UNIVERSITY FARM A MANUAL OF NORMAL HISTOLOGY AND ORGANOGRAPHY BY CHARLES HILL, B. S., M. S., PH.D., M. D. PRESIDENT AND PROFESSOR OF ANATOMY, CHICAGO HOSPITAL COLLEGE OF MEDI- CINE; PROFESSOR OF HISTOLOGY AND EMBRYOLOGY, CHICAGO VETERI- NARY COLLEGE; FORMERLY ASSISTANT PROFESSOR OF HISTOLOGY AND EMBRYOLOGY AT THE NORTHWESTERN UNIVERSITY MEDICAL SCHOOL, CHICAGO Fourth Edition, Thoroughly Revised PHILADELPHIA AND LONDON W. B. SAUNDERS COMPANY mo UNIVERSITY OF CALIFORNIA LIBRARY BRANCH OF THE COLLEGE OF AGRICULTURE Copyright, 1906, by W. B. Saunders Company. Revised, reprinted, and recopyrighted January, 1909. Reprinted September, 1910. Revised, reprinted, and recopyrighted July, 1914. Reprinted September, 1915. Revised, reprinted, and recopyrighted September, 1917 Copyright, 1917, by W. B. Saunders Company Reprinted May, 1920 PRINTED IN AMERICA PRESS OF W. B. SAUNDERS COMPANY PHILADELPHIA PREFACE TO FOURTH EDITION. THE; cordial reception given this text is much appreciated by both the author and the publishers. In issuing a fourth edition the chapters have again been carefully reviewed and the known facts to date in elementary histology have been properly recorded. The new introduction is written wholly for the elementary student, to arouse in him an interest in the subject matter of the text, and to show the close relation of normal histology to kindred important sciences. The text on spermatogenesis has been enlarged and the known facts stated in as clear and brief a manner as possible. Some of the figures have been replaced with new ones, and minor changes have been introduced throughout the text wherever the subject matter could be improved. In this, as well as in former editions, the author has kept in mind his original fundamental purpose, to issue a clear and concise text that could be used as a basis on which the instructor might " build and complete his ideal elementary course in his- tology." _ CHARLES Hiu<. CHICAGO, lu,.. PREFACE THIS manual is written in the interest of element- ary students. The fundamental facts in histology have therefore been presented in as clear and concise a manner as possible, and theories advanced only to simplify the facts and aid the memory in their retention. The figures have been selected with considerable care and are intended to illus- trate the salient points of the text. They are to be studied as critically as the text, and to further facilitate such a study the descriptive terms are placed on the figures rather than in foot-notes. The oral cavity deserves more attention than is usually given this subject. Neglect of proper care of teeth is a common failing, and the cause may be traced directly to a lack of knowledge of their structure and function. This chapter has therefore been enlarged. The author is greatly indebted to Professor Frederick B. Noyes, of the Northwestern University Dental School, for con- tributing most excellent figures on this subject. His critical essays form the basis for the descriptive part of this chapter. The author believes most thoroughly in the laboratory method of study. He believes, too, that the laboratory work should precede the class- ii 12 PREFACE. room work, for which this manual is written. Laboratory technique, however, is so extensive a* subject that a laboratory text, or the teacher's personal outlines, should be used for this particular work. In conformity with this view, only the funda- mental principles of laboratory technique are out- lined in the text. Lastly, the subjects treated have been made funda- mental and brief, that the teacher in charge may supplement the chapters by collateral work as may fit the particular course offered. It is therefore a basis on which the instructor may build and com- plete his ideal elementary course in histology. CHARLES HILU CONTENTS. PAGE. INTRODUCTION ,,..,., 17 Protoplasm 7.7 CHAPTER I. DEVELOPMENT 26 The Cell 34 CHAPTER II. TISSUES , 49 Epithelial Tissue 49 Supporting Tissue 64 Connective Tissue 66 Cartilage 74 Bone 77 Muscular Tissue 85 Nervous Tissue 94 CHAPTER III. CIRCULATORY SYSTEM, BLOOD, MARROW, AND LYMPHATIC ORGANS 108 Heart 108 Arteries and Veins 109 Blood 1 16 Marrow 121 Lymphatic System 125 Thymus Gland 129 Spleen 132 CHAPTER IV DIGESTIVE SYSTEM 136 Mouth . . . .' 136 Teeth 143 Tongue. . . 172 Pharynx 180 Esophagus : 182 Stomach 184 Small Intestine 194 Large Intestine 200 14 CONTENTS. PAGE CHAPTER V. DIGESTIVE GLANDS 207 Salivary Glands 207 Pancreas 212 Liver 216 CHAPTER VI. ORGANS OF RESPIRATION ; ... 231 Larynx 231 Thyroid Gland , 235 Parathyroids 238 Trachea and Bronchi 239 Lung 242 CHAPTER VII. THE URINARY ORGANS 253 Suprarenal Glands 253 Kidneys 257 Ureters 269 Urinary Bladder 271 CHAPTER VIII. REPRODUCTIVE ORGANS IN MALE 275 Testicles 275 Penis 289 Prostate Gland 297 CHAPTER IX. REPRODUCTIVE ORGANS IN THE FEMALE 301 Ovaries 301 Fallopian Tubes 313 Uterus 317 Pregnancy 327 Mammary Gland 332 CHAPTER X. THE SKIN 337 Hairs 343 Nails 349 Glands of skin , 356 CONTENTS. 15 CHAPTER XI. PERIPHERAL NERVE TERMINATIONS 361 ^Motor Nerve Endings 361 Sensory Nerve Endings 363 CHAPTER XII. SPINAL CORD . 37I CHAPTER XIII. THE BRAIN 3 8 4 Medulla ^35 Summary of Tracts, their Origin and Terminations 394 Pons 394 Cerebellum 398 Cerebral Cortex 403 Neuroglia 406 Blood-vessels of Central Nervous System 409 CHAPTER XIV. THE EYE 41 1 Tunica Externa 414 Tunica Media 418 Tunica Interna 42 1 Refracting Media 428 Blood-vessels 430 CHAPTER XV. ORGAN OP HEARING 437 Development of Labyrinth 45 1 CHAPTER XVI. OLFACTORY ORGAN 453 CHAPTER XVII. LABORATORY DIRECTIONS 456 Preparation of Material 456 Preparation of Elastic Fibers 466 Standard Fixing Solutions , . 472 Standard Stains 475 INDEX .,.,., 479 A MANUAL OF NORMAL HISTOLOGY AND ORGANOGRAPHY. INTRODUCTION. PROTOPLASM. IN all this material world, with its complexity of products, there are but two forms of material things namely, living matter or protoplasm and lifeless or dead matter. Protoplasm is not life. We do not know what life is, but whatever it is, we know it is not a material substance. We cannot see, feel, taste, touch, or weigh life, but we can do all these things with protoplasm. Spencer defines life as the "continuous adjustment of internal rela- tions to external relations" an acceptable concept. Protoplasm always reflects life, and it is therefore regarded as the physical basis of life. Protoplasm, whether in plant or in animal, shows a uniformly related structure and has many identical character- istics. It is a colorless, transparent, jelly-like sub- stance, of an albuminoid nature, resembling the 1 8 NORMAL HISTOLOGY AND ORGANOGRAPHY. white of an egg. It differs from all lifeless matter in being able to reproduce itself, repair a wasted or depleted condition, develop, and grow. The sharpest kind of a line divides this living matter from dead matter, and yet we know that the closest relations and interrelations do exist. Even in our own phys- ical bodies these two forms prevail, for the outer layer of the skin, the hair and nails, the liquid part of blood and lymph, the fibers of ligaments and tendons, the lime of bone, all are lifeless or dead matter. While today we firmly believe that living matter develops only from pre-existing living mat- ter, we know that dead matter is constantly and unceasingly being incorporated into living matter, actually transformed into living matter through the mysterious elaboration of this selfsame living sub- stance. Thousands of tons of dead matter are thus daily converted into living matter, throughout the animal and vegetable kingdoms, on land and in the seas. Were this constructive force nature's only process, a most unfortunate condition would soon prevail, with a dearth of the one form and a great surplus of the other the living. But we recognize a reverse current equally strong whereby the "dust shall return to the earth as it was and the spirit to the God who gave it." That mysterious and in- sidious enemy Death is absolutely necessary that man, or any other living being, may live. It is the duty of the medical profession to divert this return current as far as possible that humanity, one and all, may enjoy threescore and ten happy years. Protoplasm, so intimately associated with life, has PROTOPLASM. 19 been subjected to all forms of analyses. The chemist tells us that protoplasm, whether animal or plant, yields the following elements: carbon, hydrogen, oxy- gen, nitrogen, and some sulphur. He is unable to tell us the combining relation of these elements in protoplasm, for the obvious reason that in his anal- yses protoplasm as living substance is destroyed and life has departed. But it is of interest to know that these elements, combined in some mysterious and unknown way, make this marvelous substance liv- ing matter. It is also of interest to reflect briefly upon the individual peculiarities of these elements. Carbon is a solid, exists free in nature, and remark- able for its allotropic forms, it being found as coal, or graphite, or diamond. Its combining power with other elements is extensive, and its durability is well known. Hydrogen is a gas. It is the lightest known substance. It is practically never found free in nature. Its combinations with other ele- ments are many, forming often very stable com- pounds, the most common being its union with oxy- gen to form water. Oxygen is a gas and found free in nature, forming nearly 20 per cent, of the atmos- phere. Its combining power is perhaps the most extensive of all the elements, forming many stable oxides. Nitrogen is also a gas and found free in nature, forming about 79 per cent, of the atmosphere. Unlike oxygen, its combining power with other ele- ments is very weak, and when it does so combine the substances formed are very unstable. Nitro- gen, therefore, is one of the chief elements in our explosives. Sulphur, like carbon, is remarkable for 20 NORMAL HISTOLOGY AND ORGANOGRAPHY. its allotropic forms. Very small amounts of sulphur are found in living matter. Protoplasm is a restless substance, its granules manifesting a slow ameboid movement, which be- comes accelerated with increased physiological ac- tivity. At the end of a day's labor and toil it is not only exhausted but actually depleted, requiring repair and the replacing of its lost particles, which is accomplished by some marvelous, subtle, intrinsic power so characteristic of living matter. Food is the raw material utilized for this purpose, and it naturally follows that the essential elements of food, whether for plants or animals, must be identical with those found in protoplasm namely, carbon, hydro- gen, oxygen, nitrogen, and some sulphur. This is universally the case, as we find these present in the necessary food products, such as carbohydrates, hy- drocarbons, albuminoids, and proteins. As a mat- ter of fact, the diet of man is largely the protoplasm of some other living organism, animal or vegetable, or both. Huxley, in his lecture on the "Physical Basis of Life," very fittingly outlines this transmu- tation of the elements of protoplasm when he says that in order to replace the ' ' number of grains of protoplasm and other bodily substances wasted in maintaining my vital processes ... I shall prob- ably have recourse to the substance commonly called mutton . . . and the subtle influence to which it will then be subjected will convert the dead proto- plasm into living protoplasm and transubstantiate sheep into man. Nor is this all. If digestion were a thing to be trifled with, I might sup upon lobster PROTOPLASM. 21 and the matter of life of the Crustacea would undergo the same wonderful metamorphosis into humanity. And were I to return to my own place by sea and undergo shipwreck the Crustacea might and prob- ably would return the compliment and demonstrate our common nature by turning my protoplasm into living lobster." The medical student is here reminded of the con- stant warfare being waged between many forms of living matter. We acknowledge today that most of the diseases to which mankind is an heir and a victim are nothing else than vicious attacks upon us by some form of living matter. Tuberculosis, pneumonia, diphtheria, typhoid, tetanus, cholera, bubonic plague, and many other serious and fatal diseases are now directly traced to invasions of mi- cro-organisms, truly living things. Self-preservation is nature's first law and it is God's law. Protective selfishness and the preservation of a race, whatever be the sacrifice, is a fundamental principle deeply implanted in every form of living organism. There is, therefore, no mercy shown in this warfare. There is no hope of peace in this conflict, nay, not even a respite, and sooner or later practically each one of us must fall a victim to the enemy's invasion. The discovery of the germ origin of disease has placed the study of medicine on a new and substantial scientific basis. Not satisfied with merely a defensive program, this warfare, by means of preventive med- icine, has assumed the offensive to such a degree that the disease scourges which in the past sometimes de- cimated a people are today an impossibility. 22 NORMAL HISTOLOGY AND ORGANOGRAPHY. Histology is the science that treats oi cells and their products, therefore it is largely a study of protoplasm. The fundamental principles of thera- peutics are based upon the action of drugs on proto- plasm, Pathology involves a recognition of micro- scopic changes, other than normal, in living matter. Physiology has much to say about the actions and products of this same substance. Embryology traces the developmental history of protoplasm to form tissues, organs, and a new living being. It is this great importance of protoplasm, and partic- ularly its relation to histology, that has prompted these introductory remarks. CHAPTER I. DEVELOPMENT. The human body is composed of related structural units that may be grouped in a series of gradually increasing complexity. The simplest structural unit is the cell, which is defined as a spacially limited mass of protoplasm capable under certain conditions of assimilation, growth, and reproduction. It is a microscopic unit and forms the physical basis of life. The next grade of units is a tissue, which consists of a complex of similarly differentiated cells and their derivatives. Embryonic tissues are mostly cellular, while in some tissues of the adult body, such as bone and cartilage, the cell products predominate. The next higher grade of units is an organ, which struc- turally consists of a complex of tissues forming a body with a definite internal structure and external form. Lastly, the highest structural unit is a system, such as the nervous system, digestive system, respi- ratory system, a series of which, collectively, make up the human body. Histology is the branch of science that treats of cells and their derivatives. These cells, in the adult, are modified according to their intrinsic qualities, en- vironment, function, and varied experiences, which enable us to classify them and ultimately place them in a few elementary groups. The ovum, or starting-point of every individual, is a cell. Human embryology comprises its intra- 23 24 NORMAL HISTOLOGY AND ORGANOGRAPHY. uterine development. Ontogeny is a broader term, which includes not only embryology, but the develop- mental history of an individual .up to old age or the senile condition. There is another form of develop- ment, much slower, but just as certain as ontogeny. This affects not only the individual, but, collectively, every member of the animal group. In the study of races there is ample evidence that structural changes Fig. i. Formation of the polar bodies in the ova of Asterias gla- cialis (Hertwig): ps, polar spindle; pb' t first polar body; pb", second polar body; n, nucleus returning to condition of rest. have slowly but gradually taken place. This broader developmental history, or history of a race, is known as phytogeny. It is closely interwoven with the ontogenetic development, so much so, that the latter in large part repeats the former, or one's phylogenetic history is repeated in the ontogeny. In development, therefore, the phylogenetic or intrinsic qualities of a cell are important factors. These factors constitute heredity. There is further DEVELOPMENT. 25 evidence that these factors are lodged in the chro matin of the cell nucleus. Environment is the other great factor that brings about a structural modification. It is between environment on the one hand, and heredity on the other, that a specialization and differentiation of cells, tissues, and organs is produced. A cell is a spacially limited mass of protoplasm which, under certain conditions, will assimilate, grow, and reproduce itself. A tissue is a complex of simi- larly differentiated cells and their derivatives. An organ is a complex of tissues, forming a body with a definite internal structure and external form. Fig. 2. Portions of the ova of Asterias glaciah's, showing the ap- proach and fusion of the spermatozoon with the ovum (Hertwig): a, fertilizing male element; 6, elevation of protoplasm of egg; &', &", stages of fusion of the head of the spermatozoon with the ovum. Ovulation and Maturation. The ova develop in the ovaries and are differentiated very early during embryonic life. The estimated number of ova in each ovary is 35,000. It is a remarkable phenomenon that for many years these units show no attempt at devel- opment or cell division. At the age of puberty one or more of these cells pass periodically from the human ovaries, approximately every twenty-eight days in the 26 NORMAL HISTOLOGY AND ORGANOGRAPHY. Malt protniilcus. Female Pronuclciis. non-pregnant woman. This process is known as ovulation, and continues up to the time of the meno- pause, which appears generally at the age of forty-five. As soon as the ovum is liberated from the ovary, a mitotic divis- ion of the nucleus takes place, and the ovum extrudes or pro- duces what is called the first polar body. This is a form of bud- ding, the polar body receiving one-half the original nucleus of the ovum. Without delay, a second division of the nucleus in the ovum takes place, and a second polar body is produced. This is also an equal division of the nucleus, but this time there is a Fig. 3 . A, fertilized ovum of reduction of One-half *S* (Hertwig): the male and the female pronucleus are approaching; in the number of chro- B they have almost fused; C, ovum of mosomes, and may be ^^^^^ion of fertilization Segmen- tation nucleus. 27 called a reduction mitosis, as distinguished from all preceding and all succeeding divisions, which are called somatic mitosis. The significance of this re- duction has led to many theories. During the process the nucleus loses its membrane, is much re- duced in size, and is now known as the female pro- nucleus. All these preliminary changes are known as maturation of the ovum. Without fertilization, further development of the ovum does not seem possi- ble in higher forms, and the cell is invariably lost. Fertilization. By this is meant the union of a spermatozoon with the ovum, or, more technically, the union of a male and a female pronucleus. This Fig. 4. Diagram of the division of the frog's egg (Hertwig): Aj stage of the first division. B, stage of the third division. The four segments of the second stage of division are beginning to be divi- ded by an equatorial furrow into eight segments; p, pigmented surface of the egg at the animal pole; pr, the part of the egg which is richer in protoplasm; d, the part which is richer in deutoplasm; sp, nuclear spindle. union takes place in the upper part of the oviduct. Maturation always precedes fertilization. But in lower forms experiments upon unfertilized eggs in the absence of spermatozoa have resulted in the development of embryos, or larvae, and in a few in- 28 NORMAL HISTOLOGY AND ORGANOGRAPHY. stances adult animals. This interesting result may be obtained by adding certain salts to the sea-water in which the eggs of marine animals normally de- velop, or in the case of the frog by puncturing with a needle the outer layer of the unfertilized egg. Professor Loeb, who inaugurated these experiments, is of the opinion that oxidation is thereby stimu- lated, which is followed by an accelerated protoplas- mic activity that initiates a normal development of these ova without the process of fertilization. If a spermatozoon enters the ovum before the polar bodies are extruded, the spermatozoon remains inert within the cell until maturation is completed. The ovum, thus reinforced, enters upon an aggressive growth, a phenomenon quite in contrast with its preceding history. Fig. 5. Cleavage in egg of frog, i to 16 cell stage. Segmentation or Cleavage. Following fertiliza- tion the ovum multiplies rapidly by mitosis. The DEVELOPMENT. 29 union of male and female nuclei restores the reduced number of chromosomes, which remain constant and usually even in number for every succeeding division. In certain insects an odd number of chromosomes appears, in which case the embryo develops into a male. By repeated divisions a spherical mass of cells is produced, known as the morula stage. Fig. 6 Blastula of triton taeniatus: //*, segmentation cavity; rz, mar- ginal zone; dz t cells with abundant yolk (Hertwig). Blastula. The spherical mass quickly develops into a hollow sphere, lined by a single layer of cells, and is then a blastula. The cavity of the blastula is the segmentation cavity. Gastrula. A more vigorous growth seems to take place at one point of the blastula, producing lateral pressure and an invagination at that place so as to form a two-layered cup-like structure in some eggs a blastoderm known as the gastrula . The gastrulae vary considerably according to the different forms of cleavage. It is an established fact that all metazoa pass through the morula, blastula, and 30 NORMAL HISTOLOGY AND ORGANOGRAPHY. gastrula stages, respectively, in the course of their development. The cup-like cavity of the gastrula is known as the archenteron or codenteron, and is des- tined to develop into the alimentary canal. The pore or external opening of the coelenteron is called the blastopore. The gastrula has two layers of cells : an outer, the ectoderm, and an inner, the entoderm. The cells of Segmentation cavity. Ectoderm. I Blaslula. Blastopore gaslrula. Fig. 7. Sections through a blastula and a gastrula of amphioxus. the entoderm are much larger than the cells of the ectoderm and there is thus a structural difference. In cleavage that results in a two-layered blastoderm the term hypoderm is used, which is thus morpho- logically equivalent to the entoderm. The two-layered gastrula is rapidly invaded by a third layer of cells, the mesoderm, which develops between the first two layers and ultimately fills that cavity. This cavity, which is the segmentation cavity of the blastula, permanently disappears. The origin of the mesoderm has long been a con- tested question. The favored theory seems to be that for higher forms, at least, it develops from the hypoderm. PLATE I. i, 2, 3, Diagrams illustrating the segmentation of the mammalian ovum (Allen Thomson, after von Beneden). 4, Diagram illustrating the relation of the primary layers of the blastoderm, the segmentation- cavity of this stage corresponding with the archenteron of amphioxus (Bonnet). Outer cell. PLATE I. Outer cells Inntrcells. Inner cells. Inner cells Outer cells. Out* cells. The mesoderm, although at first a solid mass of cells, is an invaginated fold in which a cavity soon appears, having an inner layer of cells that affiliates closely Fig. 8. Sagittal section through an egg of triton (after the end of gastrulation) : ak, outer germ-layer; ik, inner germ -layer; dz, yolk- cells; dl and vl, dorsal and ventral lips of the coelenteron; ud, ccelen- teron; d, vitelline plug; w, middle germ-layer (Hertwig). with the hypoderm cells, and an outer layer that ap- plies itself to the ectoderm. The hypoderm with its mesoderm layer is known as the splanchnopleure, while the ectoderm and its mesoderm is the so- Neural groove. Somite. Enloderm, Notocord. Fig. 9 . Transverse section of chick embryo 22 hours old. matopleure. The new cavity thus produced is the 32 NORMAL HISTOLOGY AND ORGANOGRAPHY. ccelom, or body cavity, and is destined to become the pleural and peritoneal cavities. The mesoderm on Axial zone. / Neural canal. Somite. Lateral zone. Cavity within somite. Lateral plates for gut-tract. Vitelline vein. Fig. 10. Transverse section of a sheep embryo, 17 J days (Bonnet). Fig. n. Rabbit embryo of the ninth day, seen from the dorsal side (after Kolliker). The stem-zone (stz) and the parietal zone (pz) are to be distinguished. In the former 8 pairs of primitive segments have been established at the side of the chorda and neural tube; ap, area pellucida; rf, medullary groove; vh, fore-brain; ab, eye-vesicle; mh, mid-brain; hh, hind-brain; uw, primitive segment; stz, stem-zone; pz, parietal zone; h, heart; ph, pericardial part of the body cavity; vd, margin of the entrance to the head-gut (vordere Darmp}orte\ seen through the overlying structures ; a/, amniotic fold ; vo, vena ompha- lomesenterica. each side of the neural canal becomes symmetrically DEVELOPMENT. 33 blocked by means of a longitudinal fold and many transverse folds; thus many segments or joints are produced, known as myotomes or mesoblastic somites. From these develop bone, voluntary muscle, and the dermis of the skin. While these progressive changes are going on in the mesoderm, a longitudinal dorsal groove develops in the ectoderm. By a median fusion of the margins of the groove, it is transformed into a longitudinal canal, the neural canal, which develops into the brain and spinal cord. This brief embryonic growth has been productive in specialization and differentiation of cells. In no case will the cells of one germ layer reproduce, re- place, or function for the cells of any of the other layers. As derivatives of these three germ layers we are able to give the following table (Minot, " Em- bryology, " 1903)- A . ECTODERM. B. MESODERM. C. ENTODERM. I. Epidermis. i. Mesothelium. i. Notochord. (a) epidermal appen- (a) epithelium of per- dages. itoneum, pericar- (b) lens of eye. dium, pleura, uro- genital organs. (b} striated muscles. 2. Epithelium of 2. Mesenchyma. 2. Epithelium of (a) cornea. (a) connective tissue, (a) digestive tract, (b) olfactory cham- smooth muscle, esophagus, stom ber. pseudo - endothe- ach, liver, pan (c) auditory organ. (d) mouth (oral glands), lium, fat-cells, pigment cells. (b) blood. creas, small intes tine, yolk-sack large intestine (enamel organ), (c) blood-vessels. cecum, vermix (hypophysis^. (j Malpighian 2Q r g f r - inal layer. away quite regularly in thin lamellae. The deeper layers are ar- ranged to form papil- lae that interlock with connective tissue pa- pillae and thus not only anchor the epithelium to the subjacent tissue, but increase the absorbing surface by approximating a larger number of epithelial cells to the underlying blood and lymph capillaries. Fig. 2 1 . Section of epidermis of skin from palm-surface of finger. 54 NORMAL HISTOLOGY AND ORGANOGRAPHY. (c) Stratified columnar, ciliated or non-ciliated. This is found in the olfactory mucous membrane, the first part of many gland ducts, palpebral conjunc- tiva, portions of the male urethra, vas deferens, and portions of the larynx. General Considerations. The epithelial cells are simpler and more embryonic than the cells of the other tissues. They are continually multiplying throughout life, to re- place the superficial layers that are con- stantly exfoliating from the surfaces. If any of these surfaces are injured, the cells marginal to the injury repair the loss by a gradual growth cover- ing the denuded sur- face. As a consequence of this mitotic activity, Fig. 22. Section of stratified epithe- lium from esophagus. we find these cells fre- quently in pathological growths as epithelial growths or epithelioma. If the tumor is malignant it is a carcinoma or cancer. It is a remarkable fact that, in the adult, epithelium is able to produce cells only of its own kind, i. e., squamous cells produce squamous epithelioma, and columnar cells columnar epithelioma. Epithelial tissue, however, is easily modified, as is evidenced by calloused hands, pro- duced by heavy labor, and the cornification of nails, hair, horns, and teeth. TISSUES. 55 Since the blood supply never penetrates epithelial layers, it is evident that the superficial layers of cells receive less nourishment and ultimately die, which, perhaps, accounts for the constant exfoliation. It is also evident that anything that will increase the blood supply will increase the nourishment, as fric- tion, massage, and hot applications. The nour- ishment, at best, is not very good, which explains the Epithelium of cornea. Substantia propria of cornea. Fig. 23. Section showing corneal epithelium of the eye of pig. ease with which skin grafts are made. Epithelial cells will live for twenty-four hours or more in normal salt solutions, and will even multiply in favorable culture media. The nerve termination among the epithelial cells is an important relation which, in large part, controls their metabolism. A disturbed nervous system may impair or even cause a destruction of epithelial cells. 56 NORMAL HISTOLOGY AND ORGANOGRAPHY. Cilia are exclusively confined to epithelial cells. There are three theories to account for the motion of cilia: 1 . The contraction may be intrinsic in the wall of the cilium. This theory is supported by the fact that the cilium or flagellum of a spermatozoon will show motility when severed from the rest of the cell. 2. Contraction of the base where the cilia are attached. The cilia will continue to vibrate if a fragment of the cell protoplasm remains attached to them. 3. The cilia are supposed to be hollow tubes with walls of unequal elasticity. By forcing the proto- plasm rapidly into these tubes ciliary motion is produced. Pseudopodia are produced in this man- ner, and the morphological relation of pseudopodia and cilia is a close one. The one great physiological action of epithelium seems to be to secrete fluids. Consequently epithe- lium is found lining all cysts wherever the cyst is located, in the ovary, the skin, or in connection with the alimentary tract. Conversely, a cyst may be formed wherever epithelium is found. Lastly, it is of the greatest importance that stu- dents should be able to recognize epithelial cells. The facts to be remembered are : 1 . That they line surfaces. 2. That they appear in compact layers. 3. The oval or round distinct nucleus, usually rich in chromatin. 4. The regularity of the cells, i. e., they are of one pattern, either squamous, columnar, ciliated, or cubical. TISSUES. 57 5. They stain deeply with nuclear stains. 6. Their chief function is to secrete. 7. The absence of blood- and lymph- vessels. 8. The presence of free nerve endings. While this is of no diagnostic microscopic value, it is physiolog- ically an important relation to bear in mind. In wounds and old sores the epithelial border is the most sensitive part and should be carefully manipu- lated to avoid inducing pain. Glands. Much literature has been contributed the last years relative to the proper conception as to what constitutes a gland. The prevailing opin- ion seems to be that any structure which secretes or puts out a product that is not used directly in the metabolism of the body should be called a gland. If the fluid is a waste, the product is an excretion; if it has a utility, it is a secretion. Accordingly, mucous and synovial and serous membranes are glandular structures as well as the liver, the pan- creas, or the kidney. Furthermore, the simplest form of a gland is a single secreting cell situated apart by itself, and such unicellular glands are quite common in invertebrates and are represented in man by the goblet cells found in mucous membranes. Epithelial cells are the chief secreting cells of the body, and these cells, therefore, form the glandular tissue of all glands except the lympho-glandulcz, which is a connective- tissue production. The lymph glands thus constitute a class entirely by themselves as distinguished from all other forms, which may be called epithelial glands. As one of the important functions of lymph glands is to con- tribute white blood-corpuscles and thus scatter its 58 NORMAL HISTOLOGY AND ORGANOGRAPHY. own cells, they may also be called dehiscent or cyto- genic glands, a term applicable to the testes and ovaries, which are epithelial glands that perform a similar function by putting out their own cells in the form of spermatozoa and ovules. Numerous goblet cells are found in the simple epithelium lining the stomach and intestines, and are particularly abundant in the lower part of the bowel. These cells func- tion as glands and secrete mucus for the protection of the surface. In case of irritating media, such as undigested food or poisons, extensive mucus is poured out over the surface, thus protecting the delicate inner lining. In some cases of consti- pation this mucous secretion is impaired. Salts or drugs that increase the functional activity of these cells correct such complication. On the other hand, too extensive a secretion may be corrected by drugs, as opiates, that inhibit the physiological action of these cells. Constipation may be due to inertness of the musculature of the intestinal wall, in which case other remedies correcting this disturbance are indicated massage, hydrotherapy, and drugs that act on the musculature. Physiologically, many gland cells are either mu- cous or serous. In mucous cells the mucus secretion collects at one extremity of the cell as a clear, glisten- Serous gland. Mucous gland. Fig. 24. Skin and simple alveolar glands from the salamander. Tissues. 59 ing drop. The cytoplasm and nucleus are crowded to the opposite end. In serous cells the nucleus is more centrally placed, and the serous secretion is stored up as minute granules distributed through- out the cytoplasm, more especially in that por- tion of the cell lining the free surface. Some Crypt. Parietal cell. Chief cell. Fig. 25. Simple tubular gland from stomach. glands are mucous, some are serous, and some are mixed. Mucous Membrane. A mucous membrane con- sists of a lining of epithelial cells, basement mem- brane, and membrane propria. The basement mem- brane is largely an elastic cellular secretion on which 60 NORMAL HISTOLOGY AND ORGANOGRAPHY. the epithelial cells rest, although at times flattened connective-tissue cells seem to enter into its forma- tion. The membrana propria is a connective-tissue production consisting of connective-tissue cells, fibers, and blood- and lymph- vessels. Mucous mem- Demilune of Heidenhain. Fig. 26. Cells from different glands : a, Pancreas; b, submaxillary gland; c, liver. branes line cavities or tubes that communicate with the surface of the body, such as the alimentary canal, respiratory tract, and urogenital system. Serous Membrane. A serous membrane has the same histo- logical elements as the mucous membrane. The epithelial lining is simple squamous, and these cells secrete a serous fluid, more viscid and more of a lubricant than mucus. Serous membranes enclose cavities that do not com- municate with the surface of the body, as the pleural, pericardial, and peritoneal cavities and cavities of joints, forming in the latter case synovial membranes. Sheaths or bursae of tendons have serous membranes. Cell empty of secretion. Fig. 27. Goblet or mucous cells from intes- tine. TISSUES. 61 As to form, epithelial glands are classified as i. Simple. (a) Simple tubular gastric glands, sweat glands, and uterine glands. alveolar Stmfile cdoeolarr Fig. 28. Diagram of different forms of glands. (b) Simple alveolar smallest sebaceous glands, and skin glands in amphib- ians. 2. Compound. (a) Compound tubular kidney, liver, testis. 62 NORMAL HISTOLOGY AND ORGANOGRAPHY. (6) Compound alveolar or racemose sali- vary glands, mammary gland, lung, pancreas, sebaceous glands. Glands not included in this classification are uni- cellular glands and secreting membranes, which can- not be classified as to form. Lymph glands, which are of connective-tissue origin, and like the testis or ovary, may be called dehiscent or cytogenic glands. Follicular glands, such as the thyroid gland, and the ductless glands, producing internal secretions, such as the hypophysis cerebri, thyroid gland, suprarenal gland, areas of Langerhans of the pancreas, inter- stitial cells of the testis, and corpora lutea of the ovary. The thymus gland and spleen are lymphoid organs, and therefore to be classified among the lympho glandules . The object of any anatomical classification is to simplify and correlate structural facts. From the foregoing outline it is clear that glands, according to modern views, embrace such a complex of structures that any classification, either according to origin, or form, or tissues, or even function, does not accom- plish the end in view, namely, simplicity. The dif- ficulty met with is due to the fact that our con- ception of a gland rests largely with the physiolog- ical action of gland cells rather than with any com- mon intrinsic anatomical quality. Endothelium. This term, introduced by His in 1865, is generally applied to the layer of cells that line closed cavities, such as peritoneal and pleural cavities, circulatory system, and cavities of joints. TISSUES. These cells thus form the inner layer of serous mem- branes, and while structurally they bear a close re- semblance to epithelial cells, there is nevertheless an intrinsic difference made apparent by a comparison of pathological growths from these cells called endo- thelioma, and like growths from epithelial cells called epithelioma. Endothelioma are usually slower of growth, less malignant when malignancy exists, and have a tendency to form mucoid deposits. Endothe- lial cells are mononucleated, scaly, or of the pavement variety with wavy borders, and are held together with a cement substance that re- quires special staining tech- nique to demonstrate. They impart a transparent, smooth, glistening surface to the mem- brane which they clothe. Peritoneum and Pleura. These are true serous mem- branes, the structure of which is described on page 60. Elas- tic fibers are particularly abundant in the membrana propria, giving strength to both peritoneum and pleura, so that these membranes are readily sewed in surgical cases. Physiologically these membranes are of the greatest importance : i. It is claimed that the simple pavement epithe- lial cells of the peritoneum can produce a secre- tion that clots, and in this manner adhesions are Fig. 29. Epithelium or endothelium from mesentery. Silver nitrate stain. 64 NORMAL HISTOLOGY AND ORGANOGRAPHY. quickly formed. This is of the greatest importance in preventing the spreading of an infection, as in peritonitis. 2. These cells act as phagocytes and feed upon bacteria in case of an infection. According to one view they can destroy living bacteria. A second theory is that they act as scavengers and remove only dead bacteria. Stomata and stigmata have been frequently de- scribed as minute openings in these membranes to facilitate the absorption of fluids. Stomata are said to have guard cells to regulate the size of the opening, and were supposed to be specially abundant in the peritoneal lining of the diaphragm. The exist- ence of stomata has lately been strenuously denied. The rapid absorption of peritoneal fluid is a well- established fact. If stomata are absent the ab- sorption is purely one of osmosis or dialysis. It should be remembered that drainage is along lymphatic channels and therefore from the pelvis toward the thorax. II. SUPPORTING TISSUE. The embryonic connective tissue is largely cellular, but in the adult body the intercellular substance greatly predominates and gives the characteristics on which a classification is based. The cell elements are but slightly modified from the embryonic type, but the cell products or intercellular substance be- come modified to form bone, cartilage, or connective- tissue fibers. The difference here is relatively a dif- TISSUES. 65 ference in the degree of condensation of the inter- cellular substance, being either loosely arranged as in reticular connective tissue, or more compact as in tendons, or a greater degree of condensation as in cartilage, bone, and dentine. In all these types the cellular elements are morphologically very similar, which makes it possible for one form to develop into that of another; for instance, bone is produced from cartilage, or from fibrous connective tissues. The function of supporting tissue is largely a pas- sive one depending on its physical properties, and the amount of nourishment the cellular elements re- ceive is therefore a very variable quantity. Ten- dons, particularly, have a limited supply, perhaps because the cells form so small a part of these struc- tures. Nutrition is supplied from the lymph which penetrates the ground substance through clefts or minute channels placed in the intercellular material of the more condensed forms. In bone fine canals develop and anastomose to form a canalicular sys- tem, while in other forms, as mucous connective tis- sue and hyaline cartilage, the nourishing lymph seems to pass through the ground substance regard- less of lymph channels, as in these cases the latter have not been found. Blood vessels and capillaries ramify more or less freely through the matrix of supporting tissue, ex- cept in case of cartilage, where they are practically absent. Unlike epithelia, nerves may be abundant, but in no case do nerve fibers unite with the cellu- lar elements ; however, special sensory nerve endings 5 66 NORMAL HISTOLOGY AND ORGANOGRAPHV. are frequently found, particularly in the connective- tissue elements. Fat in- the human body is mostly found stored up in modified connective-tissue cells. This may occur wherever there is connective tissue, and fat cells must therefore be regarded as modified connec- tive-tissue cells. Likewise certain pigment cells and red blood corpuscles belong to this class. The supporting tissue is derived exclusively from the mesenchyma, a subdivision of the middle germ layer or mesoderm. It is divided into three classes connective tissue, carti- lage, and bone. Fig. 30. Connective-tissue cells from Fig. 31. Connective-tissue a chick embryo. cells from Wharton's jelly of the umbilical cord. 1. CONNECTIVE TISSUE, The elements of this tissue consist of cells and cell products, in the form of connective-tissue fibers, which penetrate and give consistency and support to every organ in the body. i . Connective-tissue Cells. (a) Embryonic Connective-tissue Cells. These are irregular, stellate cells found in embryos and in the umbilical cord. Those of the cord, with the matrix in which they are imbedded, form a soft, pulpy mass TISSUES. 67 known as Wharton's jelly, or mucous tissue. These cells are loosely associated in no definite order, their stellate processes interlace and sometimes appeal to come in direct contact. Their nuclei are round, or oval, or elongated, forming what is known as the spindle-shaped and pointed nucleus, often resem- bling the cigar-shaped and rounded nucleus of a plain muscle cell. The nuclei are rich in chromatin, and therefore stain heavily with hematoxylin. Blood- and lymph- vessels mingle freely with these cells; in Fig. 32. Two pigment cells from the dermis of a salamander. The pigment is in the cytoplasm. fact, this association is constant. The so-called granulation tissue in healing wounds consists of em- bryonic connective-tissue cells, always bleeds easily, because of its vascularity, and painless because of absence of nerve endings. (b) Pigment Cells. These are connective-tissue cells in which pigment is stored in the cytoplasm, never in the nucleus. The cells are extensively branched, large and flat. In amphibians and rep- tiles they are abundant in the dermis of the skin, and enable the animal to change its color, as is the case with the tree toad and the chameleon. In the human 68 NORMAL HISTOLOGY AND ORGANOGRAPHY. Fig. 33. Pigment cells from the choroid coat of the eye. These cells are of connective- tissue origin. body connective-tissue pigment cells are limited to the choroid coat and iris of the eye, to birth-moles, and to the piamater of the brain. The pigment may be of any color, the constituent being melanin, a coloring material probably derived from the blood. (c) Fat Cells. These are connective-tissue cells with a large storage of fat. The fat occupies the center of the cell as a big drop which crowds the cytoplasm and nucleus to one side, closely pressed against the cell wall, which is unusually conspicuous. The cells are large and spherical. Since fat is dissolved by alcohol, these cells in sections are distorted, polyhe- dral, and appear more like irregular spaces than any- thing else. Normal fat is not to be confounded with pathological fat found in fatty degeneration of organs. In the latter case the fat appears as little droplets diffused through the cytoplasm of the dis- eased cells, and is pro- duced at the expense of protoplasm, a destructive process or katabolism. Normal fat is a constructive process or anabolism, and is therefore a storage of food or potential energy. Its production, physiologically, is not clearly understood. It may be produced from Fat. Cytoplasm. Nucleus. Fig. 34. Normal fat cell. TISSUES. 69 a proteid diet, but its production is more easily prompted by a fatty diet and the carbohydrates. Cold prompts its production, as is clearly manifest in hibernating animals when the cold season ap- proaches, and the increased weight of animals, as a rule, during the winter season. Connective-tissue cell. Nucleus of fat cell. Fig- 35- Fat cells as they appear in sections treated with alcohol. Alcohol dissolves the fat. The usual stain for fat is osmic acid, in which fat acts as a reducing agent, precipitating black osmium. Any reducing agent will do this, as is made evident by the black color of the cork in a bottle containing osmic acid solution, fat being absent from cork. Other connective-tissue cells, as Plasma cells, Wan- dering cells, and Mast cells, are frequently described. These resemble normal constituents of blood and lymph to which they may belong. 2. Connective-tissue Products. These products may be a jelly-like substance or fibers. Connective- tissue cells are always associated with these products. There are two theories as to the production of fibers. (a) The fibers may be processes of the cytoplasm that lose their cell connection. 70 NORMAL HISTOLOGY AND ORGANOGRAPHY. (b) The connective-tissue cells may secrete a homogeneous matrix, which later becomes striated, producing fibers in a manner, as fibrin is formed in clotting blood. Connective tissue is classified according to its matrix into : i. Mucous Connective Tissue. This consists largely of embry- onic connective-tissue cells and a jelly-like matrix or ground sub- stance which gives a reaction for mucus. It is found in the um- bilical cord, where it is known as Wharton's jelly, and in embry- onic tissue. 2. White Fibrous Connective Tissue. This consists largely of white nonelastic fibers. The fibers are parallel to each other, not branched, and yield gela- Fig. 36. Reticular tissue from a lymph gland. Tendon cell. Fi g- 37- Teased tendon, showing Fig. 38. Cross section of tendon, fine wavy white fibers. tin on boiling. The fibers swell up when treated with acetic acid. They are found in tendons, the apo- neuroses, and ligaments, the fascia of muscles, the dura mater, and the fibrous capsules of many organs. TISSUES. 3. Yellow Fibrous Connective Tissue. The matrix in this consists of elastic fibers that are branched, and usually coarser than the white nonelastic fibers. They do not swell up when treated with acetic acid ^ and yield elastin on boil- ing. Like the preceding, the fibers are frequently grouped into bundles with a limited supply of blood- vessels and connective- tissue cells. This tissue is found wherever elasticity is required, as in the ligamentum nuchae and subflava, in the walls of Fig. 39. Longitudinal section of tendon. Fig, 40. a, Yellow elastic fibers from the teased ligamentum nuchae of the ox; b, Cross-section of a portion of the ligamentum nuchae of the ox. The elastic fibers are grouped in bundles with a few intervening connective-tissue cells. arteries, and in the membrana propria of the peri- toneum and pleura. 4. Reticular Connective Tissue. This is a reticu- 72 NORMAL HISTOLOGY AND ORGANOGRAPHY. lum of interlacing fibrils and is found in adenoid tissue, lymph nodes, spleen, and membrana propria of mucous membranes. Also, to a limited extent, in bone marrow. 5. Areolar Connective Tissue. This is really a mix- ture of interlacing bundles of white and yellow elastic fibers. It is found subcutaneously to the skin, to which it imparts elasticity. Areolar tissue is vascular and favors, therefore, a rapid spread of bacteria. Over bony prominences there is a limited supply of areolar tissue and the skin at these places has restricted mobility. It is at these points that the spread of an infection, such as erysipelas, is checked. General Considerations. On account of the embry- onic condition of connec- tive-tissue cells these, like Fig. 41. Elastic fibers of the mesentery. epithelial cells, are fre- quently met with in patho- logical tumors. If the tumor is malignant it is called a sarcoma, and is as fatal to life as carcinoma, or cancer. If the tumor is made up largely of fat cells, it is called a lipoma, and if the fibrous elements predominate it is a fibroma. The production of connective tissue is often nature's method of checking the spread of a disease. This tissue is produced as a wall in advance of a spreading infection, and if the bacteria are unable to penetrate this barrier, the disease soon becomes self- limiting. This accounts for the swollen infected TISSUES. 73 parts, and also for the redness, which is due to the extensive blood supply which is always associated with this tissue. Such a swollen tumor represents an induration and a congestion. In this manner a whole or a part of an organ may be affected. The connective-tissue fibers are absorbed with difficulty or not at all, and a permanent mark or scar remains as an evidence of the injury. In a healing wound, particularly if infected, these fibers are abundantly produced, and its redness is evidence of its extensive vascularity or blood supply. Later, these fibers con- tract, which occludes the blood, and then the color changes from a red to a white scar that no medical treatment can remove. Pigmentation is a most important subject. Pig- ment appears, as a rule, in the cytoplasm of cells, seldom between the cells. It is not confined to con- nective-tissue cells, but is common to epithelial cells, as the deep layer of the epidermis giving the color of races ; is found in the retina cells, where it is always black, and in hair and nails. It appears in the cyto- plasm of muscle cells, particularly in old heart muscle,where it is found near the ends of the nuclei, and is nearly always present in nerve cells, giving a gray color to this tissue. Pigmentation is an ac- companiment of many diseases, particularly skin diseases. It is also produced under the influence of light, as freckles, that can only be removed by the normal exfoliation of the epithelium. The production of pigment seems to depend upon the blood. As blood is absent from epithelium there are two theories as to the manner in which the cells obtain it: (a) they may receive it directly from the 74 NORMAL HISTOLOGY AND ORGANOGRAPHY. blood, or (6) connective-tissue cells may elaborate it and deliver it secondarily to the epithelial cells. That the pigment is not intrinsic to epithelial cells is proved by the fact that colored skin grafted on a white man soon turns white, and white skin grafted on a colored person turns black. The fruitless* at- tempts to change one's color are well known. A melanotic sarcoma 'is a pigmented connective- tissue tumor, very malignant, whose cells disseminate very rapidly throughout the body, producing every- where new tumors. These cells have their origin from normal pigmented connective-tissue cells, and therefore are supposed to come from birth-marks, or the choroid of the eye, or the piamater of the brain. The etiology of such tumors is unknown. Fortu- nately they are rare. The facts to be remembered in regard to con- nective tissues are: i. The easily stained round, or oval, or spindle-shaped nucleus. 2. The cells are loosely thrown together, not in compact layers or strata. 3. The stellate cells, although the processes are often inconspicuous and not readily detected. 4. The tissue is vascular where cells are abundant. 5. The .absence of free nerve endings as among epi- thelial cells. 6. They do not secrete as do the epi- thelial cells. 2. CARTILAGE. Cartilage is supporting tissue in which the inter- cellular substance predominates and yields chondrin upon boiling. The cartilage cells are typical con- nective-tissue cells, and occupy lenticular spaces in the matrix called lacuna. Cartilage is surrounded by a dense connective-tissue membrane called the TISSUES. 75 perichondrium, in which smaller blood-vessels ramify. Blood- and lymph-vessels are absent from the carti- lage matrix, except rarely and in places where active growth or ossification is going on they may be present. According to the structure of the matrix, cartilage is classified as, Perichondrium . Lacuna. Cartilage cell. Cartilage matrix. Fig. 42. Section of hyaline cartilage from the trachea. i. Hyaline Cartilage. This is the simplest and most common form of cartilage. The matrix appears Matrix. Cells. Lacuna. Fig 4 , TWO groups of cells from hyaline cartilage: A , .Two cells found just beneath the perichondrium; B, Four cells found deeper in the cartilage matrix. to be a homogeneous substance, although many fine interlacing fibrils are present. The lacunae near the perichondrium have their long axis parallel to the 76 NORMAL HISTOLOGY AND ORGANOGRAPHY. surface, while deeper in the matrix the long axis is often at right angles to the surface. Each lacuna contains one or more cartilage cells. The cartilage surrounding lacunae usually stains differ- Cartilase cell. Elastic fibers. ance of the matrix. This cartilage oc- curs as articular carti- lage of joints, at the end of ribs, and in the nose, the larynx, the Fig. 44. Section of elastic cartilage trachea, and bronchi. from epiglottis. 