. Digitized by the Internet Archive es ~ in 2007 with funding from Tass Microsoft Corporation a We a) : hive.org/details/essentialsofphys00patouott a teed ESSENTIALS OF PHYSIOLOGY FOR VETERINARY STUDENTS ine VTLCHET i ia ay | ah a ie J ; , ; ‘ - - > : 3 Ce tr — i. 4 ; a P 5 s | to . ; ‘ - ’ : dy 7 mi " . w . . | “7h | ‘ : . | : ‘ ° , , } ‘ * , A 4 ; ; I! ‘ ‘ 2 . ’ . ? . (‘bas ga get ‘d 90g) (‘AVAAAVHD) ‘estoFT ONY Jo wazshg snoasoN oT, ‘9s ‘AONAUANG 7. itm CG MPhy . ESSENTIALS |''@/) OF meee DO LOL OGY FOR VETERINARY STUDENTS r BY vi ve... DY NOEL PATON, M.D., B.Sc., F.R.C.P. Ep. PROFESSOR OF PHYSIOLUGY, UNIVERSITY OF GLASGOW SECOND EDITION REVISED AND ENLARGED EDINBURGH AND LONDON WILLIAM GREEN & SONS 1908 $ % = = a 5 a WwW a& oO E | aA In CUIQUE PRINTED FOR WILLIAM GREEN & SONS BY NEILL & Co., LTD. PREFACE BETWEEN the Physiology of Man and that of the Domestic Animals there is no fundamental difference, and most of our knowledge of human physiology has been acquired from experiments upon the lower animals. But while the tissues of a man, a dog, and a horse act much in the same manner, the mode of nutrition of these tissues is somewhat different, and requires special attention in the case of each. In this volume the attempt is made to give the essentials of general physiology and of the special physiology of the domestic animals in a form suitable to the requirements of Students and Practitioners of Veterinary Medicine. The book is not intended to take the place of the demonstrations and practical work from which alone physiology can be properly learned, but merely to supplement these and to focus the information derived from them. The student must take every opportunity of acquiring a really practical knowledge, and, to facilitate the more direct association of the practical and systematic study of physi- ology, throughout these pages references are made to de- scriptions of the experimental and chemical work which the student should try to do for himself or have demonstrated to him. The histological structure of the tissues and organs which is now studied practically in every school is here described only in so far as it is essential for the proper understanding of their physiology. . DNF. vii CONTENTS PART I SECTION I PROTOPLASM SECTION II THE CELL . , SECTION III Tue TIssuzEs A, THe VEGETATIVE TIssUES— Epithelium 1. Squamous 2. Columnar 3. Secreting 4, Ciliated Connective Tissues— 1. Mucoid . 2. Fibrous 3. Cartilage 4, Bone B. Tue Master Tissurs—Muscite anp NERVE— Muscle 2. In Action a. Skeletal b. Visceral 3. The Chemical Changes i in “Muscle and Soutce ‘of Energy evolved 4. Death of Muscle ix 1, At rest— Seretns, Chediistey ais Physical Gharactars : PAGE 13 x CONTENTS Nerve 1. Structure and Developinent 2. Chemistry of Nerve . 3. Physiology of Nerve SECTION IV I. Toe Nevro-MuscuntarR MECHANISM . 1. Neural Ares . sf 2. Mode of Action of Reet abel 3. Fatigue of Neuro-Muscular Mechanism . 4. The Chief Receptor Mechanisms | a. Intero-ceptive Mechanism b. Proprio-ceptive Spinal Mechanism c. Extero-ceptive Mechanism (a) Contact —1. Tactile Sense 2, Thermal Sense . 3. Taste, Sense of . (b) Distance—1. Smell, Sense of 2. Sight, Sense of 3. Hearing, Sense of d, Proprio-ceptive Mechanism of the Head ‘ 5. The Connections between the Receiving and Reacting Mechanisins II. Centrat Nervous System— A. Spinal Cord B. Medulla . (. Region of Pons Varolii D. Cerebellum E. F. Crura Cerebri and Corpora Quadrigemins Cerebrum PART II THE NUTRITION OF THE TISSUES _ SECTION V FLurips BarHine tHE TIssuEs BLoop AND LyMPH A. Blood B. Lymph PAGE 222 ee ee CONTENTS x1 CIRCULATION— PAGE 1. General Arrangement. ; . : : , . 225 2. The Central Pump—The Heart 5 : ; : . 227 3. Circulation in the Blood and Lymph Veusels f : : . 258 SECTION VI Suppty oF NovuRISHING MATERIAL TO THE BLOOD AND LYMPH, AND ELIMInaTION oF Waste MA?rTER FROM THEM Il. RESPIRATION : : ; ; : : ; : : . 294 II. Foop anp DIGESTION . t Sree: : : : : . 326 a. Food : : ‘ : : : : : ; 2 326 b. Digestion . é ; z : ; F : : 1 ook c. Absorption of Food . bee we o «le SS oe d. Fate of the Food Absorbed : : ; : : ; Xe OSE e. General Metabolism . F ‘ ; 5 P F : ;. 392 f. Dietetics . eee : ; ‘ eas at a5 ae SECTION VII INTERNAL SECRETIONS OR HORMONES— Their Production and Action . : : ; p , . 404 SECTION VIII EXCRETION OF MarTER FROM THE Bopy— 1. Excretion by the Lungs (see p. 294). 2. Excretion by the Kidneys— Urine . : : : : ; ‘ . 414 Secretion of Pigne 2k iduny: : ; : : . 423 3. Excretion by the Skin. ; : : a, eS . ++ 428 PART III RepRopvucrion . ‘ ; : ; ; : : ‘ i . 432 APPENDIX ._. a ; ; ‘ ; <« & «ee INDEX . . P , ; er : : oe . 449 ERRATA . ; ‘ ; : ; . . / : : . 464 % eo. ye 2) cea INTRODUCTION PuxyYsIOLOGY is really an older science than anatomy, for even before any idea of pulling to pieces, of dissecting the animal machine had suggested itself to our forefathers, crude specula- tions in regard to the causes and nature of the various vital phenomena must have been indulged in—speculations based upon the vivid belief in the action of spiritual agencies, and perhaps unworthy of the name of science. Still the physiology of to-day is the offspring of such speculations. Organs and Funetion.—The first great and true advance was through anatomy. As that science showed how the body is composed of distinct and different parts, it became evident that these parts or organs had separate actions or functions ; and hence arose the important conception of the co-relation of organ and function. From the early metaphysical speculations to such true inductions was a great stride, for a scientific method of advance had been established. ; Ever since this, until quite recent times, physiology has followed in the footsteps of anatomy, or, to use a more com- prehensive’ term, of morphology. The connection between organ and function having been demonstrated, the questions, Why are these various functions connected with the respective organs? why should the liver secrete bile and the biceps muscle contract ? next forced themselves upon the attention. Tissues and Function Again anatomy paved the way for the explanation. The dissecting knife and the early and defective microscope showed that the organs are composed of certain definite structures or tissues, differing widely from one another in their physical characters and appearance, and, as physiologists soon showed, in their functions. It now became xiii xiv INTRODUCTION evident why the liver secreted and the biceps contracted: the one is composed of secreting tissue and the other of contracting tissue. Cells and Funetion—Physiologists and anatomists alike devoted their energies to the study of these various tissues, and, as the structure of the microscope improved, greater and greater advances were made in their analysis, till at length Schwann was enabled to make his world-famous generalisation, that all the tissues are composed of certain similar elements more or less modified, which he termed cells, and it became manifest that the functions of the different tissues are due to the activities of their cells. The original conception of the cell was very different from that which we at present hold. By early observers it was described as composed of a central body or nucleus, surrounded by a granular cell substance with, outside all, a cell membrane. ' As observations in the structure of the cell were extended, it soon became obvious that the cell membrane was. not an essential part, and later, the discovery of cells without any distinct nucleus rendered it clear that the essential part is the cell substance, and this substance Von Mohl named protoplasm, by which name it is since generally known. Protoplasm and Funetion—So far physiology had followed in the tracks of anatomy, but now another science became her guide. Chemistry, which during the last century has advanced with enormous strides, and has thrown such im- portant light upon the nature of organic substances, now lent her aid to physiology; and morphologists having shown that the vital unit is essentially simply a mass of protoplasm, the science of life bids fair to become the science of the chemistry of protoplasm. , The prosecution of physiology on these lines is still in its infancy, but already it has*changed the whole face of the science. Physiology is no longer the follower of anatomy. It is become its leader, and at the present time, as we shall afterwards see, not only the various activities, but also the various structural differences of the different tissues, are to be explained in terms of variations in the chemical changes in protoplasm, In the study of physiology this order of evolution must be eS ‘ INTRODUCTION xv reversed, and from the study of protoplasm the advance must be made along the following lines :— 1. Protoplasm—the physical basis of life ; its activities and nature. 2. Cells.—Manner in which protoplasm forms the vital units of the body. 3. Tissues.—Manner in which these are formed by cells. Their structure, physical and chemical properties, and vital manifestations. 4, Nutrition of Tissues. a. Fluids bathing the tissues— Blood and Lymph. 6. Manner in which fluids are brought into relationship with tissues— Cireulatory System. c. Manner in which substances necessary for the tissues are supplied to these fluids— Respiratory System. Digestive System. Food, its nature and quantity. d, Chemical changes in the tissues generally — Metabolism and Heat Production. e. Manner in which the waste products of tissues are eliminated—Exeretion, Hepatic, Renal, Pulmonary, Cutaneous. 5. Reproduction and Development. ’ = TUDENTE EG 6G TORORTGS PART yal SECTION I PROTOPLASM THE first step in the study of physiology must be to acquire as clear and definite a conception as possible of the nature of protoplasmic activity in its most simple and uncomplicated form, for in this way an idea of the essential and non-essential characteristics of life may best be gained. I. Structure.—Protoplasm is a semi-fluid transparent viscous substance. It usually occurs in small individual particles— (0) fi i Ki a \) i M (c) (4 ( ( Fic. 1.—(a) Foam structure of a mixture of Olive Oil and Cane Sugar ; (d) Reticu- lated structure of Protoplasm ; (c) Reticulated structure of Protoplasm in the cell of an earth-worm (after BuTscHLi). CELLS—more or less associated, but it may occur as larger confluent masses—PLASMODIA. Sometimes protoplasm seems perfectly homogeneous, but generally a reticulated appearance can be made out even in the living condition (fig. 1), and from this it has been concluded that there is a more solid part arranged like the fibres of a sponge, or like the films of a mass of soap-bubbles, with a more I : ULityv (3) 2 VETERINARY PHYSIOLOGY fluid interstitial part. In all protoplasm, therefore, there seems to be a certain amount of organisation, and in certain eells this organisation becomes very marked indeed. Hl. Physiology.x—A knowledge of the essentials of the physi- ology of protoplasm may be gained by studying the vital manifestations of one of the simplest of living things, the yeast plant (Saccharomyces Cerevise), This plant consists of very minute oval or spherical bodies frequently connected to form chains, each composed of a harder outer covering or capsule and of a softer inner substance which has all the characters of protoplasm. Its physiology may be studied by placing a few torule in a solution, containing glucose, C,H,,O,, and urea, CON,H,, with traces of phosphate of soda, Na,HPO,, and sulphate of potash, K,SO,. Tt the vessel be kept all night in a warm place the clear solution will in the morning be seen to be turbid. An examination of a drop of the fluid shows that the turbidity is due to the presence of myriads of torule. In a few hours the few torule placed in the fluid have increased many hundred- fold. The whole mass of yeast has grown in amount by the growth and multiplication of the individual units. This power of growth and reproduction under suitable con- ditions is the essential characteristic of living matter. What are the conditions necessary for the manifestation of these phenomena of life ? 1, If the yeast be mixed with the solid constituents of the solution in a dry state no growth or reproduction occurs. Water is essential. 2. If the yeast, mixed with the solution, be kept at the freezing point no growth takes place, but this proceeds actively at about 36° C. A certain temperature is necessary for the vitality of protoplasm. In the absence of these conditions, proto- plasm is only potentially alive, and in this state it may remain for long periods without undergoing any change, as in the seeds of plants and in dried bacteria. The conditions essential for the manifestations of life being present, in order that the growth of the yeast may take place, there must be :— 2 é A PROTOPLASM 3 (a) A Supply oF MATERIAL from which it can be formed. (6) A Supply or ENErGy to bring about the construction. - (a) The chemical elements in protoplasm are carbon, hydro- gen, oxygen, nitrogen, sulphur, and phosphorus. These elements are contained in the ingredients of the solution used. If yeast be sown in distilled water, even if it be kept at a temperature of 36° C., it does not grow. (6) The energy is got by the breaking down of the sugar, C,H,.0,, into alcohol, C,H,O, and carbon dioxide, CO,. Such a breaking down of a complex into simpler molecules liberates energy, as is well seen when nitro-glycerine explodes, breaking into carbon dioxide, water, oxygen, and nitrogen— _ 2C,H,(NO,),=6CO,+5H,0+0-+6N, The energy can be used for the performance of work of any kind, as, for example, the work of building up a fresh quantity of the yeast plant out of the substances contained in the solution. The history of the yeast plant shows that protoplasm, when placed in suitable conditions, has the power of breaking down certain complex substances, and of utilising the energy liberated Sor building itself up. It is this power which has enabled living matter to exist and to extend over the earth. How does protoplasm liberate the potential energy of such substances ? The answer to this question has been given by the demon- stration by Buchner that the expressed juice of the yeast torul acts on the sugar in the same way as the living yeast. The yeast therefore manufactures something which splits the sugar. This something belongs to the group of Enzymes or Zymins which play so important a part in physiology generally. These enzymes all act by hastening reactions which go on slowly without their presence, but they do not themselves take any direct part in the reaction. Hence a very small quantity may bring about an extensive change in the substance acted upon. For the manifestation of their activity they require the presence of water and a suitable temperature—in the case of the yeast enzyme about 36° C. is the best. At lower temperatures the reaction becomes slower and is finally stopped, and at a higher temperature it is delayed and finally arrested by the destruction 4 VETERINARY PHYSIOLOGY of the enzyme. Sometimes these enzymes lead to a complete decomposition of the substance upon which they act, but this is often prevented by a checking influence exerted by the accumulation of the products of their action, e.g. by the alcohol developed from the sugar. With certain enzymes at least, the action may actually be reversed if the enzyme is brought in contact with the final products of decomposition. Thus the enzyme which splits esters into their components may cause a linking of those components to form esters. In this respect they simply aid the establishment of an equilibrium between the component substances in the reaction. The general action of the enzymes has been termed kata- lytic. It may be compared to the action of an acid in the inversion of cane sugar— C,.H,.0,; + H,0 =2 (C,H,.0,). Here the acid merely hastens a reaction which would go on slowly in the presence of water alone. The precise way in which such katalytic actions are brought about is still not quite clear, but there is evidence that the agent acts as a middle-man between the reacting substances, in the case of H,O, taking up the O and then liberating it—in the case of the decomposition of cane sugar taking up H,O and handing it on; just as in the oxidation of glucose, which occurs when it is boiled with an alkali, a metallic oxide, such as euprous oxide, may take oxygen from the air and hand it on to the glucose, thus making the oxidation more rapid. Living yeast differs from these dead substances simply in the fact that it wses the energy liberated from the glucose. In virtue of this, the yeast has the power of repair and of growth. But protoplasm is also constantly breaking down, and if yeast be kept at a suitable temperature in water without any supply of material for construction, it gives off carbon dioxide and decreases in bulk on account of these disintegrative changes. These are as essential a part of the life of living matter as the building-up changes, and it is only when they are in progress that the latter are possible, Protoplasm (living matter) is living only in virtue of its constant chemical changes, metabolism, and these changes PROTOPLASM 5 are on the one hand destructive (katabolic), on the other constructive (anabolic). Living matter thus differs from dead matter simply in this respect, that side by side with destructive changes, constructive changes are always going on, whereby its amount is maintained or increased, Hence our conception of living matter is not of a definite chemical substance, but of a substance constantly undergoing internal changes. It might be compared to a whirlpool con- stantly dragging things into its vortex, and constantly throw- ing them out more or less changed, but itself continuing apparently unchanged throughout. Hoppe-Seyler expresses this by saying: “The life of all organisms depends upon, or, one can almost say, is identical with, a chain of chemical changes.” Foster puts the same idea in more fanciful language: “We may speak of protoplasm as a complex sub- stance, but we must strive to realise that what we mean by that is a complex whirl, an intricate dance, of which, what we call chemical composition, histological structure, and gross configuration are, so to speak, the figures.” The rate of these chemical processes may be quickened or slowed by changes in the surroundings, and such changes are called stimuli. If the stimulus increases the rate of change, it is said to excite; if it diminishes the rate of change, it is said to depress. ‘Thus the activity of the changes in yeast may be accelerated by a slight increase of the temperature of the surrounding medium, or it may be depressed by the addition of such a substance as chloroform water. While the continuance of these chemical changes in proto- plasm is life, their stoppage is death. For the continuance of life the building-up changes must be in excess of or equal to the breaking-down. The evolution of energy must be sufficient for growth and maintenance. It is only the surplus over this which is available for external work. In youth the . surplus energy is largely used for growth, in manhood for work. When failure in the supply or in the utilisation of the energy-yielding material occurs, the protoplasm dwindles and disintegrates. . Death is sudden when the chemical changes are abruptly stopped, slow when the anabolic changes are interfered with. The series of changes which occur between the infliction of an incurable injury and complete disintegration 6 VETERINARY PHYSIOLOGY of the tissue constitute the processes of Necrobiosis, and their study is of importance in pathology. Ill. Chemistry.—It is impossible to analyse such an ever-. changing substance as protoplasm, and although what is left when these chemical changes are stopped can be examined, such analyses give little insight into the essential nature of the living matter. That substances of great complexity take part in the con- stant whirl is shown by the analyses of what is left after death. Five or six elements—carbon, hydrogen, oxygen, nitrogen, sulphur, and phosphorus—are present, and these are linked together. to form molecules of enormous size. Water is the most abundant constituent of protoplasm, amounting, as it does, to about 75 per cent. . The Solids, constituting the remaining 25 per cent., con- sist chiefly of a series of bodies closely allied to one another and called “chief substances” or Proteins. In addition to these, certain inorganic salts are found in the ash when protoplasm is burned, indicating the presence of PoTassIUM and CaLciIuM along with PHospHorus and SuLrHuR. The inorganic salts, and especially their kations, appear to be of considerable importance in maintaining the activity of proto- plasm, and their possible mode of action will be considered later. Small and varying quantities of Fars, and of CaRBo- HYDRATES, with traces of a number of other organic sub- stances which need not here be enumerated, are also usually present. Of these substances the Proteins alone have to be con- sidered here, since they constitute the really important part of the material. PROTEINS White of egg or the juice of meat may be taken as examples of such proteins dissolved in water with some salts. If the salts be separated, and the water carefully driven off at a low temperature, a pure protein is left. (A) Physieal Characters.—The proteins from the residue of living matter—the Native Proteins, as they may be called— have a white, yellow, or brownish:colour. In structure they PROTOPLASM - are usually amorphous, but many have been prepared in a crystalline condition, and it is probable that all may take a crystalline form. The crystals vary in shape, being usually small and needle-like, but sometimes forming larger rhombic plates. Some proteins form apparent solutions in water, others require the presence of neutral inorganic salts, others of an acid or alkali, while some are completely insoluble without a change in their constitution. All are insoluble in alcoho! and ether. When in solution, or apparent solution, the native pro- teins do not dialyse through an animal membrane. They are colloids. Other colloidal bodies reacting much like the proteins have been prepared synthetically by chemists—e.g. by heating together amido-benzoic acid and phosphoric an- hydride. Like other colloids proteins tend to coagulate, form-. ing a clot just as, for instance, silicic acid may clot when carbon dioxide is passed through its solution. The native proteins are coagulated by simply heating their solution. All proteins rotate the plane of polarised light to the left. (B) Chemistry.—Proteins. contain the following ehemical elements: carbon, hydrogen, oxygen, nitrogen, and sulphur, in about the following percentage amounts :— Cc. H. N. S. 0. 52 7 16 1 24 It is important to remember the amounts of nitrogen and carbon, since proteins are the sole source of the former element in the food and an important source of the latter. As regards the number of atoms of these elements which go to form a single molecule, information has been obtained by studying compounds with various metals. The following probable formula of the molecule of the chief protein of the white of egg is given simply to show how complex these substances are: C,,H3..N 5.0 ¢g5>: The constitution of the protein molecule has been investi- gated first by studying the products of the decomposition of the molecule by various agents, and second by attempting to build up the molecule by the synthesis of the products of disintegration. 3 VETERINARY PHYSIOLOGY 1, Products of Decomposition The native proteins in solution tend to break down into a series of simpler bodies, and this decomposition is greatly hastened by the action of acids or alkalies or of certain enzymes. A series of more and more simple molecules are thus produced which, as they decrease in complexity, give solutions which are less and Jess colloidal and less and less easily precipitated by alcohol or by neutral salts, till, finally, products are yielded which no longer give the protein tests, which are freely soluble in alcohol, and which have the characteristics of amino acids. Along with these, certain by-products are also given off. The different stages of the disintegration of the native protein molecule may be arranged as follows ! :-— lis) oes, aa be Native Proteins. 33 (8 £ S l » "| Go 1S Boe O Sb, |< O On (gael | \ Proteoses, a be O O ery HY ks : RS Peptones. iS ’ O ; | 3 | Polypeptides, B. : 7) =~ ~~ — Dipeptides. 3 3 O Plas tcgee Amino Acids 8 = O50 O a) O (mono- and di-).| % Z 3 2 =o ; S g mn — abe ook <4 VETERINARY PHYSIOLOGY and here strands of white fibrous tissue with little islands of hyaline cartilage are found. It is also found when white fibrous tissue, as tendon, is inserted into hyaline cartilage, and is really a mixture of two tissues—white fibrous tissue and cartilage. 4, Bone.—The great supporting tissue of the adult is Bong. (1) DEVELOPMENT AND SrructuRE.—Bone is formed by a deposition of lime salts in layers or lamelle of white fibrous tissue; but while some pasa as those of the cranial vault, face, and clavicle, are produced entirely in fibrous tissue, others are preformed in car- tilage, which acts as a scaf- folding upon which the for- mation of bone goes on. Intra-membranous Bone Development.—This may be well studied in any of the bones of the cranial vault where cartilage is absent (fig. 12). At the centre of ossifica- tion the matrix between the . fibres becomes impregnated Fie. 12. BEEN SERETISES Bone Develop- with lime salts, chietly the ment in the lower jaw of a fetal cat, Phosphate and carbonate. rm the les . paint is — How this deposition takes shooting out along the fibres, and on the : 5 lower surface the process of absorption is Place is . not known; and going on. Two osteoclasts—large multi- how far it is dependent on nucleated cells—are shown to the left. the action of cells has not been clearly determined. As a result of this, the con- nective tissue cells get enclosed in definite spaces, lacune, and become bone corpuscles. Narrow branching channels of communication are left between these lacune, the canaliculi, This deposition of lime salts spreads out irregu- larly from the centre into the adjacent fibrous tissue, and this advance is- preceded by a line of actively growing cells, sometimes called osteoblasts. The fully formed adult bone, hewever, is not a solid block, but is composed of a compact THE TISSUES 33 tissue outside, and of a spongy bony tissue, cancellous tissue, in- side. This cancellous tissue is formed as a secondary process. Into the block of calcareous matter, formed as above described, processes of the fibrous tissue burrow, carrying in blood vessels, lymphatics, and numerous cells. This burrowing process seems to be carried on by the connective tissue cells, which eat up the bony matter formed. In doing this they frequently change their appearance, becoming large and multi-nucleated (osteoclasts). Thus the centre of the bone is eaten out into a series of channels, in which the marrow of the bone is lodged, and between which narrow bridges of bone remain. Fic. 13.—Intra-cartilaginous Bone Developmert. A phalanx of a foetal finger showing the formation of periosteal bone round the shaft ; the opening up of the cartilage at the centre of ossification ; the vascularisation of the cartilage by the invasion of periosteum ; and the calcification of the cartilage round the spaces. ; It is by the extension of the calcifying process outwards, and the burrowing out of the central part of the bone, that the diploé and cancellous tissue are produced. Intra-cartilaginous Bone Development.—In the bones pre- formed in cartilage, the process is somewhat more complex, although all the bone is formed in connection with fibrous tissue, the cartilage merely playing the part of a scaffolding and being all removed. Where the adult bone is to be pro- duced, a minute model is formed in hyaline cartilage in the embryo, and this is surrounded by a fibrous covering, the perichondrium. In the deepest layers of this perichondrium 3 34 VETERINARY PHYSIOLOGY the process of calcification takes place, as described above, and spreads outwards, thus encasing the cartilage in an ever- thickening layer of bone (fig. 13). This was demonstrated by inserting a silver plate under the periosteum, and showing that bone was deposited outside of it. - At the same time, in thé centre of the cartilage, at what is called the centre of ossification, the cells begin to divide actively, and, instead of forming new cartilage, eat away their capsules, and thus open out the cartilage spaces (fig. 13). Into these spaces processes of the perichondrium bore their way, carrying with them blood vessels, and thus rendering the cartilage vascular (fig 13). The vas- cularisatién of the centre of the cartilage having been effected, the process of absorption extends towards the two ends of the shaft of cartilage, which continues to elongate. The cartilage cells divide and again divide, and, by absorbing the material between them, form long irregular canals running in the long axis of the bone, with trabecule of cartilage between them. Into these canals the processes of the periosteum extend, and fill them with its fibrous tissue. A deposition of lime salts takes place upon the trabecule, enclosing cells of the invading fibrous tissue, and thus forming a crust of bone, while the cartilage also becomes calcified. If this calcification of the cartilage and deposition of bone were to go on unchecked, the block of cartilage would soon be converted to a solid mass of calcified tissue. But this does not occur. For, as rapidly as the trabecule become calcified, they are absorbed, while the active changes extend farther and farther from the centre to the ends of the shaft. The centre is thus reduced to a space filled by fibrous tissue which afterwards becomes the bone marrow. The process of absorption does not stop at the original block of cartilage; but after all of this has been absorbed, the bone formed round the cartilage (the periosteal bone) is attacked by burrowing processes from inside and outside, which hollow out long channels running in the long axis of the bone. These are the Haversian spaces (fig. 14). Round the inside of these, calcification occurs, spreading inwards in layers, and enclosing connective tissue cells, until, at length, only a small canal is left, an Haversian canal, containing some TUDENTE (MED Cac TOROLTC THE TISSUES 35 connective tissue, blood vessels, lymphatics, and nerves, with layer upon layer of bone concentrically arranged around it. This constitutes an Haversian system. In this way the characteristic appearance of the shaft of a long bone is produced (fig. 14), with layers of calcified fibrous tissue, the bone lamelle, arranged as Haversian, interstitial, peripheral, and medullary lamelle. One important function performed by the cartilage is in bringing about the increase in length of the bones. In addition to the centre of ossification in the shaft, at each Fic. 14.—Cross section through part of the shaft of an adult long bone to show the arrangement in lamelle distributed as Haversian (1), interstitial (2), peripheral (3), and medullary (4). end of the bone one or more similar centres of ossification form. These are the epiphyses. Between these and the central rod of bone—the diaphysis—a zone of cartilage exists until adult life, when the bones stop growing. In this zone, the cells arrange themselves in vertical rows, divide at right angles to the long axis of the bone and form cartilage. This cartilage as it is formed is attacked by the bone-forming changes at the diaphysis and epiphyses, but the amount of new cartilage formed is proportionate to this, and thus a zone of growing cartilage continues to exist until early adult life, when epiphyses and diaphysis join and ye Osh CTMSlUlS Wid OT OO 1 36 VETERINARY PHYSIOLOGY growth in length is stopped. The rate and extent of this growth of the cartilage has an important influence on the growth of the individual. (2) CuEmistry.—The composition of adult bone is roughly as follows :— Water, 10 per cent. Solids, 90 per cent. Organic, 35 per cent.—chiefly collagen. Inorganic, 65 per cent. Calcium phosphate, 51. 4, carbonate, 11.. bs fluoride, 0-2. Magnesium phosphate, 1. Sodium salts, 1. The points to be remembered are the small amount of water, the large amount of inorganic matter, chiefly calcic phosphate, and the nature of the organic matter—collagen. (B) THE MASTER TISSUES OF THE BODY, MUSCLE AND NERVE By means of the epithelial and connective tissues the body is protected, supported, and nourished. It performs purely vegetative functions, but it is not brought into relationship with its environments. By the development of nerve and muscle the surroundings are able to act upon the body, and the body can react upon its surroundings. These tissues may therefore be called the Master Tissues, and it is as their servants that all the other tissues functionate. So far as the chemical changes in the body are concerned, © muscle is more important than nerve, for three reasons— First, it is far more bulky, making up something like 42 per cent. of the total weight of the body in man; ‘second, it is constantly active, for even in sleep the muscles of respiration, circulation, and digestion do not rest; and third, the changes going on in it are very extensive, since its great function is to set free energy from the food. So far as the metabolism of the body is concerned, muscle is the master tissue. For muscle we THE TISSUES 37 take food and breath, and to get rid of the waste of muscle the organs of excretion act. Hence it is in connection with muscle that all the problems of nutrition—digestion, respira- tion, circulation, and excretion—have to be studied. I. MUSCLE The two great functions of muscle are— To perform mechanical work. To liberate heat. 1. Muscie at Rest Structure : Chemistry and Physical Characters The first trace of the evolution of muscle is found among the infusoria, where, in certain cells, in parts of the proto- plasm, the network or cytomitoma is arranged in long parallel threads in the direction of which the cell contracts and expands. Such a development has been termed a myoid. 1. Strueture of Muscle Even. a cursory examination of mammalian muscle shows that those of the trunk and limbs, skeletal muscles, are different from those of such internal organs, as the bladder, uterus and alimentary canal, visceral muscles. The visceral muscles appear to be formed from cells similar to ordinary connective tissue cells. These elongate, acquire a covering, and their protoplasm becomes definitely longitudinally fibrillated by the arrangement of the cytomitoma. They thus become spindle-shaped cells, varying in length from about 50 to 200 micro-millimetres, A covering membrane, the sarcolemma, develops. This is thin, but tough and elastic, and it adapts itself to the surface of the cell, unless when this is excessively shortened, in which case the sarcolemma may be thrown into folds, which give the cell the appearance of cross-striping. The nucleus is usually long, almost rod-shaped, and is independent of the cytomitoma (fig. 15, a). The skeletal muscles develop from a special set of cells, early differentiated as the muscle-plates in the mesoblast 38 ” VETERINARY PHYSIOLOGY down each side of the vertebral column of the embryo. Each cell elongates. The nucleus divides across, but the cell, instead of also dividing, lengthens and continues to elongate as the two daughter nuclei again divide. The cytomitoma becomes arranged longitudinally, and a series of transverse markings appear across the cell. Lastly, a covering, the sarcolemma, develops, and the na fibre is produced (fig. 15, 6). This consists of three parts— 1. The Sareolemma is a delicate, tough, elastic membrane closely investing the fibre, and attached to it at Dobie’s lines. 2. The Musele corpuscles consist of little masses of . proto- plasm each with a nucleus, which lie just under the sarcolemma. 3. The Sareous substanee is made up of a series of longi- tudinal fibrils consisting of alternate dim and clear bands—the a b Fic. 15.—(a) Fibres of Visceral Muscle ; (b) Fibres of Skeletal Muscle to show sarcolemma, muscle corpuscles, and sarcous substance composed of fibrils showing transverse markings. former staining deeply with eosin. In the middle of the clear band is a narrow dim line, Dobie’s line. The fibres and fibrils tend to break across in the region of the clear band, showing that they are weakest at that part. The clear band differs from the dim band, not only in not taking up eosin, but also in the fact that it entirely prevents the passage of polarised light except in one position of the analysing prism, while the dim band allows polarised light to pass, whatever be the position of the prisms. A probable explanation of these facts is that the sarcous substance is made up, like other protoplasm, of a mitoma and plasma; that the mitoma is arranged in a series of longitudinal fibres, which are broader and stronger in the dim band, and lie closely applied to one another, side by side, while in the clear band they are thinner, and are separated from one another by plasma; and that at Dobie’s line there is a swelling on each fibril. asf ns. pet THE TISSUES 39 2. Chemistry of Muscle Like all other living tissues, muscle,is largely composed of water. It contains about 75 per cent. The 25 per cent. of solid constituents is made up of a small quantity, about 3 per cent., of ash, and 22 per cent. of organic substances. The ash consists chiefly of potassium and phosphoric acid, with small amounts of sulphuric and hydrochloric acids and of sodium, magnesium, calcium, and iron. The sulphuric acid is derived from the sulphur of the proteins, and a part of the phosphoric acid is derived from the phosphorus of the nucleins of muscle, and probably from other organic combinations, 1. Proteins.—Of the organic constituents, by far the greater part is made up of Proteins. These may be divided into— (a) Those soluble in neutral salt solutions. (6) Those insoluble in them. (az) The first class of bodies consists entirely of three globulins. Two of these—Myosinogen and Paramyosinogen— have the peculiar property of clotting under certain condi- tions, to form what is called Myosin, and this process, which occurs after death, is the cause of death stiffening. The post-mortem change is supposed to be brought about by the development of an enzyme, since a glycerine extract of dried muscle rapidly causes the formation of myosin. The third globulin, Myoglobulin, does not undergo this change. These three proteins are contained in the plasma—the juice which can be expressed from muscles kept near the freezing point. If the plasma is warmed it rapidly clots, just as it does post-mortem. (6) The insoluble protein of muscle, Myostromin, seems to be of the nature of a nuclein, and probably forms the framework of the fibres. It is always mixed with the collagen of the fibrous tissue of muscle, and it may be separated by dissolving it in carbonate of soda solution, from which it may be again precipitated by weak acetic acid. (Chemical Physiology.) Collagen derived from the fibrous tissue holding the muscle fibres together is also present, and yields gelatin on boiling. 40 VETERINARY PHYSIOLOGY / In addition to the proteins, small quantities of other organic substances are found in muscle. 2. Carbohydrates.—Glueose (C,H,,0,) is present in muscle, as in all other tissues. Glyeogen «(C,H,,0,)—a substance closely allied to ordinary starch, but giving a brown reaction with iodine—is always present in muscle at rest. If the muscle has been active, the amount of glycogen diminishes, being probably converted to glucose, and used for the nourishment of the tissue. (For the chemistry of the carbohydrates, see p. 344.) 3. Fat is present in small quantities in the fibres, and often in very considerable quantities in the fibrous tissue between the fibres. 4. Inosite, formerly called muscle sugar, is present in small amounts. It is not a sugar, but a benzene compound. 5. Sareolactie Acid.—Hydroxy-propionic acid— H OH O bah al H—C—C—C—O—H os. H H This dextro-rotatory isomere of ordinary lactic acid is in- creased in muscle during activity and during death stiffening. 6. Extraetives.—If dried muscle is treated with alcohol a series of bodies containing nitrogen may be extracted. The chief of these is Creatin, or methyl-guanidin-acetic acid. Guanidin C.NH(NH,), is a near ally of urea CO(NH,),, being formed by replacing the O with NH. Methyl-guanidin is produced by replacing an H in guanidin by CH,— NH CH, I HN—o_N_# If this is linked to acetic acid— H—NCH,|H 0 Was ay ee H nC eee Methy]l-guanidin , H acetic acid is produced. THE TISSUES 4I 7. The Colour of Muscle varies considerably, some muscles _ being very pale, almost white in colour—ey. the breast muscles of the fowl; others again being distinctly red, even after all the blood has been removed. This red colour is, in some cases, due _to the presence of the pigment of blood, hemoglobin, but in certain muscles it is due to a peculiar set of pigments, Myohematins, giving different reactions from the blood pigment. 3. Physical Characters of Muscle 1. Muscle is translucent during life, but, as death stiffening sets in, it becomes more opaque. 2. Muscle is markedly extensile and elastic. A small force is sufficient to change its shape, but when the distorting force is removed it returns completely to its original shape, provided always that the distortion has not overstepped the limits of elasticity. When a distorting force is suddenly applied to muscle—e.g. if a weight is suddenly attached—the distortion takes place at first rapidly, and then more slowly, till the full effect is pro- duced. If now the distorting force is removed the elasticity of the muscle brings it back to its original] form, at first rapidly, and then more slowly. (Practical Physiology.) The advantages of these properties of muscle are, that every muscle in almost all positions of the parts of the body is stretched between its point of origin and insertion. When it contracts it can therefore act at once to bring about the desired movement, and no time is lost in preliminary tightening. Again, the force of contraction, acting through such an elastic medium, causes the movement to take place more smoothly, and without jerks. Experimentally, too, it has been ascertained that a force acting through such an elastic medium produces more work than when it acts through a rigid medium. The extensibility of muscle is of value in allowing a group of muscles to act without being strongly opposed by their antagonistic group. For instance, suppose the extensor muscles of the arm were not readily extensile, when the flexors acted, a large amount of their energy would have to be employed in elongating the extensors. Similarly the elasticity of the 42 VETERINARY PHYSIOLOGY muscles tends to bring the parts back to their normal position when the muscles have ceased to contract. It must not, however, be imagined that, in all movements of one set of muscles, the antagonistic muscles are relaxed, although they may be elongated. Often they are in a state of activity so as to guide the movements which are being produced (see p. 58). Tonus of Muscle.—The tense condition of resting muscle between its points of origin and insertion is not merely due to passive elasticity, but is in part caused by a continuous con- traction kept up by the action of the nervous system. If the nerve to a group of muscles be cut, the muscles become soft and flabby and lose their tense feeling. 3. Heat Production—Muscle, like all other living prote: plasm, is in a state of continued chemical change, constantly undergoing decomposition and reconstruction. Asa result of this chemical change, heat is evolved. But the heat evolved during rest of muscle is trivial. 4, Electrical Conditions—Muscle when at rest is iso-sleetele but if one part is injured, it acts to the rest like the zine plate in a galvanic battery—becomes electro-positive ; and hence, if a wire passes from the injured to the uninjured part round a galvanometer, a current is found to flow along the wire from the uninjured to the injured part, just as, when the zinc and copper plates in a galvanic cell are connected, a current flows through the wire from copper to zinc. This is the Current of Injury (p. 63). (Practical Physiology.) 2. MUSCLE IN ACTION A, Skeletal Muscle 1. Methods of making Musele Contract Skeletal muscle remains at rest indefinitely until stimulated to contract, usually by changes in the nerves. We desire to contract our biceps: certain changes occur in our brain, these set up changes in the nerves passing to the biceps, and the muscle contracts. Can skeletal muscle be made to contract without the inter- vention of nerves—can it be directly stimulated ? To answer this, some means of throwing the nerves out of _ THE TISSUES 43 action must be had recourse to. If curare, a South American _ arrow poison, be injected into an animal—e.g. into a frog, the brain of which has been destroyed—it soon loses the power of moving. When the nerve to a muscle is stimulated, the muscle no longer contracts. But, if the muscle be directly stimulated by any of the various agents to be afterwards mentioned, it at once contracts. It might be urged that the curare leaves unpoisoned the endings of the nerve in the muscle, and that it is by the stimulation of these that the muscle is made to contract. But that these are poisoned is shown by the fact that if the artery to the leg be tied just as it enters the muscle, so that the poison acts upon the whole length of the nerve except the nerve end- ings Seta néclo. stimulation Fic. 16. —Curare Experiment to show ; : sciatic nerves exposed to curare, but of the nerve still causes muscu- nerve endings protected on the left lar contraction. Only when the side; while on the right side the curare-is allowed to act upon the curare is allowed to reach the nerve : ; endings in the muscle, nerve endings in the muscle does stimulation of the nerve fail to produce any reaction in the muscle, while direct stimulation of the muscle causes it to con- tract. This clearly shows that it is the nerve endings which are poisoned by curare, and that therefore the application of stimuli to the muscle must act directly upon the muscular fibres (fig. 16). (Practical Physiology.) Muscle, however, is more readily stimulated through its nerves, and a knowledge of the points of entrance of the nerves into muscles, the motor points, is of importance in medicine in indicating the best points at which to apply electrical stimulation (fig. 17). - Various means may be used to make the muscle contract. 1st. Various chemical substances when applied to a muscle . make it contract before killing it, while others kill it at once. Among the former may be mentioned dilute mineral acids and metallic salts. (Practical Physiology.) 2nd. A sudden mechanical change such as may be produced by pinching, tearing, or striking the muscle will cause it to contract. (Practical Physiology.) a il qt it a 44 VETERINARY PHYSIOLOGY 3rd. Any sudden change of temperature, either heating or cooling, stimulates muscle. A slow change of temperature has little or no effect. Every muscle, however, passes into a state of contraction, heat stiffening, when a sufficiently high ML triceps (long bead) M. triceps (inner head) Tinar n. { MM. fiexor carpi vlnaris M. flex. digitor. commun. profund. M. ficx. digitor. sublim. ML. flex. digitor. sublim, (digiti 1. et LL) NL flex. digit, subl. . M. Gcx pollicis longus indicis et intel” Tina te Median n. AM. nalmaris brev, - M. adductor poliic, prev. M. abductor digit: min. M. opponens pollicis M, flexor digit. min. M.opponens digit, min, M. flex. pol). brev Mm. lumbricales { = M. adductor polhec, brev. Fig. 17.—Motor Points of Arm. temperature to coagulate its protein constituents is reached. This, however, is not a true living contraction. (Practical Physiology.) 4th. Muscle may also be made to contract by any sudden change in an electric current passed through it, whether the THE TISSUES 45 current be suddenly allowed to pass into it or suddenly cut out of it, or whether it is suddenly made stronger or weaker. (Practical Physiology.) This method of stimulating muscle is constantly used in - medicine. I[t is a matter of no importance how the electricity is procured, but most usually it is obtained either— 1st. Directly from a galvanic battery or electric main; or 2nd, From an induction coil. If a galvanic battery or current from the main be used—(1) On making (closing) the current, and upon breaking (opening) the current, a contraction results. While the current is flow- ing through the muscle, the muscle uswally remains at rest; but if the current is suddenly increased in strength or suddenly diminished in strength, the muscle at once contracts. With strong currents a sustained contraction—galvanotonus—may persist while the current flows. (Practical Physiology.) It is the suddenness in the variation of the strength of the current rather than its absolute strength which is the factor in stimulating, as may be shown by inserting some form of rheonome into the circuit by which the current may be either slowly or rapidly varied. (2) If a current be made weaker and weaker, breaking ceases to cause a contraction, while making still produces it. That is, the stimulus on making is stronger than that on breaking. (3) The two poles do not produce the same effect. The negative pole or kathode—that coming from the zinc plate of the battery—causes contraction of the muscle on closing; while the positive pole or anode causes contraction at opening. This may be summarised as follows :— 1, Contraction on closing ; contraction on opening. 2. Closing contraction stronger than opening contraction. 3. Contraction at kathode on closing, at anode on opening. LCC C.0 2. CC>CO 3. CKC CAO How can these facts be explained ? 46 VETERINARY PHYSIOLOGY . Eleetrotonus A study of the influence of the current on the muscle while it is passing through it throws important light on this atts (Practical Physiology.) While the current simply flows through the muscle no con- traction is produced, but the excitability is profoundly modified. Round the kathode it becomes more easily stimulated, while round the anode or positive pole it becomes less easily stimulated. This may be expressed by saying that the part of the muscle under the influence of the kathode is in a state of katelectrotonus, of increased excitability or of more unstable equilibrium, while the part of the muscle under the influence of the anode is in a state of anelectrotonus, of decreased excitability or of more stable equilibrium. Now it is well known that any sudden disturbance of the equilibrium or balance of a series of bodies is apt to cause them to fall asunder. For instance, if from a house of cards one card is suddenly drawn out, the whole structure passes into a condition of unstable equilibrium and is apt to fall to pieces. So with a muscle, if it be suddenly made unstable, as at the kathode on closing, a breaking down occurs and a contraction results. On the other hand, suppose a house of cards is built and made extra stable by introducing some additional cards at the foundation, then if these cards are suddenly withdrawn the chances are that the house falls to bits. So with a muscle. When the current is opened the removal of the state of increased stability at the positive pole may cause a breaking down and produce the anodal opening contraction. The study of electrotonus thus explains why any sudden change in the flow of electricity through a muscle stimulates it. _ It further explains why the stimulation and contraction start from the kathode on closing and from the anode on opening; and why the closing contraction is stronger than the opening, since the sudden production of a condition of actual instability must act more powerfully than the simple sudden removal of a condition of increased stability. This law of Polar Excitation, while it applies to muscle and nerve, does not apply to all protoplasm. Thus amceba shows contraction at the anode and expansion at the kathode when a galvanic current is passed through it. THE TISSUES 47 When the galvanie current is used to stimulate muscles through the skin in man or in other living animals, the different action of the two poles is not so marked as in the excised muscle of the frog, because the current, passing through the skin above the muscle to enter the body, flows not along but rather across the muscle, and thus, under each pole applied to the skin there is on one side of the muscle the effect of an entering current—anode—and on the other, of a leaving current—kathode (fig. 18). Thus the same bit of muscle or nerve is subjected to anelectrotonus on one side and katelectro- tonus on the other, and the effects of the two poles, and there- Kathode Anode M Pi 4N ore \ me A Fie. 18.—Electrical Stimulation of human muscle or nerve to show the passage of the current across the structure, and the consequeat combination of effects under each pole. MW, making or closing the current ; B, opening or breaking the current. W, weak; M, medium; S, strong current. fore of closing and of opening, tend to be combined, although the influence of, the pole placed immediately above the muscle predominates. Hence with a strong current contraction occurs both on closing and on opening at both poles. As the current is weakened, the contraction at the kathode on opening first disappears, because the anode is not predominant. Next, con- traction at the anode on opening disappears because the anodal stimulation is so much weaker than the kathodal. Then, the contraction at the anode on closing goes because the kathode is not predominating; and, finally, the contraction at the kathode on closing also disappears. When the muscle is in 48 VETERINARY PHYSIOLOGY one stage of the degeneration which follows separation from its nerve, the anodal closing contraction (fig. 18, Anode 17) becomes much exaggerated. This is called the reaetion of degeneration. When muscle is stimulated by induced electricity (fig. 20) Electric Current. Make. Break. Make. Break. | Contraction of Muscle. | | Kathode. Anode. Kathode. Anode. Galvanic. Induced. Fic. 19.—To show separation of make and break stimuli and of anodal and kathodal effects when a galvanic current is used, and their combination when the induction coil is used. the question is much easier, for, with each make and break or each sudden alteration in the strength of the primary circuit, there is a sudden appearance and equally sudden disappearance of a flow of electricity in the secondary coil. If, therefore, wires from the Fic. 20.—Course of Electric Current in primary circuit secondary coil are led off to a muscle, each change in the primary circuit causes the sudden and _ practically simultaneous appearance and disap- pearance of an electric current in the muscle, and this of course causes a contraction. But here the effects of closing and opening the current are (lower line), and in second- ary circuit (upper line) of an induction coil. Observe that in the secondary the make (upstroke) and break (dowustroke) are combined, and that a stronger current is developed in the second- ary circuit upon breaking than upon making the primary circuit. practically fused, and hence the influ- ence of the anode and kathode, and — of closing and opening, need not — be considered (fig. 20). (Practical Physiology.) It must, of course, be remembered, that in an induction coil the opening of the primary circuit produces a more powerful current in the second- THE TISSUES 49 ary coil than the closure of the primary circuit, and therefore a more powerful stimulation of the muscle (fig. 20). 2. The Changes in Musele during Contraction I. Change in Shape The most manifest change is an alteration in the shape of the muscle. It becomes shorter and thicker, This anyone ean see by studying their own biceps muscle. Contraction of muscle, however, is not a necessary result of excitation. Thus, a part of a muscle when dipped in water may os to contract when stimulated, but may mani- — *..... fest its excitation by conducting the ~ impulse to the part of the muscle not in the water and thus making it contract. In skeletal muscle the shortening and thickening of the muscle as a whole is due to the shortening and thickening of the individual fibres and their fibrils. In these fibrils the shortening and Fic. 21.—Contraction of thickening is most marked in the dim band. The clear band also shortens, and at the same time it becomes darker till, in the fully contracted muscle, it may be as dark as the dim band (fig. 21). These appearances may best be ex- plained on the assumption that the fibrils _ are the part of the fibre which shorten and thicken, and that these fibrils chiefly Skeletal Muscle — re- laxed above, contracted below. 4 isa diagram of the change ina fibril ; B shows the shading of the clear band ; and C shows the absence fof any alteration in the influence of the two bands on_ polarised light. shorten where they are thickest—in the dim band. At the same time, by the contraction of the fibrils in the clear band, adjacent dim bands may be supposed to be pulled nearer to one another, and to cast a shadow over the clear band. That no actual chemical change takes place in either band seems to be indicated by the facts that they retain their reaction to polarised light and staining reagents. Usually the contraction of a muscle occurs simultaneously in all the fibres. This is because a nerve fibre passes to every 4 50 VETERINARY PHYSIOLOGY muscular fibre, and sets all in action together. When, as sometimes occurs in disease, the nervous mechanism acts abnormally, the muscular fibres may not all act at once, and a peculiar fibrillar twitching of the muscle may be produced. If the muscle be directly stimulated at any point, the contraction starts from that point and passes as a wave of contraction outwards along the fibres. This may be seen by sharply percussing the fibres of the pectoralis major in the — chest of an emaciated individual. The rate at which the wave of contraction travels is ascertained by finding how long it takes to pass between any two points at a known distance from one another. Its velocity is found to vary much according to the kind of muscle and the condition of the muscle. In the striped muscular fibres of a frog in good condition it travels at something over three metres per second. When the muscle is in bad condition the wave passes more slowly, and in an exhausted muscle it may remain at the point of stimulation. (Practical Physiology.) The cause of the propagation of this wave is simply the continuity of the muscle fibres. The fibres stimulated are set in action, and the evolution of energy in these stimulates the adjacent fibres, and so the contraction passes along the muscle as a flame passes down a trail of gunpowder. Contraction of Muscle as a whole may best be studied under the following heads :— lst. The course of contraction. 2nd. The extent of contraction. 3rd. The force of contraction. lst. Course of Contraction (fig. 22). By attaching the muscle (JZ) to a lever (Z),and allowing the point of the lever to mark upon some moving surface, a magnified record of the shortening of the muscle when stimulated may be obtained. A revolving cylinder covered with a smoke-blackened, glazed paper is frequently used for this purpose, and to stimulate and mark the moment of stimulation an induction coil (p.c., s.¢.), with an electro-magnetic marker (7'. M.), intro- duced in the primary circuit, may be employed. ee THE TISSUES 51 To find the duration of the contraction, a tuning fork, vibrating 100 times per second, may be made to record its Vibrations on the surface. (Practical Physiology.) A caer t— + FD ara o Aleercne “i = an “i \5i (ea | ae 5 (Pa wo ‘aia UE a = — oa’ n- LEZ ; LZ7R sa a) pa = as a ee 1 i — “x eT ee Ue ag Vb €2 he B L : mM Ss a L, k / B ie R P 5.¢, Fic. 22.—A, Method of Recording Muscular Contraction. B, Key to Parts of Apparatus. J, Muscle attached to crank lever Z. p.c., Primary circuit, and, s.c., secondary circuit of an induction coil with short circuiting key, k’, in secondary circuit, B, Galvanic cell, and, %, a mercury key for closing and opening the primary circuit. 7.M., A lever moved by an electro-magnet placed in the primary circuit and marking the moment of stimulation. In A, a tuning fork beating 100 times per second is shown recording its vibration on the drum. ~ 52 VETERINARY PHYSIOLOGY In this way such a tracing as is shown in fig. 23 is pro- duced. From this it is evident that the muscle does not contract the very moment it is stimulated, but that a short latent period supervenes between the stimulation and the contrac- tion. In the muscle of the frog attached to a lever this usually occupies about ;3,th second ; but if the change in the muscle is directly photographed without any lever being attached to it, this period is found to be very much shorter. The latent period is followed by the period of contraction. At first it is sudden, but it becomes slower, and finally stops. Its average duration in the frog’s muscle is about zésth second. » AWAWAKnn The period of relaxation follows that of contraction, . Fic. 23.—Trace of Simple Muscle Twitch and it depends essentially (1) showing periods of latency, contrac- er tion, and relaxation ; record of moment 0% the elasticity of the of stimulation (2); and a time record muscle, whereby it tends to made with a tuning fork vibrating 100 yecover its shape when the times per second (8), distorting force is removed, The recovery is therefore at first fast and then slow, and it lasts in the frog’s muscle about +35th second. The whole contraction thus lasts only about 75th second in the frog’s muscle. In mammalian muscle it is much shorter, and in the muscle of insects shorter still. 2nd, Extent of Contraction. While, as will be afterwards considered, the extent of con- traction is modified by the strength of stimulus and the state of the muscle, the total extent of contraction is primarily de- termined by the length of the muscle. If a muscle of two inches contracts to one-half its length, the amount of con- traction is one inch, but if a muscle of four inches contracts _ to the same amount, it shortens by two inches. 3rd. Foree of Contraction is measured by finding what weight the muscle can lift, and the absolute foree of a muscle may be expressed by the weight which is just too great to be ‘lifted. The lifting power of a muscle depends priwarily upon its thickness or sectional area. The absolute force of a muscle THE TISSUES 53! may therefore be expressed per unit of sectional area, In man the absolute force per 1 sq. cm. is from 5000 to 10,000 grams. The force of contraction is, however, modified by so many other conditions that no definite figure can be given. The force of contraction during different parts of the con- traction period may be recorded by making the muscle pull upon a strong spring, so that it can barely shorten. The slight bending of the spring may be magnified and recorded by a long lever, and in this way itis found that the ordinary curve of contraction gives a fair representation of the varia- tions in the force. This method of recording the force of con- traction is sometimes called the isometrie method, in distinc- tion to the isotonic method of letting the muscle act on a light lever. In clinical medicine the DYNAMOMETER is used for measuring the force of muscular contraction. (Practical Physiology.) The contraction of muscles in the body of the mammal may be studied by recording their thickening by Marey’s muscle forceps, in which a tambour placed between the limbs of a pair of forceps is pressed upon by the contraction of a muscle or group of muscles lying between the opposite limbs, and transmits the pressure to another tambour which carries a recording lever. (Practicul Physiology. ) II. The Factors modifying the Contraction 1. Kind of Fibre.—In skeletal muscles the pale fibres contract more rapidly and completely than the red fibres, which contain more sarcoplasm and nuclei. The peculiarities of the con- traction of visceral muscles will be considered later (p. 70). 2. Species of Animali—In vetebrates the contraction of the muscles of warm-blooded animals is more rapid than the con- traction in cold-blooded animals. The most rapidly contracting muscles are met with in insects. 3. State of the Muscle.—(1) Continued Exercise.—If a muscle is made to contract repeatedly, the contractions take place more and more sluggishly. At first each contraction is greater in extent, but, as the contractions go on, the extent diminishes as fatigue becomes manifest, and stimulation finally fails to call 54 VETERINARY PHYSIOLOGY forth any response. This condition is probably caused by the accumulation of the products of activity in the muscle acting as poisons upon its protoplasm, for the same phenomena may be induced by the application of dilute acids and certain other drugs, and may be removed for a time by washing out the muscle with salt solutions (fig. 24). (Practical Physiology.) (2) Temperature—If{ a muscle be warmed above the normal temperature of the animal from which it is taken, all the phases of contraction become more rapid, and the contraction is at first increased in extent, but subsequently decreased in force. If, on the other hand, a muscle be cooled, the various periods are pro- longed. At first the contraction becomes greater and more power- ful, but as the cooling process goes —— ort it becomes less and less, until ~ prin ipayeranies prcinr a finally the most powerful stimuli (1) the first trace ; (2) a trace Produce no effect. Cooling has thus after moderate exercise ; (3) a practically the same effect as fatigue trace when fatigue has been (fig. 24), (Practical Physiology.) saat (3) Many drugs modify muscular contractions, ¢g. veratrin enormously prolongs the relaxation period. (Practical Physiology.) 5. Strength of Stimulus.—A stimulus must have a certain intensity to cause a contraction. The precise strength of this minimum stimulus depends upon the condition of the muscle. The application of stronger and stronger stimuli causes the muscular contraction to become more and more rapid, more and more complete, and more and more powerful. But increase in the contraction is not proportionate to the increase in the stimulus, If the stimulus is steadily increased, the increase in contraction becomes less and less. This may be represented diagrammatically in the accompanying figure, where the con- tinuous lines represent the strength of the stimuli and the dotted lines the extent of the contractions (fig. 25). After a certain strength of stimulus has been reached, further increase of the stimulus does not cause any increase in the muscular contraction. This smallest stimulus which causes the maximum muscular contraction is called the optimum stimulus, = THE TISSUES 55 Increasing the. strength of the stimulus shortens the latent period, but lengthens the periods of contraction and relaxation. 6. Resistance to Contraction—Weight to be Lifted.—Start- ing from the extent of muscular contraction without any load it is found that small weights attached to the muscle actually increase the extent of contraction, but that greater weights Pest Con. A B c Fie, 25.—Influence of increasing the Strength of the Stimulus upon the con- traction of Skeletal Muscle. St., the stimulus; Con., the resulting contraction. A, a subminimal stimulus; B, the minimum adequate stimulus ; C, the optimum stimulus. diminish it, until finally, when a sufficient weight is applied, the muscle no longer contracts at all, but may actually slightly lengthen, because its extensibility is increased during contraction (fig. 26, a). The application of weights to a muscle causes the latent period and period of contraction to be delayed, while it renders T is a b Fic, 26.—Influence of Load on a Muscular Contraction. (a) The effect of in- creasing the load on the extent of contraction ; (b) the effect of load on the course of contraction. the period of relaxation more rapid, and an over-extension may be produced followed by a recovery resembling a small after- contraction (fig. 26, 6). (Practical Physiology.) 7. Eleetrotonus.—As already explained, the passage of a galvanic current through a muscle decreases its contractility at the anode and increases it at the kathode. 56 VETERINARY PHYSIOLOGY 8. Suecessive Stimuli—So far, we have considered the influence of a single stimulus on the shape of muscle. But in nearly every muscular action the contraction of the muscles must last much longer than 5th of a second. How is this continued contraction of muscles produced ? To understand this it is necessary to study the influence of a series of stimuli on muscle. If, to a frog’s muscle which takes -5th of a second to contract and relax, stimuli at the rate of five per second are applied, it is found that a series of simple contractions, each with an tay ar interval of ,th of a second between them, is produced (fig. 27,1). If the stimuli follow one another at the rate of ten per second, a series of simple 1 contractions is still produced, but now with no interval between them. WWW If stimuli be sent more rapidly to \/ the muscle, say at the rate of twelve 2 per second, the second. stimulus will cause a contraction before the contrac- / tion due to the first stimulus has | \ entirely passed off (fig. 27, z). The = second contraction will thus be super- Fic. 27.—Effect of a series of imposed on the first, and it is found Stimuli on Skeletal Muscle. : é (See kext.) that the second contraction is more complete than the first, and the third than the second. But while the second contraction is markedly greater than the first, the third is not so markedly greater than the second, and each succeeding stimulus causes a less and less increase in the degree of contraction until, after a certain number, no further increase takes place, and the degree of con- traction is simply maintained. When the contractions follow one another at such a rate that the relaxation period of the first contraction has begun, but is not completed, before the second contraction takes place, a lever attached to the muscle, and made to write on a moving surface, produces a toothed line. The contraction is not uniform, but is made up of alternate shortenings and lengthenings of the muscle. This constitutes “incomplete tetanus.” If the second stimulus follows the first so rapidly that the 3 THE TISSUES 37 contraction period has not given place to relaxation, then the _ second contraction will be superimposed on the first, the third on the second, and so on continuously and smoothly without any slight relaxations, and thus the lever will describe a smooth line, rising at first rapidly, then more slowly, till a maximum is reached, and being maintained at this till the series of stimuli causing the contraction is removed, or until fatigue causes relaxation of the muscle. This is the condition of “ complete tetanus” (fig. 27, 3). (Practical Physiology.) The rate at which stimuli must follow one another in order to produce a tetanus depends on a large number of factors. Anything which increases the duration of a single contraction renders a smaller number of stimuli per second sufficient to produce a tetanus, and thus all the various factors modifying a single muscular contraction, modify the number of stimuli necessary to produce a tetanus(see p. 53). D’Arsonval has shown that an alternating current with very frequent in- terruptions of about 1,000,000 per second causes no con- traction. Every voluntary contraction of any group of the muscles is probably of the nature of a tetanus; and the question thus arises, at what rate do the stimuli which cause such a tetanus pass from the spinal cord to the muscles ? In a tracing of a continued voluntary contraction, indica- tions of about ten variations per second are to be seen, while the rate of the clonic tremor of the leg which may be produced during fatigue by supporting the weight of the leg on the toes is about ten, backward and forward movement, per second, and in various morbid muscular spasms the rate is about the same. All this would seem to indicate that the number of stimuli which pass to human muscle from the central nervous system is probably about ten per second. It has, however, been found that passing a strong galvanic current into a muscle may lead to rhythmic contraction, and hence it is possible that the contractions of muscle induced by the central nervous system may be caused by a continued discharge from the nerve centres. 