Cornell University Library The original of tliis book is in tine Cornell University Library. There are no known copyright restrictions in the United States on the use of the text. http://www.archive.org/details/cu31924000337182 OUTLINES OF PHYSIOLOGY JONES 7 Cervical YertcbwB Cluvicle. Scapula. 12 Dorsal Veitebrie, Ilumenu. 6 Lumbar Vertebree Ulna. Kadius. Pelvis, Bones of the Caipun. Bones of the Meta- carpus. FUalangesof Fiiiijen. Tibia. Fibula. Bones of the Taraui. Bones of the Meta- tarsus. Fhalunges of Toe*. THE SKELETON [AmtE HolouO. OUTLINES PHYSIOLOGY By EDWARD GROVES JONES, M.D., LECTURER ON PHYSICAL DIAGNOSIS IN THE ATLANTA COLLEGE OF PHYSICIANS AND SURGEONS, AND PROFESSOR OF PHYSIOLOGY IN THE DENTAL DEPARTMENT OF THE SAME. 107 ILLUSTRATIONS ■ ^.^1. '. > ■ ^fy ' I PHILADELPHIA : P. BLAKISTON'S SON & CO. IOI2 WALNUT STREET. I9OI Copyright, 1901, P. Blakiston's Son & Co. TO DOCTOR WILLIAM S. KENDRICK, Professor of Medicine in the Atlanta College of Physi- cians AND Surgeons, THESE PAGES ARE AFFECTIONATELY DEDICATED, PREFACE. This volume has been prepared with the view of presenting, in as convenient form as possible, the essential facts of modern physiology as related to the practice of medicine. In the exe- cution of this purpose brevity has been made a prime considera- tion ; therefore, such details as are of secondary importance are omitted, theories are avoided, and conclusions are recorded without argument. There is no short road to knowledge, and it would be unfortunate should such a book as this in any way discourage extended research ; but students in college have none too much time to devote to any one subject, and any simple col- lection of pertinent facts, however brief, can, if reliable, be used to great advantage. I have endeavored, however, to make the work sufficiently exhaustive to be self-explanatory, believing that otherwise economy of expression is practiced at the expense of the reader's interest. A maximum of space has been given to those subjects which seem of most practical importance. The chemistry of the body, the special senses and embryology have not been treated in great detail. It has been thought undesirable to omit a brief anatom- ical description of the separate organs discussed. In the preparation of this volume no claim to original inves- tigation is made. The writings of various authorities have been freely drawn upon. Especial acknowledgment is due to the following authors : Howell (American Text-Book), Halliburton (Kirkes' Handbook), Flint, Verworn and Stewart. I am under obligations to Dr. J. Clarence Johnson, whose lectures have been of great value to me, and to Dr. Frank K. Boland, who has written the whole of Chapter II., read the proof sheets, and rendered other valuable assistance in connection with the work. E. G. J. Atlanta, Ga., Sept. i, 1901. INTRODUCTION. Human physiology treats of the occurrences which take place in the body of man during life. Life is indefinable. In discussing it we can at best only recount some of its invariable and characteristic phenomena. In the normal living body there is ability on the part of all tissues to perform their proper functions — functions which are termed physiological. In disease there is impairment of this ability on the part of some or all the tissues. In death there is irremediable loss of function throughout the organism, though such loss does not extend to all the tissues at the same moment of time. It therefore appears that we shall be concerned with a consideration of some of the conditions which distinguish living from dead matter, and with a description of some of the functions of the body and of its different parts. The vital processes are alike in character in plants and ani- mals, but only animal life is now under discussion. Distinguishing Characteristics of Living Matter. — It will be here considered that these are : (i) Motion, (2) irritability, (3) nutrition, (4) growth, (5) reproduction. As the character- istics of the living organism are characteristics of the living cell, attention is directed to p. 29 et seq., where the properties of cells are discussed. Functions to be Performed in the Body. — In the body there is a division of labor, whereby one organ is predominantly con- cerned in the performance of one function and others in the performance of distinctly different ones — the sum total of their activity being life and its attendant phenomena. Assuming that XU INTRODUCTION. physiological activity has as its object the preservation and prop- agation of life and the accomplishment of certain acts peculiar to the animal, the functions of the body and its different parts maybe classified as, (I.) Nutritive, (II.) Animal and (III.) Reproductive. I. The Nutritive embrace a very large part of the physio- logical processes going on in the body. They are (i) secre- tion, (2) digestion, (3) absorption, (4) respiration, (5) circu- lation, (6) metabolism, (7) excretion, (8) production of heat. Their performance occurs involuntarily and unconsciously. They make the animal and reproductive functions possible. II. The Animal are such as motion, speech, intellection, etc. Their performance may be said to be voluntary, but dependent upon the nutritive functions. III. The Reproductive involve the processes necessary to per- petuate the species. While the above remarks are meant to apply to the body as a whole, the organism consistsonly of an aggregation of specialized cells, and it is to be noted that all the functions classed above as nutritive and reproductive, and at least a part of those classed as animal, are properties of all living cells. " Vital Force." — The phenomena exhibited by living matter may be due largely to changes in the chemical and physical na- ture of protoplasm, and the ignorance of the day regarding physiological changes may be ignorance of the chemistry and physics of protoplasm. We differentiate objects in the inorganic world on chemical and physical grounds ; it may be that one form of living matter differs from another in its function on the same grounds. We know, for example, speaking broadly, that digestion takes place in accordance with known chemical laws ; that circulation is governed by the laws of hydrodynamics ; that respiration obeys laws of aerodynamics ; that absorption is de- pendent vc^ovi. osmosis ; that the skeletal movements are in ac- cordance with the principles of mechanics. INTRODUCTION. Xlll But it will be seen from time to time that many circumstances attending these processes, and many other manifestations of vital activity, cannot be explained by any known physical or chemical laws. All secretions come from blood or lymph, and yet these secretions may contain substances entirely different from any found in those fluids ; the epithelium of one gland may take out of the blood certain materials, while the apparently identical etjithelium of another gland under apparently identical condi- tions will not do so ; absorption from the alimentary canal shows numerous variations from the laboratory laws of osmosis; while, investigation of the problems of heredity and consciousness can scarcely be said to have made a beginning. It will, therefore, be necessary to use frequently such terms as " vital force," " se- lective affinity," etc., but without any clear understanding of the processes they are supposed to describe. The contributions which recent years have made to the knowl- edge of physiology seem to be the result of investigations which look to. physics and chemistry for explanation of at least some of the ' ' vital ' ' phenomena of life. It is hardly to be supposed that the application of these methods has reached its limit, and the hope may be indulged that some of the mysteries that now beset the study of physiology may be explained by perfectly well- known laws. It has been a comparatively short time since it was thought impossible to synthesize any of the so-called organic bodies. The subject of general physiology is the subject of cell physi- ology. The life of the cell is inherited ; its nutrition and activity depend on a proper supply of food (blood), proper physical sur- roundings, and (usually) proper nerve connections. Any change, Whether constructive or destructive, in the cell is effected through one or more of these agencies. Since any organ is only a collec- tion of cells and their modifications, any change (variation in activity, nutrition, etc.) in that organ can be caused only by 4n influence brought to bear on (i) the cells themselves of the INTRODUCTION. organ, (2) the blood supply, or (3) the nerve supply ; and these three factors may be said to so react upon each other that a change in one affects both the others. These are the three factors of physiological and pathological activity. CONTENTS. Introduction CHAPTER I. The Chemical Composition of the Human Body CHAPTER H. The Cell and the Elementary Tissues The Cell . The Elementary Tissues Epithelial Tissue Connective Tissue Muscular Tissue Nervous Tissue . Physiological Characteristics of Striated Muscle CHAPTER III. Secretion Salivary Glands . Gastric Glands . Intestinal Glands Pancreas . Liver Sebaceous Glands Mammary Glands Thyroid Gland . Adrenal Glands Pituitary Body . Testis and Ovary CHAPTER Foods, Digestion and Absorption . Foods IV. 17 27 27 31 31 35 42 45 45 52 55 60 65 66 69 80 81 82 83 84 84 85 85 CONTENTS. Digestion 89 Prehension ...... 92 Mastication ....■• 92 Insalivation 93 Deglutition 95 Gastric Digestion ..... 98 Intestinal Digestion 109 Large Intestine ...... 117 Absorption 122 CHAPTER The Circulation . Mechanism of Circulation . The Heart . The Arterial Circulation The Capillary Circulation The Venous Circulation The Blood The Lymph V. 131 132 132 157 169 173 179 189 CHAPTER VI. Respiration Anatomy of the Respiratory Organs . Mechanism of Respiration 19s 197 203 CHAPTER VII. Excretion By the Kidneys By the Skin 235 235 251 CHAPTER VIII. Nutrition, Dietetics and Animal Heat Nutrition Dietetics ...... Animal Heat 257 257 267 370 CONTENTS. CHAPTER IX. The Nervous System 278 The Cerebro-Spinal System 304 The Spinal Cord 305 The Encephalon 319 The Medulla Oblongata 319 The Pons Varolii . 323 The Crura Cerebri, Corpora Striata, Optic Thalami, Internal Capsule and Corpora Quadrigemini 324 The Cerebrum 328 The Cerebellum . 342 The Cranial Nerves . 344 The Spinal Nerves 364 The Sympathetic System . 366 CHAPTER X. The Senses 373 Common Sensations . 373 Special Sensations 374 The Sense of Touch . 374 The Sense of Smell 375 The Sense of Sight 376 The Sense of Taste . 38s The Sense of Hearing 386 The Production of the Voice 393 CHAPTER XI. Reproduction 396 CHAPTER I. THE CHEMICAL COMPOSITION OF THE HU- MAN BODY. "The human body contains some fifteen of the chemical ele- ments. They are oxygen, carbon, hydrogan, nitrogen, cal- cium, phosphorus, sulphur, sodium, chlorine, iron, iodine, potassium, magnesium, silicon, flourine ; to these may be added accidental traces of lead, copper and aluminium. These elements do not appear free in the organism, but unite to form various chemical combinations which are te^rmtd proxi- mate principles. The only exceptions to this statement are fur- nished by the uncombined occurrence of oxygen, nitrogen and hydrogen in the alimentary canal and of oxygen and nitrogen in the blood. These gases in the alimentary canal can scarcely be designated as parts of the organism ; while in the blood their presence zs/ree gases is comparatively insignificant. Any chem- ical element composing a part of the body may, therefore, be looked upon as being in combination with one or more of the other elements peculiar to animal tissue. The list given represents the order in which these elements belong as regards their relative quantities in the system. Oxygen constitutes more than 70% of the body weight, and carbon, nitrogen, hy- drogen and oxygen make up the main bulk of the whole body. Organic and Inorganic Bodies. — Certain of these chemical combinations cannot be made in the laboratory. When it was originally thought that none of the more complex compounds, such as urea, sugars, fats, etc. , could be artificially produced, a classification of all the chemical compounds existing in bodies 2 17 l8 CHEMICAL COMPOSITION OF THE HUMAN BODY. which evidenced phenomena of life was made into inorganic and organic, the inorganic being of definite chemical composi- tion and possessed of no power to reproduce themselves. Since that classification was made the formulae for very many of the so-called organic compounds have been determined, and it is no longer of any scientific significance, though still adhered to. Organic chemistry at present may be defined as the chem- istry of the carbon compounds, and organic substances as the union of some compound of carbon with some other element or elements ; they all contain carbon and hydrogen, if carbon diox- ide be excepted. - At least some of these possess the power of reproduction under proper circumstances, and it is these which are directly and imperatively necessary to life — /. e., to the life of protoplasm. Many organic substances are also distinguished for the complex- ity and instability of their molecules. A very large number of atoms of one element and very large number of elements may enter into the molecular formula. Those containing a large number of elements always contain nitrogen. Now nitrogen is exceedingly indifferent in its affinities, and this fact, together with the large number of atoms present in the molecule, ac- counts for the molecular instability. It will be seen that the essential part of the cell is the pro- toplasm, and it is essentially the chemistry of protoplasm which is under consideration when the chemistry of animal life is be- ing discussed. Proximate Principles.— The whole body — all the cells of which it is composed — is made up of proximate principles, i. e., of various simple and complex substances which represent the union of two or more of the 15 chemical elements peculiar to the body. The elements combine under chemical laws to form proximate principles; the proximate principles unite to form cells ; the cells collectively constitute the body. All prox- imate principles are chemical compounds, but not all chemical PROXIMATE PRINCIPLES. I 9 compounds are proximate principles. Not even are all the chemical combinations which can be made from the 15 ele- ments mentioned proximate principles — simply because all the possible combinations do not exist in the body. A proximate principle is a chemical compound existing as a part of the body. About one hundred are found in the human organism. Obviously the first great subdivision of these substances is into inorganic and organic ; but they may be further classified as (I.) Binary, or Inorganic, (II.) Ternary or Organic Non-ni- trogenized and (III.) Quaternary, Organic Nitrogenized, Pro- teid, or Albuminoid. I. The Binary Proximate Principles are so called because they contain two elements, as H^O, NaCl, etc. In such sub- stances as HjSO^, etc. , the radical, under the definition, must be considered as one element. As a rule, binary principles pass through the system unaltered ; they leave it, by whatever route, without being changed into other substances. They may be regarded as already digested, and their composition as unaffected in the alimentary canal or elsewhere. This is a statement, however, which cannot be ap- plied literally to sodium chloride and a few other substances. Some sodium chloride is decomposed to form hydrochloric acid of the gastric juice, but this is not saying that it is discharged from the body as hydrochloric acid or that it may not be dis- charged as sodium chloride finally. Some other like exceptions also occur, but the number is insufficient to invalidate the gen- eral application of the rule. Some inorganic materials are also newly formed in the body. More water, e. g. , is discharged than is ingested and the same is true of sulphates, carbonates and phosphates. The explanation is that such substances result from the decomposition of organic materials. The binary proximate principles are furnished by the ordinary foods and drinks. It is not to be understood that they are 20 CHEMICAL COMPOSITION OF THE HUMAN BODY. always taken intd the system as so many simple free binary com- pounds ; they frequently form necessary parts of organic sub- stances and cannot be separated without destruction of the identity of those substances. In such cases they are deposited in the tissues with the organic substance and when that matter has served its purpose and is ready to be discharged as effete material they are discharged with it. Much inorganic material is necessary to life, though in ordinary meals a surplus is usually consumed. Water. — The most important of the binary proximate prin- ciples is water. It is present in every tissue of the body and constitutes more than two-thirds the body weight. Not only is it taken in with ordinary food and drink, but it is formed in the body in the process of organic disassimilation. Distilled water is colorless, odorless, and tasteless, but ordinary drinking water may be said to have an agreeable taste by reason of dis- solved salts and air. It is being, continually discharged from the body and its frequent ingestion is necessary to preserve a balance. It is the universal solvent, constitutes the bulk of all the fluids of the organism, and is the vehicle of all interchange, osmotic or otherwise, in the body. In order to get an idea of its function and importance it is sufficient to conceive of its withdrawal from the tissues. They lose their physical properties, becoming hard and brittle instead of soft and flexible. The ab- straction of five or six per cent, of the water from the body, as in cholera, causes a viscid condition of the circulating fluids and subsequent violent convulsions. By its evaporation from the surface it is of very great value in regulating the body tem- perature. Sodium Salts. — Sodium chloride can scarcely be said to be of inferior importance to water. It is ingested with the ordinary foods, but usually they do not contain enough of this salt to satisfy the demands of the system and it is added as a condi- ment to articles of diet. Its withdrawal from the blood is fol- BINARY PROXIMATE PRINCIPLES. 21 lowed by grave errors of nutrition and finally probably by death. Taken with the ordinary meals it adds flavor to the food and pro- motes the flow of the various secretions. It is found in all the fluids and tissues of the body except the enamel of the teeth. Its particular office is to facilitate and govern osmotic action. It will be seen that this office is accomplished mainly, so far as the salts are concerned, by considerations of density, the less dense fluid passing through animal membranes with greater facility than the more dense. Hence the reason for preferably giving magnesium sulphate in concentrated solution, since its action is dependent upon the establishment of an osmotic cur- rent away from the blood. Oysters planted at the mouth of fresh-water streams appear fatter because more fresh than salt water is taken up by them. This action of sodium chloride is especially noticeable in connection with the red corpuscles, which swell up on addition of pure water to the blood, but shrivel when the amount of sodium chloride is raised above the normal. Consequently it is found that the proportion of this salt in the plasma is fairly constant, so much so that a watery solution of sodium chloride of the same concentration as plasma — .65 per cent. — is known universally as the "normal salt so- lution" and is the proper strength to inject directly or indi- rectly into the circulation. Sodium chloride belongs especially to the fluids of the body. It is not deposited in large quantity in the tissues, but seems to regulate the appropriation of other matters. During gastric digestion the cells of the stomach form hydrochloric acid out of the sodium chloride in the circulating blood. At that time the discharge of this salt through the urine is diminished. It is a constituent of all the excreta. Sodium sulphate, phosphate, carbonate and bicarbonate are also present in some of the body tissues and fluids. The fluids like blood, lymph, etc. , owe their alkaline reaction chiefly to sodium carbonate., Since an acid coagulates protoplasm and since acidified media will not take up carbon dioxide from the tissues 2 2 CHEMICAL COMPOSITION OF THE HUMAN BODY. the importance of the alkaline reaction of the tissues and blood is apparent. Sodium carbonate is not introduced in the food but is formed by chemical decomposition in the body. Potassium Salts. — These may be said to belong to the solids. For instance, potassium salts are present in the red corpuscles to the almost complete exclusion of sodium salts, while in the plasma an opposite condition prevails. Acid potassium phosphate is responsible for the acid reaction of the muscles in rigor mortis. The potassium salts are analo- gous in their general uses to the sodium salts. Some of them are formed in the circulating blood. Calcium Salts. ^-Q,2Xz\\xn\ is the most abundant of the metal- lic elements in the body. It is always accompanied by mag- nesium. Calcium chloride, fluoride, sulphate, phosphate and carbonate are all found in various fluids and solids of the body, but chiefly in bone. The phosphate is abundant in ordinary food and forms the chief mineral constituent of osseous tissue. Withdrawal of it from the food occasions serious nutritive disturbances, particu- larly in the bones. It is responsible for the hard texture neces- sary to proper functional activity of this tissue. It is most abundant in the bones of the lower extremities which have to support the body, while the proportion in the ribs is much less. It is largely the absence of this salt which accounts for the mis- shapen bones of rachitic individuals. Iron. — This element is found essentially in hemoglobin of the blood. Its amount is small but none the less necessary. Simple anemia is due to a lack of iron in the red corpuscles and usually prompt relief follows its administration. The quantity existing in ordinary food is commonly sufficient for the needs of the organism. Peroxide and phosphate of iron are also found in various parts of the body, but hemoglobin contains the neces- sary part of this element. The iron is in the hematin part of the hemoglobin molecule. TERNARY PROXIMATE PRINCIPLES. 23 The above are only a few of the more important binary proxi- mate principles. The behavior of all is subject to the same general laws. They cannot be built up into organic material, and consequently cannot of themselves sustain life since proto- plasm is life, and protoplasm is organic. II. The Ternary Proximate Principles are so called because they contain three and (excepting phosphorized fat) only three chemical elements — carbon, hydrogen and oxygen. These sub- stances are of animal and vegetable origin and definite chemical composition. They are divided into (A) Carbohydrates includ- ing (i) starches and (2) sugars, and (B) Hydrocarbons includ- ing (1) neutral fats and (2) fatty acids. (A) Carbohydrates were originally defined as bodies contain- ing carbon, hydrogen and oxygen, in the molecules of which bodies the number of carbon atoms is 6 or some multiple thereof, while the hydrogen and oxygen exist in the proportion to form water. From a chemical standpoint this definition is not now strictly true, but may be accepted as practically correct for pres- ent uses. 1. Starch (CgHjjOj) exists in nearly all plants, and particu- larly in the seeds of cereals such as rye, wheat, barley, rice, corn, etc. , in various roots, as potatoes, and in many fruits. The starch granule consists of an envelope of cellulose and the con- tained substance, granulose. It is a prominent article of food. It is of no value as such but is changed into dextrose (CgH,jOg). 2. The sugars are of various kinds when taken into the ali- mentary canal, but are changed to dextrose before assimilation. Cane sugar (saccharose), milk sugar (lactose) and grape sugar (dextrose, glucose) are the most important taken as food. Be- sides milk sugar the animal kingdom furnishes liver sugar (glycogen) and muscle sugar (inosite). A number of other sugars may also enter the alimentary canal, as dextrin, levulose, galactose, etc. Some of these represent products in the hydro- lytic process which results in the final conversion of all sugars 24 CHEMICAL COMPOSITION OF THE HUMAN BODY. into dextrose. This is probably the only kind of sugar that can be found in the blood or other fluids in the body except lactose in the milk. Although sugar may be stored up in the liver and muscles as glycogen and inosite, dextrose is to be regarded as the sugar of the organism. It is to be noted that practically speaking this is the form assumed previous to absorption by all the carbohydrates, both starches and sugars. The carbohydrates are all crystallizable. (B) Hydrocarbons contain the same elements as carbohy- drates, but in more variable proportions. They are relatively poor in oxygen. They are contributed by both animal and vegetable kingdoms. They are found in well nigh all animal tissues and in nuts, seeds, grains, etc. 1. The neutral fats are olein, palmitin and stearin. All animal fat is a mixture of these three. Olein is liquid ; the others are solids. They represent the union of fatty acids with glycerine. The application of heat in the presence of alkalies will decompose them into glycerine and fatty acids, the latter not remaining as acids, but uniting with the alkali bases to form soaps. The neutral fats are soluble in chlo- roform, ether, and hot alcohol. 2. The fatty acids are oleic, palmitic and stearic, but since an acid as such cannot be allowed in the organism they exist as the oleates, palmitates and stearates of the various alkali bases. In contradistinction to the neutral fats they are always com- bined. The general function of all the ternary proximate principles is to furnish energy for the running of the body machine. They are fuel for the engine. They are insufficient for the mainte- nance of life because they contain no nitrogen, and protoplasm is nitrogenous. III. The Quaternary Proximate Principles are so called because they contain four or more chemical elements. They always contain carbon, hydrogen, nitrogen and oxygen, and QUATERNARY PROXIMATE PRINCIPLES. 25 usually sulphur and phosphorus. They may in addition contain any of the remaining elements found in the body. This class cannot be discussed with satisfaction and accuracy from a chemical standpoint because their composition is indefi- nite. The provisional formula C^^Hj^fi^^N^^S may be accepted for albumin which belongs to this class. They are also called nitrogenized, or proteid, or albuminoid proximate principles. They are derived from both animal and vegetable kingdoms and are not crystallizable. They do not pass through the organism unchanged and must be converted into crystalloids before they can be absorbed from the alimentary canal. The chief articles of diet containing these substances are lean meats, eggs, milk, legu- minous vegetables, cereals, etc. It is this class of proximate principles which under proper conditions manifest the phenomena of life. They are mainly concerned in the structure of the solid tissues of the body and are also present in all the fluids. They give to any tissue an individuality which differentiates its function from that of other tissues. They are immediately necessary to the life of proto- plasm and consequently to the continuance of functional activity. Their withdrawal tells directly upon the protoplasm itself. A characteristic of these bodies is that when suitable con- ditions of heat and moisture are present they undergo putrefac- tion. Another is that they are coagulated at a temperature of 130° F. or over and by acids. Hence the necessity of a well- regulated heat balance and the perservation of an alkaline reac- tion throughout the organism generally. In the stomach and intestinal tract they are converted into peptones which being os- motic are carried away by the blood (though not as peptones) to be appropriated in large part by the living proteid tissues for their regeneration. Quaternary proximate , principles at last undergo decomposition in the body with the final production of urea, carbon dioxide, water and heat. They are the most impor- tant of the three classes. 26 CHEMICAL COMPOSITION OF THE HUMAN BODY. The nitrogenous proximate principles embrace the proteids and the albuminoids, both of which are similar in composition. Some of the best known of these are albumin of eggs, myosin of muscle, casein of milk, paraglobulin, fibrinogen and serum al- bumin of blood, pepsin of gastric juice, gelatin, mucin, chon- drin, fibrin, etc. CHAPTER II. THE CELL AND THE ELEMENTARY TISSUES. (A) THE CELL. All the tissues of the body are made up of cells and intercel- lular substance. All the cells are descended from one parent cell, called the ovum, while the intercellular substance is cre- ated through the medium of the cells. Fig. I. Nuclear membrane -.^ Linin. Nuclear fluid (matrix). . Chromatin cords (nuclear network). Nodal enlarge- ments of the chromatin. ■ Cell membrane, - Exoplasm. ~ Microsomes. - Centrosome. - Spongioplasm Hyaloplasm, :=w__ Foreign inclosures. Diagram of a cell. Microsomes and spongioplasm are only partly drawn. {Bruiaker.) A cell may be defined as an irregularly round or oval mass of protoplasm of microsopic size, enclosing usually a small indis- 27 28 THE CELL AND THE ELEMENTARY TISSUES. tinct spherical body, the nucleus. While these are the typical cell elements, cells often possess a thin wall, or surrounding membrane, and the nucleus may contain one or more smaller bodies, called nucleoli. The greater part of the cell contents is protoplasm. This is a gelatinous or semi-fluid, granular substance, transparent, and generally colorless. It is not a homogeneous mass, but is com- posed of an elastic network, called the spongioplasm, enclosing a less firm portion, the hyaloplasm. Chemically protoplasm consists of various albuminous substances, and a special nitrog- enous proteid, plastin, together with water and salts. The part played by the nucleus in the reproduction of the cell gives it great importance. It is a specialization of proto- plasm, traversed by a network of fibrils, enclosing a probably semi-fluid portion, the matrix. The name chromatin is often given to the fibrils, from their affinity for certain stains, while the matrix, which does not take these stains, is termed achromatin. The nucleus has a limiting wall or membrane, and often con- tains one or more smaller bodies, nucleoli, concerning which but little is known. A small body, the centrosome, lies usually just outside the nucleus. It is most prominent when cells are dividing or about to divide, and has been supposed to give the primary impulse to karyokinesis, but this is not certain. It has an attractive influence on the protoplasmic fibrils in the neigh- borhood producing the " attraction sphere " (see Fig. i). Nuclei are especially distinguished : (a) By their resisting the power of certain acids and alkalies {e. g., acetic acid), by which they are rendered more clearly visible under the micro- scope. This indicates some chemical difference between the protoplasm of cells and that of nuclei, since the former is de- stroyed and rendered invisible by these reagents. (/5) By staining in hematoxylin, carmine, etc. Nuclei are round or oval, and may occupy any position in the cell. They are more constant in size and shape than the PROPERTIES OF CELLS. 29 cell itself, but sometimes may occupy nearly the whole of that body. Properties of Cells. — The properties of cells in general are (i) motion, (2) irritability, (3) nutrition, (^^') growth 3,xi6. (5) reproduction. 1. Motion. — This manifestation is well illustrated in its lowest form by a fresh-water organism, the ameba, the movement of which is called ameboid. In this, the cell, which has hitherto remained smooth in outline, throws out little projections from its body, like limbs, into which the protoplasm gradually streams, thus radically changing the shape of the cell, and finally its position. Higher degrees of motion are seen in the contraction of muscle cells and the waving of cilia. 2. Irritability. — The ameboid movement of cells is spon- taneous, but motion may also be excited by external influences, viz., thermal, mechanical, nervous, chemical and electrical. 3. Nutrition includes the wonderful processes of anabolism and katabolism, by which cells take in certain foods and so change them as to nourish and build up their tissue, and throw out the parts which cannot be used. 4. Growth follows as a natural sequence of proper nutrition, and may cause a uniformly increased element ; but in higher organisms growth is usually unequal, to which phenomenon the specialization of cells is due. Thus cells assume special forms or special functions : some become nerve, others bone, some develop the power to contract, others to secrete, etc. 5. Reproduction. — By this property cells are enabled to repro- duce themselves. There are two methods, by (a) direct and {b ) indirect division. In the first the cell divides into two by the simplest method possible, the nucleus and cell protoplasm constricting in the center until two cells are formed. This is an unimportant method in the higher animal life we are study- ing. The chief manner of reproduction in animal cells is by indirect division, known as karyokinesis or mitosis. 3° THE CELL AND THE ELEMENTARY TISSUES. In the beginning of this phenomenon, the nucleus, which plays the important role, grows larger. Its chromatin greatly increases and becomes contorted so as to form a dense convolu- tion, the close skein, or spirem. Then the chromatin fibrils further thicken, but become less convoluted, forming irregularly arranged loops, the loose skein. During the formation of these Close Skein (viewed from the side); Polar field. Fig. 2. Loose Skein (viewed from above — u e., from the Mother stars (viewed from the side). Mother Star Daugbtst Star (viewed from above). Beginning, Completed. Division of the Protoplasm. Karyokinetic Figures Observed in the Epithelium of the Oral Cavity of a Salamander. The picture in the upper right-hand corner is from a section through a dividing egg of Sire- don pisciformis. Neither the centrosomes nor the first stages of the development of the spin- dle can be seen by this magnification. X 560. CSiom. Bruhaker.') skeins the nuclear membrane and the nucleoli disappear. The fibrils of the loose skein now separate at their peripheral turm into a score of loops, the closed ends of which point toward a common center — a clear space called the polar field. Seen from above these loops of chromatin make a wreath called the mother wreath ; seen from the side, they make a star, called the EPITHELIAL TISSUE. 3 1 mother star or aster. While the loose skeins are forming, deli- cate strise appear within the achromatin, so disposed as to make two cones with their bases within the polar field and directed to- ward one another, and their apices directed toward the future new nuclei. These achromatin figures constitute the nuclear spin- dle. They then arrange themselves into two daughter wreaths, or asters, similar to the mother. At tnis juncture the cell pro- toplasm begins to divide by becoming constricted in the center. The daughter stars are converted into two new nuclei, in inverse order as the original nucleus was broken up. First the loose skein forms, then close skein. Nuclear membranes and nucleoli appear, the cell protoplasm divides into two new cells, and the cycle is completed. (See Fig. 2.) Derivation of Tissues. — The primary parent cell divides into an innumerable mass of cells, which is called the blastoderm. The blastoderm soon divides into two more or less distinct layers, an outer and inner, named ectoderm and entoderm, between which a middle layer later appears, the mesoderm. All the tissues of the body develop, by specialization, from these three. (See Embryology. ) (B) THE ELEMENTARY TISSUES. Four varieties of elementary tissues are usually named, ( i '1 epithelial, (2) connective, (3) muscular and (4) nervous. I. The Epithelial Tissues. Epithelium is a tissue consisting of one or more layers of cells, covering all the free surfaces of the body. That covering the skin and mucous membranes is ( i ) epithelium proper, while that covering the serous membranes is known as (2) endothelium. I. Mucous membranes secrete a tenacious fluid known as mucus, and furnish a lining surface for all tracts with external openings, i. e., the digestive, respiratory and genito-urinary tracts. 32 THE CELL AND THE ELEMENTARY TISSUES. 2. Serous membranes secrete a watery fluid which acts as a lubricant for the walls of closed sacs to move smoothly against one another. They line those surfaces without direct external openings, such as the pleura, pericardium, peritoneum, heart and blood-vessels, synovial surfaces of joints, lymphatic spaces and vessels, etc. Epithelial tissue performs various functions in different parts of the body. In the skin, where it is known as epidermis, it protects the delicate surface of the true skin beneath ; in the alimentary and genito-urinary canals it aids in secretion and excretion ; in the respiratory tract it preserves an equable tem- perature by the moisture it produces, while in all internal parts it yields lubricants. Epithelial cells are connected together by an interstitial cement substance. They contain no blood-vessels and no nerves, being nourished by absorption through clefts of this sub- stance. The tissue usually rests upon a basement membrane, or membrana propria, which is a modification of the connective tissue beneath. Varieties. — The varieties of epithelium may be classed as follows : (I) Squamous, {a) simple, consisting of a single layer, {Jj) stratified, consisting of several layers; (II) Columnar, {a) simple, ((5) stratified; (III) Modified, (a) cilitated, (3) goblet, (f ) pigmented, (rt?) glandular, {e) neuro-epithelium. I. Squamous Epithelium. — {a) As a simple layer this occurs in but few places, lining the air sacs of the lungs, the mastoid cells, membranous labyrinth, and crystalline lens. Viewed from above, it appears as flattened, polyhedral nucleated plates like a regular mosaic. (i5) Stratified squamous epithelium is far more common. This we find covering the true skin, the cornea, mouth, lower part of the pharynx, esophagus, epiglottis and upper part of the larynx, and all the urethra in both §??;es except the membranous and penile portion in the male, EPITHELIAL TISSUE. 33 The arrangement of the cells is typified in the epidermis. The lowest layer of this variety, resting upon the membrana propria, is almost columnar in type. As they approach nearer the surface, the layers become flatter and more scale-like, and Fig. 3. Vertical Section of the Stratified Epithelium of the Rabbit's Cornea. a, anterior epithelium, showing the different shapes of the cells at various depths from the free surface ; b, a portion of the substance of cornea. {Kirkes after Klein.) possess less vitality. As the outer layer is worn away, the lower, more vigorous layers push upward to the surface to take its place. In the middle strata, where the cells are polyhedral in shape, we find the so-called prickle cells, which have minute projecting spines, by which they are connected with one another. II. Columnar Epithelium. — This type consists of column or rod-shaped cells, set upright, longitudinally striated, and con- taining oval -shaped nuclei. Ciliated epithelium is more common with this variety than with any other. Each of these cells presents, on its free sur- face, twenty or more small hair- like, protoplasmic appendages, called cilia. During life these small processes are in constant rapid motion, waving in a direction toward the outlet of the cavity in which they are found. In the genital organs they are important in bringirig together the male and female elements of reproduction, while in the respiratory tract they are concerned 3 34 THK CELL AND THE ELEMENTARY TISSUES. in aiding the passage of the mucus and in the expulsion of for- eign bodies. I. Simple columnar epithelium occurs in the alimentary tract from the stomach to the anus, mammary glands, seminal vesi- FlG. 4. Ciliated Epithelium op the Human Trachea. a^ layer of longitudinally arranged elastic fibers; b^ basement membrane; c, deepest cells, circular in form ; d, intermediate elongated cells ; e, outermost layer of cells fully developed and bearing cilia. X 35o. {Kirkes after Koiliker.) cles and ejaculatory ducts, membranous and penile portions of the urethra. This variety is found ciliated in the greater part of the uterus, and in the brain-ventricles and canal of the spinal cord. 2. Stratified columnar epithelium occurs in the last part of the vas deferens and the olfactory part of the nasal fossae. Cili- ated, it occurs in the Eustachian tube, lachrymal ducts, respira- tory part of nasal fossse, ventricle of larynx, trachea and bronchi, epididymis and first part of vas deferens. III. Modified Epithelium. — (a) The ciliated variety has been considered. (^) Goblet cells are found on all surfaces covered by columnar epithelium, but especially in the large intestine. They secrete mucin, the main constituent of mucus, which so distends the cell that it ultimately bursts and the mucus is discharged. CONNECTIVE TISSUE. 35 (^) Foreign matters, such as fat, proteid, etc. , often invade the protoplasm of epithelial cells. When these matters are col- ored, the epithelium becomes pigmented. Such cells are con- stant in the deeper layer of the epidermis, especially of certain ^'°- 5- races, and in the choroid coat of the eye. (ji") (?/««^2'-v k^}^ ^^3^ •^ J^a2feS»*"jr ^ 1- v; V * .•'' /^ ' T-!— ^' ' t **■ :/■"-■ Elastic Fibro-caktilage, Showing cells in capsules and elastic fibers in matrix. (From Veo after Cadiaf,) cells imbedded in a matrix. The cells are irregular in outline, and arranged in patches of various shapes. Fig. ti. g White Fibko-caktilage, Showing cells, a, in capsules and fibrillar matrix, 3, ( Yeo after Cadiat.) (^B) The yellow elastic type exists in the external ear, epi- glottis, cornicula laryngis, and eustachian tube. The matrix is 40 THE CELL AND THE ELEMENTARY TISSUES. composed almost entirely of fine fibers very much like the yellow variety of elastic tissue. ( C) In white fibrous cartilage the matrix is made up almost entirely of white fibrous tissue. It is found as : ( i ) inter ar- ticular Jibro-cartilage, in the semilunar cartilage of the knee- joint ; (2) circumferential, on the edges of the acetabulum and glenoid cavity; (3) connecting, between vertebrae; (4) strati- form, forming a coating to grooves on bones, through which ten- dons glide. On boiling, cartilage yields a substance, known as chondrin, which on cooling turns to gelatin. III. Bone. — Bone is a dense form of connective tissue con- stituting the skeleton or framework of the body. It serves to protect vital organs in the skull and trunk, and acts as levers in the limbs worked by muscles. The tissue is characterized by the deposit of calcareous or lime matters within its intercellular cement substance, to which its well-known hardness is due. We find in bone two distinct kinds, dense or compact, forming the outer portion, and spongy or cancellous, forming the inner portion. Microscopically bone is seen to consist of many minute longi- tudinal channels, called Haversian canals, each surrounded by concentric layers of bone called lamella, within which run still smaller longitudinal channels, called lacunce. Connecting the main canal and the lacunae, and radiating in all directions be- tween them, are other very minute channels known as canaliculi. Each Haversian canal, with its surrounding lamellae, lacunae and canaliculi, composes an Haversian system. (See Fig. 12.) Periosteum forms the membranous covering of the outer sur- face of all bones except their articular extremities. It consists of an outer and an inner layer. The outer is a dense fibrous coat protecting the more important internal structure called the osteogenetic layer, from its intimate connection with the develop- ment of bone. It possesses a rich blood supply which nour- CONNECTIVE TISSUE. 41 ishes the subjacent bone, and contains numerous cells which later become bone-forming elements — the osteoblasts. Bone marrow is the highly vascular substance found within the central cavity of the long bones and the Haversian canals. That of the adult long bone is a yellow in color, and is com- posed mainly of fat, while that occupying the spaces of cancel- lous tissue is red, profuse in blood supply, and contains but little fat. Large, multinucleated cells are found in red marrow. Fig. 12. Transverse Section of Compact Bony Tissue (of Humerus) Three of the Haversian canals are seen, with their concentric rings ; also the lacunae, with the canaliculi extending from them across the direction of the lamellae. The Haversian aper- tures were filled with air and debris in grinding down the section, and therefore appear black in the iigure, which represents the object as viewed with transmitted light. The Haversian systems are so closely packed in this section that scarcely any interstitial lamelise are visi- ble. X 150- {ICirkes ailzr Sharpey.) and are known as giant cells, or osteoclasts. They are sup- posed to be concerned in the absorption of bone tissue. Development of Bone. — According to its development from 42 THE CELL AND THE ELEMENTARY TISSUES. Fig. 13. ..sll I « ' j.n 1 P.* ' Two Fibers of Striated Muscle, In which the contractile sub- stance, wf, has been ruptured and separated from the sarco- lemma, a and j / ^, space under sarcolemma. (From Vea after Ranvier, ) the embryo, bone may be classed as : («) Endochondral, derived from the primary cartilage, hyaline in type ; (^) Periosteal, derived from the primary peripheral per- iosteum. All the bones belong to the former group, except those of the vault of the cranium (parietal and frontal) and of the face and a part of the lower jaw. The process of bone formation is a complicated one. The osteoblasts are the main agents, whether the bone be derived from cartilage or from perios- teum. These cells arrange themselves in different locations, the so-called centers of ossification, over the surfaces of the cartilaginous network or periosteal fibers, as the case may be, and soon are trans- formed into bone-cells, embedded in a matrix, which is at first soft and finally becomes ossified from the deposit of lime salts. 3. The Muscular Tissues. There are two variations of muscular tissues : (^A) Striated or Voluntary, and (^) Non-striated or Involuntary. The muscle of the heart is striated, but in- voluntary. (A) Striated muscle, or voluntary, so called because it is controlled by the will, constitutes the extensive muscular system of the skeleton, and of the walls of the abdomen, besides a few of the muscles connected with certain organs, the mid- MUSCULAR TISSUE. 43 die ear, tongue, pharynx, larynx, dia- phragm, generative organs, etc. This variety of muscle is composed of bundles of fibers, called fasciculi, each enclosed in a net -like sheath, \kvi perimy- sium. Between the fibers is a delicate cementing substance, the endomysium. A layer of areolar tissue, of variable thick- ness, known as the epimysium, surrounds the entire muscle. Each fiber consists of the sarcolemma, or investing sheath, the muscle substance, and the muscle nuclei. The sarcolemma is a tough, homogeneous, elastic mem- brane, very tightly adherent to the sub- stance of the muscle. The nuclei are oval or fusiform, and lie immediately be- neath the sarcolemma, upon the surface of the muscle substance. Seen under the microscope on longi- tudinal section voluntary muscle presents alternate light and dark transverse striae, the explanation of which is a difficult problem, for which many solutions have been offered. That of Rollett seems most plausible. According to this au- thor, striated muscle tissue is composed of darker contractile fibrillae, arranged in parallel rows of delicate spindles, with a serai-fluid, lighter portion between, called the sarcoplasm. The apposition of the spindles transversely produces the so- called transverse disks. Each spindle ter- minates in a minute spherical bead, the Cells of Smooth Muscle- tissue FROM THE Intes- tinal Tract of Rab- . BIT. (From Kr^ after Ranvier. ) A and^, muscle-cells in which di£ferentiatioii of the protoplasm can he well seen. 44 THE CELL AND THE ELEMENTARY TISSUES. apposition of which transversely produces the intermediate disks, or Krause's membrane. It is to these alternate dark rows of transverse and intermediate disks, with the lighter sarcoplasm between, that the striated appearance of voluntary muscle is due. The contractile fibrillae are arranged in bundles or muscle columns, surrounded by thick layers of sarcoplasm. On the cross-section these bundles appear in a network of sarcoplasm as minute polyhedral areas, called Cohnheim's areas or fields. The blood-vessels of striated muscles are very numerous. The larger vessels, together with the nerves, are contained within the Striated Muscular Tissue of the Heart, Showing the trelliswork formed by the short branching cells, with central nuclei. ( ] Vo.) perimysium, from which the primitive bundles are supplied by smaller branches. The lymphatic supply is scanty. Nerves are profusely distributed. (^) Non-striated, or involuntary muscle forms the coats of the (i) digestive tract from the middle of the esophagus to the anus, (2) capsule and pelvis of the kidney, ureter, bladder and PHYSIOLOGY OF STRIATED MUSCLE. 45 urethra, (3) trachea and bronchi, (4) ducts of glands, (5) gall- bladder, (6) vesiculae seminales (7) uterus, (8) blood-vessels and lymphatics, (9) iris, ciliary bodies, and eye-lids, (10) hair follicles, sweat glands, and skin of the scrotum. Non-striated muscle is made up of bundles of flat, spindle- shaped, nucleated cells longitudinally disposed. Each cell- is covered by an elastic sheath, corresponding to the sarcolemma of striated muscle. An endomysium unites the cells together, while a perimysium surrounds the bundles. Heart muscle is striated and involuntary, and is thus distin- guished from the usual form of striated muscle: (i) Its fibers are united with each other at frequent intervals by short branches; (2) the fibers are smaller and the striation is less marked; (3) the sarcolemma is absent; (4) the nuclei are situated within the substance of the fiber and not upon it. 4. The Nervous Tissues. The primary elements of the great nervous system are, (a) the cells, which originate nervous impulses, and (Ji) the fibers which transmit such impulses, the two being connected and sup- ported by (^) the neuroglia and connective tissue framework. These tissues will be described under the later discussion of the nervous system. PHYSIOLOGICAL CHARACTERISTICS OF STRIATED MUSCLE. Striated muscle possesses elasticity, tonicity, a peculiar sensi- bility, and contractility or irritability. The elasticity of muscle is not so much a property of the muscle substance as of the sarcolemma and interstitial fibrous tissue. There is of course a certain limit to this power, if ex- tended beyond which the muscle fibers become dislocated and unfitted for further use. Tonicity is the constant insensible tendency to contraction 46 THE CELL AND THE ELEMENTARY TISSUES. possessed by muscle in a nonnal and healthy state. This is seen in surgical operations in which muscles after being divided be- come permanently contracted. Muscle has a special sensibility which enables it to appreciate the force of weight, resistance, immobility and elasticity, and the sense of fatigue after long-continued exertion. General sensibility, as of pain, is but little developed. Contractility is a property possessed by striated and non- striated muscle. We see it in the former in flexing a limb, raising the eye-brows, etc., and in the latter in the variations in calibre of blood-vessels and the contraction of the uterus in labor. It is noticeable that a voluntary muscle responds so quickly to stimuli that the contraction may be said to be practically instantaneous. The subsequent relaxation occurs as soon as the stimulus is withdrawn. The contraction and re- laxation of involuntary muscle is much more sluggish. It is the phenomenon as presented in the striated variety with which we are most concerned. Muscular contractility results from a stimulus which may be transmitted from the brain through the conductors of motor impulses, as by an act of volition, or it may be produced re- flexly, or artificially. Artificially, the stimulus may be applied through the nerve supplying the muscle, or to the muscle directly, and may be mechanical, thermal, chemical or electrical. Electricity fur- nishes the most convenient form of stimulus for experimental use, because its force can be accurately regulated. Muscular contractihty can be studied in the dead organism for some time after death, especially in cold-blooded animals. The power to contract remains the same so long as the nutrition has not been disturbed beyond certain limits. Thus, when the power is lacking in muscles of a living organism from paralysis and disuse, upon minute examination it is found that chemical and physical disintegration has occurred. PHYSIOLOGY OF STRIATED MUSCLE. 47 The principal changes noted in muscle on contraction are in electrical and chemical phenomena, elasticity, temperature and form. I. The electrical changes involve the so-called " currents of rest." A galvanometer applied to a muscle removed from the body indicates the passage through it of certain electrical cur- rents. When the muscle is made to contract the galvanometric needle returns to zero (the negative variation) ; when it re- laxes the needle again indicates the passage of a current. The cause of these currents may be chemical changes due to degen- erative processes. The effects of galvanic and Faradic currents upon muscular contractions are indicated under the nervous system. II. The chemical changes are : — («) Muscle tissue, which is normally neutral or slightly alka- line in reaction, becomes acid, owing to the formation of sarcolactic acid. {b) More oxygen is taken up from the blood than when the muscle is at rest, proof of which is shown by the facts that during active muscular exercise more oxygen enters the body by respiration, and the blood leaving active muscles is poorer in oxygen. (f) More COj is produced in the muscle. The increased elimination of this gas is far in excess of the increased consump- tion of oxygen. Oxygen is stored up in the same way in the muscular substance in the intervals of activity. It is a condi- tion to be easily called into use when the metabolism incident to contractions begins. (t^) Glycogen, which is ordinarily stored up in the muscle substance, is consumed. (f) A peculiar muscle sugar, probably inosite, makes its ap- pearance. The increased output of CO^ is in striking contrast to the practically undisturbed elimination of urea, except aiitr prolonged 48 THE CELL AND THE ELEMENTARY TISSUES. exercise. An explanation of this discrepancy will be given un- der Nutrition. III. A muscle is not only elastic but extensible. A passive weight suspended from the end of a muscle will elongate it ; but when the weight is removed the muscle resumes its original length. A contracted muscle is more extensible than one at rest. Fresh muscle is perfectly elastic, i. e., it will regain its exact normal shape after contraction, elongation, etc. ; but con- tinued activity finally impairs this quality. IV. Evolution of heat accompanies muscular contraction. It will be seen later that all metabolic activity means the pro- duction of force, most of which force assumes the form of heat. The increased metabolism in muscle tissue during exercise means an increased conversion of potential energy of the proximate principles into heat and work. The heat produced represents by far the larger part of this potential energy. Of the total amount of potential energy converted, the part taking the shape of work upon conversion is greater the greater the resistance to muscular contraction. It follows that the heat is relatively diminished ; though the increased metabolism ren- ders it not absolutely so, /. e., the amount of heat actually produced is greater the greater the tension. The natural, and correct, conclusion on this ground is that when a muscle becomes fatigued the amount of potential energy taking the form of heat is increased. The heat production of muscular activity is invol- untarily made use of when a person shivers in cold weather. Given a certain amount of work to do, more heat will be evolved if it be done by a few strong contractions than by many weak ones. V. The changes in form are the most striking of those .that occur. In contracting a muscle becomes shorter and broader, the two alterations compensating each other, so that there is no change in bulk. The amount of shortening may vary all the way up to about 35% of the original length of the muscle. PHYSIOLOGY OF STRIATED MUSCLE. 4g The fresher and more irritable a muscle is the shorter it will be- come in response to a given stimulus. Up to a certain limit the stronger the stimulus the greater the shortening. Up to about 85° F. heat increases the amount of shortening. The more nearly parallel to the long axis of the muscle the fibers run, the greater the shortening in proportion to the length. Mechanism of Muscular Contraction. — The application of electricity to the nerve supplying a given muscle, by one of the various apparatus which have been devised for the purpose, shows the mechanism of muscular contraction in a graphic manner. Two varieties of phenomena may be produced by such an appa- ratus. The stimulus may be applied in the form of a single electrical discharge, when it is followed by a single muscular contraction ; or a rapid succession of discharges may be applied, producing a state of permanent, or so-called tetanic, contrac- tion. Upon the application of a single electrical discharge to a motor nerve connected with fresh muscle, there is a sudden con- traction, which is succeeded by a sudden relaxation. Under this stimulation, the muscle shortens its length about three tenths. In man, the time required for the contraction is esti- mated at .03 or .04 of a second, and for the relaxation a period a little shorter, with about .004 to .01 of a second for the inter- val between stimulation and contraction, called the latent period. Experiments have shown that when one end of a muscle is excited, a contraction occurs at that point and travels along the length of the muscle in the form of a wave, the estimated ra- pidity of which is thirty-three to forty-three feet per second. In the contraction of a muscle it is believed that shortening of the fiber takes place wherever a stimulus is received, and that this is propagated in the form of a wave, which meets in its course another wave starting from a different point of stimula- tion. 50 THE CELL AND THE ELEMENTARY TISSUES. A rapid succession of electrical impulses applied to a muscle produces a persistent, or tetanic, contraction, which is the kind that occurs in the normal physiological action of muscle. The power of the contraction is proportionate to the rapidity with which the stimuli are received. The number of stimuli received by a muscle in a state of powerful contraction is probably about twenty per second, which produces the same number of waves or vibrations in a muscle. These vibrations make a muscle sound, of a pitch corresponding to their rapidity. This can be heard in the temporal and masseter muscles by filling the ears with wax and causing the muscles to contract. Chemical Composition of Muscle. — Water represents about seventy-five parts per hundred of muscle tissue. Of the re- maining 25 parts 15 are proteid; glycogen, fat, organic and inorganic salts (chiefly potassium) constitute the remainder. When fresh muscles are subjected to pressure, there is forced out a substance, muscle plasma, which corresponds to the plasma of blood. The muscle plasma contains a substance, tnyosifiogen, analogous to fibrinogen of blood. Coagulation of the muscle plasma produces myosin, which is not unlike fibrin in some re- spects. Muscle Fatigue. — A muscle will not contract indefinitely. When it is being artifically stimulated the individual contrac- tions become progressively longer and weaker, until response finally ceases. It is said to be fatigued. The fatigue results from the consumption of the energy-pro- ducing materials at hand, but more particularly from the accu- mulation of effete products of muscular metabolism — especially of sarcolactic acid. The seat of fatigue is not, however, in the muscle itself. Nor is it in the supplying motor nerve. It seems that the waste products poison the nerve terminals in the end motorial plate, so that it acts as a block to the passage of an impulse to the muscle. It has also been shown that these same waste products PHYSIOLOGY OF STRIATED MUSCLE. 5 1 carried to the centers inhibit their power to originate eiferent impulses. Rigor Mortis. — This is a general stiifening of the musculature subsequent to death. Coagulation of the muscle plasma, with the formation of myosin, is the cause of the condition. The muscles become (a) shortened and opaque, {b ) heat is evolved, (c) they give off COj, and («, muscularis raucosce. X 15. {Stirling.) 