CORNELL UNIVERSITY THE Sflouier Uderinarg Hihranj FOUNDED BY ROSWELL P. FLOWER for the use of the N. Y. State Veterinary college 1897 Cornell University Library QM 551.S99t A Textbook of histology and microscopic 3 1924 001 037 435 The original of this book is in the Cornell University Library. There are no known copyright restrictions in the United States on the use of the text. http://archive.org/details/cu31924001037435 A TEXTBOOK OF HISTOLOGY AND MICROSCOPIC ANATOMY OF THE HUMAN BODY, INCLUDING MICROSCOPIC TECHNIQUE. BY DR. LADISLAUS |ZYMONOWICZ, A. 6- PROFESSOR OF HISTOLOGY AND EMBRYOLOGY IN THE UNIVERSITY OF LEMBERG. TRANSLATED AND EDITED BY JOHN BRUCE MacCALLUM, M. D., JOHNS HOPKINS UNIVERSITY, BALTIMORE. ILLUSTRATED WITH 277 ENGRAVINGS, INCLUDING 57 PLATES IN COLORS AND MONOCHROME. LIBRARY. LEA BROTHERS & CO., PHILADELPHIA AND NEW YORK. 1902. T Mo- ly \ Entered according to Act of Congress, in the year 1902, by LEA BROTHERS & CO., In the Office of the Librarian of Congress, at Washington. All rights reserved. S9^ ELEOTROTYPEO BY WtSTOOTT 1 THOMSON, PHIU.OA. V , |LUAM ™™^ „„,,.„,. PRESS OF PREFACE. In the translation of this work and the preparation of an American edition an effort has been made to place at the com- mand of English-speaking instructors and students a text-book which includes the best results of recent investigations. The spirit and characteristic features of the German original have been carefully retained, changes in text or illustration being made only where some definite advantage was to be attained. These changes have mostly resulted in enlargements. Thus ten of the German engravings have been replaced with thirty- five new ones, taken from various sources. These can be identified by the credits given in connection with the resjDec- tive figures. Many additions have likewise been made in the text. I am especially indebted to Dr. Florence R. Sabin for a brief description of the medulla and midbrain. Nearly all of the drawings which have been taken from the German edition were made by Dr. Baracz from material pre- pared by the author, who acknowledges the assistance also of Dr. Bochenek, Professor Browicz, and Dr. R. Krause. It has been my object throughout to trace, as far as pos- sible, the development of the organs and the histogenesis of the tissues, and it is hoped that the attention of instructors and students may be drawn to the importance of viewing Histology from this standpoint. I have endeavored, also, to emphasize the fact that in many organs it is possible to recognize struct- ural units which are repeated in a definite way and bound together by a characteristic framework. I am much indebted to Professor F. P. Mall for the kind interest he has taken in the preparation of this edition. John Bruce MacCallum. Baltimore, July, 1902. CONTENTS. PAET I. HISTOLOGY. MICROSCOPIC ANATOMY OF CELLS AND TISSUES. PAGE A. THE CELL 18 Dibect Division (Amitosis) 28 Indirect Division (Mitosis, Karyokinesis) 28 Process of Fertilization 32 B. THE TISSUES 35 I. Epithelial Tissue 37 Glandular Epithelium and Glands 46 Chorda Dorsalis 51 II. Supporting, Connecting, and Interstitial Tissues 52 1. Connective Tissue 53 (a) Embryonic Connective Tissue (Gelatinous, Mucoid Tissue) 53 (b) Areolar or Fibrillar Tissue 54 (c) White Fibrous Connective Tissue 63 (d) Elastic Connective Tissue 64 (e) Eeticulum 65 (/) Fat 66 2. Cartilage 68 (a) Hyaline Cartilage 68 (6) Elastic Cartilage 72 (c) White Fibrous Cartilage 73 3. Bone 74 III. Muscle 80 1. Smooth Muscle 81 2. Heart Muscle 83 3. Voluntary Striated Muscle (Skeletal) , . . 88 IV. Nervous Tissue 97 A. Nerve Cells 98 B. Nerve Fibres 104 V. Blood and Lymph 112 1. Blood 112 2. Lymph 120 v i CONTENTS. PAET II. MICKOSCOPIC ANATOMY OF THE ORGANS. PAGE I. CIRCULATORY SYSTEM 121 1. Blood Vascular System I 22 (a) Capillaries I 22 (6) Arteries I 23 (c) Veins 127 (d) Heart 129 2. Lymphatic System I 32 (a) Lymph-vessels 132 (6) Lymph Glands 133 (c) Peripheral Lymph Nodules 138 3. Spleen I 38 4. Thymus I 43 5. Thyroid Gland 144 6. Adrenal (Suprarenal Gland) • • ■ 147 7. Pituitary Body (Hypophysis Cerebri) 152 8. Carotid Gland (Glomus Caroticum) 153 9. Coccygeal Gland (Glomus Coccygeum) 154 II. DIGESTIVE SYSTEM (ALIMENTARY TRACT) 154 A. Mouth Cavity 155 1. Mucous Membrane of the Mouth 155 2. The Teeth 156 Development of Teeth 161 3. The Tongue 164 4. The Tonsils . . , 167 Development of Tonsils 169 5. Glands of the Mouth Cavity 170 B. Pharynx 177 C. (Esophagus 178 D. Stomach 180 E. Intestine 185 Blood-vessels, Lymph-vessels, and Nerves of Stomach and Intestine . 190 F. Pancreas 192 C. Liver 193 Gall Bladder 203 H. Peritoneum 205 III. RESPIRATORY SYSTEM 206 A. Larynx and Trachea 206 B. Bronchi and Lungs 206 IV. URINARY SYSTEM 212 A. Kidneys 212 Blood-vessels of the Kidney 217 B. Urinary Passages 221 (a) Kidney Calyces and Pelvis ; Ureter, and Urinary Bladder ... 221 (6) Urethra 223 (1) Male 223 (2) Female 223 CONTENTS. vii PAGE V. GENERATIVE (REPRODUCTIVE) SYSTEM 224 1. Male Sexual Organs 224 A. Testis 224 B. Spermatic Ducts 231 C. Accessory Glands of Male Sexual Organs 234 1. Prostate 234 2. Cowpei^s Glands 235 D. Penis 235 2. Female Sexual Organs 237 A. Ovary 237 B. Fallopian Tube 250 C. Uterus 251 Placenta 257 D. Vagina and External Female Genitals 262 VI. LOCOMOTOR SYSTEM 264 1. The Skeletal System 264 A. Bones 264 (a) Bone-marrow 265 (b) Joining together of Bones 267 (c) Development of Bones , 268 (1) Development of Bone from Cartilage 268 (2) Development of Bone from Connective Tissue . . . 273 B. Cartilages 273 2. Muscular System 274 Development of Muscles 276 VII. NERVOUS SYSTEM 278 1. Central Nervous System 278 A. Spinal Cord 278 B. Medulla, Pons, and Midbrain 287 C. Cerebral Cortex 291 D. Cerebellum 293 E. Meninges 296 F. Blood-vessels of Central Nervous System 298 2. Peripheral Nervous System . 299 A. Nerves 299 B. Ganglia 301 C. Nerve-endings 304 (1) Intra-epithelial Nerve-endings 305 (2) Nerve-endings in Connective Tissue 307 (3) Nerve-endings in Muscle 312 (a) Motor Nerve-endings 3J.2 (6) Sensory Nerve-endings 314 (4) Nerve-endings in Nervous Tissue 316 VIII. SENSE ORGANS 318 \ 1. The Skin— The Tactile Organ 318 (a) Outer Skin 318 (6) Hairs 322 (c) Nails 329 v Jii CONTENTS. PAGE (d) Glands of the Skin 331 Sebaceous Glands 331 Sweat Glands 333 (c) Vessels and Nerves of the Skin 335 (/) Mammary Gland 337 2. Visual Organ 340 (a) Eyeball 340 (1) Tunica Externa 340 (2) Tunica Media 343 (3) Tunica Interna 346 (4) Optic Nerve 354 (5) Lens 355 (6) Vitreous Body and Zonula Ciliaris 356 (7) Blood-vessels of the Eyeball 357- (8) Lymph Paths of the Eyeball 359 (9) Nerves of the Eyeball 360 (b) Protecting Organs of the Eye 361 (1) Eyelids and the Conjunctiva 361 (2) Lachrymal Apparatus 364 3. Auditory Organ 364 (a) Inner Ear 364 (1) Sacculus, Utriculus, and Semicircular Canals 365 (2) Cochlea 367 (3) Blood-vessels of the Membranous Labyrinth 373 (4) Lymph Paths in Labyrinth 375 (b) Middle Ear 375 (c) Outer Ear 376 4. Olfactory Organ 377 5. Organ op Taste 381 GENERAL MICROSCOPIC TECHNIQUE 383 1. The Microscope 333 2. The Preparation op Specimens for Microscopic Study 385 (a) Isolation and Teasing of Tissues 386 (6) Sectioning of Tissues 387 (c) Fixation of Tissues 387 (d) Hardening of Tissues. 390 (e) Decalcification of Bone 390 (/ ) Infiltration of Tissue with Celloidin and Paraffin 391 (g) Staining 395 {h) Injecting 399 SPECIAL MICROSCOPIC TECHNIQUE 401 1. The Cell 401 2. Epithelial Tissue 402 3. Connective Tissue, Cartilage, and Bone 403 4. Muscle 405 5. Nervous Tissue 406 6. Blood 407 CONTENTS. ix PAGE 7. Circulatory System 408 8. Digestive System 409 9. Organs of Respiration 410 10. Urinary and Reproductive Organs 410 11. Skeletal System 411 12. Nervous System 412 13. Skin 414 14. Eye 414 15. Ear and Nose 415 HISTOLOGY MICROSCOPICAL ANATOMY OF THE HUMAN BODY PART I. HISTOLOGY. MICROSCOPICAL ANATOMY OF CELLS AND TISSUES. Histology is the study of tissues (6 iarog, to lanov, tissue). It must therefore primarily treat of the cell as a tissue-element; then concern itself with the description of vegetable and animal tissues; and finally discuss the relations which the tissues bear to one another in all the organs. This last part of histology is also spoken of as microscopical anatomy. Our text-book, which concerns itself only with the histology of man and the animal body, is divided into two parts: the first will treat of the animal cell and tissues ; the second will make the reader acquainted with the microscopical structure of the organs. Histology takes a prominent part among the biological sciences which have developed so greatly since the discovery of the cell in the year 1838. As early as the end of the seventeenth century there were more or less definitely ex- pressed premonitions and suspicions that cells formed the elementary constituents of plants. Only in the year 1838, however, did the opinion that plants consist of cells gain general recognition after the publication of M. Schleiden. In the •' 17 18 HISTOLOGY. next year, 1839, Schwann, encouraged by the findings of Schleiden, undertook investigations on animals, and found here also a cellular structure. These two investigators considered the cell a small vesicle containing a fluid in a definite membrane. Even at this early date they thought the cell membrane and nucleus to be very important, characteristic, and constant con- stituents of the cell. So it was discovered that both animal and vegetable organisms consist of very minute elements ; and fur- ther, that all these more or less complicated structures take their origin from a single cell, — i. e., the fertilized egg. Then it was shown that on the border land between the animal and vege- table kingdoms unicellular creatures exist which form a starting-point for both kingdoms. The original conception of the cell underwent great changes in the course of the following decade. A number of years after this, when membraneless cells had been discovered, the cell membrane came to be considered as an unessential part of the cell. In the ground substance of many animal cells movements were observed, which were already known in plant cells. These evidences of life were studied, and the ground substance in animal and vegetable cells was called protoplasm. A. THE CELL. What is to-day known as a cell (cellula) is a small mass of protoplasm containing within it a nucleus. We must consider cells as elementary units ; or, since they are the bearers of the life functions, as the units of life. In reviewing the animal series, which is made up partly of organisms consisting of only a single cell (protozoa), partly of those containing a countless number of cells (metazoa), it is to be noted that the cells of the first class subserve simul- taneously different functions, while we find in the second class much differentiated cells with very diverse functions. In the most highly developed organisms we find these differentia- tions and this division of labor so strongly marked that one kind of cell cannot take on the functions of another kind. THE CELL. 19 Here the cells are joined together only for certain functions : for example, to cover and serve as a protection, to separate, to absorb, to draw together, or to conduct impulses. In unicellular organisms, on the contrary, a cell is a complex of organs which serve different functions. The essential constituents of the cell are (a) the protoplasm and (b) the cell nucleus. The nucleus may in many cases disappear, especially if the cell begins to lose its vital activity. Protoplasm is a morphological conception, and not a body capable of sharp definition chemically. By the term " proto- plasm " is not to be understood a uniform substance with con- stant physical and chemical properties, but, on the contrary, a combination of various different chemical bodies joined with one another in a truly wonderful way ; a substance which exhibits different physical, chemical, and biological properties (O. Hertwig). Protoplasm is semifluid, elastic, almost always colorless, and insoluble in water. It is not entirely homogene- ous, but shows fine granules (microsomes) and fibrils which are contained in the homogeneous ground substance. We may often observe that the cell consists at the periphery of a non-granular protoplasm (hyaloplasm), while in the inner part there is a granular protoplasm-mass (granuloplasm). These two parts of the cell are known also as ectoplasm and endoplasm. respectively. The chemical composition of protoplasm is unknown, except that its essential and most important constituent belongs to the protein substances (albuminous bodies). Besides this, proto- plasm contains globulin and albumin in small quantities, a large proportion of water, a recognizable quantity of different salts, and constantly changing products of metabolism, such as fats, cholesterin, lecithin, glycogen, sugar, etc. Living proto- plasm always has an alkaline reaction. Concerning the finer protoplasmic structure there are four different and opposing views (Fig. 1). According to one view held at the present time by only a 20 HISTOLOGY. very few investigators, protoplasm has no definite structure — i. e., it is quite homogeneous. The second is the fibril network theory, which considers the protoplasm as made up of a thread-like network and an inter- stitial substance. With regard to these strongly refractive fibrils different views are held. According to some authors (Flemming), they do not join with one another in any way ; while according to others they combine to form a sort of net- FlG. 1. Granules Nuclear membrane -•J« : '::.'. •#.•:•.*; wwv Xacleoh, im'i/i the Nuclear fluid Interfibrillur mbstance Fibrillar substance Diagram of a cell. The lower segment illustrates the fibrillar theory, the upper the granular theory, the left the foam theory. At the right the protoplasmic threads radiate from the centrosome. The nuclear network consists of nuclein, limn, and lantanin. work, so that a sponge-like structure is formed (Heitzmann, Fromman, Leydig). The less refractive and more fluid inter- stitial substance separates the fibrils from one another. The latter form the so-called filar-mass or mitom, the former the interfilar-mass or paramitom. The fibrils occur in varying quantities in the cell, are of different lengths, and often are coiled. The interstitial substance often contains more or less numerous granules. THE CELL. 21 The third place is taken by the so-called foam theory (Biitschli) ; a protoplasmic network forms a number of spaces closed in on every side. All these spaces are filled with fluid. In the angles of the foam-work fine granules (microsomes) are contained. Finally there exists the granule theory (Altmann) according to which the cells consist of fine granules which are distributed in the jelly-like intergranular substance. These granules Altmann claims to be the final elementary part of the cell, and calls them, as the bearers of life, bioblasls. According to this hypothesis, the granules play the principal role, and the inter- granular substance only an accessory part. With regard to the meaning of these two constituents of the cell, the first three theories are quite opposed to the last. According to the former, the granules of the protoplasm play a more subordi- nate role. The intergranular substance of the granule theory is identical with the essential protoplasm of the other three theories. In the protoplasm there are various substances not belong- ing to it, which we include under the name protoplasmic or cellular inclusions. To distinguish them from protoplasm, we call them deutoplasm. Their nature is not constant. They may be fat, carbohydrates, pigment granules, etc. These protoplasmic-inclusions (deutoplasm) occur in some cases in such great quantity that the protoplasm itself becomes inconspicuous and forms only a kind of network for the reserve materials and secretion stored up there, as we may notice in many egg cells and goblet cells. Fluid protoplasmic inclusions usually are present in spaces called vacuoles. These spaces are made visible by dissolving out the contents. For example, fat droplets may be dissolved in ether and the empty spaces left are plainly to be seen. The form which a mass of protoplasm or a whole cell takes on may be various : spherical, cylindrical, flat, star-shaped, spindle-shaped, or fibrillar. Cells vary in size from 3 ^u 1 to that of a bird's egg (e. g., an ostrich egg, which is a simple cell). 1 fi = a micron = 0.001 mm. 02 HISTOLOGY. The second essential part of the cell is the nucleus. This is often invisible in the living cell when the nucleus and the protoplasm have the same refractive power. They react differ- ently, however, to certain reagents. For example, acetic acid causes protoplasm to swell up and the nucleus to shrink. The nucleus is usually spherical or oval ; sometimes horseshoe-shaped, ring-shaped, or branched. The nucleus often holds a definite relation to the size of the cell. For example, the nuclei of unripe egg cells are very large. As a rule we find one nucleus in each cell. Often, however, there are more than one, and exceptionally their number may be as great as one hundred (c. g., in the giant cells of bone- marrow). Such multinucleated cells are called syncytium. The cell nucleus is not a simple structure. We are able to recognize in it at least two and often as many as six proteids which are chemically and microscopically different, namely: 1. Nuclein — chromatin ; 2. Paranuclein — pyrenin ; 3. Linin ; 4. Lantanin ; 5. Nuclear fluid (Kernsaft) ; 6. Amphipyrenin. The first two seem to be essential elements of the nucleus. 1.' Chromatin {nuclein) is the most characteristic constituent of the nucleus. It is demonstrated by its great capacity for taking up stains, and is distinguished from the other substances by the fact that it contains phosphoric acid. Chromatin occurs in the nucleus in the form of granules, fine threads, or as a network which forms the so-called chromatin network. 2. Paranuclein (pyrenin} occurs in the form of a small highly refractive sphere which forms the true nucleolus. These nucleoli are to be distinguished from chromatic enlarge- ments formed in the angles of the nuclear network. Pyrenin is distinguished from chromatin mainly by physical properties. It does not swell in water, dilute alkaline solutions, lime-water, or salt solution. Chromatin, on the contrary, swells in such THE CELL. 23 solutions, and is dissolved in stronger solutions. The unchanged nucleolus becomes even plainer after such treat- ment. Nuclein is colored better in acid stains, while para- . nuclein takes up more readily basic stains, eosin, and fuchsin. In this way these two parts can be differentiated by the so- called double staining. 3. Linin takes part in the formation of the network or framework. It is not stained by the ordinary coloring materials, and forms the so-called achromatic constituent of the nucleus. 4. Lantanin occurs often in the linin in the form of fine granules, which may be stained by acid anilin dyes, as opposed to chromatin, which takes up only basic anilin-stains. Lan- tanin is therefore called oxychromatin, while chromatin is known as basichromatin. 5. Nuclear fluid (Kernsaft) fills out the spaces between the structures formed of nuclein, paranuclein, and linin. 6. Amphipy renin is the substance which forms the nuclear membrane separating the nuclear space from the protoplasm. In large nuclei the nuclear membrane shows a plainly double contour. In chemical properties it is most nearly related to pyrenin. The nucleus may be simple or complicated in form. The most simple structure is seen in those nuclei which consist of quite compact nuclein bodies (e. g., spermatozoa). In other cases the nucleus has a more open structure, the spaces in the nuclear network being filled with nuclear fluid. Such a nuclear net- work may in its simpler forms be made up only of chromatin ; in other cases linin and lantinin are also present (Figs. 1 and 2). The resting nucleus may in certain cases appear as a vesicle surrounded by a nuclear membrane (amphipyrenin). In this is to be found a network of nuclein (chromatic) and linin (achromatic), in which granules of lantanin are scattered. It contains also a nucleolus (paranuclein) and a nuclear fluid. The third but unessential constituent of the cell, the cell skin or cell membrane, may often be lacking in animal cells. If the superficial layer of the protoplasm is distinguished from the 24 HISTOLOGY. remainder of the protoplasm lying more centrally, and is less dense, it is called ectoplasm. Such a cell contains no cell mem- brane, and is spoken of as naked. When we find a firm outer boundary for the cell, we call it crusta if there is no definite line of demarcation between it and the contained protoplasm. If, on the contrary, it is sharply marked off on its inner border, we have to do with a true cell membrane. The cell membrane may surround the whole cell, in which case it is called pellicula; or it may cover only the free surface of the cell and is then known as cvlicula. Fio. -i. Xucle Chromatin Leucocyte from the spleen of Protens [after Siedlocki 1 . The eentrosonie appears in the form of two granules. The nuclear network is distinct. The origin and manner of formation of the cell membrane are not known with certainty; for it is doubtful whether it is a secretion of the outer layer of the cell, or a modified, hardened part of the protoplasm itself. Another cell constituent which has been the object of much attention in the last few years must not be passed over. This is the so-called centrosome. Most authors consider this structure an essential part of the cell (Figs. 1 and 2). The centrosome occurs usually as one or two granules in the protoplasm, in the neighborhood of the nucleus, and may he contained in a hollowed part of it. Around the centrosome is often to be seen in the protoplasm a radiation which we call the THE CELL. attraction sphere, protoplasmic radiation, or archoplasm. The significance and relations of the centrosome during the nuclear and cell division will be spoken of later. We have considered above all the constituent parts of the cell at rest. It is necessary now to discuss briefly the living properties of the cell in so far as they can be studied by the direct help of the microscope. The reader may extend his information on this subject in more exhaustive works in which the cell is treated also from a physiological standpoint (O. Hertwig, Verworn, Bergh). The different powers and properties of the cell we can group under : 1. Those of motion; 2. Those of irritability ; 3. Those of assimilation and excretion ; 4. Those of reproduction. 1. The first function which the cell can fulfil — i. e., motility — seems to be dependent only on the protoplasm ; for portions of this separated from the nucleus are capable of motion for some time. We may speak of various kinds of motion : Fin. 3. vmm *M:m?, Lymph corpuscle of the frog, studied on a warm stage. The outline of the cell has been made at intervals of two minutes. One vacuole is to be seen. X 1500. (a) Amoeboid movement consists in the protrusion of proc- esses (pseudopodia) by the protoplasm, which draw the rest of the cell after them. The pseudopodia may also be drawn back to the cell again. If we observe under the microscope such cells or unicellular organisms which have the property of '26 HISTOLOGY. independent movement, we notice that they constantly change their form. This is seen most easily if we make outline sketches of the cell at short intervals and compare them (Fig. 3). This motility serves not only to change the location of the cell, but also to aid in the acquiring of nourishment. The pseudopodia surround the very fine foreign bodies with which they come in contact, draw them into the cell, and if they are digestible use them for the nourishment of the organism. Upon motility of this sort, a great proportion of the unicellular animals (e. g., amoeba) and many kinds of cells of higher animals ( (\ centrosome of spermatozoon. Sp A", sperm nucleus. Sp ,SY, sperm radiation. THE TISSUES. 35 egg protoplasm. The sperm cell centrosome undergoes division, so that a central spindle is formed (Figs. 14-16). At this stage of the fertilization the process of maturation is usually completed, and the egg nucleus has become vesicular (Fig. 16). The sperm nucleus now begins to swell and become also vesicular, and approaches the egg nucleus. The sperm centrosome and central spindle at the same time become closely- related to the sperm nucleus (Figs. 15, 16). Both nuclei become larger, and as they approach one another the radiation of the egg centrosome becomes less conspicuous. The sperm radiation becomes more and more prominent, spreading over the whole cell. Finally the egg radiation vanishes, since the functions of the protoplasmic striations, as well as of the egg centrosome, are after the extrusion of the two polar bodies ended (Figs. 16, 17). The radiation arising from the spermatozoon enters into combination with the nuclear framework and the last chromo- somes of the egg nucleus. At this moment the process of fer- tilization as such is completed. Both nuclei undergo the first stages of indirect division and give rise to a mother star (Figs. 18, 19). The further process is not different from the ordinary mi- totic division. This karyokinetic figure should form nuclei, of which each contains an equal number of male and female nuclear segments. The number of chromosomes in the fer- tilized egg equals the sum of the chromosomes of the ripe egg and those of the spermatozoon — that is, the original full number of chromosomes which is characteristic of the somatic cells of the animal. B. THE TISSUES. The lowest animal organisms (protozoa) are unicellular structures. Since there is only one cell, this must carry out all the life functions. More highly developed animals are made up of many cells (metazoa), which all arise by a division of one single cell — i. e., the fertilized egg. These cells are quite similar in their early embryonic state ; and there is an almost spherical, 36 HISTOLOGY. many angled form characteristic for embryonic cells. As development goes on, the cells become constantly more unlike one another — i. e., a differentiation sets in. In such a multi- cellular organism the differentiated cells no longer subserve all the life functions, as is the case with unicellular animals. There are cells capable of performing only certain duties. We see here the principle of division of labor. These cells differen- tiated in certain directions, combined to perform certain func- tions, and arranged according to certain laws, form the tissues. By a tissue we understand a complex of cells definitely arranged, differentiated in a definite direction, and combined to carry out a definite activity. Tissues consist not only of cells, but also of cell products, which we group under the term intercellular substance. This is sometimes a secretion of the cells, and sometimes a product formed by a change in the superficial part of the cell proto- plasm. It is wanting in quite early embryonic tissues and is built up in time by the cells. The various tissues unite in manifold ways to form organs — i. e., bodies of a definite internal structure, and a constant external form, which serve a special physiological function. Only exceptionally does an organ consist exclusively of one tissue, as, for example, the lens of the eye. Usually many, often all of the tissues are used in the building up of the organ, — e. g., the intestine, tbe skin, etc. The classification of tissues is one of the most difficult problems in histology. It cannot be made on a purely mor- phological basis ; for not only the form, but also the develop- ment and chemical properties of the tissue must be considered. The separation of tissues into groups according to their devel- opment and origin is not satisfactory, since the same tissue may arise in more than one way. The most generally accepted classification of tissues is the following : 1. Epithelial (and glandular) tissue; 2. Supporting and interstitial tissue ; 3. Muscular tissue ; 4. Nerve tissue. EPITHELIUM. 37 I. EPITHELIAL TISSUE. Epithelium is made up entirely of closely approximated cells consisting of cell protoplasm and nucleus. The intercellu- lar substance is reduced to a minimum, and is seen only as a cement substance joining the cells with one another. A true cell membrane is usually wanting, only a slightly denser outer sheath being present in the protoplasm. The classification of epithelial tissue depends largely upon the function which it has to fill. It covers the outer surface of the body, and lines the body spaces. In addition to this, epithelial tissue has the power of secretion and absorption, and in such a case it is called glandular epithelium (glandular tissue). Finally it is in some instances capable of receiving certain stimuli from the outer world, and transmitting them to the nervous tissue. Such a tissue forms the so-called sensory epithelium. Fig. 20. II. &- Nucleus Cett- boundary - -Nucleus .Cell boundary Diagram of flat epithelium. I. Seen from above. II. Seen from the side after trans- verse section on the line m: (a) cell boundaries as straight lines; (5) cell boundaries as wavy lines. With regard to the form of the cells, epithelium may be flat or cylindrical. Flat epithelium consists of more or less regularly polygonal cells, whose depth is very inconsiderable in comparison with the other two dimensions. Looked at on the surface, the cell boundaries are made up of straight or zigzag lines. The spherical or oval nucleus lies usually more or less in the middle of the cell. Figure 20 shows the flat epithelium viewed from above and from the side. We notice that the cell in the neighborhood of the nucleus contains more protoplasm and is thicker at this point (Figs. 20, 21). 38 HISTOLOGY. In cylindrical epithelium, on the contrary, the height exceeds the two other dimensions of the cell. The cells of cylindrical epithelium have the form of more or less long polygonal prisms or pyramids. The nucleus may be in the Fig. 21. ;3& Flat epithelial cells isolated from the oral raucous membrane of man. "X 375. middle or at either end of the cell. The centrosome lies in the protoplasm between the nucleus and the free surface of the cell, and holds often a quite superficial position. It usually is present in the form of a single or double granule. Fig. 22. Impression made by neighboring cell Mucus •Outlet ' ' &$$." h^Cell membrane f^ftfijip' — l y rotojthiNiii Xucleits Two ciliated cells and two goblet cells isolated from the frog's oesophagus. X 520. Between the flat and the higher epithelial cells there are transition forms. When all three dimensions are equal, we call them cubical epithelial cells. Cylindrical epithelium may undergo certain modifications. EPITHELIUM. 39 Fig. 23. Cilia Cuticle Basal granules Protoplasmic fibres Nucleus ■'■ i If during life it bears on the free surface moving hairs (cilia, flagella), we call it ciliated or flagellated epithelium (Fig. 22). There is sometimes on the free surface of the cell a more or less definite refractive border showing striations at right angles to the surface. These cells are called cylindrical cells with a cuticular bor- der. Finally, if the upper part of the cell is changed into mucus, so that this region of the cell is dilated in the form of a goblet, we have to do with the so-called goblet cells (Fig. 22). In ciliated cells certain details can be made out which are not always visible and whose study is attended with great difficulties. The cilia must be recognized as hair-like processes of the cell protoplasm which possess the power of moving uniformly and in one direction. Often such cilia are seen to be made up of several parts which are singly or doubly, strongly or weakly refractive. This complicated structure can be made out in the schematic representa- tion of the ciliated cell of Anodonta shown in Fig. 23. Here the cells are covered by a cuticle. Directly under this there is a row of so-called basal granules, which, according to the latest investigations (v. Lenhossek), are to be considered as centrosomes. The cilia pass through the cuticula and form in this a series of thickenings in the form of granules (Fig. 23). In the cell itself we find often in the protoplasm a series of threads which begin at the basal granules, run toward the nucleus, and make up the fibrillar structure of the protoplasm. Diagram of cilated epithelial cells. (After Apathy). 40 - HISTOLOGY. These fibres, basal granules, and cilia are joined with one another in a continuous whole. The theories concerning the function of this fibrillar struct- ure are vague and unsatisfactory. According to one view, the nucleus controls the activity of the cilia by means of the fibres extending from it to the surface. This is not tenable, because parts of the cell containing no nucleus still retain their power of ciliary movement for a considerable time. Other authors consider these fibres to be intracellular nerve-endings. Still others ascribe to the basal granules the power of causing the ciliary movement. It is probable that, in common with the protoplasmic network of every cell, the fibrillar structure pos- sesses the power of contracting. This contraction would take place mainly in the direction of the strongest fibrils, as it does in muscular tissue. And their action upon the cilia, might be compared with the action of the muscles which move hairs in the skin. The cuticular border plainly seen in the intestinal epithelium is a product of the cells. The striation is, according to the researches of R. Heidenhain, due to the entrance of fine proc- esses of the cell body into the homogeneous cuticle, and a consequent change in the refractive index of different parts of this mass. These processes may be drawn back into the cell, and in such an instance the striation disappears (see Intestines). During activity the glandular epithelium shows on its free surface a layer of fine rods, such as is seen in the convoluted tubules of the kidney. This may occur in cylindrical as well as cubical epithelial cells. There may also be often a longitu- dinal striation at the basal end, which extends more or less into the cell body. These two kinds of differentiation will be spoken of more fully in treating of the salivary glands. According to the arrangement of the cells in epithelial tissue we have : (a) simple epithelium — i. e., consisting of only one layer; and (b) stratified epithelium, consisting of many layers. This division, together with the form of the cells, gives rise to the following classification : (a) Simple (one layer) epithelium : EPITHELIUM. 41 (a) Simple flat epithelium (epithelium of the lung alveoli, the lining of the vessels, the pleural and peritoneal cavities, the pericardium, the joint cavities, the tendon sheaths, etc.) ; (fi) Simple cubical epithelium (epithelium of the small bronchi, some parts of the kidney tubules, the thyroid gland, the ducts of many glands, etc.); ciliated cubical epithelium is found in the oviduct, uterus, and fine bronchi) ; (y) Simple cylindrical epithelium (epithelium of many large gland ducts, in the intestinal canal, etc.). (b) Stratified epithelium (Fig. 24) : (a) Stratified flat epithelium or pavement epithelium. The superficial layers consist of flat cells (e. g., epithelium of the cornea, the mouth cavity, the cesophagus, the skin, etc.) ; (/3) Stratified columnar epithelium. The most superficial layer consists of columnar cells, the deepest layer of cubical or polyhedral cells (e. g., in the ureter, the urinary bladder, etc.). This is known also as transitional epithelium. The same sort of epithelium, possessing also cilia, is found in the larynx, trachea, large bronchi, vas deferens, epididymis, etc. Diagrams of epithelium: (a) nuclei at various levels; (b) stratified pavement epithe- lium ; (c) stratified cylindrical epithelium, ciliated at the right. An epithelium may consist of elements which are not all. of the same morphological significance. One often sees simple cylindrical cells, ciliated cells, goblet cells, and cells with a striated border in close association. As a transition stage between simple and stratified epithe- lium, we have an epithelium in which the same cell reaches the outer surface and also rests on the connective tissue at the base of the epithelium (Fig. 24, a). The nuclei, which in typical 42 HISTOLOGY. simple epithelium are usually all at one level, are here placed at various depths from the surface. Such cells usually bear cilia on their free surface, as, for example, in the larynx, etc. Stratified epithelium may have cylindrical cells at the base and transition forms above this, until at the surface the cells are flat. This is known as stratified flat epithelium or pavement epithelium (Fig. 24), and is characteristic of the epidermis, the mouth cavity, the oesophagus, the vagina, etc. In the epidermis the cells of the superficial layers lose their nuclei and at the same time undergo a chemical change, the so-called cornification (see Skin). Stratified epithelium may consist also of a layer of columnar cells on the surface, with or without cilia, and below this transi- tional forms, until a row of cubical or polyhedral cells at the base is reached (Fig. 24, c). Such an epithelium may be called a stratified cylindrical epithelium. We find it in the main ducts of many glands. Epithelial cells are joined together, as we have already said, by means of a cement substance, which occurs usually only in very small quantity between the cells. It is recognized always in tissues treated by silver nitrate. If the epithelium be sub- merged in a weak (0.1-1.5 per cent.) solution, the cement sub- stance enters into some sort of a combination with the reagent, which under the action of sunlight becomes dark brown or black. The surfaces of the cells which the cement substance connects are often quite smooth, but show sometimes inequalities and dej)ressions due to the pressure exerted by the cells on one another. This is seen in the epithelium of the mouth cavity (Fig. 21) and the urinary bladder. Tn the line of the cement substance there is often seen a number of rod-shaped bodies connecting the two adjacent cells. These form the so-called intercellular bridges, and can readilv'be seen, for example, in the deeper layers of the epidermis. Where the cells are isolated, the rods stand out from the surface and give rise to the term prickle cells. The prickles or rods are essentially connecting bridges passing through the cement sub- stance from one cell to the other. They are plainly processes EPITHELIUM. 43 of the cell protoplasm, and, by special methods of staining, it is sometimes possible to follow them from one cell through another into a third (Fig. 25). Between the intercellular bridges there are spaces filled with intercellular substance. These spaces can be injected from the lymph-vessels, and are therefore supposed Fig. 25. P"? '■-. V. J . . i^r~~r- kssv'v! Q^ %yP;. ifofe y - ; -^ rr- ,'^; ' sS\ /^ . From a section through the stratified pavement epithelium of the human epidermis. X 700. Some cells of the stratum spinosum are bound together by protoplasmic bridges. to have the functions of lymph spaces. This would supply the nourishing fluids which the lack of other vessels in the epider- mis makes necessary. Epithelium possesses, as a rule, neither blood- nor lymph- vessels. Only in a few places have capillary branches in the epithelium been described definitely (auditory organ — Retzius, mucous membrane of the gums in amphibia — Maurer, etc.). Nerves, on the other hand, are abundant. The flat epithelium of blood-vessels and lymph- vessels, as well as the epithelium covering the serous membranes, shows in certain places holes, the so-called stomata or stigmata. The*e ■ are fine openings in the cement substance sufficiently large to admit white blood-corpuscles. According to some authors (Arnold) these structures are not preformed, but are the result of stretching. Changes may take place in the protoplasm of the cell due to pathological processes, and give rise to appearances not at all characteristic of the normal cell. The more common of these are, the formation of vacuoles, the fatty degeneration in which 44 HISTOLOGY. small fat globules are present throughout the cell, and the so-called cloudy swelling in which the protoplasm loses its translucency and becomes filled with small granules. Cells also may become swollen, so that they lose entirely their charac- teristic appearance ; or, on the other hand, especially in har- dened specimens, cells may be much shrunken. Certain special degenerations in blood cells will be spoken of in discussing blood. Other special changes in the cell may be mentioned, such as cornification (skin, hair, nail), calcification (enamel), mucoid change (mucous glands), and fatty change (sebaceous glands, milk glands). The changes undergone by the respira- tory epithelium of the lungs and the epithelium forming the lens of the eye will be discussed later. Finally, we must not overlook the fact that epithelial cells may contain granules of pigment, as, for example, the pigment epithelium of the retina, the hairs, and the lower cells of the epidermis in darkly colored races. Between the cells of stratified epithelium we meet with nerve-endings in the form of freely terminating axis-cylinders. More will be said of this subject later. There occur also cells of a connective-tissue nature which have wandered up from lower levels. These may or may not contain pigment granules, and appear usually as stellate, much-branched structures. Finally, we find also white blood-corpuscles which have wandered in between the epithelial cells. Histogenesis of Epithelium. In the beginning, epithelial tissue has the form of a mem- brane which consists of only a single layer of cells. This may remain as it is or become thickened by an increase of its ele- ments. In the latter case, by the numerical increase of cells, the new elements either are pushed in between the old ones, all the cells lying on the connective-tissue sheath ; or the new cells form many layers, cutting off the old cells from their connection with the connective tissue. In the first case we have epithelium in which the nuclei are at different levels ; in the second, the many-layered or stratified epithelium. EPITHELIUM. 45 With further development the epithelial tissue may change superficially, giving rise to such epidermal structures as hairs, nails, claws, talons, the papillae filiformes of the tongue, etc. ; or it may be modified and grow in the deeper layers and give rise to glands. The superficial layers of a stratified epithelium which are worn away by use are replaced by cells from the deeper layers produced by mitotic division. At the place where the epithelium comes in contact with the connective tissue, there is usually to be seen a bright refractive line, which forms a boundary between the tissues. This fine structureless membrane is called the basal membrane. It cannot be said with certainty whether it is a product of the epithelial cells or of the connective tissue. In certain cases when two epithelial layers lie upon one another and are separated by a refractive boundary line, there is no doubt that this basal membrane is derived from the epithelium. The flat epithelium which arises from the middle germinal layer and clothes the joint spaces, the serous surfaces of the pleural and peritoneal cavities, the tendon sheaths, and the blood- and lymph- vessels, was for a long time considered as belonging to a separate group of cells known as endothelium. These cells were classed with connective tissue, because they have a certain similarity to the flat cells which line small spaces and lacunae in connective tissue ; and also because connective tissue is derived likewise from the middle germinal layer. In order to make the classification definite, it is best to regard these cells as epithelial cells of mesoblastic origin, so that there will be no middle group formed between epithelium and connective tissue. The main reasons for classifying these cells with epithelial tissues are the characteristic arrangement of the cells to form membranes, the small quantity of intercellular substance, and the absence of any properties which would stand in the way of their being so grouped. At the same time it must be noticed that often no sharp line can be drawn between connec- tive-tissue cells arranged like epithelium and the simple flat epithelium itself. 46 HISTOLOGY. Glandular Epithelium and Glands. Glands consist almost exclusively of epithelial tissue. In every case the most important — L e., the secreting — elements are epithelial cells. We must therefore speak here in connec- tion with epithelial tissues of the structure and classification of glands. Glandular epithelium is one possessing a secretory function. By secretion we mean the production and elimination of mate- rials which are not to be used directly in the building up of the body. These products may be made use of by the organism, in which case the process is called secretion. If, however, the materials eliminated are waste products, and of no value to the body, the process is one of excretion. If the latter are retained by the organism, they may be a menace to its welfare. These glandular functions may be carried out by a single cell, in which instance we have a unicellular gland; or there may be many cells combined to form what is known as a multicellular or true gland. As an example of unicellular glands, we have the so- called goblet cells, which were described especially as a modi- fication of the cylindrical epithelial cell. They produce mucus from their protoplasm (Fig. 22), and consist of two parts : a lower plasmatic portion, containing the nucleus ; and the upper part near the surface of the epithelium, consist- ing of mucus. If this is present in large quantities, the upper part of the cell becomes dilated or swollen, so that the whole may with some truth be compared with a goblet. The basal part of the cell usually remains thin, and often is drawn to a point. Even ciliated epithelium, or that with a striated border,' is capable of producing mucus and giving rise to goblet cells. The change always begins in the free end of the cell by the pro- duction of small bright globules, which increase in size, flow to- gether, and finally leave only a small quantity of unchanged pro- toplasm as a sort of framework to hold the mucus. At the same time there is formed on the surface a cell membrane which pre- vents the escape of the secretion. The nucleus finally is crowded EPITHELIUM. 47 into the basal end of the cell, together with a small mass of protoplasm surrounding it. When the cell is filled to the utmost with mucus the outer cell membrane breaks, and through the opening the secretion escapes to the outside, while the cell suffers a considerable reduc- tion in size, and, as it were, collapses. Usually we find goblet cells scattered here and there singly between other cylindrical epithelial cells. They are capable of undergoing many times the changes described, always reassuming their original cylin- drical form, until they finally die or degenerate. Goblet cells are distributed widely in the animal organism. Especially do we find them in the epithelium of the respiratory tract (trachea, bronchi) and in the intestinal tract (stomach, small and large intestines). In the mucous glands we find cells containing large quantities of mucus, and representing specific gland cells. We now pass on to the true glands, which consist of a few •or innumerable gland cells. They form a definite whole which is bound together by connective tissue. The gland cells are arranged beside one another to form a glandular surface, from which the secretion is poured into the gland lumen bounded by such surfaces. The lumen usually is surrounded by many •cells ; only exceptionally (liver) is it formed by two cells. Often only the deeper lying part of the gland secretes, and is called the gland body, while the parts lying near the outer surface have nothing to do with this activity or play only the subordinate role of conveying the products to the out- side — i. e., they form the ducts of the gland. Rarely the duct is absent, and then the gland secretes throughout its whole extent. The arrangement of the glandular elements gives to the gland a definite form ; and according to this and to. the shape of the lumen we make a morphological classification of glands. They may be in the form of simple cylindrical tubes (tubuU), or in that of spherical or oval sacs {alveoli). These form the tubu- lar and alveolar glands, respectively. We further divide these two groups according as they consist of one or many tubuli or 48 HISTOLOGY. alveoli, into simple and compound tubular or alveolar glands (Fig. 26). In tubular glands the simple tubule always ends blindly, and may be coiled and form a coil gland ; or it divides dichotomously and forms a simple branched tubular gland. A compound tubular gland consists of several tubules which divide and may Fig. 26. Tubular glands. Alvenlar glands. Diagram of various forms of glands: o, duct; x, simple tubule; xx, simple alveolus. become convoluted. Each of these possesses a duct which opens into the main duct of the gland. In compound glands the duct divides, while in the simple glands this is not" the case. In simple glands there may be a division of the secreting gland body, giving rise to a simple branched gland. The branches of tubular glands may anastomose with one another (e.g., in the kidney). Indeed, the anastomosis may be EPITHELIUM. 49 so great that a net-like structure results. This is known as a net-like or reticular tubular gland (liver). Most of the glands of the body are tubular. We distinguish the following : (a) Simple unbranched tubular glands : fundus glands, glands of Lieberkiihn, and the coil glands. (b) Simple branched tubular glands : pyloric glands, Brun- ner's glands, small serous and mucous glands of the oral cavity, uterine glands. (c) Compound tubular glands: salivary glands, lachrymal glands, kidneys, testes, liver, Cowper's and Bartholini's glands, and the prostate body. Similarly we distinguish between simple and compound alveolar glands. The simple ones may be branched or unbranched. Branched glands consist of many alveoli, com- bined to form an alveolar system, and opening into a duct. If many of such systems join to form a gland, we have to do with a compound alveolar gland. Here, as in compound tubular glands, many ducts open into a main duct. These may be put down as follows : (a) Unbranched simple alveolar glands : small sebaceous glands. (b) Branched alveolar glands : large sebaceous glands, and the Meibomian glands. (c) Compound alveolar glands : lungs, and mammary glands. Some authors speak of a transition form, the so-called tubulo-alveolar glands. They claim that such glands as the salivary glands have alveolar dilatations at the end of the tubuli. Some glands possess no duct, as this has in the course of development been closed. Such glands get rid of their secre- tion in two ways. In the ovary, for example, the egg cell bursts out from the Graafian follicle and comes to the outside world. This is a so-called dehiscent gland. Other glands without a duct, such as the thyroid, adrenal, hypophysis, pass their secretion into the blood which flows through them. This is what is known as internal secretion. Certain glands have both an external and an internal secretion, the functions of the 50 HISTOLOGY. two products being entirely different (e. g., the liver, the pancreas, and the testes). For a fuller discussion of internal secretion and its great influence in the general economy, the reader is referred to works on physiology, to which this subject truly belongs. Glands may also be classified according to their products into those secreting cells (ovary, and sebaceous glands), and those secreting fluids. The glands of the first class either cast out whole cells, or the cells break and their contents are secreted, the cell going to pieces and forming a part of the secretion. To this class belong the sebaceous glands, mam- mary glands, testes, ovaries, and large sweat glands. Those of the second class secrete a material from cells which do not disintegrate, but retain the power of producing this secretion many times. A sharp line of distinction cannot be drawn between these two classes, for cells secreting fluids may also under other circumstances be wholly or partially cast off them- selves. We shall now consider certain elements which go to make up glands in general. At the outer side the cells of the glandular epithelium usually are bounded by a fine membrane (membrana propria or m. basilaris). This usually shows no details of structure, and it is doubtful whether it is a product of the cells or whether it is of connective-tissue origin. In some cases it contains flat stellate cells which surround the gland body like a basket, and join together by processes. These are called basket cells. Many authors consider the membrana propria to be made up of connective-tissue elements ; others have found in it con- tractile muscle elements which have the power of drawing together and pressing the secretion out of the gland. Compound glands are divided usually by means of strands of connective tissue into lobules ; from each of which a duct emerges to pass into the main duct. Outside the membrana propria blood- and lymph-vessels and nerves are present in the connective tissue. Also we find in some glands typical smooth muscle fibres under the membrana propria. Often around the EPITHELIUM, 51 larger ducts there is a quite strongly developed layer of smooth muscle. Glands are among the most richly vascular tissues. The blood-vessels divide into fine capillaries, which surround the tubuli or alveoli, and run along the basal ends of the gland cells. The blood flowing to the gland carries with it materials used in the formation of the secretion, the gland cells being between the blood-vessels and the lumen. The constituents of the secretion may be directly taken up from the blood ; but usually they are the result of specific metabolic changes in the gland cells, some materials being, however, supplied by the blood. Also the secretion may have partly one and partly the other origin. In some glands the secretion proceeds to the lumen not only from the surface of the cells, but also through fine canals, the so-called secretory capillaries, it is carried in all directions (see Salivary Glands and Stomach). These secretory capillaries, which anastomose freely with one another to form a sort of network, open finally into the gland lumen. The materials which are secreted internally are taken up by the blood and carried to the parts of the body in which they are used. The varied appearances met with in gland cells (granular, vacuolated, striated, etc.) are due in large part to the kind of secretion present. This may be equally varied, such as mucus, bile, urine, gastric juice, ferments, sugar, etc. Likewise the appearance of the secretory cell changes according to its degree of activity. There may be various stages of functional activity shown at the same time in the cells of a tubule or alveolus. Some are filled with materials which they are about to secrete, while others are shrunken and empty on account of having discharged their products. Chorda Dorsalis. The tissue of the chorda dorsalis occupies an uncertain position in the classification. This structure is present only in the embryonic life of higher vertebrates, and is made up of 52 HISTOLOGY. tissue which, judging from its origin and chemical properties, is related to epithelium. On the other hand, the fact that it may be transformed into cartilage would seem to bring it nearer to the connective tissues. II. SUPPORTING, CONNECTING, AND INTERSTITIAL TISSUE. This group is made up of tissues whose function it in to form the supporting framework for the organs and for the body ; to join together the units which make up the organs ; and to fill up the spaces between such units and organs. A general characteristic of these tissues is the presence of a large quantity of intercellular or ground substance, so that the cellular elements are often inconspicuous. The connecting sub- stances are distributed throughout the whole body, and are classified mainly with regard to physical and chemical differ- ences in the intercellular substances. We distinguish : 1, con- nective tissue ; 2, cartilage ; and 3, bone. Usually these tissues can plainly be distinguished from one another. They are grouped together : because they are closely related both ontogenetically and phylogenetically ; because when they are near one another there is often no sharp line to be drawn between them ; and because they are capable of replacing one another. So we see, for example, that the skeleton in the different classes of animals may consist of soft connective tissue, of cartilage, or of bone. Similarly the sclera in higher animals is a connective-tissue structure, while in some fishes it is bony or cartilaginous. Also it is well known that bone may develop from cartilage, and that cartilage may develop connective-tissue fibres in its substance. All of these tissues are of mesodermal origin — i. e., they arise from the middle germinal layer (meso- derm). These tissues begin to develop from the so-called embryonic cellular tissue. This consists of round or polygonal cells with, in the beginning, no ground substance. Later the cells change their form and become spindle-shaped, or, by the formation of anastomosing processes, stellate. At this time the cells lie in a semifluid intercellular substance, which is certainly a product CONNECTIVE TISSUE. 53 of the cells themselves. At first this is homogeneous, but in further development formed elements appear in the form of fibres. After certain changes in the cellular elements and the ground substance a form is reached which belongs to one of the three main groups of connective tissues described above. In the spaces of the intercellular substance there lie various kinds of cells, whose function it is to nourish the intercellular substance. The nutritive fluids pass through the ground sub- stance from one cell to another ; and when the ground substance is of firm consistency there are special paths or canals formed. 1. Connective Tissue. To this group belong those tissues whose intercellular sub- stance (also called ground substance) is not especially firm, and contains mucin, collagen, or elastin. We may distinguish sev- eral kinds — (a) Embryonic connective tissue. (b) Areolar or fibrillar tissue : (1) Intercellular substance : (a) White connective-tissue fibrils ; ((3) Elastic fibrils ; (y) Ground substance. (2) Cells: (a) Fixed cells ; ((3) Granular cells ; (y) Wandering cells. (c) White fibrous tissue. (d) Yellow elastic tissue. (e) Reticulum. (/) Fat tissue. (a) Embryonic connective tissue (gelatinous tissue, mucoid tissue) consists of round or stellate cells which are joined by processes, between which there is a large quantity of mucus- (mucin-) holding interstitial substance (Fig. 27). Mucin may be recognized by treatment with acetic acid, with which it forms a granular precipitate. In young embryos the intercellular substance is homogeneous, while in older embryos connective- 54 HISTOLOGY. tissue fibrils begin to be formed. This gelatinous tissue is found in the umbilical cord, and also in the embryonic cutis. It is not to be considered as a separate kind of tissue, but only as an early stage in the development of the true fibrillar connec- tive tissue. A similar tissue is present in the vitreous humor of Fig. 27. Embryonic connective tissue from the subcutaneous layer of the skin of a three and a half day old chick, x 640. Two karyokinelic figures are seen. the eye, where, however, the homogeneous semifluid ground substance is very abundant and the cells have in large part disappeared. (b) Areolar or Fibrillar Connective Tissue. — The intercellular substance contains formed elements of two different kinds : the white connective-tissue fibrils, and the elastic fibres. There are also cells of various kinds present (Fig. 29). (1) Intercellular Substance. — (a) The white connective-tissue fibrils consist of collagen — i. e., when boiled they yield gelatin (glutin). These fibres run always in bundles (Fig. 30), and when they are present in large quantities are known as white fibrous CONNECTIVE TISSUE. 