58 VETERINARY PHYSIOLOGY IlJ. Mode of Action of Muscles a The skeletal muscles act to produce movements of the body from place to place, or movements of one part of the body on another. This they do by pulling on the bony framework to cause definite movements of the various joints. The muscles are arranged in opposing sets in relationship to each joint—one causing movement in one direction, another in the opposite direction—-and named according to their mode of action, flexors, extensors, adductors, abductors, etc. But in the production of any particular movement—say flexion of the forearm at the elbow—not only are the muscles manifestly causing the movement in contraction, but the opposing group, the extensors, are also in action to guide and direct the force and extent of the movement. This Co-operative Antagonism Fic. 28.—The three types of lever illustrated by the movements at the ankle joint. of groups of muscles is of very great importance, since it. explains many of the results observed in paralysis. Thus, if the extensors of the hand be paralysed, as in lead palsy, it is found impossible to clench the hand although the flexors are intact. Again, if part of the brain which causes flexion of the hand of the monkey be stimulated, and the nerve to the flexors divided, the co-operative action of the extensors brings. about an extension of the hand. In many movements some of the antagonistic muscles are relaxed under the action of their nerves (see p. 41). The muscles round the various joints act on the bones, arranged as a series of levers, of the three classes (fig. 28). Ist Class.—Fulecrum between power and weight. In the ankle this is seen when, by a contraction of the gastrocnemius, we push upon some object with the toes. 2nd Class.—-Weight between fulcrum and power. In rising 2 eee Sey See eee I = a ee THE TISSUES 59 ‘on the toes the base of the metatarsals is the fulcrum, the weight comes at the ankle, and the power on the os calcis. 3rd Class. —Power between fulcrum and weight. In raising a weight placed on the dorsal aspect of the toes by the contrac- tion of the extensors of the foot, we have the weight at the toes, the power at the tarsus, and the fulcrum at the ankle. In the other joints, actions involving the principle of each of these levers may be found. IV. Work of Muscle As a result of the changes in shape, muscle performs its great function of doing mechanical work; and the most important question which has to be considered in regard to muscle, as in regard to other machines, is the amount of work it can do. Since the work done depends upon the weight moved and the distance through which it is moved, the work-doing power of muscle is governed by the force of contraction, which determines the weight Fic. 29.—Influence of the which can be lifted, and by the amount /ensth of @ Muscle upon : the work done, to which the muscle can shorten, for this will govern the distance through which the weight may be moved. It has been already shown that the force of contraction depends chiefly upon the sectional area of a muscle. A thick muscle is stronger than a thinner one. But, on the other hand, the amount of contraction depends upon the length of the muscle, since each muscle can contract only to a fixed proportion of its original length. A glance at the diagram will at once make this plain (fig. 29). The size of the muscle is thus the first great factor which governs its work-doing power. But the many factors influenc- ing the force of muscular contraction also influence the work- doing power of the muscle (see p. 53). One factor requires special consideration, namely, the Load. We have already seen that as the load is increased the extent of contraction is diminished. 60 VETERINARY PHYSLOLOGY The following experiment will illustrate the influence of increasing the load on the work-doing power of a muscle— Load in Space through which Work in Gram, Grams. litted in mm. nm, 0 x 4:5 = 0°0 20 x 30 = 60-0 40 x 2°37 = 94-8 60 x 2°00 = 120°0 80 x 1°75 = 140°0 100 x 12 = 120°0 It will be seen that increasing the load at first increases the amount of work done, but that after a certain weight is reached, it diminishes it. There is, therefore, for every muscle, so far as its working power is concerned, an optimum load. . In studying the amount of work a muscle or set of muscles can do, the element of ¢ime must always be considered. Obviously contracting muscles will do more work in an hour than in a minute. Hence in trying to form any idea of the amount of work a muscle can do, this must be expressed in work units per unit of bulk and per unit of time. The work unit generally employed is the kilogram-metre—the work required to raise one kilogram to the height of one metre against the force of gravity. Various instruments—Ergometers—have been devised for measuring the amount of work done by various groups of muscles under different conditions. The average working capacity of skeletal muscle may be estimated as follows:—A labourer’ who raises 130,000 kilos through one metre during his eight hours of work does a good average day’s work. His muscles weigh about 25 kilos, and thus each gram of his muscle will do 5 kilogram-metres per diem, or 0°06 gram metres per second. When required, much larger amounts of work can be done for short periods. It has been calculated that in the sprint of a 100 yards race, work is done at something like 2 gram metres per second, about thirty times the rate at which a labourer’s muscles work. But to increase the rate at which work is done requires an increase in the expenditure of the energy-yielding materials in greater proportion than the increased work—just THE TISSUES 61 as to increase the speed of a ship or an engine requires an increase of coal consumption in a proportion roughly acipecpone ‘ing to the square of the increased speed. V. Heat Production in Muscle In muscle, as in other machines, by no means the whole of: the energy rendered kinetic is used for the production of mechanical work. In a steam-engine much of the energy is dissipated as heat, and the same loss occurs in muscle. If heat is given off when a muscle contracts, either the muscle itself, or the blood coming from it, will become warmer. Hence to detect such a charge some delicate method of measuring changes of temperature must be employed. The mercurial thermometer is hardly sufficiently sensitive, and, therefore, the thermo-electrical method is most generally employed. Various forms of thermopile may be used. The rise of temperature in a muscle after a single con- traction is extremely small, but after a tetanic contraction, _ lasting for two or three minutes, it is very much greater. The amount of heat produced may be calculated if (a) the weight of the muscle; (0) its temperature before and after contraction ; and (c) the specific heat of muscle, are known. The specific heat of muscle is slightly greater than that of water, but the difference is so slight that it may be disregarded. If, then, a muscle of ten grams had a tempera- _ ture of 15° C. before it was made to contract, and a temperature of 15°05° C. after a period of contraction, then 0°5 gram- _ degrees of heat have been produced ; ~e. heat sufficient to raise _ the temperature of 0:5 gramme of water through 1° C. The ' heat units employed are the small and large calorie—the small calorie the heat required to raise one gram of water | through one degree Centigrade, and the large Calorie—written with a large C—the heat required to raise a kilogram of water _ through one degree Centigrade. The amount of heat produced by muscle in different conditions varies so greatly that it is unnecessary to consider it further. | Relationship of Heat Production to Work Production.— Since it is possible to measure both the mechanical work 62 VETERINARY PHYSIOLOGY done by a muscle and the amount of energy dissipated as heat, it is possible to determine the relationship of these to one another, and thus to compare muscle with other machines as to proportion of energy which is utilised to produce work. To make this comparison it is necessary to be able to convert “work units” into “heat units,” and vice versa. It has been found that 0°45 gram-deyvrees or small calories are equivalent to 1 kilogram-metre. The proportion of work to heat is not constant. By gradually increasing the stimulus both work production and heat production are increased, but the latter is inereased more rapidly, and reaches its maximum sooner. Again, as muscle becomes exhausted, its heat production declines more rapidly than its work production. Exhausted muscle, there- fore, works more economically. If an unloaded muscle is made to contract no work is done and all the energy is given off as heat, and the same thing happens where a muscle is so loaded that it cannot contract when stimulated. But the point of practical importance to determine is— How much of the energy liberated by muscle in normal conditions is usually used for mechanical work, and how much is lost as heat? It will afterwards (p. 339) be shown that ~ all the energy of the body comes from the food, and the amount. of energy yielded by any food may be determimed by burning it in a calorimeter (see p. 341). To determine the energy used in mechanical work some form of work measurer or ergometer may be used—eg. a wheel turned against a measured resistance. By converting the work units of the work thus done into heat units, and subtracting this from the total energy of the food, the energy lost as heat may be determined, and thus the relationship between work produc- tion and heat production may be found. By experiments on men, horses, and dogs, Zuntz has found that about one-third of the energy liberated may, under favourable conditions, be avail- able for mechanical work, while two-thirds is lost as heat. The proportion of energy evolved in mechanical work and heat in normal men has been also studied by Atwater by means of the Respiratory Calorimeter (see p. 408). In these experiments only about six per cent. of the energy was used for mechanical work and the rest was lost as heat. Compared with other THE TISSUES 63 machines, such as steam-engines, muscle must be regarded as an economical worker, and it has the advantage that the heat. liberated is necessary to maintain the temperature at which the chemical changes which are the basis of life can go on. | VI. Electrical Changes in Muscle When a muscle contracts certain electrical changes occur. These may be best studied in the heart, which is a muscle which can be exposed without injury. With other muscles the injury inflicted in isolating them sets up electrical cwrrents «of injury (p. 42). Fic. 30.—To show electric current of action in a muscle (a) compared with that in a galvanic cell (5). The contracting part’of the muscle is shaded. (g) Galvanometer. If one end of a wire be brought in contact with the base of the ventricle by means of a non-polarisable electrode (in which some material which does nut act upon the muscle and is not acted upon by the muscle is in contact with it), and another wire be similarly connected with the apex, and if these wires are led off round a galvanometer, it will be found that with each contraction of the heart an electric current is set up, the one part of- the heart becoming first positive and then negative to the other part. This means that, when the contraction occurs, the part which first contracts becomes of a higher electric potential than the rest of the muscle, so that electricity flows from it to the uncontracted part in the organ, and from the uncontracting part to the contracting part in the wire round the galvano- meter. The contracting part is thus similar to the positive 64 VETERINARY PHYSIOLOGY element of a battery, the zinc; the uncontracting part to the negative element, and the wire coming from the contracting part will, therefore, correspond to the negative pole—that from the uncontracting part to the positive pole. It has now been shown that this current of action occurs along with, and does not precede, the period of contraction. The electric change in contracting muscle may be demonstrated by laying the nerve of one muscle-nerve preparation over the muscle of another muscle-nerve preparation or over the beating heart, when it will be found that the first muscle contracts with each contraction of the second, being stimulated by the current of action. (Practical Physiology.) VII. Extensibility of Muscle The extensibility of muscle is increased during contraction so that the application of a weight causes a greater lengthening than when the muscle is at rest. 3. The Chemieal Changes in Muscle and the Source of the Energy evolved _ Chemical changes are constantly going on in muscle and the study of these chemical changes in resting muscle and in the contracting muscle explains the source of the energy of muscle. Disintegration leads to the liberation of energy and construction leads to the repair of the muscles and to the storage of energy. No part of physiology is of more importance; for it is the chemical changes in muscle which give rise to the great waste products of the body, and it is to make good these losses that fresh nourishment has to be supplied. The chemical changes in muscle therefore govern both the intake ~ and output of matter from the body. By studying the question from a number of different standpoints, and by comparing the results so obtained, a fairly clear conception of the chemical changes and the source of muscular energy has been obtained. 1. Composition of Musele before and after Contraction.— The method which most naturally presents itself is to take THE TISSUES 65 two muscles or groups of muscles corresponding to one another, and to examine the chemistry of one before it has been made to contract, and of the other after it has been contracting for some time. Resting muscle is alkaline; but if an excised muscle, out- side the body, be kept contracting for some time, it becomes acid, and this acidity is due to the appearance of carbon dioxide and sarcolactic acid. Muscle in the body does not become acid, because the alkaline lymph at once neutralises the acid which is produced. Again, after contraction, the glycogen of the muscle is found to be diminished. But the most important change is that the amount of carbon dioxide, CO,, which can be ex- tracted from muscle is very greatly increased. As yet the changes, if any, in the proteins of muscle during contraction have not been fully investigated, and the results of the work accomplished on the xitroyenous extractives, which are formed by the decomposition of the proteins, are not trustworthy. They seem to indicate that these bodies are increased during muscular contraction in the excised muscle. These changes in a muscle may be diagram- matically represented as follows :— + Carbon dioxide. + Sarcolactic acid. + Nitrogenous extractives ? — Glycogen. F The results obtained -by this method of investigation are thus of considerable value, but alone they give us no clear idea of the nature of the chemical changes. 2. Respiration of Excised Muscle.—By enclosing the excised muscle in a closed space containing air of known composition, and by investigating the changes in the components of the air after the muscle has either been kept at rest for some time or made to contract, important light has also been thrown on these chemical changes. It has been found that the resting muscle constantly takes 5 66 VETERINARY PHYSIOLOGY up oxygen from the air round about it, and constantly gives off carbon dioxide. In contracting, more carbon dioxide is given off, and usually the amount of oxygen taken up is also increased (fig. 31). Here we have at once evidence that muscle seosthen and that this process of respiration is increased during muscular activity. The affinity of muscle for oxygen is very great, so great that it can actually take oxygen out of chemical combinations. If alizarin blue be injected into the vein of an animal, the blood be- comes blue, but the muscles remain colourless, having reduced the pigment to a colourless condition. When freely exposed to air after Fic. 31.—Respiration of muscle in a closed death, the blue colour re- chamber. turns. 3. Changes in the Blood passing through Musele.—For the investigation of this the hind legs of a dog have been used. The blood going to one leg, and the blood coming from the other, are collected at the same time. It is found that the blood in passing through the muscles has gained carbon dioxide and lost oxygen. If the muscles be kept contracted, it is further ascertained that the amount of carbon dioxide gained is increased, while usually the amount of oxygen taken up is also increased. This observation confirms the investiga- tions on the changes in the air surrounding a muscle. But the solid constituents of the blood are also changed. If the muscles have been contracting, the blood is found to contain sarcolactic acid probably combined with ammonia. We shall afterwards find that blood contains small quantities of glucose, C,H,,0,. As it passes through muscle it loses some of this, even when the muscle is at rest, and a much larger amount when the muscle has been active. The changes in the proteins of the blood going to and coming from muscle have not been properly investigated. Some observers have obtained resuits which seem to indicate that the amount of fat which is found in the blood is 4 y [ THE TISSUES 67 diminished as the blood passes through the muscles, but whether this diminution is greater during muscular activity has not been studied. MUS ¢ LE. ‘ GLUCOSE eT 1 OXYGEN eet re an fe is ON NSS SARCOLACTIC ACID aoe BLOOD mae oe, CORAETIG Pee Pie et See pt SS CARBON DIOKIDOE Fic. 32.—Exchanges between muscle and blood. Such direct observations on muscle and the blood nourishing it indicate that constant chemical changes are going on when the muscle is at rest. It is constantly giving off carbon dioxide and constantly consuming oxygen, glucose, and possibly fats and proteins. When doing work these chemical changes become more active. We may compare resting muscle in its chemical changes to an engine with its fires banked down. _ Active muscle is comparable to the engine with its fires in full blast. 4. Effects of Muscular Work upon the Excreta.— Another method of study has yielded results of very great value— the investigation of the effects of muscular work upon the excreta. Not only is muscle the most bulky and most constantly active tissue, but it is the tissue in which the most extensive chemical changes occur in the liberation of the energy for work and heat production; and hence, the waste products of the body are chiefly derived from muscle, and their amount and character must afford an indication of the changes in that tissue. This was long ago recognised, but the older experimenters did not sufficiently realise that the excretions are modified 'by the amount and character of the food taken, and hence their results are of little value. In studying the influence of muscular work on the excreta, food must be withheld or must be unvarying during the experiment. 68 VETERINARY PHYSIOLOGY If this precaution is taken, it is found that the excretion — of the various elements composing muscle is modified by muscular work. Attention has chiefly been directed to the variations in the output of earbon and nitrogen, the two most important elements in muscle, the former mainly appearing as carbon dioxide in the expired air, the latter as urea in the urine. It has been found — that if a fasting or underfed animal is made to do work the excretion of both these elements is increased, the carbon proportionately to the work done, the nitrogen in quantities not strictly proportionate to the work, being greater the more _underfed the animal is and the harder the work done, and being less the better nourished the animal is or the less the work that is done (fig. 33, 1). If a lean animal be fed on an exclusively protein diet, the WORK woRK woRK CO2 baal : co, N REST WORK REST WORK REST WORK ; i 2 3 Fic. 33.—To illustrate the influence of Muscular Work upon the Excretion of Carbon and of Nitrogen—(1) in a fasting or underfed animal ; (2) in an animal fed on proteins ; (8) in an animal on a normal diet. excretion of carbon and nitrogen is increased, practically proportionately to the work done (fig. 33, 2). But if the animal be well fed on an ordinary diet, contain- ing proteins, carbohydrates, and fats, the performance of muscular work increases the excretion of carbon proportion- ately to the work done, but may cause only a very slight increase in the excretion of nitrogen (fig. 33, 3). From the increased excretion of nitrogen and carbon the consumption of proteins may be calculated, since proteins contain 16 per cent. of nitrogem and 52 per cent. of carbon— ie, 34 times more carbon than nitrogen. Each gram of nitrogen excreted thus represents the breaking down of 6°25 grams of protein, and it is accompanied by 3°4 grams of carbon, If more carbon is excreted, it must come from carbohydrates:or fat. «Pte gee y we THE TISSUES 69 - _ Proceeding in this way, it is found that in the fasting animal and in the animal fed on proteins, the muscles get their energy chiefly from proteins, but that in an animal on an ordinary diet the muscles get it chiefly from the ecarbo- hydrates and fats of the food. An example of such an investigation may be given. Suppose that an animal during a period of rest excretes daily 10 grams of nitrogen, and that it then does 100,000 kgms. of work, and during the next three days the excretion of nitrogen is raised 2 grams above the 10 per diem. This means that 2x 6:25=12°5 grams of protein has been decom- _ posed. Now the amount of energy which can be liberated from 1 gram of protein has been found to be equivalent to 1738 kems. (kilogrammetres), and therefore the 12°5 grams decomposed in the experiment is sufficient to yield 21,635 kgms. of energy, about 20 per cent. of the total energy expended in the work. The rest of the energy must be derived from fats and carbohydrates. 5. A study of the ordinary diet of animals doing muscular work corroborates the conclusions arrived at by an examina- tion of the excreta. The diet of a horse on hard muscular work consists of something like the following proportions of food constituents per 1000 kilos. of body weight :— Amount. Yielding Calories. Proteins . : : 2,300 9,500 Fats . : 800 7,500 Carbohydrates . . 12,500 50,000 67,000 The energy is here expressed in heat units, Calories—the amount of heat required to raise 1 kilogram of water through 1 degree centigrade. Of the total 67,000 Calories of energy daily taken in the food, only 14 per cent. is derived from proteins, the rest comes from the carbohydrates and fats. Thus during muscular work the three great constituents of the body and of the food—proteins, fats, and carbohydrates —are broken down to liberate their energy, and apparently the muscle tends to use the non-nitrogenous fats and carbo- hydrates in preference to the proteins. Only when forced 70 VETERINARY PHYSIOLOGY to do so does it take a large proportion of its energy from these substances. It may be urged that in athletic training proteins must be a source of energy, since experience has taught that they are of such value. But their great value is as material from which the energy-liberating machine, the muscles, can be built up and increased, so that it can dispose of target and larger quantities of food. Muscle then is a machine which has the power of liberating energy from proteins, fats, and carbohydrates, but it uses proteins more especially in construction and repair. The muscles liberate energy from these substances by breaking them down into simpler molecules, just as a blow causes the disintegration of nitro-glycerine and liberates its stored energy. There is not such a direct oxidation as occurs in the coals in the furnace of an engine, for, if this were so, the consumption of oxygen would always be equivalent to the elimination of carbon dioxide and the other products of disintegration. It has, however, been shown that a frog, deprived of all free oxygen by placing it in the receiver of an air pump and then transferred through mercury to an atmosphere of nitrogen, -still continues to produce carbon dioxide. This means that its oxygen must be intramolecular, must be in the muscle molecule, like the oxygen of nitro- glycerine. Probably the presence of this oxygen is one of the causes of the instability of the molecule. The muscle then takes these substances into itself—makes them part of its molecule—assimilates them before breaking them down. It is not necessary to suppose that all the substances are equally intimately associated with the muscle protoplasm. In all probability the protein becomes much more truly a part of the muscle than the carbohydrates and fats, but with each one of them it is essential that it should come into the domain of the muscle and not simply remain in the blood and lymph, in which it cannot be used. B. Viseeral Muscles In several important respects the visceral muscles differ in their mode of action from the skeletal muscles. oe # Cm ae Le THE TISSUES 71 1. Their connection with nerves is by no means so definite and precise, for, instead of each nerve-fibre ending in a muscle- fibre, the nerves to non-striped muscle form an irregular network upon them, and the muscle-fibre appears to be capable of action, possibly before these nerves have developed in the embryo and when the influence of these nerves has been cut off in the adult. In the intestine the mode of action of the muscles is largely dominated by the plexus of nerves (see p. 385). 2. The great features of the action of visceral muscle are —lst, its tendency to sustained tonic contraction; and 2nd, its spontaneous regular rhythmic contraction and relaxation. ' 1st. The continuous slight tonic contraction is seen in all the visceral muscles ; and, while it may be increased or diminished by the intervention of nerves, it appears to be chiefly an expression of the continuous metabolism of the muscle proto- plasm. 2nd. The rhythmic contractions and relaxations are not equally manifest in all situations, nor are they so continuous ; but they are well marked in the muscles around such hollow viscera as the intestines, bladder, and uterus. Like the tonic contractions, they are to a certain extent independent of nerve action, but are influenced by it. _ These contractions recur at regular intervals of varying duration. Each contraction lasts for a considerable period— sometimes over a minute—and the relaxation is correspond- ingly long. Everything which increases the rate of chemical change increases the rapidity of the rhythm. Thus warming the muscle and the action of a galvanic current have this action. 3. When the muscle is at rest, a contraction may be pro- duced by any of the modes of stimulation which will cause the skeletal muscles to contract; and it may thus be demon- strated that the latent period is very long. 4. Unlike skeletal muscles, the extent of contraction is not increased by increasing the strength of the stimulus. The smallest available stimulus causes the maximum con- traction; but if the same stimulus is repeated at regular intervals the resulting contractions become greater and greater during the application of the first four or five stimuli, 72 VETERINARY PHYSIOLOGY so that the record of a series of contractions has a somewhat stair-like appearance. 5. A series of stimuli do not cause a tetanus, but simply increase the rapidity and force of the individual contractions. Cardiac Musele physiologically resembles other visceral muscles, but its period of contraction is shorter and its rhythm generally more rapid. 4, DEATH OF MUSCLE The death of the muscle is not simultaneous with the death of the individual. For some time after somatic death the muscles remain alive and are capable of contraction under stimulation. Gradually, however, their irritability diminishes and finally disappears. They are then dead, and neerobiotie ehanges begin. The first of these—Rigor Mortis—is a disin- tegrative chemical change whereby carbon dioxide and sarcolactic acid are set free, and, at the same time, the soluble myosinogen changes to the insoluble myosin and the muscle becomes contracted, less extensile, less elastic, and more opaque. The contraction is a feeble one, and since it affects flexors and extensors equally, it does not generally alter the position of the limbs, although it may sometimes do so. As these changes occur, heat is evolved and the muscles become warmer. The time of onset of rigor varies with the condition of the muscles. If they have been very active just before death stiffening tends to appear rapidly. It lasts for a period which varies with the species of animal and with the condition of the muscles, and as it disappears the muscles again become soft, and the body becomes limp. In all probability this latter change is due to a solution of the myosin by an enzyme like that of the stomach—pepsin— which seems to exist in all the tissues. This can act only in the presence of an acid, and the appearance of sarcolactic acid, therefore, allows it to come into play. THE TISSUES 73 Il. NERVE It is through the nerves that our surroundings act upon us, and through nerves that our muscles are made to respond appropriately to the surrounding conditions. 1. Structure and Development In unicellular organisms changes in the surroundings act directly on the cell protoplasm, e.g. an amoeba, when touched, draws itself together. But, even in these simplest organisms, certain kinds of external conditions will produce one kind of change, while others will produce a different one, as has been shown in considering unilateral stimulation (p. 15). Even among unicellular organisms—e.g. among the infusoria— animals are found in which the cell is differentiated into a receiving and reacting part. Poteriodendron, a little infusorian sitting in a cup-like frame, con- sists of a long process or cilium extending up from a cell while a contractile myoid attaches the cell to the floor of the cup. When the cilium is touched the myoid con- tracts, and draws the creature into the pro- tection of its covering. In more complex multicellular organism, ég. mm medusa, the different parts are con- nected to one another by a network of protoplasmié strands, which bring each part ; : cow iaee Fic. 34. — Poterio- into relationship with the others, and thus Dada caciitees secure co-ordinate reaction to any stimulus. trate the first stage A similar network exists and performs im- in the evolution of portant functions in the wall of the aliment- 2 NN"? vrata ary canal of vertebrates. aon In more highly organised animals, where the reaction has to be more definitely appropriate to the surrounding conditions, and where the complexity of the mechanism involved is greater, there is a development by which special conditions at special parts of the surface each lead to special reactions. This is brought about by the establishment of a nervous system—a mechanism which may be compared to a series of shunting stations between the receptive mechanism on the surface and the reacting mechan- 74 VETERINARY PHYSIOLOGY ism—the muscles, glands, etc. To form this, a part of the epithelial covering of the embryo sinks inwards as a canal composed of the surface cells, and these cells form functional — connections with the surface on the one hand and with the — reacting structures on the other. At first the cells composing this tube are undifferentiated and alike, but later some of them throw out processes towards the surface and others towards é a b ' > tee * £ Fie. 35.—To show a receiving (c) and a reacting Neuron (a), each with dendrites at its extremities, and their connection to one another through a Synapsis (8). the reacting structures, and these are connected, not by actual ~ continuity, but by coming in close relationship to one another in a series of branching processes, forming a synapsis (fig. 35). Each of the units so formed has been called a neuron; and a neuron may be defined as one of the cells with all its processes which build up the nervous system. These neurons may be Fic, 36.—(a) A Nerve Cell with Nissl’s granules ; (d) a similar cell showing changes on section of its axon, divided into the receiving and reacting series, but in structure they are alike. The shape and characters of the eells, and their position upon the processes of the neuron—the fibres—vary greatly, but they have all the following characters in common:—They are nucleated protoplasts, the protoplasm of which shows a well- marked network, in the meshes of which a material which ‘ THE TISSUES 75 stains deeply with basic stains, and which seems to be used up during the activity of the neuron, may accumulate in granules. The granules formed of this material are generally known as Nissl’s granules (fig. 36). These cells give off at least one process, which continues for some distance, as the axon. Frequently other processes are given off, which may either pass away as fibres, or may, while still in close proximity to the cell, form a branching system of dendrites. The axons end in much the same manner, so that _all the processes are essentially the same. These processes are fibrillated, and the fibrillee may be traced through the protoplasm _ of the cells (fig. 37). In many cases the dendrites show little buds or gemmules upon their course, and, according to some observers, it is through these that one neuron is brought into definite relationship at one time with one set of neurons, and at another with other adjacent neurons. There is also some evidence that the dendrites as a whole may expand and contract, and thus become connected with those of adjacent neurons. Axon.—The axon process, as it Fic. 37.— A Nerve Cell highly _ passes away from the cell, becomes magnified to show passage of a Nerve Fibre, and acquires one ag PEON NSE or two coverings. wii 1. A thin transparent membrane, the primitive sheath or -neurilemma, is present in all peripheral nerves. Between it and the axis cylinder there are a number of nuclei surrounded by a small quantity of protoplasm, the nerve corpuscles. The mode of origin of these is unknown. Fibres with only this sheath have a grey colour, and may be called non-medullated fibres. They are abundant in the visceral nerves. 2. A thick white sheath—the medullary sheath or white sheath of Schwann—which gives the white colour to most of the nerves of the body, appears somewhat late in the develop- ment of many nerve fibres. It lies between the primitive sheath with the nerve corpuscles and the axon. It is not continuous, but is interrupted at regular intervals by con- 76 VETERINARY PHYSIOLOGY strictions of the neurilemma at the nodes of Ranvier (fig. 38). It is composed of a sponge-work or felt-work of a horn-like material—neuro-keratin—the meshes of which are filled with a peculiar fatty material. The nerve fibres run together in bundles to constitute the — nerves of the body, and each bundle is surrounded by a dense fibrous sheath, the perineurium. When a bundle divides, each branch has a sheath of perineurium, and in many nerves this sheath is continued, as the sheath of Henle, on to the single fibres which are ultimately branched off from the nerve. 2. Chemistry of Nerve The chemistry of neuron cells and their processes has been deduced from a study of the chemistry of the grey matter of the brain where they preponderate, while the chemistry of the white sheath is indicated by the analyses of the white substance of the brain, which consists chiefly of medullated fibres. Fic. 38.—Pieces of two white Nerve Fibres. The grey matter contains over 80 per cent. of water. The solids consist of rather less than 10 per cent. of proteins. Two globulins, one coagulating at 47°C. and the other at 73° to 75°C., and a nucleo-proteid coagulating at 56° to 60°C., have been isolated. Lecithin and cholesterin, each to about 3 per cent., are the other important constituents. The white matter contains only about 70 per cent. of water. The proteins, similar to those in the grey matter, constitute between 7 and 8 per cent. Lecithin occurs in ~ about the same amount as in the grey matter, but cholesterin constitutes no less than 15 or 16 per cent. From the fatty material of the white sheaths various complex substances have been isolated. The most abundant of these has been called protagon. It yields stearic acid; hence it is allied to the fats, and it contains nitrogen and phosphorus. Its constitution is not known. Along with protagon, or as a result of its decomposition, lecithin occurs. THE TISSUES 77 This is a fat in which one of the acid radicles is replaced by phosphoric acid linked to cholin. Fatty acid. Glycerol | Fatty acid. Phosphoric acid. | Cholin., H H |OH | ||| Hs HO—C—C-|-N . NEURO-MUSCULAR MECHANISM I2I Between the hyaloid membrane and the choroid is the retina (Ret.). This is an expansion of the optic nerve, which enters the eye at 3 to 4 mm. to the inner side of the posterior optic axis (fig. 53). The white nerve fibres pass through the sclerotic, through the choroid, and through the retina, to form the white optic disc, and then losing their white sheath, they spread out in all directions over the front of the retina, to form its first layer—the layer of nerve Jibres (1). These nerve fibres take ie origin from a layer of nerve cells (2) behind them, forming the second layer. The dendrites of these cells arborise with the dendrites for the next set of neurons in the third layer, the internal molecular layer (3). The cells of these neurons are placed. in the next or fourth layer, the inner nuclear layer (4), and from these cells, processes pass backwards to form syn- apses in the fifth, or owter molecular layer (5), with the dendrites of the terminal neurons. These terminal neurons have their cells in the sixth or outer nuclear layer (6) of the retina, and they pass backwards and end in two special kinds of terminations in the seventh layer of the retina—the ,,, 5s. Dispret ofa Soo" rods and cones (7). These structures tion through the Retina are composed of two segments—a stained by Golgi’s method. somewhat barrel-shaped basal piece, For description, see text. : ; (From VAN GEHUCHTEN.) and a transparent terminal part which in the rods is cylindrical and in the cones is pointed. Over the central spot of the eye in man or apes there are no rods, DW 23 y Yj ZY Y Z G Y) SSS SAT ‘but the cones lie side by side, and the other layers of the retina are thinned out. The rods and cones are imbedded in the last or eighth layer of the retina—the layer of pigment cells, or tapetum nigrum. The retina in front thins out, but the tapetum nigrum, along with another layer of epithelial cells representing the rest of the retinal structures, is continued forwards over the ciliary processes and over the back of the iris. 122 VETERINARY PHYSIOLOGY The blood vessels of the retina enter in the middle of the optic nerve, and run out and branch in the anterior layer of the retina. The interior of the eye may be examined by the Ophthal- moscope, which consists essentially of a small mirror from which light can be reflected’into the back of the eye, with a smal] hole in the centre through which the observer can study the illuminated part of the chamber. (Practical Physiology.) C. Physiology The study of vision may be taken up in the following order :— 1. The mode of formation of pictures on the nerve struc- tures (retina) of the eye. (1) One eye (monocular vision). A. The method in which rays of light are focussed (di- optric mechanism). B. Stimulation of the retina. (2) Two eyes (binocular vision). 2. The conduetion of the nerve impulses from the retina to the brain. 3. The position and mode of action of the parts of the brain in which the changes are set up which accompany visual sensations (the visual centre). 1. THE MODE OF FORMATION OF PICTURES UPON THE RETINA (L.) MonocuLar VISION A. The Dioptrie Mechanism Distant Vision.—The eye may be compared to a photographic camera, having in front a lens, or lenses, to focus the light upon the sensitive screen behind (fig. 54). The picture is formed on the screen by the luminous rays from each point outside being concentrated to a point upon the screen. This is. brought about by refraction of light as it passes through the various. media of the eye—the cornea, aqueous, crystalline lens, and NEURO-MUSCULAR MECHANISM 123 vitreous. The refractive indices of these, compared with air as unity, may be expressed as follows :— Cornea . . 133 Lens . . 1:45 Aqueous - 1:33 Vitreous . 1:33 Thus light passes from a medium of one refractive index into a medium of another refractive index— 1. At the anterior surface of the cornea ; 2. At the anterior surface of the lens; 3. At the posterior surface of the lens; and at these surfaces it is bent. The degree of bending depends upon—Ist, The difference of refractive index; 2nd, Fig. 54.—To show how parallel rays are brought to a focus on the retina by refraction at the three surfaces (a), anterior surface of the cornea; (0), anterior surface of the lens ; and (c), posterior surface of the lens. The obliquity with which the light hits the surface. This will vary with the convexity of the lens—being greater the greater the convexity. The posterior surface of the lens has the greatest convexity, with a radius of 6 mm. The anterior surface of the cornea has the next greatest, with a radius of 8 mm. The anterior surface of the lens has the least, with a radius of 10 mm. A ray of light passing obliquely through these media will be bent at the three surfaces. These media, in fact, form the physiological lens, a com- 124 VETERINARY PHYSIOLOGY pound lens composed of a convexo-concave part in front, the cornea and aqueous, and a biconvex part, the crystalline lens behind. In the resting normal eye (emmetropie eye) the principal focus is exactly the distance behind the lens at which the layer of rods and cones in the retina is situated, and thus it is upon these that light coming from luminous points at a distance is focussed. Positive Accommodation.—If an object is brought nearer and nearer to the eye, the rays of light entering the eye become more and more divergent, and if the eye be set so that rays from a distance— Fic. 58.—To show that rays 1.¢. parallel rays—are focussed, then from distant and near objects yays from a nearer object will be are not focussed on theretina focussed behind the retina, and a at the same time. i : clear image will not be formed (fig. 55). This means that near and far objects eannot be distinetly seen at the same time, a fact. which can be readily demon- strated by Scheiner’s Experiment. (Practical Physiology.) Make two pin-holes in a card so near that they fall within the diameter of the pupil. Close one eye, and hold the holes in front of the other. Get someone to hold a needle against a sheet of white paper at about three yards from the eye, and hold another needle in the same line at about a foot from the eye. When the near needle is looked at the far needle becomes double (fig. 56). Fie. 56.—-Scheiner’s Experiment represents rays from the near needle and - - - - rays from the far needle. Objects may be brought nearer and nearer to the eye, and yet be seen distinctly up to a certain point, the near point of accommodation within which they cannot be sharply focussed upon the retina. This, however, requires a change in the lens arrangement of the eye, and this change, beginning when the object comes within about 6 metres (the far point of acecom- he ~—-e NEURO-MUSCULAR MECHANISM 125 modation), becomes greater and greater, till it can increase no further when the near point is reached. The change is called positive accommodation, and it consists in an increased curvature of the anterior surface of the lens. This may be proved by examining the images formed from the three refracting surfaces (Sanson’s images), when it will be found that the image from the anterior surface of the lens be- comes smaller and brighter when the eye m eae 0-8 nists Fic. 57.—Mechanism of Positive Accommoda- object. The examina- tion. The continuous lines show the parts tion of these images is in negative accommodation, the dotted lines facilitated by the use the positive accommodation. of the Phakoscope. (Practical Physiology.) Positive accommodation is brought about by contraction of the ciliary muscle (see p. 119), which pulls forward the ciliary processes to which the hyaloid membrane is attached, and thus relaxes the suspensory ligament of the lens and the front of the lens capsule, and allows the natural elasticity of the lens to bulge it forward (fig. 57). This change of positive accommodation is accompanied by a contraction of the pupil due to contraction of the sphincter pupille muscle. By this means the more divergent peripheral rays which would have been focussed behind the central ones to produce a blurred image are cut off, and spherical aberration is prevented. The muscles acting in positive accommodation—the ciliary and sphincter pupille (fig. 57, C.. and S.P.)—are supplied by the third cranial nerve (ZZ/.), while the dilator pupille is supplied by fibres passing up the sympathetic of the neck. The centre for the third nerve is situated under the aqueduct of Sylvius, and separate parts preside over the ciliary muscle and the sphincter pupille (see p. 180). The sphineter centre is reflexly called into action, and the pupil contracted—-lst, When strong light falls on the retina and stimulates the optic nerve. In this way the retina is protected against over - stimulation; 2nd, When the image upon the retina becomes blurred as the object approaches the 126 VETERINARY PHYSIOLOGY eye. At the same time the centre for the ciliary muscle is also called into play to produce accommodation. The centre for dilatation of the pupil is situated in the medulla oblongata. Like the centre of the sphincter it may be reflexly excited, stimulation of ingoing nerves causing a dilatation of the pupil when the medulla is intact (fig. 58), The dilator fibres pass down the lateral columns of the spinal cord to the lower cervical and upper dorsal region, where they arborise round cells in the anterior horn. From these, fibres pass by the anterior root of the second (2D. N.), possibly Gin / 8 mi SS == PITT SN a € Lae we wm enw oo orn IDN 2D.N. i Pe IDN Fic. 58.—Nerve Supply of the Intrinsic Muscles of the Eye (see text). also of the first and third dorsal nerves, and, passing up through the inferior cervical ganglion, run on to the superior ganglion, where they arborise round cells which send axons to the Gasserian ganglion of the fifth cranial nerve (V.), and from there the fibres pass along the ophthalmic division and its long ciliary branches to the dilator fibres of the iris (D.P.). The importance of the course taken by these dilator fibres is considerable, because diseases of the spinal cord in the lower cervical and upper dorsal region (the eilio-spinal region), and tumours in the upper mediastinum, may interfere with their action, and by stimulating cause chronic dilatation of the pupil, or by paralysing prevent dilatation of the pupil. Since the ee NEURO-MUSCULAR MECHANISM 127 dilator muscle fibres of the pupil have not been demonstrated in all animals, it has been suggested that the nerve may act by inhibiting the sphincter pupillee, but the evidence on this point is not conclusive. A peripheral mechanism exists in the muscle of the iris which may act independently of the central nervous system, as may be seen in the eye of a decapitated cat, and various drugs act directly upon it—e.g. physostigmin causing a contraction of the pupil by acting upon the nerve endings and pilocarpine by acting on the muscle fibres. Adrenalin causes a dilatation by acting on the nerve endings and atropine by acting in the same way. Imperfections of the Dioptrie Mechanism (1) Hypermetropia.—The eye is sometimes too short from before backwards, and thus, in the resting state, parallel rays are focussed behind the retina, i and in order to see even a distant object the individual has to use his positive accom- modation. As the object is approached to the eye it is b focussed with greater and greater difficulty, and the near Fic. 59.—To show the cause of Astig- point is further off than in the matism. 4, a slight curvature of : the cornea in the vertical plane ; B, emmetropic eye (fig. 58, C). more marked curvature in the hori- The long-sighted eye differs zoutal plane, leading to rays from from the slightly presbyopic b—a horizontal line being focussed in the fact that not merely gett cae ivergent, but also paralle rays, are unfocussed in the resting state. (2) Myopia.—In certain cases the antero-posterior diameter of the eye is too long, and as a result parallel rays—rays from distant objects—are focussed in front of the retina, and it is only when the object is brought near to the eye that a perfect image can be formed. In such an eye no positive accommodation is needed till the object is well within the normal far point, and the near point is approximated to the eye. 128 VETERINARY PHYSIOLOGY (3) Astigmatism is a defect due to unequal curvature of one or more of the refracting surfaces in different planes. If the vertical curvature of the cornea is greater than the hori- zontal, when a vertical line is looked at, horizontal lines will not be sharply focussed at the same time (fig. 59). B. Stimulation of the Retina 1. Reaction to Varying Illuminations.—(1) The Blind Spot. —At the entrance of the optic nerve the retina cannot be stimulated because there are no end organs in that situation. A y Wi (\ ig Al} ee: a Fic. 60.—Methods of demonstrating the Blind Spot : a, by Mariotte’s experiment ; 6, by moving a pencil along a sheet of paper. The existence of such a blind spot may be demonstrated— 1st, By Mariotte’s experiment, which consists in making two marks in a horizontal line on a piece of paper, closing the left eye, fixing the right eye on the left-hand mark with the paper held at a distance from the eye, when both marks are visible, then bringing the paper nearer to the eye, when the right-hand mark will first disappear, and, when the paper is brought still nearer, will reappear (fig. 60, a) (Practical . Physiology); 2nd, By making a mark on a sheet of paper, and with the head close to the paper moving the point of a pencil wa Peery v the sclerotic coat of the eye the shadow of NEURO-MUSCULAR MECHANISM 129 to the right for the right eye, or to the left for the left eye, when the point will disappear and again reappear (fig. 60, 0). (Practical Physiology.) The eye is blind for all objects in the shaded region. By resolving the various triangles the distance of the blind spot from the central spot of the eye may be determined (3 to 4 mm.), and the diameter of the blind spot (15 mm.) may also be ascertained. (2) The Field of Vision—The rest of the retina forward to the edge is capable of stimulation, and the whole range of objects which can be seen at one time con- stitutes the field of vision, and it may be indicated by the optical angle subtended by /.. that range of objects. As the distance from a the eye increases the field of vision extends. : (3) The layer of the retina capable of 3 stimulation is the layer of rods and cones. re This is proved by the experiment of Pur- @ 3 kinje’s images, which depends upon the fact oe that if a ray of light is thrown through the blood vessels stimulates a subjacent layer (fig. 61, ¢), and these vessels appear yg. 61.—To show that as a series of wriggling lines on the surface looked at. If the light be moved, the lines seem to move, and, by resolving the tri- angles, it is possible to calculate the distance behind the vessels of the part stimulated, and this distance is found to correspond to the thickness of the retina. The shadows of the blood vessels are not seen in ordinary vision, because they then fall upon parts of the hindmost layer of the retina is stim- ulated. (Purkinje’s Images.) a, source of light; 0b, blood vessel of retina ; c, shadow of vessel on rods and cones; d, image of shadow mentally projected on to the wall. the retina which are insensitive. (Practical Physiology.) The cones are the more: specialised elements of the retina, and they react more particularly to bright light, which soon exhausts the rods. The rods, again, react to faint illumination. This explains why it is that, when we go out into a dark night from a brightly lighted room, we at first can see nothing, but after a time, when the rods have recovered, we begin to see objects more distinctly. It also explains why it is that if we direct our eye to a small faintly shining star we may fail to see “, 130 VETERINARY PHYSIOLOGY it, because its image falls on cones alone, while it becomes visible if the eye be directed slightly from it. The rods seem incapable of giving rise to colour sensation, and when the solar spectrum is looked at in a very dim light it appears as a greyish band of illumination with the red end wanting, because the slow red vibrations fail to stimulate the rods. It is because the blue end of the spectrum is the more active in faint illumination that the illusion of a moonlight scene may be got by looking through a blue glass, while looking through a yellow glass gives the idea of sunlight and brilliant illumination. (Practical Physiology.) ; (4) Modes of Stimulation.—The rods and°cones are generally stimulated by the ethereal light vibration, but they may: be stimulated by mechanical violence or by sudden changes in an electric current. But, however stimulated, the kind of sensation is always of the same kind—a visual sensation (see p- 118). (5) Of the nature of the changes in the retina when stimu- lated we know little. But we know that— ) 1st. Under the influence of light the cells of the tapetum nigrum expand forward between the rods and cones. 2nd. A purple pigment which exists in the outer segment of the rods is bleached. Even although there is no purple in the cones, which alone occupy the sensitive central spot of the eye, this change in colour suggests that a chemical de- composition accompanies stimulation. 3rd. Electrical changes occur. (6) Fatigue.—If a bright light be looked at for some time, the part of the retina acted upon is temporarily blinded, and hence when the eye is taken off the bright light a dark spot is seen. This is sometimes called a negative after image. When coloured lights are used the phenomena of complemental colours are produced (p. 135). (Practical Physiology.) Sometimes the stimulation of the retina or of the brain neurons connected with it may last after the withdrawal of the stimulus, when a continuance of the sensation—a positive after image—is seen. This may be observed if, on opening the eyes in the morning, a well-illuminated window is looked at and the eyes closed. A persisting image of the window may be seen. , NEURO-MUSCULAR MECHANISM 131 2. The Power of Loecalising the Source or Direction of Illumination.—This may be determined in the same way as in studying the sense of touch—by finding how near two points may be stimulated and still give rise to a double sensation. 3. Colour Sensation—Physies of Light Vibration.—Physi- cally the various colours are essentially different rates of vibration of the ether, and only a comparatively small range of these vibrations stimulate the retina. The slowest acting vibrations are at the rate of about 435 billions per second, while the fastest are not more than 764 billions—the relation- ship of the slowest to the fastest is something like four to seven. The apparent colour of objects is due to the fact that they absorb certain parts of the spectrum, and either transmit onwards other parts, or reflect other parts. The vast variety of colours which are perceived in nature is due to the fact that the pure spectral colours are modified by the brightness of illumination, and by admiature with other parts of the spectrum. Thus a surface which in bright sunlight appears of a brilliant red, becomes maroon, and finally brown and black, as the light fades. Again, a pure red when diluted with all the spectrum—ie. with white light—becomes. pink as it becomes less and less saturated. (Practical Physiology.) Physiology of Colour Sensation.—1l. The peripheral part of the retina is colour blind—is incapable of acting so as to produce colour sensations. This may be shown by means of the perimeter and coloured chalks. Until the chalk is brought well within the field of vision its colour cannot be made out. As the image of the chalk travels in along the retina it is found that yellow and blue can be dis- tinguished before red and green—that is, that there is a zone of retina which is blind to red and green, but which ean distinguish blue and yellow. Only the central part of the retina is capable of being stimulated by all colours. These zones are not sharply defined, and vary in extent with the size and brightness of the coloured image (fig. 62). (Practical Physiology.) 2. While the various sensations which we call colour are generally produced by vibrations of different lengths falling 132 VETERINARY PHYSIOLOGY on the retina, colour sensations are also produced in various other ways. (a) By mechanical stimulation of the retina.. By pressing on the eyeball as far back as possible a yellow ring, or part of a ring, may often be seen. (Practical Physiology.) ena (6) Simple alternation of white and black upon the retina may produce colour sensation, as when a dise of paper marked with lines is rotated rapidly before the eye. (Prac- tical Physiology.) ; 3. By allowing different parts of the spectrum to fall upon oe oe O \\s the eye at the same time, it is ogra SOS / ¢ possible to produce either a eee a gh SS sensation of white or of some other part of the spectrum. (Practical Physiology.) To pro- duce a sensation of white from two or three different parts of Fic. 62.—Distribution of Colour Sen- the Spectrum & due propels sation in relationship to the surface of each part must be taken, of the retina (Colour Perimeter). A since different parts have differ- indicates coe ieee aici ae ent sensational activity. This — nad by white and Black: 3th may be represented by a plot blue and yellow ; and C, thecentral ting out the various parts of part capable also of stimulation by the spectrum on a curve, and sage a joining them to a central spot by means of lines. The length of the line then represents the relative sensitive activity of the particular part of the spectrum to which it passes (fig. 63). This means that by different modes of stimulation of the retina the same sensation may be produced. The sensation of orange may be produced either when vibrations at about 580 billions per second fall on the eye, or when two sets of vibrations, one about 640 and one about 560 billions, reach it. By no possible physical combination of the two is it possible to produce the intermediate rate of vibration. The sensation of eolour, therefore, depends upon the nature of the change set up in the retina, and not upon the condition producing that change. NEURO-MUSCULAR MECHANISM 133 What we call colours are particular changes in our conscious- ness which accompany particular changes in our brain neurons produced by particular changes set up in the retina, in what- ever way these changes may have been produced. 4, After looking for some time at any one colour, on remov- ing the colour another appears in its place—the complemental eolour. If the first colour is— Red, the second will be green blue ; Orange, x = blue; Green, = a pink ; Yellow, : - indigo blue; and vice versd. (Practical Physiology. ) Theories of Colour Vision.—1. From consideration of aie per- ipheral colour-blind zone of the retina and of the more limited area giving sensations only of blue and yellow when stimu- lated, and of the most limited central part giv- ing also sensations of red and green, it would seem that some special substance or substances must exist in each of Fic. 63. these areas which by its or their stimulation give rise to the various sensations. 2. Considered along with this, the phenomena of comple- mental colours suggest the possibility of there being one substance which when undergoing one change, say breaking down, produces yellow, and when undergoing another change, say building up, produces blue, and another substance which when undergoing one change produces red, and another change produces green. If such a view be correct, it becomes almost necessary to postulate the existence of another substance which when stimulated gives rise to sensations which we call white. Or there might be four different substances, one when changed giving rise to yellow, another to blue, another to red, and another to green. When the substance giving 134 VETERINARY PHYSIOLOGY the sensation of yellow is used up, then the parts stimulated by the rest of the spectrum would react to white light and give a complemental colour, and so on through the other substances. It has also been suggested that the facts may be explained on the assumption that there are ¢hree substances in the retina, one more especially stimulated by the red rays but also acted on by the others, one chiefly stimulated by the green rays, and one chiefly acted on by the blue rays. Such theories, however, do not call for consideration from the ordinary student. Colour-blindness—While everyone is colour blind at the periphery of the retina, a certain proportion of people—about 5 per cent.—are unable to distinguish reds and green, even at the centre of the retina, It is impossible to say how far this condition exists in the lower animals. (II.) BryocuLar Vision In most of the lower animals, the field of vision of each eye is separate and distinct at all times, but in the horse and dog the two eyes can be directed forwards so that the fields of vision overlap as they always do in man and in apes. When in this position, the combined action of the eyes affords a means of determining the distance and solidity of near objects. 1, Distanee of Near Objects.—As an object is approached, the two eyes have to be turned forwards by the internal recti muscles, and by the degree of contraction of these, an estimation of the distance is made. 2. Solidity of an Object.—If the object is near, a slightly different picture is given on each retina, and experience has taught us that this stereoscopic vision indicates solidity. Movements of: Eyeballs—To secure the harmonious action of the two eyes, it is necessary that they should be freely movable. Each eye in its orbit is a ball and socket joint in which the eyeball moves round every axis (fig. 64). The axis of the eye. (a) in man and in monkeys, is set obliquely to the axis of the orbit (0), and the centre of rotation is behind the centre of the ball. The movements are produced by three pairs of muscles. NEURO-MUSCULAR MECHANISM 135 1. The internal and external recti (7.2. and Ex.R.). 2. The superior and inferior recti acting along the lines indicated (S.£.). 3. The superior and inferior obliques acting in the line (S.00.). The internal rectus rotates the pupil inwards. » external ,, - - outwards. , superior ,, 3 P upwards and inwards. P ‘ { downwards and in- » inferior _,, is A l wards. ; : downwards and out- ,», superior oblique ,, ; wards. fanferior , 5 m upwards and outwards. Nose In Ob, SR a EX: Ink. S. Ob. In R. Fic. 64.—The left Eyeball in the Fic. 65.—The Movements of the Pupil Orbit, with the Muscles acting caused by the various Muscles of the upon it. (Man and Ape.) Eye. (Right Eye.) (Man and Ape.) In the horse, dog, and other similar animals the eye is set more nearly in the axis of the orbit, and the obliques do not pass backwards upon the ball, but act more purely as rotators; the superior oblique swinging the outer angle of the pupil upwards and inwards, the inferior oblique downwards and in- wards. The superior and inferior recti move the pupil more directly upwards and downwards. In the horse and other herbivora a retractor oculi muscle is inserted all round the ball inside these muscles just described, and it can retract the eye in the orbit, and at the same time pushes forward the fatty tissue to which the nictitating mem- brane is attached and thus thrusts this over the front of the eye. 136 VETERINARY PHYSIOLOGY When the eyes are allowed to sweep over a landscape or any series of objects, or when these move rapidly past the eyes or the eyes rapidly past them, as in travelling by train, the axes are directed in a series of glances to different points, and the succession of pictures thus got gives the idea of the con- tinuous series of objects. This jerking movement of the eyes " may be well seen in a passenger looking out of a Se carriage - in motion, Nervous Mechanism.—A some- what complex nervous mechan- ism presides over these various movements of the eyes. All the muscles are supplied by the third cranial nerve, except the superior oblique, which is supplied by the fourth nerve, and the external rectus, which is supplied by the | sixth nerve (fig. 66; see also fig. 95, p. 179). Fic. 66.—The Nervous Mechan- ism presiding over the combined movements of the two Eyes in man and apes. J.R., internal rectus; £.#., external rectus ; C.C., convergent centre acting on the internal recti through the nuclei of the third nerve; S.0., superior olive (centre for lateral divergence) acting on the external rectus of the same side through the nucleus of the sixth, and on the internal rectus of the opposite side through the nucleus of the third ; Z., ear. The centres for the third and fourth nerves are situated in the floor of the aqueduct of Sylvius under the corpora quadrigemina, while the centre for the sixth is in the pons Varolii (fig. 96, p. 181, and fig. 95, p. 179). The various centres are joined by bands of nerve fibres which pass between the sixth and fourth and third centres, and in part at least cross the middle line. A combined mechanism, each part of which acts harmoni- ously with the other parts, thus presides over the ocular movements, and this mechanism is controlled by impulses constantly received from the two retine, from the ear and from the brain. Thus in convergence of the optic axes the parts of the nuclei of the third nerves which supply the internal recti muscles must act harmoniously together, and hence a mechanism to direct this convergence may be NEURO-MUSCULAR MECHANISM 137 postulated. In lateral deviation of the eyes that part of the nucleus of the third nerve which presides over the internal rectus of one side acts harmoniously with the sixth nerve supplying the external rectus of the other side, and hence it may be supposed that a directing mechanism for lateral devia- tion exists possibly in the superior olive. Similarly a centre or ' centres presiding over the movements of the eyes in a vertical plain may be supposed to exist. The movements of the eyes involve not merely the con- traction of definite muscles, but also the co-ordinated inhibition of others (see p. 138). 2. CONNECTIONS OF THE EYES WITH THE CENTRAL NERVOUS SYSTEM From each eye the optic nerve extends backwards and inwards to join the other optic nerve at the chiasma. A partial crossing of the fibres takes place in the chiasma, the extent of decussation varying in different animals and being fairly extensive in the horse. From the chiasma the two optic tracts pass upwards round the crura cerebri to end in two divisions— 1. A posterior division passing to the anterior corpora quadrigemina on the same side (fig. 67, 4.C.Q.). 2. An anterior division running to the geniculate body on the posterior aspect of the thalamus opticus (fig. 73, Op. Th.). The fibres of the posterior termination of the optic tract end in synapses with neurons in the corpora quadrigemina, and the fibres of these neurons pass downwards and control the oculo-motor mechanism already described (fig. 66, p. 136). The fibres of the anterior division make synapses with other neurons in the posterior part of the thalamus, and these neurons send their fibres backwards to the occipital lobe of the brain, where they connect with the cortical neurons (fig. 67, Oce., p. 138). 3. THE VISUAL CENTRE A response to stimulation on the part of the neurons in the occipital lobe of the brain (p. 201) is the physical basis of 138 VETERINARY PHYSIOLOGY visual sensations, and hence this part of the brain is called the visual centre. Usually the visual centre is stimulated by ‘ \ j { } 4 ’R ~ i \ , : ~, —— > - ee et an or Occ Occ. L. _Fic. 67.—The Connections of the Retine with the Central Nervous System in man and apes. &, retine ; Ch., chiasma leading to optic tract; Op.Th., optic thalamus ; 4.C.Q., anterior corpora quadrigemina ; Oc.J/., oculo-motor mechanism (fig. 66) ; Occ.Z., occipital lobe of the cerebrum ; Teg., tegmentum. : changes in the chain of neurons passing from the retina and set in action by retinal changes; but direct stimulation of the occipital lobe may induce visual sensations. (c) For VIBRATION oF AIR Sense of Hearing 1, General Considerations While through the tactile mechanism differences of pressure act as stimuli, through the ear certain vibratory changes of pressure stimulate and may affect the consciousness. Even NEURO-MUSCULAR MECHANISM 139 simple organisms, devoid of any special organ of hearing, may be affected by vibratory changes, and in fish it is difficult to be certain how far such vibrations produce their effect through the ear or through the body generally. But in higher vertebrates it is chiefly through the ears that they act. In these there is a special arrangement by which the vibrations of the air are converted into vibrations of a fluid in a sac situated in the side of the head into which the free ends of neurons project (fig. 68). ENT. Fie, 68.—Diagram of the Ear. Zx.M., external meatus; 7y., tympanic mem- | brane; m., malleus; 7, incus; s., stapes; f.o., fenestra ovalis; fi7., fenestra rotunda ; Hn. 7., Eustachian tube; v., vestibule with the utricle and saccule ; s.c., semicircular canal ; Coch., cochlea. The importance of such a mechanism in the anterior part of the animal in warning it of danger or making it aware of the presence of its prey is manifest. In mammals the organ of hearing consists of an external, a middle, and an internal ear. The first is to conduct the vibrations of the air to the second, in which these vibrations produce to and fro movements of a bony lever, by which the fluid in the third is alternately compressed and relaxed. 2. External Ear The structure of this presents no point of special physio- logical interest. In lower animals the pinna is under the control of muscles, and is of use in determining the direction from which sound comes. 140 VETERINARY PHYSIOLOGY 3. Middle Ear . The object of the middle ear is to overcome the mechanical difficulty of changing vibrations of air into vibrations of a fluid. It consists of a chamber, the tympanic eavity, placed outside of the petrous part of the temporal bone (fig. 68). Its outer wall is formed by a membrane, the membrana tympani (Ty.), which is attached to a ring of bone. Its inner wall presents two openings into the internal ear—the fenestra ovalis (/.0.), an oval opening, situated anteriorly and above, and the fenestra rotunda (/.7.), a round opening placed below and behind. Throughout life these are closed, the former by the foot of the stapes, which is attached to the margin of — the hole by a membrane, and the latter by a membrane. The posterior wall shows openings into the mastoid cells, and presents a smal] bony projection which transmits the stapedius musele. The anterior wall has above a bony canal carrying the tensor tympani muscle, and below this the canal of the Eustachian tube, which communicates with the posterior nares (fig. 68, Ent.). In the tympanic cavity are three ossicles—the malleus (m.), ineus (7.), and stapes (s.), forming a chain between the membrana tympani and the fenestra ovalis. The handle of the malleus is attached to the membrana tympani, and each _ time a wave of condensation hits the membrane, it drives in the handle of the malleus. This, by a small process, pushes inwards the long process of the ineus, which thrusts the stapes into the fenestra ovalis, and thus increases the pressure in the enclosed fluid of the internal ear. The fenestra rotunda (f:7.) with its membrane acts as a safety valve. The bones rotate round an antero-posterior axis passing through the heads of the malleus and incus. They thus form a lever with the arm to which the power is applied—the handle of the malleus—longer than the other arm. The advantage of this is that, while the range of movement of the stapes in the fenestra ovalis is reduced, its force is proportionately increased. The range of movement is still further controlled by the stapedius muscle,-which twists the stapes in the fenestra. This muscle seems to act when loud sounds fall on the ear, and NEURO-MUSCULAR MECHANISM 141 when its nerve supply, derived from the facial nerve, is paralysed, such sounds are heard with painful intensity. _ If the membrana tympani is violently forced outwards by closing the nose and mouth and forcing air up the Eustachian tube, the incus and stapes do not accompany the malleus and membrane, since the malleo-incal articulation becomes unlocked. The membrana tympani is so loosely slung that it has no proper note of its own, and responds to a very large range of vibrations. By the attachment to it of the handle of the malleus it is well damped, and stops vibrating as soon as waves of con- densation and rarefaction have ceased to fall upon it. The tensor tympani muscle, supplied by the fifth cranial nerve, has some action in favouring the vibration of the membrane, and its paralysis diminishes the Fic, 69.