3. The mucous coat has an average thickness of about -^.5- in., is loosely attached to the muscular coat, and, except during gastric digestion, is thrown into longitudinal rug«. It con- sists of columnar epithelium resting upon a basement membrane, beyond (underneath) which is the capillary blood supply. Throughout the greater part of the stomach the mucous mem- brane can be shown to be divided by delicate connective tissue into numerous polygonal depressions, from the bottom of which I02 DIGESTION. extend the gastric glands. For a description of these glands and the general properties of the gastric juice, together with the mechanism of gastric secretion, see Secretion, page 60 et seq. Condition of Food on Entering Stomach. — The food has entered the stomach in the same condition in which it left the mouth. It has been more or less completely triturated by mas- tication ; the whole has been moistened, and a part dissolved by the saliva, All the materials taken in have been thoroughly mixed with each other, and some of the starch has been con- verted into sugar. The reaction is now alkaline, unless the acidity of the articles taken has been too great to be overcome by the alkalinity of the saliva — in which case there would be no amylolytic change. Excepting starch, all foods entering the stomach are chemically unaffected. It remains to see what hap- pens to the foods under the influence of gastric digestion. These changes are brought about by the gastric juice, aided by muscular movements of the stomach. Gastric Juice. — Gastric juice may be secured in several ways, but the most reliable article for experimentation is taken from a previously established gastric fistula in one of the lower animals. It is a thin, almost colorless liquid of an acid re- action, and a specific gravity of 1005 to 1009. Chemically it contains per thousand about 973 parts water and 27 solids. Organic substances compose some 17 of the 27 parts of solid matter. The organic substances are mainly mucin, pepsin and rennin. The most important inorganic constituent is free hydrochloric acid. The others are chiefly the chlorides of sodium, potassium, calcium, and ammonium, and the phosphates of iron, calcium and magnesium. Gastric juice will resist put- refaction for a long time, probably on account of the free acid. Its digestive properties are due to the proteolytic enzyme, pepsin, the milk-curdling enzyme rennin, and the free hydro- chloric acid. Hydrochloric Acid. — The amount of free hydrochloric acid GASTRIC JUICE. I03 present in normal gastric juice is from two-tenths to three-tenths of one per cent. It has been frequently claimed that the acid- ity of this secretion is due to an organic acid (lactic), but while it cannot be denied that lactic acid, from the fermentation of carbohydrates, is, or may be, normally in the stomach during digestion, yet hydrochloric acid is undoubtedly the free acid proper to the gastric juice. Digestion, however, will proceed under a proper (variable) degree of an acidity from almost any acid. Theories as to the method of production in the stomach of hydrochloric acid are noticed under Secretion. It is very prob- ably a product of the parietal cells in the so-called acid or fundic glands, but beyond the fact that it is manufactured from the neutral chlorides of the blood in the mucous membrane, nothing is definitely known. Beyond an insignificant effect in converting cane sugar into dextrose, its function is a passive one, namely, that of simply making the secretion acid, so that pepsin may act upon the proteids. Pepsin. — Pepsin is a proteolytic enzyme, the composition of which has not been determined. From the definition, it con- verts proteids into peptones. It operates only in an acid me- dium. Hence its action is contingent upon the presence of an- other constituent of the gastric juice, namely, hydrochloric acid. Pepsin is a typical enzyme, and reference to the characteristics of those bodies will avoid repetition of its properties here. Rennin. — Rennin has the property of coagulating milk. It acts upon the soluble proteid of milk (casein), changing it into an insoluble product, which is precipitated. Acids also will co- agulate casein. Milk Standing has lactic acid produced by the action of bacteria upon the lactose in it, and this acid precipi- tates the curd. The acid of the gastric juice might be sufficient to bring about this result, but the quick coagulation of milk when it is introduced into the stomach is probably not due to the acid, since neutral extracts of the gastric mucous membrane 104 DIGESTION. will themselves curdle milk. After coagulation the action of pepsin begins and the casein is converted into peptones in the usual manner. The value of the curdling process is not ap- parent. Action of Gastric Juice on Foods. (A) On Proteids. — A familiar test for the proper performance of gastric digestion is the observation of the effect of the juice in a given case upon the white of an egg (proteid). In normal gastric juice, or in a properly prepared artificial solution, the egg is seen to swell up and dissolve. This soluble proteid is now called peptone, and it differs from the proteid of the egg in certain important re- spects, to be noted later. But, although peptone is the final product of pepsin-hydrochloric action, there are certain sub- stances produced intermediate between the initial proteid and the final peptone, just as in case of the formation of maltose by ptyalin. Some of these substances have been called acid-albu- min, parapeptone, propeptone, etc. But whatever they may be, the nomenclature of Kuhne is being largely followed at present. He supposes that the first product is an acid-albumin which he calls syntonin ; that syntonin under the influence of pepsin undergoes hydrolysis, taking up water and splitting into primary proteoses ; that each of these primary proteoses takes up water and splits into secondary proteoses ; that these last undergo a similar change with the production of peptones ; so that the suc- cessive substances are proteid, syntonin (acid-albumin), primary proteoses, secondary proteoses, peptones. Peptones can be shown to be different from syntonin and the proteoses by chemical reactions. The chief object of proteo- lytic digestion is to get the proteids into a diffusible condition. Peptones differ from proteids in at least three important respects : (i) They can pass through animal membranes, that is, can be absorbed; (2) they are no longer coagulable by heat or many acids ; (3) they are capable of assimilation by the cells after. they have been absorbed. EFFECTS OF GASTRIC JUICE. 1 05 {B) On Carbohydrates. — There is no enzyme furnished by the stomach to affect any of the carbohydrates. It is true that salivary digestion proceeds in some small degree in the stomach. Saliva is swallowed with the food, and until the re- action becomes acid (which cannot be immediately), there is no reason why the conversion of starch into maltose should not proceed. It is also true that the mere acid of the gastric juice can slowly convert cane sugar into dextrose. Simple acidulated water will do the same. ( C ) On Fats. — Neither is there any fat -splitting enzyme in the gastric secretion. So far as any chemical change is concerned, the fats leave the pylorus in exactly the same condition as they entered the mouth. Their physical condition, however, under- goes some change in the stomach. The body temperature is sufficient to liquefy them, the vesicles in which the droplets are contained are dissolved, and, thus set free, they become a part of the mechanical mixture, chyme, and are made easier subjects of intestinal digestion. (ZJ) On Albuminoids. — The albuminoids are acted upon by pepsin and hydrochloric acid in much the same way as are the proteids. Taking gelatin as a type, gelatoses are formed instead of proteoses. It is stated that peptic digestion does not go fur- ther than the gelatose stage with the albuminoids, conversion into peptones taking place under the influence of trypsin. Resistance of Stomach Wall to Digestion. — It would be in- teresting to know why the stomach (or the intestine) does not digest itself. If a portion of the stomach of another animal be placed in that of a living animal, it will be digested ; or if the circulation be cut off from a limited area of the stomach, the secretion will frequently digest that part of the organ and bring about a perforation ; or further, if any living part of an animal, as the leg of a frog, be fastened in the stomach of an- other animal, it will likewise be digested. The last instance would seem to lead to the conclusion that living matter can be Io6 DIGESTION. digested, but in reality it is shown (Bernard) that the tissue is first killed by the acid, and that no digestion takes place in the alkaline intestinal juice. But why the stomach is not thus at- tacked when other living tissue is remains obscure. The most plausible theory is that the gastric epithelium is possessed of some power, mechanical or physical, the nature of which is un- known, inhibiting the action of the gastric juice, most probably hy preventing its absorption. This theory, it should be said, simply represents an advance in our probable knowledge of the subject, and does not claim to be an explanation of the phenomenon. Movements of the Stomach. — Whether the exact details of the muscular movements of the stomach be known or not, the essential fact to be remembered is that the organ is in a more or less con- tinuous state of muscular activity for several hours after the ingestion of an ordinary meal, and that this activity results in the physical disintegration of most of the solids introduced, in the thorough mixing of all the classes of foods with each other and with the gastric juice, and in the passage from time to time of such parts as have been reduced to a pultaceous condition through the pylorus into the duodenum, until finally the stomach is empty. In considering the mechanism of these movements a division of the organ into two segments, fundic and pyloric, by the sphincter antri pylorici is to be kept in mind. When food has entered the stomach the peristaltic wave of contraction begins at the splenic end and passes toward the right. This contraction is comparatively weak, is mainly evident along the greater curvature, and increases in strength as it passes toward the pylorus. Its wave-like character is due to the contraction and subsequent relaxation of successive bands of circular and oblique fibers. Regurgitation of food is prevented by a rhythmical contraction of the lower end of the esophagus, and the effect of this muscular wave (peristalsis) in the fundus is to force the MOVEMENTS OF THE STOMACH. I07 food toward the pylorus. But when the right end is reached, the rather firm contraction of the sphincter antri pylorici pre- vents the entrance into the antrum of all except the liquid or semi-liquid parts. The food, thus denied admission to the antrum, takes a course along the lesser curvature to the splenic end, then back along the greater curvature, and such parts of it as have, during this revolution, been sufficiently dissolved pass into the antrum. These revolutions continue until the fundus has been emptied. It is not to be supposed that food has been accumulating meantime in the antrum. Indeed, it is certain that muscular contractions are here much more active than in the fundus, where the movements are slow and of a rather compressing nature. It is thought that very soon after the entrance of food from the fundus the entire muscular wall of the antrum undergoes very strong contraction of a peristaltic (possibly systolic) nature, and the pultaceous parts of its contents are sent with some force into the duodenum. Those which are not sufficiently dissolved to pass the pyloric sphincter are said to excite an an ti -peristal tic movement, whereby they are thrown back into the fundus for further digestion— the sphincter antri pylorici having now relaxed. However, substances which the gastric juice and contractions cannot dissolve will finally pass the pylorus, but they are prob- ably delayed for a considerable time. This succession of movements is continued with a rapidity and regularity varying with the condition of the organ sftid the nature of its contents. They last until the organ , is emptied in part by the absorption of its contents, but mainly by their passage into the small intestine. Each circuit in the fundus probably occupies about three minutes, and gastric digestion as a whole lasts usually from two to five hours. The contraction and relaxation of plain muscle is much slower than that of striped. It is the fundus, and not the pylorus, which serves as a Io8 DIGESTION. reservoir and in which the greater part of gastric digestion occurs. The precise condition of the pyloric sphincter during gastric digestion is unknown. It may have simply an exalted degree of tonicity which does not completely close the opening and which can be overcome by pressure, or it may be tightly contracted and require a distinct nervous dispensation to effect its relaxation for the passage of fluids as well as solids. . It would seem that the length of time for which food is detained in the stomach depends more upon its physical condition than upon its chemical — that is, than upon any stage of digestion which it may have reached ; for it can be shown that fluids pass very quickly into the intestine. The secretory occurrences during these movements are of the greatest importance (see p. 60). Nerve Supply. — The stomach is supplied with pneumogastric and sympathetic fibers. These latter can be traced through the solar plexus, splanchnics and cervical ganglia to the spinal cord. They exert an inhibitory effect on the muscular tissues ; their stimulation causes relaxation. The vagus fibers are motor ; their stimulation causes contraction. But these nerves serve only to regulate the muscular movements. It is the stimulus of food in the stomach which excites gastric peristalsis. It is not stopped by section of these nerves, though it may be interfered with. This stimulation is exerted either directly upon the nerve fibers or upon the ganglia, of the stomach wall. The conditions influencing gastric digestion operate mainly through changes in the quality and quantity of gastric juice. For mention of some of these, together with the nerves con- trolling gastric secretion, see article on Secretion. Resume of Gastric Digestion. — The condition, physical and chemical, of food entering the stomach has been mentioned. Before leaving it the mass has been thoroughly mixed with gastric juice which, aided by the movements of the stomach, has softened and disintegrated and made of it a semi-fluid mechan- INTESTINAL DIGESTION. IO9 ical mixture, chyme. Some of the proteids and albuminoids have been converted into peptones. The carbohydrates have been unaffected, except so far as ( i ) the action of the swallowed saliva has continued slightly upon the starch and (2) the acid juice has converted a little cane sugar into dextrose. The/a^j are likewise unaffected, except to have the globules set free and dissolved and made a part of the general mixture. This, then, is the mass which passes, largely undigested, into the duodenum. Intestinal Digestion. Anatomy. — The small intestine extends from the pylorus to the caput coli, and is about twenty feet in length. It lies in numerous coils which are held loosely in place by a fold of peritoneum running from one side of the great abdominal ves- sels, enveloping the gut, and returning to the parietal wall on the opposite side of the vessels. The fold thus attaching the intestine to the abdominal wall is the mesentery. The distance along the mesentery from this parietal region to the gut is three or four inches, except at the beginning and end of the small intestine, where it is shorter, to bind the tube more firmly in place. The upper eight or ten inches of the small gut is called the duodenum, the next eight feet the jejunum, and the re- mainder the ileum. No anatomical peculiarity separates these parts. Their average diameter is about one and a quarter inches. Histology. — ^The wall of the intestine is in three layers, ex- ternal or serous, middle or muscular and internal or mucous. The external layer consists of the enveloping fold of peri- toneum and needs no description, except that, like serous mem- branes elsewhere, it furnishes a lubricating secretion to provide for the easy gliding of the intestines over each other and over the other viscera. The middle coat has its muscular fibers dis- posed in two layers, an external longitudinal and an internal circular. The latter is the stronger. Between the two mus- DIGESTION. cular layers is the nervous plexus of Auerbach ; between the circular layer and the mucous coat is that of Meissner. These communicate with each other by fibers of extension. The in- ternal mucous coat presents several points deserving mention. Diagram of a Longitudinal Section of the Wall of the Small Intestine. a, villi ; h, Lieberkuhn's glands ; c, tunica muscularis mucosse, below which lies Meissner's nerve plexus ; d, connective tissue in which many blood and lymph vessels lie ; e, circular muscle fibers cut across with Auerbach's nerve plexus^ below it; y, longitudinal muscle fibers; g^ serous coat. ( Yeo.") These are (i) valvulae conniventes ; (2) villi; (3) secreting glands, (a) of Brunner and {b') of Lieberkuhn ; (4) solitary and agminate glands. I. The valvulae conniventes are simply transverse folds or tucks of the entire mucous membrane, each of which extends from one-third to one-half around the circumference of the tube and projects by its middle portion sometimes to the center of the lumen. These small folds, 800 to 1000 in number, extend from about the middle of the duodenum to the beginning of the last third of the ileum and greatly increase the length of the mucous membrane over that of the gut proper. They are not effaced by the passage of food or by other circumstances, for the two sur- faces of the fold which are in apposition are bound together by loose connective tissue. The fold as a whole, however, is reely movable upward or downward in the intestine and has no endency to obstruct the canal. The only function of the val- mlse conniventes is to furnish a greater secreting surface and, Fig Portion op the Wall of the Small Intestines laid open to show the Valvul^e Conniventes. (From Veo dSier Brhiton.) 3y somewhat retarding the passage of the alimentary mass, to jLibject it for a longer time to the digestive fluids. 2. The villi are especially important in connection with ab- sorption, and their description properly belongs under that lead. They are conical elevations responsible for the velvety Fig, Vertical Section of a Villus of the Small Intestines of a Cat. a, striated border of the epithelium; ^, columnar epithelium; tr. goblet cells ; d, central ^mph-vessel ; e, smooth muscular fibers; /, adenoid stroma of the villus in which lymph orpuscles lie. {Kirkes after Klein.) 112 DIGESTION. character of the mucous membrane. They exist in great num- bers from the pylorus to the ileo-cecal valve, covering the valvulse conniventes as well as the general surface of the mucous membrane. The largest are about -^ in. long- and -^ in. in diameter at their base. They are only elevations of the mucous membrane containing a central tube, the lacteal, which is nothing but an intestinal lymphatic. The structure from without inward — that is, from the surface of the villus inward to its center — is (i) a layer of columnar epithelium resting upon a delicate basement membrane ; ( 2 ) lymphoid tissue containing abundant capillaries and connective tissue cells; (3) a thin layer of plain muscle fibers continuous from the intestinal wall ; (4) the lacteal, whose endothelial wall contains many stomata. 3. The glands of Brunner and the crypts of Lieberkuhn, or intestinal tubules, are supposed to produce the succus entericus. The former are chiefly limited to the upper half of the duode- num. The latter exist throughout the small and large intestine. For further description of them see Secretion, page 65. 4. The solitary and agminate glands are not supposed to contribute to the production of the intestinal juice. They are alike in structure, the agminate glands being only a collection of solitary glands. The former are the Peyer's patches, so important in the pathology of typhoid fever. These patches are usually about twenty in number and confined to the lower two-thirds of the ileum, where they occupy that portion of the circumference of the tube opposite the attachment of the mes- entery. Their average dimensions are i x i J4 in. They consist essentially of lymphoid tissue, the separate follicles of which are surrounded by lymphatics and penetrated by blood- vessels. They are covered by villi, but the valvulae conniventes cease at their edges. The solitary glands are more widely dis- tributed than the agminate. The chyme, having passed from the stomach to the small in- testine, encounters three digestive fluids, pancreatic juice, bile TRYPSIN. 113 and intestinal juice. These are, of course, mixed together, but none interferes with the action of the other. Pancreatic Digestion. — For a description of the pancreas and the mechanism of its secretion see page 66. The pancreatic juice has an alkaline reaction, and a specific gravity of about 1040. It quickly undergoes putrefaction, and coagulates if heated. Taken from a recent fistula, it contains of water, about 900 parts per 1000 and about 100 parts solids. Organic substances constitute the main part of the solids. The salts are the phosphates of sodium, calcium and magnesium, the chloride and carbonate of sodium. The important organic substances are the enzymes, trypsin, amylopsin and steapsin. The pancreatic juice is very comprehensive in its digestive properties — more so than any of the other secretions. Some claim that it also contains rennin. Trypsin. — Trypsin, like pepsin, converts proteids into pep- tones. Nothing positive is known of its composition, but it is possessed of the usual characteristics of enzymes regarding tem- perature, etc. It di fleers from pepsin in that its proteolytic action is more powerful and can take place in alkaline media. It will also act in neutral or weakly acid media. The opinion is advanced that while the gastric juice is capable of converting proteids into peptones, as a matter of fact it does not usually carry the process further than the proteose stage, and thus prepares the proteoses for tryptic digestion. It was seen that the successive products of pepsin-hydrochloric digestion are syntonin, primary proteoses, secondary proteoses and peptones. In tryptic digestion it seems that, in the split- ting process, the syntonin (here alkali -albumin) and primary proteose stages are omitted, and the first product is secondary proteoses, which are split into peptones. Furthermore, trypsin goes a step beyond with some of the peptones and converts them into simpler compounds, the best known of which are leucin and tyrosin. These are found normally in the intestinal canal, but 8 114 DIGESTION. the physiological importance of this conversion is not apparent. The opinion that it is a useless sacrifice of useful peptones does not seem warranted. Amylopsin. — The amylolytic enzyme, amylopsin, is identical in its action with ptyalin. For the supposed reactions taking place see page 93. This enzyme is very important, for it has been remarked that the action of ptyalin is probably rather in- consequential, and by far the greater portion of the starch, which constitutes a large part of our ordinary food, must be di- gested in the small intestine — and almost entirely by amylopsin. Steapsin. — Under the influence of steapsin neutral fats take up water and undergo hydrolysis, with the production of glyc- erin and the fatty acid corresponding to the kind of fat which is split up. In the intestine it is probable that only a part of the neutral fats are thus split into glycerin and fatty acids. The fatty acids thus formed unite with the alkaline salts to form soaps, and these soaps, aided by intestinal peristalsis, convert the remaining fats into an emulsion. The products of fat diges- tion are therefore glycerin, soaps and emulsions, all of which can be absorbed in a way to be noted later. While the emulsi- fication of fats under the influence of soaps (fatty acids and alka- line salts) is an undoubted effect, the method of procedure is unknown. It is certain that the emulsification is aided by the presence of bile, although this fluid possesses no fat-splitting enzyme. Bile in Digestion. — The bile is not, properly speaking, a di- gestive fluid, for it contains no enzyme capable of effecting di- gestive changes in any of the foods ; but it so materially affects the action of some of the other fluids that it cannot be over- looked in a discussion of intestinal digestion. The liver, its anatomy, functions, etc., are best considered elsewhere (see page 69, et seq.), and reference will here be made only to the connection of the bile with digestion. So far as the bile acids, glycocholic and taurocholic (com- SUCCUS ENTERICUS. II5 bined to form salts of sodium) are concerned, the fluid is a se- cretion, and it is these which are mainly concerned in the diges- tive process. The production of bile is continuous, but the gall bladder acts as a reservoir in which a part at least of the secretion is stored in the intervals of digestion, to be discharged in greater abundance when chyme enters the duodenum. While the action of bile in most of the digestive functions to be men- tioned is obscure, it is known to have at least these uses : 1. It promotes intestinal peristalsis. 2. It has an inhibitory effect on putrefaction in the intestinal tract. By this it is not to be understood that the bile is directly antiseptic, for it undergoes putrefaction very readily itself, but only that in some way its withdrawal from the substances passing through the alimentary canal allows their more ready disinte- gration. 3. It aids in the emulsification of fats. 4. It promotes the absorption of fats. Recently the state- ment that the bile promotes all kinds of absorption has appar- ently been successfully disproved, but it seems certain that "the bile acids enable the bile to hold in solution a considerable quantity of fatty acids, and possibly this fact explains its connec- tion with fat absorption." (American Text-Book.) The Succus Entericus. — The intestinal secretion, or succus entericus, is a product of the crypts of Lieberkuhn and Brun- ner's glands. It is scanty, of a yellow color and an alkaline re- action. Opinions vary as to what foods are affected by this fluid, but since the more recent experiments have overcome some difficulties in obtaining specimens, the conclusions based upon them seem most reliable. It is said to have no effect on proteids or fats. It contains an amyloljrtic enzyme, which aids the pancreatic juice in converting starch into maltose. It also has an enzyme, invertase, which converts cane sugar into dex- trose and levulose, as well as an alhed enzyme, maltase, which converts maltose into dextrose. The carbohydrates are absorbed Il6 DIGESTION. as dextrose, with the probable exception of lactose. It is mainly cane sugar, maltose (from starch) and lactose that are in the alimentary tract and require to be thus changed to dextrose. It is not out of place to say that ptyalin produces maltose and a little dextrose, and that the pancreatic juice and succus entericus produce maltose and considerable dextrose. The maltose is converted into dextrose during the process of absorption. It is, therefore, customary to say that the carbohydrates are ab- sorbed only as dextrose. Movements of the Small Intestine. — The effect of intesti- nal movements is to force the contents onward through the ileo-cecal valve. Here it is that typical peristalsis is found. The main factor in the passage is the layer of circular fibers. Contraction of these fibers in the upper duodenum may at least be conceived to begin upon the introduction of chyme. The contraction passes down the gut in a wave-like manner, the wave being produced by the contraction of segment after seg- ment of the circular fibers with relaxation just behind the ad- vancing contraction. The tendency of such a movement is to force the alimentary mass along the canal. The longitudinal fibers are probably chiefly concerned in changing the position of the intestine and in shortening the tube, and thus slipping the mucous membrane above the bolus, so that it can be grasped by the circular fibers. A continuation and repetition of these movements, which are slow, gentle and gradual in character, is finally effectual in passing the contents into the colon. It is not probable that anti-peristaltic movements take place normally. Nerve Supply. — Very probably the intestinal movements are naturally excited by the food and by the Me. It is probable also that these stimuli exert their influence through the gangUa of the plexuses of Auerbach and Meissner. The intestine re- ceives fibers from the right vagus and the sympathetic. The former are probably motor (contractors) and the latter inhibi- tory (dilators). Here, as in the stomach, they are probably LARGE INTESTINE. II 7 only regulator's of the movements, without being actually neces- sary to peristalsis. Large Intestine. Anatomy. — The large intestine, known as the colon, is about five feet in length and is divided into ascending, transverse and descending portions. (See Fig. 26.) The sigmoid flexure is the terminal extremity of the descending colon and empties into the rectum. The small intestine communicates with the colon at right angles a little above the beginning of the latter, leaving below the opening a blind pouch, the cecum, or caput coli. From the posterior and inner aspect of the cecum runs off the appendix vermiformis. The diameter of the colon gradually decreases from two and a half to three and a half inches in the cecum to the beginning of the rectum. The as- cending colon passes upward from its beginning in the right iliac fossa to the under surface of the liver, where it bends upon itself almost at a right angle (hepatic flexure). The transverse colon runs directly across the upper part of the abdominal cav- ity to the lower border of the spleen, where an abrupt turn downward (splenic flexure) begins the descending colon. The lower part of the descending colon occupies the left iliac fossa in the shape of the letter S, and is the sigmoid flexure. The rectum, which receives the contents of the sigmoid, is not straight, as its name indicates. It curves (i) to the right to reach the median line, (2) forward to follow the contour of the sacrum, and (3) backward in the last inch of its course. It has the shape of a dilated pouch, its lower termination at the anus being guarded by the powerful external sphincter of stri- ated muscle. Its diameter is greatest below. The vermiform appendix has the three coats common to the intestine, but its muscular coat is ill-developed. The peritoneal coat generally forms a short meso-appendix at the root of the organ. The blood supply of the organ is not abundant. It is greater in the female than in the male, a part of it coming Il8 DIGESTION. through the appendiculo-ovarian ligament. The appendix has no function. The ileo-cecal valve, guarding the opening between the large arid small intestines, is made of two folds, upper and lower, of the muscular and mucous coats, which folds project into the large intestine. The serous coat runs directly over from the small to the large intestine at their point of junction, without being folded inward upon itself, as are the others. This prevents obliteration of the folds by distention. By this ar- rangement the two portions of the gut communicate only by a buttonhole slit, which is easily opened by pressure from the di- rection of the ileum, but which pressure from the cecum tends to close more firmly. (See Fig. 26.) Structure. — The large intestine has the three usual coats. The peritoneal, however, is lacking on the posterior part of the cecum, ascending and descending colons, these parts being bound down closely and having no meso-colon. The sigmoid is entirely covered, as is the upper third of the rectum. The middle third of the rectum has no serous coat behind, being firmly held in place, while the lower third lacks this coat en- tirely. The muscular coat is peculiar, in that its longitudinal fibers are collected into three quite strong bands, evident to the eye. When the rectum is reached they spread out over the whole circumference of that part of the canal. These bands are shorter, as it were, than the wall proper, and the consequence is that the whole length of the large intestine is gathered up into a number of pouches. The mucous coat is paler than that of the small intestine, presents no villi and is rather closely adherent to the subjacent parts. In it are found glands corre- sponding in appearance to the crypts of Lieberkuhn, and they are so classed ; but they probably secrete mucus only. Some solitary lymphoid follicles also usually exist here. Changes Taking Place in the Alimentary Mass in the Large Intestine. — Most of the substances which enter the large BACTERIA IN DIGESTION. II9 intestine have resisted the action of the various digestive fluids and are on their way to be discharged in defecation. Doubtless, though, some materials undergo digestive changes in the colon, not under the influence of any secretion there formed, but of the intestinal juice with which they are incorporated on leaving the ileum. The secretion of the mucous membrane of the large intestine furnishes no digestive enzyme, and the changes going on in the alimentary mass (now feces) are chiefly due to ab- sorption. By some unknown process, however, rectal aliments of an easily digestible nature are absorbed, and that in a nutritive form. The consistence of the fecal matter increases in its pas- sage through the colon, owing to the absorption of its more fluid portions. The bile pigment is responsible for the char- acteristic color. The odor is mainly due to bacterial decomposi- tion, but partly to the secretion of the mucous membrane. Bacteria in Intestinal Digestion. — The entrance of the bile and pancreatic juice into the duodenum changes to alkaline the previously acid reaction of the chyme. But it is found that, when an ordinary mixed diet is given, the mass leaving the ileo- cecal valve has an acid (organic) reaction, and that the pro- teids have not undergone putrefaction. The alkaline medium of the upper intestine favors bacterial activity, and it would seem that proteid putrefaction would ensue. But it is supposed that in health these bacteria set up fermentative changes in the carbohydrates, with the production of organic acids which in- hibit proteid putrefaction, and account for the acid reaction at the ileo-cecal, valve. When the mass has entered the colon the acidityis soon overcome and putrefaction is the usual consequence. It can be seen how readily this delicately adjusted balance may be disturbed by errors in the proper kind and proportion of food, etc. Some of the products of bacterial activity upon carbo- hydrates and proteids are leucin, tyrosin, indol, skatol, phenol, lactic and butyric acid. The object of the production of these substances is unknown. 1 20 DIGESTION. Composition of Feces. — It seems at present that the main bulk of fecal matter is made up of substances which are contained in the intestinal secretions, and the alimentary canal is more im- portant in excretion than was formerly supposed. These sub- stances are waste matters from tissue metabolism. Besides these materials, the feces normally contain indigestible and undi- gested matters, inorganic salts, stercorin, mucus, epithelium from the intestinal wall, coloring matter and substances result- ing from bacterial activity. Stercorin is the converted form of cholesterin, a constituent of the bile. The coloring matter is from the pigment {bilirubin) of the same fluid. Of the bac- terial products the most important are indol and skatol. They represent proteid putrefaction ; they are responsible for the fecal odor ; hence the characteristic difference in the odor of the con- tents of the ileum and colon. The reaction of fecal matter varies. The amount for the average person is about four and a half ounces per day. Gases. — Hydrogen, nitrogen and carbon dioxide are found normally in the small intestines. They serve to keep the tube patulous, and avoid obstruction, and also to prevent concussion. In the large intestine bacterial activity increases the number of gases present. Here, in addition to those found in the small intestine, there are carburetted and sulphuretted hydrogen, with others at times. Movements of the Large Intestine.— The muscular contrac- tions of the colon forcing the feces onward are of the same general character as those of the small intestine, though less violent. The contents thus passed analward by peristalsis accumulate gradually in the sigmoid flexure until defecation occurs. Defecation. — The act of defecation is both voluntary and in- voluntary ;■ — voluntary in the relaxation of the external sphincter and involuntary in the peristalsis which brings the fecal matter to present at that muscle. It is probable that the rectal pouch DEFECATION. 121 does not usually contain feces, but that the desire to defecate is brought about by the entrance of the mass into it from the sig- moid. Then, if the desire be obeyed, peristalsis of the non- striated muscular coat continues, the internal sphincter of plain muscle relaxes, as does also the external of striped muscle, and evacuation takes place. Usually, by an eifort of the will, evacuation can be voluntarily prevented by maintaining the tonic contraction of the external sphincter. If the desire to defecate be disregarded, the fecal accumulation probably returns to the sigmoid, leaving the rec- tum comparatively empty. The act of evacuation is commonly aided further by voluntary contraction of the diaphragm and abdominal muscles. The lungs are filled, " the breath is held ' ' (forcing down and holding the diaphragm), and the abdominal muscles likewise contract powerfully to compress the viscera and force the feces into the rectum. Pressure on the afferent nerves of the rectum probably sets up the desire to defecate, and the contraction of its walls, as well as the relaxation of the internal sphincter, is a reflex act. The center is in the lower segment of the cord, but it is connected with the cerebrum, as is shown by emotional influences on the act. The average time occupied in the passage of the residue of an ordinary meal from the mouth to the rectum is about 24 hours. Something like 12 hours of this is thought to be spent in the large intestine. While it has been endeavored to establish clearly the separate action of each fluid with which the aliment comes in contact, it is to be remembered that they form a mixture, the combined activity of whose component parts results in the extraction of all the nutritive material from the bolus in its long journey through the gastro-intestinal tract. It can hardly be said to be still at any time during that passage, the continual peristalsis to which it is subjected facilitating both the chemical action of the enzymes and the physical phenomenon of absorption. 122 ABSORPTION. Resume of Digestion. — Digestion is a chemical process, whereby the different classes of foods are changed so as to become capable of absorption and assimilation. (i) The inorganic foods are not digested, but are simply dissolved and absorbed. (2) The proteids and albuminoids are digested in the stomach by pepsin-hydrochloric acid and in the small intestine by trypsin of the pancreatic juice. (3) The starchy carbohydrates are digested in the mouth by ptyalin, whose action is continued slightly in the stomach, and in the small intestine by amylopsin of the pancreatic juice. The sugar car- bohydrates are digested slightly perhaps in the stomach by hydrochloric acid, and in the small intestine by invertase and maltase of the succus entericus. (4) The fats are digested in the upper small intestine by steapsin of the pancreatic juice, aided by the bile. Digestion is usually completed in the small intestine, but may be continued in the colon by the passage into it of undigested particles with the intestinal secretions. All the digestive secretions are alkaline in reaction except the gastric juice. It remains to be seen how these various foods find exit from the alimentary canal to be appropriated by the cells. It may be said in a general way that they are absorbed as soon as they are digested, and therefore from that part of the alimentary canal in which they undergo this change. (C) ABSORPTION. Obviously digested materials are of no service in the vital economy until they are absorbed — first by the circulation and then by the tissues themselves. Here we will consider only their absorption from the alimentary canal, which process, in contradistinction to the other, may be termed external absorp- tion. While it is known that the laws of diffusion and osmosis out- side the body are largely responsible for absorption within the OSMOSIS. 123 organism, there are many phenomena in connection with that process which cannot be explained under these laws, and which are indeed, in some cases, at variance with them. The only explanation at present to be offered of anomalous action is to refer it to some peculiar property inherent in the cells them- selves — the epithelium in case of the alimentary canal. So profoundly important in connection with physiological activity are the laws of osmosis outside of the body, and what is known concerning the mutability of those laws inside the body, that a brief consideration of the subject seems necessary to an intelli- gent conception of many vital phenomena. Osmosis. — When two different kinds of gases are brought in contact they mingle with each other, making a homogeneous mixture. This is diie to the continual motion of their molecules. When two different kinds of liquids are brought in contact, a homogeneous mixture results for the same reason — unless the liquids be non-miscible, as oil and water. If now the liquids happen to be separated by a membrane permeable by both, the result, while it may be delayed, will be the same. If, further, these liquids hold in solution substances the molecules of which can penetrate the interposed membrane, there will likewise be an interchange of these substances, and the fluids on both sides will come ultimately to have the same composition. This pas- sage of liquids and dissolved matters through an animal mem- brane is known as osmosis. It must be remembered that in the body particularly the inter- posed membrane may be permeable to the solvent, water, and less so, or not at all, to the dissolved substances. Materials which will in solution pass through a membrane are called crystalloids ; those which will not, colloids. If simple water be on both sides of the membrane, the interchange continues because of incessant molecular motion ; but the currents equalize each other, and no alteration in volume or composition becomes apparent. But if to the water on one side there be added a solution of some crystal- 1 24 ABSORPTION. loid, as sugar, the excess of water will pass to that side. The crystalloid in solution is said to exert an osmotic pressure, and that pressure depends upon the density of the solution. In course of time, however, the crystalloid passing itself through the mem- brane, conditions of equal volume and density will be established on the two sides of the membrane, and osmosis in either direction will cease to be apparent. But if the membrane be non-penneable to the dissolved substance, an excess of water will pass to the colloid side and will continue so to pass until finally it will be inhibited by hydrostatic pressure on that side. This is taken as the measure of osmotic pressure for the colloid. All substances in solution, whether crystalloids or colloids, exert a certain osmotic pressure ; that is, they may be said to interfere with the passage of a current from their side of the membrane, and that interference depends on the number of molecules in solution, or, in other words, upon the density of the fluid. A fanciful but striking illustration refers the explana- tion to the continual molecular motion : the molecules of the dissolved substance act as a screen to protect the membrane from the water molecules, which are incessantly moving against it, and consequently, in a given time, more molecules of water will strike and pass through the membrane on the unscreened than upon the partially screened side. Evidently the number of molecules in solution (the density) has a material influence upon the escape of water from that side. Of course, since a crystal- loid finally passes to the less dense side in sufficient quantity to establish an equilibrium, the effect of its osmotic pressure is only temporary ; but while the osmotic pressure of a colloid may be less than that of a crystalloid, its effect is inclined to be perma- nent. For instance, if a hypertonic solution (one whose density is greater than that of blood serum) of sodium chloride be in- jected into the blood, the first effect is to cause an increased flow of water to the vessels, but soon enough sodium chloride passes out by osmosis to raise the density of the extra-vascular fluids, OSMOSIS. 125 and thus to cause an escape of water from the vessels. On the other hand, the osmotic pressure exerted by the proteids of the blood is comparatively small. But since they are here chiefly as colloids and tend to maintain the concentration of the circu- lating fluid, their effect is a permanent factor influencing absorp- tion into the blood-vessels. Isotonic and hypotonic solutions are those having equal and less densities respectively as compared to blood serum. Hypotonic solutions are most easily absorbed ; hypertonic least easily. Application of these principles explains the rationale of giving some medicines in dilute and others in concentrated form. As to the direction of the current, the one of greater volume may be called the endosmotic and the one of lesser volume the exos- motic. For example, the current in ordinary absorption from the alimentary canal is usually termed endosmotic, though it may be reversed, as when magnesium sulphate is given. When it is said that the greater current is from the less dense to the more dense fluid, no reference is had to the direction of the solids in solution. If there be only one solid concerned, it will be the one responsible for the difference in density, and, if it be a crystalloid, it will pass through the membrane until the density on the two sides is equal, and its direction will be opposite to that of the water. If on the side of less density there be another crystalloid in solution, but in less quantity than the solid on the side of greater density, it will pass in the di- rection of the greater current of water until conditions of equal concentration with respect to this solid are established. In the laboratory the final result in any case of dissolved crystalloid or crystalloids is two liquids absolutely identical in composition. A rectal enema, hypertonic with sodium chloride, will give up sodium chloride to the blood, but it may at the same time draw upon that fluid for urea, for example. This is suggestive when an attempt is made to explain the products of glandular secre- tion, excretion, etc. It may be that the capillary walls are 126 ABSORPTION. permeable to certain substances in certain situations and not in others. In the body it may be said that well nigh all the vital func- tions are dependent upon osmosis. There are fluids separated by animal membranes everywhere. In the alimentary canal, for instance, is a fluid containing matters fit to be absorbed ; ramifying in the wall of that canal are blood and lymph capil- laries filled with fluid ; while separating the two is an animal membrane consisting of the alimentary epithelium, a little con- nective tissue and the endothelial lining of the capillaries. These are conditions most favorable for osmosis, but the osmotic laws of the laboratory are by no means immutable in the body. From what has been said of osmosis in general, and consider- ing variations due to conditions of circulation, etc., the follow- ing facts seem clear as to absorption in the body : ( i ) The substance must be in a liquid or gaseous state ; (2) it must be diffusible ; (3) the membrane must be permeable ; (4) the greater current is toward the more dense solution ; (5) the less dense the solution the more quickly will it be absorbed ; (6) the greater the pressure in the vessels the less rapid will absorp- tion into them take place; ( 7 ) absorption is more rapid the more rapid the blood current (continually preventing " satu- ration " of the adjacent blood) ; (8) the higher the temperature the more rapid is absorption ; (9) the "vital condition" of the cells is the most important factor of all. A thorough grasp of these principles and probabilities will do much to clarify almost all the phenomena of vital activity, and many questions of a pathological nature. Absorption from the Alimentary Canal. — It has been said that all digested materials must find their way into the blood. It is to be remembered that there are two ways by which they reach the vascular circulation : first, by direct absorption into the capillaries of this system, and second, indirectly, by absorp- tion into the lymphatic circulation and passage thence to the ABSORPTION FROM DIFFERENT REGIONS. 1 27 left subclavian vein. Those lymph capillaries which are con- cerned in this absorption occupy the villi, and are called lac- teals. (^) From the Stomach. — Since all classes of foods except fats have been partly digested in the stomach, it follows that all ex- cept fats may be absorbed here. However, as a matter of ob- servation, the stomach is of much less importance in absorption than was once thought. Practically, it is found that water and salts are passed quickly on toward the duodenum and are not largely absorbed in the stomach. Sugar and peptones are also found to be absorbed rather sparingly here. All these sub- stances can undoubtedly be absorbed by the gastric mu- cous membrane, and their complete absorption is prevented only by their removal through the pylorus. It is interesting to note that alcohol and condiments, like pepper and mustard, greatly hasten absorption, either by increasing the blood flow or by directly stimulating the "vital activity" of the epithelium. (^) From the Small Intestine. — Here absorption of all classes of food is possible, and here in fact most of the foods are absorbed. The digestive influences are more active upon all the aliments, the nmcous membrane is well adapted to absorption by reason of its valvulse conniventes and its villi, and the food necessarily remains in the small intestine for a considerable time. The fats are absorbed in the upper part of the small in- testine ; for they pass into the lacteals of the villi, and these do not exist in the lower ileum. The fluids swallowed are almost completely absorbed here, but their place is taken by the in- testinal secretions. The proteids are absorbed to the extent of 85 per cent., more or less, before reaching the large intestine, and the carbohydrates almost entirely disappear. ( C) From the Large Intestines. — The absorption process in the large intestine is quite active. The passage of the mass through it is slower, and even occupies an absolutely greater time than the journey through the much longer small intestine. 128 ABSORPTION. The consistence of the contents progressively increases owing to continual absorption of the fluid portions, until the pultaceous mass received by the cecum becomes almost solid in the sigmoid. The degree of consistence may be said to be greater the longer the sojourn in the large intestine. The proteids and carbohydrates which have escaped absorption in the small in- testine are disposed of here, partly by bacterial decomposition, and do not appear as such in the feces. The absorption of easily digestible substances in solutions, such as eggs, etc., from the lower bowel, although there is no digestive enzyme there, is a matter of common observation, but one which lacks explanation. Forms in Which the Different Classes are Absorbed, i. Water and Salts. — Of course, water is absorbed in connection with all the foods as a vehicle for them, but water and salts as such have been shown to be absorbed sparingly in the stomach. They are soon conveyed to the small intestine, where their rapid disappearance ensues. However, they may be absorbed any- where in the alimentary canal. The loss of the water from the alimentary mass in the upper small intestine is compensated for by the secretions, so that the fluidity of the contents is not materially affected until the colon is reached. Here absorption of water is active, and the mass becomes more and more soUd as the rectum is approached. 2. Proteids. — It is agreed that the first object of proteid di- gestion is to render the nitrogenous foods more diffusible. It is also agreed that the end products of such digestion, so far as alimentary absorption is concerned, are proteoses and peptones ; and the natural conclusion, supported by experimental evidence, is that these represent the forms in which- the proteids are ab- sorbed. True, leucin, tyrosin, etc., further end products of proteolysis, are formed, but these cannot be absorbed. The opin- ion that proteoses and peptones are the absorbable forms of pro- teids is correct, for by far the largest part of these foods are ab- sorbed in this shape. It is supposed also that syntonin at least ABSORPTION OF DIFFERENT FOODS. 1 29 can itself be sparingly absorbed from the alimentary canal, while the phenomena of rectal absorption would point to the conclu- sion that proteid absorption in other shapes is possible. Prac- tically, however, proteoses and peptones may be regarded as the products of proteid digestion, and their production as the object of proteolysis. But, although these substances are absorbed by the blood-ves- sels, the artificial injection of them into the veins occasions un- toward effects, or at least their rejection through the organs of excretion. Furthermore, proteoses and peptones cannot be de- tected in the blood during alimentary absorption. It follows, then, that in their passage from the alimentary canal to the blood they undergo some change whereby they lose their identity and are no longer recognizable as such. It is claimed that they are con- verted into serum-albumin, and this is probably true. One ef- fect at least of the change is that they are now (in the blood) less diffusible, more complex, and consequently remain more easily a constituent part of that fluid. The proteids enter the radicles of the portal vein. 3. Carbohydrates. — The sugar of the blood is dextrose, and if carife sugar be introduced into the veins it is rejected by the urine without being changed. It may be said that, with a few exceptions, all the carbohydrates are converted into dextrose or dextrose and levulose, before entering the blood. This form of sugar is easily oxidized in the tissues. It is conveyed directly to the liver by the portal vein. 4. Fats. — The digestive end of the fats has been seen to be emulsions and soaps. They pass into the intestinal lymphatics, or lacteals. Their absorption is a mechanical process. They enter and pass through the epithelial cells and basement mem- brane of the villus. Having thus passed into the stroma of the villus, their entrance into the lacteal is easy ; for undoubtedly lymph spaces in the stroma are connected with the stomata of the central lymph capillary, and there is a more or less constant 9 130 ABSORPTION. flow of lymph through these spaces toward the lacteal. The tendency, therefore, of the fats to enter the lacteal is physically natural. It is a curious fact that the peptones and sugars, having penetrated the lining epithelium of the villus, enter the blood instead of the lymph capillaries. A number of circumstances, such as the rate of absorption, the persistent direction of the current toward the blood in the face of superior pressure, the disappearance of non-osmotic sub- stances from the canal, etc., are frequently at variance with laboratory experiments. Application of the laws of osmosis to the vital processes is seemingly subject to many variations, and explanation of many of the phenomena of absorption in the body waits upon a clearer understanding of the so-called " vital activity ' ' of the tissues. Resume of Absorption, — Absorption from the alimentary canal follows digestion and is dependent upon osmosis. The foods can be absorbed from that part of the canal in which they are digested, and speaking roughly, this may be said to occur ; but this statement is subject to modification as the result of on- ward movements of the food, (i) The inorganic foods are ab- sorbed throughout the whole gastro-intestinal tract, but mainly in the small intestine. (2) /"w/^/i/ absorption begins in the stomach, is most marked in the small intestine and is concluded in the colon. (3) Carbohydrates are absorbed slightly in the mouth and stomach, but chiefly in the small intestine and slightly in the large intestine. (4) Fats are absorbed in the villous region of the small intestine. All foods, except fats, are ab- sorbed by blood-vessels, and go almost entirely to the liver through the portal vein. The fats are absorbed by the intesti- nal lymphatics, the lacteals, and go indirectly to the blood-cur- rent through the thoracic duct into the left subclavian vein. It is claimed that while the offices of the intestinal blood and lymph capillaries are thus clearly defined as regards the food they ab- sorb, they slightly overlap each other in their action, and are consequently not mutually exclusive as regards this function. CHAPTER V. THE CIRCULATION. All the cells in the body are continually producing effete materials and must consequently be continually appropriating new matter if they are to continue to live and functionate. This new matter is supplied to the cells by the blood, and it is only for this purpose — that of a vehicle — that the blood and circula- tion exist. The circulating fluid is not depleted in health be- cause its supply of nutritive materials is being continually re- plenished by the entrance of oxygen from the lungs and of the products of alimentary digestion. Nor does it become over- charged with effete materials, because they are being continually removed by the excretory organs. Since the cells not only need to appropriate the nutritive materials furnished by the ordinary foods, and to discharge the eifete solids and liquids of metabolism, but also to appropriate and discharge gases — oxygen and carbon dioxide — two systems of circulation are provided, the systemic and the pulmonary. The systemic supplies with solid, liquid and gaseous nutriment all the tissues throughout the body ; it likewise collects, directly or indirectly through the lymph, from those tissues the solid, liquid and gaseous materials which are no longer useful and which are to be eliminated. On the other hand, the pulmonary circulation exists solely for the purpose of ridding the blood of carbon dioxide and of supplying it with oxygen. It will, there- fore, be necessary to consider (I) the mechanism of the circu- lation and (II ) the blood itself. 131 132 CIRCULATION. Fig, (I) MECHANISM OF THE CIRCULATION. The study of the mechanism of circulation may. be conven- iently divided into a consideration of the (A) heart, (B) arte- ries, ( C ) capillaries and (Z>) veins, together with whatever may be known concerning phenomena occurring in connection with the passage of blood through each. (A) The Heart. Anatomy. — The heart is bilateral, and may be considered as consisting of two separate but similar organs, each having two cavities, an upper and a lower, analogous in shape and function to the corresponding cavi- ties on the opposite side. As it is, these two organs, right and left, are applied and connected the one to the other, but there is no communi- cation between them except indi- rectly through the vessels which run to and from them. The right heart exists only for the purpose of keep- ing up the pulmonary circulation. Since the two sides of the heart act synchronously it will be unneces- sary, in describing the various move- ments, etc., to consider them sepa- rately. The human heart is a cone-shaped, hollow, muscular organ situated in the thoracic cavity behind the ster- num. Its base is in the median line and looks backward, upward and to Scheme of the Circulation. a, right, d, left, auricle ; A, right, B, left, ventricle ; i, pulmonary artery ; z, aorta; i, area of pulmonary, /(T, area of systemic, circulation: *;, the superior vena cava; G, area supply- ing the inferior vena cava, u ; d, d^ intestine ; fit, mesentric artery ; q, portal vein ; Z, liver ; A, hepatic vein. {^Landois.) THE HEART. 1 33 the right ; its apex is three inches to the left of the median line in the 5th intercostal space and looks forward, downward and to the left. Its weight in the male is 10—12 ounces, in the female some 2 ounces less. It is held in place by the great vessels which run off from its base to be attached to the posterior wall of the thorax. The heart rests by about the apical half of its posterior surface upon the upper surface of the diaphragm. The main portion of the anterior (upper) surface consists of the right ventricle ; the main portion of the posterior (under) surface con- sists of the left ventricle. The left ventricle continues farther forward than the right and constitutes the apex of the heart. From the diaphragm is sent over the surface of the heart a double fold of the serous lining of the thoracic cavity, which is here known as the pericardium. The pericardial sac is of a cone shape with its base downward on the diaphragm and its apex upward embracing the origin of the great vessels, so that the relative positions of the apex and base of the heart and peri- cardial sac are the reverse of each other. Leaving the dia- phragm the pericardium runs upward over the heart, but not in contact with its substance, until it has passed a little way beyond the base and has enveloped the vessels ; then it turns backward, folded inward upon itself, to cover closely, from base to apex, the whole extent of the viscus, and to leave an interval between it and the outer (upward) layer. The outer layer consists of an outer fibrous and an inner serous division ; it is only the serous layer which is reflected backward over the heart, the fibrous being continuous with the fibrous tissue in the vessel walls. Thus it is that the heart is covered by a slippery membrane which is enclosed by a second equally slippery. These membranes secrete a small amount of fluid for lubrication, and the friction of the beating heart is thereby reduced to a minimum. The amount of fluid in the pericardial sac is usually about one drachm. The cavities of the heart are lined by a thin serous membrane similar to the visceral pericardium called the endocardium. t34 CIRCULATION. The heart has four distinct cavities, two on each side — one at the base, the auricle, and one at the apex, the ventricle ; the latter is the larger. The Right Auricle.