55 tissue (see below). These bundles are joined together by a cement substance, which is soluble in lime-water, baryta-water, or in a saturated aqueous solution of picric acid. The fibrils themselves never divide but the bundles may branch dichoto- Connective /jy/ tissue cells ~~ Reticulum*!'^' , I LeucocyteST*'';, Reticulum of cat's lymph glaud, showing leucocytes and connective-tissue cells in its meshes, x 430. mously. The fibrils swell in acetic acid, and in solutions of sodium and potassium hydroxide, and are dissolved by boiling in dilute acids or in dilute potash. In pepsin they are digested easily, in pancreatin not. Fig. 29. Connective tissue cell Wandering cell Elastic fibre- Fibrils of white/ I fibrous connec- f . the tissue\^, Areolar connective tissue from the subeutis of a rat. X 300. (/?) Elastic fibres are found in areolar tissue in smaller quantities than the fibrils of the white fibrous tissue. They may be of different thicknesses, but always run singly without forming bundles. They often divide dichotomously (Fig. 29) 56 HISTOLOGY. and anastomose with one another to form a network. They are characterized by being highly refractive and elastic. ^ If we act upon white fibrous tissue with acetic acid or alkalies, the fibrils swell up, and on this uniform background the twisted or spiral course of the elastic fibres is often brought out with great distinctness, for the latter are not affected by these reagents. The elastin of which the elastic fibres consist is characterized in general by a resistance to ordinary reagents. Acids and alkalies do not affect it. Digestion in pepsin and boiling in water and dilute acids or alkalies are all resisted. It Fig. 30. White connective-tissue fibrils from a tendon of the mouse, treated with picric acid and teased out with needles. ■' HO0. digests, however, in pancreatin. When a great many of these elastic fibres occur together, we speak of them as elastic tissue (see below). (y) The ground substance in which these fibres are laid down is quite homogeneous, and in definitely developed connec- tive tissue is present in very small quantities. (2) Cells. — In the ground substance between the fibres we find a considerable number of cells. Two main sorts can be distin- guished, namely, the fixed connective-tissue cells, which have no power of motion, and the wandering cells, which can move from one place to another. This division is not definite, because CONNECTIVE TISSUE. 57 fixed cells sometimes become motile, and wandering cells fixed. They may therefore be classified in three groups on a morpho- logical basis. There are (a) fixed or true connective-tissue cells, ((3) granular cells, (y) wandering cells. (a) Fixed or true connective-tissue cells are always flat, usually polygonal cells, which may possess processes and have the appearance of stellate or spindle-like cells (Fig. 29). This last form is found usually in young connective tissue. Looked at from the side, they are like long, thin spindles. The border of the cell is often very thin. In the neighborhood of the nucleus Fig. 31. Tendon fibre-" — Nucleus of a tendon cell,-- surface view Ridges on cell due to pressure' Tendon cells seen „. from edge Piece of tendon from tail of white mouse. Between the bundles of connective- tissue fibrils are cells arranged in rows. Some are seen in surface view, and others in optical section, x 400. is an accumulation of finely granular protoplasm, which makes the cell thicker at that place. Where they are pressed upon by the fibres of the intercellular substance the cells sometimes show ridges and markings. Often the cells lie in rows on the bundles of fibres (e. g., in tendon) (Fig. 31), where they are disposed longitudinally. The cells may surround the bundles and form more or less complete sheaths for them. By the separation of these cells the isolation of connective-tissue bundles by the action of acetic acid can be explained. On the swelling up of connective-tissue fibres the sheaths formed of connective-tissue 58 HISTOLOGY. cells become broken. In certain places there are cells which surround the bundles and offer a great resistance to the pressure. The bundles here swell up between bands of cells and leave constrictions where the cells remain intact. In some pigmented parts of the body (skin, eye) the pro- toplasm of the fixed connective-tissue cells contains brown, black (melanin), or other colored granules. These are the so- called pigment cells (Fig. 32). Pigment granules are insoluble Fig. 32. Pigment cell from the skin of a young salamander, x 200. in water, alcohol, ether, and dilute acids. They dissolve in alkalies and lose their color in chlorine-water. They are a product of the protoplasm formed from materials taken up from the blood. Pigment cells often are found abundantly in the skin of lower animals, where they are very large and stellate, and have the power of moving themselves by means of processes. These movements are supposed by some to be under the influ- ence of the nervous system, and nerve-endings have been recog- nized in the cells (Leydig, Ballowitz, Eberth, and Gunge). Fixed connective-tissue cells may also develop within their protoplasm fine fat globules, which flow together to a large droplet and give rise to the so-called fat cells or signet-ring cells (Fig. 33). When a great many of these cells gather together, they are spoken of as fatty tissue or fat (see below). (/3) Granular Cells : 1. Plasma cells ; 2. Mast cells of Ehrlich ; 3. Clasmatocytes of Ranvier. CONNECTIVE TISSUE. 59 1. Plasma Cells (Unna).— These are cells of variable form, whose protoplasm stains characteristically in polychrome methy- lene-blue. They are found especially in the neighborhood of small blood-vessels. Two varieties usually are recognized: small plasma cells, which are similar in many ways to the ordinary lymphocytes, and large plasma cells. According to most authors, plasma cells arise from lymphocytes and later on become fixed connective-tissue cells. Fig. 33. Blood-vessel Mast cell Nucleus of "*- endothelium From the subcutaneous connective tissue of the rat. Along the vessel are found mast cells and two fat cells. X 540. 2. Mast cells may assume all the forms of plasma cells. The protoplasm is filled with round refractive granules which have a special affinity for basic aniline dyes. The granules take a deeper color than the rest of the tissue, and often assume quite a differ- ent color (metachromatic staining). Dahlia-violet stains the mast cells a characteristic reddish tint, while the other parts of the tissue are colored only faintly. The nuclei, on the other hand, take up stains only slightly, so that the nucleus-holding portion ,of the cell appears pale (Fig. 33). The nucleus may often be invisible if the darkly stained granules form a layer covering it. The term " Mastzellen," which was proposed by Ehrlich because, according to his idea, these cells appeared under the influence of better nourishment, is somewhat inappro- priate, for they are found often in senile and atrophic tissues. (50 HISTOLOGY. They seem to be in no way connected with the general nutritive condition of the animal. It is interesting to note that they have been found in equal abundance in bats before and after the winter sleep (Ballowitz). Like plasma cells, the mast cells are found usually in the neighborhood of blood-vessels. They are found also under epithelial surfaces, in the smooth muscles, mam- mary gland, and testicle. Many authors claim that the two are identical, and the differences in staining reaction they consider to be dependent on a chemical or functional condition. Some trace their origin from leucocytes ; others assert that they are true elements and essential constituents of connective tissue ; still others regard them as products of pathological change. 3. Clasmatocytes are large spindle-shaped or stellate cells, with long, irregular processes, which may be torn or cast off and be found as separate masses near the cell. Ranvier claims that they arise from leucocytes, and that in inflammation of a tissue, for example, they may again become leucocytes and form pus. They stain well with methyl-violet 5 B. Truly there is little known, and nothing with certainty, con- cerning this whole group of granular cells. Up to the present time their origin and their function are by no means clearly understood, nor, indeed, do we know in what relation the three kinds of cells stand to one another. (c) Wandering cells (Fig. 29) are really not connective- tissue cells, but leucocytes which by " diapedesis " have wan- dered through the walls of the smaller blood-vessels into the surrounding connective tissue. They are not characteristic for connective tissue, since they are found also (e. g.) in epithelium; but they occur in greater quantities in the former than in any other tissue. They possess the power of amoeboid movement, and wander freely between the constituents of other tissues. Wandering cells may undergo division in the connective tissue and increase there. They have, in common with leu- cocytes, the power of taking up certain materials (? . \%„ i»v ■„■ '■'■Ju^ '■"' -..■■::>»,'■' "^i" cartilage cell Hyaline cartilage. From a section through the thyroid cartilage of the cat. X 190. in the lower animals (cephalopods, selachians), and only in a few places in some mammals. It is noticed also in pathological new formations (endochondromata). Cartilage cells vary from 3 to 30 u in diameter. They increase usually by indirect divi- sion, but direct division has also been observed. The ground substance in the cartilage of higher animals is very abundant. If we examine a thin section of fresh hyaline cartilage, we notice that the ground substance appears quite homogeneous and structureless, and contains the so-called carti- lage spaces. Some time after death, however, and in cartilage treated with reagents (e. g., water), the cells shrink and between thern and the boundaries of the spaces there appears an empty area which allows the outlines of the cells to be plainly made 70 HISTOLOGY. out. The form of the cells corresponds accurately with that of the spaces. In the preparation of the section the cells often Ml out, leaving the spaces empty. The part of the ground substance immediately around the cell is highly refractive and has a special affinity for certain stains. This is the so-called cartilage capsule, and forms a boundary for the cartilage spaces, containing the cells. The cells have in the beginning definite cell membranes, which become thicker and firmer, and give rise to the intercellular substance. The ground substance, then, is a differentiated product of the cell protoplasm, and the most lately formed ground substance is nearest the cells. The cap- sule shows often a concentric marking. Inside the capsule there are often seen two cells, the result of a division. Each of these cells forms a new capsule around itself, which fuses with the capsule of the mother cell. As many as four or eight cells may be seen in one capsule, forming a cell group or family. These are separated only by a homogeneous thin wall. There is in the formation of so large a group an absorption of the inner layers of the capsule, in order to make room for the cells. Such cell division inside a firm capsule we call endogenous cell formation. The growth of cartilage takes place by an increase in the number of cells and a further differentiation of new ground sub- stance. These two processes we call interstitial growth. On the surface the increase of cartilage takes place by the so-called oppositional growth, by which new layers of cartilage are formed from the perichondrium. The interstitial growth takes place mainly in young cartilage. The capsules are stained deeply, as above mentioned, by such dyes as color mucin, while the rest of the ground substance remains unstained. The capsule possesses also a great resistance to the action of chromic acid and hydrochloric acid. By macera- tion in these fluids the ground substance is dissolved and the cartilage capsules remain for a time unchanged. That the ground substance is only apparently structureless can be shown by the action of certain reagents (e. g., potassium permanganate, 10 per cent, salt solution, trypsin, baryta» CARTILAGE. 71 an lime-water). In such preparations we see that it contains fibrils, running usually in parallel lines and only excep- tionally crossing one another. That they are not seen in the living tissue is due to the fact that their refractive index is nearly equal to that of the substance in which they live. The reagents cause changes which make the difference between them greater. Viewed with polarized light also, the fibrils may be demonstrated. It is to be assumed that the metabolism in cartilage is not active, because in higher animals there is only exceptionally any vascular supply to the tissue, and no visible canal system can be made out in which nourishing fluids could circulate. In lower animals, however, canals can readily be recognized with- out the use of reagents. These join the cartilage spaces with one another, and form a system by which nourishing materials may pass from one part of the tissue to another. Certain authors (Spina, Budge, Wolters, etc.) have demonstrated canals by means of special staining methods. These methods, how- ever, involve the use of materials (e. g., water, alcohol, ether, etc.) which cause shrinkage of cartilage, and it is possible that the results obtained are artifacts. Similarly it is difficult to say what parts take up the staining materials. It is possible that the fibrils present in the ground substance act as paths for the conduction of fluids from one part to another. In spite of numerous recent investigations on the subject, the problems concerning canals in cartilage are still unsolved. Cartilage possesses usually no blood-vessels. Only rarely, and in places where active growth or ossification is going on, are they present. The connective tissue and wandering cells accompanying the vessels are known as the cartilage marrow. The perichondrium consists of white connective- tissue fibrils, with only a very few elastic fibrils. These are arranged in bundles which cross in different directions. The superficial layers of cartilage usually pass over without sharp boundaries into the perichondrium. This contains blood-vessels which, under such conditions as are mentioned above, may grow into the cartilage. During appositional growth the connective-tissue 72 HISTOLOGY. fibrils of the perichondrium change into the ground substance of the cartilage and the connective-tissue cells into cartilage cells. The ground substance undergoes, in age, senile changes, such as the so-called asbestos change, calcification, and bone formation. The first change, which may be recognized by the naked eye, produces in the cartilage areas which have somewhat the appearance of asbestos. The process usually begins in the ground substance by a production of fibres arranged in parallel lines. These have nothing to do with the essential fibrillar structure of the ground substance. They appear first at a distance from the capsules, and proceed on every side toward these, which also in time suffer change. They spread slowly over the whole cartilage, and give it a white appearance. The fibres do not swell up in acetic acid, but dissolve in dilute solu- tions of sodium hydroxide and on boiling. Calcification, on the other hand, begins with the deposition of granules of calcium carbonate in the ground substance in the neighborhood of the capsules. This spreads throughout the ground substance, and appears white in reflected light and black in transmitted light. The granules dissolve in hydro- chloric acid, giving rise to bubbles of carbon dioxide. This change takes place especially in the laryngeal, tracheal, and costal cartilages, which become in consequence opaque and hard. Ossification of cartilage takes place as age advances. Its first stage is marked by an ingrowth of blood-vessels from the perichondrium (see Bone Development). Hyaline cartilage is found temporarily in embryos in places where bone is to be formed. Permanently it occurs in the epiphyses and the joint cartilages. Also, it forms a large part of the laryngeal, tracheal, and bronchial cartilages. It is found in the nose, the ribs, and in all symphyses and synchon- droses. (b) Mastic Cartilage. — Here the ground substance contains a greater or smaller number of elastic fibres, which varv sreatly in thickness and show a marked tendency to branch and form CARTILAGE. 1?> networks (Fig. 37). By means of specific staining reactions the elastic fibres can plainly be demonstrated. They give to the fresh cartilage a less transparent appearance, and cause it to have a slightly yellow color, by which it may be distinguished by the naked eye from hyaline cartilage. The elastic fibres pass over into the perichondrium at the border of the cartilage. The development of elastic fibres in cartilage has been spoken of in describing their origin in connective tissue. Fig. 37. Cartilage cell Elastic fibres- Elastic cartilage from the human ear. X 570. Elastic cartilage is found in the outer ear, in the Eustachian tube, and the sesamoid cartilages. It is found also in portions of the laryngeal cartilages ; the epiglottis, processus vocales of the arytsenoid cartilages, the cuneiform and corniculate carti- lages. (c) White Fibrous Cartilage. — Here we find in a small quan- tity of ground substance bundles of collagen-producing fibrils, which are arranged in parallel and slightly wavy lines. The homogeneous ground substance is very small in quantity and usually reduced to only that which forms the capsules around the cells. The cells themselves are not numerous, and have ;i 74 HISTOLOGY. tendency to arrange themselves in groups (Fig. 38). This cartilage occurs in the nucleus gelatinosus of the intervertebral ligaments, in the symphysis ossium pubis, in the interarticular Pig. 38. Nucleus of cartilage cell ' . gpgnn i !■■-,' ' ';■ i • , ,A.\ '' I mm Cawmle i --.'W-Wt ■ Fibril bundles '': A'. ■'•■,■! Fibrous cartilage from the ligamentum teres femoris of a dog. X 570. cartilages, and in the place of insertion of the ligamentum teres femoris. 3. Bone. Like other supporting tissues, bone possesses a large pro- portion of intercellular substance. The mineral constituents (calcium salts), whicli are connected closely with the organic parts known as ossein, produce the characteristic hardness of bone. By so-called decalcification we are able to dissolve away all the calcium salts and leave only the organic framework which shows the structure of bone completely. On the other hand, we can, by heating the bone (calcination), destroy the organic constituents, and leave a skeleton which consists of salts and likewise presents an exact picture of the bone structure. In this way it is possible to study the finer architecture of bone equally well in decalcified or in dried specimens. We distinguish compact and spongy bone substance, the former being dense and firm, the hitter resembling the skeleton PLATE IV. Q,_. Jjara^j. Fig. 39. — From a ground longitudinal section through the diaphysis of the human ulna. All canals are filled with pigment, which is here black. Haversian canals are cut longitudinally. X 90. PLATE V. 2"3 ii ">.' >^)arac Fig 40 -From a ground cross-section of the diaphysis of the human metatarsus :(«) outer ground lamelte ; (6) inner ground lamella; (c) Haversian lamella; W) lnte ™ lamelto. All canals and bone cavities are filled with coloring-matter and appear black. X 90. BONE. 75 of a sponge. The diaphyses of long bones as well as the outer parts of small and flat bones consist of compact bone substance, while the epiphyses of long bones and the middle of short and flat bones are made up of spongy bone. If we examine a longitudinal section of a bone which has been for some time macerated, we observe with low magnifica- tion broad canals which run more or less parallel to the longi- tudinal axis (Fig. 39). These are connected by transverse canals, and form altogether a complete canal system. These so-called Haversian canals are in macerated bone empty, because the blood-vessels which they contain in life have been dissolved by the macerating fluids. Everywhere in the ground substance there are spaces, the so-called bone lacunae, in which before maceration the bone cells are contained. These are arranged in rows which are more or less parallel with the Haversian canals. On examination of a cross-section of bone (Fig. 40), we notice that the Haversian canals are round and the transverse canals are cut longitudinally. Around the Haversian canals the bone lacunae are arranged in concentric rows. With higher magnification the ground substance is seen to be made up of lamellae lying in groups at various angles to one another. In compact bone we may distinguish several kinds of lamella? : 1. Special lamellae of the Haversian systems, or Haversian lamellce, are those arranged concentrically around the Haversian canals. All those lying about one Haversian canal make up what is known as an Haversian system of lamella?. The num- ber of lamellae in a system may vary from three to twenty or more, although it is usually from eight to fifteen. 2. Interstitial or intermediary lamella} are those which fill up the spaces between adjacent Haversian systems. These are divided into real interstitial lamellae, which are formed from the periosteum and run in the same direction as the outer ground lamellae ; and false interstitial lamellae, which are merely remains of Haversian systems that have been destroyed (see Skeletal System). 76 HISTOLOGY. 3. Outer ground lamella} form the outer layers of the bone, and are situated directly under the periosteum. 4. Inner ground lamellce form the boundaries of the med- ullary cavity and are arranged concentrically around it. The outer ground lamellae are in places pierced by canals which carry blood-vessels from the periosteum. These are known as Volkmann's canals. All these systems of lamellae are joined with one another by a cement substance. If this is abundant, it forms the so- called cement lines of v. Ebner, which separate the adjacent systems (Figs. 41 and 42). The structure of the intercellular substance (ground sub- stance) is fibrillar. These fibrils, which are capable of produc- ing gelatin, are joined into bundles by means of homogeneous interfibrillar cement substance. The bundles are joined in turn by interfascicular cement substance. The fibril bundles run parallel to one another and make up the lamellae. These are often so arranged that the bundles of adjacent lamellae lie at right angles to one another. An example of this is shown in a cross-section of the compact substance of a long bone (Fig. 41). The longitudinal fibril bundles are cut transversely, while those in the adjacent lamellae running concentrically are cut longitudinally. On examination with polarized light it is found that the bundles which are cut longitudinally are doubly refractive, while those that run concentrically around the Haversian canal are singly refractive. Thus in the crossing of Nicol's prisms the former appear black and the latter light (Figs. 41 and 42). This lamellated intercellular substance is found in all adult bones. The ground substance with coarser fibres is found mainly in embryos or only in special places in adults (c. g., in the points of insertion of tendons). In the intercellular substance we find bundles of connective- tissue fibrils which are quite independent of the lamellar fibrils. They pass through the lamellae from the periosteum trans- versely or diagonally (Fig. 41). These are known as Sharpens fibres, and remain only partially or not at all calcified. They BONE. 77 are found in the outer ground lamellae and in the true inter- stitial lamellae— i. e., in all lamellae which are formed from the Fig. 41. ;;: -> i v\v •■■-■>-.' ■,^,-. ■■.-,■/,•_, / •-/■■.• ■-/■-* * :- - /■ ■'* '/-.'^SiZ'"" ^'* ,B ^^'' " J ^- j ^" v "^x.'' A •- ,-^-v Fig. 42. Da.ra.c7 Figs. 41, 42. — Ground cross-section through the diaphysis of the human ulna viewed with polarized light. X 170. The entire Haversian lamellar system, together witli the neighboring interstitial and Haversian lamellae, is shown. In the centre is the Haversian canal. Around this are lamellae which contain bone spaces. Between the adjacent systems are to be seen cement lines. The dark diagonal lines at the lower right side of Fig. 41 represent Sharpey's fibres. Fig. 41, with uncrossed ; Fig. 42, with crossed Nicol's prisms. The dark cross in Fig. 42 is an appearance caused by the polarization. periosteum. We find them also in large quantities in the lamellae containing coarse fibres, spoken of above. The uncalcified Sharpey's fibres are destroyed in macerating fluids, and also in dried specimens. From the periosteum there 78 HISTOLOGY. often run elastic fibres to the lamellated bone substance. These may combine with the Sharpey's fibres or remain independent. In the intercellular substance there are small spaces (13-31 fj. long, 6-15 ft wide, 4-9 (i deep). These bone lacuna or bone cavities (formerly incorrectly called bone corpuscles) lie, as a rule, among the longitudinally disposed lamellar fibres. Their shape is variable, and is dependent on the direction of the sec- tion studied. They possess numerous very fine processes, the Fig. 43. From a section through the bone of a roebuck. The bone cavities are seen from above, and are filled with coloring-matter. In places small dots are visible, which represent the cross-sections of bone canaliculi. x 850. so-called primitive tubules or bone canaliculi, by means of which not only adjacent, but also distant lacunae, are placed in com- munication with one another. The lacunae lying near Haver- sian canals, the medullary cavity, or the surface of the bone, send canaliculi which enter the canals or the medullary cavity, or open out under the periosteum at the surface of the bone. In this way there is an anastomosis not only between all the BONE. 79 bone lacuDEe, but also between these and all the cavities which carry nourishing material in the bone. This whole canal sys- tem, can be demonstrated by filling it with colored materials (Figs. 43 and 44). The part of the ground substance immeT diately surrounding the lacunae is more resistant toward re- agents than elsewhere. By the action of concentrated acids, a preparation showing merely the canal system can be obtained, for the whole intercellular substance, with the exception of a From a section through the bone of a roebuck. The bone cavities are seen from the side. X 850. very thin layer lining the cavities, is dissolved. If a section is cut so that the lacunas can be looked into from above, small openings can be seen which represent the mouths of the small canals or processes (Fig. 43). In these lacunae lie the bone cells. These are membraneless cells, each of which fills the whole cavity. Their form corre- sponds with the cavities in which they are situated. In prepa- 80 HISTOLOGY. rations made by treatment with strong acids, the cells are usually shrunken away from the walls of the cavities. They are stellate, sending out processes into the bone canaliculi. In the lower animals the processes of neighboring cells anastomose, and also during the development of higher vertebrates the cells join with one another in the canaliculi. It is, however, certain that in adult individuals of the higher animals and man there is no such cell combination. Spongy bone substance has a quite similar minute structure, as has been described for compact bone. Its ground substance has a fibrillar structure and contains bone lacunae. There are, however, no Haversian canals and no lamellar systems. The layer masses of this tissue show a lamellar structure, of which the lamellse lie parallel to the broad surface of the mass. Other subjects in this connection, such as the vascularization and development of bone, are treated of in the section on the Skeletal System. HI. MUSCLE. This tissue is characterized by a marked contractility of the protoplasm. The power of contracting on stimulation from without is possessed by all protoplasm to a certain degree, but in muscle the contraction takes place mainly in one axis of the cell. According as the contraction is under the control of the will or not, we distinguish voluntary and involuntary muscle tissue. This is a physiological classification, but there are structural differences which allow the same division to be used histologically. It is, however, perhaps better to divide the tissues into groups on an entirely histological basis, speaking of: 1, smooth muscle ; 2, heart muscle ; and 3, voluntary striated muscle. All muscle cells contain one or more nuclei and protoplasm with a more or less highly differentiated structure. There may or may not be a cell membrane, and the cells are united by only a small quantity of cement substance. In the protoplasm there are usually to be found fibrils which may be regarded as differentiations in one direction of the primitive network of MUSCLE. 81 the protoplasm. They are associated with the power of con- tractility possessed by the cells. 1. Smooth Muscle. This tissue consists of spindle-shaped cells, usually 50-200 // long and 4-7 fi thick (Fig. 45). In the pregnant uterus they may be as much as 500 (i in length. They . do not possess a true cell membrane. In the middle of the cell, at its thickest Fig. io. Four smooth muscle cells from the stomach of a frog, isolated in 33 per cent. KOH. In the centre of each cell lies an oval nucleus, at either end of which there is a collection of granular protoplasm, x 400. part, there is an oval rod-shaped nucleus, rounded at the ends and containing one or more nucleoli. The nucleus is surrounded at both ends by granular protoplasm. In the protoplasm there can be made out a number of fibrils run- ning longitudinally. These are seen more distinctly in the Fig. 46. Nucleus- Intercellular bridges' Longitudinal section of the muscle layer of a dog's large intestine. X 530. lower animals. The differentiated fibrils are doubly refractive, and lie in the undifferentiated sarcoplasm. The smooth muscle cells usually lie close together in groups, which may be combined to form definite layers, as is seen on the muscular coats of the intestine. In cross-section the cells appear as polygonal or round areas of unequal size, on account 82 HISTOLOGY. of the fact that the section passes through different parts of the spindle-shaped cells (Fig. 47) . The smaller areas contain no nucleus, because they are sections of the small ends of the cells. Fig. 47 j Cross-section of smooth muscle from a dog's large intestine, a, cell cut at level of nucleus ; b, cell cut near the end ; c, nucleus of connective-tissue cell, < 600. The cells are joined together by a small quantity of cement substance. Intercellular bridges are also often to be made out passing across the cement substance (Figs. 46 and 48) (Kults- Fio. 48. y&™< 4X1 f\ & Cross-section of smooth muscle of a dog's large intestine, showing intercellular bridges. a, cell cut at level of nucleus; b, cell cut near the end ; d, intercellular bridges. X 800. chitzsky, Barfurth). The presence of protoplasmic bridges in smooth muscle is doubted by other authors (Schaffer, J.). The cells may be separated by maceration in dilute solutions of potas- sium or sodium hydroxide. Between the groups of cells are found blood-vessels and nerves imbedded in connective tissue. MUSCLE. 83 This tissue is found in the walls of the alimentary tract, in the respiratory, urinary, and sexual organs, in the vessel walls, in many glands, and in the skin. Fig. 49. Two heart muscle cells from the frog, isolated in KOH. In the upper cell one nucleus is to he seen, in the lower cell two. At the ends of the nuclei the granular sarcoplasm is collected, x 700. 2. Heart Muscle. This has a place midway between smooth muscle and volun- tary striated muscle. It might be called involuntary striated muscle. In higher animals it consists of short rhomboidal or cylindrical cells joined together end to end by means of a cement substance, which may be dissolved in alkalies and nitric acid. The cells usually are branched, as shown in Fig. 50. Fig. 50. From a longitudinal section through human heart muscle. Two entire cells are to he seen. The right one branches. X 500. They possess no cell membrane. In the middle of the cell there is usually one nucleus, although there may be as many as three or four. This nucleus is oval, vesicular, and surrounded by a mass of undifferentiated protoplasm, in which there is 84 HISTOLOGY. found more or less granular brown pigment. This pigment increases with age (Fig. 51). The cell substance shows darkly stained columns, which run parallel to the long axis of the cell, and are separated by unstained substance. These columns are commonly spoken of as fibril bundles, and correspond with what v. Kolliker has called " Muskelsaulchen." The unstained substance is generally- known as sarcoplasm. Careful observation and certain special methods of staining reveal a definite relation between these two parts of the cell (J. B. MacCallum). The fibril bundles are striated like those of Fig. 51. Connective^ tissue celt Stircophtsm— \p\< If Radially a fibril bundle, " "^r .'• %^V^ >j ~^W £?»!» Fl'lilteitt qriunr ■:.— : ** — J » 1 =' ' ' J t r i''''H Blood-vessels Nucleus- From a cross-section through human heart muscle. X SOU voluntary muscle, showing alternating light and dark bands. In the centre of the light band is a narrow deeply staining striation, known as Frame's membrane (Zwischenscheibe of German writers). The broader dark band is called Britcke's line, of doubly refractive substance (Querscheibe). In thin sections, especially those stained by Kolossow's method, the Krause's membranes are seen to belong to the sarcoplasm as well as to the fibril bundles. This is shown in Fig. 52. The sarcoplasm is divided into distinct disks by membranes, which horizontally are continuous with the Krause's membranes of the fibril bundles. There may be more than one of these disks between two adjacent fibril bundles (Fig. 52, A), and at the MUSCLE. 85 centre of the cell the perinuclear sarcoplasm is made up entirely of these disks. Seen in transverse section (Figs. 51 and 53), the cell consists of darkly staining masses which are cross-sections of the fibril bundles. These are band-shaped and radially arranged at the periphery of the cell, and are smaller and columnar nearer the centre. Between these there are definite disks of unstained substance. From this it will be seen that the cell protoplasm contains a network which consists of the fibril bundles and the membranes surrounding the disks of sarcoplasm. The raem- Fio. 53. -F Longitudinal section of adult hu- man heart muscle. S, sarcoplasmic disks; F, fibril bundle; K, Krause's membrane. (MacCallum.) — .F Cross-section of adult human heart mus- cle. The section is through a part of the cell either above or below the nucleus. C, central sarcoplasm mass , 8, sarcoplasmic disk ; F, fibril bundle. (MacCallum.) branes join with the fibril bundles at the lines known as Krause's membranes, and the whole forms a continuous network. With regard to the manner in which heart muscle cells in the higher mammals are joined together, two appearances com- monly met with must be mentioned. In some cases, especially where there is a certain degree of oedema in the muscle, the fibril bundles present a row of thickenings on each side of the cement line. These form what Przewoski termed the stratum granulosum terminale (Fig. 54). From each of these thicken- ings one or more fine filaments pass across the cement line to meet those from the opposite cell. The two series of filaments meet at a delicate line in the centre of the cement line. The 86 HISTOLOGY. latter line was not observed by Przewoski. In some prepara- tions this structure cannot be made out. Instead of it, there is a cement line having the characteristic step-like course. This in osmic acid preparations shows the appearance always described as caused by protoplasmic bridges (Fig. 55). Fig. 54. Fig. 55. Longitudinal section of adult hu- man heart muscle, showing the junc- tion of two cells. (MacCallum.) Longitudinal section of heart muscle from an adult dog, showing protoplasmic bridges between two cells. (MacCallum.) In the lower vertebrates the structure of heart muscle differs in many essentials from that of man and the higher mammals. In fishes the cells are small and spindle-shaped, and possess fibril bundles only around the periphery. In amphibians and reptiles the cells are still spindle-shaped and sometimes branched, but the fibril bundles are more conspicuous (Fig. 49). In birds the heart muscle cell is large and con- tains many fibril bundles. It is differentiated very much more highly than the heart cell of the lower classes of vertebrates. It is a fact worthy of notice that the heart muscle cells of cold- blooded animals are of much more primitive type than those of warm-blooded animals. MUSCLE. 87 Histogenesis of Heart Muscle. In early embryos (e. g., pigs' embryos 10-12 mm. long) the heart muscle is made up of small spindle-shaped cells lying close together. In cross-section they are round and contain an oval nucleus. The cell protoplasm contains a more or less regular network (Fig. 56). No fibril bundles are present. Fl«. oti. Cross-section of heart muscle cells of a pig's Cross-section of heart muscle cells of a pig's embryo 12 ram. long. (MacCallum.) embryo 25 mm. long. (MacCallum.) In somewhat older embryos (25 mm.) the cells are still spin- dle-shaped. Around their periphery is seen a row of dark masses, which are the cross-sections of newly formed fibril bundles (Fig. 57). These are merely an accumulation of the substance of the primitive network to form longitudinal fibril bundles. In embryos 40 mm. long fibril bundles are present not only around the periphery, but are also scattered here and there in the more central parts of the cell. In pigs 70 mm. in length the cells are no longer spindle-shaped, and have all the characteristics of adult cells. It is apparent, then, that the continuous network spoken of in the adult fibre made up of the fibril bundles and the mem- branes bounding the disks of sarcoplasm, is developed directly by a process of differentiation from the primitive protoplasmic network of the embryonic cell. The gradual acquirement of special powers of contractility is due to the progressive devel- opment of the network of contractile substance present in the beginning. 88 HISTOLOGY. 3. Voluntary Striated Muscle (Skeletal). This tissue is made up of the most highly differentiated of all muscle cells. They are long fibres, possessing a sarco- lemma or cell membrane, many nuclei, and a protoplasm con- taining fibrils with a double striation (Fig. 58). Each cell is Fig. 58. r h ! '■ I <^> Part of voluntary muscle fibre of frog. what is known as a syncytium — ;'. e., the nucleus has divided many times without corresponding division of the cell. They may be as long as 10 cm., and in small muscles may extend their entire length. The diameter of the cells varies from 30 to 60 (U. In old animals the fibres are larger than in young. When the ending of a fibre is found, it is seen to be conical or round. Often the ends are branched or forked, as in the muscle fibres of the tongue. The cross-striation is due to striae which are present in the longitudinally disposed fibrils (Fig. 60) . These can be seen in the fresh muscle. The fibril bundles are made up of what are called primitive fibrils. These are differentiated parts of the protoplasm, and have to do with the power of contraction pos- sessed by the cell. A small part of the protoplasm remains unchanged, the so-called sarcoplasm. The arrangement of the fibrils may be variable, as seen in a cross-section. They may form polygonal bundles, which are known in cross-section as Cohnheim's fields (Fig. 59). These correspond to what v. Kolliker has termed Muskelsaulchen in heart muscle. They are separated by more or less sarcoplasm, which appears as a bright network, in whose meshes the fibril bundles or Cohn- heim's fields are situated. The distinctness of the striations MUSCLE. 89 depends somewhat on the amount of sarcoplasm present. A thin layer of sarcoplasm is usually present under the sarco- lemma. Some authors who regard this as the inner sheath of the sarcolemma call it the endolemma and the outer sheath the epilemma. In the sarcoplasm under the sarcolemma there lie oval nuclei, arranged with their long axes parallel to the long- axis of the cell. They are found sometimes in the middle of the fibre, between the primitive fibrils (e. g., in amphibia), but most often are present under the sarcolemma (Fig. 59). In some animals (e. g., the rabbit) we find muscles of two kinds: the so-called red muscle cells, which contain much sarcoplasm Fig. 59. Sarcolemma Cohnhe'tm's ~? fields cojilnsra Cross-section of voluntary striated muscle fibres of a rabbit. In A the primitive fibrils are equally distributed ; in B they are grouped into Cohnheim's fields. The fine dots are transverse sections of primitive fibrils. X 1000. and possess nuclei in the interior of the cell ; and the white muscle cells, which show a more distinct striation, less sarco- plasm, and nuclei peripherally placed. The more sarcoplasm a cell has, the more slowly will it contract and the longer it will function without tiring. In many vertebrates, and in man also, the muscles are almost exclusively of the red muscle type ; some muscles, however, the so-called mixed muscles, possess both red and white fibres. Every fully developed voluntary muscle fibre in the higher vertebrates is surrounded by a sarcolemma. This is a thin, ho- mogeneous, structureless membrane, which under normal condi- tions is so closely approximated to the contents of the cell that 90 HISTOLOGY. it cannot be made out. On treatment of the tissue with water, however, the osmosis lifts the sarcolemma up so that it can be plainly seen. Also in torn, twisted, or teased cells, where the contents of the fibre have been extruded, it can be seen. It is absent in the muscle of some lower animals. The striation of the muscle fibre is due to changes in the physical properties of parts of the fibrils, in consequence of which some portions have a different refractive index and stain differently from others. The appearance changes also on high and low focussing. What appears light in high focus becomes Fig. 60. Primitive fibril Nucleus n m / f'J III / fit pi-.it, p.. a / /( ( III V-A wfck /'"'/ tit o mum! $ piStii MIIIIl(tH!l /// IliVii'i fri'tiiiieMMi** Isotropic laye '} m'ti jifniiiiicfj /J iii»itiiiii'in»«!!| .'-' liiniiiiiiiiiiiHl ■'■ ''iiiimtiiiiiinij •'i.iluiiiiiiiFMJJ l/iiWiliiiiii!!'! .. tl Itii li f £ I ri"Sil.*!L hra " se - s membrane. (iliJiiiiiiiiSiUJ \ilihiViU'n'n}t)l ffa [{'ifimiiiUiH'". *> ^ iS^fiilfilunlJU Piece of muscle fibre of the frog, broken up into fibrils. X 650. II I 1 dark on focussing to a deeper level. The following description is made from low focussing : We notice with high magnifica- tion alternating light and dark bands on the fibrils. These are of about equal thickness. The dark bands, the so-called Brucke's lines (Q), are doubly refractive — i. e., they appear light in polarized light with crossed Nicol's prisms (aniso- tropic). The bright bands, on the contrary, are singly refrac- tive, are isotropic, and appear dark in polarized light. MUSCLE. 91 It is to be observed also that there are other lines dividing these bands, so that each is bisected. The light band (isotropic) possesses in its centre a tbin, very dark, doubly refractive line, the so-called Krause's membrane (Z), which was described first by Amici. In the dark band we find usually a light line — i. e., it refracts light less strongly that the rest of the anisotropic sub- stance. This is the so-called Hensens line (h). Fig. 61. Diagram of cross-striations of a beetle's muscle. (After Eollet.) 7. with higher; II, with lower focussing of the objective ; Q, Briicke's line ; h, Hensen's line,; /, isotropic sub- stance ; N, accessory line (Nebenscheibe) ; E, isotropic suhstance (Endscheibe) ; Z, Krause's membrane. In some arthropods we meet with a still greater differentia- tion. There is a band present dividing into two equal parts the isotropic substance between Krause's membrane and Briicke's line — i. e., between Zand Q. This is called the accessory line (Nebenscheibe) (N). This quite inconstant line usually refracts light more weakly than the Briicke's membrane, but is doubly refractive (Fig. 61). 92 HISTOLOGY. In a close examination of very thin sections of voluntary muscle, especially of the red variety, the Krause's membranes can often be seen passing from one fibril to another across the sarcoplasm. Although it is much more difficult to make out than in heart muscle, on account of the small amount of sarco- plasm, it is probable that the same relation exists between fibril bundles and sarcoplasm in the two kinds of muscle. If such is the case, the explanation of the nature of Krause's membrane is possible on the hypothesis that the contractile parts of the cell form a continuous network. Concerning the nature of the other striations nothing definite is known. During a contraction of the cell these various striations undergo certain changes. They all become shorter and broader. The isotropic substance becomes very thin, and the Neben- scheiben approach Krause's membranes so closely that new striations are formed, which are known as the contraction bands. These bands usually become isotropic, while the Briicke's lines (Q) acquire a doubly refractive index. In Briicke's lines the distinction between the Hensen's line and the rest disappears. These changes can be observed best in cells in which parts of the fibre only are in contraction (so-called contraction waves). In such a fibre all the transitions from one state to the other are to be seen. Still plainer pictures can be obtained in fibres which show the so-called lateral waves (Fig. 62). This is seen in the neighborhood of a motor end plate. It appears that the Briicke's lines are active during contrac- tion, and that the isotropic substance has only elastic prop- erties and acts passively. It is certain that the striations are not an essential thing for the cell contraction. Smooth muscle contracts with no striated fibrils, but this contraction is much slower than it is in striated muscle. Supposing that the fibrils are merely a differentiated part of the primitive protoplasmic framework, the contraction may be considered as a contraction of this framework. The further differentiation of the fibrils by the appearance of striations puts them into a physical con- dition for quicker and more perfect contractions. In the primitive amoeboid cells, where the network in the protoplasm MUSCLE. 93 is simple and uniform, the contraction takes place slowly and in no definite direction. When a similar network, however, has been differentiated into longitudinal thickenings (fibrils) and these fibrils further changed physically, the contraction is quick, and takes place in the direction of the stronger strands of protoplasm — i. e., the fibrils. By the action of weak acids, Brucke's lines swell up and Krause's membranes are unchanged, so that the fibrils have the Fig. 62. Sttrcokmma Lateral contraction wave of Cassida equextru. (After Eollet.) The formation of the contraction band is well seen, at the left, as thick black lines. form of a row of beads. With stronger acids, Brucke's lines split, and disks are formed which contain Krause's membrane in their centre. The so-called Bowman's disks are formed by the action of 93 per cent, alcohol. Here the splitting is at Krause's membrane and Brucke's lines are left intact. The Krause's membranes are the most resistant of all the lines, and seem to be closely related both to the sarcoplasm and the sarcolemma. 94 HISTOLOGY. The function of these different parts of the cell is not quite clear. The sarcoplasm dissolves in water, dilute acids, and alkalies, and allows the primitive fibrils to be separated. Sar- coplasm plays some role in the nourishment, increase, and growth of the muscle fibres. It is present in large quantities around the motor nerve-endings, and serves probably to trans- mit the nervous impulse equally through the cell. Cross-striated muscle fibres are found in all the skeletal muscles, the outer muscles of the eye, muscles of the ear, pharynx, larynx, tongue, oesophagus, and those around the anus and sexual organs. All the striated muscles in vertebrates (except heart muscle) are voluntary. There are some excep- tions, however, such as the muscles of the upper part of the oesophagus, and the cremaster externus, which are not under the control of the will. Histogenesis of Voluntary Striated Muscle. The voluntary muscle of the adult body is derived from the myotomes of the embryo. In pigs' embryos 8 mm. long the myotomes are flattened bodies composed of a dorsolateral epithelium-like layer of cells, and a median mass of spindle- Fig. 63. Transverse section through the epithelial lamella of a myotome in the leg region of an embryo pig 8 mm. long. (Bardeen.) a, dividing cells ; 6, limiting capsule of external layer ; c, inner layer ; d, middle spindle-cull layer ; e, ectoderm. shaped and round cells. These have been described in detail by Bardeen. The epithelial lamella (cutis plate) is composed of three layers of columnar cells with fine fibres proceeding MUSCLE. from the external cells. These fibres join to form a limiting capsule. The middle layer is composed of spindle-shaped cells; the main layer, of columnar cells (Fig. 63). The round cells of the median muscle lamella are known as myoblasts. From these the spindle-shaped cells are derived by an elonga- tion of the body of the cell (Fig. 64). Karyokinetic figures Fig. 64. Cells from a horizontal section of muscle lamella of a pig'sembryo 8 mm. long. (Bardeen.) a, myoblast ; b, young spindle cell ; c, d, older spindle cells. are found abundantly among these cells. In the protoplasm there is to be seen only an irregular network. No fibril bun- dles are present. The cells of the epithelial lamella become converted constantly into myoblasts. For a detailed descrip- tion of the development of the myotome and its various cell Fig. 65. Fig. 66. Cross-section of voluntary muscle from the thigh of an embryo pig 25 mm. long. A, cell showing the nucleus ; B, cell show- ing sarcoplasmic disks. (MacCallum.) Cross section of voluntary muscle from the thigh of an embryo pig 45 mm. long, showing fibril bundles at the periphery of the cells. (MacCallum.) layers, the reader is referred to Bardeen's original article. The later stages in the development of the muscle cell are as follows (J. B. MacCallum): In Fig. 65 is represented a cross-section of the muscle fibres of a pig's embryo 25 mm. 96 HISTOLOGY. Ions:. This shows the cells of the muscle mass which has developed in the leg from the myotomes for that region. In the cells there is a protoplasmic network and no fibril bundles. The nucleus is large and vesicular. At about this stage the fibril bundles begin to be formed. Bardeen has pictured muscle cells from embryos of this age with distinct fibril bundles scat- tered irregularly around the periphery. In a pig's embryo 35 mm. I0112; the fibril bundles are found in nearlv all the cells. They are small and not regularly placed. In embryos 45 mm. in length there is a regular row around the periphery of the cell. The nucleus is placed centrally, and the central proto- plasm contains a more or less regular network (Fig. 66). From this on, the fibril bundles increase in number and gradu- ally fill up the entire cell. In embryos about 75 mm. long there are, in addition to the central nucleus, several peripheral nuclei. The latter are not vesicular, like the former, but stain deeply and uniformly, like adult muscle nuclei (Fig. 67). The Fig. 67. Cross-section of voluntary muscle from the thigh of an embryo pig 75 mm. in length. A, central vesicular nucleus ; B, peripheral solid nucleus. (MacCallum.) central nucleus subsequently disappears. The sarcoplasm is more and more encroached upon by the growth of the fibril bundles, and in adult muscle it occupies a very small space. Striations are noticed in the fibril bundles at their first appear- ance, and frequently the ultimate relation between Krause's NERVOUS TISSUE. 97 membrane and the sarcoplasm, which has been spoken of above, can be observed. It seems that the same hypothesis (MacCallum) is applica- ble here as was suggested for the development of heart muscle. It simplifies the conception of striated muscle very greatly to consider the fibril bundles and the membranes bounding the compartments of sarcoplasm as derived from the primitive net- work found in the muscle cells of young embryos. The later stages in this development of heart muscle and voluntary muscle differ somewhat on account of the differences in the adult tissues. But since the beginning of the differentiation is the same, the development of the power of contraction must run a somewhat similar course. If this be so, it is conceivable that the contractions in definite directions begin when the irregular network of the primitive cell becomes strengthened in these directions by an accumulation of the substance of the network to form fibril bundles. Why this should take place first around the periphery of the cell is not clear. It is true, however, not only in the development of heart muscle and voluntary muscle cells, but also in the evolution of the heart muscle cell in lower animals. IV. NERVOUS TISSUE. The essential constituents of the nervous system are nerve cells and nerve fibres. Formerly the latter were considered as separate elements, but now are recognized generally as processes of the nerve cells. It is a characteristic feature of nerve cells that at least one process proceeds from each. Usually there are many of such processes, one of which always becomes a nerve fibre, the so-called axis-cylinder process, Deiter's process, or axone. The rest are known as protoplasmic processes or den- drites. Independent nerve fibres do not exist in the animal organism. They are in every case in connection with cells. Thus the nerve cell, axone, and dendrites together form a nerve unit which is known as the neurone (Waldeyer). The nervous system is made up of such units. 7 98 HISTOLOGY. A. Nerve Cells. The nerve cells (also called ganglion cells) vary in size from 4 to 135 f in diameter in mammals, and are as large as 200 (i in fishes. Their form is variable— round, spindle-shaped, polygonal, or irregularly stellate. The processes of the cells may be divided into two main groups : (a) The axis-cylinder process (axone, neuraxone, Deiter's process) develops more quickly than the other processes, and Fig. 68. Multipolar nerve cell from the medulla of a rabbit. The axone is broken off. X 150. has always a smooth, even margin. It leaves the cell at an ele- vation known as the axone hillock, and may proceed out of the central nervous system to form an axis cylinder of one of the fibres of a peripheral nerve. On the other hand, it may end in the central nervous system by branching. It almost always sends off lateral branches, the so-called collaterals, which cause a communication to be established between the cells to which the axone belongs and other cells. NERVOUS TISSUE. 99 (b) Dendrites (protoplasmic processes) develop later than the axones. In Golgi preparations they appear much branched and with irregular outlines. Their margins are not smooth like those of the axones, but are covered with small elevations (Fig. 70). The terminal branches of the dendrites are called telodendria (Fig. 69). Fig. 69. Telodendron Dendrite ■Axone - Collateral Pyramidal cell from the cerebral cortex of the human adult. A. Bochenek.) X 150. (After a preparation by One may divide nerve cells according to their processes into uni, bi-, and multipolar cells. There is always an axone, but the number of dendrites varies. Unipolar cells are relatively rare. They are found in the nervous system of invertebrates, in the sympathetic ganglia of amphibians, in the olfactory sense cells, and in the spinal gan- glia of mammals. In these the axis-cylinder process divides 100 HISTOLOGY. dichotomously at a little distance from the cell, and gives rise to two nerye fibres, one running peripherally and one toward the central nervous system. The two fibres formed by this division separate at an angle like that of the arms of the letter Y or T (Fig. 71, d). It has been shown that the spinal ganglion cells are uni- polar in adults, but bipolar in embryos, the two embryonic processes fusing to form one in the adult (Fig. 71). Fig. 70. L- - Dendrites - - Cell body -Axone Purkinje cell from the human cerebellum, x 225. Bipolar cells (Fig. 71, a, b) are found in the spinal ganglia of fishes and in the ganglion spirale. In such cells there are two processes, one of which becomes the axone and the other a dendrite. Multipolar cells (Fig. 68) possess one axone and many den- drites. If one process runs a long distance and passes over into a nerve fibre, we have to do with a cell of the so-called Deiter's type ; while if the process runs only a short course and ends in the gray matter of the nervous system we have a cell of the Golgi type. According to Golgi, Nansen, and others, the axone is the NERVOUS TISSUE. 101 Fig. 71. © only conducting portion of the neurone, while the dendrites are nourishing organs. Most authors, however (Kamon y Cajal, van Gehuchten, Eetzius, etc.), are of the opinion that both dendrites and axones have the power of conducting impulses. According to Eamon y Cajal, the dendrites can conduct impulses only toward the cell (cellulipetal), while the axone conducts them only away from the cell (cellulifugal). In this way impulses pass from one neurone to another. It is therefore to be observed that all peripheral fibres of the sensory nerves bringing impulses from the outer world to the ganglion cells are dendrites, and such fibres as those of the motor nerves carry- ing impulses out to the muscles are axones. On the basis of investigations by newer methods (especially Golgi's) the idea has gained ground that neurones are connected with one another only by contact. But concerning this point there has been much discussion. For the last few years the neurone doctrine has been generally accepted — i. e., that the nervous system is made up of cells and cell processes forming together units or neurones which combine into systems of fibres and groups of cells. The late work of Apathy, however, throws doubt on the theory in the eyes of many investigators. Apathy finds that direct con- nections exist between ganglion cells, and claims that the nervous system cannot therefore be divided into morphological units (neurones). He makes the last elements of the nervous system not cells, but the so-called neurofibrils, which pass, according to him, without interruption from one cell to another. Even though it is possible to find certain cells joined to others, and fibrils continuous from one to the other, there is still nothing in this to disprove the neurone theory. The neurone doctrine states that the cells and fibres of the nervous system Semidiagrammatic repre- sentation of the transition from bipolar to unipolar nerve cells. 