells, present the common feature of an absolute independence of the central nervous system. Thus the evolution of the animal kingdom means essentially the evolution of the host, and must therefore be closely connected with the evolution of the central nervous system, the ruling element in the neuro-muscular syncytium. On these grounds Gaskell has used the comparative morphology of the central nervous system as a means of tracing the origin of the vertebrate from the in- vertebrate type, and has come to the conclusion that the immediate ancestor of the vertebrate must be sought in the invertebrate group presenting the most highly developed central nervous system, namely, the Arthropoda. All the complex mechanisms which are concerned in maintaining the life of the individual have apparently been developed in order to give the potentially immortal germ-cells a better chance of survival in the struggle for existence. From the broad biological standpoint, as Foster points out, all the toil and turmoil of human existence may be regarded simply as the by- play of an ovum-bearing organism. From the same standpoint one must acknowledge that the mortality of the individual, resulting from the absence of an indefinite power of multiplication among the somatic cells, must be an advantage to the race. Throughout the living world the welfare of the individual is subordinated to that of the species. With each new genera- tion there are possibilities of variation and of the production of individuals better or worse fitted for the maintenance of the race than those of the previous generation. Immortality of the individual would handicap the survival of the younger generations, and we should have the same retardation ESSENTIAL FEATURES OF SEXUAL PROCESS 1205 of progress in a race that we see in many civilised communities, where the power and the conduct of affairs are in the hands of the older members. THE FORMATION OF GERM-CELLS In multicelkilar organisms the cells which conjugate to form a new cell, capable of developing into an individual, are of two kinds. One, which has generally a certain amount of reserve material stored up in its cytoplasm, is the female element and is called the ovum. The other cell, which consists of little more than nuclear material, is the male element, and is called the spermatozoon. Both kinds of cells are derived from a mass of undifEerentiated cells, the germ epithelium, which, as we have seen, can often be traced directly back to the first divisions of the fertilised egg. The use of the reserve material in the ovum is to serve as food for the developing individual. The ovum and spermatozoon caimot be regarded as corresponding to complete cells. Before their union or conjugation both male and female germ-cells undergo certain important changes which differentiate them from the ordinary somatic cells of the indi^^dual. The essential differences between a germ-cell and a somatic cell can be best seen by a study of the nuclear changes which precede their formation. In division the nuclei of all somatic cells, whether of plants or animals, undergo a series of changes which, in their broad outlines, are identical throughout both animal and vegetable kingdoms (Fig. 555). The nucleus of the resting cell in its vegetative condi- tion is generally separated from the cytoplasm by a nuclear membrane, and contains irregular masses of a material staining deeply with basic dyes, and known as chromatin. In the cytoplasm of most animal cells may be seen a small particle known as the centrosome. When division is about to take place the clumps of chromatin arrange themselves into a filament which forms a continuous skein, the ' spireme stage.' This then breaks up into a number of segments, often V-shaped, the chromatin filaments or chromo- somes. Each of the filaments, in large nuclei, may often be seen to be com- posed of rows of granules. While this change has been occurring the nuclear membrane in most cases disappears, and the centrosome outside the nucleus divides into two parts, which travel to opposite ends of the nucleus. Round each centrosome the cytoplasm is modified and presents a radiate appearance, the aster, while joining the two centrosomes is a spindle of fine fibres, the achromatic spindle. The V-shaped segments of chromatin arrange them- selves in a circle at the equator of the spindle midway between the two centrosomes. Each of the loops then splits longitudinally, and each half travels towards one or other of the centrosomes, thus forming two daughter nuclei. The half-loops then join to form a skein, and may return to the form of a resting nucleus. These different phases in division are presented by all somatic cells, and have received the following names : (1) Prophase (the formation of the spireme and of the achromatic spindle, and the breaking up of the spireme into chromatin loops or chromosomes). (2) Metaphase (the splitting of the chromosomes). 1206 PHYSIOLOGY (3) Anaphase (the travelling of each half-chromosome to the extremity of the spindle). (4) Telophase (the retrogressive changes leading to the conversion of Pf: 1 Fig. 555. Diagram showing the changes which occur in the centrosomes and nucleus of a cell in the process of mitotic division. (Schafeb.) The nucleus is supposed to have four chromosomes. the chromatin filaments into an ordinary resting nucleus, which are accom- panied or preceded by a division of the cytoplasm across the equatorial part of the spindle). When the spireme has broken up into separate chromatin loops, it is possible to count them, and it is found that the number present in any cell is constant for the species. Thus every human somatic cell has sixteen ESSENTIAL FEATURES OF SEXUAL PROCESS 1207 cliromosomes in its nucleus. The same number is found in types so far apart as the ox, the guinea-pig, and the onion. In the mouse, the sala- mander, and the lily the number is twenty-four. Other types, such as the crustacean Artemia, are said to have as many as 168 chromosomes, while in Ascaris the cells only contain two or four chromosomes. All these changes, which are included under the term mitosis or 'karyohnesis, seem to be adapted to ensuring an equal qualitative as well as quantitative distribution of the nuclear chromatin among the daughter cells resulting from the division of any cell. As we have seen in an earlier chapter, the nucleus, by its interaction with the cytoplasm, determines the processes of assimilation \m\ \ Fig. 556. Three stages of hetcrotypc mitosis in spermatocyte of triton. (Moofe.) rt, germinal condition of chromosomes ; h, gemini arranged in quadrate loops or tetrads; c, separation of tetrads into the duplex chromosomes of the daughter nuclei. and growth of the whole cell. For the preservation of type in cell division it is therefore essential that the nuclei of the daughter cells shall be identical in all respects with the nucleus of the mother cell. Sexual reproduction involves the conjugation of two cells, with union of their nuclei. If each of these nuclei consisted of the normal number of chromosomes, the fertilised egg-cell would contain double the number characteristic of the species, and since these chromosomes would divide by splitting, the number of chromosomes in each cell would be doubled with each generation. This doubling is obviated by the fact that, in the forma- tion of the germ-cells, the ovum and spermatozoon nuclei undergo a special type of division, which leads to the reduction of the chromosomes in the sexually mature cell to one-half of the number characteristic of the species. This mode of cell division is often called ' division by reduction,' or ' hetero- type ' mitosis, or ' meiosis ' (Fig. 556). We may take as an example the development of spermatozoa. The mother cells of the spermatozoa, the 1208 PHYSIOLOOY spermatocytes, divide twice, giving rise to four daughter cells, the spermatids, each of which develops into a functional spermatozoon. In the nuclear changes preparatory to the first division, the spireme, when it breaks up, gives rise to only half the normal number of chromosomes. Thus if the somatic number of chromosomes were four we should find in the spermato- cyte, after the breaking up of the spireme, only two chromosomes. These take up their position at the equator of the achromatic spindle and then divide, but the division is effected, not by splitting of the double chromo- some, but by transverse division. Each chromosome breaks into half, one half going to each daughter cell. Since each of the reduced number of chromosomes can be regarded as made up of two normal chromosomes Primordial CTcrm-cell. Spermatogonia. Primary spermatocyte. Secondary spermatocytes. Spermatids. Spermatozoa. Division-period (the number of divi» sions is much greater) . Growth-period. M aturation-period, Fig. 557. placed end to end or joined to form a ring, as in Fig. 556, h, the division in the middle provides for a quahtative difference between the two daughter cells. If we indicate the four normal chromosomes as a, h, c, d, in ordinary somatic division each daughter cell will also contain chromosomes which may be represented as al. 61, cl, dl, and a2, &2, c2, d2. In the sperma- tocvte the two chromosomes may be represented as ah and cd. When they divide one daughter cell receives a and c, while the other daughter cell receives b and d. The second division of these daughter cells takes place generally by splitting of the filaments, so that finally four spermatids are produced (Fig. 557), each containing two chromosomes, two of them con- taining a and c, while the other two contain h and d. In the ovum, during maturation, analogous changes take place. Two successive cell divisions occur as in the formation of spermatozoa, but the daughter cells are of very unequal size. In the first division, the heterotypical division, the chromo- somes fuse in pairs and then divide as in the spermatocytes so that each of the daughter cells contains one half the somatic number of chromosomes. The larger of the two resulting cells, which retains most of the cytoplasm, ESSENTIAL FEATURES OF SEXUAL PROCESS 1209 is still called the ovum, while the smaller one is spoken of as the ' first polar body.' The ovum now divides again and throws off a second polar body, the division being of the homotypical variety. The first polar body may also divide, so that from the original ovum three cells are produced, one of which retains the greater part of the cytoplasm, while the others are extruded, and degenerate (Fig. 558). The mature ovum has, however, only half the normal number of chromosomes, so that its nucleus is equivalent to the nucleus forming the head of the spermatozoon. The only difference there- fore between the formation of ovum and spermatozoon is that in the former case three of the cells formed by the division of the primitive ovum are primordial gcnn-cel\. Ocigonia. Primary oocyte or ovarian egg. Secondary oocytes (egg and first polar body ) Mature egg and three polai bodies • Division-period fthe number of divi- sions is much greater). . Growth-period. M aturation-period. Fig. 558. abortive, whereas in the spermatozoon all four daughter cells produced from the spermatocyte remain functional. The production of these two kinds of sexual cell is represented in Figs. 557 and 558. Since the nuclei of the mature ovum and spermatozoon only contain half the normal number of chromosomes, they are generally spoken of as pro- tiuclei. FERTILISATION The essential features of fertilisation, i.e. the union of the sexual cells, are best studied in some of the lower invertebrates, such as ascaris or echino- dernis. In the latter fertilisation takes place in the sea- water, into which both ova and spermatozoa are extruded. The ovum of the echinoderm consists of a naked mass of protoplasm. Of the countless hordes of sperma- tozoa which may be in the neighbourhood of a given ovum only one as a rule enters. As soon as the spermatozoon has entered the ovum a tough membrane is rapidly formed round the latter, so preventing the entrance of any further spermatozoa. The head of the spermatozoon enters the egg, while the tail atrophies and disappears. The head of the spermatozoon enlarg(>s and assumes the character of a nucleus, the dense mass of chro- niatin broakii\g up first into a thread and then into the characteristic number 1210 PHYSIOLOGY of chromosomes (Fig. 559). The egg now contains two nuclei or pro-nuclei, exactly similar in appearance, one derived from the male and the other belonging to the egg itself. The two nuclei approach one another and join. ^ Fig. 559. Fertilisation and first division of ovum of Ascaris megalocephala. (Slightly modified from Boveri and Wilson.) A, second polar globule just formed ; the head of the spermatozoon is becoming changed into a reticular nucleus ( 5 ), which, however, shows distinctly two chromo- somes ; just above it, its archoplasm is shown : the egg- nucleus ( ? ) also shows two chromosomes, b, both pro-nuclei are now reticular and enlarged ; a double cen- trosome {a) is visible in the archoplasm which lies between them, c, the chromatin in each nucleus is now converted into two filamentous chromosomes ; the centro- somes are separating from one another, d, the chromosomes are more distinct and shortened ; the nuclear membranes have disaj^peared ; the attraction- spheres are distinct, e, mingling and splitting of the four chromosomes (c) ; the achromatic spindle is fully formed, r, separation (towards the poles of the spindle) of the halves of the split chromosomes, and commencing division of the cytoplasm. Each of the daughter cells now has four chromosomes ; two of these have been derived from the ovum nucleus, two from the spermatozoon nucleus. In many cases there is an apparent fusion of the substance of the two nuclei. In others the chromatin filaments of male and female simply lie side by side, forming a complete nucleus with the somatic number of chromo- somes. Fertilisation is rapidly followed by cell division. Each of the ESSENTIAL FEATURES OF SEXUAL PROCESS 1211 chromosomes splits longitudinally, half going to each of the daughter cells, and this process is repeated throughout the succeeding divisions which result in the formation of the new individual. Thus every cell of the body con- tains a nucleus of which exactly one half is paternal and the other maternal in origin. In ascaris it is often possible, in the first few divisions of the fertihsed ovum, to distinguish in the daughter nuclei the chromatin filaments derived from the male from those derived from the female. The strong impetus to cell division given by the process of fertihsation has naturally aroused much curiosity as to its intimate character. It might be thought that for cell division to take place a normal number of chromo- somes is essential. As against this explanation may be adduced the fact that in many animals parthenogenesis occurs. The female pro-nncleus may, under certain conditions of en\dronment or nutrition, start dividing and give rise to an embryo, each cell of which contains only half the normal number of chromosomes. In other cases of parthenogenesis only one polar body is extruded, or the second polar body joins again \\ith the female pro-nucleus. In either case the ovum contains a nucleus, with a normal number of chromosomes, which divides and produces an individual resembhng that resulting from the union of ovum and spermatozoon. It has been suggested that the impetus to division is given by the entry of the sperma- tozoon itself. In the series of divisions which precede the formation of the female pro-nucleus the centrosome of the ov^um generally disappears, whereas, in the formation of the spermatozoon, the centrosome persists ana forms the middle part of the spermatozoon. In many cases the centrosomes divide in the spermatozoon itself, so that this contains two centrosomes when it enters the egg. These two centrosomes then become the centres of attrac- tion spheres. They diverge, and between them is formed an achromatic spindle, along the equator of which the chromatin filaments of male and female pro-nuclei arrange themselves. It is doubtful, howev^er, how far the centrosome can be regarded as a permanent cell structure. In echino- derm eggs various modes of treatment will lead to the appearance of attrac- tion spheres in the cytoplasm, and even to division of the non-fertilised egg. Loeb has suggested that the action of the spermatozoon is essentially chemical in character. By alteration of the medium in which the eggs are contained, Loeb has succeeded in imitating exactly the changes which normally need the entrance of a spermatozoon for their occurrence. On immersing the eggs in a weak solution of formic or lactic acid, a membrane is formed. The eggs are then taken from the acid, placed in concentrated sea-water for a short time, and then removed to ordinary sea-water. Division rapidly occurs with