— This cavity has a small sinus running off from it anteriorly known as the auricular appendix. There Fig. 32. Interior of Right Auricle and Ventricle Exposed by the Removal of a Part OF their Walls. (From Veo after A/lgn Thompson.'] I, superior vena cava; 2, inferior vena cava; 2', hepatic veins ; 3, 3', 3", inner wall of right auricle; 4, 4, cavity of right ventricle ; 4', papillary muscle ; 5, 5', 5", flaps of tricuspid valve ; 6, pulmonary artery in the wall of which a window has been cut ; 7, on aorta near the ductus arteriosus; 8, 9, aorta and its branches; 10, ri, left auricle and ventricle. THE HEART. 135 are these openings into the right auricle : ( i ) Those for the two vencR cava, (2) the orifice of the coronary vein, (3) the foramina Thebesii, and (4) the auriculo-ventricular. The foramina The- besii are the orifices of small veins bringing blood from the Fig. 33. The Left Aukicle and Ventricle Opened and Part of their Walls Removed TO Show theik Cavities. (From Yeo ^htr Allen Thompson.) I, right pulmonary vein cut short ; i', cavity of left auricle ; 3, 3", thick wall of left ven- tricle ; 4, portion of the same with papillary muscle attached ; 5, the other papillary muscles ; 6, 6', the segments of the mitral valve, 7, in aorta is placed over the semi-lunar valves. ( Yeo. ) 136 aRCULATION. heart substance, though most of it enters by the coronary vein. The auriculo-ventricular opening is the communication between the right auricle and ventricle. The valves are the (i) coronary and (2) Eustachian. The coronary is a semicircular fold of the endocardium preventing regurgitation into the coronary vein. The Eustachian is a similar fold between the orifice of the inferior vena cava and the auriculo-ventricular opening. In fetal life it is supposed to direct the current of blood toward the foramen ovale. The fossa ovalis is a depression in the septum auriculorum represent- ing the fetal foramen ovale, and the annulus ovalis is the promi- nent margin of the same. The Right Ventricle. — This is below the right auricle. It is conical in shape. It extends nearly to the apex. The openings in this cavity are (i) the auriculo-ventricular and (2) that for \ht pulmonary artery. The auriculo-ventricular is at the base of the ventricle, is about an inch in diameter and is guarded by the tricuspid valve. The opening for the pulmo- nary artery is near the septum ventriculorum. The valves are the ( i ) tricuspid and ( 2 ) pulmonary semilunar. The tricuspid valve guards the auriculo-ventricular opening and consists of three triangular segments attached by their bases to the circumference of the orifice. Each is a double fold of endo- cardium between the layers of which, extending nearly up to the free edge, is some fibrous tissue ; this arrangement makes the free approximating edges thin and pliable, while the remainder of the cusp is thicker and stouter. The division into three separate segments is frequently not distinct at their bases. Attached to the ventricular surfaces of these cusps are the chorda tendinecB, which are delicate but stout inelastic bands; most of them are attached by their opposite ends to the papillary muscles. The columna carnem are muscular columns, some of the mare simple ridges running along the inner sur- face of the ventricular wall; some are rather thick muscular THE HEART. I37 bands attached by their two extremities to the wall ; while a third set constitute the musculi papillares, projections of mus- cular substance, more or less conical in shape, giving origin by their apices to most of the chordae tendineae. There are pri- marily only two of these muscles, but they subdivide into a large number. Some of the chordae tendineae spring directly from the muscular wall. The semilunar valves guard the opening of the pulmonary artery. They are three entirely separate segments of semi- lunar shape. They are attached by their long curved margins to the circumference of the artery just where it springs from the muscular substance of the ventricle. The three completely sur- round the orifice. They are of the same structure as the tri- cuspid. The free straight border of each cusp has running along it a very delicate band of fibrous tissue, while just in the center of this border is a small fibrous nodule, the corpus Arantii. From the corpus radiate toward the attached convex border of the leaflet delicate inelastic fibers. But on either side of the corpus Arantii is a small crescentic area in which no fibrous tissue is interposed between the two layers of the serous mem- brane. These little crescents, lunula, reach by one of their tips the corpus Arantii, by the other the attached border of the seg- ment. When the valves project into the artery during the passage of blood through the opening they do not lie in contact with the arterial wall because there is a kind of pouch in the artery behind each segment, the sinus of Valsalva. The Left Auricle. — This, like the right, presents a main cavity and an auricular appendix. The openings are ( i ) those of the four pulmonary veins and (2) the auriculo-ventricular. The two veins from the left lung often unite before they reach the heart so that there are only three pulmonary openings. The auriculo- ventricular opening is a little smaller than the one on the right. The Left Ventricle. — This is the most interesting of the car- diac divisions. 138 CIRCULATION. The openings are (i) the auriculo-ventricular and (2) the aortic. The auriculo-ventricular opening is below and to the left of the aortic orifice. The iialves are the ( i ) mitral and ( 2 ) aortic semilunar. The mitral valve guarding the auriculo-ventricular opening, resem- bles in general structure and function the tricuspid, except that it has two instead of three cusps and is thicker and stouter, as are the chordae tendinese attached to it. Smaller segments are frequently placed between the angles of the two flaps. The semilunar valves do not differ from the pulmonary semilunar, except that they are larger, thicker and stronger. The corpora Arantii are also larger. The sinuses of Valsalva are present in the aorta as in the pulmonary artery. The columnce carnea are more numerous and intricate than in the right ventricle. There are here also two primitive musculi papillares. The sep- tum ventriculorum is thicker than the septum auricularum, ex- cept toward the base of the ventricles where it is thin and fibrous. The average thickness of the walls of the several cavities is as follows : right auricle one-twelfth inch, left auricle one-eighth inch, right ventricle one-sixth inch, left ventricle one-half inch. These differences are in accord with the muscular work required of each division. Capacities of the Cavities. — The auricles are about one-third less capacious than the ventricles. The absolute capacity of any of these cavities under normal conditions is a subject of speculation, but probably the ventri- cles ordinarily expel at each systole about four ounces of blood each. They must expel equal amounts, and their capacities are at least practically the same. Structure. — In structure the heart consists of fibrous rings surrounding the orifices and of muscular fibers attached to these. The muscular fibers of the auricles and ventricles are inde- pendent of each other. The auricles have (i) a superficial set COURSE OF BLOOD. 1 39 of fibers common to both, and (2) a deep layer proper to each. The superficial set run as a rule transversely ; some of the deep set loop from the fibrous margin of the auriculo-ven- tricular opening over each auricle, while others run circularly. The ventricular fibers may be said in a very general way to be disposed in the same manner as the auricular. They compose a very intricate and complete network, the contraction of which practically obliterates the ventricular cavities. It seems that at least some of the longitudinal fibers dip into the substance of the heart at the apex to form the columnae carneae. The blood supply of the heart is furnished by the coronary arteries. Course of Blood Through the Heart and Vessels. — The right auricle receives all the venous blood from every part of the body — from the head and upper extremities through the vena cava descendens, from the trunk and lower extremities through the vena cava ascendens, and from the heart muscle through the coronary sinus and the foramina Thebesii. From the right auricle the venous blood enters the right ven- tricle through the right auriculo-ventricular opening ; thence it is passed to the lungs through the pulmonary artery. Having traversed the pulmonary capillaries and become arterialized, it converges to the two pulmonary veins in each lung; these con- vey it to the left auricle which passes it through the left auriculo- ventricular opening into the left ventricle ; being forced out of this cavity through the aortic opening into the aorta, it is dis- tributed to all parts of the system, traverses capillaries, becomes venous and is collected to be carried back to the right auricle and begin the circuit again. Ordinarily blood passes through only one set of capillaries before being returned to the heart, but the blood of the portal vein, having already passed through the capillaries in the ali- mentary canal, spleen and pancreas, must traverse the hepatic system of capillaries as well. The blood of the renal vein also 140 CIRCULATION. passes through two systems of capillaries in the kidney substance. With the exception of the pulmonary artery (and its branches) which carries venous blood, and the pulmo- nary veins which carry arterial blood, arteries normally contain arterial and veins venous blood. The departure from the rule in the nomenclature of these vessels is warranted by their struc- ture, functions, etc. Contractions of the Heart. — The ventricles operate as a pair of pumps, the action of which is somewhat modified by the presence of the auricles. Their contractions force the blood on the one hand into the lungs and on the other into the systemic circulation, while in the interval of their contractions they are filled partly by the action of the auricles. Both auricles and both ventricles contract at exactly the same time and in the same manner, so that a description of the mechanism of one side is sufficient to explain that of both. Each cavity of the heart undergoes periodic contractions when it forces out the contained blood, and periodic dilatations when it is receiving blood. The contraction is the systole and the dilatation the diastole. These terms have come to refer to the contractions and dilatations of the ventricles only, unless the adjective "auricular" be prefixed. When the heart of a living animal is exposed by opening the chest and pericardial sac it can be seen that there is a regular succession of cardiac movements followed by periods of rest. Beginning with the auricle, it is found that it receives blood from the incoming veins during most of the cardiac cycle — dur- ing all of it except the short period occupied by its systole. The auricular dastole is, therefore, much longer than the auric- ular systole. When the cavity has been filled the auricle con- tracts suddenly and forces its contents into the ventricle. Re- gurgitation into the veins may occur to a slight degree, but the contraction of the auricular fibers surrounding the venous open- ings together with the relatively large auriculo-ventricular orifice insures the passage of the blood into the ventricle. CHANGES IN HEART. I41 Previous to, as well as during, the auricular, and subsequent to the ventricular, systole, blood is entering the ventricle from the auricle. When the auricle contracts the addition of its blood to that which has already entered the ventricle completely fills it and brings on ventricular systole. The blood, prevented from regurgitating into the now dilating auricle by the auriculo- ventricular valve, forces the semilunar valve and enters the pulmonary artery (or the aorta as the case may be). As soon as the ventricle has emptied itself it begins to dilate slowly and continues to do so until another auricular systole com- pletely fills it. The ventricular diastole is also longer than its systole in health. The heart is in complete repose in all its parts from the end of ventricular to the beginning of auricular systole. During that time both cavities are in diastole, but it does not embrace the entire diastolic period of either. Changes in Shape and Size of the Heart and Arteries. — In diastole the muscular walls of the heart are soft and flaccid ; in systole they are hard and firm. During systole the cavities de- crease in size ; during diastole they progressively increase to the maximum at the beginning of the succeeding systole. There is no change at any time in the actual size of the heart muscle, but in that of the cavities. Ventricular systole charges the great arteries, already full, with an additional amount of blood, causing them to expand and lengthen ; during diastole they shorten and shrink. The changes in size, position, etc. , of the beating heart are connected almost entirely with the action of the ventricles. Their contraction makes the heart harder, smaller in circum- ference and shorter, but more pointed. The apex is also raised slightly and sweeps toward the right, while the whole organ is twisted upon itself in the same direction, and is protruded against the chest wall, notwithstanding the shortening of its long diameter. 142 CIRCULA.TION. The fact that the heart becomes harder, smaller and shorter in systole, considering that it is a hollow muscular organ, cor- responds with muscular contractions elsewhere. The twisting of the heart, the raising of the apex and its movement toward the right are mechanical consequences of the course of the ven- tricular fibers. The fact that the organ is shortened in systole, together with the protrusion of the apex against the chest wall, presupposes the locomotion of the organ forward away from the posterior 'chest wall. This is what occurs, and it is caused by the sudden distention and lengthening of the great arteries at the base. When the auricles are properly exposed they can be seen gradually to enlarge the base of the heart during their diastole, but they do not push it forward for obvious reasons. Their walls are so thinlhat their color is affected by the contained blood, the right having a dark blue and the left a red color. The apex beat, or impulse, of the heart can usually be seen or felt in the fifth left intercostal space. This beat takes place upon ventricular systole, and is caused by the locomotion forward of the heart, the hardening of the ventricles and the movement of the apex upward and to the right. The beat, when the chest wall is thin, can be seen to move slightly upward and to the right during its production ; it is also noticed that immediately around the protrusion the tissues are slightly drawn in, owing doubtless to the "suction " of the contracting ventricles. Action of the Valves. — A thorough understanding of the ar- rangement and action of the cardiac valves is of the greatest im- portance. It is through their intervention that the heart is en- abled to keep the blood in circulation — and always in the same direction. Without them the pump would be useless, so much so that the smallest lesion of a single one of them may be fol- lowed by embarrassed circulation and subsequent death. The mitral and tricuspid valves are similar in their action and function and a description of the mechanism of the former can be applied to the latter. AURICULO-VENTRICULAR VALVES. I43 Supposing that ventricular diastole has just begun, the two flaps hang loosely in the cavity of the ventricle, but probably not in contact with the wall. The blood, now continuously en- tering from the auricle, accumulates behind the flaps and floats them away from the walls, bringing their free edges toward each other, so that they are not very far apart at the beginning of auricular systole. When that systole occurs the sudden acqui- sition of the auricular contents so increases the intra-ventricular pressure as to bring together at once the free margins of the two flaps. These flaps are probably not approximated edge to edge, for thus it would seem impossible to prevent some regurgitation, but the auricular surfaces of the thin flexible portions of the free edges are apposed to each other, so that increased ventricular pressure would tend only to make the approximation firmer and regurgitation more impossible — within a certain limit. Since the forces thus holding together the apposed surfaces are equal and act one against the other, there is no strain upon these deli- cate portions of the cusps and they can, therefore, be left pliant enough to insure accurate adjustment. Those parts of the curtain, however, which intervene between the now approximated thin surface and the attachment at the auriculo-ventricular ring are subjected to considerable force and are consequently provided with fibrous tissue between the two serous layers. But without additional provision the flaps would still doubtless be forced back into the auricle during ventricular systole. This provision is in the shape of the chordce tendinecR already described. They are small but strong and inelastic, and being attached to the ventricular surfaces of the flaps from the free margins to the bases, they serve as very efficient guy-ropes to prevent reversal into the auricle. Not only do the cords pull directly away from the auricle, but, as the size of the ring de- creases with the progress of ventricular systole, the cords at- tached to the lateral halves of a flap pull them in opposite di- 144 CIRCULATION. rections so as to keep the curtain taut. The papillary muscles, from which most of the chordae tendinese spring, probably pre- vent, by their contraction, the slacking of the cords as the cavity of the ventricle becomes smaller in systole. The semilunar valves have a simpler mechanism than the auriculo-ventricular. Each is composed of three cusps attached by their convex bases to the circumference of the arterial orifice Portion of the Wall of Ventricle. d, d' , and aorta, a, b, c, showing attachments of one flap of mitral and the aortic valves; h and ^, papillary muscles; e, e' and y", attachment of the tendinous cords. (From Yco aSter Allen Tho^npson.) SEMILUNAR VALVES. I45 (Fig. 34) ; the free edge is comparatively straight when the cusp is removed from its attachments. These valves have no chordae tendinese ; they are entirely de- pendent upon their attachment to the vessel wall to prevent re- versal. When the aorta is opened the flaps are found to form three pockets between their arterial surfaces and the adjacent wall of the vessel. They are convex toward the heart. Their object is to prevent regurgitation into the ventricle, and to do so they must not themselves be reversed into that cavity. They receive some support by being attached just where the thick muscular substance of the ventricle merges into the much thin- ner arterial wall. Suppose that blood is now entering the aorta; the flaps project into that vessel, but are not in contact with its walls. Blood is always in the sinus of Valsalva behind each cusp, and, as the entering volume during ventricular systole is continuously increasing arterial pressure, the curtains probably approach each other by their free edges before the end of that systole ; at the instant of its cessation the arterial pressure becomes much greater than the ventricular and the blood in the sinuses immediately forces together the free edges of the curtains which effectually pre- vent a backward flow of the current. The approximation of the flaps is further aided by the lessening of the aortic orifice as a re- sult of contraction of the ventricle. Here, as in the case of the auriculo-ventricular valves, the flaps are not joined edge to edge but by apposition of the ven- tricular surfaces ctl the narrow flexible lunulce. These surfaces are pressed together by equal and opposite forces and do not, therefore, undergo any strain. Their flexibility provides for ac- curate adjustment except at the middle where the three cusps meet. Here is a triangular space which is just filled by the three corpora Arantii. The action of the pulmonary semilunar is the same as that of the aortic. 146 CIRCULATION. Relative Condition of the Valves. — The auriculo-ventricular valve is open except during the ventricular systole ; the semi- lunar is closed except during the same systole. Thus the auric- ulo-ventricular is open during the greater part of the cardiac cycle, and the semilunar during the remaining smaller part. When one is opened the other is invariably closed. The auric- ulo-ventricular closes to prevent regurgitation into the auricle during ventricular systole and opens as soon as the systole is fin- ished ; the semilunar closes to prevent regurgitation into the ven- tricle from the recoil in the artery following systole, and is not opened until forced by the succeeding ventricular systole. When either valve is open blood is passing through the orifice which it guards. In the first part of the period during which the auriculo-ventricular is open blood is passing through slowly but freely, while during the latter part of that period, which is occupied by the auricular systole, it rushes forcibly through ; it passes with great force through the aortic and pulmonic openings during the whole time that the semilunar valves are open. The Cardiac Cycle. — A fair idea of the succession of events taking place in connection with the movements of the heart has been gathered from preceding remarks. The term "car- diac cycle ' ' is employed to embrace this whole succession of events occurring between the ~ beginning (or ending) of any single event and the beginning (or ending) of that same event again. The point of departure may be taken anywhere, say, at the beginning of the ventricular systole ; in this case the cycle ends with the beginning of the succeeding ventricular systole and includes everything that has taken place meantime. The term " auricular cycle " or " ventricular cycle " may be similarly applied to the activity and succeeding repose of either of the cavities indicated. If we suppose the auricular cycle to begin with the contraction of the auricle that cycle will end and a second begin when the auricle begins to contract a second CARDIAC CYCLE. . 147 ime. Thus the auricular cycle (or the ventricular) is equal in ength of time to the cardiac cycle, but the terms are not syn- )nymous because the latter refers to the succession of events in the vhole heart. Fig. 35. Scheme of Cardiac Cycle. The inner circle shows the events which occur within the heart ; the outer the relation of he sounds and pauses to these events. {Kirkes after Sharpey and Cairdjiei-.} Occurrences During the Cycle. — As soon as the auricle is filled with blood from the supplying veins its systole occurs. rhe systole of the ventricle follows immediately. Then there Dccurs a " period of repose " for the whole heart — which repose is broken by the next systole of the auricle, and the same train jf events recurs. The "period of repose" just mentioned does not cover all ;he rest which any one part of the heart gets ; both auricle md ventricle rest a little longer than this. For the auricular liastole (rest) begins as soon as the auricular systole ceases, and ;his is previous to the beginning of the "period of repose " — Dy a time equal in length to the ventricular systole — and lasts mtil the end of the "period of repose " ; the ventricular dias- 148 CIRCULATION. tole begins with the "period of repose" and lasts until the auricular systole has ceased, and this is subsequent to the end of the " period of repose " — by a time equal in length to that of the auricular systole. (See Fig. 35.) Thus the auricular diastole embraces the ventricular systole and the first part of the ventricular diastole ; the ventricular diastole embraces the latter part of the auricular diastole and the whole of the auricular systole. The auricular systole embraces the latter part of the ventricular diastole ; the ventricular sys- tole embraces the first part of the auricular diastole. The two cavities are never in systole together but are in diastole together during the " period of repose " (Fig. 35). During auricular systole blood is rushing into the ventricle through the auriculo-ventricular opening; during ventricular systole blood is rushing through the aortic (and pulmonary) orifice, and is also entering the auricle (whose diastole has be- gun) through the great veins. During the "period of repose " blood is entering the auricle through the great veins and is also passing freely through the auriculo-ventricular opening into the ventricle. As soon as the auricle becomes full it contracts and passes its contents to the ventricle ; this extra amount is suffi- cient to distend the ventricle and its contraction supervenes at once. ' Length of Cycle. — Since the heart beats about 72 times per minute each cardiac cycle must occupy .83 of a second, at this rate. When the rate is doubled the cycle is of course reduced one half. If the rate be 60 each cycle will have a length of one second. It is important that when the rate is increased, it is at the expense chiefly of the diastole, the systole requiring a length of time which varies little. Thus if the rate be doubled the heart will not get half so much rest as under normal conditions. If the cardiac cycle be divided into tenths it is reckoned that the auricular systole occupies a little less than .2, the ventricu- CARDIAC SOUNDS. 149 lar systole a little less than .4, and the period of repose about .5. Reduced to their absolute values the. estimates, when each cycle occupies .8 sec, are generally reported as .1 sec. for auricular systole, . 3 sec. for ventricular systole, and . 4 for the period of repose. These periods are, of course, only approxi- mate. Sounds of the Heart. — When the ear is applied to the chest in the precordial region two sounds may be heard which are evidently connected with the action of the heart. The first is heard at the time of the apex beat ; the second follows with scarcely an appreciable interval. After the second sound there is a pause before the recurrence of the first sound. The first sound is longer, lower in pitch than the second and somewhat muffled; the second is comparatively short, high in pitch and clear. Both can usually be heard over the whole precordium, but the first is heard best toward the apex and the second toward the base. Since the first is heard at the time of the apex beat it corre- sponds in time with the ventricular systole and, from its dura- tion, must embrace practically the whole of that event. Since the second sound follows so quickly it must occupy the begin- ning of the ventricular diastole, but from its short duration must embrace only a small part of that event. (See Fig. 35.) What is the cause of the sounds ? The time of the first sug- gests that it is due directly or indirectly to the ventricular con- traction. The auriculo-ventricular valves close forcibly at the beginning of this event, and suggestion also points to them as being a factor in the production of the sound produced at that time. It is found, indeed, that the first is a compound sound in that it is produced in part by the valves and in part by other cir- cumstances ; it possesses a valvular and a non-valvular element. It has been shown by experiments that the sudden closure of the auriculo-ventricular valves is attended by a sound — and this is the main factor in the production of the first cardiac sound. ISO CIRCULATION. To it are to be added the muscular sound attending the contrac- tion of the heart, the impulse of the apex against the chest wall and possibly the vibrations of the chordae tendinese. It is the muscular contraction and the apical impulse which give to the first sound its prolonged, muffled, booming character ; were there only a valvular element this sound would probably be very similar to the second in character and duration. The second sound is simpler in its production. It is purely valvular and is caused by the sudden closure of the aortic and pulmonic semilunar valves. Succession of Events in Relation to the Sounds. — It is of prime importance, especially in pathological lesions of the valves, to know exactly what is occurring in all parts of the heart during the production of each sound and during the pause (Fig. 35). Knowing what causes these sounds and the sequence of events in the cardiac cycle this is easy. Since tht first sound is produced partly by, and is continued throughout, the ventricular systole it follows that during its pro- duction the auricle is in beginning diastole and is receiving blood from the veins ; the auriculo-ventricular opening is closed ; blood is rushing through the aortic (and pulmonic) orifice, and the semilunar valves are open. Since the second sound is caused by closure of the semilunar valves and just follows the ventricular systole, during its pro- duction the auricles are still swelling and receiving blood from the veins ; the auriculo-ventricular valves are opening and blood is beginning to enter the ventricles ; the ventricular diastole is beginning ; the semilunar valves are closed. The pause, following the second sound, lasts until the begin- ning of the following ventricular systole. During the first part of that time the auricle is receiving blood from the veins and completes its diastole ; during the latter part the auricular systole occurs ; during the whole pause the auriculo-ventricular valve is open and blood is passing into the ventricle, at first slowly, then WORK OF THE HEART. 151 forcibly ; the ventricle completes its diastole ; the semilunar valves are closed. It is to be noticed that the auricular systole is accompanied by no sound appreciable to the auscultating ear. If the foregoing circumstances be once clearly grasped there will be little difficulty in determining the location and character of at least the more common valvular lesions in disease. Attention is called to the fact that the "period of repose" and the "pause " are not identical in time. They are of about equal length, but it is seen that the pause begins and ends a little later than the period of repose. The former covers the period when no sound can be heard ; the latter when no muscu- lar contraction is in progress. In Fig. 35 the space marked " Diastole of Auricle and Ventricle " does not embrace the en- tire diastole of either. The first sound occupies a little less than .4 of the cardiac cycle; the second sound some .2, and the pause the remainder. Work of the Heart. — In estimates of the work accomplished by the cardiac contractions there are the widest variations among different observers. The figures of some are even twice those of others. The chief difficulty is in determining the amount of blood ejected at each ventricular systole. No ac- curate idea of this amount — the "pulse volume''^ — can be ob- tained from the size of the cavity when filled post mortem ; the varying conditions of pressure, etc., cannot be imitated properly. Nor is it probable that the ventricles completely empty them- selves during systole, a small, but variable, amount of blood re- maining in unobliterated spaces just below the valves. The figures below given in connection with the subject are from the American Text-book of Physiology. It is not claimed there that they are accurate. The amount of blood thrown into the arteries by the two ven- tricles at each contraction must be equal or there would be an accumulation in either the pulmonary or systemic circulation. 152 CIRCULATION. When the circulation is evenly carried on the amount of blood entering the capillaries of either system during a cardiac cycle must be equal to the amount thrown into the artery — the pulse volume — at one ventricular systole. While it is apparent that the pulse volume is subject to variations from almost numberless causes the average will be taken as 700 grams (about 3 oz.). To find the amount of work done by each ventricular systole it is necessary to multiply the pulse volume by the pressure which ejects it. It is found that the maximum pressure in the left ventricle will raise mercury in a small tube inserted into the cavity 200 mm., and in the right ventricle 77 mm. If blood be substituted for mercury it will rise from the left ventricle 2.567 m., and from the right .988 m. That is, the contraction of the left ventricle would raise the pulse volume 2.567 ra. and that of the right .988 m. The two together would raise the pulse-volume (100 gm. ) 3.555 m. Then the work done by both ventricles will be equal to 100 gm. x 3-555 m., which gives a product of 355.5 grammeters for each double ventricular systole. A grammeter is a unit of work corresponding to the foot-pound, and represents the amount of force required to raise one gram one meter. 355.5 grammeters are equal to about two foot-pounds, which may be taken as representing the amount of work done by each contraction of the ventricles ; that is, every time the ventricles contract they do enough work to raise one pound two feet, or two pounds one foot. Considering the rate of the cardiac contractions it is evident that an enormous amount of work is done in a day by this one muscle. These figures are rather below those of most authors. The work done by the auricles is so small as to be disregarded. The force exerted by the left ventricle at each contraction has been estimated at 52 pounds. This result is obtained by multiplying the weight of the pulse volume by the supposed area of the internal surface of the ventricle. INNERVATION OF THE HEART. 1 53 Ventricular Pressure. — This subject has been touched upon in the foregoing section, but besides the maximum pressure there mentioned there is also a minimum pressure in this cavity. The maximum pressure was given as 200 mm. of mercury; the minimum is found to be — 40 mm. Now, the pressure in the aorta (or any artery) varies within certain limits, but is always positive; that in the ventricle while at its maximum exceeds that in the aorta and so opens the semilunar valve, but while at its minimum it not only falls far below that of the artery, allow- ing the closure of the valve, but becomes actually negative — below the atmospheric pressure — and exerts a ' ' suction ' ' force. At this time it is below the auricular pressure and thus the auriculo-ventricular valve is opened. This negative pressure in the ventricle is due to two causes : ( i ) Aspiration of the thorax, and (2) tonicity of the heart muscle itself. The latter element is not very active ; the former is much more efficient during inspiration, and may cease to operate during expiration. Functions of the Auricles. — The maximum systolic pressure in the auricle is only some 20 mm. of mercury ; the minimum diastolic pressure is about — 10 mm. The small positive pressure is adequate for the work to be done ; for the auricle has no valve to force in systole and the ventricular pressure at that time is very little if at all above the atmospheric pressure. Its contrac- tion serves to fill the ventricle and bring on ventricular systole, but its action is not indispensable in circulation. It serves as a reservoir, providing for a much more even flow of blood from the veins than would be the case if the vessels entered the ventricle. Whether the auricle completely empties itself by systole and whether blood continues to enter from the veins during its systole are matters of dispute. Innervation of Heart. — The rhythmical contraction of the heart are tept up by ( i ) the cardiac branches of the vagus, ( 2 ) the cardiac branches of the sympathetic, (3) the intrinsic ganglia of the heart. 1 54 CIRCULATION. Accurate knowledge of the course, distribution and action of these fibers is wanting because there are so many points of dif- ference in the lower animals. The following facts seem to be true in case of the rabbit : Running upward from the inferior, middle and superior cervical sympathetic ganglia are three nerves — inferior, middle and su- perior cardiac. These, joining a branch from the pneumogas- tric at a point near its exit from the cranium, form with it the vago-sympathetic trunk, the fibers of which pass to the heart and enter superficial and deep plexuses in which are gangli- onic aggregations ; they pass thence to the heart, entering with the inferior vena cava. They thus reach the ganglion of Remak in the expansion of that vessel (sinus venosus), and run from this point to the ganglion of Ludwig between the two auricles and to the ganglion of Bidder m\!s\& left auriculo-ventricular septum. From these ganglia radiate three plexuses to the pericardium, myocardium and endocardium. The ganglia of Remak and Bidder are motor in function ; that of Ludwig is inhibitory. Stimulation of the sympathetic fibers before they join the vagus increases the frequency and diminishes the force of the heart. Stimulation of the vago-sympathetic trunk produces aug- mentation — increased force and frequency. Stimulation of the vagus before junction produces inhibition of the heart's action, increasing the force but diminishing the frequency (direct action). Just after leaving the cranial cavity the vagus receives certain fibers from the spinal accessory which run with it and are distributed to the heart. It is through these fibers that the direct inhibitory action of the loth nerve occurs. Furthermore, there has been found a nerve rising by two roots — one from the trunk of the pneumogastric and the other from the superior laryngeal — which joins the sympathetic filaments in the chest and passes to the heart. The root from the superior laryn- CARDIAC RATE. 1 55 geal probably comes from the spinal portion of the spinal accessory through the ganglion of the trunk of the vagus. This nerve is known as the " depressor vagi," or the depressor nerve of the circulation. It is a centripetal nerve. If it be cut stimulation of the peripheral end produces no effect on the heart, but stimula- tion of the central end inhibits cardiac action and lowers arterial tension. Its action is, therefore, reflex only. But this nerve must act in conjunction with the sympathetic, for it inhibits cardiac action only by stimulating the vaso -motor center to dilate the peripheral vessels. Its normal function is to adapt the heart's action to the peripheral resistance. If the splanchnic be cut the fall in blood-pressure is very slight, showing that the splanchnic area is the one most affected. In the human being the depressor nerve fibers are probably bound up in the vagus trunk. Therefore, heart action is accelerated by stimulation of the sympathetic supply ; it is augmented by stimulation of the vago- sympathetic trunk ; it is inhibited directly by stimulation of the vagus before it is joined by the sympathetic, and reflexly by stimulation of the depressor vagi. Frequency of Heart Beat. — In the normal adult the heart rate is usually given as 72 per min. for the male and 80 for the female. A very great many not abnormal circumstances cause the rate to vary. It is more rapid in tall than in short persons, and this partially explains the different rate for the sexes ; but women of the same height as men have a faster pulse than those men. The fetus\a& a cardiac rate of from 1 20 to 150, and even here the differ- ence in the sexes is evident ; so much so that the rate has been made use of as giving some indication as to the sex. During the first year it declines to about 120 and after that time gradually diminishes to adult life. At puberty the difference for the sexes becomes more marked. There is a slight acceleration in old age. The digestion of an ordinary meal increases the frequency 156 CIRCULATION. some 5-10 beats ; fasting diminishes it; nitrogenous foods in- crease it more than non-nitrogenous. Muscular exercise increases it markedly and in proportion to the violence of that exercise. Posture influences it ; for the male the average rate standing is 80, sitting 72, lying 65 ; this variation is not due entirely to the muscular exercise necessary in standing or sitting. The rate is diminished during sleep; this is a probable effect of the total freedom from muscular strain or exercise and the absence of any emotional factors. High external temperature increases it; cold diminishes it. Emotional disturbances zx& yety e.Sect\ye in usually increasing the frequency. Respiration has a marked influence. Commonly the respiratory and cardiac rates increase and decrease together. During inspiration the cardiac rate is a little more rapid ; during expiration it is a little slower. But arrest of respiration in any phase very soon slows the heart and finally, if not relieved, stops it for two reasons. First, any muscle deprived of fresh blood soon loses its power to contract, and the heart is no exception. Second, when blood is not oxygenated it will pass through the capillaries only with the greatest difficulty. As a consequence it accumulates in the arterial system, dams back upon the heart and so distends that organ as to paralyze it. Arrest of the Heart. — Asphyxia, from any reason, if prolonged, will cause the heart to stop as just described. It is the second (mechanical) reason which is chiefly operative in death from asphyxia, although the first would be equally effective in time. Engorgement, whether with or without asphyxia, often causes arrest of the heart's action. When the heart is deprived of its own blood supply, as by embolism of the coronary artery, it very soon ceases to act. It is also arrested by removal of its ordinary stimulus, the blood, passing through its cavities. This is the chief cause of death from hemorrhage ; hence the value of injections of salt solutions into the circulation. Blows in the epigastric region may cause death by arrest of the heart. It is THE ARTERIES. 157 not known whether this is an effect of direct violence to the organ or to the solar plexus through which the nervous equilib- rium is disturbed. Arrest of the heart by nervous influences seems, as already indicated, to take place through the pneumo- gastric. The heart may, therefore, be said to be stopped by any one of three causes: (i) distention, (2) loss of blood, (3) nervous disturbances. (B) The Arterial Circulation. Objects of Arterial Circulation. — The arteries exist primarily for the purpose of conveying to the capillaries the blood received from the ventricles — the object of the conveyance to the capillaries being, in the pulmonary circulation, the inter- change there of gases between the blood and air, and in the systemic circulation, the interchange of gases and nutritious and excrementitious materials between the blood and cells of the tissues. No such interchanges take place in the arteries ; the blood must reach the capillaries before they can occur, and the arteries furnish the routes by which it does so. But these vessels are so constituted as not to be simple passive tubes through which the current runs ; they have, by virtue of their structure, two very important additional offices : first, they change the forcible intermittent flow received from the ventricles into a steady constant one by the time the capillaries are reached ; second, they regulate the amount of blood going to a part at different times, thus properly distributing the fluid over the body in obedience to the varying demands of particular organs. "The movement of the blood depends on the heart, but its distribution depends on the vessels. ' ' Anatomy. — The vessel carrying all the blood away from the right ventricle to the pulmonary circulation is the pulmonary artery ; the blood to the systemic circulation leaves the left ventricle by the aorta. These divide, branch and subdivide, the divisions becoming smaller and smaller until the microscopic 158 CIRCULATION. capillaries are reached. The combined caliber of the branches progressively increases as the number becomes larger. With few exceptions the combined capacity of any two bifurcating branches is larger than that of the dividing vessel. Consequently the capacity of all the small arteries in either system is far greater than that of the vessel leaving the ventricle. The pulmonary artery carries venous blood, but by structure and office it belongs to the arterial system. It is somewhat thinner and more distensible than the aorta, but otherwise its anatomy is similar. While all the arteries are to a certain extent identical in structure, they present differences which warrant their division into three classes : ( i ) large arteries, including the common carotids and common iliacs and those larger; (2) medium arte- ries, including all between (i) and (3) ; (3) smallest arteries, including all of a diameter of ^ inch and less. They all have three coats. Fig. 36. 9 a. J ^-e. ::i-^. c:[^ ^^-^.T^^w-— - --' Transverse Section of Part of the Wall of the Posterior Tibial Artery (Man). (From K?o after SAo/^r, ) a, endothelium lining: the vessel, appearing thicker than natural from the contraction of the outer coats ; b, the elastic layer of the intima ; Cj middle coat composed of muscle fibers and elastic tissue; d, outer coal consisting chiefly of white fibrous tissue. Histology. — In the largest arteries the external coat, or tunica adventitia, is of fibrous tissue with a little plain muscular tissue \ it is no thicker than in the medium arteries. The 77iiddle coat, or tunica media, constitutes the main part of the wall. It consists chiefly of yellow elastic tissue, between the THE ARTERIES. 159 layers of which are a few plain muscle fibers. The internal coat, or tunica intima, consists of a single layer of endothelial cells upon a thin elastic supporting membrane. The cells are oval in shape, their long diameters being in the direction of the vessel. In the medium arteries, the external coat is practically iden- tical with that of the largest arteries. The middle coat, instead of presenting so much yellow elastic tissue, is made up mainly of plain muscle fibers. In the largest of this class the elastic tissue is abundant, being continued from the larger vessels, but it gradually gives place to muscular tissue as the branches be- come smaller, until before the smallest arteries are reached there is scarcely any elastic tissue left. The internal coat is the same as in the largest arteries. In the smallest arteries the external coat is the same as in the largest, except that it is thinner and disappears just before the capillaries are reached. The middle coat is of muscle fibers with no elastic tissue. The internal coat is the same as in the larger vessels. Of course, this division of the arteries is arbitrary, and they present no marked differences at the dividing lines mentioned. It is apparent that it is the middle coat which constitutes the chief difference, the elastic tissue of the large vessels giving place to muscular in the medium and smaller. It follows that the large arteries must be possessed of great elasticity and the medium and smaller of great contractility. These are, indeed, distinguishing characteristics, and of the very greatest impor- tance in the economy of the circulatory forces. Vessels called vasa vasorum penetrate the external, and to a less extent the middle coats, supplying them with nutriment ; they do not reach the internal coat. A plexus of sympathetic nerves surrounds the vessels, and fibers from it are distributed to the muscular coat. Elasticity and Contractility of Arteries. — If an amount of l6o CIRCULATION. fluid corresponding to that of the ' ' pulse volume ' ' be suddenly injected into the end of a rubber tube already distended with liquid the tube will be further distended or "pouched" by the injection, but will resume its former caliber if a correspond- ing amount of fluid be allowed to escape from the opposite end. This is, in a rough way, what happens to the arteries upon each ventricular systole. The pulse volume enters with much force the aorta (or pulmonary artery) in which the pressure is already high ; the artery is very elastic and expands under this influence, but immediately recoils with a greater pressure upon its contents. That pressure tends to force the blood along the vessel in both directions, but its return into the ventricle is effectually prevented by the now closed semilunar valves. Con- sequently it can go only toward the periphery. Now it is evident that the flow in the beginning of the aorta is intermittent; but it is found that in vessels as large as the carotid and smaller the flow has assumed a remittent character, and that it approaches nearer and nearer to being continuous as the vessels become smaller, until that condition is established when the capillaries are reached. It is the elastic coat of the aorta which allows the vessel to expand and causes it to contract upon its contents forcing them onward. It is a force, superadded to that of the ventricle, in maintaining the circulation — a force derived from the ventricle, stored there to be used during diastole. It is, furthermore, in main part, this elasticity which accounts for the conversion of the intermittent into a remittent, and later a constant, flow. If the wall of the expanded aorta reacted upon the pulse volume with the same quickness and force as the ventricle and if the same kind of reaction characterized succeeding portions of the arterial tree, the blood would be handed from one segment to another without modification of the intermittency of its current. But the contraction of the vessel here is only z. passive reaction, not due, like the heart's contraction, to the activity of striated ELASTICITY AND CONTRACTILITY. l6l muscle, and takes place in a comparatively slow manner — last- ing, in fact until the next ventricular systole. The effect of the elastic tissue, then, in the very beginning of the aorta alone would tend to convert the flow into a continuous one ; when it is considered how the blood is handed on from segment to seg- ment of the arterial system, each possessed of elasticity, it is easy to see how the intermittency of the current is gradually ab- sorbed in its passage toward the periphery. It is also easy to see how more elastic tissue is needed in the large than in the small arteries. The function of the elastic coat is, therefore, twofold : (i) It forces the blood continuously toward the periphery ; ( 2 ) it is chiefly instrumental in changing the intermittent to a constant flow — a condition very necessary in the capillary system. The contractility of the medium and smallest arteries is resi- dent in their muscular coats ; and by the term " contractility " here is not meant the passive reaction which follows distention. An inert rubber tube will react when distended, but has no inherent power of contractility, although it is usually said to "contract" under the condition just mentioned. The caliber of an artery in a normal state of tonicity may be one-twelfth inch, but by contraction of its muscular coat it may have a cali- ber of, say, one-twenty-fourth inch, or by relaxation of that coat a caliber of one-sixth inch — in both cases without any considera- tions of pressure inside it. This variation in caliber is just what is found frequently to take place. For it is evident that an organ requires different quantities of blood at different times, and the supply for the varying necessities is regulated through contraction or dilata- tion of the supplying vessel. For instance, more blood is needed in the alimentary canal during digestion, and the arteries going there are dilated. Less blood is needed in the glands in the intervals of secretion and the supplying vessels are contracted. Thus it is that the comparatively limited amount 1 62 CIRCULATION. of blood in the body is distributed most abundantly to those parts in which, for the time, physiological activity is greatest. Of course the muscular tissue is under the control of the nervous system through the vaso-motor fibers. The prominent function, therefore, of the muscular coat is to regulate the supply of blood. Arterial Tension. — If a tube open at both ends have one end inserted into, say, the common carotid artery the blood will rise for a considerable distance in the tube and remain there. If a similar tube be inserted into the internal jugular vein of the opposite side (which, under the conditions, corresponds to the artery experimented upon) the height to which the blood rises in this tube will be much less than in the former one. This shows that blood exists in the arteries and veins under a certain degree of pressure, which is 8-9 times greater in the arteries than in the veins. Furthermore, the level of the blood in the tube connected with the artery shows a fluctuation, rising and falling regularly with respiration ; it also shows a less extensive, but more rapid, fluctuation with each heart beat, the highest point of this secon- dary rise being coincident in time with the ventricular systole; the column falls then until the following systole. Bat at no time does the column in the arterial tube come near to being so low as that in the venous tube. The height of these columns is a measure of the dififerent de- grees of pressure which sustain them, and obviously the height of the column will depend on the weight of the liquid used in the tube. Owing to the quick coagulability of the blood, the great amount of friction in so high a column and to other me- chanical difficulties which beset the experiment mentioned, an instrument, the mercurial manometer (the principle of which is the substitution of mercury for blood in the tube), has been de- vised to record with considerable accuracy the pressure in dif- ferent_ parts of the circulatory system. Hence it is that the ARTERIAL TENSION. 1 63 blood pressure is usually given in terms of mercury. The aortic pressure in man is probably from 150 to 200 mm. (6-8 in.) of mercury. The Causes of Arterial Tension. — These are (i) friction in the vessels, (2) the incessant injection of the " pulse vol- ume " by the ventricle, and (3) the elasticity of the arterial walls. 1. Of course there is an element of friction opposing the entrance of blood into the aorta from the ventricle ; but this resistance is relatively much greater in a small than a large tube, and is absolutely so when a number of small tubes represent the subdivisions of the large one. Consequently, the friction in the smallest vessels is, in the aggregate, enormously greater than in the aorta. It is, as it were, reflected from the capillaries toward the heart, constituting a constant and ever-increasing impediment to the onward current. 2. There is a continuous escape of blood out of the arterial into the capillary system in spite of friction, and this would finally relieve the arterial tension were it not for the regular and frequent injection into the aorta of an amount of blood equaling that which has escaped into the capillaries since the previous systole. 3. But if these two factors alone were concerned there would be no continuous pressure in the arteries ; it would be in- termittent, and inoperative during diastole. The tube, how- ever, shows that the pressure is fairly constant, being only a little less during diastole than during systole. This is brought about by the elasticity of the arterial wall. The effect of the elastic wall on blood-pressure is illustrated by Howell as follows : Let it be supposed that, at the moment of observation, arterial tension has, for some reason, sunk very low, but that now a normal heart is forcing the usual amount of blood into the aorta. The first injection feels the influence of peripheral friction, and since the elastic wall is distended but 164 CIRCULATION. slightly it meets with little resistance in expanding the wall to a degree necessary to accommodate that amount of blood ; it is easier to distend it than to overcome friction. Succeeding in- jections for a time will find it easier to distend the arteries than to overcome the peripheral resistance, but as each injection enters it makes room for itself with greater difficulty than did the preceding one, for the vessel becomes more and more diffi- cult to distend. Thus the pressure in the vessel becomes higher and higher, and as it does so it will become easier for a part of the contained blood to overcome friction and enter the capil- laries than to distend the wall. Finally the arterial wall will become so tense as to force into the capillaries, between two systoles, an amount of blood equal to that received by the first systole. The establishment of this equilibrium marks the resto- ration of normal arterial tension. It is seen that this force is operative during diastole as well as systole. When the ventricle forces a charge of blood into the aorta a large part of its energy is expended in distending the arterial wall ; it stores up a part of its energy thus to be dealt out while it rests in diastole ; that energy is dealt out by the recoil of the elastic wall, and this recoil, lasting until the suc- ceeding systole (and representing nothing but the prolonged action of the ventricle), exerts a continuous pressure upon the arterial contents and forces a continuous current toward the pe- riphery. Upon systole the arterial pressure rises, for the system of vessels must then accommodate more blood than at any other time ; during diastole the pressure steadily declines, because blood is escaping from the vessels through the capillaries and none is entering. At the end of diastole the pressure is least because the amount of blood in the arteries is at the minimum mark. It is apparent that, in reality, there are, under normal con- ditions, only two factors to be considered as controlling arterial tension — (i) the force of the left ventricle and (2) the resistance ARTERIAL TENSION. 1 65 at the periphery. The elasticity of the arterial wall is only a means of storing up the energy of the ventricle. It furnishes a most striking example of the conservation of energy and the economy of its expenditure. When the amount of blood is constant arterial tension is chiefly governed by the condition of the capillaries and arterioles as to dilatation or contraction. The pressure in the pulmonary artery is considerably lower than in the aorta, but in general the remarks that have been made relative to the systemic circulation apply as well to the pulmonic. Pressure in Different Arteries. — The pressure is greater in the large than in the small arteries, but the difference is less than might be supposed. With an aortic pressure of 1 5 o m m . the pres- sure in the metatarsal artery is something like 125 mm. When the proximity of the great outlet — the capillaries — to the small arteries and the increase in their combined capacity over that of the aorta is considered the reason for the diminution is appar- ent. Conditions Influencing Arterial Tension. — With normal ves- sels and a normal amount of blood the degree of tension will depend, as already intimated, usually on the action of the heart and the condition of the outlet. Anything increasing the amount of blood received without at the same time increasing the amount escaping will increase the tension ; the same result will obviously follow a normal intake and a decreased output. Op- posite effects will follow opposite conditions. In ordinary tranquil respiration arterial tension increases as a result of inspiration and decreases as a result of expiration. Mus- cular exercise increases it for the supposed reason that mus- cular contractions to some extent impede the entrance of blood into the capillaries, and for the further reason that dur- ing straining the chest is compressed and tends to force the blood out of the great vessels there. The effect of hemor- rhage on arterial tension needs no comment. Various emo- 1 66 CIRCULATION. tional disturbances may diminish or augment the pressure through the nervous system. The Pulse. — By the term " pulse " is ordinarily understood the sudden incessant dilatations of the arteries corresponding to the incessant contractions of the heart. These periodic dilata- „ tions depend, in point of fact, upon fluctuations in the arterial pressure, which fluctuations depend in turn upon the injection of blood from the heart, the elasticity of the arterial walls and the resistance of friction. Variations Tracing of Blood Pressure from the normal in any of these will TAKEN WITH Fick's mano- altcr thc chatacter of the pulse. METER. i^YeO.') When the finger is applied to an artery a distinct sensation of enlargement is received just after each heart beat. But the increase in caliber is actually much less marked than the elongation of the vessel. If the radial and temporal, or better the radial and carotid, be palpated at the same time it will be discovered that the fluc- tuation due to any systole is recorded in the carotid earlier than in the radial, showing that an appreciable time is consumed in the passage of the wave from the beginning of the aorta to its peripheral divisions. It is estimated that the passage of the wave from the heart to the dorsalis pedis artery consumes about .2 second. It is not to be understood that the distention of any artery (except the very beginning of the aorta) is due to the passage through it of any part of the actual blood which entered the aorta at that systole which causes the pulse beat felt. It is only the transmission of the wave and not the body of the blood. The expansion of the arterial tree is a progressive one, passing from segment to segment in a peripheral direction. This pulse wave travels at the rate of from lo to 30 ft., while the current itself seldom exceeds i)^ ft. per second. THE PULSE. 