102 HISTOLOGY. are not separate, but go to make up morphological units, each fibre being connected with a cell. What Apathy has shown is, that these morphological units are sometimes connected, and that neurofibrils pass in the protoplasm from one to the other. The body of the nerve cell shows certain finer structures which have been the subject of much investigation in the last few years. All nerve cells possess a fibrillar structure, in the cell body as well as in the processes. The fibrils of the axone are more or less a continuation of those of the cell body. Here they run in all directions, often concentrically, and form a sort Fig. 72. Axone Axone Axon en End apparatus of axones from the trapezoid nucleus of a rabbit. (Prepared by S. Meyer's methylene-blue method.) x 700. of uetwork, especially in the middle of the cell. The proto- plasm of the cells contains numerous very fine, deeply staining granules (chromatophile). These granules are usually spindle- shaped, and are called Mssl bodies or tigroid bodies (Lenhos- sek). They are found also in the dendrites, but not in the axone hillock (Fig. 73). Their relation to the fibrils of the cell body is not yet explained. Some authors claim that they are connected closely with the fibrils, others that they are inde- pendent and lie in the spaces between the fibrils. A few authors regard them as artifacts produced on the death of the cell (Held). Some believe that they have an important trophic NERVOUS TISSUE. 103 and regulatory influence on the life and function of the nerve cell (Marinesco). It is possible that they are products of metabolism. In favor of this idea is the work of Lugaro, who found that in chronic arsenic poisoning there are definite changes in these tigroid bodies. Changes in these occur also in various diseases. Around the nucleus a network of fibrils has been recog- nized by Apathy, Bethe, and others, which are, according to them, continuous with the fibrils of the axone. nucleolus ^h\- 'ii'.\ fle Nucleus with 1 Nerve cells from the anterior horn of the spinal cord of a calf. Chromatophile granules are stained in methylene blue by the method of Nissl. X 950. A centrosome is found in spinal and sympathetic ganglion cells, but is usually not to be made out in other nerve cells. Yellow-brown pigment is found often in the protoplasm. The nucleus of the nerve cells is characteristic. As a rule, it is single, large, and vesicular. It possesses a distinct cell membrane, usually a large nucleolus, and only a small quantity of chromatin. A true cell membrane is not present. Cells which are situated peripherally, however, usually possess a secondary capsule of connective-tissue origin. 104 HISTOLOGY. B. Nerve Fibres. The continuation of the axone of a nerve cell forms the axis cylinder of a nerve fibre. This is the only essential part of the nerve fibre. All the other parts may be wanting. In a cross-section of a nerve fibre possessing all the accessory coverings we see in the centre the axis cylinder. Around this in concentric arrangement we have, from within outward, the medullary sheath, Schwann's sheath, and Henlcs sheath (Figs. 74-79). * The axis cylinder runs uninterruptedly from the nerve cell to the nerve-ending. It is characterized by being highly refractive, and possesses a fibrillar structure similar to that of the cell of which it is a process, but contains no tigroid bodies. Fro. 74. Medullary sheath Space between Schmidt- Lantermanrt Axis cylinder- From a cross-section through a nerve treated with osmie acid. X 350. Some authors regard it as quite structureless. By special methods it is seen to be made up of a large number of primi- tive fibrils (neurofibrils), between which there is a small amount of soft neuroplasm (Fig. 80). It is almost generally acknowl- edged that these neurofibrils are capable of carrying nerve impulses, and that each one is a separate conduction path (Apathy). The medullary sheath (Fig. 74) which surrounds the axis cylinder consists of myelin, a fatty, semifluid, homogeneous, highly refractive substance. Medullated fibres have a double contour, formed by the inner and outer margins of the medul- lary sheath seen in optical section. Soon after death the so-called coagulation phenomena begin. NERVOUS TISSUE. 105 The contour of the fibre becomes irregular and the myelin sheath shows interruptions in its course. With perosmic acid a characteristic picture is produced. This acid colors only the fat-containing medullary sheath. The interruptions are thus left unstained. These are called Schmidt- Lantermann lines or funnels, and fianvier's nodes (Fig. 75). Fig. 75. Piece of a medullated nerve fibre from the nervus ischiadicus of the frog. A node of Eanvier (6) and the lines of Schmidt-Lantermann (a) are shown, x 370. Fig. 76. The former appear on optical section as oblique lines run- ning down to the axis cylinder, and are therefore funnel- shaped. They thus divide the myelin sheath into cylindro- conical segments. The apices of these segments may be directed either toward the cell or away from it. Some authors regard these lines as artifacts produced by fixing reagents. Others are of the opinion that they occur normally, and that they are composed of a different substance from the myelin. The Ranvier's nodes (Fig. 75) are large annular interruptions which divide the fibres into what are known as inter- annular or Ranvie^s segments. The ab- sence of the myelin at these nodes is so distinct that the axis cylinder becomes joined with the neurilemma by a cement substance. These nodes are probably of use in the nourishment of the axis cylin- der, for at these places nutritive fluids could pass more easily into the centre of the fibre. By treating the nerve with silver nitrate and reducing this in the sunlight, we often obtain a brown striation on the axis cylinder {Fromman's silver line), which is usually considered as an artifact. This striation is especially marked at the nodes of Eanvier, and becomes less Medullated nerve fibres of a rabbit, treated with silver nitrate. The crosses of Ean- vier are shown. X 300. 106 HISTOLOGY. distinct as we proceed from these on either side. In the node itself there is a characteristic dark-brown coloration, which is due to the staining of the cement substance, which is present in the form of a ring-like sheath between the axis cylinder and Schwann's sheath. There is thus formed with the axis cylin- der a brown cross, as shown in Fig. 76. This is known as the cross of Ranvier. If we boil medullated nerve fibres in ether or alcohol, the myelin dissolves, and there remains a fine network surrounding Fig. 77. Node of Ranvier J-jfiocras^ Piece of a medullated nerve fibre from a frog, boiled in absolute alcohol. In the centre is the axis cylinder, aud around it the neurokeratin network, x 650. the axis cylinder. The substance of this network has proper- ties similar to those of keratin, not being affected by trypsin digestion. It is for this reason known as the neurokeratin net- work (Ewald and Kuhne) (Fig. 77). It is regarded by some as an artifact (v. Kolliker, Ramon y Cajal). Fig. 78. Nuclei of Henle Nucleus of Schwann'; corpuscle *m#t^ i W mmtM ^mim l A' Henle's sheath^ Axis cylinde Medullary sheath*^-* Piece of a medullated nerve fibre from the nervus radialis of man, treated with osmic acid. The nuclei of Schwann and Henle are to be seen. X 400. The Schwann's sheath or neurilemma is situated outside the medullary sheath, and is joined with the axis cylinder at the nodes of Ranvier by a cement substance. It is a very fine homogeneous membrane which shows at various places in its course nuclei surrounded by a small quantity of granular pro- toplasm (Fig. 78). These nuclei, together with the collections of protoplasm, may be termed Schwann's corpuscles. These are NERVOUS TISSUE. 107 seen in cross-section in Fig. 79, where they have a semilunar form ; while the Schwann's sheath is continuous around the whole fibre. In higher animals only one nucleus is found in each segment of Ranvier. Many authors believe that the Schwann's sheath is inter- rupted at each node of Ranvier, and joins on each side of the node with the axis cylinder, instead of being so connected by cement substance. Others go still farther, and claim that the sheath of Schwann is continuous at the nodes of Ranvier with Fig. 79. From a cross-section through the human median nerve, treated with Miiller's fluid and safranin. x 380. the so-called Mauthner's membrane or inner neurilemma, which is inside the medullary sheath and next to the axis cylinder (Fig. 80). The significance of the various coats of the nerve fibre is not clear. It is generally recognized from embryological studies that the axis cylinder is a process from a ganglion cell which has grown very much in length and possesses at its free end a globular thickening — i. e., the growing point. The sur- rounding coats are of entirely different origin, and arise from the connective tissue . Concerning the nature of the medullary sheath there are many views. According to some, each segment of Ranvier has the value of a cell, the sheaths of Schwann and Mau timer being regarded as parts of the cell membrane. The whole is considered as a connective- tissue cell, annular in shape, a part 108 HISTOLOGY. of which has been modified to form the medullary sheath, and the rest left unchanged as Schwann's corpuscles. Others regard the medullary sheath as a product of the axis cylinder. The function of the medullary sheath is mainly that of an insulator. The irritability of the nerve increases during devel- opment with the growth of the medullary sheath. Fig. 80. Node of Ran vier-- Schwann's shmthT"' Mauihner's ulieatlv- Axis cylinder*— % j — Neurofibril* **"" Cement substance Axis cylinder ^_Line of Schmidt- Lnntermann — Schwann' s corpuscle ■Medullary sheath I Schwann's sheath ft- Diagram of the structure of a medullated nerve fibre, showing two different views concerning the relations of the sheaths of Mauthner and of Schwann. Compare the right and left sides. Medullated nerves usually have a layer outside Schwann's sheath. This is of connective-tissue origin, and often shows a fibrillary structure, but is in many cases homogeneous. It pos- sesses always on its inner surface a number of flat epithelial NERVOUS TISSUE. 109 Fig. 81. i ' t'il Jl . l/l-l l.flH Nucleus cells, whose outlines can be made out by treatment with silver nitrate. This layer is known as Henle's sheath or the endo- neural sheath (Retzius). Nerve fibres of such complicated structure as that described are found in the cerebrospinal nerves. They are of variable thickness (1-20 p in diameter). Usually the longest fibres are also the thickest. The division of a medullated fibre into two, often three or four, branches takes place always at the nodes of Ranvier. As it approaches the nerve-ending it loses its sheaths. The medullated nerve fibre may lack the sheaths of Schwann and Henle, as is the case in the central nervous system, where the fibre consists of only the medullary sheath and axis cylinder. Non-medullated or sympathetic nerve fibres (Remak's fibres) are characterized by the absence of the medullary sheath (Fig. 81). In adult vertebrates such fibres are found only in the sympathetic nervous system. They are only 1 to 2 fj. in thickness, and are direct continuations of axones of sympathetic ganglion cells. Each fibre is surrounded by a covering resembling Schwann's sheath. It pos- sesses at various places nuclei surrounded by granular protoplasm. This sheath seems to be a continuation of the thin capsule that surrounds the sympathetic cells, and is of connective-tissue origin. The olfactory nerve fibres are of a still simpler type. They are very fine (less than 0.5 (i in diameter) fibres which consist of a naked axis cylinder. More often than other fibres, they are varicose and thickened in places. The bundles of these fibres are surrounded by a homogeneous sheath containing nuclei. This does not resemble Schwann's sheath, because Non-medullated (Remak's) fibre from the cervical sympa- thetic of the rabbit. X 300. 110 HISTOLOGY. the latter surrounds only single axis cylinders, and not bundles. Histogenesis of the Neurone. The following description is based on the account of the development of the neurone given by Barker, who, in turn, has used as a foundation the writings of His. As is well known, the medullary plate of the nervous system is derived from the ectoblast, by the turning in of a single layer of epithelium (Fig. 82). The neural tube is formed from this, its inner sur- Fiq. 82. Section through the medullary plate of a rabbit. Among the epithelial cells a large round germinal cell with clear protoplasm is visible. (Barker, after His.) face corresponding with the outer surface of the ectoblast. The cells increase in length and the nuclei come to lie at different levels (Fig. 83). There are formed three zones, however, the Fig. 83. Section through a rabbit's neural tube which is beginning to close. The number of epithelial nuclei is increased. (Barker, after His.) inner and outer of which are made up of the protoplasmic ends of the cells, while the middle zone contains the nuclei. The distal ends of the cells— i. e., the ends toward the lumen of the NERVOUS TISSUE. Ill Fig. 84. medullary tube — shrink, so that spaces are present between them; the proximal ends break up into irregular branches, which anastomose to form a spongy network, the neuro- spongium of His (Fig. 84). At the outside of the medullary tube this network forms a dense spongy structure known as the peripheral or marginal veil. Soon after the formation of the medullary plate large spherical cells appear between the distal ends of the epithelial cells (Figs. 82 and 83). These are the germinal cells (Keim- zellen of His). The nature of these cells is a matter of dispute. From the proximal end of each of the germinal cells there grows out a process, which, together with the cell, forms a pear-shaped structure known as the neuroblast. This becomes con- verted afterward into a nerve cell, and the process becomes its axone. The dendrites develop later on. The 1 t i n Section through the wall of a neuroblasts show a tendency to move neural tube of a rabbit oldertnan outward to the marginal veil, where that in m s- §3. Differentiation of i i t i • i i * ne * w0 ends of the epithelial cells. they are stopped. In the spinal cord {B arker, after His.) these arrange themselves parallel to the surface of the marginal veil, and on the ventral part of the cord send out processes through the marginal veil to form the ventral roots of the spinal nerves. The cell bodies become the ventral horn cells. The other neuroblasts do not send processes out of the cord. The marginal veil itself later becomes a part of the epen- dyma which is present in the white matter of the whole central nervous system. The origin of the peripheral sensory neurones is still a sub- ject of much dispute. It is agreed generally that these neu- rones are derived from the ectoblast at the edge of the 112 HISTOLOGY. medullary plate. From here the neuroblasts wander out and collect to form cell groups. The later development has been worked out by His. A process grows from each pole, one cor- responding with the axone and the other with the dendrites. The axone grows centralward and penetrates the wall of the medullary tube. Here it develops subsequently into a fibre which enters into the formation of the dorsal columns. The dendritic process proceeds in the opposite direction. By a sub- sequent change in the shape of this bipolar cell the axone and dendrites both come to be processes from one outgrowth of the cell. In other words, the cell becomes unipolar (Fig. 71). V. BLOOD AND LYMPH. Blood and lymph are properly to be considered as tissues consisting of formed elements in a fluid intercellular substance. 1. Blood. The blood of the higher animals is a red fluid, which is made up of blood plasma (intercellular substance) and formed elements (blood corpuscles, blood platelets, and various gran- ular elements). We distinguish two kinds of blood-corpuscles : red (colored) and white (colorless). Red blood-corpuscles (also known as erythrocytes) contain the red coloring material, hcemoglobin, which gives to them and to thin layers of blood a straw-yellow tint. They are in mammals almost without exception flat, round structures with- out a nucleus. The flat surfaces are depressed in the centre, giving to the cell the general form of a biconcave lens. The borders are rounded and much thicker than the centre. In optical cross-section the corpuscle is biscuit-shaped. The red corpuscles vary in size in different animals, from 2.5fi (in Moschus javanicus) to 9Ap (Elephas indicus) in diam- eter. In man they are from 7.2 to 7.8 /j. in diameter, and 1.9 (i thick at the thin middle point. Oval red corpuscles are found among mammals only in the llama and the camel. They are, however, common in lower animals. The red blood-cells of fishes, amphibians, reptiles, and birds, are oval in form and BLOOD. 113 biconvex. Each cell possesses an oval nucleus, which causes a thickening in the centre. They are much larger than in mam- mals. In Rana temporaria they are 22,a long, 15,u broad; in Salamandra maculosa, 37 n long, 23 {i broad; in Proteus san- guineus, 58 ft long and 34 fj. broad. The red blood-cell in man consists of two constituents, a protoplasmic part (stroma) and the coloring matter distributed on this — i. e., the haemoglobin. Anilin-stains, such as eosin, orange G, etc., are taken up readily by haemoglobin-containing cells. Under the influence of reagents, red blood-corpuscles change their form very quickly. In water or dilute acids they swell up and lose their haemoglobin. They are then colorless and hardly visible, and are known as blood shadows. They are decolorized similarly by the action of electricity and continued freezing. Tannic acid causes an extrusion of the haemoglobin in small globules. Salt solution stronger than " normal " causes a shrinkage of the cells from loss of water. They become irregular in outline, small sharp projections appearing everywhere; and are said to be crenated (Fig. 85, f). Fig. 85. Colored blood-cells (a-""">" Fig. 88. Small veil] ' 44 '-"TS*. Adventitia v / ^ "P2" W^ ' *' '/r^f'A \ \ - ,.. ..'4-, •:- Jfedi, I 4 ri wild ho : Endothelium Epithelium j?i., v t;,.,, fl J ' ,, .4 U, . j- Ji((i.s!ir« Put cell x Nerve Blood \ Media Bland Cross-section through a small artery and a corresponding view of a dog. X 220. of the vessel. It is colored bright yellow in any stain contain- ing picric acid (e. g., van Gieson's fluid). From fresh arteries it can be dissected out, and pieces of the membrane of a con- ARTERIES. 125 siderable size can be obtained. These have a peculiar fenes- trated appearance, clear unstainable areas being present throughout. They are due to the fact that the membrane is made up of three layers, of which the middle can be stained with magenta, and the other two not. At the fenestra the middle layer is absent. As described above, the individual Fig. 89. \~~Elastica interna Endothelial layer -Elastic fibres Media \ Bundles of smooth \ muscle cells , ~_ v- ">- +?'fc: /-V; — Elusl i at. externa Vas vasis Part of a cross-sectiou of the femoral artery of a dog. X 150. elastic fibres consist of a central stainable part and a capsule which does not take up the dye. By the fusion of many of these fibres the membrane with three coats is formed, the two outer coats corresponding with the capsules of the fibres (Mall). The media consists of circular layers of smooth muscle fibres, and the adventitia is made up of connective tissue. A medium-sized artery (Fig. 89) is composed of three dis- 126 MICROSCOPIC ANATOMY OF THE ORGANS. tinct coats: the intima, media, and adventitia. The intirna consists of a thin connective-tissue layer immediately surround- ing the endothelial tube, and Henle's fenestrated membrane or the elastica interna. Around this is a thick layer, the media, consisting largely of smooth muscle, but containing as well Fig. 90. Intima Media Adventitia ^Endothelium Connective tissue of intima —Elastic tissue \Smooth muscle / cells Nuclei of smooth ■muscle cells Elastic fibres Connective tissue Part of a cross-section of the aorta of a dog. X 140. many white fibrous and elastic connective-tissue strands. The muscle runs circularly, and is arranged in layers separated by connective tissue in the form of concentric bands of elastic tissue (Fig. 89). There are often seen longitudinal strands of muscle between the circular layers. The outermost sheath, the adventitia, is composed largely of connective tissue and muscle.- VEINS. 127 It is usually separated from the media by a more or less distinct ■elastic membrane, the elastica externa. The fibres of the adven- titia are divided roughly into two layers. The elastic fibres of the inner layer, next the media, run circularly, while those in the outer layer are longitudinal. Between the connective-tissue coats of the inner layer are longitudinal bands of smooth muscle. Vasavasorum are present in the media and adventitia. Arteries of large calibre (e. g., carotid, aorta, etc.) cannot be so distinctly divided into layers (Fig. 90). The endothelial tube is made up of short polygonal cells. The intima consists •of a subendothelial connective-tissue sheath, and the elastica interna. The subendothelial sheath is made up of white connec- tive-tissue fibrils and elastic fibres. The elastica interna is not a, firm homogeneous membrane, but is split up into several lamellae, and in some places is only a simple layer of elastic fibres. The media contains a great many membrane-like masses of •elastic tissue, and thick elastic fibres. Between these are bun- dles of smooth muscle fibres. The adventitia is similar to that of medium-sized arteries. The elastica externa is wanting in the aorta. The arteries in the skull cavity have no elastic elements in the media. This perhaps explains why they are more likely to yield to pressure than other arteries. The elastica externa is not present in them, but there are circular elastic fibres in the inner coat of the adventitia. (c) Veins. The important features which distinguish veins from arteries are the weak development of the media in the former, the small amount of elastic tissue, and the strong development of the adventitia. There is also a marked lack of uniformity in veins of the same size. The same three coats may be spoken of us in arteries, namely, intima, media, and adventitia. The intima is a connective-tissue layer containing only a few elastic fibres. In the larger vessels there are often bands of muscle running in various directions, and a layer of elastic tissue which may take on the 'form of a membrane. The latter is 128 MICROSCOPIC ANATOMY OF THE ORGANS. never as sharply marked as the fenestrated membrane of arte- ries. The media of veins is developed weakly in comparison with that of arteries (Fig. 92). It consists of a few circular muscle bands separated by thin elastic fibres. The veins of the lower extremities possess the most strongly developed media. It may Fig. 91. Endothelium,^ Elasticity ' Media Adveiititin «jfj»' ''"~^a ~e?**rx* y-.^l&W?^ Jj —Kudnts of muscle veil ii*rr^,< **#.; "* V.. -a Elastic fibres Part of a cross-section through a medium-sized vein of a dog. X 280. be quite wanting in others {e.g., vena cava superior, subclavian, veins of pia and dura mater, veins of bones, retinal veins, etc.). The adventitia, on the contrary, is usually strongly developed. It consists of white connective-tissue and elastic fibres, with often well-developed longitudinal smooth muscle bundles (Fig. 92). The valves of the veins are derived from the intima, and, like it, consist of connective tissue and elastic fibres. Their surface is covered by cells of the endothelial tube, which on the inner side toward the blood stream are long, and on the other side polygonal. On the inner surface of the valves there is under the endothelial cells a network of fine elastic fibres. The main points of distinction between arteries and veins are the following : The walls of arteries in relation to the size of their lumina are much thicker than in veins. The elastic tissue and muscle elements in the media are more strongly developed in arteries. After death the muscle of the media contracts, and in arteries throws the intima and elastica interna into folds, giving it a wavy appearance in cross-section. Veins usually contain a small quantity of blood after death; arteries are often empty. These differences are illustrated by Fig. 88. VEINS. 129 ^ All the medium-sized and large blood-vessels are supplied with small vessels (vasa vasorum) which supply their walls with blood. They run in the adventitia, and only to a small Fig. 92. Endothelial lnye Media __ — — -? Adventitia Vas vast.* — ~~ - - ■ • • •, .-• ■/.•■ i -■■■ ;-■:.:■■'' • ■- : '■ '":'. \ \ - ' v Part of a cross-section of the vena cava inferior of a dog. X 150. extent in the media. They never reach to the intima. Small blood-vessels are often surrounded by lymph capillaries, and sometimes by endothelium-lined spaces which are in communi- cation with the lymphatic system. These are called perivas- cular lymph spaces, and are found in the central nervous system, bones, etc. The vessel walls are also supplied with nerves. Medullated and non-medu Hated fibres form a network in the media, and may end in any of the accessory coats. Capillaries are sur- rounded usually by a fine network of nerve filaments. (d) The Heart. The heart is a much complicated part of the circulatory system, with walls that are made up of three main layers: 9 130 MICROSCOPIC ANATOMY OF THE ORGANS. 1. Endocardium ; 2. Myocardium; and 3. Epicardium (vis- ceral layer of the pericardium). (1) The endocardium is a connective-tissue membrane which contains smooth muscle and elastic tissue fibres. It is situated immediately outside the endothelial sac which lines the cavity of the heart. The endocardium is spoken of usually as including both the endothelial layer and the smooth muscle and elastic fibres outside it. The cells making up the endo- thelial layer are polygonal, and are continuous with the endothelial lining of the vessels. (2) The myocardium forms the main part of the heart wall. The layer is much thicker in the left ventricle than elsewhere. The finer structure of the muscle cells has already been de- scribed. By joining together laterally the branched cells form a network, the strands of which are bound together by con- nective tissue. The course of these strands of cells is not the same in different parts of the heart wall. In the auricles we find a superficial layer common to both, and a deeper layer belonging to each chamber. In the ventricles the most super- ficial layers are seen to run at right angles to the deepest. Between these there are fibres in all stages of transition. At the apex they form a whorl or vortex, disappearing from the surface in the depths. This very complicated structure is much simplified by a study of embryonic hearts by macerating methods. If hearts be taken from pigs' embryos, about 150 mm. in length, and macerated in nitric acid (commer- cial), 1 part ; glycerin, 2 parts ; water, 2 parts, the con- nective tissue binding the muscle strands together is dis- solved or destroyed. The course of the fibres may then be traced by dissection, and has been described in some detail (J. B. MacCallum). The superficial fibres are found to have their origin in either auriculoventricular ring, to wind about the heart spirally, and to end in tendons of the papillary muscles of the opposite ventricle. The deep layers also begin in the tendon of one auriculoventricular ring, pass around to the interventricular septum, cross over forward or backward in this septum, and end in the papillary muscles of the other THE HEART. 131 ventricle. Practically none of the strands of fibres begin and end in the same ventricle. It will thus be seen that the heart is made up of various layers of muscle, all of which have their origin in the tendon of the auriculoventricular ring of one side, and end in the tendon of a papillary muscle of the other side. Their fibres in passing over in the septum thus take a scroll- shaped course. In the light of this, the heart consists of several bands of muscle with tendons at each end, rolled up like a scroll or like the letter S. At the same time it is to be observed that the growing points in a very young heart are just under the endocardium. Karyokinetic figures are found there, and the cells in that region are younger than at the periphery of the heart. If the heart be then unrolled, these growing points would appear at each end of the bands of muscle that make up the heart. For a more detailed descrip- tion of this dissection of the heart, the reader is referred to the original article. In the muscle of the heart wall there is a rich network of blood capillaries, which run parallel to the fibres and send branches which surround them. Elastic tissue is found abundantly in both the auricles and ventricles. The annuli fibrosi, which consist of firm connective tissue containing elastic fibres, separate the muscle of the auricles from that of the ventricles, and form a place of attachment for those muscles. (3) Epicardium is a connective-tissue membrane rich in elastic fibres. Under it there is usually a quantity of fat, which is gathered in masses in certain places. The upper sur- face of the epicardium is covered by flat endothelial cells. The heart valves are connective-tissue structures formed by a reduplicature of the endocardium, and contain connective tissue and smooth muscle. Their surface is covered by endo- thelial cells. No blood-vessels are present in the heart valves. The pericardium is a connective-tissue membrane containing many elastic fibres, and on its free inner surface is covered by a layer of endothelial cells. 132 MICROSCOPIC ANATOMY OF THE ORGANS. The nerves are derived from the cardiac plexus, the vagus, and the sympathetic system. They are both medullated and non-medullated, partly motor and partly sensory nerves. Small ganglia are present at various places. Concerning the mode of ending of nerves in the heart, see under Nerve-endings. 2. LYMPHATIC SYSTEM. (a) Lymph-vessels. The lymph capillaries are not, like the blood capillaries, intermediate structures situated between two other systems. They form the beginning of a great lymphatic system which empties finally into the blood vascular system. The walls of the lymph capillaries consist of a flat endothelial tube, the Endothelial cells Piece of a lymph-vessel of a rabbit's mesentery. The boundaries of the endothelial cells are made visible by silver nitrate, x 235. boundaries of whose cells are irregular. The capillaries form networks that have a characteristic appearance on account of the unevenness in calibre of the vessels. There are many dilatations and constrictions, and in many places valves are present (Fig. 93). The walls of thicker lymph-vessels resemble in structure those of veins. There is an endothelial lining, an intima con- taining elastic fibres, a media consisting largely of smooth muscle, and an adventitia. The latter is made up of longitudi- nal connective-tissue bundles which contain elastic fibres and longitudinally disposed smooth muscle bundles. LYMPH GLANDS. 133 Development of Lymphatics. The problem as to the origin of the lymph-vessels is one which is not yet satisfactorily solved. While some authors believe that the lymph capillaries form a completely closed sys- tem bounded by endothelial cells, others believe that they are in open communication with tissue spaces which have no endothelial lining. According to the first view, fluids must pass by endosmosis through the walls of the lymph capillaries. The second view implies that fluids pass from the tissue spaces into the lymph capillaries through the open beginnings of the lymphatics. It has been shown by Mall that no closed system of lymphatics exists in the liver (see below). It has recently been shown by Miss Florence K. Sabin that the lymphatic sys- tem in the embryo pig develops as two blind diverticula from the veins of the cervical and inguinal regions. These grow toward the skin and widen out into four lymph sacs, from which the final lymphatics proceed. By a special growth of the lymphatics along the dorsal line, the thoracic duct is formed. (&) Lymph Glands. Lymph glands are situated in the course of the lymph- vessels, and are grouped together in various places (e. g., axilla, neck, groin, etc.). They vary considerably in size, and are usually bean-shaped. Lymph glands consist of a reticular connective-tissue net- work which contains lymphocytes (adenoid tissue). The frame- work of the organ consists of connective tissue with a few smooth muscle cells. The connective tissue is mostly of the kind known as reticulum. It forms at the surface of the gland a continuous covering, the so-called capsule. From this, leaf- like projections pass down into the substance of the gland, as shown in Fig. 94. These are called trabecular. They run in such a manner that the outer part of the gland is divided into round masses of adenoid tissue. Toward the middle of the gland they branch to form a network of connective-tissue strands, in the meshes of which are narrower masses of aden- 134 MICROSCOPIC ANATOMY OF THE ORGANS. oid tissue continuous with the round masses outside. The gland is thus divided into two zones, a cortex and a medulla. The cortex is divided by the trabeculee into follicles, while the medulla consists of much smaller masses of adenoid tissue, known as medullary cords (Fig. 04). These are directly con- tinuous with one another. The reticular connective tissue which fills the spaces between the trabecule contains very few lymphocytes in the immediate neighborhood of the trabecular and the capsule. Farther away from these, however, the lymphocytes are very numerous and Fig. 94. Li/mpk sinus Cap Medullary card. Follicles ii.!... .. .a. . . i '•'Trabeculas Medullary substance Section through a small lymph gland of a dog. v 20. make up the follicles and lymph cords. These latter masses are therefore surrounded by almost empty spaces which separate them from the trabecular and capsule. These spaces are called lymph sinuses (Fig. 96). They contain a fine reticulum, which passes over the trabecular on one side and the follicle on the other. The lymph sinus is a continuation of the lymph-vessels, and, like these, is lined with flat endothelial cells whose presence can be demonstrated by silver nitrate. This endothelium prob- ably does not form a continuous membrane. The cells are often found separated and lying freely in the lymph sinus. LYMPH GLANDS. 135 The follicles of the lymph gland consist of a dense, more or less spherical mass of lymphocytes. At its periphery next to the lymph sinus these cells are much crowded together, while in the centre there is always a more or less clear space where the lymphocytes are much less abundant. Here there are usually found an artery and one or more veins. By close observation karyokinetic figures may nearly always be found in this region. Fig. 95. ! g& •.T^« Capsule ■&W£)l _Lymph i^/^v'-'aV^ l'<~*& sinus Trabecula '""" vessel Germinal 'centre ill :■<-:'■ ■'■' : i~ m From the cortex of a dog's lymph gland. X 150. According to Flemming, the reproduction of lymphocytes takes place in the centre of the follicle, and this area is known as the germinal centre (Keimcentrum) (Fig. 95). The afferent lymph-vessel enters the gland usually at one pole, and after dividing passes through the capsule. Its walls become always thinner until, on forming the lymph sinus, they 136 MICROSCOPIC ANATOMY OF THE ORGANS. consist only of an endothelial layer. The lymph sinuses pass from the cortex to the medulla, join together, and leave the gland at the hilus by the efferent vessel. At this place the cap- sule is thick and compact, and is known as the hilus stroma. The sinus terminalis is formed at the hilus by the junction of the other lymph sinuses. The blood-vessels of the lymph gland were described in detail by Calvert, and worked out in relation to the follicle of the gland. The following account is based on his description : The Trabecule with blood-vessel Lymph sinws From the medullary substance of a oat's lymph gland. < 250. Medullary cords gland is supplied with arteries mainly from the hilus, but also to some extent from the capsule. The arteries at the hilus leave the stroma-substance and enter the trabecular, in which they run for a short distance. After leaving the trabecule they enter the medullary substance and break up into smaller arteries, sometimes sending a small branch to anastomose with the arteries of the capsule (Fig. 97, E). Other arteries at the hilus run directly into the gland substance and enter the medul- LYMPH GLANDS. 137 lary cords, giving off fine branches to form capillary plexuses around their peripheries (Fig. 97, F). These capillaries unite to form small ' veins (G), which empty into larger veins. The arteries supplying the follicles of the cortex (H) break up into many branches, which run from the centres of the Fig. 97. Composite section of three follicles and the medullary cords of the mesenteric lymphatic gland of a dog. X 50. (Calvert.) A, artery; B, medullary artery; C, follicular vein; E, artery going to the capsule; P, capillaries on the periphery of a cord; G, medullary vein ; H, follicular artery ; I, arterial capillaries in a follicle ; J, vein from capsule ; K, cord ; L, trabecula ; V, vein. follicles to the periphery, where they form capillary networks. These capillaries unite to form the vena? folliculi (C), which give origin to the larger veins of the gland returning to the hilus. It is seen that in this system there is a blood vascular unit 138 MICROSCOPIC ANATOMY OF THE ORGANS. which is repeated many times to make up the organ. It corre- sponds also in this case with the cellular unit which is repre- sented by the follicle. (e) Peripheral Lymph Nodules. Collections of lymphoid tissue are present in many organs in the form of single follicles, or many of these together. They are not so definitely connected with the lymph-vessels as the true lymph glands are. They may occur merely as a diffuse infiltration of the tissue by lymphocytes. The so-called solitary follicles of the alimentary canal are definite well-circumscribed masses of lymphocytes, possessing germinal centres, and all the characters of lymph follicles. Collections of these follicles are seen in the Peyer's patches of the small intestine. 3. SPLEEN. In the spleen, as in the lymph gland, we can distinguish a connective-tissue capsule, sending processes down into the organ to form its framework ; and adenoid tissue contained in the framework, the so-called spleen pulp. The capsule and the trabecular proceeding from it are made up of connective tissue with a considerable number of smooth muscle cells and elastic fibres. They can be easily distinguished from the corresponding structures in the lymph gland by their strong, coarse appearance. They are nearly always thicker, and the muscle cells give to them a less finely fibrous character. At the hilus the trabecular and capsule form a sheath for the blood- vessels which enter there. This sheath retains its firm fibrous character throughout the course of the veins, but with the arteries it gives place to a fine reticular tissue when these vessels by branching have become as small as 0.25 mm. in diameter. This reticular tissue contains in its meshes large quantities of lymphocytes. The arterial sheath, consisting of adenoid tissue, forms, in some animals, a continuous layer around the vessel wall. In other animals, however, it is gathered into spherical or ovoid masses which resemble the follicles of the lymph gland. These SPLEEN. 1 39 are the Malpighian corpuscles (Fig. 98). If the lymphoid tissue is equally distributed around the artery, this vessel is found in the centre of the Malpighian corpuscle. It may be placed excentrically, on account of the unequal development of this tissue. In sections of the spleen the Malpighian cor- puscles are round structures with a diameter of 0.2-0.7 mm. They are situated often at the place where an artery branches, and each shows a germinal centre (Keimeentmm), in which there is multiplication of the lymphocytes. Fig. 98. Capsule— T^r^f '?''■'","■ /* ' fi'W," flp ■..'„ , ■.. •>•> •.:■■'" .".■ T=-"'.;>^ vf£$. ■ ■ ~ i .■'•- m t -..-■ ] corpus f-^}.^;:.^. , . ,- '■•[■■.-. .. - . , ; : } ... X'Z^h t : .Trabecules mm A Hery- Spleen Part of a section through the spleen of an ape. X 60. The spleen pulp has the characteristic features of adenoid tissue. It consists largely of lymphocytes, but contains also large cells, with many nuclei containing red blood-corpuscles and pigment. Nucleated and non-nucleated red corpuscles are also found. The pigment granules which occur free or in leucocytes are formed from broken-down red blood-corpuscles. The 140 MICROSCOPIC ANATOMY OF THE ORGANS. adenoid tissue of the pulp is distinguished from that of the Malpighian bodies by the fact that the latter consists entirely of lymphoid cells. Some authors claim that the spleen is a blood-forming organ because erythroblasts are found there. Others regard it as an organ in which destruction of blood takes place, for the reason that fragments of blood corpuscles and blood pig- ment are frequently found. The character of the blood-vessels in the spleen shows some peculiarities. The arteries do not anastomose with one another; their adventitia often shows a lymphoid character — i. e., in Malpighian corpuscles. The terminal arteries show a thicken- ing of this lymphoid sheath to form the ellipsoids of the spleen. In their final divisions the arteries branch like the hairs of a brush (penicilli). The blood-vessels of the spleen are best considered together with the so-called lobule. According to Mall, the framework of the spleen, which can be demonstrated by washing out the cellular elements of a spleen macerated in water, is divided into sacs, each of which contains a spleen lobule. These lobules have a distinct relation to the blood- vessels. By injecting the vessels with celloidin or agar-agar and macerating the tissue, this relation is demonstrated. The arteries enter at the hilus, and divide into many branches, one of which enters each lobule and passes along its centre. The veins are always intimately related to the trabecule, and are always found at the periphery of the lobules. This is shown in Fig. 99. The Malpighian corpuscle usually lies at the hilus end of the lobule — i. e., at the side away from the capsule. The veins accompany the branches of the interlobu- lar trabecular into the lobule, which is divided by these branches into several compartments. The veins as well as the trabecular may be spoken of as interlobular and intralobular. The central artery of the lobule branches to supply the various compartments formed by the intralobular trabecule, and the blood is collected finally by the intralobular veins. A venous injection of the lobule fills a plexus of veins (Fig. 99, P), in whose meshes there are small areas of spleen pulp, the so- PLATE VI. Fig. 100. — From the spleen of a rabbit. The blood-vessels are doubly injected, the veins blue, the arteries red. In the centre a Malpigliian corpuscle is shown. X 100. SPLEEN. 141 called histological units (Mall). These units are, however, in communication with one another. The terminal arteries run into these units and end in the ampullce of Thoma, a small Fig. 99. Cctpszt le. Diagram of the lobule of the spleen. (Mall.) A, artery in the centre of the lobule ; V, interlobular vein within the interlobular trabecule; IV., intralobular trabeclse; L, Mal- pighian follicle ; C, intralobular collecting vein ; P, intralobular vein plexus which sur- rounds the pulp cords or histological units ; Am, ampulla of Thoma. dilatation. There is evidence to show that these ampullae communicate both with one another and with the terminal veins. According to some authors, the capillaries open directly into the veins, while others have described a system of lacuna? between them. This is known as the intermediary path for the blood. The capillaries show a funnel-like widening as they pass over into the veins (Plate VI., Fig. 100). This interme- 142 MICROSCOPIC ANATOMY OF THE ORGANS. diate space is, according to some authors, lined with epithelium, so that there is here a closed system. Most authorities, how- ever, claim that these lacunae have no wall or possess only an incomplete wall, so that the blood does not flow in a closed channel. These authors base their belief on the fact that in injecting the blood-vessels the injection mass flows also into the spleen pulp ; and because of the constant presence of red blood- corpuscles in the pulp. According to the view held by the second group of investigators, the beginnings of the veins stand in communication with spleen pulp. These endothelial cells are flattened and spindle-shaped, and show a striated structure. The nuclei project markedly into the lumen. According to Weidenreich, there is a system of spaces {spleen sinuses) in the spleen pulp which anastomose freely and open into the pulp veins (intralobular). The framework of the spleen pulp is made up of anasto- mosing fibrils which have the character of reticulum, as shown by Mall. As mentioned before, the reticulum of the capsule and trabeculse is more resistant to the action of the ordinary reagents than that of the spleen pulp. " The main strands of the reticulum accompany the interlobular venous plexus, while a more delicate network with more open meshes extends through the histological unit. In the centre of the unit the network becomes dense again, which marks the position of the terminal artery with its accompanying ellipsoid lymphatic tissue" (Mall). The reticulum surrounds the veins, and also forms a layer around the arteries, holding in its meshes the cells of the lymphoid sheath. Kyes has shown that the net- work surrounding the smaller veins is not elastic tissue, but is made up entirely of true reticulum. The reticulum of the follicles is directly continuous with that of the pulp cords. Lymphatic vessels in relation to the Malpighian follicles do not exist in the spleen. In the capsule and trabecule of the spleens of certain animals there are large lymphatic channels. These are seen also at the hilus of the oi'gan, but, according to Mall, the lymphatics are not to be observed in the spleen pulp. THYMUS. 143 Much of the above description is based on the work of Mall. For further details the reader is referred to his articles on the subject. 4. THYMUS. The thymus is a gland-like organ found only in embryos and young animals. It undergoes with age a retrogression, and at the twentieth year in man we find only a connective-tissue vestige of it. In the first years of life the thymus consists of various lobes, which are made up of lobules 0.5 to 1 cm. in diam- eter, joined together by connective tissue. These lobules con- sist of smaller lobules about 1 mm. in diameter, which are separated by connective-tissue septa. The smaller lobules are made up- of adenoid tissue, which is richer in blood-vessels and lymphocytes at the periphery than at the centre. We may thus distinguish a dark cortical substance and a light medulla. The thymus is of epithelial origin, and develops later its adenoid structure. It is an outgrowth from the foregut of the embryo, and, according to the majority of authorities, the Fig. 101. Blood-vessel ._„ ^ Connective tissue . - - ', , -. HassaVs $0. '■/f._. -■';■■. '' ^corpuxch , _j^--- i --r"^'""0-->^ Capsule ■ i'r-\ , - \ V- ■ - ■."';/-' 'S^'"""' i£> &-~^~ ' . ."■,." ■ '/■ £> . / '-'-■. -■-[■■■:: X -•- .V -. ';'<# -j* '■■'- ■• - -•' y ■ ■'■"■ ■■',- ,■' :' *.\* . -^■■3-a\ '.- ■?*■ . 4 v m< ■■■■'■ ■-■ . '-■/.- ■ -'%- .\ ^ « From a section of the hypophysis cerebri of a dog. x 300. consists of epithelial cells, concerning whose glandular nature there can be no doubt. They are round or polyhedral, and, according to some authors, are of two types. One kind of cell is of dark appearance, large and granular, and has a marked affinity for staining fluids. These are called chromophile cells. The other kind is small and clear. The two types are about equally distributed. Other authors regard the differences in appearance between the two kinds of cells to be due to post- mortem or functional changes. In the posterior part of the anterior lobe there are found CAROTID GLAXD (GLOMUS CAROTICUM). 153 alveoli which, like those of the thyroid, are filled with a colloid substance. Sometimes these alveoli are lined with ciliated epithelium. There is a close relation between the glandular elements and the blood capillaries. A dense network of vessels surrounds all the gland alveoli. The glandular nature of the organ has been further proved by physiological experiments. The internal secretion seems to have an important influence on the organism as a whole. 8. CAROTID GLAND (GLOMUS CAROTICUM). The carotid gland is a structure the size of a grain of corn, situated, in man, at the bifurcation of the common carotid artery. It is associated closely with the vessel wall, and is surrounded by a connective- tissue capsule which sends strands of tissue into the organ. The organ thus is divided by three connective-tissue septa into follicles, which are usually small round masses of cells, the so-called cell balls (Zellballen) of Schaper. These follicles, or cell balls, are made up of cells containing much protoplasm and resembling epithelial cells. They are polyhedral or round, and seem to be associated closely with the blood capillaries. They are apparently of connective- tissue origin, and are arranged in small groups in the meshes of a connective-tissue network. The true nature of this con- nective tissue has never been determined. In all probability it is largely true reticulum. In advanced age the cell groups break up, and there is a marked increase in the connective tissue and blood-vessels (Schaper). The carotid gland is supplied very richly with blood-vessels. A branch from the carotid artery enters the gland and breaks up into many small branches, each of which supplies one follicle. The capillaries formed by division in the follicle anastomose and make up a dense plexus, which is connected at the periphery of the follicle with small veins. These join with veins from other follicles, and form on the surface of the gland a venous plexus. Numerous medullated and non-medullated nerve-fibres are 154 MICROSCOPIC ANATOMY OF THE ORGANS. present in the gland, and run as fine branches into the fol- licles. Ganglion cells are rarely found. 9. COCCYGEAL GLAND (GLOMUS COCCYGEUM). This organ is situated on the arteria sacralis media. Its general structure resembles that of the carotid gland. The same polygonal epithelioid cells are found, and these stand in the same close relation to the blood-vessels. Small branches of the median sacral artery enter the gland and break up into a capillary network. In the capillaries that make this up there are peculiar dilatations mainly situated on the venous side of the network. All the glands that have been described in this section must be considered as made up primarity of two parts, namely, a connective-tissue framework, and a cellular part contained in the meshes of this framework. The cellular part is divided in each case into masses of cells (follicles or lobules), which have a similar and complete structure of their own. By the repetition of these follicles the organ is built up. The connective-tissue framework of the gland is in every case closely connected with the blood supply. The vessels follow, to a certain extent, the course of the trabecular and connective-tissue strands, which divide the organ into units. The cellular units (follicles or lobules) correspond often to blood vascular units, in which the artery usually enters the centre of the follicle and breaks up into capillaries which join with the veins at the periphery of the follicle. The arteries and veins are always as far apart in the follicle as possible. All these glands, then, are composite structures, so that in their study we should consider not only the units themselves, but also the relation of these units to one another and to the framework. II. DIGESTIVE SYSTEM (ALIMENTARY TRACT). The whole alimentary tract is lined with mucous membrane. This is a soft membrane consisting of epithelium, glands, and connective tissue. The epithelium consists of one or more PLATE VIII. Mucous gland Epithelium of mucous, membrane Bloorl- -\Unir follicles Epidermi Epithelium of mucous- membrane Cross sections , of muscle''- Lonjiitnilinal sections of muscle fibres Mucous! membranel with high 1 ' papiila;'*'^^^ Fig. no. -Section through the u __Place where stratun co me urn begins PPer lip of a two aud a half year old child. X 14. MOUTH CAVITY. 155 layers. When there are many layers the superficial cells are flattened, as in stratified epithelium. Under the epithelium there is a connective-tissue layer, the tunica propria or stratum proprium. Under this is a firm connective-tissue coat, the tela submucosa or stratum submucosum. This combines the mucosa with the underlying parts. A. MOUTH CAVITY. 1. Mucous Membrane of the Mouth Cavity. The epithelium of the mouth cavity is a stratified pave- ment-epithelium. It is not, as a rule, corneous, and the stratum granulosum and stratum lucidum are usually absent. The tunica propria consists of interlacing bundles of con- nective-tissue fibres, among which are many elastic fibres. On the surface the tunica propria forms so-called papillae, of which the highest are found in the red border of the lips and in the gums (Plate VIII.). At the border of the lips we find seba- ceous glands, but these are absent elsewhere in the mucous membrane. Everywhere in the tunica propria we find the ducts of mucous glands (glandulse buccales, palatinse, et labiales), whose bodies lie in the submucosa. These are branched tubular glands whose ducts are lined usually with stratified epithelium. The details of their structure will be discussed with the larger glands of the mouth cavity. The tela submucosa is a firm connective-tissue layer pos- sessing only very few elastic fibres. On the gums the mucosa is attached firmly to the underlying structures. Elsewhere it is more loosely connected with the submucosa. The blood- vessels form two plexuses more or less parallel to the sur- face. The deeper, which consists of larger vessels and wider meshes, lies in the submucosa ; the upper is made up of a fine meshwork of small vessels which are derived from the deeper layer and is situated in the tunica propria. From these net- works fine branches proceed to the papillae, where a capillary plexus is formed. The lymph-vessels follow a course very 156 MICROSCOPIC ANATOMY OF THE ORGANS. similar to that of the blood-vessels. The sensory nerves end in the mucous membrane in two ways : on the papillae as Krause's end bulbs, and in the epithelium as fine intra-epithelial nerve-endings (see under Nerve-endings). 2. The Teeth. The teeth in man and the higher animals are hard struct- ures, of which one part is sunk in the alveolus of the jaw (root) and the other part projects to the outside, and is called the crown of the tooth. The place of junction of the two parts is called the neck of the tooth, and this part is covered by the gum. The teeth consist of three hard substances : 1, enamel; 2, dentine; 3, cement. These substances surround a cavity in Fig. 111. Odontoblast* Connective >' tissue cells Dentine' From a longitudinal section of the crown of a milk tooth of a newborn baby. The boundary between pulp and dentine is shown, x 500. the centre of the tooth known as the pulp- or tooth-cavity. This cavity extends into the root of the tooth as the root- canal, through which vessels and nerves enter the pulp from below. The tooth pulp consists of a finely fibrous cellular con- nective tissue, and is characterized by its richness in nerves and blood-vessels. On the surface of the pulp there are large cells PLATE IX. Fig. 113.— From a longitudinal section of the lateral part of the crown of a human canine tooth. The canaliculi, filled with pigment, in some places extend outward between the enamel prisms. X •>30. PLATE X. Fig. 114. — From a cross-section of the neck of a human molar tooth. The dental canaliculi show divisions and numerous anastomoses. X 330. PLATE XI. 3 ■ Fig. 115. — From a cross-section of the root of a human molar tooth. The canaliculi, filled with violet pigment, show numerous divisions. Small interglobular spaces are to be seen in the granular sheath, y. 330. MOUTH CAVITY. 157 — the odontoblasts — forming a continuous layer (Fig. Ill), These are long cylindrical cells with the nucleus in the inner half of the cell. They each send one process, seldom more, into the dentine toward the outside. These processes form the fibres in the dentine. There are other processes sent out by the odontoblasts in the direction of the pulp. These branch and surround the pulp elements. The whole pulp is sur- rounded by dentine, which forms the main mass of the tooth. The dentine itself is covered entirely by two other coats, on the crown of the tooth by the enamel, and on the root by the •cement. These two coats meet at the neck. The dentine (substantia eburnea) is a kind of bone which is ■distinguished from ordinary bone by the fact that its cells are not situated in cavities of the ground substance. The cell bodies lie on the surface of the pulp close to the dentine, so that the dentine itself contains only, their processes, the so- called denial fibres, which lie in the dental canals. These «anals begin at the pulp surface of the dentine, and run radially toward its outer surface in a slightly curved direction, like the tetter S. At their beginning the canals are 2.5-5 u in diam- eter, but become narrower as they proceed outward, on account of division. At the outer surface of the dentine they measure •only 0.6 (j.. They give off throughout their course fine side- branches in every direction, thus joining with neighboring ■canals. These side-branches are usually 0.3-0.6 fi in diam- eter. A section cut at right angles to the course of the canals shows their relation to the side-branches. Fi°\ 112 shows that they join not only canals near one another, but also those at some distance from one another. The relation of the main dental canals, as well as of the side-branches, is characteristic for different parts of the tooth {Plates IX., X., XL). In the part near the pulp the lateral branches leave the canals at almost a right angle. In the more peripheral parts of the dentine, on the contrary, the angle is acute. In the former position the side-canals are less numerous than in the peripheral parts. In the crown of the tooth the main canals take a fairly 158 MICROSCOPIC ANATOMY OF THE ORGANS. straight course and do not often branch to form canals of the same calibre. In the neck they are slightly wavy. In the root they are more uneven, and branch frequently to form equal-sized canals (Szymonowicz). The peripheral ends of the main canals are different according to their surroundings. In the crown just under the enamel they break up into finger- like branches (Plate IX.), some of which run past the boundary line between the enamel and dentine for 10-40 ft Fig. 112. From a ground-section through the parts of the dentine, near the pulp, of a human canine tooth which has been impregnated with pigment. The dental canaliculi are cut across and are joined together by side branches. X 400. into the cement substance joining the enamel prisms (Fig. 117). Dilatations are observed often at the ends of these branches. Most of the main canals, however, end blindly at the border of the enamel. In the lower part of the tooth the main canals do not leave the dentine, but end blindly at the border of the cement — i. e., at Tomes' granular sheath. They often reach as far as the spaces of the sheath, which are filled with uncalcified ground substance (Fig. 116). Rarely they arch over and form with neighboring canals a kind of loop. The part of the ground substance immediately surrounding MOUTH CAVITY. 