BIOLOGY UBffARY AMERICAN SCIENCE SERIES— ADVANCED COURSE THE HUMAN BODY AN ACCOUNT OF ITS STRUCTURE AND ACTIVITIES AND THE CONDITIONS OF ITS HEALTHY WORKING BY H. NEWELL MARTIN, D.Sc., M.A., M.D., F.R.S. Professor of Biology in tlie Johns Hopkins University ; Fellow of University College, London; Late Fellow of Christ's College, Cambridge SIXTH EDITION. REVISED NEW YORK HENRY HOLT AND COMPANY 1890 BIOLOGY LIBRARY G Copyright, 1881. BY HOLT & Co. PREFACE TO THE SIXTH EDITION. THIS edition has been carefully revised, and many of the figures have been redrawn and improved. I owe special thanks to Professor W. Le Conte Stevens of the Packer Collegiate Institute, Brooklyn, N. Y.; and to Professor Theodore Sterling of Kenyon College,, Ohio, for calling my attention to errors printed in earlier editions: and I repeat the statement I made in the preface to the fifth edition, that a "text-book is a co-operative work, and sure to be better the more that teachers and students co-operate with the writer, or compiler, to make it what it should be." H. NEWELL MARTIN. June 11, 1890. 218913 PREFACE. IN the following pages I have endeavored to give an account of the structure and activities of the Human Body, which, while intelligible to the general reader, shall be accurate, and sufficiently minute in details to meet the requirements of students who are not making Human Anatomy and Physiology subjects of special advanced study. Wherever it seemed to me really profitable, hygienic topics have also been discussed, though at first glance they may seem less fully treated of than in many School or College Text-books of Physiology. Whoever will take the trouble, however, to examine critically what passes for Hygiene in the majority of such cases, will I think find that, when correct, much of it is platitude or truism: since there is so much that is of importance and interest to be said it seems hardly worth while to occupy space with insisting on the commonplace or obvious. It is hard to write a book, not designed for specialists, without running the risk of being accused of dogmatism, and some readers will, no doubt, be inclined to think that, in several instances, I have treated as established facts matters which are still open to discussion. General readers and students are, however, only bewildered by the production of an array of observations and arguments on each side of every question, and, in the majority of cases, the chief responsi- bility under which the author of a text-book lies is to select what seem to him the best supported views, and then to state them simply and concisely: how wise the choice of a side has been in each case can only be determined by the discoveries of the future. Others will, I am inclined to think, raise the contrary PREFACE. v objection, that too many disputed matters have been dis- cussed : this was deliberately done as the result of an experi- ence in teaching Physiology which now extends over more than ten years. It would have been comparatively easy to slip over things still uncertain and subjects as yet unin- vestigated, and to represent our knowledge of the workings of the animal body as neatly rounded off at all its contours and complete in all its details — totus, teres, et rotundus. But by so doing no adequate idea of the present state of physiological science would have been conveyed; in many directions it is much farther travelled and more completely known than in others; and, as ever, exactly the most in- teresting points are those which lie on the boundary between what we know and what we hope to know. In gross Anatomy there are now but few points calling for a suspension of judgment; with respect to Micro- scopic Anatomy there are more; but a treatise on Physiology which would pass by, unmentioned, all things not known hut sought, would convey an utterly unfaithful and untrue idea. Physiology has not finished its course. It is not cut and dried, and ready to be laid aside for reference like a specimen in an Herbarium, but is comparable rather to a living, growing plant, with some stout and useful branches well raised into the light, others but part grown, and many still represented by unfolded buds. To the teacher, moreover, no pupil is more discouraging than the one who thinks there is nothing to learn; and the boy who has "finished" Latin and " done" Geometry finds sometimes his counterpart in the lad who has " gone through" Physiology. For this unfortunate state of mind many Text-books are, I believe, much to blame: difficulties are too often ignored, or opening vistas of knowledge resolutely kept out of view: tho forbidden regions may be, it is true, too rough for the young student to be guided through, or as yet pathless for the pioneers of thought; but the opportunity to arouse the re- ceptive mental attitude apt to be produced by the recogni- tion of the fact that much more still remains to be learned — to excite the exercise of the reasoning faculties upon dis- Duted matters — and. in some of the better minds, to arouse vi PREFACE. the longing to assist in adding to knowledge, is an inesti- mable advantage, not to be lightly thrown aside through the desire to make an elegantly symmetrical book. While I trust, therefore, that this volume contains all the more important facts at present known about the working of our Bodies, I as earnestly hope that it makes plain that very much is yet to be discovered. A work of the scope of the present volume is, of course, not the proper medium for the publication of novel facts; but, while the "Human Body," accordingly, professes to- be merely a compilation, the introduction of constant ref- erences to authorities would have been out of place. I trust, however, that it will be found throughout imbued with the influence of my beloved master, Michael Foster; and on various hygienic topics I have to acknowledge a special indebtedness to the excellent series entitled Health Primers. The majority of the anatomical illustrations are from Henle's Anatomie des Menschen, and a few from Arendt's Schulatlas, the publishers of each furnishing electrotypes. A considerable number, mainly histological, are from Quain's Anatomy, and a few figures are after Bernstein, Carpenter, Frey, Haeckel, Helmholtz, Huxley, McKen- drick, and Wundt. About thirty, chiefly diagrammatic, were drawn specially for the work. Quantities are throughout expressed first on the metric system, their approximate equivalents in American weights and measures being added in brackets. H. NEWELL MAETIN. BALTIMORE, October, 1880. CONTENTS. CHAPTER I. THE GENERAL STRUCTURE AND COMPOSITION OF THE HUMAN BODY. PAGE Definitions. Tissues and organs. Histology. Zoological posi- tion of man. The vertebrate plan of structure. The mam- malia. Chemical composition of the Body 1 CHAPTER II. THE - FUNDAMENTAL PHYSIOLOGICAL ACTIONS. The properties of the living Body. Physiological properties. Cells. Cell growth. Cell division. Assimilation and repro duction. Contractility. "Irritability. Conductivity. Spon- taneity. Protoplasm. The fundamental physiological proper- ties.. 15 CHAPTER III. THE DIFFERENTIATION OF THE TISSUES, AND THE PHYSI- . OLOGICAL DIVISION OF EMPLOYMENTS. Development. The physiological division of labor. Classifica- tion, of the tissues. Undifferentiated tissues. Supporting tissues. Nutritive tissues. Storage tissues. Irritable tissues. Co-ordinating and automatic tissues. Motor tissues. Conduc- tive tissues. Protective tissues. Reproductive tissues. Or- gans. Physiological mechanisms. Anatomical systems. The Body as a working whole VU1 CONTENTS. CHAPTER IV. THE INTERNAL MEDIUM. PAGE The external medium. The internal medium. The blood. The lymph. Histology of blood. Blood crystals. Histology of lymph 39 CPIAPTER V. THE CLOTTING OF THE BLOOD. The coagulation of blood. Causes of coagulation. "Whipped blood. The buffy coat. Uses of coagulation. The fibrin factors. Artificial clot. The fibrin ferment. Exciting causes of coagulation. Relation of blood-vessels to coagulation. Composition of the blood. Quantity of blood. Origin and fate of the blood corpuscles. Chemistry of lymph 50 CHAPTER VI. THE SKELETON. Exoskeleton and endoskeleton. The bony skeleton. Segmenta- tion of the skeleton. Peculiarities of the human bony skele- ton 62 CHAPTER VII. THE STRUCTURE AND COMPOSITION OF BONE. JOINTS. Gross structure of the bones. Microscopic structure of bone. Chemical composition of bone. Hygiene of the bony skeleton. Articulations. Joints. Hygiene of the joints 86 CHAPTER VIII. CARTILAGE AND CONNECTIVE TISSUE. Temporary and permanent cartilages. Varieties of cartilage. The connective tissues. Elastic cartilage and fibro-cartilage. CONTENTS. ix PAGE Homologies of the supporting tissues. Hygiene of the develop- ing skeleton. Adipose tissue 100 CHAPTER IX. THE STRUCTURE OF THE MOTOR ORGANS. Motion in animals and plants. Amoeboid cells. Ciliated cells. >• The muscles. Histology of striped muscle. Unstriped mus- cles. Cardiac muscular tissue. The chemistry of muscular tissue. Beef -tea and Liebig's extract. . .113 CHAPTER X. . THE PROPERTIES OP MUSCULAR TISSUE. Contractility, irritability of muscle. A simple muscular con- traction. Tetanus. Causes influencing the degree of muscu- lar contraction. The measurement of muscular work. Mus- cular elasticity. Physiology of plain muscular tissue. Hy- giene of the muscles. Exercise 128 CHAPTER XL MOTION AND LOCOMOTION. The special physiology of muscles. Levers in the Body. The erect posture. Walking. Running. Leaping 143 CHAPTER XII. THE ANATOMY OP THE NERVOUS SYSTEM. Nerve-trunks. Plexuses. Nerve-centres. The cerebro-spinat centre and its membranes. The spinal cord. The spinal nerves. The brain. The cranial nerves. Ganglia and com- munications of the cranial nerves. The sympathetic system. The sporadic ganglia. The histology of nerve-fibres. The histology of nerve-cells. The structure of the spinal cord 154 CONTENTS. CHAPTER XIII. THE GENERAL PHYSIOLOGY OP THE NERVOUS SYSTEM The properties of nerve tissues. The functions of nerve-centres and nerve-trunks. Excitant and inhibitory nerves. The clas- sification of nerve-fibres. Intercentral nerve-fibres. The stimuli of nerves. General and special nerve stimuli. Specific nerve energies. All nerve-fibres are fundamentally alike. The nature of a nervous impulse. The rate of transmission of a nervous impulse. Functions of the spinal nerve-roots. The intercommunication of nerve-centres ............. . ......... 180 CHAPTER XIV. THE ANATOMY OP THE HEART AND BLOOD-VESSELS. General statement. Position of the heart. The membranes of the heart. The cavities of the heart. The heart as viewed from the outside. The interior of the heart. The valves of the heart. The arterial system. The capillaries. The veins. The pulmonary circulation. The course of the blood. The portal circulation. Arterial and venous blood. The structure of arteries, capillaries, and veins. . . ......................... 201 CHAPTER XV. THE WORKING OP THE HEART AND BLOOD-VESSELS. The beat of the heart. The cardiac impulse. Events in a cardiac period. Use of the papillary muscles. Sounds of the heart. Function of the auricles. The work done by the heart. The blood flow outside the heart. The circulation as seen with the microscope. Internal friction in the vessels. The conver- sion of an intermittent into a continuous flow CHAPTER XVI. ARTERIAL PRESSURE AND THE PULSE. Weber's schema. Arterial pressure. Modifications of arteriaf pressure, and how they may be produced. The pulse. The CONTENTS. xi PAGE rate of the blood- flow. Secondary causes of the circulation, Proofsof the circulation of the blood 233 CHAPTER XVII. THE REGULATION OF THE HEART AND BLOOD-VESSELS BY THE NERVOUS SYSTEM. The need of co-ordination in the vascular system. The intrinsic nerves of the heart. Nerves showing the heart's beat. The cardio-inhibitory mechanism. The accelerator nerves of the heart. The nerves of the blood-vessels. The vaso-motor centre. Taking; cold. Vaso-dilator nerves 247 CHAPTER XVIII. THE SECRETORY TISSUES AND ORGANS. Definition. Organs of secretion. Glands. Physical processes in secretion. Chemical processes in secretion. The mode of activity of secretory cells. Influence of the nervous system upon secretion. Summary 259 CHAPTER XIX. THE INCOME AND EXPENDITURE C;F THE BODY. The material daily losses of the Body. The daily losses of the Body in energy. The conservation of energy. Potential and kinetic energy. The energy of chemical affinity. The relation between matter lost by the Body daily and the energy spent by it. The conditions of oxidation in the living Body. The fuel of the Body. Oxidation by successive steps. The utilization of energy in the living Body. Summary 277 CHAPTER XX. FOODS. Foods as tissue-formers. The food of plants. Non-oxidizable foods. Definition of foods. Conditions which a food must xii CONTENTS. fulfill. Alimentary principles. The composition of the more important foods. Cooking. Alcohol. The advantage of a mixed diet. 293 CHAPTER XXI. THE ALIMENTARY CANAL AND ITS APPENDAGES. General arrangement. The teeth. The tongue. The salivary glands. The fauces. The pharynx. The gullet. The stomach. The histology of the gastric glands. The small intestine. The large intestine, The liver. The pancreas. ... 308 CHAPTER XXII. THE LYMPHATIC SYSTEM AND THE DUCTLESS GLANDS. Distribution and structure of lymph vessels. The thoracic duct. The serous cavities. The lymphatic glands. The movement of the lymph. The spleen. The thymus. The thyroid body. The supra-renal capsules 329 CHAPTER XXIII. DIGESTION. The object of digestion. Uses of saliva. Deglutition. The gastric juice. Gastric digestion. The chyle. The pancreatic secretion. The bile. The intestinal secretions. Intestinal di- gestion. Absorption from the intestines. The digestion of an ordinary meal. Dyspepsia < 334 CHAPTER XXIV. THE RESPIRATORY MECHANISM. Definitions. Respiratory organs. The air-passages and lungs. The pleura. The respiratory movements. The structure of the thorax. The mechanism of inspiration. Expiration. Forced respiration. The respiratory sounds. The capacity of the lungs. Hygiene of respiration. The aspiration of the thorax. Influence of respiratory movements upon the flow of blood and lymph 352 CONTEN1S. xiii CHAPTER XXV. THE CHEMISTRY OF RESPIRATION. , PAGE Nature of the problems. Changes produced in air by being once breathed. Ventilation. Changes undergone by blood in the lungs. The blood gases. Causes of color of arterial and venous blood. Laws governing the absorption of gases by liquids. The absorption of oxygen by the blood. Conse quences of the way in which oxygen is held in the blood. The general oxygen interchanges of the blood. The carbon dioxide of the blood. Internal respiration 372 CHAPTER XXVI. THE NERVOUS FACTORS OF THE RESPIRATORY MECHANISM. ASPHYXIA. The respiratory centre. Is the respiratory centre reflex? The stimulus of the respiratory centre. The cause of the respira- tory rhythm. The influence of the pneumogastric nerves upon the respiratory centre. The expiratory centre. Asphyxia. Carbon • monoxide haemoglobin. Modified respiratory move- ments 390 CHAPTER XXVII. THE KIDNEYS AND THE SKIN. The general arrangement of the urinary organs. The structure of the kidneys. The renal secretion. The mechanism of the renal secretion. The function of the renal epithelium. The skin. Epidermis and cutis vera. Hairs. Nails. Glands of the skin. Relation of nerves to sweat secretion. Hygiene of the skin. Bathing 404 CHAPTER XXVIII. NUTRITION. The problems of animal nutrition. The seat of the oxidations of the Body. Tissue-building and energy-yielding foods. The source of the energy spent in muscular work. Luxus con- sumption. The antecedents of urea. Proteid starvation and IV CONTENTS. PAGE over-feeding. The storage tissues. Glycogen. Diabetes. The history of fats. Dietetics 423 CHAPTER XXIX. THE PRODUCTION AND REGULATION OF THE HEAT OF THE BODY. Cold- and warm-blooded animals. The temperature of the Body. The sources of animal heat. Energy lost by the Body in twenty-four hours. The superiority of the Body as a machine for executing mechanical work. The maintenance of an average temperature. Local temperatures. Thermic nerves. Clothing 449 CHAPTER XXX. SENSATION AND SENSE-ORGANS, The subjective functions of the nervous system. Common sen- sation and organs of special sense. The peripheral reference of sensations. Differences between sensations. The essential structure of a, sense organ. The cause of the modality of sensa- tions. The psycho-physical law. Perceptions. Sensory illu- sions 461 CHAPTER XXXI. THE EYE AS AN OPTICAL INSTUMENT. The essential structure of an eye. The appendages of the eye. The lachrymal apparatus. The muscles of the eyeball. The anatomy of the eyeball. The structure of the retina. The refracting media of the eye. The properties of light. Accom- modation. Short sight and long sight. Hygiene of sight. Optical defects of the eye 479 CHAPTER XXXII. sy THE EYE AS A SENSORY APPARATUS. The excitation of the visual apparatus. Idio-retinal light. The parts of the retina on which light directly acts. The vision purple. Intensity of visual sensations. Duration of luminous sensations. The localizing power of the retina. Color vision. CONTENTS. xv Color blindness. Fatigue of the retina and after-images. Con trasts. Hering's theory of vision. Visual perceptions. Single vision with two eyes - . 506 CHAPTER XXXIII. THE EAR AND HEARING. The external ear. The tympanum. The auditory ossicles. The internal ear. The organ of Corti. The loudness, pitch, and timbre of sounds. Sympathetic resonance. Functions of the tympanic membrane, of the auditory ossicles, of the cochlea, and of the vestibule. Auditory perceptions 535 CHAPTER XXXIV. TOUCH, THE TEMPERATURE SENSE, THE MUSCULAR SENSE, COMMON SENSATION, SMELL, AND TASTE. Nerve-endings in the skin. The pressure sense. The localizing power of the skin. The temperature sense. Modality of skin sensations. The muscular sense. Pain. Hunger and thirst. Smell. Taste.. 556 CHAPTER XXXV. THE FUNCTIONS OF THE BRAIN AND SPINAL CORD. The special physiology of nerve-centres. The% spinal cord as a centre. Reflex actions. The least-resistance hypothesis. The education of the spinal cord. The inhibition of reflexes. Psy- chical activities of the cord. The cord as a conductor of nervous impulses. Functions of the brain in general. Func- tions of medulla oblongata. Functions of cerebellum, pons Varolii, and mid-brain. Sensations of equilibrium and func- tions of semicircular canals. Functions of the fore-brain. Hygiene of the brain * 573 CHAPTER XXXVI. VOICE AND SPEE.CH. Anatom^ of larynx. The vocal cords. Causes of the varying pitch of the voice. Range of the human voice. The' produc- tion of vowels. Consonants.. 595 XVi CONTENTS. APPENDIX. REPRODUCTION AND DEVELOPMENT. Reproduction in general. Sexual and asexual reproduction. Male reproductive organs. Female reproductive organs. Puberty, Ovulation. Menstruation. Hygiene of menstruation. Impregna- tion. Pregnancy. The foetal appendages. The intra-uteriue nutrition of the embryo. Parturition. Lactation. Feeding of infants. The stages of life. Death. THE HUMAN BODY CHAPTER I. THE GENERAL STRUCTURE AND COMPOSITION OF THE HUMAN BODY. Definitions. The living human Body may be considered from either of two aspects. Its structure may be especial- ly examined, and the forms, connections and mode of growth of its parts be studied, as also the resemblances or differences in such respects which appear when it is com- pared with other animal bodies. Or the living Body may be more especially studied as an organism presenting defi- nite properties and performing certain actions; and then its parts will be investigated with a view to discovering what duty, if any, each fulfills. The former group of studies- constitutes -the science of Anatomy, and in so far as it deals- with the Human Body alone, of IJuman Anatomy; while the latter, the science concerned with the uses — or in tech- nical language the functions — of each part is known as Pliysiology. Closely connected with physiology is the science of Hygiene, which is concerned with the conditions which are favorable to the healthy action of the various parts of the Body ; while the activities and structure of the diseased body form the subject-matters of the sciences of Pathology and Pathological Anatomy. Tissues and Organs. Histology. Examined merely from the outside our Bodies present a considerable com- plexity of structure. We easily recognize distinct parts as head, neck, trunk and limbs; and in these again smaller 2 THE HUMAN BODY. constituent parts, as eyes, nose, ears, mouth; arm, fore- arm, hand; thigh, leg and foot. We can, with such an external examination, go even farther and recognize different materials as entering into the formation of the larger parts. Skin, hair, nails and teeth are obviously different substances ; simple examination by pressure proves that internally there are harder and softer solid parts; while the blood that flows from a cut finger shows that liquid constituents also exist in the Body. The con- ception of complexity which may be thus arrived at from external observation of the living, is greatly extended bv dissection of the dead Body, which makes manifest that it consists of a great number of diverse parts or organs, whict in turn are built up of a limited number of materials; the same material often entering into the composition of many different organs. These primary building materials are known as the tissues, and that branch of anatomy which deals with the characters of the tissues and their arrange- ment in various organs is known as Histology; or, since ifc is mainly carried on with the aid of the microscope, as Microscopic Anatomy. If, with the poet, we compare the Body to a house, we may go on to liken the tissues to the bricks, stone, mortar, wood, iron, glass and so on, used in building; and then walls and floors, stairs and windows, formed by the combination of these, would answer to ana- tomical organs. Zoological Position of Man. External examination of the human Body shows also that it presents certain re- semblances to the bodies of many other animals: head and neck, trunk and limbs, and various minor parts enter- ing into them, are not at all peculiar to it. Closer study and the investigation of internal structure demonstrates further that these resemblances are in many cases not su- perficial only, but that our Bodies may be regarded as built upon a plan common to them and the bodies of many other creatures: and it soon becomes further apparent that this resemblance is greater between the Human Body and the bodies of ordinary four-footed beasts, than between it and the bodies of birds, reptiles or fishes. Hence, from THE PRIMATES. 3 a zoological point of view, man's Body marks him out as belonging to the group of Mammalia (see Zoology), which includes all animals in which the female suckles the young: .and among mammals the anatomical resemblances are closer and the differences less between man and certain apes than between man and the other mammals; so that zoologists still, with Linnaeus, include man with the mon- keys and apes in one subdivision of the Mammalia, known as the Primates. That civilized man is mentally far superior to any other animal is no valid objection to such a classifi- cation, for zoological groups are defined by anatomical and not by physiological characters ; and mental traits, since we know that their manifestation depends upon the structural integrity of certain organs, are essentially phenomena of function and therefore not available for purposes of zo- ological arrangement. Man however walks erect with the head upward, while the great majority of Mammals go on all fours with the head forward and the back upward, and various apes adopt intermediate positions; so in considering corre- sponding parts in such cases confusion is apt to arise unless a precise meaning be given to such terms as "an- terior" and "posterior." To avoid this difficulty anato- mists give those words definite arbitrary significations in all cases and these we shall use in future. The head end is always anterior whatever the natural position of the .animal, and the opposite end posterior; the belly side is spoken of as ventral, and the opposite side as dorsal; right ..and left of course present no difficulty. Moreover, that end of a limb nearer the trunk is spoken of as proximal with reference to the other or distal end. The words upper and lower may be conveniently used for the relative position of parts in the natural standing position of the animal. The Vertebrate Plan of Structure. Neglecting such merely apparent differences as arise from the differences of normal posture above pointed out, we find that man's own zoological class, the Mammals, differs very widely in its broad structural plan from the groups including sea anem- 4 THE HUMAN BODY. ones, insects, or oysters, but agrees in many points with the groups of fishes, amphibians, reptiles, and birds. These four are therefore placed with man and all other Mammals in one great division of the animal kingdom known as the Vertebrata. The main anatomical character of all vertebrate animals is the presence in the trunk of the body of two cavities, a dorsal and a ventral, separated by a solid partition, and in the adults of nearly all vertebrate animals a hard axis, the vertebral column (backbone or spine), develops in this partition and forms a central support for the rest of the body (Fig. 2,ee). The dorsal cavity is con- tinued through the neck, when there is one, into the head, and there widens out. The bony axis is also continued through the neck and extends into the head in a modified form. The ventral cavity, on the other hand, is confined to the trunk. It contains the main organs connected with the blood-flow and is thus often called the haemal cavity. Upon the ventral side of the head is the mouth open- ing leading into a tube, the alimentary canal, f, which passes back through the neck and trunk and opens again on the outside at the posterior part of the latter. In its passage through the trunk region this canal lies in the- ventral cavity. The Mammalia. In many vertebrate animals the ven- tral cavity is not subdivided, but in the Mammalia it is; a membranous transverse partition, the midriff or diaphragm (Fig. 1, z), separating it into an anterior chest or thoracic cavity, and a posterior or abdominal cavity. The alimen- tary canal and whatever else passes from one of these cavi- ties to the other must therefore perforate the diaphragm. In the chest, besides part of the alimentary canal, lie important organs, the heart, h, and lungs, lu ; the heart being on the ventral side of the alimentary canal. The abdominal cavity is mainly occupied by the alimentary canal and organs connected with it and concerned in the digestion of food, as the stomach, ma, the liver, le, the pancreas and the intestines. Among the other more prom- •nent organs in it are the kidneys and the spleen. In the dorsal or neural cavity lie the brain and spinal TEE MAMMALIAN TYPE. 5 cord, the former occupying its anterior enlargement in the head. Brain and spinal cord together form the cerebro- spinal nervous centre; in addition to this there are found in the ventral cavity a number of small nerve centres united FIG. 1.— The Body opened from the front to show the contents of its ventral cavity. Zit, lungs; /i, heart, partly covered by other things; le, le', right and l^ft liver 'lobes respectively; ma, stomach; ne, the great omentum, a membrane con- taining fat which hangs down from the posterior border of the stomach and covers the intestines. together by connecting cords, and with their offshoots form- ing the sympathetic nervous system. The walls of the three main cavities are lined by smooth, THE HUMAN BODY. moist serous membranes. That lining the dorsal cavity is the arachnoid ; that lining the chest the pleura ; that lining the abdomen the perito- neum ; the abdominal cavity is in consequence often called the per- itoneal cavity. Externally the walls of these cavities are covered by the skin, which consists of two layers : an outer horny layer called the epidermis, which is constantly being shed on the surface and re- newed from below ; and a deeper layer, called the derm is and con- taining blood, which the epider- mis does not. Between the skin and the lining serous membranes are bones, muscles (the lean of meat), and a great number of other structures which we shall have to consider hereafter. All cavities inside the body, as the alimentary canal and the air pas- sages, which open directly or indi- rectly on the surface are lined by soft and moist prolongations of the skin known as mucous mem- branes. In these the same two FIG. 2.— Diagrammatic longi . tudinal section of the body, a, laVCl'S are found as 111 the SK111, the neural tube, with its upper -, ^ , ,-, r» • -i i i -n but the superficial bloodless one is called epithelium and the deep- er one the corium. Diagrammatically we may rep- resent the human Body in longi- enlargement in the skull cavity at a'; N. the spinal cord; JV', the brain: ee. vertebrae form- ing the solid partition between the dorsal and ventral cavities; ber, opening behind into the pharynx, from which one tube leads to the lungs, I. and another to the stomach,/; h, the heart; fc, a kidney; s, the sympathetic nervous chain. From the stom- ach, /, the intestinal tube leads through the abdominal cavity to " the ali- aa' is the dorsal or neural cavity, and b and c, respectively, the thoracic and abdominal subdivi- sions of the ventral cavity; d rep- resents the diaphragm separating CROSS-SECTION OF THE BODY. 7 them; ee is the vertebral column with its modified prolon- gation into the head beneath the anterior enlargement of the dorsal cavity; / is the alimentary canal opening in front through the nose, i, and mouth, o; li is the heart, I a lung, 8 the sympathetic nervous system, and k a kidney. A transverse section through the chest is represented diagrammatically in Fig. 3, where x is the neural canal containing the spinal cord. In the thoracic cavity are seen the heart, h, the lungs, II, part of the alimentary canal, a, and the sympathetic nerve centres, sy; the dotted line on each side covering the inside of the chest wall and the outside of the lung represents the pleura. FIG. 3.— A diagrammatic section across the Body in the chest region, x. the dorsal tube, which contains the spinal cord; the black mass surrounding it is a vertebra; a. the gullet, a part of the alimentary canal; ft, the heart; sy, sym- pathetic nervous system; II, lungs; the dotted lines around them ara the pleurae; rr, ribs; st, the breastbone. Sections through corresponding parts of any other Mam- mal would agree in all essential points with those repre- sented in Figs. 2 and 3. The Limbs. The limbs present no such arrangement of cavities on each side of a bony axis as is seen in the trunk. They have an axis formed at different parts of one or more bones (as seen at U and R in Fig. 4, which represents a cross-section of the forearm near the elbow joint), but around this are closely-packed soft parts, chiefly muscles, and the whole js enveloped in skin. The only cavities in the limbs are branching tubes which are filled with liquids during life, either blood or a watery-looking fluid known as tywph. These tubes, the Uood and lymph vessels respec- 8 THE HUMAN BODY. tively, are not however characteristic of the limbs, for they are present in abundance in the dorsal and ventral cavities and in their walls. FIG. 4.— A section across the forearm a short distance below the elbo w- joint. R and U, its two supporting bones, the radius and ulna : e, the epidermis, and d the dermis of the skin; the latter is continuous below with bands of connective tissue, s, which penetrate between and invest the muscles, which are indicated by numbers; n, n, nerves and vessels. Chemical Composition of the Body. In addition to the study of the Body as composed of tissues and organs which are optically recognizable, we may consider it as composed of a number of different chemical substances. This branch of knowledge, which is still very incomplete, really presents two classes of problems. On the one hand we may limit ourselves to the examination of the chemical substances which exist in or may be derived from the dead Body, or, if such a thing were possible, from the living Body entirely at rest; such a study is essentially one of structure and may be called Chemical Anatomy. But as long as the Body is alive it is the seat of constant chemical trans- formations in its material, and these are inseparably con- nected with its functions, the great majority of which are in the long-run dependent upon chemical changes. From this point of view, then, the chemical study of the Body presents physiological problems, and it is usual to include all the known facts as to the chemical composition and metamorphoses of living matter under the name of Physio- logical Chemistry. For the present we may confine our- selves to the more important substances derived from or known to exist in the Body, leaving questions concerning the chemical changes taking place within it for consideration along with those functions which are performed in connec- tion'with them. CHEMISTRY OP THE BODY. 9 Elements Composing the Body. Of the elements known to chemists only sixteen have been found to take part in the formation of the human Body. These are carbon, hy- drogen, nitrogen, oxygen, sulphur, phosphorus, chlorine, fluorine, silicon, sodium, potassium, lithium, calcium, magnesium, iron, and manganese. Copper and lead have sometimes been found in small quantities but are probably accidental and occasional. TJncombined Elements. Only a very small number of the above elements exist in the bodyuncombined. Oxygen is found in small quantity dissolved in the blood; but even there most of it is in a state of loose chemical combination. It is also found in the cavities of the lungs and alimentary •canal, being derived from the inspired air or swallowed with food and saliva; but while contained in these spaces it can hardly be said to form a part of the Body. Nitro- gen also exists uncombined in the lungs and alimentary canal, and in small quantity in solution in the blood. Free hydrogen has also been found in the alimentary canal, be- ing there evolved by the fermentation of certain foods. Chemical Compounds. The number of these which "may be obtained from the Body is very great; but with re- gard to very many of them we do not know that the form in which we extract them is really that in which the ele- ments they contain were united while in the living Body; ;since the methods of chemical analysis* are such as always break down the more complex forms of living matter and leave us only its debris for examination. AYe know in fact, tolerably accurately, what compounds enter the Body iis food and what finally leave it as waste; but the inter- mediate conditions of the elements contained in these com- pounds during their sojourn iriside the Body we know very little about; more especially their state of combination dur- ing that part of their stay when they do not exist dissolved in the bodily liquids, but form part of a solid living tissue. For present purposes the chemical compounds existing in or derived from the Body may be classified as organic 10 THE HUMAN BODY. and inorganic, and the former be subdivided into those- which contain nitrogen and those which do not. Nitrogenous or Azotized Organic Compounds. These fall into several main groups: proteids, peptones, albu- minoids, crystalline substances, and coloring matters. Proteids are by far the most characteristic substances ob- tained from the Body, since they are only known as exist- ing in or derived from living things, either animals or plants. The type of this class of bodies may be found in the white of an egg, where it is stored tip as food for the developing chick ; from this typical form, which is called egg albumin, the proteids- in general are often called albuminous bodies. Each of them contains carbon, hydro- gen, oxygen, sulphur, and nitrogen united to form a very complex molecule, and although different members of the family differ from one another in minor points they all agree in their broad features and have a similar percentage composition. The latter in different examples appears to vary within the following limits, but it is almost impossible to get any one of them pure for analysis: Carbon 52 to 54 per cent. Hydrogen 7 to 7.5 " Oxygen 21 to 24.0 " Nitrogen 15 to 17.0 " Sulphur 0.8 to 2.0 " Proteids are recognized by the following characters : 1.. Boiled, either in the solid state or in solution, with strong ' nitric acid they give a yellow liquid which becomes orange on neutralization with ammonia. This is the xantlio-proteic test. 2. Boiled writh a solution containing subnitrate and per- nitrate of mercury they give a pink precipitate, or, if in very small quantity, a pink-colored solution. This is known as Mitton's test. 3. If a solution containing a proteid be acidulated with strong acetic acid and be boiled after the addition of an equal bulk of a saturated watery solution of sodium sul- phate, the proteid will be precipitated. PROTEIDS. PEPTONES. 11 Among the more important proteids obtained from the human Body are the following: Serum albumin. This exists in solution in the blood and is very like egg albumin in its properties. It is coagulated (like the white of an egg) when boiled, and then passes into the state of coagulated proteid which is, unlike the original serum albumin, insoluble in dilute acids or alka- lies or in water containing neutral salts in solution. All other proteids can by appropriate treatment be turned into coagulated proteid. Fibrin. This forms in blood when it " clots," either iiir side or outside of the body. . It is made by the interaction of two other proteids known &sfibrinogen an&fibrinoplastin. It is insoluble in water. Myosin. This is derived from the muscles, in which it develops and solidifies after death, causing the "death- stiffening." Globulin exists in the red globules of the blood and dis- solved in some other liquids of the body. In the blood corpuscles it is combined with a colored substance to form hcemoglobin, which is crystallizable. An allied substance, paraylobulin, is found dissolved in the blood liquid. Caxein exists in milk. It is insoluble in water but sol- uble in dilute acids and alkalies. Its solutions do not coagulate spontaneously, or like that of serum albumin on boiling. In the milk it is held in solution by the free- alkali present; when milk becomes sour the casein is pre- cipitated as the "curd." Cheese consists mainly of casein. Peptones. These are formed in the alimentary canal by the action of some of the digestive liquids upon the proteids swallowed as food. They contain the same elements as the proteids and give the xantho-proteic and Millon's reactions, but are not precipitated by boiling with acetic acid and sodium sulphate. Their great distinctive charac- ter is however their diffusibility. The proteids proper will not dialyze (see Physics), but the peptones in solution pass readily through moist animal membranes. Albuminoids. These contain carbon, hydrogen, oxy- gen and nitrogen, but rarely any sulphur. Like the 12 THE HUMAN BODY. proteids, the nearest chemical allies of which they seem to be, they are only known in or derived from living beings. Gelatin, obtained from bones and ligaments by boiling, is a typical albuminoid; as is chondrin, which is obtained similarly from gristle. Mucin, which gives their glairy tenacious character to the secretions of the mouth and nose, is another albuminoid. Crystalline Nitrogenous Substances. These are a heterogeneous group, the great majority of them being materials which have done their work in the Body and are about to be got rid of. Nitrogen enters the Body in foods for the most part in the chemically complex form of some proteid. In the vital processes these proteids are broken down into simpler substances, their carbon being partly combined with oxygen and passed out through the lungs as carbon dioxide; their hydrogen is similarly in large part combined with oxygen and passed out as water; while their nitrogen, with some carbon and hydrogen and oxygen, is usually passed out in the form of a crystalline compound, containing what chemists call an "ammonium residue.'' CO) Of these the most important is urea (Carbamide H2 >• X2), H-) which is eliminated through the kidneys. Uric acid is an- other nitrogenous waste product, and many others, such as fareatin and Tcreatinin, seem to be intermediate stages be- 'tween the proteids which enter the body and the urea and uric acid which leave it. In the bile or gall, two crystallizable nitrogen-contain- ing bodies, glycocliolic and tauroclwlic acids, are found com- bined with soda. Nitrogenous Coloring Matters. These form an arti- ficial group whose constitution and origin are ill known. Among the most important are the following: HcBmatin, derived from the red corpuscles of the blood in which a residue of it is combined with a proteid residue to form haemoglobin. Bilirubin and biliverdin, which exist in the bile ; the former predominating in the bile of man and of carnivo- FATS. CARBOHYDRATES. 13 rous animals and giving it a reddish yellow color, while biliverdin predominates in the bile of Herbivora which is green. Non-Nitrogenous Organic Compounds. These may be conveniently grouped as hydrocarbons or fatty bodies; carbohydrates or amyloids ; and certain non-azotized acids. Pats. The fats all contain carbon, hydrogen and oxygen, the oxygen being present in small proportion as compared with the hydrogen. Three fats occur in the body in large quantities, viz. : palmatin (C6iH9'606), stearin (C57HUo06), and olein (C57Hio406). The two former when pure are solid at the temperature of the Body, but in it are mixed with olein (which is liquid) in such proportions as to be kept fluid. The total quantity of fat in the Body is sub- ject to great variations, but its average quantity in a man. weighing 75 kilograms (105 pounds) is about 2.75 kilo- grams (6 pounds). Each of these fats when heated with a caustic alkali, in the presence of water, breaks up into a fatty acid (stearic, palmitic, or oleic as the case may be) and glycerine. The fatty acid unites with the alkali present to form a soap. Carbohydrates. These also contain carbon, hydrogen and oxygen, but there is one atom of oxygen present for every two of hydrogen in the molecule of each of thehi. Chemically they arc related to starch; The more impor- tant of them found in the Body are the following: Glycogen (CGH1005)' found in large quantities in the liver, where it seems to be a reserve of material answering to the starch stored up by many plants. It exists in smaller quantities in the muscles. Glucose, or grape sugar (C6HiaO«), which exists in the liver in small quantities ; also in the blood and lymph. It is largely derived from glycogen which is very readily converted into it. Inosit, or muscle sugar (C6IIi206 + 2H20), found in muscles, liver, spleen, kidneys, etc. Lactose, or sugar of milk (C,aH3aOu + H20), found in considerable quantity in milk. Organic Non-Nitrogenous Acids. Of these the most 14 THE HUMAN BODY, important is carbonic dioxide (CO*), which is the form in which by far the greater part of the carbon taken into the Body ultimately leaves it. United with calcium it is found in the bones and teeth in large proportion. Formic. Acetic, and Butyric acids also are found in the Body; stearic, palmitic, and oleic have been above men- tioned as obtainable from fats. Lactic acid is found in the stomach and develops in milk when it turns sour. A body of the same percentage composition, CsHeOs (sarcolac- tic acid), is formed in muscles when they work or die. Glycero-phosphoric acid (C3H9P06) is obtained on the decomposition of lecithin, a complex nitrogenous fat found in nervous tissue. Inorganic Constituents. Of the simpler substances en- tering into the structure of the body the following are the most important: Water; in all the tissues in greater or less proportion and forming about two thirds of the weight of the whole Body. A man weighing 75 kilos (165 Ibs.), if completely dried would therefore lose about 50 kilos (110 Ibs.) from the evaporation of water. Of the constituents of the Body the enamel of the teeth contains least water (about two per cent) and the saliva most (about 99.5 percent); between these extremes are all intermediate steps — bones containing about 22 per cent, muscles 75, blood 79. Common salt — Sodium chloride — (NaCl) ; found in all the tissues and liquids, and in many cases playing an important part in keeping other substances in solution in water. Potassium chloride (KC1); in the blood, muscles, nerves, and most liquids. Calcium phosphate (Ca32P04); in the bones and teeth in large quantity. In less proportion in all the other tissues. Besides the above, ammonium chloride, sodium and potassium phosphates, magnesium phosphate, sodium sul- phate, potassium sulphate and calcium fluoride have been obtained from the body. Uncombined Hydrochloric acid (HC1) is found in the stomach. CHAPTER II. THE FUNDAMENTAL PHYSIOLOGICAL ACTIONS. The Properties of the Living Body. When we turn from the structure and composition of the living Body to consider its powers and properties we meet with the same variety and complexity, the most superficial examination being sufficient to show that its parts are endowed with very different faculties. Light falling on the eye arouses in us a sensation of sight but falling on the skin has no such effect; pinching the skin causes pain, but pinching a hair or a nail does not: when the ears are stopped, sounds arouse in us no sensation: we readily recognize, too, hard parts formed for support, joints to admit of movements, apertures to receive food and others to get rid of wastes. We thus perceive that different organs of our Bodies have yery different endowments and serve for very distinct pur- poses; and here again the study of infernal organs shows us that the varieties of quality observed on the exterior are but slight indications of differences of property which per- vade the whole, being sometimes dependent on the specific characters of the tissues concerned and sometimes upon the manner in which these are combined to form various organs. Some tissues are solid, rigid and of constant shape, as those composing the bones and teeth; others, as the muscles, are soft and capable of changing their form; and still others are capable of working chemical changes by which such peculiar fluids as the bile or the saliva are produced. We find elsewhere a number of tissues -'com- bined to form a tube adapted to receive food and carry it through the Body for digestion, and again similar tissues 16 THE HUMAN BOD?. differently arranged to receive the air which we breathe-in,. and expel after abstracting from it part of its oxygen and adding to it certain other things; and in the heart and blood-vessels we find almost the same tissues arranged to propel and carry the blood over the whole Bodv. The working of the Body offers clearly even a more complex subject of study than its structure. Physiological Properties,, In common with inanimate objects the Body possesses many merely physical properties, as weight, rigidity, elasticity, color, and so on; but in ad- dition to these we find in it while alive many others which it ceases to manifest at death. Of these perhaps the power of executing spontaneous movements and of maintaining a high bodily temperature are the most marked. As long as the Body is alive it is warm and,. since the surrounding air is nearly always cooler, must be losing heat all day long to neighboring objects; neverthe- less we are at the end of the day as warm as at the begin- ning, the temperature of the Body in health not varying much from 37.5° C. (99° F.), so that clearly our Bodies must be making heat somehow all the time. After death this production of heat ceases and the Body cools down to the temperature in its neighborhood; but so closely do we associate with it the idea of warmth that the sensation experienced on touching a corpse produces so powerful an impression as commonly to be described as icy cold. The other great characteristic of the living Body is its power of executing movements; so long as life lasts it is never at rest; even in the deepest slumber the regular breathing, the tap of the heart against the chest-wall, and the beat of the pulse tell us that we are watching sleep and not death. If to this we add the possession of conscious- ness by the living Body, whether aroused or not by forces immediately acting upon sense-organs, we might describe it as a heat-producing, moving, conscious organism. The production of heat in the Body needs fuel of some kind as much as its production in a fire; and every time we move ourselves or external objects some of the Body is used up to supply the necessary working power, just- CELLS. 17 as some coals are burnt in the furnace of an engine for every bit of work it does; in the same way every thought that arises in us is accompanied with the destruction of some part of the Body. Hence these primary actions of keeping warm, moving, and being conscious necessitate many others for the supply of new materials to the tis- sues concerned and for the removal of their wastes; still others are necessary to regulate the production and loss of heat in accordance with changes in the exterior tempera- ture, to bring the moving tissues into relation with the thinking, and so on. By such subsidiary arrangements the working of the whole Body becomes so complex that it would fill many pages merely to enumerate what is known of the duties of its various parts. However, all the proper physiological properties depend in ultimate analysis on a small number of faculties which are possessed by all living things, their great variety in the human Body depending upon special development and combination in different tissues and organs; and before attempting to study them in their most complex forms it is advantageous to examine them in their simplest and most generalized manifestations as exhibited by some of the lowest living things or by the simplest constituents of our own Bodies. Cells. Among the anatomical elements which the liistologist meets with as entering into the composition of the human Body are minute granular masses of a soft con- sistence, about 0.012 millimeter (y^To °f an inch) in diameter (Fig. 5, e). Imbedded in each lies a central poitim, not granular and therefore different in appearance from the rest. These anatomical units are known FIG 5_Formg as cells, the granular substance being the of cells from the iJodv cell body and the imbedded clearer portion the cell nucleus. Inside the nucleus may often be distin- guished a still smaller body — the nucleolus. Cells of this kind exist in abundance in the blood, where they are known as the -white Hood corpuscles, and each exhibits of itself 18 THE HUMAN BODY. certain properties which are distinctive of all living things as compared with inanimate objects. Cell Growth. In the first place, each such cell can take up materials from its outside and build them up into its own peculiar substance; and this does not occur by the deposit of new layers of material like its own on the surface of the cell (as a crystal might increase in an evaporating solution of the same salt) but in an entirely different way. The cell takes up chemical elements, either free or com- bined in a manner different from that in which they exist in its own living substance, and works chemical changes in them by which they are made into part and parcel of itself. Moreover, the new material thus formed is not de- posited, at any rate necessarily or always, on the surface of the old, but is laid down in the substance of the already existing cell among its constituent molecules. The new- formed molecules therefore contribute to the growth of the cell not by superficial accretion, but by interstitial deposit or intussusception. Cell Division. The increase of size, which may be brought about in the above manner, is not indefinite, but is limited in two ways. Alongside of the formation and deposit of new material there occurs always in the living cell a breaking down and elimination of the old; and when this process equals the accumulation of new material, as it does in all the cells of the Body when they attain a certain size, growth of course ceases. In fact the work of the cell in- creases as its mass, and therefore as the cube of its diame- Fia. 6.— A white blood corpuscle dividing, as observed at successive intervals of a few seconds with the microscope. ter; while the receptive powers, dependent primarily upon the superficial area, only increase as the square of the di- ameter. The breaking down in the cell increases when its ASSIMILATION. 19 work does and so at last equals the reception and construc- tion. The second limitation to indefinite growth is con- nected with the power of the cell to give rise to new cells like itself. Under certain circumstances, which are not well known, the cell may become narrowed (Fig. 6) at one zone; the constriction deepens until the parts on each side of it are merely united by a narrow band which finally gives way and two cells are formed, each like the parent but for its smaller size; or the cell may divide into two or more by flat surfaces of separation, or in a way intermediate between this mode and the last (Fig. 7). In some cases new cells form in the interior of the old and are then set free from it. The new cells produced in these ways grow as the origi- J. c i,- i • FIG. 7.— Another nal cell did, and may in turn multiply in mode of ceil division. TT , , T A rounded cell elon- the same manner. Very commonly tne gates in one diame- nucleus divides before the rest of the nfrrow^nl'nSaS cell, and its parts then form the nuclei of 2&£i £tf $JS +lia nmv ppllo parts; the nucleus di- tne new ceils. vides at the same Assimilation: Reproduction. These time- two powers, that of working up into their own substance materials derived from outside, known as assimilation, and that of, in one way or another, giving rise to new beings like themselves, known as reproduction, are possessed by all kinds of living beings, w*hether animals or plants. There is, however, this important difference be- tween the two: the power of assimilation is necessary for the maintenance of each individual cell, plant, or animal, since the already existing living material is constantly breaking down and being removed as long as life lasts, and the loss must be made good if anyone is to continue its ex- istence. The power of reproduction, on the other hand, is necessary only for the continuance of the kind or race, and need be, and often is, possessed only by some of the indi- viduals composing it. Working bees, for example, cannot reproduce their kind, that duty being left to the queen-bee and the drones of each hive. The breaking down of already existing chemical com- 20 THE HUMAN BOD I. pounds into simpler ones, sometimes called dissimilation, is as invariable in living beings as the building up of new complex molecules referred to above. It is associated with the assumption of uncombined oxygen from the exterior, which is then combined directly or indirectly with other elements in the cell, as for example carbon, giving rise to carbon dioxide, or hydrogen producing water. In this wray the molecule in which the carbon and hydrogen previously existed is broken down, and at the same time energy is liberated, which in all cases seems to take in part the form of heat just as when coal is burnt in a fire, but maybe used in part for other purposes such as producing movements. The carbon dioxide is usually got rid of by the same mechanism as that which serves to take up the oxygen, and these two processes constitute the function of respiration which occurs in all living things. Assimilation and disassimilation, going on side by side and being to a certain extent correlative, are often spoken of together as the process of nutrition, a term which there- fore includes all the chemical transformations occurring in living matter. Contractility. Nutrition and (with the above-mentioned partial exception) reproduction characterize all living crea- tures; and both faculties are possessed by the simple nucleated cells already referred to as found in our blood. But these cells possess also certain other properties which, although not so absolutely diagnostic, are yet very charac- teristic of living things. Examined carefully with a microscope in a fresh-drawn drop of blood, they exhibit changes of form independent of any pressure which might distort them or otherwise mechanically alter their shape. These changes may some- times show themselves as constrictions ultimately leading to the division of the cell; but more commonly(Fig. 12*) they have no such result, the cell simply altering its form by drawing in its substance at one point and thrusting it out at another. The portion thus protruded may in turn be drawn in and a process be thrown out elsewhere ; or the rest of the cell may collect around it, and a fresh protru- *P. 48. IRRITABILITY. 21 sion be then made on the same side; and by repeating this manoeuvre these cells may change their place and creep across the field of the microscope. Such changes of form from their close resemblance to those exhibited by the micro- scopic animal known as the Amoeba (see Zoology) are called amoeboid, and the faculty in the living cell upon which they depend is known in physiology as contractility. It must be borne in mind that physiological contractility in this sense is quite different from the so-called contractility of a stretched Indian- rubber band, which merely tends to re- assume a form from which it has previously been forcibly removed. Irritability. Another property exhibited by these blood- cells is known as irritability. An Amoeba coming into contact with a solid particle calculated to serve it as food will throw around it processes of its substance, and grad- ually carry the foreign mass into its own body. The amount of energy expended by the animal under these circumstances is altogether disproportionate to the force of the external contact. It is not that the swallowed mass pushes-in mechanically the surface of the Amoeba, or bur- rows into it, but the mere touch arouses in the animal an activity quite disproportionate to the exciting force, and comparable to that set free by a spark falling into gunpow- der or by a slight tap on a piece of gun-cotton. It is this disproportion between the excitant (knotfnin Physiology as a stimulus) and the result, which is the essential character- istic of irritability when the term is used in a physiological connection. The granular cells of the blood can take foreign matters into themselves in exactly the same man- ner as an Amoeba docs; and in this and in other ways, as by contracting into rigid spheres under the influence of electrical shocks, they show that they also are endowed with irritability. Conductivity. Further, when an Amoeba or one of these ^lood-cells comes into contact with a foreign body and proceeds to draw it into its own substance, the activity ex- cited is not merely displayed by the parts actually touched. Distant parts of the cell also co-operate, so that the influ- 22 THE HUMAN BODY. ence of the stimulus is not local only, but in consequence of it a change is brought about in other parts, arousing them. This property of transmitting disturbances is known as conductivity. Finally, the movements excited are not, as a rule, ran- dom. They are not irregular convulsions, but are adapted to attain a certain end, being so combined as to bring the external particle into the interior of the cell. This capa- city of all the parts to work together in definite strength and sequence, to fulfil some purpose, is known as co-ordi- nation. These Properties Characteristic but not Diagnostic. These four faculties, irritability, conductivity, contractility, and co-ordination, are possessed in a high degree by our Bodies as a whole. If the inside of the nose be tickled with a feather, a sneeze will result. Here the feather- touch (stimulus) has called forth movements which are mechanically altogether disproportionate to the energy of the contact, so that the living body is clearly irritable. The movements, which are themselves a manifestation of contractility, are not exhibited at the point touched, but at more or less distant parts, among which those of abdomen, chest, and face are visible from the exterior; our Bodies therefore possess physiological conductivity. And finally these movements are not random, but combined so as to produce a violent current of air through the nose tending to remove the irritating object; and in this we have a manifestation of co-ordination. Speaking broadly, these properties are more manifest in animals than in plants, though they are by no means absolutely confined to the former. In the sensitive plant touching one leaflet will excite regular movements of the whole leaf, and many of the lower aquatic plants exhibit movements as active as those of animals. On the other hand, no one of these four faculties is absolutely distinctive of living things in the way that growth ~by intussusception and reproduction are. Irritability is but a name for unstable molecular equilibrium, and is as marked in nitroglycerine as in any living cells; in the telephone the influence of the voice is conducted as a SPONTANEITY. 23 molecular change along a wire, and produces results at a distance; and many inanimate machines afford examples of the co-ordination of movements for the attainment of •definite ends. Spontaneity. There is, however, one character belonging to many of the movements exhibited by amoeboid cells, in which they appear at first sight to differ fundamentally from the movements of inanimate objects. This character is their apparent spontaneity or automaticUy. The cells frequently change their form independently of any re- cognizable external cause, while a dead mass at rest and unacted on from outside remains at rest. This difference is, however, only apparent and depends not upon any faculty of spontaneous action peculiar to the living cell, but upon its nutritive powers. It can be proved that any system of material particles in equilibrium and at rest will forever remain so if not acted upon by an external force. Such a system can carry on, under certain conditions, a series of changes when once a start has been given; but it cannot initiate them itself. Each living cell in the long-run is but a complex aggregate of molecules, composed in their turn of chemical elements, and if we suppose this whole set of atoms at rest in equilibrium at any moment, no change can be started in the cell from inside; in other words, it will possess no real spontaneity. When, however, we consider the irritability of amoeboid cells, or, expressed in mechanical terms, the unstable equilibrium of their particles, it be- •comes obvious that a very slight external cause, such as may entirely elude our observation, may serve to set going in them a very marked series of changes, just as pulling the trigger will fire off a gun. Once the equilibrium of the cell has been disturbed, movements either of some of its constituent molecules or of its whole mass will continue until all the molecules have again settled down into a stable state. Bat in living cells the reattainment of this state is commonly indefinitely postponed by the reception of new particles, food in one form or another, from the exterior. The nearest approach to it is probably exhibited by the rest- ing state into which some of the lower animals, as the wheel 24: TEE HUMAN BODY. animalcules, pass when dried slowly at a low temperature; the drying acting by checking the nutritive processes, which would otherwise have prevented the reattainment of molecular equilibrium. All signs of movement or other change disappear under these circumstances, but as soon as water again soaks into their substance and disturbs the existing condition, then the so-called (( spontaneous" mcYe- ments recommence. If, therefore, we use the term spon- taneity to express a power in a resting system of particles of initiating changes in itself, it is possessed neither by living nor not-living things. But if we simply employ it to desig- nate changes whose primary cause we do not recognize, and which cause was in many cases long antecedent to the changes which we see, then the term is unobjectionable and convenient, as it serves to express briefly a phenomenon presented by many living things and rinding its highest manifestation in many human actions. It then, how- ever, no longer designates a property peculiar to them. A steam-engine with its furnace lighted and water in its boiler may be set in motion by opening a valve, and the movements thus started will continue spontaneously, in the above sense, until the coals or water are used up. The dif- ference between it and the living cell lies not in any spon- taneity of the latter, but in its nutritive powers, which enable it to replace continually what answers to the coals and water of the engine. Protoplasm. Finding all these properties possessed by a simple nucleated cell, we are naturally led to inquire upon what part of it do they depend? It is clear that if they are exhibited in the absence of any one it cannot be essen- tial to their manifestation. Now a study of the lower] forms of life shows us that these powers are independent or the cell nucleus, since we find them all exhibited by cells in which the nucleus is wanting. Moreover, in many cases not only the nucleus but all granules are absent, and yet wo find the remaining mass nutritive, reproductive, irritable, contractile, conductive, co-ordinative, and automatic. We are thus driven to conclude that in the case of the granular blood-cells, these faculties are most probably endowments PROTOPLASM. 25 of the transparent portions of the cell body, in which the granules lie imbedded. This, the really working part of the cell, is known as the cell protoplasm. The role of the nucleus and granules so often present is not yet well understood; possibly the granules in many cases represent incompletely assimilated food. What the actual chemical constitution of protoplasm is we do not know, but it is one of great complexity. All methods of chemical analysis destroy it, and what we analyze is not protoplasm, which is always alive — which is a form of matter endowed with those properties which we call vital — but a mixture of the products of its decomposition when it ceases to live. Such a mixture is often called dead protoplasm, but the phrase is objectionable as implying a contradiction. Wherever there is protoplasm there is life, and wherever we meet with life we find protoplasm, so that it has been called the " physical basis of life." The name protoplasm, is, moreover, to be regarded as a general term 'or a number of closely allied substances agreeing with one another chemically in main points, as the proteids do, but differing in minor details, in consequence of which one cell differs slightly from another in faculty. On proximate analysis every mass of protoplasm is found to contain much water and a certain amount of mineral salts; the water being in part constituent or entering'into the structure of the molecules of protoplasm, and in part probably deposited in layers between them. Of organic constituents proto- plasm always yields one or more proteids, some fats, and some starchy or saccharine body. So that the original protoplasm is probably to be regarded as containing chemi- cal " residues" of proteids, fats, and carbohydrates, com- bined with salts and water. The Fundamental Physiological Properties. All living animals possess in greater or less degree the properties con- sidered in this chapter; and since the science of physiology is virtually concerned with considering how these proper- ties are acquired, maintained, and manifested, and for what ends they are employed, we may call them the funda- mental physiological properties. CHAPTER III. THE DIFFEKENTIATION OF THE TISSUES AND THE PHYSIOLOGICAL DIVISION OF EMPLOY- MENTS. Development. Every Human Body commences its indi- vidual existence as a single nucleated cell. This cell, known as the ovum, divides or segments and gives rise to FIG. 8. — A, an ovum ; B to E, successive stages in ite segmentation until the morula, F, is produced. a mass consisting of a number of similar units and called the mulberry mass or the morula. At this stage, long before birth, ihere are no distinguishable tissues entering into the structure of the Body, nor are any organs recog- nizable. • For a short time the morula increases in size by the growth and division of its cells, but very soon new pro- cesses occur which ultimately give rise to the complex DIVISION OF LABOR 27 -.adult body with its many tissues and organs. Groups of -cells ceasing to grow and multiply like their parents begin to grow in ways peculiar to themselves, and so come to differ both from the original cells of the morula and from the cells of other groups, and this unlikeness becoming more and more marked, a varied whole is finally built up from one originally alike in all its parts. Peculiar growth of this kind, forming a complex from a simple whole, is called development; and the process itself in this case is known as the differentiation of the tissues, since by it they • are, so to speak, separated or specialized from the general mass of mother-cells forming the morula. As the differences in the form and structure of the con- stituent cells of the morula become marked, differences in property arise, and it becomes obvious that the whole cell- aggregate is not destined to give rise to a collection of in- dependent living things, but to form a single human being, in whom each part, while maintaining its own life, shall have duties to perform for the good of the whole. In other words, a single compound individual is to be built up by the union and co-operation of a number of simple ones represented by the various cells, each of which thenceforth, while primarily looking after its own interests and having its own peculiar faculties, has at the samAime its activi- ties subordinated to the good of the entireliommunity. The Physiological Division of Labor. The fundamen- tal physiological properties, originally exhibited by all the -cells, become ultimately distributed between the different modified cells which form the tissues of the fully developed Body much in the same way as different employments are -distributed in a civilized state; for the difference between the fully developed Human Body and the collection of amoeboid cells from which it started is essentially the same as that between a number of wandering savages and a civi- lized nation. In the former, apart from differences de- pendent on sex, each individual has no one special occu- pation different from that of the rest, but has all his own needs to look after: he must collect his own food and prepare it for eating, make his own clothes if he wears 28 THE HUMAN BODY. any, provide his own shelter, and defend himself from wild beasts or his fellow men. In the civilized country, on the other hand, we find agriculturists to raise food and cooks, to prepare it, tailors to make clothes, and policemen and soldiers to provide protection. And just as we find that, when distribution of employments in it is more minute a nation is more advanced in civilization, so is an ani- mal higher or lower in the scale according to the degree in. which it exhibits a division of physiological duties between, its different tissues. From the subdivision of labor in advanced communities-, several important consequences arise. In the first place, each man devoting himself to one kind of work mainly and relying upon others for the supply of his other needs, every sort of work gets better done. The man who is constantly making boots becomes more expert than one whose atten- tion is constantly distracted by other duties, and he will not only make more boots in a given time, but better ones; and so with the performance of all other kinds of work. In. the second place, a necessity arises for a new sort of indus- try, in order to convey the produce of one individual in excess of the needs of himself and his family to those at a distance who may want it, and to convey back in return the excess of their produce which he needs. The carriage of food from the country to cities, and of city produce to country districts, and the occupation of shopkeepingr are instances of these new kinds of labor which arise in civilized communities. In addition there is developed a need for arrangements by which the work of individuals shall be regulated in proportion to the wants of the whole community, such a-3 is in part effected by the agency of large employers of labor who regulate the activities of a number of individuals for the production of various articles in the different quantities required at different times. Exactly similar phenomena result from the subdivision of labor in the Human Body. By the distribution of em- ployments between its different tissues, each one specially doing one work for the general community and relying on CLASSIFICATION OF TISSUES. 29 the others for their aid in turn, every necessary work is better performed. And a need arises for a distributive mechanism by which the excess products, if any, of various •tissues shall be carried to others which require them, and for a regulative mechanism by which the activities of the various tissues shall be rendered proportionate to the needs of the whole Body at different times and under different circumstances. Accordingly, as we may classify the in- Iiabitantsof the United States into lawyers, doctors, clergy- men, merchants, farmers, and so on, we may Classify tha Tissues, by selecting the most distinctive properties of each of those entering into the construc- tion of the adult Body and arranging them into physio- logical groups; those of each group being characterized by some one prominent employment. No such classification, 3io\vever, can be more than approximately accurate, since the same tissue has often more than one well-marked physiological property. The following arrangement, how- ever, is practically convenient. 1 . Undififerentiated Tissues. These are composed of •cells which have developed along no one special line, but retain very much the form and properties of the cells form- ing the very young Body before different tissues were re- cognizable in it. The lymph corpuscle's and the colorless ^corpuscles of the blood belong to this class. 2. Supporting Tissues. Including* cartilage (gristle), &one, and connective tissue. Of the latter there are several subsidiary varieties, the two more important being white fibrous connective tissue, composed mainly of colorless in- •extensible fibres, and yellow fibrous tissue, composed mainly of yellow elastic fibres. All the supporting tissues are used in the Body for mechanical purposes: the bones and carti- lages form the hard framework by which softer tissues are supported and protected; and the connective tissues unite the various bones and cartilages, form investing mem- -branes around different organs, and in the form of fine networks penetrate their substance and support their con- stituent cells. The functions of these tissues being for the most part to passively resist strain or pressure, none of 30 THE HUMAN BODY. them has any very marked physiological property; they are? not, for example, irritable or contractile, and their mass is- chiefly made up of an intercellular substance which has been formed by the actively living cells sparsely scattered through them, as for instance in cartilage, Fig. 42,* where the cells are seen imbedded in cavities in a matrix which they have formed around them; and which matrix by its firmness and elasticity forms the functionally important part of the tissue. 3. Nutritive Tissues. This is a large group, the mem- bers of which fall into three main divisions, viz. : Assimilative tissues, concerned in receiving and prepar- ing food materials, and including — (a) Secretory tissues, composed of cells which make the digestive liquids poured into alimentary canal, and bringing about chemical or other changes in the food, (b) Receptive tissues, represented by cells which line parts of the alimentary canal and take- up the digested food. Eliminative or excretory tissues, represented by cells in the kidneys, skin, and elsewhere, whose main business it is to get rid of the waste products of the various parts of the Body. Respiratory tissues. These are concerned in the gas- eous interchanges between the Body and the surrounding air. They are constituted by the cells lining the lungs. and by the colored corpuscles of the blood. As regards the nutritive tissues it requires especially to be borne in mind that although such a classification as is here given is useful, as helping to show the method pursued in the domestic economy of the Body, it is only imperfect and largely artificial. Every cell of the Body is in itself assimilative, respiratory, and excretory, and the tissues in this class are only those concerned in the first and last interchanges of material between it and the external world. They provide or get rid of substances for the whole Body, leaving the feeding and breathing and excre- tion of its individual tissues to be ultimately looked after by themselves, just as even the mandarin described by Eobin- son Crusoe who found his dignity promoted by having STORAGE TISSUES. 31 servants to put the food into his mouth, had finally to swallow and digest it for himself. Moreover, there is no- logical distinction between a secretory and an excretory cell: each of them is characterized by the formation of cer- tain substances which are poured out on a free surface on the exterior or interior of the Body. Many secretory cells too have no concern with the digestion of food, as for example; those which form the tears and sweat. 4. Storage Tissue? . The Body does not live from hand to mouth: it has always in health a supply of food materials- accumulated in it beyond its immediate needs. This lies in part in the individual cells themselves, just as in a pros- perous community nearly every one will have some little- pocket-money. But apart from this reserve there are cer- tain cells, a sort of capitalists, which store up considerable- quantities of material and constitute what we will call the- storage tissues. These are especially represented by the- liver-cells and fat-cells, which contain in health a reserve fund for the rest of the Body. Since both of these, to- gether with secretory and excretory cells, are the seats of great chemical activity, they are all often called metabolic tissues. 5. Irritable Tissues. The maintenance, or at any rate- the best prosperity, of a nation is not fully secured when a division of labor has taken place in food.-supply and food- distribution employments. It is extreniely desirable that means shall be provided by which it may receive informa- tion of external changes which may affect it as a whole, such as the policy of foreign countries ; or which shall en- able the inhabitants of one part to know the needs of an- other, and direct their activity accordingly. Foreign min- isters and consuls and newspaper correspondents are em- ployed to place it in communication with other states and keep it informed as to its interests ; and we find also orga- nizations, such as the meteorological department, to warn distant parts of approaching storms or other climatic changes which may seriously affect the pursuits carried on in them. In the Human Body we have a comparable class of intelligence-gaining tissues lying in the sense organs, 32 THE HUMAN BODY. whose business it is to ascertain and communicate to the whole, external changes which occur around it. Since the usefulness of these tissues depends upon the readiness with which slight causes excite them to activity, we may call them the irritable tissues. 6. Co-ordinating and Automatic Tissues. Such infor- mation as that collected by ministers in foreign parts or by meteorological observers, is usually sent direct to some cen- tral office from which it is redistributed; this mere redis- tribution is, however, in many cases but a small part of the work carried on in such offices. Lotus suppose informa- tion to be obtained that an Indian chief is collecting his men for an attack on some point. The news is probably first transmitted to Washington, and it becomes the duty of the executive officers there to employ certain of the con- stituent units of the nation in such definite work as is needed for its protection. Troops have to be sent to the place threatened; perhaps recruits enlisted; food and clothes, weapons and ammunition must be provided for the army; and so on. In other words, the work of the various classes composing the society has to be organized for the common good; the mere spreading the news of the danger would alone be of little avail. So in the Body: the information forwarded to certain centres from the irritable tissues is used in such a way as to arouse to orderly activity other tissues whose services are required; we find thus in these centres a group of co-ordinating tissues, represented by nerve-cells and possibly by certain other con- stituents of the nerve centres. Certain nerve-cells are also automatic in the physiological sense already pointed out. The highest manifestation of this latter faculty, shown objectively by muscular movements, is subjectively known as the "will," a state of consciousness; and other mental phenomena, as sensations and emotions, are also associated with the activity of nerve-cells lying in the brain. How it is that any one state of a material cell should give rise to a particular state of consciousness is a matter quite beyond our powers of conception; but not really more so than how it is that every portion of matter attracts every other por- MOTOR TISSUES. 33 tion according to the law of gravitation. In the liv- ing Body, as elsewhere in the universe, we can study phenomena and make out their relations of sequence or co-existence;, but why one phenomenon is accompanied by another, why in fact any cause produces an effect, is a matter quite beyond our reach in every case; whether it be a sensation accompanying a molecular change in a nerve- cell, or the fall of a stone to the ground in obedience to the1 force of gravity. 7. Motor Tissues. These have the contractility of the original protoplasmic masses highly developed. The more important are ciliated cells and muscular tissue. The for- mer line certain surfaces of the body, and possess on their free surfaces fine threads which are in constant movement. One finds such cells, for example (Fig. 47*) lining the in- side of the windpipe, where their threads or cilia serve, by their motion, to sweep any fluid formed there towards the throat, where it can be coughed up and got rid of. Mus- cular tissue occurs in two main varieties. One kind is found in the muscles attached to the bones, and is that used in the ordinary voluntary movements of the Body. It is composed of fibres which present cross-stripes when viewed under the microscope (Fig. 53f), and is hence known as striped or striated muscular tissue. The other kind of muscular tissue is found in the walls of the- alimentary canal and some other hollow organs, and con- sists of elongated cells (Fig. 55 J) which present no cross striation. It is known as plain or unstriated muscular tissue. The cells enumerated under the heading of "undiffcr- entiated tissues" might also be included among the motor tissues, since they are capable of changing their form. 8. The Conductive Tissues. These are represented by the nerve fibres, slender threads formed by modification and fusion of cells, and having the conductivity of the amoeboid cells of the morula highly developed; that is to say, they readily transmit molecular disturbances. When its equilibrium is upset at one end, a nerve-fibre will transmit to its other a molecular movement known as a *P. 115. f P- 123 t P. 124:. 34 THE HUMAN BODY. "nervous impulse" and so can excite in turn parts distant from the original exciting force. Nerve-fibres place, on the one hand, the irritable tissues in connection with, the automatic, co-ordinating, and sensory; and on the other put the three latter in communication with the muscular, secretory, and other tissues. 9. Protective Tissues. These consist of certain cells lining cavities inside the body and called epithelial cells, and cells covering the whole exterior of the Body and forming epidermis, hairs, and nails. The enamel which covers the teeth belongs also to this group. The class of protective tissues is, however, even more artificial than that of the nutritive tissues, and cannot be defined by positive characters. Many epithelial cells are secretory, excretory, or receptive; and ciliated cells have already been included among the motor tissues, although from the fact that the movements of their cilia go on in separated cells and independently of recognizable exter- nal stimuli, they might well have been put among the au- tomatic. The protective tissues may be best defined as including cells which line free surfaces, and whose func- tions are mainly mechanical or physical. 10. The Reproductive Tissues. These are concerned in the production of new individuals, and in the Human Body are of two kinds, located in different sexes. The conjunction of the products of each sex is necessary for the • origination of offspring, since the ovum, or female pro- duct which directly develops into the new human being, lies, dormant until it has been fertilized or acted upon by the product of the male. The Combination of Tissues to Form Organs. The va- rious tissues above enumerated forming the building mate- rials of the Body, anatomy is primarily concerned with their structure, and physiology with their properties. If this, however, were the whole matter, the problems of anatomy and physiology would be much simpler than they actually are. The knowledge about the living Body obtained by studying only the forms and function? ORGANS. 35 of the individual tissues would be comparable to that at- tained about a great factory by studying separately the boilers, pistons* levers, wheels, etc., found in it, and leav- ing out of account altogether the way in which these are combined to form various machines; for in. the Body the various tissues are for the most part associated to form organs, each organ answering to a complex machine like a steam-engine with its numerous constituent parts. And just as in different machines a cogged wheel may perform very different duties, dependent upon the way in which it is connected with other parts, so in the Body any one tis- sue, although its essential properties are everywhere the same, may by its activity subserve very various uses accord- ing to the manner in which it is combined with others. For example: A nerve-fibre uniting the eye with one part of the brain will, by means of its conductivity, when its end in the eye is excited by the irritable tissue attached to it on which light acts, cause changes in the sensory nerve- cells connected with its other end and so arouse a sight sensation; but an exactly similar nerve-fibre running from the brain to the muscles will, also by virtue of its conduc- tivity, when its ending in the brain is excited by a change in a nerve-cell connected with it, stir up the muscle to con- tract under the control of the will. The different results depend on the different parts connected^with the ends of the nerve-fibres in each case, and not oil any difference in the properties of the nerve-fibres themselves. It becomes necessary then to study the arrangement and uses of the tissues as combined to form various organs, and this is frequently far more difficult than to make out the structure and properties of the individual tissues. An or- dinary muscle, such as one sees in the lean of meat, is a very complex organ, containing not only contractile mus- cular tissue, but supporting and uniting connective tissue and conductive nerve-fibres, and in addition a complex commissariat arrangement, composed in its turn of several tissues, concerned in the food supply and waste removal of the whole muscle. The anatomical study of a muscle has 36 THE HUMAN BODY* to take into account the arrangement of all these parts within it, and also its connections with other organs of the- Body. The physiology of any muscle must take into ac- count the actions of all these parts working together and not merely the functions of the muscular fibres themselves, and has also to make out under what conditions the muscle- is excited to activity by changes in other organs, and what changes in these it brings about when it works. Physiological Mechanisms. Even the study of organs added to that of the separate tissues does not exhaust the whole matter. In a factory we frequently find machines arranged so that two or more shall work together for the performance of some one work : a steam-engine and a loom may, for example, be connected and used together to weave carpets. Similarly in the Body several organs are often arranged to work together so as to attain some one end by their united actions. Such combinations are known as physiological apparatuses. The circulatory apparatus, for example, consists of various organs (each in turn composed of several tissues) known as heart, arteries, capillaries, and veins. The heart forms a force-pump by which the blood is kept flowing through the whole mechanism, and the rest, known together as the blood-vessels, distribute the blood to the various organs and regulate the supply accord- ing to their needs. Again, in the visual apparatus we find the co-operation of (a) a set of optical instruments which bring the light proceeding from external objects to a focus upon (b) the retina, which contains highly irritable parts; these, changed by the light, stimulate (c) the optic nerve, which is conductive and transmits a disturbance which arouses finally (d) sensory parts in the brain. In the pro- duction of ordinary sight sensations all these parts are con- cerned and work together as a visual apparatus. So, too, we find a respiratory apparatus, consisting primarily of two hollow organs, the lungs, which lie in the chest and com- municate by the windpipe with the back of the throat, from which air enters them. But to complete the respi- ratory apparatus are many other organs, bones, muscles, nerves, and nerve-centres, which work together to renew ANATOMICAL SYSTEMS. 37 the air in the 1-ungs from time to time; and the act of breathing is \ the final result of the activity of the whole apparatus. Many similar instances, as the alimentary apparatus, the auditory apparatus, and so on, will readily be thought of. The study of the working of such complicated mechanisms forms a very important part of physiology. Anatomical Systems. From the anatomical side a whole collection of bodily organs agreeing in structure with one another is often spoken of as a system; all the muscles, for example, are grouped together as the muscular system, and all the bones as the osseous system, and so on, without any reference to the different uses of different muscles or bones. The term system is, however, often used as equivalent to " apparatus:" one reads indifferently of the " circulatory system" or the "circulatory apparatus." It is better, however, to reserve the term system for a collec- tion of organs classed together on account of similarity of structure; and "apparatus" for a collection of organs con- sidered together on account of their co-operation to execute one function. The former term will then have an anatomi- cal, the latter a physiological, significance. The Body as a Working Whole. Finally it must all through be borne in mind that not even the most complex system or apparatus can be considered altogether alone as -an independently living part. All are united to make one living Body, in which there is throughout a mutual inter- dependence, so that the whole forms one human being, in whom the circulatory, respiratory, digestive, sensory, and other apparatuses are constantly influencing one another, each modifying the activities of the rest. This interaction is mainly brought about through the conductive and co- ordinating tissues of the nervous system, which place all parts of the Body in communication. But in addition to this another bond of union is formed by the blood, which by the circulatory apparatus is carried from tissue to tissue and organ to organ, and so, bringing materials derived in one region to distant parts, enables each organ to -influence all the rest for good or ill 38 THE HUMAN BODY. % Besides the blood another liquid, called lymph, exists in the Body. It is contained in vessels distinct from those which carry the blood, but emptying into the blood-vessels at certain points. This liquid being also in constant move- ment forms another agency by which products are carried from pavt to part, and the welfare or ill-fare of one member enabled to influence all. CHAPTER IV. THE INTERNAL MEDIUM. The External Medium. (During the whole of life inter- changes of material go on between every living being and the external world; by these exchanges material particles that one time constitute parts of inanimate objects come at another to form part of a living being; and later on these same atoms, after having been a part of a living cell, are passed out from the Body in the form of lifeless com- pounds. As the foods and wastes of various living things differ more or less, so are more or less different environ- ments suited for their existence; and there is accordingly a relationship between the plants and animals living in anyone place "and the conditions of air, earth, and water prevailing there. Even such simple unicellular animals as the amcebae live only in water or mud containing in solu- tion certain gases and, in suspension, solid food particles; and they soon die if the water be changed either by essen- tially altering its gases or by taking out of it the solid food. So in yeast we find a unicellular plant which thrives and multiplies only in liquids of certain composition, and which in the absence of organic compounds of carbon in solution will not grow at all. Each of these simple living things, which corresponds to one only of the innumerable cells composing the full-grown Human Body, thus requires for the manifestation of its vital properties the presence of a surrounding medium suited to itself: the yeast would die, or at the best lie dormant, in a liquid containing only the solid organic particles on which the amoeba lives; and the amoeba would die in such solutions as those in which yeast thrives best. 40 THE HUMAN BODY. The Internal Medium. The same close relationship between the living being and its environment, and the same cyclical interchange between the two which we find in the amoeba and the yeast-cell, occur also in even the most complex living beings^ When, however, an animal comes to be composed of many cells, some of which are placed far away from the surface of its body and so from immediate contact with the environment, there arises a new need — a necessity for an internal medium or plasma which shall play the same part toward the individual cells as the surrounding air, water, and food to the whole animal. This internal medium kept in movement, and receiving at some regions of the bodily surfaces materials from the exterior, while losing other substances to the exterior at other sur- faces, thus forms a sort of middleman between the in- dividual tissues and the surrounding world, and stands in the same relationship to each of the cells of the Body as the water in which an amoeba lives does to that animal or beer-wort does to a yeast-cell. We find accordingly the Human Body pervaded by a liquid plasma, containing gases and food material in solution, the presence of which is necessary for the maintenance of the life of the tissues. Any great change in this medium will affect injuriously few or many of the groups of cells in the Body, or may even cause their death; just as altering the media in which they live will kill an amoeba or a yeast-cell. The Blood. In the Human Body the internal medium is primarily furnished by the blood, which, as every one knows, is a red liquid, very widely distributed over the frame, since it flows from any part when the skin is cut through. There are in fact very few portions of the Body into which the blood is not carried. One of the exceptions is the epidermis, or outer layer of the skin : if a cut bo made through it only, leaving the deeper skin-layers in- tact, no blood will flow from the wound. Hairs and nails also contain no blood. In the interior of the Body the epithelial layers lining free surfaces, such as the inside of the alimentary canal, contain no blood, nor do the hard parts of the teeth, the cartilages, and the refracting media THE INTERNAL MEDIUM. 41 of the eye (see Chap. XXXI.), but these interior parts are moistened with liquid of some kind, and unlike the epi- dermis are protected from rapid evaporation. All these bloodless parts together form a group of non-vascular tis- sues; they alone excepted, wounding any part of the Body will be followed by bleeding. In many of the lower animals there is no need that the liquid representing their blood should be renewed very rapidly in different parts. Their cells live slowly, and so require but little food and produce but little waste. In a sea anemone, for example, there is no special arrangement to keep the blood moving; it is just pushed about from part to part by the general movements of the body of the animal. But in higher animals, especially those with an elevated temperature, such an arrangement, or rather ab- sence of arrangement, as this would not suffice. In them the constituent cells live very fast, making much waste and using much food, and so alter the blood in their neigh- borhood very rapidly. Besides, we have seen that in com- plex animals certain cells are set apart to get food for the whole organism, and certain others to finally remove its wastes, and there must be a sure and rapid interchange of material between the feeding and excreting tissues and all the others. This can only be brought about by a rapid movement of the blood in a definite course, and this is ac- complished by shutting it up in a closed set of tubes, and placing somewhere a pump, which constantly takes in blood from one end of the system of tubes and forces it out again into the other. Sent by this pump, the hearty through all parts of the Body and back to the heart again, the blood gets food from the receptive cells, takes it to the working cells, carries off the waste of these latter to the excreting cells ; and so the round goes on. The Lymph. The blood, however, lies everywhere in closed tubes formed by the vascular system, and does not come into direct contact with any cells of the Body except those which float in it and those which line the interior of the blood-vessels. At one part of its course, however, the vessels through which it passes have extremely thin 42 THE HUMAN BODY. coats, and through the walls of these capillaries liquid transudes from the blood and bathes the various tissues. The transuded liquid is the lymph, and it is this which forms the immediate nutrient plasma of the tissues except the few which the blood moistens directly. Dialysis. When two liquids containing different mat- ters in solution are separated from one another by a moist animal membrane, an interchange of material will take place under certain conditions. If A be a vessel (Fig. 9) completely divided vertically by such a membrane, and a solution of com- mon salt in water be placed on the side #, Suit | Sugal and a solution of sugar in water on the side- ^ c, it will be found after a time that some FIG 9 —A dia- sa^ nas S°^ in^° c au^ some sugar into #, al- tnough tnere are no visible pores in the parti- containing: two tion. Such an interchange is said to be due liquids, b and ° c, separated by to cUali/sis or osmosis, and if the process were a moist animal ,. . A ., membrane. allowed to go on for some hours the same proportions of salt and sugar would be found in the solu- tions on each side of the dividing membrane. The Renewal of the Lymph. Osmotic phenomena play a great part in the nutritive processes of the Body. The lymph present in any organ gives up things to the cells there and gets things from them; and so, although it may have- originally been tolerably like the liquid part of the blood, it soon acquires a different chemical composition. Diffusion. or dialysis then commences between the lymph outside and the blood inside the capillaries, and the latter gives up to- the lymph new materials in place of those which it has lost and takes from it the waste products it has received from the tissues. "When this blood, altered by exchanges with the lymph, gets again to the neighborhood of the re- ceptive cells, having lost some food materials it is poorer in these than the richly supplied lymph around those cells, and takes up a supply by dialysis from it. When it reaches the excretory organs it has previously picked up a quantity of waste matters and loses these by dialysis to the lymph there present, which is specially poor in such matters^ LTMPHATICS. 43 since the excretory cells constantly deprive it of them. In consequence of the different wants and wastes of various cells, and of the same cells at different times, the lymph must vary considerably in composition in various organs of the Body, and the blood flowing through them will gain or lose different things in different places. But renewing during its circuit in one what it loses in another, its aver- age composition is kept pretty constant, and, through in- terchange Avith it, the average composition of the lymph also. The Lymphatic Vessels. The blood, on the whole, loses more liquid to the lymph through the capillary walls than it receives back the same way. This depends mainly on the fact that the pressure on the blood inside the ves- sels is greater than that on the lymph outside, and so a certain amount of filtration of liquid from within out occurs through the vascular wall in addition to the dialysis proper. The excess is collected from the various organs of the Body into a set of lymphatic vessels which carry it directly back into some of the larger blood-vessels near where these empty into the heart; by this flow of the lymph, under pressure from behind, it is renewed in various or- gans, fresh liquid filtering through the capillaries to take its place as fast as the old is carried off. The Lacteals. In the walls of the alimentary canal cer- tain foodm-iterials after passing through the receptive cells into the lymph are not transferred locally, like the rest, by dialysis into the blood, but are carrried off bodily in the lymph-vessels and poured into the veins of a distant part of the Body. The lymphatic vessels concerned in this work, being frequently filled with a white liquid during di- gestion, are called the milky or lacteal vessels. Summary. To sum up: the blood and lymph form the internal medium in which the tissues of the Body live; the lymph is primarily derived from the blood and forms the immediate plasma for the great majority of the living cells of the Body; and the excess of it is finally returned to the blood. The lymph moves but slowly, but is- constantly renovated by the blood, which is kept in rapid movement, 44 THE HUMAN BODY. and which, besides containing a store of new food matters for the lymph, carries off the wastes which the various cells have poured into the latter, and thus is also a sort of sewage .stream into which the wastes of the whole Body are pri- marily collected. Microscopic Characters of Blood. If a finger be pricked, and the drop of blood flowing out be received on -a glass slide, covered, protected from evaporation, and ex- amined with a microscope magnifying about 400 diameters, it will be seen to consist of innumerable solid bodies float- ing in a liquid. The solid bodies are the blood corpuscles, and the liquid is the Hood plasma or liquor sanguinis. The corpuscles are not all alike. While currents still •exist in the freshly spread drop of blood, the great majority of them are readily carried to and fro; but a certain num- ber more commonly stick to the glass and remain in one place. The former are the red, the latter the pale or color- less Uood corpuscles. Red Corpuscles. Form and Size. The red corpuscles as they float about frequently seem to vary in form, but by a little attention it can be made out that this appearance is due to their turning round as they float, and so presenting different aspects to view; just as a silver dollar presents a different outline according as it is looked at from the front or edgewise or in three-quarter profile. Sometimes the corpuscle (Fig. 10, B] appears circular; then it is seen in full face; sometimes linear (C), and slightly narrowed in the middle; sometimes oval, as the dollar when half-way between a full and a side view. These appearances show that each red corpuscle is a circu- lar disk, slightly hollowed in the middle (or biconcave) and about four times as wide as it is thick. The average trans- verse diameter is 0.008 millimeter (-g-gVo inch). Shortly after blood is drawn the corpuscles arrange themselves in " rows, or rouleaux, adhering to one another by their broader surfaces. — Color. Seen singly each red corpuscle is of a pale yellow color; it is only when collected in masses that they appear red. The blood owes its red color to the great numbers of these bodies in it; if it be spread out in a very BLOOD. 45- thin layer it, too, is yellow. In a cubic millimeter (^ inch) of blood there are about five million red corpuscles. — Struc- ture. Seen from the front the central part of each red cor- puscle in a certain focus of the microscope appears dimmer or darker than the rest (Fig. 10, B), except a narrow band near the outer rim. If the lens of the microscope be raised, however, this previously dimmer central part becomes brighter, and the previously brighter part obscure (E).. FIG. JO.— Blood corpuscles. A, magnified about 400 diameters. The red cor' puscles have arranged themselves in rouleaux ; a, a, colorless corpuscles ; B^ red corpuscles more magnified and seen in focus ; E, a red corpuscle slightly out of focus. Near the right-hand top corner is a red corpuscle seen in three- quarter face, and at C one seen edgewise. F, G, H, I, white corpuscles highly- magnified. This difference in appearance does not indicate the presence- of a central part or nucleus different from the rest, but is an optical phenomenon due to the shape of the corpuscle, in consequence of which it acts like a little biconcave lens (see Physics). Rays of light passing through near the centre of the corpuscle are refracted differently from those passing through elsewhere; and when the microscope is so focused that the latter reach the eye, the former do notA 46 THE HUMAN BODY. and vice versa ; thus when the central parts look bright, those around them look obscure, and the contrary. There is no satisfactory evidence that these corpuscles have any enveloping sac or cell-wall. All the methods used to bring one into view under the microscope are such as would coagulate the outer layers of the substance com- posing the corpuscle and so make an artificial envelope. So far as optical analysis goes, then, each corpuscle is ho- mogeneous throughout. By other means we can, however, show that at least two materials enter into the structure of each red corpuscle. If the blood be diluted with several times its own bulk of water and be then examined with the microscope, it will be found that the red corpuscles are col- orless and the plasma colored. The dilution has caused the coloring matter to pass out of the corpuscles and dis- solve in the liquid. This coloring constituent of the cor- puscle is hcemoglobin, and the colorless residue which it leaves behind and which swells up into a sphere in the di- luted plasma is the stroma. In the living corpuscle the two are intimately mingled throughout it, and so long as this is the case the blood is opaque; but when the coloring matter dissolves in the plasma, then the blood becomes transparent, or, as it is called, laky. The difference may bo very well seen by comparing a thin layer of fresh blood diluted with ten times its volume of ten-per-cent salt so- lution with a similar layer of blood diluted with ten vol- umes of water. The watery mixture is a dark transparent red; the other, in which the coloring matter still lies in the corpuscles, is a brighter opaque red. — Consistency. Each red corpuscle is a soft jelly-like mass which can be readily crushed out of shape. Unless the pressure be such as to rupture it, the corpuscle immediately reassumes its proper form when the external force is removed. The cor- puscles are, then, highly elastic; they frequently can be seen much dragged out of shape inside the vessels when the circulation of the blood is watched in a living animal (Chap. XV.), but immediately springing back to their nor- mal form when they get a chance. Blood-Crystals. Haemoglobin is, as above shown, readily BLOOD-CRYSTALS. 47 soluble in water. In this it soon decomposes if kept in a warm room, breaking up into a proteid substance called globulin and a red-colored body, hamatin. By keeping the haemoglobin solution very cold, however, this decompo- sition can be greatly retarded, and at the same time the solubility of the haemoglobin in the water much diminished. In dilute alcohol haemoglobin is still less soluble, and so if its ice-cold watery solution have one fourth of its volume of cold alcohol added to it and the mixture be put in a re- frigerator for twenty-four hours, a part of the haemoglobin will often crystallize out and sink to the bottom of the vessel, where it can be collected for examination. The haemoglobin of the rat is less soluble than that of man, and therefore crys- tallizes out especially easily; but these haemo-- globin crystals, or, as they are often called, blood-crystals, can also be obtained from human blood. In 100 parts of FIG ^^ .ciystalS; OITh£einoglobin dry human red blood- crystals. corpuscles there are 90 of haemoglobin. The haemoglobin is the essential constituent of the red blood corpuscles, •enabling them to pick up large quantities of oxygen in the lungs and carry it to all parts of the Body. (See Ees- piration. ) Haemoglobin contains a considerable quantity of iron, •much more than any other proximate constituent of the Body. The Colorless Blood Corpuscles (Fig. 10, F, If, G). The colorless, pale, or white corpuscles of the blood are far less numerous than the red; in health there is on the ave- rage about one white to three hundred red, but the pro- portion may vary considerably. Each is finely granular and consists of a soft mass of protoplasm enveloped in no definite cell-wall, but containing a nucleus. The granules in the protoplasm commonly hide the nucleus in a fresh 48 HUMAN BODY. corpuscle, but dilute acetic acid dissolves most of them and brings the nucleus into view. These pale corpuscles belong to the group of undifferentiated tissues, and differ in no important recognizable character from the cells which make up the whole very young Human Body, nor indeed from such a unicellular animal as an Amoeba, They have the power of slowly changing their form spontaneously. At one moment (Fig. 12) a pale corpuscle will be seen as a spheroidal mass; a few seconds later processes will be seen radiating from this, and soon after these processes may be retracted and others thrust out; and so the cor- puscle goes on changing its shape. These slow amoeboid movements are greatly promoted by keeping the specimen of blood at the temperature of the Body. By thrusting out a process on one side, then drawing thJ the rest of its body up to it, and its then sending out a process again on the same side, the corpuscle can slowly change its place and creep across the field of the microscope. Inside the blood-vessels these corpuscles exe- cute quite similar movements; and they sometimes bore right through the capillary walls and, getting out into the lymph spaces, creep about among the other tissues. This emigration is especially frequent in inflamed parts, and the pus or "matter" which collects in abscesses is largely made up of white blood corpuscles which have in this way got out of the blood-vessels. The average diameter of the white corpuscles is one third greater than that of the red. The Blood Plaques. It has been recently proved that a third kind of corpuscle, the plaque, exists in circulating blood. It is much smaller than the red. When blood is drawn from the vessels, the plaques break up with great rapidity and are destroyed. Blood of Other Animals. In all animals with blood the pale corpuscles are pretty much alike, but the red corpus- cles, which with rare exceptions are found only in Verte- LYMPH. 49 brates, vary considerably. In all the class of the mammalia they are circular biconcave disks, with the exception of the camel tribe, in which they are oval. They vary in diam- eter from .002 mm.* (musk deer) to .011 mm.f (elephant). In the dog they are nearly the same size as those of man. In no mammals do the fully developed red corpuscles pos- sess a nucleus. In all other vertebrate classes the red cor- puscles possess a central nucleus, and are oval slightly biconvex disks, except in a few fishes in which they are cir- cular. They are largest of all in the amphibia. Those of the frog are 0.02 mm. (y^Vo" incn) long and 'W? mm4 broad . Histology of Lymph. Pure lymph is a colorless watery- looking liquid; examined with a microscope it is seen to contain numerous pale corpuscles exactly like those of the blood, and no doubt largely consisting of pale blood cor- puscles which have emigrated. It contains none of the red corpuscles. The lymph flowing from the intestines during digestion is, as already mentioned, not colorless, but white and milky. It is known as chyle, and will be con- sidered with the process of digestion. During fasting the lymph from the intestines is colorless, like that from other parts of the Body/ inch. \ j^ inch. i CHAPTER V. THE CLOTTING OF BLOOD. The Coagulation of the Blood. "When blood is first drawn from the living Body it is perfectly liquid, flowing in any direction as readily as water. This condition is, however, only temporary ; in a few minutes the blood be- comes viscid and sticky, and the viscidity becomes more and more marked until, after the lapse of five or six min- utes, the whole mass sets into a jelly which adheres to the vessel containing it so that this may be inverted without any blood whatever being spilled. This stage is known as that of gelatinization and is also not permanent. In a few minutes the top of the jelly-like mass will be seen to be hollowed or "cupped" and in the concavity will be seen a small quantity of nearly colorless liquid, the blood serum. The jelly next shrinks so as to pull itself loose from the sides and bottom of the vessel containing it, and as it shrinks squeezes out more and more serum. Ulti- mately we get a solid clot, colored red, and smaller in size than the vessel in which the blood coagulated but retain- ing its form, floating in a quantity of pale yellow serum. if, however, the blood be not allowed to coagulate in per- fect rest, a certain number of red corpuscles will to rubbed out of the clot into the serum and the latter will be more or less reddish. The longer the clot is kept the more serum will be obtained: if the first quantity exuded be decanted off and the clot put aside and protected from evaporation, it will in a short time be found to have shrunk to a smaller size and to have pressed out more serum; and this goes on as long as it is kept, until putrefactive changes commence. CAUSES OF COAGULATION. 51 Cause of Coagulation. If a drop of fresh-drawn blood be spread out and watched with a microscope magnifying 600 or 700 diameters, it will be seen that the coagulation is due to the separation of very fine solid threads which run in every direction through the plasma and form a close network entangling all the corpuscles. These threads are composed of a proteid substance known as fibrin. When they first form, the whole drop is much like a sponge soaked full of water (represented by the serum) and having solid bodies (the corpuscles) in its cavities. After the .fibrin tiireads have been formed they tend to shorten ; hence when blood clots in mass in a vessel, the fibrinous network tends to shrink in every direction just as a network formed of stretched india-rubber bands would, and this shrinkage is greater the longer the clotted blood is kept. At first the threads stick too'firmly to the bottom and sides of the vessel to b*e pulled awayj and thus the first sign of the contraction of the fibrin is seen in the cupping of the surface of the gelatinized blood where the threads have no solid attachment, and there the contracting mass presses out from its meshes the first drops of serum. Finally the contraction of "the fibrin overcomes its adhesion to the vessel and the clot pulls itself loose on all sides, pressing out more and more serum, in which it ultimately floats. The great majority of the red corpuscles are held back in the meshes of the fibrin, but a good many.pale corpuscles, by their amoeboid movements, work their way out and get into the serum. Whipped Blood. The essential point in coagulation being the formation of fibrin in the plasma, and blood only forming a certain amount of fibrin, if this be removed as fast as it forms the remaining blood will not clot. The fibrin may be separated by what is known as " whipping" the blood. For this purpose fresh-drawn blood is stirred up vig- orously with a bunch of twigs, and the sticky fibrin threads as they form adhere to these. If the twigs be withdrawn after a few minutes a quantity of stringy material will be found attached to them. This is at first colored red by adhering blood corpuscles: but by washing in water these 5£ THE HUMAy BODY, may be removed, and the pure fibrin thus obtained is per- fectly white and in the form of highly elastic threads. It. is insoluble in water and in dilute acids, but swells up to a transparent jelly in the latter. The "whipped" or "defi- brinated blood" from which the fibrin has been in this way removed, looks just like ordinary blood, but has lost its power of coagulating spontaneously. The BufFy Coat. That the red corpuscles are not an essential part of the clot, but are merely mechanically caught up in it, seems clear from the microscopic ob- servation of the process of coagulation; and from the fact that perfectly formed fibrin can be obtained free from cor- 'pusclcs by whipping the blood and washing the threads which adhere to the twigs. Under certain conditions, moreover, one gets a naturally formed clot containing no red corpuscles in one part of it. The corpuscles of human blood are a little heavier, bulk for bulk, than the plasma in which they float; hence, when the blood is drawn and left at rest they sink slowly in it; and if for any reason the clotting takes place more slowly or the corpuscles sink more rapidly than usual, a colorless top stratum of plasma, with no red corpuscles in it, will be left before gelatiniza- tion occurs and stops the farther sinking of the corpuscles. The uppermost part of the clot formed under these cir- cumstances is colorless or pale yellow, and is known as the "huffy coat; it is especially apt to be formed in the blood drawn from febrile patients, and was therefore a point to which physicians paid much attention in the olden times when bloodletting was thought a panacea for all ills. In horse's blood the difference between the specific gravity of the corpuscles and that of the plasma is greater than in hu- man blood, and horse's blood also coagulates more slowly, so that its clot has nearly always a buffy coat. The color- less buffy coat seen sometimes on the top of the clot must, however, not be confounded with another phenomenon. When a blood-clot is left floating exposed to the air its top becomes bright scarlet, while the part immersed in the serum assumes a dark purple-red color. The brightness of the top layer is due to the action of the oxygen of the air, USES OF COAGULATION. 53 which forms a bright red compound with the coloring mat- ter of the red corpuscles. If the clot be turned upside down and left for a short time, the previously dark bottom layer, now exposed to the air, will become bright; and the previ- ously bright top layer, now immersed in the serum, will become dark. Uses of Coagulation. The clotting of the blood is so important a process that its cause has been frequently in- Testigated; but as yet it is not perfectly understood. The living circulating blood in the healthy blood-vessels does not clot; it contains no solid fibrin, but this forms in it, sooner or later, when the blood gets by any means out of the vessels or if the lining of these is injured. In this way the mouths of the small vessels opened in a cut arc clogged up, and the bleeding, which would otherwise go on indefinitely, is stopped. So, too, when a surgeon tics up an artery be- fore dividing it, and the tight ligature crushes or tears its delicate inner surface, the blood clots where this is injured, and from there a coagulum is formed reaching up to the next highest branch of the vessel. This becomes more and more solid, and by the time the ligature is removed has formed a firm plug in the cut end of the artery, which greatly diminishes the risk of bleeding. Why Blood Coagulates. Blood plasma contains in so- lution a proteid substance, fibrinogen. Before clotting oc- curs another substance, fibrin fernient, fflrms in it from the break ing-down of some white corpuscles or, more probably, of the plaques. The ferment changes the fibrinogen into fibrin. This change only takes place when a small quan- tity of neutral salines is present: and it is much facilitated by the presence of a third substance, fibrinoplastin, or paraglobulin, which exists in solution in large amount in the blood plasma. Blood serum does not clot of itself at ordinary tempera- tures: it contains fibrinoplastin and fibrin ferment and the requisite salines, but not the fibrinogen; that which origi- nally existed in the plasma having been used up to form fibrin. The liquids found in the cavities of the Body which are 54 THE HUMAN BODY. lined by serous membranes, contain fibrinogen, fibrinoplas- tin, and salts, but little or no ferment, therefore they do not coagulate spontaneously or only imperfectly and slowly. But if a little blood serum be added to one of these liquids, coagulation quickly occurs. Artificial Clot. If serum be slightly diluted with water and kept ice-cold while a stream of carbon dioxide gas is passed through it for some hours, a white precipitate is thrown down which contains fibrinoplastin and the fibrin ferment. This precipitate after washing may be dissolved in cold water containing the merest trace of caustic potash. If the liquid moistening a serous cavity be treated in a similar way a precipitate is formed, containing fibrinogen instead of the fibrinoplastin, and but little of the ferment. If this precipitate be washed and dissolved and the solution be added to the solution of the blood-serum precipitate, no clot is formed; but if about one per cent of sodic car- bonate or other neutral salt be added to the mixture, then it clots. This shows the necessity of the salts, which is perhaps better proved in another way. If serum be put in a dialyzer (see Physics) with distilled water on the other side of the membrane, all the salts will gradually pass out from the serum into the water: as the last portions of them pass out, the fibrinoplastin and ferment, which are "colloids" (bodies which do not readily dialyze), are pre- cipitated; they may be redissolved by the addition of a- trace of caustic potash. Similarly the salts may be re- moved from the liquid obtained from a serous cavity, and the precipitated fibrinogen redissolved. If these solutions be now mixed no clot is formed; but if the salts which have been dialyzed out, or an equivalent portion of other neu- tral salts, be added to the mixture, it will clot. The Fibrin Ferment. The activity of the ferment is proved as follows: If serum be diluted with a fih^e bulk of water and then carbon dioxide gas be passed through it, fibrinoplastin will be precipitated, with little or none of the ferment. If this fibrinoplastin be added to the fibrin- ogenous liquid from a serous cavity it will not cause it to clot, or only very slowly, according as no fibrin ferment or FIBRIN FERMENT. 55 but a little is present. But if some of the ferment be added, then the mixture coagulates rapidly. The ferment may be obtained by adding a large quantity of strong al- cohol to some fresh blood serum. The alcohol precipi- tates albumin, fibrinoplastin, and the ferment. The pre- cipitate is let stay under alcohol for some months, during which time the albumin and fibrinoplastin are altered so as to become insoluble in water. The alcohol is then de- canted off and the residue treated with water which dis- solves the ferment. This solution added to the above mixture containing fibrinogen, fibrinoplastin, and salts, will make it clot. Of these four bodies which play a part in the coagula- tion of blood, the fibrinogen primarily determines the quantity of fibrin formed. The ferment acting on it, in some way turns it into fibrin, but does not itself enter into the fibrin. It is not used up in the process, and the amount of fibrin ultimately formed is the same, whether much or little ferment be present. The more ferment the quicker the clotting. The fibrinoplastin in some way makes it easier for the ferment to work. The part the salts play is obscure: probably part of them are necessary constituents of the fibrin, since it leaves' a large proportion of ash when burnt. But when present in large propor- tions they prevent coagulation, probably by hindering the formation of ferment. If fresh blood be mixed with an equal bulk of a saturated solution of magnesium sulphate (Epsom salts) or of common salt, it will not clot; but if this mixture be largely diluted with water, then clotting will take place. Exciting Causes of Coagulation. The coagulation of the blood is clearly a physico-chemical process, but it is still not satisfactorily explained why it does not occur in circulating blood inside healthy blood-vessels. It is, in fact, much easier to point out what are not the proxi- mate causes of the coagulation of drawn blood than what are. Blood when removed from the Body and received in a vessel comes to rest, cools, and is exposed to the air, from 56 THE HUMAN BODY. which it may receive or to which it may give off gaseous bodies. But it is easy to prove that none of these three things is the cause of coagulation. Stirring the drawn blood and so keeping it in movement does not preTent but hastens its coagulation; and blood carefully imprisoned in a living blood-vessel, and so kept at rest, will not clot for a long time: not until the inner coat of the vessel begins to change from the want of fresh blood. Secondly, keeping the blood at the temperature of the Body hastens coagula- tion, and cooling retards it; blood received into an ice-cold vessel and kept surrounded with ice will clot more slowly than blood drawn and left exposed to ordinary tempera- tures. Finally, if the blood be collected over mercury from a blood-vessel, without having been exposed to the air even for an instant, it will still clot perfectly. The formation of fibrin is then due to changes taking place in the blood itself when it is removed from the blood-vessels; clotting depends upon some rearrangement of the blood constituents. That the primary change is a breaking-down of the plaques (p. 48) (or, as they are also named, the corpuscles of Hayem or Osier) is probable, be- cause these plaques always do break up when blood clots; and because everything which retards or prevents the coagulation of blood also retards or prevents the breaking- up of the plaques. Moreover, when blood clots, the fibrin threads are seen first near the disintegrating plaques. They probably yield the ferment. Relation of the Blood- Vessels to Coagulation. As to the role of the blood-vessels with respect to coagulation, two views are held between which the facts at present known do not permit a decisive judgment to be made. One theory is that the vessels actively prevent coagulation by constantly absorbing from the blood some substance, as, for example, the fibrin ferment and fibrinoplastin, which may be supposed constantly to develop, and the presence of which is a necessary condition for the formation of fibrin. The other view is that the blood-vessels are passive. They simply do not excite those changes in the blood con- stituents which give rise to the formation of fibrinoplas- COMPOSITION OF THE BLOOD. 57 tin or the ferment, while foreign bodies in contact with the blood do excite these changes and so lead to coagula- tion. Whatever the part which the blood-vessels play, it is only exhibited when their inner surfaces are healthy and unin- jured. If this lining be ruptured or diseased the blood clots. Accordingly, after death, when post-mortem changes have aifected the blood-vessels, the blood clots in them; but often very slowly, since the vessels only gradually alter. If the Body be left in one position after death, the clots formed in the heart have often a marked buffy coat, be- cause the corpuscles have had a long time to sink in the plasma before coagulation occurred. In medico legal cases it is thus sometimes possible to say what was the position of a corpse for some hours after death, although it has been subsequently moved. Lymph clots like the blood, but not so firmly. The •clot formed is colorless. Composition of the Blood. The average specific grav- ity of human blood is 1055. It has an alkaline reaction, which becomes less marked as coagulation occurs. About one half of its mass consists of moist corpuscles and the remainder of plasma. Exposed in a vacuum, 100 volumes of blood yield about 60 of gas consisting of a mixture of oxygen, carbon dioxide, and nitrogen. Chemistry of Serum. Blood serum is plasma, which has lost most of its fibrinogen and gained fibrin ferment and probably some additional paraglobulin; from an analy- sis of it we can draw conclusions as to the plasma. In 100 parts of serum there are about 90 parts of water, 8.5 of proteids, and 1.5 of fats, salts, and other less-known solid bodies. Of the proteids present the most abundant are serum albumin and paraglobulin. Serum albumin agrees with egg albumin in coagulating when heated: serum when boiled sets into an opaque white mass, just as the white of an egg does. Serum albumin differs from egg albumin in not being coagulated by ether; and in the fact that although present in such large quantities in the blood, 58 THE HUMAN BODY. it is not excreted by the kidneys. The paraglobnlin is also- coagulated by heat, but may be precipitated alone by saturation of the serum with magnesium sulphate. Fats are present in the serum in small quantity except after a meal at which fatty substances have been eaten; serum ob- tained from the blood of an animal soon after such a meal is often milky in appearance from the fats present, instead of being colorless or pale yellow, and transparent as it is after fasting. The salts dissolved in the serum are mainly so- dium chloride and carbonate; small quantities of sodium,, calcium, and magnesium phosphates are also present. Chemistry of the Red Corpuscles. In these in the fresh moist state there are in 100 parts, 56 of water and 44 of. solids. Of the solids about one per cent is salts, chiefly potassium phosphate and chloride. The remaining organic solids contain, in 100 parts, 90 of haemoglobin and about & of other proteids; the residue consists of less well-known bodies. Chemistry of the White Corpuscles. These yield be- sides much water, several proteids, some fats, glycogen. (see Chap. XXVIII. ), and salts; and smaller quantities of other bodies. The predominant salts, like those of the red corpuscles, are potassium phosphates. Variations in the Composition of the Blood. Hygienic Remarks. The above statements refer only to the average composition of the healthy blood, and to its better known constituents. From what was said in the last chapter it is. clear that the blood flowing from any organ will have lost, or gained, or gained some things and lost others, when compared with the blood which entered it. But the losses and gains in particular parts of the Body arc in such small amount as, with the exception of the blood gases, to elude- analysis for the most part: and the blood from all parts being mixed in the heart, they balance one another and produce a tolerably constant average. In health, however, the specific gravity of the blood nmy vary from 1045 to 1075; the red corpuscles also are present in greater propor- tion to the plasma after a meal than before it. Healthy sleep in proper amount also increases the proportion of red BLOOD CORPUSCLES. 