167 Frequency and Regularity of the Beat. — These, of course, de- pend upon the frequency and regularity of the heart. Some conditions influencing these characteristics of cardiac action have been noticed. Sometimes, from either transitory or serious dis- orders, an otherwise regular pulse will omit a beat ; this signi- fies that the ventricle has missed a contraction in its regular time. Not always, however, does the peripheral pulse record the number of heart beats. Occasionally only every other beat, or less often every third beat, is of sufficient strength to cause a palpable pulse. Not a little information as to the condition of the heart and circulation, outside of frequency and regularity, can be obtained from the pulse ; and there have been distinguished the full- FlG. 38. Makey's Sphygmogkaph. The frame (,5, B, B) is fastened to the wrist by the straps at B, B, and the rest of the instrument lies on the forearm. The end of the screw ( V) rests on the springs (7?), the but- ton of which lies on the radial artery. Any motion of the button at A" is communicated to l^, which moves the lever (Z.) up and down. When in position, the blackened slip of glass {P} is made to move evenly by the clockwork (//) so that the writing point draws a record of the movements of the lever. ( Veo.) bounding, non-compressible, compressible, wiry, gaseous, thready and other kinds of pulse, they being supposed to give informa- tion as to the heart's action, or the peripheral resistance, or the condition of the vessel wall, or as to all of these. From me- chanical reasons which suggest themselves a ^* large " pulse often accompanies low tension, and a ''small " pulse high tension. 1 68 CIRCULATION. Pulse Tracings. — By means of the sphygmograph pulse trac- ings are taken (Fig. 38). The lever rises abruptly at the "pulse beat" and declines gradually until the next beat raises it ; thus it takes much longer for lever to fall than to rise. This instrument furnishes an accurate record of the varying arterial tension so far as the variations are due to individual contractions, and shows that, while the pressure declines in a general way throughout the whole period between pulse "beats ' ' it does not do so in a uniform manner. At a certain short in- terval after the beginning of the decline the lever is interrupted temporarily and frequently, even rises slightly again. The in- terpretation of this secondary rise of the lever is that a sec- ondary pressure wave is traversing the artery. This is called the dicrotic wave. Under normal circumstances it cannot be felt, but often in debilitated conditions when the pressure is low it can be detected by the finger ; it is then felt as a second smaller impulse quickly following the usual one, and, of course, in such case the dicrotic rise traced by the sphygmograph would be quite conspicuous. There is much discussion as to the origin of the dicrotic wave. Some hold that it is reflected like an echo from the peripheral friction and consequently travels centrally. This is hardly correct. It very probably results from a secondary im- pulse given to the blood column by the sudden closure of the semilunar valves at the end of systole and by the first quick recoil of the arterial wall upon its contents, the first (main) wave having been started by the sudden influx of blood at the beginning of systole when the artery is still expanding and the semilunar valves are still open. The condition of the muscular coat is not without its effect upon the pulse. It seems by its tonicity to prevent too great distention of the vessels. Relaxation of this coat, whether through nervous or other influence, such as external heat, causes a large, soft, compressible pulse, while contraction gives to it a small, hard, non-compressible character. THE CAPILLARIES. 169 Rapidity of the Arterial Current. — ^This diminishes pro- gressively toward the periphery. In the carotid it has been esti- mated to have a velocity of 20 in. per sec. during ventricular systole, 8.5 in. during the passage of the dicrotic wave, and some 6 in. , as an average, between that wave and the next sys- tolic acceleration. For a mean carotid velocity of 10 in. per sec. the metatarsal velocity is probably a little over 2 in. The rate must be increased by relaxed arterioles and capillaries and retarded by contraction of these vessels. The decreased periph- eral rapidity is to be expected when the large relative capacity of the small vessels is remembered, though there are conditions, both local and general, which would invalidate any calculation based upon this circumstance alone. (C) The Capillary Circulation. Histology. — As the arteries progressively decrease in size it is found that those having a diameter of ^-j- in. possess the three coats common to arteries in general, but that they are very thin ; when the diameter is reduced to -g^ in. the external coat is lost, and the middle consists of a single layer of muscular fibers ; finally this coat is lost and only the thin homogeneous membrane lined with elongated endothelial cells remains. Anatomically, these last are the capillaries. The wall is elastic and may be contractile. The diameter (not the caliber) of the capillaries varies from t^-^-^ to ^innr i'^- They are smallest in nervous and muscular substance and largest in glands and the lungs. It is thus only the largest which will admit a blood corpuscle without change of shape. The wall is exceedingly thin. The average length of a capillary tube is about ^ijj- in. Traced in either direction it is found to gradually assume the characteristics of either an arteriole or a venule. The capillaries form a true plexus, branching to form a most intricate network of tubes with no definite direction whatever. 1 70 CIRCULATION. and with no change in diameter. They penetrate all the tissues of the body except the "non-vascular" such as the hair, nails, cornea, etc., which receive their nutrition by imbibition. No- where, however, do they penetrate the ultimate anatomical elements, such as gland cells, nerve fibers, etc. Fig. Capillaries. The outlines of the nucleated endothelial cells with the cement blackened by the action of silver nitrate. {Landois.) Capacity. — The capacity of the capillary system is much larger than the arterial — probably five to eight hundred times. Physiologically, the capillaries begin where the interchange of materials between blood and tissue begins, and it is supposed that this interchange takes place only in the vessels which are designated capillaries from an anatomical standpoint. Except as regards their anatomy, the most striking characteristic of cir- culation in the capillaries is the fact that the current is no longer remittent but constant. The Capillary Flow. — .A convenient and satisfactory situation THE CAPILLARY FLOW. 171 in which to observe the phenomena of capillary circulation is the mesentery of the frog. It is scarcely possible to make out the actual walls of the capillaries or of the small veins or ar- teries in the field, but only their profiles enclosing a current of passing corpuscles. In such a field are seen small vessels with several corpuscles abreast passing either at a steady rate or with rhythmical accelerations corresponding to the beats of the heart ; the former are venules, the latter arterioles. In the capillaries the corpuscles usually pass in single file in a steady current, though dilatation often allows the passage of two or three abreast. The red corpuscles can seldom traverse a cap- illary without change of form. They are elongated so as to make one of their diameters much shorter than usual. They easily assume this shape, as a result of pressure, and as easily regain their normal outlines, when the pressure is removed. Not infrequently a corpuscle coming to a division of the channel is caught, distorted in shape, and finally, after oscillat- ing for a time, passes on in one of the currents open to it, re- gaining its form at once, or when it has passed into the larger ves- sel. In the capillaries the flow is slower than anywhere else. In one the current may be swifter than in another, and even in the same capillary the rate may be observed to vary, or a cur- rent in an opposite direction may be set up in a tube joining two others. It is to be remembered that the object of the capil- lary system is to spread the blood out, as it were, over a very large area so that it may be exposed freely to the cells ; and, while in the network as a whole there is a general tendency for the blood to pass from arterioles to venules, the plexus is so in- tricate and abundant that, except in capillaries connected directly with the larger vessels, it is not necessary for blood to pass in any one direction in any one capillary in order to ultimately reach the venous system. The corpuscles are seen not to come in contact as a rule with the confines of the tube, but to occupy the center of it only. In 172 CIRCULATION. this space, between the corpuscles and the wall, is an inert layer of plasma, the ' ' still layer. ' ' When there are several cor- puscles abreast it may be observed that the more central ones move the more rapidly. This is because the friction is least there. This element of friction accounts for the still layer of plasma — the plasma being practically the only portion of the blood which will adhere to the wall. The leucocytes move largely in contact with the wall, and therefore progress much more slowly than do the red corpuscles. They roll over repeatedly in their passage and often stick to the side of the vessel for some time before being forced on by the current. Those seen in the main stream among the reds have a continual tendency to escape into the still layer and lag behind. This gravitation of leucocytes to the periphery of the current is said to be due to their relatively low specific gravity, as well as to the adhesive character of their surfaces. Rate of Capillary Flow. — Although the rapidity of the cur- rent varies in different parts of the tube, it is apparent that the capillary current is much slower than either the arterial or ven- ous ; and this is to be expected from the physical fact of the large relative capacity of the capillary system, and from the physiological fact that here must occur the interchange of ma- terials between blood and cells. It is of no little significance, too, that the flow here is constant, for such a circumstance cer- tainly enhances the even distribution and accession of materials. The average rate of flow in a capillary is thought to be from -^^ to -^ in. per second. If the length of the ordinary capillary is -^Tj in., it is seen that with this rate the time of any corpuscle in any capillary is exceedingly brief. Causes of the Capillary Circulation. — It is a fact not to be lost sight of that the cause of the whole circulation is the force of the ventricles. That force may not be exerted directly upon the circulating column, as when it is stored up in the elastic walls of the arteries to be dealt out during diastole ; or it may THE VEINS. 173 be aided or inhibited by various circumstances, such as the valves of the veins, muscular contractions, respiration, etc.; but it is amply able to carry the current not only through the capil- laries, but through the veins back to the heart again. Its in- fluence is directly apparent in the pulse of the smallest arteries, and there is nothing in the phenomena of the capillary or venous circulation inconsistent with the transmission of its in- fluence to them. Pressure in the Capillaries. — The same forces which cause arterial pressure cause capillary pressure — ( i ) the hearf s action, {ji) friction 2xA (3) elasticity. The capillary walls have been said to be elastic ; they are continually in a distended condition from the continual reception of blood, and the reaction of their walls compresses their contents. The average capillary pressure is thought to be from 25 to 50 mm. of mercury. In depend- ent parts it is higher. That it is thus low is to be expected when the large capillary capacity is considered. Moreover, a very large part of the force of the heart has been used up in get- ting the blood out of the arterial system, and consequently less of it is available to distend the capillaries. (D) Venous Circulation. The veins exist for the purpose of getting the blood back to the heart. When it has traversed the systemic capillaries its functionjs performed until it is aerated again. The capillaries converge to vessels a little larger ; these are joined by other similar ones — all of which are the venous radicles, or venules. Uniting they form larger and larger vessels which, iilled with dark blue venous blood, make their way toward the heart. The current here is more rapid as a rule than in the capillaries but less so than in the arteries, the capacity of the veins being about four times that of the arteries. Venous blood on its way to the heart (until it has entered the large vessels near that viscus) may go by more than one route since the veins present frequent an- 1 74 CIRCULATION. astomoses. If the more direct route be blocked by muscular contraction or otherwise, the current may easily take a more cir- cuitous one. Histology. — The veins are thin and flabby as compared with the arteries. Their walls collapse on section. They possess three coats. The external coat is fibrous like that of the arteries ; near the heart are a few cardiac muscle fibers. The middle coat is chiefly composed of inelastic fibrous tissue ; it also contains some elastic fibers and non-striated muscular tissue. The internal coat is practically the same as in the arteries and capillaries ; the endothelial cells are less elongated. Thus it appears that while the vein wall is quite strong it pos- sesses neither much elasticity nor much contractility. Veins take very little active part in the circulation, and the strength of their walls would scarcely be necessary were the pressure evenly distributed as in the arteries. Vasa vasorum penetrate the middle coat. Valves of the Veins. — At frequent intervals in the course of all veins except very small ones and those in the large cavities are small folds protruding into the lumen known as the venous valves. They are quite firmly fixed, in pairs usually, and pre- vent a backward flow of the blood. They can be readily demon- strated in the extremities by rubbing the skin distally, when they produce, by the obstruction which they offer, knotted ap- pearances in the venous column. Venous Current. — The flow of blood in the veins shows nothing of the rhythmical accelerations evident in the arteries. It is more sluggish, and, other things being equal, the rapidity is directly dependent upon the rate at which blood is supplied by the capillaries. But the current cannot be uniform ; it is often interrupted by a contracting muscle or pressure from some other cause, in which case the valves prevent regurgitation and the blood must seek another way to the heart. Different VENOUS PRESSURE. 175 routes are made possible by the anastomoses. Distention from any reason naturally causes the vessel to react upon its contents and force them into whatever channel will admit them. Occa- sionally when a gland is particularly active and its supplying vessels are therefore dilated, the pulse may be transmitted to the venules. Fig. 40. A, vein with valves open. B, with valves closed ; stream of blood passing off by lateral channel. {Kirkcs after Dalton.) The speed of the venous current, in the long run, must de- pend upon the total capacity of the vessels. No estimate for the rapidity in any one part will answer for all parts or for the same part at all times. The current is more rapid near the heart than toward the periphery. Venous Pressure. — From what has been said it is evident that venous pressure is very inconstant. In the small veins it is lower than in the capillaries, and it diminishes as the heart is approached. Any circumstance favoring the flow from the capillaries increases it. It is usually high when the arterial pres- 176 CIRCULATION. sure is low and vice versa. During inspiration it is lessened in the large veins near the heart, and falls below the atmospheric pressure, so that the opening of one of these vessels is very dan- gerous lest air be sucked into the vein and reach the heart. Causes of the Venous Flow. — The one great cause is the ventricular contraction. Superadded to this are several others. Muscular contractions (in connection with the valves) serve to aid the venous circulation provided they are intermittent. The muscles compress the veins, and, since the valves prevent regur- gitation, the blood is obliged to go toward the heart ; if now the muscle relax the vein will fill again, and if another contrac- tion follow the same performance will be repeated. The value of this force is well illustrated in the fact that varicose veins of the lower extremities occur much less often in connection with occupations which require walking than when simple standing is necessary. Gravity aids in case of those veins whose course is downward. Aspiration of the thorax aids by its " suction " force. The power of muscular contraction is also possessed in some degree by the veins. Pulmonary Circulation. — The remarks made concerning the systemic circulation apply in general to the pulmonary, but it is to be remembered that in this case the arteries contain venous and the veins arterial blood. The circuit is much shorter and the capillary resistance is less ; consequently it is not surprising that the force of the right ventricle is much below that of the left — about one-third as great — and that the wall of the pulmonary artery is thinner than that of the aorta. The course of the blood is from the right ventricle to the left auricle. The Rapidity of the Entire Circulation. — So far as possible the rate of the flow in the three systems of vessels has been given. When a properly colored fluid is injected into the jugu- lar vein, whence it passes through the heart, the pulmonary cir- culation, the heart a second time and thence into the systemic circulation of the head back to the starting point (or to the op- VASO-MOTOR NERVES. I 77 posite jugular vein), the time occupied in this circuit is about 21 sec. From this it is estimated that, for the general system (where the average route is longer), some 23 sec. are consumed in the passage of any part of the blood through the entire cir- culatory apparatus. Increase of the cardiac rate increases slightly the rapidity of the blood current when the increased rate is due to physiolog- ical causes, such as muscular exercise ; the current is probably diminished in rapidity when the cardiac rate is increased from pathological causes, such as fever. Innervation of the Blood-vessels. — Frequent reference has been made to the contraction and relaxation of the muscular coats of the vessels. These movements are under the control of the vaso-motor nerves, the term vaso-motor including both vaso- constrictor and vaso-dilator nerves. They are present in the veins as well as in the arteries. Stimulation of the vaso-constric- tors lessens the vessel's caliber ; stimulation of the vaso-dilators increases it. Both kinds of fibers are often found in the same nerve trunk, as in many branches of the sympathetic or in the sciatic. Stim- ulation of such a trunk causes constriction of the vessels to which its vaso-motor fibers are distributed because the vaso-dilators are less easily irritated than the vaso-constrictors. Section of a trunk containing them causes dilatation of the vessels they supply since the " tonic " influence of the vaso-constrictors is removed. The origin and course of the vaso-motor nerves has been a subject of very great confusion among writers. The following facts are given for the supposed truth : The vaso-motor fibers reaching the vessels proceed from cells in the sympathetic ganglia, but these cells are influenced by cells in the vaso-motor centers. The chief vaso-motor center is in the medulla; subordinate centers exist in the cord, probably in the cells of the anterior cornua. Fibers from the bulbar center pass in part to the nuclei of origin of the cranial nerves, but mainly to the subordinate 12 178 CIRCULATION. cord centers. Fibers from these last named pass out with the anterior spinal roots to end in arborizations around the vaso- motor cells of the sympathetic ganglia. Thus the vaso-motor nerves are considered as belonging to the sympathetic system ; but it must not be supposed that they are not found running in cerebro-spinal trunks. Such a circumstance, however, is only a matter of expediency and has nothing to do with the character of their action. There seems no doubt that the axis-cylinders of all the spinal vaso-motor cells pass to sympathetic ganglion cells ; so that when it is said, for instance, that the vaso-motor fibers for certain parts pass out of the cord by the anterior roots of the third, fourth and fifth lumbar nerves, it is not meant that these fibers are themselves distributed to the vessels : they end by arborizing around sympathetic ganglion cells, whence fibers pass to the parts in question. The vaso-dilators differ from the vaso-constrictors in passing directly through the ganglia of the sympathetic chain, and have their cell stations ■ farther on, or even in the vessel wall itself. Vaao-motor reflexes occur from impressions conveyed either from the vessels themselves or from the general sensory surface. The latter is much the more common way. Usually the reflex constriction or dilatation is confined to the area of the nerve stimulated, but sometimes this is not so. When the vessels of one hand are contracted by its being thrown into cold water, those of the opposite hand are similarly affected. It seems that the vasomotor nerves have the power to balance the circulation in different parts ; when the superficial vessels are dilated the deep are contracted and vice versa ; when the abdominal vessels are dilated during digestion the superficial are contracted, etc. Sudden flushes or pallor of countenance are good examples of vaso-motor action, impressions being carried from the cortex (if an emotion be the cause) to the bulbar vaso-motor center. It is unnecessary to give examples of vaso-motor action since they are constantly referred to. When it is remembered that THE BLOOD. I 79 the entire physiological distribution of the blood is regulated by these fibers their importance is apparent. It is by their action that the blood flow is increased to the gastro-intestinal tract dur- ing digestion, to all glands during their activity, to muscles dur- ing exercise, and to all parts where physiological activity is in progress. It is by them also that the blood flow is decreased to parts when occasion demands a less amount. (II) THE BLOOD. Functions. — The office of the blood is to carry to the tissues nutritive materials resulting from digestion and oxygen from the lungs, to convey away from the tissues to the several organs of excretion the broken-down products of physiological activity, to convey internal secretions from one part of the body to an- other, and to aid in the equalization of temperature. It is there- fore immediately necessary to life — indeed to the life of every cell in the body, and, with the exception of such extravascular tissues as cartilage, nails, etc. , which receive their supply from neighboring tissues, it supplies nutriment directly to all the cells of the organism. Amount of Blood. — It has been estimated that the total amount of blood in the average man is about 8 per cent., or one- twelfth, of his body weight. At all times something over 80 per cent, of this amount is contained in the liver, muscles, heart and lungs. The liver contains more than the resting muscles. General Properties. — The blood is an opaque fluid of a faint characteristic odor, salty taste, and alkaline reaction. Its aver- age specific gravity is 1055. It is warmest in the hepatic veins where it has a temperature of about 107" F. In the peripheral vessels its temperature is about 99", while in the deeper ves- sels in general it probably varies from 100° to 107°. The color of arterial blood is bright red j that of venous blood, ex- cept when coming from actively secreting glands, is almost black. Plasma, free from corpuscles, is clear and it is, there- i8o CIRCULATION. fore, the corpuscles which give the blood its color. The pres- ence of oxygen in the red corpuscles accounts for the red color of arterial blood. When the oxygen is lost in the capillaries the change to the venous color takes place. It is the absence of oxygen, not Hsms. presence of carbon dioxide, which accounts for the color of venous blood. Histological Elements. — The blood is composed of (i) a clear fluid, the plasma, or liquor sanguinis, and ( 2 ) three kinds of corpuscles — the red, the white or leucocytes, and the blood- plaques or platelets. The bulk of plasma to that of corpuscles is proba.bly about 2 to i, though it is sometimes given as being only a little greater than that of the corpuscular elements. Fig. 41. A, human colored blood-corpuscles — i, on the flat; 2, .on edge; 3, rouleau of colored cor- puscles, B, amphibian colored blood-corpuscles — i, on the flat : 2, on edge. C, ideal trans- verse section of a human colored blood-corpuscle magniiied 5,000 times linear — a, b, diameter ; c, d, thickness. i^Landois.") Red Corpuscles. — These are biconcave, circular disks hav- ing a diameter of about -g^xr i"^- They probably vary less in size and shape than any other anatomical element in the body and are consequently taken as a standard of measurement for other THE BLOOD. l8l Structures. They are organized nitrogenous bodies containing also inorganic and organic non-nitrogenized materials. Their elasticity has been referred to in describing their circulation. Their specific gravity is about 1088. The biconcave surfaces cause it to appear that they have dark borders when the centers are in focus and dark centers when the borders are in focus ; but this is only because the two portions cannot be in focus at the same time. They present a bright yellow appearance when seen in comparatively small numbers, as under the microscope, the red color being evident only when they are seen in mass. In shed blood the red corpuscles show a decided tendency to accumulate surface to surface like coins of money to form rou- leaux. This results from a post mortem exudate upon their sur- faces. On exposure to the air they quickly shrink, presenting a peculiar characteristic appearance, and are said to be crenated. They have no limiting membrane and no nucleus. In the human male they exist to the number of about 5,000,000 per cu. mm. of blood; in the female the number is about 4,500,- 000. There are supposed to be some 25,000,000,000,000 in the human body. There is a significant and " compensatory " increase in the number at high altitudes where the atmosphere is rare. The biconcave surfaces increase the area for exposure to oxygen. Function of the Red Corpuscles, — It is the plasma which is concerned in conveying nutrition to the cells except so far as oxygen is concerned. It is the function of the red corpuscles to convey this gas from the lungs to the tissues. This may be said to be their only function ; while they do carry away some carbon dioxide from the cells, it is chiefly the plasma which is the vehicle of this gas. The function of carrying oxy- gen is altogether dependent upon the presence of hemoglobin, and a consideration of this substance will practically include the function of the red corpuscles. Hemoglobin. — This is the coloring matter of the blood. The l82 CIRCULATION. red corpuscle consists of a skeleton of/ supporting network, very- delicate in structure, known as the stroma, and of hemoglobin. The latter constitutes the bulk of the corpuscle, forming some 95 per cent, of the solid matter. It is of proteid composition, and may be decomposed into globin, hematin and an unknown residue. It is thought to consist of carbon, hydrogen, oxygen, nitrogen, iron and sulphur. It is present in human blood to the extent of from 125 to 150 parts per thousand. Its peculiar property is that of combining with oxygen to form oxyhemo- globin whenever it is exposed to that gas. Oxyhemoglobin is quite unstable. It readily gives off oxygen when placed in an atmos- phere devoid of oxygen, or when the oxygen pressure is dimin- ished — a condition which is shown under Respiration to exist in the capillaries. The hemoglobin left after oxygen has been given up is known as reduced hemoglobin. Any gas for which hemoglobin has a stronger affinity than for oxygen will, if present, of course, take the place of oxygen and prevent the formation of oxyhemoglobin. Carbon monox- ide or nitrous oxide will do this. The former causes asphyxia when ordinary illuminating gas is inhaled, because it forms with hemoglobin a relatively stable compound, carbon-monoxide- henioglobin, which is not broken up in the capillaries and which, if it were, would not furnish the necessary oxygen to the cells. It poisons because it prevents the appropriation of oxy- gen by the hemoglobin. It is claimed that the small part of carbon dioxide which is conveyed away from the cells by the red corpuscles is also in combination with hemoglobin. The idea has been advanced that the oxygen combines with the pigment portion and the carbon dioxide with the proteid portion of hemoglobin. The presence of iron in hemoglobin seems to be requisite to the formation of oxyhemoglobin. It exists in very small amount, but is essential. It is not free, but constitutes a part of the molecular formula of hemoglobin. It clings to the hematin molecule upon dissociation. THE BLOOD. 1 83 Hemoglobin is supposed to be the origin of the bile and urinary pigments. Development and Fate of the Red Corpuscles. — The average life of a red corpuscle is probably not very long. Their con- tinued destruction is an accepted fact, and this necessitates their continual manufacture. In a human adult the red corpuscles are formed in the red marrow of bone. In the embryo they are formed elsewhere, but by no special organs before the osseous system develops. When rapid manufacture is called for, as af- ter hemorrhage or as a consequence of other pathological condi- tions, the red marrow is greatly stimulated in respect to this function, and not infrequently the corpuscles appear in the cir- culation before they have lost their nuclei — a circumstance which under normal conditions does not occur. Is has not been shown that the spleen is concerned in forming red corpuscles. As to the destruction of these elements, it is probable that this takes place in any part of the circulation without the interven- tion of any special organ, the pigment being carried to the liver and kidneys and there discharged in the bile and urine. There is some evidence that the spleen and liver are particularly con- cerned in this destruction, but the proof is by no means conclu- sive. White Corpuscles. — The white corpuscles, or leuco- cytes, are granular in appearance, have an average diameter of ■^hi^ i"- ^Jid no characteristic shape. Their number varies with many conditions, but probably averages some 7,500 per cu, mm. of blood. Their surfaces have a more adhesive character than do those of the red corpuscles. Their appearance is indicated by their name. They are found under normal conditions in the blood, lymph, chyle, and other fluids of the body. Pus cells are dead leucocytes, and leucocytes are found to be increased in number in the blood during ordinary inflammatory processes. They are also more numerous during digestion than in the in- tervals. 1 84 CIRCULATION. According to their staining properties, their size and the shape of their nuclei they are divided into polymorphonuclear, large mono- nuclear, small mononuclear, eosinophile and franstltonal l&xcx)cytes. This division is of significance in many pathological conditions. Their most striking property is that of ameboid movement, and it is chiefly through the exercise of it that they perform their physiological duties. Fig. 42. Human Leucocvtes Showing Ameboid Movements, {Landois.) Functions of the Leucocytes. — Howell gives to the leuco- cytes a five-fold function, (i) They ingest pathogenic bacteria and thus protect the body from these organisms. (2) TYi&y aid in the absorption of fats from the intestine. (3; They aid in the absorption of peptones from the intestine. (4) They are con- cerned in blood coagulation. (5) They help to maintain the proper amount of proteid in the plasma. With respect to (2) and (3), it should be said that reference is had to leucocytes in the submucous lymphoid tissue of the alimentary canal rather than to those in the blood current during the absorptive act. As regards (5), it is thought that the leu- cocytes continually undergo disintegration in the currept, and the products of the process may keep up the normal amount of proteid in the plasma. THE BLOOD. 1 85 Origin of Leucocytes. — These corpuscles increase by cell division in the current ; their ultimate origin in the adult is probably in most part from the spleen and lymphoid tissue through- out the body, whence they reach the blood by the lymph stream. It is not impossible that the red marrow is also productive of leucocytes. In fetal life these are developed without the inter- vention of any special organ. Blood Platelets. — These are formed elements in the blood, have one-sixth the diameter of the red corpuscles, and their proportion to these latter is about i to 20. There is no evidence that they develop into corpuscles. It is not impossible that they are the nuclei, whole or broken up, of disintegrating leuco- cytes. They take part in coagulation. Composition of the Blood. — The blood, as a whole, contains of water about 790 parts, the remainder being organic and inor- ganic solids. The inorganic salts constitute only some 8 parts per thousand ; they are principally those of sodium, calcium, magnesium and potassium. Composition of the Corpuscles. — The red corpuscles contain about 680 parts water, 310 parts organic and 10 parts inorganic solids. The most important of the organic solids is hemoglobin, which constitutes about 95 per cent, of these. Besides hemo- globin there are other proteid materials, mainly globulin and fatty matters, such as lecithin and cholesterin. The corpus cles contain an excess of potassium salts and a deficit of sodium salts as compared with the plasma. The properties of hemoglo- bin have been referred to. The composition of the white corpuscle is doubtful. Their proteid is probably nuclein. They also contain lecithin, cho- lesterin and glycogen together with inorganic salts. Nothing is known of the composition of the blood-plaques. The Blood Plasma. — The plasma in a thin layer is perfectly colorless, but in quantity has a clear yellowish tinge from the presence of some unknown pigment. Its specific gravity is 1 86 CIRCULATION. about 1028. The plasma is not identical with the serum as will be seen under the discussion of coagulation. Composition of the Plasma. — When we remember that the blood is analogous to the tissues in that it contains organized cells requiring nutriment and throwing off wastes, and that it furnishes nutriment to all parts of the body and removes the products of their disassimilation, we have already an idea as to what things it contains. In a general way it may be said that the corpuscles and plasma contain the same materials, but in widely varying proportions. A discrepancy in the salts of sodium and potassium has been mentioned ; the phosphorized fats are relatively abundant in the corpuscles while the fatty acids predominate in the plasma. Briefly, the plasma contains in 1,000 parts, 920 of water and 80 of solids; of the solids 70 are organic and 10 are excrementitious and inorganic salts. Flint classifies the constituents of the plasma, and of the en- tire organism, as (i) inorganic, (2) organic saline, (3) organic non-nitrogenized, (4) excrementitious and (5) organic-nitrog- . enized. 1. The inorganic constituents need no special reference. They belong to the binary class of proximate principles, and pass through the system unchanged. 2. The organic saline constituents belong to -the ternary class of proximate principles. They are formed in the body, the lac- tates being a typical example ; these probably result from a union of lactic acid (resulting from changes in the dextrose of the blood) with a base from the decomposition of bicarbonates. The union of fatty acids with bases is another example of this class. 3. The organic non-nitrogenized constituents are derived from the food, but do not exist in the blood in large quantity. They belong to the ternary class of proximate principles, and repre- sent the oleaginous and saccharine foods. 4. The excrementitious materials are chiefly carbon dioxide, urea, urates, creatin, leucin, tyrosin, xanthin, etc. They will be noticed under Excretion. THE BLOOD. 187 5- The nitrogenized constituents correspond to the quaternary- class of proximate principles and are the iftost important in the plasma. The three chief of these are serum albumin, serum glob- ulin (^paraglobulin) and fibrinogen. Serum-albumin is supposed to be the form assumed by the largest part of the peptones in the blood; they seemingly undergo a change in being absorbed and appear in the blood chiefly as serum-albumin. This substance is supposed to furnish the bulk of proteid nourishment to the cells, it, like the other nutritive materials, being passed from the blood to the lymph and thence to the cells. It is probably identical with " serine," and exists in blood to the extent of some 53 parts per thousand. Paraglobulin, or serum globulin, has an obscure origin. It is natural to suppose that it comes from digested proteids, and this is probably correct. There is some evidence that it comes from disintegrated leucocytes. It doubtless contributes to the pro- teid demands of the cells, but whether it is appropriated as serum -globulin or changed to some other form is not determined. Fibrinogen also has an obscure origin and function. It is directly concerned in coagulation, giving rise to fibrin in a way to be described presently.; but it is not known what nutritive office it may have. It has been suggested that its origin- is in the disintegrating leucocytes. It constitutes about 3 parts in one thousand of the blood. These three constitute the main proteid substances of the plasma ; though others are mentioned they are usually these same substances under different names or supposed constituent parts of these. Fibrinogen and paraglobulin are said by some to exist together under the name of plasmine. Coagulation of Blood. — ^The clotting of shed blood is a matter of common observation. It is of prime importance in the checking of hemorrhage. When blood is received into a vessel it soon assumes ■ a jelly-like consistence, and later the mass becomes firmer and smaller. There is pressed out of it a l88 CIRCULATION. fluid very like plasma in appearance called blood serum, which accumulates on the top of the clot owing to its relatively low specific gravity. The essential part of the clotting process is the formation of fibrin. As soon as the blood is drawn, unless the containing vessel be shaken, or the fluid agitated in some other way, the corpuscles sink toward the bottom and are caught in the meshes of the fibrin as it forms. The fibrin is deposited in the shape of fine fibrils interlacing to make a close network. Fibrin con- tracts when it has been formed, and thus it is that the serum is forced out. The inclusion of the red corpuscles, and conse- quently the red appearance of the clot, is only an accident ; they remain a part of the clot only because the contraction of the fibrin cannot force them out as it does the serum ; their presence is not necessary to coagulation. The explanation of coagulation resolves itself into an explan- ation of the formation of fibrin. There is at present no abso- lutely satisfactory explanation of the process. Probably the most widely accepted view is that after blood is shed there is formed from the disintegrating leucocytes a substance called fibrin ferment, which ferment unites with fibrinogen to form fibrin. The fibrinogen and fibrin ferment are called "fibrin fac- ■ tors." Fibrin ferment does not exist in the circulating blood — at least not in sufficient quantity to cause coagulation. More- over it has become evident that clotting will not occur without the presence of calcium salts, though in what way they influence the process is not known. When blood clots the destruction of leucocytes must precede. This may be caused in the vessels by the introduction of foreign bodies into the current, or by injury to the lining epithelium of the tube. Blood does not clot in the vessels under normal con- ditions because not enough leucocytes disintegrate to form the necessary amount of fibrin ferment. The serum is the plasma minus the fibrin. THE LYMPH. 189 The relation of plasma, serum and clot is shown by the ac- companying schema. [Plasma jl^"."". Blood '■"''""Ir, . Corpuscles [ ^^°*- THE LYMPH. The lymph is a clear colorless fluid contained in the lymphatic vessels and tissue spaces. It resembles plasma in general ap- pearance and does not differ greatly from it in composition. The Lymph Vessels. — These vessels originate in at least three different ways, (i) All cells may be said to be bathed in lymph, being surrounded by that fluid lying in the irregu- larly shaped spaces between them. These spaces communi- cate with each other and finally converge to the lymph capil- laries. The intervals are called the " extravascular lymph spaces." (2) In certain situations, particularly in the nervous centers, the small blood-vessels are completely surrounded by and included in larger tubes, the ' 'perivascular lymph canals. ' ' These likewise pass on to the lymph capillaries proper. (3) The large serous cavities, like those lined by the peritoneum, pleura, tunica vaginalis, etc. , have large numbers of lymphatic radicles opening abruptly into them, or rather originating from them, and these may be considered as great extravascular lymph spaces. The course of the lymph is from the tissues to the subclavian veins, where it enters the vascular circulation. The lymphatic vessels from the right arm and the right side of the face, head and chest converge to form the ductus lymphaticus dexter, which enters the right subclavian vein at its junction with the internal jugular. The lymphatics from all other parts of the body con- verge to form the thoracic duct, which enters the left subclavian vein at its junction with the internal jugular. The thoracic duct begins by a dilated pouch lying upon the second lumbar I go CIRCULATION. Fig. 43. Diagram Showing the Course of the Main Trunks of the Absorbent System. The lymphatics of lower extremities, D, meet the lacteals of intestines, LAC, at the recep- taculum chyli, R.C., where the thoracic duct begins. The superficial vessels are shown in the diagram on the right arm and leg, S, and the deeper ones on the left arm, D. The glands are here and there shown in groups. The small right duct opens into the veins on the right side. The thoracic duct opens into the union of the great veins of the left side of the neck, T. (Veo.) THE LYMPH. I9I vertebra. This pouch receives the 1-4 lymphatic branches which have converged from the lacteals, and is called the recep- taculum chyli. The lacteals pass through the mesenteric lym- phatic glands on their way to the receptaculum chyli. The distribution of the lymphatics needs no comment when it is known that they receive the plasma which has been passed out of the vascular capillaries and thus collect fluid from well- nigh every tissue in the body. The structure of the lymphatics is quite similar to that of the veins, though they are more delicate. The lymph capillaries probably contain only a single coat like the venous capillaries. In the large vessels this thin endothelial coat is supplemented by connective tissue fibers together with some elastic and non- striated muscle fibers. They are very abundantly supplied with valves which operate in the same way as the venous valves. The vessel wall is quite elastic and has some contractile power. Lymphatic Glands. — rAU the lymphatics pass through one or more lymphatic glands on their way to the larger trunks. These bodies are not true glands. Their structure is adenoid. There are some six or seven hundred in the body, varying in size from a pinhead to a large bean. The superficial glands are especially abundant about the groin, axilla, neck and the other flexures. The deep ones are most numerous about the great vessels. The mesenteric glands are found between the folds of the mesentery. The lymphatic glands are of irregular shape and contain within their substance large numbers of lymph spaces or canals through which the incoming lymph must pass. The vasa efferentia are usually fewer in number and larger in size than the vasa affer- entia. The current must be considerably delayed in the glands. They are probably concerned in the elaboration of leucocytes of the lymphatic circulation, while their retention of toxic ma- terials — even to their own hurt — is a common pathological oc- currence. Properties and Composition of Lymph. — Lymph is a com- 192 ORCULATION. paratively clear liquid containing leucocytes. After meals the color becomes whitish from the admixture of chyle, and nu- merous fat droplets are present. Neither red corpuscles nor platelets are thought to be found in lymph except accidentally. The speciiic gravity is lower than that of the blood. Lymph coagulates when drawn, since the fibrin factors are present ; but the process is less prompt and the clot is less firm than in case of blood. In order to form an idea as to the constituents of lymph it is only necessary to say that its ultimate origin is the blood plasma, except in so far as its composition is changed during digestion. The plasma makes its way through the capillary walls out to the tissues bringing nourishment to them and removing waste products from them. In thus coming in contact with the tis- sues the plasma finds itself in the extravascular lymph spaces and its name is simply changed to lymph. It thus appears' that lymph may enter the extravascular spaces by the direct passage of plasma out of the vessels or by being excreted, as it were, from the tissue cells. In any case the constituents of lymph are not very differ- ent from those of plasma, except, of course, when intestinal digestion is in progress and chyle is introduced into the lym- phatic circulation. It contains the three plasma proteids, urea, fat, lecithin, cholesterin, sugar and inorganic salts. The proteids are less abundant than in plasma, as might be supposed when it is remembered that they possess little osmotic power. The inorganic salts are in about the same proportion in both fluids. It is significant that the amount of urea and related excre- mentitious products is more abundant in lymph than in plasma ; their source is the destructive metabolism going on in the cells to which the plasma has been supplied, this plasma finding its way back as lymph. It is by no means certain, however, that all the plasma escaping from the capillaries is carried away by the lymphatic system. Some may reenter the blood-vessels. THE LYMPH. 1 93 There is no unanimity of opinion as to the exact method of passage of plasma through the capillary walls into the lymph spaces. Some maintain that the phenomena can be explained by the ordinary physical laws of diffusion, filtration and osmosis when existing conditions of pressure, etc., are taken into consid- eration. Others hold that these laws are insufficient in them- selves to account for various occurrences in this connection, and ascribe to the capillary endothelium some active secretory power governing, or at least influencing, the outward passage of the plasma. The Flow of Lymph. — There is no organ corresponding to the heart to keep the lymph current in motion. The main causes for its direction from the extravascular spaces toward the veins in the neck is the degree of pressure to which it is subjected in those spaces as compared with the inferior, or even " negative," pressure obtaining near the terminations of the great ducts. It is known that at all times the venous pressure in the subclavian veins is low and that it may even fall below the atmospheric pressure, so that "suction" is exerted upon the lymphatic ducts where they enter those vessels. The lymph pressure in the extravas- cular spaces is estimated to be one-half the capillary blood-pres- sure. Friction and gravity (where the course of the vessels is upward) oppose the passage of the fluid. Consequently it accu- mulates in the spaces and in the smaller lymphatics until the pressure there becomes greater than the resistance of these forces, when it passes onward. Since lymph is being continually pro- duced this superior pressure in the extravascular spaces and small lymphatics is a fairly constant factor and keeps up a correspond- ingly constant current. There are two factors which are accessory to this peripheral pressure: (i) Thoracic aspiration by bringing about negative pressure in the veins in and near the chest brings about a like condition in the tributary lymphatic ducts; furthermore, the effect of aspiration makes itself felt directly upon the thoracic 13 194 CIRCULATION. duct since its greatest extent is in the thorax. (2) The valves of the lymphatics act in a similar manner to those of the veins and constitute a very necessary factor in the lymphatic circula- tion. Although the lymph flow resembles that of the venous blood, it is less regular and more sluggish, but probably not so slow as might be supposed. Properly colored solutions in- jected into the blood have been demonstrated in the lymph of the thoracic duct " in from four to seven minutes." Lymph and Chyle. — It is scarcely necessary to refer to the differences between these two fluids. Chyle is the intestinal lymph during digestion. In the intervals of digestion the con- tents of the lacteals do not differ materially from lymph in other localities. Chyle has a whitish milky appearance due to the presence of emulsified and saponified fats. Its specific grav- ity naturally depends largely upon the amount of fat ingested, but is always higher than that of ordinary lymph and lower than that of blood. Not only is there more fat in the chyle than in lymph, but the other solids are also increased. The proteid constituents are considerably more abundant. For the most part the higher specific gravity is explained by the absorption of solids in solution from the alimentary canal. Chyle is forced out of the lacteal by contraction of the non- striated muscle fibers which run along by the vessel. When re- laxation of the fibers occurs, return of chyle into the lacteal is prevented by a valve at the base of the villus. CHAPTER VI. RESPIRATION. Object. — The object of respiration is to furnish oxygen to the tissues and remove carbon dioxide from them. The inter- vention of the lungs and blood is necessary to accomplish this end. At each inspiration a certain voluifte of air is taken into the lungs, and from it, while in these organs, is removed a cer- tain amount of oxygen which enters the blood of the pulmo- nary capillaries. At each expiration there is removed from the lungs a certain volume of air, and it contains a proportion of carbon dioxide over and above that contained in the ordinary atmosphere, i. e., in the inspired air ; this carbon dioxide is re- moved from the blood of the pulmonary capillaries and enters the air in the lungs. The entrance and exit of air to and from the lungs, in obedience to movements to be noticed later, con- stitutes what is commonly called respiration ; but the mere tide of the air inward and outward is of no significance unless the interchange of oxygen and carbon dioxide take place. Internal Respiration. — Nor is this interchange of value unless another occurs in the tissues. The oxygen which has entered the pulmonary blood is conveyed by the circulation to a point where the fluid is brought into very close relationship with the tissues (namely, in the capillaries), and is here given up to the cells ; furthermore, at the same place the cells give up carbon dioxide to the capillary blood. It is only for the purpose of effecting this last interchange that there is any respiration, or any respiratory apparatus. Inspiration and expiration, the pulmonary inter- change of gases, the transportation of oxygen and carbon dioxide I9S 196 RESPIRATION. to and away from the cells, are all equally immaterial ejccept as being means to the accomplishment of this end. It would make no difference whether pulmonary respiration were kept up or not if oxygen could be introduced into the blood and carbon dioxide removed from it in some other equally efficient way. So far as the cell is dependent on the acquisition of oxygen and the re- moval of carbon dioxide, it would make no difference if there were no respiration and no circulation if these materials could be acquired and removed in some other equally efficient way. On the other hand, it were useless to keep up artificial respira- tion or to inject oxygen into the lungs if the cells, through some disability, cannot take up the oxygen furnished, or if the circu- lation cannot absorb or convey the oxygen. It is seen that, from the standpoint of the blood, the inter- change of gases in the lungs is exactly opposite to that in the tissues ; that is to say, in the lungs it loses carbon dioxide and gains oxygen, while in the tissues it loses oxygen and gains car- bon dioxide. The pulmonary interchange is properly termed external respiration in contradistinction to that in the tissues which is termed internal respiration. It is needless to comment upon the universal necessity of oxy- gen to the life of cells. Its appropriation is to be looked upon as a part of the nutritive process ; ■ and, indeed, while in the long run, cells are certainly dependent upon the nutriment fur- nished by the ordinary aliments, they will retain their vital ac- tivity for a longer time when deprived of any or all ■ of these than when deprived of oxygen alone. This gas is more imme- diately necessary to the maintenance of life than is any other substance. Since, in order to bring about internal respiration in the human being, the lungs and circulation happen to be necessary, attention will have to be directed to the respiratory phenomena taking place in both. anatomy of the respiratory organs. i97 Anatomy of the Respiratory Organs. It will be considered that the air has passed through the pos- terior nares into the pharynx and is ready to enter the larynx. The Larynx. — This lies in front of the esophagus, its upper opening communicating with the middle pharynx. It is composed of four cartilages and the muscles and ligaments which hold them together. The cartilages keep its lumen constantly open, while the muscles effect movements concerned in deglutition, respira- FiG. 44. Diagram of the Respiratory Organs. The windpipe leading down from the larynx is seen to branch into two large bronchi, which subdivide after they enter their respective lungs. (K»o.) tion and phonation. The cartilages are the thyroid, cricoid and two arytenoids. The two alae of the thyroid meet at an acute angle in front to form the Adam's apple. The cricoid is at the lower end of the larynx, completely surrounding it. The arytenoids are movable and rest upon the back of the cricoid. (Fig. 45.) The vocal cords, two ligamentous bands covered by a thin 198 RESPIRATION. layer of mucous membrane, stretch antero-posteriorly across the upper end of the larynx, while the false vocal cords, hav- ing nothing to do with phonation, and pinker in color, are above and parallel with the true cords. A small triangular leaflet of fibro-cartilage is attached by its base to the base of the tongue and to the upper anterior part of the larynx. This is the epiglottis. It fits accurately over the opening of the larynx, and during the act of deglutition is closed to prevent the entrance of food, saliva, etc. Except during deglutition the epiglottis is raised and there is free passage of air into and out of the laryngeal cavity. The vocal cords are fixed anteriorly to a point between the alse of the thyroid and posteriorly to the movable arytenoids. Intrinsic muscles have the power of so moving the arytenoids as to separate and approximate the posterior attachments of the cords and thus increase or decrease the size of the rima glottidis. During inspiration these muscles act to separate the cords and allow free entrance of air into the trachea. When this act has ceased they relax and the cords are passively approximated. The expiratory act separates the cords and they afford no obstruction to the exit of air. The inspi- ratory act, on the other hand, tends to draw the cords together and the active intervention of the muscles is necessary to keep the glottis open. The Trachea. — The trachea succeeds the larynx in the respi- ratory tract. It begins at the cricoid cartilage and extends down- ward for about four and a half inches where it bifurcates to form the right and left bronchi, one of which goes to each lung. The trachea consists of an external fibrous membrane, between the layers of which are a number oi cartilaginous rings, and an internal mucous membrane. The rings are the most striking part of the trachea. They serve to keep the canal open at all times. The inspiratory effort would otherwise collapse the walls and prevent the en- trance of air. These rings are sixteen to twenty in number, and are lacking in the posterior third or fourth of the circumference. TRACHEA AND BRONCHI. 199 OUTLINH SHOWING THE GENERAL FORM OF THE LaRYNX, TrACHEA, AND BrONCHI, AS SEEN FROM BEHIND. A, great comu of the hyoid bone ; t, superior, and f , the inferior cornu of the thyroid carti- lage ; e, epiglottis ; u, points to the back of both the arytenoid cartilages, which are sur- mounted by the corqicula ; c, the middle ridge on the back of the cricoid cartilage ; ir, the posterior membranous part of the trachea ; b, ^ , right and left bronchi. {Kirkes after Alien Thomson.) 200 RESPIRATION. They are, therefore, not true rings. The interval between their ends is filled with fibrous and nonstriped muscular tissue. The mucous membrane is lined by ciliated epithelium, and has mucous glands in its substance (Figs. 44, 45). The Bronchi. — The primitive bronchi are of the same essen- tial structure as the trachea. The right is the larger, shorter, and more nearly horizontal. This probably accounts for the more frequent lesions in the right lung. Penetrating the lung substance they divide and subdivide until each, by its ramifica- tions, communicates with every air vesicle in that lung. When Fig. 46. BrotteAiat Muse/e, J^ncMal flrteiy. Carti'lajs G/ancf aei'/ti B: c/uct. T.S, intra-pulmonary bronchus of cat. PA and PV, pulmonary artery and vein ; bronchial vein ; V, air-vesicles. {^Sterling.') the primitive bronchus has divided, the incomplete cartilagi- nous rings are replaced by irregular plates of cartilage, which are so arranged as to completely encircle the tube. These extend as far as the division of the tubes into branches -^^ in. in diameter. Surrounding the tubes in the lung substance is a circular layer AIR VESICLES. 201 of plain muscular fibers ; these cease only at the air vesicles. Elastic fibrous tissue is also present everywhere in the bronchial walls and is continued over the vesicles themselves. Bronchial tubes above -^ in. in diameter have in their walls cartilaginous plates, muscular tissue, fibrous elastic and inelastic tissue and a lining membrane of ciliated epithelium. Bronchial tubes -^ in. in diameter, and smaller, have in their walls the same elements excepting the cartilage ; but as the tubes subdivide their walls grow continuously thinner, and the inelas- tic tissue becomes less and less in amount, until it finally prac- tically disappears ; the ciliated epithelial cells gradually give place to a single layer of squamous cells in the smallest tubes. The smallest bronchial tubes, the bronchioles, are from y^-j- to 1^ in. in diameter. Of course everywhere in the walls there are vessels and nerves. The Air Vesicles. — Each bronchiole opens into a collection of air vesicles, or cells, called a pulmonary lobule. The term lobulette will be here applied to it, however, reserving the word lobule for a collection of lobulettes about \ in. in diameter. The bronchiole entering the lobulette becomes the infundibulum (Fig. 47) , a slightly dilated canal from which are given off from eight to sixteen oblong vesicles, the true air cells. The cells are a little deeper than they are wide and end in blind extremities. The diameter of the lobulette is about -j^jp-iV ^'^- > that of the vesicle about ^1^ - ' ; ^ in. It has been estimated that there are some 725,000,000 of these vesicles in the lungs and that their combined area is something over two hundred square yards. The walls of the air cells are very thin, being composed of a single layer of flattened epithelium together with highly elastic fibrous tissue. Ramifying in this latter is a most abundant supply of capillaries, which are larger here than anywhere else in the body. The physical conditions are most favorable for the exchange of gases between the blood and air, each capillary being exposed to vesicles on both sides, and the air and blood 202 RESPIRATION. being separated only by the very thin walls of the capillary and vesicle. The elastic tissue is very important in expelling the air from the cells when the inspiratory effort has ceased. For the nutrition of the bronchi and lung substance arterial blood is furnished by the bronchial artery, which enters and Fig. 47. TERMINAL BRANCH OF A BRONCHIAL TUBE, WITH ITS INFUNDIBULA AND AIR-SACS, FROM THE MARGIN OF THE LUNG OF A MONKEY, INJECTED WITH QUICKSILVER. a, terminal bronchial twig ; i h, air-sacs ; c c, infundibula. X ro. {Kirkes after E. E, Schulse. ) ramifies with the bronchi. The entire mass of venous blood passes directly from the heart through the pulmonary artery to the lungs to be arterialized, and it Is the capillaries of this artery which furnish the abundant network between the air cells. The lungs have the shape of irregular cones, their bases rest- ing on the diaphragm and their apices extending to points a little above the clavicles. They are completely separated from each other by the mediastinum and their external surfaces are covered by the pleura, a serous membrane similar to the peri- toneum and reflected from the thoracic wall. The right lung is divided by fissures into three lobes and the left into two. Superficially the lung substance is seen to be subdivided into areas about \ in. in diameter called the lobules. Each lobule is composed of a number of lobulettes as above mentioned. mechanism of respiration. 