—Transverse Section through acuteness of hearing. The Eustachian tube has a double function. It allows the escape of mucus from the middle ear, and it allows the entrance of air, so that Cartilaginous lower part of Eustachian Tube to show the Cartilaginous Arch cut across and the the pressure is kept equal on both sides of the membrana tympani. Its lower part is generally closed, but opens in the act of swallowing. It is surrounded by an arch of cartilage, to one side of which fibres of the tensor palati are attached, so that when this muscle acts in swallowing, the arch of cartilage is drawn down and flattened, and the tube opened up (fig. 69). When the Eustachian tube gets occluded, as a result of eatarrh of the pharynx, the oxygen in the middle ear is absorbed by the tissues, and the pressure falls. As a result, the membrane is driven inwards by the atmospheric pressure, and does not readily vibrate, and hearing is impaired. way in which it is pulled down and the tube opened in swal- lowing (shaded). 4. Internal Ear The internal ear is a somewhat complex cavity in the petrous part of the temporal bone, the osseous labyrinth. It is filled with fluid, the perilymph. It consists of a central space, the vestibule (V.), into which the fenestra ovalis opens. From the 142 VETERINARY PHYSIOLOGY anterior part of this, a canal makes two and a half turns round a central pillar, and then, turning sharply on itself, makes the same number of turns down again, and ends at the fenestra rotunda. This is the osseous cochlea (fig. 68, Coch.). The ascending and descending canals are separated from one an- — other, partly by a bony plate, partly by a membranous partition —the basilar membrane. At the base, the bony lamella is — broad, but at the apex its place is chiefly taken by the membrane, which measures at the apex more than ten times its width at the base. From the posterior and superior aspect of the vestibule a mechanism i a with hearing has been evolved. Three semicircular canals (fig. 70), each . with a swelling at one end, open into the vestibule. One runs in the Gas horizontal plane, and has the swell- R fo. i ing or ampulla anteriorly (fig. 70, Suge. -he.). The other two run in verti- cal planes placed obliquely to the ust middle plane, as is indicated in fig. the Semicircular Canals to one : . another. f.c., horizontalcanal; “9 The anterior or superior canal- s.c., superior canal; p.c., pos- (8¢.), has its ampulla in front and terior canal. the posterior (p.c.) has its ampulla behind. They join together, and enter the vestibule by a common orifice. In the perilymph of the bine labyrinth lies a complex membranous bag, the membranous labyrinth. In the vestibule this is divided into two little sacs, the utricle, related to the semicircular canals, and the saeeule, - related to the cochlea. They are joined together by a slender canal. From the saccule comes off a canal which runs into the cochlea upon the basilar membrane, forming a middle channel between the other two, the seala media or membranous. cochlea. This terminates blindly at the .apex. From the utricle a membranous canal extends into each of the bony semicircular canals, being provided with an ampulla, which nearly fills up the bony ampulla, while the canal portion is small, and occupies only a small part of the bony canal (fig. 68, p. 139). In the membranous cochlea the lining cells form the organ Fic. 70.—The seiisuaken of OO NEURO-MUSCULAR MECHANISM 143 of Corti (fig. 71). This is set upon the basilar membrane, and consists from within, outwards, of—1s¢. A set of elongated sup- porting cells; 2nd. A row of columnar cells, with short, stiff, hair-like processes projecting from their free border; 3rd. The inner rods of Corti, each of which may be compared to an ulnar bone attached by its terminal end, and fitting on to the heads of the outer rods; 4¢/. The outer rods of Corti, each resembling a swan’s head and neck—the neck attached to the basilar membrane, and the back of the head fitting Fic. 71.—Transverse Section through one turn of the Cochlea to show the Organ of Corti on the Basilar Membrane. S.M., scala media; S.V. scala vestibuli ; 8.7, scala tympani. into the hollow surface of the inner rods; 5th. Several rows of outer hair cells, with some spindle-shaped cells among them; 6¢i. The outer supporting cells; 7/h. Lying over the inner and outer hair cells is the membrana reticularis, resembling a net, through the meshes of which the hairs project; 8¢i. Arching over this organ is a homogeneous membrane—the membrana tectoria. The membranous labyrinth is attached to the inner wall of the bony labyrinth at certain points, through which fibres of the VIII. nerve to the terminal organs enter it. One set of fibres goes to the utricle and to the ampulle. A quite 144 VETERINARY PHYSIOLOGY independent set, the true auditory nerve, goes to the saccule and the cochlea. The membranous labyrinth has an oater fibrous coat, and inside this a homogeneous layer which is markedly thick- ened where the nerves enter it. It is lined by flattened epithelium, which becomes columnar, and is covered with stiff hair-like processes over the thickenings at the entrance of the nerves. On the top of these hair-like processes lies a little mass of calcareous nodules imbedded in a mucus-like substance, the otoliths. In the fish and some lower animals the otoliths are large structures. The terminal neurons of both the vestibule and the cochlea end in dendrites between the hair cells, and the cell of these neurons is upon their course to the medulla. 5d. Connection with the Central Nervous System The VIII. nerve is essentially double, consisting of a dorsal cochlear or auditory part, and a ventral labyrinthine or vestibular part. Cochlear Root (fig. 72).—This is the true nerve of hear-. ing. Its fibres (Coch.k.) begin in dendrites between the hair cells of the organ of Corti, have a cell upon their course, and when they enter the medulla branch into two divisions, which end either in the tuberculum acusticum or the nucleus accessorius (iV.Acc.), where they form synapses. From the cells, axons pass (a) to the oculo-motor mechanism of the’ same side and the opposite side (JV. vi.), and (6) up to the cerebrum (CB.) of the same and of the opposite side. Vestibular Root (fig. 73).—The fibres of this root take origin in dendrites between the cells of the macule in the ampullee of the semi-circular canals and of the saccule, and have their nerve cells upon their course (Ves/t.), As they enter the medulla they divide into two, forming an ascending and a descending branch. (1) The ascending branch sends fibres on to the cerebrum (CB.), and to the superior vermis of the cerebellum (CBL.). These fibres give off collaterals to the nucleus of Deiters (NV. Deit.), from the cells of which fibres pass, which divide, some running on the same side, some on the opposite side; one branch passing up to the oculo-motor NEURO-MUSCULAR MECHANISM 145 mechanism (JV. vi.), the other passing down the spinal cord to send collaterals to the cells in the grey matter. (2) The Fic. 72.—Connections of Cochlea with Central Nervous System. Coch.R., cochlear root of eighth nerve ; NV. Acc., tuberculum acusticum and nucleus accessorius sending fibres to the cerebrum (CB.) and to the oculo-motor mechanism (JV. vi.) ; CBL., cerebellum. descending branch forms connections with the medullary nuclei as it passes down. 6. The Auditory Centre and the Physiology of Hearing The qualities of sound which can be distinguished by the sense of hearing are loudness—amplitude of vibration; piteh —rate of vibration; and quality—the character of the sound given by the over-tones. The perception of this last is essentially a perception of pitch. Loudness.—It is easy to understand how the peripheral neurons in the internal ear are more powerfully stimulated 10 146 VETERINARY PHYSIOLOGY by the greater variations in the degree of pressure which are produced by more powerful aerial waves, and how the greater Oo TD wie Mire gt ~ ® . ere ete ee s ) ‘ ' , ' ' ‘ ’ ‘ og? ' = ' ‘ EYE. Cecus w MEO. wee we a 8 a ee 8 ee ee oe eee BERS nw ee eee CELLS OF ANT. HORN Ce ‘4 Fic. 73.—Connections of Semicireular Canals with Central Nervous System, Ves. R., Vestibular root of eighth nerve sending fibres upwards to OB. (cerebrum) and CBZ. (cerebellum), downwards to the centre in medulla oblongata (Med.), and to Deiters’ nucleus (NV. Dett.), from which fibres pass to the oculo-motor mechanism (NV.v7i.) and to the centres in the anterior horn of the spinal cord, stimulation of the receptive centre in the brain will be accompanied by a sensation of greater loudness. Piteh—A study of the structure of the cochlea seems to show a mechanism well suited to afford a means of estimating NEURO-MUSCULAR MECHANISM 147 the pitch of a note and the existence of over-tones. The fibres of the basilar membrane may be compared to the strings of a piano, each one of which, or each set of which, will be made to vibrate by a particular note. D. PROPRIO-CEPTIVE MECHANISM OF THE HEAD Labyrinthine Sense Just as the reflex response of the limbs-to external stimuli leads to a stimulation of the proprio-ceptive mechanism in the muscles and joints which plays an important part in guiding the subsequent movement (see p. 107), so the reflex response to visual stimuli in the muscles of the head and neck brings into play a delicate proprio-ceptive mechanism developed from the internal ear which has an important action in guiding the movements of the body as a whole, and very specially in guiding the co- ordinated movements of the eye muscles. This is the labyrinthine mechanism consisting of the utricles and the semi-circular canals. The strueture of this mechanism has been described on p. 142. The mode of action may be analysed by a study of the sensations which accompany its activity. That there is no special mechanism making us aware of uniform movement is proved by the fact that we are not conscious of whirling through space with the earth’s surface, and that in a smoothly running train we lose all sense of forward movement. It is only as the train starts or stops that we have a sensation of movement or retardation. The same thing has been demonstrated by strapping a man to a table rotating smoothly round a vertical axis and setting the table spinning. A sense of rotation is experienced as the table starts, but is lost when the movement becomes uniform. Stopping the table gives rise to a sensation of being rotated in the opposite direction. | The semicircular canals are the mechanism which act in this way. They are arranged in pairs in the twoears. The two horizontal canals are in a horizontal plane, the superior canal of one side and the posterior canal of the other are in parallel ‘planes oblique to the mesial plane of the body (fig. 74, a). The horizontal canals may be considered as forming the are 148 VETERINARY PHYSIOLOGY of a circle with an ampulla at each end. The superior canal of one side has its ampulla in front, while its twin—the posterior of the opposite side—hus its ampulla behind, and they together form the arc of a circle with an ampulla at each end (fig. 74, a). The membranous canals are very narrow, and occupy but a small part of the osseous canals. The membranous ampulle are large and almost fill the osseous ampulle (fig. 74, 0). If the head is moved in any plane, certain changes will be set up in the ampulle towards which the head is moving, and converse changes in the ampulle at the other end of the arc of the circle. If, for example, the head is suddenly turned to the right, the inertia of the endolymph and perilymph tend to make Ox : ; Fic, 74.—(a) hee of the semicircular canals on the two sides ; (b) bony and mennbranoas canal and ampulla to illustrate their mode of action. them lag behind. Thus the endolymph in the ampulla of the left horizontal canal will tend to flow into the canal, but the canal is so small that it will merely accumulate in the ampulla, and thus a high pressure will be produced (fig. 74, 6 + +). The perilymph will tend to lag behind, and a low pressure will result outside (fig 74, 6 —). The converse will take place in the opposite horizontal canal. When the movement is continued the pressures will be readjusted, and, on stopping the movement, the opposite con- ditions will be induced, and a sensation of moving in an opposite direction will be experienced. In forward movement, the two superior canals have the pressure of endolymph increased in their ampullee—in backward movement this occurs in the two posterior canals. In nodding to the right the superior and posterior canals of the right ear undergo this change. In all probability the utricle with its otoliths mounted on the top of the hair cells also plays a a i i a) N EURO-MUSCULAR MECHANISM 149 part in the labyrinthine mechanism. As the head is moved the pressure of the otolith will change from one part of the macula to another, and may thus give different stimuli for different positions of the head. The importance of this labyrinthine mechanism in muscular co-ordination is shown by the effect of its destruction. This leads to inco-ordinate movements of the eyes, head, and limbs, and to loss of tone in the muscles. When injury to the labyrinth is sudden, as when a “knock-out” blow is received on the chin, driving the condyles of the lower jaw against the petrous part of the temporal, the general loss of muscular tone may be so complete that the individual collapses. The muscles chiefly under the tonic influence of this mechanism are those of the neck and trunk, and the extensor and abductor muscles of the limbs of the same side. Just as the eye dominates the movements of the muscles of the body through the cerebrum (see p. 142), so the labyrinthine mechanism dominates the ¢onus of the muscles as a whole through the cerebellum (see p. 177). 5. THE CONNECTIONS BETWEEN THE RECEIVING AND REACTING MECHANISMS 1, THE SPINAL NERVES The connections between the peripheral receiving mechan- isms and the central nervous system, the activity of which leads to the appropriate reaction of muscles, have been in part studied in connection with the various special senses. The main connecting channels between the peripheral receiv- ing organs and the central nervous system on the one hand, and the central nervous system and the reacting structures on the other, may now be considered more generally. These connections are seen in their most typical arrange- ment in the spinal nerves, a pair of which comes off, one on each side, from each level of the spinal cord, and passes outwards between the vertebre. General Arrangement These nerves may be classified as ingoing and outgoing, and they may be divided into those connected with the 150 VETERINARY PHYSIOLOGY body wall and its “oe and those connected with the viscera. A dorsal root (PR) comes off from the dorso-lateral aspect of the cord and has a swelling upon it, the ganglion of the dorsal root. It joins a ventral root (4.#.) coming from the ventro-lateral margin. These form the spinal nerve which is distributed to the body wall. Lying in front of ‘this is a swelling or ganglion (S./.) joined to the nerve by two roots, a white ramus (W.#). and a grey ramus (G.2.); and from this a nerve extends towards the viscera (V.N.). Before this nerve reaches its final distribution it passes through another ganglion. Roots of the Spinal Nerves.—The dorsal root is the great ingoing channel to the spinal cord, and ‘the ventral root is the great outgoing channel. Section of a series of dorsal roots leads to (a) loss of sensation in the structures from which the fibres come, and (6) to a loss of muscular co-ordina- tion, as a result of cutting off the afferent impressions con- nected with the kinesthetic sense (p. 102). As a result of this section, the parts of the fibres cut off from the cells of the ganglia on the dorsal root die and degenerate (see p. 85). Therefore, if the root is cut inside the ganglion, the degeneration extends inwards. and up the dorsal columns of the cord, and if it is cut outside, the degeneration passes outwards to the periphery. Section of the ventral root causes paralysis of the muscles and other structures supplied by the outgoing fibres, and ue fibres die and degenerate. The nerve to the somatopleur or body wall (S.1.) is composed of incoming and outgoing fibres. Ist. Incoming fibres are medullated and take origin in the various peripheral sense organs. As they pass through the ganglion on the dorsal root each fibre is connected by a side branch with a nerve cell —the trophic centre of the neuron—and it then enters the spinal cord, and either passes to the dorso-lateral column, or forms synapses in the cord (see p. 87). 2nd. Outgoing fibres are medullated, and take origin from the large cells in the ventral horn of the grey matter of the cord and pass on to be connected with muscle fibres by end plates or to gland cells by less definite synapses. The nerve to the viscera or splanechnopleur (V..V.), and to NEURO-MUSCULAR MECHANISM I51 the involuntary structures in the somatopleur, contains—1s¢. Incoming Fibres.—These take origin either in definite periph- eral structures, such as Pacinian corpuscles, or in some less defined endings, and as medullated fibres pass through the various ganglia, and have their cell stations in the ganglion on the ventral root. The nerve fibres coming from Pacinian bodies and from muscles, like the similar fibres in the somatic branch, are large and are connected with large cells in the spinal ganglion, and they become myelinated at the same time. A set of smaller fibres similar to those coming from the skin in the somatic branch are also found in the visceral branch, and they seem to be connected with smaller cells in the ganglion. OTD eae, = PR ie . - ‘, = ~ oe ~ Fic. 75.—Structure of a Typical Spinal Nerve. P.#., dorsal root with ganglion; 4.R., ventral root; S./., ganglion of sympathetic chain ; W.&., its white ramus; G.R., its grey ramus; V.N., visceral nerve with collateral ganglion ; S.N., somatic nerve. 2nd. The Outgoing Fibres are characterised by their small size, take origin chiefly in a lateral column of cells, which is well developed in the dorsal region of the cord, and pass out as medullated fibres by the ventral root. From this they pass by the white root to a sympathetic ganglion, whence they may proceed in one of two different ways (fig. 75). (a) They may form synapses with cells, and fibres from these cells may pass— 1. Outwards with the splanchnic nerves; or, 2. Back into the spinal nerve by the grey root, and so down the somatic nerve to blood vessels, muscles of the hairs, sweat glands, ete. The ganglia from which fibres pass back into spinal nerve are known as lateral ganglia. 152 VETERINARY PHYSIOLOGY (6) They may pass through these ganglia on to one more peripherally situated in which they form synapses and are . continued onwards, These ganglia from which fibres do not pass back are called collateral ganglia. Before their first interruption they are termed pre-ganglionic fibres, after their interruption post-ganglionic, The various fibres after their interruption proceed as non- medullated or grey fibres to their termination, where they join a network of anastomosing fibres with cells—a sort of terminal ganglion. Many drugs have a special action on the terminal ganglia, e.g. apocodein paralyses them, while adrenalin —the extract of the medullary part of the suprarenals—stimu- lates certain of them. The interruption of fibres in PERSE or their passage through these structures, has been determined by taking advantage of the fact that nicotine in one per cent. solution when painted on a ganglion poisons the synapses but does not influence the fibres. Hence, when a ganglion is painted with nicotine, if stimulation of the fibres on its proximal side produces an effect, it is proved that the break is not in that ganglion. The arrangement of epieritie, protopathic and deep fibres in the neuron and plexuses is considered at p. 170. Distribution A, SoMATIC FIBRES (a) Outgoing Fibres—The course of these must be studied in the dissecting-room, (6) Ingoing Fibres—The fibres passing in by each pair of nerves come from zones of skin encircling the body. These are, however, interrupted by the limbs. Each limb may be considered to be an outgrowth at right angles to the trunk, composed of a pre-axial and post-axial part. B. SPLANCHNIC FIBRES (a) The Outgoing. Fibres may be divided into (fig. 76) — A. The Thoracico-Abdominal Fibres, which come out in the NEURO-MUSCULAR MECHANISM 153 aia. middle region¥o! the spinal cord and pass through the lateral ganglia of the sympathetic chain— (1) Head and Neck—These leave the spinal cord by the upper dorsal nerves and pass upwards in the sympathetic cord of the neck to the superior cervical ganglion where they Head Thorax =- f-y-- <, - =e e ‘ \ fl A A a ty --Gp--6)----=-4p—_—__—_ 6B Q)-- Epo L $ \ 4 u) : eS H * Le | ‘ : i * ‘ i : : i : airs Abdomen Pélvis” Fic. 76.—Scheme of distribution of Splanchnic Nerves. have their cell stations. From these, fibres are distributed to the parts supplied. The chief functions of these fibres are—lst. Vaso-constrictor to the vessels of the face and head; 2nd, Pupilo-dilator (see p. 126); 37d. Motor to the muscle of Miller; 4th. Secretory to the salivary glands, lachrymal 154 VETERINARY PHYSIOLOGY gland, and sweat glands. The course of these fibres is of importance in medicine, since tumours in the upper part of the thorax may press upon them. (2) Thoraa.—tThe fibres to the thoracie organs also come off in the upper dorsal nerves, have their cell stations in the stellate ganglion, and pass to the heart and lungs. (3) Abdomen.—These fibres come off in the lower dorsal and upper lumbar nerves. They course through the lateral ganglia and form synapses in the collateral ganglia of the ~ abdomen—the solar plexus and the superior and inferior mesenteric ganglia. From these they are distributed to the abdominal organs, being vaso-constrictor to the vessels, in- hibitory to the muscles of the stomach and intestine, and possibly secretory to the pancreas. _ (4) Pelvis—The fibres for the pelvis leave the cord by the lower dorsal and upper lumbar nerves, and have their cell stations in the inferior mesenteric ganglia, from which they run in the hypogastric nerves to the pelvic ganglia. They are vaso-constrictor, inhibitory to the colon, and motor to the bladder, uterus and vagina and the retractor penis. (5) Fore Limb.—These fibres, coming out by the upper and mid dorsal nerves, have their synapses in the sympathetie ganglia of the sympathetic chain, and passing back into the spinal nerves by the grey rami, course to the blood vessels, hairs, and sweat glands of the limb. (6) Hind Limb.—tThe fibres take origin from the lower dorsal and upper lumbar nerves, have their cell stations in the lateral ganglia, and pass to the leg in the same way as do the fibres to the arm. B. The Cranial and Sacral Fibres—These pass out from the upper and lower ends of the cord, and they do not pass through the lateral ganglia but have their cell station in some of the collateral ganglia. (1) The ¢hird cranial nerve carries fibres which have their synapses in the ciliary ganglion, and pass on to the sphincter pupillz and ciliary muscle. (2) The seventh nerve carries fibres through the chorda tympani to cell stations in the submaxillary and sublingual ganglia. These are secretory to the submaxillary and sub- lingual glands. NEURO-MUSCULAR MECHANISM 155 (3) The ninth nerve sends fibres to the parotid gland, which have their cell station in the otic ganglion. (4) The vagus sends inhibitory fibres to the heart, which form synapses in the cardiac plexus. It also sends motor fibres to the cesophagus and stomach, which, in some animals at least, have the cell stations in the’ ganglion of the trunk. ; (5) The nervi erigentes come off from the second and third sacral nerves, and pass to the hypogastric plexus near the bladder where the fibres have their cell stations. They are the vaso-dilator nerves to the pelvic organs, inhibit the retractor penis, and are motor to the bladder, colon, and rectum. (6) Ingoing Fibres.—The course of these from the viscera is not so clearly known; but they appear to enter the main nerve largely by the white rami. In the normal condition stimulation of their peripheral endings does not lead to modifications of consciousness, and is therefore not accom- panied by pain. But in abnormal conditions painful sensa- tions are produced. In some cases abnormal stimulation of visceral nerves leads to painful sensations referred to the cutaneous distribution of the spinal nerve with which they are connected. Thus, disease of the heart is often accompanied by pain in the distribution of the upper dorsal nerves in the left arm, with which the visceral fibres to the heart are associated. 156 VETERINARY PHYSIOLOGY 2. THE CENTRAL NERVOUS SYSTEM SPINAL CoRD AND BRAIN The anatomy and histology of each part of the central nervous system should be mastered before its physiology is studied. An outline sufficient to make the description of the physiology intelligible is all that is given here. 2oe ekki ; Pa CORPORA 4 QUABRIGEMINA. Fic. 77.—Mesial Section through the Brain and upper part of the Spinal Cord to show the positions at which the sections figured in later diagrams have been made. Cord, p. 157, fig. 78; Medulla, figs, 84 and 85 ; Pons, fig. 87, p. 178 ; Corpora, fig. 90, p. 179. A. SPINAL CORD Structure THE spinal cord is a more or less cylindrical mass of nerve tissue which passes from the base of the brain down the vertebral canal. There are two enlargements upon it, one in the cervical region, one in the lumbar region, and from these Fig. 78.—Cross Section of the Spinal Cord through the Second Dorsal Segment, to show disposition of grey and white matter. P., dorsal horn; 4., ventral horn with large cells; /.Z., intermedio-lateral horn with small cells; Z.C., Lockhart Clarke’s column of cells; P.M. and P.L. dorso-median and dorso-lateral columns; V.C., direct cerebellar tract Ase, and Desc. Ant. Lat., ascending and descending ventro-lateral or spino- ventral tracts ; B.B., basis bundles ; C.Py., crossed pyramidal tract ; 0. Py., direct pyramidal tract. On opposite side, tracts which degenerate head- wards are marked with horizontal lines ; tracts degenerating from the head with vertical lines. (After Bruce.) the nerves to the arms and legs come off. A fine central canal runs down the middle, and the two sides are almost completely separated from one another by a ventral and a dorsal mesial fissure (fig. 78). Each half is composed of a core of grey matter arranged in two processes or horns— 157 158 VETERINARY PHYSIOLOGY the ventral and dorsal horns (A. and P.)—which divide the white matter surrounding the grey into a dorsal, a lateral, and a ventral column. In the dorsal region a lateral horn of grey matter projects into the lateral column (ZZ.). The grey matter on each side is joined to that of the opposite side by bands of grey matter; the ventral and dorsal grey commissures, one below and one above the central canal. The grey matter is composed very largely of cells and synapses of neurons supported by branching neuroglia cells. The cells of the grey matter are largest and most numerous in the ventral horn, where they constitute the cells from which the majority of nerve fibres come off. In the dorsal region a group of cells in the lateral horn, the intermedio- lateral cells, give’ off visceral fibres (Z.Z.). In the dorsal region also a set of cells lie on the mesial aspect of the - dorsal horn constituting the cells of Lockhart Clarke (Z.C.). The white substance is composed of medullated nerve fibres in which the neurilemmal sheath is absent. The fibres chiefly course up and down the cord, and some run in a horizontal direction :—1. The fibres of the spinal nerves; 2. Fibres pass- ing from grey to white matter; 3. Fibres joining the two sides of the cord in front of the ventral grey commissure forming the white commissure. Functions The spinal cord is the great mechanism of reflex action, ' and the great channel of conducticn between the brain and the peripheral structures, A. REFLEX FUNCTIONS If the brain of such an animal as a frog be destroyed, the animal lies prone on its belly and immovable for any length of time; but the legs tend to be drawn up alongside the body, and the muscles are in a state of slight tonic contraction very different from the flaccid condition found after destruc- tion of the cord. The study of the spinal reflexes (p. 90) has ~ shown that the animal has the power of reflex movements with definite co-ordination of is muscles, but it has no power of balancing itself, and manifests no spontaneous movements, SPINAL CORD 159 These reflex functions of the cord in mammals have been very fully investigated by Sherrington in dogs and cats in which the cerebrum has been separated from the rest of the nervous system, and his results have been considered on p. 92. On suspending such animals in the normal horizontal position, with legs dependent, it is found that all four legs are in a state of tonic slight extension as they are in supporting the body. This he terms “ decerebration rigidity.” It appears to be due to impulses passing down from the semicircular canal. mechan- ism, which reinforce the spinal reflex arcs. That this is so is shown by the fact that section of the posterior roots of the External rotators of hip. Hamstrings. Calf muscles and extrinsic muscles of foot. Intrinsic muscles of foot. Muscles of bladder and urethra. Extrinsic muscles of foot. Intrinsic muscles of foot. Levator and sphincter ani. Fic. 79.—The Groups of Cells in the Ventral Horn of grey matter at the level of the 2nd, 3rd, and 4th sacral nerves. (From BRvuCE.) spinal nerves removes the rigidity. Stimulation of definite areas of skin at once causes the animal to strike a special attitude (p. 95). Thus stimulation of the left fore paw produces the attitude of walking, which is assumed normally when that paw reaches the ground. Stimulation of the left pinna produces the attitude assumed if the animal were turning away from the stimulus. The anatomical connection between the different levels of the cord involved in such reflexes has been demonstrated by Sherrington by keeping a dog in which the spinal cord is cut in the neck till all the down-going tracts below the point of section have completely degenerated so as to leave a clean 160 VETERINARY PHYSIOLOGY slate, and then cutting the cord at a lower level when the proprio-spinal fibres connecting the different levels of the cord degenerate. They are chiefly situated in the lateral columns of the cord. In man the cutaneous reflexes connected with various groups of skeletal muscles are definitely associated with different levels of the cord. Reflex actions in connection with various visceral museles are “also connected with the spinal cord. Many of these are complex reflexes involving inhibition of certain muscles and in- creased action of others, some visceral, some skeletal. The best marked of these are the reflex acts of micturition (p. 427), * Fic. 80.—The neuro-muscular mechanism concerned in the knee jerk, and the time of the knee jerk (4./.) compared with the time of a reflex action (A.8.). defecation (p. 376), erection, and ejaculation (p. 434). - The lumbar enlargement is the part of the cord involved. The synapses in the cord are not only capable of acting reflexly to set up definite contractions in muscles, but they also exercise a constant tonie action upon them, due to the constant inflow of incoming impressions (p. 42). When this tonic action is interfered with by any condition which interferes with the integrity of the reflex arc, the effect of directly stimulating a muscle is diminished. This is very well seen in the contraction of the quadriceps extensor femoris which occurs when the ligamentum patelle is struck sharply, causing a kick at the knee joint—the knee jerk (fig. 80). When the reflex i i — ORE 5 enw CM FP SPINAL CORD 161 arc in the lower lumbar region of the cord is interfered with, the knee jerk is diminished or is absent, and when the activity | of the are is increased, by the removal of the influence of the brain, the jerk is increased. That the jerk is probably not a true reflex is shown by the fact that the latent period is very much shorter than that of most reflex actions (fig. 80). The reflex arc, however, is necessary for the tonus. This tonus is increased by tension of the muscles and also by fatigue of the nervous system, and this condition may lead to cramp. The degeneration of special groups of cells—the anterior horn of grey matter—which follows amputation of the leg at different levels seems to indicate that the various groups of cells have definite connections with individual muscles (see fig. 79). B. ConpuctTine Patus The study of the course of the ingoing and outgoing fibres in the cord has proved to be one of great difficulty. It is possible in animals to divide the cord completely or to divide one half, or to divide any one of the white columns, and to observe any loss of muscular action which may ensue and to trace the course of degenerated fibres; but to determine what changes in the sensibility have resulted on animals, unable to give any expres- sion to their sensations, is practically impossible. On the other hand, the clinical method of carefully studying the changes in sensibility during life and determining post- mortem the exact site of the lesion which has produced these symptoms has generally proved somewhat unsatisfactory on account of the want of precision in the lesions produced by _ injury or disease of the cord. A. Ingoing Fibres.—In the cord there seems to be a sorting of fibres into those the stimulation of which can effect consciousness and give rise to sensations and those which simply produce reflex responses. Thus, those kinesthetic fibres which give rise to the muscle and joint sense run up in the posterior columns and cross above the cord, while those which are concerned with the unconscious adjustment of muscular action through the agency of the cerebellum are shunted off through the synapses in Clarke’s column to the direct cerebellar tract of the same side and the spino-ventral cerebellar tract of the opposite side. In all ; II 162 VETERINARY PHYSIOLOGY probability fibres connected with the tactile sense are similarly sorted out, and possibly even those connected with the conduc- tion of thermal and of nocuous stimuli. As regards the course of the Jngoing Fibres, the most valuable advance has been made by the study of the results of lesions of the cord in the light of the observations of Head and his co-workers on the course of these fibres in the peripheral nerves (see p. 111). While section of a cutaneous nerve destroys epicritic and protopathic sensibility, and leaves deep sensibility intact, lesions of the spinal cord are apt to interrupt the passage of the impulses concerned -with different /inds of sensation. Thus, section of a cutaneous nerve abolishes any sensation of pain from the surface of the skin, but leaves the possibility of the sensation of pain being produced by severe pressure on deep structures, while a lesion of the cord is apt to interrupt the passage of all stimuli producing pain, whether coming by epicritic, protopathic or deep fibres. A redistribution has taken place—all the various fibres carrying the impulses derived from nocuous stimulation have got shunted into one tract carrying them up to the brain. Similarly the impulses connected with thermal sensation coming by epicritic and by protopathic fibres, and those connected with the sense of touch and pressure, each get shunted into specific tracts (figs. 81 and 82). This must mean that these impulses get sorted out by passing through synapses in the cord, and the position of these synapses and fhe course of the axons running from them to the brain have been deduced partly from histological investiga- tion of the fibres of the posterior roots, partly by careful study of the symptoms and of the degenerations which follow definite lesions of the cord. J By the developmental method, by the different characters of the fibres, and by the degenerative changes, a posterior root has been divided into four main bundles : 1. A set of large fibres passing forward to form synapses with the cells of the ventral horn at the same and other levels of the cord. ». A second passing up the dorsal columns of the same side, some running right on to the top of the cord, but many entering the grey matter as they course upwards. SPINAL CORD 163 3. A third set forming synapses with the cells of Lockhart Clarke’s column. 4, A fourth ending at once in synapses with the cells of the dorsal horn or running for a short distance up the dorsal column before forming synapses. AINAESTHETIC | ae meouttal : | ] oe 4 a nd ie on --- ij Y eae an | i | r P4 me TACTILE : ! 7 a fal 2 | Pe al bl | ae THERMAL tj bs PAIN fy we % / a ae SPINAL NERVE Zs oe | OF EPICRITIC SPINAL CORD PROTO PATHIC AND INGOING MUSCLE FIBRES Fic. 81.—To show redistribution of impulses in the cord and the general course of impulses of different kinds. 1. The first set of fibres are those concerned in ordinary simple reflex action (fig. 82, 4). 9. The second set make up the dorsal columns. When 164 VETERINARY PHYSIOLOGY interrupted the result is a loss of the kinesthetic sense and a loss of the tactile sense for a short distance below the lesion, but not of the rest of the tactile sense or of the thermal sense or of pain. They form synapses in the nuclei of the dorsal columns in the medulla in which they terminate, and, from these synapses, fibres cross the middle line to run headwards to the brain in the fillet (see p. 175) (fig 82,1 and A). . of x & : . (Rad \ iS ix \ Nis ‘ ‘ . ' As ie yy BS: Y / : Be at y we Fic. 82.—To show redistribution of impulses in the spinal cord. 1 to 5, incoming fibres of dorsal root; A, tract of kinesthetic sense; B, tract of un- conscious impulses for muscular co-ordination and tone; C, same as B, but on opposite side; D, tract of impulses of pain. heat and cold; Z, tract of impulses of touch and pressure. (PAGE May.) 3. From the cells of Lockhart Clarke, with which the third set of fibres form synapses—(1) axons sweep outwards to take up a position on the dorso-lateral margin of the cord and to constitute the direct cerebellar tract which runs up to the cerebellum (fig 82,2 and B); (2) other axons cross the ventral SPINAL CORD 165 white commissure to the opposite side of the cord and come to lie on the lateral margin just ventral to the direct cerebellar tract. They may be termed the ventro-spinal cerebellar tract, and they too pass to the cerebellum, but by a different route from those of the direct tract (see p. 177) (fig. 82, 3 and C). 4, The fourth set of fibres form synapses with the cells in the dorsal horn. From these, axons cross the ventral white commissure to the opposite side of the cord, and have been traced headwards to the posterior part of the thalamus opticus. They may thus be called the spino-thalamie tract. They seem to run in two groups:—(1) Those concerned with the sensa- tions of pain, of heat and of cold cross near their point of entry and take up a position in close association with those of the ascending ventro-lateral cerebellar tract (fig. 82, D). (2) Those concerned with the tactile sense, whether of light touch or of pressure, run for a short distance up the dorsal columns, then form synapses. The fibres from these cross at a higher level than the last, and when they have crossed take up a position on the ventral margin of the cord (fig. 82, Z). As regards the side of the cord on which these fibres lie as they course upwards to the brain, it has been demonstrated, by section of one-half of the cord, that sensations of pain, temperature, and to a less extent of touch, are lost on the opposite side below the point of section, and that the kin- esthetic sense and partly the tactile sense are lost on the same side. These symptoms are explained by fig. 81. The position of the fibres in the different columns of the cord has been determined by studying the results of section of the different columns and by following out the degenerations which result. The degeneration method, or Wallerian method, is based upon the fact, that nerve fibres separated from their cell die and degenerate (see p. 85). These degenerations may be demon- strated when recent by Marchi’s method of staining, which depends upon the fact that, while the white sheath of normal fibres is not stained black when the tissue is placed in a ~ solution of chrome salt with osmic acid, it is so stained when it begins to degenerate (p. 85). When, at a. later period, the white sheaths have entirely disappeared, the degeneration is best demonstrated by Weigert’s method of staining the white 166 VETERINARY PHYSIOLOGY sheaths of normal fibres with hematoxylin, which leaves the degenerated tracts of fibres unstained. The fibres in the central nervous system do not regenerate, probably because they: are devoid of the neurilemmal sheath, B. Outgoing Fibres.—The course of these fibres is much more easily determined than that of the ingoing fibres. The effects of experimental or clinical lesions of the cord upon the muscular movements, the downward degeneration which follows such lesions, and lastly the fact that, generally speaking, these outgoing fibres get their medullary sheath at a later date than the ingoing fibres, all enable the position of the outgoing tracts to be defined. They may be grouped in four sets— 1. A very strong band of fibres lying in the dorsal part of the lateral column, just inside the direct cerebellar tract, and becoming smaller as the posterior part of the cord is reached. This is the crossed pyramidal tract (fig. 87, C.Py.), which comes from the cells of the cortex cerebri of the opposite side, and gives off collateral branches to the cells in the ventral horn of the spinal cord (fig. 41, D, p. 88). 2. Certain fibres from the cortex cerebri do not cross, but run down, some in the crossed pyramidal tract, some in the direct pyramidal tract (fig. 87, O.Py.), which runs along the margin of the ventral fissure, and extends tail- wards only into the dorsal region. These fibres decussate in the cord. 3. A set of fibres just inside the ventro-lateral ascending tract, which may be called the ventro-lateral descending tract (fig. 87, Desc. Ant. Lat.). This comes from Deiters’ nucleus (see fig. 87), and as it passes down, gives off fibres to the cells in the anterior horn of the grey matter of the cord. Deiters’ nucleus receives fibres from the cerebellum, and the ventro- lateral descending tract thus carries down impulses from that organ. 4. From the red nucleus (p. 184) some fibres pass down the — cord as the pre-pyramidal tract just ventral to the crossed pyramidal tract. C. Fibres not Degenerating beyond the Cord.—Proprio-Spinal Fibres—Round the grey matter, a band of fibres—the basis bundles (fig. 87, B.B.)\—and outside of these,. scattered through the white matter chiefly of the lateral columns, other fibres MIMMPENT OS Wihkv.eUl TORONTO UNIV MEDULLA OBLONGATA 167 degenerate in the cord and seem to be commissural between different levels of the grey matter (see p. 167). ‘Other tracts of fibres have been described, such as JLis- sauer’s tract and the septo-marginal tract, but their relations have not been satisfactorily investigated. B. THE MEDULLA OBLONGATA 1. Structure | The medulla oblongata may be regarded as the upper end of the spinal cord, which it connects with the brain (fig. 83). The cord expands and the dorsal median fissure is opened out, so that the central canal comes to the surface, and expands into a lozenge-shaped area—the floor of the fourth © ventricle. The lateral columns of the cord pass outwards tc the cerebellum to form part of its inferior peduncles— the restiform bodies. Between the lateral and the ventral columns an almond-shaped swelling, the olive, appears (fig. 85, O.). In front of this the medulla is encircled by a mass of transverse fibres—the middle peduncles of the cerebellum, or the pons Varoliw (fig. 87, P.). The floor of the fourth ventricle is constricted above by the approximation of the superior peduncles of the cerebellum (fig. 83, p.c.s.) to again become a canal. The grey matter of the cord gets broken up into separate masses, of which the most important are— 1. The nuclei of the dorsal columns—the nucleus gracilis and nucleus cuneatus (fig. 84, V.C. and JV.G.)—masses of cells and synapses in which the fibres of the dorsal columns end, and from which the upgoing fibres of the jil/e¢ start. 2. The inferior olivary nucleus (fig. 85, O.), which lies in the olive, and which is connected by bands of fibres with the dentate nucleus of the cerebellum (fig. 87, Dent.). 3. The nucleus of Deiters (fig. 87, Deit.), lying higher up in the pons Varolii, and connected with fibres from the cerebellum and from the semicircular canals (see fig. 81). 4, The nuclei of the cranial nerves, masses of cells from which the nerves take origin (fig. 86). THONOT 168 VETERINARY PHYSIOLOGY 2. Conducting Paths A. Ingoing.—1. The dorsal columns of the spinal cord terminate in two masses of grey matter on each side, the nucleus gracilis and nucleus cuneatus. From these, fibres Fic. 83.—View of the Medulla Oblongata, Corpora Quadrigemina, and the Optic Thalami from above. c.l.a., posterior columns of cord ; VI/I., XII, X. indicate the roots of these cranial nerves; p.c.t., the restiform body ; p.c.m., the middle peduncle of the cerebellum ; p.c.s., the superior peduncle of the cerebellum; ¢.g., the anterior and posterior corpora quadrigemina ; ¢.o., the optic thalamus with pulvinar (pwlv.) and ex- ternal and internal geniculate bodies behind it; e.p., the pineal body. The separation of the posterior columns of the cord and the opening out of the floor of the fourth ventricle is shown. (VAN GEUUCHTEN. ) pass forwards (7.e. towards the ventral aspect of the medulla) and cross the middle line forming the decussation of the fillet (tig. 84, #). The crossed fibres (fig. 85, #7.) then pass up in a vertical series on each side of the middle line until the pons MEDULLA OBLONGATA 169 Varolii is reached, when they spread out horizontally like a fan (fig. 87, F.) dorsally to the deep transverse fibres. Above the pons they divide into two sets (fig. 90, F)—a lateral fillet, which ends in the anterior corpora quadrigemina, and a mesial fillet, which passes on to the optic thalamus, and there ends by forming synapses. 2. The spino-thalamic tract passes up through the medulla, and with the mesial fillet ends in the thalamus. 3. The direct cerebellar tract passes up into the restiform Fie, 84 —Cross Section through Medulla Oblongata above the decussation of the Pyramids. P.M. and P.L., postero-median and postero-lateral tracts of the cord ; V.G. and N.C., nucleus gracilis and cuneatus, giving off the fillet fibres crossing at F. ; V., ascending root of fifth nerve ; G., nucleus of glossopharyngeal nerve; 4.H., anterior horn of spinal cord; P., the anterior pyramids; D.C., direct cerebellar tract; A. and D. Ant.L., ascending and descending antero-lateral tracts. (After BRUCE.) body, and so on to the superior vermis of the cerebellum (p. 182). Its fibres form synapses round cells chiefly on the opposite side. 4. The ventro-spinal cerebellar tract passes up beside the last, but it leaves it in the restiform body and courses forward, to arch back into the cerebellum round the superior cerebellar peduncle and to form synapses with the cells of the superior vermis (fig. 87, p. 173). B. Outgoing.—1. The fibres from the cerebral cortex, which 170 VETERINARY PHYSIOLOGY form in the cord the crossed and direct pyramidal tracts, pass down in the middle part of the crusta (fig. 90, P.) of the crura cerebri, and, after coursing between the superficial and deep transverse fibres of the pons (tig. 87, P.), come to lie in the ventral pyramids of the medulla (fig. 84, P.). At the lower end of the medulla most of these fibres cross (p. 174) to the lateral column of the cord; some, however, run down the direct and crossed pyramidal tracts of the same side. 2. The fibres of the descending ventro-lateral tract, coming originally from the deep nuclei of the cerebellum, take origin, pose READDCAU ETUDES LRA PPC X77 Fie. 85.—Cross Section of Medulla through the Olive. The central canal has opened out to form the floor of the fourth ventricle, 4th V., the lateral columns are passing out to form the inferior peduncles of the cerebellum ; F., fillet; O., inferior olivary nucleus; P., anterior pyramids; Rest., fibres of restiform body; V., ascending root of fifth nerve; VJZJZ. Acc.N., accessory nucleus of the eighth nerve. (After Bruce.) in part at least, in a mass of nerve cells (Deiters’ nucleus), which lies in the dorsal and lateral part of the pons Varolii (fig. 87, Deit.). C. Commissural Fibres. . 1. The basis bundles of the cord form in the medulla a strong band of fibres connecting the grey matter at different levels, the dorsal longitudinal fasciculus, 2. A set of fibres run from each olivary body across the middle line to the dentate nucleus of the cerebellum of the opposite side. MEDULLA OBLONGATA 171 3. Cranial Nerves (The physiology of these should be studied while dissecting them.) The nerves springing from and entering the medulla do not come off in the same regular fashion as do the spinal nerves, although they, like the spinal nerves, must be con- sidered as forming part of the spinal arcs. The outgoing fibres of each spring from a more or less definite mass of cells. The ingoing fibres generally form synapses with cells arranged in definite groups. In this way the so-called nuclei of the cranial nerves are formed. The position of these is indicated in fig, 86. In the cranial nerves no sharp differentiation into ventral and dorsal roots can be made out. Nevertheless they contain the same component elements as the spinal nerves, the fibres running either together or separately. 2 Mact Accessori” i Yeahs, 4 \ rw wy Rad antersores $204 f*s Fie. 86.—The Nuclei and Roots of the Cranial Nerves. (A/ter EDINGER.) Ingoing Fibres—Somatic and splanchnic fibres (p. 158) enter the medulla and have their cell stations in ganglia upon the nerves. Outgoing Fibres.—Somatic and splanchnic fibres pass out, the latter being characterised by their small size, and by forming synapses before their final distribution. The XII. (Hypoglossus) is purely a ventral root nerve, and is motor to the muscles of the tongue. The X. (Vagus), and the XI. (Spinal Accessory) are practically one nerve, consisting partly of dorsal and partly of ventral root fibres. The vagus is the great ingoing nerve from the abdomen, thorax, larynx, and gullet, while, by outgoing fibres, passing through it or through the accessorius, it is augmentor 172 VETERINARY PHYSIOLOGY for the muscles of the bronchi and alimentary canal, inhibitory to the heart, dilator to blood vessels of the thorax and abdomen, and motor to the muscles of the larynx and to the levator palati. The accessorius is ale motor to the sterno-cleido- mastoid and trapezius. The IX. (Glossopharyngeus) is essentially a dorsal root, and is the ingoing nerve for the back of the mouth, the Eustachian tube, and tympanic cavity. It transmits outgoing fibres which are motor to the stylo-pharyngeus and middle constrictor of the pharynx. The VII. (Facial) is almost purely a ventral root, trans- mitting the motor fibres to the muscles of expression, and secretory fibres to the submaxillary and sublingual glands and the glands of the mouth, It, however, carries ingoing fibres from the anterior two-thirds of the tongue. The V. (Trigeminal) is chiefly a dorsal root, but it has a distinct ventral or motor root which joins it, and carries the motor fibres to the muscles of mastication. It is the great ingoing nerve for all the face. The VI. (Abducens) supplies the external rectus of the eye. The IV. (Trochlearis) supplies the superior oblique. The III. (Oculo-motorius) supplies all the muscles of the eye except those supplied by VI. and IV. The anterior part of the nucleus consists of small cells and gives off fibres to the sphincter pupille and ciliary muscles. The fibres coming from the nuclei of these cranial nerves do not always pass out in the nerve itself. Thus, fibres from the nucleus of the III. to the orbicularis oculi pass out in the VII., while fibres for the posterior belly of the digastric which pass out in the VII. probably come from the nucleus of the XII. 4, Reflexes of the Medulla The extensive series of synapses in the medulla form arrangements by which various combined and co-ordinated movements are controlled. Thus, part of the nucleus of the vagus governs the movements of respiration, while other parts preside over the slowing mechanism of the heart. To these various reflex arrangements the name of centres has been given, REGION OF PONS VAROLII ee and their mode of action will be considered when dealing with the mechanism of circulation, respiration, and digestion. Cc. REGION OF PONS VAROLII ‘This region consists of the upper part of the medulla embraced by the transverse fibres of the middle peduncles of the cerebellum (fig. 87). Fie. 87.—Cross Section through Region of Pons, Cerebellum, and Fourth Ventricle. S.V., superior vermis; &.N., roof nucleus; Dent., dentate nucleus ; Rest., restiform body; S.P., superior peduncle of cerebellum ; Deit., Deiters’ nucleus; VZ., nucleus of the sixth nerve; F., fillet ; S.0., superior olive; D.7. and S.T7,, deep and superficial transverse fibres ; P., pyramidal fibres. (A/ter BRUCE.) Outgoing Fibres.—1. The fibres to the face muscles cross the middle line to become associated with the various nuclei of the cranial nerves. For this reason a tumour in one side of the pons may cause paralysis of the face muscles on one side and of the muscles of the rest of the body on the opposite side. 174 VETERINARY PHYSIOLOGY 2. Fibres to the limbs and trunk run down between the deep and superficial transverse fibres (fig. 87, P.). Ingoing Fibres.—The fillet fibres in the pons, instead of running up on each side of the middle line, spread out into a horizontal arrangement above the crossed fibres (fig. 87, /). The nucleus of Deiters and the nuclei of various cranial nerves lie in this region of the brain. The connections of the transverse fibres are considered on p. 177. D. CEREBELLUM Structure The cerebellum (fig. 86) or lesser brain lies above the fourth ventricle, and is joined to the cerebro-spinal axis by three ‘es > Pe eee ee ee ee ee ae wee ee Fic. 88.—Diagram of the Arrangement of Fibres and Cells in the Cortex of the Cerebellum. G.Z., molecular layer; .Z., nuclear layer; P., Purkinje’s cells sending out axons to the deeper ganglia. (After RAMON Y CAJAL.) peduncles on each side (figs. 81 and 89). It consists of a central lobe, the upper part of which is the superior vermis (fig. 87, S.V.), and two lateral lobes, each with a secondary small lobe, the flocculus. Its surface is raised into long ridge- like folds running in the horizonal plane, and is covered over with grey matter, the cortex. In the substance of the white matter forming the centre of — rae : ie CEREBELLUM 175 the organ are several masses of grey matter on each side, the most important of which are—1, the roof nucleus; and 2, the dentate nucleus (fig. 87, AN. and Dent.). The eortex may be divided into an outer somewhat homo- geneous layer (the molecular layer, fig. 88, G.Z.) and an inner layer studded with cells (the nuclear layer, N.L.). Between these is a layer of large cells—the cells of Purkinje (P.). By Golgi’s method the arrangement of fibres and cells in the cerebellar cortex has been shown to be as follows :— Fibres coming into the cortex from the white matter end either in synapses round cells in the nuclear layer, or proceed at once to the outer layer (fig. 88). From the cells in the - nuclear layer, processes pass to the outer layer and there form synapses with other cells. From these, processes pass to the cells of Purkinje, round which they arborise, and from Purkinje’s cells the outgoing fibres of the cerebellum pass into the white matter, and so to the deeper ganglia, and to Deiters’ nuclei (fig. 87). Connections The cerebellum is connected (fig. 89) :— i. With the Spinal Cord. a. Incoming Fibres—1. The direct cerebellar tract (p. 172) passes up in the restiform body to end chiefly in the superior vermis. 2. The ascending ventro-lateral tract (p. 174) passes to the cerebellum in the superior peduncle and ends in the superior vermis. 3. Fibres from the nuclei of the dorsal columns of the same side (fig. 84, p. 177) pass in the restiform body to the cerebellum. 4. Fibres from the vestibular root of the eightl nerve also pass to the cerebellum (fig. 81, p. 152). b. Commissural Fibres—Strong bands of fibres connect the inferior olive of one side with the dentate nucleus of the other. c. Outgoing Fibres——Fibres pass from the superior vermis to the deep nuclei, and, from these, fibres run to Deiters’ nuclei (fig. 87), from which fibres pass down in the descending ventro- lateral tract of the cord. ii. With the Cerebrum.—1. The fibres of the middle peduncles cross in the middle line embracing the medulla, and forming the pons Varolii, and become associated with cells from which fibres pass up in the lateral parts of the crura cerebri to the cerebral cortex (fig. 90, CU. CC., p. 187). 176 VETERINARY PHYSIOLOGY 2. The fibres of the superior peduncle, coming chiefly from the dentate nucleus and superior vermis, cross the middle line and end partly in the red nucleus of the opposite side and partly in the thalamus opticus (fig. 90, S.C.P.). The red nucleus fibres Fic, 89.—Connections of the Cerebellum with the Cerebro-spinal Axis (for explanation, see text). seem to pass upwards to the cerebrum and downwards into the pre-pyramidal tract of the spinal cord. How far the former are upward conducting and how far downward is not definitely known. The cerebellum thus constitutes the central part of one of the great nervous arcs, the ingoing fibres terminating in the CEREBELLUM 177 cortex which sends fibres to the basal ganglia, which in turn send out the outgoing fibres. Functions Removal of the cerebellum deprives the animal, for a time at least, of the power of balancing itself. This may be easily demonstrated in the pigeon (fig. 96, p. 185). But in some cases, when slowly progressing disease has destroyed the organ, no loss of equilibration has appeared, and in other cases the cerebellum has been congenitally almost absent, and yet the individual has not shown any sign of want of power of main- taining his balance. Evidently, therefore, some other part of the brain can compensate for its absence. The manner in which the cerebellum acts has been chiefly elucidated by removing parts of the organ and keeping the animals under observation for prolonged periods. If one side of the cerebellum be removed the first symptoms are (1) a tonic contraction of the muscles of the limbs of the same side by which the fore limbs may be powerfully extended, and an arching of the body with the convexity towards the side of the lesion, while the animal may be driven round its long axis to the opposite side. (2) These irritative symptoms soon pass off, and the animal then manifests inadequacy or weakness in the limbs of the affected side, so that it droops to that side, and, if a quadruped, may circle to that side. (3) After some weeks these symptoms disappear, and the loss of one side of the cerebellum is apparently completely compensated for. When in the dog compensation has been established, destruction of the cerebral cortex of the opposite side leads to a reappearance of the muscular inadequacy. Electrical stimulation of the cerebellum has yielded results somewhat difficult of interpretation, but the most recent investi- gations seem to show that stimulation of the cortex with currents strong enough to produce movements when applied to the discharging part of the cerebral cortex (see p. 209) do not produce manifest effects, but that comparatively weak currents applied to the basal ganglia do produce movements, the most manifest of which are the conjugate movements of the eyes, and the eyes and head to the side stimulated. It has been further found that powerful stimulation may 12 178 VETERINARY PHYSIOLOGY also cause flexion of the elbow of the same side and entension of the opposite elbow with entension of the trunk and lower limbs. This may be associated with the maintenance of the body in the erect position and the alternate movements of the limbs in the act of progression. Taking into consideration the fact that lesions of the posterior nerve roots cause loss of muscular co-ordination, while destruction of the ascending cerebellar tracts produces decrease of muscular tone on the same side, it may be concluded that both the kinesthetic mechanism, which plays so important a part in maintaining the balance of the body, and the labyrinthine mechanism, have an important central station in the cerebellum. It would thus appear that the cerebellum is to be regarded as a mechanism supplementary to the great cerebro-spinal mechan- ism, and that it has for its purpose more especially the muscular co-ordination and adjustment required in maintaining the balance. This it may do in one or both of two ways— 1. By receiving impulses from without, and sending impulses downwards to act upon the spinal mechanism. 2. By receiving impulses, and sending impulses upwards to the cerebrum to modify its action. Channel for such impulses exists (1) in the fibres of the pons which cross the middle line to connect with cells from which fibres pass upwards to the occipital and frontal lobes of the cerebrum (fig. 41, p. 88) and (2) in the superior peduncles, To maintain the constant muscular adjustments involved in balancing the body an arrangement whereby any dis- turbance of the equilibrium can produce an appropriate reaction is required. The ingoing impulses which are more especially of service in this way are derived from (1) the kinesthetic mechanism (see p. 170); (2) the ¢actile mechanism from the soles of the feet; (3) the eye; and (4) the labyrinthine mechanism (p. 152). When the information as to the relationship of the animal with its surroundings derived from these various sources is not concordant—e.g. when the semicircular canals indicate movement and the eyes an absence of movement—balancing CRURA CEREBRI AND CORPORA QUADRIGEMINA 179 becomes difficult. This may be readily demonstrated by setting a poker vertically on the floor, holding it in the hand, placing the forehead on the top, walking rapidly three times round it, then standing up and trying to walk out of the room. The sudden stoppage of the rotatory movement causes a disturbance in the semicircular canals giving a sense of rotation in the opposite direction, while the eyes show that no rotation is taking place. The feeling of giddiness is, however, not the cause of the loss of balanc- ing, but a mere accompaniment. (Practical Physiology.) E. THE CRURA CEREBRI AND CORPORA QUADRIGEMINA Above the pons Varolii, the two halves of the medulla diverge from one another and form the peduncles of the cerebrum Fic. 90.—Cross Section through Anterior Corpora Quadrigemina and Cerebral Peduncles. A4.S., aqueduct of Sylvius; JZJ., nucleus of third nerve ; S.C.P., superior cerebellar peduncles; ¥., mesial fillet; /, lateral fillet; P., pyramidal tract; CC., cerebello-cerebral fibres (Human). (After BRUCE.) (fig. 90, CC., P.), while posteriorly the two superior peduncles of the cerebellum come together (S.C.P.). Above these, two swellings develop on each side—the anterior and posterior corpora quadrigemina (fig. 83, p. 168). 180 VETERINARY PHYSIOLOGY The crusta, or anterior parts of each peduncle of the cerebrum, is composed, in its central part, of the pyramidal fibres passing down from the cerebrum to the spinal cord (P.), and, on each side, of the cerebello-cerebral fibres passing upwards from the pons (CC.). The posterior part, or tegmentum, contains—l1st. The fillet fibres going partly to the corpora quadrigemina, partly onwards to the thalamus opticus (7.); 2d. The nuclei of the 3rd and 4th cranial nerves; 37d. The fibres of the superior peduncles of the cerebellum which cross the middle line (S.C.\P.); and 4th. The red nuclei in which most of these fibres end. The functions of this segment of the brain are chiefly con- ducting, but the anterior corpora quadrigemina form the shunting station between the incoming fibres of the optic tract and the oculo-motor mechanism (see p. 141). F. THE CEREBRUM Structure Each crus or peduncle terminates in its half of the cerebrum (fig. 76, p. 164). As the fibres pass from peduncle to cerebrum and vice versé they come into relationship with three masses of grey matter lying in the midst of the cerebrum. These are the thalamus opticus, into which the ingoing fibres enter; the lenticular nucleus, between which and the thalamus the outgoing fibres run; and the caudate nucleus, the main part of which lies in front of the other two (fig. 91). The fibres, above these nuclei, spread out to form the corona radiata and enter a crust of grey matter, the cortex cerebri, which covers over the cerebrum, and which in the higher animals is raised into a number of folds or convolutions marked off from one another by fissures and sulci. In the lower vertebrata the differentiation of the cortex from the basal ganglia is incomplete, and it is only in the higher mammals, monkeys and man, that the cortex reaches its full physiological importance. The structure of the cortex cerebri as regards the arrangement of cells and fibres is somewhat complex, and varies greatly at different parts, but the general vee is as follows (see fig. 95) :-— CEREBRUM 181 A. Cells (fig. 95 A).—1. Plexiform layer. At the surface of the cortex is a thin layer of small irregular cells. 2. Layer of small pyramidal cells. 3, Layer of medium pyramidal cells. 4. External layer of large pyramidal cells. Some writers class layers 2, 3, and 4 as one. 5. Layer of stellate cells. This is a thin but well-defined layer of minute polymorphic cells lying rather more than halfway down the thickness of the cortex. 6. Internal layers of pyramidal cells. 7. Layer of spindle-shaped cells. B. Fibres.—The relationship of the medullated fibres to these layers is shown in fig. 95 B). 1. On the surface is a fibreless layer. Z. Under this is a thin layer of more or less horizontal fibres, the zonal layer. 3. Under this is a thicker ir- regular layer of fibres, the supra- radiary layer. 4, Next comes a band of fibres often so well developed as to make a white streak in the grey matter. This is the line of Baillarger. In the regions of the calcarine fissure in apes and in man, when it is specially marked, it is known as the layer of Gennari. 5. Radiary zone. A _ thick layer of white fibres chiefly pass- ing to and from the subjacent white substance. It should be remembered that the layer of stellate cells lie just outside Baillarger’s layer of fibres. large EXE; FACE. Fibrinogen Fibrin BLOOD AND LYMPH 205 Many eireumstanees influence the rapidity of clotting. Temperature has a marked effect ; a low temperature retarding it, a slight rise of temperature above the normal of the particular animal accelerating it. If a trace of a neutral salt be added to blood, coagulation is accelerated; but, if blood be mixed with strong solutions of salt, coagulation is prevented because the formation of thrombin is checked. Calcium salts have a marked and important action, and if they are precipi- tated by the addition of oxalate of soda, blood will not clot, apparently because thrombin cannot be formed. The injection into the blood vessels of a living animal of commercial peptones, which chiefly consist of proteoses, or of hirudin, an extract of the head of the medicinal leech, retards coagulation after the blood is shed. They appear to cause the development in the liver of some body of the nature of an anti- thrombin which checks coagulation, and if the liver be excluded from the circulation this is not developed Why is it that blood does not coagulate in the vessels and does coagulate when shed? Such a general statement is not absolutely correct, for blood may be made to coagulate in the vessels of a living animal in various ways. If inflammation is induced in the course of a vessel, coagulation at once occurs. If the inner coat of a vessel be torn, as by a ligature, or if any roughness occurs on the inner wall of a vessel, coagulation is apt to be set up. Again, various substances injected into the blood stream may cause the blood to coagulate, and thus rapidly kill the animal. Among such substances are extracts of various organs—thymus, testis, and lymph glands—which yield thrombokinase, and snake venom which seems to contain active thrombin. The injection of pure thrombin does not usually cause clotting, because an anti-thrombin is developed. Nor does blood necessarily coagulate when shed. If it is re- ceived into castor oil, or into a vessel anointed with vaseline and filled with paraffin oil, it will remain fluid for a consider- able time. Apparently some roughness in the wall of the blood vessel or of the vessel in which the blood is received is required to start the process, acting as a focus from which it can spread outwards. The advantages of coagulation of blood are manifest. By 206 VETERINARY PHYSIOLOGY means of it wounds in blood vessels are sealed and hemor- rhage stopped. Although .an important and very prominent change in the blood, clotting is really produced by change in one constituent of the plasma, which is present in very small quantities. II. Plasma and Serum These may be considered together, since serum is merely plasma minus fibrinogen. As serum is so much easier to procure, it is generally employed for analysis. Both are straw-coloured fluids, the colour being due to a yellow lipochrome. Sometimes they are clear and transparent, but after a fatty diet they become milky. They are alkaline in reaction, and have a specific gravity of about 1025. They contain about 90 per cent. of water and 10 per cent. of solids. The chief solids are the proteins—serum albumin and serum globulin (with, in the plasma, the addition of fibrinogen). The proportion of the two former proteins to one another varies considerably in different animals, but in the same animal at different times the variations are small. The globulin probably consists of at least two bodies—euglobulin precipitated by weak acid, and pseudoglobulin not so precipitated. The amount of albumin is generally greater when the body is well nourished. In most animals, they together form about 7 per cent. of the serum. The other organic constituents of the serum are in much smaller amounts, and may be divided into— 1. Substances to be used by the tissues, Glucose is the most important of these. It occurs only in small amounts—about 1 to 2 per mille. Part of it is free, but part is probably combined in organic combinations such as ‘jecorin. It is probably in larger amount in blood going to muscles than in blood coming from muscles, and this difference seems to be specially well marked when the muscles are active, Fats occur in very varying amounts, depending upon the amount taken in the food. 2. Substances given off by the tissues, The chief of these’ is urea, which occurs constantly in very small amounts in the serum—about ‘05 per cent. We shall “es Ee BLOOD AND LYMPH 207 afterwards see that it is derived from the liver, and that it is excreted in the urine by the kidneys. Creatin (p. 40), with urie acid (p. 427), and some allied bodies appear to be normally present in traces, and their amount may be increased in diseased conditions. Of the inorganic constituents of the serum the most abund- ant is chloride of sodium, but in addition sodium carbonate and alkaline sodium phosphate are also present. Calcium, potassium, and magnesium occur in very small amounts. III. Cells of Blood. 