2 Elastic Carti . lage. Elastic cartilage differs from the hyaline vari- ety in having typical interlacing, branched elastic fibers that form a dense network. This cartilage is found wherever elas- ticity is required, as in the external ear, the Eustachian tube, epi- glottis, part of ary- tenoid cartilages, and cartilag'es of Wrisberg and Santorini. 3. White Fibrous Cartilage. In this va- riety the white fibers predominate. As a rule these fibers run parallel in bundles and do not branch. A granular matrix intervenes between the fibers. Fibrous cartilage is found in the intervertebral disc, in the sym- Fig. 45 Section of white fibrocarti- lage from symphysis pubis. TISSUES. 77 physis pubis, and in the insertion of the round ligament. General Considerations. Cartilage tumors are known as chondroma, and are common. Like cartilage they are of slow growth and therefore harmless. The absence of blood accounts, in large part for the inactivity of the cartilage cells, both in the normal and pathological condition. Further- more, cartilage cells are enclosed in the matrix in a manner that inhibit their multiplication. Cartilage therefore grows by apposition or acquisition, not by intussusception, like most tissues. Cartilage slowly ossifies with age. During this process loops of blood-vessels enter the matrix from the perichondrium, and lime salts are deposited ad- jacent to the cartilage cells. This process will be -further described under bone development. The identification of cartilage is very easy, as the matrix has a marked affinity for many stains. 3. BONE. Bone is the chief supporting tissue of the body, and consists of a calcified intercellular substance, mostly calcium phosphate, and connective-tissue cells, or bone cells. Organic substance constitutes one-third the weight, and inorganic substance two- thirds the weight of bone. The bone cells, some- times called bone corpuscles, are flattened stellate cells with many slender processes, and a well defined round or oval nucleus. Like cartilage cells they lie imbedded in lenticular spaces called lacuna. These lacunae, however, communicate with adjacent la- cunae by means of numerous capillary tubes called 78 NORMAL HISTOLOGY AND ORGANOGRAPHY. canaliculi, into which extend the slender cell proc* esses. Through these canaliculi the imprisoned cells receive their nourishment and give up their waste products. Haversian System. This consists of a Haversian Haversian canal. Fig. 46. Haversian system with only one lacuna sketched. canal, containing an artery, vein, and nerve, bone lamallae concentrically arranged around the canal, and from two to six rows of concentrically arranged cells with their lacunae and canaliculi. The canals average 0.05 mm. (5-^ inch) in diameter. The canals, as a rule, run parallel with the shaft of the bone, but com- municate freely with each other. The blood penetrates as far as the Haversian canals but the lymph reaches each bone cell through the finer canaliculi. The nerve termi- nates in the wall of the blood-ves- sels and has no connection with the bone cells. Haversian systems occupy a central zone in a bony shaft. External and internal to this zone compact lamellae are present, arranged parallel to the surface. Fig. 47. Bone cell or corpuscle. The cell occupies a lacuna. TISSUES. Outer circumferen- tial lamella. , -- ' I / _' Haver sian or con* , - . ^m, >r > | centric lamella. / / \ * ' / t : x ' :> ' x I . v *> " -'- (' . v v " rf Interstitial l ' <- t-^t '-* Sr-V-.--f: wwer circumferen- tial lamella. Fig 48 Segment of a transversely ground section from the shaft of a long bone, showing all the lamellar systems. Metacarpus of man (Bohm and Davidoff). 80 NORMAL HISTOLOGY AND ORGANOGRAPHY, In the circumferential lamellae canals, called Volk- manris canals, convey blood-vessels to the Haver- sian canals. In the angular interstices, between the Haversian systems, lamellae and canaliculi are found arranged like those of the circumferential lamellae. The shafts of long bones contain a marrow cavity. At the ends the marrow cavity disappears, and the bony structure becomes spongy with many inter- stices and is then called cancellate bone. The middle of flat bones is made up of a like loose structure called diploe. Haversian canal. .. Fig. 49. Portion of a transversely ground disc from the shaft of a human femur (Bohm and Davidoff). Periosteum. The periosteum is a dense, fibrous, connective-tissue membrane that covers the bone and is derived from the perichondrium of the carti- lage. It is composed of two layers, (a) an inner layer that contains many elastic fibers and osteo- blasts, or bone-forming cells, known as the osteo- genetic layer, and (b) an outer layer of coarse, white, fibrous bundles where numerous blood-vessels ramify and send branches to the Haversian canals. TISSUES. gj The periosteum is anchored to the compact bone by means of bundles of fibers (Sharpey's fibers) that pass concentrically or parallel to the Haversian systems. Blood Supply o The compact bone is supplied with blood from the periosteum. Larger blood- vessels, called perforating vessels, pass directly through the bony shaft and supply the marrow. In removing bone, as a rib, the periosteum is not taken away, and because of the latter 's vascularity and osteogenetic layer, the removed part regenerates. On the other hand, infected marrow and diseased bone may be removed from the inner surface until a mere shell remains of the once solid shaft. If all the infection is removed a regeneration follows. Development of Bone. The development of bone is either intramembranous or endochondral. In the latter a cartilage stage intervenes, otherwise the history in each case is the same. A synopsis of endochondral development is as follows: 1 . A solid shaft of hyaline cartilage, non-vascular and without any marrow cavity. 2. In the center of this shaft the cartilage cells enlarge, their lacunae enlarge and coalesce, particu- larly along lines extending toward the ends of the bone. The rosette produced by this excavation is called the primary areola of Sharpey. 3. Lime salts are deposited in the thin walls of these spaces, making calcified cartilage. 4. Osteogenetic cells and blood-vessels from the periosteum enter the cartilage spaces. The carti- lage cells disappear with this invasion and the ex- 82 NORMAL HISTOLOGY AND ORGANOGRAPHY. cavation, begun by the cartilage cells, is -further en- larged by the bone cells. The excavated areas are now called the secondary areolce of Sharpey, the cavities having a rich blood supply quite in con- trast with the primary areolae. The marrow cavity Vesicular cartilage cells* Primary periosteal bone lamella. Periosteal bud. Periosteum. Unaltered hyaline cartilage. Fig. 50. Longitudinal section through a long bone (phalanx) of a lizard embryo. The primary bone lamella originating from the periosteum is broken through by the periosteal bud. Connected with the bud is a periosteal blood-vessel containing red blood-corpuscles (Bohm and Davidoff). is excavated and the shaft becomes longitudinally porous. Endochondral bone, therefore, develops in cartilage, not from cartilage. 5 . Osteogenetic cells attach themselves to the wall of these enlarged Haversian canals and become en- closed in lime deposits, forming thus the outer lam- ellae and outer row of bone cells of each Haversian TISSUES. system. Cells with lamellae are added centripetally to this outer row and thus ultimately complete the Haversian system, leaving a small central canal contain- ing vessels and a nerve. Ossification begins in the center of the cartilage shaft and proceeds gradually toward each end, so that all the above changes occur at one and the same time. After birth these changes go on at the ends of the bone, so long as it keeps growing. During this period the bone is made thicker by deposits from the periosteum forming the circumferential lamellae of bony shafts. These lamellae are added without the inter- vention of a cartilage stage and therefore represent intra- membranous development. Regeneration of Bone. The embryonic process of developing bone is repeated every time a broken bone heals. As a rule the carti- lage stage does not intervene. * r 1 11- Fig. 51. Longitudinal A SynopSIS Of the healing sec tion through area of rrrv>pQQ nf a Qirnnlp frartnrp ossification from long bone of human embryo IS as follows: (Huber). 84 NORMAL HISTOLOGY AND ORGANOGRAPHY. 1. Hemorrhage and clot. The fibrin of the clot tends to hold the broken ends in apposition. The parts are swollen and red, due to the influx of blood 2. Organization .of the clot. Connective- tissue cells and white corpuscles enter the clot, feed upon it and ultimately replace it, the connective-tissue cells meanwhile producing fibers. The organized clot is a more substantial fabric and more firmly holds the broken ends in apposition. 3. Osteogenetic cells enter the organized clot anc deposit lime salts, producing a primary callus. The connective fibers shrink, pulling the broken ends firmly together, producing a sensation know r n as knitting of bone. The primary callus surrounds the bone, and may even fill the marrow cavity. 4. Haversian systems are formed, uniting the broken ends. These systems appear just as de- scribed under development of bone. 5. Primary callus is absorbed and marrow cavity excavated. Bone cells called osteoclasts are sup- posed to be active factors in this absorption. General Considerations. Bone does not grow in the same sense as other tissues do. Any increase in size is due to apposition of bone lamellae upon those already formed. Accompanying and often pre- ceding bone production we usually find a destructive or excavating process. It is believed two classes of cells bring about these changes : (a) Osteoclasts that cause bone absorption, and (b) osteoblasts that en- gage in bone production. The latter are supposed to be particularly abundant in the osteogenetic layer of the periosteum. TISSUES. 85 Bone tumors are not uncommon and are called osteoma. They are of slow growth, usually as- sociated with bone, and harmless. On account of the great vascularity, a broken bone heals more rapidly than a broken tendon, or liga- ment, or a broken cartilage. An old bone is brittle and the healing process a slow one on account of the increase of earthy matter and a decrease of the organic. An infection beneath the periosteum is a felon. The periosteum is firmly attached to the bone by Sharpey's fibers and the pressure produced by an in- fection beneath it gives rise to extreme pain, which is instantly relieved by an incision. An inflam- mation in the bone is called an ostitis, while if it is located in the marrow cavity it is called an osteo- myelitis. III. MUSCULAR TISSUE. Muscular tissue consists of elongated cellular ele- ments in which contraction takes place along the long axis of the cell. This contraction is intrinsic to the muscle cytoplasm, and of this the spongioplasm seems to be the active agent. The word sarcode and its derivatives is used in describing muscle proto- plasm. This word was introduced by Dujardin, in 1835, an d was later replaced by the word protoplasm. i. Smooth, Non-striated or Involuntary Muscle. This is the simplest form of muscle tissue. The cells are mononucleated, elongated, or spindle-shaped, and vary in length from 40 to 200 />. The nucleus occupies the center of the cell, is rich in chromatin, and oval, with blunt ends, or cigar-shaped. The 86 NORMAL HISTOLOGY AND ORGANOGRAPHV. cytoplasm is longitudinally striated, the striations being due to fibrils or sarcostyles, which are, structur- ally, probably analogous to the spongioplasm. Between the fibrils there is a homogeneous sub- stance, the sarcoplasm, which is analogous to the hyaloplasm. These cells are enclosed in a delicate cement layer usually not described as a cell wall, and in which a fine interlacing reticulum has recently been described. The ends overlap each other and are held together by a delicate cement substance. Nerve-fibers from the sympathetic nervous system reach the muscle cells and terminate in small gran- ules upon the muscle cytoplasm. Nucleus Nucleus Artery Vein. Fig. 52. a, Cell from smooth muscle of intestine; b, Cross section of smooth muscle of intestine. Smooth muscle is found in the wall of the tubes of the body, and invariably in thin layers, with one exception the wall of the uterus where the muscle may be an inch in thickness. Usually, too, this muscle is laid down as an internal circular layer with an externally applied and thinner longitudinal layer. Plain muscle is found in the wall of the ali- mentary tract, trachea and bronchi, bladder, ureter, uterus, Fallopian tubes, urethra, . vas deferens, TISSUES. blood-vessels, lymph- vessels, large ducts of glands, nipple, hair-follicles, Eustachian tube, spleen, pros- tate gland, ciliary muscles, and iris of the eye. 2. Cardiac Muscle. The heart ontogenetically is a modified blood-vessel, and its muscle, therefore, has the same origin as smooth muscle. Heart muscle cells are oval or brick-shaped and mono- nucleated, the oval nuclei occupying the center of Muscle nucleus. Connective-tissue cell. pig, 23. Longitudinal section of heart muscle fibers. the cell. Longitudinal fibrils and, in addition, a fine cross striation, are present in the cytoplasm, resembling the cross stri- ation of voluntary mus- cle. This cross striation is explained under Vol- untary Muscle, and therefore will not be given here. The cells are joined together, end to end, by delicate ce- ment lines and laterally may unite with adjacent cells by means of protoplas- mic processes. In the cytoplasm adjacent to the ends of the nuclei, normally fat is frequently present and Muscle nucleus. Fig 54 _ Cross sec tion of heart muscle fibers - 88 NORMAL HISTOLOGY AND ORGANOGRAPHY. also some pigment. The latter is more prominent in old hearts, to which it imparts a brown color. At present the exact structure of the heart muscle is a disputed question. In sections, many breaks or artifacts resemble the cement lines separating adjacent cells. The longitudinal fibrils are said to penetrate the cement and thus establish a con- tinuity of protoplasm between adjacent cells. The presence or absence of a cell wall, analogous to the sarcolemma of voluntary muscle, is also disputed. 3. Voluntary Muscle. A voluntary myscle fiber is a multinucleated, greatly elongated cell, which may attain a length of 12 cm. (5 inches). These fibers are arranged parallel to each other and grouped into bundles, called fasciculi. Each fasciculus is Sarcolemma. Sarcostyles. Fig- 55- Voluntary muscle fiber. The sarcoplasm has broken, show- ing the smooth sarcolemma. surrounded by connective-tissue cells and fibers in which many blood-vessels ramify. A finer fabric of connective tissue penetrates the fasciculus and gives support to the individual fibers. The connective tissue that enters a fasciculus is called the endo- mysium, and that which surrounds a fasciculus is called the perimysium. Fasciculi are grouped into coarser bundles and these collectively make up a muscle. The muscle is in turn enveloped in a firm connective -tissue layer called the epimysium. In gross anatomy the latter constitutes the deep fascia TISSUES. 89 of muscle. In tough meat the connective-tissue element is extensively developed and the fasciculi are large and coarse. Each muscle fiber has a delicate, transparent, smooth cell wall called the sarcolemma. The oval distinct nuclei lie immediately beneath the sarco- lemma in higher vertebrates, but in lower forms and in all embryos the nuclei lie deeper in the muscle protoplasm. These nuclei have the same structure as the nuclei of any other tissue, but the cytoplasm shows a distinct and regular cross and longitudinal striation, characteristic of only one other tissue the cardiac muscle. The longitudinal striation is /ik 4jij$i&^cohnheim>s fa&f, ; ,\ |W ;_>; , S-.-.^-jA area, a bundle due to the presence of M Itg] of sarcostyles. j i- i-ii 11 J / 31 Sarcoplasm. delicate fibrillae called % ^M sarcostyles, which is an- ^/^^^ alogOUS tO the SpOngio- pfl^W\ Muscle nucleus. plasm of other cells. A %il|8l ., i v-vVf'-^'y^' "* Sarcolemma. more homogeneous and ^jSf fluid substance intervenes ^w between the SarCOStyleS, Fig. 56. Cross section of three n 1 , 7 ... voluntary muscle fibers. called sarcoplasm, and is in turn analogous to the hyaloplasm of other cells. The sarcostyles are not uniformly or evenly dis- tributed in each muscle fiber, but are grouped into bundles. In cross sections the fiber has therefore a honeycomb structure, the minute areas being known as Cohnheim's fields. A single Cohnheim field represents the cut ends of a single bundle of fibrils or sarcostyles. The cross striation is intricate and therefore more difficult to explain. This striation consists of alter- 9 NORMAL HISTOLOGY AND ORGANOGKAPHY. nating light and dark bands. The dark -bands are doubly refractive to light, or anisotropic, while the light bands are singly refractive, or isotropic. The dark bands represent a predominance of the sarco- style substance, and the light bands a predominance of the sarcoplasm. In the middle of the light band a dark line can be seen, known as Krause's membrane. In the middle of the dark band a light-colored line is present, known as Hensen's median disc. The latter disappears when a fiber contracts. KtW Sarcomere. Hensen's me- dian disc. Fig. 57. Diagram of voluntary muscle fiber; A, Fiber relaxed; B, fiber contracted. These transverse markings are all due to the dis- tribution of the sarcoplasm and the regular con- strictions of the sarcostyles. The sarcostyles are not of uniform dimensions, but at regular intervals show dilatations alternating with constrictions. The dilatations appear at regular intervals and in the same transverse plane of the muscle fiber, thus giving rise to the dark band. Krause's membrane is not a membrane, but represents minute nodal points of the sarcostyles, placed in the same trans- TISSUES. Muscle. verse plane of the fiber and in the middle of the light band. The light band represents an abundance of sarcoplasm and it is in this sectional area that the sarcostyles suffer a constriction. As Hensen's me- dian disc is a light line in the middle of the dark band there must be a deep constriction, if not a complete constriction of the sarcostyles at this point. The whole muscle fiber between the two Krause's mem- branes is called a sarcomere, and con- sists of a median dark band and the proximal halves of the adjacent light bands. A single fibril or sarcostyle between two of Krause's mem- branes is called a sarcous element. It is believed that muscular contractil- ity is particularly a function of the sarcostyles and that the sarcoplasm serves more as a storage of energy or food. As to color there are two kinds of muscle, white and red. In white meat, as the muscle of the Tendon. Fig. 58. Part of a longitudinal sec- tion through the line of junction be- tween muscle and tendon. At the line where the tendon fibrils join the sarco- lemma (a), the nuclei of the muscle are very numerous. Sublimate preparation (Bohm and Davidoff). 9 2 NORMAL HISTOLOGY AND ORGANOGRAPHY. breast of a bird, the fibers have a poor supply of sarcoplasm and a predominance of sarcostyles. In red meat the fibers are rich in sarcoplasm and have a less supply of the sarcostyle protoplasm. In the myology of man both kinds of fibers are present. The white fibers are more powerful but have less endurance; that is, if held in tetanic contraction with no interval of rest they would tire quicker than the red fibers. The pectoral muscle of birds is powerful, but would soon tire but for the interval of rest that intervenes between the strokes of the wing; that is, during its upward movement. Fig- 59- Three voluntary muscle fibers from an injected muscle, show- ing network of blood-capillaries. Blood Supply. Blood-vessels follow the con- nective tissue of a muscle, and penetrate to the individual fibers where they break up into capil- laries. These vessels run, as a rule, parallel to the fibers, forming a network with anastomosing branches. They extend in a varicose manner between the fibers in such a way that when a muscle contracts they readily adjust themselves, without breaking. Nerve Supply. Medullated nerve fibers accompany TISSUES. 93 the blood-vessels and terminate beneath the sarco- lemma in special end plates called muscle plates. These will be described under special nerve endings. Non-medullated or sympathetic nerve fibers also accompany blood-vessels, but they innervate the in- voluntary musculature of arteries and veins. Distribution. Voluntary muscles are the skeletal muscles, and make up the bulk of the body. Striated fibers are present in the upper part of the esophagus, and also constitute the platysma muscle of the skin. Union with Tendon and Bone. The muscle fibers terminate abruptly with tendon fibers. This is not a direct end-to-end union, but the tendon fibers fuse with the sarcolemma at an angle. In the same way the muscle fibers unite with the periosteum of the bone. At this point Sharpey's fibers are particularly abundant and firmly anchor the peri- osteum to the compact bone lamellae. General Considerations. A muscle tumor is called a myoma. Tumors of plain muscle are common in the wall of the uterus. They are benign, of slow growth, and usually harmless. A tumor of striated muscle fibers is very rare. The tissue is highly specialized and the fibers therefore do not multiply readily. If a muscle is injured or cut the voluntary fibers regenerate partly from the cut end and partly from free muscle nuclei that are shed into the wound, but mostly by connective-tissue repair that leaves a permanent scar. The physiological action of plain muscle is slow, producing peristaltic contractions. That of volun- tary muscle is rapid, as in the wings of insects. 94 NORMAL HISTOLOGY AND ORGANOGRAPHY. Voluntary muscles, while more powerful, tire easily. Plain muscle has a wonderful endurance. The pain produced by violent action of plain muscle is in direct proportion to the degree of contraction. Some examples are : the colicky pains of the intestine ; labor pains ; pains due to calculi in the ureter or bile duct; or the pain in appendicitis produced by con- traction of the plain muscle of the appendix. These pains have many things in common They may last for hours, they remit and recur with regularity, and they come in waves. An infection in a muscle, as a psoas abscess, bur- Telodenc Nucleus and nucleolus. Neurilemma . M edullary sheath . I Axis cylinder. Node of Ranvier. Fig. 60. Diagram of a neuron. rows in the fascia, that is, spreads along the con- nective-tissue septa, perimysium and endomysium. The quality of meat depends on the amount of con- nective tissue. In tough meat the fasciculi are coarse and perimysium abundant. In tender sir- loin the reverse prevails. IV. NERVOUS TISSUE. Nervous tissue is most highly specialized of all tissues and consists of elements called neurons. A neuron is a nerve cell with all its processes. These cells vary greatly in size; usually they are large. TISSUES. 95 They have one or more processes, no cell wall, and a distinct nucleus. The nucleus has a conspicuous nucleolus, a prominent nuclear membrane, but a small supply of chromatin. The cytoplasm is usually pigmented, the pigment being collected to one side of the cell. It is this pigment that gives nervous tissue a gray color wherever these cells are found. Fat and vacuoles are also usually found in the cytoplasm. The proc- esses of nerve cells are : Connective ... tissue. Fibrils of axial cord. Fig. 61. Transverse section through the sciatic nerve of a frog. At a and b is a diagonal fissure between two Lantermann's segments; as a result, the medullary sheath here appears double (Bohm and Davidoff). i. Axis cylinder (Deiters' process, axon, neurite, or neuraxon), which is usually a long protoplasmic process that physiologically carries an impulse away from the cell. Collaterals are nerve processes that leave the axis cylinder at right angles. They are commonly found near the nerve cells, but may ap- pear at a node of Ranvier some distance away from the nerve cell. 96 NORMAL HISTOLOGY AND ORGANOGRAPHY. 2. Dendrites, which are usually short processes, very much branched, and physiologically carry an impulse toward the nerve, cell. A collection of nerve cells constitutes a ganglion, while a nerve plexus is a reticulum or interlacing of nerve fibers. Nerve cells are classified, according to the number of their processes, into unipolar, bipolar, and multipolar. Nerve Cells. i. Unipolar nerve cells. These are nerve cells with but one process. If a nerve cell Pat cells. Artery and vein. Bipolar nerve cells. Fig. 62. Section of spinal ganglion. has but one process that process must be an axis cylinder. If a nerve cell has many processes onl} one is an axis cylinder, the others are dendrites. Unipolar nerve cells are found in the olfactory mucous membrane. They are columnar or cylindrical and each gives rise to a basal process, the axis cylinder, which remains non-medullated and extends through the cribriform plate to enter the olfactory lobe of the cerebrum. This class of nerve cells is common ir invertebrates. TISSUES. 97 2. Bipolar Nerve Cells. Bipolar nerve cells have two processes, one axis cylinder and one dendrite. Nerve cells of the spinal ganglia and ganglia of the Nerve fibers in cross section. Nucleus of nerve cell. Nucleolus. Connective-tissue cells forming a capsule around the nerve cell. Fig. 63. Two bipolar nerve cells from the spinal ganglion. cranial nerves belong to this class. These cells apparently are unipolar, but their embryology clearly shows the single process to be morphologically equivalent to two. In this particular case the long peripheral process carries an im- pulse to the cell, and this long process is therefore a dendrite. The short process that unites the ganglion with the central ner- vous system is the axis cylinder. These large bipolar cells are Fig. 64. Three ganglion cells from a spinal ganglion of a rabbit embryo. The cells are still bipolar. Their proc- esses come together in later stages, and finally form the T-shaped structure seen in the adult animal; chrome-silver method (Bohm and Davidoff). surrounded by a cap- sule of connective-tissue cells. The cells are large and the single compound process very soon divides into the two processes mentioned above. The cytoplasm 7 98 NORMAL HISTOLOGY AND ORGANOGRAPHY. of these cells has a fibrillar structure, this striation having a close relation to the fibrillae of the axis cylinder. The spinal ganglia are situated on the posterior or sensory root of the spinal nerves and within the vertebral canal. The Gasserian, geniculate, audi- tory, jugular, and petrosal ganglia of the cranial nerves are morphologically equivalent structures. The nerve cells of all these ganglia are bipolar, with the exception of a few cells said to be multipolar. In addition to nerve cells, nerve fibers and connective - tissue elements make up the histology of these gan- glia. A liberal blood and lymph supply is always present. 3. Multipolar Nerve Cells. These are nerve cells with many processes, only one of which is an axis cylinder. They constitute by far the bulk of nerve cells and are found in the brain and spinal cord and in ganglia along the sympa- thetic nervous system. The cells vary in size from 4 ^ in the granular layer of the cerebellum to 150 //, the largest nerve cells of the spinal cord. Chromatophile granules, vacuoles, fat, and a fibrillar structure is found associated with the cytoplasm. Large multipolar nerve cells, called cells of Purkinje, are found in the cerebellum and will be described with the histology of that organ. Nucleus, Fig. 65. Gan- glion cell with a process dividing at a (T-shaped proc- ess); from a spinal ganglion of the frog (Bohm and Davidoff). TISSUES. 99 Nerve Fibers. i. Medullated Fibers. Medullated nerve fibers usually consist of three parts, (a) axis cylinder, (6) medullary sheath, (c) neurilemma. An axis cylinder is a cell process that carries an impulse away from the nerve cell. It is a slender cytoplasmic process and may be very long, as is the case with Fig. 66. Ganglion cell from the Gasserian ganglion of a rabbit; stained in methylene-blue (intra vitani) (Huber). the motor fibers that come from nerve cells in the anterior horn of the spinal cord and extend, without interruption, to muscles in the distal parts of the limbs. The axis cylinder presents a longitudinal striation, a fibrillar structure, that is supposed to be continuous with the cytoplasmic striation of the 100 NORMAL HISTOLOGY AND ORGANOGRAPHY. Dendrite. Neuraxis. 'Neuraxis. Dendrite. Fig. 67. Motor neurons from the anterior horn of the spinal cord o! a new-born cat; chrome-silver method (Huber). '-n-n- Telodendrwn. Dendrite. Cell-body. Neuraxis. Fig. 68. A nerve cell with branched dendrites (Purkinje's cell), from the cerebellar cortex of a rabbit; chrome-silver method (Bohni and Davidoff). TISSUES. 101 cell body. The fibrils are imbedded in a fluid pro- toplasmic substance, the neuroplasm, and the whole surrounded by a delicate membrane, the exolemma. Implantation cone is an elevation that is sometimes Brush-like telodendrion. Main dendrite. Secondary dendrite, - Basal dendrite. Neuraxis with collaterals. Fig. 69. Pyramidal cell from the cerebral cortex of man; chrome- silver method (Bohm and Davidoff). present at the junction of the axis cylinder and cell body. The medullary sheath (white sheath of Schwann) is a covering to the axis cylinder. This sheath never 102 NORMAL HISTOLOGY AND ORGANOGRAPHY. extends to the nerve cell but begins a little distance from it. It consists of fat and neurokeratin. The latter, on burning, gives an odor of burnt bone. It Ranvier's node. Axial cord. - Medullary sheath. Nucleus. Ranvier's node. Fig. 70. Medullated nerve fibers from a rabbit, varying in thick- ness and showing internodal segments of different lengths. In the fiber at the left the neurilemma has become slightly separated from the under- lying structures in the region of the nucleus (Bohm and Davidoff). is this sheath that gives the white color to nerves and the white matter of the brain. In osmic acid prep- arations, oblique fissures appear in the medullary sheath dividing it into sections known as Schmidt- TISSUES, 103 Fibrils of axial cord. Neurilemma. Segment of Lantermann. Lantermann segments. It is claimed by some that these are artifacts. Nodes of Ranvier are con- strictions of this sheath at regular intervals of 80 to 900 ft. The smaller the fiber, the greater the distance between these nodes. Long fibers are slender, with long distance between the nodes; short fibers are coarse, with short distance between the nodes. Furthermore, in young fibers and at the distal por- tion of nerve fibers the nodes are relatively closer together. The neurilemma is a thin structureless membrane that surrounds the medullary sheath. An oval nucleus is present in this sheath, midway between the nodes of Ranvier. At each node the neurilemma is constricted and touches the axis cylinder, which in turn may be slightly thickened at this point and may give off a collateral. Medullated nerve fibers with a neurilemma are found in the cranial and spinal nerves. Medullated fibers without a neurilemma are found in the brain and spinal cord. The neurilemma gives great strength to the fibers. Its absence in the brain and cord accounts for the pulpy, soft nature of this tissue. 2. Non-medullated nerve fibers with a neurilemma, but without a medullary sheath, mingle with the medullated fibers. The sympathetic system con- Fig. 71. Longitudinal section through a nerve fiber from the sciatic nerve of a frog (Bohm and Davidoff). 104 NORMAL HISTOLOGY AND ORGANOGRAPHY. sists largely of non-medullated fibers. Terminal branched endings of an axis cylinder, called neuro- podia, have neither medullary sheath nor neurilemma. The axis cylinder, just as it leaves its nerve cell, is likewise uncovered. Nerve Trunk. The fibers that constitute a nerve are grouped into bundles called funiculi. Each funiculus is enclosed in a connective- tissue sheath, the perineurium, which sends septa, the endoneurium, in among the individual fibers. The whole nerve is enclosed in a firm con- nective-tissue sheath, the epineurium. Blood- and lymph- vessels accompany the connective-tissue elements and ramify through the nerve just as is the case in a muscle. Nerve cells with a long axis cylinder were classified by Golgi as Type I, and with a short axis cylinder as Type II. Golgi believed the former to be motor in function, and the latter sensory, a classification no longer tenable. Neuroglia tissue is a delicate sup- porting tissue of the brain and cord, consisting of cells with many fine in- terlacing branches, mossy cells, or spider cells. These cells develop from the ectoderm and are onto- genetically closely related to nerve cells. Their function is to give support, not to conduct nerve impulses. The great nerve center in the body is the cerebro- Nucleus. Fig. 72. Re- mak's fibers (non - medullated fibers) from the pneumogastric nerve of a rabbit (Bohm and Da- vidoff). TISSUES. 105 spinal system brain and spinal cord. Next comes the sympathetic system, made up of ganglia and Epineurium. Perineurium. r : fe' :&:vwS^^^^Pl\ ^ Endoneuri Funiculus. Nerve fibers in cross section. Artery and vein. Fig- 73- Cross section of nerve trunk Neuraxis of peripheral sensory neuron. Dendrite of periph- eral sen- sory neu- ron. Nerve- trunk. Spinal ganglion. -J-^ Anterior horn of gray matter of spinal cord. ' Neuraxis of peripheral motor neuron. Sympathetic ganglion. Neuraxis of sympathetic neuron. Fig. 74. Diagram to show the composition of a peripheral nerve- trunk (Huber). mostly non-medullated nerve fibers that terminate in glands or smooth muscle. Lastly, the peripheral 106 NORMAL HISTOLOGY AND ORGANOGRAPHY. system, nerve terminations formed in tissues and organs throughout the whole body. General Considerations. Nerve cells are so highly specialized that their multiplication after birth is un- known. We never, therefore, find tumors of nerve cells. If a nerve cell is cut, the axis cylinder re- moved from the nerve cell dies while the end that is still attached to the cell regenerates and may Fig- 75 Neurogliar cells: a y from spinal cord of embryo cat; 'ft, from brain of adult cat; stained in chrome-silver (Bohm and Davidoff). restore the lost part. Surgeons unite the ends of a cut nerve so that the axis cylinder may develop along the old nerve trunk which becomes a path of least resistance. In amputations the cut nerve may grow into a tumor, called a neuroma. Such a tumor would con- sist of nerve fibers and the accompanying connective- tissue elements. Injury to nerve cells, such as TISSUES. 107 brain or ganglia, heal by production of connective tissue and accompanying scar. The function of the axis cylinder is to conduct a nerve impulse. Physiologically such an impulse travels away from the cell, but experimentally it may pass in the opposite direction, as is the case when a nerve is stimulated midway in its course. The axis cylinder being made up of fibrils, it follows that such a cylinder may conduct more than one impulse, which in turn reach different centers through dif- ferent collaterals. The function of the medullary sheath is to protect and nourish the axis cylinder. Experimentally the non-medullated nerve fibers will tire quicker than the medullated. The nodes of Ranvier are points where nourishment from the blood and lymph can reach the cylinder. It is affirmed by some that the endolemma is only a lymph space surrounding the axis cylinder. The neurilemma is protective in function and gives great strength to the fibers. With nerves that ter- minate in muscle fibers the neurilemma is continuous with the sarcolemma of the muscle. Proximally the neurilemma begins where the medullary sheath takes up, always a short distance from the nerve cell, which leaves the axis cylinder uncovered as it emerges from the cell. It is affirmed that the neuron represents the ele- mentary unit of nerve tissue, and that neurons are merely in contact with each other and not in proto- plasmic continuity. This idea constitutes the neuron theory. CHAPTER III. CIRCULATORY SYSTEM, BLOOD, MARROW, AND LYMPHATIC ORGANS. HEART. The heart is a muscular organ. Its wall consists of three layers, endocardium, myocardium, and epicardium. 1. The endocardium is a serous membrane that covers the inner surface. Histologically it consists of two layers, an inner lining of simple squamous epithelial cells (endothelium or mesothelium), and an outer layer composed of connective-tissue fibers, connective-tissue cells, and smooth muscle cells. The endocardium is reflected over the heart valves where the smooth muscle is particularly abundant. 2. The myocardium is the middle layer and forms the mass of the heart wall. It consists of muscle tissue, the cardiac muscle already described (page 87) . This muscle consists of many layers that course in different directions with connective-tissue elements intervening, in which branches of the coronary blood-vessels ramify. 3. The epicardium is the outer covering, a serous membrane, and histologically similar to the endo- cardium, with a greater deposit of fat. The epi- cardium is reflected to form the pericardium, the 108 CIRCULATORY SYSTEM. 109 epithelial cells secreting a serous fluid that acts as a lubricant. ARTERIES AND VEINS. Arteries. The arteries convey blood from the heart to the capillaries, and vary in size from the aorta, the largest, down to minute structures of mi- croscopic caliber. The walls of these vessels are composed of three layers : tunica intima, media, and adventitia. i . Tunica intima is the internal coat and is a very Fig. 76. Cross section of small artery and vein; A, artery; V t vein. thin, smooth, glassy membrane, often difficult to demonstrate in sections. This is again divided into three layers, the innermost being a layer of pavement endothelial cells, outside of which we find a delicate fibrous connective-tissue fabric, the sub- endothelium, and outside of this again a layer of elas- tic fibers called the fenestrated membrane of Henle. The endothelial layer is made up of a single layer of flattened cells, held together by a cement substance and analogous to the endothelium of the peritoneum 110 NORMAL HISTOLOGY AND .ORGANOGRAPHY. and pleura already described. These cells are plas- tic, loosely attached to the subendothelium, and form a slippery surface over which the arterial blood flows rapidly. Any damage to these cells results quickly in the formation of a small blood- clot at the point of injury, from which we infer that they play a most important physiological role in their relation to the blood stream. The subendo- thelium is made up of a delicate network of elastic fibers, enclosing a few connective-tissue cells, which allows the applied endothelium a limited amount of mobility. The fenestrated membrane of Henle (called the internal limited membrane of the media by some authors) consists of a coarser elastic network of heavier elastic fibers which when peeled away as a whole presents on the exposed surface a basket- work arrangement of its fibers with numerous in- tervening elongated apertures like so many windows > hence its name. This membrane in cross-section of arteries appears as a wavy or corrugated white line encircling the artery very near to its inner surface. 2. Tunica Media. This is the middle layer of an artery, and makes up the bulk of its wall. In small arteries a considerable amount of smooth cir- cular muscle fibers is always present, while in the larger arteries circular elastic and non-elastic con- nective-tissue fibers make up its bulk. A sprinkling of connective-tissue cells may be seen, also a limited amount of longitudinal muscle and connective-tis- sue elements. A few blood capillaries and lymphatic spaces are present, which always connect with a CIRCULATORY SYSTEM. Ill coarser vascular system of the outer layer, never directly with the blood within the artery through the intima. Of course, nerve endings are found, which can only be demonstrated in specially pre- pared sections. 3. Tunica Adventitia. This is the outer layer, Endothelium. Subendothelium. Fenestrated mem- brane of Henle. Vasa vasorum. Fig- 77- Cross section of aorta. and is made up of a loose arrangement of tissues, and, taken as a whole, is less definitely defined than the media. It varies in thickness according to the lo- cation of the artery, but, as a rule, it is not so wide as the media; while in the media the connective- tissue fibers are arranged in sheets which interlace with any muscle that may be present, in the adven- titia the fibers form diagonal bundles, mostly of the 112 NORMAL HISTOLOGY AND ORGANOGRAPHY. non-elastic kind, which mingle with the adjacent are- olar tissue with which arteries are nearly always as- sociated. These bundles often serve as support to organs, by which the latter are more firmly an- chored. From such a union between the vena cava and abdominal aorta to the liver this organ receives a substantial support. The kidneys and ovaries are organs that may be cited as benefiting greatly by such a connection. A considerable amount of fat is often present in the adventitia, also connective- tissue cells, nerves, a few smooth muscle fibers, lymphatics, and blood-vessels. The latter are called vasa vasorum, and play an important part in the nourishment of the arterial wall. The vasa vasorum are sub-branches derived usually from some small branch of an adjacent artery, but may come directly from a small branch of the same artery which is given off at a higher point. As stated before, large arteries have relatively a large amount of elastic fibers and a small amount of smooth muscle. The aorta has scarcely any muscle. In the small arteries the reverse is true. The wall of large arteries is relatively thinner than that of small ones. The reverse is true of the intima. In large arteries the adventitia is also relatively scant, while in the smaller ones the adventitia may be one- half to two-thirds the thickness of the media. On account of the rigid and elastic arterial wall these vessels are usually empty after death, contracted but retain their normal shape; while veins, on the other hand, collapse and usually contain a certain amount of blood. CIRCULATORY SYSTEM. 113 Veins. These vessels convey the blood from the capillaries back to the heart. The progressive in- crease in size and the thickness of their walls is accompanied by a relative increase in blood pressure and rate of blood flow, yet nowhere is this equal to what obtains in the large arteries. Structurally we find the same layers in veins as in arteries, with the chief difference that the vein wall is much thinner. The endothelial layer of cells is supported by a very thin layer of delicate connective-tissue fibers, mostly non-elastic, while the fenestrated mem- brane of Henle is incomplete and usually difficult to demonstrate. The media, as in arteries, is the most prominent layer, but, unlike arteries, the non- elastic fibers prevail. Smooth muscle fibers, mostly circular, are often significant in this layer, while the other tissue elements are less conspicuous. The adventitia resembles more closely that found in arteries, with perhaps even less of the elastic ele- ments and more of the smooth muscle cells. Com- paring the different sizes of veins, we find an excess of elastic and muscular tissue in large veins. In the pulmonary vein the circular muscle fibers are well developed, while in the large cranial veins, such as the meningeal sinuses, muscle tissue is almost entirely absent. Veins, like arteries, therefore, show a structural variation, depending not only on size, but on location. It should be mentioned that in many superficial long veins, like those of the legs and neck, valves are present in the form of crescentic folds of the intima which function in overcoming the pressure of blood due to gravity. Those of the IT 4 NORMAL HISTOLOGY AND ORGANOGRAPHY. neck are so placed as to become functional when an animal lowers its head, as in the act of grazing. SUMMARY OF ARTERIES AND VEINS. I. Tunica intima. 1. Endothelium, simple squamous epithelial cells. 2. Subendothelial layer. *(a) White connective- tissue fibers. (b) Connective- tissue cells. f(c) Elastic connective-tissue fibers. ts. Henle's fenestrated membrane (elastic internal limiting mem- brane). Interlacing basketwork of elastic fibers. II. Tunica media. fi. Smooth muscle, circular. f2. Elastic plates and fibers, longitudinal and circular. 3. Nerves. 4. Blood capillaries, difficult to demonstrate. *5. White connective fibers. 6. Connective-tissue cells. 7. Muscle fibers longitudinal, rare. III. Tunica adventitia. *i. White connective- tissue fibers, longitudinal and oblique. 2. Connective- tissue cells. ts- Elastic connective-tissue fibers, longitudinal (external limit- ing membrane). 4. Nerves. 5. Vasa vasorum (blood-vessels). 6. Lymphatic vessels and nodes. *7. Smooth muscle fibers. It should be remembered that structural difference in large and small arteries is in keeping with their function. In small arteries or arterioles, the involun- tary muscle is conspicuous, as it is the contraction of this muscle that regulates the blood supply to an organ. In large arteries, as the aorta, the muscle is * This tissue predominates in veins, f This tissue predominates in arteries. CIRCULATORY SYSTEM. Endothelium. '" "_- .-"".;: Subendothelium. Fenestratedmem- brane of Henle, ' Media. Fig. , 78. Cross section of small artery. unnecessary and is greatly reduced, while elastic ele- ments are unusually well developed. The muscle, too, is deficient in large veins situated deep in the body, as the vena cava. Many of the smaller and more superficial veins have valves, folds of the in- tima, so arranged as to equalize the gravity press- ure of the contained blood. Without these valves the thin-walled veins would become greatly distended. If for any cause the veins ex- pand so that the valves do not act, a permanent distention with engorgement of blood follows. The veins become distorted and are spoken of as varicose veins, a condition quite common to the long saphenous veins of the lower limbs. Capillaries. These are the finer organic ramifications of the circulatory system, and unite arteries and veins. Histologically, the walls of capillaries Intima. Media. Adventilia. Vasa vasorum. Fig. 79. Cross section of vein. consist of a single layer of flattened epithelial (endothelial or mesothelial) cells. The blood courses very slowly through these in- terlacing tubes. The white cells penetrate the walls and under certain conditions even the red corpuscles Il6 NORMAL HISTOLOGY AND ORGANOGRAPHY. may do so. According to some investigators, minute pores in the epithelial wall, called stigmata and sto- mata, allow this migration. Others deny the presence of these spores, in which case the blood elements escape by passing between two adjacent epithelial cells, after which this opening closes. Flat view. Side view. THE BLOOD. The blood is derived from the mesoderm; it is a red fluid that consists of (i) a liquid portion, the plasma, and (2) solid constituents, the corpuscles. There are at least three classes of the latter, red corpuscles, white corpuscles, and platelets. i. Red corpuscles (erythro- cytes) in the mammalia are non- nucleated, circular, biconcave discs. In all the other verte- brate groups and in all embryos they are nucleated oval and biconvex cells. Each corpuscle consists of a red coloring matter, hemoglobin, and a more substan- tial fabric or reticulum, the stroma. The hemaglobin is the bearer of oxygen, is readily solu- ble in water, leaving the stroma or fabric, which is then known as a ghost corpuscle. The red corpuscles are soft and elastic and are covered by an oily film. In a fresh spread they adhere to each other by their concave surfaces forming rouleaux or " money - Rouleau. Rouleau. Fig. 80. Red blood- corpuscles from man. CIRCULATORY SYSTEM. 117 pile" rows. This is purely a physical phenomenon. As soon as the oily covering dissolves this combination disappears. The corpuscles are extremely susceptible to changes in the plasma. If water is added they will swell up and the hemoglobin begins to dissolve. With evaporation the corpuscles begin to shrink, forming minute processes and they are then said to be crenated. Evaporation of water produces an in- creased percentage of the salts in solution. This in turn ab- stracts water from the cor- puscles and the shrinking or crenated condition follows. It is estimated that the total amount of blood in man is one-thirteenth the weight of the body. The average normal male, therefore, has approximately 25,000,- 000,000,000 red corpuscles. The life period of a red Fig. 81. Crenated red blood-corpuscles from man. Fig. 82. Red blood-corpuscle of frog; a, flat view; 6, side view. corpuscle is not definitely known, but physiologists tell us it is probably from two to four weeks. Ac- Il8 NORMAL HISTOLOGY AND ORGANOGRAPHY. cordingly the daily consumption and loss is enormous, and is equaled only by as constant and regular a production of new cells. Their number in man is 5,000,000 per cubic millimeter. The following table gives the size of the red blood- corpuscle in the different groups of animals : Man 7.2 to 7.8 ft. Monkey 7.0 ft. Dog 7.5 ft. Cat 6.2 ft. Horse 5.6 ft. Chick. .. .12.1 by 7.2 p. Duck 12.9 " 8.0 ft. Tortoise. .21.2 " 12.5 ft. Snake 22.0 " 13.0/1. Frog 22.3 " 15.7 ft. Guinea-pig . . 7.5 Newt . . ..30.7 19.0 ft. White Corpuscles. These are colorless, nucleated, plastic, ameboid cells. Their number in man is from 7000 to 10,000 per cubic millimeter. They are variously classified according to the morphology of their nuclei, or the granules in the cytoplasm that Small mononuclear. . Fig. 83. White blood-corpuscles from man. take different stains, or as to their origin, or as to their function. For practical purposes they may be classified as : 1. Lymphocytes size 8 to 10 microns. ... 22 per cent. 2. Large mononuclear leucocytes. *' iotoi5 " ....4 " " 3. Polynucleated " 12 " ....74 " " (a) Mast cells 0.4 per cent. (b) Eosinophiles 2 to 4 " " (c) Neutrophiles 70 u " CIRCULATORY SYSTEM. Iig Lymphocytes are small, mononucleated, white cor- puscles, with a distinct staining nucleus and a very narrow border of cytoplasm. Amoeboid motion is, accordingly, much limited in this class. Large mononucleated leucocytes have a large vesic- ular and usually eccentric nucleus. Its chromatin occurs in scattered granules that stain less deeply with nuclear stains, while the finely granular cyto- plasm is usually abundant. These cells are gener- ally regarded as phagocytic in function. Polynuclear cells are only slightly larger than the red blood-corpuscles. The nuclei are often nodular, polymorphic; that is, are united by slender constric- tions, or are lobulated and of a variety of patterns. In a small number of these cells basophilic granules are found in the cytoplasm, which stains blue with basic stains. These are the mast cells. Another small group have eosin-staining granules, and these are the eosinophiles. The large bulk of poly nucleated white corpuscles have cy toplasmic granules that take neither acid nor basic stains, and these are the neu- trophiles. They are the white corpuscles found abun- dantly in ordinary pus and the ones that produce a general leucocytosis in such infections. The percentage of these different cells and their total number per cubic millimeter is of the greatest clinical value in blood analysis. They are often called wandering cells, as they are able to pass through the capillary wall and migrate throughout the tissues and organs. The poly nucleated form is readily recognized by multiple or fragmented nuclei. 3. Blood platelets are small, colorless, round, non- nucleated bodies about one-third the size of red 120 NORMAL HISTOLOGY AND ORGANOGRAPHY. blood-corpuscles. They are supposed to play an im- portant role in the coagulation of blood. As soon r Fig. 84. Ehrlich's leucocytic granules (from preparations of H. F. Miiller): a, Acidophile or eosinophile granules, relatively large and regularly distributed; e, neutrophile granules; /?, amphophile granules, few in number and irregularly distributed; ?, mast cells with granules of various sizes; d, basophile granules; (a, o, and e, from the normal blood; ?-, from human leukemic blood; /?, from the blood of guinea-pig) (Bohm and Davidoff). as blood is shed most of them disappear, unless spe- cial precaution is made to preserve them. They may be preserved by pricking the finger through a drop of osmic acid. Their number per cubic millimeter is from 200,000 to 600,000. According to Wright's investigations, these plate- lets represent detached portions of giant cells found in bone-marrow or in the spleen. Schafer regards them as minute cells, while others think of them as fragments of red or white corpuscles. Hemin Crystals (Teichman's crystals). These come from the hemoglobin of the blood, and when found are always a positive evidence of blood. The crystals can be obtained from clotted blood, no mat- CIRCULATORY SYSTEM. 