^^ CORNELL UNIVERSITY. 3^ . :_ THE GIFT OF ROSWELL p. FLOWER FOR THE USE OF THE N. Y. STATE VETERINARY COLLEGE 1897 CORNELL UNIVERSITY LIBRARY 3 1924 056 978 707 |l Cornell University J Library The original of this book is in the Cornell University Library. There are no known copyright restrictions in the United States on the use of the text. http://www.archive.org/details/cu31924056978707 ESSENTIALS HUMAI^ PHYSIOLOGY Printed bn Loelmer and Chalmers, Edinlmrgh WILLIAM GEEEN AND SONS January, 1903 ESSENTIALS OP HUM PHYSIOLOGY FOR MEDICAL STUDENTS D. NOEL PATON, M.D., B.Sc, F.E.C.P. Ed. SUPERINTENDENT 01' THE RESEARCH LABORATORY OF THE ROYAL COLLEGE OF PHYSICIANS 01' EDINBURGH'; LEOTURBE ON PHYSIOJ-QGY, SCHOOL OP MEDICINE OP THE ROYAL COLLEGES, EDIKBUEGH ; EXAMINER IN PHYSIOLOGY IN THE UNIVERSITY OF EDINBURGH AND I'OE THE ROYAL COLLEGE OF PHYSICIANS, EDINBURGH 1 EDINBURGH WILLIAM GREEN & SONS 5Pu&Iisf)frs 1903 T PREFACE. The object of this volume is to put before medical students as succinctly as possible the essential facts of human physiology, and to emphasise specially those parts of the science which are of cardinal importance in medicine and surgery. Physiology has now become so wide in scope that the ordinary student must limit his attention to matters which have a direct bearing upon his professional work. Fortunately the study of this limited field affords ample opportunity for cultivating the scientific methods of observation and of thought, which should be gained by the student before approaching his clinical studies. In writing this book, I have endeavoured to recognise and to adhere to these limitations, and hence, many parts of physi- ology which occupy considerable space in the ordinary text- books have been relegated to minor positions, while parts which have a direct bearing upon the study of medicine have been purposely given a prominence which their' importance when viewed from the purely scientific standpoint would hardly warrant. Physiology, like anatomy, must be studied practically. But it is impossible for students to perform more than a small number of the experiments which are the ground-work of the science, and it is the duty of the teacher to indicate such PREFACE. a course of practical exercises as it is possible for them to accomplish, and which will form a basis upon which to build a really sound knowledge. Such practical courses are apt in the student's mind to be entirely dissociated from the system- atic study of physiology. Microscopic preparations are made too often with more consideration of the methods employed, than of the structures to be studied, and experiments are performed as feats of manipulative skill, rather than in relationship to what they teach. I have attempted to bring the practical and systematic study into closer relationship, by constant references to the practical work which the student must undertake. Since no amount of reading will give the real knowledge of the histo- logical methods which are employed, and the structures revealed by these methods, or of the chemical tests which are daily more and more largely used by physicians, or of the experimental procedure by which physiology has been built up, lengthy descriptions of these methods seem uncalled for ; and hence those details which the student must master practically are here merely touched upon, and he is referred to his practical exercises. Every student in a recognised school now receives such a practical training, and the knowledge thus acquired may be refreshed by reference to those books which are used as guides during the prosecution of the practical work — such books as Schafer's " Experimental Physiology," Schafer's " Essentials of Histology," Halliburton's "Essentials of Chemical Physi- ology," and " The Practical Physiology " of Beddard, Edkin, Hill, M'Leod and Pembrey. Descriptions of apparatus have been purposely excluded. The student must make him- self practically familiar with such pieces of apparatus as he has to use himself, or to see used in demonstrations. With other apparatus he is not concerned. It is as absurd that PREFACE. vii the student of physiology should burden his memory with the construction of machines he will never see in use, as that the surgeon should attempt to learn all the various instruments which he will never require in his practice. I have pleasure in acknowledging the kindly help which I have received from many of my friends. Drs. Gibson, Bruce, GuUand, Dunlop, Boyd, and Ballantyne have given me the benefit of their criticism on those sections of the book which bear upon the parts of medical science with which each of them is specially conversant. I desire also to record my thanks to my class assistants, Dr. Goodall, Messrs. Barcroft, Kidston, and Mackenzie for help in revising the proofs, and to Mr. Graham Brown for some of the histological diagrams. D. N. P. COI^TEI^TS. Intboduction, . Protoplasm, The Cell, ^art L SECTION I. SECTION II. PAGE 1 SECTION III. The Tissues, ...... A. The Vegetative Tissues, Epithelium — 1. Stratified Squamous Epithelium, 2. Columnar Epithelium, .... 3. Glandular Epithelium, .... 4. Ciliated Epithelium, .... Connective Tissues — 1. Mucoid Tissue, .... 2. Fibrous Tissue, ..... 3. Cartilage, ; . . . 4. Bone, ..... B. The Master Tissues — Muscle and Nerve, . Muscle — 1. Physical and Chemical Characters of Muscle at Rest, 2. The Modes of making Muscle Contract, 3. The Changes in Muscle during Contraction, 4. Source of the Energy evolved by Muscle, 5. Death of Muscle, Nerve, ... . . The Neuro-Muscular Mechanism, 13 20 20 20 21 22 25 25 26 31 32 36 37 43 47 62 67 68 80 X CONTENTS. SECTION IV. The Senses, ....... fAGE 85 A. Common Sensibility, ..... 85 -B. Muscular Sense, 86 C. Special Senses, 88 Tactile Sense, . 88 Temperature Sense, Vision, ... ... 91 92 Hearing, 117 Taste, . Smell, . . . . . 123 124 SECTION V. The Central Nervous System— Spinal Nerves, ...... Spinal Cord, ...... Medulla and Cranial Nerves, .... 126 132 137 Pons Varolii, ... 142 Cerebellurrij ..... 143 Crura Cerebri and Corpora Quadrigemina, . The Cerebrum, ...... 147 148 ^art IE. THE NUTRITION OF THE TISSUES. A. — Manner in which Nourishment is sent to the Tissues. Blood and Lymph, SECTION VI. SECTION VII. Circulation op Blood and Lymph, The Circulatory System — General Arrangement, . Circxilation through the Heart, Circulation in the Blood Vessels, 163 182 182 184 213 CONTENTS. B. — Supply of Noukishing Material to Blood and Lymph, and Elimination of Waste Matter from them. SECTION VIII, Respiration, PAOE 245 SECTION IX. Food and its Digestion and Utilisation, Food, .... Digestion, Absorption, Heat Production, Hepatic Metabolism, General Metabolism,- Dietetics, 280 296 320 323 328 333 340 SECTION X. Excretion, Eenal Excretion — Urine, . The Kidney, . Renal Secretion, Cutaneods Excrbtion- Sweat Secretion, Sebaceous Secretion, Milk Secretion, . 346 349 354 354 359 361 361 SECTION XI. Eeproddction, 364 mTRODUCTIOK Physiology 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 specu- lations 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 physi- ology of to-day is the offspring of such speculations. Organs and Function. — 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 estabhshed. 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 INTRODUCTION. one another in their physical characters and appearance, and as physiologists soon showed, in their functions. It now became evident why the liver secreted and the biceps con- tracted : the one is composed of secreting tissue, and the other of contracting tissue. . Cells and Function. — 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 generalisa- tion 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 is 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, sur- rounded 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 Function. — So far physiology had fol- lowed 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 important 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 INTRODUCTION. various structural differences of the different tissues are to be explained in terms of variations in the chemical changes in protoplasm. In our study of physiology we shall reverse this order of evolution, and, starting from the study of protoplasm, we shall proceed 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 ceUs. Their structure, physical and chemical properties, and vital manifestations. 4. Nutrition of Tissues. a. Fluids bathing the tissues — Blood and Lymph. h. Manner in which fluids are brought into relation- ship with tissues — Circulatory System, c. Manner in which substances necessary for the tissues are supplied to these fluids — Respiratory System. Digestive System. Food, its nature £ind quantity. d. Chemical changes in the Tissues generally — Metabolism and Heat Production. e. Manner in which the waste products of tissues are eliminated — Excretion, Hepatic, JRenalj Pulmonary, Cutaneous. 5. Reproduction and Development. part I. 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 charactjeristics of life may best be gained. I. Structure. — Protoplasm is a semi-fluid transparent viscous substance. It usually occurs in small individual masses — 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, 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 fluid interstitial part. In all protoplasm, therefore,' there seems to be a certain amount of organisation, and in certain cells this organisation becomes very marked indeed. II. Physiology. — A knowledge of the essentials of the physiology of protoplasm may be gained by studying the vital manifestations of one of the simplest of living things, the yeast plant (Saccharomyces Cerevisse). 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 sub- stance which has all the characters of protoplasm. The physiology of this protoplasm may be studied by placing a few torulse in a solution, containing glucose, CgHiaOe, and urea, CONgH^, with traces of phosphate of soda, NaaHPO^, and sulphate of soda, NagSO^. 4 PROTOPLASM. If the vessel is kept all night in a warm place the clear solution will in the morning be seen to be turbid. An ex- amination of a drop of the fluid shows that the turbidity is due to the presence of myriads of torulse. In a few hours the few torulse placed in the fluid have increased many hundredfold. The whole mass of yeast has grown in amount by the growth and multiplication of the individual torulaa. Fig. 1. — (a) Foam structure of mixture of olive oil and cane sugar ; (6) Reticu- lated structure of Protoplasm ; (c) Reticulated structure of Protoplasm in cell of earth-worm (after Bt*TSCHLi). This power of growth and reproduction under suitable conditions is one of the chief characteristics of living matter. What are the conditions necessary for the manifestation of these phenomena of life in protoplasm ? 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 PHYSIOLOGY. 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 con- ditions, protoplasm is only potentially alive, and in this state it may remain for long periods without undergoing any change, as in the seeds of plants, bacteria in dust, &c. These conditions being present, in order that the growth of the yeast may take place, there must be : — (a.) A supply of material from which it can be formed. (&.) A supply of energy to bring about the construction. The chemical elements in protoplasm are carbon, hydrogen, 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. The energy is got by the breaking down of the sugar, CgHigOg, into alcohol, CgHeO, and carbon dioxide, COg. Such a breaking down of a complex into simpler molecules liberates energy, as is well seen when nitro-glycerine explodes by breaking into carbon dioxide, water, oxygen and nitrogen — 2(C3H,(N03)303)=6CO, + 5H,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 for building itself up. How does the living protoplasm bring about this Hberation of the potential energy of such substances as glucose ? At present the answer to this important question cannot be absolutely definite, but in all probability the yeast forms a substance which acts upon glucose, since, by applying pressure, to it, a fluid can be extracted which acts in the same way as the yeast itself We shall afterwards find that protoplasm, in many parts of the body, produces such substances, generally called Enzymes or Zymins, and that they play an important part in physiology. PROTOPLASM. Such complex substauces as sugar very readily break down and liberate energy. For instance, formate of lime breaks into carbonate of lime, carbonic acid and hydrogen, under different conditions. First, under the influence of certain bacteria, -minute living organisms resembling yeast in many particulars ; second, under the influence of some substance contained in the bacteria after they are killed ; and lastly, in the presence of finely divided iridium, rhodium, and ruthenium — i.e., the same change is brought about by a living organism, by a substance contained in it, and by a metal. Hence mere vibrations of molecules occurring in different ways and in different substances may be sufficient to bring about these changes, and however the change is brought about the result is to set free energy. Living yeast differs from these dead substances simply in the fact that it uses 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 disinte- grative 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 metabolism, constant chemical changes, and these changes are on the one hand destructive (Icataholic), on the other constructive {anabolic). Living matter thus differs from dead simply in this respect, that side by side with destructive changes, constructive changes are always going on. Hence our conception of living matter is not of a definite chemical substance, but of a substance never two seconds the same, constantly undergoing internal changes. 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 proto- PHYSIOLOGY. plasm as a complex substance, 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, histo- logical structure and gross configuration are, so to speak, the figures." The rate of these changes 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 eaxite, ; 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 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, and when failure of the supply or of the utilisation of the material used in construction occurs, the protoplasm dwindles and disintegrates. Death is sudden when the chemical changes are abruptly stopped, slow when the anabolic changes are interfered with. III. Chemistry. — It is impossible to analyse such an ever-changing substance, 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. The probable chemical formula of albumin, one of the chief substances derived from protoplasm, is CgoiHgaaNgjOggSa. Water is the most abundant constituent of protoplasm, amounting, as it does, to about 75 per cent. The solid materials, constituting the remaining 25 per cent., consist chiefly of a series of bodies closely allied to one another and called " chief substances " or Proteids. In addition to these, certain inorganic salts are found in the ash PROTOPLASM. when protoplasm is burned indicating the presence of Po- tassium and Calcium along with Phosphorus and Sulphur. Small and varying quantities of Fatty Substances, and of Carbohydrates, with traces of a number of other organic substances which need not here be enumerated, are also usually present. Proteids.^Of these substances the Proteids alone 'have to be considered here, since they constitute the really important part of the material. The Germans, in fact, talk of protoplasm as " living proteid matter," a misleading term, since the living protoplasm is so very different from dead proteids. White of egg may be taken as an example of such pro- teids dissolved in water with some salts. If the salts be separated, and the water carefully driven off at a low tempera- ture, a pure proteid is left. Physical Characters. — Proteids have a white, yellow, or brownish colour. In structure, they 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 proteids are soluble 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 alcohol and ether. When in solution or apparent solution, many of the pro- teids do not dialyse through an animal membrane. All proteids rotate the plane of polarised light to the left. Chemistry. — Proteids contain the following chemical elements : carbon, hydrogen, oxygen, nitrogen, and sulphur, in about the following percentage amounts : — C. H. N. s. o. 52 7 16 1 24 It is important to remember the amounts of nitrogen and carbon, since proteids 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 PHYSIOLOGY. 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 proteid of the white of egg is given simply to show how complex these substances are : — '^204^322-'^ 62*-'66^2- Our knowledge of the constitution of the molecule is still very imperfect, but a study of the decomposition products of the proteids seems to show that compounds of different tinds enter into their composition. The simplest bodies having the characters of proteids are the protamines which are found in the heads of spermatozoa. From the decomposition of these a set of bodies having six atoms of carbon in their molecule, and hence called the Hexone Bases, have been derived. The most important of these are : — Lysin, diamido-caproic acid. Arginin, diamido-valerianic acid linked to guanidin, and allied to creatin. (See p. 40.) Histidin, a substance of unknown constitution. These hexone bases are also formed in the decomposition of ordinary proteids, but in addition bodies belonging to the amido-acid series are liberated in varying proportions. Most of these are amido derivatives of the fatty acid series, but one is an amido derivative of the aromatic series. 1. Amido-caproic acid (Leucin). H H H H NHj I I I I I II H— C— C— C— C— C— C— 0— H H H H H H 2. Amido succinic acid. 3. Amido pyrotartaric acid. 4. Para-oxyphenyl-amido-propionic acid (Tyrosin). /\ H NH2 HO—/ \ I I II I I H H PROTOPLASM. 5. Amtnonia is also yielded in the decomposition of proteids. From these results it has been concluded that the proteids may be considered as built up of Protamine molecules yield- ing hexone bases with side chains of amido acids and aromatic compounds. As yet no scientific classification of the proteids has been accomplished, although it is highly probable that the arti- ficial classification founded on their solubilities will be found to be natural and scientific, depending on the size of the molecule. 1. Original Proteids. — These proteids, either alone, or combined with certain other substances, are constant ingre- dients of dead protoplasm, and of the fluid constituents of the body. They are distinguished from all other proteids by being coagulated on heating. There are two groups — Globulins and Albumins — the former characterised by being insoluble in distilled water, by requiring the presence of a small quantity of a neutral salt to form a solution, and by being precipitated from solution by half saturating with sulphate of ammonia. 2. Proteoses (Proteids with a less complex molecule than Albumins and Globulins). — They may be formed from albumins (albumoses) and globulins (globuloses), by the action of superheated steam, and during digestion. Under the influence of these agents, the complex molecule splits into simpler molecules and takes up water. These proteoses form a series between the original proteids on the one hand, and the peptones or simplest proteids on the other. They may be divided into two classes. (a.) Those nearly allied to the original proteids. Proto- proteoses, which are precipitated in a saturated solution of common salt, NaCl. (6.) Those more nearly allied to the peptones. Deutero- proteoses, which are not precipitated in a saturated solution of NaCl, but are precipitated by a saturated solution of sulphate of ammonia. 3. Peptones. — These are the ultimate products of the action of digestive juices in proteids. Their characteristic PHYSIOLOGY. reaction is their solubility in hot saturated sulphate or ammonium solution. They diffuse very readily through an animal membrane. If the simple peptone molecule is represented by a_ single ring, the relation of the other members of the series may be roughly represented as follows : — 0000 0000 00 00 Native Proteids. Protoproteoses. 00 Deuteroproteoses. Peptones. Proteids have a great tendency to link with other sub- stances to form a series of bodies which may be called the Conjugated Proteids. The most important of these are the Nucleins, so called because their existence was first demon- strated in the nuclei or central parts of the cells of the body. These substances may readily be split into a proteid part and into nucleic acid, a phosphorus containing material of definite composition, having an acid reaction, and containing about 10 per cent, of phosphorus. In certain places the amount of nucleic acid is large in proportion to the proteid, in others it is small. The term nuclein is usually confined to the former, nucleo-albumin tci the latter of these. From the pure nucleic acid which occurs along with protamine in the heads of sper- matozoa to the proteids free of phosphorus there is a con- tinuous series. Nucleic acid when decomposed yields phosphoric acid and a series of bodies called the Purin bodies which belong to the class of diureides, and consist of two more or less modified urea molecules linked together by the radicle usually of acrylic acid (see p. 34i8). {The tests for the Pwteids 'must he studied practically.) SECTION If. The Cell. Protoplasm occurs in the animal body as small separate masses or Cells. These vary considerably in size, but, on an average, they are from 7 to 20 micro-millemetres. The advantage of this sub-division is obvious. It allows nutrient matter to reach every particle of the protoplasm. In all higher animals each Cell has a perfectly definite structure. It consists of a mass of protoplasm, in which is situated a more or less defined body, the nucleus. Protoplasm. — This has the structure already described under protoplasm, and in different cells the reticulum or cyto- mitoma is differently arranged. In some cells there is a condensation of the reticulum, round the periphery, to form a sort of cell membrane. At some point, in the protoplasm of many cells, one or two small spherical bodies, the centrosomes, are found, from which rays pass out in different directions. For the detec- tion of these bodies special methods of staining and the use of very high magnifying powers are required. They will be again considered when dealing with the reproduction of ceUs. The cell protoplasm frequently contains granules, and these may be classified in two groups : — I. Granules forming part of protoplastn. These granules occur at the nodes of the reticulum. They may be demon- strated by various stains for which they have a special affinity. (a.) Oxyphil granules. These have a special affinity for anilin stains having an acid reaction, such as eosin. (6.) Basophil granules. These react to stains of a basic or alkaline nature, such as methylene blue. 13 14 PHYSIOLOGY. (c.) Neutrophil granules. These react to both acid and neutral stains. II. Granules not forming part of the protoplasm. ■ These may be divided into 1. Granules of material to be built into the protoplasm. Of these the most important are : — (a.) Fat Granules which may be demonstrated by stain- ing with osmic acid which blackens fat. Fig. 2. — (a) White Cell from blood to show the centrosome and nucleus ; (6) Egg Cell dividing shows reticulated structure of Protoplasm, two centro- somes and nuclear fibres in mitosis. (6.) Granules composed of small organisms or other , material taken up by the cell. 2. Granules formed by the protoplasm : — (rt.) Pigment granules. These vary in colour, some, as in certain cells, especially about the eye, being black THE CELL. 15 (meliinin), others, as in the liver cells in certain conditions, being of a brown and yellow colour. (6.) Zymogen and mucinogen granules. These will be studied afterwards. In the protoplasm, vacuoles are sometimes found, and from a study of these vacuoles in protozoa, it appears that they are often formed round material which has been taken into the protoplasm, and that they are filled with a fluid which can digest the nutritious part of the ingested particles. In certain cells protoplasm undergoes changes in shape. This may be well studied in the white cells in the blood of the frog or newt. Processes are pushed out, and these are again withdrawn, or the whole cell may gradually follow the process, and thus change its position. The processes are called pseudopodia (false feet), and the mode of movement, from its resemblance to that seen in the amoeba is called amoeboid. The part played by reticulum and hyaloplasm in these movements is not clearly understood. The pseudopodia are at first free of reticulum ; but whether the hyaloplasm is pressed out by contraction of the reticulum, or whether it actively flows out, we do not at present know. In some cells among the protozoa movements take place along some definite line, and the reticulum is arranged more or less parallel to the line of movement. Such contractile pro- cesses, from their resemblance to muscles, have been termed myoids. In other protozoa the pseudopodia may manifest a to-and-fro rhythmic waving movement which may cause the cell to be moved along, or may cause the adjacent fluid to move over the cell. Such mobile processes when per- manent have been called cilia. The activity of these movements is modified by the various factors which act upon the activity of the chemical changes in the protoplasm. Thus cooling diminishes, and finally stops them. Gentle heat increases them, but when a certain temperature is reached, they are stopped. Drying and various drugs, such as chloroform, quinine, &c., also arrest the movements. These movements are further largely governed by the condition of the surrounding medium. When an attracting i6 PHYSIOLOGY. or repelling influence acts at one side of the cell — unilateral stimulation — it may lead to movement of the cell towards it or away from it. These movements are produced by various chemical substances (chemiotaxis) or by light (phototaxis) or by electricity (galvanotaxis). If the action is towards the stimulus, it is said to be 'positive, if away from it negative. Both phenomena are well seen in the action of light upon different micro-organisms, some being attracted, others re- pelled. B. — Nucleus. Structure.— The nucleus, seen with a moderate magnify- ing power, appears in most cells as a well-defined circular or oval body situated towards the centre of the cell. (Figs. 2 and 3.) Sometimes it is obscured by the surrounding protoplasm. It has a granular appearance, and usually one or more clear refractile bodies — the nucleoli — are seen within it. It stains deeply with many reagents of a basic reaction, such as haematoxylin, carmine, methylene blue, &c. In some cells the nucleus is irregular in shape, and in some it is broken up into a number of pieces, giving the cell a multi-nucleated character. It is usually composed of a mass of fibres arranged in a complicated network, and it is these fibres which have a special aflfinity for basic stains. Between these fibres is a more fluid material which may be called the nuclear plasma. Digestion in the stomach removes the nuclear plasma, but leaves the network unacted upon. The nuclear fibres are made up of two substances, a ground substance or matrix, which has no affinity for colour- ing matter, and which is hence called achromatin substance; and a substance with a marked affinity for various dyes, set in this in a series of particles variously arranged, and distinguished as chromatin. It is the staining of this material which colours the nucleus so deeply. Tlie chromatin substance contains a large amount of nucleic acid, and its richness in phosphorus has been demon- strated by treating the cells with ammonium molybdate and THE CELL. 17 pyrogallol, wHich colours parts rich in phosphorus of a brown or black colour. The nuclear fibres vary in their arrangement in different cells. Usually they form a network, but occasionally they are disposed as a continuous skein. In nuclei, wi^h the former arrangement of fibres, swellings may be observed Avhere the fibres unite with one another — the nodal siiOeUings. The nucleoli are probably not all of the same nature, and*' some may be simply specially well-marked nodal swellings. The resting nucleus appears to be surrounded by a distinct nuclear membrane, which is, however, probably really a basket-like interlacement of the fibres at the periphery. Functions. — The part taken by the nucleus in the general life of the cell is not fully understood. 1st, It exercises an influence on the nutritive processes, since it has been observed in certain of the large cells in lower organisms, that a piece of the protoplasm detached from the nucleus ceases to grow, and, after a time, dies. Important inter- changes of material go on between the nucleus and the protoplasm. 2nd, It is the great reproductive organ of the cell, probably playing an important part in transmitting inherited characters. Reproduction of Cells — Mitosis. — Cells do not go on growing indefinitely. When they reach a certain size they generally either divide, to form two new cells, or they die and undergo degenerative changes. The reason of this is possibly to be found in the well-known physical fact that, as a sphere increases in size, the mass increases more rapidly than the periphery. Hence, as a cell becomes larger and larger, the surface for nourishment becomes smaller and smaller in relationship to the mass of material to be nour- ished. Probably the altered metabolism so produced sets up the changes which lead to the division of the cell. These changes have now been very carefully studied in a large number of cells, and it has been shown that the nucleus takes a most important part in division. In a cell about to divide, the first change is a general enlargement of the nucleus. Then the centrosome becomes double, and the two portions travel from one another. i8 PHYSIOLOGY. forming tbe polar bodies, but remain united by delicate lines to form a spindle-shaped structure. These lines may be actual fibres or they may be lines of movement in the protoplasm. The spindle passes into the centre of the nucleus, and seems to direct the changes in the reticulum (Fig. 3 a). The nuclear membrane disappears, and the nucleus is thus not so sharply marked off from the cell proto- plasm. The nucleoli and nodal points also disappear, and with them all the finer fibrils of the network, leaving only the stouter fibres, which are' now arranged either in a Fig. 3.— Nucleus in Mitosis, (a) Shows Monaster stage ; (6) Shows completed divisiim. skein or as loops with their closed extremity to one pole of the nucleus, and their open extremity to the other. The nucleus no longer seems to contain a network, but appears to be filled with a convoluted mass of coarse fibres, and hence this stage of nuclear division is called the convoluted stage. The nuclear loops of fibres break up into short, thick pieces ; and these become arranged around the equator of the spindle in a radiating manner so that when the nucleus THE CELL. 19 is viewed from one end, it has the appearance of a rosette or a conventional star. This stage of the process is, hence, often called the single star or 'monaster stage (Fig. 3 a). Each loop now splits longitudinally into two, the divisions lying side by side. The spindle continues to grow until it occupies the whole length of the nucleus. The two centrosomes are now very distinct, and from them a series of radiating strise extend out into the protoplasm of the cell (Fig. 2 'V). The next change consists in the separation from one another of the two halves of the split loops, — one half of each passing up towards the one polar body, the other half passing towards the other. It is the looped parts which first separate and which lead the way — the open ends of the loops remain- ing in contact for a longer period, but, finally, also separating. In this way, around each polar body, a series of looped fibres get arranged in a radiating manner, so that the nucleus now contains two rosettes or stars, and this stage of division is hence called the dyaster stage. The single nucleus is now practically double. Gradually in each half finer fibres develop and produce the reticular appearance. Nuclear nodes, nucleoli, and the nuclear mem- brane appear, and thus two resting nuclei are formed from a single nucleus. Between these two nuclei a delicate line appears, dividing the cell in two, and the division is accomplished (Fig. 3 h). The network of the nucleus of actively dividing cells is rich in nucleic acid, but in cells which have ceased to divide, in which the nucleus has ceased to exercise its great repro- ductive function, the amount of phosphorus — i.e., of nucleic acid, diminishes, and may be actually less than the amount in the cell protoplasm. SECTION III. The Tissues. From the protoplasm of the cells the various tissues of the body — skin, bone, cartilage, &c., are formed. The structure of these must be studied practically ; all that will be attempted here is to indicate how they are formed from the primitive cell. The human body is originally a single cell, and from this, by division, a mass of simple cells is produced. ' In the embryo, these cells get arranged in three sets — an outer, a middle, and an inner — the epiblast, mesoblast, and hypo- blast. (A.) THE VEGETATIVE TISSUES. The Vegetative Tissues are those which support, bind together, protect and nourish the body. They may be divided into the Epithelial Tissues, formed from the epiblast or hypoblast, and consisting of cells placed upon surfaces, and the Connective Tissues developed from the mesoblast, and consisting chiefly of formed material between cells. Epithelium. 1. Stratified Squamous Epithelium. — The skin and the lining membrane of the mouth and gullet are covered by several layers of cells. The deeper cells divide, and as the young ones get pushed upwards towards the surface, and away from the nourishing fluids of the body, the chemical changes are interfered with, and the protoplasm undergoes a change into a body closely allied in composition to the proteids, keratin. This substance is a hard, horny material, which is 20 THE TISSUES. well seen in the nails and hair, and in the horns and hoofs of certain animals. It first makes its appearance as a number of little masses or granules in the cells, and these run together to fill the cells, which, from pressure, become flattened out into thin scales. Fig. 4. — Stratified squamous epithelium from the cornea. Keratin forms an admirable protectiYe covering to the body, not only on account of its hardness and toughness, but because poisons cannot readily pass through it, and also, because it is not easily acted on by chemicals. Like the pro- teids it contains carbon, hydrogen, oxygen, nitrogen, and sulphur ; and the first four of these elements are in about the same proportion as in the proteids. But the sulphur is in greater proportions (3 to 5 per cent.), and readily enters into combination with various substances. Hence, lead solu- tions, which give the black sulphide of lead, colour keratin black and are largely used to dye the hair. A slightly modi- fied stratified squamous epithelium lines the urinary passages. 2. Columnar Epithelium. — The inmost set of cells in the embryo, lining the stomach and intestine, elongate at right angles to their point of attachment, and become columnar in shape. At the free border of the cells there is a peculiar layer, which looks like a double hem, and which is made up PHYSIOLOGY. of a series of short rods placed side by side. Probably this is a special development of the reticulum of the protoplasm. The great function of this form of epithelium is to take up the digested matter from the stomach and intestine, and to pass it on to the blood (Fig. 5 a). Uilllllllilllll'li mmm Fig. 5. — (a) Columnar Epithelium from the small intestine ; (6) Ciliated Epithelium from the trachea. Among these columnar cells a certain number of peculi- arly modified cells, chalice cells, are always found. They are larger than the columnar cells, and somewhat pear-shaped, being attached by their small extremity. Their protoplasm is collected at their point of attachment, while the body of the cell is filled with mucin, a clear, transparent material. 3. Glandular Epithelium.^ — A number of cells, having for their function the production of some material which is to be excreted from the cell and arranged as the lining of a depression, constitute a gland. Fie. 6. — A zymiu-secreting gland to show duct and acinus lined with secreting cells containing zymogen granules. THE TISSUES. 23 The simplest form of gland is the simple tubular — a test- tube-like depression, lined by secreting cells. Instead of being simple, the tube may be branched, when the gland is said to be racemose. In many glands the secreting epi- thelium is confined to the deeper part of the tube, the alveolus or acinus (Fig. 6), while the more superficial part is lined by cells which do not secrete, forming the duct. In many situations several simple glands are grouped together, their ducts opening into one common duct, and a compound gland results. Secreting epithelium varies according to the material it produces. (A.) Mucin Secreting Epithelium. — Many glands have for their function the production of mucin, a slimy substance of use in lubricating the mouth, stomach, intestine, &c. The acini containing such cells are usually large. The cells themselves are large, and are placed on a delicate basement membrane, which bounds th© acinus. The nucleus is situated near to the attached margm of the cell. The cells are somewhat irregular in shape, and are packed close together. Their appearance varies according to whether the gland has been at rest or has been actively secreting. In the former case, and in the fresh condition, the cells are large, and pressed closely together. Their protoplasm is filled with large shining granules. After treatment with reagents, each cell becomes distended with clear, transparent mucin, formed by the swelling and coalescence of the granules. After the gland has been actively secreting, the cells are smaller and the granules much less numerous, being chiefly situated at the free extremity of the cell, and leaving the nucleus much more apparent. This form of -epithelium, during the resting condition of the gland, takes up nourishing matter and forms this mucin- yielding substance. . During the active state of the gland, the mucin-yielder is changed to mucin, ami is extruded from the cells into the lumen of the gland. Mucin is a substance of great importance in the animal economy. When precipitated and freed from water it is white and amorphous. On the addition of water it swells 24 PHYSIOLOGY. up and forms a glairy mass. In the presence of alkalis, it forms a more op less viscous solution, and froin this solution it is precipitated by acetic acid. In composition it is a proteid linked to a molecule allied to the sugars, and hence when boiled with an acid it yields a sugar. In adult life the great function of mucin is to give to certain secretions a slimy character which renders them of value as lubricants. (B.) Zymin Secreting Epithelium. — Another form of secreting epithelium of great importance, is that which forms the various juices which act upon the food to digest it. These juices owe their activity to the presence of enzymes or zymins. A zymin-forming gland after a prolonged period of rest shows cells closely packed together, so that it is difficult to make out their borders. The protoplasm is loaded with granules which are much smaller than those seen in the mucin-forming cells, and which do not swell up in the same way, under the action of reagents. The nucleus is often obscured by the presence of these granules. When the gland has been actively secreting, the granules become fewer in number, and are confined to the free ex- tremity of the cell ; they are obviously passing out. The cell becomes smaller, and its outlines are more distinct and the nucleus more apparent. The granules which fill the cells are not composed of the active enzyme. If extracts of the living cells be made, they are inert, and it is only after the granules have left the cell, or are in the process of leaving that they become active. Hence, the granules are said to be composed of zymin-form- ing substance or zymogen. The series of changes are parallel to those described in the mucin-forming cells. During the so-called resting state of the gland, the cells are building up zymogen. When the gland is active, the cells throw off the material they have accumulated, and it undergoes a change to zymin. (C.) Excreting Epithelium does not manufacture materials of use in the animal economy, but passes waste substances out of the body. Such epithelium is seen in the kidneys, THE TISSUES. 25 sweat glands, sebaceous glands, mammary, glands, and per- haps in the liver. The cells are composed of a granular pro- toplasm, in which the presence of the material to be excreted either in its fully elaborated condition, or in process of preparation, may frequently be demonstrated — e.g., fat globules, iron containing particles, &c. These cells do not merely take up material from the blood and pass it out, but they may profoundly alter it before getting rid of it. 4. Ciliated Epithelium is another special development of the cells of the outmost and inmost layer of the embryo. The cells are usually more or less columnar, and the free border is provided with a series of hair-like processes, the cilia, which vary in size in different situations. In the living state, the cilia are in constant rhythmic motion, each cilium being suddenly whipped or bent down in one direction, and then again assuming the erect position. All the cilia of one surface work harmoniously in the same direction, aad the movement passes from the cUia of one cell to those of the next in regular order, beginning at one end of the surface and passing to the other. As a result of this constant harmonious rhythmic move- ment, any matter lying upon the surface is steadily whipped along it ; and since the ciUa usually work from the inner parts of the body to the outside, this matter is finally ex- pelled from the body. The movements of the cilia are dependent on the changes in the protoplasm, and everything which influences the rate of chemical change modifies the rate of ciliary movement. Connective Tissues. 1. Mucoid Tissue. — The cells of the niesoblast of the embryo, which at first lay in close apposition with one another, become separated, remaining attached by elongated processes. Between the cells, a clear, transparent, viscous mucin-like substance makes its appearance. This tissue is widely distributed in the embryo as a precursor of the con- nective tissues, and after birth it is still to be seen in the 26 PHYSIOLOGY. pulp of a developing tooth and in the vitreous humour of the eye (Fig. 7). Fig. 7. — Mucoid Tissue from an embryo rabbit. 2. Fibrous Tissue. — As development advances, the cells of mucoid tissue elongate and become spindle shaped, and are continued at their ends into fibres (Fig. 8). These cells are often called fibroblasts. The connec- tive tissues are thus clearly- distinguished from the Epithelia by having the formed ipaterial helween and not in the cells. Fig. 8. -Fibroblasts from young fibrous tissue. They are composed of the following parts : I. Formed material. (a) Fibres. (b) Matrix. II. Cell Spaces (Connective Tissue Spaces). III. Cells. I. Formed Material.— (A.) Fibres (Fig. 9) — 1st, Non- elastic (white fibres). These are delicate, transparent .fibrils arranged in bundles, which do not branch, and which have a mucin-like matrix between them. They are composed of a non-elastic substance, collagen. This is closely allied to the THE TISSUES. 17 proteids, and gives the proteid reactions faintly, but it does not yield tyrosin when decomposed, while it does yield amido-acetic acid. It is insoluble in cold water, but swells up and becomes transparent in acetic acid. It has a great afiSnity for carmine and stains a pink colour with it. When boiled, it takes up water to form a hydrate. Gelatin ; a substance soluble in hot water, and forming a jelly on cooling. 2nd. Elastic Fibres. These are highly refractile elastic fibres, which branch and anastomose with one another. They are composed of Elastin, a near ally of tbe proteids which is insoluble both in cold and in hot water, and is not Fig. 9. — Bundles of White Fibres with Fibroblasts and Elastic Fibres anastomosing with one another. acted on by acetic acid. It stains yellow with picric acid, and has no affinity for carmine. (B.) Interstitial Substance. — This is composed of mucus- like material. According to the arrangement of these fibres, and to the preponderance of one or other variety, various types of Fibrous Tissue are produced. When a padding is required, as under the skin and under mucous membranes, the fibres are arranged in a loose felt work to constitute Areolar Tissue. In fascia, in tendon sheaths, and in flat tendons, the fibres are closely packed together to form more or less definite 28 PHYSIOLOGY. layers. In tendons and ligaments the fibres run parallel and close together. In ordinary tendons, where no elasti- city is required, the fibres are of the white or non-elastic variety. In ligaments, when elasticity is desirable, the elastic fibres preponderate. One peculiar modification of fibrous tissue is often described as a special tissue under the name of Lymph Tissue. It is composed of a delicate network of white fibres, the interstices of which communicate with lymphatic vessels, and contain masses of simple protoplasmic, cells, leucocytes, often in a state of active division. So numerous are these that it is impossible to make out the network under the microscope, until they have been removed by washing. Lymph tissue is very widely spread throughout the body, and is of great importance in connection with nutrition. II. The spaces of fibrous tissue vary with the arrangement of the fibres. In the loose areolar tissue under the skin they are very large and irregular, in fascia they are flattened, while in tendon where the fibres are in parallel bundles they are- long channels. III. The cells of fibrous tissue (Fibroblasts) vary greatly in shape. In the young tissue they are elongated spindles, from the ends of which the fibres extend. In some of the loose fibrous tissues they retain this shape, but in the denser tissues they get squeezed upon, and are apt to be flattened and to develop processes thrust out into the spaces. In certain situations, peculiar modifications of Connective Tissue cells are to be found. (A.) Endothelium. — When cells line the larger connective tissue spaces they become flattened, and form a covering membrane, called an endothelium. Such a layer lines all the serous cavities of the body, and the lymphatics, blood vessels, and heart, which are all primarily large connective tissue spaces. To demonstrate the outlines of these cells it is necessary to stain with nitrate of sUver, which has a special afiinity for the interstitial substances, and which thus forms a series of black lines between the cells. (B.) Fat Cells. — In the areolar tissue of many parts of the body fat makes its appearance in the cells round the smaller THE TISSUES. 29 blood vessels, and when these cells occur in masses Adipose Tissue is produced. Little droplets of oil first appear, and these become larger, run together, and finally form a large single globule, distend- ing the cell, and pushing to the sides the protoplasm and nucleus as a sort of capsule (Fig. 10). Fig. 10.- -Fat Cells stained with osuiic acid, and lying alongside a small blood vessel. If the animal be starved, the fat gradually disappears out of the cell, and in its place is left a clear albuminous fluid, which also disappears, and the cell resumes its former shape. The fats are ether derivatives usually of the triatomic alcohol glycerin — ( OH C,H, \ OH (OH by the replacement of the hydrogen of the hydroxyl molecules by the radicles of the fatty acids. The most abundant fatty acids of the body are : — Palmitic Acid, CigHgiO.OH. Stearic Acid, Ci8H350,OH. Oleic Acid, CigHggO.OH. and from these the three fats — Palmitin 0,11,(0, C,,li,fi,), =G,Jl,fi,. Stearin, Olein are produced. 30 PHYSIOLOGY. It will be observed that the molecules of these fats are very rich in carbon and hydrogen, and very poor in oxygen — i.e., they contain a large amount of material capable of being oxidised, and thus capable of affording energy in the process of combustion. The fats resemble one another in being insoluble in water, but soluble in ether and in hot alcohol. As the alcohol cools, they separate out as crystals. They differ from one another in their melting point, palmitin melting at the highest and olein at the lowest temperature. Fat which is rich in palmitin and stearin, like ox fat, is thus hard and solid at the ordinary temperature of the air, while fats rich in olein, like dogs' fats, are semi-fluid at the same tempera- ture. The olein acts as a solvent for the fats of a higher- melting point. The functions of adipose tissue are twofold : — \st Mechanical. — The mass of adipose tissue under the skin is of importance in protecting the deeper structures from injury. It is a- cushion on which external violence expends itself Further, this layer of subcutaneous fat pre- vents the loss of heat from the body, being, in fact, an extra garment. Itid Chemical. — Fat, on account of its great quantity of unoxidised carbon and hydrogen is the great store-house of energy in the body. (C.) Pigment Cells. — In various parts of the eye the con- nective tissue and other cells contain a black pigment. Melanin. The precise chemistry and mode of origin of this pigment is not known. It contains carbon, hydrogen, nitrogen, oxygen, and it may also contain iron. It may be formed directly in the cells, or it may be produced by the cells from the pig- ment of the blood. Its function in the eye is to prevent the passage of light through the tissues in which it is con- tained. The cells containing the pigment are branched and in many ' cases they possess the power of movement. This is specially well seen in such cells in the skin of the frog, where con- traction and expansion may be easily studied under the microscope. By these movements the skin, as a whole, is THE TISSUES. 31 made lighter or darker in colour. The movements of these cells are under the control of the central nervous system. 3. Cartilage. — While fibrous tissue is the great binding medium of the body, support is afforded in foetal life and in certain situations in adult life by cartilage. When cartilage is to be formed, the embryonic cells become more or less oval, and secrete around them a clear pellucid capsule. This may become hard, and persist through life as in the so-caUed parenchymatous cartilage of the mouse's ear. (1.) Hyaline Cartilage. — Development, however, usually goes further, and before the capsule has hardened, the cartil- age cells again divide, and each half forms a new capsule which expands the original capsule of the mother cell, and thus increases the amount of the formed material. This formed material has a homo- geneous, translucent appear- ance, and a tough and elastic consistence, and cuts like cheese with the knife (Fig. 11). The formed material of cartilage is not a special sub- stance, but a mixture of chon- droitin-sulphuric acid with collagen in combination with , proteids. Chondroitin when decomposed yields glucosamine, a sugar-like substance containing nitrogen, and glycuronic acid. Cartilage is surrounded by a fibrous membrane, the peri- chondrium, and no hard and fast line of demarcation can be made out between them. The fibrous tissue gradually becomes less fibrillated — the cells become less elongated and more oval, as if the interfibrillar substance increased in amount and became of the same refractive index as the fibres. During old age, a fibrillation of the homogeneous- looking cartUage is brought out, especially in costal cartilage, by the deposition of lime salts in the matrix, between the fibres. It was long ago shown that in inflammation of Fig. 11. — Hyaline Cartilage covered by perichondrium. 32 PHYSIOLOGY. cartilage, this fibrillation appears ; and by digesting in salt solution, a similar structure may be brought out. The close connection of cartiliage with fibrous tissue is thus clearly demonstrated. . Such homogeneous or hyaline cartilage precedes most of the bones in the embryo, and covers the ends of the long bones in the adult (articular cartilage), forms the framework of the larynx and trachea, and constitutes the costal carti- lages. : ' , . 1 ■ (2.) .Elastic Fibro-Gartilage. — In certain situations, a specially elastic form of cartilage is developed — e.g., in the external ear, .elastic : fibres appearing in the cartilaginous matrix, and forming a network! through it. (3.) White Fibro-Gartilage. — In other situations — e.g., the intervertebral discs, a combination of the binding action of fibrous tissue with the padding action of cartilage is required ; 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 cartil- age, and is really a mixture of two tissues — white fibrous tissue and cartilage. 4. Bone. — The great sup- porting tissue of the adult is Bone. This is formfed of a deposition of lime salts in layers or lamellae of white fibrous tissue ; but while some bones, as those of the cranial vault, face and clav- icle, are produced entirely in fibrous tissue ; others are preformed in cartilage, which acts as a scaffolding upon which the formation of bone goes on. Intra membranous bone Fig. 12. — Intra-Membranous bone de- velopment in the lower jaw of a foetal cat. Above, the process of ossification is seen shooting out along the fibres, and on the lower surface the process of absorption is going on. Three osteoclasts — large multi-nucleated cells — are shown. THE TISSUES. 33 develop-ment. — This may be well studied in any of the bones of the cranial vault where cartilage is absent (Fig. 11)- At the centre of ossification the matrix between the fibres becomes impregnated with lime salts, chiefly the phosphate and carbonate. How this deposition takes place is not known ; and how far it is dependent on the action of cells has not been clearly determined. As a result of this, the connective tissue cells get enclosed in definite spaces, lacunoB, and become hone corpuscles. Narrow branching channels of communication are left between these lacunae, Fig. 12. — Intra-Cartilaginous Bone Development. 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 vascularization of the cartilage by the invasion of periosteum ; and the calcification of the cartilage round the spaces. the canaliculi. This deposition of lime salts spreads out irregularly from the centre into the adjacent fibrous tissue. The fally formed adult bone, however, is not a solid block, but is composed of a compact tissue outside, and of a spongy bony tissue, cancellous tissue inside. This cancellous tissue is formed as a secondary process. Into the block of cal- careous matter, formed as above described, processes of the fibrous tissue, with blood vessels, lymphatics, and numerous cells, burrow. 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. 34 PHYSIOLOGY. 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. It is by the extension of the calcifying process outwards, and the burrowing out of the central part of the bone that the diploe and cancellous tissue of bones is produced. Intra-cartilaginous bone development. — In the bones preformed in cartilage, the process is somewhat more com- plex, 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 produced, a minute model is formed in hyaline car- tilage in the embryo, and this is surrounded by a fibrous covering, the perichondrium. In the deepest layers of this perichondriuin the process of calcification takes place as described above, and spreads outwards, thus encasing the cartilage in an ever thickening layer of bone (Fig. 12). At the same time, in the 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. Into these spaces processes of the perichondrium bore their way carrying with them blood vessels, and thus rendering the cartilage vascular. The vascularisation of the centre of the cartilage having been effected, the process of absorption extends towards the two ends of the shaft of the bone. 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 trabeculse of cartilage between them. Into these canals the processes of the periosteum extend, and fill them with its fibrous tissue. In this fibrous tissue, the deposition of lime salts takes place upon the trabeculse, enclosing cells, 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 reduced to a solid mass of calcified tissue.- But this does not occur. For, as rapidly as the trabeculse become calcified, they get absorbed, while the active changes extend farther THE TISSUES. 35 and farther from the centre, which is thus reduced to a space filled by fibrous tissue which afterwards becomes the bone marrow (Fig. 12). The process of absorption does not stop at the original block of cartilage, which is all removed. After all of this has been absorbed, the bone formed round the cartilage 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. 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 connective tissue, blood vessels, lymphatics, and nerves, with layer upon layer of bone concentrically arranged around it. This constitutes an Haver- sian system (Fig. 13). One important function per- formed by the cartilage is in bringing about the increase in length of the bones. In addition to the centre of ossifi- cation in the shaft, at each end of the bone one or more similar centres of ossification form. These are the epiphyses. Be- tween these and the central rod of bone — the diaphysis — a zone of cartilage exists until near adult life, when the bones stop growing. In this zone the cells arrange themselves in vertical rows, divide at right angle to the long axis of the bone, and form cartilage. This cartil- age 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 Fig. 13. — Cross Section through part of the shaft of a long boue to show the arrangement in larncelse distributed as Haver- sian, interstitial, peripheral, and medullary. 36 PHYSIOLOGY. early adult life, when epiphyses and diaphysis join and 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. (5.)— 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 thus be called the Master Tissues, and it is as the servants of these tissues that all the others 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 forty- two 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 meiaboHsm of the body is concerned, muscle is the master tissue. For muscle we 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, respiration, circulation, and excretion have to be studied. I. MUSCLE. The two great functions of muscle are : — To perform mechanical work. To liberate heat. The study of the physiology of muscle may be divided into : — 1. The physical and chemical characters of muscle at rest. THE TISSUES. 37 2. The modes of making muscle contract. 3. The changes which take place in muscle during contraction. 4. The source of the energy evolved by muscle. 5. Death of muscle. 1. Physical a.nd Chemical Characters of Muscle AT Rest. The first trace of the evolution of muscle is found among the infusoria, where, in certain cells in part 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. Structure of Muscle. — Even a cursory examination of mammalian muscles shows that those of the trunk and limbs, skeletal muscles, are different froin 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 cytomi- toma. They thus become spindle-shaped cells, varying in length from about 50 to 200 micro-millimetres. The covering membrane, sarcolemma, is thin, but tough and elastic, and 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 cytomi- toma (Fig. 14 a). The skeletal muscles develop from a special set of meso- blast cells, early differentiated as the muscle plates in the mesoblast down each side of the vertebral column of the embryo. Each cell elongates. The nucleus divides across, but the cell, instead of also dividing, elongates, and continues to elongate as the two daughter nuclei again divide. The cytomitoma becomes arranged longitudinally, and a series of 38 PHYSIOLOGY. transverse markings appear across the cell. Lastly, a cover- ing develops, and the fully-formed fibre is produced (Fig. 14 &). h. Fig. 14.— (a) Fibres of Visceral Muscle ; (i) Fibres of Skeletal Muscle to show sarcolemma, muscle corpuscles, and sarcous substance composed of fibrils showing transverse markings. This consists of three parts — 1. The Sarcolem;na is a delicate, tough, elastic membrane closely investing the fibre, and attached to it at Dobie's lines. 2. The Muscle corpuscles consist of little masses of proto- plasm -with a nucleus which lie just under the sarcolemma. 3. The Sarcous substance is made up of a series of longi- tudinal fibrils, consisting of alternate dim and clear bands — the former staining deeply with eosin. In the middle of the clear band is a narrow dim line, Dobie's lixxQ.. 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 Ijght except in one position of the analysing prism, while the dim band allows polarised light to pass, whatever be the position of the prisms. The probable explanation of these facts is, that 1st, The sarcous substance is made up, like other protoplasm, of a mitoma and plasma. 2nd, 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. 3rd, At Dobie's line there is a swelling on each fibril. THE TISSUES. 39 Various descriptions of the appearances of striped muscle in different animals are given by different authors, and many theories as to the interpretation of the appearances have been advanced ; but it is needless to discuss what is still so tmcertain. 2. Chemistry of Muscle. — Like all other hving tissues, muscle is largely composed of water — containing about 75 per cent. The 25. per cent, of solid constituents is made up of a small quantity, about 1 per cent., of ash, and 24 per cent, of organic substances. The ash consists of the phosphates and chlorides of potassium, sodium, magnesium, calcium, and iron. But part of the phosphoric acid is derived from the phosphorus of the nucleins of muscle. Of the organic constituents, by far the greater part is made up of Proteids. These may be divided into — (a.) Those soluble in neutral salt solutions. (h.) Those insoluble in them. (a.) The first class of bodies consist entirely of three globulins. Two of these^^Myosinogen and Paramyosinogen, have the peculiar property of clotting under certain condi- tions, of forming what is called myosin, and. this process, which occurs after death, is the cause of death stiffen- ing. The post-morte'm 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. (6.) The insoluble proteid 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 in carbonate of soda solution. In addition to the proteids, small quantities of other organic substances are found in muscle. Glucose (CgHijOg) is present in muscle as in all other tissues. Glycogen x (CsHioOg). — A substance closely allied to 40 PHYSIOLOGY. 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 these, see p. 284.) Fat is present in small quantities. Inosite, formerly called muscle sugar, is present in small amounts. It is not a sugar, but a hydroxy-benzene-hydride, in which each carbon atom holds on to a hydroxyl molecule on one side, and on the other to a hydrogen atom. Its significance is not known. C— OH HO-Cf<;^i^C — OH HO — ck^H^Jc-OH- c— on Sarcolactic Acid.— Hydroxy-propionic acid — H OH H— C— C— C— 0— H I I H H This isomer of ordinary lactic acid is increased in muscle dtiring activity and during death stiffening. 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 is a near ally of urea, being formed by replacing the with NH thus— H N li II H^N— C— NHj H^N— C— NHj Urea. Guanidin. THE TISSUES. 41 Methyl-guanidin is produced by replacing an H by CH3 — NHCH3 H^N— C— N— H If this is linked to acetic acid — H j « N CH3H II I I II H^N— C— N— C— C— OH is produced. I- H 3. The Colour of Muscle varies considerably, some muscles being very pale, almost white, in colour — e.g., 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, hcBftioglohin, but in certain muscles it is due to a peculiar set of pigments giving different reactions from the blood pigment. 4. Muscle is translucent during life, but, as death stiffen- ing sets in, it becomes more opaque. 5. 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 produced. 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 Exercise.) 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, at once act to bring about the 42 PHYSIOLOGY. desired movement, and no time is lost in preliminary tighten- ing. 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 ipuscle 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 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 being produced. 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 tonic contraction, 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. 6. Heat Production. — Muscle, like all other living proto- plasm, is in a state of continued chemical change, constantly undergoing decomposition and reconstruction. As a result of this chemical change heat is evolved. This is a matter of considerable importance, as it explains how, even when the muscles are at rest, the body is still kept at a comparatively high temperature. 7. Electrical Conditions. — Muscle when at rest is iso- electric, but if one part is injured, it acts to the rest like the zinc 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. THE TISSUES. 43 when the zinc and copper plates in a galvanic cell are connected, a current flows through the wire from copper to zinc (Fig. 26). 2. The Modes of making Muscle Contract. Skeletal muscle remains at rest indefinitely until stimu- lated to contract, usually by changes in the nerves. We desire to contract our biceps: certain changes occur in our brain, and these set up changes in the nerves passing to the biceps and the muscle contracts. Can skeletal muscle be made to contract without the intervention of nerves — can it be directly stimulated? To answer this, some means of throwing the nerves out of 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 stimu- lated the muscle no longer contracts. But, if the muscle be directly stimulated by any of the various agents to be after- wards 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 endings in the muscle, stimulation of the nerve still causes muscular Fig. 15.— Curare Experiment to contraction. Only when the ^^°'^ ^"»*i<= "^'^™" ^^^°^^^ ,, - to curare, but nerve endings curare is allowed to act upon protected on left aide. the muscle and the nerve end- ings 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 contract. This clearly shows that it is the nerve endings which are poisoned by curare, and 44 PHYSIOLOGY. that therefore the application of stimuli to the muscle must act directly upon the muscular fibres (Fig. 15). {Practical Exercise.) 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. It will afterwards be shown that in contraction disintegra- tive changes are increased, that katabolism is in excess of anabolism, and that, therefore, contraction may be regarded as an approach to death. Hence many conditions which cause contraction may also finally cause death. 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 Exercise.) 2nd. A sudden mechanical change such as may be produced by pinching, tearing, or striking the muscle will cause it to contract. (Practical Exercise.) 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 temperature is reached. This, however, is not a true living contraction. (Practical Exercise.) 4th. Muscle may also be made to contract by any sudden ' change in an electric current passed through it, whether the current be suddenly allowed to pass into it or suddenly cut out of it, or whether it is suddenly made stronger or weaker. (Practical Exercise.) This method of stimulating muscle is constantly used in medicine. It 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 2nd. From an induction coil. If a galvanic battery is used — (1.) On making or closing the current, and upon breaking or opening the current, a THE TISSUES. 45 contraction results. While the current is flowing through the muscle, the muscle usually 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 Exercise.) (2.) If a current be made weaker and weaker breaking ceases to cause a contraction, while making still produces it. That is, it requires a stronger current to cause a contraction on breaking than on making. (Practical Exercise.) (3.) The two poles do not produce the same effect. The negative pole or cathode — that coming from the zinc plate of the batterjr — 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 cathode on closing, at anode on opening. How can these facts be explained ? Electrotonus. — A study of the influence of the current on the muscle while it is passing through it throws important light on this point. (Practical Exercise.) While the current simply flows through the muscle no contraction is produced, but the excitability is profoundly modified. Round the cathode 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 cathode is in a state of cathelectrotonus, of increased excitability or of more unstable equiUbrium, while the part of the muscle under the influence of the anode is- in a state of anelec- trotonus, of decreased excitability or of more stable equili- brium. 