59 corpuscles, and want of it diminishes their number as may be recognized in the pallid aspect of a person who has lost several nights' rest. Fresh air and plenty of it has the same effect. The proportion of these corpuscles has a great import- ance since, as we shall subsequently see, they serve to carry oxygen, which is necessary for the performance of its func- tions, all over the Body. Anosmia is a diseased condition characterized by pallor due to deficiency of red blood cor- puscles, and accompanied by languor and listlessness. It is not unfrequent in young girls on the verge of womanhood, and in persons overworked and confined within doors. In such cases the best remedies are open-air exercise and good food. Summary. Practically the composition of the blood may be thus stated: It consists of (1) plasma, consisting mainly of water containing in solution serum albumin, fibrinogen, sodium, and other salts, and extractives of which the most constant are urea, kreatin, and grape sugar; (2) red corpuscles, containing rather more than hnlf their weight of water, the remainder being mainly haemo- globin, other proteids, and potash salts; (3) white corpus-1; cles, consisting of water, various proteids, glycogen, and; potash salts; (4) the plaques', (5) gases, partly dissolved in the plasma or combined with its sodium salts, and (oxygen) partly combined with the haemoglobin of the red corpuscles. Quantity of Blood. The total amount of blood in the Body is difficult of accurate determination. It is about -jig- of the whole weight of the Body, so the quantity in a man weighing 75 kilos (165 Ibs.) is about 5.8 kilos (12.7 Ibs.). Of this at any given moment about one fourth would be found in the heart and big blood-vessels; and equal quantities in the capillaries of the liver, and in those of the muscles which move the skeleton; while the remaining fourth is distributed among the remaining parts of the Body. The Origin and Fate of the Blood Corpuscles. The white blood corpuscles vary so rapidly and frequently in number in the blood that they must be constantly in pro- 60 THE HUMAN .BOD Y. cess of alteration or removal, and formation; their number is largely increased by taking food, even more than that of the red, so that their proportion to the red rises, from 1 to 1000 during fasting, to 1 to 250 or 300 after a meal. They no doubt multiply to a certain extent by division while circulating in the blood, but the majority come from the lymphatic glands and similar structures (see Chap. XXII.) found in many parts of the Body, which con- tain many cells like pale blood corpuscles, and often in process of division. From these organs the corpuscles en- ter the lymph-vessels and are carried on into the blood. From the capillary blood-vessels many again migrate, and it is probable that these emigrants take part frequently in the repair or regeneration of injured tissues. Being un- differeiitiated and specialized to no line of work they are ready to take up any that comes to hand, and may be com- pared to the young men in a community who have not yet selected an occupation and are on the lookout for an open- ing. It is asserted by some authorities that many white corpuscles are transformed into red, but this is open to doubt. The corpuscles of nearly all invertebrate anfmals are colorless only, although the blood plasma of some con- tains haemoglobin in solution. Amphioxus, the lowest undoubted vertebrate animal (see Zoology), possesses onl} colorless corpuscles in its blood. Higher and more com- plex animals need more oxygen, and as blood plasma dis- solves very little of that gas, they develop in addition the haemoglobin-containing corpuscles which pick it up in the gills or lungs and carry it to all parts of the Body, leaving it where wanted (see Chap. XXV.). In cold-blooded vertebrates the red corpuscles are not nearly so many in proportion as in the warm-blooded, which use far more oxygen. The older view was that the mammalian red cor- puscle represented the nucleus of one of the white,- in which haemoglobin had been formed and from about which the rest of the corpuscle had disappeared. This, however, does not seem to be the case. In adults new red blood corpus- cles seem to be formed by the segregation of portions of the protoplasm of peculiar cells found in various parts of the LTMPIT. QI Body, but especially in the red marrow of bones (p. 88). In the embryo the liver, and in new-born animals the con- nective-tissue corpuscles (p. 105), form new red blood cor- puscles. How long an individual red corpuscle lasts is not known, nor with certainty how or when it disappears. There is, however, some reason to believe that a great many are de- stroyed in the spleen (see Chap. XXII.). Chemistry of Lymph. Lymph is a colorless fluid when pure, feebly alkaline, and with a specific gravity of about 1045. It may be described as blood minus its red corpuscles and much diluted, but of course in various parts of the Body it will contain minute quantities of substances de- rived from neighboring tissues. It contains a considerable quantity of carbon dioxide gas which it gives up in a vacuum, but no oxygen, since any of that gas which passes into it by diffusion from the blood is immediately picked up by the living tissues among which the lymph Hows. CHAPTER VI. THE SKELETON. Exoskeleton and Endoskeleton. The skeleton of an animal includes all its hard protecting or supporting parts, and is met with in two main forms in the animal kingdom. First as an exoskeleton developed in connection with either the superficial or deeper layer of the skin, and represented by the shell of a clam, the scales of fishes, the horny plates of a turtle, the bony plates of an armadillo, and the feathers of birds. In man the exoskeleton is but slightly developed, but it is represented by the hairs, nails, and teeth; for al- though the latter lie within the mouth, the study of devel- opment shows that they are developed from an offshoot of the skin which grows in and lines the mouth long Before birth. Hard parts formed from structures deeper than the skin constitute the endoskeleton, which in man is highly developed and consists of a great many bones and cartilages or gristles, the bones forming the mass of the hard frame- work of the Body, while the cartilages finish it off at vari- ous parts. This framework is what is commonly meant by the skeleton; it primarily supports all the softer parts and is also arranged so as to surround cavities in which delicate organs, as the brain, heart, or spinal cord, may lie with safety. The gross skeleton thus formed is completed and supplemented by another made of the connective tissue, which not only, in the shape of tough bands or ligaments, ties the bones and cartilages together, but also in various forms pervades the whole Body as a sort of subsidiary skeleton running through all the soft organs, forming net- works of fibres around their other constituents; so that it makes, as it were, a microscopic skeleton for the individual modified cells of which the Body is so largely composed, AXIAL SKELETON. 63 and also forms partitions between the muscles, cases for fi'icli organs as the liver and kidneys, and sheaths around the blood-vessels. The bony and cartilaginous framework nrith its ligaments might be called the skeleton of the organs of the Body, and this finer supporting meshwork the skeleton of the tissues. Besides forming a support in the substance of various organs, the connective tissue is also often laid down as a sort of packing material in the crevices between them; and so widely is it distributed everywhere from the skin outside to the lining of the alimentary canal inside, that if some solvent could be employed which would corrode away all the rest and leave only this tissue, a very perfect model of the whole Body would be left; something like a " skeleton leaf ," but far more minute in its tracery. The Bony Skeleton (Fig- 13). If the hard framework of the Body were joined together like the joists and beams of a house, the whole mass would be rigid; its parts could not move with relation to one another, and we would be un- able to raise a hand to the mouth or put one foot before another. To allow of mobility the bony skeleton is made of many separate pieces which are joined together, the points of union being called articulations, and at many places the bones entering into an articulation are movably hinged together, forming what are known as joints. The total number of bones in the Body is more than two hundred in the .adult; and the number in children is still greater, for various bones which are distinct in the child (and remain distinct throughout life in many lower animals) grow together so as to form one bone in the full-grown man. The adult bony skeleton may be described as con- sisting of an axial skeleton found in the head, neck, and trunk; and an appendicular skeleton, consisting of the bones in the limbs and in the arches (u and s, Fig. ]3) by which these are carried and attached to the trunk. Axial Skeleton. The axial skeleton consists primarily of the vertebral column or spine, a side view of which is represented in Fig. 14. The upper part of this column is composed of twenty-lour separate bones, each of • which is a vertebra. At the posterior part of the trunk, beneath the THE HUMAN BODY. FIG. 13.— The bony and cartilaginous skeleton. FIG. 14.— Side view of the- spinal column. VERTEBRA. 65 movable vertebrae, comes the sacrum (8 1), made up of five vertebrae, which in the adult grow together to form one bone, and below the sacrum is the coccyx (Co 1-4), consisting of four very small tail vertebrae, which in ad- vanced life also unite to form one bone. On the top of the vertebral column is borne the skull, made up of two parts, viz., a great box above which in- closes the brain and is called the cranium, and a large number of bones on the ventral side of this which form the skeleton of the face. Attached by ligaments to the under side of the cranium is the hyoid bone, to which the root of the tongue is fixed. Of the twenty-four separate vertebrae of the adult the seven nearest the skull (Fig. 14, C 1-7) lie in the neck and are known as the cervical vertebrce. These are fol- lowed by twelve others which have ribs attached to them (see Fig. 13) and lie at the back of the chest; they are the dorsal vertebra (D 1-12). The ribs (Fig. 25 *) are slender curved bones attached by their dorsal ends, called their heads, to the dorsal vertebrae and running thence round the Asides of the chest. In the ventral median line of the lat- / ter is the breast-bone or siernum (d, Fig 13). Each rib near its sternal end ceases to be bony and is composed of cartilage. These parts — skull, hyoid bone, vertebral column, ribs, and sternum — constitute the axial skeleton, and we have now to consider its parts more in detail. The Dorsal Vertebrae. If a single vertebra, say the eleventh from the skull, be examined carefully it will be found to consist of the following parts (Figs. 15 and 16): First a bony mass, C, rounded on the sides and flattened on each end where it is turned towards the vertebras above and below it. This stout bony cylinder is the "body" or centrum of the vertebra, and the series of vertebral bodies (Fig. 14) forms in the trunk that bony partition between the dorsal and ventral cavities of the body spoken of in Chapter I. To the dorsal side of the body is attached an arch — (he neural arch, A, which with the back of the body incloses a space, Fv, the neural ring. In the tube formed *P. 73. 66 THE HUMAN BODY. by the rings of the successive vertebrae lies the spinal cord. Projecting from the dorsal side of the neural arch is a long bony bar, Ps, the spinous process: and the projections of these processes from the various vertebrae can be felt through the skin all down the middle of the back. Hence the name of spinal column often given to the whole back-bone. Six other processes arise from the arch of the vertebra: two project forwards, i.e. towards the head; these, Pas, are the anterior articular processes and have a smooth surface, covered with cartilage, on their dorsal sides. A pair of sim- I's FIG. 16. FIG. 15. FIG. 15. — A dorsal vertebra seen from behind, i.e. the end turned from the head. FIG. 16. — Two dorsal vertebrae viewed from the left side, and in their natural relative positions. C, the body; A, neural arch; Fv, the neural ring; Ps. spi- nous process ; Pas, anterior articular process : Pew, posterior articular process ; ft, transverse process; Ft, facet for articulation with the tubercle of a rib; Fes, Fci, articular surfaces on the centrum for articulation with a rib. ilar posterior articular processes, 'Pai, runs back from the neural arch, and these have smooth surfaces on their ven- tral aspects. In the natural position of the vertebra, the smooth- surfaces of its anterior articular processes fit upon the posterior articular processes of the vertebra next in front, forming a joint, and the two processes are united by ligaments. Similarly its posterior articular processes form joints (Fig. 16) with the anterior articular processes of the vertebra next behind. SEGMENTATION OF SKELETON. 67 The remaining processes are the transverse, Pt, which run outwards and a little dorsally. Each of these has a smooth articular surface, Ft, near its outer end. On the "body" are seen two articular surfaces on each side: one, Fes, at its anterior, the other, Fci, at its poste- rior end, and both close to the attachment of the neural arch. Each of these surfaces forms with corresponding areas on the vertebrae in front and behind a pit into which the end of a rib fits and the rib attached in this way to the anterior part of the "body" also fitted on, a little way from its dorsal end, to the articular surface at the end of the transverse process. The Segments of the Axial Skeleton. If a dorsal verte- bra, say the first (Fig. 17), be detached with the pair of ribs, Cv, belonging to it and the bit of the sternum, S, to which these ribs are fixed ventral ly, we would find a bony partition formed by the body of the vertebra, lying between two arches which surround cavities. The dorsal cavity inclosed by the " body" and "neural . •* •'. . FIG. 17.— Diagrammatic representation arch Contained Originally of a segment of the axial skeleton. F, a , v mi vertebra; C. Ct',*ribs articulating above part Of the Spinal COrd. The with the body and transverse process of ,, -, TIT the vertebra; S, the breast-bone. The Other ring, made Up by trie lighter-shaded part between S and C is body of the vertebra dor- the rib cartilage" sally, the sternum ventrally, and the ribs on the sides, sur- rounds the chest cavity with its contents. All of these parts together form a typical segment cf the axial skeleton, which, however, only attains this completeness in the thoracic region of the trunk. In the skull it is greatly modified; and in the neck and the lower part of the trunk the ribs are either absent or very small, appearing only as processes of the vertebrae; and the sternal portion is wanting altogether. Nevertheless we may regard the whole axial skeleton as made up of a series of such segments placed one in front of 68 THE HUMAN BODY. the other, but having different portions of the complete segment much modified or rudimentary, or even altogether wanting in some regions. Parts which in this sort of way really correspond to one another though they differ in de- tail, which are so to speak different varieties of one thing,, are said in anatomical language to be homologous to one an- other; and when they succeed one another in a row, as the trunk segments do, the homology is spoken of as serial. The Cervical Vertebrae. In the cervical region of the vertebral column the bodies of the vertebraa are smaller than in the dorsal, but the arches are larger; the spinous processes are short and often bifid and the transverse pro- cesses appear perforated by a canal, the vertebral foramen. The bony bar bounding jp.rt this aperture on the ventral side, however, is in reality a very small rib which has grown into continuity with the body and true transverse process of the vertebra, al- though separate in very early life: the transverse process uiar process. proper bounds the vertebral foramen dorsally. In this latter during life runs an artery, which ultimately enters the skull cavity. The Atlas and Axis. The first and second cervical ver- tebrae differ considerably from the rest. The first, or atlas (Fig. 19), which carries the head, has a very small body, Aa, and a large neural ring. This ring is subdivided by a cord, the transverse ligament, L, into a dorsal moiety in which the spinal cord lies and a ventral into which the bony process D projects. This is the odontoid process, and arises from the front of the axis or second cervical vertebra (Fig. 20). Around this peg the atlas rotates when the head is turned from side to side, carrying the skull (which ar- ticulates with the large hollow surfaces Fas] with it. The odontoid process really represents a large piece of the body of the atlas which in early life separates from its own vertebra and grows on the axis. SACRUM. 69 The Lumbar Vertebrae (Fig. 21) are the largest of all the movable vertebrse and have no ribs attached to them. Their spines are short and stout and lie in a more horizontal Aa Fas Ta Ma Frt Pat FIG. 19. FIG. 20. FIG. 19.— The atlas. FIG. 20.— The axis. Aa, body of atlas; D, odontoid pro- cess; Fas, facet on front of atlas with which the skull articulates; and in Fig. 20, anterior articular surf ace of axis; L, transverse ligament; Frt, vertebral fora- men ; Ap, neural arch ; Tp, spinous process. plane than those ot the vertebrae in front. The articular and transverse processes are also short and stout. The Sacrum, which is represented along with the last lumbar vertebra in Fig. 22, consists in the adult of a single bone; but cross-ridges on its ventral surface indicate the Pas Pni Pa Ti Pai FIG. 21.— A lumbar vertebra seen from the left side. Ps. spinous process; Pas, anterior articular process; Pait posterior articular process ; Pt, transverse process. limits of the five separate vertebrae of which it is composed in childhood. It is somewhat triangular in form, its base being directed upwards and articulating with the under 70 THE HUMAN BODY. surface of the body of the fifth lumbar vertebra. On its sides are large surfaces to which the arch bearing the lower Pa s Fsa FIG. 22.— The last lumbar vertebra and the sacrum seen from the ventral side. Fsa, anterior sacral foramina. limbs is attached (see Fig. 13). Its ventral surface is con- cave and smooth and presents four pairs of anterior sacral foramina, Fsa, which communicate with the neural canal. Its dorsal surface, convex and roughened, has four similar pairs of pos- terior sacral foramina. The coccyx (Fig. 23) calls for no special remark. The four bones which grow togeth- er, or ankylose, to form it represent only the bodies of vertebrae, and even that imperfectly. It is in reality a short tail, although not visi- ble as such from the exterior. The Spinal Column as a Whole. The vertebral column FIG. 23.— The coccyx. THE SPINE AS A WHOLE. 71 is in a man of average height about twenty-eight inches long. Viewed from one side (Fig. 14) it presents four cur- vatures ; one with the convexity forwards in the cervical region is followed, in the dorsal, by a curve with its concav- ity towards the chest. In the lumbar region the curve has again its convexity turned ventrally, Avhilein the sacral and coccygeal regions the reverse is the case. These curvatures give the whole column a good deal of springiness such as would be absent were it a straight rod, and this is farther secured by the presence of compressible elastic pads, the inter vertebral disks, made up of cartilage and connective tissue, which lie between the bodies of those vertebrae which are not ankylosed together, and fill up completely the empty spaces left between the bodies of the vertebrae in Fig. 14. By means of these pads, moreover, a certain amount of movement is allowed between each pair of ver- tebras ; and so the spinal column can be bent to consider- able extent in any direction ; while the movement between any two vertebras is so limited that no sharp bend can take place at any one point, such as might tear or injure other- wise the spinal cord contained in the neural canal. The amount of moveme-nt permitted is greatest in the cervical region. In the case of the movable vertebra?, where the arch joins the body on each side, it is somewhat narrowed; this narrowed stalk being known as the pedicle (Ii, Fig. 16), while the broader remaining portion of the arch is its lam- ina. Between the pedicles of two contiguous vertebrae there are in this way left apertures, the intervertebral holes which form a series on each side of the vertebral column, and one of which, Fi, is shown between the two dorsal vertebras in Fig. 10. Through these foramina nerves run out from the spinal cord to various regions of the Body. The sacral foramina, anterior and posterior, are the repre- sentatives of these apertures, but modified in arrangement, on account of the fusion of the arches and bodies of the vertebras between which they lie. Sternum. The sternum or breast-bone (Fig. >24 and d, Fig. 13) is wider from side to side than dorso-ventrally. It 72 THE HUMAN BODY. consists in the adult of three pieces, and seen from the ven- tral side has somewhat the form of a dagger. The piece M nearest the head is called the handle or manubrium, and presents anteriority a notch, Id, on each side, with which the collar-bone articulates (u, Fig. 13); farther back are two other notches, Ic 1 and Ic 2, to which the sternal ends of the first and second ribs are attached. The middle piece, C, of the sternum is called the body; it completes the notch for the second rib and has on its sides others, Ic 3-7, for the third, fourth, fifth, sixth, and seventh ribs. The last piece of the sternum, P, is called the ensiform or xiphoid process; it is composed of cartilage, and has no ribs attached to it. The Ribs (Fig. 25). There are twelve pairs of ribs, each being a slender curved bone attached dorsally to the body and transverse process of a vertebra in the manner already mentioned, and con- tinued ventrally by a costal cartilage. In the case of the anterior seven pairs, the costal cartilages are attached direct- ly to the sides of the breast-bone; the next three cartilages are each attached to the cartilage of the preceding rib, FIG. 24.-The sternum while the cartilages of the eleventh and twelfth ribs are quite unattached ven- so these are called the free or ribs. The convexity of each curved rib is turned outwards so as to give roundness to the sides of the chest and increase its cavity, and each slopes downwards from its vertebral at- tachment, so that its sternal end is considerably lower than its dorsal. The Skull (Fig. 26) consists of twenty-two bones in the adult, of which eight, forming the cranium, are ar- ranged so as to inclose the brain-case and protect the auditory organ, while the remaining fourteen support f°r the THE SKULL. 73 the face and surround the mouth, the nose, and the eye- sockets. Fio. 25.— The ribs of the left side, with the dorsal and two lumbar vertebrae, the rib cartilages and the sternum. Cranium. The cranium is a box with a thick floor and thinner walls and roof. Its floor or base represents in the head (as is depicted diagrammatically in Fig. 2) that par- 74 THE HUMAN BODY. tition between the dorsal and ventral cavities which in the trunk is made up of the bodies of the vertebrae. In very early life it presents in the middle line a series of four bones, the basi-occipital, basi- sphenoid, presphenoid, and Tsp Fio. 26. — A side view of the skull. O, occipital bone; T, temporal; Pr, parie- tal; F, frontal; S, sphenoid; Z, malar; MX, maxilla; N, nasal; .E, ethmoid; Ly lachrymal; Md, inferior maxilla. basi-ethmoid, which answer pretty much to the bodies of four vertebrae, and have attached to them the thin bones which inclose the skull cavity (which may be likened to an enlarged neural canal) on the sides and top. In the CRANIAL BONES. Human Body, however, these bones very soon ankylose with others or with one another; although they remain distinct throughout life in the skulls of very many lower animals. On the base of the skull, besides many small apertures by which nerves and blood-vessels pass in or out, is a large aperture, the foramen magnum, through which the spinal cord passes in to join the brain. The cranial bones are the following: 1. The occipital bone (Fig. 26, 0) unpaired and having in it the foramen magnum. It is made up by the fusion of the basi-oc- cipital with other flatter bones. 2. The frontal bone (Fig. 26, F), also unpaired in the adult, but in the child each half is a sep- arate bone. 3. A pair of thin plate- like parietal bones (Fig. 26, Pr) which meet one another along the middle line in the top of the skull, and roof in a great part of the cranial cavity. 4. A pair of temporal bones (Fig. 26, T), one on each side of the skull beneath the parietal. On each temporal bone is a large aperture leading Above this are the paired open- . • i ings of the posterior nares, and into the ear Cavity, the essential a short way above the middle ol „ , ! t -i • , . the figure is the large median parts Of the Organs OI hearing forart?en magnum, with the bony being contained in these bones. 5. The sphenoid bone, made up atlas, on its sides, by the union of the basi-sphe- noid &i\&pre- sphenoid (lying on the base of skull in front of the basi-occipital) with one another and with flatter bones, is seen partly (Fig. 26, S) on the sides of the cranium in front of the temporals. 6. The ethmoid, like the sphe- noid, single in the adult, is really made up by the union of a single median basi-ethmoid with a pair of lateral bones. It closes the skull cavity in front, and lies between it and the top of the nasal chambers, being perforated by many 76 THE HUMAN BODY. small holes through which the nerves of smell pass. A little bit of it is seen on the inner side of the eye-socket at E in Fig. 26. Facial Skeleton. The majority of the face bones are in pairs ; two only being single and median. One of these is the lower jaw-bone or inferior maxilla (Fig. 26, Md); the other is the vomer, which forms part of the partition between the two nostrils. The paired face-bones are: 1. The maxillce, or upper jaw- bones (Mx, Fig. 26), on« on each side, carrying the upper row of teeth and forming a great part of the hard palate, which separates the mouth from the nose. 2. The pala- tine hones, completing the skeleton of the hard palate, and behind which the nose communicates by the posterior nares (Fig. 27) with the throat cavity, so that air can pass in or out in breathing. 3. The malar hones, or cheek-bones, (Z, Fig. 26), lying beneath and on the outside of the orbit on each side. 4. The nasal bones (X, Fig. 26), roofing in the nose. 5. The lachrymal bones (L, Fig. 26), very small and thin and lying between the nose and orbit. 6. The inferior turbinate bones lie inside the nose, one in each nostril chamber. The Hyoid. Besides the cranial and facial bones there is, as already pointed out, one other, the hyoid (Fig. 28), which really belongs to the skull, although it lies in the neck. It can be felt in front of the throat, just above " Adam's apple." The hyoid bone is U-shaped, with its convexity turned ventrally, and consists of a body and two pairs of processes called cornua, The smaller cornua (Fig. 28, 3) are attached to the base of the skull by long ligaments. These bone' 2^~bodyh-yoi5? ligaments in many animals are represent- great cornua ; ' si e J by bones, so that the hyoid, with them, small cornua. * J ' forms a bony arch attached to the base of the skull much as the ribs are attached to the bodies of the vertebra?. In fishes, behind this hyoidean arch come several others which bear the gills; and in the very young Human Body these also are represented, though they almost SHOULDER GIRDLE. 77 \ entirely disappear long before birth. The hyoid, then, with its cornua and ligaments answers pretty much to a gill-arch, or really to parts of two gill-arches, since the great and small cornua belong to originally separate arches present at an early stage of development. It is a remnant of a structure which has no longer any use in the Human Body; but in the young frog-tadpole parts answering to it carry gills and have clef ts • between them which extend into the throat just as in fishes. The gills are lost after- wards and the clefts closed up when the frog gets its lungs and begins to breathe by them. In the embryonic human being these gill-clefts are also present and several more- behind them, but the arches between them do not bear gills, and the clefts themselves are closed long before birth. As they have no use their presence is hard to account for; those who accept the doctrine of Evolution regard them as developmental reminiscences of an extremely remote ances- tor in which they were of functional importance somewhat as in the tadpole; of course this does not mean that men were developed from tadpoles. The Appendicular Skeleton. This consists of the shoulder girdle and the bones of the fore limbs, and the pelvic girdle and the bones of the posterior limbs. The two- supporting girdles in their natural position with reference to the trunk skeleton arc represented in Fig. 29. The Shoulder Girdle, or Pectoral Arch. This is made up on each side of the scapula or shoulder -Hade, and the clavicle or collar-bone. The scapula (8, Fig. 29) is a flattish triangular bone which can readily be felt on the back of the thorax. It is not directly articulated to the axial skeleton, but lies im- bedded in the muscles and other parts outside the ribs on each side of the vertebral column. From its dorsal side- arises a crest to which the outer end of the collar-bone is fixed, and on its outer edge is a .shallow cup into which the top of the arm-bone fits; this hollow is known as the glenoid fossa. The collar-bone (0, Fig. 29) is cylindrical and attached 78 THE HUMAN BODY. at its inner end to the sternum as shown in the figure, fit- ting into the notch represented at Id in Fig. 24. The Fore Limb. In the limb itself (Fig. 30) are thirty bones. The largest, a, lies in the upper arm, and is called FIG. 29.— The skeleton of the trunk and the limb arches seen from the front. C, clavicle; S, scapula; Oc, innominate bone attached to the side of the sacrum dorsally and meeting its fellow at the pubic symphysis in the ventral median line. the humerus. At the elbow the humerus is succeeded by two bones, the radius and ulna, c and b, which lie side by side, the radius being on the thumb side. At the distal ends of these bones come eight small ones, closely packed PELVIC GIRDLE. 79 and forming the wrist, or carpus. Then come five cylindri- cal bones which can be felt through the soft parts in the palm of the hand; one for the thumb, and one for each of the fingers. These are the metacarpal bones, and are dis- tinguished as first, second, third, and so on, the first being that of the thumb. In the thumb itself are two bones, and in each finger three, arranged in rows one after the other; these bones are all called phalanges. The Pelvic Girdle (Fig. 29). This consists of a large bone, the os innominatum, Oc, on each side, which is firmly fixed dorsally to the sacrum and meets its fellow in the middle ventral line. In the child each os innominatum consists of three bones, viz., the ilium, the ischium, and pubis. Where these three bones meet and finally ankylose there is a deep socket, the acetabulum, into which the head of the thigh-bone fits (see Fig. 13). Between the pubic and ischial bones is the largest foramen in the whole skeleton, known as the doorlike or thyroid foramen. The pubic bone lies above and the ischial below it. The ilium forms the upper expanded portion of the os innominatum to which the line drawn from Oc in Fig. 29 points. The Hind Limb. In this there are thirty bones, as in the fore limb, bat not quite similarly arranged; there being one less at the ankle than in the wrist, and one at the knee not present at the elbow-joint. The thigh bone or femur (a, Fig. 31) is the largest bone in the body and extends from the hip to the knee-joint. It presents above a large rounded head which fits into the acetabulum and, below, it is also enlarged and presents smooth surfaces which meet the bones of the leg. These latter are two in number, known as the tibia, c, or shin-bone, and fibula, d; the tibia being on the great-toe side. In front of the knee-joint is the knee-cap, or patella, b. At the distal end of the leg-bones comes the foot, con- sisting of tarsus, metatarsus, and. phalanges. The tarsus, which answers to the carpus of the fore limb, is made up of seven irregular bones, the largest being the heel-bone, or calcaneum, h. The metatarsus consists of five bones lying side by side, and each carries a toe at its distal end. In 80 THE HUMAN BODY. the great toe (or Jiallux) there are two phalanges, in each of the others three, arranged as in the fingers, but smaller. Comparison of the Anterior and Posterior Limbs. It FIG. 31. FIG. 30.— The bones of the arm. a, humerus; 6, ulna; c, radius; d, the carpus: e, the fifth metacarpal; /. the three phalanges of the fifth digit (little finger); g, the phalanges of the pollex (thumb). FIG. 31.— Bones of the leg. a, femur; 6, patella; c, tibia; d, fibula ; h, calca- neum; e, remaining tarsal bones; /, metatarsal bones; g, phalanges. is clear that the skeletons of the arm and leg correspond pretty closely to one another. Both are in fact quite alike in very early life, and their differences at birth depend upon HOMOLOGIE8 OF THE LIMBS. 81 their diverging in different ways as they develop from their primitive simplicity; as both may be regarded as modifications of the same original structure, they are ho- 4- FIG. 32.— The skeleton of the arm and leg. H. the humerus: Cd, its articular head which fits into the glenoid fossa of the scapula; (7, the ulna; R, the ra dius; O, the olecranon; Fe, the femur; P, the patella; Fi, the fibula; T, the tibia. mologous. The pelvic girdle clearly corresponds generally to the pectoral arch, the tibia and fibula to the radius and 82 TEE HUMAN BODY. ulna ; the five metatarsal bones to the five metacarpal, and the phalanges of the toes to those of the thumb and fingers. On the other hand, there is in the arm no separate bone at the elbow-joint corresponding to the patella at the knee, but the ulna bears above a bony process, the olecranon (0, Fig. 32), which at first is a separate bone and is the rep- resentative of the patella. There are in the carpus eight bones and in the tarsus but seven. The astragalus of the tarsus (Ta, Fig. 35) represents however, two bones which, FIG. 33.— Diagram showing the relation of the pectoral arch to the axial skel- eton. have grown together. The elbow-joint bends ventrally and the knee-joint dorsally. Comparing the limbs as a whole, greater differences come to light, differences which are mainly correlated with the different uses of the two limbs. The arms, serving as prehensile organs, have all their parts as movable as is consistent with the requisite FIG. Si-Diagram showingthe attach- Strength, while the lower SSJtS.thepelvicarcht° theaxial limbs, having to bear the whole weight of the Body, require to have their parts much more firmly knit together. Accordingly we find the shoulder girdle, represented red in the diagram (Fig. 33), only directly attached to the axial skeleton by the union of the inner ends of the clavicles with the sternum, and capable of considerable independent move- ment, as seen, for instance, in " shrugging the shoulders." The pelvic arch, on the contrary, is firmly and immovably HAND AND FOOT COMPARED. 83 fixed to the sides of the sacrum. The socket of the scapula, into which the head of the humerus fits, is very shallow and allows a far greater range of movement than is per- mitted by the deeper socket on the pelvis, into which the head of the femur fits. Further, if we hold the right humerus tightly in the left hand and do not allow it to move, we can still move the forearm bones so as to turn the palm of the hand either up or down: no such move- ment is possible between the tibia and fibula. Finally, in the foot the bones are much less movable than in the hand, and are arranged so as make a springy arch (Fig. 35) which bears behind on the calcaneum, Ca, and in front on the distal ends of the metatarsal bones, Os; over the crown of the arch, at Ta, is the surface with which the leg-bones Sfhl M5 FIG. 35.— The bones of the foot. Ca, calcaneum, or os calcis; Ta, articular surface for tibia on the astragalus; N, scaphoid bone; CI, CII, first and second cuneiform bones; Cb, cuboid bone; Ml, metatarsal bone of great toe. articulate and on which the weight of the Body bears in standing. The toes, too, are far less movable than the fingers, and this difference is especially well marked between the great toe and the thumb. The latter can be made to meet each of the finger-tips and so the hand can seize and manipulate very small objects, while this power of opposing the first digit to the rest is nearly absent in the foot of civilized man. In children, however, who have never worn boots, and in savages, the great toe is far more movable, though it never forms as complete a thumb as in many apes, which use their feet, as well as their hands, for prehension. By practice, however, our own toes can be made much more 84 THE HUMAN BODY. mobile than they usually are, so that the foot can to a certain extent replace the hand; as has been illustrated in the case of persons born without hands who have learned to write and paint with their toes. Peculiarities of the Human Skeleton. These are largely connected with the division of labor between the fore and hind limbs referred to above, which is carried farther in man than in any other creature. Even the highest apes frequently use their fore limbs in locomotion and their hind limbs in prehensioft, and we find accordingly that anatomically they present less differentiation of hand and foot. The other more important characteristics of the human skeleton are correlated for the most part with the maintenance of the erect posture, which is more complete and habitual in man than in the animals most closely allied to him anatomically. These peculiarities, however, only appear fully in the adult. In the infant the head is pro- portionately larger, which gives the centre of gravity of the Body a comparatively very high position and renders the maintenance of the erect posture difficult and insecure.[ The curves of the vertebral column are nearly absent, and the posterior limbs are relatively very short. In all these points the infant approaches more closely than the adult to the ape. The subsequent great relative length of the posterior limbs, which grow disproportionately fast in childhood as compared with the anterior, makes progression on them more rapid by giving a longer stride and at the same time makes it almost impossible to go on "all fours" except by crawling on the hands and knees. In other Pri- mates this disproportion between the anterior and posterior limbs does not occur to nearly the same extent. l~ In man the skull is nearly balanced on the top of the vertebral column, the occipital condyles which articulate with the atlas being about its middle (Fig. 27*), so that but little effort is needed to keep the hotid erect. 'In four-foot- ed beasts, on the contrary, the skull is carried on the front end of the horizontal vertebral column and needs special ligaments to sustain it. For instance, in the ox and sheep there is a great elastic cord running from the cervical ver- *P. 75. CHARACTERISTICS OF HUMAN SKELETON. 85 tebrae to the back of the skull and helping to hold up the head. Even in the highest apes the skull does not balance on the top of the spinal column; the face part is much heavier than the back, while in man the face parts are rela- tively smaller and the cranium larger, so that the two nearly equipoise. To keep the head erect and look things straight in the face, "like a man/' is for the apes far more fatiguing, and so they cannot long maintain that position. The human spinal column, gradually widening from the neck to the sacrum, is well fitted to sustain the weight of the head, upper limbs, etc., carried by it, and its curvatures, which are peculiarly human, give it considerable elasticity -combined with strength. The pelvis, to the sides of which the lower limbs are attached, is proportionately very broad I in man, so that the balance can be more readily maintained •during lateral bending of the trunk. The arched instep \ -and broad sole of the human foot are also very character- 1 istic. The majority of four-footed beasts, as horses, walk on the tips of tlielr toes_ and__fin£ers; and those animals, as bears and apes, which like man place the tarsus also on the ground, or in technical language are plantigrade, have a much less marked arch there. The vaulted human tar- sus, composed of a number of small bones, each of which can glide a little over its neighbors, but none of which can ' move much, is admiralty calculated to break any jar which might be transmitted to the spinal column by the contact of the sole with the ground at each step. A well-arched instep is therefore justifiably considered a beauty ; it makes progression easier, and by its springiness gives elasticity to the step. In London flat-footed candidates for appoint- ment as policemen are rejected, as they cannot stand the fatigue of walking the daily "beat." CHAPTER VII. THE STRUCTURE AM) COMPOSITION OF BONK JOINTS. Gross Structure of the Bones. The bones of the Body have all a similar structure and composition, but on ac- count of differences in shape they are divided by anato- mists into the following groups: (1) Long bones, more or less cylindrical in form, like the bones of the thigh and arm, leg and forearm, metacarpus, metatarsus, fingers and toes. (2) Tabular bones, in the form of expanded plates, like the bones on the roof and sides of the skull, and the shoulder-blades. (3) Short bones ; rounded or angular in form and not much greater in one diameter than in another, like the bones of the tarsus and carpus. (4) Ir- regular bones, including all which do not fit well into any of the preceding groups, and commonly lying in the middle line of the Body and divisible into two similar halves, as the vertebrae. Living bones have a bluish-white color and possess considerable elasticity, which is best seen in long slender bones such as the ribs. To get a general idea of the structure of a bone, we may se- lect the humerus. Externally in the fresh state it is covered by a dense white fibrous membrane very closely adherent to it and containing a good many small blood-vessels. This membrane is called the periosteum; on its under side new osseous tissue is formed while the bone is still growing, and all through life it is concerned in maintaining the nu- trition of the bone, which dies if it is stripped off. The periosteum covers the whole surface of the bone except its ends in the elbow and shoulder joints; the surfaces there which come in contact with other bones and glide over them STEUCTUBE OF A BONE. 87 in the movements of the joint have no periosteum, but are covered by a thin layer of gristle, known as the articu- Tmj Si I lar cartilage. Very early in the development of the Body the bone in fact was repre- sented entirely by cartilage; but afterwards nearly all this was replaced by osseous tis- sue, leaving only a thin carti- laginous layer at the ends. The bone itself, Fig. 36, consists of a central nearly \ cylindrical portion or shaft, •extending between the dot- ted lines x and z in the fig- ure, and two enlarged artic- ular extremities. On the upper articular ex- tremity is the rounded sur- face, Cp, which enters into the shoulder -"joint, fitting against the glenoid cavity of the scapula; and on the low- er are the similar surfaces, Cpl and Tr, which articulate with the radius and ulna re- spectively. Besides carry- ing the articular surfaces, each extremity presents sev- eral prominences. On the upper are those marked Tmj and Tm (the greater and smaller trochanters), which give attachment to muscles; and similar eminences, the external and internal con- dyles, El and Em, are seen on the lower end. Besides these, several bony ridges Cpl FIG. 36. — The right humerus, seen from the front. For description, see text. 88 THE HUMAN BODY. a and rough patches on the shaft indicate places to which muscles of the arm were fixed. Internal Structure. If the bone be divided longitudi- nally, it will be seen that its shaft is hol- low, the space being known as the medul-1 lary cavity, and in the fresh bone filled/ with marrow. Fig. 37 represents such a longitudinal section. It will be seen that the marrow cavity does not reach into the articular extremities, but there the bone has a loose spongy texture, except a thin layer on the surface. In the shaft, on the other hand, the outer compact layer is much tjie thickest, the spongy or cancellated bone forming only a thin stratum immediately -around the medul- lary cavity. To the naked eye the can- cellated bone appears made up of a trellis-work of thin bony plates which in- tersect in all directions and surround cavities rather larger than the head of an ordinary pin ; the compact bone, on the* contrary, appears to have no cavities in it until it is examined with a magnify- ing glass. In the spaces of the spongy portion lies, during life, a substance known as the red marrow, which is quite different from the yellow fatty- -c marrow lying in the central cavity of the- shaft. >scopic Structure of Bone. The shows that the compact bone contains cavities and only differs from the FIG. 37. -The humerus spongy portion in the fact that these bisected lengthwise, a, r &J £ marrow cavity; 6, hard are much smaller and the hard true bony bone; c. spongy bone; •• , n. n n d, articular cartilage, plates surrounding them much more nu- merous in proportion than in the spongy parts. If a thin transverse section of the shaft of the hu- merus be examined (Fig. 38) with a microscope magnifying HISTOLOGY OF BONE. 89 twenty diameters, it will be seen that numerous openings exist all over the compact parts of the section and gradu- ally become larger as this passes into the cancellated part, next the medullary cavity. These openings are the cross- sections of tubes known as the Haver sian canals, which ramify all through the bone, running mainly in the direc- FIG 38 —A a tranverse section of the ulna, natural size; showing the medul- lary cavity B, the more deeply shaded part of A magnified twenty diameters. tion of its long axis, but united by numerous cross or ob- lique branches as seen in the longitudinal section (Fig. 39). The outermost ones open on the surface of the bone be- neath the periosteum, and in the living bone blood-vessels run from this through the Haversian canals and convey 90 THE HUMAN BODY. materials for its growth and nourishment. The average diameter of the Haversian canals is 0.05 mm. (-g-J-^- of an inch). Around each Haversian canal lies a set of plates, or I lamellae, of hard bony substance (see the transverse section Fig. 38), each canal with its lamellae forming an Haver- sian system: and the whole bone is made up of a number of such systems, with the addition of a few lamellae lying in the corners between them, and a certain number which run around the whole bone on its outer surface. In the spongy parts of the bone the Haversian canals are very large and the intervening lamellae few in number. I Between the lamellae lie small cavities, the lacuncB, each / of which is lenticular in form, somewhat like the space which would be inclosed by two watch-glasses joined by their edges. From the lacunae many extremely fine branching ca- nals, the canaliculi, radiate and penetrate the bony la- mellae in all directions. The innermost canaliculi of each system open into the central Haversian canal; and those of various lacunae intercommu- nicating, these fine tubes form a set of passages through which liquid which has transuded from the blood-vessels in the Haversian canals can ooze all through the bone. The lacunae ami canaliculi are well seen in Fig. 39. In the living bone a granular nucleated cell lies in each lacuna. These cells, or bone corpuscles, are the remnants * of those which built up the bone, the hard parts of the lat- ter being really an intercellular substance or skeleton formed around and by these cells, much in the same way as a calcareous skeleton is formed around each Foraminifer (see Zoology) by the activity of its protoplasm. By the co-operation of all the bone corpuscles, and the union of their skeletons, the whole bone is built up. In other bones we find the same general arrangement of FIG. 39.— A thin longitudinal sec- tion of bone, magnified about 350 diameters, aa, Haversian canals. - COMPOSITION OF BONE. 91 "the parts, an outer dense layer and an inner spongy por- tion. In the flat and irregular bones there is no medullary cavity, and the whole centre is filled up with cancellated tissue with red marrow in its spaces. For example, in the thin bones roofing in the skull we find an outer and inner hard layer of compact bone known as the outer and inner tables respectively, the inner especially being very dense. Between the two tables lies the spongy bone, red in color to the naked eye from the marrow within it, and called the diploe. The interior of the vertebra? also is entirely occupied by spongy bone. Everywhere, except where a bone joins some other part of the skeleton, it is ^covered with the periosteum Chemical Composition of Bone. Apart from the bone •corpuscles and the soft contents of the Haversian canals and of the spaces of the cancellated bone, the bony sub- stance proper, as found in the lamellae, is composed of -earthy and organic portions intimately combined, so that the smallest distinguishable portion of bone contains both. 'The earthy matters form about two thirds of the total weight of a dried bone, and may be removed by soaking the bone in dilute hydrochloric acid. The organic portion left after this treatment constitutes a flexible mass, retain- ing perfectly the form of the original bone. By long boil- ing, especially under pressure at a higher temperature than that at which water boils when exposed freely to the air, the organic portion of the bone is nearly entirely con- verted into gelatine which dissolves in the hot water. Much •of the gelatine of commerce is prepared in this manner by .boiling the bones of slaughtered animals, and even well- picked bones may be used to form a good thick soup if boiled under pressure in a Papin's digester; much nutri- tious matter being, in the common modes of domestic cooking, thrown away in the bones. The earthy salts of bone may be obtained free from or- ganic matter by calcining a bone in a clear fire, which burns away the organic matter. The residue forms a white very brittle mass, retaining perfectly the shape and structural details of the original bone. It consists mainly of normal 92 THE HUMAN BODY. calcium phosphate, or bone earth (Gas, 2P04); but there is also present a considerable proportion of calcium carbonate (CaC03) and smaller quantities of other salts. Hygiene of the Bony Skeleton. In early life the bones are less rigid, from the fact that the earthy matters then present in them bear a less proportion to the softer organic parts. Hence the bones of an aged person are more brittle and easily broken than those of a child. The bones of a young child are in fact tolerably flexible and will be dis- torted by any continued strain; therefore children should never be kept sitting for hours, in school or elsewhere, on a bench which is so high that the feet are not supported. If this be insisted upon (for no child will continue it volunta- rily) the thigh-bones will almost certainly be bent over the- edge of the seat by the weight of the legs and feet, and a. permanent distortion may be produced. For the same- reason it is important that a child be made to sit straight in writing, to avoid the risk of producing a lateral curvature- of the spinal column. The facility with which the bones, may be moulded by prolonged pressure in early life is well seen in the distortion of the feet of Chinese ladies, pro- duced by keeping them in tight shoes; and in the extraor- dinary forms which some races of man produce in their skulls, by tying boards on the heads of the children. Throughout the whole of life, moreover, the bones re- main among the most easily modified parts of the Body; although judging from the fact that dead bones are the most permanent parts of fossil animals we might be in- clined to think otherwise. The living bone, however, is constantly undergoing changes under the influence of the protoplasmic cells imbedded in it, and in the living Body is constantly being absorbed and reconstructed. The expe- rience of physicians shows that any continued pressure, such as that of a tumor, will cause the absorption and dis- appearance of bone almost quicker than that of any other tissue; and the same is true of any other continued pres- sure. Moreover, during life the bones are eminently plas- tic; under abnormal pressures they are found to quickly assume abnormal shapes, being absorbed and disappearing ARTICULATIONS. 93 at points where the pressure is most powerful, and increas- ing at other points; tight lacing may in this way produce a permanent distortion of the ribs. When a bone is fractured a surgeon should be called in as soon as possible, for once inflammation has been set up and the parts have become swollen it is much more diffi- cult to place the broken ends of the bone together in their proper position than before this has occurred. Once the bones are replaced they must be held in position by splints or bandages, or the muscles attached to them will soon dis- place them again. With rest, in young and healthy per- sons complete union will commonly occur in three or four weeks; but in old persons the process of cure is slower and is apt to be imperfect. Articulations. The bones of the skeleton are joined to- gether in very various ways; sometimes so as to admit of no- movement at all between them; in other cases so as to per- mit only a limited range or variety of movement; and else- where so as to allow of very free movement in many direc- tions. All kinds of unions between bones are called articu- lations. Of articulations permitting no movements, those which unite the majority of the cranial bones afford a good exam- ple. Except the lower jaw, and certain tiny bones inside the temporal bone belonging to the organ of hearing, all the skull-bones are immovably joined together. This union in most cases occurs by means of toothed edges which fit into one another and form jagged lines of union known as sutures. Some of these can be well seen in Fig. 26* between the frontal and parietal bones (coronal suture) and between the parietal and occipital bones (lambdoidal suture)', while- another lies along the middle line in the top of the crown between the two parietal bones, and is known as the sagit- tal suture. In new-born children where the sagittal meets the coronal and lambdoidal sutures there are large spaces not yet covered in by the neighboring bones, which subse- quently extend over them. These openings are known as; fontanelles. At them a pulsation can often be felt syn- chronous with each beat of the heart, which., driving more *P. 74^ ^4 THE HUMAN BODY. blood into the brain, distends it and causes it to push out the skin where bone is absent. Another good example •of an articulation admitting of no movement, is that between the rough surfaces on the sides of the sacrum and the in- nominate bones. We find good examples of the second class of articula- tions— those admitting of a slight amount of movement — in the vertebral column. Between every pair of vertebrae from the second cervical to the sacrum is an elastic pad, the intervertebral disk, which adheres by its surfaces to the bodies of the vertebrae between which it lies, and only per- mits so much movement between them as can be brought about by its own compression or stretching. When the back-bone is curved to the right, for instance, each of the intervertebral disks is compressed on its right side and stretched a little on its left, and this combination of move- ments, each individually but slight, gives considerable flexibility to the spinal column as a whole. Joints. Articulations permitting of movement by the .gliding of one bone over another, are known as joints and all have the same fundamental structure, although the amount of movement permitted in different joints is very different. Hip-Joint. We may take this as a good example of a true joint permitting a great amount and variety of move- ment. On the os innominatum is the cavity of the aceta- bulum (Fig. 40), which is lined inside by a thin layer of -articular cartilage, which has an extremely smooth surface. The bony cup is also deepened a little by a cartilaginous rim. The proximal end of the femur consists of a nearly spherical smooth head, borne on a somewhat narrower neck, and fitting into the acetabulum. This head also is covered with articular cartilage; and it rolls in the acetabulum like a ball in a socket. To keep the bones together and limit the amount of movement, ligaments pass from one to the other. These are composed of white fibrous connective tissue (Chap. VIII.) and are extremely pliable but quite inextensible and very strong and tough. One is the cap- sular ligament, which forms a sort of loose bag all round STNOVIAL JOINTS. 95- fche joint, and another is the round ligament, which passes from the acetabulum to the head of the femur. Should the latter rotate above a certain extent in its socket, the round ligament and one side of the capsular ligament are put on the stretch, and any further movement which might dislocate the femur (that is remove the head from its socket) is checked. Covering the inside of the capsular ligament and the outside of the round ligament is a layer of flat cells, which are continued in a modified form over FIG. 40.— Section through the hip-joint. the articular cartilages and form the synovial membrane* This, which thus forms the lining of the joint, is always moistened in health by a small quantity of glairy synovial fluid, something like the white of a raw egg in consis- tency, and playing the part of the oil with which the con- tiguous moving surfaces of a machine are moistened; it makes all run smoothly with very little friction. In the natural state of the parts, the head of the femur and the bottom and sides of the acetabulum lie in close contact, the two synovial membranes rubbing together. 96 THE HUMAN BODY. This contact is not maintained by the ligaments, which are too loose and serve only to check excessive movement, but by the numerous stout muscles which pass from the thigh to the trunk and bind the two firmly together. Moreover, the atmospheric pressure exerted on the surface of the Body and transmitted through the soft parts to the outside of the air-tight joint helps also to keep the parts in contact. If all the muscles and ligaments around the joint be cut away it is still found in the dead Body that the head of the femur will be kept in its socket by this pressure, and so firmly as to bear the weight of the whole limb without dislocation, just as the pressure of the air will enable a boy's " sucker" to lift a tolerably heavy stone. Ball-and-socket Joints. Such a joint as that at the hip is called a ball-and-socket joint and allows of more free movement than any other. Through movements occurring in it the thigh can be flexed, or bent so that the knee ap- proaches the chest; or extended, that is moved in the oppo- site direction. It can be abducted, so that the knee moves outwards; and adducted, or moved back towards the other knee again. The limb can also by movements at the hip- joint be circumdticted, that is made to describe a cone of which the base is at the foot and the apex at the hip. Fi- nally rotation can occur in the joint, so that with knee and foot joints held rigid the toes can be turned in or out, to a certain extent, by a rolling around of the femur in its socket. At the junction of the humerus with the scapula is another ball-and-socket joint permitting all the above movements to even a greater extent. This greater range of motion at the shoulder-joint depends mainly on the shallowness of the glenoid cavity as compared with the acetabulum and upon the absence of any ligament answer- ing to the round ligament of the hip-joint. Another ball- and-socket joint exists between the carpus and the meta- carpal bone of the thumb; and others with the same variety, but a much less range, of movement between each of the remaining metacarpal bones and the proximal phalanx of the finger which articulates with it. Hinge-Joints. Another form of synovial joint is known POEMS OF JOINTS. 97 ^as a hinge-joint. In it the articulating bony surfaces are of such shape as to permit of movement, to and fro, in one plane only, like a door on its hinges. The joints between the phalanges of the fingers are good examples of hinge- joints. If no movement be allowed where the finger joins the palm of the hand it will be found that each can be bent and straightened at its own two joints, but not moved in any other way. The knee is also a hinge-joint, as is the .articulation between the lower jaw and the base of the .skull which allows us to open and close our mouths. The latter is, however, not a perfect hinge-joint, since it per- mits of a small amount of lateral movement such as occurs in chewing, and also of a gliding movement by which the lower jaw can be thrust forward so as to protrude the chin and bring the lower row of teeth outside the upper. Pivot-Joints. In this form one bone rotates around an- other which remains stationary. We have a good example of it between the first and second cervical vertebrae. The iirst cervical vertebra or atlas* (Fig. 19*) has a very small body and a very large arch, and its neural canal is subdi- vided by a transverse ligament (L, Fig. 19) into a dorsal and a ventral portion; in the former the spinal cord lies. The second vertebra or axis (Fig. 20) has arising from its body the stout bony peg, D, called the odontoid process. This projects into the ventral portion of the space sur- rounded by the atlas, and, kept in place there by the trans- verse ligament, forms a pivot around which the atlas, car- rying the skull with it, rotates when we turn the head from side to side. The joints on each side between the .atlas and the skull are hinge- joints and permit only the movements of nodding and raising the head. When the head is leaned over to one side, the cervical part of the .spinal column is bent. Another kind of pivot-joint is seen in the forearm. If the limb be held straight out, with the palm up and the elbow resting on the table, so that the shoulder- joint be kept steady while the hand is rotated until its back is turned upwards, it will be found that the radius has partly rolled round the ulna. When the palm is upwards and *P'69. 98 THE HUMAN BODY. the tliumb outwards, the lower end of the radius can b& felt on the outer side of the forearm just above the wrist, and if this be done while the hand is turned over, it will be easily discerned that during the movement this end of the radius, carrying the hand with it, travels around the lower end of the ulna so as to get to its inner side. The relative position of the bones when the palm is upwards is shown at A in Fig. 41, and when the palm is down at B. The former position is known as supination; the lat- ter as pronation. The elbow end of the humerus (Fig. 36*) bears a large articular surface: on the inner two thirds of this, Tr, the ulna fits, and the ridges and grooves of both bones inter- locking form a hinge-joint, al- lowing only of bending or straightening the forearm on the arm. The radius fits on the rounded outer third, Cpl, and forms there a ball-and-socket joint at which the movement takes place when the hand is turned from the supine to the prone position; the ulna forming a fixed bar around which the lower end of the radius is moved. Gliding Joints. These per- mit as a rule but little move- ment: examples are found be- tween the closely packed bones of the tarsus (Fig. 35 f) and carpus, which slide a little over one another when subjected to pressure. Hygiene of the Joints. When a bone is displaced or dislocated the ligaments around the joint are more or less torn and other soft parts injured. This soon leads to in- flammation and swelling which make not only the recogni- tion of the injury but, after diagnosis, the replacement. of the bone, or the reduction of the dislocation, difficult. FIG. 41. — A, arm in supination B, arm in pronation; if, humerus B, radius; U, ulna. * P. 87. •f P. 83. DISLOCATIONS AND SPRAINS. 9v) Moreover the u uscles attached to it constantly pull on the displaced bone and drag it still farther out of place; so that it is of great importance that a dislocation be reduced as soon as possible. In most cases this can only be attempted with safety by one who knows the form of the bones, and possesses sufficient anatomical knowledge to recognize the direction of the displacement. No injury to a joint should be neglected. Inflammation once started there is often diffi- cult to check and runs on, in a chronic way, until the syno- via! surfaces are destroyed, and the two bones perhaps grow together, rendering the joint permanently stiff. A sprained joint should get immediate and complete rest, for weeks if necessary, and if there be much swelling, or con- tinued pain, medical advice should be obtained. An im- properly cared-for sprain is the cause of many a useless ankle or kn^e. CHAPTER VIII. CARTILAGE AND CONNECTIVE TISSUE. Temporary and Permanent Cartilages. In early life P. great many parts of the supporting framework of the Body, which afterwards become bone, consist of cartilage. Such for example is the case with all the vertebrae, and with the bones of the limbs. In these cartilages subse- quently the process known as ossification takes place, by which a great portion of the original cartilaginous model is removed and replaced by true osseous tissue. Often, how- ever, some of the primitive cartilage is left throughout the whole of life at the ends of the bones in joints where it forms the articular cartilages; and in various other places still larger masses remain, such as the costal cartilages, those in the external ears forming their framework, others finishing the skeleton of the nose which is only incom- pletely bony, and many in internal parts of the Body, as the cartilage of "Adam's apple," which can be felt in the front of the neck, and a number of rings around the windpipe serving to keep it open. These persistent masses are known as the permanent, the others as the temporary cartilages. In old age many so-called permanent cartilages become calcified — that is, hardened and made unyielding by deposits of lime salts in them — without assuming the histological character of bone, and this calcification of the permanent cartilages is one chief cause of the want of pliability and suppleness of the frame in advanced life. Hyaline Cartilage. In its purest form cartilage is flexi- ble and elastic, of a pale bluish-white color when alive and seen in large masses, and cuts readily with a knife. In thin pieces it is quite transparent. Everywhere except in the CARTILAGE. 101 joints it is invested by a tough adherent membrane, the perfchondrium, which resembles in structure and function the periosteum of the bones. When boiled for a long time in water such cartilages yield a solution of chondn'n, which differs from gelatin in minor points but agrees with it in the fact that its hot watery solutions " set" or gelatinize on cooling. When a thin slice of hyaline cartilage is examined with a microscope it is found (Fig. 42) to consist of gran- ular nucleated cells, often collected into groups of two, four, or more, scattered through a homogeneous or faintly granular ground substance or matrix. Essentially, cartilage resembles bone, being made up of protoplasmic cells and a proportionately large amount of non-protoplasmic mtereel- FIG. 42. -Hyaline cartilage, c, a cell with several nuclei, and about to divide • *, a cell which has divided into two; a, a group of four cells such as would re- sult from a repetition of the division of b The granules of the matrix are represented much too coarse and conspicuous. lular substance, the cells being the more actively living part and the matrix their product. Examples of this hya- line variety (so called from its glassy transparent appear- ance) are found in all the temporary cartilages, and in the costal and articular among the permanent. They rarely contain blood-vessels except at those points where a temporary cartilage is being removed and replaced by bone; then blood-vessels run in from the perichondrium and form loops in the matrix, around which it is absorbed and bony tissue deposited. In consequence of the usual absence of blood-vessels the nutritive processes and ex- changes of material must be small and slow in cartilage, as OF THE UNIVERSITY OF 102 THE HUMAN BODY might indeed be expected from the passive and merely mechanical role which this tissue plays. Hyaline cartilage is the type, or most characteristically developed form, of a tissue found with modifications else- where in the Body. One of its other modifications is the so-called cellular cartilage, which consists of the cells with hardly any matrix, only just enough to form a thin capsule around each. This form is that with which all the car- tilages commence, the hyaline variety heing built up by the increase of the cell capsules and their fusion to form the matrix. It persists throughout life in the thin cartilaginous plate of a mouse's external ear. Other varieties of cartilage are really mixtures of true cartilage and connective tissues,. and will be considered after the latter. The Connective Tissues. These complete the. skeleton, marked out in its coarser features by the bones and car- tilages, and constitute the final group of the supporting tissues. They occur in all forms from Inroad membranes and stout cords to the finest threads, forming networks around the other ultimate histological elements of various organs. In addition to subsidiary forms, three main varie- ties of this tissue are readily distinguishable, viz., areolar, white fibrous, and yellow elastic. Each consists of fibres and cells, the fibres being of two kinds, mixed in nearly equal proportions in the areolar variety, while one kind predominates in one and another in the second of the re- maining chief forms. Areolar Connective Tissue. This exists abundantly be- neath the skin, where it forms a loose flocculent layer, somewhat like raw cotton in appearance but not so white. It is on account of its loose texture that the skin can every- where be moved, more or less, to and fro over the subja- cent parts to which it is united by this tissue. Areolar tissue consists of innumerable bands and cords interlacing in all directions, andean be greatly distended by blowing air in at any point, from whence it travels widely through the intercommunicating meshes. In dropsy of the legs or feet the cavities of this tissue are distended with lymph. From beneath the skin the areolar tissue extends all through the CONNECTIVE TISSUES. 1§3 Body between the muscles and around the blood-vessels and nerves; and still finer layers of it enter into these and other organs and unite their various parts together. It consti- tutes in fact a soft packing material which fills up the holes and corners of the Body, as for instance around the blood-vessels and between the muscles in Fig. 4. Microscopic Structure of Areolar Tissue. When exam- ined with the microscope areolar tissue is seen to consist of nucleated cells imbedded in a ground substance which is permeated by fibres. The fibres everywhere form the pre- dominant feature of the tissue (the homogeneous matrix and the cells being inconspicuous) and are of two very dif- ferent kinds. In a strict sense indeed the areolar tissue ought to be considered as a mixture of two tissues, one corresponding to each variety of fibres in it. It is charac- terized as a distinct individual by its loose texture and by the fact that the two forms of fibres are present in tolera- bly equal quantities. In many places a tissue containing the same histological elements as the areolar tissue is found in the form of dense membranes, as for example periosteum and perichondrium. White Fibrous Tissue. One of the varieties of fibres pervading the matrix of areolar tissue exists almost un- mixed with the other kind in the cords or tendons which unite muscles to the bones. This form, known as the white fibrous connective tissue, also exists fairly pure in the ligaments around most joints. Physically it is very flexi- ble but extremely tough and inextensible, so that it will readily bend in any direction but is very hard to break; when fresh it has an opaque white color. White fibrous tissue (Fig. 43) consists of a matrix, con- taining cavities in which cells lie and pervaded by bundles of extremely fine fibres. These fibres lie in each bundle tolerably parallel to one another in a wavy course (Fig. 43) and never branch or unite. Their diameter varies from 0.0005 to 0.001 millimeter (^JoT to ^-^ of an inch). Chemically this tissue is characterized by the fact that its fibres swell up and become indistinguishable when t' sated with dilute acetic acid, and by the fact that it 104 THE HUMAN BODY. yields gelatin when boiled in water. The Substance in it, and in the bones, which is turned into gelatin by such treatment is known as collagen. Glue is impure gelatin obtained from tendons and ligaments, and calfs-foot jelly, so often recommended to invalids, is a purer form of the same substance obtained by boiling the feet of calves, which contain the tendons of many muscles passing from the leg to the foot. FIG. 43. FIG. 43a. FIG. 43.— White fibrous connective tissue, highly magnified. The nucleated corpuscles, seen edgewise and appearing spindle-shaped, are sesn here and there on the surface of the bundles of fibres. FIG. 43a.— Yellow elastic tissue, magnified after its fibres have been torn apart. Elastic Tissue. This is almost invariably mixed in some proportion in all specimens of white fibrous tissue, even the purest, such as the tendons of muscles; but in certain places it exists almost alone, as for example in the liga- ments (ligamenta subflava) between the arches of the vertebra, and in the coats of the larger arteries. In quad- rupeds it forms the great ligament already referred to (p. 84), which helps to sustain the head. This tissue, in CONNECTIVE-TISSUE CORPUSCLES. 105 mass, is of a dull yellow color and extremely extensible and elastic; when purest nearly as much so as a piece of india- rubber. Sometimes it appears under the microscope to be made up of delicate membranes, but most often it is in the form of fibres (Fig. 43«) which are coarser than those of white fibrous tissue and frequently branch and unite. It is unaffected by acetic acid and does not yield gelatin when boiled. Connective-Tissue Corpuscles. The fibres of white fi- brous tissue, wherever it is found, are united into bundles by a structureless ground material known as the cement sub- stance, which also invests each bundle, or skein as we may call it, with a delicate coating. In this ground sub- stance are numerous cavities, branched and flattened in FIG. 44. — Connective-tissue corpuscles. one diameter, and often intercommunicating by their branches. In these cavities lie nucleated masses of proto- plasm (Fig. 44), frequently also branched, known as the connective-tissue corpuscles. These it is which build up the tissue, each cell in the course of development forming around it a quantity of intercellular substance, which sub- sequently becomes fibrillated in great part, the remainder forming the cement. The cells do not quite fill the cavi- ties in which they lie, and these opening into others by their offsets there is formed a set of minute tubes ramify- ing through the connective tissues ; and (since these in turn permeate nearly all the Body) pervading all the organs. In these cell cavities and their branches the lymph flows before it enters definite lymphatic vessels, and they are ac- 106 THE HUMAN BODY. cordingly known as lymph canaliculi. In addition to the fixed branched connective- tissue corpuscles there are often found other cells, when living connective tissue is exam- ined. These cells much resemble white "blood corpuscles, and probably are such which have bored through the walls of the finer blood-vessels. They creep about along the canaliculi by means of their faculty of amoeboid movement, and are known as the "wandering cells." During in- flammation at any point their number in that region is greatly increased. Subsidiary Varieties of Connective Tissue. In various parts of the Body are connective-tissue structures which have not undergone the typical development,, but have de- parted from it in one way or another. The cells having formed a non-fibrillated intercellular substance around them, development may go no farther and the mass remain permanently as the jelly-like connective tissue; or, as in the vitreous humor of the eye (Chap. XXXI.), the cells having formed the soft matrix, may disappear and leave the latter only. In other cases the intercellular substance disappears and the cells branching, and joining by the ends of their branches, form a network themselves, nucleated or not at the points answering to the centre of each originally separate cell. This is known as adenoid connective tissue. In other cases the cells almost alone constitute the tissue, becoming flattened, closely fitted at their edges,, and united by a very small amount of cement substance. Membranes formed in this way lie beneath layers of epithelium in many places and are known as basement membranes. In brain and spinal cord, protecting and supporting the nerve tissues, are found branched cells forming the neuroglia. They are not true connective tissue, but correspond to cells of the horny layer of the epidermis, shut in when the medullary canal was closed in the embryo. Elastic Cartilage, and Fibro-Cartilage. We may now return to cartilages and consider those forms which are made up of more or less tme cartilage mixed with more or INTERARTICULAR CARTILAGES. 107 less connective tissue of one kind or another. The carti- lages of the ear and nose and some others have their matrix pervaded by fine branching fibres of yellow elastic tissue, "which form networks around the groups of cartilage cells. Such cartilages are pliable and tough and possess also con- siderable extensibility and elasticity. They are known as elastic 01% from their color, as yelloiv cartilages. Elsewhere, especially in the cartilages which lie between the bones in some joints, we find forms which have the matrix pervaded by white fibrous tissue and known as fibro-cartilages. For •example the articular cartilage on the end of the lower jaw FIG. 45. — Section through the joint of the lower jaw showing its interarticular fibro-cartilage, x, with the synovial cavity on each side of it. does not come into direct contact with that covering its socket on the skull, but lying between the two in the joint (Fig. 45) is an interarticular fibro-cartilage : similar cartilages exist in the knee- joint; and the intervertebral disks arc also made up of this tissue. Both elastic cartilage and fibro-cartilage •often shade off insensibly into pure elastic or pure white fibrous connective tissue. Homologies of the Supporting Tissues. Bone, cartilage, and connective tissue all agree in broad structural charac- ters, and in the uses to which they are applied in the Body. In each of them the cells which have built up the tissue, 108 THE HUMAN BODY. with rare exceptions, form an inconspicuous part of it in its fully developed state, the chief mass of it consisting of intercellular substance. In hyaline cartilages this latter is not fibrillated; but these cartilages pass insensibly in va- rious regions of the Body into elastic or fibro-cartilages, and these latter in turn into elastic or fibrous connective tissue. The lamellae of bone, too, when peeled off a bone softened in acid and examined with a very high magnifying power, are seen to be pervaded by fine fibres. Structurally, there- fore, one can draw no hard and fast line between these tis- sues. The same is true of their chemical composition; bone and white fibrous tissue contain a substance (collagen) which is converted into gelatin when boiled in water; and in old people many cartilages become hardened by the de- posit in their matrix of the same lime salts which give its hardness to bone. Further, the developmental history of all of them is much alike. In very early life each is repre- sented by cells only : these form an intermediate substance,, and this subsequently may become fibrillated, or calcified,, or both. Finally they all agree in manifesting in health no- great physiological activity, their use in the Body depend- ing upon the mechanical properties of their intercellular substance. The close alliance of all three is further shown by the frequency with which they replace one another. All the bones and cartilages of the adult are at first represented only by collections of connective tissue. Before or after birth this is in some cases substituted by bone directly (as in the case of the collar-bone and the bones on the roof of the skull), while in other cases cartilage supplants the con- nective tissue, to be afterwards in many places replaced by bone, while elsewhere it remains throughout life. Moreover in different adult animals we often find the same part bony in one, cartilaginous in a second, and com- posed of connective tissue in a third: so that these tissues not only represent one another at different stages in the life of the same animal but permanently throughout the whole life of different animals. Low in the animal scale HYGIENE OF GROWING SKELETON. 109 we find them all represented merely by cells with struc- tureless intercellular substance : a little higher in the scale the latter becomes fibrillated and forms distinct connective tissue. In the highest Mollusks (see Zoology), as the cuttle- fishes, this is partly replaced by cartilage, and the same is true of the lowest fishes; while in some other fishes and the remaining Vertebrates, we find more or less bone ap- pearing in place of the original connective tissue or carti- lage. From the similarity of their modes of development and fundamental structure, the transitional forms which exist between them, and the frequency with which they replace one another, histologists class all three (bone, cartilage, and connective tissue) together as homologous tissues and re- gard them as differentiations of the same original struc- ture. Hygienic Remarks. Since in the new-born infant many parts which will ultimately become bone, consist only of car- tilage, the young child requires food which shall contain a Luge proportion of the lime salts which are used in building up bone. Nature provides this in the milk, which is rich in such salts (see Chap. XX. ), and no other food can thoroughly replace it. If the mother's health be such as to render it unwise for her to nurse her infant, the best substitute, apart from a wet-nurse, will be cow's milk diluted with one fourth its volume of water. Arrowroot, corn-flour, and other starchy foods will not do alone, since they are all defi- cient in the required salts, and many infants though given food abundant in quantity are really starved, since their food does not contain the substances requisite for their healthy development. At birth even those bones of a child which are most ossi- fied are often not continuous masses of osseous tissue. In the humerus for example the shaft of the bone is well ossified and so is each end, but between the shafts and each of the articular extremities there still remains a cartilagi- nous layer, and at those points the bone increases in length, new cartilage being formed and replaced by it. The bone 110 THE HUMAN BODY. increases in thickness by new osseous tissue formed beneath the periosteum. The same thing is true of the bones of the leg. On account of the largely cartilaginous and imperfectly knit state of its bones, it is cruel to encourage a young child to walk beyond its strength, and may lead to " bow-legs" or other permanent distortions. Nevertheless here as else- where in the animal body, moderate exercise promotes the growth of the tissues concerned, and it is nearly as bad to wheel a child about forever in a baby-carriage as to force it to walk beyond its strength. The best rule is to let a healthy child use its limbs when it feels inclined, but not by praise or blame to incite it to efforts which are beyond its age, and so sacrifice its healthy growth to the vanity of parent or nurse. The final knitting together of the bony articular ends with the shaft of many bones takes place only compara- tively late in life, and the age at which it occurs varies much in different bones. Generally speaking, a layer of cartilage remains between the shaft and the ends of the bone, until the latter has attained its full adult length. To take a few examples: the lower articular extremity of the humerus only becomes continuous with the shaft by bony tissue in the sixteenth or seventeenth year of life. The upper articular extremity only joins the shaft by bony con- tinuity in the twentieth year. The upper end of the femur joins the shaft by bone from the seventeen th to the nine- teenth year, and the lower end during the twentieth. In the tibia the upper extremity and the shaft unite in the twenty-first year, and the lower end and the shaft in the eighteenth or nineteenth: while in the fibula the upper end joins the shaft in the twenty-fourth year, and the lower end in the twenty-first. The separate vertebrae of the sacrum are only united to form one bone in the twenty-fifth year of life; and ti.e ilium, ischium, and pubis unite to form the os innominatum about the same period. Up to about twenty-five then the skeleton is not firmly "knit," and is incapable, without risk of injury, of bearing strains which it might afterwards meet with impunity. To let FAT- CELL 8. Ill lads of sixteen or seventeen row and take other exercise in plenty is one thing, and a good one; but to allow them to undergo the severe and prolonged strain of training for and rowing a race is quite another, and not devoid of risk. Adipose Tissue. Fatty substances of several kinds ex- ist in considerable quantity in the Human Body in health, some as minute droplets floating in the bodily liquids or imbedded in various cells, but most in special cells, nearly filled with fat, and collected into masses with supporting and nutritive parts, to form adipose tissue. In fact al- most in every spot where the widely distributed areo- lar tissue is found, there is adipose tissue in greater or less proportion along with it. Considerable quantities ex- ist for example in the subcutaneous areolar tissue, espe- cially in the female sex, giving the figure of the woman its general graceful roundness of contour when compared with that of the male. Large quantities commonly lie in the abdominal cavity around the kidneys; in the eye-sockets, forming a pad for the eyeballs; in the marrow of bones; around the joints, and so" on. Examined with the microscope (Fig. 46) adipose tissue is found to consist of small vesicles from 0.2 mm. to 0.09 mm. (TJ0- to jl^ inch) in diameter, clustered together into little masses and bound to one another by connective tissue and blood-ves- FIG at ce]lg sels which intertwine around them; supporting connective tis- in this way the little angular masses which are seen in beef suet are formed, each mass be- ing separated by a somewhat coarser partition of areo- lar tissue from its neighbors. The individual fat-cells are round or oval except when closely packed, when they become polygonal. Each consists of a delicate enve- lope containing oily matter, which in life is liquid at the temperature of the Body. Besides the oily mat- ter, a nucleus is commonly present in each fat-cell; 112 THE HUMAN BODY. and sometimes a thin layer of protoplasm forms a lin-. ing to the cell-wall. The oily matter consists of a mixture of palmatin, olein and stearin, which are compounds of palmitic, stearic and oleic acids with glycerine, three molecules of the acid being combined with one of glycerine, with the elimination of water; as for example: CsIL ) CaHi Stearic acid. Glycerine. Stearin. Water. CHAPTER IX. THE STRUCTURE OF THE MOTOR ORGANS. Motion in Animals and Plants. If one were asked to point out the most distinctive property of living animals,, the answer would probably be, their power of executing spontane- ous movements. Animals as we commonly know them are rarely at rest, while trees and stones move only when acted upon by external forces, which are in most cases readily re- cognizable. Even at their quietest times some kind of mo- tion is observable in the higher animals. In our own Bod- ies during the deepest sleep the breathing movements and the beat of the heart continue; their cessation is to an on- looker the most obvious sign of death. Here, however, as elsewhere in Biology, we find that precise boundaries do not exist; at any rate so far as animals and plants are concerned we cannot draw a hard and fast line between them with reference to the presence or absence of apparently sponta- neous motility. Many a flower closes in the evening to ex- pand again in the morning sun; and in many plants compara- tively rapid and extensive movements can be called forth by a slight touch, which in itself is quite insufficient to produce mechanically that amount of motion in the mass. The Venus's flytrap (Dionaea muscipula) for example has fine hairs on its leaves, and when these are touched by an insect the leaf closes up so as to imprison the animal, which is subsequently digested and absorbed by the leaf. The higher plants it is true have not the power of locomotion, they cannot change their place as the higher animals can; but on the other hand some of the lower animals are perma- nently fixed to one spot; and among the lowest plants many are known which swim about actively through the water in 114 THE HUMAN BODY. which they live. The lowest animals and plants are in fact those which have undergone least differentiation in their development, and which therefore resemble each other in possessing, in a more or less manifest degree, all the funda- mental physiological properties of that simple mass of pro- toplasm which formed the starting point of each individual. With the physiological division of labor which takes place in the higher forms we find that, speaking broadly, plants especially develop nutritive tissues, while animals are char- acterized by the high development of tissues with motor and irritable properties; so that the preponderance of these latter is very marked when a complex animal, like a dog or a man, is compared with a complex plant, like a pine or a hickory. The higher animal possesses in addition to greatly developed nutritive tissues (which differ only in detail from those of the plant, and constitute what are therefore often called organs of vegetative life), well-devel- oped spontaneous, irritable and contractile tissues, found mainly in the nervous and muscular systems, and forming what have been called the organs of animal life. Since these place the animal in close relationship with the sur- rounding universe, enabling slight external forces to excite it, and it in turn to act upon external objects, they are also often spoken of as organs of relation. In man they have a higher development on the whole than in any other animal, and give him his leading place in the animate world, and his power of so largely controlling and directing natural forces for his own good, while the plant can only passively strive to endure and make the best of what happens to it ; it has little or no influence in controlling the hap- pening. Amoeboid Cells. The simplest motor tissues in the adult Human Body are the amoeboid cells (Fig. 12) already de- scribed, which may be regarded as the slightly modified descendants of the undifferentiated cells which at one time made up the whole Body. In the adult they are not attached to other parts, so that their changes of form only affect themselves and produce no movements in the rest of the Body. Hence with regard to the whole frame they CILIATED CELLS. 115 can hardly be called motor tissues, and so are placed in a group by themselves under the name of undifferentiated tissues. Ciliated Cells. As the growing Body develops from its primitive simplicity we find that the cells lining some oi the tubes and cavities in its interior undergo a very re- markable change, by which each cell differentiates itself into a nutritive, and a highly motile and spontaneous portion. Such cells are found for example lining the windpipe, and a number are represented in Fig. 47. Each has a conical form, the base of the cone being turned to the cavity of the air-tube, and contains an oval nucleus, with a nucleolus. On the broader free end are a number (about thirty on the average) of extremely fine processes called cilia. During life these are in constant rapid move- ment, lashing to and fro in the liquid which moistens the interior of the passage; and as the cells are very closely packed, a bit of the inner sur- face of the windpipe examined with a microscope, looks like a field of wheat or barley when the wind blows over it. Each cilium strikes with more force in one direction than in the opposite, and as this di- rection of more powerful stroke is the same for all the cilia on any one surface, the resultant effect is that the liquid in which they move is driven one way. In the case of tho windpipe for example it is driven up towards the throat, and the tenacious liquid or mucus which is thus swept along is finally coughed or "hawked" up and got rid of, instead of accumulating in the deeper air-passages away down in the chest. These cells afford an extremely interesting example of the division of physiological employments. Each proceeds from a cell which was primitively equally motile, automatic, and, nutritive in all its parts. But in the fully developed state the nutritive duties have been especially assumed by the conical cell-body, while the automatic and contractile prop- erties have been condensed, so to speak, in that modified FIG. 47.-CUiated cells. 118 THE HUMAN BODY. portion of the primitive protoplasmic mass, which forms the cilia. These, being supplied with food by the rest of the cell, are raised above the vulgar cares of life and have the opportunity to devote their whole attention to the perform- ance of automatic movements; which are accordingly far more rapid and precise than those executed by the whole cell before any division of labor had occurred in it. That the movements depend upon the structure and com- position of the cells themselves, and not upon influences reaching them from the nervous or other tissues, is proved by the fact that they continue for a long time in isolated cells, removed and placed in a liquid, as blood serum, which does not alter their physical constitution. In cold-blooded animals, as turtles, whose constituent tissues frequently retain their individual vitality long after that bond of union has been destroyed which constitutes the life of the whole animal as distinct from the lives of its different tissues, the ciliated cells m the windpipe have been found still at work three weeks after the general death of the animal. The Muscles. These are the main motor organs ; their general appearance is well known to every one in the lean of butcher's meat. While amoeboid cells can only move themselves, and (at least in the Human Body) ciliated cells the layer of liquid with which they may happen to be in contact, the majority of the muscles, being fixed to the skeleton, can, by alterations in their form, bring about changes in the form and position of nearly all parts of the Body. With the skeleton and joints, they constitute pre- eminently the organs of motion and locomotion, and are governed by the nervous system which regulates their activ- ity. In fact skeleton, muscles, and nervous system are correlated parts: the degree of usefulness of any one of them largely depends upon the more or less complete de- velopment of the others. Man's highly endowed senses and his powers of reflection and reason would be of little use to him, were his muscles less fitted to carry out the dictates of his will or his joints less numerous or mobile. All the muscles are under the control of the nervous system, but all are not governed by it with the co-operation of will or VARIETIES OF MUSCLE. 11? -consciousness; some moving without our having any direct knowledge of the fact. This is especially the case with cer- tain muscles which are not fixed to the skeleton but sur- round cavities or tubes in the Body, as the blood-vessels and the alimentary canal, and by their movements control the passage of substances through them. The former group, or skeletal muscles, are also from their microscopic characters known as striped muscles, while the latter, or visceral mus- cles, are called unstriped or plain muscles. The skeletal muscles being generally more or less subject to the control •of the will (as for example those moving the limbs) are frequently spoken of as voluntary, and the visceral muscles, which change their form independently of the will, as invol- untary. The heart-muscle forms a sort of intermediate link ; it is not directly attached to the skeleton, but forms a hollow bag which drives on the blood contained in it and that quite involuntarily; but in its microscopic structure it resembles the skeletal voluntary muscles. The muscles of respiration might perhaps be cited as another intermediate group. They are striped skeletal muscles and, as we all know, are to a certain extent subject to the will; any one can draw a deep breath when he chooses. But in ordinary quiet breathing we are quite unconscious of their working, and even when attention is turned to them the power of control is limited; no one can voluntarily hold his breath long enough to suffocate himself. As we shall see hereafter, moreover, any one or all of the striped muscles of the Body may be thrown into activity independently of or even •against the will, as, to cite no other instances, is seen in the "fidgets" of nervousness and the irrepressible trembling of extreme terror; so that the names voluntary and involun- tary are not good ones. The functional differences be- tween the two groups depend really more on the nervous connections of each, than upon any essential difference in the properties of the so-called voluntary or involuntary muscular tissues themselves. The Skeletal Muscles. In its simplest form a skeletal, muscle consists of a red soft central part, the belly, which tapers at each end and there passes into one or more dense 118 THE HUMAN BODY. white cords which consist nearly entirely of white fibrous*, connective tissue. These terminal cords are called the tendons of the muscle and serve to attach it to parts of the bony or cartilaginous skeleton. In Fig. 48 is shown the biceps muscle of the arm, which lies in front of the humerus. Its fleshy belly, Bb, is seen to divide above and end there in two tendons, one of which, Bl, is fixed to the scapula, while the other joins the tendon of a neighboring muscle (the coraco-bracMal) and is also fixed above to the shoulder- blade. Near the elbow-joint the muscle is continued into a single tendon, B', which is fixed to the radius, but gives- an offshoot, B , to the connective-tissue membranes lying, around the elbow- joint. The belly of every muscle possesses the power of shorten- ing forcibly under certain conditions. In so doing it pulls upon the tendons, which being composed of inertensible> white fibrous tissue transmit the movement to the hard parts to which they are attached, just as a pull at one end of rope may be made to act upon distant objects to which the other end is tied. The tendons are merely passive cords and are sometimes very long, as for instance in the case of the muscles of the fingers, the bellies of many of which lie away in the forearm. If the tendons at each end of a muscle were fixed to the same bone the muscle would clearly be able to produce no- movement, unless by bending or breaking the bone; the- probable result in such a case would be that the muscle would be torn by its own efforts. In the Body, however, the two ends of a muscle are always attached to two differ- ent pieces of the skeleton, between which more or less movement is permitted, and so when the muscle pulls it alters the relative positions of the parts to which its ten- dons are fixed. In the great majority of cases a true joint lies between the bones on which the muscle can pull, and when the latter contracts it "produces movement at the joint. Many muscles even pass over two joints and can produce movement at either, as the biceps of the arm which, fixed at one end to the scapula and at the other to the radius, can move the bones at either the shoulder or MUSCLES OF ARM. 119 ?***• \ 7 -P"S$^ 03 W.ec fc, .~j •w * r cs o)^ 'Sge oU-S irnii Sg- £*§-;^- u^ji lili! illpl 2 So'y A 120 THE HUMAN BODY. elbow joint. Where a muscle passes over an articulation it is nearly always reduced to a narrow tendon; otherwise the bulky bellies lying around the joints would make them extremely clumsy and limit their mobility. Origin and Insertion of Muscles. Almost invariably that part of the skeleton to which one end of a muscle io fixed is more easily moved than the part on which it pulls by its other tendon. The less movable attachment of a muscle is called its origin, the more movable its insertion. Taking for example the biceps of the arm, we find that when the belly of the muscle contracts and pulls on its upper and lower tendons, it commonly moves only the fore- arm, bending the elbow-joint as shown in Fig. 49. The 1 io. 40.— The biceps muscle and the arm-bones, to illustrate how, under ordi- nary circumstances, the elbow joint is flexed when the muscle contracts. shoulder is so much more firm that it serves as a fixed point, and so that end is the origin of the muscle, and the forearm attachment, P, the insertion. It is clear, how- ever, that this distinction in the mobility of the points oi fixation of the muscle is only relative for, by changing the conditions, the insertion may become the stationary and origin the moved point ; as for instance in going up a rope "hand over hand." la that case the radial end of the muscle is fixed and the shoulder is moved through space by its contraction. Different Forms of Muscles. Many muscles of the Body have the simple typical form of a belly tapering to a FORMS OF MUSCLE. 121 FIG. 50.— Diagrams il- .single tendon at each end as «, Fig. 50; but others divide .at one end and are called two-headed or biceps muscles; while some are even three-headed or triceps muscles. On the other hand some muscles have no tendon at all at one end, the belly running right up to the point of attachment; .and some have no tendon at either end. In many muscles a tendon runs along one side and the fibres of the belly are attached obliquely to it: such muscles (b, ABC Fig. 50) are called penniform or feath- er-like; or a tendon runs obliquely down the middle of the muscle and has the iibres of the belly fixed obliquely on each side of it (c, Fig. 50), forming a bipenniform muscle: or even two ten- dons may run down the belly and so form a tripenniform muscle. In a few cases a tendon is found in the middle of the belly as well as at each end of it; such muscles are called digastric. A muscle Of this form (Fig. 51) is found in COn- mu^le;c,abipenmforin nection with the lower jaw. It arises by a tendon attached to the base of the skull; from there its first belly runs downwards and forwards to the neck by the side of the hyoid bone, where it ends in a tendon which passes through a loop serving as a pulley. This is succeeded by a second belly directed upwards towards the chin, where it ends in a tendon inserted into the lower jaw. Running along the front of the belly from the pelvis to the chest is a long muscle on each side of the middle line called the rectus obdonnnis : it is poly gastric, consisting of four bellies sep- arated by short tendons. Many muscles moreover are not rounded but form wide flat masses, as for example the muscle Ss seen on the ventral side of the shoulder-blade in Fig. 48. Gross Structure of a Muscle. However the form of the skeletal muscles and the arrangement of their tendons may vary, the essential structure of all is the same. Each FIG. 51. — A di- gastric muscle. 122 THE HUMAN JBODT. consists of a proper striped muscular tissue, which is its es- sential part, but which is supported by connective tissue, nourished by blood-vessels and lymphatics, and has its ac- \tivity governed by nerves; so that a great variety of things go to form the complete organ. A loose sheath of areolar connective tissue, called the | perimysium, envelops each muscle, and from this parti- tions run in and subdivide the belly into bundles orfasci- \ cult which run from tendon to tendon, or for the whole length of the muscle when it has no tendons. The coarse- ness or fineness of butcher's meat depends upon the size of these primary fasciculi, which differs in different muscles of the same animal. These larger fasciculi are subdivided by finer connective-tissue- membranes into smaller ones (as shown in Fig. 52, which represents a few pri- mary fasciculi of a muscle and the secondary fasciculi into which these are di- vided), each of which con- sists of a certain number of muscular fibres bound to- showing the secondary fasciculi of which gether by very fine con- nective tissue and envel- microscopic blood - vessels. Where a muscle tapers the fibres in the fasciculi become- less numerous, and when a tendon is formed disappear altogether, leaving little but the connective tissue. Histology of Muscle. For the present we need only concern ourselves with the muscular fibres. Each of these is from eight to thirty-five millimeters (-J- to 1J inch) long, but only from 0.034 to 0.055 mm. (T|^- to ^ inch) in diameter in its widest part, and tapering to a point at each (end. Hence in long muscles with terminal tendons, no fibre runs the whole length of a fasciculus, which may be a foot or more long, but the fasciculus is made up of many successive fibres, the narrow end of each fitting in between the ends of those which follow it. In short or penniform FIG. 52.— A small bit of muscle com- posed of four primary fasciculi. A, natural size; B. the same magnified, the primary are composed. oped in a close network of PLAIN MUSCULAR TISSUE. 123 muscles, where the fasciculi are short, the fibres may run the whole length of each of the latter. Examined carefully with a good microscope each fibre is seen to possess a very thin homogeneous sheath or envel- ope, called the sarcolemma, within which lies the contrac- tile portion of the fibre, b, which presents a striped appear- ance as if composed of darker and dimmer alternating bands (Fig. 53). During life this substance is very soft or semifluid, but after death it rapidly solidi- fies and death-stiffening, or rigor mortis, is produced. Besides the contractile sub- stance a number of oval nuclei, each sur- rounded by a little unmodified protoplasm, lie inside the sarcolemma. The latter is im perforate except at one point where the central portion (or axis cylinder, see Chap. XII.) of a nerve-fibre penetrates it, and ends in an expansion or end plate which is in immediate contact with the striated sub- stance. The larger blood-vessels of a muscle run in the coarser partitions of the connective tissue, and the finer ones lie close around each fibre but entirely outside its sarcolem- ma. Structure of the Unstriped Muscles. Of these the muscular coat of the stomach (Fig. 54) is a good example. They have no definite tendons but form expanded mem- branes surrounding cavities, so that they have no definite origin or insertion. Like the skeletal muscles they consist of proper contractile elements, with accessory connective tissue, blood-vessels, and nerves. Their fibres, however, have a very different microscopic structure. They present no striation but are made up of elongated cells (Fig. 55), bound together by a small quan- tity of cementing material. Each cell is flattened in one plane and tapers off at each end; in its interior lies an elongated nucleus with one or two nucleoli. These cells FIG. 53.— A small piece of muscular fibre highly mag- nified. At a the fibre has been crushedand twist- ed so as to tear its contents while the tougher sar- colemma, e 1 s e- where so closely applied to the rest as to be invisible, remains untorn and conspicuous. 124 THE HUMAN BODY. FIG. 54. — The muscular coat of the stomach. have the power of shortening in the direction of their long axes, and so of diminishing the capacity of the cavities in the walls of which they lie. Cardiac Muscular Tissue. This con- sists of flattened branched cells which unite to form a network, in the interstices of which blood capillaries and nerve-fibres run. The cells present transverse striations, but not so distinct as those of the skeletal mus- cles, and are said to have no sarcolemma. The Chemistry of Muscular Tissue. The chemical structure of the living muscular fibre is unknown, since all the methods of I chemical analysis at present discovered de- Compose and kill it. It contains 75 per cent of water; and, among other inorganic constituents, phosphates and chlorides of potassium, sodium, and magnesium. When at rest a living muscle is feebly alkaline, but ] after hard work, or when dying, this reaction is reversed through the formation of sarco- lactic acid (CaH^Oa). Muscles contain small quantities of grape sugar and glycogen, and of organic 55-unstriped muscie-ceiis. CHEMISTRY OF MUSCLE. 125 nitrogenous crystalline compounds, especially kreatin I (C4H9N302). As in the case of all other physiologically ac- ' tive tissues, however, the main post mortem constituents of the muscular fibres are proteid substances, and it is proba- ble that like protoplasm itself (p. 24) the essential con- tractile part of the tissue consists of a complex body con- taining proteid, carbohydrate and fatty residues; and that during muscular worktKTsls broken up, yielding proteins , / \ -carbonjik^lde, sarcolactic acid, and probably other things; for this hypothetical substance, which has never yet been isolated, the name inonen^ has been proposed. The main proteid substance obtained from muscles is that known as myosin, which is prepared as follows. Perfectly fresh and still living muscles are cut oat from a frog which has just been killed by destruction of its brain and spinal •cord, a proceeding which entirely deprives the animal of •consciousness and the power of using its muscles, but leaves these latter unaltered and alive for some time. The •excised muscles are thrown into a vessel cooled below 0° C. by a freezing mixture and so are frozen hard before any great chemical change has had time to occur in them. The solidified muscles are then cut up into thin slices, the bits •thrown on a cooled filter and let gradually warm up to the "freezing point of water, with the addition of some ice-cold 0.5 per cent solution of common salt. Gradually a small quantity of a tenacious liquid filters through, which is at first alkaline to test-paper but soon sets into a jelly and » becomes acid. The coagulation and the acidity are due to I the breaking up of the muscle substance into the myosin ( and other bodies referred to above. At first the jelly is transparent, but soon the myosin becomes opaque and \ shrinks just like blood fibrin, squeezing out a quantity of I muscle serum, and remaining itself as the muscle clot. Myosin thus prepared is insoluble in water and in saturated solution of common salt, but soluble in five or ten per cent watery solutions of the latter substance. When boiled it is turned into coagulated proteid (p. 11) and becomes in- soluble in dilute acids, though myosin itself dissolves in them easily, being at the same time converted into 126 THE HUNAN BODY. another proteid, called syntonin, which was formerly con- sidered to be the original proteid yielded by the muscles. Syntonin is insoluble in water but soluble in dilute acids and alkalies and its solutions are not coagulated by boiling. Beef Tea and Liebig's Extract. From the above stated facts it is clear that when a muscle is boiled in water its myosin is coagulated and left behind in the meat: even if cooking be commenced by soaking in cold water, the myosin still remains as it is insoluble in cold water. Beef tea as ordinarily made, then, contains little but the flavoring matters and salts of the meat and some gelatin, the former making it deceptively taste as if it were a strong solution of the whole meat, whereas it contains but little of the most nutritious proteid portions, which in an insipid shrunken form are left when the liquid is strained off. Various pro- posals have been made with the object of avoiding this and getting a really nutritive beef tea; as for example chopping the raw meat fine and soaking it in strong brine for some hours to dissolve out the myosin; or extracting it with dilute acids which turn the myosin into syntonin and dissolve it, at the same time rendering it non-coagulable by heat when subsequently boiled. Such methods, however, make unpalatable compounds which invalids, as a rule, will not take. Beef tea is a slight stimulant but hardly a focd and cannot be relied upon to keep up a patient's strength for any length of time. Liebig^s extract of meat is essentially a very strong beef tea; containing much of the flavoring substances of the meat, nearly all- its salts and the crystal- line nitrogenous bodies, such as kreatin, which exist in. muscle, but hardly any of its really nutritive parts. From its stimulating effects it is often useful to persons in feeble health, but other food should be given with it. It may also be used on account of its flavor to add to the " stock''' of soup and for similar purposes; but the erroneousness of the common belief that it is a highly nutritious food can- not be too strongly insisted upon. Under the name of liquid extracts of meat other substances have been prepared LIQUID EXTRACT OF MEAT. by subjecting meat to chemical processes in which it un- dergoes changes similar to those experienced in digestion: the myosin is thus rendered soluble in water and uncoagu- lable by heat, and such extracts if properly prepared are highly nutritious. The flavor may be improved by adding a little of Liebig's extract if desired. CHAPTER X. THE PROPERTIES OF MUSCULAR TISSUE. Contractility. The characteristic physiological property of muscular tissue, and that for which it is employed in the Body, is the faculty possessed by its fibres of shortening forcibly under certain circumstances. The direction in which this shortening occurs is always that of the long axis of the fibre, in both plain and striped muscles, and it is accompanied by an almost equivalent thickening in other diameters, so that when a muscle contracts it does not shrivel up or diminish its bulk in any appreciable way; it simply changes its form. When a muscle contracts it also becomes harder and more rigid, especially if it has to over- come any resistance. This and the change of form can be well felt by placing the fingers of one hand over the biceps muscle lying in front of the humerus of the other arm. When the muscle is contracted so as to bend the elbow it can be felt to swell out and harden as it shortens. Every schoolboy knows that when he appeals to another to " feel his muscle" he contracts the latter so as to make it thicker and apparently more massive as well as harder. In statues the prominences on the surface, indicating the muscles be- neath the skin, are made very conspicuous when violent effort is represented, so as to indicate that they are in vigor- ous action. In a muscular fibre we find no longer the slow, irregular, and indefinite changes of form seen in the undif- ferentiated cells of early development; this is replaced by a precise, rapid, and definite change of form in one di- rection only. Muscular tissue represents a group of cells in the bodily community, which have taken up the one spe- cial duty of executing changes of form, and in proportion MUSCULAR IRRITABILITY. as they have fewer other things to do, they do that one better. This contractility of the muscular fibres may be best con- ceived by considering each to possess two natural shapes; one, the state of rest, in which the fibres are long and nar- row; and the other the state of activity, in which they are- shorter and thicker: under certain conditions the fibres tend to pass, with considerable force, from their resting to their active form, and in so doing they move parts attached to their tendons. When the state of activity passes off the fibres suffer themselves to be passively extended again by any force pulling upon them, and they so regain their rest- | ing shape; and since in the living Body other parts are * nearly invariably put upon the stretch when any given muscle contracts, these by their elasticity serve to pull the latter back again to its primitive shape. No muscular fibre is known to have the power of actively expanding after it has contracted: in the active state it forcibly resists ex- tension, but once the contraction is over it suffers itself readily to be pulled out to its resting form. Irritability. With that modification of the primitive protoplasm of an amoeboid cell which gives rise to a mus- cular fibre with its great contractility, there goes a loss of other properties. Nearly all spontaneity disappears; muscles are not automatic like native protoplasm or ciliated cells; 1 except under certain very special conditions they remain at! rest unless excited from without. The amount of external change required to excite the living muscular fibre is, however, very small; muscle tissue is highly irritable, a>| very little thing being sufficient to call forth a powerful contraction. In the living Human Body the exciting force, or stimulus, acting upon a muscle is almost invariably a nervous impulse, a molecular movement transmitted along the nerve-fibres attached to it, and upsetting the equili- brium of the muscle. It is through the nerves that the will acts upon the muscles, and accordingly injury to the nerve of a part, as the face or a limb, will cause paralysis of its muscles. They may still be there, intact and quite ready to work, but there are no means of sending commands to them, and so they remain permanently idle. Although a 130 THE HUMAN BODY. nervous impulse is the natural physiological muscular stimulus it is not the only one known. If a muscle be exposed in a living animal and a slight but suddenjtap be given to it, or a jiot bar be suddenly brought near it, or an electric shock be senT through it, or a drop of glycerine or oTsolutioirof ammonia be placed on it, it will cbnlract; so that in addition to tlTe natural nervous stimulus, muscles are irritable under the influence of mechanical, thermal, electrical, and chemical stimuli. One condition of the ef- ficacy of all of them is that they shall act with some sud- denness; a very slowly increased pressure, even if ultimately very great, or a very slowly raised temperature, or a slowly increased electrical current passed through it, will not ex- cite the muscle; although far less pressure, warmth, or electricity, more rapidly applied would stimulate it power- fully. It may perhaps still be objected that it is not proved that any of these stimuli excite the muscular fibres, and that in all these cases it is possible that the muscle is only excited through its nerves. For the various stimuli named above also excite nerves (see Chap. XIII), and when we ap- ply them to the muscle we may really be acting first upon the fine nerve-endings there, and only indirectly and through the mediation of these upon the muscular fibres. That the muscular fibres have a proper irritability of their own, independently of their nerves, is, however, shown by the action of certain drugs — for example cur^ud, a South American Indian arrow poison. When this substance is introduced into a wound, all the striped muscles are apparently poisoned, and the animal dies of suffocation because of the cessation of the breathing movements. But the poison does not really act on the muscles themselves: it kills the muscle nerves, but leaves the muscle intact; and it kills the very endings of the muscle-nerves right down in the muscle fibres themselves. Now after its administra- tion we still find that the various non-physiological stimuli /eferred to above make the muscles contract just as powerfully as before the poisoning, so we must conclude that the muscles themselves are irritable in the absence of all nerve stimuli — or, what amounts to the same thing, when MUSCULAR IRRITABILITY. 131 all their nerve-fibres have been poisoned. The experiment also shows that the contractility of a muscle is a property belonging to itself, and that its contracting force is not something derived from the nerves attached to it. The nerve stimulus simply acts like the electric shock or sudden blow and arouses the muscle to manifest a property which it already possesses. The older physiologists seeing that muscular paralysis followed when the nervous connection between a muscle and the brain was interrupted, concluded that the nerves gave the muscles the power of contracting. They believed that in the brain there was a great store of a mysterious thing called vital spirits, and that some of this was ejected from the brain along the nerve to the muscle, when the latter was to be set at work, and gave it its working power. We now know that such is not the case, but that a muscle fibre is a collection of highly irrita- ble material which can have its equilibrium upset in a definite way, causing it to change its shape, under the influ- ence of slight disturbing forces, one of which is a ner- vous impulse ; and since in the Body no other kind of stimulus usually reaches the muscles, they remain at rest when their nervous connections are severed. But the muscles paralyzed in this way can still, in the living Body, be made to contract by sending electrical shocks through them. Physiologically, then, muscle is a contractile and I irritable, but not automatic tissue. / A Simple Muscular Contraction. Most of the details r concerning the physiological properties of muscles have \ been studied on those of cold-blooded animals. A frog's \ muscle will retain all its living properties for some time j after removal from the body of the animal, and so can be experimented on with ease, while the muscles of a rabbit or cat soon die under those circumstances. Enough has, however, been observed on the muscles of the higher ani- mals to show that in all essentials they agree with those of the frog or terrapin. AVhen a single electric shock is sent through a muscle it rapidly shortens and then, if a weight be hanging on it rapidly lengthens again. The whole series of phenomena 132 THE HUMAN BODY. from the moment of stimulation until the muscle regains ) its resting form is known as a "simple muscular contraction'' I or a " tioitch." It occupies in the frog about one tenth oi a second and is separable into three portions. First, there elaspes a time, after the stimulation and before the com- mencement of the shortening, which is known as the " lost | time" or the period of latent excitement. This lasts about one hundredth of a second, and represents the time during which molecular changes preparatory to the contraction are taking place in the muscle fibres. Then follows the short- ening, at first slow, then rapid, then slower again up to a maximum, and occupying rather more than half of the re- maining time; the elongation occupies the remainder of the time taken up in the contraction. In warm-blooded ani- mals, the duration of a simple muscular contraction is even less than one tenth of a second and all its stages are quick- ened. In any given animal, cold increases the time taken » in a muscular contraction and also impairs the contractile power, as we find in our own limbs when "numbed" with cold. Moderate warmth on the other hand (up to near the point at which heat-rigor is produced) diminishes the duration of the contraction; so that t4ie molecular changes in a muscular fibre are clearly eminently susceptible to slight changes in its environment. The contractility of a muscle does not depend upon a vital force, entirely distinct from ordinary inanimate forces, but upon an arrangement of its material elements, which is only maintained under cer- tain conditions and is eminently modifiable by the sur- roundings. Physiological Tetanus. It is obvious that the ordinary movements of the Body are not brought about by such tran- sient muscular contractions as those described in the last paragraph. Even a wink lasts longer than one tenth of a second. Our movements are, in fact, due to more prolonged contractions which may be described as consisting of several \ simple twitches fused together, and known as " tetanic- 1 contractions"; it might be better to call them "compound ' contractions," since the word tetanus has long been used by pathologists to signify a diseased state, such as occurs- MUSCULAR CONTRACTION. 133 in strychnine poisoning and hydrophobia, in which most of the muscles of the Body are thrown into prolonged and powerful involuntary contractions. If, while a frog's muscle is still shortening under the in- fluence of one electric shock, another stimulus be given it, it will contract again and the neAV contraction will be added on to that already existing, without any period of elongation occurring between them. While the muscle is still con- tracting under the influence of the second stimulus a third electric shock will make it contract more, and so on, until the muscle is shortened as much as is possible to it for that strength of stimulus. If now the stimuli be repeated at the proper intervals, each new one will not produce any further shortening, but, each acting on the muscle before the effect of the last has begun to pass off, the muscle will be kept in a state of permanent or " tetanic" contraction; and this can be maintained, by continuing the stimuli, until the organ begins to get exhausted or "fatigued" and it then commences to elongate in spite of the stimulation. When our muscles are stimulated in the Body, from the nerve-centres through the nerves, they receive from the Lit- ter about 20 stimuli in a second, and so are thrown into tetanic contractions. In other words, not even in the most rapid movements of the Body is a muscle made to execute a simple muscular contraction; it is always a longer or a shorter tetanus. When very quick movements are executed, as in performing rapid passages on the piano, the result is obtained by contracting two opposing muscles and alternately strengthening and weakening a little the, tetanus of each. Causes affecting the Degree of Muscular Contraction Thc extent of shortening which can be called forth in a muscle varies with the stimulus. In the first place, a single stimulus can never cause a muscle to contract so much as rapidly repeated stimuli of the same strength— since in the latter case we get, as already explained, several simple contractions such as a single stimulus would call forth, piled on the top of one another. With very power- ful repeated electrical stimuli a muscle can be made to 134 THE HUMAN shorten to one third of its resting length, but in the Body the strongest effort of the will never produces a contrac- tion of that extent. Apart from the rate of stimulation, the strength of the stimulus has some influence, a greater stimulus causing a greater contraction; but very soon a I point is reached beyond which increase of stimulus pro- / duces no increased contraction; the muscle has reached / its limit. The amount of load carried by the muscle (or the resistance opposed to its shortening) has also an influ- ence, and that in a very remarkable way. Suppose we have a frog's calf-muscle, carrying no weight, and find that with a stimulus of a certain strength it shortens two milli- meters (^ inch). Then if we hang one gram (15.5 grains) on it and give it the same stimulus, it will be found to con- tract more, say four or five millimeters, and so on, up to the point when it carries eight or ten grams. After that an increased weight will, with the same stimulus, cause a less contraction. So that up to a certain limit, resistance to the shortening of the muscle makes it more able to shorten: the mere greater extension of the muscle due to the greater resistance opposed to its shortening, puts it into a state in which it is able to contract more powerfully. Fatigue diminishes the working power of a muscle and rest restores it, especially if the circulation of the blood be going on in it at the same time. A frog?s muscle cut out of the body will, however, be considerably restored by a period of rest, even although no blood flows through it. The Measure of Muscular Work. The work done by a muscle in a given contraction, when it lifts a weight verti- cally against gravity, is measured by the weight moved, mul- tiplied by the distance through which it is moved. In the above case when the muscle contracted carrying no load it did very little work, lifting only its own weight;' when loaded with one gram and lifting it five millimeters it did five gram-millimeters of work, just as an engineer would say an engine had done so many kilogrammeters or foot- pounds. If loaded with ten grams and lifting it six millimeters it would do sixty gram-millimeters of work. Even after the weight becomes so great that it is lifted MUSCULAR WORK. 135 through a less distance, the work done by the muscle goes on increasing, for the bigger weight lifted more than com- pensates for the less distance through which it is raised. For example, if the above muscle were loaded with fifty grams it would maybe lift that weight only 1.5 millime- ters, but it would then do seventy-five gram-millimeters -of work, which is more than when it lifted ten grams six millimeters. A load is, however, at last reached with which the muscle does less work, the lift becoming very little indeed, until at last the weight becomes so great that the muscle cannot lift it all and so docs no work when stimulated. Starting then from the time when the muscle •carried no load and did no work, we pass with increasing weights, through phases in which it does more and more work, until with one particular load it docs the greatest •amount possible to it with that stimulus: after ihat, with increasing loads less work is done, until finally a load is readied with which the muscle again does no work. What is true of one muscle is of course true of all, and what is true of work done against gravity is true of all muscular work, so that there is one precise load with which a beast •of burden or a man can do the greatest possible amount of work in a day. With a lighter or heavier load the distance through which it can be moved will be more or less, but the actual work done always less. In the living Body, how- over, the working of the muscles depends so much on other things, as the due action of the circulatory and respiratory systems and the nervous energy or "grit" (upon which the stimulation of the muscles depends) of the individual man or beast, that the greatest amount of work obtainable is not a simple mechanical problem as it is with the excised muscle. Influence of the Form of the Muscle on its Working Power. The amount of work that any muscle can do de- pends of course largely upon its physiological state; a healthy well-nourished muscle can do more than a dis- eased or starved one; but allowing for such variations the work which can be done by a muscle varies with its form. The thicker the muscle, that is the greater the number of 136 THE HUMAN BODY. fibres present in a section made across the long axes of the fasciculi, the greater the load that can be lifted or the other resistance that can be overcome. On the other hand, the extent through which a muscle can move a weight in- creases with the length of its fasciculi. A muscle a foot in length can contract more than a muscle six inches long, and so would move a bone through a greater distance, provided the resistance were not too great for its strength. But if the shorter muscle had double the thickness, then it could lift twice the weight that the longer muscle could. We find in the Body muscles constructed on both plans; some to have a great range of movement, others to overcome great resistance, besides numerous intermediate forms which cannot be called either long and slender or short and thick;, many short muscles for example are not specially thick, but are short merely because the parts on which they act lie near together. It must be borne in mind, too, that many apparently long muscles are really short stout ones — those namely in which a tendon runs down the side or middle of the muscle, and has the fibres inserted obliquely into it. The muscle (gastrocnemius) in the calf of the leg for instance (Fig. 50, b) is really a short stout muscle, for its working length depends on the length of its fasciculi and these are short and oblique, while its true cross-section is that at right angles to the fasciculi and is very large. The force with which a muscle can shorten is very great. A frog's muscle of 1 square centimeter (0.39 inch) in section can just lift 2800 grams (98.5 ounces), and a human muscle of the same area more than twice as much. Muscular Elasticity. A clear distinction must be made between elasticity and contractility. Elasticity is a physi- cal property of matter in virtue of which various bodies tend to assume or retain a certain shape, and when re- moved from it forcibly, to return to it. When a spiral steel spring is stretched it will, if let go, "contract" in a certain sense in virtue of its elasticity, but such a contrac- tion is clearly quite different from a muscular contraction. The spring will only contract as a result of previous distor- tion; it cannot originate a change of form, while the tnus- MUSCULAR ELASTICITY. 