GIFT OF Pacific Coast •Tmirmal of* TJiiT*st* no* OUTLINES OF PHYSIOLOGY JONES AND BUNCE Cranium. — 7 Cervical Vertebras Clavicle. Scapula. 12 Dorsal Vertebras. Hi 5 Lumbar Vertebrne Ilium. Ulna. Kadius. Pelvis. Bones of the Carpus. Bones of the Meta- carpus. Phalanges of Fibers. Femur. Patella. Tibia. Fibula. Bones of the Tarsus. Bones of th« Meta- tarsus. Pholunge* of Toer THE SKELETON (»rr«« HOLDER). OUTLINES OF PHYSIOLOGY BY EDWARD GROVES JONES, A. B., M. D, PROFESSOR OF SURGERY IN ATLANTA SCHOOL OF MEDICINE AND ALLEN H. BUNCE, A. B., M. D, ASSOCIATE PROFESSOR OF PHYSIOLOGY IN THE ATLANTA SCHOOL OF MEDICINE THIRD EDITION, REVISED 111 ILLUSTRATIONS v v.nii li PHILADELPHIA P. BLAKISTON'S SON & CO. 1012 WALNUT STREET 1912 -.,.: .'.' • - «.,*•• BIOLOGY LIBRARY D COPYRIGHT, 1912, BY P. BLAKISTON'S SON & Co. vrr^orj THE. MAPLE* PRE8S*YORK» PA TO DOCTOR WILLIAM S. KENDRICK PROFESSOR OF MEDICINE IN THE ATLANTA SCHOOL OF MEDICINE THESE PAGES ARE AFFECTIONATELY DEDICATED 743555 PREFACE TO THIRD EDITION IN revising this work my aim has been to fulfill the original purpose of the book in so far as possible. Dr. Jones said in the preface to the first edition: "This volume has been prepared with a view of presenting, in as convenient form as possible, the essential facts of modern physiology as related to the practice of medicine." This object has been kept in mind in the preparation of the third edition. In this edition the Introduction, the chapters on The Cell, The Elementary Tissues and on the Blood have been entirely rewritten and new illustrations added. A new chapter, with illustrations, on The Physiological Characteristics of Muscle, has been added. The chapter on Secretion has been rearranged. New subject matter has been added to the chapter on The Physiology of Digestion and Absorption. Other minor changes have been made throughout the book where such were deemed advisable. Especial acknowledgment is due to Dr. Robert G. Stephens for his valuable work in preparing the second edition. In the preparation of this edition, no claim to original inves- tigation is made. I have freely used the works of other authors when necessary. ATLANTA, GA. ALLEN H. BUNCE vn PREFACE TO FIRST EDITION 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 practised 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 anatomical 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: Ho well (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. ATLANTA, GA. E. G. J. ix CONTENTS. PAGE INTRODUCTION xv CHAPTER I. THE CELL i CHAPTER II. THE ELEMENTARY TISSUES 7 The epithelial tissues 7 The connective tissues n The muscular tissues 19 The nervous tissues 22 CHAPTER III. / PHYSIOLOGICAL CHARACTERISTICS OF MUSCLE 23 CHAPTER IV. SECRETION . 27 Sebaceous glands 30 Mammary glands 31 Thyroid glands 32 Adrenal glands 33 Pituitary body 34 Testis and ovary 34 CHAPTER V. THE BLOOD 35 CHAPTER VI. THE CIRCULATION OF THE BLOOD 41 The heart 42 Circulation in blood-vessels 46 xi Xll CONTENTS PAGB Structure of the blood-vessels 47 The lymph 57 CHAPTER VII. THE PHYSIOLOGY OF DIGESTION AND ABSORPTION 63 Foods 63 Digestion 68 Prehension 70 Digestion in the mouth 71 The salivary glands and their secretion 71 Deglutition 78 Digestion and absorption in the stomach 81 The gastric glands 85 Digestion and absorption in the intestines 96 The small intestine 96 The large intestine 117 Absorption in general * . . . 122 Absorption from the alimentary canal 126 CHAPTER VIII. RESPIRATION 131 Anatomy of the respiratory organs 132 Mechanism of respiration 138 CHAPTER IX. NUTRITION, DIETETICS AND ANIMAL HEAT 171 Nutrition 171 Dietetics 181 Animal heat 184 CHAPTER X. EXCRETION 192 The kidneys 192 The skin 208 CHAPTER XI. THE NERVOUS SYSTEM 214 The cerebro-spinal system 235 CONTENTS Xlll PAGE The spinal cord 237 The encephalon 251 The medulla oblongata 251 The pons varolii 255 The crura cerebri 256 The cerebrum 260 The cerebellum 274 The cranial nerves 275 The spinal nerves 295 The sympathetic system 297 CHAPTER XII. THE SENSES 304 Common sensations 304 Special sensations 305 The sense of touch 305 The sense of smell 306 The sense of sight 307 The sense of taste 316 The sense of hearing 317 The production of the voice 324 CHAPTER XIII. REPRODUCTION 327 INDEX 365 INTRODUCTION THE science which treats of the structure, function and organi- zation of living forms, both vegetable and animal, is called biology. That branch of biology which describes animal life exclusively is termed zoology, while that branch which de- scribes vegetable life exclusively is termed botany. The study of the form of organisms, both vegetable and animal, is termed morphology. Morphology is further divided into (i) histology, which treats of the formed elementary con- stituents of organisms, and (2) anatomy, which treats of the parts and organs of the organism. After the form and structure of an organism has been studied, the next step is the study of the work which the organism has to perform. This study of the vital phenomena, or life, of the organism is called physiology. Physiology may be either animal or vegetable. Human physiology is that branch of physiology which treats of the vital phenomena occurring in man. The structural unit of the body is the cell. Myriads of cells are grouped together to form organs. An organ may be denned as a group of cells combined together to perform some special function, e. g., the kidney is an organ whose special function is the secretion of urine. The organs are further grouped together to form systems. Thus we have the circulatory system com- posed of the heart, arteries, veins, and capillaries. Now the study of the function which the circulatory system has to per- form is the physiology of circulation. Likewise we may sub- divide physiology into the physiology of the nervous system, xv XVI INTRODUCTION the physiology of the digestive system, the physiology of the respiratory system, etc. Thus we have seen the relation physiology bears to the other sciences dealing with life and the special part human physiology plays in the whole of physiology. In the following pages we have endeavored to give a brief outline of human physiology. OUTLINES OF PHYSIOLOGY,:- CHAPTER 'I: •••'•'"••- ',' THE CELL. ALL the tissues of the body are made up of cells and inter- cellular substance. All the cells are descended from one parent cell, called the ovum, while the intercellular substance is created through the medium of the cells. Nuclear membrane. Linin. Nuclear fluid, (matrix) . Nucleolus. Chromatin cords (nuclear network). Nodal enlarge- ments of the chromatin. Cell membrane. -V Exoplasm. Microsomes. Centrosome. ^// ^ ' ^J J/r 7 Spongioplasm. Hyaloplasm. --==—-- Foreign inclosures. FIG. i. — Diagram of a cell. Microsomes and spongioplasm are only partly drawn. (Brubaker.) A cell, which is the histologic unit of the body, may be denned as an irregular round or oval mass of protoplasm of microscopic size, enclosing a small indistinct spherical body, the nucleus. 2 THE CELL The essential parts of the cell are, (i) the cystoplasm, whicn is a special name given to the protoplasm forming the cell-body and (2) the nucleus, which is a small round or oval body embedded in the cytoplasm. A great many cells are surrounded oy a' cell-wall or cell- membrane, but this cannot be regarded as one of- the essential elements since all cells do not possess such rnem'jr&nes. (1) The Cytoplasm. — This is a gelatinous or semi-fluid, granular substance, transparent and generally colorless. Chem- ically it consists of water and salts, together with various organic substances, called proteids, which are complex combinations of carbon, hydrogen, oxygen and nitrogen, and sometimes phosphorus and sulphur. The proteids of the cytoplasm con- tain little phosphorus, while those of the nucleus are rich in it. The cytoplasm does not always present the same structural appearance since its constituents vary in their condition and arrangement. In some cells it has a clear homogeneous appear- ance, while in others it contains fine spherical particles which give it a granular structure. When these granules are large and clear, and are surrounded by denser areas they give to the cytoplasm an alveolar outline. But most frequently the cyto- plasm contains in its structure a meshwork of threads or fibrils which give it a reticular appearance. This network of fibrils is called the spongioplasm which encloses a less firm portion, the hyaloplasm (Fig. i). However, in all these varieties, the cytoplasm has both an active and a passive structure. In young granular cells the active substance is represented by small spherical particles, called microsomes (Fig. i). These are not always evenly dis- tributed throughout the cytoplasm, but are grouped in an area near the nucleus, while the area next the cell-wall is almost free from granules. The dense inner area is called the endoplasm, while the clear outer area is called the exoplasm (Fig. i). (2) The nucleus, which is the second essential part of a THE VITAL PHENOMENA OF CELLS 3 typical cell, is a small round or oval body contained within the cell. It is usually surrounded by a distinct nuclear membrane, except during division. Nuclei play an important role in cell reproduction and cell nutrition. They are characterized by their affinity for certain stains, e. g., hematoxylin. The substance of the nucleus, the karyoplasm, may be divided into two parts — the nuclear fibrils which form an irregular reticu- lum, and the nuclear matrix which forms the intervening semi- fluid mass. The nuclear fibrils, when properly stained, are found to consist of minute irregular masses of a deeply colored substance, called chromatin, in recognition of their affinity for certain stains. The chromatin particles are supported within delicate and colorless threads of linin. The nuclear matrix, which is semi-fluid in character and which occupies the spaces between the nuclear fibrils, possesses a very weak affinity for the stains used to color the chromatin. Hence, it usually appears clear and untinted. Chemically, the chromatin con- tains a substance nuclein, which is rich in phosphorus. The nucleolus ordinarily appears as a small spherical mass among the nuclear fibrils. It is supposed to be of little signifi- cance in so far as the vital phenomena of the cell are concerned. The Vital Phenomena of Cells. — The vital phenomena of the cell include all those processes and changes which it under- goes during its life and which take place in the performance of its various functions. They include (i) metabolism, (2) growth, (3) reproduction and (4) irritability. (i) Metabolism includes all those processes by which the cell is enabled to select from the various substances furnished it and convert them into its own substance, and, secondly, those proc- esses whereby the cell is enabled to cast off the waste products set free by its activity. The first process, that by which the cell takes the simple substances furnished it and converts them into its complex compounds, is called anabolism, or construc- tive metabolism. 4 THE CELL The process whereby the cell breaks up these complex com- pounds formed by anabolism and discharges them from its substance, is called katabolism, or destructive metabolism. A good example of anabolism is that by which the vegetable cells take such substances as carbon dioxide, water and inorganic salts and prepare food-material for the nutritive and katabolic processes in animals. (2) Growth is the natural sequence of the nutritive changes effected by metabolism and may be unrestricted and equal in all directions. However, this is not usually the case as is shown by the fact that cells are so intimately associated with other structural elements as to influence and modify their growth. These result in unequal growth, to which the specialization of cells is due. Examples of the unequal growth of cells are shown in the columnar cells of epithelium, the neurones of nervous tissue, the fibers of muscle tissue, etc. (3) Reproduction may be regarded as the culmination of the activities of the cell, for by this process the cell loses its individuality and continues its life in that of its offsprings. There are two methods by which cells may reproduce them- selves, (i) by direct cell division or amitosis and (2) by indirect cell division or mitosis. (4) Irritability is that property of cells whereby they are enabled to respond to stimuli, i. £., to change their form and shape in response to these stimuli. The various stimuli which affect cells may be mechanical, thermal, nervous, chemical or electrical. Cell Division. — (i) Direct cell division, or amitosis, is the simplest form of cell division. In this form of cell division the nucleus and protoplasm constrict in the middle until two new cells are formed. This form of cell division does not occur in the higher animals except as a secondary process. (2) In the higher animals cell division takes place chiefly by the indirect or mitotic method. This may be described CELL DIVISION briefly as follows: In the beginning of this phenomenon, the nucleus, which plays the most important role, grows larger. Its chromatin greatly increases and becomes contorted so as to form a dense convolution, the close skein, or spireme. Then the chromatin fibrils further thicken, become less convoluted and form irregularly arranged loops, the loose skein. During Close Skein Mother stars (viewed from the Loose Skein (viewed from side) ; Polar field, (viewed from above — i. e., from the the side). pole). Mother Star Daughter Star (viewed from above). Beginning. Completed. Division of the Protoplasm. FIG. 2. — 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 Siredon pisciformis. Neither the centrospmes nor the first stages of the de- velopment of the spindle can be seen by this magnification. X 560. (From Brubaker.} the formation of these skeins the nuclear membrane and the nucleoli disappear. The fibrils of the loose skein now separate at their peripheral turns into a score of loops, the closed ends of which point toward a common center — a clear space called the polar field. When seen from above these loops of chromatin make a wreath, called the mother wreath; when seen from the 6 THE CELL side, they make a star, called the mother star or aster. While the loose skeins are forming, delicate striae appear within the achromatin, so disposed as to make their bases within the polar field and directed toward one another, and their apices directed toward the future new nuclei. These achromatin figures con- stitute the nuclear spindle. They then arrange themselves into two daughter wreaths, or asters, similar to the mother star. At this juncture the cell protoplasm begins to divide by becom- ing constricted in the center. The daughter stars are converted into two new nuclei, in the inverse order to that by which the original nucleus was broken up. Nuclear membranes and nucleoli appear, the cell protoplasm divides into two new cells, and the cycle is completed. Derivation of Tissues. — The primary parent cell divides into an innumerable mass of cells, which is called the blasto- derm. The blastoderm soon divides into two more or less distinct layers, an outer and an inner, named ectoderm and entoderm, between which a middle layer develops, the meso- derm. From these three primary layers all of the various tissues of the body are later developed. (See Embryology.) CHAPTER II. THE ELEMENTARY TISSUES. THE tissues which make up the various organs and parts of the body may be divided into the following groups: (i) Epi- thelial tissue, (2) connective tissue, (3) muscular tissue and (4) nervous tissue. The Epithelial Tissues. The epithelial tissues include those which form the covering for the body, the lining of the digestive canal, the respiratory tract and the genito-urinary tract. They also constitute the derivatives of the epidermis, such as nails, hair, sebaceous glands, and the lining of the glands connected with the digestive and genito-urinary systems. These tissues are composed of cellular and intercellular elements and perform various functions in different parts of the body. In the skin where they constitute the epidermis, they protect the delicate surface of the true skin beneath; in the alimentary and genito-urinary canals they aid in secretion and excretion; in the respiratory system they preserve an equable temperature, while in all internal parts they yield lubricants. These tissues are characterized by the preponderance of the cellular over the intercellular elements. The intercellular structure consists of a cement substance which holds the cells together and through which the food for the cells is absorbed. They contain no blood-vessels and no nerves. The tissue usually rests upon a basement membrana, or membrana propria, which is a modification of the connective tissue beneath. 8 THE ELEMENTARY TISSUES Varieties. — The varieties of epithelium may be classed as follows: (i) Squamous, (a) simple, consisting of a single layer, (b) stratified, consisting of several layers; (2) Columnar, (a) simple, (b) stratified; (3) Modified, (a) ciliated, (b) gob et, (c) pigmented, (d) glandular, (e) neuro-epithelium. (i) Squamous Epithelium.— (a) Simple squamous epithe- lium consists of a single layer of cells which, when viewed from FIG. 3. — From a section of the lung of a cat, stained with silver nitrate. N, Alveoli or air-cells, lined with large, flat, nucleated cells, with some smaller polyhedral nucleated cells. (Halliburton after Klein and Noble Smith.) above, appear as flattened polyhedral nucleated plates like a regular mosaic. It occurs in but a few places, lining the air sacs of the lungs, the mastoid cells, the membranous labyrinth and crystalline lens (Fig. 3). (b) Stratified squamous epithelium is composed of several layers of epithelial cells placed upon one another. The deepest layer, which rests upon the basement membrane, is composed of irregularly columnar cells which have their nuclei near the EPITHELIAL TISSUES 9 lower border of the cells. As they approach the surface the layers become flatter and more scale-like and possess less vitality. As the outer layers are worn away the lower more vigorous layers push upward to the surface to take their place. In the middle strata, where the cells are polyhedral in shape, we find the layer of prickle cells which have minute projecting FIG. 4. — Vertical section of the stratified epithelium of the rabbit's cornea. a, anterior epithelium, showing the different shapes of the cells at various denths from the free surface; b, a portion of the substance of cornea. (Kirkes after Klein.) spines by which they are connected together. This layer is sometimes called the stratum spinosum. Just below this is the stratum germinativum, or germinating layer. (2) Columnar Epithelium. — (a) The simple columnar va- riety consists of a single layer of column or rod-shaped cells, set upright, longitudinally striated and containing oval-shaped nuclei. This variety is found in the lining of the stomach and intestines. (b) In the stratified columnar variety the single layer of cells is replaced by several layers and the superficial elements alone are typical. This type is found in the vas deferens. Ciliated, it occurs in the Eustachian tube, lachrymal ducts, respiratory part of the nasal fossae, ventricle of larynx, trachea and bronchi, epididymis and first part of vas deferens. (3) Modified Epithelium. — (a) Ciliated epithelium is more common with the columnar variety than with any other. Each 10 THE ELEMENTARY TISSUES of the ciliated epithelial cells presents on its free surface 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 FIG. 5. — Ciliated epithelium of the human trachea. a, layer of longitudinally arranged elastic fibers; b, basement membrane; c, deep- est cells, circular in form; d, intermediate elongated cells; e, outermost layer of cells fully developed and bearing cilia. Xsso. (Kirkes after Kolliker.) they are found. In the genital organs they are important in bringing together the male and female elements of reproduction, while in the respiratory tract they are concerned in aiding the passage of the mucus and in the expul- sion of foreign bodies. (b) Goblet cells are found on all sur- faces covered by columnar epithelium, but especially in the large intestine. They secrete mucin, the main constit- uent of mucus, which so distends the cell that it ultimately bursts and sets free its contents. (c) Pigmented epithelium is ordinary epithelium, the protoplasm of which has become invaded and colored by foreign matter, such as fat, proteid, etc. Such cells are constant in the deeper layers of the epidermis, especially of FIG. 6.— Goblet cells. (Halliburton after Klein.} CONNECTIVE TISSUES II certain races, as the negro. It is also found in the choroid coat of the eye. (d) Glandular epithelium may be columnar, spherical or poly- hedral in shape. It is found lining the terminal recesses of secreting glands. The protoplasm of the cell usually contains the material which the gland secretes. (e) N euro -epithelium is the name given to that covering those parts toward which the nerves of special sense are directed,, and is epithelium of the highest specialization. It occurs in the retina, the membranous labyrinth and in the olfactory and taste cells. The Connective Tissues. All these tissues are developed from the same embryonal elements, but present varieties differing widely in appearance and properties. They are characterized by the preponderance of the inter-cellular over the cellular elements. The physical characteristics of these tissues are very important and depend mostly upon the intercellular elements. Their purpose in the animal economy is to furnish a supporting and connecting framework for the body. In the embryonal state the inter- cellular substance is semi-fluid and gelatinous. Later, in adult connective tissue it becomes more definitely formed, although it is still soft. In adult areolar tissue the intercellular substance becomes tough and yielding. When this intercellular sub- stance becomes impregnated with calcareous salts we have bone. However, during all these changes in the intercellular substance little or no change has taken place in the cellular structure — the bone-corpuscle, the cartilage-cell, the tendon- cell and the connective-tissue cell all being essentially identical. The divisions of connective tissue are: (i) Mucous Tissue, (2) Reticular Tissue, (3) Fibrous Tissue, (4) Adipose Tissue, (5) Cartilage, (6) Bone. 12 THE ELEMENTARY TISSUES (i) Mucous Tissue.— This is the most immature form of connective tissue and consists of a loose protoplasmic network having a gelatinous intercellular substance. It is found in FIG. 7. — Tissue of the jelly of Wharton from umbilical cord. a, Connective-tissue corpuscles; b, fasciculi of connective-tissue fibers; c, spherical cells. (Halliburton after Frey.) FIG. 8. — Retiform tissue from a lymphatic gland, from a section which has been treated with dilute potash. (Halliburton after Schafer.} Wharton's jelly in the embryo and in certain tumors, known as myxoma. (2) Reticular Tissue. — This is composed chiefly of a net- work of connective-tissue cells which enclose a mass of lymphoid CONNECTIVE TISSUES 13 elements. It forms the connecting layer beneath the skin, the submucous and subserous tissues, and the layer between the muscles. It receives its name on account of the areolae or spaces within its substance, which admit the adjacent parts to move easily upon one another. It consists of white and yellow fibers in about an equal proportion. (3) Fibrous Tissue. — This variety includes all the more usual forms of connective tissue found in the various parts of the body. It may be further subdivided into: (a) White fibrous tissue, (b) yellow elastic and (3) loose fibrous or areolar tissue. FIG. 9. — Bundles of the white fibers or areolar tissue partly unravelled. (Kirkes after Sharpey.} (a) White fibrous tissue is composed of groups or bundles of fibers which have a wavy longitudinal striation. It is tough and inelastic and forms ligaments, tendons and membranes in various parts of the body. Chemically this tissue is composed of a complex albuminoid substance, collagen. Upon being treated with acetic acid the fibers become swollen and trans- parent and finally invisible. 14 THE ELEMENTARY TISSUES (b) Yellow elastic tissue is composed of bundles of long, regu- lar and branched fibers. It is characterized by its marked elasticity. It is found in the vocal cords, longitudinal coat of the trachea and bronchi, inner coat of blood-vessels, especially the large arteries, and in some ligaments. Its yellow-tinted fibers are seen in parallel waves and are larger than those in the white tissue. They sometimes form a web-like layer, as in the fenestrated layer of Henle in the arteries. FIG. 10. — Elastic fibers from the ligamenta subflava. Xaoo. (Halliburton after Sharpey.) FIG. 1 1 . — Group of fat-cells (F c) with capillary vessels (c). (Kirkes after Noble Smith.) (4) Adipose Tissue. — This tissue exists in nearly all parts of the body except the subcutaneous tissue of the eyelids, the penis and scrotum, the nymphae and in certain parts of the lungs. It is nearly always found within the meshes of areolar tissue, where it forms lobules of fat. Fatty matter in the form of oily tissue is found in the brain, liver, blood and chyle. The tissue is densest beneath the skin, especially of the abdomen, CONNECTIVE TISSUES around the kidneys, between the furrows on the surface of the heart and in bone marrow. It has a rich blood supply. (5) Cartilage.— Those tissues in which the intercellular substance has undergone condensation until it appears homo- geneous are classified as cartilage. Consequent upon the differences exhibited by the intercellular matrix it is divided into the following varieties: (a) Hyaline, (b) elastic and (c) fibrous. (a) Hyaline cartilage is of firm consistence, considerable elasticity and is pearly blue in color. It is enveloped in a fibrous membrane, the perichondrium, from the vessels of which it derives its nutrition. It is composed of cells, irregular in outline and arranged in patches of various shapes, which are embedded in a homogeneous matrix. The articular surfaces of bones, the costal cartilages, and the larger cartilages of the larynx, trachea and bronchi, and, also, those of the nose and Eustachian tube are formed of this variety. In the em- bryo this cartilage forms nearly the whole of the future bony skeleton. (b) Elastic cartilage is characterized by the presence of an abundance of elastic fibers in the matrix. These resemble those found in the yellow variety of elastic tissue. This variety of cartilage is found in the external ear, epiglottis, cornicula laryngis and Eustachian tube. (c) Fibrous cartilage is characterized by the presence of a large amount of white fibrous tissue in the matrix. It combines the toughness and flexibility of fibrous tissue with the firmness and elasticity of cartilage. It is found chiefly in the interver- FIG. 12. — Section of hyaline cartilage. From the end of a growing bone, showing a decrease in the intercel- lular substance compared with the number of cell-elements, which are arranged in rows. (Yeo.) i6 THE ELEMENTARY TISSUES tebral disks, the symphyses and interarticular disks of certain joints, and lining bony grooves for tendons. Chemically, cartilage is complex, consisting of a mixture of FIG. 13. — Elastic fibro-cartilage, Showing cells in capsules and elastic fibers in matrix. (From Yeo after Cadiat.) collagen, chondro-mucoid and albuminoid substances. On boiling, it yields a substance known as chondrin, which on cooling turns to gelatin. FIG. 14. — White fibro-cartilage, Showing cells, a, in capsules and fibrillar matrix, b. (Yeo after Cadiat.) (6) Bone. — Bone is a dense form of connective tissue con- stituting the skeleton or framework of the body. It serves to CONNECTIVE TISSUES 17 protect vital organs in the skull and trunk and acts as levers which are worked by the muscles in the limbs. The tissue is characterized by the deposit of calcareous or lime salts within its intercellular substance, to which its well-known hardness is due. Most bones may be divided into an outer layer of compact bone and an inner layer of spongy or cancellated bone. FIG. 15. — Transverse section of compact bony tissue (of humerus). Three of the Hayersian canals are seen, with their concentric rings; also the lacunae, with the canaliculi extending from them across the direction of the lamellae. The Haversian apertures were filled with air and debris in grinding d9\vn the section, and therefore appear black in the figure, which reoresents the object as viewed with transmitted light. The Haversian systems are so closely nacked in this section that scarcely any interstitial lamellae are visible. X 150. (Kirkes after Sharpey.) Microscopically bone is seen to consist of numbers of osseous layers or lamellae, arranged as, (a) circumferential lamella which are arranged parallel to the inner and outer surfaces of the bone, (b) Haversian lamellae which are arranged concentrically around the Haversian canals and (c) interstitial lamellae, which are arranged irregularly so as to fill in the spaces which the other l8 THE ELEMENTARY TISSUES lamellae do not fill. The Haversian canals are minute longi- tudinal channels, each surrounded by its lamellae within which run still smaller longitudinal channels, called lacunae. Connect- ing the main channel and the lacunae, and radiating in all direc- tions between them are other very minute channels known as canaliculi. Each Haversian canal with its surrounding lamellae, lacunae and canaliculi composes an Haversian system. A fibrous membrane, the periosteum, forms the outer covering of all bones except when they are covered with cartilage. It consists of two layers, an outer fibrous and an inner fibro-elastic layer. However, during the period of development a third layer, the osteogenetic layer, lies to the interior. It possesses a rich blood supply which nourishes the subjacent bone, and contains 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. It may be divided into two classes: (i) Red bone marrow and (2) yellow marrow. In early childhood all the marrow in the bones is red or has a reddish tint, but in adult life we find two kinds — the red and the yellow. (i) Red bone marrow is classed as one of the blood-forming organs since it plays an important role in the formation of the blood. When stained and examined under the microscope it is found to consist of a delicate connective-tissue reticulum which supports the blood-vessels and contains in its meshes numerous cells. On the outside of the marrow, next to the bone, we find a thin fibrous-tissue coat, the endosteum, which lines the medullary cavity and extends into the larger Haversian canals. The more numerous of the cells found in the red marrow are, (a) the myelocytes, which are very numerous and contain several different varieties of cells, (b) the eosinophiles, which are few in number, but which are conspicuous by the presence of coarse granules within the cytoplasm, which are MUSCULAR TISSUES colored intensely by acid stains, such as eosin, (c) the giant cells, which are very large, but contain only one nucleus, (d) the erythroblasts, which are nucleated red blood-cells. In addition to these, the red marrow contains mast-cells, fat- cells and osteoclasts, or multinuclear giant-cells. (2) Yellow bone marrow is formed from red marrow by the infiltration of fat-cells which convert it into adipose tissue. When examined in section yellow marrow resembles ordinary fat- tissue, consisting chiefly of large com- pressed spherical fat-cells which are supported by a reticulum of connective tissue. Yellow marrow is found in all the adult long bones, except at their extremities. The Muscular Tissues. The chief characteristic of muscular tissue which distinguishes it from all other tissues is its marked contractility. This variety of tissue may be divided into three large groups: (i) Striated muscle, (2) cardiac muscle and (3) smooth muscle. (i) Striated or voluntary muscle makes up the greater part of all the skeletal muscles by means of which all voluntary movements are made. In addition to this, it constitutes the walls FIG. 16. — Two fibers of striated muscle, In which the contractile substance, m, has been rup- tured and separated from the sarcolemma, a and s; p, space under sarcolemma. (From Yeo after Ranvier.) of the abdomen, and a 20 THE ELEMENTARY TISSUES few of the muscles connected with the middle ear, tongue, pharynx, larynx, diaphragm, and generative organs. This group of muscular tissue is composed of bundles of fibers, each fiber of which is derived from a single cell which has many nuclei. Each fiber is enclosed in a thin, homogeneous, elastic membrane, the sarcolemma. The fibers are composed of a semi-fluid and viscous material which is called the muscle plasma. The muscle plasma consists of two elements, the fibrils FIG. 17. — Striated muscular tissue of the heart, Showing the trelliswork formed by the short branching cells, with central nuclei. (Y**,) and the sarcoplasm. The fibrils which are long and thread- like, running the entire length of the fiber, consist of alternating light and dark segments which fall together in the different fibrils and give the muscle its characteristic striated appear- ance. The sarcoplasm, which varies greatly in the striated muscle of different animals, fills in the space between the fibrils. From a study of comparative physiology it is assumed that the fibrils are the contractile element of the muscle fiber, while the sarcoplasm serves a general nutritive function. MUSCULAR TISSUES 21 Striated muscular tissue is very richly supplied with blood-vessels. The larger arteries and their accompanying veins enter the muscle along connective tissue septa and then break up into smaller^ branches and, finally, into a capillary network which supplies the individual muscle fibers. They are also supplied with lymphatics which occupy the clefts in the connective-tissue septa around the fibers. There are also definite lymph- vessels which accompany the blood- vessels within the muscle. This tissue is also supplied with both motor and sensory nerves, by means of which the stimuli are carried to and from the mus- cle fibers. (2) Heart muscle occupies an inter- mediate position between the striated voluntary muscle and the non-striated involuntary muscle tissue. It is charac- teristic in that it is striated and invol- untary. The following is a brief sum- mary of its chief distinguishing features: (1) Its fibers are united with each other at frequent intervals by short branches, (2) its fibers are smaller and their stria- tion is less marked than in voluntary muscle, (3) it has no sarcolemma, and (4) its nuclei are situated within the substance of the fiber and not upon it. (3) Smooth or involuntary muscle occurs in bundles and thin sheets chiefly in viscera and blood-vessels. Its general FIG. 18. — Cells of smooth muscle-tissue from the intestinal tract of rabbit. (P'rom Yeo after Ranvier.} A and B, muscle-cells in which differentiation of the protoplasm can be well seen. 22 THE ELEMENTARY TISSUES distribution may be outlined as follows: (i) It is found in the digestive tract from the middle of the esophagus to the anus. (2) in the capsule of the pelvis of the kidney, (3) in the trachea and bronchi, (4) in the ducts of glands, (5) in the gall-bladder, (6) in the vas deferens and seminal vesicles of the male repro- ductive organs, (7) in the uterus, vagina and oviducts of the female reproductive organs, (8) in the blood-vessels and lym- phatics, (9) in the iris, ciliary bodies and eye-lids and (10) in the hair follicles, sweat glands, and skin of the scrotum and in some other places throughout the body. The structural unit of smooth muscle is the fiber-cell, which is a delicate spindle with its nucleus usually situated nearer one end than the other. The nuclei of the fiber-cells are usually elongated and oval. These fiber-cells are held together by a delicate connective-tissue network which is composed of both white and elastic fibers. Smooth muscle is very poorly supplied with blood-vessels in comparison to striated muscle. The blood-vessels run along the connective-tissue septa and small branches are distributed to the fiber-cells. The lymphatics, also, follow the connective-tissue septa. The nerves which supply the smooth muscle are of sympathetic fibers. The Nervous Tissues. These tissues will be considered under the chapter on the Physiology of the Nervous System. CHAPTER III. PHYSIOLOGICAL CHARACTERISTICS OF MUSCLE. WHEN a muscle is acted upon by a weight it extends quite readily, but as soon as the weight is removed the muscle re- sumes its normal shape. This illustrates the extensibility and elasticity of muscular tissue. The muscles all over the body are in a constant state of elastic tension, which causes them to be of greater value as a support to the body skeleton. A muscle which is in a state of elastic tension contracts more readily and forcibly than one which is relaxed. Under ordinary conditions a muscle receives the stimulus which causes it to contract through its motor nerve from the central nervous system. If this nerve be cut, the muscle is paralyzed. However, it has been demonstrated that a muscle which has its nerve cut may still be made to contract by applying an artificial stimulus, as an electrical shock. But such a muscle would still have its nerve endings in the muscle undestroyed, and hence, this would not prove that the muscle has independent contractility. Still, if the nerve is severed and the nerve endings are destroyed, e. g., by a drug, we find that the muscle will still respond to an electrical stimulus. This shows that muscular tissue has independent irritability. Hence, striated muscular tissue possesses independent contractility, by which is meant that its power of shortening is due to active processes developed in its own tissue, and independent irritability, by which is meant that it may enter into contraction by artificial stimuli applied directly to its own substance. If we isolate a muscle and stimulate it, we get a simple con- traction. If the end of this muscle is attached to a lever con- 23 24 PHYSIOLOGICAL CHARACTERISTICS OF MUSCLE nected with a revolving drum, we get a simple muscle curve (Fig. 19). The time required for a simple contraction varies with the muscles of different animals, and also with different muscles of the same animal. After the muscle is stimulated (Fig. 19), an appreciable time elapses, the latent period, before it contracts, which is about 3-^ second. Then the muscle passes into the stage of contraction, during which time the lever rises. Immediately it relaxes and elongates and the lever again descends to the base line. The whole contraction occupies about T\) second. FIG. 19. — Simple muscle curve. (Halliburton.) Those factors which modify the character of a simple muscle curve, are, .(a) the strength of the stimulus, (b) the amount of the load, (c) the influence of fatigue, (d) the effect of temperature and (e) the effect of veratrine. (a) A stimulus which is just strong enough to produce a con- traction is called a minimal stimulus. As the 'strength of the stimulus is increased the amount of the contraction, which is represented by the height of the curve, is increased. This con- tinues until a certain point is reached, the maximal stimulus, PHYSIOLOGICAL CHARACTERISTICS OF MUSCLE 25 then an increase in the stimulus produces no increase in the contraction. (b) As the weight of the load is increased the contraction becomes less until a weight is reached which the muscle is unable to raise. Also, the latent period is longer with a heavy than with a light load. (c) If we apply a series of successive stimuli to a muscle we notice that at first the contractions improve with each successive stimulus which is due to the beneficial effect of contraction. Later the contractions get less and less. As the contractions get less, the period of contraction becomes longer, the latent period is increased and the period of relaxation becomes very much longer. As the period of relaxation becomes longer, the muscle fails to return to its normal length before a second stimulus arrives, so that the original base line is not reached at all. This condition is known as contracture. (d) By varying the temperature of a muscle we find that it causes a variation in the extent and duration of its contractions. Thus, by beginning at o° C. and increasing the temperature, we find that the contractions increase up to 5°-9° C. and then decrease up to i5°-i8° C. After this point is reached they again increase reaching their maximum at 26°-3o° C. This maximum is much greater than the first maximum which was reached at 5°-9° C. As the temperature is still increased, the contractions decrease rapidly until at about 37° C. irritability is entirely lost. If the temperature is increased to about 42° C. heat rigor makes its appearance due to the coagulation of the muscle plasma. (e) Veratrin is an alkaloid which exerts a peculiar effect upon the contraction of muscle. By injecting it into an animal before the muscle is removed the following effects are noted: (i) The phase of shortening is not altered, but the period of relaxation is very much prolonged, and (2) there is a secondary rise in the curve of relaxation. 20 PHYSIOLOGICAL CHARACTERISTICS OF MUSCLE Effect of Two or More Successive Stimuli. — If a muscle receives two successive stimuli a sufficient length of time apart, two curves of contraction are produced, the second being a little higher than the first (beneficial effect of contraction) . How- ever, if the second stimulus arrives before the period of relaxation is complete, a secondary rise is produced which is called superposition or summation of effects. If the two stimuli occur u FIG. 20. 7, Two successive submaximal contractions. //, A series of contractions induced by 12 induction-shocks in a second. ///, Marked tetanus induced by rapid shocks. (Landois.) close enough together, the result will be one curve which is greater than either would have produced separately. This is called summation of stimuli. If, instead of just two stimuli, a number of stimuli are applied very close together, we get the effect shown in (II). If these stimuli occur still closer together the effect shown in (III) is produced which is called tetanus. When the stimuli occur so as to allow partial relaxation between each stimulus, (II) the effect is called incomplete tetanus, but when no relaxation occurs as in (III) the effect is complete tetanus. CHAPTER IV. SECRETION. Secretion and Excretion. — Ordinarily the product of glan- dular activity is spoken of as a secretion. On the one hand, glands may take from the blood substances which are preformed in that fluid, which would accumulate and produce detrimental effects if not removed, and which are discharged from the body. On the other hand, glands may form out of materials furnished by the blood substances which are peculiar to that gland's activ- ity, which have an office to perform in the economy, which do not accumulate on removal of the gland, and which are not dis- charged from the body. The product in the first case is an excretion, in the second case a secretion. But when it comes to naming an exclusively excretory or an exclusively secretory gland, the task is found to be practically impossible. Probably the most typical excretion of the body is the urine, yet there are in the urine substances, like hippuric acid, etc., which are undoubtedly formed by the kidney, and which do not preexist in the blood. The succus entericus, e. g., would seem as typical a secretion as it is possible to find, but not infrequently it contains urea when the activity of the kidney is impaired, to say nothing, under normal conditions, of the water and salts which are taken as such from the blood. The liver is notable in its secreto- excrementitious action. While the desirability of thus separat- ing the glands into secretory and excretory and their products into secretions and excretions is granted, the impossibility of such a division is apparent. It is possible in most cases to apply the distinction to the separate constituents of the product of a particular gland, but 27 28 SECRETION not to the product as a whole. In view of these facts, attention will be given in this chapter to several glands which manifestly produce excretions as well as secretions. The action of the kidney and sweat glands is so predominantly excretory that they are treated separately. In what follows the term " secretion" cannot always be taken as meaning a true secretion, for it is customary and convenient to speak of the " secretion of urine," for example. Glands. — If we conceive of a single layer of secreting epi- thelial cells supported by a thin basement membrane, and then this structure invaginated or folded in upon itself, so that the two layers of epithelium face each other with a greater or less interval between them, with the basement membrane constitut- ing the external support for both, we will have in mind the essential structure of a gland proper. The invaginated cells are the gland cells, and the interval between the two layers of cells is the lumen. Whether the invaginated structure sends off from itself secondary or tertiary folds similar to the original, or whether the lumen of any of these folds is in the shape of a simple tube or sac, or both, is immaterial. They may all be considered as identical in nature with the original invaginat'.on and only modifications of its architecture. However, these modifications are more or less distinguished by names. Those which become complex by numerous branch- ings of the involuted tube are usually termed compound, as opposed to a single simple fold; glands are further classified, as tubular, racemose, or tubulo-racemose, according as the termination of the lumen has the shape of a tube, or sac, or both. Thus a simple or a compound gland may belong to any one of the three last-named varieties. The crypts of Lieberkuhn are simple tubular glands. The glands of Brunner are usually described as compound tubulo-racemose structures. In a compound gland that portion which communicates with the surface is called the duct and is supposed not to be con- GLAND SECRETION 29 cerned in actual secretion, but simply in carrying the product away from the secreting terminal ramifications of the subdivi- sions of the involution — which terminations are called acini or alveoli. It follows, of course, that a collection of acini may dis- charge their secretion into the main duct by a smaller duct — that is, that the gland may have various subdivisions of the duct proper. Furthermore secretions are classified as external when they are discharged upon a surface communicating with the external air, such as the alimentary canal, or skin, and internal when they are discharged upon surfaces not in communication with the exterior, such as blood-vessels. Both external and internal secretions are liquid or semi-liquid in character, for they must contain water as a vehicle for the salts and organic substances which are present in all of them and which, in fact, distinguish them from one another. Glands in general have been divided into serous and mucous by Heidenhain, according as the secreted fluid is watery and thin, or viscid and stringy from the presence of mucin. This division is further warranted by histologic differences in the cells concerned in each kind of secretion. The cells in a serous gland are small and finely granular, and are in close apposition to each other. Those of mucous glands are larger, almost square and are definitely separated. Many glands contain both kinds of cells, but since their secretion contains mucin, such glands are usually spoken of as belonging to the mucous variety. It will be seen that the salivary glands illustrate these varieties. Gland Secretion. — Underneath the basement membrane of a gland (that is, on the side opposite the epithelial cells) ramifies an abundant network of blood and lymph capillaries. This an- atomical arrangement favors osmotic transudation from the ves- sels, especially since the pressure in the vessels is normally greater than in the acini and ducts of the gland. Numerous experiments, however, prove the inadequacy of simple osmosis 30 SECRETION to explain all the processes of glandular secretion, especially those connected with the presence of organic constituents; while the undoubted presence of secretory nerves (besides the vaso-motor nerves to the vessels) would seem to give a priori evidence that the glandular epithelium takes some active part in the formation of the secretion. Such an office is granted to these cells, but whether it is of chemical, or a physical, or a "vital" character is not evident. The physiology of the salivary glands, the gastric and intestinal glands, the pancreas and liver is taken up under the chapter on Digestion in which they are vitally concerned. Sebaceous Glands. The sebaceous glands (see Hair-follicles) are chiefly associ- ated with hair-follicles and, existing wherever hair is to be found, cover well-nigh the whole cutaneous surface. They are of the simple or compound tubular type, and discharge their secretion into the hair-follicle near its outer extremity. The alveoli are lined by several layers of cuboidal epithelial cells. The cells of the layer nearest the lumen contain fatty matter, and are thought to form the secretion by breaking down and being thrown off themselves. Their place is taken by cells from the deeper layers, which undergo similar changes and disintegrate. Composition and Properties of Sebum. — Chemically sebum is largely made up of fatty matters. It also contains choles- terin, which is in combination with a fatty acid. It forms a thin coating over the cutaneous surface, accounting for the normal oiliness of the skin. It also contributes to the characteristic softness of the hairs, and prevents their breaking off from brittle- ness. Its presence over the body surface may have some influ- ence in regulating the loss of heat by evaporation. Cerumen, smegma and the secretion from the Nabothian glands are only modified forms of sebum, and the structures producing these secretions belong to the class of sebaceous glands. MAMMARY GLANDS 31 Mammary Glands. Structure. — The mammary glands are two in number in the human being, and are loosely attached to the great pectoral muscles. They are rudimentary in both sexes until puberty, and in men throughout life. At puberty the gland in the female enlarges markedly, but is never fully developed before preg- nancy. At this time the gland vesicles make their appearance, and the rudimentary ducts come to be more and more ramified. These ramifications do not reach their full development, how- ever, until lactation begins. The skin covering the areola of the nipple is dark, especially during pregnancy, and much thinner than over other parts. The dark color is due to a deposit of pigment. The mammary gland belongs to the compound tubulo-race- mose type, and consists of fifteen or twenty lobes bound together by areolar connective tissue. Each lobe is made up of a num- ber of lobules, containing the alveoli or secreting portions. The secretion from all the alveoli and lobules of a lobe converges to a single duct, which discharges its contents upon the surface of the nipple without anastomosis with any duct. There are, therefore, some fifteen or twenty ducts thus opening upon the surface. Each of them has a dilatation beneath the nipple, and it is in these sinuses largely that the milk accumulates during lactation. When lactation has ceased the ducts retract, the sin- uses disappear, the alveoli undergo retrograde changes, and the whole gland is inclined to become flabby and pendulous. It does not regain after pregnancy the firmness which character- ized it before. Secretion of Milk.— After parturition the first discharge from the gland is colostrum, a liquid resembling milk in some respects. In two or three days the true milk appears. Besides water and salts, all the constituents of milk are formed by the cells of the mammary gland. During the period of gestation the 32 SECRETION cells lining the alveoli are flat and have only a single nucleus. When they begin to secrete they increase in height, the nuclei divide and that portion of the cell toward the lumen undergoes fatty degeneration. This fatty material is extruded into the lumen and apparently constitutes a part of the secretion. The liquid constituents taken out of the blood probably hold the pro- teid and carbohydrate portions in solution, while the fatty par- ticles constitute the fat of the milk. Thus secreted, the liquid accumulates in the ducts and sinuses until removed by the infant or otherwise. The fact that the secretion of milk in woman is in- fluenced by emotions of fear, grief, etc., is strong evidence of a nervous control of the procedure, but proof of secretory fibers to the cells has not been established. The quantity of food required by the mother during the time the child is nursed is increased, but no particular kind of food seems to be especially required. The larger demand for liquids is marked, however, and when the quantity of milk is increased by a large ingestion of liquids, the solids in the secretion are not relatively diminished. Composition and Properties of Milk.