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. If, for instance, 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 is suddenly 46 PHYSIOLOGY. made unstable as at the cathode on closing, disintegration or katabolic changes are apt to be set up and a contraction results. If, on the other hand, a house of cards is built and made extra stable by introducing some additional cards at the foundation, and if these cards be 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 disintegra- tion 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 cathode 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. When muscle is stimulated with induced electricity 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 I. disappearance of a flow of electricity I in the secondary coil. If, therefore, ' wires from the secondary coil are led off to a muscle, each change in the primary circuit causes the sudden and practically simultaneous appearance and disappearance of an electric cur- rent in the muscle, and this of course causes a contraction. But here the effects of closing and opening the current are practically fused, and hence the influence of the anode and cathode, or of closing and opening, need not be considered. {Practical Exercise.) It must, of course, be remembered, that the opening of the primary circuit produces a more powerful current in the Fig. 16. — Course of Electric Current in primary circuit (lower line), and in secondary cir- cuit (upper line) of an induction coil. Ob- serve that in the sec- ondary the make and break are combined. THE TISSUES. 47 secondary coil, and therefore a more powerful stimulation of the muscle than the closure of the circuit (Fig. 1 6). When the galvanic current is used to stimulate human muscles, the different action of the two poles is not so marked as in the excised muscle of the frog because the current flows not along but rather across the muscle, and thus, under each pole applied to the skin, we have on one side of the muscle the effect of an entering current — -anode — and on the other, of a leaving current — cathode. Hence with a strong current contraction occurs both on closing and on opening at both poles, but as the current is weakened n. / ( I < \ \ . . . I, '. H ^*\\. t t + t t + 4 -!. -i. i 4- i- MB Mb -J\ /v J\ __J\ : A. J\ /V_ ___A /V Fig. 17. — Electrical Stimulation of human muscle or nerve to show the passage of the current across the structure, and the consequent combination of effects under each pole. the contraction at the cathode on opening and at the anode on closing gradually disappear. When the muscle is in one stage of the degeneration which follows separation from its nerve, the anodal closing contraction becomes much exag- gerated. This is called the reaction of degeneration. (Practical Exercise.) 3. The Changes in Muscle during Contraction. I. Change in Shape. The most manifest change is an alteration in the shape 48 PHYSIOLOGY. Fig. of • the muscle. It becomes shorter and thicker. This anyone can see by studying their own biceps muscle. 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 iibrils the shortening and 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. These appearances may best be explained on the assumption that the fibrils are the part of the fibre which shorten and thicken, possibly by a flow of fluid into them, and that these fibrils chiefly 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 change takes place in the clear band seems to be indicated by the fact that it retains its reactions with polarised light and staining re- agents. Usually the contraction of a muscle occurs simultaneously in all the fibres. This is because every nerve fibre passes to muscular fibre, and sets them all in action together. When, as sometimes occurs in disease, the nerve fibres become implicated, 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 apart from its nerve, 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 18. — Contraction of Skeletal Muscle — re- laxed above, con- tracted below. A is a diagram of the change in u fibril ; B shows the shading of the clear band ; and G shows the absence of any altera- tion in the influence of the two bands on polarised light. THE TISSUES. .49 pectoralis .major in. the chest of. an emaciated individual. The rate at which the .wave , of contraction travels is ascer- tained 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 conditioii 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 Exercise.) 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 gun- powder. Contraction of Muscle . as a whole may best be studied under the following heads — 1st. The course of contraction. 2nd. The extent of contraction. 3rd. The force, of contraction.-. Course of Contraction. By attaching the muscle to a lever 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 in- duction coil with an electro-magnetic marker introduced with the primary circuit may be employed. 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 Exercise.) In this way such a tracing as is shown in Fig. 19 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 E 5° PHYSIOLOGY. period supervenes between the stimulation and the con- traction. In the muscle of the frog attached to a lever this usually occupies about T^^jth second ; but if the change in the muscle is directly photo- graphed without any lever being attached to it, this period is found to be very much shorter. The latent period is fol- lowed by the period of con- traction. At first it is sud- den, but it becomes slower and finally stops. Its aver- age duration in the frog's , muscle is about y^^th second. Fig. 19.— Trace of Simple Muscle Twitch showing periods of latency, contraction, and relax- ation, and a time record made with a tuning fork vibrating 100 times per second. 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 amount of contraction is primarily determined by the length of the muscle. If a muscle of two inches contracts to one half its length the amount of contraction is one inch, but of a muscle of four inches contracts to the same amount, it shortens by two inches. The period of relaxation follows that of contraction, and it depends essentially on the elasticity of the muscle whereby it tends to recover its shape, when the distorting force is removed. The recovery is therefore at first fast and then slow, and it lasts in the frog's muscle about ^^th second. The whole contraction thus lasts only about y\yth second in the frog's muscle. In mammalia muscle it is much shorter, and in the muscle of insects shorter still. The Force of Contraction is measured by finding what weight the muscle can lift, and the absolute force of a muscle may be expressed by the weight which is just too great to be lifted. The lifting power of a muscle depends primarily upon its thickness or sectional area. The absolute force of a muscle may therefore be expressed per unit and sectional area. In man, the absolute force per 1 sq. cm., is from 5000 to 10,000 grams. The force of contraction is, THE TISSUES. 51 however, modified by so many other conditions, that no definite figure can be given. The foroe of contraction during different parts of the contraction 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 recorded by a long lever, and in this way it is found that the ordinary curve of contraction gives a fair representation of the variations in the force. This method of recording the force of contraction is sometimes called the isometric method, in distinction 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 Exercise.) II.— The Factors modifying the Contraction. 1. Kind of Fibre. — The peculiarities of the contraction of visceral muscles wiU be considered later ; but in skeletal muscles the pale fibres contract more rapidly and completely than the red fibres. 2. Species of Animal. — In vertebrates the contraction of the muscles of warm-blooded animals is more rapid than the contraction 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 re- peatedly the contractions take place more and more slowly. At first each contraction is greater in extent, but, as the contrac- ~~^ tions go on, the extent dimin- Fie. 20.— Influence of continued [g\^Qg ^g fatigue becomes mani- Exerciae on Skeletal Muscle r , j n n .- i.- r-i —1 The first trace • 2 A '' ^ finally stimulation tails trace after moderate exer- tO Call forth any response. This ciae; 3. A trace when condition is probably caused by fatigue has been induced. ^^^ 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 52 : PHYSIOLOGY. for a time by washing out the muscle with salt, solution (Fig. 20). {Practical Exercise.) 2. Temperature. — If a muscle be warmed above the nor- mal temperature of the animal from which it is taken, all the phases of contraction become more rapid, but the contraction is less complete and less powerful. If, on the other hand, a muscle be cooled, the various periods are prolonged. , At first the. contraction becomes greater and more powerful, but as the cooling process goes on, it becomes less and less, until finally the most powerful stimuli produce no effect. Cooling has thus practically the same effect as, fatigue (Fig. 20). {Practical Exercise.) 4. Strength of Stimulus. — A stimulus must have a certain intensity to cause a ■ contraction. The precise strength of ^y\ /I ^ ^if\ r r 1 I 1 1 J 1 St. — Con. A S Fig. 21.— Influence of increasing the Strength of Stimulus upon the contrajction of Skeletal Muscle. St., the stimulus ; Con., the resulting contraction. A, A subminimal stimulus ; B, The minimum adequate stim- ulus ; C, The optimum stimulus. this minimuvi stim- ulus depends upon the condition of the muscle. The appli- cation of stronger and stronger stim- uli cause the muscu- lar contraction to become more and more rapid, more and more complete, and more and more powerful. But in- crease 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 accompany- ing figure where the continuous line represents the strength of the stimulus and the dotted line the extent of the con- traction (Fig. 21). 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 niuscular contraction is called the opti/mum stimulus. THE TISSUES. 53 Increasing the strength of the stimulus shortens the latent period, but lengthens the periods of contraction and relaxa- tion. (Practical Exercise.) 5. Resistance to Contraction — Weight to be Lifted. — Starting 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 greater weights diminish it, until finally, when a suflScient weight is applied, the muscle no longer contracts at all (Fig. 22 a). The application of weights to a muscle causes the latent period and period of contraction to be delayed, while it renders the period of relaxation more rapid, and an over exten- sion may be produced followed by a recovery resembling a small after-contraction (Fig. 22 6). (Practical Exercise.) 6. Successive 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 yVth of a second. W u. b. Fig. 22. — Influence of Load on a Muscular Contraction, (a) The effect of increasing the load on the extent of contraction ; (6) the effect of load on the course of contraction. 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. I If to a- frog's muscle, which takes ^-^th 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 interval of ^th of a second between them are produced. If the stimuli follow one another at the Tate of ten per second, a series of simple contractions is ■still produced, but' now with no interval between them. If stimuli be sent more rapidly to the muscle, say at the 54 PHYSIOLOGY. rate of twelve per second, the second stimulus will cause a contraction before tke contraction due to the first stimulus has entirely passed off. The second contraction will thus be superimposed on the first, and it is found 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 contraction is simply maintained (middle line, Fig. 23). When the contractions follow one another at such a rate that the relaxation period of the first con- traction has begun, but is not com- pleted, 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 uni- form, but is made up of alternate shortenings and lengthenings of the muscle. This constitutes "incom- plete tetanus." If the second stimulus follows the first so rapidly that the con- traction 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 a relaxation of the muscle. This is the condition of "complete tetanus" (lowest line. Fig. 23). {Practical Exercise.) 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 /\ l\ l\. ^v"^"" Tig. 23.— Effect of a series of Stimuli on Skeletal Muscle. (See text.) THE TISSUES. 55 single contraction renders a smaller number of stimuli per second necessary 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. 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 pro- duced 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 spasm the rate is about the same. All this would seem to indicate that the number of stimuli which pass to human muscle from the brain in one second is probably about ten per second. III. — Mode of Action of Muscles. 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. In relationship to each joint the muscles are arranged in opposing sets — one causing movement in one direction, another in the opposite direction — and named according to their mode of action, flexors, extensors, adductors, abductors, &c. But in the production of any particular inovement — 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 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, 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 stimu- 56 PHYSIOLOGY. lated, and the nerve to the flexors divided, the co-operative action of the extensors brings about an extension of the hand. ' The muscles round the various joints act on the bones, arranged as a series of. levers, of the three classes (Fig. 24). 1st Class.- — Fulcrum between power and weight. In the ankle this is seen when, by a contraction of the gastro- cnemius, we push upon some object with the toes. ,.>. Fig. 24. — The three types of lever illustrated by the movements at the ankle joint. I 2nd Class. — Weight^ between fulcrum and power. In rising 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 contraction 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. Muscle Work. As a result of the changes in shape, muscle performs its great function of doing mechanical work ; and the most impor- tant question which has to be considered in regard to muscle, as in regard to other machines, is the amount of worh it can do. The work-doing power of muscle is governed by the force of contraction and by the amount 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 THE TISSUES. 57 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. The size of, the, muscle is thus . the first great factor which governs its work-doing power. But the many factors influencing the force of mus- cular contraction also influence the work-doing power of the muscle (see p. 51). ; One factor requires special con- sideration, namely, the Load. < We have already seen that as the load is increased the extent of its contraction is diminished. The following experiment will illustrate the influence of increasing the load on the work-doing power of a muscle : — ri Fig. 25. — Influence of the length of a Muscle upon the work done. Load in Grams. Space through which lifted in mm. Work in Gram mm. X 4-5 z= 00 20 X 3 = 60-0 40 X 2-37 94-8 60 X 200 = 1200 SO X 1-75 = 1400 100 X 1-2 ■ 1200 It will be seen that increasing the load at first increases the amount of work done, but after a certain weight is reached, 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 time must always be con- sidered. Obviously contracing 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. 58 PHYSIOLOGY. 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 as to increase the speed of a ship or an engine requires an increase of coal consumption in a proportion roughly corresponding to the square of the increased speed. V. Heat Production. 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 change some delicate method of me'&,suring changes of temperature must be employed. The mercurial thermometer is hardly sufficiently sensitive, and, therefore, the thermo-electrical method is most generally employed. 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 — about 0-14 or 0-18° C. To find the amount of heat produced — (a) the weight of the muscle ; (6) its temperature before and after contraction ; and (c) the specific heat of muscle, must be ascertained. The specific heat of muscle is slightly greater than that of water, but the difference is so slight that it may .be con- THE TISSUES. 59 sidered the same. If, then, ten gramme of muscle had a temperature of 15° C. before it was made to contract, and a temperature of 15-05° C. after a period of contraction, then 05 gramme degree of heat have been produced. 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 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 do this the relationship of the work unit to the heat unit must be knoAvn — One kilogram metre is equal to '45 gramme degree. The proportion of work to heat is not constant. Thus, gradually increasing the stimulus increases both work production and heat production, but the latter is increased 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 stimu- lated. But the point of practical importance to determine is — How nmch of the energy usually liberated by muscle in normal conditions can be used for mechanical work, and how much is lost as heat? By experiments on men, horses, and dogs, Zuntz has found that about ^rd of the energy liberated may, under favourable conditions, be available for mechanical work, while |rds is lost as heat. Compared with other 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. 6o -.PHYSIOLOGY. VI; Electrical Changes. When a muscle contracts certain electrical changes bceur. These may be best studied in the heart, which is a muscle which can be exposed with- out injury. With other muscles the injury inflicted in isolating them sets up electrical cur- FlG. 26.— To show electric current of action in a rents of injury. muscle compared with that in a galvanic Xf One end of a "*"• wire be brought in contact with the base of the ventricle by means of some substance of the same electric potential ' as the muscle, and another wire be similarly connected with the apex, and if these wires are led off round a galvano- meter, it will be found that with each contraction of the heart an electric current is set up, the one part bf the heart 'becoming first positive and then negative to the other part. (Practical Exercise.) This means that when the contraction occurs the part which first contracts becomes of a higher electric potential to the rest of the muscle, so that electricity flows from it to the uncontracted part in the organ, and from the unconfcracting part to the contracting part in the wire round the galvanometer. The Contracting part is thus similar to the positive 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. ' Visceral Muscles. — In several important respects the visceral • muscles differ in their mode of action from the skeletal muscles. Their connection with nerves is by no means so definite THE TISSUES. 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, both before these nerves have developed in the embryo and when the influence of. these nerves has been cut off in the adult. The great features of the action of visceral muscle are-^ 1st, its tendency to sustained tonic contraction ; and 2nd, its spontaneous regular rhythmic contraction and relaxation. 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 largely inde- pendent, and an expression of the continuous metabolism of the muscle protoplasm. 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 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 nmscle and the action of a galvanic current have this action. 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 demonstrated that the latent period is very long. Unlike skeletal muscles the extent of contraction is not increased by increasing the strength of the stimulus. The smallest available stimulus causes the maximum contraction ; 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. A series of stimuli do not cause, a tetanus, but simply increase the rapidity and force of the individual contractions. When the muscles are arranged round hollow viscera. 62 PHYSIOLOGY. the rhythmic contraction, starting at one end, travels along the group of muscle-fibres as a regular wave — the peristaltic wave — and thus drives onwards the contents of the tube. The rate at which this wave travels varies very considerably. Cardiac Muscle physiologically resembles other visceral muscles, but its period of contraction is shorter and its rhythm generally more rapid. 4. Source of the Energy Evolved by Muscle. We know that chemical changes are constantly going on in muscle as in all living matter, and an investigation of these chemical changes in resting muscle and in contracting muscle alone can elucidate the source of the energy of muscle. 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 Muscle before and after Contraction. — The method which most naturally presents itself would be to take two muscles or groups of muscles corresponding to one another, and to examine the chemistry of one before it had been made to contract, and of the other, after it had been contracting for some time. Eesting muscle is alkaline; but if an excised muscle, outside the body, be kept contracting for some time, it becomes acid, and this acidity is due to the appearance of 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, COj, which can be extracted from muscle is very greatly increased. THE TISSUES. 63 As yet the changeSj, if any, in the proteids of muscle during contraction have not been fully investigated, while the results of the work accomplished on the nitrogenous extractives which are formed by the decomposition of the proteids are not trustworthy. They seem to indicate that these bodies are increased during muscular contraction in the excised muscle. 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 com- ponents of the air after tlie 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 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. Here we have at once evidence that muscle breathes, 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 methylene blue or alizarin blue are injected into the vein of an animal, the blood becomes blue, but the muscles remain colourless, having deoxidised the pigment and reduced it to a colourless condition. When freely exposed to air after death, the blue colour returns. 3. Changes in Blood passing through Muscle. — Observa- tions on excised muscle carry us no further, but by ascertaining the changes in composition which the blood undergoes in passing through a muscle, further information may be gained. For this purpose the two hind limbs 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 64 PHYSIOLOGY. carbon dioxide gained is increased, while usually -the amount of oxygen taken up is also increased. This observa<- tion thus confirms the investigations 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 sacrolactic acid probably combined with ammonia. We shall afterwards find that blood contains small quantities oi glucose, CgH^jOe. 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 proteids of the blood going to and coming from muscle have not been properly investigated. Some observers have obtained results which seem to indicate that the amount of fat which is found in the blood is diminished as the blood passes through the muscles, but whether this diminution is greater during muscular activity has not been studied. Such direct observations on muscle and the blood nourish- ing it indicate that constant chemical changes are going on when the muscle is at rest. It , is constantly giving off car- bon dioxide and constantly consuming oxygen, glucose, and possibly fats and proteids. When doing work these chemical changes become more active. We may compare resting muscle as to 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. — An- other 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, in the liberation of the energy for work and heat production, very extensive chemical changes occur, 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 this tissue. This was long ago recognised, but the older experimenters THE TISSUES. 65 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. 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 carbon and nitrogen thrown off, the former mainly as carbon dioxide in the expired air, the latter as urea in the urine, and 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 emaciated the auimal is and the harder the work, and being less the better nourished the animal is or the less the work that is done. If a lean animal be fed on an exclusively proteid diet, the excretion of carbon and nitrogen is increased when it works practically proportionately to the work done. But if the animal be well fed on an ordinary diet, contain- ing proteids, carbohydrates and fats, the performance of muscular work increases the excretion of carbon proportion- ately to the work done, but may cause practically no increase in the excretion of nitrogen. From the increased excretion of nitrogen and carbon the consumption of proteids may be calculated, since proteids contain 16 per cent, of nitrogen and 52 per cent, of carbon — i.e., 3'4 times more carbon than nitrogen. Each gram, of nitrogen thus represents the breaking down of 6'25 grams, of proteid, and it should be accompanied by 3' 4 grams, of carbon. If more carbon is excreted, it comes from carbo- hydrates or fat. Proceeding in this way, it is found that in the fasting animal and in the animal fed on proteids, the muscles get their energy from proteids, but that in an animal on an ordinary diet the muscles get it chiefly from carbohydrates and fats. By ascertaining the amount of heat liberated in the com- 66 PHYSIOLOGY. bustion of a definite quanity of proteids, carbohydrates or fats when oxidised to the compounds formed in the body, it is possible to determine the energy value of each of these, and it is found that — 1 gram, of proteid yields 41 kilog. degs. or 1738 kgm. 1 „ carbohydrates 4-1 „ ,, or 1738 ,, 1 „ fat 9-3 „ „ or 3943 „ If the amount of work done in kilogrammetres during any period is known, and if the increase in the excretion of nitrogen is ascertained, the amount of proteid decomposed and the energy evolved in its decomposition is known, and this may be compared with the total energy required for the work. Thus, if a man during the period of rest excretes daily 10 grams, of nitrogen, and he 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 2 X 6 '2 5 grams, of proteid has been decomposed, an amount 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 the fats and carbohydrates. A study of the ordinary diet of men doing muscular work corroborates the conclusions arrived at by an examination of the excreta. In this country the diet of a labourer consists in something like the following proportions of food con- stitutents : — Amount. Yielding Calories. Proteids, . . . 130 533 Fats, .... 100 930 Carbohydrates, . . 500 2050 Of the total 3513 Calories — kg. degrees — of energy daily taken in the food, only 1 5 per cent, is derived from proteids, the rest comes from the carbohydrates and fats. Thus during muscular work the three great constituents of the body and of the food — proteids, fats, and carbo- hydrates — are broken down to liberate their energy, and apparently the muscle tends to use the non-nilrogenous fats and carbohydrates in preference to the proteids. Only when forced to do so does it take a large proportion of its energy from these substances. THE TISSUES. 67 It may be urged that in athletic training proteids must be a great source of energy, since experience has taught that they are of such value. But their value is rather as material from which the energy liberating machine, the muscles, can be built up and increased, so that it can dispose of larger and larger quantities of food. Muscle then is a machine which has the power of liber- ating energy from proteids, fats, and carbohydrates, but it uses proteids chiefly in construction and repair. The muscles liberate energy from these substances by breaking them down into simpler molecules with the evolu- tion of energy, just as a blow causes the disintegration of nitro- glycerine. 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 must always be equivalent to the elimination of carbon dioxide and the other products of dis- integration. It has, however, been shown that a frog, de- prived of all free oxygen by placing it in the receiver of an air pump, may be transferred through mercury to an atmosphere of nitrogen, and that it continues to produce carbon dioxide. This means that the oxygen must be intra- molecular, must be in the muscle molecule, like the oxygen of nitro-glycerine ; and probably the presence of this ox5'gen 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 proteid 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. 5. 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 contrac- tion under stimulation. Gradually, however, their irritability 68 PHYSIOLOGY. diminishes, and finally disappears. They are then dead, and necrobiotic changes commence. The first of these — Rigor Mortis — is a disintegrative 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 is not generally able to alter the position of the limbs. 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 coagulated proteid by the pepsin zymin 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. II.— 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. In unicellular organisms changes in the surroundings act directly on the cell protoplasm. An amoeba when touched draws itself together. But, even in these simplest organ- isms 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. 16). Among unicellular organisms somewhat higher than amoeba — the infusoria — animals are found in which the cell is differentiated into a receiving and reacting part. Poteriodendron, a little infusiorion sitting in a cup-like frame, consists of a long process or cilium extending up from the cell body, while a contractile myoid attaches the THE TISSUES. 69 Fig. 27. — Poteriodendron to illustrate first stage in the evolution of a neuro-muscular sys- tem. cell body to the floor of the cup. When the cilium is touched the myoid contracts, and draws the creature into the protection of its covering. But 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 develop- ment by which special conditions at special parts of the surface each lead to a special reaction. This is brought about by the establishment of what may be compared to a series of shunting stations between the re- ceptive mechanism on the surface and the reacting mechanism,' the muscles, glands, &c. To form this a part of the original covering sinks inwards as a canal composed of the surface cells, and this remains in functional connection with the surface on the one hand and with the reacting structures on the other. At first the cells forming this tube are undifferentiated and alike, but they throw out certain processes towards the surface and others towards 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, a synapsis. 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 divided into the receiving and reacting series, but in structure they are alike. The shape and characters of the cells, and their position upon the processes of the neuron — the fibre — varies 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 stains deeply with basic stains, and which seems to be used up during the activity of the neuron may accumulate. The 70 PHYSIOLOGY. granules formed of this material are generally known as Nissl's granules. All these cells give off at least one process, which continues for some distance as the axon or axis cylinder process of a nerve fibre. 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 T Fis. 28. — To show a receiving and a reacting neuron, and their relationship to one another. ^stem of dendrites. All fibres appear to end in much the same manner, so that the dendrites and the axis cylinder process are essentially the same. These processes are fibrillated, and the fibrillse may be traced through the proto- plasm of the cells. In many cases the dendrites show little bnds or gemmules upon their course, and, according to some ^^. 1^ ITlG. 29. — (a) A nerve tell with Nissl's granules ; (6) A similar cell showing changes on section of its axon. observers, it is through these that one neuron is brought at one turn into definite relationship with certain other 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 associated with those of adjacent neurons. The axis cylinder process, as it passes away from the cell, acquires one or two coverings. THE TISSUES. 71 1. A thin transparent membrane is present in all peri- pheral nerves, and has been called the neurilemma. Between it and the axis cylinder a series of nuclei sur- rounded by a small quantity of protoplasm, forming the so- called nerve corpuscles, are found at intervals. Their mode of origin is unknown. Fibres with this sheath alone have a grey colour, and may be called non-medullated fibres. They are abundant in the visceral nerves. Fig. 30. — A nerve cell highly magnified to show passage of processes through the protoplasm. 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 development of the nerve fibre, between the primitive sheath and nerve corpuscles and the axis cylinder. This sheath is not continuous, but is interrupted at regular intervals at the nodes of Kanvier (Fig. 31). 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, from which certain chemical substances have been isolated, the relationships of which to one another are little under- 72 PHYSIOLOG V. stood. 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 phftsjihorus. Its con- stitution is not known. Along with protagon, or as a result of its decomposition, lecithin occurs. This is a fat in which Fig. 31. — Pieces of two white nerve fibres. one of the acid radicles is replaced by phosphoric acid linked to cholin, a hydroxy-ethyl-tri-methyl-ammonium" hydroxide. CH, OM H H I I I ifi C — C— OH CH3/ The chief interest of this substance is that it is toxic, and in all probability some of the symptoms occurring in degener- ative changes of the nervous system are due to its presence. It is closely allied to muscarin, a very powerful vegetable poison. Cholesterin may also be obtained in considerable amounts fr6m the fatty substance of the white sheath. Like the glycerine of ordinary fats it is an alcohol ^Caa H^g.OH — and- it is capable of linking with fatty acids. It is very soluble in hot alcohol, and crystallises out on cooling in character- istic square plates, with a notch out of one corner. The great function of the cell is to preside over the nutrition of the neuron. If any part of the neuron is cut off from its connection with the cell, it dies and degener- ates. In the white nerve fibres this degeneration begins in the white sheath, which breaks down into globules, and Undergoes chemical changes, so that it is more readily stained with osmic acid. This is taken advantage of in Marchis' method for tracing degenerated fibres. The axis cylinder next breaks up, and the nerve corpuscles pro- THE TISSUES. 73 liferate and increase, and absorb the remains of the white sheath and of the axis cyhnder, so that after about twenty days nothing is left of the nerve but the primitive sheath, filled with the nucleated protoplasts. After a time these nucleated structures begin to throw out processes along the course of the degenerated fibre, and these processes, joining together, regenerate the nerve, so that frequently when the detached end is united to the central part, the nerve is very rapidly capable of performing its functions again. The cell of the neuron appears to have the power of accumulating a reserve of material as the Nissl's granules, for it has been found that after continued action these granules diminish in amount. The nucleus, too, would seem to have the power of giving off material for the nourishment of the neuron, since in conditions of excessive activity it has been found shrunken and distorted. Whether the cells play any other part in the physiology of the neuron is not known. But the cell is also dependant for its proper nutrition upon the condition of the rest of the neuron. When the axon is cut, the chromatin of the cell nucleus decreases and the nucleus becomes displaced to one side, and ultimately • the whole cell degenerates (see Fig. 29). Inter-relationship of Neurons. — These neurons form a most intricate labryinth throughout all parts of the body, and more especially throughout the central nervous system. By their dendritic terminations each is brought into relationship with many others, and hence there is a con- tinued interaction between them, the activity of any one influencing the activity of many others. In this way the constant activity of the nervous system, which goes on from birth to death, during consciousness and in the absence of consciousness, is kept up. It is unnecessary and gratuitous to invoke the conception of automatic action on the part of any portion of the nervous system. Throughout life these neurons are con- stantly being acted upon from without, and activity ■ once started by any stimulus must necessarily set up a stream of action which will be co-existent with life. This implies that a neuron is capable of stimulation, 74 PHYSIOLOGY. that, like all other protoplasm, it reacts to changes in. external conditions. Means of Stimulation. — Just as with muscle, so with nerve, any sudden change excites it to activity — be this change a mechanical one, as in pinching a nerve, or a change in the temperature, or in the chemical surroundings, of the neuron, or in the electric conditions in its neighbour- hood. All that has been said of the stimulation of muscle, applies to the stimulation of nerve (see p. 44). The condition of the neuron modifies the effect of the stimulus, and the condition of other neurons modifies the ultimate result of the stimulus on the body. The condition of the neuron is modified by all the factors influencing the activity of protoplasm. It is depressed by a low temperature, increased by a higher temperature, up to, a certain point. During the flow of an electric current it is increased in the neighbourhood of the negative pole,, decreased around the positive pole. It is influenced by many chemical substances, some of which increase its excitability in small doses, and diminish it in larger doses,, some again even in the smallest dose depress its activity. Manifestations of the Activity of Neurons. — So far as is at present known, the activity of neurons is not accompanied by any manifest change — though it is possible that move- ments of the dendrites or of the gemmules upon them may occur. The activity of neurons is made manifest — 1st, By their action upon other structures — e.g., muscles, glands, &c., either (a) directly, or (6) indirectly, through other neurons ; 2nd, By electric changes in the neuron; and Brd, By changes in the consciousness. The activity of the outgoing neurons is manifested by changes in the muscles or other structures to which they go; while the activity of ingoing neurons is made manifest by the action of the same structures, induced by the stimu- lation of outgoing neurons by the ingoing neurons, or by modifications in the state of consciousness, which may be a simple brief sensation, or, by the implication of a number of other neurons, may develop into a series of sensations, or, a line of thought. THE TISSUES. 75 Conduction. — It is found that if a neuron is stimulated at any one point, some time elapses before the result of the stimulation is made manifest, and that the farther the point stimulated is from the structure acted upon, the longer will be this latent period. This of course indicates that the change, whatever it is, does not develop simultaneously throughout the neuron, but, starting from one point, be it one end or the middle, travels along. The rate at which it travels may be determined — 1st, By stimulating a nerve going to a muscle at two points at a known distance from one another, and measuring the difference of time which elapses between the contraction resulting from stimulation at each. {Practical Exercise.) 2nd, By taking advantage of the fact that the active part of a neuron — like the active part of a muscle — is electro-positive or zincy to the rest, and by finding how long after stimulation at one point this electric change reaches another point at a measured distance off. The rate of conduction varies considerably, everything ' stimulating protoplasmic activity accelerating, and every- thing depressing protoplasmic activity, diminishing it. Under normal conditions in the fresh nerve of the frog, the nerve change travels about 33 metres per second. The extent of nerve involved in conduction may be measured, if the rate of passage is known, by finding how long the electric change takes to pass any one point — ^just as, knowing the rate at which a procession is moving, its length may be calculated by finding how long it takes to pass a particular spot. It is found that about 20 mm. of nerve is involved in the change. By using these electric changes as an index of nerve action, it has been found that when a neuron is stimulated in the middle, the change set up travels in both directions, although its result is made manifest by the action of the structure on which it normally acts. Classification of Nerve Fibres. — Since a nerve normally conducts in one direction, and since the passage of impulses along it are made manifest by changes in the structure to which it goes, it is therefore possible to classify nerve fibres ac- 76 PHYSIOLOGY. cording to whether they conduct to or from the central nervous system, and according to the structure upon which they act. To find out the direction of conduction and the special mode of action of any nerve, two methods of investigation are employed — 1st, The nerve may be cut and the results of section studied. 2nd, The nerve may be stimulated and the result of stimulation noted. Usually these methods are used in conjunction, first, the nerve is cut, and, when the changes thus produced have been noted, the upper end and the lower end of the cut nerve are stimulated. It is, of course, only if a nerve is constantly transmitting impulses that section reveals any change. If the nerve is not constantly active, stimulation alone will teach anything of its functions. , Section of certain nerves produce a change in the action in muscles, glands, &c., or, if the nerve is not constantly act- ing, stimulation of the peripheral end of the cut nerve causes some change in the activity of these structures. Stimulation of the central end of such nerves produce no effect. These nerves therefore conduct impulses from the central nervous system outward, and they may be called efferent or outgoing nerves. Many of these nerves produce an increase on the activity of the parts to which they go, but others diminish or inhibit activity. The former class may be called augmentor nerves, the latter inhibitory nerves. The augmentor nerves may further be divided into groups according to the structures upon which they act. Those act- ing on muscle may be called motor nerves, those acting to cause secretion from a gland, secretory nerves, those acting to constrict blood vessels, vaso constrictor nerves. The inhibitory nerves may be similarly sub-divided into musculo-inhihitory, vaso-inhibitory and secreto-inhibitory nerves. Section of another set of nerves may produce loss of sen- sation in some part of the body. When the peripheral end of the cut nerve is stimulated no result is obtained. When the THE TISSUES. n central end is stimulated, sensations or some kind of action results. Such nerves obviously conduct to the central nervous system, and are called afferent or ingoing'. Those which, when stimulated, give rise to sensations are called sensory, those which give rise to some action are called excito-reflex, because the action which results is produced by what is called reflex action. As an example of such a nerve we may take the branches of the fifth cranial nerve which pass to the conjunctiva of the eye. When the conjunctiva is touched — i.e., when this nerve is stimulated, the orbicularis palpebrarum is brought into action through the seventh cranial nerve, and the eye is closed. The conjunctival branch of the fifth cranial nerve is thus an excito-motor nerve. When the branches of the lingual to the tongue are stimulated the result is a free flow of saliva, through the action of the seventh nerve and the secretory branches of the glosso-pharyngeal. The lingual nerve is thus excito- secretory. Further stimulation of the nerves from any part — e.g., by a mustard blister — causes relaxation of the vessels, and such afferent nerves may be called excito-vaso-inhibitory. A consideration of these various examples shows that there is no hard and fast distinction between sensory and excito- reflex nerves. Many produce both sensation and reflex action. Some may at one time produce sensation alone at another reflex action alone. Many nerves of the body contain both afferent and efferent nerve fibres, and are called mixed nerves. Factors modifying conduction. — Speaking generally, we may say that all factors which diminish protoplasmic activity diminish nervous conduction. Thus cooling a nerve lowers its power of conduction, gently heating it increases it. Various drugs which diminish protoplasmic activity — e.g., chloroform, &c., diminish conduction. The electric current acts upon the conductivity of nerve just as it does on the excitability of muscle, increasing it round the kathode or negative pole, diminishing it round the anode or positive pole. This influence on conductivity is one of the pheno- mena of electrotonus. 78 PHYSIOLOGY. Fatigue which has so marked an effect on the propagation of the wave of contraction along a muscle does not manifest its results upon the conductivity of nerve. In fact it is impossible to produce any of the phenomena of fatigue in nerve fibres. What is the nature of the " impulse " which passes along a nerve ? It is due to changes in the axis cylinder, since this without its sheath can conduct impulses. Secondly, it is dependant on the vitality of the nerve. Death of the nerve at once stops the transmission of an impulse. We may at once dismiss the idea that it is a flow of electricity. Electricity travels along a nerve at a much higher velocity than the nerve impulse. Two possibilities remain. The impulse may be of the nature of a molecular vibration, such as occurs in a stetho- scope which conducts sound vibration, or it may consist of a series of chemical changes such as cause the activity of protoplasm generally. In considering this matter it must be remembered that the amount of energy evolved in a nerve impulse need not be great. All it has to do is to start the activity of the part to which it goes. Hence if chemical changes are the basis of the impulse, these may be extremely small in amount and difficult to detect. As a matter of fact, the evidence of chemical changes in nerve fibre is entirely wanting. No change in reaction, no heat production, and no phenomena of fatigue can be demonstrated. The passage of excitation from one neuron to others occupies a very appreciable time. This may be readily demonstrated in what is called reflex action, as examples of which the drawing up of the leg when the sole is tickled, or the winking of the eye when the eyeball is touched, may be taken. In each of these the end of an ingoing neuron is excited, the change passes in and sets up changes in out- going neurons, which act upon muscles. In the case of the eye, about '06 seconds elapses between the touching of the eye and the resulting " wink." Knowing the rate at which nerve changes pass along THE TISSUES. 79 nerves, and knowing the length of the ingoing and of the outgoing neurons, the time taken in the passage of the change along these is readily calculated. In a reflex wink it is about "01 second. Hence only ^th of the total " latent time " of the reflex action is occupied in the passage of the change along the neurons, and 'OS second, or fth of the whole is taken up in the passage of the change from one neuron to another. Obviously the synapsis between the dendrites of the neurons offers a resistance, and this resistance varies with the con- dition of the neurons involved, possibly with the condition of the dendrites which form the synapses. Not only does the time of the resulting action vary with the state of the neurons, but the extent also varies. If the toe of a frog deprived of its brain is pinched, it draws up the leg, but if a dose of strychnine is first adminis- tered, even touching the toes causes a violent spasm of every muscle in the body. If, on the other hand, a dose of bromide of potassium has been administered, or if ice be put on the back of the animal, much more powerful stimulation is required to produce any reaction. For these reasons it is never easy to predicate the ultimate result of any stimula- tion of the nervous system. But, other things being equal, the strength of stimulus applied to the first neuron — that is the extent of excitation in it — directly effects the result, on other neurons. {Practical Exercise.) How the activity of one neuron acts upon other neurons is not quite clear. Actual continuity is not 'essential, mere physiological association of terminal dendrites appears to be sufficient. But mere anatomical propinquity alone is not sufficient. Every neuron is probably in equally close dendritic con- nection with hundreds of other neurons, but stimulation of the first neuron may lead at one time to changes in one set, at another time to changes in another set of these connected neurons. The mechanism by which neurons are brought into touch is not yet fully under- stood, for the action of gemmules is no more than an hypothesis. 8o . PHYSIOLOGY. THE NEURO-MUSCULAR MECHANISM. The study of the physiology of muscle and nerve leads to the consideration of how the neuro-muscular mechanism acts, how through it the co-relationship of the animal with its surroundings is maintained. This interaction of the body on the surroundings may be considered first as regards the simpler reactions — the so-called rejiex actions, and secondly as regards the more complex reactions commonly classified as voluntary actions. A. Simple Reactions. Reflex Action. — This has already been considered shortly in dealing with the stimulation of outgoing neurons through their synapses with ingoing neurons. It has been shown that considerable time is occupied in the passage of the stimulus and that the resulting action depends upon the condition of the neurons. In the ordinary reflex action the synapses involved are situated in the spinal cord, and ■ the various neurons are arranged as follows : — 1. Peripheral Neuron — ^Ingoing (Fig. 32 A). — These start in a dendritic expansion at the periphery, and enter the cord by the posterior roots of the spinal nerves. In these roots they are connected with cells by lateral branches. When they enter the cord they pass to the posterior portion, and divide into (a) branches running for a short distance down the cord ; (6) branches running right up to the top of the spinal cord, to end in synapses round masses of cells — the nuclei of the posterior columns. From these collateral branches are given off which pass forward, and, for the most part, end in synapses adjacent to cells placed in the front part of each side of the cord. B. Outgoing (Fig. 32 B). — From these cells are given off the fibres which pass out in the anterior roots of the spinal nerves to muscles, glands, and other reacting struc- tures. The action of these neurons is controlled and modified by two other series of central neurons. THE TISSUES. 8i 2. Central Neurons — 1. Cerebral — A. Ingoing (Fig. 32 0) — {a) Lower Series, These start from the cells in the nuclei of the posterior columns, and, crossing the middle line, run up to the base of the great brain, where they end in synapses round other cells in the thalamus opticus. (6) Upper Series, From . these cells, processes pass up to the surface of the great brain, to end in synapses with the cells situated there. B. Outgoing (D). — These start in the cells of the cortex cerebri, and pass down to the upper part of the spinal cord, where most of them cross and run down the lateral column of the spinal cord, giving off collaterals which end in synapses round the cells in the anterior horn of grey Fig. 32. — To show the three Arcs in the Central Nervous System. matter. Those which do not cross run down the anterior column of the cord for some distance, and end by crossing and becoming associated with the cells in the anterior horn. Cerebellar — A. Ingoing (^). — Some of the collaterals of the peripheral ingoing neuron end in synapses round a mass of nerve cells at the side of the grey matter of the spinal cord — the cells of Lockhart Clarke. From these cells fibres extend up at the side of the cord to the lesser brain or cerebellum, to form directly or indirectly, synapses round the cells in this organ. Outgoing (F). — From cells near the surface of the cere- bellum, and in the roof nucleus axons extend to the medulla oblongata, where they end in synapses round a mass of cells — the nucleus of Deiters, and from these G 82 PHYSIOLOGY. cells, fibres extend down the lateral columns of the spinal cord, and give off collaterals to the cells of the anterior horn of grey matter. Other neurons pass to the cerebrum. The nervous system may thus be considered as built up of three sets of arcs. 1st, The Spinal arcs, consisting of the peripheral ingoing neurons and the peripheral outgoing neurons. These arcs are not only at the level of the cord at which the ingoing neuron enters, but at various- levels above and below this point. 2nd, The Cerebral arcs, consisting of the peripheral, the lower and the upper ingoing neurons, and the central out- going neurons and the peripheral outgoing neurons. 3rd, The Cerebellar arcs, consisting of peripheral ingoing neurons, the cerebellar ingoing neurons, the outgoing cere- bellar neurons, either direct to the cord or through the cerebrum, and the peripheral outgoing neurons. The simplest manifestation of reflex action is to be seen in the effect of pinching the toe of a frog in which the brain has been destroyed so that the spinal arcs can act without inter- ference. If the pinch is gentle the foot is simply drawn up. If the pinch is stronger a more extensive movement occurs, and if the leg is held firmly the opposite limb is drawn up. If a piece of paper dipped in acetic acid is laid on the animal's flank, extensive and well co-ordinated movements are made with one or bath limbs to remove the irritant. In all cases the act is a definite and co-ordinated one, bring- ing about an appropriate though inevitable reaction of the animal to its surroundings. This is due to the passage of an impulse inwards, which sets up changes in the outgoing neurons. With a very gentle stimulus the effect manifests itself at the same level of the cord, but with more powerful stimuli other levels of the cord are acted upon and a more exten- sive movement results. Generally speaking, stimulating one hiad limb causes a movement first in the limb stimulated, then in the fore limb of the same side, then in the opposite fore limb, and lastly in the opposite hind limb. {Practical Exercises.) In reflex action the shortest arc is usually the line of least resistance along which the nerve change travels, and more distant arcs are less readily brought into play. But THE TISSUES. 83 in various more complex reflex actions certaLa special arcs are associated, apparently as the result of having been educated to act together, and whether this association is an inherited one, or whether it has been acquired, there seems to be a tendency for nerve action, which has once travelled by a definite route, to take the same route again. Definite channels of communication connecting ingoing impulses with definite outgoing reaction are thus established. The spinal reflexes are modified — 1st, By the condition of the neurons (p. 79) ; 2nd, By stimuli from adjacent areas. The reflex act of sneezing, set up by stimulation of the nasal mucous membrane may be checked by firmly pressing over the bridge of the nose ; 3rd, By the upper arcs. Spinal reflexes are increased when the cord is separated from the brain. These upper arcs may also act reflexly — i.e., inevitably. Thus stimulating certain areas of skin in the dog leads to a reflex scratching in which the body is bent and balanced, while the hind leg performs complex move- ments. Here the balancing action of the cerebellar arc, and also the cerebral arc are both involved. But since the action is obviously inevitable, it is classed as a reflex. B. More Complex Reactions. It has been seen that the spinal reflexes are not absolutely inevitable since they are modified by the condition of the nervous structures involved. When the more complex reactions in which the cerebral arc is involved are studied, the resulting action appears to be less inevitable, and to be influenced by the sensation and other changes in consciousness which accompany it. It has, in fact, been assumed that the state of consciousness is the determining factor in the result, and hence such actions have been called voluntary. But since in such conditions as sleep-walking and hypnotism the most complex and selective actions are performed without the intervention of consciousness, it must be admitted that this metaphysical phenomenon i's not an integral part of the response of the nervous system. On the other hand, we know that the character of the PHYSIOLOGY. reaction to any stimulus is largely dependent upon the state of the nervous centres. Just as a touch produces a differ- ent effect in a frog poisoned with strychnine, and in one under the influence of bromide of potassium, so a sudden noise may produce a totally different reaction upon a person with a fatigued brain or a brain poisoned with alcohol, and upon one with the brain in a good state of nutrition. Not only does the temporary state of nutrition thus modify the result of a stimulus, but the paths of action previously opened and defined through the centres also have a marked influence. These paths may have been formed in past generations and inherited from the parents. In young fowls, as soon as they are hatched, the acts of walking and of pecking are at once performed, and in many families particular gestures or expressions follow certain modes of stimulation in many different individuals without the con- sciousness of the person being involved. They are inherited cerebral reflexes. The paths may also have been developed in the individual «,s the result of previous activities of the nervous mechanisni. For if a given action has once followed a given stimulus it aiways tends to follow it again. This, in fact, is the basis of all rational education — to open up paths in the nervous system by which the most suitable response may be made to any given stimulus ; and to prevent the formation of paths by which inappropriate reaction may be produced. It is very important to recognise clearly the influence of these factors upon the conduct of the individual — the nutrition of the brain at any moment, and the inherited and acquired tendencies in particular directions — since various abnormalities in moral and social conduct may be explained by reference to them, and since cases of so-called insanity are frequently dependent upon them. With the relationship of consciousness to these reactions of the nervous system we shall not deal at present. Con- sciousness is a purely metaphysical conception and we do not know its relationship to cerebral action further than that we have no evidence that consciousness can manifest itself apart from such cerebral activity. SECTION IV. THE SENSES. In order that each particular kind of change in our sur- roundings should produce its appropriate reaction, it is essential that the different kinds of changes should act in different ways— that the contact of gross matter, changes of temperature, vibrations of the air, vibrations of the ether, and various chemical changes should each produce a special effect. To secure this, special peripheral developments of neurons have been evolved which react more particularly to each of these special kinds of change, and with these peripheral neurons particular parts of the brain are connected and associated. In this way the mech- anisms of the senses are produced. A. Common Sensibility. Before dealing with the special senses the phenomena of common sensibility may be considered. By this is meant a series of somewhat indescribable but quite definite sensations by which our attention is directed to the state of the body as a whole or of various parts of the body. The ordinary sensations of thirst and hunger are examples of these, sensa- tions which, although due to changes in the mouth, throat, or stomach give us information as to the general needs of the body. Such sensations may be considered as normal and physiological. But when abnormal conditions exist in 85 86 PHYSIOLOGY. certain localities they produce sensations such as tickling, tingling, &c., which generally lead to an endeavour to remove the abnormal stimulus. These modifications of consciousness produced by this mechanism of common sensibility may be small or great according to the strength of the stimulus, and according to the state of the central nervous system, and when excessive the sensation produced is called pain. It must be recog- nised that pain is a purely relative term, and that conditions which in one individual wUl cause pain will not cause it in another, while stimuli which will produce what are called painful sensations when the nervous system is debilitated may give rise to sensa- tions not considered as painful when the nervous system is normal. The mechanism of common sensibility and pain is not acted on by the same stimuli in all parts of the body. The mouth and throat are chiefly affected by abstinence from fluids, the stomach by absence of food. The intestine appears to give rise to sensation only when abnormally stimulated. In the skin the mechanism of common sensibility is so closely associated with the mechanism of the tactile and temperature senses that it is diflScult to differentiate them. Abnormal stimulation of the skin produces painful sensations very readily, while similar changes in other tissues — e.g., muscles, cause no modifica- tion of consciousness. The nerve channels by which the changes producing common sensibility and pain are transmitted to the central nervous system appear to be distinct from those connected with the tactile and other senses, since common sensibility may persist while the tactile sense is lost. B. Muscle and Joint Sense. Closely allied to the mechanism of common sensibility is the arrangement by which we are made aware of the movements and thus of the position of various parts of the body. A double mechanism is here involved, 1st, SENSES. 87 a mechanism stimulated by the contraction of the muscles ; and 2nd, a mechanism acted on by movements at the joints. Muscle Spindles. — Among the fibres of muscles are found long fusiform structures containing a few embryonic muscle fibres, and into each of these a meduUated nerve passes and breaks up into a non-medullated plexus round the fibres. Organs of Golgi are swellings in the tendons near the muscle fibres into which a meduUated fibre enters, and losing its white sheath forms a plexus of fibrils with varico- sities upon them. Varicose terminations of axons surrounded by fibrous tissues are found in the synovial membranes and round joints. Through these mechanisms information is transmitted to the central nervous system as to the position and move- ments of the various parts, and this, although not necessarily modifying the consciousness, is of service in guiding the movements. When the consciousness is affected, valuable information as to the conditions of the surroundings may be gained. In estimating the weight of bodies, these sensa- tions are much used. The body is taken in the hand, and by determining the amount of muscular contraction required to support or raise it, the weight is estimated. The shape and size of objects is also determined by this sense in con- junction with the sense of touch. If we touch a book on the table we can form an idea of its shape and size by estimating the distance through which the hand may be moved in different directions. In the dark the distance of objects is also judged by estimating the extent of movement of the hand and arm necessary to touch them. The sensations derived from the joint and muscle senses are often lost or impaired, and the condition of the mechanism may be tested by ascertaining the smallest difference of weight which can be appreciated. With moderate weights of about one pound of difference of about wth can usually be detected in the normal condition. (Practical Exercise.) 88 PHYSIOLOGY. G. Special Senses. I. Tactile Sense. The Tactile Corpuscles which consist of a naked branching varicose termination of an axon surrounded by sheaths of fibrous tissue are situated in" the papillse of the true skin. The study of the sense of touch may be approached by touching the table and analysing the manner in which the conclusion is arrived at that a table is touched. We conclude it is a table because the surface is hard and sjnooih. This judgment may or may not be correct. But even in saying that the body we touch is hard and smooth, we are also forming judgments from the sensations experi- enced. When we say the surface is hard we mean it resists pressure, and when we say it is smooth we mean that the skin of our finger is uniformly touched and not pressed upon at certain points as it would be if the surface were rough. Or it may be that we draw our finger over the table and feel a continuous contact and not the series of contacts which we should experience were the surface rough. The determination of the resistance to pressure implies the power of distinguishing differences of pressure. The determination of a continuous contact instead of a series of local contacts implies the power of determining or localising the part or parts touched, and the ability to distinguish between a continuation of contact and a succes- sion of contacts implies the power of differentiating contacts in time. The tactile sense may thus be best studied under these three heads — 1 . The Power of distinguishing differences of Pressure. — Variations of pressure in time and space are alone dis- tinguished. We live under a pressure of 760 mm. of mercury, but this gives rise to no sensation. Any sudden increase or diminution of pressure, however, leads to a marked change of sensation, but a slow change causes a SENSES. 89 lesser modification of consciousness. If a part of the body is uniformly pressed up, as when a finger is immersed in mercury, the sensation of pressure is felt as a ring at the surface of the mercury, where the greater pressure of the mercury joins the lesser pressure of the air. (Practical Exercise.) For these reasons in testing the acuteness of the pressure sense over the surface of the body, these two factors must be kept in mind, and the rate at which the pressure is varied, and the pressure on adjacent part of skin must be kept uniform. The part of the body being supported so that the muscular sense cannot come into play, dif- ferent weights may be applied to ascertain the smallest difference of weight which can be distinguished. (Practical Exercise.) The pressure sense varies in diiferent parts of the body, being most acute where the nerve terminations are most abundant, and less acute where they are fewer. Over the points of the finger with a weight of about 1 gram, a diflfer- ence of about 10 per cent, can be distinguished, but over the leg the difference may be much greater. Everything which diminishes the activity of protoplasm diminishes the acuteness of the pressure sense, and therefore when the skin is cold the sense is much less acute than when it is warm. Again, since the sensation is a modification of conscious- ness, it is modified by the state of the central nervous system. This is readily fatigued, and hence if the tests are applied for too long a period at one time, the acuteness of perception diminishes. 2. The Power of Localising the place of Contact. — Where the tactile organs are abundant, the power of distinguishing accurately the point touched is better than in places where these are more scattered. For this reason, if two contacts are made at the same time, these may be very close together in the former situation, and each of them may be localised and felt as distinct from the other, whereas in the latter situation, they would be felt as a single contact. The power is therefore tested by determining how near two points of 90 PHYSIOLOGY. contact may be brought to one another, and still cause a double sensation. This may be done by means of a pair of compasses, and in using them it is necessary to observe certain precautions. First, the two points must touch the skin simultaneously. Second, they must touch it lightly and with equal pressure each time. Third, they must not be worked steadily from close together to further apart, or vice versa, but must be used now in the one way now in the other. (Practical Exercise.) In this way it is easy to demonstrate that over the tips of the fingers where tactile organs are very abundant the two points of contact will give rise to a double sensation when no more than 2 mm. apart, while over the back of the hand, they do not give a double sensation till they are about 30 mm. apart. Over the thigh and back they must be no less than 50 to 70 mm. Both peripheral mechanism and the central nervous system are involved in this locahsation, and hence the power varies with the condition of the skin as regards temperature, &c., and with the state of the nervous system. 