203 Mechanism of Respiration. Respiration implies the more or less regular entrance and exit of air to and from the lungs. The entrance is inspiration ; the exit expiration. Now, tha thorax is a closed cavity, not- withstanding the fact that the lungs have an opening (the trachea) by which they communicate with the external air ; and, so far as the simple ingress and egress of air is concerned, the question of pulmonary respiration resolves itself into one of pure mechanics. • The lungs may be looked upon as a bag (or two bags) in the thoracic cavity. Inspired air does not enter the thoracic cavity, but this bag which is in it. This fact is of the greatest importance. Furthermore, the lungs are everywhere in contact with the thoracic wall by their pleural surfaces. They are composed very largely of highly developed elastic tissue, but are perfectly pas- sive themselves. That is to say, they possess no power of ex- pansion except in obedience to extraneous influences. As found in the thorax they possess a contractile power, but only because certain forces have put their elastic tissue on the stretch, and the contraction is a simple effort of the tissue to return to the condi- tion which characterized it before it was subjected to the expand- ing force. Before birth there is no aif in the lungs, and this is the only time when the elastic tissue is not on the stretch. The bronchioles and air cells are collapsed, but the thorax is contracted and the pulmonary and thoracic walls are in contact by their respective pleural surfaces. When the child is born an inspiration fills the lungs and they are never thereafter devoid of air. They collapse to a certain extent and leave the thoracic wall when the chest is opened, but cannot empty themselves entirely because the walls of the bronchioles collapse before all the air can escape. This collapse of the lungs when the chest wall is opened shows that the lung structure is in a constant state of tension, which tension has 204 RESPIRATION. always a tendency to empty the lungs, but cannot do so because the thorax can contract only so far, and when its contraction has reached its limit, for the lung to contract farther would mean a separation of the pulmonary and thoracic walls and the formation of a vacuum between them. The additional reason above given, namely, the collapse of the bronchioles before all the air can escape, is inoperative under normal conditions and need not be considered. Causes of Respiratory Movements. — Seeing that the lung structure has always a tendency to empty itself of air, it follows that inspiration cannot be dependent upon the lung itself Granting, from the physical conditions present, that the lungs and thorax must expand and contract together, the expansion of the lungs in inspiration is a consequence and not a cause of the thoracic expansion, and the contraction of the lungs in expira- tion is a cause and not a consequence of thoracic contraction. This statement as to expiration applies only to ordinary tran- quil respiration, as will be seen later. Speaking broadly then, inspiration is an active and expiration a passive process. That is, inspiration occurs as a result of the activity of certain muscles which operate to expand the thorax, and expiration as a conse- quence simply of the cessation of activity on the part of those muscles and the passive contraction of the lung tissue. The relation of the thorax and lungs and the action of each in respiration may be illustrated. Suppose a bellows, which, say for some mechanical reason, cannot completely collapse and which is itself air-tight, to contain a thin rubber bag communi- cating by a tube with the external air ; suppose the bag con- forms in general outline to the shape of the bellows, and under a moderate degree of distention completely fills the cavity of the bellows when the latter is collapsed as far as possible. Now, it being understood that the bag was somewhat distended to cause it to fill the bellows, and that all air has been allowed to escape by a temporary opening from between the walls of the two and RESPIRATORY MOVEMENTS. 205 the bellows itself made air-tight afterward, it follows that unless the bellows can contract the bag will remain distended and will not leave the bellows wall, although it will have a constant ten- dency to do so. It is also apparent that, since the bag exerts a continual compressing effect on its contents, the pressure inside it will be greater than that outside between it and the bellows wall. Under these conditions there will be a constant tendency on the part of the bellows to collapse, and some active force will be necessary to expand it ; when it is made to expand the con- tained bag will expand with it. Suppose the expansion to be stopped at a certain point and the bellows held (to prevent con- traction) ; it is obvious that now the pressure inside the bag is greater, while that outside between its walls and those of the bel- lows is less, than when the expansion began ; that is, the bag has become distended more and is exerting a greater compressing effect upon its contents. If now the bellows be simply released, both the bag and the bellows will contract and the former will empty itself so far as the latter will allow, but when the bellows has reached the limit of its contraction the bag also ceases to contract, although it remains in a constant state of tension. If at any time air be admitted to the bellows proper the bag will at once collapse. This illustration can be applied to the mechanical principles obtaining in ordinary respiration. The bellows is the air-tight thorax which cannot contract beyond a certain point ; the rubber bag is the elastic lungs under constant tension, communi- cating by the trachea with the external air and following, or being followed by, the movements of the thorax ; the pressure in the bag and between it and the bellows wall represents the intrapulmonary and intrathoracic pressures respectively. It will be noticed later that this illustration does not go quite far enough to explain a few of the phenomena of expiration, but it could very easily be made to do so. Inspiration. — Any force which expands the thorax aids in in- 2o6 RESPIRATION. spiration ; and any muscles which increase any of the thoracic diameters expand the thorax. The diameters increased are chiefly the (i) vertical ^Vi^ (2) antero-posterior. The vertical is increased by descent of the diaphragm, which descent is caused by its contraction, since, owing to the intra- thoracic "pull " exerted upon it, it is normally vaulted upward. The antero-posterior diameter is increased chiefly by the eleva- tion of the ribs. Since these bones, attached posteriorly to the spinal column, run not only forward but also downward to join the sternum by the costal cartilages, it follows that the elevation of their anterior ends will increase the diameter in question. Muscles of Inspiration. — Elevation of the ribs is effected by a number of muscles. The three scaleni are attached above to the cervical vertebrae and below to the first and second ribs ; their action elevates not only these ribs but the whole anterior chest wall. The action of the intercostales externi is still a subject of dis- pute in connection with the physiology of respiration. These muscles are attached externally to the adjacent borders of the ribs, and thus occupy the intercostal spaces. Their fibers are directed downward and forward, and the effect of contraction of any single intercostal muscle would be to approximate the two ribs to which it is attached; but if it can be assumed that the first rib is fixed, then, from the direction of their fibers, the external intercostals will render the ribs more nearly horizontal by raising their an- terior movable extremities. It seems that the first rib is pre- vented from descending, probably by the simultaneous contraction of the scaleni. The intercostales interni have a direction almost at right angles to that of the externi ; the sternal portions of these act from the sternum and also elevate the anterior extremi- ties of the ribs. The levatores costarum are attached to the transverse processes of the dorsal vertebrae and to the upper borders of the ribs posteriorly. The transverse processes are fixed points and the ribs are movable on their spinal articulations. Contrac- INSPIRATION AND EXPIRATION. 207 tion of these muscles is, therefore, very efficient in elevating the anterior ends of the ribs. The action of the diaphragm is the most notable of the mus- cular phenomena connected with respiration, and it deserves to be called the "muscle of respiration.''' These are the muscles which are chiefly concerned in ordi- nary inspiration. Their combined action also increases slightly the transverse diameter of the chest. But there are certain others, known as auxiliary muscles of inspiration, which are called into play during profound or forced inspiration. Their action is evident from their attachments — all operating chiefly to in- crease the antero-posterior diameter. They are the serratus posticus superior, stemo-mastoideus, levator anguli scapula, tra- pezius, pectoralis minor, pectoralis major (j:ostal portion) , serratus magnus, rhomboidei and erectores spina. It will be noticed that several of these which usually take their fixed points on the chest, as for example, the stemo-mastoideus, pectorales, etc., must, in order to aid inspiration, take their fixed points at their other extremities. Expiration. — When the force which expands the chest during inspiration ceases to operate, expiration follows. Not only does the elastic (i) lung tissue force out the air, but the (2) thoracic walls, by their costal cartilages and their intercostal tissues, are themselves elastic, and this elasticity, aided by the (3) "tone'" of the muscles which have been put lipon the stretch during inspiration and which are now seeking to return to their normal condition, tends to restore the thorax to the dimensions it had previous to the inspiratory act. So far no actual muscu- lar contraction has been brought into play, and it is here as- sumed that none is usually concerned in the expiratory act of ordinary tranquil respiration. Some maintain that the costal portions of the intercostales interni particularly are expiratory in quiet breathing ; they do contract and the ribs approach each other during the act, but it 2o8 RESPIRATION. is probable that they serve only to maintain the proper degree of tension of the intercostal tissues. Although the elastic reaction of the lung tissue during expira- tion operates together with the elasticity of the thoracic wall in diminishing the antero-posterior diameter of the chest, it is chiefly effective in diminishing the vertical diameter by raising the diaphragm. It exerts a certain " suction ' ' upon that muscle causing it to arch upward in following the contracting lungs. It is seen, therefore, that during inspiration the chest wall and diaphragm exert " suction " upon the lungs, causing them to fol- low, and during expiration the lungs exert " suction " upon the chest wall and diaphragm, causing them to follow. Forced Expiration. — It is evident that, while ordinary expi- ration is a passive process, a person can voluntarily force out of his lungs more air than is ordinarily expelled, as in singing, blowing, talking, etc. This is effected by certain muscles whose contraction diminishes the thoracic capacity, chiefly by depressing the ribs and elevating the diaphragm. Those which depress the ribs are the intercostales interni, infracostales and triangularis sterni. Those which elevate the diaphragm do so by compressing the abdominal contents and forcing them up against that muscle. They are the obliquus externus, obliquus intemus transversalis and rectus abdominis. These depress the chest wall as well. Rhythm of Respiration. — Under ordinary conditions inspira- tion and expiration follow each other in a regular rhythmical fashion. iSome hold that an interval follows inspiration before expiration begins, but this is probably not correct. Indeed, it is doubtful if there be an interval following expiration, though it will be here considered that there is a brief one. Expiration is a little longer than inspiration. The inspiratory act is of uni- form intensity throughoiit, while the expiratory act gradually diminishes in intensity as it approaches completion — a circum- stance to be expected from the physical conditions causing it. RESPIRATORY SOUNDS. 209 After every six to ten respiratory acts a more profound (sigh- ing) inspiration than usual is taken, the effect being a more thorough changing of the pulmonary contents. Coughing, sneezing, hiccoughing, laughing, etc., all interfere with rhyth- mical respiration. Modified Respiration. — In coughing and sneezing a profound inspiration precedes a violent convulsive contraction of the ex- piratory muscles. Sighing is an expression on the part of the tissues that more oxygen is needed and that, therefore, the con- tents of the lungs must be more completely changed. Yawning is a phenomenon similar to sighing, but may not represent de- ficient oxygenation, as when it occurs from contagion. Except in the contraction of different facial muscles, sobbing and laughing are identical from a respiratory standpoint ; in both there is a succession of quick contractions of the diaphragm. Hiccough is an involuntary contraction of the diaphragm accompanied by closure of the glottis. It takes place during inspiration. In hawking the glottis is open and a continuous expiratory current is sent through the narrowed passage between the base of the tongue and the soft palate. Snoring occurs with the mouth open ; the current of air throws the uvula into vibration and produces the characteristic sounds. Sounds of Respiration. — When the ear is applied to the chest there is heard during inspiration a breezy expansive sound of slightly increasing intensity throughout, and ceasing abruptly at the end of the act. Immediately begins the expiratory sound, very short, lower in pitch than the inspiratory, and gradually decreasing in intensity until it is lost before expiration is more than one-fourth finished. When listening over a large bronchus this sound is prolonged and has a higher pitch than usual. Respiratory sounds are more pronounced in the female than in the male chest, owing to the predominance of costal breathing in the former sex. Rate of Respiration. — The respiratory rate sustains a fairly 14 2IO RESPIRATION. constant relation to the cardiac rate, the ratio being about one to four. This makes the average number of respirations about eighteen per minute for adults. In a general way this rate is sub- ject to variations from the same causes as that of the pulse. Any appreciable fall in the amount of oxygen in the inspired air will increase the number of respirations for obvious reasons. The frequency and depth usually bear an inverse ratio to each other. Types of Respiration. — ( i ) Costal respiration is that carried on by the chest walls ; (2) diaphragmatic, that effected by the diaphragm. In the former type movements of the thorax are concerned ; in the latter, movements of the abdomen. Accord- ing as the movements in costal respiration are more pronounced ■in the upper or lower segment of the chest, that type is subdi- vided into (fl) superior costal and {F) inferior costal. In young children the diaphragmatic, or abdominal, type pre- vails ; in adult males a combination of the inferior costal and abdominal ; in adult females the superior costal. The last cir- cumstance is probably due in part to the mode of dress in civil- ized countries, and in part to the provision against encroachment of the uterus upon the abdominal cavity during pregnancy. Intrapulmonary and Intrathoracic Pressure. — It is evident that during inspiration the pressure inside the lungs (intrapul- monary) is less than the ordinary atmospheric pressure ; this, in fact, is the immediate cause of the entrance of air. It is also evident that during expiration the intrapulmonary pressure, owing to the compressing effect of the lung tissue and the tho- racic walls, is greater than the outside atmospheric pressure ; this is the immediate cause of the exit of air. In both acts the air rushes in or out, as the case may be, in an effort to maintain the same pressure inside the lungs as exists in the surrounding atmosphere. It is convenient to call the pressure which is less than atmospheric negative, and that which is greater positive pressure. The intrapulmonary pressure is negative during inspiration and PULMONARY CAPACITY. 211 positive during expiration. Now, owing to conditions already referred to, as the chest and lungs expand during inspiration, the pressure between the adjacent walls of the two (intrathoracic) becomes less and less and reaches a minimum at the end of that act. Furthermore, owing to the continuous "pull" of the elastic lungs upon the chest walls the intrathoracic pressure re- mains negative even at the end of expiration. But it can be made to become positive under forced action of the expira- tory muscles, as in coughing, blowing, etc. The constantly increasing negative condition of intrathoracic pressure is evi- denced by a drawing in of the intercostal tissues during inspira- tion ; when the pressure assumes a positive character, as in the expiratory acts of the pulmonary emphysema, these tissues bulge outward. Pulmonary Capacity. — It is evident that the most forcible expiration cannot completely empty the lungs of air. The air remaining after such an effort is the residual air. It amounts to about 100 cubic inches. But in ordinary respiration at the end of the expiratory act there is more than loo cubic inches of air in the lungs, because in such cases all the air possible is not forced out. In fact about 200 cubic inches usually remain ; this consists of the residual plus another 100 cubic inches, which is called the reserve or supplemental air. It can be forced out, but is not in tranquil respiration. The amount of air which is taken into the lungs by an ordinary respiratory act amounts to about 20 cubic inches, and is termed tidal air. It is the only volume used in quiet breathing. At the end of the inspiratory act in tranquil respiration it is obvious that the expansion may continue still farther, and a certain amount of air, over and above the tidal air, be taken into the lungs. The maximum amount which can be so inspired (beyond the tidal) is about no cubic inches, and is the complemental air. It is seen, then, that the entire lung capacity is equal to about 330 cubic inches. But the residual air cannot under any cir^ 212 RESPIRATION. cumstances be called into use, and consequently the vital capac- ity x^ equal to the total capacity minus the residual air (loo cubic inches), or 230 cubic inches. It is the volume which can be expelled by the most forcible expiration after the most for- cible inspiration. The capacity of the trachea and larger bronchi is known as the bronchial capacity, and amounts to about 8 cubic inches. The quantity of air in the small bronchioles and air vesicles is increased by inspiration and decreased by expiration ; it is called alveolar capacity, and at the end of ordinary expiration amounts to about i^o cubic inches< Quiet inspiration increases it to about 180 cubic inches. All these estimates, of course, represent only an average. The vital capacity is increased by stature, by any occupation which calls for active physical work and by various other con- ditions. Composition of Air. — Ordinary atmospheric air contains, in round numbers, about 21 parts of oxygen to 79 parts of nitro- gen. These two gases make up the main bulk of the atmos- phere. In addition, the atmosphere always contains a little carbon dioxide (about .04 per cent.), ammonia, moisture, or- ganic material, dust, nitric acid, etc. All except the oxygen and nitrogen are of minor importance in respiration when they are not present in amounts beyond the usual. It will be seen that the striking difference between inspired and expired air is in the proportions of oxygen and carbon dioxide. Diffusion in the Lungs. — The expired air contains much more COj and much less O than the inspired air. The interchange of gases between the alveolar air and the blood is responsible for the difference. The question is what forces cause the O of the air to enter the alveoli and the CO^ to leave it. As might be supposed, the air escaping during the first part of expiration differs very little in composition from the inspired air, for it has been occupying the DIFFUSION IN THE LUNGS. 213 upper air passages where no interchange occurs. The bronchial capacity is only about one-third large enough to accommodate the tidal air, and consequently the greater part of it must come from lower down in the lung structure, and the COj in the expired air continuously increases until the end of the act. At each inspira- tion at least two thirds of the tidal air must pass into the small bronchi, or lower. Thus it is that inspiration and expiration themselves, taking into and bringing out of the vesicles (or at least the bronchioles) air fresh with O and air vitiated with COj, aid very materially in keeping constant the composition of the alveolar air. In the second place, the cardiac movements have a similar ef- fect, each systole decreasing the size of the heart and inducing a fresh atmospheric current toward the deep alveoli, and each dias- tole forcing a like current of vitiated air toward the trachea. This force is not inconsequential. In the third place, the diffusibility of gases under known phys- ical laws, without the aid of any such movements as have been described, is an occurrence in connection with the phenomenon in question. Every gas, under ordinary atmospheric conditions, exerts a certain pressure. In every mechanical mixture of gases (such as the atmosphere) each individual gas exerts a part of the total pressure — a part proportional to its percentage in that mixture. This has been called the "partial pressure " of that gas. Since O is present in ordinary atmosphere to the extent of 21 parts per hundred, the partial pressure of oxygen in the atmosphere is -f^ of the total pressure. Now, in the air of the alveoli O is present to a less extent than 2 1 parts per hundred, and consequently its partial pressure in that situation is less than in the trachea and bronchi. The re- sult is that O continually makes its way from the point of higher pressure (trachea and bronchi) toward the point of lower pres- sure (alveoli). The tendency is thus to establish a uniform partial pressure throughout the whole respiratory tract ; but 214 RESPIRATION. this is never done during life because the partial pressure above is being continually increased by the introduction of new O, and below is being continually diminished by the removal of that gas from the alveoli by the blood. In case of CO^ opposite conditions prevail. This gas is being continually introduced into the alveolar air from the blood, and consequently it is present there in much larger quantities than in the trachea and bronchi, which contain newly inspired air. The partial pressure, therefore, of CO^ in the alveoli is much higher than in the upper respiratory passages, and a continual current of it diffuses upward to equalize the pressure ; this is never accomplished, however, for reasons of similar nature to those keeping up the constantly unequal pressure of O. These three factors — respiratory and cardiac movements and the natural diffusion of gases — are, therefore, in continual opera- tion to get O to and CO^ away from the alveoli. Under their in- fluence the composition of the alveolar air remains fairly uniform. Alterations of Air in the Lungs. — These are chiefly : (a) Loss of oxygen, ((5) gain of carbon dioxide, (c) elevation of temper- ature, ((/) gain of water, (f) gain of ammonia, (/) gain of organic matter, (g) gain of nitrogen, (A) loss of (actual) vol- ume. The capital changes are loss of O and gain of COj. (a) Loss of Oxygen. — The air in passing through the lungs loses of O nearly 5 per cent, of its total volume. That is, whereas on entering it contains 2 1 parts, on leaving it contains only about 16 parts per hundred of this gas. Nearly 25 per cent, of the total volume of O inspired, therefore, is lost in the lungs. When the respirations are 18 to the minute, and 20 cu. in. of air are inspired at each breath, the amount inspired in an hour will be 21,600 cu. in. Since a little more than one-fifth of this air is O, and since only one-fourth of the inspired O is consumed, the total amount necessary for an hour will be about 1,100 cu. in. This allows, however, for no muscular, digestive or other activ- ity, and the amount actually necessary is larger than this. CHANGES OF AIR IN THE LUNGS. 215 The circumstances which call for an increase in O almost in- variably cause an increase in the output of COj. (3) Gain of Carbon Dioxide. — The amount of CO^ in inspired air is about .04 part per hundred (4/100 per cent.); the amount in expired air is something more than 4 parts per hundred. In round numbers then, the air in passing through the lungs gains of CO2 4 per cent, of its entire volume. This is in periods of rest from exercise, digestion, etc. The total amount discharged in one hour is, on an average, about 1,000 cu. in. This estimate should probably be raised to 1,200 cu. in. for ordinary activity, and varies according to many conditions, some of which are rapidity and depth of respiration, age, sex, digestion, diet, sleep, exercise, moisture, temperature, season, integrity of the nerve supply, etc. The subjoined table from Kirkes' Physiology compares the composition of inspired and expired air. Inspired Air. Expired Air. 20.96 vols, percent. 16.03 vols, per cent. 79 79 0.04 " " 4.4 " variable. saturated. '* that of body {36° C.) Oxygen Nitrogen . Carbonic acid-. Watery vapor. . Temperature Conditions Influencing Output of CO^. — When the rapidity of respiration is increased, the depth remaining constant, the per- centage of CO2 in the expired air is reduced because more air is respired, but the total quantity in any given time is increased. The same result follows an increased depth and a constant rate. With a diminished rapidity and increased depth more CO is exhaled than under opposite conditions. The amount of CO^ exhaled is small in very young infants. But soon the output begins to increase, and in males continues to do so up to about thirty years ; there is then a slight decrease up to sixty, and afterward a considerable decrease to death. 2l6 RESPIRATION. In the female the output is less than in the male.^ In the former sex the increase is said to cease at puberty and to remain con- stant until the menopause, after which time it increases to sixty and diminishes subsequently. During digestion the quantity is considerably increased. This is probably due to the muscular activity of the alimentary tract, to glandular metabolism and to changes taking place in the food products. As to diet, it may be said in general that the exhaled CO^ is increased in quantity by the taking of nitrogenized foods, tea and coffee. The influence of sleep is to diminish the output. Muscular exercise is very efficient in increasing the amount of COj exhaled ; in fact, this explains partly the variations in con- nection with sex, digestion, sleep, etc. A high degree of moisture increases the exhalation, as does a rise in body temperature. A rise in external temperature, how- ever, has an opposite effect. The output is increased in spring and decreased in autumn. When the efferent nerve supplying a part is severed the pro- duction of COj in that part is at once diminished. The consumption of O and the exhalation of CO, bear a fairly constant relation to each other — any condition increasing one in- creasing the other, and vice versa. The facts, therefore, which have been mentioned as governing the exhalation of COj may be applied to the consumption of O. (f) Gain in Temperature. — When the body temperature is normal and the external atmospheric temperature about 70° F., it is found that air inspired through the nose and expired through the mouth has its temperature raised from 70° to about 95°; the rise is less when inspiration takes place through the moiith. The last air of expiration is warmer than the first. This gain of heat while the air is in the lungs needs no explanation when it is remembered that the average temperature of the tissues with which it is in contact is 98.5° F., or higher. AMOUNTS OF O CONSUMED AND CO, EXHALED. 21 7 {d') Gain of Water. — ^This water is in the form of vapor. It is natural that the air should absorb water from the moist sur- faces with which it is in contact. The capillary network with which it is in close relation supplies moisture to the mucous membrane not only of the alveoli but of the entire respiratory tract. One or two pounds of water are eliminated thus daily. (tf) Gain of Ammonia. — Ammonia is exhaled in small quan- tity by the lungs. It is insignificant except in cases of sup- pressed kidney action. (/) Gain of Organic Matter. — The quantity of organic mat- ter exhaled by the lungs is inconsequential (unless ventilation be bad), but such exhalation does occur to a small extent. It gives the odor to the breath. (^) Gain of Nitrogen. — The exhalation of this gas by the lungs is of no respiratory importance. The amount is said to be -rir^V *^^ amount of oxygen consumed. An occasional loss of nitrogen has been observed. (Ji) Decrease of (^Actual') Volume. — When the external tem-. perature is below about 90° F. the volume of expired air is a little greater than that of the inspired air, because of the in- crease of temperature it undergoes in passing through the lungs. But the actual volume of the expired air, when reduced to the same temperature as the inspired, is found to be always a little less than that of the latter. It is estimated that from Tnr"Ftr "^ the total volume of the inspired air is thus lost in respiration. Besides the substances mentioned as being exhaled from the lungs, it is well known that odorous emanations proceed from them after garlic, onions, turpentine, alcohol, certain drugs, etc., have been taken into the stomach. Relation Between Oxygen Consumed and Carbon Dioxide Exhaled. — A given volume of O will combine with carbon to form the same volume of CO^ ; or the amount of O in a given volume of CO^ is equivalent to that volume when set free from the carbon. A cubic foot of O will unite with carbon to form 2l8 RESPIRATION. a cubic foot of COj ; or a cubic foot of CO^ will yield, on dis- sociation, a cubic foot of O. This being the case, if all the O consumed in the lungs were exhaled therefrom in the form of CO^, the amount of CO, ex- haled would just equal the amount of O consumed. But the amount of consumed O is about 5 per cent, of the inspired air, while the amount of exhaled CO^ is only about 4 per cent, of the expired air. It follows, therefore, that i per cent, of the volume of inspired air is not represented by the COj exhaled from the lungs and skin. The relation between the consumed O and the exhaled CO^ is usually expressed as the " respiratory quo- tient " — the division of the latter by the former giving the quo- tient. This quotient is made to vary by many circumstances, though for any considerable period its average is about the same. While it has been stated that the O absorbed and the COj pro- duced vary together usually, they are in a certain measure inde- pendent of each other. For CO^ does not result from the im- mediate union of O with carbon of the carbohydrates and fats, but may be stored in the shape of complex compounds, which may later split up with the formation of CO,, either by oxida- tion or by intra-molecular cleavage. Furthermore, more O is necessary to oxidize (that is, to form carbon dioxide) some molecules than others. A fat requires considerably more O to produce CO^ than does a carbohydrate ; so that the kind of food in store would also affect the respiratory quotient. With respect to the O which, in the long run, is not repre- sented in the CO^ exhaled from the lungs and skin, it is certain that when various of the proximate principles are broken down at least a part of it is appropriated by hydrogen to form water. Source of Exhaled Carbon Dioxide. — The increase of CO, in expired air over the small amount contained in inspired air is derived from the venous blood circulating through the lungs. It exists in that blood under a constant tension, as is demonstrated by its escape when the blood is placed in a vacuum. The total CONDITION OF CO,^ IN THE BLOOD. 219 amount escapes when the blood intact is placed in vacuo : when the corpuscles alone are so treated they yield up all their CO^, though it is small in amount ; but the plasma alone in vacuo yields a less amount than when it contains corpuscles. If, now, corpus- cles be added to the plasma the total amount of CO^ is forthcom- ing. The corpuscles must, therefore, act as an acid causing the liberation of this gas from the plasma. It is probably the hemoglobin, or oxyhemoglobin, which has this effect, though in the laboratory the phosphates and certain proteids of the corpus- cles produce a like reaction when brought in contact with the carbonates and bicarbonates of soda. Condition of CO^ in the Blood. — About 5 per cent, of the total amount of CO^ in venous blood is in simple solution in the plasma; about 75-85 per cent, is in loose chemical combination in both corpuscles and plasma; the remaining 10-20 per cent, is in comparatively stable combination in the plasma. Of the 75-85 per cent., by far the largest part is in the plasma, prob- ably in a condition of loose association with sodium to form carbonates and bicarbonates ; the small part in the corpuscles may exist in a similar state, but it is now thought to exist in combination with the proteid portion of hemoglobin. The total 75-85 per cent, in corpuscles and plasma is so loosely com- bined that the mere diminution in pressure in the lungs is prob- ably sufficient to liberate it. The 10-20 per cent, in firm chemical combination is that part which cannot be extracted from plasma alone in vacuo, but which is dissociated on the ad- dition of an acid, or corpuscles, or hemoglobin, etc. It may be that as the blood passes through the lungs there is set free, in the formation of oxyhemoglobin, an acid which immediately unites with the bases holding the COj in combination — the lib- eration of the latter being the consequence. The O being thus in the air vesicles, and the COj thus free, or set free, in the blood, with the very thin animal membrane con- sisting of the vesicular and capillary walls between them, it re- 220 RESPIRATION. mains to be seen what forces are concerned in the interchange of these gases. It has been noted that only one-fourth of the O entering the lungs in the air is taken up by the blood ; so it is to be remembered that not all the COj entering the lungs in the venous blood is taken up by the air. Interchange of Oxygen and Carbon Dioxide in the Lungs. — The condition of ' ' partial pressure ' ' of gases in mixtures has been mentioned. Each gas exerts a pressure in proportion to its percentage in the mixture, and this is called its ' ' partial pressure. ' ' Now, the extraction of O and CO, from the blood by placing it in a vacuum shows that both these gases exist in the blood under a certain degree of tension. The tension of a gas in solution being only the pressure nec- essary to keep it in solution, it follows that if the pressure be diminished the gas will partly escape. If an atmosphere con- taining, say, O at a certain partial pressure be brought in con- tact with a fluid containing O at a certain tension, unless the partial pressure of the O in the air be equal to its tension in the fluid there will be an escape of the gas from the point of higher to the point of lower pressure or tension. If the partial pressure of the gas be less in the atmosphere than its tension in the fluid, the current will be from the latter to the former and vice versa. This will be the case whether the media are in actual contact or separated by an animal membrane. This is the condition which obtains in the pulmonary alveoli. The partial pressure of O in the alveolar air is much greater than the tension of O in the blood ; consequently the current is from the air to the blood. The tension of CO^ in the venous blood is much greater than the partial pressure of the CO^ in the alveolar air ; consequently the current is from the blood to the air. But, here, as in the last analysis of almost all physiological phenomena, it is found that, while these purely physical laws are certainly concerned in the pulmonary interchange of gases. ALTERATIONS OF BLOOD IN THE LUNGS. 221 they are insufficient to explain the occurrences in full. For the blood will take from the alveolar air more than enough O to establish an equilibrium of tension and partial pressure ; the ten- sion of O in arterial blood is higher than its partial pressure in alveolar air. So it is found that the alveolar air will remove more than enough CO, to establish a similar equilibrium of this gas. It is known that the avidity (chemical) of corpuscles for O to form oxyhemoglobin causes the blood to appropriate more O than it would otherwise do, but even then we are driven to the usual ultimatum of ascribing some peculiar office to the living epithelium of the intervening membrane. Condition of Oxygen in the Blood. — Almost all the oxygen is conveyed in the blood by the red corpuscles, where it exists in rather unstable composition with hemoglobin (probably with its pigment portion) under the name of oxyhemoglobin. Only a comparatively small part is held in solution by the plasma. Dissociation of oxyhemoglobin occurs when the pressure is suf- ficiently reduced. Alterations in Blood in Passing Through the Lungs. — The sum total of the changes taking place in the blood as it passes through the lungs is represented by the term arterialization. In general, it may be said that the blood undergoes changes exactly opposite to those of the air in circulating through the pulmonary structure, and reference to the list of substances gained and lost by the air will suggest the main alterations in the blood. Of course the most striking phenomena are the loss of CO^ and the gain df O. In loo volumes of arterial or venous blood there are found to be, on an average, 60 volumes of O and COj. This total remains approximately constant, though the relative amount of each gas varies according as the blood is venous or arterial, and in venous blood under the influence of several conditions to be mentioned. In arterial blood the O will represent about 20, and the CO^ about 40, of the total 60 volumes per hundred of gas. In ordinary venous blood the O 22 2 RESPIRATION. will represent about 7 volumes less (13), and the CO^ about 7 volumes more (47), of the total 60. In both venous and arte- rial blood there is an insignificant amount of nitrogen, which is usually present to the extent of i . 5 volumes per hundred. The proportion of gases is about the same in arterial blood taken from any part of the system. In blood coming from ac- tively secreting glands the ratio of O to CO, is nearly the same as in arterial blood ; in fact, such blood may have a red (arterial) instead of a blue (venous) color. This is because during ac- tivity blood is sent to the gland in increased amount to furnish materials for secretion, while the demand for oxygen is not rel- atively increased in that gland. Besides the changes which are apparent on referring to the alterations in the air in passing through the lungs, there are cer- tain other general characteristics which distinguish arterial from venous blood. The most noticeable is color'. Venous blood is changed in the lesser circulation from a dark blue, or black, to a bright red. This is due to the formation of oxyhemoglobin. The change of color does not occur when the appropriation of O is interfered with, as when air is excluded from the lungs, or when carbon monoxide is inhaled. Again, there is every reason to believe that venous blood coming from different organs differs in composition according to the special materials which have been extracted from it by those organs ; the portal blood during digestion must certainly be different in composition from the general venous blood, and so it may be conceived that the blood coming from no two different sets of capillaries is identi- cal. When all this meets in the right side of the heart and is sent thence into the lungs it has a nearly uniform composition, and needs only to receive O before it can supply the wants of any particular tissue in the body. Arterial blood is also more coagulable than venous. Internal Respiration. — It has been said that the object of external respiration and the transportation of O and CO2 is to INTERNAL RESPIRATION. 223 make internal respiration possible. Oxygen, leaving the alveoli in a manner already described, enters the blood and at once combines with hemoglobin of the red corpuscles to form oxy- hemoglobin. A small portion of the O is used up by the cor- puscles in transit, with the production of CO2 and other meta- bolic materials — the corpuscles requiring O in their metabolism just as do other cells. But by far the largest portion is carried to the capillaries, where it is taken up by the cells. At the same time the cells give up to the blood CO2 — a result of their meta- bolic activity. The blood, having thus given up its O, is changed in color, and carries the CO2 back to the lungs to be exhaled. To furnish O and to remove CO^ is the only object of respira- tion. Living tissue exposed to an atmosphere containing O will consume O and exhale COj even if no blood be circulating through it. The exact manner in which a cell uses O is not apparent. It is evidently an oxidation process which produces COj, and O is directly necessary to this process. But the amount of CO^ produced in any given time may not correspond to the amount of O consumed in that time ; it may be greater or less. "It is probable that during rest O is utilized to some extent in oxidations which are not at once carried to their final stage and in which relatively little CO2 is formed ; hence during activity comparatively little O is required to cause a final disin- tegration of the now partially broken down substances, and thus to give rise to a relatively large formation of CO2 " (Reichert). The absorption of O is to be looked upon as a part of the nutritive process just as the absorption of proteid, e. g., and CO^ as one of the products of destructive metabolism just as urea. There is small probability that the O unites directly with the carbon of any of the proximate principles — although this is the final result. Interchange of Oxygen and Carbon Dioxide in the Tis- sues. — Here applic^tioQ of the principles governing the inter- 224 RESPIRATION. change of these gases in the lungs applies. It is found that the tissues act as very strong reducing agents upon oxyhemo- globin, setting free the O. Now the tension of O in the arterial capillaries is much higher than in the tissues ; in fact, it is prac- tically nothing in the latter situation, for the O enters so quickly into combination that there is very little to be found here at any time. Consequently physical laws encourage the passage of this gas out of the capillaries into the tissue. On the other hand, the tension of COj in the tissues is much higher than in the blood, and the same physical laws encourage a current of COj toward the blood. Nevertheless, these laws do not explain all the phenomena of interchange ; the activity of the cells is an important agent, though their influence may be of a chemical nature only. Cutaneous Respiration. — Cutaneous respiration in man is in- significant and not essential to life. The skin absorbs a little O and exhales a little more CO^. It is estimated by Scharling that the skin performs about -j'^ of the respiratory function. Death following the covering of the body surface with an impermeable coating is not due to interference with cutaneous respiration. Ventilation. — Persons breathing in a confined space gradually consume the O and increase the CO^ of the atmosphere. When the amount of O has been decreased to fifteen parts per hundrea it is insufficient for the respiratory demands. When the CO,^ is increased to . 07 part per hundred the air becomes disagree- able and close ; this is not, however, from the accumulation of COj so much as from organic emanations and disagreeable odors from the body, clothing, etc. It is only that the amount of COj serves as an indication of the extent of accumulation of these materials that the amount .079^ is fixed as the limit beyond which it ought not to be present. This percentage of CO^ in air free from emanations, etc., is not deleterious. Since 1,200 cu. in. of O are consumed per hour, about 15 cu. ft. will be necessary for a day ; and since the 1,200 cu. in. con- ABNORMAL RESPIRATION. 225 sumed represent only about one-fourth of the O inspired, 60 cu. ft. will be necessary for inspiration during twenty-four hours. This amount represents some 300 cu. ft. of atmospheric air — which an ordinary person must have in that time. But this estimate allows nothing for increased respiratory ac- tivity, which inevitably occurs from some of the numerous con- ditions influencing it. It is found that in prisons and other in- stitutions of confinement it is not safe to allow each person less than 1,000 cu. ft. of atmospheric air. In crowded houses, where this space per individual cannot be obtained, it is necessary, in order to avoid unpleasant results, to change the air continuously, or at frequent intervals. Natural and artificial means are em- ployed to accomplish this end. Respiration of Various Gases.— The inhalation of pure oxygen is not deleterious unless it be under higher tension than in atmos- pheric air, when it becomes a local irritant. The blood will not, however, appropriate more than the usual amount. Nitrous oxide will sustain respiration for a time, but soon pro- duces unconsciousness and asphyxia, probably because it unites so firmly with the hemoglobin of the corpuscles. Hydrogen may be inhaled with impunity if it contain also oxygen in the atmos- pheric proportion. Carbon monoxide is poisonous because it unites with hemoglobin to the exclusion of oxygen and will not dissociate itself. Sulphuretted, phosphoretted and arseniuretted hydrogen are destructive of hemoglobin and consequently poison- ous. Pure carbon dioxide cannot be inhaled for any length of time. Abnormal Respiration. — The term eupnea is used to describe normal, tranquil breathing. Apnea is suspended respiration. Hyperpnea is exaggerated respiration. Dyspnea is labored breathing. Asphyxia is essentially a want of O characterized by convulsive respirations, and later by irregular shallow breath- ing. The last two named deserve some attention. Dyspnea may be due to either a deficiency of O or an excess IS 226 RESPIRATION. of COj in the blood. When an animal is made to breathe in a small, confined space the amount of O soon becomes insufficient, even though the amount of COj in the blood be not increased. Again, if an animal be caused- to breathe air containing the usual amount of O and a large amount of COj, it will suffer from dyspnea also. In either case the manifestations are prac- tically the same — slow, deep and labored respiration. In car- diac disease, hemorrhage, pulmonary diseases, etc., the dyspnea Fig. 48. The Heart in the First Stage of Asphyxia. The left cavities are seen to be distended ; the left ventricle partly overlaps the right ; La, left auricle; Lv, left ventricle; u, aorta; jJ, a,' pulmonary artery; p.v, pulmonary vein; r.a, right auricle; r.Ti, right ventricle; v,c.d, descending vena cava; v.c.a, ascending vena cava, {Kirkes after Sir Georffe Johnson.) is from a lack of O in the tissues, because of enfeebled action of the heart, deficient quantity of blood, insufficient exposure of the blood in the lungs, etc. Asphyxia may be looked upon as exaggerated dyspnea. The labored breathing of dyspnea becomes convulsive, and finally collapse ensues. Respiration becomes shallow, consciousness is lost, the pupils are dilated, opisthotonus develops, the re- flexes disappear, and at last the heart stops beating. The skin ASPHYXIA. 227 and mucous membranes become blue from non-oxygenation of the blood. Asphyxia from submersion is harder to overcome than from simple deprivation of air outside the water. Resusci- tation is extremely doubtful when a person has been submerged as long as five minutes. While the phenomena of dyspnea and asphyxia are referable to the lungs, it is not the need of air in these organs, but of O in the tissues, which gives rise to the symptoms. The non-oxygen- ated blood in asphyxia will not circulate through the capillaries Fig. 49. The Heart in the Final Stage of Asphyxia. The letters have the same meaning as in Fig, 48; in addition^/.c:, represents the puImo> nary capillaries. The right auricle and ventricle, and the pulmonary artery, are fully dis- tended, while the left cavities of the heart and the aorta are nearly empty, (JCirkes after Sir George Johnson.') except with the greatest difficulty, and the result is that it accu- mulates in the arterial system, dams back upon and distends the heart, so that this organ is finally paralyzed and ceases to beat. This is the cause of death from asphyxia. Effect of Respiration on Blood-Pressure. — The lowest blood- pressure is just after the beginning of inspiration, from which time it increases during inspiration to reach its maximum 228 RESPIRATION. just after the beginning of expiration ; it gradually decreases from this time to the minimum just after the beginning of in- spiration. The general efifect, then, of inspiration is to increase blood-pressure and of expiration to decrease it. This remark applies to general arterial tension. Taking inspiration, the increase in arterial tension is, in its last analysis, due to the larger amount of blood sent into the arterial system at each venticular systole. The explanation is Fig. 50. Carotid Blood-Pressure Tracing of Dog. Vagi not divided ; I^ inspiration ; E^ expiration. {Stirling.) somewhat complex, but if the mechanics of respiration be under- stood it may be made satisfactory. It was seen that the lungs are contained in an air-tight cavity, the chest, and that they expand with the chest because of negative pressure ("suction") exerted upon them. The heart is also a hollow organ situated in this cavity ; it has con- nected with it, and lying also in the thoracic cavity, large vessels communicating with smaller extra-thoracic vessels. The heart and these great thoracic vessels are elastic and distensible. Con- sequently the expansion of the thorax also expands them slightly and tends to draw blood from the extra-thoracic into the intra- thoracic vessels and heart ; in fact inspiration is one of the main forces causing a flow of venous blood toward the heart. Now all this, so far as it goes, tends to keep the blood out of the RESPIRATION AND BLOOD PRESSURE. 229 extra-thoracic vessels, and so to contradict the statement that inspiration increases arterial tension. But, remembering that we are dealing with arterial tension and that our effort is to prove that more blood is sent into the aorta during inspiration than during expiration, it is of value to note that since the walls of the aorta are more resistant than those of the venae cavae there is less expansion of the former than of the latter during inspiration, and consequently less tendency for the arterial blood to regurgitate into the thoracic aorta than for the venous blood to enter' the thoracic venge cavse. The same expanding force dilates the pulmonary capillaries, pulmonary artery and pulmonary veins — the artery least of these. Taking it for granted that more blood is being received by the right side of the heart from the incoming venae cavae, the somewhat dilated pulmonary artery receives more from the right ventricle ; the pulmonary capillaries are more dilated than the artery and this fact greatly encourages (by a suggestive "suction") the in- creased flow from the pulmonary artery ; they, therefore, receive more blood than usual. The pulmonary veins, being likewise dilated, exert "suction " upon the capillaries, and thus receive and pass on to the heart a larger supply of blood than usual. The heart, receiving more blood, must send more into the aorta, thereby increasing arterial tension in - the extra-thoracic vessels, unless, by expansion of the chest, the thoracic aorta be so dilated as to accommodate the increased amount — which is not true. Then, finally, the validity of this argument will hinge on the relative dilatation of the thoracic aorta and of the thoracic venae cavae. If the veins be less dilated by inspiration than the artery, then they will receive an increase of blood which will not com- pletely occupy the increase of space in the dilated thoracic aorta, and there will be a backward " suction " made upon the contents of the arterial tree with a consequent decrease in pressure ; but a condition just opposite to this seems to obtain. During expiration contrary conditions in general are operative 230 RESPIRATION. with contrary results. The intra-pulmonary vessels and heart are compressed, but the veins and capillaries more than the aorta, with the result that less blood reaches the heart than during in- spiration, and the thoracic aorta being, relatively to the thoracic venae cavse, more dilated now than during inspiration can easily accommodate the decreased amount of blood which it receives. Of course expiration increases venous pressure in the veins which enter the thorax back as far as the valves. . The reason the pressure does not rise with the beginning of inspiration is because a short time is consumed in filling the flac- cid intra-pulmonary veins, and the first increase of blood is de- layed for that purpose instead of passing on to the left side of the heart. Similarly, the pressure continues to rise for a short time after expiration has begun because the large veins are being emptied by pressure during this time and their contents are reaching the heart and being forced into the aorta. Movements of the diaphragm and abdominal muscles during respiration also lend themselves to create like changes in arterial pressure, but the main factors are intra-thoracic. The fact that the cardiac rate is increased during inspiration and decreased during expiration may also have to do with the variations in pressure. All the foregoing remarks relative to arterial tension are meant to apply to tranquil respiration. During forced inspiration, or forced expiration, the results may be modified, or even reversed, by circumstances not necessary to mention. Nervous Mechanism of Respiration. — Although the muscles of respiration are of the striated variety, it is by no effort of the will that the movements are kept up. They belong to the class known as automatic ; that is, they are, up to certain limits, under the control of the will, but recur in a regular, coordinate and orderly manner without the active intervention of volition. Res- piration represents the activity of a self-governing apparatus. These movements constitute a finely coordinated set of contrac- RHYTHM OF RESPIRATION. 23 1 tions — contractions which are regulated by means of afferent and efferent nerves under the supervision of the respiratory center. The respiratory center is in the lower part of the medulla ob- longata. Destruction of the encephalon above, or the cord below, the center does not arrest respiration. It is bilateral — a center for each side — and these are more or less independent of each other, but are so intimately connected by commissural fibers that any impression made upon one usually produces a like effect upon the other. Each half presides over the lungs and respiratory muscles of its own side, but acts synchronously with its fellow of the opposite side. Furthermore, each of these lateral centers may be regarded as consisting of two parts, one for inspiration and one for expiration. Stimulation of the in- spiratory center not only strengthens the inspiratory act, but also accelerates respiration. Stimulation of the expiratory center strengthens expiration and also retards the respiratory rate. The accelerator portion of the center seems more sensitive than the inhibitory, and the result of stimulation of the whole center is therefore quickened respiration. Subsidiary respiratory centers are said to exist in the tuber cinereum, optic thalamus, corpora quadrigemina, pons Varolii and spinal cord ; but the existence of at least some of these is doubtful. Rhythm of Respiration. — What agency excites the center to keep up the respiratory movements with such regularity is a matter of interest. The chief circumstances which seem to af- fect the rate and rhythm are (i) the will, (2) emotions, (3) com- position of the blood and (4) afferent impressions. I, 2. The effect of the will and emotions are too apparent to call for comment, i and 2 are properly included in 4. 3. A deficiency of O or an excess of CO, in the blood will increase the rate. Increase in temperature of the blood, as in fever, will produce a similar effect. 232 RESPIRATION. 4. The most important of these agencies is found in afferent impressions conveyed to the center. The fibers carrying these impressions are chiefly in the pneumogastric, glossopharyngeal, trigeminal and cutaneous nerves. Of these the pneumogastric is by far the most important. Section of a single pneumogastric is followed by variable respiratory disturbances which usually disappear in les^ than an hour. Section of both nerves is followed, after a shortr interval of increased respiratory activity, by slow and powerful '"'inspira- tions, by forced expiration and an appreciable interval be- fore the next inspiration. Irritation of the central end of the cut nerve by a very weak current seems to stimulate the inhibi- tory part of the center, for the rate' is slowed, the expirations are strenuous and the inspirations weak. When the current is increased to a moderate strength opposite results are ob- tained, the accelerator portion of the center being stimulated. These facts show that the pneumogastrics possess both in- spiratory and expiratory fibers, and that the former are stimu- lated more by a moderate current and the latter more by a very weak one. The rhythm of respiration, therefore, includes the regular sequence oi inspiratory and expiratory movements upon each other. Now what is it that, under normal conditions, irritates the termi- nals of the pneumogastrics and causes them to convey iflspiratory and expiratory impressions ? It has been held that a change in the composition of the alveolar air — an accumulation of carbon dioxide — irritates the nerve terminals and explains the convey- ance of the inspiratory impressions, while the stretching of the lung tissue originates the expiratory impressions. Others ascribe both inspiratory and expiratory impressions to lung mojiements — movements of inspiration exciting expiratory fibers, and move- ments of expiration exciting inspiratory fibers. These observers cite the fact that artificial inflation and aspiration excite expira- tion and inspiration respectively. NERVOUS CONTROL OF RESPIRATION. 