13? ole can actively contract or change its shape when a stimulus .acts upon it, and that without being previously stretched. It does not merely tend to return to a natural shape from which it has been removed, but it assumes a quite new natural shape, so that physiological contractility is a differ- ent thing from mere physical elasticity; the essential differ- ence being that the coiled spring or a stretched band only gives back mechanical work which has already been spent on it, while the muscle originates work independ- ently of any previous mechanical stretching. In addition to their contractility, however, muscles are highly elastic. If a fresh muscle be hung up and its length measured, and then a weight be hung upon it, it will stretch just like a piece of indian -rubber, and when the weight is removed, provided it has not been so great as to injure the muscle, the latter will return passively, without any stimulus 01 .active contraction, to its original form. In the Body all the muscles are so attached that they are usually a little stretched beyond their natural resting length; so that when a limb is amputated the muscles divided in the stump shrink away considerably. By this stretched state of the Testing elastic muscles two things are gained. In the first place when the muscle contracts it is already taut, there is no " slack" to be hauled in before it pulls on the parts .attached to its tendons: and, secondly, as we have already seen the working power of a muscle is increased by the presence of some resistance to its contraction, and this is always provided for from the first, by having the origin and insertion of the muscle so far apart as to be pulling on it when it begins to shorten. Physiology of Plain Muscular Tissue. What has hith- erto been said applies especially to the skeletal muscles ; but an the main it is true of the unstriped muscles. These also are irritable and contractile, but their changes of form are much more slow than those of the striated fibres. Upon stimulation, a longer period of latent excitement elapses before the contraction commences, and when finally this takes place it is extremely slow, very gradually attain- ing a maximum and then gradually dying away again. 138 THE HUMAN BODY. There seems in fact to be some connection between that ar- rangement of the contractile substance which shows itself under the microscope as striation and the power of rapid contraction, since we find that the heart, which is not a skeletal or voluntary muscle but yet one that contracts rap- idly, agrees with these in having its fibres striated. Hygiene of the Muscles. The healthy working of the muscles needs of course a healthy state of the Body gener- ally, so that they shall be supplied with proper materials- for growth and repair and have their wastes rapidly and efficiently removed. In other words, good food and pure air arc necessary for a vigorous muscular system, a fact which trainers recognize in insisting upon a strict dietary, and in supervising generally the mode of life of those who are to engage in athletic contests. The muscles should also not bo exposed to any considerable continued pressure since this interferes with the flow of blood and lymph through them. As far as the muscles themselves are directly concerned, exercise is the necessary condition of their best develop- ment. A muscle which is permanently unused degenerates and is absorbed, little finally being left but the connective tissue of the organ and a few muscle fibres filled with oil- drops. This is well seen in cases of paralysis dependent on injury to the nerves. In such cases the muscles may themselves be perfectly healthy at first, but lying unused for weeks they rapidly alter and, finally, when the nervous, injury has been healed, the muscles may be found incapa- ble of functional activity. The physician therefore is often careful to avoid this by exercising the paralyzed muscles daily by means of electrical shocks sent through the part. The same fact is illustrated by the feeble and wasted condi- tion of the muscles of a limb which has been kept for some time in splints. After the latter have been removed it is only slowly, by judicious and persistent exercise, that the long idle muscles regain their former size and power. The great muscles of the "brawny arm" of the blacksmith or wrestler illustrate the reverse fact, the growth of the mus- cles by exercise. Exercise, however, must be judicious; re- MUSCULAR EXERCISE. 139 peatedly continued until exhaustion it does harm; the period of repair is not sufficient to allow replacement of the parts used in wofk, and the muscles thus waste under too violent exercise "as with too little. Rest should alternate with work, and that regularly, if benefit is to be obtained. Moreover violent exercise should never be suddenly undertaken by one unused to it, not only lest the muscles suffer but be- cause muscular effort greatly increases the work of the heart, not only because more blood has to be sent to the muscles themselves, but they produce great quantities of carbon dioxide which must be carried off in the blood to the lungs for removal from the Body, and the heart must work harder to send the blood faster through the lungs, and at the same time the breathing be hastened so as to renew the air in those organs faster. The least evil result of throwing too violent work on the heart and lungs in this way, is repre- sented by being " out of breath," which is advantageous insomuch as it may lead to a cessation of the exertion. But much more serious, and sometimes permanent, injuries of either the circulatory or respiratory organs may be caused by violent and prolonged efforts without any previous train- ing. No general rule can be laid down as to the amount of exercise to be taken ; for a healthy man in business the minimum would perhaps be represented by a daily walk of five miles. Varieties of Exercise. In walking and running the muscles chiefly employed are those of the lower limbs and trunk. This is true also of rowing, which when good is performed much more by the legs than the arms: especially since the introduction of sliding seats. Hence any of these exercises alone is apt to leave the muscles of the chest and arms imperfectly exercised. Indeed no one exercise em- ploys equally or proportionately all the muscles: and hence gymnnasia in which various feats of agility are practiced, so as to call different parts into play, have attained a great popularity. It should be borne in mind however, that the legs especially need strength; while the upper limbs, in which delicacy of movement, as a rule, is more desirable than power, do not require such constant exertion; and the 140 THE HUMAN BOUY. fact that gymnastic exercises are commonly carried on in- doors is a great drawback to their value. When the weather permits, out-of-door exercise is far better than that carried on in even the best ventilated and lighted gymnasium. For those who are so fortunate as to possess a garden there is no better exercise, at suitable seasons, than an hour's daily digging in it; since this calls into play nearly all the muscles of the Body: while of games, the modern one of lawn tennis is perhaps the best from a hygienic view that has ever been introduced, since it not only demands great muscular agility in every part of the Body, but trains the hand to work with the eye in a way that walking, running, rowing and similar pursuits do not. For the same reasons baseball, cricket, and boxing are excellent. Exercise in Infancy and Childhood. Young children have not only to strengthen their muscles by exercise but also to learn to use them. Watch an infant trying to con- vey something to its mouth, and you will see how little control it has over its muscles. On the other hand the healthy infant is never at rest when awake; it constantly throws its limbs around, grasps at all objects within its reach, coils itself about, and so gradually learns to exer- cise its powers. It is a good plan to leave every healthy child, more than a few months old, several times daily on a large bed or even on a rug or carpeted floor, with as little covering as is safe and that as loose as possible, and let it wriggle about as it pleases. In this way it will not only enjoy itself thoroughly, but gain strength and a knowledge of how to use its limbs. To keep a healthy child swathed all day in tight and heavy clothes is cruelty. When a little later the infant commences to crawl, it is safe to permit it as much as it wishes; but unwise to tempt it when disinclined. The bones and muscles are still iee- ble and may be injured by too much work. The same is true of commencing walking. From four or five to twelve years of arre almost any form of exercise should be permitted, or even encouraged. At this time, however, the epiphyses of many bones are not firmly united to their shafts and so anything tending to MUSCULAR EXERCISE. 141 throw too great a strain on the joints should be avoided. After that up to commencing manhood or maidenhood any kind of outdoor exercise for healthy persons is good, and girls are all the better for being allowed to join in their brothers' sports. Half of the debility and general ill-health •of so many of our women is the consequence of deficient ex- ercise during early life; and the day, which fortunately seems approaching, which will see dolls as unknown to, or ,as despised by, healthy girls as healthy boys, will see the be- ginning of a great improvement in the stamina of the female portion of our population. Exercise in Youth should be regulated largely by sex; not that women are to be shut up and made pale, delicate, and unfit to share the duties or participate fully in the pleasures of life; but the other calls on the strength of the young woman render vigorous muscular work often unad- Tisable, especially under conditions where it is apt to be iollowed by a chill. A healthy boy or young man may do nearly anything; but until twenty-two or twenty-three very prolonged effort is unadvisable. The frame is still not firmly knit or as capable of endurance as it will subsequently become. Girls should be allowed to ride or play out-door games in moderation, and in any case should not be cribbed in tight stays or tight boots. A flannel dress and proper lawn-tennis shoes are as necessary for the healthy and safe •enjoyment of an afternoon at that game by a girl as they 4ire for her brother in the base-ball field. Rowing is excel- lei.t for girls if there be any one to teach them to do it prop- erly, with the legs and back and not with the arms only, as women are so apt to row. Properly practiced it strengthens the back and improves the carriage. Exercise in Adult Life. Up to forty a man may carry on safely the exercises of youth, but after that sudden ef- forts should be avoided. A lad of twenty-one or so may, if trained, safely run a quarter-mile race, but to a man of forty-five it would be dangerous, for with the rigidity of the cartilages and blood-vessels which begins to show itself about that time appears a diminished power of meeting a 142 THE HUMAN BODY. sudden violent demand. On the other hand the man of thirty would more safely than the lad of nineteen or twenty undertake one of the long-distance walking matches which have lately heen in vogue; the prolonged effort would be less dangerous to him, though a six days' match with its attendant loss of sleep cannot fail to be more or less dan- gerous to any one. Probably for one engaged in active business a walk of a couple of miles to it in the morning and back again in the afternoon is the best and most avail- able exercise. The habit which Americans have everywhere acquired, of never walking when they can take a horse-car, is certainly detrimental to the general health; though the extremes of heat and cold to which we are subject often render it unavoidable. For women during middle life the same rules apply: there should be some regular but not violent daily ex- ercise. In Old Age the needful amount of exercise is less, and it is still more important to avoid sudden or violent effort. Exercise for Invalids. This should be regulated under medical advice. For feeble persons gymnastic exercises are especially valuable, since from their variety they permit of selection according to the condition of the individual; and their amount can be conveniently controlled. Training. If any person attempts some unusual exer- cise he soon finds that he loses breath, gets perhaps a " stitch in the side," and feels his heart beating with un- wonted violence. If he perseveres he will probably faint — or vomit,as is frequently seen in imperfectly trained men at the end of a hard boat-race. These phenomena are avoided by careful gradual preparation known as "train- ing." The immediate cause of them lies in disturbances of the circulatory and respiratory organs, on which excessive work is thrown. CHAPTER XI. MOTION AND LOCOMOTION. The Special Physiology of the Muscles. Having now considered separately the structure and properties in gen- eral of the skeleton, the joints, and the muscles, we may go on to consider how they all work together in the Body. The properties of a muscle for example are everywhere the same, but the uses of different muscles are very varied, by reason of the different parts with which they are connected. Some are muscles of respiration, others of deglutition; many are known as flexors because they bend joints, others as extensors because they straighten them, and so on. The determination of the exact use of any particular muscle is known as its special physiology, as distinguished from its general physiology, or properties as a muscle without refer- ence to its use as a muscle in a particular place. The uses of those muscles forming parts of the physiological mechan- isms concerned in breathing and swallowing will be studied hereafter; for the present we may consider the muscles which co-operate in maintaining postures of the Body; in producing movements of its parts with reference to one another; and in producing locomotion or movement of the whole Body with reference to its environment. In nearly all cases the striped muscles carry out their functions with the co-operation of the skeleton, since nearly all are fixed to bones at each end and when they contract primarily move these, and only secondarily the soft parts attached to them. To this general rule there are, however, exceptions. The muscle for example which lifts the upper eyelid and opens the eye arises from bone at the back of the orbit, but is inserted, not into bone, but into the eyelid 144 THE HUMAN BODY. directly ; and similarly other muscles arising at the back of the orbit are directly fixed to the eyeball in front and serve to rotate it on the pad of fat on which it lies. Many facial muscles again have 110 direct attachment whatever to bones, as for example the muscle (orbicular is or is) which surrounds the mouth-opening and by its contraction nar- rows it and purses out the lips; or the orbicularis palpe- brarum which similarly surrounds the eyes and when it contracts closes them. Levers in the Body. When the muscles serve to move "bones the latter are in nearly all cases to be regarded as levers whose fulcra lie at the joint where the movement takes place. Examples of all the three forms of levers recognized in mechanics are found in the Human Body. Levers of the First Order. In this form (Fig. 56) the fulcrum or fixed point of support lies between the *' weight" P W FIG. 56.— A lever of the first order. F, fulcrum; P. power; W, resistance or weight. or resistance to be overcome, and the "power" or moving force, as shown in the diagram. The distance PF, from the power to the fulcrum, is called the "power-arm;" the distance FW is the weight-arm. When power-arm and weight-arm are equal, as is the case in the beam of an ordi- nary pair of scales, no mechanical advantage is gained, nor is there any loss or gain in the distance through which the weight is moved. For every inch through which P is de- pressed, W will be raised an equal distance. When the power-arm is longer than the other, then a smaller force at P will raise a larger weight at W, the gain being propor- tionate to the difference in the lengths of the arms. For example if PF is twice as long as FW, then half a kilo- gram applied at P will balance a whole kilogram at W, and MOTION AND LOCOMOTION. 145- just more than half a kilogram would lift it; but for every centimeter through which P descended, W would only be lifted half a centimeter. On the other hand when the weight-arm in a lever is longer than the power-arm, there is loss in force but a gain in the distance through which the weight is moved. Examples of the first form of lever are not numerous in the Human Body. One is afforded in the nodding move- ments of the head, the fulcrum being the articulations between the skull and the atlas. When the chin is elevated the power is applied to the skull, behind the fulcrum, by small muscles passing from the vertebral column to the occiput; the resistance is the excess in the weight of the part of the head in front of the fulcrum over that behind it, and is not great. To depress the chin as in nodding does not necessarily call for any muscular effort, as the head will fall forward of itself if the muscles keeping it erect cease to work, as those of us who have fallen asleep during a dull discourse on a hot day have learnt. If the chin however be depressed forcibly, as in the athletic feat of suspending one's self by the chin, the muscles passing from the chest to the skull in front of the atlanto-occipital artic- ulation are called into play. Another example of the em- ployment of the first form of lever in the Body is afforded by the curtsey with which a lady salutes another. In curtseying the trunk is bent forward at the hip-joints, which form the fulcrum; the weight is that of the trunk acting as if all concentrated at its centre of gravity, which lies a little above the sacrum and behind the hip-joints; and the power is afforded by muscles passing from the thighs to the front of the pelvis. Levers of the Second Order. In this form the weight or resistance is between the power and the fulcrum. The power-arm PF is always longer than the Weight-arm WF, and so a comparatively weak force can overcome a consid- erable resistance. But it is disadvantageous so far as re- gards rapidity and extent of movement, for it is obvious that when P is raised a certain distance JFwill be moved a less distance in the same time. As an example of the employ- 146 THE HUMAN BODY. ment of such levers (Fig. 57) in the Body, we may take the act of standing on the toes. Here the foot represents the lever, the fulcrum is at the contact of its fore part with F FIG. 57.— A lever of the second order. F, fulcrum; P, power; TF, weight. The arrows indicate the direction in which the forces act. the ground ; the weight is that of the Body acting down through the ankle-joints at Ta, Fig. 58; and the power is the great muscle of the calf acting by its tendon inserted into the heel-bone (Ca, Fig. 58). Another example is afforded by holding up the thigh when one foot is kept raised from the ground, as in hopping on the other. Here the fulcrum is at the hip- joint, the power is applied at the VTO. 58.— The skeleton of the foot from the outer side. Ta, surface with which the leer-bones articulate ; Ca, the calcaneum into which the tendon (tendo Achillis) of the calf muscle is inserted : Mb, the metatarsal bone of the fifth digit; N, the scaphoid bone; CI, CII, CT/7, first, second, and third cuneiform bones; Cb, the cuboid bone. knee-cap by a 'great muscle (rectus femoris) inserted theie and which arises from the pelvis; and the weight is that of the whole lower limb acting at its centre of gravity, which will lie somewhere in the thigh between the hip and LEVERS IN THE BODY. 147 knee joints, that is between the fulcrum and the point of application of the power. Levers of the Third Order. In these (Fig. 59) the power is between the fulcrum and the weight. In such levers the weight-arm is always longer than the power- arm, so the power works at a mechanical disadvantage, but swiftness and range of movement are gained. It is the lever most commonly used in the Human Body. For example, when the forearm is bent up towards the arm, the fulcrum is the elbow- joint, the power is applied at the insertion of the biceps muscle (Fig. 49*) into the radius (and of another muscle not represented in the figure, the brachialis anticus, into the ulna), and the weight is that W F Fia. 58. — A lever of the third order. JF", fulcrum ; P, power ; IF, weight. of the forearm and hand, with whatever may be contained in the latter, acting at the centre of gravity of the whole somewhere on the distal side of the point of application of the power. In. the Body the power-arm is usually very short so as to gain speed and range of movement, the mus- cles being powerful enough to still do their work in spite of the mechanical disadvantage at which they are thus placed. The limbs are thus made much more shapely than would be the case were the power applied near or beyond the weight. It is of course only rarely that simple movements as those described above take place. In the great majority of those executed several or many muscles co-operate. The Loss to the Muscles from the Direction of their Pull. It is worthy of note that, owing to the oblique direc- *P. 120. 148 THE HUMAN BOD 7. lion in which the muscles are commonly inserted into the* bones, much of their force is lost so far as producing move- ment is concerned. Suppose the log of wood in the dia- gram (Fig. 60) to be raised by pulling on the rope in the direction a; it is clear at first that the rope will act at a great disadvantage; most of the pull transmitted by it will be exerted against the pivot on which the log hinges, and only a small fraction be available for elevating the latter. But the more the log is lifted, as for example into the position indicated by the dotted line, the more useful will be the direction of the pull, and the more of it will be spent on the log and the less lost unavailingly in merely increas- ing the pressure at the hinge. If we now consider the ac- tion of the biceps (Fig. 49) in flexing the elbow-joint, we see similarly that the straighter the joint is, the more of FIG. 60.— Diagram illustrating the disadvantage of an oblique pull. the pull of the muscle is wasted. Beginning with the arm straight, it works at a great disadvantage, but as the fore- arm is raised the conditions become more and more favor- able to the muscle. Those who have practiced the gym- nastic feat of raising one's self by bending the elbows when hanging by the hands from a horizontal bar, know practi- cally that if the elbow-joints are quite straight it is very hard to start; and that, on the other hand, if they are kept a little flexed at the beginning the effort needed is much POSTURES. 149 less; the reason being of course the more advantageous direction of traction by the biceps in the latter case. Experiment proves that the power with which a muscle can contract is greatest at the commencement of its short- ening, the very time at which, we have just seen, it works at most mechanical disadvantage; in proportion as its force becomes less the conditions become more favorable to it. There is however, it is clear, nearly always a considerable loss of power in the working of the skeletal muscles, strength being sacrificed for variety, ease, rapidity, extent, and elegance of movement. Postures. The term posture is applied to those posi- tions of equilibrium of the Body which can be maintained for some time; such as standing, sitting, or lying, com- pared with leaping, running, or falling. In all postures the condition of stability is that the vertical line drawn through the centre of gravity of the Body shall fall within the basis of support afforded by objects with which it is in contact; and the security of the posture is proportionate to the extent of this base, for the wider it is, the less is the risk of the perpendicular through the centre of gravity fall- ing outside of it on slight displacement. ,j The Erect Posture. This is pre-eminently character- istic of man, his whole skeleton being modified with refer- ence to it. Nevertheless the power of maintaining it is only slowly learnt in the first years after birth, and for a long while it is unsafe. And though finally we learn to stand erect without conscious attention, the maintenance of that posture always requires the co-operation of many muscles, co-ordinated by the nervous system. The influ- ence of the latter is shown by the fall which follows a severe blow on the head, which may nevertheless have frac- tured no bone and injured no muscle: the "concussion" of the brain, as we say," stuns" the man, and until its effects have passed off he cannot stand upright. In standing with the arms straight by the sides and the feet together the centre of gravity of the whole adult Body lies in the articulation between the sacrum and the last lumbar verte- bra, and the perpendicular drawn from it will reach the 150 THE HUMAN BODY. ground between the two feet, within the basis of support afforded by them. With the feet close together, however, the posture is not very stable, and in standing we com- monly make it more so by slightly separating them so as to increase the base. The more one foot is in front of the other the more sway- ing back and forward will be compati- ble with safety, and the greater the lateral distance separating them, the greater the lateral sway which is possi- ble without falling. Consequently we see that a man about to make great movements with the upper part of his \ /^\ Body, as in fencing or boxing, or a sol- dier preparing for the bayonet exercise, always commences by thrusting one foot forwards obliquely, so as to increase his basis of support in both directions. The ease with which we can stand is largely dependent upon the way in || which the head is nearly balanced on the top of the vertebral column, so that but little muscular effort is needed to keep it upright. In the same way the trunk is almost balanced on the hip- ll|[| joints: but not quite, its centre of grav- ity falling rather behind them; so that just as some muscular effort is needed to keep the head from falling forwards, FIG. 6i.— Diagram ii- some is needed to keep the trunk from ™c*mb?«!S toppling backwards at the hips. In a the similar manner other muscles are called klip1" thfjXs into pky at otner joints: as between the rigid and the Body erect. vertebral column and the pelvis, and at the knees and ankles; and thus a certain rigidity, due to muscular effort, extends all along the erect Body: which on account of the flexibility of its joints could not other- wise be balanced on its feet as a statue can. Beginning (Fig. 61) at the ankle-joint, we find it kept stiff in standing WALKING 151 tty the combined and balanced contraction of the muscles passing from the neel to the thigh, and from the dorsum of the foot to the sAin-bone (tifaa). Others passing before and behind the knee-joint keep it from yielding; and so at the hip-joints: and others again lying in the walls of the abdomen and along the vertebral column, keep the latter rigid, and erect on the pelvis; and finally the skull is kept m position by muscles passing from the sternum and ver- tebral column to it, in front of and behind the occipital condyles. Locomotion includes all movements of the whole Body in space, dependent on its own muscular efforts: such as walking, running, leaping, and swimming. Walking. In walking the Body never entirely quits the ground, the heel of the advanced foot touching the ground in each step before the toe of the rear foot leaves it. The .advanced limb supports the Body, and the foot in the rear at the commencement of each step, propels it. Suppose a man standing with his heels together to com- mence to walk, stepping out with the left foot; the whole Body is at first inclined forwards; the movement taking place mainly at the ankle-joints. By this means the cen- tre of gravity would be thrown in front of the base formed "by the feet and a fall on the face result, were not simulta- neously the left foot slightly raised by bending the knee and then swung forwards, the toes just clear of the ground and, in good walking, the sole nearly parallel to it. When the step is completed, the left knee is straightened and the sole placed on the ground, the heel touching it first and, the base of support being thus widened fiom before back, a fall is prevented. Meanwhile the right leg is kept straight, but inclines forwards above with the trunk when the latter advances, and as this occurs the sole gradually leaves the ground, commencing with the heel. When the step of the left leg is completed the great toe of the right alone is in contact with the support. With this a push is given which sends the trunk on over the left leg which is now kept rigid, except at the ankle-joint; and the right knee being bent that limb swings forwards, its foot just 152 THE HUMAN BOD Y. clearing the ground as the left did before. The Body is meanwhile supported on the left foot alone, but when the right completes its step the knee of that leg is straightened and the foot thus placed, heel first, on the ground. Mean- while the left foot has been gradually leaving the ground, and its toes alone are at that moment upon it: from these' a push is given, as before with the right foot, and the knee being bent so as to raise the foot, the left leg swings for- wards at the hip-joint to make a fresh step. During each step the whole Body sways up and down and also from side to side. It is highest at the moment when the advancing trunk is vertically over the foot sup- porting it, and then sinks until the moment when the ad- vancing foot touches the ground, when it is lowest. From this moment it rises as it swings forward on this foot, until it is vertically over it, and then sinks again until the other touches the ground; and so on. At the same time, as its weight is alternately transferred from the right to the left foot and vice versa, there is a slight lateral sway, commonly more marked in women than in men, and which when ex- cessive produces an ugly "waddling" gait. The length of each step is primarily dependent on the length of the legs; but can be controlled within wide lim- its by special muscular effort. In easy walking, little mus- cular work is employed to carry the rear leg forwards after it has given its push. When its foot is raised from the ground it swings on like a pendulum; but in fast walking the muscles passing in front of the hip-joint, from the pel- vis to the limb, by their contraction forcibly carry the leg forwards. The easiest step, that in which there is most economy of labor, is that in which the limb is let swing freely, and since a short pendulum swings faster than a longer, the natural step of short-legged people is quicker than that of long-legged ones. In fast walking the advanced or supporting leg also aids in propulsion; the muscles passing in front of the ankle-joint contracting so as to pull the Body forwards over that foot and aid the push from the rear foot. Hence the fatigue and pain in front of the shin which is felt in RUNNING AND LEAPING. 153 •prolonged very fast walking. From the fact that each foot reaches the ground heel first, but leaves it toe last, the length of each stride is increased by the length of the foot. Running. In this mode of progression there is a mo- ment in each step when both feet are off the ground, the Body being unsupported in the air. The toes alone come in contact with the ground at each step, and the knee-joint is not straight when the foot reaches the ground. When the rear foot is to leave the support, the knee is suddenly straightened, and at the same time the ankle-joint is ex- tended so as to push the toes forcibly on the ground and give the whole Body a powerful push forwards and upwards. Immediately after this the knee is greatly flexed and the foot raised from the ground, and this occurs before the toes of the forward foot reach the latter. The swinging leg in each step is violently pulled forwards and not suf- fered to swing naturally as in walking. By this the rapid- ity of the succession of steps is increased, and at the same time the stride is made greater by the sort of one-legged leap that occurs through the jerk given by the straighten- ing of the knee of the rear leg just before it leaves the ground. Leaping. In this mode of progression the Body is raised completely from the ground for a considerable period. In a powerful leap the ankles, knees, and hip- joints are all flexed as a preparatory measure, so that the Body assumes a crouching attitude. The heels, next, are raised from the ground and the Body balanced on the toes. The centre of gravity of the Body is then thrown forwards, and simultaneously the flexed joints are straightened, and by the resistance of the ground, the Body receives a propul- sion forwards; much in the same way as a ball rebounds from a wall. The arms are at the same time swung for- wards. In leaping back, the Body and arms are inclined in that direction; and in jumping vertically there is no lean- ing either way and the arms are kept by the sides. CHAPTER XII. ANATOMY OF THE NERVOUS SYSTEM. K erve-Tnmks.' In dissecting the Human Body numer- ous white cords are found which at first sight might be taken for tendons. That they are something else however soon becomes clear, since a great many of them have no connection with muscles at all, and those which have usually enter somewhere into the belly of the muscle, instead of be- ing fixed to its ends as most tendons are. These cords are nerve-trunks: followed in one direction each (Fig. 62) will be found to break up into finer and finer branches, until the subdivisions become too small to be followed without the aid of a microscope. Traced the other way the trunk will in most cases be found to increase by the union of others with it, and ultimately to join a much larger mass of different structure, and from which other trunks also spring. This mass is a nerve-centre. That end of a nerve attached to the centre is naturally its central, and the other its distal or peripheral end. Nerve-centres, then, give origin to nerve-trunks; these latter radiate all over the Body, usually branching and becoming smaller and smaller as they proceed from the centre; they finally become very small, and how they ultimately end is not in all cases cer- tain, but it is known that some have sense-organs at their terminations and others muscular fibres. The general ar- rangement of the larger nerve-trunks of the Body is shown in Fig. 62. Physically a nerve is not so tough or strong as a tendon of the same size; it may readily be split up into- longitudinal strands, each of which consists of a number of microscopic threads, the nerve-fibres, bound together by connective tissue. ANATOMY OF NERVOUS SYSTEM. 155 FIG. 62.— Diagram illustrating the general arrangement of the nervous system. 156 THE HUMAN BODY. Plexuses. Very frequently several neighboring nerve- trunks send off communicating branches to one another, each branch carrying fibres from one trunk to the other. Such networks are called plexuses (Fig. 65*), and through the interchanges taking place in them it often happens that the distal branches of w nerve-trunk contain fibres which it does not possess as it leaves the centre to which it is connected. Nerve-Centres. The great majority of the nerves take their origin from the brain and spinal cord, which together form the great cerebro- spinal centre. Some, however, com- mence in rounded or oval masses which vary in size from that of the kernel of an almond down to microscopic dimensions, and which are widely distributed in the Body. Each of these smaller scattered centres is called a ganglion, and the whole of them are arranged in three sets. A considerable number of the largest are united directly to one another by nerve-trunks, and also give off nerves to various organs, especially to the blood-vessels and the viscera in the thoracic and abdominal cavities. These ganglia and their branches form the sympathetic nervous system, as distinguished from the eerebro-spinal nervous system consisting of the brain and spinal cord and the nerves springing from them. Of the remaining ganglia some are connected with various eerebro-spinal trunks near their origin, while the rest, for the most part very small and connected with the peripheral branches of sympathetic or other nerves, are known as the sporadic ganglia. The Cerebro-Spinal Centre and its Membranes. Ly- ing in the skull is the brain and in the neural canal of the vertebral column the spinal cord or spinal marrow, the two being continuous through the foramen magnum of the occipital bone and forming the great eerebro-spinal nerve- centre. This centre is bilaterally symmetrical throughout except for slight differences on the surfaces of parts of the brain, which are often found in the higher races of mankind. Both brain and spinal cord are very soft and easily crushed; the connective tissue which pervades them being of the deli- cate retiform variety; accordingly both are placed in nearly MEMBRANES OF THE NERVE-CENTRES. A completely closed bony cavities and Are also enveloped by membranes which give them consistency and support. These membranes are three in number. Externally is the dura mater, very tough and strong .and composed of white fibrous and elastic connective tissues. In the cranium this dura mater adheres by its outer surface to the inside of the skull, serving as the periosteum of its bones; this is not the case in the vertebral column, where the dura mater forms a loose sheath around the spinal cord and is only attached here and there to the sur- rounding bones, which have a sep- arate periosteum of their own. The innermost membrane of the cerebro- spinal centre, lying in immediate contact with the proper nervous parts, is the pia mater, also made up of white fibrous tissue inter- ivoven with elastic fibres, but less •closely than in the dura mater, so as to form a less dense and tough membrane. The pia mater con- tains many blood-vessels which break up in it into small branches before entering the nervous mass beneath. Covering the outside of the pia mater is a layer of flat closely fitting cells, a similar layer lines the inside of the dura mater, and these two layers are described as the third membrane of the cere- bro-spinal centre, called the arach- noid. In the space between the two layers of the arachnoid is con- tained a small quantity of watery 157 FIG. 63.— The spinal cord and medulla oblongata. A. from the ventral, and B, from the dorsal aspect ; C to H, cross-sections at different levels. 158 THE HUMAN BOD Y. cerelro-spindl liquid. The surface of the brain is folded and the pia mater follows closely these folds; the arachnoid often stretches across them : in the spaces thus left between it and the pia mater is contained some of the cerebro-spinal liquid. The Spinal Cord (Fig. 63) is nearly cylindrical in form, being however a little wider from side to side than dorso- ventrally, and tapering off at its posterior end. Its aver- age diameter is about 19 millimeters (f inch) and its length 0.43 meter (17 inches). It weighs 42.5 grams (H ounces). There is no marked limit between the spinal cord and the brain, the one passing gradually into the other (Fig. 70*), but the cord is arbitrarily said to com- mence opposite the outer margin of the foramen magnum : from there it extends to the articulation between the first and second lumbar vertebrae, where it narrows off to a. slender filament, the filum terminate (cut off and repre- sented separately at B' in Fig. 63), which runs back to the end of the neural canal behind the sacrum. In its course the cord presents two expansions, an upper, 10, the cer- vical enlargement, reaching from the third cervical to the first dorsal vertebrae, and a lower or lumbar enlargement^ 9, opposite the last dorsal vertebra. Running along the middle line on both the ventral and the dorsal aspects of the cord is a groove, and a cross-sec- tion show^s that these grooves are the surface indications of fissures which extend deeply into the cord (C, Fig. 64) and nearly divide it into right and left halves. The anterior fissure (1, Fig. 64) is wider and shallower than the posterior, 2. The transverse section, (7, shows also that the substance of the cord is not alike throughout, but that its white superficial layers envelop a central gray substance arranged somewhat in the form of a capital H. Each half of the gray matter is crescent-shaped, and the crescents are turned back to back and united across the middle line by the gray commissure. The tips of each crescent are called its horns or cornua, and the ventral, or anterior cornu, on each side is thicker and larger than the posterior. In the cervical and lumbar enlargements the *P. 169 SPINAL CORD. 159 proportion of white to gray matter is greater than else- where ; and as the cord approaches the medulla oblongata its central gray mass becomes irregular in form and begins to break up into smaller portions. If lines be drawn on the transverse section of the cord from the tip of each horn of the gray matter to the nearest point of the surface, the white substance in each half will be divided into three por- FIG. 64.— The spinal cord and nerve-roots. A, a small portion of the cord seen from the ventral side ; B. the same seen laterally ; C, a cross-section of the cord ; 7), the two roots of a spinal nerve ; 1. anterior (ventral) fissure ; 2, poste- rior (dorsal) fissure ; 3, surface groove along the line of attachment of the ante- rior nerve-roots : 4, line of origin of the posterior roots; 5, anterior root fila- ments of a spinal nerve ; 6. posterior root filaments ; 6'. ganglion of the poste- rior root ; 7, 7', the first two divisions of the nerve-trunk after its formation by the union of the two roots. tions: one between the anterior fissure and the anterior cornu, and called the anterior white column ; one between the posterior fissure and the posterior cornu, and called the posterior white column; ^rhile the remaining one lying in the hollow of the crescent and between the two horns is the lateral column. In addition to this a certain amount of white substance crosses the middle line at the bottom of 160 THE HUMAN BODY. the anterior fissure; this forms the anterior white commis- sure. There is no posterior white commissure, the bottom of the posterior fissure being the only portion of the cord where the gray substance is uncovered by white. Running .along the middle of the gray commissure, for the whole length of the cord, is a tiny channel, just visible to the unaided eye; it is known as the central canal (canalis cen- The Spinal Nerves. Thirty-one pairs of spinal nerve- trunks enter the neural canal of the vertebral column through the intervertebral foramina (p. 71). Each di- vides in the foramen into a dorsal and ventral portion known respectively as the posterior and anterior roots of the nerve (6 and 5, Fig. 64), and these again subdivide into finer branches which are attached to the sides of the cord, the posterior root at the point where the posterior and late- ral white columns meet, and the anterior root at the junc- tion of the lateral and anterior columns. At the lines on "which the roots are attached there are superficial furrows on the surface of the cord. On each posterior root is a spinal ganglion (6 , Fig. 64), placed just before it joins the an- terior root to make up the common nerve-trunk. Imme- diately after its formation by the mixture of fibres from both roots, the trunk divides into a small posterior primary find a larger anterior primary branch (7' 7 D, Fig. 64). The former branches of the spinal nerves go for the most part to the skin and muscles on the back, while the anterior primary branches form a series of plexuses from which the nerves for the sides and ventral region of the neck and trunk, and for the limbs, arise. The various spinal nerves are named from the portions of the vertebral column through the intervertebral foramina of which they pass out; and as a general rule each nerve is named from the vertebra in front of it. For example the nerve passing out between the fifth and sixth dorsal verte- brae is the "fifth dorsal" nerve, and that between the last dorsal and first lumbar vertebrae, the "twelfth dorsal." In the cervical region, however, this rule is not adhered to. The nerve passing out between the occipital bone and the THE SPINAL NERVES. 161 atlas is called the "first cervical" nerve, that between the- atlas and axis the second, and so on; that between seventh cervical and first dorsal vertebrae being the "eighth cervi- cal " nerve. The thirty-one pairs of spinal nerves are then thus distributed: 8 cervical, 12 dorsal, 5 lumbar, 5 sacral, and 1 coccygeal; the latter passing out between the sacrum and coccyx. Since the spinal cord ends opposite the upper lumbar vertebrae while the sacral and coccygeal nerves pass out from the neural canal much farther back, it is clear that the roots of those nerves, on their way to unite in the foramina of exit and form nerve-trunks, must run obliquely backwards in the spinal canal for a considerable distance. One finds in fact the neural canal in the lumbar and sacral regions, behind the point where the spinal cord has tapered off, occupied by a great bunch of nerve-roots forming the so-called " horse's tail" or cauda equina. Distribution of the Spinal Nerves. It would be out of place here to go into detail as to the exact portions of the Body supplied by each spinal nerve, but the following general statements may be made. The anterior primary branches of the first four cervical nerves form on each side- the cervical plexus (Fig. 65) from which branches are sup- plied to the muscles and integument of the neck: also to- the outer ear and the back part of the scalp. The anterior primary branches of the remaining cervical nerves and the- first dorsal form the brackial plexus, from which the upper limb is supplied. The roots of the trunks which form this plexus arise from the cervical enlargement of the spinal cord. From the fourth and fifth cervical nerves on each side, small branches arise and unite to make the phrenic nerve (4', Fig. 65) which runs down through the chest and ends in the diaphragm. The anterior primary branches of the dorsal nerves, ex- cept part of the first which enters the brachial plexus, form no plexus, but each runs along the posterior border of a rib and supplies branches to the chest-walls, and the lower ones to those of the abdomen also. The anterior primary branches of the four anterior lum- bar nerves are united by branches to form the lumbar 162 THE HUMAN BODY. plexus. It supplies the lower part of the trunk, the but- tocks, the front of the thigh, and medial side of the leg. The sacral plexus is formed by the anterior primary branches of the fifth lumbar and the first four sacral nerves, which unite into one great cord and so form the FIG. 65.— The cervical and brachial plexuses of one side of the Body. sciatic nerve, which is the largest in the Body and, running down to the back of the thigh, ends in branches for the lower limb. The roots of the trunks which form the sacral plexus arise from the lumbar enlargement of the cord. THE BRAIN. 163 The Brain (Fig. 66) is far larger than the spinal cord and more complex in structure. It weighs on the average about 1415 grams (50 ounces) in the adult male, and about 155 grams (5.5 ounces) less in the female. In its simpler forms the vertebrate brain consists of three masses, each with subsidiary parts, following one another in series from before back, and known as the fore-brain, mid- brain, and hind-brain respectively. In man the fore-brain, A, weighing FIG. 66. — Diagram illustrating the general relationships of the parts of the brain. A, fore-brain ; 6, mid-brain ; B, cerebellum ; C, pons Varolii ; D, me- dulla oblongata ; B, C, and D together constitute the hind-brain. about 1245 grams (44 ounces), is much larger than all the rest put together and laps over them behind. It consists mainly of two huge convoluted masses, separated from one another by a deep median fissure, and known as the cerebral hemispheres. The immense proportionate size of these is very characteristic of the human brain. Beneath each cerebral hemisphere is an olfactory lobe, inconspicuous in man but often larger than the cerebral hemispheres, as in most fishes. Buried in the fore-brain on each side arc two large gray masses, the corpora striata and optic tlialami. The mid-brain forms a connecting isthmus between the two other divisions and presents on its dorsal side four 164 THE HUMAN BODY. hemispherical eminences, the corpora quadrigemina. On its ventral side it exhibits two semicylindrical pillars (seen under the nerve /Fin Fig. 70*), known as the crura cere- bri. The hind-brain consists of three main parts: on its dorsal side is the cerebellum, B, Fig. 66, consisting of a right, a left, and a median lobe ; on the ventral side is the \)o ns Varolii, C, Fig. 66, and behind the medulla oNongata, D, Fig. 66, which is continuous with the spinal cord. In nature the main divisions of the brain are not sepa- rated so much as has been represented in the diagram for FTG. 67.— The brain from the left side. Cb, the cerebral hemispheres forming the main bulk of the fore-brain ; Cbl, the cerebellum ; Mo, the medulla oblon- gata ; P, the pons Varolii ; *, the fissure of Sylvius. the sake of clearness, but lie close together as represented in Fig. 67, only some folds of the membranes extending between them; and the mid-brain is entirely covered in on its dorsal aspect. Nearly everywhere the surface of the brain is folded, the folds, known as gyri or convolutions, being deeper and more numerous in the brain of man than in that of the animals nearest allied to him; and in the human species more marked in the higher than in the lower races. The brain like the spinal cord consists of gray and white nervous matter but somewhat differently arranged, for while the brain, like the cord, contains gray matter in * P. 169. CEREBRAL VENTRICLES. 165 its interior, a great part of its surface is also covered with, it. By the external convolutions of the cerebellum and the cerebral hemispheres the surface over which this gray substance is spread is very much increased (see Fig. 68). The Ventricles of the Brain. The minute central canal of the spinal cord is continued into the brain and expands there at several points into chambers known as the ventricles. Entering the medulla oblongata it approaches its upper surface and dilates into the fourth ventricle, which has a very thin roof, lapped over by the cerebellum. From the front of the fourth ventricle runs a narrow pas- VI FIG. 68.— A vertical section across the cerebral hemisph,eres. Ccl*. the corpus callosurn : VI, the anterior end of the right lateral ventricle: the gray mass on its exterior is the corpus striatum. On the left side the superficial gray matter covering the convolutions is shaded. sage (Her a tertio ad quartum ventriculum) which enters another dilatation lying in the middle :Kne near the under side of the fore-brain (just above the two small rounded masses seen between the nerves // and /// in Fig. 70) and known as the third ventricle. From the third ventricle two apertures (the foramens of Monro) lead into the first and second, or lateral ventricles, one of which lies in each 1GG THE HUMAN BODY. of the cerebral hemispheres. The front ends of these two ventricles are seen in the vertical transverse section of the brain represented in Fig. G8. The ventricles contain a small amount of cerebro- spinal liquid and are lined by epithelium which is ciliated in early life. Note. A frequent cause of apoplexy is a hemorrhage into one of the lateral ventricles; the outpoured blood accu- mulating and pressing uppn the cerebral hemispheres their functions are suppressed and unconsciousness produced. When a person is found in an apoplectic fit therefore the best thing to do is to leave him perfectly quiet until medi- cal aid is obtained: for any movement may start afresh a bleeding into the ventricle which had been stopped by clots formed in the mouths of the torn blood-vessels. Sections of the Brain. Having got a general idea of the parts composing the brain, the best way to complete a knowledge of its anatomy is to study sections taken in various directions. Two such are given in Figs. 68 and G9. Fig. 69 represents the right half of a vertical section of the brain, taken from before back in the middle line and viewed from the inner side. Above, the knife has passed between the two cerebral hemispheres, in the longitudinal fissure, without cutting either, and the convoluted inner surface of the right one is seen. The sickle-shaped mass lower down, Ccl1 to Ccl* represents the cut surface of a connecting band of white nervous tissue called the corpus callosum f which runs across the middle line from one cerebral hemisphere to the other and puts them in communication. SL the septum lucidum, is a thin membrane which forms the inner wall of the lateral ventricle of the hemisphere. Between the two septa lucida on the sides (in the natural position of the parts) and the corpus callosum above is inclosed a narrow space known as the fifth ventricle. It is, however, quite different from the remaining cerebral ventricles, not being a continuation of the canalis centralis of the spinal cord. The space beneath the septum lucidum and the back part of the corpus callosum is the third ventricle, which, lying in the middle line, has been laid open in the MEDIAN SURFACE OF THE BRAIN. 167 section. It is deep from above down but narrow from side to side. From its under side a prolongation runs down to //, the pituitary body ; behind,, the aqueduct of Sylvius, A, is seen passing back from the third to the fourth ven- tricle, Vq. At FM is the aperture (foramen of Monro) leading into the right lateral ventricle. Crossing the third ventricle and putting the two halves of the fore-brain in direct communication are three small commissures, Coa, Cn FIG. 69.— The right half of the brain as seen on its median side after a section made through the organ in the middle line. Vq, fourth ventricle ; Mo, medulla oblongata ; P, pons Varolii ; II, optic nerve ; H, pituitary body ; Coa, ante- rior commissure ; FM, foramen of Monro leading from the third ventricle, in the cavity of which the lower end of the line SM lies, to the right lateral venr tricle ; Com, soft commissure, running from side to side of the third ventricle, divided ; Cop, posterior commissure ; Lq, corpora quadrigemina ; A, aque.- duct of Sylvius or iter a tertio ad auartum ventriculum ; Cbl, cerebellum ; Cct'-oci*, corpus callosum; SI, septum lucidum; II1, the divided optic com1 missure. Com, and Cop, known respectively as the anterior, the median (or soft), and the posterior. The mass seen bound- ing a great part of the side of the third ventricle and united to its fellow by the soft commissure is the optic thalamus. Above the aqueduct is the small median body CV called the pineal gland, which contains no nervous tissue, but has an interest as being, according to Descartes, 1G8 THE HUMAN BODY. the scat of the soul. Behind it come the corpora quactH- gemina, Lq, and above the fourth ventricle the cerebellum, Cbl, showing the primary and secondary fissures on its surface which give its section a branched appearance known as the arbor vitce. Mo is the medulla oblongata, and P the pons Varolii. The canalis centralis of the spinal cord is represented leading back from the fourth ventricle. Fig. 68 represents a vertical transverse section of the brain taken through the fore part of the corpus callosum (CcP) and altogether in front of the third ventricle. It shows the foldings of the cerebrum and its superficial layer of gray substance; the anterior ends of the lateral ventri- cles, VI, with a gray mass, the corpus striatum lying be- neath and on the outer side of each. If the section had been taken a little farther back the optic tUalami would have been found reaching the floor of each ventricle. The Base of the Brain and the Cranial Nerves. Twelve pairs of nerves .leave the skull by apertures in its 'base, and are known as the cranial nerves. Most of them spring from the under side of the brain, and so they are best studied in connection with the base of that organ, which is. represented in Fig. 70. The first pair, or olfactory nerves, spring from the under sides of the olfactory lobes, /, and pass out through the roof of the nose. They are the nerves of smell. The second pair, or optic nerves, II, spring from the optic thalami and corpora quadrigemina and, under the name of the optic tracts, run down to the base of the brain where they appear passing around the crura cerebri as represented in the figure. In the middle line the two optic tracts unite to form the optic commissure (seen in section at // in Fig. 69) from which an optic nerve pro- ceeds to each eyeball. Behind the optic commissure is seen the conical stalk of the pituitary body or hypophysis cerebri (II in Fig. 69), and still further back a pair of hemispherical masses, about the size of split peas, known as the corpora albicantia. All the remaining cranial nerves arise from the hind- brain. The third pair (motor es oculi) arise from the front of the pons Varolii, and are distributed to most of the THE CRANIAL NERVES. 169 muscles which move the eyeball and also to that which lifts the upper eyelid. The four-sided space bounded by the optic tracts and commissure in front and the third pair of nerves behind, and having on it the pituitary body Fio. 70.— The base of the brain. The cerebral hemispheres are seen over- lapping all the rest. 7, olfactory lobes ; II, optic tract passing to the optic commissure from which the optic nerves proce~ed ; ///, the third nerve or motor oculi ; IV, the fourth nerve or patheticus ; V, the fifth nerve or trigeminalis; VI, the sixth nerve or abducens ; VII, the seventh or facial nerve or portio dura ,' VIII, the auditory nerve or portio mollis ; IX, the ninth or glosso- pharyngeal ; X, the tenth or pneumogastric or vagus ; XL the spinal acces- sory ; XII, the hypoglossal ; ncl, the first cervical spinal nerve. and the corpora albicantia, lies beneath the third ventricle, so that a probe pushed in there would enter that cavity. The fourth pair of nerves, IV (pathetici), arise from the front part of the roof of the fourth ventricle. From there, 170 THE HUMAN BODY. -\^ each curls around a cms cerebri (the cylindrical mass seen beneath it in the figure, running from the pons Varolii to enter the under surface of the cerebral hemispheres) and appears on the base of the brain. Each goes to one muscle of the eyeball. The fifth pair of nerves, V (trigeminales), resemble the spinal nerves in having two roots; one of these is much larger than the other and possesses a ganglion (the Gasse- rian ganglion) like the posterior root of a spinal nerve. Beyond the ganglion the two roots form a common trunk which divides into three main branches. Of these, the ophthalmic is the smallest and is mainly distributed to the muscles and skin over the forehead and upper eyelid; but also gives branches to the mucous membrane lining the nose, and to the integument over it. The second division (superior maxillary nerve) of the trigeminal gives branches to the skin over the temple, to the cheek between the eye- brow and the angle of the mouth, and to the upper teeth;, as well as to the mucous membrane of the nose, pharynx, soft palate and roof of the mouth. The third division (inferior maxillary) is the largest branch of the trigemi- nal; it receives some fibres from the larger root and {ill of the smaller. It is distributed to the side of the head and the external ear, the lower lip and lower part of the face, the mucous membrane of the mouth and the anterior two thirds of the tongue, the lower teeth, the salivary glands, and the muscles which move the lower jaw in mas- tication. The sixth pair of cranial nerves ( VI, Fig. 70) or ab- ducentes arise from the posterior margin of the pons Va- rolii, and each is distributed to one muscle of the eye- ball. The seventh pair (facial nerves), VII, appear also at the posterior margin of the pons. They are distributed to most of the muscles of the face and scalp. The eighth pair (auditory nerves) arise close to the facial. They are the nerves of hearing and are distributed entirely to the internal ear. The ninth pair (glossopharyngeals), IX, arising close to- THE CRANIAL NERVES. 171 the auditories, are distributed to the mucous membrane of the pharynx, the posterior part of the tongue, and the middle ear. The tenth pair (pneumogastric nerves or vagi), JT, arise from the sides of the medulla oblongata. Each gives branches to the pharynx, gullet and stomach, the larynx, windpipe and lungs, and to the heart. The vagus runs farther through the Body than any other cranial nerve. The eleventh pair (spinal accessory nerves), XI, do not arise mainly from the brain but by a number of roots at- tached to the lateral columns of the cervical portion of the spinal cord, between the anterior and posterior roots of the proper cervical spinal nerves. Each, however, runs into the skull cavity alongside of the spinal cord and, getting a few filaments from the medulla oblongata, passes out along with the glossopharyngeal and pneumogastric nerves. Out- side the skull it divides into two branches, one of which joins the pneumogastric trunk, while the other is distrib- uted to muscles about the shoulder. The twelfth pair of cranial nerves (liypoglossi)', XII, arise from the sides of the medulla oblongata; they are distributed mainly to the muscles of the tongue and the hyoid bone. Deep Origins of the Cranial Nerves. The points re- ferred to above, at which the various cranial nerves appear on the surface of the brain, are known as their superficial origins. From them the nerves can be traced for a less or greater way in the substance of the brain until each is fol- lowed to one or more masses of gray matter, which con- stitute its proper starting-point and are known as its deep origin. The deep origins of all except the first and second and part of the eleventh lie in the medulla oblongata. The Ganglia and Communications of the Cranial Nerves. Besides the Gasserian ganglion above- referred to, many others are found in connection with the cranial nerves. Thus for example there is one on each of the main divisions of the trigeminal, two are found on each pneumogastric and two in connection with the glosso- pharyngeal. At these ganglia and elsewhere, the various 172 THE HUMAN BODY. nerves often receive branches from neighboring cranial or spinal nerves, so that very soon after it leaves the brain hardly any one remains free from fibres derived from other trunks except the olfactory, optic, and auditory nerves. This often makes it difficult to say from where the nerves of a special part have come; for example, the nerve-fibres going to the submaxillary salivary gland from the trigemi- nal leave the brain first in the facial and only afterwards enter the fifth; and many of the fibres going apparently from the pneumogastric to the heart come originally from the spinal accessory. The Sympathetic System. The ganglia which form the main centres of the sympathetic nervous system lie in two rows (s, Fig. 2, and sy, Fig. 3), one on either side of the bodies of the vertebrae. Each ganglion is united by a nerve-trunk with the one in front of it, and so two great chains are formed reaching from the base of the skull to the coccyx. In the trunk "region ,these chains lie in the ventral cavity, their relative position in which is indicated by the dots sy in the diagrammatic transverse section re- presented on p. 7 in Fig. 3. The ganglia on these chains are forty-nine in number, viz., twenty-four pairs, and a single one in front of the coccyx in which both chains terminate. They are named from the regions of the vertebral column near which they lie; there being three cervical, twelve dorsal, four lumbar, and five sacral pairs. Each sympathetic ganglion is united by communicating branches with the neighboring spinal nerves, and near the skull with various cranial nerves also; while from the gan- glia and their uniting cords arise numerous trunks, many of which, in the thoracic and abdominal cavities, form plexuses, from which in turn nerves are given off to the viscera. These plexuses frequently possess numerous small ganglia of their own; two of the most important are the cardiac plexus which lies on the dorsal side of the heart, and the solar plexus which lies in the abdominal cavity and supplies nerves to the stomach, liver, kidneys, and intes- tines. Many of the sympathetic nerves finally end in the walls of the blood-vessels of various organs. To the naked HISTOLOGY OF NERVES. 173 •eye they are commonly grayer in color than the cerebro- spinal nerves. The Sporadic Ganglia. These, for the most part very minute, nerve-centres are found scattered in nearly all parts of the Body. They are especially abundant in the neighborhood of secretory tissues and about blood-vessels, while a very important set is found in the heart. Nerves unite them with the cerebro-spinal and sympathetic cen- tres, and probably many oH them belong properly to the .sympathetic system. The Histology of Nerve-Fibres. The microscope shows that in addition to connective tissue and other accessory parts, such as blood-vessels, the nervous organs contain tis- .sues peculiar to themselves and known as nerve-fibres and nerve-cells. The cells are found in the centres only; while the fibres, of which there are two main varieties known as the white and i\\Qgray, are found in both trunks and cen- tres; the white variety predominating in the cerebro-spinal nerves and in the white substance of the centres, and the gray in the sympathetic trunks and the gray portions of the central organs. If an ordinary cerebro-spinal nerve-trunk be examined it will be found to be enveloped in a loose sheath of areolar connective tissue, which forms a packing for it and unites it to neighboring parts. From this sheath, or pcrineurium, bands of connective tissue penetrate the nerve and divide it up into a number of smaller cords or funiculi, much as a muscle is subdivided into fasciculi; each funiculns has a sheath of its own called the neurilemma, composed of several concentric layers of a delicate membrane, with- in which the true nerve-fibres lie. These, which would be nearly all of the white kind, consist of extremely delicate threads, about 0.0125 millm. (-3-5^7 inch) in diameter, but frequently of a length which is in proportion very great. Each nerve-fibre in fact is continuous from a nerve-centre to the organ in which it ends, so that the fibres, e.g. which pass out through the sacral plexus and then run on through the sciatic nerve and its branches to the skin of the toes, are three to four feet long. If a perfectly fresh nerve-fibre 174 THE HUMAN BODY. be examined with the microscope it presents the appear- ance of a perfectly homogeneous glassy thread; but soon it acquires a characteristic double contour (Fig. 71) from the- coagulation of a portion of its substance. By proper treat- ment with reagents three layers may be brought into view.. Outside is a fine transparent envelope (1, Fig. 72) called the primitive sheath j inside this is a fatty substance,, 2r FIG FIG. 72. FIG. 71.— White nerve-fibres soon after removal from the Body and when they have acquired their double contour. FIG. 72. — Diagram illustrating the structure of a white or medullated nerve- $bre. 1, 1, primitive sheath ; 2, 2, medullary sheath ; 3, axis cylinder. forming the medullary sheath (the coagulation of which gives the fibre its double border), and in the centre is a core, the axis cylinder, 3, which is clearly the essential part of the fibre, since near its ending the primitive and medullary sheaths are frequently absent. At intervals of about one millimeter (^ inch) along the fibre are found nuclei. These are indications of the primitive cells which have elongated and formed an envelope for the axis cylin- HISTOLOGY OF NERVE-CELLS. 175 der, which itself is a branch given off by a nerve-cell, in some centre. The medullary sheath is interrupted half way between each pair of nuclei at a point called the node, which is the boundary between two of the enveloping cells. In the course of a nerve-trunk its fibres rarely divide; when a branch is given off some fibres merely separate from the rest, much as a skein of silk might be separated out at one end into smaller bundles containing fewer threads. Gray Nerve-Fibres. Some of these are merely white fibres which near their peripheral ends have lost their medullary sheaths ; others have no medullary sheath throughout their whole course, and consist merely of an axis cylinder (often striated) and nuclei, with or without a primitive sheath. Such fibres are especially abundant in the sympathetic trunks; and they alone form the olfactory nerve. In the communicating branches between the sym- pathetic ganglia and the spinal nerves both white and gray fibres are found; the former are cerebro-spinal fibres pass- ing into the sympathetic system, while the gray fibres origi- nate in the sympathetic system and pass to the membranes and blood-vessels of the spinal cord and spinal column. Another group of gray nerve-fibres may be called nerve- fibrils: they are extremely fine, and result from the sub- division of axis cylinders, close to their final endings in many parts of the Body, after they have already lost both primitive and medullary sheaths. Many fine gray fibres exist in the nerve-centres. The Histology of Nerve-Cells. So far as our knowl- edge at present goes, the only structures known with cer- tainty to be connected with the central ends of nerve-fibres are nerve-cells, and these latter may therefore be regarded as the central organs of the nerve-fibres. So many nerve- fibres have been traced into continuity with nerve-cells,, that it is pretty certain all arise in this way. At 1, Fig. 73, is shown a typical nerve-cell such as may be found in an anterior horn of the gray matter of the spinal cord. It consists of the cell body, or cell protoplasm, in which is a large nucleus containing a nucleolus. From the body of the cell arise several branches, the great ma- 176 THE HUMAN BODY. jority of which are granular and divide frequently in a forking or " dichotomous" manner. These are known as the "proto- plasmic" branches of the cell, and probably serve merely to absorb nourishment for it. One branch,, however, a, gives off at right angles smaller filaments, but still maintains its individuality and ultimately becomes the axis cylinder of a nerve-fibre. Its side branches probably put it in anatomi- cal continuity with other nerve-fibres and other nerve-cells. FIG. 73. — 1, Nerve-cell from anterior horn of gray matter of spinal cord; a, axis-cylinder process. 2, Cell from posterior horn of spinal cord. Nerve-cells from the posterior hor-n of the gray matter of the spinal cord (2, Fig. 73) also possess numerous granu- lar protoplasmic processes, and a nerve-fibre process (b): but this, instead of continuing directly into an axis cylinder, breaks up into a network of fine branches which frequently unite with one another and also, no doubt, with fibrils from neighboring cells. It is almost certain MINUTE STRUCTURE OF SPINAL CORD. 177 that one or more of these fibrils is continued to form the axis cylinder of a nerve-fibre in a dorsal root of one of the spinal nerves. As we shall learn later, the dorsal roots are concerned in carrying impulses from the outer world to the spinal cord; the anterior roots in conveying impulses from the nerve- centres to the organs (muscles, glands, etc.) of the body. Therefore, in general terms, we may speak of the type of nerve-cell 1, Fig. 73, as a motor nerve-cell; and the type of cell 2, Fig. 73, as a sensory nerve-cell. Both varieties of cells are found in the gray matter of the brain. In the sympathetic and sporadic ganglia somewhat simpler forms of nerve-cells occur. Nerve-Centres consist of white and gray nerve-fibres, of nerve-cells, and of connective tissue and blood-vessels ar- ranged in different ways in the different centres. Ganglia are collections of nerve- cells and nerve-fibres, some of the latter being connected with the cells, while others seem merely to pass through the ganglion on their way to other parts. As an illustration of the structure of a more com- plex nerve-centre we may study the spinal cord. Histology of the Spinal Cord. If a thin transverse sec- tion of the spinal cord be examined with a microscope it will be found that enveloping the whole is a delicate layer of connective tissue, the pia mater. Fine bands of it ramify through the cord, supporting the nervous ele- ments; some of the coarser of these are represented at 6, 7, and elsewhere in Fig. 74; but from these still finer proc- esses arise, as represented at d and e in Fig. 75. The ultimate finest tissue supporting the nervous elements di- rectly, is the neuroglia (p. 106). In the white columns, the cord (Fig. 75) will be seen to 'be mainly made up of medul- lated nerve-fibres which run longitudinally and therefore appear in the transverse section as circles, with a dot in the centre, which is the axis cylinder. At 1) in Fig. 75 these fibres are represented, the intermediate connective tissue being omitted, while at e this latter alone is repre- sented in order to show more clearly its arrangement. At 178 THE HUMAN BODY. the levels of the nerve-roots horizontal white fibres are found (9 and 10, Fig. 74, and a, Fig. 75) running into the gray matter, and others exist at the bottom of the anterior fissure, running from one side of the cord to the other. In the gray substance the same supporting network of con- nective tissue is found, but in it the majority of the nerve- fibres are non-medullated, and at certain points nerve-cells, FIG. 74, — A thin transverse section of half of the spinal cord magnified about 10 diameters. 1, anterior fissure ; 2. posterior fissure ; 3, canalis centralis : 8, pia mater enveloping the cord ; 6, 7, bands of pia mater penetrating the cord and supporting its nerve elements ; 9, a posterior root : 10, bundles of an ante- rior root ; a, b, c, d, e, groups of nerve-cells in the gray matter. such as are totally absent in the white substance, are found. One collection of these nerve-cells is seen at e in Fig. 74, and others are represented at a, d, /, and elsewhere. The nerve-fibres in the gray matter are for the most part branches of these cells (see Fig. 73), and as they unite with one another freely they form a structurally continuous HISTOLOGY OF SPINAL COED. 179 network through the whole gray substance. The fibres of the anterior roots of the spinal nerves enter the gray mat- ter and there become continuous with the unbranched pro- cess of a nerve-cell; the ending of the posterior root-fibres is not quite certain, but they appear to break up and join the gray network directly, without the intervention of a cell. In any case the fundamental fact remains that every nerve-fibre joining the spinal cord is directly or indirectly in continuity with the gray network and so with all the FIG. 75.— A small bit of the section represented in Fig. 74 more magnified, a, a bundle of fibres from an anterior root passing through the white substance on its way to the gray. Towards the right of the figure the nerve-fibres of the anterior column have been omitted so as to render more conspicuous the sup- porting connective tissue, d and e. Elsewhere the nerve-fibres alone are repre- sented; c, enveloping pia mater. other fibres of all the spinal nerves. From the sides of the gray substance fibres continually pass out into the white portion and become medullated; some of these enter the gray network again at another level and so bring parts of the cord into especially close union, while others pass on into the brain. At the top of the neck, moreover, the gray matter of the cord is continuous with that of the .medulla oblongata and through it with the rest of the brain, so that nervous disturbances can pass by anatomi- •cally continuous paths from one to the other. CHAPTER XIII. THE GENEKAL PHYSIOLOGY OF THE NEKV- OUS SYSTEM. The Properties of the Nervous System. General Con- siderations. If the finger of any one unexpectedly touches a very hot object, pain is felt and the hand is suddenly snatched away; that is to say, sensation is aroused and cer- tain muscles are caused to contract. If, however, the- nerves passing from the arm to the spinal cord ha^e been divided, or if they have been rendered incapable of activity by disease, no such results follow. Pain is not then felt on touching the hot body nor does any movement of the limb occur; even more, under such circumstances the strongest effort of the will of the individual will be unable to cause any movement of his hand. If, again, the nerves of the limb have uninjured connection with the spinal cord, but parts of the latter, higher up, between the brain and the point of junction of the nerves of the brachial plexus with the cord, are injured, then a sudden contact with the hot body will cause the arm to be snatched away, but no pain or other sensation due to the contact will be felt, nor can the will act upon the muscles of the arm. From the com- parison of what happens in such cases (which have been observed over and over again upon wounded or diseased persons) with what occurs in the natural condition of things, several important conclusions may be arrived at: 1. The feeling of pain does not reside in the burnt part itself; although that may be perfectly normal, no sensa- tion will be aroused by any external force acting upon it, if the nervous cords uniting it with the centres be pre- viously divided. 2. The hot body has originated some change which, pro- GENERAL PROPERTIES OF NERVES. 181 pagated along the nerve-trunks, has excited a condition of the nerve-centres ivhich is accompanied by a sensation, in this particular case a painful one. This is clear from, the fact that the loss of sensation immediately follows division of the nerves of the limb, but not the injury of any of its other parts; unless of such a character as to cut off the supply of blood, when of course the nerves soon die, with the rest. Even, however, some time after tying the vessels which carry blood to a limb one can observe in experiments upon the lower animals that sensibility is still retained if the nerves be not directly injured. 3. Wlien a nerve in the skin is excited by a burn or other- wise it does not directly call forth muscular contractions; for if so, touching the hot body would cause the limb to be moved even when the nerve is divided high up in the arm, and as a matter of observation and experiment we find that no such result follows if the nerve-fibres have been cut in any part of their course from the burned .part to the spinal marrow. It is therefore through the nerve-centres that the change transmitted from the excited part of the skin is reflected or sent back, to act upon the muscles. 4. The last "deduction makes it probable that nerve-fibres must pass from the centre to muscles as well as from the skin to the centre. This is confirmed by the fact that if the nerves of the limb be divided the will is unable to act upon its muscles, showing that these are excited to con- tract through the nerves. That the nerve-fibres concerned in arousing sensation and muscular contractions are differ- ent, is shown also by cases of disease in which the sensi- bility of the limb is lost while the power of voluntarily moving it remains, and by other cases in which the reverse is seen, objects touching the hand being felt while it can- not be moved by the will. We conclude then that cer- tain nerve-fibres when stimulated convey something (a nervous impulse) to the centres, and that these when ex- cited may radiate impulses through other nerve-fibres to distant parts, the centre serving as a connecting link be- tween the fibres which carry impulses from without in, and those which convey them from within out. 182 THE HUMAN BODY. 5. Further we conclude that the spinal cord can act as an intermediary between the fibres carrying in nervous im- pulses and those carrying them out, but that sensations can- not be aroused by impulses reaching the spinal cord only, nor has the Will its seat there ; volition and consciousness are dependent upon states of the brain. This follows from the unconscious movements of the limb which follow stimula- tion of its skin after such injury to the spinal cord as pre- vents the transmission of nervous impulses farther on (showing that the cord is a reflex centre), and from the absence, in such cases, of sensation in the part whose nerves have been injured, and the loss of the power of voluntarily causing its muscles to contract. 6. Finally we conclude that the spinal cord in addition to being a centre for reflex actions serves to transmit nerv- ous impulses to and from the brain ; a fact which is con- firmed by the histological observation that in addition to the nerve-cells, which are the characteristic constituents of nerve-centres, it contains the simply conductive nerve- fibres, many of which pass on to the brain. In other words the spinal cord, besides containing fibres which enter it from, and pass from it to, peripheral parts contains many which unite it to other centres ; and connect the various centres in it, as those for the arms and legs, to- gether. This is true not only of the spinal cord but of the brain (which contains many fibres uniting different centres in it), and probably of all nerve-centres. The Functions of Nerve-Centres and Nerve-Trunks. From what has been stated in the previous paragraphs it is clear that we may distinctly separate the nerve- trunks from the nerve-centres. The fibres serve simply to convey im- pulses either from without to a centre or in the opposite •direction, while the centres conduct and do much more. Some, as the spinal cord, are merely reflex centres, and have nothing to do with states of consciousness. A man with his spinal cord cut or diseased in the dorsal region will kick violently if the soles of his feet be tickled, but will feel nothing of the tickling, and if he did not see his legs would not know that they were moving. Eeflex centres F UNCTIONS OF NER VE- CENTRES. 183 moreover do not act, as a rule, indifferently and casually, but rearrange the impulses reaching them, so as to pro- duce a protective or in some way advantageous result. In other words, these centres, acting in health, commonly co- ordinate the incoming impulses and give rise to outward- going impulses which produce an apparently purposive result. The burnt hand or the tickled foot, in the absence of all consciousness, are snatched away from the irritant; and food chewed in the mouth excites nerves there which act on a centre which causes certain cells in the salivary glands to form and pour into the mouth more saliva. In addition tp the reflex centres we have others, placed in the brain, which, when excited, cause in us various states oi consciousness, as sensations, emotions, and the will ; con- cerning these centres of consciousness our physiological knowledge is still very incomplete; what we know about them is based rather on psychological than physiological •observation. The brain also contains a great many im- portant reflex centres, as that for the muscles of swallow- ing, an act which goes on perfectly without our conscious- ness at all. In fact if we pay attention to our swallowing we fail to perform it as well as if we let the nervo-muscular apparatus alone. To complete the statement of the func- tions of the nerve-centres we must probably add two other groups. The first of these is that of the automatic centres, which are centres excited not directly by nerve-fibres con- veying impulses to them, but in other ways. For example the breathing movements go on independently of our con- sciousness, being dependent on stimulation of a nerve-centre in the brain by the blood which flows through it (see Chap. XXVI.); and tlie beat of the heart is also dependent (Chap. XVII.) upon nerve-centres the excitant of which is un- known. The final group of nerve-centres is represented by certain sporadic sympathetic and cerebro-spinal ganglia which are not known to be either reflex, automatic, or con- scious in function. They may be called relay and junc- tion centres, since in them probably an impulse entering by one nerve-fibre excites a cell, which by its communi- cating branches arouses many others, and these then send 184 THE HUMAN BODY. out impulses by the many nerve-fibres connected with them. By such means a single nerve-fibre can act upon an extended region of the Body. In other cases it seems likely that a feeble nervous impulse reaching an irritable nerve-cell excites changes in this comparable to those pro- duced in a muscle when it is stimulated; and the cell by its discharge sends on reinforced nerve impulses along its other branches or one of them. Excitant and Inhibitory Nerves. The great majority of the nerve-fibres of the Body when they convey nervous impulses to a part arouse it to activity; they are excitant fibres. There is, however, in the Body another very im- portant set which arrest the activity of parts and which are known as inhibitory nerve-fibres. Some of these check the action of central nervous organs, and others the work of peripheral parts. For instance taking a pinch of snuff will make most persons sneeze; it excites centrally acting fibres in the nose, these excite a centre in the brain, and this in turn sends out impulses by efferent fibres which cause various muscles to contract. But if the skin of the- upper lip be pinched immediately after taking the snuff, in most cases the reflex act of sneezing, which the Will alone- could not prevent, will not take place. The afferent im- pulses conveyed from the skin of the lip have " inhibited"" what we may call the " sneezing centre;" and afford us therefore an example of inhibitory fibres checking a centre. On the other hand, the heart is a muscular organ which goes on beating steadily throughout life; but if the branches of the pneumogastric nerve going to it be excited, the beat of the heart will be stopped; it will cease to work and lie in a relaxed resting condition: in this we have an instance of an inhibitory nerve checking the activity of a peripheral organ. Classification of Nerve-Fibres. Nearly all the nerve- fibres of the Body fall into one of two great groups corre- sponding to those which carry impulses to the centres and those which carry them out from the centres. The former are called afferent or centripetal fibres and the latter efferent or centrifugal. Since the impulses reaching the centres through the afferent fibres generally cause sen- CLASSIFICATION OF NERVE-FIBRES, 185 sations they are often called sensory fibres', and as many of those which carry out impulses from the centres excite movements, they are frequently called motor fibres; but these last names are bad, since even excluding inhibi- tory nerves, many afferent fibres are not sensory and many efferent are not motor. We may distinguish as subdivisions of afferent fibres — the following groups. 1. Sensory fibres proper, the excitement of which is followed by a sensation when they are con- nected with their brain-centre, which sensation may or may not give rise to a voluntary movement. 2. Refiex fibres, the excitation of which may be attended with con- sciousness but gives rise to involuntary efferent impulses. Thus for example light falling on the eye causes not only a sensation but also a narrowing of the pupil, which is en- tirely independent of the control of the "Will. No absolute line can, however, be drawn between these fibres and those of the last group: any sudden excitation, as an unexpected noise, will cause an involuntary movement, while the same sound if expected would cause a movement or not accord- ing as was willed. 3. Excito-motor fibres. The excitation of these when reaching a nerve-centre causes the stimula- tion of efferent fibres, but without the participation of consciousness. During fasting for instance bile accumu- lates in the gall-bladder and there remains until some semi-digested food passes from the stomach into the intes- tine. This is acid, and stimulates nerves in the mucous membrane lining the intestine, and these convey an im- pulse to a centre, which in consequence sends out impulses to the muscular coat of the gall-bladder causing it to con- tract and expel its contents into the intestine: but of all this we are entirely unconscious. 4. Centro-inhibitory fibres. Whether these exist as a distinct class is at present doubtful. It may be that they are only ordinary sensory or reflex fibres and that the inhibition is due only to the in- terference of two impulses reaching a central organ at the same time and impeding or hindering the full production of the normal result of either. In efferent nerve-fibres physiologists also distinguish 186 THE HUMAN BODY. several groups. 1. Motor fibres, which are distributed to the muscles and govern their contractions. 2. Vaso-motor fibres. These are not logically separable from other motor fibres; but they are distributed to the muscles of the blood- vessels and by governing the blood-supply of various parts, indirectly produce such secondary results as entirely over- shadow their primary effect as merely producing muscular contractions. 3. Secretory fibres. These are distributed to the cells of the Body which form various liquids used in it, as the saliva and the gastric juice, and arouse them to ac- tivity. The salivary glands for instance may be made ta form saliva by stimulating nerves going to them, and the same is true of the cells which form the sweat poured out. upon the surface of the Body. 4. Trophic nerve-fibres. Under this head are included nerve-fibres which have been supposed to govern the nutrition of the various tissues, and so to control their healthy life. It is very doubtful, how- ever, if any such nerve-fibres exist, most of the facts cited to prove their existence being otherwise explicable. For instance shingles is a disease characterized by an eruption on the skin along the line of certain nerves which run be- tween the ribs; but it may be dependent upon disease of the vaso-motor nerves which control the blood-supply of the part. In other cases diseases ascribed to injury of trophic nerves have been shown to be due to injury of the sensory nerves of the part, which having lost its feeling, is exposed to injuries from which it would other- wise have been protected. On the other hand it may be said that secretory nerves are trophic nerves in the true sense of the word, since when excited they cause the se- cretory cells to live in a special way (p. 269) and produce substances which when unacted upon by their nerves they do not form. But if we call secretory nerves trophic we must include also under that name all other efferent nerves; the nutritive processes going on in a muscular fibre when at work are different from those in the sama fibre when at rest, and the same is true of all other cells the activity of which is governed by nerve-fibres. 5. Peri- pherally-acting inhibitory nerves. NERVE STIMULI. 18? Intercentral Nerve-Fibres. These, which do not convey impulses between peripheral parts and nerve-centres, but connect one centre with another, form a final group in ad- dition to efferent and afferent nerve-fibres. Many of them connect the sporadic and sympathetic ganglia with one another and with the cerebro-spinal centre, while others place different parts of this latter in direct communication ; as for instance different parts of the spinal cord, the brain and the spinal cord, and the two halves of the brain. These fibres are of very great importance, but as yet their course is imperfectly known. General Table. We may physiologically classify nerve- fibres as in the following tabular form which is founded upon the facts above stated. Xerve-fibres. «< Peripheral. Afferent. Efferent. I Sensory. I Reflex. ] Excito-motor. [ Inhibitory? I* Motor. Vaso-motor. •I Secretory. Trophic? [ Inhibitory. . ( Exciting. Intercentral. j Inhibitory. The Stimuli of Nerve-Fibres. Nerve-fibres, like mus- cular fibres, possess no automaticity; acted upon by certain external forces or stimuli they propagate a change, which is known as a nervous impulse, from the point acted upon to their ends; but they do not generate nervous impulses when left entirely to themselves. Normally, in the living Body the stimulus acts 011 a nerve-fibre at one of its ends, being either some change in a nerve-centre with which the fibre is connected (efferent nerves) or some change in an organ attached to the outer end of the nerve (afferent fibres). Experiment shows, however, that a nerve can be stimulated in any part of its course; that it is irritable all through its 188 THE HUMAN BODY. extent. If, for example, the sciatic of a frog be exposed in the thigh and divided, it will be found that electric shocks applied at the point of division to the outer half of the nerve stimulate the motor fibres in it, and cause the mus- cular fibres of the leg to contract: and similarly such shocks applied to the cut end of the central half irritate the affe- rent fibres in it, as shown by the signs of feeling exhibited by the animal. In ourselves, too, we often have the oppor- tunity of observing that the sensory fibres can be stimulated in their course at some distance from their ends. A blow at the back of the elbow, at the point commonly known as the "funny bone" or the "crazy bone," compresses the ulnar nerve there against the subjacent bone, and starts nervous impulses which make themselves known by severe tingling pain referred to the little and ring fingers to which the nerve is distributed. This shows not only that the nerve-fibres can be irritated in their course as well as at their ends, but also that sensations do not directly tell us where a nerve-fibre has been excited. No matter where in its course the impulse has been started we unconsciously refer its origin to the peripheral end of the afferent nerve. General and Special Nerve Stimuli. Certain external forces excite all nerve-fibres, and in any part of their course. These are known as general nerve stimuli; others act only on the end organs of nerve-fibres, and often only on one kind of end organ, and hence cannot be made to excite all nerves: these latter are commonly known as special nerve stimuli. In reality they are not properly nerve stimuli at all; but only things which so aifect the irritable tissues at- tached to the ends of certain nerve-fibres as to make these tissues in^turn excite the nerves. For example light itself will not stimulate any nerve, not even the optic: but in the eye it effects changes (apparently of a chemical nature) by which substances of the nature of general nerve stimuli are produced and these stimulate the optic nerve-fibres. The ends of the nerves in the skin are not accessible to light nor are the proper end organs on which the light acts there present, so light does not lead to the production of nervous impulses in them: but the optic nerve without NERVE STIMULI. 189 its peculiar end organs would be just as insensible to light as these are. Similarly the aerial vibrations which affect us as sounds, do not stimulate directly the fibres of the audi- tory nerve. They act on terminal organs in the ear, and these then stimulate the fibres of the nerve of hearing, just -as they would any other nerve which happened to be con- nected with them. General Nerve Stimuli. Those known are (1) electric •currents : an electric shock passed through any part of .any nerve-fibre, powerfully excites it. A steady current passing through a nerve' does not stimulate it, but any sudden change in this, whether an increase or a decrease, •does. A very gradual change in the amount of electricity passing through a nerve in a unit of time will not stimii' late it. (2) Mechanical stimuli. Any sudden pressurp or traction, as a blow or a pull, will stimulate a nerve- fibre. On the other hand steady pressure, or pressure very slowly increased from a minimum, will not excite the nerve. (3) Thermal stimuli. Any sudden heating or cool- ing of a nerve, as for instance bringing a hot wire close to it, will stimulate; slow changes of temperature will not. (4) Chemical stimuli. Many substances which alter the nerve- fibre chemically, stimulate before killing it; thus dipping the cut end of a nerve into strong solution of common salt will excite it, but very slow chemical change in a nerve fails to stimulate. In the case of all these general stimuli it will be seen that as one condition of their efficacy they must act with considerable suddenness. On the other hand too transient influences have no effect. An electric shock sent for only 0.0015 of a second through a nerve does not stimulate it: apparently the inertia of the nerve molecules is too .great to be overcome by so brief an action. So, also, too strong sulphuric acid and many other bodies kill nerves immediately, altering them so rapidly that they die without being stimulated. Special Nerve Stimuli. These as already explained act only on particular nerves, not because one nerve is es- sentially different from another, but because their influence 190 THE HUMAN BODY. is excited through special end organs which are attached to some nerves. These stimuli are — (1) Changes occurring in central organs, of whose nature we know next to nothing, but which excite the efferent nerve-fibres connected with them. The remaining special stimuli act on afferent fibres through the sense-organs. They are — (2) Light, which by the intervention of organs in the eye excites the optic nerve. (2) Sound, which by the intervention of organs in the ear excites the auditory nerve. (3) Heat, which through end organs in the skin is able, by very slight changes, to excite certain nerve-fibres: such slight changes of temperature being efficient as would be quite incapable of acting as general nerve stimuli without the proper end organs. (4) Chemical agencies. These when extremely feeble and incapable of acting as general stimuli, can act as special stimuli through special end organs in the mouth and nose (as in taste and smell) and probably in other parts of the alimentary tract, where very feeble acids and alkalies seem able to excite certain nerves, and reflexly through them excite movements or stir up the cells concerned in making the digestive liquids; for example the contraction of the gall-bladder already referred to. (5) Mechanical stimuli when so feeble as to be inefficient as general stimuli. Pressure on the skin of the forehead or the back of the hand, equal to .002 gram (.03 grain) can be felt through the end organs of the sensory fibres there, but would be quite incapable of acting as a general stimulus in the absence of these. It will be noticed as regards the special stimuli of afferent nerves that many of them are merely less degrees of general stimuli; the end organs in skin, mouth,and nose are in fact excited by the same things as nerve-fibres, but they are far more irritable. In the case of the higher senses, seeing and hearing, however, the end organs seem to differ entirely in property from nerve-fibres, being excited by sonorous and luminous vibrations which, so far as we know, will in no degree of intensity directly excite nerve-fibres. To make an end organ for recognizing very slight pressures we may imagine all that would be needed SPECIFIC NERVE ENERGIES. 191 would be to expose directly a very delicate end branch of the axis cylinder, and this seems in fact to be the case in the nerves of the transparent exposed part of the eyeball, if not in many other parts of the external integument of the Body. But as axis cylinders are quite unirritable by light or sound a mere exposure of them would be useless in the eye or ear, and in each case we find accordingly a very complex apparatus developed, whose function it is to convert these modes of motion which do not excite nerves into others which do. We might compare it to a cartridge, which contains not only "irritable" gunpowder but highly "irritable" detonating powder, and the stimulus of the blow from the hammer which would not itself ignite the gunpowder, acting on the detonating powder (which represents an "end organ"), causes it to give off a spark which in turn excites the gunpowder, which answers to the nerve-fibre. Specific Nerve Energies. We have already seen that a nervous impulse propagated along a nerve-fibre will give rise to different results according as different nerve-fibres are concerned, Traveling along one fibre it will arouse a sensation, along another a movement, along a third a se- cretion. In addition we may observe readily that these different results may be produced by the same physical force when it acts upon different nerves. Radiant energy, for example, falling into the eye causes a sensation of sight, but falling upon the skin one of heat, if any. The different results which follow the stimulation of different nerves do not then depend upon differences in the physical forces exciting them. This is still further shown by the fact that different physical forces acting upon the same nerve arouse the same kind of sensation. Light reaching the eye causes a sight sensation, but if the optic nerve be irritated by a blow on the eyeball a sensation of light is felt just as if actual light had stimulated the nerve ends. So too when the optic nerve is cut by the surgeon in ex- tirpating the eyeball, the patient experiences the sensation of a flash of light; and the same result follows if an electric 192 THE HUMAN BODY. •shock be sent through the nerve. Different nerves excited by the same stimulus produce different results, and the same nerve excited by different stimuli gives the same re- sult. How are these facts to be explained? The first explanation which suggests itself is that the various nerves differ in their properties: that electricity -applied to a motor nerve causes a muscle to contract, and to the optic nerve a visual sensation, and to the lingual nerve a sensation of taste, because nervous impulses in the motor, optic, and lingual nerves differ from one an- other. This was the view held by the older physiologists; and that supposed peculiarity of a muscular nerve by which its irritation caused a muscular contraction, and that of the optic nerve in consequence of which its excitation caused a sensation of sight, and so on, they called the specific energy •of the nerve. Seeing further that when a motor nerve was cut and its peripheral stump pinched the muscles connected with it contracted, but that when its central end was pinched no sensation or other recognizable change followed, while exactly the reverse was true of a sensory nerve, they Relieved that afferent nerves differed essentially from effe- rent nerves, inasmuch as the latter could only propagate impulses centrifugally and the former only centripetally. Now, however, we have much reason to believe that this view is Avrong and that all nerve-fibres are exactly alike in their physiological properties, and can carry nervous im- pulses either way. The differences observed depend in fact not on any differences in the nerve-fibres, but on the -different parts connected with their ends; in other words on the different terminal organs excited by the impulses •conveyed by the fibre. A motor fibre is one which lias at its peripheral end a muscular fibre, and a centrifugally trav- eling impulse reaching this will cause it to contract; but the cells connected with its central end are not of such a nature as to give rise to sensations when centripetally traveling impulses reach them, or to transmit these to other efferent fibres and so cause reflex movements; and therefore when a motor fibre is stimulated in the middle of its course the outward-going impulse causes a movement, while the cen- THE SIMILARITY OF NERVE-FIBRES. traveling impulse starting at the same time, gives- no direct sign of its existence. Similarly for a sensory nerve such as the optic; if it be stimulated somewhere be- tween the brain and the eyeball the centrally traveling impulse will cause a sensation of light, by exciting the brain-centre connected with it, but the outward traveling impulse not reaching muscular fibres or other parts which it can arouse to activity, remains concealed from our no- tice. In other words the so-called specific energy of a nerve-fibre depends upon the terminal organs on which it can act and not on any peculiarity of the nerve-fibre itself. Proofs that all Nerve-Fibres are Physiologically Alike, (1) The most marked difference between nerve-fibres is* obviously that between efferent and afferent, and the mi- croscope fails entirely to show any differences between the- two. Some motor and some sensory fibres may be bigger or less than others, some may be white and others may be- gray, but such differences are secondary and have no cor- relation with the function of the nerve. We can recognize no constant difference in structure between the two. (2) The physical properties and chemical composition of motor and sensory nerves agree in all known points. (3) When a nerve, say a motor one, is stimulated half way between the centre and a muscle, a nervous impulse, as we call it, is propagated to the muscle, which impulse starts at the point stimulated and travels at a definite rate to the mus- cle, the contraction of which latter evidences its arrival. Now starting at the same moment from the same point, and traveling at the same rate, is a change in the electrical properties of the nerve which can be detected by a good galvanometer and which is called the "negative variation." Since this negative variation and the nervous impulse co- exist at any given moment in a particular point of the- nerve and disappear from it together, we conclude that the negative variation is a change in the electrical properties of the nerve dependent on that internal movement of its molecules which constitutes a nervous impulse. It is an externally recognizable physical sign, and the only known one, of the existence of the nervous impulse as it travel* 194 THE HUMAN BODY. along the fibre. If the muscle were cut away from the end of the nerve we could still detect that a nervous impulse had traveled from the point of stimulation to that where the fibres were divided, by tracking the negative variation. Now if we examine the part of the nerve on the central side of the stimulated point we find that a negative varia- tion (and hence we conclude a nervous impulse) travels that way too; it starts at the same moment as the efferent nega- tive variation and travels in the same way, but the impulse of which it is a sign produces no more effect than the efferent impulse would after the muscle had been cut away; for it does not reach any muscular fibre, or sensory or reflex centre, which it can arouse to activity. (4) The following experi- ment has been put forward as even more conclusive. If the tail of a rat be amputated close to the body of the ani- mal and then be transplanted to the back of the rat, and its tip be there stitched beneath the skin, it will, in many cases, continue to live in this new position, although it is "upside down." In such circumstances, it has been found that after a time the transplanted tail is sensitive when pinched. It has been argued that this phenomenon proves one of two things: either that originally afferent (or sen- sory) nerve-fibres of the tail which normally carried im- pulses from its tip now convey them from its stump to the tip imbedded in and grown into the wound; or that efferent (or motor) nerve-fibres which formerly conveyed motor or efferent impulses down the tail now carry them up, and transmit them to sensory fibres in the skin or subcutaneous tissue with which they have become anatomically connected. If this were a correct statement of the facts it would be of great force. But we know now that the axis cylinder of every nerve-fibre is but the prolonged branch of a nerve- cell; and that such branches when divided grow rapidly again towards the periphery along the connective-tissue paths marked out by the sheaths of the peripheral portions of divided nerves. It is more probable that the afferent cutaneous nerve-fibres, cut in making the skin incision and in the implantation of the tail-tip, send outgrowths from their cut ends into the inverted tail and become its sensory SIMILARITY OF ALL NERVE-FIBRES. 195 nerves, than that afferent fibres become efferent, or efferent afferent, under such conditions. This argument, as one favoring the belief in the essential identity of all nerve- fibres, must therefore be given up. Afferent and efferent nerve-fibres differ in no ob- servable property. They are alike in faculty, and their different names simply imply that they have different ter- minal organs. Just as all muscles are alike in general physiological properties, and differ in special function according to the parts on which they act, so are all nerve- fibres alike in general physiological properties, and differ in special function only because they are attached to spe- cial things. The special physiology of various nerves will hereafter be considered in connection with the working of yarious mechanisms in the Body. If it be true that the great subdivisions of afferent and efferent fibres have identical properties, it follows that this is a fortiori true of the minor subdivisions of each, and that auditory, gus- tatory, and optic nerve-fibres are all alike, and all identi- cal with motor and vaso-motor and secretory nerve-fibres; and that the nervous impulse is in all cases the same thing, varying in intensity in different cases and in the rate at which others follow it in the same fibre, but the same in kind. To put the case more definitely: Light outside the •eye exists as ethereal vibrations, sound outside the ear as vibrations of the air (commonly). Each kind of vibration acts on a particular end organ in eye or ear which is adapted to be acted upon by it, and in turn these end organs excite the optic and auditory nerve-fibres; these in consequence transmit impulses, which reaching different parts of the brain excite them; the excitement of one of these brain-centres is associated with sonorous and of the other with visual sensations. The nervous impulse in the two cases is quite alike, at least as to quality (though it may differ in quantity and rhythm) and the resulting difference in quality of the sensations cannot depend on it. The •quality differences in these cases must be products of the central nervous system. If we had a set of copper wires we might by sending precisely similar electric currents through 196 THE HUMAN BOD Y. them produce very different results if different things were* interposed in their course. In one case the current might be sent through water and decompose it, doing chemical work ; in another, through the coil of an electro-magnet and raise a weight; in a third, through a thin platinum wire and develop light and heat; and so on, the result depending on the terminal organs, as we may call them, of each wire. Or on the other hand we might gen- erate the current in each wire differently, in one by a Daniell's cell, in a second by a thermo-electric machine, in a third by the rotation of a magnet inside a coil, but the currents in the wires would be essentially the same, as the nervous impulses are in a nerve-fibre. No matter how they have been started, provided their amount is the same, whether they shall produce similar or dissimilar results, depends only on whether they are connected with similar or dissimilar end organs. The Nature of a Nervous Impulse. Since between sense-organs and sensory centres, and these latter and the muscles, nervous impulses are the only means of communi- cation, it is through them that we arrive at our opinions- concerning the external universe and through them that we are able to act upon it; their ultimate nature is there- fore a matter of great interest, but one about which we unfortunately know very little. We cannot well ima- gine it anything but a mode of motion of the molecules of the nerve-fibres, but beyond this hypothesis we cannot go far. In many points the phenomena presented by nerve- fibres as transmitters of disturbances are like the phenom- ena of wires as transmitters of electricity, and when the phenomena of current electricity were first observed there was a great tendency, explaining one unknown by another,, to consider nervous impulses merely as electrical currents. The increase of our knowledge concerning both nerves and electric currents, however, has made such an hypothesis almost if not quite untenable. In the first place, nerve- fibres are extremely bad conductors of electricity, so bad that it is impossible to suppose them used in the Body for that purpose; and in the second place, merely physical con- FUNCTIONS OF THE SPINAL ROOTS. 197 tinuity of a nerve-fibre, such as would not interfere with the passage of an electric current, will not suffice for the transmission of a nervous impulse. For instance if a damp string be tied around a nerve, or if it be cut and its two moist ends placed in contact, no nervous impulse will be trans- mitted across the constricted or divided point, although an electrical current would pass readily. An electrical shock may be used like many other stimuli to upset the equilibrium of the nerve molecules and start a nervous im- pulse, which then travels along the fibre, but is just as dif- ferent from the stimulus exciting it, as a muscular contrac- tion is from the stimulus which calls it forth. The nerves, however, have certain interesting electrical properties from the study of which we learn some little about a nervous impulse. As already mentioned whenever a nervous impulse is started in a nerve an electrical change, known as the " negative variation" or "action current," is started at the same time, from the same point, and travels along the nerve at the same rate. Hence we conclude that the new internal molecular arrangement in a nerve-fibre which constitutes its active as compared with its resting state, is also one which changes the electrical properties of the fibre. It is an outward sign and the only known one of the internal change. Now it is found that the action current travels along the nerve (in the frog) at the rate of 28 meters (92.00 feet) in a second and takes .0007 second to pass by a given point: accordingly at any one moment it extends over about 18 mm. (0.720 inch) of the nerve- fibre. Moreover, when first reaching a point it is very feeble, then rises to a maximum and gradually fades away again. Taking it as an indication of what is going on in the nerve, we may assume that the nervous impulse is a molecular change of a wavelike character, rising from a minimum to a maximum, then gradually ceasing, and about 18 millimeters in length. The Rate of Transmission of a Nervous Impulse. This can be measured in several ways, and is far slower than that of electric currents. It agrees as above stated with that of the negative variation, being 28 meters (92.00 feet) 198 THE HUMAN BOD Y. per second in the motor nerves of a frog. In man it is somewhat quicker, being 33 meters (108.24 feet) per second, that is, about T^- of the rate of the transmission of sound- waves in air at zero. Functions of the Spinal Nerve-Roots. The great ma- jority of the larger nerve-trunks of the Body contain both afferent and eiferent nerve-fibres. If one be exposed in its course and divided in a living animal, it will be found that irritating its peripheral stump causes muscular contractions, and pinching its central stump causes signs of sensation, showing that the trunk contained both motor and sensory fibres. If the trunk be followed away from the centre, as it breaks up into smaller and smaller branches, it will be found that these too are mixed until very near their end- ings, where the very finest terminal branches close to the end organs, whether muscular fibres, secretory cells, or sen- sory apparatuses, contain only afferent or efferent fibres. If the nerve-trunk be one that arises from the spinal cord and be examined progressively back to its origin, it will still be found mixed, up to the point where its fibres sepa- rate to enter either a ventral or a dorsal nerve-root. Each of these latter however is pure, all the efferent fibres leaving the cord by the ventral or anterior roots, and all the afferent entering it by the posterior or dorsal. This of course could not be told from examination of the dead nerves since the best microscope fails to distinguish an afferent from an efferent fibre, but is readily proved by experiments first performed by Sir Charles Bell. If an anterior root be cut and its outer end stimulated, the mus- cles of the parts to which the trunk which it helps to form is distributed, will be made to contract, and the skin will be made to sweat also if the root happen to be one that contains secretory fibres for the sweat-glands. On the other hand, if the central end of the root (that part of it attached to the cord) be stimulated no result will follow, showing that the root contains no sensory, reflex, or excito-motor fibres. With the posterior roots the reverse is the case: if one of them be divided and its outer end stimulated, no observed result follows, showing the absence of all efferent fibres; COMMUNICA TION OF NEB VE- CENTRES. 199 but stimulation of its central end will cause either signs -of feeling, or reflex actions, or both. We might compare a spinal nerve-trunk to a rope made up of green and red threads with at one end alLthe green threads collected into one skein and the red into another, which would represent the roots. At its farthest end we may suppose the rope divided into finer cords, each of these containing both red and green threads down to the very finest branches consisting of only a few threads and those all of one kind, •either red or green, one representing efferent, the other afferent fibres. The Cranial Nerves. Most of these are mixed also, but with one exception (the fifth pair, the small root of which is efferent and the large gangliated one afferent) they do not present distinct motor and sensory roots, like those of the spinal nerves. At their origin from the brain most of them are either purely afferent or efferent, and the mixed char- acter which their trunks exhibit is due to cross-branches with neighboring nerves, in which afferent and efferent fibres are interchanged. The olfactory, optic, and audi- tory nerves remain, however, purely afferent in all their •course, and others though not quite pure contain mainly -efferent fibres (as the facial) or mainly afferent (as the glosso-pharyngeal). The Intercommunication of 3STerve-Centres. From the -anatomical arrangement of the nervous system it is clear that it forms one continuous whole. No subdivision of it is isolated from the rest, but nerve-trunks proceeding from the centres in one direction bind them to various tissues .and, proceeding in another, to other nerve-centres; which in turn are united with other tissues and other centres. Since the physiological character of a nerve-fibre is its con- ductivity— its power of propagating a disturbance when once its molecular equilibrium has been upset at any one point — it is obvious that through the nervous system any one part of the Body, supplied with nerves, may react on all other parts (with the exception, of such as hairs and nails and cartilages, which are not known to possess nerves) and excite changes in them. Pre-eminently the nervous system 200 THE HUMAN BOD T. forms a uniting anatomical and physiological bond through the agency of which unity and order are produced in the activities of different and distant parts. We may compare it to the Western Union Telegraph, the head office of which in New York would represent the brain and spinal cord; the more important central offices in other large cities, the sympathetic ganglia; and the minor offices in country stations the sporadic ganglia; while the tele- graph-wires, directly or indirectly uniting all, would corre- spond to the nerve-trunks. Just as information started along some outlying wire may be transmitted to a central office, and from it to others, and then, according to what happens, to it in the centre, be stopped there, or spread in all c>irec- tions, or in one or two only, so may a nervous disturbance reaching a centre by one nerve-trunk merely excite changes in it or be radiated from it through other trunks more or less widely over the Body and arouse various activi- ties in its other component tissues. In common life the very frequency of this uniting activity of the nervous sys- tem is such that we are apt to entirely overlook it. We do not wonder how the sight of pleasant food will make the mouth water and the hand reach out for it; it seems as we say " natural" and to need no explanation. But the eye itself can excite no desire, cause the secretion of no saliva, and the movement of no limb. The whole complex result depends on the fact that the eye is united by the optic, nerve with the brain, and that again by other nerves with saliva-forming cells, and with muscular fibres of the arm; and through these a change excited by light falling inta the eye is enabled to produce changes in far removed or- gans and excite desire, secretion and movement. In cases- of disease this action exerted at a distance is more apt to ex- cite our attention : vomiting is a very common symptom of certain brain diseases and most people know that a disor- dered stomach will produce a headache ; while the pain consequent upon the hip-disease of children is usually felt, not at the hip-joint bnt at the knee. CHAPTER XIV. THE ANATOMY OF THE HEART AND BLOOD- VESSELS. General Statement. During life the blood is kept flow- ing with great rapidity through all parts of the Body (ex- cept the few non-vascular tissues already mentioned) in definite paths prescribed for it by the heart and blood- vessels. These paths, which under normal circumstances it never leaves, constitute a continuous set of closed tubes (Fig. 76) beginning at and ending again in the heart, and simple only close to that organ/ Else-" where it is greatly branched, the most numerous and finest branches (I and a) being the capillaries. The' heart is essentially a bag with muscular walls, internally divided into four chambers (d, a, e, /). Those at one end (d and e) receive blood from vessels opening into them and known as the veins. From there the blood passes on to the remain- ing chambers (g and/) which have very powerful walls and, forcibly contract- ing, drive the blood out into vessels (i and b) which communicate with them and are known as the arteries. The big arteries divide into smaller; these into smaller again (Fig. 77) until the branches be- come too small to be traced by the unaided eye, and these smallest branches end in the capillaries, through which the blood flows and enters the commencements of the veins; FIG. 76.— The heart and blood-vessels dia- grammatically r e p r e- sented. 202 THE HUMAN BODY. and these convey it again to the heart. At certain points in the course of the blood-paths valves are placed, which prevent a back-flow. This alternating reception of blood a.t one end by the heart and its ejection from the other go nadu del FIG. 77.— The arteries of the hand, showing the communications or anasto. moses of different arteries and the fine terminal twigs given off from the largei trunks; these twigs end in the capillaries which would only be visible if mag> nified. R, the radial artery on which the pulse is usually felt at the wrist ; £7, the ulnar artery. on during life steadily about seventy times in a minute, and so keep the liquid constantly in motion. The vascular system is completely closed except at twro points where the lymph-vessels open into the veins (p. 329); there some lymph is poured in and mixed directly with the bLood. Accordingly everything which leaves the blood POSITION OF THE HEART. 203 must do so by oozing through the walls of the blood-vessels, and everything which enters it must do the same, except matters conveyed in by the lymph at the points above mentioned. This interchange through the walls of the vessels takes place only in the capillaries, which form a sort of irrigation system all through the Body. The heart, arteries, and veins are all merely arrangements for keeping t lie capillaries full and reneAving the blood within them. It is in the capillaries alone that the blood does its phy- siological work. The Position of the Heart. The heart (h, Fig. 1) lies in the chest immediately above the diaphragm and oppo- site the lower two thirds of the breast-bone. It is conical in form with its base or broader end turned upwards and projecting a little on the right of the sternum, while its narrow end or apex, turned downwards, projects to the left of that bone, where it may be felt beating between the cartilages of the fifth and sixth ribs. The position of the organ in the Body is therefore oblique with reference to its long axis. It does not, however, lie on the left side as is so commonly supposed but very nearly in the middle line, with the upper part inclined to the right, and the lower (which may be easier felt beating — hence the common belief) to the left. The Membranes of the Heart. The heart does not lie bare in the chest but is surrounded by a loose bag composed of connective tissue and called the pericardium. This bag, like the heart, is conical but turned the other way, its broad part being lowest and attached to the upper surface of the diaphragm. Internally it is lined by a smooth serous mem- brane like that lining the abdominal cavity, and a similar layer (the visceral layer of the pericardium) covers the out- side of the heart itself, adhering closely to it. Each of the serous layers is covered by a stratum of flat cells, and in the space between them is found a small quantity of liquid which moistens the contiguous surfaces, and diminishes the friction which would otherwise occur during the movements of the heart. Internally the heart is also lined by a fibrous membrane, 204 THE HUMAN BODY. covered with a single layer of flattened cells, and called the endocardium. Between the endocardium and the visceral lay er of the pericardium the bulk of the wall of the heart lies and is made up mainly of striped muscular tissue (differing somewhat from that of the skeletal muscles) ; but connective tissues, blood-vessels, nerve-cells, and nerve-fibres are also abundant in it. Note. Sometimes the pericardium becomes inflamed, this affection being known as pericarditis. It is extremely apt to occur in acute rheumatism, and great care should be taken never, even for a moment, except under medical advice, to expose a patient to cold during that disease, since any chill is then especially apt to set up pericarditis. In the earlier stages of pericardiac inflammation the rubbing surfaces on the outside of the heart and the inside of the pericardium become roughened, and their friction produces a sound which can be recognized through the stethoscope. In later stages great quantities of liquid may accumulate in the pericardium so as to seriously impede the heart's beat. The Cavities of the Heart. cs On opening the heart (see diagram. Fig. 78) it is found to be subdivided by a longitudinal parti- tion or septum into completely separated right and left halves, the partition running from about the middle of the base to a point a little on the right of the apex. Each of the chambers on the sides FIG. 78 -Diagram representing a section Of the Septum is again through the heart from base to apex. incompletely divided transversely, into a thinner basal portion into which veins open, known as the auricle, and a thicker apical por- tion from which arteries arise, called the ventricle. The heart thus consists of a right auricle and ventricle and a EXTERIOR OF THE HEART. 205 left auricle and ventricle, each auricle communicating by an auricula- ventricular orifice with the ventricle on its own side, and there is no direct communication whatever through the septum between the opposite sides of the* heart. To get from one side to 'the other the blood must leave the Ade Asi FIG. 79. — The heart and the great blood-vessel attached to it, seen from the side towards the sternum. The left cavities and the vessels connected with them are colored red; the right black. Ats, left Auricle ; Adx and As, the right and left auricular appendages ; Vd, right1 ventricle ; Vs, left ventricle ; Aa, aorta ; Ab, innominate artery ; Cs, left common carotid artery; Ssi, left sub- clavian artery ; P, main trunk of the pulmonary artery,- and Pd and Ps, its branches to the right and left lungs ; c-s, superior vena cava ; Ade and Asi, the right and left innominate veins ; pd and ps, the right and left pulmonary veins; crd and crs, the right and left coronary arteries. heart and pass through a set of capillaries, as may readily be seen by tracing the course of tlio vessels in Fig. 76. The Heart as seen from, its Exterior. When the heart is viewed from the side turned towards the sternum (Fir. 79) the two auricles, Aid and As, are seen to be separated 206 THE HUMAN BODY. by a deep groove from the ventricles, Vd and Vs. A more shallow furrow runs between the ventricles and indicates the'position of the internal longitudinal septum.-- On the FIG. 80.— The heart viewed from its dorsal aspect. Aid, right aTj_rlole , cf, inferior vena cava; az, azygos vein ; Fc, coronary vein. The remauhr.g letter? of reference have the same signification as in Fig. 79: , dorsal aspect of the heart (Fig. 80) similar points may be noted, and on one or other of the two figures the great vessels opening into the cavities of the heart may be seen. The pulmonary artery, P, arises from the right ventricle, INTERIOR OF THE HEART. 