159 the canals is harder and more resistant than the rest, and is known as Neumann's dental sheath. The ground substance itself has a structure finely fibrous, like that of ordinary bone. The fibrils are joined to form bundles, which run mainly in the long axis of the tooth. In the dentine of the crown there is, near the outer enamel surface, a layer of so-called interglobular spaces. These are large or small spaces of irregular shape, situated in the calcified Fig. 116. Cement Tomes' granular sheath Dentine uSarcvT Part of a cross-section through a human incisor tooth in the region of the root. X 360. ground substance and filled with a soft substance which corre- sponds with the uncalcified substance of the dentine (Plate IX.). The dental canals pass through these spaces without inter- ruption. These spaces are an indication of the unequal and incomplete calcification of the dentine. In the lower parts of the tooth we find in the outer part of the dentine the so-called Tomes' granular sheath, which is nothing more than a layer of small interglobular spaces (Fig. 116). 160 MICROSCOPIC ANATOMY OF THE ORGANS. The enamel (substantia adamantina), which is the hardest of all animal tissues, contains only 3-5 per cent, of organic sub- stance. It is soluble in dilute acids without residue. It con- sists of the so-called enamel fibres, which appear in the form of hexagonal prisms, and are on this account known as enamel prisms. These extend from the surface of the dentine to the Pig. 117. / Longitudinal ground-section through the apex of a canine tooth from a three and a half year old boy. The entrance of the dental caualiculi between the enamel prisms and the course taken by the latter are shown. X 135. free surface of the enamel, and are thicker at the outer end than near the dentine. They usually appear to be structure- less, but under the influence of certain reagents they acquire a striated appearance. They usually run radially and their course is slightly wavy. They lie pressed together, and joined with one another by a small amount of cement substance. The MOUTH CAVITY. 161 enamel prisms are in general arranged in parallel rows, but there may also be bundles of prisms running diagonally and at angles to one another (Fig. 117). The surface of the enamel is covered by a very thin (about 1 fi thick) structureless membrane, the cuticula dentis. The cement {substantia ossea) (Fig. 116) is a true bony tissue, which in young teeth as a rule possesses Haversian systems and bone lacunae. These lacuna? are wanting in the neck of the tooth. The lamellated structure is seldom observed. Large numbers of Sharpey's fibres are present. Blood-vessels and nerves reach the tooth through the pulp cavity. Small arteries enter the pulp and break up into numerous branches. These form a network with oblong meshes which extend up to the odontoblast layer as a capil- lary plexus (Lepkowski). Lymph-vessels are not known in the pulp of the tooth. The nerves enter the pulp in several bundles, which run mainly in the centre, giving out numerous branches. These fibres form a network which runs toward the periphery. The fibres lose their medullary sheath and extend as fine non-medul- lated fibres between the odontoblasts, to end freely in small swellings (Retzius). Development of Teeth. In the beginning of the seventh week of foetal life the epi- thelium covering the edge of the jaw grows into the deeper- lying connective tissue in the form of a ridge — the so-called dental ridge. In the third month round thickenings occur on the labial side of this ridge, which form the beginnings of the milk teeth (Fig. 118). At the same time certain changes take place in the connective tissue. It projects into the lower side of the thickenings in the dental ridge, and forms in each thick- ening a dental papilla or tooth germ. In consequence of this invagination the epithelium forms a sort of mantle for the dental papilla. The epithelial covering forms the starting-point for the enamel, and is known as the enamel organ. It later separates off from the dental ridge by a gradual narrowing of it 162 MICROSCOPIC ANATOMY OF THE ORGANS. its connection with it. The place of junction which remains is called the neck of the enamel organ (Fig. 119). In the region of the neck the dental ridge grows downward into the connec- tive tissue on the lingual side of the milk tooth, and forms another ridge, in which thickenings occur. Into these the : papillae of the permanent teeth grow, so that in the fifth month of foetal life there are present the beginnings of both milk and 'permanent teeth (Fig. 119). Certain changes take place in the enamel organ. The cells bordering on the tooth papilla, the so-called inner enamel cells, Fio. 118. JZpithel'min of oral cavity Connective tissue -Outer enamel crl/x Enamel pulp -Dental papilla An early stage in the development of a tooth in a pig's embryo. X 240. become higher, while the outermost layer of cells, the outer enamel cells, become more flat. The cells between these two layers form the enamel pulp (Fig. 119). In the latter region the intercellular substance increases in amount; the cells become stellate and anastomose with one another. As growth goes on, the enamel pulp becomes gradually less in quantity, and finally vanishes almost entirely. Meanwhile the connective tissue around the tooth forms a capsule, the so-called tooth sac. The development of the hard tissues of the tooth begins with the dentine. This is a product of the connective-tissue cells which lie on the surface of the dental papilla, and are known as odon- toblasts. These are columnar cells arranged in a layer. The dentine begins as a thin homogeneous membrane, the membrana MOUTH CAVITY. 163 prceformativa, which lies between the odontoblasts and the inner enamel cells. This membrane is converted into dentine and in the beginning is a non-fibrillar structure. The development of dentine starts at the apex of the tooth papilla. The odonto- blasts send processes out into the fine canals which are formed in the dentine, and these processes become the dental fibres. Fig. 119. Inner Dental^ papilla V An advanced stage in the development of a tooth in a three and a half months human embryo. X 65. Calcium salts are laid down in the fibrillar ground substance in layers. Numerous small areas where calcification is incom- plete or absent form the interglobular spaces. Soon after the beginning of the dentine formation the devel- opment of the enamel starts. In the region of the future crown 164 MICROSCOPIC ANATOMY OF THE ORGANS. of the tooth the inner enamel cells develop a cuticle-like bor- der. Toward the dentine the so-called Tomes' processes are sent out, which give rise to the enamel prisms. Finally, calci- fication takes place from the centre to the periphery, both in the prisms and, in the cement substance joining them. The enamel cells disappear, the cuticle is pushed to the surface and forms the dental cuticle. The development of the cement, which is a product of the inner wall of the tooth sac, takes place later as a sort of peri- osteal bone formation. 3. The Tongue. The tongue is an organ consisting largely of striated muscle. Its mucous membrane, which is a continuation of that lining the mouth cavity, is differentiated in certain places Fig. 120. Horny epithelial) Secondary papilla Epithelium Tunica propi Two filiform papillae from the anterior part of the human tongue. X 80. in a characteristic fashion. In most animals certain parts of it possess a distinct corneous layer, but the most essential struct- ures are the so-called papillce. In man there are three kinds of these : MOUTH CAVITY. 165 Papillae filiformes ; Papillae fungiformes ; Papillae circumvallatae. The papilla filiform.es (Fig. 120) are 0.7-3 mm. in length, round or pointed, and covered by a layer of cornified flat stratified epithelium. This in some animals (e. g., cats) forms a sharp, pointed projection of hard epithelial cells. The tunica propria under the epithelium shows several (five to twenty) small papillae, the secondary papilla, which correspond with the vascular papillae of the skin. The filiform papillae are distributed over the entire upper surface of the tongue. The papilla fungiformes (Fig. 121) are 0.7-1.8 mm. in length, and have a round form which suggests that of a small Fig. 121. Secondary jittjiillfi m mm tSiMuscle fibres Perpendicular section through a papilla fungiformis of the human tongue. X 45. mushroom. They are present mainly on the anterior part of the tongue, and are distinguished easily from the other papilla by their red color, which is due to their thin epithelial covering and rich blood supply. They are covered with epithelium similar to that of the mouth cavity, and show many secondary papillae. 166 MICROSCOPIC ANATOMY OF THE ORGANS. The papillae vallatce or circumvallatce (Fig. 122) are so named on account of being surrounded by a sort of trench. They are about nine or ten in number in man, and are arranged in two lines which diverge forward from the foramen caecum at the back of the tongue. They thus form a V-shaped line, with the apex behind and the arms forward. They resemble the Fig. 122. Furrow around papilla Secondary papilla Wall Perpendicular section through a papilla vallata of the human tongue, x x, taste buds, v 37. papillse fungiformes somewhat in general form, but are consid- erably larger than these, usually measuring 1-2 mm. in diam- eter and 1 mm. in height. They are usually sunken in the mucous membrane and surrounded by a groove and a wall. The latter is somewhat lower than the papilla. Only the upper surface possesses secondary papillse ; the side walls remain free from them. The latter, however, show the end apparatus of the nerves of taste, the so-called taste bulbs. These are some- times found also in the wall on the opposite side of the trench. Their intimate structure is described in the section on Sense MOUTH CAVITY. 167 Organs. Into the trench numerous serous glands (v. Ebner's) open (Fig. 122). At the side of the tongue of some animals (mainly the rabbit) there is found another kind of papilla, the papilla foliata. There is in the rabbit a white area about 1 cm. long, situated on each side of the posterior part of the tongue. It is made up of many papillae foliatae somewhat resembling the circumvallate papillae, separated from one another by trenches or furrows. They are covered with stratified epithelium, and on their adjacent sides are many taste bulbs (see under Organs of Taste). ' The submucosa of the tongue is firm at the tip and along its dorsal surface, but looser elsewhere. The muscles of the tongue are cross-striated. Their arrange- ment will be found in works on gross anatomy. In the frog the muscle fibres are frequently seen to branch. Between the muscle bundles there are glands, fat, and intramuscular con- nective tissue. The lymphoid tissue of the tongue is spoken of under " Lingual tonsils." The blood-vessels are spread out in a capillary network under the epithelium, which is especially well developed in the papillae. The lymphatics have a similar course. The nerves end in part freely between the epithelial cells, and partly in various terminal end organs (Krause's end bulbs, Meissner's taste corpuscles and taste buds). 4. The Tonsils. The adenoid tissue is well developed around the borders of the mouth cavity, forming an organ which Waldeyer has called the lymphatic pharyngeal ring. This tissue may be divided into three main masses, that which is in the tongue {lingual tonsils), that associated with the palate (palatine tonsils), and that situated in the pharynx (pharyngeal tonsils). The lingual tonsils (folliculi linguales) are situated in that part of the tongue between the circumvallate papillae and the epiglottis. They are round masses of adenoid tissue lying in the upper part of the tunica propria, easily visible to the naked 168 MICROSCOPIC ANATOMY OF THE ORGANS. eye, and slightly projecting above the surface of the mucosa. In the centre of these masses is a deep depression, known as Fig. 123. Epithe- lium Tunica propria Lymph nodule' Oblique section '■■,--"■." i^*!!! Vhs. v '•■■ J -t>. _> BE m '..' »!> ; of duct— — ,-r— -,., of mucous gland Muscle fibres cut~. trans- versely Section through a lingual follicle in man. j. crypt. X ^Adenoid tissue Jkratat : .1 50. the crypt. This is a blind canal lined, like the surface of the tongue, with stratified epithelium (Fig. 123). It is dis- tinguished from the epithelium of the tongue, however, by Fig. 124. Leucocytes wand through epitheli in. Epithelium Adenoid tissue in tunica propria ' \ the presence of places where the lymphocytes have pushed their way between the epithelial cells to reach the surface PLATE XII. Epithelium of pharynx Mucous glands< Fig. 125. — Section through a dog's tonsil. At x x there are seen leucocytes which have wandered out from the follicles. X 15. MOUTH CAVITY. 169 (Fig. 124). The lymphocytes which escape at these places form the salivary corpuscles of the saliva. The adenoid tissue under the epithelium is divided into follicles which resemble those of the lymph glands, possessing a germinal centre and a dense periphery. The ducts of the mucous glands of the root of the tongue often open into the crypts. The palatine tonsils have a structure similar to that of the lingual tonsils, with the exception of being much larger and possessing ten to twenty follicles and a number of crypts (Plate XII., Fig. 125). The follicles are situated at about the same level as the tunica propria and possess usually very dis- tinct germinal centres. The epithelium covering them is in many places pierced partly or completely by an encroachment of the adenoid tissue. The crypts always contain a number of lymphocytes and may be branched. The tonsils can easily be seen at the pillars of the fauces, and when inflamed may become so large that they almost or quite meet in the median line. The pharyngeal tonsils lie at the upper part of the pharynx, mainly in the naso-pharynx. Their structure is essentially the same as that of the palatine tonsils. The crypts are clothed often with ciliated epithelium, and are five or six in number. Into these open the mixed glands, which form a distinct layer under the follicles. There is here also a migration of lympho- cytes through the epithelial covering of the organ. It is these tonsils which on hypertrophy form the so-called adenoids which often are found in children. Development of Tonsils. The development of lingual tonsils begins, according to Stohr, in the eighth month of foetal life. Leucocytes wander out from the veins of the tunica propria and infiltrate the loose connective tissue around the ducts of the mucous glands. The further growth of the adenoid tissue thus formed takes place by the continued migration of leucocytes and by mitotic divi- sion of these. The wandering of leucocytes from the lingual 170 MICROSCOPIC ANATOMY OF THE ORGANS. tonsils into the mouth cavity through the epithelium begins early. It may be noticed in the eighth month of foetal life, and increases after this. The palatine tonsils arise, according to His, in a depression which represents the space between the second and third bran- chial arches. This is clothed by the mucous membrane of the mouth cavity. The crypts are formed by the downward growth of solid masses of cells from the epithelium (Stohr), a process which occurs at the end of the fourth month in the life of the human fetus, and continues throughout the entire fetal life and for the first year or two after birth. The solid masses of cells later on become hollow and give rise to the blind canals or crypts of the adult organ. In the connective tissue of the mucous membrane leucocytes begin to gather from the blood- vessels during the third month. This continues up to the time of birth, and it is only during the first year after birth that definite follicles with germinal centres are to be found. 5. Glands of the Mouth Cavity. Under this heading are to be discussed the large salivary glands whose ducts open into the mouth cavity — i. e., the parotid, submaxillary, and sublingual glands, as well as the smaller glands which are named according to their situation. All the glands of the mouth may be divided, according to their products, into : 1, Serous glands, which secrete an albuminous serous fluid ; 2, Mucous glands, which produce a mucin-con- taining secretion ; and 3, Mixed glands, which simultaneously secrete both kinds of fluid. All these glands are tubular. The smaller are simple branched tubular, while the larger are compound tubular glands. The latter are capable of division into larger and smaller lobules, which are separated by connective tissue (Fig. 126). Each lobule contains ducts which divide in its interior. The small lobules correspond with simple branched tubular glands. The ducts in the lobules are more or less curved, so that MOUTH CAVITY. 171 in a section they are cut at various angles. The main ducts, which open into the mouth cavity, are covered by one or two layers of cylindrical epithelium. In the connective tissue which makes up their outer sheath, we often (submaxillary duct) find smooth muscle fibres running longitudinally. The main duct divides into many smaller branches (interlobular ducts), which are lined with a single layer of cubical or cylin- drical epithelium. Each of these smaller ducts passes over into a salivary duct (intralobular duct), which is made up of cylin- drical epithelium, whose cells are characterized by the fact that Fig. 126. ^Salivary duct ■'/-Interlobular ducts ^^^-iuV^/ Connective tissue between lobules From a section through a dog's parotid gland. Several lobules are to be seen. - 22. their basal ends are plainly striated. This striation is due to small granules in the protoplasm, which are arranged in rows (Figs. 127 and 130). While the interlobular ducts are always present in the connective tissue between the lobules, the salivary ducts or intralobular ducts are in the lobule itself. The intra- lobular divides in the lobule, and each division passes over into a so-called intercalary part, or intermediate duct, which is a tube lined with low cubical epithelium (Figs. 129 and 130 ; Fig. 127). Many authors have ascribed to the intralobular ducts secretorv functions, while the interlobular ducts conduct the 172 MICROSCOPIC ANATOMY OF THE ORGANS. secretion. The same secretory function has been supposed to be possessed by the intercalary part. The intercalary part finally passes over into the main glandular tubes. The latter are blind tubes, consisting of a glandular epithelium limited on the out- side by a fibrillar membrana propria, on whose inner surface there are branched cells surrounding the epithelia cells. These are of doubtful origin, and are known as basket cells. The gland cells of a serous tubule at rest possess a proto- plasm filled with highly refractive granules. The nucleus is small, shrunken, and irregular in outline. During secretion Fio. 127. Salivary duct cat^ ^'~""TttP rr '' "" lT* 1 tangential]}} *^~ ** W Intermediate ductx.^: %\ ■■'■J-^. )-XZ &WjWi$m! : ;■{(■■' 1} if iff From a section through a human parotid gland, x 450. the cells decrease in size, and the protoplasm, especially in the part near the membrana propria, becomes free from granules. At the inner side the cells still contain a few granules, while the outer part has a plainly reticular structure. The nucleus becomes round and shows a distinct chromatin network (Fig. 130). The mucous cells have an appearance varying with the con- dition of their functional activity. The empty cells — *. e., those which have been active and have begun to rest — are small and contain a granular protoplasmic network. The round or oval nucleus lies near the membrana propria and possesses a well- MOUTH CAVITY. 173 marked chromatin network. During the formation of mucus the granules increase in size, and finally are converted into fluid material. The meshes of the protoplasmic network become wider as the mucus fills them. The cell grows in size and has a clear, transparent appearance. The nucleus becomes irregular and is pressed into a corner of the cell or against the membrana propria. In the immediate neighborhood of the nucleus there is a small quantity of unchanged protoplasm. During active secretion the mucus escapes from the cell, and the granular protoplasm near the nucleus increases in amount. The Fig. 128. Serous cells of a demilune cut i^' tangentially Cuiuit'etiL'e tissue uith blood-i ^Secreting mucous cell — Empty mucous cell Secretory, capillaries Demilunes of Gianuzzi From a section through a human sublingual gland. (Preparation by E. Krause.) X r>60. nucleus becomes oval and the chromatin framework more distinct, and we have again the appearance of an empty cell (Fig. 128). A mucous cell which has just emptied out its secretion, and a serous cell are very similar in appearance. Usually cells in the same tubule are found in different stages of secretion, so that their appearance is very different. Sometimes a whole tubule is made up of one kind of cell, but in a great many glands both serous and mucous cells are present in the same tubule, and we have then to deal with mixed glands. The parotid in all animals is a purely serous gland ; also the sub- maxillary of rabbits, and the small glands in the region of the 174 MICROSCOPIC ANATOMY OF THE OMGANS. circumvallate papillae of the tongue are serous glauds. The pure mucous glands are usually small and scattered throughout the mouth cavity. The submaxillary and sublingual belong to the mixed glands. In order to study the cell arrangement in both serous and mucous glands, we shall consider that of a typical mixed gland, human submaxillary, in which both are present. A diagram of such a gland is given in Fig. 129. Here one can see an intermediate part of the tube entering in one place serous tubules; in another place it enters a mucous tubule, which becomes composed at the end of serous cells. To the right of the diagram is an intermediate duct entering a mucous tubule which ends blindly. At the end of this there is a cap-like mass of cells resembling serous cells. In section they have the form of a half-moon, and are known as the demilunes of Gianuzzi. The significance of these cells is doubtful. Ac- cording to R. Heidenhain, they are young gland cells which take the place of mucous cells which have disintegrated. No evidence of mitotic or amitotic division has ever been observed in these cells. Other authors regard them as entirely separate secreting cells, which have nothing to do with the mucous cells; while some think they are merely mucous cells which have discharged their secretion. There are sometimes to be observed in these cells the so-called secretory canals or capillaries, which are a continuation of the lumen of the tubule between neigh- boring cells. They are found often in serous tubules, and are sometimes much branched. They possess no wall of their own, and are demonstrated most easily by Golgi's method, in which the whole lumen is filled with the black precipitate. It is highly probable that the demilunes of Gianuzzi have the power of secreting an albuminous fluid ; and if this is the case, it is necessary to consider all those mucous glands which contain these cells as mixed glands (R. Krause). The salivary glands are richly supplied with blood-vessels. The larger vessels run in the connective tissue between the lobules. Here they break up into fine branches, enter the lobules, and surround the tubules with a thick capillary net- ipK : ^/S o :( ^ r^5>->. ; ; H ®.. 5«^ s ; % I ■©: / ^ 1 W£i If © i^ fin ■' Szymonowicz, Histology J. Baropi ad naU del. Lith . Anst. v. Werner S.Winter, Frankfurt fM. Fig. 129. ^Intermediate duct Striated border A = Goblet cells B = Wandering cells G = Nuclei of epithelial cells D = Basal membrane Bj = Intercellular spaces F = Cells of the reticular connective tissue Fig. 129. — Diagram of human submaxillary glandV After E. Krause. Fig. 130.— From a section through a human submaxillary gland. Stained by Biondi'6 method. X 600. After a preparation by E. Krause. Fig. 131. — From a longitudinal section through a villus of a cat's intestine, x 100. wj boots. •d90D(\?. \YjVuS\ w=*V«-l *=* r A ^Mm!I0^ .Itoiliata »'i[>ui>iH '(d r>»ri!*:}8 .bnjjy Y,n:f[i/r,iml>i* ajunud fi d*jUoirfi m>f)-.>'jK £ nnvj r i - .■¥ x .001 v .'jiiitaftai s'lin ,1; Id s.r/lli/ £ rfgnoidl noit ■« iKuibiiJiaiioi i> rmnM- .est .01:1 -. it:x .oil M'i MOUTH CAVITY. 175 work, which is separated from the gland cells by a thin mem- brana propria. Non-medullated nerve fibres form a network at the mem- brana propria, which is pierced by the small branches. They become thickened in a varicose manner around the surfaces of the gland cells. Having described briefly the general characteristics of the glands of the mouth cavity, a few words will be of aid con- cerning the peculiarities of each of these glands. The parotid is a purely serous gland in man and in most animals (Figs. 126 and 127). The secretory capillaries are seen very plainly between the gland cells. The submaxillary is in man and in the majority of animals a mixed gland. In the rabbit it is purely serous. In man it contains more serous than mucous tubules (Fig. 130). The main duct has in the connective tissue a number of longitud- inally disposed smooth muscle fibres. The framework of the submaxillary gland consists of a well-marked capsule with strands of connective tissue extending from it into the gland, dividing it into lobules. Each acinus is surrounded by a deli- cate basement membrane which has a distinctly fibrillar struct- ure (Flint). These basement membranes are continuous with a delicate fibrillar membrane enclosing each lobule. Elastic fibres have been found surrounding the acini of the mucous type. These are absent in serous alveoli. The ducts of the submaxillary have been studied by Flint by means of the corrosive methods. In general the ducts divide like the branches of a tree. The intralobular ducts lie in the centre of the lobule. These pass on into the intercalary ducts, into which the acini empty. The lumen of the acinus has a dilated appearance, like an ampulla, at the end of the intercalary duct (Fig. 132). From three to six ampullae empty into each intercalary duct. Development of the Submaxillary. — The gland appears at a fairly early date as a mass of large epithelial cells arranged partly in columns which represent the developing ducts and alveoli. At the ends of these columns there are knob-like 176 MICROSCOPIC ANATOMY OF THE ORGANS. swellings showing numerous karyokinetic figures. A capillary plexus of blood-vessels develops around the masses of epithe- lial cells. The columns of cells divide many times, and a lumen is formed in them continuous with that of the duct. The interlobular connective tissue develops in connection with the ingrowing blood-vessels. The nerves in the submaxillary are numerous. Some end in Pacinian corpuscles (Krause); some supply the blood-vessels ; while most of them terminate in the secreting alveoli. These latter pierce the basement membrane and form a rich arbores- cence around the alveolar cells (Berkley). Fig. 132. Corrosion specimen of ducts of submaxillary gland of dog. (Flint.) The ducts were injected with celloidin injection mass, and the tissue dissolved away. The sublingual gland, contains no entirely serous .tubules. It is a mixed gland, but in man is in large part mucous. The cells of the intralobular ducts are not striated, as in some of the other glands. The intercalary ducts are narrow, and are lined with a low cubical epithelium. The main ducts are clothed with cylindrical epithelium, and break into many small branches whose walls are made up of cubical cells. These lead to still smaller branches, which end at the demilunes of Gianuzzi in secretory capillaries (Fig. 128). The small glands, which are distributed widely over the mouth and tongue, are tubular and branched, sometimes simple, and sometimes compound. The body of the gland is situated always in the submucosa, often extending down between the muscles. According to their location, we have: glanduhe labiales, buccales, palatinse, linguales, etc. According to their products, PHARYNX. 177 we may distinguish serous, mucous, and mixed glands. They possess neither intercalary nor intralobular ducts. The ducts often are covered at their mouths with ciliated epithelium. Serous glands are found only in the tongue, in the region of the circumvallate papillae. These are called v. Ebner's glands. The ducts open in the furrows surrounding the circumvallate papillae. In these glands also secretory capillaries may be present. Small mixed glands have the structures described for the sublingual gland. Secretory capillaries are plainly to be made out. To these belong the labial and buccal glands, and those glands at the under side of the tip of the tongue, described by Blandin and Nuhn. The palatine glands and the glands at the root of the tongue are purely mucous. B. PHARYNX. The mucous membrane of the pharynx resembles that of the mouth cavity. We find here also a stratified epithelium and a tunica propria with papillae. The stratified epithelium of the nasopharynx is converted in the region of the nasal cavi- ties into a many-layered ciliated epithelium, which is continu- ous above with the ciliated cylindrical epithelium of the nasal mucous membrane. The tunica propria of the pharynx is supplied richly with adenoid tissue, which in places is collected to form the pharyn- geal tonsils. Under the tunica propria there is a layer of elastic fibres running longitudinally, the elastic limiting layer, which is continued down to the oesophagus, where it gradually disap- pears. It lies, for the most part, on the inner surface of the pharyngeal muscles, and sends strong bands of elastic fibres into the intermuscular septa (J. Schaffer). In these places the submucosa is wanting, and the mucous glands extend down and branch between the muscle bundles. In the laryn- geal part the elastic limiting layer is separated from the mus- cle, and here there is a distinct submucosa, in which the glands lie. 12 178 MICROSCOPIC ANATOMY OF THE ORGANS. The outer muscle layer (the constrictors of the pharynx) consists of striated muscle. C. (ESOPHAGUS. In the wall of the oesophagus can be distinguished : mucosa, submucosa, muscularis, and tunica adventitia. The mucosa is similar in structure to that of the mouth cavity. It possesses, however, a thin layer not found in the oral mucous membrane, the so-called muscularis mucosas. This lies at the edge of the tunica propria, between it and the sub- mucosa, and consists of longitudinally disposed smooth muscle cells. Only in the lower half of the oesophagus is it a complete layer. Fig. 133. Epitlielium* Mucous gland^ Mucosa Submucosa Muscularis mm **B|fe \Blood-v jssel i 1% i fm % HE ' K^ .■' Qr?' ! V .Blood- Outer muscle layer — *" ~~ serosa Section through the stomach wall of man (pyloric region). X 14. The gastric mucosa has in the recent state a gray or grayish-red color. The surface is uneven, and possesses certain small depressions, the foveolce e/astrtcce, into which the gas- PLATE XIV. Epithelium of surf/ice--*- ■ ._•-■ ^ -^ l-^WA'fecH : ,1 >l^-;r-MA§rl iJ,.,.M.| .H&-~\ '■- "^'l'",*' .*> - Tunica propria— •-' Gastric crypt Bodies of glands Fundus of ijland Fig. 135. — From a section through the human gastric mucous membrane in the region of the fundus. X 250. STOMACH. 181 trie glands open. In the region of the pylorus there are small folds, called the plicae, villosce. Further, the whole surface often is divided by furrows into polygonal fields, which condition is known as the status mamillaris. This is said to be due to an unequal development of the gastric glands. The mucosa consists, as in the oesophagus, of epithelium, tunica propria, and muscularis mucosae. The epithelium covering the surface of the mucosa is a single layer of cylindrical cells. The protoplasm of that half of the cell toward the surface usually is clear or contains very fine granules, while that of the half next the membrana pro- pria is made up of large, coarse granules. The oval or round nucleus lies generally in the coarsely granular part of the cell. The cells only exceptionally possess a cuticle as in the intestine. At the cardiac end of the stomach the single layer of cylindrical cells passes abruptly over into the epithelium of the oesophagus. Under the epithelium is the tunica propria, which is a loose connective-tissue layer containing a considerable number of leucocytes. The lymphocytes form in some places groups similar to the solitary follicles of the intestine. In the tunica propria are situated all the gastric glands, of which we distin- guish three kinds : Most widely distributed are the true gastric glands ( gl. gas- tricai propria?). These are known also as fundus glands or peptic glands. They are distributed over the whole fundus and body of the stomach, and appear as simple tubular glands (Fig. 135). These often branch, take a slightly curved course, and traverse the whole thickness of the tunica propria as far as the muscularis mucosae. Usually many of these open into one foveola, which is as deep as one-third the thickness of the mucosa, and which may be considered as the gland duct. In the glands one can distinguish a neck and a body. The latter ends blindly, and the lumen of the gland is everywhere quite narrow. The epithelium lining the true gastric glands is made up of 182 MICROSCOPIC ANATOMY OF THE ORGANS. two kinds of cells, the chief cells and the parietal cells (R. Heidenhain). The chief cells, also called adelomorphous cells (Rollett), form the largest part of the gland. These are round or cubical, the form and size depending on their functional activity. During a period of fasting and at the beginning of digestion they are large, while after digestion has proceeded for a certain length of time they become much smaller. In the fresh condition they contain numerous highly refractive gran- ules, which, as in other glands (pancreas, parotid, etc.), disap- pear in the outer zone of the cell during secretion. These granules are supposed by most authorities to consist of a sub- stance, pepsinogen, which is converted into pepsin. The parietal cells (delomorphous), also known as oxyntic cells, are larger and more conspicuous than the chief cells. Fig. 136. Parietal cell Tunica propria. Chief cell- Transverse sections of glands from the fimdus of a mouse. X 300. They are not regularly arranged in the gland tubules, but are scattered here and there in the rows of chief cells. In the neck of the ■ gland they are usually very numerous, and may lie in rows like the chief cells. They are generally only sparingly present in the gland body. Here they are pressed out by the chief cells against the membrana propria, so that they seem to be at the periphery of the tubule. A cross-section of the tubules gives an accurate idea of the relation of these cells to one another (Fig. 136). The parietal cells are round or STOMACH. 183 polygonal, finely granular cells, containing one or two spher- ical nuclei. They are smallest in fasting and increase in size during digestion. In the fresh state they are clearer than the chief cells, while in fixed preparations they are much darker and less clear than these. They show a special affinity for such stains as eosin, Congo-red, neutral carmine, etc. Those parietal cells which are not situated directly on the gland lumen are connected with it by a secretory duct, which breaks up into a number of secretory capillaries. These sur- round the cells like a basket-work, and also project into its interior. The cells which are situated along the edge of the gland lumen do not possess a duct, as their secretory capillaries empty directly into the gland lumen (Figs. 137 and 138). Fig. 138. Longitudinal section of a fundus gland of a mouse. Golgi impregna- tion, x 125. From the fundus glands of a mouse. Basket-shaped plexuses of capillaries are seen to surround three oxyntic cells and to open into the gland lumen. X 600. Golgi's method is of special service in the investigation of these capillaries. During digestion they are wider, being filled with secretion. It is supposed generally that the parietal cells have the property of secreting the acid contained in the gastric juice. The pyloric glands {gl. pyloricce) are distinguished from the 184 MICROSCOPIC ANATOMY OF THE ORGANS. fundus glands by the facts that they. branch more frequently ; that they take a more curved course; and that the foveolse into which they open are very deep. Besides these things, they consist entirely of chief cells. Between the fundus and the pylorus there is a transition zone or intermediate zone in which both forms of glands are present. This is not definite, for the parietal cells are found frequently in man even in the region immediately around the pylorus. In many cases no part of the stomach is free from them. The so-called cardiac glands are present in that region of the stomach around the oesophageal orifice. They are com- pound tubular glands, whose elements closely resemble those of the pyloric glands. Parietal cells seldom are found. In this region, as well as in the pyloric area, there are found not infre- quently cells resembling those of the intestine — i. e., cells with a striated cuticle, and also goblet cells. These tubules, resem- bling Lieberkuhn's glands, do not extend so deeply in the tunica propria as the cardiac glands. The membrana propria which limits the epithelial layer of the mucosa, is a thin membrane on whose inner surface there are often to be observed flat branched cells. Where the glands lie close to one another the tunica propria is very ineonsjiicuous. Under the tunica propria is the muscularis mucosas, which consists of smooth muscle cells crossing one another, but arranged usually in two or three layers parallel to the surface. The tunica submucosa consists of fine connective tissue which contains a considerable number of elastic fibres. Fat cells, blood-vessels, and ganglion cells are seen also. The latter belong to the so-called Meissner's plexus, which is present throughout the alimentary canal. The true muscle coat of the stomach, the muscularis, con- sists of three layers. The fibres of the innermost sheath run obliquely ; the middle coat is circular, while the outermost layer is disposed longitudinally. A thickening of the inner and middle layers forms the sphincter pylori. The serosa consists of a thin layer of connective tissue cov- ered by a layer of endothelial cells (see Peritoneum). PLATE XV. l WS'^^k>M:M "Epithelium of surface ^«I .^ ;: i' ; iiif y'-'.i ' m r . ;', ; ";.;s i ."- '■'.■*•;'."- ,••'• Pyloric glands -i. ; -:'-J-'«Y m I '/')'J r'^'i ■ '■'■■< Oblique section of pyloric glands Solitary follicles fer ='.-..: -JiasrcLta- ' -ATuscularis mucoste Fig. 139. — From a section through the human gastric mucous membrane in the pyloric region, x 100. PLATE XVI. ViUn Gland of Lieberl'tthh : 'S'ir Br tinner's gla Blood-vessel- Circular muscle laye Ganglion cells of Auerbach' s plexus Longitudinal muscle layer 2^K5p" Fig. 140. — From a longitudinal section through the duodenum of a cat. X 34. PLATE XVII. ■ . •:■',' "'-X.\ r-,-v\'.'fj -.■ JP .*■'■' V.'Vi eft *'*!*• ^ l' 1 * ' , ' 1 * j1 — Axis of allu mPm V'* 7 r i> t 7 *\ i-ell V''">'\ ; V.:-.';S». 1 Villus epithelium Smooth muscle cells Gland of Lieberkuhn Oblique sections of glands ^^u^;^^ol;-^^x; JfuscHtaris mucosa; Jarii.j Fin. 1 J 1.— Section through the mucous membrane of ;i cut's jejunum, x 115. Histology plates INTESTINE. 1 85 E. INTESTINE. In the intestine we can distinguish the same number of coats as in the stomach (Fig. 140), namely, mucosa, submu- cosa, muscularis, and serosa. The surface of the mucosa is nowhere in the intestine smooth. It possesses two kinds of inequalities, whose function is to increase the area of the surface. There -are ring-like folds of the whole mucous membrane, the so-called valvules conniventes (plicae conniventes Kerkringii), which are developed especially in the upper part of the intestine. Besides these there are the villi, which are folds simply of the epithelial layer and the tunica propria, the muscularis mucosa continuing in a straight line below them. These are found only in the small intestine (Plate XVII.). They reach a height of 0.2-1 mm., and vary considerably in form according to the region of the small intes-' tine in which they occur. In the duodenum they are leaf-like ; in the jejunum and ileum cylindrical and somewhat thickened at the end. They lie more closely together in the duodenum than elsewhere. We find also, in the intestine, cavities in the form of simple tubular glands (Lieberkuhn's glands) which enter the depths of the tunica propria at the bases of the villi. These are longer in the large, than in the small, intestine. The mucosa of the whole intestine consists of a single layer of epithelium, a tunica propria, and a muscularis mucosae. The cells of the epithelial layer (Figs. 131 and 143) are cylindrical with a finely granular protoplasm, often containing many kinds of granular inclusions. The nucleus of each cell is oval and lies usually in the lower half. The sides of the cell show no definite cell membrane, while at its free surface it shows a char- acteristic finely striated border. These are known as border cells. The opposite end of the cell often runs to a point, and is separated from the underlying tissues by a thin homogeneous basal membrane. The epithelium of the glands is not essentially different from that of the villi. The cells are somewhat lower and the striated border is not so well marked. Among these epithelial 186 MICROSCOPIC ANATOMY OF THE ORGANS. cells we find both on the villi and in the glands mucus-pro- ducing cells, the so-called goblet cells (see under Epithelium). The cells full of secretion possess no true cell membrane, but only a thickened ectoplasm, which undergoes no mucoid change and corresponds with the crusta of F. Schulze (Fig. 131). These goblet cells are unevenly distributed, but are especially abundant in the large intestine. It is not fully understood whether the goblet cells are a different kind of cell or a modified form of the cylindrical cells. Some authors claim that every young cylindrical cell has the power of changing into a goblet cell, and that a cylin- drical cell is really a resting goblet cell. Most writers, how- ever, believe that the two kinds of cells are separate and distinct, and that there is only a superficial resemblance between the resting goblet cell and the cylindrical cell. Many hold that mucus can be produced by any of these cells. In fasting, the number of goblet cells increases ; likewise during active digestion, as also in poisoning with pilocarpine, they become more numerous. In connection with the regeneration of these cells, Bizzozero has observed that many karyokinetic figures are found in the glands, and almost none on the villi. Thus Lieberkiihn's glands seem to be a place of regeneration for epithelium which has been destroyed by oversecretion. Bizzozero claims that these new cells are pushed up to the villi from the glands, and the differences in form of the cells is due to their age. On this theory may be explained the great abundance of goblet cells in the large intestine. Those on the villi are destroyed quickly, and must be replaced by cells formed in the glands. This condition is not present in the large intestine, owing to the lack of villi there, and the goblet cells accumulate. The epithelial cells are joined together by protoplasmic bridges, which are best seen in horizontal sections of the epithelial layer — i. e., cross-sections of the cells. Between the bridges are spaces which can be demonstrated by treatment with silver nitrate. They are filled with the so-called cement substance. The main function of the intestinal epithelium is PLATE XVIII. Brunnersy fcj..s.r\'!i: •;• J, :•;;: .: |j.-; ,j._i;s ;: :;.--v.;' : -v tj.i.--, r" t4* Fwidu . *- '' * ^ g , I*: &ffi* i - ; - jf* >"• '■ 'if ;*" 5' ? 1 pN &■ *> Sotftarv fb^'c/e — ,, , ..; • ->-'% J^,'£: -"V'^V.vVj- 1:S;;ir7.J^\^K^J Muscularis mucosae m>'. Br miner's ghuulsi ■■ : : V : -;^"' W *. 1! f "'^ Si.tbmucosa- MuseulaH$" " ; _ -£$*:■* ^-X^i '; Fig. 142. — From a section through a cat's duodenum. The entire submucosa, Brunner's glands, and the adjacent parts of the mucosa, are shown, x 100. INTESTINE. 187 that of absorption, which can best be observed in the digestion of fat. By treatment with osraic acid preparations can be made which show all stages of this process. In what form the fat enters the cells is unknown, but it is probable that it is not as an emulsion, but as fatty acids formed by combination with the bile salts. In the epithelial cells the fatty acids are converted again into neutral fats. It then appears in the intercellular Fig. 143. Tunica ]>roi Blood capilla b**Nucleus of tramlo'iiHi cell _! Nucleus of smooth muscle cell ^»i Central lymph space ^>Goblet>cells Longitudinal section through the end of a villus from the small intestine of a cat. X 450. spaces in the form of fine globules which pass through the basal membrane. From here it reaches the lymph spaces of the parenchyma of the villus, and finally enters the central chyle vessel or lacteal. This power of fat absorption is not possessed by the epithelium of the large intestine. The second important function of the intestinal epithelium, namely, that of secretion, is carried out in great part by the 188 MICROSCOPIC ANATOMY OF THE ORGANS. goblet cells which produce mucus. It is probable, however, that the other cells of the glands of Lieberktihn secrete a spe- cific substance which is a constituent of the succus entericus. The tunica propria consists of a reticular connective tissue which contains a varying number of lymphocytes and other leucocytes. These are in some places collected in masses 1-2 mm. in diameter, which lie either singly {solitary follicles) or are grouped together {Peyer's patches or agmin- ated follicles). The solitary follicles are distributed through- out the whole alimentary canal, but are found especially in the intestine. Their development begins always in the tunica propria and extends through the whole mucosa down to the muscularis mucosae. They often cause a bulging on the sur- face of the epithelium, and not infrequently break through the muscularis mucosae to enter the submucosa. The villi and glands usually are distorted in these regions. In the submu- cosa there is a smaller resistance to the growth of the follicle, and it comes therefore to have a flask-like form, with the large end in the submucosa and the neck in the mucosa. The struct- ure of the solitary follicles is similar to that of the follicles of a lymph gland. A germinal centre is always present, and the newly formed lymphocytes proceed from this place out to the periphery of the follicle. There they enter the lymphatics or at the surface escape between the epithelial cells into the lumen of the intestine. The Peyer's patches (Fig. 144) are met with in the ileum, more particularly near its junction with the jejunum. These are oval, and sometimes several centimetres in length. They may consist of as many as sixty follicles lying so close to one another that they usually are compressed and deformed. Often adjacent follicles coalesce, so that the follicle thus formed seems to have two or more germinal centres, as is seen in the appendix vermiformis. The follicles reach the surface of the intestine and are covered by the columnar epithelium, but there are seldom found villi immediately on them. The submucosa is separated from the tunica propria by the muscularis mucosas, which is a thin laver of smooth muscle INTESTINE. 189 fibres, the innermost of which run circularly, while the outer fibres take a longitudinal course. From the inner layer muscle fibres run between the Lieberkiihn's glands into the villi. Fig. 144. Lymph nodides Submucosa Muscularis Transverse section through a Peyer's patch from a cat's small intestine. X 25. These are supposed on contraction to shorten the villi and to aid in forcing the chyle, etc., from the villi to the large lymph- atic vessels. The submucosa consists of firm connective tissue, with glands only in the region of the duodenum. These are the so-called Brunner's glands (Figs. 140 and 142). They are branched tubular glands whose entire bodies are situated in the sub- mucosa, and whose ducts pierce the muscularis mucosae and open between, or into Lieberkiihn's glands. These are occasion- ally found, not only in the duodenum, but also in the pyloric end of the stomach, just as pyloric glands are sometimes found in the duodenum. The Brunner's glands are recognized easily by the fact that they break through the muscularis mucosae and into the submucosa. The cells of Brunner's glands are cylin- 190 MICROSCOPIC ANATOMY OF THE ORGANS. drical, finely granular, and much like those of the pyloric glands. During secretion they are smaller and less clear than when no food is being digested. The blind ends of the gland tubules are dilated often like those of alveolar glands. Around the tubules there is to be seen a structureless basement membrane. The muscularis consists of an inner circular and an outer longitudinal layer of smooth muscle fibres. In the large intes- tine the outer layer is very thin in general, but is thickened in three strong flat bands, which are called the teenies coli. In certain places the circular layer is thickened also, especially at the opening of the rectum, where it forms a strong circular band, the musculus sphincter ani internus. The different regions of the intestine are distinguished easily from one another microscopically. The duodenum is character- ized by the presence of Brunner's glands and leaf-like villi ; the jejunum and ileum, by the absence of these and the presence of columnar villi. The ileum can usually be distinguished from the jejunum by the greater abundance of lymphoid tissue. The large intestine is characterized by the complete absence of villi, the abundance of goblet-cells, and the dispo- sition of the external muscle coat. Blood-vessels, Lymph-vessels, and Nerves of the Stomach and Intestine. The arrangement and relation of the blood-vessels in the stomach and large intestine are so similar that they may be described together. In the small intestine the presence of villi causes a considerable difference. The arteries enter the intestinal wall from the outside and pass through the outer layers to the submucosa. On the way small branches are given off to the peritoneum and the muscularis, to form capillary networks in these regions. In the submucosa the arteries break up to form a network of large vessels parallel to the surface. From these arteries branches pierce the muscu- laris mucosae and form a second finer network in the tunica propria, which gives off branches to make up a capillary plexus PLATE XIX. II Fig. 145.— Blood-vessels and lymphatics of stomach. (F.Mali.) M, mucosa; M\, nius- cularis mucosse ; 8, submucosa ; / and 0, circular and longitudinal muscles. A, blood- vessels ; B, microscopic anatomy ; C, lymphatics. X 70. )k r > H X INTESTINE. 191 which surrounds the gland tubules, and passes over into a venous network which is situated in the tunica propria. From here the veins enter the submucosa, where they join to form large vessels which leave the intestine by paths similar to that taken by the arteries in entering. From the submucosa also other branches from the large arteries pass downward into the muscular coats. The relations of the vessels of the stomach as demonstrated by F. P. Mall are shown in Figs. 145 and 146. In the small intestine there are small arteries proceeding from the subglandular network to enter the villi. One or sometimes two arteries run in the centre of the villus to its end, giving out on the way side branches which form a capillary network. The branches of this network join near the per- iphery of the villus to form veins, which descend to join the subglandular plexus of veins. The Brunner's glands are surrounded by a network of capil- laries derived from the submucous branches. The lymph folli- cles gain their blood supply partly from the submucous branches and partly from the plexus in the tunica propria. The beginning of the chyle vessels is between the glands in the stomach and large intestine, and in the axis of the villi in the small intestine. In the upper part of the villus the lymph- vessels end blindly and show a certain degree of anastomosis. These join to form the central chyle vessel or lacteal. Around the Lieberkiihn's glands there are numerous lymphatics, which form a thick network below. This is in combination with a second coarser network in the submucosa. The efferent vessels pass through the muscularis, collecting the fluid from numerous lymphatics in the muscle and from a lymphatic plexus between the muscle layers. The lymph-vessels leave the intestine between the two layers of the mesentery. Around the follicles the lymph-vessels form a network with sinus-like dilatations. The nerves of the alimentary canal arise mainly from the sympathetic system. The non-medullated fibres enter at the mesenteric border, pierce the external muscle layer, and form a peculiar plexus between this and the internal muscle coat. This is called the plexus myentericus or Auerbach's plexus. Where 192 MICROSCOPIC ANATOMY OF THE ORGANS. the fibres making up the large meshwork come together, there are enlargements consisting of many multipolar ganglion cells, from which new non-medullated fibres proceed. These cells are to be observed in sections from any part of the alimentary canal, as large cells with much protoplasm staining brightly in eosin, and a large vesicular nucleus with well-marked nucleolus. From this plexus branches are sent into the submucosa, where they form a second network, finer and more delicate than the first, known as Meissner's plexus. In this the meshes are smaller, the fibre bundles more delicate, and the cell groups not nearly so large. From this plexus fibres run throughout the submucosa, and end also in the muscularis mucosae and the mucosa. They extend into the villi and end under the-epi- thelium in small swellings. F. PANCREAS. The pancreas is, like the salivary glands, a compound acino- tubular gland divided by connective-tissue septa into lobules. Two ducts, the duct of Wirsung (ductus pancreaticus) and the duct of Santorini (ductus pancreaticus accessorius), conduct the external secretion to the intestine. These are lined with a single layer of cylindrical epithelium, which is surrounded by connective tissue containing small mucous glands. The inter- lobular ducts pass directly into narrow intermediate ducts (Fig. 147) lined with flat epithelial cells. The latter pass over into the secreting end tubules of the gland. The glandular cells of these end tubules resemble those of the serous tubules, as, for example, in the parotid gland. They are rounded cells with highly refractile granules on the side toward the lumen of the tubule. These are called zymogen granules. The nucleus lies in the outer non-granular part of the cell. The number of zymogen granules, as well as the relation between the inner granular zone and the outer clear part of the cell, varies according to the condition of the gland. During digestion the granules gradually vanish and the cell becomes clear. In fasting, the granules, on the contrary, increase in number, and the granular inner zone takes up PANCREAS. 193 more than half of the cell. The granules thus seem to be a stage in the formation of the secretion. Here, as elsewhere in serous glands, secretory capillaries are present. In the secreting cell a structure has been described by M. Nussbaum, in amphibians, as the Nebenlcem (paranucleus). This is a small body lying between the nucleus and membraua propria in the non-granular part of the cell. It is oval or twisted in form, and is stained easily. In animals that have fasted for some time it seldom is found. The function and significance of this structure are entirely unknown. Fig. 147. termediate duct Centroacincir cells From a section of a dog's pancreas. X 175. In the centre of the end tubules one often finds flat cells, the so-called centro-acinar cells (Langerhans). These must be considered as a continuation of the epithelium of the inter- calary ducts into the lumen of the gland tubules (Fig. 148). The tubules are surrounded by a membrana propria, which contains basket cells. The processes of these are intimately connected with the gland cells. In the centre of each lobule there can be observed with low powers of the microscope light-staining areas. These were described first by Langerhans, and are known generally as the 13 194 MICROSCOPIC ANATOMY OF THE ORGANS. islands of Langerhans. Their structure lias been described by E. L. Opie. According to him, they are most numerous in the splenic end of the pancreas. He describes them as " com- posed of cells having the same origin as those of the glandular acini," and richly supplied with blood capillaries. In injected specimens the capillaries stand out from the surrounding tissues like a glomerulus. It has been shown fairly conclusively that the islands of Langerhans have to do with the internal secre- tion of the gland and the control of the storing up and excretion of sugar (Opie). The framework of the pancreas consists of a network of white and elastic connective-tissue fibres. The interlobular septa contain blood-vessels and ducts, which, however, do not run side by side, as in the salivary glands, but enter the gland and the lobules at different points (Flint). The lobules are marked off by interlacing connective-tissue fibrils, and with these the basement membranes surrounding the acini are continuous. The lobules are polyhedral in shape, but do not possess a hilus, as in the salivary glands. The framework of the islands of Langerhans is made up of fine interlacing fibrils supporting the groups of cells and the capillaries. This structure is shown in a drawing by Flint (Fig. 149). The nerves of the pancreas are almost entirely non-medul- lated. They enter the gland with the arteries and ramify between the epithelial cells of the alveoli. Small ganglia have been observed in their course. G. LIVER. The liver is a compound tubular gland in which the tubules are joined by numerous anastomoses. It consists of many lobules, which are separated by a continuation into the gland of the connective tissue of the capsule of Glisson, which surrounds the whole organ. This is known as the interlobular connective tissue. The lobules have the form of rounded or polygonal prisms, and in section appear usually as polygonal fields, which in some animals (pig, camel) are marked off very definitely by a strongly developed connective-tissue framework (Fig. 150). LIVER. 