233 Stimulation of the superior laryngeal, as when foreign bodies accidentally enter the larynx, excites violent expiration. The glosso-pharyngeal contains afferent fibers especially impor- tant in arresting respiration — at any stage whatever — during the act of deglutition. Stimulation of the sensory fibers of the trigeminal in the nose, as by irritating vapors, may arrest respiration. Irritation of the cutaneous nerves in general, as by cold or hot water, slapping, etc. , stimulates respiratory movements. There are, of course, running from the cortex to the respira- tory center intra-cranial fibers whereby the organ of the will makes its presence felt in respiration. But when all the afferent nerve connections are severed, respiration continues with modified rhythm and rate, at least for a time. It is thought that, under these conditions, it is the cir- culation through the center of blood deficient in oxygen which causes the cells to discharge ; that is, after every inspiration and subsequent expiration there is not another inspiration until the blood has become sufficiently deoxygenated, or charged with carbon dioxide, to irritate the respiratory center. We may conclude that " the rhythmical discharges from the center are due primarily to an inherent quality of periodic ac- tivity of the nerve cells constituting the respiratory center, and maintained by the blood, and that the rhythm, rate, and other characters of these discharges may be affected by the will and the emotions, by the composition, supply and temperature of the blood, and by various afferent impulses. The chief factors are the quantities of O and COj, in the blood, and the impulses con- veyed from the lungs by the fibers of the pneumogastric nerves. ' ' (Am. Text-Book.) The efferent nerves of respiration control the muscular move- ments of that act. They are chiefly the /«««/, hypoglossal and spinal accessory controlling the respiratory movements about the face and throat ; the pneumogastric going to the larynx ; the phrenic to the diaphragm ; certain of the spinal nerves. 234 RESPIRATION. To the lungs proper fibers are distributed by the vagus, the dorsal spinal and the sympathetic nerves. Besides the expiratory and inspiratory fibers already noticed, the vagus supplies the lungs with broncho-motor, general sensory, trophic and secre- tory (mucous) fibers. The sympathetic furnishes trophic and vasomotor fibers, which latter come from the cord by the roots of the dorsal nerves mentioned to join the sympathetic ganglia. CHAPTER VII. EXCRETION BY THE KIDNEYS AND SKIN. Excretion of the various foods after they have discharged their several functions in the body is effected mainly by the kidneys, skin, lungs and alimentary canal. The excretory action of the last two named is considered under Respiration and Di- gestion. Attention is again called to the fact that it is impos- sible to differentiate strictly between a secretory and excretory fluid. The urine is as typical of the excretions as any fluid to be found. But it will be convenient to speak of the "secre- tion ' ' of urine when reference is made to the act of separating its constituents from the blood. The Kidneys. Anatomy. — The kidneys, one on each side of the body, are behind the peritoneum in the lumbar regions. The right is usu- ally a little lower and a little lighter' than the left. The hilum from which the ureter springs looks inward and forward. The kidney, as found behind the peritoneum, is covered with a con- siderable amount of fat, but the substance proper of the organ is closely surrounded by a somewhat resistant fibrous capsule which in health can be easily stripped away. At the hilum the capsule is continued inward to line the pelvis, infundibula and calyces. The kidney belongs to the class of compound tubular glands. If it be cut into two halves by an incision passing through the two borders (and, therefore, through the hilum) an idea of its gross divisions is obtained. The renal substance is seen to be 23s 236 EXCRETION BY THE KIDNEYS AND SKIN. divided into an outer layer, known as the cortical substance, and an inner, or pyramidal, portion. Internally the incision reveals a cavity into which the ureter opens. This is the pelvis. Tracing the divisions of the pelvis toward the kidney sub- LONGITUDINAL SECTION THROUGH THE KlDNEY, THE PeLVIS OF THE KiDNEY, AND A Number of Renal Calyces. {Ytotq. Brubaker,^i\xx Tyson.) A, branch of the renal artery ; U, ureter; C, renal calyx; i, cortex; i', medullary rays; I '', labyrinth, or cortex proper; 2, medulla; 2', papillary portion of medulla, or medulla proper; 2", border layer of the medulla; 3, 3, transverse section through the axes of the tubules of the border layer; 4, fat of the renal sinus; 5, 5, arterial branches; * transversely coursing- medulla rays in column of Bertin. structureTof the'kidney. 237 stance, it is found to be continued by three short canals, one to- ward the upper, one toward the lower and one toward the cen- tral portion of the organ. These are the three infundibula. Each infundibulum, passing outward, subdivides into two or three, or more, short cylinder-like canals which receive the apices of the pyramids. These are the calyces, each of which receives the apex of one or more pyramids. The urine thus escaping from the pyramidal tubules passes in succession through the calyces, infundibula, pelvis, and thence into the ureter. •***^ir****»"''^"^ ^> Cortex. \ Boundary or 7 Smarginal /zone. LS, of a pyramid of Malpighi ; PF, pyramids of Ferrein ; RA, branch of renal artery with an interlobular artery; RV^ lumen of a renal vein receiving: an interlobular vein; VR, vasa recta; PA, apex of a renal papilla; b, b, embrace the bases of the lobules. i^Stirling.') 238 EXCRETION BY THE KIDNEYS AND SKIN. The cortical substance constitutes the outer layer of the kid- ney and is about i inch thick. It is reddish and granular in appearance. From it pass in between the Malpighian pyra- mids columns known as the columns of Bertin. The cortical substance contains the glomeruli and convoluted tubules to- gether with blood-vessels and lymphatics supported by connec- tive tissues. The pyramidal substance, also called the medullary sub- stance, consists of a number of pyramids, about 12-15, whose bases look outward and rest on the cortical substance and whose apices look inward and are received into the calyces. These are called the pyramids of Malpighi. They contain uriniferous tubules, vessels, etc., supported by connective tissue. It will be seen that these tubes converge and join each other in passing from the base to the apex of the pyramid, so that the very large number entering the base is represented by only 10-25 at the apex. Thus it is that the Malpighian pyramid is divided into a number of smaller pyramids. These latter are the pyramids of Ferrein, and correspond in number to the number of tubes radi- ating from the apex of the larger pyramid. The medullary substance is marked by striae which have the direction of the tubules and which are caused by them. Its consistence is iirmer and its color is darker than that of the cortical substance. Malpighian Bodies. — These are scattered throughout the cor- tical substance, and are y^ - ^^ inch in diameter. They consist of a bunch of capillaries in the shape of a ball, the glomerulus, surrounded by the extremity, or rather the beginning, of one of the renal tubules. At the point where the tubule joins the Mal- pighian tuft it is constricted ; running then over the glomerulus it reaches the afferent artery and the efferent vein on the oppo- site side ; when it has reached these vessels it is reflected over the whole network of capillaries so that really the tuft is outside the tube, but practically it is covered by a double layer of the tube wall. A space, the beginning of the tubule, is left between STRUCTURE OF THE KIDNEY. 239 these two layers and into it the glomerular secretion passes. The outer layer is the capsule of Bowman (or Miiller). Both layers consist of a single stratum of flattened epithelial cells ; those of the inner layer are applied closely to the glomerulus and are thought to be very important in secretion. The incoming artery Fig. 53. Fig. 54- Transverse Sectiow of a Developing Malpighian Capsule and Tuft (Human). X 300. From a fetus at about the fourth month ; a, flattened cells growing to form the cap- sule ; by more rounded cells continuous with the above, reflected round c, and finally en- veloping it; c, mass of embryonic cells which will later become developed into blood-vessels. {Kirkes after W. Pye.) Epithelial Elements of a Malpi- ghian Capsule and Tuft. With the commencement of a urinary tu- bule showing the afferent and efferent vessels ; a, layer of flat epithelium forming the cap- sule ; b, similar, but rather larger epithelial cells, placed in the walls of the tube; t, cells, covering the vessels of the capillary tuft ; d, commencement of the tubule, some- what narrower than the rest of it. {Kirkes after W, Pye,) breaks up to form the capillary tuft ; the corresponding outgoing vein has a smaller caliber than the artery. The vein, having left the glomerulus, breaks up into a secondary network around the convoluted tubes. This arrangement of the Malpighian body furnishes a most favorable opportunity for the passage of sub- 240 EXCRETION BY THE KIDNEYS AND SKIN. Stances out of the blood current iiQto the beginning of the tube. Uriniferous Tubules. — These begin at the glomeruli and end at the apices of the Malpighian pyramids. From their tortuous course in the cortical portion they are there called convoluted tubules, in contradistinction to the straight Vdhes, of the medullary portion. This, however, is only a general division ; further the distinctions are to be noted. The constricted portion of the tube where it leaves the glomer- ulus is the ( I ) neck ; passing away from the neck the tubule becomes very tortuous and is known as the ( 2 ) primary convo- luted tubule, which, having run for a variable distance, becomes narrow near the base of the pyramid, and taking a comparatively straight course downward enters the pyramid under the name of the (3) descending limb of Henle's loop; some of these run nearly as far as the apex, but most of them near the base or middle of the pyramid turn upward forming thus (4) Henle's loop and beginning the (5) ascending limb of Henle's loop; the tube having reentered the cortical substance becomes con- voluted again, (6) secondary convolution, which, by a less tortuous continuation, the (7) intermediate tube, communicates with the collecting tubules, or the (8) straight tubes of Bellini ; these last beginning in the cortex, and receiving in their course large numbers of intermediate tubes, enter the base of the pyr- amid and run in a nearly straight direction toward the apex. About 100 of these straight tubes entering at the base join in their course downward until at the apex they are represented by a single tube. These collections constitute the pyramids of Ferrein ; there are about 12-18 pyramids of Ferrein to each Malpighian pyramid, and as many tubal oriiices at the apex. The so-called zigzag and spiral tubules are here considered parts of the first and second convoluted tubules. (See Fig. 55.) Before they reach the collecting tubules the tubes vary in diam- eter from yttVo' ^° ttot ^"'^'^ '> ^'^ collecting tubules progressively Fig. 55. Diagram of the Sections of Uriniferous Tubes. A, cortex limited externally hy the capsule; a, subcapsular layer not contaimng'.Malpi- ghian corpuscles; a\ inner stratum of cortex, also without Malpighian capsules; B, bound- ary layer; C, medullary part next the boundary layer; i. Bowman's capsule of Malpighian corpusclle ; 2, neck of capsule ; 3, first convoluted tubule ; 4, spiral tubule ; 5^ descending limb of Henle's loop ; 6, the loop proper ; 7, thick part of the ascending limb ; 8, spiral part of ascending limb ; 9, narrow ascending limb in the medullary j-ay ; 10, the zigzag tubule ; zi. the ^^^ecppd convoluted tubule; 12^ the junctional tubule; 1x3, the collecting tubule of the medullary ray; 14, the^collecting tube of the boundary layer ; 15, duct of Bellini. {Kirkes 2iSt&: Klein.) 16 242 EXCRETION BY THE KIDNEYS AND SKIN. increase in diameter from -g^ to -j-J-j inch. The cells lining the convoluted and intermediate tubules are inclined to the pyramidal shape. Their bases present the appearance of fibers at right angles to the basement membrane ( hence ' ' rod- ded " cells), while their opposite extremities are granular. The tubes of Henle are lined by flattened epithelium for the most part. The division is somewhat arbitrary, but the secreting portion of the tubules is supposed to be confined to the cortical substance, while the tubes of the medullary substance only carry awiay the fluid. Blood Supply. — The renal artery, having entered the hilum, divides into branches, two of which usually enter each column of Bertin. Running upward in these columns the branches give off small arterial twigs to the substance of the column. When a point opposite the bases of the Malpighian pyramids is reached each branch follows the convex base of the pyramid to which it is adjacent, the one branch going in an opposite direction to the other. Each meets a corresponding branch from the other side of the pyramid, and thus a convex arterial arch covers the base of the pyramid, from which arch branches go in- ward to supply the medullary substance and outward to furnish branches to the glomeruli. The arrangement of the vessels in relation to the Malpighian bodies has been noticed. In the glomerulus the capillaries do not form a true anastomosis, but this is not true of the network surrounding the convoluted tubes. Mechanism of Urinary Secretion. — Histologists have been unable to demonstrate the presence of distinct secretory fibers for the glomerular or tubal cells. This leaves the mechanism of secretion to be explained by ( i ) the vascular supply and by ( 2 ) the "vital activity" of the cells — both operating in conjunction with osmosis. Irritation of a certain part of the floor of the fourth ventricle STRUCTURE OF THE KIDNEY. 243 occasions certain marked changes in the quantity and quality of the urine ; section of the upper dorsal cord temporarily arrests the secretion ; mental emotions, such as fright, anxiety, etc., also Blood-Vessels of the Kidney. Af capillaries of cortex; S, of medulla; a, interlobular artery ; i, vas afiferens; 2, vas efferens ; i, e, vasa recta ; VV, interlobular vein ; S, orig^in of a stellate vein ; i, i. Bowman's capsule and glomerules ; P^ apex of papilla ; C, capsule of kidney ; e, vasa recta from lowest vas efferens. [^Stirling.') 244 EXCRETION BY THE KIDNEYS AND SKIN. modify the flow. All these circumstances, and many others, indicate some control over the activity of the kidneys by the nervous system ; but. that influence is probably exerted only through vaso-constrictor and vaso-dilator fibers to the vessels. Assuming for the present that nearly all the constituents of urine preexist in the blood and are simply taken out of the cir- culation in the kidney, it may be stated that, for most part, the water and inorganic salts are extracted by the cells of the Mal- pighian bodies, while the urea and related organic solids are removed by the cells of the convoluted tubes ; so that the specific gravity of the fluid is raised in passing down the tubes. While the histology of the kidney, and especially the arrange- ment of the glomeruli, is most favorable for the exercise of simple osmosis, and while this process is doubtless mainly re- sponsible for the phenomena which occur, it seems highly prob- able that the cells themselves modify osmotic action by taking an active part in the secretion of urine. They undoubtedly exercise a selective affinity accounting for the different materials handled by the glomeruli and the tubes. Moreover, morpholog- ical changes in the tubal cells during activity have been micro- scopically demonstrated. Vesicles are described as forming in the body of the cell, approaching the lumen, bursting and discharging their contents, — which are supposed to include the urea and such other materials as may be here extracted from the blood. As regards the elimination of water and inorganic salts by the glomerular epithelium, it must also be admitted that the cells take some obscure but active part. Were this only an osmotic process the amount eliminated would vary exactly as the pres- sure. While usually a rise in renal blood-pressure is accom- panied by an increased flow of urine and a fall by a correspond- ingly decreased flow, the rule does not always hold good. For instance, compression of the renal vein raises the pressure but does not increase the amount of urine. URINE. 245 Another fact, which seems almost if not quite as invariable as the effect of blood-pressure, is that the amount of urine varies directly as the amount of blood passing through the kidney, in- dependently of the pressure ; and these two facts constitute about all that is definitely known concerning the local conditions affecting the amount of urine. Whether diuretics increase the urinary flow by simply drawing water from the tissues into the blood and thus increasing the amount and pressure, or by stim- ulating the cells of the glomeruli to increased functional activity is a matter as yet undetermined. Properties and Composition of Urine. — When an ordinary amount of liquid is ingested and when the skin is moderately active the urine, in normal conditions, has a clear reddish am- ber color and a specific gravity of about 1020. The more fluid ingested the paler will be the color and the lower the specific gravity ; the more active the skin the higher will be the color and specific gravity. The urine is diluted in the first case and concentrated in the second. The fact is, the amount of solids (represented by urea) to be eliminated in 24 hours remains approximately the same, and those solids will cause a high or low specific gravity according as little or much water is eliminated with them. The average amount of urine for a day is 2 or 3 pints. Normally it has an acid reaction from the presence, not of a free acid, but of acid salts — chiefly acid sodium phosphate. The odor is not disagreeable on ejection, but decomposition soon begins and a characteristic offensive, ammoniacal odor develops. The kidney is the most important excretory organ in the body and the large number of urinary constituents is not surprising. The chief organic constituents are urea, uric acid, hippuric acid, xanthin, hypoxanthin, creatinin, phenol, indican, oxalic acid, lactates, etc. The phosphates, nitrates, sodium chloride, and car- ion dioxide are the chief inorganic materials. Urea is the most important of the organic constituents. It 246 EXCRETION BY THE KIDNEYS AND SKIN. contains a large amount of nitrogen. Nearly all of it is re-, moved from the body by the kidneys, and double nephrectomy means death from its retention. Its formation is constant and its removal necessary. Its presence in the blood seems to be the normal stimulus exciting the activity of the cells of the con- voluted tubes. Whether urea is produced directly in the tissues, or whether only certain substances antecedent to it are there formed, it cannot be doubted that it is the chief iinal product of nitroge- nous ingesta and nitrogenous disassimilation. It is practically the only way in which the nitrogen of proteid foods can escape from the body. It exists not only in the blood but in the lymph, vitreous humor, sweat, milk, saliva, etc. It has been stated that the taking of large quantities of liquids lowers the specific gravity of the urine by diluting it ; this is true, but the actual amount of urea is increased somewhat by such a procedure. It is not surprising that the quantity of urea is largely increased when much nitrogenous food is taken, and that it is greatly de- creased by an exclusively vegetable diet. Anything, like exer- cise, which will increase actual tissue metabolism, will increase the output of urea, whilp anything retarding tissue metabolism, like alcohol, will decrease the output. The average amount of urea for 24 hours is 3^0 to 4^0 grains. Formation of Urea. — Seeing that urea is the typical end prod- uct of the physiological oxidation of the proteids, it becomes of interest to determine, if possible, where urea formation takes place. It is known that the 'liver is very active in producing this substance ; but it is not alone by this organ that urea is formed. At the present time the prevailing opinion is that, for the most part,, the proteids under destructive metabolism in the tissues do not reach the urea stage of transformation, but are converted into ammonia compounds (which differ very slightly from the urea in chemical composition), and these compounds are conveyed by the blood to the liver, where the slight URIC ACID. 247 change necessary to make them urea is effected under' the in- fluence of this organ. Ammonium, carbamate seems the typical compound, but ammonium carbonate and others are probably likewise converted. Artificial circulation of these compounds through the liver gives rise to urea ; removal of the liver in- creases the ammonia compounds and decreases the urea in the urine ; ammonia compounds are normally very much more abundant in the portal blood than in the arterial, but when the liver is removed they are evenly distributed throughout the cir- culation, and the animal dies in a few days of symptoms which can be aggravated by administration of the ammonia compounds ; — all of which circumstances go to show that it- is ammonia com- pounds which the tissues produce, and that they are changed to urea in the liver. Still, removal of the liver does not suspend entirely the out- put of urea. Consequently this substance must be formed elsewhere, but by what organs is unknown. It is not impossible that it is formed to some extent in all organs where proteid dissociation is progressing. This is practically, if not really, the case in health at any rate, even under the theory above mentioned. It is to be noted that urea is not fully oxidized ; it can be oxidized outside the body. Thus the heat-producing capacity of the proteids is not completely utilized. If they have been broken down in the body into substances simpler than urea, then • the amount of heat liberated in such dissociation is consumed in building up the urea molecule to be discharged. Uric acid is combined in normal urine to form the urates of sodium, potassium, magnesium, calcium and ammonium. The urate of sodium is by far the most abundant of these, and, be- sides urate of potassium, only traces of the others are found. Free uric acid in human urine is pathological. The urates, like urea, come ultimately from oxidation of the nitrogenous constituents of the body. They are not formed in the kidney. 248 EXCRETION BY THE KIDNEYS AND SKIN. but pass out as such from the blood. About g-14 gr. are dis- charged daily. The amount is ihcreased in gout. In some animals uric acid takes the place of urea, none of the latter being formed. In these cases it is manufactured by the liver from ammonia compounds. This does not, however, seem to be the origin of uric acid in human urine. It has been looked upon as unconverted urea, /. e., as a product antecedent to urea ; but at present such does not seem to be the case. A theory that it is the end product of the destruction of certain materials- in the nuclei of cells has considerable support. Hippuric acid exists in the urine as hippurates. It differs from most of the other urinary constituents in being formed in the kidney ; it does not preexist in the blood; The daily output of this substance is about lo grains, though the amount may be considerably increased on a vegetable diet: The benzoic acid of vegetables seems to be synthesized into hippuric. In proteid disassimilation some benzoic acid may be produced and elimi- nated in this shape. The various lactates are not formed by the kidney, but pass unchanged into it from the blood: The lactic acid from which they are formed probably results from the transformation; of dextrose. Creatinin is normally present in the urine. It is a nitrog- enous body differing from creatin only by a molecule of water. It is eliminated to the extent of about 75 grains per day. A part comes from proteid destruction in the body, and ' another part is said to come directly, without metabolism, from creatin which is a constituent of ordinary meat. It is not formed in the kidney. Xanthin, hypoxanthin, etc., are to be regarded as organic nitrogenous excreta allied to uric acid and resulting in some way from proteid metabolism. They are regarded by some as hav- ing the same probable origin as uric acid, viz. , the disintegration of cell nuclei. URINARY CONSTITUENTS. 249 The inorganic constituents scarcely deserve separate mention. It is tlirough the kidney that the largest variety and quantity of inorganic materials are discharged. Certain of these are con- stant, but the wide variety of such materials taken into the ali- mentary canal accounts for the same wide variety in the urine. The proportion of inorganic substances in the blood is approxi- mately constant — kept so by the removal of any excess by the kidneys chiefly. Sodium chloride is eliminated thus to the extent of about 151 grains daily. The sulphates are unimportant. About 25 grains are excreted daily. The phosphates are more important, the acid sodium phosphate being mainly responsible for the acid re- action of the urine. Nitrogen and carbon dioxide are the chief gases to be found. The color of urine is due to a substance, urochrome, which is probably formed from hemoglobin. Some mucus from the bladder is also in the urine. Variation in Amount and Composition of Urine. — " Its con- stitution is varying with every different condition of nutrition, with exercise, bodily and mental, with sleep, age, sex, diet, res- piratory activity, the quantity of cutaneous exhalation, and in- deed with every condition which affects any part of the system. There is no fluid in the body that presents such a variety of con- stituents as a constant condition, but in which the proportion of these constituents is so variable " (Flint). Prolonged bodily exercise will increase the amount of urea, but the urine is generally decreased in quantity because perspi- ration is more active. The young child discharges relatively much more urea and urine than the adult. The female dis- charges relatively more urine, but less urea, than the male. Digestion increases the urinary flow. Climate and season act chiefly through increasing or diminishing cutaneous activity. Emotions of various kinds may give rise to an abundant flow of pale urine. Discharge of Urine.— On leaving the pelvis of the kidney the 2 50 EXCRETION BY THE KIDNEYS AND SKIN. urine enters the ureters and passes through them to the bladder, whence it is discharged per urethram. The ureters run, one from each kidney, downward and slightly inward behind the peritoneum, a distance of some i8 inches to the base' of the bladder. In the female the cervix uteri lies be- tween the two ureters just before they enter the bladder. They penetrate the bladder wall obliquely, their course therein being nearly an inch long. The effect of this arrangement is that dis- tention of the bladder closes the opening more closely instead of causing regurgitation into the ureter. The ureter is com- posed of three coats. The outer is fibrous, the middle muscu- lar, and the internal mucous. The bladder serves as a reservoir for the urine until such time as it is convenient for it to be evacuated. This organ, when empty, lies deep in the pelvis in front of the rectum in the male and of the uterus of the female. When moderately distended it will hold about a pint, has an ovoid shape and rises to the brim of the pelvis. It also has three coats. The outer is peri- toneal, and covers the posterior and small parts of the lateral and anterior surfkces only. Its lower limit posteriorly is the entrance of the ureters. The middle layer is muscular. The fibers, which are non-striped, are disposed in three sheets. Their contraction compresses the contents from all directions. Em- bracing the neck (outlet) of the bladder is a thick band of plain muscle tissue known as the sphincter vesicce. The tonic contrac- tion of this muscle prevents the continual escape of urine. The inner coat of the bladder is mucous. It is rather thick, and loosely adherent to the subjacent muscular coat except over the corpus trigonum where it is closely attached. The corpus trigo- num is a triangular body of fibrous tissue just underneath the mucous membrane ; its apex is at the origin of the urethra, and its other angles are at the vesical openings of the ureters. Absorption from the intact mucous membrane of the bladder takes place very sparingly, if at all. Abrasions of the membrane DISCHARGE OF URINE. 25 1 from any cause allow absorption to occur ; and this fact may be made use of to locate lesions giving rise to hematuria. Iodide of potassium injected into the bladder can be detected in the saliva if the bladder is the source of the blood. Micturition. — When the bladder has become moderately full the desire to expel its contents arises. The act of micturition involves relaxation of the sphincter vesicce and contraction of the muscular walls of the bladder aided by the abdominal muscles and those of the urethra. A slight contraction of the abdomi- nal muscles compresses the bladder ; after a short interval the sphincter relaxes and allows the stream to pass out through the urethra. When the act has been begun contraction of the blad- der will suffice to nearly empty the organ, but complete evacu- ation is finally brought about by a series of convulsive contrac- tions on the part of the muscles of the abdomen. The center controlling the reflex nervous phenomena of mic- turition is opposite to the fourth lumbar vertebra in the spinal cord. The Skin. Functions. — The functions of the skin from a physical stand- point are sufficiently apparent. It furnishes protection to the underlying parts, preserves the general contour of the body, affords lodgment for afferent nerve terminations, and thus estab- lishes relations between ourselves and our surroundings. As an organ of excretion it is very important, and in fact essential to life. While various organic and inorganic materials, such as urea and CO,, are thus discharged from the body, their amount is more or less inconsequential, and it appears that it is the action of the skin as a regulator of heat "excretion" which is vital. It furnishes one of the three chief routes for the discharge of water from the body, and it will be seen that it is largely through the output of water that the output of heat is regulated. So necessary is the skin in this respect that the covering with im- permeable substances of as much as half the body surface is fol- lowed by death. 252 EXCRETION BY THE KIDNEYS AND SKIN. The skin excretions are contained in the products of the se- baceous and sweat glands. These products correspond altogether to neither the secretions nor the excretions, and the sebaceous glands have been described under the head of secretion. It is to be remembered, however, that the sweat usually represents part Fig. 57. ^ St) ratum corneum. Stratum lucidun Stratum granulosura. Stratum Malpighii. Vertical Section of the Human Epidermis. The nerve-fibrils, n, b, stained with gold chloride. {Landois.) of the sebaceous as well as the sudoriparous secretion, because the mixture of the two is a physical necessity. It is the water of the sweat which is the most important excretion from the skin, although the elimination of CO^ and inorganic salts, and STRUCTURK OF THE SKIN. 253 especially of urea in some pathological conditions, is not to be overlooked. Structure. — The skin consists of an external covering, the epidermis, with its modifications, hair and nails, and of the cutis vera. Imbedded in the cutis vera are sebaceo.us and sweat glands and hair-follicles. (Fig. 58. ) Epidermis. — The epidermis consists of at least four layers of epithelial cells. From above downward these are (i) the stratum corneum, (2) the stratum lucidum, (3) the stratum granulosum, (4) the rete mucosum or Malpighii. All these except the stratum corneum have a fairly constant thickness. The stratum corneum is thick or thin according to location and de- gree of exposure, and its cells are flat and horny. The lowest cells of the rete mucosum are columnar. From this last-named layer the cells pass gradually upward, and as gradually assume the shape of the horny layer. The horny cells are thrown off and their place is taken by others from beneath. (Fig. 57.) Hairs are to be found on almost all parts of the cutaneous surface. They consist of a bulb and a shaft. A depression of the skin involving both epidermis and cutis vera constitutes the hair-follicle in which the bulb rests. A projection at the bottom of the follicle corresponds to a papilla, and upon it the bulb is placed. The shaft has an oval shape in cross section. It is composed of fibrous tissue, outside which is a layer of imbricated cells. Nails consist of a superficial layer of horny cells and a deeper one corresponding to the rete mucosum. The root of the nail is received into the matrix — a specialized portion of the cutis vera. Cutis Vera. — The cutis vera is tough but elastic. It rests upon cellular and adipose tissue. Its structure is areolar with some non-striated muscle fibers. Projecting from the cutis vera into the epidermis are minute conical elevations, the papillce. Many of them contain sensory nerve terminals. 254 EXCRETION BY THE KIDNEYS AND SKIN. Sweat Glands. — Practically the whole cutaneous surface con- tains sweat glands. Some two and a half millions are thought to exist in the skin of the average individual. They are particularly Fig. 58. Vertical Section of Skin. A, sebaceous gland openinj into hair-follicle : B, muscular fibers ; C, sudoriferous or sweat-gland; D, subcutaneous fat; E, fundus of hair-follicle, with hair-papilla. (Kirke after Klein. ) SECRETION OF SWEAT. 255 abundant in the skin of the pahns of the hands and soles of the feet. They belong to the simple tubular type, and consist of a secreting portion and an excretory duct. The secreting part lies just underneath the true skin and, as a whole, resembles a small nodule ; however, the nodule consists of an intricate coiling of the tube itself which is of tolerably uniform diameter throughout. It curls upon itself some 6-12 times and ends by a blind ex- tremity. It is lined by epithelial cells. The duct passes away from the glandular coil, runs through the cutis vera in a comparatively straight course and assumes a spiral shape as it traverses the epidermis to open obliquely on the surface. With the ducts of the larger glands are connected a few non-striped muscular fibers which may aid in the discharge of the secretion. (Fig. 58.) Properties and Composition of Sweat. — The secretion is colorless, has a slight characteristic odor, and a salty taste. Its specific gravity is about 1003-4, and its reaction is usually acid when just discharged. It contains a large proportion of water, a little urea and fatty matter, and quite a quantity of inorganic salts of which the chief is sodium chloride. All the constituents in health are of subsidiary importance except the water. Under average conditions of temperature and exercise the amount se- creted in 24 hours is about 2 pounds. But the quantity is very variable — as much so as the urine, and may be said in a general way to vary inversely as the urinary secretion. Mechanism of the Secretion of Sweat. — Sweat is produced continuously, though up to a certain point it passes off as vapor, or ' ' insensible perspiration. ' ' Beyond that point it accumulates on the skin as an evident fluid and becomes "sensible perspira- tion. ' ' Whether it escape as sensible or insensible perspiration, it is secreted as 3, fluid. The activity of the cells lining the glandular coils in separat- ing sweat from the blood is undoubted. Distinct secretory fibers are distributed to them, and through the influence of these fibers 256 EXCRETION BY THE KIDNEYS AND SKIN. the glands will secrete sweat even without an increase in the blood supply. But usually a determination of blood to the sur- face means an increase of perspiration. This occurs during vio- lent exercise, e. g. However, that the production of sweat is not altogether dependent on this factor is shown by profound sweat- ing in shock, nausea and like conditions when the skin is pale and cold, and by dryness of the flushed skin in febrile diseases. Furthermore, experiments on inferior animals have revealed fibers which influence the secretion of sweat without affecting the blood flow. Practically, in health, the only conditions which increase the flow of perspiration are muscular exercise and a high external temperature. Of these, exercise probably works through the nerve centers ; external heat does not stimulate the glands directly, but irritates the cutaneous terminations of afferent fibers which convey impressions to the sweat centers, whence messages are sent out by secretory fibers to the glandular epithe- lium and their activity begins. In both cases there is accom- panying dilatation of the superficial vessels under the influ- ence of the vaso-dilator fibers. It is supposed that the chief center is in the medulla oblongata and that secondary centers exist in the lumbar region of the cord. The amount of COj eliminated by the skin is inconsiderable in the human being. CHAPTER VIII. NUTRITION, DIETETICS AND ANIMAL HEAT. (A) NUTRITION. All the processes which have so far been considered — diges- tion, absorption, secretion, circulation, respiration, etc. — have a single object, viz., the nutrition of the cells of the body. The ultimate source of all nutriment is, of course, food and oxygen. The oxygen has been followed from the lungs to the tissues as oxyhemoglobin of the blood. The various foods have been seen to disappear from the digestive tract and to be con- veyed to the tissues by the great nutritive fluid, some in recog- nizable and some in unrecognizable form. If, now, we shall be able to discover in what way these different materials thus fur- nished the cells are utilized and appropriated by them, and in what condition they subsequently escape from the system, the study of nutrition will have been rendered much clearer. The intake is through the lungs and alimentary canal ; the output is mainly by the lungs, skin, kidneys and intestines. To show for the changes which take place while the food is in the body there is the growth of the body, the maintenance of tissue integrity, secretion, heat, motion and nervous energy. It may be said at once, however, that the exact method of appropriation of nutritive material by the tissues is a subject of speculation, since it involves the question of life itself; and we shall have to be content with recounting some of the conditions influencing and some of the phenomena attendant upon the process. Metabolism. — The general process of nutrition involves 17 257 258 NUTRITION, DIETETICS AND ANIMAL HEAT. breaking down and building up ; it is both destructive and con- structive. It may be said that no cell at two different periods of its existence is made up of exactly the same intrinsic particles. Some of its substance is continually reaching the worn-out stage, becomes effete and must be discharged. To take its place other material must be appropriated and built up into an essential part of the substance of the cell. The changes — of destruction and construction, of disassimilation and assimilation — are described under the term metabolism, which means "change." It is evi- dent that constructive metabolism (anabolism) and destructive metabolism (katabolism) are directly opposite processes. Met- abolic activity is certainly influenced, and largely governed, by certain physical, chemical and electrical laws, but they are in- sufficient to explain all the attendant phenomena. Problems Involved in the Nutritive Process. — Since the actual changes occurring and the method of their production cannot be understood, the question of nutrition resolves itself into a con- sideration of the final fate of the various aliments, of their rel- ative value in nutrition, of conditions influencing the process, and of the explanation of certain facts connected with the de- struction of the food-stuffs, particularly the production of heat. The change which the foods finally undergo in the body is one of oxidation. It is therefore chemical changes which give rise to physical activity. Oxidation is accompanied by the pro- duction of heat. The same sum total of heat is developed when a piece of iron rusts completely away in five years as when it is consumed in an atmosphere of oxygen in five minutes. In both cases it is oxidized. In the cell oxidation is continually going on with the production of heat and of certain excrementitious (oxidation) products depending on the kind of proximate prin- ciple oxidized. Fate of Different Foods in the Organism. — In the first place, the foods may be divided, into (I.) those which /aw through the organism unchanged 3.nA (II.) those which lose their identity and FOODS IN NUTRITION. 259 are discharged as bodies different from those which entered. The first class includes the inorganic foods ; the second the organic nitrogenized and non-nitrogenized. (I.) Attention has been given to tht inorganic foods ythe^K they are discussed as binary proximate principles. Reference should be made to p. ig et seq. for a discussion of the most im- portant of them. Only a few undergo in the body reactions which alter their identity. They may be regarded as already digested and, in fact, when dissolved, ready for discharge from the body. They are, however, useful and necessary constituents of the body, and if they do not take a considerable active part in nutrition, their favorable influence on that process makes them essential to health. The inorganic foods may be dismissed with a repeti- tion of the statement that they are largely introduced in connec- tion with the proteid foods from which they cannot be separated without destruction of the proteid molecule. Indeed, all the proteid food introduced, whether animal or vegetable, contains inorganic constituents as a part of the molecule, and these seem as necessary to nutrition as do the organic constituents. The inorganic and organic enter, are deposited, and seem to be dis- charged together. The few reactions which the inorganic foods undergo in the body do not materially affect the supply of energy. (II. ) The proieids, carbohydrates and hydrocarbons are all con- sumed in the organism, none (unless they have accidentally es- caped digestion) being discharged as they entered. I. The nitrogenous foods are changed into peptones in the alimentary canal, undergo some unknown change in their ab- sorption therefrom, appear in the blood as the proteid constitu- ents of that fluid, and are offered to the tissues through the me- dium of the lymph. The complex proteid molecule is broken down into simpler but more stable ones. These end products are carbon dioxide, water and urea, together with some sulphates 26o NUTRITION, DIETETICS AND ANIMAL HEAT. and phosphates, the production of which is comparatively imma- terial. The urea is distinctive. Heat, which is equivalent to so much energy, is evolved in the oxidation process. It is probable that not all the proteid, under the ordinary diet, is actually built up into cell substance. A part of it seems to be destroyed without being transformed into protoplasmic material, but the destruction always takes place through the agency of the cells, and the end products are always the same, whether disassimilation of the proteid occurs with or without its becoming an intrinsic part of the cell. Nitrogenous Equilibrium — Circulating and Tissue Proteid. — The fact, however, that the characteristic function of the ni- trogenous foods is to furnish protoplasmic material should not be lost sight of. A certain amount is necessary to maintain "nitrogenous equilibrium"; that is, to keep the intake of nitrogen up to the output. When nitrogenous food is with- drawn there continues to be a discharge of urea, which is the chief nitrogenous excretion and the amount of which represents the amount of nitrogenous disassimilation in the body. The urea eliminated under these conditions must represent the actual destruction of cell substance, and, since the supply is zero and the output is considerable, there is not a state of nitrogenous equilibrium ; the animal is suffering destruction of its proto- plasm without a compensatory constructive process. On the other hand, the supply of nitrogenous material may be, and usually is, in excess of the demands of the cells for the actual regener- ation of their substance. This excess may be termed ' ' circu- lating proteid,'''' and is that just referred to as being oxidized under the influence of the cells, but without being transformed into protoplasm. That part of the nitrogenous supply which is built up into a part of the cell has been called " tissue pro- teid. ' ' Whether any given molecule of proteid food pass through the system as circulating or tissue proteid is only an accident — provided the supply be above the demand of the cells for tissue FOODS IN NUTRITION. 261 proteid ; these demands are the first to be supplied by the nitrog- enous material at hand. From this it is not to be inferred that the exigencies of nutri- tion will be met as well without as with circulating proteid. When the diet consists of just enough proteid to supply the tissue wastes and of ample carbohydrate and hydrocarbon ma- terials, the nutritive process is impaired. It seems necessary to perfect health that the supply of nitrogenous food be sufficient to allow for the oxidation of some of it as circulating proteid in a manner analogous to oxidation of the non-nitrogenized organic materials. Life can be maintained on nitrogenous food alone, but it is obvious that when this is done the amount of circulat- ing proteid must be enormously increased so that it may be oxi- dized to furnish energy for the body ; for those substances, the oxidation of which corresponds to oxidation of the circulating proteids and which furnish the main supply of energy for doing work (viz., the carbohydrates and hydrocarbons), are now with- drawn from the economy. It follows, conversely, that the in- gestion of carbohydrates and hydrocarbons lessens the amount of proteid necessary to nutrition. The albuminoids, such as gelatin (not meant to be included under the term "nitrogenous " foods, though they contain nitro- gen), cannot take the place oi tissue proteid ; they may be burnt in lieu of the circulating proteids and supply energy just as the and carbohydrates and fats do. They differ in this respect from the organic non-nitrogenous foods, but cannot sustain life. It is to be remembered that any excess of proteid or albumi- noid food is not discharged as such in the excreta, but undergoes oxidation, the end products of which are always the same, water, carbon dioxide and urea, or related substances ; the development of heat is also an invariable accompaniment of their destruction. While a person may live on proteid food, the amount nec- essary taxes the digestive and excretory organs to such an ex- tent that life is probably shortened. Since the total amount of 262 NUTRITION, DIETETICS AND ANIMAL HEAT. urea is discharged by the kidney, that organ, under an excess of proteid diet, is particularly prone to degenerative changes of a most serious nature. 2. The carbohydrates enter the blood from the alimentary canal as dextrose, are conveyed to the liver and converted into glycogen, which is stored up there to be dealt out to the blood gradually, after being reconverted into dextrose. Dextrose exists in the blood for a short time only, being converted into other substances, but its final oxidation is effected by the tissues. Its end products are carbon dioxide and water, with heat. Sugar (dextrose) injected into the blood soon disappears. It is thought by some to be converted into alcohol in the blood and then oxidized. At any rate, the formation of the end products just mentioned is the final fate of the carbohydrates, through what- ever splitting processes the sugar molecule may pass before it is converted into these substances. The removal of the pancreas occasions diabetes mellitus, and the inference is that this gland gives off to the blood some internal secretion which splits up the sugar molecule in the blood. How this lesion causes the disease in question is not clear, but the retention of a small part of the gland enables the oxidation of sugar by the tissues to proceed in the proper way and it is not discharged in the urine. Value of the Carbohydrates in Nutrition. — The distinctive function of the carbohydrates is to act as fuel for the body ma- chine ; they are burnt up to supply heat, and heat represents energy. Hydrogen and oxygen exist already in the proportion to form water— one of the end products — and only enough O is required to unite with the carbon of the carbohydrates to form COj — the other end product. The burning (oxidation) of a carbohydrate outside the body results in the formation of CO2 and HjO and the elimination of heat, which last, if properly utilized, can be converted into energy — the power to do work. The result of the oxidation of a carbohydrate in the body is the FOODS IN NUTRITION. 263 same. Since this class of food is easily handled by the alimen- tary canal, requires little extra O for its destruction, and is very abundantly supplied by the vegetable world, it is the most eco- nomical from digestive, absorptive, respiratory and financial standpoints. Carbohydrates may also be deposited as adipose tissue as will be seen presently. 3. The fats have the same general office in nutrition as the carbohydrates, viz., the furnishing of energy by their oxida- tion. They leave the alimentary canal by way of the lacteals, are conveyed by the blood to the tissues and there oxidized with the formation of carbon dioxide and water and the liberation of heat. Though more O is necessary to burn up the fat than the carbohydrate molecule, oxidation of the fat is attended with the liberation of the greater amount of heat — /. e., of energy. This would seem to indicate that it would be more economical to eat fats to the exclusion of carbohydrates, since a smaller quantity of the former will supply the requisite amount of energy. This is theoretically true, but considerations of di- gestion render it not practically so ; fats tax the digestive appa- ratus much more than carbohydrates. The fat deposited in the body — the adipose tissue — whatever may be its source, is to be looked upon as so much stored-up energy. When the supply of blood is cut off it is the first part of the organism to be consumed. A fat animal will survive starvation longer than a lean one. The individuality, the functional activity, and the properties involved in regeneration of protoplasm are ultimately depend- ent upon its nitrogenous characters. The other constituents are more or less passive. The oxidation of fats and carbohy- drates, however, takes place under the influence and through the agency of the cells. It is scarcely necessary to add that neither fats nor carbohydrates, nor both together, are sufficient to sustain life; for life is embodied in protoplasm and protoplasm must have nitrogen, which element these foods cannot furnish. 264 NUTRITION, DIETETICS AND ANIMAL HEAT. Formation of Adipose Tissue. — The adipose tissue in the body is not the result of direct deposition of the oleaginous foods. The amount of fat taken on in a given time by some animals, as hogs, is often far in excess of the quantity of fat in the ingesta. Adipose tissue is, under normal conditions, the result always of changes due to protoplasmic activity. It is formed by the tissues chiefly from the carbohydrates, but also to a less extent from the proteids. The chemical changes by which sugar is converted into fat are as yet undetermined, but there are so many evidences of an increase in body fat upon an excess of carbohydrate food that the fact itself that this class of foods is the main source of fat is no longer disputed. As regards the formation of fat irora proteids, it is thought that the molecule is split up into a nitrogenous molecule, which is discharged as urea, and a non-nitrogenous, which at once, or after undergoing other changes, is deposited as fat. Experi- mental observations demonstrate that the liver produces glyco- gen on a purely proteid diet. Since glycogen is a carbohy- drate, and carbohydrates are the chief source of body fat, it is not improbable that the non-nitrogenous molecule of the proteid dissociation takes the form of glycogen and is later con- verted into, fat after the manner, whatever it may be, of the glycogen introduced in carbohydrate form. When the carbon discharged is less than the carbon ingested the deficit is thought to be retained to form fat, which is deposited as a reserve to be used whenever its oxidation may become necessary as a supply of energy. It follows that to reduce body fat the carbohydrates should be largely interdicted, while to increase it they should be taken in excess. In human beings proper regulation of the diet is more efficacious in reducing than increasing the amount of adipose tissue. Adipose Tissue a Reserve Supply of Energy. — The carbohy- drates and fats are preeminently the energy-producing foods. CONDITIONS INFLUENCING METABOLISM. 265 and of these the carbohydrates, for reasons indicated, are the more .important. They not only furnish energy which is immediately used up in running the machinery of the body, but they deposit, or attempt to deposit, a reserve supply to protect the proteid portions of the organism against accidents of temporary deprivation of food, demands for an unusual amount of energy, malnutrition from various causes, etc. — sav- ings laid for the proverbial rainy day. This reserve supply takes the form first of glycogen, which is soon used up, meeting as it were only the demands of the hour, and second of fat, which begins to be drawn upon when the glycogen is exhausted, and which lasts for a length of time depending upon its amount. Conditions Influencing Metabolism. — Regular exercise is undoubtedly favorable to the nutrition of any part, as, e. g. , the muscles, the brain, etc. Exercise may mean increased disas- similation, but if so it also means increased assimilation. With regard to muscular exercise of average severity and reasonable duration, the results of cellular activity seem at first a little sur- prising, but are really to be expected if the concluding remarks of the previous paragraph are true. The amount of urea under such exercise is not appreciably increased — which means that disassimilation in the protoplasm of the muscle cells is not in- creased. This remark holds good, however, only when the sup- ply of sugars, starches and „fats is abundant ; if they are not present in sufficient quantity to meet the increased demand for energy-supplying materials, then the proteids must be oxidized to furnish it, and the urea discharge is increased. In striking con- trast to the constant output of urea is the largely increased out- put of COj, representing oxidation of the carbohydrates and fats. During sleep the nitrogenous output is not materially dimin- ished, while that of CO2 is markedly less. This is explained by the fact that there is less energy needed and correspondingly less 266 NUTRITION, DIETETICS AND ANIMAL HEAT. oxidation of the energy-producing materials. Proteid metabo- lism is undisturbed. A low external temperature does not increase the output of urea ; it increases the output of CO2. These two facts together mean again that only the carbohydrates and fats are being oxi- dized in increased amount. This increased oxidation, the effect of which is^ to maintain the normal body temperature, is usually dismissed with the statement that it is a reflex nervous act. It is claimed by Johannson that the CO2 output is not increased until shivering occurs (Reichert). That being the case, the increase is explained on the ground of increased energy and heat production incident to muscular exercise, and shivering assumes the dignity of a physiological factor in keeping up the temperature of the body. This is perfectly reasonable when it is remembered how effective active muscular exercise is in keeping the body warm. But the fact that a person when cold shivers and is restless invol- untarily does not allow us to escape the unsatisfactory "reflex action ' ' explanation of the phenomenon in question. Within ordinary and reasonable limits proteid metabolism is undisturbed ; it is still being protected by the fats and carbohydrates. During starvation nothing is supplied from the outside world except oxygen, and the animal must live on the materials al- ready in his body. The glycogen is first consumed ; it is the surplus on hand; but at best it is all gone in a very few days. Then the fat stored up as adipose tissue is drawn upon ; it is the reserve fund ; but it is likewise soon consumed ; the animal be- comes progressively emaciated. When this is exhausted the tissue proteid is attacked ; this is the capital and is the last to be touched ; but there must be heat and at least some energy, and there is no other source. When the proteid capital has at last been so impaired that it can no longer furnish heat to maintain the body temperature and energy to carry on the necessary or- ganic functions, the organism is physiologically bankrupt and as- signment follows — death is at hand. REQUISITES OF DIET. 267 (B) DIETETICS. The appetite, under normal conditions, may be depended upon to regulate both quantity and quality of diet in a fairly satisfac- tory manner. Different peoples require different proportions and amounts of the proximate principles, and the same is true of any given individual for varying conditions of temperature, ex- ercise, etc. But in any case the object of eating is to prevent the loss, in aggregate, of proteid tissue, fat, etc., — to replace the wastes, and that in the most convenient and economical way. When the ingesta exceed the excreta the animal is gaining in weight ; when opposite conditions obtain he is losing ; when there is a balance between the two the body equilibrium is being maintained. Determination of the Requisities of a Diet. — The usual method of determining, in a scientific manner, the requisites of a normal diet for persons in general is to estimate the amount of the various excretions from the bodies of a limited number of per- sons in health, and from this knowledge to calculate the amount and kind of food which will supply the demands in the most satisfactory way, it being assumed that these excretions represent the normal and necessary metabolism going on in the body. The results of such examination are found to correspond with the actual demands of the system. It has been seen that the organism demands some fifteen or more chemical elements for use to keep itself in good running order ; it has been seen also that its demands, so far as quantity is concerned, are chiefly confined to carbon, hydrogen, oxygen and nitrogen. The other elements deserve no attention here, since they (excepting sodium chloride) are unconsciously intro- duced with the ordinary foods in amounts sufficient to satisfy the requirements of the system. Moreover, the air we breathe and the water we drink furnish an ample supply of hydrogen and oxygen when to this supply is added the quota of these ele- ments contained in the necessary quantities of other aliments. 268 NUTRITION, DIETETICS AND ANIMAL HEAT. So, therefore, if we can fix upon a diet which will furnish the requisite amounts of carbon and nitrogen no attention need be paid to the other elements. The supply of the others may be said to regulate itself if the supply of carbon and nitrogen be regulated. The object, then, of food may be said to be the replacement of carbon and nitrogen— the carbon and nitrogen in the excreta. Of these two elements, carbohydrates and fats will furnish only carbon ; proteid food will furnish both. Amount of C and N Necessary. — It is found that the dally discharge of nitrogen is about i8 grams, and of carbon about 281 grams. These are the amounts, therefore, which must be sup- plied by food. We may accept, as representing the proteid molecule in general, the formula C^fi^^fi^^^^. Then it is evi- dent that an amount of proteid food which would furnish the necessary 18 grams of nitrogen would furnish only 72 grams of car- bon — only about one-fourth enough. If, now, the proteid food be increased to supply 281 grams of carbon, the system will have to handle four times as much nitrogen as it needs ; and this is a tax to the digestive apparatus and the excretory organs, partic- ularly the kidney — a tax which is rendered unnecessary by the availability of the carbohydrates and fats as food. These contain abundance of carbon, and it is far better to eat only enough pro- teid food to supply the 18 grams of nitrogen, and make up the deficit of carbon with non-nitrogenized articles of diet. One can supply all the demands by eating nitrogenous food alone, and life will be preserved indefinitely perhaps, but the prediction would be warranted that in such a case the person would probably die prematurely — as a result of kidney or liver disease. Articles Which will Supply the Necessary Amounts of C and N. — The conclusion (modified) of Moleschott is that the average man needs daily about 120 grams of proteid, 90 grams of fat, and 320 grams of carbohydrate food, estimated dry; and that with this, in the usual state in which such food is taken, he will RKQUISITES OF DIET. 269 consume unconsciously, or as a result of craving, some 30 grams of salt? and 2,800 grams of water. These proportions are sup- posed to satisfy the demands of the system in an economical way. The estimates of Ranke vary somewhat from this as indi- cated in the subjoined table which shows also the balance kept up in the body. Income. Expenditure. Foods. Nitrogen. Carbon. Excretions. Nitrogen. Carbon. Proteid 100 gm. Fat 100 " Carbohy- drates 250 " 0.0 " 00" S3.ogm. 79.0 " 93.0 " Urea 31-5 g™- Uric acid 0.5 " Feces Respiration (CO2) } 14.4 I.I 0.0 6.16 10.84 208.00 15s " 225.0 " iS-5 225.00 The actual amounts of given substances which it is necessary to eat in order to supply the requirements of these estimates depend, of course, on the composition of those substances, and would have to be settled by reference to a table giving analyses of the common articles of diet. Two pounds of bread and |^ pound (when uncooked) of lean meat, together with water and salt, will supply the demands ; but this is an unusual diet. Or 1 pound of meat, i pound of bread and J^ pound of butter, or other fat, with water and salt is probably preferable. In any case if nutrition is to be properly performed the diet must be varied. It could not be held that the above supply of food would keep a person indefinitely in good health. His de- mands for nitrogen and carbon are always approximately the same, but the organism revolts at being supplied with them from exactly the same source for any considerable length of time. It need scarcely be added that any condition, such as exer- cise, temperature, etc. , which increase the excreta, calls for a larger supply of ingesta. Ordinary exercise is allowed for in the estimates just given. 