— Human milk has a specific gravity of about 1030, and is not so white or so opaque as cow's milk. Besides water, its chief constituents are fats, lec- ithin, cholesterin, casein and lactose, of which the two last named are the most important. Casein is the main proteid constituent. Lactose is very abundant, and is responsible for the sweet taste and for a large part of the nutritive value of the fluid. Thyroid Gland. The thyroid gland consists of two glandular masses united by an isthmus of the same structure. It lies in front of the trachea at the lower end of the larynx. It consists of a large number of vesicles bound together by connective tissue. Each vesicle is lined by cuboidal epithelial cells, which secrete a semi-gelatin- ous substance, colloid. THYROID AND ADRENAL GLANDS 33 It has long been known that the removal of the whole thyroid gland, including the parathyroid, occasioned marked interfer- ence with nutrition and other changes, the chief of which are disturbances of muscular coordination, possibly convulsions, emaciation, apathy, and subsequent death. There is no duct connected with the gland, and the secretion is therefore an in- ternal one. Very little is known of it except that it is necessary to the maintenance of life. If a very little of the gland be left, or if, after its complete removal, a small bit of it be transplanted in some other part of the body, or if the animal be fed on the thyroid extract or the fresh gland, the characteristic symptoms do not ensue. The muscular disturbances direct the attention to the central nervous system when an attempt is made to explain the occur- rences and it is not improbable that the effect of the thyroid secretion is in some way exerted upon or through the central system. It seems generally agreed that the thyroid does dis- charge a secretion into the blood and that it is the withdrawal of some part of that secretion from the circulation which is re- sponsible for the remarkable train of symptoms sequent upon its removal. This essential constituent is regarded by some as being an agent which destroys certain toxic principles in the blood, by others as being requisite to the metabolic functions in the body without destroying anything. Baumann has isolated from the gland substance a material containing a large propor- tion of iodine, to which he gives the name iodothyrin, and it is very probable that this is one, at least of the beneficial substances in the thyroid secretion. Adrenal Glands. The adrenal gland or suprarenal capsules, rest ng upon the upper ends of the kidneys, are ductless glands whose removal is followed by weakness, impaired nutrition and disturbances in the circulation. Death usually supervenes in two to four days. 3 34 SECRETION These bodies must produce an internal secretion which is re- moved by way of the adrenal veins. It may destroy toxic sub- stances in the blood. A solution injected into the circulation certainly affects the middle wall of the vessels, causing contrac- tion, and a heightened pressure. The heart is also notably inhibited. It is not thought that the effect on the vessels is brought about through the vaso-motor nerves, but by direct excitation of the muscular substance. Little in fact is known about the secretion, except that it is necessary to life. Abel has isolated an alkaloid, epinephrine, which is claimed to be the active principle. These glands are the seat of lesions in Addison's disease, and many cases of this malady are at least favorably influenced by the use of adrenal extract. Pituitary Body. The pituitary body lying in the sella turcica on the superior surface of the sphenoid bone, also produces an internal secretion of physiological value. Its removal is regarded as causing death. Howell has shown that injection of extract from the posterior division occasions a rise of temperature and slowing of the heart. Its situation makes satisfactory experiments very difficult. Testis and Ovary. The testes and ovaries, though not probably true glands, also may produce an internal secretion of obscure physiological value. It is not essential to life. Injections of extracts from these bodies are claimed to have a remarkable stimulating effect upon the nervous and muscular systems. In mental and physical disturbances occasionally following removal of the ovaries, gyne- cologists often find administration of the ovarian extract to be beneficial. CHAPTER V. THE BLOOD. General Characteristics. — The blood is a red, opaque and viscid fluid having a characteristic stale odor and a salty taste. The blood is heavier than water, having a specific gravity in the adult male of 1.041 to 1.067, tne average being about 1.055. The reaction of the blood is neutral. The nature of the diet, either meat or vegetable, causes this neutrality to turn to either an acid or an alkaline reaction. The blood temperature is that of the body. In the periphery it is about 99° F.; in deeper vessels it varies from 100° F. to 107° F. ; and in the hepatic veins it is about 107° F. The Function of the Blood.— The most important physio- logical functions of the blood are: (i) It carries to the tissues food-stuffs after they have been digested, (2) it transports to the tissues oxygen which it has absorbed from the air in the lungs, (3) it carries off from the tissues the waste products of metabolism, (4) it transmits the internal secretions of glands to the differ- ent parts of the body and (5) it aids in equalizing the body temperature. Quantity and Distribution of the Blood. — The quantity of the blood in the body is estimated at about 7.5 per cent, of body weight. A man weighing 150 pounds has a fraction over eleven pounds of blood, which is about one-twelfth of the body weight. The distribution is generally given as, one-fourth in the heart, large arteries, lungs, and veins; one-fourth in the liver; one- fourth in the muscles attached to the skeleton; and the other one-fourth variously distributed to the other organs of the body. 35 36 THE BLOOD Composition of Blood. The blood is composed of a fluid part, the plasma, in which float a great mass of small bodies, the blood corpuscles. The plasma may be denned as the blood minus the corpuscles. These are of three varieties: (i) The red corpuscles, or erythrocytes, (2) the white corpuscles, or leucocytes and (3) the blood platelets, or thrombocytes. The plasma is a thin slightly yellowish fluid with a specific gravity of 1.026 to 1.029. Hence, the bright red color of the blood is due to the red corpuscles which are held in suspension in the plasma. The proportion of plasma to cor- puscles is about two to one (Ho well). Plasma. Chemically, plasma is composed of water and about 10 per cent, of solids, together with oxygen, carbon dioxide and nitrogen. A thousand parts of plasma contain: (Halliburton.) Water 902 . 90 Solids 97 . 10 Proteins: i. Yield of fibrin 4.05 2. Other proteins 78 .84 Extractives (including fat) 5-66 Inorganic salts 8.55 The most important solids are the proteins, the chief of which are: (i) Fibrinogen, (2) serum globulin, and (3) serum albumin. Fibrinogen belongs to the globulin class of proteins, but differs from serum globulin and may be separated from it. Fibrinogen is the least abundant of the proteins. Serum globulin and serum albumin form the chief proteins of the plasma. They may be separated by the use of neutral salts. The extractives are substances other than proteins which may be extracted from the dried residue by the use of water, alcohol, or ether. The principal extractives are fats, sugar, lecithin, cholesterin, lactic acid and urea. RED BLOOD CORPUSCLES 37 The most abundant salt of the plasma is sodium chloride. It forms from 60 to 90 per cent, of the total mineral matter of plasma. Potassium chloride is present in much smaller amount. Other salts are the carbonates, sulphates and phosphates. Corpuscles. Suspended in the plasma of the blood we have a cellular formed element moving and functionating. This element is the corpus- cular element and is composed of (a) the red blood corpuscles, (b) the white blood corpuscles, (c) the blood platelets. (a) Red Blood Corpuscles or Erythrocytes. General Description. — The red blood corpuscles are circular, bi-concave discs with rounded edges. They are from 7 to 8 micra in diameter and 2 micra in thickness, so can only be seen with the aid of the microscope. When looked at singly they appear to have a yellowish-green color, collectively they are red. Number. — In males there are about 5,000,000 red cells to a cubic millimeter; in females about 4,500,000. The proportion of reds to whites is one white to every 500 red. Origin and Destruction. — The red corpuscles are continu- ally being destroyed in the body. It appears that this destruction occurs principally in the liver. As the red cells are thus de- stroyed it is natural to look for a place of manufacture. In the embryo we find that this generation takes place in the liver and in the spleen; in the adult it seems that the manufacture takes place only in the red marrow of the bones. The red corpuscles are formed from colored, nucleated cells called hemoblasts. Constituents of Red Blood Corpuscles.— The red blood cor- puscles are made up of 65 per cent, water and 35 per cent, solids. The principal solid constituents are (a) hemoglobin (oxyhemo- 3» THE BLOOD globin) 87-95 Per cent.; (b) stroma, composed of fat, lecithin, and cholesterin; (c) and salts, principally potassium chloride, and potassium phosphate. Hemoglobin. — Hemoglobin is the coloring matter of the red cells, and is composed of (i) hematin, a pigment containing iron; and (2) globin, a proteid. Hemoglobin is of great physio- logical importance, because of its ability to unite with oxygen and thus form oxyhemoglobin. By it the blood carries its FIG. 21. A, human colored blood corpuscles — i, on the flat; 2, on edge; 3, rouleau of colored corpuscles. B, amphibian colored blood corpuscles — i, on the flat; 2, on edge. C, ideal transverse section of a human colored blood-corpuscle magnified 5,000 times linear — a, b, diameter; c, d, thickness. (Landois.) oxygen from the lungs to the tissues. It also unites to some extent with carbon dioxide and it is thus that carbon dioxide is brought from the tissues. We find oxyhemoglobin chiefly in the arterial blood, while in venous blood we find both hemoglobin and oxyhemoglobin. In asphyxiated blood we find only hemoglobin. The stroma is the colorless framework of the corpuscles after BLOOD PLATELETS. 39 the coloring matter is dissolved out. The hemoglobin is ensnared in the stroma. (b) White Blood Corpuscles or Leucocytes. General Description. — The white blood corpuscles or leu- cocytes are large, colorless, nucleated cells with no general form, but which are capable of changing their form by ameboid movement. Number. — The number of leucocytes varies from seven to ten thousand per cubic millimeter. Function. — The white corpuscles are not under the control of the central nervous system, but are controlled by some chemo- taxic force. They are able to go and come by ameboid move- ment through the stromata of capillary walls and wander here and there in the tissues. It is this that gives them their name of wandering cells. White blood corpuscles are of importance from a physiological standpoint, because of this ability to wander. They can trans- fer undissolved substances from one part of the body to another and can destroy and remove foreign substances and hurtful microorganisms. The power they have of ingesting foreign substances is called phagocytosis. They will migrate in large numbers and sur- round a foreign object and endeavor to remove it from the tissue. They have the power of liquefying tissue and it is this lique- fied tissue mixed with the dead bodies of white corpuscles that is known as pus. (3) Blood Platelets. These are colorless discs about one-third to one-fourth the size of red blood corpuscles. Some claim for them the full value of blood cells, while some claim they are the nuclear re- mains of destroyed leucocytes. They are about 635,000 to one 40 THE BLOOD cubic millimeter of blood. As to their function little is known. Some claim they play an important part in the coagulation of the blood. Nothing definite is known of their origin. The Coagulation of the Blood. When blood is allowed to stand after being shed it rapidly becomes more viscous and later sets into a firm jelly. Later, as the fibrin contracts, a clear straw colored fluid, the serum, is set free. The formation of fibrin is the essential factor in coagulation. It is contained in the plasma in the form of fibrinogen. The relation of plasma, serum and clot is shown by the fol- lowing table: ( Serum Plasma < _., . ^ Corpuscles • J CHAPTER VI. THE CIRCULATION OF THE BLOOD. General. — We have seen that the composition of the blood fits it for its function of carrying food stuffs to the tissues and remov- ing the products of combustion; but, for the blood to exercise these offices, it is necessary that it be in communication with the outside world and the tissues. The movement it makes through its network of vessels in order to carry products from the exterior to the interior and from the interior to the exterior is what is meant by circulation. Pulmonary and Systemic Circulation. — Two systems of circulation are generally distinguished. The first is the pul- monary, and is the circulation of the blood through the lungs in order to get rid of carbon dioxide and to get a fresh supply of oxygen by aeration. The second is the systemic and is the circulation through the great masses of body tissue in order, by means of the lymph, to supply the tissues with different solid, liquid, and gaseous nutritive material and take from the tissues the products no longer needed but which must be eliminated. These systems are also called respectively the lesser and greater circulation. Discovery. — The circulation of the blood was an unknown fact up to 1628 when the discovery of its movements was made and proved by Sir William Harvey, an English physician promi- nent in his time and now famous for this discovery. The Circulatory Apparatus. — The blood circulates through a series of closed tubes known as blood-vessels, which divide up, ramify, and go to all parts of the body. These vary from 42 THE CIRCULATION OF THE BLOOD large, macroscopical vessels to tiny, little, hair-like tubes that cannot be seen with the naked eye. The central organ of the circulatory system is the heart. From this lead off the arteries, these in turn connect with the capillaries, and these with the veins, which lead back to the heart. I. THE HEART. The heart is a hollow, muscular organ divided by a muscular septum into two distinct compartments designated for conveni- ence, the right and left heart. The right side, and similarly the left, is divided by a muscular septum into two chambers, the upper called the auricle and the lower the ventricle. There is an opening between the right auricle and the right ventricle and one between the left auricle and the left ventricle and each open- ing is guarded and can be closed by a thin membranous flap called a valve. Situation. — The heart is located in the thoracic cavity be- hind the sternum. It is placed in a diagonal position and its base is in the middle line and looks backward, upward, and to the right. Its apex is three inches to the left of the median line, a half inch internal to the nipple, and in the fifth intercostal space. Covering and Lining.— A serous sac, called the pericardium, covers the heart. It hugs the muscle of the heart closely, com- pletely enveloping the organ, then turns back on itself leaving a space between the outer layer and the layer next to the muscle. In this space is a fluid which acts as a lubricant. The heart is lined by a membrane called the endocardium, which is composed of epithelial tissue. Structure.— The muscle of the heart is striated, but contrary to the usual rule, is involuntary in its action. The muscle fibers run circularly, obliquely, and some in the form of the figure eight, thus giving the power to contract and pump the blood on into the circulation. THE HEART 43 Contraction. — The physiological contraction of the cardiac muscle is called systole, the relaxation is called diastole. The contraction of the heart starts at the mouth of the veins and, with a uniform rhythm glides along through the auricles and along to the ventricles, each part relaxing as the rhythmic con- traction passes on. The whole time of contraction, from one FIG. 22. — Scheme of cardiac cycle. The inner circle shows the events which occur within the heart ; the outer the rela- tion of the sounds and pauses to these events. (Kirkes after Sharpey and Gairdner.) beginning in the veins to another beginning, is called the cardiac cycle. It lasts about .86 second. The cycle may be divided thus; the auricles contract (systole) and ventricles are relaxed (diastole) which occupies .16 second; the ventricles contract (systole) and the auricles are relaxed (diastole) and this occupies .3 second; both auricles and ventricles then rest and this occupies .4 second. Number of Beats. — In an adult the heart beats on an average of 72 times per minute, in children it is higher. The frequency of beat is influenced by age, sex, disease, drugs, physical causes and digestion. 44 THE CIRCULATION OF THE BLOOD Valves and Openings. Right Auricle. — Leading off from the right auricle anteriorly and superiorly is a sinus that bears the name of the auricular appendix. It is a little hollow pouch capable of distention with blood. Opening into the right auricle we find the coronary veins, the two venae cavae, and the auriculo-ventricular opening. Guard- ing these openings are valves to prevent the backward flow of the blood current. Right Ventricle. — Opening into the right ventricle are the pulmonary artery and the right auricle. The tricuspid valve guards the auriculo-ventricular opening. It is composed of three triangular shaped membranes attached to the base of the circumference of the opening and the apices of the triangles coming together when closed. The semi-lunar valves guard the pulmonary opening. They are three entirely separate segments of semi-lunar shape and are attached by their long curved margins to the circumference of the artery just where it springs from the muscular substance of the ventricles. Left Auricle. — Like the right auricle, this cavity has a small sinus leading off from it anteriorly and superiorly — the auricular appendix. The openings into the left auricle are the four pul- monary veins and the left ventricle. Left Ventricle. — This ventricle has the thickest walls and does the most work of any of the chambers of the heart, because it forces the fresh arterial blood out into the aorta and thence through the entire systemic circulation. The aorta and the left auricle open into this ventricle. The aortic semi-lunar valves guard the aortic opening. They are three distinct semi-lunar shaped membranes to close the aortic opening at the end of the systole. The mitral or bicuspid valve closes the left auriculo-ventricular opening. It is some- VALVES AND OPENINGS 45 what like the tricuspid except that it has only two flaps instead of three. Functions of Valves. — The valves are arranged at the open- ings of the different chambers of the heart so the blood will be forced in a constant direction. When the auricles are at systole the auriculo-ventricular valves are open thus letting the flow of blood go from auricles to ventricles; but as soon as auricular diastole and ventricular systole begin these valves shut and the blood is kept from flowing backward into the auricles. Then the semi-lunar valves are open and the blood is forced into the aorta and pulmonary artery. When ventricular diastole begins these semi-lunar valves closed and thus blood is prevented from running back into the heart from the arteries. Work of the Heart. — The work done by the heart is equal to the weight of a column of blood multiplied by the height or dis- tance to which this column is carried by the heart force. The column of blood is that amount that is sent by a single contrac- tion of the heart and the height to which it is carried is equal to the pressure in the aorta and pulmonary arteries. The amount of blood thrown into the aorta at each con- traction of the ventricles weighs about 87 grams (about 3 oz.) and the height to which it is forced is about 1.5 meters or 5 feet in man. In estimating the work of a machine the English express the result in foot pounds. The French in grammetres. A foot pound is -the energy expended in raising a unit weight (i Ib.) through a unit distance (i ft.). A grammetre is the force ex- pended in raising one gram one meter. Thus the work of the left ventricle at each contraction is 130.5 grammetres (or 15 foot pounds). Add 45 grammetres as the work done by the right ventricle in contracting. If the heart beats 72 times per minute it will, in twenty-four hours, do 18,000 kilogramme-metres of work. 46 THE CIRCULATION OF THE BLOOD Sounds of the Heart.— Listening to the heart's action through the thoracic wall we hear two distinct sounds. The first is a slightly elongated sound and comes immediately after the beat of the radial pulse. It is characterized by the syllable lub. The cause of this sound is supposedly the closure of the auriculo- ventricular valves combined with the sound made by the con- tracting muscle. It can best be heard over the apex of the heart. The second sound is shorter and sharper than the first and is heard just before the impulse of the radial pulse. It is char- acterized by the shorter syllable dap. The cause of this sound is supposedly the closure of the aortic semi-lunar valves along with those of the pulmonary artery. It is best heard in the right second intercostal space, as the aortic current transmits it. Certain diseases affect the heart valves and the sounds then depart from the normal. Thus it is of importance to know the cause and sound of the normal vibrations so as to detect the diseased conditions. Heart Innervation.- The nerves that inhibit the action of the heart are the two vagi ; cutting these results in an increase of the frequency of the heart beats. The nerves that accelerate the action of the heart are the nervi accelerantes, which are branches of the sympathetic system. Stimulation of these causes increase in force and fre- quency of heart beats. II. CIRCULATION IN BLOOD-VESSELS. Taking the heart as a central station for supplying force, we find the blood current constantly going from a place of higher pressure to a place of lower pressure. The highest pressure is in the muscular center, the heart. Blood-vessels connect with both auricles and ventricles. Those CIRCULATION IN BLOOD-VESSELS 47 connecting with the ventricles and carrying blood away from the heart are called arteries and the pressure in these is high, but lower than in the heart. Those vessels connecting with the auri- cles and carrying blood back to the heart are called veins and the pressure is lowest of all in these. The minute vessels that connect the arteries and veins and collect waste from and supply nutritive material to the lymph stream are called capillaries. The pressure in these is lower than in the arteries but higher than in the veins. The blood is thus kept in motion, constantly going from place of higher to lower pressure. FIG. 23. — Tracing of blood The completed circulation is thus: — pressure taken with Prick's (Beginning with the right auricle of manometer- (Y™^ the heart.) The two venae cavse pour venous blood into the right auricle and it in turn empties its contents into the right ventricle. From here the blood is driven into the pulmonary artery (carrying venous blood) to be aerated in the lungs. From the lungs it comes by pulmonary veins (carrying arterial blood) to the left auricle. This is the lesser or pulmonary circulation. From the left auricle the blood goes into the left ventricle and from here it is forced into the aorta and thus into the systemic arteries, then through the capillaries to the veins and back by means of the venae cavae into the right auricle. The complete cycle in man takes about twenty-two seconds. STRUCTURE OF THE BLOOD-VESSELS. Arteries. — The arteries have three coats: (i) the external coat called the tunica adventitia, which is composed of fibrous tissue with a little plain muscular tissue; (2) middle coat or THE CIRCULATION OF THE BLOOD tunica media, composed of yellow, elastic tissue; and (3) the inner coat or tunica intima, composed of epithelium on base- ment membrane. Veins. — The veins also have three coats, the external, middle and internal, as the arteries; but the middle coat is composed chiefly of inelastic, fibrous tissue. Thus the veins lack the elasticity and contractility given to the arteries by the middle coat. The Capillaries. — As the arteries get smaller we find them still com- posed of the three above named coats. Finally, though, in the minutest vessels we find only the innermost layer remaining. These one-coated vessels are the capillaries, and they have only one layer of epithelial cells on a basement mem- brane. This is in order to render possible the interchange of material between the blood current and the lymph stream, so the tissues may be nourished and the waste prod- FIG. 24 -Scheme of the uctg removed> circulation. a, right, b, left, auricle; A, right, B, left, ventricle; i, pulmonary IMPORTANCE OF ARTERIAL ELASTICITY. If an amount of fluid corre- sponding 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 by the liquid injected ; but if a artery; 2, aorta; i, area of pulrno nary, K, area of systemic, circula- tion; o, the superior vena cava; G, area supplying the inferior vena cava; u; d, d, intestine; m, mesen- teric artery; q, portal vein; L, liver; h, hepatic vein. (Landois.) IMPORTANCE OF ARTERIAL ELASTICITY 49 like amount of fluid be allowed to escape at the other end the tube will resume its original caliber. Thus the pulse volume enters with much force the aorta or pulmonary artery; the artery is very elastic and expands under this influence, but immedi- ately recoils with a great pressure on the contents. The pressure tends to force the blood along the vessel in both directions, but its return into the ventricle is effectually prevented by the close of the semi-lunar valves. Consequently it can go only toward the periphery. FIG. 25. — Transverse section of part of the wall of the posterior tibial artery. (Man.) (From Yeo after Shafer.} a, endothelium lining the vessel, appearing thicker than natural from the contrac- tion of the outer coats; b, the elastic layer of the intima; c, middle coat composed of muscle fibers and elastic tissue; d, outer coat consisting chiefly of white fibrous tissue. 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 carotids the flow has resumed a remittent character. The smaller the vessel the nearer the flow becomes continuous until this condition is established in the capillaries. It is the elastic coat of the arteries that allows them to expand and then contract on the contents forcing them onward. Furthermore it is this elasticity that causes the inter- mittent and remittent flow to become continuous. So the function of the elastic coat is two-fold; first, it forces the blood current continuously toward the periphery, and second, it is chiefly the cause of the change from an intermittent flow to a constant flow, which is of so much importance in the capillaries. 4 50 THE CIRCULATION OF THE BLOOD Rate of Flow. — The velocity of the blood current is equal to the volume flowing through a determined section in one second divided by the cross section. The rate is determined by the pressure, the friction in the vessels, and the cross section of the vessels. The combined cross section of the capillaries is greater than the combined cross section of the arteries or the veins, so the rate of flow must be greater in the arteries and veins than in the cap- illaries. The friction is greater in the smaller vessels than in the larger which retards the flow. The pressure is greater in the arteries than in the capillaries and veins. From these facts it is evident that the velocity is greater in the arteries than in the capillaries and veins, but increases in the veins as compared to the capillaries. In the large arteries the rate is 200-400 m.m. per second, in the capillaries, 6-8 mm. and in the large veins it is but little less than in the arteries. Valves in the Veins. — At frequent intervals in the course of the veins are found small folds of membrane protruding into the lumen of the vessels. The flow of the blood in the veins is more sluggish than in the arteries, because, as we have seen, the pres- sure lessens in the veins while gravity and friction tend to cause a stoppage. These protruding folds of the mucous membrane or valves found in the veins aid in the circulation by overcoming gravity and preventing a backward flow of blood, by holding the blood until a fresh impulse can impel it forward. They are found in pairs and are most abundant in the veins of the extremities where gravity impedes the onward flow of the current. Capillary Importance. — The capillaries are the smallest blood-vessels and the most important as to function. Being of only one thickness of epithelium and in direct touch with the lymph flow, we can readily see that the food products brought by the arterial blood can be exchanged here for waste brought bv INNERVATION OF VESSELS 51 the lymph. The flow in the capillaries is constant, as we have seen, and we can see the importance of this when we take into consideration how rapidly the tissues use oxygen and how necessary is a constant increasing supply, and how essential it is to remove the carbon dioxide poisons. Innervation of Vessels. — The blood-vessels are controlled by the sympathetic nervous system by means of the vaso-motor FIG. 26. A, vein with valves open. B, with valves closed; stream of blood passing off by lateral channel. (Kirkes after Dalton.) nerves. These compose both the vaso-constrictors, or the nerves causing the vessels to contract, and the vaso-dilators, those causing the vessels to dilate. The entire physiological dis- tribution of blood is regulated by the vaso-motor system of nerves. It is by their means that the blood is increased to all parts of the body where physiological activity is going on, as when the gastro- intestinal tract is active during digestion, a muscle in motion, or glands in activity. Paralysis of the vaso-constrictors causes blushing, paralysis of the dilators causes pallor as from fright. 52 THE CIRCULATION OF THE BLOOD Outside influences will cause the constrictors to act, as cold; while alcohol will cause dilators to act and paralyzes the constrictors. The chief vaso-motor center is in the medulla oblongata, while subordinate centers exist in the cord. The vaso-motor fibers reaching the vessels proceed from ganglia in the sympa- thetic system, but these ganglia are influenced by the cells in the vaso-motor center. FIG. 27. — Capillaries. The outlines of the nucleated endothelial cells with the cement blackened by the action of silver nitrate. (Landois.) Amount of Blood Important.— When there are small losses of blood from slight injuries the entire vascular system contracts and the current supplying this diminished area is sufficient; but at times the loss of blood is so great that the amount remain- ing is not sufficient to carry on a complete circulation. Unless remedied this results in death. In such cases of great loss the deficit may be supplied by a normal salt solution, thus giving an amount of fluid sufficient to maintain the heart action. But in PULSE 53 cases where as much as two-thirds of the blood is lost, the injec- tion of fluid does no good. The amount to cause the heart's action to continue may be supplied, but the amount of hemoglo- FIG. 28. — Interior of right auricle and ventricle exposed by the removal of a part of their walls. (From Yeo after Allen Thompson.} inner wall flaps of been cut; 7, on aorta near the ductus arteriosus; 8, 9, aorta and its branches; 10, u, left auricle and ventricle. As- i, superior vena cava; 2, inferior vena cava; 2', hepatic veins; 3, 3', 3", inr of right auricle; 4,4, cavity of right ventricle; 4', papillary muscle; 5, 5', s"> tricuspid valve; 6, pulmonary artery in the wall of which a window has be bin necessary for life is lost and this cannot be supplied, phyxiation is the result. Pulse.— If a finger be placed on any artery in the body there 54 THE CIRCULATION OF THE BLOOD will be transmitted to it a perceptible impulse. This impulse is what is called the pulse. It is caused by the force of the heart's FIG. 29. — The left auricle and ventricle opened and part of their walls removed to show their cavities. (From Yeo after Allen Thompson.} i, right pulmonary vein cut short; i', cavity of left auricle; 3, 3", thick wall of left ventricle; 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. action against the elastic arterial wall, and the subsequent con- traction of this wall against the current it contains. The impulse is an index to the condition of the circulation. PULSE 55 Its frequency normally in an adult is about 72 times per minute, in children it is higher, and it is more frequent in woman than in man. Its frequency is affected by age, sex, exercise, disease, drugs, and psychical causes, as fear, joy, sorrow, etc. We feel FIG. 30. — 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 g, paoillary muscles; e, e' and/, attachment of the tendinous cords. (From Yeo after Allen Thompson.) the pulse to learn several things: — (i) Its frequency, which tells how many times the heart is beating. (2) Its tension, which is the state of the arterial walls and is 50 THE CIRCULATION OF THE BLOOD the resistance offered in peripheral vessels. We judge the ten- sion by the force necessary to obliterate the impulse. (3) Regularity, which tells whether the heart is regular in either its force or rhythm. (4) Its strength, which tells as to the force with which the heart is acting. (5) Its length, whether the beat is long or slow and continuous. FIG. 31. — Dudgeon sphygmograph. (6) The condition of the vessel wall, whether sclerotic or not. In the study of the pulse an instrument called the sphygmograph is used, which receives the impulse from a beating artery and transmits it by means of a finely adjusted lever to a smoked sur- face of paper. Thus a graphic representation of the impulse is given, the height to which the writing end of the lever goes denot- ing the force of the impulse of the heart beat at the time of the writing. THE LYMPH 5 7 THE LYMPH. The lymph is a clear colorless fluid contained in the lymphatic vessels and tissue spaces. It resembles plasma in general appearance 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 irregularly shaped spaces between them. These spaces communicate with each other and finally converge to the lymph capillaries. 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 (lperivascu!ar lymph canals." These likewise pass on to the lymph capillaries proper. (3) The large serous cavities, like those lined by the peritoneum, pleura, tunica vagin- alis, 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 vertebra. This pouch receives the 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 THE CIRCULATION OF THE BLOOD FIG. 32. — 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 receptaculum 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. (Yeo.) THE LYMPHATIC GLANDS 59 out of the vascular capillaries and thus collect fluid from well- nigh every tissue in the body. The structure of the lymph-vessels 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. — All 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 qfferentia. 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 materials — even to their own hurt — is a common patho- logical occurrence. Properties and Composition of Lymph. — Lymph is a com- paratively clear liquid containing leucocytes. After meals the color becomes whitish from the admixture of chyle, and numerous fat droplets are present. Neither red corpuscles nor platelets are thought to be found in lymph except accidentally. The specific gravity is lower than that of the blood. Lymph coagu- lates when drawn, since the fibrin factors are present; but the 60 THE CIRCULATION OF THE BLOOD process is less prompt and the clot is less firm than in the 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 tissues 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 different from those of plasma, except, of course, when intestinal digestion is in progress and chyle is introduced into the lymphatic circula- tion. 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 excrementi- tious 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. 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- THE FLOW OF LYMPH 6 1 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," pres- sure 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 pres- sure, 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 ac- cumulates in the spaces and in the smaller lymphatics untU 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 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 circulation. 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 injected into the blood 62 THE CIRCULATION OF 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 gravity 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 con- stituents 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 lacteals 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 VII. THE PHYSIOLOGY OF DIGESTION AND ABSORPTION. FOODS. IT is evident that all the tissues of the body are continually undergoing "physiological wear" — that the materials of which they are intrinsically composed are being changed into effete matter and discharged from the system. This is a process going on in the substance of every cell in the body, and obviously, for these cells to continue to live and functionate, there must be a continual appropriation of new matter to take the place of the materials which have served their physiological purpose, and are of no further value to the body. This supply of material is made directly to the tissues by the blood, but lest this fluid be impover- ished, it must in turn be furnished with an approximately constant quantity of nutritive matter. The ultimate source of that matter is in the food which we eat. However, it must pass through the processes of digestion and absorption before it can be utilized by the tissues. This conception of a food must be understood to embrace all substances contributing, either directly or indi- rectly, to body nutrition, including, therefore, the oxygen of the air as well as all articles usually classed as drinks. An animal whose weight remains about the same must eat and digest a certain quantity of food to keep up the body temperature, to supply mechanical energy, and to repair the wastes which are continually going on in the body. An animal which is growing and increasing in weight must eat enough not only to supply the demands just mentioned, but also to form the new tissue. The articles we eat, besides being largely insoluble, differ very materially in their composition from any substances found as 63 64 THE PHYSIOLOGY OF DIGESTION AND ABSORPTION parts of the body tissues. Even those undigested substances most closely resembling living tissue will not be utilized by the cells when presented to them by being injected into the blood. All the articles which we use for food must undergo a special process, called digestion, before they can be absorbed by the tissues. Seat of Hunger. — Food is taken into the body in obedience to an expressed want on the part of the system. The desire for food — the sensation of hunger — is referred, in a rather indefinite way, to the stomach. That sensation is ordinarily satisfied by the introduction of food into the stomach. However, this does not necessarily mean that its seat is in that organ, since removal of the stomach by no means prevents hunger. But, if nutritious material be introduced in sufficient quantity into the circulation, as by rectal enemata, hunger is relieved. The true seat of this sensation is undoubtedly in the cells themselves, it being simply a call from them for more material to take the place of their worn-out constituents. Cold weather demands an increase in the amount of food, as also do physical and psychical activity, certain drugs, etc. Seat of Thirst. — The demand of the cells for water is referred to the fauces and throat, but this is no more the seat of thirst than is the stomach of hunger. The taking of water into the mouth alone will not quench thirst, except in so far as absorp- tion may take place from its mucous membrane. But, if water in sufficient amount be placed into the circulation in any way satisfaction ensues. Next to the demand for oxygen, that for water is the most imperative which comes from the tissues ; that is, they can live much longer without solid food than without water. The amount necessary is manifestly subject to many con- ditions, such as external moisture and temperature, exercise, etc. Classification of Foods. — A very large number of substances are taken into the alimentary canal as food; but examination reveals that all such materials contain one or more of a very few classes of food stuffs. These may be divided as follows: FOODS 65 I. Water. II. Inorganic or mineral salts. III. Carbohydrates. IV. Fats. V. Proteids. I. Water is scarcely looked upon as food in the common acceptation of the term, but it is quite as necessary to cell life as any of the other classes. It is found in all foods and in all tissues and fluids of the body. It forms about 70 per cent, of the entire body weight and acts as a solvent upon various ingre- dients of the food, liquefying them and rendering them capable of absorption. II. The mineral salts which are chiefly necessary for nutrition are: Of sodium and potassium. Chlorides Phosphates Sulphates Carbonates Phosphates j . > Of calcium and magnesium. Carbonates J Of these salts, sodium chloride, or common table-salt, is the most important and abundant in the foods we eat. It is present in nearly all the tissues and fluids of the body, especially the blood. Of the other salts, those of calcium exist in the largest quantity in the body. They are especially important on account of the part they play in the formation of the bones, teeth and cartilages. The remaining salts exist in larger or smaller quantities in the tissues and fluids of the body. III. The carbohydrates include principally the starches and sugars. They are of definite chemical composition containing carbon, hydrogen and oxygen, but no nitrogen. The hydrogen and oxygen which they contain are always in the proportion to form water, i. e., two atoms of hydrogen to one of oxygen. The 5 66 THE PHYSIOLOGY OF DIGESTION AND ABSORPTION starches are found chiefly in wheat, corn, oats and other grains; also in potatoes, peas, beans and in the roots and stems of many plants, and in some fruits. Starch is found in a pure state, as a white powder, in arrowroot and corn-starch. The sugars are of several kinds, the principal being: cane sugar, beet sugar, maple sugar, grape sugar which is found in grapes, peaches and other fruits, and malt sugar which is obtained from malt. These are all obtained from vegetable tissue, however a few are found in or formed by the animal organism, as glycogen, dextrose and lactose. They are the cheapest foods from financial and digest- ive standpoints and constitute the main bulk of articles eaten. They contain more oxygen than do the fats, and are more easily oxidized and converted into heat and muscular energy. In fact, their great physiological value lies in the ease with which they are burned up in the body. They furnish the main part of the fuel necessary to the running of the animal mechanism. They may also be converted into fatty tissue by the body. IV. The fats are ingested with both animal and vegetable diets. They are compounds of carbon, hydrogen and oxygen. The principal fats are stearin, palmatin, margarin and olein. These exist in varying proportions in the fat of animals, in the various vegetable oils and in milk, butter, lard and in other foods and vegetable substances. The fats contain no nitrogen, and, like carbohydrates, their great physiological value lies in the fact that they are destroyed in the organism to produce energy, whether in the form of heat or muscular exercise. They are handled and converted less readily by the system than the carbohydrates, and consequently tax the digestive powers more. But it is found that, weight for weight, they are more efficient in the production of energy than are the carbohydrates. They also furnish fuel for the running of the body mechanism. V. The proteids form a large part of all living organisms and are absolutely necessary to animal life. They are very stable compounds and are found in both animal and vegetable foods. FOODS 67 They contain carbon, hydrogen, oxygen and nitrogen, together with, usually, a small quantity of sulphur and phosphorus. They occur in the form of casein in milk and cheese, myosin and syntonin in muscle, vitellin in the yolk of eggs, glutein in flour, legumin in peas, beans and lentils, and in some other forms. Proteids may be used by the body to produce heat and energy, but being more stable in composition than carbohydrates and fats, they are more often used to build up tissue. In fact the proteids are absolutely essential to life while this is not true of carbohydrates and fats, since the proteids must be used to build up new cells to take the place of those being constantly worn out and eliminated. The animal foods which are richest in proteids are lean meat, milk, eggs, cheese and all kinds of fish, while the vegetable are wheat, beans, peas and oatmeal. It has been found that the animal proteid foods are split up and digested much more easily than are the vegetable. Hence the great majority of the people rely upon the animal foods for their supply of proteid material which is necessary to life. The composition of a few of the more important articles used as food is shown by the following tables.* Milk: Woman, Cow, Per cent. Per cent. Protein (chiefly caseinogen) 1.7 3.5 Butter (fat) 3.4 3.7 Lactose 6.2 4.9 Salts o . 2 0.7 Eggs: Total amount of solid 13 .3 per cent. Protein 12.2 per cent. Sugar 0.5 per cent. Fats ) Lecithin | Traces. Cholesterin J Inorganic salts 0.6 per cent. *These tables are taken from Halliburton's Handbook of Physiology. 68 THE PHYSIOLOGY OF DIGESTION AND ABSORPTION Meats: Ox. Calf. Pig. Fowl. Pike. Water 76.7 75.6 72.6 70.8 79.3 Solids 23.3 24.4 27.4 29.2 20.7 Proteins 20.0 19.4 19-9 22.7 18.3 Fats 1.5 2.9 6.2 4.1 0.7 Carbohydrates 0.6 0.8 0.6 1.3 0.9 Salts 1.2 1.3 i.i i.i 0.8 Vegetable Foods: Wheat. Barley. Oats. Rice. Peas. Potatoes. Water 12 6 T? 8 12 ^ 17 T 14 8 76 .0 Protein Fat .... 12 I •4 A ii 2 .1 2 IO e • 4 2 n o 9 n 23 I •7 6 2.0 O 2 Starch 67 o 6/| Q C7 8 76 B 40 ?. 20 .6 Cellulose . . . 2 r f. 7, II 0 o 6 7 e O 7 Mineral salts . . I 8 2 .7 3 0 T 0 3 .T I O DIGESTION. Object. — Digestion is largely a chemical process. Certain physical phenomena are auxiliary. The foods not yielding en- ergy are not affected in a chemical way by digestion. They are simply dissolved, if not already in solution, and are discharged from the body in the same condition in which they entered. But the other classes of food must either be separated from innutri- tious substances with which they enter, or undergo certain changes themselves, or both, before they can be absorbed and assimilated. This necessitates a complicated digestive apparatus and the subjecting of different classes of food to different diges- tive fluids and other gastro-intestinal influences. The object of digestion is therefore twofold, first, to convert the foods into soluble materials and, second, to bring about such changes in their composition as will insure their absorption and appropria- tion by the tissues. Enzymes. — The chemical changes taking place in digestion are of a peculiar nature, in that they are effected largely by the presence of substances known as enzymes, corresponding in an DIGESTION 69 obscure way with ordinary chemical reagents. These have been called unorganized or unformed ferments, to distinguish them from such organized ferments as bacteria, yeast, fungi, etc. They are not themselves possessed of any vital activity, though formed in living organisms, like plants or animals. They are of indefinite chemical composition, contain nitrogen and are supposed to be of proteid structure. The characteristic point in their action has been supposed to be that they produce a chem- ical change without themselves being affected by that change. This is doubtless practically true, but it is found in experimental work that "a given solution of enzyme cannot be used over and over again indefinitely." It finally loses its identity. According to the foods on which they act and the effects they produce, enzymes are classified as: (i) Proteolytic enzymes, which convert proteids into soluble peptones; examples are pepsin and trypsin. (2) Amylolytic enzymes, which convert starches into sugar; examples are ptyalin and amytopsin. (3) Fat-splitting enzymes, which convert neutral fats into glycerine and fatty acids; an example is steapsin. (4) Sugar-splitting enzymes, which convert the non-absorbable (saccharose) into absorbable (dextrose) sugar; an example is invertase. (5) Coagulating enzymes, which precipitate solu- ble proteids; an example is rennin. Characteristics of Enzymes. — Some of the characteristics of enzymes are as follows: (i) They are soluble in water and in glycerine. (2) In solution they are destroyed before the boiling point is reached (140° to 180° Fahrenheit). Very low temper- atures do not destroy them, but suspend their action. (3) They never completely convert the substance upon which they act. It is supposed that the substance produced, as peptones for example, have an inhibitory action upon the enzyme. If these substances be removed as they are formed, the action of the enzyme continues. (4) The particular result is independent of 70 THE PHYSIOLOGY OF DIGESTION AND ABSORPTION the amount of the enzyme (unless it be very small) no matter how large a quantity of the substance to be acted upon is present. Manner of Action. — These enzymes are supposed to bring about their respective changes through hydrolysis — that is, by causing water to be taken up by the molecules of the affected substance and by the subsequent splitting of the newly formed molecule into two or more simpler ones. How they cause this appropriation of water is as yet undetermined. It was formerly supposed to be brought about by contact merely, and the enzymes were called catalytics; but this term offers no explanation of the real change which occurs. Digestive Processes. — The digestive processes may be con- sidered under the heads of (i) prehension, (2) mastication, (3) salivary digestion, (4) deglutition, (5) gastric digestion and (6) intestinal digestion. Prehension, mastication and deglutition cannot properly be looked upon as digestive processes, inasmuch as they involve no chemical change. They are, however, neces- sary occurrences, and cannot be disregarded. Of course, ab- sorption and " internal digestion" are supposed to follow gastro- intestinal or " external digestion," and assimilation or cell appropriation to follow absorption. Prehension. Prehension is simply the taking of food into the mouth. Its mechanism in the human adult is so familiar that it needs no description. In the sucking child it is more complex. The buccal cavity is closed posteriorly by the application of the velum palati to the base of the tongue. The tip of the tongue is applied to the hard palate, and successive portions of it (going backward) being applied in the same way leave a partial vacuum in front, and liquids are drawn into the mouth. The mechanism of drinking is the same. SALIVARY GLANDS . 