207 and very soon divides into the right and left pulmonary arteries, Pd and Ps, which break up into smaller branches and enter the corresponding lungs. Opening into the right auricle are two great veins (see also Fig. 78), cs and ci, known respectively as the upper and lower vence cavce, or "hollow" veins; so called by the older anatomists be- cause they are frequently found empty after death. Into the back of the right auricle opens also another vein, Vc, called the coronary vein or sinus, which brings back blood that has circulated in the walls of the heart it- self. Springing from the left ventricle, and appearing irom beneath the pulmonary artery when the heart is looked at from the ventral side, is a great artery, the aorta, Aa. It forms an arch over the base of the heart and then runs down behind it at the back of the chest. From the convexity of the arch of the aorta several great branches are given off, Ssi, Cs, Ab; but before that, close to the heart, the aorta gives off two coronary arteries, branches of which are seen at crd and crs lying in the groove over the partition between the ventricles, and which carry to the substance of the organ that blood which comes back through the coronary sinus. Into the left auricle open two right and two left pulmonary veins, ps and pel, which are formed by the union of smaller veins proceeding from the lungs. In the diagram Fig. 78 from which the branches of the great vessels near the heart have been omitted for the sake of clearness, the connection of the various vessels with the chambers of the heart can be better seen. Opening into the right auricle are the superior and inferior venae cavae (cs and ci) and proceeding from the right ventricle the pulmonary artery, P. Opening into the left auricle are the right and left pulmonary veins (pd and ps) and spring- ing from the left ventricle the aorta, A. The Interior of the Heart. The communication of each auricle with its ventricle is also represented diagram - matically in Fig. 78, and the valves which are present at those points and at the origin of the pulmonary artery and tli at of the aorta. Internallv the auricles are for the most 208 THE HUMAN BODY. part smooth, but from each a hollow pouch, the auricular appendage, projects over the base of the corresponding ven- tricle as seen at Adx and As in Figs. 79 and 80. These pouches have somewhat the shape of a dog's ear and have- given their name to the whole auricle. Their interior is roughened by muscular elevations, covered by endocardium, known as the fleshy columns (columnw carnece). On the inside of the ventricles (Fig. 81) similar fleshy columns are very prominent. The Auriculo-Ventricular Valves. These are known as rigid and left, or as the tricuspicl and mitral valves re- spectively. The mitral valve (Fig. 81) consists of, t \zo_flaps of the endocardium fixed by their bases to the margins of auriculo-ventricular aperture and with their edges hanging down into the ventricle when the heart is empty. These- unattached edges are not however free, Tjir^have fixed to them a number of stout connective-tissue corc|s, the?j&r(7 tendinew, wlricjb^are fixed below to muscular elevations, the and Mpl, on the interior of the- ventricle. The cords are long enough to let the valve flaps rise into a horizontal position and so close the opening be- tween auricle and ventricle which lies between them, and passes up behind the opened aorta, Sp, represented in th& figure. The tricuspid valve is like tho mitral but with three flaps instead of two. Semilunar Valves. These are six in number: threejit the mouth of the aorta, Fig. 81, and three, qnif-.fi ^jfrfi themy afTEe" month oT the^j^ulmonary artery. Each is a strong: crescentic pouch fixed by its mofe^urved border, and with its free edge turned away from the heart. When the- valves are in action these free edges meet across the vessel and prevent blood from flowing back infa fli In the middle of the free border of each valve is a little- cartilaginous nodule, the corpus Arantii, and on each side of this the edge of the valve is very thin and w^hen it meets its neighbor doubles up against it and so secures the closure. The Arterial System. All the arteries of the Body arise either directly or indirectly from the aorta or pulmo- nary artery and the great majority of them from the for- THE ARTERIES. 209 mer vessel. The pulmonary artery only carries blood to the lungs to undergo exchanges with the air in them, after it has circulated through the Body generally. After making its arch the aorta continues back through JIpl Mpm FIG. 81.— The left ventricle and the commencement of the aorta laid open. Mpm, Mpl, the papillary muscles. From their upper ends are seen the cordce tendinece proceeding to the edges of the flaps of the mitral valve. The open- ing into the auricle lies between these flaps. At the beginning of the aorta are seen its three pouch-like semilunar valves. the chest, giving off many branches on its way. Piercing the diaphragm it enters the abdomen and after supplying the parts in and around that cavity with branches, it ends 210 THE HUMAN BODY. opposite the last lumbar vertebra by dividing into the right and left common iliac arteries, which carry blood to the lower limbs. We have then to consider the branches of the arch of the aorta, and those of the descending aorta, which latter is for convenience described by anatomists as consist- ing of the thoracic aorta, extending from the end of the arch to the diaphragm, and the abdominal aorta, extending from the diaphragm to the final subdivision of the vessel. Branches of the Arch of the Aorta. From this arise first the coronary arteries (crd and crs, Figs. 79 and 80) which spring close to the heart, just above two of the pouches of the semilunar valve, and carry blood into the substance of that organ. The remaining branches of the arch are three in number, and all arise from its convexity. The first is the innominate artery (Ah, Fig. 79), which is very short, immediately breaking up into the right subcla- vian artery, and the right common carotid. Then comes the left common carotid, Cs, and finally the left subclavian, Ssi. Each subclavian artery runs out to the arm on its own side and after giving off a vertebral artery (which runs up the neck to the head in the vertebral canal of the transverse processes of the cervical vertebrae), crosses the arm-pit and takes there the name of the axillary artery. This con- tinues down the arm as the brachial artery, which, giving off branches on its way, runs to the front of the arm, and just below the elbow-joint divides into the radial and. ulnar arteries, the lower ends of which are seen at R and U in Fig. 77.* These supply the forearm a nil end in the hand by uniting to form an arch, from which branches are given off to the fingers. The common carotid arteries pass out of the chest into the neck, along which they ascend on the sides of the windpipe. Opposite the angle of the lower jaw each divides into an internal and external carotid artery, right or left as the case may be. The external ends mainly in branches for the face, scalp, and salivary glands, one great subdivision of it with a tortuous course, the temporal artery, boing often seen beating in thin persons on the side of the brow. The in- * P. 202. THE CAPILLARIES. 211 ternal carotid aitery enters tlie skull through an aperture in its base and supplies the brain, which it will be remem- bered also gets blood through the vertebral arteries. Branches of the Thoracic Aorta. These are numerous but small. Some, the intercostal arteries, run out between the ribs and supply the chest-walls; others, the bronchial arteries, carry blood to the lungs for their nourishment, that carried to them by the pulmonary arteries being brought there for another purpose; and a few other small branches are given to other neighboring parts. Branches of the Abdominal Aorta. These are both large and numerous, supplying not only the wall of the •posterior part of the trunk, but the important organs in the abdominal cavity. The larger are — the cceliac axis which supplies stomach, spleen, liver, and pancreas; the superior mesenteric artery which supplies a great part of the intes- tine; the renal arteries, one for each kidney; and finally the inferior mesenteric artery which supplies the rest of ^the intestine. Besides these the abdominal aorta gives off very many smaller branches. Arteries of the Lower Limbs. Each common iliac di- vides into an internal and external iliac artery. The former mainly ends in branches to parts lying in the^pelvis, but the latter passes into the thighs and there takes the name of the femoral artery. At first this lies on the ven- tral aspect of the limb, but lower down passes back round the femur, and above the knee-joint (where it is called the popliteal artery) divides into the anterior and posterior iibial arteries which supply the leg and foot. The Capillaries. As the arteries are followed from the heart their branches become smaller and smaller, and finally cannot be traced without the aid of a microscope. Ulti- mately they pass into the capillaries, the walls of which are simpler than those of the arteries, and which form very close networks in nearly all parts of the Body; their immense number compensating for their smaller size. The average diameter of a capillary vessel is .016 mm. (^-gVir inch) so that only two or three blood corpuscles can puss through it abreast, and in many parts they are so close THE HUMAN BODY, that a pin's point could not be inserted between two of them. It is while flowing in these delicate tubes that the- blood does its nutritive work, the arteries being merely supply- tubes for the capillaries. The Veins. The first veins arise from the capillary net- works in various organs, and like the last arteries are very small. They soon increase in size by union and so form FIG. 82.— A small portion of the capillary network as seen in the frog's web when magnified about 2o diameters, a, a small artery feeding the capillaries ; v, v, small veins carrying blood back from the latter. larger and larger trunks. These in many places lie near or alongside the main artery of the part, but there are many more large veins just beneath the skin than there are large arteries. This is especially the case in the limbs, the main veins of which are superficial and can in many persons be THE VEINS. 213 seen as faint blue lines through the skin. Fig. 83 repre- sents the arm at the front of the elbow-joint after the skin and subcutaneous areolar tissue and fat have been re- — Pt bas FIG. 83. — The superficial veins in front of the elbow- joint. S', tendon of biceps muscle ; Bi, brachialis internus muscle ; Pt, pronator teres muscle ; 1, median nerve ; 2, 3, 4, nerve-branches to the skin ; B, brachial artery with its small accompanying veins ; cep, cephalic vein ; bas, basilic vein ; m', median vein ; *, junction of a deep-lying vein with the cephalic. moved. The brachial artery, B, colored red, is seen lying tolerably deep and accompanied by two small veins (vena eomites) which communicate by cross-branches. The greafc 214 THE HUMAN BODY. median nerve, 1, a branch of the brachial plexus which supplies several muscles of the forearm and hand, the skin over a great part of the palm, and the three inner fingers, is seen alongside the artery. The larger veins of the part are seen to form a more superficial network, joined here and there, as for instance at *, by branches from deeper 'parts. Several small nerve-branches which supply the skin (2, 3, 4) are seen among these veins. It is from the vessel, cep, called the cephalic vein, just above the point where it crosses the median nerve, that surgeons usually bleed a patient. A great part of the blood of the^lower limb is brought back by the long saplienous vein, which can be seen running beneath the skin from the inner side of the ankle to the top of the thigh. All the blood wfiich leaves the heart by the aorta, except that flowing through the coronary arte- ries, is finally collected into the superior and inferior venae cavcB (cs and ci? vFigs. 79 and 86^), and poured into the right auricle. The 'jugular veiri$ which run down the neck, carrying back the blood which went out along the carotid arteries, unite below with tlie arm-vein (subclavian) to form on each side an innominate vein (Asi and Ade, Fig. 79) and the innominates unite to form the superior cava. The coronary-artery blood after flowing through the capillaries of the heart itself, also returns to this auricle by the coronary veins. The Pulmonary Circulation. Through this the blood gets back to the left side of the heart and so into the aorta again. The pulmonary artery, dividing into branches for each lung, ends- in the capillaries of those organs. From these it is collected by the pulmonary veins which carry it back to the left auricle, whence it passes to the left ventricle to recommence its flow through the Body generally. The Course of the Blood. From what has been said it is clear that the movement of the blood is a circulation. Starting from any one chamber of the heart it will in time return to it; but to do this it must pass through at least two sets of capillaries; one of these is connected with the aorta and the other with the pulmonary artery, and in its PORTAL CIRCULATION. 215 circuit the blood returns to the heart twice. Leaving the left side it returns to the right, and leaving the right it returns to the left: and there is no road for it from one side of the heart to the other except through a capillary network. Moreover it always leaves from a ventricle through an artery, and returns to an auricle through a vein. There is then really only one circulation; but it is not uncommon to speak of two, the flow from the left side of the heart to the right, through the Body generally, being called the systemic circulation, and from the right to the left, through the lungs, the pulmonary circulation. But since after completing either of these alone the blood is not again at the point from which it started, but is separated from it by the septum of the heart, neither is a " circulation5* in the proper sense of the word. The Portal Circulation. A certain portion of the blood which leaves the left ventricle of the heart through the aorta has to pass through three sets of capillaries before it can again return there. This is the portion which goes through the stomach, spleen, pancreas, and intestines. After traversing the capillaries of those organs it is col- lected into the portal vein which enters the liver, and breaking up in it into finer and finer branches like an artery, ends in the capillaries of that organ, forming the second set which this blood passes through on its course. From these it is collected by the hepatic veins which pour it into the inferior vena cava, which carrying it to the right auricle, it has still to pass through the pulmonary capillaries to get back to the left side of the heart. The portal vein is the only one in the Human Body which thus like an artery feeds a capillary network, and the flow from the stomach and intestines through the liver to the vena cava is often spoken of as the portal circulation. Diagram of the Circulation. Since the two halves of the heart are actually completely separated from one another by an impervious partition, although placed in proximity in the Body, we may conveniently represent the course of the blood as in the accompanying diagram (Fig. 84) in which the right and left halves of the heart are rep- 216 THE HUMAN BODY. resented at different points in the vascular system. Such an arrangement makes it clear that the heart is really two pumps working side by side, and each engaged in forcing the blood to the other. Starting from the left auricle, la, and following the flow we trace it through the left ventri- cle and along the branches of the aorta into the systemic capillaries, sc; from thence it passes back through the systemic veins, vc. Reaching the right auricle, ra, it is sent into the right ventricle, rv, and thence through the pulmonary artery, pa, to the lung capillaries, pc, from which the pulmonary veins, pv, car- ry it to the left auricle, which drives it into the left ventricle, lv, and this again into the aorta. Arterial and Venous Blood. The blood when flowing in the pulmo- nary capillaries gives up carbon diox- ide to the air and receives oxygen from it; and since its coloring mat- ter (haemoglobin) forms a scarlet FIG. 84. -Diagram of the compound with OXV^en, it flows to blood vascular system, show- * - fe . ing that it forms a single the left auricle through the pulmo- closed circuit with two pumps . „ , . , , in it, consisting of the right nary veins of a bright red color. This and left halves of the heart, -• • , • j. • ^rt *A i which are represented sepa- Color it maintains Until it reaches the systemic capillaries, but in these ifc loses m«<* o^gen to the rarronnd- S fcisSU6S and Sains m^h carbon pul" dioxide from them. But the blood coloring matter which has lost its oxygen has a dark purple-black color, and since this un- oxidized or "reduced" haemoglobin is now in excess, the blood returns to the heart by the venae cavag of a dark purple-red color. This color it keeps until it reaches the lungs, when the reduced haemoglobin becomes again oxi- dized. The bright red blood, rich in oxygen and poor in carbon dioxide, is known as "arterial blood" and the dark 8TR UCTTJRE OF THE BLOOD- VESSELS. 217 red as "venous blood:" and it must be borne in mind that the terms have this peculiar technical meaning, and that the pulmonary veins contain arterial blood and the pulmo- nary arteries, venom blood; the change from arterial to venous taking place in the systemic capillaries, and from venous to arterial in the pulmonary capillaries. The chambers of the heart and the great vessels containing ar- terial blood are shaded red in Figs. 79 and 80. The Structure of the Arteries. A large artery can by careful dissection be separated into three coats; an internal, middle and outer. The internal coat tears readily across the long axis of the artery and consists of an inner lining of flattened nucleated cells, and of a variable number of layers composed of membranes or networks of elastic tissue, outside this. The middle coat is made up of alternating layers of elastic fibres and plain muscular tissue; the for- mer running for the most part longitudinally and the latter across the long axis of the vessel. The outer coat is the toughest and strongest of all and is mainly made up of white fibrous connective tissue but contains a considerable amount of elastic tissue also. It gradually shades off into a loose areolar tissue which forms the sheath of the artery or the tunica adventitia, and packs it between surrounding parts. The smaller arteries have all the elastic elements less developed. The internal coat is consequently thinner, and the middle coat is made up mainly of involuntary mus- cular fibres. As a result the large arteries are highly elas- tic, the aorta being physically much like a piece of indian- rubber tubing, while the smaller arteries are highly con- tractile, in the physiological sense of the word. Structure of the Capillaries. In the smaller arteries the outer and middle coats gradually disappear, and the elastic layers of the inner coat also go. Finally, in the capillaries the lining epithelium alone is left, with a more or less developed layer of connective-tissue corpuscles around it, representing the remnant of the tunica adven- titia. These vessels are thus extremely well adapted to al- low of filtration or diffusion taking place through their thin walls. 218 THE HUMAN BODY. Structure of the Veins. In these the same three pri- mary coats as in the arteries may be found: the inner and middle coats are less developed while the outer one remains thick, and is made up almost entirely of white fibrous tissue. Hence venous walls are much thinner than those of the corresponding arteries, and the veins collapse when empty while the stouter arteries remain open. But the tenacity and toughness of their outer coats give the veins great strength. Except the pulmonary artery and the aorta, which pos- sess the semilunar valves at their cardiac orifices, the arteries possess no valves. Many veins on the contrary have such, formed by semilunar pouches of the inner coat, at- tached by one margin and having that turned towards the heart free. These valves, sometimes single, oftener in pairs, and sometimes three at one level, permit -blood to flow only towards the heart, for a current in that direction (as in the upper diagram, Fig. 85) presses the valve close against the side of the vessel and ====== meets with no obstruction from it. c ^==^ ff Should any back-flow be attempted, however, the current closes up the ====________ valve and bars its own passage as indicated in the lower figure. These valves are most numerous ln superficial veins and those of muscular parts. They are absent end of the vessel. in the venae cavae and the portal and pulmonary veins. Usually the vein is a little dilated opposite a valve and hence in parts where the valves are numerous gets a knotted look. On compressing the forearm so as to stop the flow in its sub- cutaneous veins and cause their dilatation, the points at which valves are placed can be recognized by their swollen appearance. They are most frequently found where two veins communicate. CHAPTER XV. THE WORKING OF THE HEART AND BLOOD. VESSELS. The Beat of the Heart. It is possible by methods known to physiologists to open the chest of a living narcotized animal, such as a rabbit, and see its heart at work, alter- nately contracting and diminishing the cavities within it and relaxing and expanding them. It is then observed that each beat commences at the mouths of the great veins; from there runs over the rest of the auricles, and then over the ventricles; the auricles commencing to dilate the moment the ventricles commence to contract. Having finished their contraction, the ventricles also commence to dilate and so for some time neither they nor the auricles are contracting, but the whole heart expanding, f The con- wn_as its systole and the relaxation as its diastole, and since theTTwo aides of the heart work synchronously7"the auricles together and the ventricles together, we may describe a whole "cardiac \ period" or " heart-beat" as made up successively of auricu- \ lar systole, ventricular systole, and pause. This cycle is / repeated about seventy times a minute; and if the whole time occupied by it be subdivided into 100 parts, about 9 of these will be occupied by the auricular systole, about '30 by the ventricular systole, and 61 by the pause: during more than half of life, therefore, the muscles of the heart are at rest. In the pause the heart if taken be- tween the finger and thumb feels soft and flabby but dur- ing the systole it (especially in its ventricular portion) be- comes hard and rigid. Change of Form of the Heart. During its systole the 220 THE HUMAN BODY. heart becomes shorter and rounder, mainly from a change in the shape of the ventricles. A cross-section of the heart at the base of these latter during diastole would be ellipti- cal in outline, with its long diameter from right to left: during the systole it is more circular, the long axis of the- ellipse becoming shortened while the dorso- ventral diameter remains little altered. At the same time the length of the ventricles is lessened, the apex of the heart approaching the base and becoming blunter and rounder. The Cardiac Impulse. The human heart lies with its- apex touching the chest-wall between the fifth and sixth ribs on the left side of the breast-bone. At every beat a sort of tap, known as the "cardiac impulse" or "apex: beat," may be felt by the finger at that point. There is, however, no actual "tapping" since the heart's apex never leaves the chest-wall. During the diastole the soft ventri- cles yield to the chest-wall where they touch it, but dur- ing the systole they become hard and tense and push it out a little between the ribs, and so cause the apex beat. Since the heart becomes shorter during the ventricular systole it might be supposed that at that time the apex would move up a little in the chest. This however is not the case, the ascent of the apex towards the base of the ventricles being compensated for by a movement of the whole heart in the- opposite direction. If water be pumped into an elastic tube, already tolerably full, this will be distended not only transversely but longitudinally. This is what happens in the aorta: when the left ventricle contracts and pumps blood forcibly into it, the elastic artery is elongated as well as widened, and the lengthening of that limb of its arch at- tached to the heart pushes the latter down towards the dia- phragm, and compensates for the upward movement of the apex due to the shortening of the ventricles. Herice if the exposed living heart be watched it appears as if during the systole the base of the heart moved towards the tip, rather than the reverse. Events occurring within the Heart during a Cardiac Period. Let us commence at the end of the ventricular systole. At this moment the semilunar valves at the orifices. PHENOMENA OF THE HEARTS BEAT. 221 of the aorta and the pulmonary artery are closed, so that no blood can flow back from those vessels. The whole heart, however, is soft and distensible and yields readily to blood flowing into it from the pulmonary veins and the venae cavae; this passes on through the open mitral and tricuspid valves and fills up the dilating ventricles, as well as the auricles. As the ventricles fill, back currents are set up along their walls and these carry up the flaps of the valves so that by the end of the pause they are nearly closed. At this moment the auricles contract, and since this contrac- tion commences at and narrows the mouths of the veins opening into them, and at the same time the blood in those vessels opposes some resistance to a back-flow into them, while the still flabby and dilating ventricles oppose much less resistance, the general result is that the contract- ing auricles send blood mainly into the ventricles, and hardly any back into the veins. At the same time the in- creased direct current into the ventricles produces a greater back current on the sides, which, as the auricles cease their contraction and the filled ventricles become tense and press on the blood inside them, completely close the auriculo- ventricular valves. That this increased filling of the ven- tricles, due to auricular contractions, will close the valves is seen easily in a sheep's heart. If the auricles be carefully cut away from this so as to expose the mitral and tricuspid valves, and water be then poured from a little height into the ventricles, it will be seen that as these cavities are filled the valve-flaps are floated up and close the orifices. The auricular contraction now ceases and the ventricular commences. The blood in each ventricle is imprisoned between the auriculo-ventricular valves behind and the semilunar valves in front. The former cannot yield on account of the cordae tendineas fixed to their edges: the semilunar valves, on the other hand, can open outwards from the ventricle and let the blood pass on, but they are kept tightly shut by the pressure of the blood on their other sides, just as the lock-gates of a canal are by the pressure of the water on them. In order to open the canal-gates water is let in or out of the lock until it stands at the same level 222 THE HUMAN BODY. on each side of them; but of course they might be forced open without this by applying sufficient power to overcome the higher water pressure on one side. It is in this latter way that the semilunar valves are opened. The contracting ventricle tightens its grip on the blood inside it and becomes rigid to the touch. As it squeezes harder and harder, at last the pressure on the blood in it becomes greater than the pressure exerted on the other side of the valves by the blood in the arteries, the flaps are pushed open, and the blood begins to pass out: the ventricle continues its con- traction until it has obliterated its cavity and completely emptied itself. Then it commences to relax and blood immediately to flow back into it from the highly stretched arteries. This back current, however, catches the pockets of the semilunar valves, drives them back and closes the valve so as to form an impassable barrier; and so the blood which has been forced out of either ventricle cannot flow directly back into it. Use of the Papillary Muscles. In order that the con- tracting ventricles may not force blood back into the auricles it is essential that the flaps of the mitral and tricuspid valves be maintained horizontally across the open- ings which they close, and be not pushed back into the auricles. At the commencement of the ventricular sys- tole this is provided for by the cordae tendineae, which are of such a length as to keep the edges of the flaps in appo- sition, a position which is farther secured by the fact that each set of cordae tendineae (Fig. 81*) radiating from a point in the ventricle, is not attached around the edges of one flap but on the contiguous edges of two flaps, and so tends to pull them together. But as the contracting ven- tricles shorten, the cordae tendineae, if directly fixed to their interior, would be slackened and the valve-flaps pushed up into the auricle. The little papillary muscles prevent this. Shortening as the ventricular systole proceeds, they keep the cordae taut and the valves closed. Sounds of the Heart. If the ear be placed on the chest over the region of the heart during life, two distinguish- able sounds will be heard during each cardiac cycle. They are known respectively as the first and second sounds of the * P. 209. EVENTS IN A CARDIAC CYCLE. 223 heart. The first is of lower pitch and lasts longer than the second and sharper sound: vocally their character may be tolerably imitated by the words lubb, diip. The cause of the second sound is the closure, or as one might say the " clicking up," of the sem'ilunar valves, since it occurs at the moment of their closure and ceases if they be hooked back in a living animal. The origin of the first sound is still uncertain: it takes place during the ventricular systole and is probably due to vibrations of the tense ventricular wall at that time. It is not due, at least not entirely, to the auriculo-vcntricular valves, since it may still be heard in a beating heart empty of blood, and in which there could be no closure or tension of those valves. In various forms of heart disease these sounds are modified or cloaked by additional " murmurs" which arise when the cardiac orifices .are roughened or narrowed or dilated, or the valves ineffi- cient. By paying attention to the character of the new .sound then heard, the exact period in the cardiac cycle at which it occurs, and the region of the chest-wall at which it is heard most distinctly, the physician can often get impor- tant information as to its cause. Diagram of the Events of a Cardiac Cycle. In the following table the phenomena of the heart's beat are rep- resented with reference to the changes of form which are seen in an exposed working heart. Events in the same vertical column occur simultaneously; on the same horizon- tal line, from left to right, successively. Auricular Sys- tole. Commence- ment of Ven- tricular Sys- tole. Ventricular Systole. Cessation of Ventricular Systole. Pause. Auricles Dilatin Dilating Ventricles and emptying. Dilating and filling. filling. and filling. Dilating and filling. Dilating Impulse filling. and emptying. and filling. Auriculo - ventric- ular valves Semilunar valves. Sounds Closing. . Closed. Closed. Closed. . Closed. Open. Opening.1 Closing. Second Open. Closed. sound. 224 THE HUMAN BODY. Function of the Auricles. The ventricles have to da the work of pumping the blood through the blood-vessels. Accordingly their walls are far thicker and more muscular than those of the auricles; and the left ventricle, which has to force the blood over the Body generally,, is stouter than the right, which has only to send blood around the com- paratively short pulmonary circuit. The circulation of the blood is in fact maintained by the ventricles, and we have to inquire what is the use of the auricles. Not un- frequently the heart's action is described as if the auricles first filled with blood and then contracted and filled the ventricles; and then the latter contracted and drove the blood into the arteries. From the account given above, however, it will be seen that the events are not accurately so represented, but that during all the pause blood flows, on through the auricles into the ventricles, which latter are already nearly full when the auricles contract; this con- traction merely completing their filling and finishing the- closure of the auriculo-ventricular valves. The real use of the auricles is to afford a reservoir into which the veins may empty while the comparatively long-lasting ventricular contraction is taking place: they also largely control the amount of work done by the heart. If the heart consisted of the ventricles only, with valves; at the points of entry and exit of the blood, the circulation could be maintained. During diastole the ventricle would fill from the veins, and during systole empty into the ar- teries. But in order to accomplish this, during the systole the valves at the point of entry must be closed, or the ven- tricle would empty itself into the veins as well as into the arteries; and this closure would necessitate a great loss of time which might be utilized for feeding the pump. This is avoided by the auricles, which are really reser voirs at the end of the venous system collecting blood when the ventricular pump is at work. When the ven- tricles relax, the blood entering the auricles flows on into them: but previously, during the -fW of the cardiac cycle occupied by the ventricular systole, the auricles have accumulated blood, and when they at last con- FUNCTION OF THE AURICLES. 225 tract they send on into the ventricles this accumulation. Even were the flow from, the veins stopped during the auricular contraction this would be of comparatively little consequence, since that event occupies so brief a time. But, although no doubt somewhat lessened, the emptying of the veins into the heart does not seem to be, in health, stopped while the auricle is contracting. For at that mo- ment the ventricle is relaxing and receives the blood from the auricles under a less pressure than it enters the latter from the veins. The heart in fact consists of a couple of "feed-pumps" — the auricles — and a couple of " force- pumps" — the ventricles; and so wonderfully perfect is the mechanism that the supply to the feed-pumps .is never stopped. The auricles are never empty, being supplied all the time of their contraction, which is never so great as to obliterate their cavities; while the ventricles contain no blood at the end of their systole. The auricles also govern to a certain extent the amount of work done by the ventricles. These latter contract with more than sufficient force to completely drive out all the blood contained in them. If the auricles contract more powerf ully and empty themselves more completely at any given time, the ventricles will contain more blood at the commencement of their systole, and have pumped out more ut its end. "Now as we shall see in Chapter XVII., the contraction of the auricles is under the control of the nervous system; and through the auricles the whole work of the heart. In fact the ventricles represent the brute force concerned in maintaining the circulation, while the auricles are part of a highly developed co-ordinating mechanism, by which the rate of the circulation is governed according to the needs of the whole Body at the time. The Work Done by the Heart. This can be calculated with approximate correctness. At each systole each ven- tricle sends out the same quantity of blood — about 180 grams (6.3 ounces); the pressure exerted by the blood in the aorta against the semilunar valves and which the ventricle has to overcome is about that which would be ex- eried on the same surface by a column of mercury 200 226 THE HUMAN BOLT. millimeters (8 inches) high. The left ventricle therefore drives out, seventy times in a minute, 180 grams (6.3 ounces) of blood against this pressure. Since the specific gravity of mercury is 12.5 and that of blood may for prac- tical purposes be taken as 1, the work of each stroke of the ventricle is equivalent to raising 180 grams (6.3 ounces) of blood 200 X 12.5 = 2500 millim. (8.2 feet)- or one, gram 450 meters (one ounce 51.66 feet); or one kilo- gram 0.45 meters (one Ib. 3.23 feet). Work is measured by the amount of energy needed to raise a definite weight a given distance against gravity at the earth's surface, the unit, called a kilogr ammeter, being either that necessary to* raise one kilogram one meter, or, called & foot-pound, that necessary to raise one pound one foot. Expressed thus the work of the left ventricle in one minute, when the heart'* rate is seventy strokes in that time, is 0.45 X 70 — 31.50 kilogrammeters (3.23x70 = 226.1 foot-pounds); in one hour it is 31.50 X 60 = 1890 kilogrammeters (226.1 X 60 = 13,566 footpounds); and in twenty-four hours 1890 x 24 = 45,360 kilogrammeters (325,584 foot-pounds). The pres- sure in the pulmonary artery against which the right ventricle works is about -J of that in the aorta; hence thie ventricle in twenty-four hours will do one third as much work as the left, or 15,120 kilogrammeters (108,528 foot pounds) and adding this to the amount done by the left, we get as the total work of the ventricles in a day the immense amount of 60,480 kilogrammeters (434,112 foot-pounds). If a man weighing 75 kilograms (165 ibs.) climbed up a mountain 806 meters (2644 feet) high his skeletal muscles would probably be greatly fatigued at the end of the ascent, and yet in lifting his Body that height they would only have performed the amount of work that the ventricles of the heart do daily without fatigue. The Flow of the Blood Outside the Heart. The blood leaves the heart intermittently and not in a regular stream, a quantity being forced out at each systole of the ventri- cles: before it reaches the capillaries, however, this rhythmic movement is transformed into a steady flow as may readily be seen by examining under the microscope thin trans- BLOOD FLOW AS SEEN WITH THE MICROSCOPE. 22? parent parts of various animals, as the web of a frog's foot, a mouse's ear, or the tail of a small fish. In consequence of the steadiness with which the capillaries supply the veins the flow in these is also unaffected, directly, by each beat o* the heart; if a vein be cut the blood wells out uniformly, while a cut artery spurts out not only with much more force, but in jets which are much more powerful at regu- lar intervals corresponding with the systoles of the ven- tricles. The Circulation of the Blood as Seen in the Frog's Web. There is no more fascinating or instructive phenomenon than the circulation of the blood as seen with the micro- scope in the thin membrane between the toes of a frog's hind limb. Upon focusing beneath the epidermis a net- work of minute arteries, veins and capillaries, with the blood flowing through them, comes into view (Fig. 82*). _ The arteries, a, are readily recognized by the fact that the flow in them is i'ascest and irom larger to smaller branches^ "Th*e latter are seen ending- in capillaries, which form networks, the channels of wTi in n n.re a 1 1 r ft*"'! y equal in size. While in the veins arising from the capillary the flow is from smaller_tq larger -toaaks. and slower than in the arteries but faster than in the capillaries. The reason of the slower flow of the capillaries is that their united area is considerably greater than thai, of the arteries supplying them, so that the same quantity of blood flowing Jjirongh them in a given time, has a wider channel to flow in and moves more slowly. The^jirca of the veins is smaller than^thtit of the capillaries but ffreater"~trian that of thejirteries, and hence the rate of niovementTrTEKcm ia^jal^Zmtermediate. Almost always when an artery divides, the area of its branches is greater than that of the main trunk, and so the arterial current becomes slower and slower from the heart onwards. In the veins on the other hand, the area of a trunk formed by the union of two or more branches is less than that of the branches together, and the flow becomes quicker and quicker towards the heart. But even at the heart the united cross-sections of the veins entering the auricles are greater than those of *P. 212. 228 THE HUMAN BODY. the arteries leaving the ventricles, so that, since as much blood returns to the heart in a given time as leaves it, the rate of the current in the pulmonary veins and the venae cavae is less than in the pulmonary artery and aorta. We may represent the vascular system as a double cone, widening from the ventricles to the capillaries and narrowing from the latter to the auricles. Just as water forced in at a narrow end of this would flow quickest there and slowest at the widest part, so the blood flows quickest in the aorta and slowest in the capillaries, which form together a much wider channel. The Axial Current and the Inert Layer, If a small artery in the frog's web be closely examined it will be seen that the j*ate of flow is not the same in all parts of it. In the centreTs a very'rapid current carrying along alTthe red corpuscles and known as tne axial stream, while "near the Tv-nll of flip yaffil thfl flnw 1*? mucn slower, as indicateOythe rate at which the pale blood corpuscles are carried along in it. This is a purely physical phenomenon. If any liquid be forcibly driven through a fine tube which it wets, water for instance through a glass tube, the outermost layer of the liquid will remain motionless in contact with the tube; the next layer of molecules will move faster, the next faster still; and so on until a very rapid current is found in the centre. If solid bodies, as powdered sealing-wax, be suspended in the water, these will all be carried on in the central faster current or axial stream, just as the red corpuscles are in the artery. The white corpuscles, on account of their power of executing independent amoeboid movements and their consequent irregular form, get fre- quently pushed out of the axial current, so that many of them are found in the inert layer. Internal Friction. It follows from the above-stated facts that there is no noticeable friction bejhween the blood and thfi lininor nf fTia ypageLJ-.Tn-nngfr whlVh ^ flows; Since the outermost blood layer in contact with the wall of the vessel is almost motionless. But there is very great fric- tion between the different concentric layers of the liquid, since each of them is moving at a different rate from those THE CAPILLARY CIRCULATION. 229 in contact with it on each side. This form of friction is known in hydro-dynamics as " internal friction" and it is of great importance in the circulation of the blood. In- ternal friction increases very fast as the calibre of the tube through which the liquid flows diminishes: so that with the same rate of flow it is disproportionately much greater in a small tube than in a larger one. Hence a given quantity of liquid forced in a minute through one large tube, would experience much less resistance from in- ternal friction than if sent in the same time through four or five smaller tubes, the united transverse sections df which were together equal to that of the single larger ine. In the blood-vessels the increased total area, and consequently slower flow, in the smaller channels partly counteracts this increase of internal friction, bat only to a comparatively slight extent; so that the internal friction, and conse- quently the resistance to the blood -flow, is far greater in the capillaries than in the small arteries, and in the small ar- teries than in the large ones. Practically we may regard the arteries as tubes ending in a sponge: the united areas of all the channels in the latter might be considerably larger than that of the supplying tubes, but the friction to be overcome in' the flow through them would be much greater. The Conversion of the Intermittent into a Continuous Plow. Since the heart sends blood into the aorta inter- mittently, we have still to inquire how it is that the flow in the capillaries is continuous. In the larger arteries it is not, since we can feel them dilating as the "pulse" by ap- plying the finger over the radial artery at the wrist, or the temporal artery on the side of the brow. The first explanation which suggests itself is that since the capacity of the blood-vessels increases from the heart to the capillaries, an acceleration of the flow during the Tentricular contraction which might be very manifest in the vessels near the heart would become less and less obvi- ous in the more distant vessels. But if this were so, when the blood was collected again from the wide capillary sponge into the great veins near the heart, which together 230 THE HUMAN BODY. -d" are very littie bigger than the aorta, we ought to find a pulse, but we do not: the venous pulse which sometimes occurs having quite a different cause, being due to a back- flow from the auricles, or a checking of the on-flow into them, during the cardiac systole. The rhythmic flow caused by the heart is therefore not merely cloaked in the- small arteries and capillaries but abolished in them. We can, however, readily contrive conditions outside the; Body under which an intermittent supply is transformed into a continuous flow. Sup- pose we have two vessels, A and B (Fig. 86), containing; water and connected below in two ways; through the tube a on which there is a pump- provided with valves so that it can only drive liquid from ^ A to B, and through b,. )j which may be left wide open or narrowed by the clamp c, at will. If the apparatus be left at rest the water will lie at the same level, d, in each vesssel. If now we work the pump, at each stroke a cer- tain amount of water will be conveyed from A to B, and as a result of the lowering of the level of liquid in A and its rise in B, there will be immediately a return flow from B to A through the tube b. A, in these circumstances, would represent the venous system, from which the heart constantly takes blood to pump it into B, representing the arterial system; and b would represent the capillary vessels through which the return flow takes place: but, so far, we should have as intermittent a flow through the capillaries, by as through the heart-pump, a. Now imagine b to be narrowed at one point so as to oppose resistance to the back-flow, while the pump goes on working steadity. The re- sult will be an accumulation of water in B, and a fall of its level in A. But the more the difference of level in the two vessels increases, the greater is the force tending to drive water back through b to A, and more will flow back, under Fia. 86. CAUSE OF STEADY CAPILLARY BLOOD-FLOW. 231 the greater difference of pressure, in a given time, until at last, when the water in B has reached a certain level, d'y and that in A has correspondingly fallen to d" ', the current through b will carry back in one minute just so much water as the pump sends the other way, and this back- flow will be nearly constant; it will not depend directly upon the strokes of the pump but upon the head of water accumu- lated in B\ which head of water will, it is true, be slightly increased at each stroke of the pump, but the increase will be very small compared with the whole driving force; and its influence will be inappreciable. We thus gain the idea that an incomplete impediment to the flow from the ar- teries to the veins (from Bio A in the diagram), such as is afforded by internal friction in the capillaries, may bring about conditions which will lead to a steady flow through the latter vessels. But in the arterial system there can be no accumulation of blood at a higher level than that in the veins, such as is supposed in the above apparatus: and we must next con- sider if the "head of water" can be replaced by some other- form of driving force. It is in fact replaced by the elas- ticity of the large arteries. Suppose an elastic bag in- stead of the vessel B connected with the pump, "a." If there be no resistance to the back-flow the current through 1) will be discontinuous. But if resistance be interposed, then the elastic bag will become distended, since the pump sends in a given time more liquid into it than it passes back through b. But the more it becomes distended the more will the bag squeeze the liquid inside and the faster will it send that back to A, until at last its squeeze is so pow- erful that in a minute or two or five minutes it sends back into A as much as it receives. Thenceforth the back-flow through I will be practically constant, being im- mediately dependent upon the elastic reaction of the bag; and only indirectly upon the action of the pump which keeps it distended. Such a state of things represents very closely the phenomena occurring in the blood-vessels. The highly elastic large arteries are kept stretched with^ blood by the heart; and the reaction of their elasfic wallsT 232 THE HUMAN BODY, steadily squeezing on the blood in them, forces it continu- ously through the small arteries and capillaries. Thar steady flow in the latter depends thus on two factors: first the elasticity of the large arteries; and secondly the re- sistance to their emptying, dependent upon internal friction in the small arteries and the capillaries, which calls into play the elasticity of the large vessels. Were the capillary resistance or the arterial elasticity absent the blood-flow in the capillaries would be rhythmic. CHAPTER XVI. ARTERIAL PRESSURE. THE PULSE. Weber's Schema. It is clear from the statements made in the last chapter that it is the pressure exerted by the elas- tic arteries upon the blood inside them which keeps up the flow through the capillaries, the heart serving to keep the big arteries tightly filled and so to call the elastic reaction of their walls into play. The whole circulation depends primarily of course upon the beat of the heart, but this only indirectly governs the capillary flow, and since the latter is the aim of the whole vascular apparatus it is of great importance to know all about arterial pressure; not only how great it is on the average but how it is altered in different vessels in various circumstances so as to make the flow through the capillaries of a given part greater or less according to circumstances; for, as blushing and pallor of the face (which frequently occur without any change in the skin elsewhere) prove, the quantity of blood flowing through a given part is not always the same, nor is it always increased or diminished in all parts of the Body at the same time. Most of what we know about arterial pres- sure has been ascertained by experiments made upon the lower animals, from which deductions are then made con- cerning what happens in man, since anatomy shows that the circulatory organs are arranged upon the same plan in all the mammalia. A great deal can, however, be learnt by studying the flow of liquids through ordinary elastic tubes. Suppose we have a set of such (Fig. 87) supplied at one point with a pump, c, possessing valves of entry and exit which open only in the direction indicated by the arrows, and that the whole system is slightly overfilled with liquid so 234 THE HUMAN BODY. that its elastic walls are slightly stretched. These will in -consequence press upon the liquid inside them and the amount of this pressure will be indicated by the gauges; so long as the pump is at rest it will be the same everywhere (and therefore equal in the gauges on B and A), since liquid in a set of horizontal tubes communicating freely, as these do at D, always distributes itself so that the pressure upon it is everywhere the same. Let the pump c now contract once, and then dilate: during the contrac- tion it will empty itself into B and during the dilatation fill itself from A. Consequently the pressure in B, indicated by the gauge x, will rise and that in A will fall. But very rapidly the liquid will redistribute itself from B to A through D, until it again exists everywhere under the same FIG. 87.— Diagram of Weber's Schema. pressure. Every time the pump works there will occur a similar series of phenomena, and there will be a disturbance of equilibrium causing a wave to flow round the tubing; but there will be no steady maintenance of a pressure on the side B greater than that in A. Now let the upper tube D be closed so that the liquid to get from BtoA must flow through the narrow lower tubes D', which oppose con- siderable resistance to its passage on account of their fre- quent branchings and the great internal friction in them; then if the pump works frequently enough there will be produced and maintained in B a pressure considerably higher than that in A, which may even become negative. If, for WEBERS SCHEMA. 235 example, the pump works 60 times a minute and at each stroke takes 180 cubic centimeters of liquid (6 ounces) from A and drives it into B, the quantity sent in at the first stroke will not (on account of the resistance to its flow offered by the small branched tubes), have all got back into A before the next stroke takes place, sending 180 more cubic centimeters (6 oz.) into B. Consequently at each stroke B will become more and more distended and A more -and more emptied, and the gauge x will indicate a much higher pressure than that on A. As B is more stretched, however, it squeezes harder upon its contents, until at last -a time comes when this squeeze is powerful enough to force through the small tubes just 180 cubic centimeters (6oz.) in •a second. Then further accumulation in B ceases. The pump sends into it 10,800 cubic centimeters (360 ounces) in a minute at one end and it squeezes oat exactly that -amount in the same time from its other end; and so long .as the pump works steadily the pressure in B will not rise, nor that in A fall, any more. But under such circumstances "the flow through the small tubes will be nearly constant since it depends upon the difference in pressure prevailing between B and A, and only indirectly upon the pump •which serves simply to keep the pressure high in B and low in A. At each stroke of the pump it is true there will be a slight increase of pressure in B due to the fresh 180 •cub. cent. (6 oz.) forced into it, but this increase will be but a small fraction of the total pressure and so have but an insignificant influence upon the rate of flow through the small connecting tubes. Arterial Pressure. The condition of things just de- scribed represents very closely the phenomena presented in the blood-vascular system, in which the ventricles of the lieart, with their auriculo-ventricular and semilunar valves, represent the pump, the smallest arteries and the capil- laries the resistance at D', the large arteries the elastic tube B, and the veins the tube A. The ventricles con- stantly receiving blood through the auricles from the veins, send it into the arteries, which find a difficulty in emptying themselves through the capillaries, and so blood accumu- 236 THE HUMAN BODY. lates in them until the elastic reaction of the stretched ar- teries is able to squeeze in a minute through the capillaries just so much blood as the left ventricle pumps into the aorta, and the right into the pulmonary artery, in the same time. Accordingly in a living animal a pressure-gauge connected with an artery shows a much higher pressure than one connected with a vein, and this persistent differ- ence of pressure, only increased by a small fraction of the whole at each heart-beat, keeps up a steady flow from the arteries to the veins. The heart keeps the arteries stretched and the stretched arteries maintain the flow through the capillaries, and the constancy of the current in these de- pends on two factors: (1) the resistance experienced by the- blood in its flow from the ventricles to the veins, and (2) the elasticity of the larger arteries which allows the blood to accumulate in them under a high pressure, in conse- quence of this resistance. The Arterial Pressure. This cannot be directly meas- ured with accuracy in man, but from measurements made- on other animals it is calculated that in the human aorta its average is equal to that of a column of mercury 200 millimeters (8 inches) high. During the systole it rises about 5 millimeters (^ inch) above this and during the pause falls the same amount below it. The pressure in the vena? cavse on the other hand is often negative, the blood being, to use ordinary language, often "sucked" out of them into the heart, and it rarely rises above 5 millimeters (| inch) of mercury except under conditions (such as powerful mus- cular effort accompanied by holding the breath) which force blood on into the venae cavae and, by impeding the- pulmonary circulation, interfere with the emptying of the right auricle. Hence to maintain the flow from the aorta to the vena cava we have an average difference of pressure equal to 200 — 5 = 195 millimeters. (7f inches) of mercury, rising to 205 — 5 = 200 mm. (8 inches) during the cardiac systole and falling to 195 — 5 = 190 mm. (7| inches) dur- ing the pause; but the slight alterations, only about -fa of the whole difference of aortic and vena cava pressures which maintain the blood-flow, are too slight to cause appreciable ARTERIAL PRESSURE. 237 changes in the rate of the current in the capillaries. The pressure on the blood in the pulmonary artery is about J of that in the aorta. Since the blood flows from the aorta to its branches and from these to the capillaries and thence to the veins, and liquids in a set of continuous tubes flow from points of greater to those of less pressure, it is clear that the blood- pressure must constantly diminish from the aorta to the right auricle; and similarly from the pulmonary artery to the left auricle. At any point in fact the pressure is pro- portionate to the resistance in front, and since the farther the blood has gone the less of this, due to impediments at branchings and to internal friction, it has to overcome in finishing its round, the pressure on the blood always di- minishes as we follow it from the aorta to the venae cavae. In the larger arteries the fall of pressure is gradual and small, since the amount of resistance met with in the flow through them is but little. In the small arteries and capillaries the resistance passed -by is (on account of the great internal friction due to their small calibre) very great, and consequently the fall of pressure between the medium- sized arteries and the veins is rapid and considerable. Modifications of Arterial Pressure by Changes in the Rate of the Heart's Beat. A little consideration will make it clear that the pressure prevailing at any time in a given artery depends on two things — the rate at which, the vessel is filled, i.e. upon the amount of work done by the heart; and the ease or difficulty with which it is emptied, that is upon, the resistance in front. Returning to the system of elas- tic tubes with a pump represented in Fig. 87, let us sup- pose the pump to be driving as before 10,800 cub. cent. (360 oz.) per minute into the tubes B and that these latter are so distended that they drive out just that quantity in the same time. Under such conditions the pressure at any given point in B will remain constant, apart from the small variations dependent upon each stroke of the pump. Now, however, let the latter, while still sending in 180 cub. cent. (6 oz.) at each stroke, work 80 instead of 60 times a minute and so send in that time 180 X 80 = 14,400 cub. 238 THE HUMAN BODY. cent. (480 oz.) instead of the former quantity. This will lead to an accumulation in B, since its squeeze is only suf- ficient, against the resistance opposed to it, to send out 10,- 800 cab. cent. (360 oz.) in a minute. B consequently will become more stretched and the pressure in it will rise. As this takes place, however, it will squeeze more powerfully on its contents until at last its distension is such that its elasticity is able to force out in a minute through the small tubes D', 14,400 cub. cent. (480 oz.). Thenceforth, so long as the pump beats with the same force and at the same rate and the peripheral resistance remains the same, the mean pressure in B will neither rise nor fall — B sending into A in a minute as much as c takes from it, and we would have a steady condition of things with a higher mean pressure in B than before. On the other hand if .the pump begins to wrork more slowly while the resistance remains the same, it is clear that the mean pressure in B will fall. If, for example, the pump works only forty times a minute and so sends in that time 180 X 40 = 7200 cub. cent. (240 oz.) into B, which is so stretched that it is squeezing out 10,800 cub. cent. (360 oz.) in that time, it is clear that B will gradually empty itself and its walls become less stretched and the pressure in it fall. As this takes place, however, it will force less liquid in a minute through the small tubes, until at last a pressure is reached at which the squeeze of B only sends out 7200 cub. cent. (240 oz.) in. a minute; and then the fall of pressure will cease and a steady one will be main- tained, but lower than before. Applying the same reasoning to the vascular system we see that (when the peripheral resistance remains unaltered), if the heart's force remains the same but its rate increases, arterial pressure will rise to a new level, while a slowing of the heart's beat will bring about a fall of pressure. Modifications of Arterial Pressure Dependent on Changes in the Force of the Heart's Beat. Returning again to Fig. 87; suppose that while the rate of the pump remains the same, its power alters so that each time it sends 200 cub. cent. (6.6 oz.) instead of 180 (6 oz.) and so in CHANGES IF ARTERIAL PRESSURE. 239 a minute 12,000 cub. cent. (396 oz.) instead of 10,800 (360 oz. ) — the quantity which B is stretched enough to squeeze out in that time. Water will in consequence accumulate in B until it becomes stretched enough to squeeze out 12,000 -cub. cent. (396 oz.) in a minute, and then a steady pressure at a new and higher level will be maintained. On the other hand if the pump, still beating sixty times a minute, works more feebly so as to send out only 160 cub. cent. (5.6 oz.) at each stroke, then B, squeezing out at first more than it receives in a given time, will gradually empty itself until it only presses hard enough upon its contents to force 160 X 60 = 9600 cub. cent. (336 oz.) out in a minute. Similarly, if while the resistance in the small arteries and capillaries remains the same and also the heart's rate, the power of the stroke of the latter alters, so that at each beat it sends more blood out than previously, then arterial pressure will rise; while if the heart beats more feebly it Trill fall. Modifications of Arterial Pressure by Changes in the Peripheral Resistance. Let the pump c in Fig. 87 still wfrrk otoadily ocnding-1 0,800 cub. cent. (360 oz.) per min- ute into B and the resistance increase, it is clear arterial pressure must rise. For B is only stretched enough to .squeeze out in a minute the above quantity of liquid against the original resistance and cannot at first send out that quantity against the greater. Liquid will consequently ac- cumulate in it until at last it becomes stretched enough to send out 10,800 cub. cent. (360 cubic oz.) in a minute through the small tubes, in spite of the greater resistance to be overcome. A new mean pressure at a higher level will then be established. If on the contrary the resistance diminishes while the pump's work remains the same, then B will at first squeeze out in a minute more than it receives, until finally its elastic pressure is reduced to the point at which its receipts and losses balance, and a new and lower mean pressure will be established in B. system increase of the peripheral re- _ sistance by narrowing of the' small arteries will TucreiUm ara 240 THE HUNAN BODY. .jejrial pressure in all parts nearer the heart, while dilatation of the small arteries will have the contrary effect. Summary. "Wo find then that arterial pressure at any moment is dependent upon — (1) the rate of the heart's beat;, (2) the quantity of blood forced into the arteries at each beat; (3) the calibre of the smaller vessels. All of these, and consequently the capillary circulation which depends; upon arterial pressure, are under the control of the nervous system (see Chap. XVII.). The Pulse. When the left ventricle contracts it forcea a certain amount of blood into the aorta, which is already distended and on account of the resistance in front cannot empty itself as fast as the contracting ventricle fills it. As a consequence its elastic walls yield still more — it enlarges both transversely and longitudinally and if exposed in a living animal can be seen and felt to pulsate, swelling out at each systole of the heart, and shrinking and getting rid of the excess during the pause. A similar phenomenon, can be observed in all the other large arteries, for just as the contracting ventricle fills the aorta faster than the latter empties (the whole period of diastole and systole being required by the aorta to pass on the blood sent in during systole) so the increased tension in the aorta im- mediately after the cardiac contraction, drives on some of its contents into its branches, and fills these faster than. they are emptying and so causes a dilatation of them also, which only gradually disappears as the aortic tension falls before the next systole. Hence after each beat of the heart there is a sensible dilatation of all the larger arteries, known as the pulse, which becomes less and less marked at points on the smaller branches farther from the heart, but which in health can readily be recognized on any artery large enough to be felt by the finger through the skin, etc. The radial artery near the wrist, for example, will always be felt tense by the finger, since it is kept overfilled by the heart in the way already described. But after each heart-beat it be- comes more rigid and dilates a little, the increased disten- sion and rigidity gradually disappearing as the artery passes on the excess of blood before the next heart-beat. THE PULSE, 241 ( The pulse is then a wave of increased pressure started by the ventricular systole, radiating from the semilunar valves over the arterial system, and grflflp ft] ]y d i «ftppea-rl " g in the smaller branches. -fn the aorta the pulse is most marked, for the^resistance thereto^Ehe transmission onwards of the blood sentS by the heart is greatest, and the elastic tube in which it consequently accumulates is shortest, and so the increase of pressure and the dilatation caused are consider- able. The aorta, however, gradually squeezes out the ex- cess blood into its branches and so this becomes distributed over a wider area, and these branches having less resistance in front find less and less difficulty in passing it on; conse- quently the pulse-wave becomes less and less conspicuous and finally altogether disappears before the capillaries are reached, the excess of liquid in the whole arterial system after a ventricular systole being too small to sensibly raise the mean pressure once it has been widely distributed over the elastic vessels, which is the case by the time the wave has reached the small branches which supply the ca- pillaries. The pulse-wave travels over the arterial system at the rate of about 9 meters (29.5 feet) in a second, commencing at the wrist 0.159 seconds, and in the posterior tibial artery at the ankle 0.193 seconds, after the ventricular systole. The blood itself does not of course travel as fast as the pulse-wave, for that quantity sent into the aorta at each heart-beat does not immediately rush on over the whole arterial system, but by raising the local pressure causes the vessel to squeeze out faster than before some of the blood it already contains, and this entering its branches raises the pressure in them and causes them to more quickly fill their branches and raise the pressure in them; the pulse-wave or wave of increased pressure is transmitted in this way much faster than any given portion of the blood. How the wave of increased pressure and the liquid travel at differ- ent rates may be made clearer perhaps by picturing what would happen if liquid were pumped into one end of an already full elastic tube, closed at the other end. At the closed end of the tube a dilatation and increased tension 242 THE HUMAN BODY. would be felt immediately after each stroke of the pump,, although the liquid pumped in at the other end would have remained about its point of entry; it would cause the pul- sation not by flowing along the tube itself, but by giving a push to the liquid already in it. If instead of absolutely closing the distal end of the tube one brought about a state of things more nearly resembling that found in the arteries by allowing it to empty itself against a resistance, say through a narrow opening, the phenomena observed would not be essentially altered; the increase of pressure would travel along the distended tube far faster than the liquid itself. The pulse being dependent on the heart's systole, " feel- ing the pulse5' of course primarily gives a convenient means of counting the rate of beat of that organ. To the skilled touch however it may tell a great deal more, as for example- whether it is a readily compressible or " soft pulse" show- ing a low arterial pressure, or tense and rigid (" a hard pulse") indicative of high arterial pressure, and so on. In adults the normal pulse rate may vary from sixty-five to seventy-five. In the same individual it is faster when standing than when sitting, and when sitting than when lying down. Any exercise increases its rate temporarily and so does excitement; a sick person's pulse should not therefore be felt when he is nervous or excited (as the- physician knows when he tries first to get his patient calm and confident), as it is then difficult to draw correct conclusions from it. In children the pulse is quicker than in adults, and in old age slower than in middle life. The Rate of the Blood-Flow. As the vascular system becomes more capacious from the aorta to the capillaries the rate of flow in it becomes proportionately slower, and as the total area of the channels diminishes again from the capillaries to the venae cavse, so does the rate of flow quicken again, just as a river current slackens where it spreads out,, and flows faster where it is confined to a narrower channel; a fact taken advantage of in the construction of Eads' jetties at the mouth of the Mississippi, the object of which is to make the water flow in a narrower channel and so with a SECONDARY AIDS TO THE CIRCULATION. 243 more rapid current. Actual measurements as to the rate of flow in the arteries cannot be made on man, but from experiments on lower animals it is calculated that in the human carotid the blood flows about 400 millimeters ( 16 inches) in a second. In the capillaries the current travels only from 0.5 to 0.75 mm. (^ to -^ inch) in a second. The total time taken by a portion of blood in getting from the aorta through the carotid and its branches, and the capillaries, and rhen through veins to the right auricle, that is in going round the systemic circulation, is about 23 seconds — of which time about one second is spent in the capillaries; each 'portion of blood on its course from the last artery to the first vein passes through a length of capillary which on the average is 0.5 mm. (-f^ inch). The rate of flow in the great veins is about 100 mm. (4 inches) in a second, but is subject to considerable varia- tions dependent on the respiratory and other movements of the Body (see below). Secondary Causes of the Circulation. While the heart's beat is the great driving force of the circulation, certain other things help more or less — viz. gravity, compression of the veins, and aspiration of the thorax. All of them are, however, quite subsidiary; experiment on the dead Body shows that the injection of whipped blood into the aorta under a less force than that exerted by the left ventricle during life, is more than sufficient to drive it round and back by the venee cavae. Not unfrequently the statement is made in books that, probably, the systemic capillaries have an attractive force for arterial blood and the pulmonary capillaries for venous blood, but there is not the slightest evidence of the correctness of such a supposition, nor any necessity for making it. The Influence of Gravity. Under ordinary circum- stances this ma) be neglected, since in parts of the Body below the level of the heart it will assist the flow in the arteries and impede it equally in the veins, while the reverse is the case in the upper parts of the Body. In certain cases, however, it is well to bear these points in mind. A part •'* congested" or gorged with blood should if possible be 244 THE HUMAN BODY. raised so as to make the back-flow in its veins easier; and sometimes when the heart is acting feebly it may be able to drive blood along arteries in which gravity helps, but not otherwise. Accordingly in a tendency to fainting it is best to lie down, and make it easier for the heart to send blood up to the brain, bloodlessness of which is the cause of the loss of consciousness in a fainting-fit. In fact so long as the breathing continues the aspiration of the thorax will keep up the venous flow (see below), while, in the circum- stances supposed, a slight diminution in the resistance op- posed to the arterial flow may be of importance. The head of a person who has fainted should accordingly never be raised until he has undoubtedly recovered, a fact rarely borne in mind by spectators who commonly rush at once to lift any one whom they see fall in the street or elsewhere. The Influence of Transient Compression of the Veins. The valves of the veins being so disposed as to permit only a flow towards the heart, when external pressure empties a vein it assists the circulation. Continuous pressure, as by a tight garter, is of course bad since it checks all subse- quent flow through the vessel, but intermittent pressure, such as exerted on many veins by muscles in the ordi- nary movements of the Body, acts as a pump to force on the blood in them. The valves of the veins have another use in diminishing the pressure on the lower part of those vessels in many regions. If, for instance, there were no valves in the long saphenous vein (p. 214) of the leg the weight of the whole column of blood in it, which in the erect position would be about a meter (39 inches) high, would press on the lower part of the vessel. But each set of valves in it carries the weight of the column of blood between it and the next set of valves above, and relieves parts below, and so the weight of the column of blood is distributed and does not all bear on any one point. Aspiration of the Thorax. Whenever a breath is drawn the pressure of the air on the vessels inside the chest is di- minished, while that on the other vessels of the Body is un- affected. In consequence blood tends to flow into the chest. PROOFS OF THE CIRCULATION. 245 It cannot, however, flow back from the arteries on account of the semilunar valves of the aorta, but it readily is pressed, or in common language "sucked," thus into the great veins close to the heart and into the right auricle of the latter. The details of this action must be omitted until the respiratory mechanism has been considered. All parts of the pulmonary circuit being within the thorax, the respiratory movements do not influence it, except in so far as the distension or collapse of the lungs influences the calibre of their vessels. The considerable influence of the respiratory movements upon the venous circulation can be readily observed. In thin persons the jugular vein in the neck can often be seen to empty rapidly and collapse during inspiration, and fill up faster than it empties during expiration, thus exhib- iting a sort of venous pulse. Every one, too, knows that by making a violent and prolonged expiration, as exhibited for example by a child with Avhooping-cough, the flow in all the veins of the head and neck may be checked, •causing them to swell up and hinder the capillary circula-, tion until the person becomes " black in the face," from the engorgement of the small vessels with the dark-colored ve- nous blood. In diseases of the tricuspid valve another form of venous pulse is often seen in the superficial veins of the neck, since at each contraction of the right ventricle some blood is driven back through the right auricle into the veins. Proofs of the Circulation of the Blood. The older physiologists believed that the movement of the blood was an ebb and flow, to and from each side of the heart, and out and in by both arteries and veins. They had no idea of a circulation, but thought pure blood was formed in the lungs and impure in the liver, and that these partially mixed in the heart through minute pores supposed to exist in the septum. Servetus, who was burnt alive by Calvin in 1553, first stated that there was a continuous passage through the lungs from the pulmonary artery to the pulmonary veins, but the great Englishman Harvey first, in lectures delivered in the College of Physicians of London about 246 THE HUMAN BODY. 1616, demonstrated that the movement of the blood was a continuous circulation as we now know it, and so laid the foundation of modern Physiology. In his time, however, the capillary vessels had not been discovered, so that al- though he was quite certain that the blood got somehow from the final branches of the aorta to the radicles of the venous system, he did not exactly know how. The proofs of the course of the circulation are at present quite conclusive and may be summed up as follows. (1) Blood injected into an artery in the dead Body will return by a vein; but injected into a vein will not pass buck by an artery. (2) The anatomical arrangement of the valves of the heart and of the veins shows that the blood can only flow /row the heart, through the arteries and back to the heart by the veins. (3) A cut artery spurts from the end next the heart, a cut vein bleeds most from the end farthest from the heart. (4) A portion of a vein when emptied fills only from the end farthest from the heart. This experiment can be made on the veins on the back of the hand of any thin person, especially if the vessel* be first gorged by holding the hand in a dependent posi- tion for a few seconds. Select then a vein which runs for an inch or so without branching, place one finger on its distal end and then empty it up to its next branch (where valves usually exist) by compressing it from below up. The vessel will then be found to remain empty as long as the finger is kept on its lower end, but will fill immediately when it is removed; which proves that the valves prevent any filling of the vein from its heart end backwards. (5) If a bandage be placed around the arm, so as to close the superficial veins but not tight enough to occlude the deeper-seated arteries, the veins on the distal side of the bandage will become gorged and those on its proximal side empty, showing again that the veins only receive blood from their ends turned towards the capilla- ries. (6) In the lower animals direct observation with the microscope shows the steady flow of blood from the arte- ries through the capillaries to the veins, but never in the opposite direction. CHAPTER XVII. THE REGULATION OF THE HEART AND BLOOD- VESSELS BY THE NERVOUS SYSTEM. The Need of Co-ordination. Eor the safe and harmo- nious working of the circulatory apparatus it is obviously necessary that there be some mode of mutual interaction between the heart and the blood-vessels: if the heart beat and the arteries relaxed or contracted, each without any reference to the other, no orderly capillary flow c«uld be maintained. To secure such a flow, the work done by the heart and the resistance offered in the vessels must at any given moment be correlated; so that the heart shall not by too powerful action over-distend or perhaps burst the small arteries, nor the latter contract too much and so, by increasing the peripheral resistance, raise the aortic pres- sure to a great height and increase unduly the work to be done by the left ventricle in forcing open the semilunar valves. / Again, the total amount of blood in the Body is not sufficient to keep all its organs supplied with the amount needful for the full exercise of their activity at one time, and in the Body accordingly we never find all its parts hard at work at the same moment. If when one group of muscles was set at work and needed an extra blood-supply, this was attained merely by increasing the heart's activity and keeping up a faster blood-flow every- where through the Body, there would be a clear waste of force, much as if the chandeliers in a house were so ar- ranged that when a larger flame was wanted at one burner it could only be obtained by turning more gas on at all the rest at the same time; besides the big tap at the gas-meter regulating the general supply of the house, local taps at 248 THE HUMAN BODY. each, burner are required which regulate the gas-supply to each flame independently of the rest. A similar arrange- ment is found in the Body. Certain nerves control the calibre of the arteries supplying different organs and, when the latter are set at work, allow their arteries to dilate and so increase the amount of blood flowing through them while the general circulation elsewhere remains practically unaffected. The resting parts at any moment thus get just enough blood to maintain their healthy nutrition and the working parts get more; and as certain organs come to rest and others are set in activity, the arteries of the one narrow and of the others dilate ; in this way the distri- bution of the blood in the Body is undergoing constant changes, parts which at one time contain much blood at another having but little. In addition, then, to nervous organs regulathrg the work of the heart and the arteries with reference to one another, we have to consider another set of vascular nerves which govern the local blood-supply of different regions of the Body. The Nerves of the Heart. The heart gets nerves from three sources. (1) From nerve-cells buried in its own sub- stance and known as its intrinsic ganglia. (2) From the tenth pair (pneumogastrics) of cranial nerves. (3) From the sympathetic nervous system. The intrinsic ganglia keep the heart beating, and the other two sets of nerves control the rate and force of the beat. The Intrinsic Heart-Nerves. The ganglia of the heart lie for the most part in the partition between the auricles and along the line of junction of the auricles and ventricles ; a few are found also in the upper parts of the latter. From some of them arise nerve-fibres which go to the muscles of the heart, while others are connected with the endings of the extrinsic nerves reaching the organ ; and probably all communicate by a network of nerve-fibres. The heart is an automatic organ : its beat, like the movement of filaments of a ciliated cell, depends on its own structure and properties and not on anything outside itself. This is proved by the fact that the heart cut out of an animal which has just been decapitated, and entirely re- CAUSE OF THE HEARTS BEAT. 249 moved from all the rest of the body, will go on beating for some time; even the heart of a warm-blooded animal, if supplied with oxygenated blood, may be kept beating regu- larly for hours after its isolation from the rest of the body. The excised heart of a turtle or frog if kept moist will beat for days. Whether the time of its continuance be shorter or longer, the fact that the heart-beat continues after com- plete excision of the organ proves that it is not dependent on stimuli originating in other parts of the Body. In the ciliated cell we had no differentiation into muscle and nerve — its contractile and automatic parts if separated at all were not optically distinguishable and we could only speak of the cilia as still retaining both of those primitive protoplasmic properties. But in the heart, where we find distinct muscles and nerves, the question naturally arises in which of them does the automatic power reside. We have already seen (Chap. X.) that ordinary striated mus- cles possess little or no automaticity : they only contract under the influence of a recognizable stimulus, and though the muscular fibres of the heart do differ somewhat from other striated muscle, it is a priori improbable that they are automatic and we are rather led to suppose that the usual stimulus starts in the ganglion-cells of the heart, especially since we know that nerve-cells elsewhere are automatic. Experiment confirms this supposition. If an excised beating frog's heart be cut into several pieces with a sharp razor it will be found that, while bits of the auricles and the base of the ventricle go on beating, the apical portions of the ventricle lie at rest permanently or for an hour or two — not because the muscle there is dead and has lost its contractility, for these bits if excited by any extraneous muscular stimulus will still beat but — because that part of the heart possesses little automacity. Now this is just the part of the frog's heart that has no ganglion-cells of its own, while the parts that go on beating are those which possess them; hence we conclude that the normal stimulus originates in the nerve-cells of the organ. The excitant of the nerve-cells being still unknown we call them automatic. 250 THE HUMAN BODY. Under certain conditions the isolated apex of the frog's heart gives rhythmic automatic beats; cardiac muscle lias retained more automaticity than ordinary muscle, though not so much as nerve-cells. In any case the cause of the heart's beat lies in the heart itself, though controlled as to rate and force by nerves from elsewhere. Nerves Slowing the Heart's Beat. Each pneumogas- tric trunk sends several branches to the heart. Certain of these contain fibres which when excited slow, or even alto- gether stop, the beat of the heart and are hence known as the cardio-inliibitory fibres. If one pneumogastric trunk be divided as it runs down the neck and its peripheral, or lower, end be stimulated feebly the heart's beat becomes less frequent, while a more powerful stimulation will completely stop it for a few seconds, as if its muscles were suddenly paralyzed. If the experiment be performed upon a narcotized animal, the heart of which is at the same time exposed by opening the chest, it will be seen that during the stoppage the heart lies flabby and relaxed in diastole: the excitation of the nerve does not stop the heart's beat, as might perhaps be supposed, by keeping it in a state of permanent tetanic contraction, but it annuls its contractions and throws it into a state of rest ; the nerve-fibres concerned are not excitant but inhibitory, stopping instead of calling forth the activity of the part on "which they act. Whether their influence is exerted di- rectly on the muscular fibres of the heart or upon it 3 in- trinsic ganglia, abolishing their automatic activity and so cutting off the stimuli which normally radiate from them to the muscles, is not certainly known, but the former view is probably the correct one. The pneumogastric fibres seem to govern the nutrition of the heart muscle; the stoppage produced by their stimulation is nearly always temporary and followed by more rapid and powerful beats. These cardio-inhibitory fibres originate in a collection of nerve-cells in the medulla oblongata known as the cardio- inhibitory centra This centre is automatic and always in a state of slight excitation, feebly stimulating the fibres proceeding from it and slightly slowing the heart's beat. CABDIAC INHIBITION. 251 This is shown by the fact that if both pneumogastric nerves be cut in the neck the heart at once begins to beat a little faster than before; the brake, so to speak, has been taken off it. The Influence upon Arterial Pressure of Inhibiting the Heart. If the heart be entirely stopped arterial pres- sure will of course fall very rapidly, since the distended arterial system will go on emptying itself through the capil- laries into the veins, without receiving any fresh supply at its cardiac end. So too if the heart be made to beat slower, but with the same force in each stroke, it follows from the facts pointed out in the last chapter (p. 238) that arterial pressure will fall to a new and lower level, at which the elastic arteries are only stretched enough to squeeze out in a minute as much as they receive. As a matter of fact, when the heart is made to beat slower by weak pneumo- gastric stimulation each beat is usually a little more power- ful than before. However, this extra force is usually not sufficient to compensate entirely for the slower rate and so the general arterial pressure falls. Use of the Cardio-Inhibitory Mechanism.. Although the cardio-inhibitory centre is automatic and always in a state of slight activity it is also greatly under the control of afferent nerve-fibres reaching it which can arouse it to a much greater activity, and so reflexly control the heart's beat. If a frog be rendered insensible and its abdominal cavity opened, it will be found that one or two smart taps on the intestine will cause the heart to stop in diastole. If, however, the pneumogastric nerves, or the spinal cord, or the posterior roots of the spinal nerves, or the communicating branches between the sympathetic nerves of the abdomen and the spinal nerves, be previously cut, then striking the intestine has no influence upon the heart; nor has it if the cardio-inhibitory centre in the medulla oblongata be pre- viously destroyed. We thus get evidence that the mechani- cal stimulation of the intestinal nerves stops the heart re- flexly through the < pn£umogflg^kisr-t^ie afferent impulses traveling from the sympathetic into the spinal nerves and passing then up the spinal cord to the cardio-inhibitory 252 THE HUMAN BODY. centre, where they are reflected as efferent impulses down the pneumogastric trunks to the heart. In man and other mammals similar arrangements exist, the afferent fibres pass- ing from the alimentary canal through the solar plexus (p. 172} which lies behind the stomach. It is by exciting them and so reflexly stopping the heart, that men are sometimes killed by a severe blow on the abdomen or even occasionally by a large draught of very cold water, the sudden cold acting as a thermal stimulus, through tlie walls of the stomach, on the nerve-fibres outside. A hot and very thirsty person requir- ing a big drink should therefore not take too cold water — or if he does, swallow it only a mouthful at a time. The blood-vessels of the alimentary canal are very numer- ous and capacious and form one of the largest vascular tracts of the whole Body, and through the reflex mechanism above described we see how they may control the heart's beat. Probably if the heart is beating too frequently and keeping up too high a pressure in them, the sympathetic nerve- fibres in their coats are stimulated and then, reflexly, through the cardio-inhibitory centre slow the heart's beat and lower the general arterial pressure ; and so we get one co-ordinating mechanism by which the heart and blood-ves- sels are made to work in unison. Some other afferent nerves are also known to be in con- nection with the cardio-inhibitory centre. For instance, some persons are made to faint by a strong odor, the olfac- tory nerves exciting the cardio-inhibitory centre and stop- ping or greatly slowing the heart. Deaths from the admin- istration of chloroform are also usually brought about in the same way, the vapor stimulating the sensory nerves of the air-passages which then excite powerfully the cardio-inhibi- tory centre and stop the heart. The Accelerator Nerves of the Heart. These originate in the spinal cord, from which they pass by communicating branches to the lowest cervical and upper dorsal sym- pathetic ganglia and thence to the heart. When stimu- lated they cause the heart to beat quicker, but under what conditions they are employed in the physiological working of the Body is not known. NERVES OF THE BLOOD-VESSELS. 253 The Nerves of the Blood- Vessels. The arteries, as already pointed out, possess a muscular coat composed of fibres arranged across them, so that their contraction will narrow the vessels. This coat is most prominent in the smaller vessels, those of the size which go to supply separate organs, but disappears again in the smallest branches which are about to divide into capillaries for the individual tissue elements of an organ. These vascular muscles are under the control of certain nerves called vaso-motor (p. 186) and these latter can thus govern the amount of blood reaching any organ at a given time. The vaso-motor nerves of the arteries are, like those of the heart, intrinsic and extrinsic. The intrin- sic fibres originate from ganglion-cells in the coats of the arteries or lying alongside them, while the extrinsic origi- nate from cells in the cerebro-spinal centre, from which they commonly pass into the sympathetic system before they reach the vessels. The intrinsic ganglia, like those of the heart, are automatic and tend to keep the muscular coats of the arteries in a constant state of feeble contrac- tion so that, apart from their physical elasticity, the arteries always hold a certain grip on the blood. The contraction, however, is as a rule persistent and steady, or tonic, instead of rhythmic like that of the heart, although slow rhythmic contractions have been seen to occur in some arteries. The difference probably depends rather on the kind of muscle concerned in each case than on the ganglion-cells, since plain muscular tissue, such as is found in the arteries, con- tracts so slowly and remains contracted so long when excited, that stimuli reaching it at intervals which would give a rhythmic beat in cardiac muscle, would keep the arterial per- manently contracted or tetanized. As in the heart, the activity of the arterial intrinsic nervous mechanism is under the control of extrinsic nerves, certain of which, the vaso-constrictors, answer to the accelerator nerves of the heart and increase the activity vof the intrinsic ganglia, while others, corresponding to the cardio-inhibitory fibres, check the activity of the intrinsic vascular nerves. /The VaSO-MotOr CentrQ. Tl^ Vfl.gQ^nr>gfrinf.m- gyf.^^^ arterial nerves are nearly always in a state of slight activity, 254 THE HUMAN BODY, keeping the arteries more constricted than they would be under the influence of their intrinsic nerves alone. Accord- ingly if they are cut, or paralyzed, in any region of the Body its arteries dilate and it becomes flushed with blood. Those of the external ear, for example, run in the cervical sympa- thetic, from the lower part of the neck where they leave the spinal cord, until they reach the arterial branches for the ear and run along the smaller twigs to it. If, therefore, the cervical sympathetic be divided on one side in an anaesthetized rabbit, the ear on that side becomes red and warm from the dilatation of its arteries and the extra amount of blood flowing through it. If, however; that end of the cut nerve still attached to the ear be excited electri- cally or otherwise, the ear arteries contract gradually until their passage is almost closed up, and the whole organ be- comes cold and very pale. Although these vaso-constrictor fibres are thus shown to pass through the cervical sympa- thetic, other experiments show that they really originate in a group of nerve-cells in the medulla oblongata, and from there run down the spinal cord to the lower part of the neck, where they pass out in the anterior roots of some spinal nerves and reach the sympathetic syste^m. The same is true of nearly all extrinsic vaso-constrictor nerve-fibres in the Body. Some few possibly arise from centres -in the spinal cord, but the great majority come primarily from the medulla oblongata, and the collection of nerve-cells there from which they spring is known as the vaso-motor centre; a better name would be the vaso-constrictor centre. The Control of the Vaso-Motor Centre. The vaso- motor centre is automatic; that is to say it maintains 'a certain amount of activity of its own, independently of any stimuli reaching it through afferent nerve-fibres. Never- theless, like nearly all automatic nerve-centres, it is under reflex control, so that its activity may be increased or les- sened by afferent impulses conveyed to it. Nearly every sen- sory nerve of the Body is in connection with it; any stimu- lus giving risp f.f) pn.in, for example, excites it, and thus constricting the arteries, increases the peripheral resist- ance to the blood-flow and raises arterial pressure. On VA80-DILATOR NERVES. 255 the other hand, certain fibres conveying impulses from the heart inhibit the centre and dilate the arteries^ Jlpwer blood-pressure, and diminish the resistance to be overcome "hy frfrfi frfiart. These fibres run in branches of the pneumo- gastric, and are known as the depressor fibres, or in certain animals, for example the rabbit, where they are all collected into one branch, as the depressor nerve. If this nerve be divided and its cardiac end stimulated no effect is pro- duced, but if its central end (that still connected with the rest of the pneumogastric trunk and through it with the medulla oblongata) be stimulated, arterial pressure gradually falls; this result being dependent upon a dilata- tion of the small arteries, and consequent diminution of the peripheral resistance, following an inhibition of the vaso-motor centre brought about by the depressor nerve. Through the depressor nerve the heart can therefore influ- ence the calibre of the small arteries and, by lowering aortic pressure, diminish its own work if need be. Blushing. The depressor nerves control a great part of the vaso-motor centre, and so can bring about dilatation of a large number of arteries — their influence is called into play when general arterial pressure is to be lowered, but is useless for controlling local blood-supply. This is man- .aged by other afferent nerves, each of which inhibits a small part only of the vaso-motor centre, governing the arteries of a limited tract of the Body; the dilatation of these increases the amount of blood flowing through the particular region to which they are distributed, but does not affect the total resistance to the blood-flow sufficiently to influence noticeably the general pressure in the arterial system. In blushing, for example, under the influence of an emotion, that part of the vaso-motor centre which sup- plies constrictor nerves to the arteries of the skin of the neck and face, is inhibited by nerve-fibres proceeding from the cerebrum to the medulla oblongata, and the face and neck consequently become full of blood and flush up. Quite similar phenomena occur under other conditions in many parts of the Body, although when not visible on the surface we do not usually call them blushes. The mucous 256 TEE HUMAN BODY. membrane lining the empty stomach is pallid and its ar- teries contracted, but as soon as food enters the organ it becomes red and full of blood; the food stimulating afferent nerve-fibres there, which inhibit that part of the vaso-motor centre which governs the gastric arteries. Taking Cold. This common disease is not unfrequent- ly caused through undue reflex excitement of the vaso- motor centre. Cold acting upon the skin stimulates, through the afferent nerves, the region of the vaso-motor centre gov- erning the skin arteries, and the latter become contracted,, as shown by the pallor of the surface. This has a two-fold influence — in the first place, more blood is thrown into in- ternal parts, and in the second, contraction of the arte- ries over so much of the Body considerably raises the gen- eral blood-pressure. Consequently the vessels of internal parts become overgorged or " congested," a condition which readily passes into inflammation. Accordingly prolonged exposure to cold or wet is apt to be followed by catarrh or inflammation of more or less of the respiratory tract caus- ing bronchitis, or of the intestines causing diarrhoea. In fact the common summer diarrhoea is far more often due to a chill of the surface, causing intestinal catarrh, than to- the fruits eaten in that season which are so often blamed for it. The best preventive is to wear, when exposed to great changes of temperature, a woolen or at least a cotton garment over the trunk of the Body; linen is so good a conductor of heat that it permits any change in the exter- nal temperature to act almost at once upon the surface of the Body. After an unavoidable exposure to cold or wet the thing to be done is of course to maintain the cutaneous circulation; for this purpose movement should be persisted in, or a thick dry outer covering put on, until warm and dry underclothing can be obtained. ' For healthy persons a temporary exposure to cold, as a plunge in a bath, is good, since in them the sudden contrac- tion of the cutaneous arteries soon passes off and is suc- ceeded by a dilatation causing a warm healthy glow on the surface. If the bather remain too long in cold water,, how- ever, this reaction passes off and is succeeded by a "more VASO-DILATOR NERVES. 257 persistent chilliness of the surface, which may even last all day. The bath should therefore be left before this occurs, but no absolute time can be stated, as the reaction is more marked and lasts longer in strong, persons, and in those used to cold bathing, than in others, j I Vaso-Dilator Nerves. / We have already seen, in the case orthe stomach, one method by which a locally increased blood-supplv may be brought about in an organ while it is at work. Usually, however, in the Body this is managed in another way; by vjy^dil^ or paralyze, not the vaso-motor centre, but the intrinsic nerv- ous supply of the blood-vessels. The nerves of the skeletal muscles for example contain two sets of fibres: one motor proper and the other vaso-dilator. When the muscle con- tracts in a reflex action or under the influence of the will both sets of fibres are excited; so that when the organ is set at work its arteries are simultaneously dilated and more blood flows through it. Quite a similar thing occurs in the sali- vary glands. Their cells, which form the saliva, are aroused to activity by special nerve-fibres; but the gland nerve also contains vaso-dilator fibres which simultaneously cause a dilatation of the gland artery. Through such arrange- ments the distribution of the blood in the Body at any moment is governed: so that working parts shall have abundance and other parts less, while at the same time the general arterial pressure remains the same on the average; since the expansion of a few small local branches but little influences the total peripheral resistance in the vascular sys- tem. Moreover, commonly when one set of organs is at work with its vessels dilated, others are at rest with their arteries comparatively contracted, and so a general average blood-pressure is maintained. Few persons, for example, feel inclined to do brain work after a heavy meal: for then a great part of the blood of the whole Body is led off into the dilated vessels of the digestive organs, and the brain gets a smaller supply. On the other hand, when the brain is at work its vessels are dilated and often the whole head flushed: and so excitement or hard thought after a meal is very apt to produce an attack of indigestion, by diverting 258 THE HUMAN BODY. the blood from the abdominal organs where it ought to be at that time. Young persons, whose organs have a super- abundance of energy enabling them to work under unfavor- able conditions, are less apt to suffer in such ways than their elders. One sees boys running actively about after eating, when older people feel a desire to sit quiet and ru- minate— or even to go to sleep. CHAPTER XVIII. THE SECRETORY TISSUES AND ORGANS. Definition. In a strict sense of the terms every pro- cess in which substances are separated from the blood, whether they be altered or unaltered, is "secretory" and every product of such a separation is a " secretion;" in this sense secretions would be separable into three classes. (1) Liquids or gases transuding on free surfaces of the Body, whether external or internal; (2) the liquids (lymph) moistening the various tissues of the Body directly, filling the interstices between them and not contained in definitely limited cavities ; (3) all the solid tissues of the Body since, after an early period of embryonic life, they are built up from materials derived from the blood. Secretions would thus come to include all constituents of the Body except the blood itself but, while it is well to bear in mind that the whole Body is in such a way derived from the blood, in practice the term secretion is given a narrower connota- tion, the solid tissues and the lymph being excluded; so that a secretion is a material (liquid or gaseous) derived from the blood and poured out on a free surface, whether that of the general exterior or that of an internal cavity. Such true secretions fall into two classes; one in which the product is of no further use in the Body and is merely separated for removal, as the urine; and one in which the product is intended to be used, for instance as a solvent in the diges- tion of food. The former group are sometimes distin- guished as excretions and the latter as secretions proper, but there is no real difference between them, the organs and pro- cesses concerned being fundamentally alike in each case. A better division is into transudata and secretions, a transuda- 260 THE HUMAN BODY. tion being a product which contains nothing which did not previously exist in the blood, and then in such quantity as might be derivable from it by merely physical processes; while a secretion in addition to transudation elements con- tains a specific element, due to the special physiological activity of the secretory organ; being either something which does not exist in the blood at all or something which, existing in the blood in small quantity, exists in the secre- tion in such a high proportion that it must have been actively picked up and conveyed there by the secretory tissues concerned. For instance, the gastric juice contains free hydrochloric acid which does not exist in the blood; and the urine contains so much urea that we must suppose its cells to have a peculiar power of removing that body from the liquids flowing near them. This subdivision is also justifiable on histological grounds ; wherever there is a secreting surface it is covered with cells, but these where transudata are formed (as on the serous membranes) are mere flat scales, with little or no protoplasm remaining in them, while the cells which line a true secreting organ are cuboidal, spherical, or columnar, and still retain, with their high physiological activity, a good deal of their primi- tive protoplasm in a but slightly modified state. Organs of Secretion. The simplest form in which a secreting organ occurs (A, Fig. 88) is that of a flat membrane provided with a layer of cells, a, on one side (that on which the secretion is poured out) and with a network of capil- lary blood-vessels, c, on the other. The dividing mem- brane, 1), is known as the basement membrane and is usually made up of flat, closely fitting connective- tissue corpuscles; supporting it on its deep side is a layer of connective tissue, d, in which the blood-vessels and lymphatics are supported. Such simple forms of secreting surfaces are found on the serous membranes but are not common; in most cases an extended area is required to form the necessary amount of secretion, and if this were attained simply by spreading out plane surfaces, these from their number and extent would be hard to pack conveniently in the Body. Accordingly in most cases, the greater area is attained by folding the FORMS OF GLANDS. 261 FIG. 88.— Forms of glands. A, a simple secreting surface ; a, its epithelium ; 6. basement membrane ; c, capillaries ; B, a simple tubular gland ; C, a secret- ing surface increased by protrusions ; E, a simple racemose gland ; D and oj IO • O CD oi TH 1 o CO • TH O} O ri 0.0 O 0 1 o • i> i> -*• 2 IO * TH t> O C3 R CO O O 10 10 0 ^d"* iO OO CO "H^ '3 co co co ^d^ OO i 5 o S w CO 2 ^* *o N H O 03 10 10 0 0 o s §00' os' co' 2 CD TH o Oi to B g ^ ^ ^ a 8 000 00 O 00 _G TH co co 0^ TH CO fiw £u 0 TH ^ CO to IO IO IO O O 10 £M .— .' — ^ ^_" *^ ^J js^ c3 TH TH CO C" ^ CO 0 CO 3 ^ S : § 1 j x™' *^ t/i ^) 1 g 1 •§«• 1 1 I S I 11 s ^ C3 p ^-^ ^1 H 6 £ P S o THE DAILY LOSSES OF THE BODY. The living Body thus loses daily in round numbers 4 kilograms of matter (9_Jbs.) and, since it is unable to create new matter, this loss must be compensated for from the exterior or the tissues would soon dwindle away alto- gether; or at least until they were so impaired that life came to an end. After death the losses would be of a differ- ent kind, and their quantity much more dependent upon surrounding conditions; but except under yery unusual circumstances the wasting away would still continue in the dead Body. Finally, the composition of the daily wastes of the living Body is tolerably constant; it does not simply lose a quantity of matter weighing so much, but a certain amount of definite kinds of matter, carbon, nitrogen, oxygen, and so on; and these same substances must be restored to it from outside, in order that life may be continued. To give a stone to one asking for bread might enable him, if he swallowed it, to make up the weight of matter lost in twenty-four hours; but bread would be needed to keep him alive. The Body not only requires a supply of matter from outside, but a supply of certain definite kinds of matter. The Losses of the Body in Energy. The daily expendi- ture of matter by the living Body is not the only one: as continuously it loses in some form or another energy, or the power of doing work; often as mechanical work ex- pended in moving external objects, but even when at resti energy is constantly being lost to the Body in the form of | heat, by radmtion and conduction to surrounding objects, by the evaporation of water from the lungs and skin, and by removal in warm excretions. Unless the Body can make energy it must therefore receive a certain supply of it also from the exterior, or it would very soon cease to- carry on any of its vital work ; it would be unable to move and would cool down to the temperature of surround- ing objects. The discoveries of this century having shown that energy is as indestructible and uncreatable (see Phy- sics) as matter, we are led to look for the sources of the supply of it to the Body; and finding that the living Body daily receives it and dies when the supply is cut off, we no 280 THE HUMAN BODY. longer suppose, with, the older physiologists, that it works by means of a mysterious vital force existing in or created by it; but that getting energy from the outside it utilizes it for its purposes— for the performance of its nutritive and other living work — and then returns it to the exterior in what the physicists know as a degraded state; that is in a less utilizable condition. While energy like matter is in- destructible it is, unlike matter, transmy.table; iron is always iron and gold always gold; neither can by any means which we possess be converted into any other form of matter; and so the Body, needing carbon, hydrogen, oxygen, and nitro- gen to build it and to cover its daily losses, must be sup- plied with those very substances. As regards energy this is not the case. While the total amount of it in the uni- verse is constant, its form is constantly subject to change — and that one in which it enters the Body need not be that in which it exists while in it, nor that in which it leaves it. Daily losing heat and mechanical work the Body does not need, could not in fact much utilize energy, supplied to it in these forms; but it does need energy of some form and in amount equivalent to that which it loses. The Conservation of Energy. The forms of energy yet discovered are not nearly so numerous as the kinds of matter. Still we all know several of them; such as light, heat, sound, electricity, and mechanical work; and most people nowadays know that some of these forms are inter- convertible, so that directly or indirectly we can turn one into another. In such changes it is found that a definite amount of one kind always disappears to give rise to a certain quantity of the other; or, in other words, that so much of the first form is equivalent to so much of the second. In a steam-engine, heat is produced in the fur- nace; when the engine is at work all of this energy does not leave it as heat; some goes as mechanical work, and the more work the engine does the greater is the difference be- tween the heat generated in the furnace and that leaving the machine. If, however, we used the work for rubbing two rough surfaces together we could get the heat back again, and if (which of course is impossible in practice) THE CONSERVATION OF ENERGY. 281 we could avoid all friction in the moving parts of the machine, the quantity thus restored would be exactly equal to the excess of the heat generated in the furnace over that leaving the engine. Having turned some of the heat into mechanical work we could thus turn the work back into heat again, and find it yield exactly the amount which seemed lost. Or we might use the engine to drive an elec- tro-magnetic machine and so turn part of the heat liber- ated in its furnace first into mechanical work and this into electricity; and if we chose to use the latter with the proper apparatus, we could turn more or less of it into light, and so have a great part of the energy which first became conspicuous as heat in the engine furnace, now manifested in the form of light at some distant point. In fact, starting with a given quantity of one kind of energy, we may by proper contrivances turn all or some of it into one or more other forms; and if we collected all the final forms and retransformed them into the first, we should have exactly the amount of it which had disappeared when the other kinds appeared. This law, that energy can change its form but that its amount is invariable, that it cannot be created or destroyed but simply transmuted, is known as the law of the Conservation of Energy (see Phy- sics), and, like the indestructibility of matter, lies at the basis of all scientific conceptions of the universe, whether concerned with animate or inanimate objects. Since all forms of energy are interconvertible it is con- venient in comparing amounts of different kinds to express them in terms of some one kind, by saying how much of that standard form the given amount of the kind spoken of would give rise to if completely converted into it. Since the most easily measured form of energy is mechanical work this is commonly taken as the standard form, and the quantities of others are expressed by saying how great a distance against the force of gravity at the earth's sur- face a given weight could be raised by the energy in ques- tion, if it were all spent in lifting the weight. The units of mechanical work being the kilogrammeter, or the foot- pound, the mechanical equivalent of any given kind of energy 282 THE HUMAN BODY. is the number of kilogrammeters or foot-pounds of work its unit quantity would perform, if converted into mechanical work and used to raise a weight. For example the unit quantity of heat is that necessary to raise one kilogram of water one degree centigrade in temperature; or sometimes, in books written in English, the quantity necessary to warm one pound of water one degree Fahrenheit. When there- fore we say that the mechanical equivalent of heat is 423 kilogrammeters we mean that the quantity of heat which would raise one kilogram of water in temperature from 4° C. to 5° C. would, if all turned into mechanical work, be able to raise one kilogram 423 meters against the attrac- tion of the earth; and conversely that this amount of me- chanical work if turned into heat would warm a kilogram of water one degree centigrade. The mechanical equiva- lent of heat, taking the Fahrenheit thermometric scale and using feet and pounds as measures, is 772 foot-pounds. Potential and Kinetic Energy. At times energy seems to be lost. Ordinarily we only observe it when it is doing work and producing some change in matter : but sometimes it is at rest, stored away and producing no changes that we recognize and thus seems to have been destroyed. Energy at work is known as Tcinetic energy; energy at rest, not producing changes in matter, is called potential energy. Suppose a stone pulled up by a string and left suspended in the air. We know a certain amount of energy was used to lift it; but while it hangs we have neither heat nor light nor mechanical work to represent it. Still the energy is not lost; we know we have only to cut the string and the weight will fall, and striking something give rise to heat. Or we may wind up a spring and keep it so by a catch. In winding it up a certain amount of energy in the form of mechanical work was used to alter the form of the spring. Until the catch is removed this energy re- mains stored away as potential energy: but we know it is not lost. Once the spring is let loose again it may drive a clock or a watch, and in so doing will perform again just so much work as was spent in coiling it; and when the watch has run down this energy will all have been turned POTENTIAL ENEEGT OF CHEMICAL AFFINITY. 283 into other forms — mainly heat developed in the friction of the parts of the watch against one another: but partly also in. producing movements of the air, a portion of which we can readily observe in the sound of its ticking. The law of the conservation of energy does not say, then, that either the total potential or the total kinetic energy in the universe is constant in amount: but that the sum of the two is inva- riable, while constantly undergoing changes from kinetic to potential and vice versa : and from one form of kinetic to another. The Energy of Chemical Affinity. Between every two •chemical atoms which are capable of entering into combi- nation there exists a certain amount of potential energy; ivhen they unite this energy is liberated, usually in the form of heat, and once they have combined a certain amount of kinetic energy must be spent to pull them apart again; this being exactly the amount which was liberated when they united. The more stable the compound formed the more kinetic energy appears during its formation, and the more must be spent to break it up again. One may imagine the separated atoms as two balls pushed together by springs, the strength of the spring being proportionate to the de- gree of their chemical affinity. Once they are let loose and permitted to strike together the potential energy pre- viously represented by the compressed springs disappears, and in its place we have the kinetic energy, represented by the heat developed whon the balls strike together. To pull them apart again, against the springs, to their original positions, just so much mechanical work must be spent as is the equivalent of that amount of heat which appeared when they struck; and thus kinetic energy will again be- come latent in breaking up the compound represented by the two in contact. The energy liberated in chemical com- bination is the most important source of that used in our machines: and also of that spent by the living Body. The Relation between the Matters Removed from the Body daily and the Energy Spent by it. A working locomotive is, we know, constantly losing matter to the -exterior in the form of ashes and gaseous products of com- 284 THE HUMAN BODY. bustion, the latter being mainly carbon dioxide and water vapor. The engine also expends energy, not only in the form of heat radiated to the air, but as mechanical work in drawing the cars against the resistance offered by fric- tion or sometimes, up an incline, by gravity. Now the en- gine-driver knows that there is a close relationship between the losses of matter and the expenditure of energy, so that he has to stoke his furnace more frequently and allow a greater draft of air through it in going up a gradient than when running on the level. The more work the en- gine does the more coals and air it needs to make up for its gi eater waste. If we seek the cause of this relation- ship between work and waste, the first answer naturally is that the engine is a machine the special object of which is to convert heat into mechanical work, and so the more- work it has to do the more heat is required for conversion, and consequently the more coals must be burnt. This, however, opens the question of the source of the heat — of all that vast amount of kinetic energy which is liberated in the furnace; and to answer this we must consider in what forms matter and energy enter the furnace, since the' energy liberated there must be carried in somehow from outside. For present purposes coals may be considered as consisting of carbon and hydrogen, both of which sub- stances tend to forcibly combine with oxygen at high tem- peratures, forming in the one case carbon dioxide and in the other water. The oxygen necessary to form these com- pounds being supplied by the air entering the furnace, all the potential energy of chemical affinity which existed be- tween the uncombined elements becomes kinetic, and is liberated as heat when the combination takes place. The energy utilized by the engine is therefore supplied to it in the form of potential energy, associated with the uncom- bined forms of matter which reach the furnace. Once the carbon and hydrogen have combined with oxygen they are no longer of any use as liberators of energy; and the com- pounds formed if retained in the furnace would only clog it and impede farther combustion; they are therefore got rid of as wastes through the smoke-stack. The engine,. SOURCES OF ENERGY. 285 m short, receives uncombined elements associated with potential energy ; and loses combined elements (which have lost the energy previously associated with them) and kinetic energy: it so to speak separates the energy from the mat- ter with which it was connected, utilises it, and gets rid of the exhausted matter. The amount of kinetic energy liberated during such chemical combinations is very great; n kilogram of carbon uniting with oxygen to form car- bon dioxide sets free 8080 units of heat, or calories. Dur- ing the combination of oxygen and hydrogen to form water even more energy is liberated, one kilogram of hydro- gen when completely burnt liberating more than thirty-four thousand of the same units. The mechanical equivalent of this can be calculated if it is remembered that one heat unit = 423 kilogrammeters. Turning now to the living Body we find that its income and expenditure agree very closely with those of the steam- engine. It receives from the exterior substances capable of entering into chemical union; these combine in it and liberate energy; and it loses kinetic energy and the products of combination. From the outside it takes oxygen through the lungs, and oxidizable substances (in the form of foods) through the alimentary canal; these combine under the conditions prevailing in the living cells just as the carbon and oxygen, which will not unite at ordinary temperatures,, combine under the conditions existing in the furnace of the engine; the energy liberated is employed in the work of the Body, while the useless products of combination are got rid of. To explain, then, the fact that our Bodies go on working we have no need to invoke some special mysterious power resident in them and capable of creating energy, a vital force having no relation with other natural forces, such as the older physiologists used to imagine. The Body needs and gets a supply of energy from the exterior just as the steam-engine does, food and air being to one what coals and air are to the other; each is a machine in which energy is liberated by chemical combinations and then used for special work; the character of which depends upon the peculiarities of mechanism which utilizes it in each case, 286 THE HUMAN BODY. and not upon any peculiarity in the energy utilized or in its source. The Body is, however, a far more economical machine than any steam-engine; of all the energy liberated in the latter only a small fraction, about one eighth, is use- fully employed, while our Bodies can utilize for the perform- ance of muscular work alone one fifth of the whole energy supplied to them; leaving out of account altogether the nutritive and other work carried on in them, and the heat lost from them. The Conditions of Oxidation in the Living Body. Al- though the general principles applied in the Body and the steam-engine for getting available energy are the same, in minor points obvious differences are found be- tween the two. In the first place the coals of an engine are oxidized only at a very high temperature, one which would be instantly fatal to our Bodies which, although "warm when compared writh the bulk of inanimate objects, are very slow fires when compared with a furnace. Chem- istry and physics, however, teach us that this difference is quite unimportant so far as concerns the amount of energy liberated. If magnesium wire be ignited in the air it will become white-hot, flame, and leave at the end of a few seconds only a certain amount of incombustible rust or magnesia, which consists of the metal combined with oxygen. The heat and light evolved in the process repre- sent of course the energy which, in a potential form, was associated with the magnesium and oxygen before their combination. We can, however, oxidize the metal in a differ- ent way, attended with no evolution of light and no very per- ceptible rise of temperature. If, for instance, we leave it in wet air it will become gradually turned into magnesia with- out having ever been hot to the touch or luminous to the eye. The process will, however, take days or weeks; and while in this slow oxidation just as much energy is liberated as in the former case it now all takes the form of heat; and instead of being liberated in a short time is spread over a much longer one, as the gradual chemical combination takes place. The slowly oxidizing magnesium is, therefore, at no moment noticeably hot since it loses its heat to surrounding SOURCE OF BODILY ENERGY. 287 •objects as fast as it is generated. The oxidations occurring in our Bodies are of this slow kind. An ounce of arrow- root oxidized in a fire, and in the Human Body, would liberate exactly as much energy in one case as the other, but the oxidation would take place in a few minutes and ;at a high temperature in the former, and slowly, at a lower temperature, in the latter. In the second place, the engine differs from the living Body in the fact that the oxidations in it all take place in a small area, the furnace, and so the temperature there becomes very high; while in our Bodies the oxidations take place all over, in each of the living cells; there is no one furnace or hearth where all the energy is liberated for the whole and transferred thence in one form or another to distant parts: and this is another reason why no one part of the Body attains a very high temperature. The Fuel of the Body. This is clearly different from that of an ordinary engine: no one could live by eating coals. This difference again is subsidiary; a gas-engine requires different fuel from an ordinary locomotive; and the Body requires a somewhat different one from either. It needs as foods, substances which can, in the first place, be -absorbed from the alimentary canal and carried to the various tissues; and, in the second, can be oxidized at a low temperature in the blood or tissues, or can be con- verted by the living cells into compounds which can be .so oxidized. With some trivial exceptions, all substances which fulfill these conditions are complex chemical com- pounds, and to understand their utilization in the Body we must extend a little the statements above made as to the liberation of energy in chemical combinations. The general law may be stated thus — Energy is liberated whenever chemi- cal union takes place : and whenever more stable compounds •are formed from less stable ones, in which the constituent atoms ivere less firmly held together. Of the liberation by simple combination we have already seen an instance in the oxidation of carbon in a furnace; but the union need not be an oxidation. Everyone knows how hot quicklime becomes when it is slaked; the water combining strongly with the lime, and energy being liberated in the form of 288 THE HUMAN BODY. heat during the process. Of the liberation of energy by the breaking down of a complex compound, in which the atoms are only feebly united, into simpler and stabler ones, we get an example in alcoholic fermentation. During that; process grape sugar is broken down into more stable com- pounds, mainly carbon dioxide and alcohol, while oxygen is at the same time taken up. To pull apart the carbon, hydrogen and oxygen of the sugar molecule requires a cer- tain expenditure of kinetic energy; but in the simultaneous formation of the new and stabler compounds a greater amount of energy is set free, and the difference appears as heat, so that the brewer has to cool his vats with ice. It is by processes like this latter, rather than by direct com- binations, that most of the kinetic energy of the Body is obtained; the complex proteids and fats and starches and sugar taken as food being broken down (usually with con- comitant oxidation) into simpler and more stable com- pounds. Oxidation by Successive Steps. In the furnace of an engine the oxidation takes place completely at once. The- carbon and hydrogen leaving it, if it is well managed, are- each in the state of their most stable oxygen compound. But this need not be so: we might first oxidize the carbon so as to form carbon monoxide, CO, and get a certain amount of heat; and then oxidize the carbon monoxide farther so as to form carbon dioxide, COs, and get more heat. If we add together the amounts of heat liberated in each stage, the sum will be exactly the quantity which would have been obtained if the carbon had been completely burnt, to the state of carbon dioxide at first. Every one who has studied chemistry will think of many similar cases. As the process is important physiologically we may take an- other example; say the oxidation of alcohol. This may be burnt completely and directly, giving rise to carbon dioxide and water — CsHeO + Oe = 2C02 + SILO 1 Alcohol. 6 Oxygen. 2 Carbon dioxide. 3 water. But instead of this we can oxidize the alcohol by stages, UTILIZATION OF ENERGY IN THE SODT. 289 getting at each stage only a comparatively small amount of heat evolved. By combining it first with one atom of oxy- gen, we get aldehyde and water — CsHeO + 0 = CsEUO -f H20 1 Alcohol. 1 Oxygen. 1 Aldehyde. 1 Water. Then we add an atom of oxygen to the aldehyde and get acetic acid (vinegar) — + 0 = 1 Aldehyde. 1 Oxygen. 