270 NUTRITION, DIETETICS AND ANIMAL HEAT. (C) ANIMAL HEAT. The Temperature. — The average temperature of the human body, taken under the tongue, is 98.5° F. It varies in different parts, the mean being about 100°. The metabolic activity in different parts of the body is changeable, and consequently the heat production in all parts is not the same. The fact that the temperature is nearly identical throughout the body is due to the distribution of heat, which distribution is mainly effected through the agency of the circulating fluids. The rectal temperature is a little higher than that obtained in the mouth. The temperature of arterial is higher than that of venous blood. The warmest blood is in the hepatic veins ; the coolest is that which has just passed through the most exposed peripheral parts, as the helix of the ear. The mean body tem- perature is a little lower in the morning than in the evening, in the female than in the male, on a restricted than on an abun- dant diet, in cold than in hot climates, and, in general, in condi- tions of diminished than of exalted metabolic activity. But in health these variations are of trivial importance and do not represent a sweep of more than 2 ° F. The body temper- ature may be looked upon as being a fairly constant quantity. It varies scarcely at all with variations of external temperature, so long as the heat-regulating apparatus is in order. An external (dry) temperature of 212" F., or the extremely low temperature of some regions, can be borne with very slight fluctuations in the temperature of the body. The actual limits of internal tem- perature consistent with the preservation of life are given by Flint as 83° and 107° F. These temperatures cannot be long endured. The fundamental fact to be kept constantly in mind is that there is a continual production and a continual dissipation of heat, in ways to be indicated presently. These two processes are known as thermogenesis and thermolysis respectively. The HEAT AND FORCE. 271 preservation of the proper balance between heat production and heat dissipation is known as thermotaxis. Supply of Heat and its Relation to Force. — It is a matter of common observation that the burning (oxidation) of any sub- stance, as a piece of wood or an article of diet, is accompanied by the evolution of heat. It is also known that heat may be converted into force — may be made to do work. The burning of a fat or a sugar produces COjand H^O ; the burning of a pro- teid produces CO^ and H,0, and additional substances. The final products, and the amount of heat evolved, are precisely the same whether the oxidation be rapid or slow. Now, the oxida- tion of food is exactly what occurs in the human organism, though that of the proteids is not completely effected ; CO2 and HjO are produced from them, and the "additional substances " mentioned are represented by urea. This process then, is the source of body heat. To the supply thus furnished may be added a little from reactions between inorganic materials in the body, from warm foods and drinks, and from friction in the vessels, joints, etc. The foods thus possess a certain potential energy, an energy which may be converted directly or indirectly into heat, or its equivalent. The potential energy of the foods keeps up the body temperature and supplies force for doing work. It is con- verted into heat and kinetic energy. Kinetic energy is working energy, and is represented in the body chiefly by muscular con- tractions. But, since this kinetic energy has its source in the transformation of proximate principles, and since kinetic energy and heat are mutually convertible, it may be assumed that all the potential energy of the foods is converted into heat. The kinetic energy may be taken as representing so much heat, and the total production of heat (including kinetic energy) as representing the total production of energy. Or, to state the case differently, the potential energy of the food is converted into heat, a part of which appears as kinetic energy. By far the largest part of this 272 NUTRITION, DIETETICS AND ANIMAL HEAT. potential energy, however, is converted directly into heat. Not more than one-fifth of the heat produced in the body can be utilized to do work, and a part of that work is actually converted indirectly into heat, and contributes to the total heat of the body, by overcoming friction incident to respiration, circula- tion, movements of the joints, muscles, etc. Potential Value of Foods. — It is estimated that the oxidation in the body of one gram oi fat produces 9,300 calories of heat, I gram oi carbohydrate 4,100 calories, and one gram oi proteid 4,100 calories. These figures represent the potential energy of the several foods. Fats, it is seen, produce, weight for weight, more than twice as much energy as other foods, but reasons have been given why they cannot be used exclusively. A calorie is the kmount of heat necessary to raise i gram of water 1° C. A grammeter is the amount of energy necessary to raise i gram i meter. Now since heat and work are only different forms of energy, these two units — calorie and gram- meter — have each equivalents in terms of the other. One calorie equals 424. 5 grammeters ; that is, the force represented by one calorie will raise one gram 424. 5 meters. The terms kilocalorie, or kilogramdegree, and kilogrammeter are used sometimes, and represent 1,000 times the calorie and grammeter respectively. Total and Specific Heat. — The temperature of a body indi- cates nothing as to the quantity of the heat it contains. The degree of heat requires only a thermometer to determine it, but the quantity depends on the temperature, the weight and the specific heat of the substance in question. Specific heat is analogous to specific gravity. Water is taken as the standard in both cases. If it require only . 5 calorie to raise I gram of a certain substance 1 degree C. , the specific heat of that substance is said to be .5. The specific heat of the body is . 8 ; that is, whereas it requires a certain amount of heat to raise 150 pounds of water to a certain temperature, it would require only . 8 as much to raise a human ■ body weighing the same to THERMOGENESIS. 273 the same temperature. To find the total heat in calories in any body it is only necessary to multiply the weight (in grams) by the specific heat and by the temperature C. Estimates made by calorimetry from these data and from the potential value of the different foods give the total daily heat production as about 2,500,000 calories for the average individual. This is equal to about 1,400 calories per hour per kilo weight. The English heat unit is the pound-degree F. It is the amount of heat necessary to raise i pound of water i degree F. Its mechanical equivalent is the force necessary to raise i pound 772 feet. The estimates just given in the metric system when translated to English nomenclature give the total heat produc- tion for 24 hours as about 8,400 pound-degrees, or 2.5 per hour per pound weight. These figures are given as only approxi- mate and are subject to change by many causes, such as sex, cardiac and respiratory activity, internal and external temper- ature, exercise, digestion, age, nervous influences, the body weight, etc. Thermogenesis. — Thermogenesis, or the production of heat, is the result of activity on the part of tissues, nerves and cen- ters. Now, the potential energy of the foodstuffs is the ultimate source of all bodily heat no matter how it may be manifested, and it is evident from what has been said already that all the tissues of the body are heat-producing tissues, because oxidation processes go on in them all. But muscular tissue seems to be endowed with special heat-producing capabilities, so much so that it is said to generate heat as a specific product, and not as a mere incident of its metabolism. Muscle will reproduce heat when entirely at rest — when the nutritive metabolic changes are practically nil. The process seems to be regulated in accord- ance with the needs of the economy by means of a nervous mechanism, making the production of heat analogous to secre- tion. Separation of a muscle from its nerve does not stop thermogenesis, but markedly interferes with it in that part. The 18 2 74 NUTRITION, DIETETICS AND ANIMAL HEAT. existence of distinct thermogenic nerves has not been demon- strated. The existence of specific thermogenic centers seems certain. Some of them increase and some decrease thenno- genesis. The general thermogenic centers are in the spinal cord. Cen- ters increasing thermogenesis are probably in the caudate nuclei of the corpora striata, the optic thalami, pons and medulla. Irritation of these regions causes a rise in temperature. The location of the thermo-inhibitory centers is a matter of specu- lation. The general thermogenic centers in the cord prob- ably maintain a fairly constant production of heat indepen- dently, but they are subservient to encephalic centers which excite them to increased or decreased activity by reason of certain impressions, cutaneous or otherwise, which they have received. Thermolysis. — About 85 per cent, of animal heat, discharged as such, is lost by radiation and evaporation from the skin; about 12 per cent, is dissipated in the lungs by evaporation and in warming the inspired air ; the remainder is discharged in the urine and feces (disregarding the small amount which goes to warm ingested articles). Heat is radiated from the body just as from a hot stove. Radia- tion is affected by the conductivity of the surrounding medium. For instance, in media of water and air of the same temperature the radiation is greater in water, because it is a better conductor of heat. Evaporation from the skin is of very great importance in in- creasing heat dissipation. 582 calories of heat are consumed when one gram of water is vaporized ; and when this evaporation takes place on the skin the heat is abstracted largely from the body. This is said to represent nearly 15 per cent, of the total heat dissipation. Hence the value of perspiring in hot weather. Evaporation also takes place from the moist surfaces of the lungs, and, moreover, when, as is usually the case, the inspired air is THERM OT AXIS. 275 cooler than the lung structure a certain amount of heat is con- sumed in wanning it. But it is not to be inferred that thermolysis takes place from the body just as from an inanimate object and that no "or- ganic ' ' process is involved. On the other hand, it is intimately connected with and influenced by circulation, respiration, secre- tion and other functions. When there is a tendency for the body temperature to rise, the circulation becomes more active and sends more blood to the periphery to be cooled ; respiration is augmented, causing a greater abstraction in the lungs ; the secretion of sweat, for instance, is increased. There may be distinct thermolytic centers. Conditions Influencing Heat Dissipation.— These have been suggested in a previous section. Heat dissipation is greater in proportion to weight in small than in large animals because the radiating surface is relatively larger. It is less in the female than in the male because she has, as a rule, a larger proportion of subcutaneous fat, which is a poor conductor of heat. It is less when the body is covered with clothing which is a poor con- ductor of heat than when the covering conducts heat readily. It is increased when the internal temperature is raised and when the external temperature is lowered. Any general increase in vascular or respiratory activity increases heat dissipation for reasons already given. When the external temperature is high and the air is dry evaporation is more abundant, and conse- qently heat dissipation is greater than when the air is already impregnated with moisture. Hence the oppressiveness of a high external temperature with high humidity. In fever heat dissipation is usually increased, but to a less degree than heat production. Thermotazis. — Thermotaxis is the regulation of heat produc- tion and heat dissipation so that the temperature of the body may remain the same. It is evident that there is frequently a transient increase or decrease of thermogenetic activity ; unless 276 NUTRITION, DIETETICS AND ANIMAL HEAT. there be a corresponding change in thermolytic activity the tem- perature will be disturbed. The temperature of the body is not necessarily raised when thermogenesis is increased, or lowered when thermogenesis is decreased ; for thermolysis may be, and in health is, correspond- ingly increased or diminished. Conversely, a change in ther- molysis does not necessarily mean an opposite change in the body temperature. Alterations which do occur in the temper- ature are the result of the improper regulation of the heat at hand. For instance, fever may result from average thermo- genesis and deficient thermolysis ; from increased thermogenesis and thermolysis when the latter is increased less than the former ; from diminished thermogenesis and thermolysis when the latter is diminished less than the former, etc. A subnormal tempera- ture is caused by opposite conditions. The temperature remains constant when thermogenesis and thermolysis are normal, or when they are increased or decreased correspondingly. Thermotactic activity is the result of changes in the tempera- ture of the blood, or of cutaneous impressions. A rise in the tem- perature of the blood excites thermolysis, as indicated. A cold atmosphere increases thermolysis, but at the same time it makes impressions on the cutaneous nerves which, when carried to the centers, excite thermogenesis and thus compensation is estab- lished. A cold bath lowers the temperature because thermolysis is increased more than thermogenesis. There is increased radi- ation because of the relatively increased difference in the tem- perature of the body and of the surrounding medium. On the other hand, the cold contracts the capillaries, diminishing the amount of blood exposed to the cooling influence of the water and decreasing the amount of sweat ; but these influences tend- ing to inhibit thermolysis are not equal to those augmenting it. However, in health, thermotaxis prevents the disturbance of the balance between thermogenesis and thermolysis to any great ex- tent, and the temperature cannot be lowered very much. These THERMOTAXIS. 277 are only examples of the reciprocal relations maintained between the production and dissipation of heat, a disturbance of which relations is prevented under normal conditions by thermotaxis. Any change in one process is followed at once by a compensa- tory change in the other. CHAPTER IX. THE NERVOUS SYSTEM. General Functions of the System as a Whole. — The nervous system is the most delicately organized part of the animal body. Its sensory terminations receive impressions which are conducted to the centers ; it conveys impulses from the centers to the different parts of the body, controlling and regulating their action. Connecting, as it does, all parts of the organism into a coordinate whole, it is the only medium through which impressions are received, and is the only agency through which are regulated movement, secretion, calorification and all the processes of organic life. This system, ramified throughout the body, connected with and passing between its various organs, serves them as a bond of union with each other, as well as with the brain. The mind influences the corporeal organs through the instrumentality of this system, as when volition calls them into action ; on the other hand, changes in the organs of the body may affect the mind through the same channel, as when, for instance, pain is mentally perceived when . the finger is burned. Thus it is that the nervous system becomes the main agent in what is known as the " life of relation " ; for without some medium for the transmission of its mandates, or. some means of receiving those impressions which external objects are capable of exciting, the mind would be completely isolated, and could hold no communion with the external world. It should not be understood, however, that the pervous system cannot operate independently of mental influence. All those manifestations of nervous activity connected with \ the perform- 278 GENERAL FUNCTIONS. 2 79 ance of the so-called " organic functions " of life, as digestion, circulation, etc., are not directly influenced by volition ; indeed an essential character of these functions is that they are com- pletely removed from the influence of the will ; to be conscious subjectively of their performance is an evidence of abnormality. The first step in every voluntary act is a mental change, in which the act of volition consists. If this mental change be of such nature as to direct its influence upon a muscle, or a particular set of muscles, the contraction of those muscles immediately supervenes, so as to bring about the predetermined voluntary act. But the influence of the will could not possibly be exerted upon those muscles except through intervention of the nerves. Furthermore, a certain mental state, in cases of common or special sensation, is induced by an impression made upon certain bodily organs. But in no case could the mental state be pro- duced unless a particular part of the nervous system were present to convey the impression received to the center capable of rec- ognizing it. If the hand be burned pain is felt, but were the nerves not present to convey the impression made by the heat no degree of temperature could make the mind cognizant of injury. When light is admitted to the eye a corresponding mental sensation is produced, but for the production of this the integrity of the optic nerve is a necessary condition. It will be gathered from the foregoing remarks that the nerv ous system is not only capable of conveying communications, but that it has the power, in certain of its divisions, of receiving impressions and of giving rise to stimulating influences — that is, that it is capable of generating a peculiar power known as " nerve force." It thus becomes the seat of distribution of en- ergy to all the cells. These generating parts of the system are the reservoirs of force — force which has been derived from the cells and is distributed to them. This nervous force, having its origin in the living activity of the cells, is the highest manifesta- tion of vital energy. 28o THE NERVOUS SYSTEM. The nervous structure is divided into two great systems : 1. The Cerebro-Spinal System consists of the brain, the spinal cord and all the nerves which run off from these. This system is especially concerned with the functions of relation, or of animal life. It presides over general and special sensation, over voluntary movements, over intellection, over all conscious activity, and over all other functions which are peculiar to the animal. It is by this system that we know of and deal with the other great system. It is also called the Animal, or Inorganic, System. 2. The Sympathetic, Organic, Ganglionic or Vegetative System is especially connected with the functions relating to nutrition — functions similar to those occurring in the vegetable kingdom. It presides over all organic life — over all uncon- scious activity. While the operations over which this system holds sway are quite different from those under the supervision of the cerebro-spinal system, it must not be concluded that the two are not anatomically and physiologically related. Neither is independent of the other, as was once thought, but both are parts of the same great apparatus. Divisions of the Nervous Substance as a Whole. — The nervous matter, irrespective of the two systems, may be studied as consisting of two divisions. The first is made up of cells ; the second of tubes, or fibers. Although the tissue may be thus divided into nerve cells and nerve fibers, the present conception of the arrangement of the nervous substance is that these two are only different parts of the same element known as the neuron, supported by tissue elements known as neuroglia, which, though not identical with connective tissue, is comparable to it in its function of support. The neuron, thus considered, consists of a protoplasmic body which sends out a number of branchijjg proc- esses called dendrites, one of which becomes the axis cylinder. While, therefore, it is to be understood that the cell and the fiber in the nervous system are both portions of an identical whole, a NERVE FIBERS. z8l description of them as separate parts is warranted for the sake of convenience and by differences in their general characteristics. The nerve cells are the only*organs capable, under any cir- cumstances, of generating nerve force. As a rule they are stimu- lated to generate this force by the reception of an impression through the nerve fiber, but they may in some cases be directly excited by mechanical, electrical or chemical means. They also frequently act as conductors, as will be seen later. Under no circumstances can nerve fibers generate force. Their office is exclusively to conduct impressions and impulses, and they usually receive these impressions and impulses at their terminal extremities in the case of afferent nerves, and from the centers in the case of efferent nerves ; but in many instances they may be stimulated in any part of their course. Some fibers are incapable of being thus directly stimulated. The nerves of special sense are insensible to direct stimulation. Nerve Fibers. — Nerve fibers are of two kinds : (^) white or medullated fibers and (^) gray or non-medullated fibers. The non-medullated fibers possess the conducting element alone, while the medullated possess certain accessory anatomical ele- ments. (^) Each medullated fiber has (i) an external enveloping membrane called the neurilemma, or the primitive nerve sheath, or the sheath of Schwann ; (2) an intermediate substance known as the myeline sheath, or the white substance of Schwann, or the medullary substance ; (3) a central fiber, the true conducting element, which usually goes under the name of the axis cylinder, or axione. The sheath of Schwann is analogous to the sarcolemma of muscle fibers. It is a structureless protective membrane, some- what elastic, and presents oval nuclei with their long diameter corresponding to the direction of the fiber. This sheath is wanting over the medullated fibers in the white substance of the brain and spinal cord. 282 THE NERVOUS SYSTEM. Fig. 59. Node of Ranvier. Primitive sheath. ii'S] Nerve corpuscles. ' Axis cylinder. White substance of Schwann. — Node of Ranvier. Scheme of a Medullated Nerve-Fiber of a Rab- bit Acted on .by OsMic Acid. The incisures are omitted. > 400. {JLandois.) It is the white substance of Schwann which gives to the nerve its peculiar whitish appearance. This is a fatty substance of a semi-fluid consistence. It fills the tube made by the sheath of Schwann and surrounds the axis cylin- der. It is wanting at the origin of the fibers in the centers and at their periph- eral distribution. It is probably not necessary to conductivity. In fresh nerves this substance is strongly refrac- tive, and the optical effect produced by its varying thickness in the center and at the edges is the appearance of dark borders. It easily coagulates into an opaque mass. The idea that the niye- line sheath acts as an insulator lacks supporting evidence. The theory that it is nutritional is plausible ; but no suf- ficient difference in the medullated and non-medullated fibers in this respect has been found to establish the theory as a fact. At certain points in the course of medullated fibers there are seen constrictions called the nodes of Ranvier. At these points the medul- lary substance is wanting and the sheath of Schwann is in contact with the axis cylinder. It is not improbable that these nodes furnish a tnode of access for the nutrient plasma. Certain it is that they are most numerous where physio- logical activity is supposed to be most active. NERVE FIBERS. 283 The axis cylinder is composed of a large number of primitive fibrillse. This band occupies about one- fourth the diameter of the tube and is the true conducting element, as is shown by its invariable presence, its continuity and other considerations equally con- clusive. It is demonstrated under the microscope with difficulty in fresh speci- mens. It is directly connected with a nerve cell, and is the essential part of the fiber. The process of the cell which becomes the axis cylinder is not, as was once thought, unbranched, but itself sends off " collaterals " in the gray sub- stance. These collaterals, however, do not actually join any other nerve cell or fiber. The average diameter of meduUated fibers is about ^-j^ in., though all are said not to preserve the same diameter throughout their course. (^) The non-meduUated fibers (fib- ers of Remak) seem to be simple axis cylinders without the other anatomical elements peculiar to medullated fibers. They make up a large part of the trunks and branches of the sympathetic sys- tem, and represent the filaments of origin and distribution of all nerves. They are thought by some to possess a neurilemma. They are pale gray in color. Nerve Trunks. — The above remarks apply to a single nerve fiber. These Fig. 60. Non-Medullated Nerve- Fiber. Vagus of dog. h, fibrils ; «, nucleus ; /, protoplasm sur- rounding it. {Siirling.) 284 THE NERVOUS SYSTEM. fibers seldom run an extended course alone, but are bound together in large numbers to make a nerve trunk. This trunk is composed of a number of bundles of fibers, and is sur- rounded by a connective tissue membrane known as the epineu- rium ; the separate bundles, or funiculi, are surrounded each by a similar membrane called the perineurium; while inside the funiculi, between the primitive fasciculi, is a delicate supporting Fig. 61. Transverse Section of a Nerve, (Median.) ep, epinerium ; fe^ perinerium; ed^ endonerium. {Landois.) tissue known as the endoneurium, or the sheath of Henle. In con- nection with this sheath there are nuclei belonging to the con- nective tissue and to the nerve fibers themselves. The sheath be- gins where the nerve fibers emerge from the white portion of the centers, is interrupted by the ganglia in the course of the fibers, branches as the bundle branches, and is lost before the terminal distribution is reached. It is seldom found surrounding single fibers. It is likewise rare for capillaries to penetrate it and reach NERVE CENTERS. 285 the fibers themselves. There are numerous lymph spaces around the individual fibers as well as around the fiiniculi. In situations where the nerves are well protected, as in the cranium, the amount of fibrous tissue in the trunks is small, but where opposite condi- tions prevail, as in muscular substance, this tissue is largely in- creased in amount as regards both that which surrounds the trunk and that which is sent in between the fiiniculi and fibers. This tissue has ramifying in it a network of fibers known as nervi nervorum. The blood supply is not large. Individuality of Nerve Fibers. — It is to be remembered that so far as can be determined every nerve fiber, having entered a trunk, proceeds without interruption |to the part to which it is finally distributed, whether that part be the skin, or a viscus, or a muscle, or a gland, or some organ of special sense, or another nerve cell, or what not. Collections of fibers forming bundles run together in the same trunk, may leave that trunk together, may send out part of their fibers to another bundle or trunk, or may receive other fibers from other funiculi ; but everywhere the relation of the primitive fibers to each other is simply one of contiguity. However, as the axis cylinder approaches the seat of its final distribution, it breaks up into several fibrillae, such divisions always taking place at the nodes of Ranvier. Nerve Centers. — The nerve centers include the gray matter of the brain and cord and the ganglia in both the cerebro-spinal and sympathetic systems. These centers have a gray color due to the presence of a pigmentary substance in the cells and sur- rounding tissue. The ganglionic centers are simple collections of nerve cells with their usual accessory elements — myelocytes, intercellular granular matter, delicate membranes covering some of the cells, connective tissue elements, blood-vessels and lym- phatics. Nerve Cells. — These are irregular in shape and may be uni- polar, bipolar or multipolar. They also vary much in size. The unipolar cell has a single prolongation which becomes the axis 286 THE NERVOUS SYSTEM. cylinder. Bipolar cells are prolonged in two directions, and may be looked upon as simply protoplasmic enlargements of the nerve fiber. This cell is frequently covered by a connective tissue en- velope which is continuous in both directions with the sheath of Schwann. Multipolar cells have three or more prolongations, one of which always becomes continuous with the axis cylinder and is called the axis-cylinder process, the neuraxon, or the axione. Fig. 62. Nerve process or axione. Neurilemma. Neurilemma. . Nerve-cell. A, efferent neuron ; B, afferent neuron. (Bruiaker.) The other poles branch in various irregular directions like the limbs of a tree, and are hence called dendrites. They also go under the name of protoplasmic prolongations. Some of these unite the cells to contiguous cells by interlacing with, but not NEUROJNS. 287 actually joining, similar poles from those cells. The multipolar cells in the anterior cornua of gray matter of the cord are said to be larger in size and to present more poles than corresponding cells in the posterior columns. The diameter of nerve cells varies from jYtis '•'^ 7Tir ^^- '^^^ nucleus is usually single, and most cells have no true surrounding membrane. If a nerve fiber be followed toward the center which gives it origin it will be found first to lose its sheath and later its medullary substance ; this medullary substance may con- tinue for some distance after the sheath is lost, as in the white substance of the encephalon, but never penetrates the gray sub- stance proper. Every nerve fiber is connected with a cell by that cell's axis-cylinder prolongation. Certain retrograde changes take place in the neurons in old age — morphological changes agreeing with the physiological de- crease in energy-producing power at that time. The cell body becomes smaller, the dendrites atrophy, and the axiones diminish in mass. Nerve "fatigue" can also be demonstrated by the microscope. The nuclei of the sheath are flattened, the proto- plasm is shrunken and vacuolated and the nucleus is crenated. The quantity and quality of the food may be perfect, but the power of the cell to utilize it is impaired, and this means dimin- ished physiological power. Communication Between Different Neurons. — Every neuron is anatomically independent of every other neuron. There is no actual joining of fibers or dendrites — simply an interlacement of the end arborizations. This is illustrated in Figs. 63 and 64. In the latter the afferent fiber is joined to no cell except G, one of the cells of the spinal root ganglion. Its end arborizations simply interlace with the dendrites of the motor cell M. C. and cause it to send out an efferent impulse to the muscle M. Furthermore, there are frequent relays in the transmission of nerve messages. By no means do all the fibers from the motor area of the brain pass themselves out as parts of the anterior roots. 288 M.C Reflex Action: Old Idea. {Kirkes.) Fig. 64. Reflex Action; Modern Idea. {Kirkes.) Fig. 65. S.C, |M DiAuKARi uF AW Element OF THE Motor Path. U.S, upper segment ; L. S. lower segment; C.C, cell of cerebral cortex ; S.C, eel! of spinal cord, in anterior cornu; M, the muscle; S, path from sensory nerve roots. {ICirAes after Cowers.) 19 The relay service is illustrated in Fig. 65. Here again, it is seen that there is no actual joining of the neurons. Whenever it is said that a nerve cell is "joined" to another, or that the axis cylinder of a cell "joins" another cell, no actual continuity of tissue is meant. Different neurons communicate only by contiguity. . Peripheral Nerve Terminations. — Nerves terminate peripherally ( i ) in muscles, (2) in glands, (3) in special or- gans connected with the senses of sight, hearing, smell and taste, ( 4 ) in hair- follicles, (s) in simple free extremities passing between epithelial and other cells, and (6) in several kinds of so-called tactile corpuscles. The motor nerves passing to voluntary muscles form first a ' ' ground plexus ' ' for each group of muscle bundles — this plexus being made of the axis-cylinder fibrillse. From this plexus fibrils pass to form an ' ' intermediary plexus ' ' corresponding to each muscle bundle. These fibrils are still meduUated, and when a branch from the intermediary plexus enters a muscle fiber its sheath becomes continuous with the sarcolemma of that fiber, and the axis- cylinder fibrils form a network on the sur- face of the muscle fiber. This is called an end motorial plate. It contains a number of nuclei, and sends off from its under sur- face fine fibrillae which are said to pass between the muscular fibrillse which make 290 THE NERVOUS SYSTEM. up the fiber. Sensory fibers are somewhat scantily distributed to the voluntary muscles. In plain muscle tissue the motor nerves are distributed after the same general manner as in the striped muscles, though with some diflferences. Here the fibers are not medullated, and Fig. 66. U r'r--;-'' — -End-plate. iscle nucleus. Termination of a Nervk-Fiber in End-Plate of a Lizard's Muscle. (Stirling;.) primitive fibrils passing from the intermediary plexus finally enter the nuclei of the muscle cells. Medullated fibers have been traced to the cells of glands, but not farther. It is thought by some that, having formed a plexus, non-medullated fibers pass in to terminate in the nu- cleoli of the gland cells, though such endings have not been demonstrated. The peripheral distribution of nerves connected with the Special senses will be discussed elsewhere. The remaining methods of termination g,bove noted apply to afferent nerves. It is claimed that a very large number of sen- sory nerves terminate in hair-follicles. If such be the case it will account for sensory terminations in by far the greater part of the cutaneous surface. It is supposed that nerve fibrillae form a plexus beneath the true skin and send branches thence to the NERVE FIBERS. 291 Fig. 67. follicles, though the exact mode of termination is a question of some obscurity. Terminations between epithelial cells are probably more com- mon than any other method of sensory distribution. The fibers, having passed to the surface of the skin or mucous membrane, lose everything excepting the axis- cylinder, which, dividing into minute ramifications, passes, by means of these fibrillse, among the epithelial cells. This mode of termination is held by some to prevail in the glands. It certainly prevails in parts other than the skin and mucous membranes. Sensory nerves further terminate in (a) the corpuscles of Pacini or Vater, (Jb) the end bulbs, or tactile corpuscles, of Krause,(^c') the tactile corpuscles of Meissner, (d) the tactile menisques, and (i?) the cor- puscles of Golgi. (a) The Pacinian corpuscles are oval elongated bodies. Each corpuscular body has a length of about -^ of an inch, and is about half as broad. It is made up of a number of concentric layers of connective tissue ■ in- a hyaline ground substance, and is attached by a pedicle to the nerve whose termination it is. Through this Vatbr's or Pacini's Corpuscle. pedicle passes a single (occasion- «, stalk; ^.nerve-fiber entering it; c,d, ,, ,^ _, , . , connective-tissue envelope; *?, axis-cylin- ally more) nerve fiber which, der v-ith its end divided at /: (zwL.) 292 THE NERVOUS SYSTEM. piercing the several concentric layers constituting the cor- puscle, gradually loses its myeline substance and runs longi- tudinally through the center of the body to terminate at the distal end of the central cavity in a knob-like enlargement. These corpuscles are found in great abundance on the palmar and plantar surfaces of the hands and feet, being far more numerous on the first phalanx of the index finger than elsewhere. About six hundred are said to be present in each hand and foot. They are also to be found on the dorsal surfaces of the hands and feet, over parts of the forearm, arm and neck, in the nipples, in the substance of muscles, in all the great plexuses of the sym- pathetic system, and in numerous other situations. These bodies can- not be considered true tactile cor- puscles because they are situated be- neath the skin ; neither can they be positively said to have any " special sensory ' ' function, such as the ap- preciation of temperature, weight, etc. End Bulb prom Human Conjwc- .^. .pj^^ ^^^^ ^^-^^^ ^^ KlUVLSe TivA, Treated with Osmic Acid, ^ ■' Showing Cells on Core. (From exist in great number in the con- ye. .fter Lang^orik.) junctiva, the gkus penis and clitoris, tf, nerve fiber ; 3, nucleus of sheath ; 1 .. 1 . . . .-, nerve fiber within core; , pyramids and their discussion ; 01, olive, Gr, restiform body ; FK, posterior root ; AR, anterior root ; j:, crossed, and z, direct pyramidal tracts. {^Landois.) 312 THE NERVOUS SYSTEM. found to be the columns of Turck and the crossed pyramidal tracts, and these are the only parts of the cord known so to act. Impulses to the upper segment of the cord may be conveyed by either of these columns, but impulses to the lower segment must follow the crossed pyramidal tract, since the column of Turck ceases to exist in the dorsal region. Only some 3-7 per cent, of motor fibers from the cortex are thought to enter the columns of Turck. The others decussate in the medulla and enter the crossed pyramidal tracts. In any case motor impulses originat- ing in the brain and so conveyed are manifested on the side opposite their cerebral origin, since the fibers in both these tracts decussate in passing downward. It is a well known pathological fact that lesions of motor areas in the brain, or section of one lateral half of the cord, are followed by paralysis on the side opposite the lesion. Following a motor fiber (A, Fig. 74) through the anterior root of a spinal nerve, it is found to originate from one of the large multipolar cells (3) in the anterior cornu of gray matter. Around these anterior horn cells (i, 2, 3, 4) arborize the end filaments of fibers which have come down through the cord from the brain. Some fibers have come down in the uncrossed pyramidal tract (column of Turck) on the side opposite the cells i, 2, 3, 4, and crossed over to the same side through the anterior white com- missure approximately on a level with the cells ; others have de- cussated in the medulla, and come' down in the crossed pyramidal tract on the same side as the cells. In both cases the fibers or- iginated in the brain on the side opposite the cells around which they arborize in the cord. This is the connection which exists between the brain and the anterior root fibers. Not all fibers in the anterior nerve roots are thus prolonged upward in the pyramidal tracts. The number of fibers in these roots is much larger than in the pyramidal tracts, and conse- quently some of them must end (originate) directly in the cells of the anterior cornua. Furthermore, it seems that some fibers MOTOR PATHS IN THE CORD. 313 pass from the anterior nerve roots directly into the pyramidal tracts without being interrupted by motor cells. Fig. 74. Course of Nerve-Fibers in Spinal Cord. {Kirke ^itsr Schcifer.) The column of Turck and the crossed pyramidal tract are, therefore, the motor paths in the cord. Fibers entering the cord by the posterior roots send prolonga- tions both upward and downward in the gray matter of the cord, and communicate by end arborizations with the small sensory cells in the posterior cornua and with cells in several other lo- calities. (See Figs. 74, 81.) Reference to Fig. 74 will show that the connection of the anterior nerve fibers with the gray matter of the cord is simple, while that of the posterior is com- paratively complex. I, 2, 3, 4 are anterior horn cells. Each of 314 THE NERVOUS SYSTEM. these gives rise to an efferent fiber, one of which (^) is shown distributed to a muscle {M). Each of these cells also is sur- rounded by the end arborization of a fiber (/") from the cortex. A fiber from the posterior root is also shown. It originates Fig. 75. Transverse Section Through Half the Spinal Cord, Showing the Ganglia. A, anterior cornual cells; B, axis-cylinder process of one of these going to posterior root; C, anterior (motor) root; D, posterior (sensory) root; E, spinal ganglion on posterior root; F, sympathetic ganglion; G, ramus communicans ; H, posterior branch of spinal nerve; I, anterior branch of spinal nerve; a, long collaterals from posterior root fibers reaching to anterior horn; /', short collaterals passing to Clarke's column ; c, cell in Clarke's column sending an axis-cylinder process [d) to the direct cerebellar tract ; e, fiber of the an- terior root ; /, axis cylinder from sympathetic ganglion cell, dividing into two branches, one to the periphery, the other towards the cord ; £, fiber of the anterior root terminating by an arborization in the sympathetic ganglion; h, sympathetic fiber passing to periphery. (A'/r/t^ after Ramon y Caj'al.) in a cell of the sensory ganglion {G). It bifurcates, one branch going to the surface {S), the other enters the cord and itself bifurcates. The branch (E^ is short and arborizes around a small cell (i^J in the posterior cornu, from which a new axis SENSORY PATHS IN THE CORD. 315 cylinder arises to arborize around the anterior horn cell (4). The other branch ( D) travels upward in the posterior column of the cord. A collateral (5) is seen going to the anterior horn cell (2), one to the posterior horn cell {P^ and another to a cell (C) in the inner base of the posterior cornu (in Clarke's col- umn) ; from Can axis-cylinder enters the direct cerebellar tract. The main fiber (8) may terminate in the gray matter of the cord above, or in the medulla. Impressions brought thus to the cord are carried to the opposite side and pass up through the gray matter in most part. The fibers decussate at no particular point, but throughout the length of the cord. However, some fibers bearing sensory impressions pass to the column of Goll and thus upward, while some also go to the encephalon by the direct cerebellar fasciculi and the columns of Burdach. Ex- perimentally, decussation of sensory fibers is demonstrated ( i ) by longitudinal section of the spinal cord in the median line, which is followed by anesthesia on both sides below the section ; and (2) by horizontal section of one-half of the cord, which is followed by anesthesia on the opposite side below the section. It is claimed that pain and temperature sensations decussate at once on reaching the gray matter, while sensations of touch, pressure and equilibration pass up on the same side until the medulla is reached. Some afferent fibers are probably not continued upward to the brain either directly or indirectly. It thus appears that we have no very accurate knowledge of the sensory paths in the cord. The gray matter seems princi- pally concerned ; but the columns of Goll and Burdach and the direct cerebellar fasciculi also convey afferent impressions. For both motor and sensory paths to the cortex see p. 337. The columns of Burdach have been said to present no degen- eration secondary to section. Trophic centers for their fibers must, therefore, exist both above and below any given point of section. It is found that the fibers constituting these columns pass in and out along the cord between cells in different planes 3l6 THE NERVOUS SYSTEM. and acting as longitudinal commissural fibers. In locomotor ataxia the characteristic symptom is inability to coordinate the muscular movements — especially of the lower extremities ; the characteristic lesion has been found to be in the columns of Bur- dach. This is of importance in determining the function of these columns, and, in fact, leads to the conclusion that their fibers assist in regulating and coordinating the voluntary movements. This opinion is further supported by the connection of these fibers with the cerebellum, which contains the center for muscular coordination — if such a center exist. The sense oi pressure and the so-called muscular sense are probably connected with the fibers of this column, and these may be the only sensory impres- sions conveyed through the columns of Burdach. The anterior fundamental fasciculi, the anterior radicular zones, and the mixed lateral paths degenerate in neither direc- tion after section, their trophic cells existing at both extremities. They connect cells in the gray matter of the cord. Functions of the Spinal Cord. — These are (i) conduction, (2) transference, (3) reflex action, (4) augmentation, (5) coordination, (6) inhibition of reflex acts, (7) special cen- ters (Collins and Rockwell, modified). 1. Conduction. — This has been referred to in discussing the white columns of the cord. This function makes it possible for the brain to receive impressions from and send impulses to the periphery. It is to be remembered that most of these impres- sions and impulses are interrupted by spinal nerve cells in their passage between brain and periphery. 2. Transference. — An impression reaching the gray matter of the cord may be transferred (not as in typical reflex action) so as to be felt in an entirely different region from that in which the irritation takes place. Hip joint disease often gives pain in the knee alone. 3. Reflex Action. — The cord may act as a center without the cooperation of the brain. Indeed, by no means do muscular REFLEX ACTION. 317 movements cease immediately on removal of the encephalon if the cord and its nerves be left intact. An animal so mutilated possesses no sensation or volition, but for a time the sensory- nerves will continue to convey impressions and the motor nerves irripulses. Under these conditions impressions (as of heat) are conveyed to the cord by the afferent nerves ; the gray matter of the cord receives the impressions and generates motor force which is sent out through the corresponding efferent nerves, and movements result. This is reflex action. The impression is reflected through the cord and manifested in motion without the intervention of sensation or volition. Reference to Figs. 74 and 81 shows how reflex action is anatomically possible through the cord connections. Typical reflex action requires anatomically (i) something to produce an impression, (2) a nerve terminal to receive it, (3) a centripetal fiber to convey it, (4) a center to receive and transform it, (5) a centrifugal fiber to convey it to the periphery and (6) a muscle to contract. This remark applies to reflex action connected with the cord, but by common consent reflex action is not limited to the cord and its connections. If reflex action be defined as any involuntary manifestation of nerve force consequent upon the reception of an impression (general or special) by a nerve center, the term must be made •to include such phenomena as intestinal peristalsis, contraction and dilatation of the pupil, certain mental operations, etc. In reality most reflex acts are of a complex nature, involving asso- ciated action on the part of several neurons and being manifested frequently at several points. For example, a foreign body in the larynx causes reflexly not only closure of the glottis, but also the convulsive muscular contractions incident to coughing. The realm of reflex action is obviously a wide one.* It may be said that ordinary reflexes are usually under the direction of the cord, but this does not imply that the brain may not be concerned. Pricking the sole of the foot of a sleep- ing person will cause him to draw up his leg without the interven- 3l8 THE NERVOUS SYSTEM. tion of consciousness. Probably were he awake the withdrawal would still be a reflex ; but he would certainly be conscious of the pain, though after the act ^f withdrawal was accomplished. Nor is reflex action by any means limited to the cerebro-spinal system. Either of the two systems, or both, may be concerned. Now in order for reflex movements to occur, there must be a transference of impressions received by sensory cells to cells capable of giving origin to motor impulses. The cells communi- cate by their collaterals, which may be short or long, depending on the distance between the cells concerned. Cells in the gray matter of the cord are ' ' connected ' ' by such fibers, and they run largely in the white matter of the cord joining cells on different planes. They constitute the larger part of the anterior fundamental fasciculi, the anterior radicular zones, and the mixed lateral tracts, and it is these paths which are mainly con- cerned in reflex action of the cord. 4. Augmentation., — Sensory fibers, on reaching the cord, send prolongations both upward and downward in the gray matter. These prolongations, by their end arborizations, seem to com- municate indirectly with several motor cells. In the simplest reflex movements connected with the spinal cord the muscular activity is limited to the area corresponding to the distribution of the afferent nerve which has been irritated ; but if the irrita- tion be sufficiently increased other muscles in the same locality, or the corresponding muscles on the opposite side of the body, or even the whole musculature, may be thrown into action. This is explained on the ground that under favorable conditions of central excitability, strength of peripheral irritation, etc. , the afferent impression is disseminated by collaterals throughout a large area of the cord (for example), and a large number of ef- ferent cells are made to discharge. The reflex excitability of the cord is markedly increased by the administration of such drugs as strychnin. An animal so poisoned will be thrown into the most violent convulsions by so slight a sensory impression as THE ENCEPHALON. 3I9 a simple breath of air. Removal of the encephalon in inferior animals also exaggerates reflex excitability. 5. Coordination. — ^This has been referred to under the columns of Burdach. Coordination is " a repetition of ordinary reflex acts for our daily lives." No effort is necessary to coor- dinate the muscular movements of deglutition, respiration, walk- ing, etc. These movements may be performed when the cere- brum is removed. 6. Inhibition of Reflex Acts. — This is not a function of the cord proper, but is directed by the cerebrum. A great many reflex movements may be inhibited by an act of the will, providing always they are due to contraction of striped muscle. The reflex acts of coughing or sneezing, or those resulting from tickling, for example, can be largely controlled. These are usually performed as reflex cord acts, but the brain may evidently assert its superiority over the cord and inhibit them. 7. Special Centers. — In the gray matter of the cord are found various centers for distinct acts such as defecation, parturition, micturition, etc. These are all connected with each other and with the encephalon and obey the usual laws of reflex action. THE ENCEPHALON. The encephalon is situated within the cranial cavity and is commonly called the brain. Its gross divisions are the medulla oblongata, the pons Varolii, the cerebellum, and the cerebrum. All the other divisions are in a measure subordinate to the cere- brum, though each division has individual functions. The human brain weighs about 49 J^ ounces in the male and about 44 in the female. The Medulla Oblongata. Anatomy. — The medulla oblongata, or bulb, joins the upper extremity of the spinal cord and extends to the pons above. It has a pyramidal shape, lies in the basilar groove of the occipital bone, and is slightly flattened antero-posteriorly. It is about an 320 THE NERVOUS SYSTEM. inch and a quarter in length, half an inch thick, and three quarters of an inch broad above. The anterior and posterior median fissures of the cord are continued upward in the medulla ; the central canal terminates in the inferior angle of the fourth ventricle. The anterior columns appear to be continuous with the anterior pyramids of the medulla. These pyramids are situ- ated just lateral to the anterior median fissure. The innermost Fig. 76. Floor of the 4TH Ventricle and the Connections of the Cerebellum. On the left side the three cerebellar peduncles are cut short ; on the right the connections of the superior and inferior peduncles have been preserved, while the middle one has been cut short. I, median groove of the 4th ventricle with the fasciculi teretes ; 2, the strias of the auditory nerve on each side emerging from it ; 3, inferior peduncle ; 4, posterior pyramid and clava, with the calamus scriptorius above it ; 5, superior peduncle ; 6, fillet to the side of the crura cerebri ; 8, corpora quadrigemina, [Landois.) fibers of the pyramids are the continuations upward of the crossed pyramidal tracts, and are seen to decussate in the median line ; the outermost fibers are the prolongations of the uncrossed pyramidal tracts. The olivary bodies, oval in shape, are just ex- ternal to the anterior pyramids separated from them by a groove. The restiform bodies make up the postero-lateral portion of the THE MEDULLA OBLONGATA. 32 1 medulla, and are external to the olivary bodies. They contain fibers from the columns of Burdach, and contribute largely to the formation of the inferior peduncles of the cerebellum. The restiform bodies, diverging as they ascend, form the lateral boundaries of the inferior division of the fourth ventricle. Be- neath the olivary bodies, and between the anterior pyramids and the restiform bodies, are the lateral fasciculi, or the funiculi of Rolando. They constitute the upward prolongation of all the antero-lateral portion of the cord which does not go to the for- mation of the anterior pyramids. Their chief importance is in the fact that they contain the centers for respiration. The pos- terior pyramids are sometimes called tht funiculi graciles. They join the restiform bodies and pass to the cerebellum. The fourth ventricle deserves particular attention. It is a cavity on the posterior aspect of the pons and medulla extending from the upper limit of the former to a point on the latter oppo- site the lower border of the olivary body. It has the shape of two isosceles triangles placed base to base. The apex of the inferior triangle is at the calamus scriptorius, and its lateral boundaries are the diverging restiform bodies. The superior peduncles of the cerebellum form the lateral boundaries of the superior triangle. The inferior triangle is covered by the cerebellum ; the superior by the valve of Vieussens, which stretches between the superior peduncles. This ventricle com- municates above with the third ventricle by the aqueduct of Sylvius, or the iter a tertio ad quartum ventriculum ; below, with the central canal of the cord and with the subarachnoid space. The floor of the ventricle presents a longitudinal median fissure and numerous small elevations indicating the position of the nuclei of origin of certain of the cranial nerves. The gray matter of the medulla has the same general distri- bution as that in the cord, but is by no means so regular in its disposition. The direction of the white fibers is not so uniform as in the cord. They run not only longitudinally, but trans- 322 THE NERVOUS SYSTEM. versely to connect the lateral halves, and in other directions to connect various centers situated in this part of the encephalon and to connect the medulla with other parts of the brain. The following is the relation of the columns of the cord to the medulla : The direct and crossed pyramidal tracts pass to the encephalon constituting, in the medulla, the anterior pyramids — the direct, having decussated below, occupying here the outer portion of the pyramid, and the crossed decussating in the medulla and oc- cupying the inner portion of the pyramid. Those columns concerned in reflex action, the anterior funda- mental fasciculi, the anterior root zones, and the mixed lateral tracts do not continue farther upward than the gray matter of the medulla. The columns of Goll are continuous with the funiculi graciles. The columns of Burdach and the direct cerebellar fasciculi pass to the cerebellum through the restiform bodies of the medulla. Functions. — The functions of the medulla are ( i ) conduction, (2) reflex action, (3) to furnish centers for special acts. 1 . As a conductor the medulla is absolutely necessary as a means of connection between the brain and cord. Sensory impressions to and motor impulses from the brain must all pass through by this route. 2 . As a reflex nerve center the medulla also resembles the cord, though impressions reflected through this organ are frequently much less simple than those reflected through the cord. Reflex action in the medulla is dependent on (3), to be noticed now. 3. The most important center presiding over coordinated move- ments is that for respiration. The encephalon may be cut away down as far as the medulla, and life will continue for a certain time. It is also true that the medulla itself may be gradually cut away from above downward until a certain point is reached, when res- THE PONS VAROLII. 323 piration suddenly ceases. Likewise the spinal cord may be cut away upward till this point is reached, when the same result will follow. This is the true respiratory center, and is situated at the site of origin of the vagi. Its destruction is followed by an immediate suspension of respiration and consequent death by asphyxia, though there is no manifestation of the distress usually accompanying this condition. The sense of want of air is sim- ply lost. There is one of these centers for each side, but they act synchronously, being connected by commissural fibers. Prob- ably the usual mode of stimulation of the respiratory center is by afferent impressions, but it may also be stimulated directly, as by deoxygenated blood. ' Mutilation of the medulla, on account of the presence of this center, is followed by the nearest approach to instantaneous death, and the respiratory center has, therefore, been called the ' ' vital spot, ' ' though death from any cause can- not be instantaneous. Some other reflex centers are for deglutition, sucking, secretion of saliva, vomiting, coughing, sneezing, dilatation of the pupil, se- cretion of sweat, secretion of glycogen, etc. Typical of these is the reflex act of sneezing, in which case impressions are conveyed to the medulla by the nasal branches of the fifth nerve. Additional centers in the medulla are those which preside over inhibition and acceleration of the heart, vasomotor centers for the vessel walls, and centers for special senses like hearing and taste. There is also said to be here a center controlling the production of heat by the tissues. The Pons Varolii. Anatomy. — The pons is situated just above the medulla ob- longata at the base of the brain, and is frequently called the great commissure, for the reason that it contains white fibers con- necting the two lateral halves of the cerebellum and the differ- ent portions of the cord and medulla with the parts of the brain above. It resembles the cord in having its white matter situ- ated externally, while within its substance are a number of 324 THE NERVOUS SYSTEM. collections of gray- matter. The longitudinal fibers are con- tinuations upward of fibers from the olivary bodies and the an- terior pyramids of the medulla and also of parts of the poste- rior and lateral columns of the cord. They pass through the crura cerebri to the brain. Functions. — The anatomical structure and situation of the pons at once suggest that its function is to transmit motor im- pulses from and sensory impressions to the cerebrum. The gray centers, however, indicate a further function of this organ. It is found that the removal of all parts of the enceph- alon above the pons does not deprive an animal of voluntary motion and general sensibility. It will be seen later that the integrity of the cerebrum is essential to any intellectual opera- tion, and manifestly, under the conditions mentioned, there can be no voluntary motion which indicates any degree of intelli- gence ; but the fact remains that the animal can perform movements which are different from the reflex movements de- pending on the presence of the cord when all other parts of the cerebro- spinal axis have been removed. The pons is apparently "an organ capable of originating impulses giving rise to vol- untary movements, when the cerebrum, corpora striata and optic thalami have been removed, and it probably regulates the automatic voluntary movements of station and progression." (Flint.) Nor can it be doubted that an animal thus mutilated feels pain. It is probable that the sensory impression is received by some of the gray centers in the pons itself, but not being con- veyed to the cerebrum, is not remembered. The Crura Cerebri, Corpora Striata, Optic Thalami, Internal Capsule and Corpora Quadrigemina. It will be well before discussing the cerebrum to consider briefly other collections of gray and white matter in the neigh- borhood of the upper part of the pons. THE BASAL GANGLIA. 