'JI Digestion in the Mouth. Mastication. — The object of mastication is to grind up the food so that it may be swallowed more easily and the various digestive fluids, particularly the saliva and gastric juice, may have more ready access to its parts. The proper mastication of the food is an important factor in its complete digestion later on. Mechanically, mastication is effected by the action of the lower jaw, aided by the tongue, lips and cheeks. This remark presumes of course that the teeth are intact. Lateral and antero-posterior movements of the lower jaw combine with its simple elevation to compress and grind the food between the teeth. The muscles which depress the lower jaw are the digas- tric, mylohyoid, geniohyoid and platysma. Those which elevate it are the temporal, masseter, internal and external pterygoids. The attachments of the external pterygoids are such that by their simultaneous action the mandible can be thrown forward and, by their alternate contraction, from side to side. The tongue is active during mastication in carrying the mass of food to this or that part of the buccal cavity so that it may be ground up completely. It also gives accurate information as to the size (of the mass) and stage of mastication. The cheeks, as is shown in facial palsy, are quite important in keeping the food from between them and the teeth. The lips prevent the escape of liquids from the mouth, in addition to assisting in prehension. The Salivary Glands and their Secretion. The first of the digestive juices with which the food comes in contact is the saliva which is the mixed secretion of the large salivary glands and the various smaller mucous and serous glands which open into the mouth cavity. The chief salivary glands are three in number on each side of the mouth — the parotid, submaxillary and sublingual. Besides these, there 72 THE PHYSIOLOGY OF DIGESTION AND ABSORPTION are, throughout the buccal mucous membrane, a number of smaller glands of similar structure contributing to the formation of saliva. The parotid gland is situated just beneath and in front of the lobe of the ear; the submaxillary beneath the mandible about the center of the base of the submaxillary tri- angle, and the sublingual beneath the mucous membrane of the mouth, just lateral to the lingual frenum. FIG. 33. — Cells of the alveoli of a serous or watery salivary gland. (Brubaker after Yeo.) A, after rest; B, after a short period of activity; C, after a prolonged period of activity. The duct from the parotid, Stenson's duct, runs beneath the mucous membrane of the cheek to a point opposite the second upper molar tooth, where is its opening into the mouth. The duct from the submaxillary, Wharton's duct, discharges the secretion from that gland into the mouth by the side of the fre- num of the tongue. The secretion from the sublingual reaches the mouth by a number of small ducts (Rivinus) which open also by the side of the frenum, and sometimes as well by a larger duct, Bartholin's, which runs parallel with Wharton's and empties near it. Histology. — In structure the salivary glands have been shown to be of the compound tubular variety, the secreting part being tubular. The parotid is a serous gland, the other two are usually said to be mucous, though they contain both serous and mucous cells. The ducts subdivide into smaller ducts and tubes, until a distinct tubule is distributed to every acinus and becomes the SALIVARY GLANDS 73 lumen of that acinus. The whole arrangement resembles the branchings of a tree. The flow from these glands is greatly increased by mastica- tion. From the parotid the flow is much more abundant on that side upon which mastication takes place. During activity it can be shown that the granules of the serous cells accumulate toward the lumen of the acinus while the outer segment of the cells becomes comparatively clear. It is supposed that this is an essential step in the production of the organic constituents of FIG. 34. — Section of a mucous gland. (Brubaker after Lavdowsky.) A, in a state of rest; B, after it has been for some time actively secreting. the secretion — that the granules contain either the ptyalin or the substance necessary to its formation. It is also supposed that at the same time that ptyalin is being thus produced and discharged, very active constructive changes are occurring in the clear zone of the cells. During activity some at least of the mucous cells seem to break down, but it is probable that the granules in the cell protoplasm become converted into mucin, which, being extruded, seem to destroy the cell itself. Composition and Properties of Saliva. — While it is possible to draw certain distinctions between the saliva from the different glands, these distinctions are comparatively unimportant, so far 74 THE PHYSIOLOGY OF DIGESTION AND ABSORPTION as digestion is concerned; for the secretions from the three pairs of glands become mixed in the mouth, and it is their combined effect which, in any particular case, is observed. Saliva con- tains in 1,000 parts about 994 of water, the remaining six parts being organic and inorganic solids. These solids are chiefly mucin, ptyalin, albumin and salts. The salts are mainly the chlorides of sodium and potassium, the sulphates of potassium, the phosphates of potassium, sodium, calcium and magnesium, and sulphocyanide of potassium. The mucin gives the ropy consistence to the fluid and serves a mechan- ical purpose only. The sulphocyanide of potassium is unusual in the body secretions and its presence here is interesting. It may represent an end product of proteid metabolism. The true digestive value of saliva is due to ptyalin, an amylolytic enzyme. Were it not for the presence of epithelial cells in suspension, saliva would be clear and transparent. Its reaction is alkaline, its specific gravity is about 1004 to 1008, and the average amount of daily secretion is about 2} pounds. The parotid saliva is much more watery and mixes much more readily with the food than the submaxillary and sublingual, which latter is mucilaginous and gives to the bolus a glairy coating. The sublingual saliva is thicker and more viscid than the submaxillary. Nerve Supply. — The connection of the nervous system with salivary secretion deserves particular attention, since the phe- nomena presented under its influence are typical, and, if not explanatory of occurrences elsewhere in the body, are at least very suggestive. Each one of the three glands is supplied with both cerebro- spinal and sympathetic fibers. Each one of them has three kinds of nerve fibers, secretory, vaso-dilator and vaso-constrictor. The secretory and vaso-dilator reach the gland in the cerebro- spinal trunks; the vaso-constrictor in the sympathetic. The vaso-constrictors and vaso-dilators are distributed to the walls SALIVARY GLANDS 75 of the blood-vessels, and influence secretion indirectly only by increasing or diminishing the amount of blood going to the glands. The secretory fibers exert their influence directly upon the gland cells. It is claimed also that the secretory fibers are divided into sets controlling the production of the energy-yielding constituents and sets controlling the production of water and salts. The parotid gland receives its cerebro-spinal fibers through a branch of the fifth nerve, but when they are traced backward it can be shown that they are in the tympanic branch of the ninth, and pass from this branch to the small superficial petrosal nerve and thence to the optic ganglion — from which ganglion they run to the parotid gland by way of the auricula-temporal branch of the third division of the fifth. The cervical sympathetic also sends fibers to this gland. The submaxillary and sublingual glands are supplied by the same nerves. Their cerebro-spinal fibers leave the brain by way of the facial, follow the chorda tympani as far as a short distance beyond its junction with the lingual nerve, and then leave it to reach the submaxillary ganglion and run thence to the submaxillary and sublingual glands. These glands receive sympathetic fibers from the superior cervical ganglion. Influence of Nerve Supply.— Taking the parotid as an ex- ample, it is found that stimulation of its cerebro-spinal fibers produces an abundant watery flow of saliva; the gland becomes decidedly redder, pulsation is sometimes apparent, and it is evident that the amount of blood is locally increased. When the sympathetic supply of the parotid is stimulated, the secretion is inhibited or reduced to a minimum, the gland becomes pale and the amount of blood in it is evidently diminished. Similar corresponding results are occasioned in the submax- illary and sublingual glands by stimulation of the chorda tympani and the sympathetic fibers. It would seem at first, in the light of the vascular changes accompanying stimulation of the two supplies to all these glands, 76 THE PHYSIOLOGY OF DIGESTION AND ABSORPTION that the resultant phenomena could be explained entirely by variations in the amount of blood, and that the nervous system influences their secretion only by contraction and dilatation of the vessels. However, a number of circumstances, which it is un- necessary to relate here, prove that the secretory fibers exert an influence directly upon the cells themselves, causing them to secrete. The mere distribution of these fibers to the gland cells presupposes some such function on their part ; and it can actually be shown that the secretion can be increased when the blood supply is cut 'off, or without dilatation of the vessels. Such action, however, is of course only temporary, for the materials for secretion must be supplied by the blood. The exact method of termination of the secretory fibers has not been determined. It is probable that they end between and around the cells and do not penetrate their substance. Section of the chorda tympani causes a continuous flow of saliva from the submaxillary and sublingual glands for several weeks. This has been termed paralytic secretion, and is sup- posed to be due to the fact that the chorda fibers do not them- selves run directly to the glands, but are distributed to sym- pathetic ganglia (the submaxillary or others in the gland sub- stance). Section of the chorda, then causes, degeneration of its fibers only as for as these ganglia, and their cells are thought to be subject, in some obscure way, to continuous irritation during the period for which the paralytic secretion continues. Function. — The function of this secretion is twofold, (a) mechanical and (b) chemical. (a) From a mechanical standpoint (i) it facilitates phona- tion, mastication and gustation by maintaining a proper degree of moisture in the mouth; (2) its more watery parts (parotid) mix with the food, dissolving part of it, so that it may be more easily masticated and swallowed while its more viscid parts (submaxillary and sublingual) spread over the surface of the bolus to aid in deglutition. SALIVARY GLANDS 77 (6) From a chemical standpoint, the function of the saliva is to convert starch into sugar. It does this through the agency of its enzyme, ptyalin, which conforms to the characteristics of enzymes already noted. Maltose (C12H22O11+H2O) is the form of sugar produced, but there are several intermediate sub- stances formed before maltose finally results. The starch mole- cule (C6H10O5) was formerly supposed to simply appropriate a molecule of water to form dextrose (grape sugar, glucose, C6H12O6), but it is now thought that there is a succession of hydrolytic changes with the production of dextrin and maltose. That is, the starch molecule appropriates a molecule of water; this new molecule splits into a certain kind of dextrin and mal- tose; the dextrin left itself appropriates water and splits up into another kind of dextrin and maltose; this last dextrin goes through a similar process with a like result, until finally only maltose is produced. Some dextrose may be produced. It will be seen under gastric digestion that mineral acidity will also convert starch into sugar, but in this case the form of sugar is dextrose. The effect of temperature on the action of enzymes has been noticed. The optimum for ptyalin is 100° Fahrenheit. The reaction of saliva is alkaline and its effect on starch is stopped by an acid medium, since the enzyme is thereby destroyed. However, ptyalin has been shown to act even a little better in perfectly neutral than in alkaline solutions (Chittenden). The action of this substance on starch is very much facilitated if the starch be cooked; in fact, its action on uncooked starch is so slow that probably it is inconsequential in digestion. Cooked starch becomes hydrated, and furthermore has its cellulose capsule removed from the granulose, both of which circumstances make it much more susceptible to salivary influences. However, it must be admitted that the practical effect of pty- alin in digestion is not very considerable in the mouth mainly because the food is not kept in the mouth long enough. How- 78 THE PHYSIOLOGY OF DIGESTION AND ABSORPTION ever, large quantities of saliva are swallowed with the food and it continues its action in the stomach while the food is stored in the cardiac end and only ceases its activity when the food is thoroughly mixed with the acid gastric juice. The conversion of starch into sugar is continued and concluded in the small intestine. Deglutition. The act of deglutition is commonly divided into three periods, depending upon the part through which the food is passing. During the first period the bolus passes from the mouth through the isthmus of the fauces, during the second through the pharynx, and during the third through the esophagus into the stomach. A brief reference to the anatomy of these parts is necessary. Fauces. — The isthmus of the fauces is the opening at the back of the mouth, bounded below by the base of the tongue, above by the soft palate and uvula, and laterally by the pillars of the fauces, between which are the tonsils. The anterior pillars are easily seen when the mouth is opened widely, and consist of the palatoglossi muscles with their covering mucous membrane. The posterior pillars approach each other more nearly than the anterior, and consist of the palatopharyngei muscles and their covering mucous membrane. Pharynx. — The pharynx extends from the basilar process of the occipital bone above about four and a half inches down- ward. It communicates with the posterior nares, the mouth, the Eustachian tubes, the larynx and esophagus. The tube is made up of two coats, an external muscular and an internal mucous. The muscular coat consists of the three constrictors and the stylopharyngeus. The mucous coat is covered in its upper part with columnar ciliated and in its lower part by pave- ment epithelium. Esophagus. — The esophagus runs a course of about nine inches from the end of the pharynx, at a point behind the cri- DEGLUTITION 79 coid cartilage, to the stomach, which it enters a little to the left of the median line. The coats of the esophagus are two, an external muscular and an internal mucous. The external coat has its fibers disposed in two layers, longitudinal and cir- cular. The circular layer is internal. In the upper third of the esophagus the fibers of the muscular coat are all striped, but at the beginning of the middle third they begin to give place to plain fibers, and these latter progressively increase, to consti- tute virtually the whole muscular coat at the diaphragm. The internal mucous coat is lined by squamous epithelium, and, except during the passage of substances through the esophagus, is thrown into longitudinal folds. The outside fibrous tissue attaches the whole esophagus to the surrounding tissue. Mechanism of Deglutition. — The first period of deglutition is voluntary but automatic, like respiration. The morsel of food is forced toward and through the fauces by the tongue, which presses from before backward against the hard palate, with the bolus above it. That the tongue is mainly concerned in this act is shown by inability to swallow when this organ is absent, unless the food be pushed far back into the mouth by the finger or other means. The mechanism of the second period is much more complex. The food must pass through the pharynx into the esophagus, and must not be allowed to enter any of the other openings com- municating with the pharynx. The larynx especially is to be protected. Since the air enters through the posterior nares above the isthmus and must enter the larynx in front of the esoph- agus, it follows that the current of air would cross the current of food if swallowing and respiration took place together. Con- sequently respiration is suspended during deglutition. As soon as the food has passed the fauces, the elevators of the hyoid raise that bone, and with it the larynx. It is at the same time pulled a little forward, and since the pharynx is attached to the larynx posteriorly, the former necessarily follows the movement of the 8o THE PHYSIOLOGY OF DIGESTION AND ABSORPTION latter, and is thus slipped under the base of the tongue and the entering bolus. With elevation of the larynx the superior con- strictor of the pharynx contracts upon the food, and passes it quickly to the grasp of the middle constrictor, which in turn hands it to the inferior constrictor and thence to the esophagus. The posterior nares are protected by contraction of the pos- terior pillars and the superior constrictor. The laryngeal opening is protected by the epiglottis. When the tongue is forced back and the larynx raised the natural effect would be to fold the epiglottis down over the laryngeal opening. At the same time contraction of the pharyngeal muscles draws to- gether the sides of the larynx and aids in closing the glottis. Furthermore, the vocal cords fall together (as they always lie except during inspiration — and inspiration is now suspended). The third period passes the food through the esophagus into the stomach by contraction from above downward of successive portions of its muscular wall. Contraction of the longitudinal fibers draws the mucous membrane above the bolus. Then the circular fibers, contracting in successive segments from above downward, force the bolus before them. These movements are continued until the food reaches the stomach. The time con- sumed in swallowing a given article is about six seconds. This is the mechanism which carries all materials through the alimentary canal from the esophagus to the anus. It is called peristalsis, or vermicular (worm-like) action. Nervous Control. — While nearly all the muscular tissue con- cerned in deglutition is of the striated variety, the whole proc- ess, except the first, which is automatic, must be considered as reflex. The mechanism of deglutition is one of the best ex- amples of finely coordinated muscular action to be found. The afferent fibers concerned are from the 5th, pth, and loth, and the superior laryngeal branch of the last. The efferent fibers are from the 5th, yth, pth, loth, and i2th. The center for the re- flex is supposed to be far forward in the medulla. DIGESTION AND ABSOEPTION IN THE STOMACH 8 1 It ought to be added that the Kronecker-Meltzer theory of deglutition assails with considerable plausibility the mech- anism of deglutition as above given. In a word, this theory holds that when the bolus of food rests upon the dorsum of the tongue, and the tip of that organ prevents, by its apposition to the hard palate, the escape of the food forward, the mylohyoids contract with great force, compress the food, and it escapes by the route of least resistance, which is backward. It is thus shot into the esophagus, and the contraction of the pharyngeal muscles only supplements that of the mylohyoids. Digestion and Absorption in the Stomach. Anatomy. — The stomach is situated beneath the diaphragm in the upper part of the abdominal cavity, and is moored by the esophagus and folds of the peritoneum. Its general shape has been compared to that of the bagpipe. Its large, or fundic, end is to the left; its small, or pyloric, to the right. By far the greater part of the organ is to the left of the median line. A very considerable portion is to the left of the esophageal opening. Except when distended, its anterior and posterior walls hang in an approximately vertical direction, and are usually in contact by their mucous surfaces. Its greatest length when moderately distended is about fourteen inches, its transverse diameter about five inches, and its capacity about five pints. At the point where the anterior and posterior walls meet inferiorly, the great omentum (the peritoneum from the two walls) is given off. This is the greater curvature and has the gastro-epiploica-dextra and the gastro-epiploica-sinistra arteries running along it between the two folds of the omentum. Where the anterior and posterior walls meet superiorly, the stomach is joined by the lesser omentum, the two layers of which are continued in front and behind as the serous covering of the stomach. This is the lesser curvature, and has the gastric and pyloric branch of 6 82 THE PHYSIOLOGY OF DIGESTION AND ABSORPTION FIG. 35. — Human alimentary canal. a, esophagus; b, stomach; c, cardiac orifice; d, pylorus; e, small intestine;/, biliary duct; g, pancreatic duct; h, ascending colon; *', transverse colon; /, descending colon'. k, rectum. (Collins & Rockwell.) DIGESTION. AND ABSORPTION IN THE STOMACH 83 the hepatic arteries running along it between the two layers of the lesser omentum. The large left hand portion of the stomach cavity is called the fundus or greater pouch. The opposite portion of the cavity is called the lesser pouch or antrumpylori. At one end is the cardiac or esophageal opening, at the other the pyloric. Histology. — The coats of the stomach walls are four. From without inward these are the (i) peritoneal, or serous, (2) mus- cular, (3) sub-mucous and (4) mucous. 1. The peritoneal coat covers the whole of the organ except- ing an inconsiderable linear area, where the two layers of the lesser (gastro-hepatic) omentum join it along the lesser curvature, and a similar area along the greater curvature, where the serous coats of the anterior and posterior walls leave the organ to form the great omentum. This coat is simply a fold given off from the peritoneum to envelop the stomach in practically the same manner as the other abdominal viscera. Its structure is that of serous membranes in general. 2. The muscular coat, varying in thickness from -£$ in. over the fundus to ^ in. at the pylorus, is disposed in three layers, (a) external longitudinal, (b) middle circular and (c) internal oblique. The longitudinal fibers are continued from the corre- sponding fibers of the esophagus. They are marked along the lesser curvature, but not very distinct over other parts. The circular fibers are not abundant to the left of the esophageal opening. They progressively increase toward the right, and at the pyloric opening constitute a distinct and powerful muscular ring, the pyloric sphincter, which, projecting into the lumen presents a more or less flat surface on the duodenal side to pre- vent the regurgitation of food. The oblique fibers are supposed to be continuous with the circular fibers of the esophagus. They extend over the greater pouch from a point just to the left of the esophageal opening to a point on the greater curvature, about the junction of the middle and pyloric thirds. Here, at the right 84 THE PHYSIOLOGY OF DIGESTION AND ABSORPTION hand limit of the oblique fibers, the stomach is said during diges- tion to be considerably constricted, so that a temporary sphincter is established. This point is the point of separation between the fundus and the antrum pylori, and is sometimes called the sphincter antri pylorici. The fibers of the muscular coat are of the plain variety, as is all the gastro-intestinal muscular tissue Serosa. — FIG. 36. — V. S. Wall of human stomach. E, epithelium; G, glands; Mm, muscularis mucosae. X 15. (Stirling.) from the lower end of the esophagus to the external sphincter. 3. The submucous coat consists of loose fibre-elastic con- nective tissue which allows free movement between the muscular and mucous coats. It contains rather large blood-vessels and a nerve-plexus, the plexus of Meissner. 4. The mucous coat has an average thickness of about ^ GASTRIC GLANDS 85 in., is loosely attached to the submucous coat, and, except during gastric digestion, is thrown into longitudinal rugae. It consists of columnar epithelium resting upon a basement mem- brane, 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 extend the gastric glands. The Gastric Glands. In the mucous membrane of the stomach are found two kinds of glands. According to their relative position with reference to the two ends of the stomach they are called fundic and pyloric. It is to be noted, however, that neither of these divisions is confined strictly to that portion of the stomach which its name would seem to indicate. According to their secretion the glands are called acid and peptic. The fundic and acid, and the pyloric and peptic are considered to be identical. But attention is called to the fact that while peptic (pyloric) glands secrete pepsin only, the acid (fundic) secrete both acid and. pepsin. Structure. — Some of the gastric glands are simple tubules, while others may be bifurcated, so that two (or more) tubules communicate with the surface by a single canal. They may all, however, be classified as belonging to the simple tubular variety. They have a deep secreting portion and a superficial non-secret- ing portion. The latter is lined by columnar epithelium, and is the duct proper. The former is lined by cuboidal epithelium which discharges its secretion into the lumen, this lumen being only a continuation of the duct. These cuboidal cells are called peptic cells because they produce pepsin, or its forerunner, pepsinogen. The fundic (acid) glands are found to have lying close to the basement membrane a number of large cells at inter- vals between the cuboidal cells and not extending outward to the 86 THE PHYSIOLOGY OF DIGESTION AND ABSORPTION central lumen. They are thought to communicate with the lumen by capillary ducts, which may even penetrate their substance. They are supposed to secrete hydrochloric acid, FIG. 37. — Vertical section of the gastric mucous membrane. g, g, pits on the surface; p, neck of a fundus-gland opening into a duct, g; x, pari- etal, and y, chief cells; a, v, c, artery, vein, capillaries; d, d, lymphatics, emptying into a large trunk, e. (Landois.) and are called acid cells from this fact, or parietal cells from their position. (See Fig. 37.) Method of Secretion. — When food is ingested gastric move- GASTRIC GLANDS ments very soon begin, carrying the food in this direction or that, as described later. At the same time, the gastric mucous mem- brane changes from a pale pink to a congested red, and soon drops of gastric juice begin to appear. They run to the depend- P"iG. 38. — Section of the pyloric mucous membrane. (Landois.) ent portions of the cavity and become incorporated with the alimentary mass. It is believed that if the gastric movements did not occur, this secretion would be limited for fifteen or thirty 88 THE PHYSIOLOGY OF DIGESTION AND ABSORPTION minutes to a very small area, namely, that with which the food is in contact. But it is comparatively general because the move- ments bring practically all parts, at least of the fundic mucous membrane, in contact with the food before this time has elapsed. The idea is that up to fifteen or thirty minutes after the introduc- tion of food, the glands are made to secrete by direct mechanical stimulation of the food, and after this time the secretion becomes general, whether mechanical irritation becomes general or not. It ought to be added, however, that in recent years secretion by mechanical stimulation has been denied, and the denial is supported by good evidence. Besides direct proof by experi- ments, it is shown that this early secretion occurs without mechanical irritation, as when food is chewed and made to pass through an esophageal fistula, or even by the sight of food. These observers (Pawlow) state that food introduced into the stomach through a fistula produces absolutely no flow if the animal experimented upon does not know of the introduction. Under this view the secretion is a distinct reflex, the impressions being carried to the center by afferent nerves distributed to the mouth, or by nerves of special sense. Whether as a reflex or as a result of mechanical stimulation, the fact remains undisputed that the flow begins a few minutes after the introduction of food, and lasts until gastric digestion is completed. After a time it is supposed that chemical changes in the food itself further stimulate the gastric glands, through their influence on the secretory nerves. These stimulating chemical products are not developed alike from all foods; and the conclusion is warranted that some substances do not undergo gastric digestion so readily as others. Ordinary bread and the whites of eggs, for example, are said not to develop them. It has been further shown that fats, oils, etc., actually develop substances which chemically inhibit gastric secretion. There appears also to be a kind of chemical regulation of the amount GASTRIC GLANDS 89 and quality of juice, according as much or little, or a varying acidity, is needed in the digestion of the substance in the stomach. Conditions influencing digestion operate mainly by producing changes in the quantity or quality of gastric juice, and these changes in turn are largely effected through the nervous system. Fever, overeating, depressing emotions, strenuous physical or mental exercise, etc., decrease the secretion and correspond- ingly interfere with digestion. Changes During Activity.— Like the salivary cells, the cu- boidal peptic cells can be shown to undergo changes during secretory activity. When at rest they contain abundant gran- ules, but during secretion these granules disappear, first from the base and later from well-nigh the whole cell. The granules are supposed to contain pepsin, or rather pepsinogen, for it is thought that pepsin is not formed by the cell directly, but is made out of pepsinogen, which is the product of the peptic cells, probably under the action of hydrochloric acid. The rennin is also sup- posed to exist in the cells as some preliminary material cor- responding to pepsinogen. This material may be termed rennin zymogen. Changes in the acid cells during activity also occur, but are more obscure than those in the peptic cells. The source of hydrochloric acid is a decomposition of the neutral chlorides of the blood and the union of the chlorine thus liberated with hydro- gen, but how or why this occurs is not explained by phenomena so far observed. Secretory Nerves. — While it has been impossible to demon- strate secretory fibers to the cells of the gastric glands, such fibers must exist in the vagus. Section of it (and the sympathetic), however, does not entirely stop the secretion, but incidents re- ferred to in a preceding section, such as secretion at sight of food, or when food is chewed and not swallowed, certainly 90 THE PHYSIOLOGY OF DIGESTION AND ABSORPTION point to an influence of the central system over secretion. Of course the sympathetic fibers to the vessel walls are indirectly concerned. Condition of Food on Entering Stomach.— The food enters 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 acid- ity 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. Properties and Composition of Gastric Juice. — The secre- tion of the glands of the stomach is called gastric juice. Gas- tric 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 reaction, and a specific gravity of 1005 to 1009. Chemically it contains per thousand about 973 parts water and 27 solids. Proteid substances compose some 17 of the 27 parts of solid matter. These substances are mainly mucin, pepsin and rennin. The most important non-nitrogenous 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. The amount of gastric juice secreted in twenty-four hours is from six to fourteen pounds. Gastric juice will resist putrefaction for a long time, probably on account of the free acid. Its digestive GASTRIC GLANDS 9! properties are due to the proteolytic enzyme, pepsin, the milk- curdling enzyme rennin, and the free hydrochloric acid. Hydrochloric Acid. — The amount of free hydrochloric acid present in normal gastric juice is from two-tenths to three-tenths of one per cent. It has been frequently claimed that the acidity of this secretion is due to lactic acid, but while it cannot be denied that lactic acid, from the fermentation of carbonydrates is, or may be, normally in the stomach during digestion, yet hydrochloric acid is undoubtedly the free acid proper to the gas- tric juice. Digestion, however, will proceed under a proper (variable) degree of an acidity from almost any acid. 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 coagulate casein. Milk when left standing at ordinary tem- perature has lactic acid produced by the action of bacteria upon the lactose in it, and this acid precipitates 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 ex- tracts of the gastric mucous membrane will themselves curdle milk. After coagulation the action of pepsin begins and the 92 THE PHYSIOLOGY OF DIGESTION AND ABSORPTION casein is converted into peptones in the usual manner. The value of the curdling process is not apparent. 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 reaction. 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. (B) On Carbohydrates. — There is no enzyme furnished by the stomach to affect any of the carbohydrates. It is true that GASTRIC GLANDS 93 salivary digestion proceeds in some small degree in the stomach. Saliva is swallowed with the food, and until the reaction 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. (D) 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 another animal, it will likewise be digested. The last instance would seem to lead to the conclusion that living matter can be 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 intes- 94 THE PHYSIOLOGY OF DIGESTION AND ABSORPTION tinal juice. But why the stomach is not thus attacked 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 unknown, inhibit- ing the action of the gastric juice, most probably by preventing its absorption. "A nearer approach to an explanation seems to have been attained in the discovery of an antipeptic and antitryptic action of the stomach and intestinal mucosa. This action is probably due to antienzymes which are found throughout the whole animal scale and occur not only in the intestinal tract, but also in cells of other organs.'' (Tigerstadt.) 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 continuous 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, f undic and pyloric, by the sphinc- ter 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 GASTRIC GLANDS 95 of this muscular wave (peristalsis) in the fundus is to force the 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 con- tractions 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 nature, and the pultace- ous 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 anti-peristaltic 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 and 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 contrac- tion and relaxation of plain muscle is much slower than that of striped. 96 THE PHYSIOLOGY OF DIGESTION AND ABSORPTION It is the fundus, and not the pylorus, which serves as a 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, that 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. 86-88). Nerve Supply. — The stomach is supplied with pneumogastric and sympathetic fibers. The 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. Digestion and Absorption in the Intestines. The Small Intestine. Anatomy. — The small intestine extends from the pylorus to the caput coli, and is about twenty feet in length. It lies in DIGESTION AND ABSORPTION IN THE INTESTINES 97 numerous coils which are held loosely in place by a fold of perito- neum running from one side of the great abdominal vessels, 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 intes- tine, 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 remainder 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 four layers, serous, muscular, submucous and mucous. The serous layer consists of the enveloping fold of peritoneum and needs no description, except that, like serous membranes elsewhere, it furnishes a lubricating secretion to provide for the easy gliding of the intestines over each other and over the other viscera. The muscular coat has its muscular fibers disposed in two layers, an external longitudinal and an internal circular. The latter is the stronger. Between the two muscular layers is the nervous plexus of Auerbach. Between the circular layer and the mucous coat is the submucous layer which contains the nerve plexus of Meissner. These communicate with others by fibers of extension. The mucous coat presents several points deserving mention. 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 1,000 in number, extend from about the middle of the duodenum to the beginning of the last 7 98 THE PHYSIOLOGY OF DIGESTION AND ABSORPTION 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 surfaces of the fold which are in apposition are bound together by loose connective tissue. The fold as a whole, however, is freely movable upward or downward in the intestine and has no tendency to obstruct the canal. The only function of the val- vulae conniventes is to furnish a greater secreting surface and, FIG. 39. — Diagram of a longitudinal section of the wall of the small intestine. a, villi; b, 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;/, longitudinal muscle fibers; g, serous coat. (Yeo.) by somewhat retarding the passage of the alimentary mass, to subject it for a longer time to the digestive fluids. 2. The villi are especially important in connection with absorption, and their description properly belongs under that head. They are conical elevations responsible for the velvety character of the mucous membrane. They exist in great num- bers from the pylorus to the ileo-cecal valve, covering the valvulae conniventes as well as the general surface of the mucous mem- brane. The largest are about -fa in. long and T^ m- m diameter at their base. They are only elevations of the mucous membrane containing a central tube, the lacteal, which is nothing but an DIGESTION AND ABSORPTION IN THE INTESTINES 99 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 FIG. 40. — Portion of the wall of the small intestine laid open to show the valvulae conniventes. (From Yeo after Brinton.} 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 endo- thelial wall contains many stomata. FIG. 41. — Vertical section of a villus of the small intestines of a cat. a, striated border of the epithelium; b, columnar epithelium; c, goblet cells; d, central lymph- vessel ; e, smooth muscular fibers;/, adenoid stroma of the villus in which lymph corpuscles lie. (Kirkes after Klein.) 3. The glands of Brunner and the crypts of Lieberkuhn, or intestinal tubules, are supposed to produce the succus entericus. 100 THE PHYSIOLOGY OF DIGESTION AND ABSORPTION The former are chiefly limited to the upper half of the duodenum. The latter exist throughout the small and large intestine. 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 iXiJ 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 distributed than the agminate. The chyme, having passed from the stomach to the small in- testine, encounters three digestive fluids, pancreatic juice, bile and intestinal juice. These are, of course, mixed together, but none interferes with the action of the other. The Pancreas. — The pancreas is a large gland lying in the upper part of the abdominal cavity behind the stomach. It has the general shape of a hammer, its head being embraced by the bend of the duodenum and its opposite extremity reaching to the spleen. It weighs some four or five ounces, and is about seven inches long. Its duct, the duct of Wirsung, usually joins the common bile duct just where this latter penetrates the wall of the duodenum, so that the bile and pancreatic juice enter the small intestine together. Sometimes the two ducts do not join, and sometimes a second smaller duct from the pan- creas penetrates the duodenum a little below the larger one. The duct of Wirsung traced backward divides and subdivides until its final ramifications end in the alveoli, or secreting portions. THE PANCREAS IOI Histology. — This is a compound tubular gland. The cells in the alveoli are of the serous type and are granular toward the central lumen. During activity they undergo changes Very similar to the salivary cells, the non-granular zone toward the basement membrane increasing and extending and the granular zone becoming correspondingly smaller. Here, as in the sali- 4 FIG. 42. — One saccule of the pancreas of the rabbit in different states of activity. (From Brubaker after Yeo.) A , after a period of rest, in which case the outlines of the cells are indistinct and the inner zone — *'. e., the part of the cells (a) next the lumen (c) — is broad and filled with fine granules. B, after the gland has poured out its secretion, when the cell outlines (d) are clearer, the granular zone (a) is smaller, and the clear outer zone is wider. vary glands, it is believed that the granules are made from the clear protoplasm, and contain the enzymes or their formative materials. The formative material in all these glands is given the name of zymogen, although the zymogen in a particular gland may have a particular name, as pepsinogen, the forerunner of pepsin, or trypsinogen, the forerunner of trypsin. Properties and Composition of Pancreatic Juice.— The pancreatic juice is a colorless liquid, alkaline in reaction, and has a specific gravity of about 1040 if taken from a recent fistula. It coagulates when heated and is prone to putrefaction on exposure. With a specific gravity of about 1040, it contains per thousand about 900 parts water, the remainder being different solid food materials in solution. These constituents are a proteid and 102 THE PHYSIOLOGY OF DIGESTION AND ABSORPTION three very important digestive ferments, trypsin, steapsin and amylopsin. - The phosphates and carbonates are plentiful and give the fluid its alkaline reaction. 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 differs from pepsin in that its proteolytic action is more powerful and can take place in alkaline media. It will also act in/neut^'aJ or weakly acid media. The opinion is ad- vanced that while ihe gastric juice is capable of converting pro- ftjids into pfeptones; as & 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 splitting 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 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. This enyzme is very important, for it has been remarked that the action of ptyalin is probably rather incon- sequential, and by far the greater portion of the starch, which constitutes a large part of our ordinary food, must be digested 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- INTERNAL PANCREATIC SECRETION 103 erine 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 in glycerine 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 digestion are therefore glycerine, soaps, and emulsions, all of which can be absorbed in a way to be noted later. While the emulsification of fats under the influence of soaps (fatty acids and alkaline 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. Method of Secretion. It can be shown that the secretion begins to be discharged into the duodenum very soon after the entrance of food into the stomach, and continues as long as intestinal digestion is in progress. Consequently the flow will be intermittent if the meals are far enough apart. It is almost certain that the secretion is a reflex act as a result of impressions upon the mucous membrane of either the stomach or the duo- denum. The acidity of the gastric juice seems to be the natural stimulus and to exert its influence upon the duodenal mucous membrane. This is not incompatible with the early flow after the ingestion of food, for it will be seen later that at least a small quantity of that food passes quickly to the duodenum and carries gastric juice with it. The composition of the secretion seems to be influenced in some degree by the character of the food. It is interesting that oils increase the pancreatic flow. Nerve Supply. — The pancreas has, besides vaso-motor fibers to its vessels, distinct secretory fibers, like those of the salivary glands. These fibers probably run in both the sympathetic and the vagus. Internal Pancreatic Secretion. Circumstantial evidence leaves scarcely any doubt that the pancreas produces some sub- stance which is discharged into the blood and markedly influ- IO4 THE PHYSIOLOGY OF DIGESTION AND ABSORPTION ences nutrition. Removal of the gland is followed by death from inanition in two or three weeks; and previous to that sequel the most striking phenomenon is marked glycosuria, with the ordi- nary symptoms of diabetes mellitus. Retention of a compara- tively small portion of the gland obviates this condition. Sugar does not exist normally in the blood, and this internal secretion may contain some ferment which effects its consumption. The Liver. The liver is the largest gland in the body. Its function is to produce bile, glycogen and urea. Anatomy, — The liver is situated in the upper part of the abdominal cavity, chiefly in the right hypochondrium. Its FIG. 43. — The under surface of the liver. g. b., gall bladder; h. d., common bile duct- h. a., hepatic artery; v. p., portal vein; /. q., lobulus quadratus; /. s., lobulus spigelii; /. c., lobulus caudatus; d. v., ductus venosus; u. v., umbilical vein. (Kirkes after Noble Smith.) weight in the average adult is about four and a half pounds. It is covered, except for a small area behind, by peritoneum, processes of which run from it at several points and constitute its supporting ligaments. The proper coat of the liver lies THE LIVER 105 underneath the peritoneum, and at the transverse fissure is continued into the gland as a sheath, embracing the structures entering there and ramifying with them in their distribution. This is the capsule of Glisson. It is fibrous in structure, is closely attached to the liver substance, and rather loosely ad- herent to the structures which it envelops. The walls of the portal vein are seen collapsed on section, while those of the hepatic veins, which are not surrounded by Glisson's capsule, and which are closely adherent to the gland substance, stand well open. A general idea of the liver's anatomy is obtained by noting that it has five lobes, five fissures, five ligaments and five struc- tures passing through the transverse fissure. The lobes are right, left, caudate, quadrate and Spigelian. The fissures are trans- verse, umbilical, that for the ductus venosus, the fossa for the vena cava and the fossa vesicalis. The ligaments are coronary, right lateral, left lateral, round and suspensory or longitudinal. The structures passing through the transverse fissure are the portal vein, the hepatic artery, the hepatic duct, the nerves and the lymphatics. Blood-vessels. — Of the two blood-vessels entering the fissure the portal vein is decidedly the larger. It has collected the blood from the abdominal organs by the radicles of its tribu- taries, the gastric, splenic, superior and inferior mesenteric veins, while the hepatic artery is a branch of the celiac axis. These, having been distributed in a manner to be noted presently, dis- charge their blood into the radicles of the hepatic veins, which, usually three in number, enter the ascending vena cava, where that vessel passes through the liver behind. Again, it is to be remembered that these two vessels, as well as the nerves and lymphatics, are enveloped in the vagina, or capsule of Glisson. The portal vein and the hepatic artery give off branches to the capsule of Glisson, constituting the vaginal plexus. The portal vein, still ensheathed, then divides and subdivides until io6 THE PHYSIOLOGY OF DIGESTION AND ABSORPTION its branches run directly between the lobules, and are called in- terlobular veins. These direct subdivisions of the portal vein are not the only interlobular veins, however. Those branches of this vein which were given off to the capsule of Glisson, having received the corresponding branches from the hepatic artery, also here run between the lobules and make part of the FIG. 44. — Diagram of the portal vein. (pv) arising in the alimentary tract and spleen (s) and carrying the blood from these organs to the liver. (From Brubaker after Yeo.) interlobular plexus. The interlobular veins, thus surrounding the lobules and having lobules on either side of them, giving off in both directions branches (lobular branches) which penetrate the lobules, to break up into capillaries. The capillaries finally converge to three or four small radicles, which in turn unite to form a small vein in the center of the lobule. This is the in- THE LIVER 107 tralobular vein, which at the base of the lobule joins the sub- lobular vein. These sublobular veins join each other to form FIG. 45. — Section of lobule of liver of rabbit in which the blood capillaries and bile canaliculi have been injected. (From Yeo after Cadiat.) a, intralobular vein ; b, interlobular veins; c, biliary canals beginning in fine capillaries. hepatic veins, which become larger and larger until they have collected all the blood which has entered the liver. They finally enter the ascending vena cava. 108 THE PHYSIOLOGY OF DIGESTION AND ABSORPTION But what has become of the hepatic artery? As soon as it has entered the sheath, it gives off branches to the capsule forming part of the vaginal plexus and entering into the vaginal branches of the portal vein just before these run between the lobules. It also furnishes branches to the wall of the portal vein, to the wall of the larger divisions of the artery itself, and to the hepatic duct. Histology of a Lobule. — The liver is made up of a large number of lobules about one-twenty-fifth of an inch in diameter, separated by vessels, nerves and radicles of the hepatic duct. Such a lobule in certain of the lower animals has a distinct poly- gonal shape, but in man the outlines are not clear. In the lobule are the hepatic cells, ovoid in shape, possessed of small granules and one or two nuclei. They are disposed in columns radiating from the central intralobular vein. These cells belong to the epithelial type, and the liver is not essentially different from other glands, such as the salivary, except in the complexity of its arrangement. The analogy is established by the origin of the bile ducts in the lobules between the cells. Bile Ducts.— It is not difficult to demonstrate the interlobu- lar ducts, but to follow them as such into the lobule is less easy. However, there is no doubt at all that they do originate between the hepatic cells. It is probable that here they have no distinct lining membrane, but are mere tubular intercellular spaces, into which the bile is poured and carried into the interlobular duct. Typically a liver cell has one of these bile capillaries on one side and a blood capillary on the other, and while this rela- tion does not always hold good, every cell does communicate with both kinds of capillaries. The interlobular bile ducts con- sist of epithelium resting upon a very thin basement membrane. As they increase in size they gain fibrous inelastic and elastic tissue, and the largest, some non-striated muscular elements. Gradually as the ducts become larger the lining epithelium changes from the columnar to the pavement form. Mucous glands exist in the largest ducts. The interlobular ducts join THE LIVER ICQ each other and gradually increase in size as they merge from all parts of the liver, to leave its substance in two divisions — one from the right and one from the left lobe. These two unite to form the hepatic duct which, running a course of about one and a half inches, is joined at an acute angle by the cystic duct to form the common bile duct, or the ductus communis chole- dochus. This last penetrates obliquely the duodenal wall and Branch of portal vein. Large interlocular bile duct. Interlobular con- nective tissue. FIG. 46. — From a horizontal section of human liver. X4O. Three central veins, cut transversely, represent each a center of as many hepatic lobules, that at the periphery are but slightly defined from their neighbors. Below and to the right of the section the lobules are cut obliquely and their boundaries cannot be distinguished. (From Stohr.) discharges the bile into the intestine. The cystic duct has its origin at the apex of the gall bladder, and is about one inch long. The common bile duct has an average length of three inches. (See Fig. 43.) Gall Bladder.