195 The lobules are usually to be seen plainly on the surface of the liver. Each lobule shows a radial arrangement of the liver cells. These columns of liver cells radiate from the central vein in the middle of the lobule, and are separated from one another by blood capillaries. Both the capillaries and the columns of cells anastomose frequently with one another. The gland cells of the liver are polyhedral, membraneless structures, whose protoplasm is fibrillar and contains fine gran- ules. In the periphery of the cell the protoplasmic network is dense and possesses small meshes, while near the centre the meshwork becomes looser and more open. The protoplasm often Beginning of interme- — diate duct Connective tissue From a section of a cat's pancreas. X 580. contains fat and bile droplets, glycogen, and pigment granules. The cells contain usually one, but often two, round nuclei. Between the liver cells run the bile capillaries in such a manner that they touch always two or more cells (Fig. 151). There is thus always a part of a cell between each bile capillary and the nearest blood capillary. When many cells surround a bile capillary, it may be compared with the lumen of a sali- vary gland tubule. When the capillary is between only two cells, it appears as a small groove in each cell (Fig. 151). When more than two cells touch the capillary, the latter is situated at the angles of the cells. The bile capillaries possess no distinct wall of their own, 196 MICROSCOPIC ANATOMY OF THE ORGANS. except that formed by the liver cells between which they are situated. It seems that the bile capillaries begin in the interior of the cell in canals like those of the secretory capillaries in the parietal cells of the fundus of the stomach (Fig. 152). According to Browicz, the beginning is in the nucleus, for he succeeded in finding bile droplets there. In favor of the intra- protoplasmic origin of secretory capillaries is the fact that secretory vacuoles in the protoplasm are in connection with the bile capillaries. - Fig. 149. ■»cz. im s/^: 1 :^ Framework of a lobule of tbe human pancreas, showing the connective tissue of an island of Langerhans. (Flint.) By means of Golgi's method, it is possible to demonstrate the course of the bile capillaries and the presence of secretory vacuoles. The latter begin as small droplets of bile in the cell, which on reaching a certain size become discharged into the bile capillaries between the cells. They represent only transi- tory structures depending on the activity of the cell. Others hold that these are stable intracellular bile paths, which often contain bile and sometimes do not (Browicz). In mammals the bile capillaries anastomose with one another, forming a network in which the liver cells lie. The LIVER. 197 latter are surrounded by the capillaries, for these run along many surfaces of the cells. These capillaries join to form interlobular bile ducts, which are lined with low cubical epithelial cells possessing a refractile cuticular border. At the outside of these there is a homogeneous membrana propria. The wall of the large bile ducts consists of a single layer of cylindrical epithelium and a connective-tissue capsule. m ■■■< ... .- -■-- ■-■■ Fig. 350. &. Transverse section of a lobule from a pig's liver, showing the vena centralis in the centre, and the interlobular connective tissue around the whole lobule. X 35. The interlobular connective tissue is, as we have said, a continuation inward of the fibrous capsule of Glisson, which consists of fibrous and elastic connective tissue. Only a very little of this tissue enters the lobule itself. Here the framework is made up of the so-called " Gitterfasern " of Oppel. These are fine radially arranged fibrils surrounding the blood capillaries, and are entirely identical with the true reticulum described by Mall. The liver contains blood-vessels from two sources (Fig. 156). The arterial blood from the hepatic artery forms only a small part 198 MICROSCOPIC ANATOMY OF THE ORGANS. of the circulation. The larger part is venous blood entering the liver from the vena portse. This blood not only brings the materials to be stored up in the liver, but also nourishes those parts of the liver not reached by the arteries. Fig. 151. lj^&^ /-'. fcvT. V,'' From a thin section through the liver of a siredon. «, blood capillary. The small passages are bile capillaries, x 325. The vena portce (Fig. 155) divides in the interlobular con- nective tissue into branches — interlobular veins — which form at the periphery of the liver lobule a capillary network, in the meshes of which lie the columns of liver cells. The capillaries Fig. 152. Nucleus Much dilated bile passage Liver cell with two nuclei, from a human liver in which there is a damming back of the bile. The intracellular bile passages are much dilated. (Preparation by Browicz.) proceed from all sides of the lobule toward the central or intra- lobular vein. The central veins in turn open into the sublobu- lar veins, which run along the bases of the lobules. Many LIVER. 199 of these sublobular veins unite to form the hepatic veins which carry the blood into the inferior vena cava (Fig. 156). The arterial blood supply of the liver is much smaller. Fig. 153. Nucleus 'acuoles Liver cell from a dog. In the nucleus a haemoglobin crystal is to be seen ; in the Tacuoles of the cell body brown needle-like crystals of methaemoglobin are found. The latter are due to the entrance of fluid haemoglobin into the liver cells after intravenous haemoglobin injection. (Preparation by Browicz.) X 700. The branches of the hepatic artery break up in the interlobular connective tissue, and there form small networks around the larger bile ducts and enter the liver lobule in a direction similar Fig. 154. Bile capillaries in the liver lobule of a rabbit. (Chrome-silver method apillaries X 80. to that taken by the venous capillaries. Some of these enter the venous capillaries and some proceed as far as the centre of the lobule to empty into the central vein. The capillary net- 200 MICROSCOPIC ANATOMY OF THE ORGANS. work surrounding the bile ducts in the interlobular connective tissue forms veins which enter the interlobular veins. It will be seen from the above descriptions that there are in the liver two units, a secretory and a blood vascular unit. The former is quite definite, and has for its centre one of the small interlobular connective-tissue spaces in which an interlobular bile duct is present: In these spaces there is also usually an artery and one or more veins. The periphery of the secretory unit varies considerably in outline, but can always be marked by lines drawn between all the nearest central veins. It thus takes in parts of at least three and sometimes several liver lobules. The bile capillaries of these lobules run in different directions toward the ducts into which they empty, so that those of one liver lobule may belong to many secretory units. The blood vascular unit is less definite, for the organ is built up around the venous system more than the arterial. Taking the arterial system as a centre, the vascular unit would be much like that described for the biliary system. With the veins, however, a much more definite unit is formed, which can be taken in two ways according as we consider the entry or the exit of the blood. Taking the interlobular veins as a central point, units can be mapped off which include parts of various liver lobules, as in the secretory unit. If, however, the central vein be considered as the centre, the unit would correspond exactly with what is known generally as the liver lobule. This is shown in Figs. 155 and 156. The liver lobule itself is a unit formed by the division of the organ by connective-tissue septa. The framework of the liver includes the parenchyma in the form shown by these lobules. As shown in Fig. 153, the blood pigments contained by the body and nucleus of the liver cell may, under certain condi- tions, become crystallized. In connection with the walls of the intralobular blood capil- laries may here be mentioned the stellate cells of v. Kupffer. At first these were considered as perivascular connective-tissue cells, but in later years it has been determined that they belong to the endothelial coating of the intralobular blood capillaries. PLATE XXI. Fig. 155. — Blood-vessels of three liver lobules of a rabbit. In the centre of each lobule is a central vein ; at the periphery, the interlobular veins. X GO. Fig. 156.— Diagram of the liver. Three lobules (I., II., III.) are to be seen. The bile passages are black, the arteries red. and the veins blue. r. i., vena interlobularis ; A, duct. The direction of the circulation is indicated. LIVER. 201 They are large, finely granular cells, possessing phagocytic properties. They are found containing foreign materials, and red and white blood-corpuscles. The lymph-vessels form a thick plexus in the capsule of the liver, which sends branches into the interlobular connective tissue. From between the lobules fine lymph-vessels proceed along the intralobular capillaries, not as closed channels, but as perivascular lymph spaces. These surround the blood capil- laries and stand in close relation with them. The relations of the lymphatics of the liver have been studied by F. P. Mall, upon whose description the following account is based : The forcing of a colored fluid into the bile duct causes an injection of the liver lymphatics. This is accomplished through the perivascular lymph spaces sur- rounding the blood capillaries. The walls of the blood capil- laries consist of a layer of interlacing reticulum fibrils, upon which is placed an incomplete layer of the endothelial cells of v. Kupffer. The capillary walls are thus quite porous, and there is but little resistance to the passage of fluids from the capillaries into the perivascular spaces. By filling the blood- vessels of the liver with a colored injection mass, an injection of the perivascular spaces and lymphatics is also brought about. The perivascular space communicates directly with what Mall terms the perilobular space which exists between the liver cells at the periphery of the lobule and the interlobular connective tissue. The perilobular space in turn communicates with the lymph radicals by means of the interlobular connective-tissue spaces. " There are no direct channels connecting the peri- vascular and perilobular spaces with the lymphatics proper other than the ordinary spaces between the connective-tissue fibrils of the capsule of Glisson" (Mall). The nerves of the liver are in large part non-medullated. They form plexuses in the interlobular connective tissue around the blood-vessels and bile passages. Some of the branches from these networks end in the interlobular struct- ures, while others enter the lobules, to accompany the bile 202 MICROSCOPIC ANATOMY OF THE ORGANS. capillaries and end between the liver cells. At their extreme ends the nerve filaments show varicosities (Berkley). The hepatic, cystic, and common bile ducts are the larger channels concerned in the conduction of the bile to the intes- tine. They consist of a mucosa, submucosa, and muscularis. The mucosa consists of a single layer of columnar epithelium and a tunica propria which contains small saccular mucous glands and a few smooth muscle fibres. The submucosa is a thin connective-tissue layer. The muscularis has been studied in the whole extra-hepatic biliary system by Hendrickson. According to him, all these ducts possess a distinct transverse longitudinal and diagonal layer of smooth muscle arranged in a somewhat plexiform manner. In the folds of the cystic duct known as the Heisterian valve, muscle is also present. The transverse fibres of the cystic duct run in a circular direction in the valve, as though the wall at this level had been invagin- ated. Most of the longitudinal fibres continue down the duct, but a few turn into the valve almost at risfht angles. The diagonal fibres do not at all enter the valve. At the entrance Fig. 157. Macerated duodenal portion of the common bile duct of man. All of the intestinal coats have been removed. S, sphincter fibres. (Hendrickson.) x 5. of the common bile duct into the intestine at the duodenal papilla an accumulation of the smooth muscle takes place, to form a sphincter. Fig. 157, taken from Hendrickson's work, shows the arrangement of the muscle fibres in the sphincter. At the junction of the duct of Wirsung (If) and the common bile duct (B) there is a circular disposition of the fibres forming the LIVER. 203 sphincter (S). From this, certain fibres (X) run down along the sides of the intestine. Others (K) run from one side of the common bile duct to the other surrounding the duct of Wirsung. Gall Bladder. The gall bladder has been studied recently by M. T. Sudler, and the following account is based largely on the description given by him. The wall consists of the following coats : mucous, fibro-muscular, subserous, and serous. The mucous coat is somewhat corrugated on its surface, the folds corresponding with ridges in the underlying fibro-muscu- lar coat. They are covered by a single layer of columnar epi- thelial cells. No goblet cells are present. Fat droplets have been observed in these cells after chyle absorption. A few mucous glands are found in the tunica propria of the mucosa. The fibro-muscular coat is composed of a framework of con- nective tissue in which bands of smooth muscle are laid down. According to Hendrickson, there are no definite layers of muscle in the gall bladder. Others have described three indefinite layers, of which the thickest runs transversely. The portion of the fibro-muscular coat just beneath the mucosa is made up almost entirely of connective tissue. It corresponds with the submucosa of many organs. In it there are solitary lymph follicles and many blood- and lymph-vessels. The subserous coat is made up of interlacing bands of elas- tic tissue fibrils. The serous coat is the reflection of the perito- neum on the surface of the gall bladder. The blood-vessels penetrate the bladder wall and divide in the fibro-muscular coat near the subserous layer. Arterial branches are given off to the mucosa, in which there is formed a fine network. Fine branches run also to the subserous and serous coats. The veins collect in the fibro-muscular coat. Over the surface of the gall bladder run large lymphatics, which are derived from the liver and from the coats of the gall bladder. In the subserous layer there is a network of irregu- lar lymph channels which receive the lymph from a plexus of 204 MICROSCOPIC ANATOMY OF THE ORGANS. smaller lymphatics in the submucous tissue (Sudler). These are shown in Fig. 158. The nerves of the gall bladder are both medullated and non- medullated. Sympathetic fibres, according to Huber, supply the blood-vessels and smooth muscle of the wall. Dogiel has Fig. 158. Reconstruction of wall of a dog's gall-bladder. (Sudler.) A, artery ; V, vein ; L, lym- phatic. X 60. described ganglion cells in this situation. According to Huber, medullated sensory fibres are found near the large arteries and distributed to the mucous membrane. The development of the various structures in the liver has not been worked out thoroughly. The lobule is formed late in the growth of the embryo. The portal and hepatic veins are at first at opposite ends of the organ, and the regions of tissue around them have nothing to do with one another. Later on, by a shifting of some sort, and a new formation of vessels, they come to have the intimate relation seen in the adult liver. Much of this change has taken place already in the human embryo by the fourth week. As the diverticula grow out from the mid-gut of the embryo to form the first rudiments of the liver a primitive bile duct is established. By a branching of this the large ducts of the organ are formed, but it is uncertain whether the bile capil- PERITONEUM. 205 laries are formed from these or have a separate origin and later become connected with them. This subject was worked over by Hendrickson. According to this author, the capillaries cannot be demonstrated by the Golgi method in pigs' embryos less than 5 cm. in length. In these only a few appear imme- diately around the large branches of the portal vein. In human embryos 5 cm. in length the network of capillaries is considerably more extensive. In older embryos the main capil- laries gain side branches, and those encircling different portal branches finally meet. The meshes of the network in the places where it first appears are smaller than where they are subse- quently formed. This is due to the division of the older meshes by side branches. In some of the older embryos the capillaries are seen to be continuous with a larger vessel in the region of the interlobular vein, which probably represents the interlobu- lar bile duct. H. PERITONEUM. The peritoneum lines the whole abdominal cavity and is reflected over the organs contained therein. As it passes out to the organs (e. g., the intestine) it forms a double layer, known ■as the mesentery, and on the surface of the organs themselves it is spoken of as the tunica serosa. The peritoneum is a thin membrane consisting of a connec- tive-tissue layer and a single layer of flat endothelial cells. The latter cover the free surface, and are usually polygonal in outline. The cement lines between the cells can be made out readily in specimens treated with nitrate of silver ; and by special methods, especially that proposed by Kolossow, struct- ures which generally are understood to be protoplasmic bridges can be demonstrated. The outlines of the cells are often wavy or quite irregular. The connective-tissue layer consists of interlacing connec- tive-tissue bundles, containing numerous elastic fibres and con- nective-tissue cells. The peritoneum is bound to the underlying parts by means of a connective tissue rich in fat and elastic fibres. The so-called subserous connective tissue is developed more strongly in some places than in others. In the intestine and 206 MICROSCOPIC ANATOMY OF THE ORGANS. liver it is so scarce that one cannot distinguish it as a separate layer, and the serosa seems to be a part of the organ upon which it lies. The blood supply of the peritoneum is made up of an extensive capillary network. The lymph-vessels can be seen especially well in the mesentery of an animal which has recently had a fatty meal. Here they stand out as a white net- work of anastomosing vessels. The nerves are non-medullated, and end either freely or in the form of Pacinian corpuscles (see Nerve-endings). III. RESPIRATORY SYSTEM. A. LARYNX AND TRACHEA. The mucous membrane of the larynx, like that of the tra- chea, consists of a ciliated epithelium, whose cilia move in the direction of the pharyngeal cavity. In the true vocal cords and on the posterior surface of the epiglottis is found a strati- fied pavement epithelium. In these places the tunica propria forms no papillae. The tunica propria is a connective-tissue sheath, containing elastic fibres and leucocytes, which vary in quantity in different places. Solitary follicles are not often seen. At the border of the epithelial cells is a basement membrane (membrana propria). This represents a thickening of the subepithelial connective tissue. In the tunica propria there are many smooth muscle cells, which in the posterior part of the tracheal wall are strongly developed and join together the ends of the C-shaped cartilage rings. The submucosa contains a number of branched tubular mucous glands, which are largest in the posterior wall of the trachea, and here often penetrate into the muscle layer. The cartilaginous framework of the larynx and trachea is made up of hyaline cartilage, with the exception of the carti- lage of Wrisberg and Santorini, and the median part of the thyroid cartilage, which are made up of elastic cartilage. The blood- and lymph-vessels form wide networks parallel to the surface. The nerves show small ganglia in their course, X X id < -J H ggj \'^M BRONCHI AND LUNGS. 207 and end partly under and partly in the epithelium. On the lower surface of the epiglottis there are small taste buds present. B. BRONCHI AND LUNGS. The trachea divides to form the bronchi, of which the largest are quite similar in structure to the trachea. The mucous membrane is thrown into longitudinal folds, and covered on the surface with a many-layered ciliated epithelium containing a considerable number of goblet cells (Fig. 159). The mucous membrane of the smaller bronchial branches con- sists of a single layer of ciliated epithelium. The tunica pro- pria consists of connective tissue with elastic fibres and leuco- cytes. The smooth muscle cells here form a circular layer. The mucous glands break through this muscle layer, and are first absent in bronchial twigs as small as 1 mm. in diameter. This is also about the place where cartilage ceases to exist in the bronchi. In larger bronchi the cartilage has the form of half rings, while in smaller branches it usually appears as irregular plates which are arranged on all sides of the wall. Toward the outside the cartilage masses are surrounded by a fibrous mem- brane which contains elastic tissue, blood-vessels, and nerves. In bronchioli 0.5 mm. in diameter the cartilage and mucous glands are absent, and the mucous membrane consists of a single layer of ciliated epithelial cells, among which are mingled many goblet cells. The muscle layer surrounds these bronchioles as a circular sheath, and during contraction throws the surface into longitudinal folds. By a division of the bronchioles there are formed the respi- ratory bronchioles, from which thin-walled diverticula, the alveoli, are developed (Fig. 160). These are covered by the so- called respiratory epithelium. The epithelium at the beginning of the respiratory bronchioles is ciliated, and becomes gradually cubical and then flat. The respiratory epithelium, which con- sists of flat non-nucleated cells, begins in the form of small islands among the low cubical epithelium of the respiratory bronchioles. W. S. Miller has given us a new conception of the lobule 208 MICROSCOPIC ANATOMY OF THE ORGANS. of the lung and ,is relation to the blood-vessels. The following notes are based on his work. He speaks of the last division of the bronchus before it breaks up into the lung parenchyma as the terminal bronchus. From this a number of other passages Fig. 160. Alveolar I ducts \ _ Respiratory bronchiole n Bronchiole- Section through the lung of a cat. The respiratory bronchiole divides to form two alveolar ducts. X 50. lead, which are connected by a central chamber, known as the vestibulum (Fig. 161). These passages open into the atria, which communicate by means of the air-sac passages with the various air sacs. Around the periphery of the air sacs are the air cells. This can be seen in Fig. 161. The terminal bronchus (average diameter, 0.4 mm.) con- tains smooth muscle cells in its walls. It is lined with columnar epithelium. Between this and the atria are the BRONCHI AND LUNGS. 209 vestibula, which are 0.2 mm. in diameter. Three to six of these arise from the end of each terminal bronchus. Smooth muscle fibres do not extend beyond the vestibule, but are found surrounding it like a sphincter. Several atria communicate with each vestibule. These are thin-walled chambers resembling the air sacs in their possession of a network of blood capillaries. Opening from each atrium are two or more air-sac passages, which average 0.143 mm. in diameter. The atria and air-sac passages contain no muscle cells. The air sacs are irregular in shape, with an average size of 0.511 mm. by 0.313 mm. The Fig. 161. Terminal bronchus of a mammalian lung. (Miller.) s, air sac; A, atrium; B, terminal bronchus ; V, vestibule ; P, air-sac passage. The artery is shaded and the vein is in outline. walls are thin, and are made up of capillaries and a little con- nective tissue covered by flat epithelium. Irregular, thin- walled diverticula from the air sacs are the air cells. These are lined with cells of two kinds: delicate irregular cells lying over the blood-vessels, and small flat polygonal cells over the meshes of the capillary network. Similar air cells may arise from the bronchus and from the atrium. Those on the bronchi have an average diameter of 0.047 mm., and those on the atrium and air sac 0.113 mm. The respiratory bronchiole of some authors leads into two 14 210 MICROSCOPIC ANATOMY OF THE ORGANS. or more terminal bronchi, which are the same as the alveolar ducts. The alveolar sacs or infundibula correspond with Miller's air sacs. The term alveolus is applied to the air cells (Schulze, Kolliker). The interlobular connective tissue contains many elastic fibres, and often a considerable amount of pigment, such as coal-dust breathed into the lungs. These foreign particles are carried away by the lymphatics to the lymph glands at the base of the bronchi, where usually they are retained. The pleurae consist of connective tissue containing a good deal of elastic tissue, and are covered on their free surface by flat endothelial cells. The pulmonary artery, carrying venous blood to the lungs, breaks up into many branches, which accompany the bronchi. Fig. 162. Part of a section of an injected lung from a rabbit, x 300. Tbe alveoli are seen from the surface ; at a an alveolus is cut through. The arterial end twigs form a capillary network which sur- rounds the alveoli (Fig. 162). One terminal twig usually supplies several alveoli. From this capillary network venous branches proceed to the bases of the bronchi and carry out the arterial blood to the pulmonary vein. During its passage through the capillary network surrounding the alveoli the venous blood absorbs the oxygen of the air through the walls BRONCHI AND LUNGS. 211 of the alveoli, and gives out in turn gases which are to be eliminated. The gases pass through the vascular epithelium, the connective tissue between the vessels and the wall, the basal membrane, and the respiratory epithelium. The arterial blood supply to the lung is accomplished by the bronchial arteries. These break up into small branches, which supply the bronchi, the interlobular connective tissue, and the walls of the pulmonary vessels. There are numerous anastomoses between the bronchial and pulmonary systems of blood-vessels. A part of the blood of the bronchial arteries thus leaves the lungs through the bronchial veiny, and a part through the pulmonary veins. The branches of the pulmonary artery follow the bronchus to a point beyond the terminal bronchus. As has been described by W. S. Miller, the branches at this point divide to send twigs to each atrium. From these a capillary network is formed, which surrounds the air sacs and air cells. On the peripheral side of the air sacs and air cells the capillaries gather to form the veins, which remain at the outside of the lobule, as shown schematically in Fig. 161. The network which is shown in Fig. 162 is the richest capillary plexus in the body. It is thus seen that the lobule of the lung forms also a blood vascular unit, with the artery in the centre and the veins at the periphery. An exception to this is formed by two small veins arising from near the end of the terminal bronchus. Lymphatics in the lung and bronchi have been studied by Miller. In the bronchus the lymph-vessels form a network which extends as far as the end of the terminal bronchus. Here branches are sent to the pulmonary artery, to the two small veins in this region, and to the veins that run to the pleura. Nerves. — Nerve fibres follow the bronchi into the lung sub- stance. These consist of both medullated and non-medullated fibres. The sympathetic fibres show small ganglia in their course. These nerves innervate the muscles and mucous mem- branes of the bronchi, and also the walls of the blood-vessels. 212 MICROSCOPIC ANATOMY OF THE ORGANS. No nerve-endings have been found in the walls of the air sacs. Berkley has described arborizations of fine fibrils upon and between the cells of the alveoli. IV. URINARY SYSTEM. A. KIDNEYS. The kidney is a compound tubular gland ; but it may be considered as an alveolotubular structure, since the urinary tubules are dilated at their ends to form the capsules of Bow- man. There can be distinguished in this organ a medullary and a cortical substance, a marked difference existing between the two parts, in the course and structure of their tubules (Figs. 163 and 164). The medullary substance consists of a number of cone-like divisions, the so-called Malpighian pyramids, whose apices extend down into the pelvis of the kidney as papillae. In man the number of these pyramids varies from seven to twenty. In many other mammals there is only a single pyramid and one papilla. These pyramids are made up of straight tubules extending radially from the apex of each papilla to the border of the cortex. From the medulla the straight tubules extend up into the cortex in conical masses, known as the pyramids of Ferrein, or medullary rays. It will be noticed that the Mal- pighian pyramids are many times as large as the pyramids of Ferrein, that their apices point in different directions, and that their bases are approximated. Further, the pyramids of Fer- rein are situated in the cortical region, while the Malpighian pyramids make up the medulla. Each tubule has its origin in the cortex in the region between the medullary rays, in a sac, the capsule of Bowman, into which is pushed a mass of blood capillaries, the glomerulus. The capsule of Bowman with the tubule may be compared with a rubber tube possessing at the end a bulb, the wall of which has been invaginated from the outside by a body repre- senting the glomerulus. The space in the invaginated sac is the beginning of the lumen of the urinary tubule. The por- tion of the tubule next to the Bowman's capsule is known as the PLATE XXIII. -Pelvis of kifhtet/ Fig. 163.— Longitudinal section through a. part of an ape's kidney, x 13. PLATE XXIV. IP '£ C€%\W Q;'' ^''gfeP ■ Dt Interlobular )» '#/,?: SV?S diarize: Fig. 164. — From a longitudinal section through the cortex of an ape's kidney. X 55. Two medullary rays are seen, and between them the Malpighian corpuscles and convoluted tubules. An artery runs through the centre. PLATE XXV. rteria } interlob Vena \ ulari.i '-—Vena \ iuteiiob* Arteria j ularis Fig. 165. — Diagrammatic representation of the course of the urinary canals (left) and the ..-■• -/kidney vessels (right). The arteries are red, the veins blue ; capsules of Bowman, convoluted tubules I. order and loops of Henle are black ; convoluted tubules II. order and collecting tubules, gray. I., II., III., IV., four kidney lobules: «, vas afferens ; e, vas ert'crens. 1, Bowman's capsule; 2, convoluted tubule I. order; IS, descending limb of loop of Henle; 4, ascending limb of loop of Henle ; o, convoluted tubule II. order ; 6, 7, collecting tubules; H. papillary duct. KIDNEYS. 213 convoluted tubule of the first order. At the beginning of this there is a slight constriction. After taking a very tortuous course between the medullary rays, and forming what is called the labyrinth of the kidney, these convoluted tubules become much narrower and enter the pyramids of Ferrein. Here they take a straight course as far as the border of the medulla, and then turn abruptly on themselves, become considerably thicker again, and proceed upward toward the surface of the kidney, always remaining, however, in the medullary rays. This straight tubule is known as Henle's loop, of which there are the descending and the ascending arms (ramus descendens et ascendens). The ascending arm of Henle's loop passes over into the intermediate tubule or convoluted tubule of the second order, which leaves the medullary ray and takes a tortuous course in the labyrinth similar to but much shorter than that pursued by the convoluted tubules of the first order. From the labyrinth the canal passes back into the medullary ray as the connecting tubule. Similar tubules enter the pyramids of Ferrein from all sides and open into the larger collecting tu- bules, which run down through the medulla and join near the apex of the Malpighian pyramid to form the so-called papil- lary ducts, which open out in the area cribrosa of the papilla in from ten to twenty orifices. Each of these different parts of the urinary canal has a characteristic structure. Everywhere there is a single row of epithelium with a fine structureless membrane, the membrana propria. The capsule of Bowman is related to the glomerulus in such a way that the latter is covered closely by the inner wall of the capsule, while the outer wall passes over into the wall of the convoluted tubule of the first order (Fig. 166). The walls of the capsule are made up of a layer of flat epithelial cells with a membrana propria composed of reticulum (Mall). The cap- sule, of Bowman, together with the glomerulus, forms what is known as the Malpighian corpuscle, which has a diameter of from 130 to 220 (i. A reconstruction of the glomerulus of a human kidney has been made by W. B. Johnston by the Born 214 MICROSCOPIC ANATOMY OF THK ORGANS. wax-plate method. In this he found that the afferent vessel breaks up into five branches. These form a network of capil- laries which anastomose in such a manner that three main groups are formed : a median and two lateral groups. Capil- laries from these in leaving the glomerulus form two main branches, which join to make up the efferent vessel. Fig. 166. >. Afferent and efferent vessels t Convo- • luted { tubules Bowman's eapsnle, outer part Beginning of From a section through the cortex of an ape's kidney. A Malpighian corpuscle, together with the beginning of the urinary canal, is shown. ■ 350. The convoluted tubule of the first order (38-41* : « in diam- eter) is lined with cubical epithelial cells. Near the capsule of Bowman we find a transition from flat cells to the cubical type. The protoplasm of the cubical cells is finely granular, and shows in the part of the cell toward the lumen a definitely striated appearance. In the rest of the cell the granules are arranged radially in rows. The bordei's between the epithelial cells usually cannot be made out (Fig. 167). KIDNEYS. 215 The part these cells take in the secretion of urine, and the changes that take place in them during this process, have not definitely been made out. In secretion the cells become lower and the lumen of the tubule wider than during rest. Secretory capillaries have not been demonstrated. The descending arm of Henle's loop is a thin-walled tube made up of flat epithelial cells, whose nuclei bulge out into the lumen. The cells are so arranged in the tubule that in a lon- gitudinal section they alternate on the two sides — that is, two cells are never opposite one another. In cross-section the canal is not unlike a blood capillary. The membrana propria usually Fig. 167. Cross-section of a convoluted tubule from the kidney of a rabbit. The boundaries of the epithelial cells cannot be seen. Only three nuclei are shown. The rod-like structure is plainly visible, x 1100. is seen distinctly, and the whole diameter of the canal is from 9 to 15 p. The ascending arm of the loop of Henle is considerably thicker, being 25 ^ in diameter. The epithelial cells are cub- ical, and the size of the lumen narrow in relation to the thick- ness of the walls. The transition from the flat cells to the cubical takes place usually in the lower part of the descending arm of the loop. The cubical cells show the striation spoken of in the convoluted tubules. The convoluted tubules of the second order are much shorter and have a wider lumen than those of the first order. The canals are 39-46^ in diameter. The epithelial cells are low, and show a finely granular and striated appearance. 216 MICROSCOPIC ANATOMY OF THE ORGANS. The connecting tubules, collecting tubules, and the ductus papillares have clear, transparent cells, showing no striated structure. In the beginning they are cubical, but as the canal widens into the papillary ducts they become columnar (Fig. 168). The nucleus is always spherical, and sharply Kig. 168. Ab. H, From a trausverse section through the base of a pyramid of an ape's kidney. S. R., col- lecting tubule ; Ab. H., descending limh of Henle's loop; Af. H., ascending limb of Henle's loop; Bl. , blood-vessels ; Bd., interstitial connective tissue. X 500. marked off. The diameter of the papillary ducts is as much as 100 [i. Zimmermann found in the cells of all regions of the canal a double centrosome lying near the free surface of the cell. The cortical substance may be divided into kidney lobules. These consist of all those Malpighian corpuscles and tubules which go to form one medullary ray. At the boundaries of each lobule there run the interlobular vessels. This is the secretory unit, and its periphery is formed by the beginnings — ■i. e., the capsules of Bowman — of all the tubules which empty finally into the collecting ducts that run in the medullary rays (Fig. 165). There is more or less overlapping in this lobular division, and there is no definite separation of the lobules. KIDNEYS. 217 Besides this secretory unit, there is a blood vascular unit, which is made up of all those glomeruli and vessels which are con- nected with each interlobular artery and vein (Fig. 165). The connective tissue of the kidney is not abundant, but is found in greatest quantity in the papilla?. It surrounds the membranse propriae of the urinary tubules and the capsules of Bowman, and carries with it the blood-vessels. The whole kidney is surrounded by the tunica albuginea, a fibrous mem- brane containing smooth muscle. Mall, some years ago, stated that the framework of the kidney is made up of interlacing connective-tissue fibres, which are differentiated at the borders of the tubules to form basement membranes. Such membranes appear in ordinary specimens to be homogeneous, but by methods of digestion (pancreatin) they can be shown to be fibrillar in structure. This has been confirmed by the work of Ruhle, Disse, and v. Ebner. A later publication by Mall shows that the true basement membrane is destroyed by pan- creatin digestion. This leaves only a framework of connective tissue, as stated above. Specimens were obtained by macerat- ing in cold saturated sodium bicarbonate solution, in which not only this framework, but also a membrane closely associated with the epithelial cells was demonstrated. These membranes are neither elastic tissue nor reticulum. Mall suggests that they are possibly identical with the membranes of elastic fibres. Blood-vessels of the Kidney. The kidney derives its blood supply from the branches of the renal artery. The relations between these and the calyces and pyramids of the kidney have been described by Brodel. According to him, about three-fourths of the blood which enters the hilum of the kidney by four or five arterial branches, flows through the anterior subdivisions of these branches, while one-fourth is carried through the smaller pos- terior divisions. This is shown in Fis;. 171. The anterior branches supply the anterior pyramids and the anterior part of the posterior pyramids ; while the remainder of the organ is supplied through the posterior branches. The blood supply 218 MICROSCOPIC ANATOMY OF THE ORGANS. of the two poles of the kidney is derived from the main artery. Single arteries run to each pole and break up into three branches : a posterior, an anterior, and a median branch. Between these large arteries there is no anastomosis ; and it will be seen that at one place between the anterior and pos- terior arterial fields there is a comparatively non-vascular zone, marked by the dotted line in Fig. 171. Brodel has pointed out also the surgical importance of this fact. The branches of these arteries run to the kidney substance between the pyramids as the interlobar arteries. At the boundary between the medulla and cortex these bend over and run for a short distance parallel to the surface of the kidney Fig. 169. Glomerulus from an injected human kidney, showing vas efferens and vas afferens. x 160. (Figs. 165 and 170). In this way an arterial arch is formed, made up of the arcuate arteries. From the convex side of these arteries small branches proceed radially toward the kid- ney surface. These are the so-called interlobular arteries. They give off in all directions lateral twigs, which carry blood to the Malpighian corpuscles (Fig. 165). These are the vasa afferentia, which enter the capsules of Bowman and break up into many branches to form the glomerulus (see above). The blood is carried away from each glomerulus through the vas efferens. Many vasa efferentia together break up to form a capillary network in the region of the medullary rays. The tubules of the medullary rays and the tubuli contorti lie in the KIDNEYS. 219 meshes of this network. From it arise small veins which open into the interlobular veins. These run parallel with the inter- lobular arteries, and at the boundary between the medulla and cortex open into the arcuate veins. Into the most peripheral Fig. 170. The renal artery and the distribution of its branches in relation to the pelvis. (Brodel.) Anterior view of a left kidney. There are six main branches seen entering the kidney substance. Only one of these (the third) passes posterior to the pelvis at the hilum ; also small arteries coming from the upper and lower main branches are seen to pass posterior to the upper and lower calyces. All the rest of the arteries pass anterior to the pelvis and its calyces. The small branches to the cortex of the anterior portion of the kidney have not been drawn, iu order that the large branches and the pelvis might appear more distinctly. part of the interlobular veins there run small veins from the surface. These possess radial, star-like tributaries on the surface, and are known as the stellate veins of Verheyn (Fig. 165). The medullary substance is supplied partly by capillary branches from the cortex, and partly from the arteriolar rector. The latter are branches partly from the vasa efferentia of the 220 MICROSCOPIC ANATOMY OF THE ORGANS. more deeply lying glomeruli, and partly from the interlobular or arcuate arteries. The meshes of the capillary network which arises from these two sources and supplies the medulla are elongated and surround the collecting tubules. The capil- laries collect to form the venulce rectce, which end in the arcuate veins. It must be noted also that the vessels of the kidney parenchyma are in communication with those of the perirenal fat by means of the vessels of the kidney capsule. A collateral Fig. 171. ;/ Transverse section through the middle of the same kidney (Fig. 170), seen from above. (Brodel.) The anterior branch of the artery supplies about three-quarters of the kidney substance, while the posterior branch supplies only one-quarter. circulation is thus possible. There are also direct communi- cations between the arteries and veins of the kidney (Hoyer, Steinach, etc.). According to Brodel, the collecting veins form anastomoses around the bases of the pyramids and around the necks of the calyces. The lymphatics form a superficial plexus in the capsule and a deep plexus, the vessels of which leave the kidney at the hilum. Anastomosing lymphatic spaces have been observed connecting the two plexuses. The nerves accompany the blood-vessels into the kidney, where they form plexuses around the uriniferous tubules. According to Azoulay and Berkley, they penetrate the mem- brana propria, and end by knob-like thickenings on the sur- faces of the epithelial cells. PLATE XXVI. 11V' theli V- Blood-vessels Fibrous layer Fat cells Fig. 172. — Part of a transverse section of a dog's ureter. X 110. URINARY PASSAGES. 221 B. URINARY PASSAGES. (a) Kidney Calyces, and Pelvis ; Ureter and Urinary Bladder. In all of these parts of the apparatus which conducts urine from the kidney we find the walls made up of the fol- lowing layers : 1, mucosa ; 2, submucosa ; 3, muscularis ; 4, fibrosa. The mucosa consists of an epithelium and a tunica propria. The former is the so-called transitional epithelium, and is quite similar in all parts of the canal, so that in pathological condi- tions of the tract where the cells appear in the urine it is diffi- cult to say from what part these cells are derived. The cells differ, however, in the various layers. The uppermost layer consists of large cubical or somewhat flattened cells ; the middle layers of cylindrical, pyriform, spindle-shaped, or polygonal cells ; and the deepest layer of relatively small cubical or irreg- ularly oval cells. The cells of the first two layers often possess processes which extend between neighboring cells. Those of the outer row may contain more than one nucleus (Fig. 173). Fig. 173. End of epithelial cell ? jj,s?T^>- si* • " r-i ($3>5^r!, (jfs ~*M\ Qj j,<1^_^Nucleua of connective-tissue ^/r& m*- ■©.;- ^^M0 ^"f stroma From a section of the ovary of a human embryo in the third month. X 540. The structure of the glandular part of the organ and the individual egg follicles can best be understood by a study of the development of the ovary. The egg follicle' arises from the germinal epithelium. The first part of the development occurs in embryonic life, while the ripening of the egg does not take place until puberty. The cells of the germinal epithelium increase by division, and some of them develop into large cells rich in protoplasm, with large nuclei and nucleoli. These are called the primordial ova (Fig. 183, A). The germinal epi- thelium grows together with the primordial ova into the under- lying stroma (Fig. 183, B), and gives rise to the column-like 240 MICROSCOPIC ANATOMY OF THE ORGANS. structures, the Schlauche of Pfluger. The primordial ova are thus collected into groups, called egg nests (Eiballen). The egg nests are divided into smaller cell groups by the ingrowth of connective tissue. In each of these primordial follicles we rind at least one ovum, and often three or four, which are surrounded by a layer of germinal epithelium cells, the follicular cells. Later, each primordial follicle contains only one ovum, partly because the others disintegrate, and partly because a follicle containing more than one ovum is usually split up by connective tissue into as many follicles as there are ova. The follicular cells tend to increase greatly in number. Further changes which usually occur in post-embryonal life consist in the great increase in the follicular cells by karyo- Fig. 184. " -~'. >>v-' 1 ' ~~ . -~z~. %■- -^- ■■-. . *^. ,. "^--^ Stroma ovarii t'khl cular •pithelium From a section through the cortex of an ape's ovary, x 150. kinesis and the production by these of several layers around the ovum (Fig. 184). In the layers of follicular cells there occur during the growth of the follicle certain changes. The ovum increases in size and there is developed around it a deli- cate membrane — the zona pelludda — which, according to some, is a product of the follicular cells, while others hold that it arises from the ovum itself. At the same time the egg proto- i OVARIES. 241 plasm stores up in itself nourishing material in the form of a granular substance, so that the greater part is converted into the so-called deutoplaxm. Thin layers around the nucleus and at the periphery of the ovum remain unchanged. The deuto- plasm and protoplasm together form the yolk The excentrically lying nucleus of the ovum is spherical, clear, and vesicular, and possesses a distinct nuclear membrane with a double contour. On account of this structure, the nucleus is known also as the germinal vesicle (vexicula germ- Fig. 185. Them folliculi Meinbrana__ granulosa f'f Antrum f.'V* folliculi | '£ with liquor ^;,y folliculi i'U; Cumulus fci v- oophorus h • Ovum vith V zona pellu- \ ckla, yermi- ~ nal vesicle, and germi- nal spot lift;- -W m Blood-_ vessel ■■^■Vr'W.- stftii 'MSB. & Section through a Graafian follicle from an ape's ovary, x 90. vnativa). In the chromatin network is present a distinct nucleolus, which is called also the germinal spot (macula germ- inativa), and in which amoeboid movements have been observed (Nagel). At the same time, changes take place in the follicle, begin- ning in a collection of serous fluid between the follicular cells (liquor folliculi). This is contained in a cavity, which gradually becomes larger, and is known as the antrum folliculi. The fluid is due partly to a transudation from the vessels snrround- ifi 242 MICROSCOPIC ANATOMY OF THE ORGANS. ing the follicle, and partly to a liquefaction of certain of the follicular cells. In consequence of the increase in this fluid the ovum is pushed to one side (Fig. 185), and the whole Graafian follicle (folliculus oophorus vesiculosus) becomes as large as 5 mm. in diameter, and is seen bulging from the surface of the ovary. The follicular epithelium lining the interior of the follicle in many layers is known as the stratum- granulosum (membrana granulosa). At one place it forms a hill-like mass, which con- tains the ovum (Fig. 185), and is known as the cumulus oophorus or discus proligerus. At this period the membrana pellucida surrounding the ovum becomes thicker and shows a radial striation, which was at first thought to be due to a system of pores running through the membrane. Later investigators (Paladino and Retzius) claim that the striation is caused by the passage of fine processes of the follicular cells through the zona pellucida, after the manner of protoplasmic bridges. In this way there is established a close connection between the ovum and the follicular cells, which is of importance in the nourish- ment of the egg cell. Between the ovum and the zona pellucida there is a small space, known as the perivitelline space. Thus the ovum may turn inside the zona pellucida. Sabotta has described the zona pellucida in the mouse as a quite homogeneous membrane with- out any striation whatever, and disputes also the existence of a perivitelline space. Outside the zona pellucida there is a layer of cylindrical fol- licular cells arranged radially. These form the so-called corona radiata. The whole Graafian follicle is surrounded by a con- nective-tissue capsule, the theca folliculi. Between this and the follicular epithelium there is a structureless basal membrane (membrana propria folliculi, Glashaut). In the theca folliculi there are to be distinguished two layers: the tunica interna, consisting of round or spindle-shaped cells; and the tunica externa, which is made up of circularly disposed connective- tissue fibres. The formation of the Graafian follicle begins before puberty, OVARIES. 243 and often some stages are found to have been completed in the newborn and in foetuses. The above-described ovum, however, is not yet capable of being fertilized. In order to reach this stage, it must undergo the ripening processes, which consist in the so-called reduction of chromosomes. The extrusion of both polar bodies in lower animals has been discussed in treat- ing of fertilization in general. In higher animals (including man) the ripening takes place in the ovary. The second polar body is extruded shortly before the bursting of the follicle and the escape of the ovum. The theca folliculi come in contact with the tunica albuginea of the ovary as the follicle moves to the surface. The cover- ings of the follicle become gradually thinner, but the true reason for the rupture of the follicle is not clear. It is probable that many forces act simultaneously. The increase in the liquor folliculi, the marked congestion which is characteristic of the tissues in ovulation, the swelling of the ovary, and possibly the contraction of smooth muscle contained in the stroma, may help in this process. At the same time the walls of the follicle at the place of bursting become thin and atrophic on account of the obliteration of blood-vessels by pressure. Meanwhile the connection between the ovum and the cells of the discus proligerus and membrana granulosa becomes looser, and finally disappears, so that the ovum comes to lie in the liquor fol- liculi. During the bursting of the follicle the liquor folliculi as well as the ovum is cast out into the peritoneal cavity. After the ovum has escaped, there is always a certain amount of blood which fills up the empty follicle. This becomes a closed cavity containing a blood-clot, which begins to undergo organization. This is known as the corpus hcernor- rhagicum. The organization takes place by a formation of fibrin, and the ingrowth of the so-called lutein cells from the periphery of the follicle. The origin of the lutein cells is not clearly understood. They were described first in 1827 by v. Baer, who considered them as a derivative of the theca interna cells. Later on, Bischoff studied this subject, and came to the conclusion that they were derived from the fol- 244 MICROSCOPIC ANATOMY OF THE ORGANS. licular epithelium making up the membrana granulosa — i. e., from the epithelium. There are many adherents to each of these views, but the balance of evidence seems to be in favor of v. Baer's theory. Other theories have been advanced, but have gradually been abandoned. J. G. Clark has studied the subject, and believes that the lutein cells are specialized connective-tissue cells derived from the theca interna. Accord- ing to him, they appear in the inner layers of the follicle wall when a differentiation into theca interna and externa is beginning. Whatever the origin of the lutein cells may be, it is certain that the corpus haemorrhagicum is invaded on all sides by large yellow cells containing fatty granules {lutein) ; and that by this invasion the blood-clot is replaced by a definite cellular tissue, the whole making up the corpus luteum (Fig. 186). The lutein cells give to the body a yellowish color, and often there are found orange-red lisematoidin crystals, which are the remains of the blood-clot. According to Clark, the lutein cells in the growing follicle increase at the expense of the cells of the theca interna, and there is also present a network of true reticulum stretching from the theca externa among the lutein cells and collected into a membrane next the tunica granulosa to form the membrana propria folliculi. When the follicle ruptures, this membrane is broken through by the growth of lutein cells and blood-vessels. As soon as the corpus luteum has reached its highest develop- ment, certain changes take place in the cells and the retro- gression begins. Fatty degeneration in the lutein cells is fol- lowed by an increase in the connective tissue. The septa become thicker and all the connective tissue of the corpus luteum shrinks to form a firm, compact body, which is known as the corpus albicans or c. fibrosum. This becomes always more contracted, like scar tissue, and finally undergoes hyaline degeneration and is lost in the ovarian stroma. We. distinguish corpora lutea vera and corpora luiea spuria according to whether they arise from follicles whose eggs have become fertilized or not. There is no difference in the intimate OVARIES. 245 structure of these, but the corpora lutea vera, in consequence of the marked hypersemia of the ovary during pregnancy, are larger. The corpora lutea vera as well as the corpora albicantia resulting from them remain longer in the ovary, because their retrogression is slower than in the corpora lutea spuria. Fig. 186. a^- V^'v J '.« ^Lutein cells Part of a corpus luteum of a bitch . X 300. It is to be noted that only a small proportion of the ova in the ovary become ripe. According to Henle, of about 72,000 ova in the ovaries of one individual, only 400 arrive at maturity. The rest undergo degenerative changes which represent an entirely physiological process, known as follicular atresia. This depends on a series of changes not only in the organ itself, but also in the follicular epithelium and the theca. ^46 MICROSCOPIC ANATOMY OF THE ORGANS. In the beginning a chromatolysis or karyolysis takes place in the nucleus. The chromatin becomes granular, and finally is dissolved and the nuclear membrane disappears. On the other hand, the nucleus may undergo simple atrophy. In the cell body, at the same time, fatty degeneration sets in, and the pro- toplasm becomes gradually liquefied. The zona pellucida swells, and finally is dissolved. These changes in the ovum are followed immediately by similar degenerations in the folli- cular cells. The absorption and disappearance of the dead cells are brought about mainly by phagocytic wandering cells. This destruction of cells often leads to a new formation of tissue in the theca interna, consisting in the production of a fibrillar connective-tissue scar (Schottlander). Among the cells sometimes are found karyokinetic figures (Flemming). The blood-vessels of the ovary arise on the arterial side from the ovarian and the uterine arteries. Branches of these enter the medulla through the hilum, and take a characteristic tor- tuous, corkscrew-shaped course. They divide many times, and the smaller branches diverge to the peripheral part of the medulla, where they form a rich plexus. From this, branches enter the cortex, and, spreading through the stroma, form capillary networks in the theca folliculi. The lymph-vessels surround the Graafian follicle with a network and leave the ovary through numerous wide trunks in the hilum. The nerves, partly medullated and partly non-medullated, enter the ovary through the hilum, following the course of the blood-vessels, in whose walls a great many fibres end. Other fibres reach the germinal epithelium and surround the follicles with dense networks. According to Retzius, and others, the nerve fibres do not enter the follicle, while Riesc and v. Herff have found the nerve-endings between the follicular epithelial cells. Among the rudimentary organs found in the neighborhood of the ovary and derived from the Wolffian body are the epoophoron (parovarium, organ of Rosenmiiller) and the paroophoron. The first lies in the broad ligament at the hilum GENITO -URINARY SYSTEM OF THE EMBRYO. 247 of the ovary, and has the form of many coiled blind tubules lined with ciliated epithelium. The paroophoron lies more medially, and consists of similar convoluted canals. The first is homologous with the epididymis, and the second with the paradidymis in the male. Genito-urinary System of the Embryo. The first part of the genito-urinary system to appear in the embryo is the Wolffian duct. The origin of this duct is doubt- ful. According to some authors (Hensen, v. Spee), it is derived from the ectoblast. Others believe it arises from the meso- blast ; His and Kowalewsky, from the middle plate ; and Remak, Kolliker, and Waldeyer, from the lateral plate of the mesoblast. Rensen, Dansky, and others derive it from the coelomic epithelium. It is at first a solid rod of cells, which subsequently develops a lumen lined with epithelium-like cells. Tubules develop from this duct and form the Wolffian body. This embryonic organ was observed first, in 1759, by Wolff, who considered it the embryonic stage of the permanent kid- ney. Ratlike (1825) first used the term Wolffian body in connection with this organ in birds, and called the correspond- ing organ in mammals, Oken's body. Jacobson, in 1824, termed it the primordial kidney, and recognized that it excreted uric acid, which was carried into the allantois. The Wolffian body of mammalian embryos is a somewhat pyriform body symmetrically placed in the abdominal cavity. In early embryos it is, next to the liver, the largest abdominal organ. It consists of a tubular and a glomerular part. The glomeruli are situated medially, while the coiled tubules form the largest part of the organ. These come off from the Wolff- ian duct at right angles to it, and after a considerable coiling are connected with the glomeruli by means of end dilatations similar to the Bowman's capsules of the permanent kidney. In the human embryo the tubules have a somewhat S-shaped course. In the pig's embryo, on the contrary, the tubules are much convoluted. Their exact course has been 'determined (MacCallum) by means of injections into the Wolffian duct, and 248 MICROSCOPIC ANATOMY OF THE ORGANS. by the construction of wax models after the method of Born. In genera], there are two parts in the tubule, a secreting and a collecting segment. This was recognized first by Joh. Miiller. In the pig the collecting tubule possesses two convoluted parts, while the secreting portion is a large loop in the central part of the organ. The epithelium is characteristic in these two parts, being low and cubical in the collecting segment, and columnar in the secreting portion. The blood supply of the Wolffian body is derived directly from the aorta. This has been worked out in pigs' embryos (MacCallum). The arteries enter at the medial border of the gland and break up to form the glomeruli. From these many efferent arteries proceed in a radial manner toward the periph- ery. Around the tubules they form a fine capillary network, which empties into three series of veins. Two of these run on the periphery of the organ toward the medial border, over the dorsal and ventral surfaces, respectively. The other series of veins leaves the Wolffian body by the same path as that taken by the arteries in entering. A distinct blood vascular unit can be observed. At a certain stage in the development of the embryo, which differs in different species, the Wolffian body begins to undergo retrogression. The tubules degenerate, and the glomeruli become occluded. The anterior tubules alone in the male remain connected with the Wolffian duct, and grow in size and complexity to form the head of the epididymis. The tail of the epididymis and the vas deferens are derived from the Wolff- ian duct. The posterior tubules of the Wolffian body form the paradidymis or organ of Giralde. In the female the Wolffian duct degenerates. The anterior part persists usually with the parovarium. When the whole duct persists, it is known as Gartners canal. The Wolffian body in the female persists in its anterior (sexual) part as the parovarium (epoophoron, organ of Kosenmiiller). The tubules making this up increase considerably in size. The posterior tubules (renal part) disappear with the exception of a few tubules, known as the paroophoron. QEN1T0 -URINARY SYSTEM OF THE EMBRYO. 