3. The Power of distinguishing Contacts in Time. — If the finger be brought against a toothed wheel rotated slowly, the contacts of the individual teeth will be separately felt. But if the wheel is made to rotate more and more rapidly, the separate sensations are no longer felt, but a continuous sense of contact is experienced. This indicates that, if stimuli follow one another sufficiently rapidly, the sensations pro- duced are fused. From this it 1^^ l^^ k^^^k^ ]r^ is obvious that the sensation lasts longer than the stimulus — the contact. The duration of the sensation depends upon the degree of Fig. 36.— Relationship of Sensa- Stimulation of the peripheral tion to Stimulus, with weak tactile organs. When the stim- and strong stimuli. , . , . , , , . uli are strong, as in the contact of the finger with the rough edge of a toothed wheel the sensations last a considerable time, and thus stimuli follow- r-KKKU ing one another at about 500 times per second, cause a SENSES. 91 fusion of sensation. On the other hand, if a vioh'n string is made to vibrate against the finger, the stimuli are weak, and the resulting sensation of short duration, and hence stimuli may follow one another with greater rapidity, say to 1200 per second, and still be distinguished as separate sensations. II. Temperature Sense. Heat like light is physically a form of vibration of the ether. The temperature sense depends upon the fact that Avhen heat is withdrawn from the body we have one kind of sensation which we call cold, and when heat is added to our body another sensation which we call hot. This de- pends upon the temperature of our body in relationship to the surroundings, and not merely on the temperature of surrounding bodies. If three basins of water are taken, one very hot, one very cold, and one of medium temperature, and if a hand be placed, one in the very hot and one in the very cold water for a short time, and then transferred to the basin with water at a medium temperature, the water will feel hot upon the hand that has been in the cold water, and cold to the hand that has been in the hot water. (Practical Exercise.) The rate at which heat is abstracted or added is the governing factor in causing the sensation, a sudden change of temperature stimulates far more powerfully than a slow change. For this reason the thermal conductivity of sub- stances in contact with the skin has an influence upon the sensation. If a piece of iron and a piece of flannel side by side be felt, the first will feel cold, the second will not, because the first has high thermal conductivity, the second has not, and thus the former abstracts heat more rapidly than the latter. Certain parts of the skin are stimulated by the withdrawal of heat to give rise to sensations of cold, while others are stimulated by the addition of heat. This may be demonstrated by taking the cold point of a pencil and passing it over the back of the hand when it wiU be felt as cold only at certain points, such points have been called cold spots, while similar 92 PHYSIOLOGY. spots stimulated by the addition of heat are called hot spots. {Practical Exercise.) The acuteness of the temperature sense may be tested by finding the smallest difference of temperature which can be distinguished. This may be done by taking two test tubes in which thermometers have been placed, and filling them with water at slightly different temperatures, and then applying them to different parts of the skin. If the tempera- ture is very low or very high, differences of temperature are not readily distinguished, and in fact painful sensations may take the place of temperature sensations. But between 15° and 50°C. the power of distinguishing differences of temperature is fairly constant, but it varies in different parts of the body. Over the cheek as small a difference as •2°C. can be appreciated, while on the back the difference must be as great as ■9°C. {Practical Exercise.) The temperature sense is independent of the tactile sense. The one may be lost and the other retained. It is probable that the nerve endings in the deeper layers of epithelium are connected with the temperature sense. But although independent, the tactile and thermal senses influence one another. A cold body placed on the skin feels heavier than a warm body, as may be shown by placing first a cold penny and then a warm penny on the skin of the forehead. III. Vision. While the addition to and withdrawal from the surface of the body of the slower waves of ether which are the basis of heat, act upon the special nerve terminations in the skin to give rise to sensations of heat and cold, a certain range of more rapid vibrations act specially upon the nerve endings in the eye to produce molecular changes which in turn affect the centres in the brain, and give rise to changes in con- sciousness which we call light. The range of vibrations which can act in this way is comparatively limited, the slowest being about 435 billions per second, the most rapid about 764 billions. The more rapid vibrations, which are SENSES. 93 capable of setting up chemical changes, as in photography, do not act upon the eye. The action of light upon the protoplasm of lower organ- ism has been already considered (page 16), and it has been seen that it may be general or unilateral producing the phenomenon of positive or negative phototaxis. In more complex animals special sets of cells are specially set aside to be acted on by light, and these are generally set in a bed of pigmented cells to prevent the passage of light through the protoplasm. Such an accumulation of cells constitutes an eye, and in the simpler organisms such an eye can have no further function than to enable the presence or absence of light or various degrees of illumination to produce their effects. But in the higher animals these cells are so arranged that certain of them are stimulated by light coming in one direc- tion, others are stimulated by light coming in another, and while the former are connected with one set of synapses in the brain, the latter are connected with another. Thus light coming from one point will stimulate one set of cells which will excite one part of the brain, and light from another will act upon other cells which will excite another part of the brain, and thus not merely the degree of illumi- nation but the source of illumination becomes distinguish- able. It is by this arrangement that it becomes possible to form ideas of the shape of external objects. One directs the eye to the corner of the ceiling, and the idea that it is a corner is due to the fact that three different degrees of illumination are appreciated, and that these can be localised — one above, one to the right, and one to the left. One set of cells is stimulated to one degree, another set of cells to another degree, and a third set of cells to a third degree ; and the different stimulation of these different sets of cells leads to a different excitation of separate sets of cells in the brain ; this is associated with the perceptions of the three parts differently illuminated. From the previous training of the nervous system we are taught to interpret this as due to a corner. But this interpretation is simply a judgment based 94 PHYSIOLOGY. upon the sensations, and it may or may not be right. Thus instead of actually looking at a corner we may be looking at the picture of one. From the very first it must be remembered that the modi- fication of our consciousness which we call vision is not directly due to external condition, but is due to changes set up in our eye by these external states. We do not perceive the object we are looking at, but simply the changes in our brain produced by changes in the eye set up by rays of light coming from the object. Usually such changes are set up by a certain range of vibrations of the ether, but they may be set up in other ways — e.g., by the mechanical stimulation of a blow on the eye ; but, however set up, they give rise to the same kind of changes in consciousness — visual sensations. This fact has been formulated in the doctrine of specific nerve energy, that different varieties of stimuli, applied to the same organ of sense, always produce the same kind of sensation, and the converse that the same stimulus applied to different organs of sense produces a different sensation for each also holds good. The visual mechanism not only gives the power of appre- ciating the degree and source of illumination, but also of appreciating colour. Physically the different colours are simply different rates of vibration of the ether, physiologi- cally they are different sensations produced by different modes of stimulation of the eye. The slowest visible vibrations produce changes accompanied by a sensation which we call red, the most rapid vibrations produce different changes which we call violet. But, as will be afterwards shown, the sensation of a red or of a violet may be produced by other modes of stimulating the eye. The visual mechanism in this way gives a flat picture of the outer world, and from this flat picture we have to form judgments of the size, distance, and thickness, of the bodies looked at. The idea of size depends upon the extent of the eye-cells stimulated by the light coming from a body. If a large surface is acted upon, the body seems large, if a small SENSES. 95 surface the body seems small. But the extent of eye-cells acted on depends not merely upon the size of the object, but also upon its distance from the eye. Hence, our ideas of size are judgments based upon the size of the picture in the eye, and the appreciation of the distance of the object. The distance of an object, when over fifty or sixty yards from the eye, is judged by the modifications in its shading and colour due to the condition of the atmosphere. A range of hills will at one time be judged to be quite near, at another time to be distant, and since the estimation of the size of an object depends upon the judgment of its distance, the estimation we make of the size of such objects as a range of hills is often most erroneous. When objects are nearer to the eye, a special mechanism comes into play to enable us to determine their distance. The thickness or contour of an object is also largely a judgment based upon colour and shading. When a cube is looked at, we judge that it is a cube because of the degree of illumination of the different sides — degrees of illumination which may be reproduced in a flat picture of such a cube. When the object is near the eyes, by using the two eyes together a means of determining solidity comes into action. When the manner in which we gain knowledge of our surroundings by vision is analysed, it must be admitted that the dictum " seeing is believing " has at best an unsubstantial physiological basis and that most of the points we say we see about anything — e.g., its size, distance, and contour, are largely judgments formed by us upon a flat picture produced in the cells of the eye, which flat picture has in turn led to these changes in our brain which are accompanied by the changes in our consciousness upon which our judgment has to act. Any defect in the visual mechanism must, and does, lead to defects in the mental picture formed, and the accuracy of the judgment will depend upon the accuracy of the picture, and upon the previous experience and training of the nerve structures involved. But, further, while the parts of the brain connected with 96 PHYSIOLOGY. the visual sense are usually stimulated by changes in the cells of the eye, they may be directly stimulated, and when this is the case, a sensation of light apparently in front of the eye, is experienced, because the centres are naturally always stimulated by such illumination. Such a sensation is called an illusion. 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 (diop- tric mechanism). B. The method in which the retina is stimulated. (2.) Two eyes (biaocular vision). 2. The conduction 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. 1. Monocular Vision. A. The Dioptric Mechanism. Before attempting to study the physiology of the eye, the student must dissect an ox's or a pig's eye, and then make himself familiar with the microscopic structure of the various parts. The eye may be described as a hollow sphere of fibrous tissue, the posterior part, the sclerotic (Scl.), being opaque ; the anterior part, the cornea (0.), being transparent and form- ing part of a sphere of smaller diameter than the sclerotic. Inside the sclerotic coat is a loose, fibrous layer, the choroid (Gh.), the connective tissue cells of which are loaded with mel- anin, "a black pigment. This is the vascular coat of the eye — the larger vessels running in its outer part, and the capil- SEIVSES. 97 Fig. 34. — Section through the Eye. laries in its inner layer. Anteriorly, just behind tlie junction of the cornea and sclerotic, it is thickened and raised in a number of ridges, the ciliary processes {Gil. M.), running from behind forward and terminat- ing abruptly in front. In these the ciliary muscle is situated. It consists of two sets of non-striped muscular fibres — first, radiating fibres, which take origin from the sclerotic just behind the corneo- sclerotic junction, and run backwards and outwards to be inserted with the bases of the ciliary processes ; second, cir- cular fibres which run round the processes just inside the radiating fibres. The choroid is continued forward in front of the ciliary processes as the iris to the pupil and in it are also two sets of non-striped muscular fibres — ^first, the cir- cular fibres, a well-marked band running round the pupil, and called the sphincter pupili (Sph. P.) muscle ; second, a less well-marked set of radiating fibres, which are absent in some animals, and which constitute the dilator pupili muscle {D.P.). That part of the eye in front of the iris is filled by a lymph-like fluid, the aqueous humour, while the part behind is occupied by a fine jelly-like mucoid tissue, the vitreous humour. The vitreous humour is enclosed in a delicate fibrous capsule, the hyaloid meTnbrane, and just behind the ciliary processes this membrane becomes tougher, and is firmly adherent to the processes, so tliat it is difficult to strip it off. It passes forward from the processes as the suspen- sory ligatnent (S.L.), and then splits to form the lens capsule. In this is held the crystalline lens, a biconvex lens, with its greater curvature on its posterior aspect, and characterised by its great elasticity. Normally it is kept somewhat pressed out and flattened between the layers of the capsule, but if the suspensory ligament is relaxed its natural elasticity 98 - PHYSIOLOGY. causes it to bulge forward. This happens -when the ciliary muscle contracts and pulls forward the ciliary processes with the hyaloid membrane. Between the hyaloid membrane and the choroid is the retina. 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. The white nerve fibres pass through the sclerotic, through the choroid, and through the retina, to form the white (yptic 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 fibres. These nerve fibres take origin from a layer of nerve cells 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 molecula,r layer. The cells of these neurons are placed in the next or fourth layer, the inner nuclear layer, and from these cells, processes pass backwards to form synapses in the fifth, or outer molecular layer, with the dendrites of the terminal neurons. These terminal neurons h.ave their cells in the sixth, or outer nuclear layer, of the retina, and they pass backwards and end in two special kinds of terminations in the seventh layer of the retina — the rods and cones. These structures are composed of two segments — a somewhat barrel-shaped basal piece, and a transpS|,rent terminal part which in the rods is cylindrical and in the cones is pointed. Over the central spot of the eye there are no rods, 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 stops abruptly in front at the ora serrata, 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. 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 eye may be compared, to a photographic camera, SENSSS. 99 having in front a lens, or lenses, to focus the light upon the sensitive screen behind (Fig. 34a). 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 vitreous. The refractive indices of these, compared with air as unity may be expressed as follows : — Cornea, . Aqueous, 1-33. 1-33. Lens, Vitreous, 1-45. 1-33. Fig. 34a. — To show how parallel rays are brought t() a focus on the retina by refraction at the three surfaces a, b, and c. 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 — :1st, The difference of refractive index. 2nd, 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 PHYSIOLOGY. 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 a compound lens composed of a convexo-concave part in front and a biconvex part behind, and this lens has the distance behind the lens at which parallel rays are focussed as its focal distance. In the rest- ing normal eye (the emmetropic eye) this 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. If an object is brought nearer and nearer to the eye, the rays of light entering the eye become more and more diver- gent, and if the eye be set so that rays from a distance — i.e., ^ — ^ parallel rays — are focussed, then '-"" Tl'^^^~~->t ^^y^ from a nearer object will be "'-KL '^^^^^"" focussed behind the retina, and a clear image will not be formed. riG. 35-To show that Rays rpj^j^ ^^^^^^ ^^^^ ^^^^ ^^^ ^^^ Irom diBtant and near . i t • i objects are not foonssed objects Cannot be distmctly seen on the rt-tina at the same at the samo time, a fact which *""''• can be readily demonstrated by Scheiner's experiment. {Practical Exercise.) 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 some one to hold a ""' '"";/r.:..:.v--.:r..-— needle against a white sheet of paper at about Tj o,! c, u ■ . T^ ■ . three yards from the Fig. 36. — Schemer s Experiment. -,,■,-, eye, and hold another needle in the same line at about a foot from the eye. When the far needle is looked at the near needle becomes double. It is found practically that objects at a greater distance than 6 metres may be considered as " distant" and that they are focussed on the retina. Obj^ects may be brought nearer and nearer to the eye, and yet be seen distinctly up to a certain point, the near point SENSES. lOI of vision 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 vision), becomes greater and greater till it can increase no further when tlie 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 when it will be found that the image from the anterior surface of the lens becomes smaller and brighter (Sanson's images). (Practical Exercise.) Positive accommodation is brought about by contraction of the ciliary muscle which pulls forward the ciliary processes to which the hyaloid membrane is attached, and thus relaxes the suspensory ligaments of the lens and the front of the lens capsule, and allows the natural elasticity of the lens to bulge it forward. This change of positive accommodation is accompanied by a contraction of the pupil due to contraction of the sphincter pupili muscle. By this means the more diver- gent peripheral rays which would have been focussed behind the central ones are cut off, and spherical aberration is prevented. The muscles acting in positive accommodation — the ciliary and sphincter pupili {G.M. and S.P.) — are supplied by the third cranial nerve (///.), while the dilator pupili 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 pupili. The sphincter centre is reflexly called into action. 1st, When the light falls in the retina and stimulates the optic nerve. In this way the retina is protected against over Fig. 37. — Mechanism of Positive Accommodation. PHYSIOLOGY. Stimulation. 2nd, When the image upon the retina be- comes blurred as the object approaches the eye and the rays become more divergent. By this means the centre for the ciliary -muscle is also called into play. 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 any ingoing nerve causing a' dilatation of the pupil when the medulla is intact (Fig. 38). The fibres pass down the lateral columns of the spinal cord to the lower cervical and upper dorsal region where they arborise round the cells of the anterior horn: From these, fibres pass by the anterior root of the second (2 lJ.l!i>j possibly 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 gang- lion of the fifth cranial nerve, and from there the fibres pass along the ophthalmic division and its long ciliary branches to the pupil {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 cilio-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 dilator fibres of the pupil have not been demonstrated in all animals, it has been suggested that those fibres may act by inhibiting the sphincter pupili, but the evidence on this point is not conclusive. The non-striped muscle of the iris, like non-striped muscle ICG Fig. 38. — Nerve Supply of the Intrinsic Muscles of tlie Eye (see text). SENSES. 103 elsewhere, may act independently of nerve fibres as may be seen in tlie eye of a decapitated cat. Further, various drugs seem to act directly upon them — e.g., physostigmin causes a contraction, while atropin causes a dilatation. The power of positive accommodation varies at different ages, being greatest in the young child in which the lens is most convex. Years. 10 7 20 30 40 50 60 _10 U 22 40 100 Pig. 39. — To show Variations in the power of Positive Accommodation throughout life. The distance of the near point in cms. is represented in the accompanying figure, while the range of accommodation as age advances will be the difference between the near point and the far point at which the rays are parallel. After about sixty years of age not even parallel rays can be focussed on account of the flattening of the lens. This is the condition of Presbyopia. Imperfections of the Dioptric Mechanism — Myopia. — In certain individuals 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 accom- modation is needed till the object is well within the normal limit ; and the near point is approximated to the eye. To enable distant objects to be seen it is necessary to provide concave glasses by which the parallel rays are rendered divergent (Fig. 40c). Hypermetropia. — The eye of a considerable number of people is too short from before backwards, and thus, in the resting state, parallel rays are focussed behind the retina, and to see even a distant object the individual has to use I04 PHYSIOLOGY. Fig. 40.— To illustrate Myopia and Hypermetropia. his positive accommodation. As the object is approached to the eye it is focussed with greater and greater difficulty and the near point is further off than in the emmetropic eye (Fig. 406). The long-sighted eye differs from the slightly presbyopic in the fact that not merely divergent, but also parallel rays, are unfocussed in the resting state. The condition is corrected by using convex glasses which render the rays con- vergent and, therefore, capable of being focussed upon the retina of the shortened eye. Astigmatism is a defect due to unequal curvature of the 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. To correct this condition the lesser curvature in any particular plane must be made as great as the greater curvature in the other plane, and this is done by placing a cylindrical or part cylindrical glass in front of the eye so that this curvature is in front of the lesser curvature and thus equalises it with the other curvature (Fig. 41). Fig. 41. — To show the cause of AstigmatiBm. B. Stimulation of the Retina. 1. Reaction to Varying Illuminations.— The Blind Spot. —At the entrance of the optic nerve the retina cannot be stimulated because there are no end organs in that situation. The existence of such a blind spot may be demonstrated 1st, by Mariottes' 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 cross and bring^ing SENSES. 105 the paper nearer to the eye when the spot will first dis- appear and then re-appear (Fig. 42a). (Practical Exercise.) 2nd, by making a mark on a sheet of paper, and with the head close to the paper moving the point of a pencil to the right for the right eye, or to the left for the left eye when the point will disappear and again re-appear. {Practical Exercise.) A Fig. 42. — Methods of demonstrating the Blind Spot, (a) By Mariotte's Experiment ; (b) by moving a pencil along a sheet of paper. 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 (1'5 mm.) may also be ascertained. By fixing the head close to the paper, moving the point of the pencil out till it disappears and then moving it in different directions and marking when it re-appears the shape of the bhnd spot may be mapped out. It is never quite circular and often shows rays extending from its edge which are due to the blood vessels. (Practical Exercise). The rest of the retina forward to the ora serrata is capable of stimulation, but when the eye is directed forwards the extent of retina stimulated is influenced by the eyebrow cutting off rays from above and thus preventing the lower io6 PHYSIOLOGY. part of the retina being stimulated to its margin, and by the nose intercepting rays from the nasal side, thus protecting the outer part of the retina. The field of Vision, that is the optical angle subtended by the whole range of objects which can be seen at one time, is investigated by the perimeter, an instrument which can readily be made by describing the arc of a circle upon a black board or sheet of paper, placing the eye opposite the centre and directing it to a mark upon the circumfer- ence and then bringing a piece of chalk up the line until it is seen. (Practical Exercise.) The angle A jB is the measure of the vertical field of vision, and it will be observed how it is constricted by the eyebrow. The verti- cal angle amounts to about 130°; 60° in the upper field and 70° in the lower field. The horizontal field worked Fig. 43.— The Field of Vision, and the method of investigating it by the Perimeter. . , out in the same way gives about 150°, of which no less than 90° are on the outer side and only 60° on the inner. Of all parts of the retina the central spot is the most sensitive to differences of illumination. The layer of the retina capable of stimulation is the layer of rods and cones. This is proved by the experiment of Purkinje's images. It depends upon the fact that if a ray of light is thrown through the sclerotic coat of the eye the shadow of the blood vessels stimulates a subjacent layer, and these vessels appear as a series of wriggling lines on the surface looked at. If the light is moved the lines seem to move, and, by resolving the triangles, 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 (Fig. 44). (Practical Exercise.) S£JVSES. 107 Modes of Stimulation. — The rods and cones are generally stimulated by the ethereal light vibra- tion, but they may be stimulated by mechanical violence or by sudden changes in an electric current. But, however stimulated, the kind of sensa- tion is always of the same kind — a visual sensation. Of the nature of the changes in the retina when stimulated we know little. But we know — 1st, That under the influence of light the cells of the tapetum nigrum expand forward between the rods and cones. 2nd, That 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 occiapy the sensitive central spot of the eye, this change in colour suggests that a chemical decomposition accompanies stimulation. 2. The power of localising the source or direction of illumination has now to be considered. Its acuteness 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 Fig. 44. — To show that the hindmost layer of the retina is stimu- lated. {Purhinje's Images.) to "a double sensa- tion. Over the cen- tral spot two points may be as near to one another as about four micro- millimetres, and still two sensations be experienced.. This is determined by finding tb e small- est optical angle which can be subtended by, say two stars, with- out their images being fused. This angle varies from seventy- FiG. 45. — The Power o£ localising the Som:oe Illumination on different parts of the retina. of io8 PHYSIOLOGY. three to fifty seconds in different individuals, and this corre- sponds to from 5-31 to 3"65 micros on the retina (Fig. 45). Over the central spot the centres of the cones are about this distance from one another, and it would seem that, to get a double sensation, two cones must be stimulated. On passing to the more peripheral part of the retina, where the cones are more scattered, the power of localising decreases, and larger and larger optical angles must be subtended by the two objects — e.g., the points of a pair of compasses, in order that both may be seen. {Practical Exercise.) 3. Colour Sensation — Physics of Light Vibration. — Phy- sically 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 rela- tionship 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 admixture with other parts of the spectrum. Thus a surface which in bright sunlight appears of a brilliant red, be- comes maroon and finally, brown and black, as the light fades. Again, a pure red when dil- uted with all the spectrum — i.e., with white light, becomes pink as it becomes less and less saturated. Physiology of Colour Sensa- tion. — 1. 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 Fig. 46. — Distribution of Colour Sensatiou in relationship to the surface of" the retina {Colour Perimeter). S£NSES. 109 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 can 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. 2. While the various sensations which we call colour are generally produced by vibrations of different lengths falling 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 Exercise.) (6.) Simple alternation of white and black upon the retina may produce colour sensation, as when a disc of paper marked with lines, is passed rapidly before the eye. (Prac- tical Exercise.) 3rd, By mixing different parts of the spectrum, some intermediate part or white may be produced. This may be done by colouring the surface of a top with different colours, or by means of a sheet of glass allowing the image of one wafer to fall on another of a different colour. (Practical Exercise.) 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 colour therefore depends upon the nature of the change set up in the retina, and not upon the condition producing that change. 4. After looking for some time at any one colour, on PHYSIOLOGY. removing the colour another appears in its place ^ — the complemental colour. If the first colour is — Ked, the second will be green blue. Orange, „ „ blue. Green, „ „ pink. Yellow, „ „ indigo blue, and vice versa. {Practical Exercise.) 1. From consideration of the peripheral colour blind zone of, the retina and of the zone giving sensations only of blue or yellow when stimulated, it must be concluded that some- thing exists in certain areas of the retina which when acted upon gives rise to a sensation of blue or yelloAV, red or green. 2. The arrangement of the various parts of the retina in their reactions to colour, and the phenomenon of comple- mental colours suggests the possibility of there being one substance which when undergoing one change produces blue, and when undergoing another change produces yellow, and another substance which when undergoing one change pro- duces red, and another change produces green ; or that there are four different substances, one when changed giving rise to yellow, another to blue, another to red, and another to green. When the substance giving 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 com- plemental colour, and so on through the other substances. 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. While every one 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. Individuals who manifest this condition are often able to name colours fairly accurately, but when asked to match a piece of red wool from a number . of others, they tend to put beside it green wools. The condition is of great import- ance to engine drivers and seamen. {Practical Exercise.) Colour blindness for yellow and blue is much rarer. SENSES. B.— BINOCULAR VISION. By the fact that there are two eyes instead of only one, the following advantages are attained : — 1. The Field of Vision is increased ; but is not doubled, as is shown in Fig. 47, where the two eyes are directed to a spot A, and where the field of vision of the right eye is indicated by continuous lines, that of the left eye by dotted ^^^( \ ) ^'L./rv Fig. 47. — The Field of Vision in Binocular Vision. lines. The two fields greatly overlap, and the central part is common to the two eyes. (Practical Exercise.) In animals where the eyes are placed laterally, the two fields are independent. 2. A mechanism is afforded for the determination of the distance of near ohjects, because as an object is approached, the two eyes have to be turned inwards by the internal recti muscles, and by the degree of contraction of these, an estimation of the distance is made. The im- portance of this may be demonstrated by fixing a stick vertically, rapidly walking up to it with one eye shut, and endeavouring to touch it with the finger. {Practical Exercise.) 3. A means of determining the solidity of an object is afforded, because if the 'object is near, a slightly different PHYSIOLOGY. ^v rv^ Fig. 48. — Stereoscopic Vision. picture is given on each retina, and experience has taught us that this stereoscopic vision indicates sohdity (Fig. 48). But in order that with the two eyes single vision may occur, the eyes must be directed to the same place, so that the image of that place falls on each central spot. If this does not occur, double vision results. If the central spot of one eye corresponds to the central spot of the other, certain points in each retina will have corresponding points in the other which will be stimulated by the same part of the picture when the eyes are working together (Fig. 49). Movements of Eyeballs. — To secure this harmonious action of the two retinse, it is necessary that the eyes Each eye in its orbit is a Fig. 49. — Corresponding Areas of the two Retinse in Binocular Vision. should be freely movable. ball and socket joint in which the eyeball moves round every axis. The eye is set obliquely to the axis of the orbit, and the centre of rotation is behind the centre of the ball. The movements are pro- duced by three pairs of mus- cles (Fig. 50). V 1. The internal and external recti {I.B.. and Ex. R!). 2. The superior and inferior recti acting along the lines in- FiG. 60. -The Eyeball in the dicatcd (S.E.). Orbit, with the Muscles act- o T^l, o. ihe superior ing upon it. , ^ obliques acting in the line (S. ob.). The internal rectus rotates the pupil inwards. „ external „ ,, „ outwards. „ superior „ ,, „ upwards and inwards. ■ r • ( downwards and out- " ^^''°' " " " wards. and inferior SENSES. "3 In 01. txff.^ liR. Fig. 51.— The Movements of the PupU caused by the various Muacles of the Eye. (Eight Eye.) rni • UT .. i xi -if downwards and ine superior oblique rotates the pupil -j i a „ inferior „ „ „ upwards and inwards. In directing the eyes to the right, the external rectus of the right eye acts along with the internal rectus of the left. In directing the eyes straight upwards, the superior rectus and inferior oblique of each eye act together, and in looking downwards, the in- ferior rectus and superior oblique come into play. When a distant object is looked at, the axes of the two eyes may be considered as parallel ; but as an object is approached to the eye, the axes converge. It is not possible by voluntary effort to diverge the optic axis or to rotate the eyes round antero-posterior axes. 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 suc- cession of pictures thus got gives the idea of the continuous series of objects. A somewhat complex nervous mechanism presides over these various movements of the eyes. All the muscles are supplied by the third cranial nerve, except the superior oblique which is sup- plied by the fourth nerve, and the Fig. 52. external rectus which is supplied by the sixth nerve (Fig. 5 2). The centres for the third and fourth nerves are situated in the floor of the aqueduct of Sylvius under the corpora quad- Tbe Nervous Mechanism presiding over the combined movements of the two Eyes. 114 PHYSIOLOGY. rigemina, while the centre for the sixth is in the pons Varolii. 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 move- ments, and this mechanism is controlled by impulses constantly received from the two retinae from the ear and from the brain. [OpTh^^ Occ Fig. 53. — The Connections of the Eetinse with the Central Nervous System. 2. Connections of the Eyes with the Central Nervous System. From each eye the optic nerve extends backwards and in- wards to join the other optic nerve at the chiasma. From the chiasma the two optic tracts move upwards round the crura cerebri to end in two divisions — SENSES. 115 1. A posterior division passing to the anterior corpora quadrigemina on the same side (Fig. 53, A.C.Q.). 2. An anterior running to the geniculate body on the posterior aspect of the thalamus opticus (Fig. 53, Op. Th.). A partial crossing of the fibres takes place in the chiasma — fibres from the middle and internal part of the retina decussating, those from the outer part remaining on the same side. For this reason, when the right optic tract is cut, it leads to blindness of both retinae on the outer part of the right eye and on the inner and middle part of the left eye. Thus objects on the left side of the field of vision are not seen. The fibres of the posterior termination of the optic tract end in" S3mapses with neurons in the corpora quadrigemina, and the fibres of these neurons pass downwards and control the oculo-motor mechanism already described. The fibres of the anterior division also 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. 53, Occ). When the right occipital lobe is destroyed, blindness on the outer part of the right retina and on the inner and middle part of the left retina results — the individual is blind for all objects in the left half of the field of vision. 3. The Visual Centre. A response to stimulation on the part of the neurons in the occipital lobe of the brain appears to be the physical substra- tion of our visual sensations, and hence this part of the brain is called the visual centre. Usually the visual centre is stimulated by changes set up 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, as is sometimes seen in the early part of an epileptic fit. The visual centre of each side must be regarded as a chart of the opposite field of vision, each part corresponding to a particular part of the field. The two centres acting together give the whole field of vision. Since the blind spot is not represented in the centre, it is not perceived ii6 PHYSIOLOGY. in the field of vision. The centre is said to rectify the inverted image found on the retina, but this simply means that as a result of experience, we have learned that changes in, say the lower part of the retinae and in the corresponding part of the visual centres are produced by light from above the head. Since the retinal changes differ simply according to the degree of illumination and the rate of the ethereal waves, and since the part of the retina acted on is determined by the direction of the rays, we can be conscious only of different changes in different parts of the visual centre depen- dent on the changes set up in the retinae. We have there- fore only the means of getting a flat picture of what we look at, but no special arrangement for having different sensations according to the distance of an object or according to whether it is flat or in relief. Thus the means of determining the form and size of objects by the retinae and visual centres is very limited. To gain knowledge of the size, distance, or contour of an object we have to combine certain other sensations, or certain past experiences with the visual sensation, and form a judg- ment or concept of what we are looking at. It is not then wonderful that erroneous judgments are fre- quently made. The possibility of such may be indicated by one or two examples of what are called modified perceptions. If an imaginary line be divided into two equal parts, and if a series of dots are put along the one half, that part will appear longer than the other. If a black wafer on a white ground, and a white wafer on a black ground be looked at, the latter will appear larger than the former. If a square be ruled with par- allel diagonal lines, and if short vertical and horizontal lines be placed alternately upon them, they will no longer appear parallel. Fig. 54.— To illustrate Optical Illusions. SENSES. 117 ' IV. — Hearing. While through the sense of touch we are made aware of differences of pressure, through the sense of hearing certain vibratory changes of pressure specially affect the consciousness. Even simple organisms, devoid of any special organ of hearing, may be affected by vibratory changes. 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 verte- brates it is chiefly through the ear 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. In the human subject the organ of hearing consists of an external, a middle, and an internal ear. External Ear. — The structure of this presents no point of special physiological interest. In lower animals the pinna is under the control of muscles, and is of use in determining the direction in which sound comes. 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 cavity, placed outside of the petrous part of the temporal' bone. Its outer wall is formed by a membrane, the mem- brana tympani {Ty.), which is attached to a ring of bone. Its inner wall presents two openings into the internal ear — the fenestra ovalis (/.o.), an oval opening, situated anteriorly and above, and the fenestra rotunda {f-r.)., a round opening placed below and behind. Throughout life these are closed, the former by the foot of the stapes, the latter by a mem- brane. The posterior wall shows openings into the mastoid cells and presents a small bony projection which transmits the stapedius muscle. The anterior wall has above a bony canal carrying the tensor tympani muscle, and below this the canal of the Eustachian tube which communicate with the posterior nares (Fig. 55, E.N.T.). In the tympanic cavity are three ossicles — the malleus (m.), incus (i.), and stapes (s.), forming a chain between the mem- brana tympani and the fenestra ovalis. The handle of the ii8 PHYSIOLOGY. malleus is attached to the membrana tyjnpani, 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 incus which thrusts the stapes into the fenestra ovalis, and thus increases the pressure in the en- closed fluid of the internal ear. The fenestra rotunda 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 of which the arm to which the power is applied — the handle of the malleus — is 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 z%.n LnT. Fig. 55. — Diagram of the Bar. Stapedius muscle which twists the stapes in the fenestra. This muscle seems to act when loud sounds fall on the ear, and when its nerve supply, derived from the facial nerve,' is paralysed, such sounds are heard with painful intensity. If the membrana tympani isviolentlyforced outwards by clos- ing the nose and mouth and forcing air up the Eustachian tube, the incus and stapes do not accompany the malleus and mem- brane, 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 SENSES. 119 the malleus it is well damped, and stops vibrating as soon as waves of condensation 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 mem- brane, and its paralysis diminishes the 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 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. 56). When the Eustachian tube gets occluded, as a result of catarrh of the pharynx, the oxygen in the middle ear is ab- sorbed 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. Internal Ear. — The internal ear is a somewhat complex cavity in the petrous part of the temporal bone, the osseous labyrinth. It consists of a central space, ^"*- ^^- — "^^ ^"^^ the vestibule (F), into which the fenestra Ind th° mod" o! ovalis opens. From the anterior part of this, opening when a canal makes two and a-half turns round a swallowing, central piUar, 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 (Goch.). The ascending and descending canals are separated from one another, 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 three semi-circular canals (S.G.) open, each with a swelling at one end. One runs in the horizontal plane, and has the swell- 120 PHYSIOLOGY. ing or ampulla anteriorly. The other two run in the vertical planes indicated in the diagram, the anterior being called the superior canal, and having its \J~' ampulla in front, the posterior having >^ y H its ampulla behind. They join to- .,-'''\ i) gether, and enter the vestibule by a "^••^/c.. -^W:'' common orifice. ^ ^ The bony labyrinth is filled with Fig. 57.— The Relationship a lymph-like fluid,the perilymph, and of the Semi-Circular j^ -^ y^^^ ^ complex membranous bag, (Janals to one another. , . , the membranous labynnth. In the vestibule this is divided into two little sacs, the utricle and the saccule, 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 scala media or membranous cochlea. This terminates blindly at the apex. From the utricle a membranous canal extends into each of the bony semi-circular canals, being provided with an ampulla, which nearly fills up the bony ampuUa, while the canal portion is small, and occupies only a small part of the bony canal. In the membranous cochlea the lining cells form the organ of Oorti. This is set upon the basilar membrane, and consists from within, outwards, of — \st, A set of elongated supporting 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. Mh, 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 into the hollow surface of the inner rods. bth, Several rows of outer hair cells, with some spindle- shaped cells among them. Qth, The outer supporting cells. *7th, Lying over the inner and outer hair cells is the mem- brana reticularis, resembling a net, through the meshes of which the hairs project. 8th, Arching over this organ is a homogeneous membrane — the membrana tectoria. SENSES. 121 The membranous labyrintli is attached to the inner wall of the bony labyrinth at certain points through which fibres of the auditory nerve enter it. A set of fibres goes to the utricle, a set to each of the ampullae, and a set to the saccule and the cochlea. The mem- branous laby- rinth has an outer fibrous coat, and in- side this a homogenous layer which is markedly thickened where the nerves enter it. It is lined by flattened epithelium, which become columnar, and is covered with stiff hair-like pro- cesses over the thicken- ings at the entrance of the nerves. The terminal neurons of both the vestibule and the cochlea end in den- drites between the hair cells, and the cell of each is upon its course to the medulla. The auditory nerve is essentially double, consist- ing of a dorsal, or cochlear, and a ventral or vestibular part. Cochlear Root (Fig. 5 8a). — This is the true nerve of hearing. Fig. 58. — (a) The Connections of the Cochlear Root of the Eighth Cranial Nerve ; (6) The Connections of the Vestibular Root of the same {modified from Schdfer), Its fibres (a) begin in PHYSIOLOG Y. dendrites between the cells of the organ of Corti, have a cell upon their course, and when they enter the medulla branch into two divisions, one of which (6) passes to arborise with a group of cells lying dorsally in the tuber- culum acusticum {T.Ac^, from which fibres pass to the opposite side (6^) to be connected with the superior olive, and to run up to the anterior corpora quadrigenina, and possibly to the thalamus, and thence to the cortex. The other division ends in synapses in the nucleus accessorius (c), from which the fibres constituting the trapezium {d^ pass, some across to the opposite side to have the same connections as those of the first series, some up the same side, some to the superior olive {s.o^ of the same side through which they aie connected with the nucleus of the sixth nerve and the oculo-motor mechanism (Fig. 65). Vestibular Root (JFig. 586). — The fibres of this root take origin in dendrites between the cells of the maculae, and have their nerve cell upon their course (a). As they enter the medulla they divide into two, forming an ascending and a de- scending branch. The ascending branch sends fibres on to the cerebrum (6^), probably to the thalamus, and to the superior vermis of the cerebellum Q?). These fibres give off col- laterals to the nucleus of Deiters (6^), from the cells of which fibres pass (6*), which divide, some running on the same side, some on the opposite side ; one branch passing up to the oculo-motor mechanism, the other passing down the spinal cord to send collaterals to the cells in the grey matter. The descending branch forms connections with the medullary nuclei (c) as it passes down (Fig. 65). Physiology of Hearing. — The qualities of sound which can be distinguished by the sense of hearing, are loudness — amplitude of vibration ; pitch-rate of vibration ; and quahty — the character of the sound given by the over-tones. The perception of this last is essentially a perception of pitch. It is easy to understand how the peripheral neurons in the external ear are more powerfully stimulated by the greater variations in the degree of pressure which are pro- duced by more powerful aereal waves, and how the greater SENSES. 123 stimulation of the receptive centre in the brain will be accompanied by a sensation of greater loudness. A study of the structure of the cochlea at first seems to show a mechanism well suited to afford a means of estimat- ing the pitch of a note and the existence of overtones. 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 set in vibration by a particular note. Unfortunately, for this theory at least one case has been described in which the perception of pitch and quality was unimpaired in spite of complete destruction of the cochlea. The power of distinguishing differences of pitch varies in different individuals. It is more acute for notes of a moderate rate of vibration, from 46 to 4000, than for very slow or very fast vibrations. The range of perception of pitch also varies greatly, some people hearing notes as low as twenty vibrations per second, and others hearing them up to 4!0,000 per second. V. — Taste. Mechanism. — The nerve endings connected with the sense of taste are disposed in barrel-shaped structures in the epithelial covering of the mouth. These taste bulbs are most abundant at the back of the tongue, on the sides of the large circumvallate papillae which form the prominent V-shaped line on the posterior part of the dorsum. Each is composed of a covering of elongated cells like the staves of a barrel, enclosing a set of spindle-shaped cells with which the dendrites on the end of the nerve fibres are closely associated. These nerve fibres pass to the brain in the fifth nerve. Several cases of complete loss of taste have been recorded in which the root of the fifth nerve alone was destroyed. It is usually thought that the glosso - pharyngeal also carries nerve fibres connected with taste, but the conclur sion is based on anatomical rather than upon physiological evidence. Physiology. — As to the way in which this mechanism is 124 PHYSIOLOGY. stimulated our knowledge is very imperfect. In order to act the substance must be in solution. The strength of the sensation depends on the concentration of the solution, upon the extent of the surface of the tongue acted upon, upon the duration of the action, and upon the temperature of the solution. If the temperature is very high or very low the taste sensation is impaired by the feelings of cold or heat. It is most difficult to classify the many various taste sensations which may be experienced, but they may roughly be divided into four main groups : — 1. Sweet. 3. Acid. 2. Bitter. 4. Saline. Whether different sets of terminations react specially to each of these is not known, but it has been found that sub- stances giving rise to the sensation which we call bitter act best on the back of the tongue, while substances producing sweet or acid sensations act on the sides and front. Again, chewing the leaves of gymneTna sylvestre abolishes sensations of sweet and bitter, but does not interfere with those of acid and saline, and leaves the tactile sense unimpaired. On the other hand, cocaine paralyses the tactile sense before it inter- feres with the sense of taste. The sense of taste is very closely connected with the sense of smell, and, when the latter is interfered with, many sub- stances seem tasteless which under normal conditions have a marked flavour. VI.— Smell. Mechanism. — Over the upper part of the nasal cavity the columnar epithelial cells are devoid of cilia, and between them are placed spindle-shaped cells (Fig. 59,01. c), which send their processes down through the mucous membrane, and through the cribriform plate of the ethmoid into the olfactory bulb. Here they form synapses with other neurons (01. GL), the axons of which pass to the base of the olfactory tracts. Some fibres (a) pass straight on to the optic thalamus, while others (6) form synapses with other neurons, the fibres of which, passing round the fornix, end by arborising round cells in SENSES. 125 the corpora mamilaria (CM.), which send processes on to the thalamus. There is some evidence that the fibres to the cortex end in the hippocampus. Physiology. — To act upon the olfactory mechanism the substance must be volatile, and must be suspended in the In this condition infinitesmal' quantities of such sub- r air. Fig. 59. — The Connections of the Olfactory Fibres {modified from Schafer). stances as musk are capable of producing powerful sensations. The mucous membrane must be moist, and this is secured by the activity of Bowman's glands, situated in the mucous membrane. These are under the control of the fifth cranial nerve, and section of this leads indirectly to loss of the sense of smell. SECTION V. Central Nervous System. .SPINAL NERVES. Having studied the various organs of special sense and the muscles by which we can react to the impressions received through them, we may now consider the general characters of the connecting nerves between the central nervous system, and the various structures involved. These nerves may be classified as ingoing and outgoing, and they V.N. Fig. 60. — Structure of a Typical Spinal Nerve. P.R., Posterior Root with ganglion ; A.R., Anterior Koot ; S.I., Ganglion of Sympathetic Chain ; W.B., its White Kamus ; G.S., its Grey Ramus ; V.N., Visceral Nerve with collateral ganglion ; S.N., Somatic Nerve. may roughly be divided into those connected with the body wall and its appendages and those connected with the viscera. General Structure. — The arrangement of fibres may best be understood by studying the constitution of one of the typical spinal nerves coming off from, say the dorsal region, of the spinal cord (Fig. 60). 126 CENTRAL NERVOUS SYSTEM. 127 A posterior root (P.R.) coming off from the postero-lateral aspect of tlie cord and haying a swelling upon it — the ganglion of the posterior root — joins an anterior root (A.R.) coming from the antero-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.l.) joined to the nerve by two roots, a white ramus (W.R.) and a grey ramus (G.R.) ; and from this a nerve extends towards the viscera {V.N.). Before this nerve reaches its final distribution it passes through another ganglion. The nerve to the somatopleur or body wall (^S.N.) is composed of incoming and outgoing fibres. 1st, Incoming fibres are medullated and take origin in the various peri- pheral sense organs. As they pass through the ganglion on the posterior 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, passing to the postero- lateral column, it breaks up into an ascending branch and a descending branch. 2nd, Outgoing fibres, which are medul- lated, take origin from the large cells in the anterior 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 splanchnopleur and to the involuntary structures in the somatopleur likewise contain: — 1st, Incoming Fibres. — These take origin either in definite peripheral structures such as Pacinian corpuscles, or in some less defined structures, and as medullated fibres pass through the various ganglia, and, so far as is at present known, have their cell stations in the ganglion on the posterior root. 2nd, The Outgoing Fibres 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 anterior root. From this they pass by the white root to the sym- pathetic ganglion whence they may proceed in one of two different ways. (a.) They may form synapses with cells, and from these cells fibres may pass — 1 . Outwards with the splanchnic nerves ; or, 128 PHYSIOLOGY. 2. Back into the spinal nerve by the grey root and so down the somatic nerve to blood-vessels, hairs, and sweat glands, &c. The ganglia from which fibres pass back into spinal nerve are known as lateral ganglia. (b.) They may pass through these ganglia on to one more peripherally situated in which they form a synapsis and are continued onwards. These ganglia from which fibres do not pass back are called collateral ganglia. The various fibres after one interruption proceed to their termination where they break up into a network of anasto- mosing fibres with cells — a sort of terminal ganglion. Before their first interruption they are termed pre-ganglionic fibres, after their interruption post-ganglionic. The interruption of fibres in ganglia, 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 fibre. 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. General Distribution. A. — Somatic Fibres. (a.) Outgoing Fibres. — The course of these must be studied in the dissecting room. (b.) Ingoing Fibres — Cutaneous Fibres. — ^The fibres pass- ing 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, which are innervated as shown in Fig. 61. B. — Splanchnic Fibres. (a.) The Outgoing Fibres may be sub-divided as follows : — 1. Head and Neck. — These leave the spinal cord by the upper five dorsal nerves and pass upwards in the sympathetic cord of the neck to the superior cervical ganglion where they CENTRAL NERVE. 129 have their cell stations. From these, fibres are distributed to the parts supplied. The chief functions of these fibres are — 1st, Vaso-constrictor to the vessels of the face and head ; 2nd, Pupilo-dilator (see p. 102) ; 3rd, Motor to the muscle of Miiller ; 4 th, Secretory to the salivary glands, lachrymal gland, and sweat glands. 2. Thorax. — The fibres to the thoracic organs also come off in the five upper dorsal nerves, have their cell stations in Fia. 6I0. — Distribution of Cutaneous Nerves in Human Subject. the stellate ganglion, and pass to the heart and lungs. On the heart they act as augmentors (see p. 212), and on the lungs as vaso -constrictors (see p. 231). 3. Abdomen. ^These fibres come off by the lower six dorsal and upper three 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 I30 PHYSIOLOGY. mesenteric ganglia. From these they are distributed to the abdominal organs, being vaso-constrictor to the vessels, inhibitory 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 four 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. Arm. — These fibres, coming out by the fourth to the tenth dorsal nerves, have their synapses in the sympathetic ganglia of the sympathetic chain, and passing back into the spinal nerves by the grey rami, course to the blood vessels, veins, and sweat glands of the limb. 6. Leg. — The fibres take origin from the eleventh dorsal to the third 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. Certain outgoing fibres to the viscera and involuntary structures pass directly without going through the sym- pathetic ganglia. 1. The third cranial nerve carries fibres which have their synapses in the ciliary ganglion, and pass on to the sphincter pupili 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. 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 oesophagus 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 CENTRAL NERVE. 131 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. B. Ingoing Fibres. — The course of these from the viscera is not so clearly known ; but they appear to enter the cord largely by the white rami. In the normal condition stimula- tion of their peripheral endings does not lead to modifications of consciousness, and is therefore not accompanied by pain. But in abnormal conditions painful sensations are produced. In some cases, abnormal stimulation of visceral nerves leads to painful sensations referred to the cutaneous distribu- tion 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. Roots of the Spinal Nerves. — The posterior root is the great ingoing channel to the spinal cord, and the anterior root is the great outgoing channel. Section of a series of posterior roots leads to loss of sensation in the structures from which the fibres come, and, as a result of cutting off the afferent impressions, to a loss of muscular co-ordination, since the muscle sense, which is essential for the guidance of the combined action of muscle, is abolished. As a result of this section, the part of the fibres cut off from the cells of the ganglia on the posterior root die and degenerate. Therefore if the root is cut inside the ganglion, the degenera- tion extends inwards up the posterior columns of the cord, and if it is cut outside, the degeneration passes outwards to the periphery. Section of the anterior root, causes paralysis of the muscles and other structures supplied by the outgoing fibres, and these fibres die and degenerate. CENTRAL NERVOUS SYSTEM. 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. 132 PHYSIOLOGY. A.— SPINAL COED. The spinal cord is a more or less cylindrical mass of nerve tissues which passes down the vertebral canal. It terminates in a pointed extremity at the level of the 1st lumbar vertebra. There are two enlargements upon it, one in the cervical region, one in the lumbar region, and from these the nerves to the arms and legs come off. A fine central canal S).C. Asc.kDcx Anh Lal- FiG. 62. — Cross Section of the Spinal Cord through the second Dorsal Segment, to show disposition of grey and white matter. P., Posterior Horn ; A., Anterior Horn with large cells ; I.L., Intermedio-lateral Horn with small cells ; L.C., Lockhart Clarke's Column of Cells ; P.M. and P.L., Postero- median and Postero-lateral Columns ; D.C., Direct Cerebellar Tract ; Asc. and Desc. Ant. Lat., Ascending and Descending .Antero-lateral Tracts ; B.B., Basis Bundles; C.Py., Crossed Pyramidal Tract; O.Py., Direct Pyramidal Tract. On opposite side tracts which degenerate upwards are marked with horizontal lines ; tracts degenerating downwards with vertical lines. (After Beuce.) runs down the middle, and the two sides are almost com- pletely separated from one another by an anterior and a posterior mesial fissure (Fig. 62). Each half is composed of a core of grey matter arranged in two processes or horns — the anterior and posterior horns — which divide the white matter surrounding the grey into a posterior, a lateral and an anterior column. In the dorsal region a lateral horn of grey matter projects into the lateral column. The white matter is composed of white nerve fibres, the grey matter CENTRAL NERVE. 133 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 anterior 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 give off visceral fibres. In the dorsal region also a set of cells lie on the mesial aspect of the posterior horn consti- tuting the cells of Lockhart Clarke. Functions. The spinal cord is the great mechanism of reflex action, and the great channel of conduction between the brain and the peripheral structures. A.^ — Reflex Function. If the brain of such an animal as a frog is destroyed, the animal lies for any length of time prone on its belly and immovable. If the skin is pinched the limb is withdrawn, and if a piece of paper dipped in acetic acid is placed on its flank, definite co-ordinated movements are made to remove it. The animal has the power of reflex movement with definite co-ordination of its muscles, but it has no power of balancing itself, and manifests no obviously spontaneous movements. The reflexes connected with various groups of skeletal muscles are definitely associated with different levels of the cord. The reflex movement of the foot on tickling the sole is connected with the part of the cord from which the 1st, 2nd, and 3rd sacral nerves come off. The reflex of the cremaster muscle with the 1st, 2nd, and 3rd lumbar ; that of the abdominal muscles with the 8th to the 12th dorsal ; of the epigastric muscles with the 4 th to the 7 th dorsal ; and of the scapular muscles with the 5th cervical to the 12th dorsal. By taking advantage of these reflexes the condition of the cord at different levels may be studied. Reflex action in connection with various visceral muscles are also connected with the spinal cord. Many of these are complex reflexes involving inhibition of certain muscles and increased action of others, some visceral, some skeletal. The 134 PHYSIOLOGY. best marked of these are the reflex acts of micturition, defsecation, erection, and ejaculation. The lumbar enlarge- ment is the part of the cord involved. The cord is not only capable of reflexly setting up definite contractions in muscles, but it seems to exercise a constant tonic action upon them, and when this tonic action is interfered with, the effect of directly stimulating a muscle is altered, being either increased or diminished. This is very well seen in the contraction of the quadriceps extensor femoris which occurs when the ligamentum patellae is struck sharply Bxternal 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. Fig. 62a. — The Groups of Cells in the Anterior Horn of grey matter at the level of the second, third, and fourth Sacral Nerves. (From Bbucb.) causing a kick at the knee joint — the knee jerk. When the reflex arc in the cord in the lower lumbar region is interfered with, the knee jerk is diminished or is absent, and when the activity of the arc is increased by the removal of the influence of the brain, the jerk is increased. That the jerk is not a reflex is shown by the fact that the latent period is very much shorter than that of a reflex action. The various groups of cells in the anterior horn of grey matter appear to have definite connections with individual muscles (see Fig. 62a). CENTRAL NERVE. 135 This has been shown by investigations of the degeneration of cells which follows partial amputations. B. — Conducting Paths. Outgoing and ingoing fibres chiefly pass down the side of the cord upon which they act or from which they come. Section of one side of the cord leads to loss of the so called voluntary movements below the point of section on that side and loss of sensation over the same parts of the body. Since there is, at least sometimes, a slight loss of voluntary power and of sensation on the opposite side, it has been concluded that some of both sets of fibres decussate in the cord. Fig. 626. — To show the course of upgoing (dotted line) and downgoing fibres (continuous line) in the spinal cord. The two kinds of fibres run in different strands of the cord, and these have been defined by different methods, the most satisfactory of which is that depending upon the fact that nerve fibres degenerate when cut off from their cells, and that they, generally speaking, conduct in the direction in which they degenerate, although, as has been seen in the fibres of the posterior roots of spinal nerves, this is not always the case. If the spinal cord be cut across, certain tracts of fibres de- generate upwards, others degenerate downwards, while some do not degenerate to any great distance from the point of section. A. Fibres degenerating upwards (See Fig. 62). — 1. The fibres making up (he posterior columns which are derived from the posterior roots of the spinal nerves. A few of these fibres degenerate downwards, because the fibres, of the posterior roots bifurcate when they enter the cord, the one division passing right up to the top of the cord, the 136 PHYSIOLOGY. other passing down for a short distance. The ascending fibres of the posterior columns for the most part -end in synapses in the upper end of the cord, in the nucleus gracilis and nucleus cuneatus. From these fresh fibres pass upwards to the cerebrum. 