— The gall bladder has an oval shape with its large end forward. It is on the under surface of the liver, the ^peritoneum running over (or rather under) it. It has a mucous 110 THE PHYSIOLOGY OF DIGESTION AND ABSORPTION lining and the remainder of its structure is chiefly fibrous. A little plain muscular tissue may exist. Its capacity is about one and a half ounces. Mucous glands are found in its lining, as in that of the large ducts, and these are responsible for the mucin of the bile. Hepatic Nerves. — With regard to the exact destination of the nerves entering the liver, little is known. Evidence going to establish the termination of fibers in the cells, that is, the existence of distinct secretory fibers, is meager. There is little doubt that secretory fibers for the glycogenic function of the liver do exist. It is known that fibers from the vagus, phrenic and solar plexus enter the fissure, but they cannot be followed farther than the ramifications of Glisson's capsule between the lobules. Of course, vaso-motor fibers go to the vessels, as else- where. Fibers acting similarly go also to the muscular tissue of the large ducts and of the gall bladder. The contraction of the gall bladder is thought to be reflex, afferent impressions being conveyed by the vagus from the mucous membrane of the duodenum. Hepatic Lymphatics. — The lymphatics are abundant, and those not originating in the connective tissue are thought to originate by perivascular canals surrounding the blood-vessels of the lobules. The fact that when the outflow of bile is occluded it passes, not into the vascular, but into the lymphatic circulation is a curious circumstance. It may be due to the absence of a definite wall for the intralobular ducts and their comparatively free communication with the lymphatics in those localities. Properties and Composition of Bile. — Human bile is of a dark greenish-red color, has a bitter taste and is practically odor- less when fresh. It undergoes putrefaction easily, but is not coagulable by heat. It is viscid, chiefly on account of the mucin it contains. It has an alkaline reaction, and a specific gravity of about 1030. Besides water, which constitutes more than ninety per cent, of its bulk, it contains the sodium salts of tauro- THE LIVER III cholic acid and glycocholic acid (the biliary salts), cholesterin, bilirubin, lecithin, fats, soaps, mucin and various inorganic salts, such as sulphates, carbonates, phosphates, etc., and a quantity of carbon dioxide. The quantity of bile secreted in twenty-four hours is about two and a half pounds. In human bile sodium taurocholate largely predominates over glycocholate. These are formed as acids by the liver cells, are absorbed in their passage down the intestine, and are pre- sumably those parts of the bile which are concerned in its digestive action, particularly in the absorption of fats. So far as these constituents are concerned, the bile is a typical secretion. Cholesterin, on the other hand, seems to be simply removed from the blood by the liver cells, and is discharged in the feces, where, however, it exists in a slightly changed form, stercorin. It is thought to be held in solution by the bile acids, glycocholic and taurocholic. So far as this constituent is concerned, there- fore, the bile is a typical excretion. It is produced in many of the body tissues, and no function has been discovered for it. Bilirubin is the characteristic coloring matter of the human bile ; that of herbivorous animals is biliverdin, and a little of this latter is also present in human bile. These pigments originate from hemoglobin. It is supposed that when the red corpuscles break down, "the hemoglobin is brought to the liver, and then under the influence of the liver cells is converted into an iron-free compound, bilirubin, or biliverdin." (Howell.) The lecithin is probably an end product of physiological activity in the tissues, and is apparently an excretion. The mucin gives the fluid its viscid character. The production of bile is continuous, but this does not mean that its discharge into the duodenum is continuous, for in the intervals of digestion it is not admitted (freely at least) into the intestine, but regurgitates from the ductus communis choledo- chus through the cystic duct into the gall bladder, which acts as a reservoir until its contents are needed. The secretion is more 112 THE PHYSIOLOGY OF DIGESTION AND ABSORPTION active, however, during intestinal digestion than at other times. This appears to be reflex, but may be simply a result of the in- creased amount of blood passing through the portal vein to the liver during that period, for the whole alimentary canal is con- gested while digestive activity is in progress. Again, it is known that the best cholagogue is bile itself, and some of the bile is absorbed in its passage down the intestine. Its presence in the blood may account for the accelerated flow. Method of Secretion and Discharge.— The bile is a product of the liver cells. How they receive their normal stimulus is obscure. But it is reasonable to suppose that a larger supply of blood means a more abundant secretion. Such an increase of blood supply occurs during digestion. The cells discharge the bile into the bile capillaries, which pass it onward either to the intestine directly, or, during the intervals of digestion, to the gall bladder. When food enters the duodenum, a reflex influence causes the wall of the gall bladder to contract and compress its contents. The only out- let is through the cystic duct into the common duct, and thence into the duodenum. This reflex does not take place until food has entered the duodenum, and of different foods it is found that proteids (peptones) and fats are the most efficient stimuli. The secretion of bile is not stopped by ligation of either the portal vein or the hepatic artery, showing that both of these vessels contain bile materials. But it would be unreasonable to suppose that the blood of the portal vein does not furnish the bulk of secreting material. Glycogenic Function. — The formation of glycogen is con- nected with nutrition, but will receive some notice here. This is an internal secretion. It is produced by the liver cells, and can be demonstrated in their substance by the microscope and by chemical reagents. It can also be shown to increase markedly THE LIVER 113 after eating, and to decrease notably when eating is refrained from for some time. Glycogen is a carbohydrate very similar to starch, and when ingested it is acted upon by the same enzymes and undergoes the same conversions. Furthermore, the amount of glycogen in the liver is very greatly increased by restricting the diet to carbohydrate foods and is lessened considerably below the nor- mal (that is, its amount on a mixed diet), but is not reduced to zero, when proteids alone are taken. This points to the con- clusion that the source of glycogen is carbohydrates, but that it can be formed to some extent from proteids. Let it be said now that practically all carbohydrates are converted by digestion into maltose, or maltose and dextrin and furthermore that during absorption these sugars are converted into dextrose or dextrose and levulose. It is customary to assume that the digestion of a carbohydrate means its conversion into dextrose (glucose, levu- lose). It is, then, this sugar which is carried to the liver by the portal vein. We may say that the formula for dextrose is C6H]2O6 and for glycogen C6H10O5, though neither of these formulae is probably exactly correct. It will be seen, therefore, that the abstraction of one molecule of water (H2O) from dextrose will produce glyco- gen, and this is the change which the liver cells are supposed to effect. Again, when the conversion of dextrose into glycogen has taken place, the glycogen is stored up in the liver cells, to be given off continuously to the blood only in such quantities as the system may demand. The liver thus becomes a warehouse for the storage of all the carbohydrates. It will be seen under Nutrition that the carbohydrates furnish the chief material to be burned up in the body for the purpose of liberating heat and furnishing energy, and if they should be consumed as soon as they enter the circulation, there would be not only an unnecessary waste during Iheir quick consumption, but also an unfortunate lack of energy-producing materials before 114 THE PHYSIOLOGY OF DIGESTION AND ABSORPTION another meal. This storing up brings about a kind of conserva- tion of energy and an economical regulation of its distribution. The amount of sugar in the circulation at any time is very small, and a single carbohydrate meal may, by the action of the liver, be made to supply the carbohydrate demands of the tissues for a considerable period. Now, it was just said that the sugar of the blood is dextrose; if the dextrose of the portal blood is converted into glycogen to be stored up, it must be reconverted into dextrose before it can leave the liver, since it leaves by the blood. The cells do effect the second conversion, and this is the second part of the glycogenic function. It may be that the liver cells produce an enzyme corresponding to ptyalin, which converts the glycogen. Dex- trose does not normally exist in the liver cells. At the very moment of its formation it is carried away by the blood. The fact that the liver can form glycogen out of proteids shows, of course, that the nitrogen is eliminated from the proteid mole- cule in some way. A carbohydrate molecule is left to be oxidized in the usual manner. This is thought to be the initial step in the final consumption of proteids in nutrition. The fats have no influence on glycogen formation. Glycogen also exists in other parts of the body, particularly in the voluntary muscular substance. The cells of the tissue in which it is found must also have a glycogenic function. Urea Formation. — But the liver has another function besides the production of bile and glycogen, and that is to form urea. It will be seen later that the chief end product of proteid metab- olism is urea, and that it is eliminated almost entirely by the kidneys. The liver is much more active in the production of this substance when the portal blood is charged with digested materials, but it also forms urea in fasting animals. The liver must, therefore, be capable of forming urea from some of the products of digested foods. With reference to its formation in fasting animals, suffice it to say here that it seems that as long as THE INFLUENCE OF THE BILE ON DIGESTION 115 proteid metabolism goes on in other tissues, there are produced, in those tissues materials (ammonia compounds) which, when carried to the liver, are converted by it into urea. Further notice will be given to this phase of the subject under Nutrition. The liver cells produce urea; it enters the blood, is carried to the kidneys and eliminated by those organs. In the mechanism of its production and discharge from the liver, it thus corresponds to the internal secretions, though urea is distinctly an excretion. It must not be supposed, however, that the liver is the only organ producing urea. There are other organs which certainly produce it, while there are those who maintain that it is produced directly wherever proteid metabolism is in progress. The Influence of the Bile on Digestion The bile is not, properly speaking, a digestive fluid, for it contains no enzyme capable of effecting digestive changes in any of the foods; but it so materially affects the action of some of the other fluids that it cannot be overlooked in a discussion of intestinal digestion. So far as the bile acids, glycocholic and taurocholic (com- 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 Il6 THE PHYSIOLOGY OF DIGESTION AND ABSORPTION 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 Secretion of the Intestines. The intestinal secretion, or succus entericus, is a product of the crypts of Lieberkuhn and Brunner's glands. It is scanty, of a yellow color and an alkaline reaction. Opinions vary as to what foods are affected by this fluid, but since the more recent experiments have overcome some difficulties in obtaining speci- mens, the conclusions based upon them seem most reliable. It is said to have no effect on proteids or fats. It contains an amylo- lytic enzyme, which aids the pancreatic juice in converting starch into maltose. It also has an enzyme, invertase, which converts cane sugar into dextrose and levulose, as well as an allied enzyme, maltase, which converts maltose into dextrose. The carbohydrates are absorbed as dextrose, with the probable excep- tion 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 absorbed only as dextrose. Movements of the Small Intestine.— The effect of intestinal movements is to force the contents onward through the ileocecal LARGE INTESTINE II 7 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 segment of the circular fibers with relaxation just behind the advancing con- traction. 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 intes- tine 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 move- ments, which are slow, gentle and gradual in character, is finally effectual in passing the contents into the colon. It is not probable that antiperistaltic movements take place normally. Nerve Supply. — Very probably the intestinal movements are naturally excited by the food and by the bile. It is probable also that these stimuli exert their influence through the ganglia of the plexuses of Auerbach and Meissner. The intestine receives fibers from the right vagus and the sympathetic. The former are probably motor (contractors) and the latter inhibitory (dilators). Here, as in the stomach, they are probably only regulators of the movements, without being actually necessary to peristalsis. The Large Intestine. Anatomy. — The large intestine, known as the colon, is about five feet in length and is divided into ascending, trans- verse and descending portions. 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. Il8 THE PHYSIOLOGY OF DIGESTION AND ABSORPTION 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 ascending 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 di- rectly across the upper part of the abdominal cavity 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 through the appendiculo-ovarian ligament. The appendix has no function. The ileo-cecal valve, guarding the opening between the large and 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 arrangement the two portions of the gut communicate only by a buttonhole slit, LARGE INTESTINE Iig which is easily opened by pressure from the direction of the ileum but which pressure from the cecum tends to close more firmly. Structure. — The large intestine has the usual three 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 adher- ent to the subjacent parts. In it are found glands corresponding 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 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 absorp- tion. By some unknown process, however, rectal aliments of an easily digestible nature are absorbed, and that in a nutritive 120 THE PHYSIOLOGY OF DIGESTION AND ABSORPTION 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 character- istic color. The odor is mainly due to bacterial decomposition, 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 (proteid) reaction, and that the proteids 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 carbohy- drates, with the production of acids which inhibit proteid putre- faction, and account for the acid reaction at the ileo-cecal valve. When the mass has entered the colon the acidity is 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 carbohydrates and proteids are leucin, tyrosin, indol, skatol, phenol, lactic and butyric acid. The object of the production of these substances is unknown. 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 undigested matters, inactive salts, stercorin, mucus, epithelium from the intestinal wall, coloring matter and substances resulting from bacterial activity. Stercorin is the converted form of cholesterin, a constituent of the bile. The coloring matter is from the pigment (bilimbiri) of the same fluid. Of the bacterial products LARGE INTESTINE 121 the most important are indol and skatol. They represent pro- teid putrefaction; they are responsible for the fecal odor; hence the characteristic difference in the odor of the contents 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 carbureted 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 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 is 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 effort 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 122 THE PHYSIOLOGY OF DIGESTION AND ABSORPTION 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. ABSORPTION IN GENERAL. 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 absorption. While it is known that the laws of diffusion and osmosis out- side the body are largely responsible for absorption within the organism there are many phenomena in connection with that process which cannot be explained under these laws, and which ABSORPTION IN GENERAL 123 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 due 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. B ut if to the water on one side there be added a solution of some crystal- loid, as sugar, the excess of water will pass to that side. The crystalloid in solution is said to exert an osmotic pressure, and 124 THE PHYSIOLOGY OF DIGESTION AND ABSORPTION that pressure depends upon the density of the solution. In course of time, however, the crystalloid passing itself through the membrane, conditions of equal volume and density will be estab- lished on the two sides of the membrane, and osmosis in either direction will cease to be apparent. But if the membrane be non- permeable 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 Oi 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 injected 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 extravascular fluids, and thus to cause an escape of water/row the vessels. On the other hand, the osmotic pressure exerted by the proteids of ABSORPTION IN GENERAL 125 the blood is comparatively small. But since they are here chiefly as colloids and tend to maintain the concentration of the circulating fluid, their effect is a permanent factor influencing absorption into the blood-vessels. Isotonic and hypotonic solutions are those having equal and less densities respectively as compared to blood serum. Hypo- tonic 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- molic. 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 numbrane 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 direction 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 secretion, excretion, etc. It may be that the capillary walls are permeable to certain substances in certain situations and not in others. In the body it may be said that well-nigh all the vital functions 126 THE PHYSIOLOGY OF DIGESTION AND ABSORPTION 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; rami- fying in the wall of that canal are blood and lymph capillaries filled with fluid; while separating the two is an animal membrane consisting of the alimentary epithelium, a little connective tissue and the endothelial lining of the capillaries. These are condi- tions 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 following facts seem clear as to absorption in the body: (i) The substance must be in a liquid or gaesous 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 rapidly will absorption into them take place; (7) absorption is more rapid the more rapid the blood current (continually preventing "saturation" of the adjacent blood) ; (8) the higher the temperature the more rapid is absorp- tion; (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 absorption into the lymphatic circulation and passage thence to the left subclavian vein. Those lymph ABSORPTION FROM THE ALIMENTARY CANAL 127 capillaries which are concerned in this absorption occupy the villi, and are called lacteals. (A) From the Stomach. — Since all classes of food except fats have been partly digested in the stomach, it follows that all except fats may be absorbed here. However, as a matter of observation, the stomach is of much less importance in absorp- tion 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 substances can undoubtedly be absorbed by the gastric mucous 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. (B) From the Small Intestine. — Here absorption of all classes of food is possible, and here in fact most of the foods are ab- sorbed. The digestive influences are more active upon all the aliments, the mucous membrane is well adapted to absorption by reason of its valvulae conniventes and its villi, and the food necessarily remains in the small intestine for a consideraole time. The fats are absorbed in the upper part of the small intestine; for they pass into the lacteals of the villi, and these do not exist in the lower ileum. The fluids swallowed are almost com- pletely absorbed here, but their place is taken by the intestinal 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. The consistence of the contents progressively increases owing to 128 THE PHYSIOLOGY OF DIGESTION AND ABSORPTION 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 carbohy- drates which have escaped absorption in the small intestine 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 com- mon 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 mate- rially affected until the colon is reached. Here absorption of water is active, and the mass becomes more and more solid as the rectum is approached. 2. Proteids. — It is agreed that the first object of proteid diges- tion 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 opinion that proteoses and peptones are the absorbable forms of proteids is correct, for by far the largest part of these foods are absorbed in this shape. It is supposed also that syntonin at least can itself be sparingly absorbed from the alimentary canal, ABSORPTION FROM THE ALIMENTARY CANAL 1 29 while the phenomena of rectal absorption would point to the conclusion that proteid absorption in other shapes is possible. Practically, 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- vessels, the artificial injection of them into the veins occasions untoward effects, or at least their rejection through the organs of excretion. Furthermore, proteoses and peptones cannot be detected 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 effect 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 cane 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 flow of lymph through these spaces toward the lacteal. The 9 130 THE PHYSIOLOGY OF DIGESTION AND ABSORPTION 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 substances 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. CHAPTER VIII. 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 volume 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 pulmonary 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 removed 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, constitutes what is commonly called respiration ; but the mere tide of the air inward and out- ward is of no significance unless the interchange of oxygen and carbon dioxide takes 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 to and away from the cells, are all equally immaterial except as being means to the accomplishment of this end. It would make 132 RESPIRATION 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 interchange 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 carbon dioxide. The pulmonary interchange is properly termed external res- piration 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 furnished by the ordinary aliments, they will retain their vital activity for a longer time when deprived of any or all of these than when deprived of oxygen alone. This gas is more immediately 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. It will be considered that the air has passed through the pos- terior nares into the pharynx and is ready to enter the larynx. ANATOMY OF THE RESPIRATORY ORGANS 133 The Larynx. — This lies in front of the esophagus, its upper opening communicating with the middle pharynx. It is com- posed 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 degluti- FIG. 47. — 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. (Yeo.) tion, respiration 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. 48.) The vocal cords, two ligamentous bands covered by a thin layer of mucous membrane, stretch antero-posteriorly across the upper end of the larynx, while the false vocal cords, having nothing to do with phonation, and pinker in color, are above 134 RESPIRATION FIG. 48. — Outline showing the general form of the larynx, trachea, and bronchi, as seen from behind. h, great cornu of the hyoid bone; t, superior, and t', the inferior cornu of the thyroid cartilage; e, epiglottis; a, points to the back of both the arytenoid cartilages, which are surmounted by the cornicula; c, the middle ridge on the back of the cricoid cartilage; tr, the posterior membranous part of the trachea; b, b', right and left bronchi. (Kirkes after Allen Thomson.) THE TRACHEA 135 and parallel with the true cords. A small triangular leaflet of fibre-cartilage is attached by its base to the base of the tongue and to the upper anterior part of the larynx. This is the epi- glottis. 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 laryn- geal cavity. The vocal cords are fixed anteriorly to a point between the alae 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 inspiratory 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 of cartilaginous rings, and an inter- nal 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 entrance of air. These rings are sixteen to twenty in number, and are lacking in the posterior third or fourth of the circumference. They are, therefore, not true rings. The inter- val between their ends is filled with fibrous and non-striped mus- cular tissue. The mucous membrane is lined by ciliated epithe- lium, and has mucous glands in its substance (Figs. 47, 48). 136 RESPIRATION 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 the primitive bronchus has divided, the incomplete cartilaginous Bronchial Muscfe. Bronchia/ flrtery. G fa not acini fe of net FIG. 49. T. S., intra-pulmonary bronchus of cat. PA and PV, pulmonary artery and vein; bv, bronchial vein; V, air vesicles. (Stirling.) 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 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 AIR VESICLES 137 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 become 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 T^j- to -fa 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 J in. in diameter. The bronchiole entering the lobulette becomes the infundibulum (Fig. 50), 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 -^VrV ^n>> *nat °f the vesicle about 2oo~7lo in- ^ nas 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 ex- change of gases between the blood and air, each capillary being exposed to vesicles on both sides, and the air and blood 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 138 RESPIRATION blood is furnished by the bronchial artery, which enters and 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. Pig. 50 — 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; b b, air-sacs, c c, infundibula. Xio. (Kirkes after E. E. Schulze.) 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. Super- ficially the lung substance is seen to be subdivided into areas about J in. in diameter called the lobules. Each lobule is com- posed of a number of lobulettes as above mentioned. 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, the thorax is a closed cavity, not- MECHANISM OF RESPIRATION 139 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 passive themselves. That is to say, they possess no power of expansion 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 condition which characterized it before it was subjected to the expanding force. Before birth there is no air in the lungs, and this is the only time when the elastic tissue is not on the stretch. The bronchi- oles and air cells are collapsed, but the thorax is contracted and the pulmonary and thoracic walls are in contact by their respec- tive 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 always a tendency to empty the lungs, but cannot do so because the thorax can contract only so far, and when its con- traction 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 140 RESPIRATION reason above given, namely the collapse of the bronchioles be- fore all the air can escape, is inoperative under normal condi- tions 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 expiration is a cause and not a consequence of thoracic contraction. This statement as to expiration applies only to ordinary tranquil respiration, as will be seen later. Speaking broadly then, inspira- tion 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 commu- nicating 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 the bellows itself made air-tight afterwards, 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 tend- ency to do so. It is also apparent that, since the bag exerts a continual compressing effect on its contents, the pressure inside INSPIRATION 141 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, communicating by the trachea with the external air and following, or being fol- lowed by, the movements of the thorax; the pressure in the bag and between it and the bellows wall represents the mtrapulmo- nary 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- spiration; and any muscles which increase any of the thoracic diameters expand the thorax. The diameters increased are chiefly the (i) vertical and (2) antero-posterior. The vertical is increased by descent of the diaphragm, which descent is caused by its contraction, since, owing to the intra- 142 RESPIRATION thoracic "pull" exerted upon it, it is normally vaulted upward. The antero-posterior diameter is increased chiefly by the ele- vation 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 oj 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 wal'. 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 ex- ternal intercostals will render the ribs more nearly horizontal by raising their anterior movable extremities. It seems that the first rib is prevented from descending, probably by the simulta- neous 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 extremities 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. Contraction 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 " EXPIRATION 143 ^These are the muscles which are chiefly concerned in ordinary 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, sterno-mastoideus, levator anguli scapula, tra- pezius, pectoralis minor, pectoralis major (costal portion), serratus magnus, rhomboidei and electores spince. It will be noticed that several of these which usually take their point on the chest, as, for example, the sterno-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 upon the stretch during inspira- tion and which are now seeking to return to their normal con- dition, tends to restore the thorax to the dimensions it had pre- vious to the inspiratory act. So far no actual muscular con- traction has been brought into play, and it is here assumed that none is usually concerned in the expiratory act of ordinary tran- quil 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 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 144 RESPIRATION 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, blow- ing, talking, etc. This is effected by certain muscles whose con- traction 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 internus transversalis and rectus abdominis. These depress the chest wall as well. Rhythm of Respiration. — Under ordinary conditions inspi- ration and expiration follow each other in a regular rhythmical fashion. Some 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 throughout, while the expiratory act gradually diminishes in intensity as it approaches completion — a circum- stance to be expected from the physical conditions causing it. 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 rhythmical respiration. Modified Respiration. — In coughing and sneezing a profound RATE OP RESPIRATION 145 inspiration precedes a violent convulsive contraction of the expiratory 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 deficient 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 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 subject 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. 10 146 RESPIRATION 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. According as the movements in costal respiration are more pronounced in the upper or lower segment of the chest, that type is subdivided into (a) superior costal and (b) inferior costaL In young children the diaphragmatic, or abdominal, type prevails; 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 civilized countries, and in part to the provision against encroachment of the uterus upon the abdominal cavity during pregnancy. Intrapulmonary and Intrathoracic Pressure. — It is evi- dent that during inspiration the pressure inside the lungs (intra- pulmonary) 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 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 PULMONARY CAPACITY 147 "pull" of the elastic lungs upon the chest walls the intrathoracic pressure remains negative even at the end of expiration. But it can be made to become positive under forced action of the expiratory 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 100 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- cumstances be called into use, and consequently the vital capac- ity is equal to the total capacity minus the residual air (100 cubic inches), or 230 cubic inches. It is the volume which can be ex- pelled by the most forcible expiration after the most forcible inspiration. 148 RESPIRATION 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 150 cubic inches. Quiet inspiration increases it to about 1 80 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 conditions. Composition of Air. — Ordinary atmospheric air contains, in round numbers, about 21 parts of oxygen to 79 parts of nitrogen. These two gases make up the main bulk of the atmosphere. In addition, the atmosphere always contains a little carbon dioxide (about .04 per cent.), ammonia, moisture, organic 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 CO2 and much less O than the inspired air. The inter- change of gases between the alveolar air and the blood is re- sponsible for the difference. The question is what forces cause the O of the air to enter the alveoli and the CO2 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 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 CO2 in the expired air continuously increases until the end of the act. At each inspira- DIFFUSION IN THE LUNGS 149 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 CO 3, aid very materially in keeping constant the composition of the alveolar air. In the second place, the cardia'c movements have a similar effect, each systole decreasing the size of the heart and inducing a fresh atmospheric current toward the deep alveoli, and each diastole 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 at- mosphere is i^ of the total pressure. Now, in the air of the alveoli O is present to a less extent than 21 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 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 CO2 opposite conditions prevail. This gas is being 1 50 RESPIRATION 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 CO2 in the alveoli is much higher than in the upper respiratory passages, and a continual curreht 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 CO2 away from the alveoli. Under their influence the composition of the alveolar air remains fairly uniform. Alterations of Air in the Lungs. — These are chiefly: (a) Loss of oxygen, (b) gain of carbon dioxide, (c) elevation of tem- perature, (d) gain of water, (e) gain of ammonia, (/) gain of organic matter, (g) gain of nitrogen, (h) loss of (actual) volume. The capital changes are loss of O and gain of CO2. (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 21 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 ac- tivity, and the amount actually necessary is larger than this. The circumstances which call for an increase in O almost in- variably cause an increase in the output of CO2. (b) Gain of Carbon Dioxide. — The amount of CO2 in inspired air is about .04 part per hundred (4/100 per cent.); the amount ALTERATIONS OF AIR IN THE LUNGS in expired air is something more than 4 parts per hundred. In round numbers then, the air in passing through the lungs gains of CO 2 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. Oxygen 20.96 vols. per cent. 16.03 v°ls- Per cent. Nitrogen Carbonic acid Watery vapor Temperature 79 vols. per cent. 0.04 vols. per cent, variable, variable. 79 vols. per cent. 4.4 vols. per cent, saturated, that of body (36° C.). Conditions Influencing Output of CO2. — When the rapidity of respiration is increasing, the depth remaining constant, the percentage 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 CO2 is exhaled than under opposite conditions. The amount of CO2 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. In the female the output is less than in the male. In the former 152 RESPIRATION 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 CO2 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 CO2 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 CO2 in that part is at once diminished. The consumption of O and the exhalation CO2 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 CO2 may be applied to the consumption of O. (c) Gain in Temperature. — When the body temperature is normal and the external atmospheric temperature aboutyo0 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 mouth. 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. OXYGEN CONSUMED AND CARBON DIOXIDE EXHALED 153 (d) Gain of Water. — This water is in the form of vapor. It is natural that the air should absorb water from the moist surfaces 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. (e) Gain of Ammonia. — Ammonia is exhaled in small quan- tity by the lungs. It is insignificant except in cases of suppressed kidney action. (/) Gain of Organic Matter. — The quantity of organic matter 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. (g) Gain of Nitrogen. — The exhalation of this gas by the lungs is of no respiratory importance. The amount is said to be ito— sV *ne amount of oxygen consumed. An occasional loss of nitrogen has been observed. (h) 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 increase 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 ^Wo °f tne 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 CO2; or the amount of O in a given volume of CO2 is equivalent to that volume when set free from the carbon. A cubic foot of 0 will unite with carbon to form 154 RESPIRATION a cubic foot of CO2; or a cubic foot of CO2 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 CO2, the amount of CO2 exhaled 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 CO2 is only about 4 per cent, of the expired air. It follows, therefore, that i per cent, of the vol- ume of inspired air is not represented by the CO2 exhaled from the lungs and skin. The relation between the consumed O and the exhaled CO2 is usually expressed as the " respiratory quotient" — the division of the latter by the former giving the quotient. This quotient is made to vary by many circum- stances, though for any considerable period its average is about the same. While it has been stated that the O absorbed and the CO2 produced vary together usually, they are in a certain measure independent of each other. For CO2 does not result from the immediate 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 CO2, either by oxida- tion or by intramolecular cleavage. Furthermore, more O is necessary to oxidize (that is, to form carbon dioxide) some mole- cules than others. A fat requires considerably more O to pro- duce CO2 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 represented in the CO2 exhaled from the lungs and skin, it is certain that when various of the food stuffs are broken down at least a part of it is appropriated by hydrogen to form water. Source of Exhaled Carbon Dioxide. — The increase of CO2 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 CONDITION OF CO2 IN THE BLOOD 155 by its escape when the blood is placed in a vacuum. The total amount escapes when the blood intact is placed in vacuo: when the corpuscles alone are so treated they yield up all their CO2, though it is small in amount ; but the plasma alone in vocuo yields a less amount than when it contains corpuscles. If, now, corpus- cles be added to the plasma the total amount of CO2 is forth- coming. 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 CO2 in the Blood. — About 5 per cent, of the total amount of CO2 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 com- bination with the proteid portion of hemoglobin. The total 75-85 per cent, in corpuscles and plasma is so loosely combined that the mere diminution in pressure in the lungs is probably 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 wh'ich is dissociated on the addition 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 CO2 in combination — the liberation of the latter being the consequence. The O being thus in the air vesicles, and the CO2 thus free, or set free, in the blood, with the very thin animal membrane con- 156 RESPIRATION sisting of the vesicular and capillary walls between them, it remains 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 CO2 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 mixture has been mentioned. Each gas exerts a pressure in proportion to its percentage in the mixture, and this is called its "partial pres- sure." Now, the extraction of O and CO2 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 CO2 in the venous blood is much greater than the partial pressure of the CO2 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, CONDITION OF OXYGEN IN THE BLOOD 157 they are insufficient to explain the occurrence 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 CO2 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. Dis- sociation of oxyhemoglobin occurs when the pressure is suffi- ciently 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 CO2 and the gain of O. In 100 volumes of arterial or venous blood there are found to be, on an average, 60 volumes of O and CO2. 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 con- ditions to be mentioned. In arterial blood the O will represent about 20, and the CO2 about 40, of the total 60 volumes per hundred of gas. In ordinary venous blood the O will represent 158 RESPIRATION about 7 volumes less (13) and the CO2 about 7 volumes more (47) of the total 60. In both venous and arterial blood there is an insignificant amount of nitrogen, which is usually present to the extent of 1.5 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 CO2 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 activity blood is sent to the gland in increased amount to furnish materials for secretion, while the demand for oxygen is not relatively increased in that gland. Besides the changes which are apparent on referring to the alterations in the air passing through the lungs, there are certain 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 the 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 diges- tion 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 identical. 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 coagulaUe than venous. Internal Respiration. — It has been said that the object of external respiration and the transportation of O and CO2 is to OXYGEN AND CARBON DIOXIDE IN THE TISSUES 159 make internal respiration possible. Oxygen, leaving the alveoli in a manner already described, enters the blood and at once com- bines with hemoglobin of the red corpuscles to form oxyhemo- globin. A small portion of the O is used up by the corpuscles in transit, with the production of CO2 and other metabolic mate- rials— 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 CO., — a result of their metabolic activity. The blood, having thus given up its O, is changed in color, and carries the CO? back to the lungs to be exhaled. To furnish O and to remove CO2 is the only object of respira- tion. Living tissue exposed to an atmosphere containing O will consume O and exhale CO2 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 CO2, and O is directly necessary to this process. But the amount of CO2 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 prob- able 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 compara- tively little O is required to cause a final disintegration 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 CO2 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 food stuffs — although this is the final result. Interchange of Oxygen and Carbon Dioxide in the Tis- sues.— Here application of the principles governing the inter- change of these gases in the lungs applies. It is found that 160 RESPIRATION 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 CO2 in the tissues is much higher than in the blood, and the same physical laws encourage a current of CO2 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 CO2. It is estimated by Scharling that the skin performs about -^ 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 CO2 of the atmosphere. When the amount of O has been decreased to fifteen parts per hundred it is insufficient for the respiratory demands. When the CO2 is increased to .07 part per hundred the air becomes disagreeable and close; this is not, however, from the accumulation of CO2 so much as from organic emanations and disagreeable odors from the body, clothing, etc. It is only that the amount of CO2 serves as an indication of the extent of accumulation of these materials that the amount of .07 per cent, is fixed as the limit beyond which it ought not to be present. This percentage of CO2 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- sumed represent only about one-fourth of the O inspired, 60 ABNORMAL RESPIRATION l6l 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 continuou'sly, 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 atmospheric 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. Sulphurreted, 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 of CO2 in the blood. When an animal is made to breath in a small, confined space the amount of O soon becomes insufficient, ii 162 RESPIRATION even though the amount of CO2 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 CO2, it will suffer from dysp- nea also. In either case the manifestations are practically the same — slow, deep and labored respiration. In cardiac disease, hemorrhage, pulmonary diseases, etc., the dyspnea is from a lack of O in the tissues, because of enfeebled action of the heart, FIG. 51. — 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; l.v. left ventricle; a, aorta; p.a. pulmonary artery; p.v. pul- monary vein; r.a. right auricle; r.v. right ventricle; v.c.d. descending vena cava; v.c.a. ascending vena cava. (Kirkes after Sir George Johnson.) 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 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- RESPIRATION AND BLOOD-PRESSURE 163 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. 52. — The heart in the final stage of asphyxia. The letters have the same meaning as in Fig. 51; in addition, p. c. represents the pulmonary capillaries. The right auricle and ventricle, and the pulmonary artery, are fully distended, while the left cavities of the heart and the aorta are nearly empty. (Kirkes 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 just after the beginning of expiration; it gradually decreases from this time to the minimum just after the beginning of in- spiration. The general effect, then, of inspiration is to increase 164 RESPIRATION 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 arte- rial system at each ventricular systole. The explanation is some- what complex, but if the mechanics of respiration be under- stood it may be made satisfactory. FIG. 53. — Carotid blood-pressure tracing of a dog. Vagi not divided; I, inspiration; E, expiration. (Stirling.) It was seen that the lungs are contained in an air-tight cavity, the chest, and that they expand with the chest because of nega- tive pressure (" suction") exerted upon them. The heart is also a hollow organ situated in this cavity; it has connected with it, and lying also in the thoracic cavity, large vessels communicat- ing with smaller extrathoracic vessels. The heart and these great thoracic vessels are elastic and distensible. Consequently the expansion of the thorax also expands them slightly and tends to draw blood from the extrathoracic into the intrathoracic 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 extrathoracic vessels, and so to contradict the statement that inspiration increases arterial tension. But, remembering that we are dealing with arterial tension RESPIRATION AND BLOOD-PRESSURE 165 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 venae cavae. 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 increased 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 extrathoracic 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 opera- tive with contrary results. The intrapulmonary vessels and heart are compressed, but the veins and capillaries more than the aorta, with the result that less blood reaches the heart than l66 RESPIRATION during inspiration, and the thoracic aorta being, relatively to the thoracic venae cavae, 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 flaccid intrapulmonary 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 intrathoracic. 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 inspira- tion, 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. Respiration represents the activity of a self-governing apparatus. These movements constitute a finely coordinated set of contrac- tions— contractions which are regulated by means of afferent and efferent nerves under the supervision of the respiratory center. NERVOUS MECHANISM OF RESPIRATION 167 The respiratory center is in the lower part of the medulla oblongata. 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 sensi- tive 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 affect 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 CO2 in the blood will increase the rate. Increase in temperature of the blood, as in ever, will produce a similar effect. 4. The most important of these agencies is found in afferent impressions conveyed to the center. The fibers carrying these mpressions are chiefly in the pneumo gastric, glosso-pharyngeal, 1 68 RESPIRATION trigeminal and cutaneous nerves. Of these the pneumogastric is by far the most important. Section of a single pneumogastric is followed by variable res- piratory disturbances which usually disappear in less than an hour. Section of both nerves is followed, after a short interval of increased respiratory activity, by slow and powerful inspira- tions, by forced expiration and an appreciable interval before the next inspiration. Irritation of the central end of the cut nerve by a very weak current seems to stimulate the inhibitory 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 obtained, the accelerator portion of the center being stimulated. These facts show that the pneumogastrics possess both inspiratory and expiratory fibers, and that the former are stimulated more by a moderate current and the latter more by a very weak one. The rhythm of respiration, therefore, includes the regular sequence of inspiratory and expiratory movements upon each other. Now what is it that, under normal conditions, irritates the terminals of the pneumogastrics and causes them to convey inspiratory 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 conveyance of the inspiratory impressions, while the stretch- ing of the lung tissue originates the expiratory impressions. Others ascribe both inspiratory and expiratory impressions to lung movements — movements of inspiration exciting expiratory, fibers, and movements of expiration exciting inspiratory fibers. These observers cite the fact that artificial inflation and aspira- tion excite expiration and inspiration respectively. Stimulation of the superior laryngeal, as when foreign bodies accidentally enter the larynx, excites violent expiration. The glosso-pharyngeal contains afferent fibers especially impor- NERVOUS MECHANISM OF RESPIRATION 169 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 movement. There are, of course, running from the cortex to the respira- tory center intracranial fibers whereby the organ of the will makes its presence felt in respiration. But when all the afferent nerve connections are severed, respi- ration continues with modified rhythm and rate, at least for a time. It is thought that, under these conditions, it is the circulation 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 CO2 in the blood, and the impulses conveyed 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 facial, 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. To the lungs proper fibers are distributed by the vagus, the dorsal spinal and the sympathetic nerves. Besides the expiratory 1 70 RESPIRATION 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 vaso-motor fibers, which latter come from the cord by the roots of the dorsal nerves mentioned to join the sympathetic ganglia. CHAPTER IX. NUTRITION, DIETETICS AND ANIMAL HEAT. NUTRITION. ALL the processes of the body as digestion, absorption, secre- tion, 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 condi- tions influencing and some of the phenomena attendant upon the process. 