1 Acetic acid. And finally we may oxidize the acetic acid so as to get car- bon dioxide and water — C2H402 + 04 = 2C02 + 2H20 We get, in both cases, from one molecule of alcohol, two of carbon dioxide and three of water; and six atoms of oxygen are taken up. In each stage of the gradual oxidation a certain amount of heat is evolved; and the sum of these is exactly the amount which would have been evolved by burning the alcohol completely at once. The food taken into the Body is for the most part oxi- dized in this gradual manner; the products of imperfect combustion in one set of cells being carried off and more completely oxidized in another set, until the final pro- ducts, no longer capable of further oxidation in the Body, are carried to the lungs, or kidneys, or skin, and got rid of. A great object of physiology is to trace all intermediate compounds between the food which enters and the waste products which leave; to find out just how far chemical degradation is carried in each organ, and what substances are thus formed in various parts: but at present this part of the science is very imperfect. The Utilization of Energy in the Living Body. In the steam-engine energy is liberated as heat; some of the heat is used to evaporate water and expand the resulting steam; and then the steam to drive a piston. But in the living Body it is very probable (indeed almost certain) that a great part of the energy liberated by chemical transfer- 290 THE HUMAN BODJ. mations does not first take the form of heat; though some of it does. This, again, does not affect the general prin- ciple: the source of energy is essentially the same in both cases; it is merely the form which it takes that is dif- ferent. In a galvanic cell energy is liberated during the union of zinc and sulphuric acid, and we may so arrange matters as to get this energy as heat; but on the other hand we may lead much of it off, as a galvanic current, and use it to drive a magneto-electric machine before it has taken the form of heat at all. In fact, that heat may be used to do mechanical work we must reduce some of it to a lower temperature: an engine needs a condenser of some kind as well as a furnace; and, other things being equal, the cooler the condenser the greater the proportion of the whole heat liberated in the furnace which can be used to do work. Now in a muscle there is no condenser; its temperature is uniform throughout. So when it contracts and lifts a weight, the energy employed must be liberated in some other form than heat — some form which the muscu- lar fibre can use without a condenser. Summary. The living Body is continually losing mat- ter and expending energy. So long as we regard it as working by virtue of some vital force, the power of gener- ating which it has inherited, the waste is difficult to account for, since it is far more than we can imagine as due merely to wear and tear of the working parts. When, however, we consider the nature of the income of the Body, and of its expenditure, from a chemico-physical point of view, we get the clue to the puzzle. The Body does not waste because it works but works because it wastes. The working power is obtained by chemical changes occurring in it, associated with the liberation of energy which the living cells utilize; and the products of these chemical changes, being no longer available as sources of energy, are passed out. The chemical changes concerned are mainly the breaking down of complex and unstable chemical com- pounds into simpler and more stable ones, with concomi- tant oxidation. Accordingly the material losses of the SOURCES OF ENERGY IN THE BODY. 291 Body are highly or completely oxidized, tolerably simple chemical compounds; and its material income is mainly uncombined oxygen and oxidizable substances, the former obtained through the lungs, the latter through the alimen- tary canal. In energy, its income is the potential energy of uncombined or feebly combined elements, which are capable of combining or of forming more stable compound^ and its final expenditure is kinetic energy almost entirely in the form of mechanical work and heat. Given oxygen, all oxidizable bodies will not serve to keep the Body alive and working, but only those which (1) are capable of ab- sorption from the alimentary canal and (2) those which are oxidizable at the temperature of the Body under the influ- ence of protoplasm. Just as carbon and oxygen will not unite in the furnace of an engine unless the "fire be lighted" by the application of a match but, when once started, the heat evolved at one point will serve to carry on the conditions of combination through the rest of the mass, so the oxidations of the Body only occur under special con- ditions; and these are transmitted from parent to offspring. Every new Human Being starts as a portion of protoplasm separated from a parent and affording the conditions for those chemical combinations which supply to living matter its working power : this serves, like the energy of the burning part of a fire, to start similar processes in other portions of matter. At present we know nothing in physi- ology answering to the match which lights a furnace; those manifestations of energy which we call life are handed down from generation to generation, as the sacred fire in the temple of Vesta from one watcher to another. Science may at some time teach us how to bring the chemical con- stituents of protoplasm into that combination in which they possess the faculty of starting oxidations under those conditions which characterize life; then we will have learnt how to strike the vital match. For the present we must be content to study the properties of that form of matter which possesses living faculties; since there is no satisfactory proof that it has ever been produced, within 292 THE HUMAN BODY. our experience, apart from the influence of matter already living. How the vital spark first originated, how mole- cules of carbon, hydrogen, nitrogen and oxygen first united with water and salts to form protoplasm, we have no scien- tific data to ground a positive opinion upon, and such as we may have must rest upon other grounds. CHAPTER XX. FOODS. Foods as Tissue Formers. Hitherto we have considered foods merely as sources of energy, but they are also re- quired to build up the substance of the Body. From birth to manhood we increase in bulk and weight, and that, not merely by accumulating water and such substances, but by forming more bone, more muscle, more brain, and so on, from materials which are not necessarily bone or muscle or nerve tissue. Alongside of the processes by which com- plex substances are broken down and oxidized and energy liberated, constructive processes take place by which new complex bodies are formed from simpler substances taken -as food. A great part of the energy liberated in the Body is in fact utilized first for this purpose, since to construct com- plex unstable molecules, like those of protoplasm, from the simpler compounds taken into the Body, needs an expendi- ture of kinetic energy. Even after full growth, when the Body ceases to gain weight, the same synthetic processes go on ; the living tissues are steadily broken down and con- stantly reconstructed, as we see illustrated by the condition of a man who has been starved for some time, and who loses not only his power of doing work and of maintaining his bodily temperature but also a great part of his living tissues. If a^ain fed properly he soon makes new fat and new muscle and regains his original mass. Another illustration of the •continuance of constructive powers during the whole of life is afforded by the growth of the muscles when exercised properly. Since the tissues, on ultimate analysis, yield mainly car- bon, hydrogen, nitrogen and oxygen, it might be supposed •a priori that a supply of these elements in the uncombined state would serve as material for the constructive forces of the Body to work with. Experience, however, teaches us 294 THE HUMAN BODY. that this is not the case, but that the animal body requires,, for the most part, highly complex compounds for the con- struction of new tissue elements. All the active tissues yield on analysis large quantities of proteids which, as pointed out in Chapter L, enter always into the structure of protoplasm. Now, so far as we know at present,* the animal body is unable to build up proteids from simpler com- pounds of nitrogen, although when given one variety of them it can convert that one into others, and combine them with other things to form protoplasm. Hence proteids are an essential article of diet, in order to replace the proteid of the living cells which is daily broken clown and elimi- nated in the form of urea and other waste substances. Even albuminoids (p. 11), although so nearly allied to pro- teids, will not serve to replace them entirely in a diet; a. man fed abundantly on gelatin, fats, and starches, would starve as certainly, though not so quickly, as if he got no nitrogenous food at all; his tissue waste would not be made- good, and he would at last be no more able to utilize the energy-yielding materials supplied to him, than a worn-out steam-engine could employ the heat of a fire in its furnace. So, too, the animal is unable to take the carbon for the construction of its tissues, from such simple compounds as- carbon dioxide.* Its constructive power is limited to the utilization of the carbon contained in more complex and lass stable compounds, such as proteids, fats, or sugars. Nearly all the tissue-forming foods must therefore con- sist of complex substances, and of these a part must be proteids, since the Body can utilize nitrogen for tissue formation only when supplied with it in that form. The bodies thus taken in are sooner or later broken down into simpler ones and eliminated; some at once in order to yield energy, others only after having first been built up into part of a living cell. The partial exceptions afforded by such losses to the Body as milk for suckling the young, or the albuminous and fatty bodies stored for the same purpose in the egg of a bird, are only apparent ; the chemi- * There is some reason to believe that some few of the lower ani- mals which contain chlorophyl can manufacture proteids and utilize carbon dioxide. FOOD OF PLANTS. 295 cal degradation is only postponed, taking place in the body of the offspring instead of that of the parent. In all cases animals are thus, essentially, proteid consumers or wasters, and breakers down of complex bodies; the carbon, hydro- gen and nitrogen which they take as foods in the form of complex unstable bodies, ultimately leaving them in the simpler compounds, carbon dioxide, water, and urea; which are incapable of either yielding energy or building- tissue for any other animal and so of serving it as food. The question immediately suggests itself, How, since animals are constantly breaking up these complex bodies and can- not again build them, is the supply kept up ? For exam- ple, the supply of proteids, which cannot be made artificially by any process which we know, and yet are necessary foods for all animals, and daily destroyed by them. The Food of Plants. As regards our own Bodies the question at the end of the last paragraph might perhaps be answered by saying that we get our proteids from the flesh of the other animals which we eat. But, then, we have to account for the possession of them by those animals; since they cannot make them from urea and carbon dioxide and water any more than we can. The animals eaten get them, in fact, from plants which are the great proteid formers of the world, so that the most carnivorous animal really de- pends for its most essential foods upon the vegetable king- dom; the fox that devours a hare in the long run lives on the proteids of the herbs that the hare had previously eaten. All animals are thus, in a certain sense, parasites; they only do half of their own nutritive work, just the final stages, leaving all the rest to the vegetable kingdom and using the products of its labor; and plants are able to meet this demand because they can live on the simple compounds of carbon, hydrogen, and nitrogen eliminated by animals, building up out of them new complex substances which animals can use as food. A green plant, supplied with am- monia salts, carbon dioxide, water, and some minerals, will grow and build up large quantities of proteids, fats, starches, and similar things; it will pull the stable compounds eli- minated by animals to pieces, and build them up into com- 296 THE HUMAN BODY. plex unstable bodies, capable of yielding energy when again broken down. However, to do such work, to break up stable combinations and make from them less stable, needs a supply of kinetic energy, which disappears in the process being stored away as potential energy in the new compound; and we may ask whence it is that the plant gets the supply of energy which it thus utilizes for chemical construction, since its simple and highly oxidized foods can yield it none. It tas been proved that for this purpose the green plant uses the energy of sunlight: those of its cells which contain the substance called chlorophyl (leaf green) have the power of utilizing energy in the form of light for the perform- ance of chemical work, just as a steam-engine can utilize heat for the performance of mechanical work. Exposed to light, and receiving carbon dioxide from the air, and water and ammonia (which is produced by the decomposition of urea) from the soil, the plant builds them up again, with the elimination of oxygen, into complex bodies like those which animals broke down with fixation of oxygen. Some of the bodies thus formed it uses for its own growth and the formation of new protoplasm, just as an animal does; but in sunlight it forms more than it uses, and the excess stored up in its tissues is used by animals. In the long run, then, all the energy spent by our Bodies comes through millions of miles of space from the sun; but to seek the source of its supply there would take us far out of the domain of Physiology (see Astronomy). Non-Oxidizable Foods. Besides our oxidizable foods, a large number of necessary food materials are not oxidiza- ble, or at least are not oxidized in the Body. Typical in- stances are afforded by water and common salt. The use of these is in great part physical: the water, for instance, dissolves materials in the alimentary canal, and carries the solutions through the walls of the digestive tube into the blood and lymph vessels, so that they can be carried from part to part; and it permits interchanges to go on by diffusion. The salines also influence the solubility and chemical interchanges of other things present with them. Serum albumen, the chief proteid of the blood, for example, NON-OXIDIZABLE FOODS. 29T is insoluble in pure water, but dissolves readily if a small quantity of neutral salts is present. Besides such uses the non-oxidizable foods have probably others, in what we may call machinery formation. In the salts which give their hardness to the bones and teeth, we have an example of such an employment of them: and to a less extent the same may be true of other tissues. The Body, in fact, is not a mere store of potential energy but something more — it is a machine for the disposal of it in certain ways; and, wherever practicable, it is clearly advantageous to have the purely energy-expending parts made of non-oxidizable mat- ters, and so protected from* change and the necessity of frequent renewal. The Body is a self -building and self- repairing machine, and the material for this building and repair must be supplied in the food, as well as the fuels, or oxidizable foods, which yield the energy the machine ex- pends; and while experience shows us, that even for ma- chinery construction, oxidizable matters are largely needed, it is nevertheless a gain to replace them by non-oxidizable substances when possible; just as if practicable it would be advantageous to construct an engine out of materials which would not rust, although other conditions determine the use of iron for the greater part of it. Definition of Poods. Foods may be defined as sub- stances which, when taken into the alimentary canal, are- absorbed from it, and then serve either to simply material for the growth of the Body, or for the replacement of matter which has been removed from it, either after oxidation or without having been oxidized. Foods to replace matters which have been oxidized must be themselves oxidizable; they are force generators, but maybe and generally are also tissue formers; and are nearly always complex organic sub- stances derived from other animals or from plants. Foods to replace matters not oxidized in the Body axe force regu- lators, and are for the most part tolerably simple inorganic compounds. Among the force regulators we must, how- ever, include certain organic foods which, although oxidized in the Body and serving as liberators of energy, yet produce effects totally disproportionate to the energy they set free. 298 THE HUMAN BOD Y. and for which effects they are taken. In other words, their influence as stimuli in exciting certain tissues to liberate •energy, or as inhibitory agents checking the activity of parts, is more marked than their direct action as force gen- erators. As examples, we may take condiments: mustard and pepper are not of much use as sources of energy, al- though they no doubt yield some; we take them for their stimulating effect on the mouth and other parts of the alimentary canal, by which they promote an increased flow of the digestive secretions or an increased appetite for food. Thein, again, the active principle of tea and coffee, is taken for its stimulating effect on the nervous system, rather than for the amount of energy which is yielded by its own oxidation. Conditions which a Food must Fulfill. (1) A food must contain the elements which it is to replace in the Body: but that alone is not sufficient. The elements leav- ing the Body being usually derived from the breaking down of complex substances in it, the food must contain them either in the form of such complex substances, or in forms which the Body can build up into them. Free nitrogen and hydrogen are no use as foods, since they are neither cxi- dizable under the conditions prevailing in the Body (and consequently cannot yield it energy), nor are they capable of construction by it into its tissues. (2) Food after it has been swallowed is still in a strict sense outside the Body; the alimentary canal is merely a tube running through it, and so long as food lies there it does not form any part of the Body proper. Hence foods must be capa- ble of absorption from the alimentary canal; either directly, or after they have been changed by the processes of diges- tion. Carbon, for example, is useless as food, not merely because the Body could not build it up into its own tissues, but because it cannot be absorbed from the alimentary canal. (3) Neither the substance itself nor any of the products of its transformation in the Body must be inju- rious to the structure or activity of any organ. If so it is a poison, not a food. Alimentary Principles. What in common language we ALIMENTARY PRINCIPLES. 299 commonly call foods are, in nearly all cases, mixtures of several foodstuffs, with, substances which are not foods at all. Bread, for example, contains water, salts, gluten (a proteid), some fats, much starch, and a little sugar; all true foodstuffs: but mixed with these is a quantity of cellulose ) (the chief chemical constituent of the walls which surround vegetable cells), and this is not a food since it is incapable of absorption from the alimentary canal. Chemical exami- nation of all the common articles of diet shows that the actual number of important foodstuffs is but small: they are repeated in various proportions in the different tilings we eat, mixed with small quantities of different flavoring substances, and so give us a pleasing variety in our meals; but the essential substances are much the same in the fare of the workman and in the "delicacies of the season." These primary foodstuffs, which are found repeated in so many different foods, are known as ' ' alimentary principles;" and the physiological value of any article of diet depends on them far more than on the traces of flavoring matters - which cause certain things to be especially sought after and so raise their market value. The alimentary principles may be conveniently classified into proteids, albuminoids, hydrocarbons, carbohydrates, and inorganic bodies. Proteid Alimentary Principles. Of the nitrogenous foodstuffs the most important are proteids: they form an essential part of all diets, and are obtained both from animals and plants. The most common and abundant are myosin and syntonin which exist in the lean of all meats; €gg albumen; casein, found in milk and cheese; gluten and vegetable casein frpm various plants. Albuminoid Alimentary Principles. These also con- tain nitrogen, but cannot replace the proteids entirely as foods; though a man can get on with less proteids when he has some albuminoids in addition. The most important is gelatin, which is yielded by the white fibrous, tissue of animals when cooked. On the whole the albuminoids are not foods of high value, and the calf's-foot jelly and such compounds, often given to invalids, have not nearly the nutritive value they are commonly supposed to possess. 300 THE HUMAN BOD*. Hydrocarbons (Fats and Oils). The most important are stearin, palmatin, margarin, and olein, which exist in various proportions in animal fats and vegetable oils; the more fluid containing most olein. Butter contains a little of a fat named butyrin. Fats are compounds of glycerine and fatty acids, and any such substance which is fusible at the temperature of the Body will serve as a food. The stearin of beef and mutton fats is not by itself fusible at the body temperature, but is mixed in those foods with so much olein as to be melted in the alimentary canal. Beeswax, on the other hand, is a fatty body which will not melt in the intestines and so passes on unabsorbed; although from its composition it would be useful as a food could it be digested. It is convenient to- distinguish fats proper (the adipose tissue of animals consisting of fatty compounds inclosed in albuminous cell- walls) from oils., or fatty bodies which are not so surrounded. Carbohydrates. These are mainly of vegetable origin. The most important are starch, found in nearly all vege- table foods; dextrin; gums; grape sugar (into which starch is converted during digestion); and cane sugar. Sugar of milk and glycogen are alimentary principles of this group, derived from animals. All of them, like the fats, consist of carbon, hydrogen and oxygen; but the percentage of oxygen in them is much higher, there being one atom of oxygen for every two of hydrogen in their molecule. Inorganic Foods. Water; common salt; and the chlo- rides, phosphates, and sulphates of potassium, magnesium and calcium. More or less of these bodies, or the materials for their formation, exists in all ordinary articles of diet» so that we do not swallow them in a separate form. Phos- phates, for example, exisj; in nearly all animal and vegetable foods; while other foods, as casein, contain phosphorus in combinations which in the Body yield it up to be oxidized to form phosphoric acid. The same is true of sulphates, which are partially swallowed as such in various articles of diet, and are partly formed in the Body by the oxidation of 1 the sulphur of various proteids. Calcium salts are abun- f dant in bread, and are also found in many drinking waters. FLESH FOOUS. 301 Water and table salt form, exceptions to fae rule that in- organic bodies are eaten imperceptibly along with other things, since the Body loses more of each daily than is usu- ally supplied in that way. It has, however, been main- tained that salt, as such, is an unnecessary luxury; and there seems some evidence that certain savage tribes live without more than they get in the meat and vegetables they eat. Such tribes are, however, said to suffer especially from intestinal parasites; and there is no doubt that to civilized man the absence of salt is a great deprivation. Mixed Foods. These, as already pointed out, include nearly all common articles of diet; they contain more than one alimentary principle. Among them we find great differences; some being rich in proteids, others in starch, others in fats, and so on. The formation of a scientific die- tary depends on a knowledge of these characteristics. The foods eaten by man are, however, so varied that we cannot do more than consider the most important. Flesh. This, whether derived from bird, beast, or fish, consists essentially of the same things — muscular fibres, tendons, fats, blood-vessels, and nerves. It contains several proteids and especially mybsin; gelatin-yielding matters in the white fibrous tissue; stearin, palmatin, margarin, and olein among the fats; and a small amount of carbohydrates in the form of glycogen and grape sugar; also inosite, a. kind of sugar found only in muscles. Flesh also contains much water and a considerable number of salines, the most important and abundant being potassium phosphate. Os^ mazome is a ciystalline nitrogenous body which gives much of its taste to flesh; and small quantities of various similar substances exist in different kinds of meat. There is also more or less yellow elastic tissue rn« flesh; it is indigestible and useless as food. When meat is cooked its white fibrous tissue is turned into gelatin, and the whole mass becomes thus softer and more easily disintegrated by the teeth. When boiled much of the proteid matters of the meat pass out into the broth, and there in part coagulate and form the scum : this loss may be prevented in great part, if it is not intended to use 302 THE HUNAN BODY. the broth, by putting the raw meat at once into boiling water, which coagulates the surface albumin before it dis- solves out, and this keeps in the rest. In any case the myosin, being insoluble in water, remains behind in the boiled meat. In baking or roasting, all the solid parts of the flesh are preserved and certain agreeably flavored bodies are produced, as to the nature of which little is known. Eggs. These contain a large amount of egg albumen and, in the yolk, another proteid, known as vitellin. Also fats, and a substance known as lecithin (p. 14), which is important as containing a considerable quantity of phos- phorus. Milk contains a proteid, casein; several fats in the hitler; a carbohydrate, milk sugar; much water; and salts, espe- cially potassium and calcium phosphates. Butter consists mainly of the same fats as those in beef and mutton; but has in it about one per cent of a special fat, butyrin. In the milk it is disseminated in the form of minute globules which, for the most part, float up to the top when the milk is let stand and then form the cream. In this each fat droplet is surrounded by a pellicle of albuminous matter; by churning, these pellicles are broken up and the fat drop- lets run together to form the butter. Casein is insoluble in water; in milk it is dissolved by the alkaline salts present. When milk is kept, its sugar ferments and gives rise to lactic acid, which neutralizes the alkali and precipitates the casein as curds. In cheese-making the casein is similarly precipitated by the addition of an acid, and (the whey being pressed out) it constitutes the main bulk of cheese. Vegetable Poods. Of these wheat affords the best. In 1000 parts it contains 135 of proteids, 568 of starch, 46 of dextrin (a carbohydrate), 49 of grape sugar, 19 of fats, and 32 of cellulose, the remainder being water and salts. The proteid of wheat is mainly gluten, which when moist- ened with water forms a tenacious mass, and this it is to which wheaten bread owes its superiority. When the dough is made yeast is added to it, and produces a fermen-' tation by which, among other things, carbon dioxide gas is produced. This gas, imprisoned in the tenacious dough. VEGETABLE FOODS. 303 and expanded during baking, forms cavities in it and causes it to "rise" and make "light bread," which is not only more pleasant to eat but more digescible than heavy. Other cereals may contain a larger percentage of starch but none have so much gluten as wheat; when bread is made from them the cai bon dioxide gas escapes so readily from the less tenacious dough that it does not expand the mass properly. Corn contains in 1000 parts, 79 of proteids, 637 of starch, and from 50 to 87 of fats; much more than any other kind of grain. Rice is poor in proteids (56 parts in 1000) but very rich in starch (823 parts in 1000). Peas and beans are rich in proteids (from 220 to 260 parts in 1000), and contain about half their weight of starch. Potatoes are a poor food. They contain a great deal of water and cellulose, and only about 13 parts of proteids and 154 of starch in 1000. Other fresh vegetables, as carrots, turnips and cabbages, are valuable mainly for the salts they contain; their weight is mainly due to water, and they contain but little starch, proteids, or fats. Fruits, like most fresh vegetables, are mainly valu- able for their saline constituents, the other foodstuffs in them being only present in small proportion. Some form of fresh vegetables is, however, a necessary article of diet; as shown by the scurvy which used to prevail among sailors before fresh vegetables or lime-juice were supplied to them. The Cooking of Vegetables. This is of more importance •even than the cooking of flesh, since in most the main ali- mentary principle is starch, and raw starch is difficult of digestion. In plants starch is nearly always stored up in the form of solid granules, which consist of alternating layers of starch cellulose and starch yraniilose. The diges- tive fluids turn the starch into sugars which are soluble and can be absorbed from the alimentary canal, while starch itself cannot. Now these fluids act very slowly and imper- fectly on raw starch, and then only on the granulose; but when boiled, the starch granules swell up, and are more readily converted into the sugars, and the starch cellulose is so altered that it too undergoes that change. When starch is roasted it is turned into a substance known as soluble .starch wrhich is readily dissolved in the alimentary canal. 304 THE HUMAN BODY, There is, therefore, a scientific foundation for the common belief that the crust of a loaf is more digestible than the crumb, and toast than ordinary bread. Alcohol. There are perhaps no common articles of diet concerning which more contradictory statements have been made than alcoholic drinks. This depends upon their pe- culiar position: according to circumstances alcohol may be a poison or be useful; when useful it may be regarded either as a force regulator or a force generator. It is some- times a valuable medicine, but it does no good to the healthy body. If not more than two ounces (which would be con- tained in about four ounces of whiskey or two quarts of lager -beer) are taken in the twenty -four hours, they are completely oxidized in the Body and excreted as water and carbon dioxide. In this oxidation energy is of course liberated and can be utilized. Commonly, however, alco- hol is not taken for this purpose but as a force regulator, for its influence on the nervous system or digestive organs, and it is in this capacity that it becomes dangerous. For not only may it be taken in quantities so great that it is not all oxidized in the Body but is passed through it as alco- hol, or even that it acts as a narcotic poison instead of a stimulant, but when taken in what is called moderation there can be no doubt that the constant "whipping up" of the flagging organs, if continued, must be dangerous to their integrity. Hence the daily use of alcohol merely in such quantities as to produce slight exhilaration or to facili- tate work is by no means safe; though in disease when the system wants rousing to make some special effort, the phy- sician cannot dispense with it or some other similarly act- ing substance. In fact, as a force generator alcohol may be advantageously replaced by other foods in nearly all cases; and there is no evidence that it helps in the construction of the working tissues, though its excessive use often leads to an abnormal accumulation of fat. Its proper use is as a " whip," and one has no more right to use it to the healthy Body than the lash to overdrive a willing horse. The physician is the proper person to determine whether it Is wanted under any given circumstances. ALCOHOL. 305 If alcohol is used as a daily article of diet it should be borne in mind that when concentrated it coagulates the proteids of the cells of the stomach with which it comes in contact, in the same sort of way, though of course to a much less degree, as it shrivels and dries up an animal pre- served in it. Dilute alcoholic drinks, such as claret and beer, are therefore far less baneful than whiskey or brandy, and these are worst of all in the almost undiluted form of most " mixed drinks." For the same reason alcohol is far more injurious on an empty stomach than after a meal, When the stomach is full it is diluted, is more slowly absorbed, and, moreover, is largely used up in coagulating the proteids of the food instead of those of the lining membrane of the stomach. The old "three bottle" men who drank their port-wine after a heavy dinner, got off far more safely than the modern tippler who is taking "nips" all day long, although the latter may imbibe a smaller quantity of alco» hoi in the twenty-four hours. By far the best way, how- ever, is to avoid alcohol altogether in health. If the facts lead us to conclude, against the extremists, that it is to a certain extent a food, it is nevertheless a dangerous one; even in what we may call "physiological " quantities, or such amounts as can be totally oxidized in the Body. The Advantage of a Mixed Diet. The necessary quan- tity of daily food depends upon that of the material daily lost from the Body, and this vane? both in kind and amount with the energy expended and the organs most used. In children a certain excess beyond this is required to furnish materials for growth. Although it is impossible to lay down with perfect accuracy how much daily food any individual requires, still the average quantity may be de- rived from the table of daily losses given on page 278, which shows that a healthy man needs daily in assimilable forms about 274 grams (4220 grains) of carbon and 19 grams (292 grains) of nitrogen. The daily loss of hydrogen . which is very great (352 grams or 5428 grains), is nearly all that contained in water which has been drunk and, so to speak, merely filtered through the Body, after having assisted in the solution, and transference through it of other 306 TEE HUMAN BODY substances. About 300 grams (4620 grains) of water (containing 33.3 grams (513 grains) of hydrogen are, how- ever, formed in the Body by oxidation, and the hydrogen for this purpose must be supplied in the form of some oxidiza- ble foodstuff, whether proteid, fat, or carbohydrate. The oxygen wanted is mainly received fi'©m the air through the lungs, but some is taken in the food. Since proteid foods contain carbon, nitrogen and hydro- gen, life may be kept up on them alone, with the necessary salts, water and oxygen; but such a form of feeding would be anything but economical. Ordinary proteids contain in 100 parts (p. 10) about 52 of carbon and 15 of nitrogen, so a man fed on them alone wrould get about 3J parts of carbon for every 1 of nitrogen. His daily losses are not in this ratio, but about that of 274 grams (4220 grains) of carbon to 20 grams (308 grains) of nitrogen, or as 13.7 to 1; and so to get enough carbon from proteids far more than the necessary amount of nitrogen must be taken. Of dry proteids 527 grams (8116 grains) would yield the necessary carbon, but would contain 79 grams (1217 grains) of nitrogen; or four times more than is necessary to cover the daily losses of that element from the Body. Fed on a purely proteid diet a man would, therefore, have to digest a vast quantity to get enough carbon, and in eating and absorbing it, as well as in getting rid of the extra nitrogen which is useless to him, a great deal of unnecessary labor would be thrown upon the various organs of his Body. Similarly, if a man were to live on bread alone he would burden his organs with much useless work. For bread contains but little nitrogen in proportion to its carbon, and so, to get enough of the former, far more carbon than was utilized would have to be eaten, digested, and eliminated daily. Accordingly, we find that mankind in general employ a mixed diet when they can get it, using richly proteid sub- stances to supply the nitrogen needed, but deriving the carbon mainly from non-nitrogenous foods of the fatty or carbohydrate groups, and so avoiding excess of either. For instance, lean beef contains about \ of its weight of dry ADVANTAGE OF A MIXED DIET. 307 proteid, which contains 15 per cent of nitrogen. Conse- quently the 133 grams (204S grains) of proteid which would be found in 532 grams (1 Ib. 3 oz.) of lean meat would supply all the nitrogen needed to compensate for a day's losses. But the proteid contains 52 per cent of carbon, so the- amount of it in the above weight of fatless meat would be 69 grams (1062 grams) of carbon, leaving 205 grams (3157 grains) to be got either from fats or car- bohydrates. The necessary amount would be contained in about 256 grams (3942 grains) of ordinary fats or 460 grams (7084 gramss of starch; hence either of these, with the above quantity of lean meat, would form a far better diet, both for the purse and the system, than the meat alone. As already pointed out, nearly all common foods contain several foodstuffs. Good butcher's meat, for example, contains nearly half its dry weight of fat; and bread, besides proteids, contains starch, fats^ and sugar. In none of them, however, are the foodstuffs mixed in the physiologically best proportions, and the practice of employing several of them at each meal or different ones at different meals during the day, is thus not only agreeable to the palate but in a high degree advantageous to the Body. The strict vegetarians who do not employ even such substances as eggs, cheese, and milk, but confine themselves to a purely vegetable diet (such as is always poor in proteids), daily take far more carbon than they require, and are to be congratu- lated on their excellent digestions which are able to stand the strain. Those who use eggs, cheese, etc., can of course get on very well, since such substances are extremely rich in proteids, and supply the nitrogen needed without the necessity of swallowing the vast bulk of food which must be eaten in order +-o get it from plants directly. CHAPTER XXI. THE ALIMENTAKY CANAL AND ITS APPEN- DAGES. General Arrangement. The alimentary canal is essen- tially an involuted portion of the skin, specially set apart for absorption, and forming a tube which runs through the Body (Fig. 2); it communicates with the exterior at three points (the nose, the mouth, and the anal aperture), at which this modified skin, or mucous membrane, is con- tinuous with the general outer integument. Supporting the lining absorptive membrane are other layers which strengthen the tube, and are also in part muscular and, by their contractions, serve to pass materials along it from one end to the other. In the walls of the canal are numerous blood and lymphatic vessels which carry off the matters absorbed from its cavity; and there also exist in connection with it numerous glands, whose function it is to pour into it various secretions which exert a solvent influ- ence on such foodstuffs as would otherwise escape absorp- tion. Some of these glands are minute and imbedded in the walls of the alimentary tube itself, but others (such as the salivary glands) are larger and lie away from the main channel, into which their products are carried by ducts of various lengths. The alimentary tube is not uniform but presents several dilatations on its course; nor is it straight, since, being much longer than the Body, it is packed away by being coiled up in the abdominal cavity. Subdivisions of the Alimentary Canal. The mouth opening leads into a chamber containing the teeth and tongue, the mouth chamber or buccal cavity. This is sue- MOUTH CAVITY. 309 ceeded by i\\z pharynx or throat cavity, which narrows at the top of the neck into the gullet or oesophagus; this runs down through the thorax and, passing through the dia- phragm, dilates in the upper part of the abdominal cavity into the stomach. Beyond the stomach the channel again narrows to form a long and greatly coiled tube, the small intestine, which terminates by opening into the large intes- tine, much shorter although wider than the small, and ter- minating by an opening on ^ the exterior. The Mouth Cavity (Fig. 89) is bounded in front and on the sides by the lips and cheeks, below by the tongue, k, and above by the palate; which latter consists of an an- terior part, /, supported by bone and called the hard pal- ate, and a posterior, /, con- taining no bone, and called the soft palate. The two can readily be distinguished by applying the tip Of the tongue FIG. 89. -The mouth, nose and pha- ,1* j, p J.T i.i i rynx, with the commencement of the tO the roof Of the mOUth and /ullet and larynx, as exposed by a jj ^ •, i i i mi section, a little to the left of the me- d rawing it backwards. The dian plane of the head, a, vertebral linvrl rvilafa fm-me fVio -nnvfi oolumn ; b, gullet; c, windpipe : d, i pdiau larynx : e, epiglottis ; f. soft palate ; tion between the mouth and ft«^&»ffi~ff^ nose. The soft palate arches ^^^Wojgj^., down Over the back of the o. pg, the tubinate bones of the out- er tide of the left nostril chamber. mouth, hanging like a cur- tain between it and the pharynx, as can be seen by holding the mouth open in front of a looking-glass. From the middle of its free border a conical process, the uvula, hangs down. The Teeth. Immediately within the cheeks and lips are 310 THE HUMAN BODY. two semicircles, formed by the borders of the upper and lower jaw-bones, which are covered by the gums, except at intervals along their edges where they contain sockets in which the teeth are implanted. During life two sets of teeth are developed; the first or milk set appears soon after birth and is shed during childhood, when the second or permanent set appears. The teeth differ in minor points from one another, but in all three parts are distinguishable; one, seen in the mouth and called the crown of the tooth; a second, im- bedded in the jaw-bone and called the root or fang; and between the two, embraced by the edge of the gum, is a, narrowed portion, the neck or cervix. From differences in their forms and uses the teeth are divided into incisors, canines, bicuspids, and molars, arranged in a definite order in each jaw. Beginning at the middle line we meet in each half of each jaw with, successively, two incisors, one canine, and two molars in the milk-set; making twenty altogether in the two jaws. The teeth of the permanent. set are thirty-two in number, eight in each half of each jaw, viz. — beginning at the middle line — two incisors, one- canine, two bicuspids, and three molars. The bicuspids, or premolars, of the permanent set replace the milk molars, while the permanent molars are new teeth added on as the jaw grows, and not substituting any of the milk teeth. The hindmost permanent molars are often called the wis- dom teeth. Characters of Individual Teeth. The incisors (Fig. 90) are adapted for cutting the food. Their crowns are '•chisel - shaped and have sharp horizontal cutting edges, which become worn away by use so that they are beveled off behind in the upper row, and in the opposite direction in the lower. Each has a single long fang. The canines (Fig. 91) are somewhat larger than the incisors. Their crowns are thick and somewhat conical, having a central point or cusp on the cutting edge. In dogs, cats, and, other carnivora the canines are very large and adapted for seizing and holding prey. The bicuspids or premolars (Fig. 92) are rather shorter than the canines and their THE TEETH. 311 crowns are somewhat cuboidal. Each has two cusps, an outer towards the cheek, and an inner on the side turned towards the interior of the mouth. The fang is compressed laterally, and has usually a groove partially subdividing it FIG. 91. FIG. 92. FIG. 90.— An incisor tooth. / FIG. 91.— A canine or eye tooth. FIG. 92.— A bicuspid tooth seen from its outer side; the inner cusp is, accord- ingly, not visible. FIG. 93.— A molar tooth. into two. At its tip the separation is often complete. The molar teeth or grinders (Fig. 93) have large crowns with broad surfaces, on which are four or five projecting tubercles, which roughen them and make them better adapt- ed to crush the food. Each has usually several fangs. The milk teeth 'only differ in subsidiary points from those of the same names in the permanent set. The Structure of a Tooth. If a tooth be broken open a cavity extending through both crown and fang will be found in it. This is filled during life with a soft vascular pulp, and hence is known as the "pulp cavity" (c, Fig. 94). The hard parts of the tooth disposed around the pulp cavity consist of three different tissues. Of these one im- mediately surrounds the cavity and makes up most of the bulk of the tooth; it is dentine (2, Fig. 94); covering the dentine on the crown is the enamel (1, Fig. 94) and, on the fang, the cement (3, Fig. 94). The pulp cavity opens below by a narrow aperture at the tip of the fang, or at the tip of each if the tooth has more than one. The pulp consists mainly of connective tissue, but its surface next the dentine is covered by a layer of 312 THE HUMAN BODY. •columnar cells. Through the opening on the fang blood- vessels and nerves enter the pulp. The dentine (ivory) yields on analysis the same niate- 94.— Section through a premolar tooth of the cat still imbedded in aeden he bone of rials as bone but is somewhat harder, earthy matters con- stituting 72 per cent of it as against 66 per cent in bone. Under the microscope it is recognized by the fine dentinal THE TONGUE. 313 tubules which, radiating from the pulp cavity, perforate it throughout, finally ending in minute branches which open into irregular cavities, the interglobular spaces, which lie just beneath the enamel or cement. At their widest ends, close to the pulp cavity, the dentinal tubules are only about 0.005 millimeter (j-gVir °f an inch) in diameter. The cement is much like bone in structure and composition, possessing lacunae and canaliculi but rarely any Haversian canals. It is thickest at the tip of the fang and thins away towards the cervix. Enamel is the hardest tissue in the Body, yielding on analysis only from two per cent to three per cent of organic matter, the rest being mainly calcium phosphate and carbonate. Its histological ele- ments are minute hexagonal prisms, closely packed, and set on vertically to the surface of the subjacent dentine. It is thickest over the free end of the crown, until worn away by use. Covering the enamel in unworn teeth is a thin struc- tureless horny layer, the enamel cuticle. The Tongue (Fig. 95) is a muscular organ covered with a mucous membrane, extremely mobile, and endowed not only with a delicate tactile sensibility but with the ter- minal organs of the special sense of taste; it is attached by its root to the hyoid bone. Its upper surface is covered with small eminences or papillae, much like those more highly developed on the tongue of a cat, where they are readily felt. On the human tongue there are three kinds of papillae, the circumvaUate, the fungiform, and the fill*, form. The circumvaUate papillae, 1 and 2, the largest and least numerous, are from seven to twelve in number and lie near the root of the tongue arranged in the form of a V with its open angle turned forwards. Each is an ele- vation of the mucous membrane, covered by epithelium, and surrounded by a trench. On the sides of the papillae, imbedded in the epithelium, are small oval bodies (Fig. 155)* richly supplied with nerves and supposed to be concerned in the sense of taste, and hence called the taste buds (Chap. XXXIV.). The fungiform papillce, 3, are rounded elevations attached by somewhat narrower stalks, and found all over the middle and fore part of the upper surface of * P. 571. 314 THE HUMAN BODY. the tongue. They are easily recognized on the living tongue by their bright red color. The filiform papilla, most numerous and smallest, are scattered all over the dor- Fio. 95.— The upper surface of the tongue with part of the pillars of the fauces and the tonsils. 1, 2, circumvallate papillae; 3, fungiform papillae; 4, filiform papillae; 6, raucous glands; 7, tonsils; 8, part of epiglottis. sum of the tongue except near its base. Each is a conical eminence covered by a thick horny layer of epithelium. It is these papillas which are so highly developed on the tongues of Carnivora, and serve them to scrape bones clean THE SALIVARY GLANDS. 315 of even such tough structures as ligaments. Tamed tigers have been known to draw blood by licking the hand of their master. In health the surface of the tongue is moist, covered by little "fur/' and in childhood of a red color. In adult life the natural color of the tongue is less red, except around the edges and tip; a bright-red glistening tongue being, then, usually a symptom of disease. When the digestive organs are deranged the tongue is commonly covered with a thick yellowish coat, composed of a little mucus, some cells of epithelium shed from the surface, and numerous microscopic organisms known as bacteria; and there is frequently a "bad taste" in the mouth. The whole alimentary mucous membrane is in close physio- logical relationship; and anything disordering the sto- mach is likely to produce a "furred tongue." The Salivary Glands. The saliva, which is poured into the mouth and which, mixed with the secretion of minute glands imbedded in its lining membrane, moistens it, is secreted by three pairs of glands, the parotid, the subniaxil- lary and the suUingual The parotid glands lie in front of the ear behind the ramus of the lower jaw; each sends its secretion into the mouth by a tube known as Stenon9* dud, which crosses tho cheek and opens opposite the second upper molar tooth. In the disease known as mumps * the parotid glands are inflamed and enlarged. The submaxillary glands lie between the halves of the lower jaw-bone, near its angles, and their ducts open beneath the tongue near the middle line. The sublingual glands lie beneath the floor of the mouth, covered by its mucous membrane, between the back part of the tongue and the lower jaw-bone. Each has many ducts (8 to 20), some of which join the submaxil- lary duct, while the rest open separately in the floor of the mouth. The Fauces is the name given to the aperture which can be seen at the back of the mouth (Fig. 89), leading from it into the pharynx below the soft palate. It is bounded above by the soft palate and uvula, below by the root of the tongue, and on the sides by muscular elevations, covered by * Parotitis, in technical language. 316 THE HUMAN BODY. mucous membrane, which reach from the soft palate to the tongue. These elevations are the pillars of the fauces. Each bifurcates below, and in the hollow between its divi- sions lies a tonsil (1, Fig 95), a soft rounded body about the size of an almond, and containing numerous minute glands which form mucus. Note. The tonsils not unfrequently become enlarged during a cold or sore throat, and then pressing on the Eustachian tube (Chap. XXXIII.), which leads from the pharynx to the middle ear, keep it closed and produce tem- porary deafness. Sometimes the enlargement is permanent and causes much annoyance. The tonsils can, however, be readily removed without danger, and this is the treatment usually adopted in such cases. The Pharynx or Throat Cavity (Fig. 89). This por- tion of the alimentary canal may be described as a conical bag with its broad end turned upwards towards the base of the skull, and its narrow end downwards and passing into the gullet. Its front is imperfect, presenting apertures which lead into the nose, the mouth, and (through the •larynx and windpipe) into the lungs. Except when food is being swallowed the soft palate hangs down between the mouth and pharynx; during deglutition it is raised into a horizontal position and separates an upper or respiratory portion of the pharynx from the rest. Through this upper part, therefore, air alone passes, entering it from the posterior ends of the two nostril chambers; while through the lower portion both food and air pass, one on its way to the gullet, £, Fig. £9, the other through the larynx, d, to the windpipe, c ; when a morsel of food " goes the wrong way " it takes the latter course. Opening into the upper portion of the pharynx on each side is an Eustachian tube, g: so that the apertures leading out of it are seven in num- ber; the two posterior nares, the two Eustachian tubes, the fauces, the opening of the larynx, and that of the gul- let. At the root of the tongue, over the opening of the larynx, is a plate of cartilage, the epiglottis, e, which can be seen if the mouth is widely opened and the back of the tongue pressed down by some such thing as the handle of THE STOMACH. 317 a spoon. During swallowing the epiglottis is pressed down like a lid over the air-tube and helps to keep food or saliva from entering it. In structure the pharynx consists essen- tially of a bag of connective tissue lined by mucous mem- brane, and having muscles in its walls which, by their con- tractions, drive the food on. The (Esophagus or Gullet is a tube commencing at the lower termination of the pharynx and which, passing on through the neck and chest, ends in the stomach below the diaphragm, In the neck it lies close behind the windpipe. It consists of three coats — a mucous membrane within; next, a submucous coat of areolar connective tissue; and, outside, a muscular coat made up of two layers, an inner with tran& yerselv and an outer with longitudinally arranged fibres. In and beneath its mucous membrane are numerous small glands whose ducts open into the tube. The Stomach (Fig. 96) is a somewhat conical bag placed transversely in the upper part of the abdominal cavity. Its larger end is turned to the left and lies close cMm* , Fig. 97) a second incomplete layer of much larger oval cells, d. The glands of this second kind are the most numerous, and have been called peptic glands from the idea that they alone formed pepsin, the essential digestive ingredient of the gastric juice; this is how- ever by no means certain, peptic ceils. The peptic glands frequently branch at their deeper •ends. The Pylorus. If the stomach be opened it is seen that the mucous membrane projects in a fold around the pyloric orifice and narrows it. This is due to a thick ring of the circular muscular layer there developed, and forming a sphincter muscle around the orifice, which in life, by its contraction, keeps the passage to the small intestine closed except when portions of food are to be passed on from the stomach to succeeding divisions of the alimentary canal. Note. The cardiac end of the stomach lying immediately beneath the diaphragm, which has the heart on its upper side, over-distension of the stomach, due to indigestion or flatulence, may impede the action of the thoracic organs, and FIG. 97.— A thin section through the gastric mucous membrane, perpendi- cular to its surface, magnified about 25 diameters, a, a simple peptic gland • 6, a c »mpound peptic gland: c, a mu- cous gland ; d, oval, chief, or so-called 320 THE HUMAN BODY. cause feelings of oppression in the chest, or palpitation of the heart. The Small Intestine (Fig. 103*)f, commencing at the pylorus, ends, after many windings, in the large. It is about six meters (twenty feet) long, and about five centi- meters (two inches) wide at its gastric end, narrowing to about two thirds of that width at its lower portion. Exter- nally there are no lines of subdivision on the small intes- tine, but anatomists arbitrarily describe it as consisting of three parts; the first twelve inches being the duodenum, D, the succeeding two fifths of the remainder the jejunum, J9 ind the rest the ileum, L Like the stomach, the small intestine possesses four coats; a serous, a muscular, a submucous, and a mucous. The serous coat is formed by a duplicature of the peritoneum,, but presents nothing answering to the great omentum; this double fold, slinging the intestine as the small omentum slings the stomach, is called the mesentery. The muscular coat is composed of plain muscular tissue arranged in two strata, an outer longitudinal, and an inner transverse or circular. The submucous coat is like that of the stomach; consisting of loose areolar tissue, binding together the mu- cous and muscular coats, and forming a bed in which the blood and lymphatic vessels (which reach the intestine in the fold of the mesentery) break up into minute branches- before entering the mucous membrane. The Mucous Coat of the Small Intestine. This is pink, soft, and extremely vascular. It doec not present tempo- rary or effaceable folds like those of the stomach, but is, throughout a great portion of its length, raised up into per- manent transverse folds in the form of crescentic ridges, each of which runs transversely for a greater or less way round the tube (Fig. 98). These folds are the valvulce- conniventes. They are first found about two inches from the pylorus, and are most thickly set and largest in the upper half of the jejunum, in the lower half of which they become gradually less conspicuous; and they finally disappear altogether about the middle of the ileum. The folds serve greatly to increase the surface of the mucous mem- brane both for absorption and secretion, and they also de- tP.328. SMALL INTESTINE. 321 lay the food somewhat in its passage, since it must collect in the hollows between them, and so be longer exposed to the action of the digestive liquids. Examined closely with the eye or, better, with a hand lens, the mucous membrane of the small intestine is seen to be not smooth but shaggy, be- ing covered everywhere (both over the valvulae conniventes and between them) with closely packed minute processes, standing up somewhat like the ' pile'' on velvet, and known as the villi. Each villus is from 0.5 to 0.7 millimeters {•gV to -g^ inch) long ; some are conical and rounded, but the majority are compressed at the base in one diameter (Fig. 99). In structure a villas is somewhat complex. Covering it is a single layer of columnar epithelial cells, be- neath which the villus may be regarded as made up of a FIG. 98. - A portion of the small intestine opened to show the valvulce conni- rentes. framework of connective tissue supporting the more essen- tial constituents. Near the surface is an incomplete layer of plain muscular tissue, continuous below with a muscular layer found on the deep side of the mucous membrane. In the centre is an offshoot of the lymphatic system; some- times in the form of a single vessel with a closed dilated end, and sometimes as a network formed by two main ves- sels with cross-branches. During digestion these lym- phatics are filled with a milky white liquid absorbed from the intestines and they are accordingly called the lacteals. They communicate with larger branches in the submucous coat which end in trunks that pass out in the mesentery to join the main lymphatic system. Finally, in each villus, 322 THE HUMAN BODY. outside the lacteals and beneath the muscular layer, is a close network of blood-vessels. Opening on the surface of the small intestine, between the bases of the villi, are small glands, the crypts of Lieber- kiilm. Each is a simple unbranched tube lined by a layer of columnar cells similar to that which covers the villi and the surface of the mucous membrane between them. In FIG. 99.— Villi of the small intestine; magnified about 80 diameters. In the left-hand figure the lacteals, a, 6, c, are filled with white injection; d, blood-ves- sels. In the right-hand figure the lacteals alone are represented, filled with a dark injection. The epithelium covering the villi, and their muscular fibres, are omitted. structure they greatly resemble the mucous glands of the stomach (c, Fig. 97). In the duodenum are found other minute glands, the fjlands of Brunner. They lie in the submucous coat and send their ducts through the mucous, membrane to open on its inner side. The Large Intestine (Fig. 103*), forming the final por- tion of the alimentary canal, is about 1.5 meters (5 feet) long, and varies in diameUi from about 6 to 4 centimeters (2| to 1-J inches). Anatomists describe it as consisting of the caecum with the vermiform appendix, the colon, and the rectum. The small intestine does not open into the com- mencement of the large but into its side, some distance THE LIVER. 323 from its closed upper end, and the caecum, CC, is that part of the large intestine Avhich extends beyond the communi- cation. From it projects the vermiform appendix, a narrow tube not thicker than a cedar pencil, and about 10 centi- meters (4 inches) long. The colon commences on the right side of the abdominal1 cavity where the small intestine com- municates with the large, runs up for some way on that side (ascending colon, AC), then crosses the middle line (transverse colon, TO) below the stomach, and turns down (descending colon, DC] on the left side and there makes an S-shaped bend known as the s]igmoid flexure, 8F; from this the rectum, R, the terminal straight portion of the intes- tine, proceeds to the anal opening, by which the alimentary canal communicates with the exterior. In structure the large intestine presents the same coats as the small. The external stratum of the muscular coat is not, however, developed uniformly around it, except on the rectum, but occurs in three bands separated by intervals in which it is wanting. These bands being shorter than the rest of the tube cause it to be puckered, or sacculated, between them. The mucous coat possesses no villi or valvulae conniventes, out is usually thrown into effaceable folds, like those of the stomach but smaller. It contains numerous closely set glands much like the crypts of Lieberkiihn of the small intestine. The Ileo-Colic Valve. Where the small intestine joins the large there is a valve, formed by two flaps of the mucous membrane sloping down into the colon, and so disposed as to allow matters to pass readily from the ileum into the large intestine but not the other way. The Liver. Besides the secretions formed by the glands imbedded in its walls, the small intestine receives those of two large glands, the liver and the pancreas, which lie in the abdominal cavity. The ducts of both open by a com- mon aperture into the duodenum about 10 centimeters (4 inches) from the pylorus. The liver is the largest gland in the Body, weighing from 1400 to 1700 grams (50 to 64 ounces). It is situated in the upper part cf the abdominal cavity (le, le,' , Fig. 1), rather more on the right than on the left side and immediately below 324 THE HUMAN BODY. the diaphragm, into the concavity of which its upper sur- face fits; it reaches across the middle line above the pyloric end of the stomach. It is of dark reddish-brown color, and of a soft friable texture. A deep fissure incompletely divides the organ into right and left lobes, of which the right is much the larger; on its under surface (Fig. 100) shallower grooves mark off several minor lobes. Its upper surface is smooth and convex. The vessels carrying blood to the liver are the portal vein, Vp, and the hepatic artery; both enter it at a fissure (the portal fissure) on its under side, and there also a duct passes out from each half of the organ. Deb' Ve'Vp' FIG. 100.— The under surface of the liver, d. right, and .«?. left lobe: Vh. hepatic vein; Vp, portal vein; Vc, vena cava inferior; Dch, common bile-duct; DC cystic duct; Dh, hepatic duct; Vf, gall-bladder. The ducts unite to form ihehepatic duct, Dh, which meets at an acute angle, the cystic duct, DC, proceeding from the gall-bladder, Vf, a pear-shaped sac in which the bile, or gall, formed by the liver, accumulates when food is not being di- gested in the intestine. The common lile-duct, Dch, formed by the union of the hepatic and cystic ducts, opens into the duodenum. The blood which enters the liver by the portal vein and hepatic artery passes out by the hepatic veins, Vh, which leave the posterior border of the organ HISTOLOGY OF LIVER. 325 promoting digestion. Weak alkalies stimulate the mucous membrane of the stomach and cause it to pour forth more gastric juice. Hence the efficacy of a little carbonate of soda, taken before meals, in same forms of dyspepsia. The saliva by its alkalinity exerts such an action; and this is one reason why food should be well chewed before being swallowed; for then its taste, and the movements of the jaws, cause the secretion of more saliva. Deglutition. A mouthful of solid food is broken up by the teeth, and rolled about the mouth by the tongue, until it is thoroughly mixed with saliva and made into a soft pasty mass. The muscles of the cheeks keep this from getting between them and the gums; persons with facial paralysis have, from time to time, to press out with the finger food which has collected outside the gums, where it can neither be chewed nor swallowed. The mass is finally sent on from the mouth to the stomach by the process of deglutition, which is described as occurring in three stages. The first stage includes the passage from the mouth into the pharynx. The food being collected into a heap on the tongue, the tip of that organ is placed against the front of the hard palate and then the rest of its dorsum is raised from before back, so as to press the food mass between it and the palate, and drive it back through the fauces. This portion of the act of SWALLOWING. 337 swallowing is voluntary, or at least is under the control of the will, although it commonly takes place unconsciously. The second stage of deglutition is that in which the food passes through the pharynx; it is the most rapid part of its progress, since the pharynx has to be emptied quickly so as to clear the opening of the air-passages for breathing pur- poses. The food mass, passing back over the root of the tongue, pushes down the epiglottis; at ths same time the larynx (or voice-box at the top of the windpipe) is raised, so as to meet this and thus the passage to the lungs is closed; muscles around the aperture probably also contract and narrow the opening. The raising of the larynx can be readily felt by placing the finger on its large cartilage form- ing " Adam's apple" in the neck, and then swallowing something. The soft palate is at the same time raised and stretched horizontally across the pharynx, thus cutting off communication with its upper, or respiratory portion, lead- ing to the nostrils and Eustachian tubes. Finally, the isthmus of the fauces is closed as soon as the food has passed through, by the contraction of the muscles on its sides and the elevation of the root of the tongue. All pas- sages out of the pharynx except the gullet are thus blocked, and when the pharyngeal muscles contract the food can only be squeezed into the oesophagus. The muscular move- ments concerned in this part of deglutition are all reflexly €xcited; food coming in contact with the mucous mem- brane of the pharynx stimulates afferent nerve-fibres in it; these excite the centre of deglutition which is placed in the medulla oblongata, and from it efferent nerve-fibres proceed to the muscles concerned and (under the co-ordinating in- fluence of the centre) cause them to contract in proper se- quence. The pharyngeal muscles, although of the striped variety, are but little under the control of the will; it is ex- tremely difficult to go through the movements of swallow- ing without something (if only a little saliva) to swallow and excite the movements reflexly. Many persons after having got the mouth completely empty cannot perform the movements of the second stage of deglutition at all. On account of the reflex nature of the contractions of the 338 THE HUMAN BOD Y. pharynx, any food which has once entered it must be swal- lowed: the isthmus of the fauces is a sort of Rubicon; food that has passed it must continue its course to the stomach although the swallower learnt immediately that he was taking poison. The third stage of deglutition is that in which the food is passing along the gullet, and is compara- tively slow. Even liquid substances do not fall or flow down this tube, but have their passage controlled by its muscular coats, which grip the successive portions swallowed and pass them on. Hence the possibilit}' of performing the apparently wonderful feat of drinking a glass of water while standing upon the head, often exhibited by jugglers; the onlookers forget that the same thing is done every day by horses,, and other animals, which drink with the pharyngeal end of the gullet lower than the stomach. The movements of the oesophagus are of the kind known as vermicular or peri- staltic. Its circular fibres (p. 317) contract behind the morsel and narrow the passage there; and the constriction then travels along to the stomach, pushing the food in front of it. Simultaneously the longitudinal fibres, at the point where the food-mass is at any moment and immediately in front of that, contracting, shorten and widen the passage. The Gastric Juice. The food having entered the sto- mach is exposed to the action of the gastric juice, which is a thin, colorless, or pale yellow liquid, of a strongly acid re- action. It contains as specific elements free hydrochloric acid (about .2 per cent), and an enzyme called pepsin which, in acid liquids, has the power of converting the or- dinary non-dialyzable proteids which we eat, into the closely allied but dialyzable bodies called peptones. It also dis- solves solid proteids, changing them too into peptones. Dilute acids will by themselves produce the same changes in the course of several days, but in the presence of pepsin and at the temperature of the Body the conversion is far more rapid. In neutral or alkaline media the pepsin is inactive; and cold checks its activity. Boiling destroys it. In addition to pepsin, gastric juice contains another enzyme which has the power of coagulating the casein of milk, as illustrated by the use of "rennet," prepared from the mu- DIGESTION IN THE STOMACH. 339 cous membrane of the calf's digestive stomach, in cheese- flaking. The acid of the natural gastric juice might itself, .t is true, coagulate the casein, but neutralized gastric juice still possesses this power; and, since pure solutions of pepsin do not, it must be due to some third body, which has, however, not yet been isolated. The curdled condition of the milk regurgitated so often by infants is, therefore, not any sign of a disordered state of the stomach, as nurses commonly suppose. It is natural and proper for milk to undergo this change, before the pepsin and acid of the gastric juice convert its casein into peptone. Gastric Digestion. The process of swallowing is con- tinuous, but in the stomach the onward progress of the food is stayed for some time. The pyloric sphincter, re- maining contracted, closes the aperture leading into the intestine, and the irregularly disposed muscular layers of the stomach keep its semi-liquid contents in constant movement, maintaining a sort of churning by which all portions are brought into contact with the mucous mem- brane and thoroughly mixed with the secretion of its glands. The gelatin-yielding connective tissue of meats is dissolved away, and the proteid-containing fibres, left loose, are dis- solved and turned into peptones. The albuminous walls of the fat-cells are dissolved and their oily contents set free; but the gastric juice does not act upon the latter. Certain mineral salts (as phosphate of lime, of which there is always some in bread) which arc insoluble in water but soluble in dilute acids, are also dissolved in the stomach. On the other hand the gastric juice has itself no action upon starch, and since ptyalin does not act at all, or only imperfectly, in an acid medium, the activity of the saliva in converting starch is stayed in the stomach. By the solu- tion of the white fibrous connective tissue, that disintegra- tion of animal foods commenced by the teeth, is carried much farther in the stomach, and the food-mass, mixed with much gastric secretion, becomes reduced to the con- sistency of a thick soup, usually of a grayish color. In this state it is called chyme. This contains, after an ordi- nary meal, a considerable quantity of peptones which are 340 THE HUMAN BODY. in great part gradually dialyzed into the blood and lympha- tic vessels of the gastric mncons membrane and carried off, along with other dissolved dialyzable bodies, such as salts and sugar. After the food has remained in the stomach some time (one and a half to two hours) the chyme begins to be passed on into the intestine in successive portions. The pyloric sphincter relaxes at intervals, and the rest of the stomach, contracting at the same moment, injects a quantity of chyme into the duodenum ; this is repeated frequently, the larger undigested fragments being at first unable to pass the orifice. At the end of about three or four hours after a meal the stomach is again quite emptied, the pyloric sphincter finally relaxing to a greater extent and allowing any larger indigestible masses, which the gas- tric juice cannot break down, to be squeezed into the intes- tine. The Chyle. When the chyme passes into the duodenum it finds preparation made for it. The pancreas is in reflex connection with the stomach, and its nerves cause it to commence secreting as soon as food enters the latter; hence a quantity of its secretion is already accumulated in the intestine when food enters. The gall-bladder is distended with bile, secreted since the last meal; this passing down the hepatic duct has been turned buck up the cystic duct (Dc, Fig. 100*) on account of the closure of the common bile-duct. The acid chyme, stimulating nerve-endings in the duodenal mucous membrane, causes reflex contrac- tion of the muscular coat of the gall-bladder, and a relaxa- tion of the orifice of the common bile-duct; and so a gush of bile is poured out on the chyme. From this time on, both liver and pancreas continue secreting actively for some hours, and pour their products into the intestine. The glands of Brunner and the crypts of Lieberkuhn are also set at work, but concerning their physiology we know yery little. All of these secretions are alkaline, and they suffice very soon to more than neutralize the acidity of the gastric juice, and to convert the acid chyme into alka- line chyle, which, after an ordinary meal, will contain a great variety of things: mucus derived from1 the alimen- * P. 324. PANCREATIC DIGESTION. 341 tury canal; ptyalin from the saliva; pepsin from the sto- mach; water, partly swallowed and partly derived from the •salivary and other secretions;, the peculiar constituents of the bile and pancreatic juice and of the intestinal secretions; some undigested proteids; unchanged starch; oils from the fats eaten; peptones formed in the stomach but not vet absorbed; possibly salines and sugar which have also escaped absorption in the stomach; and indigestible substances taken with the food. The Pancreatic Secretion is clear, watery, alkaline, and much like saliva in appearance. The Germans call the pancreas the " abdominal salivary gland." In digestive properties, however, the pancreatic secretion is far more important than the saliva, acting not only on starch but, also, on proteids and fats. On starch it acts like the saliva, but more energetically. It produces changes in proteids similar to those effected in the stomach,, but by the agency of a different ferment, trypsin; which differs from pepsin in acting only in an alkaline instead of an acid medium. On fats it has a double action. To a certain extent it breaks them up, with hydration, into free fatty acids and glycerine; for example — Calls CaH 1 Stearin + 3 Water = 3 Stearic acid + 1 Glycerine. The fatty acid then combines with some of the alkali pres- ent to make a soap, which being soluble in water is capable of absorption. Glycerine, also, is soluble in water and dialy- zable. The greater part of the fats are not, however, so broken up, but are simply mechanically separated into little droplets, which remain suspended in the chyle and give it a whitish color, just as the cream-drops are suspended in milk, or the olive-oil in mayonnaise sauce. This is effected by the help of a quantity of albumin which exists dissolved in the pancreatic secretion. In the stomach, the animal fats eaten have lost their cell-walls, and have become melted by the temperature to which they are exposed. Hence their oily part float0 free in the chyme when it enters the 342 THE HUMAN BODY. duodenum. If oil be shaken up with water, the two can- not be got to mix; immediately the shaking ceases the oil floats up to the top; but if some raw egg be added, a creamy mixture is readily formed, in which the oil remains for a long time evenly suspended in the watery menstruum. The reason of this is that each oil-droplet becomes sur- rounded by a delicate pellicle of albumen, and is thus pre- vented from fusing with its neighbors to make large drops, which would soon float to the top. Such a mixture is called an emulsion, and the albumin of the pancreatic secretion emulsifies the oils in the chyle, which becomes white (for the- same reason as milk is that color) because the innumerable tiny oil-drops floating in it reflect all the light which falls on its surface. In brief, the pancreatic secretion converts starch inta sugars; dissolves proteids (if necessary) and converts them into peptones; emulsifies fats, and, to a certain extent,. breaks them up into glycerine and fatty acids; the latter are then saponified by the alkalies present. The Bile. Human bile when quite fresh is a golden brown liquid; it becomes green when kept. As formed in the liver it contains hardly any mucin, but if it makes any stay in the gall-bladder it acquires a great deal from the lining membrane of that sac, and becomes very "ropy."' It is alkaline in reaction and, besides coloring matters, min- eral salts, and water, contains the sodium salts of two nitro- genized acids, taurocholic and glychochotic, the former pre- dominating in human bile. Pettenkofer's Bile Test. If a small fragment of cane sugar be added to some bile, and then a large quantity of strong sulphuric acid, a brilliant purple color is developed, by certain products of the decomposition of its acids; the physician can in this way, in disease, detect their presence- in the urine or other secretions of the Body. Gmeliris Bile Test. The bile-coloring matters, treated with yellow- nitric acid, go through a series of oxidations, accompanied with changes of color from yellow-brown to green, then to blue, violet, purple, red, and dirty yellow, in succession. Bile has no digestive action upon starch or proteids. It DIGESTIVE ACTION OF BILE. 343 does not break up fats, but to a limited extent emulsifies them, though far less perfectly than the pancreatic secre- tion. It is even doubtful if this action is exerted in the intestines at all. In many animals, as in man, the bile and pancreatic ducts open together into the duodenum, so that, on killing them during digestion and finding emulsified fats in the chyle, it is impossible to say whether or no the bile had a share in the process. In the rabbit, however, the pancreatic duct opens into the intestine about a foot farther from the stomach than the bile-duct, and it is found that if a rabbit be killed after being fed with oil, no milky chyle is found down to the point where the pancreatic duct opens. In this animal, therefore, the bile alone does not emulsify fats, and, since the bile is pretty much the same in it and other mammals, it probably does not emulsify fats in them either. From the inertness of bile with respect to most foodstuffs it has been doubted if it is of any digestive use at all, and whether it should not be regarded merely as an. excretion, poured into the alimentary canal to be got rid of. But there are many reasons against such a view. In the first place, the entry of the bile into the upper end of the small intestine where it has to traverse a course of more than twenty feet before getting out of the Body, instead of its being sent into the rectum close to the final opening of the alimentary canal, makes it probable that it has some function to fulfill in the intestine. Moreover, a great part of the bile poured into the intestines is again absorbed from them, only a small part being pnssed out from the rectum; this seems to show that part of the bile is secreted for some other purpose than mere elimination from the Body. One use is to assist, by its alkalinity, in overcoming the acidity of the chyme, and so to allow the trypsin of the- pancreatic secretion to act upon proteids. Constipation is, also, apt to occur in cases where the bile-duct is temporarily stopped, so that the bile probably helps to excite the con- tractions of the muscular coats of the intestines; and it is said that under similar circumstances putrefactive decom- positions are extremely apt to occur in the intestinal con- tents. Apart from such secondary actions, however, the 344 THE HUMAN BODY. bile probably has some influence in promoting the absorp- tion of fats. If one end of a capillary glass tube, moistened with water, be dipped in oil, the latter will not ascend in it, or but a short way; but if the tube be moistened with bile, instead of water, the oil will ascend higher in it. So, too, oil passes through a plug of porous clay kept moist with bile, under a much lower pressure than through one wet with water. Hence bile, by soaking the epithelial cells lining the intestine, may facilitate the passage into the villi of oily sub- stances. At any rate, experiment shows that if the bile be prevented from entering the intestine of a dog, the animal eats an enormous amount of food compared with that amount which it needed previously; and that of this food a great proportion of the fatty parts passes out of the ali- mentary canal unabsorbed. There is no doubt, therefore, that the bile somehow aids in the absorption of fats^but exactly how is uncertain. Its possible action in exciting the muscles of the villi to contract will be referred to pres- ently. Bile precipitates from solution, not only pepsin, but any peptones contained in the chyme which enters the in- testine from the stomach. The Intestinal Secretions or Succus Entericus. This consists of the secretions of the glands of Brunner and the> crypts of Lieberkiihn. It is difficult to obtain pure; in- deed the product of Brunner's glands has never been obv tained unmixed. That of the crypts of Lieberkiihn is watery and alkaline, and poured out more abundantly duis ing digestion than at other times. It has no special action on starches, most proteids, or on fats; but is said to dissolve blood fibrin and convert it into peptone, and to change cane into grape sugar, a transformation the object of which is not very clear, since cane sugar is itself readily soluble and diffusible. Intestinal Digestion. Having considered separately the actions of the secretions which the food meets with in the small intestine we may now consider their combined effect. The neutralization of the chyme, followed by its conver- sion into alkaline chyle, will prevent any further action of the pepsin on proteids, but will allow the ptyalin of the INTESTINAL DIGESTION. 345 saliva (the activity of which was stopped by the acidity of the gastric juice) to recommence its action upon starch. Moreover, in the stomach there is produced, alongside of the true peptones, a body called parapeptone, which agrees very closely with syntonin (p. 126) in its properties, and this passes into the duodenum in the chyme. As soon as the bile meets the chyme it precipitates the parapeptone, and this carries down with it any peptones which, having es- caped absorption in the stomach, may be present; it also precipitates the pepsin. In consequence, one commonly finds, during digestion, a sticky granular precipitate over the villi, and in the folds between the valvulao conniventes of the duodenum. This is soon redissolved by the pancre- atic secretion, which also changes into peptones the pro- teids (usually a considerable proportion of those eaten at a meal) which have passed through the stomach unchanged, or in the form of parapeptones. The conversion of starch into grape sugar will go on rapidly under the influence of the pancreatic secretion. Fats will be split up and saponi- fied to a certain extent, but a far larger proportion will be emulsified and give the chyle a whitish appearance. Cane sugar, which may have escaped absorption in the stomach, will be converted into grape sugar and absorbed, along with any salines which may, also, have hitherto escaped. Elastic tissue from animal substances eaten, cellulose from plants, and mucin from the secretions of the alimentary tract, will all remain unchanged. Absorption from the Small Intestine. The chyme leav- ing the stomach is a semi-liquid mass which, being mixed in the duodenum with considerable quantities of pancreatic secretion and bile, is still further diluted. Thenceforth it gets the intestinal secretion added to it but, the absorption more than counterbalancing the addition of liquid, the foocl- mass becomes more and more solid as it approaches the ileo-colic valve. At the same time it becomes poorer in nutritive constituents, these being gradually removed from it in its progress; most dialyze through the epithelium into the subjacent blood and lymphatic vessels, and are carried off. Those passing into the blood capillaries are taken by 346 THE HUMAN BODY. the portal vein to the liver; while those entering the lacteals are carried into the left jugular vein by the thoracic duct. As to which foodstuffs go one road and which the other, there is still much doubt; sugars probably go by the portal system, while the fats, mainly, if not entirely, go through the lacteals. How the fats are absorbed is not clear, since oils will not dialyze through membranes, such as that lining the intestine, moistened with watery liquids. Most of them, nevertheless, get into the lacteals as oils and not as soluble soaps; for one finds these vessels, in a digesting animal, filled with white milky chyle; while at other peri- ods their contents are watery and colorless like the lymph elsewhere in the Body. The little fat-drops of the emul- sion formed in the intestine, go through the epithelial cells and not between them, for during digestion these cells are loaded with oil-droplets; as their free ends are striated and probably devoid of any definite cell-wall, it is possible that the intestinal movements squeeze oil- drops into them. The cell then passes the fat to its deeper end and, thence, out into the subjacent connective tissue. Here " wandering cells" (p. 106) pick it up and carry it into the central lacteal of a villus, where they break up and set it free. In the villus there are all the anatomical arrangements for a mechanism which shall actively suck up substances into it. Each is more or less elastic, and, moreover, its capillary network when filled with blood will distend it. If its muscular coat (p. '321) contracts and compresses it, causing its lacteals to empty into vessels lying deeper in the intestinal wall, the villus will actively expand again so soon as its muscles relax. In so doing it could not fill its' lacteals from the deeper ves- sels on account of the valves in the latter, and, accordingly, would tend to draw into itself materials from the intestines; much like a sponge re-expanding in water, after having been squeezed dry. The liquid thus sucked up may draw oil- drops with it, into the free ends of the cells and between them; and by repetitions of the process it is possible that considerable quantities of liquid- with suspended oil-drops. DIGESTION IN THE LARGE INTESTINE. 347 might be carried into the epithelial cells covering a vil- lus. The bile moistening the surface of the villus may facilitate the passage of oil, as it does through a paper filter or a plate of pi aster- of -Paris, and it is also said io stimulate the contractions of the villi; if so, its efficacy in promoting the absorption of fats will be explained, in spite of its chemical inertness with respect to those bodies,* Digestion in the Large Intestine. The contractions of the small intestine drive on its continually diminishing contents until they reach the ileo-colic valve, through which they £ve ultimately pressed. As a rule, when the mass enters the large intestine its nutritive portions have been almost entirely absorbed, and it consists merely of some water, with the indigestible portion of the food and of the secretions of the alimentary canal. It contains cellulose, elastic tissue, mucin, and somewhat altered bile pigments; commonly some fat if a large quantity has been eaten; and some starch, if raw vegetables have formed part of the diet. In its progress through the large intestine it loses more water, and the digestion of starch and the absorption of fats is continued. Finally the residue, with some excretory matters added to it in the large intestine, collects in the sigmoid flexure of the colon and in the rectum, and is finally sent out of the Body from the latter. The Digestion of an Ordinary Meal. We may best sum up the facts stated in this chapter by considering the diges- tion of a coiumon meal; say a breakfast consisting of bread and butter, beefsteak, potatoes and milk. Many of these substances contain several alimentary principles, and, since these are digested in different ways and in different parts of the alimentary tract, the first thing to be done is to con- sider what are the proximate constituents of each. We then separate the materials of the breakfast as in the f olV lowing table — * Some recent researches make it probable that a good deal of the •emulsified fat is also picked up by amoeboid connectjve-tissue cor- puscles, which push their way between the epithelial cells and thrusting processes (p. 20) into the intestine, pick up oil-droplets, and then travel back and convey their load to the lacteal. 348 TEE HUMAN BODY. I . if A i 1 SI H Z a d ^ s • •3 BJ "3 i D a: cr < I . Isl jl 1 jl § ll « tj 1 0 1° ^° * £3 |j 1 I i i» 0 6 1 £ 1 ' CO li • i> ' 1 I s. li A fi g C3 1 i 1 J i 1 i^ II n £ *S § SI Calcium phosphate, Calcium sulphate. • oT c5 5^ •§ .5^c- Sg^l1 cj O O °^ -a I * ^BB ^ »5 fl fl)^ aJ *§ X !>, « X! C ft I £ §1 || |||| sJjjLs *? 3 « 02 §1 §1 11,11 gall 0 o O O ^ '-^ I s . 1 C tn" I 1 as a | s 1 c3 55 1 -j 8.3 o 3 » 3 i 1 ^ ll I Is i 1 « M 6° s g DIGESTION OF A MEAL. 349 From such a meal we may first separate the elastin, cel- lulose, and calcium sulphate, as indigestible and passed out of the Body in the same state and in the same quantity as they entered it. Then come the salines which need no special digestion, and, either taken in solution or dis- solved in the saliva or gastric juice, are absorbed from the mouth, stomach and intestines without further change. Cane and grape sugars experience the same fate, except that any cane sugar reaching the intestines before absorp- tion is liable to be changed into grape sugar by the succus entericus. Calcium phosphate will be dissolved by the free acid in the stomach, yielding calcium chloride, which will be absorbed there or in the intestine. Starch will be partially converted into various sugars during mastication and deglutition, and the sugars will be absorbed from the stomach. A great part of the starch will, however, be passed on into the intestine unchanged, since the action of the saliva is suspended in the stomach; .and its conversion will be completed by the pancreatic secretion, and by the ptyaKii of the saliva, which will recommence its activity when the chyle becomes alkaline. | The various proteids will be par- tially dissolved in the stomach and converted into peptones, which will in part be absorbed there; the residue, with the undigested proteids, will be passed on to the intestines. There the bile will precipitate the peptones and parapep- tones and, with the pancreatic secretion, render the chyme alkaline, and so stop the activity of the gastric pepsin. The pancreatic secretion will, however, redissolve the precipi- tated peptone, and the unchanged proteids and parapeptone> and turn the latter two into peptones; these will be absorbed as they pass along the small intestine; a small quantity per- haps passing into the large intestine, to be taken up there. The fats will remain unchanged until they enter the small intestine, except that the proteiJ. cell- walls of the fats of- the beefsteak will be dissolved away. In the small intes- tine these bodies will be partially saponified, but most will be emulsified and taken up into the lacteals in that condi- tion. Gelatin, from the white fibrous tissue of the beef- 350 THE HUMAN BODY. steak, will undergo changes in the stomach and intestine and be dissolved and absorbed. The substances leaving the alimentary canal after such a meal would be, primarily, the indigestible cellulose and elastin, together with some water. But there might be in addition some unabsorbed fats, starch, and salts. To this would be added, in the alimentary canal, mucin, some of the ferments of the digestive secretions, some slightly altered bile pigments, and other bodies excreted by the large intestine. Dyspepsia is the common name of a number of diseased conditions attended with loss of appetite or troublesome digestion. Being often unattended with acute pain, and if it kills at all doing so very slowly, it is pre-eminently suited for treatment by domestic quackery. In reality, however, the immediate cause of the symptoms, and the treatment called for, may vary widely; and their detection and the choice of the proper remedial agents often call for more than ordinary medical skill. A fe\v of the more com- mon forms of dyspepsia may be mentioned here, with their proximate causes, not in order to enable people to under- take the rash experiment of dosing themselves, but to show how wide a chance there is for any unskilled treatment to miss its end, and do more harm than good. Appetite is primarily due to a condition of the mucous membrane of the stomach which, in health, comes on after a short fast, and stimulates its sensory nerves; and loss of appetite may be due to either of several causes. The sto- mach may be apathetic and lack its normal sensibility, so that the empty condition does not act, as it normally does, as a sufficient excitant. When food is taken it is a further stimulus and may be enough; in such cases "appetite comes with eating." A bitter before a meal is useful as an appetizer to patients of this sort. On the other hand, the stomach may be too sensitive, and a voracious appetite be felt before a meal, which is replaced by nausea, or even vomiting, as soon as a few mouthfuls have been swallowed; the extra stimulus of the food then over-stimulates the too irritable stomach, just as a draught of mustard and warm INDIGESTION. 351 water will a healthy one. The proper treatment in such <3ases is a soothing one. When food is taken it ought to stimulate the sensory gastric nerves, so as to excite the reflex centres for the secretory nerves, and for the dilatation of the blood-vessels, of the organ; if it does not, tre gastric juice will be imperfectly secreted. In such cases one may stimulate the secretory nerves by weak alkalies (p. 336), as Apollinaris water or a little carbonate of soda, before meals; or give drugs, as strychnine, which increase the irritability of re^ex nerve-centres. The vascular dilatation may be helped by warm drinks, and this is probably the rationale of the glass of hot water after eating which has recently heen in vogue; the usual cup of hot coffee after dinner (the desirability of which is proved by the consensus of civilized mankind) is a more agreeable form of the same aid to digestion. In states of general debility, when the stomach is too feeble to secrete under any stimulation, the adminis- tration of weak acids and artificially prepared pepsin is needed, so as to supply gastric juice from outside, until the improved digestion strengthens the stomach up to the point of being able to do its own work. Enough has probably been said to show that dyspepsia is not a disease, but a symptom accompanying many patho- logical conditions, requiring special knowledge for their treatment. From its nature — depriving the Body of its proper nourishment — it tends to intensify itself, and so should never be neglected; a stitch in time saves nine. CHAPTER XXIV. THE EESPIRATORY MECHANISM. » Definitions. The blood as it flows from the right ven- tricle of the heart, through the lungs, to the left auricle, loses carbon dioxide and gains oxygen. In the systemic circulation exactly the reverse changes take place, oxygen leaving the blood to supply the living tissues; and carbon dioxide, generated in them, passing back into the blood capillaries. The oxygen loss and carbon dioxide gain are associated with a change in the color of the blood from bright scarlet to purple red, or from arterial to venous; and the opposite changes in the lungs restore to the dark blood its bright tint. The whole set of processes through which blood becomes venous in the systemic circulation and arterial in the pulmonary — in other words the processes concerned in the gaseous reception, distribution and elimi- nation of the Body — constitute the function of respiration; so much of this as is concerned in the interchanges between the blood and air being known as external respiration; while the interchanges occurring in the systemic capillaries, and the processes in general by which oxygen is fixed and carbon dioxide formed by the living tissues, are known as internal respiration. When the term respiration is used alone, without any limiting adjective, the external respira- tion only, is commonly meant. Respiratory Organs. The blood being kept poor in oxygen and rich in carbon dioxide by the action of the liv- ing tissues, a certain amount of gaseous interchange will nearly always take place when it comes into close proximity to the surrounding medium; whether this be the atmos- phere itself or water containing air in solution. When an RESPIRATORY ORGANS. 353 •animal is small there are often no special organs for its ex- ternal respiration, its general surface being sufficient (espe- cially in aquatic animals with a moist skin) to permit of all the gaseous exchange that is necessary. In the simplest creatures, indeed, there is even no blood, the cell or cells •composing them taking up for themselves from their en- vironment the oxygen which they need, and passing out into it their carbon dioxide waste; in other words, there is .no differentiation of the external and internal respirations. When, however, an animal is larger many of its cells are so far from a free surface that they cannot transact this give- .and-take with the surrounding medium directly, and the Mood, or some liquid representing it in this respect, serves as a middleman between the living tissues and the external oxygen; and then one usually finds special respiratory or- gans developed, into which the blood is brought to replace its oxygen loss and get rid of its excess of carbon dioxide. In aquatic animals such organs take commonly the form of gills; these are protrusions of the body over which a constant current of water, containing oxygen in solu- tion, is kept up; and in which blood capillaries form a close network immediately beneath the surface. In air- breathing animals a different arrangement is usually found. In some, as frogs, it is true, the skin is kept moist and serves as an important respiratory organ, large quanti- ties of venous blood being sent to it for aeration. But for the occurrence of the necessary gaseous diffusion, the skin must be kept very moist, and this, in a terrestrial animal, necessitates a great amount of secretion by the cutaneous glands to compensate for evaporation; accordingly in most land animals the air is carried into the body by tubes with narrow external orifices, and so the drying up of "the breathing surfaces is greatly diminished; jiL-t as water in a bottle with a narrow neck will evaporate much more slowly than *ho same amount exposed in an open dish. In insects (as bees, butterflies, and beetles) the air is carried by tubes which split up into extremely fine branches and ramify all through the body, even down to the individual tissue elements, which thus carry on their gaseous exchanges 354 THE HUMAN BODY. without the intervention of the blood. But in tiie great majority of air-breathing animals the arrangement is dif- ferent; the air-tubes leading from the exterior of the body do not subdivide into branches which ramify all through it, but open into one or more large sacs to which the venous blood is brought, and in whose walls it flows through a close capillary network. Such respiratory sacs are called lungs, and it is a highly developed form of them which is employed in the Human Body. The Air-Passages and Lungs. In our own Bodies some- small amount of respiration is carried on in the alimentary canal, the air swallowed with food or saliva undergoing gas- eous exchanges with the blood in the gastric and intestinal mu- cous membranes. The amount, of oxygen thus obtained by the blood is however very trivial, a& is that absorbed through the- skin, covered as it is by its dry horny non-vascular epidermis. All the really essential gaseous interchanges between the Body and the atmosphere take place- in the lungs, two large sacs (In? Fig. 1) lying in the thoracic cavity, one on each side of the heart. To these sacs- the air is FIG. 104.— The lungs and air-pas- Conveyed through a series of sn.iivs seentrom tlie tront. On tne -r>OCC!0n.rvC, T7n4oiM-nrr 4-1-m -r»L o v-f-n tr left of the figure the pulmonary passages. JintCling til 6 pliai\nx the *08trils or mouth, & Passes Ollt Of tllis bJ the chus is seen entering the root of its ing leading into the larynx, or voice-box (a, Fig. 104), lying in the upper part of the neck (the communication of the two is seen in Fig. 89); from the larynx passes back the trachea or windpipe, #, which, after entering the chest cavity, divides into the right and left bronchi, d, e. Each bronchus divides up into smaller and smaller branches, called Iron- STRUCTURE OF THE LUNGS. 355 chial tubes, within the lung on its own side; and the smallest bronchial tubes end in sacculated dilatations, the alveoli of the lungs, the sacculations (Fig. 106) being the air-cells: the word " cell" being here used in its primitive sense of a small cavity, and not in its later tech- nical signification of a morpho- logical unit of the Body. On the walls of the air-cells the pulmonary capillaries ramify, and it is in them that the inter- changes of the external respira- Fio. 105.— A small bronchial tube, tlOn take place. «» dividing into its terminal branch- es, c ; these have pouched or saccu- StrUCture Of the Trachea latea walls and end in the saccu- and Bronchi. The windpipe l may readily be felt in the middle line of the neck, a little below Adam's apple, as a rigid cylindrical mass. It con- sists fundamentally of a fibrous tube in which cartilages are imbedded, so as to keep it from collapsing; and is lined internally by a mucous membrane covered by several layers of epithelium cells, of which the superficial is ciliated (Fig. 47)*. The cartilages imbedded in its walls are imperfect rings, each somewhat the shape of a horseshoe and the deficient part of each ring being turned backwards, it comes to pass that the deeper or dorsal side of the windpipe has no hard parts in it. Against this side the gullet lies, and the absence thereof the cartilages no doubt facilitates swal- lowing. 'The bronchi resemble the windpipe in structure. The Structure of the Lungs. These consist of the bronchial tubes and their terminal dilatations; numerous blood-vessels, nerves and lymphatics; and an abundance of connective tissue, rich in elastic fibres, binding all together. The bronchial tubes ramify in a tree-like manner (Fig. 104). In structure the larger ones resemble the trachea, except that the cartilage rings are not regularly arranged so as to have their open parts all turned one way. As the tubes become smaller their constituents thin away; the cartilages become less frequent and finally disappear; the epithelium is reduced to a single layer of cells which, though still cili- * P. 115. 356 THE HUMAN BODY. atcd. are much shorter tnan the columnar superficial cell layer of the larger tubes. The terminal alveoli (a, a, Fig. 106,) and the air-cells, #, which open into them, have walls composed mainly of elastic tissue and lined by a single layer of flat, non-ciliated epi- thelium, immediately beneath which is a very close network of capillary blood - vessels. The air entering by the bron- chial tube is thus only sepa- rated from the blood by the thin capillary wall and the thin epithelium, both of which are moist, and well adapted to permit gaseous diffusion. The Pleura. Each lung is covered, except at one point, by an elastic serous membrane which adheres tightly to it and is called the pleura; that point at which the pleura is wanting is called the root of the lung and is on its inner side; it is there that its bronchus, blood-vessels and nerves enter it. At the root of the lung the pleura turns back and lines the inside of the chest cavity, as rep- resented by the dotted line in the diagram Fig. 3. The part of the pleura attached to each lung is its visceral, and that attached to the chest-wall its parietal layer. Each pleura thus forms a closed sac surrounding &pleural cavity, in which, during health, there arc found a few drops of lymph, keeping its surfaces moist. This lessens friction between the two layers during the movements of the chest- walls and the lungs; for although, to insure distinctness, the visceral and parietal layers of the pleura are represented in the diagram as not in contact, that is not the natural condition of things; the lungs are in life distended so that the visceral pleura rubs against the parietal, and the pleural cavity is practically obliterated. This is due to the pressure FIG. 106.— Two alveoli of the lung highly magnified, b. 6, the air-cells, or hollow protrusions of the alveolus, opening into its central cavity ; c, ter- minal branches of a bronchial tube. WHY THE LUNGS DO NOT COLLAPSE. 357 oontain kreatin, a body intermediate chemically between proteids and urea. The quantity of this in muscles is prac- tically unaffected by work, and is from 0.2 to 0.4 per cent. .Since it is readily soluble and dialyzable, and therefore fit- ted to pass rapidly out of the muscles into the blood stream, it is a fair conclusion that a good deal of it is formed in the muscles daily and carried off from them. Kreatin, :too, exists in the brain, and probably there and elsewhere in the nervous system, is produced by chemical degrada- tion of protoplasm; the spleen also contains a good deal of kreatin, and so do many glands. This substance would therefore seem to be constantly produced in considerable quantities by the protoplasmic tissues generally; and since it belongs to a group of nitrogenous compounds which the .Body is unable to utilize for reconstruction into proteids, 434 THE HUMAN BODY. it must be carried off somehow. The urine, however, con- tains very little kreatin, or its immediate derivative, krea- tinin, and what it does contain depends mainly on the feeding, since it varies with the diet and vanishes during starvation; so it is probable that this substance is con- verted into urea and excreted in that form. This conver- sion must occur elsewhere than' in the muscles, which con- tain no urea; also, very little, if any, exists in the brain. Where the kreatin is finally changed into urea is doubt- ful. It may be in the kidneys by the renal epithelium, or it may be elsewhere, and the urea produced be merely picked up from the blood and passed out by the kidney- cells; or both may occur; histologically the distinctly secretory epitheliums of the convoluted parts of the tubules and of Henle's loops, differ so much as to suggest an en- tirely different function for them. On the whole, the evidence seems to show that urea i& merely separated and not produced in the kidneys; a priori this is more probable, since in the degradation of kreatin to yield urea energy is liberated and this might very well be utilized in some organ; while if the process took place in the kidney tubules the force set free would be wasted. The blood always contains urea, and renal-artery blood apparently more than renal-vein blood, which shows that, urea is removed from the blood in the kidneys. Moreover, if a mammal's kidneys be extirpated urea accumulates in its blood, which could not be the case if urea were nor- mally produced only in the kidneys; and if urea be injected into a vein it is rapidly picked up and carried off in the urine, showing that the kidney-cells have a selective power with respect to it. While the urea resulting from, further changes in the kreatin formed in the tissues is a measure of the wear and tear of their protoplasm, part of the urea excreted has probably a different source; being due to the oxidation of proteids, as energy liberators or respiratory foods, before they have ever formed a tissue. When plenty of proteid food is taken the urea excretion is largely increased and that very rapidly, within a couple of hours for example,. LUXUS CONSUMPTION. 435 and before we can well suppose the proteids eaten to have been built up into tissues, and these in turn broken down; in fact there need be, and usually is, under such circumstances no sign of any special activity of any group of tissues, such as one would expect to see if the urea always came from the breaking down of formed histological elements. This urea is thus indicative of a utilization of proteids for other than plastic purposes; and the same fact is indicated by the storage of carbon and elimination of all the nitrogen of the food (p. 444) when a diet very rich in proteid alimentary principles is taken. This luxus consumption may be com- pared to the paying out of gold by a merchant instead of greenbacks when he has an abundance of both. Only the gold can be used for certain purposes, as settling foreign debts, but any quantity above that needed for such a pur- pose is harder to store than the paper money and not so •convenient to handle; so it is paid out in preference to the paper money, which is really somewhat less valuable, as available at par only for the settlement of domestic debts. In artificial pancreatic digestions, when long carried on, two bodies, called leucin and tyrosin, are produced from proteids. It is found that when leucin is given to an animal in its food it reappears in the urine as urea; so the Body can turn leucin into that substance. Hence a possible source of some of the luxus-consumption urea is leucin produced during intestinal digestion; and this is very likely turned into urea in the liver. At any rate the liver, to which the portal vein might carry all leucin thus formed, contains urea, which no other gland does; and when the liver is greatly altered, as in phosphorus poison- ing and the disease known as acute yellow atrophy, urea almost entirely disappears from the urine. This latter fact seems to point to a final production of urea in the liver, what- ever its immediate antecedents may be; whether muscle kreatin, or intestinal leucin, or excess of peptones in the diet. The latter might perhaps be broken up there into a nitrogenous part (urea) and a non-nitrogenous part; and we shall find that a non-nitrogenous substance (glycogen) is stored in the liver. 436 THE HUMAN BODY. Proteid Starvation and Overfeeding. When an ani- mal is fed on food deficient in proteids, or containing none- of them at all, its urea excretion falls very rapidly during the first day or two, but then much more slowly until death : there is thus indicated a double source of urea, a part resulting from tissue wear and tear, and always present; and a part resulting from the breaking down of proteids not built up into tissue, and ceasing when the amount of this proteid in the Body (in the blood for example) falls below a certain limit as a result of the starvation. As the- nitrogen-starved Body wastes, its bulk of proteid tissues is slowly reduced and the urea resulting from their degrad- ation diminishes also. How well proteid built up into a tissue resists removal is shown by the facts already men- tioned (p. 432) as to the relative losses of the proteid-ricli and proteid-poor tissues in starvation. On the other hand, if an animal be taken while starving- and losing weight and have a small amount of flesh given it, it will continue to lose weight, and more urea than before will appear in the urine; increased proteid diet in- creases the proteid metamorphosis, and the animal still loses, though less rapidly than it did. A little more proteid still increases proteid metamorphosis in the body and the urea elimination, and so on for some time; but each increment of proteid in the food increases the nitrogenous metamorphosis somewhat less than the last did, untilr finally, a point is reached at which the nitrogen egesta and ingesta balance: in a dog this occurs when it gets daily 2*5- its weight of meat, and no other solid food. More food if then given is at first stored up and the animal increases in weight; but very soon the greater wear and tear of the- larger mass of tissues shows itself as increased urea ex- cretion; again the egesta and ingesta balance, and the ani- mal comes to a new weight equilibrium at the higher level. More meat now causes a repetition of the phenomenon: at first increase of tissue, and nitrogen storage; and then a cessation of the gain in weight, and an excretion in twenty- four hours of all the nitrogen taken. And so on, until th& animal refuses to eat any more. STORAGE TISSUES. 437 These facts seem, very clearly, to show that proteids can- not be built up quickly into tissues. Meat given to the starving animal has its proteids, at first, used up mainly in luxus consumption — while a little is stored as tissue, though at first not enough to counterbalance the daily tissue waste. When a good deal more proteid is given than answers to the nitrogen excretion during starvation, the animal builds up as much into living tissues as it breaks down in the vital Drocessesof these, the rest going in luxus consumption; it thus neither gains nor loses. More proteid does not all appear in the urine at once; some is used to build up new tissue, but only slowly; then, after some days, the increased metabolism of the increased flesh balances the excess of nitrogen in the diet, and equilibrium is again attained. But, all through, it seems clear that the tissue formation is slow and gradual; and so it becomes additionally probable that the increased urea excretion soon after a meal is not due to rapidly in- creased tissue formation and degradation, but to a more direct proteid oxidation. The Storage Tissues. Every healthy cell of the Body contains at any moment some little excess of material laid by in itself, above what is required for its immediate neces- sities. The capacity of contracting, and the concomitant evolution of carbon dioxide, exhibited by an excised muscle in a vacuum, seem to show that even oxygen* of which warm-blooded animals have but a small reserve, may be stored up in the living tissues in such forms that they can utilize it, even when the air-pump fails to extract any from them. But in addition to the supplies for immediate spend- ing, contained in all the cells, we find special food reserves in the Body, on which any of the tissues can call at need. These, especially the oxygen and proteid reserves, are found largely in the blood. Special oxygen storage is, however, rendered unnecessary by the fact that the Body can, except under very unusual circumstances, get more from the air at any time, so the quantity of this substance laid by is only small; hence death from asphyxia follows very rapidly when the air-passages are stopped, while, on account of the reserves laid up, death from other forms of starva- 438 THE HUMAN BODY. tion is a much slower occurrence. Proteids, also, we have learnt from the study of muscle, are probably but little con- cerned in energy-production in the tissues. Speaking broadly, the work of the Body is carried on by the oxidation of carbon and hydrogen, and we find in the Body, in corre- spondence with this fact, two great storehouses of fatty and carbohydrate foods, which serve to supply the materials for the performance of work and the maintenance of the bodily temperature in the intervals between meals, and during longer periods of starvation. One such store, that of car- bohydrate material, is found in the liver-cells; the other, or fatty reserve, is found in the adipose tissue. That such substances are true reserves, not for any special local purpose but for the use of the Body generally, is shown by the way they disappear in starvation; the liver reserve in a few days, and the fat somewhat later and more slowly, but very largely before any of the other tissues has been seriously affected. By using these accumulated matters the Body can work and keep warm during several days of more or less deficient feeding; and the fatter an animal is at the beginning of a starvation period the longer will it live; which would not be the case could not its fat be utilized by the working tissues. Hybernating animals prove the same thing; bears, before their winter sleep, are very fat, and at the end of it commonly very thin; while their muscular and nervous systems are not noticeably diminished in mass. During the whole winter, then, the energy needed to keep the heart and respiratory muscles at work, and to maintain the tempera- ture of the body, must have been obtained from the oxida- tion of the fat reserve with which the animal started. G-lycogen. It may have appeared curious to the reader that so large an organ as the liver should be sot apart for the formation of so comparatively unimportant a digestive secretion as the bile ; were this the sole use of the liver its size would certainly be hard to account for. The main function of the liver is, however, a very different one; the formation and storage of a carbohydrate called glycogen, from the abundant food materials carried through it by the portal vein after a meal ; in the times between GLYCOGEN. 439 meals this substance is then doled out gradually, arid sent round the Body in the blood. If a liver be cut up two or three hours after removal from the body of a healthy well- fed animal, and thoroughly extracted with water, it will yield much grape sugar. If. on the other hand, a perfectly fresh liver be heated rapidly to the temperature of boil- ing water, and be then pounded up and extracted, it will yield a milky solution, containing little grape sugar, but much glycogen; a substance which chemically has the same empirical formula as starch (C6H100&), and in other ways is closely allied to that body. The salivary and pancreatic secretions rapidly convert it into sugar, as they do starch; the elements of a molecule of water being taken up at the same time — CeHioOs -f H20 = Cell^Oe Glycogen. Water. Glucose. 'The same transformation is rapidly effected by ferments present in the blood and liver, and hence the first thing to be done in preparing glycogen is to heat the organ at once to a temperature high enough to destroy these ferments. Pure glycogen is a white amorphous inodorous powder, readily soluble in water, forming an opalescent milky solu- tion; insoluble in alcohol, and giving with iodine a red coloration which disappears on heating and reappears on cooling again. About four per cent of glycogen can be obtained from the liver of a well-nourished animal (dog or rabbit). This for the human liver, which weighs about 1500 grams (53 oz.), would give about 60 grams (2. 1 oz. ) of glycogen at any one moment. The quantity actually formed daily is, however, much in excess of that, since glycogen is constantly being removed from the liver and carried elsewhere, while a fresh supply is formed in the organ. Its quantity is subject, also, to considerable fluctuations; being greatest about two hours after a good meal, and falling from that time until the next digestion period commences, when it begins to rise until it again attains its maximum. If a warm-blooded ani- mal is starved glycogen disappears from its liver in the course of four or five days. Glycogen is, thus, clearly 440 THE HUMAN BODY. being constantly used up, and its maintenance in normal, quantity depends on food. The Source and Destination of Liver Glycogen. AIL foods are not equally efficacious in keeping up the stock of glycogen in the liver; fats by themselves are useless; pro- teids by themselves give a little; but by far the most is formed on a diet rich in starch and sugar; so it would seem that glycogen is mainly formed from carbohydrate materials- absorbed from the alimentary canal and carried to the- hepatic cells by the portal vein. These materials ara mainly glucose, since the starch eaten is changed into that substance before absorption. This view* of the matter is- supported by several facts. (1) Grape sugar if it exist, in the blood in above a certain small percentage passes out, by the kidneys and appears in the urine, constituting the- characteristic symptom of the disease called diabetes. In health, however, even after a meal very rich in carbohy- drates, no sugar appears in the urine; so that the large quantity of it absorbed from the alimentary canal within a* brief time under such circumstances, must be stopped somewhere before it reaches the general blood current. (2) Glucose injected into one of the general veins of an animal,, if in any quantity, soon appears in the urine; but the same- amount injected into the portal vein, or one of its radicles,, causes no diabetes, but an accumulation of glycogen in the liver. We may therefore conclude that the grape sugar absorbed from the alimentary canal is taken by the portal vein to the liver; there stayed and converted into glycogen;, which is then more slowly passed on into the hepatic veins during the intervals between meals. Thus in spite of the intervals which elapse between meals the carbo- hydrate content of the blood is kept pretty constant: dur- ing digestion it is not suffered to rise very high, nor dur- ing ordinary periods of fasting to fall very much below the average. In what form glycogen leaves the liver is not certain; it might be dissolved out and carried off as such, or previously turned again into glucose and sent on in that form; since the blood and the liver both seem to contain. GLYCOGEN. 441 ferments capable of changing glycogen into glucose the latter view is the more probable. Analyses of portal and hepatic bloods, made with the view of determining whether more sugar was carried out of the liver during fasting than into it, are conflicting. The main fact, how- ever, remains that somehow this carbohydrate reserve in the liver is steadily carried off to be used elsewhere: and animal glycogen thus answers pretty much to vegetable starch, which, made in the green leaves, is dissolved and carried away by the sap currents to distant and not green parts (as the grains of corn or tubers of a potato, which cannot make starch for themselves) and in them is again laid down in the form of solid starch grains, which are subsequently dissolved and used for the growth of the ger- minating seed or potato. Reasons have already been given, (p. 423) for believing that the carbohydrate leaving the liver is not oxidized in the blood, but first after it has passed out of that into a living tissue. Among these the muscles at least seem to get some, since a fresh muscle always contains glycogen, and even in normal amount when an animal is starved for some time; the muscle-fibres then, so to speak, calling on the balance with their banker (the liver) so long as there is any. When a muscle contracts this glycogen disappears and some glucose appears, but not an amount equivalent to the glycogen used up; so that the working muscle would appear, probably for its repair after each con- traction (see p. 431), to utilize this substance. How it is that the glycogen, which is so rapidly con- verted into grape sugar by the liver ferment after death, escapes such rapid conversion during life has not been satisfactorily answered. Two possible reasons readily sug- gest themselves; the liver ferment may be only produced by dying hepatic cells: or the glycogen in the living cell may not exist free, but combined with other portions of the cell substance so as to be protected; while, after death, post-mortem changes may rapidly liberate it in a condition to be acted upon by the ferment. Diabetes. The study of this disease throws some light upon the history of glycogen. Two distinct varieties of it 442 THE HUMAN BODY. are known; one in which sugar appears in the urine only when the patient takes carbohydrate foods; the other in which it is still excreted when he takes no such foods, and must therefore form sugar in his Body from substances not at all chemically allied to it. The most probable source of the sugar in the latter case is proteids; since some glycogen is found in the livers of animals fed on proteids only, while fats alone give none of it. In some complex way the proteid mole- cule would appear to split up in the liver into a highly nitro- genized part (urea, or an antecedent of urea) and a non- azotized part, glycogen. On this view the more severe form of diabetes would be due to an increased activity of a normal proteid-decomposing function of the hepatic cells; and sometimes the urea and sugar in the urine of diabetics rise and fall together, thus seeming to indicate a com- munity of origin. Diabetes dependent on carbohydrate food might be produced in several ways. The liver-cells might cease to stop the sugar and, letting it all pass on into the general circulation, suffer it to rise to such a percentage in the blood after a meal, that it attained the proportion in which the kidneys pass it out; or the tissues might cease to use their natural amount of sugar, and this, sent on steadily out of the liver, at last rise in the blood to the point of ex- cretion. Or the liver might transform (into glucose) and pass on its glycogen faster than the other tissues used it, -and so diabetes might arise; but this would only be tem- porary, lasting until the liver stock was used up by the rapid conversion. Artificially we can, in fact, produce diabetes in several of these ways; curari poisoning, for ex- ample, paralyzing the motor nerves, makes the skeletal muscles lie completely at rest, and so diminishes the glyco- gen consumption of the Body and produces diabetes. Car- bon monoxide poisoning produces diabetes also, presumably by checking bodily oxidation. Finally, pricking a certain spot in the medulla oUongata causes a temporary diabetes. This may be due to the fact that the operation injures that part of the vaso-motor centre which controls the mus- cular coat of the hepatic artery; this artery, then dilating, carries so much blood through the liver that an excess of FATS. 443 glycogen is turned into glucose in a given time, and carried off by the hepatic veins. If the splanchnic nerves be cut the whole arteries of the abdominal viscera dilate and no diabetes, follows, because so many vessels being dilated a great part of the blood of the Body accumulates in them, and there is no noticeably increased flow through the liver. Others, however, maintain that the "piqure " diabetes (as that due to pricking the medulla is called) is due to irritation of trophic nerve-fibres originating there, and governing the rate at which the liver-cells produce glycogen or convert it into glucose. This latter view, though perhaps the less- commonly accepted, is probably the more correct. The hepatic cells do not merely hold back glucose carried through the liver so that it is there to be washed out by a greater blood-flow, but they feed on sugar and proteids and make glycogen; and this is later converted into glucose and carried off. Glycogen is thus comparable to the zymo- gen of the pancreas and other glands (Chap. XVIII.); and the transformation of such bodies into the specific element of a secretion we have already seen to be directly under the control of the nervous system, and almost entirely or quite independent of the blood-flow. The History of Fats. While glycogen forms a reserve store of material which is subject to rapid alterations, deter- mined by meal-times, the fats are much more stable; their periods of fluctuation are regulated by days, weeks, or months of good or bad nutrition, and during starvation they are not so readily, or at least so rapidly, called upon as the hepatic glycogen. If we carry on the simile by which we compared the reserve in each cell to pocket-money (p. 31), the glycogen would answer somewhat to a balance on the right side with a man's banker ; while the fat would represent assets or securities not so rapidly realizable; as capital in business, or the cargoes afloat in the argosies of Antonio, the "Merchant of Venice." Fat, in fact, is slowly laid down in fat-cells and surrounded in these by a cell-wall, and, being itself insoluble in blood plasma or lymph, it must undergo chemical changes, which no doubt 444 THE HUMAN BODY. require some time, before it can be taken into the blood and carried off to other parts. When adipose tissue is developing it is seen that undif- ferentiated cells in the connective tissues (especially areolar) show minute oil-drops in their protoplasm. These increase in. size and, ultimately, fuse together and form one larger oil-droplet, while most of the original protoplasm dis- appears. The oily matter would thus seem due to a chemical metamorphosis of the cell protoplasm, during which it gives rise to a non-azotized fatty residue which remains behind, and a highly nitrogenous part which is carried off. In many parts of the Body protoplasmic masses are subject to a similar but less complete metamorphosis; fatty degenera- tion of the heart, for example, is a more or less extensive replacement of the proper substance of its muscular fibres by fat-droplets; and the cream of milk and the oily matter of the sebaceous secretion are due to a similar fatty degeneration in gland-cells. Moreover, careful feeding ex- periments undoubtedly show that fat can come from pro- teids; when an animal is very richly supplied with these all the nitrogen taken in them reappears in its excretions, but all the carbon does not; it is in part stored in the Body: and, since such feeding produces but little glycogen, this carbon can only be stored as fat. While there is, then, no doubt that some fat may have a proteid origin, it is not certain that all has such. During digestion a great deal of fat is ordinarily absorbed, in a chemically unchanged state, from the alimentary canal; it is merely emulsified and carried off in minute drops by the chyle to be poured into the blood: and this fat might be directly deposited, as such, in adipose tissue. There are, however, good reasons for supposing that all the fat in the Body is manufactured. The fat of a man, of a dog, and of a cat varies in the proportions of palmatin, stearin, margarin, and olein in it; and varies in just the same way if all be fed on the same kind of food, which could not be the case if the fat eaten were simply deposited unchanged. Moreover, if an animal be fed on a diet containing one kind of fat only, ORIGIN OF FATS. 445 say olein, but a very slightly increased percentage of that particular fatty substance is found in its adipose tissue, which goes to show that if fats come from fats eaten, these latter are first pulled to bits by the living cells and built up again into the forms normal to the animal; so that, even with fatty food, the fats stored up seem to be in most part manufactured in the Body. In still another way it is proved that fats can be constructed in the Body. In animals fed for slaughter, the total fat stored up in them during the process is greatly in excess •of that taken with their food during the same time. For ex- ample, a fattening pig may store up nearly five hundred parts of fat for every hundred in its food, and this fat must be m'ade from proteids or carbohydrates. Whether it <3an come from the latter is still perhaps an open question; for, while all fattening foods are rich in starch or similar bodies, there are considerable chemical difficulties in sup- posing an origin of fats from such; and it is on the whole more probable that they simply act by sparing from use .fats simultaneously formed or stored in the body, and which would have otherwise been called upon. They make glycogen, and this shelters the fats. Liebig, indeed, in a very -celebrated discussion, maintained that fats were formed irom carbohydrates. He showed that a cow gave out more butter in its milk than it received fats in its food; and Huber, the blind naturalist, showed that bees still made wax (a fatty body) for a time when fed on pure sugar; and indefinitely when fed on honey. Consequently, for a long time, an origin of fats from carbohydrates was supposed to be proved; but their possible origin from proteids (a possi- bility now shown to be a certainty) was neglected, and the Talidity of the above proofs of their carbohydrate origin is thus upset. The cow may have made its butter from proteids; the bees, fed on sugar, their wax for a time from proteids in their bodies already; and, indefinitely, when fed on honey, irom the proteids in that substance. Moreover, animals (ducks) fed on abundant rice, which contains much carbo- hydrate but very little proteid or fat, remain lean; while if some fat be added they lay up fat. 446 THE HUMAN BODY. Persons who fatten cattle for the butcher find that the foods useful for the purpose all contain proteids, carbohy- drates, and fats, and that rapid fattening is only obtained with foods containing a good deal of fat; as oilcake, milk,, or Indian corn. Taking all the facts into account we shall probably not be wrong in concluding that nearly all the bodily fat is manufactured either from fats or proteids; from fats easier than from anything else, but when much proteid is eaten some is made from it also. * Carbohydrates alone do not fatten; the animal body cannot make its pal- matin, etc. , out of them. Nevertheless they are, indirectly, important fattening foods when given with others, since, being oxidized instead of it, they protect the fat formed. Dietetics. That " one man's meat may be another man's poison" is a familiar saying, and one that, no doubt, expresses a certain amount of truth; but the difference probably depends on the varying digestive powers of indi- viduals rather than on peculiarities in their laws of cell nutrition: we all need about the same amount of proteids,. fats, and carbohydrates for each kilogram of body weight; but all of us cannot digest the same varieties of them equally well : it is also a matter of common experience that some foods have peculiar, almost poisonous, effects on certain persons. Many people are made ill by mutton, which the majority digest better than beef. The proper diet must necessarily vary, at least as to amount, with the work done; whether it should vary in kind with the nature of the Work is not so certain. Provided a man gets enough proteids to balance those lost in the wear and tear of his tissues, it probably matters little whether he gets for oxidation and the liberation of energy either fats or car- bohydrates, or even excess of proteids themselves; any one of the three will allow him to work either his brain or his muscles, and to maintain his temperature. Proteids, however, are wasteful foods for mere energy-yield ing purposes: in the first place, they are more costly than the others; secondly, they are incompletely oxidized in the Body; and, thirdly, it is probably more laborious to the system to get rid of urea than of the carbon dioxide and water, which alone are yielded by DIETETICS. 