the production of a normal larva. He suggests that the spermatozoon brings with it ferments, or other chemical substances, which excite the egg-iuicleus and cytoplasm in the same way as the chemical measures adopted for this artificial induction of segmentation. SECTION II DEVELOPMENT AND HEREDITY There is perhaps no phenomenon which is so impressive as the development from a minute speck of protoplasm, the fertihsed egg, of an individual partaking of the minutest characteristics of both its parents. An egg- cell has much the same appearance whether it belong to an echinoderm, a fish, or a man. In the process of development, by a simple repetition of a series of cell divisions, this undifferentiated protoplasm is formed into the complex organs with the potentialities and habits which distinguish the type from which the protoplasm has been derived. We cannot wonder that the intimate nature of this process has been the subject of speculation from the very birth of science. Running through these speculations are two main ideas, which have been labelled the theories of ' evolution ' and of ' epi- genesis.' By the ' evolutionists ' the egg was believed to contain an embryo fully formed in miniature, as the bud contains the flower or the chrysalis the butterfly. Development was therefore only the unfolding of something already existing. If, however, this theory be pushed to its utmost and if the egg contain a complete embryo, this must itself contain eggs for the next generation, and so on ad infinitum, a conclusion which is of course absurd. According to the theory of epigenesis, the structure of the egg is wholly different from that of the adult, its development consisting in the continual formation one after the other of new parts previously non-existent as such. There is no doubt that this view is fundamentally correct. The difiiculty with which we have to contend is the understanding of the orderly sequence and correlation of the cell divisions and differentiations which result in an adult individual of the same type as the parents. The fact that, under approximately identical conditions, one mammalian ovum will give rise to a mouse and the other to a man indicates that there must be some difference in structure, organisation, or composition of the primitive egg-cell in each case, and the theory of ' evolution ' has reappeared in later days in a some- what modified form, according to which the differentiation of the ovum is causally connected with a preformed differentiation in the nuclear struc- tures, e.g. chromosomes, of the ovum itself. We have already seen that the germ-cells in some types are separated off from the rest of the embryo at a fairly early stage. If in the two-celled stage of the frog's egg one cell be destroyed by means of a hot wire, the other cell develops to form half an embryo, thus suggesting that each cell of the two-celled embryo could give 1212 DEVELOPMENT AND HEREDITY 1213 rise only to the corresponding half of the body. This limitation of develop- ment, however, only occurs if the intact cell be left in coimection with the cell that has been injured. If, in echinoderm larvae, the cells be entirely separated, even as late as the eight-celled stage of division, each cell will give rise to a whole embryo, differing from a normal embryo only in respect of size. This difference suggests that the number of divisions that each cell can undergo is predetermined in the egg-cell itself, but shows also that the cells into which the egg divides are, at first at any rate, equipotential. We must assume therefore that the reason why one cell under ordinary circum- stances only forms one half of the embryo is that its development is regu- lated and determined by the presence of the other cell in connection with it forming the other half of the embryo. That is to say, the development of the egg involves the reaction to environment of a protoplasm of certain properties and powers of reaction. The final product of development depends (1) on the nature of the protoplasm (including the nucleus) of which the egg is composed, and (2) on the environmental conditions to which the egg is subjected during the rapid growth and multiplication attending its development. We could therefore speak of a morphology of inheritance, but the morphology would be ultra-microscopic and have relation to the molecular structure of the protoplasm of which the egg was composed. In the transmission of the potentiahties of development from parent to fertilised egg we must regard the nucleus as the essential structure. In ordinary development the spermatozoon furnishes only a nucleus and centrosome, the ovum supplying the whole of the cytoplasm. There seems, however, no grounds for assigning any directive power to the latter struc- ture. In echinoderm ova it is possible to get rid of the nucleus and then by the introduction of spermatozoa to have an individual entirely paternal in origin, which, on development, produces a larva of the paternal character. In division of the egg the only part of the cell which divides so that each daughter cell shall include an equal part of both parental germs is the nucleus. The constant number of the chromosomes in each species, and their accurate division on mitosis, suggest that the hereditary transmission of the potentialities of the cell is bound up with the chromosomes. It has been suggested that every character is located in a chromosome or part of a chromosome. If this be the case one might regard the differentiation into various tissues, which occurs in the process of development, as occa- sioned by an actual loss or degeneration of the constituent parts of one or more chromosomes. There is no doubt that many tissues do become thus differentiated at a fairly early period in development, having undergone in the process an absolute modification of their potentiahties, which must be at any rate shared by the chromosomes of their constituent cells. The extent to which this limitation of powers of development occurs varies widely in different animals. In the higher animals, such as man, epithehum will reproduce epithelium, and hver-cells will reproduce liver-cells, while nerve-cells are absolutely incapable of multiplication On the other hand, in Crustacea a whole limb may be torn off and be regenerated from the 1214 PHYSIOLOGY tissues of the stump. Destruction of the optic lens in the salamander is followed by its regeneration from the anterior part of the optic cup, a tissue which had no part in its primary formation. Worms will form a new head after decapitation. In these animals therefore the cells in many parts of the body possess the power of directed growth, if need arise, in case of injury, and are able to form tissues of many different kinds. I have mentioned the small size of the larva formed from isolated cells of the segmenting egg as a proof that the number of cell divisions of the somatic part of the developing animal is predetermined and limited. This conclusion must not be taken too absolutely. Many of the tissues, even of the highest animals, possess the power of almost unlimited regeneration by cell-multiphcation as a response to injury. Under normal conditions the growth of such tissues is limited, not by absence of power to divide, but as a result of a mutual interaction between them and the surrounding cells. We might almost speak of a struggle for existence between the various tissues of the body, which in the healthy organism results in an equihbrium, or balance of multipHcative powers. If this balance is upset by any means, such as stimulation of certain cells by the presence of intra-cellular parasites, or their destruction by irritants or other abnormal conditions {e.g. exposure to X-rays), one tissue may enter into active growth at the expense of the surrounding tissues, and the result is a morbid growth such as cancer. It is possible that in the latter case a new factor comes into play. All tissues of the body, as we have seen, begin to die from the time that they are born. They have a certain span of life, a certain limitation to their genera- tions, which results in the phenomenon of senescence, such as occurs in a culture of protozoa. In protozoa this phenomenon is the signal for rejuvena- tion by conjugation. It is possible that in cancer something of the same nature occurs. It is at any rate significant that in a rapidly growing cancer many of the dividing cells present the phenomenon of heterotype mitosis, a phenomenon which is otherwise found only in the sexual cells preparing for conjugation and for the production of a new individual. Given adequate conditions of nutrition, there seems to be no limit to the growth of cancer-cells. In mice a cancer may be transferred from one individual to another by inoculation^ and this process may apparently go on indefinitely, so that finally a mass of cancer-cells may have been produced equal in volume to many thousands of mice, and persisting long after the mouse from which it was first taken would have died under natural conditions. In sexual reproduction the new individual partakes of characteristics of both its parents. It therefore resembles neither of its parents in all details. The conjugation of the two parent cells from which it is derived has been preceded by a throwing out of half the chromosomes from each parent cell. It is therefore natural to ascribe the variations which occur among the members of one family to a qualitative difference in the chromosomes which have been ehminated in the formation of their respective egg-cells. Can we regard the chromosomes as representing separate qualities of the individual or must we assume that all qualities are represented to a greater or less DEVELOPMENT AND HEREDITY 1215 extent in every chromosome ? In the case of many quaUties, especially those which distinguish the species as apart from the individual variation or family characteristic, we must probably accept the latter idea as correct. In this case the child can be regarded as representing an arithmetical mean of both its parents. In certain respects, however, a quality seems to be transmitted from parent to oifpsring either completely or not at all. This is specially apphcable to those characteristics which have been rapidly produced by artificial selection, characters which, if artificial selection be abandoned, rapidly disappear, with reversion to the type from which the special strain was ultimately produced. The way in which these charac- teristics are transmitted was first studied by Mendel and has been formulated as Mendel's law. Mendel's first experiments were carried out on peas. On crossing a tall plant with a dwarf plant seeds were obtained from which all the plants were tall. On recrossing the plants of this generation among one another a third generation was obtained in which 25 per cent, of the plants were dwarfs and 75 per cent, were tall. Crossing the dwarf plants among themselves led to the production of dwarf plants through successive generations. Of the tall plants 25 per cent, and all their descendants con- tiimed to produce tall plants when self-fertihsed, whereas of the remaining 50 per cent, of the tall plants 25 per cent, produced dwarfs and the remaining 75 per cent, produced tall plants. On continuing the process of breeding, the dwarf plants when self-fertihsed always produced dwarfs, whereas of the tall plants 25 per cent, produced tall plants, which bred true, while the remaining 50 per cent, produced the same percentage of tall and dwarf as in the preceding generations. Mendel explained these results by the assump- tion that a character could be dominant or recessive. If both characters were present together in one plant it would partake of the dominant type ; the fact that this plant possessed the recessive character would only be shown by the results of breeding. In the case of the peas the tall character was dominant over the dwarf. Thus when the tall and dwarf pea were crossed the first generation of plants would exhibit the dominant character and be tall. In the second generation, however, 25 per cent, of the individuals would be pure dominants (D + D), 25 per cent, would be pure recessive (R + R), while 50 per cent, would be mixed (D + R). The pure dominants bred together would always give rise to nothing but pure dominants, the recessive to recessive, while the mixed type would always, as before, give rise to 25 per cent, pure dominants, 50 per cent, mixed, and 25 per cent, pure reces- sives. These results may perhaps be made clearer by the following Table : D -t- R I DR I 25% D 50% DR 25% R I I _ I D 25% D 50% DR 25% R R D D 25% D 60% DR 25% R R R 1216 PHYSIOLOGY It has been suggested that a very large number, if not all, of the charac- ters of an individual might be brought under this law. This might be done by indefinitely subdividing the characters, but the question would then become beyond the hmits of analysis or experimental investigation. There is no doubt that many qualities are subject to Mendel's law, and that their study will be of considerable assistance in guiding the efforts of our breeders and horticulturists in the formation of new varieties desirable for their value to man. In respect of many quahties the Mendelian law seems to fail. Thus in man the progeny of a cross between a white and black race are more or less intermediate between the two and vary according to the amount of black and white blood introduced in succeeding generations. Definite black and white individuals are not produced, but merely individuals of various degrees of brownness. SECTION III REPRODUCTION IN MAN THE DEVELOPMENT OF THE REPRODUCTIVE ORGANS The most marked example of chemical correlation is found in the influence exerted by the genital glands upon the other parts of the reproductive apparatus and upon the body generally. Thus castration, i.e. removal of the testes or ovaries, if carried out before the time of puberty, prevents the development of the secondary sexual characters, which normally occurs at this epoch in both sexes. Puberty denotes the period at which ripe spermatozoa and ova are produced in the testis and ovary respectively. In the human species this period is marked or preceded in the male by in- creased growth of the skeleton, by growth of the larynx, leading to a lower- ing in pitch of the voice, by the growth of hair on the face and pubes, and by the development of sexual desire. In the female we find at puberty enlargement of the breasts, attended by some growth of the mammary glands and by a moulding of the whole form, making it more fit for the bearing of children. The chief sign of puberty in the female consists in the periodic changes in the uterus, which give rise to menstruation, i.e. a flow of blood and mucus from the genital organs, lasting three to five days and repeated every four weeks. Menstruation persists so long as the ovary is functional, and is producing ripe ova. The activity of the ovary comes to an end between the forty- fifth and fiftieth year (' the climacteric ' or ' change of fife '). With the cessation of its activity menstruation also stops, and the uterus undergoes a process of atrophy. These secondary sexual charac- ters must be ascribed to the influence of chemical substances produced in the ovary and testis respectively. Castration after puberty, though not causing any change in the skeleton, which has already assumed its permanent form, brings about retrogressive changes in the other genital organs, analogous to those occurring in the female at the climacteric. In animals the pheno- mena of ' coming on heat ' or ' rut ' seem to be analogous with menstrua- tion in the human female, and like this depend on the normal activity of the ovary. They are permanently abolished by extirpation of the ovaries, but may be reinduced by implantation in the peritoneum of an ovary from another animal of the same species. This fact shows that the changes in the uterus responsible for rut, as well as for menstruation, are independent of any nervous connections between the ovaries and the rest of the body, and must therefore be brought about by the circulation in the blood of specific 1217 39 1218 PHYSIOLOGY chemical substances produced in tlie ovaries. According to some authors, the essential factors for the production of these genital hormones are the ' interstitial cells ' found both in the testes and ovaries of various animals. These interstitial cells are not, however, universally present. It has been shown that, by means of the Eontgen rays, it is possible to destroy the germ-cells in either testes or ovaries, so rendering the animal sterile. The interstitial cells, when present, are not destroyed by these rays, yet the effects on the accessory genital organs are stated to be as marked as after complete extirpation of either ovaries or testes. The chemical correlations between the ovaries and the other organs concerned in reproduction are perhaps best marked in the changes which attend pregnancy. In this case the fertihsation of the ovum by a sperma- tozoon is followed by a great development, first of the mucous membrane and later on of the muscular wall of the uterus. The mucous membrane thickens, apparently in order to form a bed for the developing fertilised ovum. With this growth of the uterus there is a corresponding growth of the other parts of the genital tract, e.g. the vagina. At the same time rapid changes take place in the mammary glands. These changes may be studied experimentally in the rabbit, in which animal gestation lasts only about twenty- nine days. In a virgin rabbit of a year old it is difl&cult with the naked eye to see any trace of the mammary gland in the tissue lying under the nipples. Each gland is limited to an area not more than 1 cm. broad, and consists entirely of ducts lined with a single layer of flattened epithehal ceUs. With the occurrence of conception a marked change takes place. Four or five days after fertihsation, when it is still impossible with the naked eye to discover any embryos in the swollen uterine horns, on reflecting the skin from the abdomen each mammary gland appears as a circular pink area, about 3 cm. in diameter. On section the gland consists of ducts which are in an active state of proHferation, their epithehal lining being two or three cells thick and presenting numerous mitotic figures. By the ninth day the whole abdomen is covered with a thin layer of glandular tissue ; by the twenty- fifth day this tissue is \ cm. in thickness and consists for the greater part of secreting alveoli, hned with cells containing numerous fat-globules. At full term the alveoli contain ready-formed milk. This hypertrophy of the mammary glands occurs during pregnancy after complete division of all possible nervous paths between the glands of the ovaries or uterus. In the guinea-pig a mammary gland has been actually transplanted to another part of the body, thus severing all its normal nervous connections, and yet it enlarged as usual during a subsequent pregnancy. Ancel and Bouin have brought forward evidence that the corpus luteum — the tissue produced in the ovary as a result of the discharge of an ovum — is intimately concerned with the growth of the mammary glands, and may indeed cause a certain degree of hypertrophy of these glands in the entire absence of any product of conception within the uterus.