2. A thin layer of fibres round the margin of the lateral column degenerate upwards, and has been traced as far as the cerebellum. Those behind con- stitute the direct or dorsal cerebellar tract. Those in front which take a somewhat different course may be called the ascending antero-lateral or ventral cerebellar tract. They both take origin in Lockhart Clarke's column of cells, which is specially well developed in the dorsal and upper lumbar region of the cord. B. Fibres degenerating downwards. — 1. A very strong band of fibres lying in the posterior part of the lateral column, just inside the direct cerebellar tract, and becoming smaller as the lower part of the cord is reached. This is the crossed pyramidal tract which comes from the cells of the cortex cerebri of the opposite side, and gives off branches to the cells in the grey matter of the cord which control the muscles supplied by them. Certain fibres from the cortex cerebri do not cross, but run down, some in the crossed pyramidal tract, some in (2) the direct pyramidal tract, which runs along the margin of the anterior fissure, and extends downwards only into the dorsal region. 3. A set of fibres just inside the antero-lateral ascending tract, which may be called the antero-lateral descending tract, comes from Deiters' nucleus (see p. 81), and gives off fibres as it passes to the grey matter of the cord control- ing the muscles. Deiters' nucleus is connected with the cerebellum, and these fibres probably carry down impulses from that organ. C. Fibres not degenerating for any distance. — Bound the grey matter a band of fibres — the basis bundles — do not degenerate far, and seem to be commissural between adjacent parts of the grey matter. Other tracts of fibres have been described, such as Lis- sauer's tract and the septo-marginal tract, but their relations have not been satisfactorily investigated. CENTRAL NERVE. 137 These degenerations are often produced by diseases and injuries of the cord, and thus the results experimentally produced on animals have been confirmed on the human subject. The development of the cord also helps to demonstrate these tracts, since it has been found that the fibres of out- going tracts become functionally active, and get their meduUary sheath later than the fibres of ingoing tracts. The crossed and direct pyramidal tracts are non-meduUated at birth, while the ingoing tracts are medullated. B.— THE MEDULLA OBLONGATA. The medulla oblongata may be regarded as the upper end of the spinal cord ; and it connects that structure with the brain. The cord expands and the posterior mesial 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 to the cerebellum to form part of its inferior peduncles — the restiform bodies. Between the lateral and the anterior columns an almond-shaped swelling, the olive, appears. Above this the medulla is encircled by a mass of transverse fibres — the middle peduncles of the cerebellum, or the pons Varolii. The floor of the fourth ventricle is con- stricted above to again become a canal by the approximation of the superior peduncles of the cerebellum (Figs. 63 and 64). The grey matter of the cord gets broken up into separate masses, of which the most important are : — 1. The nuclei of the posterior columns — the nucleus graciUs and nucleus cuneatus — masses of cells and synapses in which the fibres of the posterior columns end, and from which the upgoing fibres of the fillet start. 2. The inferior olivary nucleus which lies in the olive, and which is connected by bands of fibres with the dentate nucleus of the cerebellum. 3. The nucleus of Deiters, lying higher up in the pons Varolii, and connected with fibres from the cerebellum and from the semicircular canals (see Fig. 586). 138 PHYSIOLOGY. 4. The masses of cells from which the cranial nerves take origin (Fig. 65). Conducting Paths — A. Ingoing. — The posterior columns of the spinal cord terminate in the nucleus gracihs and nucleus cuneatus. From these fibres pass downwards and across the middle line forming the decussation of the fillet. The crossed fibres pass up in a vertical series on each side of the middle raphe's until the poDS Varohi is reached, when they spread out horizontally like a fan above the deep trans- FiG. 63. — Cross Section through Medulla Oblongata above the decussation of the Pyramids. P.M. and P.L., Postero-lateral and Postero-median tracts of the Cord. N.G. and N.O., Nucleus Cracilis and Cuneatus, giving off the Fillet Fibres crossing at F. ; V., Ascending Boot of Fifth Nerve ; C?., Nucleus of Glossopharyngeal Nerve ; A.ff., Anterior Horn of Spinal Cord ; P., The Anterior Pyramids ; D.O., Direct Cerebellar Tract ; A. and D. Ant. L., Ascending and Descending Antero-lateral Tracts. (After Bkuce.) verse fibres. Above the pons they divide into two sets — 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 direct cerebellar tract passes up into the restiform body, and so on to the superior vermis of the cerebellum. Its fibres form synapses round cells chiefly on the opposite side. 3. The ascending antero-lateral tract passes up beside the last, but it leaves it in the restiform, and courses for- CENTRAL NERVE. 139 ward, to arch back into the cerebellum round, the superior cerebellar peduncle to form synapses with the cells of the superior vermis. B. Outgoing. — 1. The fibres from the cerebral cortex, which form in the cord the crossed and direct pyramidal tracts, pass down in the middle part of the crusta of the crura, and coursing between the superficial and deep trans- verse fibres of the pons, come to lie in the anterior pyramids of the medulla. At the lower end of the medulla most of v;// AccvV Fig. 64. — Cross Section of Medulla through the Olive. The Central Canal has opened out to form the Floor of the Fourth Ventricle. 4(A V., The Lateral Columns are passing out to form the Inferior Peduncles of the Cerebellum ; P., Fillet; 0., Inferior Olivary Nucleus; P., Anterior Pyramids; Rest., Fibres of Restiform Body ; V., Ascending Root of Fifth Nerve ; 77//. Am. N., Accessory Nucleus of the Eighth Nerve ; XI., Glossopharyngeal Nerve. (After Bkoce.) these fibres cross over 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 antero-lateral tract take origin in a mass of nerve cells, which lies in the dorsal and lateral part of the pons Varolii (Deiters' nucleus). C. Commissural Fibres. (1.) The antero-lateral basis bundles in the medulla form a strong band of fibres connecting the grey matter at different levels, and known as the posterior longitudinal fasciculus. 140 PHYSIOLOGY. (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. Cranial Nerves. — (TAe 'physiology of these should he studied while dissecting them.) The nerves springing from and entering the medulla, do not come off in the same regular fashion as the spinal nerves. The outgoing fibres of each spring from a more or less definite mass of cells, while the ingoing fibres, instead of passing unaltered for a considerable distance, generally form synapses with cells arranged in definite groups. In this waj' the so-called nuclei of the cranial nerves are formed. The position of these is indicated in Fig. 65. In these cranial nerves no Fig. 65. — The Nuclei and Roots of the Cranial Nerves. (After Edinger.) sharp differentiation into anterior and posterior roots can be made out. The XII. (Hypoglossus) is purely an anterior root nerve, and is motor to the muscles of the tongue. The X. (Vagus) is essentially the posterior root of the XI. Spinal Accessory, but it transmits some outgoing fibres. It is the great ingoing nerve from the abdomen, thorax, larynx, and gullet, while, by fibres passing through the vagus or accessorius, it is an augmentor nerve for the muscles of the bronchi and alimentary canal, inhibitory to the heart, dilator to the blood-vessels of the thorax and abdomen, and motor to the muscles of the larynx and to the levator palati. The accessorius is also motor to the sterno-cleido-mastoid and trapezius. The IX. (Glossopharyngeal) is essentially a posterior root, CENTRAL NERVE. 141 j and is the ingoing nerve for the back of the mouth, the Eustachian tube, and tympanic cayity. It transmits out- going fibres motor to the stylo-pharyngeus and middle constrictor of the pharynx. The III. (Oculomotorius), IV. (Trochlearis), VI. (Ab- ducens), and VII. (Facial), along with the V. (Trigeminal), form what may be regarded as a pair. The Trigeminal is chiefly a posterior root, but it has a distinct anterior 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 VII, is almost purely an anterior root transmitting 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 in fibres from the anterior two-thirds of the tongue. The VI. supplies the external rectus of the eye. The IV. supplies the superior oblique. The III. supplies all the muscles of the eyes except those supplied by the VT. and IV. The fibres coming from the. nuclei of these cranial nerves do not always pass out in the nerve itself Thus, fibres from the III. nucleus 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. The other masses of grey matter in the m.edulla with their synapses may be regarded as minor shunting stations. The great hollow cup-like olivary nucleus is connected by fibres with the similar dentate nucleus in the opposite side of the cerebellum. The nucleus of Deiters lying at the side of the pons, and apparently connected with the vestibular root of the' VIII. or auditory nerve seems to receive fibres from the roof nucleus of the opposite side of the cerebellum, and to send fibres on down the cord (Fig. 65). Reflexes of the Medulla. — The various synapses in the medulla form arrangements by which various combined and co-ordinated movements are controlled. Thus, the nucleus 142 PHYSIOLOGY. of the vagus governs the movements of respiration, while other centres preside over the slowing mechanism of the heart. To these various reflex arrangements the name of centres has been given. ■Dent. JResf I"iG. 65. — Cross Section through Kegion of Pons, Cerebellum and Fourth Ventricle. S. v., Superior Vermis ; S.N., Roof Nucleus ; Dent., Dentate Nucleus ; Eest., Restiform Body ; S.P., Superior Peduncle of Cerebellum ; Deit, Deiters' Nucleus; VI., Nucleus of the Sixth Nerve; F., Fillet; S.O., Superior Olive ; D.T. and S.T. Deit., Deep and Superficial Transverse Fibres ; P., Pyramidal Fibres. (After Bruce.) 0. REGION OF PONS VAROLII. In the pons the cerebral fibres presiding over the move- ments of 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. CENTRAL NERVE. 143 The fillet fibres in the pons instead of running up on each side of the middle line, spread out into a horizontal arrange- ment above the crossed fibres. The outgoing fibres from the cerebrum to the cord lie between the deep and superficial crossed fibres (Fig. 65). D. CEREBELLUM. The cerebellum or lesser brain lies above the floor of the fourth ventricle, and is joined to the cerebro-spinal axis by- three peduncles on each side. It consists of a central lobe Fig. 66. — Diagram of the Arrangement of Fibres and Cells in the Cortex of the Cerebellum. Q.L., Molecular Layer ; N.L., Nuclear Layer ; P., Purkinje'a Cells. (After ElMON T Cajal.) and two lateral lobes, each with a secondary small lobe, the flocculus. Its surface is raised into long ridge-like folds running in the horizontal plane, and covered over with grey matter. In the substance of the white matter forming the centre of the organ are two masses of grey matter on each side — -1, the roof nucleus ; and 2, the dentate nucleus (Fig. 66). The cortex may be divided into an outer somewhat homo- geneous layer and an inner layer studded with cells — the nuclear layer. Between these is a layer of large cells — the cells of Purkinje. 144 PHYSIOLOGY. By Golgis' method the arrangement of fibres and cells in the cerebellum 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. 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, to the roof nuclei and Deiters' nuclei. Functions of the Cerebellum. — Kemoval of the cerebellum deprives the animal, for a time at least, of the power of balancing itself. But in some cases, when slowly progressing disease has; destroyed the organ, no loss of equilibriation has appeared, and in other cases the cerebellum has been con- geni tally almost absent, and yet the individual has not shown any sign of want of power of maintaining his balance. Evi- dently, therefore, some other part of the brain can com- pensate for the absence of the lesser brain. The manner in which the cerebellum acts has been chiefly elucidated by removing parts of the organ and keeping the animal under observation for prolonged periods. If one side of the cerebellum is removed the first symptom to be observed is a tonic contraction of the muscles of .the limbs of the sapue side by which the fore limbs may be powerfully extended, the body arches towards the side of the lesion, and the animal may be .driven round its long axis to the opposite side. These irritative symptoms soon pass off, and the animal then manifests weakness in the limbs of the affected side so that it droops to that side, and if a quadruped may circle to that side. After some weeks these symptoms disappear, and the loss of one side of the cerebellum is apparently completely compensated for. From these experiments it would appear that the cerebel- lum is to be regarded as a mechanism supplementary, to the great cerebro-spinal mechanism, and that it has for its purpose more especially the muscular adjustment required in maintaining the balance. This it may do in one or both of two ways. CENTRAL NERVE. 145 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. A channel for such impulses exists 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. To maintain the constant muscular adjustments required in balancing the body implies the existence of arrangements whereby any disturbance of the equilibrium can produce an appropriate reaction. The ingoing impulses which are more especially of service in this way are the muscular sense (see p. 87), the tactile sense from the soles of the feet, the eyes, and the impulses which pass from the semicircular canals and which are set up by acceleration or retardation of motion in any plane or planes of the body. 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, while stopping the table gives rise to a sensation of being rotated in the opposite direction. The semicircular canals form a mechanism which is capable of acting in this way. They are arranged in pairs in the two ears thus — A horizontal canal in each ear. A superior canal in one ear and a posterior in the other. Each one of the pair is in a plane parallel to the other. The two horizontal canals being on a horizontal plane, the superior of one side and the posterior of the other being in parallel planes oblique- to the mesial plane of the body. The horizontal canals may be considered as forming the L 146 PHYSIOLOGY. arc of a circle with an ampulla at each eni The superior canal of one side has its ampulla in front, while its twin — the . posterior of the opposite side — has its ampulla behind, and they together form the arc of a circle with an ampulla at each end (Fig. 67 a). The membranous canals are very small, and occupy but a small part of the osseous canals. The membranous ampulla} are large and almost fill the osseous ampullae (Fig. 676). If the head is moved in any plane certain changes will be set up in the ampulla towards which the head is moving, and converse changes in the ampulla at the other end of the arc of the circle. If, for example, the head is suddenly turned to the right .\/' a. h. Fig. 67. — (o) Arrangement of the Semi-Circular Canals on the two sides ; (6) Bony and Membranous Canal and Ampulla to illustrate their mode of action. the inertia of the endolymph and perilymph tend to make them lag behind. Thus the endolymph in the right ampulla will tend to flow into the canal, but the canal is so small that it will merely accumulate in the ampulla, and thus a large pressure will be produced. The perilymph will tend to lag behind, and a small pressure will result outside. 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 conditions will be induced. In forward movement, the two superior canals have the pressure of endolymph increased in their ampullae — in back- ward 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. CENTRAL NERVE. 147 There is also evidence that the semicircular canals assist in maintaining the tone of the skeletal muscles and , that destruction of the canals is followed by a loss of tone. 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 cere- brum, while posteriorly the two superior peduncles of the Fig. 68. — Cross Section through Anterior Corpora Quadrigemina and Cerebral Peduncles. A.S., Aqueduct of Sylvius ; III., Nucleus of ThircJ Nerve ; S.C.P., Superior Cerebellar Peduncles; F., Fillet; P., Pyramidal Tract; C.C., Cerebello-Cerebral Fibres. ' {After Bkdoe.) cerebellum join together. On these, two swellings develop on each side — the anterior and posterior corpora quadrigemina. 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, and on each side of the cerebeUo-cerebral fibres passing upwards from the pons (Fig. 68). The posterior part or tegmentum contains — 1st, the fillet fibres going partly to the corpora quadrigemina, partly onwards to the thalamus opticus ; 2nd, the nuclei of the 3rd and 4th cranial nerves ; 3rd, the 148 PHYSIOLOGY. fibres of the superior peduncles of the cerebellum which cross the middle line ; and 4 th, the red nucleus in -which most of these fibres end. The functions of this segment of the brain are chiefly conducting, but the anterior corpora quadrigemina form the shunting station between the incoming fibres of the optic tract and the oculo-motor mechanism (see p. 113). F. THE CEREBRUM. Each peduncle terminates in its half of the cerebrum. !FlG. 69. — Diagram of the Arrangement of Cells in a typical part of the Cerebral Cortex. (After Ramon t Cajal.) As the fibres pass from peduncle to cerebrum and vice versa they come into relationship with three masses of grey matter lying in the midst of the cerebrum. These are the thalamMS 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. The fibres, above these nuclei, spread out to form the CENTRAL NERVE. 149 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 (Fig. 71). The method of Golgi shows that the neurons in a typical part of the cortex are arranged in the following manner : — 1. The incoming fibres pass right up to the surface and to end in dendrites. 2. In this region are a set of small horizontal cells with numerous processes. 3. Underneath these are several rows of small pyramidal cells sending dendritic processes to the surface and their axons downwards. Fio. 70. — Diagram of Collateral Connections of different parts of the Cerebral Cortex. {After EAmon y Cajal.) 4. Below these are rows of large pyramidal cells similarly arranged. 5. Lowest of all are some irregular cells with a similar disposition of processes. This arrangement is considerably modified in certain regions of the cortex, the large pyramidal cells being best developed in those parts from which the great mass of fibres passing down to the spinal cord arise. From the various fibres collaterals come off which connect different parts of the cortex of the same side, and which also connect the cortex of one side with that of the other, and with the basal ganglia (Fig. 70). The accompanying diagram shows the more important convolutions of the cortex cerebri (Fig. 71). ISO PHYSIOLOGY. Functions of the Cerebrum — 1. General Consideration. — The functions of the cerebrum may be best understood by first contrasting the condition of animals with, and of animals without, this part of the brain. Pig. 71. — (a) Surface of the left Cerebral Hemisphere to show the arraogement of Convolutions and the situations of some of the Receiving and Discharg- ing Mechanisms ; (b) Mesial Surface of the same Hemisphere. In the frog the cerebral lobes may easily be removed. The animal sits in its characteristic attitude. When touched it jumps, when thrown into water it swims. It is a perfect reflex machine, with the power of balancing itself unimpairied. But it differs from , a normal frog in moving only when CENTRAL NERVE. 151 directly stimulated, and in showing no signs of htinger or of thirst. A worm crawling in front of it does not cause the characteristic series of movements for its capture which are seen in a normal frog. In the pigeon, removal of the cerebral hemispheres reduces the animal to the condition of a somnolent reflex machine. The bird sits on its perch, generally with its head turned back, as if sleeping. If a sudden noise is made, if light is flashed in its eye, or if it is touched, it flies off its perch and lights somewhere else. But every stimulus produces the same result. Clapping the hands and letting peas fall on the floor are both obviously heard and both produce a start, but the bird makes no endeavour to secure the peas as it would do in the normal state. In the dog, by a succession of operations, Goltz has removed the greater part of the cerebral cortex without causing paralyses of the muscles. The animal became dull and listless, and did not take food unless it was given to it. It showed no sign of recognising persons or other dogs, and did not respond in the usual way when patted or spoken to. But it snapped when pinched, shut its eyes and turned its head away from a bright light, and shook its ears at a loud sound. It did not sit still, but walked constantly to and fro when awake. It slept very heavily. In fact all the responses of the animal might be classed as reflex responses to immediate excitation. In monkeys, removal of the cerebral cortex leads to such loss of the so-called voluntary movements that all other symptoms are masked. In the decerebrated animal different stimuli do not produce distinctive reactions, but, with the cerebrum intact, there is at least the possibility of small differences in the modes of stimulation producing marked differences in the resulting action. These resulting actions are in part at least determined by the previous training and education of the brain ; for, just as in the spinal cord channels of action are formed, so in the cerebrum, if a given reaction once follows a given stimulus, it will tend to foUow it again. This training is in part heredi- 152 PHYSIOLOGY. tary. Each individual of a race is born with well established lines of action in the process of development, and throughout life these inherited channels play an important part in deter- mining the results of stimulation. But it is also largely acquired by the ■ individual, since the reception of each stimulus and the performance of a resulting action, however this be determined, tends to lay down a path which will again be followed. Not only will the previous training of the brain thus act as the directive force in the response to stimuli, but the nutrition of the brain also plays an important part. The action of a brain when well nourished and freely supplied with pure blood is often very different to that of the same brain when badly nourished or imperfectly supplied with healthy blood. Since the education of the brain really consists in developing proper responses to various stimuli, the importance of the brain being in a healthy and well nourished condition during the training is manifest. The power of differentiating various stimuli is dependent on the development of the brain, and becomes more perfect as the animal scale is ascended ; and the complexity of the cerebral action in the higher animals has its basis in the greater number of distinct impressions which have been received and reacted to. Each separate stimulus leaves its mark upon the brain, or as we may say, is stored in the brain, and each subsequent similar stimulus is sent into these channels, or is associated with the past reactions, and thus the present response is determined, what may be described as an unconsciousness judgment being made. So far, cerebral action may be considered in a purely material manner as consisting of a series of reflex acts, higher and more complex than those in the spinal cord, in which the result of the stimulus varies in the same way, but to a much greater degree, from its association with more complex past impressions. But this cerebral action is frequently accompanied by changes in the consciousness of the individual, some of which are termed simply sensations, while others may be described as trains of thought. Essentially, however, a train of thought is nothing more than a train of sensations, each CENTRAL NERVE. 153 evoked by a stimulus or by a preceding sensation, and, if this be admitted, we may say that it is by sensation alone that we are aware of consciousness, and that, therefore, the two are co-terminous. It is impossible to conceive consciousness without sensation, or sensation without consciousness. Cerebral action frequently goes on without consciousness being implicated ; but so far as we know, consciousness with- out accompanying cerebral action is unknown, and there is evidence that it is only when the action of the various parts of the cerebrum is co-ordinated that consciousness is possible. In cases of Jacksonian epilepsy, where, as a result of a small centre of irritation, a violently excessive action of the cerebral neurons, starting at some centre, passes to involve more and more of the brain, it is found that at first the patient's con- sciousness is not lost, but that, when a sufficient area of brain is involved in this excessive and unco-ordinated action, con- sciousness disappears. The study of the action of drugs which abolish conscious- ness — e.g., chloroform and morphine — on the dendrites of brain cells, suggests a physical explanation of the loss of consciousness. It is found that these drugs cause a general extension of the gemmules of all the dendrites ; and, if we imagine that the co-ordinated action of any part of the brain is secured by definite dendrites of one set of neurons corning into relationship with definite dendrites of another set of neurons by their gemmules, the want of co-ordinate relationship established by the general expansion might obviously explain the disappearance of the definite sensations which constitute consciousness. It is manifest that the range of consciousness must neces- sarily be wider where the stored impressions are most abundant, and where the present stimulus most readily calls into action these previous lines of cerebral activity. It is the implica- tion of consciousness in this part of brain action which is the basis of mental activity. How far the mental action is a mere accompaniment of the physical changes, and how far it can react upon them, is a question which cannot be dis- cussed here. But the study of the insane seems to point to the conclusion that the individual does certain things and has 154 PHYSIOLOGY. certain ideas, rather as a result of faulty brain action, than that the ideas he possesses are the cause of the actions. 2. Time of Cerebral Action. — The cerebral mechanism takes a very appreciable time to act, and the time varies with the complexity of the action and with the condition of the nervous apparatus. Of the time between the application of a flash of light to the eye or a touch to the skin and a signal made by the person acted upon when it is perceived, part is occupied by the passage of the nerve impulses up and down the nerves and in the latent period of muscle action, but something over xVth second remains, representing the time occupied in the cerebral action. {Practical Exercise.) If the observation be complicated by requiring a distinc- tion to be made between different parts stimulated, the reaction time is longer, and, the more unaccustomed the differentiation, the longer will the reaction time be. Thus in one accustomed to deal with figures, the discrimination of a series of these is more rapidly carried out than in one unaccustomed to do so. Prolonged action of the nerve centres soon leads to a prolongation of the reaction time and the same thing is produced by the action of alcohol, chloroform, and other poisons. This fatigue of the nerve mechanism is . the physical basis of that state of the consciousness which is called loss of attention. 3. Fatigue of Cerebral Mechanism. — This naturally leads to the consideration of fatigue of the cerebral mechanism. The way in which, as a result of poisons, the definite co-relationship of certain sets of neurons with certain other sets is abolished by the generalised expansion of the gemmules of the dendritis has been already dealt with. In all probability the same thing occurs in fatigue, and by interrupting the definite chain of action allows rest and recovery to supervene. But continued action leads to well- marked changes in the cell protoplasm of the neurons. The Nissl's granules diminish and the nucleus shrivels and becomes poorer in chromatin. In all reflex action, whether spinal or cerebral, it is the central part of the, mechanism which flrst becomes fatigued. If, by direct excitation of CENTRAL NERVE. I5S the central nervous system of a frog, muscular movements are caused for some timCj the stimulation ultimately fails to act ; but, if the nerves going to the muscles are stimu- lated, the muscles at once respond, showing that the central mechanism has given out before the peripheral structures. Fatigue of the central nervous system is manifest both upon the receiving and reacting mechanism ; upon the, receiving, on the physical side by prolongation of the reaction period, and on the metaphysical side by diminished power of attention. Upon the reacting side it is shown by lessened power of muscular contraction. 4. Sleep. — Fatigue of the cerebral mechanism is closely connected with sleep. As the result of fatigue, external stimuli produce less and less effect, and thus the changes which are the physical basis of consciousness become less and less marked. At the same time, by artificial means stimuli are usually excluded as far as possible. Absence of light, of noise, and of tactile and thermal stimuli all conduce to sleep. Consciousness fades away, and, as the cerebral activity diminishes, the blood vessels of the brain contract and the organ becomes more bloodless. The eyelids close, the eyeballs turn upwards, the pupils contract, and the voluntary muscles relax. The depth of sleep may be measured by the strength of the stimuli required to overcome it, and it has been found that usually sleep is deepest at about the end of an hour, and that it then rapidly becomes more and more shallow until, as the result of some stimulus, or when the brain has regained its normal condition, it terminates. In the later hours of sleep the consciousness may be temporarily aroused without the other conditions of sleep disappearing, arid as a result of this dreams may ensue. Or on the other hand; without consciousness being necessarily restored, stimuli may lead to muscular response of a perfectly definite and purposive character, and sleep-walking may occur. 5. Localisation of Functions. — The question must next be considered whether special parts of the brain are more especially connected with its three great functions — 1. The reception of stimuli. 156 PHYSIOLOGY. 2. The storing of effects, and the associating of present stimuli with these stored impressions. 3. The production of the resulting actions. 1. Reception of Stimuli. — In investigating the existence of special mechanisms, for this purpose, two methods of inquiry are available. Isi, By removing or stimulating parts of the brain in the lower animals and studying the results. Ind, By observations during life on the sensations or absence of sensation in patients suffering from disease of the brain, and the determination of the seat of the lesion after death. \st, Sensations are the usual accompaniment of the activity the receiving mechanism, but, in the lower animals, it is not possible to have a direct expression of whether sensations are experienced or not, and, therefore, we have to depend on the absence of the usual modes of response to any given stimulus in determining whether removal of any part of the brain has taken away the power of receiving impressions. But the absence of the usual response may mean, not that the receiv- ing mechanism is destroyed, but either that the reacting mechanism is out of action or that the channels of conduc- tion have been interfered with. Thus, if light be flashed in the eye of a monkey, it re? spends by glancing towards the source of illumination, and if these movements are absent this may be due to loss of the receiving mechanism, to loss of the mechanism causing the movements, or to interruption of the channels between these. Again, in the lower animals, stimulation of a part of the brain, if it be connected with the reception of impressions, may cause the series of movements which naturally follow such an impression. But these movements may also be caused by stimulating directly the reacting mechanism. When, however, removal of a part of the brain causes no loss of power of movement, and yet prevents a stimulus from causing its natural response, it is justifiable to conclude that that part of the brain is connected with reception. CENTRAL NERVE. I57 'ind. In man, the chief difficulty of obtaining information is in finding cases where only a limited part of the brain is affected. But such cases have, in many instances,, been observed. Tumours of an occipital lobe, for instance, have been found to be associated with loss of visual sensations without loss of muscular power, and thus the conclusion has been drawn that the occipital lobe is the receiving mechan- ism for stimuli from the eyes. Visual Centre. — The way in which the fibres, coming from the two retinae, are connected with each thalamus opticus and occipital lobe has been already considered, and it has been found that the optic tract passes into the geniculate bodies or the posterior aspect of the thalamus, and that a strong band of fibres called the optic radiation extends from these backwards to the occipital lobes (Fig. 53). An extensive lesion of one— say the right — occipital lobe is accompanied by no loss of muscular power but by blindness for all objects in the opposite side of the field of vision — i.e., the right side of each retina is blind. The central spot of neither eye is completely blinded because the fibres from the macula lutea only partially decussate at the chiasma. Probably each part of the occipital lobe is connected with definite parts of the two retinae. Certain it is that there is no part of this cortical mechanism connected with the blind spot, and hence this is not perceived in ordinary vision. Colour perception seems to be a less fundamental function of the visual apparatus than perception of the degree and locality of stimulation, and hence colour perception may be lost in less extensive lesions of the occipital lobes without the other functions being impaired. One of the most recently acquired functions of the mechanism is the power of appreci- ating the significance of the signs used in written language, and it is found that in small and superficial lesions this function may alone be lost. The cortex of the occipital lobe is rich in the smaller variety of cells and poor in the larger pyramidal cells, which are found in areas connected with the production of move- ments. The two occipital lobes act together, even in vision with 158 PHYSIOLOGY. one eye. The lower segments of the retina — i.e., the upper field of vision — are associated with one part of the mechanism, the right halves of the retinse with another part, and so on. There is no reason, a 'priori, why stimulation of one part of our occipital lobe should affect our consciousness, as due to light coming from above or below us. It is only as a result of experience, of training of this mechanism, working in co-operation with the mechanism connected with touch, that the state of consciousness accompanying changes in one part comes to be associated with any given direction of the source of illumination. The strength of the sensation depends upon the strength of the stimulus, and the smallest difference of sensation which can be appreciated is a constant factor of the degree of stimulation. Thus, to produce a change in visual sen- sation, the strength of the stimulus must vary by about T-Jiyth of the stimulus. The sensation lasts longer than the stimulus, and thus, if a series of stimuli follow one another at sufficiently rapid intervals, a fusion of sensation is produced. If a wheel rotat- ing slowly is looked at, the individual spokes are seen, but when it is going more rapidly, the appearance of a continu- ous surface is presented. If the light is dim this fusion takes place more readily than when the light is bright. From this it is concluded that a strong stimulus causes a more sudden and acute sensation than a weak one, and, therefore, the individual sensations are distinguished. While there is good evidence that a special loca,lised area of the cortex is connected with the reception of stimuli from the eyes, the evidence of the existence of similar areas or centres connected with the other organs of special sense, is by no means satisfactory. Auditory Centre. — Terrier, by removing the superior tempor-sphenoidal lobe in the monkey, produced no motor disturbance, but found evidence of loss of hearing in the opposite ear. He describes the monkey as pricking its ears, and looking to the opposite side when the region is stimu- lated, and he considered that this observation proves the CEJJTRAL NERVE. 159 existence of a special localised mechanism for the reception of stimuli from the ear. But, more recently, Schafer has removed these convolutions from both sides in the monkey, with aseptic precautions, kept the animal alive, and found no evidence of loss of auditory sensations. This conclusion depends upon the observation that stimulation of the audi- tory mechanism in such animals still leads to the usual muscular response. But since lower connections probably exist between the auditory nuclei in the medulla and the centres for muscular movement in the spinal cord, this observation cannot be accepted as excluding the relation- ship of the superior temporo-sphenoidal convolution with hearing. Taste and Smell. — So far satisfactory evidence of the existence of a special localised mechanism connected with these senses is wanting. Ferrier states that removal of the hippocampal convolution in monkeys leads to loss of taste and smell, and that stimulation causes torsion of the nostrils and lips, as if sensations of smell and taste were being experienced. But Schafer was unable to observe any indica- tion of loss of taste and smell in monkeys from which the temporo-sphenoidal lobe had been to a large extent re- moved. Other experimenters have observed interference with the sense of smell in destructive lesions of the hippocampal lobe, and one case at least has been described in which a tumour of the right gyrus hippocampus was associated with sensa- tions of smell. In monotremes the fibres from the olfactory lobe have been traced to the neighbourhood of the hippo- campus. Touch. — Here, again, the difficulty of drawing conclu- sions from observations on lower animals is encountered. Ferrier thought that removal of the hippocampal convolution caused loss of tactile sense, while Schafer believes that the gyrus fornicatus is the centre for this sense. It has been objected that in removing this lobe the fibres going to the areas on the outside of the cortex are apt to be injured. According to the observations of Mott, when the cortex round the fissure of Kolando — in which the mechanism for i6o PHYSIOLOGY. causing the various combinations of muscular movements is situated — is removed in the monkey, clips may be attached to the skin on the opposite side of the body without attract- ing its attention, while if they are placed on the same side they are at once removed ; and he therefore regards the Kolandic area of the brain as connected with the reception of tactile impressions. As already indicated, this centre must act as a chart of the surface of the body, stimulation of any definite part of the body leading to changes in a definite part of the centre, and these changes are accompanied by sensations referred to the part stimulated. We know nothing of the existence of centres connected with the thermal and muscular senses. 2. Storing and Association Mechanism. — The existence of special part or parts of the brain connected with the storing of impressions, so that they may be associated with present sensations, is indicated by the following considerations : — It is this association of present stimuli with past sensations which is the basis of intellectual life, and in man, where apparently the intellectual functions are most highly developed, the frontal lobes of the brain are much larger than in the lower animals. So far stimulation of these frontal lobes has failed to give indication of resulting sensa- tions or to produce muscular movements. They may be extensively injured without loss of sensation and with- out paralysis, and hence it has been concluded that in them the storing and associating functions must be chiefly located. 3. Discharging Mechanism. — ^The position of the discharg- ing mechanism for cerebral action has been definitely localised, by pathological and experimental observation, in the cerebral convolutions round, or probably chiefly in front of the fissure of Rolando. Certain lesions of this area on one side lead to want of response to cerebral action of groups of muscles on the opposite side of the body. The cerebral arc is stimulated, and acts along certain lines, possibly with CENTRAL NERVE. i6i the accompanimeiit of consciousness, in a sensation of decision as to the line of action to be taken and a desire to accomplish it, but this so-called volition is not accompanied by the appropriate muscular action. From the frequent involve- ment of the so-called volition in these actions, and from the fact that these metaphysical changes figure more largely in our consciousness than the physical changes which are their basis, we are accustomed to assume that the movements produced are the result of volition, and to speak of them as voluntary movements, and of the brain mechanism producing them as voluntary centres. There is no harm in doing so if /tnuaiva^"*- FlO. 74. — Left Hemisphere of Braiu of Chimpanzee to show Motor Areas (dotted). {From Gbunbaum Sheekington.) we remember that these centres can and do act without the involvement of consciousness, and, therefore, without volition ; but that their action generally implies the previous action of parts of the receiving and associating mechanism of the cerebrum. But certain lesions may directly stimulate these centres causing them to act without the previous action of the other cerebral mechanisms. This is seen in Jacksonian epilepsy where, as the result of, it may be, a spicule of bone or a thickened bit of membrane, one part of the cortex is from M i62 PHYSIOLOGY. time to time excited, and by its action produces movement of certain groups of muscles. Experimental observations have fully confirmed and extended the conclusions arrived at from such pathological evidence. If parts of these convolutions be excised in the monkey the animal loses the power of voluntary movement of certain groups of muscles, while if they are stimulated by electricity these groups of muscles respond. These Rolandic convolutions may be considered as a map of the various muscular combinations throughout' the body, the map being mounted so that the lower part represents the face, the middle part the arm, and the upper part the leg. Each large division is filled in so that all the various com- binations of muscular movement are represented. In these areas the lesion must be extensive to cause com- plete paralysis of any group of muscles. A limited lesion may simply cause a loss of the finer movements. Thus, a monkey with part of the middle portion of the Rolandic areas removed may be able to mov6 its arm and hand about, but may be quite unable to pick up objects from the floor of its cage. The early loss of the most recently acquired functions is well illustrated by the results of lesions of the left inferior frontal convolution — the area which presides over movements of the lips and tongue. This is the centre which has to be specially educated in the use of spoken language ; and, when this region is only slightly injured, the power of using language may be lost without there being any impairment in the power of moving the muscles. Similarly, in the hand area, while crude lesions are required to cause loss of power of moving the hand, very slight interferences with nutrition may cause loss of power of writing. Part M. NUTRITION OF THE TISSUES. SECTION VI. A. — Fluids Bathing the Tissues. BLOOD AND LYMPH. The blood carries tlie necessary nourishment to the tissues, and receives their waste products. But it is enclosed in a closed systena of vessels, and does not come into direct rela- tionship with the cells. Outside the blood vessels, and bath- ing the cells, is the lymph which plays the part of middleman between the blood and the tissues, receiving nourishment from the former for the latter, and passing the waste from the latter into the former. A. Blood. The various physical, chemical, and histological characters of blood 'must be practically investigated by the student, and a lengthy description of these points is therefore unnecessary. I. General Characters. Colour. — Blood changes its colour from purple to cinnabar red on shaking with air, showing that the pigment of the blood may exist in two conditions. Elements of Blood. — Microscopic examination shows that blood is composed of a clear fluid (Liquor Sanguinis or Plasma) in which float myriads of small disc-like red cells (Erythrocytes), and a smaller number of greyish cells (Leucocytes). The Opacity of Blood is due to these red corpuscles. When the pigment 163 i64 PHYSIOLOGY. is dissolved out of them by water, and they are thus rendered transparent, the blood as a whole becomes transparent and "laked." Specific Gravity is about 1055. It may be estimated by finding the specific gravity of a mixture of chloroform and benzole in which a drop of blood neither sinks nor floats. Taste and Smell are characteristic, and must be experi- enced. Reaction. — Blood is alkaline, and the degree of alkalinity is very constant in health. It is equivalent to about O'S per cent, of NagCOg. It is increased during digestion, and diminished after muscular exercise. Clotting or Coagulation. — In the course of about three minutes the blood when shed becomes a sohd jelly. The process starts from the sides of the vessel, and spreads through- out the blood until, when clotting is complete, the vessel may be inverted without the blood falling out. In a short time drops of clear fluid appear upon the surface of the clot, and in a few hours these have accumulated to a considerable extent, while the clot has contracted and drawn away from the sides of the vessel, until it finally floats in the clear fluid — the Serum. Clotting is due to changes in the plasma, since this fluid will coagulate in the absence of corpuscles. The change may be represented thus : — Blood Plasma Corpuscles Serum Clot The change consists in the formation of- a series, of fine elastic threads of fibrin throughout the plasma, and if red corpuscles are present they are entangled in the meshes of the network and give the clot its red colour. These threads may be readily collected in mass upon a stick with which the blood is whipped as it is shed. The red fluid blood which is left is said to be defihrinated. Fibrin is a proteid substance. It is slowly dissolved in BLOOD. i65 solutions of neutral salts. It is coagulated by heat, and is precipitated when an excess of a neutral salt is added. It is therefore a globulin. The plasma before clotting and the serum squeezed out from the clot agree in containing an albumin (serum albumin) and a globulin (serum globulin) in the same proportion in each. But the plasma contains a small quantity — about 0'2 per cent. — of another globulin (fibrinogen) which coagulates at a low temperature, and which is absent from serum. It is this which undergoes the change from the soluble form to the insoluble form in coagulation. When separated from the other proteids it can still be made to clot. The substance usually causing clotting appears to be an enzyme, which is formed by the union of a nucleo-proteid or a derivative of a nucleo-proteid with a lime salt. The enzyme may be called thrombin, and its precursor prothrom- bin. Oxalates, when added to blood, precipitate the soluble lime salts, and prevent the formation of thrombin, and thus prevents coagulation. There is some evidence that the prothrombin exists in solution in the plasma, but it may be derived from the breaking down of the cells of the blood. It is also formed from the breaking down of the cells of such tissues, as lymphatic glands and thymus. Many circumstances iniluence the rapidity of clotting. Temperature has a marked effect ; a low temperature re- tarding it, a high temperature 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, coagula- tion is prevented. Salts of lime have a marked and im- portant action. If they are precipitated 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 an extract of the head of the medicinal leech, retards coagula- tion after the blood is shed. Why is it that blood does not coagulate in the vessels and does coagulate when shed ? Such a general statement is i66 PHYSIOLOGY. not absolutely correct, for blood may be made to coagulate in the vessels of a living animal in various ways. If inflam- mation 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 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 prothrombin, although curi- ously enough the injection of pure thrombin does not usually act in this way. Nor does blood necessarily coagulate when shed. If it is received into castor oil, or into a vessel anointed wi£h vaseline and filled with paraffin oil, it wiU remain fluid for a considerable 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 means of it wounds in blood vessels are sealed and haemorrhage stopped. Although an important and very prominent change in the blood, clotting is really produced by change in one constitutent of the plasma, which is present in very small quantities. II. Plasma and Serum. These may be considered together, as 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 trans- parent, but after a fatty diet they become milky. They are alkaline in reaction, and have a specific gravity of about 1026. They contain about 90 per cent, of water and 10 per cent, of solids. The chief solids are the proteids — serum albumin and serum globulin (with, in the plasma, the addition of fibrinogen). The proportion of the two former proteids BLOOD. 167 to one another varies considerably in different animals, but in the same animal at different times the variations are small. The amount of albumin is generally greater when the body is well nourished. In man, they together form about seven per cent, of the serum. The other organic constitutents 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. It is probably in larger amount in blood going to muscles than in blood com- ing from muscles, and this difference is 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 afterwards see that it is probably derived from the liver, and that it is excreted in the urine by the kidneys. Creatin, uric acid, 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 abundant 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. Leucocytes — White Cells. — These are much less numerous than the red cells presently to be considered, and their number varies enormously in normal conditions. On an average there are about 7500 per cubic millimetre. They are soft, extensile, elastic, and viscous, and each contains 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 i68 PHYSIOLOGY. nucleus varies greatly, and from this and variations in the protoplasm, 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. 75 6 and, c). 2nd, Polymorpho-nuclear leucocytes, with a much dis- torted and lobated irregular nucleus and a finely granular protoplasm, staining 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. e JFis. 75.— Cells of the Blood, (a) Erythrocytes ; (6) Large, and (c) Small Lymphocyte ; (d) Polymorpho-Nuclear Leucocyte ; (e) Eosinophil Leuco- cyte. 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 constitutents of the blood, but appear when the activity of the bone marrow is increased in certain pathologi- cal conditions. These various forms have certain properties — Amoeboid BLOOD. 169 movement. They can under suitable conditions under- go 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 can in certain conditions creep out of the blood-vessels and wander into the tissues (Biapedesis). Phagocyte action. — The finely granular corpuscles have further the power of taking foreign matter into their interior, of forming around it a vesicle, 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. In disposing of these micro-organisms the large granular corpuscles seem also to play an important part. 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-albumin, along with two globulins and a small amount of an albumin, are found. Along with these proteid substances glycogen and a small amount of fat are present, while the chief inorganic constituents are potash salts. 2. Blood Platelets. — These are small circular or oval dis- coid bodies, about one-third the diameter of a red blood corpuscle. 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, possibly by yielding thrombin. They are present in the blood of mammals only. 3. Erythrocytes — Red Cells. — Shape. — All 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. I70 PHYSIOLOGY. 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 human erythrocytes is fairly constant — on an average 7 '6 m. in diameter. The number of red cells in health is fairly constant, about 5.000,000 in men and 4,500,000 in women, per cubic millimetre ; but in disease it is often decreased. The pale yellow colour of the individual corpuscles is due to the pigment they contain being spread out in so thin a layer. This pigment may be dissolved out by various agents : — e.g., salts of the bile acids, distilled water, dilute acids, or alkalis, &c., leaving a colourless shadow-like corpuscle, which seems to be composed of a sponge-like stroma in the interstices of which the pigment is held. The pigment constitutes no less than 90 per cent, of the solids of the corpuscles. Chemistry, — The strcmia of the erythroctes is made up of a globulin substance in connection with which lecithin and cholesterin occur in considerable quantities. Potassium is the base most abundantly present. The pigment is Hsemo* globin. In many animals when dissolved from the cor- puscles this substance tends to crystallise 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 occurs if the haemoglobin is in solution, or if it is still in the corpuscles. The addition of any reducing agent such as sulphide of ammonia or a ferrous salt also causes the red colour to be replaced by a purple hue. This is due to the fact that haemoglobin has an affinity for oxygen, which it takes up from the air, becoming of a bright red colour. If by any means this oxygen is taken away from it it becomes purple. Not only does the colour change, but the absorption spectrum also alters. The two bands in the green and yellow which are characteristic of dilute solutions of haemoglobin combined with oxygen become replaced by a single broad band in the same position. The BLOOD. 171 haemoglobin without oxygen is spoken of as haemoglobin, or reduced hasmoglobin — when combined with oxygen it is called oxyhsemoglobin. Haemoglobin is closely allied to the proteids, but differs from them in containing 0'42 per cent, of iron. Oxyhsemoglobin is a definite combination of haemoglobin with oxygen. 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 haemoglobin 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, just as it may be made to act when in solution in a test tube containing sulphide of ammonia or a ferrous salt. Haemoglobin constitutes about 13 or 14 per cent, of the blood, but in various diseases its amount is decreased. Methsemoglobin. — Haemoglobin forms another compound with oxygen — methaemoglobin. The amount of oxygen is the same, but methaemoglobin 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 for want of oxygen. It may be produced by the action of various substances on oxyhaemoglobin. Among these are ferricyanides, nitrites, and permanganates. It crystallises in the sanie form as oxyhaemoglobin, but has a chocolate brown colour. Its spectrum is also different from haemo- globin or oxyhaemoglobin, 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. It is of importance as it occurs in the urine in some pathological conditions. In all proba- bility, while the molecule of oxyhaemoglobin has the formula — /O . ^0 Hb<' I , in methaemoglobin the atoms are arranged Hb^' \o \0 Haemoglobin also combines with certain other gases. Among these is Carbon monoxide. For this gas ' haemoglobin has a greater affinity than for oxygen, so that when carbon 172 PHYSIOLOGY. monoxide hsemoglobin is once formed in the body, the blood has little power of taking up oxygen, and the animal dies of suffocation. This gas is evolved freely in the fumes from burning charcoal, and the production of the compound in the blood is the cause of death from inhaling such fumes in closed spaces. It is also found in the air of coal mines after explosions. The carbon monoxide hsemoglobin forms crystals like oxyhsBmoglobin, and has a bright pinkish red colour, without the yellow tinge of oxyhsemoglobin. Since after death it does not give up its carbon monoxide and become changed to purple hsemoglobin, the bodies of those poisoned with the gas maintain the florid colour of life. Its spectrum is very like that of oxyhsemoglobin, the bands being slightly more to the blue end of the spectrum. It may be at once distinguished by the fact that with sulphide of ammonia it does not yield reduced hsemoglobin. Decomposition of Haemoglobin.— Hsemoglobin is a some- what unstable bodj^, and, in the presence of acids and alkalis, splits up into about 96 per cent, of a colourless proteid sub- stance belonging to the globulin group and about 4 per cent, of a substance of a brownish colour called haematin. The spectrum and properties of this substance are different in acid and alkaline media. In acid media it has a spectrum closely resembling methsemoglobin, but it can at once be distinguished by the fact that it is not changed by sulphide of ammonia. Hsematin contains the iron of the hsemoglobin, and in alkaline solution it can take up and give off oxygen in the same way as hsemoglobin does. It is therefore the pigmented iron-containing part of the hsemoglobin molecule which has this reaction with 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 hcematin, hsematoporphyrin, is formed, which has no affinity for oxygen. This pigment occurs in the urine in some pathological conditions. (Prac- tical Exercise.) In the liver hsemoglobin is broken up to form bilirubin and the other bile pigments. These are iron-free, and like iron-free hsematin do not take up and give off oxygen. But BLOOD, 173 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 hsemoglobin in blood extravasations into a pigment known as hsematoidin, which is really the same as bilirubin. Hsemin — the hydrochloride of hsematin — is formed when blood is heated with chloride of sodium and glacial acetic acid. It crystallises in small steel black rhombic crystals, and its formation is sometimes used as a test of blood stains. The following table shows the relationship of these pig- ments to one another : — Relationship of Hb and its Derivations. Methsemoglobin HbOj HbCO , Hb Hsematin Globin Acid Hsematin Alkaline Hsematin Oxidised Reduced \ Contain Iron. (Hsemochromogen), Iron-free Hsematin (Hsematoporphyrin) Hsematoidin Bilirubin Iron Free. IV. — Gases of the Blood. The muscle 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 174 PHYSIOLOGY. pumps have been invented. One of the simplest methods is that of Haldane and Barcroft, which depends upon the fact that the oxygen can be driven oif from blood treated with dilute ammonia by the addition of ferricyanide of pbtassium, and that the carbonic acid is liberated by adding an acid. {Practical .Exercise.) About 60 c.c. of gas measured at 0° C. and 760 mm. pres- sure can be extracted from 100 c.c. of blood. The propori tion of the gases varies in arterial and in venous blood. Per Cent, of Gases in 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 haemoglobin. Carbon dioxide. — In the animal body the blood could dissolve about 2 J per cent, of carbon dioxide. But it may contain as much as 46 per cent. Hence the greater part of the gas must be in chemical combination. Since the proportion of carbon dioxide is greater in the plasma than in entire blood, this gas must be held in the plasma. 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. Carbonate of soda and basic phosphate of soda are therefor present together in the plasma, and if a stream ot carbon dioxide be passed through a solution of basic phosphate of soda,, it appropriates a certain amount of BLOOD. 175 soda and leaves the neutral phosphate. On the other hand, if the amount of carbon dioxide is small, the phosphoric acid again seizes on the soda, and turns out the ca,rbon dioxide. Thus the soda is the carrier of carbon dioxide in the blood plasma. The blood, in passing through the tissues which are loaded with this gas, gains carbon dioxide, some of the basic phosphate of soda being changed to the less alkaline, neutral phosphate. When the lungs are reached the blood is exposed to air, poor in carbon dioxide, and thus the phos- phoric acid is able to turn it out. In fact, the carriage of carbon dioxide and its excretion are effected by a struggle between that gas and phosphoric acid for the soda 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 there- fore 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 proteids has not been fully investigated. Undoubtedly they are partly derived, somewhat indirectly as we shall afterwards see, from the proteids of the food. Very probably, too, they are in part derived from the tissues! But the significance of the two proteids, albumin and globulin, and their variations has not yet been elucidated. The glucose is derived from the carbohydrates and possibly from the proteids of the food, and during starvation it is constantly produced in the liver and poured into the blood. The fats are derived from the fats and carbohydrates and possibly from the proteids of the food. The urea and other waste constituents are derived from the various tissues. B. Of the Cells — I. Leucocytes. — These are formed in the lymph tissue and in the red marrow of bone. 1. Lymph Tissue (see p. 28) is very widely distributed in the body, occurring either in patches of varying shape and size, or as regular organs, the lymphatic glands. 176 PHYSIOLOGY. These are placed on the course of a lymphatic vessel, 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. Eound some of the lymphatic glands of certain animals large blood spaces are seen, and these glands are called hsemolymph glands. They are intermediate between lymphatic glands and the spleen. Thymus. — In foetal life and in the early years of child- hood, a large much lobulated mass of lymph tissue lies in the anterior and upper part of the thoracic cavity, but it shrivels as age advances. In germ centres, wherever situated, the development of lymphocytes goes on, and the cells pass away in the lymph or blood stream. In certain pathological conditions the formation of these cells is abnormally increased and a lymphocytosis, or increase in their number in the blood, results. 2. Bone Marrow.— The structure of bone marrow is con- sidered below, but it may be stated here that young leuco- cytes in the condition of mitosis are abundant, and that they seem to 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. II. Erythrocytes. — These are formed after birth in the red marrow of bone. Marrow consists of a fine fibrous tissue with large blood capillaries or sinuses running in it. In the fibrous tissues are numerous fat cells, and generally a considerable number of multi-nucleated giant cells. In addition to these are the young leucocytes, leucoblasts, which used to be called the proper cells of the bone marrow, and lastly young nucleated red cells, the erythroblasts. After haemorrhage, the formation of these becomes unusually active, and may implicate parts of the 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. BLOOD. 177 VI.— Total Amount of Blood in Body. This was formerly determined by bleeding an animal, measuring the amount of blood shed, and determining the amount of haemoglobin contained in it ; then washing out the blood-vessels, and after measuring the amount of fluid used, determining the amount of haemoglobin in it to ascer- tain the amount of blood it represented. By this method the amount of blood was found to be about -fv of the body weight. Haldane and Lorrain Smith have devised a method which can be applied to the living man. It depends upon the fact that after a 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 contains 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 volume of the blood is about ^V^li of the weight of the body. VII.— Distribution of the Blood. Koughly speaking the blood is distributed somewhat as follows : — Heart, lungs, large vessels, . . . \ Muscles, ...... \ Liver, ...... \ Other organs, ... • i VIII.— Fate of 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 proteids we know nothing. They must be used up in the construction of the tissues, but experimental evidence of this is wanting. The glucose and fat are undoubtedly used up in the tissues. 178 PHYSIOLOGY. The urea and waste products are excreted by the kidneys. The fate of the salts is not fully worked out. The chlorides are partly excreted by the kidneys and are partly split up to form the hydrochloric acid required for stomach diges- tion. The phosphates and sulphates are excreted in the urine, but whether they are also used in the tissues is not known. The leucocytes break down in the body — but when and how we do not know. We shall afterwards find that they are greatly increased in number after a meal of pro- teids, and since the increase is only very transitory they must be broken down, probably to feed the tissues. It would thus seem that a leucocyte lives in the blood only ■ for a short time. 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 fort- night 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 Haemolysis. — The process - of breaking down old erythrocytes and eliminating their pig- ment is often called the prpcess of haemolysis. Certain organs appear to be specially connected with this, but the precise part played by each of them is not very clearly under- stood. 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 of bile pigment, a derivative of haemoglobin, in the liver cells ; 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 (p. 308) may 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 has little power of storing iron. The spleen is generally said to have a similar action. This organ is composed of a fibrous capsule containing non- BLOOD. 179 striped muscle and a sponge work of fibrous and muscular trabeculae, in the interstices of which is the spleen pulp. The branches of the splenic artery run in the trabeculse, and twigs leaving these trabeculse pass out and are covered with masses of lymph tissue forming the Malpighian corpuscles. Beyond these the vessels open into a series of complex sinuses ifrom which the blood is collected into channels, the venous sinuses, which carry it back to branches of the splenic vein in the trabeculse. The pulp is thus made up of processes of fibrous tissue and- of a set of branching endothelial like cells, the spaces between which are filled with blood. So far no decrease in the number of i erythrocytes in the blood leaving the organ has been recorded. In the cells of the spleen .pulp, yellow pigment and simple iron compounds are frequently seen, indicating that haemoglobin is being broken down. But the idea that the spleen plays an im- portant part in the actual breaking down of erythrocytes seems to be negatived by the fact that when blood .is in- jected, the cells are broken down no faster in an animal with the spleen than in an animal from which the spleen has been removed. The spleen is to be regarded rather as a grave of dead erythrocytes than as an organ actively engaged in destroying them. 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. Lymph Glands and Haemolymph Glands. — When erythrocytes break down or haemoglobin is injected the pigment accumulates in these, and they therefore . probably act as the gi-aves of old erythrocytes in the same way as the spleen. B. Lymph. 1. Characters of Lymph. — The Ijonph 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 i8o PHYSIOLOGY. 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. The lymph is derived partly from the blood and partly from the tissues. It 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 pericar- dium, 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 leucocytes. In reaction it is alkaline. The specific gravity varies according to its source. Apparently the cause of the non-coagulation of such lymph is the absence of cells from which thrombin 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 leucocytes, and it coagulates readily. Chemically, lymph resembles blood plasma in which the proteids are in smaller amount, but the inorganic salts in the same proportion as in blood plasma. The amount of solids varies in lymph from different organs. The lymph of the limbs contains only about two to three per cent, of proteids, that of the intestine from four to six, and that of the liver from six to eight per cent. In the lymphatics coming from the alimentary canal dur- ing starvation, the lymph has the characters above described. But after a meal it has a milky appearance and is called chyle. 