1. Leucoeytes—White Cells.— These are much less numerous than the red cells, and their number varies enormously in normal conditions. On an average there is one to every 400 or 500 red cells. They are soft, extensile, elastic, and sticky, and each con- tains a nucleus and a well-developed double centrosome. In size they vary considerably, some being much larger than the red cells, some slightly smaller. The character of the nucleus varies greatly, and from this and from variations in the proto- plasm, they may be divided into four classes, 1st. Lymphocytes.—Cells with a clear protoplasm and a more or less circular nucleus. Some are very small, while others are larger. They constitute about 20 to 25 per cent. of the leucocytes (fig. 102, 6 and c). 2nd. Polymorpho-nuelear leucocytes, with a much-distorted and lobated irregular nucleus and a finely granular protoplasm, whose granules stain with acid and neutral stains. These constitute about 70 to 75 per cent. of the leucocytes. 3rd. Eosinophil or oxyphil leucocytes, with a lobated nucleus like the last, but with large granules in the protoplasm which stain deeply with acid stains. From 1 to 4 per cent. of the . leucocytes are of this variety. 4th. Basophil leucocytes, practically absent from normal blood, with a lobated nucleus and granules in the protoplasm, staining with basic stains. Myelocytes are large leucocytes with a large circular or oval nucleus and a finely granular protoplasm. They are not normal constituents of the blood, but appear when the 208 VETERINARY PHYSIOLOGY | activity of the bone marrow is increased in certain patho- logical conditions. These various forms have certain properties—(a) Amosboid movement. They can, under suitable conditions, undergo certain changes in shape, as may be readily seen in the blood of the frog or other cold-blooded animal. The motion may consist simply of the pushing out and withdrawal of one or more processes (pseudopodia), or, after a process is extended, the whole corpuscle may follow it and thus change its place, or the corpuscle may simply retract itself into a spherical mass. As a result of these movements the corpuscles, in certain con- Z f Fic. 102.—Cells of the Blood. a, erythrocytes ; 6, large, and c, small lym- phocyte ; d, polymorpho-nuclear leucocyte.; ¢, eosinophil leucocyte. ditions, creep out of the blood vessels and wander into the tissues (Diapedesis). (6) Phagocyte action —The finely granular leucocytes and the lymphocytes have further the power of taking foreign matter into their interior, and of thus digesting it. By this devouring action, useless and effete tissues are removed and dead micro-organisms in the body are taken up and got rid of. This scavenger action of the leucocytes is of vast importance in pathology. Chemistry of Leucocytes.—The nucleus is chiefly made up of nuclein, and in the protoplasm a nucleo-protein, along with two globulins and a small amount of an albumin, are found. Along with these protein substances glycogen and a small BLOOD AND LYMPH 209 amount of fat are present, while the chief inorganic con- stituents are potassium salts. 2. Blood Platelets.—These are sinall circular or oval dis- eoid bodies, about one-third the diameter of a red blood cor- puscle. Some observers have stated that they contain a central nucleus. They are very sticky and mass together when blood is shed and adhere to a thread passed through blood or to any rough point in the lining of the heart or vessels. They there form clumps, and from these clumps fibrin threads are seen to shoot out. They thus appear to play an active part in clotting. They are present in the blood of mammals only. Their source is not definitely known, but it has been suggested that they are the extruded nuclei of developing erythrocytes. 3. Erythroeytes—Red Cells.—A1] mammals except the camels have circular, biconcave, discoid erythrocytes, which, when the blood is shed, tend to run together like piles of coins. The camels have elliptical biconvex corpuscles. A nucleus is not present in the fully-developed mammalian erythrocyte. In birds, reptiles, amphibia and fishes, the corpuscles are elliptical biconvex bodies, with a well-marked central nucleus. _ The size of the erythrocytes is fairly constant in each species of animal. In the horse they are on an average 6 to 6:5 micro-millimetres in diameter. The number of red cells in health is about 7,000,000 per cubic millimetre in the horse ; but in disease it is often decreased. The number of corpuscles per cubic millimetre is estimated by the Hemocytometer. This consists of (1) a pipette by which the blood may be diluted to a definite extent with normal salt solution, and (2) a cell of definite depth ruled in squares, each containing above it a definite small volume of blood so that the number of corpuscles in that volume may be counted under the microscope. (Practical Physiology.) The pale yellow colour of the individual corpuscles is due to a pigment held ina fine sponge like stroma which seems to form a capsule round the cell. This pigment may be dissolved out by various agents, and the action is termed Hemolysis. It may be brought about in different ways—1s¢. 14 210 VETERINARY PHYSIOLOGY By placing the erythrocytes in a fluid of lower osmotic equivalent, ze. of lower molecular concentration, than the blood plasma and corpuscles. A solution of 0°9 per cent. of sodium chloride has the same osmotic equivalent as the plasma and preserves the corpuscles unaltered; in more dilute fluid ~ the corpuscles tend to swell up by endosmosis and the pigment is dissolved out. Erythrocytes may therefore be used as a means of determining the osmotic equivalent—the molecular con- centration of a fluid. 2nd. By the action of substances which dissolve some constituent of the stroma, eg. salts of the bile acids (see p. 376), chloroform, ether, ete. 3rd. By Hemolysins. The serum of one animal contains a substance, destroyed by heating to 55° C., which is hemolytic to the blood of animals of other species, ¢.g. the serum of eels’ blood contains a powerful hemolysin for rabbits’ erythrocytes, and the serum of the dog a less powerful one. Further, by injecting the blood or the erythrocytes of one species of animal into another species, a hemolysin is developed which has a specific action on the erythrocytes of the first species. Chemistry.—The stroma of the erythrocytes is made up of a globulin-like substance, in connection with which lecithin and cholesterin occur in considerable quantities. Potassium is the base most abundantly present. The pigment is Heemoglobin. It constitutes no less than 90 per cent. of the solids of the corpuscles. In many animals, when dissolved from the corpuscles, it crystallises very readily. The crystals prepared from the human blood are rhombic plates. When exposed to air they are of a bright red colour, but if placed in the receiver of an air-pump at the ordinary temperature they become of a purplish tint. The same thing eccurs if the hemoglobin is in solution, or if it is still in the corpuscles. The addition of any reducing agent such as ammonium sulphide or a ferrous salt also causes a similar change. This is due to the fact that haemoglobin has an affinity for oxygen, which it takes up from the air, forming a _ definite compound of a bright red colour in which one mole- cule of hemoglobin links with a molecule of oxygen, HbO,, and is known as oxyhsemoglobin. Hemoglobin is closely allied to the proteins, but differs from them in containing 0°42 per cent. of iron. BLOOD AND LYMPH 211 When light from the sun is allowed to pass through solutions of blood pigments, certain parts of the solar spectrum are absorbed, and when the spectrum is examined dark bands— the absorption bands—are seen. In a weak solution of oxy- hemoglobin a dark band is seen in the green and another in the yellow part of the spectrum between Frauenhofer’s lines D and E, while the violet end of the spectrum is absorbed (fig. 103). These bands may be broadened or narrowed by strengthening or weakening the solution. When the oxygen is taken away and the purple reduced hemoglobin is formed ? RED. YELLOW. GREEN. BLUE. Carbon-monoxide Hemoglobin . Oxyhemoglobin . v Hemoglobin t Methemoglobin . Acid Heematin Reduced Alkaline Hematin . . Fic. 103.—Spectra of the more important Blood Pigments and their more important derivatives. (The Spectrum of Acid Hematin is not identical with that of Methemoglobin. ) a single broad band between D and E takes the place of the two bands (fig. 103). (Chemical Physiology.) The property of taking oxygen from the air and of again giving it up at a moderate temperature and under a low pressure of oxygen is the great function of the blood pigment in the body. The hemoglobin plays the part of a middle- man between the air and the tissues, taking oxygen from the one and handing it on to the others. (Chemical Physiology.) Heemoglobin constitutes about 13 or 14 per cent. of the blood, but in various diseases its amount is decreased. The best method of estimating its amount is by Haldane’s Hemo- globinometer. This consists of two tubes of uniform calibre, 212 VETERINARY PHYSIOLOGY one filled. with a 1 per cent. solution of normal blood saturated with CO, and another in which 20 emm. of blood to be examined, measured in a pipette, is placed in water, mixed with coal gas to saturate with CO, and then diluted till it has the same tint as the standard tube. The per- centage of hemoglobin in terms of the normal is indicated by the mark on the tube at which the fluid stands. (Chemical Physiology.) Methemoglobin. — Hemoglobin forms another compound with oxygen—methemoglobin. The amount of oxygen is the same, but methemoglobin must be acted on by the strongest reducing agents before it will part with its oxygen. When, therefore, this pigment is formed in the body, the tissues die from want of oxygen. It may be produced by the action of various substances on oxyhemoglobin. Among these are ferricyanides, nitrites, and permanganates. It crystallises in the same form as oxyhemoglobin, but has a chocolate brown colour. Its spectrum is also different from hemoglobin or oxyhemoglobin, showing a narrow sharp band in the red part of the spectrum, with two or more bands in other parts according to the reaction of the solution in which it is dissolved (fig. 103). It is of importance since it occurs in the urine in some pathological conditions. In all probability — O the molecule of oxyhemoglobin has the formula—Hb¢ | ; O O O Hemoglobin also combines with certain other gases. Among these is Carbon monoxide. For this gas hemoglobin has a greater affinity than for oxygen, so that when carbon monoxide hemoglobin is once formed in the body, the blood has little power of taking up oxygen, and the animal dies. This gas is evolved freely in the fumes from burning charcoal, is present in coal gas, and is found in the air of coal mines after explosions. Carbon monoxide hemoglobin forms crystals like oxyhemoglobin, and has a bright pinkish red colour, without the yellow tinge of oxyhemoglobin. Since after death it does not give up its carbon monoxide and become changed to purple while in methemoglobin the atoms are arranged Ho BLOOD AND LYMPH 213 hemoglobin, the bodies of those poisoned with the gas main- tain the florid colour of life. Its spectrum is very like that of oxyhemoglobin, the bands being slightly more to the blue end of the spectrum (fig. 103). It may be at once distinguished by the fact that when gently warmed with ammonium sulphide it does not yield reduced hemoglobin. (Chemical Physiology.) Decomposition of Hemoglobin.—Hemoglobin is a somewhat unstable body, and, in the presence of acids and alkalies, splits ‘ up into about 96 per cent. of a colourless protein globin belonging to the globulin group, and about 4 per cent. of a substance of a brownish colour called hematin. The spectrum and properties of this substance are different in acid and alkaline media. In acid media it has a spectrum closely resembling methemoglobin, but it can at once be dis- tinguished by the fact that it is not changed by reducing agents. In medicine it is sometimes important to distinguish between these pigments since both may appear in the urine. Hematin in alkaline solution can take up and give off oxygen in the same way as hemoglobin does. Reduced alkaline hematin or hzmochromogen has a very definite spectrum (fig. 103), and its preparation affords a ready means of detecting old blood stains. Hematin contains the iron of the hemoglobin, and it is this pigmented iron-containing part of the molecule which has the affinity for oxygen. Apparently it is the presence of iron which gives it this property, because, if the iron be removed by means of sulphuric acid, a purple-coloured substance, iron-free hematin, hematoporphyrin, is formed, which has no affinity for oxygen. This pigment occurs in the urine in some pathological con- ditions. (Chemical Physiology.) One point of great interest in the chemistry of hematin and its derivatives is that they, like the green chlorophyl of plants, yield upon decomposition bodies belonging to the pyrrhol group (see p. 9). In the liver hemoglobin is broken down to form bilirubin and the other bile pigments. These are iron-free, and, like iron-free hematin, do not take up and give off oxygen. But not only is this iron-free pigment formed from hemoglobin in the liver, but the cells of any part of the body have the faculty of changing hemoglobin in blood extravasations into a pig- 214 VETERINARY PHYSIOLOGY ment known as hematoidin, which is really the same as bilirubin. ; Heemin—the hydrochloride of hematin—is formed when © blood is heated with sodium chloride and glacial acetic acid. It crystallises in small steel-black rhombic crystals, and its formation is sometimes used as a test of blood stains. (Chemical Physiology.) The following table shows the ata ae des of these pigments to one another :-— RELATIONSHIP OF HB AND ITS DERIVATIVES. Methemoglobin HbO,——HbCO~ 4 Hb i | Heematin Globin | Contain Iron. | Acid Heematin Alkaline Hematin | | | | Oxidised Reduced (Hemochromogen) J Iron-free Heematin Hematoidin Toute (Heematoporphyrin) Bilirubin s IV. Gases of the Blood The muscles and other active tissues are constantly con- suming oxygen and constantly giving off carbon dioxide. The oxygen must be brought to the tissues by the blood, and the carbon dioxide carried away by the same medium. Various methods of carrying out the examination of the gases of the blood have been devised, and many different gas pumps have been invented in which the gases may be col- lected in the Torricellian vacuum over mercury. Haldane and Barcroft have devised a convenient method, which depends upon the fact that the oxygen can be driven off from blood treated with dilute ammonia by the addition of potassium BLOOD AND LYMPH 215 ferricyanide, and that the carbon dioxide is liberated by adding an acid. The amount of gas is estimated by measuring the increased pressure in the tube in which the gas has been given off. About 60 c.c. of gas measured at 0° C. and 760 mm. pres- sure can be extracted from 100 cc. of blood. The proportion of the gases varies in arterial and in venous blood. AMOUNT OF GASES PER HUNDRED VOLUMES OF BLOOD Arterial Blood. Venous Blood. Oxygen : ; 20 12 Carbon dioxide . 40 46 There are two ways in which gases may be held in such a fluid as the blood— 1st. In simple solution. 2nd. In chemical combination. Oxygen.—At the temperature of the body the blood can hold in solution less than 1 per cent. of oxygen. Now the amount of oxygen actually present is about 20 per cent. So that by far the greater quantity of the gas is not in solution. We have already seen that it is in loose chemical union with heemoglobin. Carbon Dioxide.—In the animal body the blood can dissolve about 24 per cent. of carbon dioxide. But it may contain as much as 46 per cent., and this is uniformly distributed between plasma and corpuscles. Hence the greater part of the gas must’ be in chemical combination. Analysis of the ash of the plasma shows that the sodium is more than sufficient to combine with the chlorine and phosphoric acid, and is thus available to take up carbon dioxide, as the carbonate Na,CO, and the bicarbonate NaHCO, Sodium carbonate and basic sodium phosphate are therefore present together in the plasma. If carbon dioxide is passed into a solution of sodium phosphate it appropriates a certain amount of the sodium, changing Na,PO, to NaHPO, This is what happens in the tissues where CO, is abundant. In the lungs, where the blood 216 VETERINARY PHYSIOLOGY is exposed to an atmosphere poor in CO,, the P, Os again seizes on the Na, turning out the COQ,. But the proteins of the blood also act in the same way as weak acids, being turned out of their combination with bases by the mass action of CO,, and thus acting as carriers of CO,. In the corpuscles the hemoglobin acts as an acid. When the amount of CO, is great, hemoglobin is turned out of its combination with bases. But when the pressure of CO, is low the hemoglobin turns it out. For this reason it is possible to remove all the CO, from whole blood in an air pump, but not possible to remove it from blood plasma. In fact, the carriage of carbon dioxide and its excretion are mainly the result of a struggle between that gas on the one hand and the proteins and phosphoric acid on the other, for the bases of the plasma. Nitrogen.—The amount of nitrogen in the blood is not in excess of what can be held in solution, and we may therefore infer that it is simply dissolved in the blood plasma. V. Source of the Blood Constituents A. Of the Plasma.—The water of the blood is derived almost entirely from the water ingested. The source of the proteins has not been fully investigated. Undoubtedly they are partly derived, somewhat indirectly as we Shall afterwards see, from the proteins of the food. Very probably, too, they are in part derived from the tissues. But the significance of the two proteins, albumin and globulin, and of their variations has not yet been elucidated. The glucose is derived from the carbohydrates and possibly from the proteins of the food, and during starvation it is con- stantly produced in the liver and poured into the blood (p. 395). The fats are derived from the fats and carbohydrates and possibly from the proteins of the food. The urea and other waste constituents are derived from the various tissues. | B. Of the Cells.—I. Leuecoeytes.—In the embryo these are first developed from the mesoblast cells generally. In extra- uterine life they are formed in the lymph tissue and in the red marrow of bone. 1. Lymph Tissue (see p. 30) is very widely distributed in BLOOD AND LYMPH 217 the body, occurring either in patches of varying shape and size, or as regular organs, the lymphatie glands (fig. 104). These are placed on the course of lymphatic vessels, and consist of a sponge-work of fibrous tissue, in the interstices of which are set the patches of lymph tissue or germ centres, each surrounded by a more open network, the sinus, through which the lymph flows, carrying away the lymphocytes, which are the characteristic elements produced, from the germ centres, Round some of the lymphatic glands of certain animals large blood spaces or sinuses are seen, and these glands are called hemolymph glands (fig. 106). They are futermodinke between lymphatic ies ca and the aes When ye rccie are 2 ler KE in £34 Rr ey 7 a e Fic. 104.—Section of a Lymph Gland. a, capsule ; b, germ centres of cortex ; c, sinuses ; d, trabecula ; e, germ centres of medulla. destroyed by hemolytic agents the pigment and the iron derived from the hemoglobin are often found abundantly in the cells in the sinuses of lymph and hemolymph glands. 2. Bone Marrow.—The structure of bone marrow is con- sidered below, but it may be stated here that young leuco- cytes or leucoblasts, in the condition of mitosis, are abundant, and that they pass away in the blood stream. They are of all varieties. In certain pathological conditions the formation of these cells is increased and a leucocytosis results (fig. 105). Il. Erythrocytes.—In the embryo these cells seem to be formed by a process of budding from the mesoblast cells, which become vacuolated to form the primitive blood vessels. Later they develop in the liver and spleen, but after birth 218 VETERINARY PHYSIOLOGY they are formed in the red marrow of bone (fig. 105). Marrow consists of a fine fibrous tissue with large blood capillaries or sinuses running in it. In the fibrous tissues are numerous fat cells (clear spaces 0 in fig. 105) and generally a considerable number of multi-nucleated giant cells or myelocytes (d). In addition to these are the young leucocytes, leucoblasts (a.g.h.), and lastly young nucleated red cells, the erythroblasts (c.). After hemorrhage, the formation of these becomes unusually active, and may implicate parts of the Fic. 105.—Section of Red Marrow of Bone. a, lymphocyte; 6, fat cell; e¢, erythroblast ; d, giant cell; e, erythrocyte ; f, erythroblast in mitosis ; g, neutrophil myelocyte; h, eosinophil myelocyte; %, eosinophil leucocyte ; 7, polymorpho-nuclear leucocyte. marrow not generally concerned in the process, and hence the red marrow may spread from the ends of the long bones, where it is usually situated, towards the middle of the shaft. The nuclei of the erythroblasts atrophy or are shed and the cells escape into the blood stream. The red marrow has the power of retaining the iron of disintegrated erythrocytes, which are often found enclosed in large modified leucocytes or phagocytes. The iron is often very abundant after a destruction of erythrocytes. BLOOD AND LYMPH 219 VI. Total Amount of Blood in the Body This was formerly determined by bleeding an animal, measuring the amount of blood shed, and determining the amount of hemoglobin contained in it; then washing out the blood vessels, and after measuring the amount of fluid used, determining the amount of hemoglobin in it to ascertain the amount of blood it represented. By this method the amount of blood was found to be about +}, of the body weight in man. Haldane and Lorrain Smith have devised a method which can be applied to the living animal. It depends upon the fact that, after an animal or person has inhaled carbon monoxide, it is possible to determine to what proportion the gas has replaced oxygen in the oxyhemoglobin. If then an individual breathes a given volume of carbon monoxide, and if a measured speci- men of blood is found to contain a definite percentage of the gas, the rest of the gas must be equally distributed through the blood, and thus the amount of blood may be deduced. By this method they conclude that the blood is about 3), of the weight of the body in the human subject. VII. Distribution of the Blood Roughly speaking, the blood is distributed somewhat as follows :— Heart, lungs, large vessels . } Muscles } Liver . z q Other organs VIII Fate of the Blood Constituents The water of the blood, constantly renewed from outside, is constantly got rid of by the kidneys, skin, lungs, and bowels. About the fate of the proteins we know nothing. They are probably used in the construction of the tissues, but experimental evidence of this is wanting. The glucose and fat are undoubtedly used up in the tissues. The urea and waste products are excreted by the kidneys. The fate of the salts is not fully worked out. The chlorides 220 VETERINARY PHYSIOLOGY are partly excreted by the kidneys and are partly split up to form the hydrochloric acid required for stomach digestion. The phosphates and sulphates are excreted in the urine, but whether they are also used in the tissues is not known. The leueoeytes break down in the body—but when and how we do not know. We shall afterwards find that they are greatly increased in nuwber after a meal of proteins, and, since the increase is transitory, lasting only for a few hours, they are probably rapidly broken down, possibly to feed the tissues. It would thus seem that a leucocyte may live for only a short time in the blood. The erythrocytes also break down. How long they live is not known. It is found, after injecting blood, that the original number of corpuscles is not reached for about a fortnight, and hence it has been concluded that the corpuscles live for that period. The experiment, however, is far from conclusive, and must be accepted with reservation. Organs connected with Hemolysis.—The process of breaking down of old erythrocytes and eliminating their pigment is often called the process of hemolysis. Certain organs appear to be specially connected with it, but the precise part played by each of them is not very clearly understood. That the liver acts in this way is indicated, first, by the fact that the blood passing from the organ during digestion contains fewer erythrocytes than the blood going to it; second, by the formation in the liver cells of bile pigments, which are derivatives of hemoglobin ; third, by the presence of pigment and of iron in simple combinations in the liver cells under certain conditions. It is possible that the reabsorbed salts of the bile acids in the portal blood dissolve the pigment out of the old erythrocytes, and that the liver cells may then act upon the liberated pigment. Under ordinary conditions the liver does not store much iron. The spleen is generally said to have a similar action. This organ is composed of a fibrous capsule containing non-striped muscle and a sponge-work of fibrous and muscular trabecule, in the interstices of which is the spleen pulp. The branches of the splenic artery run in the trabecule, and twigs pass out from these trabecule, and are covered with masses of — lymph tissue forming the Malpighian corpuscles. Beyond BLOOD AND LYMPH 221 ' these, the vessels open into a series of complex sinuses lined by endothelial cells of large size, from which the blood is collected into channels, the venous sinuses, which carry it back to branches of the splenic vein in the trabecule. The pulp is thus comparable with the blood sinuses of the hemolymph glands, and the spleen may be considered as being a still further development of the hemolymph gland (fig. 106). So far no decrease in the number of erythrocytes in the blood leaving the spleen has been recorded. In the cells of the spleen pulp and chiefly in the endothelial cells yellow pigment and simple iron compounds are frequently found, SPLEEN HZMOLYMPH Fic. 106.—To show the relationship of the Spleen to Lymph Glands and Hemolymph Glands. The black indicates lymphoid tissue ; the coarsely spotted part, lymph sinuses, and the finely dotted part, blood sinuses, (LEwIs. ) indicating that hemoglobin is being broken down. But the idea that the spleen plays an important part in the actual destruction of erythrocytes seems to be negatived by the fact that, when blood is injected, the cells are broken down no faster in an animal with the spleen intact than in an animal from which the spleen has been removed. While the spleen appears to have no action in killing and destroy- ing erythrocytes, its cells, like those of the sinuses of the Lymph Glands and Hemolymph Glands, have the power of taking up dead and disintegrating erythrocytes and storing the iron for future use in the body. These organs may, in fact, be regarded as the graves of the dead erythrocytes. 222 VETERINARY PHYSIOLOGY The non-striped muscle in the framework of the spleen undergoes rhythmic contraction and relaxation, and the organ thus contracts and expands at regular intervals of about a minute. . These movements may be recorded by enclosing the organ in an oncometer, a closed capsule connected with some form of recording apparatus. The movements are controlled by fibres leaving the spinal cord chiefly in the 6th, 7th, and 8th dorsal nerves of both sides. Strong stimulation of these causes contraction. B. LYMPH Characters of Lymph.—The lymph is the fluid which plays the part of middleman between the blood and the tissues, It fills all the spaces in the tissues and bathes the individual cell elements. These spaces in the tissues open into vessels— the lymph vessels—in which the lymph flows and is conducted through lymph glands and back to the blood through the thoracic duct (see fig. 107, p. 225). Lymph varies in character according to the situation from which it is taken and according to the condition of the animal. Lymph taken from the lymph spaces—e.g. the pericardium, pleura, or peritoneum—is a clear straw-coloured fluid. It has little or no tendency to coagulate. Microscopic examination shows that it contains few or no cells—any cells which may exist being lymphocytes. In reaction it is alkaline. The specific gravity varies according to its source, being lowest when from the limbs and highest when from the liver. Apparently the cause of the non-coagulation of such lymph is the absence of cells from which thrombokinase may be set free. If blood or white corpuscles be added to it, a loose coagulum forms. If the lymph be taken from lymphatic vessels after these have passed through lymphatic glands, it is found to contain a number of lymphocytes, and it coagulates readily. Chemically, lymph resembles blood plasma in which the proteins are in smaller amount, but the inorganic salts in BLOOD AND LYMPH 223 the same proportion as in the blood. The amount of solids varies in lymph from different organs. Lymph of Proteins, Limbs ; , mo 2-3 per cent. Intestines . ; i . 46 ,, Liver : , ‘ ‘ 6-8 ,, In the lymphatics coming from the alimentary canal during starvation, the lymph has the characters above described. But after a meal it has a milky appearance and is called ehyle. This milky appearance is due to the presence of fats in a very fine state of division, forming what is called the molecular basis of the chyle. Lymph in various diseases tends to accumulate as serous effusions in the large lymph spaces—eg. the pleura, peri- toneum, pericardium—and these effusions behave differently as regards coagulation. The following table helps to explain this (S.A. is Serum Albumin, 8.G. Serum Globulin) :— COAGULABILITY OF LYMPH, SERUM, AND EFFUSIONS. oa Serous Effusion. Serum. Coag. accel a Uncoag. Uncoag. S. A. S. A. S. A. S. A. S.A. 8. G. 8. G. S. G. S. G. S. G. Fibrinogen. Fibrinogen. | Fibrinogen. es Set Thrombin. Thrombin. | sa ane Thrombin. 2. Formation of Lymph.—Lymph is derived partly from the blood and partly from the tissues. The formation of lymph from the blood depends upon the permeability of the walls of the capillaries and the pressure of blood in the blood vessels. Thus, although the pressure in the blood vessels of the limbs is much higher than the pressure in the vessels of the liver, hardly any lymph is usually produced in the former, while very large quantities are produced in the latter —apparently because of the small permeability of the limb capillaries and the great permeability of the hepatic capil- laries. The permeability may be increased by anything which 224 VETERINARY PHYSIOLOGY injures the capillary wall. Thus the injection of hot water or of proteoses at once leads to an increased flow of lymph. While the permeability of the vessel wall is the most import- ant factor controlling lymph formation, any increase of the intra-vascular pressure of a region increases the flow of lymph, and for this reason any obstruction to the free flow of blood from a part leads to increased lymph production from that ~ area. That lymph is also formed from the tissues is indicated by the fact that the injection of substances of high osmotic equivalent into the blood—such as sugar or sodium sulphate —leads, by a process of osmosis, to a flow of fluid into the blood, so that it becomes diluted, and also to an increased formation of and flow of lymph, and this increase of water in both can be explained only by its withdrawal from the tissues, THE CIRCULATION I. GENERAL ARRANGEMENT THE arrangement by which the blood and lymph are dis- tributed to the tissues may be compared to a great irrigation system. It consists of a central force pump—the systemic heart (fig. 107, S.H.)—from which pass a series of conducting tubes—the arteries—leading off to every part of the body, and ending in CAP PH S.A. Fie. 107.—Scheme of the Circulation. S.H., systemic heart sending blood to the capillaries in the tissues, Cap. The blood brought back by veins and the exuded lymph by lymphatics, Zy., passing through glands ; blood sent to the alimentary canal, 4/.C., and from that to the liver, Liv. ; blood also sent to the kidneys, Kid. ; the blood before again being sent to the body is passed through the lungs by the pulmonic heart, P. H. innumerable fine irrigation channels—the capillaries (Cap.)— in the substance of the tissues. From these, a considerable proportion of the blood constituents is passed into the spaces between the cells as lymph. From these spaces the fluid is either passed back into the capillaries, or is conducted away in a series of lymph vessels, which carry it through lymph glands 225 15 a VETERINARY PHYSIOLOGY (Ly.), from which it gains certain necessary constituents, and finally bring it back to the central pump. | The fluid, which has not passed out of the capillaries into the tissues, has been deprived of many of its constituents, and this withdrawal of nutrient material by the tissues is made good by a certain quantity of the blood being sent through the walls of the stomach and intestine (A/.C.), in which the nutrient material of the food is taken up and added to the blood returning to the heart. At the same time, the waste materials added to the blood by the tissues are partly got rid of by a certain quantity of the blood being sent throne the liver and kidneys (liv. and Kid. ). The blood is then poured back, not at once into the great pump which sends it through the body, but into a subsidiary pump—the pulmonic heart (P.H.)—by which it is pumped through the lungs, there to obtain a fresh supply of oxygen, and to get rid of the carbon dioxide excreted into it by the. tissues. Finally the blood, with its fresh supply of oxygen from the lungs, and of nourishing substances from the ali- mentary canal, is poured into the great systemic pump— the left side of the heart—again to be distributed to the tissues. Thus the circulation is arranged so that the blood, exhausted of its nourishing material by the tissues, is replenished in the body before being again supplied to the tissues. The seetional area of this irrigation system varies enormously. The aorta leaving the heart has a comparatively small channel. If all the arteries of the size of the radial were cut across and put together, their sectional area would be many times the sectional area of the aorta. And, if all the capillary vessels were cut across and placed together, the sectional area would be about 700 times that of the aorta. From the capillaries, the sectional area of the veins and lymphatics steadily diminishes as the smaller branches join with one another to form the larger veins and lymphatics; but, even at the entrance to the heart, the sectional area of the returning tubes, the veins, is about twice as great as that of the aorta (fig. 132, p. 288). The circulatory system. may thus be compared to a stream which flows from a narrow deep channel, the aorta, into a CIRCULATION 227 gradually broadening bed, the greatest breadth of the channel being reached in the capillaries. From this point the channel gradually narrows until the heart is reached. Hence the blood stream is very rapid in the arteries where _ the channel is narrow, and very sluggish in the capillaries where the channel is wide, so that in them plenty of time is allowed for exchanges between the blood and the tissues. II. THE CENTRAL PUMP—THE HEART A. Structure A very simple form of heart exists in the ascidians. At one point on a large vessel there is a thickening in the wall composed of non-striped muscular fibres. A contraction is seen to pass from one end of this to the other at frequent regular intervals, thus forcing the fluid through the vessels. The embryonic heart in man has a similar structure. In the snail and cuttle-fish, in addition to the contracting muscular thickening, there is also a thin-walled receiving chamber into which the blood flows before it is expelled onwards. The heart is thus composed of two chambers. 1st. A receiving chamber—the auricle. 2nd, An expelling chamber—the ventricle. In fish the heart has a similar structure. But in lung- bearing animals a more complex arrangement is required, and a double heart is found, one concerned with the pro- pulsion of blood to the-system generally, and hence called the systemie heart; one propelling blood to the lungs, and hence called the pulmonie heart. In mammals, the former chamber is on the left side, the latter on the right. Each consists of a receiving and expelling chamber—an auricle and a ventricle. The walls of these chambers are essentially muscular; but this muscular layer, or myocardium, lies between two fibrous layers, the pericardium and the endocardium. The museulature of the auricles is separate from that of the ventricles, but some fibres more like ordinary visceral fibres than cardiac muscle extend from one to the other. This band of His’ plays a most important part in conducting contraction started in the auricles to the ventricles. If the 228 VETERINARY PHYSIOLOGY heart be boiled, the auricles, the aorta and the pulmonary artery may be separated from the ventricles. This is because boiling converts fibrous tissue to gelatine and dissolves it, and it is by white fibrous tissue that the auricles and great arteries are attached to the ventricles. This tissue is arranged in three rings, one encircling the opening between the right auricle and ventricle, and crescentic in shape; one, more circular in shape, encircling in common the left auriculo- ventricular and the aortic orifice, and one encircling the pulmonary opening. The auricles are attached to the auri- culo ventricular rings above, the ventricles are attached below, while the valves of the heart are also connected’ with them. The muscular fibres of the auricles are arranged in two badly-defined layers— 1st. An outer layer runs horizontally round both auricles. 2nd, An inner layer arches over each auricle, and is con- nected with the auriculo-ventricular rings. Contraction of the first layer diminishes the capacity of the auricles from side to side. Contraction of the second pulls the auricles downwards towards the ventricles, and thus diminishes their capacity from above downwards. The peculiar striped muscle fibres of the auricular wall extend for some distance along the great veins which open into these chambers. The left ventricle forms the cylindrical core to the heart, and the right ventricle is attached along one side of it. The septum between the ventricles is essentially the right wall of the left ventricle, and it bulges into the right ventricle with a double convexity from above downwards and from before backwards (figs. 108 and 110). The muscle fibres of the ventricles are arranged essentially in three layers— 1st. The outmost layer takes origin from the auriculo- ventricular rings, and passes downwards and to the left till it reaches the apex of the heart. Here it turns inwards, forming a sort of vortex, and becomes continuous with the inmost layer. 2nd. The middle layer is composed of fibres running hori- zontally round each ventricle. It is the thickest layer of a fe CIRCULATION 229 the heart, and in contracting it pulls the walls of the ven- tricles towards the septum ventriculi. 3rd. The inmost layer is continuous with the outmost layer, as it turns in at the apex. It may be considered as composed of two parts— (a) A layer of fibres running longitudinally along the inside of each ventricle from the apex upwards to the auriculo- ventricular ring. These fibres are raised into fleshy columns, the ecolumnz ecarnez. (6) A set of fibres, constituting the papillary muscles (fig. 109, P.M.), which, taking origin from the apical part of the Fic. 108.—Cross Section through the Ventricles of the Heart looking towards Auricles, to show the right Ventricle placed on the Central Core of the left Ventricle. The cusps of the Auriculo-ventricular Valves are also shown. ventricles, extend freely upwards to terminate in a series of tendinous cords (the chords tendines), which are inserted partly into the auriculo-ventricular valves, presently to be described, and partly into the auriculo-ventricular rings. The papillary muscles are merely specially modified column carnee. In many cases, actual muscular processes extend from the apex of the papillary muscles to the auriculo- ventricular ring. In the left ventricle there are two papillary muscles, or groups of papillary muscles, one in connection with the anterior wall of the ventricle, and one in connection with the posterior wall. 230 VETERINARY PHYSIOLOGY In the right ventricle there are—lst. One or more small horizontally running papillary muscles just under the pul- monary orifice, their apices pointing backwards—(fig. 109, S.P.M.). . 2nd. A large papillary muscle taking origin from the mass of fleshy columns at the apex of the ventricle (4.P.J/). 3rd. One or more papillary muscles of varying size arising from the posterior part of the apical portion of the ventricle (PP aL}. . 4th, A number of small septal papillary muscles arising from the septum. Fic. 109.—The Right Ventricle and Tricuspid Valve to show the relationship of the Papillary Muscles and Chorde Tendinex to the Cusps of the Valve. (See text.) The distribution of the chorde from these muscles will be considered in connection with the auriculo-ventricular valves. In contraction, the outmost and inmost layers of the ventricles tend to approximate the apex to the base of the ventricles, but this is resisted by the contracting middle layer. The apex tends to be tilted towards the right, the papillary muscles shorten, the column carne by their shortening and thickening encroach upon the ventricular cavity, and help to abolish it, while the auriculo-ventricular rings are drawn downwards and inwards towards the septum. CIRCULATION 231 The endoeardium forms a continuous fibrous layer, lined by endothelium, extending from the vessels over the inner aspect .of auricles and ventricles. At certain points flaps of this endocardium are developed to form the valves of the heart. In the heart, valves are situated at the entrance to and at the exit from the expelling cavities. There is thus on each side of the heart a valve between the auricles and ventricles, and a valve be- tween the ventricles and the great arteries. Auriculo - ventricular Valves.—On each side of the heart the auriculo-ven- tricular valve is formed by flaps of endocardium, which hang downwards from the auriculo-ventricular ring like a funnel into the ventricular cavity, and which are attach- ed to the apices of the papil- lary muscles by the chorde tendinee (figs. 109 and 110). On the left side of the heart there are two main cusps, forming the mitral valves (fig. 110)— Fic, 110.—Vertical Mesial Section through 4 Heart to show Aortic and Mitral Valves. 1st. An anterior on right R. V., right ventricle ; L. V., left ventricle cusp, which takes origin with papillary muscle ; L.A., left auricle ; from, and is continuous with, Ao., aorta with anterior cusp on top of the right posterior wall of septum. the aorta. It hangs down into the ventricle between the aortic and auriculo-ventricular orifices, thus dividing the ventricle into two parts, an aortic and an auricular part. This cusp is very strong, and in many animals bone is developed in it towards its base. It is composed of dense fibrous tissue, is smooth on both sides, and the chorde are inserted chiefly along its edges. 2nd. The posterior or left cusp takes origin from the back 232 VETERINARY PHYSIOLOGY part of the auriculo-ventricular ring, and hangs in the ventricle in its relaxed state against the posterior and left wall. It is smaller and less strongly made than the anterior cusp. The chorde tendinez are not only inserted into its edge, but run up along its posterior aspect to be inserted into the auriculo- ventricular ring, and they thus give the posterior aspect of the cusp a rough ridged appearance. When the papillary muscles contract, the cusps are drawn together. The edge of each cusp thins out to form a delicate border, which, when the cusps are approximated, completely seals the aperture. On the right side of the heart the auriculo-ventricular orifice is separate from the pulmonary opening, and the three cusps of the trieuspid valve are developed in connection with the crescentic opening from the auricle (fig. 108). One rises from the ring above the septum, and hangs down into the ventricle upon the surface of the septum. This cusp is small, thin, and delicate. It is attached by its lower border to the septal papillary muscles. The chief or infundibular cusp (fig. 109, I.C.) rises from the front part of the ring between the pulmonary infundibulum and the auriculo-ventricular opening. It is connected by its anterior border with the horizontal fibres . from the superior papillary muscles, and by its lower and inferior border with the chorde from the anterior papillary muscle. When these two sets of papillary muscles contract, this cusp is drawn flat against the bulging septum. The posterior cusp (P.C.) takes origin from the posterior and outer part of the ring, and hangs down into the posterior part of the ventricle. It is connected by its anterior margin with the anterior papillary muscle and by its posterior margin with the posterior papillary muscle. Contraction of these muscles therefore approximate its anterior edge to the infundibular cusp, its posterior edge to the cee cusp, and pull it towards the septum. In both the infundibular aa posterior cusps many of the chord pass up to be inserted into the auriculo-ventricular ring. Semilunar Valves.—The valves, situated at the opening of the ventricles into the great arteries, are also formed as special developments of the endocardium, Each is composed of three half-moon-shaped membranous ye CIRCULATION 233 pouches attached along their curved margin to the walls of the artery and upper part of the ventricle, and with their concavities directed away from the ventricle. In the centre of the free margin is a fibrous thickened nodule, the corpus Arantii, from which a very thin piece of membrane, the lunule, extends to the attached margin of the edges. A pouch, the sinus of Valsalva, lies behind each cusp. The arrangements of these various cusps is of importance in connection with their action (fig. 110). Aortic Valve.—The anterior cusp is largest, and lies some- Fic. 111.—Relations of the Thoracic Viscera in the Horse, C., heart ; Dp., the diaphragm ; ecxsp., in expiration ; cnsp., in inspiration, (From ELLENBERGER. ) what deeper iv the heart than the others. At each side it is attached to the aortic wall, but below it is attached to the upper part of the septum ventriculi, so that the base of the ‘sinus of Valsalva is formed by the upper part of the septum. At a somewhat higher level is a cusp which is. partly attached to the upper part of the septum, partly to the posterior wall of the aorta, where this becomes continuous with the anterior cusp of the mitral. The third cusp is still higher, and is attached to the aortic wall, where it becomes continuous with the anterior cusp of the mitral. 234 VETERINARY PHYSIOLOGY Pulmonary Valve.—''he posterior cusp is mounted on the top of the septum ventriculi, and is at a somewhat lower level than the other two. Thus, in each valve, the cusp placed lowest is mounted on a muscular cushion, the use of which will afterwards be con- sidered. Attachments and Relations of the Heart (fig. 111)—In the horse the heart hangs downwards from the vertebra] column, and the apex is in relation to the posterior end of the sternum and a little to the left. Behind the heart is in relation to the tendon of the diaphragm. All round it are the lungs, completely filling up the rest of the thorax. The heart is enclosed in a strong fibrous bag, the Perieardium, which supports it and prevents over-distension. When fluid accumulates in this bag the auricles are pressed upon and the tlow of blood into them is impeded. B. Physiology of the Heart The Cardiae Cyele Each part of the heart undergoes contractions and relaxa-— tions at regular rhythmical intervals, and the sequence of events frum the occurrences of any one event to its recurrence constitutes the cardiac cycle. . A. Frog.—tIn the frog «a contraction, starting from the openings of the veins, suddenly involves the sinus venosus, causing it to become smaller and paler. This contraction is rapid and of short duration, and is followed by a relaxation, the cavity again regaining its former size and colour, As- this relaxation begins, the two auricles are suddenly contracted and pulled downwards towards the ventricle; at the same time becoming paler, while the ventricle becomes more distended — and of a deeper red. The rapid brief auricular contraction now gives place to relaxation, and, just as this begins, the ventricle is seen to become smaller and paler, and, if held in the finger, is felt.to become firmer. This event takes place more slowly than the contraction of either sinus or auricles. » CIRCULATION 235 The chief change in the ventricle is a diminution in its lateral diameter, though it is also decreased in the antero-posterior and vertical directions. During ventricular con- UN: a traction the bulbus is seen SINUS. to be distended and to eae ! become of a darker colour, 4VALES r : ! I The ventricular contrac- ee aha tion passes off suddenly, VENTRICLE - the ventricle again be- : os ' | as coming larger and of a ee aioe deep red colour. At this Iss! moment the bulbus aortée oa contracts and becomes pale and then relaxes eee AS VS. 85 PP Fic. 112,—Scheme of the Cardiac Cycle in the Frog. S.S., sinus systole; 4.8., auri- before the next ventricu- cular systole ; V.S., ventricular systole ; lar contraction. ( Practical B.S., bulbus systole ; P., rest of all cham- Physiology.) bors, Each chamber of the heart thus passes through two phases— a contraction phase, a systole of short duration, and a longer re- laxation phase, the diastole. And the sequence of events in the frog’s heart might be schematically represented as in fig. 112. B. Mammal.—1. Rate of Reeurrence.—The rate of recurrence of the cardiac cycle varies with the animal examined. In the adult horse it is about 36 to 40 per minute. Many factors modify the rate of the heart. Rate of heart per minute in different animals :— Horse : : F 36 to 40 Ox. : : : 45 to 50 Sheep : : 70 to 80 Dog . ‘ , 90 to 100 Rabbit : . 120 to 150 1, Period of Life—The following table shows the average rate of the heart at different ages :— HORSE Newborn . 92 to 132 per minute. Under l year. . 50 to 68 = 4 years : : 5 50 to 56 a 236 . VETERINARY PHYSIOLOGY 2. Temperature of the Body.—The pulse varies with the body temperature, being increased as the temperature rises. 3. Muscular Exercise increases the rate of the heart, first by driving the blood from the muscles into the great veins (p. 291), and second, by developing substances such as CO,, which act directly upon the cardiac and respiratory mechanisms. 4, Stimulation of certain nerves—especially those of the abdomen—tends to cause a retardation in the rate of the heart (p. 256). 2. Sequence of Events.—The sequence of events making up the cardiac cycle is simpler in the mammal than in the frog. The contraction starts in the great veins which enter the auricles, and spreads down along them to these chambers. This corresponds to the contraction of the sinus in the frog’s heart. It is followed by a short sharp contraction of the auricles, which become smaller in all directions and seem to be pulled down towards the ventricles. The contraction of the auricles in mammals is not accompanied by so marked a dilatation of the ventricles as in the frog. After the auricles have fully contracted, the contraction of the ventricles begins, and immediately the auricles relax and resume their original size, _ The ventricular contraction develops suddenly, lasts for some time, and then suddenly passes off. The contraction of the ventricles is followed by a period during which both auricles and ventricles remain relaxed. This is called the pause of the cardiac cycle. The cardiac cycle in mammals may be represented as in fig, 113. 3. Duration of the Phases.—Ventricular systole lasts three times as long as auricular systole. The duration of these two phases in relationship to the pause varies very greatly. Whatever may be the rate of the heart, the auricular and ventricular systoles do not vary, but in a rapidly acting heart the pause is short, in a slowly acting heart it is long. 4, Changes in the Shape of the Chambers. 1. Awricles—These simply become smaller in all directions during systole. | i cael . Pd ’ CIRCULATION 237 2. Ventricles.—The changes in the diameters of the ventricles may be studied by fixing them in the various phases of contraction and measuring the alterations in the various diameters. The shape in dvzastole may be investigated after death stiffening has passed off and has left the walls relaxed. The condition at the end of systole may be studied by rapidly excising the heart while it is still beating and plunging it in some hot solution to fix its contraction. The condition in the early stage of systole, before the blood has left the ventricles, may be studied by applying a ligature round the great vessels and then plunging the heart in a hot solution to cause it to contract round the contained blood which cannot escape. Measurements of hearts so fixed show that at the begin- 4 Ve ‘AURICLES / ae ao VENTRICLES | : 1A5 VS. P Fic. 113.—Scheme of the Cardiac Cycle in the Human Heart. 4.8, auricular systole; V.S., ventricular systole ; P., pause. ning of contraction the antero-posterior diameter is increased, while the lateral diameter is diminished. In contracting, the lateral walls appear to be pulled towards the septum— the increase in the antero-posterior diameter being largely due to the blood in the right ventricle pressing on and pushing forward the thin wall of the conus. As the ventricles drive out their blood, both antero- posterior and lateral diameters are diminished—but the diminution in the lateral direction is the more marked. There is no great shortening in the long axis of the heart. Although the contraction of the longitudinal fibres tends to approximate base and apex, this is in part prevented by the contraction of the circular fibres. 5. Changes in the Position of the Heart.—During contraction the heart undergoes, or attempts to undergo, a change in 238 VETERINARY PHYSIOLOGY . position. In the relaxed condition it hangs downwards and to the left from its plane of attachment, but when it becomes rigid in ventricular contraction it tends to take a position at right angles to its base—Cor sese erigere, as. Harvey describes the movement. Since the apex and front wall are in contact with the chest, the result of this movement is to press the heart more forcibly against the chest wall. This gives rise to the cardiac impulse which is felt with each ventricular — systole over the precordium (fig. 111). - A Fig. 114.—Cardiograph consisting of a Receiving Tambour, with a button on the Membrane which is placed upon the Cardiac Impulse, and a Recording ~ Tambour connected with a Lever. If the chest is opened and the animal placed on its back this elevation of the apex is readily seen. If the animal is placed on its belly, so that the heart when relaxed hangs forwards, the apex is tilted back during contraction. The increased thickness of the heart from before backwards also assists, to some extent, in the production of the impulse. In character it is felt as a forward impulse of the chest wall, which develops suddenly, persists for a short period, and then suddenly disappears. _ The cardiac impulse may be recorded graphically by means CIRCULATION 239 _ of any of the various forms of eardiograph, one of the simplest consisting of a receiving and recording tambour connected by means of a tube (fig. 114). (Practical Physiology.) The form of the trace varies according to the part of the heart upon which the button is placed, but it has the character shown in fig. 115 if the button is upon the cardiac impulse. At the moment of ventricular systole the lever is. suddenly thrown up to a certain level (a to 0b). From this point it suddenly falls slightly (6.to ¢), but is maintained during the ventricular systole above the abscissa (¢ tod). At the end of the ventricular systole, as the heart falls away from the chest wall, the lever falls to its original level (d to ¢). In many _ tracings a small rise of the lever may be seen just before the great upstroke. This corresponds to the contraction of the auricles. In various diseases of the heart the cardiogram is materially a’ 6€cde Fic. 115.—Cardiographic Trace. @ to d, ventricular contraction. modified. Hence it is important to have a clear conception of the various parts of the trace. The elucidation of the various parts of the cardiogram is only possible after careful study of the other changes in the heart during the cycle. ‘6. Changes in the Intracardiae Pressure. These can be studied only in the lower animals. The most common way of determining the pressure in a cavity is to connect it to a vertical tube and to see to what height the fluid in the cavity is raised. If such a method be applied to the ventricles of the heart, the blood in the tube undergoes such sudden and enormous changes in level that it is impossible to get accurate results. The same objection applies to the method of connecting the heart with a U tube filled with mercury. When this is done the changes in pressure are so sudden and so extensive that 240 VETERINARY PHYSIOLOGY the mercury cannot respond to them on account of its inertia. Various means of obviating these difficulties have been devised. One of the best is to allow the changes of pressure to act upon a small elastic membrane tested against known pressures, A tube is thrust through the wall of the heart and connected with a tambour covered by a membrane to which a lever is attached. A. Pressure in the Great Veins (small dotted line in fig. 116).—When the auricles contract, the flow of blood from the great veins into these chambers is arrested, and, as a result, the pressure in the veins rises. As the auricles relax the blood is sucked from the veins and the pressure falls, but, as the auricles fill up, it again rises. When the ventricles relax and suck blood from the auricles, blood again flows in from the great veins and the pressure falls, again to rise as the auricles and veins are both filled up, towards the end of the pause. B. Pressure in the Aurieles (dash line in fig. 116).—At the moment of auricular contraction there is a marked rise in the intra-auricular pressure. When the auricular systole stops, the pressure falls rapidly, reaching its lowest level when the ventricles are throwing their blood into the arteries. From this point the pressure in the auricles rises until the moment when the ventricles relax, when another fall in the pressure is observed. The pressure remains about constant from this point until the next auricular contraction. C. Pressure in the Ventrieles (continuous line in fig. 116).— The intra-ventricular pressure suddenly rises at the moment of ventricular systole to reach its maximum. From this it falls, but the fall is gradual, and is interrupted by a more or less well-marked period during which the pressure remains constant. As the ventricles relax the pressure suddenly falls to below zero, and then rises to a little above zero, at which it is maintained until the next ventricular systole. The diastolic expansion of the ventricle is in part due to the elasticity of the muscular wall, and in part to the filling of the coronary arteries which takes place only as the muscular fibres relax. D. Pressure in the Arteries (dot-dash line in fig. 116).— The aortic pressure is high throughout. There is a sudden rise as the blood rushes out of the ventricles. The pressure then CIRCULATION 241 falls, but the fall is not steady. Often it is interrupted by a more or less marked increase corresponding to the later part of _AS. V.S. itd AS. PRESSURE IN Artery. Auricle. ee en) Great Veins ae ay Ventricle. FLOW of BLOOD fiom 1. Great Veins to Auricles. 2. Auricles to Ventricles. S. Ventricles to Arteries. CLOSURE of 1. Auriculo-Ventricular Valve. 2. Semilunar Valves. NAW AL NA SOUNDS of HEART. CARDIAC IMPULSE. Fic, 116.—Diagram to show the relationship of the events in the Cardiac Cycle to one another. A.S., auricular systole ; V.S., ventricular systole ; 7., pause. the ventricular contraction. At the moment of ventricular diastole, the fall is very sharp and is interrupted by a well- 16 242 VETERINARY PHYSIOLOGY marked and sharp rise. Following this the fall is continuous till the next systolic elevation. -In the dog the extent of variation of the pressure in auricles and ventricles is roughly as follows—measured in millimetres of mercury— Left Right Right Ventricle. Ventricle. Auricle. Maximum . . +140 + 60 +30. Minimum . 3) 80 —15 — 7 These changes in the pressure in the different’ chambers are due— lst. To the alternate systole and diastole of the chambers, the first raising, the second lowering the pressure in the chambers. 2nd. To the action of the valves. 7. Action of the Valves of the Heart. A, Auriculo-ventricular (fig. 117)—These valves have been YUU Fie. 117.—State of the various parts of the Heart throughout the Cardiac Cycle. 1, auricular systole ; 2, beginning of ventricular systole (latent period) ; 3, period of outflow from the ventricle ; 4, period of residual contraction ; 5, beginning of ventricular diastole, already described as funnel-like prolongations of the auricles into the ventricles. They are firmly held down in the ventricular cavity by the chorde tendinee. When the ventricle contracts the papillary muscles pull the cusps of the valves together and thus occlude the opening between auricles and ventricles. The cusps are further pressed face to face by the increasing pressure in the ventricles, and they may become convex towards the auricles. They thus form a central core around and upon which the ventricles contract. On the left side of the heart the strong anterior cusp of the mitral valve does not materially shift its position. It CIRCULATION 243 may be somewhat pulled backwards and to the left. The posterior cusp is pulled forwards against the anterior. On the right side the infundibular cusp of the tricuspid valve is stretched between the superior and inferior papillary muscles, and is thus pulled towards the bulging septum, against which it is pressed by the increasing pressure inside the ventricles. The posterior cusp has its anterior margin pulled: forward and its posterior margin backwards, and is thus also pulled toward the septum. The septal cusp remains against the septum. The greater the pressure in the ventricle the more firmly are these cusps pressed against one another and against the septum, and the more completely is the orifice between the auricle and the ventricle closed. On the right side of the heart other factors play an import- ant part in occluding the orifice; the muscular fibres which surround the auriculo-ventricular opening contract, while the papillary muscles pull the auriculo-ventricular ring downwards and inwards throug]: the chord which are inserted into it. Nevertheless the occlusion of this orifice is apt to be in- complete when the right side of the heart becomes in the least over-distended, giving rise to a safety-valve action from the right ventricle. The auriculo-ventricular valves are open during the whole of the cardiac cycle, except during the ventricular systole (fig. 116). - B. Semilunar Valves.—Before the ventricles contract these valves are closed and the various segments pressed together by the high pressure of blood in the aorta. As the ventricles contract the pressure in them rises, until the intra-ventricular pressure becomes greater than the pressure in the arteries. Instantly the cusps of the valves are thrown back and remain thus until the blood is expelled. When the outflow of blood is completed, the cusps are again approxi- mated by the pressure of blood in the arteries. As relaxation of the ventricles occurs, the intra-ventricular pressure becomes suddenly very low, and the high pressure of the blood in the arteries at once falls upon the upper surfaces of the cusps, which are thus forced downwards and together and thus completely prevent any back-flow of blood. The prejudicial effect of too great pressure upon these cusps 244 VETERINARY PHYSIOLOGY is obviated by the lower cusp being mounted on the top of the muscular septum upon which the pressure falls—the other cusps shutting down upon this one (fig. 110). The Flow of Blood through the Heart The circulation of blood through the heart depends upon these differences of pressure in the different chambers and upon the action of the valves. A fluid always flows from a point of high pressure to a point of lower pressure. We may then consider the flow— A, From Great Veins into Auricles.—This occurs when the pressure in the great veins is greater than the pressure in the auricles (fig. 116). The pressure in the auricles is lowest at the moment of their diastole. At this time there is therefore a great flow of blood | into them, but gradually this becomes less and less, until, when the ventricles dilate, another fall in the auricular pressure takes place and another rush of blood from the great veins. occurs, Gradually this diminishes, and by the time that the auricles contract the flow from the great veins has stopped. The contraction of the mouths of the great veins, which precedes the auricular systole, drives blood from the veins into - the auricles, and, as these enter into contraction, no flow from the veins can occur and no back-flow from the auricles is possible (fig. 116). ; B. From Auricles to Ventricles—As the ventricles dilate, a _ very low pressure develops in them, and hence a great rush of blood occurs from the auricles. During the passive stage of ventricular diastole, the intra-ventricular pressure becomes nearly the same as the auricular, and the flow diminishes or may stop. When the auricles contract a higher pressure is developed, and a fresh flow of blood occurs into the ventricles. When the ventricles contract the auriculo-ventricular valves are closed, and all flow of blood from the auricles is stopped (fig. 116). U. From Ventricles to Arteries.—When the ventricles begin to contract the intra-ventricular pressure is low, while the pressure in the arteries is high and keeps the semilunar valves shut. As ventricular systole goes on the intra-ventricular pressure rises, until after about 0°03 of a second it becomes CIRCULATION 245 higher than the arterial pressure (Latent Period). Immediately the semilunar valves are forced open and a rush of blood occurs from the ventricles (Period uf Overflow). This usually lasts less than 0:2 second. If the ventricles are acting powerfully, and if the pressure in the arteries does not offer a great resistance to the entrance of blood, the ventricles rapidly empty themselves into the arteries, and the intra-ventricular pressure varies as shown in fig. 124, b, p. 267. If the heart, however, is not acting forcibly, or if the arterial pressure offers a great resistance to the entrance of blood, then the outflow is slow and more continued and in this case the trace of the intra-ventricular pressure is like fig. 124, a, with a well-marked Period of Residual Contraction. It is not so much the absolute force of the cardiac contraction or the absolute intra-arterial pressure which governs this, as the relationship of the one to the other. The heart may not be acting very forcibly, but still if the pressure in the arteries is low its action may be relatively strong. The Coronary Arteries, unlike all the other arteries, are filled during ventricular diastole. During systole they are com- pressed by the contracting muscle of the heart, and it is only when the compression is removed in diastole that blood rushes into them. This helps to dilate the ventricles. The interpretation of the various details of the Cardiogram is now rendered more easy. The ventricles, still full of blood, are suddenly pressed against the chest wall in systole. As the blood escapes into the arteries they press with less force, and hence the sudden slight downstroke (fig. 115, b to ¢). But, so long as the ventricles are contracted, the apex is kept tilted forward, and hence the horizontal plateau is maintained (c to @). The pressure of the apex disappears as the ventricles relax (e). Sounds of the Heart On listening in the region of the heart, a pair of sounds may be heard with each cardiac cycle, followed by a somewhat prolonged silence. These are known respectively as the First and Second Sounds of the Heart (fig. 116). (Practical Physi- ology.) By placing a finger on the cardiac impulse while listening 246 VETERINARY PHYSIOLOGY to these sounds it is easy to determine that the first sound occurs synchronously with the cardiac impulse—i.e. synchron- ously with the ventricular contraction. It develops suddenly, and dies away more slowly. In character it is dull and rumbling, and may be imitated by pronouncing the syllable lib. In pitch it is lower than the second sound. The seeond sound is heard at the moment of ventricular diastole. Its exact time in the cardiac cycle has been deter- mined by recording it on the cardiac tracing by means of a microphone. It develops suddenly and dies away suddenly. It is a clearer, sharper, and higher-pitched sound than the first. It may be imitated by. pronouncing the syllable dupp. According to the part of the chest upon which the. ear is placed, these sounds vary in intensity. Over the apical region the first sound is louder and more accentuated; over the base the second sound is more distinctly heard. The Cause of the Second Sound is simple. At the moment. of ventricular diastole, when this sound develops, the only occurrence which is capable of producing a sound is the sudden stretching of the semilunar valves by the high arterial pressure above them and the low intra-ventricular pressure below them. The high arterial pressure comes on them suddenly like the blow of a drum-stick on a drum-head, and, by setting aS valves in vibration, produces the sound. Aortic and Pulmonary Avreas.—The second sound has thus a dual origin—from the aortic valve and from the pulmonary valve; and it is possible by listening in suitable positions to distinguish the character of each of these. The aortic valve is placed behind the sternum at the level of the lower border of the third costal cartilage. But it is deeply situated. The aorta, passing upwards and forwards, lies in close relationship to the chest wall at the junction of the right side of the sternum and the right second costal cartilage. The sound produced by the valve is eouduoted up the aorta, and may be heard best in this “aortic area.’ On the other hand, the pulmonary valve lies in close re- lationship to the anterior chest wall—being covered only by the anterior border of the left lung—close to the edge of the CIRCULATION 247 sternum in the second left interspace. The pulmonary element of the second sound may best be heard here. , The Cause of the First Sound is by no means so simple. When it is heard, two changes are taking place in the heart, either of which would produce a sound. 1st. The muscular wall of the ventricles is contracting. 2nd. The stretching of the auriculo-ventricular valves. 1st. That the first factor plays an important part in the pro- duction of the first sound is proved by rapidly cutting out the heart of an animal, and while it is still beating—but without any blood passing through it to stretch the valves—listening to the organ with a stethoscope. With each beat the lab sound is distinctly heard. Apparently the wave of contraction, passing along the muscular fibres of the heart, sets up vibrations, and when these are conducted to the ear the external meatus picks out the vibration corresponding to its fundamental note, and thus produces the characters of the sound. 