* The hmited * According to Ancel and Bouin, in the rabbit discharge of an ovum and formation of a corpus luteum only occur as a result of copulation. The same effect may be pro- REPRODUCTION IN MAN 1219 growth of the glands, which occurs at puberty, can certainly not be ascribed to the presence of a foetus in the uterus, and must be connected with the growth of ripe ova, or, as suggested by the two French authors, with the growth of the tissue of the corpus luteum. resulting from the discharge of ova. There seem also to be obscure relationships between the activity of the sexual organs and that of certain so-called ductless glands. Thus castra- tion at an early age leads to persistence of the thymus gland, whereas normally this gland atrophies just before the sexual organs commence their functional activity. The existence of a connection between the thyroid and the ovaries has been a popular beUef for 2000 years. In many individuals the thyroid is perceptibly enlarged at each menstrual period. On the other hand, extirpation of the thyroid before puberty brings about, among the other signs of cretinism, failure of development of the ovaries, so that puberty is delayed partially or completely. We must thus regard the germ-cells not only as representing the cells from which the individuals of the new generation may be developed, but also as concerned in the formation of chemical substances which, dis- charged into their hosts, affect many or all of the functions of the latter, with the object of finally subordinating the activities of the individual to the preservation and perpetuation of the species. THE MALE REPRODUCTIVE ORGANS In all the higher animals we may divide the reproductive organs into the essential organs, which form the germ-cells, the spermatozoa and ova respectively, and the accessory organs, which have as their office the faciUta- tion of the access of the spermatozoa to the ova (fertihsation), and in the female the nutrition of the product of fertilisation during the early period of its development. The essential sexual organ of the male is represented by the testis. This is made up of a collection of convoluted tubules, the seminal tubules, which are contained in a number of compartments separated by fibrous septa. The tubules present few or no branches, each one being about 500 mm. long. The testis is formed in the first instance in the peritoneal cavity from the germinal epithelium, but early in fife leaves the abdominal cavity by the abdominal ring to lie in a pouch of skin— the scrotum. Several tubules unite to form a straight tubule, which leads by a series of duced by artificial rupture of a ripe follicle. Whenever this occms there is a develop- ment of the niaminary glands. If no impregnation has taken i)lace {e.ff. if the buck has been sterilised by ligatme of the vas deferens), the glands develop for fourteen days and then begin to atrophy. Tliis period corresponds to the period of active growth of the corpus luteum. The continued growth during the latter half of pregnancy these authors ascribe to the production of another hormone by a special glandular tissue (' myometrial gland ') which makes its appearance about the fourteenth day in the wall of the uterus, at the site of implantation of the placenta, and lasts until the end of pregnancy. 1220 PHYSIOLOGY commiiiiicating spaces, the rete testis, into the vasa eff erentia (Fig. 560). These unite to form the duct of the epididymis, which forms a mass lying at the back of the testis. The epididymis is composed of the convolutions of this single duct, which is about 20 feet long. From the lower end of the epididymis the vas deferens, a tube with thick muscular walls, leads by the abdominal ring to the base of the bladder, where it opens into the beginning of the urethra in its prostatic part. Just before it joins the urethra each vas Appendix Epididymis Tunica vaginalis Tunica albuglnea Septum Seminal tubules — Lobule Mediastinum Testis - Vas deferens Paradidymis .■^:^_ Vasa efferentia Appendix of rete testis Vas aberrans Rete testis Fig. 560. Diagrammatic representation of the course of the seminal tubules in the testis and epididymis. (After Nagel.) deferens presents a diverticulum, the seminal vesicle, which Hes along, and is attached to, the base of the bladder. The prostate itself, which surrounds the first part of the urethra, is composed of a matrix of unstriated muscular fibres, enclosing numerous branched racemo-tubular glands. From the point of entry of the vasa deferentia to its orifice, the urethra represents a common passage for the urine and for the sexual products — the semen. It passes therefore through tissues, forming the penis, which are especially adapted for the purpose of intromission, i.e. the introduction of the semen, containing the spermatozoal into the female. In the urethra we distinguish the prostatic, the membranous, and the penile portions. Into the beginning of the penile portion, the bulb of the urethra, open the ducts of the two glands of Cowper. In the penis itself the urethra is surrounded with erectile tissue, forming the corpus spongiosum, and Hes between the two corpora cavernosa, which consist of the same kind of tissue. The erectile tissue is a spongy mesh work of elastic and unstriated muscle fibres, enclosing spaces in free communication with the efierent veins of the organ. The arterioles also open into these spaces, but under normal circumstances both the arterioles and the muscle-tissues of the framework are contracted, so REPRODUCTION IN MAN 1221 that the blood trickles only slowly from the arterioles into the spaces, whence it escapes readily by means of the veins. If the muscle fibres be relaxed, so that blood can escape readily into and distend the spaces, the tissue swells and becomes harder, causing ' erection ' of the organ. In the immature testis, i.e. from birth up to puberty, the seminal tubules are filled with cells with large nuclei. Some of ttese are the spermatogonia, the mother cells of the future spermatozoa, while the others form the cells of Sertoli, whose function it is to act as nurse cells to the developing sper- matozoa. The actual formation of spermatozoa begins at puberty, when the spermatogonia divide many times to form the spermatocytes, which in their turn undergo heterotype mitosis to form the spermatids, as already described. By a modification of the latter the fully formed spermatozoa are formed. These, when mature, pass by the tubules of the testis and of the epididymis into the vas deferens, whence they make their way into the seminal vesicles. Their movement is probably facilitated by the cells lining the tubule of the epididymis as well as by the secretion of the lining membrane of the seminal vesicles. It has been noted that the spermatozoa are practically motionless while in the seminiferous tubules of the testis, but become actively motile in the vas deferens, or when mixed with prostatic secretion. It is difficult to understand how the spermatozoa are conveyed through the resistance which must be offered by the huge length of the tubule of the epididymis, unless their onward motion is facihtated by the cilia-like structures attached to some of the cells lining this tubule. The formation of the spermatozoa is continuous, though the rate at which this occurs is variable and regulated by the sexual activity of the individual. In the fully formed semen the spermatozoa formed by the testis are mixed not only with the fluid secreted by the lining membrane of the epididymis and of the seminal vesicle but also with the mucous secretions of the prostatic glands and of Cowper's glands. Nevertheless it contains spermatozoa in enormous numbers, the semen emitted at a single act of coitus containing as many as 226,(KX),()00 spermatozoa. Though the vast majority of these are probably capable of fertilising an ovimi, this act is carried out by only one — a fact characteristic of the prodigality of nature when deahng with the perpetua- tion of the type. THE FEMALE REPRODUCTIVE ORGANS The essential organ of reproduction in the female is the ovary, the seat of production of the ova. The accessory organs include the oviducts or Fallopian tubes, the uterus, in which the fertilised ovum is retained during the first nine months of its development, and the vagina, which is especially adapted for the reception of the male organ in the act of impregnation. Among the accessory organs we may also reckon the mammary glands, which undergo a special development during pregnancy, and serve for the nourishment of the young individual during the first period of extra- uterine life. 1222 PHYSIOLOGY OVULATION. At birth the ovary consists of a stroma of spindle-shaped cells, and is covered by a layer of cubical epithehum (the germ-epithehum) continuous with the endothehum lining the general peritoneal cavity. Em- bedded in the stroma, but especially numerous just underneath the epithe- lium, are a vast number of ' primordial follicles.' These are formed during foetal life by down-growths of the germinal epithehum. Of the cells pro- longed in this way from the germinal epithehum, some undergo enlargement to form the primordial ova, while the others are arranged as a single layer of flattened nucleated cells, the ' folhcular epithelium,' as a sort of capsule to the ovum. Of the primordial folhcles, about 70,000 are to be found in the ovary of the new-born child. During the first twelve to fourteen years of life they remain in a quiescent condition. With the onset of puberty one or more of the primordial follicles begin to develop. Indeed, this development may be regarded as the causative factor in the various phenomena which are characteristic of puberty in the female [v. p. 1217). The first stage in the growth of the folhcle is a prohferation of the follicular epithelium, the cells of which become cubical and are arranged in several layers round the ovum. At one point in the mass of cells surrounding the ovum a cavity appears filled with fluid, the liquor folUcuU. The epithelium thus becomes separated into two parts, i.e. the membrana granulosa, several layers thick, lining the whole folhcle, and the discus proligerus, a mass of cells attached to one side of the folhcle, in which is embedded the ovum (Fig. 561). Round the growing folhcle the stroma assumes a concentric arrangement and forms a capsule, of which the internal layer consists chiefly of spindle-shaped cells richly supplied with blood-vessels, while the outer layer — the theca externa — ^is made up of a tough fibrous tissue. With the growth of the follicle the ovum also becomes larger and surrounds itself with a distinct membrane, known as the zona 'pellucida. This membrane presents a fine radial striation, which is supposed to indicate the existence of canals through which the ovum can obtain sustenance from the surrounding cells of the follicular epithehum. The nucleus also becomes larger, and forms the germinal vesicle containing one or two well-marked nucleoU^ — the germinal spot. The mature Graafian folhcle projects from the surface of the ovary as a trans- parent vesicle about the size of a pea. (Its diameter is about 15 mm.) In the process of growth the ovum has increased from a diameter of 25^ to 200/x. Before the ovum can undergo fertihsation the double division of the nucleus, or germinal vesicle, has to take place, which leads to the forma- tion and extrusion of the two polar bodies. This process probably occurs just before or just after the discharge of the ovum from the ovary. With increasing size of the Graafian follicle the membrane covering it becomes progressively thinner. At certain periods, or under certain condi- tions, the membrane ruptures, and the ovum is discharged in the hquor folliculi, still surrounded by an adherent mass of the cells of the discus proligerus. In some animals this process of ovulation occurs at definite periods of the year. In others, such as the rabbit, the occurrence of ovula- tion depends upon coitus taking place during the period of sexual activity. REPRODUCTION IN MAN 1223 We shall have later to discuss the relation of o\nilation in the human female to the periodic changes occurring in the other parts of the reproductive apparatus. After the discharge of the ovum the remaining portions of the follicle undergo a characteristic series of changes, which result in the production - < .9 s a o Glycine . 0 3-5 0-4 0-1 10 16-5 36-0 2-0 Alanine . 4-19 2-7 2-2 0-9 2-0 2-0 2-5 0-8 21-0 3-7 Valine . 4-3 + 1-0 o-:5 1-0 1-0 0-9 Leucine . 29-04 20-0 18-7 10-5 8-0 5-6 15-0 21 1-5 11-1 Isoleucine Phenylalanine 4-24 31 3-8 3-2 3-7 2-4 3-2 0-4 1-5 3-1 Tyrosine 1-33 2-1 2-5 4-5 1-5 1-2 1-5 10-5 2-2 Serine . 7-8 0-56 0-6 0-2 0-5 0-2 0-4 1-0 Cystine . 0-31 2-5 0-7 0-6 0-5 0 Proline . 11-0 2-34 1-0 2-8 3-1 3-2 7-0 5-4 5-2 + 5-1 Oxyproline 1-04 0-2 3-0 Aspartic acid . 4-43 3-1 2-5 1-2 5-3 0-6 4-0 0-6 + 4-1 Glutamic acid 1-73 7-7 8-5 11-0 13-8 37-4 18-4 0-9 15-1 Tryptophane . + + + 1-5 + + 0 + Arginine 87-4 5-42 4-8 10-1 3-2 7-6 1-0 7-1 Lysine . 0 4-28 5-8 4-3 0-0 2-8 + 7-1 Histidine 0 10-96 2-5 2-5 1-0 0-4 + 1.1 Ammonia 1-6 2-0 5-1 0-4 1-0 animal. Thus, in the calf the brain is only -y^^ of the whole animal. In cow's milk lecithin is present in the proportion of 1*4: per cent, of the total protein. In the puppy the brain is J^ of the whole body and the proportion of lecithin to protein in the milk is 2-11 per cent. In the infant, the brain forms -^ of the body weight, while the lecithin is 3-05 per cent, of the protein of human milk. Calf Puppy Infant Relative brain weight .... Lecithin content of milk in percentage of protein ..... 