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. 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 BLOOD, i8i the limbs is mucli 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 injures the capillary wall. Thus the injection of hot water or of proteoses at once leads to an increased flow of lymph. But, while the pernaeability of the vessel wall is the most important 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 pro- duction. 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 sulphate of soda — leads to a flow of fluid into the blood so that it becomes diluted, and also to an increased flow of lymph, and this increase of water in both can be explained only by its withdrawal from the tissues. SECTION VII. CmCULATION. I. General Arrangement. The arrangement by which the blood- and lymph are . dis- tributed to the tissues may be conppared to a great irrigation system. It consists of a central force pump — the systemic heart — from which pass a series of conducting tubes — the arteries — CAP Fig. 76. — Scheme of the Circulation. S.ff., Systemic Heart sending Blood to the Capillaries in the Tissues Cap. The Blood brought back by Veins and the exuded Fluid by Lymphatics, Ly., passing through glands ; Blood sent to the Alimentary Canal, Al.C, and from that to the Liver, I/iv. ; 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. leading off to every part of the body, and ending in in- numerable fine irrigation channels — the capillaries — in the substance of the tissues. From these a considerable propor- tion of the blood constituents is passed into the spaces between the cells. From these spaces the fluid is either 182 CIRCULATION. 183 passed back into the capillaries, or is conducted away in a series of lym'ph vessels, which carry it through lymph glands, from which it gains certain necessary constituents, and finally bring it back to the central pump (Fig. 76). 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, 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 through the liver and kidneys (Fig. 76, 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 sub- sidiary pump — the pulmonic heart — by which it is pumped through the lungs, there to obtain a fresh supply of oxygen, and there 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 white corpuscles from the lymph glands, and of other nourishing sub- stances from the alimentary canal, is poured into the great systemic pump — the left side of the heart — again to be dis- tributed to the tissues. Thus the circulation is arranged so that the blood, exhausted of its nourishing material by the tissues, is re- plenished in the body before being again supplied to the tissues. The sectional area of this irrigation system varies enormously. The great artery leaving the heart has a comparatively small channel. If all the arteries 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 1 84 PHYSIOLOGY. 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 is about twice as great as that of the aorta. The circulatory system may thus be compared to a stream which flows from a narrow deep channel (the aorta) into a 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, very sluggish in the capillaries where the channel is wide, and in them, plenty of time is allowed for exchanges between the blood and the tissues. ■'o^ 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 thicken- ing in the wall oomposed 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 propulsion of blood to the system generally, and hence called the sys- temic heart ; one propelling blood to the lungs, and hence called the pulmonic 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. CIRCULATION, 185 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 musculature of the auricles is separate from that of the ventricles, but some fibres extend from one to the other. If the heart is 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 cres- cpntric in shape ; one, more circular in shape, encircling in common the left auriculo-ventricular and the aortic orifice, and one encircling the pulmonary opening. To the double auriculo-ventricular ring the auricles are attached above, the ventricles below, while the valves of the heart are also con- nected with them. The muscular fibres of the auricles are arranged in two badly defined layers — 1st, An outer layer running horizontally round both auricles. 2nd, An inner layer arching over each auricle, and 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 (Fig. 77). The muscle fibres of the ventricle are arranged essentially in three layers — i86 PHYSIOLOGY. 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 ininost layer. 2nd, The middle layer is composed of fibres running. hori-> zontally round each ventricle. It is the thickest layer of 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 — Fig. 77. — Cross Section through the Ventricles of the Heart to show the right Ventricle placed on the Central Core of the left Ventricle, . The cusps of the Auriculo-Ventricular Valves are also shown. 1st, A layer of fibres, running longitudinally along the in- side of each ventriple from the apex upwards to the auriculo-' ventricular ring. These fibres are raised into fleshy columns, the columnse carnese. 2'nd, A set of fibres, constituting the papillary muscles,' which, taking origin from the apical part of the ventricles,' extelid freely upwards to terminate in a series of tendinous cords (the chordae tendinese); 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 columnse carnese. In- CIRCULATION. 187 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 and left wall of the ventricle, called the anterior, muscle ; and one in connection with the posterior 1 wall, called the. posterior muscle. In the right ventricle there are — 1st, One or. more small papillary muscles just under the pulmonary orifice, and having a horizontal direction, their apices pointing back-, wards — the superior papillary muscle (Fig. 78). 2nd, A large, papillary muscle taking origin from the mass of fleshy columns at the apex of the ventricle. It is directed; upwards. It may be called the anterior papillary muscle. . 3rd, One or more papillary muscles of varying size arising from the posterior part of the apical portion of the ventricle and constituting the 'posterior papillary) muscle. ' 4th, A number of, small septal papillary muscles arising from the septum. The distribution of the chordae 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, .but this is resisted by the contracting middle layer. The apex tends to be tilted towards the right, the papillary muscles shorten, the columDfe carnese by their shortening and thicken- ing encroach upon the ventricular cavity, and' help to abolish it, while the auriculo-ventricular rings are drawn downwards- and inwards towards the septum. ^ The endocardium forms a continuous fibrous layer extend-; ing from the vessels over the inner aspect ' of auricles andc ventricles. At certain points flaps of this endocardium' are developed to form the valves of the heart. > As in an ordinary Higginson's syringe, so in the heart a. valve is situated at the entrance to and at the exit; from- the expelling cavity. There is thus on each side of; the; heart a valve between the auricles and ventricles, and a; valve between the ventricles and the great arteries. PHYSIOLOGY. Auriculo- ventricular valves. — On each side of the heart the auriculo-ventricular valve is formed by flaps of endocar- dium or cusps, which hang downwards from the auriculo- ventricular ring like a funnel into the ventricular cavity, and are attached to the apices of the papillary muscles by the chordse tendinese (Fig. 78). On the left side of the heart there are two main cusps, forming the mitral valves — 1st, An anterior or right cusp, which takes origin from, and is continuous with, the right posterior wall of the aorta, and 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 chordse are inserted chiefly along its edges. 2nd, The posterior or left cusp takes origin from the back 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 chordse tendinese 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. The chordse from the anterior papillary muscle are inserted into the anterior or left edge of both cusps, those from the posterior papillary muscle into the posterior or right edges of both cusps. When the papillary muscles contract, the cusps are thus drawn together. The edge of each cusp thins out to form a delicate border, which, when the cusps are approxi- mated, 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 tricuspid valve are developed in connection with the crescentic ring. 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 CIRCULATION. 189 muscles. The chief or infundibular cusp {I.G.) 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 by the chordae from the anterior papillary muscle. When these two sets of papillary muscles contract, this cusp is drawn flat against the bulging septum (Fig. 78). The posterior cusp {P.O.) takes origin from the posterior and outer part of the ring, and hangs down into the posterior - A,P M. Flo. 78. — The Tricuspid Valve to show relationship of the Papillary Muscles and Obordse Tendinese to the Cusps of the Valve. 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 will therefore approximate its anterior edge to the infundibular cusp, its posterior edge to the septal cusp. In both the infundibular and posterior cusp many of the chordae pass up to be inserted into the oiiter part of 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. I go PHYSIOLOGY. Each is composed of three half-moon-shaped membranous pouches attached along their curved ' margin to the walls of the -artery and upper part of the ventricle, and with their concavity directed away from the ventricle. In the centre of the free margin is a fibrous thickened nodule, the corpus Arantii, from which to the attached margin the edges are formed of a very thin piece of membrane, the lunule. A pouch, the sinus of Valsalva, lies behind each cusp. The arrangements of these various cusps is of importance in' connection with their action. Aortic Valve. — The anterior cusp is largest, and lies at a somewhat lower level 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 ven- triculi, 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 con- tinuous with the anterior cusp of the mitral. Pulmonary Valve. — The posterior <3usp 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 considered. Attachments and Belations of the Heart (Fig. 79). — The heart is attached to the posterior wall of the chest at the level of the 5 th to the 8th dorsal vertebrae by the great vessels coming from it. Its, plane of attachment faces forwards and downwards. From .this the heart projects as a conical mass I"iG. 79. — Mesial Section through the Thorax to show the attach- ment and' relations of the Heart. CIRCULATION. downwards, forwards, and to the left into the chest. It does not lie at' right angles to its plane of attachment, but, when not contracting, it is limp and hangs down, as shown in the diagram. Nor can it assume a position at right angles to its plane of attachment, because the front part of the heart lies against; the anterior chest wall over an area (the prajcordium) bounded to the right' by the midsternal line, above by the fourth left rib, below by the seventh left rib, and to the left by a vertical line inside the nipple line. From the oblique position of the heart it is the right side — auricle and ventricle — which is directed forwards, and it is a portion of the right ventricle which lies in relationship with the chest wall. The rest of the organ is covered by the lungs. ' The only part of the left side which can be seen from the front is the tip of the left auricular appendix, and a narrow strip of the left ventricle. Below and behind the heart lies upon the central tendon of the diaphragm to which the pericardium is attached. All round it are the lungs completely filling up the rest of the thorax. The heart is enclosed in a strong fibrous bag, the Pericardium, which supports it and prevents overdistension. When fluid accumulates in this bag the auricles are pressed upon and the flow of blood into them is impeded. B. Physiology of the Heart. Cardiac Cycle. — If any one part of the heart of a frog is watched it is seen to undergo contractions and relaxations at regular rhythmical intervals. 1. General Description. — 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 ventricles become more distended and of a deeper red. 192 CIRCULATION. The rapid brief auricular contraction now gives place to relaxation, and, just as this commences, 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. 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 contraction the bulbus is seen to be distended and to become of a darker colour. The ventricular contraction passes off suddenly, the ventricle again becoming larger and of a deep red colour. At this moment the bulbus aortse contracts and becomes pale and then relaxes before the next ventricular contraction. Each chamber of the heart thus passes through two phases. A contraction phase, a systole of short duration, and a longer relaxation phase, the diastole. And the sequence of events in the frog's heart might be schematically represented as follows : — s/ms. AUfilCLES \/£NTnia£5\ BULBUS Fig. 80. — Scheme of the Cardiac Cycle in the Frog. The events from the beginning of the contrac- tion of any cavity to the beginning of the next contraction of that cavity constitute a cardiac cycle. Usually the cardiac cycle is reckoned from the be- ginning of the systole of the sinus. In the mammalian heart the rate of recur- rence of the cardiac con- traction varies with the animal examined. In man it is in adult life about 72 per minute. The sequence of events making up the cardiac cycle is simpler than in the frog. The contraction is seen to start in the great veins which enter the auricles, and to spread down along them to these chambers. This corresponds to the contraction of the sinus CIRCULATION. 193 in the frog's heart. It is followed by a short sharp con- traction of the auricles, which become smaller in all direc- tions and seem to be pulled down towards the ventricles ; but 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 ventricle 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 man may be represented as follows : — 2. The length of the phases of ven- tricular systole lasts three times as long as auricular systole ; the auricles contract for about O'l of a-'^"*-^^- — Scheme of the Cardiac Cycle in the Human 1 .■< . . 7 Heart. A.S., Auricular systole : F.S., Ven- second, the ventricles ^^^^^ ^^^^^j^ . ^^ p^^^J for 0'3 of a second. 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. Taking the ordinary heart rate of 72 per minute, the auricular systole will last for -^th of the whole cardiac cycle, the ventricular for f ths, and the pause for |^ths. 3. Changes in Shape. 1. Auricles. — These simply become smaller in all direc- tions during systole. 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. lA r 1 1 W.3. \ \ / [4.3 1 \ p 194 PHYSIOLOGY. The heart may be rapidly excised from an animal just killed while it is still beating and fixed in any stage of contraction or relaxation. Measurements of hearts so fixed shows that at the beginning 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 thru 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. 4. Change in Position. — ^During contraction the heart undergoes, or attempts to undergo, a change in 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 — Gor 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 prsecordium (Fig. 79). 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. Since the apex is twisted to the left, the movement of the ventricle is not simply directly forward, but also from left to right. This tilting of the apex from left to right is further favoured by the direction of the muscular fibres of the ventricles which, pass from the auriculo-ventricular rings downwards and to the left. The increased thickness of the heart from before back- CIRCULATION. 195 wards also assists, to some extent, in the production of the impulse. The study of the position and characters of the cardiac impulse is of great importance in medicine. The position is determined by the relationship of the heart to the anterior chest wall and to the lungs. The boundaries of the part of the heart lying in relationship to the chest wall have been already defined, and it is at the outer and lower part of this area, a region bounded above by the fifth rib, below by the sixth rib, outside by a line drawn vertically through the nipple, and inside by a line drawn vertically midway between the nipple and the left edge of the sternum, that the cardiac impulse is felt. Normally it does not extend outwards beyond the nipple line, but frequently when the left lung is voluminous the impulse only extends out to an inch or so inside of the nipple line. In children, on account of the size of the liver, it is often felt between the fourth and fifth ribs. This impulse is often called the apex beat — but it is not the apex which presses on the chest wall but a part of the front of the right ventricle. In character it is felt as a forward impulse of the tissue occupying the fifth interspace, which develops suddenly, persists for a short period, and then suddenly disappears. In many forms of heart disease its character is markedly altered. The cardiac impulse may be recorded graphically by means of any of the various forms of cardiograph. (Practical Exercise. ) The cardiogram has a form such as shown in Fig. 82. At the moment of ventricular systole the lever is suddenly thrown up to a certain level (a to b). From this point it suddenly falls ' slightly (6 to C), but Fia.82.-CaTdiographio Trace. . o •' . . <, . a to d, Ventncular Contraction. is mamtamed durmg the ventricular systole above the abscissa (c to d). At the end of the ventricular systole, as the heart falls away from ig6 PHYSIOLOGY. the chest wall, the lever falls to its original level {A to e). In many tracings a small rise of the lever may be seen just before the great upstroke. This corresponds to the contrac- tion of the auricles. According to the instrument used and the manner in which it is applied the tracing may vary very greatly, but these main features are always brought out. In various diseases of the heart the cardiogram is materi- ally 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. 4. Changes in the intracardiac 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 is applied to 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 apphes 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 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 cavity and connected with a tambour covered by a membrane to which a lever is attached. With such an arrangement the changes in the pressure of the different cavities may be investigated (Fig. 83). A. Pressure in the Great Veins (small dotted line in Diagram). — 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 CIRCULATION. 197 relax and suck blood from the auricles, blood again flows in A.S. v.s. p. AS. 1 ::•_-. — \-- .^'' \ PHESSUEE IN Artery. Auricle Great Veins • ■"■ Ventricle. \-~'-> illlll^ \y FLOW of BLOOD ironn 1. Great Veins to Auricles. i ^m^ A 2. Auricles to Ventricles. IL. 3. Ventricles to Arteries. iiii^^ CLOSURE or 1. Auriculo-Ventricular Valve. ilP ■■■■■■iiiiiii 2. Semilunar Valves. Ik i SOUNDS of HEART. r % I . CARDIAC IMPULSE. Fig. 83. — Diagram to Bhow the relationship of the events in the Cardiac Cycle to one another. A.S., Auricular Systole ; Y.S., Ventricular Systole ; P., Pause. from the great veins and the pressure falls, again to rise as PHYSIOLOGY. the auricles and veins are both filled up, towards the end of the pause. B. Pressure in the Auricles (dash line in Diagram)'. — At the moment of auricular contraction there is a marked rise in the intra-auricular pressure. When the auricular systole stops, the pressure falls gradually reaching its lowest level when the ventricles are throwing their blood into the arteries. Sometimes there is an interruption to this descent, apparently synchronous with the closure of the auriculo- ventricular valves. 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. 0. Pressure in the Ventricles (continuous line in Diagram). — 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 pres- sure 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. D. Pressure in the Arteries (dot-dash line in Diagram). — There is a sudden rise in the aortic pressure as the blood rushes out of the ventricles. The pressure then falls, but the fall is not steady. Often it is interrupted by a more or less marked increase corresponding to the later part of the ventricular contraction. At the moment of ventricular dias- tole, the fall is very sharp and is interrupted by a well marked and sharp rise. Following this the fall is continuous till the next systolic elevation. These changes in the pressure in the different chambers are due — \st. To the alternate systole and diastole of the chambers, the first raising, the second lowering the pressure in the chambers. 271C?, To the action of the valves. 5. Action of the Valves of the Heart. A. Auriculo-Ventricular (Fig. 84). — These valves have CIRCULATION. 199 already been described as funnel like prolongations of the auricles into the ventricles. They are firmly held down in the ventricular cavity by the chordae tendinese. 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. On the left side of the heart the strong anterior cusp of the mitral does not materially shift its position. It 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 1. 2. Fig. 84. — 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 Contrac- tion ; 5, Beginning of Ventricular Diastole. 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 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 important 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 through the chordae which ^re inserted into it. 2C30 PHYSIOLOGY. Nevertheless the occlusion of this orifice is apt to be incom- plete whenever the right side of the heart becomes in the least over distended. The auriculo-ventricular valves are open during the whole of the cardiac cycle, except during the ventricular systole (Fig. 78). 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 pres- sure in the arteries. Instantly the cusps of the valves are thrown back and remain thus until the blood gushes out. When the oatflow of blood is completed the cusps are again approximated by the pressure of blood in the arteries. As relaxation of the ventricles occurs, the intra-ventricular pres- sure 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 completely prevent any backflow of blood. The prejudicial effect of too great pressure upon these cusps is obviated by the lower cusp being mounted on the top of the muscular septum upon which the pres- sure comes — the other cusps shutting down upon this one. 6. Flow of Blood through the Heart. — The circulation of blood through the heart depends upon these differences of pressure in the different chambers. 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! 83). 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 auri- cular pressure takes place and another rush of blood from the great veins occurs. Gradually this diminishes, and by CIRCULATION. the time that the auricles contract the flow from the great reins has stopped. The contraction of the mouth 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. 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. 83). C. 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 higher than the arterial pressure. Immediately the semilunar valves are forced open and a rush of blood occurs from the ventricles. If the ventricles are acting powerfully, and if the pressure in the arteries does not offer a great resistance to the entrance of blood, then the ventricles rapidly empty themselves into the arteries. 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. In the former case we get a trace of the intra-ventricular pressure like Fig. 88a, and in the latter case the trace is like Fig. 886. 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 artery is low its action may be relatively strong. The interpretation of the various details of the Cardiogram 202 PHYSIOLOGY. is now rendered more easy. The ventricles, still full of blood, are suddenly pressed against the chest wall. As the blood escapes into the arteries they press with less force, and hence the sudden slight downstroke (Fig. 82, h to c). But so long as the ventricles are contracted the apex is kept tilted for- ward, and hence the horizontal plateau is maintained (c to d) and disappears as the ventricles relax (e). 6. Sounds of the Heart. — On listening in the region of the heart with each cardiac cycle, a pair of sounds may be heard followed by a somewhat prolonged silence. These are known respectively as the First and Second Sounds of the Heart (Fig. 83). By placing a finger on the cardiac impulse while listening to these sounds it is easy to determine that the first sound occurs synchronously with the cardiac impulse — i.e., syn- chronously with the ventricular contraction. It develops suddenly, and more slowly dies away. In character it is dull and rumbling, and may be imitated hy pronouncing the syllable li5b. In pitch it is lower than the second sound. The second 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 sud- denly. It is a clearer, sharper, and higher pitched sound than the first. It may be imitated by pronouncing the syllable dtipp. 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 the valves in vibration produces the sound. CIRCULATION. 203 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 nature 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 conducted up the aorta, and may best be heard in this " aortic area." On the other hand, the pulmonary valve lies in close relationship to the anterior chest wall — being covered only by the anterior border of the left lung — close to the edge of the 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, several important changes are taking place in the heart, any of which would itself produce a sound. \st, The muscular wall of the ventricles is contracting. 'i.nd. 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. 3rd, The heart is being pressed against the chest wall. 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, placing it in the hand, and while it is still beating — but without any blood passing through it — listening to the organ with a stethoscope. With each beat the lub 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. 2nc^, The stretching of the auriculo-ventricular valves also plays a part. 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 204 PHYSIOLOGY. sound — a murmur. Again, it has been maintained that a trained ear can pick out in the first sound the note corre- sponding' to the valvular vibration. 3rc?, 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 " hits " the chest wall. All that it does is more firmly to press against it. 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 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. When these valves are diseased and fail to act properly certain continuous musical 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 occurs — 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 : — . 1st, By a narrowing of the orifices between the cavities — stenosis. 2nd, By a non-closure of the valves — incompetence. Stenosis. — If one of the auriculo-ventricular orifices is narrowed, we then hear the murmur during the period at which blood normally flows through this opening. A refer- CIRCULATION. 205 ence to Fig. 83 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 aortic or pulmonary valve is narrowed the murmur will be heard (Fig. 83) during ventricular systole. The narrowing need not be absolute. A dilatation of the artery will make the orfice relatively narrow, and will produce the same result. Incompetence. — If the auriculo-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 aortic or pulmonary valves fail to close, the blood will regurgitate into the ventricles from the arteries during ventricular diastole, and a murmur will be heard during this period. By the position of these murmurs the pathological condi- tion producing them may be determined. 7. 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., kilogrammetres. With each beat about 80 grms. of blood are thrown from each ventricle into the aorta or pulmonary artery. Thus the weight lifted is 0-008 kilos. The average resistance in the aorta may be taken at about 1'5 metres of blood. Hence with each beat the left ventricle performs 0'006 x 1-5 = 0-012 kgms. of work. The right ventricle is only one-third as strong as the left, and hence the work done by each beat is only 0-004 kgms. If the heart is beating 72 times per minute, the amount of work per minute will be 0-115 kgms., or in 24 hours 16-560 kgms. — -about one tenth of the whole work daily performed by a labourer. 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 2o6 • PHYSIOLOGY. 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 disturbances 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 hypertrophies, 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. 8. Nature of Cardiac Contraction. — The contraction of the ventricle lasts for a considerable period — 0'3 seconds. Is it of the nature of a single contraction, or of a tetanus ? By repeated induction shocks it is impossible to tetanise heart muscle. A single stimulus applied to heart muscle produces a single prolonged contraction. Again, the mode of development of the currents of action do 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 — i.e., the smallest stimulus which will make the muscle contract makes it contract to the utmost. 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 difiicult, but as it relaxes it reacts more and more readily to stimuli. In cardiac muscle, perhaps more than in any other, the stair- case increase in the extent of contraction with a series of stimuli is manifested. 9. How is the Bhythmic Contraction of the Heart main- CIRCULATION. 207 tained ? — The mechanism is in the heart itself, for the excised heart continues to beat. In considering what this mechanism is, it must be borne in mind that two distinct questions have to be investi- gated. \«t. 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 ? 1st, 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 musclar 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. Since in the mammalian heart muscular continuity between auricles and ventricles is partially interrupted, the wave of contraction is delayed at this point, and in the dying heart the contraction frequently fails altogether to pass this block, and thus the ventricles stop beating long before the auricles. 2nd, What starts each contraction at regular rhythmic intervals ? Is this, too, simply a function of the muscle, or do nerve structures play any part ? In the ascidian 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 oif 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 contracting regularly and rhythmically. These experiments clearly show that regular rhythmic contraction is a function of cardiac muscle, but they do 2o8 PHYSIOLOGY. \d not exclude the possible action of nervous structures where these exist. In the frog's heart such nervous structures do exist, and are distributed as follows : — ^ \st, In the wall of the sinus venosus there are a number of nerve cells constituting the ganglion of the sinus (Renxak's ganglion). _^ 2 TIC?, In the interauricular septum a number of nerve cells constitute the ganglion of the auricular septum. Srd, In the auriculo-ventricular groove a number of nerve cells are also found forming the auriculo-ventricular ganglion (Bidder's ganglion). In the mammalian heart nerve cells exist, but there is not the same differentiation into distinct groups. Never- theless they are abundant round the mouths of the great veins, round the edges of the inter-auricular septum, and round the auriculo-ventricular groove. In the cardiac cycle in the frog each contraction starts in the sinus. What part does the sinus take in initiating con- traction ? If a ligature be tightly applied between the sinus and auricles (Stannius Experiment), the sinus continues to beat, and the auricles and ventricles usually stop beating for a longer or shorter period. But ultimately they begin to beat again. Hence it would seem that it is not any mechanism in the sinus which is essential in starting cardiac contraction. A ligature subsequently applied between auricles and ventricles sometimes starts the one, sometimes the other, sometimes neither. Hence we see that any part of the heart has the power of originating rhythmical con- tractions, though usually these are started in the sinus. {Practical Exercise.) 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 propagation of the cardiac contraction is a function of the muscular fibres. 10. Intra-Oardiac Inhibitory Mechanism. — While there is no evidence that the nervous structures in the heart play an important part in starting or keeping up the contractions. CIRCULATION. io<) there is evidence that they exercise a checking or controlling action. If the region between the sinus and auricles in the frog's heart is stimulated by the interrupted current from an in- duction coil, the heart is slowed or stopped. {Practical Exercise.) If atropine is first applied electric stimulation is without result. (Practical Exercise.) 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. 11. Connections of the Heart with the Central Nervous System. — In the frog a branch from the vagus connects the 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 stand still. 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 ganglion. 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 gives the result of stimulating the former, but sometimes the stimulation of the latter masks this effect. (Practical Exercise.) In the mammal three sets of nerve fibres pass to the heart : — 1st, Superior cardiac branch of the vagus starts from near the origin of the superior laryngeal, and passes to the heart to end in the endocardium. 2nd, 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. 3rd, Sympathetic nerve fibres come from the superior thoracic and inferior cardiac ganglia, and also end in the superficial cardiac plexus. PHYSIOLOGY. Functions of the Cardiac Nerves. — , Fig. 85. — Connections of the Heart with the Central Nervous System. Au., Auricle ; F., Ventricle ; V.D.Q., Abdominal Vaso-dilator Centre ; C.I.O., Cardiac Inhibitory Centre ; C.A.C., Cardio- Augmentor Centre ; S.C., Superior Cardiac Branch of the Vagus ; I.C., Inferior Cardiac Branch of the Vagus ; S., Cardio Sympathetic Fibres ; V.D. Ah., Vaso-dilator Fibres to Abdomi- nal Vessels. The continuous lines are outgoing ; the broken lines are ingoing Nerves. sure, it is called the depressor nerve. B. Inferior Cardiac branch of Vagns, A. The Superior Cardiac branch of the Vagus is an ingoing nerve. Section produces no effect ; stimu- lation of the lower end causes no effect ; stimula- tion of the upper end causes pain, slowing of the heart, and a marked fall in the pressure of blood in the ar- teries. The slow- ing of the heart is a reflex effect through the in- ferior cardiac branch ; and the fall of blood pres- sure, which is the most manifest effect, is due to a reflex dilatation of the vessels of the abdomen, caus- ing the blood to accumulate there, and thus to lessen the pressure in the arteries gener- ally. On account of the action on the blood pres- -Section of the CIRCULATION. vagus or of this branch causes acceleration of the action of the heart. The nerve is therefore constantly in action.. Stimulation 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 ner- vous system by the spinal accessory, and pass to the heart to form synapses with the cells of the cardiac plexuses. 2. Centre. — The fibres arise from a centre in the medulla ob- longata, which can be stimulated to increased activity either directly or reflexly. Direct stimulation is brought about by (a) sudden anaemia of the brain, as when the arteries to the head are clamped or occluded ; (&) increased venosity of the blood, as when respiration is interfered with ; (c) the con- current action of the respiratory centre. Eeflex stimulation is produced through many nerves. In the rabbit stimulation of the 5 th cranial by the inhalation of ammonia vapours has this action, and in all animals stimula- tion of the abdominal nerves produces the same effect. This reflex stimulation of the centre is used to determine its posi- tion 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 with atropine, they cannot act. According to the observation of Gaskell, they excite in the heart anabolic changes, since the electric cur- rent 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. (b.) 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. 212 PHYSIOLOGY. (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. C. Sympathetic Fibres. — The outgoing fibres are the augmentors and accelerators of the heart's action. When they are cut, no effect is produced, therefore the centre is Fig. 86. — Simultaneous Tracing from Auricles and Ventricles. A., During Stimu- lation of the Vagus • B., During Stimulation of the Sympathetic. (From Rot and Adami.) not constantly in action ; but 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, and leave the spinal cord by the anterior roots of the 2nd and 3rd dorsal nerves passing to the stellate ganglion where they have their cell stations. From the cells in this CIRCULATION. 213 ganglion non-medullated fibres run on in the annulus of Vieussens, and from this and 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. — These fibres seem to act by prevent- ing or cutting off the inhibitory influence of the vagus when more work is required from the heart. 4. Result of Action — •■ {a.) The output of blood from the heart is increased, and the pressure of blood in the arteries is raised. (6.) The rate of the rhythmic movements of auricles and ventricles is increased. (c.) The force of contraction of auricles and ventricles is increased. The vagus is thus the protecting nerve of the heart, re- ducing its work and diminishing the pressure in the arteries. The sympathetic is the whip which forces the heart to in- creased action in order to keep up the pressure in the arteries. 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 corre- sponding somatic nerves. Ill Circulation in the Blood Vessels. The general distribution of the various vessels, arteries, capillaries, veins and lymphatics, has been already con- sidered (Fig. 76). {The structure of the walls of each must he studied practically.) 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. 214 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 made of india-rubber tubes and a Higginson's syringe. {Practical Exercise.) A. — Blood Pressure. Since the distribution of pressure is the cause of the flow of blood, we shall consider it first. 1. General Distribution of Pressure. 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. ^nd, 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 whole contractive force of each ventricle into the correspond- ing 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 capillaries 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 capil- laries to veins the channel of the blood is becoming less CIRCULATION. 215 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. Variations in Blood Pressure. A. Rhythmic Variations. — Before considering the exact measurements of pressure in these different vessels, the rhythmic variations in pressure may be considered. A. Arterial Pulse. — If the finger be placed on any artery, a distinct expansion will be felt following each ventricular systole. 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 pressure, in other cases it may be necessary to compress the artery before the pulsation is distinctly felt. If any vein be investigated in the same way it will be found that no 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. — This arterial pulse is due to — 1st, The intermittent inflow of blood. It is with each sudden inflow of 80 grms. of blood into the arterial system from the heart that the arteries expand. 2^1, The resistance to outflow from the arteries into the capillaries. If blood could flow freelj'^ 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 2i6 PHYSIOLOGY. 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 vessels. SrcZ, But to allow of their expanding to accommodate this excess of blood their walls must be elastic. It is upon these three factors th^ the arterial pulse depends. Do away with any one 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 any obstruction to the outflow of blood from the veins into the heart, this is favoured by the suction action of the heart and thorax. Hence, even if an intermittent inflow were well marked, the absence of resistance to outflow would in itself 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 im- portant 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 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 Avail expands. But since CIRCVLA TION. 2 1 7 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 its 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 took to pass 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 1 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 particular point, obviously it is fiftj' 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 there- fore corresponds to a ventricular systole, and therefore each wave must last just the time between two ventricular systoles — just the time of a cardiac cycle. There are about 70 cycles per minute — i.e., per 60 seconds ; hence each must last 0-88 second. The pulse wave takes 0-88 second to pass any place, and it travels at 30 feet per second, its length then is 26"4 feet, or about five times the length of the body. It is then an enormously long wave, and it has disappeared at the periphery Jong before it has finished leaving the aorta. 2i8 PHYSIOLOGY. 3. The Height of a Wave. — The height of the pulse wave, as of the wave at sea, depends primarily on the pressure causing 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 we pass out to the periphery, where, as already seen, it finally disappears altogether. This disappearance is due to its force becoming expended in expanding the arterial wall. 4. The Form of the Wave. — Some waves have a sharp crest, others are less abrupt. Some -waves have a steep front, others a less steep. Some waves have secondary smaller waves upon them. In a wave at sea these points can be studied with the eye, or they might be graphically recorded by marking the changes in the surface of the water at any point. To do this it would be necessary to have some surface — e.g., the side of a ship moving through the water so that the changes in the surface at each particular moment might be marked, say by something floating on the water. If the ship were at anchor a simple vertical line would be produced. If she were under weigh 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 we knew the speed of the vessel. The same method may be applied to the arterial pulse. By recording the changes produced by the pulse wave at any point in an artery the shape of the wave may be deduced. This may be done by any of the various forms of sphygmograph. {Practical Exercise.) 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 Avail. Its apparent length depends upon the rate at which the recording surface is travelling and not on the length of the wave. Its height depends in part upon the length of the recording CIRCULA TION. 219 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. 87) — • \st, That the pulse waves follow one another without any interval. Medium Pressure. Medium Pressure. K^sK^s^^^■^^^Jv^^^ Low Pressure. Fig. 87. — Three Sphygmographio 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 muecular exercise ; 2 was made after sitting still for half-an-hour ; and 3, after an hour. Ind, That the rise of the wave is much more abrupt than the fall. SrcZ, That upon the descent of the primary wave there are one or more secondary waves. One of these is constant and is often very well marked. It forms a second crest, and is hence called the dicrotic wave. Between the chief crest and this secondary cre^t, a smaller 220 PHYSIOLOGY. crest is often manifest (Fig. 87, 3), and 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 pressure changes. This may be done by taking synchronously tracings of the intra- ventricular pressure, and of the aortic pressure (Fig. 88). 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 pressure 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, as in the case when the intra-aortic pressure is high as com- pared with the force of the heart (Fig. 88, continuous line), there is a residual outflow which arrests the diminution in the aortic pressure, or may actually raise it, causing the pre- dicrotic wave. As this residual outflow diminishes, the aortic pressure 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 the pressure falls. This fall in pressure is indicated by the dicrotic notch. But the elasticity of the semilunar valves at once makes them again spring up, thus increasing the pressure in the aorta and causing the second crest the dicrotic wave. After this the pressure in the arteries steadily diminishes till the mean CIRCULATION. is reached, to be again increased by the next ventricular systole. The form, of the pulse wave varies according to the relationship between the arterial pressure, and the activity of the heart. If the heart is active and strong in relation to the arterial pressure, the main mass of the blood is expelled in the first sudden outflow, and the residual flow is absent or slight (Fig. 88, 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 dias- tole. The rebound of the semilunar valves is here marked and causes a very prominent dicrotic wave, while the predicrotic wave is absent. 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 Fig. 88. — Diagram to show the effect of altering the relationship between the activity of the Heart and the arterial blood pressure. • h is the curve of intraventricular pressure, and 6^ is a pulse curve vsrith an active heart and a rela- tively low arterial pressure. — a and — a' are the same with a sluggish heart and a relatively high arterial pressure. readily perceived with the finger. It is to this form of pulse that the term dicrotic is applied in medicine. On the other hand, if the ventricles are acting slowly and feebly in relationship to the arterial pressure, the initial outflow of blood does not take place so rapidly and com- pletely (Fig. 88, 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 PHYSIOLOGY. 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 niay recover after their expansion in an irregular jerky manner, and may give rise to a series of kata- crotic crests producing a polycrotic pulse (Fig. 87, 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 — \sb, The rate of the 'pulse — i.e., the rate of the heart's action. Ind, The rhythm of the pulse — i.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. Kespira- tion has a slight effect which will afterwards be considered. In pathological conditions great differences in the force of succeeding pulse waves occur. (2.) Time relationship of beats. Normally the beats follow one another at regular intervals — somewhat shorter during inspiration — somewhat longer during expiration. In pathological conditions great irregularities in this respect may occur. 3rcZ, The volume of the pulse wave.^ — Sometimes the wave is high and greatly expands the artery — some- times 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. If this is high the walls of the artery are already somewhat stretched and there- fore the pulse wave expands them further only slightly. 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 throwing a sudden CIRCULATION. large wave of blood into the arterial system at each systole. The full pulse is well seen after violent exertion, 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. 4^A, 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 nearest 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 may be roughly determined. So important, however, is this point that various instrumental methods for deter- mining it have been devised. The tension of the pulse varies directly with the force of the heart and inversely 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 can easily be forced out of the arteries into the capil- laries, 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 con- stricted and the heart's action excited. The^ tension of the pulse wave must not be confused with the mean arterial pressure. hth, 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 — (l.),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 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. 224 PHYSIOLOGY. (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 closed properly 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 intermittent 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 outflow from the capillaries is not proportionately in- resistance to outflow are Such a condition is AS I \ 1 . Ir\' p 1 \ll V, I ^'\ MoRMAL fkrnoLooicAL ., / V_/ KJ>< MVPhes- \ .-■' — ' !FiG. 89. — Tracings of the Pulse in the great Veins in relationship to the Cardiac Cycle. creased, intermittent inflow and developed, and a pulse is produced, 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. 89. Its features are to be explained as follows : — Blood is constantly flowing into the great veins, pressed on by the vis a tergo. When the auricles contract the outflow CIRCULATION. 225 from these veins into the heart is suddenly checked, and consequently the veins distend. At the moment of auricular diastole the outflow is again free and a rush of blood takes place into the distending auricles, and thus the pressure in the veins falls. But as the ventricular systole prevents blood from passing through the auricles, a second obstruction to outflow occurs, and thus a second increase in pressure is developed in the veins. At the moment when the ventricles dilate a sudden rush of blood takes place from the veins and auricles into the yentricles, and thus a sudden fall in the pressure is produced. Gradually, as the ventricles fill, the pressure in the auricles and veins increases and they are again expanded. This is the normal venous pulse. But when the auriculo- ventricular valves are incompetent a pulse of very difierent form appears, having its crest during ventricular systole and a dicrotic wave at ventricular diastole. B. 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 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 264). I I I Fig. 90a. — Tracing of Arterial Blood Pressure to show large Respiratory Varia- tions, and small Variations due to Heart Beats, a to 6 Inspiration ; 2> to a', Expiration. A pulse synchronous with the respirations may also be observed in the great veins at the root of the neck and in the cranium. With each inspiration they tend to collapse, with 226 PHYSIOLOGY. each expiration they again expand. The reason for this is that during inspiration the pressure inside the thorax becomes low and hence blood is sucked into the heart, while during expiration the intra-thoracic pressure becomes higher and thus the entrance of blood into the heart is opposed. 3. Mean Blood Pressure. 1. Distribution.-7-That the pressure is positive — greater than the pressure of the atmosphere — throughout the great part of the blood vessels is indicated by the fact that if a vessel is opened, the blood flows out of it. The force with which hlood 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. Actually to measure the pressure in arteries and veins is easy. It is only necessary to let the escaping blood act against some measured force — e.g., the force of gravity, or a column of mercury. (Practical Exercise.) Arteries. — If the pressure in the aorta, in the radial, in the dorsalis pedis, and in one of the sl6allest arteries is measured, it is found that while it is great in. the great ar- teries, about 160 mm. Hg. in the aorta, it is much less in the small arteries. And this distribution of arterial pres- sure might be plotted out as in Fig. 90. Veins. — If the pressure in I60 Fig. 90. — ^Diagram of the Distribution of Mean Blood Pressure throughout the Blood Vessels. Ar., the Arteries ; C, the Capillaries ; V., the Veins. any of the small veins, in a medium vein, and in a large vein near the heart be measured, it will be found — CIRCULATION. 327 \st. That the venous pressure is less than the lowest arterial pressure. 2'n.d, That it is highest in the small veins, and becomes lower in the larger veins ; and in the great veins entering the heart during the first part of each ventricular diastole, it is lower than the atmospheric pressure. Capillaries. — The pressure in the capillaries must obviously be intermediate between that in the arteries and in the veins. It is not so easily measured, but various methods have been employed, and have yielded results confirmatory of the above statement. We have seen that the force of the heart and the degree of peripheral resistance both modify the arterial pressure ; but 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 heart's action is diminished, and no rise in the pressure occurs. Under certain conditions, however, this compensatory action is not complete, and changes in the arterial pressure are thus brought about. 2. Factors Controlling Arterial Pressure. — (a.) Heart's action. — The influence of this may be readily demon- strated if while taking a tracing of the arterial . pressure the vagus nerve is stimulated. The heart is inhibited, less blood is forced into the arteries, and the pressure falls. 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 these are chiefly composed of muscular tissue. When this is in a state of contrac- tion, the lumen of the vessels is small and the resistance is great. When these are relaxed, the lumen of the vessels dilates, and the resistance to outflow is diminished. This 228 PHYSIOLOGY. muscular tissue, in fact, acts as a stop-cock to the flow of blood from the arteries to the capillaries. It is of great importance — \st, In maintaining the uniform pressure in the arteries. 27id, 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. This is well seen under the influence of certain drugs. If while a tracing of the arterial pressure is being taken, nitrite of amyl 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. Nitrite of amyl causes the muscular coat of the arterioles to relax, and thus by diminishing peripheral resistance permits blood to flow freely from the arteries into the capillaries. Ergot, on the other hand, has precisely the opposite effect, causing 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. The condition of the arterioles may be studied in many different ways — \st. By direct observation. 1. 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 trans- parent structures, such as the web of the frog's foot, or the wing of the bat, or the mesentery of most animals, it is possible to measure the diameter of the arterioles by means of an eye-piece micrometer, and to study their dilatation and contraction. 2ndi The engorgement of the capillaries brought about by CIRCULATION. 229 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 dUated, 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 in a case with rigid walls filled with fluid which is allowed to escape by a single tube, an increase or decrease in the size of the part due to the state of its vessels maybe observed by the rise or fall of the fluid in the tube ; or it may be recorded by means of a lever. Such an instrument is called a plethysmograph. 3rci, Again, when the arterioles to a part are dilated, and the blood is flowing freely into the capillaries, the part becomes warmer. So by fixing a thermometer to any part of the body we may from the rise or fall of the temperature draw conclusions as to the condition of the arterioles. Mh, By streaming blood through the vessels and observing 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 Exercise.) These various methods depend upon the fact that the amount of blood in the capillaries depends upon the state of the arterioles. 5th, But the state of the arterioles also influences the arterial pressure, and so, 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 appearing 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 thQ heart's action and of the rate of respiration. They appear to be due to the periodic rhythmic contraction which is a characteristic 230 PHYSIOLOGY. property of non-striped muscle fibres. This rhythmic action is better marked in some vessels than in others. If the sciatic nerve of a frog be cut, the arterioles in the foot at once dilate. If the sciatic is stimulated, the arterioles becomes smaller. This shows that the nerve is constantly drCting on the arterioles to keep them in a state of partial constriction, and that this action of the nerve may be greatly increased, and the vessels thus further constricted. The same thing happens if the anterior roots of the lower spinal nerves, from which the sciatic takes origin, be cut or stimulated. We must, therefore, conclude, first that the central ner- vous system exerts a constant tonic influence upon the arterioles, keeping them in a state of semi-contraction, and second, that this influence may be increased, and thus a con- striction of the arterioles caused, and in this way the flow of blood from arteries to capillaries obstructed and the arterial pressure raised, and third, that this influence may be dimin- ished, so that the arterioles dilate and allow an increased flow into the capillaries from the arteries, and thus lower the arterial pressure. Vaso-Motor Mechanism. — These mobile arterioles, under the control of the central nervous system, constitute a vaso- motof mechanism, which plays a most important part in con- nection 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. This vaso-motor mechanism consists of the three parts : — 1st, The contractile muscular walls of the arterioles. 2nd, The nerves which pass to them. Zrd, The portions of the central nervous system presiding over these. 1. Muscular Walls of the Arterioles. — These are composed of non-striped muscular fibres which, in certain situations manifest periodic contractions. These 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 CIRCULATION. 231 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. Since this occurs where no /laN peripheral nerve cells exist, it appears that the muscular fibres here, as elsewhere, quite apart from nerve fibres, tend to maintain themselves in a state of partial contraction, which increases and dimin- ishes in a regular rhythmic manner. Normally this idio-musQU- lar mechanism is controlled by the central nervous sys- tem. 2. 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 ; some- times, however, they are dil- ated. In no case does section of a nerve cause constriction of the arterioles. These facts prove that the vaso-motor nerves may be divided into two classes. 1st, Vaso- constrictor. 2nd, Vaso-dilator. 1. Vaso-Oonstrictor Nenres. — The fact that section of these at once causes a dilata- tion of the arterioles proves that they are constantly trans- mitting impulses from the central nervous sytem. The course of these fibres has been investigated by section and by stimulation. Fig. 91. — Diagram of the Distribution of Vaso-motor Nerves. The continu- ous line shows the Vaso-oonstrio- tors ; the dotted line the Vaso- dilators. C.N., Cranial Nerves ; Vag., Vagus ; T.S,, Thoracic Sym- pathetic; A.S., Abdominal Sym- pathetic ; N.L., Nerves to the Leg. 232 PHYSIOLOGY. 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-meduUated fibres pass either along the various sympathetic nerves to the viscera, or pass back through the grey ramus (see p. 126) into the spinal nerve, and run in it to their termination. 2. 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 is cut, no change takes place in the . vessels of the gland, but, when it is stimulated, the arterioles dilate 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 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 we may take 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 geni- tals. 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 is cut, 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 limb is warm, the sciatic nerve is stimu- lated, dilatation, rather than constriction may occur. The vaso-dilator nerves pass out by the anterior roots of the various spinal nerves, and do not pass through the CIRCULATION. 233 sympathetic ganglia, but run as medullated fibres to their termination. 3. Portions of Nervous System Presiding over the Vaso- Motor Mechanism. — As we have a set of nerves causing con- striction of the arterioles, and another set causing dilatation, we must conclude that we also have two mechanisms in the central nervous system, one a vaso-constrietor, the other a vaso-dilator. I. Vaso-Constrictor — (a.) Mode of Action. — This mechan- ism is constantly in action, maintaining the tonic contraction of the arterioles. 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. It is, therefore, capable of reflex exci- tation. 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, as in asphyxia or suffocation, this centre is stimulated and a general con- striction of arterioles with high blood pressure results. Soon, however, the centre becomes exhausted, and the general con- striction is followed by a general dilatation. (6.) Position. — In investigating the position of the centre we may take advantage of — Ist, Its constant tonic influence. Removal of the centre at once causes dilatation of arterioles. 2ndy The fact that it may be reflexly stimulated. If the vaso-constrictor centre be removed stimulation of the 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 at once prevents the production of reflex con- striction by stimulation of an afferent nerve. 234 PHYSIOLOGY. 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 study- ing the influence of reflex stimulation after the removal of each slice. It is found that at a short distance below the corpora quad- rigemina, the removal of each succeeding part is followed by a diminution in the reHex 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 longitudinal extent. 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-constrictor centres, tonic in action and capable of having their activity reflexly increased, exist aU down the grey matter of the spinal cord. Nor- mally these are under the domain of the dominant centre, but when this is out of action they then come into play. II. Vaso-Dilator — (a.) Mode of Action. — This mechanism is not constantly in action. Section of a vaso-dilator nerve does not cause vascular dilation. 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 constriction 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 CIRCULATION. 235 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 their products, 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, by causing a general constriction of the arterioles, maintains or actually raises the arterial pressure, 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 pathologically in inflammation. But the stimulus need not necessarily be applied to the . region involved in the vascular dilatation. Various states of the brain, accompanied by emotions, may reflexly 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. (&.) 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. The main factors tending to alter the arterial pressure may be briefly summarised as follows. To the physician these are of importance since he is often called upon to diminish or to raise an arterial pressure pathologically modified. I. Diminishing — i. Diminished Inflow of Blood, a. Dim- inished Kate of Heart Beat. b. Diminished Force of each Beat. ii. Diminished Peripheral Resistance (Dilatation of Arterioles). Inhibition of Yaso-Constrictor Centre or Stimula- tion of Vaso-Inhibitory Centre. 1. By Sup. Cardiac branch of Vagus. 2. Digestion. 3. Hot Bath. 4. Drugs — -e.g., Nitrite of Amyl, &c. iii. Diminished Volume of Blood. 236 PHYSIOLOGY. 1. Hsemorrhage. 2. Increased Secretion of Kidneys and Bowel. II. Increasing — i. Increased Inflow (Increased Action of Heart), ii. Increased Peripheral Kesistance (Contraction of Arterioles). A. General. — a. Venous State of Blood, h. Drugs (Ergot, . &c.). B. Local — Cold Bath, &c. iii. Increased Amount of Blood. Pressure in the capillaries. — It has already been shown that this is less than in the arteries and greater than in the veins. Like the pressure in the arteries it depends upon the two factors — \sb. Force of inflow. 2nd, Resistance to outflow. (a.) Variations in the force 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. Again, when the influence of vaso-dilator nerves is withdrawn, the capillary pressure falls below the average. But the capillary pressure may also be modified by the heart's action, in as much 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 also falls. In augmented heart action both arterial and capillary pressure are raised. (b.) Variations in resistance to outflow. — Normally the flow from capillaries to veins is free and unobstructed ; but, if the veins get blocked, the capillaries get engorged with the blood which cannot escape from them. This engorge- ment of the capillaries is very different from that caused by increased inflow. In the former case the flow through the vessels is accelerated — here the flow is slowed or may be stopped. In the former case an abundance of fresh blood is passed through the part, here the blood accumulates, gets deprived of its nourishing constituents, and loaded with waste products. In the former case the functional activity of the tissue is stimulated, here it is diminished. CIRCULATION. 237 It is, therefore, most important to distinguish between" high capillary pressure from dilated arterioles or an active heart, and high pressure due to venous obstruction. A condition very similar to that described, but producing a capillary pressure high relatively to the pressure in the arteries — though not absolutely high — ^is seen in cases of failure of the heart, 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 higher and higher, and along with this the capillary pressure becomes high in relationship to the arterial pressure ; the blood is not forced through these channels, and conges- tion of the capillaries re- sults. 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 ; and, when through heart failure the blood is not properly sucked up from the . inferior extremities, this increased pressure becomes very marked indeed. But the pressure in the capillaries ma,y also be varied by the withdrawal of water from the body, as in purgation or diuresis, or by the addition of large quantities of fluid to the blood. The venous system is, however, so capacious that very great changes in the amount of blood in the vessels may take place without materially modifying the arterial or capillary, pressure. Pressure in Veins. — In the veins the force of inflow is small ; the resistance to outflow is nil. Hence the pressure is small, and steadily diminishing from the small veins to the large veins entering the heart. c ,B V c Fig. 92. — The Changes in Blood Pressure in the Capillaries produced by increasing the arterial pressure - - , and by obstructing the venous flow — - — - — , A,, Arteries ; C, Capillaries ; F., Veins. 238 PHYSIOLOGY. The venous pressure may be modified by variation in these two factors. Constriction of the arterioles tends to lower the venous pressure, dilatation to raise it. On the other hand, increased heart's action, which so markedly tends to raise arterial pressure, diminishes the pressure in the veins, because the blood is thus more rapidly driven from veins into arteries. The heart, which in its powerful systole drives out more blood, in its diastole sucks in more. This mode of lowering venous pressure is of great importance in medicine. But venous, Hke capillary pressure, may be modified by the loss or gain of water in the capillaries. If much water is passed from the capillaries into the lymph less fluid will be passed on into the veins, and the pressure will fall. Lymphatic Pressure. — 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 lym- phatic 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. 180). 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. The rate or velocity of the flow of a fluid depends upon several factors. Isi, On 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 CIRCULATION. 239 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 td pass a point in a channel ten square yards in area, it must obviously flow with greater velocity. This may be stated in the proposition 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) as — where S is the radius squared multiplied by the constant 3 ■14. Apply this to 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 the aorta. In the venous system the sectional area steadily diminishes, although it never becomes so small as in the corre- sponding arteries, and where the great veins enter the heart it is about twice the sectional area of the aorta. 