447 the oxidation of fats and carbohydrates. Between fats and carbohydrates similar considerations lead to a use of the latter when practicable: starch is more easily utilized in the Body than fats, as shown by the manner in which it protects the latter from oxidation; and a given weight of starch fully oxidized in the Body will liberate about one half as much energy as the same amount of butter, while it costs consider- ably less than half the money. Also, starch is more easily digested than fats by most persons: children especially are apt to be fond of starchy or saccharine foods and to loathe fats; and the appetite in such cases is a good guide. As a race the American people differ very markedly from the English in their love of sweet foods of all kinds; whether this is correlated with their characteristic activity, calling for some food that can be rapidly used, is an interesting question, to which, however, it would be rash to give at present an affirmative answer. It is certain that no general rules for the best dietary for all persons can be formulated, but on broad principles the best diet is that which contains just the amount of proteid necessary for tissue repair, and so much carbo- hydrates as can be well digested ; the balance needed, if any, being made up by fats. Such a food would be the cheapest; that is the supplying of it would call for less of the time and energy of the nation using it, and leave more work to spare for other pursuits than food production — for all the arts which make life agreeable and worth living, and which elevate civilized man above the merely material life of the savage whose time is devoted to catching and eat- ing. We have high authority for saying that man does not live by bread alone; in other words his highest develop- ment is impossible when he is totally absorbed in " keeping body and soul together," and the more labor that can be spared from getting enough food the better chance has he, if he use his leisure rightly, of becoming a more worthy man. While there is, thus, a theoretically best diet, it is nevertheless impossible to say what that is for each indi- vidual; but what the general experience is may be approxi- mately gathered by taking an average of the dietaries of a 448 THE HUMAN BODY. number of public institutions in which the health of many people is maintained as economically as possible. Such an examination made by Moleschott, gives us as its result a diet containing daily — Proteids 30 grams or 465 grains. Tats 84 " or 1,300 Amyloids 404 " or 6,262 " Salts 30 " or 465 " Water 2800 " or 43,400 " People in easy circumstances take as a rule more proteids and fats and less amyloids; and this selection, when a choice is possible, probably indicates that such a diet is the better one: the proteids in the above table seem especially deficient. CHAPTER XXIX. THE PEODUCTION AND KEGULATION OF THE HEAT OF THE BODY. Cold- and Warm-Blooded Animals. All animals, so long as they are alive, are the seat of chemical changes by -which heat is liberated; hence all tend to be somewhat warmer than their ordinary surroundings, though the difference may not be noticeable unless the heat production is considerable. A frog or a fish is a little hotter than the air or water in which it lives, but not much; the little heat that it pro- duces is lost, by radiation or conduction, almost at "once. Hence such animals have no proper temperature of their own; on a warm day they are warm, on a cold day cold, and are accordingly known as cliangeaUe-temperatured (poikilo-thermous) or, in ordinary language, " cold-blooded" animals. Man and other mammals, as well as birds, on the •contrary, are the seat of very active chemical changes by which much heat is produced, and so maintain a tolerably uniform temperature of their own, much as a fire does whether it be burning in a warm 'or a cold room; the heat production during any given time balancing tKe loss a nor* mal body temperature is maintained, and usually one con- siderably higher than that of the medium in which they live; such animals are therefore known as animals of con- stant temperature (liomo-tliermous), or more commonly " warm-blooded" animals. The latter name, however, does not properly express the facts; a lizard basking in the sun on a warm summer's day may be quite as hot as a man usually is; but on the cold day the lizard becomes cold, while the average temperature of the healthy Human Body 450 THE HUMAN BODY. is, within a degree, the same in winter or summer; within the arctic circle or on the equator. Moderate warmth accelerates protoplasmic activity; com- pare a frog dormant in the winter with the same animal active in the warm months: what is true of the whole frog is true of each of its living cells. Its muscles contract more rapidly when warmed, and the white corpuscles "of its blood when heated up to the temperature of the Human Body are seen (with the microscope) to exhibit much more active amoeboid movements than they do at the tempera- ture of frog's blood. In summer a frog or other cold- blooded animal uses much more oxygen and evolves much more carbon dioxide than in winter, a.s shown not only by direct measurements of its gaseous exchanges, but by the fact that in winter a frog can live a long time after its- lungs have been removed (being able to breathe sufficiently through its moist skin), while in warm weather it dies of asphyxia very soon after the same loss. The warmer weather puts its tissues in a more active state; and so the amount of work the animal does, and therefore the amount of oxygen it needs, depend to a great extent upon the tem- perature of the medium in which it is living. With the warm-blooded animal the reverse is the case. It always keeps up its temperature to that at which its tissues live best, and accordingly in cold weather uses more oxygen and sets free more carbon dioxide because it needs a more active internal combustion to compensate for its greater loss of heat to the exterior. In fact the living tissues of a man may be compared to hothouse plants, living in an artifici- ally maintained temperature; but they differ from the plants in the fact that they themselves are the seats of the combustions by which the temperature is kept up. Since, within wide limits, the Human Body retains the same tem- perature no matter whether it be in cold or warm surround- ings, it is clear that it must possess an accurate arrangement for heat regulation; either by controlling the production of heat in it, or the loss of heat from it, or both. The Temperature of the Body. The parts of the Body are all either in contact with one another directly or, if SOURCES OF BODILY HEAT. 451 not, at least indirectly through the blood, which, flowing from part to part, carries heat from warmer to colder regions. Thus, although at one time one group of muscles may especially work, liberating heat, and at other times another, or the muscles may be at rest and the glands the seat of active oxidation, the temperature of the whole Body is kept pretty much the same. The skin, however, which is in direct contact with external bodies, usually colder than itself, is cooler than the internal organs; its temperature in health is from 36° to 37° C. (96.8-98.5° F.), being warmer in more protected parts, as the hollow of the armpit. In internal organs, as the liver and brain, the temperature is higher; about 43° C. (107° F.) in health. In the lungs there is a certain quantity of heat liberated when oxygen combines with haemoglobin, but this is more than counter- balanced by loss of the heat carried out by the expired air and that used up in evaporating the water carried out in the breath, so the blood returned to the heart by the pul- monary veins is slightly colder than that carried from the right side of the heart to the lungs. The Sources of Animal Heat. These are two-fold; •direct and indirect. Heat is directly produced whenever oxidation is taking place; so that all the living tissues at rest produce heat as one result of the chemical changes sup- plying them with energy for the maintenance of their vitality; and whenever an organ is active and its chemical metamorphoses are increased it becomes hotter: a secreting gland or a contracting muscle is warmer than a resting one. Indirectly, heat is developed within the Body by the trans- formation of other forms of energy: mainly mechanical work, but also of electricity. All movements of parts of the Body which do not move it in space or move external ob- jects, are transformed into heat within it; and the energy they represent is lost in that form. Every cardiac contrac- tion sets the blood in movement, and tin's motion is for the most part turned into heat within the Body by friction with- in the blood-vessels. The same transformation of energy oc- curs with respect to the movements of the alimentary canal, except in so far as they expel matters from the Body; and 452 THE HUMAN BODY. every muscle in contracting has part of the mechanical energy expended by it turned into heat by friction against neighboring parts. Similarly the movements of cilia and of amoeboid cells are for the most part converted in the Body into heat. The muscles and nerves are also the seats of manifestations of electricity, which, though small in amount, for the most part do not leave the Body in that form but are first converted into heat. A certain amount of heat is also carried into the Body with hot foods and drinks. The Energy Lost by the Body in Twenty-four Hours, Practically speaking, the Body only loses energy in two- forms; as heat and mechanical work: by applying conduc- tors to different parts of its surface small amounts of elec- tricity can be carried off, but the amount is quite trivial in comparison with the total daily energy expenditure. Dur- ing complete rest, that is when no more work is done than that necessary for the maintenance of life, nearly all the loss takes the form of heat. The absolute amount of this will vary with the surrounding temperature and other con- ditions, but on an average a man loses, during a day of rest, 2700 calories; that is enough to raise 2700 kilograms (5940 Ibs.) of water from 0° to 1° C. (from 32° to 33.8° F.); otherwise expressed, this amount of heat would boil 27 kilos (59.4 Ibs.) of ice-cold water. This does not quite represent all the energy lost by the Body in that time: since a small proportion is lost as mechanical work in moving the clothes and air by the respiratory movements, and even by the beat of the heart, which at each systole pushes out the chest- wall a little and moves the things in contact with it. The working Body liberates and loses much .more energy; part as mechanical work done on external objects, part as increased heat radiated or conducted from the surface, or carried off by the expired air in the quickened respirations. Every one knows that he becomes warmer when he takes exercise, and measurements made on men show that the heat produced and lost in a day of moderate work is about one third greater than that in a day of rest. The follow- ing table gives more accurate numbers — BODILY ENERGY LOST PER DAY. 453 Day of Rest. Day of Work. Restl6hrs. Sleep 8 hrs. ' RestShrs. WorkShrs. SleepSlirs! \ 2470-4 32° 1235'2 2169'6 ( 10,885Fah.-lb. \ q™ a f 14.528 Fah.-lb. The mechanical work done on the working day, repre- sented in addition an expenditure of energy of 213,344 kilogrammeters, which is equal to 502 calories. Of the ex- cess heat in the working day, part is directly produced by the increased chemical changes in the quicker working heart and respiratory muscles, and the other muscles set at work; while part is indirectly due to heat arising from increased friction in the blood-vessels as the blood is driven faster around them, and to friction of the various muscles used. The average cardiac work in twenty-four hours is about 60,000 kilogrammeters ; that of the respiratory muscles about 14,000 ; and since nearly all of both is turned finally into heat within the Body, we have 74,000 kilogrammeters of energy answering to about 174 calories (6786 Fah.-lb. units) indirectly produced in tre resting Body daily from these source's. Of 100 parts of heat lost from the resting Body, about 74.7 are carried off in radiation or conduction from the skin. 14.5 are carried off in evaporation from the skin. 5.4 " " « " " " lungs. 3.6 " " " expired air. 1.8 " " « the excretions. In a day of average work, of every 100 parts of energy lost in any form from the Body — 1-2 go as heat in the excreta. 3-4 in heating the expired air. 20-30 in evaporating water from the lungs and skin. 60-75 in heat radiated or conducted from the surfaces and in external mechanical work. The Superiority of the Body as a Working Machine. During eight hours of work, we find (table at top of page) the Body loses 2169.6 calories of energy as heat; and can do 454 THE HUMAN BODY. simultaneously work equivalent to 502 calories. So of all the energy lost from it in that time about £ may take the form of mechanical work; this is a very large proportion of the total energy expended, being a much higher per- centage than that given by ordinary machines. The best steam-engines can utilize as mechanical work only about •^Q of the total energy liberated in them and lost from them in a given time; the remainder is transmitted directly as heat to the exterior, and is lost to the engine for all useful purposes. The Maintenance of an Average Temperature. This is necessary for the continuance of the life of a warm-blooded animal; should the temperature rise above certain limits chemical changes, incompatible with life, occur in the tis- sues ; for example at about 49° C. (120° F.) the muscles be- gin to become rigid. On the other hand death ensues if the Body be cooled down to about 19° C. (66° F.). Hence the need for means of getting rid of excess heat, and of protec- tion from excessive cooling. Either end may be gained in two ways; by altering the rate at which heat is lost or that at which it is produced. As regards heat-loss, by far the mo'st important regulating organ is the skin: under ordi- nary circumstances nearly 90 per cent of the total heat given off from the Body in 24 hours goes by the skin (73 by radi- ation and conduction, 14. 5 by evaporation; see above table). This loss may be controlled — 1. By clothing; we naturally wear more in cold and less in warm weather ; the effect of clothes being, of course, not to warm the Body but to diminish the rate at which the heat produced in it is lost. 2. Increased temperature of the surrounding medium in- creases the activity of the heart and lungs. A hastened circulation by itself does not as already pointed out (p. 388) increase the general tissue activity of the Body, or the oxida- tions occurring in it, and, so, apart from the harder working heart itself, does not influence the amount of heat liberated in the Body during a given time: but the more rapid blood-flow through the skin carries more of that fluid through this cool surface and increases the loss of heat in that way. The TEMPERATURE-REGULATION IN THE BODY. 455 -quickened respirations, too, increase the evaporation of water from the lungs and, thus, the loss of heat. 3. Warmth directly dilates the skin-vessels and cold con- tracts them. In a warm room the vessels on the surface dilate as shown by its redness, while in a cold atmosphere they contract and the skin becomes pale. But the more blood that flows through the skin the greater will be the heat lost from the surface — and vice versa. 4. Heat induces sweating and cold checks it ; the heat •appears to act, partly, reflexly in exciting the sweat-centres from which the secretory nerves for the sudoriparous glands arise, and, partly, directly on those centres, which are thrown into activity, at least in health, as soon as the temperature of the blood is raised. In fever of course we may have a high temperature with a dry non-sweating skin. The more sweat there is poured out, the more heat is used up in evaporating it and the more the Body is cooled. 5. Our sensations induce us to add to or diminish the heat in the Body according to circumstances; as by cold or warm baths, and iced or hot drinks. As regards, temperature-regulation by modifying the rate of heat production in the Body the following points may be noted; on the whole such regulation is far less important than that brought about by changes in the rate of loss, since the necessary vital work of the Body always necessitates the continuance of oxidative processes which liberate a tol- erably large quantity of heat. The Body cannot therefore be cooled by diminishing such oxidations ; nor on the other hand can it be safely warmed by largely increasing them. Still, within certain limits, the heat production may be con- trolled in several ways — 1. Cold increases hunger; and increased ingestion of food increases bodily oxidation as shown by the greater amount of carbon dioxide excreted in the hours succeeding a meal. This increase is probably due to the activity into which the digestive organs are thrown. 2. Cold inclines us to voluntary exercise; warmth to muscular idleness; and the more the muscles are worked the more heat is produced in the Body. 456 THE HUMAN BODY. 3. Cold tends to produce involuntary muscular move- ments, and so increased heat production; as chattering of the teeth and shivering. 4. Cold applied to the skin increases the bodily chemical metamorphoses and so heat production. At least the tem- perature in the armpit rises at first on entering a cold bath, though the heat carried off from the surface soon overbalances its increased production. The phenomenon may, however, be explained in another way, the rise being attributed to a sudden diminution of loss from more exposed parts of the skin, dependent on contraction of the cutaneous arteries. In some cases, however, the temporary rise is accompanied by an increased excretion of carbon dioxide, which would indicate that the surface cooling does really increase the oxidations of the Body. 5. Certain drugs, as salicylic acid, and perhaps quinine, diminish the heat production of the Body. Their mode of action is still obscure. On the whole, however, the direct heat-regulating me- chanisms of the Human Body itself are not very efficient, especially as protections against excessive cooling. Man needs to supplement them by the use of clothing, fuel, and exercise. Local Temperatures. Although, by the means above described, a wonderfully uniform bodily temperature is maintained, and by the circulating blood all parts are kept at nearly the same warmth, variations in both respects do occur. The arrangements for equalization are not by any means fully efficient. External parts, as the skin, the lungs (which are really external in the sense of being in contact with the air), the mouth, and the nose chambers, are always cooler than internal; and even all parts of the skin have not the same temperature, such hollows as the armpit being warmer than more exposed regions. On the other hand, a secreting gland or a working muscle becomes warmer, for the time, than the rest of the Body, because more heat is liberated in it than is carried off by the blood flowing through. In such organs the venous blood leaving is warmer than the arterial coming to them; while the reverse is the THERMIC NERVES. 457 case with parts, like the skin, in which the blood is cooled. An organ colder than the blood is of course warmed by an increase in its circulation, as seen in the local rise of tem- perature in the skin of the face in blushing. Thermic Nerves. All nerves, such as motor or secre- tory, which can throw working tissues into activity are in a certain sense thermic nerves: since they excite increased oxidation and heat production in the parts under their con- trol. A true, purely thermic nerve would be one which increased the heat production in a tissue without otherwise throwing it into activity; and whether such exist is still undecided. Certain phenomena of disease, however, seem to render their existence probable. If we return for a moment to our former comparison of the working Body to a steam-engine, such nerves might be regarded as agencies increasing its rate of rusting without setting it at work. The oxidation of the iron would develop some heat, but by processes useless to the steam-engine, although such are, in moderation, essential to living cells; the vitality of these, even when they rest, seems to necessitate a constant, if small, breaking down of their substance. In an amoeboid cell no doubfc such processes occur quite independently of the nervous system; but in more differentiated tissues they may be controlled by it. Just as a muscle does not nor- mally contract unless excited through its nerve, although a white blood corpuscle does, so may the natural nutritive processes of the muscle-fibre in its resting condition be de- pendent on the nerves going to it. If these be abnormally excited the muscle vvill break down its protoplasm faster than it constructs it, and consequently waste; at the same time the increased chemical degradation of its sub- stance will elevate its temperature. Febrile conditions, in which many tissues waste, without any unusual manifesta- tion of their normal physiological activity, would thus bo readily accounted for as due to superexcitation of the thermic nerves. Moreover, it is found that lesions or sec- tions of the spinal cord are followed by a rise in the tem- perature of those parts of the Body supplied with nerves arising below the diseased or divided portion. Now 458 THE HUMAN BODY. division of the spinal cord in two ways tends to lower the temperature of parts below the injury: in the first place, the muscles are paralyzed and so a great source of heat is cut off; and in the second, the vaso-motor nerves traveling down from the medullary centre are cut, and hence the skin arteries behind the section dilate and carry more blood to the surface to be cooled. To explain the rise of tem- perature it has therefore been concluded that there are true thermic centres in the spinal cord, which centres, like others in that organ (Chap. XXXV.), are held in check or inhibited by brain-centres; when the controlling influence of the latter is removed the former may excite excessive oxida- tions in the tissues to which they are distributed, and so produce the rise of temperature. Recent calorimetric ex- periments seem to prove that injury of certain regions of the brain is followed by greatly increased heat production in the Body: fever may be due to greater or less paralysis of these centres. Clothing. To man, as social animal, endowed with moral feelings, clothing has certain uses in the interests of morality; but for such purposes the amount necessary is not great, as we find in many tribes living in warm climates. Except in tropical regions, however, clothing has in addi- tion an important physiological use in regulating the bodily temperature. While the majority of other warm-blooded animals have coats of their own, formed of hairs or feathers, over most of man's Body his capillary coating is merely rudi- mentary and has lost all physiological importance; and so he has to protect himself by artificial garments, which his aesthetic sense has led him to utilize also for purposes of adornment. Here, however, we must confine ourselves to clothes from a physiological point of view. In civilized societies every one is required to cover most of his Body with something, and the question is what is the best covering; the answer will vary, of course, with the climatic conditions of the country dwelt in. In warm regions, clothing, in general terms, should allow free radiation or conduction of heat from the surface; in cold it should do the reverse; and in temperate climates, with varying tem- peratures, it should vary with the season. If the surface CLOTHING. 459 of the Body be exposed so that currents of air can freely traverse it much more heat will be carried oif (under those usual conditions in which the air is cooler than the skin) than if a stationary layer of air be maintained in con- tact with the surface. As every one knows, a "draught" cools much faster than air of the same temperature not in motion. All clothing, therefore, tends to keep up the temperature of the Body by checking the renewal of the layer of air in contact with it. Apart from this, however, clothes fall into two great groups; those which are good, and those which are bad, conductors of heat. The former allow changes in the external temperature to cool or heat rapidly the air stratum in actual contact with the Body, while the latter only permit these changes to act more slowly. Of the materials used for clothes, linen is a good conductor; calico not quite so good; and silk, wool, and fur are bad conductors. Whenever the surface of the Body is suddenly chilled the skin-vessels are contracted and those of internal parts reflexly dilated ; hence internal organs tend to become congested, a condition which readily passes into the diseased state known as inflammation. When hot, therefore, the most unadvisable thing to do, is to sit in a draught, thro\v oif the clothing, or in other ways to strive to get sud- denly cooled. Moreover, while in the American summer it is tolerably safe to wear good-conducting garments, and few people take cold then? this is by no means safe in the spring or autumn, when the temperature of the air is apt to vary considerably within the course of a day. A person going out, clad only for a warm morning, may have to re- turn in a very much colder evening; and if his clothes be not such as to prevent a sudden surface chill, will get oif lightly if he only " take" one of the colds so prevalent at those seasons. In the great majority of cases, no doubt, he suffers nothing worse, but many persons, especially of the female sex, often acquire far more serious diseases. When sudden changes of temperature are at all probable, even if the pre- vailing weather be warm, the trunk of the Body should be always protected by some tolerably close-fitting garment 460 THE HUMAN BODY. of non-conducting material. Those whose skins are irri- tated by anything but linen should wear immediately out- side the under-garments a jacket of silken or woolen material. In midwinter comparatively few people take cold, because all then wear thick and nonconducting cloth- ing of some kind. CHAPTER XXX. SENSATION AND SENSE-OKGANS. The Subjective Functions of the Nervous System. Changes in many parts of our Bodies are accompanied or followed by those states of consciousness which we call sen- sations. All such sensitive parts are in connection, direct or indirect, with the brain, by certain afferent nerve-fibres called sensory. Since all feeling is lost in any region of the Body when this connecting path is severed, it is clear that all sensations, whatever their primary exciting cause, are finally dependent on conditions of the central nervous sys- tem. Hitherto we have studied this as its activities are revealed through movements which it excites or prevents; we have seen it, directly or reflexly, cause muscles to con- tract, glands to secrete, or the pulsations of the heart to cease; we have viewed it objectively, as a motion-regulating apparatus. Now we have to turn to another side and con- sider it (or parts of it) as influencing the states of conscious- ness of its possessor: this study of the subjective activities of the nervous system is one of much greater difficulty. It may be objected that considerations concerning states of feeling have no proper place in a treatise on Anatomy and Physiology; that, since we cannot form the beginning of a conception how a certain state of the nervous system causes the feeling redness, another the feeling blueness, and a third the emotion anger, all examination of mental phe- nomena should be excluded from the sciences dealing with the structure and properties of living things. But, although we cannot imagine how a nervous state (neurosis) gives rise to a conscious state (psychosis], we do know this, that dis- tinct phenomena of consciousness never come under our 462 THE HUMAN BODY. observation apart from a nervous system, and so are pre- sumably, in some way, endowments of it; we are, therefore, justified in calling them properties of the nervous system; and their examination, especially with respect to what nerve-parts are concerned with different mental states, and what changes in the former are associated with given phe- nomena in the latter, forms properly a part of Physiology. W hether masses of protoplasm, before the differentiation of definite nerve-tissues, possess some ill-defined sort of con- sciousness, as they possess an indefinite contractility before they have been modified into muscular fibres, may for the present be left undecided : though those who accept the doc- trine of evolution will be inclined to assent to the proposition. While, however, the Physiologist has a right to be heard on questions relating to our mental faculties, it is never- theless true that many laws of thought have been esta- blished concerning which our present knowledge of the- laws of the nervous system gives us no clue; the science of Psychology has thus a well-founded claim to an independent existence. But, in so far as its results are confined merely to the successions and connections of mental states, as estab- lished by observation, they are merely descriptions, and not explanations in a scientific sense: we know that so many mental phenomena have necessary material antecedents and concomitants in nervous changes, that we are justified in believing that all have such, and in continuing to seek fcr them. We do not know at all how an electric current sent round a bar of soft iron makes it magnetic; we only know that the one change is accompanied by the other; bat we say we have explained the magnetism of a piece of iron if we have found an electric current circulating around it. Similarly, we do not knoAV how a nervous change causes a mental state, but we have not explained the mental state until we have found the nervous state associated with it and how that nervous state was produced. As yet it is only with respect to some of the simplest states of consciousress that we know much of the necessary physiological antecedents, and among these our sensations are the best investigated. As regards such mental pheno- SENSATION AND ORGANS OF SPECIAL SENSE. 463 mena as the Association of Ideas, and Memory, physio- logy can give us some light; but so far as others, such as the Will and the Emotions, are concerned, it has at present little to offer. The phenomena of Sensation, therefore, occupy at present a much larger portion of physiological works than all other mental facts put together. J Common Sensation and Organs of Special Sense. A sensory nerve is one which, when stimulated, arouses, or may arouse, a sensation in its possessor. The stimulant is in all cases some form of motion, molar (e.g. mechanical pressure) or molecular (as ethereal vibrations or chemical changes). Since all our nerves lie within our Bodies as circumscribed by the skin, and are excited within them, one might a priori be inclined to suppose that the cause of all sensations would appear to be within our Bodies themselves; that the ihiug feU would be a modified portion of the feeler. This is the case with regard to many sensations; a head- ache, toothache, or earache gives us no idea of any external object; it merely suggests to each of us a particular state of a sensitive portion of myself. As regards many sensations, however, this is not so; they suggest to us ex- ternal causes, to properties of which and not to states of our Bodies, we ascribe them : iiiid so they lead us to the conception of an external universe. A knife laid on the skin produces changes in it which lead us to think not of a state of our skin, but of states of some object outside the skin; we believe we feel a cold heavy hard thing in contact with it. Nevertheless we have no sensory nerves going into the knife and informing us directly of its condition; what we really feel are the modifications of our Body produced by itr although we irresistibly think of them as properties of the knife — of some object that is no part of our Body, and not as; states of the latter itself. Let now the knife cut through the skin; we feel no more knife, but experience pain, which we think of as a condition of ourselves. We do not say the knife is painful, but that our finger is, and yet we have, so far as sensation goes, as much reason to call the knife painful as cold. Applied one way it produced local changes arousing a sensation of cold, and in another local 464 THE HUMAN BODY. changes causing a sensation of pain. Kevertheless in the one case we speak of the cold as being in the knife, and in the other of the pain as being in the finger. Sensitive parts, such as the surface of the skin, through which we get, or believe we get, information about outer things, are of far more intellectual value to us than sensitive parts, such as the subcutaneous tissue into which the knife may cut, which give us only sensations referred to conditions of our Bodies. The former are called Sense-organs proper, or Organs of Special Sense; the latter are sensitive parts, or Organs of Common Sensation. The Peripheral Reference of our Sensations. The fact that we refer certain sensations to external causes is only one case of a more general law, in accordance with which we do not ascribe our sensations, as regards their lo- cality, to the brain, where the neurosis is accompanied by the sensation, but to a peripheral part. With respect to the brain, other parts of the Body are external objects as much as the rest of the material universe, yet we locate the majority of our common sensations at the places where the sensory nerves concerned are irritated, and not in the brain. Even if a nerve-trunk be stimulated in the middle of its course, we refer the resulting sensation to its outer endings. A blow on the inside of the elbow-joint, injuring the ulnar nerve, produces not only a local pain, but a sense of ting- ling ascribed to the fingers to which the ends of the fibres go. Persons with amputated limbs have feelings in their fingers and toes long after they have been lost, if the nerve- trunks in the stump be irritated. To explain such facts we must trench on the ground of Psychology, and so they can- not be fully discussed here; but they are commonly ascribed to the rggults of experience. The events of life have taught us that in the great majority of instances the sensory im- pulses which excite a given tactile sensation, for example, have acted upon the tip of a finger. The sensation goes when the finger is removed, and returns when it is replaced; and the eye confirms the contact of the external object with the finger-tip when we get the tactile sensation in question. We thus come firmly to associate a particular region of the QUALITIES OF SENSATIONS. 465 skin with a given sensation, and whenever afterwards the nerve-fibres coming from the finger are stimulated, no mat- ter where, we ascribe the origin of the sensation to some- thing acting on the finger-tip. The Differences between Sensations. In both groups of sensations, those derived through organs of special sense and those due to organs of common, sensation, we dis- tinguish kinds which are absolutely dislinct foT* our con- sciousness, and not comparable mentally. We can never get confused between a sight, a sound, and a touch, nor be- tween pain, hunger, and nausea; nor can we compare them with one another; each is sui generis. The fundamental difference which thus separates one sensation from another is its modality. Sensations of the same modality may differ; but they shadeT imperceptibly into one another, and are com- parable between themselves in two ways. First, as regards •quality; while a high and a low pitched note are both auditory sensations, they are nevertheless different and yet intelligibly comparable; and so are blue and red objects. In the second place, sensations of the same modality are distinguishable and comparable as to amount or intensity: we readily recognize and compare a loud and a weak sound of the same pitch; a bright and feeble light of the same color; an acute and a slight pain of the same general char- acter. Our^jsej^tdpj^ aspects of ^modality, quality within the same modality, and intensity. Certain sensations also differ in what is known as the '[local sign" a difference by which we tell a touch on one •part of the skin from a similar touch on another; or an ob- ject exciting one part of the eye from an object like it, but in a different location in space and exciting another part of the visual surface. As regards modality, we commonly distinguish five senses, those of sight, sound, touch, taste, and smell; to these, temperature must be added. The varieties of common sensation are also several; for example, pain, hunger, satiety, thirst, nausea, malaise, lien etre (feel- ing " good"), fatigue. The muscular sense stands on the intermediate line between specIaTand common sensations; 466 THE HUMAN BODY. we gather by it how much our various muscles are con- tracted: and so learn the position of various parts of the- Body, on the one hand, and the resistance opposed to bodily movement by external objects, on the other. In fact, we cannot draw a sharp line between the special senses and common sensations: all the Body, we conclude from ob- servations on the lower animals, is, at an early stage of its development, sensitive; very soon its cells separate them- selves into an outer layer exposed to the action of external forces and an inner layer protected from them : and some of the former cells become especially sensitive. From them, as development proceeds, some are separated and buried be- neath the surface to become the brain and spinal cord; of those which remain superficial, some are modified so that they (in the eye) become especially excited by ethereal vi- brations; others (in the ear) become especially responsive to sound vibrations; others to slight chemical changes (in mouth and nose), and others (in the skin) to variations- in pressure or temperature. All our sensations are thus modifications of one common -. primary sensibility, represented by that of the skin, or rather by the primitive representative of the skin in such an animal as the Hydra (see Zoology). The cutaneous sen- sations, being less differentiated, shade off more readily into the common sensibility of the other living tissues than do the activities of the highly differentiated cells in the eye and ear. We find, accordingly, that while a powerful pres- sure or a high temperature acting on the skin readily arouses a sensation of pain, that this is not the case with the more specialized visual and auditory organs. Their super-ex- citement maybe disagreeable, but never passes into pain, in the ordinary sense of the word. Similarly the special skin sensations, touch and temperature, may sometimes be con- founded, while a sound and a sight cannot be: the mo- dality of the less modified skin-senses is less complete. The study of comparative anatomy and development shows that the irritable parts of our sense-organs are but special dif- ferentiations of the primary external layer of cells covering the Body when it is very young. Some of these become nerve STEUCTUEE OF SENSE-OEGANS. 467 end organs in the eye, others end organs in the ear, and so on; while others, less changed, remain in the skin as organs of touch and temperature; and so, from a general exterior surface responding equally readily to many external natural forces, we get a surface modified so that its various parts respond with different degrees of readiness to different ex- ternal forces; and these modified parts constitute the essen- tial portions of our organs of special sense. Every sense organ thus comes to have a special relationship to some one natural force or form of energy — is a specially irritable mechanism by which such a force is enabled to excite sen- sory nerves; and is, moreover, commonly supplemented by arrangements which, in the ordinary circumstances of life, prevent other forces from stimulating the nerves connected with it. Not all natural forces have sense-organs with ref- erence to them developed in the Human Body; for exam- ple, we have no organ standing to electrical changes in the same relation that the eye does to light or the ear to sound. The Essential Structure of a Sense-Organ. In^every sense-organ the fundamental part is one or more end organ&r which are highly irritable tissues (p. 31), so con- structed and so placed as to be normally acted on by some one of the modes of motion met with in the external world. A sensory apparatus requires in addition at least a brain-cen- tre and a sensory nerve-fibre connecting this with the ter- minal apparatus; but one commonly finds accessory parts added. In the eye, e.g., we have arrangements for bringing to a focus the light rays which are to act on the end organs of the nerve-fibres; and in the ear are found similar subsidiary parts, to conduct sonorous vibrations to the end apparatus of the auditory nerve. Seeing and hearing are the two most specialized senses; the stimuli usually arousing them are peculiar and quite distinct from the group of general nerve stimuli (seep. 188), while those most frequently, or naturally, acting upon our other sense-organs are not so peculiar; they are forces wiiich act as general nerve stimuli when directly applied to nerve- fibres. The end organs, however, as already pointed oufr 468 THE HUMAN BODY. (p. 190), so increase the sensitiveness of the parts contain- ing them that degrees of change in the exciting forces, which would be totally unable to directly stimulate the nerve-fibres are appreciated. These terminal apparatuses are therefore as truly mechanisms enabling changes, which would not otherwise stimulate nerves, to excite them, as are the end organs in the eye or ear. The Cause of the Modality of our Sensations. Seeing that the external forces usually exciting our different sen- sations differ, and that the sensations do also, we might at first be inclined to believe that the latter difference de- pended on the former: that brightness differed from loud- ness because light was different from sound. In other words, we are apt to think that each sensation derives its specific character from some property of its external physi- cal antecedent, and that our sensations answer in some way to, and represent more or less accurately, properties of the forms of energy arousing them. It is, however, quite easy to show that we have no sufficient logical warrant for such a belief. Light falling into the eye causes a sensation of luminosity, a feeling belonging to the visual group or modality; and, since usually nothing else excites such feelings and light entering the healthy eye always does, we come to believe that the physical agent light is some- thing like our sensation of luminosity. But, as we have already seen (p. 191), no matter how we stimulate the optic nerve we still get visual sensations; close the eyes and press with a finger-nail on one eyelid; a sensation of touch is aroused where the finger meets the skin; but the pressure on the eyeball distorts it and stimulates the optic nerve- fibres in it also, and the result is a luminous patch seen in front of the eye in such a position as a bright body must occupy in space to radiate light to that part of the expan- sion of the optic nerve. Finding, then, the same kind of sensation, a visual one, produced by the totally different causes, pressure and light, we are led to doubt if the dif- ferences of modality in our sensations depend upon the dif- ferences of the natural forces arousing them; and this doubt is strengthened when we find still other forces (p. 191} WHY OUR SENSATIONS DIFFER, 469 giving rise to visual sensations. But then, since light and pressure, electricity and cutting, all cause visual sensations, we have no valid reason for supposing that light, more than either of the others, is really in any way like our sensation of light: or that sight-feeling differs from sound-feeling because objectively light differs from sound. The eye is an organ specially set apart to be excited by light, and accord- ingly so fixed as to have its nerve-fibres far more often ex- cited by that form of force than by any other; but the fact that light sensations can be otherwise aroused shows plainly that their kind or character has nothing directly to do with any property of light. Just as by pinching or heating or galvanizing a motor nerve we can make the muscles attached to it contract, and the contraction has nothing in common with the excitant, so the visual sftr»sa.i-,iQnJ as such, is inde- pendent of the stimulus arousing it and, of itself, tells us nothing concerning the kind of stimulus which has operated. Differences in kind between external forces being thus eliminated as possible causes of the modalities of our sen- sations, ^^'Q next naturally fall back upon differences in the sense-organs themselves. They do undoubtedly differ both in gross and microscopic structure, and the fact that pres- sure on the closed eye arouses a touch-feeling where the skin is compressed, and a sight-feeling where the optic nerve is, might well be due to the fact that a peripheral touch- organ was different from a peripheral sight-organ, and the same force might therefore produce totally different effects on them and so cause different kinds of feelings. However, here also closer examination shows that we must seek far- ther. Sensation is not produced in a sense-organ, but far away from it in the brain; the organ is merely an apparatus for generating nervous impulses. If the optic nerves be divided, no matter how perfect the eyeballs, no amount of light will arouse visual sensations; if the spinal cord be cut in the middle of the back no pressure on the feet will cause a tactile or other feeling; though the skin, and its nerves and the lower half of the spinal cord be all intact. In all cases we find that if the nerve-paths between a sense-organ 470 THE HUMAN BODY. and the brain be severed no stimulation of the organ will call forth a sensation. The final production of this clearly depends, then, on something occurring in the brain, and so the kind of a sensation is presumably dependent upon brain events rather than on occurrences in sense-organs. Still it might be that something in the sense-organ caused one sen- sation to differ from another. Each organ might excite the brain in a different way and cause a different sensation, and so our sensations differ because our sense-organs did. Such a view is, however, negatived by observations which show that perfectly characteristic sensations can be felt in the absence of the sense-organs through which they are normally ex- cited. Persons whose eyeballs have been removed by the surgeon, or completely destroyed by disease, have frequently afterwards definite and unmistakable visual sensations, quite as characteristic as those which they had while still possess- ing the visual end organs. The tactile sensations felt in am- putated limbs, referred to on p. 464, afford another example of the same fact. The persons still feel things touching their legs or lying between their long-lost toes; and the sen- sations are distinctly tactile and not in any way less different from visual or auditory sensations than are the touch-feel- ings following stimulation of those parts of the skin which are still possessed. It is, then, clear_thai.the modality of our sensations is to be sought deeper than^ injjroperties of the end oi^aiiS"rrHiienn^'v^s~oTeach~ sense. Properties ""61 external: -forces-a-nd properties of periphe- ral nerve-organs being excluded as causes of differences in kind of sensation, we come next to the sensory nerve-fibres themselves. Is it because optic nerve-fibres are different from auditory nerve-fibres that luminous sensations are different from sonorous? This question must be answered in the negative, for we have already (p. 193) seen reason to believe that all nerve-fibres are alike in essential structure and that their properties are everywhere the same; thatVaJL they do is to transmit "nervous impulses" when j?xci ted, andTHat, no matter" What the excitant, these impulses are molecular movements, always alike in kind, though they may differ in amount and in rate of succession. Since, WHY OUR SENSATIONS DIFFER. 471 then, all that the optic nerve does is to send nervous impulses to the brain, and all that the auditory and gustatory and touch and olfactory nerve-fibres do is the same, and these impulses are all alike in kind, we cannot explain the differ- ence in quality of visual and other sensations by any dif- ferences in property of the nerve-trunks concerned, any more than we could attempt to explain the facts that, in one case, an electric current sent through a thin platinum wire heats it, and, in another, sent through a solution of a salt decom- poses it, by assuming that the different results depend on differences in the conducting copper wires, which may be absolutely alike in the two cases. We are thus driven to conclude that our sensations primarily differ because different central nerve-organs in the brain aro concerned in their production. That just as an efferent nerve-fibre will, when stimulated, cause a secre- tion if it go to a gland-cell, and a contraction if it go to a muscle-fibre, so an optic nerve-fibre, carrying impulses to one brain apparatus and exciting it, will cause a visual sensation, and a gustatory nerve-fibre, connected with another brain-centre, a taste sensation. In other words, our kinds of sensation depend fundamentally on the proper- ties of our own cerebral nervous system. For each special sense we have a nervous apparatus with its peripheral terminal organs, nerve-fibres, and brain-centres; and the excitement of this apparatus, no matter in what way, causes a sensation of a given modality, determined by the proper- ties of its central portion. Usually the apparatus is excited by one particular force acting first on its peripheral organs, but it may be aroused by stimulating its nerve-fibres directly or, as in certain diseased states (delirium), or under the action of certain drugs, by direct excitation of the centres. The sensations of dreams, frequently so vivid, and halluci- nations, are also probably in many cases due to direct excitation of the central organs of sensory apparatuses, though no doubt also often due to peripheral stimulation. But no matter how or where the apparatus is excited, pro- vided a sensation is produced it is always of the modality of that sense apparatus. 472 THE HUMAN BODY. While in the more specialized senses the modality of the sensation can be ascribed only to brain properties (so that we may be pretty sure that a man, the inner end of whose optic nerve was in physiological continuity with the outer end of his auditory, and the inner end of his auditory with the outer end of his optic, might hear a picture and see a symphony), yet, conceivably, differences in the rhythm or intensity of afferent nervous impulses might cause differ- ences in modality in less differentiated senses. Until quite recently it has been considered possible that tactile and temperature sensations were but extremes of one gene- ral kind of feeling; that they were of the same "modal- ity;" and comparable, for example, to the sensations of yellow and blue in the visual set of feelings. This view has now been definitely proved to be inadmissible (p. 563). The points of the skin which arouse in us the sensations of touch, heat, and cold are all distinct; each one when stimulated gives rise to only one kind of sensation, if any; and always the same kind. A heavy pressure, gradually increased, arouses sensations which pass imperceptibly from touch to pain, and this result may be due to the fact that regular and orderly afferent impulses, determined through tactile nerve-endings, excite the centre in one way; while irregular, disorderly, and violent impulses, originated when the pressure is great enough to directly excite nerve-trunks beneath the skin, may cause a different sensation; much as musical notes properly combined may cause pleasure, but if all clashed together may cause pain, although the same brain-centres are stimulated in the two cases. The pain from a heavy weight may, however, be merely due to the fact that it excites the nerves very powerfully and gives rise to impulses which radiate farther in the brain than those causing touch sensations, and so excite new centres, the modality of which is a pain sensa- tion. However differences in nervous rhythm may account for minor differences in sensation, it remains clear that the characters of our sensations are creations of our own organ- FECHNER'S LAW. 473 ism; they depend on properties of our Bodies and not on properties of external things, except in so far as these may or may not be adapted to arouse our different sensory apparatuses to activity. From the kind of the sensation we cannot, therefore, argue as to the nature of the excitant: we have no more warrant for supposing that light is like our sensation of light than that the knife that cuts us is like our sensation of pain. All that we know with certainty is states of our own consciousness, and although from these we form working hypotheses as to an external universe, yet, granting it, we have no means of acquiring any real knowl- edge as to the properties of things about us. What we want to know, however, for the practical purposes of life is, not what things are, bat how to use them for our advan- tage, or to prevent them from acting to our disadvantage; and our senses enable us to do this sufficiently well. The Psycho-Physical Law. Although our sensations are,, in modality or kind, independent of the force exciting them,, they are not so in degree or intensity, at least within cer- tain limits. We cannot measure the amount of a sensation and express it in foot-pounds or calories, but we can get a sort of unit by determining how small a difference iii sen- sation can be perceived. Supposing this smallest perceptible diffenmce to be constant within the range of the same sense, (which is not proved,) it is found that it is produced by dif- ferent amounts of stimuli, measured objectively as forces; and that there exists in some cases a relation between the two- which can be expressed in numbers. 'jTJie increase of stim- ulus necessary to produce the smallest perceptible change in a_sensation is proportional to the strength of the stimulus already acting; for example, the heavier a pressure already acting on iW skin the more must it be increased or dimin- ished in order that the increase or diminution may be felt. Expressed in another way the facts may be put thus: sup- pose three degrees of stimulation to bear to one another ob- jectively the ratios 10, 100, 1000, then their subjective ef- fects, or the amounts of sensation aroused by them, will be respectively as 1, 2, 3; in other words,, the sensation in- 474 THE HUMAN BODY. creases proportionately to the logarithm of the strength of the stimulus. Examples of this, which is known as " Weber's " or "Fechner's psycho-physical " laiv will be hereafter pointed out, and are readily observable in daily life; we have, for example, a luminous sensation of certain intensity when a lighted candle is brought into a darkroom; this sen- sation is not doubled when a second candle is brought in; and is hardly affected at all by a third. The law is only true, however (and then but approximately), for sensations of medium intensity; it is applicable, for example, to light sensations of all degrees between those aroused by the light of a candle and ordinary clear daylight: but it is not true for luminosities so feeble as only to be seen at all with diffi- culty, or so bright as to be dazzling. Besides their variations in intensity, dependent on varia- tions in the strength of the stimulus, our sensations also vary with the irritability of the sensory apparatus itself; "which, is not constant from time to time or from person to person. In the above statements the condition of the sense- organ and its nervous connections is presumed to remain the same throughout. Perceptions. In every sensation we have to carefully distinguish between the pure sensation and certain judg- ments founded upon it; weJiave to distinguish between what we really feel and what we think we feel; and very often firmly believe we do feel when we do not. The most important of these judgments is that which leads us to ascribe certain sensations, those aroused through organs of special sense, to external objects — that outer reference of our sensations which leads us to form ideas €oncerning the existence, form, position, and properties of external things. Such representations as these, founded on our senses, are called perceptions. Since these _p.1wq.yp imply some ^^ntal_aj3tivity_in_MdItion to a mere feeling, their full discussion belongs to the domain of Psychology. Phy- siology, however, is concerned with them so far as it can •determine the conditions of stimulation and neurosis under which a given mental representation concerning a sensation is made. It is quite certain that we can feel PERCEPTIONS. 475 nothing but states of ourselves, but, as already pointed out, we have no hesitation in saying we feel a hard or a cold, a rough or smooth body. When we look at a distant object we usually make no demur to saying that we perceive it. What we really feel is, however, the change produced by it in our eyes. There are no parts of our Bodies reaching to a tree or a house a mile off — and yet we seem to feel all the while that we are looking at the tree or the house and feeling them, and not merely experiencing modifications of our own eyes or brains. When reading we feel that what we really see is the book; and yet the existence of the book is a judgment founded on a state of our Body, which alone is what we truly feel. We have the same experience in other cases, for example with regard to touch. Hairs are quite insensible, but are imbedded in the sen- sitive skin, which is excited when they are moved. But if the tip of a hair be touched by some external object we be- lieve we feel the contact at its insensible end, and not in the sensitive skin at its root. So, the hard parts of the teeth are insensible; yet when we rub them together we refer the seat of the sensation aroused to the points where they touch one another, and not to the sensitive parts around the sockets where the sensory nerve impulse is really started. Still more, we may refer tactile sensations, not merely to the distal ends of insensible bodies implanted in the skin, but to the far ends of things which are not parts of our Bodies at all; for instance, the distant end of a rod held between the finger and a table. We then believe we feel touch or pressure in two places; one where the rod touches our finger, and the other where it comes in contact with the table. We have, simultaneously, sensations at two places separated by the length of the rod. If we hold the rod immovably on the table we feel only its end next the fin- ger. If we could fix it immovably on the finger while the other end was movable on the table, we would lose the sen- sation at the finger and only believe we felt the pressure where the rod touched the table. When a tooth is touched with a rod we only feel the contact at its end, unless it is 476 THE HUMAN BOD Y. loose in its socket; and then we get two sensations on touching its free end with a foreign body. This irresistible mental tendency to refer certain of our states of feeling to causes outside of our Bodies, and either in contact with them or separated from them by a certain space, is known as the phenomenon of the extrinsic refer- ence of our sensations. The discussion of its origin belongs properly to Psychol- ogy, and it will suffice here to point out that it seems largely to depend on the fact that the sensations extrinsically referred can be modified by movements of our Bodies. Hunger, thirst, and toothache all remain the same whether we turn to the right or left, or move away from the place we are standing in. But a sound is altered. AVe may find that in a certain position of the head it is heard more by the right ear than the left; but on turning 'round the reverse is the case; and half way round the loudness in each ear is the same. Hence we are led, by mental laws outside of the physiological domain, to suspect that its cause is not in our Body, but outside of it; and depends not on a condition of the Body but on something else. And this is confirmed when going in one direction we find the sound increased, and in the other that it is diminished. This implies that we have a knowledge of our movements, and this we gain through the ^muscular sense. It constitutes the reactive side of our sensory life, associated with the changes we prodtrce in external things; and is correlated and contrasted with ttej^sive~si(Ierin which other things produce sensations by acting upon us. As regards our common sensations we find something of the same kind. The more readily they can be modified by movement the more definitely do we localize them in space, though in this case within the Body instead of outside it. Hunger and nausea can be altered by pressure on the pit of the stomach; thirst by moistening the throat with water; the desire for oxygen (respiration-hunger) by movements of the chest; and so we more or less definitely ascribe these sensations to conditions of those parts of the Body. Other general sensations, as depression, anxiety, and so on, are not SEtfSOBT ILLUSIONS. 47? modifiable by any particular movement, and so appear to us rather as mental states, pure and simple, than bodily sensations. Sensory Illusions. "I must believe my own eyes" and "we can't always believe our senses" are two expres- sions frequently heard, and. each expressing a truth. No doubt a sensation in itself is an absolute incontrovertible fact: if I feel redness or hotness I do feel it and that is an end of the matter: but if I go beyond the fact of my having a certain sensation and conclude from it as to properties of something else — if I form a judgment from my sensation — I may be totally wrong; and in so far be unable to believe my eyes or skin. Such judgments are almost inextricably woven up with many of our sensations, and so closely that we cannot readily separate the two; not even when we know that the judgment is erroneous. For example, the moon when rising or setting, appears bigger than when high in the heavens — we seem to feel directly that it arouses more sensation, and yet we know certainly that it does not. With a body of a given brightness the amount of change produced in the end organs of the eye will depend on the size of the image formed in the eye, provided the same part of its sensory surface is acted upon. Now the size of this image depends on the distance of the object; it is smaller the farther off it is and bigger the nearer, and measurements show that the area of the sensitive surface affected by the image of the rising moon is no bigger than that affected by it when overhead. Why then do we, even after we know this, see it bigger ? The reason is that when the moon is near the horizon we imagine, unconsciously and irresistibly, that it is farther off; even astronomerswho know perfectly that it is not, cannot help forming this unconscious and erroneous judgment — and to them the moon appears in consequence larger when near the horizon, just as it does to less well-informed mortals. In fact we have a conception of the sky over which the moon trav- els, not as a half sphere but as somewhat flattened, and hence when the moon is at the horizon we unconsciously judge that it is farther off than when overhead. * But any 478 THE HUMAN BODY. body which excites the same extent of the sensitive surface of the eye at a great distance that another does at less, must be larger than the latter; and so we conclude that the moon at the horizon is larger than the moon in the zenith, and are ready to declare that we see it so. So, again, a small bit of light gray paper on a white sheet looks gray: but placed on a large bright green surface it looks purple; and on a bright red surface looks blue-green. As the same bit of gray paper is shifted from one to the other we see it change its color: it arouses in us different feelings, or feelings which we interpret differently, although objectively the light reflected from it remains the same. Similarly a medium-sized man alongside of a very tall one appears short, but when Avalking with a very short one, tall. Such erroneous perceptions as these are known as sensory illusions; and we ought to be constantly on guard against them. CHAPTER XXXI. THE EYE AS AN OPTICAL INSTKUMENT. The Essential Structure of an Eye. Every visual organ consists primarily of a nervous expansion, provided with end organs by means of which light is enabled to excite nervous impulses, and exposed to the access of objective light; such an expansion is called a retina. By itself, however, a retina would give no visual sensations referable to distinctly limited external objects; it would enable its possessor to tell light from darkness, more light from less light, and (at least in its highly developed forms) light of one color from light of another color; but that would be all. Were our eyes merely retinas we could only tell a printed page from a blank one by the fact that, being partly covered with black letters, (which reflect less light,) it would excite our visual organ less powerfully than the spotless white page would. In order that distinct objects and not merely degrees of luminosity may be seen, some arrangement is needed which shall bring all light entering the eye from one point of a luminous surface to a focus again on one point of the sensitive surface. If A and B (Fig. 121) be two red spots on a black surface, K, and rr be a retina, then rays of light diverging from A would fall equally on all parts of the retina and excite it all a little; so with rays starting from B. The sensation aroused, supposing the retina in connection with the rest of the nervous visual apparatus, would be one of a certain amount of red light reaching the eye; the red spots, as definite objects, would be indistinguishable. If, however, a convex glass lens L (Fig. 122) be put in front of the retina, it will cause to converge again to a single point all the rays from A falling 480 THE HUMAN BODY. upon it; so, too, with the rays from B: and if the focal dis- tance (see Physics) of the lens be properly adjusted these points of convergence will both lie on the retina, that for FIG. 121.— Diagram illustrating the indistinctness of vision with a retina alone. K, a surface on which are two spots, A and B; r r, the retina. The diverging lines represent rays of light spread uniformly over the retina from each spot. rays from A at a, and that for rays from B at ~b. The sensitive surface would then only be excited at two limited and 'separated points by the red light emanating from the spots; consequently only some of its end organs and nerve- FIG. 122.— Illustrating the use of a lens in giving definite retinal images. A, B, K, r r, as in Fig. 121. L, a biconvex lens so placed that it brings to a focus on the points a and 6 of the retina, rays of light diverging from A and B re- spectively. fibres would be stimulated and the result would be the recognition of two separate red objects. In our eyes are found certain refracting media which lie in front of the retina and take the place of the lens L in Fig 122. That portion of physiology which treats of the physical action of these media, or in other words of the eye as an optical in> strument, is known as the dioptrics of the eye. The Appendages of the Eye. The eyeball itself con- sists of the retina and refracting media, together with supporting and nutritive structures and other accessory THE EYELID8. 481 apparatuses, as, for example, some controlling the light- converging power of the media, and others regulatiflgjLhe size of the aperture (pupil) b^ which jight enters. Out- sl3F!nTl)aTniriim?cle?which bring about its movements, and other parts serving to protect it. Each orbit is a pyramidal cavity occupied by connective tissue, muscles, blood-vessels and nerves, and in great part by fat, which forms a soft cushion on which the back of the eyeball lies and rolls during its movements. The contents of the orbit being for the most part incompressible, the eye cannot be drawn into its socket. It simply rotates there, as the head of the femur does in the acetabulum. When the orbital blood-vessels are gorged, however, the eyeball may protrude (as in strangulation); and when these ves- sels empty it recedes somewhat, as is commonly seen after death. The front of the eye is exposed for the purpose of allowing light to reach it, but can be covered up by the eyelids, which are folds of integument, movable by muscles and strengthened by plates of fibro-cartilage. At the edge of each eyelid the skin which covers its outside is turned in, and becomes continuous with a mucous membrane, the conjunctiva, which lines the inside of each lid, and also covers all the front of the eyeball as a closely adherent layer. The upper eyelid is larger and more mobile than the lower, and when the eye is closed covers all its transparent part. It has a special muscle to raise it, the levator palpe- brce superioris. The eyes are closed by a flat circular mus- cle, the orbicularis palpebrarum, which, lying on and around the lids, immediately beneath the skin, surrounds the aper- ture between them. At their outer and inner angles (can- thi) the eyelids are united, and the apparent size of the eye depends upon the interval between the canthi, the eyeball itself being nearly of the same size in all persons. Near the inner canthus the line of the edge of each eyelid changes its direction and becomes more horizontal. At this point is found a small eminence, the lachrymal papilla, on each lid. For most of their extent the inner surfaces of the eyelids are in contact with the outside of the eye- 482 THE HUMAN BODY. ball but, near their inner ends, a red vertical fold of con- junctiva, the semilunar fold (plica semilunar is) intervenes. This is a remnant of the third eyelid, or nictitating membrane, found largely developed in many animals, as birds, in which it can be drawn all over the exposed part of the eyeball. Quite in the inner corner is a reddish ele- vation, the caruncula lachrymalis, caused by a collection of sebaceous glands imbedded in the semilunar fold, Opening along the edge of each eyelid are from twenty to thirty minute compound sebaceous glands, called the Meibomian follicles. Their secretion is sometimes abnormally abun- dant, and then appears as a yellowish matter along the edges of the eyelids, which often dries in the night and causes the lids to be glued together in the morning. The eyelashes are short curved hairs, arranged in one or two- rows along each lid where the skin joins the conjunctiva. The Lachrymal Apparatus consists of the tear-gland in each orbit, the ducts which carry its secretion to the upper eyelid, and the canals by which this, unless when excessive, is carried off from the front of the eye without running down over the face. The lachrymal or tear gland, about the size of an almond, lies in the upper and outer part of the orbit, near the front end. It is a compound racemose gland, from which twelve or fourteen ducts run and open in a row at the outer corner of the upper eyelid. The se- cretion there poured out is spread evenly over the exposed part of the eye by the movements of winking, and keeps it moist; finally it is drained off by two lachrymal canals, one of which opens by a small pore (punctum lachrymalis) on each lachrymal papilla. The aperture of the lower canal can be readily seen by examining the corresponding papilla in front of a looking-glass. The canals run inwards and open into the lachrymal sac, which lies just outside the nose, in a hollow where the lachrymal and superior maxillary bones (L and MX, Fig. 26)* meet. From the sac the nasal duct proceeds to open into the nose-chamber below the in- ferior turbinate bone (q, Fig. 89, p. 309). Tears are constantly being secreted, but ordinarily in *Page 74 MUSCLES OF THE EYEBALL. 483 such quantity as to be drained off into the nose, from which they flow into the pharynx and are swallowed. When the lachrymal ducts are stopped up, however, their continual presence makes itself unpleasantly felt, and may need the aid of a surgeon to clear the passage. In weeping the secretion is increased, and then not only more of it en- ters the nose, but some flows down the cheeks. The fre- quent swallowing movements of a crying child, sometimes spoken of as "gulping down his passion," are due to the need of swallowing the extra tears which reach the pharynx. FIG. 123.— The eyeballs and their muscles as seen when the roof of the orbit has been removed and the fat in the cavity has been partly cleared away. On the right side the superior rectus muscle has been cut away, a, external rec- tus; s, superior rectus; i, internal rectus; t, superior oblique. The Muscles of the Eye (Fig. 123). The eyeball is spheroidal in f orm and attached behind to the optic nerve, n, somewhat as a cherry might be to a thick stalk. On its exte- rior are inserted thelejidons of six muscles, four straight and i^Q_oblique. The straigh t muscles lie, one (superior rectus), s, above, one (inferior rectus} below, one (external rectus), OijQi3aA£A&&r®^ the eyeball. Each arises behind from the bony margin of the foramen through which the optic nerve enters the orbit. In the figure, 484 TEE HUMAN BODY. which represents the orbits opened from above, the superior rectus of the right side has been removed. The superior oblique or pulley (trochlear) muscle, t, arises behind near the straight muscles and forms anteriorly a tendon, •M, which passes through a fibre-cartilaginous ring, or pulley, placed at the notch in the frontal bone where it bounds superiorly the front end of the orbit. The tendon then turns back and is inserted into the eyeball between the upper and outer recti muscjes. The inferior oblique muscle does not arise, like the rest, at the back of the orbit, but near its front at the inner side, close to the lachrymal sac. It passes thence outwards and backwards beneath the eyeball to be inserted into its outer and posterior part. The inner, upper, and lower straight muscles, the inferior oblique, and the elevator of the upper lid are supplied by branches of, the third cranial nerve (see p. 168). The sixth cranial nerve goes to the outer rectus; and the fourth to the superior oblique. The eye may be moved from side to side; up or down; obliquely, that is neither truly vertically nor horizontally, but partly both; or, finally, it may be rotated on its antero- posterior axis. The oblique movements are always accom- panied by a slight amount of rotation. When the glance is turned to the left, the left external rectus and the right in- ternal contract, and vice versa; when up, both superior recti; when down, both the inferior. The superior oblique muscle- acting alone will roll the front of the eye downwards and outwards with a certain amount of rotation; the inferior oblique does the reverse. In oblique movements two of the recti are concerned, an upper or lower with an inner or outer; at the same time one of the oblique also always contracts. Movements of :; rotation rarely, if ever, occur alone. The natural combined movements of the eyes by which both are directed simultaneously towards the same point depends on the accurate adjustment of all its nervo-muscu- lar apparatus. When the co-ordination is deficient the person is said to squint. A left external squint would be caused by paralysis of the inner rectus of that eye, for then, ANATOMY OF EYEBALL. 485 after the eyeball had been turned out by the external rec- tus, it would not be brought back again to its median position. A left internal squint would be caused, similarly, by paralysis of the left external rectue; and probably by disease of the sixth cranial nerve or its brain-centres. Dropping of the upper eyelid (ptosis) indicates paralysis of FIG. 124.— The left eyeball in horizontal section from before back. 1, 2. junction of sclerotic and cornea; 3, cornea; 4, 5, conjunctiva: sclerotic; posterior elastic layer of cornea; 7, ciliary muscle; 10, choroid; 11,13 ciliary processes; 14, iris; 15, retina; 16, optic nerve; 17, artery entering retina m optic nerve; 18, fovea centralis; 19, region where sensory part of retina ends; 22, suspensory ligament ; 23 is placed in the canal of Petit and the line from 25 points to it ; 24, the anterior part of the hyaloid membrane; 26, 27, 28 are placed on the lens; 28 points to the line of attachment around it of the suspensory ligament ; 29, vitreous humor; 30, anterior chamber of aqueous humor; 31, posterior chamber of aqueous humor. its elevator muscle (p. 481), and is often a serious symptom, as pointing to disease of the brain-parts from which it is innervated. The Globo of the Eye is on the whole spheroidal, but consists of segments of two spheres (see Fig. 124), a portion of a sphere of smaller radius forming its anterior transparent part and being set on to the front of its posterior segment, which is part of a larger sphere. From before back it 486 THE HUMAN BODY. measures about 22.5 millimeters (^ inch), and from side to side about 25 millimeters (1 inch). Except when looking at nea/ objects, the antero-posterior axes of the eyeballs are nearly parallel, but the optic nerves diverge considerably (Fig. 123); each joins its eyeball, not at the centre, but about 2.5 mm. (TV inch) on the nasal side of the posterior end of its antero-posterior axis. In general terms the eyeball may be described as consisting of three coats and three refract- /- ing media. f\J The outer coat^ I and 3. Fig. 124, consists of the sclerotic I Vand the cornea, the latter being transparent and situated in front; the former is opaque and white and covers the back and sides of the globe and part of the front, where it is seen between the eyelids as the white of the eye. Both are tough and strong, being composed of dense connective tis- sue. The white of the eye and the cornea are also covered over by a thin layer of the conjunctiva, 4 and 5. Behind the proper connective-tissue layer, 3, of the cornea is a thin structureless membrane, G, lined inside by a single layer of epithelial cells; it is called the membrane of Descemet, or posterior elastic layer. I ty 7 The second coat consists of the choroid, 9, 10, the ciliary \^/ processes, 11, 13, and the iris. 14. The choroid consists mainly of blood-vessels supported by loose connective tissue containing numerous corpuscles^vTrich in its inner layers are richly filled with dark brown or black pigment granules. Towards the front of the eyeball, where it begins to dimin- ish in diameter, the choroid is thrown into plaits, the ciliary processes, 11, 12, 13. Beyond these it continues as the_mSj_ which forms the colored part of the eye which is ..seen through the cornea; and in the centre of this is. a circular X?^ aperture, t\\Q pupil: so the second coat does not, like the outer one, completely envelop the ball. In the iris are two sets of plain muscular fibres; a circular around the margin of the pupil and narrowing it when they contract ; the other set radiate from the inner to the outer margin of the iris and by their contraction dilate the pupil. The pigment in the iris is yellow, or of lighter or darker brown, according to the color of the eye, and more or less HISTOLOGY OF RETINA. 487 abundant according as the eye is black, brown, or gray. In blue eyes the pigment is confined to the deeper layers and modified in tint by light absorption in the anterior color- .. less strata through which the light passes. ( ~} ) The third coat of the eye, the retina, 15, is its essential "^portion, being the part in winch the light produces those changes that give rise to impulses in the optic nerve. It is a still less complete envelope than the second tunic, ex- tending forwards only as far as- the commencement of the •ciliary processes, at least in its typical form. It is extremely .soft and delicate and, when fresh, transparent. Usually when an eye is opened it looks colorless; but by taking proper precautions the natural purple color of some of its outer layers can be seen. Its most external layer, more- over, is composed of black pigment cells. On its inner surface two parts, different from the rest, can be seen in a fresh eye. One-is the point of entry of the optic nerve, 16, the fibres of which, penetrating the sclerotic and choroidj spread out in the retina. At this place the retina is whiter than elsewhere and presents an elevation, the •optic mound. . The other peculiar region is the yellow spot (macula luted], 18, which lies nearly at the posterior end of the axis of the eyeball and therefore outside the optic mound; in its centre the retina is thinner than elsewhere and so a ~pit(fQveaccntrahs), 18, is formed. This appears black, the thinned retina there allowing the choroid to be seen through it more clearly than elsewhere. In Fig. 125 is represented the left retina as seen from the front, the elliptical darker patch about the centre being the yellow spot, and the 'white circle on one side, the optic mound. The vessels of the retina arise from an artery (17, Fig. 124) Avhich runs in with the optic nerve and from which branches diverge as shown in Fig. 125. The Microscopic Structure of the Retina. A simplified stratum, continuous with the proper retina, and formed of a layer of nucleated columnar cells is continued over the ciliary processes: elsewhere the membrane has a very com- plex structure and a section taken, except at the yellow spot or the optic mound, shows ten layers, partly sensory 488 THE HUMAN BODY apparatuses and nerve-tissues, and partly accessory struc- tures. Beginning (Fig. 126) on the front side we find, first, the- internal limiting membrane, 1, a thin structureless layer. Next comes the nerve-fibre layer, 2, formed by radiating fibres of the optic nerve; third, the nerve-cell layer > 3; FIG. 125.— The right retina as it would be seen if the front part of the eyeball1 with the lens and vitreous humor were removed. fourth, the inner molecular layer, 4, consisting partly of very fine nerve-fibrils, and largely of connective tissue;, fifth, the inner granular layer, 5, composed of nucleated cells, with a small amount of protoplasm at each end, and a nucleolus. These granules, or at any rate the majority of them, have an inner process running to the inner molecular layer and an outer running to, 6, the outer molecular layer, which is thinner than the inner. Then comes, seventh, the rod^jand cow fibx&Jayer, 7, or outer granular layer; composed of thick and thin fibres on each of which is a conspicuous nucleus witli a nucleolus. Next is the thin external limiting membrane, 8, perforated by apertures through which the rods and cones, 9, of the ninth layer join the fibres of the seventh. Outside of all, next the choroid, is the pigmentary layer, 10. In addition, cer- HISTOLOGY OF RETINA. 489 tain fibres run vertically through the retina from the inner to the outer limiting membrane; they are known as the radial fibres of Midler and give off lateral branches, which are especially numerous in the molecular layers. FIG. 126.— A section through the retina from its anterior or inner surfacer 1, in contact with the hyaloid memb'ane, to its outer, 10, in contact with the choroid. 1, internal limiting membrane; 2, nerve fibre layer; 3, nerve-cell layer; 4, Inner molecular layer; 5, inner granular layer; 6, outer molecular layer; 7, outer granular layer; 8, external limiting membrane; 9, rod and cone layer; 10, pigment-cell layer. On account of the way in which the supporting and essen- tial parts are interwoven in the retina it is not easy to track 490 THE HUMAN BODY. the latter through it. We shall find, however (Chap. XXXII.). that light first acts upon the rod and cone layer, traversing all the thickness of inner strata of the retina to reach this, before it can start those changes which result in visual sensations; and it is thereiore probable that the rods and cones are in direct continuity with the optic nerve- fibres. The limiting membranes, with the fibres of Muller and their branches, are undoubtedly accessory. Each rod and cone consists of an outer and an inner segment. The outer segments of both tend to split up trans- versely into disks and are very similar, except that those of the rods are longer than those of the cones and do not taper as the latter do. The inner segments of the cones are swollen, while those of the rods are narrow and nearly cy- lindrical. Over most of the retina the rods are longer and rpus than the cpnes^ but near the ciliary processes they cease before the cones do; and in ^le yellow spot elongated cones alone are found. In this region the whole retina is. much modified; at its margin all the layers are thickened but espectnttythe nerve-cell layer, which is here six or seven thick, while elsewhere the cells are found in but one or two strata. All the fibres also are oblique, reaching in to become continuous with the cones of the central pit, which are long, slender and very closely packed. In the fovea itself all the layers, except that of the cones, thin away, and so the depression is produced. The fovea is the seat of most acute vision; when we look at an object we always turn our eyes so that the light proceeding from it shall be focussed on this spot. Where the optic nerve enters, all the layers but the nerve-fibre layer, which is very thick, and the internal limiting membrane, are absent. The blood-vessels of the retina lie in the nerve-fibre and nerve-cell layers. The Refracting Media of the Eye are, in succession from before back, the cornea, the aqueous humor, the crystalline lens, and the vitreous liumor. vThe_ aqueous Jiumor fills the space between the front of the lens., ~2S, andIEe~5ack of the cornea. This space is in- completely divided by the iris into an anterior chamber, THE REFRACTING MEDIA OF THE EYE. 491 30, and a posterior, 31 (Fig. 124). Chemically, the aque- ous humor consists of water holding in solution a small amount of solid matters, mainly common salt. The crystalline lens (28, 26, 27) is colorless, transparent, and biconvex, with its anterior surface less curved than the posterior. It is surrounded by a capsule, and tliefinner edge of the iris lies in contact with it in front. In consist- ence it is soft, but its central layers are rather more dense than the outer. The vitreous humor is a soft jelly, enveloped in a thin capsule, the hyaloid membrane. In front, this membrane splits into two layers, one of which, 22, passes on to be fixed to the lens a little in front of its edge. This layer is known as the suspensory ligament of the lens; its line of at- tachment around that organ is not straight but sinuous as represented by the curved line between 28 and 26 in Fig» 124. The space between the two layers into which the hyaloid splits is the canal of Petit. The vitreous humor consists mainly of water and contains some salts, a little albumin, and some mucin. It is divided up, by delicate membranes,, into compartments in which its more liquid portions are imprisoned. The Ciliary Muscle. Running around the eyeball where the cornea joins the sclerotic is a little vein called the canal of Schlemm; it is seen in section at 8 in Fig. 124. Lying on the inner side of this canal, just where the iris and the ciliary processes meet, there is some plain muscular tissue, imbedded mainly in the middle coat of the eyeball and forming the ciliary muscle, which consists of a radial and a circular portion. The radial part is much the larger, and arises in front from the inner surface of the sclerotic; the fibres pass back, spreading out as they go, and are inserted into the front of the choroid opposite the ciliary processes. The circular part of the muscle lies around the outer rim of the iris. The contraction of the ciliary muscle tends to pull forward (radial fibres) and press inward (circular fibres) the front part of the choroid, to which the back part of the sus- pensory ligament of the lens is closely attached. In this way the tension exerted on the lens by its ligament is diminished. 492 THE HUMAN BODY. The Properties of Light. Before proceeding to the study of the eye as an optical instrument, it is necessary to recall briefly certain properties of light. Light is considered as a form of movement of the particles of an hypothetical medium, or ether, the vibrations being in planes at right angles to the line of propagation of the light. "When a stone is thrown into a pond a series of circular waves travel from that point in a horizontal direc- tion over the water, while the particles of water themselves move up and down, and cause the surface inequalities which we see as the waves. Somewhat similarly, light-waves spread out from a luminous point, but in the same medium travel equally in all directions so that the point is surrounded by shells of spherical waves, instead of rings of circular waves traveling in one plane only, as those on the surface of the water. Starting from a luminous point light would travel in all directions along the radii of a sphere of which the point is the centre; the light propagated along one such radius is called a ray, and in each ray the ethereal particles swing from side to side in a plane perpendicular to the direction of the ray. Taking a particle on any ray it would swing aside a certain distance from it, then back to it again, and across for a certain distance on the other side; and then back to its original position on the line of the ray. Such a movement is an oscillation, and takes a certain time; in lights of certain kinds the periods of oscilla- tion are all the same, no matter how great the extent or amplitude of the oscillation; just as a given pendulum will always complete its swing in the same time no matter whether its swings be great or small. Light composed of rays in which the periods of oscillation are all equal is called monochromatic or simple light, while light made of a mixture of oscillations of different periods is called mixed or compound light. If monochromatic light is steadily emitted from a point, then, at certain distances along a ray, we come to particles in the same phase of oscillation, say at their greatest dis- tance from their position of rest; just as in' the concentric waves seen on the water after throwing in a stone we would PROPERTIES OF LIGHT. 493 along any radius meet, at intervals, with water raised most above its horizontal plane as the crest of a wave, or depressed most below it as the hollow of a wave. The distance along the ray from crest to crest is called a wave-length and is always the same in any given simple light; but differs in different-colored lights; the briefer the time of an oscillation the less the wave-length. When light falls on a polished surface separating two transparent media, as air and glass, part of it is reflected or turned back into the first medium; part goes on into the second medium, and is commonly deviated from its original course or refracted. The original ray falling on the surface is the incident ray. Let A B (Fig. 127) be the surface of separation ; a x the incident ray; and CD the perpendicular or normal to the surface at the point of incidence: a x C will then be the angle of incidence. Then the reflected, ray makes an angle of reflection with the normal which is equal to the angle of incidence; and the reflected ray lies in the same plane as the inci- dent ray and the normal to the surface at x. The re- fracted ray lies also in the same plane as the normal and the incident ray, but does not continue in its original direc- tion, #/; if the medium below A B be more refractive than that above it, the refracted ray is bent, as xd, nearer to the normal, and making with it an angle of refraction , Dxd, smaller than the angle of incidence, a.x C. If, on the contrary, the second medium is less refracting than the first, the refracted ray xg is bent away from the normal, and makes an angle of refraction, Dxg, greater than the angle of incidence. The ratio of the sine of the angle of FIG. 127.— Diagram illustrating the refraction of light. AB, surface of separation between two transparent media; CD, the perpendicular to the surface at the point of incidence, x\ ax, incident ray; xd, refracted ray, if the second medium be denser than the first; xg, refracted ray, if the second medium is less refractive than the first. The reflected ray is not repre- sented, but would make an angle with Gx equal to the angle axC. 494 THE HUMAN BODY. incidence to that of the angle of refraction is always the same for the same two media with light of the same wave- length. When the first medium is air the ratio of the sine of the angle of refraction to that of the angle of incidence is called the refractive index of the second medium. The greater this refractive index the more is the refracted ray deviated from its original course. Eays which fall perpen- dicularly on the surface of separation of two media pass on without refraction. The shorter the oscillation periods of light-rays the more they are deviated by refraction. Hence mixed light when FIG. 128.— Diagram illustrating the dispersion of mixed light by a prism. sent through a prism is spread out, and decomposed into its simple constituents. For let a x (Fig. 128) be a ray of mixed light composed of a set of short and a set of long ethereal waves. When it falls on the surface A B of the prism, that portion which enters will be refracted towards the normal ED, but the short waves more than the longer. Hence the former will take the direction xy, and the latter the direction x z. On emerging from the prism both rays will again be refracted, but now from the nor- mals Fy and G z, since the light is passing from a more to a less refracting medium. Again the ray xy, made up of shorter waves, wi1! be most deviated, as in the direction y v, and REFRACTION BY LENSES. 495 the long waves less, in the direction z r. If a screen were put at S S' , we would receive on it at separate points, v and r, the two simple lights which were mixed together in the compound incident ray a x. Such a separation of light-rays is called dispersion. Ordinary white light, such as that of the sun, is com- posed of ethereal vibrations of every rate, mixed together. When such light is sent through a prism it gives a contin- uous band of light-rays, known as the solar spectrum, reach- ing from the least refracted to the must refracted and short- est waves. The exceptions to this statement due to Frauen- hofer's lines (see Physics) are unessential for our present purpose. All of the simple lights into which the compound solar light is thus separated do not, however, excite in us visual sensations when they fall into the eye, but only cer- tain middle ones. If solar light were used with the prism. Fig. 128, certain least refracted rays between r and 8' would not be seen, nor the most refracted between v and 8 ; while between v and r would stretch a luminous band exciting in us the series of colors from red (due to the least refracted visible rays),, through orange, yellow, green, bright blue, arid indigo, to violet, which latter is the sensation aroused by the most refrangible visible rays. The still shorter waves beyond the violet can only be seen under special con- ditions; they are known mainly by their chemical effects and are called the actinic rays; the invisible waves beyond the red exert a powerful heating influence and compr.se the dark heat rays. The eye, as an organ for making known to us the existence of ethereal vibrations, has, therefore, only a limited range. Refraction of Light by Lenses. In the eye the refract- ing media have the form of lenses thicker in the centre than towards the periphery; and we may here confine our- selves therefore to such converging lenses. If simple light from a point A, Fig. 122, fall on such a lens its rays, emerging on the other side, will take new directions after refraction and meet anew at a point, a, after which they ngain diverge. If a screen, r r, be held at a it will therefore receive an image of the luminous point A. For every con- 496 THE HUMAN BODY. verging lens there is such a point behind it at which the rays from a given point in front of it meet: the point of meeting is called the conjugate focus of the point from which the rays start. If instead of a luminous point a luminous object be placed in front of the lens an image of the object will be formed at a certain distance behind it, for all rays proceeding from one point of the object will meet in the conjugate focus of that point behind. The image is inverted, as can be readily seen from Fig. 129. All rays from the point A of the object meet at the point a of the image; those from B at b, and those from intermediate points at intermediate positions. If the single lens were replaced by several combined so as to converging lens. form an optical system the general result would be the same, provided the system were thicker in the centre than at the periphery. The Camera Obscura, as used by photographers, is an instrument which serves to illustrate the formation of images by converging systems of lenses. It consists of a box blackened inside and having on its front face a tube containing the lenses; the posterior wall is made of ground glass. J[f the front of the instrument be directed on ex- terior objects, inverted and diminished images of them will be formed on the ground glass; those images are only well denned, at any one time, which are at such a distance in front of the instrument that the conjugate foci of points on them fall exactly on the glass behind the lens: objects nearer or farther off give confused. and indistinct images; but by altering the distance between the lenses and the ground glass, in common language "focusing the instru- ment," either can be made distinct. For neajLoMgcts ^e lenses muaLJia-JEarther from the surface on which the image is to be received, and for distant nearer. The reason of this may readily be seen from .Fig. 130. If the system of lenses brings the parallel rays a c and I d, pro- ceeding from an infinitely distant object, to a focus at x, REFRACTION IN THE EYE. 49? then the diverging rays/c and fd, proceeding from a nearer point, will be harder to bend round, so to speak, and will not meet until a point y, farther behind the system than x is. The more divergent the rays, or what amounts to the same thing, the nearer the point they proceed from, the .farther behind the refracting system will y be. y< *. FIG. 130.— Diagram illustrating the need of "focusing" in an optical instru >ment. The refracting media of the eye form a convergent optical system, made up of cornea, aqueous humor, lens, and vitre- ous humor. These four media are reduced to three prac- tically, by the fact that the indices of refraction of the cornea and aqueous humor are the same, so that they act together as one converging lens. The surfaces at which refraction occurs are — (1) that between the air and die cornea, (2) that between the aqueous humor and the front of the lens, (3) that between the vitreous humor and the back of the lens. The refractive indices of those media are — the air, 1; the aqueous humor, 1.3379; the lens (average), 1.4545; the vitreous humor, 1.3379. From the laws of the refraction of light it therefore follows that (Fig. 131) the rays c d will at the corneal surface be refracted towards the :normals N, N, and take the course d e. At the front of lens they will again be refracted towards the normals to that surface and take the course e f; at the back of the lens, passing from a more refracting to a less refracting medium, they will be bent from the normals N" and take the course / g. If the retina be there, these parallel rays will therefore Ibe brought to a focus on it. In the*resting condition of the natural eye this is what happens to parallel rays entering it; and, since distant objects send into the eye rays which .are practically parallel, such obiects are seen distinctly 498 THE HUMAN BODY. without any effort; all rays emanating from a point of the object meet again in one point on the retina. Accommodation. Points on near objects send into the eye [verging rays: these therefore would not come to a focus on o o J -> the retiiui but behind it, and would not be seen distinctly, did not some change occur in the eye; since we can see them quite plainly if we choose (unless they be very near indeed), there must exist some means by which the eye is adapted or accommodated for looking at objects at different distances. That some change does occur one can, also, readily prove FIG. 131.— Diagram illustrating the surfaces at which light is refracted in the eye. by observing that we cannot see distinctly, at the same moment, both near and distant objects. For example standing at a window, behind a lace curtain, we can if we choose look at the threads of the lace or the houses across the street; but when we look at the one we only see the other indistinctly; and if, after looking at the more distant object, we look at the nearer we experience a distinct sense of effort. It is clear, then, that something in the eye is different in the two cases. The resting eye, suited for dis- tinctly seeing distant objects, might conceivably be accom- modated for near vision in several ways. The refracting ACCOMMODATION. 499 indices of its media might be increased; that of course does not happen; the physical properties of the media are the same in both cases: or the distance of the retina from the refracting surfaces might be increased, for example by •compression of the eyeball by the muscles around it; how- ever, experiment shows that changes of accommodation can be brought about in the fresh excised eyes of animals, in which no such compression is possible; we are thus reduced to the third explanation, that the refracting surfaces, or some of them, become more curved, and so bring more diverging rays sooner to a focus; since a lens of smaller curvature is more converging than one of greater curvature composed of the same material. Observation shows that this is what actually happens: the corneal surface remains unchanged when a near object is looked at after a distant one, but the anterior sur- face of the lens becomes considerably more convex and the posterior slightly so. As already pointed out when light meets the separating surface of two media some is reflected and some refracted (p. 493). If, therefore, a person be taken into a dark room and a candle held on one side of his eye, while he looks at a distant object an observer can see three images of its flame h11 media °£ in his pupil, due to that part of the light reflected from the surfaces between the media. One (a, Pig. 132) is erect and bright, reflected from the convex mirror formed by the cornea; the next, 19 is dimmer and also -erect; it comes from the front of the lens. The third, c, is dim and inverted, being reflected from the concave mirror {see Physics) formed by the back of the lens. If now the observed eye looks at a near object in the same line as the distant point previously looked at, it is seen that the image •due to corneal reflection remains unchanged; that due to light from the front of the lens becomes smaller and brighter, indicating (see Physics) a greater convexity of the reflecting surface; the image from the back of the lens also becomes very slightly smaller, indicating a feebly increased curvature. 500 THE HUMAN BODY. Accommodation is brought about mainly by the ciliary muscle. In the resting eye it is relaxed and the suspensory ligament of the lens is taut, and, pulling on its edge, drags it out laterally a little and flattens its surfaces, especially the anterior, since the ligament is attached a little in front of the edge. To see a nearer object the ciliary muscle is contracted, and according to the degree of its contraction slackens the suspensory ligament (p. 491), and then the- elastic lens, relieved from the lateral drag, bulges out a little in the centre. Short Sight and Long Sight. In the eye the range of accommodation is very great, allowing the rays from points infinitely distant up to those from points about eight inches in front of the eye to be brought to a focus on the re- tina. In the normal eye par- allel rays meet on the retina when the ciliary muscle is completely relaxed (A, Fig. 133). Such eyes are emme- tropic. In other eyes the eye- ball is too long from before back; in the resting state par- allel rays meet in front of the retina ( B}. Persons with such eyes, therefore, cannot see distant objects distinctly without the aid of diverging (concave) spectacles; they are short- sighted or myopic. Or the eyeball may be too short from before back; then, in the resting state, parallel rays are brought to a focus behind the retina (C). To see even infinitely distant objects, such persons must therefore use their accommodating apparatus to increase the converging power of the lens; and when objects are near they cannot, with the greatest effort, bring the divergent rays proceeding from them to a focus soon enough. To get distinct retinal images of near objects they therefore need converging (con- vex) spectacles. Such eyes are called hypermetropic, or in common language long-sighted. FIG. 133.— Diagram illustrating the path of parallel ravs after entering an emmetropic (A), a myopic (J?), and a hypermetropic (C) eye. HYGIENE OF THE EYE. 501 Hygienic Remarks. Since muscular effort is needed by the normal eye to see near objects, it is clear why the pro- longed contemplation of such is more fatiguing than look- ing at more distant things. If the eye be hypermetropic still more is this apt to be the case, for then the ciliary muscle has no rest when the eye is used, and to read a book at a distance such that enough light is reflected from it into the eye in order to enable the letters to be seen at all, requires an extraordinary effort of accommodation. Such persons complain that they can read well enough for a time, but soon fail to be able to see distinctly. This kind of weak sight should always lead to examination of the eyes by an oculist, to see if glasses are needed; otherwise severe neu- ralgic pains about the eyes are apt to come on, and the overstrained organ may be permanently injured. Old per- sons are apt to have such eyes; but young childen frequently also possess them, and if so should at once be provided with spectacles. Short-sighted eyes appear to be much more common now than formerly, especially in those given to literary pursuits. Myopia is rare among those who cannot read or who live mainly out of doors. It is not so apt to lead to per- manent injury of the eye as is the opposite condition, but the effort to see distinctly objects a little distant is apt to produce headaches and other symptoms of nervous exhaustion. If the myopia become gradually worse the eyes should be rested for several months. Short-sighted persons are apt to have, or acquire, peculiarities of appear- ance: their eyes are often prominent, indicative of the abnormal length of the eyeball. They also get a habit of "screwing" up the eyelids, probably an indication of an effort to compress the eyeball from before back so that distant objects may be better seen. They often stoop, too, from the necessity of getting their eyes near objects they want to see. The acquirement of such habits may be usually prevented by the use of proper glasses. On the other hand "it is said that myopia even induces peculiari- ties of character, and that myopes are usually unsuspicious and easily pleased; being unable to observe many little 502 THE HUMAN BODY. matters in the demeanor or expression of those with whom they converse, which, being noticed by those of quicker sight, might induce feelings of distrust or annoy- ance." In old age the lens loses some of its elasticity and be- comes more rigid. This leads to the long-sightedness of old people, known as presbyopia. The stiffer lens does not become as convex as it did in early life, when the ciliary muscle contracts and the suspensory ligament is relaxed. A special effort of accommodation is therefore needed in order to adapt the eye to see near objects distinctly; and convex glasses are required. In all forms of deficient accommodation too strong glasses will injure the eyes irreparably, increasing the defects they are intended to relieve. Skilled advice should there- fore be invariably obtained in their selection, except per- haps in the long-sightedness of old age when the sufferer may tolerably safely select for himself any glasses that allow him to read easily a book about 30 centimeters (12 inches) from the eye. As age advances stronger lenses must of course be obtained. Optical Defects of the Eye. The eye, though it an- swers admirably as a physiological instrument, is by no means perfect optically; not nearly so good, for example, as a good microscope objective. The main defects in it are due to — 1. Chromatic Aberration. As already pointed out the rays at the violet end of the solar spectrum are more re- frangible than those at the red end. Hence they are brought to a focus sooner. The light emanating from a point on a white object does not, therefore, all meet in one point on the retina; but the violet rays come to a focus first, then the indigo, and so on to the red, farthest back of all. If the eye is accommodated so as to bring to a focus on the retina parallel red rays, then violet rays from the same source will meet half a millimeter in front of it, and cross- ing and diverging there make a little violet circle of diffu- sion around the red point on the retina. In optical instru- 0 V: OPTICAL DEFECTS OF THE EYE. 503 merits this defect is remedied by combining together lenses made of different kinds of glass; such compound lenses are called achromatic. The general result of chromatic aberration, as may be seen in a bad opera-glass, is to cau.se colored borders to ap- pear around the edges of the images of objects. In the eye TVG usually do not notice such borders unless we especially look for them; but if, while a white surface is looked at, the edge of an opaque body be brought in front of the •eye so as to cover half the pupil, colorations will be seen at its margin. If accommodation is inexact they appear .also when the boundary between a white and a black sur- face is observed. The phenomena due to chromatic aberration are much more easily seen if light containing only red and violet rays be used instead of white light con- taining all the rays of intermediate refrangibility. Ordi- dinary blue glass only lets through these two kinds of rays. If a bit of it be placed over a very small hole in an opaque shutter and the sunlight be suffered to enter through the hole, it will be found that with one accommodation (that lor the red rays) a red point is seen with a violet border, and with another (that at which violet rays are brought to a focus on the retina) a violet point is seen with a red aureole. 2. Spherical Aberration. It is not quite correct to state that ordinary lenses bring to a focus in one point behind them rays proceeding from a point in front, even when these are all of the same refrangibility. Convex lenses whose surfaces are segments of spheres, as are those of the eye, bring to a focus sooner the rays which pass through their marginal than those passing through their central parts. If rays proceeding from a point and traversing the lateral part of a lens be brought to a focus at any point, then those passing through the centre of the lens will not meet until a little beyond that point. If the retina receive the image formed by the peripheral rays the others will form around this a small luminous circle of light — such as would be formed by sections of the cones of converging rays in Fig. 122, taken a little in front of r r. This defect is found in all 504 THE HUMAN BODY, optical instruments, as it is impossible in practice to grind lenses of the non-spherical curvatures necessary to avoid it. In our eyes its effect is to a large extent corrected in the following ways — (a) The opaque iris cuts off many of the external and more strongly refracted rays, preventing them from reaching the retina, (b) The outer layers of the lens are less refracting than the central ; hence the rays • passing through its peripheral parts are less refracted than those passing nearer its axis. 3. Irregularities in Curvature. The refracting surfaces of our eyes are not even truly spherical; this is especially the case with the cornea, which is very rarely curved to the same extent in its vertical and horizontal diameters. Sup- pose the vertical meridian to be the most curved; then the rays proceeding from points along a vertical line will be brought to a focus sooner than those from points on a hori- zontal line. If the eye is accommodated to see distinctly the vertical line, it will see indistinctly the horizontal and vice versa. Few people therefore see equally clearly at once two lines crossing one another at right angles. The pheno- menon is most obvious, however, when a series of concentric- circles (Fig. 134) is looked at: then when the lines appeal- sharp along some sectors, they are dim along the rest. When this defect, known as astigmatism, is marked it causes serious troubles of vision and re- quires peculiarly shaped glasses to counteract it. 4. Opaque Bodies in the Refracting Media. In diseased eyes the lens may be opaque (cataract] and need removal; or FIG. 131 opacities from ulcers or wounds may exist on the cornea. But even in the best eye there are apt to be small opaque bodies in the vitreous humor causing muscce volit antes; that is, the appearance of OPTICAL DEFECTS OF THE EYE. 505 minute bodies floating in space outside the eye, but chang- ing their position when the position of the eye changes, by which fact their origin in internal causes may be recog- nized. Many persons never see them until their attention. is called to their sight by some weakness of it, and then they think they are new phenomena. Visual phenomena due to causes in the eye itself are called entoptic; the most interesting are those due to the retinal blood-vessels (Chap. XXXII.). Tears, or bits of the secretion of the Meibo- mian glands, on the front of the eyeball often cause distant luminous objects to look like ill-defined luminous bands or patches of various shape. The cause of such appearances is readily recognized, since they disappear or are changed after wiuking, CHAPTER XXXII. THE EYE AS A SENSORY APPARATUS. The Excitation of the Visual Apparatus. The excita- ble visual apparatus for each eye consists of the retina, the optic nerve, and the brain-centres connected with the latter; however stimulated, if intact, it causes visual sensations. In the great majority of cases its excitant is objective light., and so we refer all stimulations of it to that cause, unless we have special reason to know the contrary. As already pointed out (p. 468) pressure on the eyeball causes a lumi- nous sensation (phosphene), which suggests itself to us as dependent on a luminous body situated in space where such an object must be in order to excite the same part of the retina. Since all rays of light penetrating the eye, except in the line of its long axis, cross that axis, if we press the outer side of the eyeball we get a visual sensa- tion referred to a luminous body on the nasal side; if we press below we see the luminous patch above, and so on. Of course different rays entering the eye take different paths through it, but on general optical principles, which cannot here be detailed, we may trace all oblique rays through the organ by assuming that they meet and leave the optic axis at what are known as the nodal points of the system; these (kk', Fig. 135) lie near together in the lens. If we vrant to find where rays of light from A will meet the retina (the eye being properly accommodated for seeing an object at that distance) we draw a line from A to k (the first nodal point) and then another, parallel to the first, from k' (the second nodal point) to the retina. The nodal points of the eye lie so near together that for practical purposes we may treat them as one (kf Fig. 136), POSITION OF RETINAL IMAGES. 507 placed near the back of the lens. By manifold experience we have learnt that a luminous body (A Fig. 136) which we see, always lies on the prolongation of the line joining the excited part of the retina, a, and the nodal point, k. Hence FIG. 135.— Diagram illustrating the points at which incident rays in the eye meet the retina, x x, optic axis; fc, first nodal point; fc', second nodal point; 6, point where the image of B would be formed, were the eye properly accom- modated for it; a, the retinal point where the image of A would be formed. any excitation of that part of the retina makes us think of a luminous body somewhere on the line a A, and, similarly, any excitation of #, of a body on the line b B or its pro- longation. It is only other conflicting experiences, as that FIG. 136.— Diagrammatic section through the eyeball, nodal point. xx, optic axis; fc, with the eyes closed external bodies do not excite visual sensations, and the constant connection of the pressure felt on the eyelid with the visual sensation, that enable us when we press the eyeball to conclude that, in spite of what 508 THE HUMAN BODY. we seem to see, the luminous sensation is not due to objective light from outside the eye. The Idio-Retinal Light. The eyelids are not by any means perfectly opaque; in ordinary daylight they still allow a considerable quantity of light to penetrate the eye, as any one may observe by passing his hand in front of the closed eyes. But even in a dark room with the eyes completely covered up so that no objective light can enter them, there is still experienced a small amount of visual sensation due to internal causes. The field of vision is not absolutely dark but slightly luminous, with brighter fleeting patches trav- ersing it. These are especially noticeable, for example, in trying to see and grope one's way with the eyes open up a perfectly dark staircase. Then the luminous patches attract special attention because they are apt to be taken for the signs of objective realities; they become very manifest when any sudden jar of the Body, due for example to knocking against something, occurs; and have no doubt given rise to many ghost stories. These visual sensations felt in the absence of all external stimulation of the eyes, may for con- venience be spoken of as due to the idio-retinal light. The Excitation of the Visual Apparatus by Light. Light only excites the retina when it reaches its nerve end FIG. 137. organs, the rods and cones. The proofs of this are several. 1. Light does not arouse visual sensations when it falls directly on the fibres of the optic nerve. Where this nerve enters (p. 490) is a retinal part possessing only nerve-fibres, and this part is blind. Close the left eye and look steadily with the right at the cross in Fig. 137, holding the book FUNCTION OF RODS AND CONES. 509 •vertically in front of the face, and moving it to and fro. It will be found that at about 25 centimeters (10 inches) •off the white circle disappears; but when the page is nearer •or farther, it is seen. During the experiment the gaze must be kept fixed on the cross. There is thus in the field of Tision &Jblind spot, and it is easy to show by measurement that it lies where the optic nerve enters. When the right eye is fixed on the cross, it is so directed that rays from this fall on the yellow spot, y, Fig. 138. The rays from the circle then cross the visual axis at the nodal point, n, and meet the retina at o. If the distance of the nodal point of the -eye from the paper be /, and from the retina (which is 15 mm.) be F, then the dis- tance, on the paper, of the cross from the •circle will be to the distance of y from o as / is to F. Measurements made in this way show that the circle disappears when its image is thrown on the entry of the optic nerve, which lies to the nasal side of the yellow spot (p. 487). 2. The above experiment having shown that light does not act directly on the optic nerve-fibres any more than it does on any other nerve-fibres, we have next to see in what part of the retina those changes do first occur which form the link between light and nervous impulses. They occur in .the outer part of the retina, in the rods and cones. This is proved by what is called Purkinje's experiment. Take a oandle into a dark room and look at a surface not covered ivith any special pattern, say a whitewashed wall or a plain -window-shade. Hold the candle to the side of one eye and dose to it, but so far back that no light enters the pupil ifrom it; that is so far back that the flame just cannot be seen, but so that a strong light is thrown on the white of the eye as far back as possible. Then move the candle a little to and fro. The surface looked at will appear .luminous with reddish-yellow light, and on it will be seen FIG. 138. 510 THE HUMAN BODY. dark branching lines which are the shadows of the retinal vessels. Now in order that these shadows may be seen the- parts on which the light acts must be behind the vessels, that is in the outer layers of the retina since the vessels- lie (p. 490) in its inner strata. If the light is kept steady the vascular shadows soon dis- appear; in order to continue to see them the candle must be kept moving. The explanation of this fact may readily be made clear by fixing the eyes for ten or fifteen seconds on the dot of an " i" somewhere about the middle of this page: at first the distinction between the slightly luminous- black letters and the highly luminous white page is very obvious; in other words, the different sensations arising from the strongly and the feebly excited areas of the retina. But if the glance do not be allowed to wander, very soon the letters become indistinct and at last disappear altogether; the whole page looks uniformly grayish. The reason of this is that the powerful stimulation of the retina by the light reflected from the white part of the page soon fatigues the part of the visual apparatus it acts upon; and as this fatigue progresses the stimulus produces less and less effect. The parts of the retina, on the other hand, which receive light only from the black letters are very little stimu- lated and retain their original excitability so that, at last, the feebler excitation acting upon these more irritable parts produces as much sensation as the stronger stimulus acting upon the fatigued parts; and the letters become indistin- guishable. To see them continuously we must keep shift- ing the eyes so that the parts of the visual apparatus are alternately fatigued and rested, and the general irrita- bility of the whole is kept about the same. So, in Purkinje's experiment, if the position of the shadows remain the same, the shaded part of the retina soon be- comes more irritable than the more excited unshaded parts, and its relative increase of irritability makes up for the less light falling on it, so that the shadows cease to be perceived. It is for this same reason that we do not see the vessels under ordinary circumstances. When light, as usual, enters the eye from front through the pupil the PURKINJE'S EXPERIMENT. 511 FIG. 139. shadows always fall on the same parts of the retina, and these parts are thus kept sufficiently more excitable than the rest to make up for the less light reaching them through the vessels. To see the latter we must throw the light into the eye in an unusual direction, not through the pupil but laterally through the sclerotic. If v, Fig. 139, be the section of a retinal vessel, ordi- narily its shadow will fall at some point on the line prolonged through it from the centre of the pupil. If a candle be held opposite Z» it illuminates that part of the sclerotic and from there light radiates and illumines the eye. The sensation we refer to light entering the eye in the usual manner through the pupil, and accordingly see the surface we look at as if it were illuminated. The shadow of v is now cast on an unusual spot c, and we see it as if at the point d on the wall, on the prolongation of the line joining the nodal point, Tc, of the eye with c. If the candle be moved so as to illuminate the point V of the sclerotic, the shadow of v will be cast on c' and will accordingly seem on the wall to move from d to d'. It is clear that if we know how far I is from #', how far the wall is from the eye, and how far the nodal point is from the retina (15 mm. or 0.6 inch), and measure the distance on the wall from d to d', we can calculate how far c is from c': and then how far the vessel throwing the shadow must be in front of the retinal parts perceiving it. In this way it is found that the part seeing the shadow, that is the layer on which light acts, is just about as far behind the retinal vessels as the main vascular trunks of the retina are in front of the rod and cone layer. It is, therefore, in that layer that the light initiates those changes which give rise to nervous impulses; which is further made obvious by the fact that the seat of most acute vision is thefovea centralis, where this layer and the cone-fibres diverging from it alone are found (p. 490). 512 THE HUMAN BODY. When we want to see anything distinctly we always turn our eyes so that its image shall fall on the centre of the yellow spot. The Vision Purple. How light acts in the retina so as to produce nerve stimuli is still uncertain. Recent observa- tions show that it produces chemical changes in the rod and cone layer, and seemed at first io indicate that its action was to produce substances which were chemical excitants of nerve-fibres; but although there can be little doubt that these chemical changes play some important part in vision, what .their role may be is at present quite obscure. JHL_a. perfectly fresh retina be excised rapidly, its outer layers will be "found of a rich purple color. In daylight this rapidly bleaches, but in the dark persists even when putre- faction lias set in. In pure yellow light it also remains unbleached a long time, but in other lights disappears at different rates. If a rabbit's eye be fixed immovably and exposed so that an image of a window is focused on the same part of its retina for some time, and then the eye be rapidly excised in the dark and placed in solution of potash alum, a colorless image of the window is found on the retina, surrounded by the visual purple of the rest which is, through the alum, fixed or rendered incapable of change by light. Photographs, or optograms, are thus obtained which differ from the photographer's in that he uses light to produce chemical changes which give rise to colored bodies, while here the reverse is the case. If the eye be not rapidly excised and put in the alum after its exposure, the optogram will disappear; the vision purple being rapidly- regenerated at the bleached part. This reproduction of ft is due mainly to the cells of the pigmentary layer of the retina, which in living eyes exposed to light thrust long processes between the rods and cones. Portions of frogs' retinas raised from this bleach more rapidly than those left in contact with it, but become soon purple again if let fall back upon the pigment- cells. Experiments show, however, that animals (frogs) exposed for a long time to a bright light may have their retinas completely bleached and still see very well; they can still unerringly catch flies that come VISION PURPLE. 513 within their reach; and they can also distinguish colors, or at least some colors, as green. Moreover, the vision purple is only found in the outer segments of the rods; there is none in the cones, and yet these alone exist in the yellow spot of the human eye, which is the seat of most acute vision; and animals, such as snakes, which have only cones in the retina, possess no vision purple and nevertheless see yery well. It may be that other bodies exist in the retina which are also chemically changed by light, but the changes of which are not accompanied by alterations in color which we can ^ee; and in the absence of the vision purple seeing might be carried on by means of these, which may be less quickly destroyed by light and so still persist in the bleached retinas of the frogs above mentioned. For the present, however, the question of the part, if any, played in vision by such bodies must be left an open one. The Intensity of Visual Sensations. Light considered as a form of energy may vary in quantity; physiologically, also, we distinguish quantitative differences in light as degrees of brightness, but the connection between the in- tensity of the sensation excited and the quantity of energy represented by the stimulating light is not a direct one. In the first place some rays excite our visual apparatus more powerfully than others: a given amount of energy in the form of yellow light, for example, causes more powerful yisual sensations than the same quantity of energy in the form of violet light; and ultra-violet rays only become visible, and then very faintly, when all others are suppressed; but if they be passed through some fluorescent substance (see Physics), such as an acid solution of quinine sulphate, which, without altering the amount of energy, turns it into ethereal oscillations of a longer period, then the light be- comes readibly perceptible. Even with light-rays of the same oscillation period our sensation is not proportional to the amount of energy in the light; to the amount of heat, for example, to which it would give rise if all transformed into it. If objective light increase gradually in amount our sensation increases 514 THE HUMAN BODY. also, up to a limit beyond which it docs not go, no matter how strong the light becomes; but the increase of sensation takes place far more slowly than that of the light, in accord- ance with the psycho-physical law mentioned on page 473. If we call the amount of light given out by a single candle a, then that emitted by two candles will be 2a; and so on. If the amount of sensation excited by the single candle be A, then that due to two candles will not be 2.4, and that, by three will be far less than 3 A. If a white surface, P, Fig. 140, be illuminated by a candle at c and another else- j where, and a rod, o, be placed so as to intercept the light from c, we see clearly a shadow, since our eyes recognize the difference in luminosity of this part of the paper, reflect- ing light from one candle only? from that of the rest which is illuminated by two: that is we tell the sensation due to the stimulus a from that due to the stimulus 2a. If now a bright lamp be brought in and placed alongside, and its light be physically equal to- that of 10 candles, we cease to perceive the shadow s. That is the sensation aroused by objective light = I2a (due to the lamp and two candles) cannot be told from that due to light = 11«; although the difference of objective light is still la as before. Most persons must have ob- served illustrations of this. Sitting in a room with three lights not unfrequently some object so intercepts the light from two as to cast on the wall two shadows which partly overlap. Where the shadows overlap the wall gets light only from the third candle; around that, where each shadow is separate, it is illuminated by this and one other candle; and the wall in the neighborhood of the shadows by all three. Objectively, therefore, the difference between the deep shadow and half shadow is that between the light of one candle and that of two. The difference between the half shadows and the wall around is that between the light of two and three candles. But as a matter of sensa- tion the difference between the half shadow and the full INTENSITY OF VISUAL SENSATIONS. 515 shadow seems much greater than that between the half shadow and the rest of the wall; in other words the differ- ence, «, between a and 2a, is a more efficient stimulus than the same difference, #, between 2a and 3a. When the total stimulus increases the same absolute difference is less lelt or may be entirely unperceived. An example of this •which every one will recognize is afforded by the invisibility -of the stars in daytime. On the other hand, as the total stimulus increases or de- creases the same fractional difference of the whole is per- ceived with the same ease; i.e. excites the same amount of sensation. In reading a book by lamplight we perceive clearly the difference between the amount of light reflected from the black letters and the white page. If we call the total lamplight reflected by the blank parts Wa and that by the letters 2a, we may say we perceive with a' certain distinctness a luminous difference equal to one fifth of the whole. If we now take the book into the daylight the total light reflected from both the letters and the imprinted part of the page increases, but in the same proportion. Say the one now is 5Qa and the other 10«; although the absolute difference between the two is now 40# instead of Sa we do not see the letters any more plainly than before. The smallest difference in luminous intensity which we can perceive is about T^- viol e|f, +.V>rm|grli nvgrvo-Q blue, and indigo. The prism separates iiv n one another light-rays of different periods of oscillation (p. 494) and each ray excites in us a colored visual sensation, while all mixed together, as in sunlight, they arouse the entirely different sensation of white. If the light fall on a piece of black velvet we get still another sensation, thaljiLJilack^ in tMs^ase_theJ.