249 In both sexes a new tube is developed parallel with the Wolffian duct. This is the Mullerian duct. In the female it is connected with the peritoneal cavity, and persists as the Fallopian tube and uterus. In the male it disappears in large part. The persistence of the anterior part gives rise to the hydatid of Morgagni. The posterior part may remain as Weber's organ. In some cases the whole tube is found in the adult male, and then is known as Rathke's duct. The way in which the head of the epididymis comes to be connected with the testis tubules has been determined in pigs' embryos and in man (MacCallum). It is well known that the seminiferous tubules in some of the lower vertebi-ates (fishes, etc.) carry the sexual products over into the Malpighian cor- puscles of the urinary organ, and are taken to the outside through the urinary ducts. A somewhat similar condition has been observed in the embryos of pigs and man. The testis which develops from the peritoneal covering of the Wolffian body is at all times closely connected with this organ. Tubules develop in the testis, and at a certain period grow out through the tissue connecting the two organs, and break into the cap- sules of the Malpighian corpuscles of the Wolffian body. These tubules are very fine and form a dense network. Their lumina become continuous with that of Bowman's capsule, and in this way a communication is established between the tubules of the testis and the future epididymis and vas deferens. The ovary develops on the medial surface of the Wolffian body in the same way as the testis. The permanent kidney develops as a knob-like growth at the end of the primitive ureter, posterior and dorsal to the Wolffian body. The exact course of the development of the kidney tubules has not been worked out satisfactorily. They arise in the beginning as long diverticula from the end of the ureter, which grow out to the periphery of the organ and divide into two branches, which arch backward toward the hilum to join, after many convolutions, with the Malpighian corpuscles. The exact origin of the kidney lobule and of the various segments of the uriniferous tubule is not known. 250 MICROSCOPIC ANATOMY OF THE ORGANS. B. Fallopian Tube (Tuba Uterina Fallopii). In the walls of this tube we can distinguish the following coats : mucosa, submucosa, muscularis, and serosa. The tunica mucosa is thrown into many longitudinal folds, varying somewhat in different parts of the tube. In the ampulla they are highest and possess numerous branched accessory folds, so that the lumen seems filled with them (Fig. 187). There are, however, throughout the tube only four main folds, as can be seen more plainly in a tube taken Fig. 187. Blood-vessel Lumen »f tube Transverse section through the ampulla of the Fallopian tube of a young woman. X 25. from an embryo or newborn babe. The mucous membrane is covered on the surface with a single layer of columnar ciliated cells, the movement of whose cilia is toward the uterus (Fig. 188). The tunica propria is rich in cells and overlies a thin muscularis mucosa? composed of longitudinal smooth muscle fibres. The tunica submucosa consists of loose connective tissue, and is bounded on the outside by two layers of smooth muscle, making up the tunica muscularis. The fibres of the inner stronger layer run circularly, while the outer thin layer is longi- tudinal. The muscle lavers are thicker near the uterus than at UTERUS. 251 the ampullar end. The tunica serosa, which has the same struct- ure as the peritoneum, is joined to the muscularis by a loose connective tissue. The mucosa is supplied richly with blood- Fig. 188. Cilia ©^" Connective-tissue i ^£*>i<' $' 1 ft l? W • < i5S> \^ ceUs of stratum From a section through a fold of the mucous membrane of a human Fallopian tube. X 480. vessels. The nerves form in the tube wall a rich plexus, from which fine branches proceed to the mucosa to end in the neighborhood of the epithelial cells. C. Uterus. In the wall of the uterus there are three main coats : the mucosa (endometrium), the muscularis (myometrium), and the serosa (perimetrium). The mucosa lining the whole uterine cavity is at the time of puberty about 1 mm. thick. It is covered on its surface by a single layer of cylindrical ciliated epithelial cells, whose cilia move toward the vagina. The tunica propria possesses many connective-tissue cells and leucocytes contained in a fairly dense connective tissue. Here there are found numerous simple or dichotomously branching tubular glands, which take on usually a coiLed or corkscrew form in the deeper parts. They are lined with a single layer of ciliated cylindrical cells, 252 MICROSCOPIC ANATOMY OF THE ORGANS. Fig. 189. Jf J3ecller.fa. Normal endometrium in a patient twenty-six years of age. X 25. The mucosa is slightly thickened, its surface is wavy, and its epithelial covering o is intact. In this section it is possible to trace the glands in their continuity almost from the surface to the muscle. A few of them are practically cylindrical throughout, but the majority have a wavy contour presenting a well-defined corkscrew arrangement. Quite a number, cut just along their margin, can be recognized as little masses of epithelial cells ; c, is cut longitudinally ; 4, almost transversely. At first sight, one would think that there was a great excess of glands in the section, whereas in reality, at most, there are not more than twelve, the distances between any neighboring two being about the same. The gland epithelium is intact throughout. The stroma in the superficial portions is rather lax, in the deeper portions more compact, h indicates the line of junction between the muscle and mucosa. Its irregularity is especially noticeable. (T. S. Cullen, Cancer of the Uterus; New York, 1900.) UTERUS. 253 the ciliary current moving toward the mouth of the gland. A basal membrane (membrana propria) with a double contour limits this row of cells on the side toward the tunica propria, and is a continuation of the basal membrane of the surface epithelium. The glands probably possess no secretory function. The mucosa of the cervix uteri shows some distinguishing features. The surface is thrown into folds, known as the plicm palmalce. The mucous membrane is thicker and firmer, and possesses much higher cylindrical cells than the corpus uteri. In the region of the external os it passes over into a stratified pavement epithelium with papilla? beneath. After repeated pregnancies this pavement epithelium covers also the lower part of the cervix. The mucosa of the cervix contains, besides the glands already described, numerous glands which secrete mucus (glandulce cervicales uteri). Often the mouths of the glands become closed and there are formed retention cysts, con- taining a quantity of mucoid material and reaching the size of a pea. These were formerly known as ovula Nabothi. A submucosa in the uterus cannot be made out. The mucosa lies directly on the muscularis, and the glands reach clown so as to touch the muscle coats. The latter is the thickest layer of the uterus, and is made up of long, spindle- shaped, smooth muscle elements. In the non-pregnant uterus these are 40-60 n long, while at the end of pregnancy they reach a length of 300-600^. They are arranged in bundles, mostly running concentrically around the blood-vessels. The whole muscle layer, however, can be divided roughly into layers, which in the adult are by no means distinctly separated from one another. The exact disposition of these layers has been the cause of much discussion, and there have been many ideas ad- vanced with regard to this subject. In general, three layers can be made out : 1, a longitudinal inner layer (stratum mueosum); 2, a middle circular layer of bundles closely associated with the blood-vessels (stratum vasculare); and 3, an outer layer, in which the bundles run both longitudinally and circularly. The latter layer can be divided into two parts : an inner layer of mixed longitudinal and circular fibres (stratum supravascu- 254 MICROSCOPIC ANATOMY OF THE ORGANS. tare), and an outer layer which consists exclusively of longi- tudinally disposed elements (stratum subseromm). The middle layer or stratum vasculare is by far the thickest of these coats. The serosa is not different in structure from other parts of the peritoneum. The arteries enter the muscularis and divide mainly in the stratum vasculare into numerous branches, of which the greater part run into the mucosa and break up there into capillary networks which surround the glands and reach up to the surface epithelium. The veins form a plexus in the deeper parts of the mucosa, and then pass into the stratum vasculare, where another larger plexus is formed. The lymph-vessels form a network in the mucosa and another under the serosa. These are joined by anastomosing branches. The nerves end partly in the muscularis (see Nerve-end- ings), and partly in the mucosa, where they form thick net- works. From these, non-medullated fibres run, according to some authors, to the epithelium, where they end freely between the cells. Ganglion cells have been described in the course of these fibres. In certain phases in the life of the uterus changes take place especially in the mucosa which must be spoken of here. These changes accompany menstruation and pregnancy. In menstruation there is a certain amount of bleeding from the uterus occurring more or less regularly every twenty-eight days, and continuing throughout the life of the individual from the fourteenth to about the forty-fifth or fiftieth year. It is probable that the changes in the mucosa have to do with the reception and preservation of the ovum, since ovulation occurs at about the same time as menstruation. During the menstrual period, in the first place, there is a marked hyperemia of the uterine walls five to ten days before the flow of blood. The blood-vessels are much dilated and the capillaries become large and well marked. According to Heape, there is also an increase in the number of blood-vessels. On account of the hyperemia there are a swelling and a growth of the mucosa, UTERUS. 255 so that it attains a thickness of 6 mm. It then is called the decidua menstrualis. Changes occur also in the glands. They increase in length and become corkscrew-shaped. The increased size of the mucosa is due largely to a cellular multi- plication. Karyokinetic figures in large numbers have been observed in the menstruating uterus by Mandl, not only in the epithelium, but in the interstitial tissue as well. After these changes have occurred there is an escape of blood in the sub- epithelial layers, which is due partly to a bursting of capil- laries, and partly to a diapedesis of red corpuscles through the capillary walls. The epithelium covering these collections of blood is broken away and the blood escapes. The bleeding goes on for about four days, and then the regeneration of the mucosa begins. In the course of five to ten days the epithe- lium is quite restored and the glands regain their normal relations. Following this are a few days of rest before the next period begins. There has been considerable discussion as to the extent of the tissue destruction which takes place during menstruation. According to some, the whole mucosa is cast off at each period. Others hold that none at all is destroyed, and that pieces of the epithelium are lifted up merely to allow the blood to escape. It seems certain, however, that there is always some destruction of epithelium, and at the same time there is never a complete destruction. Parts of the gland tubules at least always remain uninjured, and from these and the surface cells that remain the whole epithelium regenerates. During pregnancy the whole uterine mucosa suffers very marked changes in its structure. At the end of this time it is nearly all lost, and forms the so-called decidua graviditatis, of which there are three parts. The decidua basalis s. serotina is the part of the mucosa to which the ovum attaches itself, and in which later the placenta is developed ; the decidua capsularis s. reflexa is that part which grows up to surround the ovum ; while the decidua vera is the tissue which lines the rest of the uterine cavity. In the part of the uterine mucosa where the decidua vera 256 ' MICROSCOPIC ANATOMY OF THE ORGANS. developes, changes take place resembling those of menstruation. At the end of the fifth month the mucosa has become more than 1 cm. thick. This is due, in the first place, to the dilata- tion of the blood-vessels and the thickening of their walls, and also to the increase in length of the gland tubules. The latter become corkscrew-shaped or tortuous in their course. The tunica propria increases in its superficial part, so that there is a firm connective tissue between the necks of the glands. In consequence of this, the whole mucosa can be divided into two zones, a superficial compact layer, and a deep spongy layer. From these connective-tissue cells, the so-called decidual cells, arise. These are very large (30-100 u), round or polygonal cells somewhat resembling epithelial elements. Each cell pos- sesses usually only one nucleus, but some may contain as many as forty nuclei (giant cells) (Fig. 192). These will be spoken of later. The decidual cells are developed esj)ecially in the compact layer, where the glands have a straight course and are separated by much connective tissue. In the spongy layer the cells form narrow septa between the saccular ends of the glands. The surface epithelium vanishes entirely, while the gland cells increase in number and become flattened to accommodate themselves to the widened gland lumina. In the second half of pregnancy changes in the decidua vera occur, which are due mainly to pressure exerted by the growing foetus and the increasing amniotic fluid. The decidua becomes gradually thinner, so that at the end of pregnancy it is only 2 mm. thick. The glandular epithelium degenerates, with the exception of that in the ends of the glands which rest on the muscle. This remains, and is the basis of the epi- thelial regeneration which takes place after pregnancy. The gland necks in the compact layer become obliterated and disap- pear about the middle of pregnancy. The gland lumina in the spongy layer, on the contrary, are converted into spaces which lie parallel to the surface of the uterine wall. The decidua reflexa (capsularis) has originally the same structure as the decidua vera ; but during the first months of UTERUS. 257 pregnancy a hyaline degeneration takes place (Minot), so that it cannot be recognized at the end of pregnancy. According to Leopold, however, it is fused with the decidua vera, and is always to be seen. The decidua serotina (basalis) iu the beginning has the same structure as the decidua vera, but becomes complicated in the course of pregnancy by the formation of the placenta. Placenta. The placenta usually is discussed in detail in the text- books of embryology, but since it consists not only of an embryonic part {placenta fcetalis), but also a maternal part which is modified uterine mucosa [placenta uterina s. ma- terna), a brief description must also be given here. The placenta fcetalis consists of a connective-tissue mem- brane, the membrana chorii, which on the surface toward the uterine wall possesses many richly branched villi. These give rise to the name chorion frondosum, which is applied to the membrane. The chorionic villi are grouj)ed in large bundles or cotyledons. After the third month the chorion comes in contact with the second foetal membrane, the amnion, and later on is connected closely with it. The amnion is a thin mem- brane which consists of an epithelial and a connective- tissue layer. The epithelial coat covers its free surface and lines the whole amniotic cavity in the form of a single layer of flat- tened cells. The connective-tissue sheath fuses with that of the chorion. Through the umbilical cord there enter the mem- brana chorii two umbilical arteries, which carry the blood of the embryo to the placenta foetalis, where they branch freely. To each cotyledon there runs one branch, which breaks up into many twigs and forms capillary networks in the villi. A part of the villi end freely, while others pass into the placenta uterina and become firmly connected with it. The latter are called the fastening villi' or Haftwurzeln. By means of these the two sides of the placenta are joined securely together, so that in the later months of pregnancy no separa- tion occurs. 17 258 MICROSCOPIC AS ATOMY OF THE ORGANS. The chorion is a connective-tissue layer covered on the side toward the uterine wall with an epithelial layer. The con- nective-tissue part shows originally the structure of embryonic connective tissue — i. e., stellate cells lying in a homogeneous Placenta . uterina Fig. 190. '!"■ *.*.;£ Gland of tents /.••■■ v. • ■>;/;. "":'->*• ,'.\" ~;-v -'.'■ ' : ' \." : Vv/ ;'.*.'. '.•'f^' > s\'>'"' ** '••.""':' - 'I i Epithelium ^ ''. i . : v'-'^* r ^* ;:i; *--- : -5'-C-^'- t> *' $!ti>tii*>.ii-^ ?~>'>\,of villus eu t ---;; faiHtenthllif Placenta x- r y k^ ■ U7J"T Proliferation island Intervillous spaces filled mth blood Villus ^yJ^Membrana chorii y ^ i?y£S^* > Transverse section through a human placenta at the second month of pregnancy. (After a preparation by Prof. Mars.) x 50. ground substance. Later it assumes the character of fibrous connective tissue. The chorionic villi appear, during the first months of their development, in the form of short protuber- UTER US. 259 ances consisting entirely of epithelium. Later they develop numerous branches which go on dividing dichotomously. They are made up of gelatinous tissue, which forms the axis, and a layer of epithelium, which covers not only the villi, but also the whole membrana chorii. In the larger stems of the villi we find, instead of the gelatinous tissue, a fibrillar connective tissue (Fig. 190). The epithelial coat is differentiated early into two distinct layers (Fig. 191). The layer touching the connective-tissue part consists of well-defined cells containing Fig. 191. Connective ti&8ue~-^$WFg& i Blood-vessel Ectoderm of villus ^B'vSePi — Proliferation i Blood-vessel with .ucleated blood cells *W ^^M : Proliferation island Syncytium 7J&~n>y ; --o-i<&^ /5'J»^ ^§8 of villus ^- — ■ - - w ~^.'.i.-^.~- -* Transverse section of a human chorionic villus at the fifth month of pregnancy. X 300. clear protoplasm, and is known as the ectoderm layer of the villus (Zellschicht of Langhans). The layer outside this con- sists of cells which are not sharply marked off from one another. It is made up of a continuous protoplasmic mass in which there are numerous nuclei. We have here to do with a syncytium, and we speak of this layer as the syncytium of the chorionic villus. These two layers are separated fairly sharply from one another, for the protoplasm of the syncytium has a special affinity for acid dyes and stains more deeply, while the nuclei are much smaller than in the ectoderm layer. Toward the middle of pregnancy (fifth month) the ectoderm of the villi begins to degenerate, so that at the end of preg- 260 MICROSCOPIC ANATOMY OF THE ORGANS. nancy it is almost entirely wanting and the villi are covered only by the syncytium. In certain places there are thickenings formed in the membrana chorii as well as in the villi. In the apices of the latter they are called cell nodes. Local thicken- ings in the syncytium are called proliferation islands (Fig. 191). Toward the end of pregnancy the syncytium also vanishes, and in its place there is a homogeneous, refrac- tive, faintly staining substance containing numerous empty spaces, and known as canalized fibrin or hyaline (Fig. 192). Fig. 192. Decidual Syncytium-™, Fibr ,:■:■*. .%^sW*-i< ofl " Oblique section of syncytium i.\ iJ ieaf .- '*: J From a section through a human placenta at the fifth month of pregnancy. X 80. This substance increases with the age of the placenta, but its origin and significance are by no means clear. Although there is no doubt that the villus ectoderm is of embryonic origin, there is still some question as to the derivation of the syn- cytium. Between the villi we find so-called intervillous spaces which contain blood. The villi are thus surrounded by blood on all sides. The views held as to the origin and significance of these intervillous spaces are still much at variance. This problem is UTERUS. 261 associated closely with, that concerning the villus ectoderm and syncytium, for the origin of the intervillous spaces is associated naturally with that of the syncytium. According to one theory, which seems to have the greatest number of supporters (Virchow, Ercolani, Leopold, Waldeyer, Keibel, and others), the intervillous spaces represent the widened capillaries from the uterine mucosa. It must be remembered that at an early stage the chorion and the decidua serotina lie with their sur- faces closely applied to one another, and the epithelial layer of the decidua is cemented to a similar layer of the chorion. In this way villi grow into the decidual tissue, in which at the same time the capillaries become dilated to a system of spaces. These surround the villi, so that they become bathed in blood. Also flat endothelial cells lining the intervillous spaces have been observed by Turner, Leojjokl, Waldeyer, and Keibel, which represent the lining cells of the capillaries. Injections made by Waldeyer support this view. Many authors who share this theory claim that the syncytium and the ectoderm of the villus have different origins. The latter they describe as foetal and the former as a part of the uterine epithelium. According to other authorities (v. Kolliker, Langhans, Hof- meyer, Minot, and others), the intervillous spaces represent the original spaces between the placenta fetalis and placenta uterina. The two parts of the placenta are joined together only by the villi. According to this theory, the intervillous spaces are interplacental cavities which originally contained no blood and became filled only when the maternal vessels 02?ened into them. Almost all the adherents to this theory claim that both layers of cells covering the villi are of foetal origin, and according to Minot's theory the syncytium is a differentiated product of the ectoderm layer beneath. The maternal part, or placenta uterina, represents the decidua basalis, which has certain characteristics that distinguish it from other deciduee. From the fifth month on, there develoj) in it large cells (giant cells) containing many nuclei. These cells are present in large numbers in the ripe jdacenta. From the side toward the placenta fcetalis more or less thick connective- 262 MICROSCOPIC ANATOMY OF THE ORGANS. tissue bands arise, the so-called septa place idee. These pass between the chorionic villi and separate them into groups or cotyledons. Only at the peripheral part of the placenta do the septa come into contact with the membrana chorii and fuse with it to form the so-called subchorionic limiting ring. The circulation of blood in the maternal placenta takes place in the following way : numerous arterial branches enter through the muscular coats of the uterus to the outer layer of the placenta uterina. During their tortuous course these vessels lose their muscle cells and elastic elements, so that the thin walls that remain consist only of an endothelial and thin connective-tissue layer, and come to lie directly on the decidual cells. After branching, the arteries enter the septa placenta?, where they empty into the intervillous spaces through openings in the septa. The veins also open into these spaces, so that instead of a capillary system between the arteries and veins we find wide lacunae, which, according to most authors, arise from the superficial blood capillaries of the uterine mucosa. The veins, whose walls, like those of the arteries, have been reduced in thickness, open into the intervillous spaces by comparatively wide orifices, which are more abundant near the middle of the cotyledons. The arteries, on the contrary, open in greatest numbers at the edges of the cotyledons, so that the blood in the intervillous spaces flows from the periphery to the centre of the cotyledons. The intervillous spaces thus contain maternal blood, while in the chorionic villi the capillary vessels under the epithelial covering are all of foetal origin. These two vascular systems never communicate directly with one another, and a mixture of foetal and maternal blood never occurs. The diffusion of gases takes place through the walls of the capillaries and through two layers covering the villi. D. Vagina and External Female Genitals. The wall of the vagina is about 3 mm. thick, and consists of four layers: the mucosa, submucosa, muscularis, and fibrosa. The mucosa is thrown into transverse folds, the so-called VAGINA AND EXTERNAL FEMALE GENITALS. 263 rugce. On their surface we find a stratified pavement epithe- lium, under which there is a thin connective-tissue tunica propria. At the external os of the uterus the fiat epithelial layers, which cover the portio vaginalis uteri, pass over into the ciliated cylindrical epithelium of the cervix uteri. The tunica propria possesses papillae which are rich in elastic fibres, and contain quite numerous masses of lymphoid tissue, often gath- ered into solitary follicles (noduli lymphatici vaginales). Ac- cording to most authoi's, the vagina contains no glands, and the mucous secretion found there is derived from the glands of the cervix uteri. The submucosa which joins the mucosa loosely with the muscularis consists of connective tissue characterized by its richness in elastic fibres. The muscularis consists of an outer longitudinal and an inner circular layer of smooth muscle cells. The latter is usually not strongly developed. The fibrosa which surrounds the muscle coat contains many elastic fibres and joins the vagina with the surrounding tissues. The blood- and lymph-vessels form many plexuses par- allel to the surface. The nerves enter the epithelial layer, where they end freely. The hymen is a membranous reduplication of the vaginal mucosa. Its inner surface is covered with epithelium, which represents that of the vagina. The outer epithelial layer is like that of the skin. The whole vestibule possesses similar epithelium, with its outer cells non-nucleated. In the labia minora there are sebaceous glands. The labia majora are covered with epithelium which is not at all different from that of the skin in other parts of the body. In the region of the clitoris and the urethral openings we find numerous mucous glands {glandular vestibulares minores). The larger glands of the vestibule (glandulas vestibulares majores s. glandular Bartholini) correspond with Cowper's glands in the male, producing a similar mucous secretion. The clitoris resembles somewhat in structure the penis. 264 MICROSCOPIC ANATOMY OF THE ORGANS. There are in it considerable masses of erectile tissue and firm elastic strands like those of the penis. The glans clitoridis is supplied richly with nerves, and besides the Meissner's and Pacinian tactile bodies there are also special genital corpuscles (see Nerve-endings). VI. LOCOMOTOR SYSTEM. Here must be considered the skeleton, and the muscles, and their mode of development. 1. THE SKELETAL SYSTEM. The bones form the essential part of the skeletal system, and in connection with these the cartilages play an important role. The structure of adult bone and cartilage as tissues has been described, but here they must be spoken of as organs. A. Bones. Bones considered as organs consist of bony tissue, perios- teum, and bone-marrow, with blood-vessels and nerves sup- plying the different parts. Each bone (here the teeth are not considered) is surrounded by a connective-tissue sheath, the periosteum, with the exception of such places as are covered by cartilage. In this firm layer of connective tissue there are two layers : an outer fibrous layer, in which there are few cells, but numerous nerve plexuses and blood-vessels ; and an inner delicate layer, poor in blood-vessels, but especially rich in elastic fibres and connective-tissue cells. At the boundary between the periosteum and the bony tissue we find a layer of cubical cells (osteoblasts), which play an important part in the regeneration and development of the bone. A more or less intimate connection is established between bone and periosteum, partly by means of blood-vessels, and partly by bundles of connective-tissue fibres (Sharpey's fibres) which run from the periosteum almost at right angles to its surface and enter the bone. BONES. 265 (a) Bone-marrow. In all bones of higher animals we find a bone-marrow. In the long bones this fills the axial cavity and enters the larger Haversian canals. In the flat bones, on the contrary, it fills up the meshes of the spongy substance. Two kinds of bone- marrow can be distinguished: red and yellow marrow. The first is found in all bones of embryos and young individuals. In the course of time it changes in some bones (e. g., the diaphyses of the long and short bones of the extremities) into yellow marrow. Only in the epiphyses of these bones, in the bodies of vertebrae, and in the flat bones, is there found red marrow in the adult. The red marrow is a lymphoid organ which is the main place of formation of the red blood-cells. The different ele- ments contained in the red marrow are the following (Fig. 197): 1. Myelocytes. — These are somewhat similar to some kinds of leucocytes. In normal blood they are not found, while in leukaemia they are very abundant. Their nuclei are very large, sometimes lobed, and surrounded by a more or less finely granular protoplasm. The nuclei stain faintly, and the proto- plasm is sometimes quite abundant. 2. Nucleated Red Blood-corpuscles. — The protoplasm is col- ored yellow on account of the haemoglobin present. The nucleus usually is placed excentrically and stains very deeply. These cells are known also as erythroblasts or normoblasts, since they are the forerunners of the erythrocytes. They vary considerably in size, very large ones being known as megalo- blasts, and small ones as microblasts. These unusual forms occur, however, more often in certain diseases. 3. Non-nucleated Red Blood-corpuscles. — These are derived from the nucleated red corpuscles. 4. Giant Cells. — These are probably modified leucocytes. They contain one or more nuclei, whose form may be round, lobed, or annular. The old theory that the multinucleated giant cells arise by a fusion of many cells is abandoned. They 266 MICROSCOPIC ANATOMY OF THE ORGANS. are derived, on the contrary, from cells with a single nucleus which has divided to form many nuclei without a correspond- ing division of the protoplasm. This group of cells is made up of the so-called osteoclasts, which play an important part in the development of bone, and are spoken of in the discussion of this subject. 5. Eosinophils are found often in bone-marrow; and also, 6. Mast-cells (y-granulations), which are found exception- ally in the blood. Some of these marrow cells contain pigment granules, which are the remains of disintegrated red blood-corpuscles. In the red marrow fat cells are not abundant, and the blood-vessels and nerves are found only in small number. The yellow or fatty marrow, which owes its color to the large proportion of fat present in it, arises from the red marrow in the diaphyses of the long bones by a diminution of the marrow elements and an increase in fat. In old or emaciated individ- uals the yellow marrow becomes reddish and resembles mucus. Such a marrow is poor in fat, and is known as gelatinous bone- marrow. The connective tissue, which occurs only in small quantities in bone-marrow, is collected at the periphery of the marrow cavity, where it forms a firm fibrous membrane, lining the whole cavity. This represents a sort of inner periosteum, and is called the endosteum. The bone, periosteum, and bone-marrow are supplied more or less richly with blood-vessels. These enter the perios- teum, and from here they pass, by means of the Volkmann's and Haversian canals through the bone to form a network of vessels in the bone-marrow. All these vessels anastomose with one another. The so-called nutrient arteries, which supply the medulla with blood, break up into numerous branches, which form a rich capillary network in the medulla. Narrow capillaries broaden out, so that in joining together they pass into small veins with very delicate walls. The veins of the bone-marrow as well as the bone possess no valves. The older idea, that the capillaries and small veins possessed no wall at all, or that they were in many places BONES. 267 broken through, so that the venous blood flowed freely in spaces of the marrow, has not been supported by recent investigations. The vein walls are exceedingly thin, but are always present. The lymph-vessels form fine capillary networks in the periosteum. The nerves are partly medullated and partly non-medullated. They enter from the periosteum into the Volkmann's and Haversian canals and reach the bone-mar- row. Some of these fibres end in Pacinian corpuscles in the periosteum. (b) Joining together of Bones. The bones are joined together either immovably (synar- throsis) or in such a way that they can move freely on one another by joints (diarthrosis). The immovable combination is effected either by ligaments (syndesmosis) or by cartilage (synchondrosis). The ligaments may consist only of fibrous connective tissue and appear very like tendons, or they may contain numerous elastic fibres (ligamentum nuchse, ligamentum flava, etc.). The synchon- drosis is formed usually by fibrous cartilage, which at the border of the bone becomes hyaline. Special note must be made of the intervertebral ligaments. These contain in their interior a gelatinous mass (nucleus pulposus, gelatinous nu- cleus), which is the softened remains of the chorda dorsalis. Their periphery, however, consists of fibrous cartilage. In joints we must consider the articular ends of the bones, the labra glenoidalia, the menisci interarticulares, and the joint capsules. The articular ends of the bone consist of hyaline cartilage, which is calcified on the side adjacent to the bone. Often they are made up of fibrous cartilage (e. g., in the sterno- clavicular and maxillary joints). The labra glenoidalia and menisci interarticulares are fibrous cartilages. In the joint capsules we distinguish an outer part (stratum fibrosurn, capsula fibrosa) and an inner part (stratum synoviale, capsula synovialis). The latter consists of loose connective tissue, which contains fat cells, vessels, and nerves, and is clothed on its inner surface 268 MICROSCOPIC ANATOMY OF THE ORGANS. by a layer of flat epithelium. This is to be considered as a serous membrane. Often there extend from the synovial mem- brane into the joint cavity the so-called synovial villi. These are found abundantly 011 the borders of the joint-surfaces, and consist of a connective-tissue axis often containing blood capil- laries and an epithelial covering. The synovial fluid (synovia) contains a few fat droplets and fragments of epithelium broken off from the joint-surfaces. (c) Development of Bones. Bony tissue develops later than any other tissue, and arises from some preformed tissue, such as hyaline cartilage or con- nective tissue. In young embryos the future skeleton exists as cartilage or connective tissue. (1) Development of Bone from Cartilage. In bones which are developed from cartilage the bony tissue is laid down in two different places, either in the interior of the cartilaginous forerunner of the skeleton (endochondral ossification), or on the surface of the cartilage (perichondral ossification, wrongly called periosteal ossification). The endochondral bone formation begins with the increase in size and number of cartilage cells through karyokinesis, so that many cells come to lie in each cartilage lacuna (Figs. 193 and 194). Certain changes then begin in the homogeneous ground substance of the cartilage. Calcium salts are laid down, so that the ground substance becomes opaque. The cartilage lacunae become large and the cells shrink. Places where such changes have taken place may be quite numerous in a bone, and are known as areas of ossification or calcifica- tion. In the long bones such centres usually appear first in the diaphysis. While this process is going on inside the cartilage certain' changes take place on its outer surface. In the deeper cel- lular layers of the perichondrium an ossification (perichondral ossification) begins. These layers of perichondral cells, richly supplied with blood-vessels, are known as osteogenous tissue. DEVELOPMENT OF BONES. 269 The ground substance becomes calcified and the cells become changed into bone cells. In this way there is formed at the border of the cartilage and perichondrium a bony layer, and the perichondrium becomes the periosteum. From the latter, buds grow in toward the areas of ossification, known as peri- osteal buds (Fig. 194). These penetrate the calcified ground Fig. 193. Hyaline cartilage Area of calcification (■apsides containing 'many cartilage cells From a longitudinal section of a finger of a three and a half months' human embryo. Two- thirds of the second phalanx is represented. At x a periosteal bud is to be seen. • 85. substance of the cartilage, whose cell capsules are broken down, allowing the cells to become free. In this way there 'gradually arises a cavity in the areas of ossification which forms a primordial or primary medullary cavity, and the first trace of the permanent marrow cavity. In this space are found blood-vessels and cellular elements, which are derived partly from cells brought in by the periosteal buds, and partly 270 MICROSCOPIC ANATOMY OF THE ORGANS. from freed cartilage cells. Some of the cells form the elements of the future bone-marrow ; a part, on the contrary, play the important role of bone-formers or osteoblasts. These are large, often-branched cells, which as a rule form a layer on the inner surface of the periosteum, and are carried into the mar- row cavity along with the periosteal buds. Thus we find in the areas of ossification, first, proliferation of the cartilage cells and a calcification of the ground substance, and then a de- Fig. 194. Cartilage celUlMMl Periosteum- The place marked x in the preceding figure with stronger magnification, x 185. struction of this cartilage by the ingrowth of periosteal buds. In long bones the marrow cavity increases in size by a gen- eral breaking down of the calcined bone. The cartilage lying at both ends of the diaphysis shows characteristic relations (Figs. 195 and 196). We may notice in this several zones which are well marked off from one another. The part most distant from the marrow cavity shows no changes, containing spindle-shaped cavities with small cells. The cells lying nearer the medullary cavity are larger and PLATE XXIX. Pcriostenm^^ I Periosteal bnd -— tV— Enlarged Endochondral m£&M *£< Blood-vessels , , ,*; -'j^f___^— ,-*-^^ T , ',-'.'-.■ ..' .t '■ •' f&, filled will* ^-z^Z' Xi,i'-&:' ":- ; '.s;' Calcifii cartila Perichondral bone \ Fig. 195. — From a longitudinal section of a finger of a four months human embryo. Only the diaphysis of the second phalanx is represented, x S5, I ■\ 1 \\ \; ,\ o„*"■§* o Q - *•_-§<=» S VS| IU L^ JSZ W K . 9ai«)8 .ov-'dui » uiiiuuri arltuoai ,; ; . " i>ooii5 bat 85»$38»KVt03 T .OO* x .'ifu;ia ^'fhanui ,tirriiw-d Cells in groups Peri- bone o, , rs °°0 O St? ^ n ' o °o9 ( Fig. 196. — From a longitudinal sectiori through the second phalanx of the finger of a seven months human embryo. Stained in hematoxylin and eosin. X 130. Bed blood corpuscles Eosinophils Reticulum f — Mitosis in nucleated red blood oorpuseles -Myelocyte Eosinophile Fig. 197.— From a section through the red bone-marrow of a rabbit. Biondi's stain, x 800. ', Q eg? a rpOp '-■ fh fl :q >0 Q 3 g' o° » ji." a. g9 -' Co CD .■;■■-■■ -i,i n ;-, »1 &> o 'O n&!5*2p&2 '^-p ©0 0«i ■- ■ f &\q6W' & ;,Q ( . 't;-^>^ fW Sy.ymonowicz. Histology c?P €% § at s ' mm, J. Barney, ad vol. del. Lith.Rn<-LvWan,a tWinfrr. FranJr/itrt ^* DEVELOPMENT OF BONES. 271 arranged in rows, or cell columns, between which there is a fibrous ground substance. The individual cells of the columns are separated by thin septa. Still nearer the medullary cavity the cell lacunae are large and flattened against one another. The septa of ground substance become thinner, and finally vanish, and the lacunae in many places coalesce to form larger cavities. The ground substance is impregnated with calcium salts and becomes opaque. The spaces in the cartilage open into the marrow cavity, which in consequence seems to have many irregular cavities leading from it. Blood-vessels grow in from the marrow cavity together with marrow and osteoblasts, which on the inner surface of the increased medullary cavity begin the formation of a bony layer. The osteoblasts gradually become surrounded by ground substance which is converted into bone, the osteoblasts themselves becoming bone cells. In consequence of the activity of the osteoblasts the whole medullary cavity is lined with a thin layer of bone (Figs. 195 and 196), and of the original solid mass of cartilage there remain only irregular pieces covered with bone. The cartilage is thus converted into a spongy bone. As already mentioned, the perichondral ossification goes on at the same time at the surface of the cartilage (Figs. 195 and 196). This is due to the activity of osteoblasts lying between the cartilage and perichondrium, and in this way bone is laid down in layers on the outside of the cartilage. By this so-called apposition the bone increases in thickness. The vessels at the surface become enclosed in the develop- ing bone in cavities which form afterward the Haversian canals. The osteoblasts contained in these form concentrically lying lamellae in the ground substance of the bone. The epiphyses of the long bones become ossified later than the diaphyses ; but the process in both cases takes place by an endochondral and a perichondral ossification. Areas of calcification are formed, into which blood-vessels grow from the surface of the cartilage or from the diaphysis. A medullary cavity is formed and the ossified borders of the diaphysis and epiphysis approach one 272 MICROSCOPIC ANATOMY OF THE ORGANS. another. These are at first separated from one another by a thin layer of cartilage, the epiphyseal line. By means of this cartilage the bone is enabled to increase in length, and not until all growth in length has ceased does the epiphyseal line disappear. In addition to this process of bone formation there is also a destruction of bone. In this process of absorption the so-called osteoclasts play an active part. These are giant cells (Fig. 198) Fig. 198. Lacuna Giant cell osteoclast) Marrow cells ■ »* JT ~~~~ , _ ■ ■•;..." .* .;*•: . ■«. \»: A «';" Giant cell 'lij_^V^ * ,gr,^ : t «,- ,« (osteoclast) •», '.♦ ; ^ W >"V4' From a longitudinal section of the femur of a rabbit's embryo, x 335. containing many nuclei and situated in small hollowed-out spaces in the bone known as Howship's lacuna: It is believed generally that these osteoclasts in some way absorb or destroy the bone after it has been formed by the osteoblasts, and that in this way the medullary cavity is increased in size. They are to be observed not only in growing bones, but also in those that are fully developed. All the bones are formed from carti- lage, with the exception of the bones of the roof of the skull, the lateral part of the skull, most of the face bones, and a small part of the base of the skull. DEVELOPMENT OF BONES. 273 (2) Development of Connective-tissue Bones. In those instances in which bones are developed in con- nective tissue certain bundles of connective tissue become cal- cified and form the ground substance of the bone. The con- nective-tissue cells arrange themselves in a layer on the surface of these bundles, and, becoming more rich in protoplasm, are converted into osteoblasts (Fig. 199). There is thus formed a bony plate by the addition of bone on the surface and at the borders of the calcified mass. This increases in thickness by Fig. 199. Bone cells Osteoblasts Blood-vessel Connective tissue Bone From a transverse section of the parietal bone of a human embryo. X 220. the deposition of new bone on the two surfaces. The older bone between these two layers becomes a spongy bone substance (diploe). In this kind of bone production the osteoclasts are particularly active, for the bones that are so formed are con- stantly undergoing changes in form and relations. These are mainly the lateral bones of the skull, the facial bones, and the upper parts of the occipital bone. B. Cartilages. The cartilages are covered with a perichondrium, with the exception of those covering the joint-surfaces and those joining together bones. In fully developed cartilages we find no blood- vessels. These, as well as the nerves, exist only in the peri- chondrium (see Cartilage tissue). 18 274 MICROSCOPIC ANATOMY OF THE ORGANS. 2. MUSCULAR SYSTEM. Large aggregations of striated muscle fibres form organs which are called muscles. These taken collectively form the muscular system. The muscle fibres are grouped together in the muscles to form bundles (Fig. 200). Around each fibre there is always a certain amount of connective tissue containing blood capillaries, and bundles of these fibres are surrounded by thicker stands of connective tissue known as the perimysium internum. These primary bundles are grouped together by Fig. 200. Perimysium internum of primary bundles mysium Muscle fibres "^^jj |^^* r **$i i^A internum Perimysium of single muscle fibres From a transverse section of the human sterno-cleido-mastoid muscle. An entire secondary bundle, surrounded by the perimysium internum, is shown. X 45. connective tissue to form secondary bundles, which in large muscles are enclosed still further to make up tertiary bundles. The whole muscle is surrounded by a thick connective-tissue capsule, the perimysium externum. This is in direct connection with all the strands of connective tissue that make up the peri- mysia interna. This can best be seen in a cross-section of a muscle stained to bring the connective tissue into prominence (e. g., with acid fuchsin and picric acid). We see that the perimysium externum sends septa into the muscle between the PLATE XXXI. J.&a.fo-'-T Fig. 201. — Piece of striated muscle from a rabbit ; blood-vessels injected red. x 80. W*-v '.••: . • w N • • ' • IF 5 Fio. 202. — From a transverse section of a striated muscle of a rabbit; blood-vessels injected red. \ 100. MUSCULAR SYSTEM. 275 secondary bundles, to join finally with the perimysia interna to make up a continuous connective-tissue framework. The parts of this framework which enter the primary bundles to surround the individual muscle fibres usually contain very few elastic fibres and no fat cells ; while the larger strands separating the primary and secondary bundles are rich in both these elements. Blood-vessels and nerves enter the muscle in the connective- tissue septa and surround the muscle fibres The blood supply shows an exceptionally rich branching of capillaries around the muscle fibres. The blood-vessels enter the perimysium and run more or less parallel to the course of the muscle fibres (Fig. 201). In the perimysium between the primary bundles fine arterial brandies proceed at right angles from the larger trunks between the muscle fibres. From these, there run again at right angles — i. e., parallel to the course of the fibres — the capillaries, which form a fine network surround- ing the individual fibres. They run in large part parallel with the fibres, and send off quite frequently fine anastomosing branches, so that the meshes of the network are for the most part rectangular or rhomboidal. Each fibre is surrounded on all sides by capillaries, as may be seen in a cross-section of an injected muscle (Fig. 202). The veins arising from the capil- laries are characterized by the presence of valves, even in the finest branches. In the red muscles of the rabbit there are sinuses in many places between the arterial and venous ends of the network (Ranvier). There is in all muscles a definite blood vascular unit, special attention to which was called by Spalteholz. Arteries can be seen entering the muscle bundles at regular intervals (Fig. 201), and sending out capillaries on all sides. The veins col- lecting the blood from these capillaries are placed quite regu- larly. The unit thus has the artery for its centre and the collecting veins at the periphery. The nerves and their endings are spoken of in the section on Nerve-endings. 276 MICROSCOPIC ANATOMY OF THE ORGANS. Development of Muscles. In the early study of the growth of muscles it was claimed by some investigators (Schwann, Valentine) that muscle fibres are built up by the fusion of many indifferent cells. Remak claimed that the muscle fibre is derived from a single cell, a view which since has gained general acceptance. The way in which such fibres are joined together in the embryo to form definite muscles is not satisfactorily understood. Certain facts, how- ever, have been obtained as to the growth of embryonic muscles. The development of the human sartorius muscle has been worked over in recent years (MacCallum). At an early stage the cells making up the muscle are small and spindle-shaped, and are scattered in loose bundles. At first there are no fibril bundles, and the nucleus is placed centrally. Subsequently the fibril bundles appear around the periphery of the cell. The cells become more numerous and increase in size until the human embryo is between 130 mm. and 170 mm. in length from vertex to breech. At this stage the bundles of cells become more compact and the cells themselves are filled with fibril bundles as in the adult. The fibres now grow in length and thickness, but no longer increase in number. In embryos smaller than 170 mm. in length there is a progressive increase in the number of fibres found in a cross-section. After this, however, the number remains approximately constant. In other words, the fibres of the human sartorius do not in- crease in number after about the first half of embryonic life. After this period the increase in size of the muscle is due to growth of the individual fibres, and not to their multiplica- tion. Marpargo has observed that in the white rat the muscle cells continue to multiply for a short time after birth. Ac- cording to Meek, hyperplasia of the muscle cells ceases at birth, and after this there are a reduction in the number of fibres and a hypertrophy of those remaining. A vascular connective tissue separates the muscle bundles to form primary and secondary groups, which, according to Bardeen, are to be considered as units. MUSCULAR SYSTEM. 277 The muscles are connected with other parts of the body almost always by means of tendons. These consist, as has been stated, of connective-tissue fibrils, which are joined together by means of interfibrillar cement substance to form primary bundles. Many of these are combined by interfascicular cement substance to make up secondary tendon bundles (Fig. 203). The char- acteristic tendon cells lie between the primary bundles. The Pig. 203. Tendon cells tissue septa tj~ Connective- Fart of a cross-section of a human tendon (popliteal muscle), x 210. secondary bundles are surrounded by loose connective tissue containing elastic fibres and joined together to form tertiary bundles. The whole tendon is surrounded by a connective- tissue capsule, the so-called peritenonium. The tendon sheath consists of connective tissue lined on the inner surface with a layer of flat cells. The intimate connection between muscle and tendon is brought about by the direct transition of the perimysium into the connective tissue of the tendon (Fig. 204). In cases in which the muscle is fastened to the periosteum or fasciae, the perimysium alone effects the union by passing over directly into the periosteum or fascia. The blood-vessels of tendons are not abundant. They run in the loose connective tissue between the tendon bundles. The lymph-vessels form a rich plexus on the surface of the tendon. The nerves end on the tendons partly by means of 278 MICROSCOPIC ANATOMY OF THE ORGANS. arborizations, the so-called Golgi's tendon spindles, or by means of Vater-Pacinian corpuscles ; some end in the vessels. Fig. 204. }i MnscJe_ nucleus f I l : A Muscle Tendon iXi A 1 feasi Tendon From a longitudinal section through the gastrocnemius muscle of a frog, showing the transition from muscle to tendon, x 200. The fascics are connective-tissue membranes whose bundles of fibrils usually form interlacing layers. They contain as a rule a great many elastic fibres. VII. NERVOUS SYSTEM. 1. CENTRAL NERVOUS SYSTEM. A. Spinal Cord. Even with the naked eye, the gray and white matter can be distinguished in a cross-section of the spinal cord. The former occupies a central position and is surrounded by white matter. The relative amount of gray matter varies in different regions of the cord. In the sacral region it is present in larger amount than is the white matter (Fig. 208). In all parts of the cord Nerve PLATE XXXII. Dorsal median Faxcic. Fascic. septum qracil. nmeat. Dorsal ^— -i Lateral Ventral Ventro-medial Ventral median Ventral White fissure column commissure Gray eommisxure [left, central canal \ Fig. 205.— Transverse section of the cervical cord of man, at the level of the sixth spinal root. X 11. Fi<;. 206.— Transverse section of the dorsal cord of man, at the level of the eleventh spinal root. X 11. PLATE XXXIII. ; yscj3*' ^°' tt '' Fir,. 207. — Transverse section of the lumbar cord of man in the region of the lumbar enlargement. X 11. m ' - ' * : - < -i Fig. 208. — Transverse section of the sacral cord of man. n?!!-. Vj t, o ij; y zr fcj) ^* -^ , ! p. £ ° ? 3 3 ■=• & § § : a) £ -^ c y. SPINAL CORD. 279 the gray substance has in cross-section roughly the form of the letter H (Figs. 205-208), and, taken as a whole, consists of two long columns laid parallel to one another and joined together by the so-called gray commissure. Each of the col- umns is thicker on its ventral than on its dorsal side. There is therefore in cross-sections a wide ventral horn and a smaller dorsal horn. In the lower cervical and the upper thoracic, regions of the cord there appears the lateral horn {tractus inter- mediolateralis). In the same regions processes of the gray sub- stance extend into the white matter in such a way that a net- like structure is formed, containing in its meshes bundles of fibres from the white matter. This is known as the formatio reticularis, and appears at the junction of the ventral and dorsal horns (Fig. 205). From the ventral surface of the ventral horn, bundles of nerve fibres run out through the white matter, forming the so- called ventral root. Similar nerve bundles are present on the dorsal side, making up the dorsal root. With small magnification there can be distinguished in the thoracic region of the cord a well-defined group of cells known as the column of Stilling- Clarke, or the nucleus dorsalis (Fig. 207). It can be observed also in the upper part of the lumbar region. It occupies a position on the median side at the base of the dorsal horn opposite the formatio reticularis. Another conspicuous structure to be observed in the gray matter throughout the whole length of the cord is the substantia gelatinosa (Rolandi). This lies at the apex of the dorsal horn, and consists of small spindle-shaped cells with less neu- roglia than other parts of the gray matter (Weigert). This is seen in preparations made by Weigert's method as a light band across the end of the dorsal horn (Fig. 212). The gray commissure is a flat band of gray matter connect- ing the two lateral gray masses. In its centre is the central canal, which runs the whole length of the cord and is continu- ous with the cavities of the brain and medulla. The diameter of its lumen is usually about 1 mm. In embryos it is lined with ciliated epithelium and surrounded by the substantia 280 MICROSCOPIC ANATOMY OF THE ORGANS. yrisea centralis. In adults it is often partly obliterated on account of the growth of ependymal cells and neuroglia fibres. The gray commissure is divided by the central canal into a dorsal and a ventral gray commissure. The white matter, as already mentioned, surrounds the gray matter, and is separated into right and left halves by the fis- sura mediana ventralis in front, and the septum medianum dorsale behind. The former is a longitudinal fissure which Fig. 210. Longitudinal sections of ' edullated fibn Medullated nerve fibres cut across White matter Nerve ceils Gray matter Glia cell iw* jPt''; A The ventral half of the ventral horn from a calf's spinal cord. Section through the cervical enlargement. X HO. extends the whole length of the cord, but never is quite deep enough to reach the gray matter. A thin strand of white matter intervenes, and is known as the white commissure (Figs. 205 and 212). Each of these halves of the white matter is divided by means of the ventral and dorsal nerve roots into ventral, lateral, and dorsal columns (Figs. 205 and 212). On the surface of the cord this division is marked by the sulci (sulcus lateralis ventralis and dorsalis). PLATE XXXV. Fig. 'ill.— Diagram of the relations iif neurones in the spinal cord. Xpi', ; ..;.iV/ y,l !>-, f ;i/i)« if / 1 /I i ,ll( ■)'/■, ?j3 nV J >>?.v,'> , .f V)Oo\rt J fibres Flo. 212. — Cross-section of spinal cord of calf in cervical region. Stained by Weigert's method. Only one-half of the gray matter, with the surrounding white matter, is given. X 25. 