171 172 NUTRITION, DIETETICS AND ANIMAL HEAT Metabolism. — By metabolism is meant those processes in the body whereby food products are appropriated, their stored-up energy utilized, and the waste discarded. Metabolism is divided into, (i) anabolism, and (2) katabolism. Anabolism is the process of building up tissue by cell appro- priation of food stuffs. Katabolism is the process of destroying tissue in order to set free energy that the organs of the body may perform their various functions. When the anabolic processes are equal to the katabolic there is no excessive storage of material, but an individual remains of uniform size, weight, and strength. If the anabolic are in excess of the katabolic processes, the excessive products are stored up in cells and an individual increases in size, weight and strength. If the katabolic processes are in excess of the anabolic there is a call on the tissues for the matter already stored there and there is a decrease in strength, weight and size. Death. — As long as a cell appropriates enough to supply the deficit caused by the destruction of material in the expenditure of energy, the cell will live ; but when the intake cannot make up for the output lost the cell ceases to functionate and this is called death. 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 consideration of the final fate of the various aliments, of their relative value in nutrition, of conditions influencing the process, and of the explanation of certain facts connected with the destruction 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 produc- tion of heat. The same sum total of heat is developed when a FOODS IN NUTRITION 1 73 piece of iron rusts completely away in five years as when it is con- sumed 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 food-stuffs. Fate of Different Foods in the Organism. — In the first place, the foods may be divided, into (I) those which pass through the organism unchanged and (II) those which lose their identity and are discharged as bodies different from those which entered. The first class includes the foods furnishing no energy; the second those furnishing energy. Only a few foods 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 foods producing no energy may be dismissed with a repetition of the statement that they are largely introduced in connection 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 inert constituents as a part of the molecule, and these seem as necessary to nutrition as do the energy furnishing con- stituents. The foods furnishing energy and those furnishing no energy enter, are deposited, and seem to be discharged both together. The few reactions which the inert foods undergo in the body do not materially affect the supply of energy. (II) The proteids, carbohydrates and hydrocarbons are all con- sumed in the organism, none (unless they have accidentally escaped 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- 174 NUTRITION, DIETETICS AND ANIMAL HEAT 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 sul- phates and phosphates, the production of which is comparatively immaterial. 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 ma- terial, 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 Proteids. — 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 "ni- trogenous equilibrium"; that is, to keep the intake of nitrogen up to the output. When nitrogenous food is withdrawn there con- tinues 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 protoplasm without a com- pensatory 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 regeneration of their substance. This excess may be termed " circulating 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 FOODS IN NUTRITION 175 nitrogenous supply which is built up into a part of the cell has been called "tissue proteid" 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 proteid; these demands are the first to be supplied by the nitrogenous material at hand. From this it is not to be inferred that the exigencies of nutrition 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 materials, the nu- tritive 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 materials. Life can be maintained on nitrogenous food alone, but it is obvious that when this is done the amount of circulating proteid must be enormously increased so that it may be oxidized 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 withdrawn from the economy. It follows, conversely, that the ingestion of car- bohydrates 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 of tissue proteid; they may be burnt in lieu of the circulating proteids and supply energy just as the carbohydrates and fats do. It is to be remembered that any excess of proteid or albumin- oid 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 develop- 176 NUTRITION, DIETETICS AND ANIMAL HEAT ment of heat is also an invariable accompaniment of their destruction. While a person may live on proteid food, the amount neces- sary taxes the digestive and excretory organs to such an extent that life is probably shortened. Since the total amount of urea is discharged by the kidney, that organ, under an excess of pro- teid 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 whatever 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 FOODS IN NUTRITION 177 CO2 — the other end product. The burning (oxidation) of a carbohydrate outside the body results in the formation of CO2 and H2O 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 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 oxidation. 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 — i. 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 digestion render it not practically so, since fats tax the digestive apparatus 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. Hence, 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 carbohydrates, 1 78 NUTRITION, DIETETICS AND ANIMAL HEAT 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. 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 con- verted into fat are as yet undetermined, but there are so many evidences of an increase in body fat upon an excess of carbo- hydrate food that the fact itself that this class of food is the main source of fat is no longer disputed. As regards the formation of fat from 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 gly- cogen 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 converted 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 CONDITIONS INFLUENCING METABOLISM 179 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, 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 por- tions of the organism against accidents to temporary deprivation of food, demands for an unusual amount of energy, malnutrition from various causes, etc. — sayings 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 glyco- gen 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 supply 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 discharged is increased. In striking con- trast to the constant output of urea is the largely increased out- put of CO 2, representing oxidation of the carbohydrates and fats. l8o NUTRITION, DIETETICS AND ANIMAL HEAT 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 oxidation of the energy-producing materials. Proteid metabolism 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 undis- turbed; 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 REQUISITES OF DIET I&I the body temperature and energy to carry on the necessary or- ganic functions, the organism is physiologically bankrupt and assignment follows — death is at hand. DIETETICS. The appetite, under normal conditions, may be depended upon to regulate both quantity and quality of diet in a fairly satisfactory manner. Different peoples require different proportions and amounts of the various food-stuffs and the same is true of any given individual for varying conditions of temperature, exercise, 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 Requisites 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 1 82 NUTRITION, DIETETICS AND ANIMAL HEAT 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 elements contained in the necessary quantities of other aliments. So, therefore, if we 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 daily discharge of nitrogen is about 18 grams (4i3) an ^he 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 decrease in energy-producing power at that time. The cell body becomes smaller, the dendrites atrophy, and the axones 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 neu- ron 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. 62 and 63. 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. 224 THE NERVOUS SYSTEM S.C. M.C: FIG. 66.— Reflex action; old idea. (Kirkes.) FIG. 67. — Reflex action; modern idea. (Kirkes.) NERVE FIBERS The relay service is illustrated in Fig. 64. Here again, it is seen that there is no actual joining of the neurons. When- ever 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 organs connected with the senses of sight, hearing, smell and taste, (4) in hair-follicles, (5) in simple free ex- tremities passing between epithelial and other cells, and (6) in several kinds of so-called tactile corpuscles. The motor nerves passing to volun- tary muscles form first a "ground plexus" for each group of muscle bundles — this plexus being made of the axis-cylinder fibrillae. From this plexus fibrils pass to form an "interme- diary plexus" corresponding to each muscle bundle. These fibrils are still medullated, 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 surface of the muscle fiber. This is called an end motorial plate. It con- tains a number of nuclei, and sends off 15 [M FIG. 68. — Diagram of an element of the motor path. U. S, upper segment; L. S, lower segment ; C. C, cell of cerebral cortex; S. C, cell of spinal cord , in anterior cornu ; M, the muscle; S, path from sensory nerve roots. (Kirkes after Cowers.) 226 THE NERVOUS SYSTEM from its under surface fine fibrillae which are said to pass between the muscular fibrillae which make 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 differences. Here the fibers are not medullated, and Nerve-fibre. End-plate. Muscle nucleus. FIG. 69. — Termination of a nerve 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 fibeis 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 above 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 NERVE FIBERS 227 a plexus beneath the true skin and send branches thence to the follicles, though the exact mode of termination is a question of some obscurity. Terminations between epi- thelial cells are probably more common than any other method of sensory distribution. The fibers, having passed to the sur- face of the skin or mucous mem- brane, lose everything excepting the axis cylinder, which, dividing into minute ramifications, passes, by means of these fibrillae, among the epithelial cells. This mode of termination is held by some to prevail in the glands. It cer- tainly prevails in parts other than the skin and mucous mem- branes. Sensory nerves further termi- nate in (a) the corpuscles of P acini or Vater, (b} the end bulbs, or tactile corpuscles, of Krause, (c) the tactile corpuscles of Meissner, (d) the tactile menisques, and (e) the corpuscles ofGolgi. (a) The Pacinian corpuscles are oval elongated bodies. Each corpuscular body has a length of about 1*2- 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. FIG. 70. — Vater's or Pacini's corpuscle. a, stalk; b, nerve fiber entering it; c, d, connective-tissue envelope; e, axis- cylinder with its end divided at /. (Landois.) 228 THE NERVOUS SYSTEM Through this pedicle passes a single (occasionally more) nerve fiber which, piercing the several concentric layers constituting the corpuscle, gradually loses its myeline substance and runs longitudinally 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 corpus- cles because they are situated beneath the skin ; neither can they be positively said to have any " special sensory" function such as the appreciation of temperature, weight, etc. (b) The end bulbs of Krause exist human conjunctiva, treated in great number in the conjunctiva, with osmic acid, showing the glans penis and clitoris, the lips, cells of core. (From Yeo and in other situations. They bear after Longworth.) -. , , , some resemblance to the corpuscles a, nerve fiber; b, nucleus of sheath; c, nerve fiber within of Pacint, but are much less elaborate core; a, cells of core. in their arrangement; the number of concentric layers is much smaller, while the contained mass is larger. The shape is spherical. From one to three medullated fibers pass from the underlying plexus to wind through the corpuscle and break up in free extremities. The sheath of the fiber is continuous with the outer covering of the corpuscle, and the medulla is gradually lost as the fiber enters FIG. 71. — End bulb from 229 NERVE FIBERS the bulb. The end bulb of Krause measures from yoV(r to of an inch in diameter. (c) The tactile corpuscles of Meissner have to do with the sense of touch, and are situated largely in the papillae of the skin covering the palmar surfaces of the hands and the plantar surfaces of the feet ; they also exist in other situations, correspond- ing in general to the distribution of the Pacinian corpuscles. The largest number is found over the distal phalanges of the fingers and toes on their palmar and plantar surfaces; they FIG. 72. — Drawing from a section of injected skin. Showing three papillae, the central one containing a tactile corpuscle, a, which is connected with a medullated nerve, and those at each side are occupied by vessels. (From Yeo after Cadiat.) diminish in number proximally from these points. They may be simple or compound according as the enclosing capsule con- tains one or more collections of nucleated cells. Their form is oblong with the long axis in the direction of the papilla. They vary in thickness with the papillae of the region in which they are located. They may have a transverse diameter of from "50 o to T4-^ of an inch, and probably in most instances occupy the secondary eminences of the papillae in which they are found. A simple papilla does not generally possess both vascular and nervous loops. (d) The tactile menisques are found in certain cutaneous regions. Nerves in the superficial layer of the skin lose their 230 THE NERVOUS SYSTEM medullary substance and divide to form arborizations which are flattened into the form of a leaf. (e) The corpuscles of Golgi are situated at the point of union of tendons with muscles, and are believed by some to have to do with the muscular sense. They are flattened fusiform bodies composed of granular substance enclosed in layers of hyaline membrane and containing nervous fibrillae. Properties and Classification of Nerve Fibers. — Nerve fibers are for the purpose of conveying messages either peripher- ally or centrally. They may be stimulated to action by anything capable of suddenly increasing their irritability. In any case the effect of the stimulus, whether normal or abnormal, is mani- fested at the peripheral distribution of the stimulated fiber. So far as most external manifestations are concerned, nerves may be classified as motor and sensory. That is to say, stimulation, for instance, of a cerebro-spinal nerve (except those of special sense) is followed, under ordinary conditions, by one of two results — there is either pain or contraction of a muscle to which the nerve is distributed. This is a typical illustration of the action of motor and sensory fibers, and the manifestation of nerve action, whether it consists in pain or motion, is a result only of the conduction of an impression of an impulse to the center or the periphery. It is to be noted that the result of thus stimulating a nerve fiber is manifested at one extremity only of that fiber, and always at the same extremity. However, since there are nerve fibers the stimulation of which is not followed by pain or motion, the division into sensory and motor fibers is not comprehensive enough to include all the fibers in the body. But since, as above stated, the only office of fibers is to conduct, and since they always conduct in a direction either toward or away from the center, all nerves may be classified as either centripetal or centrifugal. A corresponding division is into afferent and efferent. It will be seen that all motor fibers are centrifugal or efferent, but not all centrifugal or efferent EFFERENT NERVES 231 fibers are motor. It will likewise be seen that all sensory fibers are centripetal or afferent, but not all centripetal or afferent fibers are sensory. For impressions made upon the terminations, or upon the trunk, of a centripetal nerve may cause (i) pain, or some other kind of sensation; (2) special sensation, (3) reflex action of any kind; (4) inhibition. Similarly impressions made upon a centrifugal nerve may (i) cause contraction of a muscle (motor nerve) ; (2) influence nutrition (trophic nerve) ; (3) con- trol secretion (secretory nerve); (^inhibit, augment, or stop any other efferent action (Kirkes). To these two classes, efferent and afferent, should be added a third, the intercentral fibers which connect different parts of the nervous centers. Most of these even can be called either afferent or efferent. Characteristics of Efferent Nerves. — In case of these nerves a force is generated in the centers and conveyed by the nerves to the periphery, where it manifests itself in one of the ways mentioned above as characteristic of centrifugal fibers. Division of these fibers, or interference with their conductivity by disease or otherwise, renders impossible the manifestation of nervous force generated in the center, for the simple reason that the organ to which the fibers are distributed cannot receive the message intended for it. For instance, a muscle cannot, by the most persistent effort of the will be made to contract if the motor fibers running to that muscle are divided. In case, however, of division of efferent nerves, if the peripheral end be irritated, thus roughly counterfeiting normal stimulation, the ordinary effects of normal stimulation will be brought about, provided (as is usually the case) that particular nerve can be thus directly stimu- lated. Stimulation, however, of the central end of such a cut nerve produces no effect. No matter whether such efferent nerves receive their stimulus directly from the center or arti- fically, as by mechanical or electrical means, the effect is pro- duced in the end organs, whatever they may be. It is an invari- 232 THE NERVOUS SYSTEM able law to which reference has already been made, that a nerve fiber thus conducting a message in either direction is not inter- fered with by the proximity of other fibers, similar or dissimilar. Such message is not in any way imparted to a neighboring fiber or diffused through the fasciculus, but is conveyed uninterrupt- edly to its destination. It is possible that the myeline sheath has an insulating effect upon the contained axis cylinder, just as an electric wire may be insulated by non-conducting substances like silk, but this is doubtful. Interesting manifestations of motor centrifugal impulses are seen in certain movements associated with corresponding mus- cles on different sides of the body and with sets of muscles on the same side. It is almost impossible to effect certain movements with a single finger or toe without causing similar movements in other fingers and toes; a part of a muscle cannot be made to contract separately; it is doubtful if it be possible to move one eye-ball without the other, even by the most persistent practice. Other similar examples are numerous. It is quite probable that in most cases these associated movements are solely matters of habit. But the connection by commissural fibers of the cells in the centers controlling and regulating the movement of these muscles and sets of muscles would offer a not unreasonable ex- planation of the phenomena in question, since such an arrange- ment might render impossible separate and individual action by the cells thus connected. Excepting, perhaps, the movements of the eye-balls, these associated movements can be greatly modified by education. Characteristics of Afferent Nerves. — Impressions received by these fibers, although they are conveyed toward the center and must reach a center before there is any nervous manifestation, are always referred to the periphery. A most common illustration of this fact is furnished by injury to the ulnar nerve as it passes the elbow — such injury being manifested not usually by any pain at the point of infliction, but on the ulnar side of the hand where AFFERENT NERVES 233 the nerve is distributed. A person whose limb has been ampu- tated often seems to feel pain in the extremity although it has been removed from the body — such pain coming from compres- sion by the cicatrix (or otherwise) of the nerves which before the amputation were distributed to the severed limb. Here, as in the case of efferent nerves, division of the fibers between the seat of impression and the center precludes the possibility of any nervous manifestation. That is to say, no pain will be felt, no matter how great the injury be, if the sensory fibers running from the seat of injury be divided. Stimulation of the periph- eral end of a divided afferent fiber produces no effect; but stimu- lation of the central end is followed by the ordinary manifestation — by pain if the nerve stimulated be a common sensory one. This remark, of course, applies only to those nerves which can be thus directly stimulated — typically to true sensory fibers. Impressions conveyed by nerves of special sense must be re- ceived through the intervention of certain complex organs, con- sideration of which belongs elsewhere. Although a division has been made of nerve fibers into afferent and efferent, each with definite, proper and dissimilar functions so far as the direction of conduction is concerned, it has been impossible to discover any actual difference in the composition, appearance, or other properties, of the actual fibers themselves. In fact, it may be even considered as only an accident that one fiber conveys a message peripherally and another centrally — an accident dependent upon the kind of center which with the fiber is connected and the kind of termination it has in the periphery. Direction of the Current in Nerve Fibers.— It has long been understood that in no case will a fiber in situ convey a message at one time in one direction and at another in an opposite one, that no individual fiber can be both afferent and efferent; and so far as practical action is concerned this is true, but "experiment has shown that if a nerve trunk be stimulated at a given point, then the nerve impulse can be demonstrated as passing away from the 234 THE NERVOUS SYSTEM point of stimulation in both directions" (American Text-book). However, only the message traveling in the physiological direc- tion is manifest, for it is the only one which finds a suitable terminal. It is not to be concluded, however, that in any nerve trunk, as the ulnar nerve, there may not be both afferent and efferent fibers. Such, in fact, is the usual arrangement. Any nerve trunk may contain all kinds of fibers — sensory, special sensory, vaso-motor, motor, trophic, secretory — but the presence of all these does not interfere with the individuality and the individual action of each fiber. A nerve trunk containing more than one kind of fibers is called a mixed nerve. Speed of Nervous Conduction. — It is stated that afferent im- pressions are conveyed by nerves at the rate of about 120 feet per second; the rate for efferent impulses is somewhat less rapid, probably no feet. In the spinal cord tactile impressions are conveyed a little faster than in the nerves proper, and painful impressions somewhat less than one-half as fast. The rate of motor conduction in the cord is said to be one-third the rate in the nerves. It has also been demonstrated that an act of volition requires a definite time for the inception of its performance; this is stated to be about -fa of a second. The recognition of a simple impression (conveyed in the opposite direction, of course) re- quires about 2^ of a second. Furthermore, the part played by the spinal cord in reflex action (to be considered later) also con- sumes an appreciable period; this is found to be more than twelve times the period occupied in the transmission of the impression to the cord or the impulse back to the muscles. Action of Electricity Upon Nerves. — A nerve may be irri- tated in any one of several ways; but mechanical, thermal and chemical irritants, besides working injury to the tissues, are much less easily managed and regulated than is electricity. This agent may be applied time after time to a nerve trunk without causing any permanent change in its conductivity, and the THE CEREBRO-SPINAL AXIS 235 strength, time and duration of application, etc., can be accurately governed. It has been noticed that the uninterrupted flow of an electric current through a nerve is unattended by muscular contraction ; it has likewise been seen that very slow changes in the strength of the current are similarly unaccompanied by the manifestations of ordinary stimulation; but sudden changes in the strength, whether in the direction of increase or decrease, act as stimuli. However, while the passage of a constant current through a nerve does not manifest itself by contractions except at making and breaking, such a passage brings about a change in the tissue of the nerve known as electrotonus. It may be considered a state of electric tension. In the anodic area the excitability is diminished (anelectrotonus) ; in the kathodic area it is increased (katelectro- tonus). Nor is the electrotonic condition restricted to that por- tion of the nerve between the poles. Between the poles there is a point where the two influences — anelectrotonus and katelectro- tonus — meet and there is neither increased nor decreased excit- ability. With weak currents this point is nearer the anode; with strong ones nearer the kathode. A descending current diminishes the excitability of a nerve; an ascending increases it. Prolonged application of electric stimuli will exhaust nervous excitability, but it may be restored by rest, or more quickly by an opposite current. THE CEREBRO-SPINAL AXIS. The cerebro-spinal axis embraces the nervous matter in the cranial cavity and in the spinal canal, excepting the roots of the cranial and spinal nerves. This axis consists of both white and gray matter. The white matter is made up of conducting ele- ments; the gray matter consists of a number of connected ganglia. In the cord the white matter is situated externally; in the brain the gray. The encephalon is situated in the cranial cavity and consists of the cerebrum, the cerebellum, the pons Varolii, 236 THE NERVOUS SYSTEM and the medulla oblongata. These different parts are con- nected with each other and with the cord by nerve fibers, and all the cranial and spinal nerves are connected with gray matter either in the brain or in the cord, or in both. This gray matter exists for the purpose of receiving impressions and generating nerve force. Membranes. — The encephalon and cord are covered by mem- branes for protection and for the support of vessels belonging thereto. These are (i) the dura matter, (2) the arachnoid and (3) the pia mater. The dura mater is a dense fibrous structure surrounding the encephalon and adherent to the inner surfaces of the cranial bones. At certain points the two layers of which it is composed separate to form the venous sinuses. Processes of the internal layers also are sent inward between the two lobes of the cere- brum (falx cerebri), between the cerebrum and cerebellum (ten- torium cerebelli and between the lateral halves of the cerebellum (falx cerebelli). This membrane passes through the foramen magnum to cover also the spinal cord, and to follow as a sheath the spinal nerves at their foramina of exit. The arachnoid resembles the serous membranes. It covers the brain and cord underneath the dura mater without dipping into the sulci of the brain. Between it and the pia mater is what is known as the subarachnoid space containing the sub- arachnoid fluid. This fluid serves a mechanical purpose, equal- izing pressure in different parts of the cerebro-spinal axis and protecting the nervous substance from injury by concussion, etc. Besides being found in the subarachnoid space, it occupies the ventricles of the brain and the central canal of the cord, com- munication between these being furnished by a small opening at the inferior angle of the floor of the fourth ventricle. The pia mater is a very delicate structure dipping between the convolutions of nervous matter and lying in close contact with the external surface of the encephalon and cord. It is exceed- THE SPINAL CORD 237 ingly vascular; indeed its main function is to support vessels be- longing to the nervous substance underneath. Both the arach- noid and the pia mater pass out at the foramen magnum with the dura to cover the cord. The Spinal Cord. The spinal cord occupies the spinal canal and is about eight- een inches long, extending from the foramen magnum to the lower border of the first lumbar vertebra. Its distal extremity is FIG. 73. —Different views of a portion of the spinal cord from the cervical region, with the roots of the nerves. (Slightly enlarged.) In A, the anterior surface of the specimen is shown; the anterior nerve -root of its right side is divided; in B, a view of the right side is given; in C, the upper surface is shown; in D, the nerve-roots and ganglion are shown from below, i, the anterior median fissure; 2, posterior median fissure; 3, anterior lateral depression, over which the anterior nerve-roots are seen to spread; 4, posterior lateral groove, into which the posterior roots are seen to sink; 5, anterior roots passing the ganglion; 5', in A, the anterior root divided; 6, the posterior roots, the fibers of which pass into the ganglion 6'; 7, the united or compound nerve; 7', the posterior primary branch, seen in A and D to be derived in part from the anterior and in part from the posterior root. (Kirkes after Allen Thomson.) 238 THE NERVOUS SYSTEM in the shape of a slender filament known as filum lerminale, which is gray in color. The sacral and coccygeal nerves, having taken origin from the cord in the dorsal region, pass downward in the canal to find exit through the sacral and coccygeal fora- mina. This collection of nerves thus passing down is known as the cauda equina. Gross Divisions of the Spinal Cord in Section.— Cross sec- tion of the cord reveals the division of its substance into two lateral halves connected by the anterior and posterior commissures. In the center of the cord, and between these commissures, is a small opening, the central canal of the cord, communicating with the fourth ventricle above. This division of the substance of the cord into lateral halves is effected by the two median fissures, anterior and posterior. The former is the more clearly marked, and is lined thoughout with pia mater. It is bounded pos- teriorly by the anterior white commissure. The posterior median fissure is not lined with pia mater and extends anteriorly as far as the posterior gray commissure. It is to be noted that there are both anterior and posterior gray commissures, but only one white commissure (anterior), which is bounded posteriorly by the anterior gray commissure. Besides the anterior and posterior median fissures there are also on each side antero-lateral and postero-lateral fissures, mark- ing the lines of exit of the anterior and posterior roots of the spinal nerves. These are not well defined. They divide the cord into anterior, posterior and two lateral columns. Arrangement of Gray Substance. — The' disposition of the gray substance in the cord (in transverse section) is somewhat after the manner of the letter H, each lateral portion represent- ing the anterior and posterior cornua of gray matter for that side, and being connected to the corresponding portion of the other side by the commissures embracing the central canal. The anterior cornua are shorter and thicker than the posterior. From these issue the anterior and posterior roots respectively of THE SPINAL CORD 239 the spinal nerves. The cells are: (i) Those in the anterior cornu; (2) those in the posterior cornu; (3) those in the lateral aspect of the gray matter; (4) those at the inner base of the posterior cornu (Clarke's vesicular column). The gray substance is made up of cells with, of course, the usual neuroglia and blood-vessels. The cells in the anterior cornua are large in size and possess a greater number of poles than those in the posterior cornua; from their connection with the anterior (motor) spinal nerve roots they are called motor cells in contradistinction to the sensory cells in the posterior cornua which are connected indirectly with the posterior (sensory) nerve roots. Degeneration. — Nerve fibers when separated from the cells of which they are outgrowths degenerate. Fibers have been said to degenerate in the direction in which they carry messages, but this is by no means always so. For instance, the parent cells for the fibers of the posterior spinal roots are in the ganglia on those roots near the cord, and section of the root beyond the ganglion causes degeneration of its fibers peripherally — which is in the opposite direction to the passage of impressions in them. Section of the posterior root between the ganglion and cord is followed by centripetal degeneration, and there is no centrifugal degeneration. The anterior spinal root fibers are outgrowths of cells in the anterior cornua of gray matter. Section of this root anywhere occasions centrifugal degeneration (Fig 74) . Arrangement of the White Substance.— It is scarcely necessary to state that the white substance of the cord consists of nerve fibers with their usual accompaniments. It is external to the gray. The fibers are medullated, but have no sheath of Schwann. The divisions of the cord already referred to are purely ana- tomical. Physiological and pathological researches warrant the further division of the white substance of the cord into eight columns for each side. The course of all the fibers in the white 240 THE NERVOUS SYSTEM matter of the cord is by no means certain. The division here given may not be strictly correct, but it probably receives as little adverse criticism as any of the others. Classified accord- ing to the direction in which their fibers degenerate after section the paths are: (i) Degenerating downward, (a) the column of Turck and (b) the crossed pyramidal tract; (II) degenerating upward, (a) the column of Goll and (b) the direct cerebellar FIG. 74. — Diagram to illustrate wallerian degeneration of nerve-roots. (Kirkes.) tract; (III) degenerating in neither direction, (a) the anterior fundamental fasciculus, (b) the anterior radicular zone, (c) the mixed lateral column and (d) the column of Burdach. I. (a) The column of Turck occupies a position just lateral to the anterior median fissure and extends downward to the lower dorsal region. Its fibers decussate high up in the cord. This column is sometimes called the direct, or uncrossed, pyramidal tract, as distinguishing it from the other descending column. (b) The crossed pyramidal tract is external to the posterior cornu of gray matter and internal to the direct cerebellar tract. Its THE SPINAL CORD 241 fibers decussate in the anterior pyramids of the medulla oblongata. II. (a) The direct cerebellar tract occupies the outer posterior part of the lateral column. Its fibers reach the cerebellum through the inferior peduncles, after having traversed the poste- rior pyramids of the medulla. This tract exists throughout the length of the coid. (V) The column of Goll (postero-internal column) is situated posteriorly in a position corresponding to a FIG. 75. — Scheme of the conducting paths in the spinal cord at the 3d dorsal nerve. The black part is the gray matter. V, anterior, hw, posterior root; a, direct, and g, crossed, pyramidal tracts; b, anterior fundamental fasciculus; c, Goll's column; d, column of Burdach; e, anterior radicular zone; f, mixed lateral tract; h, direct cere- bellar tracts. (Landois, modified.) the column of Turck anteriorly — just lateral to the posterior median fissure. Fibers in this column extend from the upper lumbar region to the funiculi graciles of the medulla. III. (a) The anterior fundamental fasciculus lies between the column of Turck internally and the anterior cornu and anterior roots of the spinal nerves externally. Its fibers are lost in the medulla above, (b) The anterior radicular zone is external to the anterior roots of the spinal nerves and anterior to the crossed pyramidal tract and the direct cerebellar fasciculus. Its fibers are lost in the medulla above, (c) The mixed lateral column is 16 242 THE NERVOUS SYSTEM just external to the main body of gray matter and does not reach the surface of the cord. Its fibers are likewise lost in the medulla a/I* FIG. 76. — Course of the fibers for voluntary movement. ab, path for the motor nerves of the trunk; c, fibers of the facial nerve; B, corpus callosum; Nc, nucleus caudatus; G. i, internal capsule; N. I, lenticular nucleus; P, pons; N.f, origin of the facial; Py, pyramids and their decussation; Ol, olive. Gr, restiform body; PR, posterior root; AR, anterior root; x, crossed, and z, direct pyramidal tracts. (Landois.) oblongata. (d) The column of Burdach (postero-external col- umn) is situated posteriorly in a location corresponding to the MOTOR PATHS IN THE CORD 243 anterior fundamental fasciculus anteriorly — external to the column of Goll and internal to the posterior cornu. Its fibers reach the cerebellum through the inferior peduncles, having passed through the restiform bodies. Functions of the Columns. — Remarks already made touch- ing the direction of degeneration in the separate columns throw some light upon the physiological function of the fibers in each. Motor impulses pass downward from the brain through cer- tain fibers to the cells of the anterior cornua of gray matter in the cord, and are sent thence through the spinal nerves to the muscles. The paths in the cord conveying these impulses are 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 patho- logical 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. 77) 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 244 THE NERVOUS SYSTEM commissure approximately on a level with the cells; others have decussated in the medulla, and come down in the crossed pyram- idal tract on the same side as the cells. In both cases the fibers originated in the brain on the side opposite the cells around which FIG. 77.— Course of nerve fibers in spinal cord. (Kirkes after Schafer.} 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 SENSORY PATHS IN THE CORD 245 pass from the anterior nerve roots directly into the pyramidal tracts without being interrupted by motor cells. 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 localities. (See Figs. 77, 84.) Reference to Fig. 77 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 these gives rise to an efferent fiber, one of which (A ) is shown distributed to a muscle (M). Each of these cells also is sur- rounded by the end arborization of a fiber (P) from the cortex. A fiber from the posterior root is also shown. It originates in a cell of the sensory ganglion (G). It bifurcates, one branch going to the surface (S), the other enters the cord and itself bi- furcates. The branch (E) is short and arborizes around a small cell (Pj) in the posterior cornu, from which a new axis 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 (P2) and another to a cell (C) in the inner base of the posterior cornu (in Clarke's column); from C an 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- 246 THE NERVOUS SYSTEM 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 FIG. 78. — 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; b, 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 anterior root;/, axis cylinder from sympathetic ganglion cell, dividing into two branches, one to the periphery, the other toward the cord; g, fiber of the anterior root terminating by an arborization in the sympathetic ganglion; h, sympathetic fiber passing to periphery. (Kirkes after Ramony Cajal.) 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 FUNCTIONS OF THE SPINAL CORD 247 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. The columns of Burdach have been said to present no degen- eration secondary to section. Trophic centers for their fibers must, therefore, exist 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 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 Burdach. This is of importance in determining the function of these col- umns, 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 coordina- tion— if such a center exist. The sense of pressure and the so- called muscular sense are probably connected with the fibers of this column, and these may be the only sensory impressions conveyed through the columns of Burdach. The anterior fundamental fasciculi, the anterior radicular zones, and the mixed lateral paths degenerate in neither direction after section, their trophic cells existing at both extremi- ties. They connect cells in the gray matter of the cord. Functions of the Spinal Cord. — These are (i) conductions, (2) transference, (3) reflex action, (4) augmentation, (5) co- ordination, (6) inhibition of reflex acts, (7) special centers (Collin and Rockwell, modified). i. Conduction. — This has been referred to in discussing the 248 THE NERVOUS SYSTEM 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 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 impulses. 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. 77 and 80 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 re- ceive 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 (gen- eral or special) by a nerve center, the term must be made to REFLEX ACTION 249 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 associated action on the part of several neurons and being manifested fre- quently 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 sleeping person will cause him to draw up his leg without the interven- 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 of 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 cap- able 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 funda- mental fasciculi, the anterior radicular zones, and the mixed lateral tracts, and it is these paths which are mainly concerned 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 wi}h the spinal cord the muscular 250 THE NERVOUS SYSTEM 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 effer- ent 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 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 coordinate the muscular movements of deglutition, respiration, walking, etc. These movements may be performed when the cerebrum 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, provid- ing 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 251 THE ENCEPHALON. The encephalon is situated within the cranial cavity and is commonly called the brain. Its gross divisions are the medulla oblongata, the pom 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^ 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 inch and a quarter in length, half an inch thick, and three-quar- ters 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 ven- tricle. 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 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 pyr- amidal tracts. The olivary bodies, oval in shape, are just exter- nal to the anterior pyramids separated from them by a groove. The restiform bodies make up the postero-lateral portion of the 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. Beneath the olivary bodies, and between the anterior pyramids 252 THE NERVOUS SYSTEM and the restiform bodies, are the lateral fasciculi, or thefuniculi 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 FIG. 79. — 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 con- nections 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 striae 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 quadrigemma. (Landois.) posterior pyramids are sometimes called the funiculi graciles. They join the restiform bodies and pass to the cerebellum. The fourth ventricle deserves paricular 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 scriptorious, and its lateral boundaries are eht diverging restiform bodies. The superior THE MEDULLA OBLONGATA 253 peduncles of the cerebellum form the lateral boundaries of the superior triangle. The inferior triangle is covered by the cere- bellum; the superior by the valve of Vieussens, which stretches between the superior peduncles. This ventricle communicates 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- 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 occupying 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 G oil 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. i. As a conductor the medulla is absolutely necessary as a means of connection between the brain and cord. Sensory im- 254 THE NERVOUS SYSTEM pressions to and motor impulses from the brain must all pass through by this route. 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 movements is that for respiration. The encephalon may be cut away down as far as the medulla, and life will continue for a cer- tain time. It is al so ti ue that the medulla itself may be gradually cut away from above downward until a certain point is reached, when respiration suddenly ceases. Likewise the spinal cord may be cut away upward till this point is reached, when the same re- sults will follow. This is the true respiratory center, and is situ- ated 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 simply lost. There is one of these centers for each side, but they act synchronously, being connected by commissural fibers. Probably 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, vaso-motor centers for the THE PONS VAROLII 255 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 situated externally, 'while within its substance are a number of collections of gray matter. The longitudinal fibers are continuations upward of fibers from the olivary bodies and the anterior pyramids of the medulla and also of parts of the posterior 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 impulses 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 intelligence; but the fact remains that the animal can perform movements which are different from the reflex movements depending 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 voluntary move- ments, when the cerebrum, corpora striata and optic thalami have been removed, and it probably regulates the automatic voluntary movements of station and progression." (Flint.) 256 THE NERVOUS SYSTEM 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 crura cerebri, passing upward from the anterior part of the pons, diverge to run apparently underneath the corpora striata and optic thalami in the direction of the cerebral hemis- pheres. They are about j inch long and slightly broader above than below. The main bulk of each crus consists of white fibers, but a collection of 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 tegmentum are supposed to convey afferent impressions chiefly, and end for the most part in the optic thalamus, though some are continued 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 THE OPTIC THALAMI 257 are mixed. Each is separated by the anterior limb of the internal capsule into two divisions, external and internal, known respec- tively as the lenticular and caudate nuclei. (See Fig. 80.) FIG. 80. — Human brain, with the hemispheres, removed by a horizontal incision on the right side. 4, trochlear; 8, acoustic nerve; 6, origin of the abducens; F, A, L, position of the pyramidal (motor) fibers for the face, arm and leg; S, sensory fibers. (Landois.) The optic thalami, one on either side, have an oval shape and rest upon the crura cerebri between the posterior extremities of 17 258 THE NERVOUS SYSTEM 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 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. 76 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 THE BASAL GANGLIA 259 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 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 di- vision are followed by anesthesia; and that lesions affecting the entire posterior limb are followed by both paralysis and an- esthesia— these phenomena always manifesting themselves on the side opposite the lesion only. This leads to a definite conclusion ; 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 promi- nences on the dorsal surface of the pons and crura above the aque- duct of Sylvius. They contain white and gray matter. The pos- terior tubercle's 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. 260 THE NERVOUS SYSTEM 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 quadrigemina, cerebellum, medulla oblongata. This arrangement exists in the human fetus, and persists throughout life in some of the lower animals. Anatomy. — The substance of each hemisphere is divided by fissures into five lobes — (a) frontal, (b) parietal, (c) occipital, (d) temporo-sphenoidal and (e) central. The main fissures are four in number — (i) The fissures of Sylvius running from the front and under part of the brain backward, outward and upward; (2) the fissures of Rolando running from the median line near the center of the longitudinal fissure forward, outward and down- ward; (3) the parieto-occipital fissure, little of which is evident upon the surface of the brain, but which appears on longitudinal 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. 81, 82.) (a) The frontal lobe is bounded internally by the longitudinal 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 convolution, 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. (b) 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 Sylvius. Just behind the fissure of Rolando is the ascending parietal, or THE CEREBRUM 261 posterior central convolution, above, this is continuous 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 FIG. 81. — 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, ascending frontal, and B, ascending parietal convolution; Fi, suoerior, F2, middle, and Fs, inferior frontal convolutions;/!, suoerior, and/2, inferior, frontal fissures;/?, sulcus precentralis ; P, superior parietal lobule; P2, inferior parietal lobule- consisting of P2, supra-marginal gyrus, and P2', angular gyrus; ip, sulcus interpariet' alis; cm, termination of calloso-marginal fissure; O, first; Os, second; Os, third occipital convolutions; po, parietal-occipital fissure; o, transverse occipital fissure; 02, inferior longitudinal occipital fissure; Ti, first; T2, sec9nd; Ta, third, temporo- sphenoidal convolutions; ti, first; tz, second, temporo-sphenoidal fissures. (Landois.) 262 THE NERVOUS SYSTEM 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- FIG. 82. — Median aspect of the right hemisphere. CC, corpus callosum divided longitudinally; Gf, gyrus fornicatus; H, gyrus hip- pocampi; 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', parecuneus or quadrate lobule; Oz, cuneus; Po, parieto-occipital fissure; o', transverse occipital fissure; oc, calcarine fissure; pc', superior; oc", inferior ramus of the same; G, gyrus descendens; T<, gyrus occipito-temporalis lateralis (lobulus fusiformis); Ta, gyrus occipito-temporalis medialis (lobulus lingualis). (Landois.) occipital fissure and external to the median fissure. It presents three convolutions, the superior, middle and inferior. (d) The temporo-sphenoidal lobe is below the fissure of Sylvius in front of the occipital lobe. It likewise presents superior, middle and inferior convolutions. (e) The central lobe, or island of Reil, presents the gyrus forni- catus, a convolution curving around the corpus callosum; the THE CEREBRUM 263 marginal convolutions 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 downward and forward to meet the calcarine fissure, between which is the cuneus; the internal aspect of the temporal lobe, the uncinate gyrus. 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 thick- ness of the gray matter of the cortex varies from -^ to J in., being thinnest in the occipital and thickest in the front parietal 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 direc- tion there are found in succession small pyramidal, larger pyram- idal, and irregular branching cells. Fibers from the Cerebrum.— Fibers pass from each cerebral hemisphere to (a) the spinal cord, (b) 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 264 THE NERVOUS SYSTEM 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 FIG. 83. — Scheme 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 (sepa- rated from B for the sake of clearness in the scheme); D, visual tract; E, auditory tract; F, G, H, superior, middle, and inferior cerebellar peduncles; J, fibers between the auditory nucleus and the inferior quadrigeminal body; K, motor decussation in the bulb; At, fourth ventricle. The numerals refer to the cranial nerves. The sen- sory radiations are seen to be massed toward the occipital end of the hemisphere. (Am. Text-book.) tract. Furthermore, fibers from the posterior cortical area pass through the posterior one-third of the posterior division of the 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. 