2nd. The auriculo-ventricular valves are being closed and subjected on the one side to the high ventricular pressure and on the other to the low auricular pressure. If the valves be destroyed or diseased the characters of the first sound are materially altered, or the sound may be entirely masked by a continuous musical sound—a murmur. Again, it has been maintained that a trained ear can pick out in the first sound the note corresponding to the valvular vibration. The idea that the impulse of the heart against the chest wall plays a part in the production of this sound is based upon the fallacious idea that the heart “fits” the chest wall. All ‘that it does is to press more firmly against it. Mitral and Tricuspid Areas—On account of the part played by the valves in the production of the first sound, it may be considered to be double in nature—partly due to the mitral valve, partly to the tricuspid. The mitral valve element may best be heard not over the area of the mitral valve—which lies very deep in the thorax—but over the apex of the heart, as at this situation the left ventricle, in which the valve lies, comes nearest to the thoracic wall and conducts the sound thither. The tricuspid element may be best heard over the area of the valve, and in listening to it 248 VETERINARY PHYSIOLOGY it is usual to go to the right extremity of the area in order, as far as possible, to eliminate the mitral sound. The best situation to select is at the junction of the fifth right costal cartilage with the sternum. Cardiac Murmurs.—When the valves are diseased and fail to act properly, certain continuous sounds called cardiac murmurs are heard. These owe their origin to the fact that, while a current of fluid passing along a tube of fairly uniform calibre is not thrown into vibrations and therefore produces no sound, when any marked alterations in the lumen of the tube occur—either a sudden narrowing or a sudden expansion— the flow of fluid becomes vibratory, and, setting up vibrations in the solid tissues, produces a musical sound. Such changes in the calibre of the heart are produced in two ways :— lst. By a narrowing, either absolute or relative, of the orifices between the cavities—stenosis. 2nd. By a non-closure of the valves—incompetence. Stenosis.—If one of the aurieulo-ventricular orifices , is narrowed, a murmur is heard during the period at which blood normally flows through this opening. A reference to fig. 116 at once shows that this occurs during the whole of ventricular diastole, and that the flow is most powerful during the first period of ventricular diastole and during auricular systole. ( . If the aortie or the pulmonary valve is narrowed the murmur will be heard (fig. 116) during ventricular systole. The narrowing need not be absolute. A dilatation of the artery will make the orifice relatively narrow, and will produce the same result. Incompetence.—lf the aurieulo-ventricular valves fail to close properly, then, during ventricular systole, blood will be driven back into the auricles, and a murmur will be heard during this period. If the aortie or pulmonary valve fails to close, the blood will regurgitate into the ventricle from the arteries during ventricu- lar diastole, and a murmur will be heard during this period. By the position at which these murmurs are best heard the pathological condition producing them may be determined. St ie CIRCULATION 249 Work of the Heart The heart in pumping blood through it is doing work, and the amount of work may be expressed in work units—e.g. kilo- grammetres (see p. 60). The method employed to measure this in the dog is to determine, by means of the cardiometer, the amount of blood expelled at each systole, and to find the resistance against which it is expelled. This gives the factors for determining the work done at each systole, and it is easy to calculate the total work in any given period (fig. 118). Nature of Cardiac Contraction The contraction of the ven- tricle lasts for a considerable period—0°3 seconds, Is it of the nature of a single contrac- en. = of - gen : : Fie 118,—Roy’s Cardiometer to measure It is impossible to tetanise the output of Blood from the Heart. heart muscle, even by rapidly b, heart in cardiometer chamber ; repeated induction shocks. ¢, piston recorder working on lever A single stimulus applied to —-*8##8t Tubber band, @. heart muscle produces a single prolonged contraction. Again, the mode of development of the currents of action does not indicate anything of the nature of a tetanus. With each beat of the ventricles the variation in the electric potential begins at the base and travels rapidly to the apex. This passage of the contraction wave along the fibres explains the great length of the ventricular systole as a whole. There can be no doubt that each contraction of heart muscle is of the nature of a muscle twitch. In this respect heart muscle resembles non- striped muscle. : It further resembles it in that the minimum stimulus is also a maximum stimulus—.e. the smallest stimulus which will make the muscle contract makes it contract to the utmost. 250 VETERINARY PHYSIOLOGY But while this is the case the strength of stimulus necessary to call forth a contraction varies at different periods. To produce another contraction while the muscle is already in the period of contraction is difficult, but as it relaxes it reacts more and more readily to stimuli. In cardiac muscle, perhaps more than in any other, the staircase increase in the extent of con- traction with a series of stimuli is manifested. In cardiac muscle the greater the resistance to contraction the stronger the force of contraction. Hence when extra blood is poured into the heart from the veins, or when the outflow from the ventricles into the arteries is impeded, the increased strain put upon the heart muscle is met by increased contraction, and the additional work thrown upon the organ is effectually performed. Not only is this the case when temporary dis- turbances of the circulation occur, but when these disturbances are permanent, the heart adapts itself to them, and, if it has continuously to perform extra work, its muscular wall hyper- trophies, just as the skeletal muscles grow by continual use. Of course, to allow such compensation to be established, the blood . supply to the heart muscle must be sufficient, and hence, when the coronary arteries are diseased, heart failure rapidly ensues. If the coronary arteries are clamped and then relaxed, a peculiar fibrillar contraction of the heart muscle occurs, The maintenance of this rhythmic contraction and relaxation seems to depend greatly upon the presence of certain kations in the circulating blood. A due admixture of salts of sodium, potassium, and calcium is essential. For the frog’s heart. Ringer finds that the proportions which give the best results are— NaCl : ’ 0°70 per cent. KCl : ; , 0°03 ‘a CaCl : 0025 +) Since an excess of calotans salts leads to tonic contraction, and since an excess of sodium or of potassium leads to relaxation, it has been concluded that these two phases are determined by the presence of these ions. How is the Rhythmic Contraction of the Heart maintained ? The mechanism is in the heart itself, for the excised heart continues to beat. CIRCULATION 251 ‘In considering what this mechanism is, it must be borne in mind that two distinct questions have to be investigated. 1st. How does the contraction, once started, pass in regular sequence from one part of the heart to the other ? 2nd. What starts each rhythmic contraction ?_ lst. Propagation of the Wave of Contraction.—In the heart of many of the lower animals, and in the embryo of mammals, no nervous structures are to be found, and the rhythmic contraction is manifestly simply a function of the muscular fibres. Even in the heart of animals with well-marked nerve cells in the walls of the heart, and with nerve fibres coursing among the muscular fibres, the conduction of the contraction is purely a function of the muscles. For if the heart of a frog be cut across and across, so that all nerve fibres are severed, the contraction passes along it. The rate at which the contraction travels is slow, only about 10 to 15 centi- metres per second. Since in the mammalian heart muscular continuity between auricles and ventricles through the band of His is of small extent, the wave of contraction is delayed at this point, and in the dying heart, and in various pathological conditions, the contraction frequently fails altogether to pass this block, and thus the ventricles either stop beating before the auricles, or respond to every second or third auricular contraction. 2nd. Starting Mechanism of Contraction.—In the early fcetal heart no nerve structures have been found, yet it beats regularly and rhythmically. In the apex of the ventricle of the frog there are no nerve structures, yet, if the apex be cut off and repeatedly stimulated at regular intervals with galvanic making and breaking stimuli, it will, after a time, begin to contract spontaneously, regularly and rhythmically. Not only so, but if the apex be tied on to a tube, and a stream of blood passed through it, it will again start con- tracting regularly and rhythmically. These experiments clearly show that regular rhythmic contraction is a function of cardiac muscle. In the cardiac cycle in the frog each contraction starts in the sinus. What part does the sinus take in initiating contraction ? 7 252 VETERINARY PHYSIOLOGY If a ligature be tightly applied between the sinus and auricles in the frog (Stannius’ Experiment), the sinus continues to beat, and the auricles and ventricle wswally stop beating for a longer or shorter period. But ultimately they begin to beat again. Hence it would seem that it is not any special mechanism in the sinus which is essential in starting cardiac contraction. A ligature subsequently applied between auricles and ventricle sometimes starts the ventricle, sometimes the auricles, sometimes neither. Hence we see that any part of the heart has the power of originating rhythmical contractions, although usually the sinus initiates it. The sinus more than any other part of the heart has the property of rhythmic contraction. (Practical Physiology.) We have no evidence that the nerve cells in the sinus or elsewhere have anything to do with this; and so far as we at present know, the initiation as well as the propaga- tion of the cardiac contraction is a function of the wine fibres. 3rd. Intra-eardiae Nervous Mechanism.— In the frog’s heart nervous structures exist, and are distributed as follows (fig. 119) :— 1st. In the wall of the sinus venosus there is a plexus of nerve cells and nerve fibres constituting the ganglion of the sinus (Remak’s ganglion). 2nd. In the inter-auricular septum a similar plexus constitutes the ganglion of the auricular septum. 3rd. In the auriculo-ventricular groove a plexus forms the auriculo-ventricular ganglion (Bidder’s ganglion). With these intra-cardiac ganglia the terminations of the nerves to the heart form definite synapses. Nerve cells exist in the mammalian heart, but there is not the same differentiation into distinct groups. Nevertheless they are abundant round the mouths of the great veins, round the edges of the inter-auricular septum, and round the auriculo-ventricular groove. - While there is no evidence that the nervous structures play an important part in starting or keeping up the con- tractions, there is evidence that they exercise a checking or controlling action. - CIRCULATION 253 Ii the region between the sinus and auricles in the frog’s heart is stimulated by the interrupted current from an induction coil, the heart is slowed or stopped. (Practical Physiology.) If atropine be first applied electric stimulation is without result. (Practical Physiology. ) These experiments seem to indicate that there is in the heart a checking mechanism which may be stimulated by electricity, and which is paralysed by atropine. 4th. Connections of the Heart with the Central Nervous System.—In the frog a branch from the vagus connects the Fic. 119.—Scheme of the various chambers of the Frog’s Heart and of the distribution of the intracardiac nervous mechanism. central nervous system with the heart. When the branch is cut no effect is produced, showing that it is not constantly in action; but when the lower end is stimulated, the heart is generally slowed or brought to a standstill. Sometimes the effect is not marked. The reason for this is that the cardiac branch of the vagus in the frog is really a double nerve derived in part from the spinal accessory and in part from fibres which reach the vagus from the superior thoracic sym- pathetic yanglion. If the spinal accessory is stimulated, the heart is always slowed; and if the sympathetic fibres are stimulated, it is quickened. Generally stimulation of the cardiac branch containing these two sets of fibres simply 254 VETERINARY PHYSIOLOGY gives the result of stimulating the former, but sometimes the stimulation of the latter masks this effect. (Practical Physiology.) - In the mammal three sets of nerve fibres pass to the heart :— . ¥ Fin 1st. The superior cardiac branch: of the vagus starts from near the origin of the superior laryngeal nerve, and passes to the heart to end in the endocardiuin (fig. 120, S.C.). 2nd. The inferior cardiac branch of the vagus leaves the main nerve near the recurrent laryngeal, and passes to join the superficial cardiac plexus in the heart (fig. 120, ZC). 3rd. The sympathetic nerve fibres come from the superior thoracic and inferior cervical ganglia, and also end in the superficial cardiac plexus (fig. 120, S.). Funetions of the Cardiac Nerves.—A. The Superior Cardiae Branch of the Vagus is an ingoing nerve. Section produces no effect; stimulation of the lower end causes no effect; stimulation of the upper end causes slowing of the heart and a murked fall in the pressure of blood in the arteries, and it may cause pain. ‘The slowing of the heart is a reflex effect through the inferior cardiac branch; and the fall of blood ‘pressure, which is the most manifest effect, is due to a reflex dilatation of the vessels of the abdomen, causing the blood to accumulate there, and thus to lessen the pressure in the arteries generally. On account of its effect on the blood pressure, this nerve is called éhe depressor nerve. B. Inferior Cardiac Branch of Vagus.—Section of the vagus or of this branch causes acceleration of the action of the heart. The nerve is therefore constantly in action. Stimula- tion of its central end has no effect; stimulation of its peripheral end causes a slowing or stoppage of the heart. It is therefore the checking or inhibitory nerve of the heart. 1. Course of the Fibres.—These fibres leave the central nervous system by the spinal accessory, and pass to the heart to form connections with the cells of the cardiac plexuses, 2. Centre-—The fibres arise from a centre in the medulla oblongata, which can be stimulated to increased activity either directly or reflexly. (1) Direct stimulation is brought about by (a) sudden anemia of the brain, as when the arteries CIRCULATION 255 to the head are clamped or occluded; (6) increased venosity of the blood, as when respiration is interfered with; (c) the f 7 Fie. 120.—Connections of the Heart with the Central Nervous System. Au., auricle; V., ventricle; V.D.C., abdominal vaso-dilator centre ; C.I.C., cardiac inhibitory centre; C.A.C., cardio-augmentor centre ; S.C., superior cardiac branch of the vagus ; J.C., inferior cardiac branch of the vagus with cell station in the heart ; S., cardio-sympathetic fibres with cell station in the lenticular ganglion ; V.D.Ab., vaso-dilator fibres to abdominal vessels, The continuous lines are outgoing, the broken lines are ingoing nerves. concurrent action of the respiratory centre (see p. 310). (2) Reflex stimulation is produced through many nerves. In 256 VETERINARY PHYSIOLOGY the rabbit stimulation of the 5th cranial nerve by the inhala- tion of ammonia vapours has this action, and in all animals stimulation of the abdominal nerves produces the same effect. This reflex stimulation of the centre is used to determine its position in the medulla. It can be induced after removal of the brain above the medulla, but destruction of the medulla entirely prevents it. 3. Mode of Action.—These inhibitory fibres appear to act by stimulating the local inhibitory mechanism in the heart; and when this has been poisoned by atropine, they cannot act. According to the observation of Gaskell, they excite in the heart anabolic changes, since the electric current of injury is increased when they are stimulated, indicating that the difference between the living part of the heart and the injured part is increased. 4. Result of Action. (a) The output of blood from the heart is diminished, and thus less blood is forced into the arteries, and the blood pressure falls (fig. 126). (6) The rhythm of both auricles and ventricles is slowed, but the effect on the auricles is more marked than upon the ventricles, and the ventricles may show a contraction rhythm independent of that of the auricles (fig. 121, A.). (c) The force of contraction of the auricles is decreased. In the ventricles the systole becomes less complete and the cavities become more and more distended, either as a result of decrease in the force of contraction or as a mechanical result of the accumulation of blood due to the decreased output per unit of time. In the heart of the tortoise excitability and conductivity are decreased, and the auricular contraction may fail to pass to the ventricles. C. Sympathetic Fibres.—The outgoing fibres are the aug- mentors and accelerators of the heart’s action. When they are cut the heart may beat slower. When the peripheral end is stimulated, the rate and force of the heart are increased. 1. Course of the Fibres—These are small medullated fibres. They leave the spinal cord by the anterior roots of the 2nd, 3rd and 4th dorsal nerves, passing to the stellate ganglion where they have their cell stations (fig. 120). From the cells in this ganglion non-medullated fibres run on in the annulus CIRCULATION 257 of Vieussens, and from this and from the inferior cervical ganglion they pass out to the muscular fibres of the heart. 2. The Centre is in the medulla, and it may’ be stimulated by stimulating various ingoing nerves, such as the sciatic ; or it may be set in action from the higher nerve centres in various emotional conditions. 3. Mode of Action—The fibres seem to act (a) upon the L Aurtcte repr ney L Vintricle / hibit shih I fg / PTE ola / py y My Hh >, 3 Periph cut Vagosympath.stn.Col20 Fic. 121.—Simultaneous Tracing from Auricles and Ventricles. A., during stimulation of the vagus; B., during stimulation of the sympathetic. Each downstroke marks a systole, each upstroke a diastole. (From Roy and ADAMI.) muscular fibres, increasing their excitability and conduc- tivity; (6) upon the inhibitory mechanism, throwing it out of action. 4, Result of Action— (a) The output of blood from the heart is increased, and the pressure of blood in the arteries is raised. 17 258 VETERINARY PHYSIOLOGY (6) The rate of the rhythmic movements of auricles and ventricles is increased. (c) The force of contraction of auricles and ventricles is increased, It is probable that the cardiac sympathetic also carries ingoing fibres which enter the cord in the lower cervical region. The pain experienced in the arm in heart disease is generally thought to be due to the implication of these fibres leading to the sensation which is referred to the cor- responding somatic nerves (p. 151), wl The vagus is thus the protecting nerve of the heart, reducing its work and diminishing the pressure in the arteries, } The sympathetic is the whip which forces the heart to increased action in order to keep up the pressure in the arteries. III. CIRCULATION IN THE BLOOD AND LYMPH VESSELS The general distribution of the various vessels—arteries, capillaries, veins, and lymphatics—has been already con- sidered (fig. 107, p. 225). Structure (The structure of the walls of each must be studied practically.) The capillaries are minute tubes of about 12 micro- millimetres in diameter, forming an anastomising network throughout the tissues. Their wall consists of a single layer of endothelium. On passing from the capillaries to arteries on the one side, and to veins and lymphatics on the other, non-striped muscle fibres make their appearance encircling the tube. Between these fibres and the endothelium a fine elastic membrane next appears, while outside the muscles a sheath of fibrous tissue develops. Thus the three essential coats of these vessels are produced :— Tunica intima, consisting of endothelium set on the in- ternal elastic membrane. Tunica media, consisting chiefly of the visceral muscular fibres. Tunica adventitia, consisting of loose fibrous tissue. ee CIRCULATION 259 The coats of the arteries are thick; those of the veins are thin. In the large arteries the muscular fibres of the media are largely replaced by elastic fibres, so that the vessels may better stand the strain of the charge of blood which is shot from the heart at each contraction. In the veins double flaps of the tunica intima form valves which prevent any regurgitation of blood. The great characteristic of the walls of the large arteries is the toughness and elasticity given by the abundance of elastic fibrous tissue, of the small arteries the contractility due to the preponderance of muscular fibres. Physiology The circulation of blood in the vessels is that of a fluid in a closed system of elastic-walled tubes, at one end of which (the great arteries) a high pressure, and at the other (the great veins) a low pressure, is kept up. As a result of this distribution of pressure there is a constant flow of blood from arteries to veins. Many points in connection with the circulation may be conveniently studied on a model or scheme made of indiarubber tubes and a Higginson’s syringe. (Practical Physiology.) A—BLOOD PRESSURE The distribution of pressure is the cause of the flow of blood, and must first be considered. 1. General Distribution of Pressure That the pressure throughout the greater part of the blood vessels is positive—greater than the pressure of the atmos- phere—is indicated by the fact that if a vessel is opened, the blood flows out of it. Zhe force with which blood escapes is a measure of the pressure in that particular vessel. If an artery be cut, the blood escapes with great force; if a vein be cut, with much less force (fig. 122), Arteries.—If the pressure in the aorta,in the radial, in the dorsalis pedis, and in one of the smallest arteries is measured, it is found that while it is great in the great arteries—about 160 mm. Hg in the aorta—it is much less 260 VETERINARY PHYSIOLOGY in the small arteries. This distribution of arterial pressure might be plotted out as in fig. 122, Ar. Veins.—If the pressure in any of the small veins in a medium vein, and in a large vein near the heart be measured, it will be found— lst. That the venous pressure is less than the lowest arterial pressure. 2nd. That it is highest in the small veins, and -pecomngs lower in the larger veins. In the great veins entering the heart it is lower than the atmospheric pressure during the first part of each ventricular diastole (tig. 122, V.). Capillaries.—The pressure in the capillaries must obviously 160 Ar oe V. RO Fie. 122 —Diagram of the Distribution of Mean Blood Pressure throughout the Blood Vessels. Ar., the arteries ; C., the capillaries ; V., the veins. be intermediate between that in the arteries and in the veins. It is not so easily measured, but it may be approximately arrived at by finding the pressure which is required to rt the capillaries—e. g. to blanch a piece of skin. The pressure in any part of a system of tubes depends upon two factors— 1st. The force propelling fluid into that part of the tubes. 2nd. The resistance to the outflow of fluid from that part of the tubes. The pressure ‘in the Arteries is high, because with each beat of the heart about 80 grms. of blood are thrown with the CIRCULATION 261 whole contractile force of each ventricle into the corresponding artery; and because the resistance offered to the outflow of blood from the arteries into the capillaries and veins is enormous. For, as the blood passes into innumerable small vessels, it is subjected to greater and greater friction—just as a river in flowing from a deep narrow channel on to a broad shallow bed is subjected to greater friction. Thus in the arteries the powerful propulsive force of the heart and the great resistance to outflow keep the pressure high. When the eapillaries are reached much of the force of the heart has been lost in dilating the elastic coats of the arteries, and thus the inflow into the capillaries is much weaker than the inflow into the arteries. At the same time the resistance to outflow is small, for in passing from capillaries to veins the channel of the blood is becoming less broken up and thus opposes less friction to the inflow of the blood. When the veins are reached the propelling force of the heart is still further weakened, and hence the force of inflow is very small. But, instead of there being a resistance to outflow from the veins into the heart, this is favoured by the suction action of the heart during diastole, and also by the fact that the great veins, in entering the heart, pass into the thorax, an air-tight box in which during each inspiration a very low pressure is developed. What has been said of the veins applies equally to the lymphatics. 2. Rhythmie Variations in Blood Pressure Before considering the exact measurements of pressure in these different vessels, certain rhythmic variations in pressure may first be considered. A. Synehronous with the Heart Beats Arterial Pulse With each ventricular systole about 80 grms. of blood are thrown into the already full arteries, and the pressure in these vessels is suddenly raised. 262 VETERINARY PHYSIOLOGY If the finger be placed on an artery, a distinct expansion will be felt following each systole, and due to this rise of pressure. This is the arterial pulse. This expansion develops suddenly and disappears more slowly. In some cases it may be felt by simply laying the finger on the surface of the artery without exerting marked pressure, in other cases it may be necessary to compress the artery before the pulsation is distinctly felt. If a vein be investigated in the same way it will be found that no such pulse can be detected. In the capillaries too this pulse does not exist. It is best marked in the great arteries, and becomes less and less distinct as the small terminal arteries are reached. Cause of Pulse.—The arterial pulse is due to— 1st. The intermittent inflow of blood. The arteries expand from the sudden increase of pressure due to each sudden flow of 80 grms. of blood from the heart into the arterial system. 2nd. The resistance to outflow from the arteries into the capillaries. ; If blood could flow freely from the arteries into the capil- laries, then the inrush of blood from the heart would simply displace the same amount of blood into the capillaries and the arteries would not be expanded. As already indicated, the friction between the walls of the innumerable small arterioles and the blood is so great that the flow out of the arteries is not so free as to allow the blood to pass into the capillaries so rapidly as it is shot into the arteries. Hence, with each beat of the heart, an excess of blood must accumulate in the arteries. 3rd. To allow of their expanding to accommodate this excess _ of blood their walls must be elastic. It is upon these three factors that the arterial pulse depends. Do away with any of them, and the pulse at once disappears. . Why is there no Pulse in the Veins?—Their walls are elastic, but, in the first place, instead of there being an obstruction to the outflow of blood from the veins into the heart, this is favoured by the suction action of the hearteand thorax. Hence, even if an intermittent inflow were well marked, the absence of resistance to outflow would in itself FY r CIRCULATION 263 prevent the development of a venous pulse. But the inflow is not intermittent. With each beat of the heart the blood does not pass freely from the arteries into the capillaries and veins, but it only slowly escapes, just as much passing out between the beats as during the beats. Hence the most important factor in causing a pulse, an intermittent inflow, is absent. With no sudden intermittent inflow, and with no resist- ance to outflow, the development of a pulse is impossible. In certain abnormal conditions, where, from the extreme dilatation of the arterioles, the inflow into the veins is very free, and where the outflow from the part of the body is not so free, a local venous pulse may develop. Characters of the Pulse Wave.—If a finger be placed on the carotid artery and another upon the radial artery it will be felt that the artery near the heart expands (pulses) before that further from the heart. The pulse develops first in the arteries near the heart and passes outwards towards the periphery. The reason for this is obvious. The arteries are always overfilled with blood. The ventricle drives its contents into this overfilled aorta, and to accommodate this the aortic wall expands. But since the aorta communicates with the other arteries this increased pressure passes outwards along them, expanding their wall as it goes. The pulse wave may thus be compared to a wave at sea, which is also a wave of increased pressure, the only differ- ence being that, while the waves at sea travel freely over the surface, the pulse wave is confined in the column of blood, and manifests itself by expanding the walls of the arteries. It greatly simplifies the study of the pulse to regard it in this light, and to study it just as we should study a wave at sea. 1. Velocity —To determine how fast a wave is travelling we might select two points at a known distance from one another, and with a watch note how long the wave takes to puss from one to the other. So with the pulse wave, two points on an artery at a known distance from one another may be taken and the time which the wave takes to pass between them may be measured. It is thus found that the pulse wave travels at about 9 or 264 VETERINARY PHYSIOLOGY 10 metres per second—about thirty times as fast as the blood flows in the arteries. 2. Length of the Wave——To determine this in a wave at sea is easy if we know its velocity and know how long it takes to pass any one point. Suppose it is travelling at 50 feet per second, and that it takes 1 second to pass a par- ticular point, obviously it is 50 feet in length. The same method may be applied to the pulse wave. We know its velocity, and by placing the finger on an artery we may determine that one wave follows another in rapid succession, — so that there is no pause between them. Each wave corre- sponds to a ventricular systole, and therefore each wave must last, at any point, just the time between two ventricular systoles—just the time of a cardiac cycle. There are about 40 cycles per minute—ze. per 60 seconds; hence each must last 1:5 seconds. The pulse wave takes 1:5 seconds to pass any place, and it travels at 10 metres per second; its length then is 15 metres. It is then an enormously long wave, and it has disappeared at the periphery long before it has . finished leaving the aorta. 3. The Height of the Wave-—The height of the pulse wave, as of a wave at sea, depends primarily on the pressure caus- ing it, but the character of the arterial wall modifies it very largely. Thus the true height of the pulse wave in the great arteries near the heart is masked by the thickness of the arterial wall. Speaking generally, however, we may say that the pulse wave is highest near the heart, and becomes lower and lower as it passes out to the periphery, where it finally disappears altogether (fig. 128), This disappearance is due to its force becoming expended in expanding the arterial wall. 4. The Form of the Wave.—Waves at sea vary greatly in form, and the form of the wave might be graphically recorded on some moving surface such as the side of a ship by some float- ing body. If the ship were stationary a simple vertical line would be produced, but if she were moving a curve would be recorded, more or less abrupt according to her speed. From this curve the shape of the wave might be deduced, if the speed of the vessel is known. : The same method may be applied to the arterial pulse. CIRCULATION 265 By recording the changes produced by the pulse wave as it passes any point in an artery the shape of the wave may be deduced. This may be done by any of the various forms of sphygmo- graph. (Practical Physiology.) Such a tracing is not a true picture of the wave, but simply of the effect of the wave on one point of the arterial wall. Its apparent length uepends upon the rate at which Medium Pressure. 2 | WW alanallal Medium Pressure. Low Pressure. Fic. 123.—Three Sphygmographic Tracings made from the radial artery of a healthy man in the course of one hour without removing the Sphygmo- graph. 1 was made immediately after muscular exercise ; 2 was made after sitting still for half an hour ; and 3, after an hour, the recording surface is travelling and not on the length of the wave. Its height depends in part upon the length of the recording lever, in part upon the resistance offered by the instrument, in part upon the degree of pressure, with which the instru- ment is applied to the artery, and in part on the thickness of - the arterial wall. Such a trace shows (fig. 123)— 1st. That the pulse waves generally follow one another with- out any interval. 266 VETERINARY PHYSIOLOGY 2nd. That the rise of the wave is much more abrupt than the fall. 3rd. That upon the descent of the primary wave there are one or more secondary waves. One of these is constant and is very often well marked. It forms a second crest, and is hence called the dicrotic wave. Between the chief crest and this secondary crest, a smaller crest is often manifest (fig. 123, 3). From its position, it is called the predicrotic wave. Sometimes other crests appear. If the wave has only one crest it is called a one-crested or monocrotic wave. If only the dicrotic crest is well marked it is called dicrotic. If three crests are present, tricrotic ; if several crests, polycrotic. To understand the various parts of the pulse wave it is necessary to compare it with the intra-ventricular pres- sure changes. This may be done by taking synchronously tracings of the intra-ventricular pressure, and of the aortic pressure (fig. 124). | Such a tracing shows that at the moment of ventricular systole the pressure in the aorta is higher than that in the left ventricle. As ventricular systole advances the intra-ventricular pres- sure rises and becomes higher than the aortic. At that moment the aortic valves are thrown open and a rush of blood takes place into the aorta, raising the pressure and expanding the artery, and causing the upstroke, and crest of the pulse curve. As the ventricle empties itself the intra- ventricular pressure tends somewhat to fall, and, at the same time, a_fall in the intra-aortic pressure also takes place. If all the blood does not leave the ventricle in the first gush, vy. When the intra-aortic pressure is high as compared with the force of the heart (fig. 124, continuous line), there is a residual outflow which arrests the diminution in the aortic pressure, or may actually raise it, causing the predicrotic wave. As this residual outflow diminishes, the aortic pres- sure again falls and continues to fall until the moment of ventricular diastole. At this instant the intra-ventricular pressure suddenly becomes less than the intra-aortic, and the semilunar valves are forced downwards towards the ventricles, and thus the capacity of the aorta is slightly increased and i a a a CIRCULATION 267 the pressure falls. This fall in pressure is indicated by the dicrotie notch. But the elasticity of the semilunar valves at once makes them again spring up, thus increasing the pres- sure in the aorta and causing the second crest, the dicrotic wave, After this the pressure in the arteries ‘steadily diminishes till the mean is reached, to be again increased by the next ventricular systole. The form of the pulse wave varies according to the relation- ship between the arterial pressure and the activity of the heart. If the heart is active and strong in relation to the arterial AS YS. Fic. 124.—Diagram to show the relationship of the pulse wave to the cardiac eycle and the effect of altering the relationship between the activity of the heart and the arterial blood pressure. — -— -— bis the curve of intra- ventricular pressure, and -- - - - b' is a pulse curve with an active heart and a relatively low arterial pressure. — a@ and a! are the same witha sluggish heart and a relatively high arterial pressure. pressure, the main mass of the blood is expelled in the first sudden outflow, and the residual flow is absent or slight (fig. 124, dotted line). In this case there is a sudden and marked rise of the arterial pressure, followed by a steady fall till the moment of ventricular diastole. The rebound of the semilunar valves is marked in this case and causes a very prominent dicrotic wave, while the predicrotic wave is absent (fig. 123, 1). Such a condition is well seen after violent muscular exertion, and in certain fevers. In these conditions the dicrotic wave is so well marked that it can be readily felt with the finger. It is to this form of pulse that the term dicrotic is applied in medicine. 268 . VETERINARY PHYSIOLOGY On the other hand, if the ventricles are acting slowly and feebly in relationship to the arterial pressure, the initial out- flow of blood does not take place so rapidly and completely (fig. 124, continuous line), and the initial rise in the pulse is thus not so rapid. The residual outflow of blood is more marked and causes a well-marked secondary rise in the pulse curve—the predicrotic wave. In certain cases this may be higher than the primary crest, producing the condition known as the anacrotic pulse. The relatively high intra-arterial pressure here prevents the development of a well-marked dicrotic wave. In extreme cases of this kind, when the arterial walls are very tense, they may recover after their expansion in an irregular jerky manner, and may give rise to a series of kata- crotic crests producing « polycrotic pulse (fig. 123, 3). From what has been said it will be seen that a study of the pulse wave gives most valuable information as regards the state of the circulation, and the physician constantly makes use of the pulse in diagnosis. ; Palpation of Pulse.—On placing the finger on the radial artery the points to determine are— lst. The rate of the pulse—ze. the rate of the heart’s action. 2nd. The rhythm of the pulse—z.e. of the heart’s action as regards—(1) Strength of the various beats——Normally the beats differ little from one another in force—since the various heart beats have much the same strength. Respira- tion has a slight effect which will afterwards be considered (see p. 321). In pathological conditions great differences in the force of succeeding pulse waves occur. (2) Zime relation- ship of beats—Normally the beats follow one another at regular intervals — somewhat shorter during inspiration — somewhat longer during expiration. In pathological con- ditions great irregularities in this respect may occur. 3rd. The volume of the pulse wave. Sometimes the wave is high and greatly expands the artery—sometimes less high and expanding the artery less. The former condition is called a full pulse (pulsus plenus), the latter a small pulse (pulsus parvus). The fulness of the pulse depends upon two factors:— — 1st. The average tension in the arteries between the pulse beats. > | i -_ CIRCULATION 269 If this is high, the walls of the artery are already somewhat stretched, and therefore the pulse wave expands them only slightly further. On the other hand, if the average pressure is low, the arterial wall is lax, and is readily stretched to a greater extent. 2nd. The force of the heart. ‘To stretch the arterial wall to a large extent requires an actively acting heart throw- ing a sudden large wave of blood into the arterial system at each systole. The full pulse is well seen after violent exer- tion, when the heart is active and the peripheral vessels fully dilated, thus allowing a free flow of blood from the arteries and thus keeping the mean arterial pressure low 4th. Tension of the pulse. Sometimes the pulse wave is easily obliterated by pressing on the artery—sometimes con- siderable force is required to prevent it from passing. To test this, two fingers must be placed on the artery. That placed nearer the heart must be pressed more and more firmly on the vessel until the pulse wave is no longer felt by the second finger. In this way the tension or force of the pulse, the maximum systolic pressure in the artery, may be roughly determined. So important, however, is this point, that various instrumental methods for determining it have been devised (see p. 273). The tension of the pulse varies directly with the force of the heart and with the peripheral resistance. The first statement is so obvious as to require no amplification. It is also clear that if the peripheral resistance is low, so that blood ‘ean easily be forced out of the arteries into the capillaries, the arterial wall will not be so forcibly expanded as when the resistance to outflow is great. Hence a high-tension pulse is indicative of a strongly acting heart with constriction of the peripheral vessels. It is well seen during the shivering fit which so frequently precedes a febrile attack, since at that time the peripheral vessels are constricted and the heart's action excited. The tension of the pulse wave must not be confused with the mean arterial pressure (see p. 273). 5th. The form of the pulse wave may be investigated by means of the finger alone or by means of the sphygmograph. The points to be observed are— (1) Does the wave come up suddenly under the finger ? In the pulsus celer (or active pulse) it does so; in the pulsus tardus, on the other hand, it comes up slowly. The 270 VETERINARY PHYSIOLOGY former condition is indicative of an actively acting heart with no great peripheral resistance—the latter indicates that the heart’s action is weak in relationship to the arterial blood pressure. (2) Does the wave fall slowly or rapidly? Normally the fall should not be so sudden as the ascent. When the aortic valves are not properly closed the descent becomes very rapid. (3) Are there any secondary waves to be observed? The only one of these which can be detected by the finger is the dicrotic wave, and this only when it is well marked. When it can be felt, the pulse is said to be dicrotic, and, as before stated, this indicates an actively acting heart with an arterial pressure low relatively to the strength of the ventricles, B. Capillary Pulse Normally there is no pulse in the. capillaries. Their thin endothelial wall is not well adapted to bear such an inter- mittent strain. If, however, the arterioles to a district are ~ freely dilated so that little resistance is offered to the escape of blood from the arteries, and if at the same time the out- flow from the capillaries is not proportionately increased, intermittent inflow and resistance to outflow are developed, and a pulse is produced. Such a condition is seen in certain glands during activity. C. Venous Pulse ~ 1. The absence of a general venous pulse has been already explained. But just as in the capillaries so in the veins, a local pulse may develop. 2. In the veins entering the auricles a pulse occurs, but a pulse having no resemblance to the arterial pulse, although depending on the same three factors. Its form is indicated in fig. 125. Its features are to be explained as follows :— Blood is constantly flowing into the great veins, pressed, on from behind. When the auricles contract, the outflow from these veins into the heart is suddenly checked, and con- sequently the veins distend. At the moment of auricular —» . CIRCULATION 271 diastole the outflow is again free, a rush of blood takes place into the distending auricles, and thus the pressure in the veins falls. But as this is occurring, blood is shot from the ‘ventricles into the arteries, and the carotid, lying behind ‘the jugular vein, transmits its pulse to the vein as a crest. While the ventricle is contracted blood cannot pass on from the auricles, and hence it accumulates in the great vein and makes a third crest at the end of the ventricular systole. At the moment when the ventricles dilate a sudden rush of blood takes place from the veins and auricles into the ventricles, and thus a sudden fall in the pressure is produced. Gradually, AS. V.S. #2 / Fic, 125.—Tracings of the Pulse in the great Veins in relationship to the Cardiac Cycle. ——— normal venous pulse. --- «and bd venous pulse in tricuspid incompetence. as the ventricles fill, the pressure in the auricles and veins increases and they are again expanded. This is the normal venous pulse. But, if the auriculo-ven- tricular valves are incompetent, blood is forced back into the auricles and veins when the ventricles contract, and a crest develops after the carotid crest which it may replace. The height of this crest is a good index of the amount of regurgita- tion. Respiratory Variations in Blood Pressure Not only do rhythmic changes in the arterial pressure occur with each beat of the heart, but larger changes are caused 272 VETERINARY PHYSIOLOGY by the respirations—the rise in pressure in great measure corresponding to the phase of inspiration, the fall in pressure to the phase of expiration. This statement is not quite accurate, as will be seen when considering the influence of respiration on circulation (see page 311), These variations are easily seen in a tracing of the arterial pressure taken with the mercurial manometer (fig. 127, A). A pulse synchronous with the respirations may also be observed in the great veins at the root of the neck and in the venous sinuses of the cranium when it is opened. With Fic. 126.—Tracing of the arterial blood pressure to show large respiratory varia- tions, and small variations due to heart beats upon these, and the sudden fall in the pressure produced by stimulating the inferior cardiac branch of the vagus nerve. each inspiration they tend to collapse, with each expiration they again expand. The reason for this is that during inspira- tion the pressure inside the thorax becomes low and hence blood is sucked from the veins into the heart, while during expiration the intra-thoracic pressure becomes higher and thus the entrance of blood into the heart is opposed. 8. Mean Blood Pressure I. PRESSURE IN THE ARTERIES A. Methods The first investigation of the pressure in the blood vessels was made by the Rev. Stephen Hales in 1733. He fixed a J ee CIRCULATION 273 long glass tube in the femoral artery of a horse laid on its -back, and found that the pressure supported a column of blood of 8 feet 3 inches, while, when the tube was placed in a vein, only 1 foot was supported. The capillary pressure is, of course, intermediate between these two. At the present time, instead of letting the blood pressure act directly against the force of gravity, it is found more convenient, in studying the pressure in an artery, to let it A Fic. 127.—A, The Mercurial Manometer with recording float, used in taking records of the arterial blood pressure of lower animals. 2B, The Hill- Barnard Sphygmometer, for measuring the arterial pressure without opening a vessel. act through a column of mercury placed in a U tube (fig. 127, A.). To record the changes in pressure a float is placed upon the mercury in the distal limb of the tube, and this carries a writing style which records the changes upon a moving surface. With such an apparatus a record such as is shown in fig. 126 is given. The actual pressure is measured by taking the difference between the level of the mercury in the two limbs of the tube. To make the measurement it is customary to describe an abscissa when the mercury is at the same level in the two sides of the tube. The difference between this and the level of the style at any moment multiplied by two, on 18 274 VETERINARY PHYSIOLOGY account of the depression in the proximal limb which accom- panies the elevation in the distal limb, gives the blood pressure in mm. of mercury. On the record made with such an instrument the rhythmic variations in the arterial blood pressure already considered on p. 262 and p. 271 are clearly visible. (Practical Physiology.) The arterial pressure may be measured by various methods without operative interference. Some of these give the systolic pressure, 7.¢. the pressure at the maximum of the pulse wave; while others give the diastolic pressure—the pressure between the pulse waves. As shown in p. 262, the difference between these is most marked in the great arteries, and falls to zero before the capillaries are reached (fig. 128). To measure the systolic pressure it is necessary to find the \ om TATRA AA s at Fic. 128.—To show the difference between systolic, diastolic and mean blood pressure throughout the arterial system, S, systolic pressure; D, diastolic pressure ; 7, mean pressure. pressure which must be applied to an artery in order to prevent the pulse from passing. This may be done with Riva Rocci’s apparatus, by applying a bag round a limb so that it rests upon an artery. The bag is firmly strapped on by means of a broad supporting — belt, and it is connected with a pump by which the pressure within it may be raised, and with a mercurial . manometer by which the pressure applied may be measured in mm. of mercury. The pressure is then raised till the pulse beyond is no longer felt, and the column of mercury indicates the systolic pressure in the artery. (Practical Physiology.) Instead of using the arterial pulse as the index, the passage of blood into a region rendered bloodless may be used. In Gaértner’s tonometer the bag and band are applied round the finger rendered bloodless at a pressure sufficient to prevent CIRCULATION 275 the blood from passing, and then the pressure is slowly lowered till the blood passes and the finger again flushes. The height of the column of mercury at that moment gives the systolic pressure in the small arteries of the finger. The diastolic pressure may be measured by taking advantage of the fact that when the pressures inside and outside an artery are equal the pulse wave is best marked. A bag and band are applied to a limb upon an artery, and the movements of the pulse are observed in the column of mercury or on an ane- roid barometer in the Hill ' Barnard instrument, or by a Pi, 12 Sums of en» see record taken with a small yithout opening a blood vessel. a, elastic tambour on a revolving bag for arm; b, mercurial manometer ; cylinder in Erlanger’s in- © Pump; ye glass bulb; 7, rubber ball ; strument (fig. 129), Erlan- ‘@mbour with lever. ger’s instrument may also be used as the Riva Rocci for systolic pressures, anil it is therefore the most valuable instru- ment for the study of the arterial pressure in the man. (Practical Physiology.) B. Results By these methods it has been found that the systolic pressure in the brachial artery of man is about 110 mm. of mercury, while the diastolic pressure is only about 65mm. The difference between these, of course, gives the pulse pressure or tension. The force of the heart and the degree of peripheral re- sistance both modify the arterial pressure, and normally these so act together that disturbance of one is compensated for by changes in the other. Thus, if the heart’s action becomes increased and tends to raise the arterial pressure, the peripheral resistance falls and prevents any marked rise. Similarly, if the peripheral resistance is increased, the 276 VETERINARY PHYSIOLOGY heart's action is diminished, and no rise in the pressure occurs. Under certain conditions, however, this compen- satory action is not complete, and changes in the arterial pressure are thus brought about. The volume of blood has a comparatively small influence on the arterial pressure, because the veins are so large that they accommodate very varying amounts of fluid. Factors controlling Arterial Pressure (a) Heart’s Action.—The influence of this may be readily demonstrated by stimulating the vagus nerve while taking a tracing of the arterial pressure. The heart is inhibited, less blood is forced into the arteries, and the pressure falls (fig. 127). If, on the other hand, the accelerator nerve is stimulated, the increased heart’s action drives more blood into the arteries, and the pressure rises. (6) Peripheral Resistance.—The resistance to outflow from the arteries depends upon the resistance offered in the small - arteries, the walls of which are chiefly composed of visceral muscle fibres. When these fibres are contracted, the lumen of the vessels is small and the resistance is great. When they are relaxed, the lumen of the vessels dilates, and the resistance to outflow is diminished. This museular tissue of the arterioles acts as a stop-cock to the flow of blood from the arteries to the eapillaries. It is of great importance— 1st. In maintaining the uniform pressure in the arteries. 2nd. In regulating the flow of blood into the capillaries. During the functional activity of a part, a free supply of blood in its capillaries is required. This is brought about by a relaxation of the muscular coats of the arterioles leading to the part. When the part returns to rest, the free flow of blood is checked by the contraction of the muscular walls of the arterioles. ; The action of the arterioles is well seen under the influence of certain drugs (vaso-dilators and vaso-constrictors). If, while a tracing of the arterial pressure is being taken, nitrite of amy] is administered to the animal, it will be seen that the skin and mucous membranes become red and engorged with blood, while at the same time the arterial pressure falls. CIRCULATION 277 Nitrites cause the muscular coat of the arterioles to relax, and thus, by diminishing peripheral resistance, permit blood to flow freely from the arteries into the capillaries. Salts of barium have precisely the opposite effect, caus- ing the skin to become pale from imperfect filling of the capillaries, and producing a marked rise in the arterial pressure. Contraction of the muscles of the arterioles is produced, and the flow of blood from arteries to capillaries is retarded. Not only is the state of the arterioles influenced thus by drugs, but it is also affected by the internal secretions (see p. 416) from certain organs. A powerful vaso-constrictor adrenalin is produced in the medulla of the suprarenals. The condition of the arterioles may be studied in many different ways— Ist. By direct observation. With the naked eye. A red engorged appearance of any part of the body may be due to dilatation of the arteriole leading to it. It may, however, be due to some obstruction to the outflow of blood from the part. 2. With the microscope. In certain transparent structures, such as the web of the frog’s foot, or the wing of the bat, or the mesentery, it is possible to measure the diameter of the arterioles by means of an eye-piece micrometer, and to study their dilatation and contraction. 2nd. The engorgement of the capillaries brought about by dilatation of the arterioles manifests itself also in an increased size of the part. Everyone knows how on a hot day, when the arterioles of the skin are dilated, it is difficult to pull on a glove which, on a cold day, when the cutaneous arterioles are contracted, feels loose. By enclosing a part of the body in a case with rigid walls filled with fluid or with air which is connected with some form of recording tambour, an increase or decrease in the size of the part due to the state of its vessels may be registered. Such an instrument is called a plethysmograph. 3rd. When the arterioles to a part are dilated and the blood is flowing freely into the capillaries, the part becomes warmer, and by fixing a thermometer to the surface, conclu- sions as to the condition of the arterioles may be drawn. 4th. By streaming blood through the vessels and observing 278 VETERINARY PHYSIOLOGY the rate at which it escapes the changes in the state of the arterioles may be made out. This perfusion method is much used in studying the action of drugs. (Practical Physiology.) 5th. Since the state of the arterioles influences the arterial pressure, if the heart's action is kept uniform, changes in the arterial blood pressure indicate changes in the arterioles, a fall of pressure indicating dilatation, a rise of pressure, constriction. Normal State of Arterioles. —If an arteriole in some transparent tissue be examined, it will be found to main- tain a fairly uniform size, but to undergo periodic slow changes in calibre. If the ear of a white rabbit be studied, it will be seen to undergo slow changes, at one time appear- ing pale and bloodless, at another time red and engorged. During this latter phase numerous vessels appear which in the former condition were invisible. These slow changes are independent of the heart’s action and of the rate of respirations. They appear to be due to the periodic rhythmic. contraction which is a characteristic property of visceral muscle fibres. This rhythmic action is better marked in some vessels than in others. Vaso-motor Mechanism.—If the sciatic nerve of a frog be cut, the arterioles in the foot at once dilate. If the sciatic be stimulated, the arterioles become smaller. The same results follow if the anterior roots of the lower spinal nerves, from which the sciatic takes origin, be cut or stimulated. We must, therefore, conclude (1) that the central nervous system exerts a constant tonic influence upon the arterioles, keeping them in a state of semi-contraction; and (2) that this influence may be increased, and thus a constriction of the arterioles caused, and in this way the flow of blood from arteries to capillaries obstructed and the arterial pressure raised; and (3) that this influence may be diminished, so that the arterioles dilate and allow an increased flow into the capillaries from the arteries, and thus lower the arterial pressure. . These mobile arterioles, under the control of the central nervous system, constitute a vaso-motor mechanism, which CIRCULATION 279 plays a most important part in connection with nearly every vital process in the body. By it the pressure in the arteries is governed, by it the supply of blood to the capillaries and tissues is controlled, and by it the loss of heat from the body is largely regulated (p. 385). This vaso-motor mechanism consists of the three parts :— 1st. The contractile muscular walls of the arterioles with the nerve terminations in them. 2nd. The nerves which pass to them. 3rd. The portions of the central nervous system presiding over these. 1. Museular Walls of the Arterioles—The muscular fibres are maintained in a state of tonic semi-contraction by nerves passing to them, and when these nerves are divided, the muscular fibres relax. But if, after these nerves have been cut, the animal be allowed to live, in a few days the arterioles again pass into a state of tonic semi-contraction, although no union of the divided nerve has taken place. Certain drugs, eg. digitalis and the salts of barium, act as direct stimulants to these muscle fibres. It appears that the muscular fibres in the arterioles, as elsewhere, tend to maintain themselves in a state of partial contraction, which increases and diminishes in a _ regular rhythmic manner. The precise part played by the nerve terminations has not been definitely established, but certain drugs appear to act specially upon them. Thus apocodeine, while it does not prevent barium salts from constricting the vessels, prevents the constricting action of extracts of the medulla of the suprarenals, even when the nerves are cut. Hence it must be concluded that it paralyses a nervous mechanism in the arteriole wall which is stimulated by the suprarenal extract. . Normally this muscular mechanism is controlled by the nervous system. 2. Vaso-motor Nerves.— When a nerve going to any part of the body is cut the arterioles of the part generally dilate, when it is stimulated the arterioles are usually contracted ; sometimes, however, they are dilated. In no case does section of a nerve cause constriction of the arterioles. 280 VETERINARY PHYSIOLOGY These facts prove that the vaso-motor nerves may be divided- — into two classes :— 1st. Vaso-constrictor. 2nd. Vaso-dilator. A. Vaso-constrietor Nerves.—The fact that section of these at once causes a dilatation of the arterioles proves that they are con- stantly transmitting impulses from the central nervous system. Course.—Thecourse of these fibres has been investigated by section and by stimulation (fig. 130). They leave the spinal cord chiefly in the dorsal region by the anterior roots of the spinal nerves, pass into the sympathetic ganglia, where they have their cell stations, and then as non-medullated fibres pass, either along the various sympathetic nerves to the viscera, or back through the grey ramus (see fig. 75, p. 151), into the spinal nerve, and run in it to their ter- minations. B. Vaso-dilator Nerves.—A good example of such a nerve is to be found in the chorda tympani branch of the facial nerve, which sends fibres to the submaxillary and_ sub- lingual salivary glands. If this nerve be cut, no change Fic. 130.—Diagram of the Distribution of Vaso-motor Nerves. The continu- ous line shows the vaso-constrictors, the dotted line the vaso-dilators. C.N., cranial nerves; Vag., vagus ; T.S., thoracic sympathetic; A.S., abdominal sympathetic ; N.L., nerves to the leg. takes place in the vessels of the gland, but, when it is stimulated, the arterioles di- late and allow an increased flow of blood through the capillaries. These fibres, therefore, instead of increasing the activity of muscular contraction, diminish or inhibit it. They CIRCULATION 281 _-play the same part in regard to the muscular fibres of the arterioles as the inferior cardiac branch of the vagus does in regard to the cardiac muscular fibres. As examples of vaso-dilator nerves the gastric branches of the vagus carrying vaso-dilator fibres to the mucous membrane of the stomach, and the nervi erigentes carrying vaso-dilator fibres to the external genitals, may be taken. The vaso-dilator nerves of most parts of the body run side by side with the vaso-constrictor nerves; and, hence, curious results are often obtained. If the sciatic nerve of a dog be eut, the arterioles of the foot dilate. If the peripheral end of the cut nerve is stimulated, the vessels contract. But, after a few days, if the nerve be prevented from uniting, the arterioles of the foot recover their tonic contraction, and, if the sciatic nerve is then stimulated, a dilatation, and not a constriction, is brought about. The vaso-constrictor fibres seem to die more rapidly than the vaso-dilator fibres which run alongside of them. Under certain conditions the activity of the vaso-dilator fibres seems to be increased. Thus, if, when the limk is warm, the sciatic nerve is stimulated, dilatation rather than constriction may occur. Again, while rapidly re- peated and strong induction shocks are apt to cause constric- tion, slower and weaker stimuli tend to produce dilatation. Course—The vaso-dilator nerves pass out chiefly by the anterior roots of the various spinal nerves, and do not pass through the sympathetic ganglia, but run as medullated fibres to their terminal ganglia (fig. 130). Bayliss has recently shown that the vaso-dilator fibres for the hind limb leave the cord by the posterior roots, and that they are connected with the cells in the ganglia. 3. Portions of Nervous System Presiding over the Vaso- motor Mechanism.—Since a set of nerves causing constriction of the arterioles, and another set causing dilatation exist, we must conclude that there are two mechanisms in the central nervous system—one a vaso-constrictor, the other a vaso-dilator. A. Vaso-constrictor Centre.—(a) Mode of Action.—This mechanism is constantly in action, maintaining the tonic contraction of the arterioles. 282 VETERINARY PHYSIOLOGY ‘If any afferent nerve be stimulated, the effect is to increase the activity of the mechanism, to cause a general constriction of arterioles, and thus to raise the general arterial pressure. The centre is, therefore, capable of reflex excitation. In ordinary conditions so many afferent nerves are constantly being stimulated, that it is not easy to say how far the tonic action of this centre is reflex and dependent on the stream of afferent. impulses. But this centre may also be directly acted upon by the condition of the blood and lymph circulating through it. When the blood is not properly oxygenated and freed from carbon dioxide, as in asphyxia or suffocation, this centre is stimulated and a general constriction of arterioles with high blood pressure results. (6) Position—In investigating the position of the centre we may take advantage of— 1st. Its constant tonic influence. Removal of the centre at once causes dilatation of arterioles. 2nd. The fact that it may be reflexly stimulated. If the . vaso-constrictor centre be removed, stimulation of an afferent nerve no longer causes constriction of the arterioles. Removal of the whole brain above the pons Varolii leaves the centre intact. Separation of the pons Varolii and medulla oblongata from the spinal cord at once causes a dilatation of the arterioles of the body, and prevents the production of reflex constriction by stimulation of an afferent nerve. The main part, at least, of the vaso-constrictor mechanism is therefore situated in the pons Varolii and medulla oblongata. The extent of this centre has been determined by slicing away this part of the brain from above downwards, and studying the influence of reflex stimulation after the removal of each slice. It is found that at a short distance below the corpora quadri- gemina, the removal of each succeeding part is followed by a diminution in the reflex constriction, until, at a point close to and just above the calamus scriptorius, all reflex response to stimulation stops. The centre is therefore one of very considerable longi- tudinal extent. =e CIRCULATION 283 But it has been found that, if, after section of the spinal cord high up, the animal be kept alive for some days, the dilated arterioles again contract, and stimulation of afferent _ nerves entering the cord below the point of section causes a further constriction. If now another section be made further down the cord, the arterioles supplied by nerves coming from below the point of section will again dilate. This shows that secondary vaso-constrietor centres, tonic in action and capable of having their activity reflexly increased, exist all down the grey matter of the spinal cord. Normally these are under the domain of the dominant centre, but when this is out of action they then come into play. B. Vaso-dilator Centre. —(a) Mode of Action. — This mechanism is not constantly in action. Section of a vaso- dilator nerve does not cause vascular dilatation. It may be excited reflexly, but in a different manner from the vaso- constrictor mechanism. Stimulation of an afferent nerve causes a dilatation of the arterioles in the part from which it comes, and a constric- tion of the arterioles throughout the rest of the body. If a sapid substance, such as pepper, be put in the mouth, the buccal mucous membrane and the salivary glands become engorged, while there is a constriction of the arterioles throughout the body. The vaso-dilator central mechanism is not general in its action like the vaso-constrictor, but is specially related to the different parts of the body. This is a matter of the greatest importance in physiology and pathology. It explains the increased vascularity of a part when active growth is going on. The changes in the part, or the products of these, stimulate the afferent nerve. This reflexly stimulates the vaso-dilator mechanism of the part, and thus causes a free flow of blood into the capillaries, and at the same time maintains or actually raises the arterial pressure by causing a general constriction of the arterioles, and thus forces more blood to the situation in which it is required. The same process occurs in the case of the stomach during digestion, in the case of the kidney during secretion, and in the process of inflammation. Not only does peripheral stimulation act in this way, but 284. VETERINARY PHYSIOLOGY various states of the brain, accompanied by emotions, may stimulate part of the vaso-dilator mechanism, as in the act of blushing. Again, it has been shown that stimulation of the central end of the depressor nerve (superior cardiac branch of the vagus) causes a dilatation of the arterioles chiefly in the abdominal cavity, but also throughout the body generally. This is the most generalised vaso-dilator reflex known (see p. 254). (6) Position.—While the dominant vaso-constrictor centre is in the medulla, the vaso-dilator centres seem to be dis- tributed throughout the medulla and spinal cord. — II. PRESSURE IN THE CAPILLARIES This may be determined by finding the pressure required to blanch the skin or to occlude the capillaries of some transparent membrane. It has already been shown that the pressure is less than in the arteries and greater than in the veins, Like the pressure in the arteries, it depends upon the two factors— 1st, Force of inflow. 2nd. Resistance to outtlow. lst. Variations in the Foree of Inflow.—The capillary pressure may undergo marked local changes through the vaso-motor mechanism. Wherever the function of a part is active, dilatation of the arterioles and an increased capillary pressure exists, and, when the influence of vaso-dilator nerves is withdrawn, the capillary pressure falls. But the capillary pressure may also be modified by the heart’s action, inasmuch as the arterial pressure, by which blood is driven into the capillaries, depends upon this. In cardiac inhibition not only is arterial pressure lowered, but capillary pressure may also fall. In augmented heart action both arterial and capillary pressure are raised. 2nd. Variations in Resistance to Outflow.— Normally the flow from capillaries to veins is free and unobstructed; but, if the veins get blocked, or if the flow in them is retarded by gravity, the capillaries get engorged with the blood which CIRCULATION 285 cannot escape from them. This increased pressure in the capillaries is very different from that caused by increased inflow. The flow through the vessels is slowed or may be stopped instead of being accelerated, and the blood gets deprived of its nourishing constituents, loaded with waste products, and tends to exude into the lymph spaces, caus- ing dropsy. It is, therefore, most im- portant to distinguish be- tween high capillary pressure from dilated arterioles or an active heart, and high pres- sure due to venous obstruc- tion. ae eet A condition very similar . =~ to that described, but pro- A Cc Vv D ducing a canpilla ressure *I¢- 131.—The Changes in Blood Pres- 8 ‘pana I sure in the Capillaries produced by in- high relatwely to the ll creasing the arterial pressure - - - - - - j an the arterres—though not and by obstructing the venous flow absolutely high—is seen in —-—-—. A, arteries; C, capil- eases of failure of the heart, isin i as when that organ is not acting sufficiently strongly to pass the blood on from the venous into the arterial system. Here the arterial pressure becomes lower and lower, the venous pressure higher and higher, and along with this tne capillary pressure becomes high in relationship to the arterial pressure. The blood is not forced through these channels, and congestion of the capillaries and dropsy may result. The influence of gravity plays a very important part on the capillary pressure, since it has so marked an influence on the flow of blood in the veins. When, through heart failure, the blood is not properly sucked up from the inferior extremities, this increased pressure becomes very marked indeed. 