3*^ •■;:£ /. mm WM % ymonowicz, Histology pilflSI? ^ SPTNAL CORD. 283 the ventral pyramidal tract (fasciculus cerebrospinalis ventralis) (Fig. 211). Its fibres run in the main from the cerebral cortex of the same side, and end by crossing over in the ventral com- missure and forming end arborizations around the motor cells of the ventral horn. Lying lateral to this tract on each side is the ground bundle of the ventral column. It contains the axones of column cells. In the lateral column (funiculus lateralis) we find the so- called lateral or crossed pyramidal tract (fasciculus cerebro- spinalis lateralis). This tract contains centrifugal fibres arising in cells of the cerebral cortex of the opposite side. The cross- ing of the fibres takes place on the lower part of the medulla oblongata. They end by arborizations around the ventral horn cells of the same side. This column, together with the ventral pyramidal tract, forms a crossed tract, which carries practically all the motor fibres on the cord. Peripheral to the crossed pyramidal tract lies the cerebellar tract (fasciculus cerebellospinalis dorsalis), which contains fibres derived from the axones of cells in Clarke's column (nucleus dorsalis). These fibres run up to the cerebellum. Ventral to this we find the column of Gowers (fasciculus ventrolateral is Gowersi). It has its origin in cells of the columns and runs upward to the cerebellum. The rest of the lateral column, the so-called ground bundles, consist of axones having their origin in cells of the columns. These axones divide into ascending and descending branches, which run only a short distance (short paths). The function of these bundles is to join together neighboring segments of the cord. The dorsal column (funiculus dorsalis) is formed from the fibres of the dorsal root, through which the axones from spinal ganglion cells enter the cord. On entering the dorsal column each axone divides into an ascending and a descending branch. Each of these gives off many side branches {collaterals), which enter the gray matter to end in fine arborizations. We find such end arborizations from the dorsal column in the nucleus dorsalis, in the substantia gelatinosa Eolandi, and in the ventral 284 MICROSCOPIC ANATOMY OF THE ORGANS. horns in the region of the motor cell groups (reflex collaterals). Only very few of these collaterals pass through the dorsal com- missure to the opposite side. The descending branches of the dorsal column fibres run only for a short distance, while the ascending branches reach usually as far up as the medulla, where they end in the nuclei of the columns of Goll and Burdach. The fibres in their course upward tend to approach the median side of the dorsal column, while the newly entering fibres of the dorsal root are always lateral to those arising in ganglia Fig. 214. Central canal Gray matter Developing astrocyte Dorsal root Ependyma cells Ventral^ root Transverse section through the spinal cord of an eight-day chick. Left, nerve cells; right, neuroglia cells ; a and t, motor cells ; c, cells of lateral columns ; d and c, hecatero- nieric cells. X 80, lower down. Thus in cross-sections of the cord the fibres enter- ing low down (e, g., those supplying the lower extremities with sensory nerves) are situated always near the septum dorsale in the fasciculus gracilis ; while similar fibres for the upper extremities are placed quite laterally in the fasciculus cuneatus. A slight addition to the fibres contained in the dorsal column is afforded by axones from small cells on the dorsal horn. These fibres after running for a short distance in the fasciculus cuneatus sink into the gray substance. SPINAL COED. 285 The fibres making up the white matter are medullated. They all lack the sheath of Schwann, and in consequence of this show no nodes of Kanvier or segmentation. Not until we reach the roots do the fibres show a neurilemma and nodes of Eanvier. In observing a cross-section of the cord (Fig. 213), we notice a difference in thickness in the fibres. In the fascic- ulus cuneatus and the funiculus ventralis are to be found the largest fibres ; while those of the smallest diameter are seen in the fasciculus gracilis and the funiculus lateralis. In such a section it is to be noted that the great majority of the fibres are cut transversely — i. e., they run parallel to the long axis of the cord. Diagonal and transverse fibres are relatively rare. The supporting framework of the cord, as well as of the whole central nervous system, consists of neuroglia. This is of ectodermal origin, and arises in a way quite similar to the rest of the nervous system. In the study of neuroglia there are to be considered the neuroglia cells ami neuroglia fibres {glia cells and glia fibres). In the medulla of adults we find glia cells of two kinds, the so-called ependyma cells and the astrocytes (Deiters' cells). Ependyma cells are cylindrical cells bordering on the central canal. They form either a single layer or are arranged in two or three rows. In embryonic life these cells are ciliated on the surface toward the central canal, but these cilia disappear later on. Toward the surface of the cord each cell sends out a long, fili- form process (ependyma fibre), which enters the gray substance, and in the embryo reaches the surface. In post-embryonal life the ependyma fibres reach the periphery of the cord only in the region of the septum dorsale. The ependyma cells are phylogenetically and ontogenetically the oldest neuroglia cells, from which the astrocytes take their origin. A part of the cells, which arise by division of the ependyma cells, leave the region of the central canal, and, moving peripheralward in the gray and white matter, become astrocytes. The astrocytes are small nucleated cells containing little protoplasm. They are more or less stellate in outline, and owe their name to this peculiarity. According to the length 286 MICROSCOPIC ANATOMY OF THE ORGANS. of the processes, we speak of astrocytes with long rays or short rays. The glia fibres, which formerly were believed to be cell processes, are to be considered as entirely independent ele- ments. They react to certain coloring reagents quite differ- ently from the cell protoplasm or its processes, and pass through the cell body, so that their course can be followed uninter- ruptedly. They usually run through the outer layers of the cell body or lie on the cells. They are jjrobably products of the cells which have become so much emancipated from the cell body that some of them seem to have no definite con- nection with the cell. These fibres are of different thicknesses and form a dense network. A considerable aggregation of neuroglia is found around the larger nerve cells, in the region of larger vessels, and especially around the central canal [cen- tral glia-mass, substantia grisca centralis). It also occurs on the periphery of the cord {superficial glia capsule). Concerning the significance and function of the neuroglia, many theories have been advanced. According to Golgi, the neuroglia serves as a source of nourishment for the nerve cells. Ramon y Cajal claims that it has an insulating function in connection with the neurones. Weigert considers that it serves only as a supporting tissue to fill up the spaces between the neurones. According to R. Krause, the cells and fibres form paths for the circulation of lymph. For a detailed description of the medulla, pons, midbrain, and the higher centres, the reader must be referred to special text-books on the subject. 1 In the space at our disposal only a brief account can be given. 1 Barker, L. F.: Nervous System, Appleton, New York, 1899. Edinger : Bau der nervosen Centralorgane, Leipsic, 1893. v. Kolliker: Handbuch d. Gewebelehre, Bd. II., Leipsic, 1896. van Gehuchten: Anatomie du systeme nerveux de l'homme, Louvain, 1897. Sabin, F. R. : Atlas of Medulla and Midbrain, Baltimore, 1901. MEDULLA, PONS, AND MIDBRAIN. 287 B. The Medulla, Pons, and Midbrain. The brain-stem, comprising the medulla, pons, and mid- brain, is the pasasge-way between the cord, the cerebellum, and the cerebrum; and, at the same time, a great reflex centre with its own nerves, both motor and sensory. It will be con- sidered under four heads : (1) the tracts that connect the cord with the cerebrum ; (2) the tracts that connect the cerebellum with the cord and the brain ; (3) its reflex centres ; and (4) its nerves. Group 1. — Two tracts connect the cord with the brain, a sensory and a motor. The latter is called the pyramidal tract. The sensory path contains a part of the ventral and lateral columns of the cord and almost all of the dorsal columns. In entering the medulla, some of the fibres of the lateral and ventro-lateral columns of the cord curve a little dorsalward and inward, to make two bands of fibres that pass upward in the medulla on either side of the raphe. These two bands are the sensory path, here called the interolivary bundle. At the same time the central canal of the cord curves dorsal- ward and opens into the fourth ventricle. The roof of the fourth ventricle is at first a thin veil of tissue, but opposite the pons it becomes the cerebellum (Fig. 215). By this thinning out of the dorsal wall the dorsal columns of the cord are pushed outward to end in the two nuclei that make the prominences on the surface of the medulla just above the clava. From these nuclei, which represent the spinal nerves, as well as from all the sensory nuclei of the medulla and the pons, fibres curve across the brain-stem, decussate in the raphe, and enter the sensory tract. These fibres are called the internal arcuate fibres. Throughout the medulla the two bands or sheets of fibres forming the sensory path are parallel. In passing into the pons, however, the ventral fibres spread out like a fan into a horizontal sheet, which divides the pons into two parts, a •dorsal and a ventral. Here the bundle is called the medial lemniscus. In entering the midbrain the sheet curves outward 288 MICROSCOPIC ANATOMY OF THE ORGANS. and rotates partially, so that it becomes oblique. In the mid- brain the pyramidal tract lies external and ventral to the medial lemniscus, but in passing upward the sensory tract passes in front of the pyramidal tract, and the two bundles to- gether make the internal capsule which lies just external to the thalamus and is connected with the region of the cortex around the fissure of Rolando. The form of the sensory tract is emphasized, because all the other structures are related to it. In the medulla it is the medial, vertical sheet; opposite the ventral part of it is the olive ; opposite the dorsal part of it is the area of the formatio reticularis, which contains all the nerves of the region. In the pons the sensory tract forms a horizontal sheet. Ventral to it lie the pontal nuclei; while dorsal to it lie the formatio reticularis and the nerves. In the midbrain the sensory tract is an oblique sheet. It lies between the red nucleus and the formatio reticularis on the inside, and the pyramidal tract on the outside. The pyramidal tracts start from the cerebral cortex around the fissure of Rolando, and pass downward in the posterior part of the internal capsule into the peduncle or midbrain. Here the tract is a compact band of fibres external to the medial lemniscus. It passes into the ventral part of the pons, where it is broken into small bundles by the cells of the pontal nuclei. In entering, the medulla, these bundles collect into a tract that passes to the cord just ventral to the interolivary bundle. At the lower end of the medulla these fibres decussate in the raphe. Part of them enter the ventral columns of the cord, part cut through the ventral horn and enter the lateral col- umns. In the brain -stem, fibres leave the pyramidal tract, decussate in small bundles or as single fibres, and enter the motor nuclei. Group 2. — The cerebellum has three peduncles — inferior, middle, and superior. The inferior receives fibres from the cord and the medulla. The direct cerebellar tract, which is a narrow band on the surface of the cord, becomes a compact bundle in entering the medulla, and receives a group of fibres from the dorsal columns of the cord (Fig. 215). These fibres, r > H X X < X X h J (1 E it MEDULLA, PONS, AND MIDBRAIN. 289 together with bundles from the olive and the vestibular nuclei, make the inferior peduncle. It passes upward on the surface of the medulla, between the cochlear and vestibular nuclei, to the lower end of tbe pons, where it turns dorsalward, enters the cerebellum, and decussates in the roof of the fourth ven- tricle. In passing dorsalward it lies just outside the dentate nucleus, which receives the superior peduncle. The superior peduncle starts in the red nucleus. Its fibres decussate in the dorsal part of the pons, and enter the cerebellum just internal to the inferior peduncle. The fibres of the middle peduncle come from the pontal nuclei. They decussate in the pons and enter the cerebellum external to the inferior peduncle. Group 3. — In the cord the gray matter between the two horns is much broken by fibres, and is called the formatio reticularis. It is a reflex centre. In the brain-stem it is greatly developed. It occupies the dorsal half of each region. It contains countless cells and fibres. The fibres are either scattered or in more or less definite bundles. One short patli in the brain-stem is very distinct, the posterior longitudinal bundle. It receives fibres from the ventral column of the cord and lies just ventral to the central canal throughout the brain- stem. It receives descending fibres from the midbrain. Group 4- — The nuclei of the motor cerebral nerves are de- rived from the ventral horn ; of the sensory, in part from the dorsal horn. Of the motor nuclei (Fig. 217), four lie near the raphe just ventral to the central canal. They are the hypoglossal in the medulla, the abducens in the pons, and the trochlear and oculomotor in the midbrain. The fibres of all these nerves, except the trochlear, pass ventralward and emerge near the median line. The fibres of the trochlear nerve pass dorsal- ward, decussate in the velum, and pass out near the median line. The other four motor nerves, the spinal accessory, the glossopharyngeal and vagus together, in the medulla, the facial and the trigeminal in the pons, lie farther lateral and ventral. The fibres of all these nerves, except the trigeminal, pass inward toward the central canal, there turn outward and ventralward to emerge on the ventral surface in a lateral line. 19 290 MICROSCOPIC ANATOMY OF THE ORGANS. The fibres of the trigeminal pass directly to a surface origin in the lateral line. The sensory nuclei (Fig. 216) represent the dorsal horn, and lie for the most part in the dorsal part of the medulla and pons. The type of a sensory nerve in this region is to divide into a long descending and a short ascending tract, each of which is accom- panied by a nucleus. The long descending tract of the glosso- pharyngeal and vagus is the tractus solitarius, which lies in the border of the central gray matter of the lower half of the medulla. It has its own nucleus. Parallel and just internal to it is another long nucleus, the ala cinerea, belonging to the same nerves. The tracts of the vestibular and trigeminal nerves are par- allel, the former lying just dorsal to the latter (Fig. 216). The descending tract of the vestibular nerve is in the medulla, while the ascending tract enters the pons. The cells which accompany these two tracts have received three names : those opposite the descending fibres are called the median nucleus ; those opposite the ascending fibres, the siqoerior nucleus ; while a small group of cells in the angle of the two tracts makes the lateral nucleus. The lateral nucleus places the nerve in com- munication with the cerebellum. The trigeminal tract is Ions:, covering half of the pons and all of the medulla. The descend- ing tract joins with Lissauer's zone in the cord, and its nucleus joins the posterior horn. Lissauer's zone is external to the substantia gelatinosa of Rolando. The ascending fibres end in the main sensory nucleus. The cochlear nerve has no descending tract, but a long and complex ascending one. Its fibres end in a nucleus on the lateral surface of the medulla (Fig. 215). Part of the fibres pass inward, dorsal to the inferior peduncle, and decussate in the floor of the fourth ventricle as the striae acusticse ; others pass ventral to the peduncle as a compact bundle, which, as it decussates, forms the trapezoid body. The fibres of the trape- zoid body and the stria? acusticse make the lateral lemniscus, which passes upward and dorsalward through the pons to the inferior colliculus. Here many fibres end ; others pass to the PLATE XL. ;•• '/v '•.". "," C M '•■■./'.'..Si 'fh'in '{'• " •' V ■• . '' L x ■'•'>'.■'>'■ . .''•.', - ';':'.:v^',-'-4;;i;.' j ;::: * A > '•■ ' ' 'U'i 1 .; Molecular layer Layer of small pyramidal cells ■ ' ;i •■'■', ^ ■ • ', " ■, , .' ."■' i - ; Befell , '-.4 ' .' a &M Layer of large pyramidal cells ■M i. a I ■■>■■.;.. r • r & v ; .;. ... r,% ■'*■ ; . " * a :■■ &£ k i$. -'m Layer of poly- morphous cells Medullary substance Via. :>1K. — Part of » perpendicular section through the cerebral cortex of man X TO. PLATE XLI. CaZ 01, Fir,. 219— Diagram of the structure of the cerebral cortex. (Prepared by Golgi's method ; partly after Stohr.) CnZ, (iijal's cells; GZ, (iolgi cells; Pi, small pyramidal cell ; P-i, large pyramidal cell ; I'm, polymorphous cell ; Rf, Ramon's fibre; Gh, Glia cell of the superficial glia layer (cell of Kctaius) ; Gh, short-rayed cell ; Gh, long-rayed cell ; M, medulla. OEKEBltAL CORTEX. 291 cortex of the temporal lobe through the medial geniculate body and the internal capsule. The sensory path of the facial nerve — that is, the pars inter- medius — is not known. The fibres of the optic nerve enter the brain in the region of the thalamus. A part of the fibres pass downward into the border of the superior colliciilus; the rest enter the internal capsule just posterior to the pyramidal tract fibres and pass out to the occipital lobe. The fibres of the olfactory nerve have no direct connection with the brain-stem. They enter the olfac- tory bulb, beneath the frontal lobe, and pass to the cortex of the frontal and temporal lobes. Only a small part of the cortex of the brain represents the nerves of the body. In general, the area around the fissure of Rolando receives sensory impressions from the entire body and sends out the fibres of voluntary control. The special senses are represented as follows : sight, in a small part of the occipital lobe ; hearing, in a part of the temporal lobe ; and smell, in a part of the temporal and frontal lobes. All the rest of the cortex is the " great silent area," or the association centres of the brain. These areas are connected richly by fibres both with the same side and with the opposite side of the brain. These are the association paths which make the brain the organ of thought. C. Cerebral Cortex. The cerebral cortex shows certain differences in structure in different regions, into the details of which we cannot here enter. All regions have a structure which conforms to one type, which will be described. The cortex consists of gray substance in which four layers can be recognized. These pass over into one another without sharp boundaries (Figs. 218 and 219). Beginning at the outside, we meet with the following structures : 1. The Molecular Layer (Stratum Zonale).- — This is a layer which is poor in cells, but shows a finely granular and reticular structure. This is due partly to the interlacing dendrites and axones of nerve cells whose bodies are situated more deeply, but 292 MICROSCOPIC ANATOMY OF THE ORGANS. mainly to nerve fibres lying parallel to the surface, the so-called tangential fibres. In this layer we find the cells of Gajal, which are spindle-shaped, pyramidal, or stellate cells, whose processes run horizontally in every direction, giving off fibres to the sur- face of the cortex. These are considered generally as nerve cells. 2. Small Pyramidal Cell Layer. — This zone owes its name to the fact that it is made up of comparatively small cells of a pyramidal form, so arranged that the apices are directed toward the surface of the cortex, and the bases in the opposite direction. Fig. 69 represents such a cell and illustrates the relation of the axone and dendrites to the cell body. From the apex of the cell the main dendrite proceeds toward the surface of the cortex, passing through almost the entire thickness of the molecular layer. Throughout its course it gives off numerous side branches, and ends freely after many arborizations in the outer part of the molecular layer. Other smaller dendrites proceed from the lateral and basal parts of the cell. 'The axone usually emerges from the middle of the basal surface, and runs toward the medullary substance. It gives off many collaterals on the way which run parallel to the surface. 3. Large Pyramidal Cell Layer. — This is made up of cells quite similar in general form to those of the layer just de- scribed. They are, however, much larger, especially in the motor area of the cortex. The axones of these cells in part enter the pyramidal tracts of the cord. 4. Layer of Polymorphous Cells. — In this layer we find polygonal cells, which give off dendrites, and send axones into the white matter. There are found also spindle-shaped cells in this region. These cells, which are typical for individual layers, and send their axones far beyond the cortex, are known as Golgi cells type L In addition there are numerous cells whose axones never reach outside the limits of the cortex, and end not far from the cell body. These are known as Golgi cells type II. Some of these cells send their axones toward the molecular layer, instead of toward the white matter, and are known then as cells of Martinotti. The various layers are made up not only of cells, but contain CEREBELLUM. 293 also networks of medullated nerve fibres. A part of these fibres run at right angles, while others lie parallel to the surface. Among the former are the axones derived from the pyramidal cells, as well as those fibres proceeding to the cortex from lower down in the central nervous system. These fibres run in bundles through the third and fourth layers up to the layer of small pyramidal cells. They form the so-called radial bundles. The strands of fibres running parallel to the surface are de- rived from many sources. The outermost forming the tangen- tial fibres, and those contained in the small pyramidal cell layer, as well as those belonging to the so-called suprarad-ial network, represent largely the side brandies of fibres running from below up to the brain surface. The deeper fibres cross the radial bundles and form the so-called interradial bundles. Some of these run in the large pyramidal cell layers, and form there the horizontal fibre tracts of Gennari or Baillarger. The fibres of the interradial network are formed from collaterals of axones of the pyramidal cells. The neuroglia distributed unequally in the different layers of the brain consists of two elements, the glia cells and glia fibres. By means of the Golgi method the following forms of glia cells have been made out : short-rayed cells (Fig. 219, Gl 2 ), which lie in the gray matter and possess much-branched processes ; long-rayed cells (Gl 3 ), which lie mainly in the white matter and possess fine processes branched only slightly ; and arborescent cells (Gl,), which lie at the surface of the cortex and send their processes outward. Weigert's method which gives a special differentiation to the glia fibres, shows in the outer layer of the cortex a thick corti- cal sheath, composed of a rich tangentically placed plexus of glia fibres In the deeper layers of the cortex the glia fibres are not so abundant, while in the medullary substance they form again a dense network. D. Cerebellum. The layers of the cerebellum are marked off much more sharply from one another than those of the cerebrum (Figs. 294 MICROSCOPIC ANATOMY OF THE ORGANS. 220 and 221). Three main layers can be distinguished, namely, the granular, ganglionic, and molecular. 1. The granular layer lies immediately on the medullary substance. In it there are to be seen two kinds of cells: (a) small granular cells and (b) large granular cells. (a) The small granular cells form the greater proportion of this layer. They are very small multipolar nerve cells, which Fig. 222. Th, Diagrammatic representation of a longitudinal section through a convolution of the cerebellar cortex. (After v. Kolliker.) P, Purkinje's cells; p, axone of cell of Purkinje; A", granular cell ; h, axone of a granular cell ; Th, place of division of axone of granular cell. give off an axone and a few dendrites ending in claw-like arbor- izations. The axone runs outward at right angles to the sur- face and divides in the outer layer of gray matter like the letter T. The two branches run parallel to the surface of the cortex and to the long axis of the convolution, and end freely. This is shown in Figs. 221 and 222. PLATE XLII. Nuclei of nerve cells and of nenroijllu Small granular cells &* ~^w^^:i^^^^^ CS* -* " *— ■— -: '•^'*,. S ^Cv, &!l*_^2!L£Z5— j5i '»* iT^* 3Bti$ % ^Medullary substance Jjarai. i£i— Fig. 220. — Part of a perpendicular section through the adult human cerebellar cortex, x 158. PLATE XLIII. Fig. 221. — Diagram of the structure of the cerebellar cortex. (Prepared by Gold's method ; in large part after v. Kolliker. ) P, Purkinje's cells; p, axones of Purkinje's cells with returuiug collaterals; A'te, basket cells; K b, baskets which surround the bodies of the Purkinje's cells: A' small granular cells, whose axones penetrate the molecular layer and then appear in cross-section as fine dots; (jrK, large granular cells; m, small cells of the molecular layer; Mf, mossy fibres; "'''' W nerve fibres'-., ji*)*^. W.'^.^iVf^'fi'v ■'■'.' ft- Nucleated £ \;<, '- /fi's, ,. <£ ;' '. ■'■$.-■ " capsule-,.. *%' **$?®" , V'l*'--" surface aT ""*""**.£?£■ i. '* & ? ^ Longitudinally t , JS^T ch( nerve _#&re -^e ^ £;/ Transversely cut nerve fibre' r "' '"-' "=' ^^ucleus with $£— — *£/" nucleolus ■ '-Mi- •aiiiji* . ^Nucleated *&~ "^ capsule Protoplasm with concen- trically arranged granules From a transverse section of a spinal ganglion of a rabbit. X 400. of the nerve bundles passes over into the ganglion, carrying blood-vessels between the ganglion cells. Numerous capil- laries, formed from the breaking up of the arteries, surround the individual cells. 302 MICROSCOPIC ANATOMY OF THE ORGANS. We recognize two types of ganglia: the spinal ganglion type and the sympathetic ganglion type. The spinal ganglia contain in the lower vertebrates (fishes) and in the embryos of higher vertebrates bijoolar cells ; while in the adult of the latter class the cells are almost all unipolar. The cell body is usually large (40-70 /i in diameter), and con- tains a vesicular nucleus with a distinct nucleolus (Fig. 226). Yellowish-brown pigment granules are also often found. There is always present a nucleated capsule around the cells, which is probably only a continuation of Schwann's sheath. It is made up of a single layer of flat connective-tissue cells (Fig. 226). The relations of the processes of these cells and their branches have been investigated in recent years by Ramon y Cajal, Dogiel, and others. According to the results of this work, we can distinguish in the spinal ganglion two kinds of ganglion cells : one in which the cell process divides like the letter T or Y into two or three branches, which run in oppo- site directions. These branches are medullated, and run for some distance outside the ganglion. This cell belongs to type I. The cell of type II., whose jirocess breaks up into numerous branches, is confined to the ganglion. None of the branches extends beyond its limits. They break up, on the con- trary, into a plexus which surrounds the nucleated capsule of the cells of type I. From this plexus fine branches break through the capsule and surround the cell itself (pericellular plexus). One cell of type II. is related usually to many cells of type I. ; and many cells of type II. take part in the formation of the plexus around each cell of type I. Besides these nerve elements already described, there are present in the spinal ganglia, endings of nerve fibres arising in sympathetic ganglia. These fibres break up into fine branches, which surround the cells, penetrate the capsule, and give rise to a pericellular network. These sympathetic fibres are related especially to cells of type II., and by means of them to cells of type I. GANGLIA. 303 A similar structure as that described is found in the ganglion Gasseri, ganglion jugulare, plexus nodosus n. vagi, ganglion petrosum n. glosso-pharyngei, and the ganglion geuiculi n. facialis. The ganglion spirale cochleae and ganglion vestibulare are distinguished from the spinal ganglia by the fact that the cells are bipolar. The sympathetic ganglia contain multipolar ganglion cells which are smaller than those of the spinal ganglia (13-40 n in diameter). They contain pigment granules and often two nuclei, and are surrounded, like the spinal ganglion cells, by a nucleated capsule. These cells give off an axone which possesses no medullary sheath and becomes a fibre of Remak ; or may, on the other hand, become medullated and run peripk- eralward. All these fibres originating in sympathetic cells are to be regarded as cellulifugal. They end either in the smooth muscle of the intestinal walls, the vessels, the arrec- tores filorum, the iris, the corpus ciliare, etc., or in the mucous membranes, and the glands (liver, kidney, etc.), where they influence the secretory function. The dendrites, of which the sympathetic cells possess many, are short. They branch many times and form at their ends fine networks which surround other cells. Besides the cells, we find in the sympathetic ganglia nerve fibres partly medul- lated and partly non-medullated. The latter arise from the cells of the ganglion itself, while the former are medullated cerebro-spinal fibres which have passed over to the sympathetic system through the rami communicantes. These are partly sensory and partly motor fibres. The sensory ones run to the periphery and end there ; the motor end, on the contrary, in the sympathetic ganglion, where they form pericellular networks around the ganglion cells. In this way the sym- pathetic cells are influenced by the motor fibres of the cerebro- spinal system. The sympathetic nerve cells themselves, how- ever, send their axones to the periphery, where they end freely, so that the cerebro-spinal fibres may be considered as motor 304 . MICROSCOPIC ANATOMY OF THE ORGANS. fibres of the first order, while the sympathetic axones are motor fibres of the second order. It must be noted that in the sympathetic ganglia of am- phibians there are unipolar cells without dendrites. The one process is to be regarded as an axone. It is surrounded by a spiral fibre which branches and forms an end plexus around the ganglion cells. These spiral fibres are derived from cells lying at a distance, and represent motor fibres of the first order. Among sympathetic ganglia are to be considered, the ganglion ciliare, splenopalatinum, oticum, and submaxillare. C. Nerve-endings. The nerve-endings are the final terminations of individual neuroues. By means of these the nervous system is put into communication with other organs and tissues, and in the nervous system itself individual neurones or segments are joined together. It is through their agency that sensory im- pulses are sent to the central nervous system, and motor im- pulses transmitted from the nervous system to peripheral organs. We distinguish free nerve-endings in which the much- branched nerve filament receives or gives out impulses without the intervention of other tissues ; and nerve-endings connected with some specialized end apparatus. In the latter the nerve- ending is combined with other tissues to form a special struct- ure. We can classify nerve-endings according to the tissue in which thev have been formed. Thus we find nerve-endings in: 1, epithelium; 2, connective tissue; 3, muscle; and 4, nervous tissue. Finally, one may also consider nerve-endings from a physio- logical standpoint ; but here unusual difficulties present them- selves. A classification of nerve-endings according to their functions is not practicable as long as we are ignorant of the anatomical difference between centripetal and centrifugal nerve fibres. For example, in glands we do not know which endings are secretory and which are sensory. Also the division of sensory endings according to their powers of transmitting special sense impressions (temperature, pressure, pain, etc.) is NEB VE-ENDINGS. 305 not of great value. In the following description the nerve- endings will be taken up according to the tissues in which they occur. (1) Intra-epithelial Nerve-endings. We can, in the first place, distinguish free nerve-endings (Fig. 227), which innervate especially the epithelium of the mucous membranes and epidermis. The nerve fibres run in bundles in the underlying connective tissue up to the margin of the Fig. 227. >3ar«e? Vertical section through the skin of a pig's snout, which contains free intra-epithelial nerve-endings and Merkel's tactile corpuscles. Stained with gold chloride. X 300. epithelium. Here they lose their various sheaths and the naked axis cylinders pass over into the epithelium and break up into fine branches. Such fibres reach often up to the outer layers of the epithelium (J «->fyi.-j* Jdgh i* trr> .r/jjuil mjnind Jlwha 1" ui;[< dyinn/l) unit'..-.-'.— .I4i .; Fig. 241. Fat cells IG. 240.— From a cross-section of the human scalp. The hair is cut throiignout its whole length, toxylin and eosin. X 55. IG. 241.— Section through akin of adult human finger, cut at right angles to the surface. Hiemato: eosin. x 70. mm '•:-■*■'. i*'- .\ '..■'■-. ^ jf.-.,:,^ . Is • 1( r * ''. "si .V :- : \v,.-' Szymonowicz, Histology ] Barafz ai rial* dtl. litkJ.r:!. vW'trrsi kWi°1-r. Fninklu THE OUTER SKIN. 319 a compact layer, the so-called corium, and a deeper-lying loose layer, the tela subeutanea. The boundary between the connective-tissue part and the epidermis is usually uneven. This is caused by the fact that the corium immediately under the epidermis is raised into con- ical or round papilla; (Figs. 241, 243, 244). These extend into the epidermis, and are of different size in various parts of the body. The largest are in the planta pedis, vola manus, glans penis, etc., where they reach a height of 0.2 mm. In other Fig. 242. Corium Diagrammatic section through the skin. This figure serves to show how the section in Fig. 240 is cut. The line x-y gives the direction of the section. S, sweat gland ; P, papilla; M, Meissner's corpuscle. places (e. g., in the skin of the face) they are inconspicuous. We divide the papilla?, according to whether they contain loops of blood capillaries or nerve corpuscles, into vascular and nervous papilla?. The corium consists of white fibrous connective tissue, the fibre bundles of which cross one another in different directions. In the network thus formed we find connective-tissue cells of various kinds, and a plexus of elastic fibres which is denser in the deeper layers. The corium may be divided into two layers : the pars papillaris and the pars reticularis. The first, which lies imme- diately under the epidermis, owes its name to the fact that it contains the papillae ; while the pars reticularis is so named on account of the net-like arrangement of the connective-tissue bundles. These bundles cross one another in such a way that there are left rhomboidal spaces or meshes (Lange's spaces), which are filled with sweat glands or fat. The two layers of the corium pass into one another without any sharp line of demarcation. 320 MICROSCOPIC ANATOMY OF THE ORGANS. In the corium we find in certain places (e. g., in the face) striated muscle fibres, extending up to the pars papillaris. There occur also smooth muscle cells, which run in bundles parallel to the surface and form special networks in the skin of the scrotum (tunica dartos) and the nipple. The smooth muscles of sweat glands and those which are connected with hairs will be spoken of later. The subcutaneous tissue (tela subcutanea) which joins the skin to the neighboring parts is made up of interlacing con- nective-tissue strands, in the meshes of which fat is found ('Fig. 241). When the fat reaches a considerable development, this layer is spoken of as the panniculus adiposus. In a few exceptional instances the fat is entirely wanting in the sub- cutaneous tissue (e. g., in the outer ear, the scrotum, etc.). The more horizontal — i. e., parallel to the surface of the skin — the connective-tissue bundles run, the longer they are and the greater is the movability of the skin. The wrinkling of the skin is dependent on the length of these bundles. If they are short, they run more nearly at right angles to the surface of the skin, and the latter cannot be moved or thrown into folds. At the boundary between the corium and the epidermis we find a very thin structureless membrane, the so-called basal membrane. The epidermis is composed of a many-layered epithelium. Two parts in this may be distinguished : the outer one, which consists of corneous cells (horny layer, stratum corneum), and the deeper -lying part, the so-called Malpighian layer (stratum Malpighii, stratum germinativum). The latter may again be divided into many layers, which, spoken of from below up, are the stratum cylindricum, stratum spinosum, stratum granulosum, and stratum lucidum (Fig. 244). The degree of development of the horny layer and the Malpighian layer differs in various parts of the body. Usually the latter is the thicker of the two, but in the vola manus and the planta pedis the horny layer is greatly thickened. The two lower layers of the stratum Malpighii consist of prickle \0 aitnW.j* Jty.oO iaomlfi ai uoijyaa oriT .i9gaft aeniiirf ts To airat9f)iq*) »di dsuo-rdl aoit-j'w luJuosriori b moil — .8*5 ."* X .t-'ti-' •si'J "' 3- 9«'I aril zd awoda s« ,nij[a sdl lo 99&hr>< adi o) teiljn&'q ittJi'HO-Suwntu ai:).iJf wyV^T VA! T' V •- G ,A A-/ A^-^^ ^-». .men .00* r. .Vioiilua £i£ca»d lli/fus iib lo xi'iljsd 9d) 'lo m>( • oiH rigucndJ jh>;j-.\h-. fi tuo'il — .*■!■£■' Cross sections of Meissner^s corpuscles Sweat ducts p Stratu spiriom Fig. 243. — From a horizontal section through the epidermis of a human finger. The section is alm< parallel to the surface of the skin, as shown by the line z in Fig. 242. X 88. Stratum f£? jylindricum. xtercellular / O bridges ' Stratum Q papillare corn. Capillary loops Fig. 244.— From a section through the skin of the hallux of an adult human subject, x 400. mi .'.'..•;■•." i'V ^ft ...v...;! <■ v. :■'.•..' l^.v^s^sSl {?•■■:•■ ■' \ l^ ■:■ :':■,.••• -.O ;• \ v *;.!r.-s>-.--'..--V'. V >■'•••■ : i'V/?;\-A V''-'..-.-! :'.•':;';.; •"■>■.'•.-•'* l:..M:.-.v : Vi^."!'.* A.y--'-.vA to/ ^ *t '■*''■■ Szymonowicz, Histology h* C?r^-^ ? " l'^~% »"%?* 7 Borofz ad not. 3/1. u Lith.Anst,v.Werr.er iWinter, FrarMui THE OUTER SKIN. 321 cells. The lowermost layer has quite high cylindrical cells lying beside one another. The prickles directed downward, with fine fibres extending between them from the corium, as well as the presence of cement substance, provide a means of joining the epidermis firmly to the corium. The stratum spinosum, whose cells have been described above, consists of many rows of cells which fill up the free spaces between the papillae. Above this is the stratum granulosum, consisting usually of two or three rows of flattened cells. These pos- sess refractive granules, which indicate the beginning of the process of cornification. They are spoken of as keratohyaline granules, and are regarded by some authors as modified cell protoplasm, and by others as a product of the dying nucleus of the cell. This latter view finds some support in the fact that often the development of keratohyaline granules is accom- panied by a poverty of the nucleus in chromatin and its final disintegration. Above the stratum granulosum there is a refractive layer, the stratum lucidum, which consists of two or three layers of flat cells. These possess disintegrating nuclei, and contain a homo- geneous substance called eleidin, which is derived from the keratohyaline granules. The latter increase in size and coalesce to form a semifluid substance, which develops new staining re- actions. Keratohyaline stains with hematoxylin, while eleidin is colored by eosin or nigrosin. In. this layer the boundaries of the cells are often not distinct. The stratum lucidum is often wanting in places where the epidermis is thin. It forms a direct transition to the horny layer. The cells of the horny layer {stratum corneum) are like thin scales and show no remains of nuclei. The whole cell is made up of keratin, which, as opposed to eleidin and keratohyaline, can be dissolved neither in trypsin nor in pepsin. In sections treated with osmic acid the horny layer shows on its upper and under surfaces, as well as on its sides, a black boundary, which is due to impregnation of the dried layer with fat. The middle part of the horny layer cannot be blackened by osmic acid. 21 322 MICROSCOPIC ANATOMY OF THE ORGANS. The horny cells are continually rubbed off from the surface of the skin, and new cells are added from the basal layers of the stratum Malpighii. In the lowest layers of the epithelium we meet with karyokinetic figures. The young cells are pushed out by still younger cells toward the surface. The skin of the white race is in various places colored brown by the deposition of pigment (e. g., the skin of the nipple, the labia majora, the scrotum, and around the anus). Much coloring-matter is found in the skin of the negro. Here very fine pigment granules are present between and in the epi- thelial cells of the lowest layer of the stratum Malpighii, and also in the outer parts of the corium in branched connective- tissue pigment cells. The origin of the pigment is not defi- nitely known. Some authors claim that the epithelial cells have no power of producing pigment, and that the pigment granules are imported by connective-tissue cells. Other au- thors, on the contrary, hold that the cells of the epidermis are capable of producing pigment granules without the assistance of the connective-tissue cells, since it is an undoubted fact that the pigment of the retina is a product of epithelial cells. (5) Hairs. The hairs are thread-ljke structures formed from the epi- dermis, which are distributed over the whole surface of the body with the exception of the palms of the hands, the soles of the feet, the red borders of the lips, and the inner surface of the preeputium. A part of the hair (hair root) is buried in the skin ; while a part projects beyond the surface (hair shaft) (Fig. 240). The lower part of the hair root is thickened to form a rounded, knob-like structure, the hair bulb. Into this there is pushed from below a small round mass of the corium, which is called the hair papilla. In small, fine hairs, the root extends into the corium, while the roots of large hairs reach as far as the subcutaneous fat. A true hair consists of horny epithelium. The part which is situated in the skin is surrounded by several layers of epi- 3!>>mS»Si!>i iiwYVtoO aioiV'fj f'ui'ft •6-yi ^uoSS Soot \o oS'jsSmO ■lYmv, l„,o\a VJ\M)\ ('..!,.)& \08SbO •' jUr^;,> ^oot i9iuO i[Ii;* UBfimH .Joot lo 31j;ii , i")W"J ai -jbiltnt -riiiil f>fi£ -iij;if is 1<> n- >f f-j-i^---' . i' )— .utS ml .001* >' .iuv>'i I>nj5 ail^yoljujigjH o o j -"' &Mt»)«.Ul ■■Sj'i"-, { , -, | -"O I 0\ „°l^ j>ju^v.i5vft Sit mWoO <= => CJ~ c — > ■—) i- O/j'^m Yi>: ' t_> : o o < W-> O ' Tj\inS ^h'vUj^.VV' --- ■ : ZZ 5 * 3 o '.n»t»f\ ' ; ^' J^-' 5»sw <-.... S'd .dirr.ri- J,..j-i s)i fca'q irflri js 1o ^ixji )dl i'J;T«mfit I'niJ'j.H IcirifiDl.'tinyl E nnri'i— ill-!; .nr'H .orr. x iTX'.i)i:iii'iiH .qlfcOK nr'HfH Cortical substance Cuticle of root sheath Huxley's layer Hair cuticle Heinle's Outer root sheath Cells. of j ' hair follicle , urface Blood vessel) ^Hyaline layer- Blood fct-selB 1 t'ilCt t ■ Fig. 245.— Cross-section of a hair and hair follicle in lower half of root. Human scalp. Hsematoxylin and eosin. X 400. ° Q OroQP O otPOdoooOooo Medullary substance Cortical substance Hair cuticle Cuticle of root sheath Huxley's layer ^Henle's layer Outer root Sheath }Hyaline layer <=> <=, <^> o O c= ^ <=> o O ^J <= <=> <=> <=> Hair follicle , ll\ i Blood vessel Fat cells Fie. 246.— From a longitudinal section through the axis of a hair p.nd its root sheath. Human scalp. HEematoxylin and eosin. x 550. f* Sf+I .A --, ® m^mw^^^ ■ \ °ol$ (k\ ■i*.- , TO' W&ZSZ8&&. CJ1 I # &''! ^ Q- .U j-s o u o am a \ .blrd'ju I., L/,,1 ii Bn fjiis Ji, ;u 9r ft il 1Uim || dor}-,-;.-: [flfl.r„n^a.>.[ - .fft'J .;>!"■! ■fit " i)iijft i'suiiranri at bsasirrjjri •...).,. rr K >i,r>iL| OclD V. ui>.„M bn* .nl.!.., lla | 7f) ,, )M |„., w ;,i, !t « ; ,[„„j UBawd mo i1— .ISS .art Pig. 25U7 Epowychium Eleidin layer Epidermis of nail fold Epidermis surrounding nail bed K,, 2H . C)6 O^, m a=acidophile granules. b=zusutrophile " c ^basophile t?= erythrocytes —^~s \ •)u«;li a t)u> — , , ^ y - Fig. 250. — Longitudinal section through the nail, and nail fold of a child. Hematoxylin and picro-carmine hardened in Flemming's fluid. X 60. Fig. 251. — From human leuksemic blood. Methylene-blue and eosin. x*660. 1 m Mi r\\ 1% % <&.*/ I -- ' Szyrrwnowicz , Histology J Sarofz adnstM Utk.Hrat. y. Werner iWiaUr, fratil^ Subpapillarif. arterial network ^ .3 Venous network Reconstruction of blood-vessels of the skin of the human foot. (Spalteholz.) in the arteries in the middle of the cutis, and in the veins ap- pears in the fourth plexus, where valves also are found first. The lymph-vessels form a fine, close plexus spread out in the stratum papillare, from which loops are sent to the papillae. The larger vessels passing into the depths from this plexus anastomose in the stratum subcutaneum, to give rise to a second coarser network. Nerves are present everywhere in the skin, while certain regions are supplied specially (e. g., soles of the feet, palms of the hands, external genitals). Numerous forms of nerve- endings are present. We find free intra-epithelial nerve- endings, and Merkel's tactile corpuscles in the epidermis; Meissner's tactile corpuscles, and end bulbs in the papilla? ; and Vater-Pacinian, Ruffini's, and other corpuscles in the subcutis. MAMMARY GLAND. 337 (/) Mammary Gland. The mammary gland is a cutaneous gland, which is present in both sexes, and up to the beginning of puberty is not well developed. Its epithelial beginning (milk line or ridge) is seen in the first months of embryonic life. After the commence- ment of puberty the gland continues to develop in the female, but undergoes a retrogression in the male. The highest de- velopment in the female is reached at the end of pregnancy. Shortly after the birth of the child the milk secretion or lactation begins. The function of the mammary gland is thus dependent on the sexual life. Before puberty this whole organ, in both sexes, consists of connective tissue in which branched tubules are imbedded. These represent the ducts of the completely developed gland, and end blindly in saccular dilatations. In the adult female there occur branched tubular gland bodies, but it is only dur- ing pregnancy that these develop in large quantities at the sides of the branched ducts. The newly formed branches of the gland bodies possess also side twigs. The well-developed mammary gland (at the end of preg- nancy and during lactation) consists of fifteen to twenty conical lobes, which are arranged radially. Each lobe con- sists of numerous smaller lobules, which represent a large group of gland bodies lying close together. These have the form of alveoli, and lead into small ducts, which join to form the duct of the mammary gland. Before the latter opens to the outside through the nipple, it is widened to form the sinus lactiferus. Each individual lobe represents really a separate compound alveolar gland, since it opens into the nipple by an orifice of its own, the porus lactiferus. The individual lobes are separated from one another by loose connective tissue, which often con- tains a quantity of fat. The finer structure of the alveolus (Fig. 258) differs accord- ing to whether it is at rest or secreting. When at rest, the round or pyriform alveoli are small and lined with cubical granular epithelial cells. During the transition to the active 22 338 MICROSCOPIC ANATOMY OF THE ORGANS. state (at the end of pregnancy) the alveoli increase in size, nu- merous leucocytes find their way into the lumena of the alveoli, and the granular epithelial cells begin the formation of fat. These fat globules are taken up by the leucocytes, which are thus converted into colostrum corpuscles. After the birth of the child the gland cells become larger and the production of fat increases. The walls of the alveoli now consist of high cylindrical gland cells full of secretion, and also lower cells which have been emptied of their contents. The part of the Fig. 258. Membratia propria Fat globules Lumen of acinus — Tangential section of acinus Part of a transverse section of the mammary gland of a guinea-pig during lactation. X 500. cell bordering on the lumen especially undergoes fatty change. When this secretion escapes, the part of the cell containing the nucleus regenerates the whole cell. This process may take place many times. The whole cell does not disintegrate, as is the case in the sebaceous glands. Mitotic division is observed often in these cells, while, on the contrary, extruded nuclei are found free in the lumen of the alveolus. The membrana propria of the alveolus is homogeneous, and contains on its inner surface stellate basket cells, which surround the gland cells by long processes. MAMMARY GLAND. 339 In a gland which has ceased to function, the interstitial connective tissue becomes relatively more abundant and the gland alveoli tend to disappear. The ducts are lined with a single layer of cylindrical epi- thelium, which in the neighborhood of the external orifice passes over into stratified flat epithelium. Outside, the duct is clothed with a circular layer of connective tissue contain- ing elastic fibres. During the climacteric the gland undergoes involution, the alveoli and ducts decreasing greatly in number and size. The skin of the nipple and its near neighborhood is strongly pigmented. It contains large papillae and smooth muscle cells which run in part circularly around the openings of the ducts and in part longitudinally in the nipple. The skin of the region immediately around the nipple (areola) contains, besides large sweat glands, many (about twelve) sebaceous glands of considerable size, the so-called Montgomery's glands (glandulae areolares). The structure of the latter forms a transition between the sebaceous and mammary glands. They increase in size during pregnancy. The blood-vessels entering the gland parenchyma from different sides break up into a fine capillary plexus, which surrounds the gland ducts and alveoli. The lymph-vessels run in the form of capillary networks, both in the interstitial connective tissue and in the .skin of the nipple and areola. The nerves entering the mammary gland in part supply the blood-vessels, and partly end in the gland parenchyma, as in the salivary glands. In the skin of the nipple and in the end dilatations of the larger ducts there are found Meissner's and Vater-Pacinian tactile corpuscles (W. Krause). The secretion of the mammary gland — the milk — is an emulsion of fat droplets, whose size varies from 1 to 5 (i in diameter. Each fat globule is surrounded by a layer of casein, which prevents their coalescence with one another. The colostrum, which is present in the mammary gland before and in the first two or three days after the birth of the child, contains fat drops and colostrum corpuscles. These 340 MICROSCOPIC ANATOMY OF THE ORGANS. __ cells are nucleated, and include in their protoplasm many free fat globules. They are derived probably from leucocytes which have wandered into the lumen of the alveolus. Some authors regard them as gland cells which have undergone fatty change. 2. VISUAL ORGAN. The true organ of vision consists of the eyeball (bu\bus oculi) and the optic nerve. Besides these there are protective struct- ures, the eyelids and the lachrymal apparatus. (a) Eyeball. In the walls of the eyeball there are three layers : (1) Tunica externa seu fibrosa, which consists of the opaque sclera and the transparent anterior part, the cornea. (2) Tunica media seu vasculosa, which is made up of the choroidea, the ciliary body, and the iris. (3) Tunica interna, which consists of the retina. The eyeball contains in its interior the aqueous and vitreous humors and the lens crystaUina. , (1) Tunica Externa. The cornea (Fig. 259) is a membrane varying in thickness from 0.8 to 1.1 mm. In it can be made out five layers, which from in front backward are. as- follows : (1) The anterior epithelial layer (corneal epithelium) ; (2) The lamina elastica anterior; (3) The substantia propria cornese ; (4) The lamina, elastica posterior; (5) The posterior epithelial layer (corneal endothelium). (1) The most superficial sheath consists of five to eight layers of epithelial cells ; the deepest of which are cylindrical. These pass over into lower polygonal cells, which at the sur- face become flat, but are always nucleated. Regeneration takes place in the basal cylindrical cells, in which karyokinetic figures are found not infrequently. The cells are bound together by intercellular bridges, as in the skin. The lower PLATE LI. a- — - «• .Substantia f propria Fig. 259. — Vertical section tl-irouidi the cornea of a newborn child. X 200 Posterior epitht'iium VISUAL ORGAN. 341 surface of this epithelium is smooth, since the connective tissue there is wanting in papillae. At the border of the cornea this epithelium passes over into that of the conjunctiva. (2) The lamina elastica anterior (Bowman's membrane, anterior basal membrane) is strongly developed in man, vary- ing from 0.01 to 0.02 mm. in thickness. It is a homogeneous refractive membrane. By means of certain reagents (potassium permanganate) fibrillae can be demonstrated in it. The anterior surface presents minute inequalities, which correspond with projections and depressions on the under surface of the basal . cells. (3) The substantia propria forms the main mass of the cornea. It consists of connective-tissue fibrillae, which are bound together into flat lamella? by means of interfibrillar cement substance. There are in man about sixty of these lamella overlying one another and running parallel to the surface of the cornea. They are joined together by interlamellar cement substance. The fibrillae run in different directions and cross one another at various angles. A few bundles run obliquely and join the individual lamellae with one another. These are the so-called Jlbrw arcuatce. Through the entire substantia propria there runs a system of canals and spaces which contain a serous fluid. This system can be demonstrated most easily by impregnation with silver or chloride of gold. The former gives a negative j^icture — i. e., the canals and spaces are colorless on a brown background. By the gold method, on the contrary, a positive picture is obtained, in which the canal system is colored violet (Fig. 260). In the spaces lie flat connective-tissue cells possessing many processes and large nuclei. These so-called fixed corneal cells lie close to the walls of the spaces. Wandering cells also occur in the cornea. (4) The lamina elastica posterior (Descemet's membrane, posterior basal membrane) is a refractive membrane only 0.006 mm. thick. It has been described as an elastic mem- brane, but, according to Mall, does not stain by Weigert's elastic stain. 342 MICROSCOPIC ANATOMY OF THE ORGANS. (5) The corneal endothelium (posterior epithelial layer) consists of a layer of low hexagonal cells, whose protoplasm is rich in fibrils. These seem to pass from one cell to another, as in the stratum spinosum of the epidermis. The sclera has a structure similar to that of the substantia propria corneaa. It possesses, however, numerous elastic fibres, of which a part form networks. The flat connective-tissue cells lie in irregularly branched spaces. The connective-tissue fibrils are arranged in layers in such a way that those of one layer have a meridional and those of another an equatorial Fig. 260. Lymph canaliculi Corneal cell in lymph space From a horizontal section of an ox's cornea. Positive picture of the canal system demonstrated by the gold chloride method, x" 4/>0. direction. The sclera shows in certain places collections of pigment {e. g., at the border of the cornea, and in the neigh- borhood of the entrance of the optic nerve). On the inner surface of the sclera we find a loose connective tissue arranged in thin layers. This contains branched pig- ment cells and joins the sclera to the chorioidea. In sep- arating these layers a part of the connective tissue remains with the sclera, and a part adheres to the chorioidea. We dis- tinguish this connective-tissue layer as the lamina fusca slero? or lamina suprachorioidea. At the place where the optic nerve penetrates the sclera we find only a remnant of the layer in the VISUAL ORGAN. u; form of a reticular network, the so-called lamina cribrosa. The eye muscles attach themselves to the sclera in such a way that their tendons pass over into the fibril bundles of the sclera. The outer surface of the sclera borders on the con- junctiva sclerse, with which it is bound by the loose subcon- junctival connective tissue. (2) Tunica Media. In the chorioidea we distinguish several layers (Fig. 261) : 1. The lamina vasculosa is the outermost layer, and is adja- cent to the lamina suprachorioidea. It contains large blood- vessels, the branches of the venae ciliares posticse, and the arterise ciliares posticse brevis. The ground substance consists Fig. 261. Pigment layer of retina Lamiva basalts- Lamina chorio- capillaris Lamwa vasculosa Lamina supra-_ chorioidea Part of the sclera Vertical section through the chorioidea and a part of the sclera of an ape. X 440. of connective tissue with fine elastic fibre networks. In it there are veins surrounded by lymph spaces. Numerous pigment cells are present, and running along the arteries are bundles of smooth muscle cells and flat branched cells. 2. The lamina choriocapillaris lies internal to the lamina vasculosa. It consists of a small amount of ground substance containing a capillary plexus, which is more dense in the region of the macula lutea. No pigment is present. 3. The lamina basalis is a highly refractive, delicate mem- 344 MICROSCOPIC ANATOMY OF THE ORGANS. brane which lies on the inner surface of the chorioidea and borders on the pigment epithelium of the retina. The corpus ciliare is to be regarded as a process of the chorioidea, which reaches from the ora serrata to the outer- most borders of the iris. It consists of the so-called orbiculus ciliares, the processus ciliares (corona ciliaris), and the musculus ciliaris. The orbiculus ciliaris differs in structure from the cho- rioidea in that it contains no lamina choriocapillaris. The lamina basalis is thickened to form intercrossing ridges, with depressions between which are filled with retinal pigment epithelium. The vessels and muscle bundles belonging to this region run in a meridional direction. The corona ciliaris (Fig. 262) consists of seventy to eighty ridge-like processes running meridionally (processus ciliares). These are arranged around the lens, and are about 2 mm. long and 1 mm. high. They are highest at the end toward the lens. Toward the outside the ground substance of the processes border on the ciliary muscles. The inner surface, on the other hand, is covered by the lamina basalis, which rests on the pigment layer of the pars ciliaris retinae. The musculus ciliaris (Fig. 262) has the form of a flat ring about 3 mm. in thickness. It consists of smooth muscle cells, which may be divided into three groups according to the direction in which they run : 1. The outermost {meridional) part (tensor chorioidese) con- tains bundles of muscle cells which run meridionally and lie next to the sclera. They reach from the canal of Schlemm to the orbiculus ciliaris. 2. Outside these fibres there is a middle {radial) layer of the ciliary muscle. Its bundles of fibres have a radial arrange- ment, so that some of them are spread out toward the centre of the eyeball, like the rays of a fan (Fig. 262). 