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 (Fig. 93). 27ici, The suddenness of the change of pressure. This 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. Now, upon looking at a scheme of the distribution of pressure throughout the vascular system (Fig. 93), it is at once seen that in the arteries the change of pressure, from the high aortic to the low arteriole pressure, is extensive and sudden. Hence we should expect to find a rapid flow in the arteries. On the other hand, very little difference can be detected between the pressure at the arterial and at the venous side of the capillaries. Hence we expect that the rate of flow will be small in the capillaries. In the veins the differ- ence between the pressure in the small veins and in the veins 240 PHYSIOLOGY. entering the heart is small as compared with the difference between the pressure in the aorta and in the arterioles, but large when compared with the difference at the two sides of the capillary system. Hence we expect that the flow in the veins will be less rapid than in the arteries, but more rapid than in the capillaries. 3rc?, Friction. — A river runs much faster in mid-stream than along the margins, because near the banks the flow is delayed by friction. Av. c. V. 1 \ / f""""" "~'~> \ \ \ \ \ \ 1 \ \ ¥ ^ A ..A r' ~"N, Fig. 93. — Diagram of the Factors determining the Velocity of the Blood flow in Arteries, Capillaries, and Veins — Ar., C, and V. The continuous line is the Distribution of Pressure ; the fainter dotted line is the Sectional Area ; and the strong dotted line is the Friction. 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 system, the blood stream is distributed through innumerable small channels the friction is very great and the velocity of flow is small. These three factors thus act together to produce a rapid flow in the arteries, a slower flow in the veins and an extremely slow flow in the capillaries. Actual measurement of the rate of flow confirms the correctness of these conclusions. CIRCULATION. 241 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 stromuhr. The velocity of the flow in the capillaries may be measured in transparent structures by means of a microscope with an eye-piece micrometer, {Practical Exercise.) The velocity of the blood is^in the carotid of the dog about 300 mm. per sec. ; in the corre- sponding vein about half as much ; in the capillaries about . O'o to 1 mm. per sec. It is not so easy to give definite figures for the velocity of the lympb stream. Factors modifying^ velocity. — Disturbance of any of the three factors which govern the rate of flow 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 difierence 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. Blood Flow in — (a.) A^'teries. — The flow of blood in an artery is rhythmically accelerated with each ventricular systole. This is due to the pulse wave. As the wave of high pressure passes along the vessels, the blood tends to flow backwards and forwards 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. A cork floating on the surface is moved forward in front of the wave and again backwards, after the wave has passed. (b.) 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 periodic accelera- tions — 1st, With each diastole of auricle or ventricle. 2nd, With each inspiration. In aU 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 " axial " rapid and "peripheral" slow stream are, therefore, described. This 242 PHYSIOLOGY. is well seen in any small vessel placed under the microscope, and in such situations it will be found that while the erythrocytes are chiefly carried in the axial stream the leucocytes 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 stand still, these 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. 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. The thorax is to be looked upon as a suction pump of considerable power, which draws blood into the heart during inspiration. Again, the abdominal blood vessels are to be regarded as the great blood reservoir, and when the abdomi- nal muscles are tightened and the respiratory movements of the thorax are increased, as is seen in the panting which accompanies muscular exercise, 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 forced on into the left side of the heart in expiration. The blood is thus forced on into the muscles, and they, by their alternate contraction and relaxation, still further help to drive it on and to accelerate the circulation. The benefit of muscular exercise is thus manifest. When, on the other hand, some sustained muscular strain has to be undergone, the thorax is fixed and hence the pressure in 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 CIRCULATION. 243 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. But, if the effort is still further sustained, the high intra-thoracic pressure tends to prevent INTRACRANIAL CIRCUIATION PULMONARY CIRCULATION [ ABDOMINAL CIRCULiVTION — CIRCULATION IN LIMBS Fie. 94. — Scheme of the Circulation modified from Hill to illus- trate the influence of the various extra-Cardiac Factors which maintain the Flow of Blood. 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 and thus fainting may result. In man the position of the abdominal reservoir of blood at a lower level than the heart increases the work of that 244 PHYSIOLOGY. organ. In the recumbent position, when the reservoir is on the same level as the pump, the work is much easier. For this reason, in people with a weak heart, the sudden assumption of the erect position may lead to a failure of the heart and to fainting. Especially is this the case when the abdominal wall is lax so that accumulation of blood in the abdominal vessels is not prevented. On the other hand, 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. SECTION VIII. SUPPLY OF NOURISHING MATERIAL TO BLOOD AND LYMPH, AND ELIMINATION OF WASTE MATTER FROM THEM. RESPIRATION. I. EXTERNAL RESPIRATION. If an animal be placed in a closed chamber filled with ordinary atmospheric air containing by volume 79 parts of nitrogen and 2 1 parts of oxygen, and if the air is examined after a time, it will be found that the oxygen has diminished in amount, and that a nearly corresponding amount of carbonic acid has been added. The same thing occurs in aquatic animals — the water round them loses oxygen and gains carbonic acid. An animal takes up oxygen and gives off carbonic acid. This is the process of 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 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 carbonic acid 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 245 246 PHYSIOLOGY. 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 of the ter- minations of the air passages. {The structure of the various parts of the respiratory tract must be _ studied practically.) Each sac is lined by a layer of simple squamous epithelium, supported by a frame-work 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 capillaries would occupy about 75 square metres of this. Through these vessels about 5000 litres of blood would pass in twenty-four hours. Fio. 97a.— Scheme of the Distribution The larger air passages are of a Bronchiole, Infundibular supported by picCes of hyaline Passage, and Air Vesicles of ., .,,. ni, the Lung. Cartilage m their walls, but the smaller terminal passages, the bronchioles, are without this support, but are surrounded by a specially well-developed circular band of non-striped muscle — the bronchialis muscle — which governs the ad- mission of air to the infundibula and air sacs. The lungs are packed in the thorax round the heart, completely 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 cavity is simply a potential space. If the thoracic wall be punctured so that this potential pleural cavity is brought into connection with the RESPIRATION. 247 air, the lungs immediately collapse and occupy a small space posteriorly round the large bronchi. This is due to their elasticity (Fig. 98). 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 natur- ally becomes less and less, as they are expanded, greater and greater. Taking the average condition of expansion in the chest, the elasticity of the lungs 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 com- municates with the atmosphere, and this at the sea level has a pressure of about 760 mm. Hg. Dur- ing one part of re- spiration this pressure becomes a few mm. smaller, during an- other part a few mm. greater ; but the mean pressure of 760 mm. of mercury is constantly expanding the lung, and acting against a pressure of only 30 mm. of mercury, tending to collapse the lung. Obviously, therefore, the lung must be kept expanded and in contact with the chest wall. When a pleura is opened, the distribution of forces is altered, for now the atmospheric pressure tells also on the out- side as well as on the inside of the lung, and acts along with the elasticity of the organ. So that now a force of 760 mm. -)-30 mm. = 790 mm. act 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. Fig. 7^5-- 98. — Shows the Distribution of Pressure in the Thorax with the chest wall intact, and with an opening into the Pleural Cavity. ( J- ) Indicates the Atmospheric Pres- sure of 760 mm. of Mercury : 30 is the elasticity of the Lungs also in mm. Hg. 248 PHYSIOLOGY. II. Physiology. The process of respiration consists of two parts — \st, The passage of air into and out of the air sacs. Ind, The interchange of gases between the air in the air vesicles and the blood in the lung capillaries. A. Passage of air into and out of the Lungs. — This is brought about — \st, By the movements of respiration — breathing. 'ind, By diffusion of gases. The air is made to pass into and out of the lungs by alternate inspiration and expiration. I. Process 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 6f this expansion of the lungs the pressure inside hecomes less than the atmospheric pressure, and air rushes in until the pressure inside and outside again becomes equal. This can be readily shown by placing a tube in the mouth or in a nostril and connecting it Avith a water manometer. (Practical Exercise.) This expansion of the lungs can readily be determined in the vertical direction by percussion, and in the horizontal 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 below the level of the lung, a dull note is heard. If the lower edge of this resonance be determined before an inspiration, and again during it, it will be found to have descended. (Practical Exercise.) As a result of inspiration, the form of the chest is markedly modified, the change being best seen in transverse sections. In expiration the chest in transverse section is an elongated ellipse, in inspiration it becomes more circular. The change from side to side and from behind forwards is best marked towards the lower part of the chest, less marked ■RESPIRATION. 249 in the upper part. These changes may be recorded by means of a Cyrtometer. {Practical Exercise.) The change from above downwards cannot be directly seen, but it is indicated by a forward movement of the wall of the abdomen. It will be described when considering the mechanism by which it is brought about. The expansion of the chest in inspiration is a muscular act and is carried out against the following resisting forces — \st, The elasticity of the Lungs. — To expand the lungs their elastic force has to be overcome, and the more they are Fig. 98a. — Tangential, Transverse, and Vertical Mesial Sections of the Thorax in Inspiration and Expiration. expanded the greater is their elasticity. This factor there- fore plays a smaller part at the beginning than towards the end of inspiration. Ind, The elasticity of the Chest all. — The resting position 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 downwards, 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 — 25° PHYSIOLOGY. \st, How is the chest increased from above downwards ? This is due to the contraction of the diaphragm. In expiration, this dome-like muscle, rising from the vertebral column and from the lower costal margin, arches upwards, 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 extensive 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. 98 a). The existence of this complemental pleura is of great importance in cases of accumulation of fluid in the chest, for the fluid may accumulate and push down the diaphragm without materially altering the lower margin of the lungs. 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 antero-posterior diameters as a result of the mechanism which has next to be considered. Nevertheless in young children, in whom the chest wall is lax, an indrawing of the chest with each contraction of the diaphragm may occur, and may lead to permanent distortion of the thorax. 2nd, How is the chest increased in the antero-posterior and lateral diameters ? This is brought about by the elevation of the ribs which rotate round the axis of their attachments to the vertebral 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 vertebrae. The tubercle of the rib is attached to the transverse process of the upper of these vertebrae. From this the shaft of the rib projects outwards, downwards and forwards, to be attached in front to the sternum by the costal cartilage. If the rib RESPIRATION. 251 is made to rotate round its two points of attachment, its lateral margin is elevated and carried outwards, while its anterior end is carried forwards and upwards (see Fig. 99). Further, as we pass from above downwards, each pair of ribs forms the arc of larger and larger circles, and as each pair rises it takes the place of a smaller pair above. In these ways the chest is increased from before backwards and from side to side. The first pair of ribs does not undergo this movement ; the motion of the second pair of ribs is more marked, and the range of movement becomes greater and greater as we pass downwards until the floating ribs are reached. These are fixed by the abdominal mus- cles. This greater movement is simply due to the greater length of the muscles moving them. The muscles moving the ribs are chiefly the exter- nal 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 j-jq, 99._shows the Movements of in length, the second rib can the Ribs from their Position be raised say half- an -inch. in Expiration to their Posi- ™, „ , , T . , tion in Inspiration. Ihe first and second mter- costals acting on the third rib will together be two inches in length, and in contracting they can raise the third rib through half of two inches — i.e., one inch. The first, second, and third intercostals, acting on the fourth rib, are three inches in length, and can therefore raise this rib half of three or one and a-half inches, and so through the other ribs, until the floating ribs fixed by the abdominal muscles are reached. When the diaphragm takes 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. Abdominal breathing is best marked in males — thoracic in females. 252 PHYSIOLOGY. Along with tlie intercostal muscles, the levatores costarum also act in raising the ribs and in increasing the thorax in the transverse and antero-posterior diameters. These are the essential muscles of inspiration, but other muscles also participate in the act. In many individuals 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. 276). Forced 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. A forced inspiration may be made voluntarily, often it is produced involuntarily. Such forced respiration is well seen in patients suffering from heart disease, in whom the blood is not properly oxygenated, and powerful efforts are made to get as much air into the lungs as possible. In this condition every muscle which can act upon the thorax to expand it is brought into play. The body and spinal column are fixed in the erect position. The head is thrown back and fixed by the posterior spinal muscles. The arms and shoulders are fixed — usually by holding on to the sides or arms of the chair, and every muscle which can act from the fixed spine, head and shoulder girdle upon the thorax is brought into play. Nor- mally, these act from, the thorax upon the parts into which they are inserted ; now they act from, their insertion upon their point of origin. The sterno-mastoids, sterno-thyroids, and sterno-hyoids, assist in elevating the thorax. The serratus magnus, pectoralis minor, and upper 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. B. 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. RESPIRA TION. 253 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. The elasticity of the costal cartilages and — in the erect position — the weight of the chest wall cause the ribs again to be depressed, 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. There is experimental evidence that the internal intercostals contract with each expiration, and help to draw the ribs downwards. Ordinary expiration is normally thus 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. Forced 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 diaphragm 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- lumbaUs pull downwards the lower ribs and the triangularis sterni also assists in this. By this constriction of the thorax, brought about by ordinary 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. The pressure of the air outside is less than this, and thus to equalise the pressure, the air inside the chest is driven out. 11. Amount of air respired. — The amount of air respired varies according to whether the respirations are ordinary or forced. In ordinary respiration about 300 ccms. of air enter and 254 PHYSIOLOGY. leave the chest. This is called the tidal air. Its amount varies with the size and muscular development of the chest. By a forced inspiration a much larger quantity of air may be made to pass into the lungs — a quantity varying with the •Copacih, size and strength of the individual — but on an average about 1500 corns. This is called the complemental air. By forced expiration an amount of air much larger than the tidal can be expelled, an amount usually about the same as the FiG.ioo.— The amount complemcntal air and called the reserve of air respired „ j_ IpirationTnd^ T^^e total amount of air which an indi- forced inspira- vidual Can draw into and drive out of his tion and ex- lungs is a fair measure of the size and muscu- piration. j^^ development of the thorax, and it has been called the vital capacity of the thorax. This vital capacity may be measured by means of a spirorroeter. Its amount, as thus indicated, depends a good deal upon practice ; the in- strument, therefore, cannot be considered as of much practical value, and a more reliable conclusion as to the vital capacity may be arrived at by measuring the circumference of the chest in expiration and in inspiration. (Practical Exercise.) Even after the whole of the reserve air has been driven out of the chest, a considerable quantity still remains in the air vesicles, its amount depending upon the size of the chest, but averaging about 2000 ccms. This is called the residual air. This very important point must always be remembered, that the air we talee into the chest never fills the air vesicles, and that we can never drive air completely out of them. The air in them is thus not changed by the ^movements of respira- tion hut by the process of diffusion. III. Interchange of air in the lungs by diffusion of gases. — It has been shown that only the air in the trachea and bronchi undergoes exchange in mass, but that the air of the vesicles is not driven out of the chest. The renewal of this air depends upon the diffusion of gases. RESPIRATION. 255 If two gases are brought into relationship with one another they diffuse and tend to form a mixture uniform throughout. But, if at one point of a system, one gas is con- stantly being taken away and another constantly added, there will be a constant diffusion of the former towards the partwhere it is being taken up, and a constant diffusion of the latter away from tbe point from which it is being given off. Suppose a tube containing oxygen and carbon dioxide, and suppose that at one end of the tube oxygen is constantly being taken up and carbon dioxide constantly given off, a diffusion of the gases in the direction indicated by the arrows will continually go on, and thus a constant supply of oxygen will be conveyed to the bottom of the tube, while carbonic acid will constantly be cleared out. This is exactly the condi- tion in the lungs (Fig. 100a). The lower part of the tube corresponds to the air vesicles — the upper part to the air passage in which the air is constantly being exchanged by the movement of respiration. The mechanism by which the gaseous exchange in the alveoli is carried out is thus a double one. IV. — Breath Sounds. — The air as it passes into and out of the lungs pro- duces sounds, which may be heard on listening over the thorax. The character sounds are of the utmost importance in of diseases of the lungs, and must be cally. On listening over the trachea or over the bifurcation of the bronchi behind (between the 4th and 5 th dorsal vertebra), a harsh sound, something like the guttural ch (German icK), may be heard with inspiration and expiration. This is called the bronchial sound. If the ear be applied over a spot under which a mass of air vesicles lies, a soft sound, somewhat resembling the sound of gentle wind among leaves, may be heard throughout riQ. 100a.— Shows the diffusion of Oxygen into, and of Carbon Dioxide out of, the Air Vesicles. of the breath the diagnosis studied practi- 256 PHYSIOLOGY.- inspiration and for a third or less of expiration. This is called the vesicular sound. When the air vesicles become consolidated by disease the vesicular sound is lost and the bronchial sound takes its place. The cause of the vesicular character is therefore to be sought in the vesicles, infundibula or small brobchi. The cause of the bronchial sound has been determined by experiments on horses. It has already been seen that a column of fluid — and the same is true of a column of air — moving along a tube of uniform calibre, or with the calibre only slowly changing, produces no sound. Any sudden alteration in calibre produces vibration and a musical sound, as explained on p. 204. The first sharp constriction of the respiratory tract is at the glottis, and it is here that the bronchial sound is produced. If the trachea is cut below the larynx and drawn freely outwards, the bronchial sound at once stops, and the vesicular sound becomes lower and less distinct. The cause of the vesicular sound is not so satisfactorily explained. It is in part due to propagation of the bronchial sound, altered by passing through vesicular tissue ; but it is also probably due to the expansion and contraction of the air vesicles drawing in and expelling air, either through their somewhat narrow openings into the infundibula, or through the narrow opening of the infundibula into the bronchioles. The reason why the sound is best heard during inspiration may be that the sound is best conducted in the direction of the air stream. V. Rhythm of Respiration. — These movements of respiration are carried on in a regular rhythmic manner. Their rate varies with many factors; but the average number of respirations per minute in the adult male is about sixteen, or about one to every four or five beats of the heart. The rate of respirations may be modified by the will, and, therefore, in counting the respirations in a patient, it is well to prevent his being aware of what is being done; Similarly, on account of this influence of the upper part of the brain, the respirations should not be counted while the patient is excited or nervous. RESPIRATION. 257 The most important factor modifying the rate of respira- tion is the age of the individual. The following table gives the average ratio at different ages : — Under 1 year, 44 o '„ . . 26 „ 20 „ . 19 Adult, . . . . 10 The other modifications in the rate of breathing will be better understood after studying the nervous mechanism of respiration. In these respiratory movements the phase of inspiration bears a certain proportion to that of expiration. Inspiration is much more rapid than expiration (see Fig. 103). • As soon as inspiration is completed, a reverse movement occurs, which is at first rapid, but gradually becomes slower, and may be followed by a pause, during which the chest remains in the collapsed condition. The existence and duration of this pause varies much, and it may really be considered as the terminal period of expiration. Considering it in this light, we may say that inspiration is to expiration as 6 is to 7. VI. Nervous Mechanism of Respiration. — The rhythmic movements of respiration require the harmonious action of a number of muscles, and this is directed by the nervous system. The diaphragm is supplied by the phrenic nerves rising from the third and fourth, and partly from the fifth cervical nerves. Theintercostals are supplied by branches from their corresponding dorsal nerves. If the spinal cord be cut below the fifth cervical nerve the intercostal muscles cease to act. If the section is made above the third cervical nerve, the diaphragm, too, is paralysed, and the animal dies of suffocation. Respiratory Centre. — Obviously then, there is some nerv- ous mechanism above the spinal cord presiding over these muscles. Eemoval of the brain above the meduUa oblongata does not stop the respiratory rhythm. The mechanism must, therefore, be situated in the medulla oblongata. s 258 PHYSIOLOGY. If the medulla is split into two by an incision down the middle line, respiration continues, but the two sides do not always act at the same rate. The mechanism, then, is bi-lateral, but normally the two parts are connected, and thus act together. Destruction of the part of the medulla lying near the root of the vagus arrests respiration, and it may, therefore, be concluded that the nervous mechanism presiding over this act is situated here. It must not be imagined that this centre sends fibres directly to the muscles concerned in respiration. The D/apk Fig. 102. — Nervous Mechanism of Respiration. R.C., Respiratory Centre ; Cut., Cutaneous Nerves ; Ph., Phrenics ; In. C., Inter- costal Nervea ; P., Pulmonary Branches of Vagus ; S.L., Superior Laryngeal Branch of Vagus; La., the Larynx; G. Ph., Glosso- pharyngeal Nerve ; LHaph., Diaphragm. nerves passing to these come from the cells in the grey matter of the spinal cord, and it is by influencing the activity of these cells that the respiratory centre controls the act of respiration. Since expiration, when forced, is a complex muscular act, it is reasonable to suppose that the respiratory centre con- tains two parts — one presiding over inspiration, one presid- ing over expiration. While the inspiratory centre is constantly in rhythmic action, the expiratory centre is only occasionally at work. Mode of Action of Respiratory Centre. — Both parts of RESPIRATION. 259 the respiratory centre are under the control of higher nerve centres, and through these they may be thrown into action at any time, or even prevented from acting for the space of a minute or so. But, after the lapse of this period, the respiratory mechanism proceeds to act in spite of the most powerful attempts to prevent it. To determine its mode of action the influence of afferent nerves upon the centre must be considered. A. Vagus. — Since the vagus is the nerve of the respira- tory tract we should expect it to have an important influence on the centre (Fig. 103). Section of one vagus causes the respiration to become mn YYWV^ Fig. 103. — Tracings of the Respirations — Downstrokes, Inspiration ; Upstrokes, Expiration. At a one Vagus Nerve was out ; at 6 the second was divided ; at c the Upper Brain Tracts were cut off ; d and d' show the eflteot of Stimulating the Glossopharyngeal Nerve. slower and deeper ; but, after a time, the effect wears off, and the previous rate and depth of respiration is regained. Section of both vagi causes a very marked slowing and deepening of the respiration, which persists for some time, and passes off slowly and incompletely. Now, if after the vagi have been cut, the connection of the centre with the upper brain tracts is severed, the mode of action of the centre totally changes. Instead of discharging rhyth- mically it remains for a long period at rest, then the inspiratory centre discharges violently, causing a strong and prolonged contraction of the muscles of inspiration. This passes off, and again a period of rest of variable duration 26o PHYSIOLOGY. sets in, to be again interrupted by another more or less long and strong discharge. Separation of the respiratory centre from the vagi and v/pper brain tracts, brings about a loss of its rhythmic action, but does not stop its activity. The centre is auto- matic, but it owes the rhythmic nature of its action to afferent iTupulses. These afferent impulses reach it normally through the vagi, but when these are cut the upper brain takes upon itself the function of maintaining the rhythm. To investigate further this influence of the vagus it is necessary to study the effect of stimulating the nerve. Stimulation of the pulmonary branches of one vagus (vagus below the origin of the superior laryngeal) causes the respiration to become more and more rapid^ the inspiratory phase being chiefly accentuated. If the stimulus is strong respirations are stopped in the phase of inspiration. Such experiments prove that impulses are constantly travelling from the lungs to the centre whereby the rhythmic activity of the centre is maintained. How do these impulses originate in the lungs ? Appar- ently from their alternate expansion and contraction. If the lungs be forcibly inflated — e.g., with a bellows, the inspiration becomes feebler and feebler and finally stops. The nature of the gas with which this inflation is carried out is of no consequence. If on the other hand the lungs be collapsed by sucking air out of them, the inspiration becomes more and more powerful, and may end in a spasm of the inspiratory muscles. This shows that with each expiration a stimulus passes up the vagus which acts upon the inspiratory centre to make it discharge. The vagus is thus a true excito-motor nerve, making the centre act in a reflex manner. With each col- lapse of the lung the vagus is thrown into action, as the lungs expand it ceases to act, and, as a result, the inspiratory centre stops acting, the muscles of inspiration cease to con- tract, and" expiration occurs. Ordinary respiration may thus be considered as a rhythmic reflex act — but it must not be forgotten that the respira- tory centre can and does act rhythmically under the influence RESPIRATION. 261 of the higher centre, or a-rhythmically and spasmodically when these as well as the vagi are severed from it. So far the pulmonary branches of the vagus alone have been considered. But the upper part of the respiratory tract, the larynx, receives its sensory fibres from this nerve. Section of the superior laryngeal branch of the vagus does not alter the rhythm of respiration. Stimulation of the upper end of the cut nerve causes first an inhibition of inspiration and, if stronger, produces forced expiratory acts. This is well illus- trated by the very common experience of the effect of a foreign body, such as a crumb, in the larynx. The fit of coughing is a series of expiratory acts produced through this nerve (Fig. 102). Another set of visceral nerves having an important influence on the respiratory rhythm are the splanchnics. When these are stimulated inspiration is inhibited. Everyone has experienced the " loss of wind " as the result of a blow on the abdomen. The glossopharyngeal, which supplies the back of the tongue when stimulated, as by the passage of food in the act of swallowing, causes an instant arrest of the respiratory movements either in inspiration or expiration. The advan- tage of this in preventing the food as it is swallowed from passing into the trachea must be obvious (Fig. 102 ; Fig. 103, d and dl). Cutaneous Nerves. — Stimulation of the nerves of the skin stimulates the inspiratory centre and causes a deep inspiration. This is well seen when cold water is dashed upon the skin, and is more clearly demonstrated in animals with the vagi and upper nerve tracts cut across. In such animals if the skin be touched an inspiratory movement is made. The action of the respiratory centre is thus regulated by many afferent nerves. Its activity is also modified by the condition of the blood and lymph going to it. If the supply of blood to the medulla be interfered with — e.g., by ligaturing the arteries to the head, breathing becomes deeper and more laboured. The activity of the respiratory 262 PHYSIOLOGY. centre is increased in the same way if the proper exchange of gases between the blood and the air in the lungs is interfered with. This may be due either to the with- drawal of something necessary for the nutrition of the nerve cells of the centre, or to the accumulation of products of activity. One of the most important substances yielded by the blood for the nutrition of cells is oxygen. That the absence of this gas is alone sufficient to stimulate the activity of the respiratory centre is shown by the fact that if an animal is made to breathe an atmosphere of nitrogen which prevents the blood obtaining oxygen, but does not interfere with its getting rid of its carbon dioxide, increase in the activity of the respiratory centres is produced. This experiment does not however exclude the possibility that the accumulation of carbon dioxide may also stimulate the centre. If an animal be supplied with air in which there is an abundance of oxygen, but which is loaded with carbonic acid, the result is that the carbonic acid of the blood is not got rid of. In such an animal the breathing becomes quicker, deeper and more laboured ; but ultimately there is a tendency for the breathing to diminish and for the animal to sink into a state of deep coma. Since under ordinary con- ditions the amount of oxygen in the blood does not materi- ally vary, while the amount of COg may be considerably altered, it is probable that the increase of COj under certain conditions is the more frequent stimulus to the respiratory centre. The accumulation of other waste substances in the blood undoubtedly stimulates these nerve cells. Muscular exer- tion increases the activity of respiration. That this is really due to the accumulation of products of muscle waste in the blood and not to any reflex influence through the nerves is shown by the fact that it occurs in a dog when the spinal cord is cut across, and the muscles of the hind limb are made to contract by stimulating the nerves with electricity. The fact that the injection of acid into the blood of animals which are unable rapidly to neutralise it increases the respirations, suggests the possibility that RESPIRATION. 263 sarcolactic acid may be among the substances which can stimulate these nerve cells. Yet another factor of importance in modifying the activity of this centre is the temperature of the animal. We have seen that the increase in temperature accelerates the rate of the heart — so too it accelerates the rate of the respirations, and in about the same proportion. This is well seen in feverish attacks, where pulse and respiration are proportion- ately quickened so that their ratio remains unaltered. When the respiratory rate rises out of proportion to the rate of the pulse, it is usually an indication that some pulmonary irrita- tion is present. Various terms are used in describing the condition of the respiratory rhythm. The normal regular breathing of about fifteen per minute is often called the condition of ewpncea. On the other hand the diminished rate of respirations induced by distension of the lungs or by hyper-oxygenation of the blood may be called hypopncea, while the absolute stoppage sometimes produced by these conditions is called apncea. On the other hand, when the respirations are increased in rate or depth or both, the condition of hyper- pificea is produced, and when the breathing becomes laboured the state is called dyspnoea. This last condition is best seen when the supply of oxygen to the centre is diminished in various diseases. Dyspnoea is sometimes divided into an inspiratory dyspnoea and an expiratory dyspricea. In the former condition it is the inspiratory phase of respiration which is exaggerated. It is well seen in bad cases of heart disease, when the patient is in the condition described under forced inspiration (p. 252). On the other hand, in asthma the expiratory phase is more increased. The patient leans forward and in every way constricts the chest, using the muscles of forced ex- piration. The lungs and heart being packed tightly together in the air-tight thorax, and both undergoing periodic changes, neces- '264 PHYSIOLOGY. sarily influence one another. At the same time the close proximity of the respiratory and cardio-motor centres in the medulla seems to lead to the activity of one influencing the other. VII. Influence of Respiration on Circulation. — The circulation is modified in two ways by respiration. First, the rate of the heart ; and second, the arterial blood pressure undergo alterations. \st. Rate of Heart. — If a sphygmographic trace giving the pulse waves during the course of two or three respira- tions be examined, it will be found that during inspiration the heart is acting more rapidly, while during expiration its action is slowed. If the vagus is cut these changes are not seen, showing that the inspiratory acceleration is not the result simply of the larger amount of blood which enters the heart during inspiration, but is really due to changes in the cardio-motor centre — ^the accelerating part of which has its activity increased during inspiration, while the inhibitory part is more active during expiration. This may be a reflex effect from the lung through the vagus, or it may be due to the proximity of the centres in the medulla. But not only is the pulse more rapid during inspiration and slower during expiration, but the waves are smaller during inspiration and larger during expiration. The size of the wave depends much upon the pressure of blood in the arteries, and this change in the pulse thus leads to the fuller consideration of the changes in the arterial pressure due to respiration. 2nd, Changes in Blood Pressure. — If a tracing of the arterial pressure and of the respiratory movements are taken at the same time, it is found that at the beginning of in- spiration the pressure is still falling, and at the beginning of expiration it is still rising (Fig. 90a). This influence of respiration on arterial pressure is chiefly a mechanical one, depending on the variations in the pressure in the thorax during inspiration and expiration. During inspiration the pressure in the thorax falls to below the atmospheric pressure, and thus during this period the RESPIRA TION. 265 heart and great vessels are under a diminished pressure. This diminution in pressure has little influence on the thick- walled ventricles and arteries, but tells markedly on the thin- walled auricles and veins. In these there occurs a diminution in pressure, which, in the case of the vena cava, may fall below an atmospheric pressure, and as a result an increased flow of blood into these vessels from the veins outside the thorax takes place (Fig. 94). But when more blood enters the heart the activity of the organ is increased, and more blood is pumped through it into the arteries, and the pressure in these rises. This explains the great rise in arterial pressure during inspiration. During expiration the pressure in the thorax rises to above the atmospheric pressure, and thus the pressure on the vessels in the thorax is increased. This tells on the thin-walled veins and auricles, and thus the flow of blood into them is retarded (Fig. 94), and less blood passing into the heart less is pumped into the arteries, and the arterial pressure falls. This, however, does not explain the slight fall of pressure at the beginning of inspiration, or the slight rise at the beginning of expiration. To understand these, the action of the pulmonary circulation has to be taken into account. As inspiration develops the lungs are dilated, and the capillaries in them are also expanded. These expanding capil- laries require more blood to fill them. They are situated on the course of the blood from the right side to the left side of the heart, and thus blood is retained from this stream to fill them, and less blood passes on into the left side of the heart and out into the arteries, and thus at the beginning of inspiration a small fall in the arterial pressure occurs (Fig. 94). Similarly, at the beginning of expiration the lungs are compressed and their blood vessels squeezed, and thus the blood is driven out from them. Now, this blood cannot pass back into the right side of the heart, so it must pass on into the left side — thus more blood is driven into the arteries, and thus the pressure. rises. As soon, however, as the excess of blood has been squeezed out of the lungs, the contracted state of the vessels further retards the passage of 266 PHYSIOLOGY. blood to the left side of the heart, and assists in diminishing the arterial pressure. VIII. Influence of the Action of the Heart on Respira- tion. — The heart lies in the thorax surrounded by the elastic lungs. As it contracts and dilates it must alternately pull upon and compress the lungs, and thus tend to cause an inrush and an outrush of air — the cardio-pneumatic move- ment. If a simultaneous tracing of the heart-beat and of the movements of the air column be taken, it will be seen that at the beginning of ventricular systole there is a slight outrush of air from the lungs, probably caused by the blow given to the lungs by the suddenness of the sys- tolic movement. This is followed by a marked inrush of air correspond- ing to the outflow of blood from the ventricles, and caused by the fact that the contracting ventricles drawon and expand the lungs. This is followed by a slower outrush of air corresponding to the active filling of the ventricles during the strokes are Expiratory, the beginning of ventricular diastolc. downstrokes Inspiratory. ° ° . . , » Lastly, durmg the period or passive diastole, the cardio-pneumatic movements of air are in abey- ance. These cardio-pneumatic movements are of great import- ance in animals which hibernate. During their winter sleep the ordinary respirations almost stop, but a sufficient gaseous interchange is kept up by these cardio-pneumatic movements. In the examination of the heart-sounds they miist always be borne in mind, because, if there is any constriction in a small bronchus near the heart, the rush of air through this may give rise to a murmuring sound, in character very like a cardiac murmur and synchronous with the heart's action. On making the patient cough, such a murmur at once dis- FlG. 104. — To show relations of Cardio - pneumatic Movements, A to Cardiac Cycle, B. In A the up- appears. RESPIRA TION. 267 B. Interchange between the Air breathed and the Blood in the Lung Capillaries. I. Effect of Respiration upon the Air breathed. — To determine this, some method of analysing the air inhaled and the air exhaled must be employed. The following table shows the average percentage composi- tion of the air inspired and the air expired : — Per Cent, of N. 0. CO Inspired air, . . 79 21 Expired air, . . 79 17 4 i.e., about 4 per cent, of oxygen is taken from the air, and about 4 per cent, of carbon dioxide is added to it. In man the amount of car- bon dioxide given off is smaller than the amount of oxygen taken up. The proportion between the CO2 given off N 79 N 79 Zl 16 «s * Fig. 105. — Shows the Composi- tion of Inspired and of Expired Air. is called the Respiratory Quotient, and it is unity — usually about '8 to '9 — that is, for taken up thus less than every five volumes of oxygen taken up only four volumes are given off in carbon dioxide, the remainder is combined with hydrogen to form water. The various factors modifying this quotient will be considered while dealing with the extent of the respiratory changes. Expired air usually contains more water than inspired air. It is always saturated with watery vapour. Expired air also contains small amounts of organic matter, which give it its offensive odour. These may possibly be to a small extent formed in the lungs, but are to a greater extent produced by putrefactive change in the mouth and nose. It is probable that the accumulation of these products in the air is one of the great causes of the injurious effect of the foul " air " in overcrowded spaces. Expired air is usually warmer than inspired air, because usually the body is warmer than the surrounding atmosphere. When, however, the temperature of the air is higher than 268 PHYSIOLOGY. that of the body, the expired air is cooler than the inspired. This is illustrated by the figures of an experiment — Temperature of Inspired Air. 6-3° C. 17-19° C. 41° C. 44° C. Temperature of Expired Air. 30° C. 37" C. 38° C. 38-5° C. in /V /"2 N im o zo O /Z co^te COs 40 Pig. 105a.— Shows the Differ- ence in the Gases of Arterial and Venous Blood. II. Effect of Respiration on the Blood. — -To understand these changes in the air we must refer to the changes in the blood in passing through the lungs. Fig. 105a shows that the blood going to the lungs is poorer in oxygen and richer carbon dioxide than the blood coming from the lungs. Oxygen is taken by the blood from the air, carbon dioxide is given by the blood to the air. How is this effected ? The ex- tensive capillary net work in the walls of the air vesicles, if spread out in a continuous sheet would present a surface of about 75 square metres. Between the blood in the capillaries and the air in the air vesicles are two layers of living cells — 1st, The endothelium lining the capillaries. 2nd, The flattened cells lining the air vesicles. Through these cells the interchange of gases must take place. This interchange might take place in two different ways — 1st, By simple mechanical diffusion. 2nd, By some special action of the cells. If the process follows strictly the laws of simple diffusion, it is then unnecessary to evoke the activity of the cells as playing a part. But if the gaseous interchange does not strictly follow these laws, we must conclude that the cells do play a part. Whether a gas is simply dissolved or whether it be held in loose chemical combination, the amount held will depend upon the temperature of the fluid and upon the pressure of RESPIRATION. 269 the gas over the fluid. If the temperature is raised the fluid will hold less of the gas in solution, and any chemical combination will tend to split, as is seen when carbonate of lime is heated and the carbon dioxide is driven off. It is a well-known fact that if the pressure of any gas over a fluid be decreased the gas will tend to come off from the fluid, if it is increased it will be taken up by the fluid. This is well seen in the case of soda water. But the same law applies to such chemical compounds as carbonate of lime. If it is heated in ordinary air — i.e., under a low pressure of carbon dioxide, the gas is given off ; but if lime is in an atmosphere of COj the gas does not come off, but is taken up and the carbonate is formed. It will thus be seen that for every temperature there is a certain pressure of the gas at which the fluid or chemical substance will neither give off nor take up more of the gas. This may b6 determined by exposing the material in a series of chambers to air containing different proportions of the gas, and ascertaining by analyses of the air whether the gas has been given off or taken up or has remained unaltered. We know that the pressure of gas in an atmosphere depends upon the proportion present. Suppose an atmo- sphere contains 20 per cent, of oxygen, then the pressure, or partial pressure, of the oxygen is got by multiplying the percentage amount of the gas by 760 — i.&., a whole atmosphere's pressure, and dividing by 100 — 20 X 760 100 =152 mm. By ascertaining at what partial pressures of a gas, that gas is not given off or taken up by a fluid, we may determine the "tension" of the gas in the fluid whether it be in solution or in loose chemical combination. The presence of a moist membrane between the fluid and the air makes no difference to these interchanges. These facts may now be applied to the question as to whether the gaseous interchange between the air in the air vesicles and the blood is due simply to diffusion. The questions to be decided are — 270 PHYSIOLOGY. \st. What is the partial pressure of these gases in the blood going to and coming from the lungs ? 2'ncZ, What is the partial pressure of oxygen and carbon dioxide in the air vesicles ? Partial pressure of Oxygen and Carbon Dioxide in Blood. — The percentage amount of these gases tells us nothing of their partial pressure since they are in chemical combination and not in solution. Experiments made upon the subject have given very varying results, but the most recent and perfect series of observations go to show that the partial pressure of oxygen in blood coming from the lungs — i.e., in arterial blood, is about 100 mm., while the COj pressure in the same blood varies enormously, but on an average is about 20 mm. of Hg. In venous blood the oxygen pressure must be much lower — some have put it as low as 21 mm. Hg. ; while the pressure of COg is higher, something over 40 mm. Hg. Partial pressure of Gases in the Alveoli. — The air in the alveoli is not renewed by direct ventilation from without, but by a process of diffusion (p. 254). For this reason the amount of oxygen in the alveoli miist be much smaller, the amount of carbonic acid much larger, than in the respired air. By catheterisation samples of air have been withdrawn from the deeper part of the lungs and have been analysed. Such analysis tend to show that in the alveolar air there is — Oxygen about 10 per cent, at a partial pressure of 76 mm. Hg. Carbon dioxide about 4 per cent, at a partial pressure of about 30 mm. Hg. The difference of the partial pressure of these gases on the two sides of the membrane — in the alveolar air and in the blood — may be represented as follows : — Oxygen. Alveolar Air, 76 / 20? Blood Venous 140 Arterial Carbon Dioxide. 30 X / 46 Venous 0—40 Arterial RESPIRA TION. 271 This shows that when the blood reaches the lungs the distribution of pressure in the gases is such that, by the laws of diffusion, oxygen will pass from the alveolar air into the blood and carbon dioxide from blood to air; but before the blood has left the lungs the distribution is such that oxygen should, by the laws of diffusion, pass from blood to lungs and carbon dioxide from lungs to blood, which is exactly the reverse of what occurs. The passage of oxygen to the blood and the passage of carbon dioxide from the blood is much greater than could be accounted for by diffusion. We must, therefore, conclude that the exchange of gases between the alveolar air and the blood is not due entirely to diffusion, but is in part, at least, brought about by the activity of the cells lining the vessels and the alveoli. It should, however, be mentioned that, in spite of these figures, some physiologists maintain that simple diffusion will explain these interchanges, 1st, because, when the amount of oxygen in the atmosphere — i.e., when the partial pressure of falls below a certain point, the gas is no longer taken up by the blood, and ^vd, because, when the amount and pressure of COj rises, the COg is not given off from the blood. It has been ascertained by experiment that the pressure of oxygen in the air may fall to about half its usual pressure without interfering with the oxygenation of the blood. If it falls below this oxygen is not taken up. Hence it is possible for men to live at high altitudes when the ox3'^gen tension is much reduced. But under these conditions the rate of respirations and the rate of blood flow through the lungs have to be accelerated, and hence hyperpncea is apt to be induced, especially on exertion. II. INTERNAL RESPIRATION. Having seen how the blood gets its oxygen and gets rid of its carbon dioxide in the lungs, it is necessary to con- sider — ■ 1st, How it gives up its oxygen to the tissues. 2nd, How it takes carbonic acid from the tissues. 272 PHYSIOLOGY. 1st, How does blood give off oxygen to the tissues ? In studying the physiology of muscle, which may be taken as a type of all the active tissues, it was seen that oxygen is constantly being built up into the muscle molecule, and that the living tissues have such an affinity for oxygen that they can split it off from such pigments as methylene blue. The tension of oxygen in muscle is therefore always very low. We have seen that the tension of oxygen in arterial blood is nearly 100 mm. Hg., therefore, when the blood is exposed to a low tension of oxygen in the tissues, the oxygen comes off from the blood and passes into the tissues by the ordinary laws of diffusion. But it must be remembered that this takes place in three stages. (1.) The tissue elements are always taking up oxygen from the lymph, because of the very low pressure of oxygen in the protoplasm and because the protoplasm has an affinity for oxygen as for other nourishing sub- stances. (2.) As a result of this the oxygen pressure in the lymph falls and becomes lower than the oxygen pressure of the blood plasma, and thus the gas passes from the blood through the capillary walls to the lymph. How far this is simply the result of mechanical diffusion, and how far it is carried on by the vital action of the endothelium of the capillaries we do not know. (3.) As a result of the withdrawal of oxygen from the plasma, the partial pressure round the erythrocytes is dimin- ished, and the blood being at a high temperature, a dis- sociation of oxyhsemoglobin takes place, and the oxygen passes out into the plasma, leaving reduced haemoglobin in the erythrocytes. 2%^, How does the blood take up carbon dioxide from the tissues ? The tissues are constantly producing carbon dioxide. In the blood the carbon dioxide is combined with the soda of the plasma and is thus at a low tension. Hence there is a constant passage of carbonic acid from the tissues to the blood. RESPIRATION. 273 III. Extent of Respiratory Interchange. The extent of the respiratory interchange in the lungs is governed by the extent of the internal respiratory changes — i.e., by the activity of the tissues. Every factor which in- creases the activity of the metabolic changes in the tissues increases the intake of oxygen and output of carbon dioxide by the lungs. Under average conditions a normal man of 6 5 kgs. excretes about 432 litres (about 850 grras.) of COj, or 230 grms. of C, and takes up 540 litres of (about 770 grms.). The rate of exchange per unit of weight is more active in the child than in the adult. 1. Muscular Work. — Since muscle is the most abun- dant and active tissue of the body, muscular work more than anything else increases the respiratory changes (see p. 65). 2. Food. — The taking of food at once sets up active changes in the digestive apparatus. The muscular mechan- ism is set in motion, and the various glands secrete. As a result of these changes the respiratory interchange is at once increased. That the increase is really dependent upon these is shown by the facts that it is not produced by the injection of the foodstuff into the blood, aind that it is pro- duced by the taking of substances which cannot be absorbed and used in the metabolic processes of the body. But while the immediate increase in the respiratory inter- change following the taking of food is due to the increased activity of the digestive structures, there is also an increase diie to the utilisation in the body of the food taken, and whether proteids, fats, or carbohydrates, or all of these form the diet, more oxygen is consumed and more carbonic acid given off than during starvation. The proportion between the oxygen taken and the carbonic acid given off is not, how- ever, the same with all these food-stuffs. If the food-stuff is rich in carbon and poor in oxygen, a greater quantity of oxygen must be taken to oxidise it than if it is rich in oxygen. Thus while the carbohydrates contain about 53 per cent, of and 40 per cent, of C, the fats contain about 76 per cent. T 274 PHYSIOLOGY. of C and only 12 per cent, of 0, and the proteids contain 5 2 per cent, of and 5 2 per cent, of C. Hence on a carbohydrate diet the respiratory quotient (p. 267) ^~ is high, about 0"9 to 1, while in a fatty or proteid diet it is low, 07 to 0'8. 3. Temperature. — If the temperature of the body is increased, the metabolic processes become more active and the respiratory interchanges are increased. But if the tem- perature of the air round the body is elevated, the meta- bolic processes are diminished in activity, and the respiratory exchange is decreased. 4. Light. — It has been shown that light increases the metabolic changes and therefore the respiratory activity. 5. Sleep. — Since in sleep the animal is in a condition of muscular rest, since light is excluded from the eyes, and since food is not taken, the respiratory exchanges are less active during sleep than during the waking hours. Similarly, in the long winter sleep of certain animals (hibernation), these factors as well as the diminished temperature of the body cause a great reduction in the intake of oxygen and output of carbonic acid. It is thus the internal which governs the external respira- tion. Merely increasing the number or depth of the respirations has no influence on the amount of the respira- tory interchanges. Ventilation. The rate of gaseous exchange governs the necessary supply of fresh air. It has been found that if the supply is insuflScient, headache and sleepiness are apt to supervene, and experience has taught that each adult should have 2000 cubic feet of fresh air per hour. If 1000 cubic feet of space is allowed to each individual, the ordinary methods of exchange of air — through the chimneys and through chinks in windows and doors — should supply this quantity of air. Asphyxia is the condition caused by any interference with the RESPIRA TION. 27 5 supply of oxygen to the blood and tissues. It may be induced rapidly and in an acute form by preventing the entrance of air to the lungs, as in drowning or suffocation, or by causing the animal to breathe air deprived, of ■., oxygen, or by interfering with the flow of blood through the lungs. It is slowly induced in a less acute form as the muscles of respiration fail when death approaches in many different conditions. In acute asphyxia there is an initial stage of increased respiratory effort, the breathing becoming panting, and. the expirations more and more forced. The pupils are small, and the heart beats more slowly and more forcibly, while the arterioles are strongly contracted. Within a, couple of minutes a general convulsion, involving chiefly the muscles of expiration, occurs. The intestinal muscles and the mus- cles of the bladder may be stimulated, and the fseces and urine may be passed involuntarily. Then the respirations stop, deep gasping inspirations occurring at longer and longer intervals. The pupils are relaxed, and consciousness is abolished. The heart fails, and the pressure in the arteries falls, although the arterioles are still contracted. Finally the movements of the heart cease, and death supervenes. VOICE. In connection with the respiratory mechanism of many animals, an arrangement for the production of sound or voice is developed. This is constructed on the principle of a wind instrument, and it consists of a bellows, a wind-pipe, a vibrat- ing reed, and resonating chambers. In man and other mam- mals the bellows are formed by the lungs and the thorax. The trachea is the wind-pipe. The vocal cords in the larynx are the vibrating reeds, and the resonating chambers are the pharynx, nose and mouth. A. Structure. — The points of physiological importance about the structure of the larynx are the following : — 1. Cartilag^es. — The ring-like cricoid at the top of the trachea is thickened from below upwards at its posterior part and carries on its upper border two pyramidal cartilages 376 PHYSIOLOGY. These articulate with outer angle the triangular in section — the arytenoids. the cricoid by their inner angle. At the posterior and lateral crico-arytenoid muscles are attached, From their anterior angles the vocal cords arise and run forward to the thyroid. The thyroid cartilage forms a large shield which articulates by its posterior and inferior process with the sides of the cricoid so that it moves round a hori- zontal axis. To the upper and anterior part the epiglottis or cartilaginous lid of the larynx is fixed. 2. Ligaments. — The articular ligaments require no special attention. The true vocal cords are fibrous ligamentous structures which run from the arytenoids forward, to the posterior aspect of the middle of the thyroid. They con- tain many elastic fibres and are covered by a stratified squa- mous epithelium and appear white and shining. 3. Muscles. — The crico-thyroids take origin from the antero-lateral aspects of the cricoid, and are inserted into the inferior part of the lateral aspect of the thyroid. In contracting they approximate the two cartilages anteriorly, and render tense the vocal cords (Fig. 106). The crico-arytenoidei postici arise from the back of the cricoid and pass outwards to be inserted into the external or muscular pro- cess of the arytenoids. In con- tracting they pull these processes inwards, and thus diverge the anterior processes (Fig. 107). The crico-arytenoidei laterales take origin from the lateral aspects of the cricoid, and pass backwards to be inserted into the muscular process of the arytenoids. They pull these forward and so swing inwards their anterior processes, and approximate the vocal cords (Fig. 107). A set of muscular fibres run between the arytenoids — ^the arytenoidei — while other fibres run from the arytenoids up Fig, 106.— Side View of the Cartilages of the Larynx, to show the Action of the Crico-Thyroid Muscle in Stretching the Vocal Cords. RESPIRA TION. 277 to the side of the epiglottis. These help to close the upper orifice of the larynx. The thyro-arytenoid is a band of muscular fibres lying in the vocal cords and running from the thyriod to the aryten- oids. Its mode of action is not fully understood. 4. Mucous Membrane. — The mucous membrane of the larynx is raised on each side into a well-marked fold above each true vocal cord — the false vocal cord. Be- tween this and the true cord is a cavity — the ventricle of the larynx. The other folds of mucous membrane, although of importance in medicine, have no special physi- ological significance. The vocal cords increase in length as the larynx grows, and in adult life they are generally longer in the male than in the female. The interior of the larynx may be examined during life by the laryngoscope. {Practical Exercise.) B. Physiology. — When a blast of air is forced between the vocal cords they are set in vibration both wholly and in segments like other vibrating reeds, and sounds are thus produced. These sounds may be varied in loudness, pitch, and quality. The loudness or amplitude of vibration depends upon the size of the larynx and upon the force of the blast of air acting on the cords. The pitch or number of Fig. 107. — Cross Section of the Larynx, to show the Cricoid, Thyrmd, and Arytenoid Cartilages, and the Action of the Posterior and Lateral Crico- Arytenoid Muscles. vibrations per second depends upon the length and tension of the vocal cords. The greater length of the vocal cords in the male, as compared with the female, makes the voice deeper. The tension of the cords may be varied by the action of the crico-thyroid muscle. 278 PHYSIOLOGY. The power of varying the pitch of the voice differs greatly in different people. The average difference between the lowest and the highest note which the ordinary individual can produce is about two octaves. The quality of the sound depends upon the overtones which are made prominent by resonance in the pharynx, nose, and mouth. By varying the shape and size of these cavities and more especially of the mouth, the quality of sound may be considerably varied. The singing voice is often classified by its average pitch and by its quality into bass and tenor in males, and contralto and soprano in females. The true or chest voice is produced by a blast of air from the chest setting the cords into vibration, but the falsetto voice, which is generally higher in pitch than the true voice, is probably produced by vibrations of the edges of the cords. Articulate Speech. — Spoken language is produced by varying the resonating chambers of the pharynx, nose and Fig. 108. — Changes in the Shape of the Mouth in Sounding the Vowels A, I, U. mouth, and thus modifying the sounds produced in the larynx. Whispered speech is produced by setting in vibra- tion in the resonating chambers a stream of air which has not been allowed to act upon the vocal cords. In articulate speech two classes of sounds may be dis- tinguished — (1.) Vowel Sounds. These are musical sounds produced by a blast of air which is so modified by changing RESPIRA TION. 279 the shape of the mouth as to produce the well known a, e, i, 0, u. In sounding a, the mouth resembles a funnel with the wide part forward. In i, it may be compared to a flask with the belly behind and the neck forward. In u, the flask is reversed the belly being forward. In o, the mouth is intermediate between its position in u and a, and in e between a and i. 2. Consonant Sounds. — These are more of the nature of noises — irregular vibration. They are produced either — (1) at the Ivps, or (2) between the teeth and hard palate and the tongue, or (3) in the soft palate and back of the tongue. According to their mode of production they may be classi- fied into explosives, aspirates, vibratories, or nasals. Explosives, . Aspirates, Vibratory, . Nasals, . Labials. P. B. F. V. M.'" Dentals. T. D. S. L. Th. E. (English). N. Gutturals. K. a Oh. (German). R. (French). Ng. SECTION IX. Food and Digestion. I.— FOOD. The great use of food is to supply energy to the body. The muscles of the body are constantly active; they are constantly liberating energy by breaking down the complex molecules of proteids, fats and carbohydrates, and hence a constant fresh supply of such material is necessary to pre- vent the body living on its own material and wasting away. During the growth of the body the material from which it is to be built up, and the energy used in its construction, must be supplied by the food. Hence a suitable food is one which will yield the necessary amount of energy, and will supply the materials necessary for repair and for growth. But food must also supply the water and salts required to keep the various constituents of the body in solution, so that the essential chemical changes may go on. Nature of Food. Food may be divided into foods not yielding energy and foods yielding energy — the former acting chiefly as solvents, the latter acting as the source of energy. A. Food stuffs not yielding Energy. — 1. Water is the chief constituent. Since it is daily given off in large quantities by the kidneys, lungs, skin and bowels, it must daily be supplied in sufficient amounts or the chemical changes cannot go on and death supervenes. 2. Inorganic salts. — The most important of these is 280 FOOD AND DIGESTION. 281 chloride of sodium, which is essential for the maintenance of the chemical changes in the body. When it is not freely supplied in the food, it is retained in the tissues and hence animals can, when necessary, live with a comparatively small supply. 3. Salts of organic acids. The sodium and potassium salts of citric, malic, and tartaric acid, which are found abundantly in various vegetables are, when taken into the tissues, oxidised into carbonates which are strongly alkaline salts. The proteids which are decomposed in the body contain sulphur and phosphorus, and these are oxidised into sulphuric and phosphoric acids. In herbivorous animals the prejudicial effect of such acids is counteracted by the for- mation of these alkaline carbonates, which neutralise the acids. In carnivorous animals these salts are not so necessary, since ammonia salts are formed from the nitrogen of the proteids in sufficient quantity to neutralise the acids produced. Man occupies a position midway between the herbivora and the carnivora. The amount of energy yielded by the break- ing down of these salts into carbonates is so small that it is of no importance. B. Food stuffs yielding Energy. — These are complex combinations of carbon, hydrogen, and oxygen, with or without jiitrogen, sulphur, phosphorus, and iron. They are of the same nature as the materials which are found on analysis of dead protoplasm. They are commonly spoken of as the Proximate Principles of the food, and they may be classified as follows : — 1. Nitrogen-containing — Proteids and Albuminoids. 