121 ter how old the clot or stain is. Dry blood and salt, equal parts, are ground together on a glass slide, a few drops of glacial acetic acid are added and heat applied until gas-bubbles appear. The crystals are brown, rhombic, and easily recognized. Fig. 85. Crystallized hemoglobin: a, 6, Crystals from venous blood of man; c, from the blood of a cat; d, from the blood of a guinea-pig; e, from the blood of a hamster; /, from the blood of a squirrel (after Frey). MARROW. Bone marrow is either white or red. The white marrow occupies the shaft of the bone and is largely fat. The red is found in the ends of bones or can- cellated portions and is richly supplied with blood. 122 NORMAL HISTOLOGY AND ORGANOGRAPHY. Histologically, we find in marrow all the con- stituents of blood and connective-tissue elements, fibers and cells. In addition, the following are some of the more characteristic cells of this tissue : i . Hematoblasts or Nucleated Red Blood-corpuscles. These cells contain hemaglobin and a small round nucleus that stains heavily with hematoxylin. They are supposed to be the chief source of the red blood- corpuscle, in which case the nucleus must disappear either by disintegration or extrusion. 2. Marrow Cells or Myelocytes. These are large cells with a rather large nucleus that stains lightly. 3. Eosinophiles. These are destined to become the eosinophile of the blood. 4. Giant Cells (myeloplaxes or osteoclasts) . These are very large poly nucleated cells, having from ten to twenty nuclei. Cells of this class are not numerous, but extremely large (30 to 100 /w). They may be found in the fetal liver or spleen, and are very char- acteristic of developing bone. They present a finely granular proto- plasm without any cell wall. The clei ar e bunched about tais from blood- the center of the cell, and in this stains of man. - -._. f - respect they differ from the giant cells found in tuberculosis foci, in which the nuclei are found near the periphery. They multiply by mitosis, and primarily are supposed to be derived from leucocytes by endogenous division of their nuclei. These remarkable cells have usually been re- garded as the active agents in bone absorption, but recently Wright has suggested that blood platelets CIRCULATORY SYSTEM. 123 may be derived from their cytoplasm by a process of budding or by particles merely breaking away. General Considerations. There is an old saying that "a person is as old as his blood." A truer ex- pression would be that he is as old as his blood- vessels. With age, or dissipation, the blood-vessels harden, due to depositions of connective-tissue ele- ments. This impairs the free circulation of blood and the body, as a whole, suffers. The hardened condition is /':V^?>... spoken of as arteriosclerosis, or atheroma. Usually the in- tima suffers first by becoming much thickened. Later, a like disturbance takes place in the media and adventitia. As superficial scars remain permanently, and can not be eliminated, so there is no re- lief for this scar formation of the blood-vessels. Under this hardened condition a rupture of the smaller arteries is not uncommon, particularly those of the brain, as they have thinner walls. Such a disas- ter is apt to be fatal. An inflammation of the heart, as endocarditis, is apt to produce a deposit of connective tissue in the endocardium, which upon shrinking brings about defective valves, with leak- age of blood. This increases the work of the heart, and although that organ in an emergency can do Hematoblasts. Eosinophile. Marrow cell. Giant cell. Fig. 87. Cellular elements of red marrow. 124 NORMAL HISTOLOGY AND ORGANOGRAPHY. twenty times its normal work, there is of course a limit to its power, and broken compensation sooner or later follows. The vasa vasorum, that carry blood to nourish the walls of both arteries and veins, are very important structures. The coronary vessels of the heart be- long to this class and their course is quite definitely known. Our knowledge of the rest is vague. They ramify through the adventitia and to a less extent in the media. If a xft^vffi|$.f|X. .. Giant cell. $ip ' Hematoblast. Fat space. blood - clot forms within the vessel, loops from thejvasa vasorum enter the clot and assist in its organization. The endothelial cells of the intima, according to one theory, are active agents in preserv- ing the fluid con- dition of the blood ; that is, inhibiting coagulation. If these cells are injured a clot of blood quickly forms upon the injured or denuded surface. Surgeons take advantage of this principle and twist or crush the ends of bleeding vessels to check a hemorrhage. The disintegration of red blood-corpuscles is known as hemolysis, and may be produced by injecting into the circulatory system certain poisons, or mixing extravasated blood directly with these poisons. Fig. 88. Section of red marrow. CIRCULATORY SYSTEM. 125 Hemolysis occurs in various diseases and is one of the chief changes observed in making a Wasser- mann test. The identification of blood-stains is often a medico- legal problem. The corpuscles of the blood pre- serve their integrity for a remarkably long period of time, so that in a water solution of even an old clot, Capsule. Trabeculce. Germinal center. Lymph sinus - phoid cells from the me- sin an( J are f ound in no other dulla. organ. It is affirmed that these epithelial cells continue to grow after birth and may be found late in life when only remnants of the thymus is present. CIRCULATORY SYSTEM. SPLEEN. The spleen is a blood-forming organ, very vas- cular, purple in color, and with a density slightly more than that of the liver. It varies greatly in size, the average being five inches long and three inches wide. Its surfaces touch the left kidney, the cardiac end of the stomach, and the left lower aspect of the diaphragm. Its long axis follows the direction of the tenth rib. It is practically covered by the Vein. Malpighian corpuscle. Trabecula. Artery. Spleen pulp. pig. 9 ^ Portion of section of human spleen. The figure gives a general view of the structure of the spleen (Sobotta). peritoneum. The structures to be recognized are capsule and trabecula, Malpighian corpuscles, and spleen pulp. i. Capsule and Trabeculcz.^he investing peri- toneum forms a serous coat with simple squamous I3 2 NORMAL HISTOLOGY AND ORGANOGRAPHY. epithelium and connective-tissue fibers. Subjacent to this the spleen is provided with a strong cap- sule consisting of elastic fibers, connective-tissue cells and involuntary muscle. The spleen is thus not only distensible but may pulsate. From the deep surface of the capsule processes or trabeculce of connective tissue and smooth muscle pass into the substance of the spleen. From the trabeculae finer branches pass to form a fine supporting fabric for the whole organ. Fig. 96. Section of spleen. 2 . Malpighian Corpuscles. These are lymph pock- ets in the adventitia of the smaller arteries. The artery rarely passes through the center of the cor- puscle, but usually eccentric to or one side of it. The lymph corpuscle is liberally supplied with blood. 3. Spleen Pulp. This constitutes the bulk of the spleen and fills the spaces between the trabeculae. The constituent corpuscles of the blood are present in this pulp and splenic cells. The latter are slightly CIRCULATORY SYSTEM. 133 larger than white blood-corpuscles, are mononu- cleated and contain pigment and frequently red blood-corpuscles . Blood Supply. The splenic artery enters the hilum and its branches follow the trabeculae. Ultimately the smaller branches enter the spleen pulp. Beyond the Malpighian bodies the smaller arteries end in minute dilatations known as the ampulla of Thoma. Beyond these the blood flows directly into the meshes Larger fibers of a Malpighian body. Reticular fibril (G it ter las- em). Fig. 97. From the human spleen (chrome-silver method) (Bohm and Davidoff). of the spleen pulp with no other walls than the spleen cells. The veins begin in the same way as the arteries end. The capillary veins pass directly to the trabeculae and ultimately unite at the hilus to form the splenic vein which drains into the portal. General Considerations. The invasion of bacteria into the system is chiefly along the lymphatics. Each lymph node becomes a point of resistance, and 134 NORMAL HISTOLOGY AND ORGANOTHERAPY. usually enlarges many times the normal size, far in advance of the seat of infection. This is due to the absorption of the toxins. Thus the lymph nodes in the groin enlarge from an infection in the toe, those in the axilla from an infected finger, and those of the neck from a bad tooth. If these barriers break down the infection becomes systemic, a condition known in a general way as blood-poisoning. Artery to one of the ten com- partments. Intralobular \ artery. \ Interlobular trabecula. \ l Intralobular L_ trabecula. \ | \ Malpighian corpuscle. [ m | Intralobular venous spaces. Intralobulor vein. Ampulla, of Thoma. Spleen pulp cord. Interlobular -vein. 1 nt rf ilobular vein. Fig. 98. Diagram of lobule of the spleen (Mall). The function of the thymus gland is not known. Since its structure resembles the tissue of a lymph node it is reasonable to suppose that it has a like function. Recently structural changes have been observed in this organ in epileptics, but whether these changes are a cause or a consequence of the dis- ease is not known. The lymphoid tissue of the spleen no doubt CIRCULATORY SYSTEM. 135 contributes to the supply of white blood-corpuscles. The broken-down red corpuscles found in this organ have led to the further idea that the spleen is a graveyard for the worn-out red corpuscles of the blood. Leucocytes are supposed to feed upon this detritus and then migrate to the liver, where it is elaborated into the bile pigment of that organ. Anything that causes an enlargement of the lymph nodes usually causes an enlargement of the spleen. Like these nodes the spleen is capable of enormous distention, due to the abundance of elastic con- nective-tissue fibers. This is particularly so in typhoid fever, where the spleen has been known to weigh fifteen or twenty pounds. On account of the rich blood supply an injury to the spleen causes severe hemorrhage, which the pulpy condition of the organ renders difficult to check, as a suture usually does not hold. In such accidents the whole spleen has been removed with- out fatal results. Extirpation of the spleen is also justified in certain diseases of that organ CHAPTER IV. DIGESTIVE SYSTEM. The digestive system consists of alimentary canal and accessory digestive glands. The Alimentary Canal. This is a muscular tube extending through the body and measures about thirty feet in length. The following parts will be described : I. Mouth II. Pharynx. III. Esophagus. IV. Stomach. V. Small Intestine. 1. Duodenum. 2. Jejunum. 3. Ileum. IV. Large Intestine. 1. Vermiform Appendix. 2. Cecum. 3. Colon. (a) Ascending. (b) Transverse. (c) Descending. (d) Sigmoid Flexure. 4. Rectum. THE MOUTH. The mouth is limited by the lips in front, and the cheeks laterally. The arched palate forms its roof 136 DIGESTIVE SYSTEM. 137 and the tongue is attached to the movable floor, while posteriorly it opens into the pharynx through the isthmus or fauces. This cavity is lined by a con- tinuous mucous membrane, consisting of stratified mucous epithelium placed on a tunica propria. In Sinus prcecervicalis. Fig. 99. Human embryo of about twenty-eight days (His): I-V, brain-vesicles; /\ f\ / 3 , / 4 , cephalic, cervical, dorsal, and lumbar flexures; ot, otic vesicle; ol, olfactory pit; mx^ maxillary process; h\ h 2 , heart; /, l\ limbs; a Is, allantoic stalk; ch, villous chorion. the submucosa is found connective-tissue elements in which the elastic fibers predominate; also con- nective-tissue cells, mucous and serous glands, nerves, and nerve endings, blood- and lymph- vessels. 138 NORMAL HISTOLOGY AND ORGANOGRAPHY. The mucous membrane is continuous with the skin at the outer border of the lips. At this border the horny layer of skin begins, otherwise the skin and this mucous membrane are similar structures. Morphologically, the mouth cavity is to be re- garded as a part of the outside surface of the body, which, embryologically, has been included by the development of neighboring parts. At the time the neural folds are closing dorsally to form the brain and cord there develops a series of paired, ventral, facial pits. These, enumerated from before back- wards, are: the lens of the eye, the nasal pit, the mouth, and gill clefts. The tissue between the latter are called visceral arches, while that one between the anterior gill cleft and the mouth cavity is the man- dibular arch. The latter is morphologically analogous to the visceral arches. In man the gill clefts all finally close permanently, but the ectodermal em- bryonic mouth cavity ultimately unites with the embryonic foregut, thus forming the fauces which lead to the pharynx. This final perforation be- tween the mouth cavity and the foregut is paired, in lower forms, which with other embryonic relations confirms the view that the mouth cavity morpho- logically represents a median fusion of two gill clefts. During this period of development the forebrain grows ventrally and the mandibular arch grows in the same direction. The space between these structures is the beginning of the mouth and the nose, and is called the stomodeum. At this time a rounded eleva- tion, from the base of the mandibular arch, grows for- ward along the base of the forebrain. This growth DIGESTIVE SYSTEM. 139 forms part of the maxillary arch, and finally most of the upper jaw. In this manner the stomodeum Stomodeum. Nasal pit. 'Lateral protuberance. Globular protuberance. Maxillary arch. ^Mandibular arch. _ ^ ^ . ,, m _ Nasal process. Nasal pit. Lacrimal canal. Maxillary arch. Mandibular arch. Fig. 100. Development of the face of the human embryo (His): A, Embryo of about twenty-nine days; B, embryo of about thirty-four days; C, embryo of about the eighth week; D, embryo at end of the second month. becomes divided into an olfactory region and the mouth cavity proper. The stomodeum at this stage 140 NORMAL HISTOLOGY AND ORGANOGRAPHY. is a deep pentagonal cavity. Its lower boundary is formed by the mandibular arch, while laterally are to be found the maxillary processes of each side. Its upper boundary is formed by an unpaired growth called the nasofrontal or nasal process (Fig. 100.) Situated on each side of the nasal process are the nasal pits. Each pit divides the nasofrontal process into a lateral external portion called the lateral frontal protuberance , which forms the outer boundary Ventricle of cerebrum. Place from which the hy- pophysis is developed. Stomodeum. Heart. Pore gut. Notochord. Third ventricle. Fourth ventricle. Spinal cord. Fig. 10 1. Median section through the head of an embryo rabbit 6 mm. long (after Mihalkovics) of each nasal pit, and a median or central portion called the globular protuberance, which constitutes the inner boundary of each pit. The two lateral or side protuberances grow around the olfactory pits and form the alse of the nose, while the two central portions develop into the intermaxillary bone con- taining the incisor teeth and the center of the lip. By studying the text figures a correct idea of these DIGESTIVE SYSTEM. 141 relations is readily obtained. It will be seen that the line of contact between each lateral protuberance and maxillary process forms a groove, the naso-optic furrow or lacrimal groove, which later closes to form the lacrimal canal. The line of contact between each globular protuberance and the maxillary pro- cess is less close, and places each nasal pit in wide communication with the mouth. A failure of union in the latter case causes the deformity of harelip, Median triangular por- tion of palate. Dental ridge. Lateral portion of palate. Fig. 102. Roof of the oral cavity of a human embryo with the fun- daments of the palatal processes (after His). which may be double or single, depending on whether both or only one side is involved. About the fortieth day, in the human embryo, the maxillary processes have grown so far toward the median plane that they have met and united with the lateral and also the median protuberances of the nasofrontal process. The nasal pits are thus sepa- rated externally from the oral fossa. With this union the arch of the upper jaw is complete, but the 142 NORMAL HISTOLOGY AND ORGANOGRAPHY. inclosed space is in one chamber, there being no separation between the mouth and nose cavities. The formation of a palate, however, effects a separa- tion between the two. The rudiments of the palate appear as shelf-like projections from the inner or oral surface of the upper jaw. A triangular piece grows backward from the globular protuberance of the nasofrontal process, which ultimately unites with horizontal or palatal plates from the maxillary arch. In the eighth week of embryonic life, union of the palatal plates begins at their anterior extremi- ties and proceeds backward. A deficiency in the union constitutes the deformity of cleft palate. Cleft palate is therefore, embryologically, a later development than harelip. Either may occur with- out the other, but they are usually found together. The cleft of the palate usually turns to one side, passing out between the cuspid and lateral incisor teeth. A double cleft palate is Y-shaped, the center piece in front containing the incisors, and representing the anterior triangular piece of the rudimentary palate, this piece having failed to unite with the lateral palatine plates. This deficiency may involve the hard or the soft palate, or it may affect both, and even produce a cleft or bifid uvula. The completion of the palate definitely separates the nasal chambers from the mouth, the only com- munication between the two being through the posterior nares.. The permanent limitations of the mouth are thus established from a cavity that develops primarily as an ectodermal invagination. The ectoderm invests not only the mouth proper, DIGESTIVE SYSTEM. 143 but clothes at least the anterior portion of the adult pharynx. The tongue, however, is invested with en- toderm epithelium and o are the Eustachian tubes. TEETH. Morphologically, teeth are appendages of the skin, and are to be compared with such structures as hair and nails. They are thus a part of the exoskel- eton and their relation to the bones or the en- doskeleton is entirely a secondary process, for the purpose of strength and support. In many of the fishes the mouth generally is lined by simple cone- shaped teeth that serve the purpose of seizing and holding the ani- mal's prey. In man the additional function of masticating the food has greatly modified the form and structure of teeth. Dentition. There are twenty deciduous or tem- porary teeth that erupt between the ages of six months and two and one-half years. In each jaw these are, incisors 4, cuspids 2, molars 4, the dental Fig. 103. The palate and supe- rior dental arch (right side): i, Me- dian incisors; 2, lateral incisors; 3, canine; 4, first bicuspid; 5, second bicuspid; 6, first molar; 7, second molar; 8, wisdom-tooth; 9, mucous membrane of the hard palate con- tinuous behind with that of the soft palate; 10, the anteroposterior raphe of palate; n, pits on each side of the raphe perforated with the orifices of glands; 12, anterior rugosities of the mucous membrane (after Testut). h W W > < a ex j S * P IjSSl" rtrtd rt rt^rtrt oJojOj D 4)l>o;qj files'" t^ t^ . CX3 1 h W W . < a tA Q 2 5 h 5 05 |H W JS J ^3 ^3 ^J3 X J= J3 X C "C"CC *^G c'CG C O OOO O OOO O M M (| ro CNICSCNCO S < a W w 03 H Z (* X-* . .X^ . 1 |1|| -s -s PH !> T o w gfifsj |.| 5 l^|s M - ^- HH Pll 3j 2 W o^ w^< . . . .. . . o i|j b J g o -< SI "^ > yes;. gingivus, a condition which becomes of primary im- portance in the mastication of food. The fibrous mat of the gum is usually greater on the lingual side, where food is brought against the gingivus with con- siderable force. If a crown band is extended too far, or if deposits accumulate as in case of uncared-for DIGESTIVE SYSTEM. 163 teeth, these supporting fibers will be cut off and the gingivus drops down and no longer fills the inter- proximal space. In the alveolar portion the principal fibers not only spread out and radiate like a fan, but are in- clined downward from their attachment in the ce- mentum to their anchorage in the bony wall of the alveolus. Some of these fibers are tangential to the cementum and thus support the tooth against a rotary strain. The principal fibers thus perform a physical function and firmly bind the tooth to the adjacent hard and soft tissues. At the alveolar border and at the apex of the root they are so ar- ranged as to support the tooth against lateral strain, while in the rest of the alveolar portion the tangential fibers are particularly numerous and sup- port the tooth against any rotary force which may result from the mastication of food. At the gingivus line the fibers blend with the submucosa and bind the gum closely to the neck of the tooth. At the apex of the root the secondary loose variety becomes continuous with the connective tissue of the tooth pulp. The cellular elements of the peridental membrane are the fibroblasts, cementoblasts, osteoblasts, osteo- clasts y and epithelial cells, which have been called the glands of the peridental membrane. All these cells are interposed between the bundles of supporting fibers already described. i. The fibroblasts are spindle-shaped connective- tissue cells arranged in radiating rows between the fibers. They are numerous in young teeth and 1 64 NORMAL HISTOLOGY AND ORGANOGRAPHY. the root. They produce the cementum and there- fore resemble the osteoblasts of bone. They have a single deep-staining nucleus and irregular processes that fit around and between the fibers and also ex- tend into the cementum. A cementoblast may be- come enclosed in the cementum and thus form a lacuna which it completely fills, thus making a Fibro- blast. Dent in. Fig. 119. Peridental membrane next to the cementum highly mag- nified (Noyes). relatively few in old teeth. The single nucleus stains deeply, being rich in chromatin. The cells are small and, as their name implies, their function is the pro- duction of fibers that give support to the tooth. 2. The cementoblasts are flat irregular cells that fit in and adjust themselves between the fibers so as to form a single layer everywhere over the surface of DIGESTIVE SYSTEM. 165 cement corpuscle analogous to a bone corpuscle. Such an inclusion is the exception in the life history of these cells. 3. The osteoblasts are also connective-tissue cells, but cover the bony wall of the alveoli. They lie between the fibers and their function is the pro- duction of bone which anchors the supporting fibers to the alveolar wall. These cells are analogous to the osteoblasts of bone. 4. The osteoclasts are large, bone-destroying, multinucleated cells that are often called giant cells. They may also act upon and absorb the cementum and dentin. They are not constantly present in the peridental membrane but appear whenever cal- cified tissue is to be destroyed. They apply them- selves to the surface to be absorbed and by their physiological action excavate cavities in which they lie, known as Howship's lacunce. The latter may later fill in with new cementum or bone, thus leaving a permanent record of the process of absorption and repair. The absorption of the roots of deciduous teeth results from the physiological action of the osteoclasts. If for any cause, such as bacterial in- vasion, the osteoclasts fail to appear, the root of the deciduous tooth does not absorb but remains as a permanent obstruction to the developing new tooth. 5. Epithelial cells, believed by some to be remains of the enamel organ, envelop the surface of the roots and are found in both young and old teeth. Their function is not known. They appear as cords of epithelial cells that anastomose freely to form an en- veloping network, which nowhere seems to unite 1 66 NORMAL HISTOLOGY AND ORGANOGRAPHY. with the epithelium of the mucous membrane of the mouth. The structure of some of these cords re- sembles that of tubular glands, and Dr. Black has suggested that their function may be a glandular one. They not infrequently enter into the pathological conditions of the peridental membrane. Blood Supply. Usually several small blood-vessels enter the foramina at the apex of the root and pass di- rectly to the pulp. Upon reaching the pulp these ves- sels anastomose freely, form- ing an extensive blood plexus. A capillary plexus with nar- row meshes has been de- scribed between the layer of odontoblasts and the dentin, but does not penetrate the \l * latter. Both arteries and lj J veins have very thin walls and may be easily ruptured. The pulp therefore bleeds very easily when exposed. No accompanying lymphatics have been described. The peridental membrane, being a connective-tissue layer, has a very rich blood supply. Vessels enter the membrane near the apex of the root, accompanying the nerve at that place; small arterioles penetrate laterally from the Haver- sian canals of the alveolar wall, and a third supply is derived from the mucous membrane of the gum, Fig. 1 20. Showing the ar- rangement of epithelial cords or glands of the peridental membrane around the root of a central incisor (dia- gram by Dr. Black). DIGESTIVE SYSTEM. 167 vessels passing over the border of the alveolar proc- esses. This vascular condition is import ant, both in health and disease. Nerve Supply. The tooth pulp is supplied with nerve fibers from the fifth cranial nerve. Medul- lated dendrites of sensory neurons enter the pulp cavity through the apical foramen of the root. All of them lose the medullary sheath, but do so at variable distances in the pulp. The varicose den- Odontoblasts. 1 Odonto- blasts. Terminal nerve fiber. Fig. 121. Nerve termination in the pulp of a rabbit's molar, stained in methylene-blue (intra vitam): a, Odontoblasts seen in side view; b, a number of odontoblasts seen in end view, showing a termi- nal branch of a nerve fiber situated between the odontoblasts and the dentin (Huber). drites ultimately form a loose plexus immediately under the layer of odontoblasts and, therefore, practically at the periphery of the pulp. Small branches pass from this plexus to terminate between the odontoblast cells, or pass through the layer of odontoblasts, but in no case have they been traced into the dentin. The sensitive dentin is, therefore, due to an indirect irritation of these nerve endings, conveyed to the latter through the medium of the 1 68 NORMAL HISTOLOGY AND ORGANOGRAPHY. dentinal fibrils and the odontoblasts. The pulp is very sensitive to traumatic and chemical irritations, even when conveyed to it through the constituents of the dentin. It is especially sensitive to changes in temperature, heat or cold acting alike. It has no localized sensation of touch. Our knowledge of the nerve supply of the periden- tal membrane is not extensive. Both medullated and non-medullated fibers are present, the latter being a part of the sympathetic nervous system, and accompany as well as innervate the small blood- vessels. The nerve fibers enter the peridental mem- brane in the same manner and from the same sources as the blood supply, which has already been de- scribed. Attachment of Teeth. In considering the attach- ment of teeth it must be remembered that teeth are not a part of the osseous system but are dermal appendages. The phylogenetic history of this sub- ject in vertebrates is very interesting. The de- scriptive literature is extensive and many classifi- cations of the different forms of attachment have been made. Tomes, in his " Dental Anatomy/' classifies four forms of attachment: (i) by a fibrous membrane; (2) hinge-joint; (3) ankylosis; (4) in- sertion in a socket. The attachment by fibrous tissue is manifest in the scaly teeth of sharks. Each cone-shaped tooth has a flattened dermal plate. Calcified connective tissue is built into this plate, which it unites more or less fibrously to the submucous matrix of the mouth. Such teeth are practically dermal scales and have no DIGESTIVE) SYSTEM. 169 direct attachment to the bony skeleton. The hinge-joint is merely a modification of the fibrous attachment, and is found in many fishes, reaching a high degree of development in the poison fangs of snakes. The hinge is composed of connective-tissue elements. In snakes the fang has a muscular at- tachment by which the reptile is able to erect the fang. .By ankylosis is meant a direct calcified union with the bone of the jaw. Such teeth have no flattened base, but a calcified pulp which binds them firmly to the bony skeleton of the mouth. Ankylosis is confined to the teeth of certain fishes. The development of a socket is associ- ated with large teeth and a consequent strong attachment . The evolution of a socket is well represented phylogenetically in reptiles where Wiedersheim makes three classes: (i) pleurodont dentition (lacertilia) , where "the teeth are situated upon a ledge on the inner side of the lower jaw, with which they become fused basally;" (2) acrodont dentition (chameleon,) where "they lie on the free upper border of the jaw;" (3) thecodont dentition (crocodiles), where "'they are lodged in alveoli." In man all the teeth are im- bedded in well-developed alveoli of the jaw-bones. Fig. 122. Diagram illustrating the de- velopment of a socket, a, Pleurodont dentition; &, acrodont dentition; c t the- codont dentition. 170 NORMAL HISTOLOGY AND ORGANOGRAPHY. Here the function of the teeth is not only to seize and bite the food, but also to masticate it and test its quality. This change in function accounts for the heterodont dentition, which must have arisen by a modification of the simple homodont condition in which the teeth are all small, conical, and of the same size and shape. The primary arrangement of the teeth is such that those of one jaw do not usually correspond in position with those of the other, but rather with the interspaces between them. As a rule, the succession of teeth in man is nearly always reduced to two functional sets, the deciduous Dental ridge. Enamel organs. Dental ridge. Enamel organs. Neck. Dental ridge. Fig. 123. Diagram illustrating the development of the enamel organ of three teeth. teeth and the permanent teeth. Traces of an earlier set have been found, which may be spoken of as a "predeciduous" dentition, and occasionally one or more teeth appear which replace corresponding permanent teeth, and thus indicate the possibility of an extra unrecorded set. An unlimited succession of teeth takes place in nearly all vertebrates, except with mammals. Development of Teeth. The enamel of the tooth develops from the epithelium of the oral cavity. In the seventh week of fetal life the mucous epithelium covering the gums invaginates to form a dental groove. The ridge or shelf thus invaginated is called DIGESTIVE SYSTEM. 171 Epithelium. the dental ridge. Early in the third month this dental ridge pro- duces lateral proc- esses along its lin- gual side, one for each deciduous tooth. These epi- thelial processes or sacs are known as enamel organs and develop directly into the tooth enamel. A little later, during the third month, a second set of proc- esses comes from the lingual side of the dental ridge and in like manner forms the enamel organs of the permanent set of teeth. The origin of the dentin is closely associated with the enamel organs but comes from the con- nective tissue under- lying these organs. This connective tis- sue forms dental Germ for per- manent tooth. Enamel. Enamel cells. Dentin. Odontoblasts. Pulp. Fig. 124. Diagram illustrating the de- velopment of a tooth. 172 NORMAL HISTOLOGY AND ORGANOGRAPHY. papilla, which later become differentiated into the dentin and dental pulp. The developing papillae gradually become invested by the enamel organ and one by one erupt on the surface of the oral cavity either as deciduous or permanent teeth. It will be observed that the enamel organs are sac- like structures consisting of an outer and inner layer of epithelial cells. The inner layer envelops the dental papillae and is destined to form the enamel prisms. The outer layer becomes associated with an investing sheath of connective tissue, the dental sac, and serves as a temporary protection while the enamel is being formed. When the tooth erupts the outer lining of epithelial cells disappears. The tooth papillae are thus all preformed at the time of birth. They remain latent and develop regularly into the different teeth according to the table on page 144. A serious illness of a child just before their eruption may affect their healthy growth by interfering with proper nutrition, and imperfect and pitted teeth result, which often ac- counts for an early decay. THE TONGUE* The tongue is a voluntary muscular organ that occupies the floor of the mouth. In lower verte- brates the tongue is a prehensile organ. In many fishes it is covered with teeth, its function being to capture and hold prey. In frogs and toads it is covered with mucous and peculiarly modified to capture insects. In reptiles it is often bifurcated, very motile, and used to frighten an enemy. In DIGESTIVE SYSTEM. 173 woodpeckers it is barbed and clearly a prehensile organ. In man, while the organ assists in taking Fig. 125. Papillar surfaces of the tongue, with the fauces and tonsils: i, i, circumvallate papillae, in front of 2, the foramen cecum; 3, fungiform papillae; 4, filiform and conical papillae; 5, transverse and oblique rugae; 6, mucous glands at the base of the tongue and in the fauces; 7, tonsils; 8, part of the epiglottis; 9, median glosso-epi- glottidean fold (fraenum epiglottidis) (from Sappey). food, its more important function is gustatory, the taste organs being located upon its surface. For 174 NORMAL HISTOLOGY AND ORGANOGRAPHY. description the tongue may be divided into body, base, inferior surface, and dorsum. Body. This is chiefly made up of striped muscle which may be divided into intrinsic and extrinsic. A median septum divides it into two symmetrical lateral halves. Connective-tissue elements, nerves, the body of glands, and blood-vessels interlace freely with the muscle. The musculature is best studied in a beef's tongue that has been boiled, and is a subject that belongs to gross anatomy. In any section of the tongue, muscle fibers will be seen both in cross section and in longitudinal section. Base. This is the posterior wide end of the tongue that is attached to the hyoid bone. It is covered with a smooth mucous membrane, beneath which is a rich supply of lymphoid tissue. The latter con- stitutes the lingual tonsil. Mucous glands are abun- dant. Inferior Surface. This is covered with smooth mucous membrane on which open many mucous and serous glands. The surface is divided into two halves by a fibrous septum which passes to the floor of the mouth and is known as the lingual frenum. When this is abnormally short the person is said to be tongue-tied, and speech is impaired. The Dorsum. This surface is convex both from before backward and from side to side. A median depression, or sulcus, divides it into lateral halves. The sulcus apex points backward to the foramen cecum just in front of the base. This cecum is a blind pocket that marks the origin of the middle portion of the thyroid gland, and is the remnant of DIGESTIVE SYSTEM. 175 the obliterated thyroid duct. The dorsal surface is studded with three sets of papillae, to be described in detail. Papillae. i. Filiform Papillae. These are not only the smallest but by far the most numerous, and give the tongue a velvety appearance. They are arranged in divergent rows that extend outward and forward from the median sulcus. Each papilla is conical, points backward, and is covered by a thick Fig. 126. Section through two filiform papillae of tongue. layer of stratified, horny, squamous epithelium. The function of these papillae is purely prehensile. In carnivorous animals they give the tongue a rasp- like structure that serves effectually in cleaning bones. It is said that a tiger, in this way, can draw blood from a living hand. 2. Fungiform Papilla. These are less numerous, larger, and supplied with blood, which gives them a 176 NORMAL HISTOLOGY AND ORGANOGRAPHY. red color. They are most numerous at the tip and margins of the tongue. Each is like an inverted cone and has a covering of eight or ten layers of squamous epithelial cells. Many of these papillae have taste buds in their lateral walls, analogous to those to be described in the third class of papillae the circumvallate. A connective-tissue papilla oc- cupies the core of the fungiform. This core has a rich supply of blood-vessels which in fevers become congested with blood and give the dorsum a Fig. 127. Section of fungiform papilla of tongue. speckled red color, spoken of as strawberry tongue This is particularly the case in scarlet fever. 3. Circumvallate Papilla. These are by far th largest and are found just in front of the foramen cecum. They are about ten in number and are ar- ranged in the form of a letter V, with the apex point- ing backward. They resemble the fungiform pa pillae, only they are much larger. Each papilla is surrounded by a deep, narrow, circular trench or DIGESTIVE SYSTEM. 177 fossa, hence their name. The wall consists of strati- fied squamous epithelium and the core of connective tissue richly supplied with blood-vessels. From this core secondary connective-tissue papillae indent the under surface of the stratified epithelial wall. Taste Buds. These are nests of epithelial cells that lie in the lateral walls of circumvallate and Epithe- lium. pbner's gland. Fig. 128. Longitudinal section of a human circumvallate papilla (Bohm and Davidoff). fungiform papillae, and are closely associated with the sense of taste. They resemble small acorns, and are made up of columnar cells so arranged as to form a central taste canal, which in turn opens by a pore into the circumvallate fossa. Two kinds of slender epithelial cells are present, (i) tegmental or ?J& NORMAL HISTOLOGY AND ORGANOGRAPHY. cover cells, principally at the periphery of the bud, which support or ensheath (2) the gustatory or taste cells. The latter are smaller, more delicate and centrally placed, with the distal or free end bearing a small process that projects into the inner taste pore. The cells of the taste bud occupy the whole lateral wall ; that is, the base of each cell rests upon the base- ment membrane next to the connective-tissue core and the distal end extends practically to the sulcus "x.-.i^r- - .y_.- Taste buds. Fig. 129. Two foliate papillae from tongue of rabbit. of the papilla. These taste buds, as a matter of pro- tection, develop in the lateral wall rather than in the exposed dorsal surface of each papilla. The nerve fibers of the gustatory nerve are not in protoplasmic continuity with the epithelial cells of the taste buds, as is the case with the sensory cells of the olfactory region. Nerve fibers enter the taste buds and terminate in varicosities that interlace and come in contact with the gustatory cells of each DIGESTIVE SYSTEM. 179 taste bud. It is evident that a food to be tasted must first be put into solution to pass into the sulcus and stimulate the delicate processes of the gustatory cells of taste buds. Foliate Papillae. On each side of the rabbit's tongue, some distance back, can be seen a small oval patch, with diagonal grooves and ridges, resembling the side of a three-cornered file. These patches are the foliate papilla. In reality they are not papillae but alternating grooves and ridges. In transverse section, the lateral walls of the ridges will be found beset with taste buds resembling in detail those - Surface pore. Fig. 130. Section through taste bud. Fig. 131. Cells from a taste bud: a, taste cells; b, supporting cells. described in the circumvallate papillae. The rabbit, therefore, to relish his clover, should roll the leaves over these lateral patches. Glands of the Tongue. Small serous racemose glands are associated with the circumvallate pa- pillae into the fossa of which their ducts open. Glands are otherwise absent over the dorsum of the tongue. Over the other parts of the tongue both serous and mucous glands are abundantly present. Many of these are mixed serous and mucous glands. 180 NORMAL HISTOLOGY AND ORGANOGRAPHY. Blood Supply. The arteries are the lingual, which branches to form (i) the dorsal lingual artery which anastomoses freely with the tonsillar branch of the facial, and (2) the ranine artery that passes along the under surface. The veins are the ranine and the dor sails lingua that drain into the internal jugular. Nerves. These are (i) the hypoglossal, the motor nerve; (2) the lingual, from the inferior maxillary of the fifth, which is accompanied by the chorda tympani of the seventh, or facial; (3) the glosso- pharyngeal, which supplies the taste buds; (4) the internal laryngeal. Many fibers of the sympathetic system mingle with these nerves. PHARYNX. The pharynx is the common passage for both food and air. It is an expanded portion of the di- gestive tube five inches in length and with seven openings: one, the fauces from the mouth; two posterior nares; two Eustachian tubes; one to the trachea, and the orifice of the esophagus. The mucous membrane of the pharynx is lined with stratified squamous epithelium, except in the region of the posterior nares where the epithelium is ciliated. In the submucosa there is a generous supply of mucous and serous glands and lymphoid tissue. The latter is particularly abundant in the region of the posterior nares, forming in this location the pharyngeal tonsils or adenoids. In early youth the adenoids are prone to enlarge so as to obstruct normal breathing, a condition that justifies their removal. A rich supply of elastic longitudinal DIGESTIVE SYSTEM. 181 connective-tissue fibers is also present in the sub- mucosa. The submucosa is therefore capable of being greatly distended, as is the case in throat in- fections, such as diphtheria, where the congestion is so great as to interfere with respiration. The diphtheria germs and toxins are thus lifted up and walled off from the deeper structures and normal KL- Epithelium. Striated muscle. Crypt. Lymph oid nodules. Tonsillar sinus. Fig. 132. Section through the pharyngeal tonsil of man (Sobotta). blood supply. This is nature's method of eliminat- ing the disease, with the possible danger to the pa- tient of suffocation. External to the submucosa come several layers of striated muscle fibers forming the pharyngeal muscle, the description of which belongs to gross anatomy. Tonsil. The tonsils are two oval lymphoid masses imbedded in the lateral walls of the pharynx, 1 82 NORMAL HISTOLOGY AND ORGANOGRAPHY. opposite the root of the tongue and between the anterior and posterior palatine arches. This lymph- oid tissue is covered with the oral mucous mem- brane, beset with many depressions or pits known as crypts. It is along these crypts that bacteria may enter the tonsil, producing an inflammation of that organ known as tonsillitis. Stratified epithelium. Muscularis mucosa. Mucous glands in the submucosa. Circular muscle layer. Longitudinal muscle layer. Fibrous coat. Fig- I 33- Cross section of the esophagus. ESOPHAGUS. The esophagus is the part of the alimentary canal that intervenes between the pharynx and the stom- ach, and is a very muscular tube about ten inches in length. Its upper end is opposite the lower bor- der of the crycoid cartilage and the sixth cervical vertebra. The lower end or cardiac orifice is oppo- DIGESTIVE SYSTEM. jg-j site the eleventh dorsal vertebra. Two distinct constrictions are present, one at the beginning and one where it is crossed by the left bronchus. The normal distention at these points is about four- fifths inch. The wall of the esophagus may be divided into four coats: mucous, submucous, muscular, and fibrous. 1. Mucous Layer. This layer is thrown into many longitudinal folds and lined, as in the pharynx, with stratified epithelium. The tunica propria is well developed and may contain solitary lymph nodes. Tubular glands resembling those of the stomach are found in patches, particularly at the extremities of the esophagus. They are entirely confined to the mucosa and distinct from the mucous glands found in the submucosa. Their function is problematic. A muscular is mucosa is present in the esophagus just external to the tunica propria, con- sisting of longitudinally disposed smooth muscle cells. This layer becomes more prominent in the alimentary canal below the esophagus. 2. Submucosa. This layer lies just external to the muscularis mucosa and consists of loose con- nective-tissue elements, blood and lymph vessels, nerves and the bodies of mucous glands. These glands are compound racemose and are particularly abundant in the lower part of the esophagus. The ducts pass through the muscular mucosa to open on the epithelial surface. They secrete mucus for the lubrication and protection of this surface. Not in- frequently the morning vomit of mucus in chronic 1 84 NORMAIv HISTOLOGY AND ORGANOGRAPHY. gastritis comes from excessive secretion of these glands. The tissues of the submucosa are loosely held together, and sections, therefore, may tear along this layer. 3. Muscular Coat. This consists of an inner cir- cular and an outer longitudinal layer, although the fibers of each often interlace. The longitudinal layer is particularly strong, often thicker than the circular. In the upper half many striated fibers are present that are continuous with the pharyngeal voluntary muscle. The control of these fibers en- ables the dog to return food to the mouth that has passed into the upper part of the esophagus. ' In the lower half, only smooth muscle fibers are present. The longitudinal layer passes on as the longitudinal layer of the stomach and intestine, while the circular becomes the diagonal fibers of the stomach and does not pass to the intestine. Connective- tissue ele- ments interlace and strengthen the whole muscu- lature. Foods and liquids are carried along this tube by peristaltic contraction of its muscles, and in this way cattle and horses can take nourishment without lifting their heads. 4. Fibrous Coat. This consists of loose connec- tive-tissue elements, mostly white fibrous, binding the esophagus to adjacent structures. It is not well defined and often difficult to demonstrate as a distinct layer. STOMACH. The stomach varies greatly in form and position according to physiological conditions. For de- scriptive purposes it has two ends, cardiac and py- DIGESTIVE SYSTEM. 185 loric; two surfaces, dorsal and ventral; two curva- tures, greater and lesser; and two orifices, esophageal and pyloric. The most fixed point is the esophageal orifice, which is situated opposite the seventh left costal cartilage one inch from the sternal junction. The most movable portion is the pyloric end, which is situated one-half to one and one-half inches to the right of a median plane and a variable distance be- low the esophageal opening. The distance between Fig. 134. Anterior outlines of stomach. (His' model.) the two orifices is about four inches. The full length of the stomach is about ten inches, and the greatest diameter four inches, with average capacity of one quart. These dimensions are subject to great variations, The stomach wall, like that of other parts of the food canal, is made up of four layers : mucosa, submu- cosa, muscularis, and serosa. I. Mucosa. The mucous surface is uneven, due 1 86 NORMAL HISTOLOGY AND ORGANOGRAPHY. to irregular folds. The surface is beset with minute pores or circular depressions, called crypts, into which the gastric glands open. As in other parts of the food canal this layer consists of epithelium, membrana propria, and muscularis mucosa. The epithelium is simple columnar with the nucleus near the bottom of the cell, leaving a clear proximal half Crypt. Gastric glands. Muscularis mucosa. " Submucosa. Circular muscle layer. Longitudinal muscle layer. Serous coat. Fig. 135. Cross section through the wall of a stomach. to each cell that does not readily stain. The pits or crypts are lined by this same epithelium. The membrana propria has a rich supply of connective- tissue cells and extensive ramifications of blood and lymph capillaries. The muscularis mucosa is a thin muscle layer, external to the membrana propria, and consists of an inner layer of circular and an outer DIGESTIVE SYSTEM. i8 7 Crypt. Parietal cell. layer of longitudinal smooth muscle fibers. A liberal supply of connective tissue is associated with this muscle. This layer, therefore, offers resistance to an invasion of bacteria, while the muscular contrac- tion relieves pressure to the rich blood supply in the submucosa just external to it, and at the same time exerts pressure upon the gastric glands. Gastric Glands. These are widely dis- tributed and per- haps the most impor- tant structures of the mucosa. They are simple tubular, except in the pyloric region, where many of them are branched. Usually several glands open into each crypt, the latter representing a circular pit-like evagination of the epithelial surface . The wall of each gland consists of simple epithelium and two kinds of cells are present : (i) chief cells, which are by far the most numerous; these are round or cuboid cells, with central nucleus that stains blue with hematoxylin ; (2) parietal cells , which are less numerous, larger, and have a granular cytoplasm that takes the red eosin stain. They inter- Chief cell. Fig. 136. Simple tubular gland from stomach. 1 88 NORMAL HISTOLOGY AND ORGANOGRAPHY. vene with the chief cells but are placed at the pe- riphery of the glands, and communicate with the central lumen by means of a network of secreting ducts. They are most abundant in the cardiac end of the stomach and along the middle and inner third of each gland. These cells are supposed to have something to do with the se- cretion of hydrochloric acid. The cytoplasm of the chief cells contains granules of pepsinogen, which is converted into pepsin of the gastric juice. During fasting, these gran- ules accumulate, and during, or after, active secretion they become smaller and tend to disappear. The mucosa secretes a varying amount of mucus for the protec- tion of the delicate epithelial sur- face . In many forms of indigestion , and particularly in poison cases, the mucus secretion is very exten- sive and serves to keep the irrita- ting stomach contents away from the epithelial lining. The excess of mucus can be removed by stom- ach lavage. The chief difference between the pyloric and cardiac regions of the stomach is found in the mucosa. (i) The crypts in the cardiac end are shallow, while in the pyloric end the crypts frequently extend half way through the thickness of the mucosa. (2) The gastric glands Fig. 137. A num- ber of fundus glands from the fundus of the stomach of young dog, stained after the chrome - silver method, showing the system of fine canals surrounding the pa- rietal cells and com- municating with the lumen of the glands (Huber). DIGESTIVE SYSTEM, 189 are longer than the crypts in the cardiac end; towards the pyloric end the glands become shorter, tortuous, and pressed closely against the muscu- laris mucosa. Many of the pyloric glands are branched. (3) The parietal cells are numerous in Fig. 138. From a section through the junction of the human esophagus and cardia (Bohm and Davidoff). the cardiac region and practically absent in the pyloric. The pyloric mucosa, in this way, comes to resemble that of the small intestine. In addition an occasional villus or Brunner's gland may be found in the pyloric end. 2. Submucosa. The submucosa in all mucous membranes is highly vascular. Besides blood and lymph there is an abundant supply of connective- 1 90 NORMAL HISTOLOGY AND ORGANOGRAPHY. Epithelium of fold be- tween gas- tric crypt. Gastric crypt. tissue elements, largely elastic fibers, connective- tissue cells and fat cells. Nerve cells and nerve fibers, known as Meisner's plexus, are found here and in the submucosa through- out the alimentary canal. 3. Muscular is. This consists of smooth muscle and may be divided into three layers : (a) an inner sheath where the fibers run ob- liquely; this sheath is continuous with the circular layer of the esophagus; (ft) a middle circular layer which is con- tinued as the circu- lar layer of the in- testine; (c) an outer longitudinal layer continuous with the F i g i 39 . From vertical section longitudinal layer of both esophagus and intestine; nerve cells and nerve fibers form a plexus between the longitudinal and circular muscle of the whole alimentary tract, which is known as the plexus of Auerbach. 4. Serosa. This consists of a thin layer of fibrous tissue covered by simple pavement epithelial cells DIGESTIVE SYSTEM. Chief cell. - Lumen. Fig. 140. Section through fundus of human stomach in a condition of hunger (Bohm and Davidoff) Lumen. L. Mucosa. Parietal cell. Fig. 141. Section through fundus of human stomach during digestion (Bohm and Davidoff). 1 92 NORMAL, HISTOLOGY AND ORGANOGRAPHY. Papilla. Stratified epi- thelium. Submucosa. musde. and bound down to the muscularis by delicate fibrous septa. It is really a part of the peritoneum. The Stomach in Ruminants. Ruminants (ox, sheep, goat, camel, llama) all have four compart- ments for the reception and maceration of food; rumen, reticulum, oma- sum, and abomasum. The first three are morphologically dis- tensions and modifica- tions of the lower end of the esophagus, while the abomasum alone corresponds to the stomach in other ani- mals and needs, there- fore, no further de- scription here. The Rumen is by far the largest compart- ment, reaching the serous coat. enormous capacity of forty gallons in the ox. It is divided into four sac-like pouches by two muscular band-like girdles whose obvious func- tion is to contract on the contents and render assistance in the mechanical process of returning food for further mastication. Its mucous mem- brane is covered with pointed papillae 3 to 9 mm. in length, excepting where the muscular pillars are most prominent. Its epithelial lining is stratified, consisting of eight to twelve lavers of cells, the Fig. 1410. Cross section through rumen of ox. DIGESTIVE SYSTEM. 