3. The innermost {circular) portion of the muscle takes an equatorial or circular course, so that the name Midler's ring muscle is applied also to it. The iris is to be regarded as a process of the chorioidea. PLATE LII. Epithelium of cornea en ii j Conjtui Fig. 26:2. — Meridional section through the ciliary body of an ape's eye. x, sinus venosus scleras. X ^'. VISUAL ORGAN. 345 It consists of four layers : the anterior epithelium, the stroma inch's, the posterior limiting layer, and the pigment layer. 1. The anterior epithelium is made up of a simple layer of flat cells, which cover the anterior surface of the iris. In old individuals this layer is no longer to be made out. 2. The stroma iridis consists, in its anterior half, of reticular connective tissue (anterior limiting layer), and in its posterior half of loose connective tissue which contains numerous blood- vessels (vascular layer). The vessels, which here have a radial arrangement, possess no muscular sheaths, but are enclosed by a strongly developed adventitia. In this part of the iris the smooth muscle cells are collected to form the musculus sphincter pupillce and the musculus dilatator pupillw. The first is formed of bundles of fibres, which are arranged circularly around the pupillary edge of the iris in the form of bands about 1 mm. broad. The second muscle is made up of bundles of fibres running radially. The pigment which is present in the con- nective tissue of the iris stroma in varying quantity lends color to the iris. In light eyes it is not abundant. The posterior limiting layer (Bruch's membrane), which is a process of the lamina basalis, is a refractive membrane 2 fi thick. 4. The pigment layer of the iris (pars iridica retinae) pre- sents two layers of cells. The cells of the posterior layer are cubical and strongly pigmented, while those of the anterior layer are flat and contain only a little pigment. Special note must be made of those places where the cornea passes over into the sclera, and where the iris and corpus ciliare are connected with the outer coats of the eye. The sclera passes directly over into the cornea, its fibril bundles running without interruption from one coat to the other. The hardly noticeable line of separation passes obliquely backward and inward. In this region the ciliary border of the iris is attached to the outer coats of the eye. This attachment takes place by means of the so-called ligamentum pectinatum iridis, which in man is developed much less strongly than in many lower animals. The ligament is made up of a network of fibres 346 MICROSCOPIC ANATOMY OF THE ORGANS. situated in the angle of the anterior chamber, between the cornea and iris. The fibres pass over into Descemet's mem- brane, which in this region shows a fibrillar structure. There occur here also, on the one side, free connective-tissue bundles from the substantia propria corneaa, and on the other side connective tissue and elastic fibres of the intermuscular tissue of the ciliary muscle and processes from the iris stroma. These fibres form a network whose strands are covered with flat epi- thelium continuous with the corneal endothelium and the epithelium of the anterior surface of the iris. Between the strands of tissue there are free spaces, the so-called spaces of Fontana. (3) Tunica interna. The retina is the third and innermost coat of the eyeball, and contains the terminations of the optic nerve fibres. It lines the whole posterior part of the eye, and ends at the pupillary border of the iris. We can distinguish it in three zones: 1. The pars optica retinas, which extends from the place of entrance of the optic nerve, to the neighborhood of the ciliary body, where it ends in a zig-zag line, the ora serrata ; 2. The pars ciliaris retinas,, from the ora serrata to the ciliary border of the iris; and 3. The pars iridica retinas, which extends from the ciliary border to the pupillary border of the iris. 1. The pars optica retinas (Figs. 263 and 265) is the only part of the retina which is sensitive to light. It consists of several layers, the elements of which have been studied by the newer methods, such as the vital methylene-blue staining, and the Golgi impregnation. Three main layers can be made out: the outermost pigment layer, the middle layer (Gehirnschicht), and the innermost neuro- epithelial layer. The middle laver is made up of six, the neuro-epithelial layer of four sheaths, so that the retina possesses altogether eleven layers : 1. Pigment layer. 2. Layer of rods and cones ; 3. Membrana limitans externa: 4. Outer granular layer ; 5. Henle's fibre layer. Neuro-epithelial layer- VISUAL ORGAN. 347 > Middle layer (Gehiraschicht). 6. Outer reticular (molecular) layer ; 7. Outer ganglionic (inner granular) layer 8. Inner reticular (molecular) layer ; 9. Inner ganglionic layer ; 10. Nerve-fibre layer ; 11. Membrana limitans interna. We shall begin the description of the individual layers with the outermost one. The elements of the pigment sheath are usually regular hexagonal cells, which are arranged in a simple Fig. 263. Diagram of the retina, compiled by Kallius, from the work of Eam6n y Cajal. A, layer of rods and cones; B, membrana limitans externa; G, outer granular layer; D, Henle's fibre layer; E, outer reticular layer ; F, outer ganglionic layer; G, inner retic- ular layer ; S, inner ganglionic layer ; J, nerve-fibre layer ; A', membrana limitans interna ; «, Miiller's supporting cell ; 6, rods ; c, cones ; d, bipolar cell belonging to rods ; e-i, bipolar cell belonging to cones ; fc-rre, horizontal cells ; v, centrifugal nerve fibre ; o-t, ganglion cells of optic nerve ; a-e, spongioblasts (amarkrine cells) ; (-&, diffuse amakrine cells ; tj, nervous spongioblast. (From Merkel-Bonnet, Ergebnisse d. Anat. u. Entwich, Bd. II. S. 251.) layer. The somewhat flattened nucleus lies in the outer pig- ment-free half of the cell. The inner strongly pigmented part of the cell possesses long, fine, fringe-like processes, which penetrate between the outer segments of the visual cells. The pigment, in the form of small dark-brown granules and rods, may change its position under the influence of light, so that 348 MICROSCOPIC ANATOMY OF THE ORGANS. it is distributed equally throughout the cell. In consequence of this the rods and cones become surrounded by pigment granules in the region of the external limiting membrane. After exposure to stronger light the pigment moves to the outer part of the cell and collects in a thin layer there, so that the visual cells are quite free from it. The neuro-epithelial layer is formed of the visual cells. Of these, there are three layers : the layer of rods and cones, the outer granular layer, and the sheath of Henle. The external limiting membrane is derived from the supporting cells of Miiller (see below). We distinguish two kinds of visual cells, rod cells and cone cells (Fig. 263, b, c). Each rod cell consists of a rod and a rod fibre. The latter contains the nucleus. The rods are elongated cylindrical structures, about 50 u long and 2 jj. thick. They may be divided into two parts, the outer segment and the inner segment. The outer segment is cylindrical and doubly refrac- tive. It contains the visual purple, and when acted upon by cer- tain reagents breaks up into many discs. The inner segment is slightly spindle-shaped, finely granular, and singly refractive. In the outer part of the inner segment there is in most verte- brates an ellipsoidal body which shows a fibrous structure. This is the so-called ellipsoid of Krause. Each rod is continuous at its inner end with a fine fibre, the rod fibre. This ends in the outer reticular layer in a small globular thickening. Each rod fibre shows somewhere in its course a nucleated enlargement, the rod nucleus. This may occur at various levels, so that the outer granular laver contains many rows of nuclei. In some animals (cat, rabbit, guinea-pig, horse, etc.) the nucleus shows a distinct transverse striation, which is due to the arrangement of the chromatin substance in two to four plate-like segments. In man the nucleus shows a reticular structure, and only seldom do we see an indistinct cross-striation, which is due to annular thickenings of the chromatin network on the surface of the nucleus. The cone cells consist also of two parts, the cone and the cone fibre. VISUAL ORGAN. The cones are shorter than the rods, measuring only about 30 (U. Like the rods, they show an outer and an inner seg- ment. The outer segment is much shorter than that of the rod, and is slightly couical in form. It sometimes presents cross-striations. The inner segment is somewhat shorter and much thicker (6 ft) than that of the rod, and is rounded. The ellipsoid of the cone is larger than that of the rod, and lies in the peripheral part of the inner segment, occupying about two-thirds of this. Each cone is continuous with a cone fibre. At the junction of these two parts of the cone cell, immediately inside the external limiting layer, lies the cone nucleus. The cone fibres end in the outer reticular layer by means of a conical expansion, from which fine fibres spread out. The number of rods is far greater than that of the cones. They are distributed less uniformly, so that in a section taken at right angles to the surface two or three rods are found between each two cones. The rods and cones lie in a row, the lower boundary of which is the membrana limitans externa (Fig. 263, B). This membrane is a product of the Midler's fibres. Outside these lie the rod and cone fibres, together with their nuclei, forming the outer granular layer (C). This consists usually of granules crowded closely together. In the region of the macula lutea the inner segments of the rod and cone fibres are elongated, and form the so-called Ilenle's fibre layer (D), which contains no granules. The outer reticular layer (E) is made up of the thickened ends of the visual cell fibres and the end arborizations of cells whose bodies lie in the outer ganglionic layer. The main constituents of the outer ganglionic layer (inner granular layer) (F) are the bipolar ganglion cells, whose processes end in the outer and inner reticular layers. Some cells (Fig. 263, d) establish a communication between the rod cells and the optic nerve fibres in such a way that the outer arborizations come in contact with the ends of the rod cells, and the inner processes reach to the inner border of the inner reticular layer to surround the ganglion cells there. Other 350 MICROSCOPIC ANATOMY OF THE ORGANS. cells (e-i) are associated, by means of the processes which are sent into the outer reticular layer, with the broad conical ends of the cone fibres. The inner processes, on the contrary, enter the inner reticular layer, where they come into contact at various levels with the branched protoplasmic processes of the ganglion cells. Besides these cells, we find at the inner border of this layer, cells which are known as spongioblasts (W. Miiller) or para- reticular cells (Kallius). The processes of these cells end in the inner reticular layer. With these cells must be classified, according to Ramon y Cajal, those in which no axis-cylinder process is to be seen (amakrine cells, a-% and S). Some of these give off end arborizations only at certain levels (a—e, cells in which the dendrites are arranged in layers). Others, on the contrary, send their processes diffusely through the whole thickness of the inner reticular layer (£ 3). Besides the amakrine cells, we find at this level, in certain animals, cells giving off axones which pass over into optic nerve fibres. Finally, there are cells (m, /) which possess one or more main processes spreading out on the outer surface of the inner reticular layer. They resemble the so-called horizontal cells (Ramon y Cajal), which lie in the outer part of the outer ganglionic layer at the boundary of the outer reticular substance. These cells owe their name to the fact that their long axis lies parallel to the surface of the retina. They are ganglion cells whose bodies give off numerous short den- drites, branching abundantly in the outer reticular layer, and also a long, fine, horizontal axis-cylinder process, which breaks up at the end into numerous branches. Two kinds of these cells can be distinguished: the outer smaller cells (m), whose axis-cylinder processes come in contact by end arborizations with the ends of the cone fibres; and the inner large cells (I), whose long processes are connected with the end bulbs of the rod fibres. These cells join together distant parts of the retina. We find also in this region cells (K) which send out processes which end above in the outer and below in the VISUAL ORGAN. 351 inner reticular layer. The nuclei of Midler's fibres also lie at the level of the outer ganglionic layer. The inner reticular (molecular) layer (G) consists of a fine network, which is derived mainly from the branched processes of cells of the outer ganglionic layer, as well as the dendrites of cells of the inner ganglionic layer. This layer shows stria- tions parallel to the surface of the retina. This appearance is due to the fact that the end arborizations of the cells lie at dif- ferent levels (Fig. 263). Between the most external arboriza- tions of the bipolar cells (e-4) belonging to the cones, and be- tween the innermost branched dendrites of the ganglion cells (o-s), there run the fine branches of the amakrine cells. Fine side branches of the Miiller's fibres (a) also take part in this network. The inner ganglion-cell layer (Fig. 263, H) consists of multipolar ganglion cells with many protoplasmic processes, which extend toward the outside, and at certain levels of the inner reticular layer break up into fine branches. Retzius and Cajal claim that each ganglion cell branches without forming anastomoses with other cells. Dogiel believes, on the contrary, that the protoplasmic processes of all ganglion cells of the retina join with one another and form a network. The axis- cylinder process extends inward and comes to lie in the nerve- fibre layer as an independent nerve fibre. In the human retina a ganglion cell is sometimes found to be bound to another by a short bridge. These are the so-called twin cells (Dogiel, GreefF). Such a bridge may vary in length, and is only a thick protoplasmic process which is continuous with that of another cell. Only one of two cells thus con- nected possesses an axis-cylinder process, which passes over into the nerve-fibre layer. In the inner ganglion-cell layer there lie cells (t) whose dendrites pass diffusely through the whole thickness of the inner reticular layer, but have no connection with the rods and cones. The nerve-fibre layer (Fig. 263, J) contains the fibres of the optic nerve, which diverge from one another in all direc- 352 MICROSCOPIC ANATOMY OF THE ORGANS. tions at the papilla nervi optici. This layer is thickest at the place of entrance of the optic nerve (Fig. 264). It contains only naked axis cylinders. The great majority of these are centripetal fibres, which are derived from the cells of the adja- cent layer (H) of the retina. It is highly probable that there are a few (n) centrifugal fibres (Cajal), which are processes of ganglion cells situated in the brain. The greater number of these fibres form by their end arborizations a pericellular network around the cells which lie in the outer part of the inner reticular layer — i. e., around the parareticular cells Home of them, on the contrary, end freely after penetrating to the more external layers of the retina (Dogiel). The membrana limitans interna (K), which forms the innermost layer of the retina, is a product of the supporting cells of Miiller (supporting fibres, radial fibres). These supporting cells of 3fii I lev are somewhat similar to the ependymul cells of the embryonic spinal cord. They are ele- ments of an epithelial nature (of ectodermal origin), and consist of elongated cells which extend through the whole thickness of the retina. The inner end of the cell is widened into a cone- shaped body, which shows a fibrous structure (radial fibre cone). In consequence of the fusion of these conical bodies, a mem- brane is formed, the membrana limitans interna. From this place the supporting cells extend toward the outer surface. In both reticular layers delicate fibres are given off in all direc- tions. At the level of the outer reticular layer each cell pre- sents an ellipsoidal nucleus. In the outer ganglionic and outer granular layers the cells show numerous cup-like depressions on their surfaces, caused by pressure exerted by other kinds of cells. At the bases of the rods and cones is found the mem- brana limitans externa, which is formed by a membranous widening of the supporting fibres. From its surface there run fine processes, which form the so-called fibre-baskets, which surround the ba^es of the rods and cones. In the supporting tissue of the retina there are, in addition to the Midler's fibres, neuroglia cells (spicier cells), which occur abundantly in the optic nerve. J&Zj* •*■!#•. 4 "*&f, -->.': a I ymcnowicz. Histology J f>ar V z ad not, del. VISUAL ORGAN. 353 From the above description of the retina, it is seen that the light stimuli reach the brain in the following way : The rod and cone visual cells, which one may call the first neurones, receive the stimulus. From here it is transmitted to the bipolar cells of the outer ganglionic layer (second neurones), and thence to the cells of the ganglion-cell layer (third neurones), which send fibres through the optic nerve to the brain. The con- nection between these cells is by contact in the two reticular layers. The retina has a somewhat different structure in the macula lutea, the papilla n. optici (see Optic nerve), and the ora serrata. In the region of the macula lutea the middle or cerebral layer contains a yellow pigment, which is distributed diffusely, so that this part has a yellowish color on the surface. In this neighborhood the inner ganglion-cell layer is distinctly thicker, consisting of as many as nine layers of ganglion cells. The outer ganglionic layer is also wider here. The layer of rods and cones becomes poorer in rods as the macula lutea is approached, so that in this region itself only cone cells are present. In the macula lutea Henle's fibre sheath is especially well developed. In the centre of the macula lutea on its inner surface there is a depression, the fovea centralis, in which the retinal layers are distinctly thinner than elsewhere. The nerve-fibre sheath ends here, and both ganglion -cell layers disapj>ear, so that in the fundus fovea? itself only a neuro-epithelial layer is found. Owing to the entire absence of the pigmented cerebral layer of the retina, the fundus fovea? appears colorless. In the region of the ora serrata a marked decrease in thick- ness of the retina takes place in consequence of the disappear- ance of the retinal layers. The nerve-fibre and ganglion-cell layers are the first to disappear. The structure of the visual cell layer is altered and the two reticular layers are lost. The outer granular layer fuses with the outer ganglionic layer. At a certain distance from the ora serrata the rod cells dis- appear, and the cone cells change their typical character and 23 354 MICROSCOPIC ANATOMY OF THE ORGANS. become finally a single layer of cylindrical epithelium. The supporting cells of Muller are well developed here. 2. In the pars ciliaris retinae, we find only two layers of cells. Toward the outside there is pigmented epithelium, while on the inner side there is a layer of high cylindrical cells, which are derived from the neuro-epithelial layer. These cylindrical cells take the place of the layer of visual cells and the outer granular layer, which is still to be seen at the ora serrata. 3. Pars iridica retinae, see Iris. (4) The Optic Nerve. The optic nerve possesses three sheaths, which are to be regarded as continuous with the membranes of the brain. The dura mater forms the outermost sheath, the arachnoidea the second, and that which lies immediately on the nerve is derived from the pia mater and sends septa between the individual fibre bundles. Between the processes of the dura mater and the arachnoidea, and between the arachnoidea and the pia mater, there are two spaces, of which the first is in communi- cation with the subdural space, and the second with the subarachnoid space. All three sheaths are bound together by connective-tissue strands, which cross over through the spaces. At the entrance of the optic nerve into the eyeball the dural and pial sheaths pass over into the sclera. The arach- noidea, on the contrary, breaks up into fibres before it reaches the sclera, so that the subdural and subarachnoidal spaces com- municate with one another. Where the optic nerve enters the eye, the sclera and choroid are pierced and perforated, so that they are reduced to a lattice- work tissue, which we call the lamina cribrosa. The fibres of the optic nerve are medullated but possess no sheath of Schwann. As the fibres pass through the chorioid and sclera they lose their medullary sheath and pass over on the inner surface of the retina as naked axis cvlinders, which, form the optic nerve-fibre layer. In consequence of the loss VISUAL ORGAN. 355 of the myelin sheaths the nerve becomes considerably thinner on entering the eyeball. \o) The Lena. In the lens we may distinguish the substantia lentis and the lens capsule. The lens is an epithelial structure formed from the ectoderm. It consists in the beginning of cylindrical cells, Fig. 266. Capsule Lens epithelium Lens fibres- Part of a meridional section through the border of an ape's lens. X 200. which during subsequent development increase in height at the posterior surface of the lens. This increase goes on until exceedingly long cells are formed, the lens fibres. In adults the substantia lentis consists of lens fibres, which 356 MICROSCOPIC ANATOMY OF THE ORGANS. at the anterior surface are covered by a single layer of cubical lens epithelium. This reaches as far as the equator of the lens, where the cells increase in height to form lens fibres. The lens fibres are flattened hexagonal prisms, which are thickened at the posterior end. They run in a meridional direction from the anterior surface backward. A small quantity of cement substance joins the fibres together. The outer fibres in the region of the equator possess oval nuclei, while in the centre of the lens no nuclei are present. The lens capsule is a clear refractive membrane, which is thicker on its anterior (10-15 (j.) than on its posterior surface (5-7 (i). On its outer surface it shows parallel striations and is lamellated. In its behavior toward reagents it resembles neither white fibrous nor elastic tissue. It is probably partly cuticular and partly connective tissue in nature. (6) The Vitreous Body and the Zonula Giliaris. The vitreous body is made up of a tissue which contains about 98 per cent, of fluid substance, the vitreous humor. The firm parts have the form of fine intercrossing connective- tissue fibrils, connective-tissue cells of various kinds, and wandering cells (leucocytes). The entire vitreous body is surrounded by a refractile homogeneous membrane, the membrana hyaloidea, which touches on the outside the membrana limitans interna retina?. In the region of the ora serrata fine fibres run from the surface of the hyaloid membrane and the ciliary processes in a meridional direction toward the lens and insert them- selves in its capsule. The insertion of the fibres in the lens occupies a wide zone at the equator, which reaches some dis- tance on the anterior and posterior surfaces. Taken together, these {fibres zonulares) form the zonula ciliaris, which serves to hold the lens in place. The fibres of the zonula and the equatorial zone of the lens form the boundaries of a whole system of large and small spaces, the spatia zonularis (canal of Petit), which are in com- munication with the posterior chamber of the eye. Cornea Fig. 267. — Diagram of the blood-vessels of the eye, as seen in a horizontal section. (Leber, after Stohr.) Arteries red, veins blue. Course of vasa eentralia retinae: a, arteria, ai, vena centralis retina?; /3, anastomosis with vessels of outer coats; y, anastomosis with branches of short posterior ciliary arteries; 5, anastomosis with.chorioideal vessels. Course, of vasa cilia r. postic. brev. : I., arteria 1 , and Ii, veinc ciliav. postic. brev. ; II., episcleral artery; Hi, episcleral vein ; 111., capillaries of lamina choricapillaris. Course of vasa ciliar. postic. long. : 1, a. ciliar. post. longa ; :„\ eireulus iridis major cut across; .">, branches to ciliary body ; -1, branches to iris. Course of vasa ciliar. ant. : «, arte Ha, * Chloride of gold methods, 413 Chorda dorsalis, 51 Chorioid plexus, 298 Chorioidea, 343 Chorion, 258 frondosum, 257 Chorionic membrane, 257 villi, 257 Chromatin, 22 Chromatolysis, 32, 246 Chromatophile granules, 102, 103 Chromopnile cells, 152 Chrom-osmium-acetic acid, 389 Chromosomes, 29 Chyle vessels, central, 187, 191 Cilia, 26 Ciliary body, 344 movement, 26 muscle, 344 Ciliated epithelium, 39 Cinnabar gelatin, 400 Circular sheath of hair, 325 Circulation in protoplasm, 26 of placenta, 262 Circulatory system, 121 Clarke's column, 279 Clasmatocytes, 60 Claudius, cells of, 372 Clitoris, 263 Cloquet's canal, 357 Club haii-, 328 Coagulation phenomena, 104 Coccygeal gland, 154 Cochlea, 365, 367 vessels of, 373 Cochlear nerve, 290 nuclei, 290 Cohnheim's fields, 88 Coil gland, 48, 333 Collaterals, 98 Collecting tubules of kidney, 213, 216 Colloid substance in thyroid, 145 Colorless blood corpuscles, 114 Colostrum, 339 corpuscles, 338 Column of Burdach, 281 of Goll, 281 of Gowers, 283 of Stilling-Clarke, 279 Commissural cells, 282 Commissure, dorsal and ventral gray, 280 gray, 279 white, 280 Common bile duct, 202' Compound alveolar gland, 49 tubular gland, 49 Concentric corpuscles of Hassal, 143 Condenser, Abbe's, 384 Conduction of nervous impulses, 101 422 INDEX. Cone fibre, 348 dines, 348 Congo red, 397 Coni vasculosi Halleri, 231 Conjunctiva, 361 palpebralis, 362 sclerae, 362 Connecting tubules of kidney, 213, 216 Connective tissue, 53 bones, development of, 273 cells, 56 classification of, 53 histogenesis of, 61 nerve-endings in, 307 of kidney, 217 Contact relation of neurones, 101 Contraction bands, 92 of muscle, 92 Convoluted tubules of kidney, 213 Corium, 319 Cornea, 340 Corneal canals, 341 cells, 341 endothelium, 342 Cornification, 42 Corona ciliaris, 344 radiata, 242 Corpora lutea spuria, 244 vera, 244 Corpus albicans, 244 cavernosum urethrae, 235 ciliare, 344 fibrosa, 244 hsemorrhagicum, 243 Highmori, 224 luteum, 244 restiforme. See Inferior cerebellar pe- duncle. spongiosum, 235 uteri, 253 Corpuscles of blood, 112 of Grandry, 308 of Herbst, 310 of Hassal, concentric, 143 of Meissner, 310 of Ruffini, 310 of Vater-Pacini, 311 Corrosive sublimate, 389 Cortex, cerebellar, 294 cerebral, 291 of adrenal, 148 of hairs. 323 of kidney, 212 of lymph gland, 134 pyramidal cells of, 100 representation of senses in, 291 Cortical sheath of glia fibres, 293 Corti's canal, 370 organ, 369 Cotyledons, 257 Cowper's glands, 235 Crenation of blood cells, 113 Crista acustica, 365 basilaris, 368 Cross of Eanvier, 106 Crossed pyramidal tract, 283 Crusta, 24 Crypt of tonsil, 168 Cubical epithelial cells, 38, 41 Cumulus obphorus, 242 Cupola, 366 Cutaneous vessels, 335 Cuticle of hair, 323 of root sheath, 324 Cuticula, 24 dentis, 161 vaginae pili, 324 Cuticular border in epithelium, 40 Cutis, 318 plate of myotome, 94 Cylindrical epithelium, 38, 41 Cystic duct, 202 Cytoblastema, 28 DECALCIFICATION of bone, 390 Decidua, basalis, 255 capsularis, 255 graviditatis, 255 menstrualis, 255 reflexa, 255 serotina, 255 vera, 255 Decidual cells, 256 Degenerations in epithelial cells, 44 Dehiscent gland, 49 Deitei^s cells, 100, 285, 372 process, 97, 98 Delafield's hematoxylin, 396 Delomorphous cells, 182 Demilunes of Gianuzzi, 174 Dendrite, 97, 99 Dense connective tissue, 63 Dental canals, 157 canaliculi, 158 fibres, 157 gei-m, 161 papilla, 161 ridge, 161 sac, 162 sheath of Neumann, 159 Dentine, 157 cell bodies, 157 ground substance, 159 origin of, 163 Derma, 318 Descemet's membrane, 341 Descending arm of Henle's loop, 213, 215 Destruction of bone, 272 Deutoplasm, 24, 241 Development of adrenal, 150 of bile capillaries, 205 of bone from cartilage, 268 ' of bones, 268 of capillaries, 122 of cement, 164 of dentine, 163 of elastic tissue, 62 of enamel, 163 INDEX. 423 Development of fibrillar connective tissue, 61 of liver, 204 of lymphatics, 133 of muscle cells, 95 of muscles, 276 of spermatozoa, 229 of submaxillary, 175 of teeth, 161 of tonsils, 169 Diapedesis, 60, 115 Diarthrosis, 267 Master, 31 Differential staining, 395 Differentiation of cells, 36 Digestion leucocytosis, 114 of fat, 187 Digestive system, 154 Diploe, 273 Direct division, 28 Disc, tactile, 308 Discus proligerus, 242 Division of labor in cells, 36 Dorsal column, 280, 283 gray commissure, 280 horn, 279 median septum, 280 root, 279 Dorsolateral group of motor cells, 281 Double stains, 397 Doyere's hillock, 93, 313 Drum of ear, 376 Duct of Santorini, 192 of Wirsung, 192 Ductless glands, 49 Ducts of liver, 202 of mammaiy gland, 339 of salivary gland, 171 Ductuli aberrantes, 234 efferentes testis, 231 Ductulus aberrans Halleri, 234 capitis epididymis, 234 retis testis, 234 Ductus cochlearis, 367 endolymphaticus, 375 ejaculatorius, 233 pancreaticus, 192 accessorious, 192 papillaris, 215 perilymphaticus, 375 reuniens (Henseni), 365 utriculo-saccularis, 365 Dura mater, 296 cerebralis, nerves of, 297 EAR, 364 wax, 377 Elmer's, v., cement lines, 76 glands, 167, 177 hydrochloric acid, 391 Ectoderm layer, 259 Ectoplasm, 19, 24 Efferent lymph-vessel, 136 Egg.balls, 240 Egg cells of ovary, 238 nests, 240 Ehrlich's methylene-blue method, 415 Elmer's organ, 307 Ejaculatory duct, 233 Elastic cartilage, 72 connective tissue, 56, 64 fibres, 55 origin of, 62 fibrils, structure of, 64 granules of Eanvier, 63 limiting layer of pharynx, 177 membrane of Henle, 124 tissue stains, 404 Elastica externa, 127 interna, 126 Eleidin, 321 Ellipsoid of Krause, 348 Ellipsoids of the spleen, 140 Embryonic connective tissue, 53 Enamel, 160 cells, 162 fibres, 160 organ, 161 origin of, 163 prisms, 160, 164 pulp, 162 End-bulbs of nervous system, 309 Endocardium, 130 Endochondral ossification, 268 Endogenous cell formation, 70 Endolemma, 89 Endolymph, 365 Endometrium, 251 in menstruation, 254 in pregnancy, 255 Endoneural sheath, 109, 300 Endoneurium, 300 Endoplasm, 19 Endosteum, 266 Endothelium, 45 End-piece of tail of spermatozoon, 229 End-plate, 314 Endscheibe, 91 Eosinophile granulation, 116 Ependyma, 111 cells, 285 fibres, 285 Epicardium, 131 Epidermis, 320 Epididymis, 231, 232 Epilemma, 89 Epineurium, 299 Epiphyseal line, 272 Epithelium, 37, 40 classification of, 40 glandular, 46 histogenesis, 44 Epithelial cells of mesoblastic origin, 45 lamella of myotome, 94 Eponychium, 330 Epoophoron, 246, 248 Erectile tissue of penis, 236 Erlicki's fluid, 389 424 INDEX. Erythroblasts, 118, 265 Erythrocytes, 112 Essential gland cells of the testis, 229 Eustachian tube, 375 Eye, 340 blood-vessels of, 357 Eyeball, 340 lymph paths, 359 nerves of, 360 Eyelashes, 361 Eyelids, 361 Excretion, 46 External female genitals, 262 Extrusion of polar bodies, 33 FALLOPIAN tube, 250 False interstitial lamella?, 75 Fascise, 278 Fasciculus cerebellospinalis dorsalis, 283 cerebrospinalis lateralis, 283 ventralis, 283 longitudinalis medialis. See Posterior longitudinal bundle. ventrolateralis Gowersi, 283 Fastening villi, 257 Fat, 66 cells, 58 development of, 66 digestion, 187 germinal layer, 66 lobule, 66, 67 staining of, 66 tissue, 58 Female genitals, external, 262 sexual organs, 237 urethra, 223 Fenestrated membrane of Henle, 64, 124 Fertilization, 32 Florae arcuata? cornese, 341 zonulares, 356 Fibre baskets of retina, 352 layer of Henle, 349 Fibres, dental, 157 Fibril bundles of muscle, 84 Fibrillar theory of protoplasmic structure, 20 connective tissue, 54 Fibrin, 119 canalized, 260 Fibre-muscular coat of gall-bladder, 203 Fibrous cartilage, 73 Filar-mass, 20 Fissura mediana ventralis, 280 Fixation of tissues, 388 Fixed connective-tissue cells, 57 Fixing agents, 388 Flagella, 26 Flagellated epithelium, 39 Flat epithelium, 37, 41 Flemming's fluid, 389 Foam theory of protoplasmic structure, 21 Follicle, Graafian, 242 of lymph gland, 134 of ovary, 238 Follicle of tonsil, 169 primordial or primary, 240 solitary, 138 Follicular atresia, 245 cells, 240 Folliculi linguales, 167 Folliculus oophorus vesiculosus, 242 Fontana, spaces of, 346 Foramina nervina, 368 Formation of elastic fibres, 62 Formed connective tissue, 63 Formatio reticularis, 279, 289 Fossa navicularis, 223 Fourth ventricle, 287 Fovea centralis, 353 Foveola? gastricse, 180 Framework of adrenal, 150 of kidney, 217 of pancreas, 194 of spleen, 140, 142 of thyroid, 146 Free nerve-endings, 304 in connective tissue, 307 Freezing of tissues, 387 Frommann's silver line, 105 Fundus glands, 181 Funiculus cuneatus, 281 dorsalis, 283 gracilis, 281 lateralis, 283 ventralis, 282 Funnels of Schmidt-Lantermann, 105 pALL BLADDER, 203 VJ Galvanotaxis, 27 Ganglia, 301 Ganglion, 98 spirale, 373 Ganglionic layer of cerebellum, 295 Gartner's duct, 248 Gastric blood-vessels, 190 glands, 181 mucosa, 180 Gelatin, 400 Gelatinous bone-marrow, 266 nucleus, 267 substance of Rolando, 279 tissue, 259 Generative system, 224 Genital corpuscles, 264, 309 Genito-urinary system of embryos, 247 Gennari, fibre tract of, 293 Germinal cells, 111 centre, 119, 135 in the spleen, 139 epithelium, 239 spot, 241 vesicle, 241 Giant cells of bone-marrow, 265 of placenta, 261 Gianuzzi, demilunes of, 174 GiraldiS, organ of, 233, 248 Gitterfasern, 66, 197 Glands, 46 INDEX. 425 Gland body, 47 blood supply of, 51 classification of, 47, 49, 170 coil, 333 ducts, 47 fundus or peptic, 181 Krause's, 362 lachrymal, 364 mammary, 337 Meibomian, 362 of Brunner, 189 of Liberkiihn, 185 of Montgomery, 339 of mouth cavity, 170 of oesophagus, 178, 179 of skin, 331 sebaceous, 331 sweat, 333 tarsal, 361 Glandulse areolares, 33 Bartholini, 263 buccales, 176 bulbo-urethrales Cowperi, 235 ceruminosae, 376 cervicales uteri, 253 ciliares, 361 gastricse propria?, 181 labiales, 176 linguales, 176 olfactorise (Bowmann), 380 palatini, 176 pyloricae, 183 sudoriparse, 333 urethrales (Littre), 223 vestibulares majores, 263 minores, 263 Glandular epithelium, 46 Glans clitoridis, 264 penis, 237 Glashaut, 242, 325 Glia cells, 285 fibres, 286 mass, central, 286 superficial, 286 Glisson's capsule, 194 Glomerulus of kidney, 212, 213 Glomus caroticum, 153 , coccygeum, 154 Goblet cells, 39, 46, 186 Gold chloride methods, 413 Golgi cells, 109, 292 Golgi-Mazzoni corpuscles, 310 Golgi's method, 412 tendon spindles, 278 Goll's column, 281 Goose skin, 328 Gower's column, 283 Graafian follicle, 242 rupture of, 243 Grandry's corpuscles, 308 Granular cells, 58 of cerebellum, 294 sheath of Tomes, 159 Granulationes arachnoidales (Pacchioni), 297 Granulations, y, 117 in leucocytes, 116 Granule theory of protoplasmic structure, 21 Granuloplasm, 19 Granulosa, 3l4 Gray commissure, 279 matter, 278 Ground bundles of lateral column, 283 of ventral column, 283 lamellae, 76 substance of bone, 76 of dentine, 159 Growth of cartilage, 70 Gustatory cells, 382 organ, 381 pore, 381 HABENULA perforata, 369 Hsemalum, 396 Haemalum-eosin, 397 ILematein, 396 Hsematin, 120 Haematoidin, 120 Hsematoxylin, 396 Haematoxylin-eosin, 397 Haematoxylin-iron-alum, 396 Hsemin, 120 Haemoglobin, 112, 119 Hsemokonien, 118 Hairs, 322 Hair-bulb, 322 cells of auditory neuro-epithelium, 366 cuticle, 323 development of, 325 follicle, 323, 324 germ, 325 muscles of, 327 nerves of, 329 papilla, 322 root, 322 shaft, 322 sinus, 329 tactile, 329 Hardening of tissues, 390 Hassal's concentric corpuscles, 143 Haversian canals, 75 lamellae, 75 Head of spermatozoon, 227 Heart, 130 layers of muscle in, 130 muscle, 83 histogenesis, 87 nerve-endings in, 312 of lower vertebrates, 86 protoplasmic bridges in, 86 nerves, 132 valves, 132 Hecateromeric cells, 282 Heidenhain's iron-haematoxylin, 396 Heisterian valve, 202 Heliotropism, 27 426 INDEX. Henle, sheath of, 104, 109, 324 Henle's colls, 372 fenestrated membrane, 124 fibre layer, 349 loop, 213 Hensen's ductus reuniens, 365 line, 91 Hepatic duct, 202 Herbst's corpuscle, 310 Hermann's fluid, 389 Heteromeric cells, 282 Plilum of lymph gland, 136 stroma, 136 Histogenesis of blood, 118 of connective tissue, 61 of elastic tissue, 62 of epithelium, 44 of fat, 66 of heart muscle, 87 of the neurone, 110 of voluntary muscle, 94 Histology, 17 Histological units of spleen pulp, 141 Homomeric cells, 281 Horizontal cells of retina, 350 fibre tract of Gennari, 293 Horns, ventral and dorsal, 279 Horny layer of epidermis, 320 Howship's lacunae, 272 Huschke's auditory teeth, 368 Huxley, sheath of, 324 Hyaline, 260 cartilage, 68 distribution of, 72 layer of hair, 325 Hyaloid artery, 357 canal, 357 membrane, 356 Hyaloplasm, 19 Hydatid of Morgagni, 234 Hydrolaxis, 27 Hydrotropism, 27 Hymen, 263 Hyponychium, 331 Hypophysis cerebri, 152 TNCLTJSIONS, cellular, 21 1 Indirect division, 28 Inferior cerebellar peduncle, 289 Injecting, 399 Injection masses, 400 Immersion lens, 384 Inner ear, 364 blood-vessels of, 373 lymph paths of, 375 enamel cells, 162 ground lamella?, 76 Interannular segments, 105 Intercalary part of duct, 171 Intercellular bridges, 43 substance, 36 of bone, 76 of connective tissue, 54 Interfilar-mass, 20 Intergemmal nerve fibres, 383 Interglobular spaces, 159 Interlobar arteries of kidney, 218 Interlobular ducts, 171 trabecules of spleen, 140 veins of kidnev, 219 of liver, 198 of spleen, 140 Intermediary lamellae, 75 path for blood in spleen, 141 Intermediate bodies, 31 duct, 171 ducts of pancreas, 192 tubules of kidney, 213 zone of stomach, 184 Internal arcuate fibres, 287 capsule, 288 secretion, 49 Interolivary bundles of fibres, 287 Interradial bundles of fibres, 293 Interstitial cells of testis, 227 connective tissue of testis, 227 growth of cartilage, 70 lamellfe, 75 Intervillous spaces, 260 Intracellular development of blood cor- puscles, 123 Intra-epithelial nerve-endings, 305 Intrafusal muscle-fibre, 315 Intragemmal nerve-fibres, 382 Intralobular ducts, 171 trabecular 'of spleen, 140 vein, 198 veins of spleen, 140 Intestine, 185 blood-vessels of, 190 lymph-vessels of, 191 mucosa of, 185 nerves of, 191 secretions of, 187 Intima, 129 pia, 298 Iris, 344 pigment layer of, 345 Iron-hsematoxylin, 396 Irritability, 27 Islands of Langerhans, 194 proliferation, 260 Isolation of tissue elements, 386 Isotropic bands in muscle, 90, 91 TACOBSON'8 organ, 380 V _ Joining together of bones, 267 Joint capsules, 267 Joints, 267 KARYOLYS1S, 32, 246 Keimcentrum, 135 Keimzellen of His, 111 Keratin, 321 Keratohyalin granules, 321 Kidney, 212 blood-vessels of, 217 framework of, 217 INDEX. 427 Kidney, lobule of, 216 lymphatics of, 220 nerves of, 220 non-vascular zone of, 218 Kolossow's osmic-acid method, 402 Krause, ellipsoids of, 348 Krause's glands, 362 membrane, 84, 92 Kupffer's, v., stellate cells, 200 LABIA majora, 263 minora, 263 Labial glands, 177 Labium tympanicum, 367 vestibulare, 367 Labra glenoidalia, 267 Labyrinth, 364 of kidney, 213 Lahcrymal canals, 364 gland, 364 Lacteal, 187, 191 Lacunae, Howship's, 272 Lamellae in bone, 75 Lamina basalis of chorioidea, 343 choriocapillaris, 343 cribrosa, 343, 354 elastica anterior, 341 posterior, 341 fusca sclera?, 342 spiralis membranaeea, 367, 369 ossea, 367 suprachorioidea, 342 vasculosa chorioideae, 343 Lampblack gelatin, 400 Langerhans, islands of, 194 Lange's spaces, 319 Lantanin, 23 Lantermann's lines, 105 Large granular cells of cerebellum, 295 mononuclear leucocytes, 115 pyramidal cells, 292 Larynx, 206 Lateral column, 280, 283 horn, 279 lemniscus, 290 pyramidal tract, 283 vestibular nucleus, 290 waves in muscle, 92 Lemniscus lateralis, 290 medialis, 287 Lens, 355 capsule, 356 fibres, 355 of microscope, 384 Leucoblast, 119 Leucocytes, 114 classification of, 115 distribution of, 114 Leucocytosis, 114 Lieberkiihn's glands, 185 Ligamentum sacculorum 365 spirale, 367 Limbus spiralis, 367 Limiting capsule of myotome, 95 Lines of SchmidtLantermann, 105 Lingual tonsils, 167 Linin, 23 Liquor folliculi, 241 Lissauer's zone, 290 Liver, 194 blood-vessels, 197 cells, 195 development of, 204 lobule, 194 lymphatics, 201 Lobes and lobules of thymus, 143 Lobule of kidney, 216 of liver, 194 of spleen, 140 Lobuli epididymis, 231 testis, 224 Locomotor system, 264 Longitudinal bundle, posterior, 289 Long rayed cells, 293 Loop of Henle, 213 Loose connective tissue, 63 Lung, 207 blood-vessels of, 210 lymphatics of, 211 Lunula, 330 Lutein, 244 cells, 243 Lymph, 120 capillaries, 132 glands, 133 framework of, 133 nodules, peripheral, 138 in adrenal, 150 paths of eyeball, 359 of membranous labyrinth, 375 sinuses, 134 space of Tenon, 359 vessels, 132, 135 of intestine, 191 of kidney, 220 of ovary, 246 of stomach, 191 of uterus, 254 Lymphatic pharyngeal ring, 167 sheath of arteries of spleen, 140 Lymphatics, development of, 133 of liver, 201 of lung, 211 Lymphocytes, 115, 120 Lymphoid masses in intestine, 188 MACEBATION,_386 Macula acustica, 365 germinativa, 241 lutea, 353 Magenta, 404 Male sexual organs, 224 accessoiy glands, 234 urethra, 223 Mallory's elastic tissue stain, 404 Malpighian corpuscle of kidney, 213 of spleen, 138 layer of skin, 320 428 INDEX. Malpigbian pyramids, 212 Mammary gland, 337 ducts of, 339 Mantle fibres, 30 Margarin crystals, 67 Marginal veil, 111 Marina's fluid, 412 Marrow cavity, primary, 269 of bones, 265 Martinotti, cells of, 292 Mast cells, 59 Matrix cells of bair, 326 unguis, 330 Maturation of the egg, 33 Mauthner's membrane, 107 Media, 126 Medial lemniscus, 287 Median fissure, ventral, 280 septum, dorsal, 280 vestibular nucleus, 290 Mediastinum testis, 224 Medulla, 287 of adrenal, 149 of cerebellum, 296 of hairs, 323 of kidney, 212 of lymph-gland, 134 sensory tract in, 288 Medullary cavity, primary, 269 cords, 134 plate, 110 rays, 212 sheath, 104 Megaloblasts, 265 Meibomian glands, 362 Meissner's plexus, 192 tactile corpuscles, 309 Membrana basilaris, 50, 369 chorii, 257 granulosa, 242 hyaloidea, 356 limitans externa, 349 interna, 352 olfactoria, 379 prseformativa, 163 propria, 50 folliculi, 242 of glands, 172 reticularis, 372 tectoria, 372 tympani, 376 vestibuli (Eeissneri), 367 Membrane of Henle, 124 of Schwalbe, 64 Membranes of central nervous system, 296 Membranous cochlea, 367 labyrinth, 365 urethra, 223 Meninges, 296 Menisci interarticulares, 267 tactile, 307 Menstruation, 254 Merkel's corpuscles, 306 tactile cells, 307 Mesentery, 205 Metakinesis, 29 Metaphase, 31 Metazoa, 18 Methylene-blue for nerve-endings, 41* Microblasts, 265 Micrometer screw, 384 Microscope, 383 Microscopic anatomy of organs, 121 Microsome, 19 Microtome, 387 Midbrain, 287 sensory tract in, 288 Middle cerebellar peduncle, 289 ear, 375 layer of cerebellum, 295 Milk line or ridge, 337 Mitome, 20 Mitosis, 28 Mixed glands, 173 muscles, 89 Molecular layer of cerebellum, 295 of cerebral cortex, 291 Moll's glands, 361 Monaster, 30 ' Mononuclear leucocytes, 115 Montgomery's glands, 339 Morgagni, hydatid of, 234 Mossy fibres, 296 Mother star, 30 Motility of cells, 25 Motor cells of cord, 281 cerebral nerves, nuclei of, 289 nerve-endings, 312 and sensory neurones, relation of, 317 Mouth cavity, 155 glands of, 170 Mucoid tissue, 53 Mucosa of gall-bladder, 203 of intestine, 185 of stomach, 180 Mucous cells, 172 of intestine, 186 glands, 174 membrane of larynx and trachea, 206 of mouth cavity, 155 Mullerian duct, 249 Mailer's fibres, 352 fluid, 389, 412 Multicellular gland, 46 Multipolar cells, 100 Muscle, 80 bud, 314 cells, nuclei of developing, 96 changes during contraction, 92 development of, 94 fibre, intrafusal, 315 histogenesis of, 94 layers of heart, 130 nerve-endings in, 312 spindles, 314 strife, 84, 90 Muscles, 274 blood-supply of, 275 INDEX. 429 Muscles, development of, 276 of the uterus, 253 Muscularis mucosae of intestine, 188 of oesophagus, 178 of oesophagus, 179 Muscular system, 274 Musculus ciliaris, 344 Muskelsiiulchen, 84 Myelocytes, 265 Myoblasts, 95 Myocardium, 130 Myometrium, 251 Myotome, 94 NAIL, 329 bed, 329 body, 329 groove, 329 leaves, 330 root, 329 ' wall, 329 Nasal duct, 364 Nasopharynx, 177 Nebenkern, 193 Nebenscheibe, 91 Neck of enamel organ, 162 of gastric gland, 181 Negative chemotaxis, 27 Nerve cell, body of, 102 cells, 98 corpuscles of Golgi-Mazzoni, 310 genital, 309 of Buffini, 310 endings, 304 in connective tissue, 307 in muscle, 312 in nervous tissue, 316 motor, 312 sensory, 314 fibres, 103 process. See Axone. Nerves, 299 cerebrospinal, 299 sympathetic, 300 of dura mater, 297 of eyeball, 360 of hairs, 329 of the heart, 132 of the intestine, 191 of the kidney, 220 of the liver, 201 of the lung, .211 of the ovary, 246 of the pancreas, 194 of the stomach, 191 of the submaxillary gland, 176 of the uterus, 254 Nervi nervorum, 301 Nervous system, 278 central blood-vessels of, 289 peripheral, 299 tissue, 97 Neumann's dental sheath, 159 Neural tube, 110 Neuraxone. See Axone Neurilemma, 105 Neuroblast, 111 Neuroepithelial cells, 307 Neurofibrils, 101, 104 Neuroglia, 285 of cerebellum, 296 of cerebral cortex, 293 Neurokeratin network, 106 Neurone, 97 . contact relation of, 101 histogenesis, 110 theory, 101 Neuroplasm, 104 Neurosponginm, 111 Neutral dyes, 116 Neutrophile granulation, 117 Nicol's prisms, in study of bone, 76 of muscle, 90 Nissl's bodies, 102, 103 method, 406 Nodes of Eanvier, 105 Nodules, lymph-, 138 Non-medullated nerve fibres, 109 Non-vascular zone of kidney, 218 Normoblasts, 265 Nose, 377 -piece, 385 Nuclear fluid, 23 Nucleated red blood corpuscles, 265 Nuclei of cerebral nerves, 289 of developing muscle cells, 96 Nuclein, 22 Nucleolus, 22 Nucleus, 19, 22 dorsalis, 279 olivaris inferior, 288 pulposus, 267 Nuel's space, 372 Nutrient arteries of bone, 266 OBJECTIVE ocular, 384 Odontoblasts, 156, 162 (Esophageal glands, 178, 179 (Esophagus, 178 Oil immersion lens, 384 Oken's body, 247 Olfactory cells, 378 glands, 380 nerve, 291 fibres, 109 organ, 377 Optic nerve, 291, 340, 354 Oral cavity, 155 mucous membrane, 155 Ora serrata, 235, 246 Orbicularis ciliaris, 244 Orcein, 403 Organ of Girald6, 233 of Jacobson, 380 Organon spirale, 369 Organs, microscopic anatomy of, 121 Origin of blood cells, 118 of connective-tissue fibrils, 61 430 INDEX. Origin of elastic tissue, 62 Osraic acid, 388 method of Kolossow, 403 Ossein, 74 Ossification, areas of, 268 of cartilage, 72, 268 Osteoblasts, 264, 270 Osteoclasts, 266, 272 Osteogenous tissue, 268 Otokonien crystals, 366 Otolith, 366 membrane, 366 Outer ear, 376 ground lamella?, 76 Ovaries, 237 . Ovary, blood-vessels of, 246 cortex of, 238 follicles of, 238 lymph-vessels of, 246 medulla of, 238 nerves of, 246 stroma of, 239 tunica albuginea of, 238 Ovula Nabothi, 253 Ovulation, 254 Ovum, primordial, 239 Oxvchromatin, 23 Oxyntic cells, 182 PACCHIONIAN bodies, 297 Palatine glands, 177 tonsils, 169 Palpebral arteries, 363 Pal-Weigcrt method, 412 Pancreas, 192 framework of, 194 nerves of, 194 Panniculus adiposus, 320 Papilla? circumvallata?, 166 dental, 161 filiformes, 165 fungiformes, 165 of oral mucous membrane, 155 of skin, 319 of tongue, 164 Papillary duct, 213, 215 Paradidymis. 233, 248 Paraffin; 392 Paramitome, 20 Paranuclein, 22 Paranucleus, 193 Parareticular cells, 350 Parathyroid gland, 147 Parenchyma of testis, 225 Parietal cells, 182 Paroophoron, 246, 248 Parotid gland, 176 Parovarium, 246, 248 Pars cavernosa urethra?, 223 ciliaiis retina?, 346, 354 iridica retina?, 345 membranacea urethra?, 223 optica retina 1 , 346 papillaris of corium, 319 Pars prostatica urethra?, 223 reticularis of corium, 319 Pavement epithelium, 41 Peduncles, cerebellar, 288 Pellicula, 24 Pelvis of kidney, 221 Penicilli, 140 Penis, 235 blood-vessels of, 237 Pepsinogen, 182 Peptic glands, 181 Periaxial space of muscle spindle, 315 Pericardium, 132 Pericellular plexus, 302 Perichondral ossification, 268 Perichondrium, 68, 71 Perilobular space of liver, 201 Perilymph, 365 Perimetrium, 251 Perimysial sheath, 315 Perimysium externum, 274 internum, 274 Perineurium, 300 Periosteal buds, 269 Periosteum, 264 Peripheral lymph-nodule, 138 nervous system, 29!) sensory neurones, origin of, 111 veil, 111 Peritenonium, 277 Peritoneum, 205 Perivascular lymph-spaces, 129, 201 spaces in nervous system, 299 Perivitelline space, 242 Permanent tooth, dental ridge, 162 Perosmic acid, 388 Petit's canal, 356 Peyer's patch, 188 Phagocytes, 60 Phagocytosis, 116 Phalanx, 372 Pharyngeal tonsils, 169 Pharynx, 177 Phototaxis, 27 Pia mater, 297 blood-vessels of, 298 Picric acid, 398 Picrocarmine, 398 Pigment cells, 58 epithelium, 44 in the skin, 322 layer of iris, 345 Pillars of organ of Corti, 370 Pituitary body, 152 Placenta, 257 circulation of blood in, 262 fetalis, 257 uterina s. materna, 257 Placental villi, 257 Plain muscle fibres. Set' Smooth muscle. Plana semilunata, 365 Plasma cells, 59 of blood, 112 Platelets of blood, 117 INDEX. 431 Platelets, preparation for study, 408 Platinum-osrniuin-acetic acid, 389 Pleura;,, 210 Plexus annularis, 360 chorioideus, 298 myentericus, 191 of Auerbach, 191 of Meissner, 192 Plicae conniventes Kerkringii, 185 palmatte, 253 semilunares, 362 villosse, 181 Polar bodies, extrusion of, 33 radiation, 31 Polymorphonuclear leucocytes, 115 Polymorphous cells, 292 Pons, 287 sensory tract in, 288 Porus lactiferus, 337 Positive chemotaxis, 27 Posterior basal membrane, 341 epithelial layer, 342 horn. See Dorsal horn. longitudinal bundle, 289 median septum. See Dorsal median septum. root. See Dorsal root. Precapillary arteries and veins, 122 Pregnancy, 255 Preparation of specimens, 385 Prickle cells, 42 Primary bundles of muscle fibres, 274 follicles, 240 medullary cavity, 269 Primitive fibrils in muscles, 88 in nerves, 104 organ of the fat lobule, 66 ova, 239 Primordial follicles, 240 medullary cavitv, 269 ova, 2.39 Processus ciliaris, 344 Proliferation islands, 260 Prominentia spiralis, 368 Prophase, 29 Prostate, 234 blood-vessels of, 235 interstitial tissue of, 234 nerves of, 235 Prostatic urethra, 223 Protoplasm, 18, 19 Protoplasmic bridges, 43 demonstration of, 402 in heart muscle, 86 in intestinal mucosa, 186 inclusions, 21 processes, 97, 99 Protozoa, 18 Pseudopodia, 25 Pulmonary artery, 210 vein, 210 Pulp cavity, 156 of spleen, 138,139 of tooth, 156 Purkinje cells, 100, 295 Pyloric glands, 183 Pyramidal cells of cortex, 100, 292 tract, 288 crossed, 283 ventral, 283 Pyramids of Ferrein, 212 of Malpighi, 212 AUEKSCHEIBE, 84 RADIAL bundles of nerve fibres, 293 Ramus ascendens of Henle's loop, 213, 215 Ramus descendens of Henle's loop, 213, 215 Ranvier's alcohol, 386 cross, 106 nodes, 105 segments, 105 Raphe 1 of semicircular canals, 365 Rathke's duct, 249 Real interstitial lamellae, 75 Red blood corpuscles, 112 nucleated, 265 number of, 114 bone marrow, 265 muscle, 89 Reduction of chromosomes, 33 Reflex arc, 317 Reissner's membrane, 367 Remak's fibres, 109 Renal ai'tery, 217 Reproduction, 28 Reproductive system, 224 Respiratory bronchioli, 207 epithelium, 207 system, 206 Rete testis (Halleri), 225, 226 Reticular tubular gland, 49 Reticulin, 66 Reticulum, 65 Retina, 346 neuro-epithelial layer, 348 pigment sheath, 347 Retinal vessels, 357 Ripening of the egg, 33 Rod fibre, 348 Rods, 348 Rolando, gelatinous substance of, 279 Root canal, 156 of hair, 322 sheath, 324 Rosenmiiller, organ of, 246, 248 Rotation in protoplasm, 26 Rouleaux, 114 Ruffini's corpuscles, 310 Ruga? of vagina, 263 QACCULUS, 364, 365 U Saccus endolymphaticus, 375 Safranin, 397 Salivary corpuscles, 169 ducts, 171 432 INDEX. Salivary glands, 170, 175 Sarcolemma, 89 Sarcoplasm, 81, 84, 88 function of, 94 Sarcoplasmic discs, 85 Scala tympani, 367 vestibuli, 367 Schlauche of Pfliiger, 240 Schwann's corpuscles, 106 sheath, 104, 106 Sclera, 342 Sebaceous glands, 331 Secondary bundles of muscle fibres, 274 papilla; of tongue, 165 Secretion, 4(> in intestine, 187 internal, 49 Secretory canals, 174 capillaries, 51, 174 in liver cells, 196 oxyntic cells, 183 unit of kidney, 216 of liver, 200 Sectioning of tissues, 387 Sections, 344 Segments of Eanvier, 105 Semen, 227 Semicircular canals, 365 Seminal vesicles, 233 Seminiferous tubules, 225 Sense cells, 307 organs, 318 representation in cortex, 291 Sensory cerebral nerves, nuclei of, 289 epithelium, 37 neurones, peripheral, origin of, 111 nerve-endings, 314 tract in midbrain, 287 Septa placenta;, 261 Septula testis, 224 Septum, dorsal median, 280 medianum dorsale, 280 ventral median, 280 Serial sections, 294 Serosa, 205 Serous glands, 174 tubules, 172 Sertoli cells, 229 Sexual organs, female, 237 male, 224 Shadows of blood cells, 113 Sharpey's fibres, 76, 264 Sheath of Henle, 324 of Huxley, 324 Sheaths of hair root, 323 Short rayed cells, 293 Signet ring cells, 58, 67 Silent area of the brain, 291 Silver bine of Frommann, 105 nitrate staining, 402 Simple branched tubular gland, 48, 49 epithelium, 40 unbranched alveolar gland, 49 tubular gland, 49 Sinus of brain, 329 lactiferus, 337 prostaticus, 235 terminalis of lymph-gland, 136 Sinuses of lymph-gland, 134 Skeletal muscle, 88 system, 264 Skin, 318 blood-vessels, and nerves, 335 Small glands of mouth cavity, 1 76 granular cells of cerebellum, 294 mononuclear leucocytes, 115 pyramidal cells, 292 Smooth muscle, 81 nerve-endings in, 312 Solitary follicles, 138 of intestine, 188 Space of Fontana, 346 of Lange, 319 Spaces, intervillous, 260 Spatia zonularis, 356 Special microscopic technique, 401 Spermatic ducts, 231 Spermatids, 230 Spermatoblasts, 230 Spermatocytes, 230 Spermatogenesis, 229 Spermatogonia, 229 Spermatozoon, 227 development of, 229 Sphincter ani, 190 of common bile duct, 202 pylori, 184 Spider cells, 282 Spinal cord, 278 ganglia, 303 Spindle, Golgi tendon, 278 muscle, 314 nerve, 316 Spleen, 138 blood-vessels of, 140 capsule of, 138 framework of, 140 pulp of, 138 sinuses of, 142 Spongioblasts, 350 Spongy bone, 74 substance, 80 Staining of sections, 395 Stains, acid, basic, and neutral, 116 Statoliths, 366 Status mamillaris, 181 Stellate cells of v. Kupffer, 200 veins of Verheyn, 219 Stigmata, 43 Stilling-Clarke, column of, 279 Stomach, 180 blood-vessels of, 190 lymph-vessels of, 191 nerves of, 191 Stomata, 43 Stratified cylindrical epithelium, 42 epithelium, 41 flat epithelium, 42 INDEX. 433 Stratum corneum, 320, 321 cylindricum, 320 fibrosum of joint capsule, 267 gemrinativum, 320 granulosum, 242, 321 terminale, 85 lucidum, 321 Malpighii, 320 mucosum of uterine muscle, 253 spinosum, 321 subserosum of uterine muscle, 253 supravasculare of uterine muscle, 253 synoviale of joint capsule, 267 vasculare of uterine muscle, 253 zonale, 291 Stria vascularis, 368 Striae acusticse, 290 Striated muscle, 88 histogenesis, 94 nerve-endings in, 312 Striation of muscle, 84, 90 Stroma of blood cell, 113 of hilum of lymph-gland, 163 of ovary, 136 Subchorionic limiting ring, 262 Subcutaneous tissue, 320 Sublingual gland, 176 Submaxillary gland, 175 development of, 175 nerves of, 176 Submucosa of intestine, 189 of oesophagus, 178 of oral mucous membrane, 155 Subpapillary network of vessels, 335 Subserous connective tissue, 205 Substantia adamantia, 160 eburnea, 157 gelatinosa (Eolandi), 279 grisea centralis, 280 lentis, 355 ossea, 161 propria cornese, 341 Succus entericus, 188 prostaticus, 234 Sulcus lateralis dorsalis, 280 ventralis, 367 spiralis externus, 368 internus, 367 Superficial glands of oesophagus, 179 glia mass, 286 Superior cells of auditory neuro-epithe- lium, 366 cerebellar peduncle, 289 Supporting cells of Miiller, 352 of nasal mucous membrane, 379 - of testis, 229 Supraradial network, 293 Suprarenal gland, 147 Sweat glands, 333 Sympathetic ganglia, 302 nerve fibres, 109 nerves, 300 Synarthrosis, 267 Synchondrosis, 267 28 Syncytium, 22, 88 of chorionic villus, 259 Syndesmosis, 267 Synovia, 268 Synovial fluid, 268 villi, 268 TACTILE cells of Merkel, 307 1 corpuscles of Meissner, 309 of Merkel, 306 disc of Grandr/s corpuscle, 308 hairs, 329 menisci, 307 organ, 318 Taeniae coli, 190 Tail of spermatozoon, 228 Tangential fibres in cortex, 291 Tarsal glands, 361 Tarsus, 361 Taste buds, 381 bulbs, 166 organ of, 381 pore, 381 Tear sac, 364 Teasing; of tissues, 386 Technique, microscopic, 383 Teeth, 156 development of, 161 Teichmann's crystals, 120 Tela subcutanea, 319 Telae chorioidese, 298 Telodendria, 99 Telophase, 29, 32 Tendon cells, 57 fibres, 57 spindles, 278 Tendons, 277 Tenon's lymph-space, 359 Tensor chorioideae, 344 Terminal bronchus, 208 Tertiary bundles of muscle fibres, 274 Testes, 224 blood-vessels of, 227 Theca folliculi, 242 Thennotaxis, 27 Thionin, 397 Thoma, ampulla of, 141 Thymus, 143 Thyroid, 144 framework of, 146 Tigroid bodies, 102, 103 Tissues, 35 definition and classification of, 36 Tome's granular sheath, 159 processes, 164 Tongue, 164 muscles of, 167 papillae of, 164 Tonsils, 167 development of, 169 tubal, 376 Tooth cavity, 156 development of, 161 434 INDEX. Tooth, papilla, 161 pulp, 156 sac, 162 Trabecule of lymph-gland, 133 of spleen, 138, 140 , Trachea, 206 Tractus cerebello-spinalis dorsalis, 283 cerebro-spinalis lateralis, 283 ventralis, 283 intermedio-lateralis, 279 solitarius, 290 Transitional leucocytes, 115 Transition zone of stomach, 184 Trapezoid body, 290 Trigeminal nerve, 290 Triple stain of Biondi-Ehrlich, 398 True connective-tissue cells, 57 gastric glands, 181 glands, 46 Tuba uterina Fallopii, 250 Tubal tonsils, 376 Tubular glands, 47, 49 Tubuli contorti testis, 225 recti testis, 225, 226 seminiferi, 225 Tubulo-alveolar gland, 49 Tunica adnata, 224 albuginea of corpora cavernosa, 236 of ovary, 238 testis, 224 externa, 242 fibrosa testis, 224 interna, 242 serosa, 205 vaginalis communis, 224 propria, 224 vasculosa testis, 224 Tympanic cavity, 375 membrane, 376 Tyson's glands, 332 TTLTRAMARINE blue, 400 U Unbrauched alveolar glands, 49 tubular glands, 49 Unformed connective tissue, 63 Unicellular glands, 46 Unipolar cells, 99 transition from bipolar cells, 101 Units of kidney structure, 216 of liver, 200 Unna's elastic tissue stain, 404 Unstriped muscle. See Smooth muscle. Ureter, 221 Urethra, 223 Urinary bladder, 221 passages, 221 system, 212 Uterus, 251 blood-vessels of, 254 changes during menstruation, 254 pregnancy, 255 glands of, 253 lymph-vessels of, 254 Uterus masculinus, 235 muscle of, 253 nerves of, 254 Utriculus, 364, 365 prostaticus, 235 VACUOLES, 21 V Vagina, 262 Valves of the heart, 132 of veins, 128 Valvuke conniventes, 185 Varicosities of nerve fibres, 306 Vasa afferentia (kidney), 218 efferentia (kidney), 218 testis, 231 vasorum, 129 Vas deferens, 232 epididymis, 231, 232 prominens, 368 Vasoformative cells, 123 Vater-Pacini, corpuscles of, 311 Veins, 127 of adrenal, 150 of spleen, 140 precapillary, 122 Vena portse, 198 Venae emissarise, 237 Ventral column, 280, 282, 283 gray commissure, 280 horn, 279 median fissure, 280 pyramidal tract, 283 root, 279 Ventrolateral group of motor cells, 281 Ventro-median group of motor cells, 2S1 Venuke rectee, 220 Verheyn, stillate veins of, 219 Vesicula germinativa, 241 prestation, 235 seminalis, 233 Vestibular glands, 263 nerve, 290 nuclei, 290 Vestibulum of lung, 208 Villi of intestine, 185 of placenta, 257 synovial, 268 Visual cells, 348 organ, 340 Vitreous body, 356 humor, 340, 356 Volkmann's canals, 76 Voluntary muscle, 88 histogenesis of, 94 WANDERING cells, 60 Wax of ear, 377 Weber's organ, 249 Weigert-Pal method, 412 White blood corpuscles, 114 commissure, 280 connective-tissue fibrils, 54 fibrous cartilage, 73 INDEX. 435 White fibrous connective tissue, 54, 63 matter, 278 muscle, 89 substance of cerebellum, 296 Wirsung's duct, 192 Wolffian body, 247 duct, 247 Wood's metal, 401 YELLOW bone marrow, 265 elastic connective tissue, 64 Yolk, 241 ZELLSCHICHT, 259 Zenker's fluid, 390 Zona fasciculata, 148 glomerulosa, 148 pectinate, 369 pellucida, 240 reticularis, 149 tecta, 369 vasculosa of ovary, 238 Zonula ciliaris, 356 Zwischenscheibe, 84, 91 Zymogen granules, 192 Catalogue of Books PUBLISHED BY Lea Brothers & Company, 706, 708 & 710 Sansom St., Philadelphia. 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