2. Non-nitrogenous — Fats and Carbohydrates. In studying the value of these food stuffs it is necessary to consider their energy value — that is, the amount of energy which can be yielded by the decomposition of a definite quantity of each in the body. The fats and carbohydrates enter the body as such. They leave it as carbonic acid and water. The pro- teids leave it partly as carbonic acid and water, partly as urea. Such a body as glucose, CgHigOg, by being oxidised into 282 PHYSIOLOGY. COg and HgO, gives off a certain amount of kinetic energy, and the amount of energy liberated is the same whether the oxidation is direct or takes place through any of many possible sets of chemical changes. In whatever ways a chemical substance breaks down into certain final products the energy set free is always the same, and this principle is taken advantage of in determining the energy value of the food stuffs. If fats and carbohydrates are changed to carbonic acid and water in the body, and if the energy given off as they undergo this change can be measured outside the body, their energy value as foods can be ascertained. This is done by burning a definite quantity of the material and finding how many degrees a definite quantity of water is heated. This gives the energy in Aeai units, and once having got this, by Joule's law it can be converted into the equivalent for any other kind of energy, such as mechanical work. It is known that the energy required to heat one kilo- gramme of water through one degree centigrade, is sufficient to raise 423 kilogrammes of matter to the height of one metre against the force of gravity, and thus if the energy value of any material as a producer of heat is known in heat units — kilogramme degrees — by multiplying by 423 we get the value in work units — kilogramme metres (kgms.). The apparatus used for ascertaining the heat produced by the combustion of material is called a calorimeter. Many different forms are in use, but the object in each is to secure that all the heat is used in raising the temperature of a known volume of water. We may now consider the value of the three great proximate principles of the food. 1. Proteids. — The chemistry of these bodies has been already considered (p. 9). They are the "chief sub- stances " of living matter, forming about 80 or 90 per cent, of its dry residue. The molecule is one of great complexity, and contains C.H.O.N.S., and sometimes P. and Fe. It is from the proteids of the food alone that the nitrogen and sulphur required in the construction and repair of the FOOD AND DIGESTION. 283 living tissues are obtained. The carbon and hydrogen required are also contained in these substances ; and, as will be presently shown, they have a considerable energy- value. Hence Froteids form the essential organic con- stituent of the food. Theoretically it should be perfectly possible for an animal to live on proteids alone, with a suitable addition of water and salts. In estimating the actual energy value of proteids in the body a difficulty arises in the fact that, instead of being decomposed to COj, H2O, NH3, SO3, as they are during com- bustion, in the body the nitrogenous part is not broken down further than urea — CONgH^. If the energy value of the complete combustion of a definite amount of proteids is first ascertained, and then the energy value of the amount of urea derived from the same amount of proteid is determined, then by subtracting the latter from the former, the energy value of proteids in their decomposition in the body is found (see p. 286). 2. Modified Proteids (Albuminoids). — In studying the chemistry of the formed material of the various protecting, supporting, and connecting tissues, these substances have been considered (page 27). Keratin, elastin and mucin seem to be of no importance as articles of food. If taken in the food they pass through the alimentary canal practically unchanged. While raw collagen seems also to be of little use, gelatin, formed by boiling collagen, has a certain value. Although it cannot take the place of proteids, because it cannot be used for building up the living tissues of the body, it is nevertheless decomposed into urea, and in decomposing it yields the same amount of energy as the proteids. It has, therefore, a definite though restricted value as a food stuff. 3. Pats. — The chemistry of the fats has already been considered (p. 29). From the fact that they contain so little oxygen in proportion to their carbon and hydrogen, a large amount of energy is liberated in their combus- tion, and therefore they have a high energy value as food stuffs. 284 PHYSIOLOGY. One gramme of fat yields twice as much energy as the same amount of proteid or carbohydrate kgms. 4. Carbohydrates. — (!Z%e chemical reaction of the different carbohydrates must he studied 'practically.) — The carbo- hydrates, starches and sugars, form a group of bodies which do not occur largely in animals, but are abundant constitu- ents of plants. They contain carbon, hydrogen, and oxygen, the carbon atoms of the molecule usually numbering six or some multiple of six, and the hydrogen and oxygen being in the same pro- portions in which they occur in water. They are aldehydes or ketones, and derivatives from these, of the hexatomic alcohol, CgHi^Og. The simplest carbohydrates are the monosaccharids, of which dextrose, the aldehyde of mannite, is the most import- ant. This dextrose is the sugar most widely distributed in nature. It has been called glucose, grape sugar, dextrose, and blood sugar. It is the sugar of the animal body. Closely allied to dextrose in chemical composition is Icevulose, a sugar ■ which instead of rotating the plane of polarised light to the right rotates it to the left, but which in other respects behaves like dextrose. It occurs in certain plants. The other monosaccharid of importance is galactose, a sugar produced by the splitting of milk sugar. These monosaccharids when boiled with caustic potash are oxidised, and if any metallic salt be present which can readily give up its oxygen it becomes reduced, the sugar appropriat- ing the oxygen. On this depends Fehling's and many other tests for glucose. These monosaccharids, under the influence of yeast, split into ethyl alcohol and carbon dioxide. They also form crystalline compounds with phenyl hydrazin, which are called osazones, the formation of which is of great value in determining their presence. By the polymerisation of two monosaccharid molecules with the loss of water disaccharids, or double sugars, are formed. Thus, two glucose molecules polymerise to form one maltose molecule. FOOD AND DIGESTION. 285 Maltose is the sugar formed by the action of malt and other vegetable and animal ferments upon starch. By the action of dilute acids and other agents it can be split into two dextrose molecules. Dextrose, polymerising with Isevulose, yields cane sugar, and this sugar, so largely used as an article of food, can be split up with dextrose and Isevulose. Lactose, the sugar of milk, is a disaccharid composed of a molecule of dextrose united with a molecule of galactose with dehydration. It readily splits into these two monosac- charids. By further polymerisation of monosaccharids with the loss of water, molecules of great size are produced and form the set of substances known as the polysaccharids. Among the simplest of these is dextrin, produced by the polymerisation of twelve molecules of glucose with the loss of twelve mole- cules of water. 12(CeHiA-H2o) = 12 (CeHjoOs) Closely allied to dextrin is inulin. But while dextrin is formed of dextrose molecules, inulin contains Isevulose mole- cules. Both are formed from the splitting of the more complex starches. The molecule of soluble starch is built up of no less than thirty dehydrated monosaccharid mole- cules, and has a molecular weight of 9000. Ordinary starch seems to have a molecular weight of 20,000 or 30,000, and hence must be of still greater complexity. These polysaccharids are not oxidised when boiled with caustic potash, nor do they change to alcohol and carbon dioxide under the influence of yeast. In cold neutral or acid solutions they strike a blue or brown colour with iodine. By boiling with a mineral acid and by the influence of various ferments they break down, take up water, and become mono- saccharids — the starches yielding the dextroses, and inulins yielding Isevulose. 286 PHYSIOLOGY. This method of classifying the carbohydrates has great advantages in studying their digestion. By combustion the energy value of the carbohydrate is found to be about the same as the proteids. Energy value of one grm. of — Proteid, Carbohydrate, Fat, . 4'1 calories. 4-1 „ 9-3 The Sources of the Various Proximate Principles of the Food. The proximate principles are in part derived from the animal in part from the vegetable kingdoms. While some races procure their food entirely or almost entirely from the former, others depend almost entirely on the latter. The vast majority of mankind, however, use a mixture of animal and vegetable foods. The animal foods may be classified as — 1. Milk and its Products, Cream, Butter, and Cheese. 2. Flesh. 3. Eggs. 1. Milk. — The characters of human milk are considered at p. 362. Cow's milk is an important constituent of the diet. Its average composition compared with human milk is as follows : — Water, . Proteids, . Fats, Carbohydrates, Salts, Cow's. Human. 88-3 88-8 3-0 1-0 3-5 3-5 4-5 6-5 0-7 0-2 The chief proteid of milk is caseinogen, a nucleo-proteid with a very small amount of phosphorus. It is held in solution in milk, but under the influence of various agents it FOOD AND DIGESTION. 287 becomes coagulated, forming casein. In cow's milk a small amount of an albumin is also present. The fats of milk occur as small globules of varying size floating in the fluid. They are chiefly olein with smaller quantities of palmatin and stearin, and still smaller amounts of such lower fats as butyrin, capronin, and caprylin. The carbohydrate of milk is lactose, a disaccharid, which splits into dextrose and galactose. The ash of milk is rich in phosphoric acid, lime, and potash — poor in soda and iron. Butter and Cream are simply the fats of the milk more or less completely separated from the other constituents. Cheese is produced by causing the coagulation of the casein, which carries with it a large amount of the fats. If cheese is made before the removal of the cream it is rich in fats, if after the removal of the cream it is poor in fats. The composition of cheese is approximately as follows : — Water, . Proteids, . Fats, Carbohydrates, Salts, . Fat Cheese. O 35 '8 27-2 30-4 2-5 4-1 Thin Cheese. 48-0 32-7 8-4 6-8 4-1 Cheese when allowed to stand affords a suitable nidus for the growth of micro-organisms by the action of which the proteids are digested, being converted into peptones and simpler bodies, and the fats split up into glycerine and 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 protoplasmic organs as the liver and kidneys. The chemistry of muscle has been already considered, and the following table shows the close resemblance between muscle and the other organs used as food : — 288 PHYSIOLOGY. Muscle. Liver. Kidney. Water, 75-9 71-6 75-5 Proteids and A,lbumi- noids, . 20-0 19-9 18-4 Fats, .... 09 3-6 4-5 Extractives, 1-9 3-3 0-4 Ash, . 1-3 1-6 1-2 Flesh is thus a source of proteids and albuminoids, and to a smaller extent of fats. In animals specially fed the amount of fat may be enormously increased, and ordinary butchers' meat may have as much fat as proteid. The extractives include such bodies as creatin, xanthin, inosit, &c., which give the peculiar flavour to the flesh of various animals. Flesh may be preserved in various ways — e.g., by simply drying, by salting, or by smoking. The result of each of these 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 following table gives the composition of these parts and of the contents of the shell as a whole : — Water. Proteids. Fats. Extraction. Salts. White, Yolk, . 85-8 50-8 12-7 16-2 0-3 31-8 0-7 0-1 0-6 1-1 Whole Egg, . 73-7 14-8 121 0-5 1-1 In the yolk there is a very large amount of lecithin, and the great value of eggs is thus that they contain both proteids, ordinary fats, and this special fat. Speaking generally we may say that the animal food stuffs are rich in proteids and fats, but are poor in carbohydrates. Vegetable Food Stuffs. — In the food of man, vegetables play as important a part as animal products. FOOD AND DIGESTION. 289 The peculiarity of special importance in vegetables is the existence of a capsule to the cells, composed of cellulose — a substance allied to starch in its composition, but which is very resistant to the action of the human digestive juices, and thus hinders the utilisation of the cell contents. In order that these may be digested and absorbed from the stomach and intestine, this capsule must be broken down either by some preparatory treatment, or by the teeth in the act of chewing. Though of practically no value as a food stuff, it acts as a natural purgative by stimulating the intestines, and is of great value in keeping up the regular action of the bowels. I. Cereals. — From the seeds of these, meals and flours are prepared : — Water. Proteids. Fats. Carbo- hydrates. Fibre and Ash. Wheaten Flour, Oat Meal, Eice, 13-3 10-1 13-1 10-2 14-7 7-0 0-9 5-9 0-9 74-8 64-7 77-4 0'8 4-6 1-6 Ordinary white bread prepared from wheaten flour con- tains these substances in something like the following pro- portions : — - Water. Proteids. Fats. Carbo- hydrates. Fibre and Ash. White Bread . 35-6 7-1 0-2 55-5 1-4 These cereals contain a fair amount of proteid, which was for long called gluten, but which is in reality a mixture of various albumins and globulins which do not differ in their characters from the animal proteids. They are most abun- dant in the outer part of the grain, just under the capsule. While oatmeal and maize contain a fair proportion of fat, the other cereals are poor in this constituent. The chief constituents of these seeds are the carbohydrates stored as starch. U 290 PHYSIOLOGY. II. Legumens. — The seeds of the leguminosEe — peas, beans, and lentils are valuable constituents of the diet. The following table gives an idea of their composition : — "Water. Proteid. Tat. Carbo- hydrates. WoodyFibre and Ash, Pea's, Beans, Lentils, . 15-0 14-8 12-3 22-9 24-3 25-7 1-8 1-6 1-9 52-4 49-0 53-5 2-5 3-2 2-8 These seeds are specially rich in proteids — a mixture of albumin and globulin, which have been classified together as legumia. The fats are small in amount, and the carbohydrates thousfh abundant are less so than in the cereals. III. Bulbous Plants. — The underground stems of certain plants develop tuberous growths in which material for the nourishment of the plant is stored. These plants belong to different natural orders, but may here be classified together. The most, commonly employed are potatoes, turnips, and carrots. The following table shows their average com- position : — Water. Proteids. Fat. Carbo- hydrate, Fibre and Ash. Potatoes, . Turnips, . Carrots, . 75-5 87-1 89-4 2-0 1-0 1-4 0-2 0-2 0-2 20-6 9-3 7-4 1-7 2-3 0-7 Such tubers are chiefly valuable as a source of carbo- hydrates — though they also contain a small proportion of proteids. . IV. Green Vegetables. — Cabbage, cauliflower, spinach, green peas, lettuce, &c., are useful additions to the diet, though their value as a source of the proximate principle of the food is not great. The composition of cabbage may be taken as a type of the others. It contains on an average : — Water, FOOD AND DIGESTION. 291 91 per cent. N. containing substances, . . 2 '5 ,, Fat, . ' . . . . 0-3 „ Carbohydrates, . . . 4'5 ,, Fibre and Ash, . . . . 1-7 „ These vegetables are rich in the potash salts of the organic acid, the importance of which has already been discussed (p. 281). The cellulose forming the walls of the cells in the young and growing part of the leaves seems to be capable of partial digestion in the human intestine. V. Fungi. — Mushrooms and other such fungi do not constitute a sufficiently important part of the diet to require attention. VI. Fruits. — These vary considerably in composition, but are all rich in water and carbohydrates, and poor in proteids. They contain practically no fat. Their great value lies in the amount of free and combined organic acids they contain. VII. Nuts. — These, unlike the fruits, are for the most part poor in water and carbohydrates but rich in proteids and fats. Chestnuts, however, contain an abundance of carbo- hydrates, and a smaller proportion of proteids and fats. Cooking. — Few of these food stuffs are used by civilised man in a raw and unprepared condition. With the object of rendering them more palatable or more easily digested they are usually cooked. This process of cooking produces important changes in many of the foods, and its effects must ■be briefly considered. Milk and its products are practically unaltered by cooking. Flesh is cooked either by exposing it directly to heat, or by treating it with boiling water. Roasting, grilling, broiling and frying are modifications of the former method, and in all of them the heat at once coagu- lates the proteids at the outer part of the piece of flesh, and thus form a more or less impermeable covering, which prevents the escape of the juices of the meat. Hence these methods of cooking, although bringing about a burning of the outer layer of the flesh, leave all the constituents in only slightly altered proportions. On the. other hand, if a piece of flesh be put into cold 292 PHYSIOLOGY. water and boiled, the proteids, the salts, and the various extractive bodies which give it its flavour, and, as the water warms, the fats also are dissolved out, and the meat becomes poorer in these constituents, while the surrounding water becomes a soup. In this soup the dissolved proteids pre- cipitate as the temperature rises, and when the soup is cooled they rise to the top with the fats, and are generally removed as a scum. Hence soups are poor in the proximate principles of food, but rich in the extractives and salts of meat. If, however, the piece of meat to be cooked is plunged into a large quantity of boiling water, the proteids at its outer part at once coagulate and form a covering which pre- vents the loss of the nutrient material which occurs when the meat is slowly boiled. Stewing is a modification of boiling by which much of the nutrient material of meat is extracted, but this is served as a gravy. It is in vegetables, however, that cooking is of the greatest importance, since by it the cellulose envelope which encloses the digestible portions of the plant is ruptured. II.— DIGESTION. I. Structure of Alimentary Canal. The Anatomy and Histology of the Alimentary tract must be studied practically. — We shall here merely give such an outline of the various structures as will assist in the com- prehension of their physiology. The Alimentary Canal (Fig. 108a) may be divided into the 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. 1. The Mouth, provided with its teeth, and surrounded by its mobile muscular wall, with the muscular tongue lying in its floor, is the part of the canal in which the food is broken up and prepared for digestion. Into the mouth three pairs of compound glands — the Salivary Glands — open. The parotid lined entirely by enzyme secreting epithelium, opens on the side of the cheek, while the submaxillary gland, composed FOOD AND DIGESTION. 293 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. The tongue is covered with a fine fur of processes, the fili- form papillae which are of use in passing the food backwards along its surface in the act of swallowing. Posteriorly, the mouth opens into the pharynx or upper part of the gullet. On each side, between the mouth and the pharynx, is the tonsil, an almond- like mass of lymphoid tissue. The pharynx is a cavity which can be shut off above from the posterior nares by raising the soft palate, and by pulling for- ward the posterior pharyngeal wall. It is surrounded by three constrictor muscles, which, by coatracting from above down- wards, can force the food down the gullet towards the stomach. The (Esophagfus is a muscular walled tube lined by a stratified squamous epithelium. The muscles are of the visceral type, and are arranged in two layers, an outer longitudinal layer, and an inner circular layer. The Stomach is a dilatation of the alimentary canal into which the gullet opens. To the left it expands into a sac- like cardiac end, and to the right it narrows, forming the pyloric end. 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 Fig. 108a. — Diagram of the Parts of the Alimentary Canal, from Mouth to Anus. T., Tonsils ; Ph., Pharynx ; S.O., Salivary Glands ; Oe., Oesophagus ; C, Cardiac ; Py., Pyloric Portion of Stomach ; D., Duodenum ; Li., Liver; P., Pancreas; /. , Jejunum ; /., Ileum ; F., Vermiform Appendix ; Col., Colon ; R., Rectum. 294 PHYSIOLOGY. cardiac sphincter, and at the pyloric end they form a very thick and strong pyloric sphincter. The mucous membrane, which is covered by a columnar epithelium, is largely composed of tubular glands, those at the car- diac end containing two kinds of cells, the peptic and the oxyntic cells, those at the pyloric end containing peptic cells alone. The Small Intestine is a tube of about 22 feet in length. It has a double muscular coat like the stomach. The mucous membrane which is covered by a columnar epithelium, is thickly set with simple test-tube like glands — Lieberkiihns follicles — and is projected into the lumen of the tube, as a series of delicate finger-like processes, the Villi. The tissue of the villi and that between the Lieberkiihn's follicles is chiefly lymphoid, and in certain places this lymphoid 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 — the submucous layer is full of small branching glands lined by an enzyme secreting epithe- lium (Brunner's glands). The Large Intestine is about 6 feet in length. The small intestine enters it at one side, and the opening is guarded by a fold of mucous membrane which forms the ileo-coecal valve. Above the opening of the small intestine, a coecal pouch exists, and at the top of this is the vermiform appendix, a narrow tube with an abundance of lymph tissue in its wall. Below the opening of the small intestine is the colon, which after passing up the right side across the abdomen, and down the left side, takes an S like bend to. end in the rectum, which, passing forward, suddenly turns down and opens at the anus. The sudden bend is of importance in retaining the contents of the rectum. The last part of the rectum is also 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 Lieber- kiihn's follicles in which the epithelium is chiefly miicus- secreting in type. The muscular coat of the colon differs from that of the. rest of the alimentary canal, ih that the long- itudinal fibres are arranged in three bands. FOOD AND. DIGESTION. 295 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 diverticulum, the gall bladder. The Liver is a large solid looking organ, formed originally as a double out-growth from the alimentary canal. These out-growths branch, and again branch, and between them the blood coming from the mother to the foetus flows in a number of capillary channels ; and later, when the alimentary canal has developed, the blood from it is also streamed between the liver tubules. In man and other mammals, the fibrous tissue supporting the liver cuts it up into a number of small divisions, the lobules, each lobule being composed of a series of tubules arranged radially with blood vessels coursing between them. The portal vein which takes blood from the stomach, intes- tine, pancreas and spleen breaks up in the liver, 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 sup- porting tissue of the liver is supplied by the hepatic artery, which has very free communication with the terminal branches of the portal vein. The Pancreas 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. The Nerve Supply of the alimentary 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 nerves, and the oesophagus is supplied by the tenth. The stomach and intestine get their nerve fibres from two sources — above the descending colon from the vagus and 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 296 PHYSIOLOGY. which fibres pass to the muscles and glands. One of these plexuses (Auerhach's) is placed between the muscular coats — the other (Meissner's) is placed in the submucosa. II. Physiology. I. Digestion in the Mouth. A. Mastication,— In the mouth, by the act of chewing, the food is thoroughly broken up and mixed with saliva. The muscular mechanism of mastication may be here briefly indicated. Movements of Mastication. — Movements of Lower Jaw in — 1. Vertical Plane, a. Elevation. a. Temporal. /8. Masseter. 7. Internal Pterygoid, h. Depression, a. Weight of Jaw. /3. Anterior Belly of Digastric, -y. Mylo- and Genio-Hyoid. ^. Platysma. Hyoid fixed by — Omo- and Sternohyoid, Sternothyroid, and Thyrohyoid. 2. Horizontal Plane — Forwards : External Pterygoids. Backwards : Pos- terior Fibres of Temporal. To Eight : Left External Ptery- goid. Eight Temporal (Posterior Fibres). Other Muscles of Mastication — Buccinator and Orbicularis oris. B. 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). Characters. — It is a somewhat turbid frothy fluid which, when allowed to stand, throws down a white deposit consist- ing of shed epithelial scales from the mouth, leucocytes, amorphous phosphates of lime and magnesia, 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. The saliva from the parotid gland contains only about 0-4< per cent., while that from the sublingual may contain from 2 to 3 per cent. The sublingual and submaxillary saliva in man is viscous, from the presence of mucin formed in these glands, while the parotid saliva is free from mucin. In addition to mucin, traces of proteids are readily demonstrated, and with these proteids is associated the active constituent or enzyme of the saliva — ptyalin. FOOD AND DIGESTION. 297 Saliva generally contains traces of sulphocyanide of potas- sium, and in some pathological conditions this may be markedly increased. The functions of the saliva are two-fold : — 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, it would appear that this is the more important function. 2. Chemical.- — Under the action of the ptyalin of the saliva, polysaccharids, like the starches, are broken down into sugars. Like other enzyme actions the process requires the presence of water and a suitable temperature, and it is stopped by the presence of strong acids or alkalis, 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. During the short time the saliva acts on the food the conversion is by no means complete. 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 disaccharid maltose. {Practical Exercise.) 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 flow of saliva or pressure of secretion 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 are supplied — (1), by branches from the lingual division of the fifth cranial nerve, and (2), by branches of the peri- vascular sympathetic fibres coming from the superior cervical ganglion. The parotid gland is supplied by the auriculo- temporal division of the fifth and also by sympathetic fibres. 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 298 PHYSIOLOGY. chorda tympani, a branch from the seventh nerve which joins the Ungual, is cut, the reflex secretion does not occur. Stimulation of the chorda tympani causes a copious flow of watery saliya, 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, indicating that the nerve carries two distinct sets of fibres to the glands.' The fibres all undergo interruption before the glands are reached ; the fibres to the sublingual yland having their Fig. 108. — Nervous Supply of the Salivary Glands, i'ar., Parotid and S.M, and S.L. the Submaxillary and Sublingual Glands ; VII,, the Seventh Cranial Nerve, with Gh. T., the Chorda Tympani . Nerve passing to L., the Lingual Branch of Y., the Fifth Nerve, to supply the Glands below the Tongue, T, ; IX., the Glosso- pharnygeal giving oflf J.N., Jacobson's Nerve, to the 0., Otic Ganglion, to supply the Parotid Gland through Aur. T., the Auriculo-Temporal Nerve. cell station in the submaxillary ganglion, the fibres to the submaxillary gland haying theirs in a little gangHon at the hilus of the gland. This was demonstrated by painting the two ganglia with nicotine. When applied to the submaxillary ganglion it did not interfere with the passage of impulses to the submaxillary gland, but stopped those going to the sublingual. FOOD. AND DIGESTION. 299 If, when the chorda tympani is stimulated, the duct of the gland is connected with a mercurial manometer, it is found that the pressure of secretion may exceed the blood pressure in the carotid. When the perivascular sympathetics, or when the sym- pathetic 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 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, stimulation of the glossopharyngeal nerve as it comes off from the brain, acts upon the parotid gland, and since the glosso-pharyngeal is united to the small superficial petrosal which passes to the otic ganglion, by Jacobson's nerve, it is obvious that these parotid fibres take this somewhat round about course. When the sympathetic fibres alone are stimulated 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 saUvary secretion. 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 passing from before backwards, the bolus of food is driven on to the back of the tongue. When this is reached the act becomes . involuntary and reflex, and the food is forced through the pillars of the fauces into the pharynx. It is pre- 300 PHYSIOLOGY. vented from passing up into the posterior nares by the contrac- tion of the palato-pharyngeus muscle, and of the levator palati, while the orifice of the larynx is covered by the epiglottis. The larynx as a whole is pulled upwards by the stylo-hyoid and stylo-thyroid and the thyro-hyoid, while the arytenoidei and the posterior crico-arytenoids close the glottis (Fig. 109). The constrictors of the pharynx contract from above downwards, and force the food into the grasp of the oesophagus. Thus by a slow peristaltic contraction sends the food onwards to the stomach. Under normal conditions this peristalsis is not essential and it is done away with by section of the vagi. In swallowing liqiads, the peristalsis of the oesophagus is not brought into play, but the fluid is forced by the tongue down the re- laxed oesophagus into the stomach. The passage of the food down the gullet may be heard by applying a stetho- scope to the right side of the spinal and any delay caused by a stricture maj' thus be ^-\ Fig. 109. — Changes in the Mouth, Pharynx, and Larynx in the Act of Swallowing. column, determined. III. Digestion in the Stomach. Various opportunities have occurred and have been taken advantage of to study the interior of the human stomach during life. The best known investigation of the kind was undertaken by Dr. Beaumont, a Canadian physician, on the person of St. Martin, a backwoodsman, who had received a gun shot wound in the abdomen, which had left him with an opening through the front wall of his stomach. Dr. Beaumont engaged St. Martin as his servant and made a prolonged and valuable study of the changes which take place in the viscus. He found that the condition varies greatly in fasting and after feeding. A. Stomach in 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. Move- FOOD AND DIGESTION. 301 ments are not marked and the secretion is scanty, only a little mucus being formed on the surface of the lining membrane. B. 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. Vascular 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 the vaso-dilator centre in the medulla, and coming down the vagus from that centre. Section of the vagi prevents its onset. 2. Secretion. — Very rapidly a free flow of gastric juice occurs from all the glands in the mucous membrane. 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 alkalis and with proteids, and is no longer free. In addition to the HCl small quantities of inorganic salts are present. Traces of proteids may also be demonstrated, and with these two enzymes are associated — one a proteolytic or proteid digested enzyme pepsin, the other a milk curdling enzyme. Amylolytic Period. — The action of the gastric juice does not at once become manifest. For half-an-hour after the food is swallowed the ptyalin of the saliva goes on acting, and the various micro-organisms swallowed with the food grow and multiply, and thus there is a continuance of the con- version 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 firsthalf hour after a mixed meal. Proteolytic Period. — Before the amylolytic period is completed the gastric juice has begun its special action on proteids. This may be readily studied by placing some coagulated proteid in gastric juice or in an extract of the mucous membrane of the stomach and keeping it at the temperature of the body. The proteid swells, becomes 302 PHYSIOLOGY. transparent, and dissolves. The solution is coagulated on boiling — a soluble native proteid has been formed. Very- soon it is found that, if the soluble native proteid is filtered off, the filtrate gives a precipitate on neutralising, showing that an acid proteate has been produced. If the action is allowed to continue and the acid proteate precipi- tated and filtered ofP, 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 sulphate of ammonia, indicating the formation of a deutero-proteose, and, if the filtrate from this be tested, the presence of a proteid may be ■demonstrated. Peptone has been produced. (Practical Exercise.) These changes may be represented in the following table : — Coagulated Proteid. Soluble Original Proteid. Acid Proteate. Protoproteose. Heteroproteose. Deuteroproteose. Deuteroproteose. I I Peptone. Peptone. The process is one of brealiing 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 proteid in the form of peptone through the wall of the intestine. It is now known that absorp- tion is not due to diffusion, and it is more probable that the change to the simplest proteid. molecule is a necessary step FOOD AND DIGESTION. 303 in the building up of the proteid into the special protoplasm of the body of the particular animal. On certain proteids and their derivatives the gastric juice has a special action. On collagen the HCl acts slightly in converting it to gelatin. On gelatin the gastric juice acts, converting it to a gelatin peptone. On nucleo proteids it acts by digesting the proteid part and leaving the nuclein undissolved. Haemoglobin is broken down into hsematin and globin, and the latter is changed into peptone. The casein 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 lime salt of casein which exists in milk as calcic paracasein. This is insoluble and is thrown down, and a small quantity of whey albumin remains in solution. The nuclein part of the paracasein remains undigested. On Fats the gastric juice has no action, but, when these are contained in the protoplasm of cells, it sets them free by digesting the proteid 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. Digestion of the Stomach Wall. — When the wall of the stomach dies either in whole, as after the death of the animal, 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. Antiseptic Action of the Gastric 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 304 PHYSIOLOGY. exist free in the stomach, the activity of these bacteria may lead to various decompositions and to many of the symptoms of dyspepsia. Source of the Constituents of the Gastric Juice. — The hydrochloric 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 CO2 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. Nervous Mechanism of Gastric Secretion. — 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 while this is the case, the vagus also exercises a direct influence. This was proved by making a pouch of part of the stomach and fixing it to the abdominal wound so that its secretion could be collected, and by opening the oesophagus in the neck so that when food was taken it did not pass into the stomach. It was found that showing food to the dog or letting the dog swallow food caused a secretion of gastric juice if the vagi were intact, but not after they were divided. 3. Movements of the Stomach. — These have been recently 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. It is found that, soon after food is taken, a constriction forms about the middle of the stomach and slowly passes on FOOD AND DIGESTION. 305 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 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. Nervous Mechanism of the Movements. — Even after the section of all the gastric nerves, movements of the stomach may be observed, but the mechanism of these has not been fully studied. The action of the vagus and sympathetic fibres is complicated, and their influence on the wall of the stomach and the sphincters appears to be distinct from one another. Speaking generally, the vagiis seems to increase the movements, while the sympathetic fibres check them ; but the vagus seems to. inhibit the cardiac sphincter. Absorption from the Stomach. — The stomach appears to play a small part in the absorption of food. Very little water is absorbed, ■ fats are not absorbed, but sugar 1 and peptones seem to be absorbed to a considerable extent. Importance of the Stomach in Digestion. — The chief function of the stomach is as a reservoir, for the food. 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. Probably thfe antiseptic action of its secretion is of very considerable praC' tical importance. Vomiting. — Sometimes the stomach is emptied upwards through the gullet instead of downwards through the pylorus. 3o6 PHYSIOLOGY. 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. Usually the act is pre- ceded by a feeling of nausea and by a free secretion of saliva. 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 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 made to replace 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. IV. Intestinal Digestion. 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 : — A. The bile. B. The pancreatic secretion. G. The intestinal secretion. A. Bile. — 1. Characters and composition. — The bile is the secretion of the liver and it may be procured for examina- tion — (a) from the gall bladder, or (6) 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 from the walls of the bladder and the bile thus becomes concentrated. Analyses of gall bladder bile thas give no information of the composition of the bile when formed. In several cases. FOOD AND DIGESTION. 307 where surgeons have produced biliary fistulas, 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 charac- teristic smell. It contains about 2 per cent, of solids, of which more than half are organic. The most abundant solids are the salts of the bile acids — the soda salts chiefly of glycochoHc acid, and a much smaller amount of taurocholic acid. These salts are readily separated from an alcholic solution of dried bile by the addition of water free ether. They thus form crystalline bile — Platner's crystals. (Practical Exercises.) Glycocholic acid is — H Amido-ace tic .acid Hv | || \n— C— C— 0— H h/ I H linked to a body of unknown constitution, cholalic acid Taurocholic acid is — Amido-ethane-sulphuric acid \N H H C— C— S— 0— H H H 6 also linked to cholalic acid. Since both acids contain nitrogen they must be derived from proteids. That they are formed in the liver and not merely excreted by it, is shown by the fact that while they accumulate if the bile-duct is ligatured, they do not appear if the liver is excluded from the circulation. Action of Bile Salts. — 1. The bile salts are solvents of fats and fatty acids, and they thus assist in the digestion and 308 PHYSIOLOGY. 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 fseces. When this is the case the fseces have a characteristic white or grey appearance from the abundance of fat. 2. These salts keep cholesterin in solution. 3. While the salts have no action on proteids, free tauro- <;holic acid precipitates native proteids and acid proteate. In . the human intestine this is an action of no import- ance. 4. These salts are powerful hsemolytic agents, and rapidly dissolve haemoglobin out of the erythrocytes. Bile pigments. — These amount to only about 0'2 per cent. In human bile the chief pigment is an orange brown substance, bilirubin, CgjHggN^Oj, while in the bile of herbivora, biliver- din, a green pigment somewhat more oxidised than bilirubin, CggHggN^Og, is more abundant. By further oxidation with nitrous acid other pigments — blue, red, and yellow — are produced, and this is used as a test for the presence of bile pigments (Gmelin's test). (Practical Exercise.) The pigments are closely allied to hsematoporphyn and hsematoidin, and they are derived from haemoglobin. Their amount is greatly increased when haemoglobin is set free or injected into the blood. They are formed in the liver, since, when the liver is excluded from the circulation, the injection of haemoglobin 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. Cholesterin is a monatomic alcohol — CjgH^gOH — 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 crystalises 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 calculi — gall stones — which may accumulate in the gall bladder and may get caught in the bile passages, obstructing the flow of bile and FOOD AND DIGESTION. 309 leading to its absorption into the blood and to jaundice. When these stones are forced along the bile passages as a result, of muscular contraction, intense agony — biliary colic — may be produced. When they 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. {Prac- tical Exercises.) The source of the cholesterin of the bile is not definitely known. It is not an excretion of cholesterin formed elsewhere, because the injection of cholesterin does not lead to an increase in the amount in the bile. According to Naunyn's observations it is most abiindant in cases of inflammation of the bile passages, and he therefore thinks 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 small 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-proteid and mucin. — The bile owes its viscosity to 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 therefore a nucleo-proteid. In some animals a certain amount of mucin is also present. Inorgfanic Constituents. — The most abundant salt is phos- phate of calcium. Phosphate of iron is present in traces. Carbonate of soda and of calcium and chloride of sodium 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 intestines thus depends upon — 1st, the secretion of bile ; 2rid, the expulsion of bile from the bile passages. It is exceedingly difficult to separate the action of these two factors. The flow of bile in the human subject has now been studied in seveiral cases in which the surgeon has had to make a fistula into 3IO PHYSIOLOGY. 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 prolonged 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 proteid meal has the most marked effect, 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. Very often, immediately after food is taken, there is a markedly increased flow due to the stimulation of the muscles of the bile passages, but the later increase seems to be due to a true increased formation of bile. 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 flstula 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 haemolysis, —i.e., the solution of the pigment of the erythrocytes, pro- ducing an increased formation of bile pigments. Influence of nerves upon the Flew of Bile. — (a.) Expulsion of Bile. — There is good evidence that fibres pass to the muscles of the bile passages and 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. 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 FOOD. AND DIGESTION. 311 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. 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 of Bile — Bile is not a secretion of any import- ance in digestion. It has no action on proteids or carbo- hydrates and its action on fats is merely that of a solvent. 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. Digestion 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 hsemoglobin and the acids are the result of proteid dis- integration. 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. B. Pancreatic 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 second the duct is made to open on the surface of the abdomen. 1. Characters and Composition. — When obtained from a temporary fistula, immediately after the operation, the pan- creatic juice is a clear, slimy fluid, with a specific gravity often of 1030 and an alkaline reaction. It contains an abundance of a native proteid having the characters of a globulin, and the alkalinity is probably due to carbonate and alkaline phos- phate of soda. From a permanent fistula a more abundant flow of more watery secretion may be collected. 2. Action. — Closely associated with the proteids, and 312 PHYSIOLOGY. precipitated by alcohol along with them, are the enzymes upon which the action of the pancreatic juice depends. They are three in number. \st, A proteolytic enzyme — Trypsin, which in a weakly alkaline or neutral fluid, converts native proteids into peptones, and which is capable of still further breaking up these peptones into simpler non-proteid bodies. It does not cause solid proteids to swell up but simply erodes them away. The pancreatic juice brings about this breaking down in stages. T'ibrin and similar bodies first pass into the con- dition of soluble native proteids and then into deuteroproteose, while boiled egg white appears at once to yield deuteropro- teose: The deuteroproteose is then changed into peptone, and part of that peptone may split up still further into : — 1. Amido Acids — (a) Not of the Benzene Series.- — The most abundantly found is A7n,ido-cap7'oic Acid — Leucine, which, on evaporation, crystalises out in oily-looking spheres, often with radiating marks upon them. (6) Of the Benzene Series. — Phenyl-mnido-propionic Aci^d—Tyrosin. H N . I I II HO— I CgH, j— C— C— C— 0— H \' / I I \/ H H This tends to crystalise out in sheaves of acicular crystals which in certain morbid conditions are found with leucin in the urine. 2. Aynmonia and Ureides. — Methyl-guanidin-propionic Acid—^Lysatinin is of special interest because it is closely related to creatin and creatinin (p. 41). 3. Tryptophan. — With chlorine water a rose-red colour is struck, but the explanation of the formation of the pigment is not known. While trypsin has the power of splitting some of the peptone in the manner indicated, it has little opportunity of FOOD: AND DIGESTION. 313 doing so in the intestine because the proteid is rapidly absorbed as it is changed to peptone. On nucleo-proteids the trypsin acts by digesting the proteid and dissolving the nucleic acid so that it can be absorbed. On collagen and elastin the trypsin has little action ; but on gelatin it acts as upon proteids. ^nd. An Aifnylolytic Enzyme. — Amylopsin. This acts in the same way as ptyalin, but more powerfully, converting a certain part of the maltose into dextose. It acts best in a faintly acid medium. 3rd, A Fat Splitting Enzyme. — Pialyn. This breaks the fats into their component glycerin and fatty acids. The fatty acid links with the alkalis 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 pialyn can act more freely. This process of emulsification is assisted by the presence of proteid in the pancreatic juice and also by the presence of bile. That these enzymes are independent of one another is shown by many facts. By some methods one may be ex- tracted, by other methods another. That most easily destroyed and most diflScult to separate is the pialyn. As to the mode of production of these enzymes, it is known that trypsin is not formed as such in the cells, but that a fore- runner of trypsin — trypsinogen — is produced, and that this changes into trypsin after it is secreted. The intestinal secre- tion contains something which has been termed enterokinase, which has the power of bringing about this change, and in all probability the cells lining the ducts of the pancreas also pro- duce this or a similar substance. It is doubtful whether the pancreatic secretion contains any true rennin, although it produces a modified clotting of milk, under certain conditions. The influence of the pancreas in the general metabolism will be considered later. Physiology of Pancreatic Secretion. — The secretion of pancreatic juice is not constant, but is induced when the acid 314 PHYSIOLOGY. chyme passes into the duodenum. This occurs even when all the nerves to the intestine have been cut, and it appears to be due to the formation in the epithelium lining the intestine, under the influence of an acid, of a material which has been called secretin. 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 of the lining membrane of the upper part of the small intestine, made with dilute hydrochloric acid, 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 whii-e secretin seems to play so important a r6ie, it has been found that stimulation of the vagus nerve also increases pancreatic secretion, so that it must be concluded that the fluid is, to a certain extent, under the control of the nervous system. C. Secretion of the Intestinal Wall (Succus Entericus). This is formed in the Lieberkiihn's follicles of the intestine, and it may be procured by cutting tlie intestine across at two points, bringing each end of the intermediate piece to the surface, and connecting the ends from which this piece has been taken away. On mechanically irritating the mucous membrane, a pale yellow clear fluid is secreted, which con- tains native proteids and mucin, and is alkaline in reaction from the presence of carbonate of soda. Actiov:. — The succus entericus contains one enzyme which splits some disaccharids, as maltose and cane sugar, into monosaccharids, but does not seem to act on lactose. Nervous Mechanism of Secretion — So far very little is known on this point. 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 might account for this without secretion being implicated. FOOD AND DIGESTION. 31 S Bacterial Action in the Alimentary Canal. With the food, water, 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, by splitting sugars, form lactic and sometimes acetic acids. But when sufficient gastric juice is poured out for the acid to exist free, the growth of micro-organisms is inhibited, and some, at least, are killed. Others pass on into the intestine, and as the hydrochloric acid in the chyme becomes neutralised, tbe acid-for-ming organisms begin to grow, and, by splitting the sugars to form lactic or acetic acids, render the contents of the small intestine slightly acid. Towards the end of the small intestine the alkaline secretions have neutralised these acids, and in the alkaline material so produced the putre- factive organisms begin to flourish and to attack any proteid 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. Indol is a derivative of ethyl-amido-benzene. H CgHj I — N^' Amido-benzene. I-n/ •H H H H I' CfiHg I— N— C C— H Ethyl-amido-benzene. I I H H C— H Indol. II C— H CeHs I ■Nv 3i6 PHYSIOLOGY. In Skatol an H is replaced by CHj. Phenol is CeH, I -^-^ By taking embryo guinea-pigs at full time from the uterus and keeping them with aseptic precautions, it has been shown that the absence of micro-organisms from the intestine does not interfere with digestion. The most important and abundant organism present in the intestinal tract is the bacillus coli communis, which has a certain power of splitting proteids and a marked action in producing acids from sugars. 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 consider- able, probably not far short of 3000 ccms., or something considerably inore than one-half of the whole volume of the blood. Only a small amount of this is given off in the faeces, 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-hcBmal circulation. One portion of this is particularly important. The blood vessels of the intestine pass to the liver, and many substances, when absorbed in 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 or their derivatives, many alkaloids such as curarin, and in all probability a set of animal alkaloids called ptomains formed by putrefactive decom- position of proteids 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; FOOD- AND DIGESTION. 317 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 proteids. Trypsin appears to be destroyed in the alimentary canal ; but the fate of the' other pancreatic enzymes and of the enzyme of the succus entericus is unknown. 3. Bile Constituents. — 1. The bile salts are partly re-absorbed from special parts of the small intestine — glycocholate of soda 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 fate of the cholalic acid is not known, but it is supposed to be excreted in the fseces. 2. The pigments undergo a change and lose their power of giving Gmelin's reaction. They appear in the faeces 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-organism. 3. The cholesterin appears in the faeces. Faeces. 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 faeces. 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 faeces 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, 3i8 PHYSIOLOGY. but, if much undigested matter is present, water may also be added, and the consistence of the faeces maj-^ thus be varied. In the average condition they contain about 70 or 80 per cent, of water. The colour is normally brown, from the hsematin of the flesh eaten, while the sulphide of iron formed by the splitting of the hsematin 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 pigments play but a small part in colouring the faeces. In infants, before bacteria are introduced and begin to exert their reducing action, the faeces may be green from the presence of unaltered biliverdin. The reaction of the faeces varies. Usually the outside 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 faeces depends on the amount of food — a fairly average amount per diem is 150 grms. of dried solids. On a vegetable diet, from the presence of undigested cellulose, the amount is very much greater. The solids of the faeces of a feeding animal consist of the same constituents as the faeces 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 fat and the earthy soaps of the fatty acids ; and, when a vegetable diet is taken, frequently, starch, and always the cellulose of the vegetable cells, which is very slightly digested in the intestine. This cellu- lose, by stimulating the intestine, is a valuable natural purgative. The odour is due to the presence of aromatic bodies such as indol and skatol. Movements of the Intestine. — These are of two kinds — myogenic and peristaltic. The myogenic movements are slight rhythmic contractions which pass rapidly along the intestine, and are insuflScient to drive on the contents, but are probably of use in churning and mixing them. They FOOD AND DIGESTION. 319 occur when all the nerves have been divided, and when the ganglia in the walls have been poisoned with nicotine. The peristaltic movements are much more complex and powerful. They consist of a constriction of the muscles, which, starting at the upper end of the intestine, passes slowly down- wards. In front of this 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 stimu- lated 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. 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 erigehtes to the lower part of the large gut are augmentor nerves, increasing the peristalsis. Defsecation. — By the peristalsis of the intestine, the matter not absorbed from the wall of the gut is passed down and accumulates in the sigmoid flexure. The contraction of the perineal muscles generally prevents its passing into the rectum, while, if any does reach that part of the gut, it is prevented from escaping by the sharp fold which the last part of the bowel makes, and by the contraction of the strong sphincter ani. Defsecation depends primarily on the intestinal peristalsis, and without that it cannot be performed. When faeces are forced into the rectum, the mucous membrane is stimulated, and impulses are sent up to inhibit a centre in the lumber region of the cord which keeps the sphincter ani contracted, and thus the sphincter is relaxed, and the escape of faeces made possible. In some diseases of the cord this centre is stimulated, and cannot be inhibited, and thus defsecation is in- terfered with, while in other diseases when this centre has been 320 PItYSIOLOGY. destroyed, the sphincter does not contract, and faeces may escape continuously. Normally the act of defsecation 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 of the sigmoid flexure are allowed to pass into the rectum — provided that the peristalsis of the bowel is strong enough. The sphincter is relaxed, and defsecation occurs. The act is completed by the emptying of the rectum through the contraction of the perineal muscles. Absorption of Food. 1. State in which Food leaves the Alimentary Canal. — The carbohydrates generally leave the alimentary canal as monosaccharids ; but some resist the action of digestion more than others. Lactose does not seem to be broken down by any of the enzymes, but it must be 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 proteids are chiefly absorbed as peptones, possibly as proteoses. But native proteids 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 considerable extent. The fats are chiefly absorbed as soaps and as fatty acids, and it is very doubtful if they leave the gut as fats. 2. Mode of Absorption of Food.— Absorption does not occur uniformly throughout the alimentary canal. Thus, while sugar and peptones are absorbed from the stomach, water is absorbed only to a small extent. 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. FOOD AND DIGESTION. 321 2. The relative rate of absorption of different substances does not follow the law 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 blood capillaries or lacteals, osmosis and adsorption, are excluded. Absorption is stopped or diminished when the epithelium is removed, injured, or poisoned with fluoride of sodium, in spite of the fact that removal must increase the facilities for osmosis and filtra- tion. 3. Channels of Absorption. — There are two channels of absorption from the alimentary canal ; the veins which run together to form the portal vein of the liver, and the lymph- atics which run in the mesentery, and, after passing through some lymph glands, enter the receptaculuvi chyli 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 vein, and pours its contents into the blood- stream. The lymph formed in the liver also passes into the thoracic duct. 1. Proteids. — While peptones are formed from proteids by digestion, they undergo a change in the intestinal wall before passing to the tissues, and it is probable that they are taken up by the leucocytes of the adenoid tissue of the wall of the alimentary canal, since under a proteid diet these increase in number and stream away in the blood to produce a digestive leucocytosis. Pohl has shown that the leucocytes leave the intestine chiefly by the veins, and it is known that ligature of the thoracic duct does not interfere with the absorption of proteids. By breaking down in the blood- stream they probably set free the proteids for use in the tissues. When an excess of proteids is taken in the food, it is broken down in the lining membrane of the gut, its nitro- genous part forming ammonia compounds, probably carbamate of ammonia. 322 PHYSIOLOGY. >N C— 0— N^ > Its non-nitrogenous part probably yields carbohydrates, since a proteid diet may lead to the accumulation of these substances in the liver. The ammonia compounds are carried to the liver and there changed to urea, and excreted as such. Thus the entrance of an excess of nitrogen to the tissues is prevented. It has been pointed out that gastric juice does not dissolve the nucleo-proteids, but that they are dissolved by the pancreatic juice. Phosphorus is undoubtedly absorbed in organic combination, but the mode of absorption and the channels by which the blood-stream is reached have not been investigated. 2. Carbohydrates. — Although the chief monosaccharid formed in digestion is dextrose, others are also produced — Isevulose from cane sugar, and galactose from milk sugar. All these are absorbed in solution, and are carried away in the blood by the portal vein. 3. Fats. — It was for long thought that the fats are absorbed as a fine emulsion ; but the most recent investigations seem to indicate that after being split up into the component acids and glycerin, they 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, and they are passed on from the cells, through the lymph tissue of the viUi, into the central lymph vessels, and thus on through the thoracic duct to the blood-stream. Unlike the proteids and carbohydrates, they are not carried directly to the liver. Utilisation of the Food Absorbed. The food absorbed may be used immediately as a source FOOD AND DIGESTION. 323 of energy, either for (1), the construction or reconstruction of tissues ; or (2), the production of mechanical work ; or (3), of heat. Or, on the other hand, 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 con- sidered. The production of heat, and the regulation of temperature may be now considered. PRODUCTION OF HEAT. A. IVEuscle. — The production of heat in muscle has been already studied (p. 58). It has been shown that muscle from 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, and 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. The temperature of the blood coming from the liver during the period of active digestion, may be nearly 1°C. higher than that of the blood going to the organ, and, when the size of the liver and the rapidity of the blood flow through it are con- sidered, it must be admitted that during digestion it may contribute an appreciable amount of heat. 0. 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 sufiicient to yield a certain amount of heat. But it is more probable that the rise of temperature is due to the increased 324 PHYSIOLOGY. flow of blood through the organ, since a study of the blood gases in the brain, gives no indication of any marked increase of chemical change during periods of increased cerebral action. Since heat is thus constantly being produced, the tempera- ture 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. Elimination of Heat. — Heat is got rid of by three channels. A. Skin. — Since the body is generally warmer than the sur- rounding air, heat is constantly lost by conduction and radiation, and the extent of this loss depends mainly upon the difference between the temperature of the body and that of the air. 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 constriction 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 over 70 per cent, of all the heat is lost by conduction and radiation. 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. The loss of heat in this way is comparatively small — 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 freer. Of the various factors increasing sweat secretion, heat is probably the most important. 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 (p. 360). FOOD AND DIGESTION. 325 B. Respiratory Passagfes. — 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 Faeces. — Since these are warmer than the surrounding air, a certain amount of heat is lost through them. The amount is small — something less than two 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 condi- tions. 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, at once, by contrac- tion of the cutaneous vessels and diminished ' activity of the sweat glands, loss of heat is rapidly checked. 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 measure of the internal temperature. The mean daily temperature of a healthy man is : — °C. °F. Rectum, . 37-2 98-96 Axilla, . 36-9 98-45 Mouth, . 36-87 98-36 But the temperature varies throughout the course of life. °0. Infant, . . . 37o Under 25 years, . :J7-2 About 40, . 37-1 Old Age, . . . 37-2 to 37-5 It also varies throughout the course of the twenty-four hours, and since this is a matter of great importance in medicine, it has been very carefully studied by many observers. The difference is not more than -5°C. or 1°F. It is lowest in the early morning and highest in the afternoon. Under all normal conditions the temperature of man under- 326 PHYSIOLOGY. goes only small variations, because the balance between pro- duction 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 tempera- ture, 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 — e.g., in alcoholic poisoning, the change is small. Although the normal variation of temperature is so small, life may be sustained even when the temperature falls for a time to about 25°C., or rises to nearly 43°C. Cases are recorded in which it has even risen to 466°C., without death supervening. While the higher " warm blooded " animals, mammals and birds maintain a constant temperature, the lower vertebrates, " cold-blooded animals," reptiles, amphibia and fishes do not do so, and their temperature varies with that of the surround- ing 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 FOOD AND DIGESTION. 2,27 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. STORAGE OF SURPLUS FOOD. This storage takes place chiefly in three situations : (1), fatty tissue ; (2), muscle ; (3), liver. 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 fats different from those of the body leads to their appearance in those tissues. Fats are also formed from the carbohydrates of the food. Feeding experiments upon pigs and other animals 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 sucli 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 proteid of the food eaten. It must therefore have been formed from the carbohydrates. There is good evidence that in excessive proteid feeding the non-nitrogenous part of the proteid mole- cule may be stored as fat. It has recently been maintained that the evidence in favour of the formation of fats directly from proteids is unsatisfactory ; but since proteids yield carbo- hydrates, and since carbohydrates form fats, it must be admitted that proteids may be a source of fats. 2. In Muscle. — Some animals, as the salmon, store fats within their muscular fibres, but in mammals such a storage is limited in amount. The salmon also stores surplus proteid material in the muscles, and mammals too appear to do the same. How far a passive storage may occur is not known. 338 PHYSIOLOGY. but feeding experiments on mammals indicate that only a small amount of proteid can be accumulated. On the other hand, the experience of athletic training shows that the muscles may be enormously increased by the building up of the proteid of the food into their protoplasm. 3. In the Liver. — The liver is the chief storehouse of carbohydrates in all animals, and of fat in some — e.g., the cod. Relationship of the Liver to Absorbed Food and to the General Metabolism. The liver develops as a digestive gland — as a couple of diverticula from the pmbryonic gut. But early in f^tal life it comes to have important relationships with the blood going to nourish the body from the placenta. The vein bringing the blood breaks up into a series of capillaries in the young liver, and in these capillaries, for a considerable time, the development of the cells of the blood goes on. Soon the liver begins to secrete bile, while animal starch and fat begin to accumulate in its cells. Gradually the formation of blood cells stops, and the mass of liver cells become larger in proportion to the capillaries. As the fcetal 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 — Glycogenic Function. — Claude Bernard discovered that there is a constant forma- tion of sugar in the liver. On account of this constant supply, even when an animal undergoes a prolonged fast, the amount of sugar in the blood does not diminish. In starvation there are only two possible sources of this glucose — the fats and the proteids of the tissues. There is no conclusive evidence that fats can be changed to sugar in the liver. That proteids are a source of sugary substances is shown by the accumulation of a starch in the liver in animals fed upon proteids ; it is therefore probable that in starvation the proteids of the body are FOOD AND DIGESTION. 329 broken down and their non-nitrogenous part clianged to sugar. But not ouly 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 — glycogen — and gives it out as sugar as that substance is required. On a carbohydrate diet the accumulation of glycogen in the liver is very great ; but even on a proteid diet, in dogs at least, a smaller accumulation takes place. The observation that various monosaccharids are 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 more 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 that the injection of methylene blue, which poisons protoplasm but does not interfere with the action of enzymes checks the conversion, and that the conversion is increased by stimulating the nerves going to the liver, without any increase in the enzyme being produced in the liver or 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 then passes on into the general circulation, and is excreted in the urine. Every individual has a certain power of dealing with 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. Whfen the glycogen stored in the liver is changed to glucose more quickly than is required by the tissues, the glucose accumulates in the blood and is excreted in the uriHe. This is seen in Bernard's experiment of puncturing the floor of the fourth ventricle at its posterior part in a rabbit. If glycogen is abundant in the liver, but not unless, glycosuria results. Another way in which sugar may be made to appear in the urine is by injecting phloridzin. But since under the 330 PHYSIOLOGY. influence of this drug the sugar in the blood is decreased, 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. The injection of large doses of extract of the supra-renal bodies has recently been shown to cause 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 of sugar or of diminished utilisation. Eemoval of the pancreas also causes glycosuria (p. 335). 