37rd. But the pressure in the capillaries may also to a certain extent be varied by the withdrawal of water from the body, as in purgation or diuresis, or by the addition of large quan- tities of fluid to the blood. The venous system is, however, 80 capacious that very great changes in the amount of blood 286 VETERINARY PHYSIOLOGY in the vessels may take place without materially modifying the arterial or capillary pressure while affecting temporarily the venous pressure. III. PRESSURE IN THE VEINS The pressure in the veins is so low that it may best be determined by a water manometer. In the veins the force of inflow is small; the resistance to outflow is nil. Hence the pressure is small, and steadily diminishes from the small veins to the large veins entering the heart (fig. 131). . The venous pressure may be modified by variations in these two factors. Constriction of the arterioles tends to lower the venous pressure, dilatation to raise it. On the other hand, inereased heart’s action, which so markedly tends to raise arterial pressure, diminishes the pressure in the larger veins, because the blood is thus more rapidly driven from veins into arteries, and because the heart, which in its powerful systole drives out more blood, in its diastole sucks in more. | Compression of the thorax has a very marked effect in retarding the flow of blood from the great veins into the heart, and thus tends to raise the venous pressure and to lower the arterial pressure. Venous pressure may be tem- porarily modified by the loss or gain of water. IV. PRESSURE IN THE LYMPHATICS No exact determination of the lymph pressure in the tissue spaces has been made, but since there is a constant flow from these spaces through the lymphatic vessels and through the thoracic duct into the veins at the root of the neck, the pressure in the tissue spaces must be higher than the pressure in the great veins. This pressure is kept up by the formation of lymph from the blood, and from the cells of the tissues (see p. 223). - CIRCULATION 287 B.—FLOW OF BLOOD The. flow of blood, as already indicated, depends upon the distribution of pressure, a fluid always tending to flow from . the point of higher pressure to the point of lower pressure. Since a high pressure is maintained in the aorta and a low pressure in the veins entering the heart and in the cavities of the heart during its diastole, the blood must flow through the circulation from arteries to veins. (Practical Physiology.) Velocity The velocity of the flow of a fluid depends upon the width of the channel. Since in unit of time unit of volume must pass each point in a stream if the fluid is not to accumulate at one point, the velocity must vary with the sectional area of the channel. In the case of a river, in each second the same amount of water must pass through the narrowest and through the widest part of its channel. Now for a ton of water to get through any point in a channel one square yard in sectional area in the same time as it takes to pass a point in a channel ten square yards in area, it must obviously flow with greater velocity. This may be stated in the pro- position that the velocity (V) of the stream is equal to the amount of blood passing any point per second (v) divided by the sectional area of the stream (S)-— e aes: where § is the radius squared multiplied by the constant 3:14. In, the vascular system the sectional area of the aorta is small when compared with the sectional area of the smaller arteries; while the sectional area of the capillary system is no less than 700 times greater than that of the aorta. In the venous system the sectional area steadily diminishes, although it never becomes so small as in the corresponding arteries, and where the great veins enter the heart it is about twice the sectional area of the aorta (fig. 132). This arrangement of the sectional area of the stream gives rise to a rapid flow in the arteries, a somewhat slower flow in the veins, and to a very slow flow in the capillaries. 288 VETERINARY PHYSIOLOGY The suddenness of the change of pressure has a certain influence on the rapidity of flow, as is well seen in a river. If the water descends over a sudden declivity to a lower level it attains a much greater velocity than if the declivity is gentle. In the first case the change of pressure is sudden, in the second case it is slow. Hence, if, from any cause, the pressure is raised at any point, the flow will tend to be more rapid from that point onwards till the normal distribution of pressure is re- established. Friction has also a certain effect. A river runs much faster in mid-stream than along the margins, because near AR c Vv a oe ee ce Fic. 182.—Diagram of the Sectional Area of the Vascular System, upon which the Velocity of the Flow depends. AR, arteries; C, capillaries; V, veins, : the banks the flow is delayed by friction, and the more broken up and subdivided is the channel, the greater is the friction and the more is the stream slowed. When, therefore, in the capillary systeur the blood stream is distributed through innumerable small channels, the friction is very great, and this tends to dam back the blood. The velocity of flow in the arteries and veins may be measured by various methods, of which one of the best is that by means of the stromuir, an instrument by which the volume of blood passing a given point in an artery or vein in a given time may be determined. The velocity of the flow in the capillaries may be measured CIRCULATION 289 in transparent structures by means of a microscope with an eye-piece micrometer. The velocity of the blood is— Carotid of the dog about . . 3800 mm. per sec. Capillaries about - . ‘ « °0°6 to:1-m.. ; Vein (jugular) about . . 150mm. Fe It is not so easy to give definite figures for the velocity of the lymph stream. Disturbance of any of the factors which govern the rate of fiow will bring about alterations in the velocity of the blood in arteries, capillaries, and veins. Thus, an increased venous pressure, by leading to a diminution in the difference of pressure between arteries and veins, will materially slow the blood stream. Great dilatation of the arterioles will slow the blood stream in them; and increased viscosity of the blood by increasing friction with the vessel wall will also slow the stream. Special Characters of Blood Flow (a) Arteries—The flow of blood in an artery is rhythmi- cally accelerated with each ventricular systole. This is due to the pulse wave. As the wave of high pressure passes alony the vessels, the blood tends to flow forwards and backwards from it—so that in front of the wave there is an acceleration of the stream and behind it a retardation. In a wave at sea the same thing happens, and a cork floating on the surface is moved forward in front of the wave and again backwards after the wave has passed. (6) Capillaries—In the capillaries the flow is uniform. (c) Veins.—In most veins, too, it is uniform, but in the great veins near the heart it undergoes accelerations— 1st. With each diastole of auricle and ventricle. 2nd. With each inspiration. 3rd. By muscular action squeezing the blood out of the small veins. In all vessels the blood in the centre of the stream moves more rapidly than that at the periphery on account of the friction between the blood and the vessels. An “azial” rapid and “peripheral” slow stream are, therefore, described. 19 290 VETERINARY PHYSIOLOGY This is well seen in any small vessel placed under the micro- scope, and in such situations it will be found that, while the erythrocytes are chiefly carried in the axial stream, the leuco- cytes are more confined to the peripheral stream, where they may be observed to roll along the vessel wall with a tendency to adhere to it. : When from any cause the flow through the capillaries is brought to a standstill, the leucocytes creep out through the vessel wall and invade the tissue spaces, This is the process of diapedesis, which plays an important part in inflammation. SPECIAL CHARACTERS OF THE CIRCULATION IN CERTAIN SITUATIONS 1. Cireulation Inside the Cranium (fig. 133).—Here the blood circulates in a closed cavity with rigid walls, and therefore its amount can vary only at the expense of the cerebro-spinal fluid. This is small in amount, and permits of very small variations in the volume .of blood. Increased arterial pressure in the | body does not therefore increase the amount of blood in the brain, but simply drives the blood more rapidly through the organ. There seems to be no regulating nervous mechanism ‘connected with the arterioles of the brain, and the cerebral pressure simply follows the changes in the general arterial pressure. The splanchnic area is the great regulator of the supply of blood to the brain. Since the cerebral arteries are supported and prevented from distending by the solid wall of the skull, the arterial pulse tends to be propagated into the veins. In these veins the respiratory pulse also is very well marked, 2. Circulation in the Lungs.—Vaso-motor nerves seem to be absent, and hence drugs like adrenalin fail to cause a constric- tion of the arterioles). The amount of blood in the lungs is regulated by the blood pressure in the systemic vessels. 3. Circulation in the Heart Wall.—A peripheral vaso-motor mechanism is not present in the arterioles of the coronary vessels (see also p. 245). 4. Cireulation in the Spleen.—Here the blood has to flow through a labyrinth of spaces in the spleen pulp, and it is driven on by the alternate contraction and relaxation of the non-striped muscles in the capsule and trabecule (see p. 221). A a | CIRCULATION 291 EXTRA-CARDIAC FACTORS MAINTAINING CIRCULATION In considering the flow of blood through the vessels due to the distribution of pressure in arteries and veins, it must be remembered that the central pump or heart is not the only factor maintaining it (fig. 133). The thorax in the movements of respiration is a suction INTRACRANIAL CIRCUJATION -—— PULMONARY CIRCULATION ———— 4 (recom. CIRCULATION , _ —_— c- / lc Yi Fie. 183.—Scheme of the Circulation, modified from Hill, to illustrate the influ- ence of the various extra-Cardiac Factors which maintain the Flow of Blood, pump of considerable power, which draws blood into the heart during inspiration. While the auricles may be regarded as the cisterns of the heart, the abdominal blood vessels are the great blood reservoir, and the diaphragm contracting in inspiration presses the blood from this reservoir up into the thorax and heart. When, in intermittent muscular exercise, the abdominal muscles are tightened and the respiratory move- CIRCUIATION IN LIMBS 292 VETERINARY PHYSIOLOGY ments of the thorax are increased in the panting which accom- panies it, the blood is partly pressed, and partly sucked from the abdomen into the heart, and so forced on into the arteries. ' Even expiration helps in this, for the blood which has filled the vessels of the lungs in inspiration is driven on into the left side of the heart in expiration. The blood is thus forced on into the arteries and so to the muscles, and they, by their alternate contraction and relaxation, further help to drive it on to the veins where the valves prevent any back flow during relaxation and thus accelerate the circulation. The high arterial tension thus produced tends to drive the blood through the cranial vessels. The benefit of intermittent muscular exercise on the circulation is thus manifest. When, on the other hand, some sustained muscular strain has to be undergone, the thorax is fixed, and hence (1st) the pressure on the heart and thoracic organs is raised, and the increased pressure in the thorax helps to support the heart and to prevent over-distension. The abdominal vessels are also pressed upon, and the sustained contraction of the limb. muscles tends to prevent the blood flowing through them. It is thus forced to the central nervous system in which the pressure rises, and if a weak spot in the vessels is present, rupture is apt to occur. (2nd) But, if the effort is still further sustained, the high intra-thoracic pressure tends to prevent proper diastolic filling of the heart; blood is therefore not sent on from the veins into the arteries, the veins become congested and the arterial pressure falls, less blood goes to the brain, and thus fainting may result. In the “head down position,” the accumulation of blood in the dependent. parts is prevented in the head by the vessels being packed inside the skull, and in the right side of the heart by the supporting pericardium. In man the position of the abdominal reservoir of blood at a lower level than the heart increases the work of that organ. In the horizontal position, when the reservoir is on the same level as the pump, the work is much easier. » Fainting is a-sudden loss of consciousness produced by failure in the supply of blood to the brain. It is accompanied Ge Oe a CIRCULATION 293 by loss of control over the muscles, so that the individual falls to the ground. It may be induced by any sudden lowering of the arterial blood pressure, whether due to decreased inflow of blood or to decreased peripheral resistance. Decreased inflow may be caused by ;—(qa) Cardiac inhibition brought about reflexly (1) by strong stimulation of ingoing nerves, and more especially of the nerves of the abdomen; (2) by strong stimulation of the upper brain neurons accompanied by changes in the consciousness of the nature of emotions ;—(d) Failure of the heart to pump blood from veins to arteries against the force of gravity, as when the erect position is suddenly assumed by people with weak hearts. Decreased resistance to outflow through sudden dilatation of arterioles may result from changes in the upper brain neurons, accompanied by emotional states and also in digestive dis- turbances. However induced, the anemic state of the brain leads to a stimulation of the cardiac inhibitory centre and the condition is thus accentuated. The cerebral anemia is accompanied by pallor of the face. The treatment consists in depressing the head to allow the force of gravity to actin filling the central vessels and in giving diffusible stimuli to increase the action of the heart. THE TIME TAKEN BY THE CIRCULATION This was first determined by injecting ferrocyanide of potas- sium into the proximal end of a cut vein, and finding how long it took to appear in the blood flowing from the distal end. From observation in the horse, dog, and rabbit, it appears that the time corresponds to about twenty-seven beats of the heart. Stewart has investigated the rate of flow through different organs by injecting salt solution into the artery, and by detecting its appearance in the vein by the change in the electric conductivity of the contents of the vessel. SECTION VI SuppLty or NourRISHING MATERIAL TO THE BLOOD AND LYMPH, AND ELIMINATION OF WASTE MATTER FROM THEM I, RESPIRATION Ir an animal be placed in a closed chamber filled with ordinary atmospheric air which contains by volume 79 parts of nitrogen and 21 parts of oxygen, and if the air be examined ~ after a time, it will be found that the oxygen has diminished in amount, and that a nearly corresponding amount of carbon dioxide has been added. (Practical Physiology.) — The same thing occurs in aquatic animals—the water round them loses oxygen and gains carbon dioxide. An animal takes up oxygen and gives off carbon dioxide. This is the process of external respiration. A. EXTERNAL RESPIRATION I. Respiratory Mechanism In aquatic animals the mechanism by which this process is carried on is a gill or gills. Each consists of a process from the surface covered by a very thin layer of integument, just below which is a tuft of capillary blood vessels. The oxygen passes from the water to the blood; the carbon dioxide from the blood to the water. A lung is simply a gill or mass of gills, turned outside in, with air instead of water outside the integument. While in aquatic gill-bearing animals there is constantly a fresh supply of water passing over the gills, in lung-bearing animals the air in the lung sacs must be exchanged by some mechanical contrivance. The lungs consist of myriads of small thin-walled sacs attached round the funnel-like expansions in which the air passages (infundibular passages) terminate. (Zhe structure 204 ; RESPIRATION 295 of the varwus parts of the respiratory tract must be studied practically.) Each sac is lined by a layer of simple squamous epithelium, supported by a framework of elastic fibrous tissue richly supplied with blood vessels. It has been calculated that, if all the air vesicles in the lungs of a man were spread out in one continuous sheet, a surface of about 100 square metres would be produced and that the blood Eun would occupy about 75 square metres of this. Through these vessels about 5000 litte of blood would ) pass. in twenty-four hours. The larger air passages are supported by pieces of hyaline cartilage in their walls, but the smaller terminal passages, the bronchioles, are without this Fic. 134.—Scheme of the Distribu- support, and are surrounded tion of a Bronchiole, Infundibular by # specially walk develope 4 Passage, and Air Sacs of the Lung. circular band of non-striped muscle—the bronchial muscle— which governs the admission of air to the infundibula and air sacs. The lungs are packed in the thorax round the heart, com- pletely filling the cavity. They may be regarded as two compound elastic-walled sacs, which completely fill an air-tight box with movable walls— the thorax—and communicate with the exterior by the wind- pipe or trachea. No air exists between the lungs and the sides and base of the thorax, so that the so-called pleural eavity is simply a potential space. If the thoracic wall be punctured so that this potential pleural cavity is brought into connection with the air, the lungs immediately collapse and occupy a small space posteriorly round the large bronchi. This is due to their elasticity (fig. 135). The lungs are kept in the distended condition in the thoracic cavity by the atmospheric pressure within them. Their elasticity varies according to whether the organs are stretched or not. As they collapse, their elastic force naturally - becomes less and less; as they are expanded, greater and 296 VETERINARY PHYSIOLOGY greater. Taken in the average condition of expansion in which they exist in the chest, the elasticity of the excised lungs of a man is capable of supporting a column of mercury of about 30 mm. in height, so that they are constantly tending to collapse with this force. But the inside of the lungs freely communicates with the atmosphere, and this, at the sea level, has a pressure of about 760 mm. Hg. During one part of respiration this pressure becomes a few mm. less, during another part a few mm. more; but the mean pressure of 760 mm. of mercury is constantly expanding the lung, and acting against a pressure of only 30 wm. of mercury, tending to collapse the lung, 760 Fie, 135.—Shows the Distribution of Pressure in the Thorax with the chest wall ' intact, and with an opening into the Pleural Cavity. (,) indicates the atmospheric pressure of 760 mm. of mercury ; 80 is the elasticity of the lungs also in mm. of mercury. Obviously, therefore, the lungs must be kept expanded and in contact with the chest wall. When a pleural cavity is opened, the distribution of forces is altered, for now the atmospheric pressure tells also on the outside as well as on the inside of the lungs and acts along with the elasticity of the organ. So that now a force of 760 mm. + 30 mm. = 790 mm. acts against 760 mm., causing a collapse of the lungs. In the surgery of the thorax, as well as in the physiology of respiration, these points are of great importance. II. Physiology The process of external respiration consists of two parts— 1st. The passage of air into and out of the air sacs. , 2nd. The interchange of gases between the air in the air vesicles and the blood in the capillaries. RESPIRATION 297 A. Passage of Air into and out of the Lungs This is brought about— 1st. By the movements of respiration—breathing. 2nd. By diffusion of gases. The air is made to pass into and out of the lungs by alternate inspiration and expiration. I. Movements of Respiration—A. Inspiration.—During this act the thoracic cavity is increased in all directions—lateral, antero-posterior, and vertical. As the thorax expands, the air pressure inside the lungs keeps them pressed against the chest wall, and the lungs expand with the chest. As a result of this expansion of the lungs the pressure inside becomes less than the atmospheric pressure, and air rushes in until the pressure inside and outside again becomes equal. This can be shown by placing a tube in the mouth or in a nostril and connecting it with a water manometer. (Practical Physiology.) This expansion of the lungs can readily be determined in the antero-posterior direction by percussion, and in the vertical planes by measurement. By tapping the chest with the finger over the lung in the right intercostal spaces, a resonant note is produced, while if the percussion is performed behind the level of the lung, a dull note is heard. Ii the posterior edge of this resonance be determined before an inspiration, and again during it, it will be found to have passed backwards. (Practical Physiology.) As a result of inspiration, the form of the chest is markedly modified, the change being best seen in trans- verse sections. In expiration the chest in transverse section is an ellipse from above downward, in inspiration it be- comes more circular (fig. 136). The change from side to side and from above downward is best marked towards the hinder part of the chest, less marked in the anterior part. These changes may be recorded by means of a Cyrto- meter, a piece of flexible gas tubing hinged behind, so that it can be modelled to the chest. (Practical Physiology.) The change from before backwards cannot be directly seen, but it is indicated by an expansion of the wall of the 298 VETERINARY PHYSIOLOGY abdomen. It will be described when considering the mechan- ism by which it is brought about. The expansion of the chest in inspiration is a muscular act and is carried out against the following forees— 1st. The Elasticity of the Lungs——To expand the lungs their elastic force has to be overcome, and the more they are expanded the greater is their elasticity. This factor therefore plays a smaller part at the beginning than towards the end of inspiration. 2nd. The Elasticity of the Chest Wali.—The resting position ES Fic. 136.—Vertical-tangential, Transverse, and Vertical Mesial Seetions of the Thorax in Inspiration and Expiration. of the chest is that of expiration. To expand the chest the costal cartilages have to be twisted. 3rd. The Elasticity of the Abdominal Wall.—As the cavity of © the thorax increases backwards; the abdominal viscera are pushed against the muscular abdominal wall, which, in virtue of its elasticity, resists the stretching force. In studying how these changes are brought about we may consider— 1st. The Increase in the Thorax from before backwards. — This is due to the contraction of the diaphragm (fig. 136). In expiration this dome-like muscle, rising from the vertebral column and from the posterior costal margin, arches forwards, 11 eS eS CT me RESPIRATION 299 lying for some distance along the inner surface of the ribs and then curving inwards to be inserted into the flattened central tendon, to which is attached the pericardium and on which rests the heart. In inspiration the muscular fibres contract. But the central - tendon being fixed by the pericardium does not undergo ex- tensive movement. The result of the muscular contraction is thus to flatten out the more marginal part of the muscle and to withdraw it more or less from the chest wall—thus opening up a space, the complemental pleura, into which the lungs expand (fig. 136). It might be expected that this contraction of the diaphragm would pull inwards the chest wall—but this is prevented by the expansion of the thorax in the lateral and vertical diameters as a result of the mechanism which has next to be considered. 2nd. The Increase in the Chest in the vertical and lateral diameters. This is brought about by the pulling forward of the ribs which rotate round the axis of their attachments to the verte- bral column. , To understand this, the mode of the connection of the ribs to the vertebral column must be borne in mind. The head of the rib is attached to the bodies of two adjacent vertebre. The tubercle of the rib is attached to the transverse process of the hinder of these vertebre. From this the shaft of the rib projects outwards, downwards and backwards, to be attached below to the sternum by the costal cartilage. If the rib is made to rotate round its two points of attachment, its lateral margin is tilted forwards and outwards, while its lower end is carried forwards and downwards (see fig. 137). Further, as we pass from before backwards, each pair of ribs forms the are of a larger and larger circle, and as each’ pair rises it takes the place of a smaller pair above. In these ways, the chest is increased from above downwards and from side to side. The first pair of ribs does not undergo this movement; the motion of the second pair of ribs is slight, but the range of movement becomes greater and greater as we pass downwards until the floating ribs are reached, and these are 300 VETERINARY PHYSIOLOGY fixed by the abdominal muscles. This greater movement is simply due to the greater length of the muscles moving the ribs. The muscles are chiefly the external intercostal muscles, and these may be considered as acting from the fixed first rib. Now, if the fibres of the first intercostal muscle are one inch in length, the second rib can be pulled forward, say, half an inch. The first and second inter- costals acting on the third rib will together be two inches in length, and in contracting they can pull the third rib through, say, half of two inches—i.e. one inch. The first, second, and third intercostals, acting on the fourth Fic. 137. —Shows the Movements of rib, are: three inches in length, the Ribs from their Position in and can therefore pull this rib Expiration to their Position in half of three, or one and a half, Inspiration. inches, and so through the other - ribs, until the floating ribs fixed by the abdominal muscles are reached. When the diaphragm tales the chief part in inspiration the breathing is said to be abdominal in type—when the intercostals chiefly act in raising the ribs it is said to be thoracic. Along with the intercostal muscles, the levatores costarum also act in raising the ribs and in increasing the thorax in the transverse and vertical diameters. These are the essential muscles of inspiration, but other muscles also participate in the act. In many animals, even when breathing quietly, it will be seen that the nostrils dilate with each inspiration. This is due to the action of the dilatores narium which contract synchronously with the other muscles of inspiration. Again, if the larynx be examined, it will be found that the vocal cords slightly diverge from one another during inspiration. This is brought about by the action of the posterior crico-arytenoid muscles (p. 335). Foreed Inspiration—This comparatively small group of muscles is sufficient to carry out the ordinary act of inspira- tion. But, in certain conditions, inspiration becomes forced. RESPIRATION 301 A forced inspiration may be made voluntarily, often it is pro- duced involuntarily. In it every muscle which can act from the fixed spine, head and shoulder girdle upon the thorax, is brought into play. Normally, these act from the thorax upon the parts into which they are inserted; now they act from their insertion wpon their point of origin. The sterno-mastoids, sterno-thyroids, and sterno-hyoids assist in elevating the thorax. The serratus magnus, pectoralis minor, and anterior fibres of the pectoralis major, and the part of the latissimus dorsi which passes from the humerus to the three last ribs, also pull these structures upwards. The facial and laryngeal movements also become exaggerated. &. In Expiration the various muscles of inspiration cease to act, and the forces against which they contended again contract the thorax in its three diameters— The elasticity of the lungs is no longer overcome by the muscles of inspiration, and the external atmospheric pressure acting along with it drives the chest wall inwards (see p. 296). The elasticity of the costal cartilages causes the ribs again to fall back, and finally the elasticity of the abdominal wall drives the abdominal viscera against the relaxed diaphragm and again arches it towards the thorax, squeezing its marginal portion against the ribs and occluding the complemental pleura. Experimental evidence shows that the internal intercostals contract with each expiration, and help to draw the ribs downwards. Ordinary expiration is thus normally mainly a passive act, being simply a return of the thorax to the position of rest. But voluntarily, and, in certain conditions, involuntarily, expiration may be forced. Foreed expiration is then partly due to the above factors, and partly due to the action of muscles. Every muscle which can in any way diminish the size of the thorax comes into play. Chief of these are the abdominal muscles, which by com- pressing the viscera push them upwards and press the dia- phragm further up into the thorax. At the same time, by acting from the pelvis to pull down the ribs, they decrease the thorax from side to side and from before backwards. The serratus posticus inferior and part of the sacro-lumbalis 302 VETERINARY PHYSIOLOGY pull downwards the lower ribs, and the triangularis sterni also assists in this. By this constriction of the thorax, brought about by ordin- ary or by forced expiration, the air inside is compressed and the pressure raised. During ordinary expiration the highest pressure reached is about 2 to 3 mm. Hg, in forced expiration about 80 mm. in man. The pressure of the air outside is less than this, and the air inside the chest is driven out. Special Respiratory Movements.—There are several peculiar and special reflex actions of the respiratory muscles, each caused by the stimulation of a special ; district, and each having a special purpose. Coughing.—This consists of an inspira- ioe ity tion followed by a strong expiratory effort during which the glottis is constricted but is forced open repeatedly by the current of expired air. It is generally due to irrita-. es 88°83 91°6 75 | Proteins ~ t 5 ; 3:0 1:0 10 Fats . : > . ‘ 3°5 1°3 11 Carbohydrates 4°5 5°7 3 Salts ; 0°7 0°4 1 Cheese, when allowed to stand, affords a suitable nidus for the growth of micro-organisms by the action of which the proteins are digested into peptones and simpler ‘bodies, and the fats split up into glycerine and the lower fatty acids. These free fatty acids give the peculiar flavour to ripe cheese. — The lactose is in part converted into lactic acid. 2. Flesh.— Under this head may be included not only the muscles of various animals, but also such cellular organs as the liver and kidneys. When free of fat, they contain about 20 per cent. of proteins. These are chiefly native proteins, but a certain amount of collagen is also present which yields gelatin on boiling. The amount of fat may vary from almost ni in white fish to about 80 per cent. in fat bacon. In animals specially fed, the amount of fat may be enormously increased and even ordinary butchers’ meat may have more fat than protein. Flesh is thus a source of proteins and sclero-proteins, and to a smaller extent of fats. The extractives include such ~ bodies as creatin, xanthin, inosit, etc. (see p. 40), which may help to give the peculiar flavour to the flesh of various animals. Flesh may be preserved in various ways—eg. by simply — drying, by salting, or by smoking. The result of each of these La eae FOOD AND DIGESTION 335 procedures is to diminish the amount of water, and thus to increase the solids. 3. Eggs—The egg of the domestic fowl need alone be considered. The composition of the white and of the yolk naturally differs considerably. The white of egg is pee more than a solution of proteins. In the yolk there is a very large amount of lecithin (p. 397) along with ordinary fats, and a large amount of a phospho- protein; and the great value of eggs is thus that they contain both proteins, ordinary fats, and the special phosphorus containing protein and fat. The mixed contents of the egg contains a little more than 10 per cent. each of proteins and of fats. Speaking generally, we may say that the animal food-stuffs are rich in proteins and fats, but are poor in carbohydrates. _ Vegetable Food-stuffs.—The peculiarity of special importance in vegetables is the existence of a capsule to the cells, com- posed of cellulose—a substance allied in its composition to starch—or of lignin or allied substances. Cellulose is to a large extent dissolved and decomposed in the alimentary canal of herbivora, while it is practically unacted upon in man and in the carnivora. Lignin or woody matter resists digestive changes even in the herbivora, and its only value is to increase the mass of the feces, and thus to stimulate intestinal action and to act as a natural purgative. The chief vegetable foods of the herbivora are grass, hay, oats, maize, and the leguminous plants, such as peas and beans. These vary considerably in composition according to their character, the ground in which they have been grown, and the season of the year at which they are used. Further, the methods of analysis do not give absolutely definite results. The proteins are generally estimated by determining the nitrogen and multiplying by 6:25. But other nitrogen- containing substances besides proteins occur in plants, amido- acids such as asparagin, which, while they are burned in the body and may yield energy like the fats and carbo- hydrates, do not seem capable of being used like the proteins in building up the living tissues. The amount of these sub- 336 VETERINARY PHYSIOLOGY stances is generally greater in the young than in the older — parts of plants. Thus in quite young grass about 70 per cent. of the nitrogen is in proteins, and about 30 per cent. not in proteins; while in old grass about 80 per cent. of the nitrogen exists in proteins. Again, in extracting fats with ether, chlorophyll, the green colouring substance of plants and other matters are removed along with the fat. Hence it must be remembered that the proteins and fats in the following table are somewhat too high :— Average Percentage Composition. Water. | Protein. | Fat.| Fibre. | jyarnte, | Ash. Grass : 80 4 1 + 10 1 | 20 to 30 per cent. of nitrogen not in proteins. Hay . ! 15 10 2 | 26 40 7 | About 10 percent. of nitrogen not in proteins, Peas . : 14 22 2 6 53 3 About 10percent. of nitrogen not in proteins. Oats 14 12 6 9 57 2 | Less than 10 per (crushed) cent. of nitrogen not in proteins. Potatoes .| 76 2 0 1 20 1 | Over 40 per cent. of nitrogen not in proteins. The soluble carbohydrates are determined by taking the — difference between the total solids and the proteins, fats, and insoluble fibre combined. But while a rough measure of their amount is thus procured, the presence of various gummy sub- stances may somewhat vitiate the results. For these reasons no attempt is made in the table above — to give more than rough approximate results of analysis such — as will be found useful in regulating the diet of domestic animals. Cooking of food for animals has two purposes—First, to soften and burst the cell capsule of vegetable foods so as to render the contents more readily available; and second, to — destroy bacteria. OO © fi FOOD AND DIGESTION. 337 II. DIGESTION I. Structure of Alimentary Canal The anatomy and histelogy of the alimentary tract must be studied practically. We shall here merely give such an out- line of the various structures as will assist in the comprehen- sion of their physiology. The Alimentary Canal (tig. 149) may be divided into the Fic. 148.—Mesial Section through the Head of a Horse, to show the long soft palate, 7, lying against the front of the epiglottis, 7; c, the tongue ; 7, the arytenoids. (ELLENBERGER. ) mouth, the cesophagus or gullet, the stomach, the small and large intestines, and three sets of supplementary structures— the salivary glands, the liver, and the pancreas. The Mouth, provided with its teeth, and surrounded by its ‘mobile muscular wall, with the muscular tongue lying in its gue iying floor, is the part of the canal in which the food is broken up and prepared for digestion. In the horse the lips are long and prehensile, and are essential for the taking of food. Into the mouth three pairs of compound glands—the Salivary 22 338 VETERINARY PHYSIOLOGY Glands—open. The parotid, lined entirely by enzyme-secret- ing epithelium, opens on the side of the cheek, while the submaxillary gland, composed partly of acini with enzyme- secreting, and partly of acini with mucin-secreting epithelium, and the sublingual, composed entirely of mucin-secreting acini, open under the tongue (S.C.). The tongue in the horse is smooth, but in the ox, and still more markedly in the cat, it is covered with a fine fur of processes, the jiliform papilla, which are of use in passing the food backwards along its sur- — face in the act of swallowing. (For Organs of Taste, see p. 113.) Posteriorly, the mouth opens into the pharynx (Ph.) or upper part of the gullet. In the horse the soft palate is very long, reach- ing to the base of the epiglottis, and, unless during swallowing, shutting off the mouth from the pharynx (fig. 148). On each Fic, 149.—Diagram of the Parts of side. between the mouth and the Alimentary Canal, from Mouth i ‘ toAnus, 7, tonsils; Ph., pharynx; the pharynx, is the tonsil (7'), S.@., salivary glands; Oc., eso- an almond-like mass of lymph- Phagus ; C., cardiac; Py., pyloric oid tissue. The pharynx is a portion ofstomach; D., duodenum ; f : Pi Teer 1 Pi denenies s Ki heee eee which can be shut off num; J., ileum; V., vermiform above from the posterior nares appendix ; Co/., colon; #., rectum. by raising the soft palate, and by pulling forward the posterior pharyngeal wall. It is sur- rounded by three constrictor muscles, which, by contracting from above downwards, force the food down the’gullet towards the stomach. The Csophagus (0e.) is a muscular walled tube lined by a stratified squamous epithelium. The muscles, below the lowest _ constrictor of the pharynx, are of the visceral type, and are A FOOD AND DIGESTION 339 arranged in two layers, an outer longitudinal layer, and an inner circular layer. The Stomach in carnivora and in the pig is a dilatation of the alimentary canal into which the gullet opens. To the left it expands into a sac-like cardiac end (C.), and to the right it narrows, forming the pyloric end (Py.). Like the gullet, it is surrounded by visceral muscular fibres, arranged essentially in two sets. At the cardiac orifice, the circular fibres form a not very marked cardiac sphincter, and at the wacc. caec. Fie. 150.—Stomach of the Horse to show—R. es, , the esophageal part ; Fu., the fundus with true gastric glands ; 2. pyl., pyloric part. pyloric end they form a very thick and strong pyloric sphincter. ‘The mucous membrane, which is covered by a columnar epi- thelium, is largely composed of tubular glands, those at the cardiac end containing two kinds of cells, the peptic and the oxyntic cells, those at the pyloric end containing peptic cells alone. In the horse the stomach is also a single sac (fig. 150), but the cardiac end is lined by a continuation of the stratified squamous epithelium of the gullet. The opening of this into the stomach is very narrow. The true cardiac mucous mem- 340 VETERINARY PHYSIOLOGY brane is confined to the great curvature. The whole stomach is small when compared with the large intestine, being capable, when distended, of holding about 17 litres. In ruminants the stomach is divided into four parts (fig, 151). 1. The large Rumen or Paunch into which the food is first passed before rumination. It is lined by a stratified squamous epithelium. 2. The Reticulum, which communicates directly with the last, and may almost be considered a part of it, is likewise lined by stratified squamous epithelium. The surface is raised into — Fic. 151.—Stomach of a Ruminant. a, cesophagus ; 0, rumen; ¢, reticulum with cesophageal groove above ; d, abomasum ; ¢, omasum ; f, duodenum. intersecting ridges, which give it the appearance of a honey- comb, - From the opening of the cesophagus, there pass along the top of the reticulum, two longitudinal muscular folds or pillars, _ 3. The Psalterium or Omasum has its surface raised into longitudinally disposed leaves, covered by rough stratified squamous epithelium. It opens below into— 4. The True Stomach. or Omasum resembles the stomach of the pig in all essential particulars. The stomach of the ox is about fifteen times as capacious that of the horse. The Small Intestine has a double muscular coat like t Stomach. The mucous membrane, which is covered by a columnar epithelium, is thickly set with simple test-tube-li glands—Lieberkiihn’s follicles—and is projected into the lum of the tube, as a series of delicate finger-like processes, th villi. The tissue of the villi and that between the Lieberkiihn follicles is chiefly lymphoid, and in certain places this lymphoi ryt; 02 Pea eee FOOD AND DIGESTION 341 tissue is massed in nodules which are either placed singly or grouped together in the lower part of the small intestine to form Peyer's patches. In the first part of the small intestine— the upper part of the duodenum (D.)—the submucous layer is full of small branching glands lined by an enzyme-secreting epithelium (Brunner's glands). The Large Intestine. The small intestine enters it at one side, and the opening is guarded by a fold of mucous membrane which forms the ileo-cecal valve. Above the opening of the small intestine a cecal pouch exists, and at the top of this is the vermiform appendix (V.), a narrow tube with an abundance of ) lymph tissue in its wall. This is specially well developed in the rabbit. Below the opening of the small intestine is the colon (Col.). This ends in the rectum (#.), which opens at the anus. The last part of the rectum is surrounded by a strong band of muscle—the internal sphincter ani—by which it is compressed. The whole large intestine is covered by columnar epithelium, and is studded with Lieberkiihn’s follicles, in which the epithelium is chiefly mucus-secreting’ in type. There are no villi. The cecum and colon in the horse (fig. 152) are enormously developed, holding about 120 litres, or seven times as much as the stomach. The colon is divided into the double colon, which is of immense size and complexity, and the single colon, which is smaller and simpler, and which ends in the rectum. The large intestine of ruminants is much smaller per unit of weight of the animal than that of the horse. In the ox, its capacity is less than 40 litres. In all animals it is the small intestine which presents the greatest extent of surface for absorption. Into the duodenum, the bile duct and the duct of the pancreas open. The bile duct is formed by the union of the ducts from the lobes of the liver. Upon its course is a diver- ticulum, the gall bladder. The Liver (Zi.) is a large solid- looking organ, formed originally as a double outgrowth from the alimentary canal. These outgrowths branch, and again branch, and between them the blood coming from the mother to the foetus flows in a number of capillary channels. Later, when the alimentary canal has developed, the blood from it is streamed between the liver tubules. In man and other 342 VETERINARY PHYSIOLOGY mammals, the fibrous tissue supporting the liver cuts it up into a number of small divisions, the lobules, each lobule --~ In the mesentry are seen the lymphatic vessels entering the Receptaculum Chyli and Thoracic Duct. (CHAUVEAU,) Fie, 152,— Viscera of the Horse, to show the Small Intestine, R, ending in the large Colon, d, and Cecum, 0, The Stomach is seen above the Colon. being composed of a series of tubules arranged radially with blood vessels coursing between them. FOOD AND DIGESTION 343 The portal vein which takes blood from the stomach, in- testine, pancreas, and spleen breaks up in the liver (fig. 107, p. 225), and carries the blood between the lobules. From the interlobular branches, capillaries run inwards and enter a central vein which carries the blood from each lobule, and pours it into the hepatic veins which join the inferior vena cava. The supporting tissue of the liver is supplied by the hepatic artery; and the terminal branches have a very free communication with those of the portal vein. The Panereas is essentially the same in structure as the parotid gland. But in the lobules are certain little masses of epithelium - like cells closely packed together, the Islets of Langerhans (fig. 153). The Nerve Supply of the ali- mentary canal. The muscles round the mouth are supplied by the fifth, seventh, and twelfth cranial nerves. The nerve supply of the salivary glands will be considered later. The pharyngeal muscles are supplied by the ninth and tenth cranial BEEVOS, and the cesophagus Fie. 153.—Section of Panbead to show is supplied by the tenth. Acini of Secreting Cells ; a large duct ; The stomach and intestine = and in the centre an Island of Langer- get their nerve fibres from "*- two sources (fig. 76, p. 153)—above the descending colon from the vagus and the abdominal sympathetic, and below this from the nervi erigentes and abdominal sympathetic—the various fibres passing through the abdominal sympathetic ganglia. In the wall of the stomach and intestine, these nerves end by forming an interlacing set of fibres, with nerve cells upon them, from which fibres pass to the muscles and glands, One of these plexuses (Auerbach’s) is placed between the muscular coats—the other (Meissner’s) is placed in the submucosa. Aksy . . } 344 VETERINARY PHYSIOLOGY II. Physiology I. DIGESTION IN THE MOUTH A. Prehension of Food.—In the horse, solid food is taken up by the lips and bitten by the incisor teeth. If the nerves — supplying the lips are cut, it becomes impossible for the horse to graze. Water is sucked into the mouth by a pumping action — of the tongue, which acts like a piston, and if air is allowed to get in above the lips, water cannot be sucked up. In ruminants the tongue plays the important part in collect- ing the hay or grass to be bitten off with the incisors. B. Mastication—In the mouth, by the act of chewing, the food is broken up and mixed with saliva. Mastication in the horse and in ruminants is chiefly a side- to-side movement, by which the food is ground between the molar teeth. It goes on for some time in one direction, and then takes place for some time in the opposite direction. The parotid gland on the side to which the animal is chewing secretes, while the other is less active. In the horse, the pro- cess of mastication is very completely performed before the food is swallowed, the animal taking about five to ten minutes to eat a pound of corn, and fifteen to twenty minutes to eat the same — amount of hay, The teeth in herbivora grow from a permanent pulp, and hence the changes due to this constant growth give a character to the incisors by which the age may be determined. In ruminants the food is chewed later during rumination. C. Saliva.—The saliva is formed by the salivary glands (viz., the parotid, submaxillary, sublingual, and various small glands in the mucous membrane of the mouth). The quantity of saliva secreted by the horse has _ been measured by making an cesophageal fistula and collecting the boluses of food which are swallowed, and so finding the amount of fluid which has been secreted in the mouth. In one horse about thirty-six litres of saliva were produced in twenty-four hours, Characters.—It is a somewhat turbid fluid which, when allowed to stand, throws down a white deposit consisting of shed epithelial scales from the mouth, leucocytes, amorphous — FOOD AND DIGESTION 345 ealcic and magnesic phosphates, and generally numerous bacteria. Its specific gravity is low—generally about 1003. In reaction it is neutral or faintly alkaline. Chemically it is found to contain a very small proportion of solids, and the proportion of these varies with the stimulus which causes the secretion. In addition to mucin, traces of proteins are present, and with these proteins in certain animals, but not in the dog, is associated the active constituent or enzyme of the saliva—ptyalin. Saliva generally contains traces of potassium sulpho-cyanide. The functions of the saliva are twofold :— 1. Mechanical, to moisten the mouth and gullet, and thus to assist in speaking, chewing, and swallowing. Since the salivary glands are absent from aquatic mammals, and since in carnivorous animals saliva has no chemical action, it would appear that this is the important function. 2. Chemical—Under the action of the ptyalin of the saliva when this is present, polysaccharids, like the starches, are broken down into sugars. Like other enzyme actions the pro- cess requires the presence of water and a suitable temperature, and it is stopped by the presence of strong acids or alkalies, by various chemical substances, and by a temperature of over 60° C., while it is temporarily inhibited by reducing the temperature to near the freezing point. The starch is first changed into the dextrins, giving a brown colour with iodine and hence called erythrodextrins, then into dextrins which give no colour with iodine, achroodextrins, and lastly into the di- saccharid maltose (see p. 330). (Chemical Physiology). Physiology of Salivary Secretion—In order to study the physiology of salivary secretion, a canula may be inserted into the duct of any of the salivary glands and the rate of flow of saliva or the pressure of secretion may be thus measured. In this way it may be shown that the taking of food, or simply the act of chewing, and in some cases the mere sight of food, causes a flow of saliva. This shows that the process of secretion is presided over by the central nervous system. : The submaxillary and sublingual glands are supplied—(1) By branches from the lingual division of the fifth cranial nerve ; and (2) by branches of the perivascular sympathetic fibres coming from the superior cervical ganglion. The parotid gland 346 VETERINARY PHYSIOLOGY is supplied by the auriculo-temporal division of the fifth and also by sympathetic fibres (fig. 154). The influence of these nerves has been chiefly studied on the — ' submaxillary and sublingual glands, It has been found that, when the lingual ‘nerve is cut, the reflex secretion of saliva still takes place, but that, when the ~ chorda tympani (C/.7.), a branch from the seventh nerve which joins the lingual, is cut, the reflex secretion does not Vv Mil Si. Fic. 154.—Nervous Supply of the Salivary Glands, Par., parotid, and S.J/. and S.Z., the submaxillary and sublingual glands; VJZ., the seventh — cranial nerve, with Cd.7., the chorda tympani nerve, passing to Z., the lingual branch of V., the fifth nerve, to supply the glands below the tongue, 7. ; [X., the glossopharyngeal giving off J.NV., Jacobson’s nerve, to the O., otic ganglion, to supply the parotid gland through Awr.T7,, the auriculo-temporal nerve. occur. Stimulation of the chorda tympani causes a copious — flow of watery saliva, and a dilatation of the blood vessels of the glands. If atropine has been first administered the dilatation of the vessels occurs without the flow of saliva. This indicates that the two processes are independent of one another. The secreting fibres all undergo interruption before the glands are reached; the fibres to the sublingual gland having their cell. station in the submaxillary ganglion (S.JZG), — the fibres to the submaxillary gland having theirs in a little + ine @ te FOOD AND DIGESTION 347 ganglion at the hilus of the gland (S.J£). This was demon- strated by painting the two ganglia with nicotine (p. 154). When applied to the submaxillary ganglion the drug does not interfere with the passage of impulses to the submaxillary gland, but stops those going to the sublingual. If the duct of the gland be connected with a mercurial manometer, it is found that when the chorda tympani is stimulated the pressure of secretion may exceed the blood pressure in the carotid, showing that the saliva is not formed by filtration. When the perivascular sympathetics, or when the sympathetic cord of the neck is stimulated, the blood vessels of the gland constrict, and a flow of very viscous saliva takes place. On the parotid gland the auriculo-temporal nerve (Aur.7.) acts in the same way as the chorda tympani acts on the other salivary glands. But stimulation of the fifth nerve above the otic ganglion, from which the auriculo-temporal takes origin, fails to produce any effect. On the other hand, stimula- tion of the glossopharyngeal nerve (LY.) as it comes off from the brain, acts upon the parotid gland. Since the glosso- pharyngeal is united by Jacobson’s nerve (J.N.) to the small superficial petrosal which passes to the otic ganglion, it is obvious that the parotid fibres take this somewhat round- about course. — When the sympathetic fibres to the gland alone are stim- ulated, constriction of the blood vessels but no flow of saliva occurs ; but if, when the flow of watery saliva is being produced by stimulating the glossopharyngeal or Jacobson’s nerve, the sympathetic fibres are stimulated, the amount of organic solids in the parotid saliva is very markedly increased. The nerve fibres passing to the salivary glands are presided over by a centre in the medulla oblongata which acts reflexly. So long as this is intact, stimulation of the lingual or glosso- pharyngeal leads to a reflex flow of saliva. Other nerves may also act on this centre. Thus, gastric irritation, when it produces vomiting, causes a reflex stimulation of salivary secretion. According to the investigations of Pavlov the salivary glands _ react appropriately to different kinds of stimuli through their nervous mechanism. When sand or bitter or saline substances 348 VETERINARY PHYSIOLOGY are put in a dog’s mouth a profuse secretion of very watery saliva ensues to wash them out. When flesh is given a saliva rich in mucin is produced. When dry food is given saliva — is produced in greater quantity than when moist food is — supplied. Pavlov further states that the sight of different kinds of food produces a flow of the kind of saliva which their presence in the mouth would produce. II. SWALLOWING The food after being masticated is collected on the surface of the tongue by the voluntary action of the buccinators and other muscles, and then, the point of the tongue being pressed — against the hard palate behind the teeth, by a contraction of the tongue passing from before backwards, the bolus of food is driven backwards. When the posterior part of the tongue is reached the act becomes purely reflex, and the food is forced through the pillars of the fauces into the pharynx, It is prevented from passing up into the posterior nares by the contraction of the palato-pharyngeus muscle, and of the levatores palati. The larynx is pulled upwards as a whole by the stylo-hyoid and stylo-thyroid and the thyro-hyoid muscles, and the entrance of food is prevented by the closure of upper part of aperture. The arytenoid cartilages are pulled forward by the thyro-— arytenoid muscles, and approximated by the arytenoidei, while a cushion on the posterior surface of the epiglottis becomes applied to their tips, forming a tri-radiate fissure or chink through which food cannot pass. The lateral crico-arytenoids also approximate the vocal cords, and close the glottis. The constrictors of the pharynx contract from above down- wards, and force the food into the grasp of the cesophagus, and this by a slow peristaltic contraction sends the food onwards to the stomach. This peristalsis is abolished by section of the vagi, and it is generally not essential to swallowing. In swallowing liquids it is not brought into play, but the fluid is forced by the tongue down the relaxed cesophagus into the stomach. 5 The passage of the food into the stomach may be heard as a FOOD AND DIGESTION 349 gurgling sound by applying a stethoscope to the right side of the spinal column, and any delay caused by a stricture may thus be determined. _ In swallowing fluids two sounds are heard, one immediately, and one after about six seconds. Ill. DIGESTION IN THE STOMACH Within recent years the most important work on gastric digestion in the dog has been accomplished by Pavlov on dogs. His method is to make a small gastric pouch opening on the rose cularis Mucosa Fie. 155.—Diagram of Pavlov’s Pouch made on the Stomach of a Dog. surface and separated from the rest of the stomach (fig. 155). This is done by cutting out a V-shaped piece along the’ great curvature, the apex being towards the pylorus and the base being left; connected with the stomach wall. By a series of stitches the opening thus made in the stomach is closed up (top line of AAs in fig. 155), while the cut edges of the V-shaped flap are stitched together to form a tube. The one end of this is made to open upon the skin surface 4, A, and by folding in the mucous membrane the deep end is isolated from the stomach. Thus a pouch is formed still connected with the nerves and vessels of the stomach, the condition of which represents the condition of the whole stomach. 350 VETERINARY PHYSIOLOGY The condition of the stomach varies greatly in fasting and after feeding. Gastric digestion in the dog and pig will first be considered. A. Gastrie Digestion in the Dog and Pig Stomach during Fasting The organ is collapsed, and the mucous membrane is thrown into large ridges. It is pale in colour because the blood vessels are not dilated. Movements are not marked and the secretion is scanty, only a little mucus being formed on the surface of — the lining membrane. Stomach after Feeding When food is taken the blood vessels dilate, a secretion is poured out, and movements of the organ become more marked. 1. Vaseular Changes.—The arterioles dilate, and the mucous membrane becomes bright red in colour. This is a reflex vaso-. dilator effect, impulses passing up the vagus to a vaso-dilator centre in the medulla, and coming down the vagus from that centre. Section of the vagi prevents its onset. 2. Seeretion.—There is a free flow of gastric juice from all the glands in the mucous membrane. (a) Characters of Gastric Secretion—The gastric juice is a clear watery fluid, which is markedly acid from the presence of free hydrochloric acid. In the dog the free acid may amount to 0°2 per cent., but in man it is less abundant, and when the gastric juice is mixed with food the acid rapidly combines with alkalies and with proteins, and is no longer free. In addition to the HCl, small quantities of organic salts are present. Traces of proteins may also be demonstrated, and with these two enzymes are associated—one a proteolytic or protein-digesting enzyme, pepsin, the other a milk-curdling enzyme, rennin. (b) Course of Gastric Digestion—(1) Amylolytie Period.—The action of the gastric juice does not at once become manifest. For a short time after the food is swallowed, in the pig the ptyalin of the saliva goes on acting, and the various micro- organisms swallowed with the food grow and multiply, and thus FOOD AND DIGESTION 351 there is a continuance of the conversion of starch to sugar which was started in the mouth, and at the same time the micro-organisms go on splitting the sugar to form lactic acid, which may thus be regarded as a normal constituent of the stomach during the first half-hour after a mixed meal. In the dog ptyalin is absent. (2) Proteolytie Period.—Before the amylolytic period is com- pleted, the gastric juice has begun its special action on proteins. This may be readily studied by placing some coagulated protein in gastric juice, or in an extract of the mucous membrane of the stomach made with dilute hydrochloric acid, and keeping it at the temperature of the body. The protein swells, becomes transparent, and dissolves. The solution is coagulated on boiling—a soluble native protein has been formed. Very soon it is found that, if the soluble native protein is filtered off, the filtrate gives a precipitate on neutralising, showing that an acid proteate (meta-protein) has been produced. If the action is allowed to continue and the acid proteate precipitated and filtered off, it will be found that the filtrate gives a precipitate on saturating with common salt, showing that a proto-proteose has been formed. Along with this a certain amount of hetero- proteose is also formed. It is characterised by being precipitated on neutralisation and by being insoluble in distilled water. On filtering off these, the filtrate yields a precipitate on saturating with ammonium sulphate, indicating the formation of a deutero- proteose, and, if the filtrate from this be tested, the presence of a protein may be demonstrated. Peptone has been produced. (Chemical Physiology.) These changes may be represented in the following table :— Coagulated Protein. | - Soluble Native Protein. | Acid Proteate (Meta-protein). | Proto-proteose, Hetero-proteose. Deutero-proteose. Deutero-proteose. Peptone. Peptone. 352 VETERINARY PHYSIOLOGY The process is one of breaking down a complex molecule into simpler molecules, probably with hydration. The object of this was formerly supposed to be to allow of the diffusion of the protein in the form of peptone through the wall of the intestine. It is now known that absorption is not due to diffusion, and it is more probable that the change to the simplest protein molecule is a step to the more complete disintegration of the molecule which seems necessary before it can be built into the special protoplasm of the body of the particular animal. On certain proteins and their derivatives the gastric juice has a special action. On collagen the HCl acts slightly in converting it to gelatin. The gastric juice acts on gelatin, — converting it to a gelatin peptone. On nucleo-proteins it acts by digesting the protein part and leaving the nuclein undissolved. Hemoglobin is broken down into hematin aud globin, and the latter is changed into peptone. . The caseinogen calcium compound of milk is first coagulated and then changed to peptone. The coagulation is brought about by the presence of the second enzyme of the gastric juice—rennin. This may be separated from pepsin in various ways, and, unlike pepsin, it acts in a neutral medium. The change set up by it seems to be due to a splitting of the soluble calcic compound of caseinogen which exists in milk into calcic paracasein, which is insoluble and is thrown down, and a small quantity of whey albumin which remains in solution. The nuclein part of the paracasein remains undigested, The gastric juice contains an enzyme which splits Fats into — fatty acids and glycerine if they are in a very fine state of sub- division, as in milk, but it has no action on fats not so sub- divided. When fats are contained in the protoplasm of cells, — they are set free by the digestion of the protein covering. On Carbohydrates the free mineral acid of the gastric juice has a slight action at the body temperature, splitting the polysaccharids and disaccharids into monosaccharids. (c) Digestion.of the Stomach Wall—When the wall of the stomach dies either in whole, as after the death of the animal, — FOOD AND DIGESTION 353 or in part, as when an artery is occluded or ligatured, the dead part is digested by the gastric juice and the wall of the stomach may be perforated. In the living condition a substance may be extracted from the mucous membrane which antogonises the action of pepsin and may be called antipepsin. (d) Antiseptie Action of the Gastrie Juice.—In virtue of the presence of free HCl the gastric juice has a marked action in inhibiting the growth of or-in killing bacteria. The bacillus of cholera is peculiarly susceptible, and a healthy condition of the stomach is thus a great safeguard against the disease. Other organisms, while they do not multiply in the stomach, pass on alive to the intestine where they may again become active. When HCl is not formed in sufficient quantities to exist free in the stomach, the activity of these bacteria in the organ may lead to various decompositions and to many of the symptoms of dyspepsia. (e) Souree of the Constituents of the Gastrie Juice.——The hydrochioric acid is formed at the cardiac end of the stomach, This may be shown by isolating a part of the stomach so that it opens on the surface. Since the parietal or oxyntic cells are confined to this portion of the stomach, it may be concluded that they are the producers of the acid. They manufacture it from the NaCl of the blood plasma. Probably the CO, liberated in the cells seizes on some of the Na and turns out HCl. The Pepsin and Rennin are produced in the chief or peptic cells which line the glands both of the cardiac and pyloric parts of the stomach. During fasting granules are seen to accumulate in these cells, and when the stomach is active they are discharged. These granules are not pepsin but the forerunner of pepsin—pepsinogen. (f) Influence of Various Diets upon the Gastrie Juice.—This has been chiefly worked out by Pavlov on dogs with a gastric pouch (p. 349). He finds that—(1) The amount of secretion depends upon the amount of food taken. (2) The amount and course of secretion varies with the kind of food taken. Thus, with flesh the secretion reaches its maximum at the end of one 23 354 VETERINARY PHYSIOLOGY hour, persists for an hour and then rapidly falls, while with bread it reaches its maximum at the end of one hour, rapidly falls, but persists for a much longer period than in the case of flesh. (3) The digestive activity of the juices varies with the kind of food and with the course of digestion. It is higher and persists longer after a diet of bread, which is difficult to digest, than after a diet of flesh, which is more easily digested. (+) The percentage of acid does not vary markedly. When more acid is required more gastric juice is secreted. (5) The work done by the gastric glands is greater in the digestion of bread than in the digestion of flesh. _ (g) Nervous Mechanism of Gastrie Seeretion.—It has been proved that in the dog the secretion of gastric juice can go on after the nerves to the stomach have been divided, and this has been ascribed to a reflex stimulation of the nerve plexus in the — submucosa. But this mechanism plays a small part compared — with the influences of the central nervous system through the vagus. Pavlov finds that, when the vagus is cut below the origin of the cardiac nerves so that they are not acted upon, — and the animal left undisturbed for some days, stimulation of the nerve with a slowly interrupted induced current causes, after a long latent period of a minute or two, a flow of gastric juice. This vagus action may be called into play either by the contact of suitable food with the mouth or by the sight of food. This he demonstrated by making an cesophageal fistula in a dog with a gastric pouch, so that food put in the mouth escaped from the gullet and did not pass into the stomach (fig. 156). Mere mechanical or chemical stimulation of the mouth produces no effect, but the administration of meat produced it. The sight of food in a fasting dog produces after a latent period of five minutes a copious flow of gastric juice. Pavlov calls this “psychic” stimulation. It is an example of how the “distance receptor” in the eye reflexly brings about an appropriate reaction—just as the “non-distance receptor” in the wall of the stomach under certain stimuli brings about an appropriate reaction. It is somewhat rash of a physiologist who can know nothing of the relation of the psychic state to the actions with which it is associated to affirm, as Pavlov does, that the psychic change is causal. There is some evidence that the formation of gastric juice FOOD AND DIGESTION 355 is also influenced by the action of a chemical substance produced in the mucous membrane of the pyloric end of the stomach. It has been found that the injection into the blood stream of an extract of this membrane made by boiling with acid or peptone causes a production of gastric juice. In all probability the initial secretion of gastric juice is dependent on the nervous mechanism, and the secondary secretion, when NOSE EYE \ CSA . ower ~~ ae N GULLET _ SD eo FISTULA \ o ' 1/ STOMACH POUCH Fic. 156.—To show the nervous mechanism of gastric secretion and how it is reflexly induced through various ingoing channels. food is in the stomach, on the action of this substance. In the case of the pancreas such a chemical stimulant plays a very important part (p. 420). Such substances have been named Hormones. 3. Movements of the Stomach.—These have been studied by feeding an animal with food containing bismuth, and then applying X-rays, which are intercepted by the coating of bismuth, so that a shadow picture of the shape of the stomach is given (fig. 157). 356 VETERINARY PHYSIOLOGY It is found that, soon after food is taken, a constriction forms about the middle of the stomach and slowly passes on towards the pylorus. Another constriction forms and follows the first, and thus the pyloric part of the stomach is set into active peristalsis. The cardiac fundus acts as a reservoir, and, by a — steady contraction, presses the gastric contents into the more active pylorus, so that, at the end of gastric digestion, it is completely emptied. The pylorus is closed by the strong sphincter muscle, which, however, relaxes from time to time during gastric digestion to — allow the escape of the more fluid contents of the stomach. into the intestine. These openings are at first slight and A B aN) Fic. 157.—Tracings of the shadows of the contents of the stomach and intestine of a cat two hours after feeding (A) with boiled lean beef, and (B) with boiled rice to show the more rapid emptying of the stomach after the carbohydrate food. The small divisions of the food in some of the intestinal loops represent the process of rhythmic segmentation (see p. 375). (Cannon.) transitory, but as time goes on they become more marked and more frequent, and when gastric digestion is complete—usually at the end of five or six hours—the sphincter is completely relaxed and allows the stomach to be emptied. The openings are regulated by a local nervous mechanism which is brought into play by the escape of the acid gastric contents into the duodenum. This leads to an immediate closure of the pylorus, which does not again open till the contents of the duodenum have been neutralised by the alkaline secretions which are poured into it. The rate of passage from the stomach of various kinds of food has been studied by feeding cats with equal amounts of each kind of food mixed with bismuth, and FOOD AND DIGESTION 357 then, by X-rays, getting the outline of the contents of the small intestine at different periods. Carbohydrates were found to pass on most rapidly and fats most slowly (fig. 156). Unless the contents of the stomach are very fluid these movements do not _ produce a very great mixing of the food taken (fig. 158). Nervous Mechanism of Gastric Movements.—Even after the section of all the gastric nerves, movements of the stomach may be observed, but the mechanism of these movements has not been fully studied. The action of the vagus and sympathetic fibres is complicated, and their influence on the wall of the Fic. 158.—Stomach of a Dog fed successively with three different foods to show the absence of mixing. (SCHEUNEET.) stomach and the sphincters requires further investigation. ‘Speaking generally, the vagus seems to increase the move- ments, while the sympathetic fibres check them. The vagus when stimulated generally causes inhibition of the cardiac sphincter and contraction of the pylorus, but the result is not constant. Absorption from the Stomach By ligation of the pyloric end, it has been found. that the stomach plays a very small part in the absorption of food. Its chief function is to act as a reservoir. Probably the antiseptic action of its secretion is of considerable 358 VETERINARY PHYSIOLOGY importance. While it. plays a certain part in digestion its action is by no means indispensable, for it-has been removed in ~ animals and in men without disturbance of the health. ’ ~ Vomiting Sometimes the stomach is emptied upwards through the Z gullet instead of downwards through the pylorus, This act of — vomiting is generally a reflex one, resulting from irritation of the gastric mucous membrane and more rarely from stimulation of other nerves. It is a reaction to nocuous stimuli. In vomiting, the glottis is closed, and, after a forced in- spiratory effort by which air is drawn. down into the gullet, a forced and spasmodic expiration presses on the stomach, while at the same time the cardiac sphincter is relaxed through the action of the vagus, and the contents of the stomach are sent upwards. They are at first prevented from passing into the nares by the contraction of the muscles of the soft palate; but, as the act continues, these muscles are overcome, and the vomited matter escapes through mouth and nose, The wall of the stomach also seems to act, but its action is non-essential, since vomiting may be produced in an animal in which a bladder has been inserted in place of the stomach. The centre which presides over the act is in the medulla oblongata, and while it is usually reflexly called into action, it may be stimulated directly by such drugs as apomorphine. — B. Gastrie Digestion in the Horse In the horse the process of gastric digestion differs from that first described in the following particulars. In the first place, the horse has to eat a very large quantity of food in proportion to the size of its stomach, and it is found that part of the food begins to pass very rapidly through the stomach into the intestine. Colin found, when he killed a horse which in two hours had eaten 2500 grms. of hay, that. the stomach contained only 1000 grms. But while this is the ease, a small residue of the meal remains for a very long time in the stomach, and passes out only when the next mea is taken, The churning action of the stomach is less complete in the FOOD AND DIGESTION 359 horse than in the dog, and hence, when the animal has received hay, followed by oats, these are found lying more or less separate, unless the animal has taken water, when they - are more fixed. In the horse the amylolytic period is well marked, and the percentage of hydrochloric acid is never so high as in the dog. Lactic acid is always formed from the carbohydrate of the food, and on a diet of hay it may exceed the hydrochloric acid. The proteolytic action of the gastric juice of the horse is slower NW Uys Zz S ~ Fic. 159.