1 : 370 1-40 1 : 30 211 1 :7 305 We thus see that under normal conditions the young animal is supphed through its natural food with all the food-stufis in the proportions which it requires for its normal nourishment and growth. It is impossible therefore satisfactorily to replace the natural milk of an animal by that of another species. In civilised communities it is becoming more and more the custom to endeavour to feed the child with cow's milk, more or less modified, in the vain endeavour to reproduce the properties of human milk. Among all classes this involves the administering of a milk differing in its qualities and in the relative proportions of its proteins, its fats, carbohydrates, and salts, from human milk. So-called ' humanised ' milk is only a rough imita- tion of the natural mother's milk. Among the poorer classes this artificial feeding means the replacement of a natural sterile food, throwing very little work on the digestive organs of the child, by a foreign milk, very difficult THE SECRETION AND PROPERTIES OF MILK 1243 to digest, and often teeming with micro-organisms. There is no doubt that of the children dying during the first year of hfe four-fifths are murdered by this unnatural method of feeding. In some cases it is necessary to adopt artificial feeding because the mother is abnormal, and there is an insufiicient secretion of milk. It is therefore important to know what are the main dift'er- ences in composition between human and cow's milk. In human milk the caseinogen is not only absolutely but also relatively less than in cow's milk, while the latter is relatively poorer in milk sugar. Human milk is poorer in salts, especially in lime, containing only one-sixth of the amount present in cow's milk. Human milk is also said to be poorer in citric acid. The main differences may be summarised as follows : Water Proteins Fat Milk sugar Salts Caseinogen Albumin Human milk Cow's milk . 88-5 87-1 1-2 302 0-5 0-53 3-3 3-7 6-0 4-8 0-2 0-7 The caseinogen of human milk presents several points of difference from the caseinogen of cow's milk. It is less easly precipitated by acids. When coagulated by rennet it does not form a firm clot, but is thrown out in a flocculent form. It is thus much more susceptible to the action of gastric juice. Whereas the caseinogen of cow's milk generally gives a precipitate of ' pseudonuclein ' on digestion with pepsin and hydrochloric acid, a smaller or no precipitate is formed with human caseinogen. Another important advantage of human milk for the infant lies in the presence of antitoxins. It has been shown by Ehi'lich that when a female animal has been immunised against any toxin and has produced in conse- quence antitoxins in its blood, these antitoxins will, if it has young, pass over into the milk. The same passage of anti-bodies into the milk has been proved in the case of various infective disorders. The ingestion of human milk will therefore not only nourish the infant, but will provide it with a certain measure of passive immunity against possible infection by diseases to which its species is liable. THE SECRETION OF MILK. When fully formed, each mammary gland consists of fifteen to twenty lobes connected by connective tissue. Each lobe is made up of a mass of secreting alveoli which lead by narrow ducts into one large lactiferous duct. These lactiferous ducts, one from each lobe, open on the nipple, undergoing in the nipple itself an oval enlargement. Before secretion begins, the alveoli as well as ducts are lined with a cubical epithe- lium. When secretion commences a marked difference develops between the epithelium of the alveoli and that of the ducts. While the latter retains its previous character, the cells of the secreting epithelium grow in length and project into the lumen of the gland. In the innermost part of the protoplasm numerous fat globules make their appearance. If sections be made of the 1244 PHYSIOLOGY gland diuing the vaiious stages of its activity and stained by Altmann's method (acid fuchsin and picric acid)^ it will be seen that the commencement of acti\dty is marked by the growth of the innermost part of the cells and the development in these of a number of grannies (Fig. 566). These granules finally lengthen into shapes hke spirilla, while others of them form fat and become metamorphosed into fat granules. The nuclei of the cells also divide, apparently in preparation for the replacement of some cells which undergo complete degeneration and are cast off into the secretion. We know very httle about the mechanism of milk secretion. It seems impossible at present to explain the very close adaptation between the '^Oi'gi) '\) W, n Fig. 566. Sections of mammary gland of guinea-j)ig (fat granules stained black with osmic acid). A, during rest, b, during active secretion. It will be noticed that in this case the active formation of j)roducts of cell-metabolism (granules, &c.) begins with the commencement of secretion, and does not occur almost exclusively during rest, as in the salivary glands. In the mammary gland, the active growth of protoplasm, the formation of granules from the protoplasm, and the discharge of these granules in the secretion appear to go on at one and the same time. activity of the secretory cells and the needs of the infant or young animal. Two at least of the constituents of milk, caseinogen and lactose, are peculiar to this secretion. It has been assumed that the caseinogen is produced by some sort of alteration in the nucleo-proteins of the gland-cells, and that the lactose is derived in the same way from some sort of gluco-protein or gluco- nucleoprotein, but the evidence for either of these assumptions is very scanty. The growth of the mammary glands during pregnancy is largely determined by some form of chemical stimulation, the specific hormone being produced in the corpus luteum of the ovary and possibly also in the growing foetus. It has been suggested by Hildebrandt that this stimulus is inhibitory in character — inhibitory, that is to say, of secretion — and therefore tending to the con- tinuous growth of the gland-cells. With the removal of the foetus at birth the THE SECRETION AND PROPERTIES OF MILK 1245 source of the inhibitory stimulus is removed and the overgrown gland-cells enter into a condition of spontaneous activity. However this may be, there is no doubt that the secretion of the gland, once formed, is continued inde- pendently of the foetus, or indeed of any of the pelvic organs. The onset of a new pregnancy brings the secretion to a close. Removal of the ovaries in a cow is sometimes employed as a means of prolonging the secretion of milk. The only condition which is necessary for secretion to continue during six to nine months after birth is the repeated emptying of the gland, i.e. the removal of the secreted milk. The process of suckling not only removes the milk already secreted but excites the secretion of more milk. The secretion is certainly subject to nervous influences, but physiologists have not succeeded in either producing secretion by stimulation of the nerves going to the glands, or in stopping secretion by section of these nerves. Moreover the food of the animal may be varied \\dthin very wide limits without altering the composition or amount of the milk secreted, provided only that the food is sufficient in amount. The only constituent of the milk for which we have direct e%'idence of alteration by changes in the food-supply of the mother is the fat. It is well known that the composition of butter may be affected according to the food supplied to the cow. A large supply of oilcake, for instance, may result in the production of a butter which is deficient in the higher fatty acids and is therefore oily at ordinary temperatures. Abnormal fats and fatty acids, such as iodised fats or erucic acid, when administered to an animal in lactation may appear among the fats of the milk. . Not only can the secretion and composition of the milk be afiected reflexly through the nervous system, as, e.fj. under the influence of emotions, but the influence may be reciprocal. This is especially marked in the case of the pelvic organs. The act of suckhng excites tonic contractions of the uterus. Putting the child to the breast shortly after birth is therefore an important means of causing con- traction of the uterus and stopping any tendency to haemorrhage from the venous siniLses opened by the separation of the placenta and foetal mem- branes. The nursing of the child is therefore an important means of pro- curing a proper involution of the uterus after labour. Many uterine troubles among women may be ascribed to the previous neglect of this elementary duty. INDEX Absentiie. action of, 440 Absorption from connective tissues, 1019 in largo intestine, 723 of amino-acids, 749 of carbohydrates, 743 of fats, 738 of food-stuffs, 731-753 of lymph, 1019 of proteins, 744 of water and salts, 731 significance of lipoids in, 735 Absorption-coefficient of gases, 1057 Acapnia, 1101 Accelerator nerves of heart, 975 Accessory olive, 367, 382 Accommodation, 539-549 comparative physiology of, 547-549 range of, 546 Acetamide, 49 Acetic acid, 48 Aceto-acetic acid, detection of, 1123 production from fats, 793 from amino-acids, 802 Acetone. 49 bodies in urine, 1123 in diabetes, 804 Aclu-omatic spindle, 1205 Aehroodcxtrin, 68, 662 Acid albumin. 98 amides, 49 hydrolysis of proteins, 76 number of fats, 56 Acidosis. 793 in diabetes, 804 Acromegaly, 1191 Acrose, 62 synthesis of, 153 Action-current, 226 E.M.F. of, 233 Adaptation, 4 sensory. 482 visual, 572 Addisan^s disease, 1185 Adenine, 103. 778 Adipose tissue. 53 action of gastric juice on, 688 Adrenaline, 51. 1182 action on blood-pressure. 1183 on blood-vessels. 1003 on heart, 977 on nerve terminations, 278 and sympathetic sj'stem, 1183 glycosuria. SOI instability of. 1185 isomers, action of, 1183 Adsorption, by colloids, 141 Adsorption, in ferment action, 168 law of, 148 of toxins, 1033 Adventitia of artery, 870 Aerotonometers, 1058. 1068 specific surface of. 1069 Afferent autonomic fibres, 476 impulses from muscles, 345 nerves, 255 path, 328 tracts, cerebellum. 402 cerebrum, 422 After-images, 571 coloured, 576, 581 After-load, 207 Air, composition of, 1052 expired, 1052 Alanine. 48, 77, 81 deamination of, 154 Albuminates, 99 Albuminoids, 106 Albvimins, characters, 97 Albuminuria, 1121 Albumoscs, 100. 686 Alcaptonuria. 770. 1124 Alco-gels, 139 Alcohol, food- value of, 646 Alcohols, 46 polyatomic, 47 Aldehyde. 47 reactions of, 47 resin, 48 Aldol, 120, 785 condensation, 120 Aldoses, 60 Alexia, 456 Alkaline htematin, 98, 830 Allantoin, 779 ' All or none ' phenomenon, 205 Altitude, effect of, on red corpuscles 1102 on respiration. 1082 A Itinann's granules. 1 7 Aluminium. 44 Alveolar air. carbon dioxide in, 1081 composition of, 1053 sampling of. 1052 Amacrine cells. 560 Amboceptors, 1036 Amide nitrogen of proteins. 91 Amines. 49 action of. 1185 formed in putrefaction, I.")5, 1185 Amino-acids. 48, 77 absorption of, 745—749 aromatic, 84 1247 1248 INDEX Amino-acids, esters of, 80 fate after absorjition, 750 food- value of, 645 heat equivalents of, 763 in digestion, 659 interconvertibility of, 765 separation of, 80 sulphur in, 86 synthesis of, 115, 154 Amino-isobutyl acetic acid, 82 a-amino-glutaric acid, 82 a-amino-succinic acid, 82 a-amino-thiopropionic acid, 86 Ammonia, estimation in urine, 1127 production from amino-acids, 761 in acidosis, 766 Ammonia-nitrogen of proteins, 92 Amoeba, 14 Amoeboid movement, 248 Amphoteric electrolytes, 80, 149 Amylase, 68 of pancreatic juice, 706 Amyloid substance, 106 Amyloplasts, 68 Amylopsin, 706 Anacrotic pulse, 926 Anaemia, 1011 Anaerobic organisms, 26 Analgesia, 495 Anal sphincter, 732 Anaphase of mitosis, 1206 Anarthria, 454 Anelectrotonus, 265 Anisotropous substance, 182 Ankle clonus, 335 Anodal contraction, 263 Anode, 187 Anoestrum, 1226 Anterior cerebellar tract, 354, 371 commissure, 426 Antidromic fibres, 323, 999 Antigens, 1036 Antilysins, 1033 Anti-peristalsis in large intestine, 730 Antithrombin, 846 Antitoxins, 150, 847, 1032 Antipeptone, 704 Apex-beat, 903 Aphasia, 454 Apnoea, 1080, 1095 spuria, 1096 vagi, 1096 vera, 1096 Aqueduct of Sylvius, 363, 373 Arabinose, 61 Arachidic acid, 54 Archipallium, 417 Arcuate fibres, 367 Arginase, 767 Arginine, 83, 84, 91 Aristotle's experiment, 493 Aromatic compounds, 49 substances, fate of, 770-773 ' Arrest ' curves, 200, 208 Arterial pressure, 872, 874 and cardiac output, 884 in man, 875 Arteries, distensibility of, 871 flow in, 918-931 structure of, 870 Arterioles, structure of, 870 Arytenoid cartilage, 520 muscle, 521 Ascending tracts, 387 Ash, estimation in food, 614 Asparagine, 83 food- value of, 646 Aspartic acid, 82, 91 Aspergillus oryzse, 168 Asphyxia, blood-pressure changes in, 984 factors in, 984-987 in decerebrate animal, 985 stages of, 1079 Assimilation, 425 Association areas, 452 ceUs of cortex, 427 centres, 433 fibres of cerebrum, 427 sensory, 451 Astasia, 404 Asthenia, 404 Asthma, 1048 Astigmatism, 538 Asymmetric carbon atoms, 51 Ataxy, 346, 598 Atonia, 404 Attraction sphere, 16 Atwater- Benedict calorimeter, 622 Auditory area, 447 fatigue, 518 localisation, 518 nerve, nucleus of, 379, 380 ossicles, 513 mechanics of, 514 projection, 518 radiation, 423 sensations, 603 analysis of, 517 Audito-sensory area, 433 Auerbach's plexus, 469, 474 in intestine, functions of, 726, 727 in stomach, 699 Aurse in epilepsy, 439, 446 Auricle, pressure-changes in, 901 Auriculo-ventricular bundle, 893, 952 node, 952 Autonomic fibres, afferent, 476 cranial, 472 of vagus, 473 sacral, 473 system, classification of, 470 Autonomic nervous system, 466—477 Autoxidisable substances, 1106 Average error method, 484 Avogadro's hypothesis, 126 Axis cylinder, 251 Axon, 251 reflexes, 474 in vaso-dilator fibres, 999 Bacteria, action on amino-acids, 76 of putrefaction, 155 Bacterium nitromonas, 40 BaJmung, 305 in cortex, 441 Balanced reactions, 166 Balance-sheets of body, 613 Barcroffs blood-gas apparatus, 1055 Burger's method, 129 Barometric pressure, influence on alveolar COo, 1082 INDEX 1249 Basal ganglia, detinitioii of, 377 developuieut, 3t>3 liadopliilo leucocytus, 813 iiasilar mouabraue, funotioos of, olU iiatliinotropic etfeot of vagus exuitatiou, 973 liay i.-i'^i ou doprusdor roiluxes, 97;^, lUUl BayLina and Slarlmy ou intesLinal movo- uionts, 7zt3 ou dcuretin, 7uy Beats, 50S Beckterews nucleus, 381, 4Mz Btckmann's apparatus for treezing-poiut, 1219, 13U iiuhuuic acid, Ji Bell and Jlaytndie's law, 'AHo, 323 Benedict' t> respiration appaiutos, 611 1 Beiisley on islots of Laugeruans, 807 JtJeazeno ring, ^19 origin oi, 118 Benzyl alanine, 155 pyrotartarie acid, 155 Bernard d experiment on vaso-motor uerves, 98:^ Betz cells, -iZS Biciu:omate cell, 187 Bidder's ganglion, 9iU Biederinann's lluid, 2iO Bile, 713-717 acids, 715 composition of, 714 flow of, 715 functions of, 71t» pigment, origin of, 834 salts, 714, 716 cii'culation of, 717 effects of, ou lipase action, 707 functions of, 717 secretion of, 715 Biliary tistula, 715 Bimolecular reaction, l(i3 Binocular vision, 587, 592^ Biogen, 20^ Biopiior, 20 Biuret, 1115 base, 88 reaction, 93, 100 Bladder, afferent impulses from, lioo central control of, 1159 evacuation of, 1157 filling of, 1155 innervation of, 1154, 1157 musculature, 1152 rhythmic contractions of, 115(5 spiiincters of, 1153, 1159 Blind spot, 502 Blix apparatus, 197 on energy of muscular ooutraction 201 Blood, 810-807 alkalinity ol, 1007 amount in body, 854 carbon dioxide in, 1004, 1071 coagulation, 812, 839-853 effect of calcium on, 852 Historical account, 849-853 negative phase, 847 positive phase, 847 corpuscles, relative amount. 857 electrical ooaductivity, 801 fraeziug-poiut of, 8t>l Blood, gases of, 1U54 gas detorminatiou. ',111,^11-11 m. mua-. 1055 pumps, 10.>4 general composition of, 802 hiemolysis, 23, 127, 820, 834, loll, 103(1 hydiogeu ion coucuatration of, 1087 ialcing of, 820 life-history ot rod corpuaclea, 831-835 methods of pi-e venting coagulation, 839 osmotic pressure of, 801 oxygen capacity of, 806, 858 pigments, 829 syutliesis of, 829 plasma, coagulation of, 840 composition of, 804 pi'oteins ot, i>4() relative amount, 857 platelets, 830-838 pi"essuiv, alterations of, 884 apparatus, 872 in capillaries, 930 in man, 875 in vascular- system, 873, 874r-885 reaction of, 800, 1007, 1087 red corpuscles, 811, 818-835 chemistry of, 820 destruction of, 832 effect of altitude on number, 1102 enmueration of, 868 life of, 834 osmotic properties of, 819 regeneration of, 833 stroma, 821 reducing substances in, 1U87 serum, proteins of, 805 specitic gravity of, 860 variations in amount of, l(X»9-l0ll velocity of, 880-«9(.i vessels, nervous control of, 982-1005 volume, carbon monoxide method, 855 white corpuscles, 8 13-*) 17 Body as a machine, 3 temperature, diurnal variations, 1170 regulation of, 1107-1177 Bone-marrow, structure of, 815, 831 Boundary layer, 1131 Bmoiiuin ti capsule, 1 132 glands, 502 Boyle s law, 124 Brachium, superior, 377 Brain, 301-390 development of, 302 evolution of, 305, 385 stem, ascending tract-*, 387 association areivj of. 452 cortical areas, 430 descending tracts, 3S9 evolution of, 301 functions of, 383-;i87, 392-390 long paths in, 383 structure of, 301-391 lireak contraction, 192 excitation, 204 induction shook, 189 Broca's aphasia, 454 convolution. 435 Brodie'is perfusion apparatus. 9(>4 Bromine, 44 BroQohi, 1039 40 1250 INDEX Bronchial murmur, 1045 Bronchioles, 1039 innervation of, 1047 Brownian movement, 145, 146 Bruits, cardiac, 907 Bulbo-spinal animal, 393 Bulbo-spiral fibres of heart, 892 Bundle of His, 893, 952 Burch's capillary electrometer, 227 Burdon-Sanderson' s electrodes, 224 Burdach's column, 324, 352, 354 Biitschli's emulsion theory, 19 Butyric acid, 48, 54 Cadaverine, 155 Caffeine, 103. 