84 taken in conjunction with Fig. 77 illustrates the most recent ideas of the motor and sensory connections between brain and cord and the motor and sensory paths in the cord. THE CEREBRUM 265 A.O.N FIG. 84. — 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 Call goes to the cortex of the oppo- site hemisphere, while another, str, enters the corpus striatum. The axis cylinder arborizes around an anterior horn cell, 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 division ending as shown better in Fig. 77. N.G, cell in posterior cornu of the cord or posterior 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 collat- erals from it in Fig. 77. 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. 266 THE NERVOUS SYSTEM (b) 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 of the corpora striata to the cerebellum on the opposite side. The connection is crossed in all these cases. FIG. 85. — Diagram of the motor areas on the outer surface of a monkey's brain. (Landois after Horsley and Schafer.) (c) 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 tempero-sphenoidal lobes and probably the corpora striata with each other; fibers in the pos- terior connect the temporo-sphenoidal lobes with the optic thai- ami of the opposite side. (d) The arcuate fibers connect different convolutions of the same lobe and the convolutions of different lobes with each 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. THE CEREBRUM 267 FIG. 86. — Side view of the brain of man, with the areas of the cerebral convolutions according to Ferrier. (Brubaker.) The figures are constructed by marking on the brain of man, in their respective situations, 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. i, 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, b, c, d, individual and combined movements of the fingers and wrist of the opposite hand. Prehensile movements. 5 , extension forward of the opposite arm and hand; supination and flexion of the opposite forearm; 7, retraction and elevation of the opposite angle of the mouth by means of the zygomatic muscle; 8, elevation of the alae nasi and upper lip, with depression of the lower lip on the opposite side; 9, 10, opening of the mouth, with (9) protrusion and (10) retraction of the tongue; region of aphasia, bilateral action; n, retraction of the opposite angle of the mouth, the head turned slightly to one side; 12, the eyes open widely, the pupils dilate, and the head and eyes turn toward the opposite side; 13, 13', the eyes move toward the opposite side, with an upward (13) or downward (13') deviation; the pupils are generally contracted ; 14, pricking of the opposite ear, the head and eyes turn to the opposite side, and the pupils dilate widely. 268 THE NERVOUS SYSTEM 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 defin- itely determined. Motor Centers. — Electrical stimulation of the convex surface of the cerebrum shows that the anterior part is motor and the posterior part non-motor; that stimulation of the motor por- tion produces muscular contractions on the opposite side of the body, that stimulation in the same spot is always followed by the same contractions; and that when the current is quite weak the contractions are limited to distinct muscles or sets of muscles. It may be further said that while experiments establishing these facts have been largely limited to inferior animals, the deductions have been made applicable to man by pathological observations and by the fact that in different animals stimulation of anatom- ically corresponding parts is followed by corresponding results. Destruction of motor areas is followed by descending secondary degeneration of fibers through the corona radiata, internal cap- sule, crura cerebri (crusta), anterior pyramids of the medulla and the pyramidal tracts of the cord; the resulting paralysis is on the side opposite the lesion. The motor cortical zone, so far as can now be said, corresponds to the ascending frontal and parietal convolutions on either side of the fissure of Rolando, to the paracentral lobule, and possibly to a small area in front of the ascending frontal convolution. From above downward, on either side of the Rolandic fissure are areas presiding over the movements of the leg, arm and face. More specific information as regards areas controlling various movements may be obtained by reference to Fig 86. Various kinds of monoplegia (crossed) are caused by lesions, as hemorrhage, in localized parts of the motor area; there may THE CEREBRUM 269 be facial, brachial, crural, brachio-facial monoplegia, etc. There can be no doubt that from the motor cortical zone pass the fibers which constitute the pyramidal tracts of the cord. Sensory Centers. — Centers for the reception of impressions giving rise to general sensation may exist. Fibers from the tem- poro-sphenoidal and occipital lobes pass through the posterior third of the posterior division of the internal capsule, and it may, therefore, be assumed that these parts of the cerebrum are con- nected with general sensation. Special Centers. — Besides these areas for motion and general sensation, special centers certainly exist. The Optic Center is in the occipital lobe, probably in the cuneus. Removal of the right occipital lobe is followed by lefthemiopia and vice versa; removal of both causes total blindness. The Olfactory Center is probably on the inner surface of the anterior extremity of the uncinate gyms (inner extremity of the temporal lobe) . The Gustatory Center is supposed to be in the temporal lobe very near the preceding. The Auditory Center is located in the superior and middle convolutions of the temporo-sphenoidal lobe. The Center for Cutaneous Sensations cannot be strictly lim- ited, though it is said to correspond with the motor area. The Center for Muscular Sensations is thought to be in the lower parietal region. The Speech Center. — One may not be able to speak because he cannot control the muscles usually involved in such an act, or because he has no comprehension of the meaning of words, or because he is incapable of forming the idea which links the reception of the impression and the muscular act. Aphasia is the term generally applied to inability to express one's self by language. It is to be distinguished, however, from aphonia, which is simply a loss of voice. Ataxic aphasia is an inability to express ideas only by reason of muscular incoordination; a 270 THE NERVOUS SYSTEM person so affected may use words, but he cannot tell what sounds he is going to utter; his ability to receive ideas is unimpaired, and he can express his own ideas in writing. When there is inability to express ideas in writing, because of muscular inco- ordination, a condition of a graphic aphasia is said to exist. There are also cases in which a person cannot comprehend ideas ex- pressed in language and cannot express himself by either speak- ing or writing; this is known as amnesic aphasia. It is not impossible that in some instances ideas may be received and there still be an inability to express one's self in any way. It is noted that when the hemiplegia accompanying the aphasia is marked the form is usually ataxic; when there is no hemiplegia the aphasia is usually amnesic. The part of the brain presiding over speech is in the left third frontal convolution near the island of Reil. In left-handed per- sons its usual situation is almost certainly at a corresponding point on the right side. Why the center is unilateral has not been explained. It may be that it was originally bilateral, and the growth of the right has been stopped by the superior develop- ment of the left side of the brain. It is at least noticed that the right instead of the left side of the brain is heavier in left-handed persons. Fibers from this center (Broca's convolution) pass through the anterior part of the posterior division of the internal capsule to reach the left crus, leaving which they enter the pons to decussate and go to the right side of the medulla. Functions of the Cerebrum. — The superior development of the intellect in man is the most predominant characteristic dis- tinguishing him from the lower animals. That many such ani- mals are possessed of a certain degree of intelligence is not usu- ally denied; and the nature of their mental operations, though they are insignificant as compared with man's, may be admitted as identical with his. The most striking difference in the nervous system of man as compared with that of inferior animals is the large size of the cerebrum in the former. This is not sur- THE CEREBRUM 271 prising when it is admitted that in the substance of this part of the encephalon is the seat of those faculties which manifest themselves in mental operations. The seat of the changes, if they be changes, which result in mental operations is supposed to be in the frontal lobes; these are insensible and inexcitable, but severe injury to them, as by hemorrhage, is followed by a cessation of mental activity; con- genital defects also cause a corresponding decrease in the mental caliber. From what has been said it is evident that the cerebral hemis- pheres are capable of generating motor impulses and receiving im- pressions general and special; but predominating in importance over these functions is the fact that the gray substance of the cerebrum is essential to the exercise of the intellect — even to the existence of that indefinite something called the mind. It is by the cerebrum that we perceive and retain impressions, that we understand, imagine, reflect, reason and judge, and thus concoct and issue the mandates of our will. It is the link which connects our impressions and our purposeful actions. In animals upon which experiments have been made it is found that life may persist for 'a time after the removal of the hemispheres, and that, outside of the cessation of mental activity, the results are not so marked as one would on first thought sup- pose. Stupor and absence of the ordinary instinctive acts (as corresponding in a way with acts of the will in man) are noted, but voluntary motion and general sensibility are not destroyed, and may be but little interfered with. Of course there is no volun- tary motion in the sense of carrying out the behests of the will, for the organ of the will is destroyed; nor is there any record of painful impressions, for the organ of memory is absent. But the animal can pertorm various consecutive and coordinate move- ments, such as walking, swimming, etc. For example, a pigeon thus mutilated will fly when thrown irtfo the air. This does not argue any mental operation. A person does not ordinarily 272 THE NERVOUS SYSTEM apply his mind to the act of walking or standing; his mental faculties may be as completely engaged with the deepest thoughts of psychology, literature, medicine or other subjects while walk- ing as at any other time. True, he probably started with some fixed purpose to go in some particular direction to some definite place, but the act of progression does not per se require fixed at- tention on his part. So in the case of the pigeon; it does not, make up its mind to fly at all; and it will not fly without being thrown into the air, or the application of some other similar stimulus; nor does it fly in any particular direction, or to any particular place. It is reduced to the condition of a " mechan- ism without spontaneity." It can perform voluntary move- ments but cannot originate them without external inter- vention. Animals which have been subjected to the operation men- tioned undoubtedly feel pain. They move away or cry out on being burned, for example. The coordination of their move- ments and the cries contrast with the phenomena (reflex) fol- lowing such stimulation when only the cord is left. It was noted above that impressions in these cases are probably received by the gray matter of the pons and not recorded. The special senses of sight and hearing remain after the re- moval of the cerebrum. The same is probably true of taste and smell. • It would seem that the cerebrum is a kind of storehouse in which are kept all the materials necessary for the performance of all kinds of pre-determined acts, whether they manifest them- selves in speech, or thought, or muscular action. What excites these materials to activity — i. e., what excites a voulntary act — is not clear. We know certain things will usually excite a certain train of thought, or cause us to will to do or say certain things. Such phenomena are akin to, if not identical with, reflex action. These manifestations of our voluntary power are due to impres- sions conveyed by afferent fibers to the cortex; indeed it may THE CEREBRUM 273 be that every afferent fiber in the system exerts an influence thus indirectly upon the organ of the will, and the impressions conveyed by them are reflected in one's character and life. But it cannot be said that all voluntary activity is thus of a reflected nature; there is some cause other than the reception of afferent impressions which sets the will in operation. Connection Between the Brain and Intelligence. — It is claimed that a single hemisphere is capable of performing all the ordinary intellectual acts as well as both; and atrophy, or de- struction otherwise, of one hemisphere has frequently been no- ticed to entail no mental defect. But whether the mind under such conditions would be equal to the highest intellectual attain- ments is doubtful. It would seem that in health the brain unites the impressions received by the two sides (as, e. g., through the optic nerves), and the resulting idea is a single one; that is to say a person does not have two opposing ideas about the same thing the same time; the two hemispheres seem to agree. In a general way, it may be stated that the degree of intelli- gence corresponds to the weight of the brain, though to this rule there are many exceptions. It may be more properly said that the development of the intellectual faculties is greater as the area of gray matter is increased by the convolutions of the cortex. Idiots' brains are usually, though not by any means invariably, much below the average weight. A difference in intellectual vigor may be present in persons whose brain shave the same weight and even the same amount of gray matter. A difference in the quality of the gray substance may in such cases account for the varying results. It is a matter of common observation that mental exercise increases mental vigor and capacity, just as muscular exercise develops muscular strength. It is difficult to reach a conclusion as to whether there is an increase in the amount of gray substance or whether that already present is endowed with additional power. 18 274 THE NERVOUS SYSTEM The Cerebellum. Anatomy. — The cerebellum, or little brain (see Fig 79), is situated beneath the occipital lobes of the cerebrum, weighs some 5 J ounces in the male to 4^ ounces in the female, and con- sists of a central and two lateral lobes. It is composed of white and gray matter, the latter being, with the exception of the corpora dentata in the lateral lobes, situated externally. The convolu- tions on its surface are much finer than are those on the cerebral surface. It is separated from the parts above by the tentorium cerebelli, a process of the dura mater. Fibers. — The fibers passing away from the cerebellum are col- lected into three bundles on each side, known as the superior, middle and inferior peduncles. The superior peduncle has a direction forward and upward to reach the crus and optic thala- mus; fibers in it connect the cerebellum with the cerebrum. Certain of these decussate underneath the corpora quadrigemina with corresponding fibers from the opposite side, so that each side of the cerebellum is connected with both sides of the cere- brum. Attention has been called to fibers passing down from the cerebrum through the pons to the cerebellum. Fibers in the middle peduncle connect the two lateral halves of the cerebellum through the pons. Fibers in the inferior peduncle are continuous below with fibers in the posterior columns of the cord through the restiform bodies of the medulla. Function. — The only characteristic phenomenon invariably following removal of the cerebellum is an inability to coordinate the voluntary muscular movements. The foot, for example, can be raised, and the voluntary muscular act concerned in raising it may be as vigorous as ever, but the animal cannot so govern his movements as to know where he put it down. Even the coordin- ation necessary in standing is lost, and the maintenance of the equilibrium is very difficult, if not impossible. The so-called muscular sense is abolished, and, while the power to contract THE CEREBELLUM 275 the muscles remains, the animal cannot contract them in a regu- lar or coordinate manner. When it is remembered that well- nigh every voluntary act requires concerted or consecutive mus- cular movements some idea is gotten of the helpless condition sequent upon such a lesion. If it be granted that there is a cen- ter presiding over the coordination of the voluntary muscles, that center is in the cerebellum, and an animal deprived of this organ is as powerless, so far as this function is concerned, as a person is to see when the optic centers are destroyed. Its action is crossed. It has been noted already that lesions of the posterior white columns of the cord are followed by disturbances of coordination, and that the cerebellum is connected with these columns through the inferior peduncles and restiform bodies. Fibers in these columns serve only as anatomical connections by which the co- ordinating center communicates with the muscles whose move- ments it is to regulate, and of necessity any lesion of these fibers destroying that connection is followed by the loss of control of the center over the muscles. However, in degeneration of the pos- terior columns (locomotor ataxia) an effort at coordination can be made, so that progression is possible by the aid of fixed atten- tion. It is possible also that the coordinating messages are carried in such cases by the motor fibers, though in an unsatis- factory manner. It has been supposed that the cerebellum is in some way con- nected with the generative function, and this much is probably true, though the evidence submitted is not sufficient to warrant the assumption that the cerebellum is the seat of the sexual instinct. THE CRANIAL NERVES. The cranial nerves, twelve in number on each side, take their origin from some part of the encephalon, pierce the dura mater and leave the skull by various openings. They have been num- 276 THE NERVOUS SYSTEM bered from before backward in the order in which they pass through the dura mater. Their names, indicating something of their function, and corresponding to their numbers, are as follows: I. Olfactory. II. Optic. III. Motor Oculi Communis. IV. Patheticus (Trochlearis) . V. Trifacial (Trigeminus) . VI. Abducens. VII. Facial. VIII. Auditory. IX. Glosso-pharyngeal. X. Pneumogastric (Vagus). XI. Spinal Accessory. XII. Hypoglossal. The point at which one of these nerves can be seen to issue from the brain tissue is the apparent origin, while the gray nu- cleus, or nuclei, to which the fibers can be traced in the brain substance is the deep origin. First Nerve (Olfactory). Origin. — This is a nerve of special sense. Its apparent or- igin is by three roots. The internal root issues from the gyrus f ornicatus ; the middle from the under surface of the frontal lobe anterior to the anterior perforated space; the external from the temporo-sphenoidal lobe. These three roots unite to pass for- ward underneath the frontal lobe near the longitudinal fissure as the olfactory tract. The deep origin is unsettled. Course and Distribution. — Reaching the upper surface of the cribriform plate of the ethmoid, the olfactory tract expands into the olfactory bulb, from the under surface of which are given off the special nerve fibers of the sense of smell. They are about THE CRANIAL NERVES 277 twenty in number and pass through the foramina in the cribri- form plate to be distributed to the mucous membrane (Schnei- derian) of the nose in three sets — an inner to the upper third of the septum, a middle to the roof of the nares, and an outer to the superior and middle turbinated bones and the ethmoid in front of them. The fibers are non-medullated. Function. — The olfactory nerves are insensible and inexcit- able. They are concerned with the sense of smell alone, and their integrity is necessary to the preservation of that sense. They convey to the brain impressions which are recognized as odors only. Removal of the olfactory bulb in a dog is evidently followed by a loss of the sense so characteristic of the animal. Furthermore, the olfactory bulbs in lower animals are shown to be developed in proportion to the acuteness of the sense of smell. Second Nerve (Optic). Origin. — This is the nerve of sight. Its apparent origin is from the anterior part of the optic commissure. The optic com- missure occupies the optic groove on the superior surface of the sphenoid. It represents the union of the two optic tracts each of which, traced backward, is found to divide into two bands; the external takes its origin from the external geniculate body, from the pulvinar of the optic thalamus and from the superior corpus quadrigeminum; the internal comes from the internal geniculate body. These two, uniting, cross the crusta obliquely to reach the optic commissure, or chiasm. In the commissure the fibers from the inner margin of each optic tract pass to the other side of the brain, and may be called commissural fibers between the internal geniculate bodies. Some fibers anteriorly connect the two optic nerves with each other and are not prop- erly part of the chiasm, but connect the two retinae. The outer fibers of each tract pass to the nerve of the same side, while the central fibers decussate in the commissure with similar fibers 278 THE NERVOUS SYSTEM from the other tract and pass thus to the optic nerve of the op- posite side. The deep origin is indicated above. Course and Distribution. — Each optic nerve leaves the front of the optic chiasm to pass out of the cranium and enter the orbital cavity by the optic foramen. Having pierced the sclerotic and choroid coats of the ball it expands into the retina. Function. — The optic nerves have no properties other than the conveying to the brain of the special impressions of sight. Stimulation produces neither pain nor motion. Third Nerve (Motor Oculi Communis) . Origin. — The third is a motor nerve. Its apparent origin is from the inner surface of the crus just in front of the pons Va- rolii. Its deep origin is in a nucleus just lateral to the median line beneath the aqueduct of Sylvius. Here decussation with fibers from the opposite side occurs. The fibers pass forward from this place through the locus niger and tegmentum to the point of apparent origin. Course and Distribution. — Having traversed the outer aspect of the cavernous sinus, the third nerve divides into two branches which leave the cranial cavity by the sphenoidal fissure between the two heads of the external muscle of the eye. The superior division is distributed to the superior rectus and levator palpe- brae superioris ; the inferior separates into three branches, one of which is distributed to the inferior rectus, another to the internal rectus, and a third to the inferior oblique. From this last a branch is given off to the lenticular ganglion to form its inferior root. Functions. — This nerve has no function other than to supply motion to the parts to which it is distributed. It is insensible at its root, but receives filaments from the fifth in the cavernous sinus, beyond which point stimulation produces pain as well as muscular contractions. The phenomena sequent upon section of the nerve are suggested in its distribution, (i) There is ptosis, THE CRANIAL NERVES 279 or dropping of the upper lid ; for the lid is kept open by the leva- tor palpebrae superioris. (2) There is external strabismus, because the external rectus is not supplied by this nerve and is unopposed by the internal rectus, the action of which is paralyzed. Diplopia is the consequence. (3) There is inability to turn the ball except in an outward direction because the muscles producing movements on the vertical and horizontal axes are deprived of in- nervation. (4) There is inability to rotate the eye in certain direc- tions on the antero-posterior axis. The antagonist of the inferior oblique is the superior oblique, the tendency of which latter is to rotate the globe so as to make the pupil look downward and outward. When the inferior oblique is paralyzed the superior oblique is unopposed, it is impossible to rotate the ball as is usual in sidewise movements of the head, and double vision is the result. (5) There is slight protrusion of the whole ball from re- laxation of the muscles. (6) The pupil is dilated and move- ments of the iris are interfered with. Stimulation of the third nerve contracts the pupil, but when it is cut the pupil does not respond to light. The ciliary nerves controlling the movements of the iris come from the ophthalmic ganglion of the sympa- thetic; to this ganglion goes a branch from the third nerve. It is known that the action of the sympathetic cannot be divorced from that of the cerebro-spinal system; and whether this influ- ence of the third nerve is exerted directly upon the iris or in- directly through the ophthalmic ganglion is a matter of some obscurity. The fact that the action of the iris is not instanta- neous strongly suggests control by the sympathetic. The decussation under the aqueduct of Sylvius is evidenced by the reflex contraction of the pupil on the opposite side when the central end of a divided optic nerve is stimulated. The impulse is reflected through the third nerve. It is not to be understood, however, that the motor oculi is the only nerve capable of influ- encing movements of the iris. Section of the sympathetic in the neck contracts the pupil, even after section of the third. 280 THE NERVOUS SYSTEM Fourth Nerve (Patheticus). Origin. — This is a purely motor nerve. Its apparent origin is behind the corpora quadrigemina from the valve of Vieussens. The two nerves decussate above this valve. Its deep origin is just below that of the third nerve beneath the aqueduct of Sylvius. Course and Distribution. — Emerging from the valve of Vie- ussens the nerve winds around the superior peduncle of the cere- bellum and the crusta immediately above the pons, and passes forward near the outer wall of the cavernous sinus to find exit from the cranial cavity by the sphenoidal fissure. Having en- tered the orbit, it runs forward to be distributed to the orbital surface of the superior oblique. In the cavernous sinus it receives fibers from the ophthalmic division of the fifth and from the sympathetic, and occasionally gives off a branch to the lach- rymal nerve. Function. — It supplies motor power to the superior oblique muscle alone. Remembering the origin and attachment of this muscle it is not difficult to fortell the consequence of lesions of the nerve. The action of the superior oblique is to rotate the ball upon an oblique horizontal axis so that the pupil will look down- ward and outward. This movement cannot be accomplished when the nerve is cut, and the inferior oblique asserts itself un- duly to bring about an opposite effect. The ball cannot accom- modate itself to movements of the head toward the shoulder, and double vision supervenes — unless the object be brought in the involuntary line of vision of the affected eye. Fifth Nerve (Trifacial, Trigeminus) . The fifth is analogous to the spinal nerves (i) in rising by two roots, (2) in having a ganglion on its posterior root, and (3) in having a mixed function. The anterior root is small and motor ; the posterior large and sensory. Origin. — Its apparent origin is from the side of the pons above THE CRANIAL NERVES 281 the median line. The deep origin of the large, sensory root is in the pons immediately below the floor of the fourth ventricle and just internal to its marginal boundary. The small, motor root rises from a point just internal to the large root. Course and Distribution. — The two roots, taking their origin as above described, pass through the dura above the internal auditory meatus and run along the superior border of the petrous portion of the temporal bone to a point near its apex, where a large ganglion, the semilunar or Gasserian, is developed on the posterior root and occupies a depression on the bone for its re- ception. The motor root passes beneath the ganglion without being connected with it. The posterior root will be first followed to its distribution. From the anterior surface of the Gasserian ganglion are given off three branches — (i) ophthalmic, (2) superior maxillary, (3) inferior maxillary. After the inferior maxillary has left the cranial cavity it receives fibers from the small or motor root, but the other branches are composed entirely of fibers from the sensory root. i. The Ophthalmic Branch passes forward along the outer wall of the cavernous sinus, divides into three branches — (a) lachrymal, (b) frontal, (c) nasal — and enters the orbit by the sphenoidal fissure. It communicates with the cavernous sympathetic, third and sixth nerves, (a) The lachrymal branch, running along the outer wall of the orbit, reaches the lachrymal gland, gives off filaments to it and to the conjunctiva/ and pierces the tarsal ligament to be finally distributed to the integument of the upper lid. (b) The frontal branch runs along the upper wall of the orbit and separates into the supra- trochlear and supra-orbital branches. The former of these leaves the orbit in front and turns up over the bone to supply the integument of the lower forehead; the latter traverses the supra- orbital canal, escapes by the foramen of the same name, and supplies the skin as far back as the occiput as well as the peri- 282 THE NERVOUS SYSTEM cranium in the frontal and parietal regions, (c) The nasal branch, crossing to the inner wall of the orbit, enters the anterior ethmoidal foramen, passes thus into the cranium again, runs in a groove on the cribriform plate of the ethmoid and finds exit into the nose through a slit by the side of the crista galli. Here it gives off branches which supply common sensation to the mucous membrane of the fore part of the nose, and then running in a groove on the posterior surface of the nasal bone, it leaves the cavity at the lower border of that bone to supply the integument of the ala and tip of the nose. From the nasal nerve pass fibers to the ophthalmic ganglion and to the ciliary muscle, iris and cornea. 2. The Superior Maxillary Branch passes away from the Gasserian ganglion and leaves the cranium by the foramen ro- tundum. Crossing the spheno-maxillary fossa it enters the orbit through the spheno-maxillary fissure and traverses the infra- orbital canal to emerge upon the face at the infra-orbital foramen. In the cranium it gives off a meningeal branch to supply the neighboring dura mater. In the spheno-maxillary fossa it sup- plies branches (a) to the integument over the temporal and post- frontal regions and over the cheeks; (b) to the spheno-palatine ganglion ; (c) the posterior superior dental branches (generally two), which enter the posterior dental canals in the zygomatic fossa, and, passing forward in the substance of the superior max- illa, give off twigs to the fangs of the molar teeth, supplying them with sensation. In the infra-orbital canal the superior maxillary nerve gives off (a) the middle superior dental, which runs down- ward and forward in the outer wall of the antrum to reach the roots of the bicuspid teeth; (b) the anterior superior dental, which likewise runs in the outer wall of the antrum to supply the incisor and canine teeth. After its exit from the infra-orbital canal the nerve divides into palpebral, nasal and labial branches, which supply sensation to the regions indicated by their names. THE CRANIAL NERVES 283 3. The Inferior Maxillary Branch after its exit from the cranium is a mixed nerve, supplying motion to the muscles of mastication as well as common sensation to the parts presently to be noted, and special sense to a part of the tongue. Its large or sensory root comes from the Gasserian ganglion to be joined just beneath the base of the skull by the small motor root which has passed under the ganglion. Almost immediately this com- mon trunk divides into (a) anterior and (b) posterior branches, but first gives off a recurrent meningeal branch and a branch to the internal pterygoid muscle. (a) The anterior of the two divisions of the inferior maxillary nerve receives nearly the whole of the motor root and divides into branches which supply the muscles of mastication, except- ing the internal pterygoid and the buccinator. (b) The posterior division, chiefly sensory, divides into the auriculo-temporal, lingual and inferior dental branches. The auriculo-temporal branch runs backward to a point internal to the neck of the condyle of the inferior maxilla, then passing upward under the parotid gland divides into branches, which are distributed to the external auditory meatus, parotid gland, integument of the temporal region and of the ear and surrounding parts. It communicates with the otic ganglion. The lingual branch is joined by the chorda tympani, passes to the inner side of the ramus of the jaw, crosses Wharton'sduct, and is distributed to the papillae and mucous membrane of the tongue and mouth. It communicates with the facial through the chorda tympani, with the hypoglossal, and with the sub- maxillary ganglion. The inferior dental branch passes be- tween the internal lateral ligament and ramus of the jaw to enter the inferior dental foramen. Thence it traverses the dental canal in the inferior maxilla to issue at the mental foramen. Here it divides into incisor and mental branches; the former con- tinues in the bone to supply the incisor and canine teeth; the latter supplies the skin of the chin and lower lip. In its course 284 THE NERVOUS SYSTEM the inferior dental gives off the mylo-hyoid (before entering the canal) to the mylo-hyoid and anterior belly of the digastric, and dental branches to supply the molar and bicuspid teeth. Four small ganglia, usually classed as part of the sympathetic system, are connected with the three divisions of the trifacial nerve. The ophthalmic, or lenticular, ganglion is connected with the first division; the spheno-palatine or MeckePs with the second; the otic and submaxillary with the third. All these receive sensory fibers from the trifacial and motor fibers from various sources. Functions. — It is seen from the foregoing description that the trifacial is the great sensory nerve of the head and face, and the motor nerve of the muscles of mastication. The small, or motor, division has properly been called the "nerve of mastica- tion." It is insensible upon stimulation before it is joined by the third division of the sensory root. Its section causes paralysis of the muscles of mastication on that side. It cannot be doubted that the large root is exclusively sensory at its origin, and the acuteness of that sensibility, as, e. g., in the teeth, is a matter of common observation. Immediate loss of sensibility in the area of its distribution follows section, and even the cornea, which is normally exquisitely sensitive, can be touched without exciting pain. Both roots are usually cut at the same time, and besides a loss of motion and general sensibility, section of this nerve produces a decided effect upon the eye, the sense of taste, deglu- tition and the nutrition of the parts to which the nerve is distrib- uted. The flow of tears is increased, the pupil becomes tem- porarily contracted and the ball protrudes. In a few hours congestion is marked, and in a day or two the cornea sloughs and the eye is destroyed. Section ot the fifth before its lingual branch is joined by the chorda tympani from the facial causes a loss of general sensation, but not of taste, in the anterior part of the tongue; section of the lingual branch after it has received the chorda is followed by loss of general sensation and of taste. This THE CRANIAL NERVES 285 shows that the special sensibility distributed to the tongue by the lingual branch of the fifth is furnished by the chorda tympani. The fifth nerve sends filaments to give sensibility to the velum palati. The reflex act of deglutition is due to impressions carried from the velum and neighboring parts to the centers; when the fitth nerve is cut no such impressions are conveyed and the reflex act cannot be excited. Regarding nutrition it is noticed that, besides the sloughing of the cornea, there is also, about the same time, the appearance of ulcers in the mouth and on the tongue, and animals thus experi- mented upon soon die. These lesions are much less marked when the section is behind the semilunar ganglion. Explanations of this difference are not altogether satisfactory, but it is rational to suppose that section of sympathetic fibers when the nerve is cut in front of Gasser's ganglion is responsible for the dis- turbances of nutrition; for this is the system of nutrition, and changes following its section in other parts of the body are not unlike those under discussion. Why, however, the changes should be inflammatory in character is not explained by this hy- pothesis, unless it be an explanation to say that the inflammation is set up by the impairment of nutrition in these structures — the impairment resulting in part from the impoverished condition of the blood as a consequence of the inability of the animal to chew. Sixth Nerve (Abducens). Origin. — This is a motor nerve entirely. Its apparent origin is from the lower border of the pons in the groove separating it from the anterior pyramid of the medulla. Its deep origin is close to the median line beneath the floor of the fourth ventricle a little below the motor root of the fifth. Course and Distribution. — The nerve enters the cavernous sinus, runs forward to enter the orbit by the sphenoidal fissure, passes between the two heads of the external rectus, and is dis- tributed to the ocular surface of that muscle. In the cavernous 286 THE NERVOUS SYSTEM sinus it receives fibers from the first division of the fifth and from the sympathetic. Function. — The function is indicated in its distribution. It is insensible at its origin. Stimulation produces contraction of the external rectus; section causes paralysis of that muscle and consequent internal strabismus and diplopia. Seventh Nerve (Facial). Origin. — The apparent origin of the seventh is from the up- per end of the medulla in the groove between the olivary and restiform bodies. Its deep origin is in the pons beneath the floor of the fourth ventricle a little external to the nucleus of the sixth. Course and Distribution. — The seventh nerve passes out- ward and forward with the auditory nerve (on its inner side) to enter the internal auditory meatus. From their relative firmness and texture and their close relation here, the seventh and eighth nerves have been called respectively the portio dura and the portio mollis. Running between them is a fasciculus from the medulla known as the intermediary nerve of Wrisberg, or the portio inter duram et mollem ; most of its fibers join the facial in the internal auditory meatus. The facial nerve enters the Fallopian aqueduct at the bottom of the meatus and follows it to issue at the stylo-mastoid foramen, run forward in the sub- stance of the parotid gland and divide behind the ramus of the jaw into temporo-facial and cervico -facial branches. Its branches of communication are numerous, (i) In the in- ternal auditory meatus it communicates with the auditory nerve ; (2) in the aqiieductus Fallopii with the otic and spheno-palatine ganglia, with the sympathetic and with the auricular branch of the pneumogastric; (3) after leaving the stylo-mastoid foramen, with the fifth, ninth, tenth and sympathetic. Its branches of distribution are also quite numerous, (i) In the aqueductus Fallopii it gives off (a) the tympanic branch to THE CRANIAL NERVES 287 the stapedius muscle, and (b) the chorda tympani, which passes through the cavity of the tympanum and emerges by a foramen at the inner end of the Glaserian fissure to go to the lingual branch of the fifth. (2) At its exit from the stylo-mastoid foramen it gives off (a) a posterior auricular branch which, receiving a filament from the auricular branch of the tenth, is distributed to the retrahens aurem and the occipital portion of the occipito- frontalis; (b) a digastric branch to the posterior belly of the di- gastric muscle; (c) a stylo-hyoid branch to the muscle of that name. (3) On the face it divides into (a) a temporo-facial branch, which is% distributed to the muscles over the temple and upper face; and (b) a cervico-facial branch, which is distributed to the lower face and upper cervical region. Functions. — This is the motor nerve of the muscles of ex- pression, of the platysma, buccinator, digastric (posterior belly) , stylo-hyoid, the muscles of the external ear and the stapedius. Communicating freely with the fifth, it also contains sensory fibers, but it is in all probability insensible at its root. Its sec- tion causes paralysis of the muscles which it supplies, but no marked changes in sensation. The branches to the otic and spheno-palatine ganglia in the aqueductus Fallopii constitute their motor roots; the branch given off in this situation to the tenth supplies it with motor filaments, and probably also here pass sensory fibers from the tenth to the seventh. In facial paralysis when the lesion is in the aqueductus Fallopii or behind it, there is paralysis also of the muscles of the palate and uvula, the uvula is drawn to the opposite side and there is trouble in deglutition. The fibers to the azygos uvulae and levator palati pass from the aqueductus Fallopii through Meckel's ganglion. The effect of paralysis of the facial upon the superficial muscles of the face is suggested in its distribution. The brow cannot be corrugated; the eye is constantly open and there may be con- sequent inflammation from exposure; the nostril cannot be di- lated, and inspiration and possibly olf action are interfered with; 288 THE NERVOUS SYSTEM the cheek is flaccid; the lips are immobile and saliva may flow from that corner of the mouth; the buccinator is paralyzed, and there is often great difficulty in mastication because of the accu- mulation of food between the cheek and the teeth; the unop- posed action of the muscles of the opposite side greatly distort the facial features, the affected side being quite expressionless. Facial monoplegia is common; facial diplegia is very uncommon. The Chorda Tympani. — This branch of the seventh is con- cerned especially in gustation. The fibers of which it is com- posed undoubtedly come from nerve of Wrisberg. Section of the seventh involving also the nerve of Wrisberg causes not only facial palsy but also a loss of the sense of taste in the anterior two-thirds of the tongue. The sense of taste will receive later notice. Eighth Nerve (Auditory) . Origin. — This is a nerve of special sense. Its apparent origin is by two roots — one from the groove between the olivary and restiform bodies at the lower border of the pons, the other coming around the upper end of the restiform body to join the first in the groove. The deep origin of the two roots is different. That of the median root is the dorsal auditory nucleus in the floor of the fourth ventricle; that of the lateral root is mainly from the ventral auditory nucleus in front of the restiform body between the two roots. Course and Distribution. — Crossing the posterior border of the middle peduncle of the cerebellum, it enters the internal audi- tory meatus in company with the facial nerve and the nerve of Wrisberg. At the bottom of the meatus it receives fibers from the seventh, and divides into branches which pass to the cochlea, semicircular canals and vestibule. Function. — This nerve receives and conveys to the brain im- pressions produced by sound waves; it is the nerve of hearing THE CRANIAL NERVES 289 and is in all probability not sensible to stimulation in any other way. Ninth Nerve (Glosso-pharyngeal). Origin. — The apparent origin of this nerve is from the upper part of the medulla in the groove between the olivary and resti- form bodies. Its deep origin is in the lower part of the floor of the fourth ventricle above the nucleus of the tenth. Course and Distribution. — Leaving the skull by the jugular foramen, it passes forward between the internal jugular vein and the internal carotid artery, descends in front of the latter to the lower border of the stylo-pharyngeus where it curves inward, runs beneath the hyoglossus, and is distributed to the fauces, pos- terior third of the tongue, and the tonsil. It communicates with the seventh, tenth and sympathetic. Its branches of distribution go to the mucous membrane and muscles of the pharynx, the stylo-pharyngeus, the tonsil and soft palate, the circumvallate papillae and the mucous membrane at the base and side of the tongue and on the anterior surface of the epiglottis. Some of its branches join branches from the pharyngeal and external laryngeal branches of the pneumogas- tric to form the pharyngeal plexus. Functions. — It is the nerve of sensation to the pharynx and fauces and a nerve of taste to the base of the tongue. Its sensi- bility at its' root is dull, but stimulation produces no motion. Although this nerve is distributed to the mucous membrane over the base of the tongue, palate and pharynx, these parts receive the greater portion of their general sensibility from filaments of the fifth, and section of the ninth produces no marked effect upon the reflex phenomena of deglutition. The sense of taste is distributed to the anterior two-thirds of the tongue by the chorda tympani, and it has nothing to do with general sensation, while the glosso-pharyngeal, endowing the posterior third with gusta- tory power, also furnishes to it a degree of general sensibility. 19 2QO THE NERVOUS SYSTEM Tenth Nerve (Pneumogastric, Vagus). Origin. — This is a mixed nerve. Its apparent origin is from the groove between the olivary and restiform bodies below the ninth. Its deep origin is in the floor of the fourth ventricle just below that of the glasso-pharyngeal. Course and Distribution. — As it leaves the cranium by the jugular foramen it presents a ganglionic enlargement, the jugu- lar ganglion, or ganglion of the root, just below which it is joined by the accessory portion of the spinal accessory. Below the junction is a second ganglion, the ganglion of the trunk. The accessory part of the eleventh passes through this ganglion, and below unites with the vagus trunk to pass chiefly into its pharyngeal and superior laryngeal branches. The pneumogas- tric passes down the neck behind and between the internal jugu- lar vein and the internal and common carotid arteries, and sends motor and sensory fibers to the organs of voice and respiration, and motor fibers to the pharynx, esophagus, stomach and heart. The branches of the pneumogastric are numerous, (i) In the jugular fossa it gives off (a) ameningeal branch to the dura mater of the posterior fossa of the skull ; (b) an auricular branch which, traversing the substance of the temporal bone, emerges by the auricular fissure to supply the integument of the back part of the pinna and external auditory meatus. (2) In the neck it gives off (a) a pharyngeal branch, which consists mainly of fibers from the accessory portion of the eleventh and is the chief motor nerve of the pharynx and soft palate; (b) a superior laryngeal branch, which also consists mainly of fibers from the accessory part of the eleventh and is the chief sensory nerve of the larynx; it also animates the crico-thyroid muscle; (c) a recurrent laryngeal branch, which, on the right side, winds round the subclavian artery, and, on the left, round the aorta to return to the muscles of the larynx whose motor nerve it is; (d) cervical cardiac branches, which communicate with the cardiac THE CRANIAL NERVES 2QI branches ot the sympathetic and pass to the deep cardiac plexus. (3) In the thorax it gives off (a) thoracic cardiac branches, which pass to the deep cardiac plexus ; (b) anterior pulmonary branches, which go to the roots of the lungs in front; (c) pos- terior pulmonary branches, which go to the roots of the lungs behind and send some filaments to the pericardium; filaments from (b) and (c) follow the air passages through the lungs; (d) esophageal branches, which unite with fibers from the opposite nerve to form the esophageal plexus. (4) In the abdomen are the gastric branches ; those from the left nerve are distributed to the anterior surface of the stomach, and those from the right to the posterior; the right vagus is also distributed to the liver, spleen, kidneys and entire small intestine. Throughout the whole course of the pneumogastric commun- ication with other nerves, especially the sympathetic, is very free. Functions. — The root of the tenth in the medulla is purely sensory, but the nerve communicates with at least five motor nerves, and is distributed to mucous membranes and to voluntary and involuntary muscle tissue. The auricular branches contain both motor and sensory fibers, and their function is indicated in their distribution. The pharyngeal branches are mixed, re- ceiving motor filaments from the spinal accessory. Sensibility is supplied to the pharynx not by this nerve alone, but by the branches of the fifth and probably of the ninth; indeed it seems that the pharyngeal branches of the tenth have little to do with the reflex phenomena of deglutition. The superior laryngeal branches, mainly sensory, supply also motor power to the crico- thyroids. Stimulation of the filaments of these branches pre- vents the entrance of foreign bodies into the larynx by reflex closure of the glottis, and also excites movements of deglutition. Their section produces hoarseness. The recurrent, or inferior laryngeal, branches, chiefly motor, supply the muscular tissue of the upper esophagus and trachea, as well as the muscles of the larynx. Section of them causes embarassed phonation, though 292 THE NERVOUS SYSTEM the fibers thus influencing the vocal sounds come to the recur- rent laryngeal from the spinal accessory. The uses of the car- diac branches have been noticed under discussion of the heart's action. The pulmonary branches are both motor and sensory and go to the lower trachea, the bronchi and lung substance. Section of the tenth destroys the sensibility of the mucous mem- brane of the trachea and bronchi and the contractile power of the muscular fibers of the tubes. The esophageal branches are mixed, though motor fibers predominate. Food will not pass readily into the stomach -on section of the tenth because of the absence of muscular contractions in the esophagus. Influence of the Vagus on Respiration. — Section of both these nerves temporarily increases the number of respirations, which soon, however, become exceedingly slow until death ensues. Inspiration is very profound — indeed so profound as to produce rupture of some of the pulmonary capillaries with con- sequent hemorrhage and coagulation of the blood and consolida- tion of the lung in part or whole. Section of only one of the vagi is not usually followed by death. Further notice of the relation of the pneumogastric to respiration is given elsewhere. Influence of Vagus on the Stomach, Intestine and Liver.