193 inner ones being very scaly and presenting a fibrous- like structure. The submucosa is vascular, with a scattering of small mucous glands, but these form no digestive secretion. Strands of smooth muscle fibers extend into the core of each papilla of the mucosa, also a net- work of blood and lymph vessels. There are two smooth muscle layers, an inner cir- cular and an outer longitudinal, the inner layer being much the heavier. Like the esophagus, these layers show a fine cross-striation, and there is no doubt but that these layers assist in the mechan- ical process of returning the food to the mouth for a more thorough mastication. The serosa is unusually heavy and easily detected in microscopic sections. The Reticulum is the second gastric reservoir and is the smallest compartment. Its mucous surface presents a honeycomb appearance when seen from the inside, hence its name. The muscular tunic is thin, otherwise the other layers are analogous to those found in the rumen. The Omasum, or third compartment, is only a little larger than the reticulum. The mucous mem- brane is extensively folded to form large leaves ex- tending the length of the organ. Between the large leaves are smaller leaves, and again a third and a fourth series, making altogether about 400 laminae of variable sizes. These leaves bear horny papillae, being large and pointed toward the reticu- lum end and small and warty toward the omasum end. These leaves are lined with eight to twelve layers of scaly, tesselated epithelial cells, forming a rough gritty surface. A liberal supply of smooth muscle is present in the center of each leaf, also a 194 NORMAL HISTOLOGY AND ORGANOGRAPHY. network of blood- and lymph- vessels. The physio- logical action of this muscle causes adjacent leaves to rub against each other, producing a trituration of the retained food. The muscular coat is fasciculated and thin and composed of two layers that pass in dif- ferent directions. The serosa presents nothing differ- ent from the general structure of the peritoneal lining. SMALL INTESTINE, The small intestine is about twenty-four feet long, and is divided into duodenum, ten inches : jejunum and _ Villus. - Crypt of Lieberkiihn. Muscularis mucosa. r - Circular muscle layer. Longitudinal muscle layer. Serous coat. Fig. 142. Cross-section of small intestine. ileum, respectively, two-fifths and three-fifths of the remainder. About three feet from the lower end of DIGESTIVE SYSTEM. 195 Fig. 143. Portion of the wall of the small intestine, laid open to show the valvulae conniventes (Brinton). the ileum Meckel's diverticulum may be present, rep- resenting the last embryonic closure of the intestine. The intestine has the same number of layers as the stomach. The muscularis, however, consists of but two strata, an inner circular and an outer longitudinal, i. Mucosa. The mucosa is lined by simple co- lumnar epithelium in which many goblet cells are present, par- ticularly in the deeper folds. Themembrana propria and muscu- laris mucosa are iden- tical with those des- cribed in the stomach. The mucous surface of the small intestine is much increased by means of folds, of which there are three mechanisms: valvulcB conni- ventes, villi, and crypts of Lie- berkuhn. (a) ValvulcB Conniventes. These are con- centric, trans- verse, crescen- tic folds of the mucosa, that usually form two-thirds of a circle, although occasionally one forms an entire circle or even a spiral. These valves are two or three Fig. 144. Mucous membrane of the jejunum, highly magnified (schematic): i, i, Intestinal villi; 2, 2, closed or solitary follicles; 3, 3, orifices of the follicles of Lieberkiihn (Testut). 196 NORMAL HISTOLOGY AND ORGANOGRAPHY. inches long, about one-third inch broad and one- eighth inch thick. The inner surface of the small in- testine is thus thrown up in a series of shelves. This mechanism has an analogue in the typhlosole of the earthworm and the spiral valve of some fishes. (6) Villi. These are tongue-like elevations of the mucosa one-thirtieth to one-fortieth inch in height, and barely visible to the naked eye. They are found on both sides of the valvulae conniventes and on the general surface of the mucosa. Collec- tively they give the surface a velvety appearance. The villi are most numerous in the upper part of the intestine, where they number fifty to eighty to the square inch. They are longer but more slender and less numerous in the ileum, where they number forty to sixty to the square inch. Their total num- Fig. 145. a t Cross section of a villus; b, cross section of crypt of Lie- berkiihn. ber in the small intestine is estimated at 4,000,000. Each villus has a lining of simple columnar epithelium which covers a connective-tissue core. A few smooth muscle fibers enter this core from the muscularis mucosa. In addition there is a rich blood supply and a central lymphatic duct. The latter is a part of the lymphatic system of the intestine known as lacteals because of the milky SYSTEM. 197 lymph they contain after each meal. The villi de- velop as invaginations of the mucosa and are exclu- sively confined to the small intestine. (c) Crypts of Lieberkuhn. These are sometimes spoken of as intestinal glands. They consist of pits or evaginated diverticulce of the mucous epithelium that open as pores between the bases of the villi. The bottom of these crypts rests against the muscu- laris mucosa. Goblet cells are particularly numer- ous in the epithelial lining of their walls. Crypts -Central chyle-vessel of villas. .yle-vessel. Artery. -Mucosa. l-Muscularis mucosse. -Submucosa. Plexus of ' lymph-ves- sels. . Circular mus- ~~ cular layer. Plexus of ., lymph-ves- .%% sels. .-.>": V~. 7 " Long, muse. " layer with serous coat. Fig. 146. Schematic transverse section of the human small intestine (after F. P. Mall). are also present in the large intestine and are analo- gous to the shorter crypts of the stomach into which the gastric glands open. Snlitarv T.vm. __. Region of the bodies of the gas- tric glands. - -Muscularis Fig. r 5 2 - Section through fundus of cat's stomach. The blood-vessels are injected (Bohm and Davidoff). they branch into a fine capillary network which in the small intestine penetrates to the core of the villi. Other branches from this plexus supply the inner portion of the circular muscle layer. The veins lie side by side with the arteries. Often two veins accompany one artery. The blood drains into the superior and inferior mesenteric veins; the inferior joins the splenic, and the latter unites with DIGESTIVE SYSTEM. 205 the superior to form the portal vein. This blood thus ultimately passes through the liver. Lymphatics of the Alimentary Canal. The lym- phatics begin in the mucosa just beneath the epithe- lium. In the small intestine they begin with the cen- tral lymph vessel of a villus. These vessels form a network in the deeper portion of the mucosa and then pass through the muscularis mucosa to form Fig. 153. A portion of the plexus of Auerbach from stomach of cat, stained with methylene-blue (intra vitam), as seen under low mag- nification (Huber). an extensive loose plexus in the submucosa. Coarser lymphatic vessels lead from this plexus through the muscularis, where branches are received from a lymphatic plexus located between the two muscle coats. The solitary lymph nodes of the mucosa contain no lymphatics, but are encircled at their periphery 206 NORMAL HISTOLOGY AND ORGANOGRAPHY. by an extensive lymphatic network. The same is true of the lymph nodules in Peyer's patches. Nerve Supply of the Alimentary Canal. The chief nerve supply of the alimentary tract consists of sym- pathetic neurons whose nerve cells form the centers of two plexuses, (i) that of Auerbach, situated between the two layers of the muscle coat, and (2) that of Meissner in the submucosa. The latter con- tains fewer ganglia and finer fibers. The numerous small sympathetic ganglia of each plexus are united by small bundles of non-medullated nerve fibers in which a few medullated nerve fibers are present. From these plexuses the nerve innervation extends to the glands and epithelial cells of the mucosa, and to the muscularis to end in small varicosities about the smooth muscle cells. While the nerve innervation is not under will con- trol it is capable of being stimulated by cerebro- spinal nerves. Medullated nerve fibers from this system have been traced to terminal end baskets surrounding cell bodies of many of the sympathetic neurons of these plexuses* CHAPTER V. DIGESTIVE GLANDS. SALIVARY GLANDS. i. Parotid Gland (serous gland). This is a com- pound tubular gland, the largest of the salivary glands, situated in the parotid recess at the side of the head below and in front of the ear. It is a triangular mass that varies in weight from one-half ounce to one ounce or more. Its three surfaces are desig- nated as superficial, anterior and posterior. The gland is divided into lobes and lobules and inter- laced with connective-tissue elements from the parotid fascia that invests it. The parotid y or Stenson's duct, measures from one and one-half to two and one-half inches in length and one-eighth inch in diameter. It runs forward across the masseter muscle, passes around the anterior border of this muscle, pierces the buc- cinator, then forward a short distance to open on the inner surface of the cheek opposite the crown of the second upper molar. This duct is subject to injury in facial wounds or operations. The gland is an epithelial organ and consists of the excretory duct, interlobular , intralobular , and in- tercalated ducts, and the distal convoluted tubules or alveoli. The excretory duct (Stenson's) is lined by stratified epithelium near its end where it opens on 207 2 o8 NORMAL HISTOLOGY AND ORGANOGRAPHY. the mucous surface of the cheek. The rest of the duct is lined by two layers of cubical epithelium, which is invested by a firm fibrous coat or tunica propria. The interlobular ducts lie between the Acini. Fig. 154. Section through salivary gland of rabbit, with injected blood-vessels (Bohm and Davidoff). lobules and have much the same histology as the excretory duct, excepting the finer branches where the epithelium becomes simple cubical. The intra- lobular ducts are found inside the lobules and have a simple layer of tall cylindrical cells whose cytoplasm DIGESTIVE GLANDS. 209 Intralobular duct. shows a distinct longitudinal striation. The inter- calated pieces, on the other hand, are clothed by a single layer of flat, slender, often spindle-shaped cells. The epithelial lining of the acini consists of typical serous-gland cells. This is a single layer of irregular columnar or cubical cells with nuclei situated near their basal portion. When at rest the cytoplasm is filled with zymogen granules which are used up and largely disappear during the process of secretion. Mumps is a specific disease of this gland, more technically known as parotitis. 2. Sublingual Gland (mucous gland) . This is really a collection of compound tubulo-alve- olar glands. It is an elongated flattened mass one and one-half to one and three-quarter inches in length, situ- ated in the floor of the mouth, one on each side of the median plane, and limited laterally by the ramus of the mandible. Its excretory ducts, from ten to twenty in number, open separately on the summit of papillae visible to the naked eye, which are situated just laterally to the base of the frenum of the tongue. The duct system of the gland is similar to that of the parotid, with the exception of the intercalated piece, which is absent. The alveoli are less tubular than those of the parotid and are lined by simple A Iveolus. Fig. 155. From the parotid gland of man. 210 NORMAL HISTOLOGY AND ORGANOGRAPHY. columnar epithelium with the nuclei situated at the base of the cells near the basement membrane. These are the chief cells of the alveoli and they se- crete mucus, which is first stored up in the cyto- plasm in coarse granules known as mucigen, A second form of cells, less numerous, is found singly or in groups in the periph- ery of the alveoli and in close apposition to the basement membrane. On account of their shape and position they are called parietal cells, cres- cents of Gianuzzi, or demi- lunes of Heidenhain. They are finely granular, stain red with eosin, and are looked upon by some Fig. 156. Model of a small as secreting a serous fluid. n:%t%Sn g e U s al o g f la Heid- There are three theories enhain are more deeply shaded a Q their USC ! ( I ) They (Maziarski, " Anatomische Hefte, " 1901). may be considered as worn-out chief cells that have been crowded to the basement membrane after too active a secretion. (2) They may be considered as latent undeveloped cells ready to take the place of mucous cells that get lost in the process of active secretion. (3) They may be considered as normal active cells contributing constantly to the salivary secretion in a way that is at present unknown. DIGESTIVE GLANDS. 211 3. Submaxillary Gland (mixed gland). In man this gland is composed of tubules having a serous secretion and similar to those of the parotid gland, and tubules with alveolar enlargements like the sub- lingual gland that secrete mucus. Its histology, therefore, would be a repetition of what has already been described in the parotid and sublingual glands. The submaxillary gland is next in size to the pa- rotid, which it resembles in color and lobulation. It Parietal cell. Acinus. Parietal cell. Intralobular duct. Interlobular duct. Fig. 157. Section from the human submaxillary gland is placed against the inner surface of the angle of the lower jaw in close proximity to the parotid gland. It has a complete firm capsule derived from the cer- vical fascia. Connective- tissue elements from the capsule ramify between the lobes and lobules of the gland. The submaxillary, or Wharton's duct, is about two inches long, passes forward beneath the mylohyoid muscle, then along the inner side of the sublingual gland to open on the summit of a small papilla situ- 212 NORMAL HISTOLOGY AND ORGANOGRAPHY. ated in the floor of the mouth at the side of the fre- num of the tongue in close proximity to its fellow of the other side. The submaxillary gland of the rabbit is a serous gland; that of the dog and cat is mucous. The accessory salivary glands are numerous small glands of the mouth known according to their Jocation as la- bial, palatine, and lingual. They are mostly glands with mucous secretions. Branched tubular glands with serous Fig. 158. A number of alveoli from the submaxillary gland of dog, stained in chrome-silver, showing some of the fine intercellular tubules (Huber). secretion occur in the tongue, their ducts opening into the depressions of the circumval- late papillae. PANCREAS. The pancreas is a lobulated compound racemose gland analogous in its structure to the salivary glands. It is situated transversely across the pos- terior wall of the abdomen, so deep in the body and so closely associated with other organs that but little is known of its diseases. Its length varies from six to eight inches, its breadth one and one-half inches and thickness from one-half to one inch. The right extremity, or head, rests in the concavity of DIGESTIVE GLANDS. 213 the duodenum and the other end touches the spleen. It lies behind the peritoneum and, unlike the salivary glands, it is not enclosed in a fibrous capsule, and is therefore looser and softer in its texture. The pancreas develops as two evaginations of the alimentary canal and embryologically, therefore, Inter- mediate tubule. Inner granular zone of secretory cells. Fig. 159. From section through human pancreas (sublimate) (Bohm and Davidoff). has two ducts. One of these, the pancreatic duct, or duct of Wirsung, opens jointly with the bile duct on the summit of an elevated papilla, situated at the inner side of the descending portion of the duodenum. This duct extends the length of the pancreas and receives in its course the short ducts from the various lobes composing the gland. 214 NORMAL HISTOLOGY AND ORGANOGRAPHY, The second duct, or duct of Santorini, loses its connection with the alimentary canal and comes to open into the duct of Wirsung. It is a very short tube, inferior in position and secondary in impor- tance. Histologically, the ducts are lined by a mucous membrane of simple columnar epithelial cells that are morphologically continuous and analogous with the epithelial cells of the intestine. This mucous Connective tissue. r.. Centro-acinal cell. Secretory cell. Intermediate duct. Fig. 1 60. Scheme showing relation of three adjoining alveoli to excretory duct, illustrating origin of centro-acinal cells (Bohm and Davidoff). membrane is ensheathed by a coat of connective- tissue elements, fibers and cells, all of which are associated with more or less fat. The acini resemble in form and size those of the salivary glands. The parietal cells are absent. The chief cells are columnar, the nucleus near the base of the cell, and the cytoplasm loaded with zymogen granules. During physiological activity these gran- ules are greatly reduced. In addition, centro-acinal DIGESTIVE GLANDS. 215 cells are present. These are smaller and flatter than the chief cells and occupy a central position of many acini. They represent an invagination of the neck of the acinus and are best understood by re- ferring to Fig. 1 60. Areas of Langerhans.- These are oval cell masses that measure 0.2 to 0.3 mm. They are found in the lobules of the pancreas, always associated with the connective tissue but having no connection with the Pancreatic duct. Acinus. Blood capil- laries. Connective tissue. Area of Langerhans. Fig. 161. Section from an injected pancreas of the dog. pancreatic tubules. The areas are surrounded by a rich supply of coarse capillary blood-vessels. The individual cells are epithelioid, smaller than those of the acini, arid finely granular. In many respects they resemble the liver cells. It is believed that the secretions from these cells is absorbed by the blood and modifies the distribution and elimination of sugar. The areas thus have a compensatory rela- tion to the liver and may do in part the work of that organ. A marked disturbance in these areas has 2l6 NORMAL HISTOLOGY AND ORGANOGRAPHY. been observed in diabetes, but whether a cause 01 consequence is not known. LIVER. The liver is a large compound tubular gland whose terminal ducts anastomose. In this respect it differs from any other gland in the body. It develops as a ventral evagination of the intestinal wall and in close proximity to the origin of the pancreas, so that in the adult the ducts of these two organs have a Non-peritoneal Tuber omen- tale. ^isis Impressio celica. Impressio pylorica. Fig. 162. Posterior and inferior surfaces of the liver (Nancrede). common opening at the apex of a papilla, as already mentioned in the description of the pancreas. If an obstruction occurs at this opening, it is possible for the pent-up bile to invade the pancreas. The ven- tral liver diverticulum quickly bifurcates to form respectively the right and left lobes of the liver. By repeated divisions the bile system of the organ is built up, forming an elaborate anastomosis of the finer bile capillaries. It follows, therefore, that the liver cells are genetically related to the pancreatic DIGESTIVE GLANDS. 217 cells and sister cells of the columnar epithelium of the alimentary tract. The fully developed liver consists of five lobes: right lobe, left lobe, quadrate, caudate, and spigelian lobes. Five fissures: umbilical, ductus venosus, transverse, gall bladder, and vena cava fissures. Five ligaments : ligamentum teres (remnant of the umbilical vein), falciform, coronal, and two lateral ligaments. Interlobular -vein. p Interlobular bile duct. Intralobular bile duct. Liver cells. jr- Intralobular vein. Sublobular vein. Fig. 163. Diagram of liver lobule. The description of these structures belongs to gross anatomy. The liver is the largest gland in the body, weigh- ing from three to four pounds, or one-fortieth the weight of the whole body. It measures ten to twelve inches in its transverse diameter, six to seven inches in its antero-posterior, and about three inches thick at the back part of the right lobe, which is the 2l8 NORMAL HISTOLOGY AND ORGANOGRAPHY thickest part. This heavy organ is held in position not only by its ligaments but by the abdominal pres- sure, and also by the connective tissue of the vena cava, which forms a dorsal fissure between the right and left lobes. The organ is enclosed in a firm connective-tissue capsule (capsule of Glissori) , which is very dense over the lower surface in the region of the fissures, par- Interlobular vein. Blood capillaries. Intralobular vein. Cord 0} liver cells. Fig. 164. Injected blood-vessels in liver lobule. ticularly the transverse, where the blood-vessels and bile duct enter. Septa from this capsule ramify between the lobes and lobules, and finer branches interlace between the liver cells, giving everywhere support and consistency to the organ. Here, as in every organ, blood-vessels, lymph- vessels, and fat are associated with this connective-tissue fabric. The capsule is closely invested with peritoneum, except- ing a circular area bounded by the coronal ligament, DIGESTIVE GLANDS. where the capsule, and therefore the liver, is in direct apposition with the lower surface of the diaphragm. The blood supply of the liver consists of the hepatic artery, and a branch of the celiac axis, and the portal vein, formed by the junction of the splenic and supe- rior mesenteric veins. The portal vein is by far the larger vessel. These vessels accompany the bile- j, Intralobular vein. - Branch of Portal vein. " Bile duct. Branch of he- Patic artery. Fig. 165. Section through liver of pig, showing chains of liver-cells (Bohm and Davidoff). duct, and wherever one branches the others do, even to the finer terminations between the liver lobules. The duct, vein and artery form the ventral border of the foramen of Winslow. Their relation at this place is, bile duct to the right, artery to the left, and vein between and behind the other two. This rela- tion is an important one in the surgery of these parts. 220 NORMAL HISTOLOGY AND ORGANOGRAPHY. The Portal Canal. This consists of an artery, a bile duct, and a vein, with accompanying lymphatics and connective tissue. Sections of the portal canal may be found between the liver lobules where the vessels are small, or between the liver lobes where the structures are large. The bile duct can be recog- nized by the columnar simple epithelium and the artery and vein by their respective histology. In Boundary - of lobule. Fig. 1 66. Reticulum (Gitterfasern) of dog's liver (gold-chlorid method) (Bohm and Davidoff). pathological sections, where the lobules can no longer be recognized, the portal canal usually re- mains patent and is therefore a valuable criterion in the identification of this tissue. The common bile duct is formed by the junction of the hepatic and cystic ducts at the mouth of the transverse fissure, and passes downward anterior to DIGESTIVE GLANDS. 221 the foramen of Winslow to open into the descending part of the duodenum three and one-half to four inches beyond the pylorus. It passes obliquely through the intestinal wall, where it is joined by the pancreatic duct, and opens by a common orifice on the bile papilla, as already described. The common bile duct is about three inches long and one-quarter inch in diameter. The histology of the bile ducts resembles that of the gall bladder. There is on the inside a mucous mem- brane clothed with simple columnar epithelium rest- ing upon a base- ment membrane. Lymphatics. \\ Smooth muscle cells are found in tl/ vw ^ X \W&%*&& Bile duct. the membrana propriaof themu- cosa. The sub- ^ mucosa is a nar- row Vascular layer Fig. 167. Section of portal canal of liver. composed of con- nective-tissue elements. The muscularis consists of an inner circular layer and an outer longitudinal layer of smooth muscle. On the outside is a strong connective-tissue coat whose fibers are continuous with the capsule of Glisson. The passage of bile into the intestine is not a pas- sive but an active process. The smooth muscle of the bile duct contracts in a peristaltic manner, and the bile is thus expelled into the intestine, periodi- cally, in jets. This activity is normally under con- trol of the nerves, largely a reflex action of the sym- 222 NORMAL HISTOLOGY AND ORGANOGRAPHY. pathetic nerves associated with the intestine. If there is an obstruction to the free passage of bile, as in the passage of a calculus, the musculature of the ducts contracts violently and spasmodically, giving rise to the characteristic pain of plain muscle con- traction described on page 94. If the obstruction occurs in the hepatic or the common duct, the liver becomes saturated with bile which is absorbed by the blood, and jaundice follows. If the obstruction is in the cystic duct the liver does not suffer and there is no jaundice. Smaller Excretory Bile Ducts and Gall Bladder- Small bile ducts of the liver begin within the liver lobules, where they form a complex system of anas- tomosing channels or tubes called bile canaliculi. Interlobular Ducts. The bile canaliculi unite to form interlobular ducts that lie between the liver lob- ules. These unite into larger and larger ducts and converge to pass out through the transverse fissure, where five or six ducts are found. The latter unite into two short main ducts that drain respectively from the right and left lobes of the liver. The hepatic duct is formed by the union of the two main ducts at the bottom of the transverse fissure and thus receives the bile from the whole liver. It is from one to one and one-quarter inches in length and one-quarter inch in breadth, and extends downward from its origin, taking an irregular course, to its junction with the cystic duct, which unites with it to form the common bile duct. The gall bladder, with its cystic duct, is an evagina- tion of the bile duct. The gall bladder is a pear- DIGESTIVE GLANDS. 223 shaped receptacle for the retention of bile, and has a fundus, body, and neck. It is usually about three inches in length and one to one and one-quarter inches in diameter with a normal capacity of one to one and one-half fluidounces. Structurally it has an outer coat of perito- neum, a middle coat of connective -tissue elements, with a liberal mixture of smooth muscle fibers, and an inner coat of mucous membrane raised into folds and covered with simple columnar epi- thelium. The cystic duct begins at the neck of the gall bladder and extends downward to its junc- tion with the hepatic duct, with which it Fig. 1 68. Portion of gall bladder and bile ducts: i, Cavity of gall bladder; 2, cavity of calyx; 3, groove separating the calyx from the bladder; 4, promontory; 5, superior valve of calyx: 6, cystic canal; 7, common bile duct; 8, hepatic duct (Testut). forms an acute angle. It takes an irregular course and is from one and one-quarter to one and one-half inches long; therefore longer than the hepatic duct, but only about one-half its diameter. Liver Lobules. These are minute units of the liver about the size of a pinhead. They are cylindrical 224 NORMAL HISTOLOGY AND ORGANOGRAPHY. or irregularly polyhedral in shape, about 2 mm. in length and i mm. in breadth. Their arrangement is quite irregular except just beneath the capsule where they usually lie with their apices toward the surface. Each lobule has a connective-tissue in- vestment in which the finer branches of the portal Blood capil- laries. - - Cord of he- patic cells. Interlobular vessel. Fig. 169. Injected blood-vessels in liver lobule of rabbit (Bohm and Davidoff). canal ramify. This investment is particularly dense in the pig, which renders the organ in this animal very fibrous and tough, quite unfit for the market. In certain chronic liver diseases the same condition obtains, when the organ is spoken of as the " hob- nailed'' or " nutmeg" liver. DIGESTIVE GLANDS. 225 In the center of each lobule is a blood-vessel known as the intralobular vein, while the small veins be- tween the lobules are called the interlobular veins. These two sets of veins are connected by irregular blood capillaries that radiate from the periphery of the lobule to its center. They are typical capillaries whose walls consist of but a single layer of flattened 3^7^- Intralobular vein. Fig. 1 70. Human bile capillaries. The capillaries of one lobule are seen to anastomose with those of the adjoining lobule (below, in the figure) (chrome-silver method) (Bohm and Davidoff). endothelial cells. The blood in the portal vein passes to the interlobular veins, then through the capillaries to the intralobular vein. The latter opens into sublobular veins which unite to form the hepatic veins, and these in turn open into the vena cava just below the diaphragm. The arterial blood of the hepatic artery supplies the connective tissues, 15 Fig. i 7 i.-Schematic diagram of hepatic cord in transverse sec- tion. At the left the bile capillary is formed by four cells, at the right 226 NORMAL HISTOLOGY AND ORGANOGRAPHY. the walls of the bile ducts and blood-vessels, and doubtless some of this arterial blood finds its way into the liver capillaries where it blends with the venous blood from the portal vein. From the center of each lobule toward its periph- ery irregular strands of liver cells radiate ana freely anastomose with Davidoff). each other, as well as in- terlace between the blood capillaries. These are called hepatic cords and con- sist of double irregular rows of liver cells. The cords constitute the bile capillaries and unite at the periph- ery of the lobule with the bile ducts of the portal canal, situated between the lobules. The bile capillaries, there- fore, are very fine tubes lying between the liver cells that constitute its walls. These tubes anasto- mose freely with each other and are the terminal endings of the bile passages or its secreting portions. The liver, as a Bile capillaries. Fig. 172. From the human liver, showing the beginning of the bile ducts (chrome-silver) (Bohm and Davidoff). PLATE V. DIGESTIVE GLANDS. 227 Intercellular bile duct. Fig. 173. Diagram of liver cells, show- ing bile passages. gland, thus differs from all other glands (i), that the secreting tubules anastomose, and (2) that the wall of the secreting portions consists of but two cells. The liver cells have no Cell Wall. Intracellular bile passage. They are large poly- hedral or irregular epithelial cells, con- taining sometimes two, but usually one, nucleus. The cyto- plasm is granular, containing bile drops and vacuoles. The chief function of these cells is twofold : (i) to secrete bile into the bile capillaries, and (2) receive and contribute sugar to the blood capillaries. In junction with this it is affirmed that definite cyto- capillary. plaswiic or intracellu- lar channels exist, particularly for the passage of bile. These channels end in minute dilatations within the cells, from which finer passages lead to and arborize around the nucleus. According to some investigators, the finer passages may penetrate the nucleus and are then called intranuclear canals. Intercellular bile duct. Intracellular bile passage. Blood capillaries. Fig. 174. Diagram of liver cells, showing bile ducts and blood capillaries. 228 NORMAL HISTOLOGY AND ORGANOGRAPHY. Fig. 175. From preparation from the liver of a rabbit, showing the so-called stellate cells of Kupffer: a, Stellate cells; b, liver cells (Huber). Fig. 176. Part of a section through liver lobule from dog, showing stel late cells (Bohm and Davidoff). DIGESTIVE GLANDS. 2 29 Whether this intracellular system is an artifact due to manipulations or a normal condition is at present unsettled. In either case the liver cells play a deli- cate role, and a slight functional disturbance may allow the bile to escape to the blood capillaries, with jaundice as a natural sequel. In such a case there may or may not be any pain depending on the pres- ence or absence of an obstruction in the bile duct. Stellate Cells of Kupffer. These are uniformly distributed in the lobules. The cells are irregular, elongated, and end in two or three pointed projec- tions. They are smaller than the hepatic cells and are seen only after a special method of treatment. According to Kupffer these cells belong to the endo- thelium of the intralobular blood capillaries, and possess a phagocytic function. Lastly, each lobule is interlaced by a fine reticular connective fabric that comes from the connective- tissue investment of the lobule. This gives support and consistency to the lobule. Lymphatics of the Liver. These may be divided into (i) the interlobular lymphatics, which accom- pany and in some places surround the blood-vessels, and (2) sub peritoneal lymphatics on the surface of the organ which in the upper portions communicate through the ligaments with the thoracic lymphatics. Nerves of the Liver. The liver receives medullated fibers from the left pneumogastric and non-medul- lated fibers from the solar plexus. The nerves reach the organ between the two layers of the small omen- turn and accompany the portal canal, therefore enter the liver at the transverse fissure. The sympathetic 230 NORMAL HISTOLOGY AND ORGANOGRAPHY. fibers innervate the walls of the blood-vessels. The medullated pass to the liver lobules where they lose the medullary sheath and then accompany the he- patic cords or bile capillaries to ramify ultimately between and around the liver cells. CHAPTER VI. ORGANS OF RESPIRATION AND THYROID GLAND. The organs of respiration comprise the larynx, trachea, bronchi and lungs. More than forty per cent, of all deaths are directly due to diseases of this tract, which renders a thorough knowledge of this system of primary importance. These organs de- velop as a ventral median outgrowth of the fore-gut, and the mucous epithelium is therefore derived from the entoderm. The primitive connection with the alimentary canal is maintained in the adult, the upper extremity of the larynx opening on the anterior wall of the pharynx. THE LARYNX. The larynx in the male averages 44 mm. in length, 43 mm. transverse diameter, and 36 mm. antero- posterioF diameter. In the female these dimensions are 36 by 41 by 26 mm. It is a cartilaginous mus- cular tube that contains the two vocal cords, the latter being transverse folds of mucous membranes. In the wall are three single symmetrical cartilages, the thyroid, cricoid, and cartilage of the epiglottis; and three pairs, namely, two arytenoids, two corniculce laryngis (cartilages of Santorini), and two cuneiform, making in all nine pieces. The two last pairs are very 231 232 NORMAL HISTOLOGY AND ORGANOGRAPHY. small, while only the thyroid and cricoid are visible on the front and sides of the larynx. The cartilage of the epiglottis, the arytenoids, the corniculae laryn- Fig. 177. Articulations and liga- ments of the larynx, anterior view: A, Hyoid bone, with a its greater, and a' its lesser cornua; 1-5, liga- ments; 6, lateral cricothyroid artic- ulation; 7, junction of cricoid and trachea (Testut). Fig. 178. Articulations and lig- aments of the larynx, posterior view: A, Hyoid; B, thyroid, with b and b f its cornua; C, cricoid; D, arytenoids; E, cartilages of San- torini; F, epiglottis; G, trachea; 1-6, ligaments; 2, opening for su- perior laryngeal artery; 7, s junc- tion of trachea and cricoid (Tes- tut). gis, are of the elastic or yellow fibrous variety and do not tend to ossify with age. The rest are composed of the hyaline cartilage, which tends to ossify with age. Many pairs of muscle control the manipulation of ORGANvS OF RESPIRATION. 233 these cartilages, regulating the vocal cords and modulating the voice. The mucous membrane of the larynx is continuous Glands in false vocal cord. Stratified pavement .._ epithelium of true \ vocal cord. Stratified ciliated col- umnar epithelium. Glands. Muscle. Muscle. Fig. 179. Vertical section through the mucous membrane of the human larynx (Bohm and Davidoff). with that of the mouth and particularly sensitive about the upper part above the glottis. This mu- 234 NORMAL HISTOLOGY AND ORGANOGRAPHY. cous membrane is covered in the greater part of its extent with stratified columnar ciliated epithelium. The cilia are found higher up the front wall than on each side, reaching in the former to the base of the epiglottis and at the sides to a point just above the false vocal cords. Above these points the epithelium is stratified squamous, like that of the pharynx. Upon the true vo- cal cords the epi- thelium is also stratified squa- mous. Mucous glands are found everywhere in this membrane but are particu- larly abundant upon the epiglot- tis. The membrana propria, on which the epithelial cells rest, is not only very vascular but has a rich supply Fig. 1 80. Diagram to illustrate the thyro-arytenoid muscles; the figure repre- sents a transverse section of the larynx through the bases of the arytenoid carti- lages: Ary, arytenoid cartilage; p.m, proc- essus muscularis; p.v, processus vocalis; Th, thyroid cartilage; c.v, vocal cords; Oe is placed in the esophagus; m.thy.ar.i, in- ternal thyro-arytenoid muscle; m.thy.ar.e, ex- ternal thyro-arytenoid muscle; m.thy.ar.ep, part of the thyro-ary-epiglottic muscle, cut more or less transversely; m.ar.t, transverse arytenoid muscle. (Redrawn from Foster.) of elastic fibers and other connective-tissue elements. It is this tis- sue that becomes edematous and greatly swollen in infections, such as diphtheria. This is nature's method of eliminating the disease, with, however, the accompanying danger of suffocation. The vocal cords are transverse elastic ligaments ORGANS OF RESPIRATION. 235 Vein. covered with a very thin mucous membrane. They are attached anteriorly to the thyroid cartilage, close to each other, and diverge posteriorly to their attachment in the arytenoid cartilages. The glottis is the slit-like opening between the vocal cords. THE THYROID GLAND. The thyroid gland is not a part of the respiratory tract, but it is so closely associated with this tract in development and in position that it seems advisable to describe the organ in this place. The gland is a highly vas- cular body con- sisting of tw r o lat- eral lobes and connected by a transverse bar, the isthmus . The rudiments of the lobes develop from the epithe- lium of the fourth gill cleft, while the isthmus and a large part of the lobes come from the floor of the mouth, the thyroglossus duct at the base of the tongue being a remnant of this origin. The gland is therefore an epithelial organ derived from the ento- derm. In position the gland lies low down in the neck and in close apposition to the trachea. The isthmus crosses in front of the trachea and covers the second Artery. Colloid. Connective tissue. Fig. 181. Section from thyroid gland, show- ing vesicles or alveoli. 236 NORMAL HISTOLOGY AND ORGANOGRAPHY. and third cartilage ring. The lateral lobes are closely applied to the sides of the trachea and extend up- ward to cover the lower part of the thyroid cartilage. Each lobe measures about two inches in length, one and one-quarter inches in breadth, and one-half inch in depth. The gland, however, is subject to great variations both in size, form, and position. It is gen- erally larger in females, and appears to undergo a periodic enlargement during each menstruation. It reaches its maxi- mum growth at pu- berty, and is fre- quently much di- minished in size in old age. Structure. The vv -..-, -* -. f thyroid gland is in- vested by a thin layer of areolar tis- sue which not only binds it fast to the trachea, but divides Fig. 182. Portion of a cross sec- it into Small lobules tion of thyroid gland of a man; c, col- . . loid substance (Sobotta). of irregular size and form. It is a duct- less gland consisting of epithelial vesicles varying in size from .05 mm. to i mm. in diameter. These vesi- cles vary greatly in form and are grouped and held to- gether by areolar connective tissue in which many blood-vessels ramify. These vesicles are generally filled with colloid substance, a yellow viscid fluid, that in sections stains a yellowish red with eosin. They are ORGANS OF RESPIRATION. 237 lined by a simple cubical epithelium made up of two kinds of cells, (i) a smaller number of colloid cells engaged in the production of colloid, and (2) inter- vening chief cells which replace the former in case they are lost. It is affirmed by some that the two kinds of cells represent merely different stages of secretion. The thyroid, being an epithelial organ, may be the seat of a cancer. Goiter is a more common thyroid tumor, consisting of accumulations within its vesi- cles of colloid substance; or an increase of the con- nective-tissue elements; or a multiplication of the thyroid vesicles. Removal of the thyroid produces myxedema, while a congenital absence of the gland is the cause of cretinism. In the latter case, thyroid extract, regularly administered, will establish a nor- mal growth of the child. In exophthalmic goiter the thyroid gland is enlarged, but in this case the en- larged thyroid is a symptom of a more general disease involving other organs and systems. The thyroid gland may be removed and grafted almost anywhere in the body. It will readily grow in its new position and assume its normal function with good results. Vessels and Nerves. The arteries of the thyroid gland are the superior and inferior thyroid arteries on each side, to which is sometimes added a fifth vessel, the thyroidea ima. The organ has, therefore, a very rich supply of blood. The smaller vessels and capil- laries ramify in the connective-tissue elements be- tween the gland vesicles. The veins, which are also large, form an extensive plexus near the surface of the gland, from which a superior, middle, andinferior 238 NORMAL HISTOLOGY AND ORGANOGRAPHY. vein are formed on each side. Extensive lymphatics accompany the blood system. The nerves are derived from the middle and inferior cervical ganglia of the sympathetic. They accompany the blood-vessels, whose walls they innervate, sending also branches that extend close to the base of the epithelial cells. PARATHYROIDS* The parathyroids are two flattened bodies, one- fourth to one-half inch in diameter, that are con- Fig. 183. From parathyroid of man (Huber.) stantly present and placed in close proximity to the upper and posterior surface of the lobes of the thy- roid gland. They consist of solid masses of epithelial cells. Lymphoid follicles are usually closely asso- ciated with these masses. The function of these little bodies is not known; however, if the thyroid gland be removed and the parathyroid left, the effect of a complete thyroidectomy is not obtained. ORGANS OF RESPIRATION. 239 THE TRACHEA AND BRONCHI. The trachea extends from the cricoid cartilage, a point opposite the lower border of the sixth cervical vertebra, to the level of the intervertebral disc be- Thyroid cartilage. Crico-thyroid membrane. Cricoid cartilage. Thyroid gland. Epartenal bronchus. Hyparterial bronchus. Fig. 184. The trachea and bronchi. tween the fourth and fifth dorsal vertebrae, extend- ing therefore one to two inches into the chest. The 240 NORMAL HISTOLOGY AND ORGANOGRAPHY. trachea measures from four to four and one- half inches in length, and three-fourths to one inch in width. It is smallest at its commencement, and, although quite uniform in its dimensions, is usually a little wider midway between its two ends. At the lower end the trachea bifurcates to form the right and left bronchi, which pass each to the root of the corresponding lung. The right bron- chus is larger than the left and more nearly vertical, so that in looking down the trachea more of the right than of the left bronchus can be seen. The right bronchus di- vides into branches, one to each root of the three lobes of the right lung, while the left gives off two branches, one to each lobe of the left lung. Structure. In the wall of the trachea there are from sixteen to twenty C-shaped cartilage rings that make a little more than two-thirds of a cir- cle. The outer surface of these cartilages is flat, but the inner surface is convex from above downward, so as to give greater thickness in the middle than at the edges. The cartilage is of the hyaline variety and is Ciliated epi- thelium. Longitudinal elastic fbers. ^Mucous glands. Fat cells. Cartilage. Fig. 185. From longitudinal sections of trachea. ORGANS OF RESPIRATION. 241 enclosed in a periosteum. The cartilage rings are held together by a strong elastic fibrous tissue, which not only occupies the space between them but is pro- longed over their surfaces, so that each ring appears imbedded in this tissue. Each cartilage terminates abruptly behind by rounded ends, between which Stratified cili- ated columnar epithelium. iif ~~ Elastic fibers. > cut trans- versely. && 'jjj^W^ -^-r~, - S&3%Z?2*ff7-<- ^m&- MUCOS, Fig. 1 86. Transverse section through human bronchus (Bohra and Davidoff). stretches a thin layer of smooth muscle .tissue. This muscle not only unites the ends but is also found in the intervening space between the cartilage rings, along the posterior wall of the trachea. Outside of the transverse fibers are a few fasciculi having a lon- gitudinal direction. The cartilage rings of the bronchi resemble those 16 242 NORMAL HISTOLOGY AND ORGANOGRAPHY. of the trachea in being incomplete behind. The right bronchus has from six to eight rings, while the left has from nine to twelve. The left bronchus is longer but narrower. The mucous membrane is smooth and contains a considerable amount of lymphoid tissue and many mucous glands. The epithelial lining consists of long columnar ciliated cells, often very irregular and even branched at their fixed ends. One or two rows of small irregular cells intervene between the basal ends of the ciliated cells, and this epithelium is therefore stratified although very thin. The cells rest upon a basement membrane through which nerves pass to reach the sensitive epithelium. Ex- ternal to the basement membrane, in what consti- tutes the membrana propria, there is a strong layer of longitudinal elastic fibers. This tissue extends the whole length of the air passages, and gives not only great elasticity but is an obstruction to invading bacteria. A vascular submucosa, not very exten- sive, intervenes between the mucosa and the carti- lage. In this may be found the bodies of small race- mose mucous glands, the largest being in the poste- rior wall. The excretory ducts of these glands open on the inner surface. Lymphoid tissue is present both in the mucosa and the submucosa. THE LUNG. The lungs occupy the greater part of the chest cavity, of which they form an accurate mould. The right lung is the larger and has three lobes, while the left has two. Each lung is suspended freely in this ORGANS OF RESPIRATION. 243 cavity and is attached only by a small part of its flattened or mesial surface called the root. The outer surface of each lung is covered by a serous membrane, the visceral pleura, which is reflected over the chest wall, where it is called the parietal pleura. The histology of the pleura is identical with that of the peritoneum. Each lobe has one bronchus which divides rapidly into smaller bronchi. The latter, instead of having cartilage rings, are supplied with small cartilage plates. These plates are not present in bronchi whose diameters are less than o. i mm. Mucous glands are rather numerous but also disappear in bronchi less than o.i mm. in diameter. These smaller bronchi differ further from the larger ones in having a circular layer of smooth muscle that in- tervenes between the cartilage plates and the mucous membrane. The contraction of this muscle reduces the caliber of the smaller bronchi and thus regulates the amount of air that passes to the lung tissue. In asthma there is a spasmodic or more or less chronic contraction of this muscle tissue, which causes diffi- culty in breathing. The air is forced through the narrow tubes, and this brings about a dilatation of the terminal passages and ,a hypertrophy of the chest muscles, the latter being due to the forced efforts in securing sufficient air. Asthmatic patients, therefore, have resonant lungs and usually an en- larged chest. The primary trouble in this disease is not in this smooth muscle tissue, but seems to involve the nervous system and the innervating nerves. Smooth muscle is present in all the bronchi, 244 NORMAL HISTOLOGY AND ORGANOGRAPHY. even in the smallest tubes whose diameters measure 0.2 mm. Structure of the Lung. The structure of the smaller bronchi, which form a part of the lung tis- sue, has just been described. Our knowledge of the terminal air passages has recently been greatly in- creased by the work of Dr. Miller, of the University of Wisconsin, whose account will be followed. The smallest bronchioles, whose diameters are 0.2 mm., are called terminal, or respiratory bronchi. In these Epithelium. Longitiidinal elastic fibers. Involuntary muscle. Cartilage. Mucous gland. Fat cells. Fig. 187. Section from lung showing portion of small bronchus and. adjacent lung tissue. tubes the character of the epithelial lining changes. Patches of small pavement epithelial cells appear among the ciliated cells /while at the end of the res- piratory bronchus all are of the pavement variety. At this point the tube, slightly dilated, opens into three to six distinct chambers called atria. Each atrium opens into a variable number (2 to 5) of larger irregular spaces called air sacs, the walls of which have concave spherical depressions called ai? ORGANS OF RESPIRATION. 24$ cells, or alveoli. Simple pavement epithelium lines the atria and air sacs, consisting of two varieties of cells: (i) small nucleated elements, and (2) larger non-nucleated plates. The latter line the alveoli and are applied directly against the blood capillaries, while the nucleated elements intervene at the free margin of the alveoli. Before birth the air sacs are collapsed and all the pavement epithelium is com- posed of nucleated cells. With the first breath of air the air sacs become distended, not uniformly, but to form vesiculated walls, the small vesicles being the alveoli. The walls of the latter suffer greater distention, due to a less resistance offered by the opposing blood capillaries, and the nucleated cells in this region become changed to non-nucleated plates, through which an exchange of gases takes place dur- ing the functional activity of the lung. The atria and air sacs vary in size according to their distention with air. An average diameter is i.omm. for the atria, and i.o by 1.5 mm. for the air sacs. Each air sac has from six to eight air cells, or alveoli, that also vary greatly in size, an average diameter being 0.25 mm. One system of atria and air sacs constitute a lobule and is separated from adjacent lobules by in- tervening areolar tissue. The base of each lobule is directed toward the surface of the lung and the apex towards its root. These lobules can be seen in a macroscopic surface examination of the lung. The elastic fibers of the membrana propria, de- scribed in the wall of the trachea and bronchi, ex- tend to the distal end of the air passages, where they spread out to form a thin reticular fabric just ex- Respiratory bronchiole. Alveolar duct. Fig. 189. Figs. 1 88 and 189. Two sections of cat's lung (Bohm and Davidoff) ORGANS OF RESPIRATION. 247 ternal to the pavement epithelium, giving great elasticity to the air sacs and functions in the expul- sion of air during normal breathing. The term in- fundibulum is sometimes applied to the distal air passage lined by pavement epithelium. The following is a resume of the tissues found in the walls of the respiratory passages : Non-nucleated epithelial cell. Nucleated ebi- ' thelial cell. Fig. 190. Inner surface of human alveolus treated with silver nitrate, showing respiratory epithelium (after Kolliker). 1. Epithelium extends the whole length. In the trachea and bronchi this is stratified ciliated. In the atrium and air sacs it is simple pavement and made up of two kinds of cells, namely, nucleated and non- nucleated. 2. Elastic fibers extend the whole length. In the respiratory parts the fibers form a reticulum just 248 NORMAL HISTOLOGY AND ORGANOGRAPHY. external to the pavement epithelium and give elas- ticity to the lung tissue. 3. Mucous glands are found in the walls of the large passages down to tubes i .o mm. in diameter. 4. Cartilage is found in the trachea and bronchi down to tubes i.o mm. in diameter. In the trachea and larger bronchi the cartilage forms C-shaped rings, while in the smaller tubes the car- tilage appears in plates. 5. Smooth muscle is found in the larger passages down to tubes 0.2 mm. in di- ameter, or down to the respiratory parts. In the trachea and the larger bronchi this muscle is placed in the posterior wall and be- tween the ends of the cartilage rings. In the smaller bronchi it forms a circular layer between the lining epithelium and the cartilage plates. Blood Supply. The lungs, like the liver, receive blood from two sources, arterial blood through the bronchial vessels, and venous blood through the pul- monary artery. The bronchial arteries, from one to three for each lung, are much smaller than the pul- monary vessels, and carry blood for the nutrition of the lung. They arise from the aorta or from an Fig. 191. Scheme of lung lobule (after Miller): b. r., respiratory bronchiole; d. al., alveolar duct (ter- minal bronchus); a, a, a, atria; s. al., air sacs; a. p., air-cells or alveoli. ORGANS OF RESPIRATION. 