2. Relation to Fats. — Although the fats are not carried directly to the liver, like proteids and carbohydrates, they are stored 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 generally throughout the muscles and other tissues, 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 — is present in the liver cells. Lecithin 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 is so, the fat of the liver must play an important part in retaining and fixing phosphorus in the body. 3. Relation to Proteids. — Along with the intestinal wall, the liver regulates the supply of proteids to the body. A study of the chemical changes in muscle has shown that the waste of proteid 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. Hence the demand for nitrogen in the muscles is small, and for this reason, apparently, any excess of proteid in the food is decomposed into ammonia compounds in the. intestinal wall, and thesQ are built into urea in the liver. Urea, the chief waste substance excreted in the urine, is the bi-amide of carbon dioxide. FOOD. And digestion. ^ V H\ II /H i\ II A H— 0— C— 0— H ^, ,^ 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 decomposed into nitrogen, carbon dioxide and water by nitrous acid and by hypobromite of soda in excess of soda. 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 carbonate of ammonia is added to blood artificially circulated through the kidneys 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 contain- ing carbonate of ammonia; is streamed through muscles urea is not produced. That it is formed in the liver is indicated — (1), by the fact that when an ammonia salt such as the carbonate dissolved in blood is streamed through the organ, it is changed to urea ; (2), by the observation of Nencki and Sieb'er^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. Source of Urea. — Urea is produced from the proteids of the food and tissues. The manner in which excess of proteid in the food is broken down into amiponia compounds and sent to the liver has been already described. But the fact that even in starvation urea is produced, seems to indicate that the initial stages of decomposition of proteids goes* on else- where than in the intestinal wall. The fate of hsemoglobin tends to show that the whole process may be conducted in the liver cells. When hsemoglobin is set free from the corpuscles, the nitrogen of its proteid part is changed to urea, while the pigment part is deprived of its iron and excreted 332 PHYSIOLOGY. as bilirubin.. Whether the proteids 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. It is probable that lactate of ammonia is produced in muscle and that this is converted into urea in the liver. Speculations as to the way in wTiich the proteid molecule is broken down are of little value. It is one thing to show that urea may be formed in a particular way outside the body, but quite another to prove that it is formed in that particular way in living protoplasm. The nitrogen excreted is not all in the form of urea, but some is combined in ammonia salts, uric acid and the other purin bodies (see p. 348), and in creatinin. In the mam- malian 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 and toxins markedly decrease their conversion into urea and so increase their quantity in the urine. Although urea may be prepared from creatinin, there is no evidence that such a process goes on in the body. After the nitrogenous portion of the proteid molecule is split up, 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 liver may be briefly summarised as follows : — (1), It regulates the supply of glucose to the body, (a) by manufacturing it from proteids when the supply of carbohydrates is cut off, and (6) by storing it as glycogen when the supply of carbohy- drates is in excess, giving it off afterwards as required. (2), Along with the intestinal wall it regulates the supply of pro- teids to the body, by decomposing any excess, and giving off the nitrogen as urea, &c. (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 haemoglobin and retaining the iron for further use. (5), From the part it plays in the entero-hepatic circulation, it protects the body against certain poisons by excreting them in the bile. METABOLISM. 333 GENERAL METABOLISM. Having considered how the food is digested and absorbed, and how it is then either stored or at once used (a), for build- ing up and repairing the tissues, or (&), as a source of energy, the rate at which the various chemical changes go on and the factors modifying these may be dealt with. The changes in the two great constituents of the body — proteids and fats — have to be separately studied. A. Proteid Metabolism. — The amount of proteid 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 — the nitrogen ex- creted is derived entirely from the proteids in the body. Proteids contain 16 per cent, of nitrogen and hence each grm. of nitrogen excreted is derived from 6 "2 5 grms. of proteid broken down. 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 neces- sary to analyse the fisces and sweat. Since nucleo-proteids form so important a constituent of living matter, it is sometimes necessary to study the chemical changes which they are undergoing. To do this the excre- tion of phosphorus and the purin bases must be investigated. But it is difficult to arrive at reliable conclusions, because there are other phosphorus - containing substances besides nucleins in the body — e.g., the bones ; and secondly, the purin bodies all tend to be converted into urea before being excreted. B. Metabolism of Fats. — Proteids contain nearly three and a-half times as much carbon as nitrogen, and hence, when broken down, for each grm. of nitrogen excreted, 3 '4 grms. of carbon are thrown off. The carbon is chiefly excreted from the lungs as carbon dioxide, and in this form it may readily be collected and estimated. Any excess of carbon excreted over 3 '4 times the amount of nitrogen given ofl", must be derived from the fats of the body or from the fats and carbohydrates taken in the food. 334 PHYSIOLOGY. Any carbon retained in the body, apart from that in proteids, is stored ultimately as fat, and 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 Grms. Output. Proteids, . Fats, . Carbohydrates, . 100 100 400 C. 54' 92 200 N. 16 ... 16 C. N. 346 300 14 Two grms. of nitrogen or 2x6-25 = 12-5 grms. of proteids are being daily laid on. Forty-six grms. of carbon are also retained in the body, of this 3 '4 x 2 = 6 '8 are fixed as proteids, while "the rest, 39-2, must be as fat — i.e., 39-2 x 1-3 = 50-9 grms. of fat. INFLUENCE OF THE INTERNAL SECRETIONS OF ORGANS ON THE METABOLISM. 1. Thyroid Gland. — This structure is formed as a hollow outgrowth for the alimentary canal, which branches and again branches. It early loses its connection with the ali- mentary canal, and becomes cut up by fibrous tissue into a number of small more or less rounded cysts or follicles, each lined with epithelium, and filled with a mucus-like substance, which contains a nucleo-proteid, and a substance with a marked power of combining w^ith iodine. The removal of the thyroid leads to a train of symptoms which varies somewhat in different animals, but is essentially the same in nearly all. The connective tissues tend to revert to the embryonic conditions, and the amount of mucin increases. The temperature falls, muscular tremors appear, the functions of the higher nervous system become sluggish, and the animal usually dies. By administering the sub- METABOLISM. 335 Stance of the thyroid, or by giving extracts of it, most of these symptoms may be delayed or prevented. When thyroid gland or extract is given to healthy animals in • moderate doses it causes an increased metabolism of both fats and proteids, and may thus induce emaciation. It would appear as if one function of the organ is to produce an internal secretion, which regulates the rate of the meta- bolic processes in the body by increasing them when such an increase is desirable. When the thyroid is not developed, the growth and development of the individual are partially arrested, and the condition of cretinism, is produced. Atrophy of the structure in adult life causes a train of symptoms somewhat resembling those produced by its removal, and constituting the disease myxoedeTna. It has been suggested that a condition of increased activity of the action of the heart, usually accompanied by prominence of the eyes and swelling in the region of the thyroid — ex- ophthalTnic goitre — may be due either to increased activity of the structure, or to deficient action of the parathyroids. 2. Ovaries and Testes. — It is well known that removal of these organs causes characteristic changes in the animal ; a tendency to the deposition of fat being produced and the activity qf the central nervous system being somewhat modified. Several years ago Brown-Sdquard maintained that by the administration of testicular substance the general effects of atrophy of these organs might be obviated ; and more recently, as a result of clinical experience, the adminis- tration of extracts of the ovaries has been described as relieving certain of the nervous symptoms which supervene on their removal or atrophy. It has further been found that ovarian substance when given to dogs, whether male or sfemale, causes a marked increase in the rate of proteid metabolism, although no similar action was found with testicular substance. There is thus evidence that the ovaries, like the thyroid, form an internal secretion having an important action in accelerating the metabolism, and it is at least probable that the testes produce a similar substance. 3. Pancreas. — That the pancreas is the most important of the digestive glands has been for long known, but a further 33^ PHYSIOLOGY. function has more recently been demonstrated. It has been found that excision of the pancreas produces a condition of diabetes — an increase of sugar in the blood, its appearance in the urine, an increased excretion of nitrogen, and^ a general emaciation. These symptoms do not occur when the duct is tied or occluded, but they are invariable when a sufficient amount of the gland is removed. They are not prevented by the administration of pancreas, either fresh or as extracts. In this condition, sugar is formed from the proteids, since it appears after all the glycogen has been removed, and its amount is proportionate to the amount of nitrogen excreted. Normally, proteids are a source of carbohydrates in the body, and it would seem as if the pancreas in some way exercises a restraining influence on this splitting of proteids, and that the removal of the organ permits it to occur to an excessive extent. Since the only respect in which the pancreas ditfers in histological character from the parotid gland — removal of which has no effect on the metabolism — is in the presence of the islets of Langer- hans, it is probable that they are related to this function of the organ. 4. Pituitary Body. — This structure lies at the base of the mid brain, and consists of a posterior part of nervous tissue, and an anterior part derived from the alimentary canal, and consisting of masses of epithelial-like cells. Removal of this body causes in cats and dogs a fall of teniperature, lassitude, muscular twitchings, dyspnoea, and ultimately death. Injection of extracts of the substance is said to diminish these symptoms. In the healthy animal the injection of extracts of the pituitary causes an augmenta- tion of the force of cardiac contraction, and a contraction of the arterioles, and thus raises the arterial pressure. 5. Saprarenal Bodies. — -These structures lie just above the kidneys. Each consists of a tough cortex composed of epithelial-like cells arranged in columns, and a soft medul- lary part consisting of large capillary vessels, large epithelial- like cells, nerve fibres and nerve cells. Long ago Brown-S^quard stated that removal of these bodies causes great muscular weakness, loss of tone of the METABOLISM. 337 vascular system, loss of appetite, and finally death in a short time. Addison had already pointed out that a similar set of symptoms, accompanied by pigmenta- tion of the skin, is associated with diseased conditions of these organs in man. Within the last few years it has been demonstrated that injections of small quantities of the central portion of the bodies have a powerful effect on the muscular system generally, and especially on the muscles of the vascular system, causing contraction of the arterioles, and an enormous rise in the blood-pressure. As already indicated, injections of extracts of the supra- renal bodies profoundly modify the metabolism, leading to an increase of sugar in the blood and to its excretion in the urine. A decrease in the nitrogen in the form of urea and an increase of the ammonia, similar to that observed in cases of true diabetes, has also been observed. Metabolism during fasting. " When the usual supply of energy in the food is cut off, the animal liberates the energy required by oxidising its own stored material and tissues. This is shown by the fact that the animal loses weight and goes on excreting carbon dioxide, urea, and the other ■ waste products of the activity of the tissues. Several prolonged fasts have been undertaken by men, and in one or two of these careful observations were made by physiologists. It has been found that during the first day or two of a fast, the individual goes on using proteids and fats at something like the same rate as he did while taking food, but that gradually he uses less and less proteid each day. This is well shown in the case of Succi, who underwent a fast of thirty days. Day of Fast. Proteid used in Grms. Eat used. 1st. 104 Not estimated. 10th. 51 170 . 20th. 33 170 29th. 31 ". / 163 ' \ It is from the stored fats that the energy is chiefly derived, z 338 PHYSIOLOGY. and the result of this is that before death the fats of the body are almost entirely used up. The proteid-containing tissues waste more slowly and waste at different rates, the less essential being used up more rapidly than the more essential, which, in fact, live upon the former. In cats deprived of food till death supervened, the heart and central nervous system had hardly lost weight, the bones, pancreas, lungs, intestines, and skin each had lost between 10 to 20 per cent, of their weight, the kidneys, blood, and muscles between 20 to 30, and the liver and spleen between 50 to 70. The rate of waste during a fast depends upon the amount of energy required, and it is therefore increased by muscular work and by exposure to cold. When a man is kept quiet and warm, a fast of thirty days may in some cases be borne without injury. Effect of Feeding. When food is given to a fasting animal or man, the first effect is to increase the rate of wasting by calUng into action the muscles and glands concerned in digestion. The result is an immediate increase in the excretion of nitrogen and carbon, indicating an increased breaking down of proteids and fats. For this reason, to give an animal which is fasting a diet containing just the amount of nitrogen and of carbon which the animal is excreting, will not at once stop the loss of weight. Suppose, for instance, that to a fasting man using daily 30 grms. of the proteids and 160 grms. of the fats of his body, a diet containing these amounts is given, the disintegra- tion of proteids and of fats will at once rise, say, to 5 grms. of proteid and 280 of fat. Thus the result will be that, instead of his losing 30 grms. of proteid he will lose only 20 grms. per diem, and instead of 160 grms. of fat, only 40 grms. But, if the diet is sufficient to supply the energy required, in a few days the intake and output will balance, and the individual is then said to be in metabolic equilibrium, and he neither gains nor loses weight. The following table gives an idea of how this adjustment of the metabolism is reached : — METABOLISM. 339 Day. Intake. Wasted. Waste diminished by Proteid. Fat. Proteid. Fat. Proteid, Fat. 1. 30 160 2. 30 160 50 280 20 120 3. 30 160 40 240 10 60 4. 30 160 30 200 40 5. 30 160 30 160 6. 30 160 30 160 If the amount of food be further increased, a small pro^ portion of the proteids and a larger proportion of the fats are retained, and weight is gained. As already indicated, the power of storing proteids is small. Proteid Diet. — Proteids contain all the chemical elements required for the building and repair of the tissues, and from the complexity of their molecules they also supply latent energy. It is therefore theoretically possible for an animal to sustain life on proteids, and certain animals can be fed exclusively upon them. Thus Pfluger kept a dog for many months upon a purely proteid diet without injury to its health. But to supply the necessary energy in proteids alone requires the consumption of excessively large quantities. For a man to get the energy equivalent to 3000 Calories — an average expenditure per diem — he would have to eat more than seven times the usual amount of proteid. Further, it has been shown, that, when large quantities are taken, a portion is broken up in the intestinal wall and formed into urea by the liver and excreted by the kidney, and thus excessive work is thrown upon these excretory organs. While these organs usually form and excrete about 33 grms. of urea per diem, on such a diet they would have to deal with no less than 231 grms. It is therefore not advantageous to adopt a too purely proteid diet. The great use of proteids is as muscle-builders. When the muscles are in a state of constant activity they have a certain power of laying on proteid as they grow. Hence the value of proteids in muscular training. Gelatin, although undergoing digestion and absorption like the proteids, is not available as a muscle-builder. Its sole 340 PHYSIOLOGY. use is as an energy yielder, and in this respect it has a value equal to the proteids. Carbohydrate Diet. — Carbohydrates are of equal value with proteids as a source of energy, but they contain no nitrogen, and they are not available for building up and repairing the protoplasm of muscles and other tissues. Carbohydrates alone will not support life, but when added to proteids they enable the animal to do with smaller quantities of the latter. They are thus sometimes termed proteid sparers. Their use in diminishing the consumption of pro- teids is, however, strictly limited. Fat Diet. — Fats, like carbohydrates, will not support life, because they cannot be used for building up protoplasm, but like carbohydrates they are a source of energy, and they have more than twice the energy value of proteids or of carbohydrates. They are thus proteid sparers. But experiment has shown that in spite of their higher energy value they have not the same power as carbohydrates of sparing proteids. A knowledge of the part played by proteids, carbohydrates, and fats in the animal body is the ground-work of the study of Dietetics. DIETETICS. The great essentials of a diet capable of maintaining health are : — 1st, That it should supply the energy required. 2nd, That it should contain sufficient proteids to make good the waste of these substances. 3rd, That it should be capable of digestion, absorption and assimilation. I. The energy requirements vary with the mode of life and with the age and size of the individual. Size. — Other things being equal, a large man requires more energy and more proteid than a small man. For this reason the energy requirements are sometimes stated as per unit of weight, but it is more convenient to take as the standard an adult man of average weight, say sixty- five kgs. Age. — In children the metabolism is more active than in DIETETICS. 341 adults. They are more constantly in motion, and they require energy and material to build up their tissues. Weight for weight, a child thus requires even a greater supply of energy than a man. The following results, based upon observations recorded by Camerer, illustrate this : — Age, Weight in Kilos. Energy used per Kilo in Calories. Total Energy . in Calories. 4 12 30 14 30 66 91-3 57-7 42-4 1280 1730 2800 It will thus be seen that the energy requirements of children at ditferent ages may be stated in terms of the energy requirements of an adult man doing average work. -Atwater formulates this as follows : — Taking a man at . . . 1"0 A woman is equivalent to 0'8 of a man. 0-8 A girl 0-7 A child 10 to 13 0-6 6 to 9 0-5 2 to 6 0-4 „ under 2 0-3 Mode of Life. — An individual kept warm and at rest, requires much less energy than if he is required to do •muscular work and is exposed to cold. The variations in the amount of material used in different conditions is well illustrated by the experiments of Zuntz on the excretion of carbon dioxide in the dog. Resting lying, ..... „ standing, .... Forward movement (unloaded), . ,, (drawing weight), . These results are corroborated by a study of the diet COj excreted per minute in corns. 124-7 170-2 525-0 798-9 342 PHYSIOLOGY. required to maintain the weight and health of men. doing different amounts of work. From a large series of such observations Atwater concludes that the energy requirements of the diet varies as follows : — Calories. Man without muscular work, 2700 „ with light „ 3000 ,, „ moderate „ . 3500 ,, „ severe „ 4500 It must, of course, be remembered that all the energy of the constituents of the diet is not available, since a consider- able and varying proportion of the food is not digested or absorbed (p. 344). The gross energy of the diet must there- fore be well above the nett energy requirements of the body. A rough idea of these nett requirements may be arrived at by considering the average expenditure of energy in different forms. Taking the case of a man doing a moderate day's muscular work, he gives off — Calories. in external muscular work, equivalent to 350 as heat, 2600 Total, 2950 To make good this loss of energy the diet should contain more than 3000 gross Calories, and experience has shown that it is desirable that it should contain about 3500. •II. Proteids.— Many experiments have been made to determine the smallest amount of proteids upon which life can be maintained, and conflicting results have been arrived at. It has, however, been shown that by enor- mously increasing the supply of carbohydrates and fats the amount of proteids may, for a short time at least, be reduced to something like the amount which is daily wasted during fasting. But, to maintain a vigorous muscular system, much larger quantities seem to be desirable. A study of a very large series of dietaries of different races shows that, unless prevented by poverty, the average man consumes over 100 grms. of proteid per diem, and that those in muscular DIETETICS. 343 training tend to consume very much larger amounts. Bear- ing in mind the importance of proteids as muscle builders it is safe to conclude that at least 120 grms. of proteids should be allowed per adult man per diem. Carbohydrates and Fats. — The 120 grms. of proteid will yield 492 Calories of energy, which leaves about 3000 to be supplied by fats and carbohydrates. In determining the proportionate amounts of these, two factors have to be considered- — first, the relative cost, and second, the limitation of the power of digestion of each. Carbohydrates are enormously cheaper than fats as a source of energy. Margarine at 8d. per lb. will yield 435 Calories for a penny, while sugar at 2|d. per lb. will yield 1860 Calories for the same sum. But the use of carbohydrates as a source of energy is limited by the fact that in most individuals digestive disturbances are apt to supervene if more than 500 to 600 grms. are consumed. A diet should therefore not contain much more than 500 grms. of carbohydrates, which will yield 2050 Calories of energy. This leaves about 950 Calories to be supplied by fats, an amount which is nearly met by 100 grms. of fat yielding 930 Calories of energy. This quantity of fat most people can easily digest. A typical diet for an average man doing moderate muscular work would thus be — Amount. ^"^'8? X»l"« '" ' Calones. Proteids, . .120 492 Carbohydrates, . .500 2050 Fats, . . 100 930 3472 or very nearly 3500 Calories. A reference to Atwater's table gives the dietary require- ments of a woman or of a child of any age, and thus renders it easy to make out the diet of a family or public institution. III. The Diet must be capable of Digestion, Absorption, and Assimilation. — While proteids, carbohydrates and 344 PHYSIOLOGY. fats in .the alimentary canal undergo the change already described, they are frequently in an unfavourable state for the action of digestive juices when taken in the food. Thus, while the proteids of flesh are exposed to the gastric and pancreatic secretion, the proteids of many vegetables, unless carefully prepared, cooked and masticated, are protected and escape digestion and absorption. The same may be said of the crude starch of vegetables. The digestibility and value as an article of diet of the cellulose and allied substances in plants is an important question at present requiring further investigation. The fats of vegetables are also generally in a less favourable state for digestion than the fats of animals. When once digested there seems to be no difference in the absorbability and assimilabihty of the proximate principles of vegetables and animals ; a given weight of vegetable proteid may be substituted for the same .quantity of animal proteid. As a result of the difference in digestibility the avail- ability of food stuffs varies. The following table, serves to show some of these variations. Absorption of Food Stuffs. Flesh, Proteid. . 97 Per Cent. Absorbed. Pat. Carbohydrate. 95 Egg, Milk, ■ . 97 89-99 95 96 100 Bread, . , . 78 • ■ ■ 99 Potatoes Carrots ( (boiled), ;raw), . . 68 . 61 ... 92 82 The availability of almost any article of food varies with the state of the digestive organs of the individual, and with the manner in which it is prepared; for example, the mode of manufacture of the flour used in bread-making has a very marked influence upon the amount digested and absorbed. Especially in vegetable foods the thoroughness of the cooking has a most important influence on the availability of their constituents. DIETETICS. 345 The subject of Dietetics cannot be quitted without con- sidering the influence of two very universally used materials — alcohol and tea. Alcohol. — The influence of alcohol may be considered under two heads. \st, Before absorption. Moderate doses of alcohol taken along with food do not appear to interfere with the digestion and absorption of proteids, of carbohydrates or of fats. When taken apart from food and in such excessive doses as to act as an irritant and to cause catarrh of the alimentary canal, alcohol of course has a prejudicial influence. Indi, After absorption. The tissues have the power of oxidising a small quantity of alcohol, and since the com- bustion of a grm. of alcohol liberates 7 "06 Calories of energy, alcohol must be regarded as a food just in the same way as sugar is a food. The amount of alcohol which can be thus oxidised varies in different individuals, but the average man does not use more than 50 grms. per diem. " Arty alcohol taken above the amount which can be oxidised is excreted in the urine and breath, and in its passage through the body it acts as a poison to the protoplasm, first diminishing its activity and so diminishing the rate of; waste, and secondly causing the death of the protoplasm and the removal of the broken down nitrogenous constitu- ents in the urine, as urea, &c. Its poisonous action on the liver is demonstrated by the diminished building up of urea, so that a greater quantity of waste nitrogen is excreted in other forms. Its first action in checking the activity of meta- bolism, along with its influence in producing dilatation of the cutaneous vessels is taken advantage of in its use in the treatment of high temperature in some fevers. Tea, Coffee and Cocoa have as their essential constituents the substances theine and caffeine, which are methyl deriva- tives of xanthin, a diureide which is excreted in the urine. SECTION X. EXCRETION OF MATTEE FEOM THE BODY. 1. EXCRETION BY THE LUNGS (see Respiration). 2. EXCRETION BY THE KIDNEYS. A. Urine. In the urine secreted by the kidneys the water and waste nitrogen of the body are chiefly eliminated. The tests for the various constituents of the urine must be studied practically. The characters of the urine depend largely on the relative proportion of water and of solids which are excreted in it : at one time it may be very concentrated, while at another time it may be very dilute indeed. For this reason its specific gravity, which depends upon the percentage of solids in solution, varies within wide limits, being often as high as 1080 and frequently as low as 1005, but the average specific gravity is about 1020. It is possible from the specific gravity to form a rough idea of the amount of solids present, for by multiplying the last two figures by 2-22 the amount of solids per 1000 parts is given. Since the percentage of pigments in the urine varies like that of the other constituents, the colour of the urine shows wide divergencies in the normal condition. A concentrated urine has a dark amber colour, while a dilute urine may be almost colourless. Under average conditions the urine has a straw yellow colour. The reaction of urine is normally acid in man, chiefly from the presence of acid sodium phosphate, NaHgPOi, and the degree of acidity varies with the concentration. But the 346 . EXCRETION. 347 acidity of the urine may also be varied by different conditions. It is increased when there is an increased oxidation of proteids, for the sulphuric acid and phosphoric acid thus formed have to be neutralised. It is decreased by taking alkalis, or by taking such vegetable salts as the citrate, malate or tartrate of soda, because these are oxidised in the body and excreted as the carbonate. The NaIl2P04 may then be changed to NagHPO^. Urine is normally transparent ; but when it has stood for a few hours, a cloud of mucin is seen floating in it. When it becomes alkaline it is turbid from the separation as a white deposit of phosphates of lime and magnesia, as is seen in the urine of herbivora. When urine is concentrated and very acid a brick red deposit of urates tends to fall as it cools. The smell of urine is characteristic, and it may be modified by the ingestion of many different substances. II. Composition. — Since the relative amounts of water and solids vary within such wide limits, the percentage composition of urine is of little moment. , Under average conditions the water constitutes about 96 per cent., and the solids about 4 per cent. Of these solids, rather more than half are organic, rather less than half are inorganic. Since water and solids are derived from the water and soHds taken by the individual, the amounts excreted depend upon the amounts taken, and must be considered in connection with them. Thus if a man takes little fluid, he will pass little water in the urine. If he takes little food, a small quantity of solids will be excreted by the kidneys. Since excretion and inges- tion must be studied in relationship to one another, it is convenient to compare them during a definite period of time, and the natural division into days of twenty-four hours is generally adopted. Under ordinary conditions the amount of solid food taken per day does not vary very greatly, but the amount of fluids imbibed varies within much wider limits. For this reason, while the amoimt of water excreted in the urine per diem varies enormously, the amount of sohds is more fixed. In a man on an average diet, it may be stated that some- 348 PHYSIOLOGY. ^\ ,H ' ' II JH >N C NC C C— 0— H S I s H \ G C^ — _c_0— H H— N— H In some individuals and in certain conditions of the meta- bolism not yet fully understood, the amount of cystin is increased, and it then tends to crystallise out in peculiar hexagonal plates. D. Phosphorus-containing Bodies. — The phosphorus in the urine is derived partly from phosphates taken in the food, and partly from the nuclei ns of the food and tissues. Normally the phosphorus is fully oxidised to P2O5, which is linked to alkalis and earths and excreted in the urine. The most important phosphate is the phosphate of soda, NaHgPO^, which is the chief factor in causing the acidity of the urine. About one quarter of the phosphoric acid is linked to lime and magnesia, and it is these earthy phosphates which pre- cipitate when the urine becomes alkaline. When the urine becomes ammoniacal, triple phosphate is formed. It it probable that a small quantity of the phosphorus is excreted in organic compounds, but so far these have not been fully investigated. E. Chlorine-containing Bodies. — Chloride of sodium is 2 A 354 PHYSIOLOGY. the chief salt of the urine. It is entirely derived from the salt taken in the food, and its amount varies with the amount ingested. From 10 to 15 grms. are usually excreted per diem in a person on normal diet. In starvation, and still more in fever, the tissues of the body have an extraordinary power of holding on to the chlorine, and the chlorides may almost disappear from the urine. F. Carbon Dioxide.— Small amounts are present in urine, and after the administration of citrates, malates or tartarates, the amount may be considerably increased, and the urine may then effervesce strongly when an acid is added. G. Bases of the Urine. — Sodium, potassium, calcium, and magnesium occur in the urine in amounts varying with the amounts taken in the food. Generally speaking sodium is far in excess of the others, but in starvation it may fall below the potassium. Calcium and magnesium are present in nuich smaller quantities. B. Secretion of Urine. Structure of the Kidney. {This must be studied prac- tically.) — The kidney is a compound tubular gland, consist- ing of innumerable tubules, each made up of — (a.) A closed extremity or Malpighian body, consisting of an expansion at the end of the tubule — Bowman's capsule — into which a tuft of capillary vessels — the glomerulus — projects. Extending away from this is — (b.) A convoluted tubule lined by high pyramidal and granular epithelial cells. After a tortuous course, and some changes in the character of the epithelium, this opens into — (c.) A collecting tubule lined by a low transparent epi- thelium, which conducts the urine to the pelvis of the kidney (Fig. 110). The renal artery breaks up and gives off a series of straight branches — the inter-lobular arteries — 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. EXCRETION. 355 I. Secretion in the Malpighian Bodies.— A consideration of the structure of the Malpighian bodies tends to the conclu- sion that in them the water of the blood must filter out into Bowman's capsule, under the influence of the intravascular pressure. The way in which the efferent vessel breaks up into the capillary plexus between the tubules must necessarily help to maintain a high pressure in the capillary loops of the Fig. 110. — Di»gr,am of the Structure of the Kidney. — M.P., Malpighian Pyramid of the Medulla; M.U., Medullary Ray extending into Cortex ; L., Labyrinth of Cortex ; M.B., a Malpighian Body con- sisting of the Glomerular Tuft and Bowman's Capsule ; P.C.T., a Proximal Convoluted Tubule ; H.L., Henle's Loop on the Tubule ; D.C.T., Distal Convoluted Tubule ; G.T., Collecting Tubule ; R.A., Branch of Renal Artery, giving oS IL.A., Inter- lobular Artery to supply the Glomeruli and the Convoluted Tubules ; IL. V., Interlobular Artery bringing Blood back from the Cortex. glomerular tuft. That the Malpighian bodies act in this way is demonstrated by the influence of (a) the blood pressure in the kidneys, and (b) the pressure in the ureters on the flow of urine. (a.) Blood pressure in the vessels of kidney. The pressure of blood in the glomerular tufts may be increased in two ways — 356 PHYSIOLOGY. (a) by dilating the arteries, and (6) by constricting or occluding the veins. The former leads to an increased pressure on the arterial side of the capillary loops and to a more rapid flow of blood, the latter to a rise in pressure on the venous side and a slower flow of blood. In both cases the ; amount of blood in the kidney is increased, and the organ expands, but while the former condition is asso- ciated with an increased flow of urine, the latter is accompanied by a decreased or arrested flow. A high pressure in the glomeruli, with a rapid flow of hlood, increases the flow of urine. This may be produced (a) by cutting the renal nerves, which maintain a tonic con- stricting action on the arterioles, (b) by raising the general arterial pressure, and thus forcing more blood into the kidneys. The converse condition may be produced by stimulating the renal nerves, or by causing a decrease of the general arterial pressure. A fall of the carotid pressure to about 50 mm. Hg. in the dog is generally sufficient to arrest the flow of urine. The influence of such a fall of pressure is often seen in heart disease, where, as a result of this, the excretion of water by the Malpighian bodies is impeded, and dropsy supervenes. (6.) If the ureter be ligatured the formation of urine stops when the pressure rises to about 50 mm, Hg., indicating that the formation of urine in the Malpighian bodies is due to filtration under pressure. The urine formed in these bodies is alkaline, and this further supports the conclusion that it is simply an exuda- tion from the blood. That various substances in solution pass out with the water is shown by the fact that when haemoglobin is injected into the blood vessels it soon makes its appearance in Bow- man's capsules. It may also be demonstrated by taking advantage of the fact that in the frog the Malpighian bodies are chiefly if not entirely supplied from the renal artery, and the tubules from a portal ■ vein. ■ When sugar or commercial peptone are injected into the circulation, they appear in the urine, but EXCRETION. 357 when the renal arteries are ligatured, they do not appear, showing that they are passed out by the glomeruli. II. Secretion in Tubules. — The alkaline urine formed in the Malpighian bodies undergoes changes as it passes along the tubules. It becomes acid and the . various solids must be increased, for urine contains a higher proportion of these than does the blood. The blood contains only about 003 per cent, of urea, but the urine usually contains 2 per cent. It has been suggested that, this is due to absorption of water from the tubules, by which the urine formed in the Malpig- hian bodies becomes more concentrated. But there is no evidence that this takes place. I On the other hand the fol- lowing facts seem to show that solids are added to the urine by a process of secretion from the cells of the tubules. ' (a.) Uric acid crystals are frequently found in the cells of the convoluted tubules of the kidney of birds. ' (6.) 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 is stopped by cutting the spinal cord in the neck so as to lower the blood pressure, then the blue pig- ment is found in the cells of the convoluted tubules and of the ascending limb of Henle's tubule, (c.) When the Mal- pighian bodies of the frog have, been thrown out of apti6]i by ligaturing the renal arteries, the injection of urea causes a flow of urine and the excretion of that substance. • > This last experiment seems to show that the cells of the convoluted tubules ' are capable of secreting water, and that they can do so is further demonstrated by the fact that, if the formation of urine in the Malpighian bodies of a dog is stopped by cutting the spinal cord in the neck, the adminis- tration of caffeine and some other substances causes an increased flow of urine, although the blood pressure in the kidney 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 kidney 358 PHYSIOLOGY. may be stimulated to get rid of water by means of such diuretics as caffeine. C. Excretion of Urine. 1. Passage from Kidney to Bladder. — ^The pressure under which the urine is secreted is suiScient to drive it along the ureters to the bladder. If these are constricted the pressure behind the constriction rises, and may distend the ureter and pelvis of the kidney, but when, in the dog, it reaches about 50 mm. Hg. 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. 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 EXCRETION, 359 by the various muscles which can press upon the contents of the abdomen. 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. 3. EXCRETION BY THE SKIN. The skin is really a group of organs, and some of these have been already studied. {The structure of the skin tnust he studied pra,GiicaUy.) (1.) The Protective functions of the horny layer of epi- dermis and of the layer of subcutaneous fat are manifest. In man the hair has largely ceased to have the important protective function it fulfils in many of the lower animals, but the muscular mechanism by which the position of the hairs can be modified still persists. Attached to each hair follicle is a band of non-striped muscle, the arrector pili, which by contracting can erect the hair. These muscles are under the control of the central nervous system, and the nerve fibres take much the same course as the vaso-constrictor fibres of somatic nerves (see p. 321). (2.) The Sensory functions have been studied under the Special Senses, and (3.) the slight Respiratory action has been referred to under Respiration. (4.) It now remains to consider the Excretory Function 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. — The simple tubular sweat glands are exceedingly numerous. It has been calculated that a man possesses about two and a-half million, and that if spread out they would present a surface of over 1000 square metres. From these glands a considerable amount of sweat is '36o PHYSIOLOGY. 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 ccm. 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 under the influence of atropine it may occur without any sweat secretion. Nervous Mechanism.— 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 ^11 pass through the sympathetic ganglia, and back into the spinal nerves. Those to the leg and foot come from the upper lumbar region ; those for the hand and arm from the fifth to the' eighth cervical nerve, and those for the head chiefly from the medulla. The centres presiding over these nerves have not been definitely located. But they are capable of reflex stimula- tion, as when pepper is taken' into the mouth ; and of direct stimulation by a venous condition of the blood, as in the impaired oxygenation of the blood which so frequently pre- cedes death as the respirations fail. But. even after the nerves to the sweat glands are cut, the glands can be stimulated by certain drugs — e.g., pilocarpin. 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. Sweat is a clear, watery fluid, which, when pure, has. a neutral or faintly alkaline reaction, but which is generally EXCRETION. 361 acid from admixture with the sebaceous secretion. Its specific gravity is low, about 1004, and it contains less than 2 per cent, of solids, of which the chief is NaCl. Of the organic solids, urea is one of the most important. About 4 or 5 per cent, of the total nitrogen excreted from the body is thrown off by the skin in this form, and, when the action of.thg kidneys is interfered with, very considerable quantities may be eliminated. The sweat nearly always contains epi- thelial squames and oil globules derived from the sebum. B. Sebaceous Secretion. — -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 composed of proto- plasm and actively divide, 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 latter are the lanolins, which differ from ordinary fats in being partly soluble in water. Cholesterin is also present in the sebum. 0. Milk Secretion. — 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 completely filling the lumen. 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, which is known as the cholostrum. The cells left upon the basement membrane elaborate the con- stituent 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 glands and in the sinus below the nipple, and is expelled from these by the contraction of the muscular fibres in their 362 PHYSIOLOGY. walls, and by the suction of the young animal. The -etccre- tion 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. Milk. — The white colour of milk is due to its being a fine emulsion of fat globules floating in a clear plasma. Hence when the cream is removed, milk becomes less white and less opaque. Its specific gravity is about 1030, and in man and herbivorous animals its reaction is alkaline. Its composition in the human subject and in the cow is roughly as follows : — Human. Cow. Proteids, . .2-0 4-0 Fats, . . . .30 4-0 Sugar, . . o'O 4"0 Salts, . . . .0-2 0-7 Proteids. — The chief proteid is caseinogen, which exists as a calcic compound. It is of the nature of a phospho-proteid, but contains very little phosphorus and does not yield purin bases. It is not coagulated on boiling, but it splits under the action of acids and the casein is precipitated. Under the influence of rennet it splits into whey albumin and paracasein lime which is insoluble and separates out as a curd which generally holds the fat of the milk, and is. therefore white. From the curd a colourless whey contain- ing albumin and milk sugar exudes. Fats. — Olein is the chief fat, but fats of the lower fatty acids are also present. They exist in a fine state of sub- division, suspended in the milk plasma. Sugar. — The dissaccharid lactose is the sugar of milk (see p. 285). Under the action of various micro-organisms it is split up to form lactic acid, thus causing the souring of milk. Phosphorus Compounds. — In addition to caseinogen, milk EXCRETION. 363 contains other organic phosphorus compounds. Among these is lecithin and a compound which has been called phosphocamic acid, the constitution of which is not fully understood. In human milk the greater part of the phosphorus is in organic combinations, while in cow's milk the amount in inorganic compounds is much greater. The amount of iron in milk is very small, and therefore, when the child has used up the store of iron which it has in its body at birth, it is necessary to replace the milk-diet by foods containing more iron. part ffi. SECTION XI. Reproduction. So far the animal has been studied simplj 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 orgavs of reproduction must he 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 ovary to produce mature ova. When the testes have become active, the glands of the prostate increase and produce a fluid which with the sperma- tozoa form the semen. In the case of the female the maturation of the ova is accompanied by changes in the uterus. Every four weeks the superficial layers of the mucous mem- brane of the uterus break down, and are shed during the menstrual period. After the menstrual period, the mem- ' brane again regenerates. Impregnation is effected by the transmission of spermatozoa into the passages of the female. For this purpose erection of the penis is brought about reflexly through a centre in the 364 REPRODUCTION. 365 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 muscle by which the veins 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. The spermatozoon meets the ovum in the Fallopian tube, or upper part of the uterus. The ovum gets enclosed in the uterine mucous membrane, which grows round it as it regenerates after menstruation. It is unnecessary here to describe the changes in the ovum before or immediately after its conjugation with the sperma- tozoon, since they are so fully dealt with in all works oq biology. The mammalian ovum is holoblastic, and after the forma- tion of the morula, the cells get disposed in two sets, a layer of small surrounding cells, and a set of large central cells. These latter spread out at one pole to form the blastoderm, and dispose themselves in three layers, the epiblast, mesoblast, and hypoblast from which the various tissues are formed as shown below. Parts of the body derived from — I. Epiblast. — Nervous system (brain and spinal cord, peri- pheral nerves; sympathetic, epithelium of organs of special sense). Epidermis, its appendages (hair, nails), its glands (mammary, sebaceous, sudorific). (Muscular fibres of sweat glands ? ) Epithelium of mouth (except' of tongue, which is ento- dermal) and of salivary glands. Enamel of teeth. Epithelium of nose, of naso-pharynx, and all cavities and glands opening into them. [Epithelium of chorion, placenta, and amnion.] //. Mesoblast. — ^Urinary and generative organs (except epithelium of urinary bladder and urethra). '■ Muscles, voluntary and involuntary, throughout the body. Vascular and lymphatic system, with serous membranes and spleen. The skeleton and all connective tissue. 366 PHYSIOLOGY. III. Hypoblast. — Epithelium of alimentary canal from back of mouth to anus, and of all its glands ; also of Eustachian tube and tympanum. Epithelium of trachea and lungs. Epithelium of thyroid and thymus (Hassall's corpuscles). Epithelium of urinary bladder and urethra. [Notochord, allantois, and yolk-sac] By the formation of a vertical groove down the back of the blastoderm, a tube of epiblast (the neural canal) is enclosed, from which the nervous system develops. The mesoblast on each side of this splits, and the outer part, with the epiblast, goes to form the body wall (Somato- pleur), while the inner part with the hypoblast gets tucked in to produce the alimentary canal (Splanchnopleur). The developing embryo sinks into the ovum, and, as a, result of this, the somatopleur folds over it and, uniting above, encloses it in a sac — the amniotic sac, which becomes dis- tended with fluid — the amniotic fluid, in which the embryo floats during the later stages of its development, and which acts as a most effi,cient protection against external violence. Almost as soon as the ovum becomes 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. At the end of about a fort- night, the mesoblast of the embryo extends out in a number of finger-like processes into the trophoblast, and soon after- wards blood vessels shoot into these, and the chorionic villi are formed. 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 are dilating, and the capillaries form large sinuses or blood spaces. Into these the chorionic villi pass, and thus the loops of fcetal vessels hang free in the maternal blood, and an exchange of material is possible between the mother and foetus. The placenta thus formed acts as the fcetal lung, giving the embryo the necessary oxygen and getting rid of the waste carbon dioxide. It is the foetal alimentary canal REPRODUCTION. 367 supplying the necessary material for growth and development ; and it is the foetal kidney through which the waste nitro- genous constituents are thrown off. The performance of these functions by the placenta is associated with a course of circulation of the blood somewhat different to that in the postrnatal state. The blood coming from the placenta to the fcetus is collected into a single umbilical vein which passes to the liver, this it divides into the ductus venosus, passing straight through the organ, and into a series of capillaries among the cells. From these the blood flows away in the hepatic vein to the inferior vena cava which carries it to the right auricle. In this it is directed by a fold of endocardium, through the foramen ovale a hole in the septum between the auricles, and it thus passes to the left auricle, and thence to the left ventricle, which drives it into the aorta, and chiefly up to the head. From the head the blood returns to the superior vena cava, and, passing through the right auricle, enters the right ventricle, which drives it into the pulmonary artery. Before birth this artery opens into the aorta by the ductus arteriosus, while the branches to the lungs are still very small and unexpaoded. In the aorta, this impure blood from the head mixes with the purer blood from the left ven- tricle, and the mixture is sent to the lower part of the body through the descending aorta. From each iliac artery an umbilical artery passes off, and these two vessels carry the blood to the placenta. When the child is bom, the flow of blood between it and the mother is arrested, the umbilical cord being tied. 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 ductus venosus a clot forms and the vessel becomes obliterated. The ductus atreriosus 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 the embryo and in extra-uterine life is still very imperfect, and cannot be discussed here. 368 . PHYSIOLOGY. The child remains in the uterus for nine months, and at the end of that period it is expelled during labour. Labour may be 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 child is expelled through the vagina. After this 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-hvrik. 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. .^ INDEX. A- . PAGE PAGE Abdominal beservoib of blood, . 243 Basophil cells, . . 168 Accommodation, positive. . 101 Beans, .... . 290 Air, amount of respired, . . 253 Bidder's ganglion. . 208 Air passages, . . 246 Bile . 306 Allantoic arteries, . . 366 Bile, flow of, . . 309 Allantoin, . 350 Bile, mode of formation of, . 310 Alcohol, . . 345 Bile, nature of. . 311 Amnion . 365 Bile pigments, . . 308 Amoeboid movement. 15, 168 Bile salts, . . 307 Amylopsin, . 313 Biliary colic, . . 309 Antero-lateral tract, ascending . 138 Bilirubin 172, 308 Antero-lateral tract, descending '„ ■ 136 Biliverdin, . 308 Aortic valves, . . 190 Binocular vision, . Ill Apnoea, . . 263 Blind spot, . 104 Areolar tissue, . . 27 Blood . 163 Arginin, .... . 10 Blood constituents, fate of, . 177 Arteries, pressure in the, . . 198 Blood constituents, source of, . . 175 Arteries, blood flow in the. . 241 Blood, distribution of, . 177 Arteries, pulse in the, . 215 Blood flow . 241 Arterioles, normal state of, . 229 Blood gases, .... . 173 Asphyxia, . 274 Blood platelets, 169 Astigmatism, . . 104 Blood pressure, . 214 Auditory centre, . 158 Blood pressure, mean. . 226 Auditory nerve, cochlear root. . 121 Blood pressure, respiratory variations, 225 Auditory nerve, vestibular root . 122 Blood reservoir in abdomen. . 243 Auricles, pressure in the, . 198 Blood, total amount of. . 177 A uriculo- ventricular valves, . 198 Bone, . 32 Availability of food stuffs, . 344 Bone corpuscles. . 33 Axon . 70 Bone development, intra-cartUaginous, 34 Bone marrow, .... . 176 B. Bread, .... . 289 Bacterial action of the alim ENTAEY Breath sounds . 255 CANAL . 315 Bronchial sound. . 255 Basilar membrane, . . 119 Bulbous plants, . 290 Basis bundles . 136 Butter . 287 370 INDEX. 0. PAGE PAGE Canaliouli . 33 Coffee . 345 Capillaries, pressure in, . , 236 Cold-blooded animals, . 326 Capillaries, blood flow in, . 241 Collagen; . 26 Carbohydrates, .... 284, 322 Colour blindness. 108, 110 Carbon-monoxide haemoglobin, . 171 Colour sensation, . 108 Cardiac contraction, nature of, 206 Common sensibility, . . 85 Cardiac cycle, .... 191 Complemental air, . . 254 Cardiac impulse. . 194 Complemental colours. . 110 Cardiac nerves, functions of the. 210 Connective tissues, . . 25 Cardio-pneumatic movement, . . 266 Consciousness, 84, 152 Cardiograph, .... 195 Consonants, . 279 Carrots, , ... 290 Cooking . 291 Cartilage, 31 Corpora quadrigemina, . 147 Caudate nucleus, 148 Corresponding areas of the two retinae, 112 Cell, 13 Cortex cerebri, .... . 149 Cell of the neuron, function of, . 72 Corti's organ, .... . 120 Cellulose, 289 Cranial nerves . 140 Centre of ossification. 34 Cream, . ... 287 Centrosomes 13 Creatiu . 40 Cereals, 289 Creatinin, .... . 350 Cerebellar reflex arc. 82 Crura cerebri, .... . 147 Cerebellar tract direct, . .13 6, 138 Crusta, . 147 Cerebellum, .... 143 Curare . 43 Cerebellum, functions of, . 144 Cutaneous nerves on respiration. . 261 Cerebral action, time of, . 154 Cerebral mechanism, fatigue of. 154 D. Cerebral reflex arc, . 81 Death 8 Cerebrum, 148 Defaeoation, .... . 319 Cerebrum, discharging mechanism. 160 Deiters' nucleus, 187, 139 Cerebrum, functions of the, 150 Dendrites, .... . 70 Cerebrum, reception of stimuli, 156 Dentate nucleus. . 143 Cheese, 287 Descending antero-lateral tract. 136 Chemiotaxis 16 Diaphragm . 250 Cholalio acid 307 Diastole, 192 Cholesterin, . . , . 7 2,308 Dicrotic pulse wave. . 219 Cholostrum, .... 361 Diet, carbohydrate, , . 340 Chorionic villi, .... 366 Diet, fat, . 330 Chyme, . . . . 306 Diet, proteid . 339 Cilia, 15 Dietetics, .... . 340 Cilio-spinal region, . 102 Digestion in the mouth, . . 296 Circulation 182 Digestion in the stomach. . 300 Circulation in the blood vessels. 213 Digestion in the intestine. 306 Circulation in foetus. 367 Digestive secretions, fate of the. . 316 Coagulation, .... 164 Dioptric mechanism. . 96 Cochlea, membranous. 120 Dioxy-benzene, . 352 Cochlea, osseous, 119 Diureides, .... . 348 Cocoa, 345 Dreams, . 155 INDEX. 371 PAGE PAGE Dyspnoea, . 263 Fats, 29, 167 Fats, absorption, .... 322 E. Fats of food, .... 283 Fats, metabolism of. 333 Ear, internal, . 119 Fenestra ovalis. 117 Ear, middle, . 117 Fenestra rotunda, 117 Education, basis of, . 84, 152 Fibrin, . . ... 164 Eggs, . 288 Fibrinogen, . . . 165. 166 Electrotonus, . . 45 Fibroblasts, .... 26 Emmetropic eye, . 100 Fibrous tissue, .... 26 Endocardium, . . 187 Field of vision, . 106. Ill Endothelium, . . 28 Fillet 138 Entero-hsemal circulation. . 316 Flesh, 287 Enterokinase, . . 313 Flocculus, .... 143 Enzymes, 6 Flow of blood, .... 238 Eosinophil cells. . 168 Flow of blood through the heart. 200 Epiblast, . 365 Food, 280 Epithelium, . 20 Food, absorption of , . 320 Epithelium, ciliated. . 25 Foods,' availability, . 344 Epithelium, columnar, 21 Fruits, ... 291 Epithelium, excreting. 24 Fungi 291 Epithelium, glandular. . 22 Epithelium, mucin secreting. 23 G. Epithelium, stratified squamouE . 20 Epithelium, zymin secreting, . 24 GrALL STONES, .... 308 Equilibrium, . 145 Galvanotaxis, . 16 Erythroblasts, . . 176 Ganglia collateral, . 128 Erythrocytes, . 169, 176 Ganglia lateral. 128 Eustachian tube, 117, 119 Gastric juice, .... 301 Expiration, . 252 Gastric juice, antiseptic action of. 303 External ear, . . 117 Gastric juice, source of the con External intercostal muscles, . 251 stituents of, . 304 External respiration. . 248 Gelatin 27 Extra-cardiac factors maink lining Gemmules, .... 70 circulation, . 242 Gills 245 Eye, structure. . 96 Glance vision, . 113 Eyeball, muscles. . 112 Gland 22 Eyeballs, movements of, . . 112 Glossopharyngeal nerve, . 140 Eyes, connection of the, witl 1 the Glossopharyngeal on respiration. 261 central nervous system. . 114 Glucose, 167 Glycocholic acid. 307 Glycogenic function of liver, 328 F. Gmelin's test, .... 308 Facial nerve. . 141 Golgi's organs, .... 87 Faeces, .... . 317 Granules in protoplasm, . 13 Far point of vision, 101 Great veins, pressure in the, 196 Fasting, metabolism during. . 337 Green vegetables, 290 Fat cells, 28 Growth and reproduction. 5 372 INDEX. H^MATIN, HaBiuatoidin, Haematoporphyrin, Hsemin, . . . Heemoglobin, Haemoglobin, decomposition of, Haemolymph glands, Haemolysis, Haversian canal, Haversian spaces. Haversian system. Hearing, . Hearing, Physiology of, . Heart, .... Heart, attachments of the, Heart, changes in position of Heart, changes in shape of, Heart, connections with the central nervous system, . Heart on respiration, Heart, relations of the, Heart sounds, . Heart valves, . Heart, work of. Heat, elimination of, Heat, production of, . Hexone bases, . Hippuric acid, . Histidin, . Human muscles, stimulation galvanic current Hunger, . Hypermetropia, Hyperpnoea, Hypoblast, Hypoglossns, Hypopnoea, Hypoxanthin, . Impregnation, Incus, .... Indican, .... Indol Indoxyl-sulphate of potassium, Inosite, .... inspiration. by PAGE 172 173 172 173 170 172 176 178 35 35 35 117 122 184 190 194 193 209 266 190 202 188 205 324 323 10 350 10 47 85 103 263 366 140 263 350 364 117 351 315 351 40 248 Internal secretions of ovaries and testes, 335 PAGE Internal secretion of pancreas, . . 335 Internal secretion of pituitary body, 336 Internal secretion of suprarenal bodies, 336 Internal secretion of thyroid gland, . 334 . Intestinal digestion, .... 306 Intestine 294 Intestine, movements of, . . . 318 Intra-cardiac inhibitory mechanism, 208 Intra-cardiac pressure, . . 196 Islets of Langerhans, . . . 295 153, 161 . 309 21 354 357 346 134 J. Jaoksonian bpilbpst, Jaundice, .... Joint-sense, .... E. Keratin, Kidney, structure of, Kidney, tubules of, secretion in. Kidneys, excretion by, Knee-jerk L. Labour, 367 Labyrinth of ear, . . . .119 Lacunae, .... .33 Language, .... 162 Lanolins, ...... 361 Larynx, ...... 275 Latent period of muscular contrac- tion, 49 Lecithin, . .... 72 Legumens, ..... 290 Lenticular nucleus 148 Lentils, ... . . 290 Leucin, 10 Leucocytes, . . . 167, 175 Liver, 178 Liver, glycogenic function, . . 328 Liver, relation to fats, . . . 330 Liver, relation to proteids, . . 330 Liver, relationship of, to absorbed food and to the general meta- bolism, 328 Liver, summary of the function of, . 332 Localising the source of illumination, 107 Lockhart Clarke's cells, . . . 133 Lung, .... . 245 INDEX. 373 P.1GE Lymph, .... . 179 Lymph, formation of, . 180 Lymph glands, . . 179 Lymph tissue, . 28, 175 Lymphatic pressure, . . 238 Lymphocytes, . . 168 Lysin, . . 10 M. Malleus . 117 Malpighian bodies, secretion in . 355 Mariotte's experiment, . 104 Medulla oblongata, . . 137 Membrana tympani, . . 117 Membranous labyrinth, . . 120 Menstruation, . . 364 Metabolic equilibrium, . 338 Metabolism, general. . 333 Metabolism, proteid, . 333 Methaemoglobin, 171 Mesoblast, . 365 Micturition, . 358 Middle ear, . 117 Milk, .... 286, 362 Milk secretion, . . 361 Mitosis, . . 17 Mitral valve, . . 188 Modified perceptions. . 116 Monocular vision. . 96 Motor points, . 44 Mouth . 292 Mucin, .... . 23 Mucoid tissue, . 25 Murmurs, . 204 Muscle, .... . 36 Muscle and joint sense, . . 87 Muscle, cardiac, . 62 Muscle, changes in contraction. . 47 Muscle, chemistry of. . 39 Muscle, co-operative antagonist n, . 55 Muscle, colour of. 41 Muscle, contraction of, . . 49 Muscle corpuscles, . 38 Muscle, death of, . 68 Muscle, elasticity, . . 41 Muscle, electrical conditions, . 42 Muscle, energy evolved by, . 62 Muscle, fatigue. . 61 Muscle, galvanic stimulation, . Muscle, heat production, Muscle modes of making contract. Muscle, physical and chemical charac- ters of. Muscle respiration. Muscle spindles, Muscle stimulated with induced elec tricity, .... Muscle, structure of, Muscle, voluntary contraction of. Muscle, wave of contraction, . Muscle work Muscles, mode of action of. Muscles, skeletal. Muscles, visceral, Muscular exercise on circulation. Muscular strain on circulation, Muscular work upon the excreta. Myelocytes, Myocardium, Myoglobulin, Myoids, Myopia, . Myosin, . Myosinogen, Myostromin N. Neae point of vision, Nerve, .... Nerve corpuscles, Nerve fibres, . Nerve fibres, classification of, Nerve fibres, grey, . Nerve fibres, post-ganglionic, Nerve fibres, pre-ganglionic. Nerve fibres, somatic. Nerve fibres, splanchnic, . Nerve fibres, white, . Nerve impulse, nature of, . Nerves, cranial. Nerves, afferent, or ingoing. Nerves, augmentor, . Nerves, efferent. Nerves, excito-reflex, Nerves, ingoing. Nerves, inhibitory, . 374 INDEX. PAGE FAOE Nerves, mixed, ... 78 Paramyosinogen, . 39 Nerves of abdomen, . 129 Peas, . 290 Nerves of arm, . . . . 130 Peptones, . . 11 Nerves of pelvis. 130 Perfusion method, . 229 Nerves of leg, 130 Perilymph, . 120 Nerves of thorax, 129 Perimeter, . 106 Nerves, roots of the spinal, 131 Phagocyte action. . 169 Nerves, sensory, . . . . 77 Pharynx, . . 293 Nerves, vaso-conslrictor, . 231 Phenol, . 316, 351 Nerves, vaso-dilator, . 232 Phosphocarnic acid. . 363 Nervi erigentes, . . 130, 36 5,368 Phototaxis, 16 Nervous system, central, . 131 Pialyn, . . 313 Nervus abducens. 141 Pigment cells, . . 30 Nervus oculomotorius. 141 Placenta, . . 366 Nervus troohlearis, . 141 Plasma, . 166 Neurilemma 71 Plasmodia, 4 Neuro-muscular mechanism, 80 Plethysmograph, . 229 Neuron, . 69 Pleural cavity, . . 246 Neuron conduction, . . 75 Polymorpho-nuclear leucocytes. . 168 Neuron stimulation, . 74 Pons Varolii, . 137, 142 Neurons, activity of, . 74 Positive accommodation, . . 101 Neurons, inter-relationship of, . . 73 Posterior longitudinal fasioulus. . 139 Nissl's granules. . 70 Potatoes . 290 Nncleins, . 12 Poteriodendron, . 69 Nucleoli, . 16 Predicrotic pulse wave, . . 220 Nucleus, . . 16 Presbyopia . 103 Nucleus cuneatus, . 137 Pressure in the arteries, . . 226 Nucleus gracilis. . 137 Pressure in the capillaries. 236 Nuts, 291 Pressure in the veins. . 237 0. Protagon, .... . 72 Oat meal . 289 Protamines, . 10 Ocular muscles, nervous mechanisn , 113 Proteids, .... 9 CEsophagus . 293 Proteids, absorption of, . . 321 Olivary nucleus, . 137 Proteids, classification of the, . 11 Olive 137 Proteids, conjugated. . 12 Optic chiasma, . . 114 Proteids of food, 282 Optic tract, .... . 115 Proteids, original, . . 11 Organ of Corti, . 120 Proteoses, 11 Organic acids, salts of. . 281 Protoplasm, 4 Organs of Golgi, . 87 Protoplasm, chemistry, . 8 Ova . 364 Proximate principles, . 281 Oxalic acid . 352 Proximate principles of the food. Oxyhsemoglobin, . 171 sources of , . . 286 P. Ptomains, . 316 Pain, . 86 Pulmonary valves, . . 190 Pancreas, ..... . 295 Pulse, arterial, . . 215 Pancreatic secretion, 311 Pulse, capillary, . 224 INDEX. 375 PAOE PAGE Pulse, venous, . . 224 Semi-circular canals. 119, 145 Pupil, dilatation. . 102 Semilunar valves, 189, 200 Pupil, contraction, . . 101 Senses, . , . . . . 85 Pupil, sphincter centre, . . 101 Serum 164, 106 Purin bodies, . . 348 Serum albumin. 166 Purkinje's cells, . 143 Serum globulin. . 166 Purkinje's images, . . 106 Singing voice, . . . . . 278 Pyramidal tract, crossed, . . 136 Skatol . 316 Pyramidal tract, direct, . . 136 Skatoxyl-sulphate, . . 351 Pyrooatechin, . . 352 Skin, excretion of . 359 Sleep, . 155 R. Sleep-walking . 155 Reflex action, 79, 80 Smell . 124 Refraction of light, . . 99 Somatopleur, . . 366 Remak's ganglion, . . 208 Special senses, . . . . . 88 Reproduction, . . 364 Speech, articulate, . . 278 Reserve air. . 234 Spermatozoa, , . 364 Residual air, . . 254 Spherical aberration, . 101 Respiration, external. . 245 Sphygmograph, . 218 Respiration, internal. . 271 Spinal cord, . 132 Respiration, nervous mech anism of, . 257 Spinal cord, conducting paths. . 135 Respiration on circulation . 264 Spinal cord, functions. . 133 Respiration on the blood, . 268 Spinal cord, time of reflex actic m, . 134 Respiration, rhythm of, . 256 Spinal nerves, . . 126 Respiration, upon the air breathed, . 267 Spinal reflex arc, . 81 Respiratory interchange, c xtentof, . 273 Splanchnic nerves on respiratio n, . 261 Respiratory quotient. . 267, 274 Splanchnopleur, . 366 Restiform bodies, . 137 Spleen Spoken language, . 178 Retina, stimulation of. . 104 . 278 Rice, . 289 Stannius' experiment. . 208 Rigor mortis, . 68 Stapedius muscle, . 117 Rolandic convolutions. . 162 Stapes, .... Stereoscopic vision, . . 117 Roof nucleus of oerebellur n, . . 143 . 112 s. Stimuli, .... 8 Saccdlb, . . 120 Stomach . 293 Saliva, . 296 Stomach, absorption from the. . 306 Salivary glands. . 292 Stomach, digestion in the, . 300 Salivary secretion, . . 297 Stomach, movements of the. . 304 Salts of the bile acids. . 307 Storage of surplus food, . . 327 Sanson's images. . 101 Sucous entericus. . 314 Sarcolactic acid, . 40 Sulphates, ethereal of urine. . 353 Sarcolemma, . 38 Sulphates, preformed of urine, . 358 Sarcous substance, . . 38 Sulphur, neutral of urine. . 35S Scala media. . 120 Swallowing, . 29£ Scheiner's experiment, . 100 Sweat secretion, . 35£ Sebaceous secretion. . 361 Synapses, .... 6£ Secretin, . . 314 Systole . 19; 376 INDEX. T. PAOE PAGE TaOTILB CORPDSOLEa, 88 Urine, phospdate of soda of, . . 353 Tactile sense 88 Urine, phosphorus containing bodies Taste, 123 of, . . . . 353 Taste and smell, centres for, 159 Urine, pigments of, . . 352 Taste bulbs 123 Urine, purin bodies of. . 348 Taste sensations, 124 Urine, secretion of, . . 354 Taurocliolio acid, 307 Urine, sulphur containing bodies of, 352 Tea, 345 Utricle, . . 120 Tegmentum of cms, . 147 V. Temperature, .... 325 Vacuoles, . 15 Temperature on respiration. 263 Vagus nerve, . . 130, 140 Temperature sense, . 91 Vagus on respiration. . 259 28 Valves of the heart, . . 188, 189, 198 Tetanus 53 Vaso-raotor mechanism, . 230 Thalamus opticus, . 148 Vegetable food stuffs. . 288 Thirst, 86 Vegetable salts, ' . . 281 Thymus, 176 Veins, pressure in, . . 237 Tidal air, 254 Veins, pulse in. . 224 Tickling 86 Veins, flow in, . . 241 Tingling, 86 Venous blood on respiration, . . 261 Tissues, 20 Ventilation, . 274 Tissues, vegetative, . 20 Ventricle, fourth, . 137 36 Ventricles, pressure in the , . . 198 Tongue 293 Vesicular respiratory sound, . . 256 Touch 159 Vestibule, . 119 Tracts of cord 136 Vision, 92 Tricuspid valve. 1S8 Vision, binocular. . Ill Trigeminal nerve. 141 Visual centre, . . 115, 157 Trophoblast, .... 366 Vital capacity, . 254 Trypsin 312 Voice, . 275 Trypsinogen 313 Voluntary action, . 84 Turnips, 290 Voluntary motor centres. . 161 117 Vomiting, . 305 Tympanic cavity. . 117 Vowel sounds, . . . . 278 10 w. u. Warm blooded animals. . 326 Urea, 16 7,348 Wheaten flour, . . 289 Uric acid, .... 349 Whispered speech, . . 278 Urine 346 Writing, . . 162 Urine, ammonia salts of, . . 348 Urine, bases of, ... . 354 X. Urine, chloride of sodium of, . . 353 Xanthin, . 350 Urine composition, . . 347 Y. Urine, diureidea of, . 348 Yeast plant, . 4 Urine, excretion of, . . 358 Z. Urine, mucin of, . 352 Ztmins, . 6 LOEIMER AND CHALMERS, PRINTERS, EDINBURGH.