[ght-rays are so absorbed that but few are reflected to the eve and the vis- — — -^^ — ' *- nal apparatus is left at rest. Physically black repre- sents nothing: it is"lTniere~zero — the absence of ethereal vibrations; but, in consciousness, it is as definite a sensation as white, red, or any other color. We do not feel blackness or darkness except over the region of the possible visual field of our eyes. In a perfectly dark room we only feel the darkness in front of our eyes, and in the light there is no such sensation associated with the back of our heads or the palms of our hands, though through these we get no visual sensa- tions. It is obvious, therefore, that the sensation of blackness is not due to the mere absence of luminous stimuli but to the nnexcited state of the retinas, which are alone capable of being excited by such stimuli when present. This fact is a very remarkable one, and is not paralleled in any other sense. Physically, complete stillness is to the ear what darkness is to the eye; but silence impresses itself on us as the absence of sensation, while darkness causes a definite feeling of " blackness." Young's Theory of Color Vision. Our color sensations insensibly fade into one another; starting with black we can insensibly pass through lighter and lighter shades of gray 520 THE HUMAN BODY. to white: or beginning with green through darker and darker shades of it to black or through lighter and lighter to white: or beginning with red we can by imperceptible steps pass to orange, from that to yellow and so on to the end of the solar spectrum: and from the violet, through purple and carmine, we may get back again to red. Black and white appear to be fundamental color sensations mixed up with all the rest: we never imagine a color but as light or dark, that is as more or less near white or black; and it is found that as the light thrown on any given colored sur- face weakens, the shade becomes deeper until it passes into black; and if the illumination is increased, the color becomes " lighter" until it passes into white. Of all the colors of the spectrum yellow most easily passes into white with strong illumination. Black and white, with the grays which are mixtures of the two, thus seem to stand apart from all the rest as the fundamental visual sensations, and the others alone are in common parlance named "colors." It has even been suggested that the power of differentiating them in sensation has only lately been acquired by man, and a certain amount of evidence has been adduced from passages in the Iliad to prove that the Greeks in Homer's time confused together colors that are very different to most modern eyes; at any rate there seems to be no doubt that the color sense can be greatly improved by practice; women whose mode of dress causes them to pay more attention to the matter, have, as a general rule, a more acute color sense than men. Leaving aside black, white, gray, and the various browns (which are only dark tints of other colors), we may enum- erate our color sensations as red, orange, yellow, green, blue, violet and purple; between each there are, however, numer- ous transition shades, as yellow-green, bine-green, etc., so that the number which shall have definite names given to them is to a large extent arbitrary. Of the above, all but purple are found in the spectrum given when sunlight is separated by a prism into its rays of different refrangibility; rays of a certain wave-length or period of oscillation cause in us the feeling red; others yellow, and so on; for convenience COLOR VISION. 521 we may speak of these as red, yellow, blue, etc., rays; all together, in about equal proportions, they arouse the sen- sation of white. A remarkable fact is that most color feel- ings can be aroused in several ways. White, for example, not only by the above general mixture, but ^red^nd blue- green rays^ or orange and blue, or yellow and violet, taken together in pairs/^cause the se"nsation of white: such colors are called complemmtary to one another. The mixture may be made in several ways; as, for example, by causing the red and blue-green parts of the spectrum to overlap, or by painting red and blue-green sectors on a disk and rotating it rapidly; they cannot be made, however, by mix- ing pigments, since what happens in such cases is a very complex phenomenon. Painters, for example, are accus- tomed to produce green by mixing blue and yellow paints, and some may be inclined to ridicule the statement that yel- low and blue when mixed give white. When, however, we mix the pigments we do not combine the sensations of the same name, which is the matter in hand. Blue paint is blue because it absorbs all the rays of the sunlight except the blue and some of the green; yellow is yellow because it absorbs all but the yellow and some of the green, and when blue and yellow are mixed the blue absorbs all the distinc- tive part of the yellow and the yellow does the same for the blue; and so only the green is left over to reflect light to the eye, and the mixture has that color. Grass-green has no complementary color in the solar spectrum; but with purple, which is made by mixing red and blue, it gives white. Several other colors taken three together, give also the sensation of white. If then we call the light-rays which arouse in us the sensation red, a, those giving us the sensation orange b, yellow cr and so on, we find that we get the sensation white with a, I, c, d, e, /and g all together; or with b and e, or with c and /, or with a, d, and e\ our sensation white has no determinate relation to ethereal oscillations of a given period, and the same is true for several other colors; yellow feeling, for example, may be excited by ethereal vibrations of one given wave-length (spectral yellow), or by mixing red and grass-green, which THE HUMAN BODY. are due to ethereal vibrations of totally different wave- lengths; in other words a physical light in which there are no waves of the "yellow" length may cause in us the sen- sation yellow, which is only one more instance of the gen- eral fact that our sensations, as such, give us no direct information as to the nature of external forces; they are but signs which we have to interpret. The modern view of specific nerve energies (p. 191) makes it highly improbable that our different color sensations can all be due to different modes of excitation of exactly the same nerve-fibres; a fibre which when excited alone gives us the sensation red will always give us that feeling when so excited. The simplest method of explaining our color sensations would therefore be to assume that for each there exists in the retina a set of nerve-fibres with appropriate terminal organs, each ex- citable by its own proper stimulus. But we can distinguish so innumerable and so finely graded colors, that, on such a supposition, there must be an almost infinite number of different end organs in the retina, and it is more reasonable to suppose that there are a limited number of primary color sensations, and that the rest are due to combinations of these. That a compound color sensation may be very different from its components when these are regarded apart, is clearly shown by the sensation white aroused either by what we may call red and blue-green, or green and purple, stimuli acting together; or of yellow due to grass-green and red. To account for our various color sensations we may, therefore, assume a much smaller number of primary sen- sations than the total number of color sensations we expe- rience; all can in fact be explained by assuming any three primary color sensations which together give white, and regarding all the rest as due to mixtures of these in various proportions; there may be more than three, but three will account for all the phenomena, black being a sensation experienced when all visual stimuli are absent. This is known as Young's theory of color vision, and is that at present most commonly accepted. The selection of the three primary sensations is somewhat arbitrary, but they are usually regarded as red, green, and violet. It is COLOR BLINDNESS. 523 assumed that all kinds of light stimulating the end appa- ratuses give rise to all three sensations, but not necessarily in the same proportion. When all arc equally aroused the sensation is white; when the red and green are tolerably powerfully excited and the violet little, the sensation is yellow; when the green powerfully and the red and violet little, the sensation is green, and so on. In this way we can also explain the fact that all colored surfaces when intensely illuminated pass into white. A red light, for example, excites the primary red sensation most, but green and violet a little; as the light becomes stronger a limit is reached beyond which the red sensation cannot go, but the green and violet go on increasing with the intensity of the light, until they too reach their limits; and all three pri- mary sensations being then equally aroused, the sensation white is produced. Color Blindness. Some persons fail to distinguish colors which are to others quite different; when such a de- ficiency is well marked it is known as " color blindness," and, assuming Young's theory to be correct, it may be ex- plained by an absence of one or more of the three primary color sensations; observation of color-blind persons thus helps in deciding which these are. The most common form is red color blindness; persons afflicted with it con- fuse reds and greens. Red to the normal eye is red because it excites red sensation much, green some, and violet less; and a white page white, because it excites red, green, and violet sensations about equally. In a person without red sensation a red object would arouse only some green and violet sensation and so would be indistinguishable from a bluish green; in practice it is found that many persons confound these colors. Oases of green and violet color blindness are also met with, but they are much rarer than the red color blindness or "'Daltonism." The detection of color blindness is often a matter of con- siderable importance, especially in sailors and railroad officials, since the two colors most commonly confounded, red and green, are those used in maritime and railroad signals. Persons attach such different names to colors that 524 THE HUMAN BODY. a decision as to color blindness cannot be safely arrived, at by simply showing a color and asking its name. The best plan is to take a heap of worsted of all tints, select one, say a red, and tell the man to put alongside it all those of the same color, whether of a lighter or a darker shade; if red blind he will select not only the reds but the greens, especially the paler tints. About one man in eight is red blind. The defect is much rarer in women. Fatigue of the Retina. The nervous visual apparatus is easily fatigued. Usually we do not observe this be- cause its restoration is also rapid, and in ordinary life our eyes, when open, are never at rest; we move them to and fro, so that parts of the retina receive light alternately from brighter and darker objects and are alternately excited and rested. How constant and habitual the movement of the eyes is can be readily observed by trying to fix for a short time a small spot without deviating the glance; to do so for even a few seconds is impossible without practice. If any small object is steadily "fixed" for twenty or thirty seconds it will be found that the whole field of vision becomes grayish and obscure, because the parts of the retina receiving most light get fatigued, and arouse no more sensation than those less fatigued and stimulated by light from less illuminated objects. Or look steadily at a black object, say a blot on a white page, for twenty seconds, and then turn the eye on a white wall; the latter will seem dark gray, with a white patch on it; an effect due to the greater excitability of the retinal parts previously rested by the black, when compared with the sensation aroused elsewhere by light from the white wall acting on the previously stimulated parts of the visual surface. All persons will recall many instances of such phenomena, which are especially noticeable soon after rising in the morning. Similar things maybe noticed with colors; after looking at a red patch the eye turned 011 a white wall sees a blue-green patch ; the elements causing red sensations having been fatigued, the white mixed light from the wall now excites on that region of the retina only the other primary color sensations. The blending of colors so as to COLOR CONTRASTS, 525 secure their greatest effect depends on this fact; red and green go well together because each rests the parts of the visual apparatus most excited by the other, and so each appears bright and vivid as the eye wanders to and fro; while red and orange together, each exciting and exhaust- ing mainly the same visual elements, render dull, or in popular phrase "kill," one another. Contrasts. If a well-defined black surface be looked at on a larger white one the parts of the latter close to the black look whiter than the rest, and the parts of the black near the white blacker than the rest; so, also, if a green patch be looked at on a red surface each color is heightened near where they meet. This phenomenon is largely due to fatigue and deficient fixation: a region of the eye rested by the black or the green is brought by a movement of the organ so as to receive light from the white or red surface; phe- nomena due to this cause are known as those of successive contrast. Even in the case of perfect fixation, however, something of the same kind is seen; black looks blacker near white, and green greener near red when the eye has not moved in the least from one to the other. A small piece of light gray paper put on a sheet of red, which latter is then covered accurately with a sheet of semi-transparent tissue-paper, assumes the complementary color of the red, i.e. looks bluish green; and gray on a green sheet under similar circumstances looks pink. Such phenomena are known as those of simultaneous contrast, and are explained on psychological grounds by those who accept Young's theory of color vision. Just as a medium-sized man looks short beside a tall one, so, it is said, a black surface looks blacker near a white one, or a gray (slightly luminous white) surface, which feebly excites red, green, and violet sensations, looks deficient in red (and so bluish green) near a deeper red surface. There are, however, certain phenomena of simultaneous contrast which cannot be satis- factorily so explained, and these have led to other theories of color vision, the most important of which is that de- scribed in the next paragraph. Hering's Theory of Vision. Contrasts can be seen with 526 THE HUMAN BODY. the eyes closed and covered. If we look a shore time at a bright object and then rapidly exclude light from the eye, we see for a moment a positive after-image of the object, e.g. a window with its frame and panes after a glance at it and then closing the eyes. In these positive after-images the bright and dark parts of the object which was looked at retain their original relationship; they depend on the persist- ence of retinal excitement after the cessation of the stimulus and usually soon disappear. If an object be looked at steadily for some time, say twenty seconds, and the eyes be then closed a negative after-image is seen. In this the lights and shades of the object looked at are reversed. Frequently a positive after-image becomes negative before disappearing. The negative images are explained commonly by fatigue; when the eye is closed some light still enters through the lids and excites less those parts of the retina previously exhausted by prolonged looking at the brighter parts of the field of vision; or, when all light is rigorously excluded, the proper stimulation of the visual apparatus itself, causing the idio-retinal light, affects less the exhausted portions, and so a negative image is produced. If we fix steadily for thirty seconds a point between two white squares about 4 mm. (I inch) apart on a large black sheet, and then close and cover our eyes, we get a negative after-image in which are seen two dark squares on a brighter surface; this surface is brighter close around the negative after-image of each square, and brightest of all between them. This luminous bound- ary is called the corona, and is explained usually as an effect of simultaneous contrast; the dark after-image of the square it is said makes us mentally err in judgment and think the clear surface close to it brighter than elsewhere: and it is brightest between the two dark squares, just as a middle- sized man between two tall ones looks shorter than if along- side one only. If, however, the after-image be watched it will often be noticed not only that the light band between the squares is intensely white, much more so than the normal idio-retinal light, but, as the image fades aways, often the two dark after-images of the squares disappear entirely with all of the corona, except that part between them which i* BERING'S THEORY OF COLOR VISION. 527 still seen as a bright band on a uniform grayish field. Here there is no contrast to produce the error of judgment, and from this and other experiments Hering concludes that light acting on one part of the retina produces inverse changes in all the rest, and that this has an important part in producing the phenomena of contrasts. Similar pheno- mena may be observed with colored objects; in their nega- tive after-images each tint is represented by its complemen- ' tary, as black is by white in colorless vision. Endeavoring to exclude such loose general explanations as " errors of judgment/' Hering proposes a theory of vision which can only be briefly sketched here. We may put all our colorless sensations in a continuous series, passing through grays from the deepest black to the brightest white; somewhere half-way between will be a neutral gray which is as black as it is white. We may do some- thipof similar with our color sensations; as in gray we see black and white so in purple we see red and blue, and all cokrs containing red and blue may be put in a series of which one end is pure red, the other pure blue. So with red and yellow, blue and green, yellow and green. If we call to mind the whole solar spectrum from yellow to blue, through the yellow-greens, green, and blue-greens, we get a series in which all but the terminals have this in common that they contain some green. Green itself forms, however, a special point; it differs from all tints on one side of it in contain- ing no yellow, and from all on the other in containing no blue. In ordinary language this is recognized: we give it a definite name of its own and call it green. Its sim- plicity compared with the doubleness of its immediate neighbors entitles it to a distinct place in the color-sensa- tion series. There are three other color sensations which like green are simple and must have specific names of their own; they are red, blue, and yellow. Green may be pure green or yellow green or blue green, but never yellow and bluish at once, or reddish. Eed may be pure or yellowish or bluish, but never greenish. Eed and green are thus mutually exclusive; yellow and blue stand in a similar relationship. All other color sensations, as orango 528 THE HUMAN BODY. suggest two of the above, and maybe described as mixtures of them; but they themselves stand out as fundamental color sensations. Moreover, it follows from the above, that more than two simple colored sensations are never combined in a compound color sensation. Since red always excludes green, and yellow blue, we may call them anti-colors (the complementary colors of Young's theory), and are led to suspect that in the visual organ there must occur, in the production of each, processes which prevent the simultaneous production of the other, since tnere is no a priori reason in the nature of things why we should not see red and green simultaneously, as well as red and yellow. Along with our color sensations there is always some colorless from the black-white series; which we recognize in speaking of lighter and darker shades of the same color. Hering assumes, then, in the retina or some part of the nervous visual apparatus, three substances answering to the black-white, red-green, and yellow-blue sensational series, the construction of each substance being attended with one sensation of its pair, and its destruction with the other. Thus, when construction of the black-white substance ex- ceeds destruction, we get a blackish-gray sensation; when the processes are equal the neutral gray; when destruction exceeds construction a light-gray, and so on. In the other color series similar things would occur; when con- struction of red-green substance exceeded destruction in any point of the retina we would get, say, a red feeling; if so, then excess of -destruction would give green sensa- tion. The intensity of any given simple sensation would depend on the ratio of the difference between the construc- tion and destruction of the corresponding substance, to the sum of all the constructions and destructions of visual sub- stances going on in that part of the visual apparatus. A little thought will show that this can hardly be reconciled with the results expressed in Fechner's law. The intensity of a mixed color sensation would be the sum of the intensities of its factors, and its tint and shade dependent on the rela- tive proportion of these factors. When the construction BERING'S THEORY OF COLOR VISION. 529 and destruction of the red-green substance are equal no color sensation is aroused by it; and we get gray, due to those simultaneously occurring changes in the black-white substance which are always present, but were previously more or less cloaked by the results of the changes in the red-green substance. Eed and green in certain pro- portions cause then a white or gray sensation, not because they supplement one another, as on Young's theory, but because they mutually cancel; and so for other comple- mentary colors. Moreover, according to Hering, destruction of a visual substance going on in one region of the retina promotes construction and accumulation of that substance elsewhere, but especially in the neighborhood of the excited spot. Hence, wheu a white square on a black ground is looked at, destruction of the black-white substance overbalances construction in the place on which the image of the square falls, but around this construction occurs in a high degree. When the eyes are shut, this latter retinal region, with its great accumulation of decomposable material, is highly irritable and^ under the internal stimuli causing the idio- retinal light, breaks down comparatively fast, causing the corona, which may be intensely luminous; for with the closed eye the total constructive and destructive processes in the visual apparatus are small, and so the excess of de- struction in the coronal region bears a large ratio to the sum of the whole processes. The student must apply this theory for himself to the other phenomena of contrasts and negative images, as also to the gradual disappearance of differences between light and dark objects when looked at for a time with steady fixation; the general key being the principle that anything leading to the accumulation of a yisual substance increases its decompositions under stimu- lation, and vice versa. The main value of Hering's theory is that it attempts to account physiologically for phenomena previously indefinitely explained psychologically by such terms as "errors of judgment," which really leave the whole matter where it was, since if (as we must believe) mind is a function of brain, the errors of judgment have 530 ^ THE HUMAN BODY. still to be accounted for on physiological grounds, as due to conditions of the nervous system. Visual Perceptions. The sensations which light exci-tes in us we interpret as indications of the existence, form, and position of external objects. The conceptions which we arrive at in this way are known as visual perceptions. The full treatment of perceptions belongs to the domain of Psychology, but Physiology is concerned with the condi- tions under which they are produced. The Visual Perception of Distance. With one eye our perception of distance is very imperfect, as illustrated by the common trick of holding a ring suspended by a string in front of a person's face, and telling him to shut one eye and pass a rod from one side through the ring. If a pen* holder be held erect before one eye, while the other is closed, and an attempt be made to touch it with a finger moved across towards it, an error will nearly always- be made. (If the finger be moved straight out towards the pen it will be touched because with one eye we can estimate direction accurately and have only to go on moving the finger in the proper direction till it meets the object.) In such cases we get the only clue from the amount of effort needed to "accommodate" the eye to see the object distinctly. When we use both eyes our perception of dis- tance is much better; when we look at an object with two eyes the visual axes are converged on it, and the nearer the object the greater the convergence. We have a pretty accurate knowledge of the degree of muscular effort required to converge the eyes on all tolerably near points. When objects are farther off, their apparent size, and the modifi- cations their retinal images experience by aerial perspective, come in to help. The relative distance of objects is easiest determined by moving the eyes; all stationary objects then appear displaced in the opposite direction (as for example when we look out of the window of a railway car) and those nearest most rapidly; from the different apparent rates of movement we can tell which are farther and nearer. We so inseparably and unconsciously bind up perceptions of dis« tance with the sensations aroused by objects looked at, that VISUAL PERCEPTION OF SIZE. 531 we seem to see distance; it seems at first thought as definite a sensation as color. That it is not is shown by cases of persons born blind, who have had sight restored later in life by surgical operations. Such persons have at first no visual perceptions of distance: all objects seem spread out on a flat surface in contact with the eyes, and they only learn gradually to interpret their sensations so as to form judgments about distances, as the rest of us did uncon- sciously in childhood before we thought about such things, The Visual Perception of Size. The dimensions of the retinal image determine primarily the sensations on which conclusions as to size are based; and the larger the visual angle the larger the retinal image: since the visual angle depends on the distance of an object the correct perception of size depends largely upon a correct perception of distance; having formed a judgment, conscious or unconscious, as to that, we conclude as to size from the extent of the retinal region affected. Most people have been surprised now and then to find that what appeared a large bird in the clouds was only a small insect close to the eye; the large apparent size being due to the previous incorrect judgment as to the dis- tance of the object. The presence of an object of tolerably well-known height, as a man, also assists in forming con- ceptions (by comparison)as to size; artists for this purpose frequently introduce human figures to assist in giving an idea of the size of other objects represented. The Visual Perception of a Third Dimension of Space. This is very imperfect with one eye; still we can thus arrive at conclusions from the distribution of light and shade on an object, and from that amount of knowledge as to the relative distance of different points which is attainable monocularly; the different visual angles under which objects are seen also assist us in concluding that objects -are farther and nearer; and so are not spread out on a plane before the eye, but occupy depth also. Painters depend mainly on devices of these kinds for representing solid bodies, and objects spread over the visual field in the third dimension of space. Single Vision with Two Eyes. When we look at a 532 THE HUMAN BODY. flat object with both eyes wo get a similar retinal image in each. Under ordinary circumstances we see, however, not two objects but one. In the habitual use of the eyes we move them so that the images of the object looked at fall on the two yellow spots. A point to the left of this object forms its image on the inner (right) side of the left eye and the outer (right) side of the right. An object verti- cally above that looked at would form an image straight below the yellow spot of each eye; an object to the left and above, its image to the inner side and below in the left eye and to the outer side and below in the right eye; and so on. We have learned that similar simultaneous excita- tions of these corresponding points mean single objects, and so interpret our sensations. This at least is the theory of the experiential or empirical school of psychologists, though others believe we have a sort of intuition on the subject. When the eyes do not work together, as in the muscular incoordination of one stage of intoxication, then they are not turned so that images of the same objects fall on cor- responding retinal points, and the person sees double. When a squint comes on, as from paralysis of the external rectns of one eye, the sufferer at first sees double for the same reason. If a given object is looked at lines drawn from it through the nodal points reach the fovea central is in each eye. Lines so drawn at the same time from a more distant object diverge less and meet each retina on the inner side of its fovea; but as above pointed out the corresponding points for each retinal region on the inside of the left eye, are on the outside of the right, and vice versa. Hence the more distant object is seen double. So, also, is a nearer object, be- cause the more diverging lines drawn from it through the nodal points lie outside of the fovea in each eye. Most people go through life unobservant of this fact; we only pay attention to what we are looking at, and nearly always this makes its images on the two f ovese. That the fact is as above stated may, however, be readily observed. Hold one finger a short way from the face and the other a little farther off; looking at one, observe the other without moving STEREOSCOPIC VISION. 533 the eyes; it will be seen double. For every given position of the eyes there is a surface in space, all objects on which produce images on corresponding points of the two retinas: this surface is called the lioropter for that position of the eyes: all objects in it are seen single; all others in the visual field, double. The Perception of Solidity. When a solid object is looked at the two retinal images are diiferent. If a trun- cated pyramid be held in front of one eye its image will be that represented at P9 Fig. 142. If, however, it be held midway between the eyes, and looked at with both, then the left-eye image will be that in the middle of the figure, and the right-eye image that to the right. The small surface, bdca, in one answers to the large surface, V d' c a, in the other. This may be readily observed by FIG. 142 holding a small cube in front of the nose and alternately looking at it with each eye. In such cases, then, the retinal images do not correspond, and yet we combine them so as to sec one solid object.' This is known as stereoscopic vision, and the illusion of the common stereoscope depends on it. Two photographs are taken of the same object from two different points of view, one as it appears when seen by the left, and the other when looked at with the right eye. These are then mounted for the stereoscope so that each is seen by its proper eye, and the scene or object is seen in distinct relief, as if, instead of flat pictures, solid objects were looked at. Of course in many stereo- scopic views the distribution of light and shade, etc., assist, but these are quite unessential, as may be readily observed by enlarging the middle and right outline drawings of Fig. 534 THE HUMAN BODY. 142 to the ordinary size of a stereoscopic slide, and placing them in the instrument. A solid pyramid standing out into space will be distinctly perceived; if the pictures be reversed the pyramid appears hollow. The pictures must not be too different, or their combination to give the idea of a single solid body will not take place. Many persons, indeed, fail entirely to get the illusion with ordinary stereo- scopic slides. The phenomena of stereoscopic vision mili- tate strongly against the view that there are any pre- arranged corresponding points in the two retinas. The Perception of Shine. When we look at a rippled lake in the moonlight, we get the perception of a "shiny" or brilliant surface. The moonlight is reflected from the waves to the eyes in a number of bright points: these are not exactly the same for both eyes, since the lines of light- reflection from the surface of the water to each are different. The perception of brilliancy seems largely to depend on this slight non-agreement of the light and dark points on the two retinas. A rapid change of luminous points, to and fro between neighboring points on one retina, seems also to produce it CHAPTER XXXIV. THE EAR AND HEARING. The External Ear. The auditory organ in man consists of three portions, known respectively as the external ear, the middle ear or tympanum, and the internal ear or 7&fly- rinth} the latter contains the end organs of the auditory nerve. The external ear consist of the expansion seen on ihe exterior of the head, called the concha, M, Fig. 143, -and a passage leading in from it, the external auditory FIG. 143.— Semidiagrammatic section through the right ear (Czermak). AY, concha; G, external auditQ£vmeatxis ; T, tympanic membrane; P, tympanic cavity; o, oval foramenf r, Toufid foramen; R, pharyngeal opening of Eusta- chian tube; V, vestibule; .B, a semicircular canal; S, the cochlea; Vt, scala vestibuli; Pt, scala tympani; A, auditory nerve. meatus, G. This passage is closed at its inner end by the tympanic or drum membrane, T. It is lined by skin, through which numerous small glands, secreting the wax of the ear, open. 536 THE HUMAN BODY. The Tympanum. (P, Fig. 143) is an irregular cavity in: the temporal bone, closed externally by the drum mem- brane. From its inner side the Eustachian tube (E) pro- ceeds and opens into the pharynx (g, Fig. 89, page 309), and the mucous membrane of that cavity is continued ii]> the tube to line the tympanum; between this inside, and the skin outside, is the proper tympanic membrane com- posed of connective tissue. The inner wall of the tym- panum is bony except for two small apertures, the oval and round foramens, o and r, which lead into the labyrinth. During life the round aperture is closed by the lining mucous membrane, and the oval in another way, to be de- scribed presently. The tympanic membrane, T, stretched across the outer side of the tympanum, forms a shallow funnel with its concavity outwards. It is pressed by the- external air on its exterior, and by air entering the tym- panic cavity through the Eustachian tube on its inner side.. If the tympanum were closed these pressures would not be always equal when barometric pressure varied, and the- membrane would be bulged in or out according as the ex- ternal or internal pressure on it were the greater. On the- other hand, were the Eustachian tube always open the- sounds of our own voices would be loud and disconcerting, so it is usually closed; but every time we swallow it is opened, and thus the air pressure in the cavity is kept equal to that in the external auditory meatus. By holding the nose, keeping the mouth shut, and forcibly expiring,, air may be forced under pressure into the tympanum, and will be held in part imprisoned there until the next act of swallowing. On making a balloon ascent or going rapidly down a deep mine, the sudden and great change of aerial pressure outside frequently causes painful tension of the drum membrane, which may be greatly alleviated by fre- quent swallowing. The Auditory Ossicles. Three small bones lie in the tympanum forming a chain from the drum membrane to the oval_foramen. The external bone (.bIig7"i44) is the malleus or hammer; the middle one, the incus or anvil; and the internal, the stapes or stirrup. The malleus, M, has TYMPANIC BONES. 537 Mcp an upper enlargement or head, which, carries on its inner side an articular surface for the incus; below the head is a constriction, the neck, and below this two processes complete the bone; one, the long or slender process, is imbedded in a ligament which reaches from it to the front Avail of the tympanum; the other process, the handle, reaches down between the mucous membrane lining the inside of the drum membrane and the membrane proper, and is firmly attached to the lat- ter near its centre and keeps the membrane dragged in there so as to give it its peculiar concave form, as seen from the out- side. The incus has a body and two processes and is much like a molar tooth with two fangs. On its body is an articular hollow to receive the head of the malleus; its short process ( Jb) is attached by ligament to the back wall of the tym- panum; the long process stapes; on the tip of this JP1 Mm FIG. 144. — The auditory ossicles of the right ear, seen from the front. M, mal- leus; J, incus; 8, stapes: Mcp, head of the malleus ; Me, neck of ditto ; Ml, long process; Mm, handle: Jc,body, J6, short, and Jl, long process, of incus; Jpl, o& orbiculare: Scp, head of stapes. (Jl) is directed inwards to tha process is a little knob, which represents a bone (os orbiculare) distinct in early life. The stapes (S) is extremely like a stirrup, and its base (the foot- piece of the stirrup) fits into the oval foramen, to the mar- gin of which its edge is united by a fibrous membrane, allowing of a little play in and out. From the posterior side of the neck of the malleus a ligament passes to the back wall of the tympanum: this, with the ligament imbedding the slender process and fixed to the front wall of the tympanum, forms an antero-pos- terior axial ligament, on which the malleus can slightly rotate, so that the handle can be pushed in and the head out and vice versa. If a pin be driven through Fig. 144 just below the neck of the malleus and perpendicular to tha 538 THE HUMAN BODY. paper it will very fairly represent this axis of rotation. Connected with the malleus is a tiny muscle, called the tensor tympani; it is inserted on the handle of the bone beloAv the axis of rotation, and when it contracts pulls the handle in and tightens the drum membrane. Another muscle (the stapedius) is inserted into the outer end of the stapes, and when it contracts fixes the bone so as to limit its range of movement in and out of the fenestra ovalis. The Internal Ear. The labyrinth consists primarily of chambers and tubes hollowed out in the temporal bone and inclosed by it on all sides, except for the oval and round FIG. 145.— Casts of the bony labyrinth. A, left labyrinth seen from the outer side; B, right labyrinth from the inner side; C, left labyrinth from above; Fc, round foramen; Fv, oval foramen; h, horizontal semicircular canal; ha, its ampulla; vaa, ampulla of anterior vertical semicircular canal; vpa, ampulla of posterior vertical semicircular canal ; vc, conjoined portion of the two ver- tical canals. foramens on its exterior, and certain apertures on its inner side by which blood-vessels and branches of the auditory nerve enter; during life all these are closed water-tight in one way or another. Lying in the bony labyrinth thus consti- tuted, are membranous parts, of the same general form but smaller, so that between the two a space is left; this is filled with a watery fluid, called the perilympli; and the mem* Iranous internal ear is filled by a similar liquid, the endo* lymph. The Bony Labyrinth. The bony labyrinth is described in three portions, the msti&ule, the semicircular canals, and the cochlea; casts of its interior are represented from THE INTERNAL EAR. different aspects in Fig. 145. The vestibule is the central part and has on its exterior the oval foramen (Fv) inta which the base of the stirrup-bone fits. Behind the vestibule- are three bony semicircular canals, communicating with the back of the vestibule at each end, and dilated near one- end to form an ampulla (vpa, vaa, and ha). The horizon- tal canal lies in the plane which its name implies and has its ampulla at the front end. The two other canals lie vertically, the anterior at right angles, and the pos- terior parallel, to the median antero-posterior vertical plane of the head. Their amp ullary ends are turned forwards, and open close together into the vestibule; their posterior ends unite (vc) and have a common vestibular opening. The bony cochlea is a tube coiled on itself somewhat like a snail's shell, and lying in front of the vestibule. The Membranous Labyrinth. The membranous vesti- bule, lying in the bony, consists of two sacs communicating by a narrow aperture. The posterior is called the utriculus, and into it the membranous semicircular canals open. The anterior, called the sacculus, communicates by a tube with the membranous cochlea. The mem- branous semicircular canals much resemble the bony, and each ha& an ampulla; in most of their extent they are only united by a few irregular connective-tissue bands with the periosteum lining the bony canals; but in the ampulla one side of the mem- branous tube is closely adherent to its bony protector; at this point nerves enter the former. The relations of the membranous to the bony cochlea are more complicated. A section through this part of the auditory apparatus (Fig. 146) shows that its osseous portion consists of a tube FIG. 146.— A section through the cochlea in the line of its axis. -540 THE HUMAN BODY. wound two and a half times (from left to right in the right •ear and vice versa) around a central bony axis, the modiolus. From the axis a shelf, the lamina spiralis, projects and par- tially subdivides the tube, extending farthest across in its lower coils. Attached to the outer edge of this bony plate is the membranous cochlea (scala media), a tube triangular in cross-section and attached by its base to the outer side of the bony cochlear spiral. The spiral lamina and the membranous cochlea thus subdivide the cavity of the bony tube (Fig. 147) into an upper portion, the scala vestibuli, S V, and a lower, the scala tympani, ST. JBetween these lie the lamina spiralis (Iso) and the mem- f +Q. 147.— Section of one coil of the cochlea, magnified. /SF", scala vestibuli' R, membrane of Reissner; CO. membranous cochlea (scala media); Us, limbua iamince spiralis, t, tectorial membrane; ST, scala tympani; Iso, spiral lamina; Co. rods of Corti; 6, basilar membrane. branous cochlea (CO), the latter being bounded above by the membrane of Reissner (R) and below by the basilar membrane (#). The free edge of the lamina spiralis is thickened and covered with connective tissue which is hol- lowed out so as to form a spiral groove (the sulcus spiralis, ss) along the whole length of the membranous cochlea. The latter does not extend to the tip of the bony cochlea; above its apex the scala vestibuli and scala tympani com- municate; both are filled with perilymph, and the former communicates below with the perilymph cavity of the ves- tibule, while the scala tympani abuts below on the round foramen, which, as has already been pointed out, is closed by a membrane. The membranous cochlea contains cer- ORGAN OF CORTL 541 tain solid structures seated on the basilar membrane and forming the organ of Corti; the rest of its cavity is filled with endolymph, which communicates with that in the sacculus. The Organ of Corti. This contains the^nd organs of _the cochlear nerves. Lining the sulcus spiralis are cuboi- ~dal cells; on tne inner margin of the basilar membrane they become columnar, and then are succeeded by a row which bear on their upper ends a set of short stiff hairs, and con- stitute the inner hair-cells, which are fixed below by a narrow apex to the basilar membrane; nerve-fibres enter them. To the inner hair-cells succeed the rods of Corti B FIG. 148.— The rods of Corti. A, a pair of rods separated from the rest; B, a bit of the basilar membrane with several rods on it, showing how they cover in the tunnel of Corti; i, inner, and e, outer rods; 6, basilar membrane; r, reti- cular membrane. (Co, Fig. 147), which are represented highly magnified in Fig. 148. These rods are stiff and arranged side by side in two rows, leaned against one another by their upper ends so as to cover in a tunnel; they are known respectively as the inner and outer rods, the former being nearer the lamina spiralis. Each rod has a somewhat dilated base, firmly fixed to the basilar membrane; an expanded head where it meets its fellow (the inner rod presenting there a concavity into which the rounded head of the outer fits) ; and a slender shaft uniting the two, slightly curved like an italic /. .The inner rods are more slender and more numerous than the outer, the numbers being about 6000 .and 4500 respectively. Attached to the external sides of the heads of the outer rods is the reticular membrane (r. 542 THE HUMAN BODY. Fig. 148), which is stiff and perforated by holes. Exter- nal to the outer rods come four rows of outer hair-cells, connected like the inner row with nerve-fibres; their bristles project into the holes of the reticular membrane. Beyond the outer hair-cells is ordinary columnar epithe- lium, which passes gradually into cuboidal cells lining most of the membranous cochlea. The upper lip of the sulcus spiralis is uncovered by epithelium, and is known as the Iwibus lamince spiralis; from it projects the tectorial membrane (t, Fig. 147) which extends over the rods of Corti and the hair- cells. Nerve-Endings in the Semicir- cular Canals and the Vestibule. Nerves reach the ampulla of each semicircular canal, and, perforat- ing its wall, enter the epithelium lining it, which is there made of cells of two kinds (Fig. 149). Some of the cells are colum- nar (c), and each of these bears FIG. HO.— sensory epithelium on its free end a long stiff hair ora0nai,rsua!:acuie(.semicircular (*)• A branch of a nerve fibre (n) joins the other end of the cell. Between the columnar cells are more slender, stiffer nucleated supporting cells (/). The hairs of the colum- nar hair-cells are quite long, several times longer than the? cells. They do not project freely into the endolymph which fills the semicircular canals, but are imbedded in a mucus-like substance, somewhat dome-shaped, known as the cupula terminal-is, which is not represented in the figure. In the utricle and saccule are somewhat similar structures; but collected among the hairs, and imbedded in gelatinous matter, are minute calcareous particles, the ear-stones or otoliths. The Loudness, Pitch, and Timbre of Sounds. Sounds, as sensations, fall into two groups — notes and noises. Physi- PROPERTIES OF SOUND. 543 cally, sounds consist of vibrations, and these, under most circumstances, when they first reach our auditory organs, are alternating rarefactions and condensations of the air, or aerial waves. When the waves follow one another uni- formly, or periodically, the resulting sensation (if any) is a note; when the vibrations are aperiodic it is a noise. In notes we recognize (1) loudness or intensity; (2) pitch; (3) quality or timbre, or, as it has been called, tone color; a note of a given loudness and pitch produced by a trum- pet and by a violin has a different character or individu- ality in each case; this quality is its timbre. Before un- derstanding the working of the auditory mechanism we must get some idea of the physical qualities in objective sound which the subjective differences of auditory sensa- tions are signs of. The loudness of a sound d^p^n^f} n^ f>m fprfifi of the .aerial jvayej; the greater the intensity of the alternating condensations and rarefactions of these in the external auditory meatus, the louder the sound. The pitch of a note depends on the length of the waves, that is the dis^"—- y tance from one point of greatest condensation to the next, or (what amounts to the same thing) on the number of waves reaching the ear in given time, say a second. The shorter the waves the more rapidly they follow one another, and the higher the pitch of the note. When audible vibrations bear the ratio 1 : 2 to one another, we hear the musical interval called an octave. The note c on the un- accented octave is due to 132 vibrations in a second. The note c' , the next higher octave of this, is produced by 264 vibrations in a second; the next lower octave (great octave, C), by 66 ; and so on. Sound vibrations may be too rapid or too slow in succession to produce sonorous sensations, just as the ultra-violet and ultra-red rays of the solar spectrum fail to excite the retina. The highest-pitched audible note answers to about 38,016 vibrations in a second, but it differs in individuals; many persons cannot hear the cry of a bat nor the chirp of acricket, which lie near this upper audible limit. On the other hand, sounds of 544: THE HUMAN BODY. vibrational rate about 40 per second are not well heard, and a little below this become inaudible. The highest note used in orchestras is the d* of the fifth accented octave, produced by the piccolo flute, due to 4752 vibra- tions in a second; and the lowest-pitched is the El9 of the contra octave, produced by the double bass. Modern grand pianos and organs go down to Gl9 in the contra octave (33 vibrations per second) or even Au, (27£), but the musical quality of such notes is imperfect; they produce rather a "hum" than a true tone sensation, and are only used along with notes of higher octaves to which they give a character of greater depth. Pendular Vibrations. Since the loudness of a tone de- pends on the vibrational amplitude of its physical antece- dent, and its pitch on the vibrational rate, we have still £o seek the cause of timbre; the quality by which we recog- nize the human voice, the violin, the piano, and the flute, even when all sound the same note and of the same loudness. The only quality of periodic vibrations left to account for this, is what we may call wave-form. Think of the movement of a pendulum; starting slowly from its highest point, it sweeps faster and faster to its lowest, and then slower and slower to its highest point on the opposite side; and then repeats the movements in the reverse direc- tion. Graphically we may represent such vibrations by the outer continuous curved line in Fig. 150. Suppose the lower end of the pendulum to bear a writing point which marked on a sheet of paper traveling down uniformly behind it, and at such a rate as to travel the distance 0-1 in two seconds. If the pendulum were at rest the straight vertical line would be drawn. But if the pendulum were swinging we would get a curved line, compounded of the vertical movement of the paper and the to-and-fro move- ment of the pendulum, writing sometimes on one side of the line 0-1-2 and sometimes on the other. Starting at a moment when the pendulum crosses the middle, 0.. we would get described the curve 0 a\ a* az, at first separating fast from the vertical line, then slower., then returning, at first PENDULAR VIBRATIONS. 545 gradually then faster, until it crossed the vertical again, at the end of a second and commenced a similar excursion on its other side, at the end of which it would be back at 1, and in just the same position, and ready to repeat exactly the swing, with which we commenced. A pendulum thus executes similar movements in equal periods of time, or its vibrations are periodic. A full swing on each side of the position of rest constitutes a complete vibration, so the vibrational period of a second's pendulum is two seconds: at the end of that time it is precisely where it was two seconds before, and moving in the same direction and at the same rate. It is clear that by examining such a curve we could tell exactly how the pendulum moved, and also in what period if we knew the rate at which the paper on which its point wrote was moving. The vertical line 0-1-2 is called the abscissa; per- pendiculars drawn from it and meeting the curve are ordinates: equal lengths on the abscissa represent equal times; where an ordinate from a given point of the abscissa meets the curve, there the writing point was at that moment; where succes- sive ordinates increase or decrease rapidly the pendulum moved fast from or towards its position of rest, and vice versa. Simi- larly, any other periodic movement may be perfectly represented by curves; and since the form of the curve tells us all about the movement, it is common to speak of the "form of a vibra- tion," meaning the form of the curve which indicates its characters. Periodic vibrations (Fig. 150), whose ordi- nates at first grow fast, then more slowly, next dimin- ish slowly and then faster, and represented by a symme- trical curve on one side the abscissa, which is repeated FIG. 150. 546 THE HUMAN BODY. exactly on the other side of the abscissa, are known as pendular vibrations. The Composition of Vibrations. The vibrations of a second's pendulum set the air-particles in contact with it in similar movement, but the aerial waves succeed one- another too slowly to produce in us the sensation of a musical note. If for the pendulum we substitute a tuning- fork (the prongs of which move in a like way), and the fork vibrates 132 times per 1", then 132 aerial waves will fall on the tympanic membrane in that time, and we will hear the note c of the unaccented octave. If the larger con- tinuous curve in Fig. 150 represent the aerial vibrations in this case, the distance 0 to 1 on the abscissa will represent rj-J-g- of a second. Let, simultaneously, the air be set in movement by a fork of the next higher octave, c', making 2 64 vibrations per 1"; under the influence of this second fork alone, the aerial particles would move as represented by the line 0, J1, b*, and so on, the waves being half as long and cutting the abscissa twice as ofter. But when both forks act together the aerial movement will be the algebraic sum of the movements due to each fork; when both drive the air one way they will reinforce one another, and vice versa; the result will be the movement represented by the dotted line, which is still periodic, repeating itself at equal intervals of time, but no longer pendular, since it is not alike on the ascending and descending limbs of the curves. We thus get at the fact that non-pendular vibrations may be pro- duced by the fusion of pendular, or, in technical phrase, by their composition. Suppose several musical instruments, as those of an or- chestra, to be sounded together. Each produces its own effect on the air-particles, whose movements, being the algebraical sum of those due to all, must at any given in- stant be very complex; yet the ear can pick out at will and follow the tones of any one instrument. From the com- plex aerial movement it can select that fraction of it which one vibrating body produces. The air in the external auditory meatus at any given moment can only be in one state of rarefaction or condensation and at one rate ANALYSIS OP VIBRATIONS. 547 .-and in one direction of movement, this being the resultant of all the forces acting upon it; all clashing, and some push- ing one way and others another. If the resultant move- ment be not periodic it will be recognized as due to noises •or to several simultaneous inharmonic musical tones; this is commonly the case when musical tones are not united designedly, and the ear thus gets one criterion for distin- guishing movements of the air due to several simultaneous musical tones. However, a composite set of tones will give rise to periodic vibrations when all are due to vibrations of rates which are multiples of the same whole number. In such cases the movement of the air in the auditory meatus has no property except vibrational form by which the ear could distinguish it from a simple tone; when the two tuning-forks giving the forms of vibration (with rates as 1 to 2), represented in Fig. 150 by continuous lines, are sounded together, we get the new form of vibration repre- sented by the dotted line, and this has the same period as that of the lower-pitched fork; yet the ear can clearly dis- tinguish the resultant sound from that of this fork alone, .as a note of the same pitch but of different timbre; and with practice can recognize exactly what simple vibrations go to make it up. The Analysis of Non-Pendular Vibrations. If a per- son with a trained ear listens attentively to any ordinary musical tone, such as that of a piano, he hears, not only the note whose vibrational rate determines the pitch of the tone as a whole, but a whole series of higher notes, in harmony with the general or fundamental tone; this latter is the primary partial tone, and the others are secondary partial tones; nearly all tones used in music contain both. If the prime tone be due to 132 vibrations a second (c), its first upper partial is c (= 264 vibrations per second); the next is the fifth of this octave (g1 = 396 = 132 X 3 vibrations per 1'); the next is the second octave, c" (132 X 4 = 528 vibrations per 1'); the next is the major third of the c" (=132 X 5 = 660 vibrations per second = e"), and so on. The only form of vibration which gives no upper partial tones is the pendular; we may call notes due to such vibra- 548 THE HUMAN BODY. tions simple tones; and we, consequently, recognize in music- tones which are simple (such as those of tuning-forks) and those which are compound; these latter are non-pendular in form. We find, then, that the form of aerial vibrations deter- mines in our sensations the occurrence or non-occurrence of upper partial tones. It also, as we have seen, deter- mines the quality or timbre of the tone, since vibrational amplitude and rate are otherwise accounted for in sensa- tion by loudness and pitch. It can be proved, by the employment of the higher mathematics, that every periodic non-pendular movement can be analyzed (as the dotted curve of Fig. 150 may be) into a given number of pendular vibrations, that is, every compound vibration into a set of simple ones; and that every periodic non-pendular vibration can be made by the- combination of pendular. Moreover, any given compound vibration can be analyzed into but one set of simple ones;- no other combination will produce it. Consequently a vibra- tional movement of the air in the external auditory passage, producing a compound musical tone sensation, can be ex- hibited always, but only in one way, as the sum of a num- ber of simple vibrations, whose rates are multiples of that which determines the pitch of the tone. Now when the trained ear listens to a tone with the object of detecting upper partials if present, it hears then? only when the vibrations producing the tone are non-pen- dular, i.e. when upper partials, theoretically, might be ex- pected; and those heard are exactly those demanded by- theory; by the help of instruments their detection is made- easy even to untrained ears. In ordinary circumstances we do not heed secondary partial tones; we hear a note of the pitch of the primary partial and of a certain timbre; and whenever the upper partials present are different, or of different relative intensities, the timbre of the note varies. Hence it becomes probable that, just as the ear can at will follow any instrument in an orchestra, analyzing the aerial movement so as to select and follow the fraction of the whole due to that one, so it can and does analyze compound; SYMPATHETIC RESONANCE. 549 tones when proceeding from one instrument, and that the ripper partial s, not rising into consciousness as definite tones, but present as subdued sensations, give its char- acter to the whole tone and determine its timbre. It might be, however, that the composition of non-pendular vibrations from pendular was a mere mathematical possibil- ity, having no real existence in nature. Before we can ac- cept the above explanation of timbre, we must see if there is any evidence that, as a matter of fact, non-pendular vibra- tions, not only may be, but are, made up by the combination of pendular. / Sympathetic Besonance. Imagine slight taps to be ugffen to a pendulum ; n these be repeated at such intervals of time as to always help the swing and never to retard it, I the pendulum will soon be set in powerful movement. If the taps are irregular, or when regular come at such intervals as sometimes to promote and sometimes retard the move- ment, no great swing will be produced; but if they always push the pendulum in the way it is going at that instant, they need not come every swing in order to set up a power- ful vibration-; once in two, three, or four swings will do. A stretched string, such as that of a piano, is so far like a given pendulum that it tends to vibrate at one rate and no other; if aerial waves hit it at exactly the right times they soon set it in sufficiently powerful vibrations to cause it to emit an audible note. By using such strings we might hope to detect the separate pendular vibrations in any non-pendular aerial periodic movement if such really existed; certain strings would pick out the pendular com- ponent agreeing in rate with their own vibrational period and be soon set in powerful movement; while those not vibrating in the same period as any of the pendular compo- nents, would remain practically at rest, like the pendulum getting taps which sometimes helped and sometimes impeded its swing. If the dampers of a piano be raised and a note be sung to it, it will be found that several strings are set in vibration, such vibrations being called sympathetic. The human voice emits compound tones which can be mathe- matically analyzed into simple vibrations, and if the piano 550 THE HUMAN BODY. strings set in movement by it be examined, they will be found to be exactly those which answer to these pendular vibrations and to no others. We thus get experimental grounds for believing that compound tones are really made up of a number of simple vibrations, and get an additional justification for the supposition that in the ear each note is analyzed into its pendular components; and that the differ- ence of sensation which we call timbre is due to the effect of the secondary partial tones thus perceived. If so, the ear must have in it an apparatus adapted for sympathetic resonance. It may be asked why, if the ear analyzes vibrations in this way, do we not commonly perceive it? How is it that what we ordinarily hear is the timbre of a given tone and not the separate upper partials which give it this character ? The explanation is more psychological than physiological, and belongs to the same category as the reason why we do not ordinarily notice the blind spot in the eye, or the double- ness of objects out of the horopter, or the duplicity of stereoscopic images. We only use our senses in daily life when they can tell us something that may be useful to us, and we neglect so habitually all sensations which would be useless or confusing, that at last it needs special attention to observe them at all. The way in which tones are com- bined to give timbre to a note is a matter of no importance in the daily use of them, and so we fail entirely to observe the components and note only the resultant, until we make a careful and scientific examination of our sensations. The Functions of the Tympanic Membrane. If a stretched membrane, such as a drum-head, be struck, it will be thrown into periodic vibration and emit for a time a note of a determined pitch. The smaller the membrane and the tighter it is stretched the higher the pitch of its note; every stretched membrane thus has a rate of its own at which it tends to vibrate, just as a piano or violin string has. When a note is sounded in the air near such a membrane, the alternating waves of aerial condensation and rarefaction will move it; and if the waves succeed at the vibrational rate of the membrane the latter will be set in powerful sympathetic USES OF DRUM-MEMBRANE. 551 vibration; if they do not push the membrane at the proper times, their effects will neutralize one another: hence -such membranes respond well to only one note. The tym- panic membrane, however, responds equally well to a large number of notes; at the least for those due to aerial vibra- tions of rates from 60 to 4000 per second, running over eight octaves and constituting those commonly used in music. This faculty depends on two things; (1) the mem- brane is comparatively loosely and not uniformly stretched; •(2) it is loaded by the tympanic bones. The drumVmembrane is (p. 536) in the form of a shallow iunnel with its sides convex towards its cavity; in such a membrane the tension is not uniform but increases towards the centre, and! it has accordingly no proper note of its own. Further, whatever tendency such a membrane may have to vibrate rather at one rate than another, is almost com- pletely removed by "damping" it; i. e. placing in contact with it something comparatively heavy and which has to be moved when the membrane vibrates. This is effected by the tympanic bones, fixed to the drum-membrane by the Jhandle of the malleus. Another advantage is gained by the damping; once a stretched membrane is set vibrating it continues so doing for some time; but if loaded its move- ments cease almost as soon as the moving impulses. The •dampers of a piano are for this purpose; and violin- players have to "damp" with the fingers the strings they iave used when they wish the note to cease. The tym- panic bones act as dampers. Functions of the Auditory Ossicles. When the air in the external auditory meatus is condensed it pushes in the handle of the malleus. This bone then slightly rotates on the axial ligament (p. 537) and, locking into the incus where the two bones articulate, causes the long process (//, Fig. 144) of the latter to move inwards. The incus thus pushes in the stapes; the reverse occurs when air in the auditory passage is rarefied. Aerial vibrations thus set the •chain of bones swinging, and push in and pull out the base of the stapes, which sets up waves in the perilymph of the labyrinth, and these are transmitted through the mem- 552 THE HUMAN BODY. branous labyrinth to the endolymph. These liquids being chiefly water, and practically incompressible, the end of the- stapes could not work in and out at the oval foramen, were- the labyrinth elsewhere completely surrounded by bone: but the membrane covering the round foramen bulges out. when the base of the stapes is pushed in, and vice versa; and so allows of waves being set up in the labyrinthic liquids. These correspond in period and form to those in the auditory meatus; their amplitude is determined by the- extent of the vibrations of the drum-membrane. The form of the tympanic membrane causes it to trans- mit to its centre, where the malleus is attached, vibrations-, of its lateral parts in diminished amplitude but increased power; so that the tympanic bones are pushed only a little- way but with considerable force. Its area, too, is about twenty times as great as that of the oval foramen, so that force collected on the larger area is, by pushing the tym- panic bones, all concentrated on the smaller. The ossicles, also form a bent lever (Fig. 144)* of which the fulcrum is at the axial ligament and the effective outer arm of this lever is about half as long again as the inner, and so the move- ments transmitted by the drum-membrane to the handle of the malleus are communicated with diminished range, but. increased power, to the base of the stapes. Ordinarily sound-waves re^ch the labyrinth through the tympanum, but they may also be transmitted through the bones of the head; if the handle of a vibrating tuning-fork be placed on the vertex, the sounds heard by the person experimented upon, seem to have their origin inside his own. cranium. Similarly, when a vibrating body is held between the teeth, sound reaches the end organs of the auditory nerve through the skull-bones; and persons who are deaf from disease or injury of the tympanum can thus be made to hear, as with the audiphone. Of course if deafness be due to disease of the proper nervous auditory apparatus no device can make the person hear. Function of the Cochlea. We have already seen reason to believe that in the ear there is an apparatus adapted for sympathetic resonance, by which we recognize different: *P. 537. FUNCTIONS OF COCHLEA. 553 musical tone-colors; the minute structure of the membra- nous cochlea is such as to lead us to look for it there. An old view was that the rods of Corti, which vary in length,, were like so many piano-strings, each tending to vibrate at a given rate and picking out and responding to pendular aerial vibrations of its own period, and exciting a nerve which gave rise to a particular tone sensation. When the labyrinthic fluids were set in non-pendular vibrations, the rods of Corti were thought to analyze these into their pendu- lar components, all rods of the vibrational rate of these be- ing set in sympathetic movement, but that rod most whose period was that of the primary partial tone; this rod would determine the pitch of the note, and the less-marked sen- sation due to the others affected would give the timbre. The rods, however, do not differ in size sufficiently to account for the range of notes which we hear; they are absent in birds, which undoubtedly distinguish different musical notes; and the nerve-fibres of the cochlea are not connected with them but with the hair-cells. On the whole it seems probable that the basilar mem- brane is to be looked upon as the primary arrangement for sympathetic resonance in the ear. It increases in breadth twelve times from the base of the cochlea to its tip (the less width of the lamina spiralis at the apex more than compensating for the less size of the bony tube there) and is stretched tight across, but loosely in the other direc- tion. A membrane so stretched behaves as a set of separate strings placed side by side, somewhat as those of a harp but much closer together; and each string would vibrate at its own period without influencing much those on each side of it. Probably, then, each transverse band vibrates to sim- ple tones of its own period, and excites the hair-cells which lie on it, and through them the nerve-fibres. Perhaps the rods of Corti, being stiff, and carrying the reticular mem- brane, rub that against the upper ends of the hair-cells which project into its apertures and so help in a subsidiary way, each pair of rods being especially moved when the band of basilar membrane carrying it is set in vibration. The tec- torial membrane is probably a "damper;" it is soft and 554 THE HUMAN BODY. inelastic, and suppresses the vibrations as soon as the mov- ing force ceases. Function of the Vestibule and Semicircular Canals. Many noises are merely spoiled music; they are due to tones so combined as not to give rise to periodic vibrations; these are probably heard by the cochlea. If a single violent air-wave ever cause a sound sensation (which is doubtful since any violent push of an elastic substance, such as the .air, will cause it to make several rebounds before coming to rest) we perhaps hear it by the vestibule; the otoliths, there in contact with the auditory hairs, are imbedded in a tenacious gummy mass quite distinct from the proper •endolymph, and are not adapted for executing regular vibrations, but they might yield to a single powerful impulse and transmit it to the hair -cells, and through them stimulate the nerves. There is reason to believe that the semicircular canals have nothing to do with hearing; their supposed function is described in Chapter XXXV. Auditory Perceptions. Sounds, as a general rule, do not seem to us to originate within the auditory apparatus; -\ve refer them to an external source, and to a certain extent can judge the distance and direction of this. As already mentioned, the extrinsic reference of sounds which reach the labyrinth through the general skull-bones instead of through the tympanic chain is imperfect or absent. The recogni- tion of the distance of a sounding body is possible only when the sound is well known, and then not very accurately; from its faintness or loudness we may make in some cases a pretty good guess. Judgments as to the direction of a sound are also liable to be grossly wrong, as most persons have experienced. However, when a sound is heard louder by the left than the right ear we can recognize that its source is on the left; when equally with both ears, that it is straight in front or behind; and so on. The concha has per- haps something to do with enabling us to detect whether a sound originates before or behind the ear, since it collects, and turns with more intensity into the external auditory meatus, sound-waves coming from the front. By turning the head and noting the accompanying changes of sensation A UDITORT PERCEPTIONS. 555 in each ear we can localize sounds better than if the head be kept motionless. The large movable concha of many ani- mals, as a rabbit or a horse, which can be turned in several directions, is probably an important aid to them in de- tecting the position of the source of a sound. That the recognition of the direction of sounds is not a true sensation, but a judgment, founded on experience, is illustrated by the fact that we can estimate much more accurately the direc- tion of the human voice, which we hear and heed most, than that of any other sound. CHAPTER XXXIV. TOUCH, THE TEMPERATURE SENSE, THE MUSCULAR SENSE, COMMON SENSA- TION, SMELL, AND TASTE. Nerve-Endings in the Skin. Many of the afferent skin-nerves end in connection with hair-bulbs; the fine hairs over most of the cutaneous surface, projecting from the skin, transmit any movement impressed on them, with increased force, to the nerve- fibres at their fixed ends. In many animals, as cats, large, specially tactile, hairs are de- veloped on the face, and these have a very rich nerve- supply. Fine branches of axis cylinders have also been described as penetrating be- tween epidermic cells and ending there without termi- nal organs. In or immedi- ately beneath the skin several peculiar forms of nerve end organs have also been de- scribed; they are known as (1) Touch-cells; (2) Pacinian corpuscles; (3) Tactile cor- puscles; (4) End-bulbs. The Pacinian corpuscles (Fig. 151) lie in the subcutane- ous tissue of the hand and foot, and about the knee-joint; but also away from the skin on branches of the solar plexus (p. 172), so that it is doubtful if they are touch-organs. FIG. 151.— A Pacinian corpuscle, magnified. NERVE END ORGANS IN SKIN. 557 They are oval, from 1.5 to 2.5 mm. (^ to ^ inch) long, by about half that width, and have a whitish translucent appearance, with a more opaque centre. When magnified ^ach is found to consist of a core, surrounded by many concentric capsules, b. A nerve-fibre, a, enters at one end, and its axis cylinder, c, runs through the core to near the other, where it terminates in one or two little knobs, or a number of fine branches. The tactile corp u scles lie in papillae of the dermis, and are oval and about .08 mm. (-^ inch) in length. They contain a soft core, enveloped by a connective-tissue cap- FIG. 152.— Dermic papillae with tactile corpuscles. A, a corpuscle with four nerve-fibres; a, corpuscle; 6 and c, nerve-fibres. B, papilla made transparent with acetic acid to show tactile corpuscle within ; o, proper tissue of the papilla; •fc, tactile corpuscle ; c, entering nerve, d, e, nerve-fibres twining round the cor- puscle. C, a papilla, containing a tactile corpuscle, seen in optical transverse -section. sule, and separated into several masses. Two, three, or more nerve-fibres go to each corpuscle and appear to end in plates lying between each of the segments of the core. Tactile corpuscles are numerous in the skin of the hand and foot, but are rare elsewhere. This limited distribution 4>ver the surface militated against the belief that they were tactile end organs; but it has lately been found that simpler bodies, the touch-cells, of the same essential structure but receiving only one nerve-fibre each, are distributed all over the skin; the more complex, and probably more irritable, form being found where the epidermis is especially thick. 558 THE HUMAN BOLT. The end-lulls are spheroidal and about . 04 mm. (^-^ inch) in diameter. Each consists of a core, with a connective- tissue capsule, to which two or three nerve-fibres run; the- axis cylinders penetrate the core. End-bulbs are found on one or two regions of the skin, as that on the red part of the lips, in the conjunctiva, and the mucous membrane covering the soft palate, and the tongue. Touch, or the Pressure Sense. Through the skin w& mvl Irindfi of fipup0^™; touch proper, heat and cold, and pain; and we can with more or less accuracy localize them on the surface ofthe i3odyT The interior of the mouth possesses also these sensibilities. Through touch proper we recognize pressure or traction exerted on the- skin, and the force of the pressure; the softness or hard- ness, roughness or smoothness, of the body producing it; and the form of this, when not too large to be felt all over. When to learn the form of an object we move the hand -over it, muscular sensations are combined with proper tactile, and such a combination of the two sensations is frequent; moreover, we rarely touch anything without at the same time getting temperature sensations; there- fore pure tactile feelings are rare. From an evolution point of view, touch is probably the first distinctly dif- ferentiated sensation, and this primary position is still largely holds in our mental life; we mainly think of the things about us as objects which would give us certain tac- tile sensations if we were in contact with them. Though the eye tells us much quicker, and at a greater range, what are the shapes of objects and whether they are smooth, rough, and so on, our real conceptions of round and square and rough bodies are derived through touch, and we trans- late unconsciously the teachings of the eye into mental terms of the tactile sense. A person who saw but had no touch-sense would conceive solid objects very differently from the rest of mankind. The delicacy of the tactile sense varies on different parts of the skin; it is greatest on the forehead, temples, and TOUCH. 559 back of the forearm, where a weight of 2 milligr. (. 03 grain) pressing on an area of 9 sq. millim. (.0139 sq. inch) can be felt. On the front of the forearm 3 milligr. (.036 grain) can be similarly felt, and on the front of the fore- finger 5 to 15 milligr. (.07-0.23 grain). In order that the sense of touch may be excited neigh- boring skin areas must be differently pressed; when we lay the hand on a table this is secured by the inequalities of the skin, which prevent end organs, lying near together, from being equally compressed. When, however, the hand is immersed in a liquid, as mercury, which fits into all its inequalities and presses with practically the same weight on all neighboring immersed areas, the sense of pressure is only felt at a line along the surface, where the immersed and non-immersed parts of the skin meet. It was in connection with the tactile sense that the facts on which so-called psycho-physical law (p. 473) is based, were first observed. The smallest perceptible difference of pressure recognizable when touch alone is used, is about J; i.e. we can just tell a weight of 20 grams (310 grains) from one of 30 (465 grains) or of 40 grams (620 grains) from one of 60 (930 grains); the change which can just be recognized being thus the same fraction of that already act- ing as a stimulus. The ratio only holds good, however, for a certain mean range of pressures; and its existence for any has lately been denied. The experimental difficulties in determining the question are considerable; muscular sensa- tions must be rigidly excluded; the time elapsing between laying the different weights on the skin must always be equal; the same region and area of the skin must be used; the weights must have the same temperature; and fatigue of the organs must be eliminated. Considerable individual variations are also observed, the least perceptible difference not being the same in all persons. The Localizing Power of the Sliin. When the eyes are closed and a point of the skin is touched we can with some accuracy indicate the region stimulated; although tactile feelings are in general characters alike, they differ in some- thing (local sign] besides intensity by which we can distin- 5GO THE HUMAN BODY. guish them; some sub-sensation quality not rising definitely into prominence in consciousness must be present, com- parable to the upper partials determining the timbre of a tone. The accuracy of the localizing power varies widely in different skin regions and is measured by observing the least distance wJiick-inust ,s^parM§^BLJQLbiQct^ (as the blunted points of a pair of compasses) in order that they may be felt as two. The following table illustrates some of the differences observed — Tongue-tip l.lmm. (.04 inch) Palm side of last phalanx of linger 2.2 mm. (.08 inch) Red part of lips 4.4 mm. (.16 inch) Tip of nose 6.6mm. (.24 inch) Back ol second phalanx of finger 11.0 mm. (.44 inch) Heel 22.0mm. (.88 inch) Back of hand 30.8 mm. (1.23 inches) Forearm 39.6 mm. (1.58 inches) Sternum 44.0 mm. (1.76 inches) Back of neck 52.8 mm. (2.11 inches) Middle of back 66.0 mm. (2.64 inches) The localizing power is a little more acute across the long axis of a limb than in it; and is better when the pressure is only strong enough to just cause a distinct tactile sensa- tion, than when it is more power- ^ul> ^ ^s a^so VG1T readily and rapidly improvable by practice. It might be thought that this localizing power depended directly on nerve distribution; that each touch-nerve had connection with a special brain-centr^ at one end (the excitation of which caused a sensation with a characteristic local sign), and at the other end was distributed over a certain skin area, and that the larger this area the farther apart might two points be and still give rise to only one sensation. If this were so, however, the peripheral tactile areas (each being determined by the anatomical distribution of a nerve-fibre) LOCALIZATION OF TACTILE SENSATIONS. 561 must have definite unchangeable limits, which experiment .shows that they do not possess. Suppose the small areas in Fig. 153 to each represent a peripheral area of nerve distribu- tion. If any two points in c were touched we would accord- ing to the theory get but a single sensation; but if, while the compass points remained the same distance apart, or were even approximated, one were placed in c and the other on a contiguous area, two fibres would be stimulated and we ought to get two sensations; but such is not the case; on the same skin region the points must be always the same distance apart, no matter how they be shifted, in order to give rise to two just distinguishable sensations. It is probable that the nerve areas are much smaller than the tactile; and that several unstimulated must intervene between the excited, in order to produce sensations which shall be distinct. If we suppose twelve unexcited nerve areas must intervene, then, in Fig, 153, a and b will be just on- the limits of a single tactile area; and no matter how the points are moved, so long as eleven, or fewer, unexcited .areas come between, we would get a single tactile sensation; in this way we can explain the fact that tactile areas have no fixed boundaries in the skin, although the nerve distri- bution in any part must be constant. We also see why the back of a knife laid on the surface causes a continuous linear sensation, although it touches many distinct nerve .areas; if we could discriminate the excitations of each of these from that of its immediate neighbors we would get the sensation of a series of points touching us, one for each nerve region excited; but in the absence of intervening unexcited nerve areas the sensations are fused together. The ultimate differentiation of tactile areas takes place in the central organs, as will be more fully pointed out in the next chapter. Afferent nerve impulses reaching the spinal cord from a finger-tip enter the gray matter and tend to radiate some way in it; from the gray region through which they spread, impulses are sent on to perceptive tactile centres in the brain; if two skin-points are so close that their regions of irradiation in the cord overlap, then the two points touched cannot be discriminated in con- 562 THE HUMAN BODY. sciousness, since the same brain regions are excited. The more powerful the stimulus the wider the irradiation in the cord, and hence the less accurate the discriminating power. The more often an impulse has traveled, the more does it tend to keep its own proper tract through the gray matter of the cord, and get on to its own proper brain- centre alone, hence the increase of tactile discrimination with practice, for we cannot suppose it to be due to a growth of more nerve-fibres down to the skin, and a rearrangement of the old, with smaller areas of anatomical distribution. As a general rule, more movable parts have smaller tactile areas; this probably depends on practice, since they are the parts which get the greatest number of different tactile- stimulations. The Temperature Sense. By this we mean our faculty of perceiving ^cold ancLgarjnth; ,and, with the help of these sensations, of perceiving temperature differences in external objects. Its organ is the whole skin, the mucous membrane of mouth and fauces, pharynx and gullet, and the entry of the nares. Direct heating or cooling of a sensory nerve may stimulate it and cause pain, but not a true temperature sensation; and the amount of heat or cold requisite is much greater than that necessary when a temperature-perceiving surface is acted upon; hence we must assume the presence of temperature end-organs. In a comfortable room we feel at no part of the Body either heat or cold, although different parts of its surface are at different temperatures; the fingers and nose being- cooler than the trunk which is covered by clothes, and this, in turn, cooler than the interior of the mouth. The tem- perature which a given region of the temperature organ has (as measured by a thermometer) when it feels neither heat nor cold is its temperature-sensation zero, and is not associated with any one objective temperature; for hot only, as we have just seen, does it vary in different parts of the organ, but also on the same part from time to time. Whenever a skin region has a temperature above its sensa- sation zero we feel warmth; and vice versa: the sensation is more marked the greater the difference, and the more TEMPERATURE SENSATIONS. 563 suddenly it is produced; touching a metallic body, which conducts heat rapidly to or from the skin, causes a more marked hot or cold sensation than touching a worse con- ductor, as a piece of wood, of the same temperature. The change of temperature in the organ may be brought -about by changes in the circulatory apparatus (more blood flowing through the skin warms it and less leads to its cool- ing), or by temperature changes in gases, liquids, or solids in contact with it. Sometimes we fail to distinguish clearly whether the cause is external or internal; a person coming in from a windy walk often feels a room uncomfortably warm which is not really so; the exercise has accelerated his circula- tion and tended to warm his skin, but the moving outer air has rapidly conducted off the extra heat; on entering the house the stationary air there docs this less quickly, the skin gets hot, and the cause is supposed to be oppressive heat of the room. Hence, frequently, opening of win- dows and sitting in a draught, with its concomitant risks; whereas keeping quiet for five or ten minutes, until the circulation had returned to its normal rate, would attain the same end without danger. The acuteness of the temperature sense is greatest at temperatures within a few degrees of 30° 0. (86° F.); at these differences of less than.l°C. can be discriminated. As a means of measuring absolute temperatures, however, the skin is very unreliable, on account of the changeability of its sensation zero. We can localize temperature sensa- tions much as tactile, but not so accurately. Are Touch and Temperature Sensations of Different Modality ? Tactile and temperature feelings are ordina- rily so very different that we can no more compare them than luminous and auditory; and if we accept the modern modified form of the doctrine of specific nerve energies (p. 191), in accordance with which the same sensory fibre when excited always arouses a sensation of the same quality because it excites the same brain-centre, it is hard to con- ceive how the same fibre could at one time arouse a tactile, and at another a temperature sensation. It has, however, been maintained that touch and temperature feelings 564 THE HUMAN BODY. sometimes pass into one another insensibly. If a half dollar cooled to 5° C. (41° F.) be placed on a person's brow, and then two (one on the other) warmed to 37° C. (98.5° F.), he commonly thinks the weight in the two cases is equal; i.e., the temperature difference leads to errors in his pressure judgments. But this does not prove an iden- tity in the sensations; the cold half dollar may produce contraction of the cutaneous tissues, leading to compression of the tactile end organs, which is mistaken, in mental in- terpretation, for a heavier pressure. When sensations are combined in other cases, as red and blue-green to produce white, or partial tones to form a compound, we either can- not, or but with difficulty, recognize the components; in this case the person feels both the cold and pressure dis- tinctly when the half dollar is laid on him. In certain cases a person mistakes the contact of a piece of raw cotton with his skin, for the approach of a warm object; this has been taken to prove that touch and tem- perature feelings gradate into one another. However, the feeble touch of the raw cotton might well be less felt than the increased temperature of the skin, due to dimin- ished radiation when it was covered by this non-conducting substance; and the constancy with which, in the ordinary circumstances of life, we feel and discriminate clearly, on the same skin region at the same time, both temperature and touch sensations, is a strong argument against any transition of one into the other. That not merely touch and temperature sensations are distinct and have different nerves, but that hot and cold sensations also depend on the excitation of different nerve- fibres, has recently been proved. If a metal point, lightly weighted, be slowly and evenly moved along the skin, it gives rise to sensations of touch at some places, and to sensations of temperature at others. If it be a little warmer than the skin, at certain places it causes a sensa- tion of heat. These "heat-points" remain the same in the same person from day to day. If the travelling point be a little colder than the skin, it gives rise to a sensation USCULAR FEELINGS. 565 of cold as ft travels over some places. These "cold sensa- tion''' spots are different from the "warm sensation" spots, and from the "touch sensation" spots; and are constant in the same individual. Excluding pain (" abnormal sensa- tion"), there are in the skin three distinct sets of nerve- fibres: — One, when excited, arouses "touch" sensation; a second, "warm" sensation; the third, "cold" sensation. The Muscular Sense. In connection with our muscles we "have sensations of great importance, although they do not often become so obtrusive in consciousness as to arouse our attention. Certain of these feelings (muscle sensations proper] are Hrm to f.hA PY^jj-.ftt.i^ nf ftp-nsjovy nej-ves ending within the muscles themselves: the others (innervation sensations) have probably a central origin and accompany the starting of volitional impulses from brain-cells; they are only felt in connection with the vol- untary skeletal muscles. The proper muscle sensations only become marked on powerful or long-continued muscular effort (cramp, fatigue), but a lower grade of them, not distinctly perceived, proba- bly accompanies all muscular activity. The innervation feelings are of far more consequence. They accompany the slightest movement of a skeletal mus- cle, and we derive from them means of determining with, great accuracy the force and extent of the contraction willed. The belief that their origin is central mainly rests on the fact that we have sensations, not merely of executed but of intended movements. The actual nature of the movement performed is probably characterized by other contemporary sensations, as from the muscle sense proper, from pressure and folding of the skin, and so on. The innervation feelings thus stand apart and opposed to all our others as primary factors in our mental life; they represent the reactive work of the organism with respect to its environment. Some distinguished physiologists, however, deny their existence, ascribing them all to a peripheral origin, either in sensory muscle-nerves, or in skin-nerves affected when a part of the Body is moved. 566 THE HUMAN BODY. As, however, we can determine more accurately the differ- ence between two weights when we lift them than we can when they are merely laid on a hand supported by a table (see below), there are undoubtedly (apart from cramp and fatigue) true muscular sensations distinct from tactile; and that these, or some of them, are central in origin seems proved by certain phenomena observed in disease. Persons suffering from paresis, i.e. muscular weakness not amount- ing to complete paralysis, make (until they have learned to interpret correctly their sensations under the new condi- tions) entirely wrong judgments as to the extent of their movements; they think they have contracted their muscles much more than is really the case. If the muscular sense depended on stimulation of sensory nerves in the mus- cles this error could not arise, for the intensity of the sen- sation would be determined by the amount of contraction which actually occurred. We have already seen how closely muscular sensations are combined with visual in enabling us to form judgments as to the distance, size, and movements of objects. They are as closely combined in ordinary life with tactile sensa- tions; in the dark, when an object is of such size and form that it cannot be felt all over by any one region of the skin, we deduce its shape and extent by combining the tactile feelings it gives rise to, with the muscular feelings accom- panying the movements of the hands over it. Even when the eyes are used the sensations attained through them mainly serve as short-cuts which we have learned by experience to interpret, as telling us what tactile and muscular feelings the object seen would give us if felt; and, in regard to distant points, although we have learnt to apply arbitrarily selected standards of neasurement, it is probable that distance, in relation to perception, is primarily a judgment as to how much muscular effort would be needed to come into con- tact with the thing looked at. When we wish to estimate accurately the weight of an object we always, when possible, lift it, and so combine mus- cular with tactile sensations. By this means we can form much better judgments. While with touch alone just percep- COMMON SENSATIONS, 567 \ibly different pressures have the ratio 1 : 3, with the mus- -cular sense differences of ^ can be perceived. Common Sgnsations. Under this name are included the sensations which weocThot mentally attribute to the prop- erties of external objects, but to conditions of our own Bodies; of them we may here consider min, hunger, and Pain arises when powerful mechanical or thermal stimuli acton the skin, and when a sensory nerve-trunk (except the optic, auditory, or olfactory) is directly excited. Most commonly we derive the feeling through cutaneous or sub- cutaneous nerves, and hence it has been supposed that pain is not a sensation of distinct modality due to the excitation of special nerve-fibres, but is dependent on excessive excite- ment of ordinary tactile fibres; and pressure or temperature sensations do undoubtedly gradate into painful as the stimuli increase. If this be so, pain is a sort of incoordi- nate or "convulsive" sensation. We shall see in Chapter XXXV. that, when a sensory nerve is normally excited in a decapitated animal, regular purpose-like reflex movements result: but if the stimulus be very powerful, or a nerve- trunk be. directly excited, then inco-ordinate convulsions occur r the afferent impulses radiate farther in the centre and produce a new and useless result. We may suppose something similar to occur in the cutaneous nerves of an animal still possessing sensory brain-centres, if the stimuli acting on the skin are such as to excite the end organs very powerfully, or the sensory fibres directly without the inter- mediation of end organs: that a new sensation should be thus aroused, different from tactile though gradually shad- ing off into them, is a phenomenon comparable with the production of new color sensations by combinations of the fundamental ones. In such case, too, we could understand the difference of kinds of "pain" in a more general sense of the word; muscular cramp, dazzling, and disagreeably shrill or inharmoniously combined tones, might all be looked upon as inco-ordinate sensations, each with a charac- acter of its own determined by the central apparatus ex- cited. 5G8 THE HUMAN BODY. The matter cannot, however, be at present decided. The? skin seems indubitably to contain many nerve-fibres which terminate in free axis cylinders without end organs; and these may well be true pain-fibres, fitted to respond to- stronger stimuli than those which excite the tactile and thermic end organs. Certain pathological and experi- mental phenomena tend also to prove that the brain-centres concerned in the production of tactile and painful sensa- tions are different. Persons sometimes lose pain sensations and keep tactile; a gentle touch is felt as well as usual, but a powerful pinch causes no pain: in one stage of ether and chloroform narcosis the same thing is observed; the sur- geon's hand and knife are felt where they touch the skin, but cutting deeper into the tissues produces no pain. In animals a similar state of things may be produced by divid- ing the gray matter of the cord, leaving the posterior white columns intact; while, if the latter be divided and the gray substance left uninjured, there is increased sensitiveness to- pain, and probably touch proper is lost, though that is im- possible to say with certainty. Such experiments make it. pretty certain that when sensory afferent impulses reach the cord at any level and there enter its gray matter with the posterior root-fibres, they travel on in different tracks to conscious centres; the tactile coming soon out of the- gray network and coursing on in a readily conducting white fibre, while the painful first travel on farther in the gray sub- stance. It is still uncertain if both impulses reach the cord in the same fibres. The gray network conducts nerve im- pulses, but not easily; they tend soon to be blocked in it, A feeble (tactile) impulse reaching it by an afferent fibre- might only spread a short way and thnn pass out into a single good conducting fibre in a white column, and proceed to the brain; while a stronger (painful) impulse would radiate farther in the gray matter, and perhaps break out of it by many fibres leading to the brain through the white columns, and so give rise to an inco-ordinate and ill localized sensation. That pains are badly localized, and worse the more intense they are, is a well-known fact,, which would thus receive an explanation (see p. 579), SMELL. 569= Hunger and Thirst. These sensations, which regulate the taking of food, are peripherally localized in conscious- ness, the former in the stomach and the latter in the throat, and local conditions no doubt play a part in their produc- tion; though general states of the Body are also concerned. Hunger in its first stages is probably due to a condition of the gastric mucous membrane which comes on when the stomach has been empty some time, and may be temporarily stilled by filling the organ with indigestible substances. But soon the feeling comes back intensified and can only be allayed by the ingestion of nutritive substances; pro- vided these are absorbed and reach the blood, their mode of entry is unessential; the hunger may be stayed by injec- tions of food into the rectum as well as by putting it inta the stomach. Similarly, thirst may be temporarily relieved by moisten- ing the throat without swallowing, but then soon returns;, while it may be permanently relieved by water injections into the veins, without wetting the throat at all. While both sensations thus depend in part on local peripheral conditions of afferent nerves (pneumogastrio and glossopharyngeal), they may be also, and more power- fully, excited by poverty of the blood in foods and water; this probably directly stimulates the hunger and thirst brain-centres. Smell, The olfactory organ consists of the upper por- tions of the two nasal cavities, over which the endings of the olfactory nerves are spread and where the mucous membrane has a brownish - yellow color. This region (regio olfactorid) covers the upper and lower turbinate bones (o, p, Fig. 89*), which are expansions of the ethmoid (p. 75) on the outer wall of the nostril chamber, the oppo- site part of the partition between the nares, and the part of the roof of the nose (n, Fig. 89) separating it from the cranial cavity. The epithelium covering the mucous mem- brane contains two varieties of cells arranged in several layers (2, Fig. 154). Some cells are much like ordinary columnar epithelium but with long branched processes attached to their deeper ends; the others have a large * P. 309. 570 THE HUMAN BODY. nucleus surrounded by a little protoplasm; a slender exter- nal process reaching to the surface; and a very fine deep one. The latter cells have been supposed to be the proper "olfac- tory end organs, and to be connected with the fibres of the olfactory nerve, which enter the deeper strata of the epithe- lium and there divide; but it is doubtful whether both kinds of cells are not so connected. Odorous substances, the stimuli of the olfactory appa- ratus, are always gaseous and frequently act powerfully when present in very small amount. We cannot, how- ever, classify them by the sen- sations they arouse, or arrange them in series; and smells are but minor sensory factors in our mental life. We commonly refer them to external objects since we find that the sensa- tion is intensified by " sniff- ing" air powerfully into the nose, and ceases when the nostrils are closed. Their peripheral localization is, how- ever, imperfect, for we con- found many smells with tastes (see below); nor can we judge well of the direction of an odorous body through the olfactory sensations which it arouses. The nose possesses also nerves of common sensation, which are stimulated by such substances as ammonia vapor. Taste. The organ of taste is the mucous membrane on the dorsum of the tongue and possibly other parts of the boundary of the mouth cavity. The nerves concerned are the glossopharyngeals, distributed over the hind part of the FIG. 154.— Cells from the olfactory epithelium. 1, from the frog; 2, from man: a, columnar cell, with its branched deep process; 6, so-called olfactory cell; c, its narrow outer process; d, its slender central pro- fess. 3, gray nerve-fibres of the ol- factory nerve, seen dividing into fine peripheral branches at a. TASTE. 571 tongue, and the lingual brandies of the inferior maxillary division (p. 170) of the trigeminals on its anterior two thirds. On the tongue the nerves run to papillae; the circumval- late (p. 313) have the richest supply, and on these are cer- tain peculiar end organs (Fig. 155) known as taste-buds, which are oval and imbedded in the epidermis covering the side of the papilla. Each consists, externally, of a number of flat, fusiform, nucleated cells and, internally, of six or eight so-called taste-cells. The latter are much like the olfactory cells of the nose, and are probably connected with nerve-fibres at their deeper ends. The capsule formed by the enveloping cells has a small opening on the surface^ FIG. 155.— Taste-buds. each taste-cell terminates in a very fine thread which pro- trudes there. Taste-buds are also found on some of the fungiform papillae, and it is possible that simpler struc- tures, not yet recognized, consisting of single taste-cells are widely spread on the tongue, since the sense of taste exists where no taste-buds can be found. The filiform papillae are probably tactile. In order that substances be tasted they must be in solu- tion: wipe the tongue dry and put a crystal of sugar on it; no taste will be felt until exuding moisture has dissolved some of the crystal. Tastes proper may be divided into sweet, bitter, acid, and saline. Intellectually they are, like smells, of small value; the perceptions we attain through them as to qualities of external objects being of little use, -572 THE HUMAN BODY. except as aiding in the selection of food, and for that pur- pose they are not by any means safe guides at all times. Many so-called tastes (flavors) are really smells; odorif- erous particles of substances which are being eaten reach the olfactory region through the posterior nares and arouse sen- sations which, since they accompany the presence of objects in the mouth, we take for tastes. Such is the case, e.g., with most spices; when the nasal chambers are blocked or inflamed by a cold in the head, or closed by compressing the nose, the so-called taste of spices is not perceived when they are eaten; all that is felt, when cinnamon, e.g., is chewed under such circumstances is a certain pungency due to its stimulating nerves of common sensation in the tongue. This fact is sometimes taken advantage of in the practice of domestic medicine when a nauseous dose, as rhubarb, is to be given to a child. Tactile sensations play also a part jn many so-called tastes. Most persons taste bitters best with the back of the tongue and sweets towards the tip; but this is not constant. The curious interference of tastes which takes place when the acidity of a sour body is covered by adding a sweet one, which does not in any way chemically neutralize the acid (when sugar is put on a lemon for example), needs explana- tion. CHAPTER XXXV. THE FUNCTIONS OF THE BRAIN AND SPINAL CORD. The Special Physiology of Nerve- Centres. We have already studied the general physiological properties of nerves and nerve-centres (Chap. XIII.) and found that while the former are mere transmitters of nervous impulses, the latter do much more. In some cases the centres are auto- matic; they originate nerve impulses, as illustrated by the heat of the heart under the influence of its ganglia. In other cases a feeble impulse reaching the centre gives rise to a great discharge of energy from it (as when an unex- pected noise produces a violent start, due to many impulses sent out from the excited centre to numerous muscles), so that certain centres are irritable; they contain a store of energy-liberating material which only needs a slight dis- turbance to upset its equilibrium and produce many efferent impulses as the result of one afferent. Further, the im- pulses thus liberated are often co-ordinated. When mucus in the windpipe tickles the throat and excites afferent nerve impulses, these, reaching a centre, cause discharges along many efferent fibres, so combined in sequence and power as to produce, not a mere aimless spasm, but a cough which clears the passage. In very many cases the excita- tion of centres, with or without at the same time some of the above phenomena, is associated with sensations or other states of consciousness. We have now to study which of these powers are manifested by different parts of the cen- tral cerebro-spinal nervous system; and under what circum- stances and in what degree: what is known of the general functions of the sympathetic and sporadic ganglia has al- ready been stated (p. 183). 574 THE HUMAN BODY. The Spinal Cord as a Centre. The^pinaLcord, forming (except the slender sympathetic) the only direct communi- cation between the brain an^niQ&LQlih^nerves of_the Body, was considered by the older physiologists as merely a huge nerve-trunk, into which the various spinal nerves were collected on their way to the encephalon. It does, it is true, contain the paths for the conductionjjj^all those, im^. ich., originating in the cerebrum, give_risc to. \^»kuitary__jnoYements of the trunk andjimbsj also for all the_centrally traveling, impulses which give rise _tq-&cn&i- tions ascribed to^ those parts; and jt is also the path for cor- s for _ jexample, those which, originating in thcrespiratory centre,, tiajel to the phrenic and intercostal neryj^ If, however, the cord were merely collected and con- tinued nerve-roots it ought to increase considerably in bulk as it approached the skull, and this it does not do in any- thing like the required proportion; a histological examina- tion, too, shows that few, if any, of the entering fibres pro- ceed directly to the brain; they pass to the central core of gray substance, containing nerve-cells and different from anything in a true nerve-trunk, (in the gray core some fibres f anterior roots') mid in noils of T.hftn.nfpTi'n|' horns, and others (posterior roots) in that fine Ji£iwork which pervades the^ whole gray substance (p. 179); the nerve-cells by their branches are also in continuity with this network and so, ultimately, with all the nerve-fibres entering the cord. From the network fibres arise at different levels, and pass out into the white columns; some place two regions of the gray matter in intimate connection, while others go on to the brain. In order to understand physiological facts we must assume, first, that a nervous impulse entering the gray network at any point may, under certain conditions, travel all through it, and give rise to efferent impulses emerging at any level; and, on the other hand, that there are certain lines or paths of easiest propagation between different points in this network, which, the impulses keep to under ordinary conditions. Reflex Actions. When a frog is decapitated it lies down REFLEX ACTIONS OF THE COED. 575 squat on its belly instcad_of_ assuming the more erect posi- Tiion of the uninjured animal; its respiratory movements cease_^ (their centre being removed with the medulla); the hind legs at first remain sprawled out in any position into which they may happen to fall, but after a time are drawn up into their usual position, with the hip and knee joints flexed; having made this movement the animal, if protected from external stimuli, makes no other by its skeletal muscles; it has lost all spontaneity, and only stirs under the influence of immediate excitation. Nevertheless the heart goes on beating for hours; the muscles and nerves, when examined, are found to still have all their usual physiological proper- ties; and, by suitable irritation, the animal can be made to execute a great variety of complex movements. But it is no longer a creature with a will, doing things which we cannot -predict; it is an instrument which can be played upon, giving different responses to different stimuli (as dif- ferent notes are produced when different keys of a piano are struck), ajid always the same reaction to the same stimulus; so that we can say beforehand what will happen when we touch it. Such actions are called reflex or excito-motor and fall into two groups; (1) orderly or purpose-like reflexes, which are correlated to the stimulus and are often defensive, tending, for instance, to remove an irritated part from the irritating object; (2) disorderly or convulsive reflexes, not tending to produce any definite result, and affecting either a limited region or all the muscles of the body. In higher animals similar phenomena may be observed. If a rabbit's spinal cord be divided at the bottom of the neck the animal is at first thrown into a flaccid limp condition like the frog, but it soon recovers. Voluntary movements in muscles supplied from the spinal cord behind the section are never seen again; but on pinching the hind foot it is forcibly withdrawn. Men, whose spinal cord has been divided by stabs or disease below the level of the fifth cervical spinal roots (above which the fibres of the phrenic nerve, which are necessary for breathing, pass out), sometimes live for a time, but can no longer move their legs by any effort of the will, nor do they feel touches, pinches, or hot 576 THE HUMAN BODY. things applied to them; if, however, the soles of the feet be tickled the legs are thrown into vigorous movement. As a rule^ howevej^orderly reflexes are less jmarked_..and less numerous in the higher animals; in them the organiza- tion is less_nia^hinjerlike^i]ie_spinal cord being more^tjie s^vajitof the larger brain, and less capable of working without directions^ Such animals, when intact, can to a greater extent control the muscular responses which shall be made to stimuli under various conditions; they have less automatic protection in the ordinary risks of life, but a greater range of possible protection. The kujmin__spiBal QprgLcontrolled by the brain, can adapt the rej^ctiojigjiLtlie Body, with great nicety y to a vast variety of conditions ; the frog's cord by itself does this for a smaller number of possi- ble emergencies without troubling at all such brain as the animal has, but is less completely under the control of the higher centres for adaptation to other conditions. The difference being, however, but one of degree and not of kind, it is best to approach the study of the reflex actions of the human spinal cord through an examination of those exhibited by the frog. The Orderly Reflex Movements of a Decapitated Frog. For the occurrence of these the following parts must be in- tact; (a) thfi_gnd organs of sensory nerve-fibres ; (b) fibres fr^m theae to the nord- (ri pjferent nbrc^s fro cord to the muscles; (d) the part of the the afferent and efferent fibres; (e) the muscles J3o$cerne( in the movement. If the decapitated animal be suspended vertically after the shock of the operation is over, it makes a few attempts to hold its hind legs in their usual flexed position; these soon cease, the legs hang down, and tfie creature comes to rest. If one flank be now gently scratched with the point of a pencil a reflex movement occurs, limited to the muscles of that region; they twitch, some- what as a horse's neck when tickled by flies. If a pinch bo given at the same spot, more muscles on the same side come into play; a harder pinch causes also the hind leg oi that side to be raised to push away the offending object; more violent and prolonged irritation causes all the muscles DISORDERLY REFLEX MOVEMENTS. 577 of the body to contract, and the animal is convulsed. Here then we see that a feeble stimulation causes a limited and purpose-like response; stronger causes a wider radiation of efferent impulses from the cord and the con traction of more muscles, but still the movements are co-ordinated to an end; while abnormally powerful stimulation of the sensory nerves throws all the motor fibres arising from the cord into activity, and calls forth inco-ordinate spasmodic action. The orderly movements are very uniform for a given stimu- lation; if the anal region be pinched, both hind legs are raised to push away the forceps; if a tiny bit of bibulous paper moistened with dilute vinegar be put on the thigh, the lower part of that leg is raised to wipe it oil; if on the middle of the back near the head, both feet are wiped over the spot; if on one flank, the leg and foot of that side are used, and so on; in fact, by careful working, the frog's skin can be mapped into many regions, the application of acidu- Lited water to each causing one particular movement, due to the co-ordinated contractions of muscles in different combinations, and never, under ordinary circumstances, any but that one movement. The above purpose-like Teflex movements may all be characterized as defensive. but all orderly reflexes are not so. For example, in the breeding season the male frog clasps the female for several days with his fore limbs. If a male at this season be decapitated and left to recover from the shock, it will be found that gently rubbing his sternal region with the finger •causes him to clasp it vigorously. Disorderly Reflexes or Reflex Convulsions. These •come on when an afferent nerve-trunk is stimulated instead of the tactile end organs in the skin; or when the skin is very powerfully excited; or, with feeble stimuli, in certain diseased states (pathological '_. tetanus}, and under the in- fluence of certain poisons, e^ecially strychnine. If a frog or a warm-blooded animal be given a dose ot the latter drug, a stimulus, such as normally would excite only limited orderly reflexes, will excite the whole cord, and lead to discharges along all the efferent fibres so that general con- vulsions result. It has been clearly proved that, in such 578 THE HUMAN BODY. cases, not the skin, or afferent or efferent nerves, or the muscles, but the spinal cord itself is affected by the poison (at least primarily), unless unnecessarily large doses have been given. The Least-Resistance Hypothesis. In order to com- prehend reflex acts we must assume a manifold union of sensory with efferent nerve-fibres; this is anatomically afforded by the minute plexus of the gray network, which is continuous through the whole cord, and in which the fibres- of the anterior and posterior nerve-roots directly or indirect- ly end. The continuity of this network serves to explain general reflex convulsions, and the spread of an afferent impulse, or its results, through the whole cord, with the consequent emission of efferent impulses through many or all the anterior roots; but, on the other hand, it renders it difficult to understand limited and orderly reflexes, in which only a few efferent fibres are stimulated. To explain them we have to assume a great resistance to conduction in the gray network, so that a nerve impulse entering it is soon blocked and transmuted into some other form of energy; hence it only reaches efferent fibres originating near the point at which it enters, or fibres placed in specially easy communication with that. When the frog's flank is tickled, only muscles innervated from anterior roots on the same side of the body, and springing from the same level of the cord, are made to contract; when the stimulus is more powerful the stronger afferent impulse radiates farther, but mainly in directions determined by lines of conduc- tivity in the cord; e.g., to the origin of the efferent fibres which cause lifting of the hind leg to the irritated spot. These paths of easiest conduction, or of least resistance, in some cases lie in the gray matter itself, in others in inter- central or commissural fibres of the highly conductive medullated kind, which, passing out of the gray substance at one level, run in the white columns to it at another, where the efferent fibres of the muscles called into play originate. A still stronger afferent impulse radiates wider still, and, liberating energy from all the nerve-cells in the gray matter, produces a useless general convulsion. Under CONDUCTION IN THE CORD. 579 the influence of strychnine and in pathological tetanus (as observed, for example, in hydrophobia) the conductivity of the whole gray matter is so increased that all paths through it are easy, and so a feeble afferent impulse spreads in all directions. To account for the phenomena of localized skin sensa- tions (p. 559) and of limited voluntary movements we must make a similar hypothesis. If the nervous impulses enter- ing the gray network when the tip of a finger is touched spread all through it irregularly, we could not tell what region of the skin had been stimulated, for the central results of stimulating the most varied peripheral parts would be the same. From each region of the gray network of the cord where a sensory skin-nerve enters there must, therefore, be a special path of conduction to a given brain- region, producing results which differ recognizably in con- sciousness from those following the stimulation of a differ- ent skin region. The acuteness of the localizing power will largely depend on the definiteness of the path of least resis- tance in the gray matter, since while traveling in a medul- lated nerverfibre from the skin to the cord, or (in the white columns) from the gray matter of the latter to the brain, the nervous impulse is confined to a definite track. Hence anything tending to let the afferent impulse radiate when it enters the cord will diminish the accuracy with which its peripheral origin can be located. This we see in violent pains; a whitlow on the finger affects only a few nerve-fibres, but gives rise to so powerful nerve impulses that when they reach the cord they spread widely and, break- ing out of the usual track of propagation to the brain, give rise to ill-localized feelings of pain often referred all the way up the arm to the elbow. So the pain from one diseased tooth is often felt along half a dozen, or all over one side of the head. Such cases are comparable to the transforma- tion of an orderly reflex into a general convulsion when the stimulus increases. As an animal shows no spontaneous movements when its •cerebral hemispheres are removed, we conclude that the nerve impulses giving rise to such movements start in those 580 THE HUMAN BODY. parts of the brain. Thence they travel down the white columns of the cord to its gray matter, which they enter at different levels, each in the neighborhood of a centre for producing a given movement. If they there radiated far and wide no definite movement could result, for all the muscles supplied from the cord would be made to contract,, and not merely those necessary to bend the index finger, for example. We must here again, therefore, assume a path of least resistance for the propagation of nerve impulses from a given fibre coming down from the brain, to the efferent fibres going to a certain muscle or group of muscles. The- path between the two is almost certainly not direct; a co-ordinating spinal centre intervenes, and all that the brain has to do is to excite this centre, which then secures the proper muscular co-ordination. If the hand be laid flat on the table and its palm be rolled over, many muscles, in- cluding thousands of muscular fibres, have to contract in definite order and sequence. Persons who have not studied anatomy and who are quite ignorant of the muscles to be used can perform the movement perfectly; and even a skilled anatomist and physiologist, if he knew them all and their actions, could not by conscious effort combine them so well as the cord does without such direct interfer- ence. "Wejijiy^jthen^to look onjthe_cord^as_containing^a. host of co-ordinating centres for different muscles. These centres are put in nervous connection, on the one liancL. with certain regions of the skin, and, on the other, with regions of^jthe brain, and may be excited from eillimJjE the former case the movement is called jreflex: in the latter it jmayjbe reflex, or may bj^ccjmipariiprl by a feeling of "jalL^jmdjs then called voluntary^- The more accurately the required centre, and no other, is excited, the more definite and precise the movement. The Education of the Cord- Much of what is called edu^ting our tonch_or our muscles j^reaIIy~^diLi'caiio5IIb£ the^spinal^cord. A person who begins to play the piano finds at first much difficulty in moving his fingers inde- pendently; the nervous impulses from the brain to the cord radiate from the spinal centres of the muscle which it is THE INHIBITION OF REFLEX ACTIONS. 581 desired to move, to others. But with practice the indepen- dent movements become easy. So, too, the localizing power of the skin can be greatly increased by exercise (p. 560) as one sees in blind persons, who often can distinguish two stimuli on parts of the skin which are so near together as to give only one sensation to other people. Such phenomena depend on the fact that the more often a nervous impulse has traveled along a given road in the gray matter, the easier does its path become, and the less does it tend to wander from it into others. We may compare the gray matter to a thicket; persons seeking to beat a road through from one point to another would keep the same general direction, determined by the larger obstacles in the way, but all would diverge more or less from the straight path on account of undergrowth, tree trunks, etc., and would meet with consid- erable difficulty in their progress. After some hundreds had passed, however, a tolerably beaten track would be marked out, along which travel was easy and all after-comers would take it If instead of one entry and one exit we imagine thousands of each, and that the paths between certain have been often traveled, others less, and some hardly at all, we get a pretty good mental picture of what happens in the passage of nervous impulses through the gray matter of the cord; the clearing of the more trodden paths answering to the effects of use and practice. The human cord and that of the frog must not, however, be looked upon as pathless thickets at the commencement; each individual inherits certain paths of least resistance determined by the structure of the cord, which is the transmitted material result of the life experience of a long line of ancestry. The Inhibition of Reflexes. Since it is possible, as by strychnine, to diminish the resistance in the gray matter, it is conceivably also possible to increase it, and diminish or pre- vent reflexes. Such is found to be actually the case. We can to a great extent control reflexes by the will; for example, the jerking of the muscles which tends to follow tickling: and it is found that after a frog's brain is removed it is much easier to get reflex actions out of the spinal cord. Certain drugs, as bromide of potassium, also diminish reflex excita- 582 THE HUMAN BODY. bility. If a frog's brain be removed and the animal's toe be dipped into very dilute acid, it will be removed after a few seconds; the time elapsing between the immersion and the lifting of the foot is known as the reflex time; any- thing diminishing reflex excitability increases this, as the stimulus (which has a cumulative effect on the centre) has to act longer before it arouses the cord to the discharging point. If the sciatic nerve of the other leg be stimulated while the toe is in the acid the reflex time is increased, or the reflex may fail entirely to appear. This is one case of a general law, that any powerful stimulation of one sen- sory nerve tends to inhibit orderly reflexes due to the ex- citation of another. A common example is the well-known trick of pinching the nose or upper lip to prevent a sneeze. The whole question of reflex inhibition is at present very obscure. It may be due to the excitation of special fibres which inhibit reflex centres, as pneumogastric fibres do the cardio-motor; or to the fact that one nerve impulse in the cord in some cases blocks or interferes with another; or partly to both. Psychical Activities of the Cord. Sin£C__wj3--Gan__g£L quite marked reflex movements in the lower part of the Body^ of a man whose cord is divided and whaoiannot voluntarily move his lower limbs, and on questioning him. find that he feels nothing and is quite ignorant of his movements unless he sees his legs, it is most probable that the^s^inal cord in all cases is devoid of centres of conscious^ ness and volition: this is not certain, however; for there might well be a less division of physiological labor between the cord and brain of a frog, than between those of a man. Still we are entitled to good evidence before we admit that things so similar as the human cord and that of the frog pos- sess different properties. Co-ordinated movements follow- inga given stimulus, or cries emitted by an animal, will not suffice to prove that it is conscious, since we know these may occur entirely unconsciously in men, who alone can tell us of their feelings. We must look for something that resembles actions only done by men consciously. In the frog it has been maintained that we have evidence of such. If a bit of PSYCHICAL ACTIVITIES OF THE SPINAL COED. 583 acidulated paper be put on the thigh of a decapitated frog, the animal will bend its knee and use its leg to brush off the irritant; always using this same leg if the stimulus be not so strong as to produce disorderly reflexes. If now the foot be tied down so that the frog cannot raise it, after a few ineffectual efforts it will move the other leg, and may wipe the paper off with it. This it has been said shows a true psychical activity in the cord; a conscious and volun- tary employment of new procedures under unusual circum- stances. But a close observation of the phenomenon shows that it will hardly bear this interpretation; the movements of the other leg are very irregular and inco-ordinate, and rnugh resemble reflex convulsions stirred up by the pro- longed action of the acid, which goes on stimulating the skin nerves more and more powerfully. Even if new muscles came, in an orderly way, into play under the stronger stimulus, that would not prove a volitional con- scious use of them; we see quite similar phenomena when there is nothing purpose-like in the movement. Many •dogs reflexly kick violently the hind leg of the same side when one flank is tickled. If this leg be held and the tickling continued, very frequently the opposite hind leg will take on the movements, which it never does in ordinary circumstances. This is quite comparable to the frog's use of its other leg under the circumstances above described, but here it would be obviously absurd to talk of a volitional source for such a senseless movement. Putting together, then, the testimony of persons with injured spinal cords, and the observations made on them and on animals, we may tolerably safely conclude that the cord contains no centres of consciousness. There are, how- ever, persons who maintain that in such cases the cord itself feels though the individual does not, whatever that may mean; if the statement is used merely to imply that the cord is irritable (just as a muscle is) no one denies it; but it is an unnecessarily confusing method of stating the fact. The Cord as a Transmitter of Nervous Impulses. In the gray substance, as we have seen, there is reason to believe that nervous impulses can pass in all directions, 584 THE HUMAN BODY. though they usually take certain easiest roads. The mam frrain and cord. howpygf, IIP in th columns; neiiye-roots, enter the grny mn.^^ grrl froin_fiac]i_ region of the Mtej^m^diill^ white column, and_continue_to^ the ^brain.^_Qnce^ in these fibres thejjiipuiae Jhas to keep to a definitely marked out anatomical path, whickJeadsJa. a brain centre or Centres. These paths lie mainly in the lateral columns, the affe- rent (sensory) impulses tending, however, to spread into the posterior columns, and the efferent (volitional) into the anterior; the main bulk of the posterior and anterior columns seems to be made up of commissural fibres join- ing different levels of the cord. The sensory fibres for. the- most part cross soon after they enter the gray matter, and proceed onwards mainly on the side opposite to that of the nerve-root which conveyed them to the cord, while the effe- rent cross mainly in the medulla before they reach the cord; the crossing seems in neither case complete. Hemisection of the cord, therefore, causes marked, but not absoluteness of voluntary movement in muscles of the same side supplied with nerves from a part of the cord below the level of the section; and a considerable, but not complete, loss of sensi- bility on the opposite side. Impulses so powerful as to lead to feelings of pain travel mainly in the gray matter (p. 568), and they also for the most part cross the middle line soon after their entry. The Functions of the Brain in General. The brain^ at least in man and _ the-Uiigher -animals^ is the seat of^con- _ j5cjou£iie£s_aiicL jntol 1 i gen p,e ; these disappear when its blood- supply is cut off, as in fainting; pressure on parts of it, as by a tumor or by an effusion of blood in apoplexy, has the same result; inflammation of it causes delirium; and when the cerebral hemispheres are unusually small idiotcy i& observed. .The brain has, however, many other important Junctions; it is the seat of many reflex, automatic, anji co-ordinating centres, which may act as entirely apariirom. consciousness as those of the spinal cord; experiment makes it probable that psychical faculties are dependent on the fore-brain, while the rest of the complex mass has other,. CEREBRAL FUNCTIONS. 