775 diuretic action of, 1141 Calcium salts, 43 Calcium salts, in milk coagulation, t)88 eii'ect on heart, 963, 975 excretion of, 724 function in blood coagulation, 841 phosphate in urine, 1125 in starvation, 626 Calculi, biliary, 47 Callender recorder, 221 Calorific value of diet, 650 food-stuffs, 620 Calorimeter, 3 Atwater-Benedicfs, 622 Canal of Schlemm, 543 functions of, 597 Cane-sugar, 67 inversion of, in stomach, 685, 689 Cannon's shadow methods for movements of alimentary canal, 677, 697, 725 Capillary circulation, 928-931 electrometer, 174, 226 tracings, analysis of, 230 pressure, 930 wall, permeability of, 1014 Capric acid, 54 Caprylic acid, 54 Caproic acid, 54 Caput cornu posterioris, 316 Carbamide, see Urea Carbamino-acids, 80 Carbohydrate metabolism, 796-809 in starvation, 628 radical in proteins, 94 Carbohydrates, action of gastric juice on. 689 influence of, on metabolism, 636 Carbohydrates, 59-70 absorption of, 743 imbibition by, 151 synthesis of, 37, 110 Carbon assimilation, 109 importance of, 36 Carbon dioxide, assimilation of, 109 in atmosphere, 37 condition of, in blood, 1064 constancy of, in alveolar air, 1081 excretion of, 1051 tension in alveoli, 1071 tension in blood, 1065, 1071 tensions in tissues, 1064 Carbonic oxide hseraochromogen, 827 hsemoglobin, 823, 824 Cardiac cycle, sequence of events, 894 time relations, 908 Cardiac impulse, 903 murmurs, 906 muscle, factors modifying activity of, 958-966 influence of tension on, 958 physiological properties of, 955- 958 nerves, 969 output, 910-915 sound, 896 Cardinal points in schematic eye, 532 Cardiogram, 904 Cardiograph, 904 Cardio-inhibitory centre, 978 Cardiometer, 914 Cardio-pneumatic movements, 910 Carlson on heart of limulus, 946 Carotid gland, 1194 Casein, formation of, 688 hydi'olysis of, 80 Caseinogen, 91, 100 action of gastric juice on, 688 preparation of, 1239 Catacrotic pulse, 926 Catalase, 1108 Catalysers, 159 Catalysis, 159 as a surface phenomenon, 161 by formation of intermediate products. 161 of methyl acetate, 166 theories of, 160 velocity of, 162, 163 Catechol, 50 Catelectrotonus, 265 Catlicart on carbohydrate metabolism, 803 Cathodal contraction, 263 Cathode, 187 Cell, 13-17 sap, 14 structure of, 16 wall, 14, 22 composition of, 22 electrical phenomena in, 173 permeability of, 22, 23, 127 Cells, chemical changes in, 153 galvanic, see Galvanic cells growth of, 1199 histological differentiation of, 7, 31 synthesis in, 168 vital phenomena of, 25 Cellulose. 70, 646 digestion of, 722 food-value of, 646 hydrolysis of, 70 Central nervous system, 288^77 Centres in medulla, 394 Centro-acinar cells, 711 Centrosome, 16, 19, 33 Cephalin, 57 Cetyl alcohol, 47, 56 Cerebellar ataxy, 405 gait, 405 path, 388 Cerebello-olivary fibres, 369 Cerebellum, ablation of, 404 afferent tracts, 402 corpus dentatum, 373 Cerebellum, efferent tracts, 402 functions of, 397 Golgi cell, 401 INDEX 1251 Cerebellum, inferior peduncles, 369. 370, 402 middle peduncle, 402 nucleus cmbolifonuis. 378. 389 fastigii. 373, 389 globosus, 373, 389 Purkinje cells, 400 roof nuclei, 373, 383, 389, 402 stimulation of. 403 structure of. 400 superior jicduncles. 371, 389. 402, 423 vermis, 373. 400 Cerebral axis, intermediate grey matter, 382 cortex, associative functions, 451 excitability of, 435 function of layers. 431 lamination of, 427. 430 motor areas in, 436 sensory areas in, 444 structure of. 416^27 thickness of, 431 hemispheres, 416-^65 development of. 416 functions of, 434^60 tracts in. 422 localisation, historical, 435 vesicles, 297 Cerebrum, afferent tracts of, 422 association fibres, 425 commissural fibres in, 422 development of. 362 fissures of, 417 lateral ventricles. 420 lobes of. 419 l)rojcction fibres, 423 structure of, 416-133 Cerumen, 512 ' Characteristic ' of excitability, 263, 271 Chemical changes in cells. 153 " in living matter, 153-169 in muscle. 212-218 correlation of functions. 1 1 78 stimulation of muscle. 186 Chilli iotaxis. 6. 27. 498 in leucocytes. 1026 Cheyne-Stokp-1 breathing. 1096 Chitin. 65 Chlorides, estimation of. in urine. 1 129 resorption of. in kidney, 1149 Chlorine. 43 Chlorophvll. 6. 17, 37. 109, 829 f'hl()r()i)lasts. 17. 33 ( 'iiolesterol. 47. 56 ( 'lioliue. 57 Ciioiidroitin. 106 ('liniidroitiii-suiphurie acid. 106 ( 'liondronnicoid. 106 Chorda tym])ani. 472. 665 vaso-dilator fibres of, 096 (!horoid coat, 542 plexus. 375 Chromaftine sul)stanee. 11 SI Clu'omatie aberration. 537 Cliromatin. 17. 31 filaments. 31 granules. 15 Chromatolysis. 320 Chroiuopliile substance. 1181 Cliromoproteins. 101 Chroiuosoiues. 17. 31 number of. in somatic colls. 1206 (Chronograph, 194, 195 Chronotropic effect of vagus excitation. 973 Chvle. 1013 fat in. 739 Ciliary ganglion. 552 movement. 248 muscle, 545 nerves, 552 processes, 542 Ciliated epithelium. 249. r'ilio-s|)irial centre. 553 Cingulum. 417, 426 Circulating proteins. 642 Circulation, general features of. 868-873 in amphibia. 868 in fishes. 868 ferties of. 145-149 heat coagulation of. 72. ]'tO imbibition by. 151 of serum. 1 .")() Coil ids. optical properties of. 144 osmotic pressure of. 142 preeipitatiim of. 147 .surface phenomena of. 141. 147 1252 INDEX Colon, movements of, 729-732 Colostrum. 1237 Colour-blindness, 578 Colour, contrast phenomena, 581 mixers, 576 saturation, 575 vision, 574-584 Edridge-Ch-een's classification. 580 Bering's theory, 579 Toung-Helmlioltz' theory, 577 Combination tones. 510 Comma tract, 353 Commissural cells of cord. 318 fibres of cerebrum, 426 Commissure of Gitdden. 388 Commutator, sec Reverser Compasses test, 492 Complement, 1036 Complemental air, 1046 Complementary colours, 576 Compressor urethrse, 1153 Concentration cell. 171 Conchiolin, 108 Condenser, 191 discharges, use of. in excita^tion, 263 Conduction, irreciprocal, 275. 302 Cone cells, 559 function of. 574 Congo red, adsorption of. 148 colloidal properties of, 137, 143 Conjugate deviation. 438, 587 foci, 529 Conjugated proteins, 101 Conjunctival reflex, 594 Cormective tissues, action of gastric juice on. 688 Consciousness, 9 Conservation of energy in body, 3, 621 of mass. 3 Consonance, 509 Consonants, 525 Constant current, 186 flow in capillaries. 879 Constrictor fibres to limbs. 995 Contractile stress, 200. 207 tissues, 177-249 vacuole, 15, 33 Contraction, paradoxical, 283 remainder, 208 secondary, 233 voluntary, 239 wave in muscle, 204 Contrast in sensation, 483 Conus arteriosus, 892 Convection, loss of heat by. 1175 Convoluted tubules, 1132 ' secretion by, 1144 structure of, 1133 Co-ordinated movements, mechanism of, 338-348 Copper, 44 ferrocyanide cell, 125 Cornea, 542 Corpora mammillaria, 374, 375 Corpora quadiigeniina, 363, 373 Corpus Arantii. 894 callosum, 426 dentatum of corcbelhim, 373 hitoura, formation of. J 222 function of, 1218 Corpus luteum, spurium. 1224 subthalamicum, 377 striatum. 363, 421 trapezoides, 371, 381, 387 Corresponding points, 587 Cortex, histological localisation in, 430 reciprocal innervation from, 437 thickness of, 430 GmWs organ, 517 Cortical areas, sensory, 444 motor, 436 epilepsy, 438 excitation, latent period of, 436, 444 inhibition. 438, 441 motor functions, characters of, 449 Crampton's muscle, 547 Cranial autonomic fibres, 472 nerves, functions of, 410-415 nuclei of, 378, 410-415 Cra3rfish, central nervous svstem, 293 Creatine, 84, 213, 766 " Creatinine, 767 estimation of, 1128 in urine, 1116 tests for. 1117 Crescents of Gianuzzi. 664 Cretinism. 1187 Crico-arytenoid joint, 520 Cricoid cartilage, 520 Crico-thjrroid muscle. 521 Crista acustica, 605 Crossed pyramidal tract, 352 reflexes, 304 Crura cerebri. 373 development of, 363 Crusta, 374 Crystallin, 98 Crystalline lens, 543 Crystallisation of egg albumin, 73 of serum albumin, 73 Crvstalloids, 139 diffusibility of. 136 Crystals, mixed, 73 Cuorine, 57 Curare, action of, 186, 259 Current, demarcation, 233 minimal gradient of, 272 of action. E.M.P. of, 233 of injury, 170. 225, 233 of rest. 225 Cushny on renal resorption, 1147 Cutaneous end-organs, 497 sensation. 486-^-97 sensibility, classification, 495 Cutis vera, 1161 Cyanuric acid, 1115 Cysteine, 86 Cystine, 86, 91. 1124 Cytase, 70, 646 Cytolvsins, 1036 Cytoplasm, 14, 17 Cytosine, 104, 776 b : N RATIO in diabetes, 795. 803. 805. 809 Daniellce]]. 171, 186 Dark-adapted eye, 482, 572 JQ'/lr,sowvrtZ galvanometer, 228 Dead space (in lungs), 1047 Deamination, 76, 154, 762 energy changes in, 763 reversibility of, 762 INDEX 1253 Decapitate animal, 329 Decarboxylation, 155 Decerebrate dog, 396 frog, 395 pigeon, 306 rigidity, 394, 398, 449 Decidua, formation of, 1231 Decussation of fillet, 368 of pyramids, 352, 366 Deep sensibility, 345, 495 Defects of ej'c, 535 Defsecation, 731 Defence, chemical, against infection, 1030- 1038 Deglutition, 676-682 movements of lar\'nx, 678 muscles of, 678 nervous mechanism of, 681 effect of, on respiration, 677, 682 sounds, 677 studj' of, by Rontgen rays, 676, 677 stages of. 677 time-relations, 679 Deiters' cells, 516 nucleus, 381, 383, 402 connections, 404 Delirium cordis, 968 Demarcation current, 170, 225, 233 compensation of, 226 E.M.F. of, 226, 233 Demilune cells, 664 Dendrites, 301 Dental consonants, 527 Depressor reflexes, 997, 1001 DescemeCs membrane, 543 Descending tracts, 389 Detrusor urinje, 1152 Deutero-albumosc, 686 Development, 1212-1216 Dex-trins, 68 Dextrose, 63 Diabetes, 800-809 in fasting animals, 802 in man, 807 pancreatic, 804 sugar consumption in, 805 Diabetic puncture, 800 Dialysis, 136 Diamino-acids, 83, 91, 97 precipitation of, 93 Diamino-caproic acid, 155 Diamino-nitrogen of proteias, 93 Diamino-trioxydodecoic acid, S3, 84, 88 a-5-diamino-valerianic acid, 79 Diaphragm, movements of, 1041 muscles of, 1041 Diastase, 68 Diastolic arterial pressure, 874 Dichromatic vision, 578 Dienccphalon, 363, 392 Diet, normal, of man, 040-657 Dietaries, 649 Difference tones, 511 Differential blood-gas apparatus, 1050 Diffusion, 124, 131 coefHcient, 131 of solutes, 131 Digestion. 658-755 course of, 751-753 in duodenum, 752 in intestine, 702-724 Digestion, in stomach, 683-696 gastric, 685 pancreatic, 702 salivary, 661-675 Dihydroxybcnzcnes, 50 Dilatator pupilla;, 550 Dilemma, 459 Dimethylamine, 49 Dioptre, definition of, 530 Dipeptides, 89 Diphasic variation, 228 Direct cerebellar tract, 354 vision, 563 Disaccharides, 61, 67 Discus proligerus, 1223 Dissimilation, 4, 25, 26 Dissociated colloids, 138 Dissonance, 508 Distearyl-lecithin, 57 Diuretics, action of, 1140 Divers' palsy, 1103 Dominant characters, 1215 Dorsal cerebellar tract, 354 Dromotropic effect of vagus excitation, 973 Dry cell, 187 Du Bois Reymond's key, 187 Ductless glands, 1178-1196 Ductus arteriosus, 1233 Dudgeon's sphygmograph, 922 Dulcite, 62 Dyne, definition, 262 Dysoxidisable substances, 1105 Ear, analysis of sounds by. 511 external, 512 internal, 515 phj-siologj' of, 511-519 Eck's fistula, 760 Edestin, 98 composition of, 91, 92 Edkins on gastric secretion, 694 Edridge-Green, classification of colour-vision 580 Efferent nerves, 255 path, 327 projection fibres, 423 tracts, cerebellum, 402 Efficiency of machines, 234 Egg albumin, 97 composition of, 01 crystallisation of, 73 EkrlicKs side-chain theory, 1034 methylene blue experiment, 1063 Eighth nerve, nucleus of, 380, 412 Einthorcn galvanometer, 227 Elasticity of muscle, 203 Elastin, 108 action of gastric juice on, 688 Elastose, 100 Electrical capillarity, 174 changes in living tissues, 170-174 Electrical changes in muscle, 224 properties of colloids, 14;')— 149 stimulation, nature of. 270 variations, effect of temperature on, 230 variation, time-relations of, 228 Electrocardiograms, 232. 953 Electrodes, non-polarisable, 224 Electrolytes, conductivity of, 129 Elcctroh'tic dissociation, 170 40* 1254 INDEX Electrolytic solution tension, 171 Electrotonic current, 280 Electrotonus, 264 influence of intrapolar length, 268 Electro-vagogram, 1093 Eleidin, 1161 Embryo, nutrition of, 1231 Emmetropic eye, 535 Emulsion, 56 theory of protoplasm, 18 Endocardiac pressure, 896-902 negative phase, 902 Endolymph, 515 End-plate delay, 276 End-plates, 183, 275 effect of nicotine on, 277 fatigue of, 260 End-products, effect of, on ferments, 166, 167 Enemata, nutrient, va,lue of, 723 Energy balance-sheet of body, 620 exchange, 3, 26 exchanges of, in body, 620-623 sources of, 25 total daily output of. 650 transformations of, 123 Engelmanri' s contractile strings, 234 Enterokinase, 703, 705 Enteroceptive nervous system, 479 Enterograph, 817 Entoptic phenomena, 536 Enzymes, see Ferments Eosinophile leucocytes, 813 Ependyma, 362, 375 Epiblast, 33 Epicritic sensibility, 495 Epididymis, function of, 1220 structure of, 1220 Epilepsy, 438, 446 Epithelium, ciliated, 277-249 Equilibration, 399 djTiamic, 606 functions of labyrinth in, 605 static, 608 Erectile tissue. 1221 Erection, 1221 Erepsin, 720 Erg, definition, 262 Ergastoplasm, 672 Ergotoxin, action of, 1184 ' Erlanger's sphygmomanometer, 876 Erythroblasts, 832 Erythrodextrin, 68. 662 Esbacli's reaction, 95 Ester method for separation of amino-acids, 80 Esters, 46, 47, 53 mixed, 54 glyceryl, 54 Ethereal sulphates, excretion of, 769 EthylamJne, 49 Euglobulin, 866 Eustachian tube, 513 valve, 1233 Excitability, 26 Excitation, duration of current, 271 effect of intrapolar length, 268 minimal gradient, 272 Nernst's theory, 286 rate of change, 270 strength of current, 271 Excitation, time, 272 without contraction, 231 Excitatory process, nature of, 284 response, nature of, 271 Exophthalmic goitre, 1189 Extensibility of muscle, 203 Extensor reflex, 332 External auditory meatus, 512 Exteroceptive nervous system, 479 Ej^e, angle between axes in, 537 centring of, 537 constants of, in man, 533 dark-adapted, 482 filtration angle of, 597 movements, centres for, 407, 438 o^Jtical defects in, 535 Eyeball, dioptric mechanism of, 528 530 electrical changes in, 566 nutrition of, 594-597 rotation of, 586 Eyeballs, movements of, 585-588 Eye-muscles, extrinsic, 585 intrinsic, 552 innervation of, 552 Eacial nerve, nucleus of, 379, 381, 413 Eacilitation of reflexes, 305 Eseces, composition of, 755 examination of, in metabolism experi- ments, 648 Faraday-Tyndcdl phenomenon, 144 Ear point of vision, 546 Fasciculus retroflexus, 421 solitarius, 368, 378 Fasting, metabolism in, 618, 624-630 Fatigue, muscular, 208 Fat, 45, 53, 55, 784 absorption of, 738 acid number of, 56 composition of, 784 depots, 783 estimation in faeces, 649 in food, 614 formation from food, 785 from carbohydrates, 785 from proteins, 788 functions of, 784 history of, in body, 783-795 in intestinal epithelium. 740 iodine number of, 56, 784 metabolism in starvation, 630 oxidation of, 792 saponification number of, 56 solubility of, 742 stains for, 739 synthesis of, 119, 169 utilisation of, in body, 790 Fats, action of gastric juice on, 689 action of pancreatic juice on, 707 chemistry of, 53-58 influence of, on metabolism, 636 Fatty acids, 54 heat equivalents, 763 volatile, 56 Fatty degeneration, nature of, 739 Fechner's law, 485 Fehliwfs test, 64, 1123 Fenestra ovalis, 515 Fenton's reaction, 1 109 Ferment action, adsorption in, 168 reversibility of, 167, 169 INDEX 125i) Ferment action, velocity of. 103. 104 Fermentation in stomach, 684 Fermentation test, 1123 Ferments, 1.57-1 09 action on optical isomers, 1 07 adenase, 778 amylase, 158 arginase, 158, 767 as catalyse rs, 159 catalase, 1108 characters, 157 classification, 158 (leaminising, 154, 158 definition, 157 effect of end-products on, IGG enterokinase, 158 erepsin, 158 inverting, 158 isolation of. 158 laccase, 1109 lactase, 158 lactic, 158 lipase, 158, 109 maltase, 158, 108 nuclease, 776 optimum temperature for. 100 oxidases, 1108 pepsin. 158, 1G9, 085 peroxidase, 1108 properties of, 159 rennin, 088 specificity of, 100, 107 as synthetic agents, 108 sucrase, 158 trypsin, 158 urease, 158 uricolytic, 779 zymase, 158 Fertilisation, 1209-1211, 1227 artificial, 1211 Fibre layers in cortex, 430 Fibrillar of muscle, 179 Fibrin, 805 composition of, 92 ferment, 842 fate of, 847 Fibrinogen, 98, 841, 864 Fibroin, 107 Fick and Wislicenus's experiment, 038 Field of vision, 503 Fifth nerve, nucleus of, 380 Fillet, 368, 387, 423 decussation of, 368 lateral, 372, 389 Final common path. 343 First focal plane, 531 point, 531 Fischer's method for separating amino-acids, 80 Flechsig's myelination method, 319 Flexion reflex, 332, 340 Flicker phenomena, 571 Fluoride plasma, clotting of, 848 Fluorine, 44 Foci in ej'e, 533 of lenses, 529 Foetal circulation, 1233 Folin's method for ammonia estimation, 1127 for urea estimation. 1127 for uric acid estimation, 1 128 Fontana. spaces of, .'US Food, amount necessary, 649 analysis of, in raetabolLsm expcriment.s, 614 materials, utilisation of, 6 Food-stuffs, digestibility of, 649 changes in alimentar}- canal, 658-600 heat-values of, 620 history of, 750-809 proportioas necessary, 652 significance of, 642—647 Foramen of Monro, 376 Foramen ovale, 514, 1233 rotundum, 515 Fore-brain, 362, 375 Formic acid, 48, 54 Fornix. 