—Stomach of Horse fed successively on four differently coloured foods to show the distribution of the various foods in the viscus. than that of carnivora, but it is very marked, and peptones are found abundantly in the stomach at the end of digestion. In the stomach of the horse the cellulose of the food is partly decomposed, probably by the action of an enzyme in the grain, which has been described by H. J. Brown. Vomiting in the horse is very rare, and only occurs when the stomach is much over-distended. The food passes up the esophagus in small quantities, and, on account of the length of the soft palate, it cannot get into the mouth, but escapes through the nose (fig. 148, p. 337.) 360 VETERINARY PHYSIOLOGY C. Gastrie Digestion in Ruminants This is complicated by the act of rumination or chewing the cud. The food is rapidly cropped and swallowed, being passed into the reticulum and rumen. The more fluid part tends to accumulate in the former cavity. At a convenient opportunity, by contraction of these cavities and of the abdominal walls and diaphragm, a bolus of their contents is regurgitated into the cesophagus, which, by an antiperistalsis, passes it up into the mouth, where it is thoroughly masticated and mixed with saliva. It is then swallowed, and, by a contraction of the walls — of the pillars of the cesophageal groove, the third stomach is drawn close up to the cesophagus and receives the bolus. After — straining through the leaves of the omasum this enters the abomasum or true stomach, and is there subjected to ordinary proteolytic digestion. This has been studied by making a Pavlov’s pouch in the abomasum of the goat. When no food is taken the secretion is alkaline and has no peptic action, but when food is taken hydrochloric acid is secreted and peptic digestion occurs. An amylolytic enzyme converting starch to sugar seems also to be formed, and its presence is confirmed by the demonstration of its occurrence in the stomach of the pig. As the food lies in the rumen and reticulum, it is subjected to the action of bacteria, by. which the cellulose is partly decomposed, and the cell contents thus set free for the action of digestive enzymes. IV. INTESTINAL DIGESTION A. In the Dog and Pig After being subjected to gastric digestion the food is gene- rally reduced to a semi-fluid grey pultaceous condition of strongly acid reaction known as chyme, and in this condition it enters the duodenum. | Here it meets three different secretions :— Panereatie secretion. Intestinal secretion. Bile. FOOD AND DIGESTION 361 A. Panereatie Secretion The secretion of the pancreas may, in the dog, be procured by making either a temporary or a permanent fistula. In the former case the duct is exposed, and a canula fastened in it; in the latter the duct is made to open on the surface of the abdomen, a small piece of the intestinal wall with the mucous membrane round the opening of the duct being stitched to the abdominal opening. 1. Characters and Composition.— When obtained immediately from a temporary fistula, the pancreatic juice is a clear, slimy fluid, with a specific gravity of 1030 or less and an alkaline reaction. It contains an abundance of a native protein having the characters of a globulin, and the alkalinity is probably due to sodium carbonate and disodium phosphate. From a permanent fistula a more abundant flow of more watery | secretion may be collected. 2. Aetion.—Closely associated with the protein, and pre- cipitated by alcohol along with it, are the enzymes upon which the action of the pancreatic juice depends. (Chemical Physiology ) Ist. 4 Proteolytic Enzyme—Trypsin.—This, in a weakly alkaline or neutral fluid, converts native proteins into peptones, and then breaks these peptones into simpler non-protein bodies. The pancreatic juice brings about this breaking down of protein in stages. It does not cause solid proteins to swell up but simply erodes them away. Fibrin and similar bodies first pass into the condition of soluble native proteins and then into deutero-proteose, while boiled egg white appears at once to yield deutero-proteose. The deutero-proteose is then changed into peptone, and part of that peptone is then split into a series of bodies which no longer give the biuret test. These consist chiefly of the component amido-acids of which the most important are leucin and tyrosin, and of ammonia compounds (see p. 8). Amido-acetic acid linked to skatol—tryptophane—is also split off, and if chlorine water is added to a pancreatic digestion, which has proceeded for a long time, a rose-red colour is struck (see p. 430). 362 VETERINARY PHYSIOLOGY On nucleo-proteins trypsin acts by digesting the protein and dissolving the nucleic acid so that it can be absorbed. On collagen and elastin trypsin has little action; but on gelatin it acts as upon proteins. . 2nd. An Amylolytic Enzyme—Amylopsin or Diastase.—This- acts in the same way as ptyalin, but more powerfully, convert- ing a certain part of the maltose into dextrose. It acts best in a faintly acid medium. 3rd. A Fat Splitting Enzyme—Lipase.—This is the most easily destroyed and the most difficult to separate of the zymins. lt breaks the fats into their component glycerin and fatty acids. The fatty acids link with the alkalies which are present to form soaps, and in this form, or dissolved as free fatty acids in the bile, they are absorbed. But the formation of soaps also assists the digestion of fats by reducing them to a state of finely divided particles, an emulsion, upon which the lipase can act more freely. This process of emulsification is assisted by the presence of protein in the pancreatic juice and also by the presence of bile. That these enzymes are independent of one another is show by many facts. 1. Diastase does not appear till about a month after birth. 2. Diastase is taken up by dry glycerin while trypsin is: not. 3. Trypsin may be precipitated and separated by shaking with collodion. 4. Trypsin acts in 0°01 per cent. ammonia while diastase does not. 5. The proportion of the zymins varies with the character of the diet. This is well shown by experiments carried out in Pavlov’s laboratory upon dogs with pancreatic fistule. The effects of diets of milk, bread, and flesh were compared, in each e the amount of the food given containing the same amount of nitrogen (protein). The total quantity of ferment unit is got by multiplying the quantity of the juice in ce.cm. by the strength of the juice, determined by ascertaining how much of the substance is digested by unit of the secretion in unit FOOD AND DIGESTION 303 of time. The following table and fig. 159 indicate the results obtained :— Quantity of Enzyme. Diet. | ———= Proteolytic. | Amylolytic Fat Splitting. cay (ones | Seis Bread, 250 grm. : 1978 1601 | 800 Milk, 600 c.c. : é 1085 432 4334 Flesh, 100 grm._ . . 1502 648 3600 TRYPSIN DIASTASE ' G J LIPASE BREAD MILK FLESH 250GRNS 600CCS J/00GRNS Fic. 160.—To show the relative amounts of the three enzymes of the pancreatic juice formed on different diets. Bread contains a protein difficult of digestion, plenty of starch, and little fat. Milk contains an easily digested protein, and plenty of fat, but no starch; while flesh contains a com- paratively easily digested protein, no starch, and a moderate amount of fat. The first food causes a copious production of - trypsin and diastase, and little lipase. The second causes the production of less trypsin, little diastase, but most lipase. The 304 VETERINARY PHYSIOLOGY last causes a moderate production of trypsin, little diastase, and a comparatively large amount of lipase. As to the mode of production of these enzymes, it is known that trypsin is not formed as such in the cells, for the secretion direct from the acini has no tryptic action. A forerunner of trypsin — trypsinogen—is produced, and this changes into trypsin after it is secreted. The intestinal secretion contains TRYPSA, TRYPSINOGEN PANCREAS ROKINASE out | E= Fic. 161.—To show the mode of action of secretin and the action of the vagus nerve on the secretion of the pancreas and the activation of trypsinogen by enterokinase. something of the nature of an enzyme which has been termed enterokinase, which has the power of bringing about this change. It is doubtful whether the pancreatic secretion contains any true rennin, although it produces a modified clotting of milk, under certain conditions. 3. Physiology of Pancreatic Secretion.—The secretion of pancreatic juice is not constant, but is induced when the acid chyme passes into the duodenum. This occurs even when all the nerves to the intestine have been cut, and it appears from y ° * a aS FOOD AND DIGESTION 365 the investigations of Starling to be due to the formation of a material which has been called seeretin, in the epithelium lining the intestine, under the influence of an acid. This is absorbed, and, on being carried to the pancreas, stimulates it to secrete. It has been shown that the injection into the blood of an extract, made with dilute hydrochloric acid, of the lining membrane of the upper part of the sma]hintestine, leads to a flow of pancreatic juice. This secretin is not destroyed by boiling, and is soluble in strong alcohol. It is therefore not of the nature of an enzyme. But while secretin seems to play so important a réle, it has been found that stimulation of the vagus nerve, after a latent period of two minutes, increases pancreatic secretion, so that it must be concluded that the secretion of the fluid is, to a certain extent, under the control of the nervous system. The influence of the pancreas in the general metabolism will _ be considered later (p. 410). B. Seeretion of the Intestinal Wall (Sucecus Entericus) This is formed in the Lieberkiihn’s follicles of the intestine, and it may be procured by cutting the intestine across at two points, bringing each end of the intermediate piece to the surface, and connecting together the ends from which this piece has been taken away so as to make a continuous tube. On mechanically irritating the mucous membrane, a pale yellow clear fluid is secreted, which contains native proteins and mucin, and is alkaline in reaction from the presence of sodium carbonate. Aetion.—The succus entericus contains: (1) An enzyme which splits some disaccharids, as maltose and cane sugar, into monosaccharids, but does not seem to act on lactose. (2) In the intestine of animals taking milk a special zymin, lactase, which splits milk sugar. (3) Erepsin, an enzyme which seems to act more powerfully than trypsin in splitting peptones into their component non-protein crystalline constituents such as the di-amino acids and non-amino acids, eg. leucin and tyrosin. The object of this is not at present fully understood. It may be that the nitrogen of the protein is largely treated as a waste product and thrown off. But feeding experiments on dogs and rats seem to show that these non-protein derivatives of proteins 366 VETERINARY PHYSIOLOGY can be retained and built into the protoplasm of the animal. Vernon has shown that a similar proteolytic enzyme is widely distributed in the tissues, being specially abundant in the kidney. (4) Enterokinase—a zymin which, acting on trypsinogen, con- verts it into active trypsin (p. 364). Mechanism of Secretion.—The taking of food leads to a flow of intestinal secretion which reaches its maximum in about three hours ; and this flow is much greater from the upper part of the bowel than from the lower. There is some evidence that the injection of secretin calls forth this secretion, and according to some observers, the injection of succus entericus into the circulation acts in the same way. As regards the action of nerves very little is known. It has been found that, when the intestine is ligatured in three places so as to form two closed sacs, if the nerves to one of these be divided, it becomes filled with a clear fluid closely resembling lymph. The dilatation of the blood vessels may account for this without secretion being implicated. C. Bile 1. Charaeters and Composition.—The bile is the secretion of the liver, and it may be procured for examination—(a) From the gall bladder, or () from the bile passages by making a fistula into them. Bile which has been in the gall bladder is richer in solids than bile taken directly from the ducts, because water is absorbed by the walls of the bladder and the _ bile thus becomes concentrated. Analyses of gall bladder bile thus give no information as to the composition of the bile when formed. In several cases, where surgeons have produced biliary fistule, opportunities have occurred of procuring the bile directly from the ducts during life in man. Such bile has a somewhat orange-brown colour, and is more or less viscous, but not nearly so viscous as bile taken from the gall bladder. It has a specific gravity of almost 1005, while gall bladder bile has a specific gravity of about 1030. Its reaction is slightly alkaline, and it has a characteristic smell. It contains about 2 per cent. of solids, of which more than half are organic. a Se i i i i i FOOD AND DIGESTION 367 Bile Salts (Chemical Physiology).—The most abundant solids are the salts of the bile acids. In man the most -important is sodium glycocholate, but in the dog sodium taurocholate is the more abundant. These salts are readily prepared from an alcoholic solution of dried bile by the addition of water-free ether, which makes them separate out as crystals. Glyeocholic acid splits into glycin, amido-acetic acid— H,N.CH,.CO.OH, and a body of unknown constitution, cholalic acid, C,H, O;. Tanrockolic acid yields amido-ethane- sulphuric acid or taurin, H,N.CH,CO.SO,OH.—a molecule closely resembling amido- acetic saa inked to sulphuric acid and cholalic acid. Since both acids contain nitrogen they must be derived from proteins. That they are formed in the liver and not merely excreted by it, is shown by the fact that, while they accumulate in the blood if the bile duct is ligatured, they do not appear if the liver is excluded from the circulation. Action of Bile Sualts—1. The bile salts are solvents of fats and fatty acids, and they thus assist in the digestion and absorption of fats. When bile is excluded from the intestines no less than 30 per cent. of the fats of the food may escape absorption and appear in the feces. When this is the case, as in jaundice from obstruction of the bile duct, the feces have a characteristic white or grey appearance from the abundance of fat. 2. These salts keep cholesterin in solution. 3. They lower the surface tension of solutions, and in this way they may bring the fat particles into more intimate contact . with the mucous membrane. 4, While the salts have no action on proteins, free taurocholic acid precipitates native proteins and acid proteate. In the human intestine this is an action of no importance. 5. These salts are powerful hemolytic agents, and rapidly dissolve hemoglobin out of the erythrocytes. Bile Pigments.—These amount to only about 0:2 per cent. of the bile. In human bile the chief pigment is an orange-brown iron-free substance, bilirubin, C,.H,,N,O,, while in the bile of herbivora, biliverdin, a green pigment somewhat more oxidised than bilirubin, C,,H,,N,O., is more abundant. By further oxida- ‘tion with nitrous acid, other pigments—blue, red, and yellow—are 368 VETERINARY PHYSIOLOGY produced, and this is used as a test for the presence of bile pigments (Gmelin’s test). (Chemical Physiology.) The pigments are closely allied to hwmatoporphyrin and hematoidin (see p. 213), and they are derived from hemo- _globin, Their amount is greatly increased when hemoglobin is set free or injected into the blood. That they are formed in the liver is shown by the fact that, when the liver is © excluded from the circulation, the injection of hemoglobin does not cause their formation. The liver has the property of excreting not only these pigments formed by itself, but also other pigments. Thus — the liver of the dog can secrete the characteristic pigment of sheep’s bile when this is injected into its blood. Cholesterin is a monatomic alcohol — C,,H,,OH — which occurs free in small amounts in the bile. It is very insoluble and is kept in solution by the salts of the bile acids. It readily crystallises in rhombic plates, generally with a notch out of the corner. On account of its insolubility, when it is in excess in the bile or when the bile salts are decreased, it may form concretions or biliary caleuli—gall stones—which may accumulate in the gall bladder and may get caught in the bile passages, obstructing the flow of bile and leading to its absorption throughout the system. Jaundice is thus pro- duced. .When these stones are passed by the rectum, their ‘nature is readily demonstrated by breaking them up in a mortar, dissolving in hot alcohol, and allowing the solution to — cool, when the characteristic crystals separate out. (Chemical Physiology.) The source of the cholesterin of the bile is not — detinitely known. It is not an excretion of cholesierin formed elsewliere, kecause the injection of cholesterin does not lead to an increase in the amount in the bile. According to Naunyn’s observations it is most abundant in cases of inflammation of the bile passages, and he therefore thinks that it is formed by the breaking down of the epithelium lining these ducts. Fats and Lecithin.—The true fats and the phosphorus con- taining lecithin are present in smal] amounts in the bile, and apparently they are derived from the fats of the liver cells, and they may be increased in amount by the administration — of fatty food. Nucleo-protein and Mucin.—The bile owes its viscosity to TORONTO FOOD AND DIGESTION 369 the presence of a mucin-like body, which, however, does not yield sugar on boiling with an acid and which contains phos- phorus. It is precipitated by acetic acid, but the precipitate is soluble in excess. It is therefore a nucleo-protein. In some animals a certain amount of mucin is also present. (Chemical Physiology.) Inorganie Constituents.—The inost abundant salt is calcium phosphate. Phosphate of iron is present in traces. Sodium carbonate, calcium carbonate, and sodium chloride are the other chief salts. 2. Flow of Bile-——The bile, when secreted by the liver cells, may accumulate in the bile passages and gall bladder to be expelled under the influence of the contraction of the muscles of the ducts or of the pressure of the abdominal muscles upon the liver. The flow of bile into the intestine thus depends upon—1st, The secretion of bile; 2nd, the expulsion of bile from the bile passages. It is exceedingly difticult to separate the action of these two factors. The flow of bile in the human subject has now been studied in several cases in which the surgeon has had to make a fistula into the gall bladder through which all the bile secreted escaped and could be collected. The flow of bile begins in intra-uterine life before the twelfth week, and it continues without intermission through- out the whole of life, even during very prolonyed fasts. The taking of food increases the flow of bile, and the extent to which it is increased depends largely on the kind of food taken. In the dog a protein meal has the most marked eftect, a fatty meal a less marked effect, and a carbohydrate meal hardly any effect. The increased flow of bile following the taking of food does not reach its maximum till six or nine hours after the food is taken, and some observers have found that the period of maximum flow is even further prolonged. Pavlov found in dogs, in which a biliary fistula had been made leaving the opening of the bile duct in the mucous mem- brane of the intestine, that an expulsion of bile follows the taking of food and the secretion of pancreatic juice, and Starling finds that the flow of bile is increased by the in- jection of secretin. It thus tends to run parallel with the flow of pancreatic juice. 24 UnIIY 8 Ee Die be Sy LIKE hte Nhe aw KR. Le Yiu MS TSO Pe OTHOHO 370 VETERINARY PHYSIOLOGY When the individual is taking a liberal diet the secretion of bile appears to be greater than when the diet is low. . In fever there is a very marked fall in the secretion, the fluid flowing from a fistula becoming colourless and almost devoid of bile salts and pigments. Certain drugs markedly modify the formation of bile—the salts of the bile acids stimulating the liver to form more solids and to secrete more water, the salicylates acting in much the same way, and all drugs which cause hemolysis—i.e. the solution of the pigment of the erythrocytes—producing an increased formation of bile pigments. Influence of Nerves upon the Flow of Bile—(a) Expulsion of Bile.—There is good evidence that nerve fibres pass to the muscles of the bile passages and that they may cause an expulsion of bile by stimulating them to contract. (6) Secretion of Bile.—There is no evidence that nerve fibres act directly upon the secretion of bile. This appears to be governed by the nature of the material brought to the liver by the blood and by the activity of the liver cells. It is an example of function regulated by chemical substances rather than by a nerve mechanism. | 3. Mode of Formation of Bile——It has been seen that the bile salts and pigments are actually formed in the liver cells, and there is good evidence that the water of the bile is not a mere transudation but is the product of the living activity of these cells. The pressure under which bile is secreted may be determined by fixing a canula in the bile duct or in a biliary fistula and connecting it with a water manometer. In man the pressure is as much as 20 to 30 mm. Hg, while the pressure in the portal vein of the dog is only 7 to 16 mm. Hg. 4. Nature and Functions of Bile.—Bile is not a secretion of direct importance in digestion. It has practically no action on proteins or carbohydrates, and its action on fats is merely that of a solvent, and possibly by its action on the surface tension of the intestinal contents. Pavlov maintains that it activates the lipase of the pancreatic juice, and others hav found that it increases the activity of trypsin and diastase. It may thus be considered as an adjuvant to the action Ty - ey S| ro A é ‘ 5 FOOD AND DIGESTION 371 pancreatic juice. Its secretion in relationship to food does not indicate that it plays an active part in digestion. It is formed during intra-uterine life and during fasting, and it is produced many hours after food is taken, when digestive secretions are no longer of use in the alimentary canal. Diges- tion can go on quite well without the presence of bile in the intestine, except that the fats are not so well absorbed. The composition of bile strongly suggests that it is a waste product. The pigment is the result of the decomposition of hemoglobin and the acids are the result of protein disintegration. All these facts seem to indicate that bile is the medium by which the waste products of hepatic metabolism are eliminated, just as the waste products of the body generally are eliminated by the kidneys. Bacterial Action in the Alimentary Canal With the food and saliva numerous micro-organisms of very diverse character are swallowed. It has been suggested that the leucocytes formed in the lymphoid tissue at the back of the mouth and pharynx attack and destroy such organisms, but so far definite proof of this is not forthcoming. When the food is swallowed, the micro-organisms multiply for some time in the warm moist stomach, and certain of them, by split- ting sugars, form lactic and sometimes acetic acid. But when sufficient gastric juice is poured out for the hydrochloric acid to exist free, the growth of micro-organisms is inhibited, and some of them, at least, are killed. Others pass on into the intestine, and, as the acid in the chyme becomes neutralised, _ the acid-forming organisms begin to grow, and, by splitting the sugars, form lactic or acetic acid, and render the contents of the small intestine slightly acid. Towards the end of the small intestine, and more especially in the large intestine, the alkaline secretions have neutralised these acids, and in the alkaline material so produced the putrefactive organisms begin to nourish and to attack any protein which is not absorbed— splitting it up and forming among other substances a series of aromatic bodies, of which the chief are indol, skatol, and phenol. This splitting probably occurs through the liberation of 372 VETERINARY PHYSIOLOGY tryptophane—in which amido-propionic acid is linked to pyrrhol-benzene. 4». 3 OO E pee. bs a A tee sage 4 is ee Benzene. | Pyrrhol. : Amino-propionic Acid, C— CH,.CH.NH,,CO.0H By the breaking down of the amino-propionic acid, skatol— oN PO ot : | } |. x G—H Re NH is formed, and by the removal of the methyl, indol is produced Gir: N See, C—H y NH Phenol— TR Maret i a [ CoH ee 4 S77 is a further stage of disintegration. ; By taking embryo guinea-pigs at full time from the uterus and keeping them with aseptic precautions, it has been show: that the absence of micro-organisms from the intestine do not interfere with digestion. The most important and abundant organism present in th intestinal tract is the bacillus coli communis, which has | certain power of splitting proteins and a marked action in pre ducing acids from sugars. Its presence in water is generall indicative of sewage contamination. Se a FOOD AND DIGESTION 373 Fate of the Digestive Secretions 1, Water.—Although it is impossible to state accurately the average amount of the various digestive secretions poured into the alimentary canal each day, it must be very considerable, probably not far short of 3000 c.cms., or something consider- ably more than one-half of the whole volume of the blood. Only a small amount of this is given off in the feces, and hence the greater part must be re-absorbed. There is thus a constant circulation between the blood and the alimentary - canal, or what may be called an entero-hewmal circulation. One portion of this is particularly important. The blood vessels of the intestine pass to the liver, and many substances, when absorbed into the blood stream, are again excreted in the bile and thus are prevented from reaching the general circulation. Among these substances are the salts of the bile acids and their derivatives, many alkaloids such as curarine, and in all probability a set of animal alkaloids called ptomaines formed by putrefactive decomposition ~ of proteins in the gut. If, from disturbances in the functions of the liver, these are allowed to pass through that organ, the feelings of lassitude and discomfort which are associated with intestinal dyspepsia are produced. The liver thus forms a protective barrier to the ingress of certain poisons. 2. Enzymes.—Ptyalin appears to be destroyed in the stomach by the hydrochloric acid. Pepsin is probably partly destroyed in the intestine, but it seems also to be absorbed and excreted in the urine; for,on the addition of hydrochloric acid, the urine has a peptic action on proteins. Z'rypsin appears to be destroyed in the alimentary canal; but the fate of the other pancreatic enzymes and of the enzymes of the succus entericus is unknown. 3. Bile Constitwents—1. The bile salts are partly reabsorbed from special parts of the small intestine—sodium glycocholate being taken up in the jejunum and taurocholate in the ileum. The acids of these salts are also partly broken up. The glycocholic acid yields amido-acetic acid, which is absorbed and passes to the liver to be excreted as urea; while the taurocholic acid yields amido-isethionic acid which goes to the liver and yields urea and probably sulphuric acid. The 374 VETERINARY PHYSIOLOGY fate of the cholalic acid is not known, but it is supposed to — be excreted in the feces. 2. The pigments undergo a change and lose their power of giving Gmelin’s reaction, They appear in the feeces as what may be called stercobilin. It is probably formed by reduction of bilirubin in the intestines as the result of the action of micro-organisms. 3. The cholesterin is passed — out in the feces. Feeces The materials not absorbed from the intestine, whether these are derived from the food or from the alimentary canal, are thrown off from the rectum as the feces. In fasting animals” these are passed at long intervals, and consist of mucin, shed epithelium, the various products of the bile constituents, and inorganic salts. In feeding animals the amount and character ‘of the feces depends largely upon the amount and character of the food, and upon the bacteria which are growing in the large intestine. The unabsorbed material, as it passes down the large intestine, becomes inspissated from the absorption of water, but, if much undigested matter is present, water may also be added, and the consistence of the feeces may thus be varied. The colour is normally brown, from the hematin ot the flesh eaten, while the sulphide of iron formed by the splitting of the hematin compounds in the intestine may make them darker in colour. On a milk diet they are light yellow in colour, and if a large excess of fatty food is taken, or if fat is not absorbed, as in jaundice, they become clay coloured. The derivatives of the bile pence play but a small. part in colouring the feces. The reaction of the feeces varies. Usually the outeide of the mass is alkaline from the alkaline secretion of the intestine, while the inside is acid from the free fatty acids and other acids formed by the action of such acid-forming bacteria as the bacillus coli communis. The amount of solid feeces depends on the amount of food, and on a vegetable diet, from the presence | of undigested cellulose, the amount is very much greater. The ‘solids of the feces of a feeding animal consist of the same constituents as the feces in a fasting animal, with the addition of all the undigested constituents of the food— elastic and white fibrous tissue, remains of muscle fibres, often —— — ee a FOOD AND DIGESTION 375 fat and the earthy soaps of the fatty acids, and, when a vegetable diet is taken, the cellulose of the vegetable cells, and frequently starch. The cellulose, by stimulating the intestine, is a valuable natural purgative. The odour is due to the presence of aromatic bodies such as indol and skatol. Meconium is the name given to the first feeces passed by the child after birth. It is greenish-black in colour, and consists of inspissated bile and shed epithelium from the intestine. Movements of the Intestine These are of two kinds—myogenie and peristaltic. The myogenic movements are slight rhythmic contractions which pass rapidly along the intestine, and are insufficient to drive on the contents, but are probably of use in churning and mixing them. By feeding with food mixed with bismuth, and employing X-rays, Cannon finds that the contents of the small intestine get broken up into small segments. This is possibly due to these myogenic movements (fig. 157, p. 356). These movements occur when all the nerves have been divided, and when the ganglia in the intestinal walls have been poisoned with nicotine, and they are therefore due to the muscle fibres alone. The peristaltic movements are much more complex and powerful. They consist of a constriction of the muscles, which seems to be excited by the passage of the food, and may be caused by inserting a bolus of cotton-wool covered with vaseline. Starting at the upper end of the intestine, they pass slowly downwards. In front of the contraction the muscular fibres are relaxed, and thus the contracting part drives its contents into the relaxed part below. These peristaltic movements go on after the nerves to the gut are cut, but they are stopped when the ganglia in the wall of the intestine are poisoned with nicotine. It has therefore been concluded that the nerve ganglia in the intestinal wall form a local reflex mechanism, which is stimulated by the presence of foreign matter in the intestine, and which brings about the co-ordinated contraction and relaxation, which together constitute a true peristalsis. 376 VETERINARY PHYSIOLOGY But while peristalsis is thus independent of the central nervous system, it is nevertheless controlled by it. The © splanchnic nerves inhibit, while the vagus to the small intes- tine and upper part of the large gut, and the nervi erigentes to the lower part of the large gut are augmentor nerves, increasing the peristalsis. Stimulation of the sympathetic fibres which inhibit the peristalsis causes contraction of the : sphincter between the small and large intestine. As the contents of the small intestine are forced through the ileo-ceecal valve, the large intestine relaxes to receive them, — and then a series of contractions passing from below upwards —an anti-peristalsis—sets in by which the contents are very thoroughly churned. Afterwards they are forced downwards — by tonic peristaltic waves. Defseeation By the peristalsis of the intestine, the matter not absorbed — from the wall of the gut is forced down and accumulates in the part of the rectum which passes horizontally forward to end in the vertical anal canal. It is prevented from escaping into this by the sharp fold which the last part of the bowel makes, and by the contraction of the strong sphincter ani muscle. Defecation depends primarily on the intestinal peristalsis, without which it cannot be performed. When feces accumu- — late in the rectum, the mucous membrane is stimulated, and impulses are sent up to inhibit a centre in the lumbar region ~ of the cord which keeps the sphincter ani contracted, and the — sphincter is relaxed, and the escape of feces made possible. In some diseases of the cord this centre is stimulated and cannot be inhibited, and thus defecation is interfered _ with, while in other diseases, when this centre has been destroyed, the sphincter does not contract, and feces may — escape continuously. Normally the act of defecation 3 is partly voluntary and partly ~ involuntary. The voluntary part of the act consists in closing the glottis, and making a forced expiration so as to press upon — the contents of the abdomen; while at the same time the perineal muscles.are relaxed, and the rectum straightened, and thus the contents are allowed to pass into the anal canal. The j i FOOD AND DIGESTION 377 act is completed by the emptying of the canal by the con- _ traction of the levatores ani muscles. B. Intestinal Digestion in the Horse and other Herbivora In the horse intestinal digestion is of great importance. In the small intestine the processes which go on are much the same as in the carnivora, but in the huge large intestine important changes occur. 1. In the exeum some of the food remains for a very considerable time—as much as twenty-four hours. It is mixed with the secretion of the intestine and with water which the horse has drunk, and which passes rapidly to the cecum. The reactions of the contents are alkaline, and bacteria are abundant. Here a considerable quantity of protein is digested - and cellulose disappears. This disappearance of cellulose is probably largely due to the action of bacteria, and it results in the production of :— (1) Lower fatty acids, such as acetic and butyric acid, which combine with the alkalies present, and, being absorbed, are oxi- dised to carbonates, yielding possibly a small amount of energy, and being partly excreted by the lungs as CO, and partly by the urine as carbonates of sodium and potassium, which help to give the fluid its alkaline reaction. It is said that 100 parts of cellulose in decomposing will yield about 60 parts of these acids. (2) Gases, of which carbon dioxide, CO,, and marsh gas, CH,, are the chief. 2. In the double colon, which has a mean capacity of no less than 80 to 100 litres, the same processes go on as in the cecum, and when the single colon is reached, a very rapid absorption of fluid and of the products of digestion held in solution takes place, and the residue of the food is formed into the desiccated balls of feces. So rapid is absorption from the lower bowel in the horse, that the animal is readily killed by rectal injection of strychnine, and may be easily anesthetised by giving ether per rectum. The rapid absorption also allows of life being maintained on nutritive enemata, The time taken for food to pass right along the alimentary canal of the horse is considerable—probably about four days. But solid bodies given by the mouth have been found in the 378 VETERINARY PHYSIOLOGY feces after twenty-two to thirty hours. Some idea of the distribution of the contents of the alimentary canal is afforded by an observation of Colin’s :— In the Stomach . : . : 5 kilos. , Small Intestine . i Y 5 , Cecum ; TY Le : . “Lia Colon : . Soa » Some interesting observations ‘have been made upon the removal of a great part of the large intestine in the rabbit. It has been found that the digestion and utilisation of proteins is not decreased, but that the digestion of the cellulose is markedly decreased. The feces of the horse are passed in rounded yellow masses. They contain about 76 per cent. of water, 3’ per cent. of ash, largely composed of silica from the husks of grain and partly of phosphates of soda, potash, lime and magnesia, and about 21 — per cent. of organic matter, such as cellulose, lignin, and other undigested vegetable remains, unabsorbed proteins, carbo- hydrates, and fats, indol and skatol, and the various constituents — of the feeces of a fasting animal. They are acid, from the presence of organic acids, and they — contain so much gas that they float in water. The amount of feces depends upon the food taken—on an average about 15 kilos are passed per diem. The question of how far cellulose is available as a food, of — how far it acts like other carbohydrates, has not been satis- factorily demonstrated. But the most recent experiments upon rabbits and upon sheep seem to show that pure cellulose may replace starch as a protein sharer. When, however, it is eaten — mixed with the encrusting substances of the cell wall, the work of digestion seems to be so great that little of its potential energy is available. C. Intestinal Digestion of Ruminants Intestinal digestion is not so important in ruminants as it is in horses, for digestion in the stomach is much more complete. It is essentially the same in character, and cellulose — is more completely dissolved in these animals than it is in — the horse. Se aa! eS ee ABSORPTION OF FOOD 379 The feeces in the ox and cow are more fluid than in the horse, while in the sheep they contain a smaller proportion of water. In the ox the average weight of the feces per diem. is about 30 kilos. Food takes about five days to pass completely through the alimentary canal of ruminants. III. ABSORPTION OF FOOD 1. State in whieh Food leaves the Alimentary Canal.—The carbohydrates generally leave the alimentary canal as mono- saccharides ; but some resist the action of digestion more than others. Lactose seems to be broken down in the intestine only when the special lactase is present in the succus entericus, but in all cases it is broken down before it reaches the liver. Cane sugar when taken in large excess may also be absorbed, and it is then excreted by the kidneys. The proteins are absorbed as peptones, possibly as proteoses, and as the amino-acids and other crystalline compounds formed by the action of trypsin and erepsin (p. 8). Native proteins may be absorbed unchanged from the lower bowel, since it has been found that when egg white is injected into an isolated part of the rectum it disappears to a very con- siderable extent. The fats are chiefly absorbed as soaps and as fatty acids. 2. Mode of Absorption of Food.—That absorption is not due merely to a process of ordinary diffusion or osmosis is clearly indicated by many facts. (1) Heidenhain has shown that absorption of water from the intestine takes place much more rapidly than diffusion through a dead membrane. (2) The relative rate of absorption of different substances does not follow the laws of diffusion. Griibler’s peptone passes more easily through the intestine than glucose, but glucose passes more readily through parchment paper, while sodium sulphate, which is more diffusible than glucose, is absorbed much less readily. Again, as shown by Reid, an animal can absorb its own serum under conditions in which filtration into 380 VETERINARY PHYSIOLOGY blood capillaries or lacteals is excluded. In such a case osmosis cannot play a part. Absorption is stopped or diminished when the epithelium is removed, injured, or poisoned with fluoride - of sodium, in spite of the fact that this must increase the facilities for osmosis and filtration. 3. Channels of Absorption—There are two channels of absorption from the alimentary canal (see fig. 107, p. 225)— the veins which run together to form the portal vein of the ~ liver, and the lymphatics which run in the mesentery and, after passing through some lymph glands, enter the receptaculum chylit in front of the vertebral column. From this, the great lymph vessel, the thoracic duct, leads up to the junction of the subclavian and innominate veins, and pours its contents into the blood stream. The lymph formed in the liver also passes into the thoracic duct. (1) Proteins.—Peptones and the further products of their digestion are formed from proteins in digestion, but they seem to undergo a change in the intestinal wall before passing to the. tissues, since they are not found in the blood. That in some altered condition they leave the intestine by the blood and not by the lymph is shown by the fact that their absorption is not interfered with by ligature of the thoracic duct. ‘ During the digestion of proteins the number of leucocytes — is enormously increased, sometimes to more than double their previous number, and in all probability it is they which carry the products of digestion from the intestine. According to the observations of Pohl, the leucocytes are derived from the lymph tissue in the intestinal wall, but more recent experiments tend to show that they come from the bone marrow, being probably attracted to the intestine by a positive chemiotaxis. By breaking down in the blood stream they probably set free the proteins for use in the tissues. When an excess of proteins is taken in the food, it is broken down in the lining membrane of the gut, and the nitrogen is rapidly excreted in the urine as urea, and thus the entrance of an excess of nitrogen to the tissues is prevented. Its non-nitrogenous part remains available as a source of energy. It has been pointed out that gastric juice does not dissolve the nucleo-proteins, but that the pancreatic juice does so. ee Pye arene: FATE OF THE FOOD ABSORBED 381 ' Phosphorus is undoubtedly absorbed in organic combination, but the mode of absorption and the channels by which it passes from the intestine have not been investigated. (2) Carbohydrates. — Although the chief monosaccharid formed in digestion is dextrose, others are also produced— levulose from cane sugar and galactose from milk sugar. All these are absorbed in solution, and are carried away in the blood of the portal vein. (3) Fats.—After being split up into the component acids and glycerin, fats pass, as soluble soaps or as fatty acids soluble in the bile, through the borders of the intestinal epithelium. Here they appear to be again converted into fats by a synthesis of the acid with glycerin. Fine fatty particles are found to make their appearance in the cells at some distance from the free margin and to increase in size. A similar synthesis occurs even when free fatty acids are given, so the cells must be capable of producing the necessary glycerin to combine with the acids. The fats are sent on from the cells, through the lymph tissue of the villi, into the central lymph vessels, and thus on, through the thoracic duct, to the blood stream. Unlike the pro- teins and carbohydrates, they are not carried directly to the liver. IV. FATE OF THE FOOD ABSORBED The food absorbed may be— (A) used immediately as a source of energy, for (1) the Con- struction or Reconstruction of Tissues; (2) the Production of Mechanical Work; (3) the Production of Heat ; Or (B) it may be stored for future use in the body. The processes of construction and repair of the tissues and the production of mechanical work have already been considered (p. 13 et seg.), and the production of heat and the regulation of temperature may now be dealt with. I; PRODUCTION OF HEAT AND REGULATION OF -TEMPERATURE 1. Production of Heat A. Musele.—The production of heat in muscle has been already studied (p. 61). It has been shown that muscle, from 382 VETERINARY PHYSIOLOGY its great bulk and constant activity, is the main source of heat in the body. Not only may it be demonstrated that the temperature of contracting muscle rises, but it has been found that the temperature of blood coming from the muscles is slightly higher than that of blood going to them. Muscular exercise raises the temperature of the body. The shivering fit which is induced by exposure to cold is really a reflex reaction by which heat production is increased. Drugs which interfere with muscular contraction, such as curare, diminish the temperature, and young animals, before their muscular tissues become active, have a low temperature unless kept in a warm atmosphere. B. Glands.—Wherever chemical change goes on in proto- plasm, heat is liberated. Therefore in glands during activity a certain amount of heat is produced. But the production in them is trivial when compared with the production in muscle. During the period of active’digestion the temperature of the blood coming from the liver may be nearly 1° C. higher than — that of the blood going to the organ. The liver alone among glandular structures contributes an appreciable amount of heat to the body, since the amount of blood passing through the organ is large, and thus a considerable amount of heat is derived from it. : C. Brain.—Some physiologists have maintained that the fact that the temperature of the brain rises during cerebral activity indicates that the chemical changes going on are sufficient to yield a certain amount of heat. But it is more probable that the rise of temperature is due to the increased flow of blood through the organ, since a study of the gases in the blood coming from the brain gives no indication of any marked increase of chemical change during periods of increased cerebral action. 7 2. Regulation of Temperature Since heat is constantly being produced, the temperature of the body would tend to rise higher and higher, were there not some arrangement by which just as much heat is eliminated as is produced, and by which the temperature is thus kept constant. ee FATE OF THE FOOD ABSORBED , 383 Elimination of Heat.—Heat is got rid of by three channels. A. Skin.—Since the body is generally warmer than the surrounding air, heat is constantly lost by conduction, con- vection, and radiation, and the extent of this loss depends mainly upon the difference between the temperature of the body and that of the air. Radiation plays the most im- portant part when a person is sitting quiet in still air; conduction and convection when the exchange of air over the surface is rapid. The temperature of the skin is increased when, from dilatation of the cutaneous vessels, more blood is brought to the surface and conversely it is lowered by con- striction of these -vessels. The influence of variations in the temperature of the air is generally minimised in man by the covering of clothes, and in animals by the covering of fur or feathers, which retains a stationary layer of air at about 25° to 30° C. over the skin. It has been calculated that in man over 70 per cent. of all the heat is lost by conduction and radiation. . The loss of heat by radiation may be determined by finding to what extent a thin metal grill fixed at a definite distance from the surface of the. body is heated. This can be done by determining the change produced in its electric conductivity. By the evaporation of sweat, heat is rendered latent, and is taken from the body, which is thus cooled just as the hand may be cooled by allowing ether to evaporate upon it. If the amount of sweat vaporised is known, it is possible to calculate the amount of heat removed from the body. in this way. The loss is comparatively small—in man only about 14 per cent. of the whole. The extent depends upon the rapidity with which evaporation goes on, and this is governed by the amount of sweat secreted, and by the dryness and temperature of the atmosphere. Thus a warm dry climate is better borne than one which is warm and moist, since in the former the loss of heat by evaporation is so much greater. Of the various factors increasing sweat secretion, heat is probably the most important. In the lower animals the loss of heat by evaporation is much increased if the skin is wet, and the temperature of the horse may fall distinctly if the coat remains moist. 384 VETERINARY PHYSIOLOGY Since the temperature of the skin is governed by the state — of the cutaneous vessels and the amount of sweat produced by the state of the sweat glands, and since both of these are under the control of the nervous system, the elimination of heat from the skin is presided over by a nervous mechanism. B. Respiratory Passages.—By conduction and radiation and by evaporation from the respiratory passages, about 10 per cent. of the heat is got rid of in man. In the dog and some other animals, the proportion of heat eliminated in this way is considerably greater. C. Urine and Feees.—Since these are warmer than the surrounding air, a certain amount of heat is lost through them. The amount is small—something less than 2 per cent. of the whole. Temperature.—In all higher animals, the loss of heat and — the production of heat are so nicely balanced that the tem- perature of the body remains fairly constant under all con- ditions. If an extra amount of heat is produced, say in muscular exercise, it is at once eliminated by the skin, and, if the body is exposed to a low temperature, loss of heat is rapidly checked by contraction of the eutaneous vessels and diminished activity of the sweat glands. Since heat is constantly being given off, the temperature at the surface of the body is always lower than the temperature in the interior. The temperature of the rectum may be taken as a ineasure of the internal temperature. The mean daily temperature of the horse is 37° to 38° C. Under all normal conditions the temperature undergoes only small variations, because the balance between production of heat and elimination of heat is so well maintained. But under abnormal conditions the balance is frequently upset. Thus severe muscular work causes a temporary rise of tem- perature, because heat elimination does not quite keep pace with heat production. Exposure to very high temperatures may cause a slight rise of temperature, while exposure to excessive cold may cause a slight fall; but, unless in the case of those unable to use their muscles, the change is small. While the higher “warm-blooded animals,” mammals and birds, maintain a constant temperature, the lower vertebrates, FATE OF THE FOOD ABSORBED 385 “cold-blooded animals,” reptiles, amphibia and fishes, do not do so, and their temperature varies with that of the surrounding medium. But even in mammals the mechanism for the regulation of temperature is not absolutely perfect, and in every species of animal there is a limit to the power of adjustment. Mammals which hibernate become for the time “ cold-blooded animals,” and lose their power of regulating their temperature. The regulation of temperature may be effected either by modifying heat production, or by altering the rate of elimination. Heat production is voluntarily modified when muscular exercise is taken during exposure to cold, and involuntarily when muscles are set in action by a shivering fit. There is, however, no evidence of the existence of a special nervous mechanism presiding over heat production in muscle. But it is not so much by changes in the rate of heat pro- duction, as by alteration in heat elimination through the skin, that the temperature is kept uniform. The nerves to the cutaneous vessels, and to the sweat glands, are the great con- trollers of temperature. It is through failure of this mechanism ‘under the action of the toxins of micro-organisms that heat elimination is diminished, and the temperature is raised in fevers. It is not*necessary to assume that there is a special heat regulating nervous mechanism, since the nervous arrangements presiding over the vessels and glands of the skin are capable of immediately responding to change of condition calling for their intervention. ; II. STORAGE OF SURPLUS FOOD A. Since bulk for bulk fat has more than twice the energy value of proteins or carbohydrates, it is an advantage to store surplus food as Fat. This storage takes place chiefly in three situations: (1) Fatty tissue; (2) liver; (3) muscle. | 1. In Fatty Tissues—In most mammals the chief storage of surplus food is in the fatty tissues. That the fat of the food can be stored in them is shown by the fact that the administration of large amounts of fats 25 36 VETERINARY PHYSIOLOGY different from those of the body leads to their appearance in those tissues. Fats are also formed from the earbohydrates of the food. Feeding experiments upon pigs and other animals, carried out in this country by Laws and Gilbert, have definitely proved that sugary foods are changed to fat in the body and stored in that form. The following may be given as an example of such experiments. Two young pigs of a litter were taken, and one was killed and analysed. The other was fed for weeks on maize, the amount eaten being weighed and the excretion of nitrogen by the pig being determined. The animal was then killed and analysed, and it was found that the fat gained was more than could be produced from the fat and protein of the food eaten. — It must therefore have been formed from the carbohydrates. The evidence that fats may be formed from the proteims of the food is conflicting. In the ripening of cheese it is undoubted that under the influence of micro-organism proteins — are changed to fats, and in all probability the same thing occurs in the formation of the fatty adipocere in the muscles of the dead body during putrefaction. At one time it was — supposed that under the influence of such poisons as phosphorus the proteins of the cells of the mammalian tissues are changed to fat. But careful chemical examination has shown that the so-called fatty degeneration is due to accumulation of already existing fats in the affected organs. Voit fed dogs on lean beef, and found that, while all the nitrogen was discharged from the body, the carbon was retained, and he concluded that it was retained as fat. But he failed to recognise that — even lean flesh contains both fat and glycogen from which ~ the fat can be formed. At present we have no direct evidence that the fats of the body are formed from proteins. 2. In the Liver.—The liver is a storehouse of carbohydrates and fats (p. 387). Lecithin is always present in the liver, even in prolonged fasting. . 3. In Musele.—Some animals, as the salmon, store fats within their muscle fibres; but in mammals such a storage is” limited in amount. | B. Proteins may, to a small extent, be stored in muscle, especially after a fast or a prolonged illness. But in the healthy mammal it is difficult to get such a storage, except in athletic FATE OF THE FOOD ABSORBED 387 training, where the muscles may be enormously increased by the building up of the protein-derivatives of the food into their protoplasm. C. Carbohydrates are stored to a small extent in the liver and in the muscle. . Ill. THE LIVER IN RELATIONSHIP TO ABSORBED FOOD AND TO THE GENERAL METABOLISM The liver develops as a couple of diverticula from the embryonic gut, and is thus primarily a digestive gland, and in invertebrates it remains as a part of the intestine both structurally and functionally. But in mammals, early in foetal life, it comes to have important relationships with the blood going to nourish the body from the placenta (see p. 440). The vein bringing the blood from the mother breaks up into a series of capillaries in the young liver, and in these capillaries the development of the cells of the blood goes on for a considerable time. Soon the liver begins to secrete bile, while animal starch and fat begin to accumulate in its cells. Gradually the forma- tion of blood cells stops, and the mass of liver cells become larger in proportion to the capillaries. As the foetal intestine develops, the vein bringing blood from it—the portal vein—opens into the capillary network of the liver, so that, when at birth the supply of nourishment from the placenta is stopped, the liver is still associated with the blood bringing nutrient material to the tissues. 1. Relation to Carbohydrates—Glycogenie Function —Claude Bernard discovered that sugar is formed in the liver. This formation of sugar goes on throughout life, and on account of this constant supply the amount of sugar in the blood does not diminish, even when an animal undergoes a prolonged fast. In starvation there are only two possible sources of this glucose—the fats and the proteins of the tissues. There is no conclusive evidence that fats can be changed to sugar in the liver, although it is difficult to explain the large amount of sugar which is sometimes excreted in phloridzin poisoning, unless it is formed from fats. That it is not all formed from proteins is shown by the fact that the sugar which appears is sometimes greater than could be produced by the proteins broken down, as indicated by the output of nitrogen. 388 VETERINARY PHYSIOLOGY That proteins are a source of sugary substances is shown by the amount of sugar which is produced by an animal rendered ~ diabetic by removal of its pancreas and fed exclusively on proteins. It is therefore probable that in starvation the proteins of the body are broken down and their non-nitrogenous part changed to sugar. But not only does the liver manufacture sugar for the tissues in starvation, but, when the supply of sugar is in excess of the demands of the tissues, it stores it as a form of starch— glyeogen—and gives it out as sugar as that substance is re- quired. Ona carbohydrate diet the accumulation of glycogen in the liver is very great; but even on a protein diet, in dogs at least, a smaller accumulation takes place. The observation that the various monosaccharids are all stored as the same form of glycogen shows that they must first be assimilated by the liver protoplasm and then converted to glycogen, the process being one of synthesis. The way in which glycogen is again changed to sugar is doubtful. The fact that the liver, after treatment with alcohol, can change glycogen to glucose, has induced some physiologists to believe that it is by an enzyme that this conversion goes on during life. But it has been shown (1) — that the injection of methylene blue, which poisons protoplasm _ but does not interfere with the action of enzymes, checks the conversion, and (2) that stimulating the splanchnic nerves going to the liver increases the conversion of glycogen without increasing the amylolytic enzyme in the liver and blood. It is therefore probable that the conversion results from chemical changes in the protoplasm which are controlled by the nerves — of the liver. If more sugar is taken than the liver can deal with, it passes on into the general circulation, and is excreted in the urine. _ Every individual has a certain power of oxidising and storing — sugar, and most persons can dispose of about 200 grms. at a time. But the carbohydrate capacity varies greatly, and even — in the same individual it is different under different conditions. When the glycogen stored in the liver is changed to glucose — more quickly than is required by the tissues, the glucose may — be to a certain éxtent again stored in the muscles as glycogen, or it may accumulate in the blood and be excreted in the FATE OF THE FOOD ABSORBED _ 389 urine (glycosuria).. This condition is seen when the posterior part of the floor of the fourth ventricle in a rabbit is punctured. If glycogen be abundant in the liver, glycosuria results, the stimulation of the nervous system producing a too rapid conversion of the glycogen. Another way in which sugar may be made to appear in the urine is by injecting phloridzin. Under the influence of this drug the sugar in the blood is not increased. It must be concluded that it acts by causing the kidneys to excrete glucose too rapidly, so that it is not available for the tissues. But even when carbohydrates are withheld and cleared out of ‘the body, phloridzin causes glycosuria. Hence the kidneys must be made to form glucose from the protein of the blood plasma. The injection of large doses of extract of the suprarenal bodies causes a glycosuria with an increase of sugar in the blood ; but so far it is not known whether the condition is- one of increased production or of diminished utilisation of sugar (p. 406). Removal of the pancreas also causes glycemia and glycosuria (p. 410). 2. Relation to Fats.—Although the fats are not carried directly to the liver, as are proteins and carbohydrates, they are storéd in large amounts in the liver of some animals—e.g. the cod among fishes and the cat among mammals. Animals which have little power of storing fat throughout the muscles and other tissues generally, seem to have a marked capacity for accumulating it in the liver. Even in starvation the fats do not disappear from the liver, and throughout all conditions of life a fairly constant amount of lecithin—a phosphorus and nitrogen containing fat (see p. 76)—is present in the liver cells. Lecithin, in the yolk of the egg, is an intermediate stage in the formation of the more complex nucleins of living cells, and the formation of lecithin in the liver by the synthesis of glycerin, fatty acids, phosphoric acid, and cholin is probably a first step in the construction of these nucleins. If this be so, the fat of the liver must play an important part in retaining and fixing phosphorus in the body. 3. Relation to Proteins.—Along with the intestinal wall, the liver regulates the supply of proteins to the body. A 390 VETERINARY PHYSIOLOGY study of the chemical changes in muscle has shown that the waste of protein is normally small in amount, and that a great part of the nitrogen is capable of being used again if a supply of oxygen and carbonaceous material is forthcoming (see p. 69). Hence the demand for nitrogen in the muscles is small, and for this reason, apparently, any excess of protein.in the food is decomposed, either by trypsin and erepsin or by the intestinal wall, into simple nitrogenous compounds, which are changed into urea in the liver. Urea, the chief waste substance excreted in the urine, is the bi-amide of carbonic acid. ; H | Bi N—C_NC It contains 46°6 per cent. of nitrogen. It is a white substance crystallising in long prisms. It is very soluble in water and alcohol—insoluble in ether. With nitric and oxalic acids it forms insoluble crystalline salts. It is readily decom- posed into nitrogen, carbon dioxide and water by nitrous acid and by sodium hypobromite in excess of soda. (Chemical Physiology.) Urea is chiefly formed in the Liver.—That it is not produced in the kidneys is shown by the following facts:—(1) When these organs are excised, urea accumulates in the blood. (2) When ammonium carbonate is added to blood artificially circulated through the kidney of an animal just killed, no urea is formed. That it is not formed in the muscles is shown—(1) By the absence of a definite increase in urea formation during muscular activity ; (2) by the fact that when blood containing ammonium carbonate is streamed through muscles, urea is not produced. That it is formed in the liver is indicated—(1) By the fact that when an ammonium salt such as the carbonate dissolved in blood is streamed through the organ, it is changed to urea; (2) by the observation that, when the liver is cut out of the circulation, the urea in the urine rapidly diminishes, and ammonia and lactic acid take its place. The exclusion of the liver from the circulation in mammals FATE OF THE FOOD ABSORBED 391 is difficult, because, when the portal vein is ligatured, the blood returning to the heart tends to accumulate in the great veins of the abdomen. But this difficulty has been overcome by | Eck, who devised a method of connecting the portal vein with the inferior vena cava, and finally occluding the portal vein, and of thus allowing the blood to return from the abdomen to the heart. Source of .Urea.—Urea is produced from the decomposition products of proteins of the food and tissues (see p. 216). The manner in which excess of protein in the food is broken down into ammonia compounds in the intestine and sent to the liver has been already considered (p. 380). But the fact that even in starvation urea is produced seems to indicate that the initial stages of decomposition of proteins may go on elsewhere than in the intestinal wall. The fate of hemoglobin tends to show that the whole process may be conducted in the liver cells. When hemoglobin is set free from the corpuscles, the nitrogen of its protein part is changed to urea, while the pigment part is deprived of its iron and excreted as bilirubin. Whether the proteins of muscle and other tissues are thus directly dealt with, or whether the initial stages of decomposition go on outside the liver, is not known. But the wide distribution of erepsin through the tissues may indicate that the initial splitting of the protein goes on in them. The nitrogen excreted is not all in the form of urea, but some is combined in ammonia salts, in uric acid and other purin bodies (see p. 427), and in creatinin. In the mammalian body ammonia and the purin bodies can be changed into urea, and it is probable that the small amounts of these substances which appear in the urine have simply escaped this conversion. Certain drugs (alcohol, sulphonal, etc.) and toxins (diphtheria) markedly decrease their conversion into urea and so increase their quantity in the urine. Although urea may be prepared from creatin, there is no evidence that the process goes on in the body. Creatin yields the creatinin of the urine. It is probable that after the nitrogenous portion of the protein molecule is split off and got rid of, the liver has the further power of turning the non-nitrogenous part into sugar and either sending it to the tissues or storing it as glycogen. Summary of the Functions of Liver.—-The functions of the 392 VETERINARY PHYSIOLOGY liver may be briefly summarised as follows :—(1) It regulates the supply of glucose to the body (a) by manufacturing it from proteins when the supply of carbohydrates is insufficient, and (2) by storing it as glycogen when the supply of carbo- hydrates is in excess, and giving it off afterwards as required, (2) Along with the intestinal wall it regulates the supply of — proteins to the body, by decomposing any excess, and giving off the nitrogen as urea, ete. (3) It regulates, in many animals — at least, the supply of fat to the body by storing any excess. — (4) It regulates the number of erythrocytes by getting rid — of waste hemoglobin and retaining the iron for further use (see p. 213). (5) From the part it plays in the entero-hepatie — circulation, it protects the body against certain poisons by — excreting them in the bile. V. GENERAL METABOLISM Having considered how the food is digested and absorbed, and how it is then either stored or at once used (a) for building up and repairing the tissues, or (6) as a source of — energy, the rate at which the various chemical changes go on and the factors modifying them may be dealt with. The changes in the two great constituents of the body— — proteins and fats—have to be separately studied. d 1. Method of Investigating A. Protein Metabolism.—The amount of protein used in — the body is readily calculated from the amount of nitrogen excreted, since, under normal conditions, unless nitrogen in some unusual combination is being taken, it is derived entirely from the proteins in the body. Proteins contain 16 per cent. — of nitrogen, and hence each grm. of nitrogen excreted is derived — from 6°25 grms. of protein. The nitrogen is almost entirely excreted in the urine. Only a small amount escapes by the bowels and skin, and hence only — when very accurate observations are desired is it necessary to — _ analyse the feeces and sweat. B. Metabolism of Fats.—Proteins contain nearly three and a_ GENERAL METABOLISM 393 half times as much carbon as nitrogen, and hence, when they are broken down, for each grm. of nitrogen excreted, 3:4 grms. of carbon are given off. The carbon is chiefly excreted from the lungs as carbon dioxide, and in this form it may be collected and estimated. Any excess of carbon excreted, over the 3°4 times the amount of nitrogen given off, must be derived from the fats of the body or from the fats and carbohydrates taken in the food. Any carbon retained in the body, apart from that in proteins, is stored ultimately as fat. Since carbon constitutes 76°5 per cent. of fats, the amount of fat is calculated by multiplying the carbon by 1°3. The following tabular example of an investigation of the metabolism may be given :— Intake in Grams. Output. G: N. C. N. Proteins : ; : 100 54 16 sp er Fic, 168.—Diagram of the Structure of the Kidney. M.P., Malpighian pyramid of the medulla ; J. R., medullary ray extending into cortex ; L., labyrinth of cortex ; Mf.B., a Malpighian body consisting of the glomerular tuft and Bowman's capsule ; P.C.7., a proximal convoluted tubule ; H.L., Henle’s loop on the tubule; D.C.7'., distal convoluted tubule; C.7., collecting tubule ; #..4., branch of renal artery, giving off JZ. A., interlobular artery, to supply the glomeruli and the convoluted tubules ; 7Z. V., interlobular artery bringing blood back from the cortex. (2) Extending away from this is a proximal convoluted tubule (P.C.7.) lined by pyramidal and granular epithelial cells. This dives into the medulla, becomes constricted and lined by a transparent flattened epithelium, and is known as the descending limb of the looped tubule of Henle. Turning suddenly upwards 424 VETERINARY PHYSIOLOGY and becoming lined by a cubical granular epithelium, it forms the ascending limb, and reaching the cortex expands into the distal convoluted tubule, which exactly resembles the proximal (D.C.T.). Tt opens into a collecting tubule (C.T.) lined by a low transparent epithelium, which conducts the urine to the pelvis of the kidney. The renal artery breaks up and gives off a series of straight branches—the interlobular arteries (JZ.4A.)—which, as they run towards the surface, give off short side branches which terminate in the glomeruli, The efferent vein passing from these breaks up again into a series of capillaries between the convoluted tubules, and these pour their blood into the inter- lobular veins (JZ.V.). This arrangement helps to maintain a high pressure in the capillary loops of the glomerular tuft. Physiology of Secretion The marked difference in the structure and vascular supply of the Malpighian bodies and of the different parts of the. renal tubules make it probable that they perform different functions. | | Malpighian Bodies.