325 The crura cerebri, passing upward from the anterior part of the pons, diverge to run apparently underneath the corpora Fig. 77. Human Brain, with the Hrimisthekes; Rewovkd by a Horizontal 'Incision on THE Right Side. 4, trochlear; 8, acoustic nerve; 6, origin of the abducens ; F, A, L, position of the pyra- midal (motor) fibers for the face, arm and leg; S, sensory fibers. {Laiidois.) striata and optic thalami in the direction of the cerebral hemis- pheres. They are about ^ inch long and slightly broader above than below. The main bulk of each crus consists of 3a6 THE NERVOUS SYSTEM. white fibers , but a collection oi gray matter (locus niger) divides the band into a lower or superficial section, called the crusta, and an upper or deep section, called the tegmentum. There is also some gray matter in the tegmentum proper. The fibers of the teg- mentum are supposed to convey afferent impressions chiefly, and end for the most part in the optic thalamus, though sonie are con- tinued to the cerebrum through the internal capsule. The fibers of the crusta are supposed to convey efferent impulses, and pass to the corpus striatum and the cerebrum. It is evident that the function of the crura is mainly to con- duct messages to and from the parts above. It is said that the locus niger is concerned in coordination of the movements of the eye-ball and iris. The Corpora Striata, Optic Thalami and Internal Capsule are closely related and are best considered together. Each corpus striatum is pear-shaped with its large end forward and near the median line ; the posterior small extremities are divergent from each other and embrace the two optic thalami. Externally they are white ; internally white and gray elements are mixed. Each is separated by the anterior limb of the internal capsule into two divisions, external and internal, known respectively as the lenticular and caudate nuclei. (See Fig. 77.) The optic thalami, one on either side, have an oval shape and rest upon the crura cerebri between the posterior extremities of the two corpora striata. Most of their external surface is white ; internally each possesses six gray nuclei. Separating the two nuclei of the corpus striatum anteriorly, and the lenticular nucleus from the optic thalamus posteriorly, is a band of white fibers known as the internal capsule. The part between the two nuclei is the anterior limb ; that between the lenticular nucleus and the optic thalamus is the posterior limb. These limbs, joining at an obtuse angle, constitute a bend in the internal capsule which is called the genu, or knee. The fibers THE BASAL GANGLIA. 327 of the capsule pass to the frontal, parietal and occipital lobes of the cortex, and in their course to these parts they diverge to form the corona radiata. External to the lenticular nucleus is a band of white fibers known as the external capsule. In it is a longitudinal mass of gray matter, the claustrum. Fig. 77 shows the relations of these parts. Functions. — The exact function of the corpora striata is a matter of some doubt. They have been considered the great motor ganglia of the base of the brain ; but, although lesions here are followed by paralysis on the opposite side of the body, it is held that this phenomenon is due to the proximity of the internal capsule. The further fact that irritation of this organ is followed by muscular contractions does not prove that it ordinarily generates motor force, for many of the fibers from the motor cortical zone pass to or through the corpus striatum. This may be only a relay station, and the corpus may be quite subsidiary. It undoubtedly, however, is connected with motion in some way. The precise function of the optic thalami is equally obscure. The relation of these organs to the tegmenta would suggest that they have something to do with the sensory fibers on their way to the cortex. It cannot be denied that they are concerned in sensation, since their removal is followed by crossed anesthesia. They may likewise be relay stations. Each sends fibers to the cerebellum and contains one of the nuclei of origin of the optic nerve. Regarding the function of the internal capsule it may be said that its fibers are in main part prolongations from the crusta and from the gray matter of the corpora striata ; fibers also pass up- ward through it from the tegmentum and the optic thalamus. As a matter of fact, most of the fibers of the crura go directly into the corpora striata (motor) and the optic thalami (sensory), but some pass directly upward through the capsule. It is to be 328 THE NERVOUS SYSTEM. noted, however, that the capsule does not consist of these last named fibers alone, but of fibers from the corpora striata and optic thalami as well. Observations show that pathological lesions affecting the anterior two thirds of the posterior division of the internal capsule are followed by paralysis of motion ; that lesions affecting only the posterior one third of the posterior division are followed by anesthesia ; and that lesions affecting the entire posterior limb are followed by both paralysis and anesthesia — these phenomena always manifesting themselves on the side opposite the lesion only. This leads to a definite con- clusion; viz., that efferent fibers occupy the anterior two thirds and afferent fibers the posterior one third of the posterior limb of the capsule. Nothing conclusive can be said about the function of the ex- ternal capsule or of the claustrum. The Corpora Quadrigemina, two on each side, are prominences on the dorsal surface of the pons and crura above the aqueduct of Sylvius. They contain white and gray matter. The pos- terior tubercles are connected with the eighth nerve, the sen- sory tract, the temporal region of the brain, and the lateral cor- pora geniculata. The anterior tubercles are connected with the optic nerve, with the occipital region, and with the median cor- pora geniculata. The function of the anterior of these bodies is mainly con- nected with the eye ; the posterior are associated with the sense of hearing. The Cerebrum. The great size of the cerebral hemispheres in man obscures the fact that the different parts of the brain are disposed in a linear series ; these, from before backward, are, the olfactory lobes, cerebral hemispheres, optic thalami, corpora quadrigem- ina, cerebellum, medulla oblongata. This arrangement exists in the human fetus, and persists throughout life in some of the lower animals. THE CEREBRUM. 329 Anatomy. — The substance of each hemisphere is divided by fis- sures into five lobes — {a) frontal, {b') parietal, (c) occipital, {d) temporo-sphenoidal and (^) central. The main fissures are four in number — (i ) The fssure of Sylvius running from the front and FiG.S78.^ Left Side of the Human Brain (Diagrammatic). F, frontal ; P^ parietal; O, occipital; T, temporo-sphenoidal lobe; S, fissure of Sylvius; S', horizontal ; S", ascending ramus of S ; c, sulcus centralis, or fissure of Rolando ; A, as- cending frontal, and B, ascending parietal convolution; Fj, superior, F,, middle, and F3, inferior frontal convolutions; _fi, superior, and y^, inferior, frontal fissures; yj,, sulcus precentralis ; P, superior parietal lobule; Pgj inferior parietal lobule, consisting of P^, su- pra-marginal gyrus, and Pg', angular gyrus; z/i, sulcus interparietals; rm, termination of calloso-marginal fissure ; O, first; O^, second; O3, third occipital convolutions; ^o, pari- etal-occipital fissure ; o, transverse occipital fissure ; o^, inferior longitudinal occipital fis- sure; Ti, first; Tg, second; T 3, third, temporo-sphenoidal convolutions; i'^, first; if ,, sec- ond, temporo-sphenoidal fissures. {Landois.) 33° THE NERVOUS SYSTEM. under part of the brain backward, outward and upward ; ( 2 ) the fissure of Rolando running from the median line near the center of the longitudinal fissure forward, outward and down- ward ; (3) tht pariefo-occipital fissure, little of which is evi- dent upon the surface of the brain, but which appears on longi- tudinal section separating the occipital and parietal lobes; (4) the calloso-marginal fissure, also evident only on the internal aspect of the hemisphere, parallel with and above the corpus callosum. (Figs. 78, 79.) (a) The frontal lobe is bounded internally by the loAgitudinal fissure, posteriorly by the fissure of Rolando and below by the fissure of Sylvius. On its surface are seen three convolutions, approximately parallel, called the superior, middle and inferior frontal convolutions, and occupying positions which their names indicate. In addition the posterior portion of this lobe is occu- pied by the ascending frontal, or the anterior central convolution, lying just in front of the Rolandic fissure. {J)) The parietal lobe is bounded anteriorly by the fissure of Rolando, internally by the longitudinal fissure, posteriorly by the parieto-occipital fissure and below by the fissure of Syl- vius. Just behind the fissure of Rolando is the ascending pa- rietal, or posterior central convolution; above, this is continu- ous with the upper parietal convolution, below which is the inferior parietal lobule separated from the preceding by the intra-parietal sulcus. This inferior parietal lobule winds around the posterior part of the fissure of Sylvius, and is divided into the supra-marginal convolution, embracing the short arm of this fissure, and the angular convolution connecting below with the temporal lobe. (c) The occipital lobe is situated posteriorly below the parieto-occipital fissure and external to the median fissure. It presents three convolutions, the superior, middle and in- ferior. {d) The temporo-sphenoidal lobe is below the fissure of Sylvius THE CEREBRUM. 331 in front of the occipital lobe. It likewise presents superior, middle and inferior convolutions. {/) The central lobe, or island of Reil, presents the gyrus forni- catus, a convolution curving around the corpus callosum ; the marginal convolution beyond the calloso-marginal fissure from the preceding and between it and the edge of the longitudinal fissure ; the continuation of the parieto-occipital fissure running Fig. 79. Median Aspect of the Right Hemisphere. CC, corpus callo&um divided longitudinally ; Gf, gyms fornicatus ; H, gyrus hippocampi ; h, sulcus hippocampi ; U, uncinate gyrus ; cm, calloso-marginal fissure ; F, first frontal convolution ; c, terminal portion of fissure of Rolando ; A, ascending frontal ; B, ascending parietal convolution and paracentral lobule; Pi', praecuneus or quadrate lobule; Oz, cuneus ; Po, parieto-occipital fissure ; o', transverse occipital fissure ; oc, calcarine fissure ; oc', superior; oc", inferior, ramus of the same; G, gyrus descendens ; T4, gyrus occipito- temporalis lateralis (lobulus fusiformis) ; T5, gyms occipito-temporalis medialis (lobulus lingualis). {Landois.) downward and forward to meet the calcarine fissure, between which is the cuneus ; the internal aspect of the temporal lobe, the uncinate gyrus. 332 THE NERVOUS SYSTEM. Structure. — -The cerebral hemispheres are composed of white and gray matter, but here the gray matter is situated externally. To increase its amount, with economy of space, the gray matter is thrown into many convolutions, to some of which reference has been made. The sulci separating these convolutions have a depth in the average human brain of about one inch. The thickness of the gray matter of the cortex varies from J^ to \ in. , being thinnest in the occipital and thickest in the front pa- rietal region. The cells found in the superficial and deep portions of the gray matter are not uniform in size or shape. In a general way it may be said that they increase in size as the surface is left, but in addition to the comparatively large cells in the deep parts there are also numbers of small ones. Passing in the same direction there are found in succession small pyramidal, larger pyramidal, and irregular branching cells. Fibers from the Cerebrum. — Fibers pass from each cerebral hemisphere to (a) the spinal cord, (^) the cerebellum, (c) the opposite cerebral hemisphere, and {d) different parts of the same hemisphere. (a) Fibers converge from the anterior and middle (particularly the latter) parts of the cortex to pass by the corona radiata to the corpora striata, from which fibers are continued to the crusta, pons, pyramids of the medulla and pyramidal tracts of the cord ; most of these pass down through the internal capsule to reach the corpora striata. From the same regions also some fibers pass directly through the internal capsule, without connection with the corpora striata, to be actually continuous themselves with fibers which, following the same course downward, are found in the pyramidal tracts of the cord. All fibers passing from these cortical areas mentioned through the internal capsule occupy the anterior two thirds of the posterior division of that tract. Furthermore, fibers from the posterior cortical area pass through the posterior one-third of the posterior division of the THE CEREBRUM. 333 internal capsule to the optic thalamus, from which fibers pass through the tegmentum to the pons and medulla and are continu- ous with fibers from the sensory tracts of the cord. The decus- sation of all these fibers has been mentioned. Fig. 8o. i Schema of the Projection Fibers Within the Brain. (Starr.) Lateral view of the internal capsule ; A, tract from the frontal gyri to the pons nuclei, and so to the cerebellum ; B, motor tract ; C, sensory tract for touch (separated from B for the sake of clearness in the schema) ; D, visual tract ; E, auditory tract ; F, G, H, superior, middle, and inferior cerebellar peduncles ; J, fibers between the auditory nucleus and the in- ferior quadrigeminal body; K, motor decussation in the bulb; Vt, fourth ventricle. The numerals refer to the cranial nerves. The sensory radiations are seen to be massed toward the occipital end of the hemisphere. (Ajk. Text-Book.) Fig. 8 1 taken in conjunction with Fig. 74 illustrates the most recent ideas of the motor and sensory connections between brain and cord and the motor and sensory paths in the cord. (^) Fibers from the anterior portion of the frontal lobe pass through the anterior limb of the internal capsule and seem to end in the gray matter of the pons and there to communicate with the cerebellum through the middle peduncles. Fibers also pass from the temporo-sphenoidal lobes and from the caudate nuclei Fig. 8i. A.C.N Muitipolar CeUofAnt Horn. Scheme of Relationship of Cells and Fibers of Brain and Cord. {Kirkes.) Pyr cell of Rolandic area ; Ax, its axis cylinder crossing the middle line AB, to enter one of the pyramidal tracts ; the collateral Cdll goes to the cortex of the opposite hemis- phere, while another, sir, enters the corpus striatum. The axis cylinder arborizes around an anterior Horn ceil, -whence a motor fiber goes to the muscle. The axis cylinder from the spinal ganglion cell is represented as bifurcating and sending one branch to the periphery and one to the . cord ; the latter itself bifurcates, the lower di- vision ending as shown better in Fig. 74. N.G, cell in posterior comu of the cord or pos- terior column of the bulb. The distance of this cell from the point of entrance of the axis cylinder into the cord may be great or small. Note the collaterals from it in Fig. 74. I.A, decussating fiber ending at cell in optic thalamus, O.T, from which a fiber passes to the cortex. A collateral is shown passing from the ascending sensory fiber to a cell of Clarke's column, whence a fiber passes to a cell, P, of the cerebellum. THE CEREBRUM. 335 of the corpora striata to the cerebellum on the opposite side. The connection is crossed in all these cases. (^) Transverse fibers in the corpus callosum connect all parts of the two lateral hemispheres. Besides these commissural fibers there are those of the anterior and posterior white commissures. Fibers in the anterior connect the temporo-sphenoidal lobes and probably the corpora striata with each other ; fibers in the pos- terior connect the temporo-sphenoidal lobes with the optic thalami of the opposite side. (fl?) The arcuate fibers connect different convolutions of the same lobe and the convolutions of different lobes with each Fig. 82. Diagram of the Motor Areas on the Outer Surface of a Monkey's Brain. {Lan- dois after Horsley and Scha/er.) Other. Some of these are in the fornix, in the corpus callosum, and in other parts, as well as running along the concave surface of the cortex. Cerebral Localization. — There are certain cortical areas which have certain fixed functions. There are certainly such areas for motion and for the reception of impressions conveyed by the nerves of special sense ; areas for the reception of impressions conveyed by the nerves of general sensation have not been definitely determined. 336 THE NERVOUS SYSTEM. Fig. 83. Side View of the Brain of Man, with the Areas of the Cerebral Convolutions According to Ferrier. i^Brubaker.) The figures are constructed by marking on the brain of man, in their respective situa- tions, the areas of the brain of the monkey as determined by experiment, and the description of the effects of stimulating the various areas refers to the brain of the monkey, t, advance of the opposite hind limb, as in walking; 2, 3, 4, complex movements of the opposite leg and arm, and of the trunk, as in swimming; a, ^, c, the head seen from the side; 2, on edge; ^,head; tk^ middle piece ; y, tail ; e^ terminal filament ; 3, from the mouse ; 4, bothriocephalus latus ; 5, deer ; 6, mole ; y, green woodpecker; 8, black swan; 9, from a cross between a goldfinch (m.) and a canary (f.); 10, from cobitis. {Landois.) inch. Its Structure is that of a typical cell. When the ovary is de- valuing a part of its covering epithelium dips down into the sub- stance of the organ and becomes walled off by union of the sur- face cells above it. A part of this ball of epithelium becomes the ovum, and a part the Graafian follicle for that ovum. 398 REPRODUCTION. Fig. 90. The youngest ova are thus found nearest the surface of the ovary. The cell has an enveloping membrane, the vitelline membrane, a protoplasm, the vitellus, a nucleus, the germinal vesicle, and a nucleolus, the germinal spot. Outside the ovum, but not strictly a part of it, is the zona pellucida, a transparent envelope, and outside the zona pellucida a collection of cells, the corona radiata. The perivitelline space is between the ovum proper and the zona pellucida. The zona presents radial striae, which may facilitate the entrance of the spermatozoon. Ova are capable of being impregnated as long as 7—9 days after their discharge from the ovary. Their formation begins early in fetal life. The ovum pos- sesses no power of independent motion. It is passive in fecun- dation ; it is sought by the male element. Its vitellus, or yolk (protoplasm), contains nutritive non-living material, deutoplasm, whose function is to furnish food- substance to the impregnated ovum until the fetal circulation is established. Deutoplasm in the human ovum is scarcely to be distinguished from the living protoplasm, though in the ova of birds, e. g., it is clearly marked off, and constitutes the main bulk of the mature egg, since the developing embryo receives no blood from the mother. Graafian Follicles. — The Graafian follicles are directly con- cerned in the development and maturation of ova. These are small vesicles in the cortical ovarian substance surrounded by a capsule of thickened ovarian stroma, the tunica vasculosa. In- side the tunica vasculosa, lining the spherical cavity of the Ovum. (From Yeo after Robin.) a^ zona pellucida and vitelline membrane; b^ yolk ; c, germinal vesicle or nucleus ; df, germinal spot or nucleolus ; e, interval left by the retraction of the vitellus from the zona pellucida. GRAAFIAN FOLLICLES. 399 vesicle, are several layers of epithelial cells making up the membrana granulosa. The cavity is filled with an albuminous Fig. 91. ''lOMT,.,., Section of the Ovary of a Cat, Showing the Origin and Development of Graafian Follicles, (From Kft? after Caciiat.) a, germ epithelium ; /', Graafian follicle partly developed ; c, earliest form of Graafian follicle ; d, well-developed Graafian follicle ; e, ovum ; _/", vitelline membrane ; g, veins ; h, i, small vessels cut across. liquid, the liq^wrfoUiculi. At one point in its circumference the membrana granulosa is much thickened, and in this thickened portion is imbedded the ovum. The epithelial cells of the 400 REPRODUCTION. membrana completely surround the ovum, constituting the discus proligerus. The cells of the discus next the ovum have their long axes at right angles to the circumference of the egg, and this layer is the corona radiata already mentioned. The zona pellucida is just underneath the corona. Usually a Graafian follicle contains only one ovum. The follicles and their contained ova begin to be formed early in fetal life. Probably none are newly formed after the child is two years old, but they are undeveloped before puberty. It is esti- mated that some 72,000 follicles and ova exist in the two ovaries of the average woman ; but of these not more than 400 reach full development, the others undergoing retrograde changes and disappearing. Up to puberty the follicles and ova are small, but at that time some of them begin to enlarge, and at more or less regular inter- vals one of these follicles bursts and allows the escape of its con- tained ovum into the fimbriated extremity of the Fallopian tube — a process known as ovulation. Previous to its rupture the Graafian follicle has been enlarging. It is always located in the cortical part of the ovary, but it may now not only form a dis- tinct protrusion above the surface of the organ, but may by its size encroach upon the medullary portion. It may at this time have a diameter of half an inch. Meantime the more superficial part of the tunica vasculosa has been undergoing fatty degener- ation, has lost its blood supply and become very thin. Here rupture occurs, and the mature ovum, ready for impregnation, escapes upon the surface of the ovary. Corpus Luteum. — When the ovum has been extruded hem- orrhage occurs, filling the empty follicle with blood. By con- traction of the extra-vesicular adjacent tissue the walls of the Graafian follicle become folded into the cavity. Soon prolifera- tion of the cells of the follicular wall takes place into the blood clot, vascular loops are formed, and the tunica vasculosa itself becomes greatly hypertrophied. The clot later disappears and CORPUS LUTEUM. 401 the mass then has a yellowish color and is known as the corpus luteum. Whether or not the ovum that escaped from the follicle which was the antecedent of any given corpus luteum was impreg- nated, has an influence upon the growth of that corpus. If the ovum failed of fecundation the corpus luteum will reach its highest development in about fifteen days, and will then assume the character of cicatricial tissue and be absorbed in a few weeks. If the ovum was fecundated, the corpus luteum will increase in size for some three months, until it may be half the size of the ovary. At labor it has been reduced to a white cicatrix, which probably persists as a small nodule throughout life. The differences between the corpora lutea of menstrua- tion and pregnancy are shown by the following table from Dalton : Corpus Luteum of Menstruation. Corpus Luteum of Pregnancy. of Three-quarters of an inch in diameter; central clot reddish ; convoluted wall pale. Smaller ; convoluted wall Larger ; convoluted wall bright yellow ; clot still At the end three weeks. One month. Two months. Four months. Six months. Nine months. reddish. Reduced to the condition of an insignificant cicatrix. Absent or unnoticeable. Absent. bright yellow ; clot still red- dish. Seven-eighths of an inch in diameter ; convoluted ; wall bright yellow; clot per- fectly decolorized. Seven-eighths of an inch in diameter ; clot pale and fibrinous ; convoluted wall dull yellow. Still as lai^e as at the end of second month ; clot fibrinous ; convoluted wall paler. Half an inch in diameter ; central clot converted into a radiating cicatrix ; external wall tolerably thick and con- voluted, but without any bright yellow color. Maturation. — But previous to its discharge from the Graafian follicle, the ovum undergoes certain changes — a ripening process 26 Absent. 402 REPRODUCTION. — whereby it is made ready to receive and be impregnated by the spermatozoon. This maturation consists in the discharge from the cell proper of a part of its nucleus and a part of its protoplasm. The nucleus moves toward the periphery, and the perinuclear membrane is lost. As the nucleus approaches the surface of the egg it undergoes karyokinesis, and a part of it, together with a little surrounding protoplasm, is extruded and Fig. 92. Polar globutis The Fertilized Ovum, or Blastosphere. {Kirkes.) finds itself in the perivitelline space. This is the. first polar body. A second polar body is likewise later discharged by karyokinetic division. (See Fig. 92.) The object of this extrusion and the final fate of the polar bodies are matters of speculation. That portion of the nucleus which remains after the polar bodies have been thrown off finds its way back to the center of the ovum. It soon develops a cover- ing membrane, and is now the female pronucleus, ready for union with the male pronucleus. It is about the time of the completion of this process that the follicle ruptures and the discharge of the ovum — ovulation — occurs. Ovulation. — It is supposed that from puberty to the meno- pause one (or more?) ovum is discharged at tolerably regular intervals of about four weeks. It should, and usually does, enter MENSTRUATION. 403 the outer end of the Fallopian tube, to be conveyed toward the uterus. Obviously only a few, -and sometimes none, are ever impregnated. Should the ovum fail to reach the uterus and become fecundated, ectopic gestation will be the result. The patent fimbriated extremity of the tube may grasp the ovary at the time of rupture of the Graafian follicle, but this is not probable. One of the tubal fimbriae is attached to the outer extremity of the ovary and has on its surface a small linear de- pression lined by ciliated epithelium and leading to the tube. The ovum very likely in most cases drops into this depression, and the influence of the cilia is to carry it towards the tube. Menstruation. — Usually between the fourteenth and seven- teenth years of female life menstruation begins. It is a discharge of blood, epithelium and other parts of the mucous membrane of the uterine cavity, together with mucus from the glands of the uterus and vagina. About the beginning of menstrual life there are marked changes in bodily development. Graafian follicles enlarge and begin to approach the surface, ovulation is begun, and the female is capable of being impregnated. In most cases menstruation 'occurs at regular intervals of twenty-eight days. The function is suspended during pregnancy and usually during lactation. When it is first established it is frequently irregular in its occurrence for several months ; a like irregularity usually accompanies the cessation of the function between the fortieth and fiftieth years — when the menopause, or climacteric, is established. The normal female may be impreg- nated during menstrual life, but not before or after. The average length of time for which the menstrual flow con- tinues is four days. There are many exceptions in both direc- tions for different women, but the time for any one woman probably varies little under normal conditions. The discharge for each period averages some five ounces. It does not usually coagulate, on account of the presence of alkaline mucus. For five or six days preceding the flow, the uterine mucous mem- 404 REPRODUCTION. brane gradually thickens, the glands become longer and more tortuous, the connective tissue cells multiply and the blood vessels are greatly increased in size. This is apparently a prep- aration for the reception of the impregnated ovuiji. A short time before the flow begins there is hemorrhage into the subepi- thelial tissue, possibly by diapedesis, possibly by rupture. In a day or so the superjacent mucous membrane becomes disinte- grated and is discharged with the included parts of the glands. The underlying vessels, being thus exposed, rupture and the san- guineous discharge carries away the debris. For three or four days subsequent to the cessation of the flow the uterine mucosa is being repaired. The deeper layers, in- cluding the deeper portions of the glands, were not cast off", and the whole is reconstructed from the intact parts. Following the reconstructive period there is a stage of quiescence lasting some two weeks, until six or seven days prior to the next menstrua- tion. At the beginning of each menstrual flow there is general con- gestion of the pelvic viscera and mammary glands, accompanied usually by headache and a sense of pelvic oppression. The congestion and discomfort begin to disappear when the flow is established. Ovulation probably in most cases takes place just before the menstrual flow begins, but neither occurrence is dependent upon the other. Ovulation has frequently been shown to take place in the inter-menstrual period, but the congestion of the repro- ductive organs incident to menstruation probably hastens the rupture of any Graafian follicle which at that time happens to be near the completion of its development. The relations between ovulation, menstruation and impregna- tion are not definitely determined. Pregnancy lasts for ten lunar, months and dates from the time of impregnation (concep- tion), but that time cannot in any case be fixed upon with pre- cision. The vitality of the ovum is thought not to last longer IMPREGNATION. 405 than seven days unless impregnated, and if impregnation is to occur, it must take place within the first week after ovulation. Since, therefore, ovulation and menstruation usually occur to- gether, and since impregnation probably occurs about the be- ginning of menstruation, we reckon from the first day of the last menstruation 280 days forward to determine the probable time of labor. This is equivalent to adding nine calendar months and seven days to the first day of the last menstrual period. It is evident that this calculation at best gives only the approxi- mate time. While fertilization probably occurs at the time mentioned, the spermatozoon effecting fecundation may have been in the female genital tract for weeks. Its vitality here is so prolonged that the time of its deposit with reference to menstruation very probably has little to do with whether or not conception shall occur. Impregnation. — The term impregnation, or fertilization, or fecundation, is used to signify that union of the male and female sexual cells which makes possible the development of a new human being. Normally impregnation takes place in the Fal- lopian tube, and almost always in the outer third. The male element, the spermatozoon, seeks and penetrates the female ele- ment, the ovum. It is the blending of the nuclei (pronuclei) which is essential. Spermatozoa in large numbers swarm around the ovum and several at least enter the perivitelline space. Only one, however, is destined usually to enter the ovum. As it approaches the vitelline membrane, head first, the protoplasm of the ovum swells up into a prominence to meet it. The fertiliz- ing spermatozoon makes its way through the vitelline membrane, losing its tail in the passage, and becomes the male pronucleus. The female pronucleus now advances from its central position to meet the male element, and they coalesce to become the segmen- tation nucleus. Impregnation has now taken place. The seg- mentation nucleus represents anew being. It contains anatom- 4o6 REPRODUCTION. ical elements from both parents, and it is not surprising that the child should resemble both, anatomically and otherwise. The term " ovum " has so far been used to signify the unim- pregnated sexual cell discharged from the female ovary. It is also used to signify the fertilized cell, and is in fact often ap- plied without much precision to the product of conception at almost any stage of its intrauterine development. The fertilized ovum is carried through the tube to the uterus, arriving there some seven days after its fecundation. In its pas- sage it becomes covered with a coating of albuminous material. This layer is probably impervious to spermatozoa — ^which fact may account for the practical universality of fecundation in the outer part of the tube, if at all. The coating corresponds to the white of an egg, in that it penetrates the perivitelline membrane and furnishes nutritive material to the vitellus. On reaching the uterus the ovum becomes attached to and covered by the thick- ened mucous membrane of that organ in a way to be noted pres- ently. Here it remains until expelled during parturition. Segmentation. — As soon as union of male and female pro- nuclei has taken place, cleavage of the ovum begins. The nucleus (segmentation nucleus) and protoplasm divide by karyo- kinesis to form two nearly similar cells. These two divide into four, these four into eight and so on, till a large number of cells occupy the vitelline space and are all surrounded by the perivitelline membrane. As division proceeds, cells arrange themselves around others, so that the former occupy the cir- cumference and the latter the center of the vitelline cavity. Later, while the outer cells constitute a layer covering the entire inner surface of the perivitelline membrane, the inner cells group to form a mass which is in contact with the outer layer at one point only — like a ball lying in a relatively large hollow sphere. The space thus left between the two kinds of cells is called the segmentation cavity. Soon the surrounding cells become attenu- ated (Rauber's cells) and disappear. Their place, as a surround- SEGMENTATION. 407 ing envelope, is taken by some of the cells of the inner layer. This second surrounding layer is the epiblast, or ectoderm; the sur- rounded mass is the hypoblast, or entoderm. Fig. 93. Sections of the Ovum of a Rabbit, Showing the Formation of the Blastoder- mic Vesicle. (From ?Vt? after .£. Van Beneden.) a, &, c, d, are ova in successive stages of development ; Z, /, zona pellucida ; ect, ecto- meres, or outer cells ; entt entomeres, or inner cells. Before long the entoderm spreads out over a larger area, and from it and from the ectoderm is developed a layer of cells, the mesoblast, or mesoderm, which occupies a position between the 4o8 REPRODUCTION. Fig. 94. other two layers. This three-layered germ is now the blasto- dermic vesicle, or the gastntla, and its cavity is the archenteron, or celenteron. From these three germ layers are developed all the parts of the body by the formation of folds, ridges, constrictions, etc., and by various metamorphoses which have as their end the adaptation of structure to function. Derivatives of the Germ Layers. — According to Heisler these are : From the ectoderm : ( i ) The epidermis and its appendages, including the nails, the hair, the epithelium of the sebaceous and sweat glands and the epithelium of the mammary gland. ( 2 ) The infoldings of the epidermis, including the epithelium of the mouth and salivary glands, of the nasal tract and its communicat- ing cavities, of the external audi- tory canal, of the anus and anterior urethra, of the conjunctiva and anterior part of the cornea, the anterior lobe of the pituitary body, the crystalline lens and the enamel of the teeth. (3) The spinal cord and brain with its outgrowths, in- cluding the optic nerve, the retina and the posterior lobe of the pitu- itary body. (4) The epithelium of the internal ear. From the entode7-m : The epi- thelium of the respiratory tract, of the digestive tract (from the back part of the pharynx to the anus, including its associated glands, the liver and pancreas), of the middle ear and Eustachian tube, of the thymus and thyroid bodies, of the bladder and first part of the male urethra and of the entire female urethra. Impregnated Egg, With commencement of formation of embryo ; showing the area germinativa or embryonic spot, the area pellucida, and the primitive groove and streak. {Kirkes after Dalton.) DEVELOPMENT OF MESODERM. 4O9 From the mesoderm: (i) Connective tissue in all its forms, such as bone, dentine, cartilage, lymph, blood, fibrous and areolar tissue ; (2) muscular tissue ; (3) all endothelial cells ; (4) the spleen, kidney and ureter, testicle and its excretory ducts, uterus, Fallopian tube, ovary and vagina. The Embryonal Area. — Soon after the germ reaches the uterus (probably) there appears on its surface an oval whitish spot, the embryonal area. The impregnated ovum is still in the shape of a vesicle. It is from the embryonal area alone that the body is developed. The other parts are accessory. Longitu- dinal division of this area is supposed to give rise to twins of the same sex and of almost identical structure. Running in the long diameter of the embryonal area is a marking, the primitive streak, in which is a longitudinal depression, the primitive groove. (Fig. 94. ) These surface markings are caused by a thickening of the ectoderm. (Fig. 95.) Development of Mesoderm. — It is about this time that the mesoderm makes its appearance. It begins under the primitive groove and extends in all directions. It originates from both ectoderm and entoderm, and lies between them. In the median line the three layers are closely united to each other. (Fig. 95.) Af first the mesoderm does not completely embrace the germ, but is deficient opposite the embryonal area. Fig. 95 shows that the cells of the mesoderm make up a thick- ened mass near the median line, but farther away they constitute two distinct lamellae. The mass near the median line is the vertebral or axial plate. The outer of the lateral lamellae is the somatic mesoderm ; the inner is the splanchnic mesoderm. The ectoderm and somatic mesoderm unite to form the somato- pleure ; the entoderm and splanchnic mesoderm unite to form the splanchnopleure. The interval left between the so- matopleure and splanchnopleure is the celom, or body cavity. (Fig. 96.) The great serous cavities of the body are de- veloped from it. 410 REPRODUCTION. Beginning Differentiation. — It thus appears that the embryo is beginning to develop from the simple vesicle into specialized parts. We shall notice briefly the development of the body proper, and the extra-embryonic accessory structures, the umbilical vesicle. THE NEURAL CANAL. 411 amnion, allaittois and placenta. As regards the embryonic body, some of the most prominent occurrences connected with its de- vo 'i; velopment consist in the formation of the neural canal, chorda dorsalis, or notochord, and mesoblastic somites. Neural Canal. — About the fourteenth day, along underneath the primitive groove, the cells of the ectoderm become thickened 412 REPRODUCTION. to form the medullary plate. The edges of this longitudinal plate soon begin to curl up, and thus form the medullary furrow, ox groove. (Fig. 96.) The margins of the adjacent ectoderm are carried up with the curling edges, and constitute the medullary folds. Later the edges of the medullary plate meet each other, and join to form a closed canal, the neural or medullary canal. The edges of the medullary folds unite above, so that the neural Transverse Section through Dorsal Region of Embryo Chick (45 hovrs). One-half of the section is represented ; if completed it would extend as far to the left as to the right of the line of the medullary canal (il/c). A, epiblast; C, hypoblast, consisting of a single layer of flattened cells ; Mc^ medullary canal ; PVy protovertebra : Wd^ Wolffian duct ; So^ somatopleure ; Sp^ splanchnopleure ; //, pleuroperitoneal cavity ; cA, notochord ; ao^ dorsal aorta, containing blood-cells ; x/, blood-vessels of the yolk-sac. {Kirkes Sitter Fosier and Balfour.) canal comes to lie underneath the surface ectoderm. (Fig. 97.) The neural canal is the forerunner of the whole nervous system. Chorda Dorsalis. — The method of formation of the chorda dorsalis, or notochord, is very similar to that of the neural canal. It is a solid, instead of a cylindrical, longitudinal collection of cells, extending along the dorsal aspect of the celom. It is de- veloped from the entoderm. A thickening of the cells of this layer constitutes the chordal plate. Its edges curl up in a direc- tion opposite to those of the medullary plate, and carry with them SOMITES AND BODY CAVITY. 413 chordal folds of the entoderm. When the curling edges have joined to form a sohd cylinder of cells, the chordal folds unite over the ventral surface of the cylinder. Figures 96 and 97 illustrate these facts. The notochord is in the line of the future vertebral bodies, but it is not developed into any adult structure. Somites. — These are masses of cells developed from the axial plates of the mesoderm, lying parallel with and on each side of the notochord. (Fig. 97.) They are in segments, the formation of which begins in the neck and proceeds caudad and cephalad. They are sometimes called the protovertebrcs. They represent the primitive vertebrae. The body begins to assume shape and the fetal appendages to be developed at the same time. The latter are for the protec- tion and nutrition of the embryo. The essential parts of a verte- brate are a vertebral column with a neural canal above and a body cavity below it. The body cavity contains the alimentary canal. The somites representing the vertebral column and the formation of the neural canal have been noticed. Body Cavity. — At first the embryo, as represented by the embryonal area, is on a level with the remaining surface of the blastoderm. Soon, however, there appears, marking the head of the embryo and with its concavity backward, a crescentic folding- in of the blastodermic wall. It is evident on the surface as a simple furrow. This tucking-in finally surrounds the whole embryonal area, and the surface fissure, now oval, becomes deeper and deeper, until those portions of the wall which are be- ing tucked under the embryo approach each other on its ventral aspect and divide the yolk into two communicating cavities. (See Figs. 99 and 100.) The layers of the blastoderm thus folded underneath the em- bryo are the visceral plates. They form the boundaries of a cav- ity which still communicates in front, at the site of the future umbilicus, with the yolk-sac. This narrow canal is the vitelline duct, and the two cavities communicating through the vitelline 414 REPRODUCTION. duct are the future alimentary canal and the yolk-sac, or umbilical vesicle. It is to be noticed that the visceral plates embrace both somatopleure and splanchnopleure, and that it is the ectodermic Fig. 98. Diagrammatic Section showing the Relation in a Mammal between the Primitive Alimentary Canal and the Membrane of the Ovum. The stage represented in this diagram corresponds to that of the fifteenth or seventeenth day in the human embryo, previous to the expansion of the allantois ; c^ the villous chorion ; «, the amnion ; a' , the place of convergence of the amnion and reflexion of the false amnion ; a" a" , outer or corneous layer ; e, the head and trunk of the embryo, comprising the primitive vertebrae and cerebro-spinal axis; i, z, the simple alimentary canal in its upper and lower portions. Immediately beneath the right hand i is seen the fetal heart, lying in the anterior part of the pleuroperitoneal cavity ; v, the yolk-sac or umbilical vesicle ; vi^ the vitello-intestinal opening; w. the allantois connected by a pedicle with the hinder portion of the alimentary canal. {Kirkes after Quain.) layers of the splanchnopleure which finally join to form the gut tract, and the somatopleure which forms the ventral arid lateral wails of the body cavity. The gut tract has the shape of a THE FETAL MEMBRANES. 4IS Straight tube occupying the long axis of the embryo and open- ing into the umbilical vesicle. Fetal Membranes. Umbilical Vesicle. — The umbilical vesicle represents that part of the vitellus which has not been constricted off to form the gut tract. (Figs. 98, 99, 100.) It furnishes nutriment to the embryo for a short time and is then largely cut off from the body. It gradually shrivels (Figs. 104, 105), and with Figs. 99 and 100. a, chorion with villi. The villi are shown to be best developed in the part of the chorion to which the allantois is extending; this portion ultimately becomes the placenta ; i^, space be- tween the true and false amnion ; c, amniotic cavity ; d, situation of the intestine, showing its connection with the umbilical vesicle : e, umbilical vesicle ; /, situation of heart and ves. sels ; ^, allantois. {Kir/ces.) that part of the duct external to the abdomen is cast off either before or at parturition. Vessels develop in its walls and ab- sorb the nourishment in it to be conveyed to the embryo. But in the human being more satisfactory arrangements for nutrition are soon made and its iunction ceases. Amnion. — When the embryo has become depressed, as it were, into the substance of the blastoderm, and while the body cavity is being formed, the layers of the somatopleure grow up over the embryo to meet and blend dorsally. (Figs. 104, 105.) The 4i6 REPRODUCTION. two layers of which the somatopleure is composed separate, the outer forming the false amnion and the inner the true amnion. The false amnion now coalesces with the original vitelline mem- brane to constitute the false chorion. Evidently there is thus Figs, ioi and 102. Diagram of Fecundated Egg. a, umbilical vesicle; b, amniotic cavity ; c^ allan- tois. {Kirkes after Dal- ian.) Fecundated Egg with Allantois Nearly Complete. a, inner layer of amniotic fold ; 3, outer layer of ditto ; c~, point where the amniotic folds come in contact. The allantois is seen penetrating between the outer and inner layers of the amniotic folds. This figure, which represents only the amniotic folds and the parts within them, should be compared with figs. 99, 100, in which will be found the structures external to these folds. ( Kirkes after Dalton. ) fornied a closed cavity, the amniotic cavity, between the true am- nion and the body of the embryo. At first the amnion and the embryo are in close contact, but soon the cavity begins to be distended with a fluid, the liquor amnii, which increases until it reaches a considerable quantity. It affords mechanical protection to the fetus during intrauter- ine life, and at labor serves to evenly dilate the cervix. When this has been accomplished is the usual time at which the sac rup- tures and the liquor amnii escapes. It also supplies the fetal tissues with water, parts of it being swallowed from time to time. The cavity between the false amnion and the true amnion is continuous, with the body cavity at the umbilicus. Allantois. — The allantois grows out from the back part of the intestinal canal into the celom or the body cavity. (Figs. loi, THE ALLANTOIS. 417 102. ) It is of splanchnopleuric origin. It soon becomes a mem- branous sac, the walls of which are very vascular. It fills the space between the two amniotic folds and joins the false amnion. Its vessels thus reach the chorion, which is already establishing Fig. 103. •».< ve This and the two following wood-cuts are Diagrammatic Views of Sections, through the developing ovum, showing the formation of the mem- BRANES OF THE Chick, {yeo^ aher J^osier and £al/bur.) A, B, C, D, E, and F, are vertical sections in the long axis of the embryo at different periods, showing the stages of development of the amnion and of the yolk-sac ; I, II, III, and IV, are transverse sections at about the same stages of development ; i, ii, and Hi, give only the posterior part of the longitudinal section to show three stages in the formation of the al- lantois; f, embryo; y^ yolk; //, pleuroperitoneal fissure; z//, vitelline membrane; a/", am- niotic fold ; a/, allantois. vascular connections with the mother. Finally they are distrib- uted only to a certain part (placenta) of the chorion ; and as the allantoic vessels anastomose more and more freely with those of the chorion, the umbilical vesicle shrivels, as it is no 27 4i8 REPRODUCTION. longer needed. The vessels of the allantois are the two allan- toic arteries and the same number of allantoic veins. The al- lantois also receives the fetal urine. As the true placental circulation is established and the vis- ceral plates close the abdominal cavity, the allantois is constricted at the umbilicus so as to be divided into two parts. That Fig. 104. e, embryo ; a, amnion ; a' ^ alimentary canal ; vU vitelline membrane ; afy amniotic ii>Id ■ ■ac^ amniotic cavity ; y, yolk ; al^ allantois. THE CHORION. 419 outside the body shrivels and is cut away with the umbilical cord at birth, while that inside the body becomes the first part of the male and the whole of the female urethra, the bladder and the urachus. Chorion. — The chorion is the outer surrounding membrane of the embryo after the appearance of the amnion. It consists of Fig. 105. '-W—-W Diagrammatic Sections of an Embryo. Showing the destiny of the yolk-sac, ys. vi, vitelline membrane ; pp, pleuroperitoneal cavity ; ac, cavity of the amnion ; a, amnion ; a' , alimentary canal ; ys, yolk-sac, three layers. From without inward these are the original vitel- line membrane, the false amnion and the allantois. The allan- tois has been seen to extend around between the two amniotic folds and to blend with the outer. From its formation from these several membranes, the chorion evidently consists of the outer ectodermic, inner entodermic and intervening mesodermic strata. 420 REPRODUCTION. By the time the impregnated ovum reaches the uterus, the chorion (false at this time) has numerous spike-like projections, — villi — over its whole surface. (Fig. 98.) These are at first non-vascular, but soon become vascular by the projection into them of capillaries from the vessels of the allantois. These capillaries probably absorb nutrient matter secreted by the uterine glands. But at the beginning of the third month the villi become much more highly developed over a certain part of the surface of the chorion than at other points, and a more intimate relation is estab- lished between their vessels and those of the mother ; here the placenta is to be formed. The Decidua. — The decidua of pregnancy consists of the hyper- trophied mucous membrane lining the cavity of the uterus and reflected at a certain point entirely over the developing ovum. Before the ovum reaches the uterus, the mucous membrane of the latter has been undergoing changes, such are mentioned under Menstruation. If fecundation has not taken place, menstruation occurs and the mucosa is discharged under the name of the decidua menstrualis. But if conception has occurred, menstruation does not ensue and the uterine mucosa becomes much more thick and spongy. Whether or not it shall be discharged as the decidua of menstruation or be retained to form the decidua of pregnancy is probably a point which is decided while the ovum is yet in the tube. When the fecundated ovum reaches the uterus it becomes at- tached to the mucous membrane, usually a little to one side of the median line on the posterior wall. The mucous membrane extends over and completely envelops it. This reflected portion is the decidua reflexa ; that lining the whole uterine cavity is the decidua vera, while that part of the decidua vera intervening be- tween the ovum and the uterine wall is the decidua serotina and becomes the maternal part of the placenta. Of course there is at first a considerable cavity left between the reflexa and the vera, but as the embryo increases in size the THE PLACENTA. 421 Space becomes smaller and is obliterated by the end of the fifth month. After this time both vera and reflexa undergo retro- grade changes due to pressure and become closely attached to the chorion. They are discharged with the membranes at birth. Placenta. — The placenta is the organ of nutrition for the Fig. 106. DlAGKAMMATIC ViEW OF A VERTICAL TkANSVEKSE SeCTION OF THE UtERUS AT THE Seventh or Eighth Week of Pregnancy. c, c, c' , cavity of uterus, which becomes the cavity of the decidua, opening at c, c, the cor- nua, into the Fallopian tubes, and at c' into the cavity of the cervix, which is closed by a plug of mucus ; dz', decidua vera ; dr, decidua reflexa, with the sparser villi imbedded in its substance ; ds, decidua serotina, involving the more developed chorionic villi of the com- mencing placenta. The fetus in seen lying in the amniotic sac ; passing up from the um- bilicus is seen the umbilical cord and its vessels passing to their distribution in the villi of the chorion ; also the pedicle of the yolk-sac, which lies in the cavity between the amnion and chorion. {Kirkes ^hux Allen Thofnson.) 422 REPRODUCTION. fetus after about the end of the third month. Through it the ves- sels of the fetus and those of the mother are brought into most intimate relations. It has been said that the villi of the chorion in one locality be- come very highly developed. This is at the site of the reflec- tion of the decidua serotina and is the chorion frondosum. The union of these, with certain other developments, constitutes the placenta. The decidua serotina becomes very spongy. It is filled with sinuses, into which the enlarged villi of the chorion frondosum project. The sinuses are filled with maternal blood, while the capillaries of the villi contain fetal blood. There is no direct connection between the vessels of mother and child, but the thin lining of the villi and sinuses allows free interchange of materials by osmosis. It seems that the interchange is under the influence of two sets of cells, each disposed in a single layer — one belonging to the maternal and the other to the fetal part of the placenta. These layers of cells are situated on either side of the membrane of the villus. They seem to take out of the maternal blood materials needed for the nutrition of the fetus, and out of the fetal blood materials which require removal. The maternal blood performs both alimentary and respiratory functions for the fetus. The placenta as a whole is discoid in shape. Its fetal surface is concave and covered by the amnion. The mass has a diame- ter of 4-5 in., and a thickness of half an inch. The villi re- ceive blood from the allantoic or umbilical arteries ; it is returned by the umbilical vein. At labor uterine contractions detach the placenta and the decidua and expel them from the womb. The separation takes place in the deeper part of the maternal placenta, or decidua serotina, so that the mass discharged represents both the fetal and maternal portions. The vessels entering the THE UMBILICAL CORD. 423 sinuses da so obliquely ; consequently uterine contractions at birth very effectually check the hemorrhage which separation of the placenta occasions. Umbilical Cord. — The umbilical cord is made up of the ves- sels which convey blood between the placenta and fetus, to- gether with the remnants of the umbilical vesicle and allantoic stalk, all of which are held together by the jelly of Wharton, a species of connective tissue. The outgrowing allantois has developed in it the two allantoic arteries and veins. By the time the placenta is formed the allantoic stalk has become much elongated, and the allantoic vessels extend into the fetal placenta (chorion frondosum) and become now the umbilical vessels. The two veins blend to con- stitute a single umbilical vein, but the arteries remain separate. The vein enters the fetal body at the umbilicus, passes to the under surface of the liver and divides in a manner to be noted presently. After birth the intra-abdominal portion atrophies, and is the round ligament of the liver. The two umbilical arteries issue at the umbilicus. Their intra-abdominal portions are the fetal hypogastric arteries. The average length of the umbilical cord is about twenty-one inches. It appears to be twisted on account of the spiral course of its relatively long arteries. It is usually attached near the center of the fetal surface of the placenta. Condition of the Fetal Membranes at Birth. — The mem- branes discharged with the placenta at birth are, from without inward, the decidua vera, decidua reflexa, chorion and amnion. The amniotic fluid, in which the fetus floats, reaches its maxi- mum amount at about the sixth month. It is sufficient then to force the amnion closely against the chorion, covered by the decidua reflexa ; these last named (chorion and reflexa) are in turn forced everywhere against the decidua vera. The result is that all four become practically one membrane, though the union between amnion and chorion is not so close as that between the 424 REPRODUCTION. Other layers. These membranes constitute, then, a sac filled with fluid. The sac is ruptured in labor, and the child escapes through the rent. Afterwards the decidua vera and placenta are detached, and escape together as the ' ' after birth. ' ' Development of the Circulation. — The development of the circulation maybe considered in these stages: (i) Vitelline circulation, (2) placental circulation, (3) adult circulation. The heart is the propelling organ in all these. I. Vitelline Circulation. — The blood and vessels make their appearance almost as early as the primitive groove. Certain blastodermic cells are transformed into both red and white cor- puscles. They are larger than the adult's cells and both are nucleated. Blastodermic cells also group to form small tubes, which constitute the area vasculosa. At the same time meso- blastic cells develop two tubes, one along each side of the body, which soon unite to form a single one, representing the heart. It becomes enlarged and twisted upon itself, and pulsations begin in it at a very early date. The heart is in the median line and gives off two arches which unite below to form the abdominal aorta. From the arches pass branches to the area vasculosa, which now form a nearly circular plexus around the embryo. Two of these branches, larger than the others, enter the umbilical vesicle and become the omphalo-mesenteric arteries ; there are two corresponding veins. This circulation through the omphalo-mesenteric vessels and the area vasculosa does not continue long in the human being. As soon as the allantois is formed and the placental circulation begins to be set up, the omphalo-mesenteric vessels are obliterated and the place of the first circulation is taken by the second. Development of the Heart. — The tube just mentioned as rep- resenting the heart has communicating with it two veins at its lower extremity and two arteries at its upper. Soon the tube becomes twisted upon itself so that the upper (arterial) is thrown in front of the lower (venous). The loop is V-shaped and is THE PLACENTAL CIRCULATION. 425 the outline of the future ventricles. Afterward a constriction forms the auricle. At this time the heart consists of a single ventricle and a single auricle. Later the ventricular and auric- ular septa are formed. The latter appears after the former and is incomplete ; the opening left between the auricles is i!a!t fora- men ovale. 2. Placental Circulation. — As the allantois is developed and the vitelline circulation is abolished, the hypogastric arteries are given off first from the aorta, but later (with the development of the vessels of the lower extremities) they are pushed down, as it were, so that they take origin from the internal iliacs. They pass to the umbilicus and thence to the placenta by the cord. Blood is at first returned from the placenta by two um- bilical veins, but these soon fuse into one. Object of Placental Circulation. — Since the activity of the respiratory and alimentary tracts has not been established, their functions must be performed by those of the mother and the necessary materials supplied from her blood. Consequently there must be a continual passage of fetal blood to and from the placenta to discharge effete matter and to absorb nutriment. Certain modifications of the circulatory apparatus, not requisite after birth, are necessary to bring this about. Course of Fetal Circulation. — The umbilical vein containing blood enriched with oxygen and other materials enters the body at the umbilicus and passes to the under surface of the liver. Here it divides into two branches. The larger joins the portal vein and enters the liver ; the smaller is the ductus venosus, which enters the ascending vena cava. The ascending vena cava, when it enters the right auricle, therefore, contains blood from the lower extremities, blood which has come from the placenta directly through the ductus venosus, and blood which has come from the placenta indirectly through the liver. Considering that blood from the body of the fetus is venous and that blood directly from the placenta is arte- 426 REPRODUCTION. rial, the contents of the ascending vena cava are mixed when they enter the heart. The Eustachian valve, together with the direc- FiG. 107, w \\ Diagram illustrating the Circulation through the Heart and thh principal Vessels of a Fetus. (From K^ 3^8 centers for 177 Vater, corpuscles of 291 Veins, capacity of 173 current in 174 pressure in 17S structure of 174 valves of 174 Venous circulation 173 causes of . . . 176 Ventilation 224 Ventricle, left 137 right 136 Venules 171 Vermiform appendix . . . . 117 Vestibule of ear ...... 388 Villi Ill Vital force 12 Vitelline circulation 424 duct 413 Vitreous humor 382 Vocal cords . . .197, 393 sounds, varieties of ... . 394 Voice, production of . ... 393 W. Water 20 elimination of by kidney . 244 functions of in body . . 20 Wharton's duct 56 442 INDEX. White corpuscles ....... 183, chemical composition of 18 J classes of . . .184 functions of . . . . 184 origin of 185 properties of 183 Wirsung, duct of . . . .65 Wolffian bodies . . ... 430 Wrisberg, nerve of . . 355 X. Xanthin, discharge of 348 Y. Yawning 2og Z. 6r A Classified Catalogue of Books on Medicine and the Collateral Sciences, Phar- macy, Dentistry, Chemistry, Hygiene, Microscopy, Etc. «^ P. 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