420 pillars of, 370 Fourth nerve, nucleus of,' 379, 382, 41 1 Fovea centralis, 501 Frank's manometer, 897 Frank on pulse, 926 Fraunhofer''s lines, 569 V. Frey, testing hairs, 490 Frontal lobe, 417 Frog, muscles of. 1 84 Frog's heart, anatomy of, 939 Fronto-pontine fibres, 424 Fructose, 61. 64 Fundamental tone, 507 Fungiform papilla, 499 Galactolitcves, 57 Galactosamine, 105 Galactose, 64. 67 Gall-bladder. 713 innervation of, 715 Galvanic cells, bichromate, 187 concentration. 171, 172 Daniell, 171, 186 drv, 187 E.'M.F. of, 173 Leclanche, 187 positive element, 186 positive pole, 186 source of energy, 171 Galvanic excitation, law of, 264 Galvanometer, D' Arsonval, 227 Eintkoven's, 227 Ganglia, evolution of, 1293 sympathetic, 468 Ganglion habenulse, 375 Gasserian, 380, 382 Gaseous diffusion, 134 exchange in lungs, 1068-1075 secretory theory, 1073 Gaseous metabolism of s^^livary glands, 674 Gases, tension of, in liquids, 1058 Gas laws, 123 Gastric digestion, 685 fistula, 683. 690 hormone, 695 juice, 083 acid of, 684 action of, on carbohydrates, 089 action of, on fats. 689 action of. on proteins. 685 action on food-stutls, 685 ' appetite ' secretion, 691, 696 composition of, 684 1256 INDEX Gastric juice, chemical excitants, 694 determination of hvdi'ochloric acid in, 685 inverting power of, 685 rate of secretion of. 690 variations in, 690-693, 696 psychical secretion of, 691 secretion of, 690 secretin, 695 Gastrocnemius of frog, 183 Gelatin, 107 composition of, 91, 92 food value of, 645 Gels, 139 Gemmules, 20 General physiology, 13-174 Geniculate "bodies, 375. 376, 383, 388 Genital hormones, 1218 Geotasis. 27 Germ cells, 1202 chromosomes in, 1208 division of, 1205 Germinal spot, 1222 vesicle, 1222 Gierke, respiratory bundle of, 1077 Glaucoma, 597 Gliadin, composition of, 91, 92 Gliadins, 98 Globin, 825 composition of, 91 Globulin, adsorption bj^ 149 salts of. 866 Globulins, 98 Glomerulus, filtration in, 1136 of kidney, 1131 pressure of blood in, 1 1 37 Glossopharyngeal nerve, nucleus of, 414 Glottis, 521, 524 Gluconic acid, 62 Glucosamine, 64, 87, 105 Glucosazone, 63 Glucosazone crystals, 1122 Glucose, 61, 63 in blood, 796 identification of, 63, 1123 lactonic structure, 66, 67 Glucosides, 65 Glutamic acid, 80, 82, 91 Glutelins, 98 Gluten peptone, 100 Glycerides, 53 Glycerophosphoric acid, 57 Glycerol, 47, 53 Glyceryl aldehyde, 59, 122 Glyceryl esters, 53 Glycine, 78, 81, 91 ring formula of, 79 salts of, 79 ester, polymerisation of, 89 Glycogen, 69, 213, 796 preparation of, 797 formation of, 797-798 Glycoproteins. 105 Glycosuria, 800-809, 1122 alimentary, 800 Glycuronic acid, 65, 1123 Glycyl-glycine, 88 Glyoxylic acid, 94 Goitre, 1187 Golgi cells, 318, 401 corpuscles, 497 Go'gl network, 301, 309 GolW column, 324, 352 Gonads, 1204 Gotch's heart apparatus, 964 Goiuefs tract, 356, 371, 388 Graafian follicles, structure of, 1223 Gracilis experiment, 254 of frog, 184 Grahain on colloids, 139 Grape-sugar, see Glucose Graphic method, 194 Gravity, effect of, on circulation, 933 Green-blindness, 581 Grey matter of cortex, minute structure of, 427 rami, 468 Growth, 4 of cells, 1199 food requirements in, 656 Guanine, 103, 105, 778 Guanylic acid, 105 Gudden's commissure, 388 Gunzherg's reagent, 684 Gustatory area, 449 sensations, 499 Guttm'al consonants, 527 Gyrnnema sylvestre, effect on taste, 500 Haib follicles, 1162 Hairs, sensibility of, 490 Haldane-Pembrey respiration ax^paratus, 616 Hales' experiment, 872 Haploscope, 588 Haptogen, 1239 Haptophore group, 1034 Harmonic intervals, 509 Harmonics, 507 Hausmami' s method for distribution of nitrogen, 91 Hayem's fluid, 836 H^matin, 97, 101, 826 chemical relationships, 828 synthesis of, 829 Hsematinic acids, 828 Hsematoporphyrin, 825, 827 combination with iron, 830 in urine, 1121 Htemin, 825 structure of, 829 Hsemochromogen, 827, 830 Hsemocyanin, 44, 93 Hsemocytometer, 858 Hsemodiomograph, 888 Haemoglobin, 43 absorption spectrum, 823 composition of, 91, 101 crystallisation of, 73 derivatives of, 825 dissociation curve, 1060 iron in, 822 molecular weight of, 75, 144 oxygen capacity of, 822, 1059 preparation of, 820 reduction of, 822 union with oxj'gen, 1059 Hsemoglobin-crystals, 821 Hsemoglobinometer, 855, 858 Hsemolysins, 1036 Haemolysis, 23, 127, 820, 834, 1011, 1036 Hsemolytic sera, 1030 Hsemopyrrol, 829 Ha;morrhage, 1011 INDEX 1257 Head on cutaneous sensibility', 495 Head's diaphragm slip, 1089 Hearing, end-organ of, 514 Helmholtz's theory, 517 physiology of, 511 Rutherford's theory, 518 telephone theory, 518 Waller's theory, 519 Heart, action of sympathetic nerves on, 975 action of vagus, 971 changes in form, 902 diastolic filling, 909 effect of calcium on, 963, 975 muscarine on, 974 nicotine on, 974 potassium on, 903, 975 electrical variations, 230 frog's, automatic contraction, 940 human, electrocardiogram of, 953 influence of temperature on rate of, 962 inhibition of, 972 mammalian, ganglion-cells in, 949 mechanism of, 891-917 nerve fibres in, 946 nutrition of, 966 l^rimitive vertebrate, 950 rate in exercise, 1007 reflexes, 978 sounds, 905 graphic record of, 907 systolic output, 910-915 theories of inhibition of, 974 work of, 915-917 Heart-beat, causation of, 939-968 contraction wave, 947-949 electrometer records of, 948 in mammals, 949-955 myogenic hypothesis, 942 neurogenic hypothesis, 941 propagation of contraction in, 943 significance of carbon dioxide for, 905 Heart-^ltlock. 955 Heart-fibrillation, 968 Heart-lung preparation. Starling's method, 911 Heart-muscle, ' all or none ' phenomenon, 955 excitation of, 955 influence of tension on, 958 of chemical substances on, 962 physiological properties of, 955-958 refractory period of, 950 summation of stimuli in, 956 Heart-musculature, 891 Heat-coagulation, 1 50 'Heat-engines, efficionoj' of. 234 Heat-loss, regulation of, 1174 Heat, production in body. 1171 in muscle, 219 regulation, nervous mechanism of, 1176 in new-born, 1177 rigor. 208, 214 sensations, conduction in cord, 359 sexual, in animals, 1226 spots, 487 values of food-stuffs, 619 Heats of conibustion. 156 Heller's test, 95, 1122 Helmholtz resonators, 508 side wire. 189 Helweg's bundle, 353 Hemiansesthesia. 446 Hemianopia, 407, 447 Hemiplegia, 446 Henle's loop, 1132 Hensen's disc, lh2 Heredity, 1212-1216 basis of, 20 Hering's theory of colour- vision, 579 Herpes zoster, causation of, 999 Heteroalbumose, 100 Heterocyclic amino-acids, 85 Heterotj'pe mitosis, 1207 Hexachromie vision, 580 Hexone bases, 83 Hexoses, 61 classification of, 61 derivatives of, 64 reactions of, 62 Hind-brain, 362, 366-373 Hippocampal commissure, 427 Hippocampus, 419 Hippuric acid. 770, 1119 Hirudinised blood, 840, 849 His' bundle, 893, 952 Histidine, 86, 91, 104 fate of, 772 Histological difterentiation, 7, 31 localisation in cortex, 431 Histones, 92, 97, 101 Hoffmann's test, 85 Hoinans on islets of Langerhans, 807 Homogentisic acid, 51, 770, 1124 Homoiothermic animals, 1169 Hopkins' test for lactic acid, 215 for tryptophane, 94 Hopkins- Adamkieioicz reaction, 94 Hormones, genital, 1218 mammary, 1218 nature of, 1180 pancreatic, 709 Horopter, 588 HUfner's method for urea estimation, 1126 Human stomach, form and movements, 698 Hiirthle's manometer, 897 Hyaline corpuscles, 813 Hyaloid membrane, 543 Hyaloplasm, 18 Hydraemic plethora, 1009 effect on volume of urine, 1140 Hydrated proteins, 99 Hydrazones, 62 Hydrocarbons, 45 unsaturated. 46 Hydrocele fluid, 849 Hydrochloric acid in gastric juice. 684, 685 Hydiogels, 72, 140 Hydrogen, 39 Hj-diogen ion concentration, determination of, 1067 Hydrolysis. 154 of casein, 80 of proteins, 70, 80. 99 Hydroquinone. 50 Hydrosols. 139 osmotic pressure of, 14 2 properties of. 142,1 44 Hyperglycajmia, 805 Hypcrmotropia. 535 Hyperpncpa. 1079 Hypoblast. 33 1258 INDEX Hypogastric nerves, action on bladder, 1159 Hypogastric reflex, 475 Hypoglossal nucleus, 379, 415 Hypoxanthine, 103, 778 Identical points in retina, 587 Idiopathic epilepsy, 439 Ileocolic sphincter, 729 Illusions of size, 591 Image, size of, 530 Imbibition by coUoids, 151 pressure, 152 relation to constitution, 152 Iminazol, 104 ring, 86 sjTithesis of, 117 Iminazol-alanine, 86 Immunity, 1030-1038 Impregnation, nervous mechanism of, 1228 Incisura angularis, 698 Incus, 513 Indican, of urine, 1120 Indicators, 1067 Indigo as catalyser, 161 Indigo-carmine, excretion by kidney, 1146 Indirect vision, 563 Indol, fate of, 770 Induced currents, 188 Inductorium, 188, 189 Infection, cellular defence against, 1021-1029 chemical defence against, 1030-1038 Inferior cervical ganglion, 466 mesenteric ganglion, 466 oblique muscle, 585 peduncles of cerebellum, 368, 369, 370 Inflammation, 816, 1024 Infundibula, 1039 Infundibulum in brain, 375 Inhibition, 26, 306, 339 Gaskell's theory of, 974 of heart, nature of, 974 in peripheral ganglia, 475 Injury, effect of, on nerve, 274 Injury-current, 170, 225 Inner cell-lamina of cortex, 428 fibre-lamina of cortex, 428 line of Baillarger, 430 Inorganic food-stuffs, 647 Inosinic acid, 774, 776 Inosit, 214 Inotropic effect of vagus excitation, 973 Instrumental inertia, 196, 927 Insular lobe, 417 Intercostal muscles, action of, 1044 Internal capsule, 364, 376, 421, 423 Internal ear, 515 respiration, 1039 restiform body, 370 secretions, 1178-1196 Intestine, absorption in, 733 law of, 727 movements of, 725-732 pendular movements of, 725 Intestinal fistula, 718 juice, 718 characters of, 720 secretion of, 719 Tntima of artery, 870 Intra-auricular pressure, 901 Tntra-molecular oxygen, 25 Intra-ocular fluid, 596 Intra-ocular pressure, 543, 595 pressure and blood pressure, 597 Intra -thoracic pressure, 1045 Intra-ventricular pressure curve, 898 Intra-vesical pressure, 1156 Intra-vitam staining, 23 Inulin, 69 Inversion, 67, 160 Invertase, 67 of intestinal juice, 721 Invert sugar, 67 Iodine, 44 in thyroid gland, 1189 number of fats, 56 lodothyrin, 1189 lonisation, 128 Ions, 170 velocity of transport, 173 Iris, 542 as diaphragm, 537 functions of, 550 local stimulation of, 554 nerve-supply, 553 Irradiation, 339 Irreciprocal conduction, 275, 302 Iron, 42, 74 excretion of, 43 in large intestine, 724 in haemoglobin, 74 in liver, 835 Island of Reil, 420, 423 Isoamylamine, 156 Isodynamic food-stuffs, 632 Isoleucine, 82 Isomaltose, 167, 168 Isometric contraction, heat of, 221 lever, 197 Isosmotic solutions, 132 Isotonic lever, 197 solutions, 819 Isotropous substance, 182 Jacksonian epilepsy, 435, 439 Jafte's test, 1117 Joints, sensibility of, 600 Jugular ganglion, 473 Kakyokinesis, see Mitosis Karyosome, 16 Keratin, composition of, 91, 107 Keratins, 86, 107 ' Kernleiter ' model, 281, 284 Keto-acids, 49, 154 origin of, 762 Ketoses, 60 Key, kick-over, 195 Keys, 187 Kidney, adaptability of, 1149 cells, structure of, 1145 functions of glomeruli, 1135-1142 nerves of, 1134 secretion in frog, 1145 of water and salts, 1135-1142 structure of, 1131-1134 tubules, functions of, 1144 absorption in, 1147 work of, 1135 Kinaesthetic areas, 442 Knee-clonus, 335 Knee-jerk, 333 abolition of, 334 INDEX 1259 Knee-jerk, exaggerated, 451 latent period, 3.34 Knoop on deamination, 762 Kiau-.e's membrane, 179 Krogh's microtonometer, 1070 Kiihne's gracilis experiment, 254 Kymograph, 872 Labial consonants, 527 Labour, 1235 Labyrinth, evohition of, 603 functions of, 398 Labyrinthine sensation, 603 Laccase, 1109 Laerymal gland, 594 Lactalbumcn, 1240 Lactase, action of, 158, 165 in intestinal juice, 721 Lactation, 1237-1245 Lacteals, 734, 1013 Lactic acid, 49 Hopkins' test, 215 in blood, 1085 in gastric juice, 684 in muscle, 215 in urine, 216, 1085 Lactoglobulin, 1240 Lactosazone crystals, 1122 Lactose, 67 1240 LcEvorotation, 52 Lajvulinic acid, 104 Lagena, 603 Lamina cinerea, 375 Langerlians' islets in pancreas, 807 Lanoline, 56 Lardacein, 106 Large intestine, absorption in, 723 excretion in, 724 functions of, 722 movements of, 725-732 Laryngoscope, 523 Larynx, anatomy of, 521 movements of, in deglutition, 678 Latent period, 198. 207 Lateral columns of cord, 354 crico-arytenoid muscle, 522 fillet, 372, 387 nucleus in medulla, 366 in pons, 370 sympathetic ganglia, 468 Lateritious deposit, 1118 J^auric acid, 54 Law of forward direction, 310 of the intestines, 727 I^aw of specific irrital)ilit\-. 480 Jjccithin, 57 Leclanchi. cell. 187 Lemniscus, 368, 387 Lens, 543 elasticity of, 546 variation with age, 546 Lenses, formation of images by, 528 Lenticular ganglion, 552 Legumin, 98 Leguminous nodules, 41 Leucine, 82, 91 cones, 82 Leucines, oxidisability of, 1106 Leucocytes, chemiotaxis in, 1026 classification of, 813 origin of, 815 Leucopla.sts, 17 Leucosin, 98 Lever, momentum of, 197 Levulose, 64 Liebermann's reaction, 94 Life, evolution of, 5, 6, 33 laws of, 7 without oxygen, 25 Ligamentum pectinatumiridis, 543 Light, nature of, 568 reflex, 550 Lignin, 106 Lignoceric acid. ;54 Limbic lobe, 417 Liminal intensity of stimulus, 482 Limulus heart, 946 Line of Gennari, 430 Lines of direction, 531 Linin, 17 Linoleic acid, 54 Linolinic acid, 54 series, 54 Lipase of pancreatic juice, 707 in gastric juice, 689 reversibility of action of, 169 Lipines, 57 Lipoids, 23, 57 Liquor folliculi, 1223 Lissauer's tract, 324, 353, 357 Liver, bile formation in, 713 formation of urea in. 759 of uric acid in, 778 glycogen of, 797 haemolysis in, 835 lymph production in, 1015 Living matter, chemical changes in, 153-169 liOad. effect of a contraction, 203 Local reflexes, 474 sign of tactile sensation, 493 Locke's fluid, 963 Localisation of function in brain, 435 Locomotion, co-ordination of, 346 Locomotor ataxia, 346 Locus perforatus posticus, 375 Long ciliary nerves, 552 sight, 536 Longitudinal inferior fasciculus, 426 superior fasciculus, 426 Loudness of sound, 506 Ludwig's Slromi(hr. 888 manometer, 872 Luminosity of spectral colours, 569, 575 Lungs, circulation in, 935 distensibility of, 1044 exchange of gases in, 1068-1075 Lungs, vaso-motor fibres of. 935 Lutein. 1224 Lmj's nucleus. 377 Lymph, absorption of. 1019 movements of. 1017 production, 1013 effect of capillary ]ii-es.sure on, 1015 in small intestine, 735 osmotic phenomena in, 1015 properties of, 1013 role of, in nutrition. 1020 and tissue fluids, 1012-1020 Lymphagogues, 1016 Lymphatic glands. 1013 tissue, struetvire of. 815 Lymphatics, course of, 1012 1260 INDEX Lymphocyte, 813 Lysine, 83, 91 decarboxylation of, 155 Lysins, 1034 MacdonaliVs theory, 287 MacdougalV s theory, muscular contraction, 235 Macro-nucleus of paramoecium, 1200 Macrophages, 1028 Macula acustica, 399, 605 lutea, 561 Magnesium, 43 excretion in large intestine, 724 Major chord, 509 Make contraction, 192 excitation, 263 induction shock, 189 Mall on heart musculature, 891 Malleus, 513 Malpighian follicles of spleen, 1194 pyramids, 1131 Maltase, 167 of intestinal juice, 721 reversibility of action, 167 Malto-dextrin, 69 Maltose, 66, 67, 69 Mammary gland, structure of, 1243 hormones, 1218 Mannite, 62 Mannose, 61, 64 Manometer, Hiirthle's, 897 Ludtvig's, 872 Piper's, 897 MarcM's method, 320 Marey's law of blood-pressure, 980 sphygmograph, 922 Marginal lobe, 417 Marie's tract, 354 Mark-time reflex, 343 Marrow, 53 Martinotti cells, 428 Material basis of body, 36-122 Matter, exchanges of, in body, 613-657 Maximal stimulus, 193, 205 Mayer curves, 988 Mechanical changes in muscular contraction, 194-204 coagulation of colloids, 150 efficiency of muscle, 222 response of muscle, 205-211 Mechanism, 8 Meconium, 1233 Media of artery, 870 Medulla oblongata, 366-370 functions of, 392 Medullary sheath, 251 Medusa, nervous system of, 290 Meibomian glands, 594 Mciosis, 1207 Meissner's corpuscles, 497 plexus, 469, 474, 720 Mcmbrana granulosa, 1223 reticularis, 517 tectoria, 517 tympani, 512 Membranes, electrical phenomena at surface of, 173 permeability of, 22, 131 semi-pcrmcablc, 132 Membranous labyrinth, 515, 603 MendeVs law, 1215 Menstruation, 1224 relation of ovulation to, 1225 Mercurial manometer, 872 Mesencephalon, 363, 392, 394 Mesial fillet, 368, 387 Mesotartaric acid, 52 Metabolism, effect of foods on, 631-636 during starvation, 624-630 experiments, analysis of excreta, 615 general, 613-647 influence of age on, 656 of carbohydrates on, 636 of fats on, 636 of muscular work on, 637 of protein on, 631 of temperature on, 1168, 1172 methods, 614-623 of muscle, 216 of nucleo -proteins, 774-782 of proteins, 756-773 relation to body weight, 627 to surface of body, 628 Metakinesis, 9 Metal ' sols,' 140, 160 Metaphase of mitosis, 1205 Metaplasm, 16 Metaplasmic products, 18 Meta position, 50 Metaproteins, 98 Metencephalon, 362, 392, 394 Methsemoglobin, 824 Methyl-acetate, catalysis of, 166 Methylamine, 49 Methyl glucosides, 66 glycine, 84 guanidin acetic acid, 84 purines, 103, 775 Micellje, 20 Micro-nucleus of paramcecium, 1201 Microphages, 1028 Microsome, 16 Microtonometer, 1070 Micturition, 1152-1160 nerves of, 1157 reflex, 1159 Mid-brain, 362, 373 crusta, 374 functions of, 394 pes, 374 tegmentum, 374 Middle cell-lamina of cortex, 428 Milk, coagulation by pancreatic juice, 706 fats of, 119, 1239 origin of, 1245 human, composition of, 657 properties of, 1238 proteins of, 1239 in relation to growth, 657 quantity secreted, 1238 salts of, 1241 secretion of, 1237-1245 Milk-sugar, 1240 Millon's reaction, 85, 94, 100 Mineral salts, importance of, 647 Minimal difference method, 484 effective stimuli, 483 gradient, 272 stimulus, 193, 205, 482 Mitosis. 