585 non-mental, duties. If the cerebral hemispheres be re- moved from a frog, the ajnmal^ can ._. still perform^ e^or nn longer, performs it .. must _be_ aroused by an immediately acting stimulus,, and its response to this is as invariable a^^rejdicable as that of a frog with its spinal cord only. The movements which can be educed are, however, far more complex; instead of mere kicks in various directions the animal L can walk, leap, swim, get off its back on to its- feet, and so on. Similar results are observable in pigeons whose fore-brain has been removed; mammals bear the operation badly, but some, as rats, survive it several hours and then exhibit like phenomena. The creatures can move, but do not unless directly stimulated; all tlieir volitional spontaneity is lost, and, apparently, all^ perceptions also; they starTat 'aloud' noise, but do not run away as if they conceived danger; they follow a light with the eyes, but do- not attempt to escape a hand stretched forth to catch them; "they can and do swallow food placed in the mouth, but would die of starvation if left alone with plenty of it about them, the sight of edible things seeming to arouse no idea or conception. It may be doubted, perhaps, whether the animals have any true sensations; they start at sounds, avoid qpacnie objects in their road, and cry when pinched; but all these may be unconscious reflex acts f on the whole it seems morejn-obable, however, that they have sensations _but not percepjjgjtisj they feel redness and blueness, hard- ness and softness, and so on: but sensations, as already pointed OUt^tell in_bliejnSPlvfia nothing; t-1^y nrp hnt. aifrr^ which have to bejiientally interpreted as indications of ex- ternal objects: it is this interpretingpower which seems ^deficient m the animal deprived of its fore brain. Functions of the Medulla Oblongata. This contains the pgjJTg of conduction between thejoarts^of thejimin in jfront of it and^jth£_gpjnjl_cj[ird. It is also the _s_eat. _of _many important reflex and automatic centres, especiaU-£- those governing the organs immediately concerned in the maintenance of life; as the respiratory, circulatory, _andL masticatory. It may therefore be called the " nerve cen- 586 THE HUMAN BODY. tral organ of the nutritive processes. " The physiological action of most of the medullary centres has already been described; the more important are — 1. The respiratory centre (p. 390). 2. The cardio-inhibitory centre (p. 250); (there is also some reason to believe that the centre of the accelerator heart-fibres (p. 252) lies in the medulla). 3. The vaso-motor centre (p. 253). 4. The centre for the dilator muscle-fibres of the pupil (p. 486). 5. The centre for the muscles of chewing and swallowing (p. 337), which are commonly thrown into action reflexly, though they may be made to contract voluntarily. 6. The convulsive centre {p. 400). 7. The diabetic centre (p. 442). 8. The centre reflexly exciting activity in the salivary glands, when sensory nerves in the mouth are stimulated. 9. Certain centres for complex bodily movements; an animal with its medulla oblongata can execute much more complicated reflex acts than one with its spinal cord alone. Functions of the Cerebellum, Pons Varolii, and Mid- Brain. These contain paths of conduction between the fore-brain and parts behind, and many important centres, especially those concerned with the maintenance of the equilibrium of the Body in various postures and modes of locomotion. If, as has been above suggested, an animal without its cerebral hemispheres has sensations which remain untranslated into terms of external things, these sensations must have their seats in these brain-regions. The crura cerebri (p. 164) are essentially paths of conduction between the cerebral hemispheres and parts behind. Injury of one produces partial loss of sensibility and incomplete muscular paralysis on the opposite side of the Body. The anterior pair of eminences of the corpora quadrigemina are concerned with sight; stimuli reaching them through the optic nerve, there, probably, first cause visual sensations, which it is left to the fore-brain to interpret. If the latter is removed from an animal light brought in front of the eye still causes contraction of the pupil; direct irritation of the eminences in question has the same effect, and destruction of one of them causes blindness of the oppo- site eye. The functions of the posterior pair of eminences FUNCTIONS OF CEREBELLUM. 587 of the corpora quadrigemina in man are uncertain. The cerebellum is the chief organ of combined muscular move- ments; it is the main seat of what we may call acquired reflexes. Every one has to learn to stand, walk, run, and so on; at first all are difficult, but after a time become easy and are performed unconsciously. In standing or walking very many muscles are concerned, and if the mind had all the time to look directly after them we could do nothing else at the same time; we have forgotten how we learnt to walk, but in acquiring a new mode of progression in later years, as skating, we find that at first it needs all our atten- tion, but when once learnt we have only to start the series of movements and they are almost unconsciously carried on for us. At first we had to learn to contract certain muscle groups when we got particular sensations, either tactile, from the soles, or muscular, from the general position of the limbs, or visual, or others (equilibrium sensations, see below) from the semicircular canals. But the oftener a given group of sensations has been followed by a given muscular contraction the more close becomes the associ- tion of the two; the path of connection between the afferent and efferent fibres becomes easier the more it is traveled, and at last the sensations arouse the proper move- ment without volitional interference at all, and while hardly exciting any consciousness; we can then walk or skate without thinking about it. The will, which had at first to excite the proper muscular nerve-centres in accordance with the felt directing sensations, now has no more trouble in the matter; the afferent impulses stimulate the proper motor centres in an unconscious and unheeded way. Injury or disease of the cerebellum produces great disturbances of locomotion and insecurity in maintaining various pos- tures. After a time the animals (birds, which bear the operation best) can walk again, and fly, but they soon become fatigued, probably because the movements require close mental attention and direction all the time. Sensations of Equilibrium. The semicircular canals have probably nothing to do with hearing. An old view was that, lying in three planes at right angles to one another, 588 THE HUMAN BOD T. they served to distinguish the direction of sound-waves reach- ing the ear; but as the direction of oscillation of the tym- panic ossicles is the same, no matter what that of the sound- waves entering the external auditory meatus may be, such an hypothesis has no foundation. The cochlea abundantly accounts for the appreciation of notes, and such noises as are due to inharmonically combined tones; while the vestibule will suffice for other noises: and it is found that disease of the semicircular canals does not interfere with hearing, but often causes uncertainty in movements and feelings of giddiness. Experiment shows that cutting a semicircular canal is followed by violent movements of the head in the plane of the canal divided; the animal staggers, also, if made to walk; and, if a pigeon and thrown into the air, cannot fly. Ail its muscles can contract as before, but they are no longer so co-ordinated as to enable the animal to maintain or regain a position of equilibrium. It is like a creature suffering from giddiness; and similar phenomena follow, in man, electrical stimulation of the regions of the skull in which the semicircular canals lie. Such facts suggest that the semicircular canals are organs in which sensory afferent impulses, assisting in the pre- servation of bodily equilibrium, arise. The unconscious maintenance of the erect position depends on the excitation of many co-ordinating motor centres through tactile, mus- cular, visual, and aural sensations; all acting together in normal combination these enable us to stand without thought; but loss of any one, or its abnormal state, will throw the whole mechanism out of gear. Persons who have lost muscular or .tjicti]e..seiidbility:stmiA and~vvaTTTwtth diffr- culty; those who have nystagmus, (jerking 'unconscious movements of the eyeballs which cause the visual field to seem to move in space) do the same and feel giddy; and, as we have just seen, similar phenomena follow injury of the semicircular canals. How the nerves in the latter are stimulated is not certain; being filled with liquid the pressure of this on any ampulla will be increased when the head is bent so as to place that one below; and this may be the excitant; giving rise to EQUILIBRIUM SENSATIONS. 589 afferent impulses, which change the condition of the co-ordinating locomotor centres, with every position of the head. Or, movements of the endolymph in relation to the wall of the canal may be the stimulus, the current swaying the projecting hairs (Fig. 149).* Place a few small bits of cork in a tumbler of water, and rotate the tumbler; at first the water does not move with it; then it begins to go in the same direction, but more slowly; and, finally, moves at the same angular velocity as the tumbler. Then stop the tumbler, and the water will go on rotating for some time. Now if the head be turned in a horizontal plane similar phenomena will occur in the endolymph of the horizontal canal; if it be bent sidewise in the vertical plane, in the anterior vertical canal; and if nodded, in the posterior verti- cal; the hairs moving with the canal would meet the more stationary water and be pushed and so, possibly, excite the nerves at the deep ends of the cells which bear them, and gen- erate afferent impulses which will cause the general nerve- centres of bodily equilibration to be differently- acted upon in each case, tinder ordinary circumstances the results of these impulses do not become prominent in consciousness as sensations; but they sometimes may. If one spins round for a time, the endolymph takes' up the movement of the canals, as the water in the tumbler does that of the glassy on stopping, the liquid still goes on moving and stimulates the hairs which are now stationary; and we feel giddy, from the ears telling us we are rotating and the eyes that we are not; hence difficulty in standing erect or walking straight. A common trick illustrates this very well; make a person place his forehead on the handle of an umbrella, the other end of which is on the floor, and then walk three or four times round it, rise, and try to go out of a door; he will nearly always fail, being unable to combine his muscles properly on account of the conflicting afferent impulses. If a person, with eyes shut, be laid on a hori- zontal table which is turned, he can at first feel and tell the direction of the rotation; as it continues he loses the feeling, and when the movement stops feels as if he were being turned * Page 542. 590 THE HUMAN BODY. in the opposite direction. All this becomes readily intelli- gible if we suppose feelings to be excited by relative more- ments of the endolymph and the canals inclosing it. The so-called " auditory sacs" of many Mollusks (see- Zoology) are probably organs for equilibration sensations, and comparable to our semicircular canals. Functions of the Fore-Brain. Beyond the broad fact that this part of the nervous system is essentially volitional and intellectual in function, we know very little. It is. clearly not the seat of the centres of muscular co-ordination* for, after its removal, an animal can still, if properly stimu- lated, execute perfectly all its usual movements. The true motor centres lie farther back; those for the less compli- cated combinations, as bending a limb, in the spinal cord, and those for more complex, as standing, walking, or breathing, in the mid- and hind-brains: and are not auto- matic. They may be excited to activity either, reflexly, by afferent impulses, traveling in from sensory regions and associated or not with consciousness; or, directly, by im- pulses, associated with those states of consciousness which we call volitions, passing back from the fore-brain. The fore -brain, also, frequently inhibits movements which, in its absence, would be caused by discharges, reflex in nature, of the lower centres; the will is as often employed in restraining as in exciting muscular contractions. For instance, after the cerebral hemispheres have been removed from a frog, stroking the animal gently on its back will, each time, cause it to croak; the skin stimulation origi- nates afferent impulses which excite the " croaking centre" to discharge: but if the creature has its fore-brain it croaks or not as it pleases; it can then allow the co-ordinating mechanism to work freely under the stimulus, or check it; and it can also, independently of any immediate stimulus, excite voluntarily the same motor nerve-centre and croak if it chooses. We constantly meet with similar phenomena in ourselves; afferent impulses are all the time at work, tend- ing to produce one action or another; and a great part of our mental activity consists in deciding which we shall prevent and which we shall permit. The restraint thus FUNCTIONS OF THE FORE-BRAIN. 591 exercised by the fore-brain on the lower centres is at least as important as its power of exciting them; strength of character depends, perhaps, more on great inhibitory power in the fore-brain than on its initiating faculty. The intellectual powers seem mainly, if not entirely, de- pendent for their necessary material antecedents or con- comitants on the surface convolutions of the cerebral hemispheres; if these alone be removed from an animal its mental condition is much the same as if the whole fore- brain be taken away. Some simple and fundamental perceptions seem, however, to remain, having, perhaps, their seats in the deeper gray masses constituting the optic tlialami and corpora striata (p. 167); a dog, from which the greater part of the cerebral surfaces had been re- moved, after a time learnt to walk about, apparently volun- tarily, and to find and eat his food; he even learned not to take the food of other dogs after he had several times been severely bitten for so doing. But more complex perceptions were lost; before the operation, for example, he was violently terrified by peeing a man fantastically dressed; but after- wards no such things seemed to arouse in him so complex a conception as that of a strange or dangerous object. Although the fore-brain is the seat of consciousness it is itself insensible to cutting or wounding; and was long supposed to be entirely inexcitable by general nerve stimuli. It has, however, been found that tolerably powerful electri- cal currents applied to the convolutions produce, in many cases, definite movements; the nature of the movement depending upon the area stimulated. Hence an attempt nas been made to detect the functions of different parts of the cerebral hemispheres by observing the results of stimu- lating each; and provisionally we may, perhaps, assume that the brain-centres, from which volitional impulses proceed to the co-ordinating centres for the muscle-groups called into play, lie in the cerebral regions whose stimulation is followed by the movement. The animals, however, so often recover the power of executing the movement spontaneously after its supposed volitional centre has been removed that the proper interpretation of the experimental results is still 592 THE HUMAN BODY. doubtful. Stimulation of many cerebral regions is fol- lowed by no results; and that of others by movements the power of voluntarily executing which is not, even tempo- rarily, lost when that brain-part is removed; these cerebral areas have been supposed to be concerned with mental faculties other than volition, the stimulation exciting sen- sations, perceptions, or emotions; but this is still very doubtful. Localized disease of regions of the human brain has, so far, given better results than physiological experi- ment on the lower animals. The power of using words to express ideas seems intimately connected with a small area on the fore part of the left cerebral hemisphere, and to be lost (producing the condition known as aphasia) when that part is diseased; and many cases have been recorded in which a wound of the skull has been followed exactly by loss of the power of voluntarily moving those muscle groups which (accepting the results of electrical stimulation in the lower animals) might be supposed to be normally excited, through the will, from the cerebral area injured. Absence of recovery unless the brain injury is cured seems, moreover, to be the rule in man; while as we have seen this is not the casein lower animals: this may, perhaps, indicate a more precise division of physiological labor in the human brain; which is a priori probable, considering its great superiority as a mental apparatus. What the use of two cerebral hemispheres is, cannot at present be said. Injury of one produces its main effects, so far as sensation and motion are concerned, on the oppo- site side of the Body; but other faculties, as that of using (Speech, seem located on one side only; and in the brains of the higher human races the surface markings on the two sides are not perfectly symmetrical, which may indicate some difference in function. It has been suggested that in many cases we only learn to use one side of the brain, and that the other is in reserve in case of injury or disease; but the evidence is inconclusive: a good deal may, however, be said for the view that a good deal of brain in every one's skull is never used. It is there untaught but ready to be educated. IMPORTANCE OF EXERCISING THE BRAIN. 593 Movements which are commonly executed together tend to become so associated that it is difficult to perform one .alone; many persons, e.g., cannot close one eye and keep the other open. From frequent use, the paths of con- duction between the co-ordinating centres for both groups of muscles have become so easy that a volitional impulse reaching one centre spreads to the other and excites both. This association of movements, dependent on the modifica- tion of brain structure by use, finds an interesting parallel in the psychological phenomenon known as the association of ideas; and all education is largely based on the fact that the more often brain regions have acted together the more readily, until finally almost indissolubly, do they so act. If we always train up the child to associate feelings of disgust with vvrong actions and of approbation with right, when he is old he will find it very hard to do otherwise: such an organic nexus will have been established that the activity of the one set of centres will lead to an excitation of that which habit has always associated with it. The nerve-centres are throughout eminently plastic; every thought leaves its trace for good or ill; and the moral truism that the more often we yield to temptation — the more often an evil solici- tation, sensory or otherwise, has resulted in a wrong act — the harder it is to resist it, has its parallel (and we can hardly doubt its physical antecedent) in the marking out of .a path of easier conduction from perceptive to volitional centres in the brain. The knowledge that every weak yielding degrades our brain structure and leaves its trail in that organ through which man is the " paragon of animals," while every resistance makes less close the bond between the thought and the act for all future time, ought surely to "give us pause:" on the other hand, every right action helps to establish a "path of least resistance," and makes its subsequent performance easier. The brain, like the muscles, is improved and strengthened by exercise and injured by overwork or idleness; and just .as a man may specially develop one set of muscles and neglect the rest until they degenerate, so he may do with Jiis brain; developing one set of intellectual faculties and 594 THE HUMAN BODY. leaving the rest to lie fallow until, at last, he almost loses the power of using them at all. The fierceness of the battle of life nowadays especially tends to produce such lopsided mental developments; how often does one meet the business. man, so absorbed in money-getting that he has lost all power of appreciating any but the lower sensual pleasures; the intellectual joys of art, science, and literature have no charm for him; he is a mere money-making machine. One, also, not unfrequently meets the scientific man with no appreciation of art or literature; and literary men utterly in- capable of sympathy with science. A good collegiate education in early life, on a broad basis of mathematics, languages, and the natural sciences, is a great security against such imperfect mental growth; one danger in American life is the tendency to put lads in a technical college, or to start them in business before they have attained any broad general education. Another danger, no doubt, is the oppo- site one of making the training too broad; a man who knows one or two literatures fairly well, and who has mastered the elements of mathematics and of one of the observational or experimental sciences, is likely to have a better and more utilizable brain than he who has a smattering of half a dozen languages and a confused idea of all the " ologies." The habits of mental slovenliness, the illogical thinking, and the incapacity to know when a thing really is mastered and understood, which one so often finds as the results of such an education, are far worse than the narrowness apt to follow the opposite error, which is often associated with the power of accurate logical thought. Those who are deprived of the advantages of a general collegiate educa- tion may now, more easily than at any previous period, cultivate mental breadth by reading some of the many excellent general reviews and magazines, and the readable but exact popular expositions now available on nearly all subjects, which are such a feature of our age. Associating, out of working hours, with those whose special pursuits aie different from our own is almost necessary to those who would avoid such an asymmetrical development as almost amounts to intellectual deformity. CHAPTER XXXVI. VOICE AND SPEECH. Voice consists of sounds produced by the vibrations of two elastic bands, the true vocal cords, placed in the larynx, an upper modified portion of the passage which leads from the pharynx to the lungs. When the vocal cords are put in a certain position, air driven past them sets them in periodic vibration, and they emit a musical note; the lungs and respiratory muscles are, therefore, accessory parts of the vocal apparatus: the strength of the blast pro- duced by them determines the loudness of the voice. The larynx itself is the essential voice-organ: its size primarily •determines the pitch of the voice, which is lower the longer the vocal cords; and, hence, shrill in children, and usually higher pitched in women than in men; the male larynx grows rapidly at commencing manhood, causing the change com- monly known as the " breaking of the voice." Every voice, Tvhile its general pitch is dependent on the length of the vocal cords, has, however, a certain range, within limits "which determine whether it shall be soprano, mezzo-soprano, .alto, tenor, baritone, or bass. This variety is produced by muscles within the larynx which alter the tension of the vocal cords. Those characters of voice which we express by such phrases as harsh, sweet, or sympathetic, depend on the structure of the vocal cords of the individual; cords which in vibrating emit only Harmonic partial tones (p. 547) are pleasant; while those in which inharmonic partials are conspicuous are disagreeable. The vocal cords alone would produce but feeble sounds; those that they emit are strengthened by sympathetic re- sonance of the air in the pharynx and mouth, the action of 596 THE HUMAN BODY. which may be compared to that of the sounding-board of a yiolin. By movements of throat, soft palate, tongue, cheeks, and lips the sounds emitted from the larynx are altered or supplemented in various ways, and converted into articulate language or speech. The Larynx lies in front of the neck, beneath the hyoid bone and aboro the windpipe; in many persons it is prominent, causing the projection known as " Adam's apple." It consists of a framework of cartilages, partly joined by true synovial joints and partly bound together Cs FIG. 156.— The more important cartilages of the larynx from behind, t, thyroid; Cs, its superior, and Ci, its inferior, horn of the right side; **, cricoid cartilage; t, arytenoid cartilage; Pv, the corner to which the posterior end of a vocal cord is attached ; Pm, corner on which the muscles which approximate or separate the vocal cords are inserted; co, cartilage of Santorini. by membranes; muscles are added which move the car- tilages with reference to one another; and the whole is lined by a mucous membrane. The cartilages of the larynx (Fig. 156) are nine in number; three single and median, and three pairs. The largest (t) is called the thyroid, and consists of two halves which meet at an angle in front, but separate behind so as to inclose a V-shaped space, in which most of the remaining cartilages lie. The epiglottis (not represented in the figure) is fixed to the top of the thyroid cartilage and overhangs the entry ANATOMY OF LARYNX. 597 from the pharynx to the larynx (e, Fig. 89);* it may be seen, covered by mucous membrane, projecting at the base of the tongue, if the latter be pushed down while the mouth is held open in front of a mirror; and is, similarly covered, represented, as seen from behind, at a in Fig. 157. The cricoid, the last of the unpaired cartilages, is the shape of a signet-ring; its broad part (*„., Fig. 156) is on the pos- terior side and lies at the lower part of the opening between the halves of the thyroid; in front and on the sides it is narrow, and a space, occupied by the crico-thyroid mem- brane, intervenes between its upper border and the lower edge of the thyroid cartilage. The angles of the latter are produced above and below into projecting horns (Cs and Ci, Fig. 156), and the lower horn on each side forms a joint with the cricoid. The thyroid can be ro- tated on an axis, passing through the joints on each side, and rolled down so that its lower front edge shall come nearer the cricoid cartilage, the membrane there intervening being folded. The arytenoids (f, Fig. 156) are the largest of the paired cartilages; they are seated on the upper edge of the posterior wide portion of the cricoid, and form true joints with it. Each is pyramidal with a triangular base, and has on its tip a small nodule (co, Fig. 156), the cartilage of Santorini. From the tip of each arytenoid cartilage the aryteno-epiglottidean fold of mucous membrane (10, Fig. 157) extends to the epiglottis; the cartilage of Santorini causes a projection (8, Fig. 157) in this; and a little farther on (9) is a similar eminence on each side, caused by the remaining pair of cartilages, known as the cuneiform, or cartilages of Wrisberg. The Vocal Cords are bands of elastic tissue which reach from the inner angle (Pv, Fig. 156) of the base of each arytenoid cartilage to the angle on the inside of the thyroid where the sides of the V unite; they thus meet in front but are separated at their other ends. The cords are not, how- ever, bare strings, like those of a harp, but covered over with the lining mucous membrane of the larynx, a slit, called the glottis (c, Fig. 157), being left between them. It is the * Page 309. 598 THE HUMAN BODY. projecting cushions formed by them on each side of this slit which are set in vibration during phonation. Above each vocal cord is a depression, the ventricle of the larynx, (b1. Fig. 157); this is bounded above by a somewhat promi- FIG. 15?.— The larynx viewed from its pharyngeal opening. The back wall of the pharynx has been divided and its edges (11) turned aside. 1, body of hyoid ; 2, its small, and 3, its great, horns; 4, upper and lower horns of thyroid carti- lage ; 5, mucous membrane of front of pharynx, covering the back of the cricoid cartilage; 6, upper end of gullet; 7, windpipe, lying in front of the gullet; 8, eminence caused by cartilage of Santorini; 9, eminence caused by cartilage of Wrisberg; both lie in, 10, the aryteno-epiglottidean fold of mucous membrane, surrounding the opening (aditus laryngis) from pharynx to larynx, a, project- ing tip of epiglottis; c, the glottis, the lines leading from the letter point to the free vibratory edges of the vocal cords. &', the ventricles of the larynx: their upper edges, marking them off from the eminences 6, are the false vocal cords. nent edge, the false vocal cord. Over most of the interior of the larynx its mucous membrane is thick and covered by ciliated epithelium, and has many mucous glands im- bedded in it. Over the vocal cords, however, it is repre- sented only by a thin layer of flat non-ciliated cells, and MOVEMENTS OF LARYNX. 599 contains no glands. In quiet breathing, and after death, the free inner edges of the vocal cords are thick and rounded, and seem very unsuitable for being readily set in vibration. They are also tolerably widely separated behind, the aryten- oid cartilages, to which their posterior ends are attached, being separated. Air under these conditions passes through without producing voice. If they are watched with the laryngoscope during phonation, it is seen that the cords approximate behind so as to narrow the glottis; at the same time they become more tense, and their inner edges project more sharply and form a better-defined margin to the glottis, and their vibrations can be seen. These changes are brought about by the delicately co-ordinated activity of a number of small muscles, which move the cartilages to which the cords are fixed. The Muscles of the Larynx. In describing the direc- tion and action of these it is convenient to use the words front or anterior and back or posterior with reference to the larynx itself (that is as equivalent to ventral and dorsal) and not with reference to the head, as usual. The base of each arytenoid cartilage is triangular and fits on a surface of the cricoid, on which it can slip to and fro to some ex- tent, the ligaments of the joint being lax. One corner of the triangular base is directed inwards and forwards (i.e. towards the thyroid) and is called the vocal process (Pv, Fig. 156). as to it the vocal cords are fixed. The outer posterior angle (Pm, Fig. 156) has several muscles inserted on it and is called the muscular process. If it be pulled back and towards the middle line the arytenoid cartilage will rotate on its vertical axis, and roll its vocal processes forwards and outwards, and so widen the glottis; the re- verse will happen if the muscular process be drawn forwards. The muscle producing the former movement is the posterior crico-arytenoid (Cap, Fig. 188); it arises from the back of the cricoid cartilage, and narrows to its insertion into the muscular process of the arytenoid on the same side. The opponent of this muscle is the lateral crico-arytenoid, which arises from the side of the cricoid cartilage, on its inner surface, and passes upwards and backwards to tho (500 THE HUMAN BODY. muscular process. The posterior crico-arytenoids, working alone, pull inwards and downwards the muscular processes, turn upwards and outwards the vocal processes, and sepa- rate the posterior ends of the vocal cords. The lateral crico-thyroid, working alone, pulls downwards and forwards the muscular process, and rotates inwards and upwards the vocal7 process, and narrows the glottis; it is the chief agent in producing the approximation of the cords necessary for aep FIG. 158.— The larynx seen from behind and dissected so as to display some of its muscles. The mucous membrane of the front of the pharynx (5, Fig. 157) has been dissected away, so as to display the laryngeal muscles beneath it. Part of the left half of the thyroid cartilage has been cut away, co, cartilage of Santorini; CM, cartilage of Wrisberg. the production of voice. When both pairs of muscles act together, however, each neutralizes the tendency of the other to rotate the arytenoid cartilage; the downward part of the pull of each is, thus, alone left, and this causes the arytenoid to slip downwards and outwards, off the eminence on the cricoid with which it articulates, as far as the loose capsu- lar ligament of the joint will allow. The arytenoid car- tilages are thus moved apart and the glottis greatly widened TENSION CHANGES IN VOCAL CORDS. 601 and brought into its state in deep quiet breathing. Other muscles approximate the arytenoid cartilages after they have been separated. The most important is the transverse arytenoid (A, Fig. 158), which runs across from one ary- tenoid cartilage to the other. Another is the oblique ary- tenoid (Taep], which runs across the middle line from the base of one arytenoid to the tip of the other; thence cer- tain fibres continue in the aryteno-epiglottidean fold (10, Fig. 157) to the base of the epiglottis; this, with its fellow, thus embraces the whole entry to the larynx ; when they -contract they bend inwards the tips of the arytenoid car- tilages, approximate the edges of the aryteno-epiglottidean fold, and draw down the epiglottis, and so close the pas- sage from the pharynx to the larynx. When the epiglottis has been removed, food and drink rarely enter the larynx in swallowing, the folds of mucous membrane being so brought together as to effectually close the aperture be- tween them. Increased tension of the vocal cords is produced by the trico-thyroid muscles, one of which lies on each s:ie of the larynx, over the crico-thyroid membrane. Their action may be understood by help of the diagram, Fig. 159, in which t represents the thyroid cartilage, c the cricoid, a an f\ Arytenoid, and vc a vocal cord. The muscle passes obliquely backwards and upwards from About d near the front end of 4, to t, about I, near the pivot (which represents the joint be- tween the cricoid cartilage and the inferior horn of the thy- roid). When the muscle con- tracts it pulls together the ante- rior ends of t and c; either by depressing the thyroid (as represented by the dotted lines) or by raising the front -end of the cricoid; and thus stretches the vocal cord, if the arytenoid cartilages be held from slipping forwards. The antagonist of the crico-thyroid is the 603 THE HUMAN BODY. thyro-arytenoid muscle; it lies, on each, side, imbedded in the fold of elastic tissue forming the vocal cord, and passes from the inside of the angle of the thyroid cartilage in front, to the anterior angle and front surface of the ary- tenoid behind. If the latter be held firm, the muscle raises the thyroid cartilage from the position into which the crico-thyroid pulls it down, and so slackens the vocal cords. If the thyroid be held fixed by the crico-thyroid muscle, the thyro-arytenoid will help to approximate the- vocal cords, rotating inwards the vocal processes of the- arytenoids. The lengthening of the vocal cords when the thyroid cartilage is depressed tends to lower their pitch; the in- creased tension, however, more than compensates for this and raises it. There seems, however, still another method by which high notes are produced. Beginning at the bottom of his register, a singer can go on up the scale some- distance without a break; but, then, to reach his higher notes, must pause, rearrange his larynx, and begin again. What happens is that, at first, the vocal processes are turned in, so as to approximate but not to meet; the whole- length of each edge of the glottis then vibrates, and its tension is increased, and the pitch of the note raised, by increasing contraction of the crico-thyroid. At last this at- tains its limit and a new method has to be adopted. The vocal processes are more rolled in, until they touch. This produces a node (see Physics) at that point and shortens- the length of vocal cord which vibrates. The shorter string emits a higher note; so the crico-thyroid is relaxed, and then again gradually tightened as the notes sung are raised in pitch from the new starting-point. To pass- easily and imperceptibly from one such arrangement of the larynx to another is a great art in singing. There is some reason to believe that a second node may, for still higher notes, be produced at a more anterior point on the vocal cords. The method of production of. falsetto notes is uncertain; during their emission the free border of the vocal cords alone vibrates. VOWEL SOUNDS. 603 The range of the human voice is about three octaves, from e (80 vib. per 1") on the unaccented octave, in male voices, to c on the thrice accented octave (1024 vib. per 1'), in female. Great singers of course go beyond this range; basses have been known to take a on the great octave (55 vib. per 1*) ; and Nilsson in " II Flauto Magico" used to take /on the fourth accented octave (1408 vib. per 1"). Mozart heard at Parma, in 1770, an Italian songstress whose voice had the extraordinary range from g in the first accented octave (198 vib. per I") to c on the fifth accented octave (2112 vib. per 1"). An ordinary good bass voice has a com- pass from / (88 vib. per 1" ) to d" (297 vib. per I'7); and a soprano from V (248 vib. per 1") to g'" (792). Vowels are, primarily, compound musical tones (p. 548) produced in the larynx. Accompanying the primary partial of each, which determines its pitch when said or sung, are a number of upper partials, the first five or six being recogni- zable in good full voices. Certain of these upper partials are reinforced in the mouth to produce one vowel, and others for other vowels; so that the various vowel sounds are really musical notes differing from one another in timbre. The mouth and "throat cavities form an air-chamber above the larynx, and this has a note of its own which varies with its size and form, as may be observed by opening the mouth widely, with the lips retracted and tho cheeks tense; then gradually closing it and protruding the lips, meanwhile tapping the cheek. As the mouth changes its form the note produced changes, tending in general to pass from a higher to a lower pitch and suggesting to the ear at the same time a change from the sound of a (father) through 6 (more) to 66 (moor). When the mouth and throat cham- bers are so arranged that the air in them has a vibratory rate in unison with any partial in the laryngeal tone, it will be set in sympathetic vibration, that partial will be strengthened, and the vowel characterized by it uttered. As the mouth alters its form, although the same note be still sung, the vowel changes. In the above series (a, 6, 66) the tongue is depressed and the cavity forms one chamber; for a this has a wide mouth opening; for 6 it is 604 THIS KTJMAN BODY. narrowed; for 66 still more narrowed, and the lips protruded so as to increase the length of the resonance chamber. The partial tones reinforced in each case are, according to Helmholtz — 00 In other cases the mouth and throat cavity is partially sub- divided, by elevating the tongue, into a wide posterior and a narrow anterior part, each of which has its own note; and the vowels thus produced owe their character to two rein- forced partials. This is the case with the series a (man), e (there) and i (machine). The tones reinforced by reson- ance in the mouth being — The usual I of English, as in spire, is not a true simple vowel but a dipththong, consisting of a (pad) followed by e (feet) ; as may be observed by trying to sing a sustained note to the sound I; it will then be seen that it begins as a and ends as ee. A simple vowel can be maintained pure as long as the breath holds out. In uttering true vowel sounds the soft palate is raised so as to cut off the air in the nose, which, thus, does not take part in the sympathetic resonance. For some other sounds (the semi-vowels or resonant s) the initial step is, as in the case of the true vowels, the production of a laryiigeal tone; but the soft palate is not raised, and the mouth exit is more or less closed by the lips or the tongue; hence the blast partly issues through the nose, and the air there takes part in the vibrations and gives them a special character; this is the case with m, n} and ng. CONSONANTS. 605 Consonants are sounds produced not mainly by the vocal cords, but by modifications of the expiratory blast on its way through the mouth. The current may be inter- rupted and the sound changed by the lips (labials) ; or, at or near the teeth, by the tip of the tongue (dentals) ; or, in the throat, by the root of the tongue and the soft palate (gutturals). Consonants are also characterized by the kind of movement which gives rise to them. In explosives an interruption to the passage of the air-current is suddenly interposed or removed (P, T, B, D, K, G). Other con- sonants are continuous (as F, S, E), and may be subdivided into — (1) aspirates, characterized by the sound produced by a rush of air through a narrow passage, as when the lips are approximated (F), or the teeth (S), or the tongue is brought near the palate (Sh), or its tip against the two rows of teeth, they not being quite in contact (Th). For L the tongue is put against the hard palate and the air escapes on its sides. For Ch (as in the proper Scotch pronunciation of loch) the passage between the back of the tongue and the soft palate is narrowed. To many of the above pure consonants answer others, in whose production true vocali- zation (i.e. a laryngeal tone) takes a part. F with some voice becomes V; S becomes Z, Th soft (teeth) becomes Th hard; and Ch becomes Gh. (2) Resonants; these have been referred to above. (3) Vibratones (the different forms of K), which are due to vibrations of parts bounding a constriction put in the course of the air-current. Ordi- nary R is due to vibrations of the tip of the tongue held near the hard palate; and guttural R to vibrations of the uvula and parts of the pharynx. The consonants may physiologically be classified as in the following table (Foster). Explosives. Labials, without voice P* " with voice B. Dentals, without voice T. " with voice D. Gutturals, without voice K. " with voice G (hard). 606 THE HUMAN BODY. Aspirates. Labials, without voice. . . .F. " with voice V. Dentals, without voice . . . S, L, Sh, Th (hard). with voice Z, Zh (asure), Th (soft). Gutturals, without voice . Ch (loc/<). " with voice Ch. Resonants. Labial. M. Dental N. Guttural NG. Vibratories. Labial — not used in European languages. Dental R (common). Guttural R (guttural). H is alaryngeal sound: the vocal cords are separated for its production, yet not so far as in quiet breathing. The air- current then produces a friction sound but not a true note, ,as it passes the glottis; and this is again modified when the current strikes the waH of the pharynx. Simple sudden closure of the glottis, attended with no sound, is also a speech element, though we do not indicate it with a special letter, since it is always understood when a word begins with a vowel, and only rarely is used at other times. The Greeks had a special sign for it, , the soft breathing; and another, °, the hard breathing, answering somewhat to our h and indicating that the larynx was to be held open, so as to give a friction sound, but not voice. In whispering there is no true voice; the latter implies true tones, and these are only produced by periodic vibra- tions; whispering is a noise. To produce it the glottis is tolerably narrowed but the cords are not so stretched as to produce a sharply defined edge on them, and the air driven past is then thrown into irregular vibrations. Such vibra- tions as coincide in period with the air in the mouth and throat are always present in sufficient number to characterize the vowels; and the consonants are produced in the ordinary way, though the distinction between such letters as P and B, F and V, remains imperfect. INDEX. Abdomen, contents of, 4. Abdominal respiration, 366. Abducens nerve, 170. Aberration, chromatic, 502. Aberration, spherical, 503. Absorbents, 329. Absorption from intestines, 345 Absorption of gases, 380. Absorption of oxygen by blood, 382. Accelerator nerves of heart, 252. Accommodation, 498. Acetabulum, 79. Acid, acetic, 14; butyric, 14; car- bonic, see carbon dioxide ; for- mic, 14; glycero-phosphoric, 14; glycocholic, 342; lactic, 14; oleic, 13; palmitic, 13; sarco- lactic, 14, 125; stearic, 13; taurocholic, 342. Acquired (secondary) reflexes, 587. Action current (negative varia- tion), 193, 197. Actions, reflex, 182, 574. Addison's disease, 333. Adenoid tissue, 106. Adipose tissue, 111. Adrenals (supra-renal capsules), 333. Advantage of mixed diet, 305, 446. After images, 526. Ague cake, 336. Air, chemical composition of, 374. Air cells, 354. Air, changes produced in by breathing, 373. Air, complemental, tidal, etc., 365. Air passages, 353. Albumin, serum, 57. Albuminoids, 11. Albuminous bodies, 10. Alcohol, 304. Alimentary canal, 308. Alimentary principles, 298. Amo3boid cells, 114. Amoeboid movements, 21, 48. Amyloids (carbohydrates), 13, 300. Amyloids, digestion of, 335, 341, 349. Anaemia, 59. Anatomical systems, 37. Anatomy of alimentary canal, 308; of ear, 535; of eye, 479; of joints, 94; of lymphatic sys- tem, 329; of muscular system, 116; of nervous system, 154; of respiratory organs, 352; of skeleton, 62; of skin, 412; of urinary organs, 402; of vascu- lar system, 201. Animal heat, source of, 451. Anterior tibial nerve, 211. Anvil bone, 527. 608 INDEX. Aorta, 209. Apex beat of heart, 220. Appendicular skeleton, 77. Appetite, 350. Appendix vermiformis, 322. Apoplexy, 166. Aqueduct of Sylvius, 137. Aqueous humor, 490. Arachnoid, 6, 157. Arbor vitae, 168. Areolar tissue, 102. Areolar tissue, subcutaneous, 414. Arm, skeleton of, 78. Arterial blood, 216, 379. Arterial pressure, 235, 251. Arteries, distribution of, 208. Arteries, structure of, 217. Artery, axillary, 210; brachial, 210; bronchial, 211; carotid, 210; cceliac, 210; coronary, 207, 210; iliac, 210; innominate, 210; intercostal, 211; radial, 210; subclavian, 210; ulnar, 210; vertebral, 210, Articular cartilage, 94. Articulations, 63, 93. Arytenoid cartilages, 597. Asphyxia, 398. Aspiration of thorax, 244, 237. Assimilation, 19. Assimilative tissues, 30. Associated movements, 593. Association of ideas, 593. Astigmatism, 503. Astragalus, 82. Atlanto-axial articulation, 97. Atlas vertebra, 68. Auditory nerve, 170. Auditory ossicles, 536. Auditory perceptions, 554. Auriculo-ventricular valves, 208. Automatic centres, 183. Automatic movements, 23. Automatic tissues, 32. Axial current. 228. Axial ligament, 537. Axial skeleton, 63, 67. Axillary artery, 210. Axis, vertebra, 68. Axis, visual, 518. Ball and socket joints, 96. Basement membrane, 106, 260. Basilar membrane, 540, 553. Bathing, 421. Beans (as food), 303. Beat of heart, 219. Beef tea, 126. Biceps muscle of arm, 118. Bile, 342. Blackness, sensation of, 519. Bladder, urinary, 402. Blind spot, 508. Blood, 40; arterial and venous, 216, 379; composition of, 57; clotting of, 50; crystals, 47; gases of 378; histology of, 44; laky, 46; quantity of, 59; se- rum, 50. Bloodflow in capillaries, 227; in kidneys, 408; in liver, 325; rate of, 242. Blood-vessels, anatomy of, 208. Blood-vessels, nerves of, 253. Blood-vessels, structure of, 218. Blushing, 255. Bone, composition of, 91; his- tology of, 88; gross structure of, 86. Bones of face, 64; of fore-limb, 78; of hind-limb, 79; of pec- toral arch, 77; of pelvic arch, 79; of skull, 75. Brachial artery, 210. Brachial plexus, 161. Brain, anatomy of, 163; physi- ology of, 584; membranes of, 157. Bread, 302. INDEX. 609 Breast-bone, 71. Bronchial arteries, 211. Bronchial tubes, 354. Bronchus, 354. Brunner's glands, 322. Buccal cavity, 308. Buffy coat on blood clot, 52. Calcaneum, 79. Camera obscura, 496. Canals, semicircular, 539, 587 Capacity of lungs, 365. Capillaries, blood, 211, 217. Capillaries, lymphatic, 329. Capillary circulation, 227. Capsule of Glisson, 326. Carbohydrates, see amyloids. Carbon dioxide, 14; in blood, 386 ; production of in muscle, 430. Carbon monoxide haemoglobin, 398. Cardiac muscular tissue, 124. Cardiac impulse, 220. Cardiac nerves/ 248. Cardiac orifice of stomach, 317. Cardiac plexus, 172. Cardio-inhibitory nerves 250. Carotid artery, 210. Carpus, 79. Casein, 11. Cartilage, 100; articular, 94; elas- tic, 106; fibro-, 107; histology of, 101; inter-articular, 107. Cartilages of larynx, 596. Cataract, 504. Catarrh, 256. Cauda equina, 161. Cells, 17; amoeboid, 114; ciliated, 33, 115; division of, 18; diffe- rentiation of, 26 ; growth of, 18. Cement, of tooth, 311. Centre, cardio - inhibitory, 250; cerebro-spinal, 156; convulsive, 400; respiratory, 391. Centre of gravity of body, 149. Centres, nerve, general functions of, 182. Cephalic vein, 214. Cerebellum, 164, 586. Cerebral hemispheres, 163. Cerebral hemispheres, functions of, 590. Cerebro-spinal centre, 5, 156. Cerebro-spinal liquid, 158. Cervical plexus, 161. Cervical vertebras, 68. Characteristics of human skele- ton, 84. Chemical combinations, energy liberated in, 283. Chemical composition of body, 8. Chemical changes in breathed air, 373. Chemistry, of bile, 342; of blood, 57; of bone, 91; of fats, 112; of gastric juice, 338; of lymph, 61; of muscle, 124, 429; of pancreatic secretion, 264; of respiration, 372; of secretion, 264; of teeth, 312; of urine, 410; of white fibrous tissue, 103 ; of working muscle, 427. Chest. See Thorax. Chondrin, 107. Chorda tympani nerve, 271. Choroid, 486. Chromatic aberration, 502. Chyle, 340. Chyme, 339. Ciliary muscle, 491. Ciliary processes, 486. Ciliated cells, 33, 115. Circulation, 201, 214; during as- phyxia, 401; influence of re- spiratory movements on, 368; influence of nerves on, 247; portal, 216, 325; renal, 408. Circulatory organs, 201. 610 INDEX. Circumvallate papillae, 313. Classification of the tissues, 29. Classification of nerve-fibres, 184. Clavicle, 77. Clothing, 458. Coagulated proteid, 11. Coagulation of blood, 50. Coccyx, 70. Cochlea,. 539. Cochlea, functions of, 553. Caecum, 322. Coeliac axis, 211. Cold-blooded animals, 449. Collagen, 104. Collar-bone, 77. Colon, 322. Color blindness, 523. Color mixing, 521. Color vision, 519. Combustible foods, 425. Commissures, cerebral, 167. Common bile duct, 324. Common sensation, 463, 567. Complemental air, 365. Complementary colors, 521. Concha, 535. Conduction in spinal cord, 578, 583. Conductive tissues, 33. Conductivity, physiological, 21. Connective tissue, 62, 102, 106. Connective-tissue corpuscles, 105. Conservation of energy, 280. Consonants, 605. Contractile tissues, 33, 113. Contractility, 20, 128. Contrasts, visual, 525, Convulsive centre, 400. Cooking of meats, 301: of vege- tables, 303. Co-ordinating tissues, 32. Co-ordination, 22, 183. Cords, vocal, 567. Corium, 6, 413. Corn, 303. Cornea, 486. Coronary artery, 207; sinus. 207. Corpora albicantia, 168. Corpora quadrigemina, 164 Corpora striata, 163, 591. Corpus callosum, 166. Corresponding retinal points- 532. Corti, organ of, 541. Costal cartilages, 72. Costal respiration, 366. Cranial nerves, 168, 168. Cranium, 73. Crazy bone, 188. Cream, 302. Cretinism, 333. Cricoid cartilage, 597. Crura cerebri, 164. Crying, 402. Crypts of Lieberkiihn, 322. Crystalline lens, 491. Curari poisoning, 130. Cutaneous organs, 412. Cutaneous secretions, 417. Cutis vera, 413. Cystic duct, 324. Daltonism, 523. Death stiffening, 430. Defects (optical) of eye, 500, 502. Deglutition, 336. Dentine, 311. Depressor nerve, 255. Derbyshire neck, 333. Dermis, 6, 413. Descemet, membrane of, 486. Development, 26. Diabetes, 441. Dialysis, 42. Diaphragm, 4, 359. Dietetics, 446. Diet, mixed, advantages of, 305. Differentiation of the tissues, 26. Digestion, 334. Digestion of a typical meal, 347. INDEX. 611 Diploe, 91. Direction, perception of, 530, 554. Disassimilatiot, 20. Dislocation, 98, Dispersion of light, 495. Distance, perception of, 530, 554. Division of physiological employ- ments, 27. Dorsal (neural) cavity, 5. Dorsal vertebrae, 65. Drum of ear, 535. Ductless glands, 332, Duodenum, 320. Dura mater, 157. Duration of luminous sensations, 516, Dyspepsia, 350. Ear, 535. Eggs, 302. Elastic tissue, 104. Elastic cartilage, 106. Elements found in body, 9. Eliminative (excretory) tissues, 30. Emulsification, 341. Enamel, 311. Endbulbs, 556, Endocardium, 204. Endo-lymph, 538. Endo-skeleton, 62. Energy, conservation of, 280; kinetic, 282; lost from body daily, 279, 452; of chemical affinity, 283; potential, 282; muscular, source of, 427; source of in body, 283; utili- zation of in body, 289. Energy-yielding foods, 425. Enzymes, 336. Epidermis, 6, 412. Epiglottis, 316, 596. Epithelium, 6, 34. Epithelium, ciliated, 115. Equilibrium sensations, 587. Erect posture, 149. Ethmoid bone, 75. Eustachian tube, 535. Excretion, 259. Exercise, 138. Exoskeleton, 62. Expiration, 363. Expiratory centre, 397. External auditory meatus, 535. External ear, 535, External respiration, 352. Extrinsic reference of sensations, 476. Eye, anatomy of, 479; append- ages of, 480; optical defects of, 502; physiology of, 506; re- fraction of light in, 497. Eyeball, 485. Eyeball, muscles of, 483. Eyelids, 481. Facial nerve, 170. False vocal cords, 598. Fat, 13, 111. Fat, source of in body, 443. Fatigue of retina, 524. Fatty tissue, 111. Fauces, 315. Fechner's law, 473. Femoral artery, 211. Femur, 79, 88. Ferments, 336. Fibrin, 11, 51, 53. Fibrin ferment, 54. Fibrinogen, 53. Fibrinoplastin, 53. Fibro-cartilage, 106. Fibula, 79. Fick and Wislecenus, 427. Filiform papillae, 313. Flesh foods, 301. Follicles of hairs, 415. Fontanelles, 93. Foods, definition of, 297.- energy- 612 INDEX. yielding, 425 ; fleshy, 301 ; non- oxidisable, 296, 300; tissue- forming, 293, 495; vegetable, 302. Food of plants, 295. Foot, skeleton of, 79. Foramen, intervertebral,71; mag- num, 75; of Monro, 167; oval, 536; round, 536; thyroid, 79; vertebral, 68. Forebrain, 163, 591. Forelimb, skeleton of, 78. Frontal bone, 75. Fuel of Body, 287. Fundamental physiological ac- tions, 15. Fungiform papillae, 313. Funny bone, 188. Fur on tongue, 315. Gall bladder, 324. Ganglia, 156, 177; of cranial nerves, 171; of heart, 248; of spinal nerve-roots, 160; spora- dic, 173. Ganglion, Gasserian, 170. Gases of blood, 378. Gasserian ganglion, 170. Gastric digestion, 339. Gastric juice, 338. Glands, 262; cutaneous, 417; ductless, 332; gastric, 319 lachrymal, 482; lymphatic, 330; of Brunner, 332 of Lieberkiilm ; parotid, 274; salivary, 270, 315; submaxillary, 271. Glenoid fossa, 77. Gliding joints, 98. Glisson, capsule of, 326. Globe of eye, 485. Globulin, 11, 47. Glossopharyngeal nerve, 170. Glottis, 597. Glucose (grape sugar), 11. Gluten, 302. Glycerine, 13. Glycogen, 14, 438. Gmelin's test, 342. Goitre, 333. Grape sugar, 13. Great omentum, 317. Growth, 18. Gullet, 317. Haemal (ventral) cavity, 4, 6. Haematin, 12, 47. Haemoglobin, 46, 379. Hairs, 415. Hair-cells, 542. Hammer-bone, 537. Hand, skeleton of, 78. Haversian canals, 89. Heart, 203; beat of, 219; nerves of, 248. Heat production and regulation in Body, 449. Heat lost from lungs, 374. Heel-bone, 79. Hemispheres, cerebral, 163 ; func- tions of, 590. Hepatic artery, 324. Hepatic cells, 325. Hepatic duct, 324. Hepatic veins, 215, 324. Bering's theory of color vision^ 524. Hiccough, 402. Hind limb, skeleton of, 79. Hinge-joints, 94. Hip-joint, 94. Histology, 1; of adipose tissue, 111; of areolar tissue, 103; of blood, 44; of bone, 88; of car- diac muscle, 124; of cartilage, 101; of connective tissues, 103: of ear, 540; of elastic tissue, 104; of hairs, 415; of kidney, 407; of liver, 325; of lungs, 355; of lymph, 49; of nails, 417; of nerve cells, 175; of INDEX. 613 nerve-fibres, 173; of nose, 569; of plain muscular tissue, 123; of retina, 487; of skin, 412; of small intestine, 320; of spinal cord, 177; of stomach, 318; of striped muscle, 122; of teeth, 311; of tactile organs, 556; of tongue, 570; of white fibrous tissue, 103 Hollow veins, 207. Homologies of supporting tissues 107, Homology, 58; of limbs, 80. Horopter, 533. Humerus, 78, 86. Humor, aqueous, 490; vitreous, 491. Hunger, 569. Hydrocarbons. See Fats. Hydrogen, 9. Hygiene, 1; of blood, 58; of brain, 593; of clothing, 366, 458 ; of exercise, 138 ; of grow- ing skeleton, 109; of joints, 98; of muscles, 138; of respiration, 366, 375; of sight, 501; of skeleton, 92; of skin, 420; of supporting tissues, 109. Hyoid bone, 76. Hypermetropia, 500. Hypoglossal nerve, 171. Ideas, association of, 593. Idio-retinal light, 508. Ileum, 320. Ileo-colic valve, 323. Iliac artery, 210, 211. Ilium, 79. Illusions, sensory, 477. Impulse, cardiac, 220. Impulse, nervous, 196. Incus.. 537. Indigestion, 350, Inert layer, 228. Inferior laryngeal nerve, 397. Inferior maxillary nerve, 170. Inferior mesenteric artery, 211. Inferior vena cava, 207. Inhibition of reflexes, 581. Inhibitory nerves, 184. Innervation sensations, 565. Innominate artery, 210. Innominate bone, 79. Innominate vein, 214. Inogen, 125. Inorganic constituents of Body, 14. Inosit, 13. Inspiration, how effected, 359. Intensity of sensations, 473. Interarticular cartilage, 107. Intercostal arteries, 211. Intercostal muscles, 361. Internal ear, 538. Internal medium, 40. Internal respiration, 352, 386. Intervertebral disks, 71, 94. Intervertebral foramina, 71. Intestinal digestion, 340. Intestines, 320. Intra-thoracic pressure, 357. Intrinsic heart nerves, 248. Iris, 491. Irritability, 21. Irritability, muscular, 129. Irritable tissues, 31. Ischium, 79. Jaw-bones, 76. Jejunum, 320. Jelly-like connective tissue, 106. Joints, 94. , Jugular vein, 214. Kidneys, 4dfc=-HoH Kinetic energy, 282. Knee-cap or knee-pan, 79. Kreatin, 12, 434. Labyrinth, 528, Lachrymal apparatus, 482. Lachrymal bone, 76. 614 INDEX. Lacteal s, 43, 321. Lacunae, lymphatic, 329. Lamina spiralis, 540. Large intestine, 322. Laryngeal nerves, 397. Larynx, 596. Laughing, 402. Law, the psycho-physical, 473. Leaping, 153, Least-resistance hypothesis, 578. Lecithin, 14. Leg, skeleton of, 79. Lens, crystalline, 491. Lenses, refraction of light by, 495. Levers in the Body, 144. Lieberkilhn, crypts of, 322. Liebig's classification of foods, 426. Liebig's extract, 126. Ligament, suspensory, of lens, 491. Ligament, round, 95. Ligaments, 94. Light, dispersion of, 495. Light, properties of, 492. Light, refraction of, 493. Limbs, 7. Limbs, skeleton of, 77. Liquid extract of meat, 126. Liquor sanguinis, 44. Liver, 323. Local sign of sensations, 465. Local temperatures, 455. Localization of cerebral func tions, 591. Localizing powers of retina, 516. Localizing power of skin, 559. Locomotion, 151. Long saphenous vein, 214. Long sight, 500. Losses of material from Body, 277. Losses of energy daily, 452. Lower maxilla, 76. Lumbar plexus, 162. Lumbar vertebrae, 69. Lungs, 353. Lungs, capacity of, 365. Luxus consumption, 432. Lymph, 41 ; canaliculi, 106 ; chem- istry of, 61 ; hearts, 331 ; histo- logy of, 49; lacunae, 329; movement of, 331, 371 ; renewal of, 42; vessels, 43, 329. Lymphatic glands, 330. Lymphatic system, 329. Malar bone, 76. Malleus, 536. Malpighian corpuscles of spleen, 333. Malpighian layer of epidermis, 412. Malpighian pyramids of kidney, 406. Mammalia, 4. Mandible, 76. Marrow of bone, 88. Marrow, spinal (spinal cord), 177, 182, 574. Material daily losses of Body, 277. Maxilla, 76. Meatus, external auditory, 535. Mechanisms, physiological, 36. Medulla oblongata, 164, 585. Media, refracting, in eye, 490. Medullary cavity, 88. Membrane, basilar, 540, 533; of Descemet, 486; reticular, 541; tectorial, 542; tympanic, 536, Metabolic tissues, 31, 265. Metacarpus, 79. Metatarsus, 79. Microscopic anatomy. See His- tology. 1 Mid-brain, functions of, 586. Midriff. See Diaphragm. INDEX. 615 Milk, 302. Millon's test, 10. Mixed diet, advantage of, 305. Modality of sensation, 465, 468. Modiolus, 540. Morula, 26. Motion, 143. Motor organs, 113. Motor tissues, 33. Motores oculi, 168. Mouth, 308. Movements, associated, 593. Movements, respiratory, 358. Mucin, 12. Mucous membranes, 6. Mucous layer of epidermis, 412. Mulberry mass, 36. Mumps, 315. Muscae volitantes, 504. Muscle, biceps, 118; cardiac, 124; ciliary, 491; stapedius, 538; tensor tympani, 538. Muscles, chemistry of, 124; histo- logy of, 122; of eyeball, 483; of larynx, 599; of respiration, 359; physiology of, 128; skele- tal, 117; structure of, 116; visceral, 123. Muscular sense, 565. Muscular tissue, 33, 122. Muscular work, 134, 428. Myopia, 500. Myosin, 11, 125. Nails, 417. Nasal bone, 76. Negative variation, 193, 197. Nerve-cells, 175. Nerve-centres, 156, 177, 182, 573. Nerve-fibres, 33, 173. Nerve-fibres, classification of, 184. Nerve plexuses, 156. Nerve stimuli, 187. Nerves, 154; cranial, 168, 199; cardiac, 248; laryngeal, 397; respiratory, 390; secretory, 269, 271; spinal, 160; sympathetic, 156; thermic, 457; trophic, 273; vaso-dilator, 257, 271; vaso-motor, 253. Nervous impulses, 196. Nervous system, anatomy of, 154. Nervous system, physiology of, 180. Neural tube (dorsal cavity), 5. Neurilemma, 173. Nitrogenous compounds in Body, 10. Nodal points of eye, 506. Noises, 542. Non- vascular tissues, 41. Notes, musical, 542. Nucleolus, 17. Nucleus, 17. Nutrition, 423. Nutritive tissues, 30. Occipital bone, 75. Oculo-motor nerves, 168. Odorous bodies, 570. (Esophagus, 317. Olecranon, 82. Olein, 13. Olfactory lobe, 163. Olfactory nerves, 168. Olfactory organs, 569. Omentum, 317. Ophthalmic nerve, 170. Optical defects of eye, 500, 502. Optic nerves, 168. Optic thalami, 163. Organ of Corti, 541. Organs, 2, 34; of animal life, 114; of circulation, 201; of common sensation, 463; of digestion, 308; of movement, 113; of relation, 114; of respi ration, 352; of secretion, 260; of special sense, 463, 467; urin 616 INDEX. ary, 404; of vegetative life, 114. Os calcis, 79. Os innominatum, 79. Os orbiculare, 537. Os pubis, 79. Osmazome, 301. Ossicles, auditory, 536. Otoliths, 542. Oval foramen, 536. Over-feeding, proteid, 436. Over-tones (upper partial tones), 547. Ovum, 26. Oxidation by stages, 288. Oxidations in the Body, 286, 423, 427. Oxygen in the blood, 382. Oxygen consumed daily, 374. Oxyhsemoglobin, 379. Pacinian bodies, 556. Pain, 567. Palate, 309. Palate bones, 76. Pancreas, 267, 328. Pancreatic secretion, 341. Papillary muscles, 208, 221. Papillae of tongue, 313. Parapcptone, 345. Parietal bone, 75. Parotid gland, 274, 315. Partial tones, 547. Patella, 79. Patheticus, 169. Pathology, 1. Peas, 303. Pectoral arch, 77. Pelvic arch, 79. Pepsin, 338. Peptic glands, 319. Peptones, 11. Perceptions, 474; visual, 530: auditory, 554. Pericardium, 203. Perichondrium, 101. Perilymph, 538. Perimysium, 122. Perineurium, 173. Periosteum, 86. Peripheral reference of sensa- tions, 464, 476. Peristaltic movements, 338. Peritoneum, 6. Pettenkofer's test, 342. Phalanges of fingers and toes, 79, 80. Pharynx, 316. Phrenic nerve, 161, 390. Physiology, 1; of blood-vessels, 219; of brain, 584; of connec- tive tissues, 102; of digestion, 334; of ear, 550; of eye, 506; of heart, 219; of kidneys, 409; of muscles, 128; of nerves, 180; of nerve-centres, 180, 573; of nutrition, 423; of respiration, 358; of skin, 418, 558; of smell, 569; of spinal cord, 574; of taste, 570; of touch, 558. Physiological chemistry, 8. Physiological mechanisms, 36. Physiological properties, 16. Pia mater, 157. Pineal gland, 167. Pitch of notes, 543. Pitch of voice, 603. Pituitary body, 167. Pivot- joints, 97. Plastic foods, 426. Pleura, 6, 356. Plexus, 156; brachial, 161; car- diac, 172; cervical, 161 ; lum- bar, 162; sacral, 162; solar, 172. Pneumogastric nerves, 171, 396. Pons Yarolii, 164, 586. Popliteal artery, 211. Portal circulation, 215, 326. INDEX. 617 Portal vein, 324. Posterior tibial artery, 211. Postures, 149. Potatoes, 303. Potential energy, 282. Pressure, intra-thoracic, 357. Pressure sense, 558. Primates, 3. Production of heat in Body, 449. Pronation, 98. Proofs of cir .ulation, 205. Protective tissues, 34. Proteids, 10. Proteids, oxidation of, 427. Protoplasm, 24. Psychical activities of cord, 582. Psycho-physical law, 473. Ptosis, 485. Ptyalin, 335. Pulmonary circulation, 214. Pulmonary artery, 206. Pulmonary veins, 207. Pulse, 240. Purkinje's experiment, 509. Pus, 48. Pylorus, 317, 319. Pyramids of Malpighi, 406. Qualities of sensation, 465. Quantity of blood, 59. Quantity of food needed daily, 305, 448. Kadial artery, 210. Radio ulnar articulation, 97. Radius, 78. Range of voice, 603. Rate of blood flow, 242. Receptaculum chyli, 330. Receptive tissues, 30. Rectum, 322. Red blood corpuscles, 44, 59 Reduced haemoglobin, 379 Reflex actions, 182, 574. Reflex convulsions, 577. Reflexes, acquired, 587. Refracting media of eye, 490. Refraction by lenses, 495, Refraction of light, 493. Refraction in the eye, 497. Regulation of temperature, 454. Renal artery, 211. Renal organs, 402. Renal secretion, 409. Rennet, 338. Reproduction, 19. Reproductive tissues, 34. Residual air, 365. Resistance theory, 395. Resonance, sympathetic, 549. Respiration, 20, 352. Respiration, chemistry of, 379. Respiration, nerves of, 390. Respiratory centre, 391. Respiratory foods, 426. Respiratory movements, 357. Respiratory sounds, 365. Reticular membrane, 541. Retiform (adenoid) connective tissue, 106. Retina, 487. Rhythmic movements, 393. Ribs, 72. Rib cartilage, 72. Rice, 303. Right lymphatic duct, 330. Rigor mortis, 430. Rods and cones, 488, 568. Round foramen, 536. Running, 153. Sacculus, 539. Sacral plexus, 162 Sacral vertebrae, 69. Saliva, uses of, 334. Salivary glands, 315. Salivary glands, nerves of, 270. Salivin (ptyalin), 335. Santorini, cartilages of, 597 Sarcolactic acid, 14, 125. Sarcolemma, 123. 618 INDEX. Scala of cochlea, 540. Scalene muscles, 361. Scapula, 77. Sciatic nerve, 162. Sclerotic, 486. Sebaceous glands, 418. Secondary (acquired) reflexes, 587. Secretion, 259. Secretion, cutaneous, 417. Secretion, renal, 409. Secretory tissues, 30, 265. Secretory nerves, 271. Sections of Body, 6, 7. Segmentation of ovum, 26. Segmentation of skeleton, 67. Semicircular canals, 539, 587. Semilunar valves, 208. Sensations, 461; color, 519; com- mon, 463, 567; intensity of, 645; of equilibrium, 587; of hunger, 567; of thirst, 567; peripheral reference of, 464, 476; qualities of, 465. Sense-organs, 467. Sense, muscular, 565 ; of hearing, 535; of pain, 567; of sight, 506; of smell, 569; of taste, 570; of temperature, 562; of touch, 558. Sensory illusions, 477. Serous canaliculi, 329. Serous cavities, 330. Serous membranes, 6. Serum, 50, 57. Serum albumin, 11, 57. Shin-bone, 79. Shingles, 186. Short sight, 500. Shoulder-blade, 77. Shoulder-girdle, 77. Sighing, 402. Sight, sense of, 506. Sight, hygiene of, 501. Sigmoid flexure, 323. Size, perception of, 531. Skeleton, 62; appendicular, 7$ axial, 63, 67; of face, 72; of skull, 76, peculiarities of hu- man, 84; of thorax, 359. Skin, 6, 412; glands of, 417; hy- giene of, 420; nerve endinga in, 556. Skull, 72. Small intestine, 320. Smell, 567. Sneezing, 402. Solar plexus, 172. Solar spectrum, 495. Solidity, visual perception ofs 533. Sounds, 542. Sounds of the heart, 222. Sounds, respiratory, 365. Source of animal heat, 451. Source of fats, 441. Source of glycogen, 440. Source of muscular work, 427. Source of urea, 433. Sources of energy to Body, 283. Special senses, 463. Specific elements, 260. Specific nerve energies, 191. Spectacles, 501. Sphenoid bone, 75. Spherical aberration, 503. Spinal cord, 158; conduction in, 578, 583; functions of, 182, 574; histology of, 177; membranes of 157; psychical activities of, 582. Spinal accessory nerve, 171. Spinal marrow. See Spinal cord. Spinal nerves, 160. Spinal nerve-roots, 160, 197. Spleen, 332. Spontaneity, 23. Sporadic ganglia, 173. INDEX. 619 Sprains, 99. Squinting, 484. Stapedius muscle, 538. Stapes, 537. Starch, 303; digestion of, 335. Starvation, proteid, 436. Stationary air, 365. Stereoscopic vision, 533. Sternum, 71. Stimuli, muscular, 129; nervous, 187. Stimulus, 21. Stirrup-bone, 537. Stomach, 317. Stomata, lymphatic, 330. Storage tissues, 31, 437. Strabismus (squinting), 484. Structure of bone, 86. Strychnine poisoning, 577. Subclavian artery, 210. Subcutaneous areolar tissue, 414. Sublingual gland, 315. Submaxillary gland, 271, 315. Succus entericus, 344. Sudoriparous glands, 417. Superior lar}'ngeal nerve, 397. Superior maxillary nerve, 170. Superior mesenteric artery, 211. Supination, 98. Supplemental air, 365. Supporting tissues, 29. Supra-renal capsules, 333. Sutures, 93. Swallowing, 336. Sweat, 418. Sweat-glands, 417. Sweat-glands, nerves of, 270. Sweetbread, 333. Sympathetic nervous system, 5, 156, 172. Sympathetic resonance, 549. Sympathetic resonance in ear, 553. Sy no vial membranes, 95. Syntonin, 126. System, alimentary, 308; circula- tory, 201; muscular, 116, 128; nervous, 128, 154; osseous, 63; respiratory, 354; renal, 405. Systemic circulation, 215. Systems, anatomical, 37. Tactile organs, 556. Taking cold, 459. Tarsus, 79. Taste, 570. Taste-buds, 571. Tear-glands, 482. Tectorial membrane, 542. Teeth, 309. Teeth, structure of, 311. Temperature of Body, 450. Temperature, bodily, regulation of, 454. Temperature sense, 562. Temperatures, local, 455. Temporal artery, 210. Temporal bone, 75. Tension of blood gases, 382. Tendons, 118. Tensor tympani muscle, 538. Tests for proteids, 10. Tetanus, 132. Theory, resistance, 395. Theory of color vision, 519, 525. Thermic nerves, 457. Thigh-bone, 79. Thirst, 569. Thoracic duct, 329. Thorax, aspiration of, 367; con- tents of, 4; movements of in respiration, 359; skeleton of, 359. Throat, 316. Thyroid body, 333. Thyroid cartilage, 596. Thyroid foramen, 79. Thymus, 333. Tibia, 79. 620 INDEX. Timbre, 550. Tissues, 2, adenoid, 106; adipose, llljareolar, 102; assimilative, 30; automatic, 32; cartilagin- ous, 100; conductive, 33, 182; connective, 62, 102; contractile, 33, 128; co-ordinating, 32,573; elastic, 104; eliminative, 30; excretory, 30; irritable, 31; jelly-like connective,' 106 ; me- tabolic, 31, 265; motor, 33, 122; muscular, 33, 128; ner- vous, 173; nutritive, 30; pro- tective, 34; receptive, 30; re- productive, 34; respiratory, 30; retiform or adenoid, 106; se- cretory, 30, 265; storage, 31, 437 supporting, 29 ; undifferenti- ated, 29; white fibrous, 103. Tissue-forming foods, 425. Toast, 304. Tone, sensations of, 542. Tone color (timbre), 550. Tongue, 313. Tonsil, 316. Touch- organs, 556. Touch, sensations of, 558. Trachea, 354. Transudata, 259. Trigeminal nerve, 170. Trophic nerves, 273. Trypsin, 267, 341. Tunica adventitia, 217. Turbinate bones, 76. Tympanic bones, 536, 551. Tympanic membrane, 536, 550. Ulna, 78. XJlnar artery, 210. Undifferentiated tissues, 29. Upper maxilla, 76. Urea, 12, 409, 433. Ureter, 402. Uric acid, 13. Urinary organs, 402. Urine, 409. Utilization of energy in Body, 289. Utriculus, 539. Uvula, 309. Vagus nerve, 171, 396. Valve, ileocolic, 323. Valves, auriculo-ventricular, 208; of veins, 218; semilunar, 208. Valvulae conniventes, 320. Vaso-dilator nerves, 257, 271. Vaso-motor centre, 253. Vaso-motor nerves, 253. Vegetable foods, 302. Veins, 212, 218; cephalic, 214, coronary, 207; hepatic, 324; hollow, 207; innominate, 214, jugular, 214; long saphenous, 214; portal, 324; pulmonary, 207. Vena cava, 207. Venous blood, 216. Ventilation, 375; Ventral cavity, 4, 6. Ventricles of brain, 165; of larynx, 598.y Vermicular (peristaltic) move- ments, 338. Vermiform appendix, 322. Vertebrae, 63; cervical, 68; coccy- geal, 70; dorsal, 65; lumbar, 69; sacral, 69. Vertebral artery, 210. Vertebral column, 63, 70. Vertebral foramen, 68. Vertebrata, 3. Vestibule, 538. Vibrations, analysis of, 547; com- position of, 546; pendular, 544- sonorous, 542. Villi of intestine, 321. Vision, color, 519. Vision purple, 512. Vision, stereoscopic, 533. INDEX. 621 Visual axis, 518 Visual contrasts, 525. Visual perceptions, 530. Visual sensations, 506, 519; dura- tion of, 516; intensity of, 513. Vital capacity, 366. Vitreous humor, 491. Vocal cords, 597. Vocal cords, false, 598. Voice, 595. Vomer, 76. Vowels, 603. Walking, 151. Wandering cells, 106. Warm-blooded animals, 449. Water, constituent, 25. Water, percentage of, in Body, 14 Waxing kernels, 330. Weber's schema, 233. Weber's law, 473. Wheat, 302. Whipped blood, 51. White blood corpuscles, 17, 47, 59. White fibrous tissue, 103. Windpipe, 354. Wrisberg, cartilage of, 597. Wrist, 79. Xantho-proteic test, 10. Yawning, 402. Young's theory of color vision, 519. Zoological position of man, 2. Zymogen, 268. APPENDIX. KEPRODUCTION AND DEVELOPMENT. Reproduction in General. In all cases reproduction consists, essentially, in the separation of a portion of living^ matter from a parent; the separated part bearing with jt.T or inheritinfl, pertain tQr>^ encies torcpoat^ with T"mv? ^r It— jrrntifrrij t.hr lifn hint 1 1-3- nf i<- |n jrm>r In the more simple cases a/^arent merely divides into two or more pieces, each resembling itself except in size; these then grow and repeat the process; as, for instance, in the case of Amoaba (see Zoology), and our own white Wood corpuscles (p. 18). Such a process may be summed up in two words as discontinuous growth; the mass, instead of increasing in size without segmentation, divides as it grows, and so forms independent living beings. In some tolerably complex multicelrSar animals we find essentially the same thing; at times certain cells of the fresh- water Po- lype (see Zoology) multiply by simple division in the manner above described, but there is a certain concert between them: they build up a tube projecting from the side of the parent, a mouth-opening forms at the distal end of this, tentacles sprout out around it, and only when thus com- pletely built up and equipped is the young Hydra set loose on its own career. How closely such a mode of multiplica- tion is allied to mere growth is shown by other polypes in which the young, thus formed, remain permanently attached to the parent stem, so that a compound animal results. This mode of reproduction (known as gemmation or bud- ding) may be compared to the method in which many of the ancient Greek colonies were founded; carefully organ- ized and prepared at home, they were sent out with a due 2 THE HUMAN BODY. proportion of artificers of various kinds; so that the new commonwealth had from its first separation a consider- able division of employments in it, and was, on a small scale, a repetition of the parent community. In the great majority of animals, however, (even those which at times, multiply by budding,) a different mode of reproduction occurs, one more like that by which our western lands are now settled and gradually built up into Territories and States. The new individual in the political world begins with little differentiation; it consists of units, separated from older and highly organized societies, and these units at first do pretty much everything, each man for himself, with more- or less efficiency. As growth takes place development also- occurs; persons assume different duties and perform differ- ent work until, finally, a fully organized State is formed. Similarly, the body_^f oneof the higher, early sta^e oFjife, meirely^ collectionof undifferentiated cells, each capable of multiplication by (\\ vision, and rpfann- ing more or less alLits-ariginal protoplaomic--p£O4)erties: and with^no spfip.iflo in^iyHunil nndowmrnt nr function __The mass fOhnp, TTT.) then slowly rjiffprpn+iqi-pg info fliA. various tissues, each with a predominant character and duty; at the same time the majority of the cells lose their primitive powers of reproduction^) though exactly how com- pletely is a problem not yet sufficiently studied A In adult Yertebrates it seems certain that the white bloon corpuscles multiply by division: and in some cases (in the newts or tritons, for example,) a limb is reproduced after amputation, but exactly what cells take part in such restorative pro- cesses is uncertain; we do not know if the old bone corpus- cles left form new bones, old muscle-fibres new muscles, and so on; though it is probable that the undifferen- tiated tissues (which we have compared — p. 60 — to youths on the look-out for an opening in life) build up the new limb. Tn 3Tn~nmni1n nn such restoration occurs; an amputated leg may heal at the stump but does not grow again. In the healing processes the connective tissues play the main part, as we might expect; their cellular elements being but little modified from their primitive state (p. 105) can still multi- REPRODUCTION OF TISSUES. 3 ply and develop. New blood capillaries, however, sprout out from the sides of old, and new epidermis seems only to be formed by the multiplication of epidermic cells; hence the practice, recently adopted by surgeons, of transplanting little bits of skin to points on the surface of an extensive burn or ulcer. In both blood capillaries and epidermis the departure from the primary undifferentiated cell is but slight; and, as regards the cuticle, one of the permanent physiological characters of the cells of the rete mucosum is their multiplication throughout the whole of life; that is a main physiological characteristic of the tissue: the same is very probably true of the protoplasmic cells forming the walls of the capillaries. .Nerve-fibres are highly differenti- ated, yet nerves arc rapidly regenerated after division. The branch of the nerve-cell (axis cylinder) grows again; and amoeboid wandering cells make a new medullary sheath. In Mammals, muscular and glandular tissues seem never to be reproduced. We find, then, as we ascend in the animal scale a dimin- ishing reproductive power in the tissues generally: with the increasing division of physiological labor, with the changes that fit pre-eminently for one work, there is a loss of other faculties, and this one among them. The more specialized a tissue the less the reproductive power of its elements, and the most differentiated tissues are either not reproduced at all after injury, or only by the specialization of amoeboid cells, and not by a progenitive activity of survivors of the same kind as those destroyed. In none of the higher animals, therefore, do we find multiplication by simple division, or by budding: no one cell, and no group of cells used for the physiological maintenance of the individual, can build up a new complete living being; but the continuance of the race is specially provided for by setting apart certain cells which shall have this one property — cells whose duty is to the species and not to any one representative of it — an essentially altruistic element in the otherwise egoistic whole. Sexual Reproduction. Income cases, especially among insects, the specialized reproductive cells can develop, each for itself, under suitable conditions, and give rise to new 4 THE HUMAN BODY. individuals; such a mode of reproduction is called parthen- ogenesis: but in the majority of cases, and always in the higher animals, this is not so; the fusion of two cells, or of products of two cells, is a necessary preliminary to develop- ment. Commonly the coalescing cells differ considerably in size and form, and one takes a more direct share in the developmental processes; this is the egg-ceil or ovum; the other is the sperm-cell or spermatozoon. The fusion of the two is known as fertilization. Animals producing both ova and spermatozoa are hermaphrodite; those bearing ova only,, female; and those spermatozoa only, male: hermaphro- ditism is not found in Vertebrates, except in rare and doubtful cases of monstrosity. Accessory Reproductive Organs. The organ in which ova are produced is known as the ov&ry, that forming spermatozoa, as the test-is or testicle; but in different groups of animals many additional accessory parts may be devel- oped.] Thus, in Mammalia, the offspring is nourished for a considerable portion of its early life within the body of its mother, a special cavity, the uterus or womb, being provided for this purpose: the womb communicates with the exterior by a passage, the vagj.na; and two tubes, the oviducts or Fallopian tubes, convey the eggs to it from the ovaries. In addition, mammary glands provide milk for the nour- ishment of the young nTthe first months after birth. In the male mammal we find as accessory reproductive organs, vasa deferentia, which convey from the testes the seminal fluid containing spermatozoa; vesiculm seminales (not pres- ent in all Mammalia), glands whose secretion is mixed with that of the testes or is expelled after it in the sexual act; a prostate gland, whose secretion is added to the semen; and an erectile organ, the penis, by which the fer- tilizing liquid is conveyed into the vagina of the female. The Male Reproductive Organs. The testes in man are paired tubular glands, which lie in a pouch of skin called the scrotum. This pouch is subdivided internally by a par- tition into right and left chambers, in each of which a testicle lies. The chambers are lined inside by a serous membrane, the tunica vaginalis, and this doubles back (like the pleura STRUCTURE OF THE TESTIS. • — p. 356 — round the lung) and covers the exterior of the gland. Between the external and reflected layers of the tunica vaginalis is a space containing a small quantity of lymph. The testicles develop in the abdominal cavity, and only later (though commonly before birth) descend into the scrotum, passing through apertures in the muscles, etc., of the abdominal wall, and then sliding down over the front of the pubes, beneath the skin. The cavity of the tunica vaginalis at first is a mere offshoot of the peritoneal cavity, and its serous membrane is originally a part of the perito- neum. In the early years of life the passage along which the testis passes usually becomes nearly closed up, and the communication between the peritoneal cavity and that of the tunica vaginalis is also obliterated. Traces of this passage can, however, readily be observed in male infants; if the skin inside the thigh be tickled a muscle lying heneath the skin of the scrotum is made to contract reflexly, and the testis is jerked up some way towards the abdomen and quite out of the scrotum. Sometimes the passage remains permanently open and a coil of intestine may descend along it and enter the scrotum, constituting an inguinal hernia or rupture. A liydrocele is an excessive accumula- tion of liquid in the serous cavity of the tunica vaginalis. Beneath its covering of serous membrane each testis has a proper fibrous tunic of its own. This forms a thick mass on the posterior side of the gland, from which partitions or .septa (i, Fig. 160) radiate, subdivid- ing the gland into many chambers. In each chamber lie several greatly coiled seminiferous tubules, a, a, averaging in length 0.68 meters (27 inches) and in diameter only •0.14 mm. (-J-JTJ- inch). Their total number in each gland is FIG. 160.— Diagram of a vertical section through the testis. a, a, tubuli semini- f eri ; 6, yasa recta ; d, vasa efferentia ending in the coni vasculosi; e, e, epidi- dymis. /i,vas deferens. 6 THE HUMAN BODY. about 800. Near the posterior side of the testis the tubules unite to form about 20 v asa recta (b), and these pass out of the gland at its upper end, as the vasa efferentia (d), which become coiled up into conical masses, the coni vasculosi; these, when unrolled, are tubes from 15 to 20 cm. (6-8 inches) in length; they taper somewhat from their com- mencements at the vasa efferentia, where they are 0.5 mm. (-^Q inch) in diameter, to the other end where they termi- nate in the epididymis (e, e, Fig. 160). The latter is a nar- row mass, slightly longer than the testicle, which lies- along the posterior side of that organ, near the lower end of which (g) it passes into the vas deferens, h. If the epididymis be carefully unraveled it is found to consist of a tube about 6 meters (20 feet) in length, and varying in diameter from 0.35 to 0.25 mm. (-fa to -fa inch). The vas deferens (li, Fig. 160) commences at the lower part of the epididymis as a coiled tube, but it soon ceases to be convoluted and passes up beneath the skin covering the inner part of the groin, till it gets above the pelvis and then, passing through the abdominal walls, turns inwards, backwards, and downwards, to the under side of the urinary bladder, where it joins the duct of the seminal vesicle; it is about 0.6 meters (2 feet) in length and 2.5 mm. (fa inch) in diameter. The vesiculce seminales, two in number, are membranous receptacles which lie, one on each side, beneath the bladder, between it and the rectum. They are commonly about 5 cm. (2 inches) long and a little more than a centimeter wide (or about 0.5 inch) at their broadest part. The nar- rowed end of each enters the vas deferens on its own side, the tube formed by the union being the ejaculatory duct, which, after a course of about an inch, enters the urethra near the neck of the bladder. The prostate gland is a dense body, about the size of a chestnut, which surrounds the commencement of the ure- • thra; the ejaculatory ducts pass through it. It is largely made up of fibrous and unstriped muscular tissues, but contains also a number of small secreting saccules whose, ducts open into the urethra. MALE REPRODUCTIVE ORGANS. 7 The male urethra leads from the bladder to the end of the penis, where it terminates in an opening, the meatus urinarius. It is described by anatomists as made up of three portions, the prostatic, the membranous, and the spongy. The first is surrounded by the prostate gland and receives the ejaculatory ducts. On its posterior wall, close to the bladder, is an elevation containing erectile tissue (see below) and supposed to be dilated during sexual congress, so as to cut off the passage to the urinary receptacle. On this crest is an opening leading into a small recess, the utricle, which is of interest, since the study of Embryology shows it to be an undeveloped male uterus. The succeeding membranous portion of the urethra is about 1.8 cm. (f inch) long; the spongy portion lies in the penis. The penis is composed mainly of erectile tissue, i.e. tissues so arranged as to inclose cavities which can be dis- tended by blood. Covered outside by the skin, internally it is made up of three elongated cylindrical masses, two of which, the corpora cavernosa, lie on its anterior side; the third, the corpus spongiosum, surrounds the urethra and lies on the posterior side of the organ for most of its length; it, however, alone forms the terminal dilatation, OY glans, of the penis. Each corpus cavernosum is closely united to its fellow in the middle line and extends from the pubic bones, to which it is attached behind, to the glans penis in front. It is enveloped in a dense connective-tissue capsule from which numerous bars, containing white fibrous, elastic, and unstriped muscular tissues, radiate and intersect in all directions, dividing its interior into many irregular cham- bers called venous sinuses. Into these arteries convey blood, which is carried off by veins springing from them. The arteries of the penis are supplied with vaso-dilator nerves (p. 257), the nervi engentes, from the sacral plexus (p. 162). Under certain conditions these are stimulated and, the arteries expanding, blood is poured into the venous sinuses faster than the veins drain it off ; the latter are probably also at the same time compressed where they leave the penis by the contraction of certain muscles passing over them; the organ then becomes distended, rigid, and 8 THE HUMAN BODY. erect. The co-ordinating centre of erection lies in the lumbar region of the spinal cord, and may be excited reflexly by mechanical stimulation of the penis, or under the influence of nervous impulses originating in the brain and associated with sexual emotions. The corpus spongiosum resembles the corpora cavernosa in essential structure and function, j The skin of the penis is thin and forms a simple layer for some distance; towards the end of the organ it separates and forms a fold, the prepuce, which doubles back, and, becoming soft, moist, red, and very vascular, covers the glans to the meatus urinarius, where it becomes continu- ous with the mucous membrane of the urethra; in it, near the projecting posterior rim of the glans, are imbedded many sebaceous glands. Histology of the Testis and its Secretion. Each semi- niferous tubule consists of a basement membrane, sup- porting an epithelium, which in early life forms a definite lining to the tube; in the adult the cells multiply so as to nearly fill the cavity. From these cells the spermatozoa are formed. The seminal fluid is an albuminous liquid con- taining granules and spermatozoa; exam- ined with the microscope the latter are seen as rapidly moving minute objects (Fig. 161) each consisting of a broad flattened body and a slender vibratile tail or cilium; they are about 0.04 mm. (-y-J-y inch) in length. The seminal fluid is not fully formed when it leaves the seminiferous tubules; com- ' magni- pletely developed spermatozoa are first fied 350 diameters, found in the vasa recta, and even in the a, viewed from the side; 6, seen from contents of the vas deferens cells are often found which have not yet finished the pro- duction of a spermatozoon; probably, therefore, the fertil- izing liquid is first fully elaborated near the junction of the ras deferens with the duct of the seminal vesicle. The Reproductive Organs of the Female. Each ovary (0, Fig. 162) is a dense oval mass about 3.25 cm. (1.5 inches) in length, 2 cm. (0.75 inch) in width and 1.27 cm. (0.5 inch) in thickness; it weighs from 4 to 7 grams THE OVARY. 11 ^brane, and is covered externally by the peritoneum, bands of which project from each side of it as the broad ligaments (II, Fig. 162). Opening on the internal mucous membrane are the mouths of closely set, simple or slightly branched, tubular glands. The vagina is a distensible passage, extending from the uterus to the exterior; dorsally it rests on the rectum, and ventrally is in contact with the bladder and urethra. It is lined by mucous membrane, containing mucous glands, and •outside this is made up of areolar, erectile, and unstriped muscular tissues. Around its lower end is a ring of striated muscular tissue, the sphincter vagina. The vulva is a general term for all the portions of the female generative organs visible from the exterior. Over the front of the pelvis the skin is elevated by adipose tissue beneath it, and forms the mans Veneris. From this two folds of skin (I, Fig. 163), the labia majora, extend downwards :and backwards on each side of a median cleft, beyond which they again unite. On separating the labia majora a shallow genito-urinary sinus, into which the urethra and vagina open, is exposed. At the upper portion of this sinus lies the clitoris, a small and very sensitive erectile organ, resem- bling a miniature penis in structure, except that it has no corpus spongiosum and is not traversed by the urethra. J?rom the clitoris descend two folds of mucous membrane, the nympJicB or labia interna, between which is the vestibule, .a recess containing, above, the opening of the short female urethra, and, below, the aperture of the vagina, which is in the virgin more or less closed by a thin duplicature of mu- cous membrane, the hymen. Microscopic Structure of the Ovary. The main mass of the ovary consists of a dense connective-tissue stroma, containing unstriped muscle, blood-vessels, and nerves: it is covered externally by a peculiar germinal epithelium, and contains imbedded in it many minute cavities, the Graafian follicles, in which ova lie. If a thin section of an ovary be examined with the microscope many hundreds of small Graafian follicles, each about 0.25 mm. (y^g- inch) in diameter, will be found imbedded in it near the surface. 12 THE HUMAN BODY. These are lined by cells, and each contains a single ovum. Deeper in, larger follicles (7, 8, 9, Fig. 164) are seen, their cavities being distended, during life, by liquid: in these the essential structure may be more readily made out. Each has an external fibrous coat constituted by a more dense layer of the ovarian stroma; within this come several layers of lining cells (9, a, Fig. 164) constituting the membrana granulosa. At one point, b, the cells of this layer are heaped up, forming the discus prohgerus, which projects into the FIG. 164. — A section of a Mammalian ovary, considerably magnified. 1, outer capsule of ovary; 2, 3, 3', stroma; 4, blood-v'essels; 5. rudimentary Graafian fol- licles; 6, 7, 8, follicles beginning to enlarge and mature, and receding from th& surface; 9, a nearly ripe follicle which is again traveling towards the surface preparatory to discharging the ovum; a, membrana granulosa; b. discus pro- ligerus; c, ovum, with, d, germinal vesicle and. e. germinal spot. The general cavity of the follicle (in which 9 is printed) is filled with liquid during life. liquid filling the cavity of the follicle. In the discus proli- gerus the ovum, c, lies, having in it a nucleus or germinal vesicle, d, and a nucleolus or germinal spot, e. The ovum is about 0.2 mm. (T|0 inch) in diameter; its structure is better represented at A, Fig. 8*, where iHs seen to consist of a thick outer coat or cell-wall, a, called the vitelline membrane or zona pellucida; within which is the granular cell protoplasm, called here the vitellus or yelk; and in that again the germinal vesicle and spot. * Page 26. PUBERTY. 13 Puberty. The condition of the reproductive organs of each sex described above is that found in adults; although mapped out7^M;~tcr-tireerIain" extent, developed before birth and during childhood, these parts grow but slowly and remain functionally incapable during the early years of life^ then they comparatively rapidly increase in size and become physiologically active: the boy or girl becomes man or woman. / This period of attaining sexual maturity, known as pu- V' berty, takes place from the eleventh to the sixteenth year commonly, and is accompanied by changes in many parts of the Body\ Hair grows more abundantly on the pubes and genital organs, and in the armpits; in the male also on various parts of tire face. The lad's shoulders broaden; his larynx enlarges,) and lengthening of the vocal cords causes a fall in the pitch of his voice; all the reproductive organs, increase in size; fully formed seminal fluid is secreted, and erections of the p&cfis occm\) As these changes are completed spontaneous nocturnal seminal emissions take place from time to time during sleep, being usually associ- ated with voluptuous dreams. Many a young man is alarmed by these; he has been kept in ignorance of the whole mat- ter, is too bashful to speak of it, and getting some quack advertisement thrust into his hand in the street is alarmed to learn that his strength is being drained off, and that he is on the high-road to idiocy and impotence unless he place himself in the hands of the advertiser. Lads at this period of life should have been taught that such emissions, when not too frequent and not excited by any voluntary act of their own, are natural and healthy. They may, however, occur too often; if there is any reason to suspect this, the- family physician should be consulted, as the healthy activity of the sexual organs varies so much in individuals as to make it impossible to lay down numerical rules on the subject. The best preventives in any case are, however, not drugs, but an avoidance of too warm and soft a bed, plenty of muscular exercise, and keeping out of the way of anything likely to excite the sexual instincts. In the woman the pelvis enlarges considerably at puberty, 14 THE HUMAN BODY. and, commonly, more subcutaneous adipose tissue develops over the Body generally, but especially on the breasts and hips; consequently the contours become more rounded. The external generative organs increase in size, and the clitoris and nymphae become erectile. The uterus grows considerably, the ovaries enlarge, some Graafian follicles ripen, and the events treated of in the succeeding paragraphs occur. Ovulation. From puberty; during the whole child- bearing period of life, certain comparatively very large Graafian follicles may nearly always be found either close to the surface of the ovary or projecting on its exterior. These, by accumulation of liquid within them, have become distended to a diameter of about 4mm. (-J- inch); finally, the thinned wall of the follicle gives way and the ovum is dis- charged, surrounded by some cells of the discus proligerus. The emptied follicle becomes filled up with a reddish-yellow mass of cells, and constitutes the corpus luteum, which recedes again to the interior of the ovary and disappears in three or four weeks, unless pregnancy occurs; in that case the corpus luteum increases for a time, and persists during the greater part of the gestation period. Menstruation, ^y vulation occurs during the sexual life of a healthy woman "at intervals of about four weeks, and is attended with important changes in other portions of the generative apparatus. °\ The ovaries and Fallopian tubes become congested, and the fimbrise of the latter are erected .and come into contact with the ovary so as to receive any ova discharged. Whether the fimbriae embrace the ovary and catch the ovum, or merely touch it at various points and the ova are swept along them by their cilia to the cavity of the oviduct, is not certain. Having entered the Fallopian tube the egg slowly passes on to the uterus, probably moved by the cilia lining the oviduct; its descent probably takes about four or five days; if not fertilized, it dies and is passed out. In the womb important changes occur at or about the periods of ovulation ; its mucous membrane be- comes swollen and soft, and minute hemorrhages occur in its substance. Unless an impregnated ovum enters the cavity the superficial layers of the uterine mucous membrane are then MENSTRUATION. 15 broken down, and discharged along with more or less blood, constituting the menses, or monthly sickness, which com- monly lasts from three to five days. During this time the, vaginal secretion is also increased, and, mixed with the blood discharged, more or less alters its color and usually destroys its coagulating power. Except during pregnancy and while suckling, menstruation occurs at the above intervals, from puberty up to about the forty-fifth year; the periods then become irregular, and finally the discharges cease; this is an indication that ovulation has come to an end, and the sexual life of the woman is completed. This time, the climacteric or "turn of life," is a critical one; various local disorders are apt to supervene, and even mental derangement. Hygiene of Menstruation. During menstruation there is apt to be more or less general discomfort and nervous irri- tability; the woman is not qtiite herself, and those respon- sible for her happiness ought to watch and tend her with special solicitude, forbearance, and tenderness, .and protect her from anxiety and agitation. Any strong emotion, especially of a disagreeable character, is apt to check the flow, and this is always liable to be followed by serious con- sequences. A sudden chill often has the same effect; hence a menstruating woman ought always to be warmly clad, and take more than usual care to avoid draughts or getting wet. At these periods, also, the uterus is enlarged and heavy, and bein^ (as may be seen in Fig. 163) but slightly supported, and that near its lower end, it is especially apt to be dis- placed or distorted; it may tilt forwards or sideways (ver- sions of the uterus), or be bent where the neck and body of the organ meet (flexion). Hence violent exercise at this time should be avoided, though there is no reason why a properly clad woman should not take her usual daily walk. Painful menstruation (dysmenorrhwa) may be due to very many causes, but it is only within recent years that physi- cians have come to recognize how often it depends on uterine displacements, and in such cases how readily it may usually be remedied by restoring the organ to its proper position, and supporting it there if necessary. A flexion of the organ 16 THE HUMAN BOD 1. closes the passage of the cervix and is especially apt to canso pain at the menstrual period. The accumulated blood distends the uterus, which makes violent contractions to expel it; finally, if the resistance is overcome, the blood has probably already clotted and its expulsion causes more suf- fering. All this might usually be soon remedied, but the sufferer, ignorant of what is wrong, often goes on month after month until her health is undermined, hoping that the trouble will get better of itself, which it never will. To submit to the necessary examination and treatment from one of the other sex is, however, to a refined woman apt to be a more severe trial than all the physical pain; and there is no recent social movement more deserving of every en- couragement and support than that whose aim is to provide properly trained female medical attendance for women in the diseases peculiar to their sex; such as may now, fortu- nately, be found in most of our large cities. Few except physicians, and perhaps few physicians, know what an amount of relievable pain women endure in silence rather than run the risk of being forced to consult a male doc- tor. If no skilled person of her own sex is at hand the sufferer, if she do anything, is only too apt to take some of the nostrums advertised in such numbers for (S female com- plaints," or to consult a half -educated female quack of some novel "school" with a taking title. The result of doing this, or doing nothing, is often permanent valetudinarianism and a life of uselessness, to those who might be active and happy wives and mothers. The absence of the menstrual flow (amenorrhcea) is nor- mal during pregnancy and while suckling; and in some rare cases it never occurs throughout life, even in healthy women capable of child-bearing. Usually, however, the non- appearance of the menses at the proper periods is a serious symptom, and one which calls for prompt measures. In all such cases it cannot be too strongly impressed upon women that the most dangerous thing to do is to take drugs tending to induce the discharge, except under skilled advice; to excite the flow, in many cases, as for example occlusion of the os uteri, or in general debility (when its absence is CONCEPTION. 17 & conservative effort of the system), may have the most dis- astrous results. Impregnation. As the ovum descends the Fallopian tube the changes accompanying or preceding menstruation are taking place in the uterus. Its mucous membrane and all the generative organs of the woman are more or less con- gested, and her sexual emotions are commonly more easily aroused. Unless the act of coition occur this passes off, new mucous membrane is formed, and the organs return to ;a quiet state until the period of the next ovulation. If .sexual congress takes place the vagina, uterus, and oviducts are thrown into reflex peristaltic contractions; and there is frequently an increased secretion by the vaginal mucous membrane. Some of the seminal fluid is received into the uterine cavity, and there, or more frequently in the Fal- lopian tube, meets the ovum. The spermatozoa are carried &long partly, perhaps, by the contractions of the muscular walls of the female cavities, but mainly by their own ac- tivity; from observations made on various lower animals it •appears that their movements cease immediately on coming into contact with the ovum, and that one only takes part in fertilization. How this latter occurs in the mammalian ovum is not certain; observations in other groups make it probable that the male element directly fuses in 'whole or part with the protoplasmic mass of the ovum, but no .special opening has been detected in the ztna pellucida of the'' mammalian ovum, which however is traversed by numerous minute channels; some physiologists are inclined to sup- pose that material is merely passed by dialysis from the spermatozoon into the egg-cell. The fertilized ovum continues its descent to the uterine cavity, but, instead of lying dormant like the unfertilized, segments (p. 26), and forms a morula. This, entering the womb, becomes imbedded in the new, soft, vascular mucous membrane forming there, from which 'it imbibes nourish- ment, and which, instead of being c^st off in subsequent menstrual discharges, is retained and grows during the whole of pregnancy, having important duties to discharge in connection with the nutrition of the embryo. 18 THE HUMAN BODY. . Jexual congress is most apt to be followed by pregnancy if it occur immediately before or after a menstrual period; at those times a ripe ovum will likely be in the Fallopian tube, near the upper end of which it is probably fertilized in the majority of casesA There is some difference of opinion as to whether the rupture of the Graafian follicle occurs most frequently immediately before the appearance of the menstrual flow, or towards its close; but the pre- ponderance of evidence favors the latter view; if this be so coition will occur under the most favorable conditions for conception if it take place on the day following the cessation of a menstrual period. There is, however, evidence that ova are occasionally discharged at other than the regular monthly periods of ovulation. There is, therefore, no time during a woman's life from puberty to the climacteric, except pregnancy or lactation, when sexual congress may not result in impregnation The exact parts taken by the male and female elements respectively in the formation of the embryo are not known with certainty; seeing that the ovum is much larger than the spermatozoon it is not uncommon to speak of the latter as a mere stimulant or excitant, arousing the egg-cell to developmental activity; but the definite characteristics often inherited by children from the father show that the sperma- tozoon is much more than that; materials derived from it are no doubt an essential constituent of the compound mass which develops into the new human being. Pregnancy. When the mulberry mass reaches the uterine cavity the mucous membrane lining the latter grows rapidly and forms a new, thick, very vascular lining to the womb, known as the decidua. At one point on this the morula becomes attached, the decidua growing up around it. As pregnancy advances and the embryo grows, it bulges out into the uterine cavity and pushes before' it that part of the decidua which has grown over it (the decidua reflexa); at about the end of the third month this meets the decidua lining the opposite sides of the uterine cavity and the two grow together. That part of the decidua (decidua serotina) Against which the morula is first attached subsequently un- GESTATION. 19 dergoes a great development in connection with, the forma- tion of the placenta (see below) . Meanwhile the whole uterus enlarges; its muscular coat especially thickens. At first the organ still lies within the pelvis, where there is but little room for it; it accordingly presses on the bladder and rectum (see Fig. 163)* and the nerves in the neighborhood, frequently causing considerable discomfort or pain; and, reflexly, often exciting nausea or vomiting (the morning sickness of pregnancy). Later on, the pregnant womb escapes higher into the abdominal cavity, and although then larger, the soft abdominal walls more readily make room for it, and less discomfort is usually felt, though there may be shortness of breath and palpitation of the heart from inter- ference with the diaphragmatic movements. All tight garments should at this time be especially avoided; the woman's breathing is already sufficiently impeded, and the pressure may also injure the developing child. Meanwhile, changes occur elsewhere in the Body. The breasts enlarge and hard masses of developing glandular tissue can be felt in them; and there may be mental symptoms: depression, anxiety, and an emotional nervous state. During the whole period of gestation the woman is not merely supplying from her blood nutriment for the foatus, but also, through her lungs and kidneys, getting rid of its wastes; the result is a strain on her whole system which, it is true, she is constructed to bear and will carry well if in good health, but which is severely felt if she be feeble or suffering from disease. Many a wife who might have led a long and happy life is made an invalid or brought to pre- mature death, through being kept in a chronic state of pregnancy. There is a general agreement that sexual con- tinence is possible and a duty in unmarried men, but the husband rarely considers that he should put any bounds on himself beyond those indicated by his own passions; consid- eration for his wife's health rarely enters his head in this connection. The healthy married woman who endeavors to evade motherhood because she thinks she will thus pre- serve her personal appearance, or because she dislikes the trouble of a family, deserves but little sympathy; she is * P. 10 of Appendix. SO THE HUMAN BODY. trying to escape a duty voluntarily undertaken, and owed to her husband, her country, and her race; but she whose strength is undermined and whose life is made one long discomfort for the sexual gratification of her husband de- serves all aid, and it is wrong to keep silent on the subject. The professor of gynaecology in a leading medical school, gives it as his deliberate opinion that the majority of American women must at some periods of their lives choose between freedom from pregnancy or early death. He further says that he does not believe that healthy men are so organized that this matter can be regulated by them; but that the question depends on the tact and prudence of the wife. Men, however, as a rule, are not utterly selfish; they commonly err through ignorance or thoughtlessness, not knowing or realizing what a strain frequent pregnancy is on the strength of a delicate woman. A social custom so deep-rooted and ancient as the usual American and English one of married couples constantly occupying the same bed is not easily changed, yet it probably leads to much harm, and especially in this country. Whatever the reason be, there is no doubt that the physical stamina of the average English woman is considerably greater than that of her American cousin, and she bears and brings up large families with greater safety. For a husband, who has reason to believe that child-bearing will injure his wife's health, to always share her couch is a deliberate walking into temptation. Apart from pregnancy, moreover, a woman's health is often injured by frequent sexual intercourse. A physician who has unusual opportunities of knowing states that he has reason to believe that not only is the act of sexual con- gress at best, from a physical point of view, a mere nuisance to the majority of women belonging to the more luxurious classes of society after they attain the age of twenty-two or twenty-three, but that a very considerable proportion suf- fer acute pain from it such as, if frequent, breaks down the general health. A loving woman, finding her highest happi- ness in suffering for those dear to her, is very unlikely to let her husband know this, so long as she can bear it; but if the possibility is known it will not, perhaps, need much NUTRITION OF THE EMBRYO. 21 acuteness in him to discover such suffering when it exists, nor very much real affection to control himself accordingly. In the class of cases referred to, rest of the over-irritable and congested female organs is above all essential. The cause is frequently removable by simple, but skilled, treat- ment; the desirability of rendering this available to a woman in members of her own sex has already been insisted upon. Even when no pain is caused harm may be done: the presi- dent of the Gynaecological Society, in an address delivered before that body, lately stated that if either party to a coi- tion fails of the orgasm damage is apt to ensue, but espe- cially to the woman if she fail; the organs are congested from the stimulus of the sexual act and the normal final orgasm is required for their healthy relief and return to the resting condition. If this be so, it is clear that coition should be restricted to times when the woman's general state encour- .ages the orgasm, and unless she generally experiences it sexual congress should be avoided until her health is restored. The Foetal Appendages. In the earliest stages of life, those occurring in the davs imme- J FIG. 165.— The embryonal vesicle. diatelv after fertilization of the a> thinned and distended zona pel- lucida; this soon after disappears OVlim, there IS little Or no altogether; &, the blastoderm; c, / n w mi tne embryonal disk. \J_growth. # The ovum segments * into a number of cells, but the morula thus formed is little larger than the original egg-cell itself. At first it is a solid mass (F, Fig. 8, p. 26), but its cells soon recede from the centre and become arranged (Fig. 165) around a central cavity containing mainly some absorbed liquid; at one point, c (the embryonal dis&), the layer thickens, and from thence the thickening spreads, by cell growth and multiplication, over the whole sac, which is known as the embryonal vesicle; the membrane thus formed is the blastoderm, and 22 THE HUMAN BODY. consists of three cell layers; an outer or epiUast, a middle or mesoblast, and an inner or hypoblast. From this simple sac, presenting no resemblance not merely to a human being but to a vertebrate animal, the foetus is built up by cell division and modification. \ The general history of intra- uterine development" IsKQiucJi too long and too complex to enter upon here, but the formation of certain structures lost at or before birth, and associated with the protection and nourishment of the embryo, may be attempted: they are the yelk sac, the amnion, and the allantois. The developing blastoderm especially thickens in the neighborhood of the embryonal disk and there the outlines of the Body are first laid down. Along the thickening a groove appears, which FIG. 166.— A, an early blastoderm with the first traces of the primitive groovy B, the same a little later ; /, primitive groove ; d, thickened region of the blas- toderm which directly builds up the embryo. marks out the future longitudinal axis and dorsal side of the body, (A, Fig. 166). This groove elongates, its edges rise (B), and finally arch over, meet, and fuse together above it. The tube thus closed in is the rudiment of the cerebro-spinal axis; from its lining epiblastic cells the brain and spinal cord are developed, and its cavity remains throughout life as the central canal of the spinal cord and as the cerebral ventricles, except the fifth (p. 1 65). Some distance on each side of this dorsal tube the mesoblast splits into an outer leaf, adherent to the epiblast, and an inner adherent to the hypoblast; the conjoined meso-epiblastic layer is the somatopleure; the meso-hypoblastic the splanchnopleure. The proximal parts of the somatopleure (i.e., the regions nearest the central THE FCETAL APPENDAGES. 23 axis) develop into the walls of chest and abdomen; farther out it turns up and arches over the back of the embryo, and its edges, there meeting, grow together and form a bag, the am-nion, enveloping the foetus. Into this a considerable quantity of liquid is secreted, in which the foetus floats. At birth the contractions of the uterus, pressing on the amnion, drive part of it down like a wedge into the neck of the uterus, and through its liquid contents an equable pressure is exerted there, until the os uteri is tolerably dilated; the sac then normally ruptures and the "waters" escape. Some- times, however, an infant is born still enveloped in the amnion, which is then popularly known as a caul. While the amnion is developing, a semi-cartilaginous rod forms along the axis of the Body beneath the floor of the dorsal tube; this is the notochord; when it appears the young being is marked out distinctly as a vertebrate animal, having a dor- sal neural tube above an axial skeleton, and a ventral licemal tube, (p. 4), formed by the proximal regions of the somato- pleure, beneath it. The ventral tube, however, is still widely open, the points where the amniotic folds turn back being far from meeting in the future middle line of the chest and abdomen. The proximal portions of the splanchnopleure incurve to inclose the alimentary tube, which is at first straight and simple. Beyond the point where it bends in for this purpose the splanchnopleure again diverges, and incloses a small globular bag, the yelk sac, which is, thus, attached to the ventral side of the alimentary canal; it at first projects through the opening where the amniotic folds turn back, but has little importance in the mammalian embryo and is soon absorbed. The allantois is primarily an outgrowth from the ali- mentary canal, containing blood-vessels. It passes out from the Body on the ventral side where the somatopleures have not yet met, and reaching the inside of the uterus, its distal end expands there to make the main part of the placenta (see below). Its narrow proximal portion forms the umbilical cord, around which the somatopleures, incurving to in- close the belly, meet at the navel some time before birth. 24 THE HUMAN BODY. The Intra-Uterine Nutrition of the Embryo. At first the embryo is nourished by absorption of materials from the soft vascular lining of the womb; as it increases in size- this is not sufficient, and a new organ, the placenta, is formed for the purpose. The allantois plants- itself firmly against the decidua serotina (p. 18), and villi developed on it burrow from its surface into the uterine mucous membrane. In the deeper layer of this latter are large sinuses through which the maternal blood flows. Into the allantoic villi total blood-vessels run and form capil- lary networks; the blood flowing through these receives, by dialysis, oxygen and food materials from the maternal 'blood, and gives up to it carbon dioxide, urea, and other wastes. There is thus no direct intermixture of the two- bloods; the embryo is from the first an essentially separate and independent organism. The allantois and decidua. serotina becoming closely united together form the placenta, which in the human species is, when fully developed, a- round thick mass about the size of a large saucer. Parturition. -; At the end of from 275 to 280 days from fertilization of tire ovum (conception] pregnancy terminates, and the child is expelled by powerful contractions of the- uterus^ assisted by those of the muscles in the abdominal walls. ) When the child is born, it has attached to its navel the-~lmibilical cord, through which arteries run to, and veins from, the placenta. Shortly afterwards the latter is detached and expelled (both its allantoic and decidual parts), and where it is torn loose from the uterine wall large blood sinuses are left open and exposed. Hence a certain amount of bleeding occurs, but in normal labor this is speedily checked by firm contraction of the uterus. Should this fail to take place profuse haemorrhage occurs (flooding] and the mother may bleed to death in a few min- utes unless prompt measures are adopted. For a few days after delivery there is some discharge (the- lochia) from the uterine cayity: the whole decidua being broken down and carried off, to be subsequently replaced by new mucous membrane. The muscular fibres developed in the uterine wall in such large quantities during pregnancy, PARTURITION. 25 undergo rapid fatty degeneration and are aosorbed, and in a few weeks the organ returns almost to its original size. The parturient woman is especially apt to take infec- tious diseases; and these, should they attack her, are fatal in a very large percentage of cases. Very special care should therefore be taken to keep all contagion from her. There is a current impression that a pregnancy, once commenced, can be brought to a premature end, especially in its early stages, without any serious risk to the woman. It ought to be widely made known that such a belief is erroneous. Premature delivery, early or late in pregnancy, is always more dangerous than natural labor at the proper term; the physician has sometimes to induce it, as when a formed pelvis makes normal parturition impossible, or the general derangement of health accompanying the preg- nancy is such as to threaten the mother's life; but the occa- sional necessity of deciding whether it is his duty to pro- cure an abortion is one of the most serious responsibilities he meets with in the course of his professional work. Dr. Storer, an eminent gynaecologist, states emphatically, from extended observation, that " despite apparent and isolated instances to the contrary — 1. A larger proportion of women die during or in con- sequence of an abortion, than during or in consequence of child-bed at the full term of pregnancy. 3. A very much larger number of women become con- firmed invalids, perhaps for life; and — 3. The tendency to serious and often fatal organic disease, as cancer, is rendered very much greater at the so-called "turn of life," by previous artificially induced premature delivery. Daring pregnancy there is a close connection between the placenta and uterus;- nature makes preparation for the safe dissolutior. of this at the end of the normal period, but "its premature rupture is usually attended by pro- fuse haemorrhage, often fatal, often persistent to a greater or less degree for many months after the act is completed, and always attended with more or less shock to the maternal system, even though the full effect of this is not noted for 26 THE HUMAN BODY. years." The same authority states again : " Any deviation from this process at the full term" (i.e., the process, asso- ciated with lactation, by which the uterus is restored to its small non-gravid dimensions) "lays the foundation of, and causes, a wide range of uterine accidents arid disease, dis- placements of various kinds; falling of the womb downwards or forwards or backwards, with the long list of neuralgic pains in the back, groin, thighs and elsewhere that they occasion; constant and inordinate leucorrhcea; sympathetic attacks of ovarian irritation, running even into dropsy," etc. etc. There is, thus, abundant reason for bearing most things rather than the risks of an avoidable abortion; among these is one not mentioned above, but more terrible than all, insanity. Lactation. The mammary glands for several years aftei birth remain small, and alike in both sexes. Towards puberty they begin to enlarge in the female, and when fully developed form in that sex two rounded eminences, tho breasts, placed on the thorax. A little below the centre of each projects a small eminence, the nipple, and the skin around this forms a colored circle, the areola. In virgin a the areolge are pink; they darken in tint and enlarge during the first pregnancy and never quite regain their original hue. The mammary glands are constructed on the com pound racemose type (p. 262). Each consists of from fifteen to twenty distinct lobes, made up of smaller divisions $ from each main lobe a separate galactophorous duct, made by the union of smaller branches from the lobules, runs towards the nipple, all converging beneath the areola. There each dilates and forms a small elongated reservoir in which the milk may temporarily collect. Beyond this the ducts narrow again, and each continues to a separate opening on the nipple. Imbedding and enveloping the lobes of the gland is a quantity of firm adipose tissue which gives the whole breast its rounded form. During maidenhood the glandular tissue remains imper- fectly developed and dormant. Early in pregnancy it begins to increase in bulk, and the gland lobes can be felt as hard masses through the super jacent skin and fat. Even at par- turition, however, their functional activity is not fully LACTATION. 27 established. The oil-globules of the milk are formed by a sort of fatty degeneration of the gland-cells, which finally fall to pieces; the cream is thus set free in the watery and albuminous secretion formed simultaneously, while newly developed gland-cells take the place of those destroyed. In the milk first secreted after accouchment (the colostrum) the cell destruction is incomplete, and many cells still float in the liquid, which has a yellowish color; this first milk acts as a purgative on the infant, and probably thus serves a useful purpose, as a certain amount of substances (biliary and other), excreted by its organs during development, are found in the intestines at birth. Human milk is undoubtedly the best food for an infant in the early months of life; and to suckle her child is useful to the mother if she be a healthy woman. There is reason to believe that the processes of involution by which the large mass of muscular and other tissues developed in the uterine walls during pregnancy are broken down and absorbed, take place more safely to health if theTiatural milk secretion is encouraged. Many women refuse to suckle their children from a belief that so doing will in jure their personal appear- ance, but skilled medical opinion is to the contrary effect; the natural course of events is the best for this purpose, unless lactation be too prolonged. Of course in many cases there are justifiable grounds for a mother's not undertaking this part of her duties; a physician is the pioper person to decide. In a healthy woman, not suckling her child, ovulation and menstruation recommence about six weeks after childbirth; a nursing mother usually does not menstruate for ten or twelve months; the infant should then be weaned. When an infant cannot be suckled by its mother or a wet- nurse an important matter is to decide what is the best food to substitute. G-ood cow's milk contains rather more fats than that of a woman, and much more casein; the following table gives averages in 1000 parts of milk: Woman. Cow. Casein 28.0 54.0 Butter 33.5 43.0 Milk sugar 44.5 42.5 Inorganic matters 4.75 7.75 28 THE HUMAN BODY. The inorganic matters of human milk yield, on analysis, in 100 parts — calcium carbonate 6. 9; calcium phosphate 70. 6; sodium chloride 9.8; sodium sulphate 7.4; other salts 5.3. The lime salts are of especial importance to the child, which has still to build up nearly all its bony skeleton. When undiluted cow's milk is given to infants they rarely bear it well; the too abundant casein is vomited in loose coagula. The milk should therefore be diluted with half or, for very young children, even two thirds its bulk of water.. This, however, brings down the percentage of sugar and. butter below the proper amount. The sugar is commonly replaced by adding cane sugar; but sugar of milk is readily- obtainable and is better for the purpose. If used at all it should, however, be employed from the first; it sweetens much less than cane sugar, and infants used to the latter refuse milk in which milk sugar is substituted. Cream from cow's milk may be added to raise the percentage of fats to the normal, but must be perfectly fresh and only added to the milk immediately before it is given to the child. While milk is standing for the cream to rise it is very apt to turn a little sour; the amount of this sour milk carried off with the cream is itself no harm when mixed with a large bulk of fresh milk; it carries with it, however, some of the fungus whose development causes the souring, and this will rapidly de- velop and sour all the milk it is added to if the mixture be let stand. As the infant grows older less diluted cow's milk may gradually be given; after the seventh or eighth month, no addition of water is necessary. In the first weeks after birth it is no use to give an infant starchy foods, as arrowroot. The greater part of the starch passes through the bowels unchanged; apparently because the pancreas has not yet fully developed, and has not commenced to make its starch-converting ferment (p. 341). Later on, starchy substances may be added to the diet with advantage, but it should be borne in mind that they cannot form the chief part of the child's food; it needs proteids for the formation of its tissues, and amyloid foods contain none of these. Many infants are, ignorantly. half starved by being fed almost entirely on such things as corn-flour or arrowroot. STAGES OF LIFE. The Stages of Life. Starting from the ovum each human being, apart from accident or disease, runs through a life- cycle which terminates on the average after a course of from 75 to 80 years. The earliest years are marked not. only by rapid growth but by differentiating growth or de- velopment; then comes a more stationary period, and finally one of degeneration. The life of various tissues and of many organs is not, however, coextensive with that of the individual. During life all the formed elements of the Body are constantly being broken down and removed; either molecularly (i.e., bit by bit while the general size and form of the cell or fibre remains unaltered), or in mass, as when hairs and the cuticle are shed. The life of many organs, also, does not extend from birth to death, at least in a functionally active state. At the former period numerous bones are represented mainly by cartilage. The pancreas has not attained its full development; and some of the sense-organs seem to be in the same case; at least new-born infants appear to hear very imperfectly. Tha reproductive organs only attain full development at pu- berty, and degenerate and lose all or much of their func- tional importance as years accumulate. Certain organs have even a still shorter range of physiological life; the thymus, for example (p. 333), attains its fullest develop- ment at the end of the second year and then gradually dwindles away, so that in the adult scarcely a trace of it is to be found. The milk-teeth are shed in childhood, and their so-called permanent successors rarely last to ripe old age. During early life the Body increases in mass, at first very rapidly, and then more slowly, till the full size is attained, except that girls make a sudden advance in this respect at puberty. Henceforth the woman's weight (excluding cases of accumulation of non-working adipose tissue) remains about the same until the climacteric. After that there is often an increase of weight for several years; a man's weight usually slowly increases until forty. As old age comes on a general decline sets in, the rib cartilages become calcified, and lime salts are laid down. 30 THE HUMAN BODY. in the arterial walls, which thus lose their elasticity; the refracting media of the eye become more or less opaque; the physiological irritability of the sense-organs in general diminishes; and fatty degeneration, diminishing their work- ing power, occurs in many tissues. In the brain we find signs of less plasticity; the youth in whom few lines of least resistance have been firmly established is ready to accept novelties and form new associations of ideas; but the longer he lives, the more difficult does this become to him. A man past middle life may do good, or even his best work, but almost invariably in some line of thought which he has already accepted: it is extremely rare for an old man to take up a new study or change his views, philosophical, scientific, or other. Hence, as we live, we all tend to lag behind the rising generation. Death. After the prime of life the tissues dwindle (or at least the most important ones) as they increased in child- hood; it is conceivable that, without death, this process might occur until the Body was reduced to its original microscopic dimensions. Before any great diminution takes place, however, a breakdown occurs somewhere, the enfeebled community of organs and tissues forming the man is unable to meet the contingencies of life, and death supervenes. "It is as natural to die as to be born," Bacon wrote long since; but though we all know it, few realize the fact until the sum- mons comes. To the popular imagination the prospect of dying is often associated with thoughts of extreme suffering; personifying life, men picture a forcible and agonizing rending of it, as an entity, from the bodily frame with which it is associated. As a matter of fact, death is prob- ably rarely associated with any immediate suffering. The sensibilities are gradually dulled as the end approaches; the nervous tissues, with the rest, lose their functional capacity, and, before the heart ceases to beat, the individual has commonly lost consciousness. The actual moment of death is hard to define: that of the Body generally, of the mass as a whole, may be taken to be the moment when the heart makes its last beat; arterial DEATH. 31 pressure then falls irretrievably, the capillary circulation ceases, and the tissues, no longer nourished from the blood, gradually die, not all at one instant, but one after another, according as their individual respiratory or other needs are great or little. While death is the natural end of life, it is not its aim — we should not live to die, but live prepared to die. Life has its duties and its legitimate pleasures, and we better play our part by attending to the fulfilment of the one and the enjoyment of the other, than by concentrating a morbid and paralyzing attention on the inevitable, with the too frequent result of producing indifference to the work which lies at hand for each. Our organs and faculties are not talents which we may justifiably leave unemployed; each is bound to do his best with them, and so to live that he may most utilize them. An active, vigorous, dutiful, un- selfish life is a good preparation for death; when that time, at which we must pass from the realm controlled by physi- ological laws, approaches, when the hands tremble and the eyes grow dim, when "the grasshopper shall be a burden and desire - shall fail," then, surely, the consciousness of having " quitted us like men" in the employment of our faculties while they were ours to use, will be no mean conso- latior INDEX TO APPENDIX. Abortion, 25. Allantois, 23. Amenorrhoea, 16. Amnion, 23. Blastoderm, 21. Breasts, 26. Budding, 1. €aul, 23. Cervix uteri, 10. Childbirth, 24. Climacteric, 15. Clitoris, 11. Coni vasculosi, b. Corpora cavernosa, 7. Corpus luteum, 14, Corpus spongiosum, 7. Death, 30. Decidua, 18. Development of embryo, 21. Discontinuous growth, 1. Discus proligerus, 12. Displacement of uterus, 15. Dysmenorrhcea, 15. Egg-cell, 4. Embryonal disk, 21. Embryonal vesicle, 21, Epiblast, 22. Epididymis, 6. Erectile tissues, 7. Fallopian tube, 4, 9. Feeding of infants, 27. Fertilization, 4, 17. Foetal appendages, 21.