249 I / intercostal artery arid follow the bronchial tubes through the lung, to be ultimately distributed in three ways: i. They supply the bronchial lymph glands, the coats of the large blood-vessels, and the walls of the bronchial tubes, forming in the latter an outer and an inner plexus for the irrigation of the muscle coat and the mucous membrane. 2. They supply the interlobular areo- lar tissue; and 3, They spread out over the surface of the lung beneath the pleura. The bronchial veins do not have so extensive a distribution be cause some of the blood sup- plied by the bronchial arteries returns by the pulmonary veins. The superficial and deep set of bronchial veins unite at the root of the lung to drain on the right side into the large azygos and on the left into the left upper azygos vein. The pulmonary artery, which supplies the venous blood, is a very large vessel that gives branches to each lobe of the two lungs. The relation of the pul- monary artery to the bronchi is different on the two sides. On the right side the first branch of the pul- monary artery turns backward below the bronchus of the upper lobe, and then passes along the posterior A Fig. 192. Reconstruc- tion in wax of a single atri- um and air sac with the alveoli: V, Surface where atrium was cut from alveolar duct; P, cut surface, where another air sac was re- moved; A, atrium; S, air sac with air cells (alveoli) (after Miller). 25 NORMAL HISTOLOGY AND ORGANOGRAPHY. surface of this bronchus. On the left side the corre- sponding artery turns backward above the level of the first bronchial branch. The bronchus to the upper lobe of the right lung is therefore called an eparterial bronchus. All the other bronchi are be- low their respective arteries and are called hyparterial bronchi. Because of these relations it is believed that the upper lobe of the right lung has no homo- logue on the left side, and that the middle lobe on the right side is homologous to the upper lobe on the left. The pulmonary ar- tery divides with the bronchi and closely ac- companies them along their posterior or superior walls. The correspond- ing veins pass along the anterior or inferior walls. These blood-vessels are very large, often as large as the bronchial tubes, but in no case do they supply blood to the walls apex of the pulmonary lobule, the pulmonary artery breaks up into several small twigs, one for each antrum, supplying blood to an extensive capillary plexus that spreads over the surface of atria and air sacs. The capillary meshes are very dense, and the capillary tubes very large, so that the intervening spaces are barely wider than the capillaries themselves. Because of thelarge size of the lung capillaries it is possible for fine Fig. 193. Section from in- jected lung showing capillaries of an air sac. of the bronchi. At the ORGANS OF RESPIRATION. 2gl shreds from a blood clot, or emboli in the blood, to filter through and reach the left side of the heart by the pulmonary veins. If so, these emboli are quickly carried by the blood current around the aorta and up the right carotid, as the latter is the most direct route. This course takes them to the right side of the brain, in which the capillaries are narrow, and where the emboli lodge often with fatal results. Such emboli or shreds of blood clots pri- marily enter the venous system at the seat of a bone fracture, or in the walls of the uterus after parturi- tion, or from clots of blood anywhere in the system. The pulmonary veins carry blood from the pul- monary capillary plexus. Each venous radicle drains an area corresponding to several air cells or alveoli. At first these small veins take an inde- pendent course in the interlobular tissue, but after they have attained a certain size they accompany the arteries and the bronchi, and, as a rule, along the lower and front aspect of the latter. At the root of the lung there are formed two pulmonary veins on each side which open separately into the left auricle. The pulmonary veins have no valves, and unlike the veins of other organs are more capacious than their corresponding arteries. Lymphatics. The lymphatics of the lung are very extensive and accompany the two blood systems. We may therefore divide them into two sets, a bronchial and an alveolar. The bronchial consists of an elaborate, fine plexus that ramifies through the mucosa and submucosa of the bronchial tubes. This set anastomoses freely with a second plexus just ex- 252 NORMAL HISTOLOGY AND ORGANOGRAPHY. ternal to the smooth circular muscle layer of the bronchi. Lymph nodes are interpolated ever} where in these plexuses. Just beneath the pleur all over the surface of the lung, lymphatics ramii^ and drain toward the root of the lung, where they join the lymphatics located in the bronchial walls. The alveolar set accompany the pulmonary vessel These lymphatics have their origin in a plexus tha surrounds the respiratory or alveolar portions of the lungs, and then accompanies the pulmonary arteries and veins along the external surfaces of the bronchial tubes to the root of the lungs, where they ultimately unite with the bronchial lymphatics. While lym- phatic nodes are present everywhere, they are par- ticularly abundant at the root of the lung. As tuberculosis spreads along the lymphatics, the dif- ferent clinical aspects of this disease depend to a considerable extent on which of these systems be- comes involved. Nerves. The nerves of the lung come from the pneumogastric and the sympathetic, and are made up of medullated and non-medullated fibers. They enter at the root of the lung and accompany the blood-vessels to the terminal air passages, where they arborize about the lung alveoli just external to the epithelial lining. Many nerve ganglia are located along their course and many fine fibers are given off that innervate the musculature and epithelial lining of the bronchial tubes and the walls of the blood^ vessels. CHAPTER VII. THE URINARY ORGANS. r The following organs will be considered under this topic: Suprarenal glands, Kidney, Ureter, and Blad- der. The urethra will be described in connection with the generative organs. THE SUPRARENAL GLANDS. The suprarenals, morphologically, belong to the nervous system, but their close proximity to the kidneys makes it advisable to describe them here. They are two triangular flattened organs covered with fat that lie one on either side of the spine, in close proximity to the upper kidney border. The left one is slightly larger and measures from one and one-fourth to one and one-half inches from above downward, one and one-fourth inches from side to side, and one-sixth to one-eighth inch in thickness. Embryologically, the organs consist of a cortical part that develops in connection with the Wolffian body and therefore comes from the mesoderm, and a medulla which is associated with the sympathetic nervous system and is derived, at least in part, from the ectoderm. The medulla decomposes very rapid- ly after death, and the organ then resembles a cap- sule; hence the name, suprarenal capsule, is often used. 253 2$4 NORMAL HISTOLOGY AND ORGANOGRAPHY. Each suprarenal is invested in a fibrous capsule and a liberal supply of fat. The capsule contains many elastic fibers and some smooth muscle cells. The cortex shows a radial structure and has been divided into three zones, which are not very well de- fined. i. The zona glomerulosa, next to the capsule, con- sists of a row of columnar epithelial cells folded in such a way as to form oval bodies or elongated heads separated by strands of connective tissue from the cap- sule. The oval nuclei are in the middle of the cells. 2. The zona fasciculata makes up the larger por- tion of the cortex and consists of anastomosing col- umns of epithelial cells, a continua- C or lex. Httum. Fig. 194. Cross section of suprarenal gland of man. Blood-vessels. tion of the zona glo merulo sa. Each column has two rows of polygonal cells that are smaller than those of the glomerulae. 3. The zona reticularis borders on the medulla. Here the columns anastomose and freely interlace. The cells resemble those of the fasciculata. Con- nective tissue ramify between the columns, hence the radial appearance of the cortex. The medulla is coarsely vascular. The cells are smaller than those of the cortex and are grouped in round or oval masses. These cells are finely gran- ular, often pigmented, and stain a brown color, PLATE IV. DIAGRAMMATIC REPRESENTATION OF THE DEVELOPMENT OF THE GENITO- URINARY SYSTEM, THE WOLFFIAN BODY AND ITS DERIVATIVES BEING COLORED RED, THE MULLERIAN DUCT AND ITS DERIVATIVES, GREEN (Heisler): i, Indifferent type; 2, indifferent type, later stage, the Wolman and Miillerian ducts and the primitive ureter now opening into the urogenital sinus; 3, male type, lower ends of Miillerian ducts fused to form the sinus pocularis; 4, female type. PLATE THE URINARY ORGANS. 2 S5 Numerous ganglion cells are present and many nerve fibers. Blood-vessels. Each suprarenal gland receives three arteries, one each from the aorta, the phrenic, and the renal. The arteries break up into small branches, most of which enter the medulla through the hilum. Some branches ramify in the capsule Cortex, .^. . % ?OT? } -* Capsule. ~vj< * > Zona glomerulosa. -* Zona fasciculate. Zona relicularis. Medulla. Fig. 195. Section of suprarenal gland of dog. and from there enter the cortex, where they form capillaries around the columns of epithelial cells. Those that enter the medulla form a coarse plexus in this part, and then send smaller capillaries into the cortex to anastomose with those from the capsule. The veins pass out from the center, usually one from 256 NORMAL HISTOLOGY AND ORGANOGRAPHY. each organ. The vein from the right suprarenal enters the vena cava, while that from the left empties into the renal. Lymphatics accompany the blood- vessels. Fig. 196. Arrangement of the intrinsic blood-vessels in the cortex and medulla of the dog's adrenal (Fig. 17, Plate V, of Flint's article in " Con- tributions to the Science of Medicine," dedicated to Professor Welch. 1900). The nerves are exceedingly numerous and come from the solar and renal plexuses of the sympathetic, and medullated fibers from the phrenic and pneumo- gastric. They ramify freely among the ganglionic cells of the medulla and between the cells of the cor- tex, particularly those of the glomerulosa. THE URINARY ORGANS. 257 The function of the suprarenal gland is not known. Its extirpation in the dog is followed by death in a few days. There are at least three interesting clini- cal features connected with this organ : 1. Suprarenal extract, taken internally, increases arterial tension by contracting the small arterioles. The extract has the same effect when sprayed upon surfaces, therefore it is much used in nose, throat, and eye work, to check hemorrhage and reduce con- gestion. 2. The cortical cells may produce a malignant growth called a hypernephroma. The malignant cells may invade and replace the kidney. The growth usually spreads to the liver and adjacent or- gans, causing death in one to three years. 3. Addison's Disease is a chronic, usually tubercu- lar, inflammation of the suprarenal glands, fatal in one or two years. It is accompanied with a striking bronze pigmentation of the skin, and digestive and ner- vous disturbances. It is a rare disease of middle life. The above facts support the view that the organ secretes a substance that is regularly gathered up by the blood. This secretion may control, in a measure, arterial pressure. The organ is also a relay in the sympathetic nervous system which, when destroyed, as in Addison's Disease, accounts for the gastric and nervous symptoms. THE KIDNEYS. The kidneys, two in number, are compound tubular epithelial glands derived, embryologically, from the mesoderm. They are situated behind the peri- 17 258 NORMAL HISTOLOGY AND ORGANOGRAPHY. toneum, one on each side of the vertebral column and on a level with the last dorsal and the upper two or three lumbar vertebrae. They are held in position by the renal vessels, by a loose areolar tissue that surrounds them which contains much fat, and by the abdominal pressure. Each kidney measures four inches in length, two and one-half inches in breadth, and one and one-fourth to one and one- half inches in thickness. Their developmental history is rather complex and can be but briefly given here. It involves the history of the pronephros, mesonephros, and meta- nephros, three sets of excretory organs which re- place each other in the sequence in which they are mentioned. 1. The pronephros, or head kidney, develops in connection with the nephrotomes of the first three or four embryonic somites. These nephrotomes unite with a longitudinal duct, the pronephric duct, which opens posteriorly into the cloaca. With this organ fluid from the celomic or peritoneal cavity can be eliminated, and also waste products from the blood, as a tuft of blood capillaries or glomerulus is present near the peritoneal opening of each nephro- tome. This kidney is exceedingly rudimentary in mammals and functional only in larval stages of amphibians and in bony fishes. 2. The mesonephros, or Wolffian body, develops in connection with the nephrotomes posterior to those that form the pronephros. The pronephric duct becomes the Wolffian duct and drains from the peritoneal cavity in the same manner as in the head THE URINARY ORGANS. 259 kidney. The glomerulus in this case is situated in the wall of the nephrotome, which makes the meso- nephros a more efficient organ. The mesonephros is an elongated segmented organ and a permanent structure in amphibians. It is an embryonic organ in birds and mammals, in which it is replaced by the metanephros. 3. The metanephros, or permanent kidney, develops as a diverticulum from the cloacal end of the Wolffian duct. The diverticulum lengthens into a tube, the ureter. The upper or anterior end of the tube Artery. Vein. Fig. 197. Kidney of new-born infant, showing a distinct separation into reniculi; natural size. At a is seen the consolidation of two adjacent reniculi (Bohm and Davidoff). branches to form a number of smaller tubes, the uri- niferous tubules of the kidney. The surrounding mesoderm becomes condensed and vascular, inter- lacing between the tubules to form the adult kidney. The development of the urinary system is closely associated with that of the generative system, and will be referred to again when the latter is described. Structure. The hilus of the kidney is an opening through which the ureter and blood-vessels pass. On making a longitudinal section of the kidney, it will be seen that the hilus leads to an expanded fis- 260 NORMAL HISTOLOGY AND ORGANOGRAPHY. sure, the renal sinus. The pelvis is a funnel-shaped expansion of the ureter that occupies a large portion of the sinus. The contents of the sinus may be re- moved, when the exposed wall will be found to be kidney substance. Each kidney is enclosed in a smooth fibrous cap- sule of areolar tis- sue, a part of which also lines the sinus. The capsule is finely vascular and can easily be de- tached. If it ad- heres to the kidney substance it is evi- dence of disease. It is customary to describe the kid- ney as made up of two layers, an out- er, or the cortex, and an inner, the me- dulla, although there is no sharp Fig. 198. Longitudinal section through the kidney: I, Cortex; i', medul- lary rays; i", labyrinth; 2, medulla; 2', papillary portion of medulla; 2", boun- dary layer of medulla; 3, transverse sec- tion of tubules in the boundary layer; 4, fat of renal sinus; 5, artery; *, transverse medullary rays; A, branch of renal artery; C, renal calyx; U, ureter (after Tyson andHenle). line dividing the two. The medulla consists of ten or twelve separate conical masses called Malpighian, or medullary pyramids, so arranged that their bases bor- der on the cortical layer and their apices point toward THE URINARY ORGANS. 261 the renal sinus where they form papillae. The medullary substance is more dense than the cortical and is strictly striated, which is due to the radiating course of the tubules in this part. At the base of each Malpighian pyramid these tubules pass up into the cortical substance and are grouped to form cone-like masses called medullary rays, or pyramids of Capsule. Glomeruli. Pyramid of Ferrein. Fig. 199. Section through cortex and medulla of kidney. Ferrein, the apex of each being in close proximity to the periphery of the kidney. There are thus a great many medullary rays for each Malpighian pyra- mid. The portions of the kidney that intervene between the medullary rays are called the labyrinth, and consist largely of convoluted tubules. The cortical substance not only covers the bases of the 262 NORMAL HISTOLOGY AND ORGANOGRAPHY. Malpighian pyramids, but sends prolongations be- tween them down to the renal sinus. These cortical prolongations are called the columns of Bertini, of which there are as many as there are Malpighian pyramids. It is convenient to describe the kidney tubules as consisting of nine parts. Each tube commences in the labyrinth of the cortex with (i) an invaginated dilatation called Bowman's capsule. This capsule is lined by simple squamous epithelium which, when invaginated, makes two layers, the lumen of the tube being between these layers. Into the in- vaginated cavity is crowded a tuft of blood capil- laries called a glomerulus. A liberal supply of con- nective tissue blends with these capillaries. The capsule and glomerulus are frequently called a Malpighian corpuscle. At the base of the cap- sule each tube is constricted, forming (2) the neck t after which it becomes much convoluted and wide, forming (3) the proximal convoluted tubule. The cells of this part are large with very thin cell walls. The cytoplasm immediately around the nu- cleus is granular, but toward the basement membrane it is striated with lines at right angles to the mem- brane. The tube now approaches the medulla, becomes nearly straight, but rapidly narrows to form (4) the spiral portion. It now passes straight down a Malpighian pyramid, where it makes a short curve, and returning thus forms (5) the loop of Henle. This loop has a narrow descending limb with a short curve, and a wider ascending limb. The narrow descending limb and curve has simple THE URINARY ORGANS. 263 flat epithelium, so that the lumen is practically as Bowman's capsule. Glomerulus. Ascending limb of Henle's loop. Loop. Collecting tube. Fig. 200. Diagram of kidney tubule. large as that of the preceding part. The ascending 264 NORMAL HISTOLOGY AND ORGANOGRAPHY. is larger but the epithelium takes on the char- acteristic of the proximal convoluted tube, although the cells are a little smaller and may contain pig- ment granules. The ascending limb passes straight up a medullary ray, from which it emerges to again enter the cortex, where it becomes irregular in out- line forming (6) the irregular tubule, which quickly becomes much twisted and coiled to form (7) the distal convoluted part. In the irregular part the cells are very un- Con-voluted . portions, equal in size and a rod- like struc- ture of the cytoplasm is very dis- tinct, while the base- ment mem- brane is said to be ab- sent. The cells of the Neck. Fig. 201. Section of a portion of the labyrinth of the kidney cortex. distal convoluted tube are rather long, with a distinct membrane and a highly refractive appearance of the protoplasm. Near the basement membrane minute projections from adjacent cells may be seen to inter- lock. Finally, this portion terminates in (8) a short functional tubule, which leaves the cortex and enters a medullary ray to join the last part (9) , the collecting tubule. The latter passes straight through the Malpighian pyramid to open on its surface in a small pore. In its course the collecting tube receives THE URINARY ORGANS. 265 not only other junctional tubules but also unites with other collecting tubes. The junctional part is narrow but its lumen relatively large, being lined by clear flat or cubical cells. The collecting tubes are large and have a lining of simple cubical epithelium. The location of the different parts of the kidney tubules may be tabulated as follows : Portion of Tubule. Location. 1. Bowman's capsule cortical labyrinth. 2. Neck cortical labyrinth. 3. Proximal convoluted cortical labyrinth. 4. Spiral portion medullary rays. 5. Loop of Henle: (a) Descending limb medulla. (b) Loop medulla. (c) Ascending limb medulla and medullary rays. 6. Irregular part cortical labyrinth. 7. Distal convoluted cortical labyrinth. 8. Junctional tubule medullary rays. 9. Collecting tubule medullary rays and medulla. It will be observed that the walls of these tubules consist of simple epithelium ; that this is made up of pavement cells in the capsule, neck, and descending limb of Henle 's loop ; while large cubical or columnar cells with granular and rod-like structure of the cyto- plasm are found in the convoluted parts and ascend- ing limb of Henle 's loop; and a low cubical epithe- lium invests the junctional and collecting tubules. Casts. It is common in high fevers and in diseases of the kidney to find moulds of the uriniferous tubules in the urine. In high fevers blood may enter and congeal in the uriniferous tubules, the kidney secretion forces out this obstruction, which then appears in the urine as a blood cast. A clear serum mould is called a hyaline cast. In more chronic cases the cells of the tubules are carried away 266 NORMAL HISTOLOGY AND ORGANOGRAPHY. and the mould is then called an epithelial cast. The cells may have decomposed, when it becomes a granular cast. Hyaline casts often show granula- tions and are then also called granular casts. Finally, fatty degenerations may appear and form fatty casts. The recognition and identification of casts form an important subject in clinical analysis. If small pieces of kidney are treated with strong hydro- chloric acid and the detritus thus produced be ex- amined in gly- cerin, under a low magnification, many pieces of the uriniferous tubules are read- ily observed. These pieces are practically iden- tical with the epi- thelial and gran- ular casts of the diseased kidney. Blood-vessels. The kidneys are highly vascular and receive a large amount of blood in proportion to their size. The renal artery and renal vein pass through the hilum, the artery between the vein and ureter. In the renal fissure the artery breaks up into four or five branches which lie external to the pelvis of the ureter. These branches pass directly to the columns of Bertini, where they break up into smaller vessels and rise to the level of the Mal- pighian pyramids. At this level they pass across the pyramids, between the latter and the cortex, Capillary. Fig. 202. Section of a portion of a kidney medulla. URINARY ORGANS. 267 Stellate vein. forming what are called arterial arches. The ar- teries form incomplete arches across the base of the pyramid, while the accompanying veins, in this place, form complete venous arches across the base of the pyramid. From the arches interlobular arteries pass outward be- tween the me- dullary rays and among the convoluted tu- bules, taking a direct course toward the sur- face of the kid- ney. At inter- vals they give off curved short branches which pass directly to the glomerulus of a Malpig- hian corpuscle, where they break up into a spongy mass o f capillaries. A vein, smaller than the artery, emerges from the glomerulus close to the point where the artery enters. Capsule. Interlobular artery. Vas afferens. Vas efferens. Glomerulus. Arterial arch between the cortex and medulla. Pseudo-arteria recta. Arteria recta. 203. Diagram of blood supply of kidney. The artery is called 268 NORMAL HISTOLOGY AND ORGANOGRAPHY. the vas afferens and the smaller vein the vas efferens. The latter, instead of uniting with other veins to form larger trunks, as is the case in other organs, passes directly to the convoluted tubules, where it forms a dense capillary system that ramifies everywhere over the walls of these tubules. Many of the effer- ent vessels from the lowermost glomeruli, that is, those nearest the medulla, break up into pencils of straight vessels called pseudo-arteries recta, which pass directly into the medulla to form capillaries around the tubules of this part. Interlobular veins convey the blood from the kid- ney cortex to the venous arches at the base of the pyramids. Near the periphery of the kidney other veins converge to form a stellate appearance just beneath the capsule. These stellate veins receive blood from the venous arches and also connect with the veins of the capsule. The blood supply of the medulla is to a great ex- tent independent of that to the cortex, excepting that supplied by the false arteriae rectae. Branches from the concave side of the arterial arches pass directly into the medulla, where they form bunches of pencils of small parallel vessels, the arteries rectcz, which sup- ply blood to the walls of the uriniferous tubules of this part. Veins return this blood to the venous arches that lie between the cortex and medulla. These arches form veins that pass through the col-, umns of Bertini and ultimately drain into the renal vein, which passes through the hilum to join the inferior vena cava. On account of this extensive blood supply any renal disturbance is, as a rule, accompanied by a cor- THE URINARY ORGANS. 269 responding circulatory disturbance. Conversely, a disturbed circulation or an enlarged heart is indica- tive of a possible nephritis. Nerves. The nerve supply is derived from the cerebrospinal system and the sympathetic. Many of these supply the blood-vessels, which they always accompany, but some arborize among the renal tubules, particularly those of the renal cortex. THE URETERS. The ureters are two muscular tubes that conduct the urine from the kidneys to the bladder. The dilated commencement of each ureter is called the pelvis and lies with its base in the renal fissure, and extends through the hilum to the lower portion of the kidney where the ureter proper begins. Lateral expansions of this pelvis extend to and enclose the papillae of the Malpighian pyramids, on the surface of which the collecting tubules open. These ex- pansions are called calyces. The ureters measure from fourteen to sixteen inches in length, and one-fourth inch in diameter. Each ureter lies behind the peritoneum and passes downward and inward to the brim of the pelvis, and then forward and inward to the base of the bladder. The ureters are about two inches apart as they enter the wall of the bladder, through which they pass obliquely for three-fourths inch to open on the inner surface in two narrow and slit-like openings. The oblique passage through the bladder wall acts as a valve to prevent a return flow of urine. Structure. The walls of the ureter consist of an 270 NORMAL HISTOLOGY AND ORGANOGRAPHY. Stratified epithe- lium. Mucous layer. outer fibrous, a middle muscular, and an inner mucous layer. The latter has many longitudinal folds and is lined by transitional epithelium of four or five layers of cells. The superficial cells are flat, or low cubical, and may contain two nuclei. Their lower surfaces have depressions that fit upon the rounded ends of the second layer, which consists of oval or pear-shaped cells. Between the apices of the latter are two or three rows of small, irregular interstitial cells. Mucous glands have been described in the renal pel- vis and so have lymphoid nod- ules, but the presence of glands in the ureter of man is doubtful. The membrana propria is com- posed of areolar tissue which becomes gradually loose toward the muscularis . This membrane is like others of its kind, having a limited blood supply. The muscular coat is composed of smooth muscle cells and consists chiefly of a circular layer between two thin longitudinal layers, particu- larly well defined in the lower part of the ureter. The fibrous coat is relatively thick and strong, con- tributing fibrous elements that interlace the muscle tissue. Fibrous coat. Fig. 204. Cross section of the ureter. THE URINARY ORGANS. 271 The function of the ureter is an active one. A few drops of urine enter the ureter and are propelled along by the peristaltic contraction of its muscula- ture, which forces the urine in intermittent jets into the bladder. In case of overdistention the force ex- erted by this mechanism is sufficient to rupture the bladder. In case of an obstruction in the ureter, as in the passage of calculi, a violent contraction of the smooth muscle follows, accompanied by severe pains. In surgical operations ureters have been sewed into the upper end of the bladder, or even into the intestine. In the latter case the kidney usually becomes infected with bacteria from the bowel. THE URINARY BLADDER. The urinary bladder is a receptacle for the reten- tion of urine, with an average capacity of one pint, although capable of much greater distention. When empty it lies wholly within the pelvis, but if dis- tended it rises into the abdomen. When moderately filled it has a rounded form, but when completely distended it becomes egg-shaped, the larger end, called the base or fundus, being directed downward and backward toward the rectum, and its smaller end, the summit, resting against the anterior ab- dominal wall. When distended the peritoneum covers the bladder, excepting a triangular space of two inches above the symphysis pubis known as the space of Retzius. This is of surgical importance, as the bladder can be opened through this space without going through the peritoneum. The mucous membrane on the inner surface of the 272 NORMAL HISTOLOGY AND ORGANOGRAPHY. Transitional epithelium. bladder is loosely attached to the muscular is, and is slightly corrugated or folded in the contracted form of the organ. At the lower part of the bladder is found the orifice leading into the urethra, and im- mediately behind this is a smooth triangular surface called the trigone. The orifices of the ureters are found at the posterior angles of the trigone, and in the distended bladder are about one and one-half inches apart and about the same distance from the urethral orifice. When the bladder is contracted this space is diminished. An exact knowledge of these relations is important in any attempt to pass a catheter into the ureter. Histology o f the Bladder- There is a mu - cous, submu- cous, muscular, and serous coat to the bladder. The mucous mem- brane resembles that of the ureters, with which it is continuous. It is covered with a transitional epithe- lium whose cells vary according to the distention of the bladder. As a rule, epithelial cells have but very little elasticity and mucous membranes, therefore are frequently much folded . The bladder cells are capable of considerable distention, when they become very flat. When the organ contracts they accommodate themselves to this condition and become cubical. The cells of the surface layer are squamous and have Fig. 205. Section through the mucosa of the bladder. THE URINARY ORGANS. 273 concave depressions into which the rounded ends of the second layer or pear-shaped cells are adjusted. Two or more layers of irregular interstitial cells inter- vene between the apices of the pear-shaped cells. The interstitial cells divide regularly by karyokinesis and are then crowded to the surface to replace the superficial cells that normally exfoliate. There are no glands in the bladder, but solid cell projections are sometimes found that resemble glands. The blad- der is a part of the allantois, a vesicular evagination of the hind-gut. The bladder epithelium, therefore, Pavement cell. Pear-shaped cell. Pavement cells. Interstitial cells. Fig. 206. Epithelial cells from the bladder. is of hypodermic origin, while that of the ureter is from the mesoderm. A vascular submucosa intervenes between the mu- cosa and the muscularis. This is a thin layer of areolar tissue, but sufficient to give the mucosa apparent elasticity and enable it to move upon the muscularis. The muscular coat consists of smooth muscle fibers which may be divided into bundles of outer longi- tudinal fibers, a middle strong circular layer, and an imperfect inner longitudinal or diagonal stratum. 18 274 NORMAL HISTOLOGY AND ORGANOGRAPHY. At the urethral opening the middle layer is thickened to form a sphincter muscle, according to some au- thors. The bladder musculature forms a basket- work fabric, and when much distended intervals may arise in its walls which become points of weak- ness through which the mucosa may protrude, when the organ is said to be sacculated. Vessels and Nerves. The bladder is supplied with blood from the superior and inferior vesicle arteries, and in the female also from branches of the uterine artery. The veins form large plexuses, particularly around the neck, sides and base. They eventually drain into the internal iliac. The nerve supply is from the third, fourth, and sometimes the second sacral nerves, and from the hypogastric plexus of the sympathetic The latter are nearly all non-medul- lated. CHAPTER VIII. REPRODUCTIVE ORGANS IN THE MALE. Under this heading are included (i) the testes and their ducts, (2) epididymis, (3) penis, and (4) pros- tate gland. THE TESTICLES. The testes are two glandular organs for the produc- tion of spermatozoa, suspended in the scrotum by the spermatic cord. Each testicle is about one and one-half inches long, one and one-fourth inches wide, and nearly one inch thick from side to side. The corresponding dimensions of the ovary are, one and one-half by three-fourths by one-half inches. The coverings of the testes are, (i) skin, (2) dartos; these two form the wall of the scrotum. The skin is thin and pigmented. The dartos is a reddish tissue continuous with the two layers of superficial fascia of the groin. It is vascular and consists of smooth areolar tissue and smooth muscle fibers. The latter give involuntary contractility to the scrotum and produce folds or rugae in the skin. (3) Intercolum- nar fascia, which is a thin connective-tissue layer closely associated with (4) .the cremasteric fascia. The latter is continuous with the internal oblique muscle. (5) The infundibuliform fascia comes next and is a continuation downward of the fascia trans- versalis. (6) The tunica vaginalis envelops each 275 276 NORMAL HISTOLOGY AND ORGANOGRAPHY. testicle and is derived from the peritoneum during the descent of the organ. It is therefore a serous coat that has the same histology as the peritoneum, and may be divided into two parts, one the visceral portion that invests the surface of the organ, and the other the parietal portion that is reflected over the surface of the infundibuliform fascia. The interval between these portions constitutes the cavity of the tunica vaginalis, and it is in this space that hydro- Tunica vaginalis, visceral portion. Tunica vagi- nalis, parie- tal portion. Tunica albuginea. Epididymis. Vas deferens. Lobule. Fig. 207. Cross section of human testicle. cele fluid collects. (7) The tunica albuginea comes next and is a firm fibrous covering. This tunic sends fibrous cords or trabeculae into the testis, which divide the organ into lobules. It is par- ticularly dense along the posterior margin of the organ where it also invests the vas deferens, forming at this margin a mediastinum called the corpus of Highmore. (8) The tunica vasculosa is a delicate vascular layer that covers the inner surface of the REPRODUCTIVE ORGANS IN THE MALE. 277 tunica albuginea. The three tunics just mentioned form the wall or capsule of each testicle, and are so closely associated that it is difficult to distinguish one from the other. Structure. The testis is a compound tubular gland divided into three hundred to four hundred lobules. Each lobule is conical in shape with the apex directed toward the mediastinum and the base toward the surface of the organ. The lobules differ in size ac- cording to their position. Each lobule represents several coiled tu- bules which, when unraveled, aver- age two feet in length. There are at least six hundred to eight hundred of these tubules t o each testicle. Their walls are lined with stratified epithelium which is in- vested with a thin layer of connective-tissue ele- ments. The epithelium rests upon a basement membrane and may be arranged in at least three irregular groups or layers: i. A layer of cubical cells, with small nuclei, rests upon the basement membrane. The cells of this layer are called sper- matogonia. The columns of Sertoli, or sustentacular cells, also belong to this layer. These columns are elongated columnar cells that extend from the Spermatozoa. Spermatoblasts. Spermatocyles. Spermatogonia. Sustentacular cell. Fig. 208. Section of convoluted tubules of testicle. 378 NORMAL HISTOLOGY AND ORGANOGRAPHY. basement membrane inward toward the lumen of the tube. They give off lateral processes that form a reticulum about groups of young spermatozoa, to which they give both support and sustenance, according to the views of some authors. 2. Within the first layer there are one or two rows of large cells with large deep -staining nuclei. These are the spermatocytes. The latter multiply rapidly and continually to form, 3, spermatoblasts or sper- matids. The spermatids are small spherical cells and each one in due time develops into a spermato- zoon. The latter ripen regularly in groups which seem to cluster about individual sustentacular cells. The nucleus of the spermatids elongate and each little spherical cell gradually assumes the form of a mature spermatozoon. The different stages in this development can be worked out by a study of the spermatids as seen in the different tubules. In the cross section of a single tubule all the spermatids will be in the same stage of development. The spermatozoa when mature are crowded into the lumen of the convoluted tubules, where they mix with a viscid secretion which probably comes from the epithelial wall. The convoluted seminiferous tubules end blindly near the surface of the testis, where they are also said to anastomose with each other. In the other direction, each tubule becomes straight and forms the tubuli recti, which approach the mediastinum and function as excretory ducts. These erect tubules anastomose to form the rete testis. Interstitial Elements of the Testis. Like any other REPRODUCTIVE ORGANS IN THE MALE. 279 organ the testicle has a fine reticulum of connective tissue that is associated with the capsule or tunica albuginea. This reticulum consists of areolar tissue that not only intervenes between the lobules, but interlaces between the seminiferous tubules. Blood and lymph vessels are everywhere associated with this tissue. In addition to ordinary connective- tissue cells, there is associated with this reticulum patches of cells that re- semble epithelium and have yellowish granules or pigment. These are called interstitial cells, and, like the areas of Langerhans of the pan- creas, are supposed to se- crete products regularly absorbed by the blood. We can postulate a pos- sible function of these cells when we consider the function of the testi- cle and its influence on Fig. 209. Sustentacular cells ~( cells of Sertoli) of the guinea-pig (chrome-silver me- thod) ; profile view: c, c, Depres- sions in the sustentacular cells due to pressure from the sper- matogenic cells; d, basilar por- tion of sustentacular cells (Bohm and Davidoff). the body as a whole. (i) Physiologically the testis exerts a marked influ- ence on the development of the body. (2) It is ac- tively engaged in the production of spermatozoa. (3) It is essential for the act of copulation. Early castration in domestic animals is a striking evidence of the influence the testicle exerts upon development. The change manifest is both physi- cal and mental. As the infantile testis does not 280 NORMAL HISTOLOGY AND ORGANOGRAPHY. " Middle piece. Tail. produce spermatozoa, it is believed that the secre- tions from the interstitial cells react upon the devel- opment of the body as a whole. While the semi- niferous tubules function in the production of sper- matozoa, it is not clear that the accumulation of semen prompts the sexual act. For instance, the testicles sometimes do not descend into the scrotum but remain in the body cav- fry. Frequently such testi- cles are of the infantile type, that is, no semen is devel- oped ; and yet the copulation act in such males is not only possible, but the sexual crav- ings may be actually exagge- rated. The interstitial cells of such testicles are well de- veloped and the male is other- wise normal. Again, the im- potency that sometimes comes with old age is said to be due to impaired functional activity of the interstitial cells rather than to lack of spermatozoa. We have no specific medical treat- ment for such cases, extracts from normal testes having been tried without satisfactory results. Spermatogenesis and Spermatozoa. The develop- ment of spermatozoa begins in man in early youth and usually continues into old age. This phenom- enon is in marked contrast to ovulation in woman, where there is a cessation or menopause at about the End piece. Fig. 210. Human sper- matozoa, side and flat view. REPRODUCTIVE ORGANS IN THE MALE. 281 age of forty-five. The explanation offered to ac- count for this difference in the sexes is sought in the blood supply to the generative organs. There is a decrease in the nourishment to the ovaries as the menopause approaches, due to a contraction of the blood-vessels that supply the organ. Thetestes, on the other hand, have a liberal supply of blood throughout life. The development of spermatozoa has been re- corded with great accuracy, particularly in ascaris, insects, amphibians, and fishes, and there is little doubt but that the processes in mammalia are es- sentially the same. The spermatogonia cells of the testicle after repeated division produce what are called primary spermato- cytes. Each of the latter divides by somatic mito- sis to produce two secondary spermatocytes, and these divide by reduction mitosis to produce spermatids, which, in turn, are moulded into individual sperma- tozoa. From every primary spermatocyte, there- fore, four spermatozoa ultimately ripen, a detailed account of which will now be considered. Multiplication of spermatogonia does not differ from other somatic mitosis. The primary sperma- tocytes, however, show a more condensed form of chromatin, and while the usual spireme structures develop, the chromosomes pair and fuse. This is not a chance fusion, but is said to be a selective union called sy nap sis of chromosomes. This coalition ap- parently reduces the chromosomes to one-half the original number. The synapsis is usually a collateral one, sometimes end to end, and while inmost instances the union is complete, in other cases the double na- 282 NORMAL HISTOLOGY AND ORGANOGRAPHY. ture of the chromosomes remains visible. Eachpaired chromosome now divides not only by splitting lon- gitudinally, but also by a median transverse division, thus producing four fragments, each group of four fragments being known as a tetrad. The number of tetrads are thus equal to one-half the original num- ber of chromosomes. The tetrad groups now ar- range themselves in the equatorial plane of the Four chromosomes. f6/\V Primary spermalocytes. Two tetrads ----- Somatic mitosis. Secondary spermatocytes. Reduction mitosis. Two diad groups in , each cell. f I T I Young spermatozoa. Fig. 2ioa. Diagram of spermatogenesis. spindle, and then two fragments or chromosomes from each tetrad pass to opposite poles of the spindle to form two daughter cells or the secondary spermatocytes. The chromosomes of each daughter cell, therefore, form in groups of twos instead of fours, and each group is now called a diad. The number of diads is equal to the number of tetrads, REPRODUCTIVE ORGANS IN THE MALE. 283 and therefore equal to one-half the original number of chromosomes. A new spindle quickly forms in the secondary spermatocytes, the diads take an equatorial position and then separate to form monads, each daughter cell or spermatid receiving an equal number. The number of monads is equal to the number of diad groups and therefore equal to one-half the original number of chromosomes. The division of tetrad groups to form diads is usually considered an equational or somatic process, while the division of the diad groups to form monads is looked upon as a reduction process. The splitting of the tetrads is then interpreted as a longitudinal division of the chromosomes, and that of the diads as an end-to-end division. Each spermatid ulti- mately moulds to form ripe spermatozoa and thus every primary spermatocyte produces four sperma- tozoa. Sex Determination. Cytological and experimental work in recent years have revealed facts which show that certain chromosomes play an important part in the determination of sex. In the grasshopper (Steno- bothrus viridulus) the cells of the male have seven- teen and the cells of the female eighteen chromo- somes. In case of the primary spermatocytes, each with seventeen chromosomes, when synapsis occurs, one chromosome is left without a mate. This odd one is called the accessory chromosome, and can be recognized by its condensed form, heavy staining qualities, and its position near the nuclear mem- brane. When the primary spermatocyte divides, this accessory or univalent chromosome remains un- divided within one of the daughter cells, thus mak- 284 NORMAt HISTOLOGY AND ORGANOGRAPHY. ing two kinds of secondary spermatocytes. When the latter divide the accessory chromosome also di- vides, thus giving rise to two kinds of spermatozoa in equal numbers, one-half of them having eight and one-half nine chromosomes. The mature ova have no such complications, and each one has nine chro- mosomes. When fertilization takes place two com- binations are possible. Should a spermatozoon with eight chromosomes fertilize the egg, then a male de- velops with somatic cells that have seventeen chro- mosomes, and if one with nine is used, then a female is produced with somatic cells having eighteen chromosomes. In some cases the accessory chromosome has a small mate in synapsis. Two classes of spermatozoa develop, one-half with a large and the other half with a small chromosome. The class with the large chromosome produces females, and that with the small produces males. Two kinds of spermatozoa, differing in quality and quantity of chromatin material, have been des- cribed in the whole animal phylum, even in man, and it seems to be proved that the quality of the sper- matozoon in these forms determines the sex. But there are many forms of life, particularly in plants, where parthenogenetic and other asexual develop- ment prevail and where sex cycles arise from fac- tors other than spermatozoa. In some of these the quantity and quality of food are sex factors, while in others we do not know the determining agents. Structure of Spermatozoa. A spermatozoon is a minute cell, about 0.055 mm. long and consisting of a nucleus or head, a middle piece or body, and a vibra- REPRODUCTIVE ORGANS IN THE MALE- 285 tile tail. In man the head is a flattened ovoid, ap- pearing pear-shaped or pointed in one view and rounded in another. It contains the nucleus and stains heavily with nuclear dyes. It measures about 0.0045 mm. long, 0.0025 mm. broad, and 0.0015 mm. thick. The middle piece, or body, is cylindrical in man, and measures about 0.006 mm. in length, and o.ooi mm. in thickness. In some animals, as the rat, a spiral thread can be seen coiled about the periphery, whilst through its center a slender filament seems to pass and be continuous with the central filament of the tail. This filament ends in a terminal enlarge- ment placed in close proximity to the nucleus and known as the terminal globule. The tail is about 0.045 mm. long and tapers toward the extremity, ending in an extremely delicate fiber, the end piece. Again, in the rat this end piece seems to be the terminal part of the central filament of the middle piece, which thus extends the whole length of the tail. The spermatozoon is propelled forward by a spirally lashing movement of the tail, similar to the movement of cilia. Movement of cilia, however, ceases as soon as they are removed from their cell, which is not the case with the tail of a spermatozoon, in which motion seems to be an in- trinsic quality. So long as spermatozoa remain in the male passages they are inert, but become active as soon as expelled. Spermatozoa differ a great deal in the different species of animals. They are very hardy cells, and in the female passages may live for days and even weeks. In some domestic birds, as turkeys, they 286 NORMAL- HISTOLOGY AND ORGANOGRAPHY. live at least a month, while in bats copulation takes place in the fall and fertilization follows in the spring. Excretory Ducts of the Testis. These ducts are the tubuli recti, rete testis, vasa efferentia, epididy- mis, and vas deferens. Tubuli Recti. The seminiferous tubules, towards the mediastinum, unite at acute angles to form a Vas deferens. Epididymis (globus major). Vasa efferentia. Tubuli recti. Rete testis. Epididymis (globus minor). Fig. 211. Diagram of human testicle, longitudinal section. series of short parallel straight tubes called the tu- buli recti. They are clothed by a simple layer of low cubical epithelium. The rete testis consists of a reticulum of tubules formed by an anastomosis of the tubuli recti in the mediastinum. They are lined by simple columnar epithelium. The vasa efferentia are tubules that lead from the REPRODUCTIVE ORGANS IN THE MALE. 287 upper portion of the rete testis through the tunica albuginea to the epididymis. There are about fif- teen of them. They are lined by simple columnar ciliated epithelium, and represent tubules that em- bryologically belong to the mesonephros. Outside of the epithelium is a thin investment of areolar tissue in which smooth muscle cells interlace. The epididymis is a very much coiled canal, about twenty feet long, formed by the confluence of the vasa efferentia. At the upper and posterior border of the testis the vasa efferentia and ep ididymis form a globular mass of tubules called the glo- bus major. At the lower and posterior bor- der there is a smaller mass of coiled tubules formed by the epididymis, and called the globus minor. The epididymis is lined by simple epithelium which is ciliated in most places, but interposed are patches of nonciliated cells. The latter form small areas that resemble glands. External to this epithelium there is a thin layer of smooth muscle fibers which blends with vascular areolar tissue. The va,s deferens begins at the lower margin of the globus minor and is a direct continuation of the Fig. 212. Cross section of epididymis. 