1207 heterotypc, 1207 INDEX 1261 Mitral cells, 421 valve, 893 Modality of sensation, 479 Molecular layer of cortex, 428 Molecules, energy in solution, 123 size of, 140, 143 MoliscJi's test, 64, 94 Molybdic acid as catalyser, 162, 163 Monakow's bundle, 353, 389 Mono-amino-dibasic acids, 82 Mono-amino-monobasic acids, 81 Mono-araino-nitrogen in ])roteins, 93 Monochromatic patches, 575 vision, 579 Monomolecular reaction, 163 Monophasic variation, 230 Monosaccharides, 61 Moore's test, 63 Morphotic proteins, 642 Moss fibres, 401 Motor aphasia, 454 area, lamination. 431 cells of cord, 318 centres, ablation of, 440 end-plates, 183 sensibilit}'^, 446 ' Motor points,' 269 somatic nuclei, 378 Mountain sickness. 1099 Jlovcmcnt, co-ordinated, 338 and sensation, 177—609 sensation? of, 598-602 Movements of alimentary canal, 676, 697, 725 of deglutition. 676 of large intestine, 729 of small intestine, 725 of stomach, 697 Mucin, action of gastric juice on, 688 Mucins, 105 Mucoids, 105 Mucous glands, 663 Midler's law of specific irritability, 255, 480 Multirotation. 05 Murexidc test, 1118 Muscao volitantcs, 537 Muscarine, action on heart. 974 Muscle, afferent nerves, 336 anistropic substance. 182 action of vcratrin. 211 arrested contraction. 200 break contraction. 192 chemical changes in, 212-218 chemical stimulation of. 186 clotting of. 213 composition. 212 contractile stress. 200 contract ion-remainder. 208 demarcation current. 170, 225, 233 efficiency of, 222, 639 elasticity of. 203 electrical variation, effect of tcmi)era- ture. 230 excitation of. 185-193 excitation without contraction, 231 extcnsihilitv of. 203 heat -production in. 219-223 independent excitability. 185 injurv-cmrent, 170, 225. 233 involuntary, 180, 243 of invertebrates. 248 isotropous substance, 182 Muscle, longitudinal striat ion. 180 make contraction, 192 mechanical response of, 205-211 metabolism of, 216 production of carbon-dioxide, 217 products of activity, 216-218 reciprocal innervation of, 355 red, 180 reflex tone, 398 respiratory cjuotient, 217 stimulation by constant current, 186, 192 thickening of, 199 utilisation of energy, 221 voluntary, 177 structure of, 179 white, 181 Muscle-current, 170, 225, 228. 229 Muscle-curve, correction of, 197, 208 Muscle fibre, 179 reversal of stripes, 182 Muscle-fibrillK, 179 Muscle-plasma, 212 Muscle-sound. 240 Muscle-spindle, 601 Muscle-twitch, 192, 194. 198 Muscle-wave, 204 Musical notes, vibration frequencies, 510 Muscular aft'erents, 600 contraction, ' all or none ' phenomenon. 255 Engehnann' s theorj', 234 effect of drugs, 211 :,i of fatigue, 208 of load, 203 of salts, 210 of temperature, 207 of tension, 222 latent period of, 198 MacdouqalVs theory, 235 mechanical changes in, 194-204 nature of. 234-238 optical method, 1 98 osmotic theory, 235 point of stimulation, 194 Schdfer's theory, 182 strength of stimtilus, 205 time-relations of, 198 voluntary, 239 electrical changes in, 242 record of. 241 contractions, summation of, 206 energy, source of. 637 exercise, effect on circulation. 1006-1008 movements, co-ordination of, 345 relaxation, 198 sense, 600 psychology of, 601 sensibiiitv, paths, 446 tone, 333", 449 relation to labyrinthine sens;itions, 606 work, energy exchanges in, 639 effect on metabolism, 637 Muscularis mucosa^ of small intestine. 735 ^lusculi papillares. 894 Musculns vocalis. 522 Mj'elencephalon. 362. 392 Myelin sheath, 251-252 Myclination, 252 method, 319 1262 INDEX Myelocytes, 816 Mylohyoid of frog, 184 Myogen. 213 fibrin, 213 Myogenic movements of intestine, 726 Myolisematin, 213 Myopia, 535 Myosin, 98, 213 Myosin fibrin, 213 Myosinogen, 212 Myristic acid, 54 Myxcedema, 1187 Nasal consonants, 527 Near point, 539, 546 Nectocysts, 33 Negative after-image, 572 polarisation of nerve, 280 variation, 233 ventilation, 1091 Negativity, 229 Neopallium, 417 evolution of, 419 Nerve, core model, 281 double conduction, 250 electrotonic current, 280 electrotonus, 265 endoneurium, 252 excitability, 'characteristic' of, 263, 271 effect of temperature on, 273 fatigue in, 259, 285 galvanic excitation, 262 human, stimulation, 268 law of forward direction in, 310 negative polarisation, 282 neurilemma, 252 nodes of Ranvier, 252 non-meduUated, 252 polarisation in, 280 positive polarisation, 282 primitive sheath, 251 unipolar excitation, 268 Nerve-axon, 251 Nerve-block, 266 Nerve-cell, automaticity of, 314 functions of, 312 Golgi net, 301, 309 Nissl's granules, 300 pericellular network, 308 structure of, 300 Nerve-centres in medulla, 393 Nerve-current, MacdonaWs theory, 173 Nerve fibres, 250-287 excitation of, 262 size of, 252 Nerve fibres, structure, 250 Nerve-impulse, 253 conditions affecting, 258 effect of temperature on, 258 electrical changes accompanying, 256 influence of drugs on, 260 velocity of, 253 Nerve-injury, effect of, 274 Nerve-trunk, composition of, 327 Nerves, cranial, 410 nuclei of, 378, 410^15 grafting of, 255 Nervi erigentes, functions of, 473, 996, 1154, 1158, 1228 Nervous system, evolution of, 288-296 ganglia, 293 of medusa, 290 of vertebrates, 297 Neural canal, 297 Neurilemma, 251 Neurine, 57 Neuroblasts, 298 Neuro-fibrillar network, 428 Neuro-fibrils, 296 continuity of, 307 Neuroglia, 297 Neurokeratins, 107, 251 Neuro-muscular junction, 183, 275 spindles, 601 Neuron, definition of, 295, 301 Neurons, continuity of, 296 structure, 296 Neuropilem, 296 Neutrophile leucocytes, 813 Nicol's prism, 51 Nicotine, effect on end-plates, 277 on heart, 974 method, 472 Ninth nerve, nucleus of, 379, 414 Nissl's granules, 301 Nitrates, 40 Nitrification of sewage, 40 Nitrifying organisms, 40 Nitrogen, 39 endogenous, in urine, 758 estimation of by KjeldahVs method, 1126 excretion in starvation, 629, 633 exogenous, in urine, 758 in food, 614 output, 756 requirements of body, 633 source of, 39 Nitrogenous constituents of urine, 757 equilibrium, 616, 631, 653 Nociceptive stimuli, 341 Nodal point, 531, 533 Nceud vital, 1077 Non-polarisable electrodes, 224 Normoblasts, 834 Nuclear sap, 17 Nucleic acid, 101 Nuclein, 97, 101 hydrolysis of, 774 metabolism of, 774-782 Nucleins, fate of, 777 formation in body, 777 Nucleolus, 16 Nucleoplasm, 17 Nucleoprotein, decomposition of, 102 Nucleoprotein. digestion of, 104 Nucleus, 14, 27, 31 ambiguus, 379 oi Bechterew, 381, 402 branched, 31 caudatus, 363 cuneatus, 367 emboliformis, 373 effect of removal, 28 fastigii, 373 function of, 27 globosus, 373 gracilis, 367 lenticularis, 363 necessity for growth, 30 INDEX 1263 Nucleus of Luys, 377 of Rolando, 3(57 relation to cytoplasm, 27 Nutrition, mechanisms of, (313, 809 Occipital lobe, 417 Occipito-frontal fasciculus, 426 Oculo-motor nucleus, 379, 407, 410 Occoid, 818 (Esophagus, inhibition of, in swallowing, 080 (Estrus, 1226 Ohm's law of auditory analysis, 51 1 Oleic acid, 55, 56 Oleyl-lecithin, 57 Olfactie, 504 Olfactometer, 503 Olfactory ajiparatus, 421 area, 448 bulb, 421 glomcrali, 421 lobe, 417, 448 mucous membrane, 502 sensations, 498, 502 tract, 421 tubercle, 421 Olivary body, 367, 382 Olive, 3G8, 382 Olivo-cerebellar fibres, 389 Olivo-spinal tract, 353, 391 Oncometer, 992 Optic axis, 531 chiasma, 388, 406 disc, 561 nerve, decussation, 387, 406 efferent fibres in, 500 radiation, 423 thalamus, 363, 375, 383, 395, 420 nuclei of, 377 tracts, 387, 406, 447 Optical activity, 51 axis of lens, 528 centre of lens, 529 defects in eye, 535 isomers, action of ferments on, 167 Optimum temperature for ferments, 160 Opsonic index, 1038 Opsonins, 1037 Ophthalmometer, 541 Ophthalmoscope, 554-557 Optograms, 565 Ora serrata. 561 Orcin reaction for pentoses, 61, 94 Organ of Corli, 517 Organic compounds in body, 45 Organic sensations, 598-609 Organic synthesis, mechanism of, 109 Organisation. 6 Ornithine. 79, 83 decarboxylation of, 156 Ortho position, 50 Osamines, 63 Osazones, 47, 62, 1123 Osmometer, 143 Osmotic pressure, 123, 133 Bargcr's method, 129 Beckvmniis methotl, 129, 130 and boiling-point, 129 of colloids, 143 of electrolytes. 1 28 freezing-point method, 129 by htcmolysis, 127 Osmotic pressure, Hamburger'' s method, 127 measurement of, 125-130 Ijy plasmolysis, 127 of proteins, 75, 142 of serum proteins, 142 and transport of water, 133 and vapour teasion, 129 Osseous labyrinth, 515, 604 Osteoporosis, 626 Otic ganglion, 473 Otolith organ, 399, 605, 608 Otoliths, function of, 608 Outer cell lamina of cortex, 427 Outer fibre-lamina of cortex, 427 Outer line of BuiUargrr, 430 Overtones, 507 Overton's theory, 23 Ovulation, 1222 Ovum, 1205 maturation of, 1209 Oxalate crj^stals, 1125 Oxidases in tissues, 1108 Oxidation. 157, 1063, 1105-1109 by indigo, 161 mechanisms of, 1105-1109 seat of, in body, 1063 Oxyacids, 49 origin of, 761 Oxygen, 39 absorption of, in lungs, 1062 ' active,' 1107 avidity of tissues for, 1063 capacity of blood. 1056 effect of changes in tension of, 1099 lack, in asphj^xia, 985 production of lactic acid in, 1085 tension in alveoli. 1062, 1070 in blood, 1071 in blood by CO method, 1074 Oxyhaemoglobin, 821 absorption spectrum of, 823 crystals, 821 dissociation of, 1060 influence of acids on reduction of, 1064 ])-uxyphen}'l alanine, 84 Oxyproliue, 85 Pacinian corpuscles, 497 Pain impulses, path of in cord, 359 sense, 494 Palmitic acid. 54 Pancreas, extii^pation of. 804 internal secretion of, 806 structural changes accompanying secre- tion. 711 Pancreatic diabetes, 804 fistula. 707 juice. 702 action on carbohydrates, 706 fats. 707 milk, 706 proteins, 703 activation of, 705 by calcium salts, 706 composition. 702 secretion of, 707 variations in, 710 secretion.effect of acids in duodenum, 708 regvdation of, 710 secretin, 709 Pangene, 20 1264 INDEX Para position, 50 Paracasein, 1240 Paracentral lobe, 417 Parados cold, 488 Paradoxical contraction, 283 Paraffins, 45 Paragiobulin, 98 Paramoecium, conjugation of, 1201 Paramucin, 105 Paramyosinogen, 98, 213 Paranuclein, 100, 688 Paraplasm, 16 ParathjToids, functions of, 1189 Parietal lobe, 417 Parotid gland, 663 innervation of, 666 Parthenogenesis, 1211 Partition coefficient. 24 Parturition, 1233-1236 nervous mechanism of, 1236 Passive movement, 600 Pawlow's gastric fistula, 690 Pelvic visceral nerve, 473 action on bladder, 1157 Pendular movements of intestine, 725 Pendulum myograph, 195, 196 Pentachromic vision, 580 Pentamethylene diamine, 155 Pentosanes, 61, 106 Pentoses, 61, 105 Pentosuria, 61 Pepsin, action of, 685 Peptone, food value of, 645 Peptones, 100 fractionation of, 685 Peptonised blood, 840, 849 Pericardium, use of, 894, 910 Perilymph, 515 Perimeter, 563 Peripheral ganglia, inhibition in, 475 nerve nets, 469 reflexes, 474 Peripolar zone, 268 Peristalsis, 727 Permeability of membranes, 22, 125, 131 Peroxidases, 1108 Pes, 374 Pette7iJcofer's respiration apjmratus, 618 Pfeffer's cell, 125 Pfiicger's law, 267 Phagocytes, 1022 Phagocytosis, 816, 1022 Phakoscope, 540 Phenylalanine, 85, 91 Phenylethylamine, 156 Phenylglucosazone, 63 Phenylhydrazine test, 123, 162 Phenyllactosazone, 68 Phenylmaltosazone, 67 Phloridzin diabetes, 801 Phloroglucin reaction for pentoses, 01 Phonation, pressure, 523 Phosphates, estimation of, 1129 excretion in large intestine, 724 by kidney, ] 147 in urine, 1] 13 Phosjihatides, 57 Phospholipincs, 57 Phosphoprotein, 100 ' Phosphoprotcins. action ofgastric juice on, 68 Phosphorus, 43, 57 Photochemical substances in retina, 567 Photo-hajmatachometer, 889 Phototaxis, 27 Phrenic nerve, electrical variations in, 242 Phrenology, 434 Physiology, definition of, 1, 7 general, 13 Physiological heat- values, 621 Pick on fractionation of proteoses, 085 Picric acid, 51 Pilomotor nerves, 469 Pineal gland, 1193 Pinna, function of, 511 Piqure diabetes, 800 Pituitary body, extirpation of, 1191 structure of, 1190 extract, effect of, 1192 Placenta, functions of, 1232 Plain muscle, 180, 243 chemical stimulation of, 246 contraction, time-relations of, 243 double innervation of, 247 influence of temperature on, 247 mechanical stimulation of, 246 stimulation of, 244 Plasma, muscle-, 212 blood, 811, 857 Plasmalmut, 21 Plasmolysis, 22, 23, 127 Plasmosome, 16 Plasome, 20 Plasteins, 169 Plastids, 17, 33 Plethora, 1009 Plethysmograph for kidney, 993 Pleura, 1040 permeability of, 136 Pleural cavity, negative pressure in, 1045 Pneumogastric nerve, see Vagus PoliVs reverser, 188 Poikilothermic animals, 1169 Polar bodies, 1209 Polar zone, 268 Polarimeter, 51 Polarisation, 51, 173, 224 by colloids, 145 in nerve, 280 Polarised light, 51 Polarising current, 265 Polymorphonuclear leucocytes, 813 Polymorijhous layer of cortex, 428 Polypeptides, 89, 90, 99, 704 action of trypsin on, 89 reactions of, 89 synthesis of, 88 Polysaccharides, 62, 68 Pons Varolii, 371 antero-lateral tract, 372 functions of, 394 pedal portion, 372 structure, 370 tegmentum, 372 Portal system in birds, 761 Position, sensations of, 603-609 Positive after-images, 571 polarisation of nerve, 282 ventilation, 1091 Posterior columns of cord, 354 crico-arytenoid muscle, 521 longitudinal bundle, 369, 370, 372, 381, 389, 408 INDEX 1265 Posterior root-ganglion, development, 298 Postganglionic fibres, 472 Postural reflexes, 449 tonus, 449 Potassium, 43 Potential energy of compounds, 156 Precipitins, 10o6 Precuneus, 417 Preganglionic fibre, 471 Pregnancy, 1230-1230 Pre-pyramidal tract, 353 Presbyopia, 546 Pressor reflexes, 1001 Pressure impulses in cord, 359 sense. 489 slope in vascular S3'stem, 877 Primary coil, 189 Primitive sheath of nerve, 251 Primordial follicles, 1222 utricle, 14, 127 Principal focus of lens, 531 plane, 531 point, 531, 533 Projection fibres of cerebnim, 422 Projicient sense organs, 293, 384 Proline, 85, 91 fate of, 772 Pro-nuclei, 1209 Proojstrum, 1226 Prophase of mitosis, 1205 Propionic acid, 48, 54 Proprioceptive system, 397, 479, 598 Propriospinal fibres, 351 Pro-sccretin, 709 Prostate, 1221 Prosthetic group, 101 Protamines, 92, 97, 101 Proteins, 45. 71 absorption of, 744 amide nitrogen in, 93 ammonia nitrogen in, 93 amount of nitrogen in, 616 biuret reaction, 93 carbohydrate radicle in, 94 classification of, 97-108 colour reactions of, 93 composition of, 71, 90 conjugated, 101 constitution of, 90 copper compounds, 75 crystallisation of. 72 derivatives of, 98 diamino-nitrogen, 93 disintegration products of, 81 distribution of nitrogen in molecule. 91 effect of. on metabolism, 631, 653 halogen derivatives, 98 heat coagulation of, 72, 95 hj^dratcd, 98 hycbolysis, 76, 99 hydrolysis by enzymes, 76 in urine, 1121 metabolism of, 756-773 effects of variation in diet, ()33 endogenous, 767 Foliiis theorv. 644 in starvation^ 630, 633 Pflugcr's theory, 642 Voit-s theory, 642 minimum requirement, 654 molecule, structure of, 76, 88 Proteins, synthesis of, 88 molecular weight of, 74 osmotic pressure of, 75 oxidation of, 764 physical characters, 72 precipitation bj' metallic salts, 94 putrefaction of, 76 salting out, 96 salts of, 72 separation of, 95 specific dynamic effect of, 636 sulphur in, 74 synthesis of in body, 113-119 tests for, 93 Proteose, food value of, 645 Proteoses, 100 fractionation of, 687 hydrolytic products, 687 Prothrombin, 843 Protocercbrom, 294 Protopathic sensibilitj-, 495 Protoplasm, 15 constituents of, 36 ph5'sical condition of, 20 salts of, 43 surface tension in, 21, 24 theories of structure, 17 ultra-microscopic structure. 20 Proximate constituents of the bod}\ 45 Psalterium, 427 Pseudo-globulin, 866 Pseudo-ious, 149 Pseudomucin, 105 Pseudo-nucleiu, 100. 088 Pseudopodia, 14 Pseudo-reflexes, 474 Psychical secretion of gastric juice. 