— Stimulation of the pneumogastric causes contraction of the stom- ach; but since the contraction is not immediate, the impulse is probably carried to it by fibers of the sympathetic running with the gastric branches of the tenth. When the vagus is cut during digestion in the stomach the contractions of the muscular wall are impaired and the sensibility of the organ is abolished. Secretion is interfered with, but not stopped. Section of the vagus seems also to impair intestinal secretion and movements, but it is not improbable that this is because sympathetic fibers joining the vagus high in the neck are dis- tributed with it to the intestine. Simple division of the pneumogastrics inhibits the formation of glycogen in the liver; but when the central ends of the cut THE CRANIAL NERVES 293 nerves are stimulated there is an increased production of sugar even to the point of glycosuria. The irritation is probably re- flected through the sympathetic; indeed it is not supposed that the vagi are concerned in the glycogenic function of the liver, ex- cept reflexly; its section only prevents the conduction cephalad of the impressions which usually give rise to a secretion of glycogen. The connection of the vagus with the kidneys, spleen and suprarenal capsules is obscure. Eleventh Nerve (Spinal Accessory). Origin. — This nerve consists of a cranial portion, accessory to the tenth, and a spinal portion. The apparent origin of the cranial root is from the side of the medulla just below the vagus. Its deep origin is in the medulla to the posterior and outer side of the nucleus of the ninth. The apparent origin of the spinal portion is by several filaments from the side of the cord as low down as the sixth cervical nerve. Its deep origin is from a column of cells in the anterior cornu of gray matter of the cord. Course and Distribution (Accessory Portion). — Passing out to the jugular foramen it is joined by the spinal portion, and sends a few filaments to the ganglion of the root of the tenth; then leaving the spinal portion it finds exit from the cranium by the jugular foramen, passes over the ganglion of the trunk of the tenth (adherent to it), and is, continued chiefly in the pharyngeal and superior laryngeal branches of that nerve (Gray), but in the recurrent laryngeal as well. Spinal Portion. — Running upward between the two roots of the spinal nerves the spinal portion enters the cranial cavity by the foramen magnum, passes outward to the jugular foramen, where it joins the accessory portion to separate from it on pass- ing through that foramen. After leaving the skull it takes a course backward, pierces the sterno-mastoid, crosses the occipital triangle and terminates in the trapezius. It gives branches to the sterno-mastoid and to the cervical plexus. 294 THE NERVOUS SYSTEM Functions. — Both roots of this nerve are purely motor, but communication with other nerves gives it a degree of sensibility. The fibers from the medulla (accessory) go exclusively to the muscles of the larynx and pharynx, which those from the cord (spinal) go exclusively to the sterno-mastoid and trapezius; and section of either root separately is followed by phenomena corresponding to these facts. When both roots are divided there is loss of voice, disturbance of deglutition, loss of cardiac inhibition and partial paralysis of the sterno-mastoid and tra- pezius. The loss of voice and disturbance in deglutition are explained by the distribution of the fibers of the eleventh with the pharyngeal and laryngeal branches of the tenth. The loss of the power of the vagus to inhibit cardiac action is because the fibers of the tenth which convey the inhibitory impulses are re- ceived from the spinal accessory. The sterno-mastoid and trapezius are only partially paralyzed because they receive motor fibers also from the cervical plexus. Twelfth Nerve (Hypoglossal). Origin. — This nerve supplies motion to the tongue. Its apparent origin is by 10-15 filaments in the groove between the anterior pyramid of the medulla and the olivary body. Its deep origin is in the floor of the fourth ventricle under the lower border of the fasciculus teres. Course and Distribution. — The nerve passes through the anterior condyloid foramen in two bundles which unite to form a common trunk below. Running downward in company with the internal carotid artery and internal jugular vein, it reaches a point opposite the angle of the jaw, then runs forward, crosses the external carotid, lies on the hyoglossus and is continued for- ward in the genio-hyoglossus to the tip of the tongue. It communicates with the tenth, sympathetic, first and second cervical and the lingual branch of the fifth. Its branches of distribution are (i) meningeal to the dura THE CRANIAL NERVES 295 mater in the posterior fossa of the skull; (2) descendens hypo- glossi, which, running downward across the sheath of the great vessels, meets branches of the second and third cervical nerves to form a loop from which are supplied the sterno-hyoid, the omo-hyoid and the sterno-thyroid muscles; (3) thyro-hyoid to the muscle of that name; (4) muscular to the muscular sub- stances of the tongue and to the styloglossus, hyoglossus, genio- hyoid and genio-hyoglossus muscles. Functions. — This nerve possesses no sensibility at its root, but receives sensory fibers from anastomoses with other nerves. Its stimulation, therefore, causes movements of the tongue and some pain. Section of both nerves causes difficult deglutition, loss of power over the tongue and consequent disturbances in mastication and articulation. When the twelfth is affected in hemiplegia the tongue, on protrusion, deviates to the affected side because it is pushed out by the genio-hyoglossus. It will be seen from the foregoing that, classified according to their properties at their roots, the I., II. and VIII. are nerves of special sense; the III., IV., VI., XI. and XII. are motor; the X. is sensory; and the V., VII. and IX. are mixed. It is to be remem- bered, however, that most of these (excepting the nerves of special sense) are mixed in their distribution by reason of the reception of fibers from other nerves. The term " mixed" in the above classification is used as meaning the association of special sensory fibers with motor or common sensory fibers as well as the association of these latter with each other. The VII. is classed as a mixed nerve only by allowing that the intermediary nerve of Wrisbery is to be considered a part of it. Its own proper root is purely motor. THE SPINAL NERVES. The spinal nerves, thirty-one on each side, are so called from the fact that they originate in the spinal cord and escape from the spinal canal by the intervertebral foramina. Eight pairs come 296 THE NERVOUS SYSTEM from the cervical region of the column, twelve from the dorsal, five from the lumbar, five from the sacral, and one from the coccygeal. They are numbered according to their foramina of exit. Each nerve rises by two roots — an anterior which can be traced to the anterior cornu of gray matter and a posterior which goes (apparently) to the posterior cornu — and these emerge B. FIG. 87. A. bipolar cell from spinal ganglion of a 4^ weeks' embryo (after His) . n, nucleus ; the arrows indicate the direction in which the nerve processes grow, one to the spinal cord, the other to the periphery. B, a cell from the spinal ganglion of the adult; the two processes have coalesced to form a T-shaped junction. (Kirkes.) respectively from the antero-lateral and postero-lateral fissures of the cord. Before leaving the spinal canal these two roots join to pass through the corresponding ihtervertebral foramen as a single tiunk which, however, just beyond that foramen divides into anterior and posterior branches to be distributed to the anterior and posterior parts of the body. The posterior root (inside the spinal canal) is sensory, and has a ganglion developed upon it. The fibers of the posterior THE SPINAL NERVES 297 root are outgrowths of cells in the ganglion of that root, as indi- cated in Fig. 87. This accounts for the arborization of the differ- ent fibers around cells in the cord instead of an actual connection with them. These facts should not be lost sight of though it is customary to speak of an efferent fiber as passing directly to a cord cell itself. The anterior root is entirely motor except for a degree of "recurrent" sensibility which is due to the presence in it of posterior root fibers which have passed backward from the point of junction of the two probably to supply the mem- branes of the cord. The common trunk is, of course, mixed, as are the anterior and posterior branches passing from it. These spinal nerves are distributed to the muscles of the trunk and extremities, to the integument of almost the entire body and to some mucous membranes; and from what has been said in speaking of the cord about the connection between it and these nerves, and their connection through it with the higher centers, it is evident that they are most important factors which, acting under the guidance of the sensorium, on the one hand, tell of the condition of the organism — its relations and environ- ments— and, on the other, control the voluntary movements of the body. The spinal nerve fibers come in part directly from the brain and in part from the gray cells of the cord. THE SYMPATHETIC SYSTEM. The sympathetic has been separated from the cerebro-spinal system only for the sake of convenience. The former sends filaments to the latter and receives both motor and sensory fibers in return, while the cooperation of the two systems, regulating in harmony all the physiological processes going on in the body, is too evident to be questioned. The sympathetic system is remarkable for the number of ganglia connected with it. These may be divided into (a} those along the vertebral column, as the thoracic, (6) those in close 298 THE NERVOUS SYSTEM proximity to the viscera and from which those viscera are to be directly supplied, as the semilunar, and (c)' terminal ganglia which the fibers reach just before final distribution, as the car- diac, intestinal, etc. The sympathetic is, therefore, frequently known as the ganglionic system. Arrangement. — There is on each side of the spinal column, extending from the lenticular ganglion above to the ganglion impar below, a chain of ganglia all of which are united to each other and to the ganglia of the opposite chain by commissural fibers. From these ganglia go fibers to form numerous plexuses and to be distributed to the various parts. In the skull there are four of these ganglia, the otic, ophthalmic, submaxillary and spheno-palatine or Meckel's; in the cervical region there are three ; in the dorsal twelve ; in the lumbar four ; in the sacral four or five; and in front of the coccyx the single ganglion impar. Connections between the cranial nerves and cranial sympa- thetic ganglia have already been noted. The cervical ganglia are of special interest as furnishing the chief sympathetic supply to the heart. The thoracic or dorsal ganglia give rise to the sympathetic supply for the great abdominal viscera. From the sixth, seventh, eighth and ninth springs the great splanchnic nerve, which passes through the diaphragm to the semilunar ganglion. This is the largest of the sympathetic ganglion, and is sometimes called the abdominal brain. It has been inaccurately called the center of the sympathetic system. The two ganglia occupy positions on opposite sides of the celiac axis, and give rise to fibers which supply most of the abdominal viscera. The tenth and eleventh thoracic ganglia give rise to the lesser splanchnic nerve. From the last thoracic springs the renal splanchnic nerve. The radiating fibers from the semilunar ganglia form the solar plexuses for the two sides. The lumbar ganglia give off fibers to form the aortic lumbar and hypogastric plexuses. THE SYMPATHETIC SYSTEM 299 The sacral and coccygeal ganglia supply the pelvic vessels. Properties. — The ganglia and nerves are slightly sensitive. Contraction of involuntary muscular tissue follows stimulation — not immediately, but after a considerable interval, and the sub- sequent relaxation is tardy. Some of the ganglia are dependent for power upon their fibers from the cerebro-spinal system, while others seem capable of acting independently, at least for a time. Functions. — Little is known of the functions of the sympa- thetic except as regards efferent fibers. They are distributed in general to the non-striped musculature of the circulatory appara- tus and of the viscera, to secreting glands and to the heart. The heart furnishes the only example of a direct sympathetic supply to striated muscle. The sympathetic has a very definite effect upon secretion, nutrition and the local production of heat. Sec- tion of the sympathetic fibers going to any part causes hyper- emia, an increased amount of secretion (sweat, e. #.), and a rise of temperature in that part. The last two conditions are caused by the first, and it in turn is due to a paralysis of the muscular coat of the vessels, allowing an abrogation of their usual tonic condition and, consequently, dilatation and an increased amount of blood with exaggerated nutritive activity. This statement confronts us with the question of vaso-motor action. Vaso-motor Phenomena. — By vaso-motor nerves is meant those fibers which convey to the muscular coat of the vessel walls impulses causing them to contract and decrease the caliber, or to relax and increase it. Those causing contraction are called vaso-constrictors ; those causing relaxation vaso-dilators. It is mainly through the operation of vaso-motor nerves that the sympathetic system influences nutrition in a particular part, though all vaso-motor fibers are not confined to the sympathetic cords. However, it is not through the operation of the vaso- motor nerves alone that the sympathetic lays claim to be the "system of nutrition," for all the parts to which its other fibers are distributed contribute also very materially to nutrition, 300 THE NERVOUS SYSTEM though perhaps in not so direct a manner as do the muscular coats of the arteries. While intestinal peristalsis, the secretion of many glands, as, for example, the production of glycogen, bile, etc., cannot be shown to be absolutely dependent on sympathetic connections, yet all these processes — nutritive in nature — have their normal activity seriously impaired by withdrawal of the sympathetic influence. The chief vaso-motor center is in the medulla, though acces- sory centers exist also in the cord; all vaso-motor fibers pass out from these centers and leave the cerebro- spinal axis with the cranial or spinal nerves. The most usual mode of action of the vaso-motor nerves is reflex, as when the mucous membrane of the stomach becomes hyperemic upon the introduction of food; or when the salivary secretion increases during mastication, or even sometimes at the sight or thought of food; or when emotions are evidenced by paling or blushing. Raising blood-pressure by stimulating the vaso-constrictors and lowering it by stimulating the vaso-dilators are simply mechanical results, and require no comment. Sleep. — Sleep is closely associated with vaso-motor action. Every part of the body has a function to perform, but it must have some rest from that performance or it will begin to act in- efficiently and finally cease altogether. For most organs these periods of rest occur at approximately uniform intervals, as in case of the stomach, heart or respiratory muscles; but notably in case of the involuntary muscles these periods of repose have no regularity — i. e., a person exercises them at no regular time except by accident of occupation or otherwise. But, in any case, there comes a time when repose must be had, for during activity the destructive processes far exceed the constructive, and in order for the balance to be preserved there must be a time when the opposite is true. THE SYMPATHETIC SYSTEM 301 Now we may say that it is the function of the brain to furnish consciousness — if we can allow that consciousness embraces all the various manifestations of nerve force peculiar to the brain. For the brain to suspend this function at frequent intervals like the heart (e. g.) would be manifestly impossible if one is to do any consecutive work depending upon this organ. The brain works longer, and must, therefore, rest longer at a time than most of the other organs of the body. True, so far as the volun- tary muscles are concerned they rest best probably when the brain is resting, but the latter condition is not a necessary one for the maintenance of their physiological integrity. This repose of the brain — this temporary abolition of the cerebral functions — is sleep. While, of course, the activity of that organ during wakefulness may be increased or diminished by volition, and it may, therefore rest from a comparative standpoint — as when one ceases to think actively upon a subject and becomes mentally list- less— still the brain can never, under such circumstances, rest properly, and sleep finally becomes imperative. Vascular Phenomena of Sleep. — Coma is analogous to sleep in that consciousness is lost; but in this case the brain is con- gested and the condition is unnatural. It was long supposed that this was the vascular condition during natural sleep, but appli- cation of the physiological principles prevailing in other parts of the body would rather presuppose a condition of cerebral anemia; for the brain receives blood for two purposes — first, to supply nutrition to the nervous substance, and second, to bring supplies which, by the action of the brain cells, may be con- verted into nerve force — and during sleep only the first of these purposes is to be served. This is true in case of glands, muscles, etc., during their intervals of repose. As a matter of fact, the cerebral vessels are contracted and there is much less blood in the brain during sleep than during consciousness. Dreams. — In explanation of the phenomena of dreams and somnambulism, it is said that what we call sleep may occur in 302 THE NERVOUS SYSTEM one part of the brain and not in another, or in different degrees in different parts of the nervous centers. "In the former case [dreams] the cerebrum is still partially active; but the mind products of its action are no longer corrected by the reception, on the part of the* sleeping sensorium, of impressions of objects belonging to the outer world; neither can the cerebrum, in this half-awake condition, act on the centers of reflex action of the voluntary muscles, so as to cause the latter to contract — a fact within the painful experience of all who have suffered from night- mare. In somnambulism the cerebrum is capable of exciting that train of reflex nervous action which is necessary for pro- gression, while the nerve center of muscular sense (in the cere- bellum?) is presumably fully awake; but the sensorium is still asleep, and impressions made on it are not sufficiently felt to rouse the cerebrum to a comparison of the difference between mere ideas or memories and sensations derived from external objects" (Kirkes). Relation Between the Cerebro-spinal and Sympathetic Systems. — A brief resume may help to clarify the association between the two systems. i. Anatomically. — The two are developed from the same em- bryological tissue ; the vaso-motor sympathetic fibers obey centers in the medulla and cord, and must, therefore, be connected with those centers either directly or indirectly; characteristic small medullated fibers pass at intervals from the cord through the roots into the sympathetic ganglia; they send fibers each to the trunks of the other to be distributed directly, or to form plexuses and then be distributed together; their fibers are found together in all organs which receive cerebro-spinal nerves (unless they be non-vascular) ; in some of these organs just named the sym- pathetic fibers are there only as vaso-motor nerves, while in others, as glandular structures like the liver and salivary glands, sym- pathetic fibers are distributed to the gland cells themselves, and both have a definite but associated influence on secretion. THE SYMPATHETIC SYSTEM 303 2. Physiologically. — The physiological relation is best indi- cated by examples. A great many, if not all, the sympathetic ganglia seem to receive their power to generate nerve force from the cerebro-spinal system; there can be no proper nutrition of the parts animated by cerebro-spinal fibers without the associated ac- tion of vaso-motor sympathetic fibers — not even of the nerve cells and fibers themselves; in reflex action the afferent impression may be conveyed by a cerebro-spinal fiber and reflected through a sympathetic, or vice versa; when one hand is thrust into hot or cold water the temperature of the opposite hand may be raised or lowered, impressions having been carried to the center by cerebro-spinal and reflected by sympathetic fibers, not only to the immersed hand, but to the other as well; food is taken into the mouth, impressions are carried by nerves of common sensation to the brain and are reflected through the sympathetic system, an increased amount of blood is thereby sent to the sali- vary glands and an increased secretion supervenes; one smells savory articles and the mouth waters, etc. Examples could be multiplied ad infinitum to establish the cooperation existing between the two systems. What has been incidentally and indirectly said on this point in considering secre- tion, digestion, circulation, respiration, etc., serves to emphasize their connection. CHAPTER XII. THE SENSES. IT is evident from preceding remarks that it is through the intervention of the nervous system that we have a "sense" of existence, of the existence and condition of different parts of our bodies and of our relations to the external world. The knowl- edge we thus obtain is based upon sensations of various kinds, all of which are carried to the centers by afferent fibers. Such sensations maybe what are termed (A) Common, or (B) Special, including (i) Touch, (2) Smell, (3) Sight, (4) Taste, (5) Hearing. It is to be remembered that the seat of sensation is in the brain, and not in any organ which primarily receives or conveys the im- pression. We do not in reality see with the eye or hear with the ear; these are only complex organs so arranged that rays of light or sound waves produce upon them such impressions as, when transmitted to the sensorium, will give rise to the sensations of sight or hearing. (A) COMMON SENSATIONS. As regards the uses of the fibers conveying impressions which result in these sensations, they (unless it be those concerned with tactile impressions) are distinct from those of special sense. That is to say, the fibers of the olfactory, optic, gustatory and auditory nerves do not convey general impressions; but it is al- most certain that fibers conveying tactile impressions convey also painful impressions — and the sensation of pain is taken as typical of common sensations. It is known that very painful impressions sometimes overcome tactile sensibility, and that 304 THE SENSE OF TOUCH 305 very frequently tactile sensibility remains in parts which receive no painful impressions, as, e. g., under anesthesia by cocain; but it may be that the power in the same fiber to convey, in the first case tactile, and in the second painful impressions is destroyed without destroying its power to convey the other. The varieties of common sensation are too numerous to even mention. Thirst, hunger, fatigue, discomfort, satiety, etc., are everyday examples, as are also the desire to urinate or defecate. Numerous subdivisions of the sensation of pain might be men- tioned, such as itching, burning, aching, etc. The so-called muscular sense — by which we become aware of the condition, relation, coordination and degree of activity or repose of the muscles — will be considered as belonging here. (B) SPECIAL SENSATIONS, i. The Sense of Touch. The sense of touch is closely related to common sensation. Its distribution over the body is as uniform as that of common sensation, but it is most highly developed in those parts where general sensibility is most marked (as in the skin), and attains its highest degree of perfection only in those situations in which tactile corpuscles exist, for example, on the palmar surfaces of the tips of the fingers. The teeth, hair, nails, etc., are rather surprisingly endowed with tactile sensibility. Leaving pain and the muscular sense as part of general sensibility, the sense of touch may be considered under two heads — (a) Tactile Sensi- bility proper and (b) Temperature. (a) Tactile sensibility proper is most marked where the epidermis over the papillae is thin. When the epidermis is re- moved and the cutis is touched there is pain instead. Tactile sensibility is much decreased where the epidermis is thickened, as over the heel. The terminal tactile organs have been de- scribed in connection with afferent nerves. They are chiefly 20 306 THE SENSES the end bulbs of Krause and the tactile corpuscles of Meiss- ner. (See Figs. 71 and 72.) Besides, tactile impressions are re- ceived by the free extremities of afferent nerves situated over the body at large. Numbness from cold is due to interference with cutaneous circulation — upon which the sense of touch is directly dependent. It is almost impossible to distinguish mere touch from pressure. Acuteness. — How the sense of touch is capable of develop- ment by practice is well illustrated in the case of many blind persons. They learn to read with 'comparative facility by pass- ing the hand over raised letters; or they frequently make the sense of touch take the place of the lost sense in other almost incredible ways. The acuteness of this sense in different por- tions of the body has been made the subject of observation by touching two different parts in the same region with finely pointed instruments and noting how near the points can be brought together and still be recognized as two. This distance is found to vary from -^ inch on the tip of the tongue to i\ inches in the dorsal region. (b) It is not improbable that there are special nerve endings concerned in the reception of temperature impressions, though this has not been definitely proven. Decisions as to tempera- ture are only relative; the surface temperature of the part upon which the impression is made is the standard, and one can only tell absolutely whether the object is hotter or colder than the skin, and, within certain limits, approximate how much hotter or colder. The delicacy of the temperature sense agrees with that of touch as regards the thickness or absence of the epidermis. 2. The Sense of Smell. Regarding the mechanism of olfaction it is found that one of the first conditions necessary is the presence of particular cells. Between the epithelial cells of the mucous membrane to which the olfactory fibers are distributed are delicate spindle-shaped THE SENSE OF SIGHT 307 cells known as olfactory cells, and to them pass the terminal filaments from the olfactory bulbs. These cells are stimulated by contact with odorous substances, and from them go, by way of the nerve fibers, impressions which are recognized as odors of different kinds. The olfactory fibers are the only ones which will convey such impressions. True, the same substance may, at the same time, excite other sensations, as of pain or taste, but the impressions giving rise to these latter sensations are con- veyed by different fibers altogether. The substances which ex- cite olfaction must come in actual contact with the nerve terminals and to do this must be dissolved in the mucus of the nasal mucous membrane; hence dryness of the nasal cavities (as in the first stage of nasal catarrh) interferes with olfaction. It is also said that odorous substances introduced in solution into the nasal cavities will not excite the sense of smell, but that they must be introduced by a current of air. Whether an odor is pleasent or unpleasant is largely a relative matter; odors most disgusting to some animals are not offensive to others. This same difference may also hold good among different men. Impairment of the sense of taste, for some reason, follows a loss of the sense of smell. 3. The Sense of Sight. It is not intended to go into a detailed consideration of the sense of sight, but some remarks on the normal eye and its action are in order. Protection of the Ball.— The orbital cavity has a pyramidal shape with its base forward. It contains the eye-ball, its mus- cles, some adipose tissue and most of the lachrymal apparatus. Above the orbit, the eye-brows prevent a flow of perspiration from the forehead on to the lid, and also shade the eye to some extent. The lids, when closed, entirely obscure the balls and protect them in front. On their free borders are rows of hairs (eye-lashes) curling away from the globe and shading and pro- 308 THE SENSES tecting it from dust. The lids are closed by the orbiculares pal- pebrarum and opened by the levatores palpebrarum superiores. In the ordinary closing of the lids only the upper one is moved, but the lower one is raised in forcible contraction of the orbicu- laris. Intervention of the will is not necessary to the action of these muscles, though they are striated. Except during fatigue, the eyes are kept open involuntarily, but when the cornea is touched no effort of the will can prevent contraction of the or icu- laris palpebrarum. During sleep the globes are rotated upward. The Lachrymal Apparatus.— This consists of the lachry- mal glands, canal, duct and sac, and the nasal duct. The secretion of the lachrymal gland keeps the cornea and conjunc- tiva constantly bathed in a thin fluid. It is situated in the orbital cavity at its upper and outer portion. Its secretion is discharged upon the conjunctiva by several little ducts. The excess of se- cretion is carried into the riose through the nasal duct. Near the inner canthus is a small opening in each lid; these openings are the orifices of the lachrymal canals, which canals join at the inner angle of the eye to form the lachrymal sac ; the sac is con- tinued below as the nasal duct, opening into the inferior meatus of the nose. The secretion of tears is much diminished during sleep. The influence of the nervous system on lachrymal secre- tion is well known. Emotional disturbances operate through the sympathetic to increase the flow. Irritation of the mucous membrane of the nose or eye is followed by a like result. Movements of the Ball.— The capsule of Tenon, a fibrous membrane outside the sclerotic, holds the ball loosely in place. A small amount of adipose tissue behind the globe is never ab- sent. Movements of the ball are effected through the action of the internal and external recti, the superior and inferior recti, and the superior and inferior oblique ; of these, all but the two last named arise from the apex of the orbital cavity. The recti are inserted into the sclerotic just back of the cornea. The superior oblique runs along the inner aspect of the orbital MOVEMENTS OF THE BALL 309 cavity to a point near the supero-internal angle; here it becomes tendinous, passes through a nbro-cartilaginous ring, and then turns backward and outward to be inserted into the sclerotic between the superior and external recti just behind the center of the globe. The inferior oblique arises just within the orbital FIG. 88. — Lateral view of the muscles of the eye-ball. 6, optic net (Landois.) 5, trochlea or pulley of the superior oblique muscle, 12 ; 6, optic nerve; 8, superior, 9, inferior, and 12, external rectus; 13, inferior oblique. cavity near the anterior inferior angle, and passes around the anterior part of the globe to be inserted in the sclerotic just below the superior oblique. The effect these muscles have upon the movements of the ball is indicated by their origin and attachment. The external and internal recti rotate it outward and inward, the superior and inferior recti upward and downward. The superior and inferior oblique antagonize each other. The former rotates the globe so 310 THE SENSES that the pupil is directed outward and downward; the latter so that it looks outward and upward. The associated action of all these muscles can produce almost any variety of movements, and no effort of the will is necessary to properly associate them when it is desired to direct the line of vision toward a certain object. For instance, when it is desired to look at an object on the right it takes no distinct voluntary effort to contract the external rectus of the right eye and the internal rectus of the left. It will be seen later that vision for the two eyes is normal only when impressions are made upon exactly corresponding parts of the two retinae, so that they may act as a single organ; and for this to be done not always the same movements are called for in both balls. Anatomy of the Ball. — The eye-ball is a globular body con- sisting of several coats enclosing refracting media. Of these coats the external is the sclerotic, dense and fibrous, covering the posterior five-sixths of the organ and continuous with the cornea, which covers the anterior one-sixth. It is not well sup- plied with blood-vessels. The cornea is transparent, and upon its external surface are several layers of delicate nucleated epithe- lium; underneath this layer of cells is a thin membrane, the an- terior elastic lamella, which is a continuation of the conjunc- tiva. The substance proper of the cornea is composed of pale interlacing fibers among which are connective tissue corpuscles and quite a quantity of fluid. These fibers are continuous from the sclerotic, but they lose their opacity at the corneo-sclerotic margin. On the posterior surface of the cornea is the trans- parent elastic membrane of Descemet, a part of which, at the circumference of the iris, passes into the ciliary muscle. The cornea is very sensitive, but contains no blood-vessels. Next inside the sclerotic is the choroid coat of the eye. It does not lie under the cornea, but is confined to the sclerotic area of the ball. Behind the optic nerve penetrates it, and in front it is connected with the iris. The choroid is very vascular. Its color is dark brown on account of the abundance of pigment ANATOMY OF THE BALL 311 in the cells on the inner surface of the membrane. Anteriorly the choroid is folded in upon itself to form the ciliary processes, which project inward around the margin of the crystalline lens. The ciliary muscle is important in accommodation. It is in the shape of a muscular ring surrounding the margin of the choroid just outside the ciliary processes. In front it is attached FIG. 89. — Diagram of a vertical section of the eye. (From Yeo after Holden.) i, anterior chamber filled with aqueous humor; 2, posterior chamber; 3, canal of Petit; a, hyaloid membrane; b, retina (dotted line); c, choroid coat (black line); d, sclerotic coat; e, cornea; f, iris; g, ciliary processes; h, canal of Schlemm or Pontana; i, ciliary muscle. to the line of junction of the cornea and sclerotic and to the liga- ment on the anterior surface of the iris; behind it is lost in the substance of the choroid. Its contraction, therefore, compresses the vitreous humor and relaxes the suspensory ligament of the lens. The iris is a circular veil hanging in front of the lens. It presents a perforation a little to the nasal side of its center, the pupil. It is attached in the corneo-sclerotic line. It contains circular and radiating fibers. The iris divides the space between the cornea and lens into two chambers, anterior and posterior 312 THE SENSES — the latter of which is very small. The " color of the eyes" depends on the color of the anterior surface of the iris; its pos- terior surface has a constant dark purple hue. The size of the pupil is subject to variations to be noted later. Inside the choroid is the retina, which is that part of the eye capable of receiving impressions of sight. Anteriorly it reaches nearly to the ciliary processes. Externally it is in contact with the choroid, and internally with the hyaloid membrane of the vitreous humor. It is penetrated by the optic nerve a little within and below the center of the posterior hemisphere. Just external to the point of entrance of the nerve is the macula lutea, a small yellow area in the center of which is the fovea centralis ; this last is exactly in the axis of distinct vision. Nine layers of cells are usually described as composing the retina. From without inward they are (i) the pigment layer, (2) rods and cones, (3-6) the four granular layers, (7) nerve cells, (8) expansion of fibers of the optic nerve, (9) the limitary membrane. Of these, the most im- portant is the layer of rods and cones. The rods, or cylinders, extend through the thickness of the membrane and have between them, at intervals, flask- shaped bodies, the cones. At the macula lutea only the cones exist. Elsewhere the rods are more abun- dant than the cones. The length of the cones is about half that of the rods, and they occupy the inner aspect of the membrane. The layer of nerve cells presents cells communicating on the one hand with the rods and cones and on the other with fibers of the optic nerve. The rods and cones are the only parts of the retina possessing special sensibility, impressions being con- veyed from them to the brain by the optic nerve. The fibers of the second nerve, composing one layer, are pale and transpar- ent. The blood supply of the retina is from the arteria cen- tralis retinae, which enters the optic nerve just before it expands, and, running in its substance, is distributed as far as the ciliary processes anteriorly. The Crystalline Lens is a biconvex transparent body situ- OCULAR REFRACTION 313 ated just behind the iris. Its function is to refract the rays of light, and its action in this respect is similar to such lenses in optical instruments. It is held in place by the suspensory liga- ment. Its anterior convexity is more marked than its posterior. It is enveloped by a thin transparent capsule. The Suspensory Ligament is a continuation of the anterior layer of the hyaloid membrane of the vitreous humor. When this layer reaches the edge of the lens (coming forward) it divides into two parts, one passing in front of and the other behind that body; the divisions are continuous respectively with the anterior and posterior portions of the capsule of the lens. The ligament supports the lens. The Aqueous Humor is behind the cornea and in front of the lens and suspensory ligament. The iris has been said to separate this cavity into anterior and posterior chambers com- municating through the pupillary opening. The aqueous hu- mor is colorless and perfectly transparent. It serves to refract the rays of light, having for that purpose the same index as the cornea. The Vitreous Humor occupies about the posterior two-thirds of the globe, and is back of the lens and suspensory ligament surrounded by the delicate hyaloid membrane. It is of a gelat- inous consistence, and is divided into numerous compartments by very delicate membranes radiating from the point of entrance of the optic nerve. It is a transparent refracting medium. Ocular Refraction. — In order for the image of an object to be distinct the rays passing from it must fall on a single portion of the retina, viz., the fovea centralis. The sensibility of the retina to light decreases in passing away from the fovea. All rays would not meet on the retina unless they were refracted; and for this purpose there are the cornea, the aqueous humor, the lens and the vitreous humor. The surfaces of the cornea and lens are the most important of these. Since the two surfaces of the cornea are parallel, the external surface alone is concerned 314 THE SENSES in refraction. The center of distinct vision (fovea) is in the axis of the lens precisely in the plane upon which the rays of light are brought to a focus by the refracting media. Refraction by the cornea alone would focus the rays behind the retina; hence the necessity of convex lenses before the eye after operations for cataract. Rays leaving the cornea are refracted by the anterior surface of the lens, by its substance to a certain extent, and again by its posterior surface, the normal mechanism being such that all rays are focused on the fovea. The rays cross each other after refraction, and the image is inverted, but the brain takes no notice of this fact, and objects are seen in their natural positions. Accommodation. — Accommodation means a change in the convexity of the lens, whereby images are focused on the retina, whether the object be far away from or near the eye. Rays of light from distant objects strike the eye practically parallel, and we may assume that there is a certain "passive" condition of the refracting media which will bring such rays to a focus at the proper point. But when the object observed is near the eye a change in the arrangement of the media, or of the convexity of their surfaces, is necessary to prevent the focusing of the rays behind the retina. The desired end is accomplished by increas- ing the convexity of the lens. When the ciliary muscle is "pas- sive" the capsule compresses the lens, decreasing its convexity to a minimum; from the attachments of this muscle, already noted, its contraction is attended by a relaxation of the suspensory ligament, which in turn relieves in some degree the compression of the capsule upon the lens and allows its antero-posterior diam- eter to increase; the result is increased convexity of the lens. When distant objects are looked at the lens become flatter as a result of contraction of the suspensory ligament, which contrac- tion is a consequence of the relaxation of the ciliary muscle. Accommodation for distant objects seems a passive process entirely. The ciliary muscle is the "muscle of accommodation." REACTION TO LIGHT 315 The contraction of the pupil for near objects is not, properly speaking, a part of accommodation. Then, granting special sensibility to the retina and optic nerve the formation and appreciation of an image is simple. Rays of light having passed through the cornea and aqueous humor are admitted by the pupil to pass through the lens and vitreous hu- mor. By all these objects they are refracted so that they cross each other and fall upon the retina, producing an inverted image there. The size of the pupil, other things being equal, is regu- lated by the intensity of the light, the opening being contracted to admit less when the light is strong. Myopia, Hyperopia and Presbyopia. — Sometimes the antero- posterior diameter of the eye-ball is too long and the rays of light are brought to a focus in front of the retina. Such a con- dition is known as myopia; the person will be near-sighted. He brings objects near his eyes so that the rays may have a greater divergence and thus be focused farther back. Or the rays may be scattered by placing concave lenses before the eyes. Some- times, too, the antero-posterior diameter may be too short and the rays come to a focus behind the retina. Such a condition is known as hyperopia ; the person will be far-sighted. He holds objects far away from his eyes that the rays from them may strike the ball with less divergence and thus be focused farther forward. Or the same end may be accomplished by placing convex lenses before the eyes. In old age the lens becomes flattened and accom- modates itself less easily. This tends to focus light behind the retina and objects have to be held far away from the eye. This is known as presbyopia. Its remedy is the same as that for hyperopia. Reaction to Light. — Regarding the reaction of the pupil to light, it is evident that this is mainly a reflex nervous phenom- enon, though direct light will cause the muscular tissue of the iris to contract. The direct influence of the third nerve on the action of the iris has been referred to under a consideration of 316 THE SENSES that nerve. Reflexly, the pupil is contracted by light by the conveyance of an impression to the brain through the optic fibers, a message is sent to the proper center, and a stimulus is reflected through the third nerve to the sphincter of the iris causing it to contract. When the optic nerve is cut the circuit is broken, and movements of the iris do not occur from the admission of light. Practically, then, when much or little light reaches the retina the pupil contracts or dilates, as the case may be, in an effort to keep the amount constant. Binocular Vision. — It is evident that when a person looks at an object two images are formed — one on each retina — but they are combined in his consciousness and he sees but one object. If one of the balls be thrown out of the proper axis, by pressure, e. g., objects appear double. The same is true in strabismus, at least until the person has grown accustomed to the defect. In normal vision the rays from an object are formed on the fovea centralis of each eye — that is, upon corresponding points which are, for each, the centers of distinct vision. 4. The Sense of Taste. In order that gustatory sensation may be exercised it is neces- sary (i) that there be specially endowed nerves and nerve centers; (2) that the nerve terminals be excited by sapid (tastable) ma- terials; (3) that these substances be in solution. It has already been seen that the special nerves of taste are (a) the chorda tym- pani distributed to the anterior two-thirds of the tongue, and (&) the glosso-pharyngeal to the posterior third of that organ. It is probable that only the dorsum of the tongue, the lateral parts of the soft palate, the uvula and the upper pharynx are concerned in gustation. On the tongue are found special papillae, (i) the circumvallate, large and few in number, near the base of the organ, and (2) the fungiform, about 200 in number, over the remaining area. The circumvallate and some of the fungiform THE SENSE OF HEARING 317 papillae contain taste beakers, true gustatory organs. They are ovoid collections of cells beneath the epithelial covering of the mucous membrane. Sapid substances enter these beakers in solution and come in contact with the taste cells, which are connected with the filaments of the gustatory nerves. Thus are produced specific impressions which are conveyed to the gustatory center, and the sense of taste is excited. The limited distribution of the taste beakers makes it impossible that they should be the only organs capable of receiving special gustatory impressions. The taste center has been indefinitely located in the uncinate gyrus near the olfactory center. Since it is necessary to the tasting of substances that they come in actual contact with the taste organs, and since to do so they must be in solution, it follows that dryness of the mouth interferes with, or abolishes, this sense. The most marked tastes are the sweet, bitter, saline, and al- kaline. The more delicate flavors involve also the special sense of smell, and it has been seen that dissociation of the two kinds of impressions is often impossible. Taste is also subject to variations by reason of education, age, association, caprice, etc. Bitters are most easily appreciated at the back, salts and sweets at the tip, and acids at the sides at the tongue. 5. The Sense of Hearing. The ear consists of a complicated apparatus for the purpose of the reception of special impressions which are appreciated by the brain as sounds. Anatomically it consists of the external, the middle and the internal ear; the last contains the essentials of the auditory apparatus, the external and middle divisions serving only to concentrate the sound waves upon the parts of the internal. The External Ear.— This consists of the pinna and the ex- ternal auditory canal. The pinna is the external visible por- 318 THE SENSES tion, and consists of the large cavity, the concha, into which the external auditory canal opens externally; of two prominent ridges partly surrounding the concha, the helix outside and the antehelix internal to this; and of a nbro-cartilaginous process projecting backward in front of the concha, the tragus. The FIG. 90. — Scheme of the organ of hearing. AG, external auditory meatus; T, tympanic membrane; K, malleus with its head (h) , short process (kf) and handle (m) ; a, incus, its short process (x) and its long process united to the stapes (s) by means of the Sylvian ossicle (Z); P, middle ear; o, fenestra ovalis; r, fenestra rotunda; x, beginning of the lamina spiralis of the cochlea; pt, scala tympani, and vt, scala vestibuli; V, vestibule; S, saccule; U, utricle; H, semicircular canals; TE, Eustachian tube. The long arrow indicates the line of traction of the tensor tympani; the short curved one, that of the stapedius. (Landois.) external auditory canal runs inward and slightly forward from the concha to terminate at the membrana tympani, or drum. Its inner part is in the petrous portion of the temporal bone; its external part is nbro-cartilaginous in structure. The whole is lined by integument. The Middle Ear (Tympanum). — This is a cavity at the bottom of the external auditory canal in the petrous portion of THE INTERNAL EAR 319 the temporal bone, containing ossicles for the conduction of sound waves to the internal ear. The cavity communicates, through the Eustachian tube, with the pharynx, and this is its oniy direct connection with the external air, though it does com- municate with the mastoid air cells. It is lined by mucous membrane. The membrana tympani, separating it from the external auditory canal, is fibrous in structure. It is lined ex- ternally by skin and internally by mucous membrane. The three ossicles of the middle ear are the malleus, incus and stapes. The malleus, shaped like a hammer, is attached in a vertical direction to the upper radius of the membrana tym- pani, and articulates by its head with the incus. The incus has the shape of an anvil; its base articulates with the malleus, while its small extremity curves downward to articulate with the neck of the stapes. The base of the stapes is applied to the membrane covering the fenestra ovalis. The tensor and laxa- tor tympani are attached to the neck of the malleus; the sta- pedius to the neck of the stapes. These bones constitute a chain, which conveys the vibrations of the membrana tympani to the fenestra ovalis. The Internal Ear (Labyrinth). — This consists of a series of cavities in the petrous portion of the temporal bone lined by a peculiar membrane. When the bony substance surrounding these cavities is carefully removed it is found that that portion immediately outside them is harder than the adjacent structure. This constitutes the bony labyrinth, while the membrane inside the bony walls is the membranous labyrinth. The bony labyrinth consists of the vestibule, cochlea and semicircular canals. The vestibule occupies the mid-portion of the labyrinth, and is that part with which the middle ear com- municates by the fenestra ovalis; it communicates also with the cochlea and semicircular canals, and on its internal aspect are openings for the entrance of some of the branches of the audi- tory nerve. The cochlea, shaped like a snail shell, runs off 320 THE SENSES from the front of the vestibule, winds about two and a half times around a cone-shaped central axis — the modiolus — and ends FIG. 91. I, Transverse section of a turn of the cochlea; II, A, ampulla of a semicircular canal with the crista acustica; a, auditory cells, p, provided with a fine hair; T, otoliths; III, scheme of the human labyrinth; IV, scheme of a bird's labyrinth; V, scheme of a fish's labyrinth. (Landois.) in a blind apex. The canal of the cochlea is partially separated into two compartments by a bony plate, the lamina spiralis. TERMINATION OF AUDITORY NERVE 321 The basilar membrane completes the septum and divides the lumen of the cochlea into two canals, the scala tympani and the scala vestibuli, corresponding in name to the tympanic and vestibular openings of the cochlea. The semicircular canals, three in number — superior, external and posterior — de- scribe arches from the posterior aspect of the vestibule, com- municating by both their extremities with that cavity. The membranous labyrinth consists of a special membrane lying inside the bony labyrinth and corresponding in general outline to the walls of the cavity. It is, however, separated from the walls by perilymph, and encloses a similar fluid, the en- dolymph. It covers the sides of the lamina spiralis in the cochlea and completes the septum, besides following the wall proper; and on one side it sends a distinct process from the tip of the a mina spiralis to the wall of the canal, so that there are in reality three divisions of the lumen of the cochlea. This process is the membrane of Reissner, and the third canal is the scala media the true membranous cochlea. (See Fig. 91.) Termination of Auditory Nerve. — The membranous laby- rinth, containing and being suspended in fluid, receives the term- inal filaments of the eighth nerve as well as all the sonorous vibra- tions intended for that nerve. When the auditory nerve has reached the base of the internal auditory meatus it enters the internal ear by two divisions, one for the vestibule and semicir- cular canals and the other for the cochlea. The vestibular portion again subdivides, sending one branch to the utricle and superior and horizontal semicircular canals, and another to the saccule and posterior semicircular canal. The fibers of the eighth nerve spread out over the inner surface of the membrane to end in a way somewhat obscure. The membrane is lined internally by epithelium whose character differs in different areas. In the region of distribution of the vestibular portion of the nerve the cells are of two kinds, hair cells and rod cells. From the inner ends of the hair cells ciliated processes project into the en- 21 322 THE SENSES dolymph; to their outer ends pass the axis cylinders of the nerve fibers, though the exact mode of connection is not clear. The rod cells are much more numerous than the hair cells, but their precise connection with audition is not apparent. Upon the basilar membrane are the rods of Corti. They con- sist of two sets of pillars of varying length, slanting toward each other, thus leaving at their base a space which becomes a canal by a longitudinal succession of these pillars. There are sup- posed to be about 4,500 elements in the outer and 6,500 in the inner set of these rods. Intimately associated with the pillars are large numbers of hair cells with which the auditory nerve filaments may communicate; it is certain that these filaments are closely connected in some way with the pillars. Functions of the Semicircular Canals. — The use of these is obscure. Their destruction is not followed by interference with hearing, although auditory filaments are distributed to some parts of them. Curiously enough, however, this lesion is one of the three chief ones interfering so markedly with equilibrium — the phenomena following it being not unlike those sequent upon lesions of the cerebellum and the posterior white columns of the cord. Functions of the Cochlea. — While the exact mechanism of the production of auditory impressions is unknown, there seems to be no doubt that such mechanism takes place almost entirely in the cochlea, and that fibers which convey to the auditory cen- ters impressions of sound are distributed to the organ of Corti therein. That is to say, loss of the sense of hearing supervenes upon destruction of this part of the internal ear. In physics it is known that for a sound, for example of a piano string, to be heard the membrana tympani must vibrate in unison with the sonorous vibrations of the cord; that is, "consonating bodies" repeat sonorous vibrations, giving them their proper pitch and quality. It has been supposed that the thousands of rods of Corti, of varying length and size, in the Cochlea are made to FUNCTIONS OF THE COCHLEA 323 vibrate separately or in correctly associated collections (like the strings of a harp), and thus reproduce communicated vibrations, and so give rise to impressions which, conveyed by the auditory nerve to the center, are there recognized as sounds of different degrees of intensity, pitch and quality. This theory may be true, but its correctness is probably beyond the range of experi- mental proof. While the usual mode of conduction of sound waves to the cochlea is through the external ear, they may reach it in other ways, as through the bones of the head, or through the Eustach- ian tube. Nor is the integrity of the membrana tympani actually necessary to the production of sound; although practically speak- ing a person in whom this organ is destroyed is deaf, he can hear if the ossicles can in some way be placed in vibration by sound waves, as by the intervention of an artificial membrane. In- deed it has already been seen that none of the parts of the ex- ternal or middle ear are actually necessary to hearing. They are only accessory conveniences for the better t.ansmission of impressions to the filaments of the auditory nerve. The (so-called) tensor and laxator tympani muscles make tense or lax the membrana tympani, thus influencing the rapidity and amplitude of its vibrations, and therefore the pitch and in- tensity of the sound. The stapedius prevents too great move- ments of the stapes. The free communication of the air in the tympanum with that in the mastoid cells and pharynx insures an approximately constant internal pressure upon the membrane, and thus precludes accidents which would otherwise interfere with its proper vibration. The auditory center in man is in the first and second temporal convolution of the temporo-sphenoidal lobe. Briefly then, the physiology of hearing is as follows: Sound waves collected by the pinna enter the external auditory canal and impinge upon the membrana tympani. The drum is thus set to vibrating and communicates its movements to the ossicles, 324 THE SENSES which in turn hand them over through the fenestra ovalis to the fluids of the internal ear, through which media they reach the auditory filaments, are conducted to the brain and given proper recognition. The Production of the Voice. The production of the voice is not connected with the special senses, but its consideration will be introduced here for the sake of convenience. The Larynx is the organ of voice. It is a cavity closed ex- cept for its openings above and below. It conisists of four car- tilages— cricoid, thyroid and two arytenoid — joined together by ligaments and muscles. The vocal cords are attached poste- riorly to the bases of the movable arytenoid cartilages and ante- riorly to the angle between the alae of the thyroid. The muscles serve to move the cartilages and thus to separate or approximate and to render lax or tense the vocal cords. Production of Sound. — The human voice is produced by vibrations of the vocal cords, which vibrations are set up by currents of expired air. Movements of the Vocal Cords. — These are those taking place (i) in respiration and (2) during vocalization. j, i. In Respiration. — When the cords are "passive" they are approximated anteriorly and separated posteriorly, so that the interval between them (rima glottidis) is triangular. This interval becomes a little wider during inspiration and a little narrower during expiration. 