288 NORMAL HISTOLOGY AND ORGANOGRAPHY. Fibrous coat. Longitudinal muscle. Circular muscle. Membrana propria. Simple epi iheliunt. canal of the epididymis. It is a duct about twelve inches long, but when unraveled and extended it is eighteen to twenty inches in length. At first it is rather tortuous, but soon becomes straight and ascends along the inner border of the epididymis to pass directly to the external abdominal ring, taking a vertical course and forming a part of the spermatic cord. It then passes through the inguinal canal, and reaching the internal abdomi- nal ring, turns quickly down- ward and inward to the side of the bladder upon which it descends, curving backward and downward to the neck of the bladder, where it enters the urethra Fig. 213. Cross section of vas deferens. through the pros- tate gland. In its abdominal course it lies external to the peritoneum, and along the bladder wall it arches between the latter and the ureter. Along this wall it becomes -sacculated and near its terminus gives off a lateral, enlarged, and sacculated diverticulum, the seminal vesicle. The distal end beyond the opening of the seminal vesicle, is a narrow straight tube called the ejaculatory duct. Structure. The wall of the vas deferens has three REPRODUCTIVE ORGANS IN THE MALE. 289 coats, an inner mucous, a middle muscular, and an outer fibrous. The mucous membrane generally presents two or three longitudinal folds and is lined with simple columnar epithelium. According to some investigators it may be ciliated in places and even resemble the transitional epithelium of the ureters. The membrana propria resembles that of other mucous membranes. No glands are present. The muscular layer is of the smooth variety. It con- sists of a strong inner circular and an outer longi- tudinal layer. Near the epididymis an extremely thin layer of longitudinal muscle fibers is present inside of the circular layer. The fibrous layer con- sists of loose areolar tissue, with which are asso- ciated blood and lymph vessels. Paradidymis. This consists of a set of branched tubules that leads off as blind diverticulae from the canal of the epididymis or the vas deferens. There is one diverticulum or several of them. The length of these tubules when unraveled varies from two to twelve inches, and histologically they resemble the structure of the vasa efferentia. Morphologically the paradidymis is analogous to the paroophoron found in the broad ligament of the ovary, the origin of both being associated with the development of the tubules of the mesonephros. Hydatid of Morgagni. There are two of these bodies. One of them, more constant than the other, lies usually between the globus major and the testi- cle and is called the sessile hydatid. It is a small cone-shaped body of epithelial cells and represents the peritoneal end of Muller's duct, the analogue of 19 290 NORMAL HISTOLOGY AND ORGANOGRAPHY. the fimbriated end of the Fallopian tube in the female. The other is less constant and lies usually just external to the globus major and is called the stalked hydatid. It is an epithelial body and repre- sents vestiges of the peritoneal end of the pronephric or Wolffian duct. The stalked hydatid is also present in the female, where it resembles a small cyst closely associated with one of the fimbriae of the Fallopian tube. The distance passed by the spermatozoa, before being eliminated by the urethra, is approximately twenty-four feet. The chief ducts and their lengths are: seminiferous tubules, each two feet long; epi- didymis, twenty feet; and vas deferens, two feet. The spermatozoa are themselves perfectly inactive in making this passage. During the copulation act they are discharged probably from the whole length of the vas deferens by peristaltic contraction of this duct, and not only from the seminal receptacle as formerly supposed. The supply of spermatozoa is extensive. If each testicle has eight hundred seminiferous tubules, each two feet long, then there are sixteen hundred feet of epithelial lining for each organ engaged in the production of spermatozoa. The semen consists of a fluid part, secreted mainly by accessory reproductive glands, and cell elements or spermatozoa that develop in the testes. In man there are about sixty thousand spermatozoa to each cubic millimeter of semen. Vessels and Nerves. The spermatic artery supplies the tubules of the testicles and the epididymis with blood directly from the abdominal aorta. It is a REPRODUCTIVE: ORGANS IN THE MALE. 291 long, slender artery that joins the spermatic cord as the latter passes through the inguinal canal. As the vessel approaches the testicle, it sends branches to the epididymis and then divides into other branches that ramify among the seminiferous tubules. The vas deferens receives a slender branch from one of the vesical arteries. This is called the artery of the *vas deferens, and reaches as far as the testis, where it anastomoses with the spermatic artery. The spermatic veins begin in the testis and epi- didymis and pass out at the posterior border of the organ, where they unite into large veins that form a plexus along the spermatic cord. Inside the abdo- men this plexus unites to form a single trunk, the spermatic 'vein, which on the right side opens into the vena cava, and on the left side into the renal veins. The lymphatics are very extensive and accompany the veins. They terminate in the lymphatic glands which encircle the large blood-vessels in front of the vertebral column. The nerves are derived from the sympathetic system. There is a spermatic plexus that accom- panies the spermatic artery, and some fibers from the hypogastric plexus that accompany the artery of the vas deferens. THE PENIS. The penis is a vascular organ composed principally of two corpora cavernosa, one corpus spongiosum, which encloses the urethra, and the glans, which is really the distal end of the corpus spongiosum. The integument of the penis is very thin and loosely 292 NORMAL HISTOLOGY AND ORGANOGRAPHY. attached. It is devoid of fat and hair and darker in color than the skin generally. Over the glans it is redoubled in a loose fold, the prepuce or foreskin. The inner layer of this fold is attached firmly to the base of the glans or cervix, and from there it becomes closely adherent to the glans as far as the orifice of the urethra, where it meets the mucous membrane of the latter. Over the glans it is red, thin, and moist, and beset with numerous large vesicular and Corpora caver nosa. Prepuce. Dorsal artery. Corpora cavernosa. Corpus spongiosum Urethra. Fig. 214. Cross section of penis: a, through the glans; 6, through the body. nerve papillae, but devoid of glands, excepting around the cervix, where large sebaceous glands are numerous, called glands of Tyson, which secrete a white, waxy, odoriferous substance, the smegma. The corpora cavernosa are two parallel cylindrical masses of erectile tissue that lie in the dorsum of the penis. They blend together in the anterior portion, and toward the root of the penis diverge to become firmly attached to the pubic and ischial rami. The anterior extremity of the corpora cavernosa is cov- ered by the glans penis. Structure of Corpora Cavernosa. There is a median REPRODUCTIVE ORGANS IN THE MALE. fibrous septum between the two corpora cavernosa which becomes thin anteriorly and incompletely separates the two bodies. There is an external fibrous investment, very strong and elastic. This is composed mostly of longitudinal bundles of white fibers with interlacing elastic fibers. These fibers are intimately associated with the median septum and also with connective-tissue trabeculae that ramify through the substance of the cavernous bodies. The substance of the latter is called erectile tissue and is of a spongy nature. The trabeculae anastomose and interlace freely to form a multitude of interstices or cavernous spaces. These are filled with venous blood, and are really a complex system of veins lined by a layer of flattened epithelium as in other veins. In the anterior portion of the penis the venous labyrinth of one corpus cavernosum intercommunicates with that of the other through the incomplete septum. In the erectile condition the corpora cavernosa are distended with blood which is carried away by two sets of veins, the one set joining the prostatic plexus and the pudendal veins, and the other draining into the dorsal vein of the penis. The arterial blood is supplied mainly by branches of the pudic arteries, but the dorsal artery of the penis sends a few branches through the fibrous sheath, particularly in the forepart of the organ. The arteries ramify in the trabeculae and terminate in minute capillary branches that open into intertrabecular spaces. Some of the smaller arteries project into the spaces, forming peculiarly twisted or looped vessels called helicine arteries. 294 NORMAL HISTOLOGY AND ORGANOGRAPHY. The corpus spongiosum is also composed of erectile tissue, but is a single cylindrical body that lies below and between the corpora cavernosa. Its posterior extremity is much enlarged and rounded, and is called the bulb. This lies in the ventral portion of the root of the penis just in front of the triangular ligament. Anteriorly the corpus spongiosum forms the glans penis, which caps the corpora cavernosa. The border of the glans is rounded and projecting and is called the corona glandis, behind which is a constriction of the penis, the cervix. In the whole of its extent the corpus spongiosum encloses the urethra. Structure of Corpus Spongiosum. This resembles the erectile tissue of the corpora cavernosa, and like the latter is distended with blood during erection, but is less rigid. The venous labyrinth is a finer meshwork and the trabeculae and fibrous tunic is much thinner. In the glans the meshes are particu- larly small and uniform. Plain muscle fibers enclose the urethra and also form a part of the external coat. Urethra in the Male. The male urethra extends from the bladder to the end of the penis, in length about eight and one-half inches. Its walls are in apposition, excepting during the passage of urine or semen. The urethral cleft in the glans is vertical; in the body of the penis it is transverse ; and through the prostatic portion near the bladder it is crescentic. It is lined by a mucous membrane, external to which is a double layer of smooth muscle fibers, the inner fibers disposed longitudinally and the outer circular. For descriptive purposes the urethra is divided into REPRODUCTIVE ORGANS IN THE MALE. 295 a prostatic portion, a membranous portion, and a spongy or penile portion. 1. The prostatic portion is about one and one- fourth inches in length and passes through the prostate gland. This is the widest portion of the urethra and passes vertically from the neck of the bladder to the triangular ligament of the perineum. In cross section the canal is crescentic with its con- vexity turned forward. The lining membrane pre- sents longitudinal folds, and along the posterior wall is a prominent median ridge which gives rise to the crescentic form of the urethra when seen in sec- tions. This ridge is called the crista, or verumon- tanum. The longitudinal groove on each side of the crista is called the prostatic sinus, which is pierced by numerous orifices of the prostate gland. In the middle of the crista is the orifice of a blind recess, and at the lateral margins of this are the slit-like openings of the seminal or ejaculatory ducts. The median or blind recess is a cul-de-sac which passes upward and backward for a distance of one-fourth to one-half inch, and is called the sinus pocularis or masculine uterus. It represents embryologically the fused ends of Miiller's duct, and is therefore mor- phologically equivalent to the female uterus. The epithelium of this part of the urethra resembles that of the bladder and is of the transitional variety. In the sinus pocularis it is said by some to be simple ciliated like that of the uterus. 2. The membranous portion is about three-fourths inch in length and lies between the two layers of the triangular ligament. It is the narrowest part of the 296 NORMAL HISTOLOGY AND ORGANOGRAPHY. Opening of ureter. Prostate gland. Sinus pocularis. Ejaculatory duct orifice. Urethra, membranous portion. Cowper's gland. Urethra, spongy portion. Glands of Littre. Fig. 215. Diagram of male bladder and urethra, front view. Ureter. Vas deferens. Seminal vesicle. Ejaculatory duct. Prostate gland. Cowper's gland. Corpus caverno- sum (bulbus portion). Corpus spongio- sum. Fig. 2 1 6. Diagram of male bladder and urethra, posterior view. urethra, not more than one-fifth inch in diameter, and curves so as to be directed downward and slightly forward beneath the pubic arch. The epi- thelium in this part varies. Por- tions of it resem- ble that of the bladder, but more often it presents the appearance of p s e u do-stratified with two or three layers of nuclei. 3. The spongy portion is by far the longest and most variable in length and direc- tion. Its length is about six inches, and its entire course is in the corpus spongio- sum. The epithe- lial lining near the meatus is strati- REPRODUCTIVE ORGANS IN THE MAI^E. 297 fied squamous, and directly continuous with the skin. The rest of this portion is lined by columnar pseudo-stratified epithelium with two or more rows of nuclei. The whole length of the urethra, excepting its distal end, is beset with small racemose mucous glands called glands of Littre. These vary much in size, some of them being sacculated. Most of them open in the floor of the urethra, their ducts passing obliquely for- ward through the lining membrane. In urethral infec- tions these glands become involved, as a rule, which increases the difficulty of eliminating the disease. The Urethra in the Female. The female urethra is about one and one-half inches in length, and cor- responds to the male urethra between the bladder and the opening of the ejaculatory ducts. It is directed downward and forward parallel to the an- terior wall of the vagina, to which it is attached. The transverse diameter of the closed tube is about one-fourth inch, but it is capable of great disten- tion, sufficient to admit the index finger. The ex- ternal orifice, or meatus, is a vertical slit with prom- inent margins, on which may be seen the orifices of two small glands, called Skene's glands. The latter are subject to infection in urethral disturbances and often give rise to severe irritations. THE PROSTATE GLAND. The prostate gland is a muscular as well as glandu- lar organ that surrounds the prostatic portion of the male urethra. It atrophies in the adult after cas- 298 NORMAL HISTOLOGY AND ORGANOGRAPHY. tration, and remains undeveloped if the testicles are removed in infancy, which supports the view that it is an accessory organ of generation. Its size varies considerably, but its average transverse or longest diameter is one and one-half inches, its antero-posterior diameter about three - fourths inch, and its vertical diameter one and one-fourth inches. Since the urethra, and also the ejaculatory ducts, pass through the organ, the gland on this account may be divided into three lobes. The wedge-shaped portion that lies between these ducts and the cervix of the bladder is called the middle Artery. Vein. Gland epithelium. Prostatic bodies. *tj~77>"-j -- ' Connective tissue. ^ .;:><* ^ Fig. 217. Section from the prostate gland lobe, and the rest of the gland is spoken of as the lateral lobes. It is the latter that often hypertrophy in old age and are removed in prostatectomy. The gland lies in close apposition to the rectum, and with the finger in the latter it can readily be palpated. The prostate is a compound tubulo-alveolar gland whose ducts open into the prostate portion of the urethra. Smooth muscle fibers not onlv surround the REPRODUCTIVE) ORGANS IN TH^ MALE. 299 organ, but interlace radially toward its center, form- ing a network in whose meshes the glandular parts are located. Areolar tissue and blood-vessels accom- pany the muscle tissue. The alveoli of the glands are lined by simple columnar epithelium, which sometimes show two rows of nuclei. These alveoli contain a serous acid coagulum and usually oval laminated concretions called prostatic bodies. The latter are more numerous in old men. The numer- ous excretory ducts unite to form twelve to fifteen collecting tubes which open into the urethra, most of them into the prostatic sinus. These ducts are lined by simple columnar epithelium, except near their terminations where it is transitional. The organ dorsal or in front of the urethra is mostly smooth muscle tissue. In old people the prostate gland frequently hyper- trophies and produces urethral stricture with reten- tion of urine. Prostatectomy or the removal of the lateral lobes, usually corrects this defect, but is a serious operation on account of the commonly feeble condition of these patients. Vasectomy, a much simpler operation, sometimes gives satisfactory re- sults, but is not to be relied upon. Cow per 1 s glands are a pair of small oval bodies about the size of a pea, situated in the space between the triangular ligaments and in close proximity to the membranous portion of the urethra. They are compound tubulo-alveolar mucous glands lined by simple cubical epithelium. Their excretory ducts, one for each gland, are one and one-half inches long 300 NORMAL HISTOLOGY AND ORGANOGRAPHY. and run forward near each other to open into the floor of the bulbous portion of the male urethra. In the female the analogue of these bodies is called the glands of Bartholin. They open into and lie in close apposition to the female urethra. They may be palpated in the lateral walls of the vestibule of the vagina. CHAPTER IX. REPRODUCTIVE ORGANS IN THE FEMALE. Under this head will be described the ovaries, Fallopian tubes, uterus, vagina, and mammary ^land. THE OVARIES. The ovaries are two dehiscent glandular organs that develop from the mesoderm in close apposition to the mesonephros. Each ovary measures about one and one-half inches in length, three-fourths inch in breadth, and nearly one-half inch in thickness. In early fetal life the ovaries lie close to the kidneys, but later they pass down into the pelvis where they lie in close proximity to the iliac fossa. The exact position varies considerably, but in the majority of cases they will be found placed against the side wall of the pelvis with their long axis parallel to that of the body. Each ovary is held in position by a suspen- sory ligament, which is a peritoneal fold that passes downward from the brim of the pelvis and contains the ovarian vessels and nerves, and also by the ova- rian ligament, which passes to the uterus and is really a reduplication of the broad ligament. The Fallopian tube partly encircles the ovary and also contributes to its support. Capsule of the Ovary. The external surface of the 301 302 NORMAL HISTOLOGY AND ORGANOGRAPHY. ovary is of a pale color and in early life is smooth and even, but in later life it becomes rough and marked by pits and scars. This is caused by the rupture of Graafian follicles and the expulsion of ova. It is covered by an epithelium which is continuous with that of the peritoneum, but differs from the latter Uterus. Isthmus. Fallopian tube. Ampulla. Parovarium Stalked hydatid. Labta minor a. Ureth- ra} orifice, Labia majora. Fig. 218. Diagram of female genitalia. in being lined by cubical cells instead of the. simple pavement variety. This ovarian epithelium is the germinal epithelium of embryos, from which the ova and the other epithelial cells of each Graafian follicle are derived. The germinal epithelium rests upon a rather dense investment of fibrous tissue, analo- REPRODUCTIVE ORGANS IN THE FEMALE. 303 gous to a like structure in the testicle, and is there- fore called the tunica albuginea. This is not a dis- tinct tunic but rather a condensed part of the ova- rian matrix. It is not well defined, and is difficult to demonstrate in sections. The medulla is practically the core of the ovary and consists of a fibro-muscular matrix well supplied Young follicle with ovum. Primordial Germinal epithelium. Ovum "with follicular epithelium. Fig. 219. Section from ovary of adult dog. At the right the stellate figure represents a collapsed follicle with its contents. Below and at the right are seen the tubules of the parovarium (copied from Waldeyer). with blood- and lymph- vessels. In this substance may be found connective-tissue cells, connective- tissue fibers and a limited supply of smooth muscle fibers. The Cortex. Between the medulla and the cap- sule is the cortex. It is a broad zone, not well de- fined, in which are found the same elements as in the 304 NORMAL HISTOLOGY AND ORGANOGRAPHY. medulla, and in addition Graafian follicles in different stages of development, and in older ovaries also cor- pora lutea. Young Graafian Follicles. Early in embryonic life, when the ovary is clothed with the germinal epi- thelium which later becomes the epithelium of the capsule, epithelial buds or strings of epithelial cells push their way into the ovarian cortex. These buds soon lose their connection with the germinal epithe- lium and form little groups or nests of cells known as young Graafian follicles. In each follicle one cell takes a central posi- tion and is the egg cell or ovum, des- tined, under proper conditions, to de- velop into a new be- ing. The ovum in- creases rapidly in size, receiving pro- tection and possibly nourishment from the investing cells. The repro- ductive cells, both ova and spermatozoa, can thus be traced directly from the germinal epithelium, which is of mesodermic origin and closely related to the pavement epithelium of the peritoneum. The Graafian follicles occupy the cortical layer of the ovary. They are all formed during embryonic life, and whatever influence environment has upon the offspring, that influence leaves its impression not upon the origin of the reproductive cells, but upon their later development. At time of birth it is esti- Germinal epithelium. Young Graafian follicle. Fig. 220. Section from ovary of young dog. REPRODUCTIVE ORGANS IN THE FEMALE. 305 mated that there are thirty-five thousand eggs or ova to each ovary. Only a small number of these ripen and become discharged as mature eggs. The extrusion of these eggs from the ovary is known as ovulation, and in woman is supposed to occur during the menstruation period, one from each ovary. Menstruation in a normal woman extends generally over a period of thirty-two years, between the ages of thirteen and forty-five. If thirteen eggs ovulate yearly from each ovary, there will be a possible total Follicular cavity. Ovum. Nucleolus. Nucleus. Ovum. Nucleolus. Nucleus. Fig. 221. Young Graafian follicles: a, follicle with one layer of epithe- lial cells; b, follicle with two layers of epithelial cells. of eight hundred and thirty-two that may ripen dur- ing the life of a woman, allowing no interruption for pregnancies. After the menopause, ovulation is supposed to cease. The Ripe Graafian Follicle. It has already been stated that only a small number of the young ova ripen and ovulate. These mature in the following manner : The ovum of such follicles occupies a cen- tral position where it accumulates food and grows into a large spherical cell. The investing epithelium 306 NORMAL HISTOLOGY AND ORGANOGRAPHY. forms at first a single layer of cells. These remain small and multiply rapidly, forming two layers of cells between which, at one side of the follicle, a cavity appears. As the follicle grows larger this cav- ity, which is eccentric in position, becomes filled with a fluid called the follicular fluid. The ovum remains attached to the side of the follicle and be- comes surrounded by several layers of cells called the discus proligerus. The outer layer also multi- Theca. Follicular cavity. Stratum granu- losum. Discus proligerus. Ovum. Fig. 222. Ripe Graafian follicle. plies, forming eight to twelve layers of cells and is then called the stratum granulosum, to which the discus proligerus is attached. External to the stratum granulosum a connective-tissue envelope forms, called a theca. This theca develops from the ovarian stroma and consists of two layers, an ex- ternal, the theca fibrosa, and an internal, the theca vasculosa, the latter being supplied with a fine plexus of lymph- and blood-vessels. The mature REPRODUCTIVE ORGANS IN THE FEMALE. 307 Graafian follicle is thus a sphere that measures from one-twentieth to one-sixth of an inch in diameter, and lies immediately beneath the surface epithelium of the ovary. The manner in which such follicles rupture has been variously explained. One rational theory is Granular layer of large Graafian follicle. % Fig. 223. From ovary of young girl (Bohm and Davidoff). that the pressure of the accumulated f ollicular liquid obliterates the blood-vessels in the theca vasculosa next to the ovarian epithelium. This establishes a point of least resistance at this place, the follicle ruptures, and the follicular fluid with the ovum is discharged upon the surface of the ovary, the 308 NORMAL HISTOLOGY AND ORGANOGRAPHY. stratum granulosum and most of the discus pro* ligerus remaining behind in the ovary. The Ovum. The ovum has already been men- tioned as a large spherical cell with a large accumu- lation of food material. It measures 0.2 mm. in diameter and is barely visible to the naked eye. When examined under the microscope, even before the rupture of its follicle, it is found encircled by a clear substance called the zona pellucida, which upon a closer examination may be found to contain trans- verse striations, hence it has also been called zona radiata. It is not uncommon for a few of the epi- thelial cells of the dis- Corona radiata. CUS proKgerUS tO TC- ona peiiudda. main attached to this Germinal spot. layer, if so they are Germin ntc ieus^ le r called the corona radi- ata. The zona pelluci- da is a secretion from Fig. 224. The ovum. the adjoining cells, and one theory of the trans- verse striations is that they are produced by minute cellular processes from the cells that form the corona radiata; that is, the first row of epithelial cells in- vesting the ovum. It is affirmed by some that these processes are in direct communication with the sub- stance of the ovum and are the means by which elaborated food material is contributed to the latter. The zona pellucida no doubt serves to strengthen the delicate ovum after its expulsion from the ovary. The transverse striae in this membrane may serve a further purpose as primitive channels for the entrance REPRODUCTIVE ORGANS IN THE FEMALE. 309 of spermatozoa, only one of which penetrates the substance of the ovum. The substance of the ovum is known as the mtellus. It is a soft semifluid substance composed of cyto- plasm in which is deposited a liberal supply of food material called deutoplasm. The nucleus of the Primordial egg-cell. Germinal zone. Zone of mitotic division. (The number of gen- erations is much larger than here represented.) Zone of growth. Zone of maturation. Oocyte I. order. Oocyte 11. order. ^^A V I. P.B. \ f\ \ Matured ovum. ^r II. P.B. Fig. 225. Scheme of the development and maturation of an ascaris ovum (after Boveri): P. B., Polar bodies. (From "Ergebn. d. Anat. u. Entw.," Bd.-I.) ovum is called the germinal 'vesicle, which is placed to one side of the cell. The nucleus is unusually large, about 0.05 mm. in diameter, and has all the characteristics of an ordinary cell nucleus. This was first described by Purkinje in the ovum of birds in 1835. There is a well-defined nuclear membrane 310 NORMAL HISTOLOGY AND ORGANOGRAPHY. which encloses a clear nuclear substance in which there is a limited amount of chromatin, and the nu- cleus, therefore, does not stain heavily. The nucleo- lus, on the other hand, is very prominent and is called the germinal spot. Not infrequently two nucleoli may be found. There is some doubt whether a cell membrane to the ovum is present before fer- Primordial sexual cell. Spermatogonia . \ Spermatocyte I. order. Spermatocytes II. order. Spermatids. Zone of proliferation. (The generations are much larger.) Zone of growth. Zone of maturation. Fig. 226. Schematic diagram of spermatogenesis as it occurs in ascaris (after Boveri). (" Ergebn. d. Anat. u. Entw., " Bd. I.) tilization. After fertilization such a membrane appears and is called the mtelline membrane. As a rule each Graafian follicle contains one ovum ; in rare cases follicles are found with two and even with three ova. When a Graafian follicle ruptures and an ovum is REPRODUCTIVE ORGANS IN THE FEMALE. 311 expelled, a great activity is at once manifest in the ovum, whether it is fertilized or not. The nucleus, which is near the margin of the ovum, divides in a few hours, extruding what is termed the first polar body. This is normal cell division, or mitosis. A second division quickly follows, resulting in a second polar body. Meanwhile the first polar body may also divide. This second division results in a reduc- tion of one-half the number of chromosomes, and the nucleus thus reduced is called the female pro- nucleus, which is now ready to unite with the male pronucleus of the spermatozoon and complete the process of fertilization. The phenomenon manifest in the extrusion of the polar bodies is known as maturation of the ovum, and seems to be an attempt on the part of -the ovum to develop into a new indi- vidual without the process of fertilization; that is, parthenogenetically. If the ovum is not fertilized, it shows no further activity and is lost. If the ovum is fertilized it continues to divide regularly and in a short time develops into the embryo. The developmental history of ova is full of interest. They are very numerous and develop so very early in embryonic life. During childhood they grow large and accumulate a liberal storage of food, while the sister epithelial cells that form the Graafian fol- licle remain small and multiply rapidly to form the ripe follicle. This latent condition extends over a period of fifteen to forty years. When the ripe fol- licle finally ruptures and the ovum is eliminated, a rapid segmentation quickly follows resulting in the extrusion of the polar bodies. This is followed by a 312 NORMAIv HISTOLOGY AND ORGANOGRAPHY. second passive period, unless fertilization takes place, when the ovum rapidly develops into a new being. By far the large majority of the ova remain undeveloped in the ovarian cortex, where they seem to pass merely a passive existence. We have no explanation of these phenomena beyond attributing them to heredity, the nature of which is still highly speculative. The Corpora Lutea. A corpus luteum is the modi- fied Graafian follicle after its rupture and discharge Surface of ovary. Epithelial cells. Connective-tissue cells. Fig. 227. Section of corpus luteum. of the ovum. This follicle remains permanently in the cortex of the ovary as a scar. When the rupture takes place the follicular cavity fills up with an exudate and an infusion of blood from the rup- tured blood-vessels. This coagulum is quickly in- vaded by white blood-corpuscles, connective-tissue cells from the theca, and epithelial cells from the stratum granulosum. The corpus luteum thus ulti- mately shows a uniform distribution of epithelial REPRODUCTIVE ORGANS IN THE FEMALE. 313 cells and connective- tissue cells. If the ovum be- comes fertilized and pregnancy follows, the corpus luteum continues to grow until it becomes many times larger than the original Graafian follicle, causing a rounded elevation at that point on the surface of the ovary. This kind is called a true corpus luteum. On the other hand, if the ovum is not fertilized the corpus luteum shrinks and becomes smaller than the original follicle. This kind is called a false corpus luteum. The corpus luteum is at first well defined by the investing follicular theca, but after a time its limits are less distinct, so that as age advances the ovarian stroma becomes gradually pervaded with cells like those of the corpora lutea. THE FALLOPIAN TUBES. The Fallopian tubes are two ducts for the passage of ova from the ovary to the uterus. They differ from the ducts of other glandular organs in being detached from the organs whose secretions they con- vey. They are from four to five inches long and pass almost horizontally outwards from the fundus of the uterus. When they reach the ovary they ascend along the pelvic floor and nearly encircle each organ, passing up the external and down the internal or mesial margins. Each tube is enclosed in the free margin of the broad ligament, which is a peritoneal fold that also contains the round ligament of the uterus, the ovary, parovarium, and numerous blood- and lymph- vessels. For descriptive purposes each duct is divided into an isthmus, an ampulla, a neck, and a fimbriated ex- 314 NORMAL HISTOLOGY AND ORGANOGRAPHY. tremity. The isthmus is smooth and round, about one inch in length, and opens into the fundus of the uterus by a small orifice that will barely admit an ordinary bristle. It is a straight and narrow part of the duct, about 2 to 3 mm. in diameter. The am- pulla encircles the ovary and is at least twice the size of the isthmus. It is also less firm to the touch, Longitudinal muscle. Ciliated ^ epithelium Fig. 228. Cross section of ampulla of Fallopian tube. being flabby while the isthmus is cord-like. The neck is an annular constriction between the ampulla and fimbriated extremity, the latter being a' funnel- shaped expansion of the ovarian end of the tube, which terminates in a number of irregular processes called fimbrice. The fimbriae vary considerably in size and number. Many of them are branched, and REPRODUCTIVE ORGANS IN THE FEMALE. 315 one is particularly long and attached to the upper end of the ovary. Structure. The Fallopian duct is a muscular tube lined the whole length by a mucous membrane clothed with simple ciliated columnar epithelium. This mucous membrane is thrown up into longitudi- nal folds that are very broad and numerous in the wide portions of the tube and in the narrow portions less conspicuous. It is continuous on the one hand Longitudinal muscle. Circular muscle. Ciliated epithelium. Fig. 229. Cross section of isthmus of Fallopian tube. with the mucous membrane of the uterus, and at the other end of the tube with the serous lining of the peritoneum, being one example of a direct conti- nuity of a mucous and a serous membrane. Glands, so numerous in the mucous membrane of the uterus, are absent in the Fallopian tube. The mucosa rests upon a thin vascular submucosa composed of areolar tissue. External to the sub- mucosa there is a muscular coat consisting of a thick inner circular layer and a thin outer longitudinal 31 6 NORMAL HISTOLOGY AND ORGANOGRAPHY. layer of smooth muscle fibers. Externally the tube, is practically enclosed by the peritoneum, forming a serous covering. Embryologically, each Fallopian tube represents a Mlillerian duct, which is derived from the mesoderm. In the male the Wolffian duct develops into the vas deferens. This duct, which is rudimentary in the female, is called Gartner's duct, and lies parallel to the Fallopian tube, between the latter and the round ligament. The round ligament extends from the uterus to the internal abdominal ring in nearly the same position as the vas deferens does in man. Fertilization, as a rule, takes place in the upper part of the Fallopian tube. In cases of tubal preg- nancy the ovum does not reach the uterus but finds lodgment in the tube. The much-folded mucous membrane allows considerable distention, but ulti- mately the rapidly growing embryo ruptures the tube, with serious complications resulting from in- ternal hemorrhage. Usually the ova pass down the tube on the corresponding side, but it is possible for the ova from one ovary to pass down the tube of the opposite side. Experimentally, the right ovary and the left tube may be removed in the dog and the ani- mal still become pregnant. The ovaries have a marked influence on the devel- opment and mentality of a woman and their removal, prior to the menopause, is followed by deleterious results, much the same as the removal of the testes in the male. While extracts from certain organs, such as the thyroid gland and the suprarenal bodies, have specific medicinal properties, extracts from the REPRODUCTIVE ORGANS IN THE FEMALE. 31? ovaries give no satisfactory results. Its potency is manifest only by the living organ in the performance of its normal function. The ovary or fragments of it will readily grow in other parts of the body, and has been successfully grafted from one animal to an- other. The Parovarium, or Epoophoron. The organ bearing this name lies in the broad ligament lateral to the ovary and between the latter and the tube. It consists of a number of closed epithelial tubules which can usually be seen by holding this part of the ligament up against the light. Embryologically they represent the upper portions of the Wolffian duct and some of the attached tubules of the meso- nephros, and correspond to the vasa efferentia in the male. The paroophoron represents vestiges of tubules similar to the parovarium, situated in the broad liga- ment below the ovary. They correspond to the paradidymis in the male. Being lined by epithelial cells, either of these or- gans may develop into parovarian or paroophoron cysts, the former being more common. THE UTERUS. The uterus, or womb, is a hollow muscular organ, with thick walls, placed in the pelvic cavity between the bladder and rectum. In case of pregnancy it receives and nourishes the ovum and later expels the fetus at the end of pregnancy. During gestation, and also periodically during menstruation, it is sub- ject to marked physiological and structural changes. 318 NORMAL HISTOLOGY AND ORGANOGRAPHY. It is therefore an organ in which great activity is manifest during the greater part of adult life. The fully developed virgin uterus is a pear-shaped organ, flattened from before backward, free above, Fig. 230. Arrangement of uterine muscle, as seen from in front after removal of serous coat (Helie). and connected below with the vagina, into which its lower extremity projects. Its average dimensions are, three inches in length, two inches in breadth at its upper and widest part, and one inch in thickness. REPRODUCTIVE ORGANS IN THE FEMALE. For descriptive purposes it is divided into fundus, body, and neck, or cervix. The fundus is the broad convex upper end that lies above the attachment of the Fallopian tubes. It is chiefly this part that expands in case of pregnancy. The body is the part between the fundus and neck. This part tapers downward with convex sides. The Fig. 231. A, Isolated muscle-elements of the non-pregnant uterus; B, cells from the organ shortly after delivery (Sappey). neck, or cervix, is about one inch long, cylindrical, and projects into the anterior part of the upper end of the vagina. The projecting portion is called the vaginal part, and has a transverse oval aperture, called the os uteri, which communicates with the cavity of the uterus. The latter is a triangular 320 NORMAL HISTOLOGY AND ORGANOGRAPHY. cavity, so flattened that the anterior and posterior uterine walls touch each other. The base of the cavity is in the fundus and is convex downwards. The two Fallopian tubes open into the upper angles each by a small aperture that will barely admit a bristle. The cavity tapers gradually toward the cervix, where it becomes constricted to form the in- ternal os The peritoneum covers the fundus and Mucosa. Gland. Muscular layer Serous layer. Fig. 232. Cross section of wall of uterus. body of the uterus, and posteriorly extends down- ward to clothe the upper posterior wall of the vagina. It is then reflected back over the rectum, forming a a sac called the pouch of Douglas. This makes it possible to open the peritoneal cavity by a puncture through the upper posterior vaginal wall, an operation which establishes free drainage to the female pelvis. REPRODUCTIVE ORGANS IN THE FEMALE. 321 Anteriorly the peritoneum does not cover the whole uterus, but at the junction of the body with the neck, it is reflected back over the bladder wall form- ing the utero-vesical pouch. Structure. The histology of the uterus resembles that of the Fallopian tubes, and the layer of the one is continuous with that of the other. Embryologi- cally, these two structures, and also the vagina, de- velop from the Mullerian ducts, the uterus and Muscular is. Fig. 233. Diagonal section of the uterine mucosa. vagina representing the fused lower ends of Muller's ducts. The whole uterus, including the epithelial lining, is therefore of mesodermic origin. The uter- ine wall is composed of a mucosa, muscular, and serous layer. The Fallopian tube has a submucosa which is absent in the uterus. The mucosa, or endometrium, is the inner layer and is lined by simple columnar ciliated epithelium, which at the external os changes to the stratified variety of the vagina. It has a rich supply of 21 322 NORMAL HISTOLOGY AND ORGANOGRAPHY. branched tubular mucous glands which, in the cer- vix, are very large and have a tendency to become sacculated. These glands extend radially as far as the muscularis, and some of them may even pene- trate a short distance into the muscle coat. The gland ducts are lined by ciliated epithelium, while in the deeper portions cilia are absent and the epi- thelium becomes simple cubical, resembling a glandu- lar type. Most of these glands take a tortuous or spiral course, and are separated from each other by an interstitial tissue composed of connective-tissue cells. These cells are of the embryonic type, rich in chromatin and therefore stain heavily with nuclear dyes. The relative amount of interstitial and glandular tissue in a normal uterine mucosa should be approximately equal parts. The connective tissue predominates in interstitial endometritis, and the glandular tissue in adenitis. The whole uterine mucosa is unusually thick and very vascular. In a mature woman it is normally subject to marked periodic changes resulting from menstrual condi- tions, which reach a high degree of complexity in case of pregnancy. The action of the cilia tend to produce a downward movement of the uterine secretions and, therefore, opposite to the upward movement of spermatozoa. The Muscular Layer. The muscularis is an unusu- ally thick layer of smooth muscle cells which in the non-pregnant uterus measure forty to sixty mi- crons, while at the end of pregnancy the cells measure four hundred to six hundred microns in length. These muscle cells are arranged in bundles with a considerable amount of connective-tissue fibers and REPRODUCTIVE ORGANS IN THE FEMALE. 323 cells interlacing them, imparting strength and elas- ticity to the uterine wall. There has been consid- erable discussion as to the exact disposition of the different layers of this musculature which, in a gen- eral way, may be divided into three strata: (i) an inner layer of longitudinal fibers, by some called the muscularis mucosa; (2) a middle circular layer, and (3) an outer thin layer of fibers that run diago- nally or somewhat irregularly. The inner layer is much the thickest; none, however, is sharply de- fined. The serous coat is the peritoneal lining which con- sists of connective- tissue elements and an invest- ment of simple pavement epithelium. Vessels and Nerves. The arteries that supply the uterus are arranged in two pairs, the uterine and ovarian. The uterine artery is a branch of the an- terior division of the internal iliac. It reaches the upper portion of the vagina, and then ascends in a very tortuous manner along the lateral border of the uterus to the fundus, where it divides into two branches, one of which anastomoses with the ovarian artery and the other supplies the Fallopian tube. From the ascending portion many side branches are given off which penetrate the uterine wall and ramify in the muscle tissue and the mucosa. These branches are very tortuous so that the uterus can expand in pregnancy without breaking the vessels. The veins are very large and have no valves. They form large sinuses mostly along the lateral walls, from which the blood is collected into two trunks: (i) the uterine vein accompanies the uter- 324 NORMAL HISTOLOGY AND ORGANOGRAPHY. ine artery and empties into the internal iliac vein ; (2) vessels communicate with the ovarian or pam- piniform plexus which drains through the ovarian veins. The lymphatics begin in the interstitial substance of the mucosa, and uniting with lymphatics from the muscular is, emerge to form a rich plexus just beneath the serous covering. This plexus drains along two channels: (i) by lymphatic vessels that accompany the uterine veins; (2) vessels that ac- company the ovarian veins. The blood and lymph drainage is therefore in two directions. That of the f undus is toward the ovary, and that of the body and cervix is in the opposite direction along the uterine vessels. This is of clinical importance in the spread of infections. The nerves are non-medullated fibers from the in- ferior hypogastric plexus, and medullated from the third and fourth sacral. The non-medullated sup- ply the muscle while the medullated fibers have been traced to the mucosa, where they form a plexus from which fibers pass to the surface epithelial cells. An- other set arborize about the mucous gland cells. Sympathetic-nerve ganglia are associated with the non-medullated fibers. Menstruation. This consists of a hemorrhagic and mucous discharge from the uterus, which recurs about every twenty-eight days in the non-pregnant woman between the ages of thirteen and forty-five. It is accompanied by more or less severe systemic disturbances of a neurotic nature, and also by in- creased activity of the glandular system as a whole, REPRODUCTIVE ORGANS IN THE FEMALE. 325 Enlargement of the thyroid gland usually accom- panies the menstrual flow. From five to ten days before the menstrual flow begins there is a marked hyperemic condition of the C Fig. 234. Uterus during menstruation, cut open to show the swelling of the whole organ, and particularly the mucous membrane: A, Mucous membrane of cervix; B, C, mucous membrane of corpus, much thickened; D, muscular layer; E, uterine opening of tube; F, os internum (the mu- cous membrane tapers down to these openings) (Courty). uterine wall. The congestion of blood causes a marked swelling and growth of the uterine mucosa, so that it attains a thickness of 6.0 mm. This mu- cosa is then called the decidua menstrualis. After these changes have occurred the menstrual flow be- 326 NORMAL HISTOLOGY AND ORGANOGRAPHY. gins and usually lasts for four days. This results in a complete or partial exfoliation of the superficial part of the mucous membrane of the uterine fundus and body, but does not involve the cervix. The exfoliation begins at the internal os and advances progressively toward the fundus. The restoration of the mucosa proceeds in the same order, from be- low upward, and in the course of five or six days the mucous membrane is restored. The epithelial lining regenerates from the free ends of the mucous glands that did not partake in the exfoliation. The uterus is thus a seat of great physiological activity. During at least one-half the menstrual period of twenty-eight days there are marked struc- tural changes manifest in the uterine mucosa. In such an active organ pathological disturbances are naturally of frequent occurrence. Menstruation and ovulation are related phenom- ena, and yet there is evidence that neither one de- pends on the other. Pregnancies may occur before the menstrual period is inaugurated. Even at the early age of nine years pregnancy has been reported ; also in mature women after confinement, but before menstruation has reappeared, pregnancy may occur. A woman who does not menstruate does not become pregnant, as a rule, but there are exceptions. Ovu- lation, therefore, may go on without menstruation, and there is evidence that menstruation may prevail without ovulation. For a further discussion of this subject, see Raymond's " Human Physiology." REPRODUCTIVE ORGANS IN THE FEMALE. 327 PREGNANCY. During pregnancy the mucous membrane of the uterus becomes modified into a membrane called the decidua gramditatis. This membrane may be divided into (i) the decidua serotina or basalis, that part of the mucosa to which the ovum is attached and in which the placenta develops; (2) the decidua re- flexa, that which envelops the ovum, and (3) the decidua vera, the part that lines the rest of the uterus. In the early stages of pregnancy the changes in the uterine mucosa resemble those of the decidua men- strualis. At the end of the first half of pregnancy the decidua serotina is i cm. thick. The epithelial lining has disappeared and two layers can be recog- nized: (i) a superficial compact layer, and (2) a deep spongy layer. The compact layer consists of con- nective-tissue elements and some very large pig- mented cells called the decidua cells. These cells usually have one large nucleus, but some of them may be polynucleated. The cells are thirty to one hundred /* in diameter, and oval or elongated, re- sembling epithelial cells, although they are supposed to develop from the interstitial connective-tissue cells of the normal mucosa. They are of diagnostic value in uterine curetments where they become a probable evidence of pregnancy. In post-mortems they have no medico-legal significance, as these cells may be found in the decidua menstrualis. In the spongy layer the connective-tissue cells form septa between the flattened and sacculated as well as tor- 328 NORMAI, HISTOLOGY AND ORGANOGRAPHY. tuous mucous glands. Blood-vessels also form a plexus in the spongy layer. The decidua reflexa disappears during the first months of pregnancy, while the decidua serotina enters into the formation of the placenta. Placenta. The placenta is a vascular organ for the nourishment of the fetus, and serves the purpose of bringing the fetal and maternal blood into closest proximity without actually blending. The organ Villi. .:- ' ^' ~'2J^4- *"*';-*-* ^* ^> ^s^ Compact layer of de- -* + ffx*r- "^r^ ' .~~ -Vt T'. cidua with decidual i~- ~ ^ .*~ ^~ ^ r7 ^- "* '."'^^L^-- ' Spongy layer of de- cidua -with gland spaces. Fig. 235. Section of the decidua serotina at the margin of the placenta. may be divided into an embryonic part, the placenta jcetalis, and a maternal part, the placenta materna. The latter is the modified uterine mucosa or decidua serotina. The fertilized ovum usually finds lodgment in the fundus of the uterus. Very early it becomes en- closed in an envelope of its own production called the chorion. This chorion has an outer epithelial REPRODUCTIVE ORGANS IN THE FEMALE. 329 Blood capillaries. Zellknoten. layer and an inner connective-tissue layer, the latter being vascular. It is this vascular chorion that enters into intimate relations with the uterine mucosa to form the placenta, the former the fetal part and the latter the maternal part. These parts become intimately associated. The chorion very early produces a large number of villi which invade the mucosa, where they ultimately become large and much branched, like the root of a tree. These are the chorionic villi and belong to the fetal pla- centa. Each villus consists of a connective- tissue core with an epithelial lining. The fetal blood cir- culates through the connective- tissue core while the maternal blood bathes the external sur- faces of the villi. The terminal ramifications of each villus becomes firmly anchored iii the uterine mucosa, while the branched lateral twigs float freely in the maternal blood spaces or intervillus sinuses. These villi serve a double purpose. They attach the fetal placenta firmly to the uterus, and establish a close relation between fetal and maternal blood whereby the embryo receives proper nourishment. Upon closer examination each villus should present in cross section an outer layer of simple squamous Syncytium or protoplasmic coat. Epithelial cells. Fig. 236. Cross section of two human chorionic villi at end of pregnancy. 330 NORMAL HISTOLOGY AND ORGANOGRAPHY. epithelial cells and a core of connective-tissue cells in which two or more small capillary blood-vessels ramify. The epithelium of the villi undergoes great alterations and may entirely disappear, to be re- placed by isolated accumulations of large round nuclei that stain intensely with nuclear dyes, and that form protuberances on the surfaces particularly of the large villi. These are called zellknoten, or cell knots. Their origin and significance is doubtful. In the earlier months of pregnancy the epithelial in- vestment of each villus is clothed externally by a continuous protoplasmic mass, called the syncytium, containing small and irregularly scattered nuclei. It is generally supposed that the syncytium repre- sents the modified and disintegrated uterine epi- thelium and is therefore of maternal origin. Some embryologists affirm that in some villi there is a membrane external to the syncytium which mor- phologically represents the epithelial wall of the uterine blood-vessels. The maternal blood, how- ever, very soon breaks through the capillary spaces of the uterine mucosa and enters the intervillus sinuses clothed by the syncytium. The fetal cir- culation is a closed system and nowhere is there a direct intermingling of fetal and maternal blood. The- oretically the exchange of gases, in the early stages of development, takes place through (i) the epi- thelial wall of the maternal capillaries; (2) the uter- ine epithelial lining, probably represented by the syncytium; (3) the epithelial lining of the chorion, and (4) the epithelial wall of the fetal blood capilla- ries. The first of these membranes is not always PLATE VI. B2*1 O p "^cra M ct- 3 O >P O g ^ (D &:<"*-. hrj ^ a- . ff a rj 3 CD HH tr* p a* <->- O o p <5 ^ 3|~i.a REPRODUCTIVE ORGANS IN THE FEMALE. 331 present, as maternal blood ruptures this wall. The second investment, or syncytium, disintegrates. The third also disappears, at least in parts, or be- comes so thin that it can scarcely be detected. Ulti- mately, therefore, the fetal and maternal blood is practically separated by only one membrane, the epithelium of the fetal capillaries. The maternal placenta does not differ structurally from the histology of the decidua already described except in degree of complexity. There is an inter- nal compact portion and a deeper spongy layer. The latter rests against the uterine muscularis and is very vascular. Numerous blood- vessels penetrate the compact layer to open freely into the intervillus sinuses already de- scribed. These blood-vessels usually take a very tortuous course, and they are thus able to adjust themselves to contractions and expansions of the uterine wall. Decidual cells are especially conspicuous in the compact layer of the placenta. These cells are sometimes present in the spongy layer but never in the chorionic villi or fetal portions of the placenta. The fetal blood reaches the placenta through the umbilical cord. There is regularly present two ar- teries and one vein, imbedded in a gelatinous con- nective-tissue matrix known as Wharton's jelly. The blood in the arteries is carried to the placenta and is venous. That in the vein returns from the placenta and is arterial. After birth, when the pla- 33 2 NORMAL HISTOLOGY AND ORGANOGRAPHY. centa comes away, it is always at. the expense of the uterine mucosa, which leaves a raw, bleeding surface. The uterine muscles at once contract, reducing the uterine cavity and checking the hemorrhage. A normal mucous membrane at once regenerates, the ciliated epithelium and mucous glands developing from remnants of glands that were not entirely ob- literated by the placental growth. As a rule regular menstruation is inaugurated when lactation ceases, but there are exceptions to this. A second preg- nancy may follow without any intervening menstrual period, but this is rare. THE MAMMARY GLAND* The mammary gland is a skin gland that is present in both sexes. In the second month of embryonic life there is a linear thickening of the skin, extending from each axilla to the groin, and at regular inter- vals in this ridge a series of mammary glands develop in many vertebrates. In the human race only one pair is produced, which represents the fourth or fifth pair of this series. In rare cases accessory mam- mary glands are found in man both above and below the normal pair. In childhood the mammary gland is identical in both sexes, but with approaching puberty it enlarges in the female, reaching its highest development at the end of pregnancy. The menopause brings about a retrogression and shrinkage of the organ. The gland is therefore to be considered an accessory sexual organ. The mammary gland is a segregation of fifteen to REPRODUCTIVE ORGANS IN THE FEMALE. 