691 Ptyalin, 601 Puberty, changes at, 1217 Pulmonary circulation, 935-938 ventilation, 1046 Pulse, 918-928 anacrotic wave, 926 catacrotic, 926 clinical features of, 928 dicrotic wave, 921, 923, 925 in veins, 933 percussion wave, 923 pre-dicrotic wave, 923 primarj'- wave, 923 Pulse-pressure, 875 Pulse-rate, in man, 981 Pulse-wave, velocity of, 923 nature of, 918 Pulvinar, 376 Pupil, 543 contraction of. 550 dilatation of. 551 Purine bases, 102 in fa?ces, 754 in urine, 780 metabolism, 774-7S2 ring, 102, 775 Purkinje cells of cerebellum, 400 fibres of heart, 952 figures, 562 Putreseine, 156 Pyloric canal, 698 sphincter, 699 vestibule, 698 Pylorus, nervous mechanism for opening, CO') 1266 INDEX Pylorus, opening of, 697 Pyramidal cells of cortex, 427 decussation, 352, 366 tracts, 352, 366, 423 Pyi'imidine, 103 bases, 775 Pyrogallol, 50 Pyrrol ring, 85 synthesis of, 118 apyrrolidin carboxylic acid, 85 Pyruvic acid, 49, 762 origin of, 762 Qtjadki-tjbates, 1118 Quellung, 151 Racemic compounds, 52 Rami communicantes, 468 Reaction, velocity of, 162, 163 of blood, 987, 1087 of degeneration, 269 time, 457 Reactions, balanced, 166 reversible, 166, 169 Recapitulation, Law of, 13 ' Receptor ' substance, 276 action of adrenaline on, 278, 1184 Recessive characters, 1215 Reciprocal innervation from cortex, 437 of antagonistic muscles, 335 Recti muscles, 585 Red-blindness, 578 corpuscles, see Blood marrow, 834 nucleus, 377, 383 ' Red reflex,' 555 Reduced eye, 533 Referred pain, 476 Reflex action, Bahnung, 305 block in, 305 delay in, 303 facilitation of, 305 fatigue of, 304, 343 general characters of, 303 inhibition of, 306, 339 in spinal animal, 329, 332 localisation, 303 reinforcement, 341 successive spinal induction, 344 summation, 304 arc, 178, 295 movements, 177 scratch-, 332, 343 time, 304 tone, 398 Refractive indices in eye, 533 Refractory period, 273 Eegnault and Reiset's respiration apparatus, 617 Reissner's membrane, 516 Eemak's ganglion, 940, 942 Renal excretion, 1110-1160 mechanism, 1149-1151 Rennin, 688 Reproduction, 1199-1245 in man, 1217-1245 in metazoa, 1202 in protozoa, 1200 Reproductive organs, development of, 1217 female, 1221 Reproductive organs, male, 1219 Residual air, 1047 Resonants, 527 Resonators, 507 Respiration, 1039-1109 apparatus, Benedict's, 617 Haldane's, 616 Pettenlcofer's, 618 Regnault and Reisef, 617 Ztintz and Geppert, 618 chemistry of, 1051-1075 effects of changes in air on, 1098-1 104 effect on circulation, 937 of deglutition on, 681 of division of vagi, 1089 movements, co-ordination of, 1076 rate of, 1041 reflex regulation of, 1089-1097 ' Respiration of swallowing,' 677 Respiratory centre, 1077 automaticity of, 1078 chemical excitants of, 1079-1081, 1087 inhibitory action of vagus on, 1092 spinal, 1078 stimulation of, by acids, 1087 by oxygen lack, 1083 exchange, total, 616 movements, chemical regulation of, 1079-1089 Head's method, 1089 mechanics of, 1039-1050 regulation of, 1076-1097 muscles, 1043 quotient, 640, 1051 effect of foods on, 641 in diabetes, 809 in hibernation, 794 in muscular work, 217 sounds, 1045 Restiform body, 368, 402 Rete mucosum, 1161 Reticulin, 107 Retina, chemical changes in, 564 development of, 558 physical changes in, 564 structure of, 559 Retinal changes in vision, 558-567 image, path of rays, 534 induction, 582 Retractor lentis, 548 penis muscle, 247. 1229 Retrograde degeneration, 320 Reverser, PohVs, 188 Reversibility of ferment action, 166, 168 Revertose, 167, 168 Rheocord, 192 Rheonome, 270 Rheoscopic frog, 233 Rheotaxis, 1228 Rhinccephalon, 421 Rhodopsin, 559, 565 Rhombencephalon, 362 Ribs, movements of, 1042 Ricin, 1031 Rigor mortis, 208, 214 Rima glottidis, 521 Ringer's fluid, 963 INDEX 1267 RUter- Valli law, 274 Kods of Corti, 51(j Rolandic fissure, 419 Roof nuclei of cerebellum, 383, 402 Rubro-spinal tract, 353, 389 Buffird's organs, 497 Saccharic acid, 62 Saccharose, 07 Saccule, 515 Saccus endol3'mpliaticus, 004 Sacral autonomic fibres, 473 Salicylic acid, 50 Saliva, comjiosition of, 601 energy involved in secretion, 074 secretion of, 003 effect of nerves on, GOO Salivary secretion, energy changes in. 074 effect of metabolites, 075 histological changes in, 070 pressure, 6158 theories of, 608 Salivary digestion, 062 in stomach, 603 glands, innervation of. 005 changes accompanying secre- tion, 008-073 double ncrve-suppl}'. 073 electrical changes in, during secretion, 072 Salmin, composition of, 91 Salt hunger, 047 solutions, absorbability of. 738 Salts, absorption of, 753 Saponification, 40, 50 number, 50 Sarcolactic acid, 215 Sarcolcmma, 179 Sarcomere, 179 Sarcoplasm, 179 Sarcosine, 84 Sarcostyles. 179. 181 Sai'cous elements, 181 Sartorius of frog, 1 83 Scala media, 515 Scatol, fate of, 770 Schafers theory of contraction, 182, 236 Schciner's experiment, 589 Schematic eye, 532 Schlcnim, canal of, 543 functions, 596 Schivann's sheath, 251 Scleroproteins. 106 Sclerotic coat of eye, 542 Scratch reflex, 332, 343 Sebaceous glands, 1162 Sebum. 1162 Second wind, 1008 Secondary' contraction, 233 Secretin, TO9. 1179 action on intestinal secretion, 720 liver, 716 Secretion, energy changes in. 674 Semicircular canals, 399, 515 destruction of, 606 functions, 605 Semilunar ganglion, 400 valves, structure of, 893 Semi-permeable cell, 125 membranes, 125, 132 Sensation, extent of stimulus, 4S2 locali.sation of, 481 modality of, 479 physiology of, 478-009 projection of, 481 quantitative study of. 482—485 relation to stimulus. 478-485 Sensations, organic. 598-009 Sensori-motor areas. 444 Sensory adaptation, 482 aphasia, 454 areas of cortex, 444 association. 451 paralysis, .340 path, 357 Septo-marginal bundle, 354 Serine, 81. 91 Serous salivary glands, 663 Sertoli, cells of, 1221 Serum-albumin, 98. 865 composition of, 91 crystal! i.^ation of, 73 Serum-globulin, 98, 860 Serum-proteins, osmotic pressure of, 143 Seventh nerve, nucleus of, 379, 381, 413 visceral fibres. 472 Sex^al organs, relation to ductless glands, 1219 process, essential features of, 1199-1211 Sham feeding, 090 Shock, 300 spinal, 330 Short ciliary nerves. 552 sight, 535 Sibilant consonants. .527 Side-chain theorj', 1034 Side wire, Hehnhol/z, 189 Simultaneous contrast, 583 Sino-auricular node, 951 Sixth nerve, nucleus of. 379, 411 Size, illusions of. 591 judgment of, 590 Skin, absorption by. 1165 functions of, 1101-1 10() gaseous exchanges in, 1105 structure of, 1101 Small intestine, absorption from, 733 innervation of. 728 lymph production in. 735 movements in, 725-729 Smell, sense of, 501 sensations, 498, 502 Smooth muscle, see Plain muscle Soaps, 40, 55 in digestion, 742 Sodium, 43 Solar plexus. 408 Soliditv. judgment of. 593 Sols. 139 Solubilitv of gas, effect of pressure on, 1057 Solute, 131 Somatic nervous S3'stem, 406 Sorbite, 62 Sound, natvne of. 505 jiitch of. 500 timbre. 506 Spaces of Fonlaiia. 543 Spastic gait, 335. 4.50 paraplegia, 335, 450 Specific dvnamic action of proteins, 630, 764! 803 irritability, law of, 255 1268 INDEX Speech, 453, 520-527 intellectual basis of, 455 mechanism of, 525 Spermaceti, 56 Spermatids, 1208 Spermatocytes, 1208 Spermatozoa, development of, 1208 Spherical aberration, 537 Sphincter puiDillse, 550 trigoni, 1153 urogenitalis, 1153 Sphingomyeline, 57 Sphygmographs, 922 Spinal animal, 329, 393 Spinal cord, aiJerent path, 328 anterior cerebellar tract, 354 columns, 356 ascending tracts, 352, 354 Biirdach's column, 324 cells in, 318 of columns, 318 central canal, 300 Clarke's column, 318 collaterals in, 324, 351, 357 comma tract, 353 commissural cells, 318 conduction in, 351-360 ^ crossed pyramidal tract, 353 descending tracts, 352 development, 299 direct cerebellar tract, 354 pyramidal tract, 352 dorsal cerebellar tract, 354 effect of poisons, 347 efferent path, 327 endogenous fibres, 352 Golgi cells, 318 Galls'' column, 324 Gower's tract, 356 Helweg's bundle, 353 hemisection, 359 lateral basis bundle, 371 lateral columns, 354 Lissauer's tract, 324, 352, 357 Mane's tract, 354 MonaJcow's bundle, 353 motor-cells, 318 olivo-spinal tract, 353 pain impulses, 359 path of impulses, 357 posterior columns, 354 postero-external column, 324 jjre-pyramidal tract, 353 pyramidal tracts, 352 as reflex centre, 322 rubro-spinal tract, 353 sensori-motor path, 357 septo-marginal bundle, 354 structure, 315 thalamico-spinal tract, 353 tracts, methods of tracing, 319 trophic functions, 349 ventral cerebellar tract, 354 vestibulo-spinal tract, 353 Wallerian degeneration' 352 white matter, arrangement, 351 Spinal dog, 331 shock, 330 Spino-tectal tract, 356 Spino-thalamic tract, 356 Spiral ganglion, 517 Spiral lamina, 517 Splanchnic nerve, 996 Spleen, function of, 1195 rhythmic contractions of, 1195 structure of, 1194 Spongin, 108 Spongioblasts, 252, 298 Spongioplasm, 18 Staircase phenomenon, 245 in heart muscle, 956 Stannius' ligature, 940 Stapedius muscle, 514 Stapes, 513 Starches, 68 Starch, digestion by saliva, 662 hydrolysis of, 68, 99 molecular weight, 68 soluble, 68 structure of, 69 Starvation, carbohydrate metabolism in, 628 fat metabolism in, 630 loss in various organs, 625 metabolism in, 624-030 protein metabolism in, 630, 633 Static ataxy, 346 Stearic acid, 54 Stearyl-lecithin, 57 Stellate ganglion, 466 Stepping reflex, 332 Stercobilin, origin of, 715 Stereoisomerism, 52, 60, 78 Stereosomers, action of ferments on, 167 Stereoscope, 593 Stereoscopic vision, 593 Sternzellen, 816 Stimulation, electrical, nature of, 270 of human nerve, 268 Stimuli, summation of, 245 Stimulus, intensity of, 482 liminal, 482 locus of, 339 maximal, 193, 205 minimal, 193, 205 threshold, 483 Stokes -Adams disease, 955 Stomach, digestion in, 683-696 innervation, 700 movements of, 697 secretory nerves of, 691 sphincters, 701 Stratum granulosum, 1161 Stratum lueidum, 1161 Stria terminalis, 377 Striae acousticse, 379, 380 Striated muscle, structure of, 179 of invertebrates, 248 String galvanometer, 228 Siromuhr, Ludwig's, 888 Starling's, 888 Structural basis of body, 13 Strychnine, effect on cord, 347 spasm, electrical variation, 242 Sturin, composition of, 91 Sublingual gland, 663 Submaxillary ganglion, 473, 666 gland, 663 Substantia gelatinosa of Rolando, 316 nigra, 374, 377, 383 Substrate, 164 effect of ferments on, 167 m INDEX 1269 Subthalamic region, 377 Successive contrast, 5815 spinal induction, 344 iSuccus entcricus, .see Intestinal juici Sugar in urine. 11 22 formation of, from fat, 794 from protein. 112^ in plants. Ill utilisation of. in body, 790 Sugars, chemistry of. (KMiS oxidisability of. in body. I KXi Sulphocyanate in saliva. (W\ Sulphur, 42 excretion of. 7H9 test for jjrotcins, Sfi. 94 Summation. 245 of muscular conti'actions. 20(> of sensation, 482 of stimuli. 245. 272 in heart muscle. 95() tones, 510 •Supplemental air. 1046 Suprarenal bodies. J181-llS(i Surface tension. 21. 24 in colloids. 141 Suspensory ligament. 543 Swallowing — -sfi' 1 )egliitition Sweat, amount and pro])erties. Il(i3 glands. 1 103 loss of heat by. 1 1 7 functions. 474 svstem. 969. 975 T.\CHYl'N(E.\, 1080 Tactile localisation. 489 sensations. 489 sense area. 444 paths. 444 Tiilbot'" law. 572 Tartaric acid. 52 Taste, 498 area, 448 buds. 499 nerves of. 500 sensation, classification. 499 Tears, comi)osition of, 594 Tecto-spinal tract. 391 Tenon, capsule of, 585 Tension, efieet of on bliidder. 1 1 ~Ai on lii^art. 958 on muscle. 201 'I'ensions of gases in liquids. 1059 Tensor tymi)ani muscle. 514 Tenth nerve, nucleus of. 414 Testis, structure of, 1219 Tetanus. 207 toxin, action on cord, 348 Tetrachromic vision. 580 Tetrapeptidcs. 89 Tegmentum of jions. 372 of mid-brain. 374 Tcichnidnn'-i crystals. 825 Telencephalon, 363 Telophase of mitosis, 1206 T(Mnperature. action of, on heart, 962 on muscle, 207 effect of, on metabolism, 1168, 1172 on electrical variations in mu.scle, 230 limits of, for life, 6 sense, 486 adaptation of, 488 Temi)oral lobe. 417 Temporo-iJontine fibres, 424 Tendon phenomena, 333 use of, 336 reflexes. 333 Thalamenccphalon. 392, 395 Thalamico-spinal tract, 353 Thalamo-cortical tract, 422 Thalamo-frontal fibres, 423 Thalamo-spinal tract, 391 Theobromine, 103, 775 Thermo-electric couple, 220 Thermogenic centres. 1177 Thermopile, 219 Thermotaxic system, 1 1 77 Thigmotaxis, 27 Thiophenc test. 215 Third nei-ve. functions of, 552 nucleus of. 379. 382. 4(i.S. 41u visceral fibres in. 472 Tltiii/- Valid fistula, 718 Threshold .stimulus, 483 Thrombin. 842 Thrombogen. 844 Thrombokinasc. 844 Thymine. 104. 776 Thymus, structure and functions of. 1193 Thyroid gland. 1186 extirpation of. 1 187 extract, efifectsof. 1188 structure of, 1186 Tidal air, 1046 Tissue-fibrinogen. 102. 846 Tissue-nroteins. 642 Tone in muscle. 333 in heart, 958 Tonus, postural, 449 Tortoise heart. *^10 Touch discrmination. 491 impulses in cord, 359 projection of. 493 sense. 489 spots. 489 Toxins, adsor])tion of, 1(*34 bacterial. 10.30 mode of action. 10.30 Toxoids. 1034 To.xones. 1032 Toxophore group. 1034 Tmnbc-Hnlnii curves. 988 Trapezium of pons. 371. 381. 3S7 Triclu'omats, anomalous. 581 Trichromatic vision. 57s 'i'ricuspid valve. 893 Trigeminal nerve, nucleus of. 38(t Trigemino-thalamic tract. 387 Trimcthylamine. 49 Tripeptides. 89 Trii>l«- phosphate. 1114. 1125 Tritocerebrom. 294 I'rwnvirr'-t test, (i2 Trophoblast, 1231 1270 INDEX Trypsin, 703 action on polypeptides, 704 on proteins. 703 velocity of action, 166. 705 Trypsinogen. 703 Tryptophane. 85. 91. 94 fate of, 772 Tuber cinereiim. 363. 375 Tympanic membrane, 512 movements of. 513 Tympanum, 512 Tyrosinase. 1108 Tyrosine, 50, 84, 91 in urine. 1125 Uffehnan7Vs reagent, 215 Ultra-microscope, 145 Umbilical cord, 1232 Uncinate fasciculus, 426 Uncus, 419 Unipolar excitation, 269 law of, 269 Unstriated muscle, 243 propagation of wave in, 246 Uracil, 104, 776 Urate deposit, 1124 Urates, 1118 Urea, 1115 estimation, 1126 Folin's method, 1127 hypobromite method, 1126 fermentation of, 1115 origin of, 759 output in starvation, 630 on protein diet, 758 preparation from urine, 1117 Ureter, contractions of. 1152 Uric acid, 103, 1117 crystals, 1125 daily amount, 1119 endogenous, 780 estimation, 1128 excretion, 779 in gout, 781 origin of. 777 preparation, 1117 production in birds, 760 structure of, 775 synthesis, 103 tests, 1118 Uricolytic ferment, 779 Urinary constituents, estimation of, 1126-- 1130 deposits, 1124 Urine, abnormal constituents in, 1121-1125 acetone in, 1123 ammonia in, 1115 average composition. 1112 bases of, 1114 chlorides of, 1112 composition of, 1110-1130, 1142 on various diets, 780 conditions of glomerular filtration, 1138 freezing-point. 1111 inorganic constituents of, 1112 lactose in, 1123 neutral sulphur in, 1113 organic constituents of,(1115 oxyacids in, 11231 phosphates of, 1113 Urine, pigments of, 1120 pressure of, in ureter, 1137 reaction of, 1111 secretion of, 1131-1151 in glomerulus, 1135 influence of colloids, 1137 kidney volume, 1139 specific gravity of, 1111 sugar in, 1122 sulphates of, 1113 Uriniferous tubule, course of, 1131 tubule, functions of, 1134 Urobilin, 1121 Urochrome, 1120 Uroerythrin, 1121 Urorosein, 1121 Urotropine, 1127 Uterus, changes in, during pregnancy, 1 230 nerve-supply, 1228 Utricle, 515 primordial, 14, 127 Uvea, 561 Vacuole, contractile, 15 Vago-glossopharyngeal nucleus, 379, 414 Vagus, 473 action on auricles, 972 on heart, 971 on intestines, 729 on oesophagus, 682 on respiration, 1089 on stomach, 701 distribution in abdomen, 700 nucleus of, 414 proof of inspiratory fibres in, 1092 tonic action on heart, 975 Valerianic acid, 54 Valine, 81 Valve of Vieussens, 362, 371 Valves of heart, see Heart Variation, monophasic, 230 Vasa afferentia. 1133 efferentia, 1133 recta, 1133 Vascular area of chick, 831 system, influence of capacity of, on circulation, 880 tone, effect of central nervous svstom on. 982 peripheral, 990 Vaso-constrictor fibres, course of, 994 Vaso-dilatation, criteria of, 991 Vaso-dilatation by metabolites, 1002 Vaso-dilator fibres in nerve-trunks, 997 nerves, 996 Vaso-motor centre, action of acids on, 989 location of. 983 variations in activity of. 983 centres in cord, 989 impulses, path of, in cord, 300 nerves, course of, 990 reflexes, 1000 Vegetable food, utilisation of, 649 Veins, blood-flow in, 932-934 capacity of, 883 distensibility of, 871 structure oi\ 870 valves in, 933 Velocity of reaction, 162, 103 in vascular system, 879 Venous pressure in man, 877 INDEX 1271 Venous outflow, determination of, 994 pulse, 933 iu heart block, 955 Ventilation, 1104 Ventral cerebellar tract, 354 Ventricles, capacity of, 891 pressure in, 890 Veratrine, action of, on muscle, 2\ 1 Vesicular murmur, 1045 Vestibular nerve. Inciters' nucleus, 381, 402 functions of, 398 nucleus of, 380, 413 Vestibule, 515, 604 Vestibulo-cerelx'llar fibres, 370, 389 Vestibulo-apinal tract, 353, 391 Vibrative consonants, 527 Vtcq (VAzyr's bundle, 388, 421 Villus, changes in, during protein absorption, 745 Villus, structure of, 734 Viscera, innervation of. 473 sensibility of, 494 Visceral nervous system, 400—477 Vision, 528-597 Visual adaptation. 572 angle, 534. 589 axis, 561 centre, 447 fatigue, 572 judgments, 589-593 localisation, 589 path, 387, 406, 447 purple, 559, 565 reflexes, 406-409 sensation, Weber's law, 570 sensations, 568-584 stimuli, time relations, 571 Visuo-psj'chic area, lamination, 431 Visuo-sensory area, lamination, 431 Vital capacity, 1046 Vitalism, 8 Vitamines, 652 Vitellins, 100 Vitreous humour, 532 Vocal cords, 521 Voice, 520-527 VolhanVs method for chlorides, 1129 Voluntary contraction, 239 electrical changes in, 242 muscle, 177 Vowel sounda, 525 Wallerian" degeneration. 36i- method, 320 i Warm points, 486 Water, absorption of, 733 estimation in food, 014 neces.sity for, 6 and dissolved substances, passage across membranes, 131-138 rigor, 231, 945 Weber's law, 485, 491 in vision, 570 Wernicke's aphasia, 454 WeyVs test, 1117 White rami. 468 sensation of, 576 Word- blindness, 455 Work, relation to stimulus, 26 Xanthine, 103, 778 Xanthoproteic reaction, 93 Xylose, 61, 104 Yeasts, action on amines, 77 on carbohydrates, 64, 65, 67 Yellow spot, 561 Young-Helmholtz theory, 577 Zein, 98 food-value of, 646 Zincative, 229 Zollner's lines, 592 Zona fascicidata, 1181 glomerulosa, 1181 pellucida, 1222 reticulata, 1181 Zonula ciliaris, 543 Zonule of Zinn, 543 Zooid, 818 Zymins, 671 Zymogen granules, 671 HALr-AxrvxK, HA^•^!0^' &> co ltd. 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