2. In Vocalization. — The production of sound in the larynx involves an approximation of the cords and an increase in their tension. They are made more nearly parallel by the approach of the arytenoids to each other, and the rima glottidis assumes the shape of a mere chink. The tenser the cords, the higher the note produced; usually also the closer the cords are brought to- THE PRODUCTION OF THE VOICE 325 gether, the higher the note. The range of the voice depends principally on the degree of tension which the cord can be made to assume. Varieties of Vocal Sounds. — These are mainly (i) monoto- nous, (2) transitional, (3) musical. 1. In monotonous sounds the notes have all nearly the same pitch, as in reading. 2. In transitional sounds there is a gradual change in the tension and approximation of the cords, so that the notes become successively higher or lower, as in the howling of a dog. 3. In musical sounds the vocal cords have a definite number of vibrations for each successive note — a number corresponding to the production of that note in the musical scale. The range of the average human voice is from one to three octaves. The highest and lowest notes of females are about one octave higher than the corresponding notes of males. The chief difference between male and female voices is, therefore, one of pitch; but they also differ materially in tone. The difference in pitch is a result of the different length, and therefore the dif- ferent rate of vibration, of the cords in the two sexes. The female cords are about two-thirds the length of the male. Before puberty the male larynx resembles the female, but at that period the alae of the thyroid becomes more prominent in the male and the cords increase in length, thus accounting for the change of voice. In old age control of the musculature of the larynx is partly lost, the cords become altered and the cartilages ossify. These circumstances make the voice weak and unsteady. Speech. — Modifications and alterations of the sounds pro- duced in the larynx during and after their production result, under the influence of the sensorium, in articulate speech. These modifications are made chiefly by the tongue, teeth, and lips. The speech sounds are divided into vowels and consonants. The distinction is that the vowel sounds are generated in the 326 THE SENSES larynx, while the consonant sounds are produced by alterations in the current of air above the larynx, and cannot be pronounced except constantly with a vowel. The current is modified mainly by the tongue and teeth in the formation of linguals and dentals, by the cavity of the nose in case of nasals, and by changes in the shape and size of the oral cavity in the production of other sounds. Nervous Supply of the Larynx. — The superior laryngeal branch of the teeth is the sensory nerve, which guards the glottis to prevent the entrance of foreign bodies. Impressions made on the filaments of this nerve are reflected through the medulla and inferior laryngeal branch of the tenth to the muscles which close the glottis. The inferior laryngeal also innervates the muscles that vary the tension of the cords, and the superior laryn- geal keeps the mind informed of the state of these muscles and of the necessity for forced expiration or coughing. CHAPTER XIII. REPRODUCTION. VERY many facts in our knowledge of reproduction depend on observations made upon lower animals, but there is sufficient analogy between the known facts connected with human repro- duction and development and those of the same stages in other groups of beings to enable us to present, as at least approxi- mately accurate, certain broad principles regarding the process as it pertains to the human race. In order that a human being may be brought into existence it is necessary that there be a union of the male element, the sper- matozoon, and the female element, the ovum. Both these sexual cells are developed from epithelium — the spermatozoon from that of the seminiferous tubules of the male, and the ovum from the germinal layer of the ovary. In what follows reference will be had to reproductive proc- esses in the human being. Spermatozoa. — Human spermatozoa (Fig. 92) are elongated bodies, about one five-hundredth of an inch in length, and con- sist of three parts, head, mid-portion and tail. The last-named part is about four-fifths the length of the entire spermatozoon. The head is egg-shaped and much the thickest part of the ele: ment. A slender filament, the axial fiber, extends throughout its length from head to tail and projects slightly beyond the latter. Spermatozoa are possessed of wonderful vitality. They live for several weeks in the genital passages of the female. In the male genital passages they may live for months in a quiescent state. The nucleus is the fertilizing agent. Spermatozoa are also 327 328 REPRODUCTION remarkable for their power of locomotion, which is effected by lashings and rotary movements of the tail. Ova.— The ovum (Fig. 93), or female sexual cell, is the largest cell to be found in the human body. Its diameter is about T ^-5- of an inch. Its structure is that of a typical cell. When the ovary d k m FIG. 92. — Spermatozoa. i, human(X 600), the head seen from the side; a, on edge; k, head; m, middle piece;/, tail; e, terminal filament; 3, from the mouse; 4, bothriocephalus latus; 5, deer; 6, mole; 7, green woodpecker; 8, black swan; 9, from a cross between a gold- finch (m) and a canary (f.) ; 10, from cobitis. (Landois.) is developing a part of its covering epithelium dips down into the substance of the organ and becomes walled off by union of the surface cells above it. A part of this ball of epithelium becomes the ovum, and a part the Graafian follicle for that ovum. The youngest ova are thus found nearest the surface of the ovary. The cell has an enveloping membrane, the mtelline membrane, a GRAAFIAN FOLLICLES 329 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 re- ceives 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 ves- icle, are several layers of epithelial cells making up the membrana granulosa. The cavity is filled with an albuminous liquid, the V FIG. 93. — Ovum. (From Yeo after Robin.) a, zona pellucida and yitelline mem- brane; b, yolk; c, germinal vesicle or nucleus; d, germinal spot or nucleolus; e, interval left by the retraction of the vitellus from the zona pellucida. 330 REPRODUCTION liquor folliculi. 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 membrana FIG. 94. — Section of the ovary of a cat, showing the origin and development of Graafian follicles. (From Yeo after Cadiat.} a, germ epithelium; b, 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. completely surround the ovum, constituting the discus proligems. The cells of the discus next the ovum have their long axes at CORPUS LUTEUM 331 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 at 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 degenera- tion, 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 the mass then has a yellowish color and is known as the corpus luteum. 332 REPRODUCTION 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 high- est 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 differ- ences between the corpora lutea of menstruation and pregnancy are shown by the following table from Dalton: Corpus Luteum of Menstruation. Corpus Luteum of Pregnancy. At the end of three weeks. One month. Two months. Four months. Six months. Nine months. Three-quarters of an inch in reddish; convoluted wall pale. diameter; central clot Smaller; convoluted wall bright yellow; clot still reddish. Reduced to the condition of an insignificant cicatrix. Absent or unnoticeable. Absent. Absent. Larger; convoluted wall bright yellow; clot still reddish. Seven-eights of an inch in diameter; convoluted; wall bright yellow; clot perfectly decolorized. Seven-eights of an inch in diameter; clot pale and fibrin- ous; convoluted wall dull yellow. Still as large as at the end of second month; clot fibrin- ous; 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. O VITIATION 333 Maturation. — But previous to its discharge from the Graafian follicle, the ovum undergoes certain changes— a ripening process — 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 pro- toplasm. 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. 95. — The fertilized ovum, or blastophere. (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. 95.) 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 pronudeus, 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 334 REPRODUCTION intervals of about four weeks. It should, and usually does, enter 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 nmbriated 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 toward 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 prob- ably 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 MENSTRUATION 335 five or six days preceding the flow, the uterine mucous mem- brane gradually thickens, the glands become longer and more tortuous, the connective tissue cells multiply and the blood-ves- sels are greatly increased in size. This is apparently a prep- aration for the reception of the impregnated ovum. 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 menstruation. 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 Graanan 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 336 REPRODUCTION 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. At is ap- proaches the vitelline membrane, head first, the protoplasm of the ovum swells up into a prominence to meet it. The fertilizing spermatozon 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 a new being. It contains anatom- SEGMENTATION 337 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 nu- cleus (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 perivi- telline membrane. As division proceeds, cells arrange them- selves around others, so that the former occupy the circumference 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 attenuated (Rauber's cells) and disappear. Their place, as a surrounding envelope, 338 REPRODUCTION .is taken by some of the cells of the inner layer. This second surrounding layer is the epiblast, or ectoderm; the surrounded mass is the hypoblast, or entoderm. ent. FIG. 96. — Sections of the ovum of a rabbit, showing the formation of the blastodermic vesicle. (From Yeo after E. Van Beneden.) a, b, c, d, are ova in successive stages of development; Z, p, zona pellucida; ect, ectomeres, or outer cells; ent, 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 DERIVATIVES OF THE GERM LAYERS 339 mesoblast, or mesoderm, which occupies a position between the other two layers. The three-layered germ is now the blasto- dermic vesicle, or the gastrula, 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 communi- cating cavities, of the external audi- tory canal, of the anus and anterior urethra, of the conjunctiva and an- terior part of the cornea, the ante- rior lobe of the pituitary body, the crystalline lens and the enamel of the teeth. (3) The spinal cord and brain with its outgrowths, including the optic nerve, the retina and the posterior lobe of the pituitary body. (4) The epithelium of the internal ear. From the entoderm: The epithe- lium 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. FIG. 97. — Impregnated egg. With commencement of forma- tion of embryo; showing the area germinaiiva or embryonic spot, the area pellucida, and the primitive groove and streak. (Kirkes after Dalton.) 340 REPRODUCTION From the mesoderm: (i) Connective tissue in all its forms, such as bone, dentine, cartilage, lymph, blood, fibrous and areo- lar 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 on 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. Longi- tudinal 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. 97.) These surface markings are caused by thickening of the ectoderm. (Fig. 98.) 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. 98.) At first the mesoderm does not completely embrace the germ, but is deficient opposite the embryonal area. Fig. 94 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 somatopleure; the entoderm and splanchnic mesoderm unite to form the splan- chnopleure. The interval left between the somatopleure and splanchnopleure is the celom, or body cavity. (Fig. 98.) The great serous cavities of the body are developed from it. Beginning Differentiation. — It thus appears that the embryo BEGINNING DIFFERENTIATION 341 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, amnion, allantois and placenta. As regards the embryonic body, some of the most prominent occurrences connected with its 342 REPRODUCTION development 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 to form the medullary plate. The edges of this longitudinal plate soon begin to curl up, and thus form the medullary furrow, or groove. (Fig. 99.) The margins of the adjacent ectoderm CHORDA DORSALIS 343 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 canal comes to lie underneath the surface ectoderm. (Fig. 100.) 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. FIG. 100. — Transverse section through dorsal region of embryo chick (45 hours) . 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 (Me). A, epiblast: t7, hypoblast, consisting of a single layer of flattened cells; Me, medullary canal; Pv, protovertebra ; Wd, Wolffian duct; So, somatopleure; Sp, splanchnopleure; pp, pleuroperitoneal cavity; ch, notochord; ao, dorsal aorta, containing blood-cells; v, blood-vessels of the yolk-sac. (Kirkes after Foster and Balfour.) It is a solid, instead of a cylindrical, longitudinal collection of cells, extending along the dorsal aspect of the celom. It is developed 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 chordal folds of the entoderm. When the curling edges have joined to form a solid cylinder of cells, the chordal folds unite over the ventral surface of the cylinder. Figures 99 and 100 344 REPRODUCTION 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. 100.) They are in segments, the forma- tion of which begins in the neck and proceeds caudad and cepha- lad. They are sometimes called the protovertebrce. 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 protection and nutrition of the embryo. The essential parts of a vertebrate 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 being tucked under the embryo approach each other on its ventral aspect and divide the yolk into two communicating cav- ities. (See Figs. 102 and 103.) The layers of the blastoderm thus folded underneath the em- bryo are the visceral plates. They form the boundaries of a cavity 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 duct are the future alimentary canal and the yolk-sac, or umbil- ical vesicle. It is to be noticed that the visceral plates embrace both somatopleure and splanchnopleure, and that it is the BODY CAVITY 345 ectodermic layers of the splanchnopleure which finally join to form the gut tract, and the somatopleure which forms the ventral and lateral walls of the body cavity. The gut tract has the FIG. 101. — 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 seven- teenth day in the human embryo, previous to the expansion of the allantois; c, the villous chorion; a, the amnion; a', the place of convergence of the amnion and re- flexion 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, i, 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; M, the allantois connected by a pedicle with the hinder portion of the alimentary canal. (Kirkes after Quain,) shape of a straight tube occupying the long axis of the embryo and opening into the umbilical vesicle. 346 REPRODUCTION 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. 101, 102, 103.) It furnishes nutriment to the embryo for a short time and is then largely cut off from the body. It gradually shrivels (Figs. 107, 108), and with that part of the duct external to the abdomen is cast off either before FIGS. 1 02 AND 103. a, chorion with villi. The yilli are shown to be best developed in the part of the chorion to which the allantois is extending; this portion ultimately becomes the placenta; b, space between 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 vessels; g, allantois. (Kirkes.) or at parturition. .Vessels develop in its walls and absorb 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 function 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. 107, 108.) The two layers of which the somatopleure is composed separate , THE ALLANTOIS 347 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 formed 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 the fluid, the liquor FIG. 104. — Diagram of fecundated egg. a, umbilical vesicle; b, amniotic cavity; c, allan- tois. (Kirkes after Dai- ton.) FIG. 105. — Fecundated egg with allantois nearly complete. a, inner layer of amniotic fold; b, outer layer of ditto; c, point where the amniotic folds come in con- tact. 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.) amnii, which increases until it reaches a considerable quantity. It affords mechanical protection to the fetus during intrauterine life, and at labor serves to evenly dilate the cervix. When this has been accomplished is the usual time at which the sac ruptures 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. 104, 105). It is of splanchnopleuric origin. It soon becomes a mem- 348 REPRODUCTION 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 vascular connections with the mother. Finally they are distrib- FIG. 106. — This and the two following wood-cuts are diagrammatic views of sections, through the developing ovum, showing the formation of the membranes of the chick. (Yeo, after Foster and Balfour.} A, B, C, D, E, and F, are vertical sections in the long axis of the embryo at differ- ent 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 iii, give only the posterior part of the longitudinal section to show three stages in the formation of the allantois; e, embryo; y, yolk; pp, pleuroperitoneal fissure; vt, vitelline membrane; a/, amniotic fold; a/, allantois. 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 longer needed. The vessels of the allantois are the two allantoic THE ALLANTOIS 349 arteries and the same number of allantoic veins. The allantois also receives the fetal urine. As the true placental circulation is established and the visceral PP- FIG. 107 < e, embryo; a, amnion; a', alimentary canal; vt, vitelline membrane; a/, amniotic fold; ac, amniotic cavity; y, yolk; al, allantois. plates close the abdominal cavity, the allantois is constricted at the umbilicus so as to be divided into two parts. That outside the body shrivels and is cut away with the umbilical 350 REPRODUCTION 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. 108. — Diagrammatic sections of an embryo. Showing the destiny of the yolk-sac, ys. vt, vitelline membrane; pp, pleuroperit- oneal cavity; ac, cavity of the amnion; a, amnion; a', alimentary canal; ys, yolk-sac. three layers. From without inward these are the original vitelline membrane, the false amnion and the allantois. The allantois has been seen to extend around between the two amni- otic 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. THE DECIDUA 351 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. 101.) 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 established between their vessels and those of the mother; here the placenta is to be formed. The Decidua. — The decidua of pregnancy consists of the hypertrophied 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 oc- curred, 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 attached 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 between 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 reflex and the vera, but as the embryo increases in size the 352 REPRODUCTION 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 FIG. 109. — Diagrammatic view of a vertical transverse 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 cornua, into the Fallopian tubes, and at ff into the cavity of the cervix, which is closed by a plug of mucus; dv, decidua vera; dr, decidua reflexa, with the sparser villi imbedded in its substance; ds, decidua serotina, involving the more developed chorionic vili of the commencing placenta. The fetus is seen lying in the amniotic sac; passing up from the umbilicus 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 after Allen Thomson.) the chorion. They are discharged with the membranes at birth. Placenta.— The placenta is the organ of nutrition for the THE PLACENTA 353 fetus after about the end of the third month. Through it the vessels 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 become very highly developed. This is at the site of the reflec- tion of the decidua serotina and is the chorion f rondo sum. 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 per- forms 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 receive 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 sinuses do so obliquely; consequently uterine contractions ' at birth very 23 354 REPRODUCTION effectually check the hemorrhage which separation of the pla- centa occasions. Umbilical Cord. — The umbilical cord is made up of the vessels 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 constitute a single umbilical vein, but the arteries remain sep- arate. 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 card 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 other layers. These membranes constitute, then, a sac VITELLINE CIRCULATION 355 filled with fluid. The sac is ruptured in labor, and the child escapes through the rent. Afterward the decidua vera and placenta are detached, and escape together as the " after birth." Development of the Circulation. — The development of the circulation may be 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 corpuscles. 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 mesoblastic 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 pulsa- tions 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 outline of the future ventricles. Afterward a constriction 356 REPRODUCTION 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 the 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 develop- ment 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- bilicial 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 arterial, the contents of the ascending vena cava are mixed when PLACENTA L CIRCULATION 357 they enter the heart. The Eustachian valve, together with the direction of the entering current, causes the blood from the FIG. no. — Diagram illustrating the circulation through the heart and the principal vessels of a fetus. (From Yeo after Cleland.) a, umbilical vein; b, ductus venosus; /, portal vein; e, vessels to the viscera; d, hypogastric arteries; c, ductus arteriosus. ascending vena cava to pass through the foramen ovale into the left auricle. Blood from the upper extremities (impure) enters the right 358 REPRODUCTION auricle through the descending vena cava. The Eustachian valve and the direction of the current here again cause this blood to enter the right ventricle. There is supposed to be very little mingling of blood from the two venae cavae as it passes thus through the right auricle. At the same time the blood which has entered the left auricle through the foramen ovale, augmented slightly by blood from the ill-developed pulmonary veins, passes into the left ventricle. The ventricles now contract simul- taneously. Blood from the right ventricle (impure) passes in small part through the pulmonary artery to the lungs, but chiefly through a tube, the ductus arteriosus, into the descending part of the aortic arch. Blood from the left ventricle (mixed) enters the aorta and goes to the system at large. The vessels going to the head and upper extremities are given off from the aortic arch before it is joined by the ductus arteriosus. Since the ductus arteriosus contains impure blood, the supply going to the upper extremities is purer than that going to the lower. Of the blood which passes down the aorta a part leaves by the hypogastric arteries, to go again to the placenta, while the other part is distributed to the trunk and lower extremities. It thus appears that the liver is the only organ of the fetus which receives pure blood, and that the head and upper extrem- ities are better provided for in this respect than are the lower parts. This may account for the relatively large liver of the fetus, and for the fact that the upper extremities are better developed than the lower. The ductus arteriosus, ductus venosus, foramen ovale, Eusta- chian valve, hypogastric (umbilical) arteries and the umbilical vein are the organs which distinguish the placental circulation, and they all partially disappear after birth, as will be imme- diately seen. 3. Adult Circulation. — The circulation as it exists in the % THE SKELETON 359 adult has been described. It is only necessary to see what changes mark its establishment. When the child is born detachment of the placenta, or ligation of the cord, stops the placental circulation. The first noticeable effect comes from the consequent deoxygenation of the blood. The respiratory center is stimulated and the child gasps to fill the hitherto collapsed lungs with air. Owing to the diminished resistance in the expanded lungs, the pulmonary artery begins to carry most of the blood from the right ventricle, and the ductus arteriosus commences to atrophy. Before birth, too, the Eustachian valve becomes less distinct and the foramen ovale partly closes. At labor a kind of valve guards the opening of the foramen ovale and allows the escape possibly of a little blood from the right into the left auricle, but none in the opposite direction. It commonly closes about the tenth day of extra- uterine life. The ductus arteriosus is reduced to the condition of an impervious fibrous cord between the third and tenth days after birth. The hypogastric arteries, umbilical vein and ductus venosus are closed between the second and fourth days. That part of each hypogastric artery between the internal iliac and the upper lateral part of the bladder remains in adult life as the superior vesical artery; the part between this point and the umbilicus is that which atrophies. The umbilical vein remains as the round ligament of the liver. The ductus venosus is represented by a fibrous cord in the fissure for the ductus venosus in the liver. The Skeleton.— The appearance of the notochord and of the protovertebrae, or somites, has been observed. The notochord becomes a thin line of soft cartilage, around which the bodies of the vertebra are developed, though it does not itself become those bodies. The protovertebrae were seen to lie longitudinally on either side of the notochord. These grow around the neural canal dorsally and the notochord ventrally to form the vertebra?. 360 REPRODUCTION From them also are developed the muscles and skin of the back. The cranium is developed as a modification of the vertebral column. All the bones are in early fetal life cartilaginous or membran- ous. Centers of ossification appear at one or more points in each bone. The bones of the extremities are not at first separate. They bud out from the upper and lower parts of the trunk, to be sub- divided later. Nervous System. — The origin of the nervous system has been indicated in describing the neural canal. The mesodermic cells multiply and fill the tube, until only the canal of the spinal cord is left. Headward the neural canal terminates in a dilated extremity, which soon becomes divided into three vesicles, an- terior, middle and posterior. From these are developed the different parts of brain. Some of these parts develop much more rapidly than others, and we thus account for the predomin- ant size of the cerebrum. At first there are no cerebral convolu- tions, but later the cavity of the cranium seems too small for the brain and the characteristic infoldings occur. The eye is formed by the projection of the optic vesicle from the side of the anterior brain vesicle. The internal ear is formed by the projection of the auditory vesicle from the posterior brain vesicle. The alimentary canal is formed by being pinched off from the mesodermic layer of splanchnopleure. It communicates for some time by means of the vitelline duct with the umbilical vesicle. When cut off from the latter it is a straight tube, occupy- ing the long axis of the body just in front of the vertebral column, and is divided into the foregut, hindgut and a central part. Later it communicates above with the pharynx and mouth and opens below upon the external body surface (anus). The liver FETAL DEVELOPMENT 361 and pancreas are developed from prolusions from the sides of the duodenum. The bladder has been seen to be that part of the allantois which is constricted off and remains in the body. The lungs are developed from the esophagus and at first lie in the abdominal cavity; but the formation of the diaphragm fixes them in the thorax. The kidneys are developed from the Wolman bodies. These bodies are embryonic structures only. Each is a tube lying parallel to the vertebral column on either side of it. This tube consists of a collection of tubules, which unite to form a common excretory duct. This duct joins the corresponding one from the opposite side to empty into the alimentary canal opposite the allantoic stalk. Outside the Wolman bodies are two other ducts, the ducts of Miiller. They also enter the intestine. The Wolman body finally gives place to the kidney, from which the ureter is developed. In the female the ducts of Miiller become the tube, uterus and vagina. In the male they atrophy. Just behind the Wolman bodies are developed the ovaries or the testes, as the case may be. The development of a few of the organs has thus been simply referred to. Satisfactory explanation of these procedures can be given only in extended works on embryology, and this section may be closed with the subjoined table of development, which is abbre- viated from one by Heisler : First Week. — Segmentation and passage of ovum to uterus. Second Week. — Ovum in uterus. Decidua reflexa present. Entoderm and ectoderm layers formed — also mesoderm. Em- bryonal area, primitive streak in primitive groove. Chorion and villi. Amnion folds. Umbilical vesicle partly formed. 362 REPRODUCTION Vascular area. Two primitive heart tubes. Gut tract partly formed. Third Week. — Body indicated. Dorsal outline concave. Vitelline duct. Amnion. Allantoic stalk. Visceral arches. Heart divides. Vitelline circulation begins. Gut tract still connected with umbilical vesicle. Liver evagination begins. Anal plate. Pulmonary protrusion. Wolffian bodies. Neural canal. The brain vesicles. Optic and otic vesicles. Olfactory plates. Notochord. Fourth Week. — Flexion of body. Yolk-sac largest size. Somites well formed. Allantois grows. Vitelline circulation complete. Allantoic vessels developing. Pharynx, esophagus, stomach and intestine differentiated. Pancreas begins. Pul- monary protrusion bifurcates, Ventral roots of spinal nerves. Limb buds apparent. Fifth Week.— Umbilical vesicle begins to shrink. Cord longer and spiral. Length of fetus two-fifths of an inch. Primitive aorta divides into aorta and pulmonary artery. Intes- tine shows loops. Bronchi divided. Ducts of Miiller. Epider- mis. Olfactory lobe. Eyes move forward. Limb buds seg- ment. Digitation indicated. Sixth Week. — Umbilical vesicle shrunken. Amnion larger. Vitelline circulation supplanted by allantoic. Teeth indicated. Duodenum, cecum. Rectum. Larynx. Genital folds and ridges. Dorsal roots of spinal nerves. Eye-lids. Lower jaw and clavicle begin to ossify. Vertebrae and ribs cartilaginous. Fingers separate. Seventh Week. — Body and limbs well defined. Heart septa complete. Transverse and descending colon. Nails indicated. Cerebellum indicated. Muscles recognizable. Ossification in cranium and vertebrae begins. Eighth Week. — Head somewhat elevated. Parotid gland. Gail bladder. Mullerian ducts unite. Genital groove. Mam- FETAL DEVELOPMENT 363 mary glands begin. Sympathetic nerves. Nose discernible. Additional centers of ossification. Ninth Week. — Weight, three-fourths of an ounce. Length, one and a quarter inches. Pericardium. Anal canal. External genitals begin to indicate sex. Ovary and testis distinguishable. Kidney characteristic. External ear indicated. Third Month. — Weight, four ounces. Length, two and three-quarter inches. Chorion frondosum. Placental vessels. Tonsil. Stomach rotates. Vermiform appendix. Liver large. Epiglottis. Ovaries descend. Testes in false pelvis. Hair and nails. Development of different parts of brain. Limbs have definite shape. Fourth Month. — Weight, seven and three-quarter ounces. Length, five inches. Head one-fourth of entire body. Germs of permanent teeth. Distinction of external genitals well marked. Spinal cord ends at end of coccyx. Eye-lids and nostrils closed. Sixth Month. — Weight, two pounds. Length, twelve inches. Amnion at maximum size. Trypsin in pancreatic secretion. Air vesicles. Eye-lashes. Lobule of ear characteristic. Seventh Month. — Weight, three pounds. Length, fourteen inches. Meconium. Ascending colon. Testes at internal rings. Cerebral convolutions evident. Differentiation of mus- cular tissue. Eighth Month. — Weight, four to five pounds. Length, sixteen inches. Body more plump. Ascending colon larger. Testes in inguinal canal. Skin brighter color. Nails project beyond finger tips. Ninth Month. — Weight, six to seven pounds. Length, twenty inches. Meconium dark green. Testes in scrotum. Labia majora'in contact. Spinal cord ends at laSjtl lumbar vertebra. Ossification centers completed. r ' INDEX. Abducens nerve, 285 Absorption, from stomach, 81 from intestines, 96 Accommodation, ocular, 314 Adipose tissue, 14 formation of, 178 value of, 179 Adrenal glands, 33 Afferent nerves, 233 Air, amount necessary, 161 alterations of, in lungs, 150 cells, 137 composition of, 148 diffusion of, in lungs, 148 vesicles, 137 Albuminoids, 175 Allantois, 347 Alveoli, capacity of, 147 Ammonia compounds antecedents of urea, 204 Amnion, the, 346 Amniotic cavity, 347 Amylopsin, 102 Anabolism, 172 Animal heat, 184 loss of, by evaporation, 189 radiation of, 189 relation of, to force, 185 source of, 185 specific, 187 total, 187 Antehelix, 318 Anterior chamber of eye, 312 elastic lamella, 310 fundamental fasciculus, 241 radicular zone, 241 Aphasia, 269 Aqueous humor, 313 Arachnoid, 236 Archenteron, 339 Arteria centralis retime, 312 Arterial circulation, 47 effect of respiration on, 164 Arteries, 47 elasticity of, 48 Arytenoid cartilages, 133, 324 Asphyxia, 162 Auditory canal, external, 317 center, 323 Auditory nerves, 288 terminations of, 321 Auerbach, plexus of, 97 Auricle, left, 44 right, 44 Axis cylinders, 222 Bacteria in digestion, 120 Bartholin's duct, 72 Bellini, straight tubes of, 197 Bertin, columns of, 195 Bile ducts, 108 Bile, in digestion, 105 functions of, 115 properties and composition of, no Bilirubin, in Binocular vision, 316 Bladder, 207 absorption of, 207 structure of, 207 Blastoderm, 339 Blood, the, 24, 35 alterations of, in lungs, 157 amount in body, 35 arterialization of, 150 coagulation of, 40 color of, 35 composition of, 36 functions of, 35 plasma, 36 platelets, 39 serum, 36 365 366 INDEX Bone, 1 6 Bone marrow, 18 Bowman's capsule, 196 Brain, the, 251 membranes of, 236 Breathing (see Respiration) Broca's convolution, 270 Bronchi, 136 capacity of, 148 Bronchioles, 137 Burdach, columns of, 247 Capillaries, the, 48 Capillary, importance, 50 Carbohydrates, 6, 176 final, products of, 176 value of, in nutrition, 176 Carbon, amount in excreta, 182 dioxide, amount exhaled, 154 amount in blood, 157 condition of, in blood, 155 discharge, 151 gain of, in lungs, 150 inhalation of, 161 interchange of, in lungs, 156 source of, exhaled, 154 monoxide, inhalation of, 161 Cardiac cycle, 43 length of, 43 Cartilage, 15 hyaline, 15 white fibrous, 15 yellow elastic, 15 Cauda equina, 238 Cecum, 117 Celenteron, 339 Celom, 343 Cell, i properties of, 3 structure of, 2 Centrifugal nerves, 230 Centripetal nerves, 230 Cerebellum, the, 274 anatomy of, 274 fibers of, 274 function of, 274 peduncles of, 274 Cerebral localization, 268 Cerebro-spinal axis, 235 system, 216 Cerebrum, the, 260 Cerebrum, cells of, 263 convolutions of, 263 fibers of, 263 fissures of, 260 functions of, 270 lobes of, 260 motor centers in, 268 paths from, 268 sensory centers in, 262 paths to, 270 special centers in, 269 Cerumen, 32 Cervical ganglia, 298 Cholesterin, in Chorda dorsalis, 343 tympani, 288 Chordal folds, 343 plates, 343 Chorion, 350 frondosum, 350 Choroid coat of eye, 310 Chyme, 100 Ciliary muscle, 311 processes, 311 Circulation, the, 41 pulmonic, 41 systemic, 41 Circumvallate papillae, 316 Claustrum, 258 Cochlea, 322 Colloids, 123 Colon, 117 Colostrum, 31 Common bile duct, 109 Complemental air, 147 Conjunctiva, 310 Connective tissue, 1 1 Convoluted tubules, 197 Cornea, 310 Corona radiata, 258 Corpora, quadrigemina, 2 59 striata, 256 Corpus luteum, 331 Corti, rods of, 322 Coughing, 144 Cranial nerves, 275 Creatin, 205 Cricoid cartilage, 133 Crossed pyramidal tracts, 245 Crura cerebri, 256 Crystalloids, 123 INDEX 36? Cutaneous respiration, 160 sensations, center for, 269 Cutis vera, 210 Cystic duct, 109 Death, 172 Decidua menstrualis, 351 of pregnancy, 351 Defecation, 121 Deglutition, 78 mechanism of, 79 nervous control of, 80 Dendrites, 216, 222 Descemet, membrane of, 310 Descendens hypoglossi, 295 Diet, amount of, 182 determination of, 181 necessary constituents of, 182 Dietetics, 181 Diffusion in lungs, 148 Digestion, 68 gastric, 81 intestinal, 96 object of, 68 processes in, 70 Direct cerebellar tract, 241 Discus proligerus, 330 Dreams, 301 Ductus arteriosus, 358 communis choledochus, 109 venosus, 359 Dura mater, 236 Dyspnea, 161 Ear, the, 317 drum, 318 external, 317 internal, 319 middle, 318 Ectoderm, 338 Efferent nerves, 280 Eighth nerve (see Auditory) Elasticity of arteries, 48 Electrical stimulation of nerves, 234 Elementary tissues, 7 derivation of, 7 varieties of, 8 Eleventh nerve (see Spinal accessory) Embryonal area, 340 Encephalon (see Brain) Endocardium, 42 Entoderm, 339 Enzymes, 68 characteristics of, 69 classification of, 69 mode of action of, 70 Epiblast, 338 Epidermis, 209 Epiglottis, 135 Epinephrine, 34 Epithelial tissue, 7 varieties of, 8 ciliated, 9 columnar, 9 glandular, n modified, 9 neuro, n squamous, 8 stratified, 8 Esophagus, 78 Eupnea, 161 Eustachian valve, 357 Excretion, 197 Expiration, 143 causes of, 143 forced, 144 effect of, in blood pressure, 165 Expired air, composition of, 151 External capsule, 258 Eye-ball, anatomy of, 310 movements of, 308 protection of, 307 Facial nerve, 286 Fats, as foods, 66 end products of, 177 Fauces, 78 Feces, composition of, 120 Fecundation, 336 Ferrein, pyramids of, 195 Fertilization, 336 Fetal membranes, 346 Fibrin, 40 Fibrous tissue 13 Fifth nerve (see Trifacial) Filum terminale, 238 First nerve (see Olfactory) Foods, 63 classification of, 64 fate of in body, 173 how absorbed, 128 potential energy of, 185 368 INDEX Foods, where absorbed, 128 Fourth nerve (see Patheticus) ventricle, 252 Fovea cen trails, 312 Fungiform papillae, 316 Funiculi graciles, 252 of Rolando, 252 Gall bladder, 109 Gases in intestine, 127 Gastric glands, cells of, 85 nerve supply of, 89 structure of, 85 varieties of, 85 juices, action of on foods, 92 properties and composition of, 90 secretion of, 86 Gastrula, 339 Glands, 27 adrenal, 33 agminate, 100 intestinal, 96 gastric, 86 mammary, 31 of Brunner, 99 of Lieberkuhn, 99 parotid, 71 salivary, 71 sebaceous, 30 secretion in, 2 9 solitary, 100 sublingual, 71 submaxillary, 71 sweat, 210 thyroid, 32 Glisson's capsule, 105 Glomeruli, renal, 195 Glosso-pharyngeal nerve, 289 Glycocholic acid, in Glycogen, formation of in liver, 112 Golgi, corpuscles of, 230 Goll, column of, 247 Gustatory center, 269 Graanan follicles, 329 Hairs, 210 Haversian canals, 17 systems, 18 Hawking, 145 Hearing, sense of, 317 Heart, anatomy of, 42 beats of, 43 contractions of, 43 development of, 355 diastole of, 43 innervation of, 46 sounds of, 46 systole of, 43 valves and openings of, 44 work of, 45 Heat of body (see Animal heat) Hemoglobin, 38 Henle, loops of, 197 sheath of, 220 Hepatic artery, 105 Hiccough, 145 Hippuric acid, 205 Hunger, seat of, 64 Hydrochloric acid, 90 Hydrogen, inhalation of, 161 Hyperopia, 315 Hyperpnea, 161 Hypoblast, 338 Hypoglossal nerve, 294 Hypoxanthin, 205 Ileo-cecal valve, 116 Impregnation, 336 Incus, 319' Infundibula, 137 Innervation of vessels, 51 Inspiration, 141 causes of, 142 effects of on blood pressure, 164 muscles of, 143 Inspired air, composition of, 151 Interlobular veins, 106 Internal capsule, 259 respiration, 158 Intestinal glands, 99 Intestine, digestion in, 96 divisions of, 97 movements of, 121 nerve supply of, 117 structure of, 119 Intralobular veins, 106 Intrapulmonary pressure, 140 Intrathoracic pressure, 146 Iris, the, 310 Katabolism, 172 INDEX 369 Kidney, blood supply of, 199 structure of, 192 Kinetic energy, 186 Krause, end bulbs of, 228 Labyrinth, bony, 319 membranous, 321 Lacrymal apparatus, 308 duct, 308 glands, 308 sac, 308 Lactates, discharge of, 205 Lacteals, 98 Large intestine, 117 digestive changes in, 119 divisions of, 117 movements of, 121 structure of, 119 Larynx, 133, 324 nerve supply of, 326 Laughing, 145 Lenticular ganglion, 284 Leucocytes (see White corpuscles} Lieberkuhn, crypts of, 99 Liquor amnii, 347 sanguinis (see Blood plasma) Liver, anatomy of, 104 histology of, 108 lymphatics of, no nerve supply of, no vessels of, 105 Lumbar ganglia, 298, Lungs, 138 capacity of, 147 Lymphatic glands, 59 Lymph, 57 course of, 57 flow of, 6 1 properties and composition of, 59 Lymph vessels, origin of, 57 Macula lutea, 312 Malleus, 319 Malpighian bodies, 195 pyramids, 195 Mammary glands, 31 Mastication, 71 Maturation, 32 Meckel's ganglion, 284 Medulla oblongata, 251 Medulla oblongata, centers in, 254 functions of, 253 gray matter of, 253 pyramids of, 251 relation of cord tracts to, 253 white fibers of, 253 Meissner, corpuscles of, 229 plexus of, 97 Membrana tympani, 319 Menstruation, 334 Mesoblast, 340 Mesoderm, 340 Metabolism, 171 conditions influencing, 179 Micturition, 207 center for, 208 Milk, human, 32 Mitral valve, 45 Mixed lateral column, 241 Motor oculi communis, 278 paths from cerebrum, 266 Muscular contractions, 23 physiological characteristics, 23 tissues, 19 Myopia, 315 Nails, the, 210 Nasal duct, 308 Nerve cells, 221 centers, 221 fibers, 217 action of electricity upon, 234 afferent, 232 classification of, 230 degeneration of, 239 directions of currents in, 233 efferent, 231 individuality of, 221 medullated, 217 non-medullated, 219 properties of, 230 rates of conduction in, 234 terminals, 225 between epithelial cells, 227 in bulbs of Krause, 228 in Golgi's corpuscles, 230 in glands, 226 in hair-follicles, 226 in Meissner's corpuscles, 229 in Pacinian corpuscles, 227 in plain muscle, 226 370 INDEX Nerve terminals, in striped muscle, 225 in tactile memsques, 229 trunks, 219 Nervous system, the, 214 development of, 360 divisions of, 216 general functions of, 214 Neural canal, 342 Neuroglia, 216 Neurons, 216 communication between, 223 Ninth nerve (see Glosso-pharyngeal) Nitrogen, amount necessary, 182 Nitrogenous equilibrium, 174 Nitrous oxide, inhalation of, 161 Nutrition, 171 Olfactory bulb, 276 cells, 276 center, 269 nerve, 276 Olivary bodies, 251 Omphalo-mesenteric vessels, 355 Ophthalmic ganglion, 284 Optic center, 269 commissure, 277 nerve, 277 thalami, 258 functions of, 259 tracts, 277 Osmosis, 123 Otic ganglion, 284 Ova, 328 Ovary, secretion of, 34 Ovulation, 333 Oxidation in the body, 171 Oxygen, amount consumed, 153 amount in blood, 157 condition of in blood, 157 entrance of into tissues, 159 loss of in lungs, 148 Pacini, corpuscles of, 227 Pancreas, anatomy of, 100 histology of, 101 internal secretion of, 103 nerve supply of, 103 secretion in, 103 Pancreatic juice, 101 Partial pressure of gases, 156 Patheticus nerve, 280 Pepsin, 91 Peptones, 92 Pericardium, 42 Periosteum, 18 Perspiration, 212 Pharynx, 132 Pia mater, 236 Pinna, 317 Pituitary body, 34 Placenta, 352 Placenta! circulation, 356 Pneumogastric nerve, 290 influence of on respiration, 168 stomach and intestines, 168 Pons Varolii, 255 functions of, 255 Posterior chamber of eye, 312 Prehension, 70 Presbyopia, 315 Pronucleus, female, 233 male, 336 Proteids as foods, 66 circulating, 174 final products of, 174 tissue, 174 Proteoses, 92 Pro to vertebrae, 344 Ptyalin, 74 Pulse, the, 53 Pupil, the, 311 Ranvier, nodes of, 218 Reaction of pupil, 315 Receptaculum chyli, 57 Rectum, 118 Red corpuscles, 37 Reflex action, 248 Refraction, ocular, 313 Reil, island of, 262 Renal tubules, 197 Rennin, 91 Reproduction, 327 Reserve air, 147 Residual air, 147 Respiration, 131 abnormal, 161 afferent nerves of, 167 center for, 167 costal, 146 cutaneous, 160 INDEX 371 Respiration, diaphragmatic, 146 effect of, on blood pressure, 163 efferent nerves of, 169 external, 132 influence of vagus on, 167 internal, 131, 158 mechanism of, 138 modified, 144 nervous control of, 166 object of, 131 organs of, 132 rate of, 145 rhythm of, 144 sounds of, 145 types of, 146 Restiform bodies, 251 Retina, 312 Rolando, fissures of, 260 Saliva, functions of, 71 properties and composition of, 73 Salivary glands, 71 histology of, 72 nerve supply of, 74 secretion in, 76 Salts, 65 Schwann, sheath of, 217 white substance of, 218 Sclerotic coat of eyes, 310 Sebaceous" glands, 30 Second nerve (see Optic nerve) Secretion, 27 external, 29 internal, 29 paralytic, 76 Segmentation, 337 cavity, 337 nucleus, 337 Semicircular canals, 322 Semilunar valves, 44 Sensations, common, 304 special, 305 Senses, the, 304 Serum-albumin, 36 -globulin, 36 Seventh nerve (see Facial) Sighing, 145 Sight, sense of, 307 Sigmoid flexure, 117 Sixth nerve (see Abdttcens) Skin, excretion by, 208 functions of, 208 structure of, 209 Sleep, 300 vascular phenomena of, 301 Smegma, 30 Sneezing, 144 Snoring, 145 Sobbing, 145 Sodium salts, 65 functions of, 65 Solar plexus, 298 Somatopleure, 340 Somites, 344 Speech, 325 center, 369 Spermatozoa, 327 Spheno-palatine ganglion, 284 Spinal accessory nerve, 293 cord, 237 columns of, 240 commissures of, 238 cross section of, 238 degeneration in, 240 functions of, 247 gray matter in, 238 motor paths in, 243 sensory paths in, 245 special centers in, 250 nerves, 295 Splanchnic nerves, 298 Splanchnopleure, 340 Starvation, effects of, 180 Steapsin, 102 Stenson's duct, 72 Stercorin, in Stomach, the, 81 histology of, 83 movements of, 94 nervous supply of, 96 Straight tubules (renal), 197 Striated muscle, 19 characteristics of, 19 Stapes, the, 319 Sublobular veins, 107 Submaxillary ganglion, 284 Succus entericus, 116 Supplemental air, 147 Suspensory ligament, 313 Sweat glands, 216 Sweat, properties, composition of, 2 12 372 INDEX Sweat, secretion of, 212 Sylvius, aqueduct of, 253 fissures of, 267 Sympathetic system, 216 Syntonin, 92 Tactile sensibility, 305 acuteness of, 306 Taste beakers, 317 sense of, 316 Taurocholic acid, in Temperature impressions, 306 of body, 184 Tenon, capsule of, 308 Tenth nerve (see Pneumo gastric) Testes, secretion of, 34 Thermogenesis, 188 Thermotaxis, 190 Third nerve (see Motor oculi com- munis} Thirst, seat of, 64 Thoracic duct, 57 ganglia, 298 Thorax, 138 Thyroid cartilage, 133 gland, 32 Tidal air, 147 Touch, sense of, 305 Trachea, 135 Tragus, 318 Trie us pi d valve, 44 Trifacial nerve, 280 Trigeminal nerve, 280 Trypsin, 102 Turck, column of, 245 Twelfth nerve (see Hypoglossal) Tympanum, 318 Umbilical cord, 354 vesicle, 346 Urea, 202 daily discharge of, 203 formation of, 203 Ureters, 206 Uric acid, 204 daily discharge of, 204 Urine, constituents of, 202 discharge of, 206 salts of, 206 properties of, 202 secretion of, 199 variations in amount of, 206 in composition of, 206 Uriniferous tubules, 197 secretory* changes in, 202 Vaginal plexus, 105 Vagus nerve (see Pneumo gastric) ValvulEe conniventes, 97 Vaso-motor nerves, 51 centers for, 51 Vater, corpuscles of, 227 Veins, 48 valves of, 50 Ventilation, 160 Ventricle, left, 44 right, 44 Vermiform appendix, 118 Vestibule of ear, 319 Villi, 98 Vitelline, circulation, 355 duct, 344 Vitreous humor, 3-13 Vocal cords, 133, 324 sounds, varieties of, 325 Voice, production of, 324 Water, 65 elimination of by kidney, 201 Wharton's duct, 72 White corpuscles, 39 Wirsung, duct of, 100 Wolffian bodies, 361 Wrisberg, nerve of, 280 Xanthin, discharge of, 205 Yawning, 145 Zymogen, 101