333 twenty separate compound tubulo-alveolar ' glands which open separately on the nipple by an equal number of pores. These glands are arranged radi- ally and enclosed by a variable supply of fat and connective tissue in such a way that it is possible to divide the breast into fifteen to twenty lobes, which may be further divided into lobules. Each pore leads to a narrow vertical tube, the lactiferous duct, which widens just below the base of the nipple to form a receptacle called the milk sinus, be- yond which it again becomes a narrow tube. The latter be- comes branched to form interlobular ducts that open into distal dilata- tions or alveoli, which constitute the secreting por- tions of the gland. The lactiferous ducts and sinus are lined with simple columnar epithelium, which becomes strati- fied near the orifices where it is directly continuous with the stratified epithelium of the skin. The finer structure of the alveoli varies according to the func- tional activity of the organ. During lactation the alveoli are distended with milk. The cells of the Lactiferous duct. Milk sinus. Alveoli. Fig. 238. Diagram of one-half of mammary gland, dissected to show gland. 334 NORMAL HISTOLOGY AND ORGANOGRAPHY. simple glandular epithelium become distended with the products of secretion that consist of granules and deposits of fat. The granules liquefy and along with the fat globules are discharged into the alveoli as milk. Many particles of fat are taken up by migra- ting white corpuscles, called phagocytes, which mix with the secretion and thus become converted into the colostrum corpuscles of early lactation. The gland cells after secretion accumulate a second sup- ; cells. Connective tissue Gland alveoli. Fig. 239. Section of a portion of the mammary gland. ply, and this process is repeated many times. The secreting cell does not disintegrate as is the case in the sebaceous glands of the skin. When the gland is not engaged in the secretion of milk, many of the alveoli shrink up and disappear, while the remaining ones become much reduced in size, and the gland as a whole is smaller. The cells of the alveoli become columnar, resembling the cells that line the ducts. The epithelium rests upon a REPRODUCTIVE ORGANS IN THE FEMALE. 335 basement membrane and a membrana propria, the latter containing basket cells whose processes mingle with the glandular epithelium. The interstitial tissue just external to the alveoli is composed of connective-tissue cells that stain heavily with nuclear dyes, while in the intervening spaces between the alveoli, connective-tissue fibers and fat cells are abundant. A supply of plain muscle fibers intervene and surround the ducts in the nip- ple. The fibers placed longitudinally function in the erection of the nipple, while the circular ones constrict the ducts. The nipple does not Epithelial Connective-tissue 1 1 . . 1 r . cells. cells. develop until after birth. Its normal po- sition is in the fourth in- tercostal space, about four inches from the sternum. It is clothed with Stratified pig- Fig. 240. Section through two alveoli mented epithelium and devoid of hair follicles and sweat glands. The skin immediately around the nipple is also pigmented, forming an areola with numerous small papillae, giving a rough or wrinkled appearance. Besides large sweat glands, twelve or more large sebaceous glands, called glands of Mont- gomery, are present in this area. These glands open at the apices of the small papillae just mentioned, and are usually considered as accessory milk glands. Vessels and Nerves. The arteries that supply tl^ breasts are the long thoracic, the internal mammary, 336 NORMAL HISTOLOGY AND ORGANOGRAPHY. and the intercostals. These anastomose freely and approach the gland from all directions. The veins are equally extensive. They accompany the arteries and bear the same names. Lymphatics are very numerous and form extensive lymph spaces around the alveoli of the gland. For the most part they drain to the lymph glands in the axilla, but from the deeper part of the breast they drain along the course of the internal mammary artery. CHAPTER X. THE SKIN. The skin covers the entire body and is directly continuous with the mucous membranes of the ali- mentary canal and urogenital organs at their external orifices. It contains sensory nerve endings and in the deeper layers there is a liberal supply of both blood- and lymph- vessels. It is the chief factor in regulating body temperature, and is an efficient mechanical protection to the deeper tissues, while the sweat and sebaceous glands render it an important excretory organ. Hairs and nails represent modi- fications of the superficial layers. It varies consid- erably in thickness, being 4 mm. thick on the palms of the hand and 0.5 mm. over the back and shoul- ders. The color is imparted by pigmentation and the blood supply. The color is characteristic of races and variable in the different parts of the body as well as subject to modification depending on age and dis- ease. The skin moves freely upon the deeper tissues, excepting over bony prominences where it is more firmly attached. On the palm of the hand and the sole of the foot it is also bound down to the subjacent tissues. The external surface presents in places numerous permanent ridges which correspond with rows of underlying papillae, and which in criminals are utilized for the purposes of identification. The 22 337 338 NORMAL HISTOLOGY AND ORGANOGRAPHY. hair follicles appear with regularity in external de- pressions practically all over the body, forming dis- tinctive patterns. Cutaneous blood-vessels and Sweat gland. Corneum. Stratum lucidum. Stratum granulosum. ? Malpighian layer. Papilla of dermis. Dermis. Fat cells. > Sweat gland. Fig. 241. Section of skin through palmar surface of fingers. tendons form ridges and lines readily detected by the unaided ye, and over which the skin is freely movable. THE SKIN. 339 Structurally the skin or integument consists of two chief strata that may be subdivided into layers in the following manner: I. Epidermis epithelial layers derived from the ectoderm. 1. Corneum or horny layer, superficial epithelial plates. 2. Stratum lucidum, absent where the skin is thin. 3. Stratum granulosum, absent where the skin is thin. 4. Malpighian or germinal layer, nucle- ated growing cells. II. Dermis or corium, connective-tissue elements from the mesoderm. 1. Papillary layer. 2. Reticular layer. Epidermis. The horny layer of the epidermis forms the outer covering of the skin and consists of several layers of scaly epithelial cells in which the nuclei have disappeared. The cells are dead and constantly exfoliating superficially, while new strata are regularly added from below. Bacteria are usually present in its external parts, and in surgi- cal operations, therefore, the skin is thoroughly scrubbed, a process that removes most of this layer and renders the field of operation practically sterile. At birth the horny layer is less compact and of a red color. It is then called the vernix caseosa, which exfoliates in a few days, when the complexion 340 NORMAL HISTOLOGY AND ORGANOGRAPHY. changes to that of the particular race to which the child belongs. The strata lucidum and granulosum are two thin layers that lie between the cortieum and the Mal- pighian layers and are best developed in the sole of the foot and the palm of the hand. Each consists of two or three rows of epithelial cells. The stratum lucidum overlies the stratum granulosum and is a refractive layer consisting of cells with disintegrating J-^ ^j=i_ - 1__^~. t!~r=r = '-^2^,' Corncum or horny layer. Stratum lucidum. Stratum granulosum. Malpi^hian or germinal layer. Fig. 242. Section of epidermis of skin from palm surface of finger. nuclei, and possessing a homogeneous substance called eleidin. The latter is colored with eosin but does not take nuclear dyes. The cells that compose the stratum granulosum possess many granules called keratohyalin granules, which are regarded as products of cell disintegration. These granules in- crease in size and coalesce to form the semifluid sub- stance called eleidin of the stratum lucidum. The granules take nuclear dyes; the eleidin does not. THE SKIN. 341 The Malpighian layer is made up of growing epi- thelial cells and constitutes the deeper parts of the epidermis. Its lower surface is beset with numerous depressions that receive connective-tissue papillae from the dermis. The epidermis and dermis thus interlock by means of an extensive system of papillae from each layer. The Malpighian layer is thicker than the horny layer, excepting in the sole of the foot and the palm of the hand. It consists of ten to fifteen layers of epithelial cells. The cells in the lower row are columnar and are so arranged that their long axis is vertical to that surface. In colored races these cells are pigmented and impart to the skin the par- ticular color of the race. Pigmentation has been discussed on page 73, to which the reader is re- ferred. The other cells of the Malpighian layer are cubical or flattened and so placed that their long axis lies parallel to the surface of the body. The cells of the deeper strata have numerous minute short processes and have been called prickle cells. These processes form intercellular bridges, which give rise to a complex system of minute intercellular channels that permit more freely the passage of nourishment. These cells are constantly dividing and adding new strata to the horny layer that is ex- foliating at the same rate. The dermis, corium, or cutis vera, is of connective- tissue origin and lies just beneath and intimately associated with the epidermis. It may be divided into a papillary portion, next to the epidermis, and a deeper reticular portion which shades off into the subdermal fascia. The papillary portion consists of 342 NORMAL HISTOLOGY AND ORGANOGRAPHY. vascular and nerve papillae that fit into depressions on the lower surface of the epidermis. The two layers of the dermis pass into each other without any sharp line of demarcation. In both layers there is an abundance of connective-tissue fibers, both elastic and non-elastic, forming what has been termed areo- lar tissue. These fibers form bundles that interlace to produce a network, particularly in the reticular or deep portions. In the meshes of this reticulum are to be found the bodies of the sweat glands, and a variable amount of adipose tissue, while hair follicles with their sebaceous glands find lodgment in the dermis with greater regularity. It is the dermis , and particularly the areolar tissue, that gives elas- ticity and mobility to the skin. The epidermis is not very elastic, consequently wrinkles of the epi- dermis are formed when the fat is absorbed, and also in old age, by shrinkage of the areolar tissue. A variable amount of muscle is everywhere present in the dermis. Smooth muscle is associated with the hair follicles, forming the arrector pili muscle. In the face and neck voluntary muscle fibers may be traced into the papillary layer, while a third set of muscle elements is associated with the sweat glands. The latter is of the smooth variety and will be de- scribed along with the sweat glands. The dermis everywhere is very vascular. Blood- and lymph- vessels ramify freely through it, but in no case do they enter the epidermis. Nerve fibers, on the other hand, enter the Malpighian layer of the epidermis and arborize around and between the epithelial cells. In the dermis these fibers form an THE SKIN. 343 Medulla. extensive nerve plexus, from which some terminal fibers proceed to the hair follicles, and others to special nerve papillae in the dermis. These and other nerve terminations will be described as peripheral nerve endings in another chapter. HAIRS. The hairs are distributed practically over the whole surface of the body, with the exception of the palms of the hand, the soles of the feet, and the red border of the lips. They are distributed with con- siderable regularity, as a rule one hair for each fol- licle, but there may be two and sometimes three. The part of the hair buried in the skin is called the root, and the part that projects beyond the surface is the shaft. The lower part of the root is thickened to form the hair bulb, into which is pushed from below a vascu- lar connective-tissue projection called the hair papil- la. The root is inserted deep in the skin, usually reaching the subdermal elements. It is placed diagonally to the surface and becomes enclosed in a specially modified wall made up of several layers de- rived partly from the epidermis and partly from the dermis. Most hairs are composed of three layers : an outer cuticle, a middle cortical, and a central portion, the Cortical layer. Hair cuticle. Fig. 243. Portion of a hair. 344 NORMAL, HISTOLOGY AND ORGAN OGRAPHY. medulla. In thin and light hairs the medulla is usu- ally absent. The hair cuticle, or outer hair membrane, is made up of structureless transparent epithelial scales that overlap each other in the direction of the distal end of the hair. A hair feels smooth, therefore, if pulled through the fingers from the root to the free end. These scales overlap each other sometimes to such an extent that the cuticle has the appearance of being stratified. The scales are derived from epi- thelial cells that have become cornified, and they are thus closely related to the horny epithelial plates of the epidermis. The cortical substance forms the main bulk of the hair and lies just beneath the cuticle. The cortex consists of spindle-shaped nucleated cells which show a distinct fibrillar structure, giving the whole hair the appearance of being longitudinally striated. Pigment granules are deposited in these cells and between them, to which the hair owes most of its color. Numerous small spaces filled with air are frequently formed between the cells of the cortical layer, and these give a white color to hairs that have a scanty supply of pigment. Hairs that have en- tirely lost their pigment and have none of these air spaces, are gray but not white. The medullary substance forms the axis of the hair and may be absent, but is usually present in thick hairs. It is made up of nucleated cubical epithelial cells forming two or three rows in thick- ness. Pigment is also present in the medullary cells. THE SKIN. 345 Hair Follicles. The hair follicles are the pits in the skin occupied by the roots of the hairs. These pits are placed diagonally to the surface, and in the scalp where the skin is thick they are at least half an inch in length. It is estimated that the normal scalp has about one hundred and twenty thousand of these follicles, or an average of eight hundred to the square inch. Each follicle is really a minute tubular de- Longitudinal connective-tissue fibers. Circular fibers. Glassy membrane. Outer root sheath. Henle's layer. Huxley's layer. Inner root sheath. Hair shaft. Fig. 244. Cross section of a hair follicle. pression or invagination of the skin, and its wall is therefore made up of constituents from both the epi- dermis and the dermis. These layers may be tabu- lated as follows : I. Outer tunic. 1. Connective-tissue fibers arranged longi- tudinally. 2. Connective-tissue fibers, circular. 3. Glassy membrane. 346 NORMAL HISTOLOGY AND ORGANOGRAPHY. II. Outer root sheath; resembles Malpighian layer of the epidermis. III. Inner root sheath. 1. Henle's layer, non-nucleated ele- ments. 2. Huxley's layer, nucleated cells. 3. Root sheath, structureless membrane. The outer tunic is derived from the dermis and is of connective-tissue origin. Externally there is a layer of connective-tissue fibers arranged longitudi- nally, in which may be found a few connective- tissue cells and a delicate plexus of nerve fibers. The non-elastic connective-tissue fibers predominate, but the elastic variety is also present. Internal to the longitudinal fibers is a compact circular layer of non-elastic connective-tissue fibers, and internal to this is the glassy membrane, a very thin hyaline sheath often difficult to find. The outer tunic in- vests the lower half of- the root sheath. This tunic, in the so-called tactile hairs of many mammals, has a rich nerve innervation and a liberal blood supply. In such hairs nerve fibers have been traced to the glassy membrane, while others apparently penetrate to tactile cells in the outer root sheath. The root sheaths encase the root of the hairs and are derived from the epidermis, being therefore of ectodermal origin. The outer root sheath is a direct continuation of the Malpighian layer and diminishes in thickness toward the bottom of the follicle. It is composed of nucleated epithelial cells which possess intercellular bridges and a fibrillar protoplasm. This sheath always stains heavily with nuclear dyes, THE SKIN. 347 The inner root sheath is less conspicuous, and in good sections will be found to consist of an outer layer of two rows of non-nucleated elements, representing cornified epithelial cells and called Henle's layer, and Hair shaft. Epidermis. Sebaceous gland. Fat cells. Hair papilla. Fig. 245. Cross section of the human scalp. internal to this about two rows of nucleated cells, called Huxley's layer. These layers are absent from the upper half of the follicle. Internal to Huxley's layer, and in direct contact with the root of the hair, is the root sheath, which has much the same structure as the hair cuticle. Many scaly plates are imbricated 348 NORMAL HISTOLOGY AND ORGANOGRAPHY. upon each other and interlock with those of the hait cuticle in such a manner that if a hair is pulled out the root sheath comes away with it, the break taking place along Huxley's and Henle's layers. The hair papilla indents the lower end of the root and is of connective-tissue origin. It has a rich blood supply which contributes nourishment to the adjacent epithelial cells of the root which are con- stantly dividing. It is this cell division that brings about the growth of a hair. If the papilla is de- stroyed the hair dies. When a hair is pulled out with its root the papilla and some adjacent epithelial cells usually remain uninjured. The epithelial cells in due time reproduce a new hair. The arrector pili muscle consists of bundles of smooth muscle fibers that pass obliquely downward from the upper surface of the dermis to be inserted in the connective-tissue tunic of the hair follicle near its lower extremity. The insertion of these fibers is always on the side toward which the hair inclines, so that when the fibers contract the root is drawn to a vertical position and the hair becomes erect. The hair on the scalp grows approximately at the rate of twelve inches a year, or one inch a month. The average duration of a hair is about four years. Many vertebrates, as horses and cattle, shed their hair annually, every spring, a phenomenon called moulting. In mankind the hair of the scalp is con- stantly dropping out and being replaced by growths of new shafts. Occasionally hair grows where it normally does not belong and for cosmetic effect requires removal. This is done by electrolysis, THE SKIN. 349 which consists in passing an electric needle down along the root of each hair to the hair papilla, which is then destroyed by a weak current of electricity. The hair is then readily removed and does not re- turn. The loss of hair on the scalp is due to a va- riety of causes, many of which we cannot explain. It often accompanies a prolonged illness, such as typhoid fever, and is then doubtless due to a general emaciation resulting in lack of proper nourishment of the scalp. The loss of hair in such cases is only temporary. Certain neurotic diseases result in a permanent loss and the same may be attributed to some germ diseases of the scalp that infest and de- stroy the hair papillae. In other cases baldness seems to be hereditary. It naturally follows that a healthy condition of the scalp will contribute to a rich growth of hair. Regular massage with a stiff brush no doubt accelerates the blood flow and thus brings about a better nourishment and growth to the hair. The natural preservation of hair after death is well known. In Egyptian mummies the hair is well preserved even to its natural color. The hair is thus an important factor in the identification of unknown deceased persons. THE NAILS. The nails are epidermal structures that are mor- phologically analogous to the hoofs and claws of lower animals. Each nail may be divided into a body, the part that is exposed, and the root that is hidden from view and lies in a fold of the skin. The 350 NORMAL HISTOLOGY AND ORGANOGRAPHY. lateral margins are also covered by a fold of the skin called the nail wall. The nails have a pink color imparted by the subjacent blood, excepting near the root, where there is an opaque area called the lunula. The lunulae diminish in size from the thumb to the little finger. Each nail rests upon a very vascular dermis which has been called the nail bed or matrix. This connective- tissue bed has many fine longitudinal ridges and alternating grooves which fit closely into correspond- ing grooves and ridges on the lower surface of the nail. Each nail consists of two parts: a deep soft stratum that represents the Malpighian layer of the epidermis, and an external hard cornified layer that Body of nail. Fig. 246. Thumb nail. Body of nail. Root of nail. Mantle. Phalanx. Nail bed. Fig. 247. Longitudinal section of nail. represents the horny layer. The former consists of nucleated polygonal prickle cells which fill the fur- rows and cover the nail bed several cells deep. It is affirmed that the cells of this Malpighian layer, in the distal part of the nail, do not produce any of the overlying horny material, but that growth of the nail THE SKIN. 351 Nail wall. Fig. 248. Cross section of nail. is exclusively due to epithelial proliferation from the Malpighian layer at the root of the nail and from that part directly under each lunula already de- scribed. A stratum granulosum is present in the upper portion of the matrix and absent in the other portions of the nail. The external cor- nified layer consists of flat epithelial scales in which rem- nants of a nucleus may frequently be found. These scales are derived from epithelial cells and overlie each other, forming hardened lamellae called nail leaves. The hoof of the horse corresponds to the finger- nail of man, and is divided for descriptive purposes into the wall, the sole and the frog. The part which is visible when the foot rests on the ground is the wall, while the sole and frog are invisible in this position. As the human nail rests on a grooved matrix, so the inner surface of hoof wall is extensively folded into leaf-like structures which interlock or digitate with like growths from the enclosed con- nective tissue, those from the wall being called horny or insensitive lamina, and those from the connective tissue or dermis the sensitive or vascular lamina. Horny Lamina. These are known collectively as the kerqphyllous tissue, and clothing the inner sur- face of the wall dovetail with the sensitive laminae 352 NORMAL HISTOLOGY AND ORGANOGRAPHY. like interlocking leaves of two books. Each lamina extends approximately from the upper and inner margin of the hoof to its plantar border. There are from five to six hundred of these laminae in each foot and they all increase in width from above to below. In a horizontal section of the hoof these laminae appear like so many papillae (Fig. 2480). From such a sec- tion it will be seen that along the sides of each lam- ina there are about sixty secondary folds, called lamel- la, by which the surf ace between sensitive and in- sensitive laminae is enormOUSlv Itt- creased. These secondary leaves establish a fine se- ries of longitudinal grooves along the lateral sides of each lamina, as seen in Fig. 2486. The surface lining of each horny fold consists of a single layer of cubical or low colum- nar epithelial cells analogous and continuous with the germinal layer of the skin. The cells are rich Horn tubes, epi- thelial cells. Horn matrix, epi- thelial cells. Insensitive lamina, epithelial tissue. Secondary lamina or lamella lined by malpighian layer of epithelial cells. Sensitive lamina con- nective tissue. Blood-vessel. Fig. 2480. Horizontal section of hoof of horse. THE: SKIN. 353 in chromatin and are doubtless capable of active multiplication. A few scaly epithelial cells are always found in the body of each lamella, but in the substance of each lamina the tissue appears to be compact and of a fibrous variety. It is this compact tissue that is called collectively the insen- sitive lamina. This fibrous tissue can be traced outward to the bases of the laminae, where it mingles with and is ultimately lost in the epithelial horn wall of the hoof. While nuclei are absent, the tissue should be regarded as made up of scaly epi- Horny tubes and wall matrix. Secondary laminae or lamella. Fig. 2486. Diagram of horizontal section through wall of hoof. thelial plates so arranged as to give it a fibrous ap- pearance very similar to the stratum lucidum of the human skin. Vascular Lamina. These structures, collectively known as the podophyllous tissue, are leaf-like growths of the dermis, which interlock very snugly with the horny laminae and lamellae just described. They form an expansive fibrous and vascular tissue uniting the distal phalanx with the horny epidermal laminae of the hoof. They are also called the sensi- tive lamina, and, while the nerve endings in them have 23 354 NORMAL HISTOLOGY AND ORGANOGRAPHY. not been worked out very carefully, it is reasonable to suppose that we have much the same structure as in the human nail-bed, such as free nerve endings, end-bulbs, and perhaps Rufini corpuscles. Hoof Horn. Like bone, this is tubular, re- sembling Haversian systems, but, unlike bone, it consists of compact layers of epithelial cells. As the human nail develops from the germinal epithelium at the root of the nail, so the wall of the hoof de- velops from similar epithelium that covers the coronary cushion situated at the upper margin of the hoof wall. This cushion has an abundance of epithelial papillae, and from the surface of these papillae cells proliferate to form the wall of the hoof tubes, while from the epithelium at the bases of these papillae cells proliferate to form the hoof matrix. Thus, the hoof horn is exclusively an epithelial tissue, composed of flattened, scaly cells, with often an easily detected nucleus, cemented together com- pactly, their protoplasm being replaced by keratin granules, a protein-like substance very insoluble and containing 4.23 per cent, sulphur. The horn tubes extend downward from the papillae of the coronary cushion and are parallel to each other. These tubes are smaller near the surface of the hoof and become larger in the deeper portion. The scaly cells of the tube wall are placed with their flat surfaces facing the tubes, that is, their long axes are perpendicular, while the long axis of the matrix cells are horizontal. This conforms to their origin, the former prolifer- ating from the sides of the vertical coronary papillae, while the latter come from the horizontal coronary THE SKIN. 355 surface between the bases of these papillae. Frag- ments of epithelial cells may usually be found in the lumen of these tubes. The laminae just described provide an enormous surface of contact between the inner face of the wall and the external surface of the pedal bone. It is estimated that this surface is equal in area to eight or ten square feet in each hoof, and its chief function is doubtless to furnish support to the body weight of the horse. The sensitive laminae thus act as an extensive and delicate cushion, tempering the jar sustained in walking or running. An inflammation of the sensitive laminae is known as laminitis, a malady not uncommon in the horse. The normal growth of the hoof is estimated at nearly one-half inch a month. Just how the horny laminae move imperceptibly downward past the softer lami- nated structure is a subject of much speculation among veterinarians, but one on which opinions differ. It seems to me any sliding process is difficult to explain and that the solution sought is one of cell growth. During embryonic development it is easy to conceive of a rapid multiplication of the germinal epithelium, that is, the cells that form the membrane clothing the insensitive laminae. Such a growth produces lateral pressure and accounts for the extensive folding of these laminated structures. In the adult foot the cells of the germinal layer show nuclei rich in chromatin, and being epithelial cells their multiplication con- tinues through life. The horny laminae, lying exter- nal to this layer, doubtless owe their origin, as well as their constant and regular growth, to the cells of 356 NORMAL HISTOLOGY AND ORGANOGRAPHY. the germinal epithelium. In fact, embryologically this layer is to be considered as a part of the horny laminae rather than interposed between the horny and the soft laminae, as is done by most authors. The hoof wall, on the other hand, grows exclusively from the epithelial surface of the coronary cushion, and its downward progress is synchronous and uni- form with the growth of the horny laminae, as de- scribed above. The provisional horn that appears after removing a part of the hoof wall, surgically or otherwise, is explained as a cell proliferation of the germinal epithelium. If the human nail is removed this germinal epithelium is torn, but enough remains to proliferate epithelial cells in a few days which cor- nify to form a thin provisional nail analogous to the provisional hoof in a like injury to the horse's foot. THE GLANDS OF THE SKIN. Sweat glands are coiled simple tubular glands dis- tributed over the whole surface of the skin, with the exception of the inner surface of the prepuce, the glans penis, and the red borders of the lips. In the axilla and around the anal opening they are excep- tionally large and often branched. They are most numerous on the palm of the hand and the sole of the foot, where they number two thousand seven hundred to the square inch. On the forehead there are one thousand two hundred, and on the cheek about five hundred to the square inch, while over the back they are the least numerous. Their total num- ber over the whole body has been estimated at nearly two million four hundred thousand, which, with an SKIN. 357 average length of three-fourths of an inch, makes the united length approximately twenty-eight miles. This vast secreting surface is constantly secreting moisture, either as insensible or sensible perspiration. The amount of this perspiration within a given time Fig. 249. Under surface of the epidermis, separated from the cutis by boiling. The sweat glands may be traced for a considerable part of their length; a, Sweat gland ; b, longitudinal ridge; c, depression, d, cross ridge (Bohm and Davidoff). varies considerably, but in the average person in good health it is estimated at about two pints every twenty-four hours. The excretory ducts open on the surface of the skin by numerous sweat pores along the crests of the epidermal ridges. These pores may be seen with a low magnification or ordinary hand lens. The duct is spirally twisted in the stratum corneum and enters the dermis between two dermal papillae; that is, at the apex of an epidermal papilla. In the dermis it takes a sinuous or nearly straight course and pene- 358 NORMAL HISTOLOGY AND ORGANOGRAPHY. trates to the lower stratum of the skin, or even deeper, to the subcutaneous connective tissue. This distal end is very much coiled and constitutes the secreting portion of the gland. In the epidermis the duct has no other wall than the epithelial cells of the various layers through which it passes, but in the dermis the wall is composed of a single layer of short cubical cells outside of which there is a delicate basement mem- brane. The secreting portion is also lined by simple epithelium, but the cells are larger and have a finely granular protoplasm. Between the gland cells and the base- 250. Cross section of deep portion of sweat gland. ment membrane there is found in the larger glands, a single layer of non-striated muscle cells arranged longitudinally. This muscle is derived from the ectoderm, while the other musculature of the body comes from the meso- derm. The muscle of the sweat glands probably aids these glands in expelling their products of secre- tion. Non-medullated nerve fibers of the sympa- thetic system form a delicate network just external to the basement membrane called the epilamellar plexus. From this plexus delicate fibers pass through the basement membrane to ramify between the gland cells, where they end in clusters of small terminal granules. The physiological activities of the sweat glands are thus directly under the control of the nervous system and do not depend on the SKIN. 359 blood supply, a fact that may also be demonstrated by physiological experiments. Sebaceous glands are associated with hair follicles, into which they pour their contents. They are also found on the red borders of lips, the labia minora, the glans and prepuce, where hairs are absent. They are simple branched alveolar glands that se- crete an oily substance called sebum. This is a Hair follicle. Hair follicle. Fig. 251. Model of a sebaceous gland with a portion of the hair follicle, reconstructed by Bern's wax-plate method (Huber). fluid at the temperature of the body, keeps the skin soft and flexible, and also supplies a natural dressing for the hair. In the scalp there may be ah exces- sive secretion of sebum which dries and exfoliates with the horny epidermis as dandruff. Each hair follicle has two or more sebaceous NORMAL HISTOLOGY AND ORGANOGRAPHY. Fig. 252. Section of two alveoli of a sebaceous gland. glands that vary in size from 0.2 to 0.5 mm. The excretory duct is short and wide and opens into the upper third of the follicle. This duct is lined by stratified epithelium that is directly continuous with the outer root sheath of the hair follicle. The cells of the alveoli are very large and contain fat globules that vary in size and give a reticular appearance to the cytoplasm. The nuclei are rela- tively small. The cells completely fill the alveoli so that the latter appears to be solid. The cells disintegrate and change directly into secre- tion, which is then poured into the follicle as sebum. The renewal of lost cells takes place by constant proliferation of basal cells.. It is quite common in the scalp to find sebaceous cysts, or wens, which result from an occlusion of the duct and an enlargement of the gland. These cysts are lined by a simple layer of epithelium and fillec with a white, waxy, or semisolid fluid quite analo- gous to the sebum. Wens are of slow growth anc cause no disturbance, unless they get very large or become infected by being carelessly opened. The radical cure consists in their complete removal, in- cluding the epithelial wall. CHAPTER XI. PERIPHERAL NERVE TERMINATIONS. Physiologically, nerve endings may be classified as motor or sensory. MOTOR NERVE ENDINGS* (The Telodendria of Nerve Fibers in Muscle Tissue.) i . In Striated Muscle. The nerve endings in stri- ated muscle are called muscle-end plates, or sole plates. In the higher vertebrates these are found in the muscle sarcoplasm just beneath the sarcolemma of each muscle fiber. A motor nerve fiber as it ap- proaches its termination becomes much branched so as to innervate from ten to twenty muscle fibers. The axis cylinder enters the sarcoplasm where it im- mediately terminates in a web-like, flat end-brush with numerous dilatations. The axilemma, or Henle's sheath, is continuous around the brush. The me- dullary layer stops short at the level of the sarco- lemma; that is, at the surface of the sarcoplasm. The neurolemma is continuous with the sarcolemma of the muscle fiber. The adjacent sarcoplasm of the muscle fiber is granular and a liberal supply of muscle nuclei is also present in the proximity, which results in an elevation of the muscle fiber at the point of nerve contact known as Doyer's elevation. 362 NORMAL HISTOLOGY AND ORGANOGRAPHY. 2. In non-striated and in heart muscle the nerve termination is more simple. The muscle is supplied with neurons from the sympathetic system, most of which are of the non-medullated variety. These fibers branch repeatedly to form an extensive Nerve-end brush. So-called gran- ular sole. Muscle fiber. Fig. 253. Motor endings in striated voluntary muscles. From Pseudopus Pallasii. As a consequence of the treatment the arborescence is shrunken and interrupted in its continuity. The end plate is in con- nection with two nerve branches (Bohm and Davidoff). primary plexus surrounding the muscle bundles. From this plexus non-medullated fibers, that is, just the.as^s cylinders, penetrate the heart muscle, or the involuntary muscle, where they anastomose to form a delicate secondary plexus, from which lat- eral short twigs pass to end in minute dilatations or granules upon the muscle cells. PERIPHERAL NERVE TERMINATIONS. 3 6 3 SENSORY NERVE ENDINGS. (Telodendria of Dendrites.) The sensory nerve terminations are essentially the terminals of dendrites as distinguished from the motor plates which are the terminals of axones. The cell bodies of these sensory neurons are found in the spinal and cranial ganglia, often at a consid- erable distance from the sensory termination. In 1 $ Fig. 254. Motor nerve ending on heart muscle cells of cat; methylene- blue stain (Huber, De Witt). Fig. 255. Motor nerve ending on involuntary non- striated muscle cell from intestine of cat; methy- lene-blue stain (Huber, De Witt). this case the dendrite is a long one, medullated, and structurally identical with an axis cylinder or axone. The nerve impulse is, however, normally carried toward the nerve cell, while in axones the impulse goes the opposite way. As stated in another place, such sensory neurons have a long dendrite that ex- tends peripherally and -a short axone that passes centrally; that is, to the spinal cord or brain. 364 NORMAL HISTOLOGY AND ORGANOGRAPHY. These sensory endings form telodendria or end- brushes that vary in complexity according to the tissue elements that take part in their formation. i. Free Sensory Nerve Endings. These are the simplest forms of nerve endings and occur in epi- Stratum corneum. Nerve fibers in the epi- dermis. Stratum Malpighii. I L Papilla. . Nerve fiber. Fig. 256. Nerves of epidermis and papilUe from ball of cat's foot (Bohm and Davidoff). thelial tissues and in some parts of the connective tissues. The sensory nerve fibers near its termina- tion repeatedly branch, the latter retaining the medullary sheath. The branches appear always at the nodes of Ranvier. From this coarse plexus a PERIPHERAL NERVE TERMINATIONS. 365 finer non-medullated system of branches appear which innervate the epithelium and terminate in varicosities, discs, or minute granules that lie in appo- sition to epithelial cells. Similar free terminations occur in tendons, and liga- Connective '-tissue capsule. Fig. 257. Tactile cells from the bill of a duck. Nerve fiber. __ Epithelial cells. ments, and other fi b r o u s connective tissue. 2. Tactile Cells. These are also called Grandry's corpuscles, and may be found in the duck's bill just beneath the gum epithelium. The cells are of epithelial origin, oval, and meas- ure about 50 //in diameter. One to five cells are surrounded by a connective-tis- sue capsule. These cells are superposed on each other, with their long axes always parallel to the surface of the bill. A me- dullated nerve fiber may be traced to the cap- sule, which it penetrates and then becomes non- - Nucleus of lamellae. End-cell of core. Lamellae. Axis-cylinder in core. Cubic cells of core. Termination of medul- lary sheath. Axis-cylinder of nerve-fiber. Medullary sheath of nerve-fiber. Neurilemma and sheath, of Henle. Fig. 258. Corpuscle of Herbst from bill of duck (Bohm and Davidoff). 366 NORMAL HISTOLOGY AND ORGANOGRAPHY. medullated. The latter terminates in tactile discs that are interposed between the tactile cells. A group of three cells will have two discs ; five cells will have three discs. 3. Corpuscles of Herbst. These are much larger bodies and may also be found in the bills of aquatic birds, in close association with the tactile cells just described. They are ovoid bodies 75^ wide and 150 p. long. There is an inner core sur- rounded with con- nective-tissue lamel- lae. The core con- tains the axis cylin- der, which is thick- ened at the end and is encased between two rows of cells that seem to have the same function as Grandry's corpus- cles . The nerve fiber enters at the end of the corpuscle and becomes non-medullated only after reaching the inner core. 4. Meissner's Corpuscles. These are found beneath the epidermis of man, particularly of the hand and foot, and occupy the dermal papillae of the dermis. Fig. 259. Meissner's tactile corpus- cle; methylene-blue stain (Dogiel, "In- ternat. Monatsschr. f. Anat. u. Phys.," vol. ix). PERIPHERAL NERVE TERMINATIONS. 367 They are oval bodies and approximately the same size as the corpuscles of Herbst. There is a thin connective-tissue capsule and a loose complex core. One or more medullated nerve fibers enter at the lower end of the corpuscle. These soon become non-medullated and their axones then make a vari- able number of spiral turns which interlace, branch, and are beset with many granules or varicosities. One or more axis cyl- inders occupy the center of the core. 5. Genital Cor- puscles. These are oval or round bodies located in the mucosa, just beneath the epi- thelium of male and female geni- talia. Their size varies from o.i mm. to 0.4 mm. They are sur- rounded by a thick fibrous capsule and each cor- puscle is innervated by one to ten medullated nerve fibers. The latter become non-medullated after passing through the capsule, and the axones then form a complex core quite analogous to that of Meissner's corpuscle. In fact, the two are very similar structures. Fig. 260. Genital corpuscle from the glans penis of man; methylene-blue stain (Dogiel,"Arch. f. mik. Anat.," vol. XLI.) Lamella. Granular cove. Axis cylinder Fig. 261. Pacinian corpuscle from mesentery. Fig. 262. Neuromuscular nerve-end organ from the intrinsic plantar muscles of dog; from teased preparation of tissue stained in methylene- blue. The figure shows the intrafusal muscle fibers, the nerve fibers and their terminations; the capsule and the sheath of Henle are not shown (Huber and De Witt, "Jour. Comp. Neurol.," vol. vn). PERIPHERAL NERVE TERMINATIONS. 3 6 9 6. Pacinian Corpuscles. These are oval bodies and the larger ones are easily visible to the naked eye, being over 2 mm. long. Structurally they seem related to the corpus- cles of Herbst. They are found in the dermis of the hand and foot, particularly along the lower surface of the fingers and toes. They are also found in the joints, the peritoneum, pleura, pericar- dium, and are especially abun- dant in the mesentery. The greater portion of the corpuscle consists of concentric lamellae of connective-tissue origin. B e- tween these flat endothelial cells intervene. A granular core forms the axis of the corpuscle, in the center of which the axis cylinder maybe traced. Usually one large nerve fiber goes to each corpuscle. After entering the core this forms a plexus of fine branches and becomes non-med- ullated. 7. Tendon and Muscle Spin- dles. A tendon spindle is an ex- pansion of tendon bundles en- closed in a well defined connec- tive-tissue sheath. The nerve fiber enters the middle of the spindle, divides re- 24 Fig. 263. Neuroten- dinous nerve-end organ from rabbit ; teased preparation of tissue stained in methylene- blue (Huber and De Witt, "Jour. Comp. Neurol.," vol. x). 370 NORMAL HISTOLOGY AND ORGANOGRAPHY. peatedly, becomes non-medullated and finally ter- minates in varicosities or expanded clavate ends. The muscle spindle is a collection of delicate muscle fibers enveloped in a dense perimysium sheath, the whole being innervated by sensory nerve endings much as in tendon spindles. The sensory endings both in tendon and muscle transmit the sensation of tension which becomes the basis for coordinate movements. CHAPTER XII. THE SPINAL CORD. The spinal cord is an organ composed largely of neurons, with which are associated blood-vessels, connective-tissue elements, and a limited amount of epithelial and muscle cells. It represents the ter- minal portion of the cerebrospinal axis and is a direct continuation of the encephalon. It is one of the first organs to develop in the embryo where it makes its appearance as a dorsal ectodermal groove. This neural groove gradually closes to form a canal which lies at first just beneath the ectoderm and later becomes encased in connective-tissue layers and the bony axial skeleton. The cord is bilaterally symmetrical and flattened dorso-ventrally. It presents two enlargements: (1) the upper or cervical, which extends from the third cervical vertebra to the second thoracic and corresponds to the origin of the nerves of the arm, and (2) the lower or lumbar, which extends from the ninth thoracic vertebra to the terminal cone at the level of the body of the second lumbar vertebra. The lumbar enlargement marks the origin of the nerves of the leg. From the apex of the terminal cone there extends a slender rudimentary prolonga- tion, the filum terminate, which, with the spinal nerves of this region, is called the cauda equina. The 372 NORMAL HISTOLOGY AND ORGANOGRAPHY. spinal cord, therefore, does not extend the whole length of the vertebral canal but only to the level of the second lumbar vertebra. Its length is about eighteen inches, its diameter one-half inch or less. Membranes of the Cord or Meninges. i. The Dura. This is a thick strong membrane composed of Blood capillaries in white matter. Spinal ganglion Posterior root of spinal nerve Ligamentum denticulatum. A nterior root of ' spinal nerve. Posterior root. Anterior root. Dorsal spinal ganglion. Dura mater. Arachnoid. Pia mater. Blood-vessels. Fig. 264. Portion of spinal cord and membranes dissected white fibrous tissue which forms the outer covering of the cord. Many blood-vessels find lodgment in this membrane. It is analogous and continuous with the dura of the brain, the most striking differ- ence being that the dura of the cord does not form THE SPINE. 273 the internal periosteum of the vertebral canal. Be- tween the dura and the vertebrae is a space called the epidural space, which is filled with areolar tissue, fat, and a plexus of spinal veins. 2. The Pia. This is a thin connective-tissue layer that lies close to the surface of the cord, dips into the anterior fissure, and also sends fibers or trabeculae into the cord substance. Many small blood-vessels accompany this layer. 3. The Arachnoid. This is a delicate membrane between the other two, but much nearer the dura. Its external surface is clothed with a single layer of flat epithelial cells which secrete a serous fluid. The arachnoid is therefore a serous membrane. Fissures. i. Posterior Median Fissure. This is a median dorsal fissure that extends the whole length of the cord. It is extremely narrow but deep, as it 'penetrates to the central gray matter, being inti- mately connected with the two sides in its course. The single septum is derived from the neuroglia tis- sue, and not from the pia, which sends no prolonga- tion of any kind into it. 2. Anterior Median Fissure. This also extends the whole length of the cord. It is shallower but wider than the posterior fissure and does not quite reach the central gray matter. The pia forms a fold into this fissure with which is associated many blood-vessels. The two median fissures or clefts divide the cord into a right and a left half, each practically identical with the other. 3. Poster o-lateral Groove. This is a shallow de- pression on each side of the posterior fissure and 374 NORMAL HISTOLOGY AND ORGANOGRAPHY. marks the entrance into the cord of the dorsal roots of the spinal nerves. In a like position, anteriorly, is the exit of the anterior roots of the spinal nerves, but there is no depression or groove as in the case of the posterior roots. The two roots of the spinal nerves divide each half of the spinal cord into three regions or major tracts known as the posterior, tferior horn ell. issed Pyram- ial column. Igi cell of osterior horn, red cerebel- ir column. Column cells. Igi's commis- ural cells, 'ers's column. Motor cells. Collate? of en Pyra, colun Collatet endin the gi matte Direct Pyramidal column. Fig. 265. Schematic diagram of the spinal cord in cross section after von Lenhossek, showing in the left half the cells of the gray matter, in the right half the collateral branches ending in the gray matter (Huber). lateral, and anterior columns. The posterior column is limited by the posterior fissure and the posterior roots, the lateral is the region between the roots, and the anterior lies between the anterior root and the anterior fissure. Spinal Neryes. Thirty-one pairs of nerves arise from the side of the cord. These are classified into THE SPINE. 375 8 cervical, 12 dorsal, 5 lumbar, 5 sacral, and i coc- cygeal. Each nerve is attached to the cord by a dorsal and a ventral root. Each root, before uniting with the cord, breaks up into secondary bundles and spreads out like a fan, making a continuous linear attachment. The dorsal ganglion is located upon the dorsal within the vertebral canal, near the union of the two roots. Gray Matter of the Cord. The gray matter of the cord is centrally located and takes the form of a capital letter H. The gray matter in each lateral half resembles a crescent which is joined to the op- posite side by an isthmus, in the center of which is the central canal. The latter is usually obliterated in the adult man, and is filled as well as surrounded by the gelatinous substance of Rolando, which is a reticular structure. That part of the isthmus above the central canal is called the posterior gray com- missure, while the gray matter ventral to the canal is the anterior commissure. Each crescent may be divided into a posterior, lateral, and anterior horn. The anterior horn is the largest and the lateral horn is the smallest. The posterior horn is pointed and approaches near to the posterior lateral groove. The apex of this horn is called the zona terminalis. At the base of this apex there is a reticular substance called the zona reticu- laris, while next to this and apparently capping the posterior horn is a gelatinous mass, similar to that which surrounds the central canal, called the sub- stantia gelatinosa of Rolando. The nerve cells of the posterior horn are irregularly 376 NORMAL HISTOLOGY AND ORGANOGRAPHY. distributed and some of them are particularly small and have a stellate appearance. At the base and mesial surface of this horn there is a group of large nerve cells called the column of Clarke. This column extends from the second lumbar up through the dorsal region of the cord to the cervical. In the cervical region it is absent; however, Sailing's nu- cleus of this region may represent a remnant of the column of Clarke. At the base of the dorsal horn, deeper down and lateral to Clarke's column, another group of nerve cells may be found that is called Waldeyer's central cell column. This is reciprocal with Clarke's column; that is, in the dorsal region where Clarke's column is conspicuous, only rem- nants of Waldeyer's tract can be found, while in the other regions of the cord this cell tract is particu- larly prominent. The lateral horn is a small lateral prominence at the side of the gray crescent. In the substance of this, a small collection of nerve cells may be found, while just beneath this the gray matter cuts into the white matter, forming processes called the processus reticularis. The anterior horn is not only large but presents a blunt, rounded appearance. The nerve cells of this horn are very large and have been classified according to their position into antero-mesial, postero-mesial, antero-lateral, and postero-lateral. The axis cylin- der of most of these cell bodies goes to form the an- terior root of the spinal nerves. They therefore carry only motor impulses. White Matter of the Cord. The white matter of THE SPINE. 377 the cord consists of medullated nerve fibers. Most of these fibers have no neurilemma, and the cord there- fore is soft and pulpy, in contrast to the nerve trunks whose fibers have a neurilemma which with the connective-tissue elements make nerves tough and strong. The white matter practically encloses the gray. The fibers which compose it vary con- Postero-lateral horn. Posterior horn. " Posterior fissure. Lissaur's marginal ground bundle. Comma tract. Pia mater. Direct cerebellar tract. Cower s's tract. Processus reticularis. Lowenthal's tract. Anterior commissure. Anterior fissure. Direct pyramidal tract. Fig. 266. Cross section of the spinal cord, dorsal region, i, Zona terminalis; 2, zona reticularis; 3, substantia gelatinosa of Rolando; 4, stellate cells of posterior horn; 5, column of Clarke; 6, Waldeyer's central cell column; 7, cells of lateral horn; 8, central canal; 9, antero- mesial cells; 10, postero-mesial cells; n, antero-lateral cells; 12, pos- tero-lateral cells. siderably in size, both large and small being mixed up together. In sections of the adult healthy cord no evidence of definite tracts of fibers can be seen. We know, however, that longitudinally arranged groups of fibers run a definite course, have definite connections, and carry impulses that result in defi- nite sensations and actions. The physiological em- 378 NORMAL HISTOLOGY AND ORGANOGRAPHY. deuce of this fact is experimental and positive. A nerve fiber detached from its cell body dies. In this way tracts will degenerate in the cord, some above and some below a transverse cut. The em- bryological evidence rests on equally positive facts. In development certain tracts acquire medullary Postero-lateral horn. Posterior fissure. A nterior horn. Lowenthal's tract. Anterior root of spinal nerve. Anterior fissure. Fig. 267. Cross section of the spinal cord, lumbar region, i, zona terminalis; 2, zona reticularis; 3, substantia gelatinosa of Rolando; 4, stellate cells of posterior horn; 5, column of Clarke; 6, Waldeyer's cen- tral cell column; 7, cells of lateral horn; 8, central canal; 9, antero- mesial cells; 10, postero-mesial cells; n, antero-lateral cells; 12, postero- lateral cells. sheaths earlier than others. This fact has greatly extended our knowledge of the white matter in the cord. The pathological evidence is a third factor. Certain diseases produce degenerate lesions in the cord. Proper interpretations of these lesions have enabled us to map out definite nefve tracts in the cord, and to determine their relations as well as pos- 379 sible functions. The information evolved from these sources makes a classification of tracts, in the cord, possible. Tracts of the Cord. POSTERIOR REGION. 