__(a) It has been shown by injecting into the blood vessels acid fuchsin, which is colourless in alkaline solution and red in acid solution, that the urine formed in these bodies is alkaline in reaction and that it becomes acid as it passes down the convoluted tubules. (6) It is also known that these bodies are thrown out of action by decreasing the flow of blood through the kidneys. The rate of flow of blood depends upon the blood pressure in the renal arteries and the dilatation of the renal arterioles. It may be measured by measuring the amount of blood passing out by the renal vein or by enclosing the kidney in an oncometer—a closed vessel connected with a recording tambour—so that changes in the volume of the kidney are recorded (see p. 249), or by a combination of these methods. Section of the renal nerves derived from the 11th, 12th, and 13th dorsal nerves causes a dilatation of the renal arterioles, an expansion of the kidney, and an increased flow of urine. Stimulation of these nerves has the opposite effect. A fall in the general arterial pressure to about 50 mm. Hg in the EXCRETION OF MATTER FROM THE BODY 425 dog generally causes a decreased flow of blood through the kidney and practically stops the flow of urine, although the tubules, as will be presently shown, still act. (c) In the frog the renal arteries supply the Malpighian bodies, while portal veins from the posterior end of the animal supply the convoluted tubules. Ligature of the renal arteries stops the flow of urine, but the flow may be again induced when urea is injected. (2) Even when this flow is induced, if dextrose or egg albumin or peptone are injected into the blood, substances which in the normal frog appear in the urine, they are not excreted. These observations seem to show that the Malpighian bodies have to do chiefly with the excretion of water and of certain abnormal constituents of the blood, and that their activity depends upon the rate of blood-flow through them. That it is not due to a mere physical filtration under pressure is indicated by experiments, in which the blood pressure in the kidneys of a dog was raised by injecting large quantities of blood from another dog, without the flow of urine being increased. That it is a-selective action of the epithelium seems to be proved by the passage into the urine of such large molecules as those of egg albumin and hemoglobin and of various pigments such as carmine. The fact that, if a cannula connected with a manometer be placed in the ureter, the secretion of urine stops when a pressure of about 50 mm. Hg is reached, is not opposed to the view that the formation of urine from the glomeruli is an active secretion. In the salivary gland, where the formation of saliva is undoubtedly due to the vital action of the cells, the same result follows obstruction of the duct. The point of practical importance is that the secretion of water takes place chiefly through the Malpighian bodies, and that this is reduced or stopped by a fall in the general arterial pressure, such as occurs in heart failure. The decreased excretion of water may lead to the development of dropsy. The Tubules.—The alkaline urine formed in the Malpighian bodies undergoes changes as it passes along the tubules. It becomes acid and the various solids are increased, for urine 676; * VETERINARY PHYSIOLOGY contains a higher proportion of these than ‘does the blood, e.g. the blood contains only about 0°03 per cent. of urea, but the urine usually contains 2 per cent, The addition of solids in the tubules is further proved by the following facts :-— (a) Uric acid crystals are frequently found in the cells of the convoluted tubules of the kidney of birds. (0) Heidenhain, — by injecting a blue pigment—sulph-indigotate of soda—into — the circulation of the rabbit, demonstrated that the cells of — the convoluted tubules take it up and pass it into the urine. In the normal rabbit the whole of the kidney and the urine become blue. But, if the formation of urine in the Malpighian bodies be stopped by cutting the spinal cord in the neck so as to lower the blood pressure, then the blue pigment is found in the cells of the convoluted tubules and of the ascending limb— of Henle’s tubule. (c) When the Malpighian bodies of the frog have been thrown out of action by ligaturing the renal arteries, the injection of urea still causes a flow of urine and the excretion of urea by the tubules. When the portal veins which supply the tubules were ligatured on one side, it was found that less urine was formed on the ligatured than on the unligatured side. These last experiments show that the cells of the convoluted tubules are capable of secreting water as well as solids, and that they can do so is further demonstrated by the fact that, if by cutting the spinal cord in the neck the formation of urine in the Malpighian bodies of a dog is stopped, the administration of caffeine and of some other substances causes an increased flow of urine, although the blood pressure in the kidneys is not raised. This is taken advantage of in cases of heart disease, when the secretion of urine is almost arrested from low arterial pressure, and when dropsy is rapidly advancing. Until the heart is toned up, the kidneys may be stimulated to get rid of water by means of such diuretics as caffeine. It is quite possible that under certain conditions the cells of the renal tubules absorb water and possibly the other con- stituents of the urine, but the weight of the evidence we at present possess is against the idea that they exercise the function to any great extent. EXCRETION OF MATTER FROM THE BODY 427 EXCRETION OF URINE 1. Passage from Kidney to Bladder.—The pressure under which the urine is secreted is sufficient to drive it along the ureters: to the bladder. It these are constricted the pressure behind the constriction rises, and may distend the ureters and the pelvis of the kidney, but when it reaches about 50 mm. Hg in the dog, the secretion of urine is stopped. The muscular walls of the ureters show a rhythmic peristaltic contraction, which must also help the onward passage of the urine to the bladder. 2, Micturition.—As the urine accumulates in the urinary bladder the rhythmic contraction of the non-striped muscle becomes more and more powerful. These contractions are chiefly excited by the fibres of the nervi erigentes of the second and third sacral nerves, although fibres passing down from the inferior mesenteric ganglion also probably act either in exciting or inhibiting them in certain animals. The backward passage of the urine into the ureters is prevented by the oblique manner in which these tubes pass through the muscular coat of the bladder. _ The passage of urine into the urethra is at first prevented either by the oblique manner in which the urethra leaves the bladder, or more probably by the contraction of a strong band of non-striped muscle, the sphincter trigonalis. This muscle or the striped fibres which surround the membranous part of the urethra are under the control of a centre in the lumbar enlargement of the spinal cord, and the expulsion of urine must be preceded by their relaxation. In some cases of inflammation of the spinal cord the increased activity of the centre may prevent the expulsion of urine, while later in the disease, when the nerve structures have been destroyed, the urine is not retained and dribbles away on account of the absence of the tonic contraction of the muscles. The expulsion of the last drops of urine is carried out by the rhythmic contraction of the bulbo-cavernous muscle, while the peristaltic contraction of the bladder wall is assisted by the various muscles which press upon the contents of the abdomen and of the bladder. 428 VETERINARY PHYSIOLOGY In man, in early life, micturition is a purely reflex act, and — in the dog it is perfectly performed when the spinal cord is cut in the back. As age advances the reflex mechanism comes to be more under the control of the higher centres, and the activity of the sphincters may be increased or abolished as circumstances indicate. 38. EXCRETION BY THE SKIN The skin is really a group of organs, and some of these have been already studied. (Zhe structure of the skin and its appendages must be studied practically.) (1) The Protective functions of the horny layer of epidermis, with its development in hair and nail, and of the layer of — subcutaneous fat, are manifest. Hair.—Attached to each hair follicle is a band of non-striped — muscle, the arrector pili, which can erect the hair by con- tracting. These muscles are under the control of the central nervous system, and the nerve fibres have been demonstrated in the cat to take much the same course as the vaso-constrictor fibres of somatic nerves (see p. 280). A hair after a time ceases to grow, and the lower part in the follicle is absorbed and the hair is readily detached. From the cells in the upper part of the follicle a new down-growth occurs, a papilla forms, and the hair is regenerated. In many of the lower animals this process occurs twice a year. (2) The Sensory functions have been studied under the Special Senses. (3) The Respiratory action of the skin in mammals is of — little importance. (4) The Exeretory Funetion of the Skin—Three sets of glands develop in the skin—sweat glands and sebaceous glands, which are common to both sexes and are constantly active— and mammary glands, which are active in the female during the period of suckling. A. Sweat Secretion—1l. Strueture.—The sweat glands are simple tubular glands coiled up in the subcutaneous tissue oii EXCRETION OF MATTER FROM THE BODY 429 with ducts opening on the surface of the skin. The secreting epithelium somewhat resembles that of the convoluted tubules of the kidney. It has been calculated that a man possesses about two and a half million sweat glands, and that if spread out they would present a surface of very great extent. 2. Funetions.—From these glands a considerable amount of sweat is poured out; but to form any estimate of the daily amount is no easy matter, since it varies so greatly under different conditions. Probably about 1000 ¢.cm. is an average amount. When poured out, sweat usually evaporates, and is then called insensible perspiration, but when large quantities are formed, or when, from coldness of the surface, or of the air, or from the large quantity of watery vapour already in the air, evaporation is prevented, it accumulates, and is called sensible perspiration. A free secretion of sweat is usually accompanied by a dilatation of the blood vessels of the skin, but this may be absent, and it may occur without any sweat secretion—eg. under the influence of atropine. 3. Nervous Mechanism of Sweat Secretion—The sweat glands are under the control of the central nervous system. This may be very conveniently studied in the cat, in which animal the sweat glands are chiefly in the pads of the feet. If a cat be put in a hot chamber it sweats on the pads of all its feet. But if one sciatic nerve be cut the foot supplied remains dry. If the cat be placed in a warm place and the lower end of the cut sciatic stimulated, a secretion of sweat is produced. These sweat-secreting fibres all pass through the sympathetic ganglia, and back into the spinal nerves. Those to the hind leg come from the upper lumbar region, those for the forelimb from the lower cervical nerves, and those for the head in the sympathetic of the neck and partly in the fifth cranial nerve. The centres presiding over these nerves are distributed down the medulla and cord. They are capable of (a) reflex stimula- tion, as when pepper is taken into the mouth ; and (6) of direct stimulation by a venous condition of the blood, as in the impaired oxygenation of the blood which so frequently precedes death as the respirations fail. But even after the nerves to the sweat glands are cut, the 430 VETERINARY PHYSIOLOGY glands can be stimulated by certain drugs—eg. pilocarpine. The action of heat seems also to be chiefly peripheral, setting — up an unstable condition of the gland cells so that they respond more readily to stimulation. 4. Chemistry of Sweat.—Sweat from the horse is a sherry- coloured fluid, which, when pure, has a neutral or faintly alkaline reaction. Its specific gravity is about 1020 in the horse, and it contains about 5°5 per cent. of solids, of which 5 per cent, are inorganic and about 0°5 organic. Potassium — is the most abundant base. Chlorides are present in small amounts. The chief organic substances present are proteins —some globulin and some albumin. Fat is also present, probably derived from the sebaceous secretions, and it com- bines with the potassium to form a soap, | B. Sebaceous Seeretion.—The sebaceous glands are simple racemose glands which open into the hair follicles, and their function is to supply an oily material to lubricate the hairs. This secretion is produced by the shedding and breaking down of the cells formed in the follicles of the glands. Those lining the basement membrane are in a condition of active division, but the cells thrown off into the lumen of the follicle disintegrate and become converted into a semi-solid oily mass, which consists of free fatty acids and of neutral fats of glycerine and of cholesterin, These cholesterin fats are the lanolins, which differ from ordinary fats in being partly soluble in water. Free cholesterin is also present in the sebum. . C. Milk Seeretion—1. Physiology.—Before pregnancy occurs the mammary glands are largely composed of fibrous tissue, with a large amount of fat, in which run the branching tubules of the glands as small solid blocks of cells. } As pregnancy advances these tubules grow outwards and increase, and the cells begin to divide, some remaining attached to the basement membrane, some coming to lie in the middle of the tubules. These latter undergo a fatty change and break down, and they are shed in the first milk which is secreted, the eolostrum. The cells left upon the basement membrane elaborate the constituents of milk, and the presence of fat — globules in their protoplasm is very manifest. The milk, after being secreted, collects in the ducts of the EXCRETION OF MATTER FROM THE BODY 431 glands and in the sinus below the nipple, and is expelled from these by the contraction of the muscular fibres in their walls, and by the suction of the young animal. The ezeretion of milk from the ducts is directly under the control of the nervous system, but the evidence as to the way in which the central nervous system influences the secretion of milk is by no means satisfactory. Clinical experience shows that it is profoundly modified by nervous changes, but so far stimulation of the nerves to the glands has not yielded definite results. The chemistry of milk is considered upon p. 332, and the way in which fats and proteins are formed is dealt with on p. 385. The production of the disaccharid, lactose, and of the phospho-protein, caseinogen, implies a synthetic process in the -protoplasm of the cells of the mammary glands. The comparative composition of the milk of the cow, mare, and sheep is given below :— Cow. Mare. Sheep. ] Water : ; oa | 88 91 to 93 87 Protein , : : | 3 1 to2 4 to 5 Fats . : . ot 3 to 4 1 to 1°5 4 to 5 Sugar... ; a 4 to5 4to6 3 to4 Salts . Z ‘ | 07 0°3 0°6 PART III SECTION IX REPRODUCTION So far the animal has been studied simply as an individual. But it has also to be regarded as part of a species, as an entity which has not only to lead its own life, but to transmit that life to offspring. The various problems of reproduction have been already studied by the student in connection with biology, and it is here sufficient to indicate some of the main points in the physiology of the process in mammals. (The structure of the organs of reproduction must be studied practically.) While the individual is actively growing, the reproductive organs are quiescent; but when puberty is reached, they begin — to perform their functions—the testes to produce spermatozoa, the ovaries to produce mature ova. The removal of the sexual organs in the young animal leads to arrest in the development of the special sexual characters, — especially in the male, in which these characters are generally best marked. Simple ligature of the vasa deferentia has not this effect, although the tubules of the testes do not develop, and it therefore seems that the interstitial cells of the organs produce a hormone which causes the development of the sex characteristics. ‘ The genital gland in both sexes is formed from a longitudinal thickening or ridge at the posterior part of the cclom or peritoneal cavity. Over this ridge the endothelium thickens and becomes epithelial-like in structure. Groups of cells grow down into the tissue below. In the ovary one of these cells in a group takes a central position and forms the ovum, while the other cells get arranged ~ 432 REPRODUCTION 433 around it to form the zona granulosa, the whole group con- stituting a Graafian follicle. In the testis the groups of cells form seminiferous tubules, in which the spermatozoa or male elements are developed. Ovary.—In the adult the ovaries are oval structures covered by a columnar germinal epithelium. In the stroma are seen Graafian follicles in different stages of development. The central cell, the ovum, enlarges. The nucleus becomes promi- nent, and the nucleolus is also large. The protoplasm becomes . encased in a transparent capsule—the zona pellucida. The cells of the zona granulosa multiply, and fluid, the liquor folliculi, appears among them, dividing them into a set attached to the capsule of the follicle and a set surrounding the ovum. When the follicle is ripe it projects on the surface of the ovary, and finally bursts, setting free the ovum into the peritoneal cavity. The ovum escapes into the peritoneal cavity and passes into the trumpet-shaped fimbriated upper end of the Fallopian tube, through which it reaches the uterus. Testis—In the adult this is enclosed in a dense fibrous capsule—the tunica albuginea. Posteriorly this is thickened, and forms the corpus Highmori. From this processes extend - and form a supporting framework. In the spaces are situated the seminiferous tubules, which open into irregular spaces in the corpus Highmori—the rete testis, from which the efferent ducts, vasa efferentia, pass away and join together to form the vas deferens, In the seminiferous tubules the spermatozoa are produced. Some of the lining cells divide into two, each forming a support- ing cell and a spermatogen. The latter divides and subdivides till a group of cells lie on the top of the supporting cell. These are the spermatoblasts. In each spermatoblast the nucleus elongates and passes to the attached extremity, the protoplasm decreases in amount, and a long cilium develops from the free end, and the spermatozoon is thus produced. Semen.—When the testes have become active, the glands of the prostate increase and produce a fluid which, with the spermatozoa, forms the semen. The Cstrous Cycle.—In carnivora and herbivora alike, the adult female has cycles of sexual activity, the cestrous cycles, which are accompanied by changes in the genital cers The 2 434 VETERINARY PHYSIOLOGY eycle may be divided into three periods: (1) The anestrous stage, during which the genital organs are at rest, the uterus and Fallopian tubes small, and the formation of Graafian follicles in the ovary quiescent; (2) the prowstrous stage, during which there i3 a rapid ripening of one or more Graafian follicles and a congestion and swelling of the Fallopian tubes and uterus, with sometimes hemorrhages into the uterine mucous mem- brane ; (3) the estrous stage, during which coition is performed, and during which the Graafian follicle ruptures and sheds the ovum (ovulation). This in some animals—e.g. ferrets—occurs as the result of coition ; in others—horse, cow, etec.—indepen- — dently of it. Sometimes the ovum is not shed, and then it and the rest of the Graafian follicle atrophy. Impregnation is effected by the transmission of spermatozoa into the genital tract of the female. For this purpose erection of the penis is brought about retlexly through a centre in the lumbar enlargement of the cord, the outgoing nerves being the nervi erigentes, which dilate the arterioles, and the internal pudics supplying the transversus perinei and bulbo-cavernous muscles by which the veins of the penis are constricted. The semen is ejected by a rhythmic contraction of the bulbo-cavernous and other perineal muscles, an action which is also presided over by a centre in the lumbar region of the cord (p. 160). The spermatozoon meets the ovum in the Fallopian tube or upper part of the uterus. DEVELOPMENT 1. Early Stage It is unnecessary here to describe the changes in the ovum before or immediately after its conjugation with the spermato- zoon, since they are so fully dealt with in all works on biology. The mammalian ovum is holoblastic, that is, undergoes com- plete segmentation, and forms a mulberry-like mass of cells (fig. 163, A.). The cells then get disposed in two sets, a layer of small surrounding cells and a set of large central cells — (fig. 169, B.). The former constitute the Ectoderm and take — part in forming the processes or primitive villi by which the ovum becomes attached to the maternal mucous membrane. REPRODUCTION 435 The latter spread out at one pole to form the blastoderm (fig. 170, A) and dispose themselves in three layers—the epiblast, Fic. 169.—Ovum after segmentation, showing the formation of the Ectoderm (a) and Endoderm (4). From the cells of the latter the Blastoderm is formed. (ELLENBERGER. ) mesoblast, and hypoblast (fig. 170, B and C). From these layers the various parts of the body are derived as follows:-— I. Epiblast.— Nervous system, epidermis and appendages. Epithelium of the mouth, nose, naso-pharynx, and all cavities and glands opening into them, and the enamel of teeth. II. Hypoblast.—Epithelia (a) of the alimentary canal from the back of the mouth to the anus and of all its glands; (4) of the A . Fig. 170.—To show 4, the spreading out of the Endoderm cells to form the Blastoderm; B, the formation of Epiblast and Hypoblast ; and C, of Mesoblast. In Band C the ectoderm is not seen. (ELLENBERGER.) Eustachian tube and tympanum; (c) of the trachea and lungs; (d) of the thyroid and thymus; and (e) of the urinary bladder and urethra. III, Mesoblast.—All other structures. By the formation of a vertical groove down the back of the blastoderm, a tube of epiblast cells (the neural. canal) is enclosed, from which the nervous system develops by the = y ; ‘ 436 VETERINARY PHYSIOLOGY conversion of some of the cells into neurons, and others into neuroglia cells (fig. 171). The mesoblast on each side of this splits, and the outer part, with the epiblast, goes to form the body wall (Somatopleur), rome), Nt NAL NN i) AN (OLIN RK iY N Fic. 171.-—Transverse section of more Fic. 172. — Longitudinal Section advanced Blastoderm, to show through Embryo to show it sinking Epiblast, Mesoblast, and Hypo- down into ovum and the formation of blast, formation of Neural Groove the amnion, am. In the Mesoblast and splitting of the Mesoblast. round, al., the allantois, the blood . vessels grow out to form the placenta, while the inner part with the hypoblast gets tucked in to pro- duce the alimentary canal (Splanchnopleur) (fig. 171). The developing embryo sinks into the ovum, and, as a ~ result of this, the somatopleur folds over it and, uniting All. Fic. 173.—Schematic section through the pregnant uterus of the Mare to show the large allantoic sac, 4J/., filled with fuid surrounding the amniotic sae ; Am., the fiuid in which the fcetus floats. above, encloses it in a sac—the amniotie sae (fig. 172, am.), which becomes distended with fluid—the amniotie fluid, in which the embryo floats during the later stages of its develop- ment, and which: acts as a most efficient protection against external violence. ——————— a REPRODUCTION 437 In the pig, horse and in ruminants, the connection of the foetal blood vessels with the maternal structures is not very intimate, and when the young are born the fetal part of the placenta separates from the maternal part, which is thus not shed. Hence such animals are called non-deciduata. FO ayy 7. x . “ — » “SS fe . ~ / " Abt f , SRE tem era / Zz, h , J, “a C A Fie, 174.—Schematie section of one cornu of the uterus of a ruminant at an early stage of gestation to show the elongated umbilical vesicle, A, and allantois, B, and the embryo in the amniotic sac, C. In Rodents, Insectivora, Apes, Man and Carnivora, the associa- tion is so intimate that at birth the maternal part of the placenta is shed along with the foetal. Hence these are called Deciduata. 2. Attachment to the Mother The ovum gets enclosed in the uterine mucous membrane, which grows round it as the decidua reflexa (fig. 175, D.R.). Almost as soon as the ovum is embedded in the maternal mucous membrane, it becomes surrounded by a nucleated mass of protoplasm—the trophoblast, formed of the cells of the ectoderm, and this probably transfers nourishment from the mother to the ovum. Very early the mesoblast of the embryo extends out in a number of finger-like processes into the trophoblast layer, and soon afterwards blood vessels shoot into these, and the ehorionie villi are formed (fig. 176). The precise origin of the first blood vessels in these is not known, but ultimately they are derived from the allantoic arteries which pass out from near the posterior end of the hind gut. As the villi grow, the blood vessels in the maternal mucosa (fig. 175, D.S.) dilate, and the capillaries form large sinuses or blood spaces. Into these the chorionic villi pass, and thus the loops of foetal vessels hang free in the maternal blood, and an exchange of material is possible between the mother and foetus. This development goes on specially at one 438 VETERINARY PHYSIOLOGY or more parts of the surface of the ovum, and the Placenta is thus formed. This acts as the foetal lung, giving the embryo the necessary oxygen and getting rid of the waste carbon dioxide. It is the foetal alimentary canal supplying the necessary material for growth and development; and it is the foetal kidney through which the waste nitrogenous constituents are thrown off. In the mesoblast, through which the allantoic arteries pass out, Fie, 175.—Longitudinal Section through the human uterus and ovum at the fifth week of pregnancy. D.S., decidua serotina, which will become the placenta; D.R., decidua reflexa; D.V., the uterine mucous membrane called the decidua vera. a vesicle filled with fluid, and at first communicating with the posterior gut, is developed (figs. 172, al/., 173 and 174). This is the allantois. In man it never attains any size, but in many of the lower animals it spreads all round and encloses the amnion. 3. Foetal Circulation The performance of these functions by the placenta is associated with a course of circulation of the blood somewhat different to that in the post-natal state (fig. 177). The blood coming from the placenta to the foetus is collected into a single umbilical vein, w.v., which passes to the liver, /. This divides into the duetus venosus, d.v., passing straight through the organ, and into a series of capillaries among the cells. From r —— tl REPRODUCTION 439 these the blood flows away in the hepatic vein to the inferior vena cava, p.v.c., which carries it to the right auricle. In this it is directed by a fold of endocardium, through the foramen ovale, 7-0.,a hole in the septum between the auricles, and it thus passes to the left auricle, and thence to the left ventricle, J.v., which drives it into the aorta, a.a., and chiefly up to the head, ant.a. From the head the blood returns to the superior vena cava, a.v.c., and, passing through the right auricle, enters the right ventricle, 7.v., which drives it into the pulmonary artery, p.a. Before birth this artery opens into the aorta by the ductus arteriosus, d.a., while the branches to the lungs are still very small and unexpanded. In the aorta, this impure blood from the head mixes with the purer blood:from the left ventricle, and the mixture is sent to the lower part of the body yg. 176.—Longitudinal Section through the descending aorta, through the tip of a villus of po.a, From each iliac artery, 7.a,, the human placenta, covered by Dr ant art its trophoblast layer, and contain- Pe STUOLY, ¥.d., Passcs ing a loop of blood vessels, and off, and these two vessels carry projecting into a large blood the blood in the umbilical cord, sinus, J.V.S. in the maternal u.c., to the placenta. eacateei When the fcetus is born in the deciduata, the placenta is pressed upon by the contraction of the uterus, and the flow of blood between child and mother is arrested. As a result of this the respiratory centre is no longer supplied with pure blood and is stimulated to action. The lungs expand and the blood flows through them. In the non-deciduata the same result is brought about by the separation of the foetal from the maternal placenta. In the ductus venosus a clot forms and the vessel becomes obliterated. The ductus arteriosus also closes up, and the foramen ovale is occluded. The circulation now takes the normal course in post-natal life. Our knowledge of the differences between the physiological processes in intra-uterine and in extra-uterine life is still very imperfect, and the subject cannot be further discussed here. 440 VETERINARY PHYSIOLOGY 4, Gestation and Delivery The child remains in the uterus for nine months, and at the end of that period it is expelled during labour. Labour may be Fic. 177.—Scheme of Circulation in the Feetus. w.v., umbilical vein; d.v., ductus venosus ; p.v.c., inferior vena cava pouring blood through the right auricle and through the foramen ovale, f.0., into the left heart ; a.v.c., superior vena cava bringing blood from the head to pass through the right side of the heart, and through the ductus arteriosus, d.a. ; pt.v., portal vein, The degree of impurity of the blood is indicated by the depth of shading. divided into three stages. In the first stage the uterus passes into contractions at intervals, and the lower part or cervix is dilated. In the second stage the contractions become stronger, and with the help of the contractions of the abdominal muscles, the young are expelled through the vagina either enclosed in REPRODUCTION 441 the membranes or in the deciduata free of the membranes. In the deciduata, the uterus is usually quiescent for a short time, and then contractions supervene, and the placenta and lining of the uterus are expelled as the after-birth. These uterine contractions are presided over by a nerve-centre in the lumbar enlargement of the cord, and in all probability the nervi erigentes play an important part in their production. — —— AP PN DT Xx SOME ELEMENTARY FACTS OF ORGANIC CHEMISTRY Tue following elementary facts may help the student who has neglected the study of the outlines of Organic Chemistry in understanding the chemical problems of physiology. Organic compounds are built round the four-handed carbon atom | a; ae | When each hand links to the one-handed hydrogen atom, MrtTHANE— H eon is formed. | H By taking away a hydrogen atom from two Methane molecules and linking these molecules together ErHane— Fo eed H—C—C—H is produced, ae H H By further linking more and more of these molecules together, similar molecules containing three, four, five or more carbon atoms are produced, When each carbon has its due proportion of hydrogen atoms it is saturated, but if two hydrogen atoms are let go, the unoccupied hands of the carbon may join and form an unsaturated molecule, thus :— Ethane becomes Ethylene r H H-b—t_# When one hydrogen atom is taken away and the molecule has a hand ready to link with some other substance a radical is constituted, and these are known as Merayt,Eruyt, etc. 443 444 : APPENDIX Alcohols.— When the two-handed oxygen atom,—O—linked to the vacant hand of the radical, an alcohol is formed, e.g.— H (H) H—C—C—O—(H) __ Ethy! Alcohol. Hu When the terminal carbon is thus oxidised a Primary Alcohol is — formed—but if a middle carbon atom is oxidised, a Secondary Alcohol — is produced— H OH H Hb bw Secondary Propyl Alcohol. . hud But the oxidation may involve more than one carbon atom and tna the Polyvalent Alcohols are produced— OH hy OH | H—0—b-¢—H Glycerin. Lamy : H ‘H - Aldehydes or Aldoses.—When, from a Primary Alcohol the two hydrogens marked above in brackets are removed, the vacant hand of the oxygen links to the vacant hand of the carbon to form an Aldehyde or Aldose— ‘ H | H—C—C=0 Ethyl Aldehyde. Lot H (H) Ketones or Ketoses.—These are formed in the same way from oe Secondary Alcohols, thus :— HOH Acetone, the Ketose of Secondary ee I = Propyl Alcohol. Acids. 8 the hydrogen of the Aldehyde marked in brackets is rosa ' by hydroxyl—OH, an acid is produced— J H: 0 [i] H6-60-H Acetic acid. H: The carboxyl group to the right of the dotted line is characteristic of thet acids, N =e Ss - ~~ APPENDIX | 445 The oxidation may be carried on at each end of the line and the divalent acids are thus produced Oo: 0 | ll H—O—C—C—0—H. Oxalic acid. If in the radical of one of these acids a hydrogen is replaced by hydroxyl —OH, an oxy-acid is formed, thus :— H—C—C—C—O—H __ Propionic acid. may be converted to the two Lactic acids called (a) and (8) oxy-propionic acid, according to the carbon which is oxidised and Similar oxy-acids are formed from the divalent acids. Cycitic CoMPoUNDS An important series of carbon compounds contain a ring of six carbons, each with an unsatisfied affinity, thus :— When each hand holds a hydrogen, Benzene is formed. a These hydrogens may be replaced by various molecules giving rise toa large series of different compounds. 446 APPENDIX If the ring contain less than six arbuin atoms it is called heterocyclic. 3 One of the most important of these is Pyrrhol— Cc—C ll c OC YY N | H which occurs linked to a benzene ring in certain important constituents of : the protein molecule. “A NITROGEN-CONTAINING COMPOUNDS Ammonia.—The three-handed Nitrogen by linking with three hydro- gens forms Ammonia, H af a H—N—H “a If one of these hydrogens is removed, Amidogen, which can link with other molecules, is produced. a Amino Acids.—If one of the hydrogen atoms in the radical of an a aci d is replaced by amidogen a mon-amino acid is formed, thus :— { H O H Leal 2 . ‘ N—C—C—O—H Amino acetic acid. aX. i When two hydrogen atoms are thus replaced, a di-amino acid is produced— ; NH, oa , nt —¢ ot --O—H _ Di-amino propionic acid. i i Amides.—If the amidogen molecule takes the place of the hydroxyl, in the carboxyl of an acid an amide results, thus :— ‘a H—C—O—H Formic acid. | _-,H : H—C_NC Formamide. H we APPENDIX 447 From the divalent carbonic acid— O \| 1050-05 is formed— O H Tl H N—C_N H H the important substance urea, Urea molecules may link together — (a) By dropping hydrogens, when Biuret is produced. O° 2 HO H TE eee Ee H aro aee | (b) By holding on to an intermediate radical of an acid, e.g. an unsatu- rated three carbon acid. These are Diureides, of which the most important is Uric Acid— ee Pelli H—N-—-C : Pek O=C : C--N-H ; ‘ ne [i ll? ye=0 H_N--C-LNZH INDEX ABDUCENS nerve, 136, 172 Abomasum, 340 Absorption of carbohydrates, 381 channels of, 380 of fats, 381 of food, 379 _ of proteins, 380 from the stomach, 357 Accessorius, 171 Accommodation, positive, 124 et seq. range of, 124 Acetone, 444 Achroo-dextrin, 345 Acid proteate, 351 sulphur in the urine, 418 Acids, divalent, 445 in gastric juice (see Gastric juice) organic, 445 Acinus, 22 Acrylic acid, 416, 447 Action, reflex, 90 et. seq. latent period of, 90 voluntary, 98 Adipose tissue, 28 Adrenalin, 406 £rotonometer, 316 AKthalium septicum, 15 Air (see Complemental, etc. ) Air vesicles in the lungs, 295 Albumin, 10 Albumose, 11 Alcohols, chemistry of, 444 polyvalent, 444 primary, 444 secondary, 444 Aldehydes, 444 Alimentary canal, 337 et seq. bacterial action in, 371 development of, 436 nerve supply of, 343 physiology of, 344 structure, 337 Alizarin blue, use in muscle experi- ments, 319 Allantoic arteries, 438 Allantoin, 417 Allantois, 438 Alloxantin, 417 Amble, 101 Amides, 9, 446 Amido-benzene, 8 Amido-ethane-sulphuric acid (tauro- cholic acid), 367 Amido-isethionie acid, 373 Amino-propionic acid, 420 Aunitotic division, 19 Ammonia, 446 Ammonio-magnesium phosphate, 416 Ammonium salts in urine, 416 Amniotic fluid, 436 sac, 436 Ameeboid movement, 208 Amount of respired air, 302 Ampulla, 142, 148 Amy] nitrite and influence on blood ressure, 276 Amylolytic period of gastric diges- tion, 350 Amylopsin, 362 Anabolic phenomena, 5 Anacrotic pulse, 268 Analysis of alveolar air, 318 Anelectrotonus, 46 Annulus of Vieussens, 256 Anterior corpora quadrigemina, 169, 179 pyramids of the medulla, 170 Antitoxin, 411 Anus, 341 Apex beat, 238 Apocodeine, 152, 279, 405 Appendix, vermiform, 341 Aqueous humour, 120 Arc, cerebellar, 89 cerebral, 88 spinal, 87 Areolar tissue, 27 Arteries, 258 pressure in, 259, 272, 276 et . ; Arterioles, methods of studying their condition, 276 normal state of, 278 walls (muscular) of, 279 Arytenoid cartilages, 323, 348 Asphyxia, 322 Assimilation, 398 29 449 450 Association, mechanism cerebral, 198 Astigmatism, 128 Atropin, 405 effect on heart, 253 effect on salivary secretion, 346 effect on sweat secretion, 429 Auditory centre, 196 nerve, 144 et seq. cochlear root of, 144 vestibular root of, 144 Auerbach’s plexus, 343 Auricle, 227 musculature of, 227 pressure in, 240 Auriculo-ventricular valves, 231 Availability of food-stuffs, 398 Axon, 75 BAcILuLus coli communis, 372 Bacterial action in the alimentary canal, 371 Barcroft and Haldane’s method for estimating gases in the blood, 214 Barfoed’s solution, 330 Barium salts, influence on blood pres- sure, 277 Barnard Hill’s sphygmometer, 275 Barotaxis, 15 Bases of the urine, 421 Basilar membrane, 142 Basis bundles, 166 Basophil leucocytes (see Leucocytes) Benzene compounds, 445 Benzoates in the circulation, 418 Bi-amide of carbonic acid (see Urea) Bidder’s ganglion, 252 Bile, 366 et seq. ‘action of salts of, 367 as a hemolytic agent, 367 crystalline, 367 duct, 341 effect of drugs on, 370 flow of, 369 influence of nerves on flow of, 370 mode of formation of, 370 nature of, 370 pigments, 367 pigments, fate of, 374 pressure under which secreted, 370 Biliary calculi, 368 colic (see Colic) Bilirubin, 213, 367, 391 Biliverdin, 367 Binocular vision, 134 Bladder, urinary, 427 Blastoderm, 435 Blind spot, 128 Blood, 202 e¢ seq. coagulation of, 203 INDEX Blood, constituents, fate of, 219 constituents, source of, 216 defibrinated, 203 E distribution of, 219 flow, 287 flow in arteries, 289 flow in capillaries, 289 flow in veins, 289 gases, 214 plasma, 202 platelets, 202, 209 serum, 203 specific gravity, 202 sugar, 330 total amount in body, 219 Blood corpuscles (see Leucocytes, etc. ), 207 et seq. Blood-pressure, 259 et seq. geveral distribution, 259 in kidney, 424 mean, 272 method of measuring, 272 respiratory changes in, 271, 311 tracing, 272 variations in, 261 Blood vessels, circulation i in, 258 coats of, 258 Bone, 32 et 869. chemistry of, 36 development, intra-cartilaginous, 33 development, intra-membranous, marrow, 217 - structure of, 32 et seq. Bowman’s capsule, 423 glands, 116 Brain (see Cerebrum, etc.) Breath sounds, 304 Brodie, on extract of suprarenals, 405 Bronchial sound, 304 Brown-Séquard, on administration of testicular substance, 408 Brown-Séquard, removal of the supra- renals, 404 Brunner’s glands, 341 CAISSON disease, 319 Calamus scriptorius, 282 Calcification of cartilage, 35 Calculi, urinary, 417 Calories, 328 r) definition of, 69 Calorimeter, 328 Canal, Haversian, 34 Canaliculi, 32 Canals, semicircular, 142 a physiology of, 147 e Cancellous tissue of bone, 33 Cane sugar, 331 INDEX _ Canter, 102 Capillaries, 225, 258 pulse in, 270 pressure in, 260, 276, 284 et seg. Capronin, 333 Capsule, internal, 181 Carpylin, 333 Carbohydrates, action of gastric juice on, 352 as diet, 396 as they leave the alimentary canal, 379 definition of, 330 energy value of, 332 requirement, of 401 tests for, 330 et seq. their conversion to fat, 386 Carbon dioxide in blood, 215 in the urine, 422 Carbon monoxide hemoglobin, 212 Cardiac contraction, nature of, 249 Cardiac cycle, 234 duration of phases of, 236 Cardiac end of stomach, 339 _ glarfds of stomach, 339 impulse, 238, 247 muscle, 72 plexus, superficial, 254 branches of the vagus, 254 ef seq., 276 Cardiogram, 239 Cardiograph, 239 Cardiometer (Roy’s), 249 Cardio-pneumatic movements, 312 during hibernation, 313 Cartilage, 30 hyaline, 30 parenchymatous, 30 Cartilages of the larynx, 323 Casein (see Milk) Caseinogen, 333, 431 Catalysis, 4 Caudate nucleus, 180 Cells, chalice, 22 fat, 28 nerve, 74 igment, 30 sepectiastions of, 17 Cellulose, 335 Central spot of the eye, 129 Centre, auditory, 196 for dilator pupille, 125 for smell, 115, 196 for sphincter pupillew, 125 for vomiting, 358 respiratory, 306 taste, 196 touch, 197 vaso-constrictor, 281 visual, 193 451 Centre, voluntary, 199 Centrosome, 13 . Cerebellar tracts, 169 | Cerebellum, 174 et seg. connections of, 175 cortex of, 175 functions of, 177 grey matter of, 175 peduncles of, 167, 175 removal of, 177 superior vermis of, 174 Cerebral cortex, 180 action of, 183 action of drugs on, 188 action, time of, 189 Cerebrum, 180 et seq. differentiation of stimuli of, 187 functions of, 183 rey matter of, 181 ocalisation of functions, 191 peduncles of, 180 removal of, 191 Chemiotaxis, 15 Chest voice (see Voice) Chlorine-containing bodies in urine, 421 Cholalic acid, 367 Cholera bacillus, action of gastric juice on, 353 Cholesterin, 76, 210, 368, 874, 430 Cholin, 77 Chondroitin, 31 Chorda tympani, 280, 346 Chord tendinz, 229, 231 Chorionic villi, 437 Choroid, 119 Chromatin, 16 Chromo-proteins, 12 Chyle, 223 Chyme, 360 Cilia, 15, 25 Ciliary muscle, 119, 125 processes, 119 Cilio-spinal region, 126 Circulation, the, 225 et seq. factors, extra-cardiac, in, 291 e¢ seq. foetal, 438 in blood vessels, 258 in kidney, 424 in the heart wall, 290 inside the cranium, 290 Circulation in the lungs, 290 respiration, influence on, 310 Claude Bernard—formation of sugar in the liver, 387 Coagulation of the blood, 203 e¢ seg. advantages of, 205 factors, influencing, 205 of milk, 383 (see also Souring of milk) 452 * Cochlea, 142 Cold spots, 111 Collagen, 26, 39 action of hydrochloric acid on, ie action of trypsin on, 362 Collodion as a means of separating trypsin, 362 Colloids, 7 ‘ Colon, 341 Colostrum, 430 Colour, 131 blindness, 134 sensation of, 131 Colours, complemental, 133 Columne carne, 229 Complement, 413 Complemental air, 303 Conducting paths (see Spinal cord, etc.) Conduction of heat from skin, 383 Conduction of nerve impulse, 80 et seq. Conductivity, thermal, 110 Connective tissues (see Tissues) Consciousness, 98 Contraction of the pupil, 120, 125 Convection of heat from skin, 383 Convoluted stage of mitosis, 18 Convoluted tubules of the kidney, 424 Convolutions, cerebral, 180 Cooking, eftects of, on food, 336 methods of, 386 of vegetables, 336 Co-operative antagonism of groups of muscles, 58 Cord, spinal (see Spinal cord) Cornea, 119 Corona radiata, 180 Coronary arteries, 245 Corpora quadrigemina, 179 Corpus Arantii, 233 Highmori, 433 Corpuscles, blood, 207 et seg. bone, 32. ~ muscle, 38 nerve, 75 tactile, 108 Corti, organ of, 143 Coughing, 302 Cranial nerves, 171 et seq. Creatin, 40, 206, 334, 391, 417 Creatinin, 391, 417 Cretinism, 408 Cricoid cartilage, 323 Crossed pyramidal tract, 166 Crura cerebri, 179 et seq. crusta of, 180 Curare, experiment, 43 Current of action, 68 injury, 63 INDEX Cutaneous nerves, influence on re- spiration, 310 Cyrtometer, 297 Cystin, 420 Cytomitoma, 13 Cytotoxins, 413 DecussaTIon of the fillet, 168 of the pyramids, 170 of the optic nerves, 137 Defecation, 376 Degeneration, Nissl’s, 86 reaction of, 48 Deiters’ nucleus, 167, 170, 174, 175 Delivery, 440 Dendrites, 75 Dentals (see Consonants) Dentate nucleus of the cerebellum, 175 Depressor nerve (see Cardiac branches of vagus) Deutero-proteose, 11, 351, 361 Development, 434 et seq. Developmental method of staining nerve tracts, 162 Deviation of eyes, 137 Dextrin, 331, 345 Dextrose, 331 Diabetes, 389, 405 Diamido acids, 9, 365, 446 Diapedesis, 208, 290 Diaphragm, 298 Diaphysis, 35 , Diastole of heart, 235, 247 Diastolic pressure, 275 Dicrotic notch, 267 wave, 266, 267 Diet, 397 et seq. during muscular work, 69 for horse, 400 for ox, 401 for sheep, 402 for milk cows, 402 Dietetics, 397 Diffusion of gases in lungs, 308, 313, 317 Digestion, 337 et seq. fate of the secretions of, 373 intestinal, 360 in mouth, 344 in stomach, 349 of stomach wall, 352 Dilatores narium, 300 pupille, 120 pupille, course of fibres, 120 Dioptric mechanism, 122 Dioxybenzene, 420 Diphtheria bacillus, 411 toxin, 411 Direct or dorsal cerebellar tract, 169, 175 pyramidal tract, 170 -= INDEX Disaccharids, 331 Discharging mechanism (cerebral), 199 Distinction of differences of pressure, 109 of place of contact, 109 of time contacts, 109 Diureides, 416, 447 Divalent acids, 445 Dobie’s line, 38 Dorsal columus of spinal cord, 163 columns, nuclei of, 88 Ductless glands, 404 et seq, Ductus arteriosus, 439 arteriosus, closure of, 439 venosus, 439 : venosus, obliteration of, 439 Duodenum, 341 Dyaster stage of mitosis, 19 Dynamometer, 53 Ear, anatomy of, 138 et seq. external, 139 internal, 141 et seq. middle, 140 e¢ seg. Eck’s fistula, 391 Ehrlich’s side chain theory, 413 Elastin, 27, 329 action of trypsin on, 361 Electrode, non-polarisable, 63 Electrotonus, 46 Eleventh nerve (sce Spinal accessory) - Emmetropia, 124 ' Enulsification, 362 Endocardium, 231 Endolymph, 148 Endothelium, 28 Energy requirements of animals, 399 Eotore modifying, 400 Energy value of food, 326, 332 determination of, 328 Entero-hemal circulation, 373 Enterokinase, 364, 366 Enzymes, 3 Eosinophiles (sce Leucocytes) Epiblast, 435 parts developed from, 435 Epicritic sensibility, 112 Epiglottis, 323 Epiphysis, 35 Epithelium, ciliated, 25 columnar, 21 excreting, 24 glandular, 22 mucin-secreting, 22 simple squamous, 20 stratified squamous, 20 transitional, 21 zymin-secreting, 24 Erection, 434 mechanism of, 434 453 Erepsin, 365, 390 Ergograph, 99 Ergometer, 62 Erlanger’s B.P. apparatus, 275 Erythrocytes, 202, 209 et seq., 217 chemistry of, 210 fate of, 220 Erythro-dextrin, 345 Ethane, 443 Ethereal sulphates in urine, 418 Ethylene, 443 Euglobulin, 206 Eustachian tube, 141 Excretion, effect of muscular work on, 67 by the kidneys, 414 et seg. by the lungs (see Respiration) by the skin, 428 Exophthalmic goitre, 408 Expiration, 301 forced, 301 Expired air, 314 Eye, 119 et seg. anatomy of, 119 e¢ seg. connection with central nervous system, 137 emmetropic, 124 hyaloid membrane of, 120 nervous mechanism of movement 136 physiology of, 122 Eyeballs, movement of, 134 FACIAL nerve, 172 Feces, 374 in fasting animals, 374 in feeding animals, 374 in young, 374 Fainting, 292 Fallopian tube, 433, 434 Far point of vision, 124 Fasting, metabolism during, 393 Fat cells, 28 Fatigue of cerebral mechanism, 189 of muscle, 53 Fats, 29, 206 in diet, 397 energy value of, 329 gastric juice on, 352 metabolism of, 392 as they leave the alimentary canal, 379 Fatty acids, 29 tissue, storage in fattening, 385 Feeding, effect of, 394 Fehling’s test, 330 Fenestra ovalis, 140 rotunda, 140 Ferrier, experiments on monkey, 196 brain of 454 Fibres, elastic, 27 nerve, 75 non-elastic, 26 non-medullated nerve, 75 pre-gapglionic and _post-gangli- onic fibres, 152 splanchnic, 152 et seq. somatic, 152 Fibrin, 204 Fibrinogen, 204 Fibroblasts, 25, 27 Fibro-cartilage, elastic, 31 white, 31 Fibrous tissue, 25 Field of vision, 129 Fifth cranial nerve (see Trigeminal) Filiform papille, 338 Fillet, 168 et seg. Fissure of Rolando, 199 Flesh, 334 Flocculus, 174 Floor of the fourth ventricle, 167 Fluoride of sodium, action on epi- thelium, 380 Feetal circulation, 438 Food, 326 et seq. absorption of, 379 et seq. animal, 332 effect of, on respiratory inter- change, 321 fate of absorbed, 381 nature of, 326 vegetable, 335 Foot of horse, 103 Foramen ovale, 439 occlusion of, 489 Formamide, 446 Formic acid, 446 Foster, changes in protoplasm, 5 Fourth cranial nerve (see Trochlearis) Frauenhofer’s lines, 211 Frog’s heart, 234 Functions of the spinal cord, 158 GALACTOSE, 330 Gall-bladder, 341 Gall-stones, 368 Gallop, 101 Galvanic current, muscle by, 45 Galvanotaxis, 15, 16 Galvanotonus, 45 Ganglia, collateral, 152 inferior mesenteric, 154 lateral, 151 superior cervical, 153 superior mesenteric, 154, Gases of the blood, 214- Gaskell, nervous mechanism of the heart, 256 stimulation of INDEX Gastric digestion, amylolytic period, 350 ; proteolytic period, 351 in carnivora, 349 in horse, 358 in ruminants, 360 Gastric glands, 339 Gastric Juice, 350 action on gelatin, 352 antiseptic action, 353 influence of diet on, 353 source of constituents, 353 Gelatin, 27, 329 action of trypsin on, 362 Gemmules, 75, 189 Generative organs, 482 et seq. Geniculate bodies, 193 Germ centres, 217 Germinal epithelium, 433 spot, 433 vesicle, 433 Gestation, 440 Giant cells, 218 Glands, compound, 22 racemose, 22 simple tubular, 22 Globin, 214 é‘ Globulin, 10 Globulose, 11 Glomerulus of kidney, 423 Glossopharyngeal nerve, 172, 348 influence on respiration, 309 Glucosamine, 23, 31 Glucose, 40, 327 - of blood, 206, 216 Glycerin, 444 Glycero-phosphates in urine, 421 Glycocholic acid, 367 Glycocoll, 418 Glycogen, 40, 332 Glycogenic function of the liver (see Liver) Glyco-proteids, 12, 23 Glycosuria, 389 Glycuronie acid, 31 Gmelin’s test, 8368, 374 . Golgi, organs of, 107 Goltz, experiments on cerebral cortex, 185 _Graafian follicle, 433 Granules, Nissl’s, 75 Gravity, influence on capillary sure, 285 Grey matter (see Cerebrum, etc.) Growth in length of bone, 35 Griibler’s peptone, 379 Guanidin, 40 pres- HaMATIN, 213 Hematoidin, 214, 368 INDEX Hematoporphyrin, 213, 368, 422 Hemin, 214 ‘Heemochromogen, 213 Hemocytometer, 209 Hemoglobin, 41, 210 action of hydrochloric acid on, 352 decomposition of, 213 reduced, 211 ‘ Hemoglobinometer, Haldane’s, 211 Hemolymph glands, 217, 221 Hemolysis, 220, 413 Hair, 428 : Haldane, experiments on air contain- ing carbon monoxide, 316 and Barcroft, method for gases in blood, 214 and Lorrain Smith, method for total blood, 219 Hassall, corpuscles of, 409 Haversian canals, spaces, etc., 34 Hearing, 138 et seq. Heart, 227 et seq. changes in shape, 236 changes in position, 237 connection with central nervous system, 253 et seq. connection in frog, 253 connection in mammal, 254 effect of atropin on, 253 electrical stimulation of, 253 fibrous rings of, 228 of frog, 234 physiology of, 234 pulmonic, 226, 227 relations of, 234 sounds of, 245 et seq. structure, 227 e¢ seq. systemic, 225, 227 valves (see Valves) work of, 249 Heat elimination, 383, 385 elimination, nervous mechanism in, 385 F - elimination from skin, 383 elimination from respiratory pas- sages, 384 elimination in urine and feces, 384 production, 381, 385 production in the brain, 382 production in glands, 382 production in muscle, 381 units, 328 Henle’s loop, 423 Hemisection of spinal cord, 161 Hemispheres, cerebral (see Cerebrum) Hetero-proteose, 351 Hiccough, 302 High tension pulse, 269 Highmori, corpus, 433 Hippocampal convolution, 197 455 Hippuric acid, 418 - Histones, 12 peat segmentation, 434 oppe-Seyler, changes in protopl Hoof, iof, Oe as Hormones, 355, 404 Hot spots, 111 Hyaline cartilage, 30 Hyaloid membrane, 120 Hyaloplasm, 13 Hydrocarbons, unsaturated, 443 saturated, 443 Hydrochloric acid, 350 Hydroxyl molecule, 446 Hypermetropia, 127 Hypnosis, 190 Hypoblast, 435 parts developed from, 435 Hypoglossal nerve, 171 Hypoxanthin, 417 ILEO-CHCAL valve, 341 Illumination, degree of, 117 source of, 117 Immune body, 411 Immunity, 411 Impregnation, 434 Incompetence (cardiac), 248 Incus, 140 Indican, 418 Indol, 371, 375, 418 Indoxyl, 418 Induced electricity, muscle by, 48 Inferior oblique muscle, 135 _ olivary nucleus, 167 Inflation of lungs, artificial, 309 Infundibula, pulmonary, 232, 295 Inhibition, intra-cardiac mechanism, 254 Inhibitory action of vagus, 254 Inhibitory nerves, 82 Inorganic salts in food, 327 Inosite, 40, 334 Inspiration, 297 forced, 300 forces opposing, 298 Intercostal muscles, external, 300 Internal capsule, 181 Internal secretion, 404 secretion, effect on blood pressure, 277 Intestines, 340 et seq. absorption from, 379 et seq. digestion in carnivora, 360 in horse, 377 in ruminants, 378 movements of, 375 nerve supply of, 343 secretion of wall, 365 stimulation of 456 Intra-cardiac pressure, changes in, 239 pressure, method of determining, Inulin, 331 Iodothyrin, 407 Iris, 119 Islets of Langerhans, 343 Isometric method of measuring mus- cular contraction, 53 Isotonic method of measuring muscular contraction, 53 JACOBSEN’S nerve, 347 Jecorin, 206 Joule’s law, 328 Juice, gastric, 350 et seq. pancreatic, 361 KATABOLIC changes, 5 Katacrotic crests, 268 Katelectrotonus, 46 Kennedy’s experiments on nerve cross- ing, 79 Keratin, 12, 21, 329 Ketones, 444 Knee jerk, 160 Kidney, 423 e¢ seq. circulation in, 424 physiology, 424 structure, 423 Lapour, 440 Labyrinth of ear, 141, 142 * Lactase, 365 Lactate of ammonia, 390 Lacteals, 380 Lactic acid, 445 Lactose, 331 Lacune, 32 Levulose, 330 Lamelle, Haversian, 35 interstitial, 35 medullary, 35 peripheral, 35 Langerhans, islets of, 343, 411 Langley, experiment on vagus and sympathetic nerves, 79 Lanolins, 430 Large intestine, 341 ; Larynx, cartilages of, 323 mucous membrane of, 324 muscles of, 324 nervous mechanism of, 325 physiology of, 325 position in deglutition, 348 structure of, 323 Law, Pfliiger’s, 81 of polar excitation, 46 ; Lecithin, 76, 333, 335, 368, 386, 389 Lens, crystalline, 120 INDEX Lenticular nucleus, 180 Lesions of the cerebral cortex, 199 Leucin, 8, 361, 365 Leucoblasts, 217, 218 Leucocytes, 202, 207 et seq. chemistry of, 208 fate of, 220 Leucocytosis, 217 digestion, 380 Levatores costarum, 300 Lieberkiihn’s follicles, 340, 365 Light, 116 Liquor folliculi, 433 sanguinis, 202 Lipase, 362 Lipochrome, 206 Lissauer’s tract, 167 Liver, 341 ef sey. development of, 387 formation of urea in, 390 glycogenic fuuction, 387 relation to fats, 389 relation to genéral metabolism, 387 relation to proteins, 389 storage of carbohydrates, 387 summary of functions, 391 Localisation of functions, 191 Lockhart Clarke’s column, 89, 168, 164 Lorrain Smith and Haldane’s method for total blood, 219 Loudness of sounds, 145, 325 Lungs, 294 air sacs, 294 capillaries, 294 elasticity of, 295 interchange between air blood in, 813 physiology of, 296 et seq. Lymph, 222 e¢ seq. chemistry of, 222 in disease, 223 formation of, 223 vessels, 223 Lymphatic glands, 217, 221 Lymphatics, pressure in, 261, 286 Lymphocytes, 30, 207 Lymphoid tissue, 30, 216 and Maize, 335 Malic acid, 327 Malleus, 140 Malpighian bodies, 424 bodies, secretion in, 425 corpuscles, 220 Maltose, 331, 345 Mammary glands, 430 Marchi’s method of nerve-staining, 85, 165 Mastication, 344 | | - Mott, views on the Rolandic area, 197 INDEX 457 Maternal attachment of ovum, 437 McFie, experiments with extract of ' suprarenal, 405 Meconium, 375 Medulla of kidney, 423 Medulla oblongata, 167 et seq. commissural fibres of, 170 conducting paths in, 168 grey matter of, 167 pyramids of, 170 reflexes of, 172 structure of, 167 Medullary sheath, 75 Meissner’s plexus, 343 Melanin, 30 Membrana tympani, 140 Membranous labyrinth, 142 Mesial fillet, 169 Mesoblast, 435 parts developed from, 435 Metabolic equilibrium, 395 Metabolism, general, 4, 392 during fasting, 393 of fats, 392 protein, 392 “method of investigating, 392 Methemoglobin, 212 Methane, 443 Micturition, 427 Milk, 333, 430 composition of cow’s, 334 fats, 431 phosphorus compounds of, 431 proteins of, 431 secretion of, 431 souring of, 333 sugar of, 432 Mitosis, 17 Mitral area, 247 valve, 231 Molecular layers of retina, 121 Monamino acids, 8, 446 Monaster stage of cell division, 19 Monosaccharids, 330 Motor nerves (see Nerves) points, 43 Mouth, 337 Mucin, 12, 23, 329, 368, 407 secreting epithelium, 22 Mucoid tissue, 25 Mucous membrane of large intestine, | 341 of small intestine, 340 of stomach, 339 of uterus, 437 Murexide test, 417 Murmurs, cardiac, 248 simulation of, by breath sounds, 313 | Muscle, 37 et seq. absolute force of, 52 application of weights to, 55 carbohydrates of, 40 cardiac, 72 Shae in blood passing through, changes in shape, 49 chemical changes in, 64 chemistry of, 39 death of, 72 duration of contraction, 52 electrical changes in, 638 electrical condition of, 42 extensibility of, 41, 64 fatigue, 53 heat production in, 42, 61 latent period of contraction, 52 minimum stimulus, 54 optimum load, 60 optimum stimulus, 54 physical characters of, 41 proteins of, 39 respiration of excised muscle, 65 skeletal, 37 spindles, 107 stimulation by galvanic current, 47 stimulation by induced _ elec- tricity, 48 storage of food in, 386 structure of, 37 successive stimuli, 56 temperature effects, 54 visceral, 37, 70 visceral, staircase contraction of, 71, 72 voluntary contraction, 57 wave of contraction, 50 work, 59 F Muscles, co-operative antagonism of, 58 Muscular sense (see Sense), 107 work, effect on excreta, 67 Myelocytes, 207 é : Myogenic movements of intestine, 375 Myoglobulin, 39 Myohematin, 41 Myoids, 14, 37 Myopia, 127 Myosin, 39 Myosinogen, 39 Myostromin, 39 Myxcedema, 408 Naunyn’s observations on cholesterin, 368 Near point of vision, 124 Necrobiosis, 6 Nerve cells, 74 458 Nerve cells, function of, 85 centres (see Centres) corpuscles, 75 fibres, 75 fibres, medullary sheaths of, 75 fibres, non-medullated, 75 specific energy of, 118 Nerves, 73 et seq. afferent, 82 auditory, 144; cochlear root of, 144 ; vestibular root of, 144 augmentor, §2 efferent, 82 excito-motor, 83 excito-reflex, 83 excito-secretory, 83 factors modifying conduction in, 81 inhibitory, 82 - mixed, 83 motor, 82 nature of impulse in, 84 physiology of, 77 rate of conduction in, 80 secretory, 82 sensory, 83 spinal (see Spinal nerves) vaso-constrictor, 82, 280 vaso-dilator, 280 Nervi erigentes, 155, 281, 376, 427 Nervous mechanism, intra-cardiac, 252 system, 156 e¢ seq. Neural canal, 435 Neurilemma, 75 Neuroglia, 436 Neuro-keratin, 76 Neuro-muscular mechanism, 87 e¢ seq. fatigue of, 99 Neurons, 436 classification of, 81 excitability of, 78 extent of excitability of, 83 ingoing, 79 manifestation of activity of, 78 outgoing, 79 stimulation of, 78 Neutral sulphur in urine, 420 Nicotine, effect of painting ganglia _ with, 152, 347 Nictitating membrane, 120 Ninth nerve, 155, 172 Nissl’s degeneration, 86 granules, 75, 85, 189 Nitrites, effect on blood pressure, 277 Nitrogen in the blood, 216 } in the urine, 415 ef seq. Nodal swellings, 17 Nodes of Ranvier, 76 Nuclei of the cranial nerves, 171 of the posterior columns, 88 INDEX Nucleins, 11 Nucleo-protein in the urine, 422 Nucleolus, .16 Nucleo-proteins, 369 action of gastric juice on, 352 action of trypsin on, 362 Nu-leus, 16 e¢ seq. cuneatus, 168 gracilis, 168 OccrPITAL lobe, 193 Oculomotor nerve, 136, 172 Esophagus, 338 application of a stethoscope to, 348 peristaltic action of, 348 Cistrous cycle, 433 Oleic acid, 29 Olein, 29, 333 Olfactory bulb, 116 tracts, 116 Olive, the, 167 Omasum, 340 Ophthalmoscope, 122 Opsonins, 413 Optic chiasma, 137 disc, 120 nerve, 120 radiation, 193 thalamus (see Thalamus) Organ of Corti, 143 Organic acids as salts in food, 327 Organs of Golgi, 107 Osazones, 330 Osmosis, 380 Osmotic pressure, 210 Ossification, process of, 32 e¢ seg. centre of, 34 Otic ganglion, 347 Ovary, 408, 432 Ovum, 433 Oxalates, 204 Oxalic acid, 422, 445 Oxy-acids, 445 : Oxygen in blood, 215 ‘ Oxyhemoglobin, 210 Oxyntic cells, 339 Oxyphil leucocytes, 297 Pacers of horse, 100 _ Pain, sensation of, 106 Palmitic acid, 29 Palmitin, 29, 333 Pancreas, 343, 410 removal of, 388, 410 ' Pancreatic secretion, 361 ; secretion, action of, 361 secretion, character of, 361 physiology of, 364 Papillary muscles, 229 _ Paracasein, caleic, 383, 352 INDEX 459 Paradoxical contraction, 81 Paralysis of laryngeal nerves, 325 Paramyosinogen, 39 cate Boag 408 | Parotid gland, 338 nerve supply, 347 reflex stimulation of, 347 Partial pressure of oxygen and carbon dioxide in blood, 315 pressure of gasesin air vesicle, 317 Passage of carbon dioxide from tissues to blood, 320 of oxygen from blood to tissues, 319 Pavlov, effect of diet on gastric juice, pancreatic fistulae, 362 Penis, 434 Pepsin, 72, 353 fate of, 373 source of; 353 Pepsinogen, 353 Peptic glands, 339 Peptone, 11, 205, 351, 361 Perfusion method of studying action of drugs, 278 Pericardium, 227 Perichondrium, 31 Perilymph, 142, 148 Perineurium, 76 Peripheral resistance, 276 Peristalsis of bladder wall, 427 of intestine, 375 nervous mechanism of, 375 of wall of ureter, 427 wave of, 71 Petrosal nerve, small superficial, 347 Peyer’s patches, 341 Pfliiger’s experiments on protein diet, 395 - law of contraction, 81 Phagocyte action, 203 Pharynx, 338 nerve supply of, 343 Phenol, 372, 418 Phenylhydrazin, 330 Phloridzin, injection of, 387 poisoning, 389 Phosphocarnic acid, 333 Phospho-protein, 431 Phosphorus-containing bodies in the urine, 420 e¢ seg. | Phototaxis, 15 Phrenic nerves, 306 Pigment cells, 30 Pigments of the urine, 421 Pillars of the fauces, 338 Pilocarpine, effect on sweat glands, | 430 Pitch of sounds, 146, 325 Pituitary body, 406 injection of extracts of, 406 Placenta, 438 Plasma, 202 et seg. Plasmodia, 1 Plethysmograph, 277 Pleura, complemental, 299 Pleural cavity, 295 Plexus, hypogastric, 154 Auerbach’s, 343 Meissner’s, 343 Pohl’s observations on leucocytosis, 380 Polar excitation, law of, 46 Polycrotic waves, 266 pulse, 268 ss gona of monosaccharids, 33 Polymorpho-nuclear leucocytes (see Leucocytes) Polysaccharids, 331 Pons Varolii, 173 Portal circulation, 343 vein, 343 Positive accommodation, 124 et seg. accommodation, varying power of, 124 Posterior corpora quadrigemina, 179 Poteriodendron, 73 Precipitins, 413 Predicrotic wave, 266 Preformed sulphate in urine, 418 Preganglionic fibres, 152 Presbyopia, 127 Pressure, arterial, 272 arterial, factors controlling, 276 Primitive nerve sheath, 75 Propionic acid, 445 Prostate gland, 433 _ Protagon, 76 Protein metabolism, 392 _ Proteins, 6 et seq. as they leave the alimentary canal, 379 of blood, 204, 206 conjugated, 11 conversion into fat, 386 diet, 395 energy value of, 328 native, 10 requirements, 400 sparers, 397 | Proteolytic period, 351 Proteoses, 11 Prothrombin, 204 Protopathic sensibility, 111 Protoplasm, 1 ef seq. cell, 13 Proto-proteoses, 11, 351 460 Proximate principles, 327 sources of, 332 *Pseudoglobulin, 206 Pseudopodia, 208 . Ptomaines, 373 Ptyalin, 345 fate of, 373 Puberty, 432 Pulmonary valve, 234 Pulse, anacrotic, 268 arterial, 262 capillary, 270 cause of, 262. characters of wave, 268 form of wave, 264, 269 height of wave, 264 high tension, 269 length of wave, 264 palpation of, 268 rate of, 268 rate, 268 rhythm, 268 tension, 269 velocity of wave, 263 venous (see Venous pulse) volume of, 268 Pulsus celer, 269 parvus, 268 plenus, 268 tardus, 269 Pupil, 119 contraction of, 125 Purin bodies, 391, 416 bodies, endogenous genous, 417 Purkinje’s cells, 175 images, 129 Pyloric end of the stomach, 339 Pyramidal tracts, 166 Pyramids of the medulla, 170 Pyrocatechin, 420 and e€xo- QUADRIURATES, 417 RapDIATION of heat from skin, 383 Radicals, organic, 443. Ranvier, nodes of, 76 Reaction, 99 of degeneration, 48 Receptaculum chyli, 380 Reception of stimuli, 191 Recti muscles (eye), 135 Rectum, 341 Recurrent laryngeal nerve, 254, 325 Red marrow of bone, 217, 218 Red nuclei, 176, 180 Reduced alkaline hematin, 213 Reflex action, 87, 90 F function of the spinal cord, 158 Refraction of light, 122 | INDEX Reid on absorption, 379 Relation of hemoglobin and derivations, 214 Remak’s ganglion, 252 Rennet, 333 Rennin, 352 source of, 853 Reproduction, 432 Reserve air, 303 Residual air, 303 contraction period of (see Systole) Respiration, 294 ef seq. effect on air breathed of, 313 effect on the blood, 314 at high altitudes, 307, 318 influence of heart’s action on, 310 internal, 319 mechanism of, 294 et seq. movements in (special), 302 nervous mechanism of, 306 physiology of, 296 rhythm of, 305 Respiratory centre, 306 action of, 306 effect of absence of oxygen on, 307 effect of accumulation of waste products in the blood, 307 effect of carbon dioxide on, 307 effect of temperature of animal on, 310 E modifications in activity of, 307 Respiratory interchange, extent of, its 320 . interchange, factors modifying, 320, 321 7 quotient, 314 quotient, effect of diet on, 321 Restiform bodies, 167 Rete testis, 433 Reticulum, 340. Retina, 121 ef seq. blood vessels of, 122 nature of changes in, 130 stimulation of, 128 rods and cones of, 121 Retractor oculi, 135 : Rhythmic contraction of the heart, 250 contraction, propagation of, 251 contraction, starting mechanism of, 251 Rigor mortis, 72 Riva Rocci B.P. apparatus, 274 Rolandic area, 199, 200 - Rolando (see Fissure) Roof nucleus of the cerebellum, 175 Roots of the spinal nerves, 150 Roy’s cardiometer, 249 Rumen, 340 SACCHAROMYCES cerevise, 2 Saccule, 142 Saliva, 344 functions of, 344 influence of chorda tympani in secretion of, 346 physiology of, 345 reflex stimulation of secretion, 346 stimulation of sympathetic effect on, 346 Salivary glands, 338 glands, nerve supply, 345 Salts of the bile acids, 367, 371 fate of, 373 Sanson’s images, 125 Sarcolactic acid, 40, 65 Sarcolemma, 37, 38 Sarcous substance, 38 Scala media, 142 Schifer’s investigations of - functions, 196 Schwann, white sheath of, 75 Sciatic nerve, 281 Sclerotic, 119 Sclero-proteins, 12, 329 Sebaceous glands, 430 Secretin, 365 Secretion, internal, 404 et seq. brain | Sectional area of circulatory system, © 226 Semen, 433 Semicircular canals, 142 ef seg. canals, physiology of, 147 Semilunar valves, 232 valves, action of, 243 Seminiferous tubules, 433 Sensation, 105 colour, 131 physiology of colour, 131 production of colour, 132 of hunger, 106 of pain, 106 of smell, 115 of taste, 113 of thirst, 105 visual, 138 Sense (The Senses), 104 ef seg. of acceleration and retardation of motion, 147 joint and muscle, 107 et seq., 147 special, 108 ef seq. of smell, 115 tactile, 108 of taste, 113 temperature, 110 visual, 116 et seq. Sensibility, common, 105 Septo-marginal tract, 167 Serum, 203 et seq. | Serum albumin, 204, 206 globulin, 204, 206 | Seventh cranial nerve, 172 (see Facial) Sexual organs, development of, 432 organs, removal of, 432 Side chain theory (see Ehrlich) Sighing, 302 Sinus of Valsalva, 233 Sixth nerve (see Abducens) Skatol, 371, 372, 875, 418 Skatoxyl-sul phate of potassium, 418 Skin, excretion by, 428 Sleep, 190 respiratory changes during, 321 Small intestine, 340 Smell, 115 centre for, 196 mechanism of, 116 physiology of, 116 Snake toxin, 411 Sneezing, 302 Somatopleur, 150, 436 | Sound perception, 145 | Sounds of the heart (see Heart) | Space, Haversian, 34 Specific nerve energy, 118 | Spectrum analysis, 211 | Spermatoblasts, 433 _ Spermatogen, 433 Spermatozoa, 433 development of, 433 | Sphincter ani, 341, 376 pupille, 120 trigonalis, 427 Sphygmograph, 265 tracings, 265 Sphygmometer, 274 Spinal accessory nerve, 171 cord, 157 et seq. cord, conducting paths in, 161 cord, dorsal columns of, 163 cord, grey matter of, 158 cord, reflex function of, 158 cord, section of one side of, 161 ‘* Spinal dog,” 92 Spinal nerves, 149 nerves, roots of, 150 Splanchnic area, 290 influence on respiration, 309 Splanchnopleur, 150, 436 Spleen, 220 Spot, blind, 128 Staircase contraction, 250 Stannius’ experiment, 252 Stapedius muscle, 140 Stapes, 140 Starches, 330 Stearic acid, 29 Stearin, 29, 333 Stenosis, 248 462 Stercobilin, 374 Stewart’s method of estimating time taken in cirtulation, 293 * Stimulus, 99 Stomach, 339 of horse, 339 of ruminant, 340 during fasting, 350 during feeding, 350 importance in digestion, 357 movements of, 355 nervous mechanism of, 357, rate of passage of food from, 356 vascular changes, 350 Storage of surplus food, 385 Storing mechanism (cerebral), 198 Stromulir, 288. Sublingual gland, 338 gland, verve supply, 346 Submaxillary gland, 338 gland, nerve supply, 346 Succus entericus, 365 - entericus, action of, 365 entericus, nervous mechanism of secretion, 366 Sulcus centralis, 199 Sulphur-containing bodies urine, 418 e¢ seq. Superior oblique muscle, 135 Suprarenal bodies, 404 extract of, 277, 279, 389 medulla, 404 metabolism after injection of, 405 Suspensory ligament of eye, 120 Swallowing, 348 Sweat, chemistry of, 430 evaporation, 383 glands, 428 nervous mechanism of secretion of, 429 sympathetic fibres to the heart, 256 Synapsis, 74 System, Haversian, 35 Systole of heart, 235, 237 latent period, 245 - period of overflow, 245 period of residual contraction, 245 Systolic pressure, 274 in the TACTILE sense, 108 Tapetum nigrum, 121 Tartaric acid, 327 Taste, 113 et seq. bulbs, 113 centre for, 196 mechanism of, 113 physiology of, 114 INDEX Taurocholic acid, 367 Tegmentum, 180 * Temperature, 382, 384 ef seq. : - in cold-blooded animals, 384 in hibernating animals, 384 ~ -in warm-blooded animals, 384 2 regulation, 382 e¢ seq. ; “ variations in, 384 ‘ p Temporo-sphenoidal lobe, 197 h- Tension of gas in a fluid, 315 of the vocal cords, 325 Tensor tympani muscle, 140, 141. Tenth nerve (see Vagus) : Testis, 408, 433 sg Tetanus, complete, 57 incomplete, 56 Thalamus opticus, 180, 193 Thermal sense, 110 : Thermotaxis, 16 ; Third nerve, 154, 172, 136 call Thrombin, 204 ; ; Thrombokinase, 204 Thymus gland, 205, 409 etfect of castration on, 410 removal of, 410 Thyroid cartilage, 323 _ gland, 406 administration of extracts of, — 406 . removal of (thyroidectomy), 406 me Tidal air, 303 ; Tissue, adipose, 28 areolar, 27 _ cancellous, 33 ; connective, 25 et seq. fibrous, 25 lymph, 30 mucoid, 25 vegetative, 20 et seq. Tongue, 338 Tonometer, 274 Tonsil, 338 ~N Torricellian vacuum, 214 Touch, centre for, 197 — Toxic action, 411 ef seq. ; Trabecule, 34 Tricrotic pulse, 266 <: Tricuspid valve, 232 a, area, 247 a Trigeminal nerve, 172 Trochlearis nerve, 172 Trophoblast, 437 Trot, 101 Trypsin, 361 fate of, 873 Trypsinogen, 364 Tryptophane, 361 ‘Tubular glands of stomach, 339 Tubules, secretion in kidney, 425 Tunica albuginea, 433 . INDEX Twelfth nerve, 171 | Tympanic cavity, 140 Typhoid toxin, 412 Tyrosin, 8, 361, 365 UMBILICAL arteries, 439 veins, 438 Unilateral stimulation, 15, 16 Urates, 417 Urea, 329, 390, 392, 415 - of blood, 216 effect of drugs on formation of, 391 source of, 391 ’ Urethra, 427 Uric acid, 207, 417, 426, 447 Urinary bladder, 427 nerves of, 427 Urine, 414 et seq. excretion of, 427 method of estimating solids in, 414 micro-organisms in, 415 - non-urea nitrogen in, 416 of herbivora, 415 pigments of, 421 secretion of, 423 Urobilin, 421 Urochrome, 421 Uroerythrin, 422 ’ Uterus, 437 centre for contractions of, 441 Utricle, 142 VACUOLES, 13 Vagus, 155, 172 cardiac branches, 254 ef seq. _ frog’s, 253 _ gastric branches, 281 ' effect on respiration of section, 307 et seq. effect on respiration of stimulation, 308 et seq. Valsalva, sinus of, 233 Valve, ileo-cecal, 341 Valves of the heart, 231 et seg. action of, 242 aortic, 233 auriculo-ventricular, 231 Mitral and Tricuspid) mitral, 231 pulmonary, 234 semilunar, 232 (see Aortic and Pulmonary) Vas deferens, 433 deferens, ligature of, 432 Vasa efferentia, 433 Vaso-constrictor centre, 282 centres, secondary, 283 (see 463 Vaso-constrictor nerves, 280 Vaso-dilator centre, 283 nerves, 280 ; Vaso-motor centre (see Centre) mechanism, 278 nerves, 278 nerves, section of, 278 Vegetables, as food, 335 Veins, pressure in, 240, 261, 286 umbilical (see Umbilical) ‘Vena cava, inferior (feetal), 439 superior (foetal), 439 Venous pulse, 270 - pulse, normal, 271 Ventricles of the heart, 227 contraction of, 280 left, 228 muscular structure of, 228 pressure in, 240 right, 230 Veratrin, effect on muscular contrac- tion, 54 Vermiform appendix, 341 : Vernon on erepsin in the tissues, 366 Vesicular sound, 304 Vestibule of the ear, 141 Vestibular root of the eighth nerve, 144, 175 Vibrations, light, 131 Vieussens, annulus of, 256 Villi, chorionic, 437 of intestine, 340 Vision, 116 et seq. binocular, 134 far point of, 124 field of, 129 monocular, 122 e¢ sey. near point of, 124 theories of colour, 133 Visual centre, 137, 193 Vital action of endothelium of capil- laries, 318 Vital capacity of the thorax, 303 Vitreous humour, 120 Vocal cords (false and true), 323 Voice, 323 et seq. Volition, 199 Voluntary actions, 98 centres, 199 Vomiting, 358 in horse, 359 centre, 358 WALK, 100 Wallerian degeneration method, 165 Waste, rate of (during fasting), 393 Water in food-stuffs, 326 Weigert’s method of nerve-staining, 85, 165 Whey-albumin, 333, 351 — 464 Jers INDEX 5 vi . \ XANTHIN, 334, 417 ows nulosa, 433 ; X rays in the examination of the pellucida, 433 — oe stomach, 355 . Bunt on exertion of CO, in the de Be of the intestine, Le eae et BO P . ; ne grand on muscle worl, \ i: Zymin, * “381 + oe Seas » Yawnine, 302 ; _ secreting epithelium, 1.) Yeast, 2, 330 . 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