1 . Column of Goll. This lies adjacent to the dorsal fissure and extends the whole length of the cord. Its fibers arborize about nerve cells in the nucleus gracilis of the lower region of the medulla. 2. Column of Burdach. This extends parallel to the column of Goll between the latter and the pos- terior horn of gray matter. It extends the whole length of the cord and its fibers arborize about nerve cells in the nucleus cuneatus of the medulla, adjacent to the nucleus gracilis. The column of Goll becomes wider in the upper portions of the cord and that of Burdach narrower. This is due to nerve fibers that gradually pass into the column of Goll from the col- umn of Burdach on their way to the brain. 3. Comma Tract. This is a small tract found in the column of Burdach, and represents sensory fibers from the posterior roots of the spinal nerves that pass down the cord for a short distance, and then turn into the anterior horn of gray matter to arborize about nerve cells of this region. 4. Lissaur's Marginal Ground Bundle. This is a small commissural tract placed near the surface of the cord just lateral to the entrance of the posterior root fibers. It is formed by some of the fibers of this root. The tract extends the whole length of the cord, but each individual fiber runs but a short distance and then turns inward to arborize about nerve cells of the posterior horn. 380 NORMAL HISTOLOGY AND ORGANOGRAPHY. These four tracts are composed almost entirely of nerve fibers that enter the cord by the posterior roots of the spinal nerves. They are therefore sen- sory tracts and forward impulses toward the brain. The termination of the posterier root fibers may be enumerated as follows: 1. Column of Goll, Burdach, comma tract, Lis- saur's tract. 2. Arborize about the cells in Clarke's column. 3. Arborize about the cells in posterior horn. 4. Arborize about the cells in anterior horn on same and opposite side. Lateral Region. The tracts of this region are: (i) Direct Cerebellar ; (2) Go wers's, or Ascending Antero- lateral; (3) Crossed Pyramidal; (4) Lowenthal's, or Antero-lateral Descending ; (5) Mixed Lateral. 1. The Direct Cerebellar. This is a band of fibers that lies at the surface of the cord just lateral to the dorso-lateral groove. Its nerve fibers are derived from the cells of the column of Clarke, consequently the tract extends from the last dorsal up to the medulla on the same side. It is an ascending or sensory tract, and, as it is traced upward, becomes wider from the acquisition of axones from the col- umn of Clarke. It enters the cerebellum through the inferior peduncle and finally terminates in the Cerebellar cortex of the superior worm on both sides, chiefly the opposite. 2. Cowers' 's tract lies just in front of the direct Cerebellar and also at the surface of the cord. It is an ascending or sensory tract and some of its fibers are supposed to reach the cerebellum by passing THE SPINE. 381 through the formatio reticularis of the medulla, and making a backward turn through the superior med- ullary velum of the same side. Other fibers of this tract have been traced to the corpora quadrigemina, the thalamus, substantia nigra, and the lenticular nucleus of the cerebrum. The fibers of this tract extend the whole length of Posterior roof. Posterior fissure. Lissaur's marginal ground bundle. Comma tract. Pia mater. Direct pyramidal tract. A tUerior root of spinal nerve. Anterior fissure. Fig. 268. Cross section of spinal cord cervical region, i, Zona ter- minalis; 2, zona reticularis; 3, substantia gelatinosa of Rolando; 4, stellate cells of posterior horn; 5, column of Clarke; 6, Waldeyer's cen- tral cell column ; 7, cells of lateral horn; 8, 'central canal; 9, antero-me- ,sial cells; 10,^ postero-mesial cells; n, % antero-lateral cells; 12, postero- lateral cells. the cord and probably have "their origin in the cells of the posterior horn. 3. The crossed pyramidal tract is a large and well-defined bundle that lies just beneath the direct cerebellar; that is, between this and the posterior horn. Below the point where the direct cerebellar 382 NORMAL HISTOLOGY AND ORGANOGRAPHY. begins, the crossed pyramidal comes to the surface of the cord. The fibers of this tract have their origin in the large pyramidal cells of the cerebrum in the region of the area of Rolando. Traced downward from this source, they cross in the lower part of the medulla to the opposite side of the cord, making at this point the motor decussation. As it descends the tract gradually diminishes in size, due to the fact that fibers leave it to arborize around the large motor cells of the anterior horn. In this way the entire tract is ultimately exhausted near the lower extremity of the cord. It is thus a descending or motor tract that governs the opposite side of the body from where it has its origin. 4. LowenthaVs tract is closely associated with Gowers's. Its position is anterior and mesial to Gowers's, encroaching some upon the anterior region of the cord. The fibers of this tract are supposed to come from cells in Deiters' nucleus of the medulla which may be regarded as an internode between the cerebellum and the medulla. This tract descends as far as the lumbar region and its fibers are supposed to arborize around the motor cells of the anterior horn of the spinal cord. 5. The mixed lateral bundle represents the re- mainder of the lateral region. Its fibers probably come from cells in all parts of the gray matter of the cord, and from cells on the opposite side of the cord. At intervals these fibers ultimately reenter the gray matter and arborize about nerve cells. It is there- fore an intersegmental or commissural tract, both sensory and motor. THE SPINE. 383 ANTERIOR REGION. 1. Direct Pyramidal Tract. This is a small well- defined tract that lies next to the antero-median fissure. As a rule this tract can only be traced down to the middle of the dorsal region. These fibers originate jointly with those of the crossed pyramidal tract; that is, from the large pyramidal cells of the cerebral cortex. The fibers, however, do not cross in the medulla but pass directly down the cord on the same side. The fibers cross in the cord at inter- vals in its course, making use of the anterior com- missure to reach the opposite side, where they ar- borize around the cells of the anterior horn in a manner like those of the crossed pyramidal. It thus follows that the motor cells on one side of the cerebrum control the muscular contraction on the opposite side of the body, either through the crossed or the direct pyramidal tract or through both. 2. The anterior ground bundle is really a part of the mixed lateral tract already described. It is composed of ascending and descending fibers that have both their origin and their termination in the gray matter of the cord and is therefore an inter- segment al or commissural tract. 3. Anterior White Commissure. This is composed of medullated nerve fibers passing parallel to the gray commissure between the latter and the bottom of the anterior fissure. It is a decussation of fibers of a mixed variety, many of them being derived from the direct pyramidal tract as already stated. CHAPTER XIII. THE BRAIN. The brain, or encephalon, develops jointly will the spinal cord, and represents the anterior extremit} of the cerebrospinal axis. The average weight of the brain is about forty-eight ounces, while the cord weighs less than one ounce. Like the cord it is an organ in which all the elementary tissues may be found but in which the neurons predominate. During embryonic growth the brain and cord are a hollow tube and this cavity is never obliterated, but remains in the brain as its ventricles. Develop- mental history further discloses the fact that the brain, like the cord, is made up of definite segments or joints called neuromeres. These primary units are soon replaced by three larger vesicles called primary fore-brain, mid-brain, and hind-brain. It is generally affirmed that the first of these repre- sents 3 neuromeres, the second 2 neuromeres, and the third 6 neuromeres. Later the primary fore- brain divides to form the cerebrum and the 'tween- brain, while the hind-brain also divides to form the cerebellum and medulla. In the adult brain, there- fore, five divisions may be recognized, each of them presenting a central canal or cavity as designated in the table below : 384. THE BRAIN. 385 Primary Divisions. Secondary Divisions. Cavity. Fore-brain or prosen- f T " Jelencephalon or cerebrum. Lateral ventricle. cenhalon 1 2 ' * halamencephalon, or dien- Third ventricle. ' ( cephalon, or 'tween-brain. Mid-brain. 3. Mesencephalon or mid-brain. Aqueduct of Sylvius. Hind-brain or rhomb- f 4> Metencephalon or cerebel- Upper part of fourth pnrenhalnn mm. ventricle. ' ( 5. Myelencephalon or medulla. Fourth ventricle. The brain is thus a hollow multiple organ. Its central cavity is lined with a serous membrane, the ependyma, which secretes a serous fluid, the cerebro- spinal fluid. The outer vascular investments are the meninges, which serve as a delicate packing between the brain wall and the bony vault of the cranium. The Meninges. These are three connective-tissue investments to the brain that are practically identical and continuous with those already described inclos- ing the cord. It will be sufficient, therefore, to men- tion the points wherein these membranes slightly differ. The dura of the brain forms the periosteum of the investing bones, while each segment of the vertebral column has its own periosteum. Several broad prolongations of the brain dura extend between the different divisions of the brain. These are the ten- torium between the cerebrum and the cerebellum; the falx cerebri which dips into the great fissure be- tween the two lobes of the cerebrum, and the falx cerebelli, which is a small median septum between the cerebellar hemispheres. At the basal skull fora- mina, the dura accompanies the cranial nerves and is continuous with the areolar sheaths of these nerves. The pia is composed mostly of areolar tissue and 386 NORMAL HISTOLOGY AND ORGANOGRAPHY. small blood-vessels. It is the nourishing tissue of the brain and clothes its entire surface, dipping down to the bottom of fissures, and sending strands, associated with blood-vessels, into the brain sub- stance. The brain pia is more vascular than that of the cord. The arachnoid is a w T eb-like membrane between the dura and the pia, but much nearer the dura. The space beneath the dura is called the subdural space, and is small. That between the arachnoid and pia is larger and is called the subarachnoid space. The former has a little serous fluid, and the latter is filled with lymph and some cerebrospinal fluid. This fluid reaches the external surface of the brain through a small foramen or pore in the thin roof of the fourth ventricle. Trabeculae intervene between all these membranes. THE MEDULLA. The medulla is about one inch in length, and rep- resents the portion of the brain next to the spinal cord. Its lower extremity is at the lower margin of the foramen magnum. From this point it passes upward in nearly a vertical direction to its upper ex- tremity at the lower border of the pons. Being the nerve center for the large cranial nerves, the medulla is the most vital part of the brain, and the best pro- tected. Its lower portion resembles the cord, having the same fissures and grooves. The upper portion is expanded in such a manner as to bring its cavity or fourth ventricle to the dorsal surface. This ex- panding process has carried the dorsal tracts later- THE BRAIN. 387 ally, leaving a very thin roof, consisting of the ependyma and a vascular pia, to cover the ventricle. The lower half of the fourth ventricle is found in the upper half of the medulla, while the upper half of this ventricle extends into the pons region and is overlaid by the cerebellum. The central canal of the cord opens into the lowest point of this ventricle and therefore extends through the lower half of the medulla, but nearer its dorsal surface. The lower Valve' of Vieussens. Middle peduncle of the cerebellum. Area acustica. Trigonum vagi. Calamus scriptorcs. Pineal body. Superior quadrigeminal body. Inferior quadrigeminal body. Crus cerebri. Superior peduncle of the cerebellum. Eminentia teres. Stria, acustica. Restiform body. Trigonum hypoglossi. Clava. Rolandic tubercle. Funiculus gracilis. Funiculus cuneatus. Fig. 269. Dorsal view of 'the medulla, pons and mid-brain. - half is therefore spoken of as the closed medulla while the upper part, that has the ventricle, is called the open medulla. Two ridges, the funiculus gracilis and funiculus cuneatus, may be recognized on the dorsal surface of the medulla, and represent the continuation of the columns of Goll and Burdach. The funiculus gra- cilis terminates anteriorly in a blunt expansion 388 NORMAL HISTOLOGY AND ORGANOGRAPHY. called the claua. On the Fig. 270. View, from below, of the connection of the principal nerves with the brain: I', the right olfactory tract; II, the left optic nerve; IF, the right optic tract (the left tract is seen passing back into i and e, the internal and external corpora geniculata); III, the left oculomotor nerve; IV, the trochlear; V, V, the large roots of the trifacial nerves; + -}-, the lesser roots (the + of the right side is placed on the Gasserian ganglion); i, the oph- thalmic; 2, the superior maxillary; and 3, the inferior maxillary divi- sions; VI, the left abducens nerve; VII, VIII, the facial and auditory nerves; IX-XI, the glossopharyngeal, pneumogastric, and spinal accessory nerves; XII, the right hypoglossal 'nerve; C lt the left suboccipital or first cervical nerve (Nancrede). dorsal aspect of the open medulla is found the restiform body, which passes in- to the inferior pedun- cle of the cerebellum and represents fiber tracts, the most im- portant being the di- rect cerebellar tract. The lower half of the fourth ventricle is V-- shaped and its apex is called the calamus scriptoriuSy from its resemblance to a pen. In its floor three tri- angular areas are found, called trigo- num vagi, trigonum hypoglossi, and area acusticcz. It is in these areas that we find, respectively, the origin of the tenth, twelfth, and eighth cranial nerves. The stria acusticce are transverse ridges in the floor of this ventri- cle, extending across its middle part from the median sulcus to THE BRAIN. 389 the lateral margins, and represent nerve fibers car- rying impulses from the eighth cranial nerve. On the lateral surface of the medulla a prominent oval elevation appears called the olivary body, which represents a crescent collection of nerve cells. Just dorsal to the olivary body is the continuation of the dorsal groove of the cord, and it is from this groove that fibers of the ninth, tenth, and eleventh cranial nerves emerge. Near the anterior extremity, at the lower margin of the pons, is the superficial origin of the seventh and eighth nerves. The origin of these nerves corresponds to the entrance into the cord of the posterior root of the spinal nerves. Just median or ventral to the olive is a groove that cor- responds to the points of exit of the anterior root of the spinal nerves. From this groove the fibers of the twelfth cranial nerve escape. The ventral region of the medulla presents a median fissure, the continuation of the anterior fissure of the cord. The upper end of this fissure forms a pit at the lower margin of the pons, called the foramen cecum. Just lateral to this cecum, and curving around the pons, is the superficial origin of the sixth pair of cranial nerves. On each side of the median fissure is a longitudinal ridge called the pyr- amids which represents the fibers of both the crossed and the direct pyramidal tract. Near the lower ex- tremity of the medulla the fissure seems partly obliterated by ridges recrossing. These represent pyramidal fibers crossing to form the crossed pyr- amidal tract, and constitute, therefore, the motor decussation. Just below the olive curved striae may 390 NORMAL HISTOLOGY AND ORGANOGRAPHY. be seen, that appear to come from the median fissure and sweep around the olive and enter the cerebellum through the restiform body and inferior peduncle. These are called the superficial arcuate fibers, and many of them come from the nerve cells of the olive of both the same and the opposite sides. Sections of the Medulla. Cross Sections of the Closed Medulla. These verify the surface markings Median raphe. Central canal. Funiculus gracilis. Nucleus gracilis. Funiculus cuneatus. Nucleus cuneatus. Spinal root of fifth nerve. Sub slant ia gelati- nosa Rolando. Deep arcuate fibers. Mesial olive. Decussation of fibers. Superficial arcuate. Olive nucleus. Arcuate nucleus. Anterior Pyramids. Fig. 271. Cross section of the closed medulla. already described. In the dorsal region is the funiculi gracilis and cuneatus, fiber tracts of the columns of Goll and Burdach. Beneath these are the nuclei of gracilis and cuneatus, nerve cells around which arborize the telodendria of the fibers of the columns of Goll and Burdach. From these nerve cells axones spring that sweep downward and across to the oppo- site side, and in crossing form the sensory decussa* THE BRAIN. 391 lion. It thus happens that the sensory impulses also cross and reach the opposite side of the brain. Ex- ternal to the nucleus gracilis is the substantia gelati- nosa of Rolando, a continuation of that of the cord. Just external to this is a cross section of nerve fibers, the ascending root of the fifth nerve. The central area of each half of the section shows a large number of scattered nerve fibers interlacing and in cross sections. This area is called the formatio reticularis. Anterior to it, a collection of nerve cells represents the olivary body, median and dorsal to which a second and smaller collection of cells consti- tute the mesial olivary nucleus. The bulk of the anterior portion shows a cross section of the pyra- mids. Some of these fibers may be seen to sweep to the opposite side, thus making the motor decus- sation. In doing so they seem to pass over in large alternate bundles, rather than uniformly, as is the case with the sensory decussation. At the anterior surface on each side of the median fissure and sweep- ing around the pyramids are the superficial arcuate fibers, and also a collection of nerve cells, the arcuate nucleus. In cross sections of the open medulla the resem- blance to that of the cord is less distinct. The thin roof of the fourth ventricle is usually broken, leaving a dorsal expanded cleft. The lateral margin or remnant of the roof is called the lingula. Just be- neath the floor and close to the median sulcus are the nerve cells of the twelfth nerve. Lateral but in close proximity to these are the nerve cells of the tenth nerve. The axones from these cells may be 392 NORMAL HISTOLOGY AND ORGANOGRAPHY. traced through the substance of the medulla to their ventral exit. The superficial origin of the twelfth nerve is just anterior to the olive, and the fibers of the tenth nerve may be traced to their superficial origin just dorsal to the olive. In serial cross sec- Nucleus am- ' 'juus. Substantia gelatinosa Rolando. Descending root of -nerve V. Nucleus later a Us. A nt. lat. ascend^ ing tract. Olive. Superficial arcuate fibers. Arcuate nucleus* Anterior pyramids. Fig. 272. Cross section of the open medulla (composite drawing). tions it will be seen that the nuclei of the other cra- nial nerves, from the sixth to the twelfth, lie in the floor of the fourth ventricle. The fasciculus solitarius is a bundle of nerve fibers, cut in cross section, and placed just lateral to the THE BRAIN/ 393 nucleus of the tenth nerve. This bundle represents fibers from the ninth and tenth nerves. Just in- ternal or mesial to this bundle are a few cells called' the solitary nucleus. This is probably a motor nu- cleus of the ninth and tenth nerves. Lateral to the solitary fasciculus are the fibers of the large restiform body on their way to the cerebellum. The posterior longitudinal bundle is a tract of nerve fibers that appears in cross section just an- terior to the nucleus of the twelfth nerve, and lies in close apposition to the median plane. The formatio reticularis occupies the greater part of the center of each lateral half of the medulla, and presents the same appeareance as in sections of the closed med- ulla. Likewise the cells of the olivary body, which in the open medulla form a large conspicuous nu- cleus that takes the form of a U with wavy sides, and with the open extremity turned inward and upward. Nerve fibers from its hilum sweep across to the oppo- site side and some curve around to join the super- ficial arcuate fibers of the same side. The arcuate fibers are divided into the deep and the superficial set. The deep set come from the nuclei cutieatus and gracilis and from the sensory nuclei of the cranial nerves. From this source they arch to the opposite side and then turn to pass upward in the brain stem, where they form the middle fillet. The superficial set may be divided into an anterior and a posterior group. The anterior group originate in the nuclei gracilis and cuneatus, accompany the deep set to the opposite side of the medulla, where some of them become superficial in the anterior mesial fissure, then curve around the anterior pyra- 394 NORMAL HISTOLOGY AND ORGANOGRAPHY. mid in the superficial border of the medulla, and finally enter the restiform body and the cerebellum through the inferior cerebellar peduncle. Others become superficial in the antero-lateral groove lateral to the anterior pyramid, passing also to the cerebel- lum, through the inferior cerebellar peduncle. The posterior group originate in the nuclei gracilis and cuneatus and pass directly forward and up- ward in the cerebellar peduncle of the same side to terminate in the cerebellum. All the arcuate fibers carry sensory impulses. The arcuate nucleus is a collection of nerve cells interposed in the anterior superficial arcuate fibers at a point just anterior to the pyramids of the medulla. SUMMARY OF TRACTS, THEIR ORIGIN AND TERMINA- TIONS. Columns. 1. Column of Goll . . 2. Column of Burdach 3. Comma tract . . . 4. Lissaur's tract . . 5. Direct cerebellar . 6. Gowers's tract . . 7. Lowenthal 8. Lateral ground bundle. 9. Crossed pyramidal . . 10. Direct pyramidal . : . 11. Ant. ground bundle . . Origin of Axones. Cells of dorsal ganglion. Cells of posterior horn. Same as column of Goll. Dorsal ganglion . . . . Dorsal ganglion . . . . Cells of col. of Clarke . Cells of post, horn Deiters' nucleus . Cells of cord . . . Cerebral cortex . Cerebral cortex . Cells of cord . . . Terminations. i. Nucleus gracilis. i. Nucleus cuneatus. i. Cells of post. horn, i. Cells of post. horn, i. Cerebellum. 1. Cerebellum. 2. Corp. quadrigemina. 3. Thalamus. 4. Substantia nigra. 5. Lenticular nucleus, i. Cells of ant. horn, i. Cells of cord. i. Cells of ant. horn, i. Cells of ant. horn. i. Cells of cord. The sensory tracts are Nos. i, 2, 3, 4, 5, 6. The motor tracts are Nos. 7, 9, 10. The mixed tracts are Nos. 8, n. THE PONS. The pons represents the anterior basal portion of the hind-brain. It is an oval body, one inch long, one inch thick, and about one and one-half inches broad. It is a junctional piece between the medulla THE) BRAIN. 39S and the mid-brain and the overlying cerebellum. The upper half of the fourth ventricle is confined to its dorsal aspect; that is, between the pons and the cerebellum. Viewed from the ventral surface it pre- sents the appearance of a rhomboid with striations that pass transversely and become constricted later- ally to form the middle peduncles of the cerebellum. Nucleus of nerve VI. Restiform body. Arcuate fibers. Descending root of nerve V. Nucleus of nerve VII- s\ ' .v\ "v y : _ '- * ^ --v'jr > s'?S5SsS_ Middle peduncle of cerebellum. Nuclei pontis. Floor of fourth ventricle. Posterior longitudinal bundles. Ascending root of nerve VII. Formatio reticularis. Median raphe. Pyramidal bundles. Transverse fibers of pons. Fig. 273. Cross section through the lower part of the pons. The fifth cranial nerve, with its large sensory root and its small motor root, is attached to the ventral aspect of the pons, nearer its upper than its lower border. The anterior pyramids seem to enter the pons from below, and emerge above the pons, where they become lost in the crura cerebri. In a transverse section the pons presents the fol- lowing parts: 396 NORMAL HISTOLOGY AND ORGANOGRAPHY. I. White matter. 1. Transverse fibers (a) superficial, (ft) deep (trapezium). 2. Longitudinal fibers (a) superficial (an- terior pyramids), (fe) deep. 3. Posterior longitudinal bundle. 4. Fibers of fifth, sixth, seventh, and eighth cranial nerves. 5. Formatio reticularis. 6. Median raphe. 7. Fillet mesial and lateral. II. Gray matter. 1. Nucleus pontis. 2. Superior olive. 3. Nuclei the origin of fifth, sixth, seventh, and eighth cranial nerves. Transverse Fibers. The superficial and the deep- set of transverse fibers of the pons pass into the cere- bellum through the middle peduncle. Some of the fibers are commissural between the two halves of the cerebellum, while others connect with the nuclei pontis on the same side or the opposite side. In the lower portion of the pons, near the medulla, the deep- set are called the trapezium, on account of their trapezoid arrangement. Longitudinal Fibers. The superficial set repre- sents mostly longitudinal bundles of the anterior pyramids that interlace the transverse fibers. The deep-set are near the dorsal aspect of the pons and comprise at least three groups: (i) the posterior longitudinal bundle near the median raphe in which THE BRAIN. 397 are found fibers from the antero-lateral column of the cord; (2) the lemniscus or fillet, a continuation of the sensory decussation; (3) the fasciculus teres, just dorsal to the posterior longitudinal bundle, and which contain fibers of the seventh cranial nerve. Fibers of the cranial nerves pass through the medulla from their nuclei in the dorsal portion to their various su- perficial exits on the ventral surface. The formatio re- ticularis is similar to that described in the medulla. The median raphe is also a continuation of that described in the medulla. Gray Matter of the Pons . The nuclei pontis are nerve cells that are scattered among the superficial transverse fibers and are nodal points forming connections between the medulla, cerebellum, and higher brain centers. The superior olive lies in the formatio reticularis and is seen only in the lower portions of the pons. The nuclei of the cranial nerves are found in the dorsal aspect, most of them just beneath the floor of the fourth ventricle. Nerve- fiber layer. Fig. 274. Section through the hu- man cerebellar cortex vertical to the sur- face of the convolution. Treatment with Muller's fluid (Bohm and Davidoff). 398 NORMAL HISTOLOGY AND ORGANOGRAPHY. THE CEREBELLUM. The cerebellum is next in size to the cerebrum and overlies the fourth ventricle. It is characterized by transverse curved sulci which divide it into lamellae, giving the organ a foliate appearance. A cross sec- tion of the lamellae shows a central core of white matter with a gray cortex, giving the section the appearance of a branching tree, hence the name arbor mtcz. A section taken in this plane presents the following layers : 1. Molecular layer on the outside. (1) Small cortical cells. (2) Stellate cells. (3) Cells of Purkinje. 2. Granular layer. 1 i ) Granular cells . (2) Large stellate cells. 3. Medullary substance core of nerve fibers. (1) Centrifugal neuraxes from Purkinje cells. (2) Centripetal neuraxes. (a) Mossy fibers. (b) Climbing fibers. (3) A few ganglion cells forming the central gray nucleus. Molecular Layer. The small cortical cells are found in all parts of this layer, but more especially near its periphery. They are multipolar cells and but little is known of the distribution of their neu- raxes. The stellate cells are evenly distributed, and of particular interest are their neuraxes. The latter pos- sess two types of collaterals. One set forms branches among the cortical cells, while a second class branches Stellate cell. Neuraxis Large of cell of stellate granular Tolodendrion of collateral cell. layer, of climbing fiber. K g I h *1 <3 I B ce/i. Molecular layer Granular layer. Medullary layer 400 NORMAL HISTOLOGY AND ORGANOGRAPHY. at a level with the Purkinje cells, where it forms a basket-like net around the bodies of these cells. The cells of Purkinje are the largest nerve cells in the body (about 6o// in diameter or seven times the diameter of a red blood-corpuscle). They form a single row of cells, placed with considerable regularity some distance apart, along the inner margin of the molec- ular layer. Their neuraxes arise from the basal end of the cell body and extend through the granular layer and enter the medulla as the centripetal fibers. The other extremity of the cell body passes into one or more prominent dendrites that arise toward the Neuraxis. Fig. 276. Cell of Purkinje from the human cerebellar cortex. Chrome- silver method (Bohm and Davidoff). periphery of the cerebellum. These dendrites branch freely in one plane, like an ivy growing against the wall, and this plane is always at right BRAIN. 401 Neuraxis. angles to the lamellae of the cerebellum, and therefore sections of the cerebellum should be made in this plane. Granular Layer. This layer is densely packed with nerve cells of two varieties. The granular cells are most numerous, small, and have only a few dendrites that end in hook-like telodendria. The neuraxes from these cells pass vertically into the molecular layer, where many of them form a T-- shaped division, the two end branches passing par- allel with the laminae and therefore into a plane vertical to that of the den- drites of the Purkinje cells. Large stellate cells form the second variety of this layer. They are few in number and lie close to the mo- lecular. Their dendrites branch in all directions and their neuraxes form telo- dendria among the granu- lar cells. The medullary substance may be divided into centri- petal fibers, those that carry nerve impulses toward the granular and molecular layers, and centrifugal fibers; those that carry impulses in the opposite direction. The latter are the neuraxes of the cells of Purkinje, The centripetal fibers are the mossy fibers, that form mossy telodendria in the granular layer, and also so- called climbing fibers that pass through the granular Claw-like telodfn- drion of dendrite. Fig. 277. Granular cell from the granular layer of the human cerebellar cortex. Chrome-silver method (Bohm andDavidoff). Layer of polym- orphous cells. Fig. 278. Portions of vertical section of human cerebral cortex, treated by the Golgi method. The figure shows the arrangement of the different cells of the cerebral cortex (Sobotta). 402 Layer of small Pyramidal cells. Layer of large pyramidal cells. THE BRAIN. 403 layer and connect with the dendrites of Purkinje cells, up which they seem to climb. Collaterals are given off in their passage through the granular layer. The central gray nucleus forms the central core of each lateral cerebellar hemisphere. It forms a cap- sule of gray matter from whose hilum many nerve fibers pass, the majority to enter the superior cerebel- lar peduncle. THE CEREBRAL CORTEX. The cerebrum is such an extensive and complicated organ that only a description of the cortex in the region of the fissure of Rolando will be given here. From without inward this region presents, rather indistinctly, the following layers: (T) molecular layer; (2) small pyramidal cells; (3) large pyr- amidal cells; (4) polymorphic cells; (5) medullary substance of nerve fibers. ' It is to be borne in mind that this cortex presents many fissures and minor folds into which the pia dips, and that a transverse section is any plane that is vertical to the folded surface. i., The molecular or outer layer is composed chiefly of nerve fibers which interlace in all directions but which have chiefly a direction parallel to the external surface. The chief dendrite of the pyramidal cells terminates in this layer in tuft-like telodendria, and also ascending neuraxes, mostly from the polymor- phous cells. The cells of this layer are few, and have been described as polygonal, spindle-shaped, and triangular, or stellate. Their neurons are nearly all confined to this layer, the axones of only a few reach down to the deeper layers. 404 NORMAL HISTOLOGY AND ORGANOGRAPHY. 2. The layer of small pyramidal cells is not well defined and usually not so broad as the molecular layer. The nerve cells have a triangular body, the apex being directed toward the surface of the cortex. From this apex a primordial dendrite ascends and Brush-like telodendrion. Main dendrite. Secondary dendrite. Basal dendrite.' Neuraxis with collaterals- Fig. 279. Large pyramidal cells from the human cerebral cortex, Chrome-silver method (Bohm and Davidoff). gives off a number of branches that end in terminal filaments or telodendria in the outer layer, frequently at the brain surface. Several short dendrites arise THE BRAIN. 405 ~ d from the basal surface of the cell body where also the axone is attached. The latter passes toward the medullary substance, and near the cell body is pro- vided with col- laterals that connect with adjacent neu- rons. The layer of large pyramidal cells comprises a broad area. The cells meas- ure 20 p. to 30 fJt in diameter, be- ing twice as large as those of the preced- ing layer. In all other re- spects the cells of this layer re- semble the small pyram- idal cells. Their dendrites and axones also occupy the same relation as those of the preceding layer. 4. The layer of polymorphic cells usually includes if--/ Fig. 280. Schematic diagram of the cerebral cortex: a, Molecular layer with superficial (tan- gential) fibers; b, striation of Bechtereff-Kaes; c, layer of small pyramidal cells; d t stripe of Baillarger; e, radial bundles of the medullary substance; /, layer of polymorphous cells (Bohm and Davidoff). 406 NORMAL HISTOLOGY AND ORGANOGRAPHY. a few large pyramidal cells, and it is not well defined from the preceding layer. There is present in this layer: (i) multipolar cells with short neuraxes (Golgi cells) whose dendrites project in all directions ; and (2) cells with slightly branched dendrites and with neuraxes passing toward the surface of the brain where they terminate in the molecular outer layer. The cells of this layer are triangular or spindle-shaped and vary considerably in size. 5. The medullary substance is composed of a mass of nerve fibers that take a radial course and in which we can detect no structural difference. Physio- logically, however, we can divide them into four classes as follows : i , projecting or centrifugal fibers which indirectly connect the cerebral elements with the periphery of the body; that is, they carry im- pulses away from the nerve center; 2, commissural fibers that connect corresponding parts of the two cerebral hemispheres through the corpus callosum; 3, association fibers that connect different parts of the same hemisphere ; 4, centripetal fibers or terminal fibers, those that come from cell bodies in the same or the opposite hemisphere, or in some more distant nerve center, and that ultimately arborize about nerve elements in the cerebral cortex. In a strict sense the second and the third class fall under either the centrifugal or the centripetal group. THE NEUROGLIA. The neuroglia tissue represents a special form of supporting elements found exclusively in the central nervous system and in the retina. It develops from THE BRAIN. 407 the ectoderm while all other supporting tissues are derived from the mesoderm. The development of neuroglia cells is closely associated with the origin of neurons, as both are derived from epithelial cells that lie primarily near the central canal of the nervous system. The embryonic neuroglia cells are called spongioblasts, while those that develop into true nerve elements are called neuroblasts. Both kinds pass out very early and per- manently lodge in the gray matter. The neuroglia ele- ments appear as cells with many ra- diating, slender pro- cesses that are usu- ally unbranched, and because of their peculiar ap- pearance have been called spider cells or mossy cells, or astro- cytes. The cell bodies contain but very little protoplasm, and their shape is modi- fied according to their surroundings, being triangu- lar, or quadrangular, or polyangular, the protoplas- mic processes arising from their angles. According to the length of their processes attempts have been made to classify them as short-rayed astrocytes, pos- Fig. 281. Neurogliar cells: a, from spinal cord of embryo cat; b, from brain of adult cat; stained in chrome-silver (Huber). 408 NORMAL HISTOLOGY AND ORGANOGRAPHY. sessing a few short processes, and long-rayed astro- cytes, having many long, slender processes. The former appear among the nerve cells only, the latter are found both in the gray and the white matter. Fig. 282. Typical neuroglia cells, from cross section of the white mat- ter of the human spinal cord, stained after Benda's selective neuroglia tissue staining method (Huber, "Studies on Neuroglia Tissue," Vaughan Festschrift, 1903). Not infrequently detached processes may be found and processes that can be traced directly through the bodies of adjacent astrocytes. The neuroglia thus forms a delicate web-like fabric that interlaces THE BRAIN. 409 the whole central nervous system, to which it gives substance and support. It is to be remembered that supporting tissue of mesodermic origin does the same thing, especially in the cord. The connective-tissue elements usually accompany the nutrient blood- vessels. BLOOD-VESSELS OF THE CENTRAL NERVOUS SYSTEM, The spinal cord receives its blood supply from a plexus of blood-vessels in the pia mater. There is an anterior median artery just in front of the anterior fissure. Some two hundred branches from this vessel pass at right angles directly into the fissure and enter the gray matter, where each divides into a right and left branch that enclose the central canal. Each arterial branch ultimately bifurcates, just in front and external to the cell column of Clarke, forming minute ascending and descending terminals, which become lost in an extensive capil- lary system of the central gray matter. The white matter receives its blood supply from a plexus of vessels situated on the dorsal and lateral surfaces of the- cord. From this system small branches enter the cord anywhere and form capillaries among the nerve fibres; that is, supplies blood to the white matter. The gray matter has a more liberal blood supply than the white matter. The brain substance also receives its blood supply from a plexus of blood-vessels in its pia. The capil- laries are particularly numerous and closely meshed wherever the nerve cells are segregated, that is, in the ganglion centers. In the cerebellum the gran- 410 NORMAL HISTOLOGY AND ORGANOGRAPHY. ular layer is the most vascular. The arterioles have thin walls, and in old age may become brittle and may easily rupture. No lymphatic vessels with definite walls have been discovered in the central nervous system. The blood-vessels are, however, surrounded by peri- vascular spaces which probably function as lymph channels. CHAPTER XIV. THE EYE. The eyes begin to develop during the fourth week of embryonic life, and appear then as a pair of lateral e vagina tions of the fore-brain. A pair of vesicles are thus formed called the primary optic vesicles. When the latter reach the ectoderm an invagination of these vesicles takes place, like pushing in one side of a hollow rubber ball. The cavity of the primary optic vesicle becomes obliterated by this process, and a new vesicle forms, called the secondary optic vesicle. It will be observed that the cavity of this vesicle is practically the same as would be pro- duced by an invagination of the brain wall. Later it will be seen that this cavity corresponds to the space occupied by the vitreous humor of the adult eye, while its wall becomes the retina. The stalk that connects this vesicle to the brain is the optic stalk, in which later optic nerve fibers appear. At the time the secondary optic vesicle is forming there is a disc-like thickening of the adjacent ecto- derm, which soon invaginates and becomes con- stricted as an ectodermal vesicle. This is the lens, which later takes a position at the mouth of the secondary optic vesicle. The latter presents, at this stage, a fissure in its ventral surface called the cho- roid fissure. Connective-tissue cells migrate through 412 NORMAL HISTOLOGY AND ORGANOGRAPHY. this fissure and fill the cavity of the secondary vesicle. Fig. 283. Part of a section through the head of an early human embryo, showing the connection of the primary optic vesicles with the fore-brain (His): olf, Olfactory area of epiblast; ch., part of fore-brain which gives rise to cerebral hemispheres; th, thalamencephalon; p.o.v., primary optic vesicles. Fig. 284. Three successive stages of development of the eye, showing formation of secondary optic cup and crystalline lens in human embryos of 4 mm. (A), 6 mm. (#), and 8 mm. (C) (Tourneux): a, a, primitive optic vesicles; b, external layer of secondary optic cup (future pigment layer of retina) ; c, inner layer of cup (retina proper) ; d, lens pit (thick- ened and depressed ectoderm) ; e, lens vesicle. These cells form the vitreous humor, while the choroid THE EYE. 413 fissure closes and permanently disappears. The ex- ternal coats of the eye that is, the sclera and the choroid develop from the surrounding connective tissue. Fig. 285. Plastic representation of the optic cup with lens and vitre- ous body (Hertwig): ab, Outer wall of the cup; ib, its inner wall; h, cavity between the two walls, which later disappears entirely; Sn, fun- dament of the optic nerve (stalk of the optic vesicle with a furrow on its lower surface); aus, optic (choroid) fissure; gl t vitreous body; /, lens. The parts of the adult eyeball may be tabulated as follows : I. Tunica externa. 1. Sclera. 2. Cornea. . II. Tunica media. 1. Choroid coat. 2. Ciliary body. 3. Iris. III. Tunica Interna. 1. Retina. 2. Pigment membrane. The refracting media, or transparent media of the eye traversed by a ray of light, are: i. The cornea. 414 NORMAL HISTOLOGY AND ORGANOGRAPHY. 2. Aqueous humor. 3. Lens. 4. Vitreous humor. TUNICA EXTERNA. i . The sclera is a dense connective-tissue covering of the eye that terminates anteriorly in the cornea. It is of interest to note that, in birds of prey, horny plates develop in the sclera for the better protection Canal of Schlemm. Iris. Cornea. Aqueous humor. Lash. Iris. Tunica exierna. Blind spot. Optic nerve. Ciliary body. Ora serrata. Fig. 286. Diagram of the eye. of the eye. Posteriorly the sclera is perforated by the entrance of the optic nerve. Connective-tissue fibers, known as the lamina cribrosa, pass across this point and interlace the optic fibers, while others sweep backward along the optic nerve as its external envelope. The sclera consists of interlacing bundles of con< THE EYE. 415 nective-tissue fibers closely felted together. The tendons of the ocular muscles are continuous with the scleral fibers. The external scleral surface is clothed with a layer of flattened endothelial cells which belong to the capsule of Tenon. The latter is a loose connective-tissue fabric that invests the eye- ball, and is so intimately connected with the eye muscles that coordinate movement of the arti- ficial eye, or glass shell, is made possible after the enucleation of an eye. Pigmentation is regularly present at the corneal margin and the surface next to the choroid. This inner pigmented scleral sur- face is lined by a layer of flattened endothelial cells, forming a sepa- rate membrane and called by some the lam- ina fusca; generally, Fig. 287. Section of the cornea of the however, it is regarded as the outermost layer of the choroid and known as the lamina choroidea. 2. The Cornea. The cornea is inserted into the sclerocorneal junction in which is found an annular venous sinus, the canal of Schlemm, which may ap- pear as a single canal or as several canals. The cornea is a perfectly transparent medium and free Substantia Propria. 41 6 NORMAL HISTOLOGY AND ORGANOGRAPHY, from red blood corpuscles, the nearest blood supply being that of the sclerocorneal margin in the region of the canal of Schlemm. Histologically the cornea is made up of five layers : i, the anterior epithelium; 2, the anterior elastic membrane, or Bowman's membrane; 3, the ground substance, or substantia propria; 4, Descemet's membrane; 5, the endothelium of Descemet's membrane. The corneal epithelium is of the stratified squamous variety, a little thicker near the corneal margin than at its center and in the human eye is composed of five layers of cells. It is related to the epidermis of the skin, the cells being provided with short prickles that are very difficult to demonstrate. This epithe- lium forms an efficient and important protection to the front of the eye. The anterior elastic membrane measures 8 p in thickness, about the width of a red blood corpuscle, and becomes thinner towards the sclerocorneal junction. It is a compact layer of con- nective-tissue fibrils and is regarded by some as a basement membrane to the overlying epithelium. Nerve fibers penetrate this membrane to connect with the corneal epithelial cells. The substantia propria constitutes the bulk of the cornea. It con- sists of bundles and lamellae of connective-tissue fibrils, and peculiarly flattened cells called corneal corpuscles. The fibrils of each lamella are cemented together and run parallel to each other and to the corneal surface, but so arranged that those of ad- jacent lamellae cross at an angle of about twelve degrees. The lamellae are also cemented to each other. The THE EYE. 417 corneal cells have irregular processes and lie in special cavities called corneal spaces, in which are also found a varying number of leucocytes. These spaces seem to be part of a complicated lymphatic system, and communicate with each other by means of a complex system of canals. While blood does not irrigate the cornea, lymph does, freely and extensively. The posterior elastic or Descemet's membrane resembles the Lymph cannliculi. % Corneal space. Fig. 288. Corneal spaces of a dog (Bohm and Davidoff). anterior elastic membrane, and may be separated into shreds of fine, elastic, connective-tissue fibrils. The endothelium of Descemet's membrane is com- posed of low, hexagonal cells forming a single layer. It will be found that Descemet's membrane with its investing endothelium is reflected to form the an- terior layer of the iris, enclosing therefor the anterior portion of the aqueous chamber. 27 41 8 NORMAL HISTOLOGY AND ORGANOGRAPHY. TUNICA MEDIA. i. The Choroid Coat. The choroid is the vascular tunic of the eye, and may be divided into four layers. From without inward these are named: i, lamina suprachoroidea ; 2, lamina vasculosa Halleri; 3, lamina choriocapillaris ; 4, glassy layer, or vitreous membrane. This entire tunic is derived from the Solera. . Lamina supra- choroidea. Lamina vascu- losa Halleri. Lamina chorio- capillaris. Glassy layer. Fig. 289. Section through the human choroid (Bohm and Davidoff). mesoderm and is largely composed of connective- tissue elements. The Lamina Suprachoroidea. This layer is closely applied to the sclera, and is composed of a loose fabric of areolar tissue in whose meshes are con- nective-tissue cells and lymph spaces lined with endothelium, known as perichordeal lymph spaces. THE EYE. 419 Pigment cells are also present. The lamina vascu- losa is the broadest layer and is also composed of areolar tissue. The blood-vessels constitute its principal portion, and they are so distributed that the larger ones, the veins, occupy its outer portions. The lamina choriocapillaris consists chiefly of capil- Lens. Anterior epithelium. Iris. Pars ciliaris iridical. Cornea, Canal of Petit. Processus ciliares. Canal of Schlemm. Ciliary muscle. Conjunctiva. ' Fig. 290. Section through the ciliary body. lary vessels that are particularly abundant in the region of the macula lutea, or yellow spot of the retina. In other respects this layer resembles the lamina suprachoroidea, except that pigment cells are absent. The glassy or vitreous membrane is but 2 fi thick, homogeneous, clothes the inner choroid 420 NORMAL HISTOLOGY AND ORGANOGRAPHY. surface, and also forms a lining membrane against which the pigment cells of the retina are applied. 2. The Ciliary Body. The ciliary body is that por- tion of the tunica media extending between the ora serrata of the retina and the base of the iris. On the inner surface of this body there are about seventy meridional folds called the ciliary processes. Second- ary folds and processes appear on and between the primary folds, while the whole surface is clothed with two rows of epithelial cells, the pars ciliaris retina. Of these the outer layer is deeply pig- mented and represents the outer layer of the second- ary optic vesicle, while the inner layer is non- pigmented and develops from the inner layer of the optic vesicle. The greater bulk of the ciliary body is made up of smooth muscle tissue called the ciliary muscle, or muscle of accommodation. This muscle may be divided into three portions. The outer por- tion is made up of meridional fibers. The middle division of radial fibers have their origin near the canal of Schlemm, from which they spread out like a fan. The inner portion is near the base of the iris and the fibers are circular. The combined action of these fibers is to pull the choroid coat forward and inward and thus slacken the tension on the suspen- sory ligament of the lens, as this ligament joins with the epithelial cells of the ciliary body as well as with the hyaloid membrane that encloses the vitreous humor. Under this condition the lens becomes more convex and the eye is focused to near objects. 3. The Iris. The iris is a pigmented circular cur- tain that occludes the rays of light from the periphery THE BYE. 421 of the lens. The circular opening in the iris is the pupil. Three layers may be recognized in the iris, enumerated from before backward, as follows: i anterior endothelium; 2, stroma with sphincter muscle ; 3 , pigment epithelium, or pars iridica retinae. The anterior endothelium is a single layer of cells that is continuous with the posterior endothelium of the cornea. The stroma forms the bulk of the iris and is very vascular and muscular. Large pigment cells are present and a fine reticular tissue. Smooth muscle fibers, the sphincter muscle of the pupil, encircle the pupil. Along the posterior surface radial fibers probably function as a dilator muscle of the pupil. The posterior epithelium, or pars iridica ciliaris, is a direct continuation of the pars ciliaris retinae and extends to the margin of the pupil. It is composed of two layers of cells and both, in this case, are pigmented. TUNICA INTERNA* The inner tunic is the retina of the eye, which may be divided into ten layers, named from within out- ward as follows : 1 . Internal limiting membrane. 2. Layer of nerve fibers. 3. Ganglion cell layer. 4. Inner molecular layer. 5. Inner nuclear layer. 6. Outer molecular layer. 7. Outer nuclear layer. 8. External limiting membrane. 9. Rods and cones. 10. Pisment laver. 422 NORMAL HISTOLOGY AND ORGANOGRAPHY. (i) The internal limiting membrane is a very deli- cate homogeneous layer formed by lateral expansions of processes of neuroglia elements. It is closely ap- plied to (2) the optic nerve fibers. The latter are non- medullated and radiate toward the entrance of the op- tic nerve, composed of both centrifugal and centrip- etal axones. The latter arise from (3) the ganglionic cells, that are irregularly distributed along the inner r/S> 4Urf '@a '9'4*&av 5 8 Unipolar nerve cells, 96 Ureters, 269 function of, 271 structure of, 269 Urethra in female, 94 in male, 292 Urinary organs, 253 Uterus, 317 masculine, 293 muscular layer, 322 structure of, 321 vessels and nerves of, 323 Utriculus, 440 VACUOLES, 39 Valvulae conniventes, 195 Vas deferens, 285 structure of, 286 Vasa efferentia, 284 vasorum, 122, 124 Vascular laminae, 351, 353 Veins, 113 Vermiform appendix, 202 Vernix caseosa, 339 Verumontanum, 293 Vestibule, 440 Villi, chor ionic, 329 of small intestine, 196 Visceral arches, 138 Vitelline membrane, 310 Vitellus, 309 Vitreous humor, 412, 430 membrane of, 419 Vocal cords, 234 Volkmann's canals, 78 Voluntary muscle, 88 distribution of, 93 WALDEYER'S central cell column, 376 Wandering cells, 69, 119 Washing tissues, 458 Weigert-Pal stain, 477 Weil, layer of, 159 Wens, 360 Wharton's duct, 70, 211 jelly, 67, 331 White blood-corpuscles, 118 fibrous cartilage, 76 connective tissue, 70 matter of pons, 395 of spinal cord, 376 muscle, 91, 92 Winslow's foramen, 219 Wirsung's duct, 213 Wolffian body, 258 duct, 258 494 INDEX YEU,OW elastic connective tissue, ZELLKNOTEN, 330 Zenker's fluid, 474 Zona pellucida, 308 radiata, 308 reticularis, 375 terminalis, 375 Zymogen, 209 granules of pancreas, 214 UNIVERSITY OF CALIFORNIA BRANCH OF THE COLLEGE OF AGRICULTURE THIS BOOK IS DUE ON THE LAST DATE STAMPED BELOW ^s SI&& ;^ ' gEC^ W* /IP Wr 27 "40 u " D LIBRARY APR tow DUE JUN5 1 3 8EP20'50 3rn-9,